Thèses : « Doped Nanocrystals »

1

Sutton, Rebecca Suzanne. « Dual-emitting Cu-doped ZnSe/CdSe nanocrystals ». Thesis, Kansas State University, 2015. http://hdl.handle.net/2097/19047.

Texte intégral

Résumé :

Master of Science
Department of Chemistry
Emily McLaurin
Cu-doped ZnSe/CdSe core/shell nanocrystals were synthesized using the growth doping method. Upon shell growth, the nanocrystals exhibit dual emission. The green luminescence peak is assigned as band edge emission and the broad, lower energy red peak is due to Cu dopant. Although, the oxidation state of Cu in the nanocrystals is debated, the emission is explained as recombination of a hole related to Cu²⁺ with an electron from the conduction band. The emission changed in the presence of dodecanethiol. Generally, the band edge emission intensity decreases and the Cu emission intensity increases. One explanation is the thiol acts as a hole trap, preventing hole transfer to the conduction band. Samples were obtained with varying amounts of Cd²⁺. In the presence of larger amounts of Cd²⁺, the nanocrystals had “thicker shells”, and both the band edge and Cu emission were less sensitive to thiol. The sensitivity likely decreased because the shelled, larger nanocrystals have fewer surface defects resulting in more available electrons.

Styles APA, Harvard, Vancouver, ISO, etc.

2

PINCHETTI, VALERIO. « Advanced Spectroscopy of Interface Engineered, Doped and “Electronically” Doped Colloidal Semiconductor Nanocrystals ». Doctoral thesis, Università degli Studi di Milano-Bicocca, 2018. http://hdl.handle.net/10281/199097.

Texte intégral

Résumé :

I nanocristalli colloidali a semiconduttore (NC) sono materiali processabili da soluzione che, dalla loro scoperta 30 anni fa, hanno attirato l’attenzione in campo scientifico e tecnologico per le loro proprietà ottiche ed elettriche. Infatti, i NC hanno un ampio range di potenziali applicazioni, che vanno dalle sorgenti luminose, alle celle solari, al bioimaging fino all’informazione quantistica. Ciò è dovuto alla profonda conoscenza e controllo delle loro proprietà elettroniche che si è raggiunto. Infatti, queste ultime si possono modificare controllando la dimensione, la composizione ma anche formando eterostrutture o introducendo impurezze, cioè drogando i NC. A causa dell’ampia varietà di NC che si possono sintetizzare, molti dubbi sui processi fotofisici sottostanti le proprietà ottiche macroscopiche rimangono ancora irrisolti. Dunque, mi sono focalizzato sullo studio di tre sotto-classi di NC: 1) a interfaccia ingegnerizzata; 2) drogati e 3) drogati elettronicamente. Dopo un breve ‘stato dell’arte’ della scienza dei NC colloidali (Capitolo 1), nel secondo Capitolo riporto una studio dettagliato dell’interazione fra i portatori di carica eccitati e l’interfaccia ingegnerizzata dei Dot-in-Bulk core/shell NC, che sono caratterizzati da emissione di fotoluminescenza (PL) sia dagli stati di core che da quelli di shell. Tramite misure di PL ultraveloce, dimostro che la caratteristica struttura all’interfaccia è la motivazione ultima da cui scaturisce la capacità di avere una doppia emissione radiativa, aggiungendo un ulteriore parametro nella chimica dei NC con il quale è possibile modificare le loro proprietà ottiche. Nel Capitolo 3, propongo una nuova strategia di sintesi che permetta di avere NC contenenti tutti un esatto numero di atomi droganti, evitando la distribuzione Poissoniana tipica dei contemporanei metodi di drogaggio. A questo scopo, uso cluster metallici monodispersi come semi di nucleazione per la sintesi dei NC e tramite analisi elementali ed ottiche mostro che effettivamente ogni NC sintetizzato contiene un solo cluster metallico e quindi un numero preciso di impurezze. Il drogaggio può essere considerato ‘isovalente’ nel caso in cui l’impurezza abbia lo stesso stato di ossidazione del semiconduttore, o ‘elettronico’ nel caso questa introduca una carica netta nella matrice ospitante. Il drogante isovalente più noto per i NC II-VI è il Mn2+. La sua configurazione elettronica d5 è caratterizzata da proprietà magnetiche uniche che, in strutture confinate quanticamente porta alla formazione di polaroni. Nel Capitolo 4, mostro come la formazione di polaroni tocca l’energia degli eccitoni tramite misure di PL risonante, ottenendo anche una stima precisa dell’intensità di campo magnetico generata solo dagli ioni Mn2+. Nel Capitolo 5, mostro come la risposta magnetica tipica del Mn2+ si può ottenere anche con l’argento, che è un drogante elettronico in quanto può assumere solo lo stato di ossidazione +1. L’argento però introduce uno stato nel gap energetico del semiconduttore ospitante che partecipa alla ricombinazione radiativa diventando, in modo transiente, un Ag2+ paramagnetico. Tramite misure di dicroismo circolare magnetico, dimostro che NC drogati con impurezze non magnetiche di argento possono assumere comportamenti paramagnetici attivati otticamente. Infine, nel Capitolo 6 ho focalizzato l’attenzione sui NC non tossici di CuInS2. I processi fotofisici alla base del meccanismo di emissione sono ancora dibattuti. A questo scopo, ho eseguito misure di PL risolta in temperatura e di spettroelettrochimica per studiare le dinamiche intrinseche ed estrinseche di questa classe di NC colloidali di ultima generazione.
Semiconductor colloidal nanocrystals (NCs) are solution-processable materials that have focused scientific and technological attention thanks to their tunable optical and electrical properties. Colloidal NCs have indeed wide applicative perspectives that span from light-emitting diodes, to lasers, from solar cells to luminescent solar concentrators, from bioimaging to quantum information. Such a large range of potential NCs technologies is warranted by the unique knowledge and control that has been achieved over the years about their electronic properties. Specifically, the optical and electric properties of these nanomaterials have been tuned by either controlling their size, composition and shape, or producing multicomponent heterostructures and introducing few atoms of a different chemical element, i.e. doping the NCs. Because of the vast gamut of possibilities that colloidal NCs offer, many questions on the elusive charge carrier dynamics underlying the macroscopic observations are still unanswered. In this picture, my work points toward three different sub-classes of NCs: i) interface engineered NCs; ii) doped NCs and iii) ‘electronic’ doped NCs. After a brief review about the ‘state of the art’ of the colloidal NC science (Chap. 1), in Chap. 2 I show a detailed investigation on the interaction between the photoexcited charge carriers and the engineered interface of Dot-in-Bulk core/shell NC, which are featured by radiative recombination from both the core and shell states. I demonstrate that their uncommon dual emission is due to the peculiar interface structure between the compositional domains and that a fine tuning of the optical properties can be also achieved by modifying the interfacial potential profile. In Chap. 3, I propose a novel synthetic approach to overcome the intrinsic Poisson distribution characteristic of the up-to-date NC doping strategies that are based on stochastic distribution of impurity ions in the NC ensemble. To this aim, I use monodispersed metal cluster as seeds for the NC nucleation in the synthesis reaction flask. By mean of combined optical and elemental analysis, I show that the copper clusters composed of exactly four atoms are indeed embedded in the semiconductor matrix, giving monodispersed doped NCs. Semiconductor doping can be further distinguished in ‘isovalent’ doping, in which the impurity has the same oxidation state of the host compound, and ‘electronic’ doping, given by ions which introduce a net charge in the surrounding matrix. The most known ‘isovalent’ dopant for II-VI NCs is Mn2+. Its d5 configuration is featured by unique magnetic properties that, in quantum confined nanomaterials lead to the formation of magnetic polarons. In Chap. 4, I reveal how polaron formation affects the exciton energy by mean of resonant PL measurements, offering a precise estimation of the intensity of the internal magnetic field generated by the Mn2+ spins. In Chap. 5, I report how the magnetic response typical of Mn2+ is reproduced by introducing silver, which is an electronic dopant for II-VI semiconductors, since it can only assume the +1 oxidation state. However, it introduces an electronic level in the forbidden energy gap of the host semiconductor that participates to the radiative recombination and therefore transiently switches to the paramagnetic +2 state. By mean of magnetic circular dichroism experiments I demonstrate that in NCs doped with nonmagnetic silver dopants, the paramagnetic response is completely optically activated. Finally, in Chap. 6 I focused the attention on non toxic, ternary CuInS2 colloidal NCs. The photophysical processes underlying their emission mechanism are, however, still under debate. To address this gap, I carried out temperature-controlled photoluminescence and spectro-electrochemical experiments to unravel the intrinsic and extrinsic charge carrier dynamics of this last-generation class of colloidal N

Styles APA, Harvard, Vancouver, ISO, etc.

3

Kriegel, Ilka. « Near-infrared plasmonics with vacancy doped semiconductor nanocrystals ». Diss., Ludwig-Maximilians-Universität München, 2013. http://nbn-resolving.de/urn:nbn:de:bvb:19-164558.

Texte intégral

Résumé :

Plasmonics with heavily doped semiconductor nanocrystals (NCs) is an emerging field in NC science. However, impurity doping of NCs remains far from trivial and is, as yet, dominated by a low chemical control over the incorporated dopant atoms. An appealing alternative is vacancy doping, where the formation of vacancies in the structure is responsible for an increased carrier density and elegantly circumvents the issues related to impurity doping. Due to high carrier densities of around 10^21cm^(-3) localized surface plasmon resonances (LSPRs) in the near infrared (NIR) are expected, and as such highlighted to close the gap between conventionally doped NCs and noble metal nanoparticles. Copper chalcogenide NCs, namely copper sulfide (Cu2-xS), copper selenide (Cu2-xSe), and copper telluride (Cu2-xTe), are an attractive example of vacancy doped semiconductor NCs, with spectra dominated by intense NIR resonances. Within this study thorough experimental evidence has been given to prove the plasmonic nature of those NIR resonances. By presenting typical plasmonic characteristics, such as refractive index sensitivity of the LSPR, its intrinsic size dependence, plasmon dynamics, or interparticle plasmon coupling, the LSPRs in copper chalcogenide NCs have unambiguously been identified. The chemical nature of vacancy doping turns out to deliver an additional, highly attractive means of control over the LSPR in vacancy doped copper chalcogenide NCs. Through chemical tailoring of the copper vacancy density via controlled oxidation and reduction, as shown in this study, a reversible tuning of the LSPR over a wide range of frequencies in the NIR (1000-2000 nm) becomes feasible. This highlights copper chalcogenide NCs over conventional plasmonic materials. Notably, the complete suppression of the LSPR uncovers the excitonic features present only in the purely semiconducting, un-doped NCs and reveals the unique option to selectively address excitons and highly tunable LSPRs in one material (bandgap Eg~1.2 eV). As such, copper chalcogenide NCs appear to hold as an attractive material system for the investigation of exciton plasmon interactions. Indeed, a quenching of the excitonic transitions in the presence of the developing LSPR is demonstrated within this work, with a full recovery of the initial excitonic properties upon its suppression. A theoretical study on the shape dependent plasmonic properties of Cu2-xTe NCs reveals a deviation from the usual Drude model and suggests that the carriers in vacancy doped copper chalcogenide NCs cannot be treated as fully free. On the other hand, the Lorentz model of localized oscillators appears to account for the weak shape dependence, as observed experimentally, indicating an essential degree of localization of the carriers in vacancy doped copper chalcogenide NCs. Taken together, this work delivers a huge step toward the complete optical and structural characterization of plasmonic copper chalcogenide NCs. The advantages of semiconductor NC chemistry have been exploited to provide access to novel plasmonic shapes, such as tetrapods that have not been feasible to produce so far. A precise size, shape and phase control presents the basis for this study, and together with a thorough theoretical investigation delivers important aspects to uncover the tunable plasmonic properties of vacancy doped copper chalcogenide NCs.

Styles APA, Harvard, Vancouver, ISO, etc.

4

Nordin, Muhammad N. « Magneto optical study of undoped and doped PbS nanocrystals ». Thesis, University of Surrey, 2013. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.606691.

Texte intégral

Résumé :

Magneto optical studies of colloidal PbS nanocrystals (NCs) have been undertaken to determine their fundamental properties. Measurements including absorption, photoluminescence (PL) and PL lifetime decay are presented along with their dependence upon temperature, magnetic field, and magnetic dopant concentration for undoped and doped PbS NCs. Temperature dependence of undoped PbS NCs, recorded from 300 K down to 3 K, displays a Stoke shift increasing from ~7 5 meV to ~125 meV which is fitted using a three-level rate equation model, supported by PL lifetime decay measurements, that indicate energy separation of ~ 6.0 ± 0.3 meV between the two optically active levels within PbS NCs. Magneto optical studies of undoped PbS NCs at low temperature using a field sweep from -7 Tesla to 7 Tesla are presented. Analysis of the magneto-PL data yields a degree of circular polarization (DCP) of 33% at 7 T and 2 K. Further analysis predicts an excitonic g-factor, gu for the -4 nm diameter PbS NCs of ....().54 by taking account of random orientation of PbS NCs. Using this value of g~x the expected Zeeman splitting at 7T ,ΔEzeeman , is calculated to be ~0.22 meV. Optical studies of PbS NCs with TIFffCNQ molecule showed modification of the PL spectra and PL lifetime. It is proposed that the quenching effect on the PL of PbS-TTF in the range of 900 nm to 1300 nm is due to a charge transfer mechanism . The PL obtained from PbS-TCNQ solutions display a second emission peak centred at ~700 nm which is directly related to the TCNQ concentration. A study of the optical properties of Mn-implanted PbS NCs was undertaken and compared with that of undoped PbS NCs. The PL spectra of all Mn-implanted PbS NCs showed a significant red shift of the PL peak compared to undoped PbS NCs. Based on fitting of a Brillouin function to the difference in the temperature dependent Stokes shift between the Mn-implanted and undoped PbS NCs an effective exchange field of ~81 T is predicted.

Styles APA, Harvard, Vancouver, ISO, etc.

5

Lounis, Sebastien Dahmane. « The influence of dopant distribution on the optoelectronic properties of tin-doped indium oxide nanocrystals and nanocrystal films ». Thesis, University of California, Berkeley, 2015. http://pqdtopen.proquest.com/#viewpdf?dispub=3686398.

Texte intégral

Résumé :

Colloidally prepared nanocrystals of transparent conducting oxide (TCO) semiconductors have emerged in the past decade as an exciting new class of plasmonic materials. In recent years, there has been tremendous progress in developing synthetic methods for the growth of these nanocrystals, basic characterization of their properties, and their successful integration into optoelectronic and electrochemical devices. However, many fundamental questions remain about the physics of localized surface plasmon resonance (LSPR) in these materials, and how their optoelectronic properties derive from their underlying structural properties. In particular, the influence of the concentration and distribution of dopant ions and compensating defects on the optoelectronic properties of TCO nanocrystals has seen little investigation.

Indium tin oxide (ITO) is the most widely studied and commercially deployed TCO. Herein we investigate the role of the distribution of tin dopants on the optoelectronic properties of colloidally prepared ITO nanocrystals. Owing to a high free electron density, ITO nanocrystals display strong LSPR absorption in the near infrared. Depending on the particular organic ligands used, they are soluble in various solvents and can readily be integrated into densely packed nanocrystal films with high conductivities. Using a combination of spectroscopic techniques, modeling and simulation of the optical properties of the nanocrystals using the Drude model, and transport measurements, it is demonstrated herein that the radial distribution of tin dopants has a strong effect on the optoelectronic properties of ITO nanocrystals.

ITO nanocrystals were synthesized in both surface-segregated and uniformly distributed dopant profiles. Temperature dependent measurements of optical absorbance were first combined with Drude modeling to extract the internal electrical properties of the ITO nanocrystals, demonstrating that they are well-behaved degenerately doped semiconductors displaying finite conductivity at low temperature and room temperature conductivity reduced by one order of magnitude from that of high-quality thin film ITO.

Synchrotron based x-ray photoelectron spectroscopy (XPS) was then employed to perform detailed depth profiling of the elemental composition of ITO nanocrystals, confirming the degree of dopant surface-segregation. Based on free carrier concentrations extracted from Drude fitting of LSPR absorbance, an inverse correlation was found between surface segregation of tin and overall dopant activation. Furthermore, radial distribution of dopants was found to significantly affect the lineshape and quality factor of the LSPR absorbance. ITO nanocrystals with highly surface segregated dopants displayed symmetric LSPRs with high quality factors, while uniformly doped ITO nanocrystals displayed asymmetric LSPRs with reduced quality factors. These effects are attributed to damping of the plasmon by Coulombic scattering off ionized dopant impurities.

Finally, the distribution of dopants is also found to influence the conductivity of ITO nanocrystal films. Films made from nanocrystals with a high degree of surface segregation demonstrated one order of magnitude higher conductivity than those based on uniformly doped crystals. However, no evidence was found for differences in the surface electronic structure from one type of crystal to the other based on XPS and the exact mechanism for this difference is still not understood.

Several future studies to further illuminate the influence of dopant distribution on ITO nanocrystals are suggested. Using synchrotron radiation, detailed photoelectron spectroscopy on clean ITO nanocrystal surfaces, single-nanoparticle optical measurements, and hard x-ray structural studies will all be instructive in elucidating the interaction between oscillating free electrons and defect scattering centers when a plasmon is excited. In addition, measurements of temperature and surface treatment-dependent conductivity with carefully controlled atmosphere and surface chemistry will be needed in order to better understand the transport properties of ITO nanocrystal films. Each of these studies will enable better fundamental knowledge of the plasmonic properties of nanostructures and improve the development of nanocrystal based plasmonic devices.

Styles APA, Harvard, Vancouver, ISO, etc.

6

Clark, Maurice Tzeng Y. « Growth and characterization of nitrogen doped nanocrystalline diamond films ». Auburn, Ala., 2006. http://hdl.handle.net/10415/1313.

Texte intégral

Styles APA, Harvard, Vancouver, ISO, etc.

7

Kriegel, Ilka [Verfasser], et Jochen [Akademischer Betreuer] Feldmann. « Near-infrared plasmonics with vacancy doped semiconductor nanocrystals / Ilka Kriegel. Betreuer : Jochen Feldmann ». München : Universitätsbibliothek der Ludwig-Maximilians-Universität, 2013. http://d-nb.info/1046503316/34.

Texte intégral

Styles APA, Harvard, Vancouver, ISO, etc.

8

Chen, Xiaobo. « Synthesis and Investigation of Novel Nanomaterials for Improved Photocatalysis ». Case Western Reserve University School of Graduate Studies / OhioLINK, 2005. http://rave.ohiolink.edu/etdc/view?acc_num=case1117575871.

Texte intégral

Styles APA, Harvard, Vancouver, ISO, etc.

9

ROSINA, IRENE. « Exploiting Cation Exchange Reactions in Doped Colloidal NIR Semiconductor Nanocrystals : from synthesis to applications ». Doctoral thesis, Università degli studi di Genova, 2020. http://hdl.handle.net/11567/1019427.

Texte intégral

Résumé :

Colloidal quantum dots (CQDs) have tunable optical properties through manipulation of their size, shape, and surface chemistry. Among pholuminescent QDs, near-infrared (NIR) emitting ones are of particular interest since they can be used in several applications, from the labeling in living tissues, to the integration in commercial optoelectronic devices, like photovoltaics for solar energy conversion or photodetectors from visible to the near-infrared and mid-infrared. In addition, the exciting promise of CQDs is that is associated with easy and low-cost device fabrication process. In fact, solution-based techniques like spin-coating, dip coating and ink-jet printing are typically used for solution CQDs readily to be used in large-area processing techniques. Thus, to obtain an ink solution of nanocrystals (NCs) ready to be used in device fabrication process, in this thesis, cation exchange (CE) reactions have been used as a convenient tool to finely transform NCs directly in solution or deposited as thin films. These reactions allow to substitute a fraction or all “host” metal cations of pre-synthesized NCs with new “guest” cations while preserving both NCs’ size, shape and, typically, crystal structure. Depending on the miscibility of the reactant and product materials, and on the kinetics of the CE reaction, different types of nanostructures can be accessed ranging from alloy NCs, doped systems, dimers, core@shell (or core@graded-shell) heterostructures even with elaborated architectures (i.e., quantum wells, multiple-cores@shell). Unlike ion substitution in solids, cation exchange at nanoscale results in fast reaction rate and an easy modulation of the thermodynamics through selective ion coordination in solution. This study provides an overview of the CE on semiconductor NCs, in particular on II-VI, I-III-VI2 and III-VI compounds. We first explore the exchange between cadmium chalcogenides and mercury ions to produce Cd1-xHgxTe CQDs which can be potentially employed in NIR photodetectors and photovoltaic devices. Our developed synthesis is a result of a wide systematic investigation process, in which we varied specific physical parameters, such as the reaction temperature, the feed molar ratio of the precursor and the solvent. More specifically, these aspects were studied to have control on the size, shape, surface composition and crystalline phase after mild conditions of annealing into stable connected crystals. This peculiarity could be exploited to boost the photogenerated charges diffusion in polycrystalline photoconducting films fabricated by means of an ink of NCs solution. Additionally, another aspect studied was the surface passivation of Cd1-xHgxTe colloidal NCs, in order to understand how to optimize the charge transfer efficiency among the nanocrystals. The carrier transport in QD devices differs fundamentally from band transport in bulk semiconductors. In nanocrystal film it is of fundamental relevance to improve the mobility of the photogenerated charges. Noteworthy, the granularity of the system and the consequent coupling between adjacent dots can produce additional physical parameters, as charge recombination. The carrier diffusion length can be limited by trapping sites1. To overcome these limitations, post-synthetic strategies that couple the high quality NCs solutions with ideal properties (band gap, absorption, monodispersivity) and high-quality films (quantum dot packing, passivation, and absorptive/conductive properties) are necessary. Indeed, to improve the inter-NCs conductivity in a NC film, ligand exchange and stripping procedures are widely used, with the aim of replacing insulating surfactants with more conductive species. These procedures have some drawbacks, for example metal cations can desorb from the surface of the NCs during the stripping. On the contrary, here we will show how our nano heterostructures (NHCs) enable to avoid the post-process ligand stripping and to perform the final annealing step in milder conditions. Above these considerations, CE can be exploited to address NCs solution through surface uniformity from the nano- to the macroscopic scale. This is the first step toward electronic coupling between the separate building blocks of nanocrystals. Apart from III-V QDs, we shifted our research activity on valid alternative material which do not contain toxic heavy metals such as Cd, Pb, As or Hg, and that offer a high flexibility for tuning band gap in the NIR window. In chapter 5, the results about the study of a III-V system are reported. Thus, we studied InP system, which is probably the only one that could provide a compatible emission color range similar to that of Cd-based QDs but without intrinsic toxicity. Nevertheless, the synthesis of III-V NCs, due to their covalent-bond character, is limited by long reaction times or an uncontrollably fast nucleation that may lead to the formation of amorphous or bulk compounds. The role of our work is to explore the reported InP synthesis and to further improve the luminescent properties of these systems Here we study the effect of different parameter (molar concentration in reaction mixture, the use of different phosphorous precursors) to enhance the control over the particle size and size distribution. After that, we studied different Sulphur source precursors to obtain InP@ZnS core@shell NCs with high quantum Yield (QY). In the last chapter, we describe also I-III-VI2 system as CuInS2 for photoluminescence modulation. In this Chapter Copper Indium Sulfide nanocrystals are prepared using a single-step heating up method relying on the low thermal stability of ter- dodecanethiol used as stabilizing agent, solvent, and sulfur precursors. The obtained particles exhibit an emission varying from 710 to 940 nm. This range depends on the extent of the heating time (pre-heating) before the threshold temperature of 230°C for the growth process of ternary semiconductor NCs such as of CIS nanocrystals. Afterwards we report on the procedure for the growth of a ZnS shell, which enables a blueshift of the PL emission wavelength with respect to those of their parent CIS, due to the widening of the band gap for the entrance of zinc ions into the CIS structures.

Styles APA, Harvard, Vancouver, ISO, etc.

10

Archer, Paul I. « Building on the hot-injection architecture : giving worth to alternative nanocrystal syntheses / ». Thesis, Connect to this title online ; UW restricted, 2007. http://hdl.handle.net/1773/8520.

Texte intégral

Styles APA, Harvard, Vancouver, ISO, etc.

11

Ruhge, Forrest. « EFFECT OF GERMANIUM DOPING ON ERBIUM SENSITIZATION IN THE ERBIUM DOPED SILICON RICH SILICA MATERIAL SYSTEM ». Master's thesis, University of Central Florida, 2006. http://digital.library.ucf.edu/cdm/ref/collection/ETD/id/2933.

Texte intégral

Résumé :

The continued size reduction in electronic integrated circuits has lead to a demand for on-chip high-bandwidth and low loss communication channels. Optical interconnects are considered an essential addition to the silicon electronics platform. A major challenge in the field of integrated Si photonics is the development of cost effective silicon compatible light sources. This thesis investigates the sensitization of group IV doped silica films emitting at 1.535μm for applications as silicon compatible light sources. Thin erbium-doped silica films containing excess silicon and germanium were deposited using a multi-gun sputter system. The composition of the deposited materials was verified by Rutherford Backscattering Spectrometry. Samples from each deposition were annealed in a controlled atmosphere tube furnace at temperatures between 500ºC and 1100ºC for 30 minutes. The photoluminescence spectra from the visible to the near-infrared region were acquired while pumping either near or far from the Er3+ absorption lines. Under both excitation conditions all samples annealed at temperatures below 1000ºC show clear emission at 1.535μm from Er3+ ions in the host material. In the current literature this is attributed to exciton mediated excitation of the Er3+. By contrast, in these studies indirect excitation was observed for samples annealed at temperatures well below the onset of nanocrystal nucleation and growth (between 500ºC and 1000ºC), suggesting excitation via small clusters or lattice defects. These findings could have significant implications in the further development of group IV sensitized silicon compatible gain media.
M.S.
Other
Optics and Photonics
Optics

Styles APA, Harvard, Vancouver, ISO, etc.

12

Kaleli, Buket. « Towards Silicon Based Light Emitting Devices : Photoluminescence From Terbium Doped Silicon Matrices With Or Without Nanocrystals ». Master's thesis, METU, 2009. http://etd.lib.metu.edu.tr/upload/3/12610614/index.pdf.

Texte intégral

Résumé :

In this study, silicon (Si) rich silicon dioxide (SiO2) films and terbium (Tb) embedded in three different Si containing films has been produced by e-beam evaporation and magnetron sputtering techniques. Post deposition annealing was done for different temperatures and durations to study its effect on both Si nanocrystal formation and Tb luminescence. It was verified by X-ray diffraction technique (XRD) that Si nanocrystals were formed in Si rich matrices. Energy dispersive X-ray (EDS) spectroscopy analysis was carried out to determine the relative concentrations of the atoms inside the produced films. X-ray photoelectron spectroscopy (XPS) gave the evidence of different bonding structures inside the Tb-Si-O containing films. Depth profile measurements were carried out to analyze changes in the relative concentration during sputtering of the layers after annealing of the Tb containing film. Luminescence characteristics of Si nanocrystals and Tb3+ ions were studied by photoluminescence (PL) spectroscopy. It was observed that Tb3+ luminescence enhanced by an energy transfer from Si nanocrystals and trap levels in a matrix. This result supplies valuable information about the excitation paths of Tb3+ ion the way of intense luminescence.

Styles APA, Harvard, Vancouver, ISO, etc.

13

Muckel, Franziska [Verfasser], et Gerd [Akademischer Betreuer] Bacher. « Transition metal doped colloidal semiconductor nanocrystals : from functionality to device development / Franziska Muckel ; Betreuer : Gerd Bacher ». Duisburg, 2018. http://d-nb.info/1155722787/34.

Texte intégral

Styles APA, Harvard, Vancouver, ISO, etc.

14

Muckel, Franziska Elisabeth [Verfasser], et Gerd [Akademischer Betreuer] Bacher. « Transition metal doped colloidal semiconductor nanocrystals : from functionality to device development / Franziska Muckel ; Betreuer : Gerd Bacher ». Duisburg, 2018. http://d-nb.info/1155722787/34.

Texte intégral

Styles APA, Harvard, Vancouver, ISO, etc.

15

Crockett, Brandon. « Mechanistic insights into structure-property relationships of doped metal oxide nanocrystals produced from a continuous growth synthesis ». Thesis, University of Oregon, 2019. http://hdl.handle.net/1794/24556.

Texte intégral

Résumé :

Colloidal metal oxide nanocrystals have tremendous potential to solve some of the world’s biggest problems in energy storage and harvesting, medicine, catalysis, electronics, and information technology. Colloidal metal oxide nanocrystals display unique size- and structure-dependent properties that differ from the bulk materials. The incorporation and utilization of these nanomaterials into modern technology hinges upon chemists’ abilities to synthesize the nanomaterials with atomic-level precision and control over size, morphology, composition, and surface chemistry. To this end, advances in synthetic development hold the keys to providing high-performance nanomaterials to solve global problems. This dissertation focuses on new synthetic approaches to producing doped-metal oxide nanocrystals, using a continuous growth synthesis. The synthesis allows for nanocrystals to be grown layer-by-layer at nearly the atomic level, much akin to atomic layer deposition in solid state chemistry and living polymerizations in polymer chemistry. This layer-by-layer growth allows for metal oxide nanocrystals to be synthesized with angstrom-level control over size, composition, and distribution of dopant atoms. This level of structural control has produced significant advancements in the investigations of structure/property relationships. With In2O3 as a model system, in this dissertation nanocrystals are shown to exhibit composition-dependent optical properties, size-dependent electrical properties, and dopant distribution-dependent plasmonic properties. This dissertation begins with a brief introduction outlining current challenges chemists face in synthesizing metal oxide nanocrystals. The advances in continuous growth synthesis developed in the Hutchison laboratory are then discussed. Additionally, the technological relevance of In2O3 (the focus material of this dissertation) is highlighted. The following chapters demonstrate new investigations only made possible through the continuous growth synthesis. First, improvements in nanocrystal composition is demonstrated, through the doping of In2O3 nanocrystals with a variety of transition metal dopant atoms, and the dopant atom incorporations are shown to be stoichiometric, and the dopants are homogenously distributed. Next, nanometer-level control over Sn-doped In2O3 is demonstrated, in order to relate thin film resistivity to nanocrystal diameter. Finally, control over radial distribution of dopants is demonstrated in Sn-doped In2O3 and highlights the striking influence the dopant distributions exhibit on the plasmonic properties of the nanocrystals. This dissertation contains previously published and unpublished co-authored material.
2021-04-30

Styles APA, Harvard, Vancouver, ISO, etc.

16

Seregin, Vladimir Victor. « Part I, Fabrication and surface modification of composite biomaterials based on silicon and calcium disilicide Part II, Synthesis and characterization of erbium doped silicon nanocrystals encapsulated by aluminum and zinc oxides / ». Fort Worth, Tex. : Texas Christian University, 2006. http://etd.tcu.edu/etdfiles/available/etd-04252006-145309/unrestricted/seregin.pdf.

Texte intégral

Styles APA, Harvard, Vancouver, ISO, etc.

17

Kim, Changsu. « Optical, laser spectroscopic, and electrical characterization of transition metal doped ZnSe and ZnS nano- and microcrystals ». Birmingham, Ala. : University of Alabama at Birmingham, 2009. https://www.mhsl.uab.edu/dt/2009r/kim.pdf.

Texte intégral

Résumé :

Thesis (Ph. D.)--University of Alabama at Birmingham, 2009.
Title from PDF title page (viewed Feb. 3, 2010). Additional advisors: Renato Camata, Derrick Dean, Chris M. Lawson, Andrei Stanishevsky, Sergey Vyazovkin. Includes bibliographical references (p. 133-140).

Styles APA, Harvard, Vancouver, ISO, etc.

18

DE, TRIZIO LUCA. « Polymer nanocomposites for illumination : towards warm white light ». Doctoral thesis, Università degli Studi di Milano-Bicocca, 2013. http://hdl.handle.net/10281/41175.

Texte intégral

Résumé :

The aim of this PhD project, also called “warm white light project”, is to propose an alternative solid state lighting device that works using the same principles of color-conversion WLEDs. The idea is to create a polymer based nanocomposite that could work as a transparent glass in off-mode and as a “lamp” in on-mode. The new concept is to spatially decouple the light source and the converters in order to avoid all the thermal problems that affect the current LEDs (in which the color is “generated” within the chip and the epoxy case). To do so, a blue LED, made of Indium gallium nitride (InGaN), is coupled to a bulk polymer nanocomposite sheet where the actual conversion of light takes place. For emitting white light it is necessary that the nanocomposite would not only convert partially the blue light into warmer colors (like yellow and red) but also diffuse the blue light outside in order to have a color mixing.

Styles APA, Harvard, Vancouver, ISO, etc.

19

Gul, Sheraz. « Synthesis, Optical and Structural Characterization, and Exciton Dynamics of Doped ZnSe Nanocrystals, and, Simultaneous X-ray Emission Spectroscopy of Two Elements Using Energy Dispersive Spectrometer ». Thesis, University of California, Santa Cruz, 2014. http://pqdtopen.proquest.com/#viewpdf?dispub=3630692.

Texte intégral

Résumé :

Doped semiconductor quantum dots (QDs) comprise an important subclass of nanomaterials in which a small quantity of impurity is added intentionally, adding another degree of freedom to alter their size-dependent physical and electronic properties. Intense, tunable, long lived and stable photoluminescence make them quintessential candidates for many opto-electronic applications including solid-state lighting, display devices and biomedical imaging. ZnSe QDs, which are blue-emitting fluorophores, were doped with Cu⁺¹ to redshift their photoluminescence (PL) to green region of the visible spectrum. These Cu-doped ZnSe QDs were then codoped with Al³⁺, Ga³⁺ and In³⁺ to improve the PL quantum yield (QY) by eliminating the defect states originating from charge imbalance created by aliovalent doping. Codoping also resulted in further redshifting of the PL, covering most of the visible spectrum, making them potential candidates for use in solid-state lighting and as optical down converters in next generation light emitting diodes (LEDs). To better understand the optical properties of these materials, local structure around the luminescent centers was investigated by extended X-ray absorption fine structure (EXAFS). Cu was found to occupy a distorted tetrahedral site with the codopant residing in a substitutional Zn site. Based on the structural information obtained by EXAFS, density functional theory calculations (DFT) were performed to get a clear picture of the energy levels associated with the electronic transitions. Furthermore, the dynamics studies of the exciton and charge carriers were carried out to get deeper insight of the various photophysical processes involved. The fluorescence lifetime was increased approximately 10 times after doping.

The multielectron catalytic reactions often involve multimetallic clusters, where the reaction is controlled by the electronic and spin coupling between metals and ligands to facilitate charge transfer, bond formation/breaking, substrate binding, and release of products. A method was developed to detect X-ray emission signal from multiple elements simultaneously to probe the electronic structure and sequential chemistry that occurs between the elements. A wavelength dispersive spectrometer based on the von-Hamos geometry was used, that disperses Kβ emission signals of multiple elements onto an area detector, and enables an XES spectrum to be measured in a single-shot mode. This overcomes the scanning needs of the Rowland circle spectrometers, and the data is free from temporal and normalization errors, and therefore ideal to follow sequential chemistry at multiple sites. This method was applied to MnO_x based electrocatalysts, and the effect of Ni addition was investigated. Electro-deposited Mn oxide catalyses oxygen-evolution reaction (OER) and oxygen-reduction reaction (ORR) at different electrochemical potentials under alkaline condition. Incorporation of Ni reduced the low valent Mn component resulting in higher average oxidation state of Mn in MnNiOx under ORR and OER conditions, when compared to MnO_x under similar conditions. The reversibility of the electrocatalyst was also found to improve by the inclusion of Ni.

Styles APA, Harvard, Vancouver, ISO, etc.

20

Liu, William K. « Electron spin dynamics in quantum dots, and the roles of charge transfer excited states in diluted magnetic semiconductors / ». Thesis, Connect to this title online ; UW restricted, 2007. http://hdl.handle.net/1773/8588.

Texte intégral

Styles APA, Harvard, Vancouver, ISO, etc.

21

Icli, Kerem Cagatay. « Core-shell Type Nanocrystalline Fto Photoanodes For Dye Sensitized Solar Cells ». Master's thesis, METU, 2010. http://etd.lib.metu.edu.tr/upload/12612328/index.pdf.

Texte intégral

Résumé :

Aim of this work is to construct dye sensitized solar cells employing core shell type nanocrystalline FTO/TiO2 photoanodes. Fluorine doped tin dioxide (FTO) nanoparticles were synthesized under hydrothermal conditions. Homogeneously precipitated SnO2 nanoparticles were dispersed in aqueous solutions containing NH4F as fluorine source and heat treated at 180oC for 24 hours. X-Ray analysis revealed that particles show rutile type cassiterite structure. Particles had 50 m2/g specific surface area measured by BET. Particle size was around 15-20 nm verified by XRD, BET and SEM analysis. Electrical resistivity of the powders measured with four point probe technique was around 770 ohm.cm for an F/Sn atomic ratio of 5, which showed no further decrease upon increasing the fluorine content of solutions. Thick films were deposited by screen printing technique and SEM studies revealed that agglomeration was present in the films which decreased the visible light transmission measured by UV-Visible spectrophotometry. TiO2 shell coating was deposited by hydrolysis of ammonium hexafluorotitanate and TiCl4 aqueous solutions. Efficiency of FTO nanoparticles was enhanced upon surface treatment where best result was 4.61 % for cells treated with TiCl4. Obtained photocurrent of 22.8 mA/cm2 was considered to be very promising for the future work. Enhancement v in efficiency was mostly attributed to suppressed recombination of photoelectrons and it is concluded that improved efficiencies can be obtained after successful synthesis of FTO nanoparticles having lower resistivity values and deposition of homogeneous shell coatings.

Styles APA, Harvard, Vancouver, ISO, etc.

22

Mikulec, Frederic Victor 1971. « Semiconductor nanocrystal colloids : manganese doped cadmium selenide, (core)shell composites for biological labeling, and highly fluorescent cadmium telluride ». Thesis, Massachusetts Institute of Technology, 1999. http://hdl.handle.net/1721.1/9358.

Texte intégral

Résumé :

Thesis (Ph.D.)--Massachusetts Institute of Technology, Dept. of Chemistry, 1999.
Includes bibliographical references.
This thesis describes the characterization and applications of nanometer sized semiconductor (or quantum dot) colloids produced by chemical means. The nanocrystals are synthesized by pyrolysis of organometallic precursors in the coordinating solvent trioctylphosphine oxide (TOPO). The important developments that have contributed to this method are discussed. Manganese doped CdSe nanocrystals are synthesized using a manganese and selenium containing organometallic compound. Chemical etching and electron paramagnetic resonance (EPR) experiments reveal that most of the dopant atoms lie near the surface within the inorganic lattice. Results from fluorescence line narrowing (FLN) and photoluminescence excitation (PLE) spectroscopies show that doped nanocrystals behave as if they were undoped nanocrystals in an external magnetic field. The nanocrystal surface is initially passivated by dative organic ligands. Better passivation and optical properties are achieved by growth of a large band gap semiconductor shell that provides both a physical and an energetic barrier between the exciton and the surface. (CdSe)ZnS (core)shell are prepared with control over both core and shell sizes. The composite nanocrystals are characterized by absorption, emission, X-ray photoelectron spectroscopy (XPS), transmission electron microscopy (TEM), small angle X-ray scattering (SAXS), and wide angle X-ray scattering (W AXS). The maximum quantum yield is achieved when the core is protected from oxidation by a complete shell; thicker shells show no further increase in quantum yield values, due to defects caused by the large lattice mismatch. Exchange of surface TOPO ligands for mercaptocarboxylic acids produces (core)shell nanocrystals that, when treated with base, are soluble in water and remain fluorescent. Established protocols are used to link these water-soluble nanocrystals to the biomolecules avidin or biotin, producing useful fluorescent labels. Stable phosphine tellurides are prepared using hexapropylphosphorus triamide (HPPT). This precursor is used to prepare CdTe nanocrystals that display room temperature quantum yields up to 70%. The CdTe growth is investigated by absorption and emission spectroscopy. CdTe nanocrystals are characterized by TEM and WAXS.
by Frederic Victor Mikulec.
Ph.D.

Styles APA, Harvard, Vancouver, ISO, etc.

23

Samadi, Khoshkhoo Mahdi [Verfasser], et Marcus [Akademischer Betreuer] Scheele. « Tin-doped Indium Oxide (ITO) Nanocrystal Superlattices (Surface Chemistry, Charge Transport, and Sensing Applications) / Mahdi Samadi Khoshkhoo ; Betreuer : Marcus Scheele ». Tübingen : Universitätsbibliothek Tübingen, 2018. http://d-nb.info/1198973072/34.

Texte intégral

Styles APA, Harvard, Vancouver, ISO, etc.

24

Skrzypczak, Ulrich Verfasser], Gerhard [Akademischer Betreuer] Seifert, Jan Christoph [Akademischer Betreuer] [Goldschmidt et Grant V. M. [Akademischer Betreuer] Williams. « Rate equation analysis of nanocrystal-enhanced upconversion in neodymium-doped glasses / Ulrich Skrzypczak. Betreuer : Gerhard Seifert ; Jan Christoph Goldschmidt ; Grant V. M. Williams ». Halle, Saale : Universitäts- und Landesbibliothek Sachsen-Anhalt, 2015. http://d-nb.info/1072072750/34.

Texte intégral

Styles APA, Harvard, Vancouver, ISO, etc.

25

Geszke-Moritz, Malgorzata. « Synthesis of stable and non-cadmium containing quantum dots conjugated with folic acid for imaging of cancer cells ». Thesis, Vandoeuvre-les-Nancy, INPL, 2011. http://www.theses.fr/2011INPL066N/document.

Texte intégral

Résumé :

Les Quantum Dots (QDs) sont des particules cristallines de semi-conducteur ou du métal de forme sphérique et de dimension nanométrique. L'intérêt majeur des QDs réside dans leur grande adaptabilité à de nombreuses applications biologiques.Le but de mon travail était de développer une nouvelle classe de QDs de faible toxicité afin de les utiliser pour la bio-imagerie des cellules cancéreuses. Pour cela, il est nécessaire de préparer des sondes hydrosolubles, photostables, biocompatibles, de luminescence élevée et possédant une faible toxicité. La synthèse des cœurs de type ZnS and ZnSe dopés au manganèse ou au cuivre et stabilisés par l’acide 3-mercapropropionique ou par le 1-thioglycérol a été réalisée par la voie hydrothermale. Les techniques analytiques de caractérisation utilisées sont la spectroscopie UV-visible, la spectroscopie de fluorescence, la diffraction des rayons X (XRD), la spectroscopie photoélectronique de rayon X (XPS), la microscopie électronique à transmission (TEM), la diffusion dynamique de la lumière DLS, la spectroscopie infra-rouge (IR), et la résonance paraélectronique (RPE). La toxicité des QDs a été déterminée sur des cellules cancéreuses. Les différents test de cytotoxicité (MTT, XTT et ferrous oxidation-xylenol orange) ont été réalisés. Finalement, les QDs de type ZnS:Mn conjugués à l’acide folique ont été utilisés pour la bio-imagerie des cellules cancéreuses par le biais d’une excitation biphotonique
Semiconductor QDs are tiny light-emitting crystals, and are emerging as a new class of fluorescent labels for medicine and biology. The aim of this work was to develop a new class of non-toxic QDs probes with essential attributes such as water dispersibility, photostability, biocompatibility, high luminescence and possible excitation with low-energy visible light, using simple processing method. Such nanoprobes could be used for bio-imaging of cancer cells. In the performed studies, I focused on ZnS and ZnSe QDs as they are cadmium-free and might be excited biphotonically.The synthesis protocols of ZnS and ZnSe QDs doped with two ions such as Mn or Cu and stabilized by 3-mercaptopropionic acid or 1-thioglycerol were established, followed by NCs characterization (diameter, surface charge, photophysical properties, …) using analytical techniques such as spectrophotometry UV-vis, fluorimetry, X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), transmission electron microscopy (TEM), dynamic light scattering (DLS), infra-red analysis (FT-IR), thin layer chromatography (TLC) and electron paramagnetic resonance (EPR). The cytotoxicity of synthesized bare and conjugated NPs was evaluated on cancer cell lines using MTT, XTT and ferrous oxidation-xylenol orange assay.Finally, chosen well fluorescent and weakly toxic types of as-prepared and characterized QDs were used for bio-imaging of cancer cells. In these experiments, FA-functionalized NCs were excited biphotonically. The performed experiments showed the potential of QDs as cancer cells fluorescent markers and that they accumulate around the cell nuclei

Styles APA, Harvard, Vancouver, ISO, etc.

26

Silva, Estelina Lora da. « Electronic properties of doped silicon nanocrystals ». Master's thesis, 2010. http://hdl.handle.net/10316/13812.

Texte intégral

Résumé :

No presente trabalho estuda-se, através de modelação computacional baseada na teoria do funcional da densidade, os efeitos de impurezas em nanocristais de silício passivados com hidrogénio. São calculados os níveis de energia das impurezas intersticiais mais propícias à dopagem de tipo-n e p para os sistemas con nados, nomeadamente os três primeiros elementos pertencentes ao grupo dos alcalinos, Li, Na e K, e os três primeiros halogéneos, F, Cl e Br. Observou-se que à temperatura ambiente estas impurezas não contribuem com portadores de carga (electrões ou lacunas) para os estados HOMO nem LUMO. Este facto resulta do con namento da superfície e do meio dieléctrico fraco existente no nanocristal. Os níveis de energia para o P, B e o Duplo Dador Térmico do modelo da cadeia O5 foram também calculados por forma a comparar o comportamento destes nos sistemas con nados com o comportamento, bem estabelecido, na matéria extensa. Níveis de energia profundos no hiato também foram observados para estas impurezas.

Styles APA, Harvard, Vancouver, ISO, etc.

27

Hazarika, Abhijit. « Photophysical Properties of Manganese Doped Semiconductor Nanocrystals ». Thesis, 2015. http://etd.iisc.ernet.in/2005/3675.

Texte intégral

Résumé :

Electronic and optical properties of semiconducting nanocrystals, that can be engineered and manipulated by various ways like varying size, shape, composition, structure, has been a subject of intense research for more than last two decades. The size dependency of these properties in semiconductor nanocrystals is direct manifestation of the quantum confinement eﬀect. Study of electronic and optical properties in smaller dimensions provides a platform to understand the evolution of fundamental bulk properties in the semiconductors, often leading to realization and exploration of entirely new and novel properties. Not only of fundamental interests, the semiconductor nanocrystals are also shown to have great technological implications in diverse areas. Besides size tunable properties, introduction of impurities, like transition metal ions, gives rise to new functionalities in the semicon-ductor nanocrystals. These materials, termed as doped semiconductor nanocrystals, have been the subject of great interest, mainly due to the their interesting optical properties. Among diﬀerent transition metal doped semiconductor nanocrystals, manganese doped systems have drawn a lot on attention due to their certain advantages over other dopants. One of the major advantages of Mn doped semiconductor nanocrystals is that they do not suﬀer from the problem of self-absorption of emission, which quite often, is consid-ered detrimental in their undoped counterparts. The doped nanocrystals are known to produce a characteristic yellow-orange emission upon photoexcitation of the host that is relatively insensitive to the surface degradation of the host. This emission, originating from an atomic d-d transition of Mn2+ ions, has been a subject of extensive research in the recent past. In spite of the spin forbidden nature of the specific d-d transition, namely 6A1 −4 T1, these doped nanocrystals yield intense phosphorescence. However, one major drawback of utilizing this system for a wide range application has been the substantial inability of the community to tune the emission color of Mn-doped systems in spite of an intense eﬀort over the years; the relative constancy of the emission color in these systems has been attributed to the essentially atomic nature of the optical transition involving localized Mn d levels. Interestingly, however, the Mn emission has a very broad spectral line-width in spite of its atomic-like origin. While the long (∼ 1 ms) emission life-time of the de-excitation process is well-studied and understood in terms of the spin and orbitally forbidden nature of the transition, there is little known concerning the process of energy transfer to the Mn from the host in the excitation step. In this thesis, we have studied the ultrafast dynamic processes involved in Mn emission and addressed the issues related to its tunability and spectral purity. Chapter 1 provides a brief introduction to the fundamental concepts relevant to the studies carried out in the subsequent chapters of this thesis. This chapter is started with a small preview of the nanomaterials in general, followed by a discussion on semiconducting nanomaterials, evolution of their electronic structure with dimensions and size as well as the eﬀect of quantum confinement on their optical properties. As all the semiconducting nanomaterials studied in the thesis are synthesized via colloidal synthesis routes, a separate section is devoted on colloidal semiconducting nanomaterials, describing various ways of modifying or tuning their optical properties. This is followed by an introduction to the important class of materials “doped semiconductor nanocrystals”. With a general overview and brief history of these materials, we proceed to discuss about various aspects of manganese doped semiconductor nanocrystals in great details, highlighting the origin of the manganese emission and the associated carrier dynamics as well as diﬀerent reported synthetic strategies to prepare these materials. The chapter is closed with the open questions related to manganese doped semiconductor nanocrystals and the scope of the present work. Chapter 2 describes diﬀerent experimental and theoretical methods that have been employed to carry out diﬀerent studies presented in the thesis. It includes common experimental techniques like UV-Vis absorption spectroscopy, steady-state and time-resolved photoluminescence spectroscopy used for optical measurements, X-ray diﬀraction, trans-mission electron microscopy and atomic absorption spectroscopy used for structural and elemental analysis. Experimental tools to perform special studies like transient absorption and single nanocrystal spectroscopy are also discussed. Finally, theoretical fitting method used to analyse various spectral data has been discussed briefly. Chapter 3 deals with the dynamic processes involved in the photoexcitation and emission in manganese doped semiconductor nanocrystals. For this study, Mn doped ZnCdS alloyed nanocrystal has been chosen as a model system. There are various radiative and nonrdiative recombination pathways of the photogenerated carriers and they often compete with each other. We have studied the dynamics of all possible pathways of carrier relaxation, viz. excitonic recombination, surface state emission and Mn d-d transition. The main highlight of this chapter is the determination of the time-scale to populate surface states and the Mn d-states after the photoexcitation of the host. Employing femtosecond pump-probe based transient absorption study we have shown that the Mn dopant states are populated within sub-picosecond of the host excitation, while it takes a few picoseconds to populate the surface states. Keeping in mind the typical life-time of the excitonic emission (∼ a few ns), the ultra-fast process of energy transfer from the host to the Mn ions explains why the presence of Mn dopant ions quenches the excitonic as well as the surface state emissions so eﬃciently. Chapter 4 presents a study of manganese emission in ZnS nanocrystals of diﬀerent sizes. By varying the size of the ZnS host nanocrystal, we show that one can tune the Mn emission over a limited range. In particular, with a decrease in host size, the Mn emission has been observed to red-shift. We have attributed this shift in Mn emission to the change in the ratio of surface to bulk dopant ions with the variation of the host size, noting that the strength of the ligand field at the Mn site should depend on the position of the Mn ion relative to the surface due to a systematic lattice relaxation in such nanocrystals. The ligand field aﬀects the emission wavelength directly by controlling the splitting of the t2 and e levels of Mn2+ ions. The surface dopant ions experience a strong ligand field due to distorted tetrahedral environment which leads to larger splitting of these t2 and e states. We further corroborated these results by performing doping concentration dependent emission and life-time studies. In Chapter 5 addresses two fundamental challenges related to manganese photolumines-cence, namely the lack of a substantial emission tunability and presence of a very broad spectral width (∼ 180-270 meV). The large spectral width is incompatible with atomic-like manganese 4T1 −6 A1 transition. On the other hand, if this emission is atomic in nature, it should be relatively unaﬀected by the nature of the host, though it can be manipulated to some extent as discussed in Chapter 3. The lack of Mn emission tunability and spectral purity together seriously limit the usefulness of Mn doped semiconductor nanocrystals. To understand why the Mn emission tunability range is very limited (typically 565-630 nm) and to understand the true nature of this emission, we carried out single nanocrystal imaging and spectroscopy on Mn doped ZnCdS alloyed nanocrystals. This study reveals that Mn emission, in fact, can vary over a much wider range (∼ 370 meV) and exhibits widths substantially lower (∼ 60-75 meV) than reported so far. We explained the occur-rence of Mn emission in this broad spectral range in terms of the possibility of a large number of symmetry inequivalent sites resulting from random substitution of Cd and Zn ions that leads to diﬀering extent of ligand field contributions towards the splitting of Mn d-levels. The broad Mn emission observed in ensemble-averaged measurements is the result of contribution from Mn ions at diﬀerent sites of varying ligand field strengths inside the NC. Chapter 6 presents a synthetic strategy to strain-engineer a nanocrystal host lattice for a controlled tuning of the ligand field eﬀect of the doped Mn sites. It is realized synthesizing a strained quantum dot system with the structure ZnSe/CdSe/ZnSe. A larger lattice parameter of CdSe compared to that of ZnSe causes a strain field that is maximum near the interface, gradually decreasing towards the surface. We control the positioning of Mn dopant ions at diﬀerent distances from the interface, thereby doping Mn at diﬀerent predetermined strain fields. With the help of this strain engineering, we are able to tune Mn emission across the entire range of the visible spectrum. This strain induced tuning of Mn emission is accompanied by life-times that is dependent on the emission energy which has been explained in terms of perturbation eﬀect on the Mn center due to the strain generated inside the quantum dot. The spectacular emission tuning has been explained by modelling the quantum dot system as an elastic continuum containing three distinct layers under hydrostatic pressure. From this modelling, we found that the strain is max-imum at the interface and decreases continuously as one goes away from the interface. We also show that the Mn emission maximum red shifts with increasing distance of the dopants from the maximum strained region. In summary, we have performed a study on the photophysical processes in manganese doped semiconductor nanocrystals. We have emphasized in understanding of diﬀerent dynamic processes associated with the manganese emission and tried to understand the true nature of manganese emission in a nanocrystal. This study has brought out some new aspects of manganese emission and opened up possibilities to tune and control manganese emission by proper design of the host material.

Styles APA, Harvard, Vancouver, ISO, etc.

28

Yi, Guang-Shun, et Gan-Moog Chow. « Rare-Earth Doped LaF₃ Nanocrystals for Upconversion Fluorescence ». 2005. http://hdl.handle.net/1721.1/30391.

Texte intégral

Résumé :

Upconversion fluorescent nanocrystals, Yb-Er, Yb-Ho and Yb-Tm co-doped LaF₃ were chemically synthesized. The average particle size was 4.4 nm with a narrow size distribution of ± 0.3 nm. Under the 980 nm NIR excitation, the green, red and blue emission bands from these nanocrystals were observed, respectively. These nanocrystals have potential applications as bio-probes and displays.
Singapore-MIT Alliance (SMA)

Styles APA, Harvard, Vancouver, ISO, etc.

29

Boyer, John-Christopher. « Synthesis and spectroscopy of upconverting lanthanide-doped nanocrystals ». Thesis, 2006. http://spectrum.library.concordia.ca/9236/1/boyer_john%2Dchristopher_2006.pdf.

Texte intégral

Résumé :

In the past two decades it has been widely demonstrated that the optical properties of select inorganic materials may be modified by changing either their size or shape on the nanometer level (sub-nanometer to 100 nm length scale). Much of the early research on this topic focused on semiconducting nanocrystals, where it has been effectively demonstrated that reducing particle size below their Bohr radius produced a characteristic blue shift of the band gap absorption. A little over a decade after the initial work on semiconducting nanocrystals, the first scientific articles on insulating nanocrystals doped with lanthanide ions started to appear. While numerous studies have focused on examining the luminescence generated by exciting with ultraviolet (UV) light, very few have examined the upconversion phenomenon in nanocrystalline materials. Upconversion is the generation of higher energy light from lower energy radiation typically through the use of lanthanide ions doped into a solid state host. Much of the interest in upconverting nanocrystalline materials is due to their prospective application as fluorescent biological labels. In this thesis the synthesis, spectroscopic, and upconversion properties of lanthanide-doped nanocrystalline materials will be discussed. We report on our efforts to date to achieve viable upconversion luminescence from Ho 3+ doped nanocrystalline Y 2 O 3 and Gd 3 Ga 5 O 12 prepared via the combustion synthesis. These studies have determined that, while upconversion occurs in Y 2 O 3 bulk samples, it is severely reduced or nonexistent in the nanocrystal samples. This behaviour is attributed to the presence of high vibrational energies, 1500 and 3350 cm -1 , due to adsorbed atmospheric CO 3 2- and OH - anions, respectively, on the surface of the nanocrystals. A substantial increase in the upconversion efficiencies was observed in the case of the garnet (Gd 3 Ga 5 O 12 ) nanocrystals due to considerably less surface contamination. The effect of Yb 3+ Co-doping on the upconversion luminescence in the Gd 3 Ga 5 O 12 sample will also be introduced. We also evaluate the spectroscopic properties of lutetium oxide nanocrystals doped with trivalent europium (Lu 2 O 3 :Eu 3+ ) prepared by the same combustion synthesis technique. These results are compared and contrasted to those of a bulk Lu 2 O 3 :Eu 3+ sample. In the case of Lu 2 O 3 we observe significant changes in the luminescence behaviour that we attribute to the vastly different particle sizes of the two different materials. Finally, we present a new procedure for synthesizing NaYF 4 :Er 3+ , Yb 3+ nanoparticles that are capable of colloidal dispersion in non-polar organic solvents. The highly luminescent nanoparticles are synthesized via the thermal decomposition of trifluoroacetate precusors in a mixture of oleic acid and octadecene. The Er 3+ , Yb 3+ and Tm 3+ , Yb 3+ doped cubic NaYF 4 nanocrystals exhibit green/red and blue upconversion luminescence, respectively under 980 nm laser excitation with low power densities while colloidally dispersed. A brief discussion on our current attempts and future efforts towards modifying the nanoparticles surface will also be given.

Styles APA, Harvard, Vancouver, ISO, etc.

30

Lin, Xi-Cong, et 林熙琮. « Synthesis and Optical Properties of Mn2+ Doped ZnS Nanocrystals ». Thesis, 2006. http://ndltd.ncl.edu.tw/handle/33984445122788205353.

Texte intégral

Résumé :

碩士
國立彰化師範大學
光電科技研究所
94
Mn2+ doped ZnS nanocrystals were synthesized by chemical method. The structure, morphology and optical properties of these nano-particles were studied by the means of X-ray diffraction (XRD), transmission electron microscope (TEM), photoluminescence (PL) and photoluminescence excitation (PLE) measurements. The XRD patterns showed the structure of the ZnS:Mn nanocrystals is zinc-blende. From the XRD and TEM results, the size of these narocrystals was estimated at about 3 ~4 nm. From the PL spectra, two emission peaks were observed. One is the orange emission (about 590 nm) which is due to the 4T1-6A1 characteristic transition in the Mn2+ ions, the other is the broad blue emission (from 410~500 nm) which can be attributed to the S vacancies in the ZnS nanocrystals. The PL spectra also showed that the emission intensity of samples increases as the Mn2+ concentration increases up to 4 % and then decreases. The surfactant effect on the luminescence intensity was also studied. It was found that the adding of surfactant in the synthesis process can increase the luminescence intensity efficaciously. The energy gap of the ZnS:Mn nanocrystals was determined by the PLE spectra. It was found to increase as the crystal size decreases. This blue shift of the PLE peaks is due to the quantum confinement effect of nanocrystals. Irradiation-induced luminescence enhancement effect was observed. The luminescence intensity of fresh samples was increased under irradiation by 325 nm He-Cd laser beam. From the temperature-dependent PL spectra, a blue shift of the Mn2+ ions emission peaks was observed as the temperature increased. This phenomenon could be attributed to the variation of crystal-field strength. The temperature dependence of the PL spectra of samples with surfactant was different to that without surfactant. The luminescence intensity of samples with surfactant increases as the temperatire increases. However, for the samples without surfactant, the luminescence intensity deccreaes as the temperature increases.

Styles APA, Harvard, Vancouver, ISO, etc.

31

Lin, Chen-Han. « Nanocrystals Embedded Zirconium-doped Hafnium Oxide High-k Gate Dielectric Films ». Thesis, 2011. http://hdl.handle.net/1969.1/ETD-TAMU-2011-08-9884.

Texte intégral

Résumé :

Nanocrystals embedded zirconium-doped hafnium oxide (ZrHfO) high-k gate dielectric films have been studied for the applications of the future metal oxide semiconductor field effect transistor (MOSFET) and nonvolatile memory. ZrHfO has excellent gate dielectric properties and can be prepared into MOS structure with a low equivalent oxide thickness (EOT). Ruthenium (Ru) modification effects on the ZrHfO high-k MOS capacitor have been investigated. The bulk and interfacial properties changed with the inclusion of Ru nanoparticles. The permittivity of the ZrHfO film was increased while the energy depth of traps involved in the current transport was lowered. However, the barrier height of titanium nitride (TiN)/ZrHfO was not affected by the Ru nanoparticles. These results can be important to the novel metal gate/high-k/Si MOS structure. The Ru-modified ZrHfO gate dielectric film showed a large breakdown voltage and a long lifetime. The conventional polycrystalline Si (poly-Si) charge trapping layer can be replaced by the novel floating gate structure composed of discrete nanodots embedded in the high-k film. By replacing the SiO2 layer with the ZrHfO film, promising memory functions, e.g., low programming voltage and long charge retention time, can be expected. In this study, the ZrHfO high-k MOS capacitors that separately contain nanocrystalline ruthenium oxide (nc-RuO), indium tin oxide (nc-ITO), and zinc oxide (nc-ZnO) have been successfully fabricated by the sputtering deposition method followed with the rapid thermal annealing process. Material and electrical properties of these kinds of memory devices have been investigated using analysis tools such as XPS, XRD, and HRTEM; electrical characterizations such as C-V, J-V, CVS, and frequency-dependent measurements. All capacitors showed an obvious memory window contributed by the charge trapping effect. The formation of the interface at the nc-RuO/ZrHfO and nc-ITO/ZrHfO contact regions was confirmed by the XPS spectra. Charges were deeply trapped to the bulk nanocrystal sites. However, a portion of holes were loosely trapped at the nanocrystal/ZrHfO interface. Charges trapped to the different sites lead to different detrapping characteristics. For further improving the memory functions, the dual-layer nc-ITO and -ZnO embedded ZrHfO gate dielectric stacks have been fabricated. The dual-layer embedded structure contains two vertically-separated nanocrystal layers with a higher density than the single-layer embedded structure. The critical memory functions, e.g., memory window, programming efficiency, and charge retention can be improved by using the dual-layer nanocrystals embedded floating gate structure. This kind of gate dielectric stack is vital for the next-generation nonvolatile memory applications.

Styles APA, Harvard, Vancouver, ISO, etc.

32

Hsu, Yi-Husan, et 徐憶瑄. « Synthesis and characterization of near-infrared light triggered lanthanide-doped upconversion nanocrystals ». Thesis, 2015. http://ndltd.ncl.edu.tw/handle/08626558123607409893.

Texte intégral

Résumé :

碩士
中山醫學大學
應用化學系碩士班
103
This study mainly discusses the synthesis of NaYF4/LiYF4 nanoparticles containing Tm3+/Yb3+. The upconversion efficiency of these nanoparticles correlated to the equivalent of activator or base during the syntheses is also demonstrated. The TEM images of the particles prepared by autoclave under lower temperature show that most of the particles are irregular (AC7 and AC14). In the meantime, the particles could not show the upconversion efficiency under 980 nm excitation. In order to improve the diameter and the upconversion efficiency of the nanoparticles, we used the heating mantle for the synthesis of the nanoparticles. The nanoparticles with upconversion efficiency and diameter less than 100nm are successfully synthesized. To study the relationship between the equivalent of the activator / base and the upconversion efficiency of the nanoparticles, we increased the equivalent of the activator. The result indicated that the upconversion efficiency was not enhanced by increasing the equivalent of the activator. However, the increasing the equivalent of the base ( LiOH / NaOH) during the synthesis resulted in the enhanced upconversion efficiency of the nanoparticles. The further addition of Y(CH3CO2)3 and base (LiOH / NaOH) to the synthesized NaYF4/LiYF4:Yb,Tm nanoparticles by the heating mantle led to the formation of new nanoparticles. The TEM images of the nanoparticles show that the shapes of the nanoparticles transformed from hexagon to rod (L1S、L3S、N1S、N3S). The analysis of the length-to-width (aspect ratio, AR) of the rod (L1S (AR=3.90), L3S (AR= 3.77); N3S (AR=3.73), N1S (AR=2.64)) showed that the rod with the higher AR value exhibited the effective upconversion efficiency.

Styles APA, Harvard, Vancouver, ISO, etc.

33

Sabergharesou, Tahereh. « Magnetic and Structural Investigation of Manganese Doped SnO_2 and In_2 O_3 Nanocrystals ». Thesis, 2013. http://hdl.handle.net/10012/7874.

Texte intégral

Résumé :

Diluted magnetic semiconductor oxides (DMSOs) have received great attention recently due to their outstanding applications in optoelectronic and spintronic devices. Ever since the initial observation of ferromagnetism at room temperature in cobalt-doped titania, extensive effort is concentrated on preparation of transition metal doped wide band gap semiconductors, especially Mn- doped ZnO. Compared to Mn-doped ZnO, magnetic interactions in SnO! and In!O! semiconductors have been underexplored. SnO! and In!O! semiconductors have many applications, owing to their high charge carrier density and mobility as well as high optical transparency. Investigation on electronic structure changes induced by dopants during the synthesis procedure can effectively influence magnetic interactions between charge carriers. In this work, a combination of structural and spectroscopic methods was used to probe as-synthesized SnO! and In!O! nanocrystals doped with Mn!! and Mn!! as precursors. X-ray absorption near edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) spectroscopy are powerful techniques to explore formal oxidation state of manganese dopant, electronic environment, number of nearest neighbors around the absorbent, and bond lengths to the neighboring atoms. Analysis reveals the presence of multiple oxidation states in the doped nanocrystals, and establishes a relation between !"!! ratio and expansion or contraction of lattice parameters. !"!! Although doping semiconductors are crucial for manipulating the functional properties, the influence of dopants on nanocrystals structure is not well understood. Nanocrystalline films prepared from colloidal Mn-doped SnO! and In!O! nanocrystals through spin coating process exhibit ferromagnetic behavior in temperatures ranging from 5 K to 300 K. Magnetic transformation from paramagnetic in free-standing Mn-doped nanocrystals to strong ferromagnetic ordering in nanocrystalline films is attributed to the formation of extended structural defects, e.g., oxygen vacancies at the nanocrystals interface. Magnetic circular dichroism (MCD) studies clearly show that Mn!! occupies different symmetry sites in indium oxide, when bixbyite and rhombohedral In!O! nanocrystals (NCs) are compared.

Styles APA, Harvard, Vancouver, ISO, etc.

34

« Synthesis and characterization of transition-metal-doped zinc oxide nanocrystals for spintronics ». Thesis, 2007. http://library.cuhk.edu.hk/record=b6074400.

Texte intégral

Résumé :

A simple bottom-up-based synthetic strategy named a solvothermal technique is introduced as the primary synthetic approach and its crystal growth mechanism is scrutinized. N-type cobalt-doped ZnO-based DMS nanocrystals are employed as a model system, and characterized by a broad spectrum of advanced microscopic and spectroscopic techniques. It is found that the self-orientation growth mechanism, imperfect oriented attachment, is intimately correlated with the high-temperature ferromagnetism via defects. The influence of processing on the magnetic properties, such as compositional variations, reaction conditions, and post-growth treatment, is also studied. In this way, an in-depth understanding of processing-structure-property interrelationships and origins of magnetism in DMS nanocrystals are obtained in light of the theoretical framework of a spin-split impurity band model. In addition, a nanoscale spinodal decomposition phase model is also briefly discussed.
Following the similar synthetic route, copper- and manganese-doped ZnO nanocrystals have been synthesized and characterized. They both show high-temperature ferromagnetism in line with the aforementioned theoretical model(s). Moreover, they display interesting exchange biasing phenomena at low temperatures, revealing the complexity of magnetic phases therein.
Spintronics (spin transport electr onics), in which both spin and charge of carriers are utilized for information processing, is believed to challenge the current microelectronics and to become the next-generation electronics. Nanostructured spintronic materials and their synthetic methodologies are of paramount importance for manufacturing future nanoscale spintronic devices. This thesis aims at studying synthesis, characterization, and magnetism of transition-metal-doped zinc oxide (ZnO) nanocrystals---a diluted magnetic semiconductor (DMS)---for potential applications in future nano-spintronics.
The crystal growth strategy demonstrated in this work not only provides a more convenient approach to directly tailor magnetic properties of advanced multifunctional spintronic materials on a nanometer scale but also contributes to a deeper insight into the microscopic origin of magnetism in wide-band-gap oxide DMSs.
Wang, Xuefeng.
"August 2007."
Adviser: J. B. Xu.
Source: Dissertation Abstracts International, Volume: 69-02, Section: B, page: 1230.
Thesis (Ph.D.)--Chinese University of Hong Kong, 2007.
Includes bibliographical references.
Electronic reproduction. Hong Kong : Chinese University of Hong Kong, [2012] System requirements: Adobe Acrobat Reader. Available via World Wide Web.
Electronic reproduction. [Ann Arbor, MI] : ProQuest Information and Learning, [200-] System requirements: Adobe Acrobat Reader. Available via World Wide Web.
Abstract in English and Chinese.
School code: 1307.

Styles APA, Harvard, Vancouver, ISO, etc.

35

Lee, Sheng-Kai, et 李勝凱. « Study on CoSi2 nanocrystals in Ge-doped dielectric layer for nonvolatile memory ». Thesis, 2008. http://ndltd.ncl.edu.tw/handle/33062508031317007089.

Texte intégral

Résumé :

碩士
國立交通大學
電子工程系所
96
In recent years, digital life has attracted great importance for Taiwan，s electronics market. Then he portable electronic products have been applied widely, such as digital cameras, notebooks, hand-carry USB memories, a chip on credit card , PDA, GPS, memory card, MP3 audio players and so on. However, these portable electronic products are based on the nonvolatile memory (NVM) due to the need of low working power and portability. In a conventional nonvolatile memory (NVM), charge is stored in a ploy-silicon floating-gate (FG). However, it suffers some limitations for continual scaling down of the device structure. In FG memory, the electrons which injected from channel to the poly-silicon trapping layer influence the shift of threshold voltage in the memory. Then it can be defined through the difference of threshold voltages as logic “0” & “1”. Nevertheless, the definition fails if the tunneling oxide provides a leakage path after repeatedly performing write/erase cycling. On other hand, the oxide will produce some defect after repeat impact during electrons the write/erase cycle because the whole structure of FG is semiconductor. All of the charge stored in FG will be trapped into trapping layer or be lost from trapping layer with leak path which was formed with defects. FG structure will have reliability problem when device scale down to nano-meter level. Among the Metal Silicide, cobalt-silicide (CoSi2) has been widely used as a contact source due to the lowest resistivity value ( 10-20 ~μΩcm) and good thermal stability. In this thesis, CoSi2 films were sputtered and we choose rapid thermal annealing (RTA) and sputter system in order to reduce process cost because of temperature controlling and reduce thermal budget because of diffusion reducing. Co-sputtering approach was used to deposit the mixed cobalt, silicon and germanium film. After rapid temperature oxidation (RTO), Novel cobalt silicide nanocrystals embedded in the dielectrics which are doped with Ge have been formed. The charge storage effect of this novel trapping layer have also been investigated by capacitance-voltage (C-V), current density-voltage (J-V) measurement. Transmission Electron Microscopy (TEM), Secondary Ion Mass Spectrometer (SIMS) and X-ray photoelectron spectroscopy (XPS) have been used to analyze formation of the cobalt-silicide nanocrystals. In addition, the structure formed by co-sputtering the Co target with SiO2, Si3N4 target and Al2O3 target have also been demonstrated in this work. The approach also shows good charge storage ability. The charge storage mechanism of various dielectrics has also been revealed by related material analysis.

Styles APA, Harvard, Vancouver, ISO, etc.

36

Chang, Chia-Wei, et 張家瑋. « Down- and Up-converted Visible Luminescence Properties of Er3+-doped Y2Ti2O7 Nanocrystals ». Thesis, 2009. http://ndltd.ncl.edu.tw/handle/80944779317381649090.

Texte intégral

Résumé :

碩士
國立中正大學
光機電整合工程所
97
Er3+-doped Y2Ti2O7 thin films with the thickness of ~375 nm thin films were fabricated by the sol-gel spin-coating method. A pyrochlore phase ErxY2-xTi2O7 was observed with a strong (222) preferred orientation while the annealing temperature exceeded 800 °C. Below 800 °C annealing, thin films exhibited amorphous structure. The average visible transmittance calculated in the wavelength range 200–1100 nm of the Er3+-doped Y2Ti2O7 thin films annealed at 400 to 900 °C reduced from ~87 to ~77% because of the increase of grain size and surface roughness. The variation of refractive indexes and optical band gaps of Er3+-doped Y2Ti2O7 thin films strongly depend on the Er3+ concentrations and annealing temperatures. Higher annealing temperatures result in the increase of refractive indexes but the decease of optical band gaps. In addition, higher Er3+ concentrations lead to a decrease in both refractive indexes and optical band gaps. The variation of these basic optical properties can be attributed to the evolution of grain size, crystallinity, lattice constant, and composition of Er3+-doped Y2Ti2O7 thin films. Because the competition between the [OH-], concentration quenching effect, as well as the diversity and symmetry of Er3+ lattice sites, the Er3+ (5%)-doped Y2Ti2O7 thin films annealed at 700 °C for 1 h possessed the largest intensity of ~1.5μm PL and FWHM ~ 60 nm. Er3+-doped Y2Ti2O7 nanocrystals with pyrochlore phase were fabricated by the Pechini sol-gel method and the average crystal size increased from ~34 to ~46 nm under 800 to 1000 °C/1 h annealing. The amorphous Er3+-doped Y2Ti2O7 nanocrystals was obtained at ≦700°C annealing temperature. The Er3+-doped Y2Ti2O7 nanocrystals possess the dual down- and up-converted luminescent properties, which convert the 380 and 980 nm photons to the visible green light (~526, and ~547 nm; 2H11/2→4I15/2 and 4S3/2→4I15/2) and red light (~660 nm; 4F9/2→4I15/2). For both high (10 mol%) and low (5 mol%) Er3+ doped concentration, the mechanism of up-converted green light is two-photon excited-state absorption; however, much stronger intensity of red light relative to green light is observed for sample with high Er3+ doped concentration (10 mol%), attributed to the reduced distance between Er3+-Er3+ ions resulting in the enhancement of the energy-transfer up-conversion and energy-transfer cross-relaxation mechanisms.

Styles APA, Harvard, Vancouver, ISO, etc.

37

Chiu, Yi-Shan, et 邱意珊. « A study on preparation and luminescence properties of Er3+-doped Y2Ti2O7 nanocrystals ». Thesis, 2010. http://ndltd.ncl.edu.tw/handle/23018698376874802380.

Texte intégral

Résumé :

碩士
國立中正大學
光機電整合工程所
98
Er3+-doped Y2Ti2O7 nanocrystals were fabricated by the sol-gel method. While the annealing temperature exceeds 757 °C, amorphous pyrochlore phase Er3+-doped Y2Ti2O7 transfers to well-crystallized nanocrystals, and the average crystal size increases from ~70 to ~180 nm under 800 to 1000 °C/1 h annealing. The Er3+-doped Y2Ti2O7 nanocrystals absorbing the 980 nm photons can produce the up-conversion (526, 547, and 660 nm; 2H11/2→4I15/2, 4S3/2→4I15/2 and 4F9/2→4I15/2, respectively) and Stokes luminescence (1528 nm; 4I13/2→4I15/2). The IR PL decay curve is single-exponential for Er3+ (5 mol%)-doped Y2Ti2O7 nanocrystals but slightly nonexponential for Er3+ (10 mol%)-doped Y2Ti2O7 nanocrystals. For both 5 and 10 mol% Er3+ doping concentrations, the mechanism of up-converted green light is the two-photon excited-state absorption; however, much stronger intensity of red light relative to green light was observed for sample with 10 mol% Er3+ doping concentration. This phenomenon can be attributed to the reduced distance between Er3+-Er3+ ions, resulting in the enhancement of the energy-transfer up-conversion and cross-relaxation mechanisms.

Styles APA, Harvard, Vancouver, ISO, etc.

38

Chien, Yungchuan, et 簡勇全. « Up and Down-converted visible luminescence properties of Er3+ doped Gd2Ti2O7 nanocrystals ». Thesis, 2012. http://ndltd.ncl.edu.tw/handle/34303679092975627005.

Texte intégral

Résumé :

碩士
國立中正大學
光機電整合工程研究所
100
Er3+-doped Gd2Ti2O7 nanocrystals were fabricated by the sol-gel method. While the annealing temperature exceeds 800 °C, amorphous pyrochlore phase Er3+-doped Gd2Ti2O7 transfers to well-crystallized nanocrystals, and the average crystal size increases from ~70 to ~180 nm under 800 to 1200 °C/1 h annealing. The Er3+-doped Gd2Ti2O7 nanocrystals absorbing the 980 nm photons can produce the up-conversion (526, 547, and 660 nm; 2H11/2→4I15/2, 4S3/2→4I15/2 and 4F9/2→4I15/2, respectively) and Stokes luminescence (1528 nm; 4I13/2→4I15/2). The IR PL decay curve is single-exponential for Er3+ (5 mol%)-doped Gd2Ti2O7 nanocrystals but slightly nonexponential for Er3+ (10 mol%)-doped Gd2Ti2O7 nanocrystals. For both 5 and 10 mol% Er3+ doping concentrations, the mechanism of up-converted green light is the two-photon excited-state absorption; however, much stronger intensity of red light relative to green light was observed for sample with 10 mol% Er3+ doping concentration. This phenomenon can be attributed to the reduced distance between Er3+-Er3+ ions, resulting in the enhancement of the energy-transfer up-conversion and cross-relaxation mechanisms.

Styles APA, Harvard, Vancouver, ISO, etc.

39

Ju, Ling. « Synthesis and Investigation on Phase Transition of BaTiO3 and Cr3+-Doped BaTiO3 Nanocrystals ». Thesis, 2009. http://hdl.handle.net/10012/4773.

Texte intégral

Résumé :

Various sizes of BaTiO3 and Cr3+-doped BaTiO3 nanocrystals were synthesized through hydrothermal and solvothermal methods. The applied solvents water, ethanol and benzyl alcohol lead to nanoparticles with average sizes of 200, 10 and 5 nm, respectively. The nanocrystals were treated with trioctylphosphine oxide to remove surface-bound dopant ions, and colloidal free-standing nanocrystals smaller than 10 nm were obtained by using oleic acid as a dispersant surfactant. The tetragonal-to-cubic phase transition at room temperature of undoped nanocrystalline BaTiO3 has been investigated by powder X-ray diffraction (XRD) and Raman spectroscopy. The size effect of nanoscale BaTiO3 is observed that the tetragonal phase becomes unstable with decreasing particle size. However, we found that ferroelectric tetragonal structure persists to some extent even for particles at 5 nm. The successful substitution of Ti4+ with Cr3+ in the host BaTiO3 lattice for all three sizes was achieved at different Cr3+/Ti4+ molar ratios. The dopant is found to significantly promote the phase transition, even dominate over the size effect. Ligand-field electronic absorption spectroscopy suggests a subtle change of the octahedral coordinated Cr3+ environments between particles at 5 and 10 nm, confirming the structural differences. Preliminary magnetic measurement indicates Cr3+ as isolated paramagnetic ions without any chromium clusters or oxides. The ability to rationally manipulate the ferroelectric properties of BaTiO3 by size and dopants, in combination with possible ferromagnetism induced by incorporating paramagnetic transition metal ions, opens up new opportunities for modern multiferroic materials in information storage technology.

Styles APA, Harvard, Vancouver, ISO, etc.

40

Vetrone, Fiorenzo. « Luminescence spectroscopy of Er³⁺ doped inorganic nanocrystals : an investigation into their upconversion properties ». Thesis, 2005. http://spectrum.library.concordia.ca/8450/1/NR04038.pdf.

Texte intégral

Résumé :

This thesis presents a detailed investigation into the spectroscopic properties of inorganic nanocrystals doped with trivalent rare earth ions. We focus on their upconversion luminescence, emission of radiation at higher energy than the pump wavelength, and evaluate the fundamental mechanisms of upconversion in the nanocrystal. We evaluate the spectroscopic properties of sesquioxide nanocrystals doped with trivalent erbium (M 2 O 3 :Er 3+ , where M = Y, Lu, or Sc) prepared by the propellant synthesis technique. Characteristic green, red, and near-infrared Er 3+ emission is observed following excitation with 488 nm in all samples under investigation. The overall luminescence intensity of the sesquioxide nanocrystals is lower compared to the microcrystalline material (bulk) as a result of the presence of high vibrational energies, 1500 and 3350 cm -1 , due to adsorbed CO 3 2- and OH - anions, respectively, which significantly increase the rate of multiphonon relaxation. The garnet (Gd 3 Ga 5 O 12 :Er 3+ ) nanocrystals, however, have considerably less surface adsorbed species, which consequently increases the luminescence intensity drastically. The upconversion of red (n exc = 650 nm) and near-infrared (n exc = 800 or 980 nm) radiation into UV, blue, green, and red emission is studied for Er 3+ ions doped in various sesquioxide (Y 2 O 3 , Lu 2 O 3 , and SC 2 O 3 ) and garnet (Gd 3 Ga 5 O 12 ) nanocrystals over a wide range of temperatures and dopant concentration is investigated. We present, for the first time, upconversion in a trivalent rare earth (RE 3+ ) doped nanocrystalline material, specifically Y 2 O 3 :Er 3+ . We show that replacing the Y 3+ cation has significant consequences on the upconversion. The upconverted luminescence of Lu 2 O 3 :Er 3+ nanocrystals have intensities that are 100x greater compared to identically doped nanocrystalline Y 2 O 3 :Er 3+ . Furthermore, Sc 2 O 3:Er 3+ nanocrystals show an enhanced red emission, which is greater than Y 2 O 3 :Er 3+ nanocrystals (with identical Er 3+ concentration) due to the smaller unit cell resulting in increased interaction between Er 3+ ions. The upconversion is observed to be dependent on the method of preparation. We explore nanocrystalline Y 2 O 3 :Er 3+ prepared via the propellant synthesis technique and a controlled hydrolysis synthesis (or wet chemical synthesis) where we observed quite diverse upconversion behavior attributed to the vastly different morphological properties of the two different nanocrystalline materials. Additionally, we investigate the effect of Yb 3+ co-doping on the upconversion luminescence of Y 2 O 3 :Er 3+ nanocrystals prepared via the two distinct synthesis routes, and observe a significant change in the mechanisms of upconversion. In the sesquioxides, the upconversion properties of the nanocrystalline material are diverse from the bulk counterpart. Finally, we attempt to ascertain if any spectroscopic changes occur in nanosized Lu 2 O 3 :Nd 3+ , Y 2 O 3 :Sm 3+ and Y 2 O 3 :Dy 3+ prepared via combustion synthesis. In all cases, the size of the particles affects the luminescence behavior

Styles APA, Harvard, Vancouver, ISO, etc.

41

Zheng, Hairong. « Dynamical process of rare earth ions doped in nanocrystals embedded in amorphous matrices ». 2003. http://purl.galileo.usg.edu/uga%5Fetd/zheng%5Fhairong%5F200305%5Fphd.

Texte intégral

Styles APA, Harvard, Vancouver, ISO, etc.

42

Lin, Guang Chun, et 林廣春. « Up-converted Visible Luminescence Properties of Er3+ and Yb3+ Co-doped Y2Ti2O7 Nanocrystals ». Thesis, 2011. http://ndltd.ncl.edu.tw/handle/40667102353356229108.

Texte intégral

Résumé :

碩士
國立中正大學
光機電整合工程研究所
99
The (Er3+, Yb3+)-codoped Y2Ti2O7 nanocrystals were synthesized by the sol-gel method in this work. We were focused on the fluorescent properties of different annealing temperature and different dopent concentrations of ytterbium ions. Er3+ (5%) & Yb3+ (5, 10, 15, 20%)-codoped Y2Ti2O7 nanocrystals with particle size of 50~120 nm were synthesized by the sol-gel method. The strong green band centered at 547 nm (4S3/2 → 4I15/2) and red band centered at 678 nm (4F9/2 → 4I15/2) visiable emission and weak blue band centered at 409 nm (2H9/2 → 4I15/2) emission were observed in codoped nanoparticles under the 980 nm laser diode excitation. The intensity of red band and blue band were increasing by concentration of ytterbium, but the intensity of green was decreased. The sample of (Er3+ 5%, Yb3+ 5%)-codoped Y2Ti2O7 nanocrystals have the best of brightness under 1000 ℃ annealing by photoluminescence spectroscopy measurement. The longest life time (1010 μs) at red band (678 nm) was observed in (Er3+5%, Yb3+5%)-codoped Y2Ti2O7 under 900 ℃ annealing. The mechanism of up-converted red light and green light are two-photon excited state absorption, and the blue light is three-photon excited state absorption.

Styles APA, Harvard, Vancouver, ISO, etc.

43

Wu, Ping-jung, et 巫秉融. « Growth and Characterization of Phosphorus Doped Silicon Nanocrystals Embedded in Silicon Nitride Matrix ». Thesis, 2013. http://ndltd.ncl.edu.tw/handle/25193996218502516239.

Texte intégral

Résumé :

博士
國立中央大學
材料科學與工程研究所
101
For development of next generation crystalline Si solar cells, one of the new concepts for heterojunction solar cells is using wide bandgap materials as emitter layers due to the higher light transmittance. Silicon nanocrystals (Si-NCs) embedded in a wide bandgap dielectric matrix, such as SiOx, SiNx, and SiCx could be used for this purpose. Moreover, for increasing the conductivity of Si-NCs, incorporating the dopants into Si-NCs has also been demonstrated. So far, the doped Si-NCs in SiOx or SiCx matrix have been used for device fabrication. However, there are few studies for discovering the properties of the doped Si-NCs embedded in SiNx, even if SiNx has a higher bandgap than SiCx and better conductivity than SiOx theoretically. In this dissertation, we first utilized electron cyclotron resonance chemical vapor deposition (ECRCVD) rather than sputtering or inhomogeneous implantation process to grow homogeneous Si-rich SiNx films doped with phosphorus atoms. The initial doping concentration was controlled by changing the introduced [PH3]/[SiH4] gas flow ratio. The as-grown phosphorus doped Si-rich SiNx thin films were then annealed for the formation of phosphorus doped Si-NCs. The effects of phosphorus additives on the Si crystallization behavior in Si-rich SiNx films were investigated. From the experimental results, existence of phosphorus enhances phase separation of Si-rich SiNx and Si crystallization rate. As the phosphorus content within the as-grown Si-rich SiNx thin film increases, the Si-NC size in the Si-NCs/SiNx film increases under the same annealing conditions. In addition, the bonding configurations of phosphorus atoms have been investigated and we could speculate that the phosphorus atoms would probably position at three regions according to the analysis results, including Si-NCs, boundaries between Si-NCs and SiNx, or SiNx matrix. Furthermore, observation of the Fano interference and improvement in conductivity of Si-NCs/SiNx films as a function of the phosphorus content within the as-grown Si-rich SiNx thin film provide evidences for the phosphorus activation, that is, the phosphorus atoms could position at the substitutional sites within Si-NCs. Owing to the achievement of Si-NCs n-type doping, Si-NCs/c-Si heterojunction solar cells have also been realized with a best power conversion efficiency of 8.6%.

Styles APA, Harvard, Vancouver, ISO, etc.

44

Chen, Jian-Hao, et 陳建豪. « Studies of photoluminescent properties in Eu-doped Ca(Ti,Zr)O3 provskite nanocrystals ». Thesis, 2017. http://ndltd.ncl.edu.tw/handle/2d6p6q.

Texte intégral

Résumé :

碩士
國立清華大學
物理學系
105
In this thesis, we discuss the relationship between the changes of perovskite structure of europium-doped Ca(Ti,Zr)O3 and its fluorescence properties. Europium-doped Ca(Ti,Zr)O3 were prepared by a sol-gel method and annealed at 800°C in O2. The structure and particle size were investigated by X-ray powder diffraction (XRD). The XRD results show that the particle size is dependent on the Ti/Zr ratio. The extended x-ray absorption fine structure results show that Zr and Ti are in similar chemical environments. The Eu L3-edge EXAFS result exhibits reduced symmetry when Zr dopant atoms are present in the CaTiO3 host structure. Photoluminescence measurements show that the fluorescence intensity increases with the asymmetry of local environment surrounding Eu3+.

Styles APA, Harvard, Vancouver, ISO, etc.

45

Chen, Hsiang-Yun. « Energy Transfer Dynamics and Dopant Luminescence in Mn-Doped CdS/ZnS Core/Shell Nanocrystals ». Thesis, 2012. http://hdl.handle.net/1969.1/148167.

Texte intégral

Résumé :

Mn-doped II-VI semiconductor nanocrystals exhibit bright dopant photoluminescence that has potential usefulness for light emitting devices, temperature sensing, and biological imaging. The bright luminescence comes from the 4T1→6A1 transition of the Mn2+ d electrons after the exciton-dopant energy transfer, which reroutes the exciton relaxation through trapping processes. The driving force of the energy transfer is the strong exchange coupling between the exciton and Mn2+ due to the confinement of exciton in the nanocrystal. The exciton-Mn spatial overlap affecting the exchange coupling strength is an important parameter that varies the energy transfer rate and the quantum yield of Mn luminescence. In this dissertation, this correlation is studied in radial doping location-controlled Mn-doped CdS/ZnS nanocrystals. Energy transfer rate was found decreasing when increasing the doping radius in the nanocrystals at the same core size and shell thickness and when increasing the size of the nanocrystals at a fixed doping radius. In addition to the exciton-Mn energy transfer discussed above, two consecutive exciton-Mn energy transfers can also occur if multiple excitons are generated before the relaxation of Mn (lifetime ~10^-4 - 10^-2 s). The consecutive exciton-Mn energy transfer can further excite the Mn2+ d electrons high in conduction band and results in the quenching of Mn luminescence. The highly excited electrons show higher photocatalytic efficiency than the electrons in undoped nanocrystals. Finally, the effect of local lattice strain on the local vibrational frequency and local thermal expansion was observed via the temperature-dependent Mn luminescence spectral linewidth and peak position in Mn-doped CdS/ZnS nanocrystals. The local lattice strain on the Mn2+ ions is varied using the large core/shell lattice mismatch (~7%) that creates a gradient of lattice strain at various radial locations. When doping the Mn2+ closer to the core/shell interface, the stronger lattice strain softens the vibrational frequency coupled to the 4T1→6A1 transition of Mn2+ (Mn luminescence) by ~50%. In addition, the lattice strain also increases the anharmonicity, resulting in larger local thermal expansion observed from the nearly an order larger thermal shift of the Mn luminescence compared to the Mn-doped ZnS nanocrystals without the core/shell lattice mismatch.

Styles APA, Harvard, Vancouver, ISO, etc.

46

Liu, Chung-Chan, et 劉仲展. « Syntheses and characterizations of gallium arsenide nanowires and carbon nanohelices doped with silicon nanocrystals ». Thesis, 2005. http://ndltd.ncl.edu.tw/handle/31374960691585498961.

Texte intégral

Styles APA, Harvard, Vancouver, ISO, etc.

47

Yu-Ping, Li, et 李御賓. « A Study on Up-converted Luminescence Properties of Er3+-doped Gd2Ti2O7 Nanocrystals and Films ». Thesis, 2014. http://ndltd.ncl.edu.tw/handle/51643065022876576680.

Texte intégral

Résumé :

碩士
國立中正大學
光機電整合工程研究所
102
The purpose of this paper is to change the different doping concentration and the process temperature to synthesize of Er3+-doped Gd2Ti2O7 powders and films, and to explore its fluorescence properties　and characteristics of thin films. Er3+-doped Gd2Ti2O7 nanocrystals were fabricated by the sol-gel method.Under the 980 nm laser excitation of the Er3+ (10 mol%) – doped Gd2Ti2O7 powder with annealing more than 800 ℃ can produce the up-conversion (405, 520, 545, and 650 nm; 2H9/2→4I15/2, 4H11/2→4I15/2, 4S3/2→4I15/2 and 4F9/2→4I15/2, respectively) and Stokes luminescence (1534 nm; 4I13/2→4I15/2).And the XRD shows that while the annealing temperature exceeds 800 °C, amorphous pyrochlore phase Er3+-doped Gd2Ti2O7 transfers to well-crystallized nanocrystals. The average crystal size can be observed by SEM that it increases from 80 nm to 200nm while the annealing temperature increases from 800 to 1200 °C for an hour. We discuss optical properties and roughness of the Er3+ (0, 10, 20, 100 mol%)-doped Gd2Ti2O7film in different concentration and processing temperature.And then we coat colloidal phosphor on the glasses by spin coating. It is found that the average roughness is at 0.1 nm to 0.214 nm, and we use the method (by R.Swanepoel) to obtain the relations of film refractive index, optical band gap and packing density between the process temperature and doping concentration.

Styles APA, Harvard, Vancouver, ISO, etc.

48

Foxy et 邱智偉. « A study on preparation and luminescence properties of Er3+, Tm3+,Yb3+-doped Y2Ti2O7 nanocrystals ». Thesis, 2012. http://ndltd.ncl.edu.tw/handle/hg2h7s.

Texte intégral

Résumé :

碩士
國立中正大學
光機電整合工程研究所
100
Abstrct The (Er3+, Tm3+, Yb3+)-codoped Y2Ti2O7 nanocrystals were synthesized by the sol-gel method in this work. We were focused on the fluorescent properties of different annealing temperature and different highest common factor a concentrations. Er3+ &Tm3+& Yb3+ -codoped Y2Ti2O7 nanocrystals with particle size of 32~124 nm were synthesized by the sol-gel method. The green band centered at 547 nm (4S3/2 → 4I15/2) and red band centered at 660 nm (4F9/2 → 4I15/2) visiable emission and blue band centered at 488 nm (1G4→3H6) emission were observed in codoped nanoparticles under the 980 nm laser diode excitation. The total photon intensity were increasing by lower factor a and highest annealing temperature. The sample of (Er3+ 0.5%, Tm3+0.5%, Yb3+ 1%) -codoped Y2Ti2O7 nanocrystals have the best of brightness under 1000 ℃ annealing by photoluminescence spectroscopy measurement. The mechanism of up-converted red light and green light are two-photon excited state absorption, and the blue light is three-photon excited state absorption.

Styles APA, Harvard, Vancouver, ISO, etc.

49

Pitto-Barry, Anaïs, P. J. Sadler et Nicolas P. E. Barry. « Dynamics of formation of Ru, Os, Ir and Au metal nanocrystals on doped graphitic surfaces ». 2015. http://hdl.handle.net/10454/11261.

Texte intégral

Résumé :

Yes
The fabrication of precious metal (ruthenium, osmium, gold, and iridium) nanocrystals from single atoms has been studied in real-time. The dynamics of the first stage of the metal nanocrystallisation on a doped (B,S)-graphitic surface are identified, captured, and reported.
We thank the Leverhulme Trust (Early Career Fellowship No. ECF-2013-414 to NPEB), the ERC (Grant No. 247450 to PJS), EPSRC (EP/F034210/1 to PJS).

Styles APA, Harvard, Vancouver, ISO, etc.

50

Möller, Björn [Verfasser]. « Optical spectroscopy on single and coupled microspheres doped with semiconductor nanocrystals / vorgelegt von Björn Möller ». 2005. http://d-nb.info/997823259/34.

Texte intégral

Styles APA, Harvard, Vancouver, ISO, etc.

Thèses sur le sujet « Doped Nanocrystals »

Créez une référence correcte selon les styles APA, MLA, Chicago, Harvard et plusieurs autres