Добірка наукової літератури з теми "Nanocrystals - Surface Defects"

Semiconductor zero-dimensional nanocrystals – quantum dots (QDs) – have been increasingly used in various fields of opto- and nanoelectronics in recent decades. This is because of the exciton nature of their luminescence, which can be controlled via the well known quantum-dimensional effect. At the same time, at small nanocrystall sizes, the influence of the surface on the optical and structural properties of nanocrystals increases significantly. The presence of broken bonds of surface atoms and point defects – vacancies and interstial atoms – can both weaken the exciton luminescence and create new effective channels of radiant luminescence. In some cases, these surface luminescence becomes dominant, leading to optical spectra broadening up to the quasi-white light. The nature of such localized states often remains unestablished due to the large number of the possible sorts of defects in both of QD and its surrounding. In contrast to exciton luminescence, which can be properly described within effective-mass approximations, the optical properties of defects relay on chemical nature of both defect itsself and its surrounding, what cannot be provided by “hydrogen-type coulomb defect” approximation. Moreover, charge state and related to this lattice relaxation must be taken into account, what requires an application of atomistic approach, such as Density functioal theory (DFT). Therefore, this review is devoted to the study of surface (defect) states and related luminescence, as well as the analysis of possible defects in nanocrystals of semiconductor compounds A2B6 (CdS, CdZnS, ZnS), responsible for luminescence processes, within ab initio approach. The review presents the results of the authors' and literature sources devoted to the study of the luminescent characteristics of ultra-small (<2 nm) QDs.

Nanotechnology is playing a greater role in biomedical engineering. Microphotonic technology is on another side, having faster growth with more requirements. The nanocrystals are a part of nanotechnology which uses silicon for manufacturing. These silicon nanocrystals have the optical property mostly used in microphotonic devices. Silicon nanocrystals are of biocompatibility with less toxicity. Therefore, the advancement in the silicon nanocrystal helps develop more microphotonic devices for biological purposes. One critical factor of silicon nanocrystal is the surface defects or surface imperfections. Surface passivation is the method employed for rectifying this disadvantage of silicon nanocrystal. Another major thing is that silicon nanocrystals are size dependent. So proper variation on the surface is required for yielding high performance of the nanocrystal. After characterizing the surface of the silicon nanocrystal, ion bombardment can occur. Nickel is a lustrous white chemical element which is less reactive when it is of a smaller size. So ion bombardment of nickel ion on the surface of the silicon nanocrystal can be done to improvise the performance of the microphotonic devices. Nearly there is an excess of 20 a.u. of photoluminescence intensity yielded. The relative fluorescence is also increased by 150 a.u. This research work enhanced the silicon nanocrystal using ion bombardment of nickel ion, which increased energy traps resulting in more intensities.

The near-infrared to near-infrared (NIR-to-NIR) photoluminescence of nanocrystals has outstanding advantages in biological imaging. NaGdF4:Nd3+ core nanocrystals and NaGdF4:Nd3+@NaGdF4 core/shell nanocrystals with different shell thicknesses were synthesized by a simple solvothermal method. The obtained nanocrystals were characterized by transmission electron microscopy (TEM) and X-ray diffraction (XRD) analysis. The phase of all nanocrystals is hexagonal. NaGdF4:Nd3+ core nanocrystals have an average size of 6 nm. By controlling core–shell ratio for 1:2 and 1:3, we obtained NaGdF4:Nd3+@NaGdF4 core/shell nanocrystals with average sizes of 10 nm and 11 nm, respectively. When excited at 808 nm, strong NIR emission was observed. The emission peaks at ∼860 nm, ∼1060 nm and ∼1330 nm correspond to the transitions from the 4F3/2 statetothe 4I9/2, 4I11/2 and 4I13/2 state of Nd3+ ions, respectively. The emission intensity of NaGdF4:Nd3+@NaGdF4 core/shell nanocrystals is stronger than that of the core. The intensity increases with the increase of shell thickness. The shell improves the luminous efficiency by reducing surface defects. The decay time of Nd3+ emission in NaGdF4:Nd3+@NaGdF4 core/shell nanocrystal is longer than that in NaGdF4:Nd3+ core, indicating that the shell isolates effectively the emitting ions (Nd3+)from the quenching defects. With the increase of shell thickness, the decay time becomes longer. Within a certain range of shell thickness, thicker shell can protect the emitting Nd3+ ions on the surface of core nanocrystals more effectively.

It has been observed that the defect centers on the surface play a crucial role in the conductivity behavior of ZnO. Above 300 °C only surface defects can be visible in EPR spectra for ZnO nanocrystals which indicate p-type conductivity.

In recent years, rare earth doped upconversion nanocrystals have been widely used in different fields owing to their unique merits. Although rare earth chlorides and trifluoroacetates are commonly used precursors for the synthesis of nanocrystals, they have certain disadvantages. For example, rare earth chlorides are expensive and rare earth trifluoroacetates produce toxic gases during the reaction. To overcome these drawbacks, we use the less expensive rare earth hydroxide as a precursor to synthesize β-NaYF4 nanoparticles with multiform shapes and sizes. Small-sized nanocrystals (15 nm) can be obtained by precisely controlling the synthesis conditions. Compared with the previous methods, the current method is more facile and has lower cost. In addition, the defects of the nanocrystal surface are reduced through constructing core–shell structures, resulting in enhanced upconversion luminescence intensity.

Abstract We have fabricated Eu3+-doped ZnO (ZnO:Eu) nanocrystals (NCs) surface-modified with organic molecules such as dodecylamine (DDA) and tri-n-octylphosphine oxide (TOPO) and have studied their optical properties. In the ZnO:Eu NCs without surface modification, strong broad photoluminescence (PL) band due to surface defects is observed, so the Eu3+ PL peaks are not observed because they are covered by the strong surface-defect PL band. In the DDA-capped ZnO:Eu NCs, the Eu3+ PL and the exciton PL are observed because of suppression of the surface-defect PL caused by the inactivation of the surface defects. Contrary to expectations, the surface modification with TOPO suppressed not only the surface-defect PL but also the exciton PL. As a result, the ZnO:Eu NCs with red PL due to the Eu3+ ions have been successfully prepared. We discuss the influence of surface modification by DDA and TOPO on optical properties.

The growth of InGaN layers on hybrid SiC/Si substrates with orientations (100), (110), and (111) by the HVPE method was studied at temperatures that wittingly exceed the temperature of InN decomposition onto nitrogen atoms and metallic In (1000oC). On substrates with orientations (110) and (111), the formation of InGaN whisker nanocrystals was observed. The shape and growth mechanisms of nanocrystals were investigated. It is shown that nanocrystals nucleate on the (111) surface only inside V-defects formed at the points where screw dislocations exit onto the surface. On the (110) surface, nanocrystals are formed only on pedestals that arise during the film growth. An explanation is given for the difference in the growth mechanisms of nanocrystals on substrates of different orientations. Keywords: InGaN, heterostructures, SiC on Si, silicon, whisker nanocrystals, nanostructures, atomic substitution method

The growth of InGaN layers on hybrid SiC/Si substrates with orientations (100), (110), and (111) by the HVPE method was studied at temperatures that wittingly exceed the temperature of InN decomposition onto nitrogen atoms and metallic In (1000C). On substrates with orientations (110) and (111), the formation of InGaN nanocrystals was observed. The shape and growth mechanisms of nanocrystals were investigated. It is shown that nanocrystals nucleate on the (111) surface only inside V-defects formed at the points where screw dislocations exit onto the surface. On the (110) surface, nanocrystals are formed only on pedestals that arise during the film growth. An explanation is given for the difference in the growth mechanisms of nanocrystals on substrates of different orientations.

ABSTRACTFully inorganic lead halide perovskite nanocrystals (NCs) are of interest for photovoltaic and light emitting devices due to optoelectronic properties. Understanding the surface chemistry of these materials is of importance as surface defects can introduce trap-states which reduce their functionality. Here we use Density Functional Theory (DFT) to model surface defects introduced by Pb2+ on a CsPbBr3 NC atomistic model. Two types of defects are studied: (i) an under-coordinated Pb2+ surface atom and (ii) Pb2+ atomic or molecular adsorbents to the NC surface. From the DFT calculations we compute the density of states (DOS) and absorption spectra of the defect models to the pristine fully-passivated NC model. We observe that for the low surface defect regime explored here that neither (i) or (ii) produce trap-states inside of the bandgap and exhibit bright optical absorption for the lowest energy transition. From the models studied, it was found that the Pb2+ atomic absorbent provides broadening of the conduction band edge, which implies chemisorption of Pb2+ to the NC surface. At higher defect densities it would be expected that Pb2+ atomic absorbents would introduce trap-states and degrade the opto-electronic properties of these materials.

Durante la mia tesi di dottorato mi sono occupata dello studio spettroscopico di nanocristalli colloidali di semiconduttore e in particolare della correlazione tra le loro superfici e la fotofisica, che ho studiato per mezzo di spettroelettrochimica (SEC) e spettroscopia ottica in atmosfera controllata. Più precisamente, ho studiato e modellizzato il comportamento di diversi tipi di nanocristalli sottoponendoli a variazioni controllate delle condizioni ossidanti/riducenti dell’ambiente che li circonda, con l’obiettivo di implementarne l’uso in sensori ottici di ossigeno. L’elevato rapporto superficie-volume tipico dei nanocristalli fa sì che la loro fotoluminescenza sia fortemente influenzata da un’ampia distribuzione di stati introdotti da difetti superficiali. Un portatore catturato da una trappola in superficie, infatti, non è più disponibile per la ricombinazione radiativa, e abbassa la resa quantica del nanocristallo. Tramite SEC, si può applicare una differenza di potenziale elettrochimico (EC) a un sottile film di nanocristalli depositati su un substrato trasparente e conduttivo. La fotolumiscenza del campione viene eccitata e raccolta sia in continua sia temporalmente risolta tramite appositi rivelatori. L’applicazione di una differenza di potenziale EC negativo corrisponde ad aumentare il livello di Fermi del nanocristallo, riempiendone gradualmente i difetti superficiali e attivando l’intrappolamento di lacune. La fotoluminescenza che ne risulta dipende dall’effetto dominante tra lo spegnimento della stessa dovuto all’intrappolamento di lacune e l’aumento derivante dalla soppressione dell’intrappolamento di elettroni. Per ciascun materiale ho effettuato misure di SEC ed esperimenti in atmosfera controllata, per arrivare alla realizzazione di diversi tipi di sensori ottici di ossigeno (Pressure Sensitive Paints, PSPs), usati per monitorare il flusso di ossigeno nei pressi di superfici complesse o miniaturizzate. Tipicamente consistono in un mezzo poroso in cui è disperso un cromoforo organico, la cui emissione cambia in modo inversamente proporzionale alla pressione di ossigeno. Ho utilizzato nanocristalli di perovskite (cesio-piombo-bromo, CsPbBr3) per realizzare un’alternativa completamente inorganica alle PSP tradizionali, basate su un aumento di segnale sotto pressione ridotta. Questo approccio però non è ottimale in applicazioni in cui è necessario rilevare grandi quantità di ossigeno (a pressione ambientale, per esempio). Un avanzamento in questo senso è fornito dalla PSP ‘inversa’ che ho realizzato tramite nanoplatelet di seleniuro di cadmio (CdSe), che diversamente dai materiali tradizionali per PSP sono in grado di illuminarsi maggiormente, invece che di spegnersi, in presenza di ossigeno. Nonostante il vantaggio offerto dal materiale a comportamento inverso, sia le PSP inverse sia quelle tradizionali si basano su una misura radiometrica di intensità luminosa, la quale però può cambiare anche in seguito a variazioni di temperatura, o degradazione indotta da UV, il che comporta la necessità di complesse procedure di calibrazione. Un miglioramento importante che ho introdotto nel corso del mio dottorato è rappresentato dall’impiego di nanocristalli bi-emissivi core/shell di seleniuro di cadmio/solfuro di cadmio (CdSe/CdS), in grado di sostenere contemporaneamente eccitoni di core e di shell, la cui ricombinazione radiativa porta a fotoluminescenza a due colori (rosso e verde) anche con basse potenze di eccitazione. É importante notare che i due canali emissivi presentano una risposta opposta all’ossigeno, il che mi ha permesso di realizzare una PSP raziometrica e intrinsecamente calibrata, con elevata sensibilità sia a livello di ensemble sia di singola particella.
The main research theme of my PhD has been the spectroscopic investigation of colloidal semiconductor nanocrystals (NCs), with a focus on the correlation between their surfaces and their photophysics, and was conducted by means of spectroelectrochemistry (SEC) and optical spectroscopy under controlled atmosphere. Specifically, I aimed to understand and model the NCs behavior in a changing oxidative/reducting environment, with the ultimate goal to implement their use as active material in optical oxygen pressure sensors. The high surface-to-volume ratio typical of NCs causes their photoluminescence (PL) efficiency to be strongly affected by a broad distribution of surface defect states. If captured by a surface trap, a photogenerated electron (or hole) becomes unavailable for the radiative recombination, thus lowering the overall PL efficiency of the NCs. By means of SEC, an electrochemical (EC) potential can be applied to a thin film of NCs deposited onto a transparent and conductive substrate, whose PL is excited and collected via dedicated instruments for either continuous or time-resolved measurements. The application of a negative EC potential corresponds to raising the Fermi level of the NCs, thus gradually filling the surface defects and activating their hole-trapping capability. The PL intensity is thus determined by the competition between the quenching effect of hole withdrawal and the brightening effect of suppressed electron trapping. For each material system I performed side-by-side SEC measurements and spectroscopic experiments under controlled atmosphere, and eventually demonstrated different types of optical oxygen pressure sensors, also called pressure-sensitive paints (PSPs), i.e, all-optical probes for monitoring oxygen flows in the vicinity of complex or miniaturized surfaces. They typically consist in a porous binder embedding an oxygen sensitive chromophore, whose PL intensity changes accordingly to the oxygen partial pressure. By employing cesium lead bromide (CsPbBr3) perovskite NCs, I realized an all-inorganic alternative to traditional organic PSPs, based on the increase of their PL intensity under reduced oxygen pressure. This approach relies on the disappearance of the signal in presence of oxygen, which means it may not represent the best approach when high oxygen concentrations (for instance, at atmospheric pressure) need to be detected. In this thesis, I demonstrated how to overcome this issue by realizing a novel-concept, inorganic ‘reverse’ PSP, with cadmium selenide (CdSe) nanoplatelets (NPLs) as active material, since their PL intensity increases with the oxygen concentration. Although the SEC and optical measurements under controlled atmosphere allowed me to understand and model the unusual benefit of an oxidative environment on CdSe NPLs, the PSPs based on them share with the perovskite-based sensors the major drawback of providing a radiometric oxygen detection only, that is, the measurement solely relies on a change in the PL intensity of the chromophore. The PL, however, can also change as a result of a temperature variation or UV-induced degradation. In my work, I introduced a significant improvement by employing dual-emitting, core/shell cadmium selenide/cadmium sulfide (CdSe/CdS) NCs that are capable of simultaneously sustaining core and shell excitons, whose radiative recombination leads to two-color (red and green) luminescence under low-intensity power excitation. Importantly, the two emissive channels exhibit opposite responses to the oxygen pressure, which allowed me to realize an intrinsically calibrated ratiometric PSP whose sensitivity is significantly enhanced with respect to traditional reference-sensor pairs, both in ensemble and at the single particle level.

The dissertation starts from the understanding of dislocation dissipation mechanism due to the image force acting on the dislocation. This work implements a screw dislocation in solids with free surfaces by a novel finite element model, and then image forces of dislocations embedded in various shaped GaN nanorods are calculated. As surface stress could dramatically influence the behavior of nanostructures, this work has developed a novel analytical framework to solve the stress field of solids with dislocations and surface stress. It is successfully implemented in this framework for the case of isotropic circular nanowires (2D) and the analytical result of the image force has been derived afterwards. Based on the finite element analysis and the analytical framework, this work has a semi-analytical solution to the image force of isotropic nanorods (3D) with surface stress. The influences of the geometrical parameter and surface stress are illustrated and compared with the original finite element result. In continuation, this work has extended the semi-analytical approach to the case of anisotropic GaN nanorods. It is used to analyze image forces on different dislocations in GaN nanorods oriented along polar (c-axis) and non-polar (a, m-axis) directions. This work could contribute to a wide range of nanostructure design and fabrication for dislocation-free devices.

碩士
國立交通大學
光電工程系所
97
The fluctuation-dissipation theorem unveils the importance of thermal molecular motion which always exists, even in thermal equilibrium, as a fluctuation. However, the underlying information of thermal fluctuation which appears to be random noise is a huge question mark that had been overlooked for years. This thesis study focuses on the discussion about the dynamics of molecular fluctuations in a specific condensed matter- the ferroelectric liquid crystals (FLC) with and without doping of nanocrystal ZnO. The dynamics of orientation director fluctuations is governed by the material properties of the liquid crystal. In this thesis, we first derive the relation between the scattered light intensity and fluctuations of the FLC director which, through some reasonable assumption, could be described as a stochastic equation of motion. After performing autocorrelation technique to the scattered light signals, we have come to realize that the internal fluctuation is characterized by a correlation function of relevant physical quantities of the FLC system fluctuating in thermal equilibrium. The measurement results lead to the fact of improved molecular alignment and faster response time in the SSFLC cell doped with ZnO nanocrystals.

Abstract Additive Manufacturing (AM) which is also known as metal 3D printing technique is one of the promising manufacturing processes due to the capability to process a complex geometry component. This is implemented in wide range of applications in various industries such as automotive, aerospace, power plants, etc. The aging nuclear power plant components and the obsolescence of those components has become a concern in this industry, and AM has come as an alternative solution for this matter. The Board on Pressure and Technology Codes and Standards (BPTCS) and Board on Nuclear Codes and Standards (BNCS) Special Committees started to study the application of Powder Bed Fusion (PBF) technique for pressure retaining equipment made from UNS S31603. Also, later Korean International Working Group (KIWG) was also started a Task Group on Additive Manufacturing for Valves which focusing on Powder Bed Fusion (PBF) and Direct Energy Disposition (DED) process for pressure-retaining valve manufacturing especially for nuclear power plant application with the same material. However, the poor mechanical properties and performance, especially fatigue strength of AM materials become a concern due to the defects and flaws as the results of layering and multiple interfaces and welding related discontinuities. In this study, the fatigue strength of PBF and DED manufactured and Ultrasonic Nanocrystal Surface Modification (UNSM) treated UNS S31603 austenitic stainless steel was investigated.