Thèses sur le sujet « Nanojunction »

A molecular junction consists of a single molecule or self-assembled monolayer (SAM) placed between two electrodes. It has varieties of functionalities due to quantum interference in nanoscale. Although there exist issues, advantages could still appeal to scientists who wish to investigate transport properties in many aspects such as electronics, thermoelectronics, spintronics, and optotronics. Recent studies of single-molecule thermoelectricity have identified families of high-performance molecules. However, controlled scalability might be used to boost electrical and thermoelectric performance over the current single-junction paradigm. In order to translate this discovery into practical thin-film energyharvesting devices, there is a need for an understanding of the fundamental issues arising when such junctions are placed in parallel. As a first step in this direction, we investigate here the properties of two C60 molecules placed in parallel and sandwiched between top and bottom graphene electrodes. It is found that increasing the number of parallel junctions from one to two can cause the electrical conductance to increase by more than a factor of 2 and furthermore, the Seebeck coefficient is sensitive to the number of parallel molecules sandwiched between the electrodes, whereas classically it should be unchanged. This non-classical behaviour of the electrical conductance and Seebeck coefficient are due to interjunction quantum interference, mediated by the electrodes, which leads to an enhanced response in these vertical molecular devices. Except the study of thermoelectricity, on the other hand, stable, single-molecule switches with high on-off ratios are an essential component for future molecularscale circuitry. Unfortunately, devices using gold electrodes are neither complementary metal-oxide-semiconductor (CMOS) compatible nor stable at room temperature. To overcome these limitations, several groups are developing electroburnt graphene electrodes for single molecule electronics. Here, in anticipation of these developments, we examine how the electrical switching properties of a series of porphyrin molecules with pendant dipoles can be tuned by systematically increasing the number of spacer units between the porphyrin core and graphene electrodes. The porphyrin is sandwiched between a graphene source and drain and gated by a third electrode. The associated rotation of porphyrin referred to graphene plane leads to the breaking of conjugation and a decrease in electrical conductances. As the number of spacers is increased, the conductance ratio can increase from 100 with one spacer to 200 with four spacers, and further enhanced by decreasing the temperature, reaching approximately 2200 at 100K. This design for a molecular switch using graphene electrodes could be extended to other aromatic systems. As mentioned in the design of 퐶60 -based thermoelectric vertical junction with graphene layers as electrodes and porphyrin-based switch in graphene nanogap, graphene provides a two-dimensional platform for contacting individual molecules, which enables transport spectroscopy of molecular orbital, spin, and vibrational states. Next, we report single-electron tunnelling through a molecule that has been anchored to two graphene leads. It is found that quantum interference within the graphene leads gives rise to an energy-dependent transmission and fluctuations in the sequential tunnelling. The lead states are electrostatically tuned by a global back-gate due to the weak screening effect compared to the metal electrodes, resulting in a distinct pattern of varying intensity in the measured conductance maps. This pattern could potentially obscure transport features that are intrinsic to the molecule under investigation. Finally, using ensemble averaged magneto-conductance measurements, lead and molecule states are disentangled, enabling spectroscopic investigation of the single molecule. As the above describes, there are varieties of research on the charge transport properties of molecular devices. It is noticed that noise exists in all electronic devices, and the investigation on noise could help us understand more fundamental information of the device, i.e. the imperfections and configurational changes in the system, the correlation of the transmission conduction channels or even exploit the noise characteristics for biosensing. In electroburnt graphene nanogaps, our collaborators observe that 1/f noise and random telegraph noise at room temperature and 77K respectively. Here, I employ a simple one-dimensional tight binding model to gauge the effect of two-level fluctuations in the electrostatic environment in the tunnel junctions. Two types of models are investigated. Model I describes the case that the environmental traps drive the tunnel barrier locally and differently. Model II is the case that the collective effect of all the environmental traps drives the tunnel barrier synchronously. It is concluded that the 77 K data is best described by a single environmental fluctuator influencing the transmission through the tunnel barrier. This may either occur via a local perturbation of the barrier potential, or via an overall modulation of the barrier height. A 1/푓 signal emerges as more fluctuators with different lifetime 휏 are added to the environment, corresponding to the thermal activation of multiple random telegraph noises (RTNs) at room temperature.

L’une des application les plus remarquables de l’effet tunnel est le microscope à effet tunnel (STM) qui permet de cartographier spatialement et énergétiquement la répartition des électrons à la surface des matériaux avec une résolution nanométrique. Des avancées récentes permettent en outre d’exploiter la pointe du STM comme une source d’excitation locale des matériaux. Le travail de thèse présenté dans ce manuscrit vise à décrire et à modéliser les phénomènes impliqués lors d’une telle excitation. Nous présentons une modélisation des spectres d’absorption de molécule de phthalocyanine reposant sur des surfaces dans le cadre de la théorie de la fonctionnelle de densité dépendante du temps (TD-DFT). Nous montrons que l’analyse spectroscopique des transitions entre l’état fondamental et les états excités de la molécule permet de caractériser son état de contrainte. Nous mettons également en évidence une variété de spectres d’excitation selon la localisation de l’excitation de la molécule. Nous discutons la possibilité d’exploiter ce phénomène pour caractériser les transports d’énergie inter-moléculaire
One of the most remarkable applications of the tunnel effect is the Scanning Tunneling Microscope (STM), allowing to get the spatially and energetically map distribution of electrons on the surface of materials with nanometric resolution. Recent advances make it possible to exploit the tip of the STM as a source of local excitation of materials. The work presented in this manuscript aims to describe and model the phenomena involved in such excitation process. We present a modeling of the absorption spectra of phthalocyanine molecules lying on surfaces within the framework of the time-dependent density functional theory (TD-DFT). We show that spectroscopic analysis of the transitions between the ground state and the excited states of the molecule allows to characterize the stress inside the molecule. We also highlight a variety of excitation spectra depending on the location of the excitation of the molecule. We discuss the possibility of exploiting this phenomenon to characterize inter-molecular energy transport

L'objectif de cette thèse est de développer des méthodes théoriques et numériques pour calculer la diffusion d'ondes de spin et leur transport balistique à travers deux types de nanomatériaux magnétiques désordonnés de terres rares - métaux de transition, à savoir le cobalt-gadolinium et le fer-gadolinium, comme éléments constitutifs des systèmes de nanojunctions. La modélisation développée dans ce travail décrit proprement les conséquences du désordre caractéristique de ces systèmes, à savoir de type alliage et celui de type structurel. Les méthodes théoriques et numériques développées servent en particulier à explorer les attributs de ces nanojonctions comme des filtres et des éléments de transmission assistée par résonance dans des dispositifs magnoniques. La thèse développe une version dynamique et non-locale pour l'approximation du potentiel (DNLCPA) afin d'étudier la dynamique de spin des systèmes ultraminces magnétiques désordonnés Fe-Gd et Co-Gd. Les potentiels aléatoires dynamiques de diffusion sont dérivés d'une manière inédite, exploitant les propriétés de phase des excitations de spin élémentaires dans le cadre du formalisme de Dyson. La méthode théorique est ensuite développée en deux manières fondamentales différentes, pour l'appliquer convenablement aux nano systèmes désordonnés qui présentent les types de désordre alliage et structurel. L'approche DNLCPA est ensuite conjuguée avec la théorie de raccordement de phase des champs (PFMT) pour étudier le transport balistique d'ondes de spin à travers les nanojonctions Co-Gd et Fe-Gd entre des gUides d'ondes de Co et Fe respectivement. L'approche PFMT-DNLCPA donne pour la première fois une modélisation des propriétés de diffusion et de transport d'ondes de spin incidents sur les nanojonctions, elle réussit à démontrer, modéliser et à quantifier la perte d'énergie en diffusion balistique due à chaque type de désordre
The aim of this thesis is to develop theoretical and numerical methods to analyze the ballistic spin waves scattering and transport across two types of rare earth - transition metals disordered magnetic nanomaterials, namely the cobalt-gadolinium and the iron-gadolinium types, as building blocks for nanojunction systems. The theoretical computations developed in this work account properly for the consequences of the characteristic disorder present in these systems, whether alloy disorder for the former or structural amorphous-like disorder for the latter. The developed methods serve, in particular, to explore the attributes of these nanojunctions as filters and elements for resonance assisted transmission in a magnonic device. The thesis develops a novel and dynamic non-local version of the coherent potential approximation (DNLCPA), to study the spin dynamics on disordered ultrathin Co-Gd and Fe-Gd magnetic systems. The dynamic random scattering potentials are derived in a completely novel approach, exploiting the phase properties of the elementary spin excitations within the Dyson formalism. This approach is then developed in two different fundamental manners, and applied appropriately for the disordered nanosystems presenting alloy and structural disorder. The DNLCPA approach is incorporated with the phase field matching theory (PFMT) to study the spin waves ballistic transport across the Co-Gd and the Fe-Gd nanojunctions, sandwiched between Co and Fe leads respectively. This PFMT-DNLCPA method yields for the first time the description of the scattering and transport properties for the spin waves incident on the nanojunctions. Furthermore, our computations successfully demonstrate, model and quantify the diffusive energy loss in ballistic scattering due to each type of disorder

碩士
國立交通大學
電子物理系所
100
Using first-principles approaches, we have investigated the thermoelectric properties of two different systems of nano-junction. The first system is Carbon-atoms chains connecting bimetal junction, and the other one is paired metal-Br-Al junction. In Carbon-atoms chains system, we observe odd-even effects in the Seebeck coefficient of carbon-atom chains as the number of carbon atoms increases. The Seebeck coefficients show minus signs for the odd-numbered atomic chains and plus signs for the even-numbered atomic chains in low temperatures regime. The reason for the odd-even effects is that the tangent of transmission functions near the chemical potential (Fermi level) change signs as the number of carbon atoms in the junction increases. In the paired metal-Br-Al junction, we have investigated the thermoelectric properties and energy conversion efficiency. Owing to the narrow states in the vicinity of the chemical potential, the nanojunction has large Seebeck coefficients such that it can be considered an efficient thermoelectric power generator.We also consider the nanojunction in a three-terminal geometry, where the current, voltage, power, and efficiency can be efficiently modulated by the gate voltages. Such current_voltage characteristics could be useful in the design of nanoscale electronic devices such as a transistor or switch.

碩士
淡江大學
物理學系碩士班
100
One-dimensional nanostructures have been demonstrated as outstanding materials for fabricating ultrasensitive nanosensors .In order to obtain gigantic sensitivity, our research team had proposed such a new mechanism, schottky-gate effect, Schottky-contacted device can enhance the sensitivity of nanosensor devices dramatically. In this study, we deliberately introduce a point contact of nonsymmetrical Schottky junction (PCSJ). The sensitivity of this new device can be greatly enhanced due to the nano Schottky contact area. The signal sensitivity was more than 2500%, the response and reset time of UV sensing are less than 1s. This work will provide a new technology to design the nanodevice and nanosensor.

碩士
國立交通大學
材料科學與工程系所
95
This research focus on the preparation of hybrid polymer/zinc oxide (ZnO) solar cells, in which the ZnO rods grown perpendicularly on ITO substrates, as a means to provide a direct and ordered path for photogenerated electrons to the collecting electrode. We use sol-gel method to fabricate ZnO backing layer and low-temperature solution-based growth techniques to grow ZnO nanostructures. Different backing layer thickness, heat-treatment condition, growth time were investigated for solar cell application .After deposit polymer, using scanning electron microscopy, absorption spectroscopy and photovoltaic device measurements to study the morphology and device performance of the prepared structures. The dependence of the photovoltaic performance on the ZnO nanorods length and the organic layer thickness was investigated. Under AM1.5(100mWcm -2) global solar condition, the best ZnO nanorods/ poly(3-hexylthiophene) photovoltaic device yields power conversion efficiency 0.909% and a peak external quantum efficiency 22% at 460nm. The power conversion efficiency of it is tree times as large as that of ZnO film/ regioregular poly(3-hexylthiophene) bilayer system. The ZnO / polythiophene device is limited in photocurrent due to the large spacing between the ZnO fibers. This is overcome by blending PCBM into the polythiophene film and can enhance power conversion efficiency to 1.01%.

In this thesis various spin crossover complexes have been synthesized and nanostructured using various techniques. Thermal high vacuum sublimation and chemisorption through hydrosilylation have been used to fabricate the nanomaterials. Characterization of the obtained nanostructures has been performed by using surface-sensitive spectroscopic techniques (XPS and XAS). Furthermore, a spin crossover-based nanomaterial has been integrated in a nanojunction.

Electrical conduction at the single molecule scale has been studied extensively with molecular nanojunctions. Measurements have revealed a wealth of interesting physics. I3owever; our understanding is hindered by a lack of methods for simultaneous local imaging or spectroscopy to determine the conformation and local environment of the molecule of interest. Optical molecular spectroscopies have made significant progress in recent years, with single molecule sensitivity achieved through the use of surface-enhanced spectroscopies. In particular surface-enhanced Raman spectroscopy (SERS) has been demonstrated to have single molecule sensitivity for specific plasmonic structures. Many unanswered quest ions remain about the SERS process, particularly the role of chemical enhancements of the Raman signal. The primary goal of the research presented here is to combine both electrical and optical characterization techniques to obtain a more complete picture of electrical conduction at the single or few molecule level. We have successfully demonstrated that nanojunctions are excellent SERS substrates with the ability to achieve single molecule sensitivity. This is a major accomplishment with practical applications in optical sensor design. We present a method for mass producing nanojunctions with SERS sensitivity optimized through computer modeling. We have demonstrated simultaneous optical and electrical measurements of molecular junctions with single molecule electrical and SERS sensitivity. Measurements show strong correlations between electrical conductance and changes to the SERS response of nanojunctions. These results allow for one of the most conclusive demonstrations of single molecule SERS to date. This measurement technique provides the framework for three additional studies discussed here as well as opening up the possibilities for numerous other experiments. One measurement examines heating in nanowires rather than nanojunctions. We observe that, the electromigration process used to turn Pt nanowires into nanojunctions heats the wires to temperatures in excess of 1000 K, indicating that thermal decomposition of molecules on the nanowire is a major problem. Another measurement studies optically driven currents in nanojunctions. The photocurrent is a result of rectification of the enhanced optical electric field in the nanogap. From low frequency electrical measurements we are able to infer the magnitude of the enhanced electric field, with inferred enhancements exceeding 1000. This work is significant to the field of plasmonics and shows the need for more complete quantum treatments of plasmonic structures. Finally we investigate electrical and optical heating in molecular nanojunctions. Our measurements show that molecular vibrations and conduction electrons in nano-junctions under electrical bias or laser illumination can be driven from equilibrium to temperatures greater than 600 K. We observe that individual vibrations are also not in thermal equilibrium with one another. Significant heating in the conduction electrons in the metal electrodes was observed which is not expected in the ballistic tunneling model for electrons in nanojunctions this indicates a need for a more completely energy dissipation theory for nanojunctions.

Optical trapping is an important tool for studying and manipulating nanoscale objects. Recent experiments have shown that subwavelength control of nanoparticles is possible by using patterned plasmonic nanostructures, rather than using a laser directly, to generate the electric fields necessary for particle trapping. In this thesis we present a theoretical model and experimental evidence for plasmonic optical trapping in nanoscale metal junctions. Further, we examine the use of the resultant devices as ultrasmall photodectors. Electromigrated nanojunctions, or “nanogaps”, have a well-established plasmon resonance in the near-IR, leading to electric field enhancements large enough for single-molecule sensitivity in Surface-Enhance Raman (SERS) measurements. While molecule-based devices have been carefully studied, optically and electrically probing individual quantum dots in nanoscale metal junctions remains relatively unexplored. Plasmon-based optical trapping of quantum dots into prefabricated structures could allow for inexpensive, scalable luminescent devices which are fully integrable into established silicon-based fabrication techniques. Additionally, these metal-nanocrystal-metal structures are ideal candidates to study optoelectronics in ultrasmall nanocrystals-based structures, as well as more exotic nanoscale phenomena such as blinking, plasmon-exciton interactions, and surface-enhanced fluorescence (SEF). We present experimental data supporting plasmon-based optical trapping in the nanogap geometry, and a corresponding numerical model of the electric field-generated forces in the nanogap geometry. Further, we give proof-of-concept measurements of photoconductance in the resultant quantum dot-based devices, as well as challenges and improvements moving forward.

碩士
國立交通大學
電子物理系所
102
In this thesis, we have applied the maximally localized Wannier function approach[Wannier90] to a density function theory based first principle code[Quantum espresso]. In order to understand the behavior of magnetic atom in different environment, we calculate three different systems which are isolated atom system, CuCoCu nanojunction system and Co doped in Cu bulk system. We found that magnetic properties, PDOS and Maximally localized Wannier functions will be varied depends on the symmetry between the magnetic d orbital and nearest atoms around it. We also found spreads of MLWF will be changed depending on distance between MLWF and nearest atoms around it and spreads also depends on spin if it is a spin polarized case.

Thesis (Ph.D.)--University of Nebraska-Lincoln, 2008.
Title from title screen (site viewed Apr. 9, 2009). PDF text: v, 156 p. : ill. (some col.) ; 9 Mb. UMI publication number: AAT 3338831. Includes bibliographical references. Also available in microfilm and microfiche formats.

碩士
國立臺灣大學
化學研究所
104
The fundamental of molecular electronics involves electron-transport through electrode-molecule-electrode junctions. The transporting efficiency can be correlated to barrier height by tunneling decay constant. The interaction of molecular electronics with plasmons, collective oscillations of free electrons coupling to electromagnetic fields, has drawn lots of attentions. The formation of hot electrons from surface plasmon decay results in photocurrent. In addition, hot electron can also be generated from photoexcitation. We measure the tunneling phenomenon without molecules bridging between electrodes and the responses of gold and silver electrodes, solvent including octylbenzene and propylene carbonate, and electromagnetic field to tunneling decay constant by scanning tunneling microscopy break junction (STM BJ). The tunneling decay constant is reduced for silver electrodes in propylene carbonate when irradiated, that would be realized by comparing the work function of metal, polarity of solvent and by the presence of hot electrons. Following the concept of hot electrons, we investigate the increased conductance of molecules that binding to Au electrodes via head group under illumination and explain the differences in the conductance enhancement of 2,7-diaminofluorene and 4,4′-bipyridine.