Дисертації з теми "Hybrid quantum devices"

Significant research efforts have been directed towards the development of solar cells comprising blends of conjugated polymers and II-VI inorganic semiconductors (e.g. CdSe and CdS). Despite recent advances in the power conversion efficiency of such devices, the toxicity of Cd-based materials remains a concern with regard to widespread implementation. This thesis focuses on alternative (lower toxicity) InP nanocrystals for use as electron acceptors and light-harvesting materials in solution-processed polymer solar cells. In this thesis a combination of novel materials design/processing, transient absorption spectroscopy (TAS) and time-resolved photoluminescence spectroscopy (TRPL) is used to study the charge generation in InP:polymer photoactive layers. These studies are complimented by morphological characterisation of the photoactive layers as well as device studies. One aim of this thesis is the elucidation of quantitative structure function relationships that can be used to guide the design of new hybrid nanocomposite materials for photovoltaic devices. As such the data presented in this thesis helps to advance the present day understanding how hybrid solar cells work. The first chapter focuses on the synthesis of InP quantum dots (QDs) using an organometallic reaction. The aim of the work in this chapter was to prepare InP QDs with a size that provides an appropriate energy offset relative to the selected the electron donating polymer, poly(3-hexylthiophene) (P3HT). Detailed studies on the growth of InP QDs and how the reaction conditions affect the particle size are provided. The process of ligand exchange from hexadecylamine (HDA) to pyridine prior to blending with P3HT is also described. The second chapter focuses on charge transfer between the P3HT and the InP QDs which is a key process for achieving efficient photovoltaic device operation. Steady state and time-resolved photoluminescence and absorption spectroscopy were used to better understand the parameters influencing charge separation. After the blending and annealing conditions had been optimised to maximise the yield of photogenerated charges, the P3HT:InP blend was found to provide approximately twice yield of standard P3HT:PCBM blends. In addition, the decay lifetime of the polaron in P3HT:InP was found to be longer than that of P3HT:PCBM, suggesting the P3HT:InP blend is a promising active layer material for hybrid solar cells. The third chapter focuses on the fabrication and characterisation of hybrid solar cells. The fabrication conditions were optimised before carrying out detailed studies on the effect of thermal annealing. Although the device performance improved significantly with increasing annealing temperature, the net photocurrent was found to be low, compared to standard P3HT:PCBM devices, suggesting poor charge transport within the device. Nevertheless, if the charge transport can be improved, P3HT:InP still has potential to provide efficient hybrid solar cells. The last result chapter focuses on preliminary studies of quantum dot based light emitting diodes (QDLEDs) using InP QDs as light emitters. ZnO was used as electron transporting and hole blocking layer and poly(9,9-dioctylfluorene) (PFO) as a host medium and a hole transporting layer. The device structure and the PFO:InP blend composition were investigated to obtain QDLEDs with electroluminescence from the InP quantum dots. The findings suggest that ZnO plays a key role in suppressing the electroluminescence of PFO, most likely due to the hole blocking effect of the ZnO layer. Despite the low efficiencies of the InP-based QDLEDs, the results suggest that InP QDs are potential candidates for emitters in QDLEDs.

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Electrical Engineering and Computer Science, 2005.
Includes bibliographical references (p. 157-169).
Organic light emitting diodes have undergone rapid advancement over the course of the past decade. Similarly, quantum dot synthesis has progressed to the point that room temperature highly efficient photoluminescence can be realized. It is the purpose of this work to utilize the beneficial properties of these two material sets in a robust light emitting device. New deposition techniques are necessary to the realization of this goal, enabling QD organic hybrids to be created in a quick and reliable manner compatible with known device fabrication methods. With these techniques, quantum dot light emitting devices are fabricated, measured, and analyzed. The devices are of high efficiency and color saturation, and provide us with a test bed for understanding the interactions between inorganic QDs and organic thin films.
by Seth Coe-Sullivan.
Ph.D.

The emergence of optical applications, such as lasers, fiber optics, and semiconductor based sources and detectors, has created a drive for smaller and more specialized devices. Nanophotonics is an emerging field of study that encompasses the disciplines of physics, engineering, chemistry, biology, applied sciences and biomedical technology. In particular, nanophotonics explores optical processes on a nanoscale. This dissertation presents nanophotonic applications that incorporate various forms of the organic polymer N-isopropylacrylamide (NIPA) with inorganic semiconductors. This includes the material characterization of NIPA, with such techniques as ellipsometry and dynamic light scattering. Two devices were constructed incorporating the NIPA hydrogel with semiconductors. The first device comprises a PNIPAM-CdTe hybrid material. The PNIPAM is a means for the control of distances between CdTe quantum dots encapsulated within the hydrogel. Controlling the distance between the quantum dots allows for the control of resonant energy transfer between neighboring quantum dots. Whereby, providing a means for controlling the temperature dependent red-shifts in photoluminescent peaks and FWHM. Further, enhancement of photoluminescent due to increased scattering in the medium is shown as a function of temperature. The second device incorporates NIPA into a 2D photonic crystal patterned on GaAs. The refractive index change of the NIPA hydrogel as it undergoes its phase change creates a controllable mechanism for adjusting the transmittance of light frequencies through a linear defect in a photonic crystal. The NIPA infiltrated photonic crystal shows greater shifts in the bandwidth per ºC than any liquid crystal methods. This dissertation demonstrates the versatile uses of hydrogel, as a means of control in nanophotonic devices, and will likely lead to development of other hybrid applications. The development of smaller light based applications will facilitate the need to augment the devices with control mechanism and will play an increasing important role in the future.

Abstract This thesis presents a novel method for fabricating quantum dot light-emitting devices (QDLEDs) based on colloidal inorganic light-emitting nanoparticles incorporated into an organic semiconductor matrix. CdSe core/ZnS shell nanoparticles were inkjet-printed in air and sandwiched between organic hole and electron transport layers to produce efficient photon-emissive media. The light-emitting devices fabricated here were tested as individual devices and integrated into a display setting, thus endorsing the capability of this method as a manufacturing approach for full-colour high-definition displays. By choosing inkjet printing as a deposition method for quantum dots, several problems currently inevitable with alternative methods are addressed. First, inkjet printing promises simple patterning due to its drop-on-demand concept, thus overruling a need for complicated and laborious patterning methods. Secondly, manufacturing costs can be reduced significantly by introducing this prudent fabrication step for very expensive nanoparticles. Since there are no prior demonstrations of inkjet printing of electroluminescent quantum dot devices in the literature, this work dives into the basics of inkjet printing of low-viscosity, relatively highly volatile quantum dot inks: piezo driver requirements, jetting parameters, fluid dynamics in the cartridge and on the surface, nanoparticle assembly in a wet droplet and packing of dots on the surface are main concerns in the experimental part. Device performance is likewise discussed and plays an important role in this thesis. Several compositional QDLED structures are described. In addition, different pixel geometries are discussed. The last part of this dissertation deals with the principles of QDLED displays and their basic components: RGB pixels and organic thin-film transistor (OTFT) drivers. Work related to transistors is intertwined with QDLED work; ideas for surface treatments that enhance nanoparticle packing are carried over from self-assembled monolayer (SAM) studies in the OTFT field. Moreover, all the work done in this thesis project was consolidated by one method, atomic force microscopy (AFM), which is discussed throughout the entire thesis.

The present work focuses on two strategies contributing to the development of high efficiency, cost-effective photovoltaic (PV) technology for renewable energy generation: the design of new materials offering enhanced opto-electronic performance and the investigation of material degradation processes and their role in predicting the long-term reliability of PV modules in the field. The first portion of the present work investigates the integration of a novel CdTe-ZnO nanocomposite material as a spectral sensitizer component within a thin-film, hybrid heterojunction (HJ) PV device structure. Quantum-scale semiconductors have the potential to improve PV device performance through enhanced spectral absorption and photocarrier transport. This is realized via appropriate design of the semiconductor nanophase (providing tunable spectral absorption) and its spatial distribution within an electrically active matrix (providing long-range charge transport). Here, CdTe nanocrystals, embedded in an electrically active ZnO matrix, form a nanocomposite (NC) offering control of both spectral absorption and photocarrier transport behavior through the manipulation of nanophase assembly (ensemble effects). A sequential radio- frequency (RF) magnetron sputter deposition technique affords the control of semiconductor nanophase spatial distribution relative to the HJ plane in a hybrid, ZnO-P3HT test structure. Energy conversion performance (current density-voltage (J-V) and external quantum efficiency (EQE) response) was examined as a function of the location of the CdTe nanophase absorber region using both one dimensional solar cell capacitance simulator (SCAPS) and the experimental examination of analogous P3HT-ZnO based hybrid thin films. Enhancement in simulated EQE over a spectral range consistent with the absorption region of the CdTe nanophase (i.e. 400–475 nm) is confirmed in the experimental studies. Moreover, a trend of decreasing quantum efficiency in this spectral range with increasing separation between the CdTe nanophase region and the heterojunction plane is observed. The results are interpreted in terms of carrier scattering/recombination length mitigating the successful transport of carriers across the junction. The second portion of the research addresses the need for robust PV performance in commercial module as a primary contributor to cost-effective operation in both distributed systems and utility scale generation systems. The understanding of physical and chemical mechanisms resulting in the degradation of materials of construction used in PV modules is needed to understand the contribution of these processes to module integrity and performance loss with time under varied application environments. In this context, the second part of present study addresses microcracking in Si–an established degradation process contributing to PV module power loss. The study isolates microcrack propagation in single-crystal Si, and investigates the effect of local environment (temperature, humidity) on microcrack elongation under applied strains. An investigation of microindenter-induced crack evolution with independent variation of both temperature and vapor density was pursued in PV-grade Si wafers. Under static tensile strain conditions, an increase in sub-critical crack elongation with increasing atmospheric water content was observed. To provide further insight into the potential physical and chemical conditions at the microcrack tip, micro-Raman measurements were performed. Preliminary results confirm a spatial variation in the frequency of the primary Si vibrational resonance within the crack-tip region, associated with local stress state, whose magnitude is influenced by environmental conditions during the period of applied static strain. The experimental effort was paired with molecular dynamics (MD) investigations of microcrack evolution in single-crystal Si to furnish additional insight into mechanical contributions to crack elongation. The MD results demonstrate that crack-tip energetics and associated cracking crystal planes and morphology are intimately related to the crack and applied strain orientations with respect to the principal crystallographic axes. The resulting fracture surface energy and the stress-strain response of the Si under these conditions form the basis for preliminary micro-scale peridynamics (PD) simulations of microcrack development under constant applied strain. These efforts were integrated with the experimental results to further inform the mechanisms contributing to this important degradation mode in Si-based photovoltaics.

Le contexte de cette thèse est une proposition théorique par Haikka et al [1] qui vise à détecter des spins individuels avec des micro-ondes, en utilisant un micro-résonateur supraconducteur incorporant une constriction nanométrique proche du spin, refroidi au millikelvin. Les spins utilisés sont des centres NV du diamant, implantés à une faible profondeur (20nm) dans un échantillon de diamant purifié isotopiquement en ¹²C. Sa fréquence de transition est ~ 2.88 GHz. Nous démontrons la fabrication d'une grille de NV unique avec de longs temps de cohérence et bien localisée par rapport aux marques d'alignement gravées dans le diamant. Nous démontrons une méthode pour déterminer la position de centres NV par rapport à un nanofil métallique déposé sur du diamant. Nous utilisons le centre NV comme magnétomètre vectoriel [2, 3] pour mesurer le champ généré par le passage d'un courant continu à travers le fil, permettant d'inférer la position des centres NV par rapport au fil avec une précision de ~ 10 nm. Nous avons fabriqué et réalisé la caractérisation d'un résonateur LC de faible impédance 11Ω, d'un facteur de qualité interne aussi élevé que 2.10⁵ et d'une fréquence de résonance d'environ 2,93 GHz au-dessus d'un tel ensemble de NV implantés. La résistance en champ magnétique de ce résonateur n'a cependant pas été suffisante pour voir le signal de spin. [1] P. Haikka, Y. Kubo, A. Bienfait, P. Bertet, and K. Mølmer, “Proposal fordetecting a single electron spin in a microwave resonator,” Phys. Rev. A, vol. 95,p. 022306, Feb 2017. [2] J. M. Taylor, P. Cappellaro, L. Childress, L. Jiang, D. Budker, P. R. Hemmer, A. Yacoby, R. Walsworth, and M. D. Lukin, “High-sensitivity diamond magnetometer with nanoscaleresolution,”Nature Phys, vol. 4, pp. 810–816, Oct. 2008. [3] X.-D. Chen, F.-W. Sun, C.-L. Zou, J.-M. Cui, L.-M. Zhou, and G.-C. Guo, “Vector magneticfield sensing by a single nitrogen vacancy center in diamond,” EPL, vol. 101, p. 67003, Mar.2013
The context of this thesis is a proposal by Haikka et al. [1] that aims at detecting individual spins with microwaves, using a superconducting micro-resonator that incorporates a nanometric constriction located close to the spin, cooled down to millikelvin temperatures. The electron spins of choice are shallow implanted ~ 20 nm depth single Nitrogen - Vacancy (NV) centers in an isotopically purified ¹²C diamond. Their transition frequency is ~ 2.88 GHz We report the fabrication of a single NV center grid with long coherence times and well localized with respect to alignment marks etched in the diamond. We demonstrate a method to determine the position of shallow individual implanted nitrogen-vacancy (NV) centers with respect to a metallic nanowire deposited on diamond. We use the NV center as a vector magnetometer [2, 3] to measure the field generated by passing a DC current through the wire, enabling to infer the NV centers position relative to the wire with a precision of ~ 10 nm. We fabricated and performed the characterization of a LC resonator of low impedance 11Ω, internal quality factor as high as 2.10⁵ and resonance frequency of ~ 2.93 GHz on top of such an ensemble of implanted NVs. The magnetic field resilience of the resonator was however not sufficient to observe the spin signal. [1] P. Haikka, Y. Kubo, A. Bienfait, P. Bertet, and K. Mølmer, “Proposal fordetecting a single electron spin in a microwave resonator,” Phys. Rev. A, vol. 95,p. 022306, Feb 2017. [2] J. M. Taylor, P. Cappellaro, L. Childress, L. Jiang, D. Budker, P. R. Hemmer, A. Yacoby, R. Walsworth, and M. D. Lukin, “High-sensitivity diamond magnetometer with nanoscaleresolution,” Nature Phys, vol. 4, pp. 810–816, Oct. 2008. [3] X.-D. Chen, F.-W. Sun, C.-L. Zou, J.-M. Cui, L.-M. Zhou, and G.-C. Guo, “Vector magneticfield sensing by a single nitrogen vacancy center in diamond,” EPL, vol. 101, p. 67003, Mar.2013

The realization of a quantum computer, after being considered only a wishful thinking for many years, looks now more solid, with the major issues about its realization looking like “engineering problems”, whose solution is not far to be found. The most considerable open problems concerning the realization of a quantum computer concern: the sensitivity to external disturbances, the addressability of any single qubit, and the scalability of the proposed architectures. These problems may have opposite solutions, since a good scalability would require high density of qubits embedded in solid state matrices, making it more difficult to address and protect each of them. Conversely, well separated qubits realized by atoms or ions are easily addressed and protected, but make it more difficult to achieve scalability. Another aspect related to the realization of generic quantum devices (i.e. devices which exploit the quantum features of their constituents to realize some operations) is connected to the way one interfaces with them: we live in a “classical world”, and any quantum device will have to interface with it at some stage, e.g. to be controlled or measured. The problem, in this case, is keeping our classical disturbances as far as possible from the action taking place at the heart of the quantum device. However, the problem of being able to find the proper “interface” can be turned into an advantage, since macroscopic systems may posses robustness features which can be exploited for the realization of novel quantum devices where the sensitivity problem is much alleviated. A good interface can be thus provided by semi-classical systems, i.e. objects whose nature is intimately quantum, but whose features and behaviour is well framed within a classical description. Any scheme, application or device, where a semi-classical system interacts with some pure quantum object (e.g. a qubit) for realizing a quantum operation, will be referred to as hybrid. In this thesis, we have developed the idea of exploiting the robustness features of some non-linear classical systems in order to realize quantum devices where the sensitivity problem is alleviated. Hence, we have proposed a hybrid scheme for accomplishing the most basic actions related with the realization of a magnetic- based quantum device, i.e. the control of a qubit state and entanglement generation. In particular, we have developed it focusing on magnetic one-dimensional systems, i.e. spin chains, as channels connecting one or more qubits to some external control apparatus. This choice follows from the observation that classical spin chains are non-linear systems which enumerate solitons among their dynamical configurations which are solutions of their equations of motion. In fact, solitons are those particular solutions of non-linear systems which show space-time localization and shape- invariant evolution and are celebrated for their impressive properties of robustness against scattering and external disturbances. These properties make them suitable candidates for practical purposes. We have considered systems made by one-dimensional discrete lattices hosting, at each lattice site, one classical spin (classical spin chain) or a large-S spin, i.e. a quantum spin characterized by a S-value large enough for the spin to be well described by a semi-classical behaviour (large-S spin chain). A classical, or a large-S, spin chain (with its solitons) can play the role of the robust partner while the role of the fragile quantum system is played by the qubit, which is the agent of the relevant quantum operations in our hybrid quantum device.We have first introduced a method for generating solitons on discrete, classical Heisenberg chains by applying a time-dependent magnetic field to one of the chain extremities. The method has been numerically checked, revealing the actual possibility of producing soliton-like dynamical configurations running on the spin chain, which resemble the known analytical solitons of the continuous chain if their typical width is large with respect to the chain spacing. The robustness of the generated solitons has been also tested with respect to thermal noise present in the system. We have then proposed a set-up where the generated soliton acts as a magnetic signal that travels along the chain and eventually reaches a qubit and changes its quantum state. Since any unitary action on a single qubit can be represented in terms of a Zeeman interaction lasting for a precise time interval, qubit state control is usually assumed to be obtained by applying suitable sequences of external magnetic fields. Indeed, for this particular application, the spin chain needs not to be quantum and the suitable magnetic field is provided by the moving deformation of the uniform chain represented by the magnetic soliton travelling along the chain. Numerical results confirm that solitons are suitable for this task, giving the possibility to remotely control the qubit state by an appropriate choice of soliton shape and qubit-chain coupling. We have finally addressed the problem of generating entanglement between distant qubits by introducing a model where two qubits, distant and non-interacting, are locally coupled with a large-S Heisenberg chain, whose dynamics is assumed to be characterized by the presence of solitons. The aim of this study was to verify if, by properly choosing the state of the spin chain, the evolution of such a robust semi-classical system can bring the two qubits from a separable initial state to a non-separable state after a certain amount of time, i.e., if a semi-classical channel could generate entanglement between the two qubits. At variance with the previous application, the spin chain must here be considered, and consequently treated, as a quantum system in order to allow for entanglement generation/transfer, but a suitable approximation is needed to solve the chain dynamics, as accounting for the exact evolution of large-S spin chain is out of reach, even numerically. This leads us to introduce a particular set of chain states built as product of single-spin coherent states which are in one-to-one correspondence with the configurations of the classical spin chain and are thus referred to as the semi-classical states of the chain. Being able to solve the evolution of the coherent state products, allow us to complete the hybrid scheme for entanglement generation: in fact, it is shown that, choosing the chain initial state as a semi-classical state corresponding to a soliton configuration, the correlations, generated between one qubit and the corresponding spin-S, are efficiently transferred along the chain up to the other qubit, finally leaving the two qubits in an entangled state. The results about the proposed hybrid schemes, showing their effectiveness for remotely controlling qubit states and generating entanglement between distant qubits, encourage further studies opening also new perspectives for the realization of novel quantum devices based on the exploitation of the robustness features of semi-classical systems.

碩士
國立交通大學
光電工程研究所
104
In this thesis, We studied the device engineering and photophysical properties of delay fluorescent materials, quantum dots and inorganic halide perovskite light-emitting devices. In the introduction, We briefly reviewed the applications and current development of organic light-emitting diodes (OLEDs) for solid-state lighting and display. In the second chapter, We reviewed the history of OLEDs, and their operating principles and measurement methodology. In the third chapter, We investigated two novel excimer-formation materials, and studied the characteristics of the excimer emission. Careful transient photophysical measurements revealed the exciton up-conversion from triplet states to singlet states. The excimer-base OLEDs were fabricated and optimized by judicious selection of the electron transport materials. The device showed an external quantum efficiency (EQE) up to 6.5%, which is higher than the theoretical efficiency of the conventional fluorescent OLEDs. In the fourth chapter, We measured the photoluminescence (PL) quantum yield of the quantum dots (QDs). And fabricated quantum dot light-emitting devices by blending QDs with Tris(4-carbazoyl-9-ylphenyl)amine (TCTA) as an emission layer. Vertical phase separation between QDs and TCTA was observed .The device with lowest QDs concentration possessed the highest EQE of 0.62%. In the fifth chapter, We studied a series of thermally activated delayed fluorescence materials, of which the emission colors range from green to deep-blue. Large bandgap materials were selected as their host materials. We measured the temperature-dependent transient PL to analyze the delay fluorescence characteristics of these compounds. The efficient deep-blue OLEDs with International Commission on Illumination (CIE) coordinates of (0.18, 0.14) and a EQE up to 6.5% were demonstrated. In chapter six, Cesium Lead halide Perovskite films were deposited by dual source thermal evaporation. We further studied the charge transport and emission characteristics of Cesium Lead halide Perovskites. Finally, We demonstrated organic-inorganic hybrid light-emitting devices.

This dissertation develops a novel application of the resonant-infrared matrix-assisted pulsed laser evaporation (RIR-MAPLE) technique toward the end goal of conjugated-polymer-based optoelectronic device fabrication. Conjugated polymers are attractive materials that are being investigated in the development of efficient optoelectronic devices due to their inexpensive material costs. Moreover, they can easily be combined with inorganic nanomaterials, such as colloidal quantum dots (CQDs), so as to realize hybrid nanocomposite-based optoelectronic devices with tunable optoelectronic characteristics and enhanced desirable features. One of the most significant challenges to the realization of optimal conjugated polymer-CQD hybrid nanocomposite-based optoelectronics has been the processes by which these materials are deposited as thin films, that is, conjugated polymer thin film processing techniques lack sufficient control so as to maintain preferred optoelectronic device behavior. More specifically, conjugated-polymer-based optoelectronics device operation and efficiency are a function of several attributes, including surface film morphology, internal polymer chain morphology, and the distribution and type of nanomaterials in the film bulk. Typical conjugated-polymer thin-film fabrication methodologies involve solution-based deposition, and the presence of the solvent has a deleterious impact, resulting in films with poor charge transport properties and subsequently poor device efficiencies. In addition, many next-generation conjugated polymer-based optoelectronics will require multi-layer device architectures, which can be difficult to achieve using traditional solution processing techniques. These issues direct the need for the development of a new polymer thin film processing technique that is less susceptible to solvent-related polymer chain morphology problems and is more capable of achieving better controlled nanocomposite thin films and multi-layer heterostructures comprising a wide range of materials. Therefore, this dissertation describes the development of a new variety of RIR-MAPLE that uses a unique target emulsion technique to address the aforementioned challenges.

The emulsion-based RIR-MAPLE technique was first developed for the controlled deposition of the conjugated polymers poly[2-methoxy-5-(2'-ethyl-hexyloxy)-1,4-phenylene vinylene] (MEH-PPV) and poly[2-methoxy-5-(2'ethylhexyloxy)-1,4-(1-cyanovinylene) phenylene] (MEH-CN-PPV) into homogenous thin films. Therein, it was identified that target composition had the most significant influence on film surface morphology, and by tuning the concentration of hydroxyl bonds in the target bulk, the laser-target absorption depth could be tuned so as to yield more or less evaporative deposition, resulting in films with tunable surface morphologies and optical behaviors.

Next, the internal morphologies of emulsion-based RIR-MAPLE-deposited MEH-PPV thin films were investigated by measuring their hole drift mobilities using the time-of-flight (TOF) photoconductivity method in the context of amorphous materials disorder models (Bässler's Gaussian Disorder model and the Correlated Disorder model) in order to provide a quantitative measure of polymer chain packing. The polymer chain packing of the RIR-MAPLE-deposited films was demonstrated to be superior and more conducive to charge transport in comparison to spin-cast and drop-cast MEH-PPV films, yielding enhanced hole mobilities.

The emulsion-based RIR-MAPLE technique was also developed for the deposition of different classes of inorganic nanoparticles, namely un-encapsulated nanoparticles and ligand-encapsulated nanoparticles. These different classes of nanoparticles were identified to have different film growth regimes, such that either rough or smooth films were obtained, respectively. The ligand-encapsulated nanoparticles were then co-deposited with MEH-PPV as conjugated polymer-CQD hybrid nanocomposites, wherein the distributions of the constituent materials in the film bulk were identified to be tunable, from homogeneous to highly clustered. The RIR-MAPLE deposition regime determined the said distributions, that is, if the polymer and CQDs were sequentially deposited from a sectioned target or simultaneously deposited from a single target, respectively. The homogeneous conjugated polymer-CQD nanocomposites were also investigated in terms of their charge transport properties using the TOF photoconductivity technique, where it was identified that despite the enhanced dispersion of CQDs in the film bulk, the presence of a high concentration of CQDs degraded hole drift mobility, which indicates that special considerations must be taken when incorporating CQDs into conjugated-polymer-based nanocomposite optoelectronics.

Finally, the unique capability of RIR-MAPLE to enable novel conjugated polymer-based optical heterostructures and optoelectronic devices was evaluated by the successful demonstration of a conjugated polymer-based distributed Bragg reflector (DBR), a plasmonic absorption enhancement layer, and a conjugated polymer-based photovoltaic solar cell featuring a novel electron-transporting layer. These optical heterostructures and optoelectronic devices demonstrate that all of the constituent polymer and nanocomposite layers have controllable thicknesses and abrupt interfaces, thereby confirming the capability of RIR-MAPLE to achieve multi-layer, conjugated polymer-based heterostructures and device architectures that are appropriate for enhancing specific desired optical behaviors and optoelectronic device efficiencies.

Dissertation

The main motivation of this thesis is derived from the fact that physics of disordered systems like conjugated polymer has yet not achieved as concrete understanding as ordered and crystalline systems such as inorganic semiconductors. Through the work done in this thesis, several efforts have been made in order to understand basic charge transport (hopping, current injection) phenomena and photo-physical properties (photoluminescence quenching, absorption, photoconductivity) in conjugated polymer and their hybrid composites. The thesis consists of 7 chapters. Chapter 1 discusses the background knowledge and information of the general properties of conjugated polymers, quantum dots and their hybrid nanocomposites. Chapter 2 deals with the sample preparation and experimental techniques used in this thesis. Chapter 3 elaborates the temperature and field dependent anisotropic charge transport in polypyrrole. Chapter 4 presents an idea to probe and correlate disorder and transport properties using impedance and Raman spectroscopy. Chapter 5 mainly talks about the doping level dependent photophysical and electrical properties of poly(3-hexylthiophene). Chapter 6 reveals the charge transport phenomena in hybrid composites of poly(3,4-ethyldioxythiophene):polysterene sulfonate (PEDOT:PSS) and cadmium telluride quantum dots. Chapter 1: Conjugated polymers and their hybrid systems are easily processible and cost effective material having huge scope for advanced materials of the future. Although variable range hopping (VRH) is widely accepted to model charge transport in π-conjugated systems, but at very low temperatures, high fields, high carrier concentrations one need to explore other models. Conjugated polymers are anisotropic intrinsically. Therefore, anisotropic charge transport can provide basic insights about the physics of charge hopping. Quantum dots, and their hybrid nanocomposites with semiconducting polymers receiving a huge attention for light emission and photovoltaic purposes. It is important to learn about the charge injection,barrier heights, etc. in order to achieve efficient hybrid devices. Chapter 2: Synthesis of the samples, both conjugated polymers and quantum dots, and fabrication of hybrid devices is an important and integral part of this thesis. An Electropolymerization technique is used for making polymer samples on conducting substrates. This is quite interesting because one can tune doping level, disorder and thickness simultaneously. Hydrothermal process is adopted to get highly aqua-dispersible quantum dots. Samples are characterized by different techniques like Raman spectroscopy, energy dispersive spectroscopy. Photoluminescence, UV-Vis absorption, transmission electron microscopy and atomic force microscopy are used to explore several properties of the polymer and hybrid nanocomposites. Chapter 3: It is known that conjugated polymers are intrinsically one–dimensional materials. Therefore it is important to learn anisotropic behavior of these complex systems. Hence, a comparison of electronic transport to their morphology has been carried out and role of carrier density and disorder is discussed further. Both in-plane and out-of-plane charge transport is studied in electrochemically deposited polypyrrole on platinum. Strong anisotropy is observed in the system which is correlated to granular morphology. Field dependence of anisotropic conductivity is also explored. Field scaling analysis shows that all field dependent curves of conductance at different temperatures can fall on to single master curve. Glazman – Matveev model is used to describe nonlinear conduction in field dependence and nonlinearity exponent is estimated. Disorder and carrier density along with the morphological structure like length and orientation of polymer chains with stacking arrangement of different layers in PPy films play an important role in governing the anisotropy in transport properties. Chapter 4: Two different techniques, namely impedance and Raman spectroscopies are used to probe disorder and transport properties in the polypyrrole. An effort is made to correlate the transport properties to the morphology by probing disorder via two different spectroscopic techniques. Frequency dependence of both real and imaginary part has shown that disorder and inhomogeneity varies in different PPy devices, which thus affect the transport properties like conductivity and mobility. Mobility values along the thickness direction for each sample reveal the impact of disorder on out-of¬plane geometry. A circuit based on consideration of the distributed relaxation times, is successfully used to obtain the best fit for the Cole–Cole plot of various PPy devices. FWHM of the de-convoluted peaks of Raman spectra is attributed to the change in distribution of the conjugation length in the PPy films. Chapter 5: The main focus of this chapter is the qualitative exploration of different photo-physical and electrical properties of electropolymerized poly(3-hexylthiophene) and their dependence on doping level. Photoluminescence quenching, band edge shifting in absorption spectra, electrochromic effect, significant enhancement in photocurrent at optimum doping level, two relaxation behaviors in reactance spectra and presence of negative capacitance at low frequencies are distinct features which are observed in poly(3-hexylthiophene) in this work. Quenching in photoluminescence intensity is attributed to charge transfer occurring between polymer chains and dopant ions. Two semicircles in the Cole-Cole plots refer to two type of relaxation process occurring in bulk layer and at interface. Frequency response of capacitance at higher bias and lo side of frequency shows a negative capacitance due to the relaxation mechanism associated with the space-charge effect. Chapter 6: Synthesis of quantum dots and fabrication of hybrid devices is one of the catchy parts of this chapter. Huge quenching photoluminescence intensity and very high increment (~ 400 %) in photocurrent clearly depict the charge transfer at molecular level. Temperature dependent current–voltage characteristics show the absence of thermionic emission since the barrier height is more than the thermal energy of the carriers. Further analysis confirms that the charge carrier injection of ITO/PPCdTe3/Al device is controlled by tunneling processes. The hybrid system has shown a peculiar transition from direct tunneling to Fowler–Nordheim tunneling mechanism which is because of the change in shape of the barrier height from trapezoidal to triangular type with increase in applied electric field. Chapter 7: The conclusions of the different works presented in this thesis are coherently summarized in this thesis. Thoughts and prospective for future directions are also summed up.

There is a compelling need to explore different material options as well as device structures to facilitate smooth transistor scaling for higher speed, higher density and lower power. The enormous potential of nanoelectronics, and nanotechnology in general, offers us the possibility of designing devices with added functionality. However, at the same time, the new materials come with their own challenges that need to be overcome. In this work, we have addressed some of these challenges in the context of quasi-2D Silicon, III-V semiconductor and graphene. Bulk Si is the most widely used semiconductor with an indirect bandgap of about 1.1 eV. However, when Si is thinned down to sub-10nm regime, the quasi-2D nature of the system changes the electronic properties of the material significantly due to the strong geometrical confinement. Using a tight-binding study, we show that in addition to the increase in bandgap due to quantization, it is possible to transform the original in direct bandgap to a direct one. The effective masses at different valleys are also shown to vary uniquely in an anisotropic way. This ultra-thin Si, when used as a channel in a double gate MOSFET structure, creates so called “volume in version” which is extensively investigated in this work. It has been found that the both the quantum confinement as well as the gating effect play a significant role in determining the spatial distribution of the charge, which in turn has an important role in the characteristics of transistor. Compound III-V semiconductors, like Inx Ga1-xAs, provide low effective mass and low density of states. This, when coupled with strong confinement in a nanowire channel transistor, leads to the “Ultimate Quantum Capacitance Limit” (UQCL) regime of operation, where only the lowest subband is occupied. In this regime, the channel capacitance is much smaller than the oxide capacitance and hence dominates in the total gate capacitance. It is found that the gate capacitance change qualitatively in the UQCL regime, allowing multi-peak, non-monotonic capacitance-voltage characteristics. It is also shown that in an ideal condition, UQCL provides improved current saturation, on-off ratio and energy-delay product, but a degraded intrinsic gate delay. UQCL shows better immunity towards series resistance effect due to increased channel resistance, but is more prone to interfacial traps. A careful design can provide a better on-off ratio at a given gate delay in UQCL compared to conventional MOSFET scenario. To achieve the full advantages of both FinFET and HEMT in III-V domain, a hybrid structure, called “HFinFET” is proposed which provides excellent on performance like HEMT with good gate control like FinFET. During on state, the carriers in the channel are provided using a delta-doped layer(like HEMT) from the top of a fin-like non-planar channel, and during off state, the gates along the side of the fin(like FinFET) help to pull-off the carriers from the channel. Using an effective mass based coupled Poisson-Schrodinger simulation, the proposed structure is found to outperform the state of the art planar and non-planar MOSFETs. By careful optimization of the gate to source-drain underlap, it is shown that the design window of the device can be increased to meet ITRS projections at similar gate length. In addition, the performance degradation of HFinFET in presence of interface traps has been found to be significantly mitigated by tuning the underlap parameter. Graphene is a popular 2D hexagonal carbon crystal with extraordinary electronic, mechani-cal and chemical properties. However, the zero band gap of grapheme has limited its application in digital electronics. One could create a bandgap in grapheme by making quasi-1D strips, called nanoribbon. However, the bandgap of these nanoribbons depends on the the type of the edge, depending on which, one can obtain either semiconducting or metallic nanoribbon. It has been shown that by the application of an external transverse field along the sides of a nanoribbon, one could not only modulate the magnitude of the bandgap, but also change it from direct to indirect. This could open up interesting possibilities for novel electronic and optoelectronic applications. The asymmetric potential distribution inside the nanoribbon is found to result in such direct to indirect bandgap transition. The corresponding carrier masses are also found to be modulated by the external field, following a transition from a“slow”electron to a“fast” electron and vice-versa. Experimentally, it is difficult to control the bandgap in nanoribbons as precise edge control at nanometer scale is nontrivial. One could also open a bandgap in a bilayer graphene, by the application of vertical electric field, which has raised a lot of interest for digital applications. Using a self-consistent tight binding theory, it is found that, inspite of this bandgap opening, the intrinsic bias dependent electronic structure and the screening effect limit the subthreshold slope of a metal source drain bilayer grapheme transistor at a relatively higher value-much above the Boltzmann limit. This in turn reduces the on-off ratio of the transistor significantly. To overcome this poor on-off ratio problem, a semiconductor source-drain structure has been proposed, where the minority carrier injection from the drain is largely switched off due to the bandgap of the drain. Using a self-consistent Non-Equilibrium Green’s Function(NEGF) approach, the proposed device is found to be extremely promising providing unipolar grapheme devices with large on-off ratio, improved subthreshold slope and better current saturation. At high drain bias, the transport properties of grapheme is extremely intriguing with a number of nontrivial effects. Optical phonons in monolayer grapheme couple with carriers in a much stronger way as compared to a bilayer due to selection rules. However, it is difficult to experimentally probe this through transport measurements in substrate supported grapheme as the surface polar phonons with typical low activation energy dominates the total scattering. However, at large drain field, the carriers obtain sufficient energy to interact with the optical phonons, and create so called ‘hot phonons’ which we have experimentally found to result in a negative differential conductance(NDC). The magnitude of this NDC is found to be much stronger in monolayer than in bilayer, which agrees with theoretical calculations. This NDC has also been shown to be compensated by extra minority carrier injection from drain at large bias resulting in an excellent current saturation through a fundamentally different mechanism as compared to velocity saturation. A transport model has been proposed based on the theory, and the experimental observations are found to be in agreement with the model.

碩士
國立暨南國際大學
光電科技碩士學位學程在職專班
100
In recent decades, variety of the porous silicon material components was highly discussed, investigated, & intentional developed, such as photo detectors, light emitting diodes, and solar cells.As for this case study focus the surface of the silicon substrate on its electrochemical etching experiments, together utilizing the material properties of porous silicon to further improved the rate of light absorption, thereby increasing degrees of its responsively. However, the use of the current density adjustment, by etching of the different holes in its correspondences, further through the optical conductivity measurement methodology of its ongoing light reflection & measurement. In contrast, the strength characteristics of the porous silicon surface area on its volume ratio, achieved better performance in light absorption. Simultaneously, carrying out nanoparticle quantum dots, appropriately implementing correspondent holes, moreover, further highlighted the light response degrees in the optical absorption spectrums via the phenomenon of the Band-gap Alignment, as in further illustrated the porous silicon of hybrid quantum dot will significantly improve its responsiveness. In concluded the experiment & findings of this case study, I strongly believed the porous silicon of its future application by ways of porous silicon optical sensors developments will play a great assist enhancement in a new coming era.