Electrical resistance of individual defects at a topological insulator surface

Three-dimensional topological insulators host surface states with linear dispersion, which manifest as a Dirac cone. Nanoscale transport measurements provide direct access to the transport properties of the Dirac cone in real space and allow the detailed investigation of charge carrier scattering. Here, we use scanning tunnelling potentiometry to analyse the resistance of different kinds of defects at the surface of a (Bi0.53Sb0.47)2Te3 topological insulator thin film. The largest localized voltage drop we find to be located at domain boundaries in the topological insulator film, with a resistivity about four times higher than that of a step edge. Furthermore, we resolve resistivity dipoles located around nanoscale voids in the sample surface. The influence of such defects on the resistance of the topological surface state is analysed by means of a resistor network model. The effect resulting from the voids is found to be small compared to the other defects

Thermoelectric Limitations of Graphene Nanodevices at Ultrahigh Current Densities.

Graphene is atomically thin, possesses excellent thermal conductivity, and is able to withstand high current densities, making it attractive for many nanoscale applications such as field-effect transistors, interconnects, and thermal management layers. Enabling integration of graphene into such devices requires nanostructuring, which can have a drastic impact on the self-heating properties, in particular at high current densities. Here, we use a combination of scanning thermal microscopy, finite element thermal analysis, and operando scanning transmission electron microscopy techniques to observe prototype graphene devices in operation and gain a deeper understanding of the role of geometry and interfaces during high current density operation. We find that Peltier effects significantly influence the operational limit due to local electrical and thermal interfacial effects, causing asymmetric temperature distribution in the device. Thus, our results indicate that a proper understanding and design of graphene devices must include consideration of the surrounding materials, interfaces, and geometry. Leveraging these aspects provides opportunities for engineered extreme operation devices

Spatial control of electron & hole states in InAs/GaSb heterostructures

Single nanowire transistors employing three separately controlled electrostatic gates were fabricated to investigate band gap modulation in InAs-GaSb heterostructures. The aim is to show hybridization between electron and hole states over the heterojunction. Electric and thermoelectric characterization at low temperatures suggests that transport can be tuned from electrons in InAs to holes GaSb and that the relative band alignment can be altered from an inverted to a small effective gap. The band gap modulation is speculated to be caused by quantum confinement induced by the gates

Thermoelectric Limitations of Graphene Nanodevices at Ultrahigh Current Densities

Graphene is atomically thin, possesses excellent thermal conductivity, and is able to withstand high current densities, making it attractive for many nanoscale applications such as field-effect transistors, interconnects, and thermal management layers. Enabling integration of graphene into such devices requires nanostructuring, which can have a drastic impact on the self-heating properties, in particular at high current densities. Here, we use a combination of scanning thermal microscopy, finite element thermal analysis, and operando scanning transmission electron microscopy techniques to observe prototype graphene devices in operation and gain a deeper understanding of the role of geometry and interfaces during high current density operation. We find that Peltier effects significantly influence the operational limit due to local electrical and thermal interfacial effects, causing asymmetric temperature distribution in the device. Thus, our results indicate that a proper understanding and design of graphene devices must include consideration of the surrounding materials, interfaces, and geometry. Leveraging these aspects provides opportunities for engineered extreme operation devices

비공유 화학적 도핑을 이용한 단일층 그래핀 소자의 전자특성 최적화

학위논문(박사) -- 서울대학교대학원 : 자연과학대학 화학부, 2022. 8. 홍병희.2004년 그래핀은 테이프를 이용한 (고배향 열분해성) 흑연(highly oriented pyrolytic graphite; HOPG)으로부터의 박리를 통해 최초 발견되었다. 이후 수많은 연구들에 의해 그래핀이 우수한 열적, 기계적, 전기적, 광학적 특성을 지녔음이 알려졌다. 2009년에 이르러 화학기상증착(chemical vapor deposition; CVD) 방식을 이용한 다결정 그래핀의 대면적 합성이 실험적으로 가능해졌고, 이로써 그래핀이 다양한 분야에 응용될 수 있는 발판이 마련되었다. 특히 그래핀의 응용분야 중 전기전자특성을 이용한 분야가 각광을 받고 있다. 그래핀은 높은 전자이동도, 전기전도도 및 열전도도를 지닌 재료이며, 밀접결합(tight-binding; TB) 근사 모형을 이용하여 계산한, 결함이 없는 단결정 단층 그래핀의 밴드갭(band gap)은 0임이 밝혀졌다. 재료의 전자특성 조절은 전자소자로의 응용에 필수적 공정이고, 도핑은 전자특성 조절에 주로 쓰이는 방법 중 하나이다. 그래핀에 도핑 처리를 함으로써 밴드갭, 전기전도도 및 일함수와 같은 전기전자특성을 조절할 수 있다. 그래핀에 대한 도핑 방법으로는 원자 치환, 전계 인가, 분자나 금속 나노입자 등의 물리적 흡착 등이 있다. 이 중 물리적 흡착 방식은 결함 없이 간단하고 우수한 도핑 효과를 얻을 수 있어 그래핀 도핑 방법으로 널리 사용되고 있다. 본 논문에서는 화학기상증착 방식으로 합성한 그래핀의 전자특성 최적화 방법 및 전자소자로의 응용에 관한 연구를 다루었다. 그래핀의 전자특성 최적화 방식으로 물리적 흡착을 통한 비공유 화학적 도핑을 택하였으며, 도핑된 그래핀의 전자소자로의 응용 가능성에 대하여 확인하였다. 제1장에서는 그래핀의 물리적 특성 중 전기전자특성에 초점을 맞춰 설명하였다. 또한, 연구에 사용한 도핑 방법과 도핑된 그래핀의 전하 이동현상에 관하여 소개하였다. 제2장에서는 그래핀의 합성, 전사 및 도핑 방법에 관하여 서술하였다. 연구에 사용된 그래핀은 화학기상증착 방식으로 합성되었으며, 합성된 그래핀은 구리 식각 및 전사 공정을 통해 소자 연구를 위한 시편으로 제작되었다. 그래핀은 자기조립단층(self-assembled monolayer; SAM)을 형성하는 분자 외 다양한 나노물질을 이용한 물리적 흡착 방식에 의해 화학적 도핑된다. 라만 분광분석을 통해 합성 및 도핑 직후의 그래핀 시편의 품질을 평가하였고, 3 전극 시스템을 이용한 전계효과 트랜지스터를 제작하여 그래핀의 전자특성을 분석하였다. 제3장에서는 화학기상증착 방식으로 합성한 그래핀에 다양한 나노물질을 차례로 제공함으로써 화학적 도핑 효과의 변화를 나타낸 전자소자 연구를 기술하였다. 그래핀 표면에 금 나노입자를 물리적 흡착 방식으로 도핑하여 비공유 기능화하고, 이를 이용한 그래핀을 전계효과 트랜지스터 소자로 제작하였다. 제작된 소자에 존재하는 금 나노입자에 4-머캅토벤조산(4-mercaptobenzoic acid; 4-MBA) 분자를 흡착시킴으로써 자기조립단층을 형성케 한다. 이때 수은 이온을 주입하면 자기조립단층을 형성한 4-MBA 분자의 카복시기(carboxyl group)가 리간드로 작용하여 수은 이온을 포획하면서 킬레이트(chelate) 복합체를 구성한다. 각 단계의 그래핀 전계효과 트랜지스터 소자의 전자특성 분석을 통해, 각 나노물질 요소에 의해 그래핀 표면의 도핑 효과가 미세 조정됨을 알 수 있다. 본 연구를 통해 그래핀 전계효과 트랜지스터의 화학적 기능화에 대한 가능성을 확인하였다. 제4장에서는 화학기상증착 방식으로 합성한 그래핀에 n-알킬아민(n-alkylamine; H2NCn) 분자를 도입함으로써, n형 도핑된 그래핀을 이용한 열전소자 성능의 향상에 관하여 기술하였다. n-알킬아민 분자는 그래핀 표면에서 자기조립단층을 형성하고 비공유 기능화를 통해 전자를 그래핀에 제공한다. 탄소사슬 길이가 각기 다른 n-알킬아민 분자를 이용하여 도핑한 그래핀을 3 전극 시스템을 통해 분석함으로써, 서로 다른 길이의 분자를 통해 그래핀 시편의 전하운반자 농도의 조절이 가능함을 확인하였다. n-알킬아민 분자의 자기조립단층이 형성된 각 그래핀 시편 위로 산화갈륨(Ga2O3) 박막층 및 갈륨-인듐 공융합금(eutectic Ga-In alloy; EGaIn) 벌크층을 차례로 적층하여 열전소자를 제작하였다. n-알킬아민 분자의 비공유 접합에 의해 유도 갭 상태(induced-gap state)가 그래핀 열전소자(SLG//H2NCn//Ga2O3/EGaIn)에 도입되었다. 금 박막층과 n-알케인싸이올레이트(n-alkanethiolates; SCn) 분자의 접합으로 구성된 종래의 열전소자(Au/SCn//Ga2O3/EGaIn)와의 비교를 통해, 상기한 방식으로 제작된 그래핀 열전소자가 우수한 열전특성을 지니고 있음을 증명하였다.Since its first discovery as a flake-form from mechanical exfoliation of highly-oriented pyrolytic graphite (HOPG) using tape in 2004, numerous studies have shown that graphene has outstanding and extraordinary thermal, mechanical, electrical, electronic and optical properties. In 2009, large-area synthesis of polycrystalline graphene using a chemical vapor deposition (CVD) method became experimentally possible, thereby establishing a foothold for the graphene to be applied to various fields. In particular, the field of application using electrical and electronic characteristics of graphene is in the spotlight. Graphene is a remarkable material with high electron mobility, electrical conductivity and thermal conductivity. Furthermore, the pristine single-layer graphene (SLG) has zero gap, a theoretical value calculated by a tight-binding (TB) approximation model. Engineering the electronic properties of materials is an essential process for application to electronic devices, and doping is one of the methods mainly used to control electronic properties. By doping graphene, electrical and electronic characteristics such as band gap, electrical conductivity, and work function (WF) can be modified and controlled. Doping methods for graphene include atomic substitution, applying electric field, physisorption (physical adsorption) of molecules and metal nanoparticles, etc. Among those methods, the physisorption is widely used as a graphene doping method because it can obtain a simple and superior doping effect without crystallographic defects. This paper describes researches on optimization methods of the electronic properties of graphene synthesized by CVD method and its applications of electronic devices. Noncovalent chemical doping by the physisorption was selected as the optimization method of the electronic properties of graphene, and the possibility of application of the doped graphene to an electronic device was verified. Chapter 1 delineates the physical properties of graphene, focusing on the electrical and electronic properties. In addition, the doping method used in the study and the charge transfer phenomenon of doped graphene were introduced. Chapter 2 gives a detailed description of the procedure such as the synthesis, transfer, and doping methods of graphene. Graphene used in these researches was synthesized by CVD method, and the synthesized graphene was manufactured as electronic device specimens through copper etching and transfer processes. Graphene is chemically doped by the physisorption method using various nanomaterials such as molecules forming self-assembled monolayers (SAM). Through Raman spectroscopy, the quality of graphene specimens immediately after synthesis and doping process was evaluated. Moreover, the electronic properties of graphene were analyzed by a 3-electrode system using field-effect transistor (FET) devices Chapter 3 depicts a study on electronic devices showing changes in chemical doping effects by sequentially providing various nanomaterials to graphene synthesized by CVD method. Gold nanoparticles were used as dopants on the surface of graphene by physisorption for a noncovalent functionalization, and the doped graphene was manufactured as FET devices. SAM is formed by adsorbing 4-mercaptobenzoic acid (4-MBA) molecules onto gold nanoparticles on the manufactured graphene device. And then, if mercury ions are injected, a carboxyl group of 4-MBA molecules constructing SAM acts as a ligand to capture mercury ions, thereby assembling a chelate complex. Through the analyses of the electronic properties of the graphene FET devices in each step, it can be seen that the doping effect of the graphene surface is finely adjusted by each nanomaterial element. Through this study, the possibility of chemical functionalization of graphene FET devices was exactly clarified. Chapter 4 describes the improvement in the performance of graphene thermoelectric devices using n-type doping by introducing n-alkylamine (H2NCn) molecules onto SLG film synthesized by CVD method. The n-alkylamine molecules form SAM on the surface of graphene and provide electrons to graphene through noncovalent functionalization. Graphene doped by n-alkylamine molecules with different lengths of carbon chain was manufactured as FET devices and analyzed by a 3-electrode system. Graphene FET devices were proved clearly that the concentration of charge carriers of graphene specimens could be regulated by chemical doping method using each molecule. Graphene thermoelectric devices was manufactured by sequentially stacking a gallium oxide (Ga2O3) thin film layer and a eutectic gallium-indium alloy (EGaIn) bulk layer onto the n-alkylamine SAM formed on each graphene specimen. An induced-gap state was introduced into the graphene layer in graphene thermoelectric devices (SLG//H2NCn//Ga2O3/EGaIn) by noncovalent junctions of n-alkylamine molecules. Through comparison with thermoelectric devices with a conventional structure (Au/SCn/Ga2O3/EGaIn) composed of the junction of gold thin film layer and n-alkanethiolates (SCn) molecules, it was shown that the graphene thermoelectric devices produced by the above method have improved and outstanding thermoelectric properties.Cover 1 Abstract 3 Table of Contents 6 List of Tables 8 List of Figures 9 Chapter 1. Introduction to Graphene 12 1. 1. Discovery and Advancement of Graphene 12 1. 2. Crystal Structure of Graphene 16 1. 3. Band Structure of Graphene 25 1. 4. Group Theory to Analyze Graphene 40 1. 5. Chemical Doping of Graphene 47 1. 6. Properties of Doped Graphene 50 Chapter 2. Experimental 54 2. 1. Graphene Synthesis by Chemical Vapor Deposition 54 2. 2. Pre-treatment Process for Graphene Transfer 65 2. 3. Graphene Transfer Process 66 2. 4. Graphene Doping by Physisorption 69 2. 5. Raman Spectroscopic Analyses for Graphene 70 2. 6. Electronic Analyses for Graphene Field-effect Transistor 80 Chapter 3. Gold Nanoparticle-Mediated Noncovalent Functionalization of Graphene for Field-Effect Transistors 94 3. 1. Abstract 94 3. 2. Introduction 95 3. 3. Experimental 96 3. 4. Results and Discussion 101 3. 5. Conclusion 123 Chapter 4. Enhanced Thermopower of Saturated Molecules by Noncovalent Anchor-Induced Electron Doping of Single-Layer Graphene 124 4. 1. Abstract 124 4. 2. Introduction 125 4. 3. Experimental 128 4. 4. Results and Discussion 138 4. 5. Conclusion 157 Bibliography 158 Abstract in Korean 178박

Van Der Waals heterostructures:Fabrication and nanoscale and thermal characterisation via SPM methods

Since the dawn of two-dimensional (2D) era, graphene and the plethora of other atomically thin layered materials cousins, including their combinations in heterostructures, have revolutionized material science and established themselves as promising candidates for the next-generation of nanoscale devices. However, the landscape of opportunities and possible combinations is so vast, that many promising research topics remain widely unexplored. The aim of this work is to address some of these questions, contributing to the progress and understanding in three different areas: fabrication of novel 2D materials and their heterostructures; electronic properties and doping mechanisms in these at the nanoscale; and characterization of the nanoscale thermal and thermoelectric properties of 2D materials. Starting with the fabrication procedures, different techniques for mechanical exfoliation and polymeric dry transfer of 2D materials and heterostructures were explored. This research route led to the establishment of a new fabrication facility at the National Physical laboratory (NPL), and to the production of a wide variety of samples, from exfoliated flakes to complex heterostructures and devices. A selection of the produced samples was employed during this thesis to investigate the electronic and thermal properties of 2D materials via spectroscopic and advanced scanning probe microscopy (SPM) techniques. Regarding the electronic response of graphene, two routes were undertaken: first, the electronic response of different types of graphene towards humidity acting as a p-dopant was studied for the first time using Raman spectroscopy. Second, a procedure improving current methods of quantitative probe calibration in Kelvin probe force microscopy (KPFM) was developed, establishing the determination of reliable and consistent work-function values with high-resolution. This method was then employed to study the electronic properties and doping of encapsulated graphene heterostructures, providing quantitative values of the work-function of the system, as well as demonstrating the capability of KPFM as an excellent visualisation and characterisation technique for buried layers otherwise inaccessible by other methods. Finally, various thermal properties of 2D materials were studied via advanced SPM techniques: Scanning thermal microscopy (SThM) that was used for the determination of the thickness dependence of the thermal conductance of exfoliated InSe, and scanning thermal gate microscopy (STGM) that was employed to explore the thermovoltage, and thus Seebeck coefficient, variation in encapsulated graphene heterostructures with patterned constrictions. The main highlights of the work developed during this thesis are: (1) the formulated need and subsequent realisation of various approaches towards the fabrication of 2D materials and heterostructures. For this, shared expertise with other researchers and institutions, and access to different fabrication facilities were essential; and (2) the exploitation of the potential of spectroscopic and advanced SPM methods in providing reliable characterisation of the 2D material and heterostructure’s properties with nanoscale resolution. The findings of this thesis have provided new insights in a varied number of areas, and hold promise for different future applications: from single material thermocouples to graphene-gas sensors, including improved fabrication procedures for 2D materials, or even, probe-calibration and characterisation methods

Proximity Effects in Epitaxial Graphene on SiC and their Impact on Atomic-Scale Charge Transport

Es gibt erste Ansätze, die Nähe des zweidimensionalen Materials Graphen zu bestimmten Materialien auszunutzen, um die Eigenschaften von Graphen gezielt zu verändern. Um diese Ansätze jedoch in vorteilhafte Werkzeuge zur gezielten Beeinflussung der Eigenschaften von epitaktischem Graphen umzuwandeln, müssen zunächst die Proximity-Effekte in unverändertem Graphen auf SiC untersucht und die zugrundeliegenden Kopplungsmechanismen verstanden werden. Ein direkter Einfluss der Nähe zum Substrat zeigt sich in den Ladungstransporteigenschaften von epitaktischem Graphen, da charakteristische Größen wie die Ladungsträgerdichte und die Ladungsträgerbeweglichkeit stark beeinflusst werden. In dieser Arbeit wird ein umfassender Zugang verwendet, der auf der Verknüpfung von lokalen strukturellen, elektronischen und Transporteigenschaften basiert, um die Auswirkungen der Nähe des SiC-Substrates auf epitaktisches Graphen zu untersuchen. Dies ermöglicht es, lokale Variationen in der Kopplung zu identifizieren und Zugang zu den grundlegenden Mechanismen der Substrat-Graphen-Wechselwirkung zu erhalten.Approaches to exploit the proximity of the two-dimensional material graphene to certain materials in order to specifically tune graphene’s properties start to emerge. However, to turn these approaches into truly beneficial tools to engineer epitaxial graphene’s properties, it is first necessary to investigate proximity effects in pristine graphene on SiC and to understand the coupling mechanisms behind them. A direct influence of the proximity to the substrate is seen in the charge transport properties of epitaxial graphene, as characteristic quantities such as the charge carrier density and the charge carrier mobility are significantly affected. In this thesis, a comprehensive approach based on linking local structural, electronic as well as transport properties is used to investigate proximity effects of the SiC substrate on epitaxial graphene. This allows to identify local variations in the coupling and to gain access to the fundamental mechanisms of the substrate-graphene interaction.2021-10-2

Novel Multiphysics Phenomena in a New Generation of Energy Storage and Conversion Devices

The swelling demand for storing and using energy at diverse scales has stimulated the exploration of novel materials and design strategies applicable to energy storage systems. The most popular electrochemical energy storage systems are batteries, fuel cells and capacitors. Supercapacitors, also known as ultracapacitors, or electrochemical capacitors have emerged to be particularly promising. Besides exhibiting high cycle life, they combine the best attributes of capacitors (high power density) and batteries (high energy storage density). Consequently, they are expected to be in high demand for applications requiring peak power such as hybrid electric vehicles and uninterruptible power supplies (UPS). This dissertation aims to make advancements on the following two topics in supercapacitor research with the aid of modeling and experimental tools: applying various thermophysical effects to design supercapacitor devices with novel functionalities and studying degradation mechanisms upon continuous cycling of conventional supercapacitors. The prime drawback of conventional supercapacitors is their low energy density. Most research in the last decade has focused on synthesizing novel electrode materials. Although such novel electrodes lead to high energy density, they often involve complicated synthesis process and result in high cost and low power density. A new concept of inducing pseudocapacitance developed in recent years is by introducing redox additives in the electrolyte that engage in redox reactions at the electrode/electrolyte interface during charge/discharge. The first section of this dissertation reports the performance of fabricated solid-state supercapacitors composed of redox-active gel electrolyte (PVA-K3Fe(CN)6-K4Fe(CN)6). The electrochemical performance has been studied extensively using cyclic voltammetry, constant current charge/discharge and impedance spectroscopy techniques, and then the results are compared with similar devices composed of conventional gel electrolytes such as PVA-H3PO4 and PVA-KOH on the basis of capacitance, internal resistance and stable voltage window. The second section explores the utility of the thermogalvanic property of the same redox-active gel electrolyte, PVA-K3Fe(CN)6-K4Fe(CN)6 in the construction of a thermoelectric supercapacitor. The integrated device is capable of being electrically charged by applying a temperature gradient across its two electrodes. In the absence of available temperature gradient, the device can be discharged electrically through an external circuit. Therefore, such a device can be used to harvest waste heat from intermittent heat sources. An equivalent circuit elucidating the mechanisms of energy conversion and storage applicable to thermally chargeable supercapacitors is developed. A fitting analysis aids in the evaluation of model circuit parameters providing good agreement with experimental voltage and current measurements. The latter part of the dissertation investigates the factors influencing aging in conventional supercapacitors. In the first part, a new imaging technique based on the electroreflectance property of gold has been developed and applied to characterize the aging characteristics of a microsupercapacitor device. Previous aging studies were performed through traditional electrical characterization techniques such as cyclic voltammetry, constant charge/discharge, and electrochemical impedance spectroscopy. These methods, although simple, measure an average of the structures’ internal performance, providing little or no information about microscopic details inside the device. The electroreflectance imaging method, developed in this work is demonstrated as a high-resolution imaging technique to investigate charge distribution, and thus to infer aging characteristics upon continuous cycling at high scan rates. The technique can be used for non-intrusive spatial analysis of other electrochemical systems in the future. In addition, we investigate heat generation mechanisms that are responsible for accelerated aging in supercapacitors. A modeling framework has been developed for heat generation rates and resulting temperature evolution in porous electrode supercapacitors upon continuous cycling. Past thermal models either neglected spatial variations of heat generation within the cell or considered electrodes as flat plates that led to inaccuracies. Here, expressions for spatiotemporal variation of heat generation rate are rigorously derived on the basis of porous electrode theory. Detailed numerical simulations of temperature evolution are performed for a real-world device, and the results resemble past measurements both qualitatively and quantitatively. In the last chapter of the thesis, a rare thermoelectric effect called the Nernst effect has been investigated in single-layer periodic graphene with the aid of a modified Boltzmann transport equation. Detailed formulations of the transport coefficients from the BTE solution are developed in order to relate the Nernst coefficient to the amount of impurity density, temperature, band gap and applied magnetic field. Detailed knowledge of the variation of the thermoelectric and thermomagnetic properties of graphene shown in this work will prove helpful for improving the performance of magnetothermoelectric coolers and sensors

Spin-dependent phenomena and device concepts explored in (Ga,Mn)As

Over the past two decades, the research of (Ga,Mn)As has led to a deeper understanding of relativistic spin-dependent phenomena in magnetic systems. It has also led to discoveries of new effects and demonstrations of unprecedented functionalities of experimental spintronic devices with general applicability to a wide range of materials. In this article we review the basic material properties that make (Ga,Mn)As a favorable test-bed system for spintronics research and discuss contributions of (Ga,Mn)As studies in the general context of the spin-dependent phenomena and device concepts. Special focus is on the spin-orbit coupling induced effects and the reviewed topics include the interaction of spin with electrical current, light, and heat.Comment: 47 pages, 41 figure