Download or read book Electrical Transport of Topological Insulator bismuth Selenide and Thermoelectric Properties of Graphene written by Peng Wei and published by . This book was released on 2011 with total page 117 pages. Available in PDF, EPUB and Kindle. Book excerpt: The second half of this thesis is focused on graphene. Our work first reported the thermoelectric study of graphene and demonstrated the anomalous thermoelectric transport of massless Dirac fermions. As a direct consequence of the linear dispersion of massless particles, we find that the Seebeck coefficient S[subscript xx] diverges with [Special characters omitted.], where n[subscript 2D] is the carrier density. We observe a very large Nernst signal S[subscript xy] (~ 50 [Mu]V/K at 8 T) at the Dirac point, and an oscillatory dependence of both S[subscript xx] and S[subscript xy] on n[subscript 2D] at low temperatures. Our results underscore the anomalous thermoelectric transport in graphene, which may be used as a highly sensitive probe for impurity bands near the Dirac point.

Download or read book Electrical Transport and Thermal Expansion in Van Der Waals Materials written by Lei Jing and published by . This book was released on 2013 with total page 141 pages. Available in PDF, EPUB and Kindle. Book excerpt: Novel two-dimensional materials with weak interlayer Van der Waals interaction are fantastic platforms to study novel physical phenomena. This thesis describes our investigation on two different Van der Waals materials: graphene and bismuth selenide with calcium doping (Ca [subscript x] Bi [subscript 2-x] Se[subscript 3] , x as the doping level) in the topological insulator family. Firstly, we characterize the electrical transport behaviors of high-quality substrate-supported bilayer graphene devices with suspended metal gates. The device exhibits a transport gap induced by external electric field with an on/off ratio of 20,000, which could be explained by variable range hoping between localized states or disordered charge puddles. At large magnetic field, the device presents quantum Hall plateau at fractional values of conductance quantum, which arises from the equilibration of edge states between differentially doped regions. Secondly, we present our study on the electronic transport of Ca[subscript x] Bi [subscript 2-x] Se [subscript 3] thin films, which are three-dimensional topological insulators and coupled with superconducting leads. In these novel Josephson transistors, we observe different characteristic features by energy dispersion spectrum (EDS) and Raman spectroscopy, and the weak suppression in the critical current I [subscript c]. Thirdly, we explore the thermal expansion of suspended graphene. By in-situ scanning electron microscope (SEM), we measure the thickness-dependence of graphene's negative thermal expansion coefficient (TEC). We propose that there is a competitive relation between the intrinsic TEC and the friction from the substrate and the graphene. Lastly, in collaboration with Dr. Nikolai Kalugin from New Mexico Tech., we explore the graphene's application as a quantum Hall effect infrared photodetector. This graphene-based detector can be operated at higher temperature (liquid nitrogen) and wider frequency than the previous implementations of quantum Hall detector.

Download or read book Optical and Electrical Properties of Topological Insulator Bi2Se3 written by Jiajun Zhu and published by Anchor Academic Publishing. This book was released on 2017-07 with total page 91 pages. Available in PDF, EPUB and Kindle. Book excerpt: Topological insulator is one of the hottest research topics in solid state physics. This is the first book to describe the vibrational spectroscopies and electrical transport of topological insulator Bi2Se3, one of the most exciting areas of research in condensed matter physics. In particular, attempts have been made to summarize and develop the various theories and new experimental techniques developed over years from the studies of Raman scattering, infrared spectroscopy and electrical transport of topological insulator Bi2Se3. It is intended for material and physics researchers and graduate students doing research in the field of optical and electrical properties of topological insulators, providing them the physical understanding and mathematical tools needed to engage research in this quickly growing field. Some key topics in the emerging field of topological insulators are introduced.

Download or read book Electrical Transport Properties of Topological Insulators and Graphene written by Zhiyong Wang and published by . This book was released on 2014 with total page 160 pages. Available in PDF, EPUB and Kindle. Book excerpt: For single-layer MoS2 , at the valence band maxima, the band is split by 160 meV due to strong spin-orbit coupling. Spin-up and spin-down electrons reside in different bands due to the broken inversion symmetry. Valley and spin degrees of freedom of the valence bands are inherently coupled in single-layer MoS2. It is an ideal material to study the valley Hall effect.

Download or read book 2D Dirac Materials written by Desalegne Bekuretsion Teweldebrhan and published by . This book was released on 2011 with total page 120 pages. Available in PDF, EPUB and Kindle. Book excerpt: Silicon has been reaching physical limits as the semiconductor industry moves to smaller device feature sizes, increased integration densities and faster operation speeds. There is a strong need to engineer alternative materials, which can become foundation of new computational paradigms or lead to other applications such as efficient solid-state energy conversion. Recently discovered Dirac materials, which are characterized by the liner electron dispersion, are examples of such alternative materials. In this dissertation, I investigate two representatives of Dirac materials - graphene and topological insulators. Specifically, I focus on the (i) effects of electron beam irradiation on graphene properties and (ii) electronic and thermal characteristics of exfoliated films of Bi [subscript 2] Te [subscript 3] -family of topological insulators. I carried out Raman investigation of changes in graphene crystal lattice induced by the low and medium energy electron-beam irradiation (5.20 keV). It was found that radiation exposures result in appearance of the disorder D band around 1345 cm [superscript -1]. The dependence of the ratio of the intensities of D and G peaks, I(D)/I(G), on the irradiation dose is non-monotonic suggesting graphene.s transformation to polycrystalline and then to disordered state. By controlling the irradiation dose one can change the carrier mobility and increase the resistance at the minimum conduction point. The obtained results may lead to new methods of defect engineering of graphene properties. They also have important implications for fabrication of graphene nanodevices, which involve electron beams. Bismuth telluride and related compounds are the best thermoelectric materials known today. Recently, it was determined that they reveal the topological insulator properties. We succeeded in the first "graphene-like" exfoliation of large-area crystalline films and ribbons of Bi [subscript 2] Te [subscript 3] with the thickness going down to a single quintuple. The presence of van der Waals gaps allowed us to disassemble Bi [subscript 2] Te [subscript 3] crystal into the five mono-atomic sheets consisting of Te [superscript (1)] -Bi-Te [superscript (2)] -Bi-Te [superscript (1)]. The exfoliated films had extremely low thermal conductivity and electrical resistance in the range required for thermoelectric applications. The obtained results may pave the way for producing Bi [subscript 2] Te [subscript 3] films and stacked superlattices with strong quantum confinement of charge carriers and predominantly surface transport, and allow one to obtain theoretically predicted order-of-magnitude higher thermoelectric figure-of-merit.

Download or read book Topological States of Matter written by Vincenzo Parente and published by LAP Lambert Academic Publishing. This book was released on 2013 with total page 180 pages. Available in PDF, EPUB and Kindle. Book excerpt: Abstract from dissertation: My research program is focused on the study of elastic deformation and topological defects in two materials: graphene and topological insulators. In both systems at low energies the electrons have a nearly linear spectrum, i.e. they behave like relativistic fermions. This allows the study of the effects of defects and deformations on the dynamics of electrons trough the formalism of Dirac equation on curved space-time. In this setting it's possible to derive correction to observables properties of the systems like the conductivity for example. In the case of graphene I have derived the contribution to conductivity in the Born approximation of the metric arising from the so-called bumps and made a comparison with the scattering on the gauge potential arising from the elastic deformation. A particular defect, the edge dislocation, is found to be a possible responsible for the behaviour of the conductivity at low energies. The topological insulators are a class of band insulators showing gapless edge states, capable of conduction. This situation is similar to Quantum Hall Effect, both physically and formally. Indeed, as in QHE topological invariants (Chern numbers) classify the behaviour of the material. I am thus focused on the study of these material both formally, on the ground of differential geometry, and physically, studying topological defect in topological insulators. Further investigation has been devoted to the analysis of electron-phonon interaction at the surface of a 3D TI, analysing superconductive instability.

Download or read book Electronic Transport in Dirac Materials graphene and a Topological Insulator Bi2Se3 written by Sungjae Cho and published by . This book was released on 2011 with total page pages. Available in PDF, EPUB and Kindle. Book excerpt:

Download or read book Electronic Transport in Topological Insulator Nanostructures written by Seung Sae Hong and published by . This book was released on 2013 with total page pages. Available in PDF, EPUB and Kindle. Book excerpt: Topological insulators are states of quantum matter with an insulating gap in the bulk and gapless surface states. The exotic spin nature of the surface electrons, resulting in topological protection from localization, suggests unconventional applications in electronics as well as fundamental scientific interests. While these exotic states have been investigated via surface-sensitive techniques intensively, electronic transport device, crucial to realize topological electronics, has lagged behind due to material challenges in candidate materials. Topological insulator nanostructure is an attractive candidate for device applications, as the size effect and boundary conditions offer a unique way to enhance / tailor the surface electron transport. In this dissertation, we first describe the design principle of topological insulator nanomaterials, with an emphasis on bismuth selenide. Two major material challenges, dominant bulk electron contribution and low surface mobility due to surface oxidation, are discussed and the solutions via nanomaterial synthesis are achieved. Elemental doping and core-shell heterostructures are developed to suppress bulk carriers and to achieve high surface electron mobility. The high electronic mobility allows us to observe Shubnikov-de Haas oscillations originated from the surface Dirac fermions. In addition to the material development, we also investigate transport properties from helical nature of the surface electrons. 1D modes of surface electrons in bismuth selenide nanowire Aharonov-Bohm interferometers is a unique electronic state providing an opportunity to reveal helical spin nature and topological protection via transport. The helical 1D mode, directly observed near the Dirac point under half magnetic flux quantum, is robust against disorder but fragile against a magnetic field breaking time-reversal-symmetry. The newly discovered 1D helical mode is expected to open a new direction to study topological electronics, as well as future applications.

Download or read book The Theory of Thermal Thermoelectric and Electrical Transport Properties of Graphene written by Vincent Ike Ugarte and published by . This book was released on 2010 with total page 83 pages. Available in PDF, EPUB and Kindle. Book excerpt: Motivated by the experimental measurement of transport properties such as electrical and hall conductivity, thermopower and Nernst, I present a study of longitudinal and transverse transport in graphene for the dilute limit of impurities. The temperature and carrier density dependence in this system display a number of anomalous properties that can be related to three effects: 1) emergence of " chirality ", 2) vanishing density of states at the chemical potential in the ideal undoped (zero gate voltage) systems and 3) nature of scattering. In an attempt to theoretically understand these anomalous transport properties, I use the theory of quantum transport in a two-dimensional system with Unitary(lattice vacancy) and screened Coulomb(charge impurity in the underlining substrate) scatterers. I show for a system such as graphene, the type of scattering potential has a profound effect on all transport properties, even though both types of potentials induce low energy states that yield a finite density of states at zero energy. My results are compared with experimental data for both types of scatterer and I show for a single set of impurity parameters all transport properties can be reproduced to agree qualitative with the features observed in experimental data. Parts of this work have been submitted to Physical Review B and are currently in the review process.

Download or read book Experimental and Theoretical Investigation of Thermal and Thermoelectric Transport in Nanostructures written by Arden Lot Moore and published by . This book was released on 2010 with total page 420 pages. Available in PDF, EPUB and Kindle. Book excerpt: This work presents the development and application of analytical, numerical, and experimental methods for the study of thermal and electrical transport in nanoscale systems, with special emphasis on those materials and phenomena which can be important in thermoelectric and semiconductor device applications. Analytical solutions to the Boltzmann transport equation (BTE) using the relaxation time approximation (RTA) are presented and used to study the thermal and electrical transport properties of indium antimonide (InSb), indium arsenide (InAs), bismuth telluride (Bi2Te3), and chromium disilicide (CrSi2) nanowires. Experimental results for the thermal conductivity of single layer graphene supported by SiO2 were analyzed using an RTA-based model and compared to a full quantum mechanical numerical BTE solution which does not rely on the RTA. The ability of these models to explain the measurement results as well as differences between the two approaches are discussed. Alternatively, numerical solutions to the BTE may be obtained statistically through Monte Carlo simulation for complex geometries which may prove intractable for analytical methods. Following this approach, phonon transport in silicon (Si) sawtooth nanowires was studied, revealing that thermal conductivity suppression below the diffuse surface limit is possible. The experimental investigation of energy transport in nanostructures typically involved the use of microfabricated devices or non-contact optical methods. In this work, two such approaches were analyzed to ascertain their thermal behavior and overall accuracy as well as areas for possible improvement. A Raman spectroscopy-based measurement design for investigating the thermal properties of suspended and supported graphene was examined analytically. The resulting analysis provided a means of determining from measurement results the thermal interface conductance, thermal contact resistance, and thermal conductivity of the suspended and supported graphene regions. Previously, microfabricated devices of several different designs have been used to experimentally measure the thermal transport characteristics of nanostructures such as carbon nanotubes, nanowires, and thin films. To ascertain the accuracy and limitations of various microdevice designs and their associated conduction analyses, finite element models were constructed using ANSYS and measurements of samples of known thermal conductance were simulated. It was found that designs with the sample suspended were generally more accurate than those for which the sample is supported on a bridge whose conductance is measured separately. The effects of radiation loss to the environment of certain device designs were also studied, demonstrating the need for radiation shielding to be at temperatures close to that of the device substrate in order to accurately calibrate the resistance thermometers. Using a suspended microdevice like those analyzed using finite element analysis, the thermal conductivities of individual bismuth (Bi) nanowires were measured. The results were correlated with the crystal structure and growth direction obtained by transmission electron microscopy on the same nanowires. Compared to bulk Bi in the same crystal direction, the thermal conductivity of a single-crystal Bi nanowires of 232 nm diameter was found to be 3 - 6 times smaller than bulk between 100 K and 300 K. For polycrystalline Bi nanowires of 74 nm to 255 nm diameter the thermal conductivity was reduced by a factor of 18 - 78 over the same temperature range. Comparable thermal conductivity values were measured for polycrystalline nanowires of varying diameters, suggesting a grain boundary scattering mean free path for all heat carriers in the range of 15 - 40 nm which is smaller than the nanowire diameters. An RTA-based transport model for both charge carriers and phonons was developed which explains the thermal conductivity suppression in the single-crystal nanowire by considering diffuse phonon-surface scattering, partially diffuse surface scattering of electrons and holes, and scattering of phonons and charge carriers by ionized impurities such as oxygen and carbon of a concentration on the order of 1019 cm−3. Using a similar experimental setup, the thermoelectric properties (Seebeck coefficient, electrical conductivity, and thermal conductivity) of higher manganese silicide (HMS) nanostructures were investigated. Bulk HMS is a passable high temperature thermoelectric material which possesses a complex crystal structure that could lead to very interesting and useful nanoscale transport properties. The thermal conductivities of HMS nanowires and nanoribbons were found to be reduced by 50 - 60 % compared to bulk values in the same crystal direction for both nanoribbons and nanowires. The measured Seebeck coefficient data was comparable or below that of bulk, suggesting unintentional doping of the samples either during growth or sample preparation. Difficulty in determining the amorphous oxide layer thickness for nanoribbons samples necessitated using the total, oxide-included cross section in the thermal and electrical conductivity calculation. This in turn led to the determined electrical conductivity values representing the lower bound on the actual electrical conductivity of the HMS core. From this approach, the measured electrical conductivity values were comparable or slightly below the lower end of bulk electrical conductivity values. This oxide thickness issue affects the determination of the HMS nanostructure thermoelectric figure of merit ZT as well, though the lower bound values obtained here were found to still be comparable to or slightly smaller than the expected bulk values in the same crystal direction. Analytical modeling also indicates higher doping than in bulk. Overall, HMS nanostructures appear to have the potential to demonstrate measurable size-induced ZT enhancement, especially if optimal doping and control over the crystallographic growth direction can be achieved. However, experimental methods to achieve reliable electrical contact to quality four-probe samples needs to be improved in order to fully investigate the thermoelectric potential of HMS nanostructures.

Download or read book Transport of Phonons and Electrons in Thermoelectric Materials and Graphene written by Sangyeop Lee and published by . This book was released on 2015 with total page 143 pages. Available in PDF, EPUB and Kindle. Book excerpt: Understanding transport of phonons and electrons plays a critical role in developing energy conversion and information devices. Thermoelectric materials, which directly convert heat to electricity or vice versa, require both extremely low thermal conductivity and high thermoelectric power factor. However, a good understanding of low thermal conductivity is still lacking even for several good thermoelectric materials that have been studied over several decades. For the information devices, graphene has recently drawn much attention for various applications including high speed transistors due to its high electron mobility and high thermal conductivity. However, the graphene's high thermal conductivity has yet to be fully understood. There have been many studies based on diffusive-ballistic phonon transport, but no conclusive explanation for the graphene's high thermal conductivity has been drawn. In this thesis, we investigate the transport of phonons and electrons in thermoelectric materials and graphene using both first principles calculations and experimental characterizations. We start by studying phonon transport in Bi and Bi-Sb alloys using first principles calculations. A notable observation from this calculation is that a strong long-range interaction exists in Bi and Sb along a specific crystallographic direction. We further show that this long-range interaction is also found in other good thermoelectric materials, and is a key to understanding their low thermal conductivity. The long-range interaction is explained with resonant bonding which many good thermoelectric materials commonly share. The particularly strong resonant bonding in group IV-VI materials leads to the low thermal conductivity through the long-range interaction and resulting softening of optical phonons that strongly scatter acoustic phonons. We study electron transport in thermoelectric materials with two-dimensional discontinuities, such as grain boundaries. We set up an experimental system to measure thermo- and galvano-magnetic electron transport coefficients of a Bi2Te2.7Se0.3 nanocomposite sample to examine the electron filtering effect by many grain boundaries in the nanocomposite. The experimental results indicate that the nanocomposite sample exhibits the electron filtering effect and it would be possible to increase the thermoelectric power factor by engineering the potential barrier of grain boundaries. While thermoelectric applications require materials with low thermal conductivity, electronic and optoelectronic devices often require high thermal conductivity. Graphene is attractive for these applications because of its unique electrical, optical, and thermal properties. We use first-principles calculations to reveal that the phonon transport in graphene is not diffusive unlike many threedimensional materials, but is hydrodynamic due to graphene's two-dimensional features. The hydrodynamic phonon transport is demonstrated through a drift motion of phonons, phonon Poiseuille flow, and second sound, all of which are not possible in both diffusive and ballistic phonon transport.

Download or read book Transport Properties and Electrical Field Effect Study of Topological Insulator Bi Sb 2Te3 Magnetic Insulator EuIG Heterostructures written by Wei-Jhih Zou and published by . This book was released on 2021 with total page 0 pages. Available in PDF, EPUB and Kindle. Book excerpt:

Download or read book Nanomaterials for Innovative Energy Systems and Devices written by Zishan H. Khan and published by Springer Nature. This book was released on 2022-05-24 with total page 608 pages. Available in PDF, EPUB and Kindle. Book excerpt: This book covers the latest research on applications of nanomaterials in the field of energy systems and devices. It provides an overview of the state-of-art research in this rapidly developing field. It discusses the design and fabrication of nanostructured materials and their energy applications. Various topics covered include nanomaterials for perovskite solar cells, transition metal dichalcogenides (TMDs) nanocomposites based supercapacitors, battery materials and technologies, major challenges toward development of efficient thermoelectric materials for energy efficient devices, extraction and experimentation of biodiesel produced from leachate oils of landfills coupled with nano-additives aluminium oxide and copper oxide on diesel engine and many more. It has contributions from world-renowned specialists in the fields of nanomaterials and energy devices. The book will be useful for students, researchers and professionals working in the area of nanomaterials and energy systems & devices.

Download or read book Thermal Characterization of Nanostructures and Advanced Engineered Materials written by Vivek Kumar Goyal and published by . This book was released on 2011 with total page 170 pages. Available in PDF, EPUB and Kindle. Book excerpt: Continuous downscaling of Si complementary metal-oxide semiconductor (CMOS) technology and progress in high-power electronics demand more efficient heat removal techniques to handle the increasing power density and rising temperature of hot spots. For this reason, it is important to investigate thermal properties of materials at nanometer scale and identify materials with the extremely large or extremely low thermal conductivity for applications as heat spreaders or heat insulators in the next generation of integrated circuits. The thin films used in microelectronic and photonic devices need to have high thermal conductivity in order to transfer the dissipated power to heat sinks more effectively. On the other hand, thermoelectric devices call for materials or structures with low thermal conductivity because the performance of thermoelectric devices is determined by the figure of merit Z=S 2 [sigma]/ K, where S is the Seebeck coefficient, K and [sigma] are the thermal and electrical conductivity, respectively. Nanostructured superlattices can have drastically reduced thermal conductivity as compared to their bulk counterparts making them promising candidates for high-efficiency thermoelectric materials. Other applications calling for thin films with low thermal conductivity value are high-temperature coatings for engines. Thus, materials with both high thermal conductivity and low thermal conductivity are technologically important. The increasing temperature of the hot spots in state-of-the-art chips stimulates the search for innovative methods for heat removal. One promising approach is to incorporate materials, which have high thermal conductivity into the chip design. Two suitable candidates for such applications are diamond and graphene. Another approach is to integrate the high-efficiency thermoelectric elements for on-spot cooling. In addition, there is strong motivation for improved thermal interface materials (TIMs) for heat transfer from the heat-generating chip to heat-sinking units. This dissertation presents results of the experimental investigation and theoretical interpretation of thermal transport in the advanced engineered materials, which include thin films for thermal management of nanoscale devices, nanostructured superlattices as promising candidates for high-efficiency thermoelectric materials, and improved TIMs with graphene and metal particles as fillers providing enhanced thermal conductivity. The advanced engineered materials studied include chemical vapor deposition (CVD) grown ultrananocrystalline diamond (UNCD) and microcrystalline diamond (MCD) films on Si substrates, directly integrated nanocrystalline diamond (NCD) films on GaN, free-standing polycrystalline graphene (PCG) films, graphene oxide (GOx) films, and "pseudo-superlattices" of the mechanically exfoliated Bi 2 Te 3 topological insulator films, and thermal interface materials (TIMs) with graphene fillers.

Download or read book Electrical and Thermoelectrical Transport Properties of Graphene written by Deqi Wang and published by . This book was released on 2011 with total page 104 pages. Available in PDF, EPUB and Kindle. Book excerpt: Graphene is a newly discovered material. It has many excellent properties, which make the research of this new material very important not only for the fundamental physics but also for the application. This thesis is a summary of our study in the electrical and thermoelectrical transport properties of graphene. In the first part of our work, we use a layer of molecules act as the charge reservoir to control the charge environment near or on graphene, we first obtained a mobility enhancement. By set the charge back and forth between graphene and the molecules, we found the graphene mobility can be widely tuned from 4000 to 19000 cm 2 /Vs. This strongly supports that charge impurities scattering is the main mechanism for graphene mobility. In addition, the charge neutral point in graphene can also be tuned independently over a wide gate voltage. A large memory effect is also found in the graphene device addressed with molecules, which makes this system potentially important for graphene application, such as graphene FLASH memory. In the second part of our work, we studied the thermopower of graphene with a wide range of mobility, i. e. varying degree of disorders, along with electrical conductivity at different temperatures. We have proved that the transport properties in graphene are indeed caused by the Dirac fermions particles. Moreover, we have found that the Mott relation fails in the vicinity of the Dirac point in high-mobility graphene. By properly taking account of high-temperature effects, we have obtained good agreement between the Boltzmann transport theory and our experimental data. In low-mobility graphene where the charged impurities induce relatively high residual carrier density, the Mott relation holds at all gate voltages.

Download or read book Characterization of Bismuth Telluride Two dimensional Nanosheets for Thermoelectric Applications written by Lingling Guo and published by . This book was released on 2015 with total page 111 pages. Available in PDF, EPUB and Kindle. Book excerpt: Solid-state thermoelectric devices are compact, scalable, quiet, and environmentally friendly, which are widely used as thermal engines or refrigerators. Bismuth telluride (Bi2Te3) and other V-VI group chalcogenides are known as one of the best thermoelectric materials specifically for applications in a temperature environment from room temperature to 300°. Recently, the unique topological surface states were discovered in Bi2Te3 family materials, and these novel surface states are arisen from a strong spin-orbit coupling in topological insulators. Topological surface states are protected against time-reversal perturbations (i.e., non-magnetic impurities or surface defects), making the electronic transport essentially dissipation-less. Such unique transport behavior with zero energy loss provides new opportunities to enhance thermoelectric properties. Although the promise in thermoelectric properties of topological insulators have been shown in theoretical reports, there is a lack of experimental investigations for a better understanding of their basic properties. This research work focuses on the characterizations of fundamental properties of Bi2Te3 two-dimensional (2D) nanosheets. Samples were prepared via respective solvothermal synthesis and van der Waals epitaxy. The charged surface properties of Bi2Te3 2D nanosheets were investigated using kelvin probe force microscopy. The measured electrical potential difference between aminosilane self-assembled monolayer and Bi2Te3 nanosheet surfaces is found to be 650 mV, which is larger than that (400 mV) between the silicon oxide substrate and Bi2Te3 nanosheet surface. The elastic properties of Bi2Te3 2D nanosheets (i.e., Young's modulus and prestress) were acquired by analyzing the thickness dependence of 2D nanosheet deformations creating by atomic force microscopy tips. The Young's modulus by fitting linear elastic behaviors of 26 samples is found only 11.7-25.7 GPa, significantly smaller than the bulk in-plane Young's modulus (50-55 GPa). Furthermore, the thermoelectric properties of Bi2Te3 2D nanosheets were characterized in the cryostat system at a temperature range of 20-400 K. The results reveal that electrical conductivity of 2D nanosheets decreases with increasing temperature and thickness, while the measured Seebeck coefficient does not show a strong thickness dependence and the value is smaller than bulk Bi2Te3. These fundamental properties would help improve the basic understanding of topological surface states towards practical applications.

Download or read book Electrical Transport Studies Of Topological Insulator Bi2te3 Nanotubes written by Renzhong Du and published by . This book was released on 2016 with total page pages. Available in PDF, EPUB and Kindle. Book excerpt: Topological insulators (TIs) are bulk insulators with unusual gapless metallic surface states protected by time reversal symmetry. The topological surface states are expected to yield unique phenomena, such as spin-momentum locking and the suppression of non-magnetic backscattering. The surface states with a Dirac cone like dispersion relation have been observed directly using Angle-resolved photoemission spectroscopy (ARPES). Their signatures in electrical transport properties have also been reported although the transport behaviors are complicated by the fact that the samples often have dominating conduction from bulk channels due to high carriers density from impurity states. It is theoretically predicted that the surface states are robust against strong disorders, but it has not been tested experimentally. Bi2Te3, a member of the Bi2Se3 family, has been demonstrated to be a promising candidate of 3D topological insulators. In this work, various transport studies have been carried out on Bi2Te3 nanotubes with strong disorders. As essentially narrow gap semiconductors with band gaps of 100 to 300 meV, the Bi2Se3 family often have conducting bulk due to impurity doping and lattice imperfection, such as vacancies, dislocations, and defects. Several approaches have been applied to reduce bulk conductivity. The first one is to compensate bulk carriers by chemical doping or field effect, which drags Fermi level back to bang gap. Another is to fabricate nano-scale samples with enhanced surface-to-volume ratio, and effectively reduce the weight of bulk conduction. In our work we have prepared Bi2Te3 nanotubes by solution phase method utilizing Kirkendall effect. Due to the nature of this process, polycrystalline Bi2Te3 nanotubes form with extremely strong disorders. In this way, bulk carriers are strongly localized and the samples are brought from a heavily doped band semiconductor to the regime of Anderson insulator. We have demonstrated that the bulk conductance of the nanotubes is truly insulating and characterized by Mott Variable Range Hopping (VRH) mechanism. Despite the strongly disordered bulk channel, Aharonov-Bohm (AB) like oscillations in magnetoconductance have been observed on the nanotubes at low temperatures. The conductance oscillations differ from those on either ballistic or diffusive normal metals. With the combination of theoretical analysis and simulations, we show that these oscillations originate from the outer surface states of the nanotubes. Their behaviors are consistent with the prediction of the "anomalous AB effect" of topological surface states. In comparison with published work on clean TIs, we provide a direct demonstration of the fundamental aspect of topological surface states, namely their robustness to time-reversal invariant disorder. The interplay between a TI and a superconductor (SC) is another interesting topic to explore the exotic behaviors of topological surface states. For example, proximity induced superconductivity on topological insulators leads to the new phase of topological superconductivity, which serves a host of Majorana fermions. On the other hand, proximity effect can be suppressed on topological surface states, with the presence of an anomalous resistance increase. We report a systematical study of the anomalous resistance increase on Bi2Te3 nanotubes with superconducting Nb electrodes. With various measurement strategies, we show that this resistance increase behavior occurs on the nanotubes rather than from SC/TI interface effects. It is induced from the contact with superconducting Nb without requiring current flowing through the Nb. The characteristic length of the anomalous resistance increase reaches as far as 450 nm at low temperature. Gate experiments confirm that the surface states of the nanotubes are responsible for this effect. Our results indicate that a new charge correlation may establish on the surface states of Bi2Te3 nanotubes under the proximity of superconductor, suggesting further studies on this subject to explore the underlying physical origins, both theoretically and experimentally.