Thermoelectric properties of p-type (Bi2Te3) x(Sb2Te3)1-x single crystals doped with 3 wt. % Te

This is the final version of the article. Available from AIP via the DOI in this record.In the present work, thermoelectric properties of p-type (Bi 2Te3)x (Sb2Te3) 1-x single crystals doped with 3 wt. % Te are theoretically explored for various chemical compositions (x = 0.18, 0.19, 0.20, 0.22, 0.24, 0.26) in the temperature range of 290-500 K. The influence of the chemical composition in enhancing the thermoelectric figure of merit (ZT) is discussed in detail. Using the nearly-free electron approximation and the Fermi-Dirac statistics, the temperature dependences of Fermi level (E f), Seebeck coefficient (S), and electrical conductivity (σ) are successfully reproduced as reported in the experimental study of Li [Intermetallics 19, 2002 (2011)]. The thermal conductivity contributions from phonons (κph), acceptor holes (κh), and electron-hole pairs (κbp) are included by employing Srivastava's scheme, Wiedemann-Franz law, and Price's theory, respectively. By combining all three contributions of the thermal conductivity we successfully explain the experimental measurements of the total thermal conductivity as reported by Li Furthermore, it is theoretically found that among all the compositions the p-type 20%(Bi2Te3)-80%(Sb 2Te3) sample has the maximum ZT value of 1.31 at 390 K, which is also in good agreement with the experimental results obtained by Li © 2013 American Institute of Physics.Övgü Ceyda Yelgel is grateful for financial support from The Republic of Turkey Ministry of National Education through the Recep Tayyip Erdog̃an University in Rize/Turkey (Recep Tayyip Erdog̃an Üniversitesi Rektörlüg̃ü, Fener Mah. Merkez Kampüs 53100/RİZE/TÜRKİYE)

Theoretical Studies of Structural, Electronic and Optical Properties of Graphene Based Systems

The equilibrium atomic geometry, electronic properties, electronic orbital states, and optical properties of graphene and graphene based systems have been comprehensively investigated using the density functional theory (DFT) in the framework of plane wave pseudopotential. These properties have been compared with recently found experimental results and theoretical works done by tight-binding and DFT methods. In Chapter 1, a brief theoretical overview about the structural properties of graphene based systems has been provided. The basic concepts of semiconductor surfaces have been introduced for the study of graphene on InAs(111) surfaces. In Chapter 2, explanations of the essential ingredients of the density functional theory and the plane wave pseudopotential theory have been provided. The main concepts of geometry optimisation in all calculations have been explained. The theory of scanning tunnelling microscopy which gives very detailed information about geometrical structures and electronic states has been described . In Chapter 3, atomic geometry, electronic structures, and interband optical transitions for isolated monolayer graphene, bilayer graphene, ABA-stacked trilayer graphene, and graphite systems have been studied. The electron velocity and effective mass were estimated using the in-plane electronic band calculation. The modifications in the electronic properties due to increasing the number of graphene layers have been discussed. The changes in interband optical transitions have also been presented for bilayer graphene, trilayer graphene, and graphite with respect to the monolayer graphene. In Chapter 4, a detailed theoretical study of the electronic structures of ABC-stacked trilayer and N-layer graphene has been presented. The nature of the trigonal warping of energy bands slightly above the Fermi level for different layer thicknesses was examined. The orbital natures of the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) states have been investigated. In Chapter 5, the equilibrium geometry and electronic structure of multilayer graphene deposited on hexagonal boron nitride substrate have been studied. It has been found that the graphene sheet is weakly adsorbed on the boron nitride substrate. Using the in-plane electronic band calculations the carrier velocity and effective mass were estimated. The importance of the interlayer interaction and stacking patterns of multilayer graphene/boron nitride has been explained for band gap and effective mass tuning in multilayer graphene. The interband transition energies for all systems were also calculated. In Chapter 6, the atomic geometry and electronic structure of graphene on the most stable In-vacancy InAs(111)A surface have been investigated. The effect of the substrate on electronic charge re-distribution around the graphene sheet was examined. The transparency effect of graphene has also been investigated by simulating scanning tunnelling microscopy. Finally, the summary of results and the future works have been outlined in Chapter 7

Stacking domains and dislocation networks in marginally twisted bilayers of transition metal dichalcogenides

We apply a multiscale modeling approach to study lattice reconstruction in marginally twisted bilayers of transition metal dichalcogenides (TMD). For this, we develop DFT-parametrized interpolation formulae for interlayer adhesion energies of MoSe$_2$, WSe$_2$, MoS$_2$, and WS$_2$, combine those with elasticity theory, and analyze the bilayer lattice relaxation into mesoscale domain structures. Paying particular attention to the inversion asymmetry of TMD monolayers, we show that 3R and 2H stacking domains, separated by a network of dislocations develop for twist angles $\theta^{\circ}<\theta^{\circ}_P\sim 2.5^{\circ}$ and $\theta^{\circ}<\theta^{\circ}_{AP}\sim 1^{\circ}$ for, respectively, bilayers with parallel (P) and antiparallel (AP) orientation of the monolayer unit cells and suggest how the domain structures would manifest itself in local probe scanning of marginally twisted P- and AP-bilayers

Structural and electronic properties of MoS2, WS2, and WS2/MoS2 heterostructures encapsulated with hexagonal boron nitride monolayers

In this study, we investigate the structural and electronic properties of MoS2, WS2, and WS2/MoS2 structures encapsulated within hexagonal boron nitride (h-BN) monolayers with first-principles calculations based on density functional theory by using the recently developed non-local van der Waals density functional (rvv10). We find that the heterostructures are thermodynamically stable with the interlayer distance ranging from 3.425 Å to 3.625 Å implying van der Waals type interaction between the layers. Except for the WS2/h-BN heterostructure which exhibits direct band gap character with the value of 1.920 eV at the K point, all proposed heterostructures show indirect band gap behavior from the valence band maximum at the Γ point to the conduction band minimum at the K point with values varying from 0.907 eV to 1.710 eV. More importantly, it is found that h-BN is an excellent candidate for the protection of intrinsic properties of MoS2, WS2, and WS2/MoS2 structures. © 2017 Author(s)

Effects of Dimensionality Reduction for High-Efficiency Mg-Based Thermoelectrics

Over the past decade, there has been significant interest in the field of thermoelectric materials (TEs) owing to their use in clean and sustainable energy sources for cooling and/or power generation applications. Especially, Mg2XIV (XIV = Si, Ge, Sn) based TEs are promising candidates for middle-temperature range energy conversion due to their high thermoelectric performance, environmentally harmless, abundant raw materials, non-toxicity, and relatively inexpensive cost of modules. In this book chapter, we present an overview of the theoretical background of the thermoelectric transport properties (Seebeck coefficient, electrical conductivity, thermal conductivity, and thermoelectric figure of merit ZT) of magnesium-based bulk and low dimensional systems (i.e., quantum wells and quantum wires). A detailed description of the temperature-dependent Fermi level both in extrinsic and intrinsic regimes will be provided whereby it is the primary step in deriving the thermoelectric transport parameters of materials. Following the linearized Boltzmann transport equations temperature-dependent electronic transport properties (Seebeck coefficient, electrical conductivity, and electronic thermal conductivity) of materials under the energy-dependent relaxation time approximation will be defined. By employing Debye’s isotropic continuum model within the single mode relaxation time approximation including various phonon relaxation rates contributed by different scattering mechanisms the lattice contribution to the thermal conductivity will be included

Thermoelectric Properties of V-VI Semiconductor Alloys and Nanocomposites

Thermoelectric materials are materials which are capable of converting heat directly into electricity and vice versa. They have long been used in electric power generation and solid-state cooling. The performance of a thermoelectric device determined by the dimensionless figure of merit (ZT) of the material, defined as ZT = (S2 σ/κ)T, where S is the Seebeck coefficient, σ is the electrical conductivity, κ is the total thermal conductivity, and T is the absolute temperature. The total thermal conductivity consists of contribution from electrons, electron-hole pairs and phonons. Since the 1960s, the best thermoelectric material has been Bi2Te3 alloys, with a ZT of 1.0 at room temperature. In recent years, the idea of using nanotechnology has opened up the possibility of engineering materials at nanoscale dimensions to achieve higher values of ZT in other words to have more efficient thermoelectric devices. This thesis starts with a broad introduction to thermoelectricity including various thermoelectric effects and their applications. The state-of-the-art thermoelectric materials and the optimisation methods to enhance the value of ZT have also been reviewed. A systematic theoretical modelling of the thermoelectric properties of three dimensional bulk semiconductors has been presented in Chapter 2. Electronic properties (Fermi level, Seebeck coefficient, and electrical resistivity) and thermal conductivity contribution from carriers (donor electrons or acceptor holes) have been derived by using the nearly-free electron approximation and the Fermi-Dirac statistics. Other thermal conductivity contributions originated from electron-hole pairs and phonons have also been described in detail. In Chapter 3, this theoretical study is extended to two dimensional semiconducting quantum well structures bearing in mind that the Fermi level should change with the temperature as well as the quantum well width and additional interface scattering mechanisms (interface mass-mixing and interface dislocation scatterings) should be included for the definition of anharmonic scattering rate. Thermoelectric properties of n-type (Bi2Te3)0.85(Bi2Se3)0.15 single crystals doped with 0.1 wt.% CuBr and 0.2 wt.% SbI3 and p-type (Bi2Te3)x(Sb2Te3)1−x single crystals doped with 3 wt.% Te (0.18 ≤ x ≤ 0.26) have been explored in Chapter 4 and 5, respectively. It has been found that p-type Bi2Te3 based alloys showed higher values of ZT due to their larger power factor (S2σ) and smaller thermal conductivity values. These calculations have concluded that the influence of the composition range of semiconductor alloys together with its type and amount of dopant plays an important role in enhancing the ZT. In Chapter 6, a detailed theoretical investigation and comparision of the thermal conductivities of these single crystals have been reported including frequency dependence of the phonon thermal conductivity for different temperatures. In Chapter 7, based on temperature and well width dependent Fermi level, a full theory of thermoelectric properties has been investigated for n-type 0.1 wt.% CuBr doped Bi2Se3/Bi2Te3/Bi2Se3 and p-type 3 wt.% Te doped Sb2Te3/Bi2Te3/Sb2Te3 quantum well systems. Different values of well thicknesses have been considered for both types of quantum well systems to study the effect of confinement on all thermoelectric transport coefficients. It has been found that reducing the well thickness has a pronounced effect on enhancing the ZT. Compared to bulk single crystals studied in Chapter 4 and 5, significantly higher thermoelectric figure of merits have been estimated theoretically for both n- and p-type semiconducting quantum well systems. For the n-type Bi2Se3/Bi2Te3/Bi2Se3 quantum well system with taking 7 nm well width the maximum value of ZT has been estimated to be 0.97 at 350 K and for the p-type Sb2Te3/Bi2Te3/Sb2Te3 quantum well with well width 10 nm the highest value of the ZT has been found to be 1.945 at 440 K. Chapter 8 briefly recapitulates the results presented in this thesis and outlines possibilities for future work

Thermoelectric transport behaviours of n-type Mg-2 (Si,Sn,Ge) quaternary solid solutions

WOS: 000487287600015Mg2X (X=Si, Sn, and Ge) based systems have attracted widespread attention owing to their various benefits in thermoelectric applications. in particular, to date, ternary Mg2X based solid solutions have become one of the most widely investigated thermoelectric systems. However, the investigation of temperature varied thermoelectric properties of Mg2X based quaternary systems is rather limited both theoretically and experimentally. Therefore, here, we report a rigorous theoretical work of thermoelectric properties for n-type Mg2Si0.55-zSn0.4Ge0.05Biz quaternary solid solutions (z=0.02, 0.025, 0.03, and 0.035) from 300 K to 850 K. By using nearly-free-electron model together with Fermi-Dirac statistics we define Fermi level both in extrinsic and intrinsic regimes as a function of temperature. We follow Hicks and Dresselhaus' approach to calculate electronic transport properties. By performing the Debye's isotropic continuum model a detailed theoretical investigation of lattice thermal conductivity is presented among with various phonon relaxation rates. From our theoretical analysis the highest ZT is attained for Mg2Si0.53Sn0.4Ge0.05Bi0.02 solid solution as 1.14 at 850 K. (C) 2019 Published by Elsevier B.V. on behalf of Chongqing University.TUBITAK (Scientific and Technical Research Council of Turkey)Turkiye Bilimsel ve Teknolojik Arastirma Kurumu (TUBITAK) [115F387]Ovgu Ceyda Yelgel wishes to acknowledge financial support from TUBITAK (Scientific and Technical Research Council of Turkey) (Project number: 115F387)

Electronic structure of ABC-stacked multilayer graphene and trigonal warping: A first principles calculation

International Physics Conference at the Anatolian Peak (IPCAP) -- FEB 25-27, 2016 -- Ataturk Univ, Nenehatun Cultural Ctr, Erzurum, TURKEYYelgel, Celal/0000-0003-4164-477XWOS: 000561775700021We present an extensive density functional theory (DFT) based investigation of the electronic structures of ABC-stacked N-layer graphene. It is found that for such systems the dispersion relations of the highest valence and the lowest conduction bands near the K point in the Brillouin zone are characterised by a mixture of cubic, parabolic, and linear behaviours. When the number of graphene layers is increased to more than three, the separation between the valence and conduction bands decreases up until they touch each other. For five and six layer samples these bands show flat behaviour close to the K point. We note that all states in the vicinity of the Fermi energy are surface states originated from the top and/or bottom surface of all the systems considered. For the trilayer system, N=3, pronounced trigonal warping of the bands slightly above the Fermi level is directly obtained from DFT calculations.Ataturk Univ, Phys Dep

Structural and electronic properties of MoS2, WS2, and WS2/MoS2 heterostructures encapsulated with hexagonal boron nitride monolayers

Yelgel, Celal/0000-0003-4164-477X; Gulseren, Oguz/0000-0002-7632-0954WOS: 000407742400034In this study, we investigate the structural and electronic properties of MoS2, WS2, and WS2/MoS2 structures encapsulated within hexagonal boron nitride (h-BN) monolayers with first-principles calculations based on density functional theory by using the recently developed non-local van der Waals density functional (rvv10). We find that the heterostructures are thermodynamically stable with the interlayer distance ranging from 3.425 angstrom to 3.625 angstrom implying van der Waals type interaction between the layers. Except for the WS2/h-BN heterostructure which exhibits direct band gap character with the value of 1.920 eV at the K point, all proposed heterostructures show indirect band gap behavior from the valence band maximum at the Gamma point to the conduction band minimum at the K point with values varying from 0.907 eV to 1.710 eV. More importantly, it is found that h-BN is an excellent candidate for the protection of intrinsic properties of MoS2, WS2, and WS2/MoS2 structures. Published by AIP Publishing.Scientific and Technological Research Council of Turkey (TUBITAK)Turkiye Bilimsel ve Teknolojik Arastirma Kurumu (TUBITAK) [115F024]OG acknowledges the support from the Scientific and Technological Research Council of Turkey (TUBITAK) under Project No. 115F024. the numerical calculations reported in this paper were fully performed at TUBITAK ULAKBIM, High Performance and Grid Computing Center (TRUBA resources)

Structural and electronic properties of multilayer graphene on monolayer hexagonal boron nitride/nickel (111) interface system: A van der Waals density functional study

Yelgel, Celal/0000-0003-4164-477XWOS: 000370974200036The structural and electronic properties of multilayer graphene adsorbed on monolayer hexagonal boron nitride (h-BN)/Ni(111) interface system are investigated using the density functional theory with a recently developed non-local van der Waals density functional (rvv10). the most energetically favourable configuration for a monolayer h-BN/Ni(111) interface is found to be N atom atop the Ni atoms and B atom in fcc site with the interlayer distance of 2.04 angstrom and adsorption energy of 302 meV/BN. Our results show that increasing graphene layers on a monolayer h-BN/Ni(111) interface leads to a weakening of the interfacial interaction between the monolayer h-BN and Ni(111) surface. the adsorption energy of graphene layers on the h-BN/Ni(111) interface is found to be in the range of the 50-120 meV/C atom as the vertical distance from h-BN to the bottommost graphene layers decreases. With the adsorption of a multilayer graphene on the monolayer h-BN/Ni(111) interface system, the band gap of 0.12 eV and 0.25 eV opening in monolayer graphene and bilayer graphene near the K point is found with an upward shifting of the Fermi level. However, a stacking-sensitive band gap is opened in trilayer graphene. We obtain the band gap of 0.35 eV close to the K point with forming a Mexican hat band structure for ABC-stacked trilayer graphene. (C) 2016 AIP Publishing LLC