Electrostatic modulation of thermoelectric transport properties of 2H-MoTe2 (original) (raw)
Very high thermoelectric figure of merit found in hybrid transition-metal- dichalcogenides
The search for thermoelectrics with higher figures of merit (ZT) will never stop due to the demand of heat harvesting. Single layer transition metal dichalcogenides (TMD), namely, MX 2 (where M is a transition metal and X is a chalcogen), that have electronic band gaps are among the new materials that have been the focus of such research. Here, we investigate the thermoelectric transport properties of hybrid armchair-edged TMD nanoribbons, by using the nonequilibrium Green's function technique combined with the first principles and molecular dynamics methods. We find a ZT as high as 7.4 in hybrid MoS 2 /MoSe 2 nanoribbons at 800 K, creating a new record for ZT. Moreover, the hybrid interfaces by substituting X atoms are more efficient than those by substituting M atoms to tune the ZT. The origin of such a high ZT of hybrid nanoribbons is the high density of the grain boundaries: the hybrid interfaces decrease thermal conductance drastically without a large penalty to electronic conductance. Published by AIP Publishing. [http://dx.
In-Plane and Interfacial Thermal Conduction of Two-Dimensional Transition-Metal Dichalcogenides
Physical Review Applied
We elucidate the dependence of the in-plane and interfacial thermal conduction of twodimensional (2D) transition metal dichalcogenide (TMDC) materials (including MoS2, WS2, and WSe2) on the materials' physical features, such as size, layer number, composition, and substrates. The in-plane thermal conductivity k is measured at suspended 2D TMDC materials and the interfacial thermal conductance g measured at the materials supported onto substrates, both with Raman thermometry techniques. The thermal conductivity k increases with the radius R of the suspended area following a logarithmic scaling as k ~ log(R). k also shows a substantial decrease from monolayer to bilayer, but only changes mildly with further increase in the layer number. In contrast, the interfacial thermal conductance g bears negligible dependence on the layer number. But g increases with the strength of the interaction between 2D TMDC materials and the substrate, substantially varying among different substrates. The result is consistent with theoretical predictions and clarifies much inconsistence in references. This work provides useful guidance for the thermal management in 2D TMDC materials and devices.
Cross-plane thermal properties of transition metal dichalcogenides
Applied Physics Letters, 2013
Thermal conductivity predictions of herringbone graphite nanofibers using molecular dynamics simulations J. Chem. Phys. 138, 084708 (2013) Thermoelectric properties of p-type (Bi2Te3)x(Sb2Te3)1−x single crystals doped with 3wt. % Te J. Appl. Phys. 113, 073709 Effect of grain boundaries on thermal transport in graphene Appl. Phys. Lett. 102, 033104 Nonlinear thermal conductance in single-wall carbon nanotubes: Negative differential thermal resistance J. Chem. Phys. 138, 034708 Sensitivity of thermal conductivity of carbon nanotubes to defect concentrations and heat-treatment J. Appl. Phys. 113, 034312 (2013) Additional information on Appl. Phys. Lett.
Science Advances
Two-dimensional (2D) materials have enabled promising applications in modern miniaturized devices. However, device operation may lead to substantial temperature rise and thermal stress, resulting in device failure. To address such thermal challenges, the thermal expansion coefficient (TEC) needs to be well understood. Here, we characterize the in-plane TECs of transition metal dichalcogenide (TMD) monolayers and demonstrate superior accuracy using a three-substrate approach. Our measurements confirm the physical range of 2D monolayer TECs and, hence, address the more than two orders of magnitude discrepancy in literature. Moreover, we identify the thermochemical electronegativity difference of compositional elements as a descriptor, enabling the fast estimation of TECs for various TMD monolayers. Our work presents a unified approach and descriptor for the thermal expansion of TMD monolayers, which can serve as a guideline toward the rational design of reliable 2D devices.
Electronic Transport and Thermopower in 2D and 3D Heterostructures-A Theory Perspective
Annalen der Physik
In this review, we discuss the impact of interfaces and heterojuctions on the electronic and thermoelectric transport properties of materials. We review recent progress in understanding electronic transport in heterostructures of two-dimensional (2D) materials ranging from graphene to transition metal dichalcogenides (TMDs), their homojunctions (grain boundaries), lateral heterojunctions (such as graphene/MoS 2 lateral interfaces), and vertical van der Waals (vdW) heterostructures. We also review work on thermopower in 2D heterojunctions, as well as their applications in creating devices such as resonant tunneling diodes (RTDs). Lastly, we turn our focus to work in three-dimensional (3D) heterostructures. While transport in 3D heterostructures has been researched for sev-eral decades, here we review recent progress in theory and simulation of quantum effects on transport via the Wigner and non-equilibrium Green's functions (NEGF) approaches. These simulation techniques have been successfully applied toward understanding the impact of heterojunctions on transport properties and thermopower, which finds applications in energy harvesting, and electron resonant tunneling, with applications in RTDs. We conclude that tremendous progress has been made in both simulation and experiments toward the goal of understanding transport in heterostructures and this progress will soon be parlayed into improved energy converters and quantum nanoelectronic devices.
Modern society is hungry for electrical power. To improve the efficiency of energy harvesting from heat, extensive efforts seek high-performance thermoelectric materials that possess large differences between electronic and thermal conductance. Here we report a super high-performance material of consisting of MoS 2 /WS 2 hybrid nanoribbons discovered from a theoretical investigation using nonequilibrium Green's function methods combined with first-principles calculations and molecular dynamics simulations. The hybrid nanoribbons show higher efficiency of energy conversion than the MoS 2 and WS 2 nanoribbons due to the fact that the MoS 2 /WS 2 interface reduces lattice thermal conductivity more than the electron transport. By tuning the number of the MoS 2 /WS 2 interfaces, a figure of merit ZT as high as 5.5 is achieved at a temperature of 600 K. Our results imply that the MoS 2 /WS 2 hybrid nanoribbons have promising applications in thermal energy harvesting. Environmental pollution and energy shortages are two big concerns in modern society. Thermoelectric materials, which can convert waste heat in the environment to electricity, are expected to be helpful in resolving these two issues 1–4. The energy conversion efficiencies of thermoelectric materials are measured by the so-called figure of merit ZT which is defined as σ = / ZT S T k 2 , where S is the Seebeck coefficient, σ is the electronic conductance, and k (= + k k k e p) is the total thermal conductance including contributions of electrons (k e) and phonons (k p). Therefore, a high-performance thermoelectric material should have high electron conductance and low thermal conductance, i.e., electron crystals and thermal glasses. However, the ZT values of most bulk materials are very small (much less than 1.0) because their electronic and thermal properties always have the same trends 5–8. There are extensive studies to search for high ZT materials. It is reported that the ZT values of some materials are improved significantly after nanocrystallization due to drastic changes of electronic and thermal properties 9–16. For example, theoretical calculations proved that the ZT values of quasi-one nanowires have a larger increase than those of bulk and two-dimensional structures 2 nanostructured bismuth antimony telluride showed experimentally higher ZT values than the bulk because of a sharp reduction in k p 6. Beyond the intrinsic improvement, the ZT values of nanostructures can be further enhanced by various modifications, such as hybridization 10–12 , doping 13,14 , absorption 15,16 , etc. Previous theoretical studies indicated that hybrid nanostructures, such as SiGe alloys and hybrid BN/graphene nanoribbons, possess higher thermoelectric properties than single nanostructures 11,12. Even if the thermoelectric performances of nanostructures are much better than those of bulk, most of them still cannot meet the requirements for real world applications. As such, the search for high-performance thermoelectric materials for energy harvesting applications has become a main focus in the thermoelectric field. Recently, the thermoelectric properties of single-layer or few-layer transition metal dichalcogenides (TMD) MX 2 (M = Mo, W, while X = S, Se etc.) have attracted attention 17–21. MoS 2 and WS 2 are two typical TMDs, which are considered as excellent electronic materials because of direct band gaps and high carrier mobility. Electron transistors based on MoS 2 and WS 2 have been reported and show high electronic performance, while the thermal conductivities of the two nanosheets are relatively low 22–26. Therefore, MoS 2 and WS 2 monolayer should have high ZT values, which have been proven by previous theoretical studies 18,19,21. Meanwhile, some interesting MoS 2 /WS 2
Effect of Metal Doping and Vacancies on the Thermal Conductivity of Monolayer Molybdenum Diselenide
ACS applied materials & interfaces, 2018
It is well understood that defect engineering can give rise to exotic electronic properties in transition-metal dichalcogenides, but to this date, there is no detailed study to illustrate how defects can be engineered to tailor their thermal properties. Here, through combined experimental and theoretical approaches based on the first-principles density functional theory and Boltzmann transport equations, we have explored the effect of lattice vacancies and substitutional tungsten (W) doping on the thermal transport of the suspended molybdenum diselenide (MoSe2) monolayers grown by chemical vapor deposition (CVD). The results show that even though the isoelectronic substitution of the W atoms for Mo atoms in CVD-grown Mo0.82W018Se2 monolayers reduces the Se vacancy concentration by 50% compared to that found in the MoSe2 monolayers, the thermal conductivity remains intact in a wide temperature range. On the other hand, Se vacancies have a detrimental effect for both samples and more ...
Orientation dependent thermal conductance in single-layer MoS 2
Scientific Reports, 2013
We investigate the thermal conductivity in the armchair and zigzag MoS 2 nanoribbons, by combining the non-equilibrium Green's function approach and the first-principles method. A strong orientation dependence is observed in the thermal conductivity. Particularly, the thermal conductivity for the armchair MoS 2 nanoribbon is about 673.6 Wm 21 K 21 in the armchair nanoribbon, and 841.1 Wm 21 K 21 in the zigzag nanoribbon at room temperature. By calculating the Caroli transmission, we disclose the underlying mechanism for this strong orientation dependence to be the fewer phonon transport channels in the armchair MoS 2 nanoribbon in the frequency range of [150, 200] cm 21 . Through the scaling of the phonon dispersion, we further illustrate that the thermal conductivity calculated for the MoS 2 nanoribbon is esentially in consistent with the superior thermal conductivity found for graphene.
The Journal of Physical Chemistry C, 2018
Thermal transport in a material is governed by anharmonicity of crystal potential, which depends on the type of inter-atomic interaction. Using first-principles calculations, we report that lattice thermal conductivity (κ latt) and its anisotropy (κ x,y − κ z) of transition metal dichalcogenides (TMDs) increase by orders of magnitude with the change of constituent metal atom from Zr/Hf to Mo/W. This unprecedented difference in κ latt is substantiated by lower phonon group velocity, and four times larger anharmonicity of Zr/Hf based TMDs compared to Mo/W based TMDs. The sign and the absolute value of the Born effective charges, which emerges from the ionicity of the bonds, are found to be different for these two classes of materials. This leads to a significant difference in their interlayer van der Waals (vdW) interaction strengths, which are shown to be inversely related to the anisotropy in κ latt .
Electron transport and thermoelectric performance of defected monolayer MoS2
Physica E: Low-dimensional Systems and Nanostructures, 2018
Electronic and thermoelectric properties of a two-dimensional MoS 2 monolayer containing atomic defects are investigated using density functional theory. All the atomic defects have been found to exhibit endothermic nature. Electronic structure of MoS 2 shows tuneability of band gap with the atomic defects. The MoS 2 vacancy in pristine monolayer makes it magnetic and narrow band gap semiconductor. The spin-polarized character of the monolayer with defects is clearly captured by the tunneling current calculated in the STM-like setup. A relatively low thermal conductivity has been observed in monolayers with defects as compared to pristine form resulting in enhanced room temperature figure of merit as high as 6.24 and 1.30 respectively. We believe that our results open up a new window for the use of monolayer MoS 2 in electronic devices, thermal management and thermoelectric devices.
arXiv (Cornell University), 2022
We discuss the effect of the dielectric environment (substrate/bottom oxide, gate insulator, and metal gates) on electronic transport in two-dimensional (2D) transition metal dichalcogenides (TMD) monolayers. We employ well-known ab initio methods to calculate the low-field carrier mobility in free-standing layers and use the dielectric continuum approximation to extend our study to layers in double-gate structures, including the effects of dielectric screening of the electron-phonon interaction caused by the bottom oxide and the gate insulator, and of scattering with hybrid interface optical-phonon/plasmon excitations ('remote phonon scattering'). We find that the presence of insulators with a high dielectric constant may improve significantly the carrier mobility. However, scattering with the interface hybrid excitations negates this gain and degrades the mobility significantly below its free-standing value. We find that this process is dominated by long-wavelength interactions that, for the carrier sheet-density of interest, are strongly affected by the coupling with the 2D plasmons. Considering 2D layers in a double-gate geometry with SiO2 as bottom-oxide and various top-gate insulators, we find that the mobility decreases as the top-insulator dielectric constant increases (from hBN to ZrO2), as expected. However, we observe two main deviations from this trend: A high mobility is predicted in the case of the weakly polar hBN, and a mobility much lower than expected is calculated in the case of gate-insulator/TMD/bottom-oxide stacks in which two or more polar materials have optical-phonon with similar resonating frequencies. We also find that the effect of screening by metal gates is noticeable but not particularly strong. Finally, we discuss the effect of the TMD dielectric constant, of the free-carrier density, and of temperature on the transport properties of TMD monolayers.
Journal of Applied Physics
Two-dimensional group IV transition-metal dichalcogenides have encouraging thermoelectric applications since their electronic and lattice properties can be manipulated with strain. In this paper, we report the thermoelectric parameters such as electrical conductivity, Seebeck coefficients, electrical relaxation times, and the mode dependent contributions to the lattice thermal conductivity of ZrX 2 (X = S, Se, Te) from first principles methods. Our calculations indicate that due to tensile strain, the powerfactor increases while simultaneously decreasing the lattice thermal conductivity thus enhancing the thermoelectric figure of merit. Tensile strain widens the bandgap which corresponds to higher powerfactor. The lattice thermal conductivity decreases due to the stiffening of the out-of-plane phonon modes thus reducing the anharmonic scattering lifetimes and increasing the thermoelectric figure-of-merit.
2016
Transition metal dichalcogenides have potential applications in power generation devices that convert waste heat into electric current by the so-called Seebeck and Hall effects thus providing an alternative energy technology to reduce the dependence on traditional fossil fuels. In this study, the thermoelectric properties of 1T and 2HTaX2 (X= S or Se) dichalcogenide superconductors have been computed using the semi-classical Boltzmann theory. Technologically, the task is to fabricate suitable materials with high efficiency. It is found that 2HTaS2 possesses the largest value of figure of merit ZT= 1.27 at 175 K. From a scientific point of view, we aim to model the underlying materials properties and in particular the transport phenomena as mediated by electrons and lattice vibrations responsible for superconductivity, Charge Density Waves (CDW) and metal/insulator transitions as function of temperature. The goal of the present work is to develop an understanding of the superconducti...
The Journal of chemical physics, 2015
We investigate the electronic and thermal transport properties of bulk MX2 compounds (M = Zr, Hf and X = S, Se) by first-principles calculations and semi-classical Boltzmann transport theory. The band structure shows the confinement of heavy and light bands along the out of plane and in-plane directions, respectively. This results in high electrical conductivity (σ) and large thermopower leading to a high power factor (S(2)σ) for moderate n-type doping. The phonon dispersion demonstrates low frequency flat acoustical modes, which results in low group velocities (vg). Consequently, lowering the lattice thermal conductivity (κlatt) below 2 W/m K. Low κlatt combined with high power factor results in ZT > 0.8 for all the bulk MX2 compounds at high temperature of 1200 K. In particular, the ZTmax of HfSe2 exceeds 1 at 1400 K. Our results show that Hf/Zr based dichalcogenides are very promising for high temperature thermoelectric application.
Thermal and Photo Sensing Capabilities of Mono- and Few-Layer Thick Transition Metal Dichalcogenides
Micromachines
Two-dimensional (2D) materials have shown promise in various optical and electrical applications. Among these materials, semiconducting transition metal dichalcogenides (TMDs) have been heavily studied recently for their photodetection and thermoelectric properties. The recent progress in fabrication, defect engineering, doping, and heterostructure design has shown vast improvements in response time and sensitivity, which can be applied to both contact-based (thermocouple), and non-contact (photodetector) thermal sensing applications. These improvements have allowed the possibility of cost-effective and tunable thermal sensors for novel applications, such as broadband photodetectors, ultrafast detectors, and high thermoelectric figures of merit. In this review, we summarize the properties arisen in works that focus on the respective qualities of TMD-based photodetectors and thermocouples, with a focus on their optical, electrical, and thermoelectric capabilities for using them in se...
Thermoelectric and phonon transport properties of two-dimensional IV–VI compounds
Scientific Reports, 2017
We explore the thermoelectric and phonon transport properties of two-dimensional monochalcogenides (SnSe, SnS, GeSe, and GeS) using density functional theory combined with Boltzmann transport theory. We studied the electronic structures, Seebeck coefficients, electrical conductivities, lattice thermal conductivities, and figures of merit of these two-dimensional materials, which showed that the thermoelectric performance of monolayer of these compounds is improved in comparison compared to their bulk phases. High figures of merit (ZT) are predicted for SnSe (ZT = 2.63, 2.46), SnS (ZT = 1.75, 1.88), GeSe (ZT = 1.99, 1.73), and GeS (ZT = 1.85, 1.29) at 700 K along armchair and zigzag directions, respectively. Phonon dispersion calculations confirm the dynamical stability of these compounds. The calculated lattice thermal conductivities are low while the electrical conductivities and Seebeck coefficients are high. Thus, the properties of the monolayers show high potential toward thermoelectric applications.
Small, 2019
2D materials, such as transition metal dichalcogenides (TMDs), graphene, and boron nitride, are seen as promising materials for future high power/high frequency electronics. However, the large difference in the thermal expansion coefficient (TEC) between many of these 2D materials could impose a serious challenge for the design of monolayer‐material‐based nanodevices. To address this challenge, alloy engineering of TMDs is used to tailor their TECs. Here, in situ heating experiments in a scanning transmission electron microscope are combined with electron energy‐loss spectroscopy and first‐principles modeling of monolayer Mo1−xWxS2 with different alloying concentrations to determine the TEC. Significant changes in the TEC are seen as a function of chemical composition in Mo1−xWxS2, with the smallest TEC being reported for a configuration with the highest entropy. This study provides key insights into understanding the nanoscale phenomena that control TEC values of 2D materials.
Electrical and electrothermal properties of few-layer 2D devices
Journal of Computational Electronics, 2020
While two-dimensional (2D) materials have emerged as a new platform for nanoelectronic devices with improved electronic, optical, and thermal properties, and their heightened sensitivity to electrostatic and mechanical interactions with their environment has proved to be a bottleneck. Few-layer (FL) 2D devices retain the desirable thinness of their monolayer cousins while boosting carrier mobility. Here, we employ an electrothermal model to study FL field-effect devices made from transition metal dichalcogenides MoS 2 and WSe 2 and examine the effect of both electrical and thermal interlayer resistances, as well as the thermal boundary resistance to the substrate, on device performance. We show that overall conductance improves with increasing thickness (number of layers) at small gate voltages, but exhibits a peak for large gate voltages. Joule heating impacts performance due to relatively poor thermal conductance to the substrate and this impact, along with the location of the hot spot in the FL stack, varies with carrier screening length of the material. We conclude that coupled electrothermal simulation can be employed to design FL 2D devices with improved performance.