Generating sustainable sources of energy to match the needs of the ever-increasing world population and to mitigate the impacts of climate change is one of the most important scientific challenges of the 21st century. Thermal-to-electrical energy conversion through thermoelectric devices have begun to play a crucial role in fulfilling future demands for clean energy. The market for thermoelectric generators is projected to grow from USD 460 million in 2019 to USD 741 million by 2025, at a compound annual growth rate (CAGR) of 8.3% during the forecast period. Increasing demand to recover the waste heat generated by various industries has become an important facet of energy balance and green energy usage. The thermoelectric device market can be classified into Low Temperature (<80° C.), Medium temperature (80°-500° C.) and High temperature (>500° C.) regimes. The low-temperature regime includes high-power mobile consumer electronics, and wearable electronics, among others.
A method of fabricating a two dimensional thermoelectric device includes forming dissimilar atomic layers having quantum electron transport properties, and forming a well-defined interface between the dissimilar atomic layers for effecting a thermoelectric transport by exploiting a gradient in the material parameters between the layers. The resulting device defines an inclusion matrix of the dissimilar atomic layers such that the inclusion layer is confined within a matrix formed by the other atomic layer.
Thermoelectric effects result from electrical properties of materials exposed to a thermal gradient, resulting in an electron flow. Configurations herein are based, in part, on the observation that thermoelectric effects can be difficult to scale to magnitudes needed for efficient power generation. Unfortunately, conventional approaches to thermoelectric heat dissipation and electrical harvesting suffer from the shortcoming that the bulk materials having such properties do not lend well to surface applications and often employ volatile and/or environmentally insensitive materials such as heavy metals in production.
Accordingly, configurations herein substantially overcome the above-described shortcomings of bulk thermoelectric materials by providing a quantum transport design and manufacturing system for producing an ultra-efficient in-plane electronic and thermoelectric device to fulfill future demands for devices based on quantum electronics and clean energy solutions.
Traditionally, semiconductor superlattices and heterostructures have been used to construct thermoelectric devices. However, in such structures, it is experimentally difficult to achieve the efficiency predicted by the theory, since a large number of parameters have to be optimized. In this regard, two-dimensional (2D) materials such as graphene and transition-metal dichalcogenides (TMDC) have attracted tremendous attention due to their unique physical and chemical properties. Generation of a two dimensional matrix or material based on TMDCs can leverage a gradient in the material parameters to enhance the thermoelectric efficiency.
A two dimensional thermoelectric device as defined herein includes a transition-metal dichalcogenide (TMDC) heterostructure having an inclusion confined within a matrix, defining the heterostructure, such that the inclusion is based on a material suited for thermoelectric transport. The resulting monolayer structure defines the transition-metal dichalcogenide heterostructure.
The matrix of the opposed atomic layers may be formed on a silicon wafer by semiconductor fabrication techniques, among others. The TMDC heterostructure as defined herein is usually based on a matrix forming a triangular arrangement. The constituent layers define a triangular orientation of atoms forming a heterointerface between the monolayers. The heterostructure typically has a thickness not greater than 5 angstroms, only 10% of a conventional 50 angstroms for bulk materials, and is based on a layer of three atoms in a matrix arrangement. The side and top views of the matrix and triangular orientation are depicted below in
Practical applications of the two dimensional heterostructure with thermoelectric properties include disposing or deploying the heterostructure in conjunction with a heat sink of an electrical appliance. The thermoelectric heterostructure has an electrical connection to an electrical source coupled to the electrical appliance for directing electrons, based on the thermoelectric properties, to the electrical appliance. For example, a computing device such as a cellphone/table/personal device/laptop applies the thermoelectric material to a processor, which generates heat during normal operation. The thermoelectric flow produced is connected back into the battery/charging circuit of the device to supplement the battery power.
The foregoing and other objects, features and advantages of the invention will be apparent from the following description of particular embodiments of the invention, as illustrated in the accompanying drawings in which like reference characters refer to the same parts throughout the different views. The drawings are not necessarily to scale, emphasis instead being placed upon illustrating the principles of the invention.
Increasing demands for renewable sources of energy has been a major driving force for developing efficient thermoelectric materials. Two-dimensional (2D) transition-metal dichalcogenides (TMDC) have emerged as promising candidates for thermoelectricity due to their large effective masses and low thermal conductivity. In this article, we study the thermoelectric performance of lateral TMDC heterostructures within a multiscale quantum transport framework. Both n-type and p-type lateral heterostructures are considered for all possible combinations of semiconducting TMDCs: MoS2, MoSe2, WS2, and WSe2. The band alignment between the materials is found to play a crucial in enhancing the thermoelectric figure-of-merit (ZT) and power factor far beyond the pristine TMDCs.
In particular, a room-temperature ZT value of n-type WS2 with WSe2 triangular inclusions is five times larger than the pristine WS2 monolayer. p-type MoSe2 with WSe2 inclusions is also shown to have a room-temperature ZT value about two times larger than the pristine MoSe2 monolayer. The peak power factor values calculated here, are the highest amongst the gapped 2D monolayers at room temperature. Hence, 2D lateral TMDC hetero structures opens new avenues to construct ultra-efficient in-plane thermoelectric devices.
Thermoelectric devices can play a pivotal role in fulfilling future demands for clean energy. A good thermoelectric material must have a high thermoelectric figure-of-merit ZT, defined as
where T is the absolute temperature, σ is the electrical conductance, S is the Seebeck coefficient, κe is the electronic thermal conductivity, and κph is the lattice phonon thermal conductivity. In bulk materials, the value of ZT is limited by σ and S varying in inverse proportion, and κe and σ varying in direct proportion.
Hence, thermoelectricity was historically believed to be an inefficient source of energy for practical application. However, through the use of nanostructures, one could achieve a substantial increase in the value of ZT by reducing the dimensionality of the system. The density of electron states per unit volume increases in lower dimensions, thereby resulting in an enhancement in ZT. Since then, the field of thermoelectricity has focused on: a) increasing S and σ independently through quantum confinement effects, and b) decreasing Kph by systematically controlling phonon contributions. Additionally, other techniques such as band-gap engineering, carrier-pocket engineering, energy filtering, and semimetal-semiconductor transition have been developed to engineer the thermoelectric properties of nanostructures.
In the disclosed approach, the thermoelectric device 50 may take the form of two dimensional heterostructure device comprising having thermoelectric properties disposed on a heat sink of an electrical appliance such as the personal device 10. In the example arrangement, the device 50 includes opposed atomic layers defining an inclusion layer for defining a thermoelectric response, and an electrical connection 16 to an electrical appliance for directing electrons based on the thermoelectric properties to the electrical appliance. A heat sink defined on the processor 12 is disposed for thermal transfer from the electrical appliance for generating a thermoelectric current. This may be simply an aluminum panel, conductive paste, or simply direct placement on the processor surface 12.
Traditionally, semiconductor superlattices and heterostructures have been used to construct efficient thermoelectric devices. However, in such structures, it is experimentally difficult to achieve the predicted efficiency, since a large number of parameters have to be optimized. In this regard, two-dimensional (2D) materials such as graphene and transition-metal dichalcogenides (TMDC) have attracted tremendous attention due to their unique physical and chemical properties. The high degree of flexibility of 2D materials to tune the electrical and thermal properties, makes them ideal candidates for thermoelectric applications. A prototypical 2D material, graphene, exhibits a power factor (PF) value as high as 34.5 mWm-1K-2 at room temperature. However, it has limited thermoelectric applications due to the extremely high thermal conductivity (2000-4000 Wm-1K-1 for freely suspended samples at room temperature). In comparison, monolayer (1L) TMDCs maintain a very low thermal conductance due to significantly lower phonon mean free paths. Hence, TMDCs have tremendous potential to realize in-plane thermoelectric and Peltier cooling devices.
There have been several first-principles studies in the literature, calculating the thermoelectric quantities in 1L and layered TMDCs. P-type MoS2 1L and n-type WSe2 1L were observed to have maximum ZT values at room temperature and at higher temperatures, respectively. Also, bilayer MoS2 is observed to have a PF of 8.5 mWm-1K-2, which is highest amongst materials with a non-zero bandgap. Yet the conductance and ZT values observed in TMDCs are much lower than the corresponding quantities in traditional thermoelectric materials such as Bi2Te3, and phonon-glass electron crystals. There are opportunities to boost the thermoelectric performance in TMDCs through the formation of heterostructures.
Similar crystal structure and comparable lattice constants observed in MX2 (M=Mo, W; X=S, Se) monolayers have motivated the construction of lateral TMDC heterostructures. Experimentally, such structures are fabricated through multistep chemical vapor deposition techniques, one-pot synthesis, and omnidirectional epitaxy. In traditional thermoelectric materials, such as Bi2Te3, quantum confinement through the formation of heterostructures have been demonstrated to enhance the figure-of-merit. Such an enhancement can be anticipated in lateral 2D TMDC heterostructures as well.
Configurations herein present thermoelectric performance of lateral TMDC heterostructures within a multiscale quantum transport framework with inputs from first-principles calculations. Particular configurations employ triangular inclusions, as shown in
where e is the elementary charge, g(E) is the density of states, ν=|∇kEn(k)|/n is the carrier velocity, f0(E)=1/(1+e(E-μF)/kB T) is the Fermi-Dirac distribution function, μF is the Fermi level, and τ(E) is the total scattering time. The density of states g(E) is extracted from the electronic band structure obtained using the density functional theory (DFT) calculations within the local-density approximations (LDA).
To determine τ(E), we need to consider both the intrinsic and extrinsic scattering rates. According to Matthiessen's law:
where τe is the extrinsic carrier scattering time arising from the material inclusions, and τph is the total intrinsic scattering time arising from all the acoustic and optical phonon mode contributions. The intrinsic scattering rate τph is assumed to remain unaltered from the pristine 1L, a commonly used assumption while studying nanostructured thermoelectric materials.
In
In the example approach defined herein, the fabricated layers 410, 420 therefore form a two-dimensional thermoelectric material, including a transition-metal dichalcogenide (TMDC) heterostructure having an inclusion confined within a matrix defined by the heterostructure. The inclusion is based on a material for forming a thermoelectric transport. The layers define a triangular 154 orientation of atoms forming a heterointerface between the layers.
To calculate the carrier scattering time τe, the example configuration employs a multiscale quantum transport framework informed by first-principles calculations. Material inclusions break the translation symmetry of the system. Hence, the scattering in these structures can occur via both propagating (real wavevector) and evanescent modes (purely imaginary wavevector). The example of
The main results for the peak power factor and ZT values for the n-type and p-type TMDC lateral heterostructures are listed in Table I and Table II, respectively. In these tables, the notation A(B) represents that the material B inclusions are confined within the matrix of the material A. The material inclusion is considered here to be a substantially equilateral triangle.
In Table I, the peak power factor (PF) and the figure-of-merit ZT are listed for n-type monolayer (1L) TMDC heterostructures at temperatures 300 K, 500 K, and 800 K. Here, the notation A(B) represents that the material B inclusions are confined within the matrix of the material A. The material inclusion is equilateral triangle of the side length 8 nm. The density of inclusions is consider to be, nd=1012 cm. For comparison, we have listed the room temperature ZT values for pristine 1L TMDCs.
It can be observed that the n-type WS2(WSe2), and p-type MoSe2(WSe2) have the maximum ZT values at room temperature. On the other hand, n-type WS2(WSe2), n-type WS2(MoS2), and p-type MoS2(MoSe2) have larger ZT values at higher temperatures. In Tables I and II, for comparison, we have listed the room temperature ZT values for pristine 1L TMDCs. For the n-type WS2 1L we observe up to five times larger ZT value with WSe2 inclusions as compared to a pristine n-type WS2 1L. Similarly, for p-type MoSe2 with WSe2 inclusion, we observe an enhancement by a factor of two in the ZT values while compared to a pristine MoSe2 1L. In general, ZT values increase with temperature, as there is a multiplicative factor of temperature.
The calculated peak value of the PF for n-type WS2(WSe2) and WS2(MS2) 1L at room temperature is 5.977 mWK−2 m−1 and 4.565 mWK−2 m−1, respectively. These values are about twice the peak PF value ob-served in pristine TMDC 1L. Moreover, they are of the same order of magnitude as the observed PF in the traditional thermoelectric materials, such as Bi2Ti3 (5.2 mWK−2 m−1) and BiSbTe (5.4 mWK−2 m−1) crystals.
In Table I, it can be observed that n-type MoS2(WS2) and MoS2(MoSe2) have significantly lower thermoelectric values compared to a pristine MoS2 1L. Similarly, p-type WSe2(WS2) and WSe2(MoSe2) have significantly lower thermoelectric values compared to a pristine WSe2 1L (see Table II). These phenomena can be explained as a direct consequence of band alignment.
While the system and methods defined herein have been particularly shown and described with references to embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the scope of the invention encompassed by the appended claims.
This patent application claims the benefit under 35 U.S.C. § 119(e) of U.S. Provisional Patent App. No. 63/090,871, filed Oct. 13, 2020, entitled “THERMOELECTRIC DEVICE AND FABRICATION,” incorporated herein by reference in entirety.
Number | Date | Country | |
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63090871 | Oct 2020 | US |