1. Remove the plagiarism to below 15% without changing the science. You need a good knowledge of thermoelectric properties of a material. Do NOT use a software.2. Correct grammatical mistakes.
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There has been an increasing wave of research on 2D materials, with a good number of
researchers focussing on thermoelectric properties of 2D single element materials and 2D
heterostructure materials. In the 1990s, the thermoelectric properties of a large number of
nanomaterials have been widely recognized, and some recent scientists have studied this feature
more deeply 1–9. A large number of theoretical and experimental studies have described the thermal
and thermoelectric properties of graphene/h-BN heterostructure10.
The measurement techniques for in-plane Seebeck and four-probe conductivity were first
demonstrated on monolayer graphene samples in 200911–13.Anomalous Seebeck (Sxx = ΔVxx/ΔTxx)
and Nernst (Sxy = ΔVxy/ΔTxx) signals were observed, in violation of the well-accepted Mott relation
for metallic samples. This was especially seen to be true close to the charge neutrality point (CNP)
away from the highly doped degenerate limit, where electron-hole puddles were purported to be
present. In this resistive state, the carrier density is strongly inhomogeneous and also the effect of
impurities was found to be nontrivial. The manifestation of this on the thermoelectric power
(Seebeck and Nernst) was explained well using an effective medium theory14.
Wang et al. used the first-principles density functional calculations combined with the nonequilibrium Green’s function to simulate the thermal transport and thermoelectric properties of the
graphene/h-BN heterostructure in the AB stacking mode 15. They observed that thermal transport
characteristics of graphene in the heterostructure are lower than that of pure graphene, but the
Seebeck coefficient is enhanced due to the opening of the graphene band gap. The figure of merit,
ZT of the graphene/h-BN superlattice is larger than that of pure graphene, with 44% enhancement15.
From the formula 𝑍𝑇 = (𝑆^2 𝜎)𝑇/𝑘 , it can be seen that all of the factors that affect ZT are
enhanced and lead to the enhancement of ZT itself.
The Seebeck measurements across a graphene/h-BN/graphene heterostructurehas been
determined by inducing a temperature gradient between the bottom and top graphene layers and
measuring the corresponding thermoelectric voltage (∆V) across the heterostructure16. A temperature
gradient (∆T) of 39 K and thermoelectric voltage (∆V) of almost 4 mV is observed in the device,
which results in a Seebeck coefficient of -99.3 μV/K, a power factor (𝑆 2 𝜎) of 1.51 × 10–15 W/K2 ,
and a thermoelectric figure of merit of ZT = 1.05 × 10–6 for the graphene/h-BN/graphene
heterostructure. While the overall thermoelectric energy conversion efficiency of this device is small,
the relatively large Seebeck coefficient and temperature drops observed here indicate that the I–V
characteristics of 2D heterostructures can contain an appreciable thermoelectric component. 16.
The thermal and thermoelectric properties of the most structurally stable AB configuration
among the heterostructures formed by vertical stacking of graphene and h-BN have been revealed in
recent studies 15,17. Molecular dynamics simulations have been performed to study the interfacial
thermal resistance (ITR) of a graphene/h-BN bilayer system as well as its one-dimensional
counterpart, a concentric CNT/BNNT double-walled nanotube, based on the lumped capacity model.
The calculated ITR is in an order of magnitude of 10-7-10-6 Km2/W and it monotonically decreases
with temperature and interlayer/intertube coupling strength17.
Recent experiments where the graphene was processed to be extremely clean with low defect
density on an atomically smooth, high-k hexagonal boron nitride (h-BN) dielectric indeed verified
predictions that electron-electron scattering can dominate at high temperatures. This enabled the
exploration of rich physics, where strong inelastic collisions between electrons (e–e interactions)
dominate, resulting in a violation of the Mott Relation18. Further, near the CNP, a plasma of
electron–holes (unlike charge puddles shown previously in defect-dominated samples) manifested in
large Seebeck values18,19 as well as a violation of the Wiedemann– Franz Law20. Studies have show
that the thermoelectric performance of graphene can be significantly improved by using hexagonal
boron nitride (hBN) substrates instead of SiO219. The power factor times temperature, reached values
as high as 10.35 W·m−1·K−1. higher values of PF3D were reported, as large as 34.5 mW m−1 K−2
(converted from a 10.35 W m−1 K−1 (Figure 3c) powerfactor × temperature (PFT) value at 300
K).19. These large values are similar to the report of violation of the Mott Relation in clean samples
and can be ascribed to hydrody- namic electrons as well18.
Researchers have used the Molecular Dynamic (MD) simulation method to assess the thermal
conductivity of single-layer graphene based on a multilayer h-BN substrate 21. They have
demonstrated that bulk hexagonal boron nitride (h-BN) is a more appealing substrate to achieve high
performance heat dissipation in supported graphene. Notable length dependence and high thermal
conductivity have been observed in single layer graphene on h-BN substrate. At room temperature,
the thermal conductivity of h-BN-supported SLG is as high as 1347.3 ± 20.5 Wm−1 K−1, which is
about 77% of that for the suspended case, and is more than twice that of the SiO2-supported SLG.
The thermoelectric figures of merit of pristine two-dimensional materials are predicted to be
significantly less than unity22, making them uncompetitive as thermoelectric materials. Since the inplane transverse and longitudinal phonons are effectively filtered out from contributing to crossplane transport because they do not substantially alter the tunneling matrix elements, theoretical
calculations predict enhanced cross-plane thermoelectric properties in van der Waals heterojunctions,
including high ZT factors at room temperature 22. These researchers have analysed the thermoelectric
performance of monolayer molybdenum disulphide (MoS2) sandwiched between two graphene
monolayers. They observed that CP ZT can be as high as ~2.8 for the graphene/MoS2/graphene
Researchers have used quantum transport and molecular dynamics (MD) simulations to
calculate the electronic and thermal properties of polycrystalline graphene-hBN heterostructures 23.
They have estimated the thermoelectric conversion ratio and found that it remains far too low to be
useful for energy harvesting applications. The upper value of the figure of merit, 𝑍𝑇 =
𝑆 2 𝜎𝑇
found which is quite small. For 40nm average grain size and 20% hBN, ZT~1×10-4 for a carrier
concentration n=5×1012 cm-2 , which is quite small. Even for energies near the edge of the gap, where
the see beck coefficient should be maximised, the value of ZT only reaches ~1×10-2.
Other researchers observed that an effective “inter-layer phonon drag” determines the
Seebeck coefficient (S) across the van der Waals gap formed in twisted bilayer graphene (tBLG)24.
They have demonstrated that the cross-plane thermoelectric transport is driven by the scattering of
electrons and interlayer layer breathing phonon modes, which thus represents a unique “phonon
drag” effect across atomic distances. They calculated the cross-plane thermoelectric power-factor
(S2Gcp), by combining the experimentally observed magnitudes of see beck coefficient, S and
interlayer conductance, Gcp. They observed that, the maximum effective PFT = TS2Gcp/d, where d
≈ 0.4 nm is the van der Waals distance, increases with temperature, and can be as high as ≈ 0.3
Wm−1K−1 at room temperature.
In summary, several researchers have used theoretical and experimental approaches to
study the thermoelectric properties of graphene based 2D heterostructures. However, more study is
needed to fully optimise the thermoelectric performance of these materials. Besides, there is need to
explore the performance of other 2D heterostructures.
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by Edward Ma
Submission date: 03-Jan-2019 11:15PM (UT C-0700)
Submission ID: 1061432901
File name: Literature_review2.docx (39.18K)
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SIMILARIT Y INDEX
INT ERNET SOURCES
ST UDENT PAPERS
Jing Wu, Yabin Chen, Junqiao Wu, Kedar
Hippalgaonkar. “Perspectives on
Thermoelectricity in Layered and 2D Materials”,
Advanced Electronic Materials, 2018
Jingang Wang, Xijiao Mu, Xinxin Wang, Nan
Wang, Fengcai Ma, Wenjie Liang, Mengtao
Sun. “The thermal and thermoelectric
properties of in-plane C-BN hybrid structures
and graphene/h-BN van der Waals
heterostructures”, Materials Today Physics,
Int ernet Source
Phanibhusan S. Mahapatra, Kingshuk Sarkar,
H. R. Krishnamurthy, Subroto Mukerjee,
Arindam Ghosh. “Seebeck Coefficient of a
Single van der Waals Junction in Twisted
Bilayer Graphene”, Nano Letters, 2017
Int ernet Source
José Eduardo Barrios-Vargas, Bohayra
Mortazavi, Aron W. Cummings, Rafael
Martinez-Gordillo et al. “Electrical and Thermal
Transport in Coplanar Polycrystalline
Graphene–hBN Heterostructures”, Nano
Ting Li, Zhenan Tang, Zhengxing Huang, Jun
Yu. “Interfacial thermal resistance of 2D and 1D
carbon/hexagonal boron nitride van der Waals
heterostructures”, Carbon, 2016
Int ernet Source
Int ernet Source
Submitted to Indian Institute of Science,
St udent Paper
Int ernet Source
Int ernet Source
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