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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Two-dimensional (2D) materials, such as graphene, hexagonal boron nitride (hBN), phosphorene,
and transition metal dichalcogenides (TMDs) (e.g., MoS2,WS2, etc.), have
attracted the attention of researchers in the last decades due to their extraordinary properties, and
their various promising engineering applications. Graphene is well appreciated for its high electrical
[1] and thermal conductivities[2] whereas hBN is an electrical insulator with to date an unmeasured
thermal conductivity[3], [4].
The two-dimensional nanomaterials graphene and hexagonal boron nitride (h-BN) have good
thermal properties. When the two are combined together to form a planar C-BN hybrid structure or a
van der Waals heterostructure due to the differences in crystal lattice and the electronic structure
between graphene and h-BN, the new material is formed with novel thermal characteristics.
Recently, research groups worldwide have shown that by stacking two-dimensional(2D) crystals
with different properties it is possible to create multi-layer materials, the Van der Waals
Heterostructures (vdWHs), with highly-configurable electronic characteristics [5]. 2D
heterostructures, for example, graphene/h-BN, graphene/MoS2, MoS2/MoSe2, and MoS2/h-BN,
have emerged as a fascinating research topic. For instance, the applications of these 2D
heterostructures in thermal management and thermoelectric energy generation are promising due to
their attractive electrical and thermal transport properties. The technological potential of vdWHs is
stimulating worldwide research including significant investment by industry, e.g., Samsung [6].
The application of vdWHs in thermo-electronics is still in research stage and more research is needed
to understand and perfect their thermoelectric properties. Large-scale coplanar Graphene-hBN
heterostructures have been successfully fabricated using chemical vapor deposition (CVD), enabling
the possible control of periodic arrangements of domains whose sizes range from tens of nanometers
to millimetres [7]–[10]. Their charge transport properties can be, however, quite surprising, such as
the presence of a metal-insulator transition [11]–[13] and anomalous transport phenomena, that is not
fully understood[14].
The unprecedented degree of tunability over the electronic properties of vdWHs suggests they are
ideally placed to observe and exploit thermoelectric phenomena. Thermoelectric phenomena include
the Peltier effect, where an electric potential difference can cause heating or cooling of material, and
the Seebeck effect, where a temperature gradient can generate an electrical potential difference.
These effects have been exploited in many technological applications: the Peltier effect is used to
cool electronic devices from supercomputers to camping fridges and, due to the absence of moving
parts, the Seebeck effect is exploited to scavenge heat in space technology applications and remote
Highly efficient thermoelectric devices can be created by utilizing quantum phenomena such as
quantum tunnelling and charge coupling to the spin and lattice degrees of freedom [15]. Due to
their reduced dimensionality and quantum confinement effects, 2D materials provide further
enhancement of the Seebeck and Peltier coefficients.
The graphene-hBN tunnel transistor comprises two graphene electrodes separated by an insulating
hBN layer. Here, current flows by quantum tunnelling of electrons between the graphene electrodes
through the hBN tunnel barrier. Their electronic properties can be tuned by controlling the relative
alignment of the lattices of the two graphene electrodes and has promising properties for applications
in high-frequency electronics and logic devices [16].
Thermoelectric effects often rely on strong asymmetry in the electronic density of states either side
of a junction between two materials [17]. Electrostatic gating of graphene-hBN tunnel transistors
allows precise control over the alignment of the density of states between the two graphene layers.
Therefore, after the optimization of their layer and electrostatic configuration, we expect these
devices to exhibit a tremendous and tuneable thermoelectric response.
In this work we will study some important thermoelectric effects, such as Peltier effect, Seebeck
effect, the coefficient of performance and figure-of-merit (ZT).
Both the optimization of thermal and electrical transport properties are important in constructing an
efficient direct energy conversion device.
1.1.1 Thermoelectric properties
The thermal conductivity (TC), thermal resistance (TR), thermal rectification, and thermoelectric
effects of nanomaterials affect the performance of microelectronic devices. Thermoelectrics is the
field of study where a temperature gradient can be converted to electrical power and vice versa.
In a typical thermoelectric device, a junction is formed between two different types of
semiconducting materials, with one being p-type containing holes, while the other being n-type
containing electrons as the major carrier. As shown in Fig. 1a, a temperature difference ∆𝑇 at the
junction causes the carriers to flow away from the junction, leading to an open circuit voltage V and
thus forming an electrical generator. This is called the Seebeck effect.
𝑆 =
is defined as the Seebeck coefficient [18]. On the other side, when an electric current 𝐼 is passing
through the junction, both electrons and holes move away from the junction and carry heat energy 𝑄
away, thus cooling the junction, which is called the Peltier effect. The ratio of 𝑄 to 𝐼 is defined as the
Peltier coefficient.
𝑃𝑒𝑙𝑡𝑖𝑒𝑟 𝑐𝑜𝑒𝑓𝑓𝑖𝑐𝑖𝑒𝑛𝑡 =
Figure 1. Schematic illustration of the thermoelectric effect. (a) thermoelectric generatoer (Seebeck effect). (b) thermoelectric
cooler (Pertier effect)[19].
The quality factor (dimensionless figure of merit), 𝑇𝑍, is an important parameter to measure the
performance of thermoelectric devices. It is expressed as:[18]–[20]
𝑆 2𝜎
𝑍𝑇 =
Where, 𝑆 is the Seebeck coefficient, 𝜎 is the electrical conductivity of the thermoelectric material, 𝑇
is the absolute temperature, and 𝑘 is the TC. Therefore, to improve the thermoelectric conversion
efficiency of a thermoelectric device, a large Seebeck coefficient 𝑆, a high electrical conductivity 𝜎,
and a low 𝑘 are required. Figure of merit is employed to evaluate the efficiency of thermal-to-electric
energy conversion[18] of a material. For graphene, although a high Seebeck coefficient is
observed,[21] its ultra-high thermal conductivity[22] offsets its advantage in the power factor and
thus limits its application as an efficient thermoelectric material.
In future thermal management and thermoelectric applications, it is expected that new
2D hybrid and novel van der waals heterostructures based on 2D materials with much large figure of
merit will be discovered, leading to their values to be much higher than 3, and making heat-toelectricity energy generation commercially viable. To make this a reality, more systematic studies
are needed to further understand the fascinating properties of this class of materials and to seek their
novel thermoelectric applications.
The electrical conductivity (σ) and electron thermal con- ductivity (ke) are proportional because of
the Wiedemann-Franz law, which suggests optimization should be focused around increasing the
Seebeck coefficient.
the efficiency of the thermoelectric technology is still limited by currently available thermoelectric
materials. The priority aim in thermoelectric industry is to achieve a higher ZT, the dimensionless
thermoelectric figure of merit. There are key thermoelectric properties determining the efficiency
ZT: Seebeck coefficient, electrical conductivity, and thermal conductivity.
The efficiency of a thermoelectric system is measured through a dimensionless quantity referred to
as the thermoelectric figure of merit
1.1.2 Graphene/h-BN van der Waals heterostructures
Suspended graphene requires the aid of a substrate during its use and migration. Hexagonal boron
nitride is the best substrate for graphene. The initial introduction of h-BN as a substrate is due to hBN and graphene having similar lattice structures [23]–[28], as the lattice mismatch between the two
is only 1.7%. Different from substrates such as Si, SiC, or SiO2 [29]–[32] h-BN has the atomic level
of a flat surface, very low roughness, almost no dangling bonds on the surface, and weaker van der
Waals forces than graphene; there are smaller effects on the performance of graphene, and it
provides outstanding performance [33], [34]. The different structure and combination of the two
have special significance for controlling the electrical, magnetic, and thermal properties of composite
materials[35]. This is a very important application value for the preparation of micro-nanoscale
functional devices. The research on the double-layer graphene/h-BN heterostructure has become
1.2 Challenge/problem
Globally, there is a need for fast, efficient, and small electronics. Besides, Power dissipation in
modern integrated circuits has become a big problem because it limits the performance of
electronics. However, thermoelectric-energy-conversion techniques can be used to save this energy
and improve the performance of devices. Two-dimensional (2D) materials show great promises for
application in electronic devices and may solve these problems. My study which focus on 2D
materials will contribute to this goal.
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by Edward Ma
Submission date: 03-Jan-2019 11:13PM (UT C-0700)
Submission ID: 1061432688
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