Tóm tắt tiếng anh khảo sát tính chất nhiệt Điện của một số vật liệu cấu trúc hai chiều, graphene và tựa graphene

Thus, the study aims to control the conduction state and increase the S coefficient of thin film materials such as graphene and graphene - like structures through the stimulation of ext

MINISTRY OF EDUCATION AND TRAINING

CAN THO UNIVERSITY

SUMMARY OF PH.D THESIS

Specialty: Theoretical and Mathematical Physics

CODE: 9440103

NGUYEN THI KIM QUYEN

INVESTIGATING THE THERMOELECTRIC PROPERTIES IN TWO-DIMENSIONAL MATERIALS, GRAPHENE AND GRAPHENE-

LIKE STRUCTURES

CAN THO 2024

THE WORK HAS BEEN COMPLETED AT

CAN THO UNIVERSITY

Main scientific supervisor: Assoc Prof Dr Vu Thanh Tra

Vice scientific supervisor: Assoc Prof Dr Huynh Anh Huy

The doctoral thesis was evaluated by the Board of

Examiners at the basis level

Meeting at:

At: on the date

Reviewer 1:

Reviewer 2:

Confirmation of review by the Chairman of the Board

Libraries for more information on the thesis:

- Learning Resource Centre, Can tho University

- National Library

LIST OF PUBLICATIONS International journal

1 Quyen, N T K., Tra, V T., & Truong, T V (2020)

Tight-binding description for the electronic band structure of

penta-graphene Semiconductor Science and Technology, 35, 095037

DOI: 10.1088/1361-6641/ab98d9

2 Quyen, N T K., Hanh, P N H., Loan, P T K., V T., & Truong,

T V (2021) Effect of electric fields on the electronic and thermoelectric properties of zigzag buckling silicene nanoribbons

Advances in Natural Sciences: Nanoscience and Nanotechnology,

12, 035002

DOI: 10.1088/2043-6262/ac204b

National fournal

1 Quyên, N T K., Hạnh, P N H., Trà, V T (2021) Tính chất điện

tử của Hexagonal Chromium Nitride Tạp chí Khoa học Trường

Đại học Cần Thơ, Tập 57, Số 6A

DOI: 10.22144/ctu.jvn.2021.173

Research projects

1 By Nafosted (National Foundation for Science and Technology Development): Theoretical investigation of the thermoelectric

coefficients in two dimensional materials (Graphene and Graphene

- like materials) Code: 103.01-2018.338

Duration: 2019-2021

Coordinator: PGS TS Vu Thanh Tra

Members:

Dr Huynh Anh Huy

PhD Nguyen Thi Kim Quyen

Master Nguyen Thi My The

Master Thai Thanh Lap

2 Bu unit (Kien Giang University): Theoretical study of the energy bands of Chromium Nitride (CrN) - a typical material of the rocksalt structure group - by the Tigh binding method

Code: A2020-KTCN-38

Duration: 2020-2021

Coordinator: PhD Nguyen Thi Kim Quyen

Member: Master Nguyen Thien Nhan

of materials has increasingly affirmed its great potential in practical applications However, to put these materials into useful applications,

the survey results show that the quality factor ZT of the device must be greater than 2 Thus, the problem is how to improve the ZT factor of this

group of single-layer materials and ensure the conditions for production application

In addition, the figure of merit ZT of the material is calculated by the function

corresponding to the temperature T of the material To improve the

thermoelectric properties of materials, the two preferred options are to reduced the thermal conductivity or increase the S factor of the material

in different ways In which case, if the first option is to reduce the

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thermal conductivity to increase the thermoelectric properties of the material, then two other problems arise: how to reduce both the thermal conductivity of phonons and the thermal conductivity of electrons, because the thermal conductivity of the material is synthesized from these two components However, according to the Wiedemann-Franz-Lorenz law, the thermal conductivity will be proportional to the electrical conductivity and temperature in metals Therefore, if we want

to reduce the thermal conductivity due to the contribution of electrons, the electrical conductivity will decrease, which will lead to a decrease in the S factor, which is undesirable Therefore, to reduce the thermal conductivity, the chosen option is to reduce the thermal contribution of the crystal lattice This means the selected materials will be semiconductors instead of the original metallic ones Meanwhile, if

considering the second option of increasing the S coefficient to increase the ZT factor of the material simultaneously, it is completely possible Previous studies have shown that the S coefficient has a relationship

with the band gap of the material Thus, if the band gap of the material

can be controlled, controlling the S coefficient and the ZT factor of the

material is completely proactive

Thus, the study aims to control the conduction state and increase

the S coefficient of thin film materials such as graphene and graphene -

like structures through the stimulation of external electric fields (including the vertical and transverse electric fields) By using the tight binding (TB) method to construct the hamiltonian model and the non-equilibrium Green’s formalism, based on Landauer’s formalism, the study will build a model to investigate the thermoelectric properties of two-dimensional materials, graphene, and graphene-like structures

- Investigation of the electronic and thermoelectric properties of penta graphene

1.3 Research objects and content

The research topic is aimed at two-dimensional materials with a thickness of one atomic layer, specifically BL-AGNRs, and a group of graphene-like structures including penta graphene, BSiNRs, and h-CrN The calculation of the energy bands and the electronic and thermoelectric properties (specifically the S coefficient) of these materials is specifically as follows:

- Construct the modeling for graphene and graphene - like structures, such as BL-AGNRs, BSiNRs, penta graphene, and h-CrN

- Investigation of the energy bands and the electronic properties

of materials without and with the influence of external electric fields

- Investigation of the energy bands and the electronic properties

of materials without and with the influence of external electric fields

- Evaluate the effect of each type of electric field on the S

coefficient

1.4 New research contributions

The thesis achieved the following results:

- Building a model to calculate the electronic and thermoelectric properties of materials using the TB method, specifically as follows: (i)

a model to calculate the energy bands of BL-AGNRs under the simultaneous influence of the vacancy and an external electric field; (ii)

a model to calculate the energy bands and the S coefficient of penta graphene and BSiNRs without and with the influence of an external electric field; (iii) a model to calculate the band structure of h-CrN according to atomic orbitals

- Evaluate the influence of each type of electric field (vertical and transverse) on controlling the band gap as well as the conduction state of the material

- Using the external electric field to control the S coefficient of the material, in particular: (i) for penta graphene: the S coefficient

without the vertical electric field is 5361 V / K  and larger than 67 times

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for graphene; under the impact of the vertical electric field, the S coefficient decline are 4883 V / K  and 3760 V / K  , correspond to

Vt = 0.5 and V t = 1.5, respectively; (ii) for BSiNRs: without fields, the S

coefficient about 0.06 mV / K, with the values of voltage are V s = 0.5

and V t = 0.5, this coefficient raise to 0.37 mV / K and 0.7 mV / K,

respectively Especially, under the effect of both fields, the S coefficient

of this material strongly increases and reaches the value at 1.05 mV / K

1.5 Scientific and practical meaning

The thesis has proposed some remarkable results, as follows:

- Constructing the modeling of the energy bands of some dimensional materials, including BL-AGNRs, penta graphene, BSiNRs, and h-CrN

two Successfully investigated the changes in the energy bands and the band gap of the material with and without the existence of an external electric field From there, the influence of the external electric field on the band structure and the conduction state of the material were shown

- Evaluate the effect of each type of electric field on the S coefficient of each material in the thin film group

In practice, these results contribute to giving a full picture of the energy bands and the conduction state of the material in the absence and presence of an external electric field At the same time, based on the evaluation of the electric field impaction, the thermoelectric coefficient

is also shown more clearly This will contribute to providing useful information for selecting stimuli suitable for each type of material, which will improve the heat transfer coefficient, and aim at applications

in the thermoelectric industry as well as the semiconductor industry - transistors in the future

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Figure 3 The gap size as a function of different defect positions corresponds to the odd and the even positions: (a), (b) M = 13; (c), (d) M = 19

Specifically, the results in Fig 4 show that under the combination

of the vacancy and the perpendicular electric field, the Egap1 band gap and the conductance states of BL-AGNRs have positive changes Depending on the model and different potential values, the influence on the material's band structure is different Materials can be controlled to change from semiconductor-insulators to metals by using different values in the electric field This is a positive signal when using the

vacancy and external electric field as a stimulus to adjust the S-factor as

well as the thermoelectric properties of materials

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The nearest and next nearest interactions between carbon atoms used in the TB calculation model are shown in Fig 6

Figure 6 Convention of the nearest and the next nearest hopping parameters

The results of examining the energy bands of penta graphene according to the interaction parameters, overlap factor, and onsite energy are shown in Figs 7, 8, and 9

Figure 7 The band structure of penta graphene with different hopping

parameters: (a) Model with one parameter t; (b) Model with two parameters t and t’; (c), (d), (e), (f) Model with the first and second interactions.

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RESULTS AND DISCUSSION

A INVESTIGATION OF THE EFFECT OF COMBINING VACANCY AND EXTERNAL ELECTRIC FIELD ON BL- AGNRS

Using the TB method to construct the modeling for BL-AGNRs under the effect of the vacancy, as presented in Fig 1:

Figure 1 Modeling of Bl-AGNRs: (a)-(b) hopping parameters; (c)-(d) the cases

Calculating for two cases of BL-AGNRs vacancy defects: Type 1

is a vacancy defect in an odd position (for example, atom 3 - VC = 3),

whereas type 2 is a vacancy defect in an even position (for example,

atom 4 - VC = 4), as shown in Fig 1(c)-(d)

In addition, the vertical electric field is also used to calculate the simultaneous influence of a vacancy defect and an external electric field

on controlling the conduction state of the material

in building the model In addition, with the buckling structure due to the

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Figure 3 The gap size as a function of different defect positions corresponds to the odd and the even positions: (a), (b) M = 13; (c), (d) M = 19

Specifically, the results in Fig 4 show that under the combination

of the vacancy and the perpendicular electric field, the Egap1 band gap and the conductance states of BL-AGNRs have positive changes Depending on the model and different potential values, the influence on the material's band structure is different Materials can be controlled to change from semiconductor-insulators to metals by using different values in the electric field This is a positive signal when using the

vacancy and external electric field as a stimulus to adjust the S-factor as

well as the thermoelectric properties of materials

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(a) perfect model; (b) defective model with VC = 3; (c) defective model with

VC = 6; (d) investigation of band gap as a function of V t potential

B INVESTIGATION OF THE ELECTRONIC STRUCTURE AND THE THERMOELECTRIC PROPERTIES

OF PENTA GRAPHENE

For convenience of calculation, the sheet of penta graphene is divided into cells, numbered sequentially from (0) to (8), as shown in Fig 5

Figure 5 Symbols of cells in a penta graphene sheet

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The nearest and next nearest interactions between carbon atoms used in the TB calculation model are shown in Fig 6

Figure 6 Convention of the nearest and the next nearest hopping parameters

The results of examining the energy bands of penta graphene according to the interaction parameters, overlap factor, and onsite energy are shown in Figs 7, 8, and 9

Figure 7 The band structure of penta graphene with different hopping

parameters: (a) Model with one parameter t; (b) Model with two parameters t and t’; (c), (d), (e), (f) Model with the first and second interactions.

in building the model In addition, with the buckling structure due to the

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existence of sp hybrid atoms, the overlap factor and the onsite energy

of carbon atoms also play an important role in solving the band structure problem of this material The set of structural parameters of penta graphene obtained is shown in Table 1

Table 1 The structural parameters of penta graphene (unit eV)

In addition, the vertical electric field is also used as an external stimulus to control the band gap of the material At that time, two electrodes will be applied from top to bottom and from bottom to top, so that only the bottom layer and the top layer are directly affected by the electric field; the middle layer is the layer of sp3 hybridized carbon atoms that will not be affected by the electric field The results of the energy bands and density of states (DOS) of the material under the influence of the electric field are shown in Fig 10

Figure 10 The band structure (blue color) and DOS (red color) under the effect

of an external electric field: (a) V t 0V ; (b) V t  0.15V; (c)V t 0.3V;

(d) E gap as a function of V t

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The results show that under the effect of a vertical electric field, the energy levels in the conduction band will separate and tend to be pulled toward the Fermi level Meanwhile, the energy levels in the valence band seem to have not changed This leads to a decrease in the band gap in the structure of the material In Fig 10(d), the band gap is plotted as a function of the applied voltage values The band gap reaches its maximum value without the field, but when the vertical electric field

is used, the gap size almost linearly decreases with the applied voltage values

In particular, the change in the band gap under the influence of

a vertical electric field leads to an interesting change in the thermoelectric properties of this material, specifically: the S coefficient

of the material changes as follows: when the field is not applied, the S coefficient of the material reaches the value S 5136V K/  However, when we increase the applied voltage value, respectively

0.5 V and 1.5 V, the magnitude of the S coefficient obtained at this time

is 4883V K/ and 3760V K/ , as presented in Fig 11 This is consistent with the results on the influence of the external electric field

on the band gap of the material as well as the previous research on the

relationship between the band gap and the S coefficient of the material

Figure 11 The S coefficient of penta graphene with different voltages