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` VIETNAM NATIONAL UNIVERSITY, HANOI VIETNAM JAPAN UNIVERSITY NGUYEN HOANG HUNG DFT STUDY ON GAS ADSORPTION OF DICHALCOGENIDE MONOLAYERS: TOWARDS GAS SENSING APPLICATION h MASTER'S THESIS ` VIETNAM NATIONAL UNIVERSITY, HANOI VIETNAM JAPAN UNIVERSITY NGUYEN HOANG HUNG h DFT STUDY ON GAS ADSORPTION OF DICHALCOGENIDE MONOLAYERS: TOWARDS GAS SENSING APPLICATION MAJOR: NANOTECHNOLOGY CODE: 8440140.11QTD RESEARCH SUPERVISORS: Specially Appointed Professor, Dr DINH VAN AN Prof Dr YOJI SHIBUTANI Hanoi, 2021 ` ACKNOWLEDGMENT First of all, I would like to express my special thanks to my supervisors Specially Appointed Professor, Dr Dinh Van An and Prof Dr Yoji Shibutabi Thank you for all your thorough and supportive instructions, your courtesy, and your encouragement This thesis absolutely could not be conducted well without your dedicated concerns Second of all, I would like to show my gratitude to the MNT program and VNU Vietnam Japan University (VJU) professors, lecturers and staff for all of your support Third of all, I would like to express my warm thanks to my seniors, Pham Trong Lam, Ta Thi Luong, Pham Ba Lich for significant advices from my first step in VJU to the last days Last but not least, I would like to send thanks from the bottom of my heart to Japan International Cooperation Agency (JICA) and VJU for providing me with h financial support during my study in VJU ` CONTENTS h CONTENTS LIST OF TABLES LIST OF FIGURES LIST OF ABBREVIATIONS INTRODUCTION CHAPTER LITERATURE REVIEW 1.1 Overview toxic gases 1.2 Two-dimensional (2D) material-based toxic gas sensors 1.3 2D Transition Metal Dichalcogenides (TMDCs) and monolayer MoTe2 1.3.1 2D TMDCs 1.3.2 Monolayer MoTe2 based sensor CHAPTER METHODOLOGY 2.1 Density Functional Theory 2.1.1 Kohn-Sham method 2.1.2 DFT computational scheme 2.1.3 vdW-DF method 2.2 VASP 2.3 Bader Charge Analysis 2.4 Computational Method CHAPTER RESULTS AND DISCUSSION 3.1 Monolayer MoTe2 properties 3.1.2 Geometrical properties 3.1.2 Electronic properties 3.2 Adsorption mechanism of toxic gases on monolayer MoTe2 3.2.1 Adsorption configuration 3.2.2 Adsorption profile 3.3 Charge transfer 3.4 Charge density difference 3.5 Electronic structures 3.6 Work function CONCLUSION FUTURE PLANS REFERENCES i ii iv 2 7 12 12 12 15 16 17 18 19 20 20 20 21 23 23 29 34 46 49 52 54 55 56 ` LIST OF TABLES Table 1.1 Adsorption characteristics of toxic gases on some typical 2D materials Table 1.2 Adsorption characteristics of common gases on MoTe2 monolayer 11 Table 3.1 The structure parameters of monolayer MoTe2 21 Table 3.2 The band gap and spin orbit splitting values comparting by non-empirical vdW functionals .22 Table 3.3 The values of bond-length (l), the distance between the lowest atom of molecule and the nearest atom of absorbent (da), the angle between molecule and the absorbent surface () and the bond angle () of favorable configurations 28 Table 3.4 Summary of the adsorption energy, adsorption distance and response length for toxic gases adsorption on monolayer MoTe2 34 Table 3.5 Bader charge analysis of the CO – MoTe2 system 35 Table 3.6 Bader charge analysis of the CO2 – MoTe2 system .37 Table 3.7 Bader charge analysis of the NO – MoTe2 system 39 Table 3.8 Bader charge analysis of the NO2 – MoTe2 system 41 Table 3.9 Bader charge analysis of the NH3 – MoTe2 system 43 h i ` LIST OF FIGURES h Figure 1.1 The molecular structure of CO2 Figure 1.2 The molecular structure of CO .2 Figure 1.3 The molecular structure of NO2 Figure 1.4 The molecular structure of NO Figure 1.5 The molecular structure of NH3 Figure 1.6 Illustration of the charge transfer mechanism of MoS2 to gas molecules Figure 1.7 Illustration some synthesis methods for few-layer TMDCs (a) MBE (b) CVD (c) Metal-organic CVD Figure 1.8 The illustration of MoTe2 FET sensor (a) Optical microscope image (b) AFM topography image d) High resolution TEM image (e) SAED pattern taken were reported by Feng’s group……………………………………………………………….9 Figure 2.1 DFT computational scheme .16 Figure 3.1 (a) side view (b) top view of the hexagonal monolayer MoTe2 and (c) the considered adsorption sites on monolayer MoTe2 20 Figure 3.2 The Band structure and DOS of the monolayer MoTe2 with (a) and without (b) SOC taken into account 23 Figure 3.3 The C≡O bond-length (l), the distance between C atom and the nearest atom of absorbent (da), the angle between molecules and the absorbent surfaces () of CO-MoTe2 favorable configurations 24 Figure 3.4 The favorable configurations of CO2-MoTe2 top view (a) and side view (b) with the adsorption distance (d), the C=O bond length (l), the distance between C atom and the nearest Te atom of absorbent (da), the bond angle () 25 Figure 3.5 Top view (a) and side view of the favorable configurations of NO-MoTe2 (b) with the adsorption distance (d), the N O bond length (l), the distance between N atom and the nearest atom of absorbent (da), the angle between molecules and the absorbent surfaces () 25 Figure 3.6 Top view (a) and side view of the favorable configurations of NO2-MoTe2 (b) with the adsorption distance (d), the N O bond length (l), the distance between O atom and the nearest Te atom of absorbent (da), the bond angle O N O () 26 Figure 3.7 The favorable configurations of NH3-MoTe2 top view (a) and side view (b) with the adsorption distance (d), the N-H bond length (l), the distance between C atom and the nearest Te atom of absorbent (da), the bond angle N-H-N () 27 Figure 3.8 The adsorption energy Ead as a function of distance dz between (a) CO (b) CO2 (c) NO (d) NO2 (e) NH3 and monolayer MoTe2 achieved by differerent vdW-DFs 33 Figure 3.9 The comparison of the magnitude of adsorption energies and the adsorption distance of toxic gases adsorption on monolayer MoTe2 achieved by optB88 vdW functional 33 ii ` Figure 3.10 Comparison of the charge transfer of the toxic gases adsorbed on monolayer MoTe2 achieved by optB88 vdW functional 46 Figure 3.11 Charge density difference for CO – MoTe2 system achieved by optB88 vdW functional Top view (left) and side view (right) (isosurface level = 0.0002) 47 Figure 3.12 Charge density difference for CO2 – MoTe2 system achieved by optB88 vdW functional Top view (left) and side view (right) (isosurface level = 0.0002) 47 Figure 3.13 Charge density difference for NO – MoTe2 system achieved by optB88 vdW functional Top view (left) and side view (right) (isosurface level =0.0022) .48 Figure 3.14 Charge density difference for NO2 – MoTe2 system achieved by optB88 vdW functional Top view (left) and side view (right) (isosurface level: 0.0006) 48 Figure 3.15 Charge density difference for NH3 – MoTe2 system achieved by optB88 Top view (left) and side view (right) (isosurface level = 0.0004) 48 Figure 3.16 BAND-DOS of CO – MoTe2 calculated by optB88 50 Figure 3.17 BAND-DOS of CO2 – MoTe2 calculated by optB88 .51 Figure 3.18 BAND-DOS of NH3 – MoTe2 calculated by optB88 .51 Figure 3.19 BAND-DOS of NO – MoTe2 calculated by optB88 52 Figure 3.20 BAND-DOS of NO2 – MoTe2 calculated by optB88 .52 Figure 3.21 The variation in work function () for different gas adsorption systems.53 h iii ` LIST OF ABBREVIATIONS 2D DFT GGA LDA VASP PAW vdW vdW-DFs TMDCs BAND DOS KS HF VBM CBM FET Two-dimensional Density Functional Theory Generalized gradient approximation Local density approximation Vienna Ab initio Software Package Projector Augmented Wave van der Waals van der Waals density functionals Transition metal dichalcogenides Energy band structure Density of state Kohn-Sham Hartree-Fock Valence Band Maximum Conduction Band Minimum Field Effect Transistor h iv ` INTRODUCTION The continuous increase in human ambition for development has led to environmental devastation The toxic air pollutant which is mainly sourced from coalfired power plants, industry, refineries, building materials like asbestos, tobacco smoke, and chemicals like solvents as well as from transportation, is one of the biggest issues in today's era These gas pollutants may cause climate change, ozone pollution, harmful direct and indirect human health and even threaten food security Therefore, the detection of toxic gases has become extremely essential for environmental controlling, human safety and manufacturing monitoring products In general, a proficiency of the gas sensor is decided by several factors, that include, (1) selectivity, (2) sensitivity (3) faster response and recovery time, (4) reversible i.e., return to its original state after gas is removed (5) low cost Recently, it can be seen that the gas sensor is generally based on the semiconducting metal oxide which is high sensitivity and low cost However, there are some drawbacks such as the high operating temperature requirement, large power consumption and low selectivity h Hence, the exploration for a novel material to produce a proficiency gas sensor that can operate at room temperature is broadened ` CHAPTER LITERATURE REVIEW 1.1 Overview toxic gases The toxic gases have a direct and indirect effect on human health as well as the environment In our work, we investigate the five most effective candidate gases: CO, CO2, NO, NO2, NH3 a) Carbon dioxide (CO2): CO2 is a “greenhouse gas” described as the worst climate pollutant It was proved that high concentration of carbon dioxide is responsible for a variety of health effects when inhaled which include headaches, dizziness, increased heart rate, etc Carbon dioxide mainly produced from burning fossil fuels (coal, natural gas, and oil), land-use changes and industrial processes In structure, CO2 is a flat molecule with sp hybridization, carbon linked to two oxides by a double bond with the bond length equal to 1.163 Å [1] h Figure 1.1 The molecular structure of CO2 b) Carbon monoxide (CO): Carbon monoxide is a colorless, odorless, tasteless, flammable, high toxicity gas that has a main source from burning carbon-based fuel of vehicles, regime, etc High concentration of carbon monoxide entering your bloodstream immediately combined with hemoglobin produces carboxyhenmologin, which prevents carrying oxygen in blood, leading to serious nervous system damage, or even death For chemical properties, CO is linked by the triple bond including one bond and bond with the bond length around 1.128 Å [2] Figure 1.2 The molecular structure of CO ` Te 6.25102 0.25102 36 Te 6.25641 0.25641 37 Te 6.25408 0.25408 38 Te 6.24824 0.24824 39 Te 6.2481 0.2481 40 Te 6.25117 0.25117 41 Te 6.25136 0.25136 42 Te 6.25091 0.25091 43 Te 6.24821 0.24821 44 Te 6.25364 0.25364 45 Te 6.25076 0.25076 46 Te 6.25129 0.25129 47 Te 6.25412 0.25412 48 Te 6.25228 0.25228 49 N 6.16843 1.16843 50 H 0.60737 -0.39263 51 H 0.60682 -0.39318 52 H 0.60684 -0.39316 h 35 - 0.01054 45 ` Charge transfer to gas molecules Q (e) 0.42 0.3705 0.37 0.32 0.26455 0.27 0.22 0.17 0.12 0.07 0.02492 0.0385 0.02 -0.03 CO CO CO CO22 NO NH3 NO NO -0.01054 NH3 NO2 Figure 3.10 Comparison of the charge transfer of the toxic gases adsorbed on monolayer MoTe2 achieved by optB88 vdW functional 3.4 Charge density difference The charge density difference of the adsorption system is defined as h ∆𝜌 = 𝜌𝑐𝑜𝑚𝑝𝑙𝑒𝑥 − 𝜌𝑠𝑢𝑏𝑡𝑟𝑎𝑡𝑒 − 𝜌𝑔𝑎𝑠 (9) with 𝜌 is the charge stored in CHGCAR file achieved from self-consistent calculation The charge density difference provides information about the charge accumulation and charge depletion regions after the adsorption processing The charge difference of adsorption systems is shown in Figure 3.11, Figure 3.12, Figure 3.13, Figure 3.14, Figure 3.15 The pink color implies the charge accumulation while the cyan color implies the charge depletion or in the other word charge density increasing and charge density decreasing, respectively As can be seen that the both MoTe2 monolayer and molecules are significantly polarized because of the charge redistribution during the adsorption process Consequently, the electrostatic interactions play a role in the attractive interaction By careful comparison of the isosurface level of the different density of charge, the polarization is found to be stronger in N-based gas cases than in the C-based gas cases which is consistent with 46 ` the adsorption energies trend obtained above (Figure 3.9) The adsorption energies of NH3, NO, NO2 cases are -212, -256, -586 meV, respectively; larger than these values for CO (-137 meV), CO2 (-205 meV) cases In addition, the depletion region near the Te surface layer, especially large for NO and NO2 case, could be one proof of charge acceptor characteristics of gas molecules Furthermore, the significant depletion region near the adsorbent surface combined with the remarkable accumulation region around the NO2 molecule (Figure 18) might provide the evidence for the large amount of its accepted charges h Figure 3.11 Charge density difference for CO – MoTe2 system achieved by optB88 vdW functional Top view (left) and side view (right) (isosurface level = 0.0002) Figure 3.12 Charge density difference for CO2 – MoTe2 system achieved by optB88 vdW functional Top view (left) and side view (right) (isosurface level = 0.0002) 47 ` Figure 3.13 Charge density difference for NO – MoTe2 system achieved by optB88 vdW functional Top view (left) and side view (right) (isosurface level =0.0022) h Figure 3.14 Charge density difference for NO2 – MoTe2 system achieved by optB88 vdW functional Top view (left) and side view (right) (isosurface level: 0.0006) Figure 3.15 Charge density difference for NH3 – MoTe2 system achieved by optB88 Top view (left) and side view (right) (isosurface level = 0.0004) 48 ` 3.5 Electronic structures To better understand the effects of gas adsorption on the electronic properties of monolayer MoTe2, we examine the BAND-DOS of adsorption systems shown in Figure 3.16, Figure 3.17, Figure 3.18, Figure 3.19, Figure 3.20 It can be seen from the band structures that when nonmagnetic CO, CO2, NH3 are exposed to adsorbent, neither the valence bands nor conduction bands of monolayer MoTe2 is significant influenced due to the impurity states contributed by gas molecules located away from the Fermi level In contrast, the flat impurity states are clearly witnessed in the band gap when the magnetic NO, NO2 adsorb on monolayer MoTe2 It means that the adsorption of CO, CO2 and NH3 not change the band structure near the Fermi level, and has no considerable effect on the electronic structure while the adsorption of NO, NO2 significantly modify the electronic structure of MoTe2 In detail, NO shows one spin-down state at 0.7 eV and two spin-up states at directly Fermi level and 0.4 eV while NO2 shows one spin-down in the band gap at around 0.1 eV above the Fermi level Consequently, the band gap of these system now is narrower to approximately 0.5 eV (0.1 eV) for NO (NO2) case h To confirm and further analyze the modification of electronic structure of MoTe2 upon the gas molecules adsorption, DOS is calculated and described side by side with the band structures As mentioned in 3.1.2, the states of the pristine MoTe2 in around CBM and VBM are mainly contributed by Mo d orbitals For non-magnetic CO, CO2, NH3 adsorptions, consistently with the band structure, there are no substantial modifications around Fermi level Meanwhile, the adsorption of CO produces two impurity states at around eV and -4 eV which are mainly contributed by p orbital of C The adsorption of CO2 dominates two DOS peaks, one peak at around eV contributed by both C and O p orbitals and one peak at around -4 eV mainly contributed by p orbital of both oxides The adsorption of NH3 contributes to the area of states surrounding -2 eV which is mainly composed by the N p orbital All of these lead to the hybridization of atom contributed orbitals and the d orbitals of MoTe2 For magnetic NO, NO2 adsorption, the spin-polarized DOS is presented in Figure 23, Figure 24 The adsorption of NO induces two up-spins DOS peaks at 49 ` around Fermi level and one down-spin DOS peak at around 0.7 eV, which result consistently the impurity states in band structure, are mainly attributed by the N p orbital Similarly, the adsorption of NO2 induces a down-spin DOS peak near the Fermi level which is mainly contributed by the O p orbital Overall, the adsorption of CO, CO2, NH3 does not significantly affect the electronic structure while the adsorption of NO, NO2 has a notable influence on the electronic structure which is consistent with their small (large) adsorption energies, respectively h Figure 3.16 BAND-DOS of CO – MoTe2 calculated by optB88 50 ` Figure 3.17 BAND-DOS of CO2 – MoTe2 calculated by optB88 h Figure 3.18 BAND-DOS of NH3 – MoTe2 calculated by optB88 51 ` Figure 3.19 BAND-DOS of NO – MoTe2 calculated by optB88 h Figure 3.20 BAND-DOS of NO2 – MoTe2 calculated by optB88 3.6 Work function In addition, we have calculated the work function of the toxic gases adsorption on monolayer MoTe2 and compared those results with the pristine monolayer MoTe2 The work function () might be defined as the minimum energy required to eject an 52 ` electron from the surface of material to vacuum The work function was reported that relate directly to the conductivity of materials, especially the low dimension systems [42] [43] = 𝑉∞ − 𝐸𝐹 (10) As described in Figure 3.21, the work function of MoTe2 upon all gas adsorptions is larger than the pristine MoTe2 except the NO case due to the fact that it shifts the Fermi level upward to the conduction band (Figure 23) Meanwhile, the value of NO2 adsorption on the monolayer MoTe2 increase significantly to 5.472 eV, thus, the variation in for NO2 with respect to pristine MoTe2 is much higher than other gases This is consistent with a large amount of charge transfer from the monolayer MoTe2 to NO2, which clearly lowers the Fermi level Notably, the different of NO, NO2 direction shown in Figure 3.21 lead to the selectivity of NO2 from NOx gases Overall, the work function sensor based on monolayer MoTe2 is not only highly sensitive to NO2 gas but also selective h Figure 3.21 The variation in work function () for different gas adsorption systems 53 ` CONCLUSION In summary, we perform a comprehensive DFT study on favorable adsorption configuration, adsorption mechanism (adsorption energy, adsorption distance), electronic structures of the toxic gas molecules (CO, CO2, NO, NO2 and NH3) absorbed on the monolayer MoTe2 Meanwhile, the influence of vdW interaction was comparatively captured by non-empirical vdW functionals (revPBE, optPBE, optB88, optB86 and vdW-DF2) The results show that all the gases were physiosorbed on the monolayer MoTe2, therein, the adsorption energies of N-based gases were larger than those for C-based gases Changing the vdW functionals did not influence the geometry of molecules and the favorable adsorption sites; Nevertheless, it significantly varied the adsorption energy and adsorption distance The order of adsorption energy was evaluated, C-based gases: vdW-DF2< revPBE
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