A first-principles study of propanol adsorption on Monolayer Molybdenum Diselenide (MoSe2)

A first-principles study of propanol adsorption on Monolayer Molybdenum Diselenide (MoSe2) investigate the adsorption mechanism of propanol gas on the surface of monolayer MoSe2 by the quantum simulation method. The images of the potential energy surfaces for configuration of adsorbate on MoSe2 surface were investigated using the Computational DFT-based Nanoscope tool to explore the most stable configurations and diffusion possibilities.

A first-principles study of propanol adsorption on Monolayer

Molybdenum Diselenide (MoSe2)

Luong Thi Theu1, Tran Thi Nhan2, Tran Quang Huy3, Van An Dinh1,4

1Institute of Applied Technology, Thu Dau Mot University, Binh Duong, Vietnam

2Faculty of Fundamental Sciences, Hnoi University of Industry, Hanoi, Vietnam

3Faculty of Physics, Hanoi Pedagogical University 2, Hanoi, Vietnam

4Department of Precision Engineering, Graduate School of Engineering, Osaka

University, Osaka, Japan

ABSTRACTS

It is desirable to seek new sensors capable of detecting VOCs at low concentrations in the early stages of cancer Because of their high surface area to volume ratio, 2D TMDs are predicted to

be appealing materials for gas sensitive sensors In this work, we investigate the adsorption mechanism of propanol gas on the surface of monolayer MoSe2 by the quantum simulation method The images of the potential energy surfaces for configuration of adsorbate on MoSe2

surface were investigated using the Computational DFT-based Nanoscope tool to explore the most stable configurations and diffusion possibilities The optPBE-wdW functional was used for discussing the adsorption mechanism, electronic structure, and charge transfer It is found that by using optPBE-vdW functional, the propanol-adsorbed MoSe2 is physisorbed with a pretty high energy of 436 meV, which indicates that MoSe2 is considerably sensitive with this gas The charge transfer between the substrate and VOCs was also addressed

Keywords: Adsorption, VOCs, propanol, monolayer MoSe2, DFT

Nghiên cứu nguyên lý ban đầu của hấp phụ khí propanol lên đơn lớp

Molybdenum Diselenide (MoSe2)

Luong Thi Theu1, Tran Thi Nhan2, Tran Quang Huy3, Van An Dinh1,4

1Institute of Applied Technology, Thu Dau Mot University, Binh Duong, Vietnam

2Faculty of Fundamental Sciences, Hnoi University of Industry, Hanoi, Vietnam

3Faculty of Physics, Hanoi Pedagogical University 2, Hanoi, Vietnam

4Department of Precision Engineering, Graduate School of Engineering, Osaka

University, Osaka, Japan

TÓM TẮT

Chúng ta mong muốn tìm ra các cảm biến mới có khả năng phát hiện hợp chất hữu cơ dễ bay hơi ở nồng độ thấp trong giai đoạn ung thư sớm Do tỷ lệ diện tích bề mặt trên thể tích cao, vật

liệu kim loại chuyển tiếp dichalcogenides hai chiều được dự đoán là vật liệu hấp dẫn cho các

cảm biến nhạy khí Trong bài báo này, chúng tôi nghiên cứu cơ chế hấp phụ của khí propanol trên bề mặt của MoSe2 đơn lớp bằng phương pháp mô phỏng lượng tử Hình ảnh thế năng lượng

bề mặt cho cấu hình của chất hấp phụ trên bề mặt MoSe2 đã được khảo sát bằng cách sử dụng công cụ Nanoscope dựa trên tính toán lý thuyết hàm mật độ để khám phá cấu hình ổn định nhất

và khả năng khuếch tán Phiếm hàm optPBE-wdW được sử dụng để thảo luận về cơ chế hấp phụ, cấu trúc điện tử và sự truyền điện tích Bằng cách sử dụng phiếm hàm optPBE-vdW chúng tôi thấy rằng, hấp phụ propanol trên MoSe2 là hấp phụ vật lý với năng lượng khá cao khoảng 436 meV, điều này cho thấy MoSe2 rất nhạy với khí này Sự dịch chuyển điện tích giữa chất nền và khí cũng được thảo luận

Từ khóa: Hấp phụ, hợp chất hữu cơ bay hơi, propanol, đơn lớp MoSe2, lý thuyết hàm mật độ

1 Introduction

Volatile organic compounds (VOCs), which are found in the breath of people with early-stage cancer, are considered as new cancer biomarkers for diagnostic purposes [1-4] It has been shown that early diagnosis increases the chances of survival in patients with various types of cancer [5,6] Breath analysis might provide the foundation of a non-invasive technique for early cancer detection by gas sensitive sensors

Traditional semiconductor metal oxide based gas sensors have good selectivity and sensitivity [7], but they generally require high operating temperatures, which lead in high power consumption and inducing deviation in results 2D transition metal dichalcogenides (TMDs) have attracted a lot of interest as a chemical sensing device alternative to traditional metal oxides Because of its outstanding semiconductor characteristics, this material family has been found with applications in a variety of disciplines including photocatalysis, optoelectronics, electronics, spintronics, and gas sensors [8-12] Among the 2D TMD materials, monolayer (ML) MoSe2 displays appealing electronic properties including high surface-to-volume ratio, stability, high conductivity, and layered structure, and the capacity to adjust the number of layers [13-15]

Therefore, MoSe2 is expected to be a promising semiconductor option for the design and manufacturing of high-performance gas sensing devices [16-20]

In the present study, by first principle simulation method, we explore the mechanism, structural and electronic properties of MoSe2 monolayer adsorbed on the surface of propanol gas using the non-empirical van der Waals (vdW) density functional The most favored configuration

of the gas-MoSe2 adsorption system was explored by optimizing the geometric structure and computing the interaction energy with the Computational DFT based Nanoscope tool [21-23] The adsorption energy profile was calculated in detail with the optPBE-vdW functional to evaluate the propanol sensitivity of MoSe2 In addition, the influence of gas adsorption on the electronic structure, as well as the charge transfer mechanism between the substrate and the gas molecule, are carefully considered with the appropriate optPBE-vdW functional

2 Computational Method

The adsorption mechanism of propanol toward the surface of a MoSe2 4x4 supercell was studied theoretically using Density Functional Theory (DFT) [24-26] Ab initio calculation simulation was done on High Performance Computers using the density functionals approach implemented in Vienna Ab initio Simulation Package (VASP) [27, 28] The van der Waals interaction is accounted for by using the optPBE-vdW functional [29, 30] Automatic scanning using a Computational DFT-based Nanoscope [21-23] was used to find the most optimal adsorption geometries of the adsorbate on the MoSe2 substrate, and the adsorption energy profile A cutoff energy of 500 eV for the plane-wave basis set and a 4x4x1 Gamma-centered k-point mesh corresponding to the 4x4 super-cell were utilized A vacuum thickness of 20 Å was applied to eliminate the interaction between layers The structures were all thoroughly relaxed until the maximum residual Hellmann-Feynman force acting on each atom was less than 0.01 eV/Å

The adsorption energy Ead is calculated by the following equation:

where Egas+sub, Egas, and Esub are the total energies of the gas-MoSe2 system, isolated gas molecule, and isolated MoSe2, respectively Bader charge analysis was used to estimate the Charge Differential Density (CDD) of the most favorable adsorption configuration The charge

density difference is calculated using the equation  = AB−A−B [31], in which ρAB is the

total charge of the system, ρA and ρB are the charges are two separate systems The visualization

using in this paper is VESTA, which was developed by K Momma and F Izumi [32]

3 Results and Discussions

3.1 Geometrical structure of propanol and the stable position of adsorption system propanol-MoSe 2

Propanol is an organic compound with the molecular formula CH3CH2CH2OH that belongs

to the alcohol functional group Figure 1 illustrates the molecular structure of propanol, with carbon, hydrogen, and oxygen atoms represented by the brown, light pink, and red spheres, respectively Propanol concentrations in the breath of patients with lung, liver, and stomach cancers have been reported [1, 2, 4], that play a critical role in early cancer detection

Figure 1 Molecular structure of propanol

Figure 2 Top (a) and side (b) views of propanol adsorption on MoSe 2 The Se and Mo atoms are

represented by the green and purple spheres, respectively

By carefully optimizing the geometry structures of the gas-MoSe2 adsorption systems using the Nanocope tool, the most stable configuration of the propanol-MoSe2 adsorption systems was explored, as shown in Figure 2 This tool has been used to simulate the adsorption of organic gases on other 2D materials, such as brophene [22] and silicene [23] It is discovered that the most preferred adsorption site is the gas oriented parallel to the substrate, approximately 2.7 Å from the material's surface to the gas's lowest atomic position When the structures of gas and material were compared before and after adsorption, the bond angles and bond lengths showed very little variations in both This suggests that MoSe2 is a highly stable sensor material

3.2 Binding potential, adsorption energy profile of propanol onto MoSe 2

The stable positions of adsorbent can be determined by the minima of the potential energy surface (PES) When PES equals zero, the adsorption system is in the most equilibrium state Because of the periodicity in the ML MoSe2 lattice structure, it is only necessary to compute PES

of unitcell instead of supercell Figure 3 shows the potential energy surface per unitcell of propanol gas on ML MoSe2 The color gradient represents the PES value, with brighter colors exhibiting a higher PES values and vice versa According to this convention, the blue and black regions correspond to favorable adsorption areas Unfavorable adsorption areas are represented

by the yellow and orange colors Diffusion allows gas molecules to move between adsorption regions Because the minimum energy difference between the propanol adsorption regions is 15 meV, the adsorption regions will be linked, indicating that MoSe2 is quite sensitive to this gas

Figure 3 The 3D projected Potential Energy Surface for the adsorption of propanol on the surface of

MoSe 2

Table 1 shows the adsorption characteristics of propanol on MoSe2 with the optPBE-vdW

functional The optimal adsorption distance dz (dc) is the distance from the lowest atom (center

of mass –COM) of the gas molecule to the surface of MoSe2 It was discovered that the propanol-adsorbed MoSe2 is physisorbed with a pretty high energy of 436 meV The adsorption

distance between the lowest atom and the substrate is 2.631 Å, the response length l is 9.867 Å,

and the recovery time τ is quite small, 20 microseconds According to the findings, MoSe2 is extremely sensitive to propanol vapor

Table 1 Adsorption energy (Ead), equilibrium height of the lowest atom (dc), equilibrium height of the massed center (dz), response length (l), and recovery time (τ) for the adsorption of propanol on MoSe2

using optPBE-vdW functional

3.3 Electronic band structure and charge transfer of propanol gas on MoSe 2

monolayer

3.3.1 Electronic band structure

To investigate the nature of propanol adsorption on MoSe2, we computed the band dispersion and density of states along high symmetry k-points We observed that ML MoSe2 exhibits a direct band gap involved to the Brillouin zones' K-K shift with the band gap value of approximately 1.418 eV, in good agreement with previous calculations [33, 34] Based on the calculation results shown in Figure 4, the electronic band structure of MoSe2 shows a decrease in the band gap by about 7.8 meV Decreasing a band gap might lead to the increasing of electrical conductivity, which implies the possibility of detecting VOCs by monitoring the conductance of MoSe2 upon exposure to a breath containing VOCs

Molecule Adsorption

properties

optPBE-vdW

Propanol

E ad (meV) -436

Figure 4 Band structures and density of states (DOS) of propanol on MoSe 2 (optPBE-vdW functional)

The dashed lines represent Fermi level DOS is in units of state/eV

3.3.2 Charge transfer mechanism

Bader charge analysis was used to determine charge transfer The amount and direction of charge transfer may be determined by comparing the charge before and after adsorption of a VOC molecule After adsorption, the positive sign indicates electron accumulation, which is represented by the yellow color, whereas the negative sign shows electron depletion, which is represented by the blue hue

Figure 5 Top (a) and side (b) views of charge density difference (CDD) by the adsorption of propanol on

monolayer MoSe 2

We discovered that, propanol acts as an electron acceptor from substrate The amount of charge transferred about 0.4e are higher by far than its chemical-adsorption on single-layer

stanane in previous work[35] The donated charge is believed to change the resistivity of the adsorbent, MoSe2, making it a selective and sensitive material to propanol gas

4 Conclusion

We have investigated the interaction between freestanding ML MoSe2 and propanol gas by the quantum simulation based on the Density functional theory with the use of the optPBE-vdW functional The picture of PES, favorable adsorption configuration, electronic structure, and charge transfer were computed Upon propanol gas adsorption, a decrease in the band gap by about 7.8 meV appears in the electronic structure of MoSe2, resulting in changeable conductivity

It is also found the charge transfer of 0.4 electrons between VOC and material, suggesting a possibility of the use of MoSe2 material in the sensing devices, especially in VOC sensors

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