Germanene as potential material for sensor of toxic gases CO2, SO2, and CH4: A DFT study

Germanene as potential material for sensor of toxic gases CO2, SO2, and CH4: A DFT study show that gases of CO2, SO2, and CH4 are physically adsorbed on germanene via a charge transfer mechanism. The physisorption of these gas molecules on germanene opens a band gap at the Dirac point of germanene. The different adsorption behaviors of gas molecules on germanene provide a feasible way to extend germanene for gas sensors.

Germanene as potential material for sensor of toxic gases CO2, SO2,

and CH4: A DFT study

Nguyen Trung Hieu1, Nguyen Duy Khanh1, Le Vo Phuong Thuan1, Vo Duy Dat1, Hoang Van Ngoc1, Nguyen Thanh Tung1, Huynh Thi Phuong Thuy1, Mai Quang Vinh1, Tran Thi

Hong Anh1, Mai Thi Hao1, and Vo Van On1,*

1Group of Computational Physics and Simulation of Advanced Materials, Institute of Applied

Technology, Thu Dau Mot University, Binh Duong Province, Viet Nam

*Corresponding at onvv@tdmu.edu.vn

Abstract

The adsorption of common gas molecules (CO2, SO2, and CH4) on germanene is studied using the density functional theory The structural characteristics of gases adsorbed germanene are analyzed in the adsorption energy as a function of adsorption distance The results show that gases of CO2, SO2, and CH4 are physically adsorbed on germanene via a charge transfer mechanism The physisorption of these gas molecules on germanene opens a band gap at the Dirac point of germanene The different adsorption behaviors of gas molecules on germanene provide a feasible way to extend germanene for gas sensors

Keywords: germanene, adsorbtion, first-principles calculations, toxic gases

1 Introduction

The single-layer structure of graphite was theoretically discovered and named graphene in

1947 by Wallace P R [1] However, it was not until 2004 that graphene was mechanically exfoliated from graphite, and it has attracted enormous experimental and theoretical interest [2,3] due to unusual properties [4] One of the most important graphene applications is the gas sensor, thanks to its large surface area, low electrical noise, and too high electron mobility [5,6] On the other hand, pristine graphene's sensitivity is limited by common gas molecules' physisorption [7] This obstacle can be overcome by introducing defect and substitutional doping graphene to modify its electronic properties [8–10], leading to numerous challenges [11,12] Recently, both silicene and germanene have attracted increasing attention due to their analogs of 2D structures

to graphene, which allows them to be a potential replacement for graphene and overcome graphene's limitations While silicene was successfully grown on Ag [13–16], Ir [17], and ZrB2

[18] substrates, germanene was reported to be grown on Pt(111) [19] Germanane, multilayer hydrogen-terminated germanene, has also been synthesized and then mechanically exfoliated to a single layer onto SiO2/Si surface [20] Both silicene and germanene show outstanding properties similar to those of graphenes, such as high carrier mobility[21], ferromagnetism [22], half-metallic [23], quantum hall effect [24], and a topological insulator [25] Interestingly, because of its buckled honeycomb structure [26], silicene exhibits a significantly higher chemical reactivity

than graphene, showing much stronger adsorption of atoms [27–33] and molecules [34–38] than graphene with good potential applications on new silicene based nanoelectronic devices [26], Li-ion storage batteries [30], hydrogen storage [31], catalysts [32], thin-film solar cell absorbers [33], helium [35] and hydrogen [36] separation membranes, molecule sensor and detection However, silicene on metal substrates does not exhibit these properties, and the properties of silicene on low dimensional materials, e.g., graphene, is considerably closer to pristine silicene [39] Recently, atom adsorption on germanene is also studied [37,38] Similar to silicene, atoms bind much stronger to germanene than graphene, which is mainly caused by the sp2–sp3

hybridization of the Ge atom However, little attention has been focused to molecule adsorption

on germanene In this study, a systematic investigation of CO2, SO2, and CH4 molecules' adsorption behavior on germanene is performed to propose germanene as a promising sensor for these toxic gases

2 Computational Methods

All calculations based on the density functional theory were implemented in the Vienna Ab Initio Simulation Package (VASP) [40] Because the van der Waals functionals are expected to

be better than van der Waals correction schemes [41,42], the van der Waals interaction was taken into account in all calculations by employing the PBE - vdW functional [43–45] for the aim to produce the results in better agreement with experiment [46,47] The adsorption configuration, the potential energy surface (PES), and adsorption energy profile were calculated To eliminate the interaction between two adjacent periodic images, a vacuum layer of 20 Å was added into the

4 x 4 supercell of the pristine germanene Cutoff energy of 450 eV for the plane-wave basis set and a 3x3x1 Gamma centered k-point mesh was utilized to yield the energy convergence All structures were fully relaxed until the maximum Hellmann-Feynman force acting on each atom

is less than 0.03 eV/Å The charge transfer between Ge substrate and CO2, SO2, and CH4 gases was calculated based on the Bader charge transfer scheme [48]

3 Results and discussion

3.1 Structure Stability

The structural optimization processes achieved the optimal configurations of the CO2, SO2, and CH4 on germanene The adsorption at top, valley, hollow, and bridge sites were considered for each molecule on the germanene surface to find the most favorable structure corresponding to the lowest total energy The most stable adsorption configurations are shown in Fig.1, where the CO2 molecule is preferably located on the top site of germanene In contrast, the carbon atom is bonded with the Ge atom, the two oxygen atoms, from a straight angle The SO2 molecule prefers to stay at the valley site Consequently, the two oxygen atoms interact with Ge atoms obtuse angle The tetrahedral structure's symmetry allows the CH4 molecule to stay nearly at the center of the hollow site, with three hydrogen atoms pointing parallelly in the in-plane direction and one hydrogen atom pointing along the out-of-plane direction

Figure 1: The top- and side-views of the most stable configurations of the CO2, SO2, and CH4 gases adsorbed on germanene The purple, red, yellow, and white balls are carbon, oxygen,

sulfur, and hydrogen atoms

To evaluate the adsorption nature of gas molecules on germanene, the adsorption energy is defined as follows:

Ead = Etotal- Egas-EGermanene

Etotal, Egas, and EGermanene are the ground-state total energy of the total system, the gas molecule, and pristine germanene As the definition adopted here, negative adsorption energy exhibits that the process is exothermic while the magnitude signifies thermodynamic stability Based on the magnitude of Ead, the equilibrium distances between gas molecules and germanene’s surface (adsorption distance) are defined As shown in Fig.2, the adsorption energy strongly depends on the adsorption distance Therefore, it is very important to consider the adsorption distance in a large range of values to accurately define the minimum adsorption energy The adsorption energies of CO2, SO2, and CH4 on germanene are -0.131 meV, -0.285 meV, and -0.144 meV, respectively The corresponding adsorption distance between CO2, SO2, CH4, and germanene surface is 3.413 Å, 3.628 Å, and 3.367 Å, respectively The very small low adsorption energies confirm the physical nature of molecule adsorption on germanene

Figure 2: Adsorption energy profiles of adsorption configurations: a) CO2, b) SO2, c) CH4

The molecule adsorption shifts the CO2, SO2, and CH4 closer to Ge atoms, causing orbitals' splitting near the Fermi level As a result, the germanene bandgap is significantly modified, depending on the adsorption distance and adsorption molecule Due to the larger atomic radius of

S compared with C, the bandgap of CO2/Ge increases from 4.3 meV to 29.3 eV of SO2/Ge, while the bandgap of CH4/Ge is only 1.3 meV However, this theoretical prediction also depends on the calculation methods For comparison, the adsorption energy (Ead), adsorption distance (d), and the bandgap (Eg) from the current study and other published works are presented in table 1 There is a noticeable difference in the bandgap calculated by GGA + DFT-D2, and DFT + NEGF methods, which reveal 11 meV [49], and 0.00 eV [50], respectively However, the DFT + NEGF method results in the same band gap increasing from CO2/Ge to SO2/Ge

Table 1: Energy adsorption (Ead), adsorption distance (d), band gap (Eg) of germenene after

adsorbed gas molecules

CO2/Ge -0.131 a 0.42 d, 0.1 e 3.413 a 3.32 e 4.3 a 0.00 d

11 e

SO2/Ge -0.285 a 1.56 d 3.628 a 29.3 a 140 d

CH4/Ge -0.144 a -0.114 b,

0.231 d

3.367 a 3.36 b 1.3 a 3.2 b, 3.9 d

a Current study by GGA + DFT-D2

b GGA DFT-D2 [51]

c DFT + nonequilibrium Green’s function (NEGF) [52]

d GGA + DFT-D2 [50]

e GGA + DFT-D2 [49]

3.2 Electronic properties

The charge transfer between gas molecules and Ge p-orbitals near the Dirac points can induce a local electric potential to break the symmetry of sub-lattice in germanene Consequently, some band gap is created at the Dirac points, as shown in Fig 3, and the shape of Dirac points is nearly unchanged While CO2 and CH4 molecules' adsorption induce some meV bandgap, the SO2

molecule causes a more recognizable bandgap of nearly 0.3 eV Although the form of both the highest occupied molecular orbitals (HOMO) and lowest unoccupied molecular orbitals (LUMO)

is almost unchanged in three adsorption cases, there are some changes at higher orbitals of the conduction band This signifies some weak hybridization between orbitals of gas and the Ge p-orbitals, especially in the case of SO2 molecule The orbital interaction is elucidated by considering the difference in the density of state (DOS) between Ge-gas and Ge systems, as shown in Fig.4 It can be seen in Fig 4(c) that the blue peak (total DOS) and the red peak (DOS

of Ge) within the energy levels ranging from -1 eV to 1 eV almost overlap each other, indicating very weak orbital interaction Meanwhile, the discrepancy of these peaks in Ge/CH4 and Ge/SO2

is bigger, leading to a wider bandgap The more remarkable discrepancy between the blue and red peaks occurs at the same energy levels lower than -2 eV or higher than 2 eV However, these orbital interactions do not affect the magnitude of the bandgap

Figure 3: Electronic band structure of a) CO2/Germanene ; b) SO2/Germanene and c)

CH4/Germanene

Figure 4: Density of states (DOS) in the adsorption of a) CO2/Germanene, b) SO2/ Germanene and c) CH4/Germanene

At energy levels lower than -4 eV or higher than 4 eV, single high blue peaks indicate the asymmetric distribution of charge near the adsorbing gas molecules As the charge distribution reveals the bonding character, the charge density difference (CDD) was calculated and presented

in Fig 5 There is a slight electron depletion around the CO2 molecule together with a small electron accumulation near the C atom, and a larger electron concentration between C and Germanene's surface, indicating the weak covalent nature of C-Ge bonding It is worthy to notice that the charge transferred from Germanene to CO2 molecule, -0.037e, is mainly concentrated on

C atoms signifying the weak interaction between C-2p and Ge-2p orbitals It can be seen in Fig 5(b) that the electron density on C and O atoms is nearly the same Moreover, the electron concentration in the area between SO2 molecule and Germanene is also small Therefore, the charge transfer from Ge to SO2 is small, which is about 0.027e The charge transfer to CH4 is -0.175e, indicating the ionic character of the bonding between Ge and CH4 molecule Therefore,

the electron is concentrated mostly on the CH4 molecule, as shown in Fig 5(c), and a smaller electron density is found on the Germanene

Figure 5: Charge density difference (CDD) in the a) CO2/Germanene, b) SO2/ Germanene and c)

CH4/Germanene compounds, in which the yellow and blue iso-surfaces represent electron gain and depletion, respectively

4 Conclusion

The first-principles calculations are performed to investigate the structural and electronic properties of germanene adsorbed with several small gas molecules, including CO2, SO2, and

CH4 In contrast to graphene, all gas molecules are predicted to bind weakly to germanene’s surface due to the hybridized sp2-sp3 bonding of Ge atoms It was found out that all three gas molecules are physisorbed on germanene through the weak charge transfer mechanism Besides, the sizable band gaps of 1.3 to 29.3 meV are opened at the Dirac point of germanene under CO2,

SO2, and CH4 adsorptions, while only slight modification of the Dirac cone shape is observed The negative charge transferred from germanene to all three gases indicates that CO2, SO2, and

CH4 are electron acceptors in the germanene adsorption Overall, different adsorption behaviors

of gas molecules on germanene provide a feasible way to extend germanene for a wide range of practical applications, such as gas sensors

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