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,* Group 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 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], halfmetallic [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 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 x 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] 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 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 Sample CO2/Ge SO2/Ge CH4/Ge Adsorption Energy (eV) -0.131 a 0.42 d, 0.1 e -0.285 a -0.144 a 1.56 d -0.114 b, 0.231 d Adsorption distance (Å) 3.413 a 3.32 e 3.628 a 3.367 a 3.36 b 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] Band gap (meV) 4.3 a 0.00 d 11 e 29.3 a 140 d 1.3 a 3.2 b, 3.9 d 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 porbitals, 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 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 eV However, these orbital interactions not affect the magnitude of the bandgap 10 Figure 3: Electronic band structure of a) CO2/Germanene ; b) SO2/Germanene and c) CH4/Germanene 11 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 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 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, 12 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 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, 13 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 References [1] Wallace P R 1947 The Band Theory of Graphite Phys Rev 71 622–34 [2] Geim A K and Novoselov K S 2007 The rise of graphene Nat Mater 183–91 [3] Castro Neto A H, Guinea F, Peres N M R, Novoselov K S and Geim A K 2009 The electronic properties of graphene Rev Mod Phys 81 109–62 [4] Novoselov K S, Geim A K, Morozov S V, Jiang D, Zhang Y, Dubonos S V, Grigorieva I V and Firsov A A 2004 Electric Field Effect in Atomically Thin Carbon Films Science vol 306 pp 666 LP – 669 [5] Choi J H, Lee J, Byeon M, Hong T E, Park H and Lee C Y 2020 Graphene-Based Gas Sensors with High Sensitivity and Minimal Sensor-to-Sensor Variation ACS Appl Nano Mater 2257–65 [6] Schedin F, Geim A K, Morozov S V, Hill E W, Blake P, Katsnelson M I and Novoselov K S 2007 Detection of individual gas molecules adsorbed on graphene Nat Mater 652– [7] Leenaerts O, Partoens B and Peeters F M 2008 Adsorption of H2O, NH3, CO, NO2, and NO on graphene: A first-principles study Phys Rev B 77 125416 [8] Zhang Y-H, Chen Y-B, Zhou K-G, Liu C-H, Zeng J, Zhang H-L and Peng Y 2009 Improving gas sensing properties of graphene by introducing dopants and defects: a firstprinciples study Nanotechnology 20 185504 [9] Ni S, Li Z and Yang J 2012 Oxygen molecule dissociation on carbon nanostructures with different types of nitrogen doping Nanoscale 1184–9 [10] Dai J and Yuan J 2010 Adsorption of molecular oxygen on doped graphene: Atomic, electronic, and magnetic properties Phys Rev B 81 165414 [11] Terrones H, Lv R, Terrones M and Dresselhaus M S 2012 The role of defects and doping in 2D graphene sheets and 1D nanoribbons Reports Prog Phys 75 62501 [12] Tian W, Li W, Yu W and Liu X 2017 A Review on Lattice Defects in Graphene: Types, Generation, Effects and Regulation ed H D Ngo Micromachines 163 [13] Feng B, Ding Z, Meng S, Yao Y, He X, Cheng P, Chen L and Wu K 2012 Evidence of Silicene in Honeycomb Structures of Silicon on Ag(111) Nano Lett 12 3507–11 14 [14] Vogt P, De Padova P, Quaresima C, Avila J, Frantzeskakis E, Asensio M C, Resta A, Ealet B and Le Lay G 2012 Silicene: Compelling Experimental Evidence for Graphenelike Two-Dimensional Silicon Phys Rev Lett 108 155501 [15] Chen L, Liu C-C, Feng B, He X, Cheng P, Ding Z, Meng S, Yao Y and Wu K 2012 Evidence for Dirac Fermions in a Honeycomb Lattice Based on Silicon Phys Rev Lett 109 56804 [16] Chen L, Li H, Feng B, Ding Z, Qiu J, Cheng P, Wu K and Meng S 2013 Spontaneous Symmetry Breaking and Dynamic Phase Transition in Monolayer Silicene Phys Rev Lett 110 85504 [17] Meng L, Wang Y, Zhang L, Du S, Wu R, Li L, Zhang Y, Li G, Zhou H, Hofer W A and Gao H-J 2013 Buckled Silicene Formation on Ir(111) Nano Lett 13 685–90 [18] Fleurence A, Friedlein R, Ozaki T, Kawai H, Wang Y and Yamada-Takamura Y 2012 Experimental Evidence for Epitaxial Silicene on Diboride Thin Films Phys Rev Lett 108 245501 [19] Li L, Lu S, Pan J, Qin Z, Wang Y, Wang Y, Cao G, Du S and Gao H-J 2014 Buckled Germanene Formation on Pt(111) Adv Mater 26 4820–4 [20] Bianco E, Butler S, Jiang S, Restrepo O D, Windl W and Goldberger J E 2013 Stability and Exfoliation of Germanane: A Germanium Graphane Analogue ACS Nano 4414–21 [21] Bechstedt F, Matthes L, Gori P and Pulci O 2012 Infrared absorbance of silicene and germanene Appl Phys Lett 100 261906 [22] Wang X-Q, Li H-D and Wang J-T 2012 Induced ferromagnetism in one-side semihydrogenated silicene and germanene Phys Chem Chem Phys 14 3031–6 [23] Wang Y, Zheng J, Ni Z, Fei R, Liu Q, Quhe R, Xu C, Zhou J, Gao Z and Lu J 2012 Halfmetallic silicene and germanene nanoribbons: towards high-performance spintronics device Nano 07 1250037 [24] Ma Y, Dai Y, Niu C and Huang B 2012 Halogenated two-dimensional germanium: candidate materials for being of Quantum Spin Hall state J Mater Chem 22 12587–91 [25] Si C, Liu J, Xu Y, Wu J, Gu B-L and Duan W 2014 Functionalized germanene as a prototype of large-gap two-dimensional topological insulators Phys Rev B 89 115429 [26] Kara A, Enriquez H, Seitsonen A P, Lew Yan Voon L C, Vizzini S, Aufray B and Oughaddou H 2012 Corrigendum to “A review on silicene—New candidate for electronics” [Surf Sci Rep 67 (2012) 1–18] Surf Sci Rep 67 141 [27] Lin X and Ni J 2012 Much stronger binding of metal adatoms to silicene than to graphene: A first-principles study Phys Rev B 86 75440 [28] Sivek J, Sahin H, Partoens B and Peeters F M 2013 Adsorption and absorption of boron, 15 nitrogen, aluminum, and phosphorus on silicene: Stability and electronic and phonon properties Phys Rev B 87 85444 [29] Sahin H and Peeters F M 2013 Adsorption of alkali, alkaline-earth, and 3$d$ transition metal atoms on silicene Phys Rev B 87 85423 [30] Tritsaris G A, Kaxiras E, Meng S and Wang E 2013 Adsorption and Diffusion of Lithium on Layered Silicon for Li-Ion Storage Nano Lett 13 2258–63 [31] Wang J, Li J, Li S-S and Liu Y 2013 Hydrogen storage by metalized silicene and silicane J Appl Phys 114 124309 [32] Li C, Yang S, Li S-S, Xia J-B and Li J 2013 Au-Decorated Silicene: Design of a HighActivity Catalyst toward CO Oxidation J Phys Chem C 117 483–8 [33] Huang B, Xiang H J and Wei S-H 2013 Chemical Functionalization of Silicene: Spontaneous Structural Transition and Exotic Electronic Properties Phys Rev Lett 111 145502 [34] ệzỗelik V O and Ciraci S 2013 Local Reconstructions of Silicene Induced by Adatoms J Phys Chem C 117 26305–15 [35] Hu W, Wu X, Li Z and Yang J 2013 Helium separation via porous silicene based ultimate membrane Nanoscale 9062–6 [36] Hu W, Wu X, Li Z and Yang J 2013 Porous silicene as a hydrogen purification membrane Phys Chem Chem Phys 15 5753–7 [37] Hu W, Xia N, Wu X, Li Z and Yang J 2014 Silicene as a highly sensitive molecule sensor for NH3, NO and NO2 Phys Chem Chem Phys 16 6957–62 [38] Feng J, Liu Y, Wang H, Zhao J, Cai Q and Wang X 2014 Gas adsorption on silicene: A theoretical study Comput Mater Sci 87 218–26 [39] Berdiyorov G R, Neek-Amal M, Peeters F M and van Duin A C T 2014 Stabilized silicene within bilayer graphene: A proposal based on molecular dynamics and densityfunctional tight-binding calculations Phys Rev B 89 24107 [40] Kresse G and Furthmüller J 1996 Efficient iterative schemes for ab initio total-energy calculations using a plane-wave basis set Phys Rev B 54 11169–86 [41] Ramalho J P P, Gomes J R B and Illas F 2013 Accounting for van der Waals interactions between adsorbates and surfaces in density functional theory based calculations: selected examples RSC Adv 13085–100 [42] Vlaisavljevich B, Huck J, Hulvey Z, Lee K, Mason J A, Neaton J B, Long J R, Brown C M, Alfè D, Michaelides A and Smit B 2017 Performance of van der Waals Corrected Functionals for Guest Adsorption in the M2(dobdc) Metal–Organic Frameworks J Phys Chem A 121 4139–51 16 [43] Dion M, Rydberg H, Schröder E, Langreth D C and Lundqvist B I 2004 Van der Waals Density Functional for General Geometries Phys Rev Lett 92 246401 [44] Román-Pérez G and Soler J M 2009 Efficient Implementation of a van der Waals Density Functional: Application to Double-Wall Carbon Nanotubes Phys Rev Lett 103 96102 [45] Klimeš J, Bowler D R and Michaelides A 2009 Chemical accuracy for the van der Waals density functional J Phys Condens Matter 22 22201 [46] Hamada I and Otani M 2010 Comparative van der Waals density-functional study of graphene on metal surfaces Phys Rev B 82 153412 [47] Berland K, Cooper V R, Lee K, Schröder E, Thonhauser T, Hyldgaard P and Lundqvist B I 2015 van der Waals forces in density functional theory: a review of the vdW-DF method Reports Prog Phys 78 66501 [48] Henkelman G, Arnaldsson A and Jónsson H 2006 A fast and robust algorithm for Bader decomposition of charge density Comput Mater Sci 36 354–60 [49] Xia W, Hu W, Li Z and Yang J 2014 A first-principles study of gas adsorption on germanene Phys Chem Chem Phys 16 22495–8 [50] Wang Y, Ji W, Zhang C, Li S, Li F, Li P, Ren M, Chen X, Yuan M and Wang P 2016 Enhanced band gap opening in germanene by organic molecule adsorption Mater Chem Phys 173 379–84 [51] Liu G, Luo W W, Wang X, Lei X L, Xu B, Ouyang C Y and Liu S B 2018 Tuning the electronic properties of germanene by molecular adsorption and under an external electric field J Mater Chem C 5937–48 [52] Monshi M M, Aghaei S M and Calizo I 2017 Doping and defect-induced germanene: A superior media for sensing H2S, SO2, and CO2 gas molecules Surf Sci 665 96–102 17 ... 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... 2013 Stability and Exfoliation of Germanane: A Germanium Graphane Analogue ACS Nano 4414–21 [21] Bechstedt F, Matthes L, Gori P and Pulci O 2012 Infrared absorbance of silicene and germanene Appl... 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] Results and discussion