Organic gas adsorption on vacancy defect Germanene: A DFT study show that acetone, toluene and propanol are physisorbed on germanene via van der Waals interactions. The physisorption of gas molecules on germanene opens a band gap at the Dirac point of germanene. Overall, the different adsorption behaviors of organic gas molecules on vacancy defected germanene provide a feasible way to exploit chemically modified vacancy defected germanene for a wide range of practical applications, such as gas sensors and spintronic devices.
Organic gas adsorption on vacancy defect Germanene: A DFT study Nguyen Trung Hieu, Vo Van On* Computational Physics Group and Simulation of Advanced Materials and - Institute of Application Development- Thu Dau Mot University Abstract The adsorption of common organic gas molecules (acetone, toluene and propanol) on vacancy defected germanene is studied with density functional theory The results show that acetone, toluene and propanol are physisorbed on germanene via van der Waals interactions The physisorption of gas molecules on germanene opens a band gap at the Dirac point of germanene Overall, the different adsorption behaviors of organic gas molecules on vacancy defected germanene provide a feasible way to exploit chemically modified vacancy defected germanene for a wide range of practical applications, such as gas sensors and spintronic devices I Introduction In recent years, two-dimensional (2D) materials have attracted enormous interest of researchers due to their excellent potential properties for applying in nanotechnology [1–3] Among them, graphene has been widely used in various areas [4–7] However, integrating graphene into current semiconductor technology is still a large challenge, because it’s inapplicable in silicon/germanium-based semiconductor industry Many efforts have been made to explore graphenelike 2D materials [8–14], including silicene, germanene, and stanene, which are expected to overcome the diffculty of application of graphene in future Nowadays, germanene is regarded as one of the most emergent and novel 2D material in this nano materials family A Nijamudheen et al have also reported that buckling plays a very important role in electronic and chemical properties of germanene [15] They have concluded that, buckling enhances chemical reactivity of germanene to form hydrogenated germanene with a direct bandgap, called germanane Another study reveals that, value of this direct bandgap for germanane is 3.2 eV, which meets the requirement of semiconductor device fabrication [16] This buckled configuration of germanene is a consequence of mixed sp2 - sp3 hybridization [17] In addition, germanene is becoming a leader among them, due to its quantum spin Hall effects (QSHEs) [18–19] Furthermore, germanene exhibits high charge carrier mobility, which is expected to apply in high-speed and lowenergy-consumption feld effect transistors [20–21] In addition, germanene sheets have been successfully fabricated on the Pt (111), Al (111), Ag (111), and Au (111) experimentally [22–25] Functionalized germanene has also been considered for application of energy storages [19] Therefore, germanene-based materials are promising candidates for electrodes in electric double-layer capacitors (EDLCs) During the synthesis process, the presence of structural defects is unavoidable; hence it is necessary to examine the influence of defects on the structural, electronic and transport properties of these materials Previous studies by Yang at al [26] indicated that defects 18 and adsorption of metal atoms can improve the quantum capacitance of germanene However, the effect of combination of vacancy and adsorption on the quantum capacitance of germannene has not been investigated for supercapacitors In this study, a systematic investigation of acetone, toluene and propanol molecules’ adsorption behavior on vacancy defect germanene is performed with density functional theory calculations II Computational methodology All calculations based on the density functional theory were performed using the Vienna ab initio simulation package (VASP) with PAW potential[27,28] Because the van der Waals functionals are expected to be better than van der Waals correction schemes[29-32] we take the van der Waals interaction into account in all calculations by employing the opt PBE - vdW functional[32] for the aim to produce the results in better agreement with experiment[33] 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 silicene A cutoff energy of 450 eV for the plane-wave basis set and a 3x3x1 Gammacentered K-point mesh were 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 III Results and discussion 3.1 Structure Stability Before going to explore about the electronic properties of vacancy defected germanene, firstly, we would like to discuss about structure is most stable after vacancy defected germanene has an adsorption of gas molecules Fig depicts two possible adsorbent sites at germanene In this paper, we focus on the adsorption of organic gas molecules on Ge31A configuration We need to identify the model which the guest molecules to approach uninhibitedly toward the ribbons to explore the most preferred configuration We in turn determine the most appropriate configuration for each gas molecule: acetone, toluene and propanol Figure shown the stable structure of the vacancy defect germanene after adsorbed gas molecules including a) Acetone (Ge31A-Acetone), b) Toluene (Ge31A-Toluene) and c) Propanol (Ge31A-Propanol) 19 Figure 1: The two stable configurations of vacancy defect germanene For the comparison of most stable configuration, we first calculated adsorption energy (Ead) of considered configurations Following relation has been used for calculating the Ead, Ead = Etotal- Egas-EGermanene Where Etotal , EGermanener and Egas are the total energy of the gas molecule, pristine germanene, and gas molecule adsorbed on vacancy defected germanene, respectively As per the definition adopted here, negative adsorption energy exhibits that process is exothermic in nature while the magnitude signifies thermodynamic stability Numerical results indicate that adsorption energy of those samples decreased gradually from -0.29 eV to -0.42 eV with the order Ead(Propanol) > Ead(Acetone) > Ead(Toluene) In adsorption structures, sample adsorption of Toluene possesses the highest stability It is obvious that Toluene adsorption is the most stable one Figure 2: The stable structure of the vacancy defect germanene after adsorbed gas molecules including a) Acetone, b) Toluene and c) Propanol 20 Figure shows the change of adsorption energy with different adsorption distances As shown in Figure 3, we see that the adsorption distance of the samples is about 3.2 Å so it is possible that the adsorption of the gases under consideration to vacancy defected Germanene is more physical than chemical adsorption Figure 3: Adsorption energy profile of a) acetone, b) toluene and c) propanol gas-adsorbed vacancy defect germanene 3.2 Electronic properties After discussing the structural stability in previous section, the electronic properties of vacancy defected germanene to be discussed in present section for revealing sensing capability towards acetone, toluene and propanol To further study the effects of adsorbed organic gas molecules on the electronic properties of vacancy defected Germanene, the band structures, electronic density of states (DOS) and charge density difference are calculated for the systems with the adsorption Fig gives the energy band diagrams of different systems, respectively K, G and M are the high symmetric k points of the Brillouin zone corresponding to the systems above 21 Figure 4: Band structure diagrams of a) vacancy defect germanene and b) acetone, c) toluene and d) propanol gas-adsorbed vacancy defect germanene After adsorption of gas molecules, the band gap of germanene has differences between samples Band gap of propanol adsorption sample has the largest band gap about 0.052 eV It is easy to see that propanol adsorption enlarges the vacancy defected germanene (0.0402 eV) while acetone and toluene narrow the band gap The band gap for the defected germanene vacancy after the adsorption of acetone and toluene is 0.0374 eV and 0.0267 eV, respectively The very weak interaction between gas molecules and germanene is also reflected by the sharp peaks in the DOS of all three samples shown as Fig 22 Figure 5: Total density of states (DOS) of a) acetone, b) toluene and c) propanol gas-adsorbed vacancy defect germanene On the other hand, as see from Fig that all three cases, there is an overlap between the DOS lines This confirms the connection between the gas molecules and the substrate To clearly know the bonding character between molecule and germanene, we calculated the charge density difference (CDD) for all adsorption system, and the isosurface of CDD is depicted in Fig It can be seen that there is only a subtle accumulation of electrons in gas atoms and a depletion of electrons in its three nearest Ge neighbors The Gas molecule obtains electrons from germanene Similar to gas adsorbed graphene, such a subtle charge transfer indicates that gas molecule is physisorbed on germanene through weak vander Waals interaction 23 Figure 6: Charge density difference (CDD) of a) acetone, b) toluene and c) propanol gasadsorbed vacancy defect germanene In order to scrutinize the transfer of charges that happen between the chief component and the gas molecules at atomistic levels, surface assimilating or adsorption features like Bader charge transfer (Q) The Bader charge transfer (Q) furnishes us a pathway to realize the charge transfer that occurs between the base component (Ge31A) and the gas molecules (acetone, toluene and propanol) and their associated direction of charge traversal [34-39] The estimated measure of Q for vacancy defected Germanene and organic gas molecules (acetone, toluene and propanol) are 0.082628 e, -0.06147 e and -0.075697 e, respectively The negative magnitude observed for all the adsorption cases confrms the direction of charge traversal to be from vacancy defected Germanene to organic gas molecules Moreover, the magnitude of Q is noticed to be higher for toluene sample IV Conclusions The first-principles calculations are performed to investigate the structural, energetic and electronic properties of vacancy defected germanene adsorbed with several organic gas molecules (including acetone, toluene and propanol) In contrast to graphene, all organic gas molecules considered bind weak to germanene surface due to the hybridized sp2-sp3 bonding of 24 Ge atoms We found that all three organic gas molecules is physisorbed on vacancy defected germanene via van der Waals interaction In addition, sizable band gaps of (0.0402 eV) are opened at the Dirac point of vacancy defected germanene through propanol adsorptions meanwhile, it narrows when adsorbing the other two gases Overall, the different adsorption behaviors of organic gas molecules on vacancy defected germanene provide a feasible way to exploit chemically modified germanene for a wide range of practical applications, such 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Anota, M.S Villanueva, E.S.M Castro, Adsorption and possible dissociation of glucose by the [BN fullerene-B6 ]— magnetic nanocomposite silico studies, Appl Nanosci 8, 455– 465 (2018) 27 ... gases under consideration to vacancy defected Germanene is more physical than chemical adsorption Figure 3: Adsorption energy profile of a) acetone, b) toluene and c) propanol gas- adsorbed vacancy. .. propanol gas- adsorbed vacancy defect germanene After adsorption of gas molecules, the band gap of germanene has differences between samples Band gap of propanol adsorption sample has the largest...and adsorption of metal atoms can improve the quantum capacitance of germanene However, the effect of combination of vacancy and adsorption on the quantum capacitance of germannene has not