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. 2021 Jul 21;6(30):19546–19552. doi: 10.1021/acsomega.1c01862

Ozonation of Group-IV Elemental Monolayers: A First-Principles Study

Lokanath Patra †, Geeta Sachdeva †, Ravindra Pandey †,*, Shashi P Karna ‡,*
PMCID: PMC8340093  PMID: 34368540

Abstract

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Environmental effect on the physical and chemical properties of two-dimensional monolayers is a fundamental issue for their practical applications in nanoscale devices operating under ambient conditions. In this paper, we focus on the effect of ozone exposure on group-IV elemental monolayers. Using density functional theory and the climbing image nudged elastic band approach, calculations are performed to find the minimum energy path of O3-mediated oxidation of the group-IV monolayers, namely graphene, silicene, germanene, and stanene. Graphene and silicene are found to represent two end points of the ozonation process: the former showing resistance to oxidation with an energy barrier of 0.68 eV, while the latter exhibit a rapid, spontaneous dissociation of O3 into atomic oxygens accompanied by the formation of epoxide like Si–O–Si bonds. Germanene and stanene also form oxides when exposed to O3, but with a small energy barrier of about 0.3–0.4 eV. Analysis of the results via Bader’s charge and density of states shows a higher degree of ionicity of the Si–O bond followed by Ge–O and Sn–O bonds relative to the C–O bond to be the primary factor leading to the distinct ozonation response of the studied group-IV monolayers. In summary, ozonation appears to open the band gap of the monolayers with semiconducting properties forming stable oxidized monolayers, which could likely affect group-IV monolayer-based electronic and photonic devices.

1. Introduction

The discovery of graphene1−3 has inspired extensive interest in two-dimensional (2D) materials due to their unique properties and promising applications in electronics and optoelectronics, including transistors,4,5 sensors,6,7 energy storage and conversion devices,8,9 and light-emitting devices.10,11 Theoretical and experimental studies show that the group-IV elements Si, Ge, and Sn can also form graphene-like structures, namely silicene, germanene, and stanene. The first hypothesis of these layers can be traced back to 1994,12 but the experimental synthesis13−15 became possible only in recent years. Fabrications of these layers on various substrates have been reported.13−16 These monolayers stabilize with buckled structures, unlike the planar graphene.17 However, they resemble the electronic characteristic of graphene, i.e., an energy band crossing at the Dirac cone and linear dispersion around the crossing,18 which prevents their use in the semiconductor industry.

Opening a sizable band gap without change in the structure is critical for the application in nanoelectronics and is probably one of the most important and tantalizing research topics19 for the graphene family. Recent studies show that the band gap in these layers can be opened by the adsorption of small molecules on the surface.20−22 Of particular importance is the interaction with the oxidizing species in the air, which can significantly affect their electronic properties, and therefore their applications. This simultaneously has led to a fundamental question about the effect of environmental conditions on their physical and electronic properties. Ozone (O3) being a common oxidation precursor in the atmosphere, a fundamental understanding of its adsorption and dissociation in contact with a new class of materials is important from the health and environmental point of views as well as their effects on devices.23,24 It is also important to note that O3 is widely used in the semiconductor industry for substrate cleaning,25,26 fabrication processes,27,28 and ultrathin oxide formation.27,29−31 Therefore, a detailed and reliable understanding of O3 interaction with the emerging 2D nanomaterials of the group-IV elements is important for a fundamental understanding as well as their commercial applications.

Ozonation of graphene has been recently investigated on atomic layer deposited (ALD) samples,35 in which trimethylaluminum (TMA)/H2O treatment of ozone-exposed highly oriented pyrolytic graphite (HOPG) films led to the formation of alumina layer on the surface. Combining the experimental observation with spin-polarized density functional theory (DFT) calculations in local density approximation (LDA), the authors concluded that the ozone exposure facilitates the formation of epoxy groups on the HOPG surface. Furthermore, the LDA calculations described O3 interaction in terms of physisorbed O3, which needed to overcome an energy barrier of 0.47 eV to dissociate into a molecular oxygen and an oxygen atom with the dissociation energy of 0.72 eV. In contrast, a recent Perdew–Burke–Ernzerhof (PBE)-density functional theory (DFT) study predicted20 the ozone-exposed stanene to form a physisorbed O3-stanene complex accompanied by substantial charge transfer (∼0.6e) from the monolayer to the O3 molecule without any changes in the latter. This is somewhat surprising since such a charge transfer, in general, results in the dissociation of a molecule.17 We believe the results obtained in ref (20) are a consequence of the constrained optimization method used in the PBE-DFT method, which limits the movement of O atoms on the surface. As discussed in the following sections, the present results based on the state-of-the-art calculations show O3 to form an epoxide-like complex followed by its dissociation in atomic oxygens with a barrier of 0.27 eV on stanene. For the remaining members of the group-IV elemental monolayers (i.e., silicene and germanene), the effect of ozone exposure has not been reported in the scientific literature.

In general, the nature of the chemical bonding of the elemental monolayers governs their interaction with O3, i.e., the robustness of σ bonds and out-of-plane π bonds17 make graphene to be highly stable against oxidation under ambient conditions. The same is not true for other elemental layers due to sp2–sp3 hybridized bonds at the surface. However, no detailed understanding exists. Here, we present the results of the group-IV monolayer–O3 interaction investigated with the use of the state-of-the-art theoretical approach that combines density functional theory with the climbing image nudged elastic band (CINEB)32 method. The calculated results are discussed in terms of the electronic structure, chemical bonding, and dissociation pathway of pristine and oxidized monolayers.

2. Results

2.1. Ozone

Table S1 [Supporting Information (SI)] lists the structural parameters of molecular O3 calculated at the PBE-DFT level of theory. The singlet spin state is energetically preferred over the triplet spin state by 0.35 eV. The calculated bond length of 1.28 Å and the bond angles of 118.2° are consistent with previously published results including experiments.33,34

2.2. Pristine Monolayers

Figure S1 (SI) shows a single-atom-thick model honeycomb lattice of the group-IV elemental monolayers, and Table S2 (SI) lists the calculated structural parameters associated with the equilibrium configurations of elemental monolayers. The calculated structural parameters of the group-IV elemental monolayers are found to be consistent with previous reports.20−22,35 The calculated band structures for graphene, silicene, germanene, and stanene are also given in Figure S2 (SI). Graphene is a near-zero band gap material, whereas other monolayers have a finite and small band gap (Table S2) as also reported previously.36−39 Note that silicene, germanene, and stanene possess buckled structures due to weak π bonds between neighboring atoms in the lattice, unlike atomically flat graphene.

2.3. Interaction with Monolayers

To investigate the interaction of the group-IV elemental monolayers with O3, we begin with the noninteracting configuration consisting of O3 at a vertical distance of 4.5 Å above the surface. The path describing the O3 interaction with the monolayers was calculated in the framework of the CINEB method, which samples the configuration space consisting of various intermediate configurations and their corresponding energies as O3 approaches the surface. Analysis of the transition path in terms of the energy barrier and dissociation energy provides insight into the stability of the group-IV elemental monolayers against ozonation. It is worth mentioning that O3 is allowed to rotate as it approaches the surface, yielding the transition path to be independent of the initial orientation of the molecule.

Figure 1 displays the transition path describing O3 interaction with the group-IV elemental monolayers in which zero of the energy is taken to be the energy of the noninteracting configuration and the first calculation step to be the noninteracting configuration for all cases. In this way, the number of calculation steps along the x-axis can be considered as the one-dimensional reaction coordinate that is varied from the noninteracting to the final oxidized configuration. The details of the initial, intermediate, and final configuration of O3 interacting with the elemental monolayers are listed in Table 1. The oxidized configurations formed by the surface-supported dissociation of O3 into atomic oxygens are shown in Figure 2.

Figure 1.

Figure 1

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Transition pathway of O3 (red) approaching the surface of (a) graphene (black), (b) silicene (blue), (c) germanene (gray), and (d) stanene (green). Zero of the energy is taken to be the noninteracting system consisting of O3 at a distance of 4.5 Å above the monolayer surface.

Table 1. Calculated Binding Energy (EBinding), Dissociation Energy (Edissociation), Bond Lengths (RX–O), Bond Angle (<X–O–X), and Band Gap (Eband gap) of the Initial, Intermediate, and Final Configurations Describing O3 Interaction with the Group-IV Elemental Monolayersa.

O3 complexes graphene silicene germanene stanene
initial (noninteracting) configuration TEinitial (eV) –177.16 –101.08 –88.22 –76.82
RX–O (Å) RC–O = 4.5 RSi–O = 4.5 RGe–O = 4.5 RSn–O = 4.5
physisorbed configuration TEphysisorbed (eV) –179.68      
Ebinding (eV) –0.29      
RX–O (Å) RC–O = 2.9      
intermediate configuration (O3 → O + O2) TEintermediate (eV) –179.82 –105.73 –90.82 –79.75
Ebinding1 (eV) –0.43 –4.65 –2.60 –2.93
E(barrier height)1 (eV) 0.68      
RX1–O (Å) RC–O = 1.56 RSi–O = 1.71 RGe–O = 1.82 RSn–O = 2.04
final (oxidized) configuration (O3 → O + O + O) TEfinal (eV) –176.47 –110.83 –92.49 –81.50
E(barrier height)2 (eV) 2.53   0.45 0.27
Edissociation (eV) 1.21 –9.75 –4.43 –4.58
RX2–O (Å) RC–O = 1.47 RSi–O = 1.72 RGe–O = 1.86 RSn–O = 2.06
<(X–O–X) (deg) 55 82 84 87
Eband gap (eV) 0.64 0.48 0.24 0.10
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a

Edissociation = (TEinitial – TEfinal) and Ebinding = (TEmonolayer+O3 – TEmonolayer – TEO3). TE is the total energy of the system.

Figure 2.

Figure 2

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Top and side views of the final oxidized configuration of (a) graphene, (b) silicene, (c) germanene, and (d) stanene.

To benchmark the results of the present study, we first consider graphene which is known to be highly stable against oxidation.35,40−42 Our result shows that O3 is physisorbed at about ∼2.9 Å above the surface with the binding energy of 0.29 eV (Figure 1a). In general, physisorption is a weakly adsorbed state that leads to negligible charge transfer between the molecule and surface. Following this, a small amount of charge (∼0.05e) transfers from graphene to O3 molecule in the physisorbed state as displayed by the charge density difference plot. In Figure S3 (SI), the accumulation of charges at O sites can only be seen at a very small isovalue of 1/100 of its maximum value. Furthermore, the physisorbed O3 does not modify the energy band characteristics of the pristine graphene, where bands associated with oxygen appear with minimal dispersion in the k-space (Figure S4 (SI)).

Following physisorption, O3 encounters an energy barrier of 0.68 eV to go through an intermediary reaction which yields a dissociated O atom forming an epoxide (i.e., C–O–C) group on the surface and an O2 molecule. This intermediate reaction occurs because the O–O2 bond is relatively weak with O2 being a covalent σ bond in O3. In the intermediate configuration, Bader’s charge analysis shows a charge transfer of 0.6e from graphene to the chemisorbed oxygen with a bond length of ∼1.6 Å (Table 1). Subsequently, O2 dissociates into atomic oxygens by overcoming an energy barrier of 2.53 eV. The resulting oxidized configuration consists of epoxide groups in which atomic oxygens are bonded at the bridge site of two adjacent C atoms with a bond length of 1.47 Å (Figure 2a). The nature of the bond appears to be polar covalent, with oxygen acquiring a charge to be ∼0.6e in the oxidized configuration. The epoxide formation on the surface is accompanied by a band gap opening of up to 0.64 eV (Figure S5 (SI)). It is important to note that the final oxidized configuration is higher in energy (∼1.6 eV) than the intermediate (O + O2) configuration, as shown in Figure 3.

Figure 3.

Figure 3

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O3 interaction with the group-IV elemental monolayers: Relative energies of the intermediate (O3 → O + O2) and final dissociative (O3 → O + O + O) configurations. Zero of the energy corresponds to the noninteracting configuration.

Unlike graphene, silicene is predicted to have nearly zero resistance to ozonation as O3 approaching the surface dissociates into atomic oxygens without encountering an energy barrier (Figure 1b). The resulting oxidized configuration is stable with a dissociation energy of 9.75 eV and consists of siloxane (Si–O–Si) groups with R(Si–O) of 1.72 Å and A (Si–O–Si) of 82° (Figure 2b). Germanene, on the other hand, responds to ozonation in a slightly different way; O3 readily dissociates into an O2 molecule and an atomic O requiring no energy barrier (Figure 1c). However, subsequent dissociation of O2 molecule to atomic oxygens requires overcoming an energy barrier of 0.45 eV. Similar to silicene, the oxidized germanene configuration is stable with the binding energy of 4.43 eV with an equilibrium value of R(Ge–O) of 1.86 Å and A(Ge–O–Ge) of 84°. The charge density plots displayed in Figure 4 show the preference of O atoms to form Ge–O–Ge groups via partial electrostatic interactions in both intermediate and the final oxidized configurations.

Figure 4.

Figure 4

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O3 interaction with germanene: The charge density difference plots of (a) intermediate (O + O2) configuration and (b) final oxidized configuration. The isosurface value is 1/50th of the maximum isosurface value of each case. The cyan and yellow colors indicate the charge depletion and accumulation, respectively. Atomic color codes: Ge (gray) and O (red).

The response of stanene to ozonation is similar to that predicted for germanene. First, O3 dissociates into an O2 molecule and an atomic O without any energy barrier (Figure 1d). Later, O2 dissociates into atomic oxygens requiring an energy barrier of 0.27 eV. In the oxidized configuration, O atoms form Sn–O–Sn configurations (Figure 2d) with R(Sn–O) of 2.06 Å and A(Sn–O–Sn) of 87°. The binding energy of the oxidized configuration is 4.58 eV with respect to pristine stanene and atomic oxygens. It is worth mentioning that dissociated O atoms are in the same plane in contrast to the underlying buckled silicene (or germanene or stanene) and their bond lengths follow the trend displayed by the corresponding diatomic molecules (Table S3 (SI)). For example, the bond lengths of CO, SiO, GeO, and SnO molecules are calculated to be 1.18, 1.66, 1.96, and 2.01 Å, respectively, at the PBE-DFT level of theory.

3. Discussion

It is well known that O3 has longer and substantially weaker bonds than O2 due to the coupling of a pair of electrons in orbitals on each of the terminal atoms representing a weak π interaction within the molecule.43 Therefore, O3 interaction with the group-IV elemental monolayers first induces the bond breaking in the molecule yielding O and O2 before dissociating further into atomic oxygens. Since the bond-breaking process also depends on the nature of the substrate, we observe notable differences in the way O3 interacts with the group-IV elemental monolayers. The present calculation shows a high degree of selectivity toward ozonation, distinguishing the monolayers from each other. For example, as O3 approaches the surface, the intermediary reaction (O3 → O + O2) encounters an energy barrier for graphene, but not for its high-Z homologs, silicene, germanene, or stanene. The subsequent O2 dissociation reaction encounters an energy barrier for the group-IV elemental monolayers except for silicene.

Overall, silicene is predicted to exhibit a higher degree of ozonation among the group-IV elemental monolayers and its predicted preference can be understood in terms of the electronegativity difference between O and the group-IV atoms. The Pauling electronegativity values of O, C, Si, Ge, and Sn atoms are reported to be 3.44, 2.55, 1.9, 2.01, and 1.96, respectively. Therefore, graphene forms a polar covalent bond with atomic oxygens with the electronegativity difference being 0.9, while the rest of the monolayers form an ionic bond with atomic oxygens at the surface. Analysis of the Bader charges further confirms the nature of the bonding since the calculated charges associated with O atoms chemisorbed on graphene, silicene, germanene, and stanene are −0.6, −1.6, −1.12, and −1.06e, respectively. This is similar to what is calculated for diatomic molecules with Bader’s oxygen charges of −0.07, 0.61, 0.41, and 0.39e in CO, SiO, GeO, and SnO, respectively, at the PBE-DFT level of theory (Table S3 (SI)). Furthermore, the instantaneous dissociation of O3 over silicene, germanene, and stanene can be understood by analyzing the frontier orbital energies of O3 and the group-IV elemental monolayers. The lowest occupied molecular orbital (LUMO) of the ozone molecule lies below the highest occupied molecular orbital (HOMO) of Si, Ge, and Sn atoms (Figure S6 (SI)), allowing a spontaneous charge transfer to the LUMO of the molecule when it approaches the surface. The LUMO level of O3 is −6.72 eV, whereas the HOMO levels of C, Si, Ge, and Sn are calculated to be −9.3, −6.5, −6.3, and −5.6 eV, respectively.

Analysis of the density of states (DOS) further affirms the distinction between graphene and other elemental monolayers. Near the Fermi region (<−2 eV), nonzero DOS primarily associated with p-states for silicene (or germanene or stanene) appears in contrast to the nearly zero DOS for graphene (Figure 5a). The finite DOS near the Fermi region then facilitates charge transfer to the O3 molecule whose LUMO lies quite close, as displayed in Figure 5b for the noninteracting configuration. Subsequently, electron transfer in going from the noninteracting to the oxidized configuration shifts the Fermi level below the Dirac point, making these elemental monolayers to be hole-doped.3 Overall, graphene appears to act as an electron-donating system with a delocalized π-cloud, whereas Si-, Ge-, and Sn- monolayers exhibit electron transfer to atomic oxygens, leading to predominantly ionic X–O–X (X = Si, Ge, Sn) bonds at the surface. Furthermore, the calculated results predict a substantial opening of the band gap in oxidized configurations; the band gaps are 640, 480, 240, and 100 meV for the oxidized configurations of graphene, silicene, germanene, and stanene, respectively [Figure S5 (SI)]. The corresponding band gaps in the pristine monolayers are ∼0, 2, 18, and 72 meV.

Figure 5.

Figure 5

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Calculated total and atom-projected density of states of (a) pristine monolayers, (b) noninteracting configurations, and (c) oxidized configurations consisting of O3 and the group-IV elemental monolayers. Zero is taken to be the Fermi level.

In general, reactions with a negative value of the dissociation energy together with the DFT energy barrier a few tenths of electronvolts are likely to proceed spontaneously at room temperature.44 Based on this criterion, our calculations identify silicene to be readily oxidizable followed by stanene and germanene upon ozonation. On the other hand, graphene, with an energy barrier of ≈0.7 eV, appears resistant to ozonation, with O3 preferring physisorption on its surface.

The dissociation of O3 to O + O2 is followed by subsequent dissociation of the O2 molecule on group-IV elemental monolayers. It is worth comparing the findings of the present study with previously published results on O2 interaction with the group-IV elemental monolayers.17 A PBE-DFT calculations45 reported graphene to be stable against oxidation with a dissociation barrier of about 2.71 eV. The present study predicts a dissociation barrier of 2.53 eV for graphene oxidation. For silicene, also the results of our calculations agree with a previous PBE-DFT study38 predicting no dissociation barrier21 to oxidation. For germanene, our calculated energy barrier of 0.45 eV is comparable to the previously reported value of 0.6 eV,22 also obtained from a similar calculation. It is also worth pointing that the calculated results predict positive dissociation energy for O2 on the graphene surface since the oxidized configuration is higher in energy than noninteracting configurations as shown in Figure 1. In this regard, graphene oxidation appears to be similar to that predicted for its isoelectronic BN monolayer,23 which also shows a positive value of O2 dissociation.17

4. Summary

We have performed a systematic and detailed investigation of O3 interaction with the group-IV elemental monolayers, namely graphene, silicene, germanene, and stanene, using DFT in combination with the CINEB approach. The results show that, except graphene, the monolayers are highly reactive to ozonation. We find that in the case of graphene, the dissociation of O3 to O + O2 encounters an energy barrier of ∼0.7 eV, and further dissociation of O2 to atomic oxygens faces an energy barrier of 2.53 eV. Silicene, on the other hand, readily dissociates O3 to atomic oxygens forming Si–O–Si bonds on the surface without any energy barrier. This can be explained in terms of a large electronegativity difference between Si and O atoms, which facilitates the formation of the Si–O bond over the O–O bond. For germanene and stanene, the results predict a two-step dissociation process: the first step involves a spontaneous dissociation of O3 to O + O2 followed by the second step for O2 dissociation to O atoms with an energy barrier of 0.45 and 0.27 eV (germanene and stanene), respectively. For the monolayers considered, dissociated oxygen atoms prefer the bridge site to form X–O–X (X = C, Si, Ge, Sn) epoxy-type configurations. Finally, the oxidized monolayers are predicted to be semiconducting with a finite band gap, suggesting potential modification in the properties of nanoscale devices based on these elemental monolayers, except graphene, under ozone exposure.

5. Computational Method

DFT calculations were performed using the projected augmented wave (PAW)46 pseudopotentials as implemented in the Vienna ab initio simulation package (VASP).47 The exchange–correlation functional form given by PBE within the generalized gradient approximation (PBE-GGA)48 together with the van der Waals (vdW) correction term49 were used. A plane-wave basis set with an energy cutoff of 520 eV was employed for all our calculations. The energy convergence was set to 10–6 eV, and atomic positions were optimized until the Helman–Feynman force acting on each atom was less than 0.01 eV/Å. The Monkhorst–Pack k-point grid50 of size (10 × 10 × 1) was adopted for the sampling of the Brillouin zone.

A (3 × 3 × 1) periodic supercell was used to simulate the 2D monolayer which was separated by ∼18 Å vacuum layer in the z-axis direction (i.e., perpendicular to the surface) to minimize the interaction of its periodic images. The spin–orbit (SO) interaction terms were included only for the elemental Sn monolayer considering that SO terms become important for heavier atoms.51,52 Atomic charges of interacting complexes were calculated using Bader’s atoms-in-molecules approach.53

The CINEB method32 was used to determine the minimum energy path of transition for a given molecule interacting with a surface. It is based on the static treatment of a set of intermediate configurations called images along the transition path which connects the predetermined initial and final states representing the endpoint configurations of a system. These images are connected with springs to form an elastic band. The system is then relaxed toward the transition path where all of the physical forces perpendicular to the path must be zero allowing this method to determine a saddle point on the energy surface.32 We note that the potential energy maximum along the minimum energy path is referred to as the saddle point energy yielding the activation energy barrier associated with the adsorption/dissociation process. The CINEB method is computationally efficient as it only requires evaluation of the interaction energy and the first derivative of the energy with respect to coordinates and has been applied successfully to investigate the reaction path between the initial and final products of a given reaction.32

Acknowledgments

The authors thank Drs. Sangam Chatterjee and Max Seel for fruitful discussions. Computational resources at Michigan Technological University with the SUPERIOR high-performance computing cluster were utilized. The research at MTU was partially supported by the Army Research Office (ARO) through Grant no. W911NF-14-2-0088.

Supporting Information Available

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acsomega.1c01862.

  • Structural parameters for ozone and pristine monolayers, electronic structure of the pristine monolayers, charge density difference plots of ozone with monolayers, electronic structure of the ozone–graphene system, electronic structures of monolayers after ozonation, molecular orbital energy levels of ozone, and group-IV atoms (PDF)

The authors declare no competing financial interest.

Supplementary Material

ao1c01862_si_001.pdf (560.6KB, pdf)

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