⇦Chinese Physics Letters, 2017, Vol. 34, No. 1, Article code 017302 Band Gap Adjustment of SiC Honeycomb Structure through Hydrogenation and Fluorination * Yu-Feng An(安玉凤)1, Zhen-Hong Dai(戴振宏)1**, Yin-Chang Zhao(赵银昌)1, Chao Lian(廉超)2, Zhao-Qing Liu(刘兆庆)3 Affiliations 1Computational Physics Laboratory, Institute of Opto-electronic Information Science and Technology, Yantai University, Yantai 264005 2Beijing National Laboratory for Condensed Matter Physics, and Institute of Physics, Chinese Academy of Sciences, Beijing 100190 3National Natural Science Foundation of China, Beijing 100085 Received 22 October 2016 *Supported by the Program for New Century Excellent Talents in Universities of China under Grant No NCET-09-0867.
**Corresponding author. Email: zhdai@ytu.edu.cn Citation Text: An Y F, Dai Z H, Zhao Y C, Lian C and Liu Z Q 2017 Chin. Phys. Lett. 34 017302 Abstract Previous calculations show that the two-dimensional (2D) silicon carbide (SiC) honeycomb structure is a structurally stable monolayer. Following this, we investigate the electronic properties of the hydrogen and fluorine functionalized SiC monolayer by first-principles calculations. Our results show that the functionalized monolayer becomes metallic after semi-hydrogenation or semi-fluorination, while the semiconducting properties are obtained by the full functionalization. Compared with the bare SiC monolayer, the band gap of the fully hydrogenated system is increased, in comparison with the decrease of the gap in the fully fluorinated case. As a result, the band gap can be tuned from 0.73 to 4.14 eV by the functionalization. In addition to the metal–semiconductor transition, hydrogenation and functionalization also realize a direct-indirect semiconducting transition in the 2D SiC monolayer. These results provide theoretical guidance for design of photoelectric devices based on the SiC monolayer. DOI:10.1088/0256-307X/34/1/017302 PACS:73.20.At, 73.20.Hb, 73.22.-f, 68.43.Bc © 2017 Chinese Physics Society Article Text Graphene, a planar monolayer honeycomb structure, has attracted tremendous attention due to its unique properties such as high carrier mobility, ballistic electronic transport, and the anomalous quantum Hall effect.[1,2] Electrons in graphene behave as massless Dirac fermions.[3] Since graphene was obtained by the exfoliated technique in 2004, great efforts have been devoted to two-dimensional (2D) graphene-like structures[4] such as boron nitride hexagonal structure, silicon carbide honeycomb structure and graphene-like BC$_{3}$.[5-17] Silicon, the same as carbon, belongs to an IV-group element in the element periodic table. Based on the theoretical calculations, the honeycomb structure of silicon lying in a plane is less stable than the structure with buckling. However, the plane 2D silicon carbide monolayer in the honeycomb structure is found to be stable,[12] and it is a direct semiconductor with an energy gap of 2.54 eV. It is very profound to tune the band gap of 2D SiC monolayer by some feasible methods such as hydrogenation, fluorination, metal doping, defect, and impregnated with other flakes.[18-24] Although the structure is similar to graphene's configuration, the 2D SiC monolayer shows different properties. For example, it is a wide-band-gap semiconductor compared to the metallic zero-band-gap electronic structure of graphene. The 2D SiC processes two species of atoms, Si and C, locating at A and B sub-lattice, respectively, while both sub-lattices of graphene are occupied by C atoms. The structural differences between 2D SiC and graphene show that for chemical functionalization, one should consider the preferred configuration when 2D SiC is fully or partly decorated with such as H or F atoms. In this Letter, via hydrogenation, fluorination, semi-hydrogenation, and semi-fluorination, we tune the structural and electronic properties of 2D SiC monolayer. It is revealed that the fully hydrogenated and/or fluorinated SiC structures are semiconductors, and for the system with full fluorination the energy gap is reduced sharply compared with the gap of the pure 2D SiC structure. It is shown that the configurations of SiC decorated with H and/or F atoms on two sides are more stable than that decorated on only one side. In addition, all of the semi-hydrogenated and semi-fluorinated SiC structures are metallic materials. All the calculations have been carried out by the Vienna ab initio simulation package (VASP)[25] based on density functional theory (DFT).[26] The exchange-correlation potential has been approximated by the functional of generalized gradient approximation (GGA) of Perdew–Burke–Ernzerhof (PBE).[27] The cutoff energy of the plane basis is set to be 500 eV and Monkhorst–Pack $k$-points mesh of $11\times11\times1$ is used.[6] We use a vacuum space of 15 Å to avoid fictitious interactions between adjacent two layers. All atoms are fully relaxed until the forces acting on each atom are less than 0.01 eV/Å and the convergence criterion for total energy is chosen to be $1.0\times10^{-5}$ eV. Figure 1(a) shows the unit cell of the pristine 2D SiC monolayer, and we perform the hydrogenation and fluorination on this structure. In the following we discuss the electronic and structural properties of the SiC structure in the cases of full hydrogenation on one side or both sides, semi-hydrogenation, semi-fluorination, full fluorination on one side and both sides, and the mixed functionalization of hydrogenation and fluorination. As shown in Fig. 1(b), the 2D pristine SiC is an indirect-gap semiconductor with a band gap of 2.54 eV by our calculations, and the corresponding optimized bond length of Si–C is 1.79 Å. The electron localization function between Si and C atoms shown in Fig. 1(c) approaches 1, which implies a strong localization of the Si–C bond, complying with the covalent binding properties of the Si–C bond.
cpl-34-1-017302-fig1.png
Fig. 1. (a) Top view of the optimized structure of the 2D SiC monolayer. The dashed line is the unit cell. Si and C are represented by brown and blue balls, respectively. (b) The calculated band structure and PDOS of the pristine SiC structure. (c) The local electron density distribution which reflected the bonding mechanism of SiC, and plot is obtained by cutting slabs in (001) and (110) planes, respectively.
We first discuss the results of the fully hydrogenated 2D SiC monolayer. Due to the presence of two species of atoms in 2D SiC, two kinds of configurations for fully hydrogenated system were considered, as shown in Figs. 2(a) and 2(d). They are labeled as All-H(a) and All-H(b), respectively. To study the stability of the hydrogenation SiC sheet, we define the formation energy $\varepsilon _{\rm f}$ as $$ \varepsilon _{\rm f} =(E_{\rm hydro} -E_{\rm SiC} -E_{\rm H})/n_{\rm H}, $$ where $E_{\rm hydro}$, $E_{\rm SiC}$ and $E_{\rm H}$ are the total energies of hydrogenated SiC structure, pristine SiC and isolated H atom, respectively, and $n_{\rm H}$ represents the number of hydrogen atoms in the unit cell. For the All-H(a) structure, the hydrogen atoms adsorbed on Si and C atoms are on different sides, and the obtained formation energy $\varepsilon _{\rm f}$ is $-$3.88 eV. The distance between Si and C planes is found to be 0.57 Å, and the Si–C, Si–H, and C–H bond lengths are 1.89 Å, 1.50 Å, and 1.11 Å, respectively. Obviously, the Si–C bond length of the hydrogenated structure is slightly larger than that of the pristine 2D SiC sheet. From Fig. 2, we can see that the Si–H bond length is larger than the C–H bond length due to the different electronegativities. The All-H(a) structure belongs to a semiconductor with a direct band gap of about 4.12 eV, which is larger than that in the pristine SiC structure. The valence-band maximum (VBM) and conduction-band minimum (CBM) are both located at the ${\it \Gamma}$ point, which realizes the transition from indirect semiconductor to direct semiconductor, and is different from the transition from semimetal to semiconductor in the functionalized graphene.[28] In the pristine 2D SiC structure, the VBM is mainly contributed to by Si-$p_z$ orbitals and the CBM comes mainly from the C-$p_z$ orbitals, as shown in Fig. 1(b). For the All-H(a) structure, unlike the pristine SiC, most states of both C-$p_z$ orbitals and Si-$p_z$ orbitals are occupied due to the bonding between C-$p_z$ orbitals and H-$s$ orbitals as well as Si-$p_z$ orbitals and H-$s$ orbitals, respectively. The electron localization density distribution (ELDD) is plotted in Fig. 2(c). It can be described in the form of an isogram in real space with values ranging from 0 to 1. The region with 0 is a low electron density area, the region close to 0.5 is an area with uniform electron gas and the region with 1.0 indicates the strong covalent electrons or lone-pair electrons. In the region between Si and C, a high ELDD ($\sim$0.9) lies in the middle of the bond, which indicates strong covalent electron states, complying with the PDOS results. For the bonds of Si–H and C–H, a high ELDD lies in the position of H atoms.
cpl-34-1-017302-fig2.png
Fig. 2. (a) Top view and lateral view of the optimized structure of the hydrogenated 2D SiC All-H(a). Si, C, and H atoms are represented by blue, brown, and pink balls, respectively. (b) The calculated band structure and PDOS of the All-H(a) structure. (c) The electron localization density distribution (ELDD) of All-H(a) obtained by cutting slabs in (001) and (110) planes, respectively. (d) Top view and lateral view of the optimized structure of the hydrogenated 2D SiC All-H(b). (e) The calculated band structure and PDOS of the All-H(b) structure. (f) The ELDD of ALL-H(b).
We also calculate the configuration of the fully hydrogenated SiC on one side (All-H(b)), as shown in Fig. 2(d). The formation energy $\varepsilon _{\rm f}$ is about $-$3.18 eV, which indicates that the system of All-H(b) is less stable than the system of All-H(a) structure. Our results show that it is a semiconductor with an indirect band gap of about 3.55 eV. The band gap of All-H(b) is also larger than those in pristine SiC and All-H(a). Most states of C-$p_z$ orbitals and Si-$p_z$ orbitals are occupied like All-H(a). Then we consider the configurations that H atoms are only absorbed on the top side of Si or C site, as shown in Figs. 3(a) and 3(d), respectively. We first consider semi-hydrogenation by placing H atoms on C sites, labeled as SiC-H. The pristine planar structure is distorted when H atoms are absorbed on C sides. The distance between Si and C plane is about 0.64 Å. The bond lengths of Si-C and C-H are 1.89 Å and 1.13 Å, respectively, and the corresponding formation energy $\varepsilon _{\rm f}$ is $-$1.23 eV. Band structure and PDOS are plotted in Fig. 3(b), which shows that the system is metallic. The peak of Si-$p_z$ orbitals is located at the Fermi level, indicating that the state near the Fermi level is mainly contributed to by Si-$p_z$ orbitals. This result is in contrast with the case of the pristine SiC structure, in which the states of Si-$p_z$ orbital mainly contribute to the highest occupied VBM. In Fig. 3(c), there is a high ELDD value ($\sim$0.9) centered on the middle of the bond of Si–C, which indicates that the Si–C bond forms a covalent bond, being consistent with the PDOS in Fig. 3(b). In addition, for C–H bonds, a high electron density distribution prefers to locate on H atoms.
cpl-34-1-017302-fig3.png
Fig. 3. (a) Top view and lateral view of the optimized structure of the semi-hydrogenated configuration SiC-H. (b) The calculated band structure and PDOS. (c) The bonding mechanism of SiC-H reflected from ELDD cutting slabs in (001) and (110) planes, respectively. (d) Top view and lateral view of the optimized structure of the semi-hydrogenated configuration H-SiC. (e) The calculated band structure and PDOS. (f) The bonding mechanism of H-SiC reflected from ELDD.
Figure 3(d) shows the optimized geometry for the semi-hydrogenation on Si sites (labeled as H-SiC). Si atoms and C atoms are not in a plane like previous cases, and the distance between Si and C planes is 0.39 Å, which is smaller than that in SiC-H. The Si–C bond length of 1.54 Å is larger than that of the C–H bond. Because the electronegativity of C is larger than that of Si, the formation energy $\varepsilon _{\rm f}$ of this system is $-$1.03 eV, smaller than $\varepsilon _{\rm f}$ of the SiC-H configuration, indicating that the SiC-H system is more stable than H-SiC. As a result, both kinds of semi-hydrogenation are exothermic. Band structure and PDOS plotted in Fig. 3(e) show that the H-SiC system is also metallic. Most states of C-$p_z$ orbitals are located near the Fermi level and make the largest contribution to states around the Fermi level. In Fig. 3(f), a high ELDD value ($\sim$0.9) centers on the middle of the bond of Si–C, indicating that the Si–C bond forms a covalent bond, a high electron density distributes on H atoms. From the above, we can see that full hydrogenation can tune the band gap from an indirect one to a direct one, and semi-hydrogenation can tune the band structure from semiconducting to metallic. Next, we also consider two kinds of full fluorination like full hydrogenation, labeled as All-F(a) and All-F(b), which correspond to the F atoms absorbed on Si and C on one side and two sides, respectively. We define the formation energy of fluorination as $$ \varepsilon _{\rm f} =(E_{\rm fluor} -E_{\rm SiC} -E_{\rm F})/n_{\rm F}, $$ where $E_{\rm fluor}$, $E_{\rm SiC}$ and $E_{\rm F}$ are the total energies of fluorinated 2D SiC structure, pristine 2D SiC, and isolated F atom.
cpl-34-1-017302-fig4.png
Fig. 4. (a) Top view and lateral view of the optimized structure of fluorinated configuration All-F(a). Si, C, and F atoms are represented by the blue, brown, and white balls, respectively. (b) The calculated band structure and PDOS of All-F(a). (c) The bonding mechanism of All-F(a) reflected from ELDD. (d) Top view and lateral view of the optimized structure of fluorinated configuration All-F(b). (e) The calculated band structure and PDOS for All-F(b). (f) The bonding mechanism of All-F(a) reflected from ELDD.
For All-F(a), the formation energy $\varepsilon _{\rm f}$ is $-$4.57 eV, the distance between Si and C-plane is 0.51 Å. The Si–C, Si–F, and C–F bond lengths are 1.87 Å, 1.61 Å, and 1.44 Å, respectively. The Si–C bond length is larger than that in pristine SiC. Band structure and PDOS are plotted in Fig. 4(b). It is found to be a semiconductor with a direct gap of 1.81 eV, which is smaller than that of the pristine SiC structure; both the VBM and CBM are located at the ${\it \Gamma}$ point. In Fig. 4(c), the ELDD values between Si and C are close to 0.9, indicating that the bond of Si–C forms a covalent bond, which is consistent with the orbital hybridization in Fig. 4(b). For Si–F and C–F, a high ELDD ($\sim$0.7) distributes around F atoms. We also calculate the configuration of fully fluorinated SiC on one side, as shown in Fig. 4(d), which is named as All-F(b). The formation energy of this structure is $-$1.5 eV, which indicates that the All-F(b) system is less stable than the All-F(a) system. In Fig. 4(e), it shows that this system is also a direct semiconductor, and the corresponding band gap is 0.73 eV, much smaller than the gaps of the All-F(a) configuration and the pristine 2D SiC.
cpl-34-1-017302-fig5.png
Fig. 5. (a) Top view and lateral view of the optimized structure of hydrogenated configuration SiC-F. (b) The calculated band structure and PDOS of the SiC-F structure. (c) The bonding mechanism of All-F(b) reflected from ELDD. (d) Top view and lateral view of the optimized structure of hydrogenated configuration F-SiC. (e) The calculated band structure and PDOS for F-SiC structure. (e) The bonding mechanism of F-SiC reflected from ELDD.
As stated in previous cases, the full fluorination of SiC structures is exothermic and their band gap is reduced sharply. In the following we present detailed investigations of the semi-fluorinated SiC structure. We first discuss the results of semi-fluorination by placing F atoms on C sites, labeled as SiC-F, as shown in Fig. 5(a). Optimized geometry shows that after placing F atoms on C sites, SiC surface becomes distorted and the distance between Si and C planes is 0.39 Å. Bond lengths of Si–C and C–F are 1.82 Å and 2.08 Å, respectively. The formation energy $\varepsilon _{\rm f}$ of SiC-F is $-$1.26 eV. The system is metallic, as shown in Fig. 5(b). We can also see that the state of the Fermi level is mainly contributed to by C-$p_z$ orbitals and F-$p_z$ orbitals. In Fig. 5(c), the ELDD values are close to 0.9 between Si and C atoms, indicating Si–C bond forming a covalent bond. For C-F, a high ELF ($\sim$0.7) distributes on F atoms. Another configuration of semi-fluorination is that F atoms are absorbed on Si sites, labeled as F-SiC, as shown in Fig. 5(d). The distance between Si and C planes is about 0.40 Å, and bond lengths of Si–C and Si–F are 1.83 Å and 1.63 Å, respectively. The formation energy $\varepsilon _{\rm f}$ of F-SiC is $-$2.20 eV, which is larger than that of SiC-F, indicating that the F-SiC system is more stable than the SiC-F system. Band structure and PDOS are plotted in Fig. 5(e). The system is also metallic and the states of Fermi level are mainly contributed by C-$p_z$ orbitals. In Fig. 5(f), there is a high ELDD value ($\sim$0.9) between Si and C, indicating that the Si–C bond forms a covalent bond. For Si-F, a high ELDD ($\sim$0.7) distributes on F atoms.
cpl-34-1-017302-fig6.png
Fig. 6. (a) Top view and lateral view of the optimized structure of the mixed functionalized configuration H-SiC-F. Si, C, H, and F atoms are represented by blue, brown, pink, and white balls, respectively. (b) The calculated band structure and PDOS of the H-SiC-F structure. (c) The bonding mechanism of H-SiC-F reflected from ELDD. (d) Top view and lateral view of the optimized structure of the mixed functionalized configuration F-SiC-H. (e) The calculated band structure and PDOS of F-SiC-H. (f) The bonding mechanism of F-SiC-H reflected from ELDD.
In addition, we also introduce H atoms to semi-fluorinated SiC structure or introduce F atoms to semi-hydrogenated SiC structure. It is found that the systems, labeled as H-SiC-F and F-SiC-H, are more stable than the other configuration, as shown in Figs. 6(a) and 6(d). The distance between Si and C planes is 0.59 Å in the H-SiC-F structure. The bond lengths of Si–C, Si–H, and C–F are 1.91 Å, 1.50 Å, 1.44 Å, respectively. The formation energy $\varepsilon _{\rm f}$ is $-$5.22 eV. The H-SiC-F system is a direct semiconductor with a band gap of 2.70 eV, which is slightly larger than that of the pristine 2D SiC. PDOS shows that most of the Si-$p_z$ orbitals and C-$p_z$ orbitals contribute to the valence band due to the bonding between C-$p_z$ orbitals and F-$p_z$ orbitals, and the bonding between Si-$p_z$ orbitals and H-$s$ orbitals. In Fig. 6(c), a high ELDD distribution centered on the middle of the bond reflects that Si and C form a strong covalent bond. For C–F and Si–H bonds, a high ELDD ($\sim$0.9) distributes on F atoms and a high ELDD ($\sim$1) distributes on H atoms. For the F-SiC-H structure, the distance between Si-plane and C-plane is 0.57 Å. The bond lengths of Si–C, Si–F, and C–H are 1.86 Å, 1.61 Å, and 1.11 Å, respectively. The F-SiC-H system is a direct semiconductor with a band gap of 3.93 eV. The formation energy $\varepsilon _{\rm f}$ is $-$7.63 eV, much larger than that of H-SiC-F, indicating that the F-SiC-H system is more stable than the H-SiC-F system. In summary, we have systematically studied the electronic properties via hydrogenation and/or fluorination of the SiC monolayer. The system of semi-hydrogenation and semi-fluorination of the SiC monolayer are found to be metallic and the SiC structure decorated with H atoms and/or F atoms are found to be semiconductor with direct gap. It is found that the band gaps of two configurations of the fully fluorinated SiC structure are reduced and the band gaps of the other semiconductor are enlarged. Among this system of semiconductor, only All-H(b) is an indirect semiconductor. As a result, the indirect semiconducting-metallic-direct semiconducting transition is realized in the 2D SiC structure by functionalization. These results provide theoretical guidance for design of photoelectric devices based on the SiC monolayer. References Experimental observation of the quantum Hall effect and Berry's phase in grapheneElectronic Confinement and Coherence in Patterned Epitaxial GrapheneThe electronic properties of grapheneElectric Field Effect in Atomically Thin Carbon FilmsHydrogen Storage Capacity Study of a Li+Graphene Composite System with Different Charge StatesFirst-principles study of defects and adatoms in silicon carbide honeycomb structuresFermi surface nesting and magnetic quantum phase transition in graphenelike BC 3 : A first-principles studyNature and evolution of the band-edge states in MoS 2 : From monolayer to bulkPrediction of a Dirac state in monolayer TiB 2 Semiconducting Layered Blue Phosphorus: A Computational StudyElectrical contacts to monolayer black phosphorus: A first-principles investigationTwo- and One-Dimensional Honeycomb Structures of Silicon and GermaniumPenta-graphene: A new carbon allotropeTuning the electronic and mechanical properties of penta-graphene via hydrogenation and fluorinationTiC 2 : a new two-dimensional sheet beyond MXenesThermal exfoliation of stoichiometric single-layer silica from the stishovite phase: insight from first-principles calculationsA New Silicon Phase with Direct Band Gap and Novel Optoelectronic PropertiesHydrogen on graphene under stress: Molecular dissociation and gap openingConductance of telescoped double-walled nanotubes from perturbation calculationsElectronic and magnetic properties of a BN sheet decorated with hydrogen and fluorineBand structure engineering of graphene by strain: First-principles calculationsFirst-principles study of strain-induced modulation of energy gaps of graphene/BN and BN bilayersTuning the band gap and magnetic properties of BN sheets impregnated with graphene flakesEfficient iterative schemes for ab initio total-energy calculations using a plane-wave basis setInhomogeneous Electron GasGeneralized Gradient Approximation Made SimpleThird conformer of graphane: A first-principles density functional theory study
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