Adsorption of Metal Atoms on SiC Monolayer

: The electronic, magnetic, and optical behaviors of metals (M = Ag, Al, Au, Bi, Ca, Co, Cr, Cu, Fe, Ga, K, Li, Mn, Na, Ni) adsorbed on the SiC monolayer have been calculated based on density functional theory (DFT). The binding energy results show that all the M-adsorbed SiC systems are stable. All the M-adsorbed SiC systems are magnetic with magnetic moments of 1.00 µ B (Ag), 1.00 µ B (Al), 1.00 µ B (Au), 1.01 µ B (Bi), 1.95 µ B (Ca), 1.00 µ B (Co), 4.26 µ B (Cr), 1.00 µ B (Cu), 2.00 µ B (Fe), 1.00 µ B (Ga), 0.99 µ B (K), 1.00 µ B (Li), 3.00 µ B (Mn), and 1.00 µ B (Na), respectively, except for the Ni-adsorbed SiC system. The Ag, Al, Au, Cr, Cu, Fe, Ga, Mn, and Na-adsorbed SiC systems become magnetic semiconductors, while Bi, Ca, Co, K, and Li-adsorbed SiC systems become semimetals. The Bader charge results show that there is a charge transfer between the metal atom and the SiC monolayer. The work function of the K-adsorbed SiC system is 2.43 eV, which is 47.9% lower than that of pristine SiC and can be used in electron-emitter devices. The Bi, Ca, Ga, and Mn-adsorbed SiC systems show new absorption peaks in the visible light range. These results indicate that M-adsorbed SiC systems have potential applications in the ﬁeld of spintronic devices and solar energy conversion photovoltaic devices


Introduction
Since the successful preparation of graphene [1], there has been a surge in research into two-dimensional (2D) materials, including 2D WS 2 [2,3], GaN [4][5][6], BN [7,8], black phosphorus [9,10], ZnO [11,12], SiC [13,14], etc. SiC is a third-generation semiconductor material with a wide band gap, high electron saturation drift rate, high breakdown field strength, high thermal conductivity, high radiation resistance, etc. It has a wide range of applications in solar cells, high-frequency high-power devices, and high-temperature electronic devices. Two-dimensional SiC has the advantages of high electron mobility, chemical stability, and high catalytic activity and is often used to make photocatalysts [15]. Based on the first-principles approach, 2D SiC has been predicted to have a graphene-like honeycomb structure and can exist stably as a semiconductor material with a direct band gap of 2.52-2.87 eV [16,17]. Chabi et al. have successfully prepared SiC nanosheets with an average thickness of 2-3 nm through a catalytic carbon thermal reduction method and ultrasonic pretreatment process [18]. Two-dimensional SiC has great potential in the field of nanoelectronic devices, but there are some problems in photocatalysis. Two-dimensional SiC is only responsive to partially visible light [19], so it is necessary to reduce the band gap and improve the absorption efficiency of visible light. Current methods to effectively modulate the band structure include doping [20,21], stacking [22,23], adsorption [24,25], heterojunctions [26][27][28], etc.
The adsorption of metal atoms is one of the most important means to modulate the properties of 2D materials. The adsorption of different atoms on the surface of 2D materials can modulate the optical, electrical, and magnetic properties of 2D materials. Nie et al. have studied the adsorption of 3D transition metals on the SnO monolayer [29]. They found that 3D transition metal adsorption induced magnetism and achieved ntype and p-type doping. Guo  WSSe monolayer by adsorbing Fe, Co, and Ni atoms and developed its applications in gas sensors and single-atom catalysts [30]. Cui et al. have studied the adsorption of transition metals on the Pd 2 Se 3 monolayer [31]. They found that the adsorption of transition metals improved light absorption in the ultraviolet, visible, and infrared regions. Xu et al. have predicted the magnetism of the SnSe 2 monolayer after the adsorption of transition metals and found that the adsorption of Ti atoms can endow the SnSe 2 monolayer with perpendicular magnetic anisotropy [32]. In this paper, the electronic structure, magnetic, and optical properties of 15 metal atoms adsorbed on SiC monolayer have been calculated using the first-principles approach. The influence of the M atoms on the properties of the SiC monolayer is analyzed according to the band structure, work function, and light absorption spectra, and the application prospects of M-adsorbed SiC systems in the field of spin devices and photovoltaic devices are explored.

Computational Details
The electronic, magnetic, and optical behaviors of M-adsorbed SiC systems have been investigated in the Vienna ab initio calculation simulation package (VASP) [33,34] using density functional theory (DFT) [35,36]. The electron-ion interactions are performed using the Perdew-Burke-Ernzerhof (PBE) form of the generalized gradient approximation (GGA) approach [37]. The exchange-correlation interactions are performed using the projector-enhanced wave (PAW) approach [38]. Dispersion corrections are considered by Grimme's DFT-D3 method [39]. The plane wave cutoff energy is 400 eV, the Monkhorst-Pack scheme [40] grid in the Brillouin zone is 4 × 4 × 1, and the vacuum space is 20 Å. During structural relaxation, the convergences of the force and self-consistent energy are 1 × 10 −2 eV Å −1 and 1 × 10 −5 eV, respectively. The optical properties are considered according to the frequency-dependent dielectric response theory, including the local field effects in the random-phase-approximation (RPA) method [41].

Results and Discussion
The pristine SiC has a graphene-like structure with an alternating arrangement of C and Si atoms, and its lattice parameter is 3.1 Å with a bond length of 1.78 Å. From the band structure and density of state (DOS) of Figure 1b,c, it can be seen that pristine SiC is a nonmagnet semiconductor with a direct bandgap of 2.5 eV, and the conduction band minimum (CBM) is mainly contributed by the hybridization of the p-state of Si and C, while the valence band maximum (VBM) is mainly contributed by the 2p-state of C. The VBM and CBM are not at the same high symmetry point, indicating that the pristine SiC is an indirect bandgap semiconductor. These results are consistent with previous reports [42], indicating that our computational method is reliable.
In order to study the stability of metal adsorption on the SiC system, we constructed four adsorption models for each type of metal, as shown in Figure 1a. The adsorption sites were located above a Si atom, above a C1 atom, above a C2 atom, and above the Si-C bond. Adsorption energy (E ads ) was used to characterize the stability of the adsorption system, which can be calculated using the following formula: where E M-SiC is the total energy of the M-adsorbed SiC systems, including the interaction energy between the metal atom and the SiC monolayer; E SiC is the energy of the pristine SiC monolayer; µ M is the chemical potential of an isolated metal atom. As listed in Table 1 It can be seen that the E ads of all systems are negative, indicating that the systems are stable. Different metal adsorption has different adsorption sites that are the most stable. The most stable adsorption site for Ag is located at S C2 , while for Co, Cr, K, Li, Mn, Na, and Ni, it is at S H .   In order to investigate the effect of metal adsorption on the electronic properties of SiC systems, we studied the band structures of different metal-adsorbed silicon carbide   In order to investigate the effect of metal adsorption on the electronic properties of SiC systems, we studied the band structures of different metal-adsorbed silicon carbide systems, as shown in Figure 2. It can be seen that, except for the Ni-adsorbed SiC system, the spin-up and spin-down of other systems do not overlap, indicating that these systems all exhibit magnetism. Among them, the adsorption of Ag, Al, Au, Cr, Cu, Fe, Ga, Mn, and Na atoms on SiC systems result in a magnetic semiconductor, and the bandgaps of 0.521 eV (Ag), 0.659 eV (Al), 0.837 eV (Au), 0.199 eV (Cr), 0.705 eV (Cu), 0.734 eV (Fe), 0.640 eV (Ga), 0.494 eV (Mn), and 0.442 eV (Na), respectively. However, the adsorption of Ni on the SiC system leads to a non-magnetic semiconductor with a bandgap of 1.754 eV. The band gaps of the systems after adsorption are all smaller than those of the unadsorbed systems. Interestingly, the Bi, Ca, Co, K, and Li-adsorbed SiC systems exhibit semimetallic characteristics, indicating that they can be used as sensitive components in magnetic materials, electrodes, or electronic devices. Furthermore, Figure 3 describes the spinpolarized charge density of these magnetic systems. In addition to the magnetic distribution of Li and K-adsorbed SiC systems mainly distributed on the SiC monolayer, it can be clearly seen that the magnetic distribution of other systems mainly lies on the adsorbed metal and the atoms underneath it. The magnetic moments of the M-adsorbed SiC systems are 1. all exhibit magnetism. Among them, the adsorption of Ag, Al, Au, Cr, Cu, Fe, Ga, Mn, and Na atoms on SiC systems result in a magnetic semiconductor, and the bandgaps of 0.521 eV (Ag), 0.659 eV (Al), 0.837 eV (Au), 0.199 eV (Cr), 0.705 eV (Cu), 0.734 eV (Fe), 0.640 eV (Ga), 0.494 eV (Mn), and 0.442 eV (Na), respectively. However, the adsorption of Ni on the SiC system leads to a non-magnetic semiconductor with a bandgap of 1.754 eV. The band gaps of the systems after adsorption are all smaller than those of the unadsorbed systems. Interestingly, the Bi, Ca, Co, K, and Li-adsorbed SiC systems exhibit semimetallic characteristics, indicating that they can be used as sensitive components in magnetic materials, electrodes, or electronic devices. Furthermore, Figure 3 describes the spin-polarized charge density of these magnetic systems. In addition to the magnetic distribution of Li and K-adsorbed SiC systems mainly distributed on the SiC monolayer, it can be clearly seen that the magnetic distribution of other systems mainly lies on the adsorbed metal and the atoms underneath it. The magnetic moments of the M-adsorbed SiC systems are 1.00 μB (Ag), 1.00 μB (Al), 1.00 μB (Au), 1.01 μB (Bi), 1.95 μB (Ca), 1.00 μB (Co), 4.26 μB (Cr), 1.00 μB (Cu), 2.00 μB (Fe), 1.00 μB (Ga), 0.99 μB (K), 1.00 μB (Li), 3.00 μB (Mn), and 1.00 μB (Na), respectively. This indicates that the adsorption of metal atoms can modulate the band structure and magnetic properties of SiC monolayers, so the M-adsorbed SiC systems can be applied to the production of spintronic devices.    Charge transfer is an important parameter for describing the interaction between substrate material and the adsorbed atoms. The charge density difference (CDD) c clearly see the charge transfer and distribution, and the CDD of M-adsorbed SiC syste can be calculated using the following formula:

Adsorption Style
where Δρ is the CDD; ρM-SiC is the charge density of the M-adsorbed SiC systems; ρSiC the charge density of the pristine SiC monolayer; and ρM is the charge density of an i lated metal atom. The CDD of M-adsorbed SiC systems is studied in Figure 4 of this s tion. It can be seen that there is a significant charge transfer between the metal atoms a the SiC monolayer. For the Ag, Au, Cu, K, and Ni-adsorbed SiC systems, the adsorb atom is the acceptor, and the SiC monolayer is a donor. For other M-adsorbed SiC system the adsorbed atom is the donor, and the SiC monolayer is the acceptor. Bader charges [4 45] are used to accurately describe the amount of charge transfer. After calculation, amount of charge transfer for various metals to the SiC monolayer are +0.446|e| (A −0.588|e| (Al), +0.319|e| (Au), −0.109|e| (Bi), −0.766|e| (Ca), −0.110|e| (Co), −0.560| (Cr), +0.023|e| (Cu), −0.280|e| (Fe), −0.292|e| (Ga), +1.455|e| (K), −0.867|e| (L −0.468|e| (Mn), −0.391|e| (Na), and +0.045|e| (Ni), respectively.
The work function is a crucial parameter for evaluating the electron emission perf mance of optoelectronic materials, which can be calculated using the following formul where Ф, Evacuum, and EFermi represent work function, vacuum level, and Fermi level, spectively. We have studied the work functions of various metals-adsorbed SiC monol ers and presented the results in Figure 5. It can be seen that the work function of prist SiC is 4.8 eV, and the work function of the M-adsorbed SiC systems fluctuates after a sorption. Interestingly, apart from Bi-adsorbed SiC, the work functions of all other adsorbed SiC systems are lower than that of the pristine SiC of 3.58 eV (Ag), 4.23 eV (A Charge transfer is an important parameter for describing the interaction between the substrate material and the adsorbed atoms. The charge density difference (CDD) can clearly see the charge transfer and distribution, and the CDD of M-adsorbed SiC systems can be calculated using the following formula: where ∆ρ is the CDD; ρ M-SiC is the charge density of the M-adsorbed SiC systems; ρ SiC is the charge density of the pristine SiC monolayer; and ρ M is the charge density of an isolated metal atom. The CDD of M-adsorbed SiC systems is studied in Figure 4 of this section. It can be seen that there is a significant charge transfer between the metal atoms and the SiC monolayer. For the Ag, Au, Cu, K, and Ni-adsorbed SiC systems, the adsorbed atom is the acceptor, and the SiC monolayer is a donor. For other M-adsorbed SiC systems, the adsorbed atom is the donor, and the SiC monolayer is the acceptor. Bader charges [43][44][45] are used to accurately describe the amount of charge transfer. After calculation, the amount of charge transfer for various metals to the SiC monolayer are +0.446|e| (Ag), −0.588|e| (Li), −0.468|e| (Mn), −0.391|e| (Na), and +0.045|e| (Ni), respectively. The work function is a crucial parameter for evaluating the electron emission performance of optoelectronic materials, which can be calculated using the following formula: where Φ, E vacuum , and E Fermi represent work function, vacuum level, and Fermi level, respectively. We have studied the work functions of various metals-adsorbed SiC monolayers and presented the results in Figure 5. It can be seen that the work function of pristine SiC  One of the important indicators for evaluating the performance of photoelectro devices is light absorption. The optical properties of matter are represented by the tra verse dielectric function ε(ω) [46,47].
where ε1(ω) and ε2(ω) are the real and imaginary parts of the dielectric function, a   One of the important indicators for evaluating the performance of photoelectronic devices is light absorption. The optical properties of matter are represented by the transverse dielectric function ε(ω) [46,47].
where ε1(ω) and ε2(ω) are the real and imaginary parts of the dielectric function, and ω is the photon frequency. The ε2(ω) can be obtained by dipole transition amplitude from the valence band (occupied states) to the conduction band (unoccupied states), while the ε1(ω) can be obtained from the Kramers-Kronig relationship. In additional, the absorption coefficient α(ω) can be obtained from the ε1(ω) and ε2(ω) [48]: One of the important indicators for evaluating the performance of photoelectronic devices is light absorption. The optical properties of matter are represented by the transverse dielectric function ε(ω) [46,47].
where ε 1 (ω) and ε 2 (ω) are the real and imaginary parts of the dielectric function, and ω is the photon frequency. The ε 2 (ω) can be obtained by dipole transition amplitude from the valence band (occupied states) to the conduction band (unoccupied states), while the ε 1 (ω) can be obtained from the Kramers-Kronig relationship. In additional, the absorption coefficient α(ω) can be obtained from the ε 1 (ω) and ε 2 (ω) [48]: (5) Figure 6 shows the light absorption spectra of different metals adsorbed on the SiC monolayer. The pristine SiC mainly absorbs in the ultraviolet region and hardly absorbs in the visible light range, indicating that SiC can be used as a UV photodetector, but its application in the visible light range is limited. After metal adsorption, the absorption peak in the ultraviolet region is enhanced. The Bi, Ca, Ga, and Mn-adsorbed SiC systems show new absorption peaks in the visible light range. The Cu-adsorbed SiC system shows a strong absorption peak at 352.1 nm. These indicate that the systems can be used for solar energy conversion photovoltaic devices. (5) Figure 6 shows the light absorption spectra of different metals adsorbed on the SiC monolayer. The pristine SiC mainly absorbs in the ultraviolet region and hardly absorbs in the visible light range, indicating that SiC can be used as a UV photodetector, but its application in the visible light range is limited. After metal adsorption, the absorption peak in the ultraviolet region is enhanced. The Bi, Ca, Ga, and Mn-adsorbed SiC systems show new absorption peaks in the visible light range. The Cu-adsorbed SiC system shows a strong absorption peak at 352.1 nm. These indicate that the systems can be used for solar energy conversion photovoltaic devices.

Conclusions
The electronic, magnetic, and optical behaviors of the metals (M = Ag, Al, Au, Bi, Ca, Co, Cr, Cu, Fe, Ga, K, Li, Mn, Na, Ni) adsorbed SiC systems have been calculated based on the first-principles. The binding energy results show that the most stable adsorption sites are SC2 for Ag atoms, SH for Co, Cr, K, Li, Mn, Na, and Ni atoms, and SC1 for Al, Au, Bi, Ca, Cu, Fe, and Ga atoms. All the M-adsorbed SiC systems are magnetic except for the Ni-adsorbed SiC system. The magnetic distribution of Li and K-adsorbed SiC systems is mainly distributed on the SiC monolayer, while the magnetic distribution of the other systems mainly lies on the adsorbed metal and the atoms underneath it. The band gap is smaller in the M-adsorbed SiC systems compared to the pristine SiC. The Ag, Al, Au, Cr, Cu, Fe, Ga, Mn, and Na-adsorbed SiC systems are magnetic semiconductors with band gaps of 0.521 eV (Ag), 0.659 eV (Al), 0.837 eV (Au), 0.199 eV (Cr), 0.705 eV (Cu), 0.734 eV (Fe), 0.640 eV (Ga), 0.494 eV (Mn), and 0.442 eV (Na), while SiC becomes semimetal after adsorption of Bi, Ca, Co, K, and Li atoms. The Bader charge results show that the adsorbed atom is more readily charged in the Ag, Au, Cu, K, and Ni-adsorbed SiC systems, while the SiC monolayer is more readily charged in the other M-adsorbed SiC systems. The work function of the K-adsorbed SiC system is 2.43 eV, which is 47.9% lower than the work function of pristine SiC and can be used in an electron emitter device. After metal atom adsorption, the absorption peak of the M-adsorbed SiC systems in the UV region is enhanced, and new absorption peaks in the visible range appeared for the Bi, Ca, Ga, and

Conclusions
The electronic, magnetic, and optical behaviors of the metals (M = Ag, Al, Au, Bi, Ca, Co, Cr, Cu, Fe, Ga, K, Li, Mn, Na, Ni) adsorbed SiC systems have been calculated based on the first-principles. The binding energy results show that the most stable adsorption sites are S C2 for Ag atoms, S H for Co, Cr, K, Li, Mn, Na, and Ni atoms, and S C1 for Al, Au, Bi, Ca, Cu, Fe, and Ga atoms. All the M-adsorbed SiC systems are magnetic except for the Ni-adsorbed SiC system. The magnetic distribution of Li and K-adsorbed SiC systems is mainly distributed on the SiC monolayer, while the magnetic distribution of the other systems mainly lies on the adsorbed metal and the atoms underneath it. The band gap is smaller in the M-adsorbed SiC systems compared to the pristine SiC. The Ag, Al, Au, Cr, Cu, Fe, Ga, Mn, and Na-adsorbed SiC systems are magnetic semiconductors with band gaps of 0.521 eV (Ag), 0.659 eV (Al), 0.837 eV (Au), 0.199 eV (Cr), 0.705 eV (Cu), 0.734 eV (Fe), 0.640 eV (Ga), 0.494 eV (Mn), and 0.442 eV (Na), while SiC becomes semimetal after adsorption of Bi, Ca, Co, K, and Li atoms. The Bader charge results show that the adsorbed atom is more readily charged in the Ag, Au, Cu, K, and Ni-adsorbed SiC systems, while the SiC monolayer is more readily charged in the other M-adsorbed SiC systems. The work function of the K-adsorbed SiC system is 2.43 eV, which is 47.9% lower than the work function of pristine SiC and can be used in an electron emitter device. After metal atom adsorption, the absorption peak of the M-adsorbed SiC systems in the UV region is enhanced, and new absorption peaks in the visible range appeared for the Bi, Ca, Ga,