Theoretical Investigation of the Prospect to Tailor ZnO Electronic Properties with VP Thin Films

The atomic and electronic structure of vanadium phosphide one- to four-atomic-layer thin films and their composites with zinc oxide substrate are modelled by means of quantum chemistry. Favorable vanadium phosphide to ZnO orientation is defined and found to remain the same for all the structures under consideration. The electronic structure of the composites is analyzed in detail. The features of the charge and spin density distribution are discussed.

Electronic properties of ZnO are strongly affected by the synthesis conditions and method. This fact is associated with the point defects (oxygen/zinc vacancy and oxygen/zinc interstitials) acting as dopants and influencing physical and chemical characteristics of material [21][22][23][24]. ZnO doping enhances its physical properties-namely, electric conductivity [25], and transparency [26]-and decreases the electron work function [27]. Ferromagnetic properties [28,29] may also occur in doped ZnO while the pristine material is non-magnetic.
n-doping of ZnO is usually reached by XIII group elements (i.e., B [30], Al [31,32], Ga [33,34], In [35]) as well as transition metals such as Ti [36]. On the other hand, XV group elements (N [37,38], P [39,40] and Sb [41]) are promising p-type dopants substituting oxygen atoms in ZnO structure. ZnO doped by transition metal atoms arouses great interest due to the opportunity to obtain diluted magnetic semiconductors (DMS) for new device applications.
Besides the doping of ZnO with different elements of periodic table, the formation of thin films-based composites is another popular way to tune its properties. For instance, synthesis and enhanced photocatalytic properties have been recently reported for ZnObased composites with graphene [42][43][44][45]. Another way to improve ZnO photocatalytic activity is using MXenes, a promising family of materials defined by M n+1 X n T x composition where M is an early transition metal, X is carbon and/or nitrogen atom and T represents Nanomaterials 2021, 11, 1412 2 of 11 the surface-terminating functional groups [46][47][48]. Thus, investigations of zinc oxide-based metamaterials obtained by its doping as well as growing thin films of transition metals compounds on ZnO substrate are a promising direction of modern materials science.
Transition metal phosphides (TMP), one more promising family of two-dimensional transition metal compounds, have gained significant research interest due to their unique properties and catalytic activity in hydrogen evolution reaction [49][50][51][52][53]. Some of them have even been predicted to be comparable with Pt (111) surface [54]. Extensive theoretical studies of M 2 P monolayers have shown them as promising candidates for catalysis and electrode materials [55][56][57]. The most recent study of tetragonal VP monolayer reveals its half-metallicity and interesting optical properties [58].
The present paper aims to show how ZnO electronic structure changes when forming nanoscale composites with VP thin films. First, thin films of vanadium phosphide with various thickness and composition are characterized by means of density functional theory. After that, ZnO/VP stacking, electronic and magnetic properties are discussed.

Computational Methods
All quantum chemical calculations were performed within the framework of density functional theory using the plane wave basis set and projector-augmented wave method [59,60], as implemented in Vienna Ab-initio Simulation Package [61][62][63][64]. GGA-PBE spin-polarized exchange-correlation functional [65] and Grimme correction [66] for van der Waals interactions were used for electronic and structural optimization. The residual forces acting on atoms being less than 10 −3 eV/Å were used as stopping criteria for cell vectors and geometry optimization. Monkhorst-Pack k-point first Brilloin zone sampling [67] was used with k-point mesh containing 12 × 12 × 6 points along three translation vectors for bulk ZnO and VP calculations. When calculating the slabs and interfaces, the vacuum interval of 15 Å was used to guarantee the absence of interactions between slab images in periodic boundary conditions. For these structures, 12 × 12 × 1 k-point mesh was used.
The surface energy for all slabs was estimated as: where E sur f , E sc , E uc , n, S correspond to the surface energy, total energy of the surface supercell, total energy of ZnO unit cell, number of unit cells in the supercell, and the area of ZnO slab unit cell, respectively. The most favorable orientation of VP slab with respect to ZnO surface was determined by comparing stacking energies of each configuration estimated using the equation: where E comp , E ZnO , E VP correspond to the total energies of composite, pure ZnO slab and pure VP slab, respectively.

Results and Discussions
At the first step, the correspondence of the ZnO (0001) surface and vanadium phosphide hexagonal lattices was proved. The zinc oxide hexagonal unit cell belongs to the space group P6 3 mc with lattice parameters a = b = 3.25, c = 5.21 Å [68] while the VP hexagonal unit cell belongs to the space group P6 3 /mmc with lattice parameters a = b = 3.180, c = 6.220 Å [69]. A set of free-standing ZnO (0001) slabs with the number of atomic layers varying from 7 to 12 were modelled. It was found that the values of E surf are close to each other and lie in the range of 1.854 to 1.883 J/m 2 . Thus, the one with the smallest number of atoms was used as the surface unit cell for further calculations. Next, VP slabs cut from the bulk crystal with the number of layers decreasing from four to one were modelled.
Lattice parameter a as well as the corresponding magnetic moments for VP are presented in Table 1. Structural parameters of bulk VP are in good agreement with experimen-tal data [69]. The structure stoichiometries correspond to the number of each element's atomic layers. Thin films of two or more layers are close to the original bulk structure while monolayers demonstrate fluctuations of a parameter which can be explained in terms of structural instability. The stoichiometric compositions of VP thin films are characterised by larger magnetic moments on vanadium atoms caused by the V dangling bonds while their non-stoichiometric counterparts have magnetic moments close to zero ( Figure 1 illustrates atomic structure for stoichiometric and non-stoichiometric bilayer of VP). In this work, we mainly focus on stoichiometric structures as V-terminated surfaces possessing larger magnetic moments. Magnetic catalysts are considered to be environmentally friendly as they can be easily and completely separated from reactants using an external magnet without any loss, unlike other heterogeneous catalysts requiring filtration, centrifugation and other techniques that might be quite sophisticated [70]. It is also known that not only charge transfer but also spin transfer may occur when the molecule is adsorbed on magnetic surface, enhancing its catalytic properties [71] and expanding the area of potential applications in spintronic devices [72]. Non-stoichiometric ones are presented both for the reference and as an intermediate step of thin films formation. number of atoms was used as the surface unit cell for further calculations. Next, VP slabs cut from the bulk crystal with the number of layers decreasing from four to one were modelled.
Lattice parameter a as well as the corresponding magnetic moments for VP are presented in Table 1. Structural parameters of bulk VP are in good agreement with experimental data [69]. The structure stoichiometries correspond to the number of each element's atomic layers. Thin films of two or more layers are close to the original bulk structure while monolayers demonstrate fluctuations of a parameter which can be explained in terms of structural instability. The stoichiometric compositions of VP thin films are characterised by larger magnetic moments on vanadium atoms caused by the V dangling bonds while their non-stoichiometric counterparts have magnetic moments close to zero (Figure 1 illustrates atomic structure for stoichiometric and non-stoichiometric bilayer of VP). In this work, we mainly focus on stoichiometric structures as V-terminated surfaces possessing larger magnetic moments. Magnetic catalysts are considered to be environmentally friendly as they can be easily and completely separated from reactants using an external magnet without any loss, unlike other heterogeneous catalysts requiring filtration, centrifugation and other techniques that might be quite sophisticated [70]. It is also known that not only charge transfer but also spin transfer may occur when the molecule is adsorbed on magnetic surface, enhancing its catalytic properties [71] and expanding the area of potential applications in spintronic devices [72]. Non-stoichiometric ones are presented both for the reference and as an intermediate step of thin films formation.  The manifold of composite structures considered included different VP film orien- The manifold of composite structures considered included different VP film orientations with respect to ZnO (see Figure 2 for the notations: A_top_B corresponds to the atom A of VP being on top of the atom B of ZnO; A_hex represents hexagonal hollow site below the atom A of VP).
The [P_top_Zn:V_hex] configuration of VP/ZnO composite was found to have the lowest stacking energy for both V 4 P 4 /ZnO and VP monolayer/ZnO structures (−1.273 eV and −1.167 eV, respectively, see Table 2). This configuration is also characterized by the largest values of magnetic moments, and the VP monolayer possesses the largest among all (2.285 µ B ). According to the common trend in stacking energies for one-and four-layer VP films, only [P_top_Zn:V_hex] configuration was constructed for two-and three-layer ones.  Table 2). This configuration is also characterized by largest values of magnetic moments, and the VP monolayer possesses the largest amo all (2.285 µB). According to the common trend in stacking energies for one-and four-la VP films, only [P_top_Zn:V_hex] configuration was constructed for two-and three-la ones.     Composite formation leads to the shifting and broadening of DOS peaks, which is more prominent for the VP monolayer in VP/ZnO composite while VP thickness up to three layers leads to the change mostly in the ZnO valence zone (see Figure 4). However, the levels of zinc oxide thin film above the Fermi level are much less affected (see insets in Figures 3C and 4C). Figure 5 demonstrates element-resolved PDOS for V4P4/ZnO structure. While Zn and O states are highly hybridized, V contribution is dominating for VP and PDOS of P are almost negligible. Figure 6, similarly to Figure 3C,D, demonstrates more prominent redistribution of ZnO valence band states and less that of its conduction band.      For the reference, non-stoichiometric configurations of one and three-layer thick VP/ZnO hybrid structure were modelled (see Table 3). The calculated stacking energies revealed that favorable configuration of VP and ZnO slabs' mutual arrangement remains the same ([P_top_Zn:V_hex]). These values, however, should not be compared to those obtained for stoichiometric structures directly as uniform adsorption of a whole P layer is required to turn from one to another. In addition, the charge and spin density distributions were analyzed. The negative charge on VP slab demonstrates the electron transferred to it from the ZnO slab ( Figure  7). Composite formation leads to the shifting and broadening of DOS peaks, which is more prominent for the VP monolayer in VP/ZnO composite while VP thickness up to three layers leads to the change mostly in the ZnO valence zone (see Figure 4). However, the levels of zinc oxide thin film above the Fermi level are much less affected (see insets in Figures 3C and 4C). Figure 5 demonstrates element-resolved PDOS for V 4 P 4 /ZnO structure. While Zn and O states are highly hybridized, V contribution is dominating for VP and PDOS of P are almost negligible. Figure 6, similarly to Figure 3C,D, demonstrates more prominent redistribution of ZnO valence band states and less that of its conduction band.
For the reference, non-stoichiometric configurations of one and three-layer thick VP/ZnO hybrid structure were modelled (see Table 3). The calculated stacking energies revealed that favorable configuration of VP and ZnO slabs' mutual arrangement remains the same ([P_top_Zn:V_hex]). These values, however, should not be compared to those obtained for stoichiometric structures directly as uniform adsorption of a whole P layer is required to turn from one to another. In addition, the charge and spin density distributions were analyzed. The negative charge on VP slab demonstrates the electron transferred to it from the ZnO slab (Figure 7). revealed that favorable configuration of VP and ZnO slabs' mutual arrange the same ([P_top_Zn:V_hex]). These values, however, should not be comp obtained for stoichiometric structures directly as uniform adsorption of a w required to turn from one to another. In addition, the charge and spin density distributions were analyzed. charge on VP slab demonstrates the electron transferred to it from the ZnO 7).  Table 4. The same non-uniform trend is observed for both charge and spi as the number of layers increases. The latter is generally in agreement wi culated for pristine VP slabs. The amount of charge transfer estimated by the AIM (Bader) method [73][74][75] is listed in Table 4. The same non-uniform trend is observed for both charge and spin distribution as the number of layers increases. The latter is generally in agreement with values calculated for pristine VP slabs. According to Figure 8, which demonstrates spin density spatial distribution, the topmost V layer gains the most of the magnetic moment while the magnetism in deeper-lying V atoms is rather quenched with the increase in the number of VP layers in the composite. According to Figure 8, which demonstrates spin density spatial distribution, the topmost V layer gains the most of the magnetic moment while the magnetism in deeper-lying V atoms is rather quenched with the increase in the number of VP layers in the composite.

Conclusions
The atomic and electronic structure of VP thin films was calculated and the possibility of VP/ZnO composite formation was proven by quantum chemical modelling. Configuration characterized by phosphorous atoms being atop the Zn ones and vanadium atoms placed above the hexagon centre was found to be favourable for all structures considered regardless of the number of VP layers and stoichiometry of structure. The valence band is mostly formed by the ZnO slab while VP states are more prominent

Conclusions
The atomic and electronic structure of VP thin films was calculated and the possibility of VP/ZnO composite formation was proven by quantum chemical modelling. Configuration characterized by phosphorous atoms being atop the Zn ones and vanadium atoms placed above the hexagon centre was found to be favourable for all structures considered regardless of the number of VP layers and stoichiometry of structure. The valence band is mostly formed by the ZnO slab while VP states are more prominent in the conduction band. Zinc and oxygen states are highly hybridized whereas VP DOS rises mainly from vanadium atoms. The topmost V atoms are visibly spin-polarized which opens opportunities for various applications of these structures in spintronics as magnetic substrates for organic molecules or metal complexes adsorption and in catalysis as magnetic catalysts that can be removed from the solution with external magnet. These applications are to be further investigated.