J. Cent. South Univ. (2017) 24: 270-275

DOI: 10.1007/s11171-017-3427-2

First-principle study of electronic structure and optical properties of Au-doped VO₂

HUANG De-wei(黄德伟)¹, ZHAO Cui-hua(赵翠华)¹, CHEN Jian-hua(陈建华)²,

LI Yu-qiong(李玉琼)², LI Wei-zhou(李伟洲)¹

1. College of Materials Science and Engineering, Guangxi University, Nanning 530004, China;

2. Guangxi Colleges and University Key Laboratory of Minerals Engineering, Guangxi University,Nanning 530004, China

Central South University Press and Springer-Verlag Berlin Heidelberg 2017

Abstract: The electronic structure and optical properties of VO₂ and Au-VO₂ were studied using density functional theory. The calculation results show that the interaction between Au and O is stronger than that between V and O. There exists not only the covalent bonding but also ionic bonding in Au—O bond. The band gap of Au-VO₂ is smaller than that of VO₂, while the dielectric constant, conductivity, and intensity of optical absorption of Au-VO₂ are larger than those of VO₂.

Key words: Au-VO₂; electronic structure; optical property; First-principle

1 Introduction

Vanadium dioxide is the most amazing technologically thermochromic material because of the reversible metal-to-insulator transition (MIT). The crystalline phases will be quite different when the temperature changes. It is the monoclinic phase (M₁) and infrared-transmitting semiconductor or narrow gap insulator at a low temperature, which transforms to the infrared-reflecting tetragonal rutile structure phase (R) at a critical temperature near 340 K [1–5]. It becomes metallic state conductor above the T_MIT (metal–insulator transition temperature). There is another intermediate phase between the monoclinic phase (M₁) and rutile (R), which is the insulator monoclinic phase (M₂) [6]. The transition takes place in the arrangement of the atomic structure and the properties like optical and electronic [7], and on the time scale of 10^-7–10^-5 s for nano-scale devices [8]. The V atoms of the M₁-VO₂ are the formation of Zig-zag chain V–V along the C_b axis, but there are two formations in the M₂-VO₂ along the C_b axis. One is the V atomic chains in pairs but does not tilt (relative to the C_b axis), another is the V atomic chains in tilt but is not in pairs [9]. These fascinating characteristics make VO₂ a further application in thermodynamic window coating, smart optoelectronic switcher, memory material, actuators, photonics technologies, and diffractive elements, and so on [10–13]. However, some drawbacks in properties and practical applications need to be resolved, such as high critical temperature, low visible transparency, and limited solar-energy modulation efficiency [14]. The last two are calculated by

   (1)

where φ_lum represents the spectral sensitivity of the light–adapted eye; φ_sol represents the solar irradiance spectrum for an air mass of 1.5 corresponding to the position 37° above the horizon [14]. It must have been a widely application in our life when these drawbacks are overcome, especially the critical temperature near to room temperature (298 K). Phase transition temperature can be affected by temperature, stress, doping, nonstoichiometry, graining, or a combination of all factors. The studies have shown that some characteristics have different change when some higher-valence ions (Nb⁵⁺, Mo⁵⁺, W⁶⁺) were doped to the VO₂ [15], such as the crystalline structure, color, Mott phase transition, optical properties, and electrical properties, and the T_MIT can be reduced. On the contrary, the T_MIT can be increased when low-valence ions (Al³⁺, Cr³⁺, Ga³⁺) were doped. Therefore, doping can decrease or increase the phase transition temperature and increase the visible transparency [16, 17]. The studies also showed that W is one of the best effective dopant to decrease the transition phase temperature at a certain content, and the decrease of phase transition temperature will keep the linear relationship with the doping contents performance, which could be interpreted by Peierls-like phase transition, but it is an irreversible process [18]. LI et al [19] showed that purely Mg-doped can increase the bandgap ((3.9±0.5) eV) per unit of atom ratio Mg/(Mg+V). Doped VO₂ exhibits different characteristics when some nonmetal elements like F, Cl, Br and I were doped to VO₂ compared to metal elements. For instance, the phase transition temperature of the F-doped VO₂ is 308 K at 2.93% F in VO₂. The halogen-doped VO₂ can decrease the bandgap, which is different from Mg-doped widening the bandgap. Cl-doped VO₂ at V is the best one to achieve good VO₂ thermochromic energy-saving material [20, 21]. The glass cover with F-doped VO₂ not only increases the solar-heat shielding ability but also modifies a comfortable color, which still keeps excellent solar modulation abilities and a suitable visible transmittance. It had been demonstrated that the phase transition temperature will decrease when oxygen vacancy is introduced into the VO₂ thin film [22].

The underlying mechanism of the MIT in VO₂ is considered the Mott-Hubbard-like driven by electron- electron correlation or Peierls-like driven by electron- phonon interactions, which continues debating. However, a lot of research had shown that doping is the effective way to change the band gap and trigger the phase structure change. Au is the most conductivity metal element. KIM et al [23] studied the conductance control in VO₂ nanowires by physical vapor deposition. The results demonstrated that Au-doped VO₂ has good conductance control. In this work, electronic structures and optical properties of pure VO₂ and Au-doped VO₂ were studied using the first-principle calculations, including energy gap, density of states, Mulliken population, dielectric function, conductivity and optical absorption. The emphasis was put on the mechanism of the electronic structures and optical properties changes of VO₂ caused by Au doping.

2 Computational methods and models

2.1 Computational methods

The electronic structures and optical properties of pure VO₂ and Au-doped VO₂ were studied using the periodic DFT calculations implemented in the CASTEP code, using the generalized-gradient-approximations (GGA) in the form of the Perdew–Wang 91 (PW91) exchange-correlation functional. A simplified version of the rotationally invariant LDA+U method was used, where the adopted values of U in the LDA +U were set at 2.5 eV for the vanadium d electrons. Calculations were performed as nonmagnetic. The energy cutoff was set at 340 eV, and a 1×1×1 uniform Monkhorst-Pack k-point grid was used for M₁ phase. The convergence tolerances for geometry optimization calculations were set to the maximum energy change of 2×10^–5 eV/atom, the maximum displacement of 2×10^–4 nm, the maximum stress of 0.1 GPa and the maximum force of 0.08 eV/nm.

2.2 Computational model

The M₁ phase VO₂ possesses a space group of P21/c. The lattice constants are 5.734 , 4.517 , 5.375 , respectively. Each V atom coordinates with adjacent six O atoms and one V atom, while each O atom coordinates with adjacent three V atoms (Fig. 1). The model of doping Au was obtained from optimum unit cell and was modeled using a supercell approach (2×2×2), which is shown in Fig. 2. For convenient for comparing, the electronic structure and optical properties of undoped VO₂ were obtained by calculating supercell VO₂.

Fig. 1 Cell model of VO₂

Fig. 2 Model of Au–VO₂

3 Results and discussion

3.1 Energy band structure and density of states

Figure 3 show the band structures of VO₂ (Fig. 3(a)) and Au–VO₂ (Fig. 3(b)). Band gaps of VO₂ and Au–VO₂ are found to be 0.115 eV and 0.013 eV, respectively, which are smaller than the theoretical value (0.7 eV) [24]. Smaller band gap values are caused by DFT in GGA functional, which is insufficient to process inter-electron exchange interaction and does not affect the analysis of energy gap results [25]. However, the band gap of VO₂ after doping with Au becomes smaller. TIAN et al [26] reported that the band gap of VO₂ after W doping decreased from 1.103 eV to 0.643 eV. REN et al [20] reported that the band gap of VO₂ after F doping also decreased. The smaller band gap suggests that the doped VO₂ has a higher metallicity.

Fig. 3 Band structures of supercell VO₂ (a) and Au–VO₂ (b)

The character of energy bands can be evaluated by the density of states. Figure 4 shows the total densities of states of VO₂ and Au–VO₂. Figure 5 presents partial densities of states of V atom for VO₂ and Au atom for Au–VO₂. The curves of total density of states for VO₂ and Au–VO₂ are very similar. The partial densities of state of V atom are mainly made up of four groups. The first group between –67 eV and -65 eV is from V 4s orbital. The second group between –40 eV to –37 eV is from V 4p orbital. The third group between –7 eV to 0 eV is mainly made up of V 3d orbital. The conduction band is made up of V 3d orbital. The partial densities of state of Au are made up of two groups. The first group between –0.2 eV and –0.7eV is mainly composed of Au 5d orbital with few Au 6s orbital. The conduction band is made up of Au 5d orbital.

Fig. 4 Total densities of states for VO₂ and Au–VO₂

Fig. 5 Density of states of supercell Au–VO₂ and VO₂

Figure 6 shows DOS results of interactions between V atom and O atom for VO₂, and between Au atom and O atom for Au–VO₂. It is clear that near Fermi level electrons of V—O bond are mainly from O 2p orbital and V 3d orbital; those of Au–O bond are mainly from O 2p orbital and Au 5d orbital. DOS of Au 5d for Au—O is larger than that of V 3d for V—O bond, showing that Au is more active than V. In addition, the overlap of O 2p and Au 5d orbital is larger than that of O 2p and V 3d orbital. These results show that the interaction between O and Au is stronger than that between O and V.

Fig. 6 DOS results of O atom and V atom for VO₂, O atom and Au atom for Au–VO₂

3.2 Mulliken population analysis

Table 1 shows Mulliken bond population values of O—V bond (VO₂) and Au—O bond (Au-VO₂). It is very clear that the population values of Au—O bond is much smaller than that of V—O bond, which shows that covalent bond between Au and O is weaker than that between O and V. According to Fig. 6, the interaction between O and Au is stronger than that between O and V, indicating that O—Au contains the stronger ionic bond.

Table 1 Mulliken bond population of VO₂ and Au–VO₂

3.3 Optcial properties

Figure 7 shows the dielectric functions of VO₂ and Au-VO₂. The formula of dielectric function is shown as follows:

                         (2)

where ε₁(ω)=Reε(ω); ε₂(ω)=Imε(ω); Reε(ω) and Imε(ω) are real and imaginary parts of dielectric function, respectively; ω is frequency. According to Fig. 7, the dielectric function of Au-VO₂ is very different from VO₂. The dielectric peak of Au-VO₂ shifts towards the low energy compared toVO₂, and the peak intensity of the imaginary part increases; while that of the real part decreases.

Figure 8 shows the conductivities of VO₂ and Au–VO₂. It is very obvious that the conductivity of Au–VO₂ is larger than that of VO₂, especially those around 1000 nm (imaginary part). The conductivity of polarization current σ_p and the dielectric function ε(ω) are related through Eq. (3):

                                (3)

According to Eq. (3), the conductivity is proportional to the imaginary part of dielectric function [27], which is in good agreement with the results of dielectric function (Fig. 7) and also with those of band structures in which band gap of Au–VO₂ is smaller (Fig. 3). The conductivity of Au–VO₂ increases with a decrease in band gap. When light is irradiated to a material, the photon energy larger than the band gap can excite electron-hole pair and cause an increase of carrier concentration, resulting in increasing conductivity.

Fig. 7 Dielectric functions of VO₂ (a) and Au–VO₂(b)

Fig. 8 Conductivities of VO₂ (a) and Au–VO₂ (b)

Figure 9 shows the absorption spectra of VO₂ and Au–VO₂. The intensity and corresponding wavelength of absorption peak for Au–VO₂ are larger than those of VO₂. The larger wavelength of absorption peak for Au–VO₂ means a red-shift relative to VO₂. Therefore, the band gap of Au–VO₂ is smaller than that of VO₂, which is in good agreement with the results of band structure (Fig. 3). On the other hand, optical absorption coefficient is proportional to the imaginary part of dielectric function [27, 28], which is also in well agreement with our results. In this study, the peak intensity of the imaginary part of dielectric function for Au–VO₂ was found larger, resulting in the increase of intensities of the optical absorption.

Fig. 9 Optical absorption of VO₂ (a) and Au–VO₂ (b)

4 Conclusions

The electronic structure and optical properties of VO₂ and Au–VO₂ were studied using density functional theory. The electronic structures and optical properties of VO₂ and Au–VO₂ are different. The interaction between Au and O is stronger than that between V and O. There exists not only the covalent bonding but also ionic bonding in Au—O bond. The band gaps of VO₂ and Au–VO₂ are 0.115 eV and 0.013 eV, respectively. In other words, when Au is doped in the VO₂, the band gap of VO₂ becomes small, while the dielectric constant, conductivity, and intensity of optical absorption get large.

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(Edited by YANG Hua)

Cite this article as: HUANG De-wei, ZHAO Cui-hua, CHEN Jian-hua, LI Yu-qiong, LI Wei-zhou. First-principles study of electronic structure and optical properties of Au-doped VO₂ [J]. Journal of Central South University, 2017, 24(2): 270-275. DOI: 10.1007/s11171-017-3427-2.

Foundation item: Project(2014GXNSFAA118342) supported by Guangxi Natural Science Foundation, China; Project supported by Open Foundation of Guangxi Key Laboratory for Advanced Materials and Manufacturing Technology, China; Project supported by High-level Innovation Team and Outstanding Scholar Program in Guangxi Colleges (the second batch), China

Received date: 2015-09-21; Accepted date: 2016-03-22

Corresponding author: ZHAO Cui-hua, PhD; E-mail: xiaocui2000110@163.com