Nanostructured VO2 Materials and Modulation of Their Properties: Comparison
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Please note this is a comparison between Version 2 by Conner Chen and Version 1 by Woong-Ki Hong.

The morphology of VO2 depends on synthesis methods, which are primarily categorized solution- and gas-phase-based synthesis methods. For example, sol-gel process and hydrothermal synthesis are representative solution-based chemical approaches, while pulsed laser deposition (PLD), sputtering method, and chemical vapor deposition (CVD) are gas- or vapor-phase synthesis techniques. In previous reports, various techniques for the fabrication of nanostructured VO2 materials have been described in detail.

  • vanadium dioxide
  • phase transition
  • nanostructure
  • property modulation

1. Introduction

The complex interplay between charge, spin, orbital, and lattice degrees of freedom results in the novel electronic and magnetic phenomena in strongly correlated materials (SCMs), as an interesting class of materials in condensed-matter physics [1]. Among SCMs, vanadium dioxide (VO2) has attracted considerable attention, due to the reversible and dramatic changes in conductance and transmittance during metal–insulator transition (MIT), which is a first-order phase transition accompanied by a crystal structure change from a low-temperature monoclinic phase to a high-temperature rutile phase at near-room-temperature (Tc ~ 340 K) [2,3][2][3]. VO2 is a tetragonal rutile (R) structure with space group P42/mnm and lattice constants a = b ≈ 4.55 Å and c ≈ 2.85 Å above Tc, whereas it is a monoclinic M1 structure with space group P21/c and lattice constants a ≈ 5.75 Å, b ≈ 4.53 Å, c ≈ 5.38 Å, b = 122.6° [4]. According to the band theory proposed by Goodenough, the vanadium (V) 3d orbitals are split into σ* (eg) symmetry and π* (t2g) symmetry states, and the t2g states are further split into two dπ orbitals and one d‖ orbital [5]. In the R structure, the Fermi level falls between the π* band and the d‖ band, whereas in the monoclinic structure, the d‖ band is split into two energy bands (d‖ and d‖*), and a forbidden band with the bandwidth of approximately 0.7 eV between the d‖ band and the π* band is formed [5].
The driving mechanisms behind the MIT in VO2 have been a topic of controversy for decades whether the transition is driven by electron–electron correlations (Mott transition) or by a structure distortion (Peierls transition). Recently, a collaborative Mott-structural transition mechanism in the phase-transition process has also been proposed as an alternative to the two abovementioned mechanisms of the MIT, because both the structural and electron-correlation aspects are important for describing the MIT behavior in VO2 [6,7][6][7]. Park and co-workers studied a series of epitaxial VO2 films with different deposition temperatures to understand the cooperation effect between Peierls and Mott transitions in VO2 [6]. They proposed the diagram of band structures, which provides insights into the role of the strain and multivalent V states on the phase transition of VO2 [6]. In addition, they inferred electronic band structures corresponding to insulating M1 + M2 coexisting phases and metallic M1 and R phases, on the basis of experimental results through hydrogen incorporation in VO2 [8].

2. Synthesis of Nanostructured VO2 Materials and Modulation of Their Properties

2.1. Synthesis Methods of Nanostructured VO

2

The morphology of VO2 depends on synthesis methods, which are primarily categorized solution- and gas-phase-based synthesis methods. For example, sol-gel process and hydrothermal synthesis are representative solution-based chemical approaches, while pulsed laser deposition (PLD), sputtering method, and chemical vapor deposition (CVD) are gas- or vapor-phase synthesis techniques. In previous reports [3,12,13,14[3][9][10][11][12],15], various techniques for the fabrication of nanostructured VO2 materials have been described in detail. The advantages and limitations for some of these synthesis methods are summarized in Table 1. Sol-gel or hydrothermal approaches have been used to synthesize nanostructured VO2, mainly for the application of thermochromic smart windows. Meanwhile, PLD, sputtering, and CVD have been used to fabricate high quality VO2 thin films or single-crystals for the application of MIT-related devices. The various nanostructures (e.g., nanowire, nanorod, nanobeam, nanosheet, nanoparticle, and nanoplate), as well as thin films, can be fabricated by using these synthesis methods.
Table 1.
Synthesis methods of nanostructured VO
2 [3,12,13,14,15].
[3][9][10][11][12].
Synthesis Method Advantages Limitations
Sol-gel
  • Simple, cheap, and precise control of variables
  • Creation of amorphous materials
  • High chemical reactivity of precursors
  • Post-deposition heat treatment
  • Long preparation and curing times
  • Poor adhesion
Hydrothermal

synthesis
  • Easy regulation of size, shape, and composition
  • Higher chemical purity
  • Expensive autoclaves
  • Longer reaction times
  • Poor adhesion
PLD 1
  • Growth of compatible and consistent films
  • No restrictions on the types of PLD targets
  • Slow process and high cost preparation
  • Not suitable for vast area deposition
  • Material loss due to evaporation
Sputtering
  • Deposition of stable and uniform films
  • Easy deposition of hybrid materials
  • Good film adhesion
  • Expensive and sophisticated equipment
  • Low purity
CVD 2
  • Growth of nanostructures with various shapes
  • High-quality crystalline
  • Small-scale crystals at low cost and in a short time
  • Rapid and inexpensive method
  • Numerous growth parameters: growth time, temperature, precursors, ambient conditions, and substrate, etc.
  • Influences of growth substrates: crystal size, shape, and orientation, etc.
1 PLD: pulsed laser deposition. 2 CVD: chemical vapor deposition.

2.2. Modulation of Physical Properties of Nanostructured VO

2

In recent years, considerable efforts have been devoted to manipulate physical properties (e.g., electrical and optical properties) of nanostructured VO2 materials for a variety of applications, such as optical switches, smart window coating, Mott transistors, memristors, sensors, and thermal actuators [12,15][9][12]. Most recently, Shi et al. [16][13] demonstrated the effective phase management of the metallic R phase and insulating phases of monoclinic (M1, M2) and triclinic (T) structures in single-crystalline VO2 microbeams through stoichiometry engineering [16][13]. The VO2 microbeam actuators showed a clear laterally asymmetric configuration and evolution of domains and deflection with increasing temperature. The formation of a radially asymmetric M2-T-M1 domain pattern led to the initial bending at the beginning of the heating stage and with a further increase in temperature, the oxygen-deficient side was gradually occupied by R domains (the oxygen-rich side is occupied by M2 domains). At 60 °C, the entire VO2 beam was transformed into the pure R phase of the straight state. As mentioned in ref. [16][13], the stoichiometry engineering, which was used to selectively stabilize all the three insulating phases (M1, T, M2) in single-crystalline VO2 microbeams, may open opportunities for designing and controlling phase inhomogeneity and domains of VO2. In addition to stoichiometry engineering, the ability to control domain structures and phase transitions of VO2 by strain or stress may lead to a deeper understanding of the correlated electron materials exhibiting the MIT, superconductivity, and magnetoresistance [15,17,18][12][14][15]. Cao et al. [19][16] demonstrated the manipulation of ordered arrays of metal (M) and insulator (I) domains along single-crystal VO2 microbeams by strain engineering, where uniaxial external stress was used to engineer M-I domains and to observe the Mott transition at room temperature [19][16]. In the stress–temperature phase diagram), when the M phase fraction η = 1 at high temperatures and high compressive stresses, the system was in pure M phase, while it was in pure I phase when η = 0 at low temperatures and high tensile stresses. The coexistence of M and I phases with the spatial arrangement and relative fraction was shown at intermediate temperatures and stresses. The epitaxial VO2 nanostructures grown on single-crystal substrates can be strongly affected by the lattice mismatch with substrate or crystal orientations, resulting in determining the relationship between the stress and strain [17,20][14][17]. The results show that substrate-dependent strains in the VO2 films result in different MIT temperatures. This suggests an enhanced ability to manipulate the MIT properties of VO2 by using lattice strain control through the implementation of a microstructured buffer layer in heteroepitaxial oxide thin films. More recently, Shin et al. [22][18] demonstrated core-shell heterostructure-enabled stress engineering on MIT, providing accommodation of uniform axial stress and control of the phase-transition pathway and properties in VO2 nanobeams. In this previous study [22][18], core-shell VO2-Al2O3 (CS-VO2) nanobeams exhibited a simple and direct M1–R phase-transition pathway at a lower temperature without the appearance of metastable intermediate phases (M2 or T), compared to pristine VO2 nanobeams with an M1–M2–R transition pathway. These results provide the unique insight that the formation of uniform stress states through core-shell architectures can be applied to the design of phase-transition paths and physical properties for VO2-based device applications using the MIT process. Meanwhile, doping in VO2 has attracted much attention as an effective way for its electrical and optical modulation for electronic and optical device applications [5,23,24,25][5][19][20][21]. Shao et al. [5] reviewed previous works by Yoon et al. [26][22] and Zou et al. [27][23]: (1) a two-step insulator (M-VO2)-to-metal (HxVO2)-to-insulator (HVO2) modulation as the hydrogen concentration increases in nano-sized Pt-island-decorated VO2 layers during annealing the samples at 120 °C, under forming gas containing 5% hydrogen gas; (2) a facile approach to hydrogenate monoclinic VO2 in an acidic solution under ambient conditions, by placing a small piece of low-work function metal (Al, Cu, Ag, Zn, or Fe) on the VO2 surface. Recently, Chet et al. [24][20] modulated the insertion/extraction of hydrogen into/from the VO2 lattice at room temperature through a solid electrolyte-assisted gating control, resulting in tristate phase transitions that enable the control of light transmittance. Strelcov et al. [25][21] proposed a new high-yield method of doping VO2 nanostructures with aluminum, which could provide possible stabilization of the monoclinic M2 phase for realization of a purely electronic Mott transition field-effect transistor. According to previous reports [28[24][25],29], uniaxial stress and doping can stabilize the M2 phase at ambient conditions. In the schematic diagram depicting phase transformations of VO2 phases by metal-ion dopants, dopants of higher oxidation states (M = W6+, Nb5+, and Mo6+) lower the transition temperature, whereas dopants of lower oxidation states (M = Cr3+, Al3+, Fe3+, or Ga3+) stabilize the M2 and T phases of VO2 at room temperature [25,30,31][21][26][27]. This behavior shows the influences of reduction and oxidation of the V4+ ions, respectively, in which the oxidation effect is similar to the effect of application of uniaxial stress along the [110] direction of the R phase [25][21].

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