Lele Fan

Strain Dynamics of Ultrathin VO2 Film Grown on TiO2 (001) and the Associated Phase Transition Modulation

Tuning the metal insulator transition (MIT) behavior of VO2 film through the interfacial strain is effective for practical applications. However, the mechanism for strain-modulated MIT is still under debate. Here we directly record the strain dynamics of ultrathin VO2 film on TiO2 substrate and reveal the intrinsic modulation process by means of synchrotron radiation and first-principles calculations. It is observed that the MIT process of the obtained VO2 films can be modulated continuously via the interfacial strain. The relationship between the phase transition temperature and the strain evolution is established from the initial film growth. From the interfacial strain dynamics and theoretical calculations, we claim that the electronic orbital occupancy is strongly affected by the interfacial strain, which changes also the electron-electron correlation and controls the phase transition temperature. These findings open the possibility of an active tuning of phase transition for the thin VO2 film through the interfacial lattice engineering.

Growth and phase transition characteristics of pure M-phase VO2 epitaxial film prepared by oxide molecular beam epitaxy

VO2 epitaxial film with large size has been prepared by oxide-molecular beam epitaxy method on Al2O3 (0001) substrate. The VO2 film shows a perfect crystal orientation, uniformity, and distinct metal-insulator phase transition (MIT) characteristics. It is observed that the MIT character is closely associated with the crystal defects such as oxygen vacancies. By controlling the growth condition, the MIT temperature can be tuned through modifying the content of oxygen vacancies. The role of the oxygen vacancies on the phase transition behavior of this VO2 film is discussed in the framework of the hybridization theory and the valence state of vanadium.

Control of the Metal-Insulator Transition in VO2 Epitaxial Film by Modifying Carrier Density

External controlling the phase transition behavior of vanadium dioxide is important to realize its practical applications as energy-efficient electronic devices. Because of its relatively high phase transition temperature of 68 °C, the central challenge for VO2-based electronics, lies in finding an energy efficient way, to modulate the phase transition in a reversible and reproducible manner. In this work, we report an experimental realization of p-n heterojunctions by growing VO2 film on p-type GaN substrate. By adding the bias voltage on the p-n junction, the metal-insulator transition behavior of VO2 film can be changed continuously. It is demonstrated that the phase transition of VO2 film is closely associated with the carrier distribution within the space charge region, which can be directly controlled by the bias voltage. Our findings offer novel opportunities for modulating the phase transition of VO2 film in a reversible way as well as extending the concept of electric-field modulation on other phase transition materials.

Decoupling the Lattice Distortion and Charge Doping Effects on the Phase Transition Behavior of VO2 by Titanium (Ti4+) Doping

The mechanism for regulating the critical temperature (TC) of metal-insulator transition (MIT) in ions-doped VO2 systems is still a matter of debate, in particular, the unclear roles of lattice distortion and charge doping effects. To rule out the charge doping effect on the regulation of TC, we investigated Ti4+-doped VO2 (TixV1-xO2) system. It was observed that the TC of TixV1-xO2 samples first slightly decreased and then increased with increasing Ti concentration. X-ray absorption fine structure (XAFS) spectroscopy was used to explore the electronic states and local lattice structures around both Ti and V atoms in TixV1-xO2 samples. Our results revealed the local structure evolution from the initial anatase to the rutile-like structure around the Ti dopants. Furthermore, the host monoclinic VO2 lattice, specifically, the VO6 octahedra would be subtly distorted by Ti doping. The distortion of VO6 octahedra and the variation of TC showed almost the similar trend, confirming the direct effect of local structural perturbations on the phase transition behavior. By comparing other ion-doping systems, we point out that the charge doping is more effective than the lattice distortion in modulating the MIT behavior of VO2 materials.

Synchrotron radiation study of VO2 crystal film epitaxial growth on sapphire substrate with intrinsic multi-domains

The growth behavior of VO2 crystal film deposited on Al2O3 (0001) monocrystalline substrate by pulsed laser deposition was investigated by high-resolution synchrotron radiation X-ray diffraction (XRD). φ-scan XRD confirmed the in-plane epitaxial matching relation. Furthermore, fine structures observed in the φ-scan indicated that each main peak contained two additional satellites in both the inclined (220) plane and some other vertical planes. A growth model for this observation was proposed based on the intrinsic multi-domain growth of the VO2 crystal at the interface. This observation will give some insights in VO2 epitaxial growth on the hexagonal substrate system.

Metal–insulator transition in V1−xWxO2: structural and electronic origin

The driving mechanism of the metal-insulator transition (MIT) in VO(2) has always attracted attention, in particular with regards to understanding if and how the doping mechanism may tune the MIT transition temperature. However, due to the lack of detailed local structural information, in this oxide the underlying MIT mechanism is still matter of debate. In this contribution on the V(1-x)W(x)O(2) system, we attempt to clarify the origin of the MIT induced by tungsten doping. Combining W L(3)-edge and V K-edge extended X-ray absorption fine-structure (EXAFS) spectroscopy, the local structures around both V and W have been obtained. The data point out the occurrence of internal stress along the V-V chains induced by doping. It reaches a critical value that remains constant during the transition. The main effect of the internal stress on the vanadium local structure has also been identified. Actually, upon increasing the dopant concentration, the tilt of the V-V pairs towards the apex oxygen atoms in the VO(6) octahedron decreases while the V-V bond lengths remain unchanged. The electronic structure has also been investigated by O K-edge X-ray absorption near-edge structure (XANES) spectroscopy. Actually, at high doping concentrations the interaction of O(2p) and the V d(∥) state increases, while the hybridization of O(2p) and V π* decreases. The O(2p)-V(3d) hybridization is therefore an essential parameter correlated with the decreasing transition temperature in the V(1-x)W(x)O(2) system.

Spectroscopic analysis of phase constitution of high quality VO2 thin film prepared by facile sol-gel method

VO2 thin films with large-area were prepared on Al2O3 substrates by a simple sol-gel method. After an annealing treatment under low vacuum condition, all the VO2 films showed a preferred growth direction and exhibited excellent semiconductor-metal transition (SMT) characteristics. The structure and electrical properties of the obtained VO2 films were investigated systematically. Raman spectra, X-ray diffraction and X-ray absorption spectra measurements pointed out that the VO2 film on Al2O3(101¯0) substrate showed a M1 phase instead of M2 phase as reported in previous studies. Based on the experiment results, it was suggested that the strained structure of oriented VO2 films could be a mechanism for the formation of the intermediate M2 phase, whereas it is difficult to access the pure M2 phase of undoped VO2 films. VO2 film on Al2O3101¯0 substrate showed a lower SMT temperature compared to VO2 film on Al2O3 (0001), which can be mostly attributed to the differences of both lattice mismatch and thermal stre...

Few-layer graphene growth on 6H-SiC(0001) surface at low temperature via Ni-silicidation reactions

Few-layer graphene (FLG) has been prepared by thermal annealing of SiC crystal via the surface Ni-silicidation reactions. Results reveal that the temperature plays an important role for the final FLG quality and the optimized annealing temperature is about 800 °C. The investigation of surface morphology and microstructure for the FLG sample indicates that after the rapid cooling, the carbon atoms will segregate to form the FLG layer and the NiSix particles will congregate on the top surface. The mechanism of the FLG formation on SiC surface assisted by the Ni ultra-thin layer is briefly discussed based on the experimental results.

Comprehensive studies of interfacial strain and oxygen vacancy on metal–insulator transition of VO2 film

As a typical strong correlation material, vanadium dioxide (VO2) has attracted wide interest due to its particular metal-insulator transition (MIT) property. However, the relatively high critical temperature (T c) of ~68 °C seriously hinders its practical applications. Thus modulating the phase transition process and decreasing the T c close to room temperature have been hot topics for VO2 study. In the current work, we conducted a multi-approach strategy to control the phase transition of VO2 films, including the interfacial tensile/compressive strain and oxygen vacancies. A synchrotron radiation reciprocal space mapping technique was used to directly record the interfacial strain evolution and variations of lattice parameters. The effects of interfacial strain and oxygen vacancies in the MIT process were systematically investigated based on band structure and d-orbital electron occupation. It was suggested that the MIT behavior can be modulated through the combined effects of the interfacial strain and oxygen vacancies, achieving the distinct phase transition close to room temperature. The current findings not only provide better understanding for strain engineering and oxygen vacancies controlling phase transition behavior, but also supply a combined way to control the phase transition of VO2 film, which is essential for VO2 film based device applications in the future.

Thermally driven V2O5 nanocrystal formation and the temperature-dependent electronic structure study

The temperature sensitive crystallization behavior of amorphous V2O5 films on glass and silicon substrates was investigated. Results indicated that after annealing in oxygen ambience, the as-sputtered amorphous V2O5 films on glass substrates dramatically transformed to standing β-phase V2O5 nanorods and flat-lying nanoslices, while large V2O5 particles with α-phase were obtained on Si substrates. Thermally driven surface diffusion was considered as the crystal growth mechanism and different crystallization behavior on the glass or silicon substrate was attributed to the distinct initial surface microstructures. The temperature dependent electronic properties of V2O5 on glass were investigated by synchrotron radiation, which clearly shows the anisotropic structure characteristics of crystallized V2O5 compound.