Xuefei Wu

Rebound effect of IMT properties by different doping form in Si-doped vanadium dioxide films

Vanadium dioxide is a promising material for THz modulations due to its remarkable insulator-to-metal transition (IMT) properties. Silicon-doped VO2 films, exhibiting excellent IMT properties with giant modulation amplitude and tunable phase transition temperature, greatly adapt in this area. In this paper, we report on a rebound effect of the IMT in Si-doped VO2 films. As the silicon dopants are increasingly introduced into VO2 films, the IMT is first tuned to lower temperature and then is anomalously shifted to higher temperature. This rebound effect is confirmed by crystal structure, valence concentration, and surface morphology. We attribute this rebound behavior to the interstitial and substitutive doping of Si atoms. Due to their distinct impactions on the crystallite, IMT properties of the VO2 films are depressed initially and recovered later.

Influence of infrared optical properties by transformation of the crystal structure in Al-doped vanadium dioxide films

Some infrared-active phonons in VO2 films suppress their modulation performance in the infrared region. Al-doped VO2 films, due to transforming VO2 crystal into the M2 phase, promptly eliminate absorption peaks in far-IR/THz bands and present widely modulating properties. Furthermore, we found high-frequency shifts of phonon vibration modes in Raman spectra by Al doping, indicating the stronger V-O bonds as the evidence of VO2 crystalline modification. However, although the high-frequency shifts and peak broadening were observed in V-O-V bending modes, mid-infrared spectra as the other phonon characterization show that its resonances are less involved, which is different from the remarkable variation of THz phonons. We attribute the difference to the distinct origins of phonon vibrations. As Al doped into films, the group-rotational peaks were rapidly erased with crystalline deformation whereas the high-frequency bending modes only slightly changed.

Tunable mid-infrared patch antennas based on VO2 phase transition

Abstract The metal-insulator phase transition in vanadium oxide makes it an attractive material for developing reconfigurable infrared optoelectronic components. In this paper, we present a tunable mid-infrared plasmonic patch antenna array based on vanadium oxide. The antennas consist of a circular gold patch array separated from a metallic ground plane by a film of vanadium dioxide. As the insulator-to-metal phase transition is thermally triggered, the resonances of the antenna array redshift with reduced absorbance before they are eventually switched off. The measured tuning range is about 10% of the resonant frequency, and the modulation depth in reflection is as high as 50%. A hysteresis loop in the tuning behavior is also observed. The XRD and XPS characterizations reveal a polycrystalline and multi-phase vanadium oxide. Our demonstrated tunable patch antennas hold promise for optical switching and modulating in mid-infrared applications.

Effects of annealing time on the application of vanadium dioxide films in smart windows

Vanadium dioxide (VO2) films have great potential applications in photoelectric switching, storage devices, terahertz modulators and smart windows, due to the abruptly insulator-metal phase transition (IMT) near room temperature. In this research, vanadium oxide films were deposited by DC reactive magnetron sputtering in different annealing time of 450°C on glass substrates. As for electrical properties, the increasing of annealing time turns out sheet resistance increases at first, and then decreases in insulating phase, vice versa in metallic phase. In optical properties, the visible transmittance of VO2 films initially drops with annealing time prolonging, afterwards the transmittance slightly recovers. Differences between the electrical and optical are due to the grain size. Moreover, VO2 film annealing 15 min presents excellent visible transmittance, highly near-IR modulation efficiency (about 92% at a wavelength of 1100nm) and the lowest phase transition temperature (55.7°C). This result indicates that an appropriate annealing ambient can facilitate the application of VO2 film in smart windows.

THz transmittance and electrical properties of silicon doped vanadium dioxide films tuning by annealing temperature

Silicon doped vanadium dioxide (VO2) films were successfully prepared on high purity Si(111) substrate. Confirmed by X-ray diffraction, all samples showed a preference orientation of (011) direction. Introducing silicon led grain sizes decreasing comparing to undoped VO2 film, and this result induced a narrow hysteresis width in MIT performance. Furthermore, silicon doped VO2 films annealing in different temperature presented different phase transition properties. In the electrical, a higher annealing temperature resulted in a decrease of sheet resistance and lowering the transition temperature. In terahertz optical transmittance, silicon doped VO2 films keep an excellent modulation ratio, indicating a great potential in the application of terahertz modulator devices.