Zehua Huang

Optimization of metal-to-insulator phase transition properties in polycrystalline VO2 films for terahertz modulation applications by doping

Vanadium dioxide (VO2), due to its well-known metal–insulator phase transition (MIT), is a promising candidate to realize optical modulation devices operating at terahertz (THz) frequencies. Moreover, the application of VO2 on modulation devices requires a narrow hysteresis width associated with a significant change in its optical properties, which is quite challenging in Si-based polycrystalline VO2 films. In this paper, by doping high-valence metal ions (W6+ or Nb5+) into polycrystalline VO2 films, a narrowed hysteresis width and a decreased phase transition temperature are observed. Intriguingly, these doped VO2 films always maintain a high THz field modulation depth despite the low phase transition temperature resulting from the usage of either W or Nb dopants. To sum up, the optimized VO2 film with 6.5% Nb doping deposited on high-purity silicon substrates exhibits the best MIT characteristics with a giant field THz modulation depth of 62.5%, a small hysteresis width down to 4.8 °C and a low phase transition temperature of around 31.1 °C, which is very excellent for practical applications. Furthermore, we synthetically investigate the influences of W and Nb doping on the microstructures and MIT characteristics of the polycrystalline VO2 films. Doping Nb5+ and W6+ ions has similar effects through a similar mechanism. In addition, the excellent balance between the THz modulation ability and phase transition temperature is related to the high crystallinity degree of the doped films. The annealing process may play a key role in this peculiar case. These results show that the excellent MIT properties of the polycrystalline VO2 films can be effectively tailored by our distinctive preparation method, which provides a feasible solution to the design and fabrication of VO2 films with suitable MIT properties for THz devices.

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.

Carbon nanotube-graphene junctions studied by impedance spectra

Two kinds of carbon nanotube (CNT)-graphene structures, vertical CNT-graphene and paralleled CNT–graphene, were fabricated to investigate the geometrical effect on the transport properties of the CNT–graphene junctions by using AC impedance spectra. The results demonstrated that the geometrical structure showed obvious impact on the resistance rather than the capacity of the junction. It is proposed that the difference caused by the geometrical structure may be associated with the dangling bonds terminated by –OH or –COOH of the open-ended CNTs. The unsymmetrical chemical bonds will increase the dipole moment in CNTs, which enhance the interaction between vertical CNTs and graphene and reduce the contact resistance.

Preparation and Characterization of LiTaO3 Films Derived by an Improved Sol-Gel Process

In this paper, an improved sol-gel method was suggested to obtain high-concentration LiTaO3 precursor solution for simplified experimental conditions and thicker films, by mixing lithium acetate and tantalum ethoxide in a 1, 2-Propylene glycol solution. Compared to traditional methods, the process was done without weak acidic solution and absolute dry experimental condition. Results of a comparative study of LiTaO3 thin films derived by the improved sol-gel process and a traditional process using 2-methoxy ethanol as solvent were presented. Nano-crystalline LiTaO3 films with rhombohedral structures were formed in both methods after annealing at 650 ° for 5 min. The thickness of each LiTaO3 layer coated onto the substrate increased from 25 nm to 110 nm when 2-methoxy ethanol was replaced by 1, 2-Propylene glycol. LiTaO3 films with a stronger preferential orientation were obtained in 1, 2-Propylene glycol due to its higher boiling point and slower volatilization rate. On the other hand, the diffraction peak intensity of LiTaO3 thin films prepared using 1, 2-Propylene glycol was weaker than that of the films prepared using 2-methoxy ethanol due to decreased times of annealing.

Broadband terahertz modulator based on graphene metamaterials

Tunable complementary split ring resonators (CSRRs) based on monolayer graphene are presented in terahertz regime. By applying different gate voltage, the Fermi level and optical conductivity of monolayer graphene pattern can be changed. Here, we employ a numerical simulation to study the interaction of light with graphene CSRRs. The results indicate that the extinction in transmission becomes stronger, and the resonance frequency presents blue shift with higher Fermi level of the graphene pattern. Three pronounced resonant peaks appear which can be modulated dynamically in the range of 1-2THz and 3-7THz, and realizing dynamic broadband terahertz modulation, the modulation depth exceeds 85% at all three resonant peaks, the highest modulation depth reaches 98.8% at 7.47THz.

High thermochromic performance of Fe/Mg co-doped VO2 thin films for smart window applications

In the field of energy-efficient smart windows, vanadium dioxide (VO2) is a promising material due to its reversible metal–insulator transition. However, the development of VO2-based smart windows has been restricted by several drawbacks in performance, including high phase transition temperature (TC), low luminous transmittance (Tlum), insufficient solar energy modulation ability (ΔTsol) and unpopular color. In this paper, Fe/Mg co-doping is proposed for the first time to improve the thermochromic performance of VO2 films. Interestingly, the Fe/Mg co-doped films can exhibit outstanding thermochromic performance with a balanced solar regulation efficiency of 12.8%, suitable luminous transmittance up to 42.1%, and low phase transition temperature around 38.2 °C, which is very promising for practical applications. In addition, the color of films is modified to increase their brightness and lighten the brown color. Different from previous work on co-doped VO2 focusing on W6+ and low-valence (<4+) elements, Fe3+/Mg2+ co-doping is found to have the synergistic combination of the advantages of single doping with Fe and Mg by getting rid of the neutralization between charge carriers e− and h+. Such a feature is especially desired for optimizing the thermochromic performance with co-doping methods. This study provides a new solution for improving the optical properties of VO2 films for smart window applications.

Graphene/Organic Semiconductor Heterojunction Phototransistors with Broadband and Bi‐directional Photoresponse

A graphene-semiconductor heterojunction is very attractive for realizing highly sensitive phototransistors due to the strong absorption of the semiconductor layer and the fast charge transport in the graphene. However, the photoresponse is usually limited to a narrow spectral range determined by the bandgap of the semiconductor. Here, an organic heterojunction (C60 /pentacene) is incorporated on graphene to realize a broadband (405-1550 nm) phototransistor with a high gain of 5.2 × 105 and a response time down to 275 µs. The visible and near-infrared parts of the photoresponsivity (9127 A W-1 @650 nm and 1800 A W-1 @808 nm) come from the absorption of the organic layer and the graphene, respectively. For the first time, a bi-directional (positive and negative) photoresponse is demonstrated at different wavelengths, due to the opposite charge transfer direction of the photoexcited carriers enforced by the unique band alignment. Such tunability will enable new functionalities such as large-scale real-time optical image and infrared focal plane array detection in the future.

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.

Synthesis and characterization of lithium tantalate thin films fabricated by sol-gel method

Multilayer lithium tantalate thin films have been successfully prepared on Pt/Ti/SiO2/Si(100) substrate using sol-gel and spin-coating method. Polycrystalline perovskite films were obtained through pyrolysis at 400°C and subsequent calcination at various temperatures from 500°C to 700°C for 1h, which were optimized by means of X-ray diffraction, Raman spectroscopy and thermal analysis. The morphology on the top surface and fractured cross section of LT films was observed by a field-emission scanning electron microscope. The 300 nm thick LT film annealed at 650°C exhibited a dielectric constant of 19.8 and a loss tangent of 0.06 at10KHz.