Abstract-Synthesis of novel rambutan-like graphene@aluminum composite spheres and non-destructive terahertz characterization

Zhongbo Yang, Shuanglong Feng, Wei Yao, Jiaguang Hanc, Huabin Wang

https://pubs.rsc.org/en/Content/ArticleLanding/2019/RA/C8RA09129C#!divAbstract

Graphene reinforced Al (graphene@Al) spheres were synthesized using microwave plasma chemical vapor deposition technique in which H2, CH4, and Ar were used as the reduced gas, carbon source, and plasma enhancement gas, respectively. The obtained graphene@Al spheres presented a rambutan-like structure and had a graphene shell wrapped on the sphere surface, which was proved by scanning electron microscopy, X-ray diffraction, X-ray photoelectron spectroscopy, and Raman spectroscopy. The thickness of the graphene shell on the Al sphere is difficult to be characterized by conventional techniques. However, it was successfully measured with a sophisticated terahertz (THz) time-domain spectroscopic technique. To the best of our knowledge, neither have graphene@Al spheres been synthesized before nor has a THz-based technique been exploited to characterize the thickness of a shell structure. Therefore, the present work sheds useful insights on both the rational synthesis and non-destructive characterization of graphene reinforced functional structures.

Abstract-Terahertz transmittance and metal-insulator phase transition properties of M2 phase VO2 films induced by Cr doping

Chunhui Ji, 

Zhiming Wu,  

Xuefei Wu, 

Jun Wang,  

Xianchao Liu, 

Jun Gou, 

Hongxi Zhou,

Wei Yao, 

Yadong Jiang, 

https://www.sciencedirect.com/science/article/pii/S0169433218313849

In this work, we investigate the terahertz transmittance and metal-insulator phase transition properties of M2 phase VO₂ films induced by Cr doping. Firstly, Cr-doped VO₂films were successfully produced on silicon substrates using DC reactive magnetron sputtering and then fully characterized by XRD, XPS, Raman shift and SEM tests. These results demonstrate the formation of M2 phase and the effect of Cr doping on composition, crystal structure and surface morphology of VO₂ films. Compared with undoped VO₂, Cr doping significantly enhances the amplitude modulation in the frequency of phonon-absorption peaks by the suppression of infrared-active phonon modes. More concretely, the amplitude modulation is largely increased from 29.6% to 39.0% at 8.25 THz and from 24.1% to 37.1% at 9.33 THz, respectively. This enhancement strongly contributes to the development of VO₂-based devices in high THz range. In addition, Cr doped VO₂ films exhibit outstanding metal-insulator phase transition properties with very narrower hysteresis width (8.0 °C) as well as smaller transition sharpness (3.4 °C) than undoped film, which is ascribed to the increase of heterogeneous nucleation site density and the transformation of crystal structure from M1 phase to M2 phase, respectively. This work indicates that VO₂ films with suitable Cr doping concentration have great potential for THz modulation applications.

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

Chunhui Ji,  Zhiming Wu,  Xuefei Wu,  Haoqian Feng,  Jun Wang,  Zehua Huang,  Hongxi Zhou,  Wei Yao,  Jun Gou  and  Yadong Jiang 

http://pubs.rsc.org/en/content/articlelanding/2018/tc/c7tc05536f#!divAbstract

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 associating with a significant change in optical properties while it is quite challenging for 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 high THz field modulation depth despite the low phase transition temperature with using 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 around 31.1 °C, which is very excellent for practical application. Furthermore, we synthetically investigate the influences of W and Nb doping on the microstructures and MIT characteristics of polycrystalline VO2 films. Doping Nb5+ and W6+ ions has similar effects through the similar mechanism. In addition, the excellent balance between THz modulation ability and phase transition temperature is related to the high crystallinity degree of doped films. The annealing process may play a key role in this peculiar case. These results show that the excellent MIT properties of 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.