Guangmei Zhai

Preparation and characterization of SiC@CNT coaxial nanocables using CNTs as a template

Novel one-dimensional SiC@carbon nanotube (CNT) coaxial nanocables have been successfully fabricated on a large scale by using a carbothermal chemical vapor deposition method. Sol–gel-derived silica xerogels containing commercial multi-wall carbon nanotubes (MWCNT) were used as silicon and carbon sources. The obtained product was characterized by SEM, HRTEM, Raman spectroscopy and XRD. The coaxial nanocables have been found to be composed of a 40–100 nm diameter carbon nanotube as the core, surrounded by a thick SiC outlayer. The inner nanotube corresponded to the multi-walls of the carbon nanotube with a lattice spacing of 0.34 nm. The PL spectrum revealed that the SiC@CNT nanocables have two broad emission bands centered at 461 nm and 573 nm which can be attributed to the quantum confinement effect and the morphology effects. The morphology of the product was tuned by simply altering the reaction temperature. In the formation of SiC@carbon nanotube coaxial nanocables, it was proposed that CNTs acted as a template to confine the reaction, which resulted in the continuous SiC outlayer growth on the CNT surface to form SiC@CNT coaxial nanocables.

The evolution of a GaN/sapphire interface with different nucleation layer thickness during two-step growth and its influence on the bulk GaN crystal quality

The role of the nucleation layer thickness on the GaN crystal quality grown by metal organic chemical vapor deposition is explored. The surface morphologies of a low-temperature GaN nucleation layer (NL) investigated by Atomic Force Microscopy shows the nuclei grain size increases with increasing thickness. After annealing, island-like morphologies of the low-temperature GaN NL are obtained. Increasing the NL thickness is beneficial for obtaining larger island size, however, the uniformity of the island size is deteriorated. The high-resolution X-ray diffraction analysis reveals that bulk GaN crystal properties are closely connected with NL thickness, which can be well explained by the dislocation generation and propagation process in the GaN films. All the obtained results indicate that the NL thickness effectively controls the size and density of the islands and thus determines the crystal properties of GaN films.

Enhanced device performance and stability of perovskite solar cells with low-temperature ZnO/TiO2 bilayered electron transport layers

The instability of perovskite films is a major issue for perovskite solar cells based on ZnO electron transport layers (ETLs). Here, ZnO nanoparticle (NP)- and ZnO sol–gel layers capped with low-temperature processed TiO2, namely ZnO/TiO2 bilayered films, have been successfully employed as ETLs in highly efficient MAPbI3-based perovskite solar cells. It is demonstrated that these ZnO/TiO2 bilayered ETLs are not only capable of enhancing photovoltaic performance, but also capable of improving device stability. The best device based on the ZnO/TiO2 bilayered ETL exhibits an efficiency of ∼15% under standard test conditions and can retain nearly 100% of its initial efficiency after 30 days of atmosphere storage, showing much higher device performance and stability compared to those devices based on ZnO single-layer ETLs. Moreover, it is found that perovskite films and devices prepared on the single ZnO sol–gel ETLs are much superior to those deposited on the single ZnO NP-ETLs in both stability and performance, which can be ascribed to fewer surface hydroxyl groups and much smoother surface morphology of the ZnO sol–gel films. The results pave the way for ZnO to be used as an effective ETL of low-temperature processed, efficient and stable PSCs compatible with flexible substrates.

Enhanced light extraction efficiency of a InGaN/GaN micro-square array light-emitting diode chip

A InGaN/GaN micro-square array light-emitting diode (LED) chip (micro-chip) has been successfully fabricated by the focused ion beam (FIB) etching technique, which can reduce ohmic contact degradation in the fabrication process of three-dimensional (3D) structure devices. Our results show that the micro-chip exhibits a similar current–voltage performance compared to the corresponding InGaN/GaN planar LED chip (planar-chip). At the driving current of 20 mA, the output power of the micro-chip is improved by 17.8% in comparison to that of the planar-chip. A relatively broad emission and enhanced emission intensity in the perpendicular direction are obtained in angular-resolved EL (AREL) measurements for the micro-chip. Three-dimensional finite difference time domain (FDTD) simulations have also proven enhanced emitted optical energy distribution. The enhancement mechanism is correlated to the increased light extraction efficiency (LEE) of the micro-chip, mainly owing to more photons from the exposed MQWs surfaces that can be efficiently extracted by the micro-square array.