We demonstrate a highly selective and reversible NO resistive gas sensor using vertically aligned MoS (VA-MoS) flake networks. We synthesized horizontally and vertically aligned MoS flakes on SiO/Si substrate using a kinetically controlled rapid growth CVD process. Uniformly interconnected MoS flakes and their orientation were confirmed by scanning electron microscopy, x-ray diffraction, Raman spectroscopy and x-ray photoelectron spectroscopy. The VA-MoS gas sensor showed two times higher response to NO compared to horizontally aligned MoS at room temperature. Moreover, the sensors exhibited a dramatically improved complete recovery upon NO exposure at its low optimum operating temperatures (100 °C). In addition, the sensing performance of the sensors was investigated with exposure to various gases such as NH, CO, H, CH and HS. It was observed that high response to gas directly correlates with the strong interaction of gas molecules on edge sites of the VA-MoS. The VA-MoS gas sensor exhibited high response with good reversibility and selectivity towards NO as a result of the high aspect ratio as well as high adsorption energy on exposed edge sites.

A nucleation controlled one‐step process to synthesize MoS2–MoO3 hybrid microflowers using vapor transport process and its application in efficient NO2 sensing at room temperature are reported. The morphology and crystal structure of the microflowers are characterized by scanning electron microscope (SEM), Raman, X‐ray diffraction (XRD), and X‐ray photoelectron spectroscopy techniques. A cathodoluminence mapping reveals that the core of the microflower consists of MoO3, and the flower petals as well as nanosheet are composed of a few layers of MoS2. Further, the MoS2–MoO3 hybrid microflower sensor exhibits a high sensitivity of ≈33.6% with a complete recovery to 10 ppm NO2 at room temperature without any extra stimulus like optical or thermal source. Unlike many earlier reports on MoS2 sensor, this advanced approach shows that the sensor is exhibited a low response time (≈19 s) with complete recovery at room tepmerature and excellent selectivity toward NO2 against various other gases. The efficient conventional sensing of the sensor is attributed to a combination of high hole injection from MoO3 to MoS2 and modulation of a potential barrier at MoS2–MoO3 interface during adsorption/desorption of NO2. It is believed that the modified properties of MoS2 by such composite could be used for various advanced device applications.

A comprehensive analysis of oxygen chemisorption on epitaxial gallium nitride (GaN) films grown at different substrate temperatures via RF-molecular beam epitaxy was carried out. Photoemission (XPS and UPS) measurements were performed to investigate the nature of the surface oxide and corresponding changes in the electronic structure. It was observed that the growth of GaN films at lower temperatures leads to a lower amount of surface oxide and vice versa was observed for a higher temperature growth. The XPS core level (CL) and valence band maximum (VBM) positions shifted towards higher binding energies (BE) with oxide coverage and revealed a downward band bending. XPS valence band spectra were de-convoluted to understand the nature of the hybridization states. UPS analysis divulged higher values of electronic affinity and ionization energy for GaN films grown at a higher substrate temperature. The surface morphology and pit structure were probed via microscopic measurements (FESEM and AFM). FESEM and AFM analysis revealed that the film surface was covered with hexagonal pits, which played a significant role in oxygen chemisorption. The favourable energetics of the pits offered an ideal site for oxygen adsorption. Pit density and pit depth were observed to be important parameters that governed the surface oxide coverage. The contribution of surface oxide was increased with an increase in average pit density as well as pit depth.

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