Transport property variation in O₂ plasma treated graphene and related enhancement in NH₃ sensing.

Real-time toxic gas mapping in complex urban environments have become increasingly possible with improvements in data analysis and network infrastructures. Hindering this is the cost and operation requirements of commercial gas sensors, requiring sensors with high sensitivity and selectivity that are robust and capable of operating at room temperature. Transition metal oxide-based sensors are of historical significance in the production of commercial gas sensors due to their low cost and high selectivity to target gases. The low inherent conductivity of metal oxides, however, requires operating temperatures higher than 150 °C, limiting their operation to controlled environments. To overcome this limitation, heterostructures have been formed between graphene and transition metal oxides, seeking to couple the conductivity of graphene with the reactivity of transition metal oxides. Among these transition metal oxides, manganese dioxide exhibits unique properties that can be leveraged to improve gas sensing. Its wide variety of synthesized structural polymorphs (1x 1 tunnel (β ), 1x 2 tunnel (α ), spinel (ɣ ), and layered (δ )) allow for control over the available reactive surface area to enhance gas response. By utilizing defect rich δ -phase, the reactivity of the material can be improved. Here we present a δ -MnO₂/epitaxial graphene/silicon carbide heterostructure for use as a room temperature gas sensor. We confirm the composition through Raman spectroscopy and surface morphology through scanning electron microscopy and atomic force microscopy. We then demonstrate its room-temperature detection by testing against NO₂, NH₃, IPA, and CH₃OH at room temperature. [2020-0144]

Strong enhancement in a photoacoustic signal due to plasmonic absorption in Au nanostructures was measured using piezotransistive GaN microcantilevers. A pulsed 790 nm laser focused on the Au metallization of the piezotransistor resulted in a much larger photoacoustic signal compared to the non-metallized areas. Upon deposition of a 5 nm Au layer, the photoacoustic signal increased significantly for both previously metallized and non-metallized areas, while 2 nm Ni deposition decreased the photoacoustic signal, confirming the role of Au nanostructures in facilitating plasmonic absorption. Infrared microscopy images covering the boundary of Au metallized and non-metallized surfaces indicated a much larger rise in temperature of the former region with laser exposure, explaining the generation of photoacoustic signals through plasmonic absorption.

Nonlinear oscillations in micro- and nanoelectromechanical systems have emerged as an exciting research area in recent years due to their promise in realizing low-power, scalable, and reconfigurable mechanical memory and logic devices. Here, we report ultralow-power mechanical memory operations utilizing the nonlinear oscillation regime of GaN microcantilevers with embedded piezotransistive AlGaN/GaN heterostructure field effect transistors as highly sensitive deflection transducers. Switching between the high and low oscillatory states of the nonlinear oscillation regime was demonstrated using a novel phase-controlled opto-mechanical excitation setup, utilizing a piezo actuator and a pulsed laser as the primary and secondary excitation sources, respectively. Laser-based photoacoustic excitation was amplified through plasmonic absorption in Au nanoparticles deposited on a transistor. Thus, the minimum switching energy required for reliable memory operations was reduced to less than a picojoule (pJ), which translates to one of the lowest ever reported, when normalized for mass.

Surface work function (SWF) measurements using a piezotransistive III–nitride cantilever has been demonstrated on multiple surfaces. The minimum detectable surface potential change of 10 mV was achieved with a signal to noise ratio of 3. This method was applied to determine the surface potential changes due to exposure of 5 ppm NO ₂ in graphene and In ₂ O ₃ thin film, simultaneously with conductivity changes. The potentiometric measurements yielded 100 and 80 mV potential changes in SWFs of graphene and In ₂ O ₃ respectively, which matches very well with experimental data published earlier indicating the efficacy of this readily miniaturizable measurement technique.