Mechanical memory operations in piezotransistive GaN microcantilevers using Au nanoparticle-enhanced photoacoustic excitation.

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.

Voltage triggered near-infrared light modulation using VO2 thin film.

Development of compact and fast modulators of infrared light has garnered strong research interests in recent years due to their potential applications in communication, imaging, and sensing. In this study, electric field induced fast modulation near-infrared light caused by phase change in VO2 thin films grown on GaN suspended membranes has been reported. It was observed that metal insulator transition caused by temperature change or application of electric field, using an interdigitated finger geometry, resulted in 7% and 14% reduction in transmitted light intensity at near-infrared wavelengths of 790 and 1550 nm, respectively. Near-infrared light modulation has been demonstrated with voltage pulse widths down to 300 µs at 25 V magnitude. Finite element simulations performed on the suspended membrane modulator indicate a combination of the Joule heating and electric field is responsible for the phase transition.

H2 Detection Using Plasmonically Generated Surface Photoacoustic Waves in Pd Nanoparticle-Deposited GaN Microcantilevers.

Detection of H2 using plasmonic amplification of surface photoacoustic (SPA) waves generated in Pd nanoparticle-deposited GaN piezotransistive microcantilevers has been investigated using a pulsed 520 nm laser. Using 1.5 nm thickness of the Pd functionalization layer, H2 detection down to 1.5 ppm was demonstrated with a high signal-to-noise ratio, underscoring the feasibility of sub-ppm level detection using this novel sensing method. Adsorption of H2 in Pd nanoparticles (NPs) changes their plasmonic absorption spectra because of Pd lattice expansion, in addition to changing their work function. The high sensitivity exhibited by the SPA-based H2 detection method is attributed to a combination of changes in the plasmonic spectrum and work function of Pd NPs and was observed to be a strong function of Pd thickness, biasing conditions, and probe laser power. A comparison of the SPA-based detection technique with traditional chemidiode and chemiresistor sensors, integrated in the functionalized piezotransistor, indicated a superior detection performance of the former.

Investigation of AlGaN/GaN HFET and VO2 Thin Film Based Deflection Transducers Embedded in GaN Microcantilevers.

The static and dynamic deflection transducing performances of piezotransistive AlGaN/GaN heterojunction field effect transistors (HFET) and piezoresistive VO2 thin films, fabricated on GaN microcantilevers of similar dimensions, were investigated. Deflection sensitivities were tuned with the gate bias and operating temperature for embedded AlGaN/GaN HFET and VO2 thin film transducers, respectively. The GaN microcantilevers were excited with a piezoactuator in their linear and nonlinear oscillation regions of the fundamental oscillatory mode. In the linear regime, the maximum deflection sensitivity of piezotransistive AlGaN/GaN HFET reached up to a 0.5% change in applied drain voltage, while the responsivity of the piezoresistive VO2 thin film based deflection transducer reached a maximum value of 0.36% change in applied drain current. The effects of the gate bias and the operation temperature on nonlinear behaviors of the microcantilevers were also experimentally examined. Static deflection sensitivity measurements demonstrated a large change of 16% in drain-source resistance of the AlGaN/GaN HFET, and a similarly high 11% change in drain-source resistance in the VO2 thin film, corresponding to a 10 µm downward step bending of the cantilever free end.

Photoacoustic Detection of H2 and NH3 Using Plasmonic Signal Enhancement in GaN Microcantilevers.

Photoacoustic (PA) detection of H2 and NH3 using plasmonic excitation in Pt- and Pd-decorated GaN piezotransistive microcantilevers were investigated using pulsed 520-nm laser illumination. The sensing performances of 1-nm Pt and Pd nanoparticle (NP) deposited cantilever devices were compared, of which the Pd-coated sensor devices exhibited consistently better sensing performance, with lower limit of detection and superior signal-to-noise ratio (SNR) values, compared to the Pt-coated devices. Among the two functionalization layers, Pd-coated devices were found to respond only to H2 exposure and not to NH3, while Pt-coated devices exhibited repeatable response to both H2 and NH3 exposures, highlighting the potential of the former in performing selective detection between these reducing gases. Optimization of the device-biasing conditions were found to enhance the detection sensitivity of the sensors.

Direct measurement of K+ ion efflux from neuronal cells using a graphene-based ion sensitive field effect transistor.

A graphene-based ion sensitive field effect transistor (GISFET) has been developed and investigated in terms of its ion sensing performance. The GISFET sensor was found to demonstrate a high detection sensitivity enabling direct measurement of K+ ion efflux from live cells. The sensing performance of the GISFET was directly compared to that of a commercial Si ISFET and very similar detection results were obtained, highlighting the promise of the GISFET sensor for ion-sensing applications. Additionally, fabrication of a GISFET array containing 25 devices using a CMOS compatible photolithographic process was demonstrated, which resulted in good uniformity across the array and high ion sensing properties of the devices, underlining their application potential for simultaneous multi-well testing with small sample volume.

Impact of oxygen plasma treatment on carrier transport and molecular adsorption in graphene.

Impact of plasma treatment on graphene's transport properties and interaction with gas molecules has been investigated with Raman, X-ray photoelectron spectroscopy, and Hall measurements. Experimental results indicate the formation of nanocrystalline domains and the enhanced fraction of adsorbed oxygen following oxygen plasma treatment, which correlates with a significant reduction in carrier mobility and an increase in carrier density. The oxygen plasma treated graphene was found to exhibit much stronger sensitivity toward NH3 molecules both in terms of magnitude and response rate, attributable to increased domain edges and oxygen adsorption related enhancement in p-type doping. The carrier mobility in plasma exposed graphene was modeled considering both ionized impurity and short-range scattering, which matched well with the experimentally observed mobility.