We demonstrate a new optical approach to generate high-frequency (>15 MHz) and high-amplitude focused ultrasound, which can be used for non-invasive ultrasound therapy. A nano-composite film of carbon nanotubes (CNTs) and elastomeric polymer is formed on concave lenses, and used as an efficient optoacoustic source due to the high optical absorption of the CNTs and rapid heat transfer to the polymer upon excitation by pulsed laser irradiation. The CNT-coated lenses can generate unprecedented optoacoustic pressures of >50 MPa in peak positive on a tight focal spot of 75 μm in lateral and 400 μm in axial widths. This pressure amplitude is remarkably high in this frequency regime, producing pronounced shock effects and non-thermal pulsed cavitation at the focal zone. We demonstrate that the optoacoustic lens can be used for micro-scale ultrasonic fragmentation of solid materials and a single-cell surgery in terms of removing the cells from substrates and neighboring cells.

Hysteresis, which is induced by both extrinsic and intrinsic causes, is often observed in molybdenum disulphide (MoS2) field-effect transistors (FETs), and several extrinsic hysteresis effects have been reported in unpassivated bottom-gate MoS2 device structures. In this study, interface-trap-induced hysteresis and other electrical properties are examined. We experimentally investigate thermally activated trap charges near a silicon-dioxide (SiO2)-MoS2 interface that gives rise to hysteresis in a multilayer MoS2 FET in a temperature region of 10–300 K. The threshold voltage (VTH) and field-effect mobility (μFE) decrease with the increase in temperature, regardless of the gate-bias sweep direction. The hysteresis that coincides with the trend of subthreshold swing increases sharply above T = 150 K as the released charges from interface traps become dominant over the fixed charges. Based on a temperature-dependent hysteresis analysis, we discussed the activation energy of interface traps and maximum interface trap density of the fabricated multilayer MoS2 FET.

approaches. While addressing the limitations of the electrical counterparts (e.g., massive amounts of radio frequency (RF) cabling and electromagnetic (EM) interference), a photoacoustic approach for gene rat ing high-amplitude ultrasound has created new opportunities.Active research on efficient photo acoustic materials has started with the development of photoacoustic contrast mediums (e.g., exogenous contrast particles) for contrast enhancement. [3][4][5] Recent advances in nanofabrication technologies have allowed to develop more efficient photoacoustic contrast mediums, [3] such as metal nanoparticles having enhanced optical absorption due to optical resonance. Later, by coating such nanoparticles on a flat substrate, layered structures were developed for generating high-amplitude ultrasound. Among many nanoparticles, gold nanoparticles were widely used owing to strong resonance. [6] In fact, these initial efforts to increase photoacoustic conversion were focused on increasing light absorption, while the critical role of thermal expansion for photoacoustic generation were often overlooked.To enhance thermal expansion for photoacoustic generation, researchers have started to fabricate composite materials, composed of light-absorbing materials mixed with polymers with high thermal expansion coefficients. These polymer-based composites significantly enhance photoacoustic conversion efficiency. Among many polymers, polydimethylsiloxane (PDMS) is exclusively used because of its high thermal expansion, optical transparency, and acoustic impedance comparable to that of water. [7][8][9][10][11] Photoacoustic pulses generated with PDMSbased composites have the characteristics of high amplitude and high frequency, which have enabled interesting applications such as cavitation-based therapy [12] and imaging. [13] For photoacoustic composites, there are a myriad of light-absorbing nanomaterials such as metal absorbers [6] and carbon nanomaterials (e.g., carbon black (CB), [14] carbon nanotube (CNT), [7,10] carbon nanofiber (CNF), [9] candle soot (CS), [8] and reduced graphene oxide (rGO) [15] ). These light-absorbing materials of nanometer sizes have an advantage over microscale absorbing materials [7] in facilitating heat transfer from the light absorbers into the surrounding mediums. Such facilitated heat transfer is another key factor that contributes to photoacoustic energy conversion.In this Review, we will start with general considerations of photoacoustic generation in nanomaterial composites. Various photoacoustic composites enabling interesting applications will be introduced. In addition, recent efforts will be reviewed from an application perspective. Also, we will discuss outlook and future directions.Photoacoustic pulses generated by pulsed laser irradiation have the characteristics of high frequency and wide bandwidth, which are desirable for imaging and sensing. Efficient photoacoustic composites have been developed for fabricating photoacoustic transmitters capable of generating highamplitude ultrasound....