(457b) Piezotronics and Piezo-Phototronics in Semiconductor Nanowires for Active Electronics and Optoelectronics

New technologies for developing electronics and optoelectronics with tunable/adaptive functionalities and performance are critical to emerging applications in wearable technology, communication, pervasive computing, human-machine interfacing and biomedical diagnostics, in which the active and adaptive interactions between devices and stimuli from the ambient or host (e. g. the human body) are essential. Mechanical stimuli are ubiquitous and abundant in the environment for interacting with or controlling these electronics and optoelectronics. This is, however, not facile to implement using the state-of-the-art technology using semiconductor materials such as silicon, which rely on electrically-gated modulation of charge carriers by externally-applied voltage to perform intelligence-bearing operations. Piezoelectric effect has been widely utilized for electromechanical sensing, actuating and energy harvesting, which produces polarization charges in response to mechanical deformation in materials lacking inversion symmetry or with polarization domains. Conventional piezoelectric materials such as PZT and PVDF are insulators and hence not feasible for constructing functional electronics/optoelectronics. The effect of mechanically-induced polarization on electronic transport and optoelectronic processes of charge carriers in piezoelectric materials has consequently been long overlooked.

Semiconductor materials such as ZnO, GaN and CdS also possess piezoelectric properties but are not as extensively utilized in piezoelectric sensors and actuators due to their relatively small piezoelectric coefficients. The coupling among piezoelectric polarization, semiconductor properties and optical excitation in piezoelectric semiconductor nanowires has resulted in both novel fundamental phenomenon and unprecedented device applications, leading to the increasing research interests in the emerging fields of piezotronics and piezo-phototronics. For wurtzite-structured semiconductor nanowires which have non-central symmetry, the piezoelectric polarization charges can effectively tune/control the charge transport across interface/junction (piezotronic effect) and modulate the optoelectronic processes, such as generation, separation, diffusion and recombination, of charge carriers (piezo-phototronic effect). Utilizing piezotronic effect, novel nanoelectronic devices such as force/pressure controlled sensors, logic units and memories have been demonstrated. Large-scale vertical nanowire transistor array has also been developed based on piezotronic effect for tactile imaging. Based on the principle of piezo-phototronic effect, optoelectronic devices such as solar cells, photon detectors and light emitting diodes fabricated using wurtzite nanowires show enhanced and tunable performance, which is not readily available in existing technologies. Large-scale array of piezoelectric nanowire light-emitting diode has also been demonstrated for mapping and imaging the distribution of applied strain through the output of light-emission.

The objective of this talk is to introduce the fundamentals of piezotronics and piezo-phototronics using ZnO nanowire as the material platform, and to give an updated progress of their applications in energy science and sensing. These studies provide the novel approach for modulating device characteristics by tuning the junction/contact properties, which has been unavailable in conventional technologies without modifying the interface structure or chemistry, and may open up opportunities in enabling technology advances in sensing, adaptive human-electronics interfacing, robotics, biomedical therapy, prosthetics, optical MEMS, efficient solid-state lighting and more.