Ferroelectret nanogenerators were recently introduced as a promising alternative technology for harvesting kinetic energy. Here we report the device’s intrinsic properties that allow for the bidirectional conversion of energy between electrical and mechanical domains; thus extending its potential use in wearable electronics beyond the power generation realm. This electromechanical coupling, combined with their flexibility and thin film-like form, bestows dual-functional transducing capabilities to the device that are used in this work to demonstrate its use as a thin, wearable and self-powered loudspeaker or microphone patch. To determine the device’s performance and applicability, sound pressure level is characterized in both space and frequency domains for three different configurations. The confirmed device’s high performance is further validated through its integration in three different systems: a music-playing flag, a sound recording film and a flexible microphone for security applications.

The search for higher-performance photothermal microactuators has typically involved unavoidable trade-offs that hinder the demonstration of ubiquitous devices with high energy density, speed, flexibility, efficiency, sensitivity, and multifunctionality. Improving some of these parameters often implies deterioration of others. Photothermal actuators are driven by the conversion of absorbed optical energy into thermal energy, which, by different mechanisms, can produce mechanical displacement of a structure. We present a device that has been strategically designed to show high performance in every metric and respond to optical radiation of selected wavelength bands. The device combines the large energy densities and sensitivity of vanadium dioxide (VO₂)–based actuators with the wavelength-selective absorption properties of single-walled carbon nanotube (SWNT) films of different chiralities. SWNT coatings increased the speed of VO₂ actuators by a factor of 2 while decreasing the power consumption by approximately 50%. Devices coated with metallic SWNT were found to be 1.57 times more responsive to red light than to near-infrared, whereas semiconducting SWNT coatings resulted in 1.42 times higher responsivities to near-infrared light than to red light. The added functionality establishes a link between optical and mechanical domains of high-performance photoactuators and enables the future development of mechanical logic gates and electronic devices that are triggered by optical radiation from different frequency bands.

The curvature of ${\text{VO}}_2$ -coated silicon microcantilevers was measured as the temperature was cycled through the coating’s insulator-to-metal transition (IMT), which drives the curvature change mainly through the strain generated during this reversible structural transformation. The films were grown by pulsed laser deposition (PLD) on heated substrates. Cantilever tip displacement was measured for a $130\mu \text{m}$ long cantilever as the temperature was changed by recording the deflection of a laser beam, and the curvature change and estimated film stress were calculated from this data. A change in curvature of over $2000{\text{m}}^{-1}$ was observed through the narrow temperature range of the IMT, with a maximum rate of $\sim 485{\text{m}}^{-1}$ per degree. Estimated recoverable stress was $\sim 1\text{GPa}$ through the transition region. These results suggest applications in actuator devices with reduced dimensions, including submicron lengths, multifunctional capabilities, and possibly with higher operational frequencies than other thermally actuated devices.

Vanadium dioxide exhibits a well-known insulator-to-metal transition during which several of its physical properties change significantly. A hysteresis loop develops for each of them as the material is heated and then cooled through the transition. In this work ${\text{VO}}_2/{\text{SiO}}_2$ samples were maintained—by heat sinking—at a selected temperature within the heating branch of the hysteresis loops for resistance and near-infrared transmittance, while brief thermal excursions of the ${\text{VO}}_2$ film were caused by either voltage pulses applied to the film or laser light pulses irradiating the film. These pulses had durations from milliseconds to a few seconds and the resulting drops in resistance or transmittance were easily and repeatably measurable without appreciably affecting their new values. A sequence of equal-duration pulses (for either equal-voltage or equal-irradiation pulses) caused the resistance and infrared transmittance to continue to drop, each time by a smaller amount, and larger energy pulses were required in order to cause drops comparable with the initial one. The ability of the film to change the values of the measurands in this manner with additional pulses was maintained up to a limit defined by the outer hysteresis curve for the measurand in question. The results presented show that a plurality of memory “states” in ${\text{VO}}_2$ can be established or “written” either by voltage pulses or by light pulses applied to the material, and queried or “read” by resistance or transmittance readings, or both. These states were found to remain stable for at least several hours, as long as temperature was kept constant, and are expected to persist indefinitely under this condition. In the all-optical case, if the same light beam is used for writing and reading the memory state, the device is an optical analog of a memristor.