Surface acoustic waves (SAWs) propagating on piezoelectric substrates offer a convenient, contactless approach to probing the electronic properties of low-dimensional charge carrier systems such as graphene nanoribbons (GNRs). SAWs can also be used to transport and manipulate charge for applications such as metrology and quantum information. In this work, we investigate the acoustoelectric effect in GNRs, and show that an acoustoelectric current can be generated in GNRs with physical widths as small as 200 nm at room temperature. The positive current in the direction of the SAWs, which corresponds to the transportation of holes, exhibits a linear dependence on SAW intensity and frequency. This is consistent with the description of the interaction between the charge carriers in the GNRs and the piezoelectric fields associated with the SAWs being described by a relatively simple classical relaxation model. Somewhat counter-intuitively, as the GNR width is decreased, the measured acoustoelectric current increases. This is thought to be caused by an increase of the carrier mobility due to increased doping arising from damage to the GNR edges.

The spatial variation of thermal emission from large area graphene grown by chemical vapor deposition, transferred onto SiO2/Si substrates and fabricated into field effect transistor structures, has been investigated using infra-red microscopy. A peak in thermal emission occurs, the position of which can be altered by reversal of the current direction. The experimental results are compared with a one dimensional finite element model, which accounts for Joule heating and electrostatic effects, and it is found that the thermal emission is governed by the charge distribution in the graphene and maximum Joule heating occurs at the point of minimum charge density.

The acoustoelectric current in graphene has been investigated as a function of illumination, using blue (450 nm) and red (735 nm) light-emitting diodes (LEDs), and surface acoustic wave (SAW) intensity and frequency. The measured acoustoelectric current increases with illumination, more than the measured change in the conductivity of the graphene, whilst retaining a linear dependence on the SAW intensity. The latter is consistent with the interaction between the carriers and SAWs being described by a relatively simple classical relaxation model suggesting that the change in the acoustoelectric current is caused by the effect of the illumination on the electronic properties of the graphene. The increase in the acoustoelectric current is greatest under illumination with the blue LED, consistent with the creation of a hot electron distribution.

The acoustoelectric current in graphene nanoribbons, with widths ranging between 350 nm and 600 nm, has been investigated as a function of illumination. For all nanoribbon widths, the acoustoelectric current was observed to decrease on illumination, in contrast to the increase in acoustoelectric current measured in unpatterned graphene sheet devices. This is thought to be due to the higher initial conductivities of the nanoribbons compared to unpatterned devices.

Surface acoustic waves (SAWs) are elastic waves that can be excited directly on the surface of piezoelectric crystals using a transducer, leading to their exploitation for numerous technological applications, including for example microfluidics. Recently, the concept of SAW streaming, which underpins SAW microfluidics, was extended to make the first experimental demonstration of ‘SAW swimming’, where instead of moving water droplets on the surface of a device, SAWs are used as a propulsion mechanism. Using theoretical analysis and experiments, we show that the SAW swimming force can be controlled directly by changing the SAW frequency, due to attenuation and changing force distributions within each SAW streaming jet. Additionally, an optimum frequency exists which generates a maximum SAW swimming force. The SAW frequency can therefore be used to control the efficiency and forward force of these SAW swimming devices. The SAW swimming propulsion mechanism also mimics that used by many microorganisms, where propulsion is produced by a cyclic distortion of the body shape. This improved understanding of SAW swimming provides a test-bed for exploring the science of microorganism swimming, and could bring new insight to the evolutionary significance for the length and beating frequency of swimming microbial flagella.

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