Abstract
Studying nanomaterial piezoelectricity and triboelectricity is attractive for energy and sensing applications. However, quantitative characterisation of electromechanical effects in nanomaterials is challenging due to practical limitations and possible combination of effects, resulting in contradicting reports at times. When it comes to piezoelectricity at the nanoscale, piezoresponse force microscopy (PFM) is the default characterisation tool. In PFM the converse piezoelectric effect is measured - the conversion from electrical signal to mechanical response. However, there is an underlying desire to measure the direct piezoelectric effect - conversion of mechanical deformation to an electrical signal. This corresponds to energy harvesting and sensing. Here we present time-resolved open-circuit conductive atomic force microscopy (cAFM) as a new methodology to carry out direct electromechanical characterisation. We show, both theoretically and experimentally, that the standard short-circuit cAFM mode is inadequate for piezoelectric characterisation, and that resulting measurements are governed by competing mechanisms. We apply the new methodology to nanowires of GaAs, an important semiconductor, with relatively low piezoelectric coefficients. The results suggest that time-resolved operation distinguishes between triboelectric and piezoelectric signals, and that by measuring the open-circuit voltage rather than short-circuit current, the new methodology allows quantitative characterisation of the vertical piezoelectric coefficient. The result for GaAs nanowires, ∼ 1–3 pm V−1, is in good agreement with existing knowledge and theory. This method represents a significant advance in understanding the coexistence of different electromechanical effects, and in quantitative piezoelectric nanoscale characterisation. The easy implementation will enable better understanding of electromechanics at the nanoscale.
Highlights
Three distinct electromechanical effects are manifested in piezoelectric semiconductor nanowires: i) the high aspect ratio allows large elastic deformations, enhancing the piezoelectric effect [3], describing changes in surface polarisation due to applied strain; ii) increased surface-to-volume ratio enhances interfacial effects such as triboelectricity [9, 10], relating to surface charge transfer upon contact with a dissimilar material; iii) the combination of semiconducting and piezoelectric properties results in a unique electromechanical phenomenon coined the piezotronic effect [6, 11], whereby the height of a semiconductor energy barrier for charge carrier transport is changed due to mechanical pressure
Alongside the NWs, parasitic growth dominates the silicon surface, on which the NWs were grown by molecular beam epitaxy (MBE)
The origin of our discussion is the following question: can direct piezoelectric generation of a single NW be reasonably measured in an AFM apparatus? Our answer is - yes it could
Summary
The search for sustainable and ubiquitous energy sources [1], combined with the several decades long interest in semiconductor nanowires [2] (NWs) has brought focus to a topical niche - that of piezoelectric semiconductor NWs. The search for sustainable and ubiquitous energy sources [1], combined with the several decades long interest in semiconductor nanowires [2] (NWs) has brought focus to a topical niche - that of piezoelectric semiconductor NWs This field, largely pioneered by Wang et al [3,4,5], offers potential applications in sensing, energy harvesting and logic [6,7,8]. The work presented here aims to distinguish these effects in conductive atomic force microscopy (cAFM)
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