Much recent attention has been focused on the development of field-effect transistors based on low-dimensional nanostructures for the detection and manipulation of molecules. Because of their extraordinarily high charge sensitivity, InAs nanowires present an excellent material system in which to probe and study the behavior of molecules on their surfaces and elucidate the underlying mechanisms dictating the sensor response. So far, chemical sensors have relied on slow, activated processes restricting their applicability to high temperatures and macroscopic adsorbate coverages. Here, we identify the transition into a highly sensitive regime of chemical sensing at ultralow concentrations (<1 ppm) via physisorption at room temperature using field-effect transistors with channels composed of several thousand InAs nanowires and ethanol as a simple analyte molecule. In this regime, the nanowire conductivity is dictated by a local gating effect from individual dipoles, leading to a nonlinear enhancement of the sensitivity. At higher concentrations (>1 ppm), the nanowire channel is globally gated by a uniform dipole layer at the nanowire surface. The former leads to a dramatic increase in sensitivity due to weakened screening and the one-dimensional geometry of the nanowire. In this regime, we detect concentrations of ethanol vapor as low as 10 ppb, 100 times below the lowest concentrations previously reported. Furthermore, we demonstrate electrostatic control of the sensitivity and dynamic range of the InAs nanowire-based sensor and construct a unified model that accurately describes and predicts the sensor response over the tested concentration range (10 ppb to 10 ppm).
Keywords: InAs; ethanol; isotherm; nanowire; physisorption; sensor.