Improving the selectivity of a metal oxide nanowire gas sensor using a microhotplate/FET platform

Summary form only given: Although metal oxide nanowires have been shown to be sensitive transducer materials when employed in gas sensors, e.g. such nanowire-based sensors have been shown to detect gaseous analytes at target concentrations of 1 μmol/mol and lower; they suffer from the deleterious issue of cross-sensitivity to a broad range of gases. To address this issue, a new sensor device structure is detailed that includes both an integrated heater and back-gate structure. In particular, these device features provide for control over the sensor operating temperature and the magnitude of an electric-field that is applied in a transverse direction relative to horizontal-lying transducer nanowire(s). Modulation of either of these two physical parameters, temperature and transverse electric-field, is known to affect the gas-sensing behavior of metal oxide nanowires in an analyte-dependent fashion. Such analyte-dependent response behavior can be utilized as a “fingerprint” to enable the determination of the identity of an exposed analyte through statistical analysis of the modulated sensor output.To elaborate on how such temperature and electric-field control is achieved, the described sensor utilizes a microfabricated, suspended membrane structure with an integrated platinum heater that allows for low-power and rapid temperature control. This structure has been commonly termed a "microhotplate". Embedded within this microhotplate is a back-gate electrode that is used to apply a transverse electric-field in order to modulate the free carrier density within the bulk of the nanowire. This back-gate control operates in a similar manner as compared to semiconductor nanowire field-effect transistor (FET) devices. In order to ensure that the nanowires are affected in a significant fashion by applied back-gate biases, which correlate to the strength of the transverse electric-field, single-crystalline tin oxide (SnO
₂
) nanowires that have been shown to possess high field-effect mobilities (>; 100 cm
²
/V·s) are proposed to be utilized as the sensor transducer material. These SnO
₂
nanowires are grown using the vapor-liquid solid (VLS) process. Briefly, the VLS nanowire growth process relies on the use of a catalyst nanoparticle, e.g. Au, to serve as both the nanowire nucleation and axial growth site. Owing to the high temperature of this growth process, ≈ 900°C, low-defect, single-crystalline SnO
₂
nanowires have been shown to be able to be synthesized. Once grown, these SnO
₂
nanowires may be employed in chemiresistive sensors by utilization of the device structure. Simple drop-casting of a nanowire suspension may be used to deposit the nanowires on the sensor device substrate; this deposition method is utilized for the device. An alternate deposition method, termed "contact printing," may also be utilized. Nanowire contact printing is a shear-force based transfer method that provides for the deposition of a large number (tens or more) of nanowires in a parallel, uniaxially aligned configuration. The advantage of this method is that these nanowires may be easily be contacted and operated in parallel so as to reduce the effect of wire-to-wire variability by averaging out variations over many discrete nanowires. Overall, this modulated nanowire sensor approach presents a practical method for the utilization of nanowire sensors in real-world, multi-chemical detection problems where sensor selectivity is of prime importance. Critically, the presented temperatureand electric-field based modulation method is rapidly adjustable, and as such provides for a more dynamic control over the nanowire sensor selectivity as compared to the commonly-used method of metal nanoparticle functionalization of nanowire surfaces.