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

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2 Author(s)
Dattoli, Eric N. ; Mater. Meas. Lab., Nat. Inst. of Stand. & Technol. (NIST), Gaithersburg, MD, USA ; Benkstein, K.D.

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 (SnO2) nanowires that have - een shown to possess high field-effect mobilities (>; 100 cm2/V·s) are proposed to be utilized as the sensor transducer material. These SnO2 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 SnO2 nanowires have been shown to be able to be synthesized. Once grown, these SnO2 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.

Published in:

Semiconductor Device Research Symposium (ISDRS), 2011 International

Date of Conference:

7-9 Dec. 2011