Article Cite This: ACS Sens. 2018, 3, 128−134
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Control of the Intrinsic Sensor Response to Volatile Organic Compounds with Fringing Electric Fields Alex Henning,†,#,¶ Nandhini Swaminathan,†,¶ Yonathan Vaknin,† Titel Jurca,‡,§ Klimentiy Shimanovich,† Gil Shalev,∥,⊥ and Yossi Rosenwaks*,† †
Department of Physical Electronics, School of Electrical Engineering, Tel-Aviv University, Ramat-Aviv 69978, Israel Department of Chemistry and §Cluster for the Rational Design of Catalysts for Energy Applications and Propulsion, University of Central Florida, Orlando, Florida 32816, United States ∥ Department of Electrical and Computer Engineering and ⊥Ilse-Katz center for Nanotechnology, Ben-Gurion University of the Negev, POB 653, Beer-Sheva 84105, Israel
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ABSTRACT: The ability to control surface−analyte interaction allows tailoring chemical sensor sensitivity to specific target molecules. By adjusting the bias of the shallow p−n junctions in the electrostatically formed nanowire (EFN) chemical sensor, a multiple gate transistor with an exposed top dielectric layer allows tuning of the fringing electric field strength (from 0.5 × 107 to 2.5 × 107 V/m) above the EFN surface. Herein, we report that the magnitude and distribution of this fringing electric field correlate with the intrinsic sensor response to volatile organic compounds. The local variations of the surface electric field influence the analyte−surface interaction affecting the work function of the sensor surface, assessed by Kelvin probe force microscopy on the nanometer scale. We show that the sensitivity to fixed vapor analyte concentrations can be nullified and even reversed by varying the fringing field strength, and demonstrate selectivity between ethanol and n-butylamine at room temperature using a single transistor without any extrinsic chemical modification of the exposed SiO2 surface. The results imply an electric-field-controlled analyte reaction with a dielectric surface extremely compelling for sensitivity and selectivity enhancement in chemical sensors. KEYWORDS: intrinsic sensor response, fringing electric field, sensor selectivity, chemical sensor, volatile organic compounds, kelvin probe force microscopy hemical sensors based on an open gate field-effect transistor (FET) architecture have been thoroughly investigated since their introduction in 1970 by Bergveld.1 To date, the most promising chemical sensors for mass production and integration into mobile electronics comprise devices based on silicon-on-insulator (SOI) technology,2−4 as well as on nanostructured silicon such as fin FETs5 and nanowire FETs.6,7 In general, chemical sensor sensitivity can be enhanced by independently improving the extrinsic response, mainly determined by the signal-to-noise ratio and the transducer gain, and the intrinsic response that depends on the analyte− surface interaction. Most commonly, the sensor dielectric is functionalized with organic linker molecules8,9 to enhance the intrinsic sensitivity and achieve sensor chemoselectivity (i.e., to discriminate between different analytes) at room temperature (RT). Sensor surface reactivity with specific analytes can be enhanced or suppressed depending on the reactive binding sites of these linker molecules. A sensor array consisting of different units, each with a unique surface termination, outputs a characteristic signal pattern upon interaction with a target molecule serving as an electronic nose (e-nose). Such an e-nose
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can be used to monitor indoor air quality,10 ascertain food freshness,11 and detect diseases based on breath analysis.12−14 However, organic surface modifications complicate the device fabrication process and their long-term stability is uncertain, especially after prolonged and repetitive exposure to the analyte and ambient air.15 The electrostatically formed nanowire (EFN) sensor is a multiple gate FET based on the four-gate transistor introduced in the early 2000s.16,17 The top gate dielectric of the EFN device is exposed to the ambient and functions as a “molecular gate”18 capable of detecting femtomolar concentrations of biomolecules in liquid 4 and parts per million (ppm) concentrations of organic vapors19,20 at RT. In our previous work, we demonstrated that the applied voltages control the cross-sectional area and position of the conducting channel confined to nanometer dimensions within the Si body4,19,21 of the EFN transistor resulting in a tunable dynamic range (e.g., Received: October 9, 2017 Accepted: December 26, 2017 Published: December 26, 2017 128
DOI: 10.1021/acssensors.7b00754 ACS Sens. 2018, 3, 128−134
Article
ACS Sensors ∼26 to 2030 ppm for ethanol)22. The EFN device geometry and bias configuration influence the transducer sensitivity (i.e., the extrinsic response);21 however, their effect on analyte interaction with the EFN sensor surface (i.e., the intrinsic response) has not yet been explored. Herein, we use EFN devices of the same structure and demonstrate control of the intrinsic sensor response to short chain organic molecules with fringing electric fields whose strength is adjusted (from 0.5 × 107 to 2.5 × 107 V/m) with the junction gates of the EFN transistor in operando. Kelvin probe force microscopy (KPFM) elucidates this novel concept by revealing the electric-field-controlled buildup of an adsorbate on the sensor surface with submicrometer spatial resolution. The effect of surface electric fields on the SiO2 dielectric breakdown of silicon transistors exposed to different environments has been thoroughly studied since the 1960s.23−26 Electric fields below 108 V/m are rather too small to cause stress-induced leakage current at RT27−29 but have been reported to affect vapor analyte adsorption on surfaces of silica30 and semiconductors (mainly metal oxides),31−33 and references therein. However, to the best of our knowledge, fringing-field-controlled chemical sensing has not yet been studied with FET-based chemical sensors. We examine this principle for vapor phase chemical detection at RT and demonstrate selectivity between ethanol (CH3CH2OH) and nbutylamine (CH3CH2 CH2CH2NH2) using a single transistor without any additional (extrinsic) surface modification.
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100 ppb concentration level using a photoionization detector (ppbRAE 3000, RAE Systems, San Jose, USA) as the reference sensor. Vapors of ethanol and n-butylamine were sensed independently with the same device in a dry nitrogen atmosphere. The applied junction gate bias was sustained well below the built-in potential of ∼0.7 V to maintain a p−n junction leakage current (reverse saturation current) of below ∼1 nA. The mean square error of the sensor response was calculated from fluctuations (noise) of the measured current and threshold voltage, uncertainties in the flow rate and the analyte concentration inside the gas chamber, as well as from the deviations of current and threshold voltage before/after sensor recovery (see S4 of the SI). We refer to our previous work that provides additional details on the signal-to-noise ratio of the EFN sensor.20 Kelvin Probe Force Microscopy. KPFM was carried out with a commercial atomic force microscopy (AFM) setup (Dimension Edge, Bruker, Billerica, USA) inside a nitrogen glovebox with
