Relative Differential Resistance Research Articles

Photo-Induced Effects in Graphene Channels When Interfaced with Quantum Dot Array

Field effect transistors with graphene channels were interfaced with semiconductor quantum dot (QD) array. Electrical characteristics and opto-electronic behaviour of the elements were assessed. Relative photo-resistance (the difference in channel resistances under dark and under light conditions and normalized by the channel’s resistance) was observed as a function of drain-source potential. Graphene – a mono layer thick crystalline form of carbon - portrays high conductivity, chemical inertness, mechanical robustness and unusual dispersion relations [1]. Characteristics of free-standing, mono and bi-layer graphene have been studied when deposited over nano-pore array of anodized aluminum oxide (AAO) substrate. One may postulate that QDs, filling the pores, will have a profound effect on the graphene channel. Such arrangement led to the realization of the first visible surface plasmon laser [2]. The field effect transistors (FET) were fabricated on oxidized silicon wafers. They were composed of graphene channels and two metal electrodes, used as drain and source electrodes. The silicon wafer served as a back gate electrode. Contacts to the graphene were found Ohmic. Aluminum, was deposited on the oxidized silicon wafer and was anodized according to our previous recipe. Core-shell semiconductor CdSe/ZnS QDs were imbedded in the anodized aluminum pores and the semi-transparent graphene was deposited on top of the anodized layer. Unlike ordinary graphene channels, the current between the drain and source, Ids, decreased as a function of gate bias, Vgs. When illuminated by a 50 mW/cm2 white light source, the channel exhibited a clear relative differential resistance at Vds=0.3 V. This behavior was attributed to a negative differential resistance effect. [1] A. C. Ferrari, J. C. Meyer, V. Scardaci, M. Lazzeri, F. Mauri, S. Piscance, D. Jiang, K. S. Novoselov, S. Roth and A. K. Geim, “Raman Spectrum of Graphene and Graphene Layers”, Phys. Rev. Lett., 97, 187401 (2006). [2] R. Li, A. Banerjee and H. Grebel, “The possibility for surface plasmon lasers”, Optics Express, 17, 1622-1627 (2009). [3] S. W. Lee, A. Kornblit, D. Lopez, S. V. Rotkin. A. Sirenko, H. Grebel, “Negative Differential Resistance: Gate Controlled and Photoconductance Enhancement in Carbon Nanotube Intra-connects”, Nano Letters, DOI: 10.1021/nl803036a (2009).

Detection of ammonia, 2,4,6-trinitrotoluene, and common organic vapors using thin-film carbon black-metalloporphyrin composite chemiresistors

Thin-film chemiresistive composites of octaethylporphine-based transition-metal complexes (Ph(M), M=Co, Cu and Zn) and carbon black (CB) have been fabricated and tested as chemical vapor sensors. The sensing performance of such sensor composites was compared to the sensing performance of composites of metallophthalocyanines (Phtc(M)) and CB. The relative differential resistance response of Ph(M)/CB sensor films upon exposure to organic vapors, such as n-hexane, n-heptane, n-octane, iso-octane, cyclohexane, toluene, ethyl acetate and ethanol, was dependent on the nature of the metal center. An array of chemiresistive Ph(M)/CB vapor sensors therefore provided discrimination between the organic vapor analytes that had different polarities, specifically classifying non-polar vapors, aprotic polar vapors and protic polar vapors. However, discrimination was not observed for analytes that had mutually similar polarities. The Ph(M)/CB sensors showed reversible responses toward ammonia, NH3(g), at concentrations below the 8h permissible exposure level (50ppm). Ph(M)/CB composites exhibited a slightly larger resistance response than Phtc(M)/CB composites, consistent with the Ph(M) species having less π-stacked molecular aggregates, resulting in an increase in the number of adsorption sites relative to the Phtc(M)/CB composites. Resistance responses with a signal-to-noise ratio value of ∼900 were obtained upon exposure to vapor pulses saturated with 2,4,6-trinitrotoluene.

Composites of carboxylate-capped TiO 2 nanoparticles and carbon black as chemiresistive vapor sensors

Titanium (IV) dioxide (TiO 2) nanoparticles (NPs) with a 1–5 nm diameter were synthesized by a sol–gel method, functionalized with carboxylate ligands, and combined with carbon black (CB) to produce chemiresistive chemical vapor sensor films. The TiO 2 acted as an inorganic support phase for the swellable, organic capping groups of the NPs, and the CB imparted electrical conductivity to the film. Such sensor composite films exhibited a reproducible, reversible change in relative differential resistance upon exposure to a series of organic test vapors. The response of such chemiresistive composites was comparable to, but generally somewhat smaller than, that of thiol-capped Au NPs. For a given analyte, the resistance response and signal-to-noise ratio of the capped TiO 2-NP/CB composites varied with the identity of the capping ligand. Hence, an array of TiO 2-NP/CB composites, with each film having a compositionally different carboxylate capping ligand, provided good vapor discrimination and quantification when exposed to a series of organic vapors. Principal components analysis of the relative differential resistance response of the sensor array revealed a clear clustering of the response for each analyte tested. This approach expands the options for composite-based chemiresistive vapor sensing, from use of organic monomeric or polymeric sorbent phases, to use of electrically insulating capped inorganic NPs as the nonconductive phase of chemiresistive composite vapor sensors.

Detection of organic vapors and NH3(g) using thin-film carbon black–metallophthalocyanine composite chemiresistors

Thin-film chemiresistive vapor sensors have been fabricated using composites of carbon black (CB) and metallophthalocyanines. The resulting sensors exhibited large, rapid, and reversible relative differential resistance changes upon exposure to a series of test organic vapors. The individual sensor responses, and collective sensor array properties for classification of a series of test organic vapors, were comparable to those of chemiresistor arrays made from composites of conductors and insulating organic polymers. In addition to displaying resistance responses to volatile organic compounds, the CB/metallophthalocyanine composite sensors exhibited a high sensitivity towards trace levels of NH_3(g) under ambient temperature and pressure conditions. No degradation in sensor response was observed after nearly 12 h of repeated exposure to 30 parts per million of NH_3(g) in air. Chemiresistors formed from composites of CB and zinc phthalocyanines demonstrated responses that were comparable to those of commercial NH_3(g) sensors. The CB/metallophthalocyanine composites thus broaden the types of materials that can be used for this type of low-power chemiresistive gas sensing, and broaden the types of analytes that can be sensitively detected to include inorganic gases as well as organic vapors.

Cover Picture: Vapor Sorption and Electrical Response of Au‐Nanoparticle– Dendrimer Composites (Adv. Funct. Mater. 6/2007)

AbstractThe cover shows chemiresistors and mass‐sensitive vapor sensors coated with Au‐nanoparticle/dendrimer composites. The Au nanoparticles provide the film with electrical conductivity and the dendrimers control the chemical selectivity, as reported by Nadjedja Krasteva and co‐workers on p. 881. Analyte sorption follows a combined Henry–Langmuir model, and measurements reveal that sorption‐induced increase in film resistance scales linearly with the concentration of analyte sorbed in the film. The background shows a silicon wafer with lithographically defined microelectrode structures for chemiresistor fabrication.Films comprising Au nanoparticles and polyphenylene dendrimers (first and second generation) are deposited onto transducer substrates via layer‐by‐layer self‐assembly and characterized by atomic force microscopy and X‐ray photoelectron spectroscopy. Their sorption behavior is studied by measuring the uptake of solvents from the vapor phase with quartz crystal microbalances (QCMs). The resistance of the films is simultaneously monitored. Both sensor types, QCMs and chemiresistors, give qualitatively very similar response isotherms that are consistent with a combination of Henry‐ and Langmuir‐type sorption processes. The sorption‐induced increase in relative differential resistance scales linearly with the amount of analyte accumulated in the films. This result is in general agreement with an activated tunneling process for charge transport, if little swelling and only small changes in the permittivity of the film occur during analyte sorption (a first‐order approximation). The relative sensitivity of the films to different solvents decreases in the order toluene ≈ tetrachloroethylene > 1‐propanol ≫ water. Films containing the larger second‐generation dendrimers show higher sensitivity than films containing first‐generation dendrimers.

Vapor Sorption and Electrical Response of Au‐Nanoparticle– Dendrimer Composites

AbstractFilms comprising Au nanoparticles and polyphenylene dendrimers (first and second generation) are deposited onto transducer substrates via layer‐by‐layer self‐assembly and characterized by atomic force microscopy and X‐ray photoelectron spectroscopy. Their sorption behavior is studied by measuring the uptake of solvents from the vapor phase with quartz crystal microbalances (QCMs). The resistance of the films is simultaneously monitored. Both sensor types, QCMs and chemiresistors, give qualitatively very similar response isotherms that are consistent with a combination of Henry‐ and Langmuir‐type sorption processes. The sorption‐induced increase in relative differential resistance scales linearly with the amount of analyte accumulated in the films. This result is in general agreement with an activated tunneling process for charge transport, if little swelling and only small changes in the permittivity of the film occur during analyte sorption (a first‐order approximation). The relative sensitivity of the films to different solvents decreases in the order toluene ≈ tetrachloroethylene > 1‐propanol ≫ water. Films containing the larger second‐generation dendrimers show higher sensitivity than films containing first‐generation dendrimers.

Chemiresistors for Array-Based Vapor Sensing Using Composites of Carbon Black with Low Volatility Organic Molecules

Chemically sensitive resistors have been fabricated from composites of carbon black and low volatility, nonpolymeric, organic molecules such as propyl gallate, lauric acid, and dioctyl phthalate. Sorption of organic vapors into the nonconductive phase of such composites produced rapid and reversible changes in the relative differential resistance response of the sensing films. Arrays of these sensors, in which each sensing film was comprised of carbon black and a chemically distinct nonpolymeric organic molecule or blend of organic molecules, produced characteristic response patterns upon exposure to a series of different organic test vapors. The use of nonpolymeric sorption phases allowed fabrication of sensors having a high density of randomly oriented functional groups and provided excellent discrimination between analytes. By comparison to carbon black−polymer composite vapor sensors and sensor arrays, such sensors provided comparable detection limits as well as enhanced clustering and enhanced resolu...

Vapor Sensing Using Polymer/Carbon Black Composites in the Percolative Conduction Regime

To investigate the behavior of chemiresistive vapor sensors operating below or around the percolation threshold, chemiresistors have been formed from composites of insulating organic polymers and low mass fractions of conductive carbon black (CB, 1-12% w/w). Such sensors produced extremely large relative differential resistance changes above certain threshold vapor concentrations. At high analyte partial pressures, these sensors exhibited better signal/noise characteristics and were typically less mutually correlated in their vapor response properties than composites formed using higher mass fractions of CB in the same set of polymer sorption layers. The responses of the low-mass-fraction CB sensors were, however, less repeatable, and their nonlinear response as a function of analyte concentration required more complicated calibration schemes to identify and quantify analyte vapors to compensate for drift of a sensor array and to compensate for variability in response between sensor arrays. Because of their much larger response signals, the low-mass-fraction CB sensors might be especially well suited for use with low-precision analog-to-digital signal readout electronics. These sensors serve well as a complement to composites formed from higher mass fractions of CB and have yielded insight into the tradeoffs of signal-to-noise improvements vs complexity of signal processing algorithms necessitated by the use of nonlinearly responding detectors in array-based sensing schemes.

Nano-Electromechanical Displacement Sensing Based on Single-Walled Carbon Nanotubes

We present a nano-electromechanical system based on an individual single-walled carbon nanotube (SWNT) demonstrating their potential use for future displacement sensing at the nanoscale. The fabrication and characterization of the proposed nanoscaled transducer, consisting of a suspended metal cantilever mounted on top of the center of a suspended SWNT, is presented and discussed. The displacement of the nanoscale cantilever is detected via the electromechanically induced change in conductance of the strained SWNT. A relative differential resistance sensitivity (for a metallic SWNT) of up to 27.5%/nm was measured and a piezoresistive gauge factor of a SWNT of up to 2900 was extracted.

Environmental temperature-independent gas sensor array based on polymer composite

This paper reports the effect of gas temperature on the response of sensor array based on polymer–carbon black composite. Temperature-controlled chemical vapors of chloroform, methanol, benzene, acetone, and cyclohexane were injected into the sensor array equipped with a micro heater. The temperature of the sample gas was varied (20, 40, and 60 °C) and the resistance change was measured with the sensor films thermostatted at the temperature of 25 and 45 °C, using embedded micro heater. The maximum relative differential resistance of polymer composite was severely affected by the gas temperature, and the response pattern of the sensor array also changed with the gas temperature. Consequently, the chemical vapors controlled at the temperature of 20 and 40 °C were recognized differently in principal component space, even though the same concentration of chemical vapor was injected. However, when the polymer detector was maintained at a temperature of 45 °C, radial-response patterns for the gas temperature of 20 and 40 °C showed similar shape and were recognized identically in the projected principal component space.

Comparison of analytical methods and calibration methods for correction of detector response drift in arrays of carbon black-polymer composite vapor detectors

The responses of 15 carbon black-polymer composite chemiresistors have been analyzed during exposure to eight different analytes ( n-hexane, tetrahydrofuran, ethanol, ethyl acetate, cyclohexane, n-heptane, n-octane, and isooctane) in random order at low concentration (0.5% of the vapor pressure of analyte at room temperature) over 4 months (8000 total analyte exposures) of data collection. Data were collected for periods during which the array was continuously exposed periodically to analytes and after long periods during which no analyte exposures had been performed. All but the most difficult separation tasks (for example, discrimination between low concentrations of straight-chain hydrocarbons) could be performed robustly over the entire 4 month time period based only on the use of a decision boundary formulated from an initial training set of 200 exposures, indicating the sensor drift had minimal effect on system performance in such classification tasks. For the remaining classification tasks, modeling the dynamics of sensor drift either through a linear regression or Fourier transform decomposition of the individual relative differential resistance responses versus time of each sensor yielded little improvement in classification performance, indicating that external events were largely responsible for changes in sensor response versus time. Six analytes that were not treated as unknowns for a binary separation task were individually treated as calibrants whose response was intermittantly used to renormalize the response of the sensor array. A simple linear sensor-by-sensor calibration scheme proved effective at restoring the classification performance of difficult binary separation tasks to the performance that was observed in the initial training set period. Calibrants that were mutually similar to the analytes being differentiated tended to be more effective than calibrants that were very chemically different from the analytes of interest. Evaluation of various calibration protocols indicated that an optimal tradeoff existed between the number of calibration exposures and the frequency of calibration periods. Condition-based calibration, in which calibration was only performed when the classification model exhibited a decline in classification performance below a predetermined threshold value, was observed to be superior to a time-based calibration approach or to interval-based, cyclic calibration protocols for this set of analytes exposed under the chosen analysis conditions.

Detection of Organic Mercaptan Vapors Using Thin Films of Alkylamine-Passivated Gold Nanocrystals

Alkylamine-capped gold nanocrystals have been used as chemically sensitive resistors for the detection of volatile organic mercaptan vapors. Thin (<1 × 10-4 cm) films of dodecylamine-capped, 6−7 nm diameter Au nanocrystals produced dc electrical resistances of 10 kΩ to 10 MΩ when deposited onto interdigitated Au/Cr electrodes. These chemiresistive vapor detectors displayed a reversible increase in dc electrical resistance when exposed to non-thiol-containing vapors such as water, acetone, or toluene delivered at a constant fraction (0.05) of their vapor pressure, with relative differential resistance responses to these vapors of ≈5% under such conditions. In contrast, the amine-capped Au nanocrystal films exhibited a much larger, irreversible decrease in resistance upon exposure to vapors possessing the thiol (−SH) functionality, including H2S, CH3SH, and propanethiol. Optical spectroscopic and transmission electron microscopy data indicated that the thiols displaced the amine caps, allowing the gold cores to move closer together and in some cases enter into contact, thereby lowering the film resistance. Consistently, the resistance of such films did not decrease upon exposure to octanethiol. For CH3SH, the rate of resistance change under repeated experimental conditions allowed extraction of the concentration of analyte over the range 4 ppb to 1.5 ppm in air.

Characterization of the Temporal Response Profile of Carbon Black−Polymer Composite Detectors to Volatile Organic Vapors

The relative differential resistance responses of carbon black−poly(ethylene-co-vinyl acetate) (PEVA) composite vapor detectors were evaluated in response to short rise time (<2 ms for a 17 ms pulse length) square pulses of acetone, n-hexane, methanol, 2-propanol, or toluene, in a background of synthetic air. The use of ultrathin films, along with a rapid vapor delivery system, facilitated measurement of the rapid time response available from this exemplary carbon black−polymer composite chemiresistive film for the detection of common organic vapors. Detectors formed from very thin (<200 nm) PEVA−carbon black composites produced steady-state responses within 17 ms upon exposure to methanol and produced steady-state responses within 90 ms upon exposure to toluene, acetone, and n-hexane. In accord with Fickian diffusion, the response times of the relative differential resistance of PEVA−carbon black detectors to analyte exposures were proportional to the square of the film thickness, l, in the range 510 ≤ l ≤ 5700 nm. Additionally, the relative differential resistance versus time profiles of PEVA−carbon black detectors were well fit by a simple finite difference model based on Fickian analyte diffusion, using a single analyte diffusion coefficient, for a variety of different film thicknesses and analyte concentrations.

Preparation and properties of vapor detector arrays formed from poly(3,4-ethylenedioxy)thiophene-poly(styrene sulfonate)/insulating polymer composites

Poly(3,4-ethylenedioxy)thiophene-poly(styrene sulfonate) (PEDOT-PSS) was used as the conductive component in a matrix of chemically different insulating polymers to form an array of vapor detectors. Such composites produced larger relative differential resistance responses when exposed to polar analytes than did the corresponding carbon black filled polymer composite detectors. However, the PEDOT-PSS composites produced smaller responses than carbon black composites when exposed to nonpolar analytes. The resolving power of a PEDOT-PSS detector array was compared to that of a carbon black composite array for a broadly construed set of organic vapors. The PEDOT-PSS array exhibited better, on average, discrimination between pairs of polar analytes and polar/nonpolar analytes than did the carbon black composite array. The carbon black composite array out-performed the PEDOT-PSS array in discriminating between nonpolar compounds. The addition of PEDOT-PSS composites to an array of carbon black composite detectors therefore can produce improved overall discrimination in a vapor sensor system when used in tasks to differentiate between of a broad set of analyte vapors.

An investigation of the concentration dependence and response to analyte mixtures of carbon black/insulating organic polymer composite vapor detectors.

The responses relative to an air background of carbon black/polymer composite vapor detectors have been determined as a function of the concentration of a homologous series of alcohols (n-CnH2n+1OH, 1 < or = n < or = 8), a homologous series of alkanes (n-CnH2n+2, 5 < or = n < or = 10 and n = 12, 14), and a set of diverse solvent vapors. In all cases, the steady-state relative differential resistance responses, delta R/Rb, of the carbon black/polymer composite vapor detectors were well-described by a linear relationship with respect to the analyte partial pressure, at least over the tested concentration range (P/P degree = 0.005-0.03, where P degree is the vapor pressure of the analyte). When two vapors in air were simultaneously presented to the detectors, the delta R/Rb response, relative to an air background, was the sum of the delta R/Rb values obtained when each analyte was exposed separately to the carbon black/polymer composite detectors under study. Similarly, when an analyte was exposed to the detectors on top of a background level of another analyte, the delta R/Rb values of the array of detectors were very close to those obtained when the test analyte was exposed to the detectors only in the presence of background air. The initial training requirements from the array response output data of such detectors are minimized because the delta R/Rb response pattern produced by the analyte of concern can be associated uniquely with that odor, under the conditions explored in this work.

Differential Detection of Enantiomeric Gaseous Analytes Using Carbon Black−Chiral Polymer Composite, Chemically Sensitive Resistors

Carbon black-chiral polymer composites were used to provide diagnostic differential resistance responses in the presence of enantiomers of chiral gaseous analytes. Vapors of (+)-2-butanol and (-)-2-butanol, (+)-α-pinene and (-)-α-pinene, (+)-epichlorohydrin and (-)-epichlorohydrin, and methyl (+)-2-chloropropionate and methyl (-)-2-chloropropionate were generated and passed over a chemically sensitive carbon black-poly((R)-3-hydroxybutyrate-co-(R)-3-hydroxyvalerate) (77% butyrate) composite resistor. Each enantiomer of a pair produced a distinct relative differential resistance change on the chiral detector, whereas both enantiomers of a set produced identical signals on achiral carbon black-poly(ethylene-co-vinyl acetate) (82% ethylene) detectors.