Gas Sensing Mechanism in Chemiresistive Cobalt and Metal-Free Phthalocyanine Thin Films (original) (raw)
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Journal of the American Chemical Society, 2009
The sensitivities of metallophthalocyanine (MPcs: M ) Co, Ni, Cu, Zn, and H 2 ) chemiresistors to vapor phase electron donors were examined using 50 nm MPc films deposited on interdigitated electrodes. Sensor responses were measured as changes in current at constant voltage. Analytes were chosen to span a broad range of Lewis base and hydrogen bond base strengths. The MPc sensor responses were correlated exponentially with binding enthalpy. These exponential fits were consistent with the van't Hoff equation and standard free energy relationships. Sensor recovery times were found to depend exponentially on binding enthalpy, in agreement with the Arrhenius equation. Relative sensitivities of all MPcs were compared via two-way ANOVA analysis. Array response patterns were differentiated via linear discriminant analysis, and analyte identification was achieved over a range of concentrations with 95.1% classification accuracy for the strong binding analytes. The ability to distinguish among different analytes, regardless of their concentration, through normalization of the responses to a reference sensor is particularly noteworthy.
Chemical Papers, 2012
We have investigated the morphology, charge transport, and gas-sensing characteristics of thin films of cobalt phthalocyanine (CoPc) deposited on glass and sapphire substrates, using molecular beam epitaxy (MBE). CoPc films deposited on glass were found to be highly disordered. The ambient oxygen was found to be chemisorbed and created deep trap states, which led not only to hysteretic current-voltage (J-V) characteristics but also reduce the charge mobility. These properties render them unsuitable for gas-sensing. On the other hand, films deposited on sapphire were polycrystalline, which was attributed to an improved molecule-substrate interaction. The physically sorbed oxygen only created shallow traps, and the J-V characteristics were non-hysteretic, rendering them suitable for gas-sensing applications. It was demonstrated that the ultrathin (20 nm) CoPc films deposited on sapphire acted as highly sensitive and selective sensors for chlorine present in the w Cl concentration rang...
The Journal of Physical Chemistry B, 2006
The electrical properties of 50 nm thick metallophthalocyanine films, prepared by organic molecular beam epitaxy (OMBE) on interdigitated electrodes, were studied with DC current-voltage measurements and impedance spectroscopy. The transition from Ohmic behavior at low voltages to space-charge-limited conductivity (SCLC) at higher voltages depends on the metal electrode (Pt, Pd, and Au), but does not correlate with the work function of the electrode. Impedance spectroscopy studies show the coexistence of low-and high-frequency traps in the thin film devices, and the contribution of low-frequency traps associated with Ohmic behavior diminishes at higher bias. Although device resistances are strongly influenced by the electrode material, and vary by a factor of over 300, the relative chemical sensor responses on exposure to dimethyl methylphosphonate (DMMP), methanol, water, or toluene vapors are similar for CoPc on Pt, Pd, and Au electrodes when these devices are operated in the SCLC regime at room temperature. When the devices are operated at voltages where the low-frequency interfacial traps are filled, the sensor response to analyte becomes uniform and reliable regardless of the specific interfacial electrode contact.
Effect of electrode geometry on gas sensitivity of lead phthalocyanine thin films
Sensors and Actuators B: Chemical, 1992
The effect of electrode geometry on the gas sensitivity of lead phthalocyanine (PbPc) thin films has been studied. The material PbPc has been chosen for this study because it has a high sensitivity ( -ppm) to oxidizing gases and may be suitable for use in a commercial solid-state chemical sensor. PbPc films of varying thickness (up to 1.5 pm) are vacuum sublimed across ultrathin (0.1 pm) coplanar pairs of gold electrodes lying on a sapphire substrate. The change of the electrical conductance of these devices in atmospheric nitrogen dioxide (NO,) is measured, over a range of electrode separations for several film thicknesses and temperatures, by a computer-controlled automated system. The temperatures are chosen around 160 "C, at which a peak sensitivity of PbPc to nitrogen dioxide has been observed. The transient and steady-state responses of these devices are analysed in terms of diffusion-rate limited and reaction-rate limited models modified from previous work on thick porous metal oxide semiconducting films. Experimental results show that the response time of the device at temperatures from 130 to 190 "C is insensitive to both the electrode separation and film thickness. Furthermore, the response time, typically 120 s, decreases with increasing nitrogen dioxide concentration (l-9 ppm). These observations are consistent with the reaction-rate limited rather than the diffusion-rate limited mode1 of mass transport in thin PbPc films. The activation energy of the PbPc films is determined from the temperature dependence of the device conductance, and found to be (0.15 + 0.1) eV; as expected, it is independent of electrode separation and film thickness. The film conductance in air increases with increasing reciprocal electrode separation, but does not follow the linear relationship predicted from the model. Furthermore, the steady-state response (defined as the fractional change in conductance) of the PbPc films to atmospheric nitrogen dioxide shows a gradual but systematic increase with electrode separation, whereas the model predicts a constant value. The deviation of the experimental results from theory suggests the presence of considerable film inhomogeneity. The nature of this deviation is consistent with a film in which both the electrical conductivity and density of adsorption sites are considerably lower near the substrate surface than elsewhere. This observed heterogeneity in the gas sensitivity of thin sublimed PbPc films makes it more difficult to characterize their behaviour and may reduce their commercial viability as chemical sensors. 092%4005/92/%5.00
Gas sensing behavior of metal-phthalocyanines: Effects of electronic structure on sensitivity
Chemical Physics, 2018
Cu-phthalocyanine (CuPc) and five of its metal variants (M = Mg, Mn, Co, Ni, and Zn) have been studied by density functional theory (DFT) methods for sensing five volatile organic compoundsisoprene, acetone, ammonia, methanol, and methane. Having performed experimental validation of the methods, interaction energies, binding configurations, changes in charge distributions and energy levels after interaction, and interaction barriers were studied to account for the sensitivity trend across the analytes for CuPc, while also providing supporting data from experiments carried out herein. It is also demonstrated that relatively simple calculations of interaction barrier can result in quick screening of metal-Pc variants for an analyte without any extensive experimental or computational efforts. Present literature lacks such detailed studies for these materials and analytes. Thus, this work would consolidate the understanding of sensing phenomena at the electronic structure level that could be useful for emerging technologies in gas sensing.
Organic Electronics, 2012
Charge transport and gas sensing characteristics of cobalt phthalocyanine films deposited along (ATB) and perpendicular (PTB) to the natural twin boundaries of (0 0 1) LaAlO 3 substrate have been investigated. The charge carrier mobility of ATB films ($5 cm 2 V À1 s À1 ) is five orders of magnitude higher compared to that of PTB films ($7 Â 10 À5 cm 2 V À1 s À1 ), suggesting that twin boundaries acts like a template for ordering of molecules. The ATB films on exposure to ammonia showed a reversible increase of resistance, with fast response and recovery. In contrast PTB films showed same sensitivity, but exhibits base resistance drift along with sluggish response.
MRS Proceedings, 1997
In order to realize a high-sensitivity, low temperature operable NO2 gas sensor, thin films of at-form copper phthalocyanine (α-CuPc) have been deposited by vacuum sublimation. In this study, we have attempted to improve the gas-sensing characteristics through a modification of the film microstructure. Firstly, the gas sensitivity is remarkably increased by an insertion of higher-sensitive layer (vanadyl Pc film) between the α-CuPc film and the glass substrate in the low gas concentration range. Secondly, a reversibility in cycles of gas doping and dedoping is improved by film deposition on hydrofluoric acid-treated substrate. It is found from atomic force microscope analyses that this phenomenon may be closely related to a modification of the film microstructure.
Analyte chemisorption and sensing on n- and p-channel copper phthalocyanine thin-film transistors
The Journal of Chemical Physics, 2009
Chemical sensing properties of phthalocyanine thin-film transistors have been investigated using nearly identical n-and p-channel devices. P-type copper phthalocyanine ͑CuPc͒ has been modified with fluorine groups to convert the charge carriers from holes to electrons. The sensor responses to the tight binding analyte dimethyl methylphosphonate ͑DMMP͒ and weak binding analyte methanol ͑MeOH͒ were compared in air and N 2 . The results suggest that the sensor response involves counterdoping of pre-adsorbed oxygen ͑O 2 ͒. A linear dependence of chemical response to DMMP concentration was observed in both n-and p-type devices. For DMMP, there is a factor of 2.5 difference in the chemical sensitivity between n-and p-channel CuPc thin-film transistors, even though it has similar binding strength to n-and p-type CuPc molecules as indicated by the desorption times. The effect is attributed to the difference in the analyte perturbation of electron and hole trap energies in n-and p-type materials.