NANO LETTERS
Gas Sensor Based on Metal-Insulator Transition in VO2 Nanowire Thermistor
2009 Vol. 9, No. 6 2322-2326
Evgheni Strelcov,† Yigal Lilach,‡ and Andrei Kolmakov*,† Department of Physics, Southern Illinois UniVersity at Carbondale, Illinois 62901-4401, and Center for Nanoscience and Nanotechnology, The Hebrew UniVersity of Jerusalem 91904, Israel Received March 3, 2009; Revised Manuscript Received April 22, 2009
ABSTRACT Using temperature driven sharp metal-insulator phase transition in single crystal VO2 nanowires, the realization of a novel gas sensing concept has been tested. Varying the temperature of the nanowire close to the transition edge, the conductance of the nanowire becomes extremely responsive to the tiny changes in molecular composition, pressure, and temperature of the ambient gas environment. This gas sensing analog of the transition edge sensor radiometry used in astrophysics opens new opportunities in gas sensorics.
The two main factors, which define the performance of gas sensors, are their receptor and transduction functions. The first defines the interaction of the analyte molecules with the surface of the sensing element: its rate, selectivity, and reversibility. The second is the mechanism of conversion of this interaction into the output signal. Among many of the modern chemical sensing platforms based on electromechanical,1-3 optical,4-6 and conductometric nanosensors (see most recent reviews7-13 and references therein), the latter is most thoroughly studied and is believed to be the closest one to real world application due to reasonable costeffectiveness ratio. The receptor-transduction functions of such a sensor (for instance, chemiresistor made of semiconducting metal oxide nanowire) are based on the charge transfer mechanism between a nanowire surface and an analyte molecule followed by formation of charge-induced depletion/accumulation layer in the nanowire. Since for thin enough nanowires (10-100 nm) the nanostructure’s radius and the width of the depleted layer are comparable, the adsorption/desorption of analyte molecules (receptor function) effectively modulates the cross section of the conducting channel (transduction function). Because the charge exchange reaction is a base of the above receptor-transduction scheme, the detection ability of the aforementioned nanowire sensors is low beyond the class of strong redox molecules and in general requires elevated temperatures (∼200 °C) to have reasonable reaction rate. In this communication, we are using very sharp temperature driven metal-insulator phase transition in VO2 single crystal quasi-one-dimensional nano- and mesostructures (VO2 NW), * To whom correspondence should be addressed. E-mail: akolmakov@ physics.siu.edu. † Southern Illinois University. ‡ The Hebrew University of Jerusalem. 10.1021/nl900676n CCC: $40.75 Published on Web 05/13/2009
2009 American Chemical Society
to test a principally different gas sensing concept. The idea of this approach is based on recent advancements in modern radiometric sensing technology used in observational astronomy and on developments of ultrasensitive transitionedge sensors (TES) in particular (see recent review14). In these resistive type devices, the active sensing element is made of superconducting material whose operating temperature is set to an extremely narrow region (transition edge) between superconductive and normal states. Thus, tiny variations of the sensor’s temperature due to incoming electromagnetic radiation will result in a drastic change of the resistance. Currently TES are adopted as state of the art energy resolving detectors of single photons in a broad spectral range for astrophysics studies.15 However, the usage of superconducting sensing elements is experimentally demanding and expensive since it requires cryogenic temperatures. On the other hand, VO2 is an actively studied strongly correlated oxide that exhibits a temperature driven metal-insulator transition (MIT) at TMIT ∼ 68 °C in the bulk form where conductivity of the sample increases several orders of magnitude (alternatively, the notation SMT (semiconductor-metal transition) is used).16-22 The transition is accompanied by a crystalline structure change from monoclinic at low temperature insulating phase to a tetragonal one in a metal state. This sharp change in conductivity of VO2 has been proposed as a working principle for uncooled TES microbolometers23-26 and critical temperature sensors.27 However, further progress in implementation of this approach in radiometry was hampered by transition broadening and hysteresis effects observed for polycrystalline VO2 films and bulk solids.28-35 Recently, high quality single crystal VO2 NWs have been synthesized36-47 that are domain-free and therefore in
principle should have superior phase transition properties.37,38 In addition, it has been shown that the MIT in VO2 NWs can be tuned to lower temperature by appropriate doping36 or by axial stress.37,38 Therefore, such a nanowire indexed as a thermistor, can be a sensitive indicator of the delicate equilibrium between Joule heat dissipated inside the sensing element and the rate of heat transfer from the detector to the environment.48 Single crystallinity, high surface-to-bulk ratio of VO2 NWs combined with their small size and thermal capacitance make these nanostructures an ideal platform for uncooled TES-like sensors. The latter leads to a promising novel transduction principle in gas sensorics. VO2 NWs and mesoscopic ribbons were synthesizes following the protocols reported in refs 39 and 41. Briefly, nanostructures were grown on SiO2/Si substrates via vapor solid method at 1000 °C at 13 Torr and 3 sccm flux of Argon (see Supporting Information for NW characterization). Individual nanowires were mechanically removed from the SiO2 film using manual micromanipulator and placed onto the device substrate with predeposited metal leads. The substrate was either microhot plate chip (µhp) or a SiO2/Si wafer. The MIT in these devices was induced by Joule heat using programmed ramps of the DC bias voltage across the nanostructure. Since the resistivity of the NWs in the metallic phase was found to be rather low (r ∼ 10-4 Ωcm), a limiting resistor of 11 kΩ was employed in series to prevent the nanothermistor from damage and the current amplifier from overloading. During the structural transition from monoclinic (semiconducting) to tetragonal (metal) phase (and vise versa), the lattice constant changes ∼1% along the nanowire length. As a result of this structural phase transition, the nanowire, if embedded into the substrate or clamped by the electrodes, experiences a significant axial tensile or compressive stress37,38,44 that can modify TMIT and the sharpness of the MIT transition. To ensure stable TMIT and unperturbed contacts between the NWs and the gold electrodes within a broad temperature range, a droplet of liquid alloy (Ga-In-Sn eutectics, 99.99%, Alfa Aesar) was employed as a “soft contact” media. Having the two ends of the nanowire immersed in the liquid microdroplets (Figure 1a), the nanowires and mesostructures were suspended over the substrate in the device. The latter design is favorable for selfheated nanostructure, as it reduces the thermal mass of the nanothermistor device and eliminates parasitic thermal losses to the substrate.48 To elucidate the spatial evolution of the conducting and insulating phases during the MIT, the optical imaging, coupled with electrical measurements and wider microscopic VO2 single crystal ribbons (Figure 1c-e), were used. The latter characterization was performed using Nikon Eclipse L150 microscope (200-1000× magnification) equipped with Nomarski prisms and polarizer filters to enhance phase contrast imaging. The realization of the truly TES mode for VO2 NW sensor, where the temperature is set exactly at TMIT, appears to be experimentally challenging task due to thermal instabilities. In addition, hysteresis effects impede the implementation of the negative feedback to stabilize the temperature. The Nano Lett., Vol. 9, No. 6, 2009
Figure 1. (a) The design and principle of operation of VO2 NW MIT gas sensor. PG and PL indicate heat fluxes dissipating in to the gas environment and metal contacts correspondingly. (b) Realization of the sensor using thin long nanowire with its ends immersed in the Ga-In-Sn liquid microdroplets. The electrodes of the NIST µhp served as the base for the liquid contacts. (c) Alternatively, wide microscopic VO2 ribbons were employed when the imaging of the metal (M) and insulator (I) domains dynamics was required. (d,e) The appearance and evolution of the M domains (dark) with increasing the Joule heat (optical images were taken with 1000× magnification, scale bars correspond to 10 µm).
Figure 2. I(V) scans of a self-heated NW at different Ar pressures show typical onset of MIT (forward direction) as well as the typical scatter in transition voltage. The cooling (metal-to-insulator) part of the curve is hysteretic with V-MIT being beyond the bias voltage region shown in the figure.
simplest practical solution would be to use periodic thermal cycling of the nanowire around MIT. To reduce the thermal inertia (and therefore the sensor’s response time) and the power consumption of the sensor, we employed the self-heating mode of operation, where the Joule heat, released in the NW during the bias ramp, was used to trigger MIT directly (Figure 2). This approach was recently used to study size effects on VO2 NW metal-insulator transition,44 current-induced domain wall dynamics in VO2 NW,36 and gas sensing with metal oxide nanowires.48,49 In this operation mode, the NW acts as a preheated thermistor whose temperature (and thus resistance) depends on the delicate balance between the incoming Joule heat and outgoing heat fluxes.44,48 The former is manipulated through the DC bias ramps, whereas the latter ones are determined mainly by the type of the ambient gas, its temperature, 2323
pressure, and heat dissipation into the metal contacts. Thus, any variations in the thermal conductivity of the ambient gas will be recorded as shifts in the transition voltage for MIT. One of the significant advantages of this transduction principle is its inherent independence on the chemical reactivity of a gas, what allows detection of chemically inert gases. Figure 2 demonstrates I(V) forward scans of the VO2 single crystal mesoscopic ribbon suspended over the substrate at four different ambient pressures of Ar. (Due to qualitative similarity between the results obtained on the nanowires and mesoscopic ribbons, we will use the general notation NW unless the difference is specifically stated. Ribbons were used for combined electrical and optical studies.) Forward voltage ramps from 0 to 10 V cause an increase of Joule heat released in the NW that leads to its temperature rise and finally to transition at a certain V+MIT transition voltage. The ramping rate was tested between ca. 1 and 5 V/s. No significant difference in performance of the device was found for this frequency range. The observed insensitivity of V+MIT and hysteresis effect to the ramping rate is due to the fact that thermal equilibrium in the nanostructure establishes faster (in the order of fraction of millisecond) with respect to the ramping rate. The latter, along with the optical observations of the domains dynamics (Figure 1c-e and Figure S4 of Supporting Information) is in line with the previous results from ref 38 obtained for low frequency domain. With the increase of the Ar concentration, the thermal losses to environment raise which results in forward shifts of the transition voltage to higher values. The sharpness of the MIT, potentially perfect reproducibility of the V+MIT and small thermal inertia of the single crystal nanowire thermistor open a promising detection scheme for these TES-like gas sensors. (In our routine measurements, the variation of the V+MIT was in the order of 0.1 V. This value can be further improved by optimizing the design of the device (see supporting material).) The proposed detection mode is essentially an adaptation of the lock-in technique where the periodic ramping of the thermistor’s bias serves as a reference waveform (Figure 3a). In this approach one can set a narrow “band” of bias voltages where the onset of MIT takes place and monitor a “phase shift” signal ∆V+MIT as a function of the changes in the environment. When the V+MIT band is set narrow enough, any tiny changes in composition or concentration of the gas will induce the shift in V+MIT beyond the band with a concomitant sharp drop of the output signal. In our measurements, we employed a simplified version of this approach by continuously ramping the bias applied to the nanowire with a frequency ca. 1 Hz and automatically detecting the transition voltage V+MIT through the onset of the current jump. The V+MIT was monitored as a function of time and analyte partial pressure. The response to analyte admission ∆V with respect to initial background pressure (in our case ∼1 Torr) was defined as a sensor signal (Figure 3 a). A typical gas sensing measurement of this type is shown in the Figure 3b. As can be seen, as the sensor is exposed to the gas, the transition voltage V+MIT increases concomitantly with Ar pressure increase, compensating the growth of the 2324
Figure 3. (a) The proposed gas detection scheme uses the bias onset of the MIT as a sensing signal; (b) the response of the self-heated VO2 microribbon sensor to three different pressure helium pulses.
thermal losses into the gas ambient. Upon removal of the Ar, the sensor readings return to their background value. These data in the Figure 3 were obtained on a few micrometer wide but submicrometer thin single crystal VO2 ribbon. This mesoscopic thermistor achieved a maximum sensitivity level ca. S ∼ 10-3 V/Pa to light gases at low pressure range, which can be further improved at least by 2 orders of magnitude via optimizing the dimensions of the nanowire (see Supporting Information). To elucidate the details of the transduction mechanism, dependences of the sensors signal as a function of the gas pressure and its molecular mass have been measured. Four different gases were tested in this mode: hydrogen, helium, air, and argon in the range of pressures 1.5-560 Torr. The dependence of the sensor signal on the concentration of the gas is depicted on the Figure 4a. The sensor response ∆V increases with concentration not linearly but obeys a squareroot dependence. On the other hand, the transition power PMIT depends linearly on the gas pressure (Figure 4b). This behavior of the meso-thermistor can be explained in terms of a simple heat-balance evaluations assuming that the temperature of the self-heated suspended NW has a linear gradient from highest T in the center to the ambient T0 at the electrical leads.48 The Joule power, generated in such a NW is dissipated via three channels: heat transfer to the ambient gas, to the metal contacts, and via radiation losses. The latter channel is negligible in the practical temperature range compared to the first two and one can get P ) IV )
4Sκ(T - T0) +υ L
R S*(T - T0)p ) B + Rp 8T0 Mπ (1) Nano Lett., Vol. 9, No. 6, 2009
Figure 4. (a) Voltage response of the self-heated NW to gas pulses for four different gases as a function of pressure; (b) transition power vs gas pressure graph for the self-heated NW and four different gases (the slope of this graph defines the R parameter); (c) The experimental data for the R parameter proving its linear dependence on the reciprocal square root of the molar mass of the gases.
Where S is the cross-sectional area of the NW, κ is its thermal conductance, T is the temperature at its center, T0 is the contacts temperature (room temperature), L is the NW’s length, S* is its surface area, υ ∼ 4 is a numerical parameter which depends on accommodation thermal coefficient and gas specific heat ratio, R is the universal gas constant, M and p are molar mass and pressure of the ambient gas, respectively. The first summand of the formula is the term of the thermal losses into the metal leads and the second is the Knudsen term,50 which describes heat dissipation to the gas ambient. Since the parameters B and R depend on the geometry of the nanowire thermistor and type of gas, eq 1 predicts the linear dependence of the transition power with the gas pressure, which is consistent with the experimental data (Figure 4b). On the other hand, under constant bias conditions with R being the NW’s resistance in the semi+ conductor at the onset of MIT PMIT ) IVMIT ) VMIT2/R ) + BMIT + RMITp. Hence, VMIT ) (BMITR + RMITRp)1/2 and under condition p0 , p the thermistor response becomes ∆V ≈ √BMITR + RMITRp - √BMITR
(2)
In spite of the simplifications used in the model, there is a satisfactory agreement between experimental data for ∆V and PMIT as a function of pressure (Figure 4a, b) and corresponding fitting curves. The mass dependent parameter R can be extracted from the slope of the transition power PMIT versus gas pressure (Figure 4b). The model also predicts that R should be inversely proportional to the square root of the molar mass of the gases. Corroboration of this prediction can be found in Figure 4c (we used a molar mass of 29 × 10-3 kg/mol for air). The same set of measurements was performed on several nanowires including those for which an external microhot plate heater was used. The nanowires significantly different in size (so that their width varies from ca. 100 nm up to few micrometers) were tested, yet the measurements on all of them yielded qualitatively similar results with all of the dependencies easily fitted into the above model. To summarize, the proposed simple heat balance evaluations, being applied to VO2 MIT-based nanowire thermistor, adequately explain all major experimental data and dependences. The latter indicate that there is not any Nano Lett., Vol. 9, No. 6, 2009
significant chemical interaction between tested gas molecules and nanowire surface and the proposed interpretation of the transduction mechanism in terms of thermistor is correct. In the frame of selected detection scheme, the sensitivity of the sensor S can be defined as S≡
RR dV ) dp B/R + p
(
)
1/2
(3)
and therefore the sensor will have higher sensitivity at lower ambient pressure, and for lighter gas, longer nanowire with smaller diameter (see Supporting Information). The latter define the optimization strategy for fabrication and deployment of TES-like gas sensing nanothermistors. As an example, the suspended nanowire having the width, thickness, and length to be 80 nm, 40 nm, and 300 µm, correspondingly, would have the sensitivity to helium S ∼ 1 V/Pa in the pressure range 10-100 Pa (see Supporting Information). Assuming the achievable experimental scatter in the V+MIT value to be in the order of 10-2 V, one will be able to detect 10-2 Pa pressure changes with VO2 nanothermistor. Such a performance places these nanoscopic sensors ahead of the closest commercial analogs such as Pirani, and thermocouple pressure gauges. In conclusion, a new type of nanoscopic gas sensor was proposed and tested for which the pressure dependent onset of metal-insulator transition in single crystal suspended VO2 NW was used as a sensor signal. The proposed TES-like nanothermistors are able to operate in a wide pressure range and offer potentially excellent sensitivity to a large moiety of chemically inert and reactive gases. The performance of the sensor can be further improved by decreasing the nanowire diameter, increasing its length (see Supporting Information) and doping with W to reduce the transition temperature. Another possible direction to increase the sensitivity and selectivity of the proposed sensor will be via improving the coupling between surface redox reaction and MIT. This can be done by functionalizing the surface of the VO2 NW with a catalyst, which promotes exothermic redox reaction. The latter will trigger MIT in self-heated NWs at much lower concentrations of the reactants. Finally, in addition to gas sensors, these kinds of devices can be used 2325
as nanoscopic ultrasensitive anemometers and nanobolometers. Acknowledgment. Authors thank Dr. D. Meier and Dr. S. Semancik (NIST) for providing us with calibrated micro hot plates. We are thankful to Professor Martin Moskovits (UCSB) and Professor David Cobden (UW) for fruitful discussions. The research is partially supported through ACS PRF#45842-G5 and Caterpillar Inc. 21305 (supervised by Dr. S. Zemskova) research grants. Supporting Information Available: This material is available free of charge via the Internet at http://pubs.acs.org. References (1) Hansen, K. M.; Thundat, T. Methods 2005, 37 (1), 57–64. (2) Lange, K.; Rapp, B. E.; Rapp, M. Anal. Bioanal. Chem. 2008, 391 (5), 1509–1519. (3) Wang, Z. L. Mater. Today 2007, 10 (5), 20–28. (4) Epstein, J. R.; Walt, D. R. Chem. Soc. ReV. 2003, 32 (4), 203–214. (5) Rakow, N. A.; Suslick, K. S. Nature 2000, 406 (6797), 710–713. (6) Naddo, T.; Che, Y. K.; Zhang, W.; Balakrishnan, K.; Yang, X. M.; Yen, M.; Zhao, J. C.; Moore, J. S.; Zang, L. J. Am. Chem. Soc. 2007, 129 (22), 6978+. (7) Patolsky, F.; Zheng, G. F.; Lieber, C. M. Anal. Chem. 2006, 78 (13), 4260–4269. (8) Chen, P. C.; Shen, G. Z.; Zhou, C. W. IEEE Trans Nanotechnol. 2008, 7 (6), 668–682. (9) Comini, E.; Baratto, C.; Faglia, G.; Ferroni, M.; Vomiero, A.; Sberveglieri, G. Prog. Mater. Sci. 2009, 54 (1), 1–67. (10) Kolmakov, A. Int. J. Nanotechnol. 2008, 5 (4-5), 450–474. (11) Pearton, S. J.; Kang, B. S.; Gila, B. P.; Norton, D. P.; Kryliouk, O.; Ren, F.; He, Y. W.; Chang, C. Y.; Chi, G. C.; Wang, W. M.; Chen, L. C. J. Nanosci. Nanotechnol. 2008, 8 (1), 99–110. (12) Kolmakov, A.; Chen, X. H.; Moskovits, M. J. Nanosci. Nanotechnol. 2008, 8 (1), 111–121. (13) Law, M.; Sirbuly, D. J.; Yang, P. D. Int. J. Nanotechnol. 2007, 4 (3), 252–262. (14) Irwin, K. D.; Hilton, G. C. Transition-Edge Sensors. In Cryogenic Particle Detection; Enss, C., Ed.; 2005; Vol. 99, pp 63-149. (15) Wei, J.; Olaya, D.; Karasik, B. S.; Pereverzev, S. V.; Sergeev, A. V.; Gershenson, M. E. Nat. Nanotechnol. 2008, 3 (8), 496–500. (16) Morin, F. J. Phys. ReV. Lett. 1959, 3 (1), 34–36. (17) Kim, H. T.; Chae, B. G.; Youn, D. H.; Maeng, S. L.; Kim, G.; Kang, K. Y.; Lim, Y. S. New J. Phys. 2004, 6. (18) Eyert, V. Ann. Phys. 2002, 11 (9), 650–702. (19) Qazilbash, M. M.; Brehm, M.; Chae, B. G.; Ho, P. C.; Andreev, G. O.; Kim, B. J.; Yun, S. J.; Balatsky, A. V.; Maple, M. B.; Keilmann, F.; Kim, H. T.; Basov, D. N. Science 2007, 318 (5857), 1750–1753. (20) Cavalleri, A.; Dekorsy, T.; Chong, H. H. W.; Kieffer, J. C.; Schoenlein, R. W. Phys. ReV. B 2004, 70 (16), 161102(R). (21) Calatayud, M.; Silvi, B.; Andres, J.; Beltran, A. Chem. Phys. Lett. 2001, 333 (6), 493–503.
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Nano Lett., Vol. 9, No. 6, 2009
