Langmuir 1989,5,797-805
797
Interactions of NH3 and O2with the Surfaces of Chlorogallium Phthalocyanine Thin Films: Microcircuit Photoconductivity and Quartz-Crystal Microgravimetry Studies S. Waite,f J. Pankow, G. Collins, P. Lee, and N. R. Armstrong* Department of Chemistry, The University of Arizona, Tucson, Arizona 85721 Received October 26, 1988 The chemisorption of Ozand NH3on the surface of G a P d 1 vacuum-deposited thin films hns been studied by a combination of photoconductivity changes on interdigitated array microcircuits (MC) and mass uptake on quartz-crystal microbalances (QCM). Measurement of photoconductivity versus dark conductivitychanges gives a higher sensitivity to changes caused by the adsorption of monolayer (or less) levels of these gases. The simultaneous monitoring of mass changes and photoconductivity changes shows clearly that there exists more than one type of chemisorption site for both 02 and NH3 and that these molecules compete for at least one of these sites. The response of the GaF’d1-coated microcircuit to NH3in room temperature air is poor, because of this competition with 02.Photoelectrochemical modification (A$ deposition at submonolayer coverages) of the Pc-coated microcircuit is possible, however, which then provides for a reasonable room temperature response to “3, presumably by producing a unique chemisorption site for this molecule.
Introduction We have recently begun to explore the effects of chemisorbed molecules on the dark conductivity and photoCondensed-phase organic molecules (polymers or moconductivity of thin Pc films such as the tetravalent and lecular aggregates) generally have electrical conductivities trivalent metal Pc chlorogallium phthalocyanine (e.g., that place them in the class of insulators or nearly insuTiOPc, GaPc-C1) vacuum deposited on microcircuit (MC) lating semicondtlctors.’ It is well-known, however, that interdigitated electrode arrays, which maximize the intheir conductivities can be increased by several orders of fluence of near-surface compositional changes on their magnitude through the incorporation or chemisorption of electrical properties.“ The GaPc-Cl thin films reported electron-accepting or -donating molecular When conductivity measurements are made in such a way as to maximize the effects of conduction in the near-surface (1) (a) Meier, H. Organic Semiconductors; Verlag Chemie: Berlin, region of a thin film of the molecular material, the chem1974. (b) Simon, J.; Andre, J.-J. Molecular Semiconductors; Springerisorption of even submonolayer amounts of an electronVerlap: New York, 1985; pp 73-148. (c) Stilinsh, E. A. Organic Molecular accepting or -donating molecule can have a measurable Crystals; Springer-Verlag: New York, 1980. influence on the c o n d ~ c t i v i t y . ~It ~ is this effect of (2) (a) Marks, T. J. Science 1986,227,881. (b) Dirk, C. W.; Inabe, T.; Schoch, K. F.; Marks, T. J. J. Am. Chem. SOC.1983,105,1539. (c) Diel, chemisorption on the near-surface conductivity which is B. N.; Inabe, T.; Lyding, J. W.; Schoch, K. F.; Kannewurf, C. R.; Marks, the basis for the use of molecular thin films as chemical T. J. J. Am. Chem. SOC.1984,106,7748. (d) Inabe, T.; Gaudiello, J. G.; sensors, e.g., the chemiresistor. These same molecular Maguel, M. K.; Lyding, J. W.; Burton, R. L.; McCarthy, W. J.; Kammewurf, C. R.; Marks, T. J. J. Am. Chem. SOC.1986,108,7595. (e) Diel, B. materials can be used in gravimetric sensors such as surface N.; Inabe, T.; Lyding, J. W.; Schoch, K. F.; Kannewurf, C. R.; Marks, T. acoustic wave (SAW) devices, where changes in electronic J. J. Am. Chem. SOC.1983,105,1551. (0Nohr, R. S.; Kuznesof, P. M.; properties can also have a profound effect on the apparent Wynne, K. J.; Kenney, M. E.; Siebernmann, P. G. J. Am. Chem. SOC. 1981,103,4371. (g) Lineky, J. P.; Paul,T. R.; Nohr, R. S.; Kenny, M. gravimetric response to a chemisorbed analyte.3aJo E. Imrg. Chem. 1980,19,3131. Nohr, R. S.; Wynne, K. J. J. Chem. SOC., Our own interest in phthalocyanine thin films grew out Chem. Commun. 1981, 1210. of their response to light, in photoelectrochemid studies (3) (a) Snow, A. W.; Barger, W. R.; Klusty, M.; Wohltjen, H.; Jarvis, of thin films of tetravalent and trivalent metal P C ’ S . ~ ~ J ~N. L. Langmuir 1986,2,513. (b) Wohltjen, H.; Barger, W. R.; Snow, A. W.; Jarvis, N. L. ZEEE Trans. Electron. Devices 1986,32, 1170. These films showed reasonable energy conversion effi(4) (a) Wright, J. D. Mater. Sci. 1987,13, 294. (b) Chadwick, A. V.; ciencies but showed a large sensitivity to incorporated Dunning, P. B. M.; Wright, J. D. Mol. Cryst. Liq. Cryst. 1986,134,137. (c) van Ewyk, R. L.; Chadwick, A. V.; Wright, J. D. J. Chem. SOC.,Farimpurity “dopants”. Several investigators have advocated aday Trans. 1 1981,77,73. (d) Wright, J. D.; Chadwick, A. V.;Meadow, that the photovoltaic response of thin-film molecular B.; Miesik, J. J. Mol. Cryst. Liq. Cryat. 1983, 93, 315. materials is entirely due to an “impurity-doped”interfacial (5) (a) Collins, R. A.; Mohammed, K. A. Thin Solid Films 1986,145, region, which is the only area where exciton dissociation 133. (b) Wilson, A.; Collins, R. A. Phys. Statue. Solid A 1986,98,633. (6) Honeybourne, C. L.; Ewen, R. S.J. Phys. Chem. Solids 1983,44, and charge carrier formation is fa~ored.’~J~ Of particular 215, 833. interest is the competition between electron-accepting (7) Bott, B.; Jones, T. A. Sene. Acctuators 1984,5, 43. impurity dopants, such as 02,and nominally electron-rich (8)Doug, P.; Price, M. G. J. Chem. SOC.A 1969, 236. (9) Sakaguchi, M.; Ohta, M. J. Solid State Commun.1986, 61, 130. molecules, such as NHB. Both molecules may interact with (10) Ricco, A. J.; Martin, S. J.; Zippeman, T. E. Sens. Actuators 1985, (a) mrdinatively unsaturated metal centers which are part 8, 319-333. of the Pc, (b) the benzoid portion of the molecule (espe(11) (a) Rieke, P. C.; Armstrong, N. R. J. Am. Chem. SOC.1984,106, 47. (b) Klofta, T.; Rieke, P. C.; Linkous, C. A,; Buttner, W. J.; Nanthacially for 02,a weak charge-transfer interaction), or (c) kumar, A.; Mewborn, T. D.; Armstrong, N. R. J. Electrochem. Soc. 1986, defects, e.g., molecular fragments of the Pc molecule 132,2134. (c) Klofta, T.; Daneiger, J.; h e , P.; Pankow, J.; Nebesny, K. and/or metal impurities incorporated during deposition W.; Armstrong, N. R. J. Phys. Chem. 1987,91,5646,5651. (12) Sims, T. D.; Pemberton, J. E.; h e , P.; Armstrong, N. R. Chem. of the thin film.
* Author to whom correspondence should be addressed. Present addrese: Department of Chemistry, University of Utah.
0743-7463/89/2405-0797$01.50/0
Mater. 1989,1, 26. (13) (a) Popovic, 2.D. J. Chem. Phys. 1982, 77, 498; 78, 1552. (b) Martin, M.; Andre, J.-J.; Simon, J. J. Appl. Phys. 1983, 54, 2792. (c) Leempoel, P.; Fan,F.-R. F.;Bard, A. J. J.Phys. Chem. 1983,87, 2948.
0 1989 American Chemical Society
798 Langmuir, Vol. 5, No. 3, 1989 here may not be the optimum choice for Pc-based sensors. It is demonstrated, however, that the sensitivity toward molecules which are potential analytes for any Pc-based chemiresistor can be greatly enhanced when photoconductivity versus dark conductivity measurements are made on molecular materials such as these Pc's. The trivalent and tetravalent metal Pc's have demonstrated a large difference between dark and photoconductivity (asmuch as 10o0:1), in contrast to some of the more traditional divalent metal Pc's, where the contrast may be at most 2:l."J2J4 Through the combined use of microgravimetric sensors coated with the same Pc film as the MC, it is possible to compare the chemisorption process of O2 and NH3 on the Pc surface. Combinations of the photoconductive GaPc-Cl thin film with metal surface modifiers can produce other chemisorption sites for gas-phase molecules such as NH,, which may eventually allow us to develop real selectivity into these sensor materials.
Waite et al.
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(14)(a) Lee, P.; Pankow, J.; Danziger, J.; Nebesny, K. W.; Armstrong, N. R. In Deposition and Growth, Frontiers for Microelectronics; Rubeloff, G., Ed.; Am. Inst. of Physics: New York, 1988; No. 167,p 376. (b) Armstrong, N. R.; Lee, P.; Pankow,J.; Danziger, J.; Nekny, K. W. In Photoelectrochemistry and Electrosynthesis on Semi-conducting Materiakr; Ginley, D., Armstrong, N. R., Nozik, A., Ede.; Electrochemical Society: Pennington, NJ, 1987.
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Experimental Section The interdigitated gold electrode arrays were produced at Motorola, Mesa, AZ. Each array consisted of 59 gold fiigers, ca. lo00 A high, 3.2 pm wide, and with a 3.2-~mfinger center to finger center spacing. The active length of the interdigitated array was 1.3 mm, yielding a geometric active area of ca. 5 X cm2. The effective area used in the calculation of conductivities ( u ) or photoconductivities (a*), which are discussed below, was the product of the perimeter length around each of the array fiigers and the height of each finger (Aefi = 7.5 X 10" cm2). Because this area was small,it was oftan difficult to get an a m a t e measure of dark conductivity of the pure Pc films. Many previous studies with interdigitated circuits have used a much larger area,resulting in more easily measured dark conductivitie~.~~ The arrays used in this study were created by a standard lithographic procedure which depositied the Au layer over a thin chromium film on a single-crystalsapphire substrate. Scanning electron microscopy of the finished arrays demonstrated sharp metal edges with less than 10% deviation from 90" angles at all of the interfaces. Immediatelyprior to insertion intothe vacuum for Pc deposition, the circuits were cleaned in a radio frequency plasma chamber (Harrick Scientific) to remove traces of hydrocarbon, chlorine, fluorine, etc., which Auger electron spectroscopy showed were present on the as-received circuits. We have found this step and others described below to be critical to the formation of Pc thin films with low dark currents, a fact which may not have been apparent in previous studies involving chemiresistors." Vacuum deposition of the GaPc-Cl was carried out in two different chambers. The first chamber was a traditional sublimation vessel with a base pressure of lo4 Torr." Sublimation temperatures were ca. 300 "C, while the microcircuit came up to a temperature of ca. 110 OC during the deposition. GaPCCl films deposited by using this system were an average 240 equivalent monolayers thick (ca. 82 nm). Thinner (ca. 7-11 nm), more compact GaPc-Cl films were deposited on these same microcircuits in an ultra-high-vacuum (UHV) deposition chamber (base pressure ca. lo4 Torr) which uses effusion sources and shutter assemblies to direct beams of the molecular material at a variety of substrates (Figure 1A). This system has recently been described.14 In this system, the MC was held at a temperature of 77 "C during pump down and bake out cycles (and at temperatures described in the text during the Pc deposition steps). Holding the MC above ambient temperatures during pump down helps create films with much lower dark current (in the absence of dopants) and high photo-to-dark current contrast, versus Pc-MC aeaemblies created in the more traditional vacuum environments. A QCM was also occasionally coated with Pc simultaneouslywith
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Figure 1. (A) Schematic of the UHV PC deposition chamber
showing the positioning of the effusion sources (only one of which was used in this study), the microcircuit (MC),the quartz-crystal microbalance (QCM), the illumination sources external to the systems, and some of the control electronics. (b) Schematic of the microcircuitovercoated with the Pc thin f i b and subsequently decorated with Ago nuclei by photoelectrochemical metal deposition. the MC, so that absolute Pc surface coverage could be known (in equivalentmonolayers) and so that the QCM could be used along with the MC in subsequent vacuum and atmosphere chemisorption/sensing experiments. Exposure to dopant gases in the UHV system was controlled by standard leak values. After the MC and QCM combinations were brought up to atmosphere and back to UHV several times (see text), they were removed and placed in the atmospheric pressure-sensingchamber. This chamber was an all stainless steel system based on a UHV 2.75-in. conflat flange and nipple assembly and associated tubing, with gas-entrance and exit ports, and a wide-anglequartz view port for illumination of the microcircuit. All of the surfaces exposed to the flowing g- were stainlesssteel, with the exception of the microcircuit and its associated TO-99 socket (Bakelite). Temperaturesfor the chamber were controlled between 20 and 110 "C,monitored by means of a thermocouple situated adjacent to the MC-QCM region of the cell. The inner volume of the cell was ca. 100cm3. After assembly of the chamber, the entire system was purged with atmosphericpressure nitrogen at 65 "C for 17 h. Illumination of the MC was carried out by several different light sources. Photocurrent yield spectra were obtained with a 450-W xenon arc lamp (Oriel) monochromatized with either a Jobin-Yvon H-20 or H-1OIR halographic grating monochromator. The photon output of the monochromator was normalized with a calibrated photodiode assembly or a thermopile to correct the photocurrent spectra for lamp/monochromator wavelength variations. There is considerableuncertainty in the photocurrent yield spectra in the region near 800-900 nm because of the power fluctuations of the arc lamp source in this region, which are difficult to compensate for. For photocurrent spectra, the light source was chopped and the photocurrent yield demodulatedand measured by using a potentiostat i/V converter and lock-in amplifier. The potentiostat was also used to control the potential drop between the interdigitated array electrodes (as in Figure 1A).
Langmuir, Vol. 5, No. 3, 1989 799
O2 and NHs Chemisorption on GaPc-C1 Films
The other light sources were either (a) a 25-mW He-Ne laser (Spectra-Physics), (b) a 5-mW He-Ne laser (Hughes),or (c) an 820-nm or 85onm GaAs/GaAlAs diode laser (Spectra-diodeLabs) with 0.5-W peak power (60-ns pulse width) and 5-mW average power. The pulse control circuit for thia diode laser was housebuilt and provided the control logic for the laser driver. For most applications,the laser was kept defocused so that it illuminated the entire MC. A pulse train was produced with 10000-300000 pulses/s (the laser is effectively a CW source for these experimenta). All laser sources were subsequently modulated at 14 Hz, and the correspondingphotocurrent was measured by the same demodulation scheme described above. Occasionally, the light sources were left unmodulated and the dark current and photocurrent measured with a Keithly 485 picoammeter. The QCMs used in this study were 10-MHz AT-cut quartz crystals with silver coatings on the front and back surfaces. The Pc coating was applied to only one surface, leaving the back of the crystal exposed to the analyte gas. The response of the uncoated silver surfaces to analyte gases such aa NH3was therefore corrected (see text). The frequency shift of the QCM was measured by means of an HP 5327 C or an HP 5394 A frequency counter. The mass sensitivity of this QCM was calculated to be 4 ng.Hz-1-cm-2for small molecules such as O2 and NH3. Gas compositionswere varied by adjusting the flow rates of the analyte gas (primarilyO2and NH3)in the presence of various flow rates of N2 or air. All gases used were Matheson 99.999% purity (Nz), 99.99% purity anhydrous NH3,and 99.5% (02).N2was passed over copper turnings and molecular sieves prior to introduction into the sensing cell. O2 and NH3 were passed over molecular sieves. GaPc-Cl was synthesized according to previously published procedures and purified by Soxhlet extraction in a variety of solvents, followed by repeated sublimation (2-3 times).11*12 Electrochemical modification of the GaPc-C1-coated microcircuit and QCM was carried out with M Ag+ in a pH 1(0.1 M HN03) solution. The MC was positioned under a drop of electrolyte (Figure 1B)extruded from a silver-platedsyringe which also acted as the counter and reference electrode. In some experiments, the syringe was occasionally accompanied by small separate counter and reference electrodes inserted into the electrolyte drop. Potentials were held at a point where Ag+ Ago reduction would occur photoelectrochemically but where reduction in the dark was not significant,as recently described.12
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Results and Discussion Photocurrent Yield Spectra. Figure 2a shows the photocurrent yield spectrum (in a stream of flowing, room temperature, atmospheric pressure nitrogen) of an 82-nm G a P d 1 film vaccum deposited on the microcircuit. The photocurrent yield has been corrected for power fluctuations in the lamp source and is presented as incident light quantum efficiency (electrons per photon). The potential difference between the interdigitated electrodes was 2-4 V corresponding to an electric field gradient across the Pc film of 6.3 X lo3 to 1.3 X lo4 V-cm-'. Parts a and b of Figure 2 were taken with a monochromator whose throughput was limited to wavelengths lower than 820 nm. Figure 2c was taken with a monochromator with better near-Et throughput. Subsequent experiments have shown that this does not alter the conclusions drawn about the behavior of the Pc films in the 500-800-nm region. As we have shown earlier, the Q-band absorbance region for Pc's such as GaPc-C1, AlPc-C1, TiOPc, VOPC, etc., is extensively broadened and red-shifted in the condensed phase and actually represents two or more different three-dimensional stacking patterns of the adjacent Pc rings with different levels of photoactivity.'J1JZJ6 The photocurrent yield spectrum on the MC is somewhat (16)(a) Loutfy, R. 0.;Hor, A. M.; DiPaola-Baranyi, G.; Haiao, C. K. J.Zmuging Sci. 1985,29,116. (b) Griffthe, C. H. Mol. Cryst. Liq. Cryst. 1979, 33, 149. (c) Ziolo, R. F.; Griffitha, C. H.; Daltlon J. Chem. SOC., Chem. Commun. 1980,2300. (d) Ogorodnik, K.2.Opt. Spektrosc. 1976,
39, 396.
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Photocurrentyield spectra of GaPc-ClIMCcomposites in (a) flowing, room temperature,atmospheric pressure NP,(b) flowing, room temperature air, and (c) N2 + ca. 1ppt NH3after several days of use and temperature excursions to 95 O C and back to room temperature. Figure 2.
different than the absorbance spectrum of comparable thin films on glass or other smooth substrates, which is consistent with the fact that different crystalline morphologies are expected after deposition on substrates with different roughness and different composition. The Pc film shows photocurrent production in the entire wavelength region between 550 and 850 nm with a maximum response in the region in the vicinity of 780 nm. The absorbance region near 800 nm is principally due to a slipped stack phase of GaPc-C1 (versus the cofacial, linear stack, which absorbs in the 600-nm region9 and has clearly been optimized for the Pc film on the MC with respect to the phase corresponding to absorbance in the 600-nm region. We have recently demonstrated that the phase absorbing near 800 nm has a higher overall photoactivity by a fador of at least lox, so that the response of Figure 2a should be viewed acc~rdingly.'~J~ The photocurrent yield spectrum taken in room temperature air (Figure 2b) shows the same basic response, with slightly higher quantum efficiencies at all wavelengths and a higher relative photoconductive response in the 550-700-nm region. Because the measured photoconductivity is so critically dependent upon alignment of the MC in front of the illumination source and the power variations of the source, the absolute differences in photoconductivity may not be significant. The relative differencesin the 550-700-nm region may be, however, and this effect is under investigation. Following exposure of the MC to NP, and NH3 in N2 at 80-95 OC for several hours, the photocurrent yield spectra of Figure 2c were observed. A clear blue shift in the photocurrent yield spectrum is observed, in contrast to the transmission optical absorbance spectra of comparable thickness GaPc-C1 thin films on transparent substrates, where annealing in N2produces only slight spectral shifts. We conclude from these results that phase transformations
800 Langmuir, Vol. 5, No. 3, 1989
Waite et al.
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can occur in the Pc film as the result of high-temperature sensor operation, especially in a very thin region near the surface, where the photoconduction process predominates (at levels not detectable by optical spectroscopies). The spectrum obtained for ca.5 ppt NHs in N2 is qualitatively similar to the N,-only spectrum, indicating no clear selectivity a t any wavelength for the effect of NH3 chemisorption on photoconduction. Unleas otherwise stated, the data presented here are generally taken from circuits which had not seen extended use a t high temperatures or for which any changes in response had stabilized. Initial Response of Pc Films to Atmosphere. Before dealing with the issue of the response of the microcircuit to NH3 and O2 at atmospheric pressure conditions, it is worth considering the response of the Pc-coated microcircuit and a similarly coated QCM to atmospheric gases immediately following deposition in UHV (Figure 3). The GaPc-Cl films in question for this experiment were grown from the effusion source at a rate of ca. 0.8 nm/min to a thickness of ca. 7 nm, while the MC was maintained at a temperature of ca. 77 "C. We have found that this is a condition which produces a denser GaPc-Cl film with a nearly undetectable dark current at room temperature, and larger photocurrent yields than for films grown on room temperature MCs in the UHV system.14 This difference in electrical properties occurs ostensibly because of better electrical contact between denser Pc film and the Au linea and possibly because of lower surface contamination of the Au-MC before and during the deposition process. Following this deposition process, the circuit was allowed to cool to room temperature, whereupon the vacuum system was slowly vented ta atmosphere while the photocurrent, dark current, and the response of the Pc-coated QCM were monitored. Venting of the vacuum system was carried out slowly through a precision leak valve. The base pressure was initially 2 X lo* Torr, and through regions a, b, and c of Figure 3, the pressure rose to 1 atm. While still a t low pressures, leas than 10-6Torr (region a), no QCM frequency shifts (and only small fluctuations in photoconductivity) were noted. In region b, as the pressure rose to approximately the 10-3-Torrrange, a small QCM frequency shift
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Figure 3. Photocurrent yield of a GaPc-Cl-coated MC (632.8 nm) and frequency shifts of a similarly coated QCM immediately after deposition at ca. lo4 Torr and during the first venting to atmosphere. Region a corr88po11c18to preaaures less than 1 PTorr, region b corresponds to ca. 10" Torr pressures, and region c completes the vent to atmospheric pressure.
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L_0 10 Figure 4. Room temperature photocurrent response (MC, 632.8 nm) and frequency shift (QCM) of a G a P d 1 thin film (ca. 10 nm) upon (a) introduction of 1.6 ppt NHSin the flowing stream of N2and (b)removal of a 4.8 ppt NHSsteady-state flow in the Nzstream. was noted (ca. 2 Hz), and a rise in photocurrent (from ca. 100 to 200 PA) was noted. Finally as the leak valve was opened wider and as atmospheric pressure was achieved in region C, a sharp shift in QCM frequency was noted (ca. 15 Hz) accompanied by a more gradual decline in photocurrent. The photoconductivity at steady state had by then declined from u = 2.0 X l0-g f2-I cm-I at 2 X l0-g Torr to u = 6.7 X iT1cm-I at atmospheric pressure. These changes in mass and photoconductivity were reversible through several evacuation/vent-to-atmospherecycles, but eventually some irreversible small increases in dark conductivity were observed. Similar results were obtained when the system was vented with pure 0% Similar changes in photoconductivity were observed for other Pc's, such as T ~ O P C . ' ~ JThere ~ are clearly complicated surface chemical processes involved during initial exposure of the GaPeCl surface to atmospheric adsorbates. The decline in photoconductivity is consistent with the role that O2 plays in determining the response of the MC at room temperature as described below.
Microcircuit Response at Room Temperature to NH3. After removal from the UHV deposition chamber for the last time, the MC and QCM were placed in the atmospheric pressure sensing cell and, after this exposure to atmosphere, purged with nitrogen for periods of several hours. It was found that this was necessary before the introduction of ammonia to ensure stable and predictable responses to the analyte gas. If the circuit was not purged with nitrogen, the introduction of 0.1 ppt levels or higher of NH3caused an immediate drop in the photoconductivity and dark conductivity, followed by a slow rise to a steady value, which could take hours to achieve. Figure 4a shows the photocurrent (MC) and microgravimetric (QCM) response following the introduction of 1.6 ppt NH3 in N2. This particular response was typical of the thin UHV-prepared GaPc-C1 films (7-11 nm), where (16) Lee, P. A. M.S.Thesis, University of Arizona, 1988.
Langmuir, Vol. 5, No. 3,1989 801
O2and NH3 Chemisorption on GaPc-C1 Films the light source was modulated (632.8 or 830 nm) and where the photocurrent response was processed by demodulation with the LIA with a 30-s time constant. The QCM response shows an immediate, monotonic uptake of NH,, reaching a near-steady-state surface coverage within 3-5 min. The MC response, in contrast, actually shows a small initial decrease in photoconductivity, followed by an increase with a longer time constant than for the QCM. As discussed further below, we believe the initial changes in photocurrent response to be due to competition between the NH, molecule and residual chemisorbed O2(left in the N2 stream) for sites on the GaPc-C1 surface which influence photoconductivity. This transient drop in photoconductivity was only seen with the first introduction of any concentration of NH,. Further changes in NH3 level showed only the monotonic photoconductivity increases. If the circuit was purged with N2 again for several hours, the first reintroduction of NH3 produced the transient photocurrent decrease. Trace O2 in the N2 apparently collects to near saturation coverages on the GaPc-C1 surface. The implication from this data is that there is more than one type of NH3 chemisorption site (which may also be occupied by 02), only one of which plays a role in modulating the population of photogenerated charge carriers. The data in Figure 4b lend further support to this argument. In this experiment, the NH3 concentration was raised to a constant 4.8 ppt, and a steady-state response of both the MC and QCM had been achieved. A t time t = 0, the NH, flow into the N2 stream was stopped. Allowing for a small period to clear the sensing chamber of residual NH3, the QCM returned within 2-3 min to near the original frequency observed prior to the introduction of NH3, followed by a smaller increase in frequency lasting several minutes more. The photoconductivity response, however, showed a slow monotonic decline with a time constant much longer than for the QCM and without the initial rapid change which would have paralleled the change in mass. It appears that the majority of the chemisorbed NH,, lost in 2-3 min from the Pc-coated QCM surface, does not play a significant role in altering the photoconductivity of the GaPc-C1 thin film. Similar data were obtained when the photocurrents from the MC were measured directly with a picoammeter with a much smaller time constant than was used to obtain the data in Figure 4. The delay in MC response cannot be attributed to the time constant of the measurement circuitry. Correction must be made for the possible chemisorption of NH3 to the backside of the QCM since this was silver coated and not covered with the GaPc-C1 layer. Control experiments were conducted on an uncoated QCM over the same NH, concentration ranges as were used for the Pc-coated QCM experiments. Over the range 0 4 ppt NH3, the mass uptake of the QCM was such as to produce a frequency shift of ca. 20 Hz. The adsorption and desorption rates for NH3 were much faster (less than l min to steady state) than for the Pc-coated QCM, so that the data presented here are not perturbed by the rates of these processes. After correction of the frequency shifts, the total mass change of the Pc-coated QCM was converted to an effective surface coverage of NH3 on the GaPc-C1 surface. A change in NH, concentration from 0 to 5 ppt corresponded to a change in population of chemisorbed molecules of ca. 4 X lo* mol/cm2. The chemisorption of one monolayer of NH3 on a closest-packed silver surface (as mol/cm2. Given the an example) would be ca. 1 X uncertainties in surface roughness (a factor of 5-1OX is reasonable from SEM data) and coverage of chemisorption
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Figure 5. Photocurrent and dark current yield versus change in NH3 in N2 concentration at room temperature for (a) illumination at 600 and 800 nm and (b) illumination at 632.8 nm at powers of 8.3 and 25 mW (ca. 0.83 and 2.5 w/cmz). S is the sensitivity of the response in nanoamps of current change per ppt change in NH3 concentration. sites, it is reasonable to conclude that the chemisorbed NH3 represents an equivalent monolayer or less in surface coverage. It was of interest to explore the way in which the use of the photoconductive response of the MC increased the sensitivity to a simple molecule like NH3 in nitrogen. Figure 5 shows the dark current and photocurrent responses as a function of NH, concentration for two different wavelengths and several different light intensities for a GaPc-C1 (ca. 82 nm)/MC not subjected to the annealing discussed in Figure 2. The relative sensitivities are indicated by the slopes of those plots given in nanoamps/p& per thousand change in NH3 concentration. With our microcircuit design and small area, the dark currents for the GaPc-CI films are sufficiently low that the detection limit for NH3 is ca. 1 ppt. Illumination at either 600 or 800 nm (Figure 5a) obviously increases the sensitivity to NH3concentration changes, as would be predicted from the photocurrent yield spectra, which show that for the as-prepared circuits, the photocurrent yields in the 800-nm region are greater than those in the 600-nmregion. Consequently, comparable slopes are obtained in Figure 5a for light sources which differ in intensity by almost a factor of 3. At low light intensities, the sensitivity, S = photocurrent yield change per ppt change in NH, concentration, increases linearly with intensity at all wavelengths but eventually becomes nonlinear, as shown in Figure 5b. The data taken with a He-Ne laser source are shown, where the increase in light intensity from 0 to ca. 5 mW produced a near linear change in S, while the increase from 8.3 to 25 mW produces only a small (ca. 6%) increase in S. The absolute magnitude of the photocurrents measured does approximately linearly scale with the light intensity in this range, confirming that only the near-surface photoconductivity of the GaPc-C1 thin films is affected by the chemisorption of NH3 At the highest light intensities, the photocurrent is dominated by the subsurface photoconductivity, and since the light is absorbed first in the
802 Langmuir, Vol. 5, No. 3, 1989
Waite et al.
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Figure 7. Photocurrent yield and QCM response versus time at room temperature for (a) exposure to 1.6 ppt NHS and (b) removal of a 5.2 ppt NH3 stream from flowing air.
near-surface region, the surface photogeneration sites appear to have been saturated by available photons, hence a change in NH3 concentration only slightly affects a charge carrier population which is near a maximum allowable level in the near-surface region. From a signal processing viewpoint, there is merit in increasing the absolute magnitude of the currents to be measured, thereby decreasing the minimum detectable NH3 levels. A better use of the light, however, is to defocus it over a larger circuit area, which will increase the number of photoexcitable surface states to which the molecule in question can absorb and increase the signal intensity. This is the focus of our present studies. Increase of sensor area will, of course, increase the sensitivity using just the dark conductivity mode, but we estimate that a minimum of 2-3 orders of magnitude increase in sensitivity is achievable by using the photoconductive mode. Competition between NH3 and O2 for Chemisorption Sites. Figure 6 shows the response of the MC to NH3 and O2 a t room temperature and a t 98 OC. These data were obtained in the 830-nm region by using the diode laser operated as a CW source. As expected, the increase in temperature causes the photocurrent response to increase. The photoconductivity, u*, is dependent upon temperature through the relationship u* = a,,* exp(-AE*/kT) (1)
more rapid rise and fall to steady-state responses seen at this temperature. The data for pure O2 in nitrogen (Figure 6b) impact upon the response of the MC to NH3 in air (discussed below). At room temperature, the exposure of the freshly N2-purged MC to O2 actually causes a slight decrease in photoconductivity. This is similar to behavior discussed earlier, for the cases where the UHV-deposited MC was first exposed to atmosphere. Increase of the temperature to 98 "C, however, causes the photoconductivity in O2 to increase with a sensitivity that exceeds that for NH3 exposures. It should be noted that after exposure of the MC to Oz at 98 OC for any length of time a purge of the circuit in pure N2, or N2 with NH3, at least overnight, is necessary to reestablish the response of the MC to NH3 as before. The interaction with O2at this higher temperature is much less reversible than at room temperature. These results are consistent with our previous studies, which showed that doping G a P d 1 films at ca. 100 OC with O2for periods of hours caused detectable, permanent changes in photoelectrochemical behavior, whereas doping at lower temperatures for the same time periods did not produce such changes.Ilc Changes in phatoelectrochemical behavior could be produced with room temperature exposures but could take weeks to months of exposure. Such treatments have also been shown to increase the concentration of free-spin components detected by ESR experiments with powdered GaPc-C111cJ7 and other Pc powders.'J8 Figures 7 and 8 deal with the response of the MC and the QCM to NH3 in dry air rather than nitrogen, so that the full effect of the competition between NH3 and O2 could be determined. Figure 7 represents the response a t room temperature to the introduction of 1.6 ppt NH3and the cessation of a flow consisting of 5.2 ppt NHB. The QCM response to the introduction of NH3 is not as rapid as was the case when nitrogen was the fill gas. The total mass change, after correction for NH3 adsorption of the
where AE* represents the threshold energy necessary to move a photogenerated charge out of a trap site. AE* does not represent the band gap energy, as would be the case for dark conduction.'J4 We have determined that the activation energy for photoconduction in GaPc-C1 films varies from 0.1 to 0.2 eV, while the activation energy for dark condition (in UHV-deposited, pure films) is greater than 1.5 eV (the approximate band gap energy).14 Since raising the temperature affects subsurface as well as surface photoconduction, the slope of the photocurrent vs NH3 concentration plot decreases at 98 "C, since there are obviously more subsurface than surface sites which control photoconduction. The sticking coefficient of NH3 on the G a P c C l surface is also likely lower, as indicated by the
(17)Klofta, T. K. Ph.D. Dissertation, University of Arizona, 1986. (18) Boas, J. F.; Fielding, P. E.; McKay, A. G. A u t . J. Chern. 1974, 27, 7.
Langmuir, Vol. 5, No. 3, 1989 803
Chemisorption on GaPc-C1 Films
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Figure 9. Photocurrent yield spectra (this time plotted as incident light quantum efficiency, in arbitrary units) of (a) a GaPc-Cl/MC in flowing N2(room temperature) and (b) the same circuit following decoration with Ago nuclei as described in the
text. backside of the QCM, was less than half the change seen for NH3 introduced at the same concentration in N2. Of in that wavelength region, to be principally decorated with particular interest is that the MC response showed very Ago nuclei. Illumination in the 800-nm region shows AgO different behavior from the case for NH3 in nitrogen, in that the photoconductivity change was small and actually decoration of some of the first phase and decoration of a platelet phase responsible for a large portion of the light decreased rather than showing an increase. This continued absorbance in that region. to be the case with further additions of higher concentrations of NH3 in air; the photoconductivity decreased Figure 9a shows the photocurrent action spectrum for slightly even though there was clear indication of NH3 a freshly vacuum deposited G a P d l / M C (Pc film thickuptake on the QCM. Figure 7b shows the result of ceasing ness = ca. 80 nm). This circuit was prepared in the vacNH3 flow after a concentration of 5.2 ppt had been uum system with a base pressure of ca. 1 X lo4 Torr, with achieved. The QCM response showed a fast desorption the MC only slightly above room temperature during the of NH3 followed by a slower loss, while the microcircuit deposition. We recognize, as discussed above, that this showed a slow monotonic increase in photoconductivity does not provide for the optimum density of the PC film with time. on the MC but has qualitatively the same response as for At higher temperatures (88 OC, Figure 8), the adsorption a film annealed while growing. The photocurrent yield and desorption of NH3 was more easily detected in the spectrum was obtained after the circuit had been purged changes of the photocurrent response of the MC. The with flowing N2 for over 8 h, and N2 flowed over the room QCM was difficult to stabilize at these higher temperatemperature MC while the spectrum was recorded. There tures, so that these data are not included. Introduction is a clear dominance of the photocurrent response in the of 1.2 ppt NH3 causes a change in the photoconductivity 760-900-nm region. The structure in the spectrum should of the MC over a period of 3-4 min, consistent with the not be taken too literally, as discussed in the Experimental time constant of the response for NH3 in nitrogen at this Section. This kind of accentuated response in this temperature. However, the photoconductivity decreased wavelength region is similar to that seen in Figure 2a. Figure 10 shows that the response of the MC toward NHS as at room temperature in air. Cessation of a 5.5 ppt flow of NH3 also showed a return to steady state within 4-5 in N2 with 632.8-nm illumination (5 mW) gives a slope of min. Similar, but much smaller, changes in dark con0.7 nA/ppt change in NH3 concentration. ductivity were noted for these same exposures. This type Following this control experiment, the MC was removed of response for NH3 in air (decrease in dark conductivity) from the gas-handling cell and subjected to photoelectrohas been cited for other Pc thin films as chemiresist~rs.~*~ chemical deposition of Ago from a M, 0.1 M HN03 The introduction of NH3 has appeared to cause a subsolution, as shown in Figure 1B. Upon contact of the stantial decrease in dark conductivity, but the reasons for solution drop to the surface of the GaPc-Cl/MC, a this have not previously been fully explored. steady-state dark current response of ca. 9 nA was obModification of the GaPc-Cl Surface with Elecserved. Ideally, this dark current response should be trodeposited Silver. We have recently demonstrated the negligible since it represents electrodeposition of Ago at possibility of photoelectrodepositing silver nuclei on the sites which may be dark conductive. It may be due to (a) surface of GaPc-C1 films.12 The distribution of the silver exposed sites on the gold-bonding pads of the MC or on nuclei population on different crystal morphologies deportions of the wire bonds which unavoidably come in pends upon the wavelength of illumination. Illumination contact with the electrolyte (acceptable) or to (b) pore sites in the 600-650-nmregion causes the block/prismatic phase in the Pc film which allow penetration of the electrolyte of GaPc-C1, responsible for most of the light absorbance down to the gold fingers of the MC (less acceptable). The
804 Langmuir, Vol. 5,No. 3, 1989
Waite et al. '4
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mations in supramolecular orientation of the adjacent Pc molecules. The presence of Ago nuclei, however, is significant in changing the response of the Pc film toward "3.
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flowing air stream.
He-Ne laser was turned on to the MC, and a steady-state photocurrent of ca. 14 nA was achieved. A total of 26.9 mC/cm2 (using the active area of the circuit as described in the Experimental Section) was transferred corresponding to the deposition of ca. 0.2 equivalent monolayer of A$ on the active area of the MC. Since the photoactive area of the Pc thin film on the MC is uncertain, this surface coverage is likely to be in error, and it is possible that there exist local regions with larger Ago nuclei than a 0.2-monolayer coverage would suggest. The MC was then rinsed with triply distilled water, dried, and reinserted into the gas-dosing cell. Following an overnight purge of this modified circuit with N2 at 90 "C, the photocurrent action spectrum of Figure 9b was obtained. There is a considerable relative difference in the intensity of response in the 520-650-nm region, and the spectrum now takes on the appearance of many of the thin-film absorbance spectra seen for GaPc-Cl filmson a variety of substrates11J2but is not as blueshifted as Figure 2c. The act of silver deposition might have (a) caused phases of GaPc-Cl, initially present within the film, to be activated to participate in the photoconduction process or (b) actually caused a phase transformation in the near-surface region which produced a new photoactive morphology of the Pc. Dodelet and co-workers have recently shown that solution exposures of AlPc-Cl and related Pc films can cause morphological transformations that change photocurrent yield spectra.lg Even though such transformations have not been seen for GaPc-Cl thin films, it is possible that solution exposure accompanied by Ago deposition does indeed lead to near-surface transfor-
Upon introduction of NH3 in the ppt concentration range, there was a considerable difference in photocurrent response observed. Firstly, the rate of response (compared with Figures 2, 3, 6, and 7) was shortened by a factor of ca. lox. Upon introduction of the lowest concentration of NH3 possible with our system (200-400 ppm), the MC photocurrent response rose to its steady-state value in less than 30 s. It was also clear from this experiment that the sensitivity to NH3 was greatly enhanced. Experiments in progress deal with a much lower concentration range of NH3. The overall sensitivity toward NH3 in N2 at 632.8 nm was increased as shown in Figure lob. The slope now was increased, for the same illumination intensity, to 3.7 nA/ppt NH3 concentration change. When the same range of NH3 concentrations was examined in room temperature air (Figure lOc), the response to NH3 now was measurable, and photocurrent changes were positiue rather than negative, compared to Figures 7 and 8. The presence of silver on the GaPc-C1 surface clearly has a profound effect on the response of this type of chemiresistor.
Conclusions A proposed mechanism for the effects that O2and NH3 have on the conductivity and photoconductivity of GaPc-C1 thin films is shown in Figure 11. Dark conductivity is presumed to occur through thermal ionization of the bulk organic material, with a band gap energy of ca. 2.0 eV or greater, or through the ionization of impurity levels incorporated within the bulk and/or near-surface region^.^?^^ Photoconductivity is initially trap dominated with activation energies of 0.1-0.5 eV depending upon Pc film growth parameters (base pressure, substrate temperature, etc.). It is anticipated, however, that many of the sites which control dark conduction will also affect photoconduction,so'that adsorption of molecules like NH3 will produce a qualitatively similar change in both u and u* (as in Figure 5). Molecular species which constitute electron-accepting or hole-accepting traps modulate the flow of charge carriers in the Pc film and are presumed to exist at their highest concentrations in the near-surface region. The influence of O2 a t atmospheric pressure on the conductivity and photoconductivity of phthalocyaninethin films has been previously investigated and has been generally shown to increase both, presumably by acting as an electron holepair dissociation site (especially for excitons created as the result of an absorbed p h ~ t o n ) . ' ~ ~The *~~~'~ presence of O2has been shown to be especially important (19) BBlanger, D.; Dodelet, J. P.; Dao, L. D.; Lombos, B. A. J. Phys. in the formation of rectifying junctions between Pc's and Chem. 1984, 88 4288. Gray, D.; Cote, R.; Marques, R.; Dodelet, J. P.; Lawrence, M. F.; Gravel, D.; Langford, C. H. In P ~ o t o e l e c t r o c ~ e m ~ ~ ~ r ymetals.138 These results are compounded by the fact that and Electrosynthesis on Semiconducting Materials; Ginley, D.; Nozik, small phase transformations are possible by virtue of A.; Armstrong, N. R.; Honda, K.; Fujishima, A.; Sakata, T.; Kawas, T., gas-phase reactions and annealing of the thin films at Eds.; Electrochemical Society: Pennington, NJ, 1967.
Langmuir 1989,5, 805-808 temperatures near 100 OC (Figure 2 and ref 14 and 15) and apparently by solution exposures/electrodeposition,as in Figure 9. The surface chemistries of phthalocyanine thin films are obviously complex and require additional studies beyond the scope of this paper. Nevertheless, there are several phenomena which have been observed for which we can formulate hypotheses as to the mechanism for the interaction of small molecules with these surfaces. We leave aside for the moment the question of the interaction of water vapor with the GapC-C1 surface, since the effects seen in Figure 3 have been observed both with atmosphere and pure O2 exposures. The chemisorption of O2 with most Pc surfaces, including the GaPc-C1 surface, is weak, and it is therefore not surprising that the effect of O2 on the photoconductivity response in Figure 3 is not seen until 10-3-Torr pressures are achieved.20 We assume that the first adsorbed O2occupies a true surface site (Figure 11,site A), which apparently assists in the photogeneration of charge, possibly through an exciton dissociation mechanism as has been previously p r o p o ~ e d . ~At J ~higher pressures we hypothesize that the chemisorbed O2undergoes diffusion to sites just below the surface (site B in Figure 11, also previously proposed),'lz1 where ita principle role is to act as a trap site, thereby lowering the overall photoconductivity. The activation energy for photoconduction in atmosphere for several Pc's is ca. 0.1-0.5 eV,14consistent with the idea that energetically shallow traps control the flow of charge in the near-surface region, after its generation. Prolonged purging with N2 probably removes some, but not all, of these trap sites and apparently lowers the O2level on the surface (A sites) to trace levels. The presence of such 02-filled,A sites is confirmed with the first introduction (20) (a) Dahlberg, S.C . Appl. Surf. Sci. 1982, 14, 47. (b) Dahlberg, S. C.; Mueser, M. E.Surf. Sci. 1979, 90,1.
(21) Bonham, J. Aust. J. Chem. 1978,31, 2117.
805
of NH3 as a small photoconduction drop. Further introduction of NH3 appears to affect photoconduction through the action of diffusion of the NH3 molecule into a chemisorption/trap site B and either displacement of the molecule responsible for the creation of this trap (O,?) or charge donation to the molecular entity which constitutes the trap. Prolonged exposure of the Pc thin film to O2in the atmosphere apparently creates more of the type A sites, which enhance photoconduction and are then acted upon by NH3 to decrease the photoconductivity (Figure 7). At elevated temperatures, the action of NH3 is apparently to act primarily upon those sites responsible for charge generation, thereby lowering the net photoconductivity. Because of the diversity of surface states on these and other Pc thin films, the development of chemical sensor applications of these materials probably requires a combination of (a) controllable introduction of "impurities", which provide a selective chemisorption site for analyte molecules of interest, and (b) better control over the ordering of Pc molecules or other cyclic conjugation molecular semiconductor materials, on both the MC and QCM, so as to provide a smaller range of chemisorption sites and therefore some hope of understanding the nature of their effect on the conduction and photoconduction processes. Research in progress using electron spin resonance spectroscopies and thermal desorption techniques seeks to identify the energetics of such chemisorption sites on better ordered molecular semiconductort h i n - f h surfaces.
Acknowledgment. We wish to thank Fred Hickernell of Motorola for the generous donation of the microcircuits used in these and other recent studies. This work was assisted by support from the National Science Foundation (CHE 86-18181) and by the Materials Characterization Program-University of Arizona. Stimulating discussions with J.-P. Dodelet are also gratefully acknowledged. Registry No. GaPc-Cl, 19717-79-4;02,7782-44-7;NHS, 7664-41-7.
Deactivation of Naphthalene and Pyrene Derivatives Bound to Aerosol OT Reverse Micelles by Fumaronitrile and Acrylonitrile M. V. Encinas* and E. A. Lissi Departamento de Quimica, Facultad de Ciencias, Universidad de Santiago de Chile, Santiago, Chile
C. M. Previtali and J. Cosa Departamento de Qulmica y Flsica, Universidad Nacional de Rio Cuarto, 5800 Rio Cuarto, Argentina Received July 27, 1988. I n Final Form: January 13, 1989 Fluorescence quenching of cationic and anionic derivatives of naphthalene and pyrene by fumaronitrile and acrylonitrile in AOT reverse micelles was investigated as a function of the water content. For donors bound to the micellar interface, quenching rate constants ( k ~were ) ~evaluated ~ in terms of the interfacial quencher concentration. For both fumaronitrile (a diffusion-controlled quencher) and acrylonitrile (whose quenching rates are determined by the solvent polarity), the quenching rate constants obtained are considerably smaller than those obtained in homogeneous solvents, and they increase when the water/AOT ratio increases. For ppenetetrasulfonic acid (sodium salt) at high water/AOT ratios, the quenching rate constant obtained in terms of the quencher concentration in the water pool is a factor of 2 smaller than that obtained in bulk water and slightly increases when the water/AOT ratio increases.
Introduction The fluorescence quenching in reverse micelles and or water in oil microemulsions has been extensively studied
and reviewed.'$ The results obtained in these systems are generally discussed in terms of two limiting cases, based on whether or not the probe is associated with the mi-
0743-7463/89/2405-0S05$01.50/0@ 1989 American Chemical Society
