Anal. Chem. l W 2 , 64, 1845-1850
1845
Detection of Formic Acid Vapor: Inelastic Electron Tunneling Spectroscopy of Infused AI-A1203-M-Au Junctions Ursula Mazur, X. D. Wang, and K. W. Hipps' Department of Chemistry, Washington State University, Pullman, Washington 99164-4630
Inelastlcelectrontunnelng spectra are reportedfor AI-A1203M-AU Junctlons[M = nothlng or M = 1 nm of Pb] externally doped wtth formlc acld vapor In humld air. As expected, the specks observed In the tunnel Junctlonare formate Ion and hydroxyl Ion. Spectra of the formate Ion can be observed over a wlde range of formlc acld concentratlon, wRh a lower llmtt well below 50 ppm In alr. The spectra observed for externally doped Junctlonsare essentially the same as for Junctlonadoped wHh formlc acld durlng devlce fabrlcatlon. The uttrathln Pb layer plays a crttlcal role In determlnlng the sensltlvlty of the devlce for gao-phase fonnlc acld. Our procedure for produclng gabpermeable gold electrodes may have appllcatlon In many different types of solld state sensors. We flnd that tho Infudon-hduced Increase In Junctbnredstance (as a functlon of tlme) Is almost completely dependent upon the humldlty of the carrler gas (alr). Thls resldance Increase Is very well described by a angle exponentlal term. Scannlng tunnellng mlcroscopy Images of AI-A120s-Au Junctlonsand of ACAI2OrPb( 1nm)-Au Junctlonsare presentedand discussed In the context of the observed Infuslon.
INTRODUCTION Inelastic electron tunneling spectroscopy (IETS) is now two decades old.' It is an all-electronic (photon-free) technique that provides vibrationalz-5 and electronic2*- spectra of microscopicquantities of material (S1014molecules). The object of study, a metal-insulator-metal sandwich, can be made small enough to be integrated with other electronic circuitry on a single chip. In fact, tunneling spectroscopy is the only broad band spectroscopic technique wherein the sample and the entire spectrometer might be placed in a single integrated circuit. Thus, it would seem natural that IETS be applied to sensor development. There have been, however, several barriers to its application in analysis. The first and most obvious problem relates to how these devices are conventionally constructed. Usually, the material of interest is added during device fabrication-after insulator growth and before top metal deposition. This procedure is completely impractical for sensor applications. The second major problem is that the spectral line width in IETS is very dependent on sample temperature during the measurement process. For conventional devices, the full width is about 5 kT. For electronic transitions, this is not a significant impediment. In the vibrational region of the spectrum, (1)Lambe, J.; Jaklevic, R. C. Phys. Reo. 1968,165, 821. (2)Hansma, P. K. Tunneling Spectroscopy; Plenum Press: New York, 1982. (3) Hansma, P. K. Phys. Rep. C 1977,30, 145. (4)Weinberg, W. H. Vib. Spectra Struct. 1982,11, 1. (5) Vibrational Spectroscopy of Molecules on Surfaces; Yates, J. T., Madey, T. E., Eds.; Plenum Press: New York, 1987. (6) Hipps, K.W.; Mazur, U. J . Am. Chem. SOC.1987,109,3861. (7)Hipps, K.W.; Mazur, U. Surf. Sci. 1989,207, 385. (8)Hipps, K.W.; Mazur, U. J. Phys. Chem. 1987,91,5218. (9)Hipps, K. W. J . Phys. Chem. 1989,93,5958. 0003-2700/92/0384-1845$03.00/0
however, significant loss in selectivity results if the sample is measured at temperatures higher than about 30 K. In recent years, it has begun to appear that the fabrication problem can be solved and that the 5-kT barrier is surmountable.+l2 The CN stretching band, for example, can be clearly identified in the presence of hydrocarbon interference at measurement temperatures above 100 K-a temperature easily reached using inexpensive and compact refrigerators. Moreover, the 5-kT barrier may be circumvented, in part, by the use of deconvolution methods. An alternative solution involves changing the bandwidth of the tunneling experiment. If focus is placed on electronic transitions located in the near-IR region of the spectrum (many of which have intrinsic widths of 1000 cm-l or more) the 5-kT barrier has little significance at room temperature.11 Another practical approach is to utilize tunnel diodes as chemical fuses. In this mode, the device would be constructed with a chemically active element in the barrier region. Exposure to a particular reagent would trigger chemical changes in the barrier which would appear as a change in device impedance and/or capacitance. The fabrication problem can also be solved. Jaklevic and Gaerttner first reported external doping of A1-A12O3-Pb tunnel diodes.13 These authors demonstrated that small organic molecules in the presence of water vapor would penetrate the Pb top metal of completed junctions. These vapor-infused diodes gave tunneling spectra comparable to those obtained from conventionally doped devices. In subsequent work2J3J4theyshowed that premade Al-A1203P b junctions could be infused with various species from either water vapor or aqueous solution. In addition, they demonstrated hydrogenation of Al-Al203-Pd devices.'5 Other groups reproduced Jaklevic's Al-Al203-Pb results and were able to demonstrate that water vapor played a significant role in the P b infusion process.l&l8 The picture that emerges is that water vapor etches the grain boundaries (of Pb) and thereby provides a path for entry of the add-species into the M-I-Pb device. Unfortunately, the very process that allows P b to be infused makes it a very poor choice for a sensor electrode. Pb films of appropriate thickness are converted to oxy hydroxides over a period of hours in wet air. A chemically stable, but infusible, top electrode is needed. In some of Jaklevic's work, he mentions that Sn and Au topped junctions can be infused, but he never presents spectra. Recently, the variation of resistance and capacitance with time of Au-topped junctions exposed to humid air was (10)Deconoolution of Absorption Spectra; Blass, W. E., Halsey, G. W., Eds.; Academic Press: New York, 1981. (11)Hipps, K.W.; Peter, S. L. J . Phys. Chem. 1989,93,5717. (12)Hipps, K.W.; Mazur, U. J.Phys. Chem. 1992,96,1160. (13)(a) Jaklevic, R. C.; Gaerttner, M. R. Appl. Phys. Lett. 1977,30, 646. (b) Jaklevic, R. C.; Gaertner, M. R. Appl. Surf. Sci. 1978,1 , 479. (14)Jaklevic, R. C.Appl. Surf. Sci. 1980,4 , 174. (15)Gaerttner, M. R.; Jaklevic, R. C. Surf. Sci. 1979,517. (16)Nelson, W. J.; Walmsley, D. G.;Bell, J. M. Thin Solid Films 1981, 79,229. (17)Heiras, J. L.;Adler, J. G. Appl. Surf. Sci. 1982,10, 42. (18)Mallik,R. R.;Pritchard,R.G.;Oxley,D.P.;Horley,C. C.; Comyn, J. Thin Solid Films 1984,112, 193. 0 1992 American Chemical Society
1848 ANALYTICAL CHEMISTRY, VOL. 64, NO. 17, S€PTEMBER 1, 1002
reported.lg These reporta suggecrted that IETS might be obtainable from portfabrication doped (infuaed) Al-Al203Au junctions. Since gold is nearly an ideal material in terms of ita thermal and environmental stability, we felt that infusible Au-topped junctiom might wrve as solid-state W M O ~ . Our fimt study of infwion into AI-A1203-A~ junctiom concerned the thiocyanate ion in nonaqueous aolventa.12 We showed that the tunneling spectrum of SCNcould be olwsrved after e x p i n g them devices to thiocyanate concentrations 81) low 81) 0.6 pM for periods of 6 min or lese. We noted in that study that the very sensitive and reliable response we obrved could be due to a specific interaction between thiocyanate ion and gold, e.g. reactive etching of grain boundaries. In the current work, attention is directed toward the infusion of epecies that do not react with gold. Specifically, we will study the infusion of formic acid through gold, and modified gold, electrodes. Formic acid was chosen because, in addition to itR lack of reactivity with gold, it has been studied extensively by conventional tunneling methods.1-6*20 That is, tunneling spectra of devices fabricated by depositing metal onto formic acid chemisorbed on alumina are available. Moreover, formic acid has been used in studies of infusion through leadPJ6 We will addrese the following questions: (1)Cangold filmof theorder of 20nm in thickneee be reliably used for infusion studies of small molecules that do not react with gold? (2) Can the gold layer be made more permeable without compromising ita electrical continuity? (3) What causes the observed riee in device resistance with infusion time and does it foilow a simple functional form?
EXPERIMENTAL SECTION bagentr. Gold, lead, and aluminum were greater than 99.99+% purity. Formicacid (HCOZH)solutionswere prepared by diluting Baker brand reagent-grade (89% in water) formic acid in distilled water. Apparatus. Alldeviceswerefabricatedinadiffusion pumped vacuum system. The pump was fitted with a liquid nitrogen trap, and a system base pressure of 3 X lo-' could be obtain in 1h. This is a 14411. bell jar system having removable staiirless steel shields and substrate poaitioner allowing three different materials to be deposited on three different substrates without breaking vacuum. The inelasticelectrontunnelingspectrometerused in thisstudy has been described elsewhere.21 The scanning tunneling microscopeis alsoone of our own design,and its constructionhas been described elsewhere.22 Procedure. Aluminum was deposited from high-purity tungsten wire filaments. Gold was deposited from an aluminacoated basket. We found that extended ( 1month) use of these baskets produced contaminated films and unreliable resulta. Frequent replacement of these sources is advised. Pb was depositedfrom tungstendimpleboats. The substrates used were Corning brand glass slidesthat had been cleaned in a nitric acid and hydrogen peroxide bath. Four tunnel diodes were prepared on a single substrate in the following manner. First, about 100 nm of aluminum was deposited in the form of a 2-in.-long and 1-mm-widestrip. This deposition was performed at pressures less than 5 X lo-' Torr. The A1 was oxidized for 7-8 min in a 100 mTorr oxygen plasma formedby a 400-V ac discharge. The vacuum chamber was then opened to air for about 30 8. The bell jar was reevacuated, and the top electrode(s)was deposited. On each substrate half the area was coated with 1nm of Pb while the other half was masked
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(19)Bellineham,J. R.;Adkins, C. J.; Phillips, W. A. Thin Solid Films 1991,J98,85~ (20)Sleigh, A. K.;Taylor, M.E.;Adkins,C. J.; Phillips, W. A. J. Phys.: Condens. Matter 1989,1,1107. (21)(a) Hipps, K.W. Reu. Sei. Instrum. 1987,58,285.(b)Hippe, K. W.;Mazur, U.Reu. Scr. Instrum. 1988,59,1903. (22)Hipp, K. W.;Fried, G.; Fried, R. Rev. Sci. Instrum. 1990,6J, 1869.
An M-I-M' chemical sensor
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Flguro 1. Cartoon of an lnfudbb ACA120s-M-Au tunmi dkek.
from the Pb source. Then, four thin gold strips (1mm wide by 2 cm long)were depositedacrossthe oxidizedaluminumelectrode such that two covered the masked area and two covered the ultrathin lead film. Thus, each substrate provided two matched pairs of junctions suitable for determining the effecta of the Pb deposition step. The Pb and Au deposition steps were carried out at between 5 X 10" and 2 X 10" Torr. The Pb waa deposited at a rate of 0.2 nm/s, and the gold was deposited at 0.05 nm/s. We were careful to use the smallest possible gold source to minimize radiative heating of the sample during the gold deposition. The sampleswere than allowed to cool for 1h before breaking vacuum and removing them from the vacuum system. This cool down was essential to the production of stable devicee on the Pb/Au side of the substrate. A cartoon of tha resulting device is presented in Figure 1. These devices were found to have a four terminal resistanceof between 1and 20 0 at the time they were removed from the chamber. The junction resistances remained stable for many days if they were stored in vacuum or an inert atmosphere. On the other hand, allowing them to sit in air for several hours generally resulted in an increase in resistance to several hundred ohms (vide infra). The 16-nmthick pure gold electrodes had a strip resistance of about 35 0. The Pb(1 nmI-Au(l5 nm) electrodes had a strip resistance of about 60 Q. The thick gold films had smaller resistances. The process of infusion had no measurableeffect on the Au or Pb-Au film resistance. InfuRion with formic acid was carried out as follows. First, electricalcontacts were made to completed 1-mm2junctions by the use of indium solder. The junctions were then mounted on a sample rod that had a vacuum connector appropriate to mate toeither our doping chamber or a standard 100-Lhelium dewar. The sample was sealed into the chamber,and air saturated with the vapor of an aqueoussolution of formicacid was slowly pawed through the chamber. The junction resistances were measumd at 6-min intervals until the infusion was terminated (usually 8 I min). The concentrationsof formic acid uegd were adjwte I a provide vapor-phaseacid concentrationsof 0,50,250,1200, and 7000 ppm. In a few cases, we also allowed junctions to stand in the laboratory ambient atmosphere for a controlled length of time. The resistances of these junctions were also recorded M a function of time. Once the infusion was terminated, the devices were then immersed in liquid helium and tunnelingspectra were measured. The tunnelingintensitiesreported are proportionalto (dW/dP) taken at fixtd phase and constant modulationvoltage. Thelockin phase was set 90 dag out from the null setting obtained at an applied bias voltage of 118 mV (the AI-0 stretching band). The IETS data presented haw not been manipulated-they haw not been smoothed or baseline corrected. The accuracy of the band positions depends on the modulation voltage (resolution) used. The half width at l/e height of the observed peab (in cm-1) is given approximatelyby't [(2,379*+ (7,1Vm)*+ 6']1/*, where Tis in Kelvin, V,,, is the modulation voltage in millivolts, and 6 is the intrinsic half width at l/e height for the transition.11
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ANALYTICAL CHEMISTRY, VOL. 64, NO. 17, SEPTEMBER 1, l9Q2 1847 1000,
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TIME (min) Fl@urr2. Resistance versus exposure time for AI-Al2O3-Pb( 1 nm)Au( 15 nm)tunnelJunctionsexposed to various concentratlonsof formic acid canled In alr saturated with water vapor at 20 OC. The smooth lines are best flts of In (R) = mt b to the data points. m = (0.13 f O.Oll/min.
+
All our spectra were taken at 4.2 K where, for a sharp line, the modulation width is the dominant factor when V,, > 1.5 mV. All modulation amplitudes will be reported as rms values and exceed 1.5 mV. Scanning tunneling microscopy images were obtained of AlA1203-Pb(l nm)-Au and Al-Al203-Au junctions deposited on mica prior to infusion. These were prepared in pairs (as were those made for IETS) so the material layers were identical except for the presence or absence of 1nm of Pb. Images were obtained in air with electrochemically etched tungsten or gold-plated tungsten tips. The sample was biased at -100 mV relative to the tip, and the feedback current was maintained constant at 0.5 nA. For these experiments, the preparation procedure was identical to that used for spectral studies, except that a mica substrate was used.
Figure 3. Resistance versus exposure time for AI-Al2O3-Pb(l nm)Au(15 nm)and AI-Ai203-A~15nm)tunnel junctions exposed to varbus conditions at 20 O C . The smooth llnes are best fits of in (R) = mt b to the data points.
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RESULTS Resistance versus Infusion Time. Figure 2 is a (logarithmic) plot of junction resistance versus time for typical devices of the form Al-A1203-Pb(l nm)-Au(lB nm) exposed to varying concentrations of formic acid vapor. Clearly, In (R) varies linearly with time and with the same rate for all the acid concentrations studied. Averaged over 12 samples, we found that In (R(t)IR(O))= 0.13t. Because the log of the resistance increased at the same rate for junctions infused with pure water, we must conclude that all of the resistance increase is determined by the infusion of water. The amount of water vapor does a f f m the resistance increase. Comparison of resistance data for water-saturated air and ambient air (Figure 3) clearly shows that increasing the vapor pressure of water increasesthe rate of increaseof log ( R ) . These results are simple and highly reproducible for the Pb-treated barrier. If we instead consider Al-A1203-Au(15 nm) devices, the picture is considerably different. While the In (R)vs t plots are linear, they have few other common features. For example, as shown in Figure 3, junctions doped with pure water and 50 ppm formic acid vapor (in air saturated with water vapor at 20 "C) do not have the same rate of increase of In (R). Moreover, the results vary from day to day. Often, the slope of the In (R)curve might vary by a factor of 2 or more with different batches of the same device. In all cases In (R) increased much more slowly for pure gold topped junctions than for the PbIAu junctions on the same substrate. Reference Tunneling Spectra (Blanks). Figures 4 and 5 document the tunneling spectra obtained at various stages
0
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Figure4. Tunnellng spectra obtalned from AI-Ai203-Au Junctions: (1) after 45-min exposure to alr saturated with water vapor at 20 O C ; (2) after 3 M i n exposure to relathrely dry room air; and (3) Immediately measured after fabrication.
in the sample preparation if formic acid is excluded from the process. The three pairs of traces shown in Figures 4 and 5 were obtained from Al-Al203-Aut 15 nm) and Al-Al203-Pb(1 nm)-Au(l5 nm) junctions that were measured (1) after 45-min exposure (in the doping chamber) to air saturated with water, (2) after 30 min of exposure to room air, and (3) immediately after withdrawal from the vacuum system. It is important to note that these spectra were taken from pairs-junctions formed on the same substrate-that experienced identical conditions after the P b layer was, or was not, deposited. All these spectra were obtained with 2.5-mV modulation amplitude and are plotted on the same scale (pV) but have been arbitrarily shifted up or down for the purposes of display. Note also that noise levels may vary since the spectra are the result of coadding a variable number of scans. The most intense features are due to alumina vibrational bands and to surface hydroxyls.'-5 Junctions Infused with Formic Acid. The most notable difference between the PbIAu topped junctions and the Au
ANALYTICAL CHEMISTRY, VOL. 64, NO. 17, SEPTEMBER 1, 1992
1848 2 .E - 6
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(cm-1) Figure 5. Tunneling spectra obtained from AI-Ai203-Pb(l nm)-Au junctions: (1)after 45-min exposure to air saturated wlth water vapor at 20 O C ; (2) after 30-min exposure to relattvely dry room air; and (3) immedlately measured after fabrication. Energy
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Figure 8. Tunneling spectra obtained from a pair of AI-Ai203-M-Au
junctions after 45-min exposureto 50 ppm of formic acM in air saturated with water vapor at 20 O C . The upper spectrum was obtained from a junction that had 1 nm of Pb deposited before the gold layer was applied. I n the lower trace, only gold was deposited. Spectra were obtained at 4.2 K with 2.5-mV modulation; the box height is 2.0 pV. only ones is that formic acid will not reliably infuse through Au only top layers 15 nm or more thick. Figure 6 contrasts the spectra obtained from a matched pair of junctions exposed to 50 ppm of formic acid for 45 min. While the junction made with 1nm of Pb coated by 15 nm of Au shows a clear formate ion spectrum, its mate shows no sign of formate in ita tunneling spectrum. Even for junctions exposed to 7000 ppm of formic acid, no formate spectrum was seen in the case of Au only topped junctions. On the other hand, junctions coated first with 1 nm of P b and then with 15 or 20 nm of Au showed consistent and reliable infusion. Figure 7 displays typical results obtained from Pb(1 nm)-Au(l5 nm)-topped tunnel junctions exposed to various concentrations of formic acid. Similar results were obtained with Pb(1nm)-Au(2O nm) fiims. Clearly, these overlayers are highly permeable to formic acid.
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Energy (cm-1) Flgure 7. Ai-Ai203-Pb( 1 nm)-Au( 15 nm) tunnel Junctionsinfused wlth varying concentrations of formic acid. The upper and middle traces resulted from infusion of formic acM for 30 min at concentrations of 7000 and 250 ppm, respectively. The lower trace resulted from 45 min of infusion wlth 50 ppm formic ackl. Spectra were obtained at 4.2 K wlth 2.5-mV modulation; the box height is 2.5 pV.
If the gold layer thickness is further increased to 25 nm, infusion is severely limited and is often completely inhibited. Thus, there is a maximum gold thickness beyond which the P b pretreatment is ineffective. We know that thicker P b layers make for more easily infused Au films of greater mean thickness. However, we did not do a detailed study of the effect of changes in the Pb/Au ratio upon infusion rate.
STM Images. Figures 8 and 9 present the scanning tunneling microscopy images obtained from matched pairs of A1-Al2O3-M-Au junctions prepared on mica and having Au layers 15 or 25 nm thick. It should be emphasized that many images were taken from many spots on three different preparations of samples with 15-, 20-, and 25-nm gold thickness. The pictures shown are representative of this large data set. It is immediately apparent from consideration of Figures 8 and 9 that the 1-nm-thick P b underlayer causes a considerable roughening of the gold film. Because the measured peak to valley excursions are of the order of 15 nm in the case of Au(15 nm) films and since grain size is very small in the Pb(1 nm)-Au(l5 nm) films, it is easy to imagine a high density of breaks and channels through the top layer down to the oxide. In fact, the topography of Pb(1 nm)Au(15 nm)-topped tunnel junctions was hard to measure well by scanning tunneling microscopy. We frequently encountered noisy and unstable images that we associated with tunneling over small regions of exposed oxide. Since the barrier height over the oxide is considerably different than over gold, delays in the feedback circuit can result in tipsurface contact as the tip moves past regions of exposed oxide. As the gold thickness increases to 25 nm, the smaller grain size induced by the Pb underlayer is still apparent, but the peak to valley variation is now significantly less than the average film thickness. A pronounced smoothing of the grain edges makes it clear that the additional gold is blanketing the rough features, much as snow covers rough terrain. We also note that the STM images of the 25-nm-thick gold layers were always very quiet, and no unstable regions were ever encountered.
ANALYTICAL CHEMISTRY, VOL. 64, NO. 17, SEPTEMBER 1. 1992
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DISCUSSION The measured resistance of a two layer M-I-M' tunnel diode can he approximated23 as In ( R ) = b A(do
