The Journal of
Physical Chemistry
Registered in US. Patent Office
0 Copyright, 1981, by the American Chemical Society
VOLUME 85, NUMBER 16
AUGUST 6,1981
LETTERS A Novel Top Metal-Adsorbate Reaction Observed by Tunneling Spectroscopy Ursula Mazur, Stephen D. Wllliams, and K. W. Hipps’ Department of Chemistry and Chemical Physics Program, Washington State University, Pullman, Washington 99 164 (Received: April 24, 1981; In Final Form: June 1, 1981)
Top metal deposition during tunnel diode fabrication is generally believed to be a minor perturbation on species adsorbed on the alumina surface of Al-AlO,-M tunnel diodes. This Letter reports the novel observation of a complete chemical reaction between adsorbed ferrocyanide ion and gold top metal to produce a surface species similar to gold(1) dicyanide. Reduction of the gold(II1)tetracyanide ion to Au(CN)~by the alumina surface is also reported.
Introduction Since its discovery by Jaklevic and Lambe,lP2 inelastic electron tunneling spectroscopy (IETS) has been used to obtain the vibrational spectra of molecules and ions adsorbed on metal oxide surfaces. The technique has been well r e ~ i e w e d . ~Spectroscopic ?~ information is obtained by measuring the electrical properties of metal-insulator-metal tunnel diodes. The insulator is a thin ( 20 A) film of metal oxide, with an adsorbed layer (I1monolayer) of the species under study. The commonly used tunnel diodes are sandwichlike structures of the type, All AlO,/adsorbate/M, where M = Pb, Ag, and Sn. Modifications of the adsorbate during diode fabrication have, to date, been limited to chemical3+‘ and redox’s8 processes associated with adsorption on the oxide surface. The reduction of TCNQ’ to TCNQ- and of ferricyanide to ferrocyanide8 are examples of oxide-dependent redox processes. Shifts in vibrational band positions as a function of top metal have been observed, but are generally due to dipole-image dipole interactions between the adsorbate N
*Alfred P. Sloan Fellow. 0022-3654/81/2085-2305$01.25/0
and top metaLgJO To our knowledge, no cases of reactive modification of the adsorbate by the top metal have been reported. This Letter reports a top-metal-dependent redox process which proceeds to the extent that the spectrum of the original adsorbate is no longer observable. Alumina doped with ferricyanide and held at room temperature during gold deposition yields a strong tunneling spectrum similar to that of gold(1) dicyanide. Gold deposition at 77 K reduces the efficiency of the reaction to the extent that the spectra of both the oxide-reduced ferricyanide and the (1) Jaklevic, R. C.; Lambe, J. Phys. Reu. Lett. 1966, 17, 1139. (2) Lambe, J.; Jaklevic, R. C. Phys. Reu. 1968, 165, 821. (3) Hansma, P.K. Phys. Rep. 1977,30C, 146. (4) Wolfram, T.,Ed. “Inelastic Electron Tunneling Spectroscopy”; Springer Verlag: New York, 1978. (5) McBride, D. E.; Hall, J. T. J. Catal. 1979,58, 320. (6) Brown, N.M.; Floyd, R. B.; Walmsley, D. G. J. Chem. SOC.,Faraday Trans. 2 1979, 75, 261. (7) Korman, C. S.;Coleman, R. V. Phys. Reu. B. 1977, 15, 1877. (8)Hipps, K. W.; Mazur, U. J . Phys. Chem. 1980,84, 3162. (9) Kirtley, J.; Hansma, P. K. Phys. Rev. B 1975, 12,531. (10) Kirtley, J.; Hansma, P. K. Phys. Reu. B 1976, 13, 2910.
0 1981 American Chemical Society
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The Journal of Physical Chemistty, Vol. 85, No. 16, 198 1
Letters
k t >
I-
H
m Z W IZ
H
La
10Lara ENERGY
2La00 Ccm-
1>
Figure 1. Low-resolution tunneling spectra obtained from AI-AI0,-M diodes doped with aqueous K,Fe(CN),: (a) M = Pb; substrate held at (b) 77 K, and (c) 300 K during gold deposition (M = Au).
top metal reaction product are observed with equal intensity.
Experimental Section KAU(CN)~ was obtained from Fischer and was multiply recrystallized from water before use. KAu(CN)~was prepared by the method given in ref 11. The IR and R m a n spectra of these materials agreed well with reported spectra.11-14 K4Fe(C15N)6 and K4Fe(13CN)6 were prepared as discussed in ref 8. The diodes with lead, silver, and tin top metals were fabricated as previously described.8 Gold top metal diodes were made in the following way: An A1 film about 1 mm X 5 cm X -2000 A was evaporated onto a glass microscope slide. This film was exposed to a 50 X torr O2ac discharge for about 2 min to grow the oxide film. The substrate was removed from the vacuum system and doped with a solution of K,Fe(CN),, K4Fe(CN)6,KAU(CN)~, or KAu(CN)& The slide was mounted on a holder that could be cooled to 77 K, and returned to the vacuum system. The gold top metal film was evaporated with the substrate held at room temperature or at 77 K, at pressures below lo4 torr. In the case of cooled junctions, the completed diodes were allowed to warm to at least 15 "C before the vacuum was broken and the slide removed. All other junctions had top metals deposited at room temperature. Tunneling spectra were obtained with a previously described s p e ~ t r o m e t e r . ~Reported J~ energies are accurate to f 3 cm-'. For the lead-topped diodes samarium-cobalt magnets were used to suppress the lead superconductivity. A smooth polynomial curve was subtracted from each spectrum to remove the rising background and to clarify the presentation. The strong broad peak near 950 cm-' is due to A1-0 motion. (11)Jones, L. H.; Smith, J. M. J. Chem. Phys. 1964,41, 2507. (12) Jones, L. H. J. Chem. Phys. 1957,27,468. (13)Jones, L.H.Spectrochirn. Acta 1963,19,1675. (14)Jones, L.H.; Kressin, I. K. J. Chem. Phys. 1965,43, 3956. (15)Mmur, U.;Hipps, K. W. J. Phys. Chem. 1979,83,2773.
ra
120121
24121121
E N E R G Y < o m - 1> Figure 2. Low-resolution tunneling spectra obtained from AI-AIO,-Au type diodes: (a) K,Fe('3CN)6 doped junction with gold deposited at 300 K; (b) K4Fe(CI5N), doped junction with gold deposited at 300 K; (c) KAu(CN), doped junction with gold deposited at 77 or 300 K. KANCN), doped junctions prepared at 300 K gave resutts identical with spectrum C.
Results Figure 1 displays the spectra obtained from diodes of the type Al-AlO,-Fe(CN):--M prepared by ferricyanide doping prior to top metal (M) deposition. Figure l a shows the results obtained from lead-topped junctions. This spectrum has been extensively analyzed and identified as due to the ferrocyanide ion.8J6 Figure l b presents the spectrum obtained from diodes topped with gold at 77 K. In addition to the ferrocyanide bands, new bands at 306 and 2120 cm-' are present. Figure ICdisplays the results obtained from junctions topped with gold at 300 K. This spectrum contains no bands due to iron complexes. Clearly, the gold top metal is reacting with the adsorbed ferrocyanide and destroying it. Further, the extent of reaction depends on deposition conditions and not on the mere presence of a gold top layer. Parts a and b of Figure 2 show the spectra obtained from junctions topped with gold at 300 K and doped with isotopically substituted ferrocyanide. High-resolution spectra of the 200-600-cm-l region showed well-defined shifts of the 306-cm-l band to 301 and 296 cm-l for the 16Nand 13C complexes, respectively. Further, the 2120-cm-l band shifts to 2082 and 2072 cm-'. These isotopic shifts clearly identify the principal features seen in Figures ICand 2 as due to CN motions. Figure 2c is a typical spectrum obtained from KAU(CN)~ or KAu(CN)~ doped [from acetone] junctions topped with gold at 300 K. Aside from a small (5 cm-l) increase in the CN-stretching frequency, gold(1) dicyanide doped junctions yield spectra identical with that shown in Figure IC. Figure 3 portrays the spectra obtained from lead-topped diodes doped with KAU(CN)~ [Figure 3a] and KAu(CN)~ [Figure 3b] from acetone and methanol solution, respectively. As is the case for gold-topped junctions, the tun(16)Hipps, K. W.;Mazur, U.; Pearce, M. s. Chem. Phys. Lett. 1979, 68,433.
The Journal of Physical Chem/stty, Vol. 85, No. 16, 1981
Letters
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TABLE I : Vibrational Mode Energies (cm-l ) for Gold Cyanide Complexes tunneling
u3
m m
. 2125
2140
(u
2141-2159 4CN)
H
Q \
>
Tu
4 55
a
.
I '
452 4MC) 427 4MC)
..1
2189-2207 4CN) 462 V(MC) 4 59 4MC) 4 50 4MC) 422 s (MCN) 415 6 (MCN)
413c 350 (sh) 306
m
1 ramer
2[21121m 1>
308
Reference 12.
Discussion The reactive formation of a gold cyanide complex in the barrier region of an iron hexacyanide doped tunnel diode during room temperature gold deposition is proven by the following: (1)isotopic shifts of the observed bands identify them as due to CN motions (parts a and b of Figure 2), and (2) junctions doped with KAU(CN)~ give basically the same spectra as those doped with ferrocyanide. The equivalence of gold(1) and gold(II1) complex tunneling spectra obtained with the same top metal, and the variation in spectra with top metal bring into question the oxidation and coordination state of the gold cyanide species formed during the deposition process. We will demonstrate that the species formed is similar to gold(1) dicyanide. There are three commonly known gold cyanides, gold(1) cyanide, gold(1) dicyanide anion, and gold(II1) tetracyanide
Reference 11.
298 s(MCN) See text and Figure
4.
ENERGY < a m Figure 3. Low-resolution tunneling spectra obtained from AI-AI0,-Pb type diodes: (a) doped with KAu(CN), from acetone solution; (b) doped with KAu(CN), from methanol.
neling spectra obtained from the gold(1) and gold(II1) salts are the same. The spectra obtained from lead-topped junctions, however, are not identical with the spectra obtained from gold-topped diodes. Surprisingly, the intensities of gold cyanide spectra obtained with gold top metal are much greater than those obtained with lead top metal. The broad and weak doublet near 450 cm-l seen in the lead-topped junctions (Figure 3) is absent in the Au-topped diodes (Figures 1 and 2). A broad shoulder near 350 cm-l in Figures ICand 2 is absent in Figure 3. Also, the CNstretching mode shifts by 15 to 20 cm-l with change in top metal. Table I reports the tunneling transition energies observed for Au(CN)< with lead and gold top metal. Also shown are the known fundamental energies of the gold dicyanide and gold tetracyanide ions. Figure 4 presents a comparison of tunneling and Raman spectra obtained from Au(CN)-. The Raman spectra are of 5 and 1 M LiAu(CN), in water, and were kindly provided by L. H. Jones from work originally published in ref 13. The tunneling (IETS) spectrum is a medium resolution scan of a lead-topped junction.
368 6 (MCN) 305 6 (MCN)
I
IETS
-
t t-
m
Z U
+Z
H
I
I \
RAMAN
300 400 500 E N E R G Y < c m - 1)
Figure 4. Medium-resolution tunneling spectrum obtained from KAu(CN), doped junctions topped with lead as compared to solution phase Raman spectra of LiAu(CN),. The Raman spectra were provided by Dr. L. H. Jones and were originally published in ref 13.
anion. AuCN is an insoluble solid and is not amenable to tunneling studies. The CN-stretching motion of this polymeric material occurs at 2217 cm-', slightly higher than the CN-stretching frequency of the tetracyanide.l'J' The mean CN-stretching frequency of the dicyanide, 2150 cm-l, occurs near the CN stretch observed in the tunneling spectra of lead-topped gold cyanide doped junctions (see Table I). In the case of gold-topped junctions, this band is further shifted to lower energy. If one considers the large dipole derivative associated with the CN-stretching motion (17)El-Sayed, M. A.; Sheline, R. K. J.Inorg. Nucl. Chem. 1968,6,187.
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The Journal of Physical Chemktty, Vol. 85, No. 16, 1981
of the gold dicyanide anion,12a significant dipole-image dipole interaction with the top metal is expected. Au(CN)4-, on the other hand, has a much smaller dipole derivative and little image dipole shift will r e ~ u l t . ~ - ' ~ While the above statements about the image dipole shift are justified by detailed calculations, a simpler argument yields similar results. The shift in observed OD stretching frequency in going from lead- to gold-topped junctions is approximately equal to the shift seen in lead-topped surfaces relative to the free s ~ r f a c e .Taking ~ the Pb to Au shift of the gold cyanide CN-stretching band to be 15 cm-', and assuming that the intrinsic CN-stretching frequency of the gold cyanide surface species is top metal independent, a top metal free CN frequency of about 2155 cm-I is expected. This is in good agreement with IR and Raman results for the dicyanide.12 The similarity of the 5 M solution Raman spectrum and the medium-resolution tunneling spectrum seen in Figure 4 also provides compelling evidence that the surface species formed in leadtopped diodes is the dicyanide anion. Thus, adsorption of Au(CN)~-occurs reductively. This behavior parallels that of ferricyanidesJ6 and is similar to the reduction of gold cyanides by carbon.ls The absence of bands near 450 cm-l in the case of the gold-topped junctions (Figures ICand 2) is consistent with a surface species similar to gold dicyanide. The very broad band seen in the 1 M solution phase Raman spectrum shown in Figure 4 would be hard to identify in the tunneling spectrum. Further, the down shift in CN-stretching motion reported in Table I is consistent with the calculated dipole-image dipole shift expected for Au(CN)~-. The remaining inconsistencies between lead- and gold-topped junctions are the enhanced intensity of all bands and the presence of a shoulder near 350 cm-l. The intensity enhancement observed here has no parallel in the tunneling literature and we cannot explain it at this time. The shoulder near 350 cm-' could be due to the G(AuCN) motion, or to extended interactions with the gold metal layer. While the two inconsistencies described above make assignment of the gold plus ferrocyanide reaction product somewhat uncertain, we feel that the evidence presented is only consistent with the formation of a gold cyanide complex in which gold has an oxidation state less than or equal to 1. The existence of apparently fractionally valent (18)McDougall, G. J.; Hancock, R. D.; Nicol, M. J.; Wellington, 0. L.; Copperthwaite, R. G. J. S. Afr. Inst. Min. Metall. 1980, 344.
Letters
gold cyanide polymers on carbon surfaces has been well documented,ls but the absence of vibrational data prevents a comparison with the present results. The process which produces the spectrum seen in Figure IC, therefore, is similar to Fe(CN)$- nAu nAu(CN)2- X (1) The form of eq 1is only approximate, and is not intended to eliminate the possibility of gold dicyanide aggregates or polymers having a mean gold oxidation state of less than one. The absence of iron related bands in the 300 K goldtopped barrier (Figure IC)indicates that X in eq 1is not an iron cyanide species. While it is likely that the iron(I1) ions are reduced to iron(0) and migrate into the gold metal layer during deposition, it cannot be demonstrated by tunneling spectroscopy. In any case, there is no indication of intermediate (Figure lb) or product (Figure IC)complexes of iron. The question of reaction mechanism for this metal exchange reaction can be partially addressed at this time. The ability to limit the extent of reaction by substrate cooling during the deposition process indicates that this is an activated process occurring during Au deposition and is not simply due to gold contacting adsorbed ferrocyanide. One expects, therefore, some correlation between the kinetic energies of deposited atoms and the extent of chemical reaction. If we assume a Boltzmann distribution in energy for the incident atoms and a pressure of torr, the average kinetic energies for Pb, Ag, Sn, and Au are 931, 1250, 1370, and 1660 cm-l, respectively. Further, the ferrocyanide doped diodes show increasing chemical perturbations due to top metal in the same order.8
+
.
-
+
Conclusions
A novel top metal reaction with adsorbed ferrocyanide ion has been observed. During the deposition process, iron is reduced and displaced from the ferrocyanide complex with the production of a surface species similar to Au(CN)2-. Substrate cooling to 77 K greatly reduces, but does not eliminate, this reaction. Gold(II1) tetracyanide is reduced to gold(1) dicyanide upon adsorption from methanol solution on alumina. Acknowledgment. We express our thanks to the National Science Foundation for supporting this work under Grant DMR 7820251. We also thank Dr. L. H. Jones for providing the Raman spectra shown in Figure 4.
