J . Phys. Chem. 1985,89, 862-867
862
Adsorption of Tetracyanoethylene on a Nickel( 111) Surface Studied by Auger Electron Spectroscopy, Thermal Desorption Spectroscopy, and Raman Spectroscopy Fu-Ming Pan,+ John C. Hemminger,*+and S. Ushiodat Department of Chemistry, and Department of Physics, University of California, Irvine, Iruine. California 9271 7 (Received: September 6, 1984)
We have studied the adsorption, subsequent chemistry, and charge transfer characteristics of tetracyanoethylene (TCNE) on a Ni(ll1) surface under ultrahigh vacuum (UHV) conditions. In these experiments we have employed laser Raman scattering, thermal desorption spectroscopy, and Auger electron spectroscopy. At room temperature, a stable film that is more than one monolayer thick can be grown on the surface. Desorption from this film begins at 680 K. Desorption is complete by 900 K leaving a clean Ni(ll1) surface. Our experiments lead us to postulate the direct desorption of a negatively charged species of the stoichiometry of TCNE from the room temperature film. Low-temperature exposure to a clean Ni( 111) surface results in condensation of TCNE on top of the above-described film. We have identified the (TCNE)- anionic radical within the condensed layers via the observation of the characteristic Raman active modes and the associated excitation profiles of this species. Heating the condensed layer results in desorption of neutral TCNE in competition with strong chemisorption on top of the multilayer film. The resulting chemisorbed TCNE desorbs at -650 K.
Introduction
results, and discussion are given in the following sections.
Tetracyanoethylene (TCNE), (CN),C=C(CN),, is a very well-known strong *-Lewis acid, as a result of the cumulative electron withdrawal by the four cyano groups. TCNE forms colored charge transfer complexes with many organic r-Lewis bases and undergoes complete charge transfer with alkali metals to form TCNE-. It also reacts easily with certain organometallic complexes and is bonded to the transition metals through either the C=C "double bond" or nitrile nitrogen.'-3 Many elaborate studies on TCNE have been carried out since the initial preparation of TCNE was published by Cairns and co-workers in 1958.4-9 These include studies of the electronic absorption spectra of its charge transfer complexes as well as the assignments of vibrational modes of neutral TCNE and the TCNE- anionic radical. Charge transfer is generally believed to be an important interaction between adsorbates and metal surfaces. Knowing the extent of charge transfer would allow one to understand more about the electronic structure of an adsorption system. One method to determine the degree of charge transfer from electron donor to acceptor is to measure the Raman vibrational frequency shifts of the components of a complex. In the case of complexes of TCNE or TCNQ (tetracyancquinodimethane), the degree of charge transfer has been shown to be linearly proportional to the vibrational frequency TCNE is such a strong electron acceptor that its adsorption on various substrates could be a good probe of the electron donation properties of different substrate surfaces. Because the assignments of Raman vibrational modes of both neutral TCNE and anionic TCNE- are well established and because the electronic absorption specta of most TCNE complexes fall within the visible range, we believed that we could take advantage of charge transfer induced resonance Raman scattering to study the adsorption of TCNE on various substrates. Bearing these comments in mind, we engaged in our experiments of adsorption of TCNE on a Ni( 1 11) surface using Auger electron spectroscopy (AES), thermal desorption spectroscopy (TDS), and Raman scattering spectroscopy. It has been found that when TCNE reacts with Ni(0) organometallic complexes containing isocyanides, it is bonded to Ni atom through the C = C bond., However, when it reacts with Ni[P(O-~-tosyl)~]~, TCNE was not olefin b ~ n d e d .Because ~ of the planar and symmetrical structure of TCNE, and the fact that there is no steric hindrance for TCNE to approach the N i ( l l 1 ) surface, we expected that TCNE would lie flat on Ni( 111) surface, and partial or complete charge transfer might occur. The details of the experiments, 'Department of Chemistry. *Department of Physics.
Experiments Adsorption of TCNE on the Ni( 111) surface was carried out in an ultrahigh vacuum chamber, which has a base pressure of torr and is equipped with a Varian cylindrical mirror 1X analyzer (CMA) for Auger electron spectroscopy (AES) and a UTI-lOOC mass analyzer. The UTI-lOOC mass analyzer was mainly used to perform thermal desorption experiments (TDS) and was interfaced with an LSI-11 microcomputer in order to obtain multiple mass thermal desorption spectra. The analog mass intensity signal was digitized by a voltage-to-frequency converter.I2 A 2-mm-thick and 1-cm-diameter circular Ni( 111) crystal was mechanically polished with 0.05-pm alumina powder. A tantalum wire spotwelded on the back side of the crystal was used as the electrical heating wire. The crystal could be cooled to 130 K with liquid nitrogen. The N i ( l l 1 ) surface was cleaned by Ar ion bombardment followed by annealing at 1000 K before experiments were performed. Oxidation of carbon residues on the Ni surface at 650 K with 5 X lo-* torr of oxygen was also often used to clean the Ni surface when the quantity of carbon residue was small. A clean Ni surface AES spectrum obtained in this work is shown in Figure 1, and, for comparison, two other TCNE covered Ni(1 l l ) AES spectra are included in the same figure. For a clean Ni( 11 1) surface, the AES spectra showed a signal ratio of C (272 eV) to Ni (102 eV) of less than 10%. In order to understand how much area of our clean Ni( 111) sample surface was covered by (1) Rettig, M. F.; Wing, R. M. Inorg. Chem. 1969, 8, 2685. (2) Kawakami, K.; Ishii, K.; Tanaka, T. Bull. Chem. SOC.Jpn. 1975.48,
1051. (3) Tolman, C. A. J . Am. Chem. SOC.1974, 96,2780. (4) Cairns, T. L.; Carboni, R. A.; Coffman, D. D.; Engelhardt, V. A,; Heckert, R. E.; Little, E. L.; McGreer, E. C.; McKusick, B. C.; Middleton, W. J.; Scribner, R. M.; Theobald, C. W.; Winberg, H. E. J . Am. Chem. SOC. 1958, 80, 2775. (5) Michaelian, K. H.; Rieckhoff, K. E.; Voigt, E. M. Spectrosc. Lett. 1977, I O , 99. (6) Moore, J. C.; Smith, D.; Youhne, Y.; Devlin, J. P. J. Phys. Chem. 1971, 75, 325. (7) Jeanmaire, D. L.; Suchanski, M. R.; Van Duyne, R. P. J . Am. Chem. Soc. 1975, 79, 1699. (E) Hinkel, J. J.; Devlin, J. P. J . Chem. Phys. 1973, 58, 4750. (9) Matsuzaki, S.; Mitsuishi, T.; Toycda, K. Chem. Phys. Lett. 1982, 91, 296. (10) Kamitsos, E. I.; Risen, W. M., Jr. J . Chem. Phys. 1983, 79, 5808. (1 I ) Stanley, J.; Smith, D.; Latimer, B.; Devlin, J. P. J . Phys. Chem. 1966, 70, 2011. (12) Dahlgren, D.; Arnold, J.; Hemminger, J. C. J . Vac. Sci. Techno/.,A 1983, I , 81.
0022-365418512089-0862$01.50/0 0 1985 American Chemical Society
The Journal of Physical Chemistry, Vol. 89, No. 5, 1985 863
Adsorption of TCNE on Ni( 111)
6
0
= C(272eV)/Ni(102eV)
A
= C (272eV)/ Ni(848eV)
T E M P E R A T U R E = 180K 0
2 5 a CK
w
t-
'0
\ A
0 0
w v
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U
z
0
a
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2
0
3
v)
0 0 0
B
v)
60eV
0
200
w
a
400
600
I
000
E L E C T R O N ENERGY ( e V ) Figure 1. (a) AES spectrum for clean Ni(ll1). The AES ratio of C (272 eV) to Ni (102 eV) is less than 10%. (b) AES spectrum of approximately one monolayer of TCNE adsorbed on Ni( 111). (c) AES spectrum following room temperature saturation exposure of TCNE to a Ni(ll1)
surface. this amount of carbon residue, we carried out one monolayer adsorption of naphthalene on the Ni( 111) sample at room temperature. An AES signal ratio of C (272 eV) to Ni (102 eV) for this saturated adsorption was determined to be 3. Therefore we decided that this 10% ratio of a cleaned N i ( l l 1 ) surface corresponded to about 3% of a monolayer, and it is reasonable to regard this cleaned N i ( l l 1 ) sample surface as satisfactorily clean for experiments. The Auger ratio of C (272 eV) to N (380 eV) for TCNE on N i ( l l 1 ) is about 1.6, as shown in Figure 1. TCNE was purchased from Alfa Chemical Co. It was recrystallized in chlorobenzene twice and then sublimed under vacuum overnight before it was used in experiments. In order to avoid any reactions of TCNE with a metal gas handling line and leak valves, the exposure of the colorless purified TCNE onto the Ni( 111) crystal was carried out through a glass doser, which was made of only Pyrex glass, teflon valves, and viton O-rings. A capillary with inner diameter of 0.1 mm inside the glass doser was the only path that TCNE gas molecules could leak through into the UHV system. The doser provided a flow rate of about 1 X lOI4 molecules s-l when the pressure on the non-UHV side of this capillary was 1 torr. The pressure in the UHV chamber torr under these dosing conditions. A 2-mmremained 1 X thick circular glass capillary array disk with I-cm diameter was epoxy sealed on the end of an extension glass tubing and was used to direct the TCNE molecular flow toward the Ni( 111) surface. The capillary array has orifices 20-pm in diameter and is 50% open surface area. Under typical dosing conditions, the glass doser was warmed to about 70 OC with a heating cord, and the pressure in the UHV chamber was maintaining at 1 X 104-2 X lo4 torr. The Ni( 11 1) surface was 2 cm away from and faced toward the capillary array disk. Hence the background of the thermal desorption spectra was minimized as much as possible. Since it is difficult to determine local pressure around the capillary array disk, we use exposure time and exposure pressure read with the ion gauge, instead of Langmuir, to express the relative exposure quantity of gas molecules. AES experiments, which we carried out, showed this to be a reliable method of repeating a given exposure. In the thermal desorption experiments, the Ni( 111) surface was faced toward the mass spectrometer, and a heating rate of 8 K/s was used. An Arf ion laser (Spectra Physics 171) was used as the radiation source in the Raman scattering experiments. A Spex 1403 double monochromator equipped with two holographic gratings and a room temperature, blue-sensitive photomultiplier (Hammamatsu R464) was used to disperse and detect the scattered radiation. The Spex 1403 spectrometer was interfaced with an LSI-11 microcomputer, and a CAMAC system connected the
2
0
0
200
400
600
800 1000
EXPOSURE T I M E (sec) Figure 2. The dependence of the AES intensity ratios of C (272 eV) to Ni Auger peaks on the exposure time. The exposures were carried out
as described in the text. photon-counting electronics with the microcomputer. The Spolarized incident radiation was focussed with a cylindrical lens in order to minimize local heating effects on the Ni surface. The focused line on the Ni( 111) surface was -0.5 mm wide and 5 mm high. The scattered radiation was collected in a cone centered on the surface normal. The accumulation time necessary to obtain a reasonable signal-to-noise ratio was 60 to 120 s for each data point, and the resolution of the Raman spectra in this work is 12 cm-I.
Results and Discussion Auger Spectra and TDS. Figure 2 is a plot of the AES intensity ratio of carbon (272 eV) to nickel vs. exposure time for TCNE adsorption on Ni( 111) at 200 K. A similar plot has been obtained for room temperature adsorption but the ratio of C (272 eV) to Ni (848 eV) saturates at about 2 (see Figure IC). According to Figure 2 and TDS data to be discussed, we conclude that more than one monolayer of TCNE can be adsorbed on the Ni( 11 1) surface at room temperature and that this film is stable at room temperature under UHV conditions. TCNE has a vapor pressure torr at 20 OC,I3therefore, it is unlikely that the second of 2 X monolayer would remain on the Ni surface without a strong interaction with the underlayer adsorbate in our UHV chamber with a base pressure of 1 X torr. The slope in Figure 2 after the first break is small, but becomes larger after the second break. a. Room Temperature Adsorption. Thermal desorption spectra at low coverage show very interesting results. For coverages up to room temperature saturation, we only observe desorption of mass 28 and 14. Figure 3 shows a set of TDS for different exposure times a t room temperature monitoring mass 28. Mass 14 exhibits a similar TDS as mass 28, with an intensity of about 10% of that of mass 28. This ratio amounts to the intensity ratio of mass 14 to mass 28 observed in our UTI-100C mass spectrometer when N 2 gas is dosed into the UHV system. Therefore the observation of mass 28 in the TDS obviously comes from the desorption at 680,720,805, and 875 K of N2 molecules. In Figure 3, four peaks develop in order with increasing exposure time. We did not observe the desorption of any carbon containing species. However, AES spectra taken following TDS experiments showed the surface to be clean of carbon and nitrogen! Dissolution of carbon residues into the Ni crystal has been ruled out. Multiple adsorption-desorption cycles can be carried out without observing any resegregation of carbon to the surface. In similar experiments (13) Boyd,
R.H.J . Chem. Phys. 1963, 38, 2529.
The Journal of Physical Chemistry, Vol. 89, No. 5, 1985
864
Pan et al.
I'
CRYSTAL TEMPERATURE ( K )
CRYSTAL TEMPERATURE ( K )
Figure 3. Thermal desorption spectra monitoring mass 28 (N2)following different room temperature exposures of TCNE to the Ni( 1 1 1) surface. A heating rate of 8 K/s was used. The chamber pressure during the
exposure was 5 2.4
2 c
2.0
a
torr. Exposure times are as indicated.
X
1
'
I
0
A
= C(272eV)/Ni(848eV) = N(380eV)/Ni(848eV) = C(272eV)/N(380eV) h
A
1.6
:
0
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0
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CRYSTAL TEMPERATURE ( K ) Figure 4. Temperature dependence of the Auger intensity ratios following a room temperature exposure of TCNE to the Ni(ll1) surface.
with naphthalene monolayers on Ni( 1 1l ) , which had smaller amounts of carbon on the surface, very little loss of carbon due to dissolution into the crystal was oberved as a result of the TDS experiment. In an effort to understand why we did not observe the desorption of carbon containing species and to correlate the desorption temperatures of nitrogen with coverage, we monitored the composition of the TCNE adsorbate on the Ni( 11 1) surface at different temperatures with AES. Figure 4 shows that the AES intensity ratio of carbon (272 eV) to nitrogen (380 eV) remains constant at about 1.6 as a function of temperature up to the maximum temperatures of our experiments. The desorption processes are clearly indicated by the drops in the AES ratios for carbon (272 eV) to Ni (848 eV) and nitrogen (380 eV) to Ni (848 eV). Thus, the desorption of carbon and nitrogen from the Ni(1 l l ) surface occurs with the stoichiometry of TCNE as the temperature of the crystal is increased. We are drawn to the conclusion that carbon indeed leaves the surface with nitrogen during the desorption process. The lack of carbon containing species in the desorption following room temperature adsorption is possibly due to the desorption of negative ions, which we would not detect with our quadrupole mass analyzer in the present configuration. Herron and ~ e w o r k e r shave ' ~ studied the formation of negative ions as gas molecules passed by a hot metallic filament. (14) Herron, J. T.; Rosenstock, H. M.; Shields, W. R. Nature (London) 1965,206, 61 1.
(15) Miller, F. A.; Sala, 0.;Devlin, P.;Overend, J.; Lippert, E.; Luder, W.; Morse, H.; Varchmin, J. Spectrochim. Acta 1964, 20, 1233.
Figure 5. Measurement of the current collected at a copper braid placed in front of the Ni(ll1) crystal surface as a function of temperature: (a) following adsorption of TCNE; (b) clean Ni(ll1) surface. The inset
shows the AES spectrum following the exposure just before the temperature ramp. They found that TCNE generated only TCNE- anionic radicals with a hot platinum filament (900-1600 "C) without producing any other negatively charged fragments. Considering that we might have stable negative ions produced in the desorption process, we thermally desorbed the TCNE layer from the Ni( 11 1) surface and measured the current between the Ni crystal and a copper braid, mounted close to the front of the crystal and attached via a feedthrough to an electrometer. Figure 5 shows the results of such an experiment for a clean Ni( 1 11) surface and following an exposure of TCNE. In the region of the desorption temperature of NZ,we found an obvious current change. The rising background after 900 K is caused by thermionic electron emission from the Ni crystal. Although changes in work function with coverage make the interpretation of this experiment somewhat uncertain, we feel that the combined evidence points toward direct desorption of negative ions. The identity of the negative ions is uncertain, but the average stoichiometry must be that of TCNE, (C6NJn. As just discussed we are drawn to the conclusion that the N, observed as mass 28 during the desorption experiments is not a direct desorption product. It is possible that the N, which we observe in the desorption is the result of reactions of the initially desorbed species on the chamber walls. The formation of a negative ion as a desorption product is not really unreasonable since the desorption occurs at high temperature, near the onset of thermionic electron emission, and the precursor to the desorbed species may have a reasonably large electron affinity (e.g., TCNE electron affinity is 2.8 eV). We are presently conducting experiments to detect the negative ion desorption by modifying our quadrupole mass analyzer. b. Low-Temperature Adsorption. We have also carried out experiments with adsorption at 200 K and below. Low-exposure, TDS experiments give the same results as those with room temperature adsorption. That is, a film (more than one monolayer) is formed which is completely removed during desorption without desorption of any stable neutral species but rather by the direct desorption of a negatively charged species. When higher exposures are used, at low adsorption temperature, additional TCNE can be deposited on the surface. Under these high-exposure conditions, in addition to the high-temperature mass 28 peaks which are characterisitc of desorption from the above-mentioned film, we observe the direct desarption of neutral TCNE as mass 128. Figure 6 shows TDS results following a 600-s exposure of TCNE at 200 K. The sharp desorption peak which occurs at -250 K is from physisorbed TCNE molecules. This will be discussed more in light of the Raman spectroscopy results of the next section. In addition, there is a high-temperate mass 128 desorption state which has a peak at -650 and -870 K. This is a surprisingly high temperature and would correspond to a desorption activation energy of 40-50 kcal/mol, assuming first-order desorption and
Adsorption of TCNE on Ni( 111) 1
u
>
I
The Journal of Physical Chemistry, Vol. 89, No. 5, 1985 865 I
I
1
T C N E gas
-
~A (TCNE phy)
(( Ob)) MASS 128 28
! I
-B
v)
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(1) TS230K
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C R Y S T A L T E M P E R A T U R E (K) Figure 6. Thermal desorption spectra monitoring mass 28 and mass 128 following exposure of TCNE to the Ni( 11 1) surface at 200 K. The heating rate was 8 K/s.
I
S
, I
I *
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- 200 sec TCNE, 7.0 x IO-''
torr
I 5 0 sec TCNE, 1.5 x IO-' ZOO sec TCNE, 2.5 x IO-' 650 sec TCNE, 1.7 x IO-'
torr torr torr
------ 600 sec TCNE,
2.1 x IO-'
torr
Figure 8. The temperature dependence of the multilayer TCNE film formed by exposure of TCNE to the Ni( 111) crystal at temperatures below 230 K.
V
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- INCIDENT WAVELENGTH = 4 5 7 9 i LASER POWER = 5 0 M W
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400
600
100
800
CRYSTAL TEMPERATURE ( K ) Figure 7. Thermal desorption spectra monitoring mass 128 following various exposures of TCNE to the Ni(ll1) surface at 200 K. The heating rate was 8 K/s. Exposures are as indicated in the figure.
a preexponential factor of 1013s-l.16 This is a strongly chemisorbed state of TCNE. While we have no direct evidence that would preclude assigning the 650 K TCNE desorption to recombination of fragmented TCNE, we consider this to be unlikely for such a complex molecular species. Figure 7 shows mass 128 TDS spectra for several different exposures of TCNE. The physisorbed TCNE desorption peak obviously grows in before the more strongly bonded chemisorption state. The most likely explanation of this is that the chemisorbed state is formed from the physisorbed TCNE, during the temperature ramp of the TDS experiment. That is, there is a significant activation barrier to formation of the chemisorbed state. We have also carried out annealing experiments in which the adsorption was done at low temperatures followed by annealing to temperatures just below the onset of desorption of the physisorbed state (-230 K). Such annealing did not increase the amount of TCNE in the 650 K desorption peak even when the exposures below that of saturation of the chemisorption state were used. Thus, the rate of conversion from physisorbed to chemisorbed TCNE becomes appreciable only for temperatures near or at which the rate of desorption of the physisorbed species is also significant. Since we have no quantitative information about the preexponential factors the for two (16) Redhead,
D.A. Vacuum 1962, 12,203.
Figure 9. Raman spectrum of a multilayer film of TCNE on the Ni( 111) surface at 190 K. The transition at -2240 cm-' is the unresolved m N stretch doublet of neutral TCNE. The excitation wavelength was 547.9 nm. Only every third data point is shown for the Xl spectrum (solid line) for clarity.
competing processes (desorption of the physisorbed molecule and chemisorption) we cannot make any statements comparing the two activation energies. Our present picture of the temperature dependence of the thick multilayer formed by adsorption of TCNE at low temperature is shown schematically in Figure 8. Initially, TCNE adsorbs to form the film labeled B in the schematic. This film is more than one monolayer thick and results in the desorption of a negatively charged species (possibly TCNE-) upon heating to high temperature. On top of the B layer, a thick multilayer of physisorbed (condensed) TCNE can be grown by adsorption at temperatures below -230 K. The condensed TCNE is refered to as A in Figure 8. During the temperature ramp of a thermal desorption experiment, part of the condensed layer desorbs as neutral TCNE at -250 K. Competing with the desorption at 250 K is the chemisorption of some of the TCNE from the condensed layer (to form species C). As is shown in Figure 8 this must occur as the result of an interaction between the physisorbed TCNE molecules and the underlying B layer, since the Ni substrate is completely covered by the B layer. Further heating results in desorption of neutral TCNE at 650 and 870 K followed by breakup of the B layer and desorption of a negatively charged
866
The Journal of Physical Chemistry, Vol. 89, No. 5, 1985
1542cm-I
:
ELECTRONIC
-
40
30
:
ABSORPTION u4
,
\
-I
w
a
4500 4700 4900 5100
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INCIDENT WAVELENGTH A Figure 10. Excitation profiles of the Raman scattering transitions observed from a film of TCNE on the N i ( l l 1 ) surface at 200 K. All signals are normalized to that at 514.5 nm.
species a t temperatures above 680 K. Raman Spectra. In the Raman scattering experiments, we focused our attention on the C=C stretching mode and the C%N stretching modes. For neutral TCNE, the C=C stretching A, mode (vz) occurs at 1568 cm-', and the G N stretching A, mode ( Y J and B, mode (uI9) occur at 2236 and 2248 an-',respectively. Figure 9 shows the Raman spectrum of TCNE adsorbed, at low temperature, on the Ni( 111) surface. The A, mode at 1568 cm-I is seen clearly; the A, and B3, modes at 2236 and 2248 cm-I are not resolved and appear as one merged peak due to the lowresolution (12 cm-') used for this measurement. The small peak at 1530 cm-' is a combination mode of the C-C stretching mode (1282 cm-') v20 and the (CN)-C-(CN) rocking mode (254 cm-') vZ2.l5 In addition to those neutral TCNE Raman modes just mentioned, three weaker peaks are observed in the region between 1400 and 2300 cm-'. They occur at 1426, 1542 and 2198 cm-'. The peaks at 1426 cm-' and the peak at 2198 cm-' are the Raman active modes of the TCNE- anionic radical (C-C stretch and C=N stretch)." Our experiments also showed a characteristic TCNE- radical C-C stretching A, mode (Y,) at 540 cm-' (not shown in Figure 9). The evidence of the existence of the TCNE- anionic radical is further supported by the Raman excitation profile. TCNEhas an electronic absorption maximum around 430 nm, and this allows us to use our Ar+ ion laser to investigate the resonance Raman scattering excitation profile. Four exciting wavelengths were selected. They are 514.5,488.0,476.5, and 457.9 nm. Figure 10 shows the measured excitation profile. The relative intensities for different exciting wavelengths are normalized to the intensity a t 514.5 nm. The 1426-cm-' mode is strongly enhanced by the use of shorter wavelength incident radiation. For comparison, we also normalized the electronic absorption spectrum from ref 7 to the absorption a t 514.5 nm, and the result is similar to the Raman excitation profile for the 1426-cm-' mode. At this point we are not prepared to make a definitive assignment of the peak at 1542 cm-l. The origin of the peak at 1542 cm-' is still not clear to us. This peak has no counterpart in the Raman spectrum for either TCNEO or TCNE-. It is possible that (17) Determination of the vibrational red shift of v2 mode in a completely charge transferred TCNE- anionic radical from its neutral counterpart is still argumentative. Different alkaline salts of TCNE give different Raman frequencys of the u2 mode due to electron back-donation from the TCNE- anion to the cations of the salts. For example, CsTCNE has the largest red shift ever known, which is 212 cm-' (ref 6). However, recently it has been suggested that this large red shift was due to the u2 mode in a TCNE- dimer, (TCNE-)2 (ref 9). Thus our measured frequency of 1426 cm-'is in the region which is presently assigned t o the TCNE- C-C stretch mode.
Pan et al. it is a red-shifted v 2 mode due to partial charge transfer between the physisorbed TCNE molecules and the adsorbate in the underlying film. The red shift of 26 cm-' from the v 2 mode of the neutral TCNE would correspond to approximately 18% charge transfer to the physisorbed TCNEO if we assume a linear shift in frequency with charge transfer, as has been shown for TCNE and TCNQ complexes.lO*llSince the resolution of our Raman spectra is 12 cm-', we are not able to tell confidently whether or not there is a corresponding shifted v 1 mode in the 2200-cm-I C=N stretch region. The intensity of TCNE- modes slowly grows with increased exposure of TCNE and increased intensity in the neutral TCNE modes. The purified TCNE crystal does not show any TCNERaman signal. The Raman intensity ratio of 1568 cm-' to 1425 cm-' quickly increases with exposure time; this also indicates that the generation of TCNE- is a result of adsorption on the Ni( 1 11) surface. The reason that TFNE- continues to grow without apparent saturation can be understood by the electron exchange reaction between TCNE- and TCNEO. The reaction, TCNE,+ TCNE? = TCNElo+ TCNEF, has been studied. For instance, a rate constant of 2.1 X lo5 L mol-' s-l for this reaction in tetrahydrofuran at room temperature was obtained by Phillips and co-workers.'* In our system, the electrons transferred from the Ni(l11) surface to TCNE molecules can, in turn, switch to other neutral TCNE molecules on neighboring sites or in upper layers. Complete charge transfer to a molecule in the upper layer will decrease the concentration of negative ions near the Ni surface, and then the tendency toward equilibrium of the surface ionization reaction between the Ni surface and TCNE molecules will allow more electrons to be transferred from the Ni surface to adsorbed layers. However, charge repulsion will eventually restrict the TCNE- concentration in the physisorbed layers. In addition the adsorption orientation may play a controlling role in the probability of charge transfer. A simplistic picture of the energetics of the formation of adsorbed TCNE- by removal of an electron from the nickel surface would involve the comparison of the work function of a Ni( 1 11) surface with the electron affinity of TCNE. The Ni( 11 1) surface work function (a) is 5.35 eV.21 The electron affinity of TCNE (E,ff)while substantial is only 2.38.19 The difference between and Eaffof 2.52 eV would appear to provide a major barrier to the formation of TCNE-. This simple argument does ignore the effect of the image on the stability of the TCNE- on the Ni( 111) surface:2 and the fact that the TCNE- may reside within the crystal surface potential so that the full work function is not appropriate here. However, these are likely to be small effects. It has been found that the adsorption of TCNE on many metallic surfaces lowers the work function by more than 1 eV.20 Thus, the film underlying the physisorbed TCNE layer (labeled B in Figure 8) could play an important role in the formation of the negative ions in the physisorbed layers by reducing the surface work function. Defects on the N i ( l l 1 ) surface may also be important in the formation of the adsorbed TCNE- species. The work function of Ni vanes depending on the crystallographic plane of the surface. The much more open Ni( 110) surface, for example, has a work function of 5.04 eVS2l Thus, a surface defect could provide an energetically more favorable site for formation of the TCNE-. The concentration of such defects should be low. However, since we do not have an independent measure of the TCNE- Raman cross section, we cannot quantify the concentration of TCNE- formed, and we thus cannot rule out defects as an
(18) Phillips, W. D.; Rowell, J. C.; Weissman, S . I. J . Chem. Phys. 1960, 33, 626. (19) Page, F. M.; Goode, G. C. "Negative Ion and the Magnetron";Wiley: New York, 1969; p 101. (20) Page, F.M.; Goode, G . C. "Negative Ion and the Magnetron";Wiley: New York, 1969; p 60. (21) "CRC Handbook of Chemistry and Physics", 62nd ed.; CRS Press: Boca Raton, El, 1981-1982; p E-80. (22) Lyo, S . K.; Gomer, R. In "Interactions on Metal Surface";Gomer, R., Ed.; Springer-Verlag: West Berlin, 1975; p 42.
The Journal of Physical Chemistry, Vol. 89, No. 5, 1985 867
Adsorption of TCNE on Ni( 1 11)
A
:
1568 cm-l
:
1530 Cm-'
V
< 300 Q,
u)
section (due to modified electron densities or orientation and symmetry effects) for the species in the room temperature film (B of Figure 8). However, we feel it is more likely that the species which makes up the film deviates drastically from TCNE (does not have Raman active modes in the 1300-1600-~m-~region).
0 :
1 5 4 2 cm-'
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u
250
CRYSTAL TEMPERATURE ( K ) F w e 11. The crystal temperature dependence of the Raman scattering intensity in the 1400-1600-~m-~region of the spectrum.
important factor in the formation of TCNE-. At temperatures above 270 K no observable Raman signal was detected in the 1300- to 1600-cm-' range ( C = C stretch region). When the sample is warmed following low-temperature adsorption the Raman peaks of both TCNEO and TCNE- all disappear abruptly at between 250 and 270 K. This is the temperature range for desorption of the physisorbed TCNE overlayer (see Figure 6). Figure 11 shows the temperature dependence of the various Raman peak intensities assigned to TCNEO and TCNE-. Thus, the film underlying the physisorbed layer (layers B and C in Figure 8) does not appear to contain either TCNE- or isolated TCNEO entities. To estimate the intensity of the Raman signal from one monolayer of neutral TCNE, we carried out a TDS experiment immediately after the Raman spectrum shown in Figure 9. If we assume that, at this saturation level exposure, the chemisorbed mass 128 peak (650 and 870 K) corresponds to approximately one monolayer, then the 1568-cm-' peak of the Raman spectrum shown in Figure 9 corresponds to 9-12 monolayers of physisorbed TCNE. This leads to an expected peak count rate of -30 cps for monolayer TCNE. This is easily observable with our apparatus. The lack of observable Raman signals from the room temperature film may be due to a significantly lower Raman cross
Summary and Conclusions Low-temperature adsorption of TCNE on Ni( 11 1) results in a multilayer structure. The molecules closest to the metal surface form a multilayer film which is stable to high temperatures and results in the desorption of negatively charged species. Physisorbed TCNE can be condensed on top of this film at temperatures below 250 K. Desorption of the physisorbed layer competes with an activated chemisorption of TCNE. The chemisorbed TCNE desorbs at -650 K and above. TCNE- has been identified in the physisorbed layer by Raman scattering. The C=C stretch mode (1426 cm-I) and the C=N stretch mode (2198 cm-') of TCNE- have been observed. In addition the Raman excitation profile of the 1426-cm-' mode matches the electronic absorption spectrum of TCNE- from the literature. We cannot rule out the importance of defect sites in the formation of the TCNE-. The formation of a stable multilayer film at room temperature and the complete desorption of this film as a negatively charged species points to the unusual chemical behavior of this adsorption system. Although we are not able to detail the structure of the "chemisorbed state" which is formed in competition with desorption of physisorbed TCNE we believe that the chemisorbed state must retain a structure similar to TCNE. Without a more detailed picture of the underlying film, it is hard to do more than speculate about the details of the bonding of the chemisorbed state. Further studies of the rmm temperature multilayer film with other surface probes and a mass spectrometer modified for negative ion detection are underway in our laboratory. We would like to stress that our results indicate that extreme care must be taken in interpreting vibrational spectroscopy experiments of other r-Lewis acids (such as TCNQ) on metal surfaces. The detection of the vibrational spectrum of the negative ion may lead one to jump to the conclusion that there is a direct charge transfer between the metal and the first layer adsorbate. The chemistry of TCNE adsorption on Ni( 11 1) is clearly much more complex than that simple picture. Acknowledgment. This work has been supported by the United States Department of Energy Contract No. DE-AT03-8 1ER10820. Registry No. TCNE,670-54-2; Ni, 7440-02-0.
