J. Phys. Chem. 199498, 297-302
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Tunneling Spectroscopy of Halogen Adlayers on Ag( 11 1) Surfaces Joachim Hossick Scbott Department of Chemical Engineering and Materials Science, University of Minnesota, Minneapolis, Minnesota 55455
Henry S.White' Department of Chemistry, University of Utah, Salt Luke City, Utah 84112 Received: August 9, 1993; In Final Form: October 26, 19930
Tunneling spectroscopy measurements performed with a scanning tunneling microscope (STM) of Ag( 111) surfaces covered with a monolayer of halogen atoms (F, C1, Br, and I) are reported. At positive sample biases, the tunneling current rises sharply from the nanoampere range well into the microampere range for all Ag(1 11)-halogen systems. The threshold voltage of this diodelike response is a function of the halogen species and correlates with vibrational energies of the respective Ag-halogen system determined from resonant Raman scattering. The minimum tunnel junction resistance is -?rh/e2, in agreement with theoretical predictions based on models of a point-contact single-atom junction or a double-barrier tunnel junction. The results are discussed in terms of a point contact formed between the tip and surface halogen atom and resonant tunneling coupled to the vibrational modes of the Ag-halogen bond.
Introduction Tunneling spectroscopy (TS) performed using a scanning tunneling microscope has proven to be a valuable method for probing local surface electronic properties. In a typical experiment, a metallic STM tip is positioned several angstroms above the surface of a conductive sample. Depending on the polarity of the bias voltage applied between the tip and the sample, Vb, electronscan tunnel from filled tip states into empty samplestates, or vice versa. The overall current density depends on the product of the tip and sample density of states and a barrier penetration factor.1J For two bare metal surfaces separated by a thin insulator3 and for small bias voltages Vb (i.e., in the energetic vicinity of the Fermi level), the tunneling current, I, is a linear function of Vb. Upon applying higher bias voltages, the onset of electron field emission is expected to occur, resulting in an exponential dependence of the current on the bias voltage.' Resonance structures in the tunneling I-V spectrum are expected whenever electrons can be injected into localized and unoccupied electronic states on the surface. If the energy of the tunneling electron matches the energy of such localized states, the tunneling current is locally maximized in the I-V spectrum. Thus, metal surface states,4 image-type field ~ t a t e s ,and ~ . ~states due to adatoms adsorbed on the ~ u r f a c e ~ may . ~ + *be identified in I-V spectra obtained with an STM. Tunneling electrons may also excite vibrationsof molecular or quasi-molecularcomplexes adsorbed on the surface.9 In STM experiments,the contribution of such processes to the overall current density is expected to be small.10 Electronic resonances, on the other hand, may yield very high current densities. Thus, the transmission probability of the tunneling electron can reach unity in double barrier tunnelingjunctions, if the energy of the tunneling electron matches the energy of virtual bound states localized in the double-barrier system." Pioneered by Tsu, Esaki, and Chang,l*J3research on such systems is a highly interesting topic and currently focused on two-dimensional and one-dimensionalsemiconductor quantum well s t r u c t u r e ~ . ~A ~ Jphenomenon ~ closely related to resonant tunneling is observed in the STM when the tip and the metal form a one-atom point contact.16-18 In this case, LangI6has shown that the resistance R is of the form (rh/eZ)A,with the constant A being dependent on the identity of the tip atom, but of order Abstract published in Advance ACS Absrracrs, December 15, 1993.
0022-3654/94/2098-0297$04.50/0
unity. Similarly,the minimum resistanceat resonance in doublebarrier tunneling junctions has been calculated to be Irh/e2 ( 12.6 kn) .ll In this report, we present tunneling spectroscopy results obtained by using an STM operated in air and at room temperature. The samples investigated in this study are Ag(1 11) surfaces covered with a complete monolayer of halogen atoms (F, C1, Br, and I). The I-V responses of these samples display a highly nonlinear response. In addition, within a particular range of bias voltage, the tunneling currents measured on the halogen-coated Ag surfaces are 1000 times larger than that observed for bare Au( 111) or oxygen-covered Ag( 1 111 samples at the same bias voltage. Resonant tunneling and chemical bonding effects are discussed as possible explanations of the observed I-V response.
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Experimental Section Our experiments were performed in air with a Digital Instruments Nanoscope I1 scanning tunneling microscope,~9in which the tip is held at virtual ground and the bias voltage Vb is applied to the sample. Mechanically-cutPt(70%)-Ir( 30%) tips were used in all imaging and spectroscopyexperiments. All images reported here were recorded in the constant-current mode using a scan rate of 8.6 Hz and consist of 400 X 400 data points, TS experiments were performed by measuring the tunneling current Ias a function of the ramped bias voltage V. The tunneling current was recorded after current-voltage conversion across a l-Ma resistor. The tip was placed in the center of a surface area which was imaged before and after each current-voltage measurement. All I-Vdata are reported without filtering, smoothing, or averaging. Before the actual I-Vmeasurement, a tip-sample distance was.established by adjusting the feedback loop of the instrument with a programmable set-point resistance R- = Vwt/ Iwt. (V,, and Iwt denote the set-point bias voltage and the setpoint tunneling current, respectively). The feedback was then disabled, and the tunneling current was measured as a function of the ramp voltage. Some of the TS measurements reported here were recorded using commercial software installed in the Nanoscope I1 STM which generates a triangle-shaped voltage ramp. The program, however, records I-V data only during the second half-cycle of Q 1994 American Chemical Society
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The Journal of Physical Chemistry, Vol. 98, No. I, 1994 '
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Energy I eV Figure 1. Auger electron spectrum of a chlorinated Ag sample.
the ramp, which starts at the positive limit of the selected voltage scan window. All other I-Vmeasurements reported were recorded with an x-y recorder, after passing the current-proportional voltage signal through a 1:l operational amplifier. The bias voltage was also recorded after passing the signal through a 1:l operational amplifier. This modification allows the I-Vresponse to be recorded during both halves of the potential cycle. Since extremely high current levels (by STM standards) were measured in these experiments, the 1-MQ resistor of the current follower was replaced in some experiments by a 10 kQ f 1% resistor, in order to increase the useful range of current measurements. Typical scan times during the TS measurements were -2 s. Our method of preparing Ag surfaces is adapted from the method developed by Zurawski et a1.,20 who adsorbed iodine on Pt samplesand demonstrated that the halogen adlayer effectively protects the sample from surface contaminants. Spherical Ag samples (1-2 mm in diameter) were prepared by melting one end ofa -2-cm pieceofAg wire (99.9985%purity, 0.5-"diameter, Aesar/Johnson Mathey) in a H2/02 flame. The spheres were further annealed in a H2 flame at 1000 K for -60 s, cooled for -20 s in air, and transferred into one of the following solutions: hydrofluoric acid (38%, Baker) to form F adlayers; hydrochloricacid (38%, Merck) to form C1 adlayers; hydrobromic acid (48%, Mallinckrodt) to form Br adlayers; and hydroiodic acid (196, Merck) to form I adlayers. After immersion of Ag for 30-300 s, the samples were rinsed with triply distilled water, dried, and mounted in a home-built specimen holder for STM studies. Au(ll1) surfaces were prepared according to the procedure described in ref 21. The formation of halogen adlayers, as well as surface contamination, was monitored by Auger electron spectroscopy (PHI 595 SAM, cylindrical mirror analyzer). As shown in Figure 1, no measurable oxygen or carbon signal is observed in the Auger electron spectrum of a C1-covered Ag surface. Similar spectroscopic analyses of Ag samples following immersion in HI, HF, or HBr solutions also did not yield any detectable oxygen or carbon signal, even after exposure of the prepared samples to ambient air for 2-3 h. Ag samples which were not immersed in a halogen solution, but which were otherwise prepared in an identical fashion, yielded a significant oxygen Auger signal immediatelyafter cooling (not shown here). These results indicate that the halogen adlayer effectively prevents the adsorption of expected contaminantssuch as oxygen. Our results are consistent with ultrahigh-vacuum (UHV) studies of Kiskinova et a1.22 and theoretical arguments of Lang et al.23 that indicate that electronegative adsorbates, such as C1, prevent the coadsorption of oxygen in UHV environments. However, after -3-h exposure to air, we were able to detect an oxygen signal on some halogencoated Ag samples. Therefore, all STM and TS studies were performed within 2 h of sample preparation.
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Results and Discussion
The structures of the halogen monolayers on Ag( 1 11) can be atomically resolved in STM and are shown in Figure 2. Since a detailed discussion of these structures is given in ref 24, only
Hossick Schott and White a brief account will be presented here. STM images of I adlayers are consistent with a nearly perfect ( d 3 X d 3 ) R 3 0 ° structure; F, C1, and Br adlayers are characterized by a parallel double-row structureof adsorbed halogen atoms. The row structureobserved by STM for C1 and Br adlayers is consistent with low-energy electron diffraction (LEED) patterns2S28 but inconsistent with the correspondingstructures derived from LEED. It is important to note that high-quality images of the halogen monolayers could only be obtained using comparatively high negative (sample) bias voltages (note the Vb values given in the figure captions). At positivevoltages,the feedbackloopof the instrument was unstable and imaging was impossible. Figure 3 shows a typical I-V curve obtained for a C1-coated Ag( 111) facet. We note several striking features of this curve. (1) The tunneling current increasessharplyabovea positive sample bias threshold. (2) Above the threshold, thecurveexhibitscurrent plateaus and/or peaks. ( 3 ) For bias voltages positive of +0.15 V, the tunneling current reaches the limit set by the 10-kQresistor. (4) The I-Vcurve exhibits a pronounced hysteresis between the upward and downward positive voltage scans. ( 5 ) For negative bias voltages the tunneling current rises significantly only at voltages below - 4 . 5 V. (See insert in Figure 3; this curve was obtained using a 1-MQ resistor in series with the tip.) We emphasizethat the qualitative features of the I-Vcurves reported here are highly reproducible. However, the positions of the current peaks and plateaus as well as the threshold voltage, above which the current rises into the microampere range, were found to vary by f0.1 V for different experiments (see below). In addition, no surface damage was observed in STM images recorded immediately after the I-Vexperiments, provided that the positive limit of the voltage scan window was kept below -0.5 V. Tunneling spectroscopy experiments performed on bare Au(1 11) surfacesunder similar set-point resistance, and scanvoltage conditions do not yield any of the unusual features in the I-V curve observed for the chlorinated Ag sample (Figure 4). Here, the tunning current is a linear function of the applied bias voltage, and the current levels are in the nanoampere range, as expected for such a smallvoltage scan window and high set-pointresistance. Note that the current levels for positive voltages differ by a factor of 1000 between the I-Vcurves shown in Figures 3 and 4. I-V measurements recorded for Ag surfaces that were not coured with a halide adlayer were quite similar to that shown for the Au sample in Figure 4. Auger spectroscopicanalysisof these samples indicated that, in the absence of the halide layer, the Ag surface is covered by at least a monolayer of 0xygen.2~ I-V curves obtained on Ag( 111) surfaces coated with other halogen species are qualitatively similar to the curve obtained for the chlorinated surface (Figure 5). However, it appears that the voltage threshold, above which the current rises into the microampere level, is significantly lower for I (Figure 5a) than for F (Figure 5b) or C1 (Figure 3 ) . This trend is more clearly apparent in Figure 6, where the voltage thresholds obtained for F, C1, Br, and I monolayers are plotted against the halogen ion electron affinity (Figure 6a) and the physical diameter of the halogen ions (Figure 6b). These plots were obtained by averaging the voltage thresholds found in 10consecutiveI-Vmeasurements performed for each of the four Ag-halogen chemisorptionsystems, with the set-point conditions being the same for the total of 40 experiments. The four tips used in this set of experiments (one for each halogen chemisorptionsystem) yielded atomic resolution images of the respective halogen monolayer, before and after recording thel-Vcurves. All thresholdvoltagedata wereobtained with a I-MQ resistor in series with the tip. It is inferred from Figure 6 that the voltage threshold is a function of the halogen adatom. We note that this conclusion does not depend of the specific tip used in the experiment. We find that the voltage threshold for the C1monolayer is always higher than the threshold for the I monolayer in any given set of experiments performed
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STM of Halogen Adlayers on Ag( 1 1 1) Surfaces
The Journal of Physical Chemistry, Vol. 98, No. I , 1994 299
Figure 2. STM images of halogen monolayers on Ag( 1 1 1). All image areas are 2.5 X 2.5 nm2. (A) F adlayer, v b = -1 135 mV, I , = 2.2 nA. (B) mv, I , = 2.3 nA. (C)Br adlayer, vb = -550 mv, it = 5.7 nA. (D)I adlayer, &, = -358 mV, it = 4.9 nA. All images were band-pass filtered once.
c1 adlayer, vb = -850
under constant set-point conditions. Our inability to image any of the halogen-coated Ag surfaces at positive bias is clearly due to the highly nonlinear I-V response in this voltage region. All of thehalogens are expected to form a predominantlyionic bond with the silver surface.30 The theoretical work of Lang and Williams3l indicates that the broadened 3p level of the chemisorbed C1atom lies -2.5 eV below the Fermi energy of the metal and is thereforetotally occupied (ionic adsorption). At zero bias, the potential energy contours between tip and sample may then be depicted as in Figure 7a. For small negative values of vb, tunneling will proceed mainly from the filled portion of the tail of the broadened 3p CI ion level into empty states of the tip. The expected current density is low because of the low density of states in the tail of the 3p level, in agreement with the I-Vcurve shown in the insert of Figure 3. For more negative values of vb, the available emitter density of states increases (Figure 7b), resulting in an increase in the tunneling current, in agreement with the I-Vcurve shown in the insert of Figure 3. This localized distribution of state density accounts for the observation that high-contrast STM images are only obtained at relatively high negative bias voltages (--1 to - 4 . 5 V). We also note that the results of Bagus and Pacchioni30 suggest that the halogen ion moves slightly toward the surface upon charging the Ag surface
positive. Conversely, negative surface charge slightly increases the Ag-halogen bond length. This phenomenon is indicated in Figure 7. It is apparent from Figure 7a,b that the potential energy contours in the tunnel junction assembled from the Ag surface, the chemisorbed halogen ion, and the tip resemble the potential energy contours of a double-barrier tunneling junction. Therefore, the following mechanism may explain the high current levels observed in the I-V curves at positive bias voltages. For small positive bias voltages, tunneling will mainly proceed from the Fermi level of the tip to empty states in the tail of the halogen 3p level. Since there are virtually no such states, the current is essentially zero and the junction is blocking. At higher positive biases, electrons may resonantly tunnel from filled tip states into empty Ag states, via virtual bound state levels in the Cl potential well (Figure 7c). Whenever the energy of the incident electrons matches the energy of such bound states, a peak in the I-V spectrum should occur, in qualitative agreement with the I-V curves in Figure 3 and 5b. Following Kalmeyer and Laughlin,ll the minimum resistznceR at resonance should be .rrfz/eZ (- 12.6 kfl). Inspection of Figures 3 and 5b reveals that the minimum res.istance observed at a current peak is in reasonable agreement with the theory.
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