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(alane) oligomers appear as 2.8 Å protrusions on Al(111). ... image contrast is both enhanced and inverted, with alane oligomers appearing as depress...
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J. Phys. Chem. B 2000, 104, 8507-8511

8507

Scanning Tunneling Microscopy Study of Alanes/Al(111): Contrasting Neat and Molecularly-Terminated W Tips Eden P. Go, Konrad Thuermer, and Janice E. Reutt-Robey* Department of Chemistry and Biochemistry, UniVersity of Maryland, College Park, Maryland 20742 ReceiVed: March 30, 2000; In Final Form: June 20, 2000

Scanning tunneling microscopy has proven to be a very powerful method for real space imaging of molecules on surfaces. Here we demonstrate how the morphological and chemical structure of the tunneling tip alters the apparent topography of STM images. When imaged with conventional W tips, isolated aluminum hydride (alane) oligomers appear as 2.8 Å protrusions on Al(111). When imaged with alane-terminated tips, STM image contrast is both enhanced and inverted, with alane oligomers appearing as depressions. We attribute this image reversal to the electronic levels of tip-adsorbed alanes, whose distance from the surface Fermi levels reduces the coherence in the tunneling junction. The absence of a bias dependence in these images suggests predominant tunneling through tip-alane LUMO states.

I. Introduction Scanning tunneling microscopy has emerged as a powerful tool for mapping surface structure, including the conformation of adsorbates. While many organic molecules1-5 and biomolecules6-8 have been molecularly resolved, image interpretation remains problematic because the tunneling mechanisms that deliver image contrast are not fully understood. Researchers have invoked several image contrast mechanisms in the interpretation of STM images of adsorbates on surfaces. These include modulation of the local work function induced by the presence of polarizable molecules,1 elastic deformation of the substrate,9 resonant tunneling,10,11 and tunneling through the molecular orbitals of adsorbates.2 These mechanisms focus on the relation of the tunneling current to the electronic structure of the adsorbate and substrate for a given tip position, with the contribution of tip-sample interaction presumed to be weak or negligible. Tip-sample interactions are utilized, however, in adsorbate manipulation experiments. Site transfer, diffusion, and rotation of adsorbates have been induced by van der Waals12 and electrostatic forces13-15 in the tunneling junction. Recently, the influence of tip-induced surface polarization on image contrast has also been reported.16 To further investigate how the tip affects image contrast, knowledge of the structure and chemical composition of the STM tip at its apex is necessary. Unfortunately, tip structure and composition are often not known, and in any event, are frequently modified by field effects in the tunneling junction, which can cause the sudden transfer of material to the tip. STM studies of O/Cu(100) and O/Ni(110),17 S/Cu(11 1 1),18 CO/Cu(221),19 S/Re(0001),20 and S/Pt(111)21 have demonstrated the dramatic changes in the image contrast of the adsorbed species due to a change in the chemical composition of the tip. While these studies have been largely restricted to atomic modifications, reports involving molecular tip modifications are few. Contrast reversal observed for perylenetetracarboxylic dianhydride (and species such as copper pthalocyanine, anthracene, and nitronaphthalene) was attributed to the trapping of these large molecular adsorbates below the tip apex.22 * Corresponding author. E-mail: [email protected].

In the present paper, we show how molecular termination of an STM tip has a dramatic effect on STM images. Aluminum hydride oligomers (alanes) adsorbed on Al(111) are imaged as protrusions by metallic W tips, indicating sufficient conductivity through aluminum-bound alanes. Chemical modification of the W tip by the surface-to-tip transfer of an alane oligomer increases spatial resolution, at the expense of “reversing” image contrast. This contrast “reversal” is interpreted in terms of the energy misalignment between tunneling levels, reducing conductivity through aluminum-bound alanes. II. Experimental Section All measurements were performed under ultrahigh vacuum (UHV) in a chamber with a base pressure 1 Å at yet lower gap resistances, reflects strong tip-sample interactions for this close scanning.24 To facilitate quantitative interpretation of STM images and for stable imaging of chemisorbates, it is necessary to increase the tip-sample separation by increasing the tunnel gap resistance.25 Raising the gap resistance to 5 GΩ corresponds

10.1021/jp001212l CCC: $19.00 © 2000 American Chemical Society Published on Web 08/15/2000

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Figure 1. STM image of Al(111) substrate, showing the step-terrace structure that extends on a mesoscale prior to reaction with atomic hydrogen. The inset is an unfiltered small-scale image of the substrate, acquired at a relatively low gap resistance of 0.7 MΩ (0.007 bias, 10 nA tunnel current) to resolve the atomic detail of the lattice. The relatively large corrugation of substrate atoms is attributed to elastic deformation due to close scanning.

to an ∼4 Å increase in the tip-sample separation, which is suitable for imaging alanes with molecular resolution. For these mild tunneling conditions (typically 1.0 V sample bias, 0.20 nA tunneling current), alanes are imaged as bright protrusions by a clean W tip. Upon decreasing the gap resistance to ∼3 GΩ, an alane oligomer will invariably transfer to the W tip while scanning. Alane-terminated W tips are thus prepared by the transfer of a surface-bound alane to the scanning W tip at reduced gap resistances (i.e., reduced tip-sample separation). We then deduce the size of the alane transferred to the tip from the spatial resolution it achieves. (Unfortunately, the transferred alanes cannot be imaged prior to transfer because many transferable surface alanes freely diffuse on the surface at room temperature.) For neat and alane-terminated W tips, we imaged surface alanes over a wide range of voltages (0.35 V < |V| < 1.5 V) and tunneling currents (0.05 nA < I < 3.0 nA). Higher voltages are precluded by the tendency of surface alanes to dissociate; lower voltages are precluded by the tendency to transfer surface alanes to the tip at the operable tunnel currents (g0.05 nA) of our microscope, which makes for unstable imaging. III. Results The sensitivity of STM images of surface alanes to the chemical termination of the STM tip is first demonstrated in large-scale STM images. As shown in Figure 2a, when the surface is imaged with a clean W tip scanning at a distance high (∼10 Å) above the surface, alanes on Al(111) are imaged as small bright features distributed in low density (∼0.01 ML) on terraces and in the higher density decoration of crystallographic steps. (The irregular step contours result from the

Figure 2. Tip dependence of STM images of the Al(111) surface covered by ∼0.01 ML of aluminum hydride oligomers. Both images were acquired with a positive sample bias of 1.0 V and a constant tunnel current of 0.20 nA (gap resistance of 5 GΩ). (A) Image acquired with a clean W tip, in which oligomers appear as protrusions (“normal” contrast). (B) Image acquired with an alane-terminated W tip, in which surface oligomers appear as depressions (“reverse” contrast).

alane-formation process, where the step edge serves as the net source of aluminum atoms.) After transferring an alane from a nearby area to the tip (by simply reducing the gap resistance to 1 GΩ), the Figure 1a region was rescanned with identical tunneling parameters as the first image. In the image acquired with an alane-terminated tip (Figure 2b), alanes appear as depressions (dark features). This dramatic contrast reversal can be reversed once more to “normal” contrast by removing the alane from the tip with bias pulsing. To investigate these dramatic tip effects on image contrast, it is instructive to consider small-scale STM images. As a reference, we begin with STM images of small alane oligomers

STM Study of Alanes/Al(111)

Figure 3. Small-scale STM images of surface alanes obtained with clean W tips with different resolving powers (apex geometries), characterized by the measured widths of crystallographic steps. (A) For a moderately sharp W tip (width ∼ 15 Å), alanes appear simply as 2.8 Å protrusions, as seen in the corresponding line profile. (B) For a sharper W tip (width ∼6 Å), alane protrusions of ∼1.1 Å height are ringed by annular depressions. Additional flicker noise in room temperature STM images of surface alanes is introduced by mobile surface alanes. Tunnel conditions: -0.8 V sample bias and 0.35 nA current.

(monomer-tetramer) obtained with clean W tips of different sharpness (apex morphology), deduced from the measured widths of monatomic crystallographic steps. Step widths were determined by fitting the step line scan (perpendicular to the step) to the form, a0 + a0/2 tanh(x - a1/w), where a0 is the step height, a1 is the step position, and w is the step width. (For this tanh function, w is the region where the step height rapidly changes from 0.25 to 0.75 of its maximum value.) For moderately sharp W tips (corresponding to a step width of ∼15 Å), alanes are imaged as 2.8 Å protrusions (Figure 3a), consistent with the expected height of a surface alane.26 In addition to these molecularly resolved alanes, species with greater surface mobility are also sensed indirectly as flicker noise in these room temperature STM images. We focus our analysis on the fixed alanes that are directly imaged at room temperature but note that the contrast of the flicker noise (“normal” or “reversed”) for mobile alanes has the same dependence on tip termination as the contrast for the fixed alanes. With sharper W tips (step width of ∼6 Å), the alane protrusions are ringed by annular depressions (Figure 3b). The “flicker” signal from mobile alanes appears streaked along the scan direction, indicating greater attraction (or pulling) of mobile alanes. Although images acquired with neat W tips show some electronic distortion with tip morphology, alanes are nonetheless imaged as protrusions with neat W tips. We next consider small-scale images of molecular alanes obtained with alane-terminated tips (“reverse” contrast). The most extreme case of contrast reversal involves the transfer of “trialane” to the tip. As shown in Figure 4a, the contrast of both surface alanes and the aluminum substrate atoms are reversed, with alanes appearing as 2.8 Å deep depressions and the substrate atoms as ∼0.5 Å dips. We deduce a triangular geometry of the tip apex (hence “trialane” termination) from the triangular appearance of the smallest surface alanes (monomers), which indicates that the surface alanes are actually

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Figure 4. Small-scale STM images of surface alanes obtained with W tips terminated with different aluminum hydride molecules showing image contrast reversal due to tip alane termination. (A) “Trialane” transfer to the tip results in complete contrast reversal, with alanes imaged as 2.8 Å depressions and substrate aluminum atoms as ∼0.5 Å depressions. The triangular appearance of the small alanes in the field of view reflects the shape of the alane-terminated tip. The triangular tip shape also leads to “geometric” contrast reversal of the substrate atoms. Tunnel conditions: -0.12 V, 2.9 nA. (B) More typical STM image obtained after transferring a larger alane to the tip. Surface alanes are imaged with “reverse” contrast, appearing as ∼0.25 Å depressions, surrounded by halos, while substrate atoms are observed with “normal” contrast. Note that the flicker noise, due to room temperature imaging of mobile alanes, also appears with “reverse” contrast with alaneterminated tips. Tunnel conditions: -1.4 V sample bias and 0.20 nA current.

imaging the tip. Contrast reversal for the substrate atoms also follows from this trimer tip termination. Previous studies with hexagonal substrates27 have shown how the registry of trimer tip atoms relative to the substrate lattice leads to inverted lattice images. While these completely inverted images are striking, they represent an extreme case. The majority of images, acquired with larger alane-terminated tips, exhibit partial contrast reversal (Figure 4b), in which alanes appear with “reverse” contrast, while substrate atoms retain “normal” contrast. In >130 images acquired with 23 different alane-terminated tips, surface alanes are imaged as depressions, while substrate atoms appear “normal.” Images acquired with different alane-terminated tips vary in the depth of the alane depressions (0.2-2.8 Å) and the intensity of the halos that surround surface alanes. Because image details are sensitive to the alane termination of the tip, we next investigated the effect of bias voltage. To avoid additional alane-to-tip transfer during these measurements, measurements were restricted to tip bias range 0.65 V e |V| e 1.5 V at 0.2 nA tunnel currents. In contrast to previous studies involving oxide-terminated tips, STM images obtained with both W tips and alane-terminated tips showed little sensitivity to these variations in tip bias. Throughout this bias range, all alanes imaged with W tips appear as protrusions, whereas all alanes imaged with alane-terminated W tips appear as depressions. IV. Discussion and Conclusions 1. Sample-to-Tip Molecular Transfer. A dramatic contrast reversal in the STM images of alane oligomers results from the spontaneous transfer of a surface alane to the STM tip.

8510 J. Phys. Chem. B, Vol. 104, No. 35, 2000 Surface alanes can spontaneously transfer to the STM tip even under relatively mild tunneling conditions, although the probability for transfer increases significantly when the gap resistance (and tip-sample separation) decreases. Unlike tip manipulation experiments,28-30 where the transfer of atom or clusters of atoms is induced by external electric fields through bias pulsing, the transfer of alane molecule to the tip is induced by the local electric field in the tunneling gap generated by the difference in the work function of the tip and the H-dosed Al(111) surface. Indeed, recent quantum mechanical calculations31,32 show that a nonactivated material transfer between a clean Al(111) surface and a tungsten tip can occur in STM experiments in the absence of an external electric field if the tip-sample separation reaches a critical value. Once a critical tip-sample separation is reached, the Al atom or cluster desorption barrier will collapse due to strong chemical interaction, resulting from the attractive interaction between the Al core and the d electrons on tungsten. Although the study involved bare Al(111), the same underlying mechanism of molecule transfer can be applied here. Under normal tunneling conditions, the electric field generated in the gap is sufficient to transfer an alane oligomer to the tip. 2. Image Contrast with Molecularly-Terminated Tips. We first summarize the contrast observed in STM images of surface alanes: (i) The overall image contrast of surface alanes depends on the tip chemical termination. Alanes appear as protrusions with neat W tips but as depressions with alane-terminated tips. (ii) Image details of surface alanes are sensitive to the structure at the tip apex. Nontopographic features such as halos and annular depressions are commonly observed in images for both W and alane-terminated tips. (iii) Regardless of tip termination, the image contrast of surface alanes is mostly insensitive to the magnitude and polarity of the tunneling voltage, at least over the voltage range for tip bias, -1.6 to -0.65 and 1.6-0.65 V, permitted by this system for tunnel currents of g0.2 nA. To understand these observations, we consider electron transfer through the complex tunneling junction that includes the W tip, a surface alane, and the Al(111) substrate, as illustrated in Figure 5. Electron transfer will depend on the positioning of the alane molecular energy levels with respect to the Fermi levels of the tip (φW ) 5.1 eV) and the substrate (φAl ) 4.1 eV). For the purpose of discussion, we consider gaseous dialane (Al2H6) and use its calculated ionization potential of 11.08 eV and electron affinity of -1.04 eV33 to approximate the location of the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) of alanes relative to the surface Fermi level. Although these gaseous HOMO and LUMO levels are far from the surface Fermi levels (the LUMO level even exceeds the vacuum level), surface alanes are nonetheless imaged as protrusions when a metallic W tip scans these adsorbates. This conductivity through alanes adsorbed on Al(111) suggests that the energy levels of alanes on Al(111) are significantly broadened and shifted toward the Fermi level from their gas-phase values to facilitate transfer of the tunneling electrons. Such a change in the energy levels of aluminum hydrides upon adsorption on Al(111) is reasonable: Because aluminum hydrides coordinate to the substrate via bridging hydride bonds, alanes undergo an effective reduction in hydrogen stoichiometry when bonded to the aluminum substrate, imparting a metallic character that allows them to be imaged as protrusions. Alanes adsorbed on the W tip, in contrast, appear to be much poorer conduits of tunneling electrons, inverting the image

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Figure 5. Schematic illustration of tunnel junctions for the interpretation of STM images: At left, the W-Al tunnel junction shows the vacuum alignment of the Fermi levels of the clean W tip and bare Al(111) surface. At center, the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) of gaseous dialane (black bars) are located with respect to the vacuum levelsfar from the Fermi level. At right is the alane-modified W-Al tunnel junction. Upon chemisorption, gaseous alane levels shift toward the Fermi level and broaden (gray shading) according to the strength of the alane-surface interaction. Greater level adjustment is expected for alanes chemisorbed on Al(111), due to the formation of alane-Al(111) bridging hydride bonds. Alanes/Al(111) are thus imaged as protrusions (normal contrast) with high conductivity W tips. For alaneterminated W tips, electrons must be transferred through levels that remain far from the Fermi level. To make up for the reduction in tunneling current through misaligned tip-alane levels, the tip must move closer to the surface, thereby imaging surface alanes as depressions (contrast reversal).

contrast of surface alanes. More discrete LUMOS and HOMOS of the tip-alanes (see Figure 5 right), can account for the reduction in tunneling through this alane-terminated tip. Because of the chemical inequivalence of Al and W atoms, it is reasonable to expect less rehybridization of alanes transferred to the W tip than adsorbed on the Al(111) surface. The lack of a bias dependence in the image contrast further suggests that the tip-alane LUMO’s are the primary conduits of the tunneling current. To maintain a constant tunneling current, the alaneterminated tip must move toward the surface, resulting in an inversion of the image contrast. The extent of this contrast reversal (i.e., the depth of the alane depressions) depends on the particular alane transferred to the tips: Smaller-alane terminated tips (with presumably the most discrete levels) yield images with greater contrast reversal (alanes appear as ∼2.8 Å depressions), whereas tips terminated by larger alanes show a “smaller” contrast reversal (alanes appear as 0.2-2 Å depressions). This sensitivity of contrast reversal to alane “size” presumably reflects variation in the location and width of the tip LUMO levels. The effect of tip shape (morphology) on the topography of alane images acquired with a metallic W tip is reminiscent of previous observations involving atomic adsorbates. Subtopographic heights and electronic modulations due to the scattering of tunneling electrons are observed in images of atomic adsorbates on metal surfaces. This nontopographic detail is very sensitive to tip geometry at the apex because of geometric influence on the wave character of the tunneling electrons. Halos, for example, can arise from tunneling by high angular momentum electrons.27 Although we cannot completely separate the influence of tip geometry and electronic structure in STM

STM Study of Alanes/Al(111) imaging of surface alanes, we make a qualitative distinction between the contrast reversal obtained with alane-terminated tips (level alignment) and electronic modulation due to tip geometric structure (electronic wave character). Because aluminum is a simple metal, and highly amenable to electronic structure calculations, there are significant opportunities for testing and extending these conclusions with rigorous calculations of the electronic levels of surface alanes and resonant tunneling through model junctions of these materials. Acknowledgment. This research has been supported in part by the NSF (CHE-9800470) and the Materials Research Science and Engineering Center (DMR-9632521). We acknowledge Jack Tossell for performing quantum chemistry calculations on dialane and for helpful discussions. References and Notes (1) Spong, J. K.; Mizes, H. A.; LaComb, J. L., Jr.; Dovek, M. M.; Frommer, J. E.; Foster, J. S. Nature 1989, 338, 137. (2) Smith, D. P. E.; Horber, J. K.; Binnig, G.; Nejoh, H. Nature 1990, 344, 641. (3) Claypool, C. L.; Faglioni, F.; Matzger, A. J.; Goddard, W. A., III; Lewis, N. S. J. Phys. Chem. B 1999, 103, 9690. (4) Giancarlo, L. C.; Flynn, G. W. Annu. ReV. Phys. Chem. 1998, 49, 297. (5) Weiss, P. S.; Eigler, D. M. Phys. ReV. Lett. 1993, 71, 3139. (6) Garcia, R.; Garcia, N. Chem. Phys. Lett. 1990, 173, 44. (7) Youngquist, M. G.; Driscoll, R. J.; Coley, T. R.; Goddard, W. A., III; Baldeschweiler, J. D. J. Vac. Sci Technol. B 1991, 9, 1304. (8) Wang, X. W.; Tao, N. J.; Cunha, F. J. Chem. Phys. 1996, 105, 3747. (9) Thibaudau, F.; Watel, G.; Cousty, J. Surf. Sci. Lett. 1993, 281, L303. (10) Han, W.; Durantini, E. N.; Moore, T. A.; Moore, A. L.; Gust, D.; Rez, P.; Leatherman, G.; Seely, G. R.; Tao, N.; Lindsay, S. M. J. Phys. Chem. B 1997, 101, 10719. (11) Tao, N. J. Phys. ReV. Lett. 1996, 76, 4066. (12) Girard, C.; Bouju, X. Chem. Phys. 1992, 168, 203. (13) Sautet, P.; Joachim, C. Ultramicroscopy 1992, 115, 42-44. (14) Richter, Y. M. S. J. Phys. Chem. 1994, 98, 2941. (15) Pai, W. W.; Zhang, Z.; Wendelken, J. F. Surf. Sci. 1997, 393, L106.

J. Phys. Chem. B, Vol. 104, No. 35, 2000 8511 (16) Ness, H.; Fischer, A. J.; Briggs, G. A. D. Surf. Sci. Lett. 1997, 380, L479. (17) Ruan, L.; Besenbacher, F.; Stensgaard, I.; Laegsgaard, E. Phys. ReV. Lett. 1993, 70, 4079. (18) Rousset, S.; Gauthier, S.; Siboulet, O.; Belin, M.; Klein, J. Phys. ReV. Lett. 1989, 63, 1265. (19) Meyer, G.; Neu, B.; Reider, K. H. Chem. Phys. Lett. 1995, 240, 379. (20) Dunphy, J. C.; Ogletree, D. F.; Salmeron, M. B.; Sautet, P.; Bocquet, M. L.; Joachim, C. Ultramicroscopy 1992, 490, 42-44. (21) McIntyre, B. J.; Sautet, P.; Dunphy, J. C.; Salmeron, M.; Somorjai, G. A. J Vac. Sci. Technol. B 1994, 12, 1751. (22) Bohringer, M.; Schneider, W. D.; Berndt, R.; Glocker, K.; Sokolowski, M.; Umbach, E. Phys. ReV. B 1998, 57, 4081. (23) Go, E. P.; Thuermer, K.; Reutt-Robey, J. E. Surf. Sci. 1999, 437, 377. (24) We estimate the absolute tip-sample separation by considering the turning point of helium atoms scattering off of a metal surface. To the 4 Å turning point (Ellis, J.; Hermann, K.; Hofmann, F.; Toennies, J. P. Phys. ReV. Lett. 1995, 75, 886), we add 1 Å (to account for the size difference between He and tip (W) atoms) and 2 Å (to move the tip from repulsive contact with the surface to a region of weaker attraction.) Under close scanning conditions (Ω ) 0.7 MΩ), we thus estimate the tip-sample separation to be ∼6 Å. (25) The gap resistance of the one-dimensional metal-vacuum-metal tunneling junction is R(Ω) ∝ e1.025φ(eV)W(Å), where W is the tip-sample separation and φ is the work function, as per Introduction to Scanning Tunneling Microscopy by C. Julian Chen (Oxford University Press: New York, 1993). We experimentally set the gap resistance R and estimate the corresponding separation W with φ ) 4.7 eV (the average work function for Al and W) and the reference value W ∼ 6 Å at Ω ) 0.7 MΩ. (26) While the geometries of chemisorbed alanes have not been determined precisely, a height of 2.8 Å is consistent with the calculated geometries of free alanes reported in: Duke, B. J.; Liang, C.; Schaeffer, H. F., III. J. Am. Chem. Soc. 1991, 113, 2884. (27) Chen, J. C. J. Vac. Sci. Technol. B 1994, 12, 2193. (28) Stroscio, J. A.; Eigler, D. M. Science 1991, 254, 1319. (29) Zeppenfeld, P.; Lutz, C. P.; Eigler, D. M. Ultramicroscopy 1992, 128, 42-44. (30) Avouris, P.; Lyo, I. W. Appl. Surf. Sci. 1992 60/61, 426. (31) Koetter, E.; Drakova, D.; Doyen, G. Surf. Sci. 1995, 679, 331333. Koetter, E.; Drakova, D.; Doyen, G. Phys. ReV. B 1996, 53, 16595. (32) Wang, F.-H.; Yang, J.-L.; Li, J.-M. Phys. ReV. B 1999, 59, 16053. (33) Calculations were carried out in GAUSSIAN95 using the 6-316* OVGF basis set.