Ultramicroscopy 42-44 (1992) 128-133 North-Holland ~ I / ~ ~ U

Manipulating atoms and molecules with a scanning tunneling microscope

P. Zeppenfeld 1, C.P. L u t z a n d D . M . E i g l e r

IBM Almaden Research Center, 650 Harry Road, San Jose. CA 95120-6099, USA

Received 12 August 1991

We report recent developments in the manipulation of atoms and molecules adsorbed on metal surfaces using a scanning tunneling microscope. We demonstrate that individual adsorbates can be positioned with atomic precision with the "sliding process" in which the tip is dragged in close proximity atop the adsorbed atom or molecule. Using this process we have successfully built simple structures from Xe atoms on the Ni(ll0) surface as well as from CO molecules and Pt atoms on a Pt(111) substrate.

1. Introduction

The scanning tunne l ing microscope (STM) has proven to be a powerful tool providing an un- p receden ted view of mat ter at the atomic level. It has fur ther afforded local electron spectroscopy. Besides its applicat ion as an analytical instru- ment , the STM can also be opera ted as a tool for the modificat ion and manipu la t ion of mat ter on the n a n o m e t e r scale. Such " n a n o - e n g i n e e r i n g " using the microscope tip can be per formed in various ways. Some examples are the mechanical scratching or inden ta t ion of the surface [1], elec- t ron-beam-assis ted etching and deposit ion [2], the f ie ld- induced transfer of material from the tip to the sample and vice versa [3], as well as the modificat ion of the substrate surface on an atomic scale by voltage pulsing [4]. A recent review of the modificat ion and manipu la t ion of nano- meter-scale objects with the STM can be found in ref. [5].

Here we report on the posi t ioning of individ- ual atoms and molecules on single-crystal sur-

L Permanent address: Institut fiir Grenzfl~ichenforschung und Vakuumphysik, KFA Forschungszentrum Jiilich, Postfach 1913, 5170 Jfilich, Germany.

faces by dragging the tunne l ing tip in close prox- imity atop the adsorbed atom or molecule. This "sl iding process" has been used previously to posit ion xenon atoms on a Ni(110) surface and to assemble simple two-dimensional s tructures from Xe atoms [6]. We demons t ra te that atomic ma- n ipu la t ion using the sliding process is not re- stricted to the physisorbed (weakly bound) xenon atoms but can also be applied to posit ion Pt adatoms and CO molecules.

2. Experiment

The exper iments were per formed with an STM conta ined in an u l t ra-high-vacuum chamber and cooled to 4 K. A P t ( l l l ) single crystal was pre- pared by heat ing in oxygen and cycles of sputter- ing with Ar ions and subsequent annea l ing at about 1000 K until no con tamina t ion could be detected in the Auger spectrum. After cooling to 4 K the sample was examined with the STM. Terrace widths of a few hundred fmgstr6m and a concent ra t ion of point defects of about 1% were observed. CO molecules were adsorbed by expos- ing the cold P t ( l l l ) sample to an ambient pres- sure of CO gas at room tempera ture . Once ad-

0304-3991/92/$05.00 © 1992 - Elsevier Science Publishers B.V. All rights reserved

P. Zeppenfeld et al. / Manipulating atoms and molecules by STM 129

sorbed on the cold P t ( l l l ) surface the CO

molecules r ema ined immobi le at their original adsorpt ion site and could be investigated at length.

3. Results and discussion

The idea that atoms or molecules could be i n t en t iona l l y posi t ioned on a surface using the tunne l ing tip arose from the observat ion that unde r cer ta in c i rcumstances adsorbates may change their site already dur ing regular imaging with the tunne l ing microscope. The sliding of the adsorbate is most likely to occur at low tunne l ing gap impedance , i.e. if the tip is scanned in close proximity above the adsorbate. This is i l lustrated in fig. 1, showing two STM images of a P t ( l l l ) surface dosed with about 1% of a monolayer CO at a t empera tu re of 4 K. The two images were taken from the same spot on the surface with the tip biased at - 0 . 1 V, but using a different tunne l - ing cur ren t for imaging, namely 1.0 n A in (a) and 10 n A in (b). The t en - t imes - sma l l e r gap impedance in (b) corresponds to the microscope tip being about 1 A closer to the surface dur ing recording as compared to (a). The main differ- ence be tween the two images is that of the four CO molecules (appear ing as the large prot rus ions in (a)) the two molecules in the upper right corner of the image have d isappeared in (b). Instead, a series of little bumps forms a trace directed towards the right corner of the image. Obviously the two CO molecules have been af- fected by the t unne l ing tip while scanning the image #1. Indeed , as the tip approaches the CO molecule the in terac t ion be tween the tip and the CO gradually increases. Being still far away from the center of the adsorbate, the tip images the lower part of the relatively large area on the surface affected electronical ly by the adsorbed

#1 The images are scanned line by line from bottom to top. For each scan line data are taken only during the move- ment of the tip from left to right (the scanning speed used in these experiments ranges from a few tens to a few hundreds of ~ngstr6ms per second). Then the tip is quickly drawn back to the left and onto the next scan line.

Fig. 1. (50×50) ~2 STM images of a Pt(lll) surface after adsorption of ~ 1% of a monolayer CO. The two pictures were taken at the same spot on the surface with the micro- scope tip biased at -0.1 V and a tunneling current of 1.0 nA (a) and 10 nA (b), respectively. The four prominent protru- sions (with a height of 0.5 A) are single CO molecules. The two molecules in the upper right corner in (a) have been moved out of the picture area by the scanning tip while

recording image (b), as explained in the text.

130 P. Zeppenfi'ld i't al. / Manipulating atoms and molecules by STM

CO molecule giving rise to one of the little bumps in the image. In a subsequent scan line the inter- action between tip and adsorbate is then strong enough to kick the CO molecule out of its pre- sent adsorption site into a new site. As a result, the CO molecule is now imaged at its new posi- tion yielding the next bump. Therefore, the se- quence of the bumps appearing in fig. lb is a direct manifestation of the moving of the CO molecule by the tunneling tip during scanning of the image. The separation and relative location of successive bumps gives the distance and direction of each individual move. Fortunately, the in- creased tunneling current, i.e. the scanning at reduced height of the tip, also enhances the reso- lution at which the P t ( l l l ) substrate is imaged [7]. As a result the trace of the moving CO molecules can be directly correlated with the atomic structure of the underlying substrate which is clearly resolved in fig. lb: the CO molecules jump between equivalent nearest-neighbor lattice sites along the close-packed (112.) direction of the P t ( l l l ) substrate. It is known from previous investigations of the C O / P t ( l l l ) system [8] that at low coverage CO occupies the on-top site (the CO is adsorbed in an upright position with the carbon atom binding to the Pt atom). It is likely, therefore, that the jumps occur between neigh- boring energetically favored on-top sites. This is exactly what is expected if the lateral force be- tween the tip and the adsorbate is just large enough to overcome the energy barrier stabilizing those sites. Hence, the force exerted on the CO molecule by the tip is a measure of the C O / P t ( I I 1 ) corrugation potential or diffusion barrier [9]. Finally, it is noticed that only two of the four molecules in fig. la actually change their position while the other two remain immobile. A clue to the understanding of this different behav- ior can be found by inspection of the local envi- ronment of the molecules. The two CO molecules unaffected by the scanning tunneling tip have a defect in their immediate vicinity (discernable as a small protrusion in fig. 1), probably an impurity atom. It is well known that adsorbates tend to bind more strongly at defects than on the bare surface. Therefore, it is conceivable that the in- teraction between tip and adsorbate is large

~ I ~ TIP / B I ~\ /

_ _ _ _ _ _ ~ . . . . . . ~ , , , J ~

@ (', ------ ~'~------ Xe)

Fig. 2. The process for sliding an adsorbate, here shown as a xenon atom. across a surface. The atom is located and the tip is place above it (1). The tip is lowered to position (2), where the force between the tip and the atom is sufficient to keep the atom below the tip when the tip is subsequently moved across the surface (3) to the desired location (4). Finally the tip is retracted to position (5) where the a to m- t i p interaction is negligible, leaving the atom bound to the surface at its new

location.

enough to overcome the barrier for moving CO on the smooth bare platinum surface but yet too small to pull them away from defect sites.

From the coincidental observation that ap- proaching the tip towards the sample may lead to the adsorbate being dragged through the force exerted by the tip, an algorithm for the controlled positioning of individual atoms or molecules can be constructed: the tip is lowered above the ad- sorbed atom or molecule to be moved until the interaction exceeds the threshold value for slid- ing, then the tip is dragged to the target position and withdrawn to its original height used for regular imaging. We have found that the adsor- bate will follow the path of the tip parallel to the surface and reliably end up at one of the binding sites closest to the point at which the tip is withdrawn. This "sliding process", illustrated schematically in fig. 2, was first applied to posi- tion individual xenon atoms on a Ni( l l0) surface [6].

Meanwhile we have studied the details of the sliding process on the Xe/Ni(110) system [10]. As a result, we find that the process can be de- scribed in terms of a constant threshold impedance or tip height, independent of the mag- nitude or sign of the applied tunneling voltage or current. Therefore we conclude that the forces responsible for the sliding are not the electric

P. ZeppenfeM et al. / Manipulating atoms and molecules by STM 131

field nor the current but rather the intrinsic (van der Waals-type) interaction between the adsor- bate and the frontmost atoms of the tip. The threshold for sliding the Xe atoms is quite sharp. Along the close-packed Ni rows running along the (1-10) direction of the (110) surface the threshold gap impedance is 4.8 Mf~ [10].

As can already be inferred from fig. 1, the sliding of adsorbates using the STM tip is not restricted to the xenon adsorbate or the Ni( l l0) surface as substrate. Indeed, fig. 3 shows three different sample structures assembled from CO molecules adsorbed on the P t ( l l l ) surface using the same process as described above. In the case of the CO molecules, the physical mechanism

governing the sliding process appears to be the same as for the X e / N i ( l l 0 ) system. Only the threshold gap impedance is smaller (below 500 kf~ for C O / P t ( l l l ) as compared to 4.8 Mf~ for X e / N i ( l l 0 ) along the (170) rows).

In addition, we have been able to slide individ- ual metal (platinum) adatoms along the P t ( l l l ) surface. The Pt adatoms in these experiments were generated by evaporation of part of the Pt sample itself, and adsorbed onto the 4 K surface. This was accomplished by applying ~ 100 V be- tween the tip and sample and then discharging the tip-to-sample capacitance by touching the tip to the sample. Vaporized Pt was deposited over an area several micrometers in diameter, with the

Fig. 3. STM images of patterns obtained by positioning individual CO molecules on the Pt(111) surface using the process illustrated in fig. 2: (a) "CO" written with CO molecules spaced at 4.8 ,~,, imaged at 0.1 V, 1 nA. (b) Hexagonal island of CO molecules arranged in a (~J- x fJ-)R30 ° structure with respect to the underlying P t ( l l l ) lattice. This image was recorded at - 0.2 V and 1 nA. (c) "Molecular Man" built from 28 CO molecules, measurir/g 45 A, from head to foot; imaged at -0 .1 V, 0.1 hA. In all three cases

the sliding of the CO was achieved at a gap impedance of 300 k~.

132 P. Zeppenfeld et al. / Manipulating atoms and molecules by STM

Fig. 4. STM image of a linear chain of four Pt atoms on top of a P t ( l l l ) surface, imaged at 0.1 V, 10 nA. Neighboring Pt adatoms are spaced at four times the nearest-neighbor distance of the underlying P t ( l l l ) substrate (i.e. at 4 × 2.77 A = 11.08 A). The Pt adatoms have an apparent height of 1.54 ,~. For comparison, the step height on the Pt(111) surface is 2.26 A. The gap impedance

for moving was 20 kl).

coverage smoothly decreasing with radius. When the surface is imaged (at - 0 . 1 V and 1 nA) we observe identical 1.3 A tall, ~ 5 ,~ diameter, cylindrically symmetric protrusions which we at- tribute to single Pt adatoms. These adatoms may be repositioned with the STM tip but the re- quired junction resistance is only ~ 20 kl), which corresponds to the tip being at or near mechani- cal contact with the surface [7]. As a result, sliding is much less reliable for Pt atoms than for the Xe and CO adsorbates; sometimes the Pt atom to be moved is transferred unintentionally to the tip or an extra atom drops from the tip onto the surface as the tip is dragged across the surface. Nevertheless, simple structures can be fabricated, such as a linear chain of 4 Pt atoms regularly spaced at exactly four times the Pt sub- strate nearest-neighbor distance as shown in fig. 4. We have also produced a close-packed linear chain of 7 Pt adatoms with the same spacing as the substrate nearest neighbors, and we have extended monatomic step edges by dragging Pt atoms one at a time along the lower terrace into a step. While we are able to pull apart pairs of

adjacent Pt adatoms into isolated adatoms, we have not been able to drag Pt atoms away from higher coordination sites, such as from a step edge.

4. Conclusion

The results presented above demonstrate that molecules and even metal atoms can be posi- tioned and arranged into rudimentary structures of our own design. It would be worthwhile to build model structures (such as the one-dimen- sional chain, which would be otherwise unobtain- able) with a scanning microscope tip and then study their structural, electronic, and possibly magnetic properties with the same instrument used to build them.

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