NANO LETTERS
A Reversible Molecular Switch Based on Pattern-Change in Chlorobenzene and Toluene on a Si(111)−(7 × 7) Surface
2006 Vol. 6, No. 4 809-814
Xuekun Lu,† John C. Polanyi,* and Jody (S. Y.) Yang Department of Chemistry and Institute of Optical Sciences, UniVersity of Toronto, 80 St. George Street, Toronto, Ontario M5S 3H6, Canada Received January 20, 2006; Revised Manuscript Received February 17, 2006
ABSTRACT A reversible molecular switch is proposed, based on an observed change in a physisorbed pattern of chlorobenzene or toluene at Si(111)−(7 × 7), from “triangles” to “circles”. Electronic excitation, at an applied surface voltage of Vs ) −2.0 V, caused molecular migration, by one atomic site, from under the tip (switch “off”). Thereafter, the adsorbate pattern reverted thermally from circles to triangles (switch “on”) across a measured activation barrier of Ea ) 0.3 eV for chlorobenzene and 0.2 eV for toluene.
Reversible pattern-change in ordered substrates has been reported in a number of instances in recent years with a view to the fabrication of molecular-scale switches. For both Si(100) and Ge(100) surfaces, phase-manipulation between c(4 × 2) and p(2 × 2) has been shown.1,2 Transformation between the two phases of the substrate was achieved by scanning at different bias voltages. Similarly, site-specific migration of Si adatoms was observed on Si(111)-(7 × 7), providing a basis for switching at selected substrate atoms.3 This paper discusses induced pattern-change in the much more labile and variable medium of adsorbate submonolayers. Brown et al.4 have reported differing states of adsorption for benzene, namely, physisorbed or chemisorbed states without, however, pattern change. The physisorbed state could be recovered from the chemisorbed state by scanning at -3 V. Additionally, adsorbate molecules have been used for switching in a number of instances by means of internal conformation-change,5-9 oxidation-state change10,11 up and down motion on a Au substrate,12 or orientational change on a Au substrate.13 Induced pattern-change (via lateral siteto-site motion) in adsorbates should, in the future, provide a rich source of molecular switching. We give an example here. Physisorbed circles and triangles are shown by STM to be reversibly interchangeable in the adsorption of chlorobenzene or toluene on Si(111)-(7 × 7). Switching involves induced migration of the adsorbate from a Si site, the current then being off at that site which is observed to go dark, followed by thermal reversion to triangles, the current at that Si site then being once more on (bright image). This work * Corresponding author. E-mail:
[email protected]. † Current address: Department of Electrical and Computer Engineering, University of Minnesota, Minneapolis, MN 55455. 10.1021/nl0601379 CCC: $33.50 Published on Web 03/09/2006
© 2006 American Chemical Society
was an outgrowth of previous studies of chlorobenzene and toluene adsorption and reaction on Si(111)-(7 × 7) in this and other laboratories.14-20 The experiments were carried out using an Omicron VTSTM in an ultrahigh vacuum (UHV) chamber with a base pressure of less than 1 × 10-10 mbar. Clean Si(111)-(7 × 7) substrates (B-doped, 0.0493∼0.0533 Ω‚cm) were prepared by flashing to 1500 K for approximately one minute. The sample was cooled by pumping liquid He through a continuous-flow cryostat. Reagent-grade chlorobenzene and toluene (Sigma Aldrich) were purified using freeze-pump-thaw cycles. Adsorbate was introduced into the chamber while the surface was held at a temperature from 50 to 100 K. The Si(111)-(7 × 7) surface has been characterized using the dimer-adatom-stacking-fault (DAS) structure model, by Takayanagi et al.21 The (7 × 7) unit cell is divided into faulted and unfaulted halves, differing in their stacking. STM images taken with a negative sample bias show that the faulted site is brighter than the unfaulted site. The unit-cell has six “corner” adatoms and six “middle” adatoms. Figure 1 shows images of two physisorbed self-assembled patterns of chlorobenzene on the Si(111)-(7 × 7) surface, together with their schematic representation. Identical patterns were found to be formed from toluene. All STM images were recorded in constant current mode at 0.2 nA using a sample bias of 1.5 V. A circle pattern (Figure 1a) formed with 0.5 monolayers (ML) of either adsorbate at 50 K, due to physisorption exclusively over Si middle-adatoms. The physisorbed species appeared as bright spots over the middle-adatom positions, forming the bright circles shown in perspective and from above in Figure 1a. Each circle consists of 12 chlorobenzene
Figure 1. STM images of physisorbed self-assembled patterns for chlorobenzene on a Si(111)-(7 × 7) surface. [Vs ) 1.5 V, I ) 0.2 nA; ∼13 nm × ∼16 nm]. (a) The circle pattern consists of physisorbed molecules over middle (M) adatoms with 0.5 monolayer coverage (ML) at 50 K. (b) Triangle pattern consisting of physisorbed molecules over unfaulted (U) sites, with 0.5 ML at 100 K. F,U(M) denotes faulted and unfaulted middle adatom coverage, whereas U(M,C) is unfaulted middle and corner adatom coverage.
Figure 2. (a f b) Induced switch by -3 V from triangle to circle pattern of physisorbed chlorobenzene. (b f c f d) Thermal reversion to triangle. All images for Si(111)-(7 × 7) at 100 K, at Vs ) 1.5 V, I ) 0.2 nA; ∼20 nm × 20 nm. The schematics next to the figures show the representative patterns of adsorbate at each stage (not to scale). In b f d molecules migrate from faulted middle (FM) to unfaulted corner (UC) sites, en route to restoring the triangle pattern of type a.
molecules. Dashed half-circles in the schematic at the right of the figure indicate two halves of circle patterns. As noted above, similar patterns have been reported previously for benzene on the Si(111)-(7 × 7) surface at 0.2 nA, the switching rate stayed constant (Figure 3a). A similar plateau in the rate of electron-induced migration has been observed Nano Lett., Vol. 6, No. 4, 2006
Figure 3. (a) Log-log plot showing induced switching rates at 80 K (triangle to circle) as a function of a tunneling current (decreasing tip-to-surface separation) at Vs ) -2.0 V. The dashed line is a least-squares fit to the data from 0.05 to 0.2 nA and corresponds to R R I n, where n ) 0.83 ( 0.12 (number of electrons needed for each switching event). (b) Logarithmic plot showing induced switching rates at 80 K (triangle to circle) as a function of negative bias (constant current of 0.2 nA requires decreasing tipto-surface separation).
by Stipe et al. for the atomic displacement rate of Si adatoms on a Si(111) surface.3 Stipe et al. explained this saturation effect in terms of a decrease in the effective tunneling voltage, due to an increase in the voltage drop between the surface adatoms and the Si bulk as the tunneling current was increased (the effective voltage being V ) IR, with the surface adatoms treated as a resistive layer). In accord with this explanation, the switching rate in the present work, as in that of Stipe et al., increased with the applied voltage; the higher the voltage the higher the current at the onset of the “plateau”. With an applied voltage of -3 V (rather than 811
Figure 4. Example of STM images (Vs ) 1.5 V, I ) 0.2 nA) showing the switching effect of the stationary tip (Vs ) -3 V for 60 s). The pattern-change is in the vicinity of the tip (highlighted rectangle).
-2 V used in plotting Figure 3a) the plateau was absent for currents up to ∼1 nA. Figure 3b shows a logarithmic plot of the switching rate from triangle to circle at 80 K as a function of bias voltage at constant current. The tip-to-surface separation was altered in order to maintain the constant current. The pattern-change from triangles to circles was also examined using a stationary STM tip in order to determine whether scanning was essential to pattern-change. The STM tip was held at a fixed position above a triangular pattern while a sample bias of -3 V was applied for 60 s. Afterward, the area was scanned once more at 1.5 V. Adsorbate molecular-transfer from a UC site to a FM site was observed only in the vicinity of the STM tip (up to a half-unit cell away, ∼27 Å). Figure 4 shows an example of STM images (a) before and (b) after the pattern-change, due to the stationary tip located over the rectangle. Switching events observed not immediately below the tip, but in its vicinity, are attributed to the propagation of tunneling electrons laterally over the surface. Such slightly nonlocal effects have been observed on Si(111) for other processes.3,25 It has also been shown for a Si(100) surface at low temperature that injected electrons can spread to some extent laterally over the surface, instead of traveling into bulk states.26 Scanning does not, therefore, appear to be required in order for electron impact to cause adsorbate migration. The circle pattern that is metastable at 100 K, will, as noted above, revert thermally to the triangle pattern. To determine the thermal-decay rate from the FM to the UC site (circles to triangles), we recorded images at Vs ) 1.5 V at intervals of a few seconds, at a reduced surface temperature (see the caption of Figure 5). The adsorbate molecular-transfer rate, dN/dt, at temperature T was described by dN/dt ) -VN exp(-Ea/kT), where Ea is the activation energy and V is a preexponential factor; V was assumed to be 1013 s-1.4 A plot of the decrease in the number of molecules on FM sites as they transferred to UC sites gave an activation energy of 0.3 eV for chlorobenzene and 0.2 eV for toluene, as shown in Figure 5. The rate of pattern change from circle to triangle could be varied by orders-of-magnitude by changing the adsorbate-plus-substrate temperature over the range 50-100 812
Figure 5. Plot showing the thermal transfer of chlorobenzene and toluene back from faulted middle (FM) to unfaulted corner (UC) site in the reverse switching process from circle to triangle, measured at intervals of 40-60 s at a surface temperature of 100 K for chlorobenzene and 70 K for toluene. N is the number of molecules counted. Slopes give Ea ) 0.3 eV (chlorobenzene) and 0.2 eV (toluene).
K. An interesting variant on this experiment (not attempted) would be to determine the average thermal decay rate to a single UC site using a tip held stationary through many cycles of adsorbate motion. The observed adsorbate pattern-switching on Si(111) constitutes, in effect, a molecular switch in the tip-surface current. The molecule, chlorobenzene or toluene, is weakly bound to a Si adatom at the surface (one electrode), with the STM tip acting as the second electrode. The on state of the switch corresponds to the molecule being over a UC site; in the off state the conducting molecule has migrated to an FM site leaving the UC site beneath the tip vacant; at constant current this is observed as a switch from a bright image over the UC site, to a dark image. Because the change in tipheight in going from this on to off state is ∼1 Å, the effect of this molecular-switch on the tunneling current would be one order of magnitude. The time required for on f off switching is limited ultimately by the time required for adsorbate migration by one atomic space. Effectively, this Nano Lett., Vol. 6, No. 4, 2006
time depends on the migration switching rate (0.01 s at >0.1 nA; ∼0.001 s at >1 nA). The time for restoring the switch thermally to its “on” state can be controlled, as noted above, by changing the substrate temperature. Experiments were also performed with positive sample bias. The incident electrons caused a stimulated reaction (black features at 1.5 V bias, characteristic of chlorination) and some desorption (clean Si) but no switching of the adsorbate to new sites. A single-atom electrical switch was first demonstrated by Eigler and co-workers.27 In their work, the atomic motion leading to switching was vertical between the STM tip and the surface. Recently Stroscio and Celotta28 have described a process of atomic switching due to forced lateral motion of a single cobalt atom on Cu(111) under the influence of tip movement. They anticipated major challenges in extending this lateral switching to semiconductor surfaces, where the corrugations are high. From the present observation of lateral motion on Si(111) it would seem that the roughness of semiconductors does not constitute a serious obstacle to switching. We have measured (see above) the barrier height to thermal switching of our organic adsorbates, chlorobenzene and toluene, to lie in the range Ea ) 0.2-0.3 eV. This is indeed about an order of magnitude greater than the estimated value for Co on Cu(111). Facile switching nonetheless occurs for physisorbed organics even at 100 K. Moreover, in contrast to the moving-tip switch in which the adsorbate is carried deliberately from site to site,28 switching by lateral migration is in the present instance observed for a voltage applied to a stationary tip. Figure 6 gives a rationale for the reversible pattern-change process. The suggested mechanism resembles that proposed on the basis of experiment and ab initio theory by Alavi et al. for the desorption of benzene from Si(111)-(7 × 7) due to the application of a similar negative voltage to the underlying surface.23,24 The schematic potential-energy curves correspond to energy levels of different physisorption sites (UC and FM) at the Si(111)-(7 × 7) surface. For the neutral adsorbate, UC is a more stable site for physisorption as compared to FM. With the sample bias of -2.0 V, a positive ionic state is thought to be created as an electron is withdrawn. The adsorbate in the temporary positive ionic state experiences a force causing migration from UC to FM. If reverse charge-transfer occurs while the adsorbate occupies this new site, it will be trapped there as indicated by the downward arrow of Figure 6. Because this adsorption site (FM) is metastable with respect to the prior site, the adsorbate reverts thermally to its original position, as observed. When an electron is withdrawn from the adsorbate creating a temporary positive state, tunneling occurs from the occupied state (HOMO) of the adsorbate to the STM tip. Tunneling from the HOMO of the adsorbate to the STM tip takes place by electron transfer from this occupied state of the adsorbate into the conduction region of the positively charged tip. A mechanism involving a temporary negative ionic state was also considered. A temporary negative ionic state could Nano Lett., Vol. 6, No. 4, 2006
Figure 6. (a) Schematic potential-energy diagram and (b) schematic representation of pattern-change at Si(111)-(7 × 7). In part a above, the black curve is RX(ad) and the red curve is for ionic RX+(ad); the adsorbate, chlorobenzene or toluene, following charge-transfer, moves from left to right along the reaction coordinate from an unfaulted corner (UC) to faulted middle (FM) Si adatom site.
be created as an electron is transferred from the Si substrate to the adsorbate. In this case, the electron from the substrate would occupy the unoccupied state (LUMO) of the physisorbed adsorbate and subsequently tunnel to the STM tip. Tunneling by this route is, however, improbable because the high energy of the unoccupied (LUMO) state of the adsorbate can be expected to have a poor overlap with the depressed conduction region of the positively charged STM tip. In conclusion, we have reported on physisorbed selfassembled patterns of both chlorobenzene and toluene on Si(111)-(7 × 7) at low temperatures, recorded by STM. We have demonstrated reversible pattern-change between circles and triangles, induced by charge-transfer in one direction and thermally in the reverse direction. Because a physisorbed molecule, chlorobenzene or toluene, is removed from the atomic site beneath the tip in the first instance (bright f dark; on f off) and returned to that site in the second (dark f bright; off f on), we have a basis for a reversible molecular switch involving current-switching by approximately an order of magnitude. Acknowledgment. We are indebted to the Natural Science and Engineering Research Council of Canada (NSERC), Photonics Research Ontario (PRO), an Ontario Centre of Excellence, the Canadian Institute for Photonic Innovation (CIPI) and the Canadian Institute for Advanced Research (CIAR) for their support of this work. We thank Drs. P. A. Sloan and I. R. McNab for helpful discussions. References (1) Sagisaka, K.; Fujita, D.; Kido, G. Phys. ReV. Lett. 2003, 91, 1461031-1461034. 813
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