VOLUME 9, NUMBER 5, MAY 2009 Copyright 2009 by the American Chemical Society
Letters
Playing Pinball with Atoms Amirmehdi Saedi,† Arie van Houselt,† Raoul van Gastel, Bene Poelsema, and Harold J. W. Zandvliet* Physical Aspects of Nanoelectronics and Solid State Physics, MESA+ Institute for Nanotechnology, UniVersity of Twente, P.O. Box 217, NL-7500AE Enschede, The Netherlands Received July 29, 2008; Revised Manuscript Received August 28, 2008
ABSTRACT We demonstrate the feasibility of controlling an atomic scale mechanical device by an external electrical signal. On a germanium substrate, a switching motion of pairs of atoms is induced by electrons that are directly injected into the atoms with a scanning tunneling microscope tip. By precisely controlling the tip current and distance we make two atom pairs behave like the flippers of an atomic-sized pinball machine. This atomic scale mechanical device exhibits six different configurations.
Attempts by physicists and engineers to push the dimensions of functional structures toward atomic length scales1 have led to several intrinsic challenges. One of them is that a tiny change in an atomic-scale device, like the displacement of a single atom, can drastically alter its properties and functionality.2 Today, the “discreteness of functionality” is one of the most important hurdles in designing robust quantum devices as we approach the atomic limit. Here, we report how a mechanical device consisting of two moving parts, * To whom correspondence should be addressed. † Both authors contributed equally to this work. 10.1021/nl8022884 CCC: $40.75 Published on Web 09/10/2008
2009 American Chemical Society
each composed of only two atoms can be controlled by an external electrical signal, while being stable and providing a variety of functional modes. The elementary building block of the structure that we have studied is an atom pair, referred to as a dimer, on the Ge(001) surface. Pt atoms that are placed on Ge(001) in ultrahigh vacuum upon heating give rise to the formation of chains3,4 with a uniform width of a single atom. Figure 1a shows several such chains. Atomic chains can exhibit a myriad of physical phenomena that are a direct result of their one-dimensional character.5-13 At low temperature, a Peierls
Figure 1. (a) An STM topograph of atomic chains on Ge(001) at 4.7 K, gap voltage -1.5 V and tunneling current 0.5 nA. Bright protrusions are the atoms that make up the atomic width chains in the image. As a result of a Peierls instability the chains exhibit a 4-fold periodicity with dimers paired together, giving a periodic low-high-high-low appearance of the atoms in the chains. (b) Top view of a regular dimer pair at 77 K, gap voltage -1.0 V and tunneling current 0.8 nA. Two terrace atoms can be seen to protrude from the Ge(001) surface indicated by the white arrows. (c,d) Two subsequent images of a dimer pair that exhibits mobility. The reconfiguration of the dimer pairs is too fast to image and shows up as a discontinuity as the tip is scanned across the chain. (e) Schematic diagram of the dimerized atomic chain and the underlying substrate.
Figure 2. The boundaries of the dimer pair at 87% maximum height measured from STM topographs. The narrow lines designate the measured boundary positions. The atoms of the dimer pairs are identified as pivot atoms (P) or revolving atoms (R). The motion of the dimers, indicated by the red arrows, resembles that of the flippers of an atomic pinball machine. The thick dashed lines designate a down-down (orange), down-up (blue) and up-up.
instability occurs which causes the dimers to form pairs.14 We have investigated the dynamics of dimer pairs that constitute atomic chains on Ge(001). In contrast to an individual atom, a dimer can exhibit a rotational mode too that leads to additional distinguishable and often energetically equivalent configurations.15 Initial experiments performed in the temperature range from 4.7 to 300 K, shown in Figure 1b, did not provide any evidence for dynamic behavior of the dimer pairs. In one very select case, however, the atomic structure of a dimer pair deviated from the periodic structure that is observed elsewhere in the chains. Figure 1c,d shows that the two center atoms of this “special” dimer pair rearrange frequently. Each discontinuity in the two images indicates a rearrangement of a dimer in the vertical direction while the scanning tunneling microscope (STM) tip was scanning across the chain. To visualize the atomic configurations that can occur, STM topographs of the dimer pair have been recorded. The boundaries of the dimer pair were defined as 87% of the maximum recorded height in an image and have been extracted from the STM topographs using an edge detection technique. Figure 2 shows a superposition of the measured boundary positions. We observe that only the two center atoms of the dimer pair move. They are always laterally 1734
Figure 3. (a) The measured flip-flop frequency of the dimers as a function of tunnel current. The frequency depends linearly on the tunnel current and passes through the origin. Each plotted data point is the average of 100 values. (b) Telegraphic signal resulting from different dimer flipping modes at 77 K, gap voltage -1.0 V, open feed-back loop, and an initial tunnel current of 1.0 nA. Slow variations in tunnel current resulting from drift of the STM tip have been corrected for in this graph. The dimer pair switches between six well-defined states, indicated by the red lines in the graph.
displaced with respect to the axis defined by the outer atoms of the dimer pair. We designate the two outer atoms as pivot atoms, marked P in Figure 2. The two center atoms are referred to as the revolving atoms, marked R in Figure 2. Three possible configurations are highlighted in Figure 2 by the dashed lines. Each of the two dimer pairs acts like the flipper that is found in a pinball machine, hence our designation of this structure as an atomic pinball machine (APM). The temporal resolution of an STM is insufficient to study the dynamic behavior of the APM. To overcome this limitation we position the STM tip close to the dimer pair, but not above one of its two symmetry axes, open the STM feedback loop, and record the tunnel current as a function of time. In literature this method is referred as I-t spectroscopy.16-20 Because the tunnel current depends exponentially on the distance between tip and surface, any reconfiguration of the dimer pair causes a detectable change of the tunnel current. The resulting telegraphic signal allows us to directly measure the frequency of the switching motion of the dimers. Figure 3a shows how the frequency of the dimer motion depends on the tunnel current. The switching frequency of the dimers shows a linear dependence on the tunnel current. This indicates that the dimer motion is a single electron process. In the literature several nice examples of (STM) electron-induced motion exists, such as the electronstimulated migration of carbon monoxide molecules and the vibrationally mediated motion of cis-2-butene molecules on Pd(110).21,22 From the slope of the curve of Figure 3a, we infer an efficiency of 1.1 × 10-9 dimer switching events per tunneling electron. Moreover, a linear extrapolation of the relation that we find between the frequency and tunnel current intersects the origin of the graph. This implies that the motion Nano Lett., Vol. 9, No. 5, 2009
Figure 4. I-t measurements of the APM at 77 K (open feed-back loop gap voltage -1.0 V). Initial tunnel currents are 1.5 nA, 0.8 nA, 0.8 nA, and 1.5 nA for (a-d), respectively. The APM flips back and forth between two well-defined current levels, indicated by red dashed lines. The decrease of the tunnel current with time in panel a results from a drift of the STM tip. Panels b-d have been corrected for this drift. Panel b shows three well-defined states, of which one is common in both observed flipping modes. Panels c and d show two different transitions between two flipping modes. The different flipping modes are indicated in Figure 5b.
Figure 5. (a) A ball model of the APM showing the four possible configurations. The interaction between the flippers is ignored. The pivot and revolving atoms are drawn in red and the two terrace atoms that were marked by arrows in Figure 1b are drawn in purple. Yellow, blue, and orange colors refer to the states highlighted in Figure 2. (b) For the tip position that is indicated in panel a, six different dimer flipping modes can occur for the four different states. (c) By including an attractive interaction between the revolving atoms during a concerted flipping of the dimer pair, two additional current levels will appear, as is outlined by red dashed line in panel b.
of the dimers is exclusively current induced. The configuration of the dimer pair in the pinball machine can thus be altered by simply injecting charge using the STM tip. The dimer pair will remain stable when the tip is retracted or when the current is switched off. In Figure 4a, the APM flips back and forth between two levels. With increasing time the tunnel current decreases due to the fact that the STM tip slightly drifts away from the surface. The I-t spectrum shown in Figure 3b demonstrates that we can distinguish between a total of three different telegraphic regimes. This proves that the APM can reside in at least six different mechanical configurations. Note that the curve shown in Figure 3b was measured while the tip was slowly drifting away from the surface. In Figure 4b-d examples of a three and two four levels time traces are depicted. The decay of the current that took place as the current traces were recorded in Figures 3b and 4b-d has been scaled to the current value at t ) 0 s. As the position Nano Lett., Vol. 9, No. 5, 2009
of the tip changes, we observe that the switching motion of the dimer pair changes between different modes. On the basis of the interpretation that we have presented so far, either of the two dimers can be arranged in only two possible ways. One would thus expect the pinball machine to have only four well-defined states. The fact that we observe six states in Figure 3b makes it necessary to develop a more sophisticated model. Figure 5a shows the four possible configurations of the APM dimer pair. These four configurations define a total of four current levels, which makes it possible to observe a total of six different telegraphic signals, as is schematically depicted in Figure 5b. We propose that the two additional states are the result of an interaction between the two revolving atoms of the dimers. We use a simple bond counting argument to make this plausible. Our observation that the two APM dimers are able to rotate implies that they 1735
are only weakly bound to the Ge(001) substrate compared to all other dimer pairs. The bonding that is needed to lock the revolving atoms of the APM pair can be made at the protruding terrace atoms that are indicated in purple in panels (a) and (c) of Figure 5. This provides the four levels seen in Figure 5a. An attractive interaction between the two revolving atoms defines two additional states that are graphically depicted in Figure 5c. Instead of being bound to the substrate atoms, the two revolving atoms are bound to one another. Although this is in principle sufficient to explain the measured current traces, a further refinement of the model could be made by no longer assuming that the pivot atoms have a fixed position. A small displacement of the pivot atoms away from the revolving atoms could facilitate the motion of the dimer pairs. The two crucial questions that still remain to be answered regarding the mobility of the dimer pair are why the particular dimer configuration that we have investigated here is mobile and how it is structurally different from other dimer pairs in the chains. One possible hypothesis is that the mobility is caused by the adsorption of residual gas atoms from the ultrahigh vacuum system, particularly hydrogen. It is known that hydrogen adsorbed on Ge(001) and Si(001) surfaces cannot be seen directly with STM.23,24 We have exposed the nanowire covered Ge surface to 500 Langmuir of hydrogen and observed no additional mobility of the dimers in the chains, thus ruling out this hypothesis. The STM images reveal no indication of a structural defect or adsorbates near the mobile dimer pair. The height and total length of the mobile dimer pair are 0.2 and 2.5 Å less than a normal dimer pair. A normal dimer pair consists of four identical atoms. The symmetric appearance of the dimers that is shown in Figure 2, implies that a single substitutional defect in the dimers where an atom is replaced by an impurity atom cannot cause the mobility. Even though it is unlikely, we cannot rule out the existence of a symmetric pair of such defects. A more likely explanation is that for this particular dimer the substrate atom underneath the two revolving atoms has been substitutionally replaced by a different atom, leading to a reduced binding of the dimers with the substrate and a sideways displacement of the dimers as they attach to neighboring substrate atoms. Unfortunately, it is not possible for STM to identify the chemical nature of this subsurface atom and no other available chemical characterization techniques has the required resolution, meaning that the chemical identification of this atom cannot be performed experimentally. Regardless of the chemical identity of the atom(s) that cause the increased mobility of the dimer pair, our observations prove unambiguously that it is possible to control an atomic scale mechanical device using a simple electrical signal. The current of the STM tip can be used to reliably influence the rate at which the dimer pair switches from one mechanical configuration to another. The precise lateral and
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vertical position of the STM tip influences between which two states the dimer pair switches. The stochastic nature of the switching process and the small quantum efficiency for dimer motion mean that the application of this particular structure will however not be straightforward. In summary, we have reported on the stimulated and controllable mobility of an atomic scale mechanical device. This atomic scale variant of a pinball machine exhibits a variety of dynamic modes that are exclusively excited by an external electrical signal. STM and I-t spectroscopy has been used to quantify the dynamics of this device. A better understanding of similar devices can shed light on the future possibilities and opportunities for the application of atomicscale devices. Acknowledgment. This work is financially supported by NANONED (TTF.6947) and the Stichting voor Fundamenteel Onderzoek der Materie (FOM, 03PR2208). References (1) Feringa, B. L. J. Org. Chem. 2007, 72, 6635. (2) Grill, L.; Rieder, K.-H.; Moresco, F.; Rapenne, G.; Stojkovic, S.; Bouju, X.; Joachim, C. Nat. Nanotechnol. 2006, 2, 95. (3) Gurlu, O.; Adam, O. A. O.; Zandvliet, H. J. W.; Poelsema, B. Appl. Phys. Lett. 2003, 83, 4610. (4) Fischer, M.; Van Houselt, A.; Kockmann, D.; Poelsema, B.; Zandvliet, H. J. W. Phys. ReV. B 2007, 76, 245429. (5) Himpsel, F. J.; Altmann, K. N.; Bennewitz, R.; Crain, J. N.; Kirakosian, A.; Lin, J.-L.; McChesney, J. L. J. Phys.: Condens. Matter 2001, 13, 11907. (6) Yeom, H. W.; Kim, Y. K.; Lee, E. Y.; Ryang, K. D.; Kang, P. G. Phys. ReV. Lett. 2005, 95, 205504. (7) Ahn, J. R.; Kang, P. G.; Ryang, K. D.; Yeom, H. W. Phys. ReV. Lett. 2005, 95, 196402. (8) Oncel, N.; van Houselt, A.; Huijben, J.; Hallb¨ack, A.-S.; Gurlu, O.; Zandvliet, H. J. W.; Poelsema, B. Phys. ReV. Lett. 2005, 95, 116801. (9) van Houselt, A.; Oncel, N.; Poelsema, B.; Zandvliet, H. J. W. Nano Lett. 2006, 6, 1439. (10) Ahn, J. R.; Yeom, H. W.; Yoon, H. S.; Lyo, I. W. Phys. ReV. Lett. 2003, 91, 196403. (11) Snijders, P. C.; Rogge, S.; Weitering, H. H. Phys. ReV. Lett. 2006, 96, 076801. (12) Hager, J.; Matzdorf, R.; He, J.; Jin, R.; Mandrus, D.; Cazalilla, M. A.; Plummer, E. W. Phys. ReV. Lett. 2005, 95, 186402. (13) Lee, G.; Guo, J.; Plummer, E. W. P. Phys. ReV. Lett. 2005, 95, 116103. (14) van Houselt, A.; Gnielka, T. J. M.; de Brugh, J. A.; Oncel, N.; Kockmann, D.; Heid, R.; Bohnen, K.-P.; Poelsema, B.; Zandvliet, H. J. W. Surf. Sci. 2008, 602, 1731. (15) Zhang, Z.; Wu, F.; Zandvliet, H. J. W.; Poelsema, B.; Metiu, H.; Lagally, M. G. Phys. ReV. Lett. 1995, 74, 3644–3647. (16) Sato, T.; Iwatsuki, M.; Tochihara, H. J. Electron Microsc. 1999, 48, 1. (17) Ronci, F.; Colonna, S.; Thorpe, S. D.; Cricenti, A.; Le Lay, G. Phys. ReV. Lett. 2005, 95, 156101. (18) Lastapis, M.; Martin, M.; Riedel, D.; Hellner, L.; Comtet, G.; Dujardin, G. Science 2005, 308, 1000. (19) van Houselt, A.; van Gastel, R.; Poelsema, B.; Zandvliet, H. J. W. Phys. ReV. Lett. 2006, 97, 266104. (20) Liljeroth, P.; Repp, J.; Meyer, G. Science 2007, 317, 1203. (21) Komeda, T.; Kim, Y.; Kawai, M.; Persson, B. N. J.; Ueba, H. Science 2002, 295, 2055. (22) Sainoo, Y.; Kim, Y.; Okawa, T.; Komeda, T.; Shigekawa, H.; Kawai, M. Phys. ReV. Lett. 2005, 95, 246102. (23) Zandvliet, H. J. W. Phys. Rep. 2003, 388, 1. (24) Hill, E.; Freelan, B.; Ganz, E. Phys. ReV. B 1999, 60, 15896.
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