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Pulsed-dosing controls self-assembly: 1-Bromopentane on Si(1 1 1)-7 × 7
Chemical Physics Letters 527 (2012) 1–6
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FRONTIERS ARTICLE
Pulsed-dosing controls self-assembly: 1-Bromopentane on Si(1 1 1)-7 7 Alon Eisenstein, K.R. Harikumar, Kai Huang, Iain R. McNab 1, John C. Polanyi ⇑, Amir Zabet-Khosousi 2 Department of Chemistry and Institute of Optical Sciences, University of Toronto, 80 St. George Street, Toronto, Ontario, Canada M5S 3H6
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Article history: Available online 10 January 2012
a b s t r a c t We have constructed a high-pressure fast-pulse dosing system for use with Scanning Tunneling Microscopy (STM). For 1-bromopentane on Si(1 1 1)-7 7 at low temperature (100 K) two physisorbed phases were found to co-exist; spaced-out molecules above corner silicon-adatoms in a one-per-corner-hole (OPCH) pattern, and circles of molecules above middle-adatoms. By tuning the parameters of high-pressure fast-pulse dosing, we can choose which of these two patterns, OPCH or circles, to chemically imprint on room temperature silicon. Ó 2012 Elsevier B.V. All rights reserved.
1. Introduction Physisorbed self-assembled molecules can be used as templates for permanently attaching patterns to surfaces by way of chemisorption; for a historical review, see Ref. [1]. Patterns formed by self-assembly can subsequently be rendered permanent by Localized Atomic Reaction (LAR) [2]; in LAR the reaction of a molecule at the surface results in a chemisorbed imprint closely adjacent to the original position of the physisorbed reagent. Localized Atomic Reaction suggests, therefore, a method of ‘molecular printing’ of a physisorbed self-assembled pattern onto a surface, employing in the first stage mobile self-assembly followed secondly by permanent chemical attachment. At moderate surface coverage, the chemical dynamics of surface reaction can be studied one-molecule-at-a-time. When adsorbates are in close proximity, cooperative effects evidence themselves including pattern formation [3–5] and ‘cooperative reaction’ [6]. The self-assembled physisorbed surface patterns with which we are concerned occur at sub-monolayer coverages. Patterns of physisorbed adsorbates at a surface can be understood if the physisorbed molecules constitute a mobile physisorbed state that seeks out favorable sites on the surface. The concentration of the mobile physisorbed molecules depends upon the ratio of the rate of attachment in the mobile physisorbed state, to the rate of desorption from that state back into the gas phase. The rate of attachment to the surface depends principally upon the pressure of gas above the surface but also upon the existing
⇑ Corresponding author. Fax: +1 416 978 7580. E-mail address: [email protected] (J.C. Polanyi). Present address: School of Biological Sciences and Applied Chemistry, Seneca College, 70 The Pond Road, Toronto, Ontario, Canada M3J 3M6. 2 Present address: Department of Chemistry, Columbia University, 3000 Broadway, New York, NY 10027, USA. 1
0009-2614/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.cplett.2011.12.052
surface coverage, while the rate of desorption depends only upon concentration of the mobile physisorbed molecules at the surface. Previous work from this laboratory showed that for sub-monolayer coverages of molecules at silicon surfaces, the rate at which a given dose was made, altered the pattern of physisorption. These prior measurements were made using background dosing, in which the impingement rates were 1014–1016 m 2 s 1 (unit-cell area of the 7 7 surface is 627 Å2, so this represents impingement rates 0.1–10 per 7 7 unit-cell per minute obtainable in the pressure range of 10 10–10 8 Torr. For 1-chlorododecane and 1-bromododecane adsorbed on Si(1 1 1)-7 7 both physisorbed monomers and physisorbed dimers were observed [7]. The ratio of monomers to dimers favored monomer formation at low pressure and dimer at high. This was understood as arising from monomer physisorption that was linear in the concentration of mobile physisorbed molecules, while dimer physisorption was quadratic. As the pressure over the surface increased, the increased concentration of mobile physisorbed molecules favored the quadratic process of dimer physisorption. For both 1-chlorododecane and 1-bromododecane, two physisorbed configurations were found, one immobile and one highly-mobile. For 1,5-dichloropentane (DCP) at the surface of Si(1 0 0)-2 1 lines of physisorbed molecules were formed [8]. The growth of these lines was initiated by a physisorbed pair of DCP molecules, but thereafter the lines grew by attachment of single molecules as the repeat unit. Higher pressure, hence higher dose rate, therefore favored formation of new dimer-initiated lines over the growth of existing lines, resulting in many short lines. Lower pressure, hence lower dose rate, favored longer lines. Some of us also studied lines propagated by 1-chloropentane pairs (CP-pairs) on Si(1 0 0)-2 1 [9]. In this case both the initiation and growth of the lines was by physisorbed pairs. In contrast to the formation of lines of DCP (see above) in which lower dose rate favored longer lines, for CP-pairs it was higher dose rate that favored longer lines (see Figure 3, Ref. [9]). This was explained by rate of
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CP-pair formation and subsequent line formation depending quadratically on the concentration of mobile CP, whereas the competing rate of desorption back to the gas phase depended only linearly on this quantity. The system examined in this work is 1-bromopentane at the surface of Si(1 1 1)-7 7. As will be shown, physisorbed 1-bromopentane has two stable phases that can co-exist in two adsorbate patterns: (1) molecules adsorb on corner-adatoms in a one-per-corner-hole fashion (OPCH) (Figure 2a) consisting of a ‘spaced-out’ pattern of physisorbed molecules adsorbed at corner-adatoms, with only one adsorbate molecule being accommodated per corner-hole of the 7 7 surface [10] and (2) molecules adsorb side-by-side as aggregates on middle-adatoms (Figure 2b), forming trimers and at higher coverages assembled trimers that give the appearance of prominent circles, similar to methyl bromide [11,12] or 1-bromopropane [13] on Si(1 1 1)-7 7. Previous examples of physisorption systems with two stable phases were previously reported from this laboratory; chlorobenzene or toluene at low temperature on the Si(1 1 1)-7 7 surface [14]. In order to explore the effect of dosing pressure and time upon patterning of surfaces, we constructed a high pressure pulsed-doser permitting us to dose the surface with pressures varying over a much larger range, 10 4–1 Torr in times varying from 100 to 1 ms, while retaining sub-monolayer coverages. Similar pulsed dosers have previously been used in other laboratories to dose solutions of molecules onto a surface in ultra-high vacuum (UHV), followed by annealing to remove residual solvent [15,16]. Several studies of large molecules deposited in a pulsed fashion on metallic and semiconducting surfaces imaged by Scanning Tunneling Microscopy (STM) have also appeared [17–23].
2. Experiment 2.1. Pulsed doser In order to increase the pressures of dosing while maintaining sub-monolayer coverages of the surface it is necessary to reduce the time of dosing proportionately. We therefore designed and built a pulsed-doser that could dose a high fore-pressure of molecules through a small orifice, in a time controlled by an electronically activated solenoid valve. The pulsed valve was obtained from Parker Hannifin (Series 9) and driven by Parker Hannifin electronics (Parker Iota One). According to the manufacturers specifications the valve has a minimum opening time of 1 ms, which can be increased to seconds in increments of 1 ms. The valve can be driven at repetition rates of up to 120 Hz, but was operated here in single shots. A schematic of the pulsed-doser is shown in Figure 1. As the pulsed valve is not rated for UHV applications, we mounted it in a secondary pulsed-valve chamber that could be separately pumped, isolated from UHV. The secondary chamber was mounted on an XYZ multistage manipulator (U.H.V. Instruments, Omniax) to enable precise alignment of the nozzle to the center of the sample (translation in X and Y) and to allow the pulsed doser to be withdrawn, when not in use (translation along Z). Although provision was made for pumping of the secondary chamber, the chamber was leak tight to UHV standards and consequently was left filled with dry-nitrogen at atmospheric pressure. This pulsed valve has a specified minimum pulse length of 1 ms, determined by the manufacturers measurement of the mechanical response of the valve, rather than by the much shorter electrical pulse applied to the solenoid. Gas expanding from the 0.75 mm orifice of the pulsed-valve passed through two further 0.75 mm channels in two conflat flanges in contact with one another, on which the pulse valve
Figure 1. Pulsed doser. Schematic representation of the pulsed doser coupled to the UHV chamber. The solenoid-based pulsed valve can be controlled from a nominal 1 ms to 1 s in increments of 1 ms. The pulsed valve is fixed to the end of a linear drive, and can be retracted by up to 10 cm.
was mounted. The total enclosed nozzle length was 1.6 cm, the closest approach of the nozzle orifice to the sample was 6 mm. For effusion through a long nozzle [24] the pressure above the sample depends linearly upon the fore-pressure of the gas, being reduced at the sample by a calculated factor of approximately 250 from the fore-pressure in the nozzle due to the separation between the sample and the orifice. 2.2. 1-Bromopentane physisorption on Si(1 1 1)-7 7 at 100 K The molecule studied here was 1-bromopentane. In all experiments the 1-bromopentane was purified by several cycles of freeze-pump-thaw before being dosed on the surface. Below we contrast results obtained at a low temperature surface (100 K) using either conventional ‘low pressure’ background dosing or localized deposition using the STM tip. The information gained at low temperature assisted in the interpretation of the room temperature (300 K) results obtained using the pulsed doser and hence high dose-rate. At low temperature we observed two patterns of physisorption coexistent on the surface. Using highpressure pulsed-dosing we were able to select which pattern we imprinted on the surface by chemisorption as described below. 2.2.1. Background dosing at 100 K (OPCH and circles) At low temperature, 1-bromopentane exhibited two physisorbed phases; ‘one-per-corner-hole’ and circles. Under a range of conditions both phases were present concurrently. Measurements were made with a variable temperature STM (RHK 300), with liquid-nitrogen as a coolant. The Si(1 1 1)-7 7 surface was maintained at a temperature of 100 K. The 1-bromopentane was background-dosed. Two distinct co-existent phases of physisorbed 1-bromopentane were formed as different domains, recognizable as two distinct patterns of physisorption described below: namely (1) one-per-corner-hole (OPCH), and (2) circles. The physisorbed 1-bromopentane molecules that comprise the OPCH pattern are immobile. In other work [10], a theoretical
A. Eisenstein et al. / Chemical Physics Letters 527 (2012) 1–6
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Figure 2. Coexistence of two physisorbed patterns of 1-bromopentane at 100 K on a single surface. Physisorption of 1-bromopentane at 100 K by background dosing gives (a) horizontal configuration (imaged as a dark feature) in a spaced-out one-per-corner-hole pattern (b) linked trimers of partially mobile vertical molecules (imaged as a bright feature), equally distributed between faulted and unfaulted halves, forming 12-member circles around a corner-hole. In the top half frame of (a), the corner-holes around which horizontal 1-bromopentane physisorbs are highlighted in white dotted circles, showing predominant one adsorbate (dark feature) per six silicon corner adatoms around a corner-hole. A rare exception where two adsorbates at the same corner-hole is highlighted in blue dotted circle. In (b), six trimers around a corner hole are highlighted in white solid triangles showing how touching trimers give the appearance of circles. The two STM images are from different domains of physisorption of a single Si(1 1 1)-7 7 crystal following dosing (4 10 9 Torr 50 s) at 100 K. Both images are 134 134 Å2, scan conditions (a) Vs = +1.5 V, Itun = 0.1 nA and (b) Vs = +1.4 V, Itun = 0.3 nA. The streakiness of vertical 1-bromopentane in (b) c.f. the chemisorbed circles of Br-atoms in Figure 4d shows that the physisorbed molecules move when traced by the STM tip.
analysis showed that the OPCH pattern resulted from indirect repulsion between adsorbates, mediated by charge transfer through the silicon surface. In conformity with our earlier study of longer-chain halocarbons [7,13] we assign the immobile physisorbed 1-bromopentane in this OPCH pattern as a horizontal configuration in which the molecule is attracted to the surface along its full length. By contrast, circles of physisorbed 1-bromopentane are made up from partially mobile (streaked) physisorbed molecules located atop middle (rather than corner) adatoms, giving the appearance of prominent circles of adsorbates as shown in Figure 2b. We ascribe these partially mobile physisorbed 1-bromopentane molecules, in conformity with earlier work [7], to a vertical configuration bound to the surface only by their physisorbed terminal Br atom. These mobile (vertical) physisorbed molecules, situated above middle-adatoms, were found to be equally distributed between faulted and unfaulted half unit-cells, consequently at sufficient coverages they formed full circles of physisorbed adsorbates, see Figure 2b. The vertical configuration of 1-bromopentane gives an apparent height of 1.8–1.9 Å greater than the horizontal configuration in our sampled STM image. This measured height difference was found to be independent of our imaging sample bias and is therefore due to topology, not to a difference in the electronic densities of states.
2.2.2. Tip-dosing at 100 K (monomers, dimers and trimers) One avenue by which the self-assembly of the observed circle patterns may proceed is by forming a connected network of trimers (see Figure 2b) atop all available middle adatoms. This would occur if trimers of adsorbates atop middle adatoms existed as building-blocks to pattern formation. We have directly imaged dimers and also trimers by ‘tip-dosing’ at the surface, suggesting that dimers and trimers constitute the favored intermediate to circle formation. We dosed 1-bromopentane on the surface using ‘tip-dosing’, following the method of Suzuki et al. [25]. The tip was first held at room temperature and exposed to 1-bromopentane at 4 10 9 Torr for 400 s. The dosed tip was then used to repeatedly scan clean Si(1 1 1)-7 7 at 100 K using a sample bias of 1.5 V, molecules were thereby deposited at the surface by the tip. The same surface area was subsequently imaged using a sample bias
of +1.0 to +2.5 V, where the tip deposition was largely inhibited. There were two advantages of tip-dosing; (1) imaging of the surface could be achieved almost immediately following deposition, whereas for background dosing typically 20 min elapsed between dosing and subsequent STM imaging because of the time required for tip-approach and to achieve stable imaging conditions, (2) a further advantage of tip-dosing was that a high local concentration of molecules could be dosed, in a manner that could not be achieved by high-pressure background dosing. The high local concentration and rapid imaging enabled us to observe some physisorbed states that were only transiently stable, namely dimers and trimers. Representative STM images obtained using tip-dosing are shown in Figure 3. The same area was imaged at two bias voltages (+1.5 V and +2.5 V). Although deposition was random the 1-bromopentane molecules adsorbed exclusively on the faulted-half unit-cell of the 7 7 surface. Three typical physisorbed structures were observed from tip-dosing and assigned as trimers, dimers and monomers of physisorbed 1-bromopentane, confined to the faulted half-unit-cell with the typical appearance of (a) trimers: bright, (b) dimers: streaked and (c) monomers: dimly-streaked. Streaked images indicate movement of adsorbate under the STM tip [26]. It is apparent that single molecules are highly mobile within a half unit cell, dimers slightly less mobile, and trimers immobile. Importantly stabilization is evident, due to adjacent adsorbate-adsorbate interaction. The two images were taken at different biases, but the streakiness of the images was unchanged. It followed that the influence from the tip was negligible and hence the observed mobility was intrinsically thermally induced at 100 K.
2.3. High pressure pulsed dosing at 300 K We have seen that at low temperature (100 K), two stable physisorbed phases of 1-bromopentane were observed, corresponding to adsorption at corner-adatoms (in a one-per-corner-hole fashion), and as trimers over middle adatoms, giving the appearance of circles at high coverage. The results of dosing 1-bromopentane at high pressure were explored for a variety of fore-pressures and dosing times at room temperature using a fixed temperature STM (RHK 400). For dosing, the Si(1 1 1)-7 7 sample was placed 6 mm below the pulse valve
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Figure 3. Aggregates of physisorbed 1-bromopentane obtained by tip-dosing. Physisorption of 1-bromopentane at 100 K from the STM tip gave monomers (black dashed triangular boundaries), dimers (white dashed triangular boundaries), and trimers (white solid triangular boundaries) observed exclusively on the faulted half-unit-cell of the Si(1 1 1)-7 7 The trimers are just distinguishable as three bright spots on a half unit cell. Monomers and dimers were also observed, but were less stable; they appeared as half-unit cells filled with streaks. Dimers are brighter than monomers. We correlate the observed streaking with molecules that are mobile within the half unit cell. Both panels are 210 210 Å2, imaged at (a) Vs = +1.5 V, Itun = 0.2 nA; (b) Vs = +2.5 V, Itun = 0.2 nA. Some further deposition from the STM tip occurred at the +2.5 V image, panel (b), giving features absent from panel (a).
Figure 4. Effect of pressure and time on pattern formation with the pulsed doser. Room temperature bromination patterns resulting from pulsed dosing using different fore-line pressures of 1-bromopentane showing a change (a)–(c) from predominantly middle-adatom bromination (partial circles and trimers) to predominantly corner-adatom bromination (OPCH). From (c) to (d) we see that a similar change can be affected by changing the time of dosing at a fixed fore-pressure. Both long time and high pressure can change the imprinted bromine patterns from OPCH to trimers and part-circles. All STM images are 150 150 Å2. (a) Predominantly brominated middle adatoms (1 ms pulse, 1.00 Torr). Total bromination is 3/unit-cell, and the ratio of middle/corner brominated adatoms is 23/1 (Vs = +3.0 V, Itun = 0.1 nA). (b) Mixture of brominated corner and brominated middle adatoms (1 ms pulse, 0.50 Torr). Total bromination is 1/unit-cell, and the ratio of middle/Corner brominated adatoms is 1/1 (Vs = +2.6 V, Itun = 0.1 nA). (c) corner-adatom bromination (1 ms pulse, 0.10 Torr). Total bromination is 0.4/unit-cell, and the ratio of middle/corner brominated adatoms is 1/9 (Vs = +3.0 V, Itun = 0.2 nA). (d) Predominantly brominated middle adatoms (10 ms, 0.10 Torr). Total bromination is 1.6/unit cell, and the ratio of middle/corner brominated (a small preference for the faulted half-unit-cell was observed). Total bromination is 1.6/unit-cell, and the ratio of middle/corner brominated adatoms is 9/1 (Vs = +2.5 V, Itun = 0.2 nA).
A. Eisenstein et al. / Chemical Physics Letters 527 (2012) 1–6
orifice. The valve was then opened for a ‘single-shot’ exposing the sample to an effusive beam of gas due to the fore-line pressure of 1-bromopentane. The fore-line pressure was varied between 10 2 Torr and 1 Torr as measured by a Convectron gauge connected to the gas fore-line (giving estimated impingement rates 1–100 per unit cell per millisecond). The pulse-valve opening time was varied between 1 and 100 ms as set by the control electronics. The base pressure in the dosing chamber was less than 10 10 Torr. During a typical pulse (1 ms, 0.1 Torr fore-pressure) the base pressure rose to 4 10 10 Torr, recovering base pressure within 10 s. Below we show the results from changing the fore-pressure of dosing while maintaining the time of the dosing at 1 ms, and changing the dose-time while maintaining the fore-pressure of dosing constant. As the observations were made at room temperature only the outcome of reaction is observed, namely brominated silicon. This patterned bromination is known to result from a prior pattern of physisorbed molecules reacting by way of Localized Atomic Reaction (LAR) [2]. The observed chemisorbed pattern of bromine atoms is indicative of the prior physisorbed pattern. By increasing the fore-pressure from 0.1 to 1.0 Torr at the same dose-time of 1 ms we markedly change the observed pattern of bromination. Images of patterns obtained for 1 ms pulse times are shown in Figure 4 with increasing pressures of 0.1 Torr (4c), 0.5 Torr (4b) and 1.0 Torr (4a). As the fore-pressure was increased the observed pattern changed from predominantly corner-adatom bromination (OPCH) to predominantly middle-adatom bromination (circles). The ratio of middle/corner brominated adatoms changes from 1/9 at 0.1 Torr, to 1/1 at 0.5 Torr and 23/1 at 1.0 Torr. As the Br coverage at silicon middle adatoms changes as a high power of pressure, the aggregation of 1-bromopentane above middle adatoms must also be high order (order >1) in pressure. By increasing the dose-time from 1 to 10 ms, while maintaining a fore-pressure of 0.1 Torr we markedly changed the observed pattern of bromination as shown in Figure 4c (1 ms, 0.1 Torr) and 4d (10 ms, 0.1 Torr). As the dose-time increased the observed pattern changed from predominantly corner-adatom bromination (OPCH) to predominantly middle-adatom bromination (trimers and partcircles).
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The OPCH pattern is formed because of repulsive interactions between physisorbed molecules atop corner-adatoms that arises because of ‘through surface’, adsorbate-adsorbate repulsion [10]. In contrast, the circle patterns are formed because of attractive interactions between partially mobile physisorbed molecules atop middle-adatoms [12,13]. At room temperature, this results in dimers, trimers and circles of chemisorbed bromine atoms.
4. Conclusion We have shown that 1-bromopentane has two physisorbed phases on Si(1 1 1)-7 7 at low temperature. The first phase is a spaced-out arrangement of immobile physisorbed molecules above corner silicon adatoms as one-per-corner-hole (OPCH). The second phase is circles above middle silicon-adatoms, stabilized by adsorbate-adsorbate attraction. With low temperature dosing we were able to observe both physisorbed phases simultaneously. Using pulsed dosing at fore-pressures of 0.1–1 Torr and fast times of dosing of 1–10 ms, we were able to select the ratio of OPCH to circles and thereby control the pattern of bromine chemisorption at the room temperature surface.
Acknowledgements We are grateful for financial support from the Natural Sciences and Engineering Research Council of Canada (NSERC), the Xerox Research Centre Canada (XRCC) and the Canadian Institute for Advanced Research (CIfAR). We thank Prof. G. Dujardin and Dr. G. Comtet for their help and advice on the design of the pulsed dose system. References [1] [2] [3] [4]
3. Discussion
[5] [6]
We imaged the brominated surfaces after room temperature dosing, observing the results of complete reaction to form patterns of bromination arising from initial physisorbed 1-bromopentane molecules. Formation of OPCH physisorbed molecules is first order in the concentration of mobile physisorbed molecules, while formation of physisorbed circles of molecules is a complex higher order process. Qualitatively, low pressure decreases the concentration of mobile physisorbed molecules favoring the low order process leading to OPCH chemisorption while high pressure achievable by pulsed dosing increases the concentration of mobile physisorbed molecules, favoring the higher order processes that lead to circle formation. Hence, the pattern of physisorption and subsequent chemisorption is controlled by the dosing pressure in the pulsed doser. We observe dependence upon dosing-time of the ratios of concentrations of reaction products, in this case OPCH to partial-circles. This dependence is characteristic of reaction products that are sampled at different times before steady-state has been reached [27]. From these results it is clear that we are still approaching steady-state after a dosing-time of 10 2 s. In previous work from this laboratory the approach to steady-state was examined for 1-chlorododecane at the same surface using lowpressure background-dosing gave a time for steady-state to be reached 100 s [7].
[7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26] [27]
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Alon Eisenstein obtained his B.Sc. and M.Sc. from BenGurion University of the Negev, Israel. During his master’s degree, under the supervision of Prof. Yishay Manassen, he designed and built a novel low-temperature scanning tunneling microscope for electron spin noise measurements. He is now completing his doctoral studies at the University of Toronto, addressing nanoscale phenomenon using scanning probe microscopy.
K.R. Harikumar obtained his B.Sc. and M.Sc. from University College, Kerala University, India and his doctorate in 1999 from the Indian Institute of Science, Bangalore, India working in the group of Prof. C.N.R. Rao. He did postdoctoral work at Cardiff University before moving to Prof. Polanyi’s group, where he currently holds the post of Senior Research Associate.
Kai Huang obtained his B.Sc. from Peking University, People’s Republic of China, before joining Prof. Polanyi’s group in 2006 as a doctoral student. He graduated with a Ph.D. in 2011. He is taking up a postdoctoral position in the group of Prof. Freund, Fritz-Haber-Institut der Max-Planck-Gesellschaft, Germany.
Iain R. McNab received his doctorate from Oxford University, was then a Research Fellow at Southampton University and later joined the Physics faculty at the University of Newcastle upon Tyne. He first worked with Prof. Polanyi as a Visiting Professor, later joining the group as a member of staff. He is currently at Seneca College, Toronto.
John C. Polanyi, following receiving his Ph.D. degree from Manchester University, then was a postdoctoral fellow at the National Research Council Laboratories in Ottawa and Princeton University. He is a professor of chemistry at the University of Toronto, with interests in the molecular motions involved in chemical reactions. This led to his current work, with his research group, on surface reaction dynamics examined a-molecule-at-atime by Scanning Tunneling Microscopy.
Amir Zabet-Khosousi obtained his B.Sc. from Sharif University of Technology, Iran and his M.Sc. and Ph.D. at the University of Toronto, working with Prof. Dhirani. Recipient of an Ontario Postdoctoral Fellowship, he worked with Prof. Polanyi from 2009 to 2011. He is currently a research scientist at Columbia University, USA.
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Chemical Physics Letters journal homepage: www.elsevier.com/locate/cplett
FRONTIERS ARTICLE
Pulsed-dosing controls self-assembly: 1-Bromopentane on Si(1 1 1)-7 7 Alon Eisenstein, K.R. Harikumar, Kai Huang, Iain R. McNab 1, John C. Polanyi ⇑, Amir Zabet-Khosousi 2 Department of Chemistry and Institute of Optical Sciences, University of Toronto, 80 St. George Street, Toronto, Ontario, Canada M5S 3H6
a r t i c l e
i n f o
Article history: Available online 10 January 2012
a b s t r a c t We have constructed a high-pressure fast-pulse dosing system for use with Scanning Tunneling Microscopy (STM). For 1-bromopentane on Si(1 1 1)-7 7 at low temperature (100 K) two physisorbed phases were found to co-exist; spaced-out molecules above corner silicon-adatoms in a one-per-corner-hole (OPCH) pattern, and circles of molecules above middle-adatoms. By tuning the parameters of high-pressure fast-pulse dosing, we can choose which of these two patterns, OPCH or circles, to chemically imprint on room temperature silicon. Ó 2012 Elsevier B.V. All rights reserved.
1. Introduction Physisorbed self-assembled molecules can be used as templates for permanently attaching patterns to surfaces by way of chemisorption; for a historical review, see Ref. [1]. Patterns formed by self-assembly can subsequently be rendered permanent by Localized Atomic Reaction (LAR) [2]; in LAR the reaction of a molecule at the surface results in a chemisorbed imprint closely adjacent to the original position of the physisorbed reagent. Localized Atomic Reaction suggests, therefore, a method of ‘molecular printing’ of a physisorbed self-assembled pattern onto a surface, employing in the first stage mobile self-assembly followed secondly by permanent chemical attachment. At moderate surface coverage, the chemical dynamics of surface reaction can be studied one-molecule-at-a-time. When adsorbates are in close proximity, cooperative effects evidence themselves including pattern formation [3–5] and ‘cooperative reaction’ [6]. The self-assembled physisorbed surface patterns with which we are concerned occur at sub-monolayer coverages. Patterns of physisorbed adsorbates at a surface can be understood if the physisorbed molecules constitute a mobile physisorbed state that seeks out favorable sites on the surface. The concentration of the mobile physisorbed molecules depends upon the ratio of the rate of attachment in the mobile physisorbed state, to the rate of desorption from that state back into the gas phase. The rate of attachment to the surface depends principally upon the pressure of gas above the surface but also upon the existing
⇑ Corresponding author. Fax: +1 416 978 7580. E-mail address: [email protected] (J.C. Polanyi). Present address: School of Biological Sciences and Applied Chemistry, Seneca College, 70 The Pond Road, Toronto, Ontario, Canada M3J 3M6. 2 Present address: Department of Chemistry, Columbia University, 3000 Broadway, New York, NY 10027, USA. 1
0009-2614/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.cplett.2011.12.052
surface coverage, while the rate of desorption depends only upon concentration of the mobile physisorbed molecules at the surface. Previous work from this laboratory showed that for sub-monolayer coverages of molecules at silicon surfaces, the rate at which a given dose was made, altered the pattern of physisorption. These prior measurements were made using background dosing, in which the impingement rates were 1014–1016 m 2 s 1 (unit-cell area of the 7 7 surface is 627 Å2, so this represents impingement rates 0.1–10 per 7 7 unit-cell per minute obtainable in the pressure range of 10 10–10 8 Torr. For 1-chlorododecane and 1-bromododecane adsorbed on Si(1 1 1)-7 7 both physisorbed monomers and physisorbed dimers were observed [7]. The ratio of monomers to dimers favored monomer formation at low pressure and dimer at high. This was understood as arising from monomer physisorption that was linear in the concentration of mobile physisorbed molecules, while dimer physisorption was quadratic. As the pressure over the surface increased, the increased concentration of mobile physisorbed molecules favored the quadratic process of dimer physisorption. For both 1-chlorododecane and 1-bromododecane, two physisorbed configurations were found, one immobile and one highly-mobile. For 1,5-dichloropentane (DCP) at the surface of Si(1 0 0)-2 1 lines of physisorbed molecules were formed [8]. The growth of these lines was initiated by a physisorbed pair of DCP molecules, but thereafter the lines grew by attachment of single molecules as the repeat unit. Higher pressure, hence higher dose rate, therefore favored formation of new dimer-initiated lines over the growth of existing lines, resulting in many short lines. Lower pressure, hence lower dose rate, favored longer lines. Some of us also studied lines propagated by 1-chloropentane pairs (CP-pairs) on Si(1 0 0)-2 1 [9]. In this case both the initiation and growth of the lines was by physisorbed pairs. In contrast to the formation of lines of DCP (see above) in which lower dose rate favored longer lines, for CP-pairs it was higher dose rate that favored longer lines (see Figure 3, Ref. [9]). This was explained by rate of
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CP-pair formation and subsequent line formation depending quadratically on the concentration of mobile CP, whereas the competing rate of desorption back to the gas phase depended only linearly on this quantity. The system examined in this work is 1-bromopentane at the surface of Si(1 1 1)-7 7. As will be shown, physisorbed 1-bromopentane has two stable phases that can co-exist in two adsorbate patterns: (1) molecules adsorb on corner-adatoms in a one-per-corner-hole fashion (OPCH) (Figure 2a) consisting of a ‘spaced-out’ pattern of physisorbed molecules adsorbed at corner-adatoms, with only one adsorbate molecule being accommodated per corner-hole of the 7 7 surface [10] and (2) molecules adsorb side-by-side as aggregates on middle-adatoms (Figure 2b), forming trimers and at higher coverages assembled trimers that give the appearance of prominent circles, similar to methyl bromide [11,12] or 1-bromopropane [13] on Si(1 1 1)-7 7. Previous examples of physisorption systems with two stable phases were previously reported from this laboratory; chlorobenzene or toluene at low temperature on the Si(1 1 1)-7 7 surface [14]. In order to explore the effect of dosing pressure and time upon patterning of surfaces, we constructed a high pressure pulsed-doser permitting us to dose the surface with pressures varying over a much larger range, 10 4–1 Torr in times varying from 100 to 1 ms, while retaining sub-monolayer coverages. Similar pulsed dosers have previously been used in other laboratories to dose solutions of molecules onto a surface in ultra-high vacuum (UHV), followed by annealing to remove residual solvent [15,16]. Several studies of large molecules deposited in a pulsed fashion on metallic and semiconducting surfaces imaged by Scanning Tunneling Microscopy (STM) have also appeared [17–23].
2. Experiment 2.1. Pulsed doser In order to increase the pressures of dosing while maintaining sub-monolayer coverages of the surface it is necessary to reduce the time of dosing proportionately. We therefore designed and built a pulsed-doser that could dose a high fore-pressure of molecules through a small orifice, in a time controlled by an electronically activated solenoid valve. The pulsed valve was obtained from Parker Hannifin (Series 9) and driven by Parker Hannifin electronics (Parker Iota One). According to the manufacturers specifications the valve has a minimum opening time of 1 ms, which can be increased to seconds in increments of 1 ms. The valve can be driven at repetition rates of up to 120 Hz, but was operated here in single shots. A schematic of the pulsed-doser is shown in Figure 1. As the pulsed valve is not rated for UHV applications, we mounted it in a secondary pulsed-valve chamber that could be separately pumped, isolated from UHV. The secondary chamber was mounted on an XYZ multistage manipulator (U.H.V. Instruments, Omniax) to enable precise alignment of the nozzle to the center of the sample (translation in X and Y) and to allow the pulsed doser to be withdrawn, when not in use (translation along Z). Although provision was made for pumping of the secondary chamber, the chamber was leak tight to UHV standards and consequently was left filled with dry-nitrogen at atmospheric pressure. This pulsed valve has a specified minimum pulse length of 1 ms, determined by the manufacturers measurement of the mechanical response of the valve, rather than by the much shorter electrical pulse applied to the solenoid. Gas expanding from the 0.75 mm orifice of the pulsed-valve passed through two further 0.75 mm channels in two conflat flanges in contact with one another, on which the pulse valve
Figure 1. Pulsed doser. Schematic representation of the pulsed doser coupled to the UHV chamber. The solenoid-based pulsed valve can be controlled from a nominal 1 ms to 1 s in increments of 1 ms. The pulsed valve is fixed to the end of a linear drive, and can be retracted by up to 10 cm.
was mounted. The total enclosed nozzle length was 1.6 cm, the closest approach of the nozzle orifice to the sample was 6 mm. For effusion through a long nozzle [24] the pressure above the sample depends linearly upon the fore-pressure of the gas, being reduced at the sample by a calculated factor of approximately 250 from the fore-pressure in the nozzle due to the separation between the sample and the orifice. 2.2. 1-Bromopentane physisorption on Si(1 1 1)-7 7 at 100 K The molecule studied here was 1-bromopentane. In all experiments the 1-bromopentane was purified by several cycles of freeze-pump-thaw before being dosed on the surface. Below we contrast results obtained at a low temperature surface (100 K) using either conventional ‘low pressure’ background dosing or localized deposition using the STM tip. The information gained at low temperature assisted in the interpretation of the room temperature (300 K) results obtained using the pulsed doser and hence high dose-rate. At low temperature we observed two patterns of physisorption coexistent on the surface. Using highpressure pulsed-dosing we were able to select which pattern we imprinted on the surface by chemisorption as described below. 2.2.1. Background dosing at 100 K (OPCH and circles) At low temperature, 1-bromopentane exhibited two physisorbed phases; ‘one-per-corner-hole’ and circles. Under a range of conditions both phases were present concurrently. Measurements were made with a variable temperature STM (RHK 300), with liquid-nitrogen as a coolant. The Si(1 1 1)-7 7 surface was maintained at a temperature of 100 K. The 1-bromopentane was background-dosed. Two distinct co-existent phases of physisorbed 1-bromopentane were formed as different domains, recognizable as two distinct patterns of physisorption described below: namely (1) one-per-corner-hole (OPCH), and (2) circles. The physisorbed 1-bromopentane molecules that comprise the OPCH pattern are immobile. In other work [10], a theoretical
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Figure 2. Coexistence of two physisorbed patterns of 1-bromopentane at 100 K on a single surface. Physisorption of 1-bromopentane at 100 K by background dosing gives (a) horizontal configuration (imaged as a dark feature) in a spaced-out one-per-corner-hole pattern (b) linked trimers of partially mobile vertical molecules (imaged as a bright feature), equally distributed between faulted and unfaulted halves, forming 12-member circles around a corner-hole. In the top half frame of (a), the corner-holes around which horizontal 1-bromopentane physisorbs are highlighted in white dotted circles, showing predominant one adsorbate (dark feature) per six silicon corner adatoms around a corner-hole. A rare exception where two adsorbates at the same corner-hole is highlighted in blue dotted circle. In (b), six trimers around a corner hole are highlighted in white solid triangles showing how touching trimers give the appearance of circles. The two STM images are from different domains of physisorption of a single Si(1 1 1)-7 7 crystal following dosing (4 10 9 Torr 50 s) at 100 K. Both images are 134 134 Å2, scan conditions (a) Vs = +1.5 V, Itun = 0.1 nA and (b) Vs = +1.4 V, Itun = 0.3 nA. The streakiness of vertical 1-bromopentane in (b) c.f. the chemisorbed circles of Br-atoms in Figure 4d shows that the physisorbed molecules move when traced by the STM tip.
analysis showed that the OPCH pattern resulted from indirect repulsion between adsorbates, mediated by charge transfer through the silicon surface. In conformity with our earlier study of longer-chain halocarbons [7,13] we assign the immobile physisorbed 1-bromopentane in this OPCH pattern as a horizontal configuration in which the molecule is attracted to the surface along its full length. By contrast, circles of physisorbed 1-bromopentane are made up from partially mobile (streaked) physisorbed molecules located atop middle (rather than corner) adatoms, giving the appearance of prominent circles of adsorbates as shown in Figure 2b. We ascribe these partially mobile physisorbed 1-bromopentane molecules, in conformity with earlier work [7], to a vertical configuration bound to the surface only by their physisorbed terminal Br atom. These mobile (vertical) physisorbed molecules, situated above middle-adatoms, were found to be equally distributed between faulted and unfaulted half unit-cells, consequently at sufficient coverages they formed full circles of physisorbed adsorbates, see Figure 2b. The vertical configuration of 1-bromopentane gives an apparent height of 1.8–1.9 Å greater than the horizontal configuration in our sampled STM image. This measured height difference was found to be independent of our imaging sample bias and is therefore due to topology, not to a difference in the electronic densities of states.
2.2.2. Tip-dosing at 100 K (monomers, dimers and trimers) One avenue by which the self-assembly of the observed circle patterns may proceed is by forming a connected network of trimers (see Figure 2b) atop all available middle adatoms. This would occur if trimers of adsorbates atop middle adatoms existed as building-blocks to pattern formation. We have directly imaged dimers and also trimers by ‘tip-dosing’ at the surface, suggesting that dimers and trimers constitute the favored intermediate to circle formation. We dosed 1-bromopentane on the surface using ‘tip-dosing’, following the method of Suzuki et al. [25]. The tip was first held at room temperature and exposed to 1-bromopentane at 4 10 9 Torr for 400 s. The dosed tip was then used to repeatedly scan clean Si(1 1 1)-7 7 at 100 K using a sample bias of 1.5 V, molecules were thereby deposited at the surface by the tip. The same surface area was subsequently imaged using a sample bias
of +1.0 to +2.5 V, where the tip deposition was largely inhibited. There were two advantages of tip-dosing; (1) imaging of the surface could be achieved almost immediately following deposition, whereas for background dosing typically 20 min elapsed between dosing and subsequent STM imaging because of the time required for tip-approach and to achieve stable imaging conditions, (2) a further advantage of tip-dosing was that a high local concentration of molecules could be dosed, in a manner that could not be achieved by high-pressure background dosing. The high local concentration and rapid imaging enabled us to observe some physisorbed states that were only transiently stable, namely dimers and trimers. Representative STM images obtained using tip-dosing are shown in Figure 3. The same area was imaged at two bias voltages (+1.5 V and +2.5 V). Although deposition was random the 1-bromopentane molecules adsorbed exclusively on the faulted-half unit-cell of the 7 7 surface. Three typical physisorbed structures were observed from tip-dosing and assigned as trimers, dimers and monomers of physisorbed 1-bromopentane, confined to the faulted half-unit-cell with the typical appearance of (a) trimers: bright, (b) dimers: streaked and (c) monomers: dimly-streaked. Streaked images indicate movement of adsorbate under the STM tip [26]. It is apparent that single molecules are highly mobile within a half unit cell, dimers slightly less mobile, and trimers immobile. Importantly stabilization is evident, due to adjacent adsorbate-adsorbate interaction. The two images were taken at different biases, but the streakiness of the images was unchanged. It followed that the influence from the tip was negligible and hence the observed mobility was intrinsically thermally induced at 100 K.
2.3. High pressure pulsed dosing at 300 K We have seen that at low temperature (100 K), two stable physisorbed phases of 1-bromopentane were observed, corresponding to adsorption at corner-adatoms (in a one-per-corner-hole fashion), and as trimers over middle adatoms, giving the appearance of circles at high coverage. The results of dosing 1-bromopentane at high pressure were explored for a variety of fore-pressures and dosing times at room temperature using a fixed temperature STM (RHK 400). For dosing, the Si(1 1 1)-7 7 sample was placed 6 mm below the pulse valve
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Figure 3. Aggregates of physisorbed 1-bromopentane obtained by tip-dosing. Physisorption of 1-bromopentane at 100 K from the STM tip gave monomers (black dashed triangular boundaries), dimers (white dashed triangular boundaries), and trimers (white solid triangular boundaries) observed exclusively on the faulted half-unit-cell of the Si(1 1 1)-7 7 The trimers are just distinguishable as three bright spots on a half unit cell. Monomers and dimers were also observed, but were less stable; they appeared as half-unit cells filled with streaks. Dimers are brighter than monomers. We correlate the observed streaking with molecules that are mobile within the half unit cell. Both panels are 210 210 Å2, imaged at (a) Vs = +1.5 V, Itun = 0.2 nA; (b) Vs = +2.5 V, Itun = 0.2 nA. Some further deposition from the STM tip occurred at the +2.5 V image, panel (b), giving features absent from panel (a).
Figure 4. Effect of pressure and time on pattern formation with the pulsed doser. Room temperature bromination patterns resulting from pulsed dosing using different fore-line pressures of 1-bromopentane showing a change (a)–(c) from predominantly middle-adatom bromination (partial circles and trimers) to predominantly corner-adatom bromination (OPCH). From (c) to (d) we see that a similar change can be affected by changing the time of dosing at a fixed fore-pressure. Both long time and high pressure can change the imprinted bromine patterns from OPCH to trimers and part-circles. All STM images are 150 150 Å2. (a) Predominantly brominated middle adatoms (1 ms pulse, 1.00 Torr). Total bromination is 3/unit-cell, and the ratio of middle/corner brominated adatoms is 23/1 (Vs = +3.0 V, Itun = 0.1 nA). (b) Mixture of brominated corner and brominated middle adatoms (1 ms pulse, 0.50 Torr). Total bromination is 1/unit-cell, and the ratio of middle/Corner brominated adatoms is 1/1 (Vs = +2.6 V, Itun = 0.1 nA). (c) corner-adatom bromination (1 ms pulse, 0.10 Torr). Total bromination is 0.4/unit-cell, and the ratio of middle/corner brominated adatoms is 1/9 (Vs = +3.0 V, Itun = 0.2 nA). (d) Predominantly brominated middle adatoms (10 ms, 0.10 Torr). Total bromination is 1.6/unit cell, and the ratio of middle/corner brominated (a small preference for the faulted half-unit-cell was observed). Total bromination is 1.6/unit-cell, and the ratio of middle/corner brominated adatoms is 9/1 (Vs = +2.5 V, Itun = 0.2 nA).
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orifice. The valve was then opened for a ‘single-shot’ exposing the sample to an effusive beam of gas due to the fore-line pressure of 1-bromopentane. The fore-line pressure was varied between 10 2 Torr and 1 Torr as measured by a Convectron gauge connected to the gas fore-line (giving estimated impingement rates 1–100 per unit cell per millisecond). The pulse-valve opening time was varied between 1 and 100 ms as set by the control electronics. The base pressure in the dosing chamber was less than 10 10 Torr. During a typical pulse (1 ms, 0.1 Torr fore-pressure) the base pressure rose to 4 10 10 Torr, recovering base pressure within 10 s. Below we show the results from changing the fore-pressure of dosing while maintaining the time of the dosing at 1 ms, and changing the dose-time while maintaining the fore-pressure of dosing constant. As the observations were made at room temperature only the outcome of reaction is observed, namely brominated silicon. This patterned bromination is known to result from a prior pattern of physisorbed molecules reacting by way of Localized Atomic Reaction (LAR) [2]. The observed chemisorbed pattern of bromine atoms is indicative of the prior physisorbed pattern. By increasing the fore-pressure from 0.1 to 1.0 Torr at the same dose-time of 1 ms we markedly change the observed pattern of bromination. Images of patterns obtained for 1 ms pulse times are shown in Figure 4 with increasing pressures of 0.1 Torr (4c), 0.5 Torr (4b) and 1.0 Torr (4a). As the fore-pressure was increased the observed pattern changed from predominantly corner-adatom bromination (OPCH) to predominantly middle-adatom bromination (circles). The ratio of middle/corner brominated adatoms changes from 1/9 at 0.1 Torr, to 1/1 at 0.5 Torr and 23/1 at 1.0 Torr. As the Br coverage at silicon middle adatoms changes as a high power of pressure, the aggregation of 1-bromopentane above middle adatoms must also be high order (order >1) in pressure. By increasing the dose-time from 1 to 10 ms, while maintaining a fore-pressure of 0.1 Torr we markedly changed the observed pattern of bromination as shown in Figure 4c (1 ms, 0.1 Torr) and 4d (10 ms, 0.1 Torr). As the dose-time increased the observed pattern changed from predominantly corner-adatom bromination (OPCH) to predominantly middle-adatom bromination (trimers and partcircles).
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The OPCH pattern is formed because of repulsive interactions between physisorbed molecules atop corner-adatoms that arises because of ‘through surface’, adsorbate-adsorbate repulsion [10]. In contrast, the circle patterns are formed because of attractive interactions between partially mobile physisorbed molecules atop middle-adatoms [12,13]. At room temperature, this results in dimers, trimers and circles of chemisorbed bromine atoms.
4. Conclusion We have shown that 1-bromopentane has two physisorbed phases on Si(1 1 1)-7 7 at low temperature. The first phase is a spaced-out arrangement of immobile physisorbed molecules above corner silicon adatoms as one-per-corner-hole (OPCH). The second phase is circles above middle silicon-adatoms, stabilized by adsorbate-adsorbate attraction. With low temperature dosing we were able to observe both physisorbed phases simultaneously. Using pulsed dosing at fore-pressures of 0.1–1 Torr and fast times of dosing of 1–10 ms, we were able to select the ratio of OPCH to circles and thereby control the pattern of bromine chemisorption at the room temperature surface.
Acknowledgements We are grateful for financial support from the Natural Sciences and Engineering Research Council of Canada (NSERC), the Xerox Research Centre Canada (XRCC) and the Canadian Institute for Advanced Research (CIfAR). We thank Prof. G. Dujardin and Dr. G. Comtet for their help and advice on the design of the pulsed dose system. References [1] [2] [3] [4]
3. Discussion
[5] [6]
We imaged the brominated surfaces after room temperature dosing, observing the results of complete reaction to form patterns of bromination arising from initial physisorbed 1-bromopentane molecules. Formation of OPCH physisorbed molecules is first order in the concentration of mobile physisorbed molecules, while formation of physisorbed circles of molecules is a complex higher order process. Qualitatively, low pressure decreases the concentration of mobile physisorbed molecules favoring the low order process leading to OPCH chemisorption while high pressure achievable by pulsed dosing increases the concentration of mobile physisorbed molecules, favoring the higher order processes that lead to circle formation. Hence, the pattern of physisorption and subsequent chemisorption is controlled by the dosing pressure in the pulsed doser. We observe dependence upon dosing-time of the ratios of concentrations of reaction products, in this case OPCH to partial-circles. This dependence is characteristic of reaction products that are sampled at different times before steady-state has been reached [27]. From these results it is clear that we are still approaching steady-state after a dosing-time of 10 2 s. In previous work from this laboratory the approach to steady-state was examined for 1-chlorododecane at the same surface using lowpressure background-dosing gave a time for steady-state to be reached 100 s [7].
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Alon Eisenstein obtained his B.Sc. and M.Sc. from BenGurion University of the Negev, Israel. During his master’s degree, under the supervision of Prof. Yishay Manassen, he designed and built a novel low-temperature scanning tunneling microscope for electron spin noise measurements. He is now completing his doctoral studies at the University of Toronto, addressing nanoscale phenomenon using scanning probe microscopy.
K.R. Harikumar obtained his B.Sc. and M.Sc. from University College, Kerala University, India and his doctorate in 1999 from the Indian Institute of Science, Bangalore, India working in the group of Prof. C.N.R. Rao. He did postdoctoral work at Cardiff University before moving to Prof. Polanyi’s group, where he currently holds the post of Senior Research Associate.
Kai Huang obtained his B.Sc. from Peking University, People’s Republic of China, before joining Prof. Polanyi’s group in 2006 as a doctoral student. He graduated with a Ph.D. in 2011. He is taking up a postdoctoral position in the group of Prof. Freund, Fritz-Haber-Institut der Max-Planck-Gesellschaft, Germany.
Iain R. McNab received his doctorate from Oxford University, was then a Research Fellow at Southampton University and later joined the Physics faculty at the University of Newcastle upon Tyne. He first worked with Prof. Polanyi as a Visiting Professor, later joining the group as a member of staff. He is currently at Seneca College, Toronto.
John C. Polanyi, following receiving his Ph.D. degree from Manchester University, then was a postdoctoral fellow at the National Research Council Laboratories in Ottawa and Princeton University. He is a professor of chemistry at the University of Toronto, with interests in the molecular motions involved in chemical reactions. This led to his current work, with his research group, on surface reaction dynamics examined a-molecule-at-atime by Scanning Tunneling Microscopy.
Amir Zabet-Khosousi obtained his B.Sc. from Sharif University of Technology, Iran and his M.Sc. and Ph.D. at the University of Toronto, working with Prof. Dhirani. Recipient of an Ontario Postdoctoral Fellowship, he worked with Prof. Polanyi from 2009 to 2011. He is currently a research scientist at Columbia University, USA.