NANO LETTERS
Transport through a Single Octanethiol Molecule
2009 Vol. 9, No. 3 1147-1151
Daan Kockmann,*,† 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, 7500 AE Enschede, The Netherlands Received December 13, 2008; Revised Manuscript Received January 17, 2009
ABSTRACT Octanethiol molecules adsorbed on Pt chains are studied with scanning tunneling microscopy and spectroscopy at 77 K. The head of the octanethiol binds to a Pt atom and the tail is lying flat down on the chain. Open-loop current time traces reveal that the molecule wags its tail and attaches to the scanning tunneling microscopy-tip resulting in a dramatic increase of the current. We measured a single molecule resistance of 100-150 MΩ.
The adsorption of molecules on surfaces has been of interest for more than a century. In the mid seventies, this interest was heavily enhanced by the first molecular electronics publication by Aviram and Ratner.1 In this landmark article they put forward the idea to use a single organic molecule as a molecular rectifier. A single molecule that has a donorspacer-acceptor like structure could behave as a p-n-junction and thus exhibit rectifying IV behavior. At that time, the so-called top-down approach was the only way of creating electronic components. However, Aviram and Ratner proposed a novel method: the so-called bottom-up approach. With this method devices/components are assembled, taking the individual molecules or atoms as a starting point. Nonetheless, it took more than two decades before the physical and chemical properties of an individual molecule could be measured successfully. In particular the introduction of scanning probe techniques2-5 in the 1980s has given an enormous boost to this field. Meanwhile, numerous papers are published on molecular electronics, demonstrating many intriguing and exotic phenomena. Single molecules, or small bundles of molecules, have been trapped between electrodes6-13 and researches have measured Coulomb gaps and Coulomb staircases,6 bistable switching,7 current rectification,8 negative differential resistance characteristics,9 Kondo resonances,10 and conductance switching.11 The use of alkanethiols in molecular electronics is now well established14 since these molecules form densely packed and well-ordered self-assembled monolayers on metal surfaces. An alkanethiol molecule basically consists of three * To whom correspondence should be addressed. E-mail: d.kockmann@ utwente.nl. † Physical Aspects of Nanoelectronics. ‡ Solid State Physics. 10.1021/nl803767c CCC: $40.75 Published on Web 01/30/2009
2009 American Chemical Society
parts: a sulfur-hydrogen head, a hydrocarbon chain of variable length, and an end group. The basic picture used to explain the adsorption of an alkanethiol on a metal substrate, ending in self-assembly, entails physisorption followed by chemisorption.15 After physisorption, the molecule binds via the sulfur atom to the surface. The H-atom is released and the alkanethiol transforms to an alkanethiolate. At low coverage, the alkanethiolate molecules are flat-lying on the surface in ordered domains, referred to as the striped or lying down phase. Upon increasing the coverage to one monolayer, the thiolate molecules will eventually convert to a standingup phase. By using nanostructured surfaces instead of normal metal surfaces as a template, one could construct or assemble more sophisticated nanostructures. Here we report on the adsorption of octanethiol molecules on atomic Pt chains. These Pt chains were grown on Ge(001) via a self-organization process. The Pt/Ge(001) surface is a particularly attractive candidate to be used as a template in molecular electronics since it predominantly consists of arrays of dimerized, perfectly straight, and virtually defectfree monatomic Pt chains.16-18 The Pt/Ge(001) substrate is exposed to octanethiol at room temperature and subsequently cooled down to 77 K. After adsorption and cooling down to 77 K, the structural and electronic properties of the adsorbed thiolate molecules were studied. We found that the head of the thiolate, that is, the sulfur atom, binds to an on-top Pt site. Next we measured the I(V) properties of the head and tail of the octanethiolate molecules separately. There appeared to be a distinct difference in the I(V) responses of both parts. Additionally, we measured open-loop I(t) transients above the adsorbed thiolate molecules. While measuring the tunneling current in the open-loop configuration, we frequently noticed that the current jumps back and forth
between the current set point value (typically 1 nA) and a much higher current value of 10-15 nA. In order to understand this dramatic increase in the tunnel current, we suggest that the octane-tail of the adsorbed molecule (initially lying flat on the Pt chain) flips upward and makes contact with the scanning tunneling microscopy (STM) tip. Hence, we can now simply extract the resistance of a single octanethiolate molecule. Ge(001) substrates (5.0 mm × 10.0 mm) were cut from nominally flat, single-side polished n-type wafers. After ex situ rinsing with isopropanol, the samples were placed into the ultrahigh vacuum low-temperature (UHV LT)-STM (Omicron). Here, they were slowly degassed. Subsequently, the samples were cleaned in situ by sputtering with 800 eV argon ions and thermal annealing to 1100 K (using a dc current) during 2 min. This in situ procedure was repeated several times before atomically clean Ge(001) surfaces were obtained. Imaging with STM at 77 K revealed that the surface predominantly consists of two different, well-ordered domain patterns: p(2 × 2) and c(4 × 2).19 Subsequently, we deposited a submonolayer amount of Pt onto the atomically clean Ge(001) surface at room temperature. Pt was evaporated onto the surface using a straightforward home-built Pt evaporator; a high-purity Pt-wire was wrapped around a W filament. Evaporation occurs simply by heating the W filament via dc current. After evaporation the Pt/Ge(001) substrate was annealed up to 1100 K for 2 min and subsequently cooled down to room temperature by radiative quenching. Following this procedure, we found large patches of Pt chains that are spaced 1.6 nm apart. A more detailed explanation on the procedure and formation of the Pt chains is given in refs 16-18. After formation of the Pt chains, the surface was exposed to octanethiol (Sigma-Aldrich). A small amount of octanethiol was introduced in the preparation chamber of our UHV system via a leak valve. This valve allows us to controllably adjust the pressure of the octanethiol in our preparation chamber. We exposed the samples to 10-16000 L. However, results shown in this paper were all acquired on samples exposed to less than 60 L. After reaching the normal background pressure again, the sample is transferred to the STM chamber for cooling down and imaging. In Figure 1 an overview STM image (25nm × 25nm) of a Pt modified Ge(001) surface after being exposed to 60 L of octanethiol is shown (A), together with a model of the surface structure (B). The surface was scanned at 77 K with a sample bias VS ) -0.90 V and a tunnel current of I ) 0.50 nA. The chains of Pt appear as “dashed bars”. The chains are comprised of Pt dimers, which are aligned in the direction of the chains. The width of a single chain is exactly one Pt atom, and the distance to neighboring chains is 1.6 nm. Clearly present in the image are many white circular spots located on top of the Pt chains. These spots were not present before exposure to octanethiol. The apparent size of the spherical features is consistent with the size of an octanethiolate molecule, namely 1.0 nm.20 Moreover, an increase of the exposure of octanethiol leads to an increase in the number density of these white spots. Therefore, we attribute these white spherical features to octanethiolate 1148
Figure 1. STM image (25 nm × 25 nm; VS ) -0.90 V; I ) 0.5 nA) of a Pt-modified Ge(001) surface after exposure to 60 L of octanethiol, recorded at 77 K (A). The octanethiol molecules (circular white spots) almost exclusively adsorb on the Pt atomic chains. In panel B, we show a model of the region enclosed by the square in panel A. Grey dumbbells are substrate dimers, dark dumbbells are Pt dimers, and the adsorbed molecule is shown in orange.
molecules adsorbed on the atomic Pt chains. The density of these white spots increases with increasing exposure to octanethiol up to about 2000 Langmuir. Between 2000-3000 Langmuir the density of white blobs on the Pt chains becomes so high that we cannot recognize the separate molecules anymore. In addition, the STM image reveals that the octanethiolate molecules almost exclusively adsorb on the Pt chains and only very rarely on the underlying troughs in between the chains. The adsorbed octanethiolate molecules were scanned at biases from -1.5 to +1.5 V. At both positive and negative sample bias, the Pt chains are easy recognizable. Regarding the adsorbed octanethiolate molecules a clear distinction can be made between negative sample bias (filled state) and positive sample bias (empty state). Figure 2 shows filled-state (A) and an empty-state (B) images (5 nm × 5 nm) of an adsorbed octanethiolate molecule on a Pt chain. In the filled-state image (VS ) -1.5 V), a large white protrusion covers virtually one and a half Pt dimer. The topographic center of the molecule is positioned on one of the Pt atoms of the fully decorated Ptdimer. More clearly this is shown in the height profile in Figure 3A; the profile in this figure is taken across the Pt chain as indicated by the red arrow. As a reference profile, we also plotted the height profile of a clean Pt chain, as indicated by the black arrow in Figure 2A. Nano Lett., Vol. 9, No. 3, 2009
Figure 2. Two STM images (5 nm × 5 nm; I ) 0.50 nA; 77 K) recorded simultaneously at the same position at different biases, VS ) -1.50 V (A) and VS ) +1.50 V (B). In the center of both images, one can clearly see the adsorbed molecule on the Pt atomic chain. At negative bias (panel A) the thiolate molecule shows up as a large protuberance on top of the chain; at positive bias (panel B) a small protuberance is accompanied by a small depression in the chain. Furthermore, when looking at the troughs flanked by the chains, there is no difference between the region near the adsorbed molecule and the regions further away from the adsorbed molecule.
Figure 3. Height profiles of Pt chains with (red) and without (black) adsorbed thiolate molecule taken from both filled state (A) and empty state (B) images as indicated in Figure 2. The center of the molecular profile is for both polarities located on top of one of the atoms of a dimer.
Simultaneously with the filled-state image of Figure 2A, we recorded an empty-state image of the same area (see Figure 2B). This image shows that the adsorbed octanethiolate now exhibits a completely different shape. The protrusion, which is much smaller in size, is located on one side of the Pt dimer. Additionally, on the other side of this dimer, the protrusion is accompanied by an even smaller depression. The height characteristics at positive sample bias of a bare Pt chain and a Pt chain decorated with an adsorbed molecule are presented in Figure 3B (black and red arrows as indicated in Figure 2B). As mentioned before, alkanethiols are well studied because of their ideal self-assembly properties on metal surfaces. The sulfur atom of the thiol headgroup binds to the surface and for a densely packed monolayer the alkane chains are standing upward due to the intermolecular Van der Waals forces. At low thiolate coverage, the alkane chains are lying flat down on the surface. On the basis of both these observations and our STM measurements, we assume that the octanethiolates are also lying flat on the Pt chains at least in the low coverage regime. In fact, if we take a closer look Nano Lett., Vol. 9, No. 3, 2009
at the region in the troughs between the Pt chains we see no difference between before and after adsorption of the octanethiolate molecules. Moreover, the region in the troughs between the Pt chains near an adsorbed octanethiolate molecule is virtually identical to a molecule-free trough region. Hence, the tail of the octanethiolate molecules must be located on the Pt chains rather than in troughs between the chains. Thus, on the basis of our STM data we tentatively conclude that the sulfur atom binds on-top to one of the atoms of the Pt dimer and the tail of the octanethiolate molecule lies flat on the Pt chain. In order to obtain more insight in the exact adsorption geometry of the octanethiolate molecule we have recorded I(V) curves on the head and the tail of the molecule. It must be noted that at this moment we do not know which part is the head and which part is the tail. We only know that one part should be the head, while the other should then be the tail. In Figure 4A, we show I(V) curves recorded on both parts of the octanethiolate molecule; the spectra were acquired as indicated by the blue circle and the red triangle in the STM image in Figure 4A. Both spectra are averaged 1149
Figure 4. I(V) characteristics taken on two different parts of an octanethiolate molecule adsorbed on a Pt chain, as indicated by the blue dot and the red triangle in the STM image of panel A. Panel B shows a detailed view of the voltage window from 0 to 0.4 V. We ascribe the curve with the highest current ratio (red) to the sulfur-side of the molecule and the blue curve to the octane-side of the molecule.
Figure 5. (A) I(t) transient recorded on an octanethiolate molecule that is adsorbed on a Pt chain. The trace is recorded at constant height. The sample bias is 1.5 V. The current jumps back and forth between the current set point (1 nA) and a much higher current of 11 nA. We have measured residence times up to 40 s and currents in the range of 10-15 nA. The current jumps are caused by the octanethiolate molecule that wags its tail and subsequently contacts the STM tip, as indicated by the schematic drawings in panels B and C.
over 30 different measurements. The tunnel current was measured between -1.5 V e VS e +1.5 V. Figure 4B shows a closer look at the region 0.0 to +0.4V. At negative sample bias, the I(V) curves measured at the two different sides of the molecule are virtually identical. However, for positive sample bias the two parts of the molecule behave differently. The region marked with the red triangle in the STM image in Figure 4 shows a much higher current than the region marked with the blue circle. After superimposing the negative sample bias STM image of Figure 4 onto its positive bias counterpart, we found that the blue circle is positioned on the small depression, similar to the one discussed in Figure 2B. We ascribe the more metallic curve (red triangle) to the sulfur-head of the molecule and the other curve (blue circle) to the tail of the octanethiolate. So while it is hard to discriminate between the thiolate and the octane part of the adsorbed molecule by just STM, the combination with scanning tunneling spectroscopy (STS) on different parts of a molecule allows one to discriminate between the two parts. Additionally, we measured the current as a function of time on top of adsorbed molecules with the feedback of the electronics in the open-loop configuration. Therefore the separation distance between the tip and the surface of the sample was kept constant during these experiments. So if there is a change in the geometry of the adsorbed molecule, which is in between the tip and the surface of the sample, this should be immediately visible in the measured current. 1150
In Figure 5A, we have plotted an I(t) transient, measured as described above, with a bias VS ) 1.5 V. Clearly shown in the graph is the dramatic increase in the current from 1 to 11 nA. After being constant for more than 10 s, the current drops back abruptly to its original value of 1 nA. During other experiments, we even measured current jumps from 1 to 15 nA and also high current residence times up to 40 s. In these experiments, we only observed a current jump upward and no return to 1 nA. Although we have measured different maximum currents and also different residence times at high current, we are dealing with the same process in all these cases. The simple fact that the current returns to its starting value (1 nA) excludes a permanent change in the geometry of the adsorbed molecule. This is confirmed by comparison of STM images taken both before and after recording the current traces. As a most likely explanation of the sudden increase/decrease in the current signal we propose the following scenario: during tunneling to the molecule, the tail of the octanethiolate molecule flips up and jumps into contact with the apex of the STM tip. Interestingly the length of the tail of the octanethiolate molecule is about the same as the width of the tunnel gap, that is, about 1 nm. Hence, the tail of the octanethiolate molecule is long enough to contact the STM tip. From our experiments, we extract a single-molecule resistance from 100-150 MΩ, which is in excellent agreement with values reported in literature (ref 14 and references therein). It should be pointed out here that at the side of Nano Lett., Vol. 9, No. 3, 2009
STM tip we have not created a chemisorbed contact since the tail of the octanethiolate detaches typically after 10-40 s. Since the resistance we measured falls in the range of the reported values for both C6 and C8-thiolates, we cannot determine whether the current flows through the whole molecule or just through a major part of the molecule. Sudden current jumps due to spontaneous flipping of an adsorbed molecule were reported previously.21,22 However, in these experiments the current changes were much smaller and the average residence time of the molecule in contact with the tip was much shorter. It should be pointed out here that it does not make sense to vary the current set point, since an increase (decrease) of the current by a factor of 10 results in a decrease (increase) of tip-substrate by only 0.1 nm. In summary, we have studied the adsorption of octanethiol molecules on a Pt-modified Ge(001) surface with STM/STS. The octanethiolate molecules adsorb almost exclusively on the Pt chains. By measuring the I(V) characteristics on both the head and tail of the adsorbed molecule, we were able to discriminate between the two different parts. Finally, during open-loop I(t) experiments above the adsorbed molecule, the molecule often wags its tail and thereby contacting the STMtip. We were able to keep contact between the tip and the molecule for tens of seconds. Throughout this contact we measured the current through the molecule resulting in a measured single-molecule resistance of 100-150 MΩ. Acknowledgment. We thank NanoNed for their financial support. References (1) Aviram, A.; Ratner, M. A. Chem. Phys. Lett. 1974, 29, 277.
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