Surfaces by Atomic Force Microscopy - American Chemical Society

J. Phys. Chem. C 2007, 111, 17275-17284

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Nanostructuring, Imaging and Molecular Manipulation of Dithiol Monolayers on Au(111) Surfaces by Atomic Force Microscopy Jian Liang,† Luis G. Rosa,† and Giacinto Scoles*,†,‡ Department of Chemistry and the Princeton Center for Complex Materials Research, Princeton UniVersity, Princeton, New Jersey 08544, and International School for AdVanced Studies and Elettra Synchrotron Laboratory, Trieste, Italy ReceiVed: August 12, 2007

In this paper, we use atomic force microscopy (AFM) to nanostructure and image 1,10-decanedithiol (DDT) and biphenyl 4,4′-dithiol (BPDT) layers on Au(111) surfaces comparing them to those prepared by selfassembly. First, layers of dithiols are self-assembled from solution onto gold surfaces and are imaged in situ with an AFM to examine the roughness of the layers. Second, 100 nm × 100 nm monolayer patches made of dithiol molecules are nanografted into a self-assembled monolayer (SAM) inert matrix made of 1-decanethiol (DT). Although nanografting of thiols routinely generates very compact layers with good height uniformity, nanostructuring of dithiols using this method always yields multilayers that form through intermolecular S-S bonds. We demonstrate in this paper two possible ways of tailoring, layer by layer, the structure of dithiols. First, we form multilayers by nanografting, using then the AFM tip to gradually shave away the top layers. In the second way, we add antioxidant to the solution while doing nanografting to suppress the oxidative coupling of the -SH groups. We found that nanografting in the presence of excess amounts of antioxidant can produce monolayers of dithiols. The so-produced DDT monolayer patches are lower than what can be calculated by the 30° tilt model, while the height of nanografted patches of BPDT closely corresponds to a vertical configuration. Finally, we use conductive-probe AFM to investigate the electron tunneling properties through BPDT multilayers. The molecules in these layers turn out to behave as conductive molecular wires and making these nanostructures good candidates for constructing molecular electronic devices.

Introduction Self-assembled monolayers (SAMs) are organic thin films grown on different substrates using molecules with a head group (typically an SH group) that has high vertical affinity to the surface (typically gold) but high lateral mobility allowing the lateral packing of the molecules to be stabilized by intermolecular van der Waals forces. R, ω-dithiol SAMs are considered promising building blocks for constructing nanodevices because the SH groups at the top of the hydrocarbon chains offer the possibility to form connections to other functional units, for example, metal or metal oxide clusters,1-7 thin films8-12 and other functional molecules.13,14 A number of dithiols, both aromatic and aliphatic ones, have been self-assembled in different nanojunctions, especially for their potential applications in molecular electronics.15,16 In the past 10 years, many papers have been published reporting the structure17-21 and spectroscopic properties22-27 of dithiol SAMs. This area is, however, still partially open because of the fact that some of those papers have generated considerable controversy. For example, the existence of a -SH group at each end of the molecule raises the debate on whether only one or both sulfur atoms would bind to the gold surface in the SAM. For the most common preparation method, in which the selfassembly takes place in solution, both laying-flat18,27 and standing-up18,23,26,27 conformations have been reported. Another * Corresponding author. E-mail: [email protected]. † Princeton University. ‡ International School for Advanced Studies and Elettra Synchrotron Laboratory.

question is whether or not the molecules would “polymerize” during self-assembly forming multilayers through intermolecular S-S bonds. In our previous study, no ordered standing-up structures were observed for hexanedithiol monolayers on Au(111) using diffraction techniques.17 Wo¨ll and co-workers28 pointed out that direct self-assembly of organodithiols on gold surfaces would result in disordered layers. They also suggested preparing the SAM by using dithiols with one of the SH groups being protected and then removing the protecting groups after the formation of the SAM. However, because of the limited commercial availability of such dithiols, it is necessary to find ways of preparing compact and ordered SAMs directly from dithiols perhaps tailoring the number of layers until reaching a monolayer if a multilayer is formed. Since a detailed investigation on the molecular orientation and adsorption kinetics of dithiols could shed considerable light on this subject, it could be very useful to visualize the dithiol thin films at the nanometer scale throughout their preparation process. In this paper, we use atomic force microscopy (AFM) to monitor the topography of the dithiol SAMs in situ during their growth, so to resolve some of the questions left open by the previous work. As reliable topographic measurements on SAMs are difficult because of the intrinsic lack of a reference level, we decided trying to solve this problem by using a nanostructuring method, called nanografting, which provides an easily available reference height for this type of measurement. In nanografting, the AFM tip is used to shear away the thiols of the monolayer from the gold substrate by applying a relatively large force load while scanning over a flat region of interest in the presence of

10.1021/jp076470y CCC: $37.00 © 2007 American Chemical Society Published on Web 10/10/2007

17276 J. Phys. Chem. C, Vol. 111, No. 46, 2007

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Figure 1. Structure and calculated dimensions of the molecules.

molecules different than those of the initial SAM, which can be the solute in a neighboring solution or the adsorbate on the tip itself (dip-pen grafting).29,30 As revealed by a lower force scan, a new patch of SAM composed of the new molecules is then formed on the exposed gold sites, surrounded by the initial SAM. There is evidence in the literature31,32 that nanografting can accelerate the kinetics of SAM growth and prepare SAMs with fewer defects than those prepared by self-assembly. Moreover, numerous experiments confirm that molecules in SAMs fabricated by nanografting assume well-predictable conformations.30 In this paper, we apply nanografting to fabricating patches of dithiol layers in an inert matrix, establishing that nanografting is a very good method for making densely packed dithiol layers with good height homogeneity. There have been a large number of theoretical33-40 and experimental41-47 papers investigating the charge transport properties through dithiol molecules. Using dithiols as molecular wires is particularly useful because both -SH groups can chemically link to the external electrodes. By adding a small bias to the electrodes and measuring the current tunneling through the gap filled with the organic medium, the conductivity of the nanometer scale metal-to-molecule-to-metal junction is obtained. In this paper, we utilize conductive-probe AFM to measure the tunneling current through the biphenyl 4,4′-dithiol (BPDT, Figure 1) patch after nanografting is performed. The resulting current is recorded for every (x, y) position. The current maps, together with the topography images acquired by the conventional AFM laser-deflection feedback, are simultaneously obtained. We found that, compared with the alkanethiol molecule, the BPDT molecule is more conductive. Interestingly, for the latter molecule, patches containing 3 layers and 2 layers have very similar conductance, which indicates that the bottleneck for the tunneling is located in the vacuum gap. Experimental Section Sample Preparation. The Au(111) surface was prepared by thermal evaporation on a mica substrate in vacuum at a background pressure of 1 × 10-7 mbar. The mica (Ruby Muscovite mica, S & J Trading) had been outgassed at 563 K for 5 h and was kept at the same temperature during the evaporation. Typically, 1000 Å of gold (CERAC, 99.999% purity) was deposited on freshly cleaved mica at the rate of 0.2-0.3 Å/s. During gold deposition, the vacuum increased to and remained at 6∼8 × 10-7 mbar. After metallization, the Aucoated mica was allowed to cool down to room temperature. The chamber was then filled with nitrogen and the sample was taken out and immediately immersed into pure 2-butanol (Sigma-Aldrich, 99.5% purity) or a thiol/2-butanol solution. Before AFM characterization, the sample was rinsed with

absolute ethanol (AAPER Alcohol and Chemical Co.) and dried by a gentle flow of nitrogen. As revealed by the AFM, the gold surface was composed of gold “mesas” with atomically flat tops as large as ∼300 nm in diameter. The flat top was, of course, the region on which we performed the nanografting experiments. All purchased chemicals were used without further purification. The 1-decanethiol (DT) was purchased from Aldrich (96% purity), and the 1,10-decanedithiol (DDT) was from Alfa Aesar (95% purity). The BPDT was synthesized in the way described elsewhere.48 Prior to use, all of the dithiols were stored in a N2 atmosphere. AFM Characterization. Nanostructuring and imaging were both carried out using a Digital Instruments MultiMode AFM (Santa Barbara, CA) with a Nanoscope IIIa controller and a current sensor with a detection limit of ∼50 fA. The process of nanoshaving and nanografting has been described extensively elsewhere.29,30 Basically, an AFM tip is used, at low force, to image the surface morphology and select a flat region. Nanoshaving is performed in pure ethanol in a liquid cell to determine the thickness of the initial SAM. In different experimental runs, 100 nm × 100 nm patches of the second molecule were nanografted in air or in solution. When nanografting in solution, the concentration of thiols is 0.1 mM. Dip-pen nanografting and current measurements were carried out under ambient conditions in an acoustic isolation box (Molecular Imaging) where the humidity was kept low at 23 ( 1% (T ) 24 ( 2 °C). When measuring the tunneling current, a new and clean conductive tip is used to visualize the region containing the nanografted patch after applying a voltage bias. Nanoshaving and nanografting were performed with commercially available V-shaped cantilevers with oxide-sharpened Si3N4 tips (NPS, Veeco Instruments) while current measurements were carried out with single-beam Pt-Ir coated Si probes (SCM-PIC, Veeco Instruments). The applied load was controlled to be less than 5 nN during imaging and current measurements unless otherwise specified, while, for nanografting, loads as large as 50 nN were used. Typical scan rates for nanografting and current measurement were 2 and 0.5 Hz, respectively. Finally, cross-sectional height and amplitude analysis were carried out on the acquired topographical and current images. The softwares used for the data processing were Nanoscope (Veeco Instruments, Version 5.12r5) and WSxM (Nanotec Electronica, Version 4.0).49 Results and Discussions (a) Self-Assembly of DDT and BPDT from Solution on Gold Surfaces. Figure 2 displays some selected images representing typical moments during the self-assembly of DDT from a 0.1 mM 2-butanol solution onto an Au(111) surface. The normal force applied by the AFM probe was controlled to

Dithiol Monolayers on Au(111) Surfaces

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Figure 2. Series of images taken during the DDT layer formation process from solution onto a Au(111) surface. Because of the drift of the AFM, the images are not focused on exactly the same region. Parts b′ and c′ are the corresponding line profiles of b and c, respectively.

remain below 1 nN when imaging, and pure 2-butanol was added to the solution when necessary to compensate for the slight evaporation of the solvent during the experiment. On top of a clean gold nugget, the surface is atomically flat with only a few steps (Figure 2a). A few minutes after the dithiol solution is injected, islands ranging from 10 to 50 nm in lateral dimension begin to appear. As the adsorption process advances, the height of these islands gradually increases. When the height exceeds half of the length of the dithiol, one of the sulfur atoms must be free from the gold surface (Figure 2b). However, the detection of the free thiol groups is not solid evidence for the ordered structure of a dithiol monolayer. The molecular chains can bend and tangle which results in a poor degree of orientation.28 With continued exposure to the thiols, the coverage of the islands increases through both nucleation and growth of previously formed nuclei. Starting from the images taken after 40 min, some islands (Figure 2c) grow up to and even higher than the full length of a molecule (1.76 nm, see Figure 1). This is different from the growth of monothiols and can be explained by the linking of two molecules through the S-S bond. Gradually, the size and height of these islands increase. After 24 h, they can cover up to a 10% area of the surface (Figure 2d) and cannot be removed or reduced in size by being rinsed with pure ethanol or 2-butanol. It is worth noting that the region excluding those islands may actually contain a multilayer of dithiols because of the lack of height reference in the images. The results suggest that, when the thin film is prepared in solution, the cross linking of dithiol molecules is not negligible because of the active -SH groups at the free ends. The oxidative coupling of the intermolecular S-S bond (see also sections b and c) causes the nucleation of molecules on top of the first layer. However, under our experimental conditions, whether the second layer starts via S-S bonds before or after the first layer is completed is still unknown. It is, in principle, possible that even when the height is only half of the standing-up, singlemolecule height some S-S bonds are already forming.

Following the same protocol, we carried out the experiment on BPDT, and the results are shown in Figure 3. Similar to DDT, the BPDT molecules also form islands that are much taller than a monolayer, indicating that BPDT can also polymerize through S-S coupling. The rate of formation of the islands is even quicker than that of DDT and after several hours the initially separated islands begin to merge into bigger ones and cover the whole surface. Wang et al.50 tried to prevent the S-S coupling of dithiols by preparing the SAMs under an inert gas atmosphere. This method is, however, questionable because in a typical N2 filled glove box the O2 level can reach up to 100 ppm,50 and although the amount is small from a macroscopic view, it may not be negligible for two-dimensional (2-D) SAMs. We test this method by self-assembling the DDT and BPDT from solution onto the gold surface in a N2 atmosphere. The solvent used had also been vigorously bubbled with N2 for 24 h before the dithols were dissolved in it. The self-assembly was allowed for 48 h, and then, the sample was taken out, rinsed with degassed solvent, and dried under N2 protection. It turned out that, even under these conditions, both dithiols do not form good monolayers (Figure 4). Instead, numerous bumps are observed on the surface indicating that polymerization still occurs. These results suggest that self-assembly of dithiol under inert gas protection is questionable probably because (1) there is always a trace amount of impurities which are enough to initiate the oxidative coupling because of the extremely limited number of molecules in a monolayer and (2) although the whole preparation process is carried out in N2, the sample is exposed to air in the later imaging by AFM (which is also commonly the case in other applications or further treatment to the sample50). Unfortunately, for both DDT and BPDT, the nucleation on the gold surface is incomplete and randomly distributed rather than ordered and compact and requires us to develop another way of fabricating fine-quality dithiol layers on gold surfaces. Nanografting, which offers the opportunity of making patches


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Figure 3. Series of images taken during the BPDT layer formation process from solution onto Au(111). Parts b′ and d′ are the corresponding line profiles of b and d, respectively.

Figure 4. (a) DDT and (b) BPDT layers prepared by self-assembly from solution onto Au(111) under N2 protection. Parts a′ and b′ are the corresponding line profiles of a and b, respectively.

of layers with accelerated kinetics and improved quality, will be applied to both dithiols in the following sections of this paper. (b) Dip-Pen Grafting in Air and Nanografting in Solution of BPDT and DDT Patches into a DT SAM. Measuring the thickness of a SAM made of linear molecules, if done with enough precision, is useful for determining the configuration of the molecules in it and that of molecules contained in patches suitably nanografted in it. If the SAM is in contact with a pure

solvent while the thiols within a nanometer-sized region are shaved away by the AFM tip under relatively high force loads, then the displaced molecules ejected into the solution become extremely diluted and have little opportunity of returning to the gold surface. The process is called nanoshaving, and the depth of the resulting “hole” measures the thickness of the SAM; the technique can be a useful tool to determine the conformation of the molecules in it.51,52 In our experiments, before nanoshav-



Figure 5. Typical height images of patches (100 nm × 100 nm) made by dip-pen nanografting in the DT matrix and the corresponding line profiles. a, a′: BPDT, ∆h ) 2.2 ( 0.2 nm. b, b′: DDT, ∆h ) 0.8 ( 0.2 nm. Histogram c is constructed on height differences between BPDT and DT obtained from 232 BPDT patches. The envelope of the histogram, the two fitted Gaussian peaks, and the sum of the two fitted peaks are shown in blue, black, and green, respectively. Histogram d is constructed on height differences obtained from 49 DDT patches. The fitted Gaussian peak is shown in black.

ing was performed, the DT monolayer had been allowed to form on the Au(111) surface for at least 48 h. The cross-sectional analysis, which makes direct depth measurements of the “holes” by producing line profiles, indicates that the thickness of the SAM is 1.4 ( 0.2 nm, in good agreement with the calculated value of 1.36 nm which is obtained by assuming that the hydrocarbon chains are fully extended and tilted by approximately 30° with respect to the surface normal (see Figure 1).53,54 A significant advantage of nanografting is that it can make compact layers of thiols on the gold surface in minutes instead

of days as required by self-assembly. The accelerated kinetics may be due to a space-confinement effect30 or to local pressures, temperatures, or both developed during the process that are high enough to “anneal” the molecules to form a compact layer. Additionally, when nanografting into a SAM matrix composed of molecules with a well-known configuration, the latter provide a height reference for the new ones in the grafted patch. In this section, we first use dip-pen nanografting in which, just before the nanofabrication, the AFM probe is immersed in a 1 mM dithiol/ethanol solution for ∼1 min and then dried with air or nitrogen for 45 min. The tip with dithiols adsorbed on it act as


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Figure 6. Three-dimensional images of (a) a BPDT patch and (b) a DDT patch within a DT matrix being shaved down by the AFM tip (normal load ≈ 8 nN). The height differences (with reference to DT) vs the number of scans of (c) the BPDT patch and (d) the DDT patch.

both the imaging tool and the reservoir for molecule transfer during nanografting. From the thickness of the matrix SAM made of DT determined above and the height difference between the patch and the matrix, the vertical height of the dithiols can be calculated. For BPDT, the tilt angle of the molecules in numerous biphenyl thiolate SAMs was determined by infrared spectra48 and diffraction studies55 to be less than 15° with respect to the surface normal. The BPDT monolayer thickness measured by nanoshaving and nanografting51 ranges from 1.12 to 1.32 nm, suggesting tilt angles ranging from 26 to 12°, respectively. The typical patterns we obtained are, however, significantly higher (lighter in the images) than the matrix (Figure 5a,a’). By considering the atomic defects on the gold surface and the imperfection and compressibility of the SAM, the error of height measurement should be no more than 0.2-0.3 nm (the step height of Au(111) is 0.23 nm). Even taking into account the deviation of the calculated model from the real nature of the SAM, the measured height difference is larger by an amount that is far outside the error limits. A reasonable explanation, which is also supported by other spectroscopic studies,56,57 is that the patch actually contains a multilayer instead

of a monolayer of BPDT. In order to confirm or contradict this hypothesis, we constructed the histogram of the height difference between the BPDT and the DT by creating and measuring more than 200 patches (Figure 5c). The envelope curve of the histogram can be fitted with two Gaussian peaks at 1.3 and 2.3 nm. The calculated thickness of a vertical bilayer and a trilayer of BPDT can be estimated at 2.3∼2.6 nm and 3.4∼3.9 nm,51,57 which is 0.9∼1.3 nm and 2.0∼2.5 nm higher than the DT monolayer, respectively, and completely consistent with our data. Although the structure of the multilayer is still poorly understood, a very large fraction of the BPDT patches we made had heights that were within the errors of those calculated for films containing 2 or 3 layers of BPDT. Dip-pen grafting of DDT in the DT matrix also results in polymerization of the dithiol molecule (Figure 5b,b’). However, from the histograms (Figure 5d), we can see that DDT is less likely to form more than 2 layers than BPDT. It is possible that the rigid structure and the π stacking of the phenyl rings in BPDT helps the molecule to form a more compact and ordered bottom layer. Consequently, the layer on top of it replicates the order and enhances the multilayer formation. On the other



Figure 7. Nanografting of BPDT and DDT patches into a DT SAM with addition of tri-n-butylphosphine. Histograms of height differences between the grafted patches and the matrix of (a) DDT and (b) BPDT are shown. (Without antioxidant: CBPDT or DDT ) 0.1 mM, Cphosphine ) 0. With equimolar antioxidant: CBPDT or DDT ) Cphosphine ) 0.1 mM. With excess antioxidant: CBPDT or DDT ) 0.1 mM, Cphosphine ) 4 mM.)

hand, the alkyl chains in the DDT layer are more flexible and less ordered because the bulky sulfur atoms prevent the chains from getting close to each other, which makes the forming of extra layers less favorable. Similar to dip-pen grafting in air, nanografting in solution also make patches containing multilayers (Figure 7). The solution being used, without deoxygenation, contains a certain amount of O2 which helps the S-S bond formation through the following overall reaction:

2R-SH + [O] f R-S-S-R

(1)

Figure 7 also indicates that in solution DDT has more chance of forming more than two layers than it does in air for reasons that are not clear at this point. (c) Tailoring the Layer-by-Layer Structure of the Dithiol Multilayers by AFM Tip Shaving in Air and by Adding Antioxidant during Nanografting in Solution. Although nanografting cannot eliminate the oxidative coupling of dithiols, the patches of layers are usually more uniform in height, which offer a better starting point for tailoring the layer-by-layer structure. First, we tried to shave away the extra layers in air with the AFM tip by applying a larger force load to the surface (but less than that in nanografting) and continuously scan the region containing the patch after dip-pen nanografting was

performed. We found that forces between 5 and 10 nN are not large enough for the tip to touch the gold but large enough to deform the top layer and gradually remove molecules from it. Figure 6 demonstrates how a BPDT patch and a DDT patch is lowered by continuous scanning. The results indicate that the patches can be trimmed down to approximately monolayer height, but to quantize the number of layers is not at all simple, suggesting that the shaving of the top layer is not always complete. For both BPDT and DDT, the height changes slow down as the patch lowers, probably because the bottom layer is surrounded by the DT matrix and is less accessible to the AFM tip. It is also worth noting that the height of the DDT patch decreases more quickly than that of BPDT, implying that the BPDT molecules are more orderly packed and harder to shave. If we consider the height of the layers when the height change reaches the plateau (the dotted lines in Figure 6c,d) being that of a monolayer, the monolayer heights of BPDT and DDT are ∼0.1 nm lower and ∼0.5 nm lower than the DT matrix, respectively, which is consistent with the results obtained by nanografting in solution in the presence of excess amounts of antioxidant (see below). Since the multilayer formation is presumably due to the oxidative coupling of the S-S bond, it is reasonable to think that the presence of some antioxidant should suppress the polymerization. Compared with the purging methods, it is also often referred to as the chemical methods.58 Weckenmann et al.59 suggested multilayer formation of BPDT on polycrystalline silver can be inhibited by adding tri-n-butylphosphine during self-assembly. The phosphine can deplete the oxygen in the solution and generate a reducing environment. We do not recommend preparing dithiol layers by self-assembly with trin-butylphosphine in ambient conditions, though, because the relatively long period of time of self-assembly would waste large amounts of antioxidant. Nanografting, with its significantly accelerated kinetics compared with self-assembly, offers exactly the right tool for fabricating dithiol layers with the oxygen scavengers. In the present paper, we report on a series of experiments carried out in solution with excess, equimolar (to the dithiol), and without tri-n-butylphosphine. The results are shown in Figure 7. Figure 7a shows that DDT patches generated by nanografting without any antioxidant have broad height distribution and do not show any clear peaks. Adding equimolar antioxidant can lower the heights a little, but the majority of the patches are still higher than that of a monolayer. The results are not surprising because oxygen has considerable solubility in alcohol (e.g., 2 mM in ethanol, at 20 °C).60 Additionally, the number of molecules in the 2-D SAM (∼104 molecules within a 100 nm × 100 nm patch) is very small, even tiny amounts of oxygen left in the solution can be sufficient to initiate the oxidative coupling of the -SH groups. Only adding an excess amount of antioxidant can produce strictly oxygen-free conditions and completely suppress the multilayer formation (it is worth noting that the tri-n-butylphosphine solution should be freshly made). The histogram then shows a sharp peak at ∼0.4 nm lower than the DT matrix, indicating that the dithiols probably form a monolayer, but with the tilt angle of the alkyl chains larger than 30° (otherwise the patch should be higher than DT because of the relatively large atomic radius of the sulfur), or with a less degree of order than the DT SAM. Similarly, Figure 7b demonstrates how the different concentrations of antioxidant affect the height distribution of the BPDT patches. The peak position when adding an excess amount of antioxidant is ∼0.1 nm lower than the DT matrix, agreeing quite


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Figure 8. Simultaneous height (a, a′) and current (b, b′) images of a BPDT patch (100 nm × 100 nm) made by nanografting in the DT matrix and the corresponding line profiles: ∆h ) 2.2 ( 0.2 nm, ∆I ) 90 ( 10 pA (bias ) +1 V). On the right, the height profile drops because it is near the edge of the gold microcrystal. When the bias changes to -1 V, the contrast of the current images reverses (c, c′), indicating the flow of current in the opposite direction.

well with the calculated value. It proves again for the BPDT it is easier than for DDT to form a monolayer with vertical molecular configuration, probably because of the rigid structure of the phenyl rings and the π stacking of the molecules. Interestingly, adding excess antioxidant after nanografting can reduce the height of the multilayer patches but cannot reach that of a monolayer. This result suggests that the molecules inside the remaining patch are probably densely packed (crystallized) and/or the S-S bonds between the bottom two layers are protected by the DT matrix, thus less accessible to the antioxidant.

(d) Simultaneous Height and Current Measurements for BPDT Patches into a DT SAM. In this section, we discuss electron tunneling measurements over BPDT multilayers. Figure 8 shows a typical pattern (named patch A) obtained by dip-pen grafting. The current images (Figure 8b,b′,c,c′) clearly show that the tunneling current through the grafted BPDT patch is substantially larger than through the DT layer. Under a bias of ( 1 V, the current difference is about 90 pA. Taking into account the fact that the tunneling distance between the tip and the gold substrate through the aromatic molecules is much longer because of the formation of the multilayer, we can safely


J. Phys. Chem. C, Vol. 111, No. 46, 2007 17283 Interestingly, after collecting the current images at biases from +1 to -1 V, the height of the patch decreased to 1.2 nm above the DT surface and became a bilayer patch, probably because of the potential, force-induced top layer desorption, or both. Surprisingly, this 0.8 nm reduction of the tunneling distance does not result in significant current intensity change from the values obtained with patch A and the initial patch B containing 3 layers. According to the tunneling mechanism, the current (I) should decay exponentially with the increment of distance (d):

I ∝ e-βd

Figure 9. Comparison of the I-V curves derived from different BPDT patches in a DT SAM. Black squares: patch A, averaged from three data points at each bias. Red circles: patch B (in a trilayer form). Green triangles: patch B (after being shaved down to a bilayer), averaged from two data points at each bias. The error bars present the standard deviation.

conclude that BPDT molecules are more “conductive” than the DT. This result is in agreement with the observation made by other researchers that conjugated molecular wires are more conductive than saturated ones with similar lengths.61,62 Although the mechanism of charge transport through a metalmolecule-metal junction has not yet been conclusively established,63 it is generally accepted that, if the energy gap between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) is small, this helps both the charge transport through the molecule and the molecule mediate electron tunneling. By drawing line profiles across the current images of patch A under different biases using the same tip, we can obtain the current difference versus bias relationship shown as the black squares in Figure 9. The bias was started at +1 V (the tip is positive with respect to the substrate) and decreased stepwise with intervals of 100 mV after capturing a current image at each bias until -1 V was reached. Then the bias was increased back to +1 V again with the same interval to complete another voltage scan. Three scans were performed, and three current images and line profiles as shown in Figure 8 were recorded at each bias. The height of patch A did not change after the current measurement indicating that the layers were not disturbed by the presence of the field. The plot is asymmetric indicating that the current increases more rapidly at negative biases and is almost constant between -0.5 and -1 V, while at positive biases the current increases at a relatively steady rate. This asymmetry suggests asymmetric alignment of the molecular orbital of BPDT close to the Fermi level of the electrodes. The current differences at ( 1 V, however, both level off at around 80 pA regardless of the polarity. The red circles in Figure 9 refer to another plot of current difference versus bias from another experimental run on a different patch (named patch B) which is higher than DT by 2.0 nm and probably contains a trilayer of BPDT. The experimental results of patch B, which were obtained using a different AFM tip (but of the same type), show current intensities that are very close to those of patch A and follow the same trend. This fact leads to the conclusion that the contact properties of the same type of tips probably are very similar to each other.

where β is the decay constant. Assuming β ) 0.4 Å-1 which is typical for aromatic thiols,61 the current should have increased by e0.4×8 ) 25 times. Instead, the current measurements with the same tip after the patch being shaved down (green triangles in Figure 9) demonstrate that the I-V curves of trilayers and bilayers of BPDT overlap within the error bars, indicating that the “resistance” of the trilayer is not significantly greater than the bilayer. This can be justified by supposing the total resistance of the junction is, actually, dominated by the value of the density of states of the top (i.e., near the vacuum gap) groups in the molecule, which is also consistent with our recent report64 on other short molecules sandwiched between the substrate and the AFM probe. The height of patch B further reduced a little to ∼0.9-1 nm higher than DT after the new current measurement but still within the acceptable error of 2 layers. Conclusions We have shown that the dithiol molecules, including the aliphatic dithiol and the aromatic dithiol, cannot form compact and smooth monolayers by self-assembly in solution in ambient conditions. The oxidative coupling of the thiol groups between molecules results in rough layers with randomly distributed islands. Purging the solution with inert gas during self-assembly cannot significantly improve the situation. Nanografting can successfully be applied to fabricate nanometer-sized patches of SAMs of dithiols with more homogeneous height and faster kinetics. To fabricate monolayer patches, we can either shave away in air the extra layers using an AFM tip or add tri-nbutylphosphine as antioxidant when nanografting is performed in solution. Continuous scanning using moderate force can lower the patch gradually until a monolayer height is reached. Controlling, however, the number of layers beyond the first remains a problem. Nanografting with antioxidant can significantly suppress the polymerization of the dithiols. Especially, in the presence of antioxidant, patches close to monolayer height can be made. The height of the BPDT monolayer prepared by this method is close to the vertically standing-up model due to the rigid structure of the phenyl rings, while the height of the aliphatic layer is lower than the stable 30°-tilted model probably because the flexible alkyl chain and the bulky S atom at the ω position hinder the ordered packing of the molecules. The electrical measurements combining nanografting and conductiveprobe AFM63 indicate that the number of layers of BPDT does not significantly affect the conductivity of these “molecular wires” because the total resistance of the molecular junction in our experiment is dominated by the contact resistance. These results provide new insight to the preparation and properties of dithiol monolayers on gold surfaces. Acknowledgment. This work was supported by the National Science Foundation (MRSEC Program) through the Princeton Center for Complex Materials (DMR 0213706) and partially supported by DOE under Grant DE-FG02-93ER45503. We

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