ARTICLE pubs.acs.org/JPCC
Break-Junction Experiments on Acetyl-Protected Conjugated Dithiols under Different Environmental Conditions ngeles Herranz,‡ M. Teresa Gonzalez,*,† Edmund Leary,†,^ Raul García,‡ Prashant Verma,† M. A § †,‡ †,§,|| Gabino Rubio-Bollinger, Nazario Martín, and Nicolas Agraït †
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Instituto Madrile~ no de Estudios Avanzados en Nanociencia (IMDEA-Nanociencia), Ciudad Universitaria de Cantoblanco, E-28049 Madrid, Spain ‡ Departamento de Química Organica, Universidad Complutense de Madrid, 28040 Madrid, Spain § Departamento de Física de la Materia Condensada, and Instituto “Nicolas Cabrera”, Universidad Autonoma de Madrid, E-28049 Madrid, Spain ABSTRACT: Using an oligo(phenylene ethynylene) prototype molecular wire, we study the optimum preparation conditions for acetyl-protected conjugated dithiols to be tested in a breakjunction experiment. Even without adding a deprotection agent, molecular junctions are formed in reasonable percentage in all experiments, both under solvent and in air. Under liquid conditions, a higher rate of junction formation is achieved by deprotecting the molecules with a specific agent. A similar rate can be achieved by performing the experiment in air without adding a deprotection agent. No significant variations of the conductance values of the molecule were observed with the change of environment or deprotection conditions.
’ INTRODUCTION Oligo(phenylene ethynylene) (OPE) compounds are prototypes of conjugated molecular wires, which can display a variety of electrical behavior by simple changes in their molecular structure, or replacement of the internal benzene ring by other active moieties.1 7 A diversity of related molecules have been synthesized2 14 and characterized in selfassembled monolayers (SAMs)1,8,15 19 and at the single molecule level.2,11,20 31 Although for this kind of molecule several binding groups have been tested,8,29,30,32 34 thiols are the most common choice, providing a covalent linkage to the electrodes, which translates into a high final molecular conductance. The main drawback of conjugated thiols is that they can oxidize to disulfides in the presence of oxygen,8 and, therefore, acetyl-protected thiols are typically used for OPE-related molecules. These can be deprotected to free thiols upon exposure to an acidic or basic environment.35 37 On the other hand, spontaneous cleavage of the acetyl group can also occur in the vicinity of a gold surface. Acetyl-protected OPE dithiols have been observed to form SAMs without the need of a deprotection agent, when a gold substrate is exposed to a high concentrated solution of the molecule for long enough periods of time.2,8,37 The aim in a break-junction experiment is to connect a single molecule between two metallic electrodes. Using an scanning tunnelling microscope (STM), a gold tip is repeatedly driven in and out of contact with a gold substrate exposed to the molecules to be studied.38 To increase the probability of forming a molecular junction, it is required to have enough molecules available in the vicinity of the electrodes, but not r 2011 American Chemical Society
necessarily ordered in a SAM. In the literature, different procedures to expose the electrodes to OPE-related molecules for a break-junction experiment have been described.2,11,20 23,26,31 However, there is a lack of a systematic comparison of the probability of forming a molecular junction, or of its properties, depending on the deprotection procedure. In this Article, we address this issue by comparing break-junction results for an acetyl-protected OPE dithiol performed in three different environments: trichlorobenzene (TCB), mesitylene:tethrahydrofuran (4:1) (Mes/THF) (two commonly used solvents), and air, with and without specific deprotection by tetrabutylammonium hydroxide (Bu4NOH). We conclude that molecular junctions are formed in reasonable percentage in all experiments, even without specific deprotection both under liquid and in air, the percentage being significantly larger in air. When deprotecting, the percentage of successful experiments under liquid increases to values similar to those in air. We also observe that specific deprotection favors single molecule junctions over those involving the π-stacking of two molecules.
’ EXPERIMENTAL SECTION Figure 1a shows the oligo(phenylene ethynylene) dithiol studied in this work. Synthesis of the molecule was performed according to the procedure reported in the literature.39 Received: April 29, 2011 Revised: July 11, 2011 Published: July 12, 2011 17973
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Figure 1. (a) Schematics of the OPE dithiol studied molecule. (b) Examples of G versus z curves resulting from individual break-junctions experiments in the presence of the OPE dithiol without adding a deprotection agent (measurement in air).
Solvents 1,2,4-trichlorobenze, mesitylene, and tetrahydrofuran, and the deprotection agent tetrabutylammonium hydroxide, Bu4NOH (as hydrated salt), were purchased from SigmaAldrich and used without further purification. Bu4NOH was kept in the refrigerator and under Ar atmosphere. When deprotecting the molecule, solvents were purged with Ar to remove oxygen and prevent disulfides formation. Break-junction experiments were performed using a homebuilt STM optimized for room temperature measurements in air or liquid environment. Our liquid cell is made of PEEK (polyetheretherketone) and fixed tight over the gold substrate with the help of a Kalrez O-ring. This leaves a circular area of the substrate 9 mm in diameter exposed. We used commercial gold substrates on quartz (Arrandee), which were cleaned in boiling ethanol, and then flame-annealed with a butane flame. All pieces of the liquid cell in contact with solutions or the substrate were also washed several times in boiling ethanol between measurements. As STM tips, we used a 0.25 mm gold wire (99.99%) also heated with a butane flame, which was freshly cut immediately before use. For the measurements under liquid without specific deprotection, around 0.15 mL of a 1 mM solution of protected OPE (OPE-p) in TCB or Mes/THF was added in the liquid cell open to air. When deprotecting the molecule, we added 3 equiv of Bu4NOH to the OPE solution kept in a flask under Ar. We determined this to be the minimum amount necessary to produce an instantaneous change of the solution color from pale yellow to bright orange, signaling that most of the OPE molecules in solution were deprotected.37 The solution of deprotected OPE (OPE-dp) is immediately added to the STM liquid cell. A flow of Ar through an outlet at the head of the microscope ensures a high concentration of Ar in the surroundings of the cell and minimizes the oxygen content in the solution during the measurement. For the measurements in air, a just-prepared gold substrate was immersed for 20 min to 1 h in the OPE-p solution. Next, the substrate was removed from the solution and directly dried under an Ar flow.
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Figure 2. 2D histograms for a clean gold substrate in air (a), for OPE-p (protected) measured in air (b), and with the substrate immersed in the TCB solution (c). (d) 1D histograms for the same sets of data used in (a) (c). The color scale shows the number of counts normalized to the number of curves included in the histograms.
During the break-junction experiment, the tip is moved vertically in and out of contact with the substrate at a constant speed of approximately 40 nm/s, in 1 pm steps. For the conductance G = I/ V evolution measurements, a bias voltage V of 150 mV was applied between the tip and the substrate. A linear current-to-voltage converter with two amplification stages allows us to record conductance values in a window from 3 10 6 G0 to 1.5 G0 (G0 = 77.5 μS). The motion of the tip and the conductance measurement are controlled by an in-house computer program to record conductance versus tip-displacement (G versus z) curves as those of Figure 1b. Typically, when moving out of contact, we move up to 2 nm after reaching our 3 10 6 G0 limit. When in contact, the piezo moves a further 1 nm after reaching 1.5 G0. These limits guarantee that a broad gold contact is formed and broken in each cycle, and that any molecular junction is broken at the end of the cycle.
’ RESULTS AND DISCUSSION Figure 1b shows several examples of G versus z curves recorded in individual break-junction experiments. Starting from the tip forming a gold nanocontact with the substrate, and moving the tip upward (increasing z), a short plateau at 1 G0 followed by a sharp G decay signals the breakage of the gold contact. Longer plateaus at lower G values indicate the formation of molecular junctions by one or a few OPE molecules attached between the tip and the substrate. Figure 2a c shows the resulting two-dimensional (2D) histograms built from several thousand G versus z curves recorded on a clean gold substrate in air (a), after exposing the substrate to a 1 mM solution of OPE-p in TCB for 20 min to 1 h (b), and when performing the experiment with the sample immersed in the OPE-p solution (c) (without adding deprotection agent). The histograms were built using bin sizes of Δlog(G/G0) = 0.03 and Δz = 0.03 nm, after shifting all traces on the z-axis to z = 0 for G = 0.5 G0 (just after the breakage of 17974
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Figure 3. 2D histograms built from the curves showing only monotonic decay (without plateaus) (a), and the remaining (with plateaus) (b f). Percentages of included curves are indicated in brackets. (b, c, and e) 2D histograms for OPE-p (protected) in air, TCB, and Mes/THF, respectively. (d and f) 2D histogram for OPE-dp (deprotected) in TCB and Mes/THF, respectively. (g) Corresponding 1D histograms for the measurements in air and TCB. Dashed lines correspond to fits of two Gaussians to the histograms.
the tip substrate gold contact). The color scale indicates the number of counts normalized to the number of curves used to build the histograms. The same color scale has been used in all of the histograms presented in this Article. In the histograms obtained in the presence of molecules, two clear prominences stand out of the linear color band produced by the Au Au tunneling G decay: one between log(G/G0) = 3 and 4.5 (p1), and the other one extending below 4.5 down to our resolution limit (p2). These prominences indicate the preferred conductance and stretching lengths of the plateaus present in individual G versus z curves as those shown in Figure 1b. This result shows that OPE-p succeeds in forming molecular junctions without the need of adding a specific deprotection agent, both in air and under liquid conditions. Figure 2d shows the 1D histograms40 for the same sets of data. For the measurements with molecules, only subtle peaks are observed over the tunneling region (especially for the measurement in TCB), which indicates that the rate of curves with plateaus (successful molecular junction formation) is low. The use of 2D histograms, adjusting the color code to highlight the lowest probability region, has the advantage of showing clear identifiable signals even for low success rates. To estimate the rate of success in forming molecular junctions in our experiments, as well as to enhance the molecular signal in the histograms, we have divided all of the measured G versus z curves into two groups. In the first group, we have separated those
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curves that only show monotonic tunneling decay. Our criterion consists of limiting the stretching length that it takes to produce a given Δlog(G/G0) decay all along the curve. For our curves, we found that a good choice of parameters was to consider as monotonic all of the curves for which a Δlog(G/G0) decay of 0.1 is produced in less than 1.2 Å in our range of interest: from log(G/G0) = 5.3 to 2.5.41,42 We note, however, that this criterion is not absolute, and variations within reasonable boundaries of these parameters values produce differences in the number of curves in the final two groups of around 5%. The same separation criterion was used in all our measurements for the comparison between the final percentages obtained under different environmental conditions. An example of the resulting groups is shown in the histograms of Figure 3 a and b, which are built by separating the curves measured for OPE-p in air, contained in Figure 2b. Figure 3c and e shows the histogram analogous to Figure 3b for the measurement performed in the TCB and Mes/THF solutions, respectively. In these 2D histograms, as well as in the corresponding 1D histograms of Figure 3g, the molecular peaks are now prominent. For the measurements under solvent without deprotection, although the percentages of curves with plateaus (indicated in brackets in the figure) are similar for both solvents, we observe that the peaks are better defined for TCB than for Mes/THF. This difference could be related to a better solubility of OPE in Mes/THF, which favors the solvation of protected molecules rather than binding to the gold surface. The dashed lines in Figure 3g correspond to the fits of two added Gaussians to the histograms. From these fits,26 we obtained the following mean conductance values in air: Gp1 = (9 ( 3) 10 5 and Gp2 = (3 ( 2) 10 6, respectively, for the peaks labeled by p1 and p2 in the figure. For the measurement under liquid, we get values for Gp1 and Gp2 within the error bars of those of air, excluding a significant effect of the environment on the OPE molecular conductance. These values are in good agreement with those reported previously for OPE dithiol and monothiol, respectively.21,26,27,31 In the case of OPE monothiol, it has been proposed that molecular junctions can be formed by two OPEs bonded to opposite electrodes through their thiols, which interact with each other via π-stacking.27 From a simple inspection to the 2D histograms of Figures 2 and 3, we observed that the length of the plateaus contributing to the second peak (p2) can be up to 0.7 nm longer than those contributing to the first (p1). This is indeed very close to the length difference expected between a π-stacking configuration (2.91 nm) and a single dithiol molecule bonded directly between the electrodes (2.07 nm). From these results, we conclude that the OPE-p molecules studied here are forming molecular junctions both by direct covalent attachment to the electrodes at both ends of the molecule and by π-stacked arrangements, in which only one end of a molecule covalently binds to the electrodes. For the rest of this Article, we will refer to these junctions as “covalent” (p1) and “π-stacked” (p2) junctions, respectively. As only spontaneous deprotection of the acetyl group is taking place in these experiments, it is likely that a fraction of the participating OPE-p have only one thiol deprotected and hence behave as OPE monothiols. It is remarkable, however, that the acetyl group at the extreme of the molecules does not prevent an efficient π π interaction. On the other hand, the hexyloxy groups at the sides of their backbone are probably favoring the formation of π-stacked junctions. It has been reported that linear side groups favor π-stacked aggregates on conjugated oligomers 17975
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Table 1. Percentages of Molecular Junction Formation for Acetyl-Protected OPE Dithiol (p) and after Deprotection with Bu4NOH (dp) in Three Different Environmentsa total
covalent
π-stacking 13 ( 4%
air
p
34 ( 8%
25 ( 9%
TCB
p
15 ( 7%
8 ( 5%
8 ( 6%
dp
37 ( 5%
24 ( 7%
16 ( 6%
p
12 ( 2%
7 ( 2%
6 ( 3%
dp
39 ( 5%
27 ( 7%
15 ( 5%
Mes/THF a
Percentages are related to the total number of measured curves. The first column summarizes the percentages of curves with plateaus in the total range between log(G/G0) = 5.3 and 2.5. The second and third columns include those curves with plateaus between log(G/G0) = 4.5 and 2.5, and between log(G/G0) = 5.3 and 4.5, respectively. Curves with plateaus in both conductance ranges are considered in both of the last two columns.
(see, for example, Hoeben et al.43 and references therein), while bulkier side groups can prevent π-stacking in this kind of molecule.31,43 Finally, we repeated our experiments adding 3 equiv of Bu4NOH to the OPE solution. The overall shape of the 2D histograms does not change, as shown in Figure 3d and f, but now the rate of curves with plateaus increases to values similar to those obtained in air. The final percentages of curves with plateaus obtained under different environmental conditions are also summarized in the first column of Table 1. In some cases, additional Bu4NOH was needed to observe a clear change of the rate of successful curves. In any case, percentages of molecular junction formation larger than the ones reported in Table 1 were not observed upon further additions of Bu4NOH. Without deprotection, the percentage of curves with plateaus is more than doubled when the experiments are performed with the sample in air instead of immersed in the molecular solution. This seems to indicate that the number of doubly deprotected molecules on the electrodes in air is significantly higher than in solution experiments. Reasons for this include the fact that, in the absence of deprotection agent, the rate of molecules spontaneously deprotected and adsorbed to the gold electrodes is low, and most of these molecules will retain one acetyl group.37 In addition, we expect to have an exchange of the molecules between the substrate and the solution,4,24,44 which further reduces the number of available molecules to form molecular junctions. This exchange process is probably favored by the high number steps and defect sites of the electrodes in a breakjunction experiment. Conversely, for the measurements performed in air, as the solvent is evaporated, molecules in solution become confined to the gold surface. In this case, the larger amount of water coadsorbed on the surface can speed up the hydrolysis of the acetate group, with the gold acting to catalyze or drive the reaction. In addition, in air, there is no exchange of the molecules with the solution. The result is that the density of doubly deprotected molecules on the surface increases significantly, and the probability of forming a molecular junction in a break-junction experiment in air is, in fact, the same as that in liquid when the molecules are deprotected with the help of Bu4NOH. To study separately the effect of the environmental conditions and deprotection on the formation of covalent and π-stacked junctions, we divided our G versus z curves further into three
Figure 4. 2D histograms built from curves showing only π-stacked junctions (a), only covalent junctions (b), and both (c), for OPE-p (protected) in air. Percentages are referred to the number of curves with plateaus. (d) Junction length distribution for the same sets of curves included in (a) (c).
groups: those curves with only plateaus between 5.3 and 4.5 (only π-stacked junctions), those with only plateaus between 4.5 and 2.5 (only covalent junctions), and those with plateaus in both intervals. The resulting histograms for the measurements performed in air are shown in Figure 4. We observe no correlation between the presence of plateaus in the two intervals (see also examples of curves showing plateaus in only one of these intervals or in both in Figure 1b). This suggests that these plateaus originate from two independent junction formation mechanisms, which can take place separately or coexist in the same junction. In the latter case, at least three molecules participate in the molecular junction: one molecule bound covalently bridging the electrodes close to their apexes, and the other two overlapping each other, and bound only by one end to opposite electrodes,27 further from the apexes of our sharp electrodes. Upon elongation of the electrodes, we see the signal of the π-stacked junction only when the covalent one breaks first. We expect the probability of this event to be low, which is in agreement with the low observed rate of curves with plateaus in both intervals (considered in both the last two columns of Table 1). When measuring OPE-p in air, a covalent junction is formed in approximately 70% of the observed junctions (groups b and c in Figure 4). Table 1 summarizes the final percentages (referred to the total number of measured curves) of observed covalent and π-stacked junctions. In liquid, we observe that the rate of junction formation increases significantly more for covalent than for π-stacked junctions, when adding a deprotection agent. We conclude therefore that the addition of deprotection favors the formation of covalent junctions over π-stacking ones. On the other hand, the increase of π-stacked junctions in air or when adding Bu4NOH can be explained by the presence of a larger amount of polar molecules (water and/or Bu4NOH), which are known to favor π π interactions (Hoeben et al.43 and references therein). It is remarkable, however, that the effect of adding deprotection agent in solution is equivalent to perform the experiment in air without deprotection. Without causing any observable deterioration of the sample molecules, performing the 17976
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The Journal of Physical Chemistry C experiment in air offers a simple procedure of increasing the rate of molecular junction formation and favoring covalent junctions over π-stacking ones. Finally, Figure 4d shows the distribution of the junction stretching length Ls, defined as the total length interval between log(G/G0) = 0.5 and 5.3. The mean Ls values (maxima of the distributions) are lower than the molecular length. This is not surprising as, in break-junctions experiments, the gold electrodes retract just after the breakage of the gold nanocontact, due both to the elastic relaxation of the stretched Au Au bonds during the pulling and to rearrangements of the Au atoms at the apexes. This retraction is reflected in the G versus z curves as a sudden decay of conductance just after the plateau at 1 G0 (see Figure 1b) and means that the real gap between the electrodes apexes is, on average, 4 5 Å longer than the recorded Δz = z z(G0) of the piezo.42,45 This problem is not present in experiments where the gold electrodes are only approached to each other, without forming and breaking a metallic contact.31 It is, however, significant that the largest observed values of Ls (those to the extreme right of the Ls distribution) reach 2 nm for covalent junctions, while extending to around 3 nm when π-stacked junctions are formed as would be expected for both kinds of junction.27,31 These Ls values probably correspond to processes in which both the gold retraction is small (or compensated during the molecular junction extension) and the participant molecules are located at the apexes of the electrodes. No significant change in the Ls values was observed with the explored environmental conditions.
’ CONCLUSIONS We have studied the rate of molecular junction formation for an acetyl-protected oligo(phenylene ethynylene) dithiol in air and two solvents (TCB and Mes/THF), with and without deprotection by Bu4NOH. The observed conductance values agree with those reported in the literature for covalent and πstacked junctions, with no significant changes with the environment or the deprotection conditions. In solution, we observed that, even without deprotection, molecular junctions are formed in an appreciable percentage. The rate increases when deprotecting the molecules, and direct binding of one molecule is favored over π-stacked junctions. It is noteworthy that approximately the same rate of molecular junction formation is achieved when performing the experiment in air without specific deprotection, showing that the presence of doubly deprotected molecules bound to the electrode surfaces increases when drying. ’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected]. Present Addresses ^
Departamento de Física de la Materia Condensada, Universidad Autonoma.
’ ACKNOWLEDGMENT This work was supported by the Spanish Ministerio de Ciencia e Innovacion through the projects MAT2008-01735, RYC-2008-03328, CTQ2008-00795, and CSD2007-0010 (Consolider-ingenio, nanociencia molecular), the CAM through the project S2009/MAT-1726 (Nanobiomagnet), S2009/PPQ-
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1533 (Madrisolar-2), and the EU through the network FUNMOLS (grant number PITN-GA-2008-212942).
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