Effect of Heteroatom Substitution on Transport in Alkanedithiol-Based

Effect of Heteroatom Substitution on Transport in Alkanedithiol-Based Molecular Tunnel Junctions: Evidence for Universal Behavior Zuoti Xie,*,†,⊥ Ioan Bâldea,*,‡,§,⊥ Stuart Oram,† Christopher E. Smith,† and C. Daniel Frisbie*,† †

Department of Chemical Engineering and Materials Science and Department of Chemistry, University of Minnesota, Minneapolis, Minnesota 55455, United States ‡ Theoretische Chemie, Universität Heidelberg, INF 229, D-69120 Heidelberg, Germany § National Institute of Lasers, Plasma and Radiation Physics, Institute of Space Science, POB MG-23, RO 077125 Bucharest, Romania S Supporting Information *

ABSTRACT: The transport properties of molecular junctions based on alkanedithiols with three different methylene chain lengths were compared with junctions based on similar chains wherein every third −CH2− was replaced with O or S, that is, following the general formula HS(CH2CH2X)nCH2CH2SH, where X = CH2, O, or S and n = 1, 2, or 3. Conducting probe atomic force microscopy revealed that the low bias resistance of the chains increased upon substitution in the order CH2 < O < S. This change in resistance is ascribed to the observed identical trend in contact resistance, Rc, whereas the exponential prefactor β (length sensitivity) was essentially the same for all chains. Using an established, analytical single-level model, we computed the effective energy offset εh (i.e., Fermi level relative to the effective HOMO level) and the electronic coupling strength Γ from the current−voltage (I−V) data. The εh values were only weakly affected by heteroatom substitution, whereas the interface coupling strength Γ varied by over an order of magnitude. Consequently, we ascribe the strong variation in Rc to the systematic change in Γ. Quantum chemical calculations reveal that the HOMO density shifts from the terminal SH groups for the alkanedithiols to the heteroatoms in the substituted chains, which provides a plausible explanation for the marked decrease in Γ for the dithiols with electron-rich heteroatoms. The results indicate that the electronic coupling and thus the resistance of alkanedithiols can be tuned by substitution of even a single atom in the middle of the molecule. Importantly, when appropriately normalized, the experimental I−V curves were accurately simulated over the full bias range (±1.5 V) using the single-level model with no adjustable parameters. The data could be collapsed to a single universal curve predicted by the model, providing clear evidence that the essential physics is captured by this analytical approach and supporting its utility for molecular electronics. KEYWORDS: molecular tunnel junctions, heteroatom substitution, single-level model, electronic coupling, quantum chemical calculations, transition voltage, universal behavior out of equilibrium

F

However, few experimental studies have focused on how these same factors affect the coupling Γ in detail.23,24,26,27 A theoretical model is required to extract εh and Γ from the I−V data for a molecular junction.23,24 Recently, a compact single-level model derived from the Landauer formalism was applied to analyze many different types of junctions; the I−V

undamental research in molecular electronics focuses on establishing clear connections between molecular structure, the ensuing electronic structure, and the current−voltage (I−V) characteristics of molecular junctions.1−21 In particular, the offset εh of the Fermi level relative to the appropriate frontier molecular orbital and the electrode− molecule coupling strength Γ (level width) are recognized as two main factors that determine the electrical properties of a typical molecular junction (Figure 1).22−27 The εh can be tuned by changing the contact terminal groups, work functions of the electrodes, and electronic structure of the molecules.4,14,28−38 © 2016 American Chemical Society

Received: October 1, 2016 Accepted: December 9, 2016 Published: December 9, 2016 569

DOI: 10.1021/acsnano.6b06623 ACS Nano 2017, 11, 569−578

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resonant tunneling. R depends strongly on heteroatom substitution; namely, it increases following the trend X = CH2 < O < S. We find the changes in R reflect the trend in contact resistance, Rc; the exponential factor β defined later does not vary appreciably with substitution. Analysis of the full I−V characteristics over ±1.5 V using the single-level model reveals that εh also does not vary significantly with substitution, but Γ, by contrast, varies strongly. These results are consistent with the low bias data; that is, β is unchanged with substitution because β relates to εh and Rc varies strongly with substitution because Rc ∝ Γ−2.24 Quantum chemistry calculations indicate that the “center of gravity” of the HOMO shifts from the metal−molecule interface to the middle of the molecules with O or S substitution, and this provides the reason for the systematic variation in Γ (and Rc). An important conclusion is that Γ reflects not only the properties of a metal−molecule interface but also the whole structure of the molecule in the junction. More specifically, a single-atom substitution in the interior of the molecule can change Γ and thus overall R. Another central result of our work is that the single-level model beautifully simulates the measured I−V curves for all junctions based on the molecules in Figure 2. Further, the I−V curves for all 10 types of junctions, when appropriately normalized, collapse onto a single universal curve predicted by the singlelevel model. This result verifies the corresponding states principle for molecular junctions25 and demonstrates that the single-level model captures the essential transport physics in these systems.

Figure 1. (A) Typical I−V characteristics of a metal−molecule− metal junction and (B) typical junction electronic structure with key parameters εh and Γ.

behavior can be accurately simulated in many cases,24−26,39,40 and a series of analytical formulas, detailed later, allow straightforward calculation of εh and Γ from the I−V curves.24,39,41 This analytical model provides an alternative to density functional theory (DFT) calculations, which have also been used to determine εh and Γ from I−V data.24,26,39,40 In this study, we employ conducting probe atomic force microscopy (CP-AFM) to examine transport in a series of molecular junctions based on aliphatic dithiol molecules of the general structure HS(CH2CH2X)nCH2CH2SH, where X = CH2, O, or S and n = 1, 2, or 3 (Figure 2). These relatively

RESULTS AND DISCUSSION Our CP-AFM setup, shown in Figure 2A, is similar to that used in our previous studies on other molecular junctions.4,45−47 Three groups of molecules (Figure 2B) were employed to measure the effects of atomic substitution on transport properties. These included regular alkanedithiols C1, C2, and C3; alkanedithiols with oxygen atoms in the chains O1, O2, and O3; and alkanedithiols with sulfur in the chains S1, S2, and S3. An additional molecule 1S was synthesized and measured for comparison with S3, which has the same approximate length but contains one S atom instead of three. Low Bias Resistance. In the case of off-resonant tunneling, it is found that low bias resistance scales exponentially with the length of the molecule according to the well-accepted empirical relationship:48,49

Figure 2. (A) Schematic representation of the CP-AFM setup. Aucoated AFM tip is brought into contact with a SAM of the substituted alkyls of various lengths on Au substrate. (B) Substituted and unsubstituted alkyl thiol molecules used in this study.

R n = R cexp(βnL0)

(1)

where Rc is the effective contact resistance, β is the tunneling decay parameter, L0 is the molecular length in repeat units, and n is the number of repeat units. This expression is convenient because it disentangles the length (exponential) dependence from the effective contact contribution Rc. The exponential length dependence shown in eq 1 is a general feature of offresonant tunneling. From the slope of a semilog plot of R = Rn versus the number of units in the chain n, one can determine the tunneling attenuation factor β, and its intercept at n = 0 gives the effective contact resistance Rc. According to eq 1, R is proportional to Rc at any molecular length, which is very different from the situation in macroscopic transport measurements on semiconductors. In those cases, Rc is normally an additive factor to the total R and becomes less important as the contacts become further apart. Equation 1 shows that because Rc is a multiplicative coefficient, it is important at any length.4,50 Resistances of the junctions were calculated from the average of

simple molecules, which are known to form good selfassembled monolayers (SAMs) on Au,42,43 provide an excellent opportunity to examine the role of heteroatom substitution on tunneling transport and, in particular, the impact on εh and Γ. We note that Waldeck and colleagues have previously examined single-molecule junctions based on thiolated oligoethers (X = O)44 but not oligothioethers (X = S). With respect to oligoethers, there are important differences between their work and ours in terms of the data sets, analysis, and conclusions, as will be described later. On the experimental side, one key distinguishing point is that with the CP-AFM technique (versus the scanning tunneling microscopy approach used by Waldeck) it is straightforward to obtain I−V sweeps over a voltage range of ±1.5 V or so, and the analysis of these data is critical to determining εh and Γ accurately. As expected, we find that the low bias resistance R for all molecules varies exponentially with length, consistent with off570


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ACS Nano approximately 150−250 I−V traces within ±0.1 V. The linear relationships in Figure 3 indicate that the data fit well within the

Figure 3. Semilog plot of resistances versus length of the molecules. Points represent the average low bias resistance from 150 to 250 traces. Error bars represent standard deviation. The inset shows the averaged β values and contact resistance of C-, O-, and Salkanedithiol chains.

off-resonance tunneling model described by eq 1. The β values were found to be 1.11, 1.19, and 1.17 per atom-unit for the C, O, and S wires, respectively, and thus, within the experimental uncertainty, β is independent of the heteroatom substitution in the alkanedithiol chain. We note that previous work by Whitesides’ group indicated that the β value of alkanethiols is insensitive to functional group or terminal contact group changes.51−54 The contact resistance Rc, however, changes markedly by altering the elemental makeup of the chain for reasons that are discussed in detail below. The results obtained from low bias (−0.1 to 0.1 V) I−V measurements are summarized in Table 1. Analysis of I−V Traces. For a more comprehensive examination of transport properties, we investigated the full I−V characteristics over the interval ±1.5 V. Figure 4A−C shows semilog plots of the average I−V curves for the unsubstituted, O-substituted, and S-substituted molecules, respectively. Each average I−V trace is nearly symmetric with respect to zero bias, and for a given voltage, the current decreases exponentially with length. It is also clear that the maximum current at 1.5 V decreases systematically upon substitution in the order anticipated from the low bias measurements, that is, ICH2 > IO > IS. The shapes of the I−V curves also appear to be very similar. We have chosen to analyze the shape of these curves by recasting the plots on new axes, |V2/I| versus V.25,55 The results are shown in Figure 4D−F.

Figure 4. (A−C) Semilog plot of average I−V curves, (D−F) transition voltage spectroscopy of Au−Cn−Au, Au−On−Au and Au−Sn−Au junctions (n = 1, 2, 3).

This type of plot gives a peak maximum, which corresponds to the point where the differential conductance is two times larger than the low bias pseudo-ohmic conductance.24,25 This approach is an alternative version of transition voltage spectroscopy (TVS), and it can be shown that the voltage at peak maximum and the transition voltage Vt (defined as the bias at the minimum of the Fowler−Nordheim plot29) are mathematically identical.25,55 Inspection of the TVS curves in Figure 4D−F shows similar Vt values for different lengths of each type of alkanedithiol. Vt exhibits only a slight shift to lower voltage when the backbone is substituted with O and S. Table 1 lists the Vt at negative and positive bias for nine different classes of junctions and demonstrates they are independent of bias polarity (|Vt−| ≈ Vt+). Furthermore, the Vt values of C-alkanedithiols are consistent with those in previous studies obtained from CP-

Table 1. Summary of the Main Results for Alkyl CP-AFM Junctionsa C1 β, Rc R/R0 |Vt−| Vt+ εh Γav I(0) I(1)

2.36 1.22 1.25 1.07 68.94 8.970 9.034

C2

C3

1.11, 1.25 × 102 7.10 × 101 1.96×103 1.26 1.15 1.27 1.13 1.10 0.99 13.06 2.23 8.978 8.953 8.977 8.953

O1 4.69 1.10 1.08 0.95 43.39 9.061 9.144

O2

O3

1.19, 1.63 × 102 2.11 × 102 6.41 × 103 1.11 1.14 1.09 1.12 0.95 0.98 6.73 1.22 9.160 9.152 9.163 9.153

S1

S2

S3

1S

3.88 × 101 1.01 0.99 0.87 13.96 8.562

1.17, 1.46×103 1.46 × 103 1.00 1.02 0.88 2.28 8.449

4.37 × 104 0.98 1.02 0.87 0.42 8.380

6.84 × 104 0.96 1.00 0.86 0.33 8.334

The β value and contact resistance Rc for C-, O-, and S-alkane, β value in per atom-unit, Rc in Ω, the resistance of the junctions with respect to R0 [R0 ≡ h/(2e2) = 12.9 kΩ], transition voltages Vt± in V, energies (εh, I(0), and I(1)) in eV, Γav (meV) obtained from eq 4 by assuming N = 100. Also included are the first relevant two ionization energies I(0) and I(1) (HOMO and HOMO−1 energies with reversed sign) computed using the outer valence Green’s function method. a

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ACS Nano AFM and break junction measurements.56−58 The transition voltages versus length of the molecules are displayed in Figure 5A and are length-independent for each set of dithiol junctions (C-alkane, O-alkane, and S-alkane).

2e2/h is the quantum conductance, and N is the number of molecules participating in the transport. Figure 5B shows the energy offset εh for all nine types of molecular junctions determined via eq 2. As shown in Table 1, the differences in the effective tunneling barrier caused by the heteroatom substitution are within 0.2 eV. The small variation in εh is likely responsible for the relative unchanging characteristic of β across the molecular sets. However, the increase in the contact resistance Rc of junctions caused by the O or S substitution is nearly 1 order of magnitude. It is clear that the effective tunneling barrier εh, which is essentially constant, cannot be responsible for the contact resistance difference of the junctions. However, based on eq 4, the calculated interface coupling Γ drastically changes with substitution (see Table 1 and Figure 5C). The strength of the interface coupling decreases systematically, ΓC‑alkane > ΓO‑alkane > ΓS‑alkane, with an exponential fall-off with increasing length in each molecular set. Thus, we ascribe the strong change in contact resistance Rc (Figure 3) to be primarily related to changes in Γ. Work Function Measurements. In view of eq 4, the number of molecules participating in charge transport and the surface coverage of the SAMs composed of the molecules investigated here are important in the analysis presented in this paper. One simple way to estimate the coverage of the monolayer is to measure the change in the work function of the alkanedithiols (10 dithiols were used in this work) chemisorbed of the metallic substrate. The change of the work function is proportional to the density of the molecules on the substrate.59 We have performed scanning Kelvin probe microscopy (SKPM) measurements for the investigated molecules chemisorbed on gold. These SKPM data indicate that the coverage of the molecules are similar. The SAM-induced changes in work function (ΔΦ) on the Au surface are shown in Figure 6. In addition, we note that, in a recent work60 based on

Figure 5. (A) Transition voltage, (B) effective tunneling barrier, and (C) average level width Γ = Γav of Au−Cn−Au, Au−On−Au, and Au−Sn−Au junctions (n = 1, 2, 3). The inset in (C) shows the coupling intercept (contact coupling, Γc).

Importantly, Vt can be employed to calculate the effective energy offset εh for the junctions using the single-level model mentioned in the introduction.24,39 For symmetric junctions (Vt = −Vt− = Vt+), the correlation between transition voltage (Vt) and effective tunneling barrier (εh) is expressed as24,25,39 eVt = 2εh / 3

(2)

and the I−V characteristics are given as I = GV

Figure 6. SAM-induced changes in work function (ΔΦ) of the Au surface.

24,39

εh2 εh2 − (eV /2)2

nuclear reaction analysis, we presented results showing that the coverages of the alkanethiols with different lengths are equal. Overall, we believe that the number of molecules in the junction is known with a good certainty (approximately ±20%). Picture of Transport Aided by Quantum Chemical Calculations. The change in the interface coupling Γ suggests that replacing CH2 with O and S may induce a significant change in the molecular orbital (or molecular orbitals; see below) dominating the charge transport. Indeed, quantum chemistry calculations confirm this expectation. At the same time, alkanedithiols and substituted alkanedithiols provide

(3)

The zero-bias conductance G = 1/R of the CP-AFM junction can be expressed as follows G = NG0

2 Γ av

εh2

(4)

where Γav = ΓΓ s t = εh G /(NG0) is the average interface coupling, Γs, Γt determined by the molecular coupling to the substrate (s) and tip (t) (Γs ≈ Γt in symmetric junctions), G0 = 572


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concentrated at opposite molecular ends (see Figure S1B,C). A qualitatively similar situation, wherein the HOMO and HOMO−1 become located at either molecular end, is encountered also in the case where an electric field is applied (see Figure S3C,D). These results are important because they demonstrate that conduction through alkanedithiols occurs through two albeit practically equivalent channels (HOMO and HOMO−1). The fact that our I−V data can be simulated with eq 3, which is valid for a single level, is due to the fact that, even in realistic situations wherein the two contacts are not perfectly identical, HOMO and HOMO−1 remain practically degenerate. They both give practically equal and additive contributions to the current, each expressed by the RHS of eq 3. Qualitatively, the oxygen-substituted molecules O1, O2, and O3 behave similarly to the parent molecules C1, C2, and C3, respectively (cf. Figure 7E,F and Figures S1E,F and S2E,F). Quantitatively, however, substitution with O causes (small) parts of the practically degenerate HOMO and HOMO−1 (see Table 1) distributions to be displaced away from the molecular ends toward the substituting O atoms. As a consequence, the contact coupling constants Γ are somewhat smaller than in the parent molecules and, hence, a somewhat higher resistance obtains (see Table 1). Waldeck and colleagues also analyzed transport in alkanedithiols and oligoether dithiols using STM.44 They recognized the presence of essentially degenerate HOMO and HOMO−1 orbitals localized on the two ends of the molecules, as shown in Figure 7. However, they concluded that transport was facilitated by the delocalized HOMO−2, not shown here, but which extends across the entire molecular backbone for both alkanedithiols and oligoether dithiols. In light of our TVS analysis, which indicates that the barrier is NOT length-dependent and that also indicates that the magnitude of the barrier is on the order of 1 eV (HOMO−2 is deeper), we favor the interpretation that transport is indeed facilitated by the HOMO and HOMO−1 orbitals. Substitution of CH2 by S turns out to be qualitatively (and hence also quantitatively) different from that by O. In the Ssubstituted molecules S1, S2, S3, and 1S (cf. Figures 8 and S4), the HOMO is no longer degenerate. The fact that conduction proceeds in these molecules through a single channel (HOMO) instead of two in the cases of C1, C2, C3 and O1, O2, O3 is one reason why their resistance is higher than those of the latter. Loosely speaking, this is responsible for an increase in Rc of the S1-, S2-, and S3-based junctions by a factor of 2 with respect to the parent molecules C1, C2, and C3. Another and more important reason is that the HOMO spatial distribution is located far away from the molecular ends (“electrodes”), and this fact considerably reduces the contact couplings Γ (cf. Figures 8B and S4B,D). The combined effect of the two aforementioned facts is that, in comparison with Osubstitution, S-substitution yields an increase in Rc larger by almost 1 order of magnitude. To further study how the physical distance between the molecular orbitals in the case of S substitution and the electrodes affects the transport properties of the junction, a molecular junction comprising molecule 1S was measured to compare with the S3 junction. These two molecules have the same terminal group and comparable lengths, but the number of S atoms is different (Figures 2 and 8). 1S has only one S in the middle of a C3 chain. Importantly, the measurements indicate that Vt does not shift, while the low bias resistance

important insight into the charge transport through molecular junctions. The relevant results of the quantum chemical calculations are detailed below. Table 1 shows that in the unsubstituted molecules (C1, C2, and C3), HOMO and HOMO−1 are practically degenerate. Both the spatial distributions of the HOMO and of the HOMO−1 are concentrated at the molecular ends (cf. Figure 7B,C and Figures S1B,C and S2B,C). We employed Gabedit61

Figure 7. Like in the parent molecule C3 (A−C), the HOMO and HOMO−1 (calculated as indicated in the main text) of O3 (D−F) are practically degenerate (cf. Table 1), and their spatial distribution is concentrated at the molecular ends (“electrodes”).

Figure 8. Unlike in the parent molecule C3 (cf. Figure 7B,C) or in the oxygen-substituted species O3 (cf. Figure 7E,F), in S3 (A,B) and 1S (C,D) the HOMO is nondegenerate (cf. Table 1), and its spatial distribution is located away from the molecular ends (“electrodes”).

to generate the spatial distributions presented in Figures 7, 8, and S1−S4). The HOMO of molecule C1 is located around one molecular end and the HOMO−1 at the other end (see Figure S1B,C). This is the consequence of the fact that, unlike the isolated C2 and C3 molecules at their optimized geometries (cf. Figures 7A and S2A), C1 does not possess any symmetry (Figure S1A). The equilibrium geometries of C2 and C3 are symmetric, and this fact means that the spatial distributions of their practically degenerate HOMO and HOMO−1 have practically equal contributions symmetrically located around the two molecular ends (cf. Figures 7A,B and S2B,C). To mimic a real configuration, wherein the geometries at the molecule−tip and molecule−substrate contacts are inherently different, in Figure S3A,B, we present the HOMO and HOMO−1 distributions of the C2 molecule having the C−S bond at one end different from the S−C bond at the other end. As visible in Figure S3A,B, while still remaining practically degenerate, HOMO and HOMO−1 become concentrated at one molecular end: HOMO at the left end, and HOMO−1 at the right end, that is, a situation similar to C1, which, even isolated, has practically degenerate HOMO and HOMO−1 573


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Figure 9. Agreement between the individual experimental I−V curves (red) and those obtained theoretically via eq 3 (black) is illustrated here for (A1−A3) Au−Cn−Au, (B1−B3) Au−On−Au, and (C1−C3) Au−Sn−Au junctions (n = 1, 2, 3). The two parameters of each junction, low bias conductance G (1/R) and the energy offset εh, that are required for I−V curve fitting (eq 3) and calculated coupling strength for each case are provided. We estimate each junction contains ∼100 molecules in parallel.

to determine both εh and Γ and, in particular, that εh is independent of molecular length, a finding that in our opinion rules out HOMO−2 assisted transport. To be sure, there are still some puzzling features of transport in these systems. Most notable is that photoelectron spectroscopy data indicate that HOMO to EF offset for alkane dithiols on Au is on the order of 4 eV,62,63 whereas we extract εh ≈ 1 eV. This discrepancy between electronic spectroscopy and transport measurements, which is not seen for oligoacenes or oligophenylenes,24,45 may reflect the stronger impact of the second electrode (which makes the difference between the UPS and CP-AFM setups). Electrode-driven energy shifts of HOMOs located close to electrodes (present case) may be different from HOMOs delocalized over the entire molecule (oligophenylenes or oligoacenes). Elucidating this issue deserves further theoretical investigation. Simulation of I−V Curves Using the Single-Level Model. Finally, an important verification of the single-level model is whether it accurately predicts the measured I−V behavior. Using the extracted values of εh and low bias G (from which we calculate Γ directly by eq 4), the I−V data for all junctions were simulated with eq 3, Figure 9 and Figure S5. Indeed, eq 3 reproduces the individual I−V curves measured for our CP-AFM junctions extremely well. Importantly, εh and G in eq 3 were not taken as adjustable parameters to achieve these

slightly increases, and therefore the coupling Γ is slightly lower for 1S than for S3 (Table 1). The main difference between 1S and S3 is the fact that the HOMO of 1S is located closer to the molecule’s center (the sulfur atom), that is, farther away from the molecular ends (“electrodes”); compare Figure 8D with Figure 8B. The comparison of 1S with S3 indicates that a longer distance between the electrodes and the HOMO density decreases the coupling and increases the effective contact resistance. We believe that our collective results above provide a relatively clear picture of transport in alkanedithiols and the heteroatom-containing variants. Inspection again of Table 1 shows that for each molecular class, εh is essentially independent of length, but Γ is strongly length-dependent. It is the exponential decrease in Γ (Figure 5C) which leads to the exponential increase in junction resistance with length. Furthermore, it is largely the strong change in Γ, not εh, with heteroatom substitution that results in the increase in resistance following CH2 < O < S. The relative localization of the frontier orbitals on the heteroatoms appears to be responsible for the trends in Γ and R for these systems. In a broad sense, our findings agree with the earlier results of Waldeck,44 who also reported increased resistance for oligoether chains. However, our data set is larger as we examined full I−V sweeps and thioethers in addition to ether chains. The I−V data allowed us 574


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ACS Nano fits. Rather, the values shown in Table 1, which were obtained analytically from eq 2 and directly from experiment, were inserted into eq 3 to directly compute the I−V traces, which indeed matched the data. Of course, this is really a selfconsistency check as eq 2 is derived from the single-level model described by eq 3. It is nevertheless important that the simulations nicely reproduce the experimental data without any need for adjustments to εh or Γ. We believe the high quality of the simulations justifies the use of the single-level model for analysis of the dithiol junctions. Furthermore, upon normalizing I and V appropriately, the rescaled I−V curves collapse to a single curve for all junctions in this study. Following our previous work, we define the reduced variables as IR = I/It and VR = V/Vt (It is the current at Vt). A theoretical law of corresponding states (LCS) has been deduced for charge tunneling (eq 5):25

IR =

single-level model provides an accurate physical picture of transport in many molecular junctions.

CONCLUSION We have reported an extensive study of the transport properties of molecular junctions based on alkanedithiols and substituted alkanedithiols. The results indicate that the tunneling attenuation factor β and the effective energy offset εh are unaffected by substitution, while the contact resistance and Γ vary greatly. Using only the estimated energy offset εh obtained via transition voltage spectroscopy and measured low bias resistance, we are able to reproduce all the I−V curves from CP-AFM measurements with a compact, analytical single-level model. Good agreement between theory and experiment indicates that the single-level model is an appropriate framework to investigate the I−V characteristics of molecular junctions and to extract two important transport factors, namely, εh and coupling Γ. The differences in εh introduced by O and S substitution were found to be weak and not responsible for the dramatic difference in the junction resistances. Rather, the difference in the interface coupling strength Γ caused by replacing select CH2 units in the alkane chain by O and S dominates contact resistance and thus the total resistance. Quantum chemistry calculations have shown how the electronic structure of the conducting orbitals are significantly tuned by substitution and provide a deeper explanation of the transport effects. Importantly, all the dithiol junctions investigated show clear universal behavior in charge tunneling despite the fact that their low bias resistance varies over more than 4 orders of magnitude, supporting the utility of our single-level model for analysis of molecular junctions.

2VR 3 − VR2

(5)

Equation 5 (the LCS) verifies that the charge tunneling through the molecular junction can be collapsed to a single universal IR−VR curve. The universal behavior held in these molecular junctions is shown in Figure 9. Altogether, 10 types of molecular junctions with a total of ∼550 individual I−V curves collapse very close to each other after normalization (Figure 10A): within statistical deviations and measurement

THEORETICAL METHODS Because of existing experimental evidence based on gated systems14 and the known impact of the electrodes’ work function,46,47 HOMO-mediated quantum chemical calculations (with basis sets of 6-311++g(d,p) quality for all atoms) focused on the two highest occupied molecular orbitals (HOMO and HOMO−1). The molecular geometries have been optimized at the DFT/B3LYP level of theory using the GAUSSIAN 09 package.66 The relevant values of the first ionization energies of the molecules investigated in the present paper I(0,1) (collected in Table 1) were computed using the outer valence Green’s function method67−69 as implemented in GAUSSIAN 09. Let us recall that, taken with opposite sign, the energies of the HOMO and HOMO−1 represent approximations of I(0) and I(1) (Koopmans theorem) and will be referred as such hereafter. The HOMO and HOMO−1 spatial distributions presented in Figures 7, 8, and S1−S4 have been obtained from natural orbital expansions69 via calculations based on the equation-ofmotion coupled-cluster singles and doubles of the ionization potentials (IP, EOM-IP-CCSD)70,71 as implemented in the CFOUR package,72 which represent the state-of-the-art of quantum chemistry for the molecular sizes considered. All quantum chemical calculations were performed on bwUniCluster and bwForCluster/JUSTUS HPC facilities.73

Figure 10. (A) Theoretical universal curve (red) of eq 5 plotted along with ∼550 experimental IR−VR curves (black) measured for 10 different types of molecular junctions. (B) Comparison of the theoretical (red) curve, eq 5, and the statistical average (black) of a statistical ensemble comprising ∼550 experimental curves analyzed. Error bars (black) represent standard deviations of the average of the rescaled I−V curves.

errors, they match the theoretical universal IR−VR curve (Figure 10B), expressed by eq 5. The finding of a LCS provides a convenient test for whether the single-level model can be applied to a given junction. It has been a significant challenge in the field of molecular electronics to uncover a simple compact model that both accurately captures I−V behavior and allows straightforward extraction of critical electronic structure parameters. The I−V characteristics of different molecular junctions can be extremely different based on differences in the electrode materials, molecular anchoring groups, and molecular electronic structure.8,24,25,33,34,45,64,65 An encouraging message of this LCS (i.e., universal behavior out of equilibrium) is that the transport behavior across different molecular platforms can be reproducibly similar. Many different types of the molecular junctions have been tested and found to fit this universal curve,25,40 including the molecular junctions described here with internal substitution, further supporting the view that the

EXPERIMENTAL METHODS Materials. Gold nuggets (99.999% pure) were purchased from Mowry, Inc. (St. Paul, MN). Evaporation boats and chromium evaporation rods were purchased from R.D. Mathis (Long Beach, CA). Silicon (100) wafers were obtained from WaferNet (San Jose, CA). Contact-mode AFM tips (DNP 10 silicon nitride probes) were 575


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ACS Nano Author Contributions

purchased from Bruker AFM Probes. The 1,5-pentanedithiol (C1) 96%, 1,8-octanedithiol (C2) 97%, 1,11-undecanedithiol (C3) 99%, 2mercaptoethyl ether (O1) 95%, 2,2′-(ethylenedioxy)diethanethiol (O2) 95%, tetra(ethylene glycol)dithiol (O3) 97%, and 2,2′thiodiethanethiol (S1) 90% used in this study were purchased from Sigma-Aldrich Company. Details of the synthesis of S2, S3, and 1S are provided in the Supporting Information. Conducting Tip and Substrate Preparation. Contact-mode AFM tips were coated by Au at base pressure ( 0 means positive voltage on the tip). All measured I−V curves gradually switch from practically linear at low biases to gradually more parabolic at higher biases. The inverse of the slope of the linear portion of the I−V characteristic was used to define a junction (low bias) resistance R. The tunneling attenuation parameter β and contact resistance Rc were extracted with high certainty from plots of the low bias resistance versus molecular length. Voltage sweeps to ±1.5 V were applied to observe the pronounced nonlinear (I−V) behavior.

⊥

Z.X. and I.B. contributed equally to this work.

Notes

The authors declare no competing financial interest.

ACKNOWLEDGMENTS C.D.F. acknowledges financial support from the U.S. National Science Foundation (CHE-1213876). I.B. acknowledges financial support provided by the Deutsche Forschungsgemeinschaft (Grant BA 1799/3-1) and partial computational support by the state of Baden-Württemberg within the bwHPC initiative and the bwHPC-C5 project provided through the associate computer services bwUniCluster and bwForCluster/ JUSTUS HPC facility.73 REFERENCES (1) Bumm, L. A.; Arnold, J. J.; Cygan, M. T.; Dunbar, T. D.; Burgin, T. P.; Jones, L., II; Allara, D. L.; Tour, J. M.; Weiss, P. S. Are Single Molecular Wires Conducting? Science 1996, 271, 1705−1707. (2) Donhauser, Z. J.; Mantooth, B. A.; Kelly, K. F.; Bumm, L. A.; Monnell, J. D.; Stapleton, J. J.; Price, D. W., Jr; Rawlett, A. M.; Allara, D. L.; Tour, J. M.; et al. Conductance Switching in Single Molecules through Conformational Changes. Science 2001, 292, 2303−2307. (3) Venkataraman, L.; Klare, J. E.; Nuckolls, C.; Hybertsen, M. S.; Steigerwald, M. L. Dependence of Single-Molecule Junction Conductance on Molecular Conformation. Nature 2006, 442, 904− 907. (4) Choi, S. H.; Kim, B.; Frisbie, C. D. Electrical Resistance of Long Conjugated Molecular Wires. Science 2008, 320, 1482−1486. (5) Nijhuis, C. A.; Reus, W. F.; Whitesides, G. M. Molecular Rectification in Metal-SAM-Metal Oxide-Metal Junctions. J. Am. Chem. Soc. 2009, 131, 17814−17827. (6) Holmlin, R. E.; Ismagilov, R. F.; Haag, R.; Mujica, V.; Ratner, M. A.; Rampi, M. A.; Whitesides, G. M. Correlating Electron Transport and Molecular Structure in Organic Thin Films. Angew. Chem. 2001, 113, 2378−2382. (7) Amdursky, N.; Marchak, D.; Sepunaru, L.; Pecht, I.; Sheves, M.; Cahen, D. Electronic Transport via Proteins. Adv. Mater. 2014, 26, 7142−7161. (8) Díez-Pérez, I.; Hihath, J.; Lee, Y.; Yu, L.; Adamska, L.; Kozhushner, M. A.; Oleynik, I. I.; Tao, N. J. Rectification and Stability of a Single Molecular Diode with Controlled Orientation. Nat. Chem. 2009, 1, 635−641. (9) McCreery, R. L.; Bergren, A. J. Progress with Molecular Electronic Junctions: Meeting Experimental Challenges in Design and Fabrication. Adv. Mater. 2009, 21, 4303−4322. (10) Aragones, A. C.; Aravena, D.; Cerda, J. I.; Acís-Castillo, Z.; Li, H.; Real, J. A.; Sanz, F.; Hihath, J.; Ruiz, E.; Díez-Pérez, I. Large Conductance Switching in a Single-Molecule Device through Room Temperature Spin-Dependent Transport. Nano Lett. 2016, 16, 218− 226. (11) Guo, X.; Small, J. P.; Klare, J. E.; Wang, Y.; Purewal, M. S.; Tam, I. W.; Hong, B. H.; Caldwell, R.; Huang, L.; O’Brien, S.; et al. Covalently Bridging Gaps in Single-Walled Carbon Nanotubes with Conducting Molecules. Science 2006, 311, 356−359. (12) Kim, Y.; Hellmuth, T. J.; Pauly, F.; Marius, B.; Scheer, E. Characteristics of Amine-Ended and Thiol-Ended Alkane Single Moecule Junctions Revealed by Inelastic Electron Tunneling Spectroscopy. ACS Nano 2011, 5, 4104−4111. (13) Artes, J. M.; Li, Y.; Qi, J.; Anantram, M. P.; Hihath, J. Conformational Gating of DNA Conductance. Nat. Commun. 2015, 6, 8870. (14) Song, H.; Kim, Y.; Jang, Y. H.; Jeong, H.; Reed, M. A.; Lee, T. Observation of Molecular Orbital Gating. Nature 2009, 462, 1039− 1043.

ASSOCIATED CONTENT S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsnano.6b06623. Experimental and theoretical details, supplementary tables and figures (PDF)

AUTHOR INFORMATION Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. *E-mail: [email protected]. ORCID

Zuoti Xie: 0000-0002-1828-0122 576


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