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Resonant Transport in Single-Diketopyrrolopyrrole Junctions Yaping Zang, Suman Ray, E-Dean Fung, Anders Borges, Marc H. Garner, Michael L. Steigerwald, Gemma C. Solomon, Satish Patil, and Latha Venkataraman J. Am. Chem. Soc., Just Accepted Manuscript • Publication Date (Web): 03 Oct 2018 Downloaded from http://pubs.acs.org on October 3, 2018
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Journal of the American Chemical Society
Resonant Transport in Single-Diketopyrrolopyrrole Junctions Yaping Zang1‡, Suman Ray2‡, E-Dean Fung1, Anders Borges3, Marc H. Garner3, Michael L. Steigerwald4, Gemma Solomon3*, Satish Patil2*, Latha Venkataraman1,4* 1
Department of Applied Physics and Applied Mathematics, Columbia University, New York
2
Solid State and Structural Chemistry Unit, Indian Institute of Science, Bangalore, India
3
Nano-Science Center and Department of Chemistry, University of Copenhagen, Copenhagen Ø, Denmark
4
Department of Chemistry, Columbia University, New York
ABSTRACT: We study single-molecule transport properties of small bandgap diketopyrrolopyrrole oligomers (DPPn, n=1-4) with lengths varying from 1 to 5 nm. At a low bias voltage, the conductance decays exponentially as a function of length indicative of non-resonant transport. However, at high bias voltage, we observe a remarkably high conductance close to 10-2 G0 with currents reaching over 0.1 µA across all four oligomers. These unique transport properties, together with density functional theory-based transport calculations, suggest a mechanism of resonant transport across the highly delocalized DPP backbones in the high bias regime. This study thus demonstrates the unique properties of diketopyrrolopyrrole derivatives in achieving highly efficient long-range charge transport in single-molecule devices.
Understanding and exploiting the electronic properties of molecules that mediate intramolecular charge transport is critical for the development of functional molecular-scale devices.1-4 However, most studies are aimed at providing structure-function relations focusing on the coherent tunneling regime in single-molecule devices where the energetic offset between the frontier molecular orbitals and the electrode Fermi energy is large. Thus, conductance typically suffers from an exponential decay with increasing molecular length3, 5-8. Some strategies to promote charge transport over greater distances include designing conjugated molecules with highly delocalized electronic states and small HOMO-LUMO gaps.9-10 For example, polyporphyrins11, oxidized oligothiophenes12 and oligoenes13-14, have all demonstrated reduced conductance decay in non-resonant transport regime compared to oligophenylenes.15 A transition from tunneling to thermalassisted hopping mechanism, which features nonexponential decay, often facilitates charge transport across long molecular wires.16 Beyond these methods, a more efficient way to obtain high conductance over long distance is resonant transport, where the frontier molecular orbital is in resonance with electrode energy. In principle, resonant transport can be accessed by increasing the bias voltage across single-molecular junctions,17-18 where the bias required correlates with the energy level alignment. Although a range of molecular sys-
tems have been examined experimentally, most have a large energy mismatch between the electrode Fermi level and the frontier molecular level, making it challenging to observe resonant transport within realistic biases. Furthermore, studies of oligomeric series have proved even more challenging as this requires a small HOMO-LUMO gap and well-defined conjugation across multiple molecular repeat units.8, 19 Here, we focus on an outstanding π-conjugated building block, diketopyrrolopyrrole (DPP), which is a strong electron acceptor used extensively in constructing low band-gap oligomers and polymers for optoelectronic devices.20-23 Coupling the DPP unit with electron donor units to form a donor-acceptor (D-A) motif is a common strategy to tune the frontier molecular orbital positions and increase effective conjugation length24-26. Given these remarkable properties, we design and synthesize a series of DPP oligomers (DPPn, n=1 to 4) and measure their transport properties in single-molecule junctions under varied applied biases. We demonstrate that at a low bias voltage, transport occurs via a non-resonant coherent tunneling mechanism. By contrast, at a bias voltage of ~0.7 V, the conductance increases significantly, reaching close to 10-2 G0 for all molecules without length dependent conductance decay. We attribute these remarkable characteristics to achieving resonant transport. Importantly, we show from experiments and density functional theory (DFT) calculations, that the bias necessary for achieving resonant transport decreases with increasing molecular length, consistent with the narrowing HOMO-LUMO gap that occurs as the length is increases. Our work highlights the contribution of fundamental electronic properties in controlling resonant transport to achieve an outstanding high conductance over long distances. The molecular structures of DPP oligomers are shown in Figure 1a. The repeating unit is comprised of a strong electron-deficient DPP core flanked by two electron-rich thiophene units forming a donor-acceptordonor (D-A-D) backbone. Synthetic procedures are detailed in the supporting information (SI). We first present results from ultraviolet-visible (UV-Vis) absorption spectroscopy and cyclic voltammetry (CV) in Figure 1b. These oligomers absorb at long wavelengths an optical gap that decreases with increasing length. Figure 1c shows clear
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and reversible redox peaks indicating an electrochemical HOMO-LUMO gap that decreases from 1.68 to 1.35 eV as the number of DPP units increases, primarily due to a shift in the reduction potential. We attribute this small gap to the decreased aromatic character and increased quinoidal character of the DPP backbone.23-24, 27
Figure 1: (a) Schematic of a single DPPn junction. (b) UV-vis absorption spectra for DPP1-DPP4 measured in chlorobenzene at a concentration of 1 µM. (c) Cyclic voltammetry (CV) measurements for DPP1-DPP4 in anhydrous CH2Cl2 solution containing 0.1 M Bu4NPF6 as the supporting electrolyte and measured under a nitrogen atmosphere with an Ag/AgCl reference and a Pt working and counter electrode.
We next report results from scanning tunneling microscope-break junction (STM-BJ) measurements carried out using a custom setup.28-29 We use a gold STM tip and substrate, form and break Au point contacts in dilute (0.1– 1 mM) molecular solutions in 1,2,4-trichlorobenzene (TCB). We measure conductance (current/voltage) as a function of the tip-substrate displacement which reveals molecule-dependent conductance plateaus signifying junction formation. Here, around 70% of DPP1 traces and close to 100 % for all longer oligomer traces show molecular plateaus. All measured traces are used to generate conductance histograms without selection. Figure 2a shows logarithmically binned onedimensional (1D) conductance histograms measured at 0.09 V for the first three oligomers. These conductance histograms show peaks around integer multiples of G0, corresponding to Au point-contacts and a peak below 1 G0 corresponding to molecular junctions. We see that the molecular conductance decreases with increasing molecular length. The conductance of DPP4 is below the instrument noise of 5×10−6 G0 at this bias and is therefore not included. We repeat these measurements by increasing the applied bias in steps. When the bias is at or above 0.65 V, we observed a dramatic increase in conductance for all four oligomers. All oligomers show a length independent conductance peak between 10−3 and 10−2 G0. The histogram for DPP1 shows an additional peak around 0.1 G0; we will discuss the origin of this peak in more detail below.
Figure 2. (a) Logarithm-binned 1D conductance histograms for DPP1-DPP3 in TCB at a tip bias of ~ 0.09 V. (b) Logarithm-binned 1D histograms at a bias of 0.65 V. (c) 2D conductance-displacement histograms for DPP3 at a tip bias of 0.65 V. (d) Conductance determined from Gaussian fits to histogram peaks of DPP1 (red), DPP2 (green), DPP3 (blue) and DPP4 (yellow) as a function of the molecular length and bias.
To verify that we form junctions across the entire DPPn molecular backbone as opposed to altering the contact locations at high bias, we create 2D conductancedisplacement histograms by overlaying all measured traces at after aligning them at 0.5 G0. This allows a direct determination of the junction elongation, which correlates with the molecular backbone length30. Figure 2c presents the 2D histogram for DPP3 obtained at 0.65 V. We see a conductance feature between 10−3 and 10−2 G0 that extends 3 nm. By accounting for the Au relaxation gap (~0.6-0.8 nm upon breaking of the Au contacts)30, this elongation compares well with the molecular length of ~3.7 nm. A similar length is also observed in the low-bias data as shown in Figure S1. By further comparing the 2D histograms across the series (Figure S1), we see a biasindependent molecular plateau length that increases with increasing backbone length. This indicates that we are indeed probing the conductance across the entire molecular backbone at both low and high biases. We next examine the conductance versus molecular length by plotting the 1D histogram peak values on a semi-logarithmic scale against the computed molecular backbone length in Figure 2d. Conductance decreases exponentially with increasing molecular length at a low bias of 0.09 V. We fit these data with a line to obtain a decay constant β of 0.18 ± 0.03 Å-1 which compares well with that determined for conjugated molecules like oxidized oligothiophene12 and oligoene14 systems and reveals efficient conjugation over extended length of DPP oligomers. More importantly, such a trend indicates that charge transport at this low bias regime is dominated by a
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Journal of the American Chemical Society non-resonant coherent tunneling mechanism. Measurements at 0.24 V look similar to those at 0.09V (Figure S2). At a higher bias of 0.45 V, although the conductance of DPP1 and DPP2 remains unchanged, the conductances of longer DPP3 and DPP4 show a large increase and are even higher than that of DPP2. Increasing the bias to 0.65 V leads to the increased high conductance for all the four oligomers, leading to a roughly length independent conductance.
Figure 3: (a) DFT-based transmission as a function of energy for DPP1-DPP4 junctions. (b) Calculated conductance determined by applying the Landauer formula to the zero-bias transmission using a bias of 0.1 V for non-resonant transport (triangle), 0.45 V for the transition regime (diamond) and 0.7 V for resonant transport (square) with DPP1 in red, DPP2 in green, DPP3 in blue and DPP4 in yellow. Since the resonance condition occurs at a larger bias for DPP1, an additional point is calculated at 1.0 V (circle).
These results are independent of the bias polarity and cannot be attributed to changes in the linker-Au contact as the SMe-Au contact has been shown to have a good stability within a large bias range31. Since these oligomers have small HOMO-LUMO gap and conjugated backbones, we attribute the bias dependent results to a bias dependent non-resonant to resonant transport transition17, 32. Although resonant transport is typically achieved by gating the molecular orbital,2, 33 it is possible that the dominant transport orbital falls within the bias window at a high applied bias.17 We perform transport calculations of molecular junctions formed with DPP1-DPP4 to rationalize these results using DFT and determine the energy dependent transmission functions using the non-equilibrium Green’s function (NEGF) formalism.34-35 The computational details are presented in the SI. The Landauer transmission are shown in Figure 3a. For all molecules, the HOMO resonance is closest to the Fermi level (EF). The energy separation between the HOMO-derived and LUMOderived resonances in these transmission curves decreases with increasing length, in agreement with the HOMOLUMO gaps determined from CV measurements. The coupling between electrode and molecular orbitals, which is reflected by the width of resonance, decreases from DPP1 to DPP4, resulting in an exponential decrease in transmission at EF as the molecular length is increased despite the molecular resonance moving closer to EF. Since the exact position of the HOMO and LUMO reso-
nances is very sensitive to the computational method (see SI), we focus here on the qualitative trends. The difference between the low- and high-bias regime becomes clear upon calculating the conductance from the Landauer expression using the zero-bias transmission functions.36 In the low-bias or non-resonant tunneling regime, the region in the transmission function near EF dominates transport. Consequently, we see the calculated conductance decreasing exponentially with increasing molecular length as shown by the empty triangles in Figure 3b. The calculated decay constant β is 0.16 ± 0.01 Å-1, which is in good agreement with the experimentally obtained value. In the non-resonant low-bias transport regime, the conductance is only weakly dependent on bias. Only in the transition from non-resonant to resonant tunneling regime does the conductance become strongly bias dependent, and the bias at which this transition occurs depends predominately on the alignment of the dominant transport orbital to EF. The non-monotonic length dependence of the conductance at 0.45 V (Figure 3b, diamonds) can be explained by the alignment between the HOMO resonance and EF. At this bias, the bias window is very close to the resonance of the longer molecules DPP3 and DPP4, whereas DPP1 and DPP2 are still in the nonresonant transport regime. Interestingly, conductance is higher for the longer oligomers than for the shorter ones, as is reproduced by our experiment (Figure 2d, diamonds). Together, the experimental and DFT results indicate that the transition from non-resonant to resonant transport dependence on the details of the levelalignment. Finally, for sufficiently large biases, the bias window includes the peaks in the transmission function regardless of length, resulting in resonant transport with a length independent conductance as seen in the experiment. In the calculation, a bias of 0.7 V is necessary to obtain resonant transport through the HOMO in DPP2, DPP3, and DPP4. DPP1, on the other hand, requires a bias of 1 V to achieve HOMO-based resonant transport resulting in a conductance of 10-1 G0. In the experiment, since the exact alignment of the molecular orbital levels to EF can vary from junction to junction, we attribute the observed conductance peak at around 10-1 G0 for DPP1 to resonant transport in a subset of junctions (~20% as shown in the Figure S3). For the longer molecules, we achieve resonant transport in all junctions because the HOMO is slightly closer to EF. We note here that the conductance maximum at resonance does not reach G0 since it requires a finite bias to achieve resonant transport as discussed in detail in the SI. In summary, we demonstrate resonant transport across a series of DPP oligomers 1-5 nm in length with a decay-less high conductance.
ASSOCIATED CONTENT Supporting Information includes additional data, synthetic methods and characterization.
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AUTHOR INFORMATION Corresponding Author *
[email protected], *
[email protected] *
[email protected] Author Contributions ‡
Y. Z. and S. R. contributed equally.
Notes The authors declare no competing financial interest
ACKNOWLEDGMENT Y.Z. was supported primarily by the National Science Foundation (DMR-1507440). S.R. acknowledges D.S. Kothari postdoctoral fellowship. S.P. thanks Department of Science and Technology, New Delhi, India for a Swarnajayanti fellowship. G.C.S. A.B. and M.H.G. received funding from the Danish Council for Independent Research Natural Sciences and the Carlsberg Foundation.
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