Hopping Transport and Rectifying Behavior in Long Donor–Acceptor

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Hopping Transport and Rectifying Behavior in Long Donor− Acceptor Molecular Wires Liang Luo,† Luke Balhorn,‡ Bess Vlaisavljevich,‡ Dongxia Ma,‡ Laura Gagliardi,‡ and C. Daniel Frisbie*,† †

Department of Chemical Engineering and Materials Science and ‡Department of Chemistry, Supercomputing Institute, and Chemical Theory Center, University of Minnesota, Minneapolis, Minnesota 55455, United States S Supporting Information *

ABSTRACT: We have developed a series of long donor (D)− acceptor (A) block molecular wires (DmAn or DmCAn: C, cyclohexane bridge; m, n = 1−4) attached to Au surfaces with lengths ranging from 3 to 10 nm in order to probe electrical rectification in the hopping regime. In each wire, the donor block was synthesized from the Au surface by stepwise imine condensation between 4,4′(5′)-diformyltetrathiafulvalene electron donors (D) and 1,4-diaminobenzene linkers, followed by the stepwise synthesis of the acceptor block using N,N′-di(4-anilino)1,2,4,5-benzenebis(dicarboximide) electron acceptors (A) and terephthaldehyde linkers. Molecular junction measurements by conducting probe atomic force microscopy (CP-AFM) revealed that the DmCA1 (m = 1−4) wires exhibited electrical rectification with current rectification ratios as high as 30 at ±1.0 V when contacted with Au-coated tips and Au substrates; DmAn wires did not rectify, suggesting electronic decoupling of the D and A blocks is necessary for diode behavior. The forward bias condition for DmCA1 corresponded to negative potential on the acceptor block and positive potential on the donor block, as anticipated. Furthermore, the rectification ratio was a function of the wire architecture, length, and measurement temperature. Density functional theory (DFT) calculations of ground state neutral and ionized electronic structures and the experimental data for DmCA1 suggest that under forward bias the rate limiting transport step in these diodes is activated hole hopping from the HOMO level of the D block to the HOMO level of the A block; that is, holeonly transport pertains and it is sensitive to energy level alignment. Under reverse bias, the rate limiting transport step is relatively insensitive to temperature, which is consistent with a change in the rate limiting mechanism from hopping to tunneling. We propose a simple energy level model that rationalizes the change in transport mechanism and we suggest how these molecular diode structures might be further improved to achieve better rectification with simultaneous hole and electron transport in the D and A blocks, respectively.

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A− zwitterions, and achieved rectification ratios ranging from sub-20 to 150.21,23−26,37 Whitesides and colleagues demonstrated current rectification with rectification ratios of 180 in self-assembled monolayers containing a single electron acceptor or donor with an aliphatic chain.27−31 Follow-up studies revealed that subtle changes in the intermolecular van der Waals interactions in the monolayer impact the rectification behavior dramatically- junctions made from an odd number of alkyl units rectify currents 10 times higher than those made from even number of alkyl units.38 In addition, a series of single molecules consisting of two weakly coupled electronic πsystems with asymmetric energy levels have been synthesized,32−36,39 and diode-like current (I)−voltage (V) characteristics (rectification ratios ∼5−10) were achieved using break junctions and STM junctions. For smaller molecules, Reed and

INTRODUCTION Molecular electronics represents an active domain in the science of nanometer-scaled systems, not only because it has been proposed as an ultimate solution to the scaling limits of conventional semiconductor technology,1−4 but especially because of its importance to the fundamental understanding of charge transport in molecular systems.5−10 Research efforts in molecular electronics are motivated by a variety of intriguing behaviors in molecular junctions, including the Kondo effect,11−13 the Coulomb Blockade,13−15 switching,16−20 and electrical rectification.21−36 Aviram and Ratner proposed a theoretical molecular diode structure, D−σ−A, in 1974,1 in which an electron donor moiety (D) connects to an electron acceptor moiety (A) through a sigma bridge (σ). Over the last 40 years, researchers have pursued many experimental approaches to achieve electrical rectification in solid-state diodes modeled on this basic structure. Ashwell and Metzger succeeded in fabricating molecular diodes using Langmuir−Blodgett films of D+−σ− © XXXX American Chemical Society

Received: July 15, 2014 Revised: October 19, 2014

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Figure 1. Molecular structures of donor-block-acceptor-block molecular wires linked to Au substrates. Both DmAn and DmCAn structures have the same end-groups.

to understand how to rationally manipulate the electronic structure of the junction, what electronic structures are most desirable for rectification, and the roles of the metal−molecule contacts.8 In the end, irrespective of these qualitative arguments, it is important to determine what diode behaviors can be observed in the hopping regime, and how they compare to results on shorter molecules in the tunneling regime. Facile stepwise synthesis of long conjugated molecular wires connected to electrodes48−50 has recently enabled the development of long molecular rectifiers with a user-defined sequence of building blocks (donors, acceptors, and σ-bridges). Using this method, Ashwell and co-workers51,52 have developed molecular wires with length of up to 10.5 nm and achieved rectification ratios as high as 160 at ±1.0 V. Systematic examination of the structuretransport relationships of such hopping transport diodes as a function of molecular length, bond architecture, temperature, and applied bias is critical in order to understand fundamental mechanisms and to improve overall performance. In this paper, we report synthesis, electrical characterization, and quantum chemical studies of the electronic structure of long donor block−acceptor block molecular wires, DmAn and DmCAn, with controlled block architectures and molecular lengths, Figure 1. We have used aryl imine condensation chemistry for the synthesis because, as we have shown previously,42,43 it allows precise control over the wire structure with a spatial resolution corresponding to the length of a monomer (∼1 nm). For DmAn wires, the donor block was synthesized from the Au surface by stepwise imine condensation between 4,4′(5′)-diformyltetrathiafulvalene (TTF) electron donors (D) and 1,4-diaminobenzene linkers, followed by the stepwise synthesis of the acceptor block using N,N′-di(4-anilino)-1,2,4,5-benzenebis(dicarboximide) (PMDI) electron acceptors (A) and terephthaldehyde linkers. TTF and PMDI were chosen as the donor and the acceptor building blocks because of their reversible electrochemical properties and because they are well-known functional donor and acceptor materials in solar cells and in solution electron transfer experiments,53 due to their strong electron donation and affinity behaviors, respectively.54−56 For DmCAn wires, the same synthesis procedure was followed except that a saturated cyclohexyl moiety (C) was inserted between the D and A blocks. We show that DmCAn wires rectify current, they function as diodes, whereas DmAn wires do not, indicating that (1) in this system some degree of electronic decoupling between the D and A blocks is necessary to observe rectification, and (2) the rectification results from the molecular architecture, not simply the metal−molecule interface charac-

Tour developed an asymmetrical Au/4-thioacetylbiphenyl/Ti, TiO2/Au junction, for which a rectification ratio of about 500 was observed at ±1.0 V.40 Later, a monolayer of nitroazobenzene, developed by McCreery and sandwiched between pyrolyzed photoresist substrate (PPF) and metal/metal oxide electrodes, exhibited a rectification ratio of 600 at ±2.0 V.16,41 Overall, these successful molecular rectifier approaches have focused predominantly on junctions that incorporate short molecules with lengths less than ∼4 nm. It appears that, in most cases, the key rate-limiting transport steps involve (nonresonant) electron tunneling. In contrast, molecular diode behavior in the hopping regime, in which the molecules are long enough that tunneling is suppressed and charges are injected into the molecular orbitals,42,43 has been much less studied. By hopping transport, we mean thermally activated, nearest neighbor hopping, as occurs in amorphous organic semiconductor films near room temperature. There is reason to believe that molecular rectification in the hopping regime may result in much higher rectification ratios than can be achieved in the tunneling regime. This idea is suggested by noting that conventional organic thin film diodes involving a donor−acceptor bilayer (e.g., in organic solar cells or photodetectors) achieve rectification ratios in excess of 104 at room temperature,44−47 far higher than what has been observed so far in molecular junctions. In these systems, energy level offsets at the D/A interface are tailored by judicious choice of the D and A materials. The films are too thick (∼100 nm) to allow electrode-to-electrode tunneling and instead charges (i.e., holes in the HOMO levels or electrons in the LUMO levels) are injected into the donor or acceptor orbitals. The hopping charges then sample the discrete, localized energy states in the film, and when they reach the D/A interface there is a clear energy barrier for both holes and electrons arising from mismatched HOMO levels (and LUMO levels) that must be overcome for transport. This barrier can be large, on the order of 10kBT at room temperature, and it can be manipulated by choosing donors and acceptors with different ionization potentials or electron affinities. In nanoscopic tunnel junctions, on the other hand, molecular orbitals mediate electrode-to-electrode tunneling, and the energy barriers reflect the energy differences between the Fermi level of the contacts and the nearest lying molecular orbitals. The transmission through the junction is a complex function of the orbital energetic positions relative to the Fermi level and the spatial extent of the orbitals. Additionally, the orbital-to-Fermi level offsets are often controlled by the nature of the molecule−metal contact, which can pin the Fermi-level with respect to the frontier orbitals. It is therefore challenging B

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Figure 2. Stepwise synthesis of DmAn monolayers on Au surfaces.

teristics. Density functional theory (DFT) calculations provide insight into the electronic structure and geometrical conformation of these wires. Systematic measurements under forward and reverse bias for DmCA1 wires suggest that the fundamental rate limiting transport step is different in the two bias regimes (as is the case in conventional Si diodes). In forward bias, the transport appears to be activated hopping, while in reverse bias it is tunneling. We propose a simple energy level model to rationalize the observed behavior. To our knowledge, this work provides only the second example of molecular diodes operating in the hopping regime, and is the first to present a comprehensive analysis of temperature and length dependent transport, allowing informed speculation on the key rate limiting transport steps under forward and reverse bias.

were synthesized according to the literature procedures, respectively. The linking molecules 1,4-diaminobenzene (2) and terephthaldehyde (4) are commercially available. The Au substrates were 1000 Å thick thin films on silicon (with a 50 Å Cr adhesion layer) prepared in a Balzers thermal evaporator at a rate of 1.0 Å/s at a base pressure of 2 × 10−6 Torr. The stepwise growth procedure of the DA block wires is shown in Figure 2. Freshly evaporated Au substrate was immersed in a solution of 1 mM 4-aminothiophenol (ATP) in Ar-purged absolute ethanol for 24 h to allow the formation of a 4-ATP self-assembled monolayer (SAM) on the Au substrate. The 4-ATP SAM sample was then immersed in a solution of 10 mM 1 in dimethyl sulfoxide (DMSO) for 24 h to result in D1. The wires were grown by immersing D1 in a solution of 20 mM 1,4-diaminobenzene (2) in ethanol for another 24 h to ensure the formation of D1N (see Figure 2). The same procedure was followed using 1 and 2 alternatively until the donor block reached the desired length Dm. The Au substrate with the Dm SAM was reacted with the acceptor 3 by immersing the substrate in a solution of 3 in DMSO (10 mM) for 24 h. The obtained DmA1-p wire was immersed in a solution of 4 in ethanol (20 mM, 0.02% HCl to activate the carbonyl) for 24 h to generate DmA1L. Alternating additions of 3 and 4 led to the formation of DmAn wires with controlled lengths. Each wire was finished by end-capping with benzaldehyde 5 to provide a consistent contact group for reproducible electrical characterization. The DmCAn wires were synthesized in a similar fashion. During imine condensation reactions, 0.02% HCl was present as an acid catalyst to ease the formation of Schiff base. Before being immersed in a new solution, it was crucial

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EXPERIMENTAL SECTION Materials. Au nuggets (99.99% pure) were purchased from Mowrey, Inc. (St. Paul, MN). Evaporation boats and Cr evaporation rods were purchased from R. D. Matthis (Long Beach, CA). Silicon (100) wafers were purchased from WaferNet (San Jose, CA). Contact mode AFM tips (DNP silicon nitride probes) were purchased from Veeco Instruments (Camarillo, CA). Absolute ethanol was purchased from Fisher Scientific. All other chemicals were purchased from Aldrich (Milwaukee, WI) and used without further purification. Stepwise Growth of Molecular Wires on Au Substrates. The donor (D) and acceptor (A) building blocks, 4,4′(5′)-diformyltetrathiafulvalene (DF-TTF, 1)57,58 and N,N′di(4-anilino)-1,2,4,5-benzenebis(dicarboximide) (4-ABI, 3),59 C

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the inset of Figure 6b. These experiments were performed with a Multimode AFM from Veeco Instruments (now Bruker Instruments) in an Ar-filled glovebox (O2 < 4 ppm). A 2 nN load force was applied to make soft contact. Au-coated tips were prepared as reported previously,42 each of which gave the same resistance of 2 GΩ for tunneling through a 1-nonanethiol SAM (an effective calibration standard). The radius of each Aucoated AFM tip was ∼50 nm, enabling contact to ∼100 molecules in each junction, as estimated from the wire surface coverage. We have examined the I−V characteristics of each molecular wire up to ±1.0 V, and the resistance was determined from the derivative dV/dI at specific biases. Temperature-dependent measurements of the I−V characteristics of the wires were performed with an environmentally controlled Molecular Imaging PicoScan/PicoSPM in the range from −25 °C (248 K) to 55 °C (333 K). Two different sample stages, a normal heating stage and a Peltier stage, were used to access temperatures above and below room temperature, respectively. Ag paste was painted on the edge of each sample to make thermal contact between the sample and the stage. All measurements were conducted under flowing N2, keeping the relative humidity of the instrument chamber below 5%. Computational Details. Geometry optimizations were performed for DmAn (m = 1−3, n = 1−3) and DmCAn (m = 1− 3, n = 1, 2) at the density functional level of theory with the B3LYP,60,61 M06-2X,62 M06-HF,62 and M1163 functionals using the 6-31G(d,p) basis set,64 as implemented in the MNGSM package in a locally modified version of the Gaussian09 software package65,66 for the neutral ground state. Additionally, optimizations were performed for the radical cations with the M06-2X, M06-HF, and M11 functionals. D4CAn (n = 1, 2) were studied with the M06-2X functional only. For comparison, the donor and acceptor groups were studied alone. Geometric parameters and electronic properties are reported, focusing on the results from the M06-2X functional with a brief discussion of functional effects. The full results with B3LYP, M06-HF, and M11 are included in the Supporting Information. The effects of applied voltage were approximated by performing geometry optimizations in a constant applied field with the B3LYP functional. Vertical electronic excitation energies were computed using time-dependent density functional theory (TD-DFT).67

that the Au substrate with each wire intermediate was immersed in pure solvent (DMSO or ethanol) for 30−60 min and thoroughly rinsed by pure solvent to remove physically adsorbed compounds. All the wires or wire intermediates were rinsed one more time with absolute ethanol and dried under a stream of Ar before measurements. SAM Characterization. Prior to junction formation and electrical measurement, the SAMs of different molecular wires on Au substrates were extensively characterized by reflection− absorption Fourier transform infrared spectroscopy (RAFTIR), ellipsometry, X-ray photoelectron spectroscopy (XPS), and cyclic voltammetry. RA-FTIR spectra were taken with a Nicolet Series II Magna-IR System 750 FTIR with a Harrick Seagull accessory for grazing incidence specular reflectance measurements. The infrared beam was incident at 84° with respect to the surface normal. A total of 2000 scans were collected at 1.0 cm−1 resolution. Monolayer thicknesses were determined by ellipsometry and XPS. Ellipsometry measurements of the polarization angles Ψ and Δ were taken as a function of wavelength (λ) between 600 and 990 nm at an incident angle of 65° from the surface normal. The indices of refraction (n) and extinction coefficients (k) were assumed to be 1.45 and 0 based on previous experience.43 XPS spectra were taken on an SSX-100 system (Surface Science Instruments) equipped with a monochromatic Al Kα X-ray source (200 W), a hemispherical sector analyzer (HSA), and a resistive anode detector, under ultrahigh vacuum ( 1) and (DA)n wires do not rectify, which is consistent with the idea that the key contribution to the rectification in DmCAn is the proper alignment of frontier orbitals in the donor and acceptor blocks; in particular, the conjugation blocking C group is apparently important to the observation of rectification. Metzger and others have proposed different mechanisms for molecular rectification;25,30,81,82 however, these theories are all developed based on tunneling. Here we have carried out DFT calculations of the highest occupied orbital energies for an isolated D2CA1 wire subjected to external electric fields in order to better understand the transport mechanisms in forward and reverse bias. Figure 11 shows the energy levels and spin orbital probability densities for the D2CA1+ cation under no bias, forward bias, and reverse bias conditions (energy levels and orbital probability densities for neutral D2CA1 are shown in Supporting Information, Figure S3; see Table S3 for tabulated energies). In the no bias case, panel A, the hole (positive charge) is localized in the SOMO (spin orbital 1), which is confined to the donor block. Here, the donor block comprises structural “subunits” A and B as shown. The other spin orbitals 2 and 3 are energetically localized to different positions of the acceptor block and are also energetically relatively near the SOMO (1). When a 1 V forward bias is applied (Figure 11B), the degeneracy of orbital 1 in panel A is broken and four distinct spin orbitals are in play. We propose that a positive hole is injected into orbital 1 which lies on molecular subunit A (a donor group) closest to the left (positive) electrode. Under the influence of the applied electric field, the hole then hops to localized states 2, 3, and 4 on subunits B, C, and D, sequentially, as shown. The key rate limiting step is then the endothermic hop (holes prefer to hop up in energy) from subunit B to C (orbitals 2−3), that is, the hop from the donor block to the acceptor block, which according to Figure 11B is about 0.35 eV uphill. More accurate TD-DFT calculations suggest that the cost for this transition is 0.27 eV (B3LYP, see Table S4 in Supporting Information), remarkably close to the 0.23 eV hopping activation energy we measured for D2CA1 under forward bias (Figure 10). When 1 V reverse bias is applied, Figure 11C, we propose that the acceptor block orbitals † and †† are simply too deep to be accessible for hole injection. We propose instead that tunneling occurs so that a hole is introduced from the right electrode onto state 1 on subunit B of the donor block. This is the rate limiting step given the relatively large tunneling

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CONCLUSION We have observed electrical rectification in molecular junctions containing long donor−acceptor block molecular wires. Forward bias corresponds to a positive potential applied to the donor block and a negative potential applied to acceptor block, as expected. Length and temperature dependent electrical measurements of these rectifying wires allow informed speculation regarding the mechanisms of transport and rectification. We propose that for D2CA1, the most rectifying wire, that the rate limiting transport step under forward bias is hole hopping across the conjugation blocking subunit C, and that under reverse bias the rate limiting step is hole tunneling from the positive electrode across the A block. We judge that transport in these wires is “hole-only”, that is, electrons are not injected into the A block because of the large workfunction of the Au contact and the very high LUMO level of the A block. Future work is aimed at having a truly ambipolar junction where both electrons and holes are injected into the wires and to systematically examine the transport and rectification properties of the wires as a function of tunable energy levels in the D and A blocks.

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ASSOCIATED CONTENT

S Supporting Information *

I−V curves of DmCA2 wires, temperature-dependent I−V curves for D2CA1 and D4CA1 wires, and a comparison for I−V curves between Ag and Au-coated tips. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest. J

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ACKNOWLEDGMENTS We thank NSF (CHE-0616427) for financial support. Parts of this work were carried out in the Characterization Facility, University of Minnesota, which receives partial support from NSF through the MRSEC program. D.M. was supported primarily by the National Science Foundation through the University of Minnesota MRSEC under Award Number DMR0819885. The computational results (L.B., B.V., D.M., L.G.) are based on work supported by the National Science Foundation under Grant CHE-1212575.

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