Large Magnetoresistance at Room Temperature in Organic Molecular

Large Magnetoresistance at Room Temperature in Organic Molecular Tunnel Junctions with Nonmagnetic Electrodes Zuoti Xie,†,§ Sha Shi,‡,§ Feilong Liu,‡ Darryl L. Smith,‡ P. Paul Ruden,*,‡ and C. Daniel Frisbie*,† †

Department of Chemical Engineering and Materials Science and ‡Department of Electrical and Computer Engineering, University of Minnesota, Minneapolis, Minnesota 55455, United States S Supporting Information *

ABSTRACT: We report room-temperature resistance changes of up to 30% under weak magnetic fields (0.1 T) for molecular tunnel junctions composed of oligophenylene thiol molecules, 1−2 nm in length, sandwiched between gold contacts. The magnetoresistance (MR) is independent of field orientation and the length of the molecule; it appears to be an interface effect. Theoretical analysis suggests that the source of the MR is a two-carrier (two-hole) interaction at the interface, resulting in spin coupling between the tunneling hole and a localized hole at the Au/ molecule contact. Such coupling leads to significantly different singlet and triplet transmission barriers at the interface. Even weak magnetic fields impede spin relaxation processes and thus modify the ratio of holes tunneling via the singlet state versus the triplet state, which leads to the large MR. Overall, the experiments and analysis suggest significant opportunities to explore large MR effects in molecular tunnel junctions based on widely available molecules. KEYWORDS: tunneling molecular junction, charge transport, magnetoresistance, unpaired charge carrier, interface dipole, singlet and triplet transmission etal−molecule−metal junctions (i.e., “molecular junctions”) provide a powerful platform for investigating the transport physics of molecules as a function of precisely controlled molecular architecture, molecular orientation, and molecule−metal linkages.1−18 In many cases these junctions are formed by making soft electrical contact to nanometer-thick self-assembled monolayers (SAMs) of functional molecules that bind in an oriented fashion to metals by specific chemistry (e.g., Au-thiol bonds).5,12,19−29 The conductance of SAM-based junctions continues to be extensively investigated, and different transport regimes5,10,30−32 and conductance phenomena (e.g., switching, rectification)12,26,33−36 have been reported. Interestingly, the number of papers that have explored magnetotransport effects in SAM-based or single-molecule junctions is still relatively small.37,38 Most of the published work concerns the Kondo effect at very low temperatures (e.g., 10 K).39−49 In Kondo experiments it is common to employ a large magnetic field, on the order of a few tesla, to observe the splitting of the Kondo resonance near zero bias across the junction.39,40,42,46−52 Some of the published data indicate a positive magnetoresistance (MR) for larger biases away from the zero bias Kondo resonance.39,40,46,47 However, to our knowledge, general MR, particularly at room temperature, has not been examined systematically in molecular junctions,

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though it is likely to provide important information about the transport physics of these systems. In this paper, we report observation of significant positive MR, typically ∼30%, at room temperature and at small magnetic fields (∼0.1 T) in SAM-based molecular tunnel junctions consisting of ∼100 parallel oligophenylene thiol molecules (OPT1, OPT2, OPT3) and two nonmagnetic gold contacts, Figure 1. Using conductive probe atomic force microscopy (CP-AFM) to form molecular junctions,30,53−55 systematic measurements of room-temperature MR as a function of molecular length (1−2 nm) reveal that the magnetic field does not affect the tunneling decay parameter, β, but does increase the molecule−metal interfacial resistance, i.e., the “contact resistance”, as determined by extrapolation of resistance versus molecular length plots. Additionally, the MR is independent of field orientation. The nonmagnetic character of both the molecule and the electrodes indicates that previously reported MR mechanisms for magnetic electrodes,14,56−59 chiral molecules,7,60,61 and magnetic molecules8,62 are not responsible for these observations. Received: June 10, 2016 Accepted: September 6, 2016 Published: September 6, 2016 8571

DOI: 10.1021/acsnano.6b03853 ACS Nano 2016, 10, 8571−8577

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Figure 1. Schematic representation of the conducting probe atomic force microscopy setup. A Au-coated AFM tip is brought into contact with a SAM of oligophenylene thiols of various lengths on a Au-coated substrate.

Figure 2. (a) Representative histogram of the low-bias resistance of OPT3 junction without (dark blue) or with (red) 0.1 T magnetic field perpendicular to the substrate. (b) Magnetoresistance versus the number of the phenyl rings of OPTn (n = 1, 2, 3) with perpendicular or parallel magnetic field. Quoted uncertainties in (a) and error bars in (b) represent one standard deviation. Data in (a) were recorded with a single Au-coated tip.

The effect may be explained by supposing that in equilibrium there is localized charge in the vicinity of the molecular linker at the interface, e.g., at the C−S bond. The Coulomb interaction couples the associated localized charge carriers with tunneling charge carriers. Thus, we propose that transmission of charge carriers through the molecules will involve singlet and triplet two-particle states, corresponding to two different nonresonant tunneling barrier heights. Effectively, these singlet and triplet states constitute separate and different transmission channels. Spin relaxation allows for transfer between these channels; however, the application of a magnetic field suppresses spin relaxation and thus reduces the combined probability of transmission. It is this effect that causes large positive MR. Importantly, the analysis is consistent with the independence of the effect on molecular length and field orientation and predicts large MR magnitudes for reasonable ratios of singlet and triplet transmissivities. Overall, our experimental results and theoretical model suggest intriguing opportunities to examine magnetotransport effects in molecules sandwiched between nonmagnetic contacts.

resistance (%) is defined as MR =

RB − R 0 , R0

where RB is the mean

resistance in the presence of the magnetic field and R0 is the mean resistance without the magnetic field. Figure 2a displays the histograms of hundreds of individual low-bias (±0.05 V) resistance measurements (∼1100 for 0 T and ∼800 for 0.1 T, tabulated without selection) for the OPT3 junction with and without a 0.1 T magnetic field perpendicular to the substrate. The means are (3.46 ± 0.06) × 106 Ω and (2.51 ± 0.03) × 106 Ω with and without the field, respectively. Here the quoted uncertainties represent 95% confidence intervals on the mean (not the measurement standard deviations). That is, there is a statistically meaningful positive MR effect with a magnitude of 38%. Figure S1 in the Supporting Information shows multiple measurements with and without the perpendicular magnetic field, highlighting the reproducibility of the effect. Furthermore, the results are independent of molecular length and field orientation, as shown in Figure 2b. The representative histogram for a parallel field is shown in Figure S2 in the Supporting Information. The average MRs of OPTs for perpendicular and parallel magnetic field orientation are (30 ± 4)% and (33 ± 3)%, respectively. Generally speaking, for short molecules (30 cm away to test resistance in the absence of the field. Approximately two-thirds of the junctions showed reproducible MR effects, as shown in Figure S1. For one-third of the junctions the MR was not reproducible upon repeated application and removal of the magnetic field. We attribute this to irreversible tip damage which caused changes in junction resistance measurements independent of the presence or absence of the magnetic field.

compact analytical result that depends only on the ratio of the singlet and triplet transmission coefficients can be obtained: MR =

(1 − KT/KS)2 8(1 + KT/KS)(KT/KS)

Results for the general case are plotted in Figure 6. As expected, MR = 0 if there is no difference in the transmission

Figure 6. Calculated magnetoresistance as a function of the ratio of the singlet and triplet transmission rate constants for four different ratios of the (singlet) transmission rate and the spin relaxation rate constants (kS = KS/ws).

rate coefficients of singlets and triplets. In all other cases, positive MR is obtained. The order of magnitude is consistent with the experimental evidence for reasonable transmission coefficient ratios. Finally, in contrast to the tunneling resistances for both singlets and triplets, the MR is completely independent of the number of phenyl rings in the limit KS, KT ≪ ws.

CONCLUSION We report significant room-temperature magnetoresistance, up to ∼30% for magnetic fields of 0.1 T, for Au-OPT-Au molecular junctions. The MR is independent of the length of the molecule and the orientation of the field. However, the resistance scales exponentially with the length of the molecule, indicating that the transport mechanism is off-resonance tunneling. Holes are identified as the relevant charge carriers. Theoretical analysis suggests that the interaction between the tunneling hole and an unpaired hole populating an interface state near the S−C bond leads to significant differences in the transmission barriers for singlet and triplet configurations. Furthermore, spin relaxation enables transitions between the singlet and triplet transmission channels. This relaxation can be suppressed by a small magnetic field, thus impacting the charge transport. A master equation model yields results that are consistent with the experimental data and provides insight into the underlying mechanism that gives rise to the magnetoresistance. Overall, these results suggest that there are significant opportunities to explore MR effects in relatively simple molecular junctions with nonmagnetic contacts. METHODS

ASSOCIATED CONTENT

Materials. Gold nuggets (99.999%) were purchased from Mowrey, 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). Contactmode AFM tips (DNP-10 silicon nitride probes) were purchased from

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsnano.6b03853. 8575

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Figure S1, average low-bias resistance with or without magnetic field perpendicular to the substrate and the corresponding histograms; Figure S2, representative histogram of the low-bias resistance of OPT3 junction with or without 50 mT magnetic field parallel to the substrate; Figure S3, high-resolution XPS spectra; and Figure S4, surface potential image and histogram for OPT1 on Au surface (PDF)

AUTHOR INFORMATION Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. Author Contributions §

Z.X. and S.S. contributed equally.

Notes

The authors declare no competing financial interest.

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