Matthieu
Koepf‡§
a,
Christopher
Koenigsmann¶‡
a,
Wendu
Ding||
a,
Arunbah
Batra
b,
Christian F. A.
Negre
c,
Latha
Venkataraman
*b,
Gary W.
Brudvig
*a,
Victor S.
Batista
*a,
Charles A.
Schmuttenmaer
*a and
Robert H.
Crabtree
*a
aDepartment of Chemistry & Energy Sciences Institute, Yale University, P.O. Box 208107, New Haven, Connecticut 06520-8107, USA. E-mail: gary.brudvig@yale.edu; victor.batisa@yale.edu; charles.schmuttenmaer@yale.edu; robert.crabtree@yale.edu
bDepartment of Applied Physics and Applied Mathematics, Columbia University, Mail Code: 4701, New York, NY 10027, USA. E-mail: lv2117@columbia.edu
cTheoretical Division, Los Alamos National Laboratory, Los Alamos, New Mexico 87545, USA
First published on 17th August 2016
The development of molecular components functioning as switches, rectifiers or amplifiers is a great challenge in molecular electronics. A desirable property of such components is functional robustness, meaning that the intrinsic functionality of components must be preserved regardless of the strategy used to integrate them into the final assemblies. Here, this issue is investigated for molecular diodes based on N-phenylbenzamide (NPBA) backbones. The transport properties of molecular junctions derived from NPBA are characterized while varying the nature of the functional groups interfacing the backbone and the gold electrodes required for break-junction measurements. Combining experimental and theoretical methods, it is shown that at low bias (<0.85 V) transport is determined by the same frontier molecular orbital originating from the NPBA core, regardless of the anchoring group employed. The magnitude of rectification, however, is strongly dependent on the strength of the electronic coupling at the gold–NPBA interface and on the spatial distribution of the local density of states of the dominant transport channel of the molecular junction.
The implementation of molecular components into functional devices will require a detailed understanding of the fundamental relationship between the intrinsic performance of the component considered and the strategy used to interface it to the surrounding device architecture. Here, we investigate the influence of metallic contacts on the rectification properties of archetypal gold–diode–gold molecular junctions.
We focus on N-phenylbenzamide (NPBA) derivatives since earlier studies have indicated that they are good candidates for intrinsic rectification.7 Furthermore, they offer a stable molecular backbone of which the substitution pattern can be systematically tailored using robust synthetic strategies. Finally, their structural simplicity makes them accessible to advanced computational analysis and combinatorial methods for a detailed understanding of their electronic properties.7–9
From an experimental perspective, the Scanning Tunneling Microscopy Break-Junction (STM-BJ)10,11 technique has enabled direct investigation of the transport properties of such derivatives.12 In these measurements, the selected molecular backbone is interfaced with nanoscopic metallic electrodes (typically gold electrodes) via specific chemical functionalities that act as anchoring groups. In the case of simple molecular wires, it has been shown that the nature of the anchoring groups can be of critical importance in determining the molecular states that govern the transport through the junction, and in controlling the strength of the electronic coupling between the backbone and the electrode materials.13
Rectification has been observed in molecular wires possessing symmetrical unsaturated backbones when different anchoring groups were employed on opposite termini of the wire, leading to a difference in the coupling to the gold leads.14,15 In molecular diode motifs with intrinsically asymmetric backbones, rectification is achieved even with symmetrical anchoring groups.16,17 Thus, a key question arises: how does coupling in symmetrically anchored metal–molecule–metal junctions influence the electronic properties and intrinsic rectification of molecular diodes such as NPBA derivatives? To address this question, we synthesized and analyzed a series of NPBA derivatives symmetrically substituted with a range of anchoring groups (Anchor–NPBA–Anchor, Anchor = CH2, NH2, or C2H4, Fig. 1).
We studied the transport properties of the gold–NPBA–gold junctions by using a combined experimental and computational approach that allows for interpretation of STM-BJ measurements based on density functional theory (DFT) calculations, the non-equilibrium Green's function (NEGF) approach and a single-state tight-binding model based on the Breit–Wigner formula.18 Our findings demonstrate that within the range of bias considered (<0.85 V) the dominant transport channel of the junctions and the rectification mechanism remain the same throughout the series of NPBA derivatives, despite the changes in anchoring groups. However, for any given bias, the anchoring group significantly affects the experimentally determined conductance and magnitude of rectification.
The conductance and I–V characteristics of the single molecule contacts are subsequently measured by forming junctions in the presence of a dilute solution of the desired molecule (1 mM–10 mM) dissolved in 1,2,4-trichlorobenzene (99%, Sigma Aldrich).19 The tip was brought into contact with the surface of the substrate until the conductance was greater than 5G0 (1G0 = 77.5 μS). The tip was subsequently withdrawn at a rate of 15 nm s−1 for a period of 125 ms and held for a period of 150 ms, while a triangular voltage ramp was applied between +1 V and −1 V. Finally, the tip was withdrawn at 15 nm s−1 for a period of 75 ms to break the junction before repeating the process for a total of ∼50000 individual traces. A data selection and sorting process, which is described in detail elsewhere,14,15 was then employed to generate the histograms of the I–V curves and determine the average I–V curve.
All derivatives led to the formation of stable junctions in STM-BJ measurements and the current profiles under both fixed and variable bias were investigated. The distribution of conductances measured under fixed bias (≤0.025 V) for junctions derived from the three NPBA derivatives is presented in Fig. 2a. In each case, the average low-bias conductance (G) is determined from a fit of the conductance distribution of the corresponding junction. The G values obtained for the three different anchoring groups are shown in Table 1. The trend is consistent with prior results12,13 as well as with the theoretically calculated zero-bias conductances (Gth) (vide infra).
Compound | G (G0) | G thb (G0) | RRc | RRcd | RRthe |
---|---|---|---|---|---|
a Derived from the statistical analysis of STM-BJ traces measured at ≤0.025 V. b Determined from DFT–NEGF calculations at zero bias. c Derived from the analysis of STM-BJ traces measured under variable bias. The values are reported for 0.80 V. d Corrected for the contribution arising from the sorting process of the individual I–V curves (cf. text). e Calculated for an applied bias of 0.80 V. | |||||
CH2-NPBA | 1.0 × 10−3 | 6.7 × 10−2 | 2.0 | 1.8 | 1.8 |
NH2-NPBA | 9.4 × 10−5 | 2.2 × 10−5 | 1.6 | 1.4 | 1.2 |
C2H4-NPBA | 8.5 × 10−6 | 1.6 × 10−5 | 1.4 | 1.2 | 1.1 |
The rectification ratio (RR) was determined from the asymmetry of the current–voltage (I–V) profile. Specifically, the RR is defined as the absolute value of the ratio of the current (I+) measured in the positive voltage sweep with respect to the current (I−) measured at the same voltage in the negative sweep: RR = |I+/I−|. For a fair comparison with the computed values (vide infra), at 0.8 V, the raw RR values can be corrected (RRc = RR − 0.2) by removing the contribution that arises from the sorting process of the individual I–V curves, which produces a negligible RR of 1.2 in non-rectifying junctions based on Z-stillbene.14Table 1 summarizes the measured RR and corrected RRc, at 0.8 V, for the NPBA backbones with varying anchoring groups. In agreement with the previous reports,14,15 we observe an increase in the RR for all junctions upon increasing the applied bias up to 0.8 V (Fig. 2b).
In the low bias regime (<0.85 V), the increase in the RR was found to correlate with the increase in conductance. The observation of a raw RR of ∼2 at 0.8 V, for the junctions derived from CH2-NPBA is remarkable in the context of other STM-BJ measurements, especially considering the simplicity of the molecular structure investigated. To the best of our knowledge, it is one of the highest values reported thus far for this method at low bias, for intrinsic rectifiers. As minimal differences in the conformation of the NPBA cores are expected within this series, the large variations of conductance and rectification behavior observed among the junctions is most likely related to the modulation of the electronic coupling between the NPBA backbone and the metallic electrodes.
To determine the RR under non-equilibrium conditions, we employed a single state tight-binding (TB) model based on the Breit–Wigner formula and computed I–V curves for the different Au–NPBA–Au junctions (see the ESI† for details). The model, parameterized from the zero-bias transmission function (Table S1†), simulates the energy shift of the dominant transport channel (ENPBA), according to the applied bias, and provides an approximation of the transport properties of the junctions under non-equilibrium conditions. The TB model provides remarkably good agreement between the RRth and RRc values (Table 1). These results confirm the critical role of the dominant transport channels identified above in the transport properties of all three junctions.
The nature of the anchoring groups significantly affects the energy alignment and broadening of the TF peaks shown in Fig. 3a, although the dominant transport channel for the three junctions is similar. Thus, the anchoring groups within this series primarily modulate the electronic coupling strength between the gold contacts and the NPBA HOMO. The coupling at the interface (as determined from the TB model) increases as follows: NH2 < C2H4 ≪ CH2, while the trend observed for both G and RR is: C2H4 < NH2 ≪ CH2. The inversion of the order for C2H4- and NH2-NPBA can be rationalized in terms of the efficiency of the coupling between gold and the specific subsets of molecular orbitals that dominate transport (here the π-based HOMO of the NPBA backbone; vide infra).
First, we focused on the junctions formed by CH2- and NH2-NPBA, which share the closest structural similarity in the set. For these junctions, we observe a strong difference in the ability of the gold leads to couple into the π-system of the NPBA core, despite the formation of formal σ bonds in each case (either dative N→Au or covalent C–Au bonds). To understand this difference, we examined the ratio of the projected density of states (PDOS) of the phenyl fragment (PDOSPh) to the adjacent Au–anchor motif (PDOSAu–Anchor). Specifically, we investigated the variation of the ratio PDOSPh/PDOSAu–Anchor as the phenyl group is rotated about the axis defined by the anchor–phenyl bond (see Fig. S6 and S7†). In this analysis, a large angle-dependent variation of the ratio PDOSPh/PDOSAu–Anchor indicates that there is a significant redistribution of the electronic density as the Au–Anchor and phenyl units are rotated with respect to each other from the equilibrium geometry. Such a redistribution can in turn be interpreted as the modulation of the electronic coupling between the Au–Anchor motif and the phenyl π-system, where the more coupled system exhibits the most homogeneously distributed density (e.g. PDOS ratio closest to 1).
In the case of CH2-NPBA, we observe a significant angle dependence of PDOSAu–Anchor/PDOSPh, with a 7-fold increase of the ratio as the angle varies from +0 to +90° relative to the equilibrium geometry (cf. Table S2†). Since a direct π–π overlap is excluded due to the pure sp3 hybridization of the benzylic carbon, this observation suggests that the strong electronic coupling can be attributed to the hyperconjugation between the quasi-covalent C–Au σ bond and the π-system of the NPBA.31 The electronic interaction between the benzylic C–Au bond and the aromatic π-system, initially postulated14,26 and recently demonstrated experimentally25 for (oligo)phenylenes, is thus well captured by our DFT analysis. On the other hand, the NH2-NPBA-based junction exhibits a much smaller angle dependence of PDOSAu–Anchor/PDOSPh with a 2-fold variation of this ratio as the angle is rotated from +0 to +90° from the equilibrium geometry. This is expected since the N→Au dative bond in NH2-NPBA is significantly more polarized than the C–Au bond, which reduces the strength of interactions through hyperconjugation.
In the case of C2H4-NPBA, a strong hybridization between the gold leads and the terminal carbons of the backbone accounts for the larger electronic coupling compared to NH2-NPBA. However, the π-system of the former remains isolated from the Au–C interface due to the additional methylene group on either side of the backbone. In this case, hyperconjugation cannot occur and the spatial distribution of the local density of states of the dominant transport channel remains strongly localized towards the center of the junction. It results in poorly conductive and virtually non-rectifying junctions in the STM-BJ measurements at low bias.
Footnotes |
† Electronic supplementary information (ESI) available: Detailed synthetic procedures and full characterization of the derivatives; description of the leads used in the theoretical studies; details of the parameters used for the single state tight binding model; representation of the DOS of the three junctions; representation of the HOMO and LUMO state of an isolated NPBA backbone; determination of the angular dependence of the PDOS of the phenyl/anchor–lead fragments; and 1H, 13C 19F NMR spectra of the derivatives. See DOI: 10.1039/c6nr04830g |
‡ These authors contributed equally to this work. |
§ Current address: Laboratoire de Chimie et Biologie des Métaux, CEA-CNRS-UGA UMR 5249, 17 avenue des martyrs 38054 Grenoble Cedex 9, France. |
¶ Current address: Department of Chemistry, Fordham University, 441 East Fordham Road, Bronx, NY 10458, USA. |
|| Current address: Department of Chemistry, Northwestern University, 2145 Sheridan Road, Evanston, IL 60208-3113, USA. |
** In the case of NH2-NPBA, a peak centred at +1.65 eV is almost equally distant to EF than compared to the dominant peak centred in the negative range (−1.58 eV). This suggests that both states might be controlling the transport properties of the junction. However, we will initially focus our analysis on the peak centred at −1.58 eV which nevertheless is the closest in energy to EF. |
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