Sequential electron transport and vibrational excitations in an organic molecule coupled to few-kayer graphene electrodes

Graphene electrodes are promising candidates to improve reproducibility and stability in molecular electronics through new electrode-molecule anchoring strategies. Here we report sequential electron transport in few-layer graphene transistors containing individual curcuminoid-based molecules anchored to the electrodes via pi-pi orbital bonding. We show the coexistence of inelastic co-tunneling excitations with single-electron transport physics owing to an intermediate molecule-electrode coupling; we argue that an intermediate electron-phonon coupling is the origin of these vibrational-assisted excitations. These experimental observations are complemented with density functional theory calculations to model electron transport and the interaction between electrons and vibrational modes of the curcuminoid molecule. We find that the calculated vibrational modes of the molecule are in agreement with the experimentally observed excitations.


Abstract
Graphene electrodes are promising candidates to improve reproducibility and stability in molecular electronics through new electrode-molecule anchoring strategies. Here we report sequential electron transport in few-layer graphene transistors containing individual curcuminoidbased molecules anchored to the electrodes via π − π orbital bonding. We show the coexistence of inelastic co-tunneling excitations with single-electron transport physics owing to an Molecular electronics promises to take advantage of the variety of built-in functionalities of single molecules to fabricate molecule-based electronic devices. 1 The advance in miniaturization of electronic components has been preceded by a plethora of interesting physics at the nanoscale brought on by the interaction between charge, magnetism and superconductivity at the single molecule limit. 2,3 The progress towards robust, room-temperature operation has, however, been limited by the instability of the molecule-gold bond in ambient conditions inherent to the high mobility of gold atoms. 4,5 Additionally, reproducibility of the molecular conductance remains an open challenge in solid-state devices due to the variability in the geometry of the molecule-electrode bonding from device to device.
Carbon-based electrodes 6,7 and in particular graphene electrodes [8][9][10][11] are attracting special interest as a viable solution for stability and reproducibility in contacting single molecules. The covalent sp 2 hybridization of the carbon lattice provides graphene electrodes with high stability even in ambient conditions. 12 Moreover, the two-dimensional character of graphene reduces the thickness of the electrodes and may therefore increase the coupling of the molecule to an underlying gate. Increasing in complexity, graphene electrodes can be combined with superconducting or ferromagnetic metals to achieve functional hybrid devices. Such geometry preserves the anchoring chemistry between molecule and electrodes. 3 When using graphene, new anchoring strategies need to be developed to substitute the traditional anchoring groups to gold (-SH, -NH). Graphene offers the possibility of covalent bonds to the molecule leading to stronger bonds stable for room temperature application. 8 In addition, aromatic anchoring groups such as pyrene or anthracene could lead to a more reproducible electrodemolecule bonding thanks to a gentler contact with the electrode that preserves a more well-defined geometry. 9,13 However, the potential of π − π stacked molecules can be limited by the variety of shapes and composition of the edges of the electrodes. 14 The molecule-electrode contact should therefore be placed far from the edge to minimize its influence.
In this work we synthesize a new curcuminoid-based molecule better suited for few-layer graphene (FLG) junctions improving on our previous study 8 by extending the transport backbone which allows the anchoring groups to sit farther from the graphene edges. We report intermediate electrode-molecule coupling (Γ) by π − π stacking of anthracene groups to FLG electrodes. We show the coexistence of inelastic co-tunneling excitations with Coulomb blockade physics. We further show that an intermediate electron-phonon coupling results in vibrational excitations in the single-electron transport (SET) and Coulomb blockade regimes. To complement our experimental observations, we perform density functional theory (DFT) calculations to model electron transport and the interaction between electrons and vibrational modes of the curcuminoid molecule.

RESULTS AND DISCUSSION
The structure of the curcuminoid-based molecule, 9ALCccmoid, is schematically shown in Fig.1(a). This molecule is synthesized by modifying the methodology reported in the literature. [15][16][17] Details of the synthesis and crystallographic information are given in the Methods section. The 9ALCccmoid molecule is made of a conjugated linear chain of eleven C atoms that contains a β -diketone group at the center of the backbone (diarylundecanoid framework). Two anthracene groups complete the chain on both sides. The crystallographic data shows that the conjugated linear backbone  Table 1 summarizes some selected interatomic distances. The length of the molecule is 2.15 nm and therefore is significantly longer than the antecedent ccmoid form 9Accm, [19][20][21][22] with a length of 1.71 nm and only seven C atoms in the chain. 8 This modification ideally brings the anchoring groups farther from the edges, facilitating the coupling of the molecule to the aromatic structure of the graphene by π − π stacking. The β -diketone moiety and the two ending anthracene groups are identical for both systems. The electroburning process is known to result in a variety of source-drain separations ranging from 1-2 nm. 8,12 The 1 The two possible tautomers have not been included in the explanation of the transport through the curcuminoid. Only the keto/enol tautomer has been used as is indicated in Figs 1 and 5. Interestingly, the crystallographic structure shows that the keto/enol tautomer is the most stable (Fig 1). In addition, DFT calculations with the B3LYP functional indicate that for the isolated molecule, the keto/enol form is 15.4 kcal/mol more stable than the diketo one (considering the C2v isomer). This calculated energy difference is even higher than the equivalent for the acetylacetone (acac) molecule (10.0 kcal/mol 18 ). Thus, the estimated interconversion barrier for acac is higher than 60 kcal/mol and should be even higher for the curcuminoid confirming the stability of the keto/enol form due to the strong hydrogen bond. purpose behind the design of 9ALCccmoid is to provide an optimal length, capable of adapting the molecule to a range of electrode separations without disturbing the conjugation of the molecule and the anchoring groups. The comparison of the two structures 9ALCccmod and 9Accm, further characterization of 9ALCccmoid and the resulting lengths of both molecules are shown in the Supporting Information.
The nanometer-spaced electrodes are fabricated by electroburning of few-layer graphene (FLG) flakes contacted by gold leads as sketched in Fig.1(b). Flakes of around 10 nm thickness are obtained by mechanical exfoliation of natural graphite and deposited onto a silicon wafer coated with 285 nm of silicon oxide. The underlying silicon substrate is used as gate electrode. Further details of the fabrication of the devices are described elsewhere. 3,8,12 Few-layer graphene flakes are selected, in contrast to single layer, to guarantee a continuous reservoir of electrons and avoid the gating of the source and drain electrodes. 23,24 The gating of the current in our devices can therefore be ascribed to the gating of the molecule itself. This is an improvement when compared with previous reports on carbon-based electrodes such as carbon nanotubes. 6 characteristics of nanoscale junctions. 3,27,28 The gate trace corresponding to the same device is measured at V = 300 mV. Except for a slow capacitive increase in current due to the high junction resistance, no gate dependence is observed in the accessible gate voltage window (−40 < V g < 40 V) as seen in the inset of Fig.1(d). This is a strong indication of an empty gap. Note that such a gate response at low temperatures stipulates the absence of small graphene quantum dots in the gap as room-temperature characterization may overlook graphene nano-islands with large charging energies that could mimic the behaviour of the target molecules 29   After molecule deposition, the current increases around two orders of magnitude and becomes asymmetric in bias. Moreover, several steps appear in the current that are more clearly seen as peaks in the dI/dV . The current level is comparable with that reported for the shorter curcuminoid derivative. 8 This picture indicates that either the anchoring is the limiting factor for the current in this family of molecules or that the effective transport pathway of the molecules is similar. Our theoretical results (see below) support the transport pathway picture.
The inset of Fig.2(a) shows the I −V g trace measured at V = 200 mV. Two high-current peaks appear at 23 V and 31 V in contrast with the flat I − V g for the empty junction in Fig.1(d). An additional resonance starts to show up below -20 V. These peaks are a fingerprint of resonant transport through a molecule coupled to the electrodes. Figure 2(b) shows an I color plot measured in sample A as a function of V and V g around the V g = 31 V resonance (for a complete color plot in V g see Supporting Information). High-current regions, a signature of sequential electron trans- port (SET), separate low-current regions where current is blocked. In the later, the charge in the molecule is stabilized; i.e., the molecule is in the Coulomb blockade regime. Figure 2(c) shows a dI/dV color plot numerically derived from a finer I measurement around the SET region shown in Fig.2(b). High-conductance SET resonances separate low-conductance regions. Additional inelastic co-tunneling excitations within the low-conductance diamonds (not visible in this contrast) point to an intermediate coupling of the molecule with the electrodes (Γ). 30 The value of Γ is estimated from the full width at half maximum (FWHM) of the dI/dV peaks at the Coulomb diamond edge. We find Γ = 10 meV (see Supporting Information for calculation), a value that is comparable to that obtained with thiol groups used for gold electrodes. 30   The absence of resonant transport could be ascribed to a lower gate coupling that shifts the resonances to higher absolute gate voltages. A representative dI/dV trace at V g = 0 is shown in Fig.3.  Table 2 lists the energy of the excitations in both samples. The origin of such excitations in 9AL-Cccmoid could be either magnetic, electronic or vibrational. The 9ALCccmoid is a non-magnetic molecule and therefore we disregard magnetic excitations as in Ref. 31 We argue below in favor of the vibrational origin of the excitations due to an intermediate electron-phonon coupling.
To gain insight on these experimental observations, we have performed DFT calculations on the curcuminoid molecule. DFT calculations of the molecular device were performed with the numerical ATK 2014.2 code 32-34 using the PBE functional adding dispersion terms through Grimme D2 approach. 35 The model structure was created from a single layer of graphene by deleting a ribbon of four carbon atoms and adding hydrogen atoms to saturate the carbon dangling bonds.
The 9ALCccmoid molecule was placed on top with one anthracene group placed close to each graphene electrode. The fully optimized structure is shown in Fig.4(a). The non-symmetric final structure is due to the presence of an energy minimum with a small relative horizontal shift of the The color scale is the direction of the transport in radians: 0 is for rightwards transport , π is for leftwards transport and π/2 is transport perpendicular to the graphene plane. The calculations show that electrons jump on the molecule through the anthracene but also through the backbone.
graphene electrodes and it was selected in order to be closer to the experimental geometry. Interestingly, the H atom in the β -diketone group can be replaced by different magnetic metallic atoms. [19][20][21][22] The small presence of the β -diketone for HOMO transport makes 9ALCccmoid a good candidate to probe magnetism at the nanoscale by providing a well conjugated backbone in close proximity to a magnetic atom.  Fig.4(b). The LUMO, the most active for transport, is mainly centered in the backbone and barely present in the anthracene groups. This picture could explain the relatively similar currents obtained for the 9ALCccmoid and the shorter 9ALCccmod curcuminoid molecules, because in both cases the transport path is essentially determined by the distance between the graphene source and drain electrodes.

Synthesis of 9ALCccmoid
All experiments were carried out in aerobic conditions using commercial grade solvents for the synthesis of 9ALCccmoid. The molecule was synthesized according to the procedure described elsewhere. [15][16][17] Acetylacetonate (0.83 g, 8.30 mmol) and B 2 O 3 (0.44 g, 6.25 mmol) were dis-solved in 10 mL of EtO 2 CMe. The reaction mixture was heated at 40 • C for 30 min. Then, a solution of 4.1 g of 3-(9-Anthryl)acrolein (17.65 mmol) and 8.12 g of tributyl borate (35.30 mmol) in 20 mL of EtO 2 CMe was added. The mixture was stirred at 40 • C for 3 h. After cooling down, an excess of n-butylamine (0.44 mL, 4.45 mmol) in EtO 2 CMe (10 mL) was introduced dropwise.
The final reaction was stirred at room temperature for 2 days. As a result, a red solid precipitate appeared. The colored solid was filtered and washed with H 2 O, MeOH and Et 2 O to remove impurities. Yield 87 %. Crystals of 9ALCccmoid were achieved by slow evaporation using THF. Physical Measurements.

Crystallography
The crystalline structure of the 9ALCccmoid derivative has been obtained using synchrotron radiation source. Data were collected on a cut piece of a red needle of dimensions 0.12 x 0.02 x 0.02 mm 3 on a Bruker D8 diffractometer on the Advanced Light Source beam-line 11.3.1 at Lawrence Berkeley National Laboratory, from a silicon 111 monochromator (λ = 0.7749 ). Data reduction and absorption corrections were performed with SAINT and SADABS, 42 respectively. The struc-ture was solved and refined on F 2 with SHELXTL suite. [43][44][45] Hydrogen atoms were all found in a difference Fourier map, included at calculated positions on their carrier atom and refined with a riding model, except that on the central diketone moiety that was refined freely with its isotropic thermal parameter 1.5 times that of the closest oxygen, O1. The main crystal parameters are summarized in Table S1 in the Supporting Information. All crystallographic details can be found in CCDC 1437644 and can be obtained free of charge from the Cambridge Crystallographic Data Centre via https://summary.ccdc.cam.ac.uk/structure-summary-form.