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Rectification effect in organic junctions based on aryl thin films covalently attached to a multilayer graphene electrode

The quantum interaction between molecules and electrode s material at molecules-electrode interfaces is a major ingredient in the electronic transport properties of organic junctions. Driven by the coupling strength between the two materials, it results mainly in a broadening and an energy shift of the interacting molecular orbitals. Using new electrodes materials, such as the recent semi-conducting two-dimensional nanomaterials, has become a recent challenge in the field of molecular-organic electronics that opens new possibilities for controlling the interfacial electronic properties and thus the charge injection properties. In this article, we report the use of atomically thin two-dimensional multilayer graphene films as base electrode in organic junctions with a vertical architecture. The interfacial electronic structure dominated by the covalent bonding between bis-thienyl benzene diazonium-based molecules and the multilayer graphene electrode has been probed by ultraviolet photoelectron spectroscopy and the results compared with those obtained on junctions with standard Au electrodes. Room temperature injection properties of such interfaces have been also explored by electronic transport measurements. We find that, despite strong variations of the density of states, the Fermi energy and the injection barriers, both organic junctions with Au base electrodes and multilayer graphene base electrodes show similar electronic responses. We explain this observation by the strong orbital coupling occurring at the bottom electrode-bis(thienyl) benzene molecules interface and by the pinning of the hybridized molecular orbitals.


INTRODUCTION
The architecture of the two-and three-terminals building blocks of organic electronic devices, such as electroluminescent diodes and organic transistors, is largely inspired by their inorganic counterparts. Practically, the control and understanding of the electrode/molecules interface's properties is crucial for reaching high performances and designing functionalities 1 , such as electrical rectification. Symmetric current density as a function of bias voltage (J-V) characteristics were widely observed in two-terminal devices and discussed in the case of single molecule junctions 2 , monolayers 3 or oligomers based-organic junctions, as in the case of a large variety of aromatic nanostructures between conducting carbon electrodes 4,5 . It has 3 been shown that to introduce asymmetry (rectification) in the electronic response of molecular and organic junctions, it is necessary to introduce an asymmetry along the transport direction 6,7 . This can be achieve either by using different contact electrodes (i.e. with different injection properties), and/or by using asymmetric anchoring moieties or molecular structures as originally proposed by Aviram and Ratner 8 . Using such artificial systems to create nano-rectifiers has been widely reported and recently reviewed 9 . The electrode/molecules coupling plays also a crucial role in defining the amplitude of the rectification ratio (RR) 9 . Molecules can be either attached to electrodes via strong (chemisorption) or weak interactions (physisorption). Depending on the nature of the interactions on both sides of the junctions, the RR has been demonstrated to vary from few tenth to few thousands 9 . Recently record RR up to 10 5 were reported in alkyl-ferrocene-based self-assembled monolayers contacted by chemisorption to a bottom electrode and physisorption to a top electrode 10 . The rectification effect was also shown to be strongly dependent upon the position of the Fc unit (i.e. the molecular structure) between the two electrodes 11 . We have shown before that based on diazonium approach oligo-BTB form high quality organic layer and lead to highly stable organic junctions. We have reported before molecular diodes based on diazonium grafting in junctions of the form of Au-BTB//Ti/Au where "-" and "//" denote the bottom interface and the top interface respectively. Those organic diodes presented large rectification ratio of 1000 and we have systematically studied the mechanism of charge transport across this layer.
In this article, we focus on the electrode's properties. New atomically thin electrodes made of two-dimensional materials, such as graphene, have been recently proposed and developed to replace more traditional metallic electrodes 12 more sensitive to oxidation and/or electromigration. Graphene, multilayer graphene (MG) and graphite were recently used as a soft, transparent and tunable electrode for contacting rubrene single crystals 13 , self-assembled monolayers [14][15][16] and even single molecules [17][18][19][20][21][22] . The strength of the chemical bonding   between a graphene/graphite electrode and molecules was demonstrated to be a key   ingredient over the global electronic response of single-molecule junctions 21 .
Here we demonstrate electrical rectification effects in the case of MG-(oligo(1-(2bisthienyl) benzene, named BTB) diazonium-based thin film//Ti/Au junctions. We show that rectification is directly linked to the strong orbitals coupling at the MG-BTB interface like in the well-studied case of Au-BTB interfaces 23 and to pinning of the hybridized molecular orbitals at top and bottom electrode/molecules interfaces. Our observation is strongly supported by a comparative analysis of those two device's interfaces by electronic transport measurements and ultraviolet photoelectron spectroscopy.  corresponds to the formation of aryl radicals 26 at the proximity of the MG electrode. In the following scans, the current strongly drops to zero as the conducting surface of the electrode becomes passivated by the growth of an oligo-BTB thin layer 27 . BTB thin films are electroactive and can be easily p-doped at a potential close to 0.5 V/SCE (saturated calomel electrode) 27 . As a consequence, the conductance of a BTB thin film can be switched and diode-like behavior with a high rectification ratio is observed in the electrochemical response of outer-sphere redox probes 28 . At the end of the process, the MG surface is fully covered by the organic film as previously detailed for graphene and graphite surfaces 29 . No trace of diazonium salt was detected by X-ray photoelectron spectrometry analysis within the resolution of the detector (not shown). The final thickness and roughness of the BTB film were determined by atomic force microscopy as shown in figure 1(c) and was found to be around 10 ± 1 nm. After electro-grafting, the MG electrode was again characterized by Raman spectroscopy.   figure 3(c). The first molecular monolayer is expected to be strongly coupled to MG and molecular orbitals can be broadened by several eV 35 and on resonance with the Fermi energy of the MG electrode 36 . In the specific case of diazonium molecules, a strong C-C chemical bond is expected between the molecule and the carbon-based surface 4 .

MG
In case of chemisorption of molecules on metals, orbital's broadening up to few eV have been calculated for instance for Co/Alq3 surfaces 36 . It then tends to decay within the molecular layer as also shown for a Co/Alq 3 interface 36

EXPERIMENTAL SECTION
Multilayer graphene transfer. MG was purchased on Ni films. The Ni/MG bilayer was first covered with a thin layer of PMMA 950K deposited by spin-coating (4000 rpm) and baked at 110°C for 10 minutes. The Ni/MG/PMMA trilayer was then gently deposited at the surface of a FeCl3 bath in order to dissolve the Ni substrate. After dissolution, the floating MG/PMMA bilayer was transferred in a deionized H2O bath for 5 minutes. MG was cleaned in successive baths following the recipe described in reference 24 (i.e. RCA clean). It was then transferred in a deionized H2O bath and then gently "fished" by the substrate. After transfer, the sample was slowly baked from room temperature to 110°C in order to evaporate the remaining water.
The PMMA film was finally removed by immersing the sample in a warm (50°C) acetone bath for 5 minutes and in an isopropanol bath for 2 minutes. Yield and reproducibility: A high percentage (80 %) of working and reproducible devices is found in the case of Au-BTB//Ti/Au junctions in agreement with previous reports 37 . The yield of working devices is reduced when MG is inserted (around 20 %). It may be explained by the relatively high peak-to-peak roughness (< 10 nm) due to folds of the MG electrode compared to a more standard evaporated Au electrode (< 1 nm) as revealed by the AFM image presented in the figure 1(c). All the J-V characteristics presented are stable over periods of several hours.

Contributions
All authors have given approval to the final version of the manuscript.