Article Cite This: J. Phys. Chem. C 2018, 122, 25102−25109
pubs.acs.org/JPCC
High-Quality Reduced Graphene Oxide Electrodes for Sub-Kelvin Studies of Molecular Monolayer Junctions Martin Kühnel,†,‡,§ Marc H. Overgaard,† Morten C. Hels,§ Ajuan Cui,† Tom Vosch,† Jesper Nygård,§ Tao Li,∥ Bo W. Laursen,† and Kasper Nørgaard*,†
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Nano-Science Center & Department of Chemistry and §Center for Quantum Devices & Niels-Bohr Institute, University of Copenhagen, Universitetsparken 5, 2100 Copenhagen, Denmark ‡ Sino-Danish Center for Education and Research, Niels Jensens Vej 2, 8000 Aarhus, Denmark ∥ School of Chemistry and Chemical Engineering, Shanghai Jiao Tong University, 800 Dongchuan RD., Minhang District, 200240 Shanghai, China S Supporting Information *
ABSTRACT: Electron transport phenomena in molecular monolayers are complex and potentially different from those of single molecules because of, for example, molecule− molecule interactions. Unfortunately, access to detailed mechanistic investigations of molecular monolayer junctions at ultralow temperatures is typically hampered by the narrow range of operating temperatures for most large-area device platforms. Here, we present a highly optimized chemically derived graphene material with a near temperature-independent conductance profile. Using this material as a conducting interlayer electrode in solid-state molecular electronic devices, we show robust and reliable large-area molecular junction operation at temperatures ranging from room temperature to below 1 K, and we demonstrate the ability to measure inelastic electron tunneling spectroscopy of a conjugated molecular monolayer at cryogenic temperatures.
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INTRODUCTION Access to low-temperature measurements is important for many studies of molecular electronic junctions, such as for identifying the charge transport mechanism1−5 and for investigating specific chemical signatures of the molecular junctions,6,7 for example, by inelastic electron tunneling spectroscopy (IETS).8 Numerous techniques have given insight into the charge transport on the single-molecule level, including scanning tunneling microscopy9,10 and mechanical break junctions,11,12 reporting charge transport phenomena such as quantum interference and vibrational mode signatures.13,14 Established platforms for room-temperature studies of molecular monolayers include conductive atomic force microscopy (AFM), eutectic gallium−indium (EGaIn),15−17 and micropore devices.18−23 The two latter approaches, in particular, offer a solution for fabricating permanent and robust molecular electronic junctions with a much larger potential for device integration than any single-molecule technique. However, while low-temperature measurements of singlemolecule junctions are readily accessible,24 studies of largearea molecular junctions at cryogenic temperatures remain largely unexplored. We have previously demonstrated solid-state molecular electronic devices based on large-area self-assembled molecular monolayers (SAMs) that utilize an ultrathin film of thermally © 2018 American Chemical Society
reduced graphene oxide (rGO) as the top electrode for roomtemperature studies of molecular monolayers.19,20,25,26 rGO serves as a protective barrier, shielding the underlying SAM against damage from a thermally evaporated Au top electrode,20,27 while in itself functioning as a top electrode, with the added benefit of high transparency for optically addressing the SAM.19 Measurements at reduced temperatures, however, revealed a very large temperature dependence on the conductivity of these films, with the transversal resistance increasing several orders of magnitude upon cooling from room temperature to sub-1 K.28 The temperature dependency was found to originate from a variable-range hopping (VRH) type of charge transport between localized aromatic domains in the rGO thin films. The term rGO covers a wide range of chemically derived graphene materials with varying properties that depend intimately on the preparation conditions. rGO is the reduced form of graphene oxide (GO), which is typically prepared by oxidizing and exfoliating a graphite precursor.29 Several different oxidation reactions have been reported, the most common being the Hummers’ method30 and, in particular, its Received: August 28, 2018 Revised: October 10, 2018 Published: October 15, 2018 25102
DOI: 10.1021/acs.jpcc.8b08377 J. Phys. Chem. C 2018, 122, 25102−25109
Article
The Journal of Physical Chemistry C
been pretreated in a base piranha solution. The GO film thickness was determined by the concentration of the GO solution, the applied volume, the spinning speed (600 rpm for 15 s, followed by 3000 rpm for 40 s), and the number of spincasting cycles. The GO thin films were reduced chemically (HI vapor, 70 °C, 20 min) and/or through thermal annealing (nitrogen atmosphere, 600 °C, 1 h). The rGO thin films were transferred to microchips by a two-step process. First, the rGO thin film was partly floated off in a 1 M NaOH solution, before being picked up again and floated completely off in an ultrapure water bath. The rGO thin film could now be transferred to any substrate by catching it from below. The rGO thin film samples (∼6 nm thick) for van der Pauw Hall effect measurements were spin-coated directly onto a silicon wafer (285 nm oxide), and a 5 × 5 mm square was patterned from the thin film. Conducting silver ink combined with a gold wire bonder was used to contact each of the four corners. Fabrication of the Microchip. The lithography and fabrication of microchips were described previously.20 In brief, gold electrodes were patterned with electron beam lithography and thermal evaporation on a silicon wafer (285 nm oxide). A dielectric layer of Al2O3 was deposited with atomic layer deposition. Well-defined microwells (2 μm in diameter) were etched in the Al2O3 layer down to the gold electrode. The resulting microwell has steep walls and gold roughness of 0.3− 0.4 nm. Solid-State Junctions. Molecular monolayers were grown from molecular solutions (20−100 μM), 1-octane thiol and 1dodecane thiol (Sigma-Aldrich) in ethanol (99% purity) and 1hexadecane thiol and OPE3 dithiol (Sigma-Aldrich) in tetrahydrofuran. Before the molecules were added, the growth solvent was purged with nitrogen (10 min). The cleaned microchips were submerged in the molecular solution and the vial was sealed with parafilm and alufoil and stored in a nitrogen box. After 48 h, the microchips were removed from the molecule solution and rinsed with the growth solvent and isopropanol, before being blown dry with nitrogen. A rGO thin film was directly transferred to the microchip, and the chip was dried in vacuum (10−3 mbar, >2 h). After rGO transfer, a 50 nm thick and 100 μm wide gold top contact was evaporated through a shadow mask. Uncovered rGO was removed in oxygen plasma for 4−10 min depending on the film thickness. Upon successful transfer of rGO, the yield of working microwell junctions exceeded 90%, based on at least 100 junctions for each molecule. Chemical, Structural, and Electrical Characterization. X-ray photoelectron spectroscopy (XPS) measurements were performed with a K-Alpha+ X-ray photoelectron spectrometer system, and AFM measurements were performed with a Bruker MultiMode 8 microscope. The Raman measurements were done using a 514 nm CW argon ion laser (CVI Melles Griot 35MAP431-200) aligned to an Olympus IX71 microscope, which was coupled to a spectrograph (Princeton Instruments SPEC-10:100B/LN_eXcelon with SP 2356, 600 grooves/ mm). The Hall effect measurements using the van der Pauw method were performed on a home-built system, utilizing a Keithley 2101 source meter and measured at 0.08 T. The room-temperature conductance measurements were performed with a home-built probe station, utilizing a Keithley 2401 source meter. Low-temperature electronic measurements were performed with a Heliox AC-V cryostat from Oxford Instruments and standard lock-in measurement techniques.
modified version.29 Similarly, a variety of approaches have been reported for the reduction of GO to rGO,29,31−33 typically based on either a chemical or a thermal treatment of the GO material. The resulting rGO is believed to consist of ordered and conducting sp2-hybridized carbon domains embedded in more disordered regions with higher sp3 and oxygen content.34 Careful optimization of the GO synthesis and subsequent reduction have recently been shown to produce a vastly superior rGO material with less defects and lower oxygen content compared with the traditional Hummers’ method.35−37 Here, we present a newly developed high-quality rGO thin film with a near temperature-independent transversal conductance for use in large-area, solid-state molecular junctions. The low-temperature charge transport mechanism of the highquality rGO thin film is investigated and the film is employed as a top electrode in SAM devices, and we demonstrate that the temperature independence is maintained also in working molecular tunneling junctions. To show the full potential of the high-quality rGO soft-top contact, we obtained vibrational information of the molecular junctions using IETS at 2.6 K.
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METHODS Graphite Oxide Synthesis. Two different GO solutions were synthesized by the modified Hummers’ method29 (GO1) and the optimized Hummers’ method35 (GO2). Both syntheses were based on the same graphite (25 mesh, Alfa Aesar). GO1. Graphite (0.5 g) and NaNO3 (0.25 g, Sigma-Aldrich) were dispersed in concentrated sulfuric acid (20 mL, 95−98%, Sigma-Aldrich). Potassium permanganate (3 g, Sigma-Aldrich) was added over 1 h, while cooling the mixture on ice. After 1.5 h with ice cooling, the temperature was raised to 35 °C and kept for 16 h, after which Milli-Q water (40 mL) was added and the temperature was raised further to 90 °C. After 15 min, the mixture was cooled to room temperature. Additional MilliQ water (100 mL) was added followed by hydrogen peroxide (4 mL, 33%, Sigma-Aldrich). After synthesis, the product was washed with Milli-Q water until reaching neutral pH. GO2. Graphite (0.5 g) was dispersed in concentrated sulfuric acid (20 mL, 95−98%, Sigma-Aldrich). Potassium permanganate (1 g, Sigma-Aldrich) was slowly added over a time span of 4 h, while cooling the mixture on ice. The cooling was continued for 16 h, after which diluted sulfuric acid (20 mL, 10%) was added over 2 h. Subsequently, Milli-Q (60 mL) water was added over a period of 6 h. The mixture was kept on ice. To dilute the mixture, ice cooled water (500 mL) was added before slow addition of hydrogen peroxide (4 mL, 33%, Sigma-Aldrich). After synthesis, the product was washed with Milli-Q water until reaching neutral pH. GO Purification. Solutions of primarily single-sheet GO in a 1:5 mixture of water and methanol were prepared and ultrasonicated to exfoliate the graphite oxide into single sheets of GO (30 min, 250 W, 50−60 Hz). Subsequently, the solutions were refined with high-speed (20 min, 8000 rpm/ 6800g) and low-speed (20 min, 2500 rpm/660g) centrifugation to remove the smallest and largest GO particles, respectively. The precipitate from the high-speed centrifugation was redispersed in a 1:5 mixture of water and methanol, and the supernatant from the low-speed was used directly for thin-film fabrication. Reduced GO Thin Films. The resulting GO solutions (0.1−0.5 mg/mL) were spin-coated onto glass wafers that had 25103
DOI: 10.1021/acs.jpcc.8b08377 J. Phys. Chem. C 2018, 122, 25102−25109
Article
The Journal of Physical Chemistry C d2I/dV2 was measured directly by the lock-in amplifier equipment.
Table 1. Oxidation and Reduction Conditions for s-rGO and hq-rGO and the Resulting Chemical and Electronic Properties
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RESULTS AND DISCUSSION We investigated several candidate rGO thin films using Raman scattering and XPS to select the rGO material with the fewest structural defects. We expected this material to display the lowest contribution from temperature-dependent VRH charge transport because of the presence of larger aromatic domains. Two variations of the oxidation reaction were used to oxidize a graphite precursor: (1) the conventional “modified” Hummers’ reaction29 (labeled GO1) and (2) a more recent GO synthesis described by Eigler, et al.35 and ourselves38 (labeled GO2). The GO2 approach improves significantly on the Hummers’ synthesis by keeping the reaction on ice throughout, while adding the oxidant (KMnO4) very slowly during the synthesis (see Methods). Because overoxidation of GO is a primary cause of structural defects in the carbon framework due to the release of CO2, maintaining the reaction temperature at 0 °C keeps this undesired side reaction at a minimum and thus produces a GO material with fewer structural defects. Thin films cast from a solution of the two synthesized GO materials were exposed to three different methods of reduction to form rGO: (i) a thermal reduction at 600 °C (T); (ii) a chemical reduction with HI vapor (HI); and (iii) a combination of chemical reduction with HI vapor followed by thermal reduction at 600 °C (HI + T), yielding a total of six different types of rGO thin films. A summary of the XPS data is shown in Figure S1 and Table S1. In general, the rGO materials prepared using the GO2 route had a lower oxygen content compared to their GO1 counterpart. The C 1s spectra of the GO showed similar oxygen content between GO1 and GO2 but with the latter having a lower percentage of more strongly bound oxygen, making it more amenable to complete reduction. The two reduction methods each proved efficient, with the thermal reduction removing the largest amount of oxygen. However, thermal reduction is known to introduce unwanted lattice defects due to decarboxylation.39 The rGO material with the lowest oxygen content was obtained based on the GO2 oxidation route, followed by both a chemical and thermal reduction (HI + T). This high-quality rGO material is provisionally labeled hq-rGO. Previous works from our group focusing on rGO materials as electrodes19,20,28 have utilized the modified Hummers’ method,29 annealed at 600 °C. For benchmarking purposes, we label this standard rGO material srGO. The XPS analysis of the two chosen rGOs is summarized in Table 1, along with the exact synthesis conditions. Raman spectroscopy supports the observation from XPS of a more highly reduced hq-rGO, that is, containing larger graphitic domains and less defects, see Figure S2 for Raman spectra and Table 1 for summary. The observed decrease of the ID/IG area ratio along with the decrease of the full width at half-maximum (fwhm)2D indicates a more graphene-like structure of the hq-rGO.40,41 Verifying that the observed differences in the chemical structure are directly correlated with the electronic performance, we measured the sheet resistance and Hall mobility with the van der Pauw method. Both the sheet resistance and the charge carrier mobility were measured for the full series of rGO materials used in the XPS study, see Table S2. From the sheet resistance measurements, we observe that the efficiency of reduction follows T < HI < HI + T and that rGO films based on the improved GO2 synthesis display lower sheet
s-rGO reaction
oxidation reduction
XPS Raman van der Pauw
C/O ratio ID/IG fwhm2D (cm−1) sheet resistance (kΩ/sq) mobility (cm2/V·s)
modified Hummers26 (GO1) 600 °C (T)
hq-rGO
9.3 2.23 1300 1062
optimized Hummers32 (GO2) HI vapor + 600 °C (HI + T) 20.9 1.63 430 3.1
