Monolayered Graphene Oxide as a Low Contact Resistance

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C: Energy Conversion and Storage; Energy and Charge Transport

Monolayered Graphene Oxide as a Low Contact Resistance Protection Layer in Alkanethiol Solid-State Devices Martin Kühnel, Søren Vermehren Petersen, Rune Hviid, Marc H. Overgaard, Bo W. W. Laursen, and Kasper Nørgaard J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b12606 • Publication Date (Web): 15 Apr 2018 Downloaded from http://pubs.acs.org on April 15, 2018

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The Journal of Physical Chemistry

Monolayered Graphene Oxide as a Low Contact Resistance Protection Layer in Alkanethiol SolidState Devices Martin Kühnel‡†, Søren V. Petersen‡§, Rune Hviid†, Marc H. Overgaard†, Bo W. Laursen*†, Kasper Nørgaard*† †

Nano-Science Center & Department of Chemistry, University of Copenhagen, Universitetsparken 5, 2100 Copenhagen, Denmark

§

iNano & Department of Chemistry, University of Aarhus, Langelandsgade 140, 8000 Aarhus C, Denmark

1 ACS Paragon Plus Environment

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ABSTRACT.

Vapor deposition of metals has long been the primary method for making contact to organic molecules in electronic devices in a fast and scalable manner. However, direct metal evaporation has proven to be the primary cause of device failure in solid-state molecular devices, due to degradation of the self-assembled molecular monolayers. The introduction of a protective interlayer between the molecular monolayer and the evaporated top-electrode greatly improves the yield of working devices but at the cost of an increased internal contact resistance that depends on the nature of the interlayer and its interface to both the organic molecules and the metal top electrode. In the present work, we investigate the performance of single layered graphene oxide as an atomically thin interlayer in solid-state molecular devices. We show that a single layered graphene oxide sheet is sufficient to protect an organic monolayer of alkanethiols from metal induced degradation and short-circuiting. Remarkably, and despite graphene oxide being an insulating material, the contact resistance in our devices with a graphene oxide as protective interlayer is similar to pure metal/molecule/metal junctions. We interpret this observation as graphene oxide effectively becoming part of the top electrode. The graphene oxide monolayer is thus a very promising candidate as protective interlayer in solid-state molecular devices.


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1. INTRODUCTION Understanding the charge-transport properties of organic molecules is one of the main challenges in the field of molecular electronics.1-3 With increasing knowledge of the transport mechanism and its influence with molecular structure, new molecules with desirable properties can be synthesized, which subsequently could lead to fully functional electronic devices based on molecular junctions. Another strong motivation that drives research into molecular electronics is the potential to solve some of the scaling limitations and heating problems facing today’s silicon based technologies.1, 4-5 Successful realization of these goals is critically dependent on the fabrication of well-defined and stable molecular junctions, where especially the molecule-electrode contact is crucial.6 Numerous techniques have given insight into the electronic properties of molecules at the single molecule level, including scanning tunneling microscopy,7 mechanically controlled break-junction8 and electron-migration.9 In general, single molecule junctions are excellent for analyzing charge-transport properties of molecules, albeit challenging when it comes to producing stable and reliable devices.10 Furthermore, statistics from hundreds or thousands of measurements are required due to the large variance in conductance, which originates from the many possible molecule-electrode configurations. This challenge can be solved by instead studying an ensemble of molecules, where the variations in individual molecule-electrode contacts are averaged out. Self-assembled monolayers (SAMs) present a quick and reliable way to produce such ensembles.11 The electronic properties of SAMs can be measured using a variety of techniques, including e.g. conducting-probe AFM (cAFM) and eutectic gallium-indium (EGaIn).12 These techniques offer great versatility and reliable characterization of SAMs formed from a large variety of molecules on conducting substrates.13-14 SAM-based molecular junctions can also be investigated using a cross-bar configuration. This method offers a scalable way to produce



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molecular tunneling devices with a metal/SAM/metal structure.15-16 Here the key fabrication step is the formation of the top electrode. While being fast, scalable and compatible with current silicon fabrication techniques, the deposition of metal top electrodes through physical vapor deposition (PVD) has been shown to destroy the underlying organic films.17 This process leads to a very low yield of working devices due to metal induced degradation of the SAM, where metal filaments penetrate the SAM, resulting in short-circuiting of the device.18 Several different approaches have been suggested to address this problem. Promising methods include spin-casting of soft conductive polymer electrodes,19 direct metal transfer,20 nano-pores,21 EGaIn,12 and the introduction of protective interlayers between the SAMs and metal electrodes.22 In 2012, we introduced a new device design for molecular solid-state devices,23 where a reduced graphene oxide (rGO) thin film was utilized as both a top electrode and protection layer. This strategy exploits the unique material and optoelectronic properties of graphene, yielding a few nanometers thin, flexible and electrically conducting protection layer or independent top electrode.24 Utilizing graphene as a transparent electrode, this property was used to demonstrate a stimuli-responsive solid-state device where switching was achieved by using light.25 However, high contact resistance and a temperature dependent hopping mechanism in the rGO thin film, set limitations for its usage as a testbed for analyzing new molecules.26 In this paper, we investigate whether the ultimate limit for protection layers, i.e. an atomically thin monolayer, can be applied in an alkanethiol molecular solid-state cross-bar junction, with the top metal electrode fabricated by PVD. We envision that reducing the thickness of the protective interlayer has the potential for solving some of the current issues encountered in the existing device structures, such as the high contact resistance. For this approach, we choose to investigate graphene oxide (GO) as protective interlayer. GO is a


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highly oxidized and electrically insulating analogue of graphene and we have previously shown that during PVD of metals, a single layer of GO is sufficient to structurally protect an otherwise highly sensitive physiosorbed Langmuir-Blodgett (LB) film composed of a fatty acid metal salt towards reorganization.27 Being fundamentally a 2D interface, GO should merely function as a tunneling barrier, as has been demonstrated for hexagonal boron nitride, another 2D wide bandgap material.28 Since GO is electrically insulating, using GO as a protective interlayer also eliminates the need for harsh plasma etching after the assembly of the device to eliminate short circuits. Being dispersible in a variety of solvents including water, GO is easy to process, and it does not suffer from wrinkle formation or folding during processing, which can be an issue for rGO thin films.29-30 Additionally, the chemistry of GO has been shown to be extremely versatile,31 which might enable chemical bonding between modified end-groups of the SAM molecules and the GO.

2. EXPERIMENTAL METHODS Materials. The alkane thiols (1-octanethiol, 1-dodecanethiol and 1-hexadecanethiol) were all purchased from Sigma Aldrich. Graphite oxide was synthesized from Graphite following a procedure described elsewhere,32 which was based on a modified version of the Hummer’s oxidation reaction.33 Briefly, graphite (0.5 g, 325 mesh, Alfa Aesar) was oxidized by dispersion in concentrated sulfuric acid (20 mL, 95%-98%, Sigma Aldrich), and adding potassium permanganate (1g, Sigma Aldrich) slowly over 4 hours while cooling the mixture on an ice bath. After 16 hours of reaction time, hydrogen peroxide (4 mL, 6%, Sigma Aldrich) was added to the solution over 40 min. The as-obtained Graphite Oxide was washed multiple times with Milli-Q water. GO was exfoliated from the Graphite Oxide using ultrasonication (30 min, 250W, 50-60 Hz). Single sheet GO was refined through a combination of high speed (10 min, 8000 rpm/6800g) and low speed (20 min, 2500 rpm/660g) centrifugation



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in order to remove unexfoliated GO flakes as well as oxidative debris.27, 34 The precipitate from the high speed centrifugation was redispersed in 1:5 H2O:MeOH, and the supernatant from the low speed was kept and used for LB transfer. Fabrication of Molecular Solid-State Devices. The solid state molecular junctions were assembled by growing SAMs of molecules onto gold electrodes inside pre-fabricated microwell devices, the fabrication of which is described elsewhere.23 In brief, gold bottom electrodes were evaporated on a silicon wafer, followed by a 25 nm aluminum oxide dielectric layer deposited through atomic layer deposition (ALD). Well-defined micropores were patterned with Electron Beam Lithography and etched with Aluminum Etchant Type D (Transene). AFM analysis of the etched microwells showed gold roughness of 0.4-0.5 nm, and very steep dielectric sidewalls. Cleaned microwell devices were immersed in a 20 µM solution of molecules in ethanol. Prior to the addition of molecules, the solvent was degassed using nitrogen. The microwell device was left in the solution for one day to ensure a homogeneous growth of the monolayer. After growth, the microwell device was rinsed with ethanol and blown dry with nitrogen. The GO protection layer was transferred to the microwell device using a Langmuir-Blodgett trough. The dilute (~0.5 mg/mL) GO dispersion in 1:5 H2O:MeOH solvent mixture was added to the LB trough. A GO monolayer film was transferred via the inverse Schäfer technique at low surface pressure (target pressure 5 mN/ m) in order to minimize the occurrence of overlapping GO flakes.35 The compression speed was 10 mm/min and the transfer speed 3 mm/min. After transfer, all samples were blown dry with nitrogen before evaporating the top electrode. Top electrodes were evaporated in an electron-beam PVD chamber under ultra-high vacuum (UHV). The samples were kept in the UHV for 15 min before metal deposition, in order to remove any residual water. A 5 nm titanium layer, followed by a 100 nm gold layer was evaporated on top of the transferred GO film. The evaporation rates were 1 Å s-1 and 2.5 Å s-1, for Ti and Au, respectively.


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Electrical and Structural Characterization. The electrical characterization of the assembled solid-state junctions, was carried-out in a home built probe station setup, utilizing a Keithley 2401 source meter. Structural characterization of the transferred GO LB film was performed with both a Bruker Multimode 8 Atomic Force Microscope and a JEOL JSM632+F Scanning Electron Microscope. GO samples coated with titanium was prepared for the X-ray Photoelectron Spectroscopy (XPS) analysis of the formation of titanium carbide. A GO solution (~0.5 mg/mL) was spin cast onto a clean silicon wafer to a thickness of 1-2 nm. 3 nm of titanium was evaporated with electron beam PVD chamber with an evaporation rate of 1 Å s-1.

3. RESULTS AND DISCUSSION For vertical molecular junctions with a metal/SAM//protection layer/metal configuration (Figure 1), the active part of the device is ideally isolated to the SAM section, assuming negligible contribution from the protection layer to the device operation. Here we prepared the molecular SAMs from three different aliphatic alkane thiols of different length, namely 1octanethiol (C8), 1-dodecanethiol (C12) and 1-hexadecanethiol (C16). The SAMs were grown on gold electrodes inside well-defined microwells of 2 µm diameter that were fabricated through standard lithography as described elsewhere.23 As such, the SAMs do not contain any stimuli-responsive behavior,25 but primarily function as tunneling barriers of different length as described in great detail in the literature.36 This choice of a well-studied molecular system allows us to investigate the properties of GO as protective interlayer as any observed features in the I-V characteristics can easily be compared to similar studies.11, 26



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Figure 1. Schematic illustration of the fabrication of a molecular junction with a monolayer GO protection layer in a solid-state device. Stage 1: Fabrication of microwells in aluminum oxide through lithography to expose the Au bottom electrode. Stage 2: Formation of a SAM of alkane thiols within the microwell. Stage 3: Transfer of GO sheets using the LB technique. Stage 4: Evaporation of a top Ti-Au composite electrode by PVD consisting of 5 nm Ti and 100 nm Au. Lower right corner is an illustration of the cross-sectional view of the assembled device.

After SAM formation, the GO protection layer was deposited. This was achieved by using the Langmuir-Blodgett (LB) technique as first described by Cote et al. and which we have previously used for surface coatings.29, 35, 37 Using this technique it is possible to control the density of the GO flakes floating on the water-air interface and can even be used to fabricate coherent and overlapping films. In this study, we assembled the GO flakes into Langmuir films with a dilute to close-packed density at the air-water interface. In this regime, the GO flakes are not overlapping. An inverse Schäfer deposition technique was used for transferring these films onto the microwell devices. The use of a monolayer LB film of GO with no overlapping GO flakes is crucial for this study, since the stacking distance in GO is 0.7 nm30 8 ACS Paragon Plus Environment

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and the length of the molecules in question ranges between 1-2 nm. A double layer of GO would therefore be expected to contribute significantly to the tunneling distance and with an exponential scaling, yield a large increase the contact resistance between the organic molecules and the metal electrode. Figure 2a shows a typical Scanning Electron Microscope (SEM) image of a LB film after transfer to the microwell device. Since the LB films were homogeneous over several square centimeters, the image was taken outside the microwell regions to avoid adversely affecting the assembled devices by the electron radiation. Here it can be seen that most of the flakes are isolated and have the same contrast (grey color) indicating a monodisperse distribution of monolayered flakes. This was confirmed by Atomic Force Microscope (AFM), see Figure 2b, where these flakes were found to be ca. 1 nm thick, as expected for monolayered GO.38 The SEM image in Figure 2a also display a very low density of double or multi layered flakes (having a dark grey color), as required. The surface morphology of the GO covered microwells was characterized by AFM as seen in Figure 2b, which shows a microwell fully covered with a monolayered GO flake before evaporation of the top metal electrode. Due to the flexibility of the GO,39 the flake follows the contours of the microwell structure perfectly, ensuring good contact between the SAM and the GO, as confirmed by the AFM phase image in Figure 2c.



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Figure 2. A. SEM image of the transferred Langmuir-Blodgett film of GO. The SiO2 surface is light grey, and the majority of the GO flakes are isolated and with the same grey color. B. AFM height image showing a microwell covered by a monolayered GO sheet. C. AFM phase image confirming that the GO sheet follows the contour of the microwell.

After transfer of the monolayered GO LB film, a 5 nm Ti / 100 nm Au top electrode was deposited by PVD through a shadow mask, see Figure 1 for a schematic representation of the fully assembled device. The completed solid-state device was then electrically characterized to evaluate the performance of GO as a protective interlayer. The I-V curves for all microwell devices were collected by measuring the generated current from an applied voltage bias, ±0.1V, across the molecular vertical junction. Figure S2 shows the resistance for all devices, extrapolated from linear fits to the collected I-V curves. Here working devices are clearly distinguished from short circuited devices, with the later having resistances no higher than ~200 Ω. We also note a spread in the resistance of working devices of approximately one order of magnitude. This can be attributed to small variations in microwell size and bottom electrode roughness, which both influence the overall current through the devices.40 The yield of working devices was generally around 10 % (specifically 11.5 %, 9.9 % and 8.3 % for C8, C12 and C16, respectively). This low yield can primarily be attributed to a combination of a low coverage of the GO LB film, which were estimated to be 58-59 % based on pixel counting, Figure S3, and relatively small GO flakes compared to the microwell size. Through analysis of SEM images, we found that most GO flakes were found to be smaller than (

Monolayered Graphene Oxide as a Low Contact Resistance

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