Ultra-thin graphene–polymer heterostructure membranes

The fabrication of arrays of ultra-thin conductive membranes remains a major challenge in realising large-scale micro/nano-electromechanical systems (MEMS/NEMS), since processing-stress and stiction issues limit the precision and yield in assembling suspended structures. We present the fabrication and mechanical characterisation of a suspended graphene–polymer heterostructure membrane that aims to tackle the prevailing challenge of constructing high yield membranes with minimal compromise to the mechanical properties of graphene. The fabrication method enables suspended membrane structures that can be multiplexed over wafer-scales with 100% yield. We apply a micro-blister inflation technique to measure the in-plane elastic modulus of pure graphene and of heterostructure membranes with a thickness of 18 nm to 235 nm, which ranges from the 2-dimensional (2d) modulus of bare graphene at 173 ± 55 N m$^{-1}$ to the bulk elastic modulus of the polymer (Parylene-C) as 3.6 ± 0.5 GPa as a function of film thickness. Different ratios of graphene to polymer thickness yield different deflection mechanisms and adhesion and delamination effects which are consistent with the transition from a membrane to a plate model. This system reveals the ability to precisely tune the mechanical properties of ultra-thin conductive membranes according to their applications

Quantum Nature of Plasmon-Enhanced Raman Scattering

We report plasmon-enhanced Raman scattering in graphene coupled to a single plasmonic hotspot measured as a function of laser energy. The enhancement profiles of the G peak show strong enhancement (up to $10^5$) and narrow resonances (30 meV) that are induced by the localized surface plasmon of a gold nanodimer. We observe the evolution of defect-mode scattering in a defect-free graphene lattice in resonance with the plasmon. We propose a quantum theory of plasmon-enhanced Raman scattering, where the plasmon forms an integral part of the excitation process. Quantum interferences between scattering channels explain the experimentally observed resonance profiles, in particular, the marked difference in enhancement factors for incoming and outgoing resonance and the appearance of the defect-type modes.Comment: Keywords: plasmon-enhanced Raman scattering, SERS, graphene, quantum interferences, microscopic theory of Raman scattering. Content: 22 pages including 5 figures + 11 pages supporting informatio

Directed self-assembly of block copolymers for use in bit patterned media fabrication

Reduction of the bit size in conventional magnetic recording media is becoming increasingly difficult due to the superparamagnetic limit. Bit patterned media (BPM) has been proposed as a replacement technology as it will enable hard disk areal densities to increase past 1 Tb in−2. Block copolymer directed self-assembly (BCP DSA) is the leading candidate for forming BPM due to its ability to create uniform patterns over macroscopic areas. Here we review the latest research into two different BCP DSA techniques: graphoepitaxy and chemoepitaxy (or chemical prepatterning). In addition to assessing their potential for forming high density bit patterns, we also review current approaches using these techniques for forming servo patterns, which are required for hard disk drive (HDD) operation. Finally, we review the current state of UV nanoimprint lithography, which is the favoured technique for enabling mass production of BPM HDDs

Improving the glial differentiation of human Schwann-like adipose-derived stem cells with graphene oxide substrates

There is urgent clinical need to improve the clinical outcome of peripheral nerve injury. Many efforts are directed towards the fabrication of bioengineered conduits, which could deliver stem cells to the site of injury to promote and guide peripheral nerve regeneration. The aim of this study is to assess if graphene and related nanomaterials can be useful in the fabrication of such conduits. A comparison is made between GO and reduced GO substrates. Our results show that the graphene substrates are highly biocompatible, and the reduced GO substrates are more effective in ncreasing the gene expression of the biomolecules involved in the regeneration process compared to the other substrates studied.Comment: 15 pages, 3 figure

PIM-1/graphene pervaporation membranes for bioalcohol recovery

Biofuels are an alternative to more traditional fuels, such as those derived from crude oil. Bioalcohols, including bioethanol and biobutanol, are produced from biomass through sugar fermentation and purification processes and thus they are a more sustainable alternative to reducing the CO2 footprint of transportation and mitigating climate change. In the short term they will find it difficult to replace hydrocarbon fuels due to direct competition with the food supply chain, although as new alternative raw materials and production processes are developed this hurdle will be overcome. The recovery of bioalcohols from fermentation broths includes a series of very challenging steps that need more attention. In this regard, membrane-based technologies with lower energy consumption, such as pervaporation (PV), have emerged as potential candidates for the replacement of energy intensive distillation operations. In this work we present the development of novel organophilic membranes based on polymers of intrinsic microporosity (PIMs) and graphene for the separation of ethanol and butanol from aqueous solutions. PIM-1 is one of the few polymers that offer selectivity for organic compounds over water [1-3]. However, excessive swelling limits its performance and the addition of graphene nanoparticles can have a positive effect in preventing it [4,5]. For the preparation of mixed matrix membranes (MMMs) PIM-1 and graphene were first synthesized. Graphene oxide (GO) was obtained from natural flake graphite via a modified Hummer’s method, functionalized with octylamine (OA) and octadecylamine (ODA), 8 and 18 carbons, respectively and subsequently reduced with hydrazine monohydrate. PIM-1 was prepared by the polycondensation of monomers 3,3,3’,3’-tetramethyl-1,1’’-spirobisindane-5,5’,6’,6’-tetrol with 2,3,5,6-tetrafluorophthalonitrile with a molecular ratio of 1:1 [6]. The preparation of freestanding membranes was done via a casting-evaporation technique using chloroform as solvent (one of the very few that dissolve PIM-1). The functionalization of GO with OA or ODA allowed its dispersion in chloroform and therefore a homogeneous casting solution was obtained. Membranes of thicknesses up to 40 µm with loadings of graphene from 0.01 to 0.5 wt.% were prepared and characterized via contact angle measurements, FTIR, TGA, and SEM. PV tests of aqueous feed solutions containing 5wt% of alcohol were performed at 65 ˚C and a pressure of 10 mbar on the permeate side of the membrane. An increase in the separation factor of ethanol and butanol over water was achieved for both amine-functionalized GO with maximum values of 7 and 40, respectively. The maximum flux achieved of ~ 2 kg m-2 h1 was obtained for membranes with graphene loadings of 0.5 wt.%. [1] Mason, C.R., et al. Polymer, 2013. 54(9), 2222-2230. [2] Žák, M., et al. Separation and Purification Technology, 2015. 151, 108-114. [3] Adymkanov, S.V., et al. Polymer Science Series A, 2008. 50(4), 444-450. [4] A. Gonciaruk, et al., Microporous Mesoporous Mater., 2015. 209, 126-134. [5] M.M. Khan, et al. J. Membr. Sci. 2013. 436, 109-120. [6] Budd, P.M., et al. Advanced Materials, 2004. 16(5), 456-459

Polarized Plasmonic Enhancement by Au Nanostructures Probed through Raman Scattering of Suspended Graphene

We characterize plasmonic enhancement in a hotspot between two Au nanodisks using Raman scattering of graphene. Single layer graphene is suspended across the dimer cavity and provides an ideal two-dimensional test material for the local near-field distribution. We detect a Raman enhancement of the order of 103 originating from the cavity. Spatially resolved Raman measurements reveal a near-field localization one order of magnitude smaller than the wavelength of the excitation, which can be turned off by rotating the polarization of the excitation. The suspended graphene is under tensile strain. The resulting phonon mode softening allows for a clear identification of the enhanced signal compared to unperturbed graphene

Plasmonic enhancement of SERS measured on molecules in carbon nanotubes

We isolated the plasmonic contribution to surface-enhanced Raman scattering (SERS) and found it to be much stronger than expected. Organic dyes encapsulated in single-walled carbon nanotubes are ideal probes for quantifying plasmonic enhancement in a Raman experiment. The molecules are chemically protected through the nanotube wall and spatially isolated from the metal, which prevents enhancement by chemical means and through surface roughness. The tubes carry molecules into SERS hotspots, thereby defining molecular position and making it accessible for structural characterization with atomic-force and electron microscopy. We measured a SERS enhancement factor of 106 on α-sexithiophene (6T) molecules in the gap of a plasmonic nanodimer. This is two orders of magnitude stronger than predicted by the electromagnetic enhancement theory (104). We discuss various phenomena that may explain the discrepancy (including hybridization, static and dynamic charge transfer, surface roughness, uncertainties in molecular position and orientation), but found all of them lacking in enhancement for our probe system. We suggest that plasmonic enhancement in SERS is, in fact, much stronger than currently anticipated. We discuss novel approaches for treating SERS quantum mechanically that appear promising for predicting correct enhancement factors. Our findings have important consequences on the understanding of SERS as well as for designing and optimizing plasmonic substrates

Designing Peptide/Graphene Hybrid Hydrogels through Fine-Tuning of Molecular Interactions

A recent strategy that has emerged for the design of increasingly functional hydrogels is the incorporation of nanofillers in order to exploit their specific properties to either modify the performance of the hydrogel or add functionality. The emergence of carbon nanomaterials in particular has provided great opportunity for the use of graphene derivatives (GDs) in biomedical applications. The key challenge when designing hybrid materials is the understanding of the molecular interactions between the matrix (peptide nanofibers) and the nanofiller (here GDs) and how these affect the final properties of the bulk material. For the purpose of this work, three gelling β-sheet-forming, self-assembling peptides with varying physiochemical properties and five GDs with varying surface chemistries were chosen to formulate novel hybrid hydrogels. First the peptide hydrogels and the GDs were characterized; subsequently, the molecular interaction between peptides nanofibers and GDs were probed before formulating and mechanically characterizing the hybrid hydrogels. We show how the interplay between electrostatic interactions, which can be attractive or repulsive, and hydrophobic (and π–π in the case of peptide containing phenylalanine) interactions, which are always attractive, play a key role on the final properties of the hybrid hydrogels. The shear modulus of the hydrid hydrogels is shown to be related to the strength of fiber adhesion to the flakes, the overall hydrophobicity of the peptides, as well as the type of fibrillar network formed. Finally, the cytotoxicity of the hybrid hydrogel formed at pH 6 was also investigated by encapsulating and culturing human mesemchymal stem cells (hMSC) over 14 days. This work clearly shows how interactions between peptides and GDs can be used to tailor the mechanical properties of the resulting hydrogels, allowing the incorporation of GD nanofillers in a controlled way and opening the possibility to exploit their intrinsic properties to design novel hybrid peptide hydrogels for biomedical applications