Free-standing graphene membranes on glass nanopores for ionic current measurements

A method is established to reliably suspend graphene monolayers across glass nanopores as a simple, low cost platform to study ionic transport through graphene membranes. We systematically show that the graphene seals glass nanopore openings with areas ranging from 180 nm2 to 20 μm2, allowing detailed measurements of ionic current and transport through graphene. In combination with in situ Raman spectroscopy, we characterise the defects formed in ozone treated graphene, confirming an increase in ionic current flow with defect density. This highlights the potential of our method for studying single molecule sensing and filtration.The authors would like to thank S. Purushothaman and K. Göpfrich for careful reading of the manuscript and V. Thacker for useful discussions. This work was supported by the EPSRC Cambridge NanoDTC, EP/G037221/1, and EPSRC grant GRAPHTED, EP/K016636/1. R.S.W. acknowledges a Research Fellowship from St. John's College, Cambridge. N.A.W.B. acknowledges an EPSRC doctoral prize award.This is the accepted manuscript. Copyright 2015 American Institute of Physics. This article may be downloaded for personal use only. Any other use requires prior permission of the author and the American Institute of Physics. The final version is available in Applied Physics Letters 106, 023119 (2015); doi: 10.1063/1.490623

Measuring the proton selectivity of graphene membranes

By systematically studying the proton selectivity of free-standing graphene membranes in aqueous solutions we demonstrate that protons are transported by passing through defects. We study the current-voltage characteristics of single-layer graphene grown by chemical vapour deposition (CVD) when a concentration gradient of HCl exists across it. Our measurements can unambiguously determine that H+ ions are responsible for the selective part of the ionic current. By comparing the observed reversal potentials with positive and negative controls we demonstrate that the as-grown graphene is only weakly selective for protons. We use atomic layer deposition to block most of the defects in our CVD graphene. Our results show that a reduction in defect size decreases the ionic current but increases proton selectivity.This is the author accepted manuscript. The final version is available from AIP via http://dx.doi.org/10.1063/1.493633

The origin of chemical inhomogeneity in garnet electrolytes and its impact on the electrochemical performance

The interface between solid electrolytes and lithium metal electrodes determines the performance of an all-solid-state battery in terms of the ability to demand high power densities and prevent the formation of lithium dendrites. This interface depends strongly on the nature of the solid electrolyte surface in contact with the metallic anode. In the garnet electrolyte/Li system, most papers have focused on the role of current inhomogeneities induced by void formation in the Li metal electrode and the presence of insulating reaction layers following air exposure. However, extended defects in the solid electrolyte induced by chemical and/or structural inhomogeneities can also lead to uneven current distribution, impacting the performance of these systems. In this work, we use complementary surface analysis techniques with varying analysis depths to probe chemical distribution within grains and grain boundaries at the surface and in the bulk of garnet-type electrolytes to explain their electrochemical performance. We show that morphology, post-treatments and storage conditions can greatly affect the surface chemical distribution of grains and grain boundaries. These properties are important to understand since they will dictate the ionic and electronic transport near the interfacial zone between metal and electrolyte which is key to determining chemo-mechanical stability

Effects of polymethylmethacrylate-transfer residues on the growth of organic semiconductor molecules on chemical vapor deposited graphene

Scalably grown and transferred graphene is a highly promising material for organic electronic applications, but controlled interfacing of graphene thereby remains a key challenge. Here, we study the growth characteristics of the important organic semiconductor molecule para-hexaphenyl (6P) on chemical vapor deposited graphene that has been transferred with polymethylmethacrylate (PMMA) onto oxidized Si wafer supports. A particular focus is on the influence of PMMA residual contamination, which we systematically reduce by H2 annealing prior to 6P deposition. We find that 6P grows in a flat-lying needle-type morphology, surprisingly independent of the level of PMMA residue and of graphene defects. Wrinkles in the graphene typically act as preferential nucleation centers. Residual PMMA does however limit the length of the resulting 6P needles by restricting molecular diffusion/attachment. We discuss the implications for organic device fabrication, with particular regard to contamination and defect tolerance.B.C.B acknowledges a College Research Fellowship from Hughes Hall, Cambridge. P.R.K. acknowledges the Lindemann Trust Fellowship. A.M. and G.R. acknowledge support by the Serbian MPNTR through Projects OI 171005 and III 45018. R.S.W. acknowledges a research fellowship from St. John’s College, Cambridge. S.H. acknowledges funding from EPSRC (GRAPHTED, Grant No. EP/K016636/1). We want to thank Dr. Sarah M. Skoff (Vienna University of Technology, Austria) for fruitful discussions.This is the author accepted manuscript. The final published version is available via AIP at http://scitation.aip.org/content/aip/journal/apl/106/10/10.1063/1.4913948

The parameter space of graphene chemical vapor deposition on polycrystalline Cu

A systematic study on the parameter space of graphene CVD on polycrystalline Cu foils is presented, aiming at a more fundamental process rationale in particular regarding the choice of carbon precursor and mitigation of Cu sublimation. CH4 as precursor requires H2 dilution and temperatures ≥1000°C to keep the Cu surface reduced and yield a high quality, complete monolayer graphene coverage. The H2 atmosphere etches as-grown graphene, hence maintaining a balanced CH4/H2 ratio is critical. Such balance is more easily achieved at low pressure conditions, at which however Cu sublimation reaches deleterious levels. In contrast, C6H6 as precursor requires no reactive diluent and consistently gives similar graphene quality at 100-150°C lower temperatures. The lower process temperature and more robust processing conditions allow the problem of Cu sublimation to be effectively addressed. Graphene formation is not inherently self-limited to a monolayer for any of the precursors. Rather, the higher the supplied carbon chemical potential the higher the likelihood of film inhomogeneity and primary and secondary multilayer graphene nucleation. For the latter, domain boundaries of the inherently polycrystalline CVD graphene offer pathways for a continued carbon supply to the catalyst. Graphene formation is significantly affected by the Cu crystallography, i.e. the evolution of microstructure and texture of the catalyst template form an integral part of the CVD process.S.H. acknowledges funding from ERC grant InsituNANO (n°279342) and from EPSRC (Grant Nr. EP/H047565/1). P.R.K. acknowledges funding from the Cambridge Commonwealth Trust and C.D. acknowledges funding from Royal Society.This is the accepted manuscript. The final version is available from ACS at http://pubs.acs.org/doi/abs/10.1021/jp303597m

Atomic layer deposited oxide films as protective interface layers for integrated graphene transfer

The transfer of chemical vapour deposited (CVD) graphene from its parent growth catalyst has become a bottleneck for many of its emerging applications. The sacrificial polymer layers that are typically deposited onto graphene for mechanical support during transfer are challenging to fully remove and hence leave graphene and subsequent device interfaces contaminated. Here, we report on the use of atomic layer deposited (ALD) oxide films as protective interface and support layers during graphene transfer. The method avoids any direct contact of the graphene with polymers and through the use of thicker ALD layers (≥100nm), polymers can be eliminated from the transfer-process altogether. The ALD film can be kept as a functional device layer, facilitating integrated device manufacturing. We demonstrate back-gated field effect devices based on single-layer graphene transferred with a protective Al2O3 film onto SiO2 that show significantly reduced charge trap and residual carrier densities. We critically discuss the advantages and challenges of processing graphene/ALD bilayer structures.We acknowledge funding from EPSRC (Grant No. EP/K016636/1, GRAPHTED) and ERC (Grant No. 279342, InsituNANO). ACV acknowledges the Conacyt Cambridge Scholarship and Roberto Rocca Fellowship. JAA-W acknowledges the support of his Research Fellowships from the Royal Commission for the Exhibition of 1851 and Churchill College, Cambridge. RSW acknowledges a Research Fellowship from St. John's College, Cambridge and a Marie Skłodowska-Curie Individual Fellowship (Global) under grant ARTIST (no. 656870) from the European Union's Horizon 2020 research and innovation programme

Environment-Dependent Radiation Damage in Atmospheric Pressure X-ray Spectroscopy

Atmospheric pressure x-ray spectroscopy techniques based on soft x-ray excitation can provide interface-sensitive chemical information about a solid surface immersed in a gas or liquid environment. However, x-ray illumination of such dense phases can lead to the generation of considerable quantities of radical species by radiolysis. Soft x-ray absorption measurements of Cu films in both air and aqueous alkali halide solutions reveal that this can cause significant evolution of the Cu oxidation state. In air and NaOH (0.1M) solutions, the Cu is oxidized towards CuO, whilst the addition of small amounts of CH3OH to the solution leads to reduction towards Cu2O. For Ni films in NaHCO3 solutions, the oxidation state of the surface is found to remain stable under x-ray illumination, and can be electrochemically cycled between a reduced and oxidized state. We provide a consistent explanation for this behavior based on the products of x-ray induced radiolysis in these different environments, and highlight a number of general approaches that can mitigate radiolysis effects when performing operando x-ray measurements.R.S.W. acknowledges a Research Fellowship from St. John’s College, Cambridge and a EU Marie Skłodowska-Curie Individual Fellowship (Global) under grant ARTIST (no. 656870) from the European Union’s Horizon 2020 research and innovation programme. This work was supported by the Office of Basic Energy Sciences (BES), Division of Materials Sciences and Engineering, of the U.S. Department of Energy (DOE) under Contract DE-AC02-05CH11231, through the Chemical and Mechanical Properties of Surfaces, Interfaces and Nanostructures program and through work performed at the Advanced Light Source and Molecular Foundry user facilities of the DOE Office of Basic Energy Sciences

In Situ Graphene Growth Dynamics on Polycrystalline Catalyst Foils

The dynamics of graphene growth on polycrystalline Pt foils during chemical vapor deposition (CVD) are investigated using in situ scanning electron microscopy and complementary structural characterization of the catalyst with electron backscatter diffraction. A general growth model is outlined that considers precursor dissociation, mass transport, and attachment to the edge of a growing domain. We thereby analyze graphene growth dynamics at different length scales and reveal that the rate-limiting step varies throughout the process and across different regions of the catalyst surface, including different facets of an individual graphene domain. The facets that define the domain shapes lie normal to slow growth directions, which are determined by the interfacial mobility when attachment to domain edges is rate-limiting, as well as anisotropy in surface diffusion as diffusion becomes rate-limiting. Our observations and analysis thus reveal that the structure of CVD graphene films is intimately linked to that of the underlying polycrystalline catalyst, with both interfacial mobility and diffusional anisotropy depending on the presence of step edges and grain boundaries. The growth model developed serves as a general framework for understanding and optimizing the growth of 2D materials on polycrystalline catalysts.St. John’s College, Cambridge (Research Fellowship), European Union’s Horizon 2020 research and innovation programme (Marie Skłodowska-Curie Individual Fellowship (Global) under Grant ID: ARTIST (no. 656870)), National Science Foundation (graduate research fellowship (DGE-1324585)), European Research Council (Grant ID: InsituNANO (no. 279342)), EUFP7 Work Programme (Grant ID: GRAFOL (project reference 285275)) , Engineering and Physical Sciences Research Council (Grant ID: GRAPHTED (project reference EP/K016636/1)), Strategic Capability programme of the National Measurement System of the U.K. Department of Business, Innovation, and Skills (project no. 119376

Nucleation control for large, single crystalline domains of monolayer hexagonal boron nitride via Si-doped Fe catalysts.

The scalable chemical vapor deposition of monolayer hexagonal boron nitride (h-BN) single crystals, with lateral dimensions of ∼0.3 mm, and of continuous h-BN monolayer films with large domain sizes (>25 μm) is demonstrated via an admixture of Si to Fe catalyst films. A simple thin-film Fe/SiO2/Si catalyst system is used to show that controlled Si diffusion into the Fe catalyst allows exclusive nucleation of monolayer h-BN with very low nucleation densities upon exposure to undiluted borazine. Our systematic in situ and ex situ characterization of this catalyst system establishes a basis for further rational catalyst design for compound 2D materials.S.C. acknowledges funding from EPSRC (Doctoral training award). R.S.W. acknowledges a Research Fellowship from St. John ’ s College. B.C.B acknowledges a Research Fellowship at Hughes Hall. A.C.-V. acknowledges the Conacyt Cambridge Scholarship and Roberto Rocca Fellowship. S.H. acknowledges funding from ERC grant InsituNANO (No. 279342). B.B., S.J.S., K.M., and A.J.P. would like to acknowledge the National Measurement O ffi ce (NMO) for funding through the Innovation, Research and Development (IRD) programme (Project No. 115948). We acknowledge the European Synchrotron Radiation Fac ility (ESRF) for provision of synchrotron radiation, and we thank the sta ff for assistance in using beamline BM20/ROBL. We would also like to acknowl- edge Prof. Bonnie J. Tyler for discussions related to the manuscript.This is the final published article. It first appeared at http://pubs.acs.org/doi/abs/10.1021/nl5046632

Low temperature growth of fully covered single-layer graphene using a CoCu catalyst.

A bimetallic CoCu alloy thin-film catalyst is developed that enables the growth of uniform, high-quality graphene at 750 °C in 3 min by chemical vapour deposition. The growth outcome is found to vary significantly as the Cu concentration is varied, with ∼1 at% Cu added to Co yielding complete coverage single-layer graphene growth for the conditions used. The suppression of multilayer formation is attributable to Cu decoration of high reactivity sites on the Co surface which otherwise serve as preferential nucleation sites for multilayer graphene. X-ray photoemission spectroscopy shows that Co and Cu form an alloy at high temperatures, which has a drastically lower carbon solubility, as determined by using the calculated Co-Cu-C ternary phase diagram. Raman spectroscopy confirms the high quality (ID/IG < 0.05) and spatial uniformity of the single-layer graphene. The rational design of a bimetallic catalyst highlights the potential of catalyst alloying for producing two-dimensional materials with tailored properties