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Structure-property characteristics of graphene materials with controlled nanoscale rippling

Final Report Summary - GRANARIP (Structure-property characteristics of graphene materials with controlled nanoscale rippling)

The scientific objective of the GraNaRip project was to elaborate and characterize corrugated graphene, as well as to investigate the electronic and gas sensing properties of graphene/nanoparticle heterostructures.
Comprehensive summary of the work performed during the project and the main results:

1) We have investigated by atomic force microscopy (AFM) and confocal Raman microscopy the properties of graphene transferred onto a Langmuir-Blodgett film of SiO2 nanoparticles. We showed that the nanoscale rippling of graphene can be modified by annealing at moderate temperatures, which introduces compressive strain into the atomically thin membrane. Both topographic and phase images revealed extended graphene regions suspended between silica nanoparticles. This gave the possibility to investigate by local indentation the elastic properties of the transferred graphene. We presented a method for the preparation and mapping of suspended graphene regions. We were able to control dynamically the local graphene morphology, which can play an important role in the development of graphene based nanomechanical devices such as switches (Nanoscale 2014, Journal of Experimental Nanoscience 2016,

2) We have shown by AFM measurements that graphitic carbon nanodisks can significantly deform as they follow the surface roughness changes of gold substrate induced by annealing. This leads to strained nanodisks, which is confirmed by confocal Raman microscopy. They support deformations as high as 22 %, which makes them interesting alternative candidates for fillers in composite materials. We observed that the field emission scanning electron microscope (FE-SEM) contrast obtained from the disks depended on the working distance at which the image was obtained. We explained this finding by the diffraction of the secondary electrons on the graphitic structure, which decreased the amount of electrons reaching the detector. This contrast alteration is likely to be observable also during the FE-SEM investigation with In-lens detection of other nanoscale crystalline materials forming homogenous or heterogenous two-dimensional nanostacks (Thin Solid Films 2014,

3) AFM and scanning tunnelling microscopy (STM) measurements revealed the nanoscale structure of gold nanoparticle-supported graphene. The rippling of graphene could be modified by annealing at moderate temperatures. Graphene was completely separated from the SiO2 or HOPG substrates. It was either directly supported by nanoparticles, either suspended between Au NPs. In spite of the relatively small nanoparticle dimensions, the studied graphene/Au NPs material showed surface enhanced Raman scattering (SERS) properties, with a maximum enhancement factor of 22 for the graphene 2D peak. The observed SERS effect depended on the laser excitation wavelength and it was attributed to the near-field enhancement around plasmonic Au NPs. These results can open a route towards tailoring the optical properties of graphene/plasmonic nanoparticle hybrid structures as advanced SERS substrates (Nanoscale 2015,

4) We showed by STM that graphene on crystalline gold nanoislands exhibits moiré superlattices, which generate secondary Dirac points in the local density of states. Several of these moiré periodicities are anomalously large, which cannot be explained by the standard lattice mismatch between the graphene and the topmost Au (111) layers. We showed for the first time by density functional theory and molecular dynamics simulations that in such cases the graphene and the interfacial metallic layer is strained, leading to distorted lattice constants. Room temperature charge localization induced by a large wavelength moiré pattern is also observed by scanning tunnelling spectroscopy (STS). These findings can open a route towards the strain engineering of graphene/metal interfaces with various moiré superlattices and tailored electronic properties for nanoscale information coding (Carbon 2016,

5) Graphene grown by chemical vapour deposition was transferred on top of flat gold nanoislands and characterized by STM and AFM. Graphene bubbles were formed with lateral dimensions determined by the size and shape of nanoislands. These graphene bubbles could be squeezed during STM imaging using bias voltages of less than 250 mV and tunnelling currents of 1 nA. Similarly, the graphene suspended over gold nanovoids was deflected 4–5 nm by the STM tip when imaging at low bias voltages (U = 30 mV). Nanoindentation measurements performed by AFM show that the squeezing of graphene bubbles occurs at repulsive forces of 20–35 nN, and such forces can result in deflections of several nanometres in suspended graphene parts, respectively. Comparing the AFM and STM results, this study revealed that mechanical forces of the order of 10^(-8) N occur between the STM tip and graphene under ambient imaging conditions and typical tunnelling parameters (RSC Advances 2016,

6) We have investigated MoS2 sheets grown by chemical vapour deposition (CVD) on highly ordered pyrolytic graphite (HOPG), as a model system with atomically clean interface. Our STM study revealed that the crystallographic orientation of MoS2 sheets is determined by the orientation of the underlying graphite lattice. The electronic properties of the MoS2 flakes have been investigated using STS. A significant modification of the electronic structure has been revealed at flake edges and grain boundaries. These features are expected to have an important influence on the performance of future nanoelectronic devices made of stacked two-dimensional materials (Carbon 2016,

7) We investigated the local electronic structure of a quadrilateral graphene quantum dot with zigzag and armchair edges using STM at room temperature. The experimental findings could be well understood by taking into account the Coulomb interaction in the quantum dot, accounting for both the measured density of state values and the experimentally observed topography patterns. The analysis of the level-spacing distribution substantiated the experimental findings as well. We also revealed the magnetic properties of the graphene nanostructure, which should be relevant in future graphene based electronic and spintronic applications (Physical Review B 2017,

8) We investigated the vapour sensing properties of different graphene-gold hybrid nanostructures. The experiments were carried out by changing the concentration and nature of the vapours (ethanol, isopropanol, and toluene), while monitoring the variations in the optical spectra near the local surface plasmon resonance of the gold nanostructures. The sample with smaller, dome-like gold nanoparticles were proved to be more sensitive to these vapours compared to the sample with slightly larger, flat gold nanoislands. We showed that the presence of graphene increases the sensitivity to ethanol and isopropanol, while it decreases to toluene exposure. These results can have potential impact on the development of new sensors based on graphene-gold hybrids.

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