Structure-property characteristics of graphene materials with controlled nanoscale rippling (GraNaRip)

Funded by the European Union
(Marie Curie Career Integration Grant
2013-2017)

Abstract: The development of novel functional nanodevices based on graphene is of great interest for many technological applications. Transferring the bunch of exceptional properties of this highly promising two dimensional carbon material (light, strong, flexible, semi-metal, etc.) is currently one of the hot topics in material science. The objective of this project is the realisation of functional graphene materials by controlling their rippling at nanoscale. A comparative study will be performed to investigate the microscopic structure and charge transport properties of graphene layers transferred onto different nanostructured surfaces. These surfaces will be prepared by ion beam irradiation techniques and by dispersing nanoparticles on appropriate substrates. Transport measurements will be performed in the view of potential gas sensing applications. We expect to identify correlations between the corrugation of graphene layers and their gas sensing property. We aim also to characterize individual nanocrystals located under the graphene sheets. Focus lies on measuring the electronic band gap of semiconducting nanocrystals by scanning tunneling microscopy and spectroscopy (STM/STS), since these techniques are sensitive to the surface states below the one-atom-thick graphene. A more comprehensive study of graphene/nanocrystal heterostructures is envisaged by STM/STS, because not only geometric effects but also electronic effects (spatially dependent doping) can influence the gas sensing properties. The project thus will contribute to enhance the knowledge on the properties of strained graphene/nanoparticle hybrid systems and additionally, will result in novel graphene based materials with own proper characteristics and potential interest to technological applications.

Project results

1. Corrugated graphene samples elaborated using SiO2 nanoparticles

Figure 1. Tapping mode AFM images of graphene on top of SiO2 NPs (annealed sample). Topographic image is shown on the left, while the corresponding phase image is displayed on the right. Low-phase areas reveal suspended graphene parts.

We have investigated by 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 (Fig. 1). 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.

Further reading: Nanoscale 6 (2014) 6030, arXiv.

2. Strained few-layer carbon nanodisks

Figure 2. AFM image of a carbon nanodisk after annealing. The line section shows that the disk is deformed upon annealing, as it follows the roughened gold surface. Vertical ( H ) and horizontal ( L ) distances between the marker lines are shown.

We have shown by AFM measurements that carbon nanodisks can significantly deform as they follow the surface roughness changes of gold substrate induced by annealing (Fig. 2). 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.

Further reading: Thin Solid Films 565 (2014) 111, REAL.

3. Structure and properties of graphene on gold nanoparticles

Figure 3. (a) STM image of graphene/Au NPs. The left part of the image shows uncovered Au NPs. Nanoparticle-supported and suspended graphene regions are marked with black and white dots, respectively. (b) Average dI/dV spectra obtained from nanoparticle-supported (black) and suspended (red) graphene regions. The corresponding Dirac points are marked with dashed vertical lines.

AFM and STM measurements revealed the nanoscale structure of gold nanoparticle-supported graphene, which 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. This was demonstrated by simulations using boundary element method on a group of 9 nanoparticles reconstructed from AFM measurements. We found that the Au NPs have dome-like morphology, which we used also for the calculation of the extinction spectrum. The simulated local surface plasmon resonance maximum is in very good agreement with the measured one.

STS measurements revealed that the local density of states of graphene depends on the spatial position. Suspended graphene regions were more p-doped than supported ones (Fig. 3). Thus, graphene was selectively doped electrostatically and formed a network of p-p' nanojunctions. Finally, optical reflectance spectra showed that the presence of graphene increased significantly the lateral scattering of the graphene/Au NPs, while the reflectance could be tuned by annealing. We suggested that besides doping effects, the nanoscale corrugation of graphene also affects the reflectance properties of graphene/Au NPs. This is highly intriguing for further theoretical and experimental investigations, since it can open a route towards tailoring the optical properties of graphene/plasmonic nanoparticle hybrid structures through their morphology

Further reading: Nanoscale 7 (2015) 5503, arXiv.

4. Mapping the nanomechanical properties of graphene suspended on silica nanoparticles

Figure 4. Simultaneously acquired PeakForce AFM images of graphene-covered nanoparticles after annealing. (a) Topography measured at a peak force of 8 nN. (b) Adhesion map: light-contrasted regions correspond to graphene suspended between nanoparticles.

We studied the structure and elastic properties of graphene grown by chemical vapour deposition and transferred onto a continuous layer of SiO2 nanoparticles. We show that the transferred graphene follows only roughly the morphology induced by nanoparticles. The graphene membrane parts bridging the nanoparticles are suspended and their adhesion to the atomic force microscope tip is larger compared to that of supported graphene parts. These suspended graphene regions can be deformed with forces of the order of 10 nN. The elastic modulus of graphene was determined from indentation measurements performed on suspended membrane regions with diameters in the 100 nm range.

 

Further reading: J Exp. Nanosci. 11 (2016) 1011, arXiv.

5. Moiré superlattices in strained graphene-gold hybrid nanostructures

Figure 5. STM images of graphene on Au(111) showing moiré superstructures with anomalously large periodicities. (a) moiré mattern with periodicity of 7.7 nm. (b) moiré pattern with periodicity of 5.1 nm. The insets show the honeycomb graphene lattice.

Graphene-metal nanoparticle hybrid materials potentially display not only the unique properties of metal nanoparticles and those of graphene, but also additional novel properties due to the interaction between graphene and nanoparticles. This study shows that gold nanoislands can be used to tailor the local electronic properties of graphene. Graphene on crystalline gold nanoislands exhibits moiré superlattices, which generate secondary Dirac points in the local density of states. Conversely, the graphene covered gold regions undergo a polycrystalline --> Au (111) phase transition upon annealing. Moreover, the nanoscale coexistence of moiré superlattices with different moiré periodicities has also been revealed. 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. Density functional theory and molecular dynamics simulations show for the first time that in such cases the graphene and the interfacial metallic layer is strained, leading to distorted lattice constants, and consequently to reduced misfit. Room temperature charge localization induced by a large wavelength moiré pattern is also observed by scanning tunneling spectroscopy. 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.

 

Further reading: Carbon 107 (2016) 792, REAL.

6. Determination of the STM tip-graphene repulsive forces by comparative STM and AFM measurements on suspended graphene

Figure 6. STM images of a graphene nanobubble measured at bias voltages of (a) U = 1000 mV, and (b) U = 100 mV. Tunneling current: I = 1 nA. (c) Height profiles taken at different bias voltages along the same line section shown with horizontal line in (a)–(b).

Graphene grown by chemical vapour deposition was transferred on top of flat gold nanoislands and characterized by scanning tunnelling microscopy (STM) and atomic force microscopy (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.

 

Further reading: RSC Advances 6 (2016) 86253, REAL.

7. STM and STS study of the MoS2 flakes grown on graphite

Figure 7. STM images of a single- and bi-layer MoS2 flake on HOPG measured at (a) U = 1.5 V, and (b) U = -1.5 V. Tunneling current: I = 0.2 nA. The insets show line cuts along black lines. The difference between the tip-sample distance at positive (+1.5 V) and negative (-1.5 V) voltage is large due to the significantly different density of states of the MoS2 at these bias voltages. Therefore, the same MoS2 flake may appear as protrusion or pit, depending on the polarity of the applied bias voltage.

Heterostructures of 2D materials are expected to become building blocks of next generation nanoelectronic devices. Therefore, the detailed understanding of their interfaces is of particular importance. In order to gain information on the properties of the graphene-MoS2 system, 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. The results are compared with results reported recently for MoS2 grown on epitaxial graphene on SiC. Our STM study revealed that the crystallographic orientation of MoS2 sheets is determined by the orientation of the underlying graphite lattice. This epitaxial orientation preference is so strong that the MoS2 flakes could be moved on HOPG with the STM tip over large distances without rotation. The electronic properties of the MoS2 flakes have been investigated using tunneling spectroscopy. 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 nanoelectronic devices. We have also demonstrated the ability of the STM to define MoS2 nanoribbons down to 12 nm width, which can be used as building blocks for future nanoelectronic devices.

 

Further reading: Carbon 105 (2016) 408, REAL.

8. Interaction effects in a chaotic graphene quantum billiard

Figure 8. (a) STM image of the investigated quantum dot (QD). Tunneling parameters: U = 200 mV, and I = 1 nA. The atomic resolution inset image shows the crystallographic orientation in the dot. (b) Height profile taken along the line section 1 in a), showing monolayer thickness. (c) dI/dU spectra measured on HOPG substrate (black line), and on the QD (red line) are compared to the theoretical results for the local density of states of the QD (dotted blue line).

We investigate the local electronic structure of a Sinai-like, quadrilateral graphene quantum billiard with zigzag and armchair edges using scanning tunneling microscopy (STM) at room temperature. It is revealed that besides the (√3×√3)R30° superstructure, which is caused by the intervalley scattering, its overtones also appear in the STM measurements, which are attributed to the Umklapp processes. We point out that these results can be well understood by taking into account the Coulomb interaction in the quantum billiard, accounting for both the measured density of state values and the experimentally observed topography patterns. The analysis of the level-spacing distribution substantiates the experimental findings as well. We also reveal the magnetic properties of our system which should be relevant in future graphene based electronic and spintronic applications.

 

Further reading: Phys. Rev. B 95 (2017) 075123, REAL.