Friction force microscopy : a simple technique for identifying graphene on rough substrates and mapping the orientation of graphene grains on copper

At a single atom thick, it is challenging to distinguish graphene from its substrate using conventional techniques. In this paper we show that friction force microscopy (FFM) is a simple and quick technique for identifying graphene on a range of samples, from growth substrates to rough insulators. We show that FFM is particularly effective for characterizing graphene grown on copper where it can correlate the graphene growth to the three-dimensional surface topography. Atomic lattice stick–slip friction is readily resolved and enables the crystallographic orientation of the graphene to be mapped nondestructively, reproducibly and at high resolution. We expect FFM to be similarly effective for studying graphene growth on other metal/locally crystalline substrates, including SiC, and for studying growth of other two-dimensional materials such as molybdenum disulfide and hexagonal boron nitride

Carbon fibre tips for scanning probe microscopy based on quartz tuning fork force sensors

We report the fabrication and the characterization of carbon fibre tips for their use in combined scanning tunnelling and force microscopy based on piezoelectric quartz tuning fork force sensors. We find that the use of carbon fibre tips results in a minimum impact on the dynamics of quartz tuning fork force sensors yielding a high quality factor and consequently a high force gradient sensitivity. This high force sensitivity in combination with high electrical conductivity and oxidation resistance of carbon fibre tips make them very convenient for combined and simultaneous scanning tunnelling microscopy and atomic force microscopy measurements. Interestingly, these tips are quite robust against occasionally occurring tip crashes. An electrochemical fabrication procedure to etch the tips is presented that produces a sub-100 nm apex radius in a reproducible way which can yield high resolution images.Comment: 14 pages, 10 figure

Atomic-scale friction : a scanning probe study on crystalline surfaces

Friction is one of the most fascinating and yet elusive phenomena in physics. Everyday life cannot be imagined in the absence of friction. It allows us to walk, to climb stairs, to sit in a chair, to stop a car, to handle tools. In technological applications friction is the evil of all motion and huge amounts of money are spent annually on energetic and mechanical losses due to friction and wear. Friction is also responsible for natural disasters like earthquakes, landslides and avalanches. Along the centuries mankind has tried to understand and control friction but in spite of the huge volume of experimental knowledge with remarkable technical applications, very little is known about the fundamental, elementary processes taking place on the atomic level at the interface between sliding surfaces. The contact between two apparently flat solids consists in reality of a very large number of micro-protrusions or asperities belonging to both contacting surfaces. Studying and understanding the processes responsible for the occurrence of friction at the buried interface of a single-asperity became the tasks of a new but rapidly expanding science called nanotribology. In the present work we have used a variable-temperature ultra-high vacuum atomic force microscope (VT-UHV-AFM) to investigate the frictional properties, namely the stick-slip behavior, of (100) and (111) crystalline diamond and (001) sodium chloride surfaces. While diamond is a technologically important material and a perfect candidate for an ideal friction experiment, NaCl is well established as a representative model, standard surface for nanotribological investigations. In order to properly simulate the interface with a single asperity at the nanoscale, sharp AFM-tips are used on atomically flat surfaces. The ultra-high vacuum conditions are an essential ingredient for well-defined and reproducible experiments. A hard, stable and sharp AFM-tip termination is of the essence for friction measurements. Therefore, using a hot-filament assisted chemical vapor deposition (HF-CVD) of diamond, a method of growing individual good quality diamond crystallites at the apexes of standard Si AFM tips was demonstrated; the resulting tips showed sharpness, hardness, stability and reliable behavior during friction measurements. We have investigated the atomic-scale friction behaviour between standard silicon nitride tips and diamond-coated tips and a specially prepared hydrogen-terminated (100) diamond sample by means of ultra-high vacuum atomic force microscopy. Tunneling experiments revealed a very flat surface and the typical atomic features (dimers) of a (2x1) surface reconstruction of the hydrogen-terminated (100) diamond sample. When using a diamond-terminated tip, for the first time atomically resolved topography, normal force error signal and lateral force maps are simultaneously obtained and perpendicular domains, hydrogen atomic positions and atomic steps between domains could be observed. This was attributed to a very sharp tip, namely one hydrogen atom-terminated tip, describing a stick-slip movement in two orthogonal directions and causing a dynamic rearrangement of the surface atoms; these results were consistent with an ab-initio electronic structure calculation which reveals the fact that the repulsive interaction between the apex H-atom at the tip and H-atoms at the surface is the essential factor that governs the atomic stick-slip behaviour Similar experiments were carried out in UHV on a (111) crystalline diamond surface with standard silicon nitride and diamond-coated tips. The friction measurements with the same diamond-coated tip on this surface led to atomic resolution: the measured periodicity is consistent with the one of the individual hydrogen atoms of the diamond surface. This reconfirmed the use of an atomically sharp tip and, similar to the results on the (100) diamond surface, we believe that the repulsion between the last H-atom of the tip and the H-atoms of the surface termination is the most important ingredient controlling the complicated two-dimensional atomic scale stick-slip behavior observed experimentally. Finally, atomic-scale friction between a silicon tip and the atomically flat (001) NaCl surface was investigated in ultra-high vacuum at various sample temperatures in the interval from 25 to 300 K. The temperature dependence of measured average friction forces is found to be in good qualitative agreement with theoretical models that consider a thermally-activated discontinuous tip movement during scanning and predict higher friction forces at low temperature. Higher mean friction values observed for two temperature values were attributed to possible changes in the tip apex configuration

Mechanical properties of freely suspended atomically thin dielectric layers of mica

We have studied the elastic deformation of freely suspended atomically thin sheets of muscovite mica, a widely used electrical insulator in its bulk form. Using an atomic force microscope, we carried out bending test experiments to determine the Young's modulus and the initial pre-tension of mica nanosheets with thicknesses ranging from 14 layers down to just one bilayer. We found that their Young's modulus is high (190 GPa), in agreement with the bulk value, which indicates that the exfoliation procedure employed to fabricate these nanolayers does not introduce a noticeable amount of defects. Additionally, ultrathin mica shows low pre-strain and can withstand reversible deformations up to tens of nanometers without breaking. The low pre-tension and high Young's modulus and breaking force found in these ultrathin mica layers demonstrates their prospective use as a complement for graphene in applications requiring flexible insulating materials or as reinforcement in nanocomposites.Comment: 9 pages, 5 figures, selected as cover of Nano Research, Volume 5, Number 8 (2012

Optical imaging of strain in two-dimensional crystals

Strain engineering is widely used in material science to tune the (opto-)electronic properties of materials and enhance the performance of devices. Two-dimensional atomic crystals are a versatile playground to study the influence of strain, as they can sustain very large deformations without breaking. Various optical techniques have been employed to probe strain in two-dimensional materials, including micro-Raman and photoluminescence spectroscopy. Here we demonstrate that optical second harmonic generation constitutes an even more powerful technique, as it allows to extract the full strain tensor with a spatial resolution below the optical diffraction limit. Our method is based on the strain-induced modification of the nonlinear susceptibility tensor due to a photoelastic effect. Using a two-point bending technique, we determine the photoelastic tensor elements of molybdenum disulfide. Once identified, these parameters allow us to spatially image the two-dimensional strain field in an inhomogeneously strained sample.Comment: 13 pages, 4 figure

Migration of latent fingermarks on non-porous surfaces:observation technique and nanoscale variations

Latent fingermark morphology was examined over a period of approximately two months. Variation in topography was observed with atomic force microscopy and the expansion of the fingermark occurred in the form of the development of an intermediate area surrounding the main fingermark ridge. On an example area of a fingermark on silicon, the intermediate region exists as a uniform 4nm thick deposit; on day 1 after deposition this region extends approximately 2µm from the edge of the main ridge deposit and expands to a maximum of ~ 4µm by day 23. Simultaneously the region breaks up, the integrity is compromised by day 16, and by day 61 the area resembles a series of interconnected islands, with coverage of approximately 60%. Observation of a similar immediate area and growth with time on surfaces such as Formica was possible by monitoring the mechanical characteristics of the fingermark and surfaces though phase contrast in tapping mode AFM. The presence of this area may affect fingermark development, for example affecting the gold distribution in vacuum metal deposition. Further study of time dependence and variation with donor may enable assessment of this area to be used to evaluate the age of fingermarks

A Review on Mechanics and Mechanical Properties of 2D Materials - Graphene and Beyond

Since the first successful synthesis of graphene just over a decade ago, a variety of two-dimensional (2D) materials (e.g., transition metal-dichalcogenides, hexagonal boron-nitride, etc.) have been discovered. Among the many unique and attractive properties of 2D materials, mechanical properties play important roles in manufacturing, integration and performance for their potential applications. Mechanics is indispensable in the study of mechanical properties, both experimentally and theoretically. The coupling between the mechanical and other physical properties (thermal, electronic, optical) is also of great interest in exploring novel applications, where mechanics has to be combined with condensed matter physics to establish a scalable theoretical framework. Moreover, mechanical interactions between 2D materials and various substrate materials are essential for integrated device applications of 2D materials, for which the mechanics of interfaces (adhesion and friction) has to be developed for the 2D materials. Here we review recent theoretical and experimental works related to mechanics and mechanical properties of 2D materials. While graphene is the most studied 2D material to date, we expect continual growth of interest in the mechanics of other 2D materials beyond graphene