C. Moura

Characterisation of chromium nitride films produced by PVD techniques

Chromium nitride thin films have been deposited on stainless steel substrates by r.f. reactive magnetron sputtering. The influence of process parameters such as substrate bias and partial pressure of reactive gas have been investigated. The characterisation of the coatings was performed by X-ray diffraction (XRD), Raman Spectroscopy (RS) and nano-indentation experiments. These studies allow to analyse the influence of deposition parameters in crystal phases, crystal orientation/texture and crystallite size. The relationship between structural defects and their characteristics with deposition conditions will also be taken into account. The presence of oxygen on the coatings surface, due to atmospheric contamination, is analysed by means of Raman spectroscopy. This optical technique can be used for the characterisation of the surface oxides at different stages of oxidation. The changes observed in Raman spectra can be correlated with process parameters. Coatings produced with an unbiased substrate showed higher tendency to surface oxidation. Increasing the nitrogen partial pressure in the working atmosphere produces changes from a hexagonal Cr2N to cubic CrN microstructure. The strain in CrN crystals increases with nitrogen content in working atmosphere. When the Cr2N phase is dominant the hardness has a relative maximum (42.2 GPa), but the highest hardness was obtained for a coating with dominant CrN phase produced with highest nitrogen flow (44.9 GPa).

Enhancing visibility of graphene on arbitrary substrates by microdroplet condensation

In order to take advantage of the enormous potential of graphene for future electronic microcircuits and other applications it is necessary to develop reliable, rapid, and widely applicable methods to visualize graphene-based structures. We report here on a microdroplet condensation technique, which allows for quick visual identification of graphene on a variety of substrates, including some which were previously considered unsuitable for the visualization of carbon layers. The technique should also be applicable to visualize artificially patterned graphene structures which are expected to be key technologically enabling components in electronic microcircuits and other applications.

Effect of nitrogen gas flow on amorphous Si–C–N films produced by PVD techniques

Abstract Si x C y N z thin films were deposited by reactive magnetron sputtering on glass and steel substrates. The films were grown in a rotation mode over a carbon and a silicon targets in a mixed Ar/N 2 atmosphere at a substrate temperature of 300 °C. The substrates were held grounded or at a negative bias of −25 and −50 V. The film characteristics were also controlled by nitrogen flow. Binary and ternary films were obtained. The films were analysed with respect to microstructure, state of chemical bonding and optical properties by Raman spectroscopy (RS) and optical transmittance. RS was used as a probe of micro-structural modifications induced by deposition conditions. The main features observed in RS spectra are the well-known D- and G-bands characteristic of amorphous carbon. The position, widths and intensity ratio of these bands are found to be dependent on the film composition. The refractive index, the absorption coefficient and also the thickness were calculated from transmittance spectra obtained between 200 and 2500 nm. The hardness and Youngs modulus of the films were measured by nano-indentation experiments. The average hardness and Youngs modulus of the produced coatings was 21 and 200 GPa, respectively.

Enhancement of graphene visibility on transparent substrates by refractive index optimization.

Optical reflection microscopy is one of the main imaging tools to visualize graphene microstructures. Here is reported a novel method that employs refractive index optimization in an optical reflection microscope, which greatly improves the visibility of graphene flakes. To this end, an immersion liquid with a refractive index that is close to that of the glass support is used in-between the microscope lens and the support improving the contrast and resolution of the sample image. Results show that the contrast of single and few layer graphene crystals and structures can be enhanced by a factor of 4 compared to values commonly achieved with transparent substrates using optical reflection microscopy lacking refractive index optimization.

Steel Spheres and Skydiver — Terminal Velocity

This paper describes the use of open source video analysis software in the study of the relationship between the velocity of falling objects and time. We discuss an experiment in which a steel sphere falls in a container filled with two immiscible liquids. The motion is similar to that of a skydiver falling through air.

Effect of substrate bias voltage on amorphous Si–C–N films produced by PVD techniques

SixCyNz thin films were deposited by reactive magnetron sputtering on glass and steel substrates. The films were grown in a rotation mode over a carbon and a silicon targets in a mixed Ar/N2 atmosphere. The substrates were held at a substrate temperature of 573 K. The argon flow was kept constant (100 sccm) and the nitrogen flow was 20 sccm or 25 sccm, in each one of the two series of produced films, resulting in a working pressure at approximately 0.5 Pa. The substrate bias varied between grounded and −100 V. The films were analysed with respect to microstructure, state of chemical bonding and optical properties by X-ray diffraction (XRD), Raman spectroscopy (RS), optical reflectance and transmittance. Raman spectroscopy was used as a probe of microstructural modifications induced by deposition conditions. The main features observed in RS spectra are the well-known D- and G-bands characteristic of amorphous carbon materials. The position, widths and intensity ratio of these bands are found to be dependent of the films deposition conditions. The refractive index, absorption coefficient, optical band gap and also the thickness were calculated from transmittance spectra obtained between 200 and 2500 nm. The residual stress of the coatings depends on deposition conditions and was calculated by measuring the substrate curvature before and after film deposition. The curvature of the samples was measured by laser triangulation in two series of two orthogonal directions. All coatings were in a state of compressive residual stress. The average hardness and Youngs modulus of the produced coatings is approximately 16 GPa and 170 GPa, respectively. The nano-hardness of the grounded produced samples presented values approximately 30% lower.

Long range energy transfer in graphene hybrid structures

We thank Nuno Peres and Bruno Amorim for informative discussions. This work was supported by FEDER through the COMPETE Program and by the Portuguese Foundation for Science and Technology (FCT) in the framework of the Strategic Project PEST-C/FIS/UI607/2011 and under grant PTDC/FIS/101434/2008. TS has been supported by Spain’s MINECO under Grant No. FIS2014-57432-P and by the Comunidad de Madrid under Grant No. S2013/MIT-3007 MAD2D-CM.

New optical techniques to improve the visibility of graphene on multiple substrates

Optical microscopy as a means to identify graphene is hampered by the low absorptivity of its monolayers and few-layer structures. However for many of the upcoming applications for graphene, it is essential to develop techniques to readily deliver images of graphene based structures. We report on two novel techniques and additionally on a well-known, but modified technique for the identification of graphene. All of the described methods employ standard optical reflection and transmission microsocopy and can be readily adapted in most laboratories. One of the novel techniques is based on the enhancement of the optical contrast by refractive index matching using oil immersion microscopy. The second technique, microdroplet condensation, exploits the hydrophobicity difference between the carbonic sheets and almost any arbitrary substrate. The third technique is a modification of the already well known technique to enhance the visibility contrast of graphene using interferometric effects by employing a Si wafer coated with a dielectric of specific thickness.

Easy process to obtain suspended graphene flakes on TEM grids

Much of the ongoing research on graphene requires free-hanging (suspended) graphene to eliminate any influence from underlying substrates. Several methods have been developed for its preparation but they are either very complex or not completely reliable. Here, we describe a simple method for the transfer of graphene single layers from glass or silicon substrates onto TEM grids. The method uses a carrier film for the transfer process. By optimizing the process yields greater than 60% were achieved. The integrity of the transferred films was confirmed using Raman spectroscopy; successful suspension of both mono- and double-layer graphene sheets was obtained.