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The possibility of modification of the local properties of hexagonal boron nitride (h-BN) by laser irradiation is investigated. Investigations conducted using both Raman spectroscopy and electrostatic force microscopy were performed. Laser light induced modifications are found to cause no structural changes. However, they impact the Raman spectra and local charge state of the material. They are also shown to be stable in time and during electrical grounding of the sample. A mechanism of photoionization of deep defects present in h-BN is proposed to explain the observed phenomenon. The discussed effect opens up a new method of nanostructuration of h-BN based planar heterostructures. [ABSTRACT FROM AUTHOR]

We present the optical and the electrical properties of GaN/AlGaN structures that can be used as tunable ultraviolet photodetectors. The photosensitivity spectrum of the structures can be tuned in the range 3.5–3.85 eV by external voltage. The photosensitivity of the sample in the 3.5 eV spectral range increases about 500 times when the reverse bias changes from -2 to -6 V. The effect can be explained by changes in the electric field in the GaN/AlGaN structure. The field is generated by the internal electric polarization and the external bias. We present also a numerical modeling of the electric field, the potential profiles, and the current flow in such structures. Our modeling shows that at low bias a two-dimensional (2D)-electron gas at the AlGaN/GaN interface screens the electric field generated by spontaneous polarization. The lack of the field stops transport of photoexcited holes. The holes that are accumulated on the interface causes fast electron recombination and reduces photocurrent. The external electric field can move the 2D-electron gas and increase photocurrent from the GaN layer. [ABSTRACT FROM AUTHOR]

Transmission electron microscopy (TEM), scanning tunneling microscopy (STM), and micro-Raman investigations of epitaxial graphene on 4H-SiC on-axis and 4° off-axis are presented. The STM images show that there is superimposed on 1×1 graphene pattern the carbon nanomesh of honeycomb 6×6 structure with the lattice vector of 17.5 Å. The TEM results give evidence that the first carbon layer is separated by 2 Å from the Si-terminated SiC surface and that subsequent carbon layers are spaced by 3.3 Å. It is also found in TEM that the graphene layers cover atomic steps, present on 4° off-axis SiC(0001) surface, indicating a carpetlike growth mode. However, a bending of graphene planes on atomic steps of SiC apparently leads to generation of stress which leads to creation of edge dislocations in the graphene layers. [ABSTRACT FROM AUTHOR]

We present the study of metalorganic vapor phase epitaxy hexagonal boron nitride (MOVPE-h-BN) and graphene/MOVPE-h-BN heterostructures under the illumination with monochromatic light. The process of illumination makes the modification visible by both the shifting of Raman lines and the emergence of local electric fields. These changes are related to the modification of the charge state of the deep defect centers and can be used to control other 2D materials within heterostructures. The scanning tunneling spectroscopy study of the graphene/MOVPE-h-BN heterostructure allowed us to observe the defect states associated with the h-BN and evaluate the effect of illumination on them as well as the electronic structure of graphene. [ABSTRACT FROM AUTHOR]

The overprints produced in inkjet technology with graphene oxide dispersion are presented. The graphene oxide ink is developed to be fully compatible with standard industrial printers and polyester substrates. Post-printing chemical reduction procedure is proposed, which leads to the restoration of electrical conductivity without destroying the substrate. The presented results show the outstanding potential of graphene oxide for rapid and cost efficient commercial implementation to production of flexible electronics. Properties of graphene-based electrodes are characterized on the macro- and nano-scale. The observed nano-scale inhomogeneity of overprints' conductivity is found to be essential in the field of future industrial applications. [ABSTRACT FROM AUTHOR]

In the present work, we demonstrated a new method of graphene films transferring applying a marker-frame instead of polymer-like films as a support. This method is an alternative that is much faster, cheaper and freely available to all. Our technique guarantees cleaner graphene surface and no polymer residues on it. Consequently, it significantly increases the applicability of graphene. For instance, one can think of using graphene as a component of nanocomposites or coupling it with other nanomaterials. As an example we showed graphene films suspended on GaN nanowires. The characterization of the properties of graphene transferred following the enhanced framemethodwas performed by Raman spectroscopy, as well as by carrying out SEM imaging and conducting TEM investigations. [ABSTRACT FROM AUTHOR]

Structural investigations of hydrogenated epitaxial graphene grown on SiC(0001) are presented. It is shown that hydrogen plays a dual role. In addition to contributing to the well-known removal of the buffer layer, it goes between the graphene planes, resulting in an increase of the interlayer spacing to 3.6 Å-3.8 Å. It is explained by the intercalation of molecular hydrogen between carbon planes, which is followed by H2 dissociation, resulting in negatively charged hydrogen atoms trapped between the graphene layers, with some addition of covalent bonding to carbon atoms. Negatively charged hydrogen may be responsible for p-doping observed in hydrogenated multilayer graphene. [ABSTRACT FROM AUTHOR]

Protection of Cu(111) surface by chemical vapor deposition graphene coating is investigated. The X-ray photoemission spectroscopy results do not reveal any signs of corrosion on graphene-coated Cu(111), and suggest perfect protection of copper surface against interaction with atmospheric gases. However, the scanning tunneling spectroscopy results show that cracks in the graphene sheet open up windows for nanoscale corrosion. We have shown also that such local corrosions are not only limited to the discontinuities but may also progresses underneath the graphene cover. [ABSTRACT FROM AUTHOR]

Graphene grown by chemical vapor deposition on 4H-SiC (0001) was studied using micro-Raman spectroscopy and atomic force microscopy (AFM). AFM revealed that the graphene structure grown on on-axis substrates has a stepped morphology. This is due to step bunching, which results from etching in hydrogen as well as from the process of graphene formation itself. It was shown by micro-Raman spectroscopy that the properties of graphene present on step edges and on terraces are quite different. Graphene on terraces is uniform with a relatively small thickness and strain fluctuations. On the other hand, graphene on step edges has a large thickness and strain variations occur. A careful analysis of micro-Raman spatial maps led us to the conclusion that the carrier concentration on step edge regions is lowered when compared with terrace regions. [ABSTRACT FROM AUTHOR]

The results of investigations of parallel magnetotransport in AlGaAs/GaAs and InGaAs/InAlAs/InP multiple quantum-well structures (MQWs) are presented. The MQWs were obtained by metalorganic vapor phase epitaxy with different shapes of the QWs, numbers of QWs, and levels of doping. The magnetotransport measurements were performed in a wide region of temperatures (0.5–300 K) and at high magnetic fields up to 30 T (B is perpendicular to and the current is parallel to the plane of the QW). Three types of observed effects are analyzed: the quantum Hall effect and Shubnikov—de Haas oscillations at low temperatures (0.5–6 K) and magnetophonon resonance at higher temperatures (77–300 K). © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Single postdielectric resonators operating on their quasi TE011 modes were used for the measurement of the surface resistance and conductivity of graphene films grown on semi-insulating SiC substrates. With this technique the surface resistance was measured with an uncertainty of ±5% and the conductivity was evaluated with an uncertainty equal to the uncertainty in determining the film thickness. The room temperature conductivity of the graphene films proved to be in the range 5×106 to 6.4×106 S/m. [ABSTRACT FROM AUTHOR]

In this letter, the authors demonstrate large size m-plane GaN substrates grown by ammonothermal method. These substrates have excellent structural quality. The concentration of threading dislocation density is below 5×104 cm-2 and the full width at half maximum for the symmetrical and asymmetrical peaks equals 16 and 19 arc sec, respectively. Also good optical quality, the energy gap-related transition is clearly observed at room temperature in photoluminescence and contactless electroreflectance spectra. GaN epilayers deposited on these substrates exhibit intrinsic narrow exciton lines which are very sensitive to the optical selection rules typical for hexagonal symmetry, proving truly nonpolar character of the material. [ABSTRACT FROM AUTHOR]

The study of epitaxial graphene layers grown on SiC by two techniques, namely, the traditional Si sublimation method and the recent chemical vapor deposition (CVD) using temperature induced shift of the Raman 2D line, is presented. The measurements of thermal shift rate of 2D line on 4 H-SiC(0001) allowed us to determine notable differences in interaction of graphene with SiC substrate. The obtained results show that graphene layers grown by Si sublimation of 4 H-SiC(0001) are pinned strongly to the substrate. In contrast, the layers of graphene grown on 4 H-SiC(0001) substrates by CVD showed much weaker pinning. It was found that the film consisting of two or three graphene layers grown by CVD was already unpinned and thus showing Raman shift expected for freestanding graphene. The obtained differences in pinning of epitaxial graphene layers are explained in terms of basic growth mechanism differences between these two methods: graphene growth by Si sublimation is a 'bottom-up' process and by CVD-a 'top-down' process. The obtained results show that the CVD method of growth is an alternative technology for producing high quality graphene structures on SiC. [ABSTRACT FROM AUTHOR]

A letter to the editor is presented in response to the article "Role of grapheme defects in corrosion of grapheme-coated Cu(111) surface" in the 2013 issue.

The authors have applied contactless electroreflectance (CER) spectroscopy to study the epi readiness of m-plane GaN substrates obtained by the ammonothermal method. It has been clearly observed that the CER resonance, which is related to the energy gap transition, appears for samples with a well-polished surface. The sharpness of this resonance is directly related to the surface quality. The broadening of energy gap transition can be used as a parameter to quantify the surface quality. For samples polished with optimal conditions, this broadening (γpol) is close to the broadening observed for the cleaved GaN surface (γclev) with m-plane orientation (150-190 vs 135 meV). The quality of the polishing process can be evaluated by analyzing the γclev/γpol ratio, where γclev/γpol=1 corresponds to an excellent polishing process. In the authors' case, this ratio has been determined to be close to 1 for well-polished samples. [ABSTRACT FROM AUTHOR]