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Gallium nitride nanowire and nanorod substrates are prospective platforms allowing to control the local strain distribution in graphene films on top of them, resulting in an induction of pseudomagnetic fields. AFM measurements performed in a HybriD mode complemented by SEM allow for a detailed visualization of the strain distribution on graphene surface. Graphene in direct contact with supporting regions is tensile strained, while graphene located in-between is characterized by lower strain. Characteristic tensile strained wrinkles also appear in the areas between the supporting regions. A positive correlation between strain gradient and distances between borders of supporting regions is observed. These results are confirmed by analysis of the Raman D’ band intensity, which is affected by an enhancement of intravalley scattering. Furthermore, scanning tunneling spectroscopy shows a local modification of the density of states near the graphene wrinkle and weak localization measurements indicate the enhancement of pseudomagnetic field-induced scattering. Therefore, we show that nanowire and nanorod substrates provide strain engineering and induction of pseudomagnetic fields in graphene. The control of graphene morphology by modification of distances between supporting regions is promising for both further fundamental research and the exploration of innovative ways to fabricate pseudomagnetic field-based devices like sensors or filters.

Integration of optically active diamond particles with glass fibers is a powerful method of scaling diamond's magnetic sensing functionality. We propose a novel approach for the integration of diamond particles containing nitrogen-vacancy centers directly into the fiber core. The core is fabricated by stacking the preform from 790 soft glass canes, drawn from a single rod dip-coated with submicron diamond particles suspended in isopropyl alcohol. This enables manual control over the distribution of nanoscale features, here – the diamond particles across the optical fiber core. We verify this by mapping the diamond distribution in the core using confocal microscopy. The particles are separated longitudinally by 12–29 μm, while in the transverse plane a separation of approximately 1.5–2.2 μm is observed, corresponding to the individual cane diameter in the final fiber, and without significant agglomeration. The fiber's magnetic sensitivity is confirmed in optically detected magnetic resonance recorded with a coiled, 60-cm-long fiber sample with readout contrast of 1.3% limited by microwave antenna coverage. Moreover, magnetic-field dependence of the NV─ fluorescence intensity is demonstrated in the absence of microwaves, allowing magnetometric applications with a large (from 0 to 35 mT) B-field dynamic range.

By using four layered graphene/gallium nitride (GaN) Schottky diodes with an undoped GaN spacer, we demonstrate highly effective gating of graphene at low bias rendering this type of structure very promising for potential applications. An observed Raman G band position shift larger than 8.5 cm-1 corresponds to an increase in carrier concentration of about 1.2x10^13 cm-2. The presence of a distinct G band splitting together with a narrow symmetric 2D band indicates turbostratic layer stacking and suggests the presence of a high potential gradient near the Schottky junction even at zero bias. The subbands characterized by the highest Raman energies correspond to the largest concentration of electrons. An analysis based on electroreflectance measurements and a modified Richardson equation confirmed that graphene on n-GaN separated by an undoped GaN spacer behaves like a capacitor at reverse bias. At least 60% of G subband position shifts occur at forward bias, which is related to a rapid reduction of electric field near the Schottky junction. Raman micromapping shows a high uniformity of gating efficiency on the surface. Therefore, our studies demonstrate the usefulness of few layer turbostratic graphene deposited on GaN for tracing electron-phonon coupling in graphene. Multilayer graphene also provides uniform and stable electric contacts. Moreover, the observed bias sensitive G band splitting can be used as an indicator of charge transfer in sensor applications in the low bias regime.

A constant height of gallium nitride (GaN) nanowires with graphene deposited on them is shown to have a strong enhancement of Raman scattering, whilst variable height nanowires fail to give such an enhancement. Scanning electron microscopy reveals a smooth graphene surface which is present when the GaN nanowires are uniform, whereas graphene on nanowires with substantial height differences is observed to be pierced and stretched by the uppermost nanowires. The energy shifts of the characteristic Raman bands confirms that these differences in the nanowire height has a significant impact on the local graphene strain and the carrier concentration. The images obtained by Kelvin probe force microscopy show clearly that the carrier concentration in graphene is modulated by the nanowire substrate and dependent on the nanowire density. Therefore, the observed surface enhanced Raman scattering for graphene deposited on GaN nanowires of comparable height is triggered by self-induced nano-gating to the graphene. However, no clear correlation of the enhancement with the strain or the carrier concentration of graphene was discovered.

The surface-enhanced Raman scattering in graphene deposited on AlxGa1−xN/GaN axial heterostructure nanowires was investigated. It was found that the intensity of graphene Raman spectra is not correlated with aluminium content. Analysis of graphene Raman band parameters, Kelvin probe force microscopy, and electroreflectance showed a screening of polarisation charges. Theoretical calculations demonstrated that plasmon resonance in graphene is far beyond the Raman spectral range. This excludes the presence of an electromagnetic mechanism of surface-enhanced Raman scattering, hence suggesting a chemical mechanism of enhancement.

Graphene has been shown to be an interesting material with various possible applications. Since graphene is transparent and has good conductivity, it is considered to be a good candidate for replacing ITO in solar cells. On the other hand, it has been shown recently that nanowire structures can substantially increase efficiency of solar cells. Therefore, possibility of using graphene as a transparent electrode on GaN nanowires (NWs) is very attractive. In this paper we focus on experimental studies of graphene grown by Chemical Vapor Deposition method, and transferred onto GaN NWs grown by Molecular Beam Epitaxy. Electron Spin Resonance (ESR) and Raman spectroscopy have been used to study the properties of graphene. ESR was measured as a function of temperature. Observation of the characteristic peak related to weak-localization allowed us to estimate the coherence length changes from about 150 nm at 5 K to 120 nm at 25 K (Fig.1). Interband and intraband scattering lengths were found to be independent of temperature and equal 150 nm and 31 nm, respectively. Interestingly, the obtained coherence length was shorter, whereas elastic scattering lengths were similar to those obtained for graphene transferred to other substrates. It was found, that presence of GaN NWs substantially increases intensity of graphene Raman spectra, by more than one order of magnitude compared to as grown CVD material. Besides, as can be seen from Raman mapping, the observed changes of spectrum parameters (energy shifts and intensity) correlated with positions of individual NWs (Fig. 2). The intensity ratio of D to G peak was high, however could not be correlated with GaN NWs pattern, which suggested large enhancement of defect spectrum rather than defect generation by NWs. The obtained results are discussed in terms of the interaction of graphene with a polar medium (GaN, with free carrier distribution along NWs).

The influence of GaN nanowires on the optical and electrical properties of graphene deposited on them was studied using Raman spectroscopy and microwave induced electron transport method. It was found that interaction with the nanowires induces spectral changes as well as large enhancement of Raman scattering intensity. Surprisingly, the smallest enhancement (about 30-fold) was observed for the defect induced D' process and the highest intensity increase (over 50-fold) was found for the 2D transition. The observed energy shifts of the G and 2D bands allowed to determine carrier concentration fluctuations induced by GaN nanowires. Comparison of Raman scattering spatial intensity maps and the images obtained using scanning electron microscope led to conclusion that vertically aligned GaN nanowires induce a homogenous strain, substantial spatial modulation of carrier concentration in graphene and unexpected homogenous distribution of defects created by interaction with nanowires. The analysis of the D and D' peak intensity ratio showed that interaction with nanowires also changes the probability of scattering on different types of defects. The Raman studies were correlated with weak localization effect measured using microwave induced contactless electron transport. Temperature dependence of weak localization signal showed electron-electron scattering as a main decoherence mechanism with additional, temperature independent scattering reducing coherence length. We attributed it to the interaction of electrons in graphene with charges present on the top of nanowires due to spontaneous and piezoelectric polarization of GaN. Thus, nanowires act as antennas and generate enhanced near field which can explain the observed large enhancement of Raman scattering intensity.