Enhanced Raman scattering and weak localization in graphene deposited on GaN nanowires

Abstract

The influence of GaN nanowires on the optical and electrical properties of graphene deposited on them was studied using Raman spectroscopy and a microwave-induced electron transport method. It was found that the interaction with the nanowires induces spectral changes and leads to a significant enhancement of the 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 us to determine the carrier concentration fluctuations induced by the GaN nanowires. A comparison of the Raman scattering spatial intensity maps and the images obtained using a scanning electron microscope led to a conclusion that the vertically aligned GaN nanowires induce a homogenous strain, substantial spatial modulation of the carrier concentration in graphene and unexpected homogenous distribution of defects created by the interaction with the nanowires. The analysis of the D and D' peak intensity ratio showed that the interaction with the nanowires also changes the probability of scattering on different types of defects. The Raman studies were correlated with the weak localization effect measured using microwave-induced contactless electron transport. The temperature dependence of the weak localization signal showed electron-electron scattering as the main decoherence mechanism with an additional, temperature-independent scattering-reducing coherence length. We attributed it to the interaction of electrons in graphene with charges present on top of nanowires due to the spontaneous and piezoelectric polarization of GaN. Thus, nanowires act as antennas and generate an enhanced near field which can explain the observed significant enhancement of the Raman scattering intensity.

Surface-enhanced Raman scattering of graphene caused by self-induced nanogating by GaN nanowire array

“…[3] Our previous studies completed on graphene deposited on gallium nitride nanowires (GaN NWs) with similar height have shown that the Raman spectra of graphene are enhanced by more than an order of magnitude, when compared to graphene deposited on a GaN epilayer. [4] This enhancement has been interpreted to be caused by Surface Enhanced Raman Scattering (SERS). There are typically two mechanisms of SERS, an electromagnetic mechanism (EM) and a chemical mechanism (CM), and it is still under debate which of them is dominant in graphene.…”

Surface-enhanced Raman scattering in graphene deposited on Al Ga1−N/GaN axial heterostructure nanowires

“…Studies of graphene deposited on vertically aligned gallium nitride nanowires (GaN NWs) are interesting not only because of possible applications in photovoltaics but also because of the presence of surface-enhanced Raman scattering in graphene. [1], [2], [3] Intensity of Raman bands has been found to be more than one order of magnitude higher than in graphene deposited on GaN epilayer. Our previous results have excluded light interference in NWs layer to be responsible for this enhancement.…”

Properties of graphene deposited on GaN nanowires: influence of nanowire roughness, self-induced nanogating and defects

“…Consequently, a high concentration of carriers on the GaN surface can be observed [ 22 – 23 ]. Previous studies of graphene on GaN NWs have shown that electric charges located on the top of the GaN NWs strongly impact Raman scattering in graphene, causing an enhancement of the spectrum [ 24 – 25 ]. Therefore, studies of graphene on NWs with different densities and variations in height might give information about the role of supporting points on graphene properties.…”

“…22,23 Previous studies of graphene on GaN NWs have shown that electric charges located on the top of the GaN NWs strongly impact Raman scattering in graphene, causing enhancement of the spectrum. 24,25 Therefore, studies of graphene on NWs with different density and variations in height might give an information about the role of the density of supporting points under graphene on its properties. For example, analysis of graphene deposited on uniformly distributed silicon nanopillars showed the dependence of graphene strain on distances between nanopillars.…”