We study the electronic properties of GaAs-AlGaAs superlattices with intentional correlated disorder by means of photoluminescence and vertical dc resistance. The results are compared to those obtained in ordered and uncorrelated disordered superlattices. We report the first experimental evidence that spatial correlations inhibit localization of states in disordered low-dimensional systems, as our previous theoretical calculations suggested, in contrast to the earlier belief that all eigenstates are localized. [S0031-9007(99) PACS numbers: 73.20.Dx, 73.20.Jc, In recent years, a number of tight-binding [1][2][3] and continuous [4] models of disordered one-dimensional (1D) systems have predicted the existence of sets of extended states, in contrast to the earlier belief that all the eigenstates are localized in 1D disordered systems. These systems are characterized by the key ingredient that structural disorder is short-range correlated. Because of the lack of experimental confirmations, there are still some controversies as to the relevance of these results and their implications on physical properties. In this context, some authors have proposed finding physically realizable systems that allow for a clear cut validation of the above-mentioned purely theoretical prediction [5][6][7][8]. Given that semiconductor superlattices (SL's) have been already used successfully to observe electron localization due to disorder [9][10][11][12][13][14], these authors have suggested SL's as ideal candidates for controllable experiments on localization or delocalization and related electronic properties [5][6][7][8].To the best of our knowledge, up to now there is no experimental verification of this theoretical prediction owing to the difficulty in building nanoscale materials with intentional and short-range correlated disorder. However, the confirmation of this phenomenon is important both from the fundamental point of view and for the possibility to develop new devices based on these peculiar properties. In this work we present an experimental verification of this phenomenon in semiconductor nanoscale materials, taking advantage of the molecular beam epitaxy growth technique, which allows the fabrication of semiconductor nanostructures with monolayer controlled perfection.We grew several GaAs-Al 0.35 Ga 0.65 As SL's and we studied their electronic properties by photoluminescence (PL) at low temperature and dc vertical transport in the dark. Indeed PL has been proven to be a good technique to study the electronic properties of disordered SL's [9-11], giving transition energies comparable with theoretical calculations of the electronic levels. The electronic states were calculated using a Kronig-Penney model that has been shown to hold in this range of well and barrier thicknesses, with precise results [15]. This allows the analysis of the experimental transition energies for PL and the ascertainment of the localization and delocalization properties of the SL's. The details of the calculations and a schematic view of the cond...

The different exfoliation routes of graphite to produce graphene by sonication in solvent, chemical oxidation and electrochemical oxidation are compared. The exfoliation process and roughening of a flat graphite substrate is directly visualized at the nanoscale by scanning probe and electron microscopy. The etching damage in graphite and the properties of the exfoliated sheets are compared by Raman spectroscopy and X‐ray diffraction analysis. The results show the trade‐off between exfoliation speed and preservation of graphene quality. A key step to achieve efficient exfoliation is to couple gas production and mechanical exfoliation on a macroscale with non‐covalent exfoliation and preservation of graphene properties on a molecular scale.

Blends of reduced graphene oxide (RGO) and poly(3-hexylthiophene) (P3HT) are used as the active layer of field-effect transistors (FETs). By using sequential deposition of the two components, the density of RGO sheets can be tuned linearly, thereby modulating their contribution to the charge transport in the transistors, and the onset of charge percolation. The surface potential of RGO, P3HT and source-drain contacts is measured on the nanometric scale with Kelvin Probe Force Microscopy (KPFM), and correlated with the macroscopic performance of the FETs. KPFM is also used to monitor the potential decay along the channel in the working FETs.