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Growing ultrathin nanogranular (NG) metallic films with continuously varying thickness is of great interest for studying regions of criticality and scaling behaviors in the vicinity of quantum phase transitions. In the present work, an ultrathin gold plasmonic NG film was grown on a sapphire substrate by RF magnetron sputtering with an intentional deposition gradient to create a linearly variable thickness ranging from 5 to 13 nm. The aim is to accurately study the electronic phase transition from the quantum tunneling regime to the metallic conduction one. The film structural characterization was performed by means of high-resolution transmission electron microscopy, atomic force microscopy, as well as x-ray diffraction and reflectivity techniques, which indicate the Volmer–Weber film growth mode. The optical and electrical measurements show a transition from dielectric-isolated gold NPs towards a continuous metallic network when t becomes larger than a critical value of tM = 7.8 nm. Our results show that the onset of the percolation region occurs when a localized surface plasma resonance transforms to display a Drude component, indicative of free charge carriers. We demonstrate that, by using a continuously varying thickness, criteria for metallicity can be unambiguously identified. The onset of metallicity is clearly distinguished by the Drude damping factor and by discontinuities in the plasma frequencies as functions of thickness. [ABSTRACT FROM AUTHOR]

In this work, we have grown nanostructured vanadium dioxide (VO2) thin films by a spray pyrolysis coating method on glass pre-heated substrates at 450 °C and studied their optical and electronic properties. As-prepared film structure and morphology, with a thickness of ~ 400 nm, were examined by X-ray diffraction, Raman spectra, and atomic force microscopy (AFM), thus revealing a polycrystalline monoclinic M1 phase. On the other hand, the electrical resistance was measured as a function of the temperature in the range of 25 to 100 °C where a clear transition from the M1 phase at T = 25 °C to the metallic one (conductive phase) at ~ 60 °C was observed. This also confirms the stability of the M1 phase for T < 60 °C. The electronic properties and band structure were additionally investigated by the local density approximation (LDA) using the first principle of the Korringa-Kohn-Rostroker (KKR). As a result, our material showed a magnetic property, specifically an antiferromagnetic stabilization of the M1 phase. [ABSTRACT FROM AUTHOR]

• Un- and Ni-doped ZnO thin films are synthesized and characterized. • Experimental and theoretical characterizations demonstrate stabilization of a small p-Schottky barrier upon Ni doping. • Reduction in p-Schottky barrier is mainly due to amelioration in the surface uniformity and modulization in the relative alignment of band edges at the interfaces. Large contact resistance at metal-substrate/ZnO heterostructure interfaces prevents achieving highly efficient device performance. Herein, we present a systematic study on the effect of Ni-doping in the reduction of the Schottky contact barrier at metal-substrate/ZnO heterostructure. To this end, Ni-doped zinc oxide (Ni:ZnO) thin films were deposited on glass substrate by a spray technique with different Ni-doping concentrations. X-ray Diffraction and Atomic Force Microscopy (AFM) measurements showed that Ni-doping enhances the surface uniformity as compared to the undoped-ZnO films and significantly decreases the roughness (RMS) from 35 to 17 nm. Conductive Atomic Force Microscopy (C-AFM) with a Bruker's platinum coated probe (Pt-Ir) tip results in the stabilization of a p-type Schottky contact with a small height barrier of ~0.4 eV, which is among the smallest values reported in literature for ZnO thin films. Our first principle calculations, which are based on the relative alignment of the band edges of the components, also confirm the reduction in the Schottky barrier height by Ni-doping in line with the experimental tendency. Both experimental and theoretical results provide a robust evidence of the potential of stabilization of a small p-type Schottky contact at metallic-substrate/ZnO interface through a Ni-doping. [ABSTRACT FROM AUTHOR]