ABSTRACT
Tin monoxide (SnO) has attracted attention due to its p-type character and capability of ambipolar conductivity when properly doped, properties that are beneficial for the realization of complementary oxide thin film transistors technology, transparent flexible circuits and optoelectronic applications in general. However, its small fundamental band gap (0.7 eV) limits its applications as a solar energy material, therefore tuning its electronic properties is necessary for optimal performance. In this work, we use density functional theory (DFT) calculations to examine the electronic properties of the Sn1-xPbxO ternary oxide system. Alloying with Pb by element substitution increases the band gap of SnO without inducing defect states in the band gap retaining the anti-bonding character of the valence band maximum which is beneficial for p-type conductivity. We also examine the properties of the SnO/PbO heterojunction system in terms of band alignment and the effect of the most common intrinsic defects. A broken gap band alignment for the SnO/PbO heterojunction is calculated, which can be attractive for energy conversion in solar cells, photocatalysis and hydrogen generation.
ABSTRACT
Interest in the Mn+1AXn phases (M = early transition metal; A = group 13-16 elements, and X = C or N) is driven by their ceramic and metallic properties, which make them attractive candidates for numerous applications. In the present study, we use the density functional theory to calculate the elastic properties and the incorporation of lithium atoms in the 312 MAX phases. It is shown that the energy to incorporate one Li atom in Mo3SiC2, Hf3AlC2, Zr3AlC2, and Zr3SiC2 is particularly low, and thus, theoretically, these materials should be considered for battery applications.
ABSTRACT
Epitaxial, highly ordered Sb:SnO2 nanowires were grown by the vapor-liquid-solid mechanism on m-, r- and a-Al2O3 between 700 °C and 1000 °C using metallic Sn and Sb with a mass ratio of Sn/Sb = 0.15 ± 0.05 under a flow of Ar and O2 at 1 ± 0.5 mbar. We find that effective doping and ordering can only be achieved inside this narrow window of growth conditions. The Sb:SnO2 nanowires have the tetragonal rutile crystal structure and are inclined along two mutually perpendicular directions forming a rectangular mesh on m-Al2O3 while those on r-Al2O3 are oriented in one direction. The growth directions do not change by varying the growth temperature between 700 °C and 1000 °C but the carrier density decreased from 8 × 1019 cm-3 to 4 × 1017 cm-3 due to the re-evaporation and limited incorporation of Sb donor impurities in SnO2. The Sb:SnO2 nanowires on r-Al2O3 had an optical transmission of 80% above 800 nm and displayed very long photoluminescence lifetimes of 0.2 ms at 300 K. We show that selective area location growth of highly ordered Sb:SnO2 nanowires is possible by patterning the catalyst which is important for the realization of novel nanoscale devices such as nanowire solar cells.
ABSTRACT
Anatase titanium oxide is important for its high chemical stability and photocatalytic properties, however, the latter are plagued by its large band gap that limits its activity to only a small percentage of the solar spectrum. In that respect, straining the material can reduce its band gap increasing the photocatalytic activity of titanium oxide. We apply density functional theory with the introduction of the Hubbard + U model, to investigate the impact of stress on the electronic structure of anatase in conjunction with defect engineering by intrinsic defects (oxygen/titanium vacancies and interstitials), metallic dopants (iron, chromium) and non-metallic dopants (carbon, nitrogen). Here we show that both biaxial and uniaxial strain can reduce the band gap of undoped anatase with the use of biaxial strain being marginally more beneficial reducing the band gap up to 2.96 eV at a tensile stress of 8 GPa. Biaxial tensile stress in parallel with doping results in reduction of the band gap but also in the introduction of states deep inside the band gap mainly for interstitially doped anatase. Dopants in substitutional positions show reduced deep level traps. Chromium-doped anatase at a tensile stress of 8 GPa shows the most significant reduction of the band gap as the band gap reaches 2.4 eV.
ABSTRACT
Controlling the number of layers of graphene grown by chemical vapor deposition is crucial for large scale graphene application. We propose here an etching process of graphene which can be applied immediately after growth to control the number of layers. We use nickel (Ni) foil at high temperature (T = 900 °C) to produce multilayer-AB-stacked-graphene (MLG). The etching process is based on annealing the samples in a hydrogen/argon atmosphere at a relatively low temperature (T = 450 °C) inside the growth chamber. The extent of etching is mainly controlled by the annealing process duration. Using Raman spectroscopy we demonstrate that the number of layers was reduced, changing from MLG to few-layer-AB-stacked-graphene and in some cases to randomly oriented few layer graphene near the substrate. Furthermore, our method offers the significant advantage that it does not introduce defects in the samples, maintaining their original high quality. This fact and the low temperature our method uses make it a good candidate for controlling the layer number of already grown graphene in processes with a low thermal budget.
