A first-principles study of the electronic and structural properties of Sb and F doped SnO2 nanocrystals

Highly Monochromatic Ultraviolet LED Based on the SnO2 Microwire Heterojunction Beyond Dipole-Forbidden Band-Gap Transition

SnO2 has been extensively applied in the fields of optoelectronic devices because of its large band gap, high exciton binding energy, and outstanding optical/electrical properties. However, its applications in ultraviolet light-emitting diodes (LEDs) are still hindered by the dipole-forbidden rule. Herein, the dipole-forbidden rule can be conquered by synthesizing Sb-incorporated SnO2 microwires (SnO2:Sb MWs), which are examined by ultraviolet photoluminescence emitting at 363.2 nm and a line width of 11.3 nm. Subsequently, a highly monochromatic ultraviolet light-emitting diode (LED) based on a SnO2:Sb MW heterojunction was constructed with a p-GaN film serving as the hole supplier. In the LED, the presence of a MgO intermediate layer can modulate carrier transport and recombination path, thus achieving band-edge optical transition in the SnO2:Sb MW. As the LED is modified using Ag nanowires, electrical properties, especially for the hole injection efficiency, were dramatically boosted, contributing significantly to the device high brightness. The LED emits at 365.9 nm and a line width of 12.4 nm. Therefore, we have realized a high-brightness and narrow-band ultraviolet LED with the shortest peak wavelength never seen in previously reported SnO2 LEDs. This work will promote the potential applications of low-dimensional SnO2 optoelectronic devices and provide an effective exemplification to overcome the dipole-forbidden rule in metal-oxide materials with "forbidden" energy gaps.

Competition and Cooperation between Fluorine and Oxygen in SnO2:F Films

The interaction of oxygen and fluorine (F&O) in an F-doped SnO2 film, prepared by regulating oxygen partial pressure and the content of doped fluorine from 2.5 at% to 10 at%, was investigated in the large perspective through characterization methods including XRD, Raman spectroscopy, photoluminescence spectroscopy, wettability measurement and a Hall effect test system. The results show that F&O’s competitive and cooperative relationship would be reflected in the structure and electrical characteristics of SnO2 films. The oxygen action is overwhelming and restricts fluorine, so a growing number of F atoms occupy the position by the order of co-edge oxygen of tin–oxygen octahedron chains > oxygen vacancies > segregation, which leads to that carrier concentration modestly increasing from ~1015 to ~1017/cm−3. As oxygen action is inadequate to restrain fluorine, more F atoms are likely to enter the SnO2 lattice in a solid-solution way to replace the O atoms at the co-edge position of the octahedron chains, causing a dramatic increase in carrier concentration from ~1016 to ~1019/cm−3. Furthermore, by continuing to weaken oxygen action, only 2.5 at% of fluorine content could bring about a carrier concentration augment from ~1016/cm−3 to ~1018/cm−3, then going up to ~1021/cm−3 by post-annealing. However, the impairment of oxygen action contributes to a more effective doping of fluorine on SnO2 film. Such mutual action between fluorine and oxygen provides a direction for highly efficient production and tunable regulation of SnO2 film on demand.

Real-space pseudopotential study of vibrational properties and Raman spectra in Si-Ge core-shell nanocrystals

We examine the vibrational properties and Raman spectra of Si-Ge core-shell nanostructures using real-space pseudopotentials constructed within density functional theory. Our method uses no empirical parameters, unlike many popular methods for predicting Raman spectra for nanocrystals. We find the dominant features of the Raman spectrum for the Si-Ge core-shell structure to be a superposition of the Raman spectra of the Ge and Si nanocrystals with optical peaks around 300 and 500 cm(-1), respectively. We also find a Si-Ge "interface" peak at 400 cm(-1). The Ge shell causes the Si core to expand from the equilibrium structure. This strain induces significant redshift in the Si contribution to the vibrational and Raman spectra, while the Ge shell is largely unstrained and does not exhibit this shift. We find that the ratio of peak heights is strongly related to the relative size of the core and shell regions. This finding suggests that Raman spectroscopy may be used to characterize the size of the core and shell in these structures.

High order forces and nonlocal operators in a Kohn–Sham Hamiltonian

Real space pseudopotentials have a number of advantages in solving for the electronic structure of materials.