Fabrication and electrical characterization of p-Sb2S3/n-Si heterojunctions for solar cells application

Antimony trisulphide (Sb₂S₃) films were prepared by thermal evaporation technique on n-type single crystal Si substrates to fabricate p-Sb₂S₃/n-Si heterojunctions. The electrical transport properties of the p–Sb₂S₃/n-Si heterojunctions were investigated by current–voltage (I–V) and capacitance–voltage (C–V) measurements. The temperature-dependent I–V characteristics revealed that the forward conduction was determined by multi-step tunnelling current and the activation energy of saturation current was about 0.54 eV. The 1/C²–V plots indicated the junction was abrupt and the junction built-in potential was 0.6 V at room temperature and decreased with increasing temperature. The solar cell parameters have been calculated for the fabricated cell as V_oc = 0.50 V, J_sc = 14.53 mA cm⁻², FF = 0.32 and η = 4.65% under an illumination of 50 mW cm⁻².     

Strain-induced quantum confinement of electron gases

Zero-dimensional electron gases have been fabricated by the strain-patterning of a GaAs/AlAs heterojunction using amorphous carbon stressors. We have used steady-state, time-resolved and temperature-dependent photoluminescence measurements to probe the occupied density of states of the electron gases. We observe a novel lateral confinement mechanism and efficient transfer of modulation-doped electrons from the regions between the stressors to the quantum dots. In finite magnetic fields we have mapped the evolution of the electron states from which we estimate the number of electrons per dot to be 15.     

Magneto-optics of zero-dimensional electron systems

Photoluminescence spectroscopy has been used to probe the occupied electron states below the Fermi energy of zero-dimensional electron systems (0DESs) in both zero and finite magnetic fields. The arrays of modulation-doped quantum dots investigated were fabricated by both reactive-ion etching and strain-confining GaAs heterojunctions with a -layer of Be present in the GaAs, in order to improve luminescence efficiency. For the etched quantum dots we show that the low magnetic field dispersion T) of the acceptor recombination line is directly related to the magnetic field dependence of the total ground-state energy of interacting electrons in the quantum dots. For the strain-confined 0DESs we have mapped the magneto-dispersion of the quantum confined electron states to reveal 15 electrons per dot.     

Highly conjugated water soluble CdSe quantum dots to multiwalled carbonnanotubes

Highly conjugated multiwalled carbon nanotube-quantum dot heterojunctions were synthesized by ethylene carbodiimide coupling procedure. The functional multiwalled carbon nanotube with carboxylic groups on sidewall could react with the amino group of L-cysteine capped CdSe quantum dots and then resulted in nanotube-quantum dot heterojunctions. Scanning electron microscopy was used to characterize the heterojunctions.     

XPS study of the formation of ultrathin GaN film on GaAs(1 0 0)

The nitridation of GaAs(1 0 0) surfaces has been studied using XPS spectroscopy, one of the best surface sensitive techniques. A glow discharge cell was used to produce a continuous plasma with a majority of N atomic species. We used the Ga3d and As3d core levels to monitor the chemical state of the surface and the coverage of the species. A theoretical model based on stacked layers allows to determine the optimal temperature of nitridation. Moreover, this model permits the determination of the thickness of the GaN layer. Varying time of nitridation from 10 min to 1 h, it is possible to obtain GaN layers with a thickness between 0.5 nm and 3 nm.     

Ni-based photocatalytic H2-production cocatalysts2

Photocatalysis is believed to be one of the best methods to realize sustainable H₂ production. However, achieving this through heterogeneous photocatalysis still remains a great challenge owing to the absence of active sites, sluggish surface reaction kinetics, insufficient charge separation, and a high thermodynamic barrier. Therefore, cocatalysts are necessary and of great significance in boosting photocatalytic H₂ generation. This review will focus on the promising and appealing low-cost Ni-based H₂-generation cocatalysts as the alternatives for the high-cost and low-abundance noble metal cocatalysts. Special emphasis has been placed on the design principle, modification strategies for further enhancing the activity and stability of Ni-based cocatalysts, and identification of the exact active sites and surface reaction mechanisms. Particularly, four types of modification strategies based on increased light harvesting, enhanced charge separation, strengthened interface interaction, and improved electrocatalytic activity have been thoroughly discussed and compared in detail. This review may open a new avenue for designing highly active and durable Ni-based cocatalysts for photocatalytic H₂ generation.     

Role of the substrate thickness for the structural properties of organic-organic heterostructures

F₁₆CuPc deposited on pentacene is characterized by the coexistence of two different configurations: F₁₆CuPc is found in the standing up phase (“s-configuration”) on top of pentacene terraces and in a lying down phase (“l-configuration”) at pentacene step edges. By combining AFM and grazing incidence X-ray diffraction we show that the ratio between F₁₆CuPc in l- and s-configurations increases with thickness of the pentacene substrate film, demonstrating the role of the pentacene steps as nucleation centers for the F₁₆CuPc l-configuration. Experiments performed with ultra-thin pentacene thicknesses disclose that the F₁₆CuPc l-configuration does not grow on top of the first and second pentacene layers, pointing to the action of long-range interactions with the substrate.     

Manganite-layer thickness-dependent photovoltaic effect of La0.9Sr0.1MnO3/SrNb0.01Ti0.99O3 p–n heterojunction

The perovskite p–n heterojunctions were fabricated by depositing La_0.9Sr_0.1MnO₃ (LSMO) layers with thicknesses ranging from 20 to 400 Å on SrNb_0.01Ti_0.99O₃ (SNTO) single-crystal substrates by laser molecular beam epitaxy (laser-MBE). The open-circuit photovoltage of the LSMO/SNTO heterojunction at room temperature increases with the increase of the thickness of LSMO layer. This result is ascribed to the increase of the carrier amount and the enhancement of the built-in electric field in the space-charge region of the LSMO/SNTO heterojunction with the increase of the thickness of LSMO layer. Furthermore, we found that the speed of photovoltaic response is almost independent of the thickness of LSMO layer in the heterojunction.     

Submillimeter wave induced resistance oscillations in ultra-high mobility two-dimensional electron systems

Submillimeter wave induced resistance oscillations in two ultra-high mobility GaAs/Al_0.24Ga_0.76As quantum well samples have been investigated by means of a backward wave oscillator and far-infrared laser at ³He temperatures. Subnikov–de Haas oscillation, submillimeter wave induced resistance oscillation, and magnetoplasmon resonance occur simultaneously in this frequency regime. The primary radiation induced resistance minimum shifts toward cyclotron resonance with increasing radiation frequency. The positions of these minima agree well with those of the magnetoplasmon resonance. The higher-order harmonics of the resistance oscillation remain around the multiples of the cyclotron resonance frequency. An in situ transmission measurement exhibits an asymmetric broadening of the cyclotron resonance, appearing as a combination of the cyclotron resonance and the magnetoplasmon resonance, but no features directly linked to the microwave induced resistance oscillation can be seen.     

The growth mechanism of pentacene-fullerene heteroepitaxial films

The growth of pentacene on C₆₀ film has been studied in real-time by low energy electron microscope. The standing-up phase overgrows on the lying-down phase at room temperature with increasing film thickness. At intermediate temperature we observed two distinct types of nucleation: an earlier nucleation of lying-down phase and a delayed nucleation of standing-up phase on the bare C₆₀ surface between islands of lying-down phase. Further thermal activation control enabled us to tune the types of nucleation and the standing-up phase without co-presence of the lying-down phase could be achieved above ∼70 °C.