Thermal Stability of CsPbBr3 Perovskite Quantum Dots Assembled with SBA-15

Nowadays, the excellent performance of metal halide perovskite quantum dots (PQDs) has been demonstrated, but the stability is still a perplexing issue. In this paper, the CsPbBr3 QDs were assembled into SBA-15 for the first time. The thermal stability and photoluminescence (PL) intensity of SBA-15@CsPbBr3 QDs were improved. The PL spectra of pure CsPbBr3 QDs have red-shift (~6 nm) with the increasing temperature. However, that of SBA-15@CsPbBr3 QDs have almost no red-shift. The PL intensity of SBA-15@CsPbBr3 QDs decreased slightly after heating and cooling for several times. By comparison, the PL intensity of pure CsPbBr3 QDs decreased more significantly. The experimental results showed that SBA-15 played a significant role in improving the thermal stability of PQDs, which will have an excellent potential for the application of PQDs in the future.

Photoluminescence properties of GaAsBi single quantum wells with 10% of Bi

A set of single quantum well (SQW) samples of GaAs1-xBix with x ~ 0.1 and p-doped GaAs barriers grown by molecular beam epitaxy was investigated by the temperature-dependent photoluminescence (PL) spectroscopy. Those GaAsBi SQW structures showed a high crystalline quality, a smooth surface and sharp interfaces between the layers and exhibited a high PL intensity and a lower than 100 meV PL linewidth of QW structures. Temperature dependence of the optical transition energy was S-shape-free for all investigated structures and it was weaker than that of GaAs. An analysis of the carrier recombination mechanism was also carried out indicating that the radiative recombination is dominant even at room temperature. Moreover, numerical calculations revealed that a higher Be doping concentration leads to an increased overlap of the electron and heavy hole wave functions and determines a higher PL intensity.

Strain Modulation of Selectively and/or Globally Grown Ge Layers

This article presents a novel method to grow a high-quality compressive-strain Ge epilayer on Si using the selective epitaxial growth (SEG) applying the RPCVD technique. The procedures are composed of a global growth of Ge layer on Si followed by a planarization using CMP as initial process steps. The growth parameters of the Ge layer were carefully optimized and after cycle-annealing treatments, the threading dislocation density (TDD) was reduced to 3 × 107 cm−2. As a result of this process, a tensile strain of 0.25% was induced, whereas the RMS value was as low as 0.81 nm. Later, these substrates were covered by an oxide layer and patterned to create trenches for selective epitaxy growth (SEG) of the Ge layer. In these structures, a type of compressive strain was formed in the SEG Ge top layer. The strain amount was −0.34%; meanwhile, the TDD and RMS surface roughness were 2 × 106 cm−2 and 0.68 nm, respectively. HRXRD and TEM results also verified the existence of compressive strain in selectively grown Ge layer. In contrast to the tensile strained Ge layer (globally grown), enhanced PL intensity by a factor of more than 2 is partially due to the improved material quality. The significantly high PL intensity is attributed to the improved crystalline quality of the selectively grown Ge layer. The change in direct bandgap energy of PL was observed, owing to the compressive strain introduced. Hall measurement shows that a selectively grown Ge layer possesses room temperature hole mobility up to 375 cm2/Vs, which is approximately 3 times larger than that of the Ge (132 cm2/Vs). Our work offers fundamental guidance for the growth of high-quality and compressive strain Ge epilayer on Si for future Ge-based optoelectronics integration applications.

Strong Photoluminescence Enhancement from Bilayer Molybdenum Disulfide via the Combination of UV Irradiation and Superacid Molecular Treatment

A direct band gap nature in semiconducting materials makes them useful for optical devices due to the strong absorption of photons and their luminescence properties. Monolayer transition metal dichalcogenides (TMDCs) have received significant attention as direct band gap semiconductors and a platform for optical applications and physics. However, bilayer or thicker layered samples exhibit an indirect band gap. Here, we propose a method that converts the indirect band gap nature of bilayer MoS2, one of the representative TMDCs, to a direct band gap nature and enhances the photoluminescence (PL) intensity of bilayer MoS2 dramatically. The procedure combines UV irradiation with superacid molecular treatment on bilayer MoS2. UV irradiation induces the conversion of the PL property with an indirect band gap to a direct band gap situation in bilayer MoS2 when the interaction between the top and bottom layers is weakened by a sort of misalignment between them. Furthermore, the additional post-superacid treatment dramatically enhances the PL intensity of bilayer MoS2 by a factor of 700×. However, this procedure is not effective for a conventional bilayer sample, which shows no PL enhancement. From these results, the separated top layer would show a strong PL from the superacid treatment. The monolayer-like top layer is physically separated from the substrate by the intermediate bottom MoS2 layer, and this situation would be preferable for achieving a strong PL intensity. This finding will be useful for controlling the optoelectronic properties of thick TMDCs and the demonstration of high-performance optoelectronic devices.

Acquisition of Optimum Co-sources of Sulfur MAA Capped-ZnS Quantum Dots Conditions for Photoluminescence Chlorine Sensor

ZnS quantum dots (QDs) has received a great attention due to its unique properties and wide applications. The objective of this work is to synthesize ZnS QDs by hydrothermal method and capped with mercaptoacetic acid (MAA) to be used as a chlorine sensor in the range from 1 to 35 mg/L. Optical, structural and morphological properties of MAA capped-ZnS QDs were investigated. MAA capped-ZnS QDs exhibited a cubic structure with an average diameter size of 8.8 nm. Photoluminescence (PL) spectra of the fabricated MAA capped-ZnS QDs revealed three basic emission peaks at 371, 423 and 486 nm due to level transitions involving of zinc vacancy, interstitial sulfur and zinc, respectively. The photostability of the MAA capped-ZnS QDs was examined after 14 months which retains 12% of the original PL intensity without any peak shift. The MAA capped-ZnS QDs PL intensity was changed linearly with the chlorine concentration in the range from 1 to 35 mg/L with correlation coefficient, sensitivity and limit of detection of 0.9782, 6.96×10-2 ppm-1 and 3.6 mg/L, respectively.

Stable and Reversible Photoluminescence from GaN Nanowires in Solution Tuning by Ionic Concentration

We report response of photoluminescence (PL) from GaN nanowires without protection in solutions. The distinct response is not only toward pH but toward ionic concentration under same pH. The nanowires appear to be highly stable under aqueous solution with high ionic concentration and low pH value down to 1. We show that the PL has a reversible interaction with various types of acidic and salt solutions. The quantum states of nanowires are exposed to the external environment and have a direct physical interaction which depends on the anions of the acids. As the ionic concentration increases, the PL intensity goes up or down depending on the chemical species. The response results from a competition of change in surface band bending and charge transfer to redox level in solution. That of GaN films is reported for comparison as the effect of surface band bending can be neglected so that there are only slight variations in PL intensity for GaN films. Additionally, such physical interaction does not impact on the PL peaks in acids and salts, whereas there is a red shift on PL when the nanowires are in basic solution, say NH4OH, due to chemical etching occurred on the nanowires.

Electrical, Photoluminescence and Optical Investigation of ZnO Nanoparticles Sintered at Different Temperatures

We report here structural, electrical, photoluminescence (PL), and optical investigations of ZnO nanoparticles. The ZnO samples are initially sintered at various temperatures (T s ) (600-1200 o C) temperatures and their size is reduced twice to nanoscale by using ball friction at 200 rpm rotational speed and 30 minutes duration. It is found that the T s do not influence the well-known peaks associated with the ZnO hexagonal structure, whereas the constants of the lattice and the average crystallite diameters are affected. Although the nonlinear area is observed for all samples in the I-V curves, the breakdown field E B and nonlinear coefficient β are moved to lower values as T s increases, while the residual voltage K r and nonlinear conductivity (σ 2 ) are increased. The empirical relations for K r , E B , and β as a function of T s are; K r = 0.004 T s – 0.487, E B = -1.786T s +2559.5 and β = -0.052 T s +75.19. On the other hand, a maximum UV absorption shift (A max ) is obtained at 412 nm, 400 nm, 384 nm, and 326 nm as the T s increases up to 1200 o C. For each sample, two different energy band gap values are obtained; the first is called the basic bandgap (E gh ) and its value above 3 eV, while the second is called the optical band gap (E gL ), and its value below 2.1 eV. Moreover, the empirical relations of them are E gh = 0.002 T s - 0.24, E gl = -0.0033 T s +5.242 and ∆E = - 0.0015 T s +5.002. Furthermore, the values of (N/m*) and lattice dielectric constant ε L are increased by increasing T s up to 1200 o C, while the vice is versa for the interatomic distance R. The dielectric loss tan δ is almost linear above 4 eV for all samples, and it decreases sharply as the T s increases. The optical and electrical conductivities σ opt and σ ele are decreased as the T s increases up to 1200 o C. Finally, the characteristic of UV band edges against the optimum value of PL intensity for the samples shows 8-continuous peaks. Furthermore, the PL intensity of the peaks is decreased by increasing T s and also by shifting the UV wave number towards the IR region.

Pulsatile therapy for perovskite solar cells

Although photovoltaics employing hybrid perovskite halides have continuously been breaking world- records of power conversion efficiency (PCE) and expectations for their industrialization are rapidly rising, long-term stability issue that has greatly hampered the commercialization of perovskite solar cells has not been resolved yet. Ion instability and trapped charges were suggested as a fundamental reason for perovskite device degradation. Here, we report a pulsatile therapy relieving the accumulation of both trapped charges and ions in the perovskite solar cell device during the middle of maximum power point tracking (MPPT) for reviving the device and prolonging its device lifetime. In the technique, reverse biases are repeatedly applied for a very short time to eliminate the charges accumulated and re-distribute the ions migrated during power harvesting without any pause of operation. Intriguingly, the therapy is not only delaying irreversible degradation, but also, restoring the degraded power right after a short reverse bias. In-situ photoluminescence (PL) and photocurrent (PC) measurements for the working device were done while applying the pulsatile therapy for studying the underlying physics. Time evolving PL intensity and PC not only revealed the steady increase of PL intensity during the therapy indicating the reduction of non-radiative recombination, but also strikingly showed the restoration of degraded PL intensity and PC right after a short reverse bias suggesting the device healing. In the long-term test, we observed outstanding improvement of device stability and total harvesting power. A model considering trap-assisted recombination has also been developed to explain the efficacy of the therapy based on defect formation during MPPT operation and defect healing by the pulsatile therapy. The unique technique will open up new possibility to commercialize perovskite materials into a real market.

Indirect to direct band gap crossover in two-dimensional WS2(1−x)Se2x alloys

In atomically thin transition metal dichalcogenide semiconductors, there is a crossover from indirect to direct band gap as the thickness drops to one monolayer, which comes with a fast increase of the photoluminescence signal. Here, we show that for different alloy compositions of WS2(1−x)Se2x this trend may be significantly affected by the alloy content and we demonstrate that the sample with the highest Se ratio presents a strongly reduced effect. The highest micro-PL intensity is found for bilayer WS2(1−x)Se2x (x = 0.8) with a decrease of its maximum value by only a factor of 2 when passing from mono-layer to bi-layer. To better understand this factor and explore the layer-dependent band structure evolution of WS2(1−x)Se2x, we performed a nano-angle-resolved photoemission spectroscopy study coupled with first-principles calculations. We find that the high micro-PL value for bilayer WS2(1−x)Se2x (x = 0.8) is due to the overlay of direct and indirect optical transitions. This peculiar high PL intensity in WS2(1−x)Se2x opens the way for spectrally tunable light-emitting devices.