We report the synthesis, crystal and electronic structures, as well as optical properties of the hybrid organic-inorganic compounds MACdX (MA = CHNH; X = Cl, Br, I). MACdI is a new compound, whereas, for MACdCl and MACdBr, structural investigations have already been conducted but electronic structures and optical properties are reported here for the first time. Single crystals were grown through slow evaporation of MACdX solutions with optimized conditions yielding mm-sized colorless (X = Cl, Br) and pale yellow (X = I) crystals. Single crystal and variable temperature powder X-ray diffraction measurements suggest that MACdCl forms a 2D layered perovskite structure and has two structural transitions at 283 and 173 K. In contrast, MACdBr and MACdI adopt 0D KSO-derived crystal structures based on isolated CdX tetrahedra and show no phase transitions down to 20 K. The contrasting crystal structures and chemical compositions in the MACdX family impact their air stabilities, investigated for the first time in this work; MACdCl is air-stable, whereas MACdBr and MACdI partially decompose when left in air. Optical absorption measurements suggest that MACdX have large optical band gaps above 3.9 eV. Room temperature photoluminescence spectra of MACdX yield broad peaks in the 375-955 nm range with full width at half-maximum values up to 208 nm. These PL peaks are tentatively assigned to self-trapped excitons in MACdX following the crystal and electronic structure considerations. The bands around the Fermi level have small dispersions, which is indicative of high charge localization with significant exciton binding energies in MACdX. On the basis of our combined experimental and computational results, MACdX and related compounds may be of interest for white-light-emitting phosphors and scintillator applications.

Optical emission from type-II ZnTe/ZnSe quantum dots demonstrates large and persistent oscillations in both the peak energy and intensity indicating the formation of coherently rotating states. Furthermore, the Aharanov-Bohm (AB) effect is shown to be remarkably robust and persists until 180K. This is at least one order of magnitude greater than the typical temperatures in lithographically defined rings. To our knowledge this is the highest temperature at which the AB effect has been observed in semiconductor structures.

Replacement of the toxic heavy element lead in metal halide perovskites has been attracting a great interest because the high toxicity and poor air stability are two of the major barriers for their widespread utilization. Recently, mixed-cation double perovskite halides, also known as elpasolites, were proposed as an alternative lead-free candidate for the design of nontoxic perovskite solar cells. Herein, we report a new nontoxic and air stable lead-free all-inorganic semiconductor Rb 4 Ag 2 BiBr 9 prepared using the mixedcation approach; however, Rb 4 Ag 2 BiBr 9 adopts a new structure type (Pearson's code oP32) featuring BiBr 6 octahedra and AgBr 5 square pyramids that share common edges and corners to form a unique 2D layered non-perovskite structure. Rb 4 Ag 2 BiBr 9 is also demonstrated to be thermally stable with the measured onset decomposition temperature of T o = 520 °C. Optical absorption measurements and density functional theory calculations suggest a nearly direct band gap for Rb 4 Ag 2 BiBr 9 . Room temperature photoluminescence (PL) measurements show a broadband weak emission. Further, temperature-dependent and power-dependent PL measurements show a strong competition between multiple emission centers and suggest the coexistence of defect-bound excitons and self-trapped excitons in Rb 4 Ag 2 BiBr 9 .

No abstract

Hot electrons established by the absorption of high-energy photons typically thermalize on a picosecond time scale in a semiconductor, dissipating energy via various phonon-mediated relaxation pathways. Here it is shown that a strong hot carrier distribution can be produced using a type-II quantum well structure. In such systems it is shown that the dominant hot carrier thermalization process is limited by the radiative recombination lifetime of electrons with reduced wavefunction overlap with holes. It is proposed that the subsequent reabsorption of acoustic and optical phonons is facilitated by a mismatch in phonon dispersions at the InAs-AlAsSb interface and serves to further stabilize hot electrons in this system. This lengthens the time scale for thermalization to nanoseconds and results in a hot electron distribution with a temperature of 490 K for a quantum well structure under steady-state illumination at room temperature.

Mixed-cation-based perovskite solar cells are investigated under conditions consistent with those found in orbit around Mars, Jupiter, and Saturn. Temperature dependent photoluminescence spectra show no evidence of a phase change demonstrating the stability of these systems. At low temperature, a barrier that impedes current flow and reduces the fill factor is apparent in the operation of the solar cell, particularly at 1 sun AM0. However, under low intensity and low temperature, where thermionic emission limits the carrier extraction, the fill factor and efficiency are recovered demonstrating the promise of these systems for deep space missions.

The temperature dependence of a InAs/AlAs0.84Sb0.16 multi-quantum-well sample is studied using continuous wave photoluminescence. An “s-shape” shift in peak energy is observed and attributed to low energy localization states. High incident power density photoluminescence measurements were performed to probe the nature of such localization. The results opposed the possibility of a type-II band structure and supported the idea of low energy localization states. The effect of such localization on hot carriers in our system was studied and an improvement in their stability due to hole mobility at elevated temperature is presented.

InAs/AlAs x Sb 1−x quantum wells are investigated for their potential as hot carrier solar cells. Continuous wave power and temperature dependent photoluminescence indicate a transition in the dominant hot carrier relaxation process from conventional phonon-mediated carrier relaxation below 90 K to a regime where inhibited radiative recombination dominates the hot carrier relaxation at elevated temperatures. At temperatures below 90 K photoluminescence measurements are consistent with type-I quantum wells that exhibit hole localization associated with alloy/interface fluctuations. At elevated temperatures hole delocalization reveals the true type-II band alignment; where it is observed that inhibited radiative recombination due to the spatial separation of the charge carriers dominates hot carrier relaxation. This decoupling of phonon-mediated relaxation results in robust hot carriers at higher temperatures even at lower excitation powers. These results indicate type-II quantum wells offer potential as practical hot carrier systems.