Reversible optical property changes in lead-free perovskites have recently received great interest due to their potential applications in smart windows, sensors, data encryption, and various on-demand devices. However, it is challenging to achieve remarkable color changes in their thin films. Here, methylamine gas (CH3NH2, MA0) induced switchable optical bleaching of bismuth (Bi)-based perovskite films is demonstrated for the first time. By exposure to an MA0 atmosphere, the color of Cs2AgBiBr6 (CABB) films changes from yellow to transparent, and the color of Cs3Bi2I9 (CBI) films changes from dark red to transparent. More interestingly, the underlying reason is found to be the interactions between MA0 and Bi3+ with the formation of an amorphous liquefied transparent intermediate phase, which is different from that of lead-based perovskite systems. Moreover, the generality of this approach is demonstrated with other amine gases, including ethylamine (C2H5NH2, EA0) and butylamine (CH3(CH2)3NH2, BA0), and another compound, Cs3Sb2I9, by observing a similar reversible optical bleaching phenomenon. The potential for the application of CABB and CBI films in switchable smart windows is investigated. This study provides valuable insights into the interactions between amine gases and lead-free perovskites, opening up new possibilities for high-efficiency optoelectronic and stimuli-responsive applications of these emerging Bi-based materials.

Lead-free halide double perovskites (HDPs) have emerged as a new generation of thermochromic materials. However, further materials development and mechanistic understanding are required. Here, a highly stable HDP Cs2NaFeCl6 single crystal is synthesized, and its remarkable and fully reversible thermochromism with a wide color variation from light-yellow to black over a temperature range of 10 to 423 K is investigated. First-principles, density functional theory (DFT)-based calculations indicate that the thermochromism in Cs2NaFeCl6 is an effect of electron–phonon coupling. The temperature sensitivity of the bandgap in Cs2NaFeCl6 is up to 2.52 meVK−1 based on the Varshni equation, which is significantly higher than that of lead halide perovskites and many conventional group-IV, III–V semiconductors. Meanwhile, this material shows excellent environmental, thermal, and thermochromic cycle stability. This work provides valuable insights into HDPs' thermochromism and sheds new light on developing efficient thermochromic materials.

Solution-processable semiconductors with antiferromagnetic (AFM) order are attractive for future spintronics and information storage technology. Halide perovskites containing magnetic ions have emerged as multifunctional materials, demonstrating a cross-link between structural, optical, electrical, and magnetic properties. However, stable optoelectronic halide perovskites that are antiferromagnetic remain sparse, and the critical design rules to optimize magnetic coupling still must be developed. Here, we combine the complementary magnetometry and electron-spin-resonance experiments, together with first-principles calculations to study the antiferromagnetic coupling in stable Cs-2(Ag:Na)FeCl6 bulk semiconductor alloys grown by the hydrothermal method. We show the importance of nonmagnetic monovalence ions at the B-I site (Na/Ag) in facilitating the superexchange interaction via orbital hybridization, offering the tunability of the Curie-Weiss parameters between -27 and -210 K, with a potential to promote magnetic frustration via alloying the nonmagnetic B-I site (Ag:Na ratio). Combining our experimental evidence with first-principles calculations, we draw a cohesive picture of the material design for B-site-ordered antiferromagnetic halide double perovskites.

Photon energy upconversion, i.e. the conversion of several low-energy photons to a photon of higher energy, offers significant potential for nano-optoelectronics and nanophotonics applications. The primary challenge is to achieve high upconversion efficiency and a broad device performance range, enabling effective upconversion even at low excitation power. This study demonstrates that core/shell semiconductor nanowire heterostructures can exhibit upconversion efficiencies exceeding what was previously reported for semiconductor nanostructures even at a low excitation power of 100 mW/cm2, by a two-photon absorption process through conduction band states of the narrow-bandgap nanowire shell region. By engineering the electric-field distribution of the excitation light inside the NWs, upconversion efficiency can be further improved by eight times. This work showcases the effectiveness of the proposed approach in achieving efficient photon upconversion using core/shell NW heterostructures, resulting in some of the highest upconversion efficiencies reported in semiconductor nanostructures. Additionally, it offers design guidelines for enhancing energy upconversion efficiency.

β-Ga2O3 is a wide bandgap semiconductor that is attractive for various applications, including power electronics, transparent conductive electrodes, etc. Electrical and optical properties of Ga2O3 are affected by the presence of dopants/contaminants and/or intrinsic defects. Here, we investigate the electrical and optical properties of transition metals like Co and Cr since they are often unintentionally present during the growth or used as intentional dopants. This is done by using magnetic resonance spectroscopy and magneto-optical characterization techniques. We determine spin- Hamiltonian parameters of the Cr3+ ground-state and first excited-state as well as the spin-Hamiltonian parameters of Co2+.

Energy upconversion via optical processes in semiconductor nanowires (NWs) is attractive for a variety of applications in nano-optoelectronics and nanophotonics. One of the main challenges is to achieve a high upconversion efficiency and, thus, a wide dynamic range of device performance, allowing efficient upconversion even under low excitation power. Here, we demonstrate that the efficiency of energy upconversion via two-photon absorption (TPA) can be drastically enhanced in core/shell NW heterostructures designed to provide a real intermediate TPA step via the band states of the narrow-bandgap region with a long carrier lifetime, fulfilling all the necessary requirements for high-efficiency two-step TPA. We show that, in radial GaAs(P)/GaNAs(P) core/shell NW heterostructures, the upconversion efficiency increases by 500 times as compared with that of the constituent materials, even under an excitation power as low as 100 mW/cm2 that is comparable to the 1 sun illumination. The upconversion efficiency can be further improved by 8 times through engineering the electric-field distribution of the excitation light inside the NWs so that light absorption is maximized within the desired region of the heterostructure. This work demonstrates the effectiveness of our approach in providing efficient photon upconversion by exploring core/shell NW heterostructures, yielding an upconversion efficiency being among the highest reported in semiconductor nanostructures. Furthermore, our work provides design guidelines for enhancing efficiency of energy in NW heterostructures.

A common way of determining the majority charge carriers of pristine and doped semiconducting polymers is to measure the sign of the Seebeck coefficient. However, a polarity change of the Seebeck coefficient has recently been observed to occur in highly doped polymers. Here, it is shown that the Seebeck coefficient inversion is the result of the density of states filling and opening of a hard Coulomb gap around the Fermi energy at high doping levels. Electrochemical n-doping is used to induce high carrier density (>1 charge/monomer) in the model system poly(benzimidazobenzophenanthroline) (BBL). By combining conductivity and Seebeck coefficient measurements with in situ electron paramagnetic resonance, UV-vis-NIR, Raman spectroelectrochemistry, density functional theory calculations, and kinetic Monte Carlo simulations, the formation of multiply charged species and the opening of a hard Coulomb gap in the density of states, which is responsible for the Seebeck coefficient inversion and drop in electrical conductivity, are uncovered. The findings provide a simple picture that clarifies the roles of energetic disorder and Coulomb interactions in highly doped polymers and have implications for the molecular design of next-generation conjugated polymers.

GaAs-based semiconductors are highly attractive for diverse nonlinear photonic applications, owing to their non-centrosymmetric crystal structure and huge nonlinear optical coefficients. Nanostructured semiconductors, for example, nanowires (NWs), offer rich possibilities to tailor nonlinear optical properties and further enhance photonic device performance. In this study, it is demonstrated highly efficient second-harmonic generation in subwavelength wurtzite (WZ) GaAs NWs, reaching 2.5 × 10⁻⁵ W⁻¹, which is about seven times higher than their zincblende counterpart. This enhancement is shown to be predominantly caused by an axial built-in electric field induced by spontaneous polarization in the WZ lattice via electric field-induced second-order nonlinear susceptibility and can be controlled optically and potentially electrically. The findings, therefore, provide an effective strategy for enhancing and manipulating the nonlinear optical response in subwavelength NWs by utilizing lattice engineering.

The ultimate goal for singlet fission is that each photo-excited singlet exciton, S1, will result in two triplet excitons with unity yield. However, the singlet fission is now recognized to be complicated, involving bright/dark excited states of different spin multiplicity. Identifying the role of such states is vital to optimize singlet fission yield but difficult due to their elusive spectral signature. Here, we develop an experimental protocol based on a refined magneto-optical probe to access the fast time evolution of various excited states. In diphenylhexatriene crystal, the S1 is found to undergo two competing processes?to form one of the two dark triplet pair intermediates having different exchange energies or to form a bright state, Sx, exhibiting excimer-like delayed photoluminescence. Our result provides a clear picture of a competition event in singlet fission, which is beneficial for the development and tailoring of singlet fission materials with high yield.

We report on optimization of growth conditions of GaAs/GaNAs/GaAs core/shell/shell nanowire (NW) structures emitting at ~1 μm, aiming to increase their light emitting efficiency. A slight change in growth temperature is found to critically affect optical quality of the active GaNAs shell and is shown to result from suppressed formation of non-radiative recombination (NRR) centers under the optimum growth temperature. By employing the optically detected magnetic resonance spectroscopy, we identify gallium vacancies and gallium interstitials as being among the dominant NRR defects. The radiative efficiency of the NWs can be further improved by post-growth annealing at 680 °C, which removes the gallium interstitials.