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Short-wave infrared (SWIR) light, 0.9–2.5 μm wavelengths, has widespread applications, including inspection processes, nighttime imaging, and machine vision. As such, there is increasing demand for practical and effective SWIR detectors. Many current SWIR photodetectors are based on high-cost materials and require cryogenic cooling. Perovskite materials, including CsPbI3, have been effectively used as photodetectors in the UV to near IR ranges, but their large bandgaps limit their use for lower energy SWIR light. In this report we introduce an all-inorganic perovskite photodetector based on CsPbI3 with heterojunction engineering for efficient and practical detection in the SWIR range at room temperature. The devices undergo a simple, solution-based fabrication process which includes spin-coating under ambient conditions and moderate annealing temperatures. Without additional cooling, the SWIR devices produce excellent results at room temperature with responsivity of 1.65 × 103 A W−1 and a specific detectivity of 8.0 × 1010 Jones under 0.28 mW cm−2 of 1310 nm light and bias of −5 V. This material shows not only high response but also high sensitivity, making it stand out in the field of SWIR photodetection with the additional benefits of low-cost production and room temperature operation. 

The movements of molecules at interfaces and surfaces are restricted by their asymmetric environments, leading to anisotropic orientational motions. In this work, in-plane orientational motions of the –C=O and –CF3 groups of coumarin 153 (C153) at the air/water interface were measured using time-resolved (TR) vibrational sum frequency generation (SFG). The in-plane orientational time constants of the –C=O and –CF3 groups of C153 are found to be 41.5 ± 8.2 and 36.0 ± 4.5 ps. These values are over five-times faster than that of 198 ± 15 ps for the permanent dipole of the whole C153 molecule at the interface, which may indicate that the two groups experience different interfacial friction in the plane. These differences could also be the result of the permanent dipole of C153 being almost five times those of the –C=O and –CF3 groups. The difference in orientational motions reveals the microscopic heterogeneous environment that molecules experience at the interface. While the interfacial dynamics of the two functional groups are similar, our TR-SFG experiments allowed the quantification of the in-plane dynamics of individual functional groups for the first time. Our experimental findings about the interfacial molecular motion have implications for molecular rotations, energy transfer, and charge transfer at material interfaces, photocatalysis interfaces, and biological cell/membrane aqueous interfaces. 

Climate change and the global energy crisis have led to an increasing need for greenhouse gas remediation and clean energy sources. The electrochemical CO2 reduction reaction (CO2RR) is a promising solution for both issues as it harvests waste CO2 and chemically reduces it to more useful forms. However, the high overpotential required for the reaction makes it electrochemically unfavorable. Here, we fabricate a novel electrode composed of TiO2 nanoparticles grown in situ on MXene charge acceptor 2D sheets with excellent CO2RR characteristics. A straightforward solvothermal method was used to grow the nanoparticles on the Ti3C2Tx MXene flakes. The electrochemical performance of the TiO2/MXene electrodes was analyzed. The Faradaic efficiencies of the TiO2/MXene electrodes were determined, with a value of 99.41% at −1.9 V (vs. Ag/AgCl). Density functional theory mechanistic analysis was used to reveal the most likely mechanism resulting in the production of one CO molecule along with a carbonate anion through ∗CO, ∗O, and activated CO22− intermediates. Bader charge analysis corroborated this pathway, showing that CO2 gains a greater negative charge when TiO2/MXene serves as a catalyst compared to MXene or TiO2 alone. These results show that TiO2/MXene nanocomposite electrodes may be very useful in the conversion of CO2 while still being efficient in both time and cost. 

The surface states of photoelectrodes as catalysts heavily influence their performance in photocatalysis and photoelectrocatalysis applications. These catalysts are necessary for developing robust solutions to the climate and global energy crises by promoting CO2 reduction, N2 reduction, contaminant degradation, and water splitting. The semiconductors that can fill this role are beholden as photoelectrodes to the processes of charge generation, separation, and utilization, which are in turn products of surface states, surface electric fields, and surface carrier dynamics. Methods which are typically used for studying these processes to improve semiconductors are indirect, invasive, not surface specific, not practical under ambient conditions, or a combination thereof. Recently, nonlinear optical processes such as electronic sum-frequency generation (ESFG) and second-harmonic generation (ESHG) have gained popularity in investigations of semiconductor catalysts systems. Such techniques possess many advantages of in-situ analysis, interfacial specificity, non-invasiveness, as well as the ability to be used under any conditions. In this review, we detail the importance of surface states and their intimate relationship with catalytic performance, outline methods to investigate semiconductor surface states, electric fields, and carrier dynamics and highlight recent contributions to the field through interface-specific spectroscopy. We will also discuss how the recent development of heterodyne-detected ESHG (HD-ESHG) was used to extract charged surface states through phase information, time-resolved ESFG (TR-ESFG) to obtain in-situ dynamic process monitoring, and two-dimensional ESFG (2D-ESFG) to explore surface state couplings, and how further advancements in spectroscopic technology can fill in knowledge gaps to accelerate photoelectrocatalyst utilization. We believe that this work will provide a valuable summary of the importance of semiconductor surface states and interfacial electronic properties, inform a broad audience of the capabilities of nonlinear optical techniques, and inspire future original approaches to improving photocatalytic and photoelectrocatalytic devices. 

The escalating global energy predicament implores for a revolutionary resolution—one that converts sunlight into electricity—holding the key to supreme conversion efficiency. This comprehensive review embarks on the exploration of the principle of generating multiple excitons per absorbed photon, a captivating concept that possesses the potential to redefine the fundamental confines of conversion efficiency, albeit its application remains limited in photovoltaic devices. At the nucleus of this phenomenon are two principal processes: multiple exciton generation (MEG) within quantum-confined environments, and singlet fission (SF) inside molecular crystals. The process of SF, characterized by the cleavage of a single photogenerated singlet exciton into two triplet excitons, holds promise to potentially amplify photon-to-electron conversion efficiency twofold, thereby laying the groundwork to challenge the detailed balance limit of solar cell efficiency. Our discourse primarily dissects the complex nature of SF in crystalline organic semiconductors, laying special emphasis on the anisotropic behavior of SF and the diffusion of the subsequent triplet excitons in single-crystalline polyacene organic semiconductors. We initiate this journey of discovery by elucidating the principles of MEG and SF, tracing their historical genesis, and scrutinizing the anisotropy of SF and the impact of quantum decoherence within the purview of functional mode electron transfer theory. We present an overview of prominent techniques deployed in investigating anisotropic SF in organic semiconductors, including femtosecond transient absorption microscopy and imaging as well as stimulated Raman scattering microscopies, and highlight recent breakthroughs linked with the anisotropic dimensions of Davydov splitting, Herzberg–Teller effects, SF, and triplet transport operations in single-crystalline polyacenes. Through this comprehensive analysis, our objective is to interweave the fundamental principles of anisotropic SF and triplet transport with the current frontiers of scientific discovery, providing inspiration and facilitating future ventures to harness the anisotropic attributes of organic semiconductor crystals in the design of pioneering photovoltaic and photonic devices.