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Abstract:

Inorganic cesium‐based metal halide perovskite (MHP) semiconductors have great potential as active layers in optoelectronic devices, such as perovskite light‐emitting diodes (PeLEDs) and perovskite lasers. However, precise control of crystal type, quality, and thickness is required to create high‐performance and reproducible devices. Vapor‐phase vacuum deposition enables fabrication of MHP thin films and devices with excellent uniformity and control over layer thickness, although a full understanding of crystal growth mechanisms and products has proved elusive. Here, conditions of vapor co‐deposition of CsBr and PbBr are related with the optical performance and atomic microstructure of resulting CsPbBr3 thin films. It is found that the structure is predominantly photoactive γ‐CsPbBr3 over a wide range of conditions, but the presence of impurity phases and Ruddlesden–Popper (RP) planar defects both degrade optical performance as quantified through measured amplified spontaneous emission (ASE) thresholds. Furthermore, the atomic structure of the dominant impurity phases is resolved: CsPb2Br5 and Cs4PbBr6. It is revealed that a small nominal excess of CsBr‐precursor flux during co‐evaporation can significantly enhance the nucleation of thin films, resulting in well‐defined grains greater than 500 nm in size and the relative suppression of RP planar defects. Such films exhibit intensified photoluminescence (PL) emission and a reduced ASE threshold of 30.9 µJ cm−2.

Abstract:

Tin-halide perovskites currently offer the best photovoltaic performance of lead-free metal-halide semiconductors. However, their transport properties are mostly dominated by holes, owing to ubiquitous self-doping. Here we demonstrate a noncontact, optical spectroscopic method to determine the effective mass of the dominant hole species in FASnI3, by investigating a series of thin films with hole densities finely tuned through either SnF2 additive concentration or controlled exposure to air. We accurately determine the plasma frequency from mid-infrared reflectance spectra by modeling changes in the vibrational response of the FA cation as the plasma edge shifts through the molecular resonance. Our approach yields a hole effective mass of 0.28m e for FASnI3 and demonstrates parabolicity within ∼100 meV of the valence band edge. An absence of Fano contributions further highlights insignificant coupling between the hole plasma and FA cation. Overall, this approach enables noncontact screening of thin-film materials for optimized charge-carrier transport properties.

Abstract:

Self-trapped excitons (STEs), which have one or both carriers spatially trapped by a lattice distortion, are associated with broadband emission and a large Stokes shift that is desirable for many applications. The fundamental physical processes that lead to their formation are difficult to observe, mainly due to the ultrafast time scales involved and the low oscillator strength of STE transitions. Here, we employ ultrafast transient absorption spectroscopy with sub-20 fs temporal resolution in the ultraviolet to study the STE formation process in a pair of lead-free double perovskites, Cs₂AgInCl₆ and Cs₂(Ag_0.6Na_0.4)InCl₆. Using first-principles calculations, we assign a broad photoinduced absorption band in Cs₂AgInCl₆ to an intraband transition in the valence band that tracks the initial 70 fs hot-hole cooling step. Furthermore, exciton-phonon coupling calculations unravel the phonon modes that couple strongly with excitons in the lowest absorption peak to cause self-trapping. The transient absorption data shows the buildup of a stimulated emission band from the STE on a 200 fs time scale and long-lived coherent oscillations corresponding to the phonons of the lattice modified by the STE formation process.

Abstract:

Alkali metals are considered as a promising alternative to conventional noble metals for plasmonic applications, offering lower optical loss and significantly reduced material costs. The recent development of a thermo-assisted spin-coating process paired with phase-shift photolithography has enabled the creation of stable nanostructured sodium, which exhibits narrow resonances in the near-infrared (NIR) region and demonstrates free electron relaxation times comparable to noble metals. Through the control of nanostructure pitch and light incident angle, the surface plasmon polariton (SPP) resonance wavelength can be tuned throughout the visible and NIR regions, making nanostructured sodium particularly attractive for nanophotonics, surface-enhanced sensing, and photocatalytic applications. In this work, we investigate hot electron dynamics in nanostructured sodium thin films on polyurethane supports by leveraging the high sensitivity of SPPs to their metal's bulk properties. Through optical transient reflectance measurements, we probe the distinct signatures of electron-electron and electron-phonon interactions in sodium at ultrafast time scales. Our results show the unique early time response of sodium that differs from those observed in noble metals, providing key insight into sodium-based plasmonics. This comprehensive understanding of hot electron dynamics will enable more efficient design and implementation of sodium in next-generation plasmonic devices and applications where hot electron processes are critical considerations.

Abstract:

Fully conjugated 3D covalent organic frameworks (COFs) are a newly emerged class of materials that expands reticular chemistry to extended electron delocalization for optoelectronic applications. To overcome the limitations of sp3‐connected 3D frameworks, the pseudo‐tetrahedral motif cyclooctatetrathiophene (COTh) has gained attention for forming fully conjugated 3D COFs. We report on a novel COTh building block, featuring functional formyl groups directly attached to the core's conjugated thiophenes. The modulation synthesis approach with mono‐functionalized inhibitors enables the formation of COTh‐1P COF, which exhibited remarkable crystallinity and permanent porosity. By following this approach and by optimizing the synthesis conditions for the solvothermal growth of thin films, we fabricated the first preferentially oriented conjugated 3D COF films on various substrates without pre‐functionalization. With these thin films, optical pump terahertz probe studies allowed us, for the first time with 3D‐fully conjugated COFs, to provide insights into the excited state and charge‐carrier dynamics of these unique organic frameworks. Low effective masses are discovered for valence and conduction bands by density functional theory simulations. The ability to create crystalline and oriented films of fully π‐conjugated 3D COTh‐based COFs on non‐modified substrates is expected to open the way for integration of such frameworks into diverse optoelectronic applications.