Semiconductors group

Abstract:

Thermal evaporation is an industrially compatible technique for fabricating metal halide perovskite thin films, without the requirement for hazardous solvents. It offers precise control over film thickness and is a good candidate for large-scale production of commercial optoelectronic metal halide perovskite devices, such as solar cells. The use of additives to passivate electronic defects in solution-processed metal halide perovskite has led to dramatic increases in device performance. However, there are a few reports of vapor-deposited films with coevaporated passivating agents. Triphenylphosphine oxide (TPPO) has been used as an effective surface passivating agent in solution-processed metal halide perovskite films. It is a promising candidate passivating agent for coevaporation, where it is beginning to be used with encouraging results. However, here we report that triphenylphosphine oxide is incompatible with thermal deposition in the same deposition chamber. Such TPPO remnants are found to result in severe suppression of the perovskite phase, long-range crystalline ordering, and optical absorption of lead halide perovskite films subsequently deposited in the same chamber. TPPO contamination persists even through repeated baking cycles, with the reduction of the contaminant to acceptable levels requiring vacuum chamber dismantling and manual cleaning. We conclude that TPPO should not be coevaporated in order to prevent the contamination of future batches.

Abstract:

Thermo-optical nonlinearities (TONL) in metasurfaces enable dynamic control of optical properties—such as transmitted power, phase, and polarization—through external stimuli like laser irradiation or temperature. Due to the inherently slow thermal dynamics of extended systems, research has primarily focused on steady-state effects, as rapid modulation is typically considered challenging. In this study, photo-driven TONL is investigated in amorphous silicon (a-Si) metasurfaces under both steady-state and, more importantly, dynamic conditions using a modulated 488 nm continuous-wave pump laser. First, a non-monotonic change is observed in transmission as a function of irradiation intensity at a wavelength red-shifted by 15 nm from the electric-dipole resonance. Specifically, transmission initially decreases by 30% before increasing by 30% as the laser intensity reaches 5 mW/ (Formula presented.). Next, it is demonstrated that TONL decouple thermal and optical response times, with the optical response being up to seven times faster than the thermal response under tested conditions ((Formula presented.) (Formula presented.) vs. (Formula presented.) (Formula presented.)). Most remarkably, it is experimentally shown that the interplay of these effects enables optical modulation at twice (100 kHz) the excitation laser's modulation frequency (50 kHz). Finally, it is shown that exploiting these unique conditions allow thermo-optical metasurfaces to intrinsically encode multiple optical states within a single modulation cycle, realizing a self-modulating photonic platform. TONL thus open new avenues for engineering active metasurfaces that combine fast, high-amplitude modulation with self-modulating optical dynamics, making them promising for next-generation optical switching, dynamic holography, optical information processing, and neuromorphic computing.

Abstract:

A novel class of semiconducting compounds, metal‐halide perovskites (MHPs), has emerged as a versatile platform for advanced optoelectronic device architectures, offering a unique combination of exceptional physical properties and facile processing. In this study, we present a monolithic high‐speed photodetector capable of directly sensing the time delay between two light pulses with a temporal resolution of at least 170 ps, corresponding to a light propagation distance of ~5 cm—making it well suited for Light Detection and Ranging (LiDAR) applications. This outstanding time resolution is achieved through a signal‐balancing detection scheme that effectively overcomes the limitations of conventional photodetectors, whose response speed is inherently limited by charge‐carrier lifetime and transit time. The device exhibits an exceptionally low noise spectral density, comparable to that of state‐of‐the‐art silicon photodiodes. The fully symmetric device stack comprises a crystalline CsPbBr3 absorber layer tens of microns thick, fabricated via a confined melt process. Comprehensive electro‐optical characterization reveals charge‐carrier lifetimes and mobilities on both microscopic and macroscopic length scales, using transient photoluminescence, time‐resolved photocurrent, time of flight, and terahertz pump–probe spectroscopy. The CsPbBr3 layer exhibits charge‐carrier lifetimes exceeding 100 ns, a microscopic electron–hole mobility of 15 ± 1 cm2 V−1 s−1, and a macroscopic non‐dispersive hole mobility of 8.5 cm2 V−1 s−1. image

Abstract:

Second-harmonic generation (SHG) is a powerful surface-specific probe for centrosymmetric materials, with broad relevance to energy and biological interfaces. Plasmonic nanomaterials have been extensively utilized to amplify this nonlinear response. Yet, material instability has constrained most studies to gold, despite the significance of plasmonic metals such as copper for catalysis. Here, we demonstrate stable and anisotropic SHG from monocrystalline copper, overcoming long-standing challenges associated with surface degradation. By leveraging an on-substrate synthesis approach that yields atomically flat and oxidation-resistant Cu microflakes, we enable reliable SHG measurements and reveal a strong cross-polarized response with C 3v surface symmetry. The SHG signal remains stable over 3 h of continuous femtosecond excitation, highlighting the remarkable optical robustness of the Cu microflakes. These results reinforce the viability of monocrystalline Cu as a robust platform for nonlinear nanophotonics and surface-sensitive spectroscopy, expanding the range of copper-based optical applications.

Abstract:

Lead halide perovskite nanocrystals have emerged as promising candidates for classical light-emitting devices and single-photon sources, owing to their high photoluminescence quantum yield, narrow emission line width and tunable emission. Judicious choice of ligands to passivate nanocrystal surfaces has proven to be critical to the structural stability and optoelectronic performance of such nanocrystals. While many ligands have been deployed, the resulting quality of the nanocrystal surface can be difficult to assess directly. Here, we demonstrate ultralow frequency Raman spectroscopy as a powerful tool to resolve surface-sensitive changes in size and ligand choice in perovskite nanocrystals. By investigating a size series of CsPbBr3 nanocrystals from the strong (5 nm) to the weak (28 nm) confinement range, we show that the line width of Raman-active modes provides a highly selective metric for surface disorder and quality. We further examine a series of 28 nm diameter nanocrystals with four different zwitterionic ligands, unravelling clear links between varying steric effects and surface quality evident from Raman analysis. Photoluminescence and THz photoconductivity probes reveal an evident correlation of charge-carrier dynamics and radiative emission yields with ligand chemistry and surface quality inferred from phonon broadening. We further show that surface defects preferentially trap hot charge carriers, which affects exciton stability and radiative emission yields. Overall, our approach offers powerful insights into optimizing nanocrystal-ligand boundaries to enhance the performance of nanoscale quantum light sources and optoelectronic devices.