Artur Pawel Herman

Two-dimensional van der Waals materials display rich nonequilibrium physics due to their reduced dimensionality and strong light–matter interactions. Layered metal phosphorus trichalcogenides (MPX3) feature bandgaps spanning the visible to ultraviolet range and exhibit a complex interplay between excitons, phonons, and defects. Here we use static and time-resolved photoluminescence and femtosecond broadband transient reflectivity to investigate the temperature-dependent ultrafast carrier dynamics in layered HgPSe3, a semiconductor with a quasi-direct bandgap near 2 eV. We identify distinct sub-bandgap photoluminescence features that become prominent at low temperature. Their temperature and fluence dependence strongly indicate radiative recombination involving localized states. The transient reflectivity map displays three features associated with interband optical transitions in the visible range, as corroborated by density functional theory band structure calculations. These features exhibit fluence-dependent decay dynamics, indicating a competition between carrier localization into sub-bandgap states, state-filling effects that limit the available electronic phase space for hot-carrier relaxation, and phonon-bottleneck-limited carrier cooling. Our results provide new insights into the ultrafast carrier dynamics of HgPSe3 and establish a framework for understanding the role of sub-bandgap states in MPX3 semiconductors, with potential implications for broadband optoelectronic and photodetection applications.

Two-dimensional (2D) lead halide perovskites are emerging as excellent materials for optoelectronic applications including light-emitting diodes, photovoltaics, and photodetectors, owing to their efficient excitonic emission originating from both self-trapped excitons (STEs) and free excitons (FEs). Recently, many efforts have been focused on enhancing the emission intensity, modulating the dominant emission mechanism, and establishing direct correlations between optical properties and underlying structural motifs. Across the hybrid organic-inorganic layers in 2D perovskites, the nanorange modulation of the density, stiffness, strain, and ionicity can be efficiently tuned by external stimuli, including the substrate-film strain. Here, we report a pressure-induced narrowing of the band gap and an enhancement of STE and FE emission in the topology of the (110)-oriented Ruddlesden-Popper (RP) perovskite ACE₂PbBr₄ (ACE = acetamidinium), accompanied by the rare phenomenon of the reversible detrapping process from STEs to FEs under compression. Specifically, the STE-related emission exhibits a 6.4-fold increase of intensity up to 2.56 GPa, while the FE emission dominates under higher pressures, up to 11.11 GPa, causing a pronounced change in the emission color, from orange-yellow to greenish-blue, across the compression range. In situ single-crystal X-ray diffraction and Raman spectroscopy reveal that these emission changes arise from the rarely observed pressure-induced reduction of lead bromide octahedral distortion and confinement in the direction of the corrugated structure and changes in amine-framework interactions, as well as pressure-induced phase transitions (PTs) occurring near 2, 5, 5.9, and 6.7 GPa. These findings elucidate the structure-property relationship in (110)-oriented RP perovskites and underscore the utility of strain engineering for realizing light-emitting materials with tailored and enhanced functionalities.

As the field of hybrid perovskites continues to expand, new methods to study their optical properties can enhance conventional approaches. In this study, we used piezo-modulation spectroscopy to investigate optical transitions in well-known perovskite films. A series of methylammonium and formamidinium perovskite layers via spin-coating and drop-casting directly onto a piezoceramic substrate were prepared. At room temperature, the piezoreflectance signal corresponding to optical transitions was successfully observed. To investigate the phase transitions in MAPbI3 in detail, measurements were performed over a wide temperature range for layers prepared using both drop-casting and spin-coating techniques. Complementary photoluminescence (PL) measurements served as a reference. The optical transition energies obtained from absorption-like (piezoreflectance) and emission-like (PL) experiments showed good agreement. The piezoreflectance method demonstrated higher sensitivity to phase transitions in standard MAPbI3 perovskite compared to PL due to its inherent sensitivity to structural phase transitions. This comparative analysis of the two techniques revealed that piezo-modulation spectroscopy provides valuable insights into the optical properties of MAPbI3 in halide perovskite semiconductors.

One of the most intriguing properties of hybrid halide perovskites is their structural softness. The interplay between organic and inorganic sublattices leads to a multitude of physical phenomena that are not observed in conventional semiconductors such as III-V materials. One among these is the exceptional band gap tunability under hydrostatic pressure. In this context, tin-based perovskites exhibit a more pronounced effect than their lead-based counterparts. Here, we report on one of the largest band gap tunabilities in this material family. The studied material, layered tin-based (4FP)₂SnI₄ perovskite, exhibits an exceptionally strong pressure sensitivity, with a band gap shift of up to -160 meV/GPa at room temperature. Combined experimental and computational studies reveal that its band gap dependence on pressure remains linear in the 0-5 GPa range, making (4FP)₂SnI₄ highly attractive for pressure-sensor applications.

Chalcogen phosphates of transition metals make up a well-known group of antiferromagnetic semiconductors with the general formula MPX₃, where M represents a transition metal and X is a chalcogen, either sulfur or selenium. Most of these compounds adopt a similar structure; however, mercury phosphochalcogenides present an exception with their unique van der Waals layered structure. Transition metal chalcogenides are highly appealing materials for photodetectors due to their exceptional optoelectronic properties. Among them, HgPSe₃, a layered van der Waals phosphoselenide, shows promise for photodetection over a broad spectral range, from visible light to X-rays. Despite this, the electronic processes governing its photoresponse remain unclear. In this study, we demonstrate a nanosecond response time of a HgPSe₃-based photodetector to visible light and gain deeper insights into the underlying charge carrier dynamics through a comprehensive investigation using complementary time-resolved experimental techniques. Our findings on the role of carrier traps provide a potential pathway for optimizing optoelectronic device performance.

As hydrostatic pressure increases, the polaron formation energy in EA₂PbI₄ increases and therefore the emission of free exciton (FE) containing a large polaron (FEP) is enhanced and redshifts by a factor of 1.5 relative to the FE emission.

Application of external pressure leads to three pressure-induced phase transitions, allows band gap engineering and induces giant enhancement of the photoluminescence efficiency of the Tz₂PbBr₄ perovskite.