Pressure Dependence Of Photoluminescence Research Articles

Optical tunning of high-luminescent iodine-substituted CsPb2(Br0.84I0.16)5 under pressure

Metal halide compounds hold significant interest for optoelectronics, given their substantial potential for solar cells and light-emitting applications. Low-dimensional (0D, 1D, and 2D) metal halide perovskites or perovskite-like compounds exhibit various compositions with soft lattices and modular optical properties. Thus, the finding of new strategies to further improve such properties is a current research challenge. In this scenario, the 2D perovskite-like compound CsPb2Br5 has attracted attention due to its structural stability and favorable optical properties. This study presents the pressure-induced structural phase transitions of the iodine substituted CsPb2(Br0.84I0.16)5. This substitution induces strong green photoluminescence in single crystals at ambient conditions. High-pressure optical and synchrotron X-ray diffraction experiments reveal a tetragonal-tetragonal structural phase transitions around 1 GPa, while the photoluminescence emission is quenched around 2.7 GPa. Additionally, a new high-pressure orthorhombic phase is identified in the iodine-doped compound beyond 4.5 GPa. Further analysis of structural features prove that the pressure dependent photoluminescence behavior is directly related to structural distortions observed upon pressure increase.

Exploring the Potential Applications of Lanthanide-Based Double Perovskite in Remote Pressure and Temperature Sensing.

Remote optical sensing with nondestructive, fast, and accurate detection capabilities is a powerful noncontact method widely used in natural, industrial, and biological fields. In this work, Cs2NaErCl6 double perovskite was synthesized via a hydrothermal method. The pressure-dependent photoluminescence (PL) lifetime of Er3+ in the range of 0-20 GPa was investigated, demonstrating its potential for pressure monitoring. The high-pressure relative sensitivity (SR) is ∼18.45% GPa-1. Temperature measurements were conducted using the fluorescence intensity ratio (FIR) of the thermal couple energy level (TCEL) and the nonthermal couple energy level (NTCEL) of Er3+ across a temperature range of 100-660 K, with a maximum SR of 5.36% K-1. By combining MXene with Cs2NaErCl6 and recording the FIR of Cs2NaErCl6 under 1550 nm excitation, the photothermal conversion temperature of MXene can be accurately determined. These findings highlight the potential of Cs2NaErCl6 for remote pressure and temperature sensing, particularly in the biomedical field.

Piezochromism in Dynamic Three-Dimensional Covalent Organic Frameworks.

Piezochromic materials with pressure-dependent photoluminescence tuning properties are important in many fields, such as mechanical sensors, security papers, and storage devices. Covalent organic frameworks (COFs), as an emerging class of crystalline porous materials (CPMs) with structural dynamics and tunable photophysical properties, are suitable for designing piezochromic materials, but there are few related studies. Herein, we report two dynamic three-dimensional COFs based on aggregation-induced emission (AIE) or aggregation-caused quenching (ACQ) chromophores, termed JUC-635 and JUC-636 (JUC = Jilin University China), and for the first time, study their piezochromic behavior by diamond anvil cell technique. Due to the various luminescent groups, JUC-635 has completely different solvatochromism and molecular aggregation behavior in the solvents. More importantly, JUC-635 with AIE effect exhibits a sustained fluorescence upon pressure increase (~3 GPa), and reversible sensitivity with high-contrast emission differences (Δλem = 187 nm) up to 12 GPa, superior to other CPMs reported so far. Therefore, this study will open a new gate to expand the potential applications of COFs as exceptional piezochromic materials in pressure sensing, barcoding, and signal switching.

Access and Capture of Layered Double Perovskite Polytypic Phase through High-Pressure Engineering

Engineering halide perovskites through external pressure is as an effective means to tune the crystal structure, thus optoelectronic properties of the material. In this work, we studied the structural and optical property evolutions of Cs4MIIBi2Cl12 (MII: Cd, Cd0.8Mn0.2, Mn) layered double perovskite (LDP) crystalline powders under high pressure. A novel polytypic phase transition was observed featuring lateral interlayer sliding of the Bi–MII–Bi trilayer units, resulting in a new LDP-12R phase. Importantly, this high-pressure induced LDP-12R crystal phase can be preserved after complete decompression and captured at ambient conditions. Moreover, the LDP samples showed a pressure-dependent photoluminescence property. Our findings exemplified a new perovskite polytype that can be accessed and captured through high-pressure processing, advocating the uniqueness of LDP materials with soft and transformable crystal lattices.

Opposite pressure impact on electron–phonon coupling in Eu2+ and Ce3+ doped AlN

This paper analyzes the influence of pressure on electron–lattice interactions in the 5d excited states of Ce3+ and Eu2+ in the AlN host based on pressure-dependent photoluminescence and photoluminescence excitation spectra.

Effects of Repetitive Pressure on the Photoluminescence of Bare and ZnS-Capped CuInS2 Quantum Dots: Implications for Nanoscale Stress Sensors

Semiconductor quantum dots (QDs) are promising materials for stress/strain sensing applications owing to their pressure-dependent photoluminescence (PL) and nanoscale size, while the impact of stress and microstructure on their optical properties still awaits in-depth investigations under more realistic loading conditions. Herein, bare CuInS2 QDs and core–shell structured CuInS2/ZnS QDs are investigated under repetitive pressure loadings to elucidate the pressure-dependent PL responses over many pressure cycles. The CuInS2/ZnS QDs not only show higher PL intensity but also exhibit a reliable and simple relationship between PL emission peak energy (EPL) and external pressure (P), which is desirable for actual application in stress/strain sensing. Specifically, the EPL–P relationship of bare CuInS2 QDs changes after the first loading–unloading cycle, while the EPL–P relationship of CuInS2/ZnS QDs shows repeatable trajectories under different cycles. This research provides experimental support for designing QD-based stress/strain sensing materials and explains how the shell and surface microstructure will affect the mechanical-luminescence responses of QDs.

Anomalous pressure-dependence in surface-modified silicon-derived nanoparticles

Surfaces can significantly alter the optical properties of nanomaterials, but they are difficult to control and their roles are hard to understand in highly reactive materials such as silicon nanomaterials. In this work, we investigate the role of the surface in controlling the optical transitions in highly luminescent silicon-derived nanoparticles. By combining high-pressure and low-temperature experiments, we experimentally correlate the anomalously intense and narrow transitions in the UV range with the surface oxides, while the visible transition and the photoluminescence (PL) are verified to originate from the Si-ligand charge transfer band. We find that the high-pressure absorption and PL depends on the rigidity of the surface ligand. This indicates that the surface plays a dominant role on the optical properties of these silicon-derived nanoparticles, and is different than other semiconductor nanomaterials, in which pressure-dependent optical transitions depend on lattice strain or phase transformations. This work presents a comprehensive understanding of the optical transitions and the effect of surface ligands and surface oxidation in these highly luminescent Si-derived nanoparticles. The new insight into the oxidation-activated and ligand-mediated transitions, and the pressure-dependent PL may help with engineering the band structure of other highly-reactive optical nanomaterials.

Study of the Photoluminescence Characteristics of 4,4′-((1E,1′E)-Quinoxaline-2,3-diylbis(ethene-2,1-diyl))bis(N,N-dimethylaniline)

A comparative investigation on the photophysical properties of a quinoxaline derivative 4,4'-((1E,1'E)-quinoxaline-2,3-diylbis(ethene-2,1-diyl))bis(N,N-dimethylaniline) (QDMA2) was performed by employing many spectroscopies. Based on the pump-dump/push-probe measurement, it is found that a solvent-stabilized charge-transfer state can participate in the relaxation of excited QDMA2 with increasing solvent polarity. Meanwhile, the aggregated QDMA2 molecules were engineered into the organic light-emitting diode test, which showed a correlated color temperature value of 1875 K. With the help of a diamond anvil cell, the pressure-dependent photoluminescence of aggregated QDMA2 shows that the intermolecular interaction can affect the color and intensity of photoluminescence through adjusting the band gap and irradiative channel of the aggregated molecules. These results are important for understanding the structure-property relationships and the rational design of functional materials for optoelectronic applications.

Multi-Site Cation Control of Ultra-Broadband Near-Infrared Phosphors for Application in Light-Emitting Diodes.

Near-infrared (NIR) phosphors are fascinating materials that have numerous applications in diverse fields. In this study, a series of La3Ga5GeO14:Cr3+ phosphors, which was incorporated with Sn4+, Ba2+, and Sc3+, was successfully synthesized using solid-state reaction to explore every cationic site comprehensively. The crystal structures were well resolved by combining synchrotron X-ray diffraction and neutron powder diffraction through joint Rietveld refinements. The trapping of free electrons induced by charge unbalances and lattice vacancies changes the magnetic properties, which was well explained by a Dyson curve in electron paramagnetic resonance. Temperature and pressure-dependent photoluminescence spectra reveal various luminescent properties between strong and weak fields in different dopant centers. The phosphor-converted NIR light-emitting diode (pc-NIR LED) package demonstrates a superior broadband emission that covers the near-infrared (NIR) region of 650-1050 nm. This study can provide researchers with new insight into the control mechanism of multiple-cation-site phosphors and reveal a potential phosphor candidate for practical NIR LED application.

Broadband NaK2Li[Li3SiO4]4:Ce Alkali Lithosilicate Blue Phosphors

Phosphors with a rigid and symmetrical structure are urgently needed. The alkali lithosilicate family (A[Li3SiO4]) has been extensively studied with a narrow emission band due to its unique cuboid-coordinated environment and rigid structure. However, here we demonstrate for the first time Ce-doped NaK2Li[Li3SiO4]4 phosphors with a broad emission band, a high internal quantum efficiency (85.6%), and excellent thermal stability. Photoluminescence indicates the Ce's preference to occupy the Na+ site, leading to a strong blue color emission with peak maxima at 417 and 450 nm. Temperature- and pressure-dependent photoluminescence reveals thermal stability and a phase transition. Moreover, the X-ray absorption near-edge structure reveals the mixing of Ce3+ and Ce4+ in the materials; this result differs from that of Eu2+-doped A[Li3SiO4] phosphors. The charge compensation process is then proposed to explain this difference. This study not only provides insights into Ce-doped UCr4C4-type phosphors but also explains the charge compensation mechanism of the aliovalent doping process.

Pressure-Tuned Core/Shell Configuration Transition of Shell Thickness-Dependent CdSe/CdS Nanocrystals.

Pressure is adopted as a "clean" tool to achieve a core/shell configuration transition of CdSe/CdS nanocrystals (NCs) from quasi-type II to type I. The pressure-dependent photoluminescence (PL) spectra demonstrate a sudden decrease in PL intensity, because of the enhanced rate of exciton-exciton annihilation of type I structured CdSe/CdS NCs. Likewise, the large decrease in the PL lifetime with pressure confirms that the electron wave function mainly localizes into the CdSe core, indicating the decreased separation of electrons and holes in type I band alignment. We propose that pressure increases the conduction band energy of the CdS shell but hardly changes that of the CdSe core with almost both unchanged valence band energies, thus ultimately increasing the conduction band offsets between the CdSe core and CdS shell to form the type I core/shell configuration. Our studies elucidate the significance of external pressure in determining the electronic and optical properties of core/shell nanomaterials.

Pressure-dependent photoluminescence of CdSe/ZnS quantum dots: Critical point of different pressure regimes

High pressure condition can alter and control the electronic structure of semiconductor quantum dots, and therefore provides novel insight for designing optoelectronic devices. Here we report the pressure-dependent photoluminescence (PL) of CdSe/ZnS core/shell quantum dots (QD) in different pressure regimes. The center of the PL spectrum has blue-shift when hydrostatic pressure increases, due to the increasing bulk modulus. However, the shifting rate becomes lower in the higher pressure regime, i.e. above the critical pressure point, mainly due to the interaction between QDs. Accordingly, the lifetimes of PL drop in the low pressure regime, due to the increased pressure-induced trapping states, then increase above the critical pressure point, indicating the interaction between dots likewise. The observed critical pressures for both cases are consistent with each other, and strongly depend on the QD concentration, which forcefully support the aforementioned interaction model.

Experimental and theoretical analysis of influence of barrier composition on optical properties of GaN/AlGaN multi-quantum wells: Temperature- and pressure-dependent photoluminescence studies

Studies of internal electric fields in GaN/AlxGa1-xN (3 nm/4 nm) multi-quantum-wells (MQWs) with x = 0.25, 0.5, and 1 are presented. The structures were grown by plasma-assisted molecular-beam epitaxy and characterized by x-ray diffraction, transmission electron microscopy, and secondary ion mass spectroscopy. Optical properties of these structures were investigated by ambient and high-pressure photoluminescence (PL) measurements. They were strongly affected by polarization-induced electric fields giving rise to a quantum confined Stark effect, depending on the composition of the AlGaN barrier. The optical emission energy redshifts by > 100 meV when the Al content in the barrier increases from x = 0.25 up to x = 1, while the pressure coefficients of the PL energy are significantly reduced in comparison with bulk GaN. The transition energies and their pressure dependencies are modelled for tetragonally strained structures with the same geometry using a full tensorial representation of the strain in the MQWs under external pressure. The same MQWs are also simulated using density functional theory calculations. Additionally, influence of blurring of well-barrier interface on transition energy has been modelled. A good agreement between experimental results and theoretical analysis indicates that nonlinear effects induced by the tetragonal strain due to the lattice mismatch between the substrates and the polar MQWs are responsible for a drastic decrease of the pressure coefficients of PL energy, and that these effects are well described by ab initio calculation procedures. Our results reflect the role of composition and geometry on the basic optical properties of nitride quantum wells.

Anomalous Pressure Characteristics of Defects in Hexagonal Boron Nitride Flakes.

Research on hexagonal boron nitride (hBN) has been intensified recently due to the application of hBN as a promising system of single-photon emitters. To date, the single photon origin remains under debate even though many experiments and theoretical calculations have been performed. We have measured the pressure-dependent photoluminescence (PL) spectra of hBN flakes at low temperatures by using a diamond anvil cell device. The absolute values of the pressure coefficients of discrete PL emission lines are all below 15 meV/GPa, which is much lower than the pressure-induced 36 meV/GPa redshift rate of the hBN bandgap. These PL emission lines originate from atom-like localized defect levels confined within the bandgap of the hBN flakes. Interestingly, the experimental results of the pressure-dependent PL emission lines present three different types of pressure responses corresponding to a redshift (negative pressure coefficient), a blueshift (positive pressure coefficient), or even a sign change from negative to positive. Density functional theory calculations indicate the existence of competition between the intralayer and interlayer interaction contributions, which leads to the different pressure-dependent behaviors of the PL peak shift.

5a-F-16 MOVPE成長AlGaInPのフォトルミネッセンスの圧力依存性II

5a-F-16 MOVPE成長AlGaInPのフォトルミネッセンスの圧力依存性II

Pressure induced photoluminescence modulation in a wide range and synthesis of monodispersed ternary AgCuS nanocrystal based on Ag2S nanocrystals.

Binary Ag2S nanocrystals (NCs) have many potential optical applications because of their low toxicity, narrow direct band gaps and near-infrared photoluminescence (PL) with high emission efficiency. However, due to its small exciton Bohr radius (2.2 nm), the PL spectra of Ag2S NCs can only be modulated below ∼1200 nm with increasing particle size. Meanwhile, ternary silver copper chalcogenides (AgCuX, X = S, Se) have also attracted increased attention in recent years. Temperature-dependent structural phase transformation leads electrical transport to exhibit fascinating transitions between p and n type conduction, which makes AgCuS and AgCuSe ideal materials for diode or transistor devices. Nevertheless, the traditional method to synthesize these materials is mainly through melting the mixture of Ag, Cu and S/Se powder under extremely high reaction temperatures (973-1373 K) and long reaction time, forming a bulk product. Therefore, the synthesis of high quality monodispersed and size-tunable AgCuS or AgCuSe NCs is still a challenge. To address these issues, in this paper, we report using Ag2S NCs as a template, a method to synthesize monodispersed and size-tunable β-AgCuS NCs via ion exchange and diffusion processes. Similarly, monodispersed β-AgCuSe NCs were also synthesized by this simple and reproducible strategy. This synthetic method opens new avenues for investigating the size-, morphology- and temperature-dependent phase transitions of these ternary AgCuS and AgCuSe materials. Thus, the corresponding electrical transport between p and n type conduction can be studied in the future. Furthermore, we systematically investigated the pressure-dependent PL properties and band gap modulation of monodispersed Ag2S NCs using in situ high pressure PL and absorption spectroscopy. We found that the PL peak of 6.0 nm for Ag2S NCs could be easily adjusted from ∼1200 to 1900 nm with increasing pressure from 0 to 5.1 GPa, which greatly extends the wavelength range of the PL peak beyond that of other approaches.

Pressure Effects on Structure and Optical Properties in Cesium Lead Bromide Perovskite Nanocrystals.

Metal halide perovskites (MHPs) are gaining increasing interest because of their extraordinary performance in optoelectronic devices and solar cells. However, developing an effective strategy for achieving the band-gap engineering of MHPs that will satisfy the practical applications remains a great challenge. In this study, high pressure is introduced to tailor the optical and structural properties of MHP-based cesium lead bromide nanocrystals (CsPbBr3 NCs), which exhibit excellent thermodynamic stability. Both the pressure-dependent steady-state photoluminescence and absorption spectra experience a stark discontinuity at ∼1.2 GPa, where an isostructural phase transformation regarding the Pbnm space group occurs. The physical origin points to the repulsive force impact due to the overlap between the valence electron charge clouds of neighboring layers. Simultaneous band-gap narrowing and carrier-lifetime prolongation of CsPbBr3 trihalide perovskite NCs were also achieved as expected, which facilitates the broader solar spectrum absorption for photovoltaic applications. Note that the values of the phase change interval and band-gap red-shift of CsPbBr3 nanowires are between those for CsPbBr3 nanocubes and the corresponding bulk counterparts, which results from the unique geometrical morphology effect. First-principles calculations unravel that the band-gap engineering is governed by orbital interactions within the inorganic Pb-Br frame through structural modification. Changes of band structures are attributed to the synergistic effect of pressure-induced modulations of the Br-Pb bond length and Pb-Br-Pb bond angle for the PbBr6 octahedral framework. Furthermore, the significant distortion of the lead-bromide octahedron to accommodate the Jahn-Teller effect at much higher pressure would eventually lead to a direct to indirect band-gap electronic transition. This study enables high pressure as a robust tool to control the structure and band gap of CsPbBr3 NCs, thus providing insight into the microscopic physiochemical mechanism of these compressed MHP nanosystems.

High-Pressure Single-Crystal Structures of 3D Lead-Halide Hybrid Perovskites and Pressure Effects on their Electronic and Optical Properties.

We report the first high-pressure single-crystal structures of hybrid perovskites. The crystalline semiconductors (MA)PbX3 (MA = CH3NH3+, X = Br– or I–) afford us the rare opportunity of understanding how compression modulates their structures and thereby their optoelectronic properties. Using atomic coordinates obtained from high-pressure single-crystal X-ray diffraction we track the perovskites’ precise structural evolution upon compression. These structural changes correlate well with pressure-dependent single-crystal photoluminescence (PL) spectra and high-pressure bandgaps derived from density functional theory. We further observe dramatic piezochromism where the solids become lighter in color and then transition to opaque black with compression. Indeed, electronic conductivity measurements of (MA)PbI3 obtained within a diamond-anvil cell show that the material’s resistivity decreases by 3 orders of magnitude between 0 and 51 GPa. The activation energy for conduction at 51 GPa is only 13.2(3) meV, suggesting that the perovskite is approaching a metallic state. Furthermore, the pressure response of mixed-halide perovskites shows new luminescent states that emerge at elevated pressures. We recently reported that the perovskites (MA)Pb(BrxI1–x)3 (0.2 < x < 1) reversibly form light-induced trap states, which pin their PL to a low energy. This may explain the low voltages obtained from solar cells employing these absorbers. Our high-pressure PL data indicate that compression can mitigate this PL redshift and may afford higher steady-state voltages from these absorbers. These studies show that pressure can significantly alter the transport and thermodynamic properties of these technologically important semiconductors.

Pressure-induced phase transition in hydrothermally grown ZnO nanoflowers investigated by Raman and photoluminescence spectroscopy

This paper reports the pressure-dependent photoluminescence and Raman spectral investigation of hydrothermally synthesized ZnO nanoflowers at room temperature. Intrinsic near-band-edge UV emission from ZnO nanoflowers is monotonously blue-shifted under pressures up to 13.8 GPa with a pressure coefficient of 26 meV GPa−1, and this pressure value is nearly 5 GPa above the transition pressure from the wurtzite to the rock salt phase for bulk ZnO. The Raman band corresponds to the wurtzite phase, the and modes were observed up to about 11 GPa from the spectra. The apparent discrepancy in the transition pressures as determined from photoluminescence and Raman studies suggests that it is a consequence of the gradual phase transition, in which the smallest nanoparticles are expected to remain in the wurtzite phase up to 13–15 GPa.

Evidence for a defect level above the conduction band edge of InAs/InAsSb type-II superlattices for applications in efficient infrared photodetectors

We report pressure-dependent photoluminescence (PL) experiments under hydrostatic pressures up to 2.16 GPa on a mid-wave infrared InAs/InAs0.86Sb0.14 type-II superlattice (T2SL) structure at different pump laser excitation powers and sample temperatures. The pressure coefficient of the T2SL transition was found to be 93 ± 2 meV·GPa−1. The integrated PL intensity increases with pressure up to 1.9 GPa then quenches rapidly indicating a pressure induced level crossing with the conduction band states at ∼2 GPa. Analysis of the PL intensity as a function of excitation power at 0, 0.42, 1.87, and 2.16 GPa shows a clear change in the dominant photo-generated carrier recombination mechanism from radiative to defect related. From these data, evidence for a defect level situated at 0.18 ± 0.01 eV above the conduction band edge of InAs at ambient pressure is presented. This assumes a pressure-dependent energy shift of −11 meV·GPa−1 for the valence band edge and that the defect level is insensitive to pressure, both of which are supported by an Arrhenius activation energy analysis.