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Temperature and pressure dependent measurements have been performed on 3.5 nm ZnS:Mn[sup 2+] nanoparticles. As temperature increases, the donor-acceptor (DA) emission of ZnS:Mn[sup 2+] nanoparticles at 440 nm shifts to longer wavelengths while the Mn[sup 2+] emission ([sup 4]T[sub 1] - [sup 6]A[sub 1]) shifts to shorter wavelengths. Both the DA and Mn[sup 2+] emission intensities decrease with temperature with the intensity decrease of the DA emission being much more pronounced. The intensity decreases are fit well with the theory of thermal quenching. As pressure increases, the Mn[sup 2+] emission shifts to longer wavelengths while the DA emission wavelength remains almost constant. The pressure coefficient of the DA emission in ZnS:Mn[sup 2+] nanoparticles is approximately -3.2meV/GPa, which is significantly smaller than that measured for bulk materials. The relatively weak pressure dependence of the DA emission is attributed to the increase of the binding energies and the localization of the defect wave functions in nanoparticles. The pressure coefficient of Mn[sup 2+] emission in ZnS:Mn[sup 2+] nanoparticles is roughly -34.3 meV/GPa, consistent with crystal field theory. The results indicate that the energy transfer from the ZnS host to Mn[sup 2+] ions is mainly from the recombination of carriers localized at Mn[sup 2+] ions. [ABSTRACT FROM AUTHOR]

Polycrystalline bulk samples of BaSn1−xCrxO3 with different Cr concentrations were prepared by standard solid state reaction method. The effects of Cr concentration on crystal structure of BaSn1−xCrxO3, as well as optical and magnetic properties were investigated. Powder X-ray diffraction (XRD) shows that each of these samples possess a perovskite structure with P m 3 ¯ m space group. The Cr ions are located at the substitution Sn sites as revealed by the XRD, Diffuse reflectance spectra and Raman spectra. Moreover, it is found that room-temperature ferromagnetism (FM) can be reversed by annealing in the nitrogen atmosphere. This can be well explained by a modified model of F-center mediated FM. [ABSTRACT FROM AUTHOR]

MnBi2Te4 can generate a variety of exotic topological quantum states, which are closely related to its special structure. We conduct comprehensive multiple-cycle high-pressure research on MnBi2Te4 by using a diamond anvil cell to study its phase transition behaviors under high pressure. As observed, when the pressure does not exceed 15 GPa, the material undergoes an irreversible metal–semiconductor–metal transition, whereas when the pressure exceeds 17 GPa, the layered structure is damaged and becomes irreversibly amorphous due to the lattice distortion caused by compression, but it is not completely amorphous, which presents some nano-sized grains after decompression. Our investigation vividly reveals the phase transition behaviors of MnBi2Te4 under high pressure cycling and paves the experimental way to find topological phases under high pressure. [ABSTRACT FROM AUTHOR]

Abstract: Zn2SiO4:Mn2+, Zn2SiO4:Eu3+ and Zn2SiO4:Mn2+,Eu3+ phosphors were prepared by a sol-gel process and their luminescence spectra were investigated. The emission bands from intra-ion transitions of Mn2+ and Eu3+ samples were studied as a function of pressure. The pressure coefficient of Mn2+ emission was found to be –25.3±0.5 and –28.5±0.9meV/GPa for Zn2SiO4:Mn2+ and Zn2SiO4:Mn2+,Eu3+, respectively. The Eu3+ emission shows only weak pressure dependence. The pressure dependences of the Mn2+ and Eu3+ emissions in Zn2SiO4:Mn2+,Eu3+ are slightly different from that in Zn2SiO4:Mn2+ and Zn2SiO4:Eu3+ samples, which can be attributed to the co-doping of Mn2+ and Eu3+ ions. The Mn2+ emission in the two samples, however, exhibits analogous temperature dependence and similar luminescence lifetimes, indicating no energy transfer from Mn2+ to Eu3+ occurs. [Copyright &y& Elsevier]

Photoluminescence measurements for pure BaSnO3 are carried out at low temperatures and high magnetic fields, down to 4 K and up to 8 T, respectively. Multiple phonon replicas are observed in the near-infrared region, which allows for a quantitative estimation of the Huang-Rhys factor, . Due to the extremely narrow linewidth, a magnetic-field–induced nonlinear blueshift of the zero phonon line (ZPL) and a small increase of the phonon energy are clearly distinguishable. The magnetic-field dependence of the ZPL energies can be interpreted in terms of diamagnetic effects of the s-like local electronic state. The electron effective mass value of 0.14m0 is extracted from the best fitting. [ABSTRACT FROM AUTHOR]

We employ polarization-resolved femtosecond optical pump–probe spectroscopy to investigate the nonequilibrium photocarrier dynamics in excitonic insulator Ta2NiSe5. The electronic dynamics, including hot carrier cooling, exciton formation, and recombination in the timescale ranging from subpicoseconds to a few tens of picoseconds, have been established from the transient reflectivity spectra, showing strong in-plane anisotropy with respect to the probe polarization. Such anisotropic photocarrier dynamics possibly arise from the crystalline orientation dependence of the excitonic polarizability. Furthermore, we find that the amplitude of coherent phonons with a frequency of 1 THz is subject to the probe polarization, whereas it is not sensitive to the pump polarization. This substantiates that the displacive excitation of coherent phonons plays a decisive role in lattice dynamics. In addition, we find that the photo-induced dielectric screening tends to suppress the amplitude of coherent phonons with increasing pump fluence, manifesting a remarkable polarization dependence. Our work provides valuable insights into the excitonic dynamics and the origin of coherent phonon generation and also may contribute to the development of polarization-sensitive photoelectric devices based on Ta2NiSe5. [ABSTRACT FROM AUTHOR]

The structure and photoluminscence (PL) properties of Pr-implanted GaN thin films have been studied. RBS/channeling technique was used to explore the damage recovery at high annealing temperature and study the dependence of the radiation damage with ion implantation direction. A complete recovery of the ion implantation damage cannot be achieved at annealing temperatures up to 1050°C. It is found that the channeling implantation results in the decrease of the damage. The PL experimental results indicate that the PL efficiency increases exponentially with annealing temperature up to the maximum temperature of 1050°C. Moreover, the PL intensity is also seriously affected by ion implantation geometries. The PL intensity for the sample implanted along channeled direction is nearly 2 times more intense than that observed from the sample implanted along random direction. The thermal quenching of PL intensity from 10 to 300 K for sample annealed at 1050°C is only 30%. [Copyright &y& Elsevier]

Pentagonal palladium diselenide (PdSe 2 ) stands out for its exceptional optoelectronic properties, including high carrier mobility, tunable bandgap, and anisotropic electronic and optical responses. Herein, we systematically investigate photocarrier dynamics in PdSe 2 ribbons using polarization-resolved optical pump-probe spectroscopy. In thin PdSe 2 ribbons with a semiconductor phase, the photocarrier dynamics are found to be dominated by intraband hot-carrier cooling, interband recombination, and the exciton effect, showing weak crystalline orientation dependences. Conversely, in thick semimetal-phase PdSe 2 ribbons, the photocarrier relaxations governed by the electron-optical/acoustic phonon scattering strongly depend on the sample orientation, albeit with a degradation in in-plane anisotropy following hot-carrier cooling. Furthermore, we analyze the correlations between photocarrier dynamics and anisotropic energy dispersions of electronic structures across a wide range in k space, as well as the contributions from the anisotropic electron-phonon couplings. Our study provides crucial insights for developing polarization-sensitive photoelectronic devices based on PdSe 2 .

The transport properties of charge carriers in MXene, a promising material, have been studied using terahertz time-domain spectroscopy (THz-TDS) to examine its potential applications in optical and electronic devices. However, previous studies have been limited by narrow frequency ranges, which have hindered the understanding of the intrinsic mechanisms of carrier transport in MXenes. To address this issue, ultrabroadband THz-TDS with frequencies of up to 15 THz to investigate the complex photoconductances of MXene (Ti 3 C 2 T x ) films with different thicknesses are employed. The findings indicate that the electronic localization is substrate-dependent, and this effect decreases with an increase in the number of layers. This is attributed to the screening effect of the high carrier density in Ti 3 C 2 T x . Additionally, the layer-independent photocarrier relaxations revealed by optical pump THz probe spectroscopy (OPTP) provide evidence of the carrier heating-induced screening effect. These results are significant for practical applications in both scientific research and various industries., (© 2023 Wiley‐VCH GmbH.)

VO 2 , which exhibits semiconductor-metal phase transition characteristics occurring on a picosecond time scale, holds great promise for ultrafast terahertz modulation in next-generation communication. However, as of now, there is no reported prototype for an ultrafast device. The temperature effect has been proposed as one of the major obstacles. Consequently, reducing the excitation threshold for the phase transition would be highly significant. The traditional strategy typically involves chemical doping, but this approach often leads to a decrease in phase transition amplitude and a slower transition speed. In this work, we proposed a design featuring a highly conductive MXene interfacial layer between the VO 2 film and the substrate. We demonstrate a significant reduction in the phase transition threshold for both temperature and laser-induced phase transition by adjusting the conductivity of the MXene layers with varying thicknesses. Our observations show that the phase transition temperature can be decreased by 9 °C, while the pump fluence for laser excitation can be reduced by as high as 36%. The ultrafast phase transition process on a picosecond scale, as revealed by the optical-pump terahertz-probe method, suggests that the MXene layers have minimal impact on the phase transition speed. Moreover, the reduced phase transition threshold can remarkably alleviate the photothermal effect and inhibit temperature rise and diffusion in VO 2 triggered by laser. This study offers a blueprint for designing VO 2 /MXene hybrid films with reduced phase transition thresholds. It holds significant potential for the development of low-power, intelligent optical and electrical devices including, but not limited to, terahertz modulators based on phase transition phenomena.

MnBi2Te4 can generate a variety of exotic topological quantum states, which are closely related to its special structure. We conduct comprehensive multiple-cycle high-pressure research on MnBi2Te4 by using a diamond anvil cell to study its phase transition behaviors under high pressure. As observed, when the pressure does not exceed 15 GPa, the material undergoes an irreversible metal-semiconductor-metal transition, whereas when the pressure exceeds 17 GPa, the layered structure is damaged and becomes irreversibly amorphous due to the lattice distortion caused by compression, but it is not completely amorphous, which presents some nano-sized grains after decompression. Our investigation vividly reveals the phase transition behaviors of MnBi2Te4 under high pressure cycling and paves the experimental way to find topological phases under high pressure., (© 2024 Author(s). Published under an exclusive license by AIP Publishing.)

Van der Waals heterostructures composed of different two-dimensional films offer a unique platform for engineering and promoting photoelectric performances, which highly demands the understanding of photocarrier dynamics. Herein, large-scale vertically stacked heterostructures with MoS 2 and ReSe 2 monolayers are fabricated. Correspondingly, the carrier dynamics have been thoroughly investigated using different ultrafast spectroscopies, including Terahertz (THz) emission spectroscopy, time-resolved THz spectroscopy (TRTS), and near-infrared optical pump-probe spectroscopy (OPPS), providing complementary dynamic information for the out-of-plane charge separation and in-plane charge transport at different stages. The initial charge transfer (CT) within the first 170 fs, generating a transient directional current, is directly demonstrated by the THz emissions. Furthermore, the TRTS explicitly unveils an intermediate free-carrier relaxation pathway, featuring a pronounced augmentation of THz photoconductivity compared to the isolated ReSe 2 layer, which likely contains the evolution from immigrant hot charged free carriers to bounded interlayer excitons (∼0.7 ps) and the surface defect trapping (∼13 ps). In addition, the OPPS reveals a distinct enhancement in the saturable absorption along with long-lived dynamics (∼365 ps), which originated from the CT and interlayer exciton recombination. Our work provides comprehensive insight into the photocarrier dynamics across the charge separation and will help with the development of optoelectronic devices based on ReSe 2 -MoS 2 heterostructures.

A systemic investigation of the terahertz (THz) transmission of La 0.67 Ca 0.33 MnO 3 film on the (001)-oriented NdGaO 3 substrate under external magnetic field and low temperature have been performed. The significant THz absorption difference between the out-of-plane and the in-plane magnetic field direction is observed, which is consistent with the electrical transport measurement using the standard four-probe technique. Furthermore, we find that the complex THz conductivities can be reproduced in terms of the Drude Smith equation as the magnetic field is perpendicular to the film plane, whereas it deviates from this model when the in-plane magnetic field is applied. We suggest that such anisotropies in THz transport dynamics have close correspondences with the phase separation and anisotropic magnetoresistance effects in the perovskite-structured manganites. Our work demonstrates that the THz time-domain spectroscopy (TDS) can be an effective non-contact method for studying the magneto-transport properties of the perovskite-structured manganites., (© 2021 Author(s).)

Pressure effects on photocarrier dynamics such as interband relaxations and intraband cooling in GaAs have been investigated using in situ time-resolved terahertz spectroscopy with a diamond anvil cell. The interband photocarrier lifetime significantly decreases by nearly two orders of magnitude as the external hydrostatic pressure is increased up to 10 GPa. Considerable pressure tuning for the intervalley scattering processes has also been observed, and the time constants under different pressures are extracted based on the three-state rate model. This work provides new perspectives on tailoring nonequilibrium carrier dynamics in semiconductors using hydrostatic pressure and may serve as the impetus for the development of high-pressure terahertz spectroscopy.

We perform femtosecond pump-probe spectroscopy to in situ investigate the ultrafast photocarrier dynamics in bilayer graphene and observe an acceleration of energy relaxation under pressure. In combination with in situ Raman spectroscopy and ab initio molecular dynamics simulations, we reveal that interlayer shear and breathing modes have significant contributions to the faster hot-carrier relaxations by coupling with the in-plane vibration modes under pressure. Our work suggests that further understanding the effect of interlayer interaction on the behaviors of electrons and phonons would be critical to tailor the photocarrier dynamic properties of bilayer graphene.

The frequency and time resolved conductivity in a photoexcited large-area monolayer tungsten disulfide (WS 2 ) have been simultaneously determined by using time-resolved terahertz spectroscopy. We use the Drude-Smith model to successfully reproduce the transient photoconductivity spectra, which demonstrate that localized free carriers, not bounded excitons, are responsible for the THz transport. Upon the optical excitation with 400 nm and 530 nm wavelength, the relaxation dynamics of the free carriers include fast and slow decay components with time constants approximately smaller than 1 ps and between 5-7 ps, respectively. The former sub-picosecond decay is attributed to the charge carrier loss induced by the exciton formation, while both the Auger recombination and the surface trapping can contribute to the slow relaxation.