X. L. Zhang

Inherent optical behavior and structural variation in Na0.5Bi0.5TiO3-6%BaTiO3 revealed by temperature dependent Raman scattering and ultraviolet-visible transmittance

Optical properties of Na 0.5Bi0.5TiO3-6%BaTiO3 (NBT-6%BT) single crystal have been studied by temperature dependent Raman and ultraviolet-visible spectra from 25 to 180 °C. With increasing the temperature, the absorption edge approximately decreases from 3.13 to 3.04 eV. Moreover, abnormal changes of phonon mode and spectral transmission are observed at 83, 106, and 150 °C, which can be unambiguously correlated with thermal evolutions of polar nano-regions and phase transition. It indicates that there is an inherent relationship between optical behavior and structural variation of NBT-6%BT, which provides a valid methodology to explore the phase transition of relaxor ferroelectric oxides.

Optical bandgap and phase transition in relaxor ferroelectric Pb(Mg1∕3Nb2∕3)O3-xPbTiO3 single crystals: An inherent relationship

We report band to band transition behaviors of relaxor ferroelectric Pb(Mg1∕3Nb2∕3)O3-xPbTiO3 (PMN-xPT) single crystals derived from temperature-dependent spectral transmittance. A typical bandgap formula with the temperature and composition ( 8 K ≤ T exp ≤ 453 K , 0.1 ≤ x ≤ 0.4 ) has been presented. Moreover, the phase diagram of PMN-xPT crystals can be well proposed, which is based on the bandgap variations and can be explained by electronic structure evolution. It reveals an intrinsic relationship between fundamental bandgap and phase transition of PMN-xPT single crystals, which pioneers an effective methodology to explore the phase transition of ferroelectric oxides.

Reversible air-induced optical and electrical modulation of methylammonium lead bromide (MAPbBr3) single crystals

The photoluminescence (PL) variations of organic-inorganic hybrid lead halide perovskites in different atmospheres are well documented, while the fundamental mechanism still lacks comprehensive understandings. This study reports the reversible optical and electrical properties of methylammonium lead bromide (MAPbBr3 or CH3NH3PbBr3) single crystals caused by air infiltration. With the change in the surrounding atmosphere from air to vacuum, the PL intensity of perovskite single crystals decreases, while the conductivity increases. By means of first-principles computational studies, the shallow trap states are considered as key elements in PL and conductivity changes. These results have important implications for the characterization and application of organic-inorganic hybrid lead halide perovskites in vacuum.

Identifying the Assembly Configuration and Fluorescence Spectra of Nanoscale Zinc-Tetraphenylporphyrin Aggregates with Scanning Tunneling Microscopy

ZnTPP (Zinc-Tetraphenylporphyrin) is one of the most common nanostructured materials, having high stability and excellent optoelectronic properties. In this paper, the fluorescence features of self-assembled ZnTPP monomers and aggregates on Au(111) surface are investigated in detail on the nanometer scale with scanning tunneling microscopy (STM). The formation of ZnTPP dimers is found in thick layers of a layer-by-layer molecular assembly on Au substrate with its specific molecular arrangement well characterized. Tip-induced luminescence shows a red shift from tilted dimers comparing with the behavior from monomers, which can be attributed to the change of vibrational states due to the intermolecular interaction and the increasing dielectric effect. The nanoscale configuration dependence of electroluminescence is demonstrated to provide a powerful tool aiding the design of functional molecular photoelectric devices.

Photoluminescence study on polar nanoregions and structural variations in Pb(Mg 1/3 Nb 2/3 )O 3 -PbTiO 3 single crystals

We report polar nanostructure and electronic transitions in relaxor ferroelectric Pb(Mg₁/₃Nb₂/₃)O₃ ₋ PbTiO (PMN-PT) single crystals around morphotropic phase boundary (MPB) region by variable-temperature (80-800 K) photoluminescence (PL) spectra and low-wavenumber Raman scattering (LWRS). The discontinuous evolution from peak positions and intensity of luminescence emissions can be corresponding to formation of polar nanoclusters and phase transitions. Six emissions have been derived from PL spectra and show obvious characteristics near phase transition temperatures, which indicates that PL spectral measurement is promising in understanding the microcosmic mechanism. The Raman mode at 1145 cm(-1) indicates that temperature dependent luminescence phenomena can be modulated by thermal quenching.

Phonon modes and local polar distortions in nanoclusters of Pb(In1/2Nb1/2)O3-Pb(Mg1/3Nb2/3)O3-PbTiO3 crystals probed by terahertz reflectance spectra and Raman scattering

Optic phonons and lattice vibrations of Pb(In1/2Nb1/2)O3-Pb(Mg1/3Nb2/3)O3-PbTiO3 single crystals near the morphotropic phase boundary (MPB) are determined by terahertz (THz) reflectance spectra in a temperature range of 5.5–300 K. Raman scattering is measured with a He–Ne laser with a wavelength of 632.8 nm as the exciting source. On cooling from 300 K, the A1 component of the splitting TO1 mode hardens, which follows the Cochran law with a critical softening temperature of 761–861 K. Moreover, the E component of the soft mode is heavily damped around 0.81 THz (27 cm − 1). Additional polar phonon modes, which are forbidden in the cubic structure, are activated due to the broken cubic symmetry in polar clusters.

Temperature dependent spectroscopic ellipsometry and Raman scattering of PbTiO 3 -based relaxor ferroelectric single crystals around MPB region

To discover intrinsic relationship between optical bandgap and structural transformations in relaxor ferroelectric single crystals, electronic band structures and dielectric functions of xPb(In1/2Nb1/2)O3-(1 − x − y)Pb(Mg1/3Nb2/3)O3-yPbTiO3 single crystals (x∼0.27–0.28, y∼0.29–0.35) around morphotropic phase boundary have been investigated by variable-temperature (200–750 K) spectroscopic ellipsometry. It was found that the discontinuous evolution from the second derivative of dielectric functions corresponds to structural transformation patterns. Using the SCP (standard critical point) model, four typical interband transitions (Ea∼2.8 eV, Eb∼3.6 eV, Ec∼4.6 eV, and Ed∼5.4 eV) can be uniquely assigned. These interband transitions are mainly attributed to the contributions from B-O bonds and multiphase coexistence. Furthermore, a modified phase diagram based on interband transition variations with the temperature and PT composition for PIMN-PT crystals was provided. In order to verify the accuracy of phase transition temperature, temperature-dependent low-wavenumber Raman scattering was used as a support. The present results provide important supports for the theoretical model, which establish a quantitative relationship between the electronic transition and structural transformation for ferroelectric oxides.

Plasmon-enhanced fluorescence of submonolayer porphyrins by silver-polymer core-shell nanoparticles

We investigate the fluorescence from submonolayer porphyrin molecules near silver-polymer core-shell nanoparticles (NPs) at a well-controlled separation distance of about 1 nm - 5 nm. When porphyrin molecules are deposited on silver NPs with the plasmonic resonance peak at about 410 nm, which matches very closely with the 405-nm excitation laser and the absorption band of porphyrin molecules, their emission intensity is found to be enhanced due to the plasmonic resonant excitation enhancement, and shows a decline as the increasing polymer shell thickness. Meanwhile, the lifetime results demonstrate that there exists the fluorescence quenching due to the charge transfer and nonradiative energy transfer losses, which is also the main reason that the maximum enhancement factor obtained in experiment is only about 2.3, although the theoretical one is above 60 according to the electric field distribution near silver NPs calculated by finite-difference time-domain method.

Temperature-dependent photoluminescence spectra and decay dynamics of MAPbBr3 and MAPbI3 thin films

The steady-state spectra and fluorescence lifetimes are investigated under vacuum for methylammonium lead bromide and iodide (CH3NH3PbBr3 or MAPbBr3, and CH3NH3PbI3 or MAPbI3) thin films by stably controlling the sample temperature in the range of 78 K to 320 K. The transformation of spectrum features and lifetime components are proved to be quite sensitive to the temperatures in accordance with the phase transition of structures. Our work demonstrates that the halide anions I- and Br- lead to remarkable differences on optical characteristics. Due to the distinct behaviors of excitons, electron-hole pairs and free carriers in decay channels, MAPbI3 has much longer lifetime and higher low-temperature fluorescence efficiency than those of MAPbBr3. The findings provide possible choices for certain perovskites under various ambient temperature conditions to display photovoltaic or luminescent advantage.

All-optical control of three-photon spectra and time asymmetry in a strongly coupled cavity polariton system

Manipulating the nature of photons emission is one of the basic tasks in quantum optics and photonics. The ever growing list of quantum applications requires a robust means of controlling the strongly coupled coherent interaction of photons and matter. Here, we investigate three-photon transmission spectra in a strongly coupled cavity polariton system and show that the correlation functions and transmitted photon stream can be optically manipulated. The dynamics of single photons and photon pairs at the polariton resonances can be changed by light from a single external coupling laser. At the “dark-state polariton,” three-photon transmission is a perfectly coherent field in contrast to the strong photon-bunching behavior of a typical cavity quantum electrodynamics system. When the detuned probe light is tuned to the “bright polariton,” the light exhibits a dramatic photon antibunching effect. Remarkably, the Fano-resonant asymmetric three-photon transmission caused by the interference between the dressed states leads to a new quantum feature that is strongly nonclassical (the third-order correlation function g(3)(0, 0) ≪ 1) and has a wide and tunable bandwidth. The dependence of the intrinsic third-order correlation and time symmetry of the photon stream on the controlled parameters is also examined. Strongly nonclassical, all-optically controllable multi-photon dynamics are very important for future quantum devices and metrology.