Liyao Zhang

DIELECTRIC STUDY IN NANOCRYSTALLINE BI4TI3O12 PREPARED BY CHEMICAL COPRECIPITATION

Bi4Ti3O12 nanoparticles were synthesized by chemical coprecipitation and crystallized in the perovskite structure after calcining at 520 °C. The dielectric properties were measured in bulk nanocrystalline compacts with different grain sizes. It was found that there were three peaks in the curves of the dielectric response as a function of temperature. The first peak shifts to higher temperature with decreasing grain size, which is considered to originate from the polarization of the ions in the conductive (Bi2O2)2+ layers across the potential barrier of the weak conductive perovskitelike layers. The second peak, contributed by the polarization of the defect dipoles on the grain surfaces, especially Bii.–Vo″ dipoles, decreases gradually in intensity and finally disappears with increasing grain size. The last one, corresponding to a ferroelectric phase transition temperature, increases at first with decreasing grain size from 56 to 25 nm, then decreases with further decreasing grain size, and the mechanism ...

A cellular automaton investigation of the transformation from austenite to ferrite during continuous cooling

A cellular automaton model has been employed to investigate the transformation from austenite to ferrite in low carbon steels during continuous cooling. An important aspect of this approach is the implementation of incorporating local concentration changes into a nucleation or growth function, which is utilized by the automaton in a probabilistic fashion. The modeling gives a visual insight into the effect of cooling conditions on this transformation. The final nucleation number, the number of ferrite grains per austenite grain, ferrite grain size and the kinetics of ferrite formation are obtained as a function of the cooling rate or the undercooling temperature

Random Lasing Action from Randomly Assembled ZnS Nanosheets

Lasing characteristics of randomly assembled ZnS nanosheets are studied at room temperature. Under 266-nm optical excitation, sharp lasing peaks emitted at around 332 nm with a linewidth less than 0.4 nm are observed in all directions. In addition, the dependence of lasing threshold intensity with the excitation area is shown in good agreement with the random laser theory. Hence, it is verified that the lasing characteristics of randomly assembled ZnS nanosheets are attributed to coherent random lasing action.

X-ray photoelectron spectroscopy of GaSb nanoparticles embedded in SiO2 matrices by radio-frequency magnetron co-sputtering

GaSb-SiO2 composite films were fabricated by radio-frequency magnetron co-sputtering. Transmission electron microscope (TEM) results exhibit that the GaSb nanoparticles were uniformly dispersed in SiO2 matrices. The sizes of nanoparticles are in the range of 4-10 nm. X-ray photoelectron spectroscopy (XPS) analysis confirms the high purity of GaSb nanoparticles contained in the SiO2 matrices and shows that the GaSb and SiO2 exist in nonstoichiometry in the composite films due to the strong affinity of Sb and Si with oxygen. Room temperature optical transmission spectra show a very large blue shift of about 2.76 eV with respect to that of bulk GaSb, suggesting the existence of quantum size effects.

Anomalous photoluminescence in InP1-xBix

Low temperature photoluminescence (PL) from InP1−xBix thin films with Bi concentrations in the 0–2.49% range reveals anomalous spectral features with strong and very broad (linewidth of 700 nm) PL signals compared to other bismide alloys. Multiple transitions are observed and their energy levels are found much smaller than the band-gap measured from absorption measurements. These transitions are related to deep levels confirmed by deep level transient spectroscopy, which effectively trap free holes and enhance radiative recombination. The broad luminescence feature is beneficial for making super-luminescence diodes, which can theoretically enhance spatial resolution beyond 1 μm in optical coherent tomography (OCT).

Bi-induced acceptor level responsible for partial compensation of native free electron density in InP1-xBix dilute bismide alloys

Deep level transient spectroscopy (DLTS) has been applied to study electron and hole traps in InPBi alloys with 2.2 and 2.4% Bi grown by molecular beam epitaxy. One donor-like trap with the activation energy of 0.45-0.47 eV and one acceptor-like trap with activation energy of 0.08 eV have been identified in DLTS measurements. For the reference sample (InP grown at the same temperature), the deep donor trap has also been observed, while the acceptor trap was not detected. According to the literature, the deep donor level found in InP(Bi) at 0.45-0.47 eV below the conduction band has been attributed to the isolated P-In defect, while the second trap, which is observed only for Bi containing samples at 0.08 eV above the valence band can be attributed to Bi clusters in InPBi. This acceptor level was proposed to be responsible for the observed partial compensation of native free electron density in InPBi layers. It is also shown that the deep donor traps are active in photoluminescence (PL). A strong radiative recombination between donor traps and the valence band are observed in PL spectra at energy 0.6-0.8 eV, i.e. similar to 0.47 eV below the energy gap of InPBi, which is determined by contactless electroreflectance.

Novel dilute InPBi for IR emitters

InPBi crystalline thin films with a bismuth concentration up to 4.8% have been successfully grown using molecular beam epitaxy for the first time. This novel material reveals strong and broad photoluminescence in the wavelength range of 1-2.5 μm at room temperature, although the absorption measurements point out a near band-gap absorption character. Various structural and optical characterization techniques are used to assess material quality and to understand the physical origins of the unexpected light emission. The InPBi is almost lattice matched to InP, making such a material very promising for InP based optoelectronics devices. The emitted light covers the telecom wavelength regime as well as other important wavelengths for gas sensing. The very broad emission spectrum of more than 600 nm promises for making super-luminescence IR diodes that have potentials to significantly enhance the spatial resolution in optical coherence tomography (OCT).

Optical properties and band bending of InGaAs/GaAsBi/InGaAs type-II quantum well grown by gas source molecular beam epitaxy

Photoluminescence (PL) properties of In0.2Ga0.8As/GaAs0.96Bi0.04/In0.2Ga0.8As quantum well (QW) grown on GaAs substrates by gas source molecular beam epitaxy were studied by varying excitation power and temperature, respectively. The type-II transition energy shifts from 1.149 eV to 1.192 eV when increasing the excitation power from 10 mW to 150 mW at 4.5 K, which was ascribed to the band-bending effect. On the other hand, the type-II PL quenches quickly along with fast redshift with the increasing temperature due to the relaxation of the band bending caused by the thermal excitation process. An 8 band k.p model was used to analyze the electronic properties and the band-bending effect in the type-II QW. The calculated subband levels and transition energy fit well with the experiment results, and two thermal activation energies of 8.7 meV and 50 meV, respectively, are deduced. Published by AIP Publishing.

Photoluminescence of InGaAs/GaAsBi/InGaAs type-II quantum wells grown by gas source molecular beam epitaxy

InxGa1-xAs/GaAs1-yBiy/InxGa1-xAs (0.20 ≤x ≤0.22, 0.035 ≤y ≤0.045) quantum wells (QWs) were grown on GaAs substrates by gas source molecular beam epitaxy for realizing the type-II band edge line-up. Both type-I and type-II transitions were observed in the Bi containing W QWs and the photoluminescence intensity was enhanced in the sample with a high Bi content, which is mainly due to the improvement of carrier confinement. The 8 band k • p model was used to analyze the electronic properties in the QWs and the calculated transition energies fit well with the experiment results. Our study shows that the proposed type-II QW is a promising candidate for realizing GaAs-based near infrared light emitting devices near 1.3 μm

Bismuth-content-dependent polarized Raman spectrum of InPBi alloy

We systematically investigate the optical properties of the InP1−x Bi x ternary alloys with 0 ≤ x ≤ 2.46%, by using high resolution polarized Raman scattering measurement. Both InP-like and InBi-like optical vibration modes (LO) are identified in all the samples, suggesting that most of the Bi-atoms are incorporated into the lattice sites to substitute P-atoms. And the intensity of the InBi-like Raman mode is positively proportional to the Bi-content. Linear red-shift of the InP-like longitudinal optical vibration mode is observed to be 1.1 cm−1/Bi%, while that of the InP-like optical vibration overtone (2LO) is nearly doubled. In addition, through comparing the Z(XX)Z and Z(XY)Z Raman spectra, longitudinal-optical-plasmon-coupled (LOPC) modes are identified in all the samples, and their intensities are found to be proportional to the electron concentrations.