Qike Jiang

Dynamics of bound exciton complexes in CdS nanobelts.

Intrinsic defects such as vacancies, interstitials, and anti-sites often introduce rich luminescent properties in II-VI semiconductor nanomaterials. A clear understanding of the dynamics of the defect-related excitons is particularly important for the design and optimization of nanoscale optoelectronic devices. In this paper, low-temperature steady-state and time-resolved photoluminescence (PL) spectroscopies have been carried out to investigate the emission of cadmium sulfide (CdS) nanobelts that originates from the radiative recombination of excitons bound to neutral donors (I(2)) and the spatially localized donor-acceptor pairs (DAP), in which the assignment is supported by first principle calculations. Our results verify that the shallow donors in CdS are contributed by sulfur vacancies while the acceptors are contributed by cadmium vacancies. At high excitation intensities, the DAP emission saturates and the PL is dominated by I(2) emission. Beyond a threshold power of approximately 5 μW, amplified spontaneous emission (ASE) of I(2) occurs. Further analysis shows that these intrinsic defects created long-lived (spin triplet) DAP trap states due to spin-polarized Cd vacancies which become saturated at intense carrier excitations.

Layered Bi2Se3 Nanoplate/Polyvinylidene Fluoride Composite Based n-type Thermoelectric Fabrics

In this study, we report the fabrication of n-type flexible thermoelectric fabrics using layered Bi2Se3 nanoplate/polyvinylidene fluoride (PVDF) composites as the thermoelectric material. These composites exhibit room temperature Seebeck coefficient and electrical conductivity values of -80 μV K(-1) and 5100 S m(-1), respectively, resulting in a power factor approaching 30 μW m(-1)K(-2). The temperature-dependent thermoelectric properties reveal that the composites exhibit metallic-like electrical conductivity, whereas the thermoelectric power is characterized by a heterogeneous model. These composites have the potential to be used in atypical applications for thermoelectrics, where lightweight and flexible materials would be beneficial. Indeed, bending tests revealed excellent durability of the thermoelectric fabrics. We anticipate that this work may guide the way for fabricating high performance thermoelectric fabrics based on layered V-VI nanoplates.

A ZnO/ZnMgO Multiple-Quantum-Well Ultraviolet Random Laser Diode

A ZnO/ZnMgO multiple-quantum-well ultraviolet (UV) random laser diode was fabricated on a commercially available n-type GaN wafer using a radio frequency magnetron sputtering system. The electroluminescence measurements revealed that the diode exhibited fairly pure UV random lasing centered at ~370 nm under sufficient forward bias at room temperature. The full-widths at half-maximum of the sharp lasing peaks are less than 0.4 nm. The device has a very low threshold current density of 4.7 A/cm2 and extremely weak visible emission.

Ultraviolet/orange bicolor electroluminescence from an n-ZnO/n-GaN isotype heterojunction light emitting diode

We fabricate an ultraviolet (UV)/orange bicolor light emitting diode (LED) based on an n-ZnO/n-GaN isotype heterojunction, which presents a sharp ultraviolet emission centered at 367 nm and a broad orange emission centered at 640 nm under forward and reverse biases, respectively. Time dependence electroluminescence (EL) measurements reveal that this device shows good stability. The electroluminescence mechanism of the bicolor light emitting diode is discussed in terms of the material properties of the interfacial layer and the luminescence properties of the device in this work.

Nanointerlayer Induced Electroluminescence Transition from Ultraviolet- to Red-Dominant Mode for n-Mn:ZnO/N-GaN Heterojunction

High-quality Mn:ZnO (MZO) film had been prepared on N-GaN coated sapphire substrates followed by postdeposition thermal annealing treatment at 700 °C. For the annealed MZO/GaN heterojunction, a 15 nm cubic structural ZnGa(2)O(4) layer was observed at the MZO/GaN interface through transmission electron microscope analysis. Through electroluminescence (EL) measurement, the formation of the nanointerface results in an EL transition from ultraviolet- to red-dominant mode for n-Mn:ZnO/N-GaN heterojunction light-emitting diodes (LEDs). The heterojunction LED showed a rectification ratio of ∼2.0 × 10(5) at ±2 V, a dark current of 3.5 nA at -2 V and a quite strong red EL with a low turn-on voltage of 3 V. On the basis of the energy band diagram, we think the EL transition from ultraviolet- to red-dominant mode is mainly due to the formation of a thin oxide blocking nanolayer at the MZO/GaN interface during the annealing process.

Structural and Optical Characterization of ZnO/MgxZn1–xO Multiple Quantum Wells Based Random Laser Diodes

Two kinds of laser diodes (LDs) comprised of ZnO/Mg(x)Zn(1-x)O (ZnO/MZO) multiple quantum wells (MQWs) grown on GaN (MQWs/GaN) and Si (MQWs/Si) substrates, respectively, have been constructed. The LD with MQWs/GaN exhibits ultraviolet random lasing under electrical excitation, while that with MQWs/Si does not. In the MQWs/Si, ZnO/MZO MQWs consist of nanoscaled crystallites, and the MZO layers undergo a phase separation of cubic MgO and hexagonal ZnO. Moreover, the Mg atom predominantly locates in the MZO layers along with a significant aggregation at the ZnO/MZO interfaces; in sharp contrast, the ZnO/MZO MQWs in the MQWs/GaN show a well-crystallized structure with epitaxial relationships among GaN, MZO, and ZnO. Notably, Mg is observed to diffuse into the ZnO well layers. The structure-optical property relationship of these two LDs is further discussed.