Zhipeng Wei

Room temperature p-n ZnO blue-violet light-emitting diodes

ZnO p-n junction light-emitting diodes (LEDs) were fabricated on c-plane Al2O3 substrates by plasma-assisted molecular beam epitaxy. Gas mixture of N2 and O2 was used as the p-type dopant, by which the double-donor doping of N2(O) can be avoided significantly. The fabricated p-type ZnO layers have a higher hole density and carrier mobility. The LEDs showed a very good rectification characteristic with a low threshold voltage of 4.0V even at a temperature above 300K. The LEDs can even emit intensive electroluminescence in the blue-violet region at the temperature of 350K. The blue-violet emission was attributed to the donor-acceptor pair recombination at the p-type layer of the LED.

Formation of p-type MgZnO by nitrogen doping

A wurtzite N-doped MgZnO film with 20at.% Mg (MgZnO:N) was grown by plasma-assisted molecular beam epitaxy on c-plane sapphire using radical NO as oxygen source and nitrogen dopant. The as-grown MgZnO:N film behaves n-type conduction at room temperature, but transforms into p-type conduction after annealed for 1h at 600°C in an O2 flow. The p-type MgZnO:N has a hole concentration of 6.1×1017cm−3 and a mobility of 6.42cm2∕Vs. X-ray photoelectron spectroscopy measurement indicates that substitution of N for O site is in forms of N atom (N)O and N molecule (N2)O for the as-grown MgZnO:N, but almost only in a form of (N)O for the annealed MgZnO:N. The mechanism of the conduction-type transition induced by annealing is discussed in the present work.

Influence of Exciton Localization on the Emission and Ultraviolet Photoresponse of ZnO/ZnS Core-Shell Nanowires.

The structural and optical properties of ZnO and ZnO/ZnS core-shell nanowires grown by a wet chemical method are investigated. The near-bandgap ultraviolet (UV) emission of the ZnO nanowires was enhanced by four times after coating with ZnS. The enhanced emission was attributed to surface passivation of the ZnO nanowires and localized states introduced during ZnS growth. The emission of the ZnO and ZnO/ZnS core-shell nanowires was attributed to neutral donor-bound excitons and localized excitons, respectively. Localized states prevented excitons from diffusing to nonradiative recombination centers, so therefore contributed to the enhanced emission. Emission from the localized exciton was not sensitive to temperature, so emission from the ZnO/ZnS core-shell nanowires was more stable at higher temperature. UV photodetectors based on the ZnO and ZnO/ZnS core-shell nanowires were fabricated. Under UV excitation, the device based on the ZnO/ZnS core-shell nanowires exhibited a photocurrent approximately 40 times higher than that of the device based on the ZnO nanowires. The differing photoresponse of the detectors was consistent with the existence of surface passivation and localized states. This study provides a means for modifying the optical properties of ZnO materials, and demonstrates the potential of ZnO/ZnS core-shell nanowires in UV excitonic emission and detection.

Ultraviolet Electroluminescence from ZnS@ZnO Core-Shell Nanowires/p-GaN Introduced by Exciton Localization.

We investigate the electroluminescence (EL) from light emitting diodes (LEDs) of ZnO nanowires/p-GaN structure and ZnS@ZnO core-shell nanowires/p-GaN structure. With the increase of forward bias, the emission peak of ZnO nanowires/p-GaN structure heterojunction shows a blue-shift, while the ZnS@ZnO core-shell nanowires/p-GaN structure demonstrates a changing EL emission; the ultraviolet (UV) emission at 378 nm can be observed. This discrepancy is related to the localized states introduced by ZnS particles, which results in a different carrier recombination process near the interfaces of the heterojunction. The localized states capture the carriers in ZnO nanowires and convert them to localized excitons under high forward bias. A strong UV emission due to localized excitons can be observed. Our results indicated that utilizing localized excitons should be a new route toward ZnO-based ultraviolet LEDs with high efficiency.

Alternative Spectral Photoresponse in a p-Cu2ZnSnS4/n-GaN Heterojunction Photodiode by Modulating Applied Voltage.

We report alternative visible and ultraviolet light response spectra in a p-Cu2ZnSnS4 (p-CZTS)/n-GaN heterojunction photodiode. A CZTS film was deposited on an n-GaN/sapphire substrate using a magnetron sputtering method. Current-voltage characteristic of the p-CZTS/n-GaN heterojunction photodiode showed a good rectifying behavior. The spectral response measurements indicate that the response wavelength of the photodiode can be tuned from ultraviolet to visible regions via applying zero and reverse bias. A band alignment at the interface of the p-CZTS/n-GaN heterojunction was proposed to interpret the spectral response of the device.

Optical and electrical properties of individual p-type ZnO microbelts with Ag dopant

P-type ZnO microbelts were successfully synthesized by thermal diffusion of Ag in a step by step thermal annealing process. The effect of the annealing temperature on the room temperature photoluminescence (RT-PL) and current versus voltage (I–V) properties of Ag-doped ZnO microbelts were studied. With the increase of the annealing temperature, the deep-level emission (506 nm) decreased and the AgZn related violet emission peak at 405 nm was observed. The substitution of Zn atoms by Ag atoms is unstable at high temperature, which might be a limitation for Ag as the dopant of ZnO. In our experiment, the optimal annealing temperature is 250 °C, at which the strong neutral acceptor bound exciton (A0X) peak appeared in the low temperature-PL spectrum. Correspondingly, the conductivity type of the ZnO microbelt transformed from n to p. The symmetrical I–V curves exhibited slightly nonlinear behaviors. The result indicated the existence of Schottky barriers between the individual Ag-doped ZnO microbelt and the In electrodes.

Investigation of Localized States in GaAsSb Epilayers Grown by Molecular Beam Epitaxy

We report the carrier dynamics in GaAsSb ternary alloy grown by molecular beam epitaxy through comprehensive spectroscopic characterization over a wide temperature range. A detailed analysis of the experimental data reveals a complex carrier relaxation process involving both localized and delocalized states. At low temperature, the localized degree shows linear relationship with the increase of Sb component. The existence of localized states is also confirmed by the temperature dependence of peak position and band width of the emission. At temperature higher than 60 K, emissions related to localized states are quenched while the band to band transition dominates the whole spectrum. This study indicates that the localized states are related to the Sb component in the GaAsSb alloy, while it leads to the poor crystal quality of the material, and the application of GaAsSb alloy would be limited by this deterioration.

Zn or O? An Atomic Level Comparison on Antibacterial Activities of Zinc Oxides.

For the first time, the influence of different types of atoms (Zn and O) on the antibacterial activities of nanosized ZnO was quantitatively evaluated with the aid of a 3D-printing-manufactured evaluation system. Two different outermost atomic layers were manufactured separately by using an ALD (atomic layer deposition) method. Interestingly, we found that each outermost atomic layer exhibited certain differences against gram-positive or gram-negative bacterial species. Zinc atoms as outermost layer (ZnO-Zn) showed a more pronounced antibacterial effect towards gram-negative E. coli (Escherichia coli), whereas oxygen atoms (ZnO-O) showed a stronger antibacterial activity against gram-positive S. aureus (Staphylococcus aureus). A possible antibacterial mechanism has been comprehensively discussed from different perspectives, including Zn(2+) concentrations, oxygen vacancies, photocatalytic activities and the DNA structural characteristics of different bacterial species.

Pressure and temperature-dependent Raman spectra of MoS2 film

Molybdenum disulfide (MoS2), a relatively new and exciting two-dimensional graphene-like material, has been attracting more and more attentions from the researchers due to its unique structural and fascinating properties. The potential application of MoS2 under high-pressure and low-temperature is expected, while the related research is few at present. In this paper, quadrilayer MoS2 was synthesized by chemical vapor deposition, and its structural properties under different pressures (0–20.7 GPa) and temperatures (10–300 K) were investigated via the Raman spectra. We find that the lattice of quadrilayer MoS2 is not damaged and the quadrilayer MoS2 exhibits good semiconductive properties under large variable pressures from atmospheric to 20.7 GPa, which is much different to its bulk and single crystalline phases. In addition, the lattice structures of the quadrilayer MoS2 are stable in 10–300 K, and the Gruneisen parameters of E12g and A1g modes are smaller than that of bulk. This study indicates that quad...

Preparation, Applications of Two-Dimensional Graphene-like Molybdenum Disulfide

Two-dimensional graphene-like molybdenum disulfide, a relatively new and exciting transition metal dichalcogenides, has a number of unique properties which have been widely studied. The structure, property and preparation methods of molybdenum disulfide are introduced in this paper in detail. In order to improve performance of molybdenum disulfide, some researchers start paying close attention to composite and decorating about molybdenum disulfide, such as graphene-MoS2, metal-MoS2, which is not mentioned in other review papers. In addition, we systematically review the application of molybdenum disulfide, which included field effect transistor, phototransistors, gas detector, digital logic transistors and biosensor. At last, we prospect for future advances in methods for preparing atomically thin layers and devices.