Jijun Ding

Enhancement of Field Emission and Photoluminescence Properties of Graphene-SnO2 Composite Nanostructures

In this study, the SnO(2) nanostructures and graphene-SnO(2) (G-SnO(2)) composite nanostructures were prepared on n-Si (100) substrates by electrophoretic deposition and magnetron sputtering techniques. The field emission of SnO(2) nanostructures is improved largely by depositing graphene buffer layer, and the field emission of G-SnO(2) composite nanostructures can also further be improved by decreasing sputtering time of Sn nanoparticles to 5 min. The photoluminescence (PL) spectra of the SnO(2) nanostructures revealed multipeaks, which are consistent with previous reports except for a new peak at 422 nm. Intensity of six emission peaks increased after depositing graphene buffer layer. Our results indicated that graphene can also be used as buffer layer acting as interface modification to simultaneity improve the field emission and PL properties of SnO(2) nanostructures effectively.

Blue-green emission mechanism and spectral shift of Al-doped ZnO films related to defect levels

The crystal structure, surface morphology, chemical state and optical properties of Al-doped ZnO films grown at different sputtering powers are studied. Results indicated that compressive stress related to defects exists in all the samples measured by X-ray diffraction. Blue-green emission mechanisms and a blue-shift were explored based on defects sites. The peak at 458 nm comes from the electron transition from interstitial Zn to the top of the valence band and transition from the conduction band to misplaced oxygen defects. The peak at 490 nm comes from the electron transition from interstitial Zn to Zn vacancies or interstitial O and transition from a complex defect level of O vacancies and interstitial Zn to the valence band. The existence of compressive stress related to the defects in Al-doped ZnO plays a significant role in the blue-green emission and blue-shift.

Semi-Transparent ZnO-CuI/CuSCN Photodiode Detector with Narrow-Band UV Photoresponse.

UNLABELLED The ZnO homogeneous pn junction photodiode is quite difficult to fabricate due to the absence of stable p-type ZnO. So exploring reliable p-type materials is necessary to build a heterogeneous pn junction with n-type ZnO. Herein, we develop a simple and low-cost solution-processed method to obtain inorganic p-type CuI/CuSCN composite film with compact morphology, high conductivity, and low surface state. The improved performance of CuI/CuSCN composite film can be confirmed based on high-rectification ratio, responsivity, and open voltage of ZnO-CuI/CuSCN photodiode UV detectors. Moreover, photodiodes with novel top electrodes are investigated. Compared with commonly used Au and graphene/Ag nanowire (NWs) electrode, poly(3,4-ethylenedioxythiophene):poly(4-styrenesulfonate) ( PEDOT PSS) electrode prepared by Meyer rod-coating technique opens one route to obtain a semitransparent photodiode. The photodiode with PEDOT PSS as the top electrode under reverse illumination has the highest photocurrent density due to higher UV transmittance of PEDOT PSS transparent electrode compared with ITO glass. The low-energy consumption, and high responsivity, UV to visible rejection ratio and air stability make this ZnO-CuI/CuSCN photodiode quite promising in the UV-A detection field.

Microstructures, surface states and field emission mechanism of graphene–tin/tin oxide hybrids

The effects of microstructures and surface states on the field emission, which are important to a good understanding of the field emission mechanism, are unclear. In this paper, the microstructures and surface states of graphene-Sn/SnO2 hybrids were analyzed, and the field emission mechanism was explored. Raman spectra and images revealed that SnO2/Sn droplets are strongly bound on graphene surface, and there exist oxygen vacancies at the surface of graphene-Sn/SnO2 hybrids. Among X-ray photoelectron spectroscopy spectra, the peak of O 1s shifts 1.6 eV toward higher binding energies in the 5 min sample with the best field emission properties, which indicates that the field emission improvement in graphene-Sn/SnO2 hybrids arises from the band-bending effect and a lower work function.

Field emission mechanism insights of graphene decorated with ZnO nanoparticles

Graphene decorated with ZnO nanoparticles (G–ZnO hybrids) were prepared on n-Si substrates. The microstructures and chemical states of these hybrids were analyzed. Based on these experimental results, a field emission enhanced mechanism of the G–ZnO hybrids related to stable oxygen groups in the form of cyclic edge ethers is proposed. G–ZnO hybrids consist of a higher ratio of C–O–C chains than in graphene structures. This causes the potential barrier the electrons have to overcome in vacuum to be further diminished, resulting in a lower work function. Consequently, both the potential barrier height and width for the electrons emission are further reduced. So electrons can easily tunnel through the full barrier width, causing a larger field emission current in the G–ZnO hybrids. Such insight into the C–O–C chains from the G–ZnO hybrids offers the prospect for understanding the physical mechanism and future novel applications as field emitters.

The novel upconversion properties of LiYbF4:Er microcrystals compared to the Na counterpart

Erbium-doped ytterbium fluoride compounds with different crystal phases and morphologies have been synthesized via a facile hydrothermal route assisted with EDTA. Tunable upconversion emissions can be obtained by replacing Na+ with Li+ in NaYbF4 microcrystals. An interesting blue shift is observed and a mechanism based on a weakened polarization effect is proposed.

Shape-selective synthesis, characterization and upconversion improvement of Yb3+/Er3+ doped LiYF4 microphosphors through pH tuning

Yttrium fluoride compounds with different crystal phases and morphologies, including rice-like orthorhombic-phase YF3:Yb3+/Er3+, and octahedral and quasi-spherical tetragonal-phase LiYF4:Yb3+/Er3+ microcrystals have been synthesized under hydrothermal conditions by tuning the pH value of the mother solution. X-ray diffraction, scanning electron microscopy, transmission electron microscopy, Fourier transform infrared spectroscopy, and photoluminescence spectra were used to characterize the samples. It is found that the pH value in the initial reaction solution has significant effects on selective adsorption of the organic additive EDTA on different crystal facets controlling the final sizes and geometries of the particles. Meanwhile the pH value in conjunction with hydrothermal reaction time affects the solubility constants and thermodynamic stability of reactants and products, further triggering the phase transformation from orthorhombic-phase YF3 to tetragonal-phase LiYF4 and controlling the nucleation and anisotropic growth of fluoride microcrystals. The possible formation mechanisms for products with various architectures have been presented. The multicolor upconversion (UC) luminescence was successfully realized in a series of Yb3+/Er3+ codoped fluoride microcrystals by excitation in the NIR region, and the UC emissions were investigated as a function of the pH value. Results show that the UC luminescence of microparticles sharply increases with pH elevation. The enhancement was attributed to Li+ interstitials in the channels of LiYF4, which is consistent with the growth mechanism of LiYF4 microcrystals, while the possibility of doping Li+ in the cation vacancies of LiYF4 cell cannot be fully excluded because of the presence of the cation defects. The study further demonstrates that the pH tuning approach can be used as an efficient tool to control the crystal phase, size and the improvement of luminescence emission for fluoride microcrystals.

Employing the plasmonic effect of the Ag–graphene composite for enhancing light harvesting and photoluminescence quenching efficiency of poly[2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylene-vinylene]

In this work, we report that the Ag-graphene composite (AGC) can effectively enhance the light harvesting and photoluminescence (PL) quenching efficiency of poly[2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylene-vinylene] (MEH-PPV). Loading the AGC on MEH-PPV leads to improved light absorption ability and PL quenching efficiency, which is due to the strong interaction between localized surface plasmon resonance (LSPR)-activated Ag nanoparticles and the MEH-PPV molecule. Control experiment reveals that the combination of graphene and Ag nanoparticles achieves superior light absorptivity and PL quenching ability compared with individual graphene and Ag NPs. The exponential shape of the Stern-Volmer plot implies that both Ag and graphene in the AGC can offer the quenching pathway for the PL quenching process. We also found that the AGC with a broader LSPR absorption range is competitive in enhancing the light absorption ability and PL quenching efficiency of the MEH-PPV-AGC composite, because it can expand LSPR-induced light harvesting and PL quenching response to a wider absorption range.

Photoluminescence investigation about zinc oxide with graphene oxide & reduced graphene oxide buffer layers

ZnO with graphene oxide (GO-ZnO) & reduced graphene oxide (rGO-ZnO) buffer layers were fabricated. Photoluminescence (PL) properties of GO-ZnO and rGO-ZnO compositions induced by oxygen vacancies defects were investigated using photoluminescence spectroscopy. The results showed that blue emission is quenched while yellow-orange emissions from GO-ZnO and rGO-ZnO compositions are significantly increased as compared to that of ZnO films. In stark contrast to enhanced yellow-orange emissions, PL spectra show three sharp, discrete emissions that characterize the dominant optical active defect, which is the oxygen vacancies and extended oxygen vacancies. Our results highlight the ability of GO & rGO buffer layers to modulate defect concentrations in ZnO and contribute to understanding the optical properties of deep-level defects, which is significant for development of long-wavelength photoelectric devices related with graphene materials.

Physical deoxygenation of graphene oxide paper surface and facile in situ synthesis of graphene based ZnO films

In-situ sputtering ZnO films on graphene oxide (GO) paper are used to fabricate graphene based ZnO films. Crystal structure and surface chemical states are investigated. Results indicated that GO paper can be effectively deoxygenated by in-situ sputtering ZnO on them without adding any reducing agent. Based on the principle of radio frequency magnetron sputtering, we propose that during magnetron sputtering process, plasma streams contain large numbers of electrons. These electrons not only collide with argon atoms to produce secondary electrons but also they are accelerated to bombard the substrates (GO paper) resulting in effective deoxygenation of oxygen-containing functional groups. In-situ sputtering ZnO films on GO paper provide an approach to design graphene-semiconductor nanocomposites.