Disentangling the Role of the SnO Layer on the Pyro-Phototronic Effect in ZnO-Based Self-Powered Photodetectors.

Self-powered photodetectors (PDs) have been recognized as one of the developing trends of next-generation optoelectronic devices. Herein, it is shown that by introducing a thin layer of SnO film between the Si substrate and the ZnO film, the self-powered photodetector Al/Si/SnO/ZnO/ITO exhibits a stable and uniform violet sensing ability with high photoresponsivity and fast response. The SnO layer introduces a built-in electrostatic field to highly enhance the photocurrent by over 1000%. By analyzing energy diagrams of the p-n junction, the underlying physical mechanism of the self-powered violet PDs is carefully illustrated. A high photo-responsivity (R) of 93 mA W-1 accompanied by a detectivity (D*) of 3.1 × 1010 Jones are observed under self-driven conditions, when the device is exposed to 405 nm excitation laser wavelength, with a laser power density of 36 mW cm-2 and at a chopper frequency of 400 Hz. The Si/SnO/ZnO/ITO device shows an enhancement of 3067% in responsivity when compared to the Al/Si/ZnO/ITO. The photodetector holds an ultra-fast response of ≈ 2 µs, which is among the best self-powered photodetectors reported in the literature based on ZnO.

Hierarchical Self-Assembly of Dipolar ZnO Nanoparticles and Microdroplets.

In this work, we investigated the orientation and the polarization of ZnO nanoparticles, which serve as building blocks of highly monodisperse microspheres, using a droplet microfluidic-assisted synthesis method. We observe, for the first time, a square lattice organization of liquid microdroplets, in a steady state, at the oil/water interface. Such square organization reveals clearly a dipolar organization of ZnO nanoparticles at the surfaces of droplets at the early stage of ZnO nanocrystal aggregation and microsphere formation. We discuss different models of organization of ZnO nanoparticles and show that the well-known tip-streaming effect in droplets in microfluidics explains the reason for the obtained dipolar droplets. The square organization is illustrated and explained.

Effect of ZnO surface morphology on its electrochemical performance.

The purpose of this paper is to bridge the gap between ZnO surface morphology and its electrochemical performance. For this reason, ZnO nanowires (NWs) of different length were synthesized using an electrochemical method. Then, the electrochemical performance of the synthesized ZnO surfaces was studied using cyclic voltammetry and electrochemical impedance spectroscopy. The electrochemical analysis results revealed that the increase of ZnO NW length contributes to the retrogression of electrochemical performance. Indeed, the electrochemical performance is mainly related to the wettability behavior of the ZnO nanowire surfaces. When the ZnO NWs length increases, the surface become more hydrophobic, therefore, charge transfers between the electrode/electrolyte decrease. To improve the electrochemical performance of ZnO, we propose a new strategy combining NWs and microsheets (µSs) for further improving the morphology. Finally, the surfaces based on the double structure of ZnO provide good propagation of charge at the surface, good transfer in the electrode, good stability, and excellent scanning ability. In the present work we intend to pave the way for achieving high electrochemical performance ZnO-based layers.

Rhodamine B Doped ZnO Monodisperse Microcapsules: Droplet-Based Synthesis, Dynamics and Self-Organization of ZnO Nanoparticles and Dye Molecules.

In the present work, droplet-based microfluidics and sol-gel techniques were combined to synthesize highly monodisperse zinc oxide (ZnO) microspheres, which can be doped easily and precisely with dyes, such as rhodamine B (RhB), and whose size can be finely tuned in the 10-30 µm range. The as-synthesized microparticles were analyzed by scanning electron microscopy (SEM), transmission electron microscopy (TEM), and confocal microscopy. The results reveal that the microspheres exhibit an excellent size monodispersity, hollow feature, and a porous shell with a thickness of about 0.6 µm, in good agreement with our calculations. We show in particular by means of fluorescence recovery after photobleaching (FRAP) analysis that the electric charges carried by ZnO nanoparticles primary units play a crucial role not just in the formation and structure of the synthesized ZnO microcapsules, but also in the confinement of dye molecules inside the microcapsules despite a demonstrated porosity of their shell in regards to the solvent (oil). Our results enable also the measurement of the diffusion coefficient of RhB molecules inside the microcapsules (DRhB=3.8×10-8 cm2/s), which is found two order of magnitude smaller than the literature value. We attribute such feature to a strong interaction between dye molecules and the electrical charges carried by ZnO nanoparticles. These results are important for potential applications in micro-thermometry (as shown recently in our previous study), photovoltaics, or photonics such as whispering gallery mode resonances.

Dye-Doped ZnO Microcapsules for High Throughput and Sensitive Optofluidic Micro-Thermometry.

The main objective of this work is to show the proof of concept of a new optofluidic method for high throughput fluorescence-based thermometry, which enables the measure of temperature inside optofluidic microsystems at the millisecond (ms) time scale (high throughput). We used droplet microfluidics to produce highly monodisperse microspheres from dispersed zinc oxide (ZnO) nanocrystals and doped them with rhodamine B (RhB) or/and rhodamine 6G (Rh6G). The fluorescence intensities of these two dyes are known to depend linearly on temperature but in two opposite manner. Their mixture enables for the construction of reference probe whose fluorescence does not depend practically on temperature. The use of zinc oxide microparticles as temperature probes in microfluidic channels has two main advantages: (i) avoid the diffusion and the adsorption of the dyes inside the walls of the microfluidic channels and (ii) enhance dissipation of the heat generated by the focused incident laser beam thanks to the high thermal conductivity of this material. Our results show that the fluorescence intensity of RhB decreases linearly with increasing temperature at a rate of about -2.2%/°C, in a very good agreement with the literature. In contrast, we observed for the first time a nonlinear change of the fluorescence intensity of Rh6G in ZnO microparticles with a minimum intensity at a temperature equal to 40 °C. This behaviour is reproducible and was observed only with ZnO microparticles doped with Rh6G.

Wettability of zinc oxide nanorod surfaces.

In this work, we have studied the wettability of zinc oxide (ZnO) nanorods grown on fluorine-doped tin oxide (FTO) by highlighting the effect of polar and non-polar ZnO facets on contact angle (CA) results. The variation in the wettability behaviors of the synthesized surfaces is mainly related to physical and chemical surface texturing which influenced the liquid drop penetration. Indeed, three main penetration states can be deduced: total, partial, and null-penetration. Where, low CA (100.9°) with high contact angle hysteresis (CAH) (13°) is observed for total penetration of the liquid drop. While, high CA (139.6°) with low CAH (7°) is observed for null-penetration. Moreover, we have found that the chemical texturing of ZnO, especially the hydrophilicity of ZnO tips, responsible for liquid drop sticking, prevents the liquid slipping over the surface. In order to promote the liquid rolling on the ZnO surface, we reported the physical modification of the ZnO structures. Therefore, the rolling of the liquid drop on the inclined surface of ZnO is achieved by using a new structure based on double scale roughness. This surface exhibits superhydrophobic behavior with a CA of 153° and CAH of 3°.

Low-temperature fabrication of layered self-organized Ge clusters by RF-sputtering.

In this article, we present an investigation of (Ge + SiO2)/SiO2 multilayers deposited by magnetron sputtering and subsequently annealed at different temperatures. The structural properties were investigated by transmission electron microscopy, grazing incidence small angles X-ray scattering, Rutherford backscattering spectrometry, Raman, and X-ray photoelectron spectroscopies. We show a formation of self-assembled Ge clusters during the deposition at 250°C. The clusters are ordered in a three-dimensional lattice, and they have very small sizes (about 3 nm) and narrow size distribution. The crystallization of the clusters was achieved at annealing temperature of 700°C.