Romain Parize

Physical Properties of Annealed ZnO Nanowire/CuSCN Heterojunctions for Self-Powered UV Photodetectors

The low-cost fabrication of ZnO nanowire/CuSCN heterojunctions is demonstrated by combining chemical bath deposition with impregnation techniques. The ZnO nanowire arrays are completely filled by the CuSCN layer from their bottoms to their tops. The CuSCN layer is formed of columnar grains that are strongly oriented along the [003] direction owing to the polymeric form of the β-rhombohedral crystalline phase. Importantly, an annealing step is found essential in a fairly narrow range of low temperatures, not only for outgassing the solvent from the CuSCN layer, but also for reducing the density of interfacial defects. The resulting electrical properties of annealed ZnO nanowire/CuSCN heterojunctions are strongly improved: a maximum rectification ratio of 2644 at ±2 V is achieved following annealing at 150 °C under air atmosphere, which is related to a strong decrease in the reverse current density. Interestingly, the corresponding self-powered UV photodetectors exhibit a responsivity of 0.02 A/W at zero bias and at 370 nm with a UV-to-visible (370-500 nm) rejection ratio of 100 under an irradiance of 100 mW/cm(2). The UV selectivity at 370 nm can also be readily modulated by tuning the length of ZnO nanowires. Eventually, a significant photovoltaic effect is revealed for this type of heterojunctions, leading to an open circuit voltage of 37 mV and a short circuit current density of 51 μA/cm(2), which may be useful for the self-powering of the complete device. These findings show the underlying physical mechanisms at work in ZnO nanowire/CuSCN heterojunctions and reveal their high potential as self-powered UV photodetectors.

Highly crystalline urchin-like structures made of ultra-thin zinc oxide nanowires

We report the synthesis of ultra-thin, monocrystalline, highly luminescent ZnO nanowires (NWs) on polystyrene (PS) beads as templates. The synthesis of these NWs on PS beads was conducted by the chemical bath deposition technique in the absence of any catalysts or additives. They have an average diameter of 15 nm (depending on the concentration of the solution) and an average length of 500 nm. Structural characterization reveals that these NWs are monocrystalline, with a hexagonal phase and grow along the [0001] direction. Photoluminescence measurements of these unannealed, ultra-thin NWs exhibit a strong ultra-violet emission at room temperature with an internal quantum efficiency of 23%. We show that the concentration of the aqueous solution plays a key role in controlling the size of the NWs.

Quantitative and simultaneous analysis of the polarity of polycrystalline ZnO seed layers and related nanowires grown by wet chemical deposition

The polarity in ZnO nanowires is an important issue since it strongly affects surface configuration and reactivity, nucleation and growth, electro-optical properties, and nanoscale-engineering device performances. However, measuring statistically the polarity of ZnO nanowire arrays grown by chemical bath deposition and elucidating its correlation with the polarity of the underneath polycrystalline ZnO seed layer grown by the sol-gel process represents a major difficulty. To address that issue, we combine resonant x-ray diffraction (XRD) at Zn K-edge using synchrotron radiation with piezoelectric force microscopy and polarity-sensitive chemical etching to statistically investigate the polarity of more than 107 nano-objects both on the macroscopic and local microscopic scales, respectively. By using high temperature annealing under an argon atmosphere, it is shown that the compact, highly c-axis oriented ZnO seed layer is more than 92% Zn-polar and that only a few small O-polar ZnO grains with an amount less than 8% are formed. Correlatively, the resulting ZnO nanowires are also found to be Zn-polar, indicating that their polarity is transferred from the c-axis oriented ZnO grains acting as nucleation sites in the seed layer. These findings pave the way for the development of new strategies to form unipolar ZnO nanowire arrays as a requirement for a number of nanoscale-engineering devices like piezoelectric nanogenerators. They also highlight the great advantage of resonant XRD as a macroscopic, non-destructive method to simultaneously and statistically measure the polarity of ZnO nanowire arrays and of the underneath ZnO seed layer.

Synthesis and properties of ZnO/TiO2/Sb2S3 core–shell nanowire heterostructures using the SILAR technique

The development of the successive ionic layer adsorption and reaction (SILAR) technique is of high interest for the integration of a semiconducting layer onto high aspect ratio nanoscale structures. We show the deposition of an Sb2S3 shell by SILAR on top of ZnO/TiO2 core–shell nanowire heterostructures, using antimony chloride and sodium sulfide as cationic and anionic precursors, respectively. The conformal anatase-TiO2 shell deposited by atomic layer deposition acts as a protective layer to chemically stabilize these heterostructures with a type II band alignment in the cationic precursor solution. The resulting Sb2S3 shell is composed of uniformly distributed Sb2S3 clusters crystallized at a relatively low temperature of 250 °C from the bottom to the top of ZnO nanowires. It is, further, of high purity, where only a very minor senarmontite-Sb2O3 phase is detected by Raman spectroscopy, and exhibits a relevant band gap energy of 1.74 eV derived from a Tauc plot in the framework of the double pass analysis. These findings reveal the high potential of the SILAR technique to form ZnO core–shell nanowire heterostructures with high uniformity at moderate temperature as well as its advantages over the most widely used chemical bath deposition technique.

Effects of Polyethylenimine and Its Molecular Weight on the Chemical Bath Deposition of ZnO Nanowires

The addition of polyethylenimine (PEI) in the standard chemical bath deposition (CBD) of ZnO nanowires has received an increasing interest for monitoring their aspect ratio, but the physicochemical processes at work are still under debate. To address this issue, the effects of PEI are disentangled from the effects of ammonia and investigated over a broad range of molecular weight (i.e., chain length) and concentration, varying from 1300 to 750 000 and from 1.5 to 10 mM, respectively. It is shown that the addition of PEI strongly favors the elongation of ZnO nanowires by suppressing the homogeneous growth at the benefit of the heterogeneous growth as well as by changing the supersaturation level through a pH modification. PEI is further found to inhibit the development of the sidewalls of ZnO nanowires by adsorbing on their nonpolar m-planes, as supported by Raman scattering analysis. The inhibition proceeds even in the low pH range, which somehow rules out the present involvement of electrostatic interactions as the dominant mechanism for the adsorption. Furthermore, it is revealed that PEI drastically affects the nucleation process of ZnO nanowires on the polycrystalline ZnO seed layer by presumably adsorbing on the nanoparticles oriented with the m-planes parallel to the surface, reducing in turn their nucleation rate as well as inducing a significant vertical misalignment. These findings, specifically showing the effects of the PEI molecular weight and concentration, cast light onto its multiple roles in the CBD of ZnO nanowires.