Chemical Bath Deposition of ZnO/ZnGa2O4 Core-Shell Nanowire Heterostructures Using Partial Chemical Conversion.

The development of innovative heterostructures made of ZnO nanowires is of great interest for enhancing the performances of many devices in the fields of optoelectronics, photovoltaics, and energy harvesting. We report an original fabrication process to form ZnO/ZnGa2O4 core-shell nanowire heterostructures in the framework of the wet chemistry techniques. The process involves the partial chemical conversion of ZnO nanowires grown via chemical bath deposition into ZnO/ZnGa2O4 core-shell nanowire heterostructures with a high interface quality following their immersion in an aqueous solution containing gallium nitrate heated at a low temperature. The double-step process describing the partial chemical conversion relies on successive dissolution and reaction mechanisms. The present finding offers the possibility to fabricate ZnO/ZnGa2O4 core-shell nanowire heterostructures at low temperatures and over a wide variety of substrates with a large surface area, which is attractive for nanostructured solar cells, deep-UV photodetectors, and piezoelectric devices.

Chemical Bath Deposition of α-GaOOH with Tunable Morphology on Silicon Using the pH Adjustment.

The growth of GaOOH by chemical bath deposition has received great attention over the past years as a first step to form Ga2O3 with the α- or ß-phases by combining a wet chemical route with thermal annealing in air. By using gallium nitrate and sodium hydroxide in aqueous solution, we show that the structural morphology of GaOOH deposits is thoroughly tunable in terms of both dimensions, density, and nature by varying the initial pH value from acidic to basic conditions. In the low-pH region associated with a low supersaturation level and where Ga3+ ions represent the dominant Ga(III) species, GaOOH microrods with a low aspect ratio and low density prevail. In the intermediate-pH region associated with a high supersaturation level and where GaOH2+ ions represent the dominant Ga(III) species, GaOOH prismatic nanorods with a high aspect ratio and high density are preferentially formed. In the high-pH region where Ga(OH)4- complexes are predominantly formed, the growth of partially crystallized GaOOH thin films with a typical thickness of about 1 µm proceeds. These findings show the correlation between the characteristics of the chemical bath and the resulting structural morphology of GaOOH deposits. They further open great perspectives to grow GaOOH and hence Ga2O3-based materials on silicon with a dedicated structural morphology using chemical bath deposition for engineering devices in the fields of gas sensing, solar-blind UV-C photodetection, and power electronics.

Interplay Effects in the Co-Doping of ZnO Nanowires with Al and Ga Using Chemical Bath Deposition.

The simultaneous co-doping of ZnO nanowires grown by chemical bath deposition is of high interest for a large number of engineering devices, but the process conditions required and the resulting physicochemical processes are still largely unknown. Herein, we show that the simultaneous co-doping of ZnO nanowires with Al and Ga following the addition of Al(NO3)3 and Ga(NO3)3 in the chemical bath operates in a narrow range of conditions in the high-pH region, where the adsorption processes of respective Al(OH)4- and Ga(OH4)- complexes on the positively charged m-plane sidewalls are driven by attractive electrostatic forces. The structural morphology and properties of ZnO nanowires are significantly affected by the co-doping and mainly governed by the effect of Al(III) species. The incorporation processes of Al and Ga dopants are characterized by significant interplay effects, and the amount of incorporated Ga dopants into ZnO nanowires is found to be larger than the amount of incorporated Al dopants owing to energetic considerations. The Al and Ga dopants are located in the bulk of ZnO nanowires, but a part of Al and Ga lies on their surfaces, their incorporation processes in the bulk being enhanced by thermal annealing under oxygen atmosphere. Eventually, the Al and Ga dopants directly affect the incorporation of hydrogen-related defects, notably by annihilating the formation of VZn-nH defect complexes. These findings present an efficient strategy to proceed with the co-doping of ZnO nanowires grown by chemical bath deposition, opening perspectives to control their electronic structure properties with a higher precision.

Optimization of the Sb2S3 Shell Thickness in ZnO Nanowire-Based Extremely Thin Absorber Solar Cells.

Extremely thin absorber (ETA) solar cells made of ZnO/TiO2/Sb2S3 core-shell nanowire heterostructures, using P3HT as the hole-transporting material (HTM), are of high interest to surpass solar cell efficiencies of their planar counterpart at lower material cost. However, no dimensional optimization has been addressed in detail, as it raises material and technological critical issues. In this study, the thickness of the Sb2S3 shell grown by chemical spray pyrolysis is tuned from a couple of nanometers to several tens of nanometers, while switching from a partially to a fully crystallized shell. The Sb2S3 shell is highly pure, and the unwanted Sb2O3 phase was not formed. The low end of the thickness is limited by challenges in the crystallization of the Sb2S3 shell, as it is amorphous at nanoscale dimensions, resulting in the low optical absorption of visible photons. In contrast, the high end of the thickness is limited by the increased density of defects in the bulk of the Sb2S3 shell, degrading charge carrier dynamics, and by the incomplete immersion of the P3HT in the structure, resulting in the poor hole collection. The best ETA solar cell with a short-circuit current density of 12.1 mA/cm2, an open-circuit voltage of 502 mV, and a photovoltaic conversion efficiency of 2.83% is obtained for an intermediate thickness of the Sb2S3 shell. These findings highlight that the incorporation of both the absorber shell and HTM in the core-shell heterostructures relies on the spacing between individual nanowires. They further elaborate the intricate nature of the dimensional optimization of an ETA cell, as it requires a fine-balanced holistic approach to correlate all the dimensions of all the components in the heterostructures.

Chemical Bath Deposition of ZnO Nanowires Using Copper Nitrate as an Additive for Compensating Doping.

The controlled incorporation of dopants like copper into ZnO nanowires (NWs) grown by chemical bath deposition (CBD) is still challenging despite its critical importance for the development of piezoelectric devices. In this context, the effects of the addition of copper nitrate during the CBD of ZnO NWs grown on Au seed layers are investigated in detail, where zinc nitrate and hexamethylenetetramine are used as standard chemical precursors and ammonia as an additive to tune the pH. By combining thermodynamic simulations with chemical and structural analyses, we show that copper oxide nanocrystals simultaneously form with ZnO NWs during the CBD process in the low-pH region associated with large supersaturation of Cu species. The Cu(II) and Zn(II) speciation diagrams reveal that both species show very similar behaviors, as they predominantly form either X2+ ions (with X = Cu or Zn) or X(NH3)42+ ion complexes, depending on the pH value. Owing to their similar ionic structures, Cu2+ and Cu(NH3)42+ ions preferentially formed in the low- and high-pH regions, respectively, are able to compete with the corresponding Zn2+ and Zn(NH3)42+ ions to adsorb on the c-plane top facets of ZnO NWs despite repulsive electrostatic interactions, yielding the significant incorporation of Cu. At the highest pH value, additional attractive electrostatic interactions between the Cu(NH3)42+ ion complexes and negatively charged c-plane top facets further enhance the incorporation of Cu into ZnO NWs. The present findings provide a deep insight into the physicochemical processes at work during the CBD of ZnO NWs following the addition of copper nitrate, as well as a detailed analysis of the incorporation mechanisms of Cu into ZnO NWs, which are considered beyond the only electrostatic forces usually driving the incorporation of dopants such as Al and Ga.

Chemical Synthesis of ß-Ga2O3 Microrods on Silicon and Its Dependence on the Gallium Nitrate Concentration.

ß-Ga2O3 microrods have attracted increasing attention for their integration into solar blind/UV photodetectors and gas sensors. However, their synthesis using a low-temperature chemical route in aqueous solution is still under development, and the physicochemical processes at work have not yet been elucidated. Here, we develop a double-step process involving the growth of α-GaOOH microrods on silicon using chemical bath deposition and their further structural conversion to ß-Ga2O3 microrods by postdeposition thermal treatment. It is revealed that the concentration of gallium nitrate has a drastic effect on tuning the morphology, dimensions (i.e., diameter and length), and density of α-GaOOH microrods over a broad range, in turn governing the morphological properties of ß-Ga2O3 microrods. The physicochemical processes in aqueous solution are investigated by thermodynamic computations yielding speciation diagrams of Ga(III) species and theoretical solubility plots of GaOOH(s). In particular, the qualitative evolution of the morphological properties of α-GaOOH microrods with the concentration of gallium nitrate is found to be correlated with the supersaturation in the bath and discussed in light of the standard nucleation and growth theory. Interestingly, the structural conversion following the thermal treatment at 900 °C in air results in the formation of pure ß-Ga2O3 microrods without any residual minor phases and with tunable morphology and improved structural ordering. These findings reporting a double-step process for forming high-quality pure ß-Ga2O3 microrods on silicon open many perspectives for their integration onto a large number of substrates for solar blind/UV photodetection and gas sensing.

The Path of Gallium from Chemical Bath into ZnO Nanowires: Mechanisms of Formation and Incorporation.

ZnO nanowires grown by chemical bath deposition (CBD) are of high interest, but their doping with extrinsic elements including gallium in aqueous solution is still challenging despite its primary importance for transparent electrodes and electronics, and for mid-infrared plasmonics. We elucidate the formation mechanisms of ZnO nanowires by CBD using zinc nitrate and hexamethylenetetramine as standard chemical precursors, as well as gallium nitrate and ammonia as chemical additives. A complete growth diagram, revealing the effects of both the relative concentration of gallium nitrate and pH, is gained by combining a thorough experimental approach with thermodynamic computations yielding theoretical solubility plots as well as Zn(II) and Ga(III) speciation diagrams. The role of Ga(OH)4- complexes is specifically shown as capping agents on the m-plane sidewalls of ZnO nanowires, enhancing their development and hence decreasing their aspect ratio. Additionally, the gallium incorporation into ZnO nanowires is investigated in detail by chemical analyses and Raman scattering. They show the predominant formation of gallium substituting for zinc atoms (GaZn) in as-grown ZnO nanowires and their partial conversion into GaZn-VZn complexes after postdeposition annealing under oxygen atmosphere. The conversion is further related to a significant relaxation of the strain level in ZnO nanowires. These findings reporting the physicochemical processes at work during the formation of ZnO nanowires and the related gallium incorporation mechanisms offer a general strategy for their extrinsic doping and open the way for carefully controlling their physical properties as required for nanoscale device engineering.

Formation mechanisms of ZnO nanowires on polycrystalline Au seed layers for piezoelectric applications.

ZnO nanowires are considered as attractive building blocks for piezoelectric devices, including nano-generators and stress/strain sensors. However, their integration requires the use of metallic seed layers, on top of which the formation mechanisms of ZnO nanowires by chemical bath deposition are still largely open. In order to tackle that issue, the nucleation and growth mechanisms of ZnO nanowires on top of Au seed layers with a thickness in the range of 5-100 nm are thoroughly investigated. We show that the ZnO nanowires present two different populations of nano-objects with a given morphology. The majority primary population is made of vertically aligned ZnO nanowires, which are heteroepitaxially formed on top of the Au (111) grains. The resulting epitaxial strain is found to be completely relieved at the Au/ZnO interface. In contrast, the minority secondary population is composed of ZnO nanowires with a significant mean tilt angle around 20° with respect to the normal to the substrate surface, which are presumably formed on the (211) facets of the Au (111) grains. The elongation of ZnO nanowires is further found to be limited by the surface reaction at the c-plane top facet in the investigated conditions. By implementing the selective area growth using electron beam lithography, the position of ZnO nanowires is controlled, but the two populations still co-exist in the ensemble. These findings provide an in-depth understanding of the formation mechanisms of ZnO nanowires on metallic seed layers, which should be taken into account for their more efficient integration into piezoelectric devices.

Well-ordered ZnO nanowires with controllable inclination on semipolar ZnO surfaces by chemical bath deposition.

Controlling the formation of ZnO nanowire (NW) arrays on a wide variety of substrates is crucial for their efficient integration into nanoscale devices. While their nucleation and growth by chemical bath deposition (CBD) have intensively been investigated on non-polar and polar c-plane ZnO surfaces, their formation on alternatively oriented ZnO surfaces has not been addressed yet. In this work, the standard CBD technique of ZnO is investigated on [Formula: see text] and [Formula: see text] semipolar ZnO single crystal surfaces. A uniform nanostructured layer consisting of tilted ZnO NWs is formed on the [Formula: see text] surface while elongated nanostructures are coalesced into a two-dimensional compact layer on the [Formula: see text] surface. By further combining the CBD with selective area growth (SAG) using electron beam-assisted lithography, highly tilted well-ordered ZnO NWs with high structural uniformity are grown on the [Formula: see text] patterned surface. The structural analysis reveals that ZnO NWs are homoepitaxially grown along the polar c-axis. The occurrence of quasi-transverse and -longitudinal optical phonon modes in Raman spectra is detected and their origin and position are explained in the framework of the Loudon's model. These results highlight the possibility to form ZnO NWs on original semipolar ZnO surfaces. It also opens the way for comprehensively understanding the nucleation and growth of ZnO NW arrays on poorly and highly textured polycrystalline ZnO seed layers composed of nanoparticles with a wide range of non-polar, semipolar, and polar plane orientations. Eventually, the possibility to tune both the inclination and dimensions of well-ordered ZnO NW arrays by using SAG on semipolar surfaces is noteworthy for photonic and optoelectronic nanoscale devices.

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.

Effects of the pH on the Formation and Doping Mechanisms of ZnO Nanowires Using Aluminum Nitrate and Ammonia.

The elucidation of the fundamental processes in aqueous solution during the chemical bath deposition of ZnO nanowires (NWs) using zinc nitrate and hexamethylenetetramine is of great significance: however, their extrinsic doping by foreign elements for monitoring their optical and electrical properties is still challenging. By combining thermodynamic simulations yielding theoretical solubility plots and speciation diagrams with in situ pH measurements and structural, chemical, and optical analyses, we report an in-depth understanding of the pH effects on the formation and aluminum doping mechanisms of ZnO NWs. By the addition of aluminum nitrate with a given relative concentration for the doping and of ammonia over a broad range of concentrations, the pH is shown to strongly influence the shape, diameter, length, and doping magnitude of ZnO NWs. Tuning the dimensions of ZnO NWs by inhibition of their radial growth only proceeds over a specific pH range, where negatively charged Al(OH)4- complexes are predominantly formed and act as capping agents by electrostatically interacting with the positively charged m-plane sidewalls. These complexes further favor the aluminum incorporation and doping of ZnO NWs, which only operate over the same pH range following thermal annealing above 200 °C. These findings reporting a full chemical synthesis diagram reveal the significance of carefully selecting and following the pH to control the morphology of ZnO NWs as well as to achieve their thermally activated extrinsic doping, as required for many nanoscale engineering devices.

Polarity-Dependent Growth Rates of Selective Area Grown ZnO Nanorods by Chemical Bath Deposition.

Polarity is known to affect the growth and properties of ZnO single crystals and epitaxial films, but its effects are mostly unknown in ZnO nanorods. To leave polarity as the only varying parameter, ZnO nanorods are grown by chemical bath deposition under identical conditions and during the same run on O- and Zn-polar ZnO single crystals patterned by electron beam lithography with the same pattern consisting of 15 different domains. The resulting well-ordered O- and Zn-polar ZnO nanorod arrays with high structural uniformity are formed on all the domains. The comparison of their typical dimensions unambiguously reveals that Zn-polar ZnO nanorods have much higher growth rates than O-polar ZnO nanorods for all the hole diameter and period combinations. The distinct growth rates are explained in the framework of the surface reaction-/diffusive transport-limited elongation regime analysis, which yields a much larger surface reaction rate constant for Zn-polar ZnO nanorods. The origin of the difference is attributed to polarity-dependent dangling bond configurations at the top polar c-faces of ZnO nanorods, which may further be affected by polarity-dependent interactions with the ionic species in aqueous solution. These findings show the relevance of considering polarity as an important quantity in ZnO nanorods.

Spontaneous shape transition of thin films into ZnO nanowires with high structural and optical quality.

ZnO nanowires are usually formed by physical and chemical deposition techniques following the bottom-up approach consisting in supplying the reactants on a nucleation surface heated at a given temperature. We demonstrate an original alternative approach for the formation of ZnO nanowire arrays with high structural and optical quality, which is based on the spontaneous transformation of a ZnO thin film deposited by sol-gel process following a simple annealing. The development of these ZnO nanowires occurs through successive shape transitions, including the intermediate formation of pyramid-shaped islands. Their nucleation under near-equilibrium conditions is expected to be governed by thermodynamic considerations via the total free energy minimization related to the nanowire shape. It is further strongly assisted by the drastic reordering of the matter and by recrystallization phenomena through the massive transport of zinc and oxygen atoms towards the localized growth areas. The spontaneous shape transition process thus combines the easiness and low-cost of sol-gel process and simple annealing with the assets of the vapor phase deposition techniques. These findings cast a light on the fundamental mechanisms driving the spontaneous formation of ZnO nanowires and, importantly, reveal the great technological potential of the spontaneous shape transition process as a promising alternative approach to the more usual bottom-up approach.

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 ß-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.

Improvement of the physical properties of ZnO/CdTe core-shell nanowire arrays by CdCl2 heat treatment for solar cells.

CdTe is an important compound semiconductor for solar cells, and its use in nanowire-based heterostructures may become a critical requirement, owing to the potential scarcity of tellurium. The effects of the CdCl2 heat treatment are investigated on the physical properties of vertically aligned ZnO/CdTe core-shell nanowire arrays grown by combining chemical bath deposition with close space sublimation. It is found that recrystallization phenomena are induced by the CdCl2 heat treatment in the CdTe shell composed of nanograins: its crystallinity is improved while grain growth and texture randomization occur. The presence of a tellurium crystalline phase that may decorate grain boundaries is also revealed. The CdCl2 heat treatment further favors the chlorine doping of the CdTe shell with the formation of chlorine A-centers and can result in the passivation of grain boundaries. The absorption properties of ZnO/CdTe core-shell nanowire arrays are highly efficient, and more than 80% of the incident light can be absorbed in the spectral range of the solar irradiance. The resulting photovoltaic properties of solar cells made from ZnO/CdTe core-shell nanowire arrays covered with CuSCN/Au back-side contact are also improved after the CdCl2 heat treatment. However, recombination and trap phenomena are expected to operate, and the collection of the holes that are mainly photo-generated in the CdTe shell from the CuSCN/Au back-side contact is presumably identified as the main critical point in these solar cells.

Selective area growth of well-ordered ZnO nanowire arrays with controllable polarity.

Controlling the polarity of ZnO nanowires in addition to the uniformity of their structural morphology in terms of position, vertical alignment, length, diameter, and period is still a technological and fundamental challenge for real-world device integration. In order to tackle this issue, we specifically combine the selective area growth on prepatterned polar c-plane ZnO single crystals using electron-beam lithography, with the chemical bath deposition. The formation of ZnO nanowires with a highly controlled structural morphology and a high optical quality is demonstrated over large surface areas on both polar c-plane ZnO single crystals. Importantly, the polarity of ZnO nanowires can be switched from O- to Zn-polar, depending on the polarity of prepatterned ZnO single crystals. This indicates that no fundamental limitations prevent ZnO nanowires from being O- or Zn-polar. In contrast to their catalyst-free growth by vapor-phase deposition techniques, the possibility to control the polarity of ZnO nanowires grown in solution is remarkable, further showing the strong interest in the chemical bath deposition and hydrothermal techniques. The single O- and Zn-polar ZnO nanowires additionally exhibit distinctive cathodoluminescence spectra. To a broader extent, these findings open the way to the ultimate fabrication of well-organized heterostructures made from ZnO nanowires, which can act as building blocks in a large number of electronic, optoelectronic, and photovoltaic devices.

Photochromism of spiropyran nanocrystals embedded in sol-gel matrices.

Spectroscopic and kinetic properties of a new photochromic medium, consisting of nanocrystals of spyropyran molecules (1,3-dihydro-1,3,3,5',6',pentamethyl-spiro[2H-indole-2,2'-[2H]pyrano [3,2-b]pyridinium] iodide) embedded in an organo-silicate sol-gel film, are presented and compared to microcrystals obtained by slow evaporation of a solvent. High photoconversion efficiencies for both kinds of crystals have been observed. In microcrystals, the photomerocyanine form absorbs at 570 nm with a fading rate of 5 h, in nanocrystals the photomerocyanine form absorbs at 535 nm with a fading rate of 41 h. Therefore, the crystalline structure of nanocrystals is different from the microcrystal one.

In situ diagnostics of the crystalline nature of single organic nanocrystals by nonlinear microscopy.

We elucidate the crystalline nature and the three-dimensional orientation of isolated organic nanocrystals embedded in a sol-gel matrix, using a polarized nonlinear microscopy technique that combines two-photon fluorescence and second harmonic generation. This technique allows the distinction between monocrystalline structures and nanoscale polycrystalline aggregates responsible for incoherent second harmonic signals.