Modeling the Elongation of Nanowires Grown by Chemical Bath Deposition Using a Predictive Approach

Abstract

The chemical bath deposition of nanowires is of high interest for a wide variety of optoelectronic, piezoelectric, and sensing devices, but a theoretical description of the elongation process is still missing despite its critical importance. By solving Fick's diffusion equations in combination with thermodynamic computations, we determine the expression of the axial growth rate of nanowires and its temporal dependence under dynamic conditions, namely in a sealed reactor where the depletion of chemical reactants occurs. The theoretical model is found to be in very good agreement with a large set of experimental data specifically collected in the case of the chemical bath deposition of ZnO nanowires. In particular, an activation energy of 198 ± 24 kJ/mol is deduced for the elongation process of ZnO nanowires, involving the energy barriers for both the dehydration process of Zn(II) species (i.e. [Zn(H2O)6] 2+ ions) and their subsequent direct incorporation onto the c-plane top faces. This shows its high potential for deeply investigating the physicochemical processes at work in the chemical bath. By using the theoretical model as a predictive approach, a complete growth diagram reporting the evolution of the length of ZnO nanowires vs effective growth time and temperature is also gained over a broad range of conditions, revealing its additional high potential for applied research and industrial purposes. The present general approach is further compatible with a broad range of chemicals in solution and of semiconducting materials grown by chemical bath deposition. predictive behavior in a broad range of CBD conditions for applied research and industrial purposes. Eventually, this general approach is of high potential for other systems involving a wide variety of chemicals in solution and of semiconducting materials grown by CBD. SUPPORTING INFORMATION Detailed proof of the dynamic growth model, pictures of the chemical bath, top-view FESEM images of the series at 90 °C with 800 mM of NH3, cross-sectonial view FESEM images of the series at 80 and 95 °C, NW length and numerical values of the series at 80, 90 and 95 °C (PDF).

Dimensional Roadmap for Maximizing the Piezoelectrical Response of ZnO Nanowire-Based Transducers: Impact of Growth Method

“…In the CBD process, the massive incorporation of hydrogen-related defects acting as shallow donors is mainly responsible for this range of charge carrier density [ 36 ]. The growth medium in water is full of hydrogen and the crystallization process resulting in the elongation of ZnO NWs through the development of their c-plane top facet basically involves a dehydration process [ 64 ]. A large number of hydrogen-related defects (e.g., interstitial hydrogen, substitutional hydrogen on the oxygen lattice site, zinc vacancy–hydrogen complexes) acting as shallow donors are thus formed systematically [ 37 ].…”

Engineering nitrogen- and hydrogen-related defects in ZnO nanowires using thermal annealing

“…The accurate knowledge on the nature and concentration of intrinsic/extrinsic point defects governing the electronic structure properties of ZnO NWs is required. In the CBD technique [7], an inherent characteristic originates from the crystallization process of ZnO NWs operating in an aqueous solution, namely in an hydrogen-rich environment [8][9][10].…”

In situ analysis of the nucleation of O- and Zn-polar ZnO nanowires using synchrotron-based X-ray diffraction

“…The leak-tight growth cell was designed to work in the most widely used CBD configuration, namely under dynamic conditions where the depletion of chemical reactants with time occurs. 43 In that configuration, neither continuous flow to introduce the chemical reactants nor mechanical stirring during the CBD process were used.…”

“…41 The elongation process of ZnO NWs is limited either by i) the surface reaction at the top polar c-face for small S values, or by ii) the diffusive transport of reactants (e.g., the limiting Zn(II) species) for larger S values. [41][42][43][44] In the vast majority of cases, a high number density of ZnO NWs on the polycrystalline ZnO seed layer results in large S values such that growth occurs in the diffusive transport-limited regime. 41 The diameter and length of ZnO NWs are thus inversely proportional to the number density.…”