Nanowire Length, Density, and Crystalline Quality Retrieved from a Single Optical Spectrum

Abstract

We propose spectral domain attenuated reflectometry (SDAR) for fast characterization of nanomaterial growth. The method is demonstrated here for zinc oxide (ZnO) nanowires (NWs) which are grown vertically in random forest fashion showing that it is not limited to well-ordered NWs. We show how SDAR can provide, on the basis of a single measured spectrum, simultaneous information on nanowire length, nanowire density (through nanowire/air filling ratio), and crystalline quality (through band gap). The robustness of the proposed method is assessed first through comparison with information obtained from SEM and XRD taken as reference. In SDAR, the process for fast extraction of NW thickness and filling ratio values makes use of the interference pattern contrast and the spectral periodicity in the reflection response which involve a best fit of the measured spectra with simple theoretical modeling based on the effective medium approach, achieved with a mean square error down to 0.1%. The results also suggest the existence of either 2 or 3 layers of different effective refractive index, hence providing insight on possible growth mechanisms.

Spatiotemporal dynamics of nanowire growth in a microfluidic reactor

“…A hydrothermal in situ method is proposed to synthesize ZnO-NWs on seeded silicon (Si) substrate within a microfluidic chip in dynamic mode instead of the static mode previously reported in the literature 13 , 24 , 25 , 45 . This method has all the advantages of a microfluidic environment.…”

“…The ZnO-NWs were grown vertically in a random forest fashion with a maximum tilting angle of 10° with respect to the direction normal to the surface, as shown in the acquired SEM images. This growth is similar to those of the NWs grown at the macroscale, where preferential growth occurs along the c axis of the ZnO Wurtzite lattice, as already confirmed by X-ray diffraction (XRD) patterns, in which the largest XRD peak corresponds to the [002] crystallographic plane of ZnO 45 . Uniform growth of NWs was achieved over a 10-mm-wide microreactor, as shown in the figure inset, which is 20× larger than the previously reported in situ growth in microchannels, which was limited to widths below 500 µm 38 – 40 .…”

“…For the first time, to the best of our knowledge, time-resolved and spatially resolved online monitoring of NWs growth within a microfluidic reactor was implemented noninvasively, hence avoiding cleaving the samples, as in conventional characterization methods such as SEM. This was achieved thanks to the integration of our recently proposed characterization method, spectral-domain attenuated reflectometry (SDAR) 45 , which is proven in this study to be very effective for noncontact, real-time characterization of NWs in situ dynamic growth within a microfluidic reactor. The obtained quantitative results were confirmed using SEM as a reference characterization tool.…”

Direct growth of ZnO nanowires on civil engineering materials: smart materials for supported photodegradation

“…Considering the substrate mechanical resistance, the classical cleavage method for monocrystalline substrates (such as Si) is not suitable here, and thus, no SEM cross-section view is available here. However, thanks to a model developed by Erfan et al 20 , the nanowire length on the tiling substrates was estimated by UV–visible measurement and is ~1.794–1.930 μm, depending on the model fitting used. This kind of surface is quite close to the ideal Si–wafer surface, i.e., a smooth and non-porous surface, which explains the almost identical morphology that was observed by SEM 13,17,20,21 .…”

“…However, thanks to a model developed by Erfan et al 20 , the nanowire length on the tiling substrates was estimated by UV–visible measurement and is ~1.794–1.930 μm, depending on the model fitting used. This kind of surface is quite close to the ideal Si–wafer surface, i.e., a smooth and non-porous surface, which explains the almost identical morphology that was observed by SEM 13,17,20,21 . The good homogeneity of the ZnO NWs was also confirmed by the UV–vis measurements, which showed good signal reproducibility all over the substrate.…”