Correlating Light Absorption with Various Nanostructure Geometries in Vertically Aligned Si Nanowire Arrays

Exploring the interactions between light and nanostructures contributes greatly to understanding and engineering nanoscale optical phenomena related to device performance. However, this often involves a compromise between uniformity and scalability. Given that optical properties, and especially light absorption, are governed by the geometries of nanostructures, this study investigated the correlation between light absorption and vertically aligned silicon nanowire (<i>v</i>-SiNW) arrays synthesized using KrF stepper lithography. Controlled growth experiments of the <i>v</i>-SiNW arrays indicated that their geometrical parameters strongly influence their corresponding light absorption properties, as confirmed by reflection measurements and finite difference time domain (FDTD) simulations, which showed specific wavelength-dependent absorption. Moreover, the extent of tapering the <i>v</i>-SiNW arrays was modulating to achieve broad absorption of visible light resulting from the gradual change in diameter and to optimize their optical characteristics, based on diameter-dependent nanophotonic resonance, for use in various applications

Control over Alignment and Growth Kinetics of Si Nanowires through Surface Fluctuation of Liquid Precursor

Control over alignment and growth kinetics of vertically aligned Si nanowire (<i>v</i>-SiNW) arrays, which were grown using chemical vapor deposition (CVD) via a metal catalyst-assisted vapor–liquid–solid (VLS) mechanism, was demonstrated by introducing a homemade bubbler system containing a SiCl₄ solution as the Si precursor. Careful control over the bubbler afforded different amounts of SiCl₄ supplied to the reactor. By varying the dipping depth (<i>D</i>_d) and tilting angle (<i>T</i>_a) of the bubbler, the SiCl₄ precursor concentration would fluctuate to different degrees. The different SiCl₄ concentrations afforded the fine-tuning of <i>v</i>-SiNW array properties like alignment and growth kinetics. The degree of alignment of <i>v</i>-SiNWs could be increased with large amounts of SiCl₄, which was caused by slight shallow depth or gentle tilting of the SiCl₄ solution in the bubbler due to an increasing degree of fluctuation and fluctuation area. The ability to control alignment and growth kinetics of <i>v</i>-SiNW arrays could be employed in advanced nanoelectronic devices

Local Liquid Phase Deposition of Silicon Dioxide on Hexagonally Close-Packed Silica Beads

Liquid phase deposition (LPD) is a useful method for the production of oxide film with low reaction temperature and production cost. With the report that the LPD of oxide films is conformally processed with uniform thickness and composition, there has been significant attention given to investigating its kinetic controls and growth mechanism on the flat surface. In this work, we explored the LPD of silicon dioxide on the hexagonally close-packed silica beads array as a nanostructured surface. The deposition and etching reactions of SiO₂ occurred locally and simultaneously on silica beads, and were distinguished from the amount of fumed silica added in LPD solution. From locally competitive reactions, we obtained the anisotropic morphology of close-packed silica beads, and proposed a mechanism for the local LPD of SiO₂ driven by nanostructured surfaces. This work contributes highly to improve metal oxide-based engineering, and also provide greater insight into the topography-driven LPD

Bioinspired Morphogenesis of Highly Intricate and Symmetric Silica Nanostructures

Biosilification is of interest due to its capability to produce a highly intricate structure under environmentally friendly conditions. Despite the considerable effort that has been devoted toward biomimetic silification, the synthesis of highly complex silica structures, as found in the structures of diatom cell walls, is still in its infancy. Here, we report the bioinspired fabrication of well-organized and symmetric silica nanostructured networks, involving phase separation and silicic acid polymerization processes, in analogy to the morphogenesis of diatom cell walls. Our approach exploits self-assembled silica spheres as a self-source of the silicic acids as well as scaffolds that, interplayed with droplets of ammonium hexafluorosilicate, direct the site-specific silification. Moreover, we have achieved multiple morphological evolutions with subtle changes in the process, which demonstrates exquisite levels of control over silica morphogenesis

Fine-Tunable Absorption of Uniformly Aligned Polyurea Thin Films for Optical Filters Using Sequentially Self-Limited Molecular Layer Deposition

Development of methods enabling the preparation of uniformly aligned polymer thin films at the molecular level is a prerequisite for realizing their optoelectronic characteristics as innovative materials; however, these methods often involve a compromise between scalability and accuracy. In this study, we have grown uniformly aligned polyurea thin films on a SiO₂ substrate using molecular layer deposition (MLD) based on sequential and self-limiting surface reactions. By integrating plane-polarized Fourier-transform infrared, Raman spectroscopic tools, and density functional theory calculations, we demonstrated the uniform alignment of polyurea MLD films. Furthermore, the selective-wavelength absorption characteristics of thickness-controlled MLD films were investigated by integrating optical measurements and finite-difference time-domain simulations of reflection spectra, resulting from their thickness-dependent fine resonance with photons, which could be used as color filters in optoelectronics

Annealed Au-Assisted Epitaxial Growth of Si Nanowires: Control of Alignment and Density

The epitaxial growth of 1D nanostructures is of particular interest for future nanoelectronic devices such as vertical field-effect transistors because it directly influences transistor densities and 3D logic or memory architectures. Silicon nanowires (SiNWs) are a particularly important 1D nanomaterial because they possess excellent electronic and optical properties. What is more, the scalable fabrication of vertically aligned SiNW arrays presents an opportunity for improved device applications if suitable properties can be achieved through controlling the alignment and density of SiNWs, yet this is something that has not been reported in the case of SiNWs synthesized from Au films. This work therefore explores the controllable synthesis of vertically aligned SiNWs through the introduction of an annealing process prior to growth via a Au-catalyzed vapor–liquid–solid mechanism. The epitaxial growth of SiNWs was demonstrated to be achievable using SiCl₄ as the Si precursor in chemical vapor deposition, whereas the alignment and density of the SiNWs could be controlled by manipulating the annealing time during the formation of Au nanoparticles (AuNPs) from Au films. During the annealing process, gold silicide was observed to form on the interface of the liquid-phase AuNPs, depending on the size of the AuNPs and the annealing time. This work therefore makes a valuable contribution to improving nanowire-based engineering by controlling its alignment and density as well as providing greater insight into the epitaxial growth of 1D nanostructures

Effects of Hydrogen Partial Pressure in the Annealing Process on Graphene Growth

Graphene domains with different sizes and densities were successfully grown on Cu foils with use of a chemical vapor deposition method. We investigated the effects of volume ratios of argon to hydrogen during the annealing process on graphene growth, especially as a function of hydrogen partial pressure. The mean size and density of graphene domains increased with an increase in hydrogen partial pressure during the annealing time. In addition, we found that annealing with use of only hydrogen gas resulted in snowflake-shaped carbon aggregates. Energy-dispersive X-ray spectroscopy (EDX) and high-resolution photoemission spectroscopy (HRPES) revealed that the snowflake-shaped carbon aggregates have stacked sp² carbon configuration. With these observations, we demonstrate the key reaction details for each growth process and a proposed growth mechanism as a function of the partial pressure of H₂ during the annealing process

Intramolecular and Intermolecular Interactions in Hybrid Organic–Inorganic Alucone Films Grown by Molecular Layer Deposition

Investigation of molecular interactions in polymeric films is crucial for understanding and engineering multiscale physical phenomena correlated to device function and performance, but this often involves a compromise between theoretical and experimental data, because of poor film uniformity. Here, we report the intramolecular and intermolecular interactions inside the ultrathin and conformal hybrid organic–inorganic alucone films grown by molecular layer deposition, based on sequential and self-limiting surface reactions. Varying the carbon chain length of organic precursors, which affects their molecular flexibility, caused intramolecular interactions such as double reactions by bending of the molecular backbone, resulting in formation of hole vacancies in the films. Furthermore, intermolecular interactions in alucone polymeric films are dependent on the thermal kinetics of molecules, leading to binding failures and cross-linking at low and high growth temperatures, respectively. We illustrate these key interactions and identify molecular geometries and potential energies by density functional theory calculations

Axon-First Neuritogenesis on Vertical Nanowires

In this work, we report that high-density, vertically grown silicon nanowires (<i>vg</i>-SiNWs) direct a new <i>in vitro</i> developmental pathway of primary hippocampal neurons. Neurons on <i>vg</i>-SiNWs formed a single, extremely elongated major neurite earlier than minor neurites, which led to accelerated polarization. Additionally, the development of lamellipodia, which generally occurs on 2D culture coverslips, was absent on <i>vg</i>-SiNWs. The results indicate that surface topography is an important factor that influences neuronal development and also provide implications for the role of topography in neuronal development <i>in vivo</i>