Three-Dimensional Metal-Oxide Nanohelix Arrays Fabricated by Oblique Angle Deposition: Fabrication, Properties, and Applications
© Kwon et.al. 2015
Received: 9 July 2015
Accepted: 25 August 2015
Published: 21 September 2015
Three-dimensional (3D) nanostructured thin films have attracted great attention due to their novel physical, optical, and chemical properties, providing tremendous possibilities for future multifunctional systems and for exploring new physical phenomena. Among various techniques to fabricate 3D nanostructures, oblique angle deposition (OAD) is a very promising method for producing arrays of a variety of 3D nanostructures with excellent controllability, reproducibility, low cost, and compatibility with modern micro-electronic processes. This article presents a comprehensive overview of the principle of OAD, and unique structural and optical properties of OAD-fabricated thin films including excellent crystallinity, accurate tunability of refractive indices, and strong light scattering effect which can be utilized to remarkably enhance performances of various systems such as antireflection coatings, optical filters, photoelectrodes for solar-energy-harvesting cells, and sensing layers for various sensors.
Over the last few decades, various nanostructures have received steadily growing interests for many applications due to their fascinating physical, optical, and chemical properties [1–4]. Novel physical properties such as size-dependent excitation or emission, quantized conductance, and metal-insulator transition have been reported to emerge when the size of structures is reduced to nanoscale dimensions [5–11]. In the field of photonics, natural or engineered nanostructures often cause novel optical phenomena based on the interaction between light and nanostructured materials with specific geometrical shape, size, orientation, and arrangement . In addition, the highly porous nature of nanostructured films having giant surface areas for chemical reactions, together with nanoscale dimensions comparable to the Debye length, can bring about innovation in various fields such as chemical sensor systems [13–17] and photoelectrodes for solar-energy-harvesting cell systems [18–24]. Besides, properties of nanostructures can be diversely tailored by controlling their sizes and structures providing tremendous opportunities to be applied in a wide variety of research fields [25–27].
Nanostructures can be classified according to the dimensionality of the nanoscale component: zero-dimensional (0D, nanoparticles, quantum dots, etc.) [9–11], one-dimensional (1D, nanowires, nanorods, etc.) [8, 13], two-dimensional (2D, nanoplates, nanoscale multilayers, etc.) [7, 28], and three-dimensional (3D, nanohelixes, various hierarchical structures) [29, 30] nanostructures. Among them, 3D nanostructures can have larger degree of freedom in design than other nanostructures by precisely controlled structural factors such as geometrical shape and size, opening possibilities for realization of multifunctionality and ultimate performance of various systems, and exploring new physical phenomena [29, 31, 32]. Therefore, fabrication of thin films based on 3D nanostructures with a precisely controlled and reproducible way is of greatest interest to many researchers. Several fabrication methods for various 3D nanostructures have been developed such as colloidal self-assembly , holographic laser lithography , phase-mask holography , layer-by-layer fabrication [36, 37], and direct writing techniques [38, 39]; however, they are somewhat complex, costly, and lacking in compatibility with conventional microelectronics fabrication processes for introducing such nanostructures into integrated circuit chips.
Oblique angle deposition (OAD), or often referred to as glancing angle deposition (GLAD), is a method for producing an array of 3D nanostructures with excellent controllability in geometrical shape, reproducibility, low cost, and compatibility with current microelectronics fabrication techniques [40–46]. By controlling deposition parameters during OAD, a variety of nanostructures can be fabricated including slanted nanorods and nanohelixes (NHs). In particular, arrays of metal-oxide NHs are promising building blocks due to their unique structural and optical properties which can be adjusted precisely to satisfy specific requirements for various future applications. In this article, recent studies on 3D nanostructures of various metal oxides fabricated by OAD are reviewed. Firstly, the fabrication principle of various nanostructures by OAD is introduced. Secondly, unique structural and optical properties of 3D NH arrays are discussed including crystallinity, tunability of refractive index, and light scattering effects. Finally, promising applications of nanostructures fabricated by OAD such as passive optical components, photoelectrodes for solar-energy-harvesting cell systems, and sensing layers for chemical sensors are reviewed.
Fabrication of 3D Nanostructured Thin Films by OAD
In addition to the three parameters, r, θ, and ϕ, determining the geometrical shape of the nanostructures, the temperature, and the surface morphology of substrates is also important as they influence the porosity and the periodicity of nanostructured thin films. Since substrate temperature affects the diffusion of depositing vapor flux on the substrate at the initial stage as well as on top of the nuclei, it is strongly related to the density of nanostructured thin films and the dimension of individual nanostructures constituting the thin film. Furthermore, introducing periodically ordered topographical patterns on the substrate can produce the periodically ordered nanostructures enabled by the formation of “enforced” nucleation sites. Figure 2d,e shows cross-sectional and top-view SEM images of Si NHs on a ZnO pre-patterned substrate, respectively. The inset of Fig. 2e shows a top view of the ZnO pre-pattern on Si substrate achieved by nanoimprint lithography . The Si NHs exhibit a periodic hexagonal close-packed array due to the periodicity of ZnO pre-patterns acting as the initial nucleation seeds. Enforcing periodically arranged nucleation sites for OAD can be a very elegant way to realize a novel optical phenomenon caused by a periodicity of 3D nanostructures [49–56].
One of powerful aspects of OAD is its ability to fabricate heterostructures consisting of various materials such as oxides, metals, and insulators with various combinations of nanostructures. Although a great deal of efforts has been made to fabricate aligned heterostructures in nanoscale by using conventional nanofabrication methods, the formation of 3D nano-heterostructures is quite difficult due to limitations in the method itself as well as available materials suitable for the method. Figure 2f shows the NH heterostructure consisting of WO3-SiO2-WO3 with different geometrical shapes. NHs of a material can be sequentially grown on NHs of another material which acts as the nucleation sites. Note that 3D nano-heterostructures based on various combinations of materials, for example, metal-oxide, oxide-oxide, semiconductor-oxide, etc., can be fabricated on demand, which can exhibit new functionalities with tailored electric, magnetic, optical, and mechanical properties [57–59].
Properties of 3D NH Arrays
A large near-single-crystalline domain with a few grain boundaries over the individual NH can ensure an excellent electrical conduction with low carrier recombination losses at grain boundaries, which can be utilized for electrodes of various devices where charge collection efficiency needs to be enhanced. Furthermore, high porosity in NH arrays allows the possibility of fabricating hybrid structures, for example, by conformal coating of individual NHs with other materials or by filling 0D or 1D nanomaterials into the pores between NHs. Such hybrid structures composed of NH arrays and other materials have a great potential to be applied for a variety of high-performance devices due to their multifunctionality and excellent design freedom. Examples of applications of NH arrays or their hybrid structures as electrodes will be introduced in the section “Applications of 3D NH arrays”.
Applications of 3D NH Arrays
Passive Optical Components
In addition to the multi-layered AR coatings with graded refractive indices, thin films consisting of alternating layers of low- and high-refractive indices fabricated by OAD can be used for high-performance optical filters and distributed Bragg reflectors  made of a single material. White LEDs based on the combination of a GaInN-based blue LED and a yellow phosphor layer suffer from a low phosphor conversion efficiency (PCE) because a significant amount of yellow fluorescence from the yellow phosphor layer is emitted toward the blue LED chip where the fluorescence is partially absorbed. Incorporated conductive dichroic-filtering contacts (DFCs) made of a single ITO by OAD was reported to multifunction as a blue-transmitting but yellow-reflecting optical filter as well as a low-resistance ohmic contact to GaN, thus improving the PCE of the white LEDs . Figure 7c shows a schematic description of a phosphor-converted white LED with a conductive DFC which has alternating stacks of low-refractive-index nanoporous ITO and high-refractive-index dense ITO, and its cross-sectional SEM image. DFC layers are composed of five-layer ITO thin film structures having clear interfaces. Figure 7d represents the emission spectra of white LEDs with three different contacts normalized with respect to the blue electroluminescence peak so that the enhancement of yellow fluorescence can be clearly seen. Compared to the LED with reference ITO contact, yellow fluorescence of LEDs with DFCs is improved due to the enhanced transmission of blue electroluminescence through the DFC and the increased reflection of the downward yellow fluorescence.
Photoelectrodes for Solar-Energy-Harvesting Cells
Sensing Layers for Various Sensors
3D nanostructured thin films fabricated by OAD exhibit high porosities and high surface-to-volume ratio, providing a great potential to be used for active layers of chemical sensors due to their giant surface areas for chemical reactions as well as easiness of in-and-out of chemical species being detected [15, 16, 70–73]. Hawkeye et al. demonstrated a photonic-crystal optical humidity sensor using OAD . Layers with alternating refractive indices having high and low density were fabricated for active layers whose spectral features shift when relative humidity changes. Beckers et al. fabricated SiO2 nanostructured films by OAD for active layers of selective alcohol sensors, successfully distinguishing methanol, ethanol, and 2-propanol . In addition to chemical sensors, an array of Cr zigzag nanosprings for a pressure sensor was fabricated by Kesapragada et al. . The Cr zigzags exhibit a reversible change in resistivity upon loading and unloading, due to a compression of the nanosprings which causes them to physically touch their neighbors, indicating their potential as pressure sensors.
This review article introduced recent studies on 3D nanostructured thin films fabricated by OAD technique, which is suitable for producing a wide range of 3D nanostructures with excellent controllability, reproducibility, and compatibility with current microelectronics processes. By tilting the substrate during OAD, the initial growth fluctuation (nuclei) forms a self-shadowed region against highly directional vapor flux, creating a self-organized nanoporous thin film. The geometrical shape of the 3D nanostructures can be tailored by adjusting the combination of three parameters (r, θ, ϕ) during the deposition. The 3D nanostructures fabricated by OAD show unique structural and optical properties including excellent crystallinity after proper annealing, accurate tunability of refractive indices down to near 1.0, and strong light scattering effects due to their large scattering cross section. These unique structural and optical features can be utilized to enhance performances of various systems when they are applied to passive optical components, photoelectrodes in solar-energy-harvesting cells, sensing layers in diverse sensors and their integrated forms—electronics noses, and other future devices.
The authors gratefully acknowledge supports by the Brain Korea 21 PLUS project for Center for Creative Industrial Materials (F14SN02D1707).
Open AccessThis article is distributed under the terms of the Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made.
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