Nanocolumnar association and domain formation in porous thin films grown by evaporation at oblique angles

Planar Microsupercapacitors Based on Oblique Angle Deposited Highly Porous TiN Thin Films

Microsupercapacitors are gaining increasing interest for energy storage in miniaturized electronic devices. However, the production of porous electrode material with standard microfabrication techniques is a big problem. Here, we report on the oblique angle deposition of highly porous and nanostructured columnar titanium nitride (TiN) films on silicon substrate using magnetron sputtering for high-performance microsupercapacitors. The intercolumnar porosity of the sputtered TiN films can be systematically controlled as a function of the oblique angle α achieved by tilting the substrate. The denser morphologies in TiN films deposited at α = 0° lead to moderate capacitive behavior in a 1 M Na₂SO₄ electrolyte solution. Meanwhile, a high areal capacitance of 17.5 mF·cm^-2 is obtained for a 60° oblique angle due to high intercolumnar porosity in films, which increases the specific surface area and facilitates easy electrolyte permeation. The electrodes also retain 88.2% of the initial specific capacitance after 10,000 charging/discharging cycles. A planar interdigitated microsupercapacitor has been subsequently fabricated based on an optimized TiN thin film serving as both an efficient electrode and a current collector. TThe device was electrochemically tested using polyvinyl alcohol (PVA)-Na2SO4 hydrogel electrolyte allowing a voltage window of 1.8 V and showed energy densities of 0.46 μWh·cm^-2 while maintaining a high-power density of 703.12 μWh·cm^-2. This work gives insight into the use of oblique angle deposition for obtaining highly porous films of other electrode materials for microsupercapacitor applications with the advantage of using a simple microfabrication process.

Solid-State Dewetting of Gold on Stochastically Periodic SiO2 Nanocolumns Prepared by Oblique Angle Deposition

Solid-state dewetting (SSD) on patterned substrates is a straightforward method for fabricating ordered arrays of metallic nanoparticles on surfaces. However, a drawback of this procedure is that the patterning of substrates usually requires time-consuming and expensive two-dimensional (2D) fabrication methods. Nanostructured thin films deposited by oblique angle deposition (OAD) present at the surface a form of stochastically arranged periodic bundles of nanocolumns that might act as a patterned template for fabricating arrays of nanoparticles by SSD. In this work, we explore this concept and investigate the effect of three different types of OAD SiO₂ thin films on the SSD of Au deposited on their surface. We demonstrate that the size and spatial distribution of the particles can be tailored through the surface morphology of these OAD film substrates. It has been found that the SSD of the evaporated Au layer gives rise to a bimodal size distribution of particles. A majority of them appeared as mesoparticles with sizes ≳100 nm and the rest as nanoparticles with ∼10 nm, respectively, located either on top of the nanocolumns following their lateral distribution (i.e., resulting from a patterning effect) or incorporated inside the open mesopores existing among them. Moreover, on the SiO₂-OAD thin films where interconnected nanocolumnar bundles arrange in the form of discrete motifs, the patterning effect gave rise to the formation of approximately one Au mesoparticle per motif, which is one of the assets of patterned SSD. The morphological, optical (i.e., plasmon resonance), and crystalline structural characteristics of Au mesoparticles suggest that the interplay between a discontinuous nanocolumnar surface acting as a template and the poor adhesion of Au onto SiO₂ are key factors for the observed template effect controlling the SSD on the surface of OAD thin films.

Kinetic energy-induced growth regimes of nanocolumnar Ti thin films deposited by evaporation and magnetron sputtering

We experimentally analyze different growth regimes of Ti thin films associated to the existence of kinetic energy-induced relaxation mechanisms in the material's network when operating at oblique geometries. For this purpose, we have deposited different films by evaporation and magnetron sputtering under similar geometrical arrangements and at low temperatures. With the help of a well-established growth model we have found three different growth regimes: (i) low energy deposition, exemplified by the evaporation technique, carried out by species with typical energies in the thermal range, where the morphology and density of the film can be explained by solely considering surface shadowing processes, (ii) magnetron sputtering under weak plasma conditions, where the film growth is mediated by surface shadowing mechanisms and kinetic-energy-induced relaxation processes, and (iii) magnetron sputtering under intense plasma conditions, where the film growth is highly influenced by the plasma, and whose morphology is defined by nanocolumns with similar tilt than evaporated films, but with much higher density. The existence of these three regimes explains the variety of morphologies of nanocolumnar Ti thin films grown at oblique angles under similar conditions in the literature.

Highly anisotropic titanium nitride nanowire arrays for low-loss hyperbolic metamaterials fabricated via dynamic oblique deposition

Hyperbolic metamaterials (HMMs) with highly anisotropic metal nanowires exhibit unique optical properties arising from their extraordinary optical anisotropy. Although metal nanowires are often fabricated by embedding noble metals such as silver in an anodic alumina membrane dielectric host, the low melting point of noble metals limits their utilization in high-temperature applications, and there are fabrication issues such as overfilling or discontinuous islands within the host pores. Thus, metal nanowires with a high melting point for HMMs and alternative fabrication techniques are desired. In this study, we fabricated a highly anisotropic nanowire array (NWA) using titanium nitride, which has a high melting point, via dynamic oblique deposition of titanium and subsequent thermal treatment in ammonia. Spectra of ellipsometric parameters were well-fitted by a Fresnel reflection theoretical model considering the optical anisotropy, in which the effective permittivity was described using effective medium theory and the Drude-Lorentz model. The out-of-plane component of the effective permittivity was negative at λ > 850 nm, whereas the in-plane component was positive, indicating that the fabricated NWA behaves as a HMM. The figure of merit in the near-infrared range was higher than that of conventional multilayer TiN HMMs. The NWA presented here is a promising candidate for HMMs in high-temperature applications due to the high melting point of titanium nitride.

Understanding the Influence of Interface Morphology on the Performance of Perovskite Solar Cells

In recent years, organo-halide perovskite solar cells have garnered a surge of interest due to their high performance and low-cost fabrication processing. Owing to the multilayer architecture of perovskite solar cells, interface not only has a pivotal role to play in performance, but also influences long-term stability. Here we have employed diverse morphologies of electron selective layer (ESL) to elucidate charge extraction behavior in perovskite solar cells. The TiO₂ mesoporous structure (three-dimensional) having varied thickness, and nanocolumns (1-dimensional) with tunable length were employed. We found that a TiO₂ electron selective layer with thickness of about c.a. 100 nm, irrespective of its microstructure, was optimal for efficient charge extraction. Furthermore, by employing impedance spectroscopy at different excitation wavelengths, we studied the nature of recombination and its dependence on the charge generation profile, and results showed that, irrespective of the wavelength region, the fresh devices do not possess any preferential recombination site, and recombination process is governed by the bulk of the perovskite layer. Moreover, depending on the type of ESL, a different recombination mechanism was observed that influences the final behavior of the devices.