Actively Tunable Metasurfaces via Plasmonic Nanogap Cavities with Sub-10-nm VO2 Films.

Actively tunable optical materials integrated with engineered subwavelength structures could enable novel optoelectronic devices, including reconfigurable light sources and tunable on-chip spectral filters. The phase-change material vanadium dioxide (VO2) provides a promising solid-state solution for dynamic tuning; however, previous demonstrations have been limited to thicker and often rough VO2 films or require a lattice-matched substrate for growth. Here, sub-10-nm-thick VO2 films are realized by atomic layer deposition (ALD) and integrated with plasmonic nanogap cavities to demonstrate tunable, spectrally selective absorption across 1200 nm in the near-infrared (NIR). Upon inducing the phase transition via heating, the absorption resonance is blue-shifted by as much as 60 nm. This process is reversible upon cooling and repeatable over more than ten temperature cycles. Dynamic, ultrathin VO2 films deposited by ALD, as demonstrated here, open up new potential architectures and applications where VO2 can be utilized to provide reconfigurability including three-dimensional, flexible and large-area structures.

Optical Limiting Based on Huygens' Metasurfaces.

Optical limiting is desirable or necessary in a variety of applications that employ high-power light sources or sensitive photodetectors. However, the most prevalent methods compromise between on-state transmission and turndown ratio or rely on narrow transmission windows. We demonstrate that a metasurface-based architecture incorporating phase-change materials enables both high and broadband on-state transmission (-4.8 dB) while also providing a large turndown ratio (25.2 dB). Additionally, this design can be extended for broadband multiwavelength limiting due to the high off-resonance transmittance and readily scalable resonant wavelength. Furthermore, our choice of active material allows for protection in ultrafast laser environments due to the speed of the phase transition. These benefits offer a strong alternative to state-of-the-art optical limiters in technologies ranging from sensor protection to protective eyewear.

Atomic Layer Deposition of Ultrathin Nickel Sulfide Films and Preliminary Assessment of Their Performance as Hydrogen Evolution Catalysts.

Transition metal sulfides show great promise for applications ranging from catalysis to electrocatalysis to photovoltaics due to their high stability and conductivity. Nickel sulfide, particularly known for its ability to electrochemically reduce protons to hydrogen gas nearly as efficiently as expensive noble metals, can be challenging to produce with certain surface site compositions or morphologies, e.g., conformal thin films. To this end, we employed atomic layer deposition (ALD), a preeminent method to fabricate uniform and conformal films, to construct thin films of nickel sulfide (NiSx) using bis(N,N'-di-tert-butylacetamidinato)nickel(II) (Ni(amd)2) vapor and hydrogen sulfide gas. Effects of experimental conditions such as pulse and purge times and temperature on the growth of NiSx were investigated. These revealed a wide temperature range, 125-225 °C, over which self-limiting NiSx growth can be observed. In situ quartz crystal microbalance (QCM) studies revealed conventional linear growth behavior for NiSx films, with a growth rate of 9.3 ng/cm2 per cycle being obtained. The ALD-synthesized films were characterized using X-ray photoelectron spectroscopy (XPS) and X-ray diffraction (XRD) methods. To assess the electrocatalyitic activity of NiSx for evolution of molecular hydrogen, films were grown on conductive-glass supports. Overpotentials at a current density of 10 mA/cm2 were recorded in both acidic and pH 7 phosphate buffer aqueous reaction media and found to be 440 and 576 mV, respectively, with very low NiSx loading. These results hint at the promise of ALD-grown NiSx materials as water-compatible electrocatalysts.

Porphyrins as Templates for Site-Selective Atomic Layer Deposition: Vapor Metalation and in Situ Monitoring of Island Growth.

Examinations of enzymatic catalysts suggest one key to efficient catalytic activity is discrete size metallo clusters. Mimicking enzymatic cluster systems is synthetically challenging because conventional solution methods are prone to aggregation or require capping of the cluster, thereby limiting its catalytic activity. We introduce site-selective atomic layer deposition (ALD) on porphyrins as an alternative approach to grow isolated metal oxide islands that are spatially separated. Surface-bound tetra-acid free base porphyrins (H2TCPP) may be metalated with Mn using conventional ALD precursor exposure to induce homogeneous hydroxide synthetic handles which acts as a nucleation point for subsequent ALD MnO island growth. Analytical fitting of in situ QCM mass uptake reveals island growth to be hemispherical with a convergence radius of 1.74 nm. This growth mode is confirmed with synchrotron grazing-incidence small-angle X-ray scattering (GISAXS) measurements. Finally, we extend this approach to other ALD chemistries to demonstrate the generality of this route to discrete metallo island materials.

Fabrication of Thin Films of α-Fe2O3 via Atomic Layer Deposition Using Iron Bisamidinate and Water under Mild Growth Conditions.

Atomic layer deposition (ALD) has been shown to be an excellent method for depositing thin films of iron oxide. With limited iron precursors available, the methods widely used require harsh conditions such as high temperatures and/or the use of oxidants such as ozone or peroxide. This letter aims to show that bis(N,N'-di-t-butylacetamidinato) iron(II) (iron bisamidinate or FeAMD) is an ideal ALD precursor because of its reactivity with water and relative volatility. Using in situ QCM analysis, we show outstanding conformal self-limiting growth of FeOx using FeAMD and water at temperatures lower than 200 °C. By annealing thin films of FeOx at 500 °C, we observe the formation of α-Fe2O3, confirming that we can use FeAMD to fabricate thin films of catalytically promising iron oxide materials using moderate growth conditions.

Fabrication of transparent-conducting-oxide-coated inverse opals as mesostructured architectures for electrocatalysis applications: a case study with NiO.

Highly ordered, and conductive inverse opal arrays were made with silica and subsequently coated with tin-doped indium oxide (ITO) via atomic layer deposition (ALD). We demonstrate the utility of the resulting mesostructured electrodes by further coating them with nickel oxide via ALD. The NiO-coated arrays are capable of efficiently electrochemically evolving oxygen from water. These modular, crack-free, transparent, high surface area, and conducting structures show promise for many applications including electrocatalysis, photocatalysis, and dye-sensitized solar cells.

Real-time observation of atomic layer deposition inhibition: metal oxide growth on self-assembled alkanethiols.

Through in situ quartz crystal microbalance (QCM) monitoring, we resolve the growth of a self-assembled monolayer (SAM) and subsequent metal oxide deposition with high resolution. We introduce the fitting of mass deposited during each atomic layer deposition (ALD) cycle to an analytical island-growth model that enables quantification of growth inhibition, nucleation density, and the uninhibited ALD growth rate. A long-chain alkanethiol was self-assembled as a monolayer on gold-coated quartz crystals in order to investigate its effectiveness as a barrier to ALD. Compared to solution-loading, vapor-loading is observed to produce a SAM with equal or greater inhibition ability in minutes vs days. The metal oxide growth temperature and the choice of precursor also significantly affect the nucleation density, which ranges from 0.001 to 1 sites/nm(2). Finally, we observe a minimum 100 cycle inhibition of an oxide ALD process, ZnO, under moderately optimized conditions.

Systematic modulation of quantum (electron) tunneling behavior by atomic layer deposition on nanoparticulate SnO2 and TiO2 photoanodes.

Ultrathin films of TiO2, ZrO2, and Al2O3 were conformally created on SnO2 and TiO2 photoelectrodes via atomic layer deposition (ALD) to examine their influence upon electron transfer (ET) from the electrodes to a representative molecular receptor, I3(-). Films thicker than 2 Å engender an exponential decrease in ET time with increasing film thickness, consistent with tunneling theory. Increasing the height of the barrier, as measured by the energy difference between the transferring electron and the bottom of the conduction band of the barrier material, results in steeper exponential drops in tunneling rate or probability. The variations are quantitatively consistent with a simple model of quantum tunneling of electrons through square barriers (i.e., barriers of individually uniform energy height) that are characterized by individually uniform physical thickness. The findings demonstrate that ALD is a remarkably uniform and precise method for modifying electrode surfaces and imply that standard tunneling theory can be used as a quantitative guide to intentionally and predictively modulating rates of ET between molecules and electrodes.