Stable Ultra-thin Silver Films Grown by Soft Ion Beam-Enhanced Sputtering with an Aluminum Cap Layer.

Ultra-thin silver films are susceptible to ambient environments and form grayish layers in the silver mirroring process. The poor wettability together with the high diffusivity of surface atoms in the presence of oxygen accounts for the thermal instability of ultra-thin silver films in the air and at elevated temperatures. This work demonstrates an atomic-scale aluminum cap layer on the silver to enhance the thermal and environmental stabilities of ultra-thin silver films deposited by sputtering with the assistance of a soft ion beam reported in our previous work. The resulted film consists of an ion-beam-treated seed silver layer of ∼1 nm nominal thickness, a subsequent silver layer of ∼6 nm thickness produced by sputtering alone, and an aluminum cap layer of ∼0.2 nm nominal thickness. Although the aluminum cap is only one to two atomic layers and likely non-continuous, it significantly improved the thermal and ambient environmental stability of the ultra-thin silver films (∼7 nm thick) without affecting the film's optical and electrical properties. The improved environmental stability is attributed to the cathodic protection mechanism and reduced diffusivity of surface atoms. The improved thermal stability is attributed to the reduced mobility of surface atoms in the presence of aluminum atoms. Thermal treatment of the duplex film also improves the film's electrical conductivity and optical transmittance by enhancing its crystallinity. The annealed aluminum/silver duplex structure has exhibited the lowest electric resistivity among the reported ultra-thin silver films and high optical transmittance similar to the simulated theoretical results.

Single-beam plasma source deposition of carbon thin films.

A single-beam plasma source was developed and used to deposit hydrogenated amorphous carbon (a-C:H) thin films at room temperature. The plasma source was excited by a combined radio frequency and direct current power, which resulted in tunable ion energy over a wide range. The plasma source could effectively dissociate the source hydrocarbon gas and simultaneously emit an ion beam to interact with the deposited film. Using this plasma source and a mixture of argon and C2H2 gas, a-C:H films were deposited at a rate of ∼26 nm/min. The resulting a-C:H film of 1.2 µm thick was still highly transparent with a transmittance of over 90% in the infrared range and an optical bandgap of 2.04 eV. Young's modulus of the a-C:H film was ∼80 GPa. The combination of the low-temperature high-rate deposition of transparent a-C:H films with moderately high Young's modulus makes the single-beam plasma source attractive for many coatings applications, especially in which heat-sensitive and soft materials are involved. The single-beam plasma source can be configured into a linear structure, which could be used for large-area coatings.

A fully transparent, flexible PEDOT:PSS-ITO-Ag-ITO based microelectrode array for ECoG recording.

Integrative neural interfaces combining neurophysiology and optogenetics with neural imaging provide numerous opportunities for neuroscientists to study the structure and function of neural circuits in the brain. Such a comprehensive interface demands miniature electrode arrays with high transparency, mechanical flexibility, electrical conductivity, and biocompatibility. Conventional transparent microelectrodes made of a single material, such as indium tin oxide (ITO), ultrathin metals, graphene and poly-(3,4-ethylenedioxythiophene)/poly(styrenesulfonate) (PEDOT:PSS), hardly possess the desired combination of those properties. Herein, ultra-flexible, highly conductive and fully transparent microscale electrocorticogram (µECoG) electrode arrays made of a PEDOT:PSS-ITO-Ag-ITO assembly are constructed on thin parylene C films. The PEDOT:PSS-ITO-Ag-ITO assembly achieves a maximum ∼14% enhancement in light transmission over a broad spectrum (350-650 nm), a significant reduction in electrochemical impedance by 91.25%, and an increase in charge storage capacitance by 1229.78 µC cm-2. Peeling, bending, and Young's modulus tests verify the enhanced mechanical flexibility and robustness of the multilayer assembly. The µECoG electrodes enable electrical recordings with high signal-to-noise ratios (SNRs) (∼35-36 dB) under different color photostimulations, suggesting that the electrodes are resilient to photon-induced artifacts. In vivo animal experiments confirm that our array can successfully record light-evoked ECoG oscillations from the primary visual cortex (V1) of an anesthetized rat.

TiO2 nanotube membranes on transparent conducting glass for high efficiency dye-sensitized solar cells.

Crack-free TiO(2) nanotube (NT) membranes were obtained by short time re-anodization of a sintered TiO(2) NT array on Ti foil, followed by dilute HF etching at room temperature. The resulting freestanding TiO(2) membranes were opaque with a slight yellow color having one end open and another end closed. The membranes were then fixed on transparent fluorine-tin-oxide glass using a thin layer of screen-printed TiO(2) nanoparticles (NPs) as a binding medium. It was found that low temperature treatment of the resulting NT/NP film under appropriate pressure before sintering at 450 °C was critical for successful fixation of the NT membrane on the NP layer. The resulting films with open-ends of NT membranes facing the NP layer (open-ends down, OED, configuration) exhibited better interfacial contact between NTs and NPs than those with closed-ends facing the NP layer (closed-ends down, CED, configuration). The cells with an OED configuration exhibit higher external quantum efficiency, greater charge transfer resistance from FTO/TiO(2) to electrolyte, and better dye loading compared to CED configurations. The solar cells with the OED configuration gave 6.1% energy conversion efficiency under AM1.5G condition when the commercial N719 was used as a dye and I(-)/I(3)(-) as a redox couple, showing the promise of this method for high efficiency solar cells.