Unusual High Hardness and Load-Dependent Mechanical Characteristics of Hydrogenated Carbon-Nitrogen Hybrid Films.

Mechanical components are exposed to a rigorous environment in a number of applications including engineering, aerospace, and automobiles. Thus, their service lifetime and reliability are always on the verge of risk. Protective coatings with high hardness are required to enhance their service lifetime and minimize the replacement cost and waste burden. Hydrogenated amorphous carbon including nitrogen-incorporated films, that are commonly deposited by plasma-enhanced chemical vapor deposition, are widely used for commercial protective coating applications. However, their mechanical hardness still falls into the moderate hard regime. This needs to be substantially enhanced for advanced applications. Here, we report the synthesis of very hard nanostructured hydrogenated carbon-nitrogen hybrid (n-C:H:N) films. The optimized n-C:H:N film displays a hardness of about 36 GPa, elastic modulus of 360 GPa, and reasonably good elastic recovery (ER) of 62.7%. The mechanical properties of n-C:H:N films are further tailored when nitrogen pressure is tuned during the growth. The realized remarkably improved mechanical properties are correlated with the films' structural properties and experimental growth conditions. We also conducted density functional theory calculations that show the trend for the elastic modulus of the amorphous carbon films with varying nitrogen concentrations matches well with experimentally measured values. Finally, we probed load-dependent mechanical properties of n-C:H:N films and found an anomalous behavior; some of the mechanical parameters, for instance, ER, reveal an irregular trend with indentation load, which we explain in the framework of the film-substrate composite concept. Overall, this work uncovers many unknown and exciting mechanical phenomena that could pave the way for new technological developments.

Sub-100 nm patterning of TiO2 film for the regulation of endothelial and smooth muscle cell functions.

Previous studies have shown that fundamental cell functions such as adhesion, proliferation, and morphology are regulated by the interaction of cells with basement membrane nano- and micron- scale surface topography. By taking the basement membrane as a guiding principle, the surface can be designed with biophysical cues in the form of surface topographies to modulate the cellular functions of vascular endothelial and smooth muscle cells, which are crucial for vascular diseases. Titanium-based materials whose surfaces are covered by a thin layer of titanium dioxide (TiO2) are utilized in several regenerative medicine applications such as vascular prosthetics. The utilization of TiO2-covered materials makes it essential to understand the interaction of cells with the TiO2 layer to control the cell response. While it has been shown by means of patterned micro- and nano-topography that it is important to regulate cell functions on non-metallic materials, it would be of interest in the field of regenerative medicine to study the cell response on patterned TiO2 surface layers. Previous studies have mostly focused on studying the cell response on random micro- and nano-roughened metallic and metal oxide surfaces as it is challenging to fabricate patterned TiO2 surface layers. Here, the biocompatibility of a method that is capable of the rational patterning of a continuous TiO2 surface layer of sub-100 nm resolution scale using thermally curable resin-based nanoimprinting was studied. The responses of human umbilical vein endothelial cell (HUVEC), such as proliferation, morphology and focal adhesions, and smooth muscle cell (SMC) proliferation and morphology to the nano-patterned TiO2 layer were investigated. Overall, the TiO2 layer with surface nano-gratings showed enhanced proliferation of HUVECs, while it significantly lowered the proliferation of SMCs as compared to the unpatterned control. The HUVECs and SMCs were shown by topography to be sensitive to the 70 nm gratings as evident by the regulation of proliferation and cell shape. A significantly lower focal adhesion density was found of HUVECs on TiO2 nano-gratings, while a significantly higher average focal adhesion size of HUVECs was seen on TiO2 nano-wells and nano-gratings, compared to the unpatterned controls.

Direct nanoimprint lithography of Al2O3 using a chelated monomer-based precursor.

Nanostructuring of Al2O3 is predominantly achieved by the anodization of aluminum film and is limited to obtaining porous anodized aluminum oxide (AAO). One of the main restrictions in developing approaches for direct fabrication of various types of Al2O3 patterns, such as lines, pillars, holes, etc, is the lack of a processable aluminum-containing resist. In this paper, we demonstrate a stable precursor prepared by reacting aluminum tri-sec-butoxide with 2-(methacryloyloxy)ethyl acetoacetate, a chelating monomer, which can be used for large area direct nanoimprint lithography of Al2O3. Chelation in the precursor makes it stable against hydrolysis whilst the presence of a reactive methacrylate group renders it polymerizable. The precursor was mixed with a cross-linker and their in situ thermal free-radical co-polymerization during nanoimprinting rigidly shaped the patterns, trapped the metal atoms, reduced the surface energy and strengthened the structures, thereby giving a ~100% yield after demolding. The imprinted structures were heat-treated, leading to the loss of organics and their subsequent shrinkage. Amorphous Al2O3 patterns with line-widths as small as 17 nm were obtained. Our process utilizes the advantages of sol-gel and methacrylate routes for imprinting and at the same time alleviates the disadvantages associated with both these methods. With these benefits, the chelating monomer route may be the harbinger of the universal scheme for direct nanoimprinting of metal oxides.

Direct imprinting of high resolution TiO(2) nanostructures.

We demonstrate a different approach to direct nanoimprint lithography of oxides, in particular TiO(2), using the metal methacrylate route which not only gives very high resolution ( approximately 20 nm) but also provides yields of approximately 100% over areas > 1 cm x 1 cm. TiO(2) was imprinted using a polymerizable liquid 'TiO(2) resin' consisting of a mixture of titanium methacrylate, ethylene glycol dimethacrylate, and azobis-(isobutyronitrile). The resin underwent free radical polymerization when imprinted using a silicon mold at 110 degrees C with pressures as low as 10 bar. Polymerization strengthens the imprinted structures, thereby giving approximately 100% yield after demolding. Heat-treatment of the imprinted structures at 400 degrees C resulted in the loss of organics and their subsequent shrinkage ( approximately 75%) without the loss of integrity or aspect ratio, and converted them to TiO(2) nanostructures as small as approximately 20 nm wide. Furthermore, our method demonstrates that large imprinted areas of sub-100-nm features can be achieved by sub-micron molds which translate into huge cost savings with the added flexibility of direct patterning of urinary as well as multi-component oxides.

Trapped charge dynamics in a sol-gel based TiO(2) high- k gate dielectric silicon metal-oxide-semiconductor field effect transistor.

We have studied the response of a sol-gel based TiO(2), high k dielectric field effect transistor structure to microwave radiation. Under fixed bias conditions the transistor shows frequency dependent current fluctuations when exposed to continuous wave microwave radiation. Some of these fluctuations take the form of high Q resonances. The time dependent characteristics of these responses were studied by modulating the microwaves with a pulse signal. The measurements show that there is a shift in the centre frequency of these high Q resonances when the pulse time is varied. The measured lifetime of these resonances is high enough to be useful for non-classical information processing.

TiO2 patterning using electro-hydrodynamic lithography.

An increasing number of technologies require the fabrication of micro- and nanostructures over large areas. Soft lithographic methods are gaining in popularity for the manufacture of low-cost micrometre and sub-micrometre structures. Increasingly, these methods developed to structure organic resists can also be used to pattern inorganic materials. Here we introduce a simple lithographic technique that is able to pattern ceramic TiO micro- and nanostructures with high fidelity. Our method makes use of an electrohydrodynamic (EHD) film instability that is controlled by a laterally modulated electric field. A spin-coated film of a stabilized metal alkoxide precursor material was patterned using EHD lithography followed by a heat treatment at 400 °C to yield crystalline TiO micropatterns. Our technique is rather general and can be extended to a number of single- and multicomponent oxide systems.