High-throughput measurements of thermochromic behavior in V(1-x)Nb(x)O(2) combinatorial thin film libraries

An Inter-Laboratory Study of Zn-Sn-Ti-O Thin Films using High-Throughput Experimental Methods

High-throughput experimental (HTE) techniques are an increasingly important way to accelerate the rate of materials research and development for many technological applications. However, there are very few publications on the reproducibility of the HTE results obtained across different laboratories for the same materials system, and on the associated sample and data exchange standards. Here, we report a comparative study of Zn-Sn-Ti-O thin films materials using high-throughput experimental methods at National Institute of Standards and Technology (NIST) and National Renewable Energy Laboratory (NREL). The thin film sample libraries were synthesized by combinatorial physical vapor deposition (cosputtering and pulsed laser deposition) and characterized by spatially resolved techniques for composition, structure, thickness, optical, and electrical properties. The results of this study indicate that all these measurement techniques performed at two different laboratories show excellent qualitative agreement. The quantitative similarities and differences vary by measurement type, with 95% confidence interval of 0.1-0.2 eV for the band gap, 24-29 nm for film thickness, and 0.08 to 0.37 orders of magnitude for sheet resistance. Overall, this work serves as a case study for the feasibility of a High-Throughput Experimental Materials Collaboratory (HTE-MC) by demonstrating the exchange of high-throughput sample libraries, workflows, and data.

Carbon-reduction as an easy route for the synthesis of VO2 (M1) and further Al, Ti doping

A low-cost and facile method to synthesize highly crystallized VO2 (M1) particles is proposed, using carbon black as the reducing agent mixed with V2O5 nanopowders comparing two types of vacuum systems for the thermal activation. In a sealed vacuum system, CO gas is generated in the first reductive step, and continues to reduce the new born VO2, until all the V (+4) is reduced to V (+3), resulting in V2O3 formation at 1000 °C. In contrast, in a dynamic vacuum system, CO gas is ejected through pumping as soon as it is generated, leading to the formation of pure VO2 (M1) at high temperatures (i.e. in the range 700 °C ≤ T ≤ 1000 °C). The evolution of the carbon content, determined by CHNS, of each sample versus the synthesis conditions, namely temperature and type of vacuum system, confirms that the transformation of V (+5) into V (+4) or V (+3) can be controlled. The characterization of the morphologies and crystal structures of two synthesized VO2 (M1) at 700 °C and 1000 °C shows the possibility to tune the crystallite size from 1.8 to more than 5 μm, with a uniform size distribution and highly crystallized powders. High purity VO2 (M1) leads to strong physical properties illustrated by a high latent energy (∼55 J g-1) during the phase transition obtained from DSC as well as high resistivity changes. In addition, with this method, dopants such as Ti4+ or Al3+ can be successfully introduced into VO2 (M1) thanks to the preparation of Al or Ti-doped nano-V2O5 by co-precipitation in polyol medium before carbon-reduction.

Approaches for the quantitative analysis of oxidation state in cerium oxide nanomaterials

Cerium oxide nanomaterials (nanoceria, CNMs) are receiving increased attention from the research community due to their unique chemical properties, most prominent of which is their ability to alternate between the Ce3+ and Ce4+ oxidation states. While many analytical techniques and methods have been employed to characterize the amounts of Ce3+ and Ce4+ present (Ce3+/Ce4+ ratio) within nanoceria materials, to-date no studies have used multiple complementary analytical tools (orthogonal analysis) with technique-independent oxidation state controls for quantitative determinations of the Ce3+/Ce4+ ratio. Here, we describe the development of analytical methods measuring the oxidation states of nanoceria analytes using technique-independent Ce3+ (CeAlO3:Ge) and Ce4+ (CeO2) control materials, with a particular focus on x-ray photoelectron spectroscopy (XPS) and electron energy loss spectroscopy (EELS) approaches. The developed methods were demonstrated in characterizing a suite of commercial nanoceria products, where the two techniques (XPS and EELS) were found to be in good agreement with respect to Ce3+/Ce4+ ratio. Potential sources of artifacts and discrepancies in the measurement results were also identified and discussed, alongside suggestions for interpreting oxidation state results using the different analytical techniques. The results should be applicable towards producing more consistent and reproducible oxidation state analyses of nanoceria materials.

Formation of a Crack-Free, Hybrid Skin Layer with Tunable Surface Topography and Improved Gas Permeation Selectivity on Elastomers Using Gel-Liquid Infiltration Polymerization

Surface modifications of elastomers and gels are crucial for emerging applications such as soft robotics and flexible electronics, in large part because they provide a platform to control wettability, adhesion, and permeability. Current surface modification methods via ultraviolet-ozone (UVO) and/or O₂ plasma, atomic layer deposition (ALD), plasmas deposition, and chemical treatment impart a dense polymer or inorganic layer on the surface that is brittle and easy to fracture at low strain levels. This paper presents a new method, based on gel-liquid infiltration polymerization, to form hybrid skin layers atop elastomers. The method is unique in that it allows for control of the skin layer topography, with tunable feature sizes and aspect ratios as high as 1.8 without fracture. Unlike previous techniques, the skin layer formed here dramatically improves the barrier properties of the elastomer, while preserving skin layer flexibility. Moreover, the method is versatile and likely applicable to most interfacial polymerization systems and network polymers on flat and patterned surfaces.

Visibly Transparent Heaters

Heater plates or sheets that are visibly transparent have many interesting applications in optoelectronic devices such as displays, as well as in defrosting, defogging, gas sensing and point-of-care disposable devices. In recent years, there have been many advances in this area with the advent of next generation transparent conducting electrodes (TCE) based on a wide range of materials such as oxide nanoparticles, CNTs, graphene, metal nanowires, metal meshes and their hybrids. The challenge has been to obtain uniform and stable temperature distribution over large areas, fast heating and cooling rates at low enough input power yet not sacrificing the visible transmittance. This review provides topical coverage of this important research field paying due attention to all the issues mentioned above.

Detecting Carbon in Carbon: Exploiting Differential Charging to Obtain Information on the Chemical Identity and Spatial Location of Carbon Nanotube Aggregates in Composites by Imaging X-ray Photoelectron Spectroscopy

To better assess risks associated with nano-enabled products including multiwalled carbon nanotubes (MWCNT) within polymer matrices, it is important to understand how MWCNT are dispersed throughout the composite. The current study presents a method which employs imaging X-ray photoelectron spectroscopy (XPS) to chemically detect spatially segregated MWCNT rich regions at an epoxy composites surface by exploiting differential charging. MWCNT do not charge due to high conductivity and have previously been shown to energetically separate from their insulating surroundings when characterized by XPS. XPS in imaging mode revealed that these conductive regions were spatially separated due to micrometer-scale MWCNT aggregation and poor dispersion during the formation of the composite. Three MWCNT concentrations were studied; (1, 4 and 5) % by mass MWCNT within an epoxy matrix. Images acquired in periodic energy intervals were processed using custom algorithms designed to efficiently extract spectra from regions of interest. As a result, chemical and electrical information on aggregate and non-aggregate portions of the composite was extracted. Raman imaging and scanning electron microscopy were employed as orthogonal techniques for validating this XPS-based methodology. Results demonstrate that XPS imaging of differentially charging MWCNT composite samples is an effective means for assessing dispersion quality.

An apparatus for spatially resolved, temperature dependent reflectance measurements for identifying thermochromism in combinatorial thin film libraries

A metrology and data analysis protocol is described for high throughput determination of thermochromic metal-insulator phase diagrams for lightly substituted VO2 thin films. The technique exploits the abrupt change in near infrared optical properties, measured in reflection, as an indicator of the temperature- or impurity-driven metal-insulator transition. Transition metal impurities were introduced in a complementary combinatorial synthesis process for producing thin film libraries with the general composition space V(1-x-y)M(x)M'(y)O2, with M and M' being transition metals and x and y varying continuously across the library. The measurement apparatus acquires reflectance spectra in the visible or near infrared at arbitrarily many library locations, each with a unique film composition, at temperatures of 1 °C-85 °C. Data collection is rapid and automated; the measurement protocol is computer controlled to automate the collection of thousands of reflectance spectra, representing hundreds of film compositions at tens of different temperatures. A straightforward analysis algorithm is implemented to extract key information from the thousands of spectra such as near infrared thermochromic transition temperatures and regions of no thermochromic transition; similarly, reflectance to the visible spectrum generates key information for materials selection of smart window materials. The thermochromic transition for 160 unique compositions on a thin film library with the general formula V(1-x-y)M(x)M'(y)O2 can be measured and described in a single 20 h experiment. The resulting impurity composition-temperature phase diagrams will contribute to the understanding of metal-insulator transitions in doped VO2 systems and to the development of thermochromic smart windows.