Spectroscopic ellipsometry modelling of thin AuSn films and nanostructures as plasmonics materials

We present a reviewof phase formation tendencies, methods forpreparation and optical properties of alloys and compounds from thebinary systems of silver or gold with metals and metalloids from thep-block of the Periodic system of elements. Reference data about thehomogeneity regions in the systems of interest, together with informationabout the crystalline structure of existing indexed compounds in them,is proposed and statistically analyzed. General background for thesynthesis of intermetallic alloys and compounds, and the tendenciesfor their preparation for plasmonic purposes are presented. The highplasma frequency, ωp of p-block metals makes theiralloys with silver and gold an interesting object of study, due tothe possibility of ωp variation over a wide intervalin the ultraviolet (UV) spectral region with a view to finding moreefficient materials for excitation of a localized surface plasmonresonance (LSPR) necessary for various applications and techniquesoperating in this part of the electromagnetic spectrum. Unlike thealloys between the noble metals Cu, Ag, and Au, which form continuousseries of solid solutions, different areas can be observed in thephase diagrams of the Ag(Au)–p-block systems, containing solidsolutions, intermetallic compounds, and heterogeneous mixtures. Theability to vary the plasma frequency of solid solutions, like thealloys between the noble metals Cu, Ag, and Au, is the reason to payattention to the compositions of the Ag(Au–p-block systemsthat fall in these regions of their phase diagrams. The analysis ofthe published results for complex permittivity shows that the additionof small amounts of conductive p-block elements to noble metals reducesthe energy gap for interband transitions and increases their plasmonicactivity in the UV spectral range. The article analyzes the relationshipbetween electrical resistivity and LSPR excitation efficiency, whichshows that the intermetallic compounds from Ag(Au)–p-blocksystems with a well-ordered crystalline structure and good conductivitylevel can be more effective materials for UV plasmonics than the boundarysolid solutions. Intermetallic compounds can be easily obtained inthe form of bulk samples, thin films, and nanoparticles with controlledsize and geometric shape. The spectral dependences of the plasmonefficiency of the intermetallic compounds, determined from their complexpermittivity functions, show that they are promising materials forexcitation of LSPR in the UV spectral region.

Interrelation between nanostructure and optical properties of oxide thin films by spectroscopic ellipsometry

Relationships among surface processing at the nanometer scale, nanostructure and optical properties of thin oxide films

Correlating nanostructure and optical properties of thin hybrid films is the crucial ingredient for designing sustainable applications ranging from structural colors in anticounterfeiting to sensors. Here, the tailoring of the refractive index of hybrid cellulose nanofibril/water‐dispersed colloidal ink thin films is presented. The authors apply scalable, layer‐by‐layer slot‐die coating for preparing the cellulose nanofibril and hybrid thin films. Making use of the mobility of the polymer chains in the colloids upon annealing, the influence of the different colloid sizes and their glass transition temperature on the refractive index of the hybrid material is shown. The complex refractive indices of the thin films are characterized by spectroscopic ellipsometry and correlated to the different nanostructures of the thin films. The authors find that post‐deposition annealing changes the colloidal nanostructure from particulate to agglomerates. Depending on the size of the colloids, imbibition of the colloids into the cellulose nanofibril template is observed. This scalable approach offers new avenues in structural color functional biomaterial hybrid layers.

The enhancement of surface sensitivity by grazing incidence geometry facilitates the investigation of nanostructures in thin films and at surfaces. The technique provides information about the surface roughness, lateral correlations, sizes and shapes of objects (such as, nanoparticles and nanostructures) positioned on top of the surface or in a region near the surface. Grazing incidence small-angle neutron scattering (GISANS) overcomes the limitations of conventional small-angle neutron scattering for extremely small sample volumes in the thin-film geometry. Although real space analysis techniques, such as atomic force microscopy, provide easy access to surface structures, reciprocal space analysis techniques, such as GISANS, provide several advantages: (i) average statistical information over the large illuminated sample surface can be detected and (ii) buried lateral structures can be probed without damage, using the variable-probed depth as a function of the incident angle. To illustrate the potential applications and challenges of GISANS, several different examples of thin nanostructured polymer films are reviewed. Nanostructures in triblock copolymer thin films are studied in the bulk as well as at the polymer-air and the silicon–polymer interface. Confined nanostructures in a dewetted diblock copolymer film are also discussed in terms of contrast and experimental settings.

Synthesis and characterization of nano-structured Se77Sb15Ge8 thin films

AuSn and AuSn2 thin films (5 nm) were used as precursors during the formation of semiconducting metal oxide nanostructures on a silicon substrate. The nanoparticles were produced in the processes of annealing and oxidation of gold–tin intermetallic compounds under ultra-high vacuum conditions. The formation process and morphology of a mixture of SnO2 and Au@SnOx (the core–shell structure) nanoparticles or Au nanocrystalites were carefully examined by means of spectroscopic ellipsometry (SE), X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), and transmission electron microscopy (TEM) combined with energy-dispersive X-ray spectroscopy (EDX). The annealing and oxidation of the thin film of the AuSn intermetallic compound led to the formation of uniformly distributed structures with a size of ∼20–30 nm. All of the synthesized nanoparticles exhibited a strong absorption band at 520–530 nm, which is typical for pure metallic or metal oxide systems.

3M Nanostructured Thin Film (NSTF) electrocatalysts and electrodes are a unique approach towards addressing key technical commercialization challenges for PEM fuel cells and water electrolyzers. NSTF electrocatalysts comprise a nm-scale catalyst thin film supported on a high aspect ratio, sub-micron crystalline organic pigment whisker [1]. The thin film electrocatalyst structure imparts substantially high oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) specific and mass activities, and high resistance to electrocatalyst dissolution and sintering induced by electrochemical cycling. The NSTF support whisker enables exceptional resistance to corrosion in fuel cell and water electrolysis applications [2, 3]. Traditional NSTF electrodes are ultra-thin (<1 µm) and ionomer-free, consisting of a single layer of catalyzed NSTF supports embedded into an ion-conducting membrane, which enables substantially high specific power densities (kW/g) in fuel cell and water electrolyzers [4, 5]. Our presentation will address recent progress in the development, characterization, and modeling of next generation NSTF ORR electrocatalysts, and performance and durability characteristics of NSTF-based fuel cell and water electrolyzer membrane electrode assemblies (MEAs). Recent developmental NSTF ORR electrocatalysts are based on two distinct thin film morphologies: nanoporous thin film (NPTF) and ultra-thin film (UTF) [6]. The formation of high activity and durable NPTF and UTF ORR electrocatalysts depends strongly on alloy composition (alloying elements, alloy mole fractions) and process-induced structure control. Tailoring of these compositional and structural properties has resulted in several electrocatalysts yielding specific activities up to 7x higher than Pt nanoparticles in MEA testing (Fig. 1A). Electrocatalyst and electrocatalyst support durability remain key barriers to wide-spread commercialization of economically-competitive PEM fuel cells and water electrolyzers. In fuel cell applications, electrocatalysts must be tolerant of many 10s of thousands of load cycles and numerous off-nominal operations including stop/starts and fuel starvation. NSTF electrocatalysts are substantially robust towards support corrosion losses. In fuel cells, NSTF catalysts have exceeded U.S. Department of Energy (DOE) Fuel Cell Support Accelerated Stress Test (AST) 2020 targets [6], and NSTF OER electrocatalysts have yielded stable electrolyzer performance for 5000 hours (Fig. 1B). NSTF ORR electrocatalyst durability depends strongly upon composition and morphology. Previous generation “whiskerette” PtCoMn/NSTF electrocatalysts achieve 30% mass activity loss after the U.S. DOE Electrocatalyst AST, while nanoporous PtNi/NSTF mass activity losses exceed 60% [6], well above the 40% DOE target. NPTF and UTF PtNi ORR electrocatalyst durability has been improved by integration of Ir, resulting in mass activity losses approaching the DOE target and substantially stable H2/Air performance at ultra-low PGM loadings (Fig. 1C). The ultra-thin, ionomer-free NSTF electrode minimizes reactant, ionic, and electronic transport distances and can enable substantially high power densities at low absolute electrode loadings and surface areas. An experimental UTF fuel cell MEA has demonstrated a specific power density of 8.1kW/gPGM with only 0.077 mgPGM/cm2 total MEA loading [6], exceeding the DOE 2020 targets of 8.0kW/gPGM and 0.125mgPGM/cm2­. With water electrolysis MEAs, current densities exceeding 15A/cm2 have been demonstrated with only 0.50mgPGM/cm2 total MEA loading­[5]. While enabling very high specific power densities, the ultra-thin traditional NSTF electrode structure also brings unique challenges. In fuel cells, traditional NSTF electrodes are susceptible to water flooding, causing larger-than-desired performance sensitivities to operating conditions. The operational robustness has been substantially addressed by both electrode-extrinsic and electrode-intrinsic approaches [7, 8]. In water electrolysis applications, H2 crossover from cathode to anode can yield higher than acceptable H2 concentrations in the O2 effluent stream. Effective H2 crossover mitigation has been developed, and H2crossover has been reduced two orders of magnitude (Fig. 1D) with little apparent impact on performance or durability. Acknowledgements We acknowledge 3M Company and the US Department of Energy, which provided funding for this work under grants DE-EE0007270, DE-SC0004192, DE-SC0007471, and NASA for funding under grant NNX12CE73P. References M. K. Debe, J. Electrochem. Soc. 160(6) F522-F534 (2013).M. K. Debe et al., J. Electrochem. Soc. 159(6) K165-K176 (2012).K. A. Lewinski at el., 228th Meeting of The Electrochemical Society, MA2015-02 1457. A. J. Steinbach et al., ECS Trans. 69(17) 291-301 (2015).K. A. Lewinski et al., 227th Meeting of The Electrochemical Society, MA2015-01 1948.A. J. Steinbach, U.S. Department of Energy Hydrogen and Fuel Cells Program Annual Merit Review and Peer Evaluation, Project FC143, June 7th, 2017, Washington, DC. Submitted. A. J. Steinbach, U.S. Department of Energy Hydrogen and Fuel Cells Program Annual Merit Review and Peer Evaluation, Project FC104, June 8th, 2016, Washington, DC.A. T. Haug, U.S. Department of Energy Hydrogen and Fuel Cells Program Annual Merit Review and Peer Evaluation, Project FC155, June 6th, 2017, Washington, DC. Submitted. Figure 1

Engineering of morphological, optical, structural, photocatalytic and catalytic properties of nanostructured CuO thin films fabricated by reactive DC magnetron sputtering

A method is developed for producing nano-structured titanium oxide thin films using H2 gas interaction with titanium thin film at a high temperature. These nano-structured thin films have been formed on a quartz crystal substrate. Titanium (Ti) thin films were deposited on the quartz crystal using a RF magnetron sputterer. The samples were placed in the oven at 500-800°C for 5 hours. The gas mixture of 1% H2 in N2 was introduced in the oven. The process of Ti annealing in the presence of H2 carves Ti films into nano-structure shapes. The process is a gas-solid interaction. Thin films were characterised using Scanning Electron Microscopes (SEM) and X-Ray Diffraction (XRD) technique. The nano structures formed have dimensions in a range of 25nm - 150nm obtained after gas carving.

In the frame of the nanoarchitectonic concept, the objective of this study was to develop simple and easy methods to ensure the preparation of polymorphic HfO2 thin film materials (<200 nm) having the best balance of patterning potential, reproducibility and stability to be used in optical, sensing or electronic fields. The nanostructured HfO2 thin films with micropatterns or continuous morphologies were synthesized by two different methods, i.e., the micropatterning of sol-gel solutions by deep ultraviolet (DUV) photolithography or the electrophoretic deposition (EPD) of HfO2 nanoparticles (HfO2-NPs). Amorphous and monoclinic HfO2 micropatterned nanostructured thin films (HfO2-DUV) were prepared by using a sol-gel solution precursor (HfO2-SG) and spin-coating process following by DUV photolithography, whereas continuous and dense monoclinic HfO2 nanostructured thin films (HfO2-EPD) were prepared by the direct EPD of HfO2-NPs. The HfO2-NPs were prepared by a hydrothermal route and studied through the changing aging temperature, pH and reaction time parameters to produce nanocrystalline particles. Subsequently, based on the colloidal stability study, suspensions of the monoclinic HfO2-NPs with morphologies near spherical, spindle- and rice-like shapes were used to prepare HfO2-EPD thin films on conductive indium-tin oxide-coated glass substrates. Morphology, composition and crystallinity of the HfO2-NPs and thin films were investigated by powder and grazing incidence X-ray diffraction, scanning electron microscopy, transmission electron microscopy and UV-visible spectrophotometry. The EPD and DUV photolithography performances were explored and, in this study, it was clearly demonstrated that these two complementary methods are suitable, simple and effective processes to prepare controllable and tunable HfO2 nanostructures as with homogeneous, dense or micropatterned structures.

Purpose – The purpose of this paper is to investigate the effect of typical morphologies of Au-Sn IMCs (intermetallic compounds) at the interfaces of solder and pads on shear properties of laser reflowed micro-solder joints. Design/methodology/approach – Sn-2.0Ag-0.75Cu-3.0Bi (SnAgCuBi) solder balls (120 μm in diameter), pads with 0.1, 0.5, 0.9 or 4.0 μm thickness of Au surface finish, and different laser input energies were utilized to fabricate micro-solder joints with Au-Sn IMCs having different typical morphologies. The joints were performed by a shear test through a DAGE bond test system. Fracture surfaces of the joints were analyzed by scanning electron microscopy and energy-dispersive X-ray spectrometry to identify fracture modes and locations. Findings – Morphologies of Au-Sn IMCs would affect shear properties of the joints remarkably. When needle-like AuSn4 IMCs formed at the interfaces of solder and pads, almost entire surfaces presented the manner of ductile fracture. Moreover, shear forces of this kind of solder joints were higher than those of joints without Au-Sn IMCs or with a nearly continuous/continuous Au-Sn IMCs layer. The reason was that the shear performance of the solder joints with needle-like AuSn4 IMCs was enhanced by an interlocking effect between solder and needle-like AuSn4 IMCs. As a nearly continuous or continuous Au-Sn IMCs layer formed, the fracture surfaces presented more character of brittle than ductile fracture. However, if an Au layer still remained under Au-Sn IMCs, the shear performance of the joints would be enhanced. Originality/value – The results in this study can be used to optimize microstructures and shear properties of laser reflowed micro-solder joints.

Photocorrosion stability and the discrepancy between optical absorption and carrier diffusion length of cuprous oxide (Cu2O) thin films are the main limiting factors for hydrogen evolution and practical applications of Cu2O photocathodes. In this paper, by nanocrystal engineering the Cu2O optical absorbing thin film, the photocorrosion stability can be significantly improved. Furthermore, palladium (Pd) nanostructures are used to both act as a cocatalysis and address the discrepancy between optical absorption and carrier diffusion length of cuprous oxide and improve the photocatalytic activity of Cu2O photocathodes. By tuning the crystal quality of thin Cu2O film and Pd nanostructures through controlling the sputtering power of Cu2O and in situ plasma of substrate, the impact of the degree of crystallinity of thin Cu2O film and Pd nanostructures on photocorrosion stability and photocatalytic activity of prepared photocathodes are investigated in detail. Systematic characterization of prepared samples by using X‐ray diffraction, high‐resolution transmission electron microscopy, and scanning electron microscope analysis indicates that improving the crystallinity of deposited Cu2O thin film and Pd nanostructures can significantly improve the photocorrosion stability and performance of Cu2O based photocathodes.

Ellipsometry is widely used to determine the thermo-optical properties of thin polymer films. However, if the thermo-optic coefficient (TOC) and the linear thermal expansion coefficient (LTEC) are to be used to determine the temperature coefficient of electronic polarizability (TCEP) in thin polymer films, their values must be determined with the greatest possible accuracy, as both have the opposite effect. In this article, we analyze changes in ellipsometric parameters resulting from changes in the thin film temperature in order to develop a data analysis method for temperature-dependent spectroscopic ellipsometry that will facilitate the accurate determination of thermo-optical parameters, including the TCEP, in polymer thin films. As practical application examples, we identified optimal spectral windows to accurately determine the thermo-optical parameters of 50 to 150 nm-thick PMMA thin films deposited on Si and SiO2 substrates. The influence of thin-film thickness on the accuracy of TOC and LTEC determination is discussed.

Synthesis and characterization of graphite nanostructure thin film using the DC-sputtering technique has been carried out. Nanostructured graphite for target of deposition using DC-Sputtering technique has been prepared by milling technique using High Energy Milling (HEM) with the variation of milling time between 50 hours until 100 hours. First, the graphite target was prepared by doing a compaction using press machine to the nanostructured graphite powder got from milling process. Secondly, a thin film of graphite was fabricated using DC-Sputtering technique. The phase identification of nanostructured graphite thin film were carried out using X-Ray Diffraction (XRD), and the surface and cross section morphology of thin film were observed using Scanning Electron Microscopy (SEM). XRD identification shows the presence of peaks of Si(100) and C(002) in all conditions of preparing powder using for target, but a shift of the angel‘s peak to the left and the decreasing of peak intensity were found. While the observation using SEM to surface morphology of thin film shows that the form of thin films are mostly homogeneous, smooth and flat at the milling time of 50-75 hours. From the SEM photograph of cross section, it is shown that there is a tendency of the more commonly found particles of droplets on the surface of thin film with the increasing of milling process against the carbon powder as a constituent of pellets for the DC-Sputtering targets, especially in the case of C/Si thin film fabricated using target prepared by milling for 100 hours, the morphology of surface was worst. Keywords: Graphite Thin Film, Nanostructure, DC-Sputtering Technique, HEM, Carbon Target.