Variable Angle Spectroscopic Ellipsometry Research Articles

The effect of CsI on the thermal and optical properties of IR transparent Se-S-Ge-Sb-As chalcogenide glasses

This study aims to investigate the effect of CsI addition on the optical and thermal behavior of Se-S-Ge-Sb-As glasses. For this purpose, the glasses with nominal compositions of Se40S20Ge20Sb5As15 with different percentages of CsI (0 < x < 0.15) were ready by the usual melt-quenching method in quartz ampoule. The glasses were identified using XRD, DSC, UV-Vis-NIR, FTIR, Raman, and IR variable angle spectroscopic ellipsometry analysis. The glass transition temperature of glasses was decreased from 288 °C to 274 °C with increasing the CsI content. The bandgap energy and refractive index of glasses in the existence of CsI were about 1.78–1.72 eV, 2.45–2.40, respectively. Reducing IR transparency, shifting to longer wavelengths of cut-off wavelength, and lowering band gap energy were significant effects of adding CsI to the glass composition.

Determination of complex optical constants and photovoltaic device design of all-inorganic CsPbBr3 perovskite thin films

All-inorganic perovskites exhibit interesting properties and unprecedented stability compared to organic-inorganic hybrid lead halide perovskites. This work focuses on depositing and characterizing cesium lead bromide (CsPbBr3) thin films and determining their complex optical constants, which is a key requirement for photovoltaic device design. CsPbBr3 thin films are synthesized via the solution method followed by a hot-embossing step to reduce surface roughness. Variable angle spectroscopic ellipsometry measurements are then conducted at three angles (45°, 55°, and 65°) to obtain the ellipsometric parameters psi (Ψ) and delta (Δ). For the present model, bulk planar CsPbBr3 layer is described by a one-dimensional graded index model combined with the mixture of one Tauc-Lorentz oscillator and two Gaussian oscillators, while an effective medium approximation with 50% air void is adopted to describe surface roughness layer. The experimental complex optical constants are finally determined in the wavelength range of 300 to 1100 nm. Furthermore, as a design example demonstration, the simulations of single-junction CsPbBr3 solar cells are conducted via the finite-difference time-domain method to investigate the properties of light absorption and photocurrent density.

(Invited) Band Line-up of High-k Oxides on GaN

GaN high electron mobility transistors (HEMTs) have been commercially available for over 10 years, however gate leakage limits their performance. The HEMT has the advantages of offering simple associated circuit design and fail-safe operation. Currently the GaN based Metal-Insulator-Semiconductor (MIS)-HEMT device is seen to demonstrate superior performance in power electronics applications over the Schottky gate counterpart, due to its inherently lower gate leakage current, together with the ability to provide larger forward gate voltage swing by engineering the threshold voltage between depletion and enhancement mode operation and also an improved gate-drain breakdown voltage. High band gap gate dielectric materials are preferable as they can provide higher tunnelling barriers for electrons and holes, which result in lower gate leakage current. On the other hand, high dielectric constant (high-k) material is also necessary for improved electrostatic control over the channel and improved on-current, which in-turn results in higher transconductance. The quality of the gate dielectric and the dielectric/GaN interface plays a central role in device performance due to potential problems arising from fixed oxide charge, border and interface traps. The leakage current issue has been mitigated using Al2O3, SiO2 and Si3N4, but comes at a cost of device transconductance degradation and undesirable threshold voltage shifts. A number of high-k dielectrics such as HfO2, ZrO2, Ta2O5, LaLuO3 and TiO2 have been investigated to resolve this issue.In this paper, engineered high-k oxide approach will be presented where Al2O3 is doped with Ti to boost oxide permittivity value while preserving band offsets, as well as Ga2O3 doped with Al to increase band gap and maintain good interface quality with GaN. X-ray photoelectron spectroscopy, inverse photoemission spectroscopy and variable angle spectroscopic ellipsometry were used to estimate the band alignment and interfacial properties. TiO2 is very attractive due to having reported k of 20-86 but has a small band gap of 3.4 eV for amorphous and 3.26 eV for anatase TiO2, being far too narrow to obtain a sufficient band offset with GaN of >1 eV. Al2O3 on the other hand has a sufficient band offset of 1.8-2 eV with AlGaN but suffers from a low dielectric constant of 7.5-9.6 depending on the growth method. The previous studies of Al2O3/TiO2 nanolaminates show favourable properties, in particular the optimum between the rather high-k (~30) and low leakage current has been observed for 30% Ti. No band offset study has been reported for TixAl1-xOy/GaN system. We will show full band alignment analysis of TixAl1-xOy/GaN fabricated by atomic layer deposition (ALD) on GaN for the range of Ti composition of up to 40%. Furthermore, a trivalent Ga2O3 is a promising oxide due to its band gap of 4.4 - 4.9 eV and a moderate permittivity of 10-14. Thermally oxidized Ga2O3 has shown valence band offset of 1.4 eV to GaN. A drawback of thermal oxidation is a growth of non-stoichiometric oxide at GaN interface reported to be as Ga(x+2)N3xO(3-3x). In contrast, ALD has been shown to produce Ga2O3 with no interfacial layer with GaN and low density of interface states of 3.62x1011 cm-2eV-1. Despite the good interface with GaN, the issue with using Ga2O3 is a small conduction band offset (< 1 eV) leading to high leakage current. In this paper, Al doped Ga2O3 have been fabricated using ALD on GaN. The results point to substantial increase of the band gap from ~4.6 eV for Ga2O3 to 5.9 eV for the 1:19 Ga:Al doped sample and a strong suppression of leakage current for Al doped Ga2O3 MIS capacitors. The results are of importance for future GaN HEMTs. Acknowledgement. The work has been conducted under the University Grants Commission - UK-India Education Research Initiative (UGC-UKIERI) joint research programme UKIERI III, project numbers IND/CONT/G/17-18/18 and F.No.184-1/2018(IC), titled “Dielectric engineering on GaN for sustainable energy applications” funded by the British Council. The authors also acknowledge UKRI GCRF GIAA award 2018/19 and “Digital in India” project no. EP/P510981/1, both funded by the EPSRC, UK.

Characterization of Europium Doped Silicon Oxide, Silicon Oxynitride, and Silicon Nitride Films Prepared By Integrated Ecr-PECVD and Magnetron Sputtering

The compatibility of Si-based light sources with mainstream metal-oxide semiconductor technology has become very interesting over the past decades due to their potential in integrated optoelectronics circuits of monolithic Si [1]. A very promising approach to improve the light emission from silicon-based materials is doping silicon with rare earth elements, and numerous research is involved in this field [1]. However, there are only a few works on europium (Eu) doped silicon-based matrix. In this work, Eu doped silicon oxide, silicon oxynitride, and silicon nitride thin films were deposited on P (100) Si substrates using integrated electron cyclotron resonance plasma enhanced chemical vapor deposition (ECR-PECVD) and magnetron sputtering. Silane (SiH4), oxygen (O2), and Nitrogen(N2) gases were used as precursors and solid Eu as the sputtering target. The integrated ECR-PECVD and sputtering system allows tunable and highly consistent Eu doping concentration, better control of the layer stoichiometry along with less hydrogen incorporation in the deposited film compared to conventional CVD systems [2]. Several thin films were investigated using room temperature photoluminescence (PL) and X-ray diffraction (XRD). A detailed investigation of the effect of annealing on the PL was performed. The annealing temperature was varied from 500 OC to 1200 OC in nitrogen and a mixture of nitrogen and hydrogen atmospheres. It was observed that the intensity of the PL spectra increased with increasing annealing temperature. Besides, the hydrogen passivation effect has been studied in this work, which shows that hydrogen passivation increases the intensity of the peak. Based on the XRD measurements, we have found that the amorphous silicon oxide structure changes to a crystalline structure by adding nitrogen in the film. Interestingly silicon oxynitride and silicon nitride films do not show any luminescence at room temperature. The influence of the precursors' gases in the atomic percentage of the thin films was analyzed by Rutherford Backscattering spectrometry (RBS). In addition to it, the thickness and the index of refraction were characterized by variable angle spectroscopic ellipsometry (VASE).Reference: 1. Lin, Z., Huang, R., Wang, H., Wang, Y., Zhang, Y., Guo, Y., ... & Li, H. (2017). Dense nanosized europium silicate clusters induced light emission enhancement in Eu-doped silicon oxycarbide films. Journal of Alloys and Compounds, 694, 946-951.2. Miller, J. W., Khatami, Z., Wojcik, J., Bradley, J. D. B., & Mascher, P. (2018). Integrated ECR-PECVD and magnetron sputtering system for rare-earth-doped Si-based materials. Surface and Coatings Technology, 336, 99-105.

Influence of Different Carbon Precursors on Optical and Electrical Properties of Silicon Carbonitride Thin Films

Silicon carbonitride (SiCN) thin films are widely used for protective hard coatings due to their superior mechanical and chemical properties such as high wear resistance, chemical and thermal stability at high temperatures, and hardness. Another scope of study of SiCN thin films is as the low dielectric constant (LKC) materials, which are desired for replacing silicon oxide (SiO2) in integrated circuits (IC). SiCN owes the interest shown to being an intermediate compound between silicon nitride (Si3N4) which is a highly transparent, wide band gap (5 eV) dielectric, and silicon carbide (SiC) with excellent mechanical performance.In this study we present the optical and electrical properties of SiCN:H thin films fabricated by electron cyclotron resonance plasma enhanced chemical vapor deposition (ECR PECVD) by using a mixture of acetylene (C2H2) or methane (CH4), silane (SiH4), argon (Ar), and nitrogen (N2) gas precursors. Samples fabricated with two different carbon sources were analyzed and compared regarding their chemical composition as well as their electrical and optical properties. The atomic composition of Si, C, N, O, and H were determined by Rutherford backscattering spectrometry (RBS) and elastic recoil detection (ERD) analysis and the chemical bonds formed in SiCN:H were analyzed through Fourier transform infrared spectroscopy (FTIR). Optical bandgap, index of refraction and extinction coefficient were analyzed by variable angle spectroscopic ellipsometry (VASE) and will be presented for different deposition conditions. The dielectric constants and dielectric breakdown voltages of the thin films were determined through current-voltage (I-V) and capacitance-voltage (C-V) measurements. Lastly, the hardness properties are explained due to varying C and H concentrations.

Optoelectronic Properties of Ce3+ Doped Silicon Oxide and Oxynitride Electroluminescent Devices

In this work, we propose a new type of silicon-based electroluminescent device. The thin film emitting layers of the device were deposited using Electron-Cyclotron-Resonance Plasma Enhanced Chemical-Vapor Deposition (ECR-PECVD) with in-situ Ce3+ doping on a P-type silicon substrate. Oxygen was gradually substituted by nitrogen to produce silicon oxynitride thin films with different layer compositions. Refractive indices extracted from variable-angle spectroscopic ellipsometry (VASE) measurements classified the thin films into two main groups, silicon oxide (SiOx) and silicon oxynitride (SiOxNy). The thin film composition was studied by Rutherford Backscattering Spectrometry (RBS), verifying the gradual increase in oxygen content. Photoluminescence (PL) spectroscopy of the emitting layer was obtained using a 375 nm laser as an excitation source. All samples were subjected to the post-deposition annealing treatment for 1 hour at different temperatures varying from 600 to 1200°C in 95% N2 and 5% H2 ambient gas environment, yielding considerably stronger blue PL emission than as-deposited ones. PL intensity of SiOx showed a sudden increase due to the formation of Ce2Si2O7 clusters when annealed at 1200°C. Internal Quantum Efficiency (IQE) and External Quantum Efficiency (EQE) were measured using an integrating sphere and a UV-Vis-NIR spectrometer. The optimum layer composition and annealing condition to produce SiOxNy thin films with maximized Ce3+ excitation efficiency were obtained. To further investigate the electrical performance of the produced samples, the thin films were coated with indium tin oxide (ITO) and aluminum (Al) on the top and bottom side of the thin film respectively. Current-Voltage (I-V) measurements showed improved charge injections in SiOxNy compared to SiOx, due to the reduction of the bandgap upon the incorporation of nitrogen.

Experimental Determination of Complex Optical Constants of Air‐Stable Inorganic CsPbI3 Perovskite Thin Films

Air‐stable inorganic cesium lead iodide (CsPbI3) perovskite thin films with a bandgap of 1.7 eV are a promising candidate for tandem cell solar cells, comprising a perovskite top cell with a crystalline silicon bottom cell. The device design and simulations are important to develop high‐efficiency photovoltaic devices. However, knowledge of complex optical constants of the CsPbI3 thin films is mandatory to complement such tasks. Herein, air‐stable inorganic CsPbI3 perovskite thin films are prepared using one‐step synthesis through a spin‐coating method. Variable angle spectroscopic ellipsometry (VASE) is then conducted at five angles (43.9°, 48.9°, 53.9°, 58.9°, and 63.9°) to obtain ellipsometric data (Ψ and Δ). The thickness nonuniformity model of the perovskite thin film combined with an effective medium approximation for describing rough surface is adopted to achieve excellent fitting. The complex optical constants of the CsPbI3 thin film are experimentally obtained in the wavelength range of 300–1200 nm. The present results open the door for design and simulations on high‐efficiency CsPbI3/c‐Si tandem solar cells.

Deep-ultraviolet aperiodic-oscillation emission of AlGaN films

In this Letter, we describe an aperiodic-oscillation emission phenomenon originating from the Fabry-Perot effect in the deep-ultraviolet backscattering fluorescence spectrum of the $c$c-plane AlGaN film, which is related to the dispersion of its ordinary refractive index near the band edge. Based on this fluorescence spectrum, the ordinary refractive index of the AlGaN film near the band edge could be directly obtained. Certainly, by means of variable angle spectroscopic ellipsometry, the ordinary refractive index of the AlGaN film could be also achieved. Comparing the results obtained by both methods, we discovered that the refractive indices are quite similar, which suggests that the aperiodic-oscillation fluorescence spectrum is also a reliable approach to capture the refractive index of anisotropic optical films.

Annealing-enhanced birefringence and aggregation in MEH-PPV: A spectroscopic ellipsometry study

We have used absorption, photoluminescence (PL), and variable angle spectroscopic ellipsometry (VASE) measurements to investigate the structural changes that take place upon high temperature annealing in spin-coated films of the prototypical conjugated polymer (CP) poly[2-(2′-ethylhexyloxy)-5-methoxy-1,4-phenylenevinylene] (MEH-PPV). Absorption and VASE measurements reveal that the birefringence of the films increases by approximately a factor of two upon heating, which indicates a significant increase in the alignment of the conjugated polymer (CP) strands within the film plane. Absorption and PL spectra indicate the formation in annealed films of interchain species having lower energy transitions. But these measurements alone do not reveal the type of interchain species formed, such as excimers or aggregates. VASE measurements were used to investigate this feature and clearly reveal a new, low energy, feature with a shoulder at 650 nm in the dispersion relations of the extraordinary (out-of-plane) extinction and absorption coefficients of the annealed films, which we assign to aggregate absorption. Thus, our work shows that VASE is sufficiently sensitive to measure aggregate absorption in CP films. In the case of the ordinary (in-plane) extinction and absorption coefficients, there is an increased amplitude of the lower energy peak upon heating, owing to increased uniaxial anisotropy, along with a broadening and a longer red tail, but the well-resolved red-shifted absorption band seen for the extraordinary absorption coefficient is not observed. Therefore, we conclude that while in-plane and out-of-plane aggregation occurs in annealed spin-coated films of MEH-PPV, aggregate absorption is only clearly observed when the aggregate electronic transition dipole is oriented preferentially in a direction perpendicular to the film plane. This conclusion is consistent with the usual observation that aggregate absorption in MEH-PPV films is not easily observed using absorption spectra alone, which are typically measured at normal incidence.

Near vacuum-ultraviolet aperiodic oscillation emission of AlN films

An accurate measurement of the refractive index is necessary for the optical design of both deep ultraviolet laser diodes and light-emitting diodes. Generally, the refractive indices along different crystallographic axes of anisotropic thin films are measured using variable angle spectroscopic ellipsometry. However, there are still some limitations concerning this method. Here we proposed a potential method to measure the band edge refractive index of wide bandgap semiconductor. An aperiodic oscillation emission phenomenon due to the Fabry-Perot effect was observed in the fluorescence spectrum of an AlN film with a thickness of 1500 nm. Based on the characteristics of the fluorescence spectrum and the definition of Fabry-Perot effect, we obtained the ordinary refractive index of the AlN thin film near the band edge directly. This refractive index measurement method is a supplement to the variable angle ellipsometry, and it is a more direct and effective method for transferred film and thinner samples to measure the fluorescence spectrum.

Ellipsometric characterization of highly non-uniform thin films with the shape of thickness non-uniformity modeled by polynomials.

A common approach to non-uniformity is to assume that the local thicknesses inside the light spot are distributed according to a certain distribution, such as the uniform distribution or the Wigner semicircle distribution. A model considered in this work uses a different approach in which the local thicknesses are given by a polynomial in the coordinates x and y along the surface of the film. An approach using the Gaussian quadrature is very efficient for including the influence of the non-uniformity on the measured ellipsometric quantities. However, the nodes and weights for the Gaussian quadrature must be calculated numerically if the non-uniformity is parameterized by the second or higher degree polynomial. A method for calculating these nodes and weights which is both efficient and numerically stable is presented. The presented method with a model using a second-degree polynomial is demonstrated on the sample of highly non-uniform polymer-like thin film characterized using variable-angle spectroscopic ellipsometry. The results are compared with those obtained using a model assuming the Wigner semicircle distribution.

Enhanced and Polarization-Dependent Coupling for Photoaligned Liquid Crystalline Conjugated Polymer Microcavities

Here we report the fabrication and optical characterization of organic microcavities containing liquid-crystalline conjugated polymers (LCCPs): poly(9,9-dioctylfluorene-co-benzothiadiazole) (F8BT), poly(9,9-dioctylfluorene) (PFO) and poly(2,7-(9,9-dihexylfluorene)-co-bithiophene) (F6T2) aligned on top of a thin transparent Sulfuric Dye 1 (SD1) photoalignment layer. We extract the optical constants of the aligned films using variable angle spectroscopic ellipsometry and fabricate metallic microcavities in which the ultrastrong coupling regime is manifest both for the aligned and non-aligned LCCPs. Transition dipole moment alignment enables a systematic increase in the interaction strength, with unprecedented solid-state Rabi splitting energies up to 1.80 eV for F6T2, the first to reach the visible spectrum; with an optical gap of 2.79 eV this also gives the highest-to-date organic microcavity coupling ratio, 65$\%$. We also demonstrate that the coupling strength is polarization-dependent with bright polaritons photoluminescence for TE polarization parallel to the transition dipoles and either no emission or weakly coupled emission from the corresponding TM polarization. The use of uniaxally aligned organic microcavities with switchable coupling strength offers exciting prospects for direct observations of ultrastrong coupling signatures, quantum simulation, polaritonics and condensation related phenomena.

Band Line-up Investigation of Atomic Layer Deposited TiAlO and GaAlO on GaN

A comprehensive study of the band alignments of TixAl1−xOy (with x = 9%, 16%, 25%, 36%, 100%) and GaxAl1−xOy (x = 5%, 20%, 80% and 95%) fabricated using atomic layer deposition on GaN has been presented using X-ray photoelectron spectroscopy and variable angle spectroscopic ellipsometry. The permittivity, k, has been found to be enhanced from ∼10 for 9% Ti in TixAl1−xOy to 76 for TiO2, however TiO2 brings an unfavorable band alignment and a small conduction band offset (<0.1 eV) with GaN. The latter has been observed for all studied TixAl1−xOy films deposited on GaN. On the other hand, GaxAl1−xOy films show a substantial increase of the band gap from 4.5 eV for Ga2O3 to 5.5 eV for x = 20% Ga and 6.0 eV for x = 5% Ga. A strong suppression of leakage current in associated GaxAl1−xOy-based metal insulator semiconductor capacitors has also been observed, showing promise for device applications.

Spectroscopic ellipsometry of inhomogeneous thin films exhibiting thickness non-uniformity and transition layers.

In this paper the complete optical characterization of an inhomogeneous polymer-like thin film of SiOxCyHz exhibiting a thickness non-uniformity and transition layer at the boundary between the silicon substrate and this film is performed using variable angle spectroscopic ellipsometry. The Campi-Coriasso dispersion model was utilized for describing the spectral dependencies of the optical constants of the SiOxCyHz thin film and transition layer. The multiple-beam interference model was used for expressing inhomogeneity of the SiOxCyHz thin film. The thickness non-uniformity of this film was taken into account by means of the averaging of the elements of the Mueller matrix performed using the thickness distribution for the wedge-shaped non-uniformity. The spectral dependencies of the optical constants of the SiOxCyHz thin film at the upper and lower boundaries together with the spectral dependencies of the optical constants of the transition layer were determined. Furthermore, the thickness values of the SiOxCyHz film and transition layer, profiles of the optical constants of the SiOxCyHz thin film and the rms value of local thicknesses corresponding to its thickness non-uniformity were determined. Thus, all the parameters characterizing this complicated film were determined without any auxiliary methods.

Surface modification of silicon solar cell using TiO2 and Ta2O5: fabrication and characterization

We report the fabrication and characterization of surface modified silicon solar cells with the deposition of amorphous tantalum oxide (Ta2O5) and crystalline titanium oxide (TiO2) nanolayers of thickness 54.9 nm and 69.82 nm, respectively, as antireflection coating (ARC) using RF-sputtering technique. The thickness of the films measured by variable angle spectroscopic ellipsometry and scanning electron microscopy is in close agreement. The transmittance measurement as a function of wavelength of incident light showed that the thin-films deposited have lowest effective reflectance in the wavelength range of 380 nm–570 nm indicating reduced light reflection and enhanced light trapping as observed from UV–Vis measurements. Illuminated current–voltage measurements showed an increase in the short circuit current density (Jsc) and an increase of 1.54% in the efficiency of the antireflection-coated cells. Results of the External Quantum Efficiency measurement as a function of wavelength for the solar cells with ARC is also presented in this paper.

Optical characterization of inhomogeneous thin films containing transition layers using the combined method of spectroscopic ellipsometry and spectroscopic reflectometry based on multiple-beam interference model

This paper presents the results of the optical characterization of inhomogeneous thin films of polymer-like SiOxCyHz and non-stoichiometric silicon nitride SiNx. An efficient method combining variable angle spectroscopic ellipsometry and spectroscopic reflectometry applied at the near-normal incidence based on the multiple-beam interference model is utilized for this optical characterization. The multiple-beam interference model allows us to quickly evaluate the values of ellipsometric parameters and reflectance of the inhomogeneous thin films, which exhibit general profiles of their optical constants. The spectral dependencies of the optical constants of the inhomogeneous SiOxCyHz and SiNx thin films are determined using the Campi–Coriasso dispersion model. The profiles of the optical constants of these films can also be determined. Furthermore, the transition layers at the lower boundaries of the characterized films are also taken into account. Spectral dependencies of the optical constants of these transition layers are also determined using the Campi–Coriasso dispersion model.

Spectroscopic Ellipsometry of Asphalt Binder: A Study of Optical Constants

Spectroscopic ellipsometry is a powerful technique for determining optical properties of any thin surface. A method for characterizing optical properties of warm asphalt binder using spectroscopic ellipsometry along with associated results has been presented in this paper. Thin films of warm asphalt binders were produced using LEADCAP and Sasobit and characterized using a variable angle spectroscopic ellipsometry. Refractive index and extinction coefficient of each of the samples (original and aged) were extracted from model fitted spectra. Clear variation in refractive index and extinction coefficient was observed among the samples before and after aging. The samples were found to be highly reflective with refractive index ‘n’ in the range 1.55–1.62 and extinction coefficient ‘k’ in the range of 0.01–0.06, both before and after aging. Reflectivity of PG 64-22 with LEADCAP exhibited the highest refractive index and the lowest extinction coefficient in visible wavelength of light among the samples. The results indicated that the highest reflection of visible light can be observed from LEADCAP-modified binder. Cracking properties and absorption coefficient of the binders are observed to have a correlation. The method reported here can be applied to any type of asphalt binder to determine its light absorption and reflection characteristics for a wide range of wavelength.

Spectroscopic and morphological study of graphene nanoplatelets thin films on Si/SiO2 substrates

In recent years, graphene nanoplatelets (GNPs) have emerged as innovative materials to be exploited for several applications. Nevertheless, the optical properties of GNPs thin films have not been studied yet. In this study, an exfoliation synthesized GNPs dispersion was dip-coated on Si/SiO2 substrates and GNPs thin films were studied by means of Variable Angle Spectroscopic Ellipsometry (VASE) in the [300–1000] nm wavelength range. Ellipsometry data was analyzed using a set of Gaussian oscillators. Moreover, the morphological and structural properties were investigated by Scanning Transmission Electron Microscopy (STEM), Scanning Electron Microscopy (SEM) and Micro-Raman Spectroscopy. Our results indicate that GNPs thin films have a lower optical density in comparison to other graphene related materials films.

Poly(2-methyl-2-oxazoline) and poly(4-vinyl pyridine) based mixed brushes with switchable ability toward protein adsorption

In this work, poly[(2-methyl-2-oxazoline)-random-glycidyl methacrylate] (PMOXA-r-GMA) and poly(4-vinyl pyridine)-block-poly(glycidyl methacrylate) (P4VP-b-PGMA) were synthesized by cationic ring opening polymerization (CROP) of 2-methyl-2-oxazoline (MOXA) and reversible addition-fragmentation chain transfer (RAFT) polymerization of 4-vinyl pyridine (4VP) followed by their random and block copolymerization with glycidyl methacrylate (GMA), respectively. Then poly(2-methyl-2-oxazoline)/poly(4-vinyl pyridine) (PMOXA/P4VP) based binary polymer brushes on silicon/glass substrate were fabricated using spin coating of the mixture of PMOXA-r-GMA and P4VP-b-PGMA copolymer solution followed by annealing process at 110 °C. Afterwards, these polymer brushes were characterized via water contact angle (WCA), X-ray photoelectron spectroscopy (XPS), variable angle spectroscopic ellipsometry (VASE), and atomic force microscopy (AFM) techniques. The results showed that the surface composition on the substrate surface could be adjusted by tuning the composition of the mixture of polymer solution used for the preparation of mixed brushes. Moreover, it also showed the swelling and de-swelling of mixed polymer brushes in a systematic way between two specific points during varying the pH of the surrounding medium from 2 to 10. Finally, the tunable behavior of PMOXA/P4VP based mixed brushes against bovine serum albumin (BSA) adsorption/desorption was investigated by fluorescein isothiocyanate-labelled BSA (FITC-BSA) assay and ellipsometry measurements. The results indicated that by changing the pH value from 2 to 10, the PMOXA/P4VP based mixed brushes could adsorb and desorb protein from very low to high amount depending upon the composition of PMOXA-r-GMA and P4VP-b-PGMA in preparing solution used for the PMOXA/P4VP based coatings, and ~89% of the adsorbed protein could be desorbed under the selected conditions.

Atomic Layer Deposition of Transparent p-Type Semiconducting Nickel Oxide Using Ni(tBu2DAD)2 and Ozone.

A novel atomic layer deposition (ALD) process for nickel oxide (NiO) is developed using a recently reported diazadienyl complex, Ni(tBu2DAD)2, and ozone. A window of constant growth per cycle is found between 185 and 200 °C at 0.12 nm/cycle, among the highest reported for ALD NiO. For films deposited at 200 °C, grazing-incidence X-ray diffraction indicates a randomly oriented polycrystalline cubic NiO phase. X-ray photoelectron spectroscopy shows good agreement with bulk NiO reference spectra and no detectable impurities. Atomic force microscopy reveals low root mean square roughness of 0.6 nm for an 18 nm thick film. The refractive index of 2.36 and an electronic bandgap of 3.78 eV, as determined by variable angle spectroscopic ellipsometry, are close to reported values for bulk and thin film NiO. Finally, fabricated Ag/NiO/n-Si/In heterojunction diodes show a current-voltage asymmetry of 1.27 × 104 at 2.3 V and an ideality factor of 3.5, confirming the intrinsic p-type semiconducting behavior of transparent NiO.