Variable Angle Spectroscopic Ellipsometry Research Articles

Electrical and Optical Characterization of Graphene Oxide and Reduced Graphene Oxide Thin Films

Despite a growing interest in graphene, an aspect which is less studied is the electrical and optical characterization of graphene oxide (GO)-based transparent conductors obtained using thermal annealing. In addition, few research works have studied the electrical properties of GO and reduced graphene oxide (RGO) films using electrical impedance measurements. In this study, electric impedance measurements are performed on GO and thermally reduced GO films dip-coated on glass substrates. The electric resistance of RGO films decreases by about two orders of magnitude compared to GO films. Moreover, optical microscopy and variable angle spectroscopic ellipsometry (VASE) were carried out on the same samples. Thermal annealing increases the optical conductivity and the absorption coefficient of GO films. Such findings could be used in many optoelectronic applications, improving future GO applicability.

Optical dielectric function and electronic band structure of dilute GaAsBi-GaAs multiquantum wells by using variable angle spectroscopic ellipsometry

ObjectivesThis letter reports about the electronic band structure of dilute GaAsBi-GaAs quantum wells (QWs) experimentally studied by variable angle spectroscopic ellipsometry (VASE) and theoretically by nextnanomat software by solving symmetric double quantum well. MethodologyThe GaAsBi-GaAs quantum wells (QWs) samples were grown by molecular beam epitaxy (MBE). X-Ray diffraction (XRD) measurements were performed to confirm the incorporation of Bismuth in GaAs. Raman measurements in all samples were performed to investigate the lattice vibration properties. Spectroscopic ellipsometry was used first time on bismide quantum wells to estimate the optical functions and critical point transition energies of the innovative GaAsBi quantum wells. A parameterized semiconductor model was used to demonstrate the dielectric function of alloys as a combination of Gaussian oscillators focused on critical points in the band structure. Then from this model, energy values of those critical points are extracted. ResultsThe bandgap was measured for QWs samples for a range of bismuth concentration (1 %, 2 % and 3 %) and bulk sample with Bismuth concentration 0 % and 5 %. PL measurements of bulk and quantum wells samples showed reduction in bandgap with the increase in Bismuth concentration. This reduction in bandgap for QW samples was also confirmed experimentally by VASE and theoretically by using nextnanomat software. ConclusionThis research work gives important electronic and optical parameters for the Bismide quantum wells to design optoelectronic devices using these innovative alloys.

Low-temperature NIR-VUV optical constants of (001) LaAlO3 crystal

The optical constants and dielectric function of (001) LaAlO3 crystal were investigated at low temperatures down to 10 K in the NIR-VUV spectral range (photon energies 0.8-8.8 eV). Reflection variable angle spectroscopic ellipsometry and transmission spectroscopy were applied. Interband transitions were examined using the Tauc plots and the critical-point analysis. At room temperature, the indirect bandgap of 5.6 ± 0.01 eV and the lowest-energy direct transition at 7.2 ± 0.03 eV were detected. On cooling to 10 K, a blueshift of ∼0.2 eV and ∼0.1 eV was observed for the indirect and direct transitions, respectively. In the transparency spectral range, the index of refraction was found to be nearly temperature-independent and vary with photon energy from 2.0 (1 eV) to 2.5 (5.5 eV). It was suggested that the excellent thermal stability of the index of refraction may be related to the revealed thermally stable interband transitions. The results are of importance for modeling and design of modern optical devices.

PECVD Silicon Nitride-Based Multilayers with Optimized Mechanical Properties

Silicon nitride (SiN) based films deposited by plasma-enhanced chemical vapor deposition (PECVD) have interesting optical, mechanical, and chemical properties. They are used for applications such as anti-reflective coatings and surface passivation layers in solar cells. Amorphous SiN-based films also are frequently used to create multilayer structures of alternatingly high-index and low-index films. This approach is very promising to fabricate narrow-bandwidth notch filters on photovoltaic cells to reflect on demand a wide variety of colors over the entire visible spectrum [1]. However, these multilayer structures need to be highly durable and mechanically stable since the lifecycle of photovoltaic cells in some applications [2] is very long, and the coated surfaces are very large. To satisfy these specific functional requirements, we need to adjust the films' optical and mechanical properties by changing the deposition parameters of a PECVD reactor. For instance, it is possible to strongly influence the chemical composition of amorphous SiN-based films, such as their stoichiometry, by tuning the gas flows [3]. This work investigates the effect of the deposition pressure, power, and source gas ratio on SiN-based monolayers during plasma deposition. We have deposited SiN and silicon oxynitride (SiON) films in an electron cyclotron resonance (ECR) PECVD reactor using a SiH4/N2/O2/Ar precursor mixture. We have measured the refractive index and the absorption of the films using a variable angle spectroscopic ellipsometry (VASE) to assess their optical quality. The mechanical properties of the films, such as residual stress and coefficient of thermal expansion, were measured ex-situ on a KLA-Tencor FLX-2320 film stress measurement system. The Young's modulus and hardness of the films were evaluated using nanoindentation. After studying the properties of monolayers, we have made numerical simulations and further characterizations to optimize the design of multilayer structures considering both the optical and mechanical properties. Through this discussion, we try to better understand the interactions taking place when amorphous SiN-based films with different structural properties and compositions are stacked on top of each other. Finally, we suggest methods to predict and control the residual stress in multilayer structures without affecting the optical properties.

Optical and Mechanical Properties of Europium-Doped Silicon Oxynitride Thin Films

Light emission from silicon-based systems has gained significant interest over the past few decades with the vision of developing an optoelectronic platform that can be integrated with the existing metal-oxide-semiconductor (MOS) technology [1]. Rare earth doping of silicon-based materials is a promising approach to fabricating efficient light emitting devices because of their excellent luminescence properties and their chemical stability [2]. Europium (Eu) ions are attractive dopants for silicon photonic applications due to their intense emission in the visible spectral region. The efficiency of such photonic devices can be influenced by the mechanical properties of the Eu-doped films. Thus, the integration of these films in photonic devices requires an understanding of their mechanical properties as well. In this work, we discuss the relationship between the photoluminescence (PL) behavior and the elasticity and hardness of Eu doped silicon oxynitride (SiON) thin films.We have fabricated thin films using an electron cyclotron resonance plasma-enhanced chemical vapor deposition system with integrated magnetron sputtering system [3] on p-type 3” Si (100) substrates. Silane (diluted in 90% argon) and nitrogen (diluted in 90% argon) were used as precursor gases, while a 99.9% pure Eu sputtering target was used as a sputtering source. Different sets of samples were fabricated by changing the sputtering power and argon flow rate to understand the effect of deposition parameters on the dopant concentration and the optical and mechanical properties. Post-deposition annealing in nitrogen and a mixture of nitrogen with hydrogen (5%) were performed over a wide range of temperatures from 600° to 1100° C. The effect of hydrogen passivation was studied as well. The atomic concentration of the constituents was identified by Rutherford backscattering spectrometry. Optical properties were investigated by variable angle spectroscopic ellipsometry (VASE) analysis. The PL spectra were obtained at room temperature with a laser diode excitation source, operating at a wavelength of 375nm. Finally, we performed nanoindentation measurements to understand the deformation behavior of SiON films with the incorporation of Eu. The indentation hardness and Young’s modulus of the films were investigated, respectively, for different Eu concentrations and different deposition parameters.Reference[1] A. Moadhen, H. Elhouichet, M. Oueslati, and M. Férid, “Photoluminescence properties of europium-doped porous silicon nanocomposites,” J. Lumin., vol. 99, no. 1, pp. 13–17, 2002, doi: 10.1016/S0022-2313(02)00322-8.[2] Z. Lin R. Huang, H. Wang, Y. Wang, Y. Zhang, Y. Gao, J. Song, C. Song, H. Li, “Dense nanosized europium silicate clusters induced light emission enhancement in Eu-doped silicon oxycarbide films,” J. Alloys Compd., vol. 694, pp. 946–951, 2017, doi: 10.1016/j.jallcom.2016.10.132.[3] J. W. Miller, Z. Khatami, J. Wojcik, J. D. B. Bradley, and P. Mascher, “Integrated ECR-PECVD and magnetron sputtering system for rare-earth-doped Si-based materials,” Surf. Coatings Technol., vol. 336, pp. 99–105, 2018, doi: 10.1016/j.surfcoat.2017.08.051.

Design and Fabrication of Multiple-Color-Generating Thin-Film Optical Filters for Photovoltaic Applications

The use of electric vehicles (EVs) can reduce greenhouse gas emissions, air pollution, dependency on fossil fuels, and their adverse health effects on humans. But, we can only utilize the full environmental benefits of EVs when they are charged with renewable energy sources with zero or low carbon emissions. As a solution, Mobarak et al. [1] suggested integrating low-cost, flexible, and thin-film copper indium gallium selenide (CIGS) solar cells directly onto the steel of all the upward-facing body parts of the vehicles. But, this integration of solar cells comes with an aesthetic drawback. Previously, colorful photovoltaics (PVs) have been designed with one-dimensional (1D) photonic crystals or various 1D and 2D metallic nanostructures for aesthetic building-integrated photovoltaics (BIPVs) [2, 3]. However, the functionality of our application differs from that of BIPV as we need maximum absorption of the solar spectrum to obtain maximum conversion efficiency. Thus, we propose replacing the anti-reflective coating (ARC) present in the solar cells with a notch filter (a narrow high-reflection region in the visible range along with high transmission for the rest of the solar spectrum) to obtain colors.High-performance notch filters with a narrow and ultra-steep notch are well known in literature [4, 5]. Generally, high-performance notch filters are designed with a minimum of 45 layers. It is challenging to use filters with many layers on solar cells due to fabrication and thickness complexities. Thus, we created designs with a maximum of 27 layers for possible integration with photovoltaics. We used OptiLayer [6] to simulate our designs and the gradual evolution technique was used to optimize the designs. We performed our simulations with a multilayer structure of alternating high and low refractive indices of 2.09 and 1.45, respectively, on top of a silicon substrate. We optimized this multilayer structure for three reference wavelengths (400 nm, 550 nm, and 700 nm) resembling three colors. Our designs have notch widths of less than 100 nm for all the reference wavelengths with an average of 70% reflection in the high-reflection region and less than 20% reflection in the high-transmission area.To fabricate our designs, we need materials that are transparent to the solar spectrum targeted by the active material of the solar cells. The materials also need to have refractive indices closer to our simulation. Thus, we chose the combination of silicon nitride and silicon dioxide as our high and low refractive index material, respectively [7, 8]. To better understand our designs’ optical characteristics, we fabricated a scaled-down version of our structure with 5-10 layers. We used electron cyclotron resonance plasma-enhanced chemical vapor deposition (ECR-PECVD) to deposit the multilayer structure on silicon wafers. To obtain the silicon nitride and silicon dioxide layers, we used a SiH4/N2/O2/Ar precursor mixture. By tuning the gas flow rate in the reactor chamber, we tuned the stoichiometry and obtained the required refractive index for each layer. To characterize the refractive index and thickness for each layer, we used variable angle spectroscopic ellipsometry (VASE). We made a detailed comparison of our simulation and fabrication results.

(Digital Presentation) Optimization of MIM Rectifiers for Terahertz Rectennas

There is a significant demand for harvesting renewable infrared (IR) energy from unused heat sources. The rectifying antenna (rectenna) device has the ability to capture alternating current (AC) IR radiation and rectify it into usable direct current (DC) electricity. Metal-Insulator-Metal (MIM) diodes have shown to be the most prominent contenders for rectenna applications. This is due to their ultra-fast current transport mechanism in the femtosecond range by means of quantum mechanical tunnelling. Optical rectification at 28.3 THz has recently been demonstrated by rectenna devices based on MInM diodes using Au/Al2O3/Ti [1] and Ti/TiO2/ZnO/Al [2] configurations. Although the results are promising, the overall conversion efficiency is quite low, 2.05 × 10-14 [1], mainly due to the poor rectification properties of the diodes. Other recent research [3-5] has focused on the combination of stoichiometric and non-stoichiometric oxides with the aim of engineering the barrier heights to achieve low dynamic resistance (R0 ), high responsivity (β0 ) and asymmetry (η0 ) at zero-bias where the results are very promising for self-biased rectennas. The most recent experimental breakthrough was achieved by Ni/NiO/AlOx/CrAu bowtie rectennas that feature a device area of 0.035 µm2, low R0 of 13 kΩ and high β0 of 0.5 A/W. The results show that 5.1% coupling efficiency and 1.7 × 10−8% power conversion efficiency can be achieved with correct optimization of oxide stack. Furthermore, the most recent theoretical study [6] shows that the β0 of the MI2M diodes can be further improved to ~ 5 A/W by keeping the impedance match between the diode and the antenna at around 100 Ω. The proposed Ti/1 nm TiO2/1 nm Nb2O5/Ti rectenna design can achieve diode cut-off frequency (fc ) of 17 THz and resistance × capacitance (RC) time constant of 9 fs assuming the diode area of 0.01 µm2. Liverpool group has demonstrated recently [7,8] the effect of resonant tunnelling in non-cascaded (Al/Ta2O5/Nb2O5/Al2O3/Al) and cascaded (Al/Nb2O5/Al2O3/Ta2O5/Al) triple insulator diode structures with an oxide thickness ratio of 1:3:1 (in nm) deposited by atomic layer deposition (ALD). The diodes exhibit superior β = 5 A/W at 0.2 V and η = 12 at 0.1 V, with a drawback of high R0 due to high barrier heights between the metal/oxide layers.In this paper, we further optimize the MInM diode configurations so that the metal/oxide barrier is significantly lowered to ~ 0.1 eV. This is achieved by using the combinations of rectenna contender oxides such as Al2O3, NiO and ZnO that have high electron affinity and low dynamic permittivity [9]. Ultra-thin (≤ 5 nm) insulating layers were fabricated using radio frequency (RF) magnetron sputtering and ALD. Metal electrodes were deposited by thermal evaporation and RF sputtering using shadow mask, photolithography and nanolithography processes. The device areas range from 100 µm × 100 µm, 1 µm × 1 µm and 100 nm × 100 nm depending on the patterning process to observe the effect of device scaling on the conduction mechanisms and direct current (DC) rectification properties. The deposited oxide layers were measured by variable angle spectroscopic ellipsometry (VASE) to ascertain their thickness, uniformity and optical constants. DC current voltage measurements were performed on fabricated diodes to evaluate key rectification parameters such as R0 , β0 , η and non-linearity (fNL ) around zero-bias. Complementary theoretical calculations were performed to substantiate the experimental results and allow comparison of different MInM diode configurations such as NiO/Al2O3, ZnO/Al2O3, NiO/ZnO and NiO/ZnO/Al2O3. This work shows that the coupling efficiency at IR cut-off frequencies can be improved by optimizing the barriers in MInM rectifiers. References. [1] Jayaswal et al.,Materials Today Energy, 7, 1-9 (2018); [2] Elsharabasy et al., IEEE J. Photovoltaics, 9, 1232, (2019); [3] Matsuura et al., Sci. Rep. 9, 1-7 (2019); [4] Weerakkody et al., ACS Appl. Nano Mater. 4, 2470–2475 (2021); [5] Belkadi et al., Nat. Commun., 12, 1–6 (2021); [6] Elsharabasy et al., Results Mater. 11, 100204 (2021); [7] Mitrovic et al., ECS Trans. 72, 287 (2016); [8] Tekin et al., Solid State Electron. 185, 108096 (2021); [9] Mitrovic et al., Materials, 14, 5218 (2021).

(Invited) Rare Earth Oxides on Wide Band Gap Semiconductors

Globally, most recent power electronics converter and device technology has been driven by the unprecedented demand seen within the automotive sector [1]. The latter demands power converters with near 100% energy efficiency, lightweight, compact and reliable. Wide band gap (WBG) semiconductor materials such as GaN and 4H-SiC have emerged as contenders to replace Si in many power electronics applications. Currently, GaN devices based on the high electron mobility transistor (HEMT) architecture are limited commercially to a maximum of 650 V [2,3]. GaN HEMTs have traditionally suffered from a poor thermal conductivity and the “current collapse” phenomenon, limiting their ability to operate within harsh environments. On the other hand, 4H-SiC has a few reliability issues that limit its ubiquitous application in the automotive sector [2]. 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. Furthermore, 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. In terms of SiC based devices, the use of SiO2 proves to be a bottleneck in exploiting full potential of SiC due to the low value of dielectric constant of SiO2 [4]. Since the oxide is subjected to an electric field that is ~2.5x the field in the semiconductor, the breakdown of SiO2 occurs first, and hence causes the breakdown of SiO2/4H-SiC based devices well below the critical electric field of SiC. This shortcoming led to the exploration of high-k dielectrics as gate stacks in 4H-SiC MIS based devices as well as for surface passivation. It is worth noting that SiO2 is the only dielectric commercially used for SiC devices largely due to the unavailability of a reliable high-k dielectric alternative. Most published results focus on use of Al2O3 and HfO2 films [5], however they have yet to become used mainstream in 4H-SiC devices.In this paper, two rare earth high-k oxides, Y2O3 and Sc2O3, both with dielectric constant >10, have been studied in terms of their suitability as gate dielectrics on GaN and 4H-SiC. The oxide films were prepared by radio frequency (RF) magnetron sputtering (Sc2O3) and ion gun sputtering (Y2O3). The substrates were cleaned with Kr ions with anode current of ~ 50 mA and accelerating voltage of 3 kV for 1-2 hours. Then metallic Y was sputtered using pressure of 1.5 x 10-5 mbar and current of 26 mA, which was followed by exposure to O2 to form Y2O3 films. Sc2O3 films were deposited using Moorfields nano-PVD (physical vapour deposition) equipment with circular Sc2O3 target of 99.99% purity using RF power of 60 W and a gas flow of 0.5 sccm for 3 nm film and 3 sccm for 40 nm film. The films were also deposited simultaneously on Si to be used as reference samples for variable angle spectroscopic ellipsometry (VASE) measurements to ascertain their thickness and optical properties. The X-ray photoelectron spectroscopy (XPS) was used to characterise comprehensively the oxide/semiconductor interface. The capacitance voltage measurements on MIS stacks were used to determine permittivity of deposited oxides. The complete band alignment of oxide/WBG semiconductor as well as comprehensive comparison to state of the art data will be presented and discussed in full paper. Acknowledgement. The UKIERI IND/CONT/G/17-18/18 and F.No.184-1/2018(IC) project funded by the British Council; UKRI GCRF GIAA award 2018/19 and EP/P510981/1, funded by the EPSRC, UK. References. [1] http://www.yole.fr/Compound_Semiconductor_Monitor_Q1.aspx. [2] Li et al., Materials 14, 5831 (2021); [3] Gonzalez et al. IEEE Trans. Ind. Electron. 67, 7375 (2020); [4] A. Siddiqui et al., J. Mater. Chem. C 9, 5055 (2021); [5] Bencherif et al., Appl. Phys. A 126, 854 (2020).

Impact of aqueous solution pH on network structure of corrosion‐induced surface layers of boroaluminosilicate glass

AbstractUnderstanding of the long‐term stability of vitrified nuclear waste glass is critical to its safe storage, but knowledge gaps remain regarding the corrosion mechanism, especially with respect to surface alteration layers whose presence changes the observed corrosion rates. Here, we report the aqueous corrosion of a model boroaluminosilicate glass at ground water relevant pHs as investigated by vibrational spectroscopy to understand the network structure of the alteration layer that we hypothesize controls the corrosion rates. Both specular reflection infrared spectroscopy and infrared variable angle spectroscopic ellipsometry were used to elucidate details of the structure of the alteration layers formed at pH 7 and pH 9. After 2312 days, the thickness of the alteration layer is found to be ∼2000 nm for corrosion at pH 7 and ∼300 nm for corrosion at pH 9. The interpretation of the vibrational spectra suggests that alteration layers formed at pH 7 and pH 9 both exhibit a porous network with "silica‐like" structures, but with differences in the bond lengths and bond angles in the glass network. Moreover, the relative abundance of the hydrous species within the porous network appears to be dependent on pH. Additionally, the spectroscopic data suggest that the B and Ca trapped in the alteration layer during the secondary corrosion of the pH 9 pre‐corroded sample in pH 3 solution are hydrated within the nano‐porous layer and not bound to the silicate network. However, the precise chemical form remains to be determined. Based on these findings, we propose that the combination of the network structure of the alteration layer and the relative abundance of hydrous species within the alteration layer act to control the transport of ionic species dissolved from the reaction front to the aqueous solution outside the glass.

Tailoring of Multisource Deposition Conditions towards Required Chemical Composition of Thin Films.

The model to tailor the required chemical composition of thin films fabricated via multisource deposition, exploiting basic physicochemical constants of source materials, is developed. The model is experimentally verified for the two-source depositions of chalcogenide thin films from Ga–Sb–Te system (tie-lines GaSb–GaTe and GaSb–Te). The thin films are deposited by radiofrequency magnetron sputtering using GaSb, GaTe, and Te targets. Prepared thin films are characterized by means of energy dispersive X-ray analysis coupled with a scanning electron microscope to determine the chemical composition and by variable angle spectroscopic ellipsometry to establish film thickness. Good agreement between results of calculations and experimentally determined compositions of the co-deposited thin films is achieved for both the above-mentioned tie-lines. Moreover, in spite of all the applied simplifications, the proposed model is robust to be generally used for studies where the influence of thin film composition on their properties is investigated.

Dependence of amplified spontaneous emission threshold of BUBD-1 thin films on molecular orientation

The dependence of the amplified spontaneous emission (ASE) threshold on molecular orientation in vacuum deposited BUBD-1 films was investigated with the plane waveguide structure. The molecular orientations measured by the variable angle spectroscopic ellipsometry and the angular-dependent p-polarized PL spectra were optimized by varying the substrate temperature during deposition. Both of the optimized ASE threshold and the gain factor were obtained at the substrate temperature of 75 °C. The lowest ASE threshold of 3.3 μJ/pulse is only one third of that at 30 °C. The maximized net gain factor of 10.55 cm−1 is 1.7 times higher than that of 6.11 cm−1 at 30 °C. The enhancement of ASE performance of BUBD-1 film was ascribed to the lower horizontal ratio and surface roughness, which increased the total reflection and decreased the light scattering of light emission in the thin film light waveguide. It is an efficient way to optimize the molecular dipole orientation to enhance the ASE properties.

Tunable emission from Eu:SiOxNy thin films prepared by integrated magnetron sputtering and plasma enhanced chemical vapor deposition

This paper reports tunable emission of trivalent (Eu3+) and divalent (Eu2+) europium (Eu) from SiOxNy films fabricated by integrated electron cyclotron plasma enhanced chemical vapor deposition and magnetron sputtering. The photoluminescence (PL) spectra of intense red emission from Eu3+ around 600 nm and blue (cyan) broadband emission from 400 to 750 nm of Eu2+ are observed under daylight conditions with the naked eye. The spectra reveal a strong dependency of the PL on the atomic concentration of the dopant and the silicon/nitrogen ratio. The atomic composition of the films is investigated by Rutherford backscattering spectrometry and elastic recoil detection analysis. Values of the refractive indices are obtained by variable angle spectroscopic ellipsometry. To confirm the presence of crystalline phases of europium silicate Eu4.67(SiO4)3O [Eu2(Si2O7)] and europium silicon oxynitride (EuSi2N2O2) distributed in the amorphous host matrix of SiOxNy, high-resolution x-ray diffraction is used. The findings of this work are promising toward the realization of europium-doped Si-based materials for photonics and lighting technologies.

Variable Angle Spectroscopic Ellipsometry Characterization of Graphene Oxide in Methanol Films

It has been widely established that solvents modify the functional groups on the graphene oxide (GO) basal plane and, thus, modify its reactivity. Despite the increasing interest in GO films, a less studied aspect is the influence of methanol on the refractive index of GO films. Herein, the Variable Angle Spectroscopic Ellipsometry (VASE) technique has been used to characterize the optical response of GO in methanol films (0.4 mg/mL) dip-coated on glass substrates. The ellipsometric data have been modeled using a Lorentz oscillator model. We have found that the energy of the oscillator at ~3.9 eV for GO in water shifts to ~4.2 eV for GO in methanol films.

Electrochemical biosensors based on peptide-kinase interactions at the kinase docking site

Kinases are important cancer biomarkers and are conventionally detected based on their catalytic activity. Kinases regulate cellular activities by phosphorylation of motif-specific multiple substrate proteins, resulting in a lack of selectivity of activity-based kinase biosensors. We present an alternative approach of sensing kinases based on the interactions of their allosteric docking sites with a specific partner protein. The new approach was demonstrated for the ERK2 kinase and its substrate ELK-1. A peptide derived from ELK-1 was bound to a gold electrode and ERK2 sensing was performed by electrochemical impedance spectroscopy. We performed a detailed analysis of the interaction between the ELK-1 peptide and the kinase on gold surfaces. Atomic force microscopy, variable angle spectroscopic ellipsometry, X-ray Photoelectron Spectroscopy, and polarization modulation IR reflection-absorption spectroscopy analysis of the gold surface revealed the adsorbed layer of the ERK2 on the peptide monolayer. The sensors showed a high level of target selectivity for ERK2 compared to the p38γ kinase and BSA. ERK2 was detected in its cellular concentration range, 0.5–2.0 μM, and the limit of detection was calculated to be 0.35 μM. Using the flexibility of peptide design, our method is generic for developing sensitive and substrate-specific biosensors and other disease-related enzymes based on their interactions.

Spectroscopic Ellipsometry Study on Tuning the Electrical and Optical Properties of Zr-Doped ZnO Thin Films Grown by Atomic Layer Deposition.

This work reports the ellipsometry analysis of atomic layer deposition (ALD) films of ZnO doped with Zr to determine parameters like free carrier concentration and mobility. Thin films of zinc oxide (ZnO) and Zr-doped ZnO of thickness ∼100 nm were prepared by atomic layer deposition on sapphire, SiO2/Si(100), and Si(100) substrates. Variable-angle spectroscopic ellipsometry was used to study their optical properties in the 0.5–3.5 eV spectral range. The optical constants were accurately obtained using a model that combines Drude and Tauc–Lorentz oscillators with Bruggeman effective medium approximations, allowing the inclusion of a roughness layer in the optical model. The effect of Zr doping (ca. 1.9–4.4 atom %) was then investigated in both as-prepared samples and samples annealed in the temperature range of 100–300 °C. All of the films exhibited good optical transparency (ca. 70–90% in the visible region). For doping levels below 2.7 atom %, the real part of the dielectric permittivity reveals a semiconductor-to-metal transition in the near-infrared (NIR) region, as the permittivity goes from positive to negative. Besides, the plasma energy increases with increasing Zr concentration, and both resistivity and carrier concentration exhibit slightly parabolic behaviors, with a minimum of ∼1.5 × 10–3 Ω cm and a maximum of 2.4 × 1020 cm–3, respectively, at the same critical Zr concentration (2.7 atom %). In contrast, the carrier mobility decreases rapidly from 76.0 to 19.2 cm2/(V s) with increasing Zr content, while conductivities and carrier mobilities worsen when the annealing temperature increases, probably due to the segregation of ZnO crystals. Finally, the optical band gap is very stable, revealing its interesting independence of substrate composition and annealing temperature, as it collapses to a single master curve when band gap energy is plotted versus free carrier concentration, following the Burstein–Moss effect. Overall, the Zr-doped ZnO films studied here would be a highly desirable system for developing thermally stable transparent conductive oxides (TCOs).

Spectroscopic ellipsometry characterization of photoinduced phenomena in Ge25Sb5Se70 films using the Cody–Lorentz parameterized model

Variable angle spectroscopic ellipsometry (VASE) was employed to examine the photoinduced effect on the optical properties of Ge25Sb5Se70 thin films, that were prepared by thermal evaporation method. The measurements were carried out at room temperature and in the UV–vis–NIR (near infrared) spectral region for photon wavelengths from 280 to 1000 nm. For data analysis, we employed linear combination consist from two Cody–Lorentz (CL) dispersion model in the entire measured photon range. The photo bleaching (increase in bandgap) was identified by this model. The behavior of the Cody–Lorentz model parameters with the time of light illumination is discussed in detail. To obtain a good fitting to the variation of fundamental CL amplitude with the time of exposer, we proposed a simple phenomenological model based on the alternation of bond formation and breaking. Good agreement between calculated and experimental data is obtained.

Optical characterization of inhomogeneous thin films with randomly rough boundaries.

An inhomogeneous polymer-like thin film was deposited by the plasma enhanced chemical vapor deposition onto silicon single-crystal substrate whose surface was roughened by anodic oxidation. The inhomogeneous thin film with randomly rough boundaries was created as a result. This sample was studied using the variable-angle spectroscopic ellipsometry and spectroscopic reflectometry. The structural model including the inhomogeneous thin film, transition layer, and identically rough boundaries was used to process the experimental data. The scalar diffraction theory was used to describe the influence of roughness. The influence of the scattered light registered by the spectrophotometer due to its finite acceptance angle was also taken into account. The thicknesses and optical constants of the inhomogeneous thin film and the transition layer were determined in the optical characterization together with the roughness parameters. The determined rms value of the heights of roughness was found to be in good agreement with values obtained using AFM. The results of the optical characterization of the studied inhomogeneous thin film with rough boundaries were also verified by comparing them with the results of the optical characterization of the inhomogeneous thin film prepared using the same deposition conditions but onto the substrate with a smooth surface.

Reduction of Electric Current Loss by Aggregation-Induced Molecular Alignment of a Non-Fullerene Acceptor in Organic Photovoltaics.

Y6 is a recently developed non-fullerene electron acceptor (NFA) with dithienothiophen[3.2-b]-pyrrolobenzothiadiazole as the central unit and improves the performance of organic photovoltaics (OPVs) in combination with many electron-donor polymers. Although Y6 has desirable electronic properties for OPVs, the origin of its superiority as an acceptor is unclear. This study empirically investigates why Y6 is an excellent acceptor by comparing Y6 with F8IC, an analogue with a similar electronic structure, in bulk heterojunction (BHJ) OPVs with various electron-acceptor concentrations. The difference in the performance between Y6 and F8IC appears only at high concentrations, suggesting that it originates from the aggregation structures of the NFAs in the BHJs. Electric current loss analysis reveals that the exciton loss and non-geminate recombination are suppressed more strongly in Y6 OPVs than in F8IC OPVs. Variable angle spectroscopic ellipsometry shows that Y6 molecules align in the in-plane direction at high concentrations, contributing to the high charge generation rate via efficient exciton collection.

Effect of thermal cycling on optical properties of e-beam deposited hafnia thin film

HfO2 thin films have gained much significance in recent years as a promising dielectric material for semiconductor electronics added to their wide applications in the field of optical filters as a high index material. The resistance of HfO2 films to impurity diffusion and intermixing at the interface as well as higher environmental stability have made these films one of the most extensively studied upon materials in laser optics, optical coatings and semiconductor domain. In the present study, Hafnium Oxide film was deposited on glass substrate using reactive oxygenated E-Beam deposition technique with in-situ quartz crystal thickness monitoring to control the film thickness and rate of evaporation. The coated substrate was optically characterized using spectrophotometer and Variable Angle Spectroscopic Ellipsometry (VASE) to determine its transmission spectra as well as optical constants. The coated sample was put under thermal stress testing in a test chamber with temperature variation from -40° to + 65° C in a cyclic manner for 7 cycles with a rate of temperature change of 5° C/minute. The coated sample was again optically characterized to investigate the effect of thermal cycling on its optical performance and physical parameters. Copyright © 2017 VBRI Press.

Dispersity within Brushes Plays a Major Role in Determining Their Interfacial Properties: The Case of Oligoxazoline-Based Graft Polymers.

Many synthetic polymers used to form polymer-brush films feature a main backbone with functional, oligomeric side chains. While the structure of such graft polymers mimics biomacromolecules to an extent, it lacks the monodispersity and structural purity present in nature. Here we demonstrate that side-chain heterogeneity within graft polymers significantly influences hydration and the occurrence of hydrophobic interactions in the subsequently formed brushes and consequently impacts fundamental interfacial properties. This is demonstrated for the case of poly(methacrylate)s (PMAs) presenting oligomeric side chains of different length (n) and dispersity. A precise tuning of brush structure was achieved by first synthesizing oligo(2-ethyl-2-oxazoline) methacrylates (OEOXMAs) by cationic ring-opening polymerization (CROP), subsequently purifying them into discrete macromonomers with distinct values of n by column chromatography, and finally obtaining poly[oligo(2-ethyl-2-oxazoline) methacrylate]s (POEOXMAs) by reversible addition–fragmentation chain-transfer (RAFT) polymerization. Assembly of POEOXMA on Au surfaces yielded graft polymer brushes with different side-chain dispersities and lengths, whose properties were thoroughly investigated by a combination of variable angle spectroscopic ellipsometry (VASE), quartz crystal microbalance with dissipation (QCMD), and atomic force microscopy (AFM) methods. Side-chain dispersity, or dispersity within brushes, leads to assemblies that are more hydrated, less adhesive, and more lubricious and biopassive compared to analogous films obtained from graft polymers characterized by a homogeneous structure.