Nikolay Borodinov

Label-Free Water Sensors Using Hybrid Polymer–Dielectric Mid-Infrared Optical Waveguides

A chip-scale mid-IR water sensor was developed using silicon nitride (SiN) waveguides coated with poly(glycidyl methacrylate) (PGMA). The label-free detection was conducted at λ=2.6-2.7 μm because this spectral region overlaps with the characteristic O-H stretch absorption while being transparent to PGMA and SiN. Through the design of a hybrid waveguide structure, we were able to tailor the mid-IR evanescent wave into the PGMA layer and the surrounding water and, consequently, to enhance the light-analyte interaction. A 7.6 times enhancement of sensitivity is experimentally demonstrated and explained by material integration engineering as well as waveguide mode analysis. Our sensor platform made by polymer-dielectric hybrids can be applied to other regions of the mid-IR spectrum to probe other analytes and can ultimately achieve a multispectral sensor on-a-chip.

Kinetics of evaporation and gel formation in thin films of ceramic precursors.

Precursors derived from the hydrolysis of organic or inorganic salts have been widely used to produce ceramic coatings for a broad variety of applications. When applying the liquid precursors to the substrates, it is extremely challenging to control the film uniformity and homogeneity. The rate of solvent evaporation at different locations is different, causing the viscosity variation and flows in the film. There is very limited knowledge about the viscosity change in evaporating ceramic precursors. Therefore, it is crucial to understand the effect of evaporation on viscosity variation in thin films and droplets. We use magnetic rotational spectroscopy to study the time dependence of viscosity in mullite precursors. A correlation between the viscosity change and evaporation kinetics is revealed. This correlation was used to relate the change of viscosity to the concentration of mullite. A master curve relating viscosity to the mullite concentration was constructed and used to propose a possible scenario of the viscosity increase during solvent evaporation.

Stability of Grafted Polymer Nanoscale Films toward Gamma Irradiation

The present article focuses on the influence of gamma irradiation on nanoscale polymer grafted films and explores avenues for improvements in their stability toward the ionizing radiation. In terms of applications, we concentrate on enrichment polymer layers (EPLs), which are polymer thin films employed in sensor devices for the detection of chemical and biological substances. Specifically, we have studied the influence of gamma irradiation on nanoscale poly(glycidyl methacrylate) (PGMA) grafted EPL films. First, it was determined that a significant level of cross-linking was caused by irradiation in pure PGMA films. The cross-linking is accompanied by the formation of conjugated ester, carbon double bonds, hydroxyl groups, ketone carbonyls, and the elimination of epoxy groups as determined by FTIR. Polystyrene, 4-amino-2,2,6,6-tetramethylpiperidine-1-oxyl, dimethylphenylsilanol, BaF2, and gold nanoparticles were incorporated into the films and were found to mitigate different aspects of the radiation damage.

Multi-Frequency Measurement of Volatile Organic Compounds With a Radio-Frequency Interferometer

We present a radio-frequency (RF) sensor and its measurement results of three volatile organic compounds (VOCs) at multiple frequency points from ~ 2 to ~ 11 GHz, which is a convenient range in our examination. The sensor is based on a simple RF interferometer and uses two coplanar waveguides (CPWs), A and B of 5 and 25 mm length, respectively, as VOC sensing electrodes. Approximately 70-nm-thick poly copolymer films are coated on CPW surfaces for VOC adsorption and concentration. It is shown that ethanol, acetone, and isopropyl (IPA) induce frequency-dependent RF responses, which are also VOC-dependent. Thus, the frequency-dependent properties provide a possible new approach for better VOC sensing selectivity. With CPW A, the limit-of-detections (LODs) are ~ 600 ppm for ethanol, ~ 270 ppm for acetone, and ~ 330 ppm for IPA at 9.29 GHz. With CPW B, the LODs are roughly four times better. These LODs are also better than most of other RF VOC sensor results. In the future work, it is promising to further improve RF sensitivity and selectivity significantly.

Electrospray Deposition of Uniform Thickness Ge23Sb7S70 and As40S60 Chalcogenide Glass Films.

Solution-based electrospray film deposition, which is compatible with continuous, roll-to-roll processing, is applied to chalcogenide glasses. Two chalcogenide compositions are demonstrated: Ge23Sb7S70 and As40S60, which have both been studied extensively for planar mid-infrared (mid-IR) microphotonic devices. In this approach, uniform thickness films are fabricated through the use of computer numerical controlled (CNC) motion. Chalcogenide glass (ChG) is written over the substrate by a single nozzle along a serpentine path. Films were subjected to a series of heat treatments between 100 °C and 200 °C under vacuum to drive off residual solvent and densify the films. Based on transmission Fourier transform infrared (FTIR) spectroscopy and surface roughness measurements, both compositions were found to be suitable for the fabrication of planar devices operating in the mid-IR region. Residual solvent removal was found to be much quicker for the As40S60 film as compared to Ge23Sb7S70. Based on the advantages of electrospray, direct printing of a gradient refractive index (GRIN) mid-IR transparent coating is envisioned, given the difference in refractive index of the two compositions in this study.

En Route to Practicality of the Polymer Grafting Technology: One-Step Interfacial Modification with Amphiphilic Molecular Brushes

Surface modification with polymer grafting is a versatile tool for tuning the surface properties of a wide variety of materials. From a practical point of view, such a process should be readily scalable and transferable between different substrates and consist of as least number of steps as possible. To this end, a cross-linkable amphiphilic copolymer system that is able to bind covalently to surfaces and form permanently attached networks via a one-step procedure is reported here. This system consists of brushlike copolymers (molecular brushes) made of glycidyl methacrylate, poly(oligo(ethylene glycol) methyl ether methacrylate), and lauryl methacrylate, which provide the final product with tunable reactivity and balance between hydrophilicity and hydrophobicity. The detailed study of the copolymer synthesis and properties has been carried out to establish the most efficient pathway to design and tailor this amphiphilic molecular brush system for specific applications. As an example of the applications, we showed the ability to control the deposition of graphene oxide (GO) sheets on both hydrophilic and hydrophobic surfaces using GO modified with the molecular brushes. Also, the capability to tune the osteoblast cell adhesion with the copolymer-based coatings was demonstrated.

Highly Conductive and Transparent Reduced Graphene Oxide Nanoscale Films via Thermal Conversion of Polymer-Encapsulated Graphene Oxide Sheets

Despite noteworthy progress in the fabrication of large-area graphene sheetlike nanomaterials, the vapor-based processing still requires sophisticated equipment and a multistage handling of the material. An alternative approach to manufacturing functional graphene-based films includes the employment of graphene oxide (GO) micrometer-scale sheets as precursors. However, search for a scalable manufacturing technique for the production of high-quality GO nanoscale films with high uniformity and high electrical conductivity is still continuing. Here we show that conventional dip-coating technique can offer fabrication of high quality mono- and bilayered films made of GO sheets. The method is based on our recent discovery that encapsulating individual GO sheets in a nanometer thick molecular brush copolymer layer allows for the nearly perfect formation of the GO layers via dip coating from water. By thermal reduction the bilayers (cemented by a carbon-forming polymer linker) are converted into highly conductive and transparent reduced GO films with a high conductivity up to 104 S/cm and optical transparency on the level of 90%. The value is the highest electrical conductivity reported for thermally reduced nanoscale GO films and is close to the conductivity of indium tin oxide currently in use for transparent electronic devices, thus making these layers intriguing candidates for replacement of ITO films.

Molecular reorganization in bulk bottlebrush polymers: direct observation via nanoscale imaging

Bottlebrush polymers are important for a variety of applications ranging from drug delivery to electronics. The functional flexibility of the branched sidechains has unique assembly properties when compared to linear block polymer systems. However, reports of direct observation of molecular reorganization have been sparse. This information is necessary to enhance the understanding of the structure-property relationships in these systems and yield a rational design approach for novel polymeric materials. In this work, we report direct visualization of bottlebrush molecular organization and the formation of nematic-type ordering in an amorphous polymer bottlebrush system, captured with plasma etching and helium ion microscopy. By observing the unperturbed structure of this material at high resolution and quantifying image features, we were able to qualitatively link experimental results with structures predicted by coarse-grained molecular dynamics simulations. The direct visualization and computation workflow developed in this work can be applied to a broad variety of polymers with different architectures, linking imaging results with other, independent channels of information for better understanding and control of these classes of materials.

Photothermoelastic contrast in nanoscale infrared spectroscopy

The contrast formation mechanism in nanoscale Infrared (IR) Spectroscopy is analyzed. The temperature distribution and elastic displacement across the illuminated T-shape boundary between two materials with different IR-radiation absorption coefficients and thermo-physical and elastic properties located on a rigid substrate are calculated self-consistently for different frequencies f ∼ (1 kHz–1 MHz) of IR-radiation modulation (fully coupled problem). Analytical expressions for the temperature and displacement profiles across the “thermo-elastic step” are derived in the decoupling approximation for f = 0 (“static limit”), and conditions for approximation validity at low frequencies of IR-modulation are established. The step height was found to be thickness-independent for thick layers and proportional to the square of the thickness for very thin films. The theoretical results will be of potential interest for applications in the scanning thermo-ionic and thermal infrared microscopies for relatively long sa...

Effect of Gamma Exposure on Chalcogenide Glass Films for Microphotonic Devices

Bulk and thermally evaporated film forms of As2Se3 and Ge23Sb7S70 were subjected to gamma irradiation and characterized periodically after exposure. Differences in the effect of exposure were observed between bulk and film forms.