Ultra-broadband absorbance of nanometer-thin pyrolyzed-carbon film on silicon nitride membrane.

Fifty percents absorption by thin film, with thickness is much smaller than the skin depth and optical thickness much smaller than the wavelength, is a well-known concept of classical electrodynamics. This is a valuable feature that has been numerously widely explored for metal films, while chemically inert nanomembranes are a real fabrication challenge. Here we report the 20 nm thin pyrolyzed carbon film (PyC) placed on 300 nm thick silicon nitride (Si3N4) membrane demonstrating an efficient broadband absorption in the terahertz and near infrared ranges. While the bare Si3N4membrane is completely transparent in the THz range, the 20 nm thick PyC layer increases the absorption of the PyC coated Si3N4membrane to 40%. The reflection and transmission spectra in the near infrared region reveal that the PyC film absorption persists to a level of at least 10% of the incident power. Such a broadband absorption of the PyC film opens new pathways toward broadband bolometric radiation detectors.

Nano- and micro-crystalline diamond film structuring with electron beam lithography mask.

Direct current plasma enhanced chemical vapor deposition (CVD) was employed to create polycrystalline diamond films from CH4/H2gaseous mixture at 98 mbar pressure and various substrate temperatures between 720 °C and 960 °C. The Si chips with patterns of periodic masked and open seeded zones were used as substrates. The mask free seeded areas evolved into polycrystalline diamond films after CVD process. The diamond crystallites of the films featured single crystal ordering individually with distinct cubic (100) or octahedral (111) facets on the film surfaces. Notably, specific growth conditions were determined for obtaining diamond films composed of the crystallites of nanometre and micrometre scale. These conditions are differing from those observed for non-pattern-prepared Si substrates. The nano-crystalline diamonds emerged within the 4.5-5 A current range, with growth conditions involving 3% CH4/H2mixture at 98 mbar. The micro-crystalline diamonds (MCDs) predominantly characterized by well-developed rectangular (100) crystal faces on the film surface were successfully grown with current settings of 5.5-6 A, under 3% CH4/H2mixture at 98 mbar. Furthermore, MCDs characterized by entirely crystalline (111) diamond faces forming CVD film surface were attained within a growth parameter range of 4.5-5.8 A, employing 3% CH4/H2mixture for certain samples, or alternatively, utilizing 5 A with a 1.5% CH4/H2mixture for others. Upon thorough evaluation, it was established that SiO2, TiO2, and Cr masks are well-suited materials for the planar patterning of both nano- and micro-crystalline diamond films, and the bottom-up approach can pave the way for the production of diamond planar structures through CVD, facilitated by electron beam lithography (EBL).

Synthesis and Electrical Percolation of Highly Amorphous Polyvinyl Alcohol/Reduced Graphene Oxide Nanocomposite.

Polyvinyl alcohol is the most commercially water-soluble biodegradable polymer, and it is in use for a wide range of applications. It shows good compatibility with most inorganic/organic fillers, and enhanced composites may be prepared without the need to introduce coupling agents and interfacial modifiers. The patented high amorphous polyvinyl alcohol (HAVOH), commercialized with the trade name G-Polymer, can be easily dispersed in water and melt processed. HAVOH is particularly suitable for extrusion and can be used as a matrix to disperse nanocomposites with different properties. In this work, the optimization of the synthesis and characterization of HAVOH/reduced graphene oxide (rGO) nanocomposite obtained by the solution blending process of HAVOH and Graphene Oxide (GO) water solutions and 'in situ' reduction of GO is studied. The produced nanocomposite presents a low percolation threshold (~1.7 wt%) and high electrical conductivity (up to 11 S/m) due to the uniform dispersion in the polymer matrix as a result of the solution blending process and the good reduction level of GO. In consideration of HAVOH processability, the conductivity obtained by using rGO as filler, and the low percolation threshold, the nanocomposite presented here is a good candidate for the 3D printing of a conductive structure.

Piezoelectric Nanogenerators Based On BaTiO3/PDMS Composites for High-Frequency Applications.

A series of highly flexible and environmentally friendly composites based on polydimethylsiloxane (PDMS) filled with 200 nm size ferroelectric BaTiO3 (BTO) particles at different concentrations (from 7 to 23 vol %) have been fabricated by a simple dispersion method. The dielectric, piezoelectric, and ultrasonic properties have been studied. The ferroelectric state of BTO was confirmed by differential scanning calorimetry and ultrasonic spectroscopy. The addition of BTO into PDMS strongly affects the dielectric properties of the composites. At low temperatures close to 160 K, the PDMS matrix exhibits a dielectric anomaly related to a dynamic glass transition, which shifts to higher temperatures as the BTO content increases due to the strong interaction between polymer chains and nanoparticles. Ultrasonic measurements demonstrate the appearance of a piezoelectric voltage signal on a thin plate of the composite with the highest available filler concentration (23 vol %) under longitudinal stress applied by a 10 MHz ultrasonic wave. As a result, at room temperature, the detected signal is characterized by output voltage and specific stored energy values of 10 mV and 367.3 MeV/m2, respectively, followed by a further increase with cooling to 35 mV at 150 K. The proposed BTO/PDMS composite system is thus a potential candidate for nanogenerators, namely, a simple, flexible, and lead-free device converting high-frequency (10 MHz) mechanical vibrations into electrical voltage.

Stable and Reusable Lace-like Black Silicon Nanostructures Coated with Nanometer-Thick Gold Films for SERS-Based Sensing.

We propose a simple, fast, and low-cost method for producing Au-coated black Si-based SERS-active substrates with a proven enhancement factor of 106. Room temperature reactive ion etching of silicon wafer followed by nanometer-thin gold sputtering allows the formation of a highly developed lace-type Si surface covered with homogeneously distributed gold islands. The mosaic structure of deposited gold allows the use of Au-uncovered Si domains for Raman peak intensity normalization. The fabricated SERS substrates have prominent uniformity (with less than 6% SERS signal variations over large areas, 100 × 100 µm2). It has been found that the storage of SERS-active substrates in an ambient environment reduces the SERS signal by less than 3% in 1 month and not more than 40% in 20 months. We showed that Au-coated black Si-based SERS-active substrates can be reused after oxygen plasma cleaning and developed relevant protocols for removing covalently bonded and electrostatically attached molecules. Experiments revealed that the Raman signal of 4-MBA molecules covalently bonded to the Au coating measured after the 10th cycle was just 4 times lower than that observed for the virgin substrate. A case study of the reusability of the black Si-based substrate was conducted for the subsequent detection of 10-5 M doxorubicin, a widely used anticancer drug, after the reuse cycle. The obtained SERS spectra of doxorubicin were highly reproducible. We demonstrated that the fabricated substrate permits not only qualitative but also quantitative monitoring of analytes and is suitable for the determination of concentrations of doxorubicin in the range of 10-9-10-4 M. Reusable, stable, reliable, durable, low-cost Au-coated black Si-based SERS-active substrates are promising tools for routine laboratory research in different areas of science and healthcare.

Black Silicon: Breaking through the Everlasting Cost vs. Effectivity Trade-Off for SERS Substrates.

Black silicon (bSi) is a highly absorptive material in the UV-vis and NIR spectral range. Photon trapping ability makes noble metal plated bSi attractive for fabrication of surface enhanced Raman spectroscopy (SERS) substrates. By using a cost-effective room temperature reactive ion etching method, we designed and fabricated the bSi surface profile, which provides the maximum Raman signal enhancement under NIR excitation when a nanometrically-thin gold layer is deposited. The proposed bSi substrates are reliable, uniform, low cost and effective for SERS-based detection of analytes, making these materials essential for medicine, forensics and environmental monitoring. Numerical simulation revealed that painting bSi with a defected gold layer resulted in an increase in the plasmonic hot spots, and a substantial increase in the absorption cross-section in the NIR range.

Nanodiamond surface as a photoluminescent pH sensor.

A systematic spectroscopic characterization of highly homogeneous water suspensions of 'buckydiamonds' comprising sp3cubic nanodiamond (ND) core covered with disordered sp2shell densely decorated with oxygen-containing groups demonstrates the excitation-wavelength-dependent photoluminescence (PL) given by at least four types of specific structures on the ND surface (hydroxyl, C=O containing ketones, carboxylic anhydrides, and carboxyl groups). PL properties of NDs suspensions possess concentration-dependent behavior revealing tendency of NDs to agglomerate. PL of NDs has been found to be strongly sensitive to pH of the environment in wide range of pH values, i.e. 2-11. We disclosed the mechanisms of pH sensitivity of the 'buckydiamond' and proved that it can serve as all-optical sensor of tiny pH variations suitable for further exploitation for pH sensing locally in the area where NDs have been delivered for any purpose, e.g. bioimaging or therapeutic needs.

A Deep Learning Approach to Organic Pollutants Classification Using Voltammetry.

This paper proposes a deep leaning technique for accurate detection and reliable classification of organic pollutants in water. The pollutants are detected by means of cyclic voltammetry characterizations made by using low-cost disposable screen-printed electrodes. The paper demonstrates the possibility of strongly improving the detection of such platforms by modifying them with nanomaterials. The classification is addressed by using a deep learning approach with convolutional neural networks. To this end, the results of the voltammetry analysis are transformed into equivalent RGB images by means of Gramian angular field transformations. The proposed technique is applied to the detection and classification of hydroquinone and benzoquinone, which are particularly challenging since these two pollutants have a similar electroactivity and thus the voltammetry curves exhibit overlapping peaks. The modification of electrodes by carbon nanotubes improves the sensitivity of a factor of about ×25, whereas the convolution neural network after Gramian transformation correctly classifies 100% of the experiments.

Hysteresis and Stochastic Fluorescence by Aggregated Ensembles of Graphene Quantum Dots.

"Blinking" behavior of fluorophores, being harmful for the majority of super-resolved techniques, turns into a key property for stochastic optical fluctuation imaging and its modifications, allowing one to look at the fluorophores already used in conventional microscopy, such as graphene quantum dots, from a completely new perspective. Here we discuss fluorescence of aggregated ensembles of graphene quantum dots structured at submicron scale. We study temperature dependence and stochastic character of emission. We show that considered quantum dots ensembles demonstrate rather complicated temperature-dependent intermittent emission, that is, "blinking" with a tendency to shorten "blinking" times with the increase of temperature. We verify "blinking" mechanism demonstrating hysteresis of the optical response under pulsed excitation timed to expected rates of dots transition to "dark" nonemitting states. Experimental results are well fitted by a simple qualitative model of transitions to the "dark" states. The obtained results suggest that this type of standardized quantum dots and even their submicron-size agglomerations can be useful as controlled fluorophores for super-resolution microscopy and, particularly, for SOFI-like microscopy.

Fragmented graphene synthesized on a dielectric substrate for THz applications.

Fragmented multi-layered graphene films were directly synthesized via chemical vapor deposition (CVD) on dielectric substrates with a pre-deposited copper catalyst. We demonstrate that the thickness of the sacrificial copper film, process temperature, and growth time essentially influence the integrity, quality, and disorder of the synthesized graphene. Atomic force microscopy and Kelvin probe force microscopy measurements revealed the presence of nano-agglomerates and charge puddles. The potential gradients measured over the sample surface confirmed that the deposited graphene film possessed a multilayered structure, which was modelled as an ensemble of randomly oriented conductive prolate ellipsoids. THz time domain spectroscopy measurements gave theacconductivity of the graphene flakes and homogenized graphitic films as being around 1200 S cm-1and 1000 S cm-1, respectively. Our approach offers a scalable fabrication of graphene structures composed of graphene flakes, which have effective conductivity sufficient for a wide variety of THz applications.

Sensitive Detection of Industrial Pollutants Using Modified Electrochemical Platforms.

Water pollution is nowadays a global problem and the effective detection of pollutants is of fundamental importance. Herein, a facile, efficient, robust, and rapid (response time < 2 min) method for the determination of important quinone-based industrial pollutants such as hydroquinone and benzoquinone is reported. The recognition method is based on the use of screen-printed electrodes as sensing platforms, enhanced with carbon-based nanomaterials. The enhancement is achieved by modifying the working electrode of such platforms through highly sensitive membranes made of Single- or Multi-Walled Carbon Nanotubes (SWNTs and MWNTs) or by graphene nanoplatelets. The modified sensing platforms are first carefully morphologically and electrochemically characterized, whereupon they are tested in the detection of different pollutants (i.e., hydroquinone and benzoquinone) in water solution, by using both cyclic and square-wave voltammetry. In particular, the sensors based on film-deposited nanomaterials show good sensitivity with a limit of detection in the nanomolar range (0.04 and 0.07 µM for SWNT- and MWNT-modified SPEs, respectively) and a linear working range of 10 to 1000 ppb under optimal conditions. The results highlight the improved performance of these novel sensing platforms and the large-scale applicability of this method for other analytes (i.e., toxins, pollutants).

Quantitative and qualitative analysis of pulmonary arterial hypertension fibrosis using wide-field second harmonic generation microscopy.

We demonstrated that wide-field second harmonic generation (SHG) microscopy of lung tissue in combination with quantitative analysis of SHG images is a powerful tool for fast and label-free visualization of the fibrosis pathogenesis in pulmonary arterial hypertension (PAH). Statistical analysis of the SHG images revealed changes of the collagen content and morphology in the lung tissue during the monocrotaline-induced PAH progression in rats. First order statistics disclosed the dependence of the collagen overproduction on time, the second order statistics indicated tightening of collagen fiber network around blood vessels and their spreading into the alveolar region. Fourier analysis revealed that enhancement of the fiber orientation in the collagen network with PAH progression was followed with its subsequent reduction at the terminating phase of the disease. Proposed approach has potential for assessing pulmonary fibrosis in interstitial lung disease, after lung(s) transplantation, cancer, etc.

The Performance of Graphene-Enhanced THz Grating: Impact of the Gold Layer Imperfectness.

We report the performance of a graphene-enhanced THz grating fabricated by depositing a gold layer on the femtosecond micromachined SiO2 substrate. The morphology of the gold plated patterned substrate was studied by scanning electron microscopy (SEM) and atomic force microscopy (AFM), while the quality of the chemical vapor deposition (CVD) graphene was evaluated by Raman spectroscopy. The electromagnetic (EM) response of the metasurface comprising the graphene sheet and the gold plated substrate was studied by THz time domain spectroscopy in the 100 GHz-1 THz frequency range. We employed the finite elements method (FEM) to model the metasurface EM response by adjusting the ac conductivity of the gold layer covering the patterned SiO2 substrate to reproduce the measured transmission/reflection spectra. The results of the numerical simulation reveal the impact of the imperfectness of the gold layer on the performance of the THz metasurface. The experimental results are well described in terms of the Drude-Smith model of metal conductivity that takes into account the anisotropic scattering of the carriers in thin metal films.

Visualizing hypochlorous acid production by human neutrophils with fluorescent graphene quantum dots.

In living organisms, redox reactions play a crucial role in the progression of disorders accompanied by the overproduction of reactive oxygen and reactive chlorine species, such as hydrogen peroxide and hypochlorous acid, respectively. We demonstrate that green fluorescence graphene quantum dots (GQDs) can be employed for revealing the presence of the hypochlorous acid in aqueous solutions and cellular systems. Hypochlorous acid modifies the oxygen-containing groups of the GQD, predominantly opens epoxide ring C-O-C, forms excessive C=O bonds and damages the carbonic core of GQDs. These changes, which depend on the concentration of the hypochlorous acid and exposure time, manifest themselves in the absorbance and fluorescence spectra of the GQD, and in the fluorescence lifetime. We also show that the GQD fluorescence is not affected by hydrogen peroxide. This finding makes GQDs a promising sensing agent for selective detecting reactive chlorine species produced by neutrophils. Neutrophils actively accumulate GQDs allowing to visualize cells and to examine the redox processes via GQDs fluorescence. At high concentrations GQDs induce neutrophil activation and myeloperoxidase release, leading to the disruption of GQD structure by the produced hypochlorous acid. This makes the GQDs a biodegradable material suitable for various biomedical applications.

Electrical impedance sensing of organic pollutants with ultrathin graphitic membranes.

In this paper we propose an original approach for the real-time detection of industrial organic pollutants in water. It is based on the monitoring of the time evolution of the electrical impedance of low-cost graphitic nanomembranes. The developed approach exploits the high sensitivity of the impedance of 2D graphene-related materials to the adsorbents. We examined sensitivity of the nanomembranes based on pyrolyzed photoresist, pyrolytic carbon (PyC), and multilayer graphene films. In order to realize a prototype of a sensor capable of monitoring the pollutants in water, the membranes were integrated into an ad hoc printed circuit board. We demonstrated the correlation between the sensitivity of the electric impedance to adsorbents and the structure of the nanomembranes, and revealed that the amorphous PyC, being most homogeneous and adhesive to the SiO2substrate, is the most promising in terms of integration into industrial pollutants sensors.

Terahertz Absorber with Graphene Enhanced Polymer Hemispheres Array.

We propose an original technique for the fabrication of terahertz (THz) metasurfaces comprising a 3D printed regular array of polymer hemispheres covered with a thin conductive layer. We demonstrate that the deposition of a thin metal layer onto polymer hemispheres suppresses the THz reflectivity to almost zero, while the frequency range of such a suppression can be considerably broadened by enhancing the structure with graphene. Scaling up of the proposed technique makes it possible to tailor the electromagnetic responses of metasurfaces and allows for the fabrication of various components of THz photonics.

0.7Pb(Mg1/3Nb2/3)O3-0.3PbTiO3 Phosphate Composites: Dielectric and Ferroelectric Properties.

Composite materials with 83 wt.% of the 0.7Pb(Mg1/3Nb2/3)O3-0.3PbTiO3 distributed in phosphate-bonded ceramics were prepared at three different pressures. A phosphate matrix comprises a mixture of an aluminum phosphate binder and melted periclase, MgO. All samples demonstrate a homogeneous distribution of the ferroelectric perovskite phase and are thermally stable up to 900 K. At higher temperatures, the pyrochlore cubic phase forms. It has been found that the density of the composites non-monotonously depends on the pressure. The dielectric permittivity and losses substantially increase with the density of the samples. The fabricated composites demonstrate diffused ferroelectric-paraelectric transition and prominent piezoelectric properties.

Rapid and delayed effects of single-walled carbon nanotubes in glioma cells.

Single-walled carbon nanotubes (SWCNTs) demonstrate a strong potential as an optically activated theranostic nano-agent. However, using SWCNTs in theranostics still requires revealing mechanisms of the SWCNT-mediated effects on cellular functions. Even though rapid and delayed cellular responses can differ significantly and may lead to undesirable consequences, understanding of these mechanisms is still incomplete. We demonstrate that introducing short (150-250 nm) SWCNTs into C6 rat glioma cells leads to SWCNT-driven effects that show pronounced time dependence. Accumulation of SWCNTs is carried out due to endocytosis with modification of the actin cytoskeleton but not accompanied with autophagy. Its initial stage launches a rapid cellular response via significantly heightened mitochondrial membrane potential and superoxide anion radical production, satisfying the cell demand of energy for SWCNT transfer inside the cytoplasm. In the long term, SWCNTs agglomerate to micron-sized structures surrounded by highly active mitochondria having parameters return to control values. SWCNTs postponed effects are also manifested themselves in the suppression of the cell proliferative activity with further restoration after five passages. These results demonstrate relative cellular inertness and safety of SWCNTs eliminating possible side effects caused by optically activated theranostic applications.

Laser Patterning of Aligned Carbon Nanotubes Arrays: Morphology, Surface Structure, and Interaction with Terahertz Radiation.

The patterning of arrays of aligned multi-walled carbon nanotubes (MWCNTs) allows creating metastructures for terahertz (THz) applications. Here, the strips and columns from MWCNTs vertically grown on silicon substrates are prepared using CO2 laser treatment. The tops of the patterned arrays are flat when the laser power is between 15 and 22 W, and craters appear there with increasing power. Laser treatment does not destroy the alignment of MWCNTs while removing their poorly ordered external layers. The products of oxidative destruction of these layers deposit on the surfaces of newly produced arrays. The oxygen groups resulting from the CO2 laser treatment improve the wettability of nanotube arrays with an epoxy resin. We show that the patterned MWCNT arrays absorb the THz radiation more strongly than the as-synthesized arrays. Moreover, the pattern influences the frequency behavior of the absorbance.

Dielectric Relaxation Spectroscopy and Synergy Effects in Epoxy/MWCNT/Ni@C Composites.

The dielectric/electric properties of the Ni@C (carbon-coated Ni)/epoxy composites and Ni@C/MWCNTs (multi-walled carbon nanotubes)/epoxy composites loaded with fixed MWCNTs amount just below the percolation threshold (0.09 vol.%) and Ni@C at different concentrations up to 1 vol.% were investigated in broad frequency (20 Hz-40 GHz) and temperature (30 K-500 K) regions. In composites with the only Ni@C nanoparticles, the electrical percolation threshold was determined between 10 and 15 vol.%. Above the percolation threshold the dielectric permittivity (ε') and the electrical conductivity (σ) of the composites loaded with Ni@C only are high enough, i.e., ε' = 105 and σ = 0.6 S/m at 100 Hz for composites with 30 vol.% Ni@C, to be used for electromagnetic shielding applications. The annealing to 500 K was proved to be an effective and simple tool to decrease the percolation threshold in epoxy/Ni@C composites. For hybrid composites series an optimal concentration of Ni@C (0.2 vol.%) was determined, leading to the conductivity absolute values several orders of magnitude higher than that of a composite filled with MWCNTs only. The synergy effects of using both fillers have been discussed. Below room temperature the electrical transport is mainly governed by epoxy resin compression in all composites, while the electron tunnelling was observed only in hybrid composites below 200 K. At higher temperatures (above 400 K), in addition to the nanoparticles redistribution effects, the electrical conductivity of epoxy resin makes a significant contribution to the total composite conductivity. The dielectric relaxation spectroscopy allows estimating the nanoparticles distributions in polymer matrix and could be used as the non-destructive and fast alternate to microscopy techniques for general polymer composite fabrication control.