Multifunctional Block Copolymers, Acting as Recycling Aids, by Atom Transfer Radical Polymerization.

Block copolymers utilizing oligomeric poly(pentylene-co-hexylene carbonate)diol modified with 2,4-diisocyanatotoluene and further with 2-bromo-N-(3-hydroxypropyl)-2-methylpropanamide were synthesized and utilized as Activators ReGenerated by Electron Transfer Atom Transfer Radical Polymerization macroinitiators to obtain a first generation of multifunctional recycling additives with poly(glycidyl methacrylate-co-butyl methacrylate-co-methyl methacrylate) side chains, which could act as chain extenders. Then, chosen additive was reacted with a radical scavenger, 3,5-ditertbutyl-4-hydroxybenzoic acid (DHBA), to obtain a second generation of reactive additives. Those copolymers had different numbers of epoxy groups per polymer chain, and different number of epoxides opened with DHBA, hence showed a range of properties, and were utilized as reactive modifiers for polylactide (PLA) extrusion melting. The first-generation modifiers caused an increase in PLA's blends relative melt viscosity, stabilized material properties, and enhanced impact strength, while the second-generation modifiers with more than 8 % of epoxide ring opened showed worse properties. However, they managed to suppress the UV degradation of PLA blend plates.

Influence of the filler distribution on PDMS-graphene based nanocomposites selected properties.

Insufficient homogeneity is one of the pressing problems in nanocomposites' production as it largely impairs the properties of materials with relatively high filler concentration. Within this work, it is demonstrated how selected mixing techniques (magnetic mixer stirring, calendaring and microfluidization) affect filler distribution in poly(dimethylsiloxane)-graphene based nanocomposites and, consequently, their properties. The differences were assessed via imaging and thermal techniques, i.a. Raman spectroscopy, differential scanning calorimetry and thermogravimetry. As microfluidization proved to provide the best homogenization, it was used to prepare nanocomposites of different filler concentration, whose structural and thermal properties were investigated. The results show that the concentration of graphene significantly affects polymer chain mobility, grain sizes, defect density and cross-linking level. Both factors considered in this work considerably influence thermal stability and other features which are crucial for application in electronics, EMI shielding, thermal interface materials etc.

Study of Structural and Optoelectronic Properties of Thin Films Made of a Few Layered WS2 Flakes.

We report a surfactant-free exfoliation method of WS2 flakes combined with a vacuum filtration method to fabricate thin (<50 nm) WS2 films, that can be transferred on any arbitrary substrate. Films are composed of thin (<4 nm) single flakes, forming a large size uniform film, verified by AFM and SEM. Using statistical phonons investigation, we demonstrate structural quality and uniformity of the film sample and we provide first-order temperature coefficient χ, which shows linear dependence over 300-450 K temperature range. Electrical measurements show film sheet resistance RS = 48 MΩ/Υ and also reveal two energy band gaps related to the intrinsic architecture of the thin film. Finally, we show that optical transmission/absorption is rich above the bandgap exhibiting several excitonic resonances, and nearly feature-less below the bandgap.

Study of optical properties of graphene flakes and its derivatives in aqueous solutions.

In this work, we study optical spectroscopy of graphene flakes and its derivatives such as graphene oxide and reduced graphene oxide in the same surfactant-free aqueous solution. We show that transmittance (T) and absorbance (A) spectra of different graphene suspension is nearly feature-less as a function of wavelength (λ) in the VIS-NIR range (350-1000 nm) except graphene oxide solution and the smallest graphene flakes, and they change linearly with concentration. The optical absorption coefficient (at 660 nm) of pure graphene solution seems to be flake-size dependent, changing from ∼730 mL·mg-1m-1 (for ∼25 µm flake size) to ∼4400 mL·mg-1m-1 (for ∼2 µm flake size), and it is several times higher than in the case of graphene oxide, which also varies with type and level of doping/defects (checked by FTIR and statistical Raman spectroscopy). Finally, we show wavelength-dependent evolution of optical absorption coefficient in the VIS-NIR range, which is roughly mimicking the A(λ) function but is strongly material-dependent. Our study could be useful for application of graphene solution in optofluidic devices, functional inks or printed flexible optoelectronics.

Wavelength- and dispersion-tunable ultrafast holmium-doped fiber laser with dual-color operation: publisher's note.

This publisher's note contains corrections to Opt. Lett.45, 956 (2020)OPLEDP0146-959210.1364/OL.383788.

Wavelength- and dispersion-tunable ultrafast holmium-doped fiber laser with dual-color operation.

We present a versatile ultrafast holmium-doped fiber laser with an intracavity Martinez compressor. The compressor enables continuous dispersion control, spectral filtering, and dual-color operation of the laser. Mode locking is supported for net cavity dispersion values ranging from highly anomalous (-1.42ps2) to net normal (0.3ps2), and wavelength tuning of the optical solitons is obtained in a 2021-2096 nm span. Dual-color pulsed operation of the laser is reached by implementing a mechanical bandstop filter within the compressor. The repetition rate offset of the two emitted frequency combs can be tuned in a 3-8 kHz range by adjusting the net cavity dispersion, or by changing the beam block diameter. We show that a relatively simple fiber resonator integrated with a Martinez compressor can serve as a highly tunable laser source.

Thermal properties of thin films made from MoS2 nanoflakes and probed via statistical optothermal Raman method.

A deep understanding of the thermal properties of 2D materials is crucial to their implementation in electronic and optoelectronic devices. In this study, we investigated the macroscopic in-plane thermal conductivity (κ) and thermal interface conductance (g) of large-area (mm2) thin film made from MoS2 nanoflakes via liquid exfoliation and deposited on Si/SiO2 substrate. We found κ and g to be 1.5 W/mK and 0.23 MW/m2K, respectively. These values are much lower than those of single flakes. This difference shows the effects of interconnections between individual flakes on macroscopic thin film parameters. The properties of a Gaussian laser beam and statistical optothermal Raman mapping were used to obtain sample parameters and significantly improve measurement accuracy. This work demonstrates how to address crucial stability issues in light-sensitive materials and can be used to understand heat management in MoS2 and other 2D flake-based thin films.

Metallic carbon nanotube-based saturable absorbers for holmium-doped fiber lasers.

In 2003, carbon nanotubes opened a new field of research on nanomaterial-based mode-locked fiber lasers. They maintain popularity in the ultrafast laser community due to their broadband operation, relatively high damage threshold, and tunable optical properties. Here we show that metallic carbon nanotube-based thin film fabricated by vacuum filtration technique can be used as a saturable absorber in holmium-doped fiber laser operating in anomalous and normal dispersion regimes. Scaling the absorbers modulation depth by adjusting the film thickness was observed. The Fourier transform limited 6.65 nm wide optical solitons in anomalous dispersion regime were generated. Utilizing stretched-pulse regime greatly improves the laser performance - 212 fs pulses reach the energy of 3.79 nJ.

Graphene-based plastic absorber for total sub-terahertz radiation shielding.

Increasing the requirements on telecommunications systems such as the need for higher data rates and connectivity via the Internet of things results in continuously increasing amounts of electromagnetic radiation in ever-higher telecommunications bands (up to terahertz). This can generate unwanted electromagnetic radiation that can affect the operation of electronic devices and human health. Here, we demonstrate that nonconductive and lightweight, graphene-based composites can shield more than 99.99% of the electromagnetic energy in the sub-THz range mainly via absorption. This contrasts with state-of-the-art electromagnetic radiation shielding materials that simply redirect the energy of the radiation from a protected area via conduction-based reflection mechanisms. This shifts the problem of electromagnetic pollution from one place to another. We have demonstrated that the proposed composites can be fabricated by industrial compatible methods and are characterized by specific shielding efficiency values that exceed 30 dB cm3 g-1, which is more than those for typical metals used today. Therefore these materials might help to solve the problem of electromagnetic environmental pollution.

CNT-based saturable absorbers with scalable modulation depth for Thulium-doped fiber lasers operating at 1.9 µm.

In this work, we demonstrate a comprehensive study on the nonlinear parameters of carbon nanotube (CNT) saturable absorbers (SA) as a function of the nanotube film thickness. We have fabricated a set of four saturable absorbers with different CNT thickness, ranging from 50 to 200 nm. The CNTs were fabricated via a vacuum filtration technique and deposited on fiber connector end facets. Each SA was characterized in terms of nonlinear transmittance (i.e. optical modulation depth) and tested in a Thulium-doped fiber laser. We show, that increasing the thickness of the CNT layer significantly increases the modulation depth (up to 17.3% with 200 nm thick layer), which strongly influences the central wavelength of the laser, but moderately affects the pulse duration. It means, that choosing the SA with defined CNT thickness might be an efficient method for wavelength-tuning of the laser, without degrading the pulse duration. In our setup, the best performance in terms of bandwidth and pulse duration (8.5 nm and 501 fs, respectively) were obtained with 100 nm thick CNT layer. This is also, to our knowledge, the first demonstration of a fully polarization-maintaining mode-locked Tm-doped laser based on CNT saturable absorber.

Charge Blinking Statistics of Semiconductor Nanocrystals Revealed by Carbon Nanotube Single Charge Sensors.

We demonstrate the relation between the optical blinking of colloidal semiconductor nanocrystals (NCs) and their electrical charge blinking for which we provide the first experimental observation of power-law statistics. To show this, we harness the performance of CdSe/ZnS NCs coupled with carbon nanotube field-effect transistors (CNTFETs), which act as single charge-sensitive electrometers with submillisecond time resolution, at room temperature. A random telegraph signal (RTS) associated with the NC single-trap charging is observed and exhibits power-law temporal statistics (τ(-α), with α in the range of ∼1-3), and a Lorentzian current noise power spectrum with a well-defined 1/f(2) corner. The spectroscopic analysis of the NC-CNTFET devices is consistent with the charging of NC defect states with a charging energy of Ec ≥ 200 meV. These results pave the way for a deeper understanding of the physics and technology of nanocrystal-based optoelectronic devices.

High accuracy determination of the thermal properties of supported 2D materials.

We present a novel approach for the simultaneous determination of the thermal conductivity κ and the total interface conductance g of supported 2D materials by the enhanced opto-thermal method. We harness the property of the Gaussian laser beam that acts as a heat source, whose size can easily and precisely be controlled. The experimental data for multi-layer graphene and MoS2 flakes are supplemented using numerical simulations of the heat distribution in the Si/SiO2/2D material system. The procedure of κ and g extraction is tested in a statistical approach, demonstrating the high accuracy and repeatability of our method.

Temperature-dependent nonlinear phonon shifts in a supported MoS2 monolayer.

We report Raman spectra measurements on a MoS(2) monolayer supported on SiO(2) as a function of temperature. Unlike in previous studies, the positions of the two main Raman modes, E(2g)(1) and A(1g) exhibited nonlinear temperature dependence. Temperature dependence of phonon shifts and widths is explained by optical phonon decay process into two acoustic phonons. On the basis of Raman measurements, local temperature change under laser heating power at different global temperatures is derived. Obtained results contribute to our understanding of the thermal properties of two-dimensional atomic crystals and can help to solve the problem of heat dissipation, which is crucial for use in the next generation of nanoelectronic devices.

Microstructural and optical characterization of TiO2 doped with ytterbium synthesized by sol-gel and Solar physical vapor deposition process.

Pure and ytterbium doped TiO2 nanopowders in anatase phase have been prepared by sol-gel method (SGM) and Solar Physical Vapour Deposition process (SPVD). The physico-chemical parameters of the nanopowders have been described based on the results of micro-structural characterization performed by X-ray diffractometry, scanning electron microscopy, atomic force microscopy, and nitrogen sorption measurements. Thus, final micro-structural properties of SGM and SPVD titania nanopowders have been compared in detail revealing significant changes in the structure and morphology of these two types of materials. Addition of ytterbium had no significant effect on above-mentioned properties, although it modifies significantly the optical properties of the investigated materials. The luminescent properties of developed material were found to be comparable to bulk oxide materials and better than these reported earlier for ytterbium doped titania. In particular it has been shown that the luminescence of SPVD nanopowders is significantly stronger than this of SGM samples.