In order to use the infrared (IR) radiation shielding materials, they should take a form of thin film coatings deposited on glass/polymer substrates or be used as fillers of glass/polymer. The first approach usually suffers from several technological problems. Therefore, the second strategy gains more and more attention. Taking into account this trend, this work presents the usage of iron nanoparticles (Fe NPs) embedded into the poly(vinylidene fluoride-co-hexafluoropropylene) (PVDF-HFP) films as the shielding material in near-infrared (NIR) and mid-infrared (MIR) region. The performed investigations show that the transmittance of copolymer films decreases with increasing content of the Fe NPs inside them. It is found that the average fade of IR transmittance for 1, 2.5, 5, 10, and 50 mg of Fe NPs is about 13%, 24%, 31%, 77%, and 98%, respectively. Moreover, it is observed that the PVDF-HFP films filled in the Fe NPs almost does not reflect the NIR and MIR radiation. Hence, the IR shielding properties of the PVDF-HFP films can be effectively tuned by the addition of proper amount of the Fe NPs. This, in turn, shows that the PVDF-HFP films filled in the Fe NPs constitute a great option for IR antireflective and shielding applications.

Infrared (IR) shielding materials are commonly used for different applications, such as smart windows or optical filters. Infrared radiation is responsible for about 50% of the energy coming from the sun. During a hot summer or cold winter a lot of energy is needed to keep the optimal temperature inside buildings and means of transport. To reduce the heat transmission and save energy IR shielding materials can be used as coatings made of polymer composites. Graphene oxide (GO) and its reduced forms have interesting IR absorption properties and might be used as a filler in a polymer matrix for IR shielding applications. Graphene oxide can be reduced by different methods. Depending on the reduction method reduced graphene oxide (rGO) with a different content of oxygen can be obtained exhibiting different properties. In this work we propose new polymer nanocomposites with poly(vinyl alcohol) as the matrix and 0.1 wt.% addition of graphene materials with different oxygen content to be used for IR shielding applications. The results show that the properties of the graphene filler strongly influence the infrared shielding properties of the obtained nanocomposites. The best IR shielding properties were obtained for the composites where rGO with the lowest oxygen content was used.

In this work, we have prepared a series of polydimethylsiloxane (PDMS) composites containing various graphene flakes loadings (0.02–2 wt%), and their broadband optical properties are being investigated. We demonstrate the tunability and evolution of transmittance and reflection spectra of the composites in a wide spectral range (0.4–200 μm) as a function of graphene content. Using these data we derive the broadband wavelength-dependent absorption coefficient (α) values. Our results show that α is roughly constant in the visible and IR ranges, and, surprisingly, is approximately one order of magnitude lower in the terahertz regime, suggesting different terahertz radiation scattering mechanism in our composite. Our material could be useful for applications in optical communication, sensing or ultrafast photonics.

Thin films of multi-walled carbon nanotubes forming random networks were produced by vacuum filtration method, and their broadband electromagnetic radiation transmittance spectra are presented. Thickness of the nanotube films was between 100 nm and 1 μm, and the transmission properties are demonstrated for the wavelength range from 300 nm to 400 μm. It is observed that transmittance is an increasing function of a radiation wavelength, and for the thickest films it almost saturates above 1 μm wavelength. To explain the experimental results in the ultraviolet–near infrared range, we employed effective medium theory (in the form of symmetric Bruggeman model) correlating properties of multi-walled carbon nanotubes with the effective dielectric function of a nanotube network. The optical properties of a single multi-walled carbon nanotube that were used for calculations were based on ordinary and extraordinary dielectric functions of bulk graphite. The proposed theoretical model has been successfully fitted to the experimental results. It has been also found that despite the fact that radiation undergoes multiple internal reflections at the film interfaces, the transmittance–thickness relation can be still described by exponential decay.

"Graphene paper" prepared by new proprietary method involving high pressure and high temperature treatment in the reduction process show new possibilities in this area. Different phase content: multilayer and single layer graphene stacks recorded in this study for RGO samples are accompanied by the specific electric and optical parameters. We have found that process temperatures above 900 degrees C play crucial role in structural and other properties. For the process temperature around 2000 degrees C we found the onset of the graphitization in the samples.

In order to characterize magnetic field (B) tunable THz plasmonic detectors, spectroscopy experiments were carried out at liquid helium temperatures and high magnetic fields on devices fabricated on a high electron mobility GaAs/AlGaAs heterostructure. The samples were either gated (the gate of a meander shape) or ungated. Spectra of a photovoltage generated by THz radiation were obtained as a function of B at a fixed THz excitation from a THz laser or as a function of THz photon frequency at a fixed B with a Fourier spectrometer. In the first type of measurements, the wave vector of magnetoplasmons excited was defined by geometrical features of samples. It was also found that the magnetoplasmon spectrum depended on the gate geometry which gives an additional parameter to control plasma excitations in THz detectors. Fourier spectra showed a strong dependence of the magnetoplasmon resonance amplitude on the conduction-band electron filling factor which was explained within a model of the electron gas heating with THz radiation. The study allows to define both the advantages and limitations of plasmonic devices based on high-mobility GaAs/AlGaAs heterostructures for THz detection at low temperatures and high magnetic fields.

Reflection spectra for the arrays of vertically aligned multi-walled carbon nanotubes (MWCNTs) in an extremely broadband spectral range (up to half a millimeter) are presented. Using Fourier transform infrared spectroscopy it is shown that in some conditions, the material generally regarded in the literature as a promising candidate for a blackbody, dramatically losses its anti-reflection properties when stepping into infrared range of the electromagnetic spectrum. The experimental results confirm that structural parameters of an array are the key factor responsible for the discussed properties and decide about the spectral extent of a low reflectance level.

Crystal Morphology and Optical Emissions of GaN Layers Grown on Si(111) Substrates by Molecular Beam Epitaxy

We present results of Raman and infrared absorption spectroscopy research on phonons in LiMnPO4-a new material for high capacitance rechargeable lithium-ion batteries. There is a significant interest in the structural and electrical properties of this material, because the battery performance depends strongly on the rate of lithium diffusion. Nanopowder of LiMnPO4 was obtained via a modified sol-gel method from salts of lithium and manganese. This method is cheap and effective so it is promising for the most popular applications.The material showed sharp phonon peaks in Raman and infrared spectra. In the Raman spectra, the strongest peak was A(g)nu(1) mode at energy 117.77 meV (950.1 cm(-1)), at 4K. At room temperature, its energy decreased (due to phonon-phonon interaction) to 117.5 meV (947.5 cm(-1)). The Gruneisen parameter found for this oscillation mode was relatively low, gamma(Ag nu 1) = 0.5, at about 300 K. Since the mode consisted mainly of the symmetric PO4 tetrahedra oscillations, the low gamma(Ag nu 1) value indicated that the temperature influenced rather Li-O and Mn-O bonds than the P-O bonds forming the LiMnPO4 structure. The thermal dependencies of the antisymmetric modes (A(g)nu(3) and A(g)nu(4)) were stronger (gamma(Ag nu 3) = 0.7, gamma(Ag nu 4) = 1.4) what suggested that these modes experienced stronger coupling.The thermal broadening of the A(g)nu(1) mode could be described in wide temperature range by exponential dependence with activation energy of 65 meV (about two times smaller than the A(g)nu(1) energy), what suggested a symmetric two-phonon decay. (C) 2011 Elsevier B.V. All rights reserved.

Cyclotron resonance in highly doped graphene has been explored using infrared magnetotransmission. Contrary to previous work, which only focused on the magneto-optical properties of graphene in the quantum regime, here we study the quasiclassical response of this system. We show that it has a character of classical cyclotron resonance, with an energy which is linear in the applied magnetic field and with an effective cyclotron mass defined by the position of the Fermi level $m={E}_{F}/{v}_{F}^{2}$.

Optical absorption between 0.4 and 4.5 eV of an InN layer grown by metalorganic vapour phase epitaxy on sapphire was measured at 296 and 12 K. The layer was also characterized by measurements of the Hall effect and of infrared reflectivity in the region of the plasma edge, which determined the concentration, mobility, and effective mass of electrons in the conduction band. The energy gap of InN was estimated to be equal to 0.9 +/- 0.2 eV. It was obtained from the spectral position of the fundamental absorption edge. Corrections to the energy gap resulting from the broadening of the fundamental absorption edge, from the Burstein-Moss shift, and from a band-gap shrinkage due to the impurity potential were included.

We found that the fine structure related to Lyman spectra of [Mn2+ (d(5)) + a hole] centers in GaAs was present only for samples with low Mn concentration. Such samples, at low temperature, did not show any hopping conductance within Mn impurity band. Magnetooptical measurements revealed that magnetic field induced splitting of the Lyman optical transitions was larger than Zeeman splitting observed for typical shallow acceptors in GaAs, like Be, Zn, and C. This experimental result proved that in the case of Mn acceptor impurity, the exchange coupling of a hole and the S = 5/2 Mn2+(d(5)) core could not be neglected, which was in accordance with the [Mn2+ (d(5)) + a hole] model of the neutral Mn center in GaAs.

The spontaneous electric fields present in GaN crystals are known to substantially influence their physical properties. However, the magnitude of these fields remains unknown, with estimates ranging from hundreds to thousands of kV/cm. We present an experimental evaluation of the built-in electric fields in MOVPE grown GaN layers on sapphire, by fair-infrared spectroscopy of shallow donor states. The observation of electric-field-allowed forbidden transitions suggests homogeneous electr ic fields of the order of 1 kV/cm.

Successful growth of the ZnO and ZnO:Mn crystals by the CVT method and their characterisation are reported. The source material was synthesized at 650 °C from oxygen and zinc vapours and subsequently – baked (as a powder) to achieve stoichiometry. Crystal growth (at ∼1100 °C, with the rate ∼1–2 mm/day) proceeded in graphite-covered quartz ampoules containing pure (6N) hydrogen or nitrogen or chlorine and a little of water vapour and carbon. The crystals, both as-grown and annealed in pure oxygen, were characterised by the measurements of: transmission spectra in energy gap and far infrared regions, photoluminescence and reflectivity spectra, electrical transport properties and EDXRF – the energy dispersive X-ray fluorescence (for Mn content determination). In the surface region – the impurities and the Mn content were studied by the secondary ion mass spectroscopy (SIMS). Manganese and native defects (VO) were investigated by electron paramagnetic resonance (EPR) and Raman spectroscopy. Results of the measurements are presented and discussed. (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Shallow donors in GaN epilayers grown by metalorganic chemical vapour deposition (MOCVD) and by molecular beam epitaxy (MBE) on sapphire, SiC and GaN substrates have been studied by selectively excited photoluminescence (SPL) and far‐infrared (FIR) absorption. The spectroscopic studies of neutral donor bound excitons allow us to determine the D0X rotational excited state spectra in a large domain of tensile and compressive strain fields depending on the substrates used for the growth. The energy transitions between the ground state and the n = 2 states of residual donors are determined from the spectra of the two‐electron replica of D0X lines in accordance with resonant electronic Raman scattering results. The comparison between FIR results and the two‐electron spectroscopy shows, that silicon is the main residual donor (Ed = 30.4 meV in strain‐free samples) in both MOVPE and MBE layers. A second unknown donor (Ed = 31.8 meV) is frequently found in GaN/SiC grown by MOCVD. A third deeper level (Ed = 33.1 meV) is rarely detected by two‐electron spectroscopy but could be associated with a weak neutral‐donor bound exciton shifted down by 0.9 meV from the main SiGa related D0X line in MBE homoepitaxial layers.

Electron paramagnetic resonance, optical absorption, luminescence and electrical studies of InP highly doped with Mn were performed. Electron paramagnetic resonance revealed presence of manganese in Mn2+ (d(5)) configuration. In optical absorption, systematic reduction of InP band gap was observed with increase in Mn content. This was correlated with increase in photoionization-type absorption band starting at 0.2 eV. Time-resolved photoluminescence measurements showed decrease in photoexcited carrier lifetime and shortening of donor-acceptor pair recombination time with increase in Mn content. Moreover, photoluminescence band was shifted to lower energies, similarly to optical band gap. In electrical transport two mechanisms of conductivity were observed. Valence band transport dominated at higher temperatures, above 160 K, and activation energy of free-hole concentration was determined as about 0.20 eV. At lower temperatures hopping conductivity, clearly related to Mn defect band, was present. All these results were consistent with assumption of creation of Mn-related defect-band at 0.2 eV above InP valence band. It was found that Mn centers responsible for this band were in configurations of either d(5) or d(5) plus a hole localized about 7 Angstrom around corresponding Mn core.

Recently, ZnO attracts a wide interest as a promising material for the application in optoelectronic devices working in the blue and ultraviolet region and (when doped with magnetic impurities) in spintronic devices. Unfortunately, the technology of good, large (e.g.: 2.5–5cm in diameter) single crystals is very difficult, even as compared with other II–VI compounds. We report on the successful growth of the ZnO crystals with a chemical vapour transport (CVT) method and on the characterisation of them. The source material is synthesised at 650°C from oxygen and zinc vapours and subsequently, baked (as a powder) to achieve stoichiometry. The crystals grow (with the rate 1–2mm per day) in the graphite-covered quartz ampoules containing pure (6N) hydrogen or nitrogen and a small amount of water vapour. The crystals, both as-grown and annealed in pure oxygen, are characterised by the measurements of photoluminescence spectra, transmission spectra, far infrared transmission, X-ray diffraction and electrical transport. The surface region is analysed by the secondary ion mass spectroscopy (SIMS).