Carbon systems of polymerized nanotubes: Crystal and electronic structures

Molecular dynamic calculations are carried out for the (P, T) phase diagram of a covalent compound of cross-linked carbon single-wall nanotubes (SWNT) and for the structures and electronic spectra of the novel crystals of polymerized carbon nanotubes. It is shown that the transformation of covalently bonded nanotubes in a close-packed conducting structure cardinally modifies their electronic properties. The P-SWNT crystal becomes semiconducting and, upon complete transformation of sp ²-hybridized carbon atoms into sp ³-hybridized ones, it becomes an insulator.     

Optimization of the calculations of the electronic structure of carbon nanotubes

A method is proposed for calculating the electronic structure and physical properties (in particular, Young’s modulus) of nanotubes, including single-walled carbon nanotubes. This method explicitly accounts for the periodic boundary conditions for the geometric structure of nanotubes and makes it possible to decrease considerably (by a factor of 10–10³) the time needed to calculate the electronic structure with minimum error. In essence, the proposed method consists in changing the geometry of the structure by partitioning nanotubes into sectors with the introduction of the appropriate boundary conditions. As a result, it becomes possible to reduce substantially the size of the unit cell of the nanotube in two dimensions, so that the number of atoms in a new unit cell of the modified nanotube is smaller than the number of atoms in the initial unit cell by a factor equal to an integral number. A decrease in the unit cell size and the corresponding decrease in the number of atoms provide a means for drastically reducing the computational time, which, in turn, substantially decreases with an increase in the degree of partition, especially for nanotubes with large diameters. The results of the calculations performed for carbon and non-carbon (boron nitride) nanotubes demonstrate that the electronic structures, densities of states, and Young’s moduli determined within the proposed approach differ insignificantly from those obtained by conventional computational methods.     

Growth,structural, mechanical,spectral and dielectric characterization of NaCl-added Triglycine sulfate single crystals

Pure and sodium chloride (NaCl)-added Triglycine sulfate (TGS) crystals were grown from aqueous solutions by slow evaporation technique. The values of concentration of dopants in the mother solution were 0.2, 0.6 and 1 mol%. The solubility of the grown samples have been found out at various temperatures. The determination of unit cell parameters was carried out by single crystal XRD method and found that the grown crystals crystallize in monoclinic structure. The dielectric characterization for the pure and NaCl-doped TGS crystals was performed by measuring the dielectric parameters like dielectric constant and dielectric loss with various frequencies in the range 10²–10⁶ Hz and with the temperatures ranging from 30 to 70 °C and this study reveals an increase of dielectric constant and loss with the increase of NaCl concentration. Studies on mechanical properties like microhardness and density of the grown pure and NaCl-doped TGS crystals were carried out. UV–Visible transmittance studies were carried out for the grown samples. A sharp fall in the transmittance is observed at 228 nm for pure and NaCl-doped TGS crystals. Atomic absorption spectroscopic (AAS) study was done on the NaCl-doped TGS crystals to ascertain the presence of Na⁺ ions in the lattice.     

Optical homogeneity,defects, and photorefractive properties of stoichiometric,congruent, and zinc-doped lithium niobate crystals

Using the laser-conoscopy method, the photorefractive light-scattering method, and the Raman light-scattering method, we have studied the structural and optical homogeneities and photorefractive properties of (i) stoichiometric lithium niobate crystals (LiNbO₃(stoich)), which were grown from a melt with 58.6 mol % of Li₂O; (ii) congruent crystals (LiNbO₃(congr)); and (iii) congruent crystals that were doped with Zn²⁺ cations (LiNbO₃:Zn; [Zn] = 0.03–1.59 mol %). We have shown that the speckle-structure of the photorefractive light scattering in all the crystals is three-layer. The shapes of the second and third layers repeat in general the shape of the first layer. We have shown that the differences that are observed between the Raman spectra, the photorefractive light scattering, and the conoscopic patterns of the examined crystals are caused by the fact that defects are distributed inhomogeneously over the volume of these crystals and that Zn²⁺ cations are incorporated inhomogeneously into the lattice. This leads to the appearance of local changes in the elastic characteristics of the crystal and to the appearance of mechanical stresses, which locally change the optical indicatrix and, correspondingly, the conoscopic pattern and the Raman spectrum.     

Aspects of crystal growth within carbon nanotubes

The comparative crystallisation and HRTEM imaging properties of simple binary halides formed by the alkali iodides MI (M = Li, K, Na, Rb and Cs) within single walled carbon nanotubes (SWNTs) are described. The most common structure type observed within SWNTs is the rocksalt archetype, although CsI was observed to form both bcc and rocksalt structure types. In SWNTs forming in the 1.2–1.6 nm diameter range, all of the incorporated halides showed preferred orientation, with the 〈100〉 growth direction predominating for rocksalt-type packing and 〈112〉 so far observed exclusively for bcc packing. Crystals with dimensions spanning 2–6 atomic layers thickness in projection invariably exhibited lattice expansions that were attributed predominantly to a net reduction in coordination at the crystal-carbon interface. The crystallisation behaviour of UCl₄–KCl and AgI–AgCl eutectic melts was compared in carbon nanotubes of different diameters and a pronounced ordering influence over the normally glassy melts was observed in narrower capillaries. HgI₂ crystallised within nanotubes with ultra-narrow (i.e., 0.8 nm) capillaries were observed to form helical 2 ×1 layer crystals. To cite this article: J. Sloan et al., C. R. Physique 4 (2003).     

Electronic structure of collapsed C,BN, and BC3 nanotubes

The electrical properties of single-wall C, BN, and BC₃ nanotubes in ideally rolled-up forms show a wide spectrum from truly metals to large band gap semiconductors. In the presence of radial deformations that collapse tubes, the electrical properties are severely modified such that metals turn into semiconductors and vice versa. Based on first-principles pseudopotential calculations, we find that metallic C nanotubes have a finite band gap if radial deformations break all mirror symmetries of the tubes, and that original finite gaps (∼0.5 eV) of semiconducting C and BC₃ tubes are closed by collapsing deformations. In BN tubes, band gaps can be tuned in the range 2–5 eV. On the other hand, the band gaps of armchair BN and zigzag BC₃ nanotubes are found to be insensitive to radial deformations. These new findings can be applied to design new types of nanotube-based functional devices using radial deformations.     

Spectroscopic and lasing properties of disordered Yb3+-doped crystals

The luminescence properties and luminescence decay kinetics of disordered Yb³⁺-doped crystals of various oxides are investigated and the possibility of their use as active laser media in the near-IR region (in the vicinity of 1 μm) under laser diode pumping is established. Three groups of disordered oxide crystals were analyzed: calcium niobium gallium garnet, yttrium-stabilized zirconia, and double tungstates andmolybdates with scheelite structure (sodium gadolinium tungstate, sodium gadolinium molybdate, and sodium lanthanum molybdate). These compounds are characterized by large integrated cross sections of energy-level transitions of rare earth ions. The effect of degree of disorder on the spectroscopic and lasing properties of disordered oxide crystals is revealed. The results of the study show the possibility of lasing in all crystals studied and application potential of Yb³⁺-doped disordered crystals for developing new laser media.     

Modification of structural,optical, thermal and mechanical properties of KDP crystals on the addition of Ni

Nonlinear optical materials are acquiring a new significance day by day with the advent of a large number of devices utilizing solid state laser sources. Owing to this technological application KDP having superior nonlinear optical property has been exploited for various applications. KDP crystals were grown from aqueous solutions added with Ni²⁺. SHG studies confirm that the title exhibits NLO property. The influence of doping Ni²⁺ on the structural, optical, thermal, NLO and mechanical properties has been studied in this present investigation. It is observed that addition of Ni²⁺ improves the optical, thermal and mechanical properties of the crystal.     

Elaboration,caracterisation et proprietes magnetiques de cristaux d'oxyde Sm2O3

Single crystals of samarium oxide Sm₂O₃ with monoclinic B structure were prepared by the Verneuil process adapted to a plasma torch. Some properties of these crystals have been studied (cleavage, hardness …) especially the magnetic properties. Paramagnetic susceptibilities measurements have been performed for various orientations of the crystal with respect to the magnetic field at constant temperatures (5, 77, 300 K) as well as at increasing temperatures (5–1000 K) and indicate an anisotropy. An interpretation of this phenomena is proposed.     

Computational investigation on alcohol nanosensors in combination with carbon nanotube: a Monte Carlo and ab initio simulation

Single-walled nanocarbons (SWNT) are common nanovehicles of interest for making biosensors more sensitive. Carbon nanotubes (CNTs) have many distinct properties, causing them to be exploited in the development of the next generation of such nanosensors. The keto–enol tautomerization is one of the most common investigated subjects of isomerism in this regard; sensors are devices that are able to detect and change the physical properties of such reactions. Some chemicals with the properties to do keto–enol tautomerization are substituted to CNTs, and the physicochemical properties are simulated. HyperChem is used as the main software to design the CNT sensor, and the main physical properties are calculated after Monte Carlo simulation. In all situations, the energy minimization has been done by MM⁺, and the fully optimized systems have transferred to Guassian98. After final optimization with 3–21 G, Hartree–Fock (HF) method, SWNT have been linked up to caffeic acid and chlorogenic acid. Then, frequency and intensity were investigated using AM1 and PM3 codes. Also, we determined some nuclear magnetic resonance parameters in the HF method and several basis sets.     

Synthesis, characterization and optical spectroscopy of Eu doped titanate nanotubes

The synthesis of Eu³⁺ doped titania nanotubes was carried out via a hydrothermal method. X-ray diffraction and transmission electron microscope analyses showed that the nanotubes were formed by rolling multilayered titania structure with a length of up to 100 nm. The Eu³⁺-doped nanotubes exhibited strong emission lines associated with the ⁵D₀→⁷F_J (with J from 1 to 4) transition of Eu³⁺ and the differences between the luminescence properties of the precursor powders and the nanotubes were studied at low temperature.     

Structural,electrical and optical properties of gamma irradiated methyl para-hydroxy benzoate single crystals

ABSTRACT

Nonlinear optical materials (NLO) have been garnering attention due to their role in optical data storage, optical communication and laser technology. Organic crystals have emerged as an extremely important class of NLO materials, since their NLO properties compare very well with traditional inorganic NLO materials like KCl, LiNbO₃, KDP (potassium dihydrogen phosphate), etc. They offer the additional advantage that they can be grown relatively inexpensively from solution close to room temperature, unlike the inorganic NLO materials which are grown from high temperature melts. In the present work, organic transparent single crystals of methyl para-hydroxy benzoate (MHB) were grown by slow evaporation solution growth technique (SEST) from aqueous solution at room temperature. The changes in structural, electrical and optical properties of gamma irradiated MHB single crystals were studied using X-ray diffraction (XRD), UV–Visible absorption spectroscopy, Photo-luminescence (PL), Fourier transform infrared (FTIR) spectroscopy and AC conductivity measurements at room temperature. The polished MHB single crystals were irradiated with gamma rays of doses 10 and 15 kilogray (kGy). From the XRD analysis, it was observed that gamma irradiation for these doses drastically decreases the crystallinity. The optical absorption constants were examined by UV-Visible absorption spectroscopy, measured over the wavelength range of 200–800?nm, at normal incidence. The optical band gap as estimated from the Tauc plot ((αhν)² vs hν) was found to be reduced with increasing gamma irradiation doses. PL spectra showed emission at wavelengths of 361?nm (3.43?eV) and 452?nm (2.74?eV), with enhanced intensities for the irradiated crystals. FTIR spectroscopy was utilised to identify the functional groups of MHB and indicated the rupture of specific types of bonds with gamma irradiation. Apart from that, the enhancement of AC conductivity with gamma irradiation was also observed for the gamma irradiated crystals.     

Characterization of fluorinated multiwalled carbon nanotubes by x-ray absorption spectroscopy

The C 1s and F 1s x-ray absorption spectra of fluorinated multiwalled carbon nanotubes with different fluorine contents and reference compounds (highly oriented pyrolytic graphite crystals and “white” graphite fluoride) were measured using the equipment of the Russian-German beamline at the BESSY II storage ring with a high energy resolution. The spectra obtained were analyzed with the aim of characterizing multiwalled carbon nanotubes and their products formed upon treatment of the nanotubes with fluorine at a temperature of 420°C. It was established that, within the probing depth (～15 nm) of carbon nanotubes, the process of fluorination occurs uniformly and does not depend on the fluorine concentration. The interaction of fluorine atoms with multiwalled carbon nanotubes in this case proceeds through the covalent attachment of fluorine atoms to graphene layers of the graphite skeleton and is accompanied by a change in the hybridization of the 2s and 2p valence electron states of the carbon atom from the trigonal (sp ²) to tetrahedral (sp ³) hybridization.     

High temperature defects in electron irradiated semiconductors HgCdTe,PbSnTe

The peculiarities of defect formation in n- and p-type conductivity HgCdTe and PbSnTe crystals after electron irradiation (2 MeV, 300 K) up to 2 × 10¹⁸ cm^-2 are examined. It has been found that irradiation results in formation of n-type conductivity crystals with final parameters that are determined by the composition of initial samples. The annealing of radiation defects occurs in the 360–470 K temperature range. It has been believed that the change of HgCdTe, PbSnTe properties after electron irradiation at 300 K are connected with formation of radiation defects, including Te vacancies.     

Electric field effects in nematic liquid crystals doped with carbon nanotubes

The aim of this paper was to investigate electric field induced effects in mixtures of nematic liquid crystals (NLCs) with positive electric anisotropies (MCL 6601 Merck) with carbon nanotubes (MWCNT from Aldrich). In planar alignment, the current–electric field dependence and the current–temperature dependence were explained by assuming a Poole–Frenkel effect (i.e. a tunnelling mechanism) and good agreement with the experimental data was obtained. Within this high field range it resulted that in planar aligned NLC–CNTs mixture the conductivity decreases when the temperature was increased. In homeotropic aligned mixture, the conduction mechanism is similar to the one occurring in a semiconductor: the conductivity increases when increasing temperature. This happens because in thin liquid crystal cells there is a possibility to realize an inner contact between nanotubes and electrodes so the mixture behaves like a semiconductor.     

Optical excitation of the Rb+ 4p-level in rubidium halides at 8 K

In the 15–19 eV range new spectral features have been resolved in the reflection spectra of RbCl, RbBr, and RbI single crystals. They are not entirely consistent with results of a recently proposed ligand field model for the Rb⁺4p- excitons.     

Periodic,quasi‐periodic,and random quadratic nonlinear photonic crystals

Quadratic nonlinear photonic crystals are materials in which the second order susceptibility χ⁽²⁾ is spatially modulated while the linear susceptibility remains constant. These structures are significantly different than the more common photonic crystals, in which the linear susceptibility is modulated. Nonlinear processes in nonlinear photonic crystals are governed by the phase matching requirements, which are determined by the reciprocal lattice of these crystals. Therefore, the modulation of the nonlinear susceptibility enables to engineer the spatial and spectral response in various three‐wave mixing processes. It enables to support the efficient generation of new optical frequencies at multiple directions. We analyze three wave mixing processes in nonlinear photonic crystals in which the modulation is either periodic, quasi‐periodic, radially symmetric or even random. We discuss both one‐dimensional and two‐dimensional modulations. In addition to harmonic generations, we outline several new possibilities for all‐optical control of the spatial and polarization properties of optical beams in specially designed nonlinear photonic crystals.     

Kinetics of generation, relaxation, and accumulation of electronic excitations under two-photon interband picosecond absorption in tungstate and molibdate crystals

Under two-photon 523.5 nm interband picosecond laser excitation, we measured the kinetics of induced absorption in PbWO₄, ZnWO₄, and PbMoO₄ crystals with 532 to 633 nm continuous probe radiation. We obtained real-time information about the dynamics of the generation, relaxation, and accumulations of electronic excitations over a wide time range (from picoseconds to hundreds of seconds) and the 77–300 K temperature range. For the studied crystals, exponential temperature-independent growth of the induced absorption (IA) with 60 ns rise time reflects the dynamics of the generation of electronic excitation. The kinetics of the IA exponential growth with temperature-dependent 3.5–11 μs time constants reflect the dynamics of energy migration between neighboring tungstate (molibdate) ions to traps for the studied crystals. The multiexponential relaxation absorption kinetics strongly depend on temperature, and the relaxation decay time of induced absorption increased from tens to hundreds of milliseconds to seconds under crystal cooling from 300 to 77 K. We found that the increase in the laser pump repetition rate (0–10 Hz) leads to the accumulation of electronic excitations. Control of the repetition rate and the number of excitations allowed us to change the relaxation time of the induced absorption by more than two orders of magnitude. Due to accumulation of excitations at 77 K, the absorption relaxation time can exceed 100 s for PbWO₄ and PbMoO₄ crystals. In the initially transparent crystals, two-photon interband absorption (2PA) leads to crystals opacity at the 523 and 633 nm wavelengths. (An inverse optical transmission of the crystals exceeds 50–55 at a 50–100 GW/cm² pump intensity.) Measured at ～1 mW probe radiation of 532 and 633 nm wavelengths, the induced absorption values are comparable with those obtained under two-photon absorption at ～5 kW pump power. An optical 2PA shutter for the visible spectral range is proposed with a variable shutting time from hundreds of microseconds to tens of seconds.     

Crystal growth and optical properties of Cr3+, Er3+, RE3+: Gd3Ga5O12 (RE=Tm,Ho, Eu) for mid-IR laser applications

In this paper we report on the optical properties of triply Cr³⁺, Er³⁺, and RE³⁺ (RE=Tm, Ho, Eu) doped Gd₃Ga₅O₁₂ crystals that were grown by the Czochralski method. Optical absorption, near-infrared (NIR), and mid-infrared (mid-IR) fluorescence spectra were characterized for the fabricated crystals and corresponding luminescence decay measurements under 654 nm excitation were also carried out. Based on the analysis of energy transfer process between Er and RE (RE=Tm, Ho, Eu) ions, the energy transfer efficiency (ETE) values were evaluated, correspondingly. From the spectral data of all the studied crystals, it is observed that the co-doped Cr³⁺ ion highly increases the absorption pump power and the three kinds of co-doped RE³⁺ ions depopulate the Er:⁴I_13/2 energy level effectively. The spectral analysis shows that titled rare earth doped crystals are promising materials for ～3.0 μm mid-IR laser applications and among them Cr,Er,Eu:GGG is relatively more suitable due to its excellent optical properties compared with others.     

Advances in vanadate laser crystals at a lasing wavelength of 1 micrometer

Sapphire, garnet and vanadate crystals are the most prominent optical materials, and vanadates play important roles in optics, especially in lasers and nonlinear optics. Neodymium‐doped yttrium vanadate (Nd:YVO₄) is representative and available commercially. Based on Nd:YVO₄, several vanadate crystals are being developed with the goal of fulfilling the need for differential applications and improvement of certain operational aspects, such as with pulsed lasers or high‐power continuous‐wave lasers. In recent years, some important effects, including energy enhancement, bistability of output performance, self‐Raman frequency shifting, etc., and some novel applications, such as quantum optics, pulsed lasers modulated by the two‐dimensional crystals, etc., have been discovered with vanadates as gain materials. In this paper, the preparation, characterization and laser applications of vanadate laser crystals at the lasing wavelength of 1 micrometer, including YVO₄, GdVO₄, LuVO₄, Gd_xY_1–xVO₄ and Lu_xGd_1–xVO₄ (0 < x < 1) doped with Nd³⁺ and ytterbium (Yb³⁺) are systematically reviewed by highlighting the most recent research progress. Their specific properties are presented, generation mechanisms of novel physical effects are discussed, new applications are given and possible future applications proposed by focusing on some potential strengths.