Optical Kerr effect exhibited by carbon nanotubes and carbon/metal nanohybrid materials

Structural modification of carbon nanotubes in combination with metallic nanoparticles is reported. An enhancement in the nonlinear optical refraction of multi-wall carbon nanotubes by the incorporation of platinum nanoparticles was observed. Comparative results were analyzed taking into account the participation of single-wall carbon nanotubes that originate a decrease in the nonlinear optical response of the multi-wall carbon nanotubes integrating a thin film. A Nd:YAG laser system featuring 532 nm wavelength with 4 ns pulse duration in a two-wave mixing experiment was employed for exploring the studied optical nonlinearities of the samples. The contribution of optical processes to mechanical characteristics dependent on high optical irradiance in carbon nanotubes was described. A variation in the mass density associated to the optically irradiated tubes allowed us to calculate the change in Young's modulus in a thin film configuration. The estimation of an opto-mechanical phenomenon was based on the evaluation of the nonlinearity of index responsible for the optical Kerr effect. According to Raman and optical evaluations, the inclusion of metallic nanoparticles in carbon structures results in a modification of surface that also gives origin to noticeable optical Kerr nonlinearities. Potential applications for developing laser-induced controlled opto-mechanical nanohybrid systems can be contemplated.     

Magnetization of carbon nanotubes

For narrow-gap carbon nanotubes, the curvature effect due to the misorientation of 2p_z orbitals dominates over electronic structures and thus magnetic properties. It significantly changes magnetization and magnetic susceptibility, and creates special structures in them. There exists a critical field direction in changing magnetism. The critical angle strongly depends on the chiral angle, the nanotube radius, and the temperature. One-dimensional carbon nanotubes are quite different from zero-dimensional carbon tori, such as in terms of special structures in magnetization, the strength of the magnetic response, and the critical angle.     

碳纳米管悬浮液的光限幅特性实验研究

 通过超声波降解法制备了多壁碳纳米管的水-表面活性剂悬浮液,测量了其对于1 064 nm,脉宽10 ns Nd:YAG调Q脉冲激光的光限幅曲线。实验发现：入射激光能量密度较低时,出射能量密度随入射能量密度的增加而线性增加;当入射能量密度为160 mJ/cm²时,出射能量不再线性增加并且逐渐趋近于光限幅器的嵌位输出值,约16 mJ,同时,对激光的透过率从71%下降到15%。通过Z扫描和探针光实验以及45°散射角下散射能量、散射率随入射激光能量变化曲线的测量,对碳纳米管悬浮液的光限幅机理进行了研究。结果表明：其限幅机理可能源于碳纳米管吸收激光能量后升华产生的膨胀的碳气泡对入射激光产生的非线性散射;另外,非线性折射对光限幅效果也有一定的作用。     

Visible and near ultraviolet photocurrent generation in carbon nanotubes

Multiwall carbon nanotubes are found to generate photocurrent in the visible and near ultra violet spectral range using a photoelectrochemical technique. Peaks in the photocurrent are observed at excitation energies in the visible region. Their electron energy loss spectra exhibit the π plasmon feature, typical of graphite layers, and a peak at lower energy. Features at energies between 0 and 4 eV have been already observed for single wall carbon nanotubes and ascribed to interband electronic transitions due to the reduced dimensionality of these systems. The present measurements suggest that the usual identification of multiwall carbon nanotubes electronic density of states with that of graphite layers is not sufficient and more theoretical investigations are necessary to shed light on this point.     

Optical absorption by atomically doped carbon nanotubes under strong atom–field coupling

We analyze optical absorption by atomically doped carbon nanotubes with a special focus on the frequency range close to the atomic transition frequency. We derive the optical absorption line-shape function and, having analyzed particular achiral nanotubes of different diameters, predict the effect of absorption line splitting due to strong atom–vacuum–field coupling in small-diameter nanotubes.     

Gas phase kinetic and optical emission spectroscopy studies in plasma-enhanced hot filament catalytic CVD production of carbon nanotubes

Optical emission spectroscopy (OES) is used as the main experimental tool for comparison with simulations of the plasma and gas phase composition during plasma-enhanced hot filament catalytic chemical vapor deposition (PE HF CCVD) growth of carbon nanotubes (CNTs). Calculated concentration of more than 45 species in model of the CVD reactor is acquired by Chemkin™ software. Study of different conditions is performed and a close relationship can be found between the nature and the growth rate of carbon nanostructures and the concentration of the active gas phase species. Moreover it is shown that significant changes in the density and morphology of the CNTs grown in the presence of NH₃ could be mainly explained by the gas phase formation of CN and HCN.     

Effect of Pt and Fe catalysts in the transformation of carbon black into carbon nanotubes

In this research carbon nanotubes and carbon nano onion-like structures were synthesized from carbon black using metal catalysts at 400 °C and 700 °C. Platinum and iron-group metals were used as catalysts for the transformation of CB into graphitized nanocarbon and the effect of both metals was compared. The synthesized products were characterized using X-ray diffraction (XRD), transmission electron microscope (TEM), high resolution transmission electron microscope (HRTEM) and Raman spectroscopy. The characterization shows that this process is very efficient in the synthesis of high quality graphitized products from amorphous carbon black, even though the process temperature was relatively low in comparison with previous studies. Distinguished graphitic walls of the newly formed carbon nanostructures were clearly visible in the HRTEM images. Possible growth difference related to the type of catalyst used is briefly explained with the basis of electron vacancies in d-orbitals of metals.     

A comparison between the mechanical and thermal properties of single-walled carbon nanotubes and boron nitride nanotubes

Carbon nanotubes (CNTs) are semimetallic while boron nitride nanotubes (BNNTs) are wide band gap insulators. Despite the discrepancy in their electrical properties, a comparison between the mechanical and thermal properties of CNTs and BNNTs has a significant research value for their potential applications. In this work, molecular dynamics simulations are performed to systematically investigate the mechanical and thermal properties of CNTs and BNNTs. The calculated Young’s modulus is about 1.1 TPa for CNTs and 0.72 TPa for BNNTs under axial compressions. The critical bucking strain and maximum stress are inversely proportional to both diameter and length-diameter ratio and CNTs are identified axially stiffer than BNNTs. Thermal conductivities of (10, 0) CNTs and (10, 0) BNNTs follow similar trends with respect to length and temperature and are lower than that of their two-dimensional counterparts, graphene nanoribbons (GNRs) and BN nanoribbons (BNNRs), respectively. As the temperature falls below 200 K (130 K) the thermal conductivity of BNNTs (BNNRs) is larger than that of CNTs (GNRs), while at higher temperature it is lower than the latter. In addition, thermal conductivities of a (10, 0) CNT and a (10, 0) BNNT are further studied and analyzed under various axial compressive strains. Low-frequency phonons which mainly come from flexure modes are believed to make dominant contribution to the thermal conductivity of CNTs and BNNTs.     

Optical and photoconductivity studies of Cu2O nanowires synthesized by solvothermal method

The solvothermal method has been employed to synthesize cuprous oxide (Cu₂O) nanowires using a precursor of cupric acetate monohydrate (CuAc₂) and ethylene glycol (EG) as the solvent. By optimizing the reaction temperature and reaction time, we have prepared Cu₂O nanowires with a diameter of approximately 7 nm and a length of several nanometers. The UV-visible absorption spectrum of the nanowires shows obvious blueshift compared to the bulk Cu₂O, which arises from the quantum confinement effect of the excitonic transition expected for Cu₂O nanowires. Here we also report the role of different excitation energies on the photoluminescence (PL) properties of the Cu₂O nanowires by steady-state and time-resolved PL spectroscopy. The decay times vary from nanoseconds to picoseconds. Decay kinetics indicates that the average lifetime 〈τ〉 of the nanowires increases with increasing excitation energy. The current-voltage (I-V) curves of the nanowires give the photocurrent density 16 times larger than the dark current density.     

Magneto-optical spectroscopy of carbon nanotubes

We review our recent optical experiments on single-walled carbon nanotubes in high magnetic fields. The data revealed magnetic-field-induced optical anisotropy as well as broadening, splittings, and shifts of interband absorption and photoluminescence peaks. Quantitative comparison with theoretical predictions based on the Aharonov–Bohm effect is presented.     

Electronic thermal conductivity of armchair graphene nanoribbons and zigzag carbon nanotubes

Through the Green's function formalism and tight-binding Hamiltonian model calculations, the temperature dependent electronic thermal conductivity (TC) for different diameters of zigzag carbon nanotubes and their corresponding unzipped armchair graphene nanoribbons is calculated. All functional temperature dependencies bear crossovers, for which, at higher temperatures, nanotubes have a slightly higher TC than their derived nanoribbons, while below that crossover, both systems exhibit a significant coincidence over a moderate range of lower temperatures. Noticeably, TC decreases with increasing the width or diameter of the corresponding systems. Also, at low temperatures TC is proportional to the density of states around the Fermi level, and thus increasing for metal or semiconductors of narrower gap cases.     

Crystalline perfection, optical and dielectric studies on l-histidine nitrate: A nonlinear optical material

Single crystals of l-histidine nitrate (LHN), a recently investigated nonlinear optical material, were grown by conventional solution technique. Crystal structure and vibrational modes of the grown crystals were confirmed by powder X-ray diffractometry and FT-Raman spectrometry, respectively. Crystalline perfection of the grown crystals was evaluated by employing an in-house developed high-resolution X-ray diffractometer (HRXRD) and it was found that the grown crystals were free from structural grain boundaries and the perfection was reasonably good. However, HRXRD could reveal the fact that the crystals contain predominantly the interstitial point defects. The birefringence was measured over a range of wavelength between 5480 and 5630 Å and it was found that its value is nearly constant and 10 times higher than that of KDP. The optical band gap was found to be ∼3.73 eV. The photoluminescence excitation and emission spectra for single crystals were recorded. The SHG efficiencies of LHN samples of different particle sizes were measured by the Kurtz and Perry technique and they removed the ambiguity in the values reported differently in the literature. Dielectric properties were studied as a function of temperature over a wide range of frequency. The optical and dielectric studies along with the crystalline perfection reveal that the LHN crystal could be a good candidate for nonlinear optical devices.     

Plasmon‐enhanced Raman scattering by suspended carbon nanotubes

We report plasmon‐enhanced Raman scattering of the order of 10³ by a metallic carbon nanotube partially suspended inside a near‐field cavity. The tube is part of a small bundle, and is interfaced with an Au nanodisc dimer using a recently developed assembly scheme based on dielectrophoretic deposition. Spatially resolved Raman measurements with two excitation wavelengths and two orthogonal polarizations confirm that the enhancement arises from a 65 nm long suspended tube segment. We show that the orientation of the tube inside the cavity can be as effective for generating enhancement as placing the nanotube precisely in a plasmonic hotspot. Position and shape of the G‐peak show that the suspended part of the tube is free of strain and doped with a Fermi energy shift ≤40 meV. (© 2014 WILEY‐VCH Verlag GmbH &Co. KGaA, Weinheim)     

Terahertz applications of carbon nanotubes

We formulate and justify several proposals utilizing unique electronic properties of carbon nanotubes for a broad range of applications to THz optoelectronics, including THz generation by hot electrons in quasi-metallic nanotubes, frequency multiplication in chiral-nanotube-based superlattices controlled by a transverse electric field, and THz radiation detection and emission by armchair nanotubes in a strong magnetic field.     

The importance of solvent properties for optical limiting of carbon nanotube dispersions

The optical limiting performances of single-walled carbon nanotube dispersions in N-methyl-2-pyrrolidone were enhanced significantly by blending a range of organic solvents or by increasing the temperature of the dispersions up to 100 °C. While both nanotube bundle size and various solvent parameters have an influence on the optical limiting responses, we verified experimentally that the surface tension of the solvent plays a more important role than the viscosity or boiling point; the appropriate solvent properties contribute to the nonlinear scattering dominated optical limiting phenomenon more than the bundle size.     

Preparation and characterization of amphiphilic multi-walled carbon nanotubes

Multiwalled carbon nanotubes (MWCNTs, average diameter 8 nm) functionalized by N-vinyl pyrrolidone (NVP) were synthesized by radical polymerization and characterized by Fourier transform infrared and Raman spectroscopies, thermogravimetric analysis and transmission electron microscopy. These NVP–MWCNTs exhibit remarkable solubility in water, ethanol and dimethyl formamide. The polyvinyl pyrrolidone can be attached onto the surface of the MWCNTs and the degree of functionalization is affected by NVP content. The functionalization causes possible grafting reaction and solid physical coating between MWCNTs and PVP.     

SERS activity of carbon nanotubes modified by silver nanoparticles with different particle sizes

A two-dimensional (2D) surface-enhanced Raman scattering (SERS) substrate is fabricated by decorating carbon nanotube (CNT) films with Ag nanoparticles (AgNPs) in different sizes, via simple and low-cost chemical reduction method and self-assembling method. The change of Raman and SERS activity of carbon nanotubes/Ag nanoparticles (CNTs/AgNPs) composites with varying size of AgNPs are investigated by using rhodamine 6G (R6G) as a probe molecule. Meanwhile, the scattering cross section of AgNPs and the distribution of electric field of CNTs/AgNPs composite are simulated through finite difference time domain (FDTD) method. Surface plasmon resonance (SPR) wavelength is redshifted as the size of AgNPs increases, and the intensity of SERS and electric field increase with AgNPs size increasing. The experiment and simulation results show a Raman scattering enhancement factor (EF) of 10⁸ for the hybrid substrate.     

Electric field-induced unzipping of hydrogenated carbon nanotubes into graphene nanoribbons

We have investigated the electric field effect on horseshoe-shape carbon nanotubes (CNTs) resulting from hydrogen adsorption on the single-wall armchair (n,n)CNTs with 6 ≤ n ≤ 16 by using the density functional theory calculations. The horseshoe-shape CNT is completely unzipped into a graphene nanoribbon upon applying a critical electric field, which decreases with increasing CNT diameter, thus enabling one to select a nanoribbon width. A simple model based on the tensile force exerted on the tube walls by the applied electric field was introduced to understand the CNT-diameter dependence of the critical field.     

Strong effects of substitute nitrogen-doping on linear optical absorption spectra of zigzag carbon nanotubes

We theoretically study effects of substitute nitrogen-doping on linear optical properties of zigzag carbon nanotubes using tight-binding model and gradient approximation. It is found that, generally, nitrogen-doping reduces the value of absorption peaks and creates a number of new absorption peaks. In addition, near the low energy range, linear optical absorption spectra of nitrogen-doped (m,0) zigzag tubes are remarkably dependent on whether m mod 3 (the remainder of dividing m by 3) is equal to 0, 1 or 2.