Gold nanorods: Synthesis and optical properties

The main results of studying the synthesis, growth mechanisms, and optical properties of gold nanorods published in the last 5–8 years are briefly reviewed. Hydrosols of gold nanorods with variable axial ratios are synthesized in the micellar solution of ionic surfactants by sead-mediated growth procedure using the stage of particle separation in the glycerol concentration gradient. Results of synthesis in systems containing one surfactant, albeit with different Ag/Au molar ratios and different amounts of gold seeding particles, agree with the published data. It is shown that, in the case of the mixture of two surfactants, the Ag/Au ratio is an efficient controlling parameter of the synthesis of nanorods with large axial ratios. The extinction and differential light scattering, spectra dynamic light scattering, and the depolarization of laser light scattering at 90° are used for the optical control of synthesis. Three fractions are observed in separated samples. One of these fractions is characterized by the only short wavelength plasmon resonance at 570 nm corresponding, in agreement with the published data, to cubic particles. Measurements of the extinction spectra of nanorods in water-glycerol mixtures reveal higher sensitivity of the longitudinal plasmon resonance to the dielectric environment relative to the transverse resonance. It is shown theoretically and experimentally that the relative shift of plasmon resonance is proportional to the relative increment of refractive index of the surrounding medium. To calculate optical properties of nanorods, we employed a model of cylinders with semispherical ends (s-cylinders) corresponding to the shape of real particles and admitting the exact solution by the T-matrix method with a computational burden that is an order of magnitude lower than that used in the discrete dipole method. The set of dependences of the longitudinal resonance wavelength on the axial ratio of different-thickness particles complies with our data and published measurements. Theoretical and experimental values of depolarization ratio I _VH/I _VV for nanorods and nanospheres with different sizes prepared with both citrate (15–46 nm) and original thiocyanate (90 nm) reduction of HAuCl₄ are compared. It is shown that the depolarization parameter of light scattered by a nanorod suspension can exceed the theoretical limit (1/3) for common dielectric particles. The measured 10%-depolarization ratio for 90-nm spheres was far beyond the set of “size-depolarization” measurements for 15–46-nm-dia particles prepared by the citrate method and is indicative of the improved spherical morphology of 90-nm particles. This assumption was confirmed by TEM data, which also revealed both the presence of a noticeable amount of nanorods with a large axial ratio and “nanowires” of about the same thickness. A new analytic calibration for determining the diameter of spherical particles (5–100 nm) by the spectral position of the sol extinction maximum is proposed.     

Preparation and optical properties of worm-like gold nanorods

A type of worm-like nanorods was successfully synthesized through conventional gold nanorods reacting with Na2S2O3 or Na2S. The generated worm-like gold nanorods comprise shrunk nanorod cores and enwrapped shells. Therefore, a gold-gold sulfide core-shell structure is formed in the process, distinguishing from their original counterparts. The formation of the gold chalcogenide layers was confirmed by transmission electron microscopy and X-ray photoelectron spectroscopy. Experimental results showed that the thickness of the gold chalcogenide layers is controllable. Since the increase of shell thickness and decrease of gold nanorod core take place simultaneously, it allows one to tune the plasmon resonance of nanorods. Proper adjustment of reaction time, temperature, additives and other experimental conditions will produce worm-like gold nanorods demonstrating desired longitudinal plasmon wavelength (LPW) with narrow size distributions, only limited by properties of starting original gold nanorods. The approach presented herein is capable of selectively changing LPW of the gold nanorods. Additionally, the formed worm-like nanorods possess higher sensitive property in localized surface plasmon resonance than the original nanorods. Their special properties were characterized by spectroscopic methods such as Vis-NIR, fluorescence and resonance light scattering. These features imply that the gold nanorods have potential applications in biomolecular recognition study and biosensor fabrications.     

Phase transformation and optical properties of Cu-doped ZnS nanorods

ZnS nanorods doped with 0-15 mol% of Cu have been prepared by simple solvothermal process. With gradual increase in the Cu concentration, phase transformation of the doped ZnS nanorods from wurtzite to cubic was observed. Twins and stacking faults were developed due to atomic rearrangement in the heavily doped ZnS nanorods during phase transformation. UV-vis-NIR absorbance spectroscopy ruled out the presence of any impure Cu-S phase. The doped ZnS nanorods showed luminescence over a wide range from UV to near IR with peaks at 370, 492-498, 565 and 730 nm. The UV region peak is due to the near-band-edge transition, whereas, the green peak can be related to emission from elementary sulfur species on the surfaces of the nanorods. The orange emission at 565 nm may be linked to the recombination of electrons at deep defect levels and the Cu(t₂) states present near the valence band of ZnS. The near IR emission possibly originated from transitions due to deep-level defects.     

Synthesis and optical properties of colloidal tungsten oxide nanorods

Thermal decomposition of W(CO)6 in oleylamine in the presence of mild oxidant Me3NO.2H2O produces tungsten oxide nanorods with diameters ranging from 3 to 6 nm. The size of nanorods can be easily varied by the employed surfactant ratio or reaction temperature. The prepared tungsten oxide nanorods exhibit strong photoluminescence (PL) peaks in 300-500 nm, which show a weak size dependency.     

One-dimensional CoPt nanorods and their anomalous IR optical properties

One-dimensional (1D) CoPt nanorods were synthesized by a galvanic displacement reaction. The morphology of the nanomaterials was characterized by transmission electron microscopy (TEM), scanning electron microscopy (SEM) and X-ray powder diffraction (XRD). Energy-dispersive X-ray spectroscopy (EDS) analysis confirmed the coexistence of Co and Pt in the 1D nanorods. Studies of cyclic voltammetry (CV) demonstrated that the 1D CoPt nanorods exhibit a better electrocatalytic property for CO oxidation than that of bulk Pt electrode does. In situ electrochemical FTIRS illustrated, for the first time, that the 1D CoPt nanorods display abnormal infrared effects (AIREs), which was previously revealed mainly on 2D film nanomaterials.     

Soft template synthesis of ZnO nanorods and their optical properties

A simple solution route was developed to fabricate monodisperse wurtzite ZnO nanorods. The as-prepared samples were 5 ??m in length and 70?C100 nm in diameter. The crystallinity, morphology, and structure of the rod-like ZnO microcrystals were examined. The crystal phases and the microstructure of the nanorods were studied by X-ray diffraction (XRD), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). Room- and low-temperature photoluminescence (PL) and Raman spectra were employed to investigate the surface states of the samples. The deep-level emission band was barely observable at both room and cryogenic temperatures.     

Synthesis and optical properties of V2O5 nanorods

A two-step method was proposed in synthesizing V2O5 nanorods on planar substrates, i.e., depositing a V2O3 thin film at approximately 220 degrees C (by heating a pure sheet of vanadium in a rough vacuum) and then heating it in air at approximately 400 degrees C. The V2O5 nanorods produced by this technique are single crystalline and could emit intense visible light at room temperature, possibly due to some defects such as oxygen vacancies which got involved during growth. This study provides a simple and low-substrate-temperature route in fabricating V2O5 nanorods on planar substrates, which might be also applicable to other metal oxides.     

Chemical sensing and imaging with metallic nanorods

In this Feature Article, we examine recent advances in chemical analyte detection and optical imaging applications using gold and silver nanoparticles, with a primary focus on our own work. Noble metal nanoparticles have exciting physical and chemical properties that are entirely different from the bulk. For chemical sensing and imaging, the optical properties of metallic nanoparticles provide a wide range of opportunities, all of which ultimately arise from the collective oscillations of conduction band electrons ("plasmons") in response to external electromagnetic radiation. Nanorods have multiple plasmon bands compared to nanospheres. We identify four optical sensing and imaging modalities for metallic nanoparticles: (1) aggregation-dependent shifts in plasmon frequency; (2) local refractive index-dependent shifts in plasmon frequency; (3) inelastic (surface-enhanced Raman) light scattering; and (4) elastic (Rayleigh) light scattering. The surface chemistry of the nanoparticles must be tunable to create chemical specificity, and is a key requirement for successful sensing and imaging platforms.     

Liquid-crystalline polymer composites with CdS nanorods: structure and optical properties

We report on the structure, uniaxial orientation, and photoluminescent properties of CdS nanorods that form stable nanocomposites with smectic C hydrogen-bonded polymers from the family of poly(4-(n-acryloyloxyalkoxy)benzoic acids. TEM analysis of microtomed films of nanocomposites reveals that CdS nanorods form small domains that are homogeneously distributed in the LC polymer matrix. They undergo long-range orientation with the formation of one-dimensional aggregates of rods when the composite films are uniaxially deformed. The Stokes photoluminescence was observed from CdS NRs/LC polymer composites with emission peak located almost at the same wavelength as that of NRs solution in heptane. An anti-Stokes photoluminescence (ASPL) in polymer nanocomposites was found under the excitation below the nanoparticles ground state. The mechanism of ASPL was interpreted in terms of thermally populated states that are involved in the excitation process. These nanocomposites represent an unusual material in which the optical properties of anisotropic semiconductor nanostructures can be controlled by mechanical deformation of liquid-crystalline matrix.     

Theory of nonlinear optical properties of small metallic spheres

We calculate the nonlinear optical properties of small metallic spheres using electromagnetic theory and assuming that the local response of the conduction electrons is the same as for a plane surface. Electromagnetic Mie-resonances cause a strong increase of the second and higher harmonics in the reflected light. Detailed results are given for the second and third harmonic generation, its dependence on the frequency and polarization of the incident light, and on the cluster size. An enhancement of the second harmonic generation by a factor of about 5000 is obtained for small spherical metallic clusters. This is in good agreement with experiments on artificially roughened metal surfaces.     

Synthesis, optical properties, and growth mechanism of blue-emitting CdSe nanorods

Blue-emitting, cubic phase CdSe nanorods with an approximate diameter of 2.5 nm and lengths up to 12 nm have been synthesized at low temperature (100 degrees C) in a single surfactant using a single-source molecular precursor. Transmission electron microscopy and dynamic light scattering measurements indicate that the nanorods are formed from self-assembly of isotropic nanoclusters. Anisotropic growth in a single surfactant appears to be favored when growth occurs below the thermal decomposition temperature of the single-source precursor.     

Experimental evidence for quantum size effects in ultrathin metallic films

In recent years it has become clear that size quantization in ultrathin metallic films can influence many physical properties but most effects are rather weak. Due to short de Broglie wavelength of electrons at Fermi level most effects can be observed in extremely thin and smooth films. Recent achievements in technology of ultrathin metallic films allow to verify early theoretical predictions. Among them the measurements of specific resistivity and electron photoemission can be used as useful and sensitive tools. Clear QSE effects showed ultrathin layers of Pb, Pb-In alloy and In deposited on Si(111)-(6×6)Au substrate. Using Reflection High Energy Electron Diffraction (RHEED) technique the layered structure of ultrathin films was examined and correlated with QSE effects.     

Effect of composition and packing configuration on the dichroic optical properties of coinage metal nanorods

When nanorods of Au, Ag and some other elements are aligned with a preferred orientation with respect to light, their optical extinction characteristics become dependent on the polarization and angle of incidence of the light. This effect is explored here and it is shown that it could potentially be exploited to produce a 'colour-change coating'. However, particle-particle interactions are also likely to occur in such coatings, with red shifting of extinction spectra occurring for end-on-end configurations of monodisperse rods, and blue shifting for side-by-side configurations. Surprisingly, the particle-particle interactions are attenuated if they are between rods of differing aspect ratios, and this offers a useful new means of control of the optical properties of coatings of nanorods.     

Nonlinear optical properties of organic molecular assemblies and fractal metallic clusters

The paper overviews progress in the development of molecular media based on organic compounds possessing a high third-order nonlinear optical susceptibility. Such media are useful in the design of elements for optical data processing devices. The_X ⁽³⁾ values of polyconjugated polymers and organic dyes are presented; these depend on the film production method and molecular assembly organization. The review discusses nonlinear optical properties of metallic fractal clusters. The nonlinear response of a molecular medium is shown to be greatly enhanced by aggregation of molecules and by their location in a local field of a metallic cluster.Novosibirsk Institute of Organic Chemistry, Siberian Branch, Russian Academy of Sciences. Translated fromZhurnal Strukturnoi Khimii, Vol. 34, No. 6, pp. 90–105, November–December, 1993.Translated by L. Smolina     

Modeling size effects on the surface free energy of metallic nanoparticles and nanocavities

Based on the rigorous consideration of the bond broken rule and surface relaxation, a model for the size-dependent surface free energy of face-centered-cubic nanoparticles and nanocavities is presented, where the surface relaxation is calculated by the BOLS relationship. It is found that the surface free energy of nanoparticles and nanocavities represents a reverse size effect-the surface free energy of nanoparticles decreases with the decrease of particle size while it rises with the shrinkage of cavities. The size effect on the surface free energy of nanoparticles and nanocavities is not evident in large size ranges, while it becomes more and more distinct with decreasing size, especially for sizes smaller than 10 nm. The present predictions are in good agreement with the available literature data.     

Synthesis and optical properties of poly (vinyl acetate)/bismuth oxide nanorods

Poly (vinyl acetate) (PVAc) loaded bismuth oxide (Bi₂O₃) nanorods were successfully prepared at ambient pressure. X‐ray diffraction (XRD) and transmission electron microscopy were used to characterize the final product. It was found that Bi₂O₃ nanorods were formed and the diameter of the rods was confined to about 8 nm. The diameter and length of formed rods were found to increase by increasing the bismuth oxide concentration in the PVAc matrix. The optical properties of the nanocomposite films were characterized from the analysis of the experimentally recorded transmittance and reflectance data in the spectral wavelength range of 300–800 nm. The values of some important parameters of the studied films are determined such as refractive index (n), extinction coefficient (k), optical absorption coefficient (α), and band energy gap (E_g). According to the analysis of dispersion curves, it has been found that the dispersion data obeyed the single oscillator of the Wemple–DiDomenico model, from which the dispersion parameters and high‐frequency dielectric constant were determined. In such work, from the transmission spectra, the dielectric constant (ε_∞) and the third‐order optical nonlinear susceptibility χ⁽³⁾ were determined. Copyright © 2010 John Wiley & Sons, Ltd.     

Universal relation for size dependent thermodynamic properties of metallic nanoparticles

The previous model on surface free energy has been extended to calculate size dependent thermodynamic properties (i.e., melting temperature, melting enthalpy, melting entropy, evaporation temperature, Curie temperature, Debye temperature and specific heat capacity) of nanoparticles. According to the quantitative calculation of size effects on the calculated thermodynamic properties, it is found that most thermodynamic properties of nanoparticles vary linearly with 1/D as a first approximation. In other words, the size dependent thermodynamic properties P(n) have the form of P(n) = P(b)(1 -K/D), in which P(b) is the corresponding bulk value and K is the material constant. This may be regarded as a scaling law for most of the size dependent thermodynamic properties for different materials. The present predictions are consistent literature values.     

Synthesis and optical properties of small Au nanorods using a seedless growth technique

Gold nanoparticles have shown potential in photothermal cancer therapy and optoelectronic technology. In both applications, a call for small size nanorods is warranted. In the present work, a one-pot seedless synthetic technique has been developed to prepare relatively small monodisperse gold nanorods with average dimensions (length × width) of 18 × 4.5 nm, 25 × 5 nm, 15 × 4.5 nm, and 10 × 2.5 nm. In this method, the pH was found to play a crucial role in the monodispersity of the nanorods when the NaBH(4) concentration of the growth solution was adjusted to control the reduction rate of the gold ions. At the optimized pH and NaBH(4) concentrations, smaller gold nanorods were produced by adjusting the CTAB concentration in the growth solution. In addition, the concentration of silver ions in the growth solution was found to be pivotal in controlling the aspect ratio of the nanorods. The extinction coefficient values for the small gold nanorods synthesized with three different aspect ratios were estimated using the absorption spectra, size distributions, and the atomic spectroscopic analysis data. The previously accepted relationships between the extinction coefficient or the longitudinal band wavelength values and the nanorods' aspect ratios found for the large nanorods do not extend to the small size domain reported in the present work. The failure of extending these relationships over larger sizes is a result of the interaction of light with the large rods giving an extinction band which results mostly from scattering processes while the extinction of the small nanorods results from absorption processes.     

Shape and electrostatic effects in optical Kerr effect spectroscopy of aromatic liquids

We present a detailed, temperature-dependent, optical Kerr effect (OKE) study of pyridine, pyridine-d(5), 2,4,6-trifluoropyridine, 2,4,6-trimethylpyridine, and 1,3,5-tris(trifluoromethyl)benzene. By combining these data with those for other aromatic liquids that we have studied previously (Loughnane, B. J.; Scodinu, A.; Fourkas J. T. J. Phys. Chem. B, 2006, 110, 5708), we are able to assess the relative importance of molecular shape and electrostatic forces in determining the form of the OKE reduced spectral density for such liquids.