Diffractive electron imaging of nanoparticles on a substrate

The observation of the detailed atomic arrangement within nanostructures has previously required the use of an electron microscope for imaging. The development of diffractive (lensless) imaging in X-ray science and electron microscopy using ab initio phase retrieval provides a promising tool for nanostructural characterization. We show that it is possible experimentally to reconstruct the atomic-resolution complex image (exit-face wavefunction) of a small particle lying on a thin carbon substrate from its electron microdiffraction pattern alone. We use a modified iterative charge-flipping algorithm and an estimate of the complex substrate image is subtracted at each iteration. The diffraction pattern is recorded using a parallel beam with a diameter of approximately 50 nm, illuminating a gold nanoparticle of approximately 13.6 nm diameter. Prior knowledge of the boundary of the object is not required. The method has the advantage that the reconstructed exit-face wavefunction is free of the aberrations of the objective lens normally used in the microscope, whereas resolution is limited only by thermal vibration and noise.     

Onset of superfluidity in4He films adsorbed on a graphite-fiber substrate

A torsional oscillator technique has been employed to investigate the onset of superfluidity in⁴He adsorbed on graphite fibers. A low temperature capacitance pressure gauge has been used to characterize the layered growth of the film as mass is added to the experimental cell. At T=0.84 K the onset of superfluidity occurs after completion of three layers. A comparison with similar data for a Mylar substrate shows that the dissipation peaks are much sharper for the graphite-fiber substrate.     

Nanolithographic self-assembly of colloidal nanoparticles

Technical Physics Letters - Methods of nanolithographic self-assembly of colloidal nanoparticles have been created, which are promising for the development of fractal nanolithography.     

Synthesis of a stable colloidal solution of PbS nanoparticles

A method has been proposed for the synthesis of stable colloidal solutions of lead sulfide nanoparticles from aqueous lead acetate and sodium sulfide solutions. The citrate ion and ethylenediaminetetraacetic acid were used as complexing agents to stabilize the solution. The solutions obtained were shown to remain stable at room temperature for at least 30 days from the instant of synthesis, depending on the initial reactant concentration. According to X-ray diffraction and dynamic light scattering data, the stability and coagulation of the nanoparticles in solution were influenced by the lead and sulfur concentration, the presence of complexing agents, and the quantitative relation between the lead and complexing agent ions in solution. Optical absorption measurements for the colloidal PbS nanoparticle solutions showed that their absorbance was a nonlinear function of reactant concentration.     

Crystalline transformation of colloidal nanoparticles on graphene oxide

Emergence of novel two-dimensional (2-D) templates, e.g., graphene oxide, has signified new intriguing opportunities to couple nanocrystals electronically to the microscopic 2-D contacts. A promising approach to uniform dispersion of inorganic nanocrystals on the 2-D interfaces is to graft them through chemical bonding. The 2-D dispersion would offer a unique opportunity to address one of the primary challenges in the field of nanotechnology: fulfilling excellent chemical and physical properties of the nanocrystals in electronic solid-state devices. In this study, we blended colloidal nanocrystals with graphene oxide in aqueous solution in attempts to bind the nanocrystals on reactive sites of the graphene oxide surface, thereby achieving uniform loading. Interestingly, the nanocrystals undergo significant crystalline transformation even under relatively moderate reaction conditions. The growth of particle size and the drastic crystalline deformation, e.g., from wurtzite CdSe to amorphous Se, appear to take place in the proximity of acidic functional groups on graphene oxide. Photocarriers also play a key role in the reaction: under room light, the transformation yielded dramatic size increase and crystalline transformation, whereas in the dark, the change was suppressed. The experimental results presented in this study provide guidelines for uniform 2-D loading of colloidal nanocrystals on graphene oxide. The findings suggest that the surface acidity be titrated for colloidal nanocrystals to deposit on the graphitic layer and to avoid unwanted changes of nanocrystal size and properties.     

Synthesis and self-assembly of colloidal nanoparticles

We describe recent developments in the synthesis of semiconductor nanoparticles, which lead to a substantial improvement of the luminescence quantum efficiency. Concerning a theoretical model for the growth of an ensemble of nanoparticles, the highest quantum efficiencies are achieved in particles that grow under conditions of a rapid exchange of monomers at the particle surface, leading to a smooth surface structure. Selective etching, core-shell formation and doping of nanoparticles are also discussed as fluorescence-enhancing preparative techniques. Examples of self-assembly of almost-uniformly-sized nanoparticles are given, which result in two-dimensional and three-dimensional superlattices, colloidal crystals and crystalline structures built-up from particles of different sizes. Finally, the self-assembled oriented attachment of quasi-spherical ZnO nanoparticles onto single-crystalline nanorods is presented.     

Photochemical synthesis of colloidal gold nanoparticles

Monodisperse gold nanoparticles protected by small organic molecules or by macromolecules with different sizes and shapes are widely used as a precursor material in various applications of gold nanotechnology. However, their preparation is still a formidable task. In this paper the use of photochemically assisted syntheses of monodisperse gold nanoparticles is summarized and some preparations by the authors’ group are introduced. These include spherical and rod-like particles, bimetallic composite nanoparticles, and syntheses using complex intramolecular photoreduction to generate the reducing agent.     

Three-dimensional reduced-symmetry of colloidal plasmonic nanoparticles

Owing to their novel optical properties, three-dimensional plasmonic nanostructures with reduced symmetry such as a nanocrescent and a nanocup have attracted considerable current interest in biophotonic imaging and sensing. However, their practical applications have been still limited since the colloidal synthesis of such structures that allows, in principle, for in vivo application and large-scale production has not been explored yet. To date, these structures have been fabricated only on two-dimensional substrates using micro/nanofabrication techniques. Here we demonstrate an innovative way of breaking symmetry of colloidal plasmonic nanoparticles. Our strategy exploits the direct overgrowth of Au on a hybrid colloidal dimer consisting of Au and polystyrene (PS) nanoparticles without the self-nucleation of Au in an aqueous solution. Upon the overgrowth reaction, the steric crowding of PS leads to morphological evolution of the Au part in the dimer ranging from half-shell, nanocrescent to nanoshell associated with the appearance of the second plasmon absorption band in near IR. Surface-enhanced Raman scattering signal is obtained directly from the symmetry-broken nanoparticles solution as an example showing the viability of the present approach. We believe our concept represents an important step toward a wide range of biophotonic applications for optical nanoplasmonics such as targeting, sensing/imaging, gene delivery, and optical gene regulations.     

Solution-processed ultraviolet photodetectors based on colloidal ZnO nanoparticles

A "visible-blind" solution-processed UV photodetector is realized on the basis of colloidal ZnO nanoparticles. The devices exhibit low dark currents with a resistance >1 TOmega and high UV photocurrent efficiencies with a responsivity of 61 A/W at an average intensity of 1.06 mW/cm(2) illumination at 370 nm. The characteristic times for the rise and fall of the photocurrent are <0.1 s and about 1 s, respectively. The photocurrent of the device is associated with a light-induced desorption of oxygen from the nanoparticle surfaces, thus removing electron traps and increasing the free carrier density which in turn reduces the Schottky barrier between contacts and ZnO nanoparticles for electron injection. The devices are promising for use in large-area UV photodetector applications.     

硅烷偶联剂在二氧化硅基片表面吸附膜的形貌

使用原子力显微镜(AFM)观测了吸附在二氧化硅基片表面的硅烷偶联剂薄膜的形貌.结果表明,在气相法吸附过程中偶联剂是以分子形态吸附在基片表面,而在液相法吸附过程中偶联剂是以分子聚合体的形态吸附在基片表面,因此通过气相法吸附在基片表面的吸附膜比通过液相法吸附在基片表面的吸附膜光滑.硅烷偶联剂在二氧化硅基片表面有化学吸附和物理吸附两种模式,吸附了硅烷偶联剂薄膜的基片表面呈现出一定的疏水性.     

Enhanced stability of enzymes adsorbed onto nanoparticles

We have discovered that the highly curved surface of C60 fullerenes enhances enzyme stability in strongly denaturing environments to a greater extent than flat supports. The half-life of a model enzyme, soybean peroxidase, adsorbed onto fullerenes at 95 degrees C was 117 min, ca. 2.5-fold higher than that of the enzyme adsorbed onto graphite flakes and ca. 13-fold higher than that of the native enzyme. Furthermore, this phenomenon is not unique to fullerenes, but can also be extended to other nanoscale supports including silica and gold nanoparticles. The enhanced stability was exploited in the preparation of highly active and stable polymer-nanocomposite films. The ability to enhance protein stability by interfacing them with nanomaterials may impact numerous fields ranging from the design of diagnostics, sensors, and nanocomposites to drug delivery.     

Interfacial friction of submonolayer helium films adsorbed on the surface of graphite

The frictional coupling between an adsorbed helium film and the surface of graphite has been studied using torsional oscillator methods and exfoliated graphite as a substrate, in the temperature range 50mK to 2K. The observed period shift is consistent with the film decoupling from the substrate as the temperature is reduced, and is accompanied by a maximum in dissipation. At coverages below registry and in the incommensurate solid region the data are well described by Debye relations with a thermally activated relaxation time. This would correspond to a vanishing of the interfacial friction for the solid monolayer as T - 0. At coverages between 0.064Ų and 0.08Å-² a double peaked structure in the dissipation is observed. The results relate to the work of Krim et al.¹ who have used a quartz microbalance to study the nanotribology of adsorbed monolayers. In the present work the use of the graphite substrate offers the possibility of studying these effects on a system with a rich and well characterised submonolayer phase diagram.     

Double layer calculations for the attachment of a colloidal particle with a charged surface patch onto a substrate

The attachment of a particle onto a substrate with a like surface charge may be greatly facilitated in the presence of an oppositely charged surface patch on the particle. Calculations are presented for the effect of various parameters on particle attachment. These include the patch charge, patch size, particle size and the ionic strength of the medium.     

Room temperature synthesis of colloidal platinum nanoparticles

Efficient preparation of stable dispersions of platinum nanoparticles from platinous chloride (K₂PtCl₄) was achieved by simultaneous addition of capping polymer material. The size of platinum nanoparticles was controlled by changing the ratio of concentration of capping polymer material to the concentration of platinum cation used. The morphology of colloidal particles were studied by means of UV-visible spectrophotometry and transmission electron microscopy (TEM). Particle size increased with low reagent concentration. The change in absorption spectra with the particle size was observed, i.e. blue shift attributed to decrease in particle size Paper presented at the 5th IUMRS ICA98, October 1998, Bangalore.     

Photoluminescence of ZnO nanoparticles prepared by a low-temperature colloidal chemistry method

ZnO nanoparticles have been synthesized by a low-temperature colloidal chemistry method using ethylene glycol as the reaction medium. Crystalline ZnO nanoparticles were formed at a temperature as low as 150 °C. The crystallite size of the ZnO nanoparticles ranged from 8 to 20 nm. The synthesized ZnO nanoparticles exhibited size-dependent photoluminescence. Photoluminescence of the ZnO nanoparticles depended largely on the post-annealing temperature. Both DL and UV emissions were suppressed at a lower post-annealing temperature of 300 °C; however they were recovered at a higher annealing temperature of 900 °C.     

Study of lanthanum-based colloidal sols formation

Lanthanum-based colloidal sols have been studied in order to synthesize lanthanum oxychloride inorganic coatings or thin layers for catalytic applications. The singularity of this process leading to microporous coatings is based on the polymerization of lanthanum acetate species in aqueous solution. Sols are prepared from lanthanum chloride modified by acetate ions. The sol formation mechanism can be explained by La³⁺ hydrolysis, giving basic species (La (OH)_X with x = 1 or 2) which condense and lead to polycondensed hydroxo ions. In the pH range of sol formation, OH^– and CH₃COO^– are ligands competitive towards La³⁺; when acetate ions are present, the condensation rate is limited by lanthanum acetate complexation. Several distributions of lanthanum hydroxide and acetate species are given and related to experimental results (pH, Fourier transform infrared (FTIR) spectroscopy, turbidity and stability of sols, SEM and TEM analysis of coatings). FTIR spectroscopy has been revealed as useful to evidence a polymerization of acetate species in sols leading to translucent gels. These gels allow the preparation of almost fully dense LaOCl coatings after thermal treatment above 450°C. These results confirm the possible polymerization of lanthanum acetate complexes during the drying step and emphasize the great effect of acetate species on the final material texture.     

Micromechanics of elastic buckling of a colloidal polymer layer on a soft substrate: experiment and theory

We study the buckling instability of a colloidal particle layer adhered to an elastic substrate using an integrated experimental and theoretical approach. Experiments using monodisperse colloid-scale spherical particles made of polystyrene and silica, show that the wavelength of the initial (critical) buckling mode is independent of particle modulus and linearly dependent on particle radius—in contradiction with the predictions of the prevailing continuum model. We developed a granular model of the particle layer using structural mechanics techniques. The granular model predicts the observed wavelength of the initial, critical buckling mode within the estimated range of parameter values for the experiment. The evolution of this mode into the post-buckling regime is examined. Results highlight the crucial role of material discreteness in the mechanical response, and the need for accurate methods of estimating parameters for the particle-scale resistances against buckling.     

GISAXS studies of self-assembling of colloidal Co nanoparticles

In this work we report on the formation of ordered monolayers (2-D) and arrays of rods (3-D) of magnetic Co nanoparticles in magnetic field perpendicular to the substrate surface. Samples were prepared by drying a droplet of colloidal solution of Co nanoparticles (10 nm diameter) on Si/Si₃N₄ substrates in magnetic field between 0.2 and 0.9 T. The samples were characterized by high resolution scanning electron microscopy (SEM), atomic and magnetic force microscopy (AFM/MFM) and grazing incidence small angle X-ray scattering (GISAXS). SEM studies of monolayers show well-ordered 2-D arrays with hexagonal symmetry of 200 nm × 500 nm in size forming a mosaic structure. Rods, about 500 nm in diameter, aligned with the field direction and forming a hexagonal pattern were obtained when higher concentration of colloid and low evaporation rate of the solvent were used. The ordering of nanoparticles in the monolayer analyzed by GISAXS is described by the local order with hexagonal symmetry. The model of close packing of hard spheres is used for ordering of particles inside the rods. Magnetic features corresponding to the 3-D arrays have been observed by MFM pointing out that all magnetic moments in the rod are oriented along the field direction.