P. John Thomas

Metal nanoparticles and their assemblies

Metal nanoparticles of varying sizes can be prepared by nphysical as well as chemical methods. They exhibit many fascinating nproperties, the size-dependent metal to nonmetal transition being an nimportant one. Metal nanoparticles capped by thiols can be organized into nordered one-, two- and three-dimensional structures and these structures nhave potential applications in nanodevices. In this context, organization nof arrays of metal nanoparticles with a fixed number of atoms assumes nsignificance.

Size-Dependent Chemistry: Properties of Nanocrystals

Properties of materials determined by their size are indeed fascinating and form the basis of the emerging area of nanoscience. In this article, we examine the size dependent electronic structure and properties of nanocrystals of semiconductors and metals to illustrate this aspect. We then discuss the chemical reactivity of metal nanocrystals which is strongly dependent on the size not only because of the large surface area but also a result of the significantly different electronic structure of the small nanocrystals. Nanoscale catalysis of gold exemplifies this feature. Size also plays a role in the assembly of nanocrystals into crystalline arrays. While we owe the beginnings of size-dependent chemistry to the early studies of colloids, recent findings have added a new dimension to the subject.

Superlattices of metal and metal-semiconductor quantum dots obtained by layer-by-layer deposition of nanoparticle arrays

Superlattices formed by arrays of Pt or Au nanoparticles have been obtained by layer-by-layer deposition by using dithiols as cross-linkers. The superlattices have been characterized by X-ray diffraction, photoelectron spectroscopy, and scanning tunneling microscopy. The core-level intensities of the metal and of the dithiol in the X-ray photoelectron spectra show the expected increase with successive depositions. The formation of such structures has been confirmed by depositing Pt and Au layers alternatively. Layers of metal and CdS nanoparticles have been deposited alternatively to obtain heterostructures.

Mesoscale organization of metal nanocrystals

Nanocrystals of metals covered by alkanethiols organize themselves in two-dimensional arrays. We discuss such arrays of metal nanocrystals at length, with focus on the dependence of the structure and the stability of the arrays on the particle diameter and the distance between the particles. Three-dimensional superstructures of metal nanocrystals obtained by the use of alkanedithiols are examined. These ordered two- and three-dimensional structures of thiolized metal nanocrystals are good examples of mesoscale self-assembly. The association of metal nanocrystals to give rise to giant clusters with magic nuclearity provides an even more graphic demonstration of mesoscale self-assembly.

Metal nanoparticles, nanowires, and carbon nanotubes

The size-dependent metal to nonmetal transition in metal nanoparticles has been investigated using photoelectron and tunneling spectroscopic techniques. Metal nanoparticles capped by thiols are shown to organize into ordered 2D and 3D structures. Single-walled nanotubes and aligned carbon nanotube bundles have been synthesized by controlling the size of metal nanoparticles produced in situ during the pyrolysis of precursors. Nanowires of gold and other metals have been produced in the capillaries of the single-walled nanotubes.

Dip-pen nanolithography with magnetic Fe2O3 nanocrystals

Dip-pen nanolithography has been employed to obtain magnetic nanopatterns of γ-Fe2O3 nanocrystals on mica and silicon substrates. The chemical and magnetic nature of the patterns have been characterized employing low-energy electron microscopy, x-ray photoemission electron microscopy, and magnetic force microscopy measurements.

Dip-pen lithography using aqueous metal nanocrystal dispersions

Dip-pen lithography has been successfully demonstrated on mica substrates employing hydrosols of polyvinylpyrrolidone-capped Pd nanocrystals as well as Au nanocrystals stabilized by tetrakishydroxymethyl phosphonium chloride. Lines of widths as small as 30 nm and various aspect ratios have been successfully drawn by this method.

Submicron particles of Co, Ni and Co-Ni alloys

Magnetic sub-micron sized particles (with diameters in the range 100–600 nm) of Co, Ni and Co-Ni alloys, protected with polyvinylpyrrolidone have been prepared in gram quantities using the polyol process. Experiments carried out with different metal precursors and starting compositions have yielded reliable routes to produce particles of the desired diameters in the 100–600 nm range. The particles were characterized with X-ray diffraction, scanning electron microscopy, energy dispersive X-ray analysis, thermogravimetric analysis and magnetic measurements. The particles are found to be stable under ambient conditions indefinitely. The coercivity values of the Co and Ni particles are ∼50% higher compared to the corresponding bulk values. The alloy particles follow a trend similar to the bulk alloys.

Effect of size on the Coulomb staircase phenomenon in metal nanocrystals

Abstract The Coulomb staircase in polymer-covered Pd and Au nanocrystals of varying diameters in the 1.7–6.4 nm has been investigated by employing tunneling conductance measurements. Charging up to several electrons is observed at room temperature in the I–V data. Small nanocrystals show charging steps exceeding 200 mV while the larger ones exhibit smaller steps. Significantly, the charging energies follow a scaling law of the form, U = A + B / d , where d is the diameter of the nanocrystal. Furthermore, the line widths in the derivative spectra also vary inversely with the diameter.

Self-assembling bilayers of palladiumthiolates in organic media

Alkylthiolates of palladium forming a homologous series (butyl to octadecyl) have been prepared and characterized using X-ray diffraction and STM. The thiolates adopt an unusual bilayered lamellar structure, whose thickness is governed by the length of the alkyl chain. These mesophases melt in the temperature range, 60° to 100°C, with the melting point increasing linearly with the thiol chain length. There is evidence to suggest that the alkyl chains are orientationally disordered especially prior to melting.