Nanosphere lithography (NSL) is an inexpensive, simple to implement, inherently parallel, high throughput, materials general nanofabrication technique capable of producing an unexpectedly large variety of nanoparticle structures and well-ordered 2D nanoparticle arrays. This article describes our recent efforts to broaden the scope of NSL to include strategies for the fabrication of several new nanoparticle structural motifs and their characterization by atomic force microscopy. NSL has also been demonstrated to be well-suited to the synthesis of size-tunable noble metal nanoparticles in the 20−1000 nm range. This characteristic of NSL has been especially valuable for investigating the fascinating richness of behavior manifested in size-dependent nanoparticle optics. The use of localized surface plasmon resonance (LSPR) spectroscopy to probe the size-tunable optical properties of Ag nanoparticles and their sensitivity to the local, external dielectric environment (viz., the nanoenvironment) is discussed in...

Nanosphere Lithography: A Versatile Nanofabrication Tool for Studies of Size-Dependent Nanoparticle Optics

The mean size of the gold (Au) core in the synthesis of dodecanethiolate-stabilized Au cluster compounds can be finely adjusted by choice of the Au:dodecanethiolate ratio and the temperature and rate at which the reduction is conducted. The Au clusters have been examined with a large number of independent analytical tools, producing a remarkably consistent picture of these materials. Average cluster and core dimensions, as ascertained by 1H NMR line broadening, high-resolution transmission electron microscopy, small-angle X-ray scattering, and thermogravimetric analysis, vary between diameters of 1.5 and 5.2 nm (∼110−4800 Au atoms/core). The electronic properties of the Au core were examined by UV/vis and X-ray photoelectron spectroscopy; the core appears to remain largely metallic in nature even at the smallest core sizes examined. The alkanethiolate monolayer stabilizing the Au core ranges with core size from ∼53 to nearly 520 ligands/core, and was probed by Fourier transform infrared spectroscopy, diff...

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Alkanethiolate Gold Cluster Molecules with Core Diameters from 1.5 to 5.2 nm: Core and Monolayer Properties as a Function of Core Size

The mean diameter of monodispersed Pd nanoparticles could be controlled from 17 to 30 A in a one-step reaction by changing the amount of protective polymer, poly(N-vinyl-2-pyrrolidone) (PVP) and the kind and/or the concentration of alcohol in the solvent. Although increasing the amount of protective polymer made the size of Pd nanoparticles smaller, the particle size appeared to have a lower limit determined by the kind of alcohol. On the other hand, monodispersed Pd nanoparticles of smaller diameter were obtained in the order methanol > ethanol > 1-propanol, indicating that a faster reduction rate of [PdCl4]2- ions is an important factor to produce the smaller particles. The particle diameter showed a minimum at around 40 vol % of alcohol in solvent. Once the monodispersed Pd nanoparticles were obtained, the larger particles with a narrow size distribution could be easily synthesized by using the stepwise growth reaction. The Pd nanoparticles obtained here had fcc structures like that of bulk Pd, althoug...

Size Control of Palladium Nanoparticles and Their Crystal Structures

Nanosphere lithography (NSL) is an inexpensive, inherently parallel, high-throughput, and materials-general nanofabrication technique capable of producing well-ordered 2D periodic particle arrays of nanoparticles. This paper focuses on the synthesis of size-tunable silver nanoparticle arrays by nanosphere lithography and their structural characterization by atomic force microscopy (AFM). The in-plane diameter, a, of Ag nanoparticles was tuned from 21 to 126 nm by systematic variation of the nanosphere diameter, D. Similarly, the out-of-plane height, b, was tuned from 4 to 47 nm by varying the mass thickness, dm, of the Ag overlayer. Experimental measurements of a, b, and interparticle spacing dip of many individual nanoparticles as a function of D and dm were carried out using AFM. These studies show (i) b = dm, (ii) dip accurately corresponds to predictions based on the nanosphere mask geometry, (iii) a, after correction for AFM tip convolution, is governed only by the mask geometry and the standard devi...

Nanosphere Lithography: Size-Tunable Silver Nanoparticle and Surface Cluster Arrays

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

Metal nanoparticles and their assemblies

THE properties of nanometre-scale metallic particles differ from those of the same material1,2 in bulk. Conduction electrons, because of their wave-like nature, can have only certain discrete values of kinetic energy or wavelength. Such 'quantum-size' effects have been observed in two-dimensional electron gases in semiconductors3,4, and in atomic-scale metallic point contacts5. Also present are 'Coulomb-charging' effects: these are purely classical in origin, and occur when the energy required to add one electron to a conducting sphere exceeds the mean thermal energy kBT. Thermal fluctuations in the total charge on the particle are then suppressed6. In theory, the combination of quantum-size and Coulomb-charging effects should cause the properties of small metallic particles to depend sensitively on whether they have an odd or even number of electrons7. Odd–even effects have been observed in experiments on tunnelling between discrete electronic levels of single metal particles8, but their influence on thermodynamic properties remains to be demonstrated. Here we report measurements of the heat capacity and electronic magnetic susceptibility of small metallic clusters. Our results show definitive evidence for odd–even effects, thus confirming that quantum and classical size effects strongly influence the thermodynamic properties of small particles.

I. I. Moiseevi

Papers

Quantum-size effects in the thermodynamic properties of metallic nanoparticles