Journal of Physics and Chemistry of Solids
A review of characterization and physical property studies of metallic nanoparticles
Introduction
Recently, many scientists [1], [2], [3], [4], [5], [6], [7], [8] have developed their efforts to the study of preparation and physical properties of metallic nanoparticles, expecting to obtain a right perspective to the essential features of quantum confined electronic systems. Classical size effect [9], [10], [11] embodies in the sample size that approaches or is smaller than the carrier mean free path therein the scattering of carriers with the surface manifests. As the size for metallic particles reduces, the uncertainty principle implies an intrinsic kinetic energywhere r is particle diameter; me and mh are the mass of electrons and photons, respectively. Whereas, an excess electron induces image charges on the particle surface introducing a Coulomb attractive potential which is proportional to e2/ϵr where ϵ is the dielectric constant. The Bohr radiusfor nanoparticles will be defined as that the attractive Coulomb energy is equal to the repulsive kinetic energy. Quantum size effect prevails the classical size effect as the particle is smaller than the Bohr radius.
Many exotic physical properties intrigue with quantum size effect such as the splitting of the continuum conduction band into discrete levels, the electromagnetic field enhancement on the surface, the magnetic properties changes from diamagnetic into paramagnetic, and from ferromagnetic into super paramagnetic. In the linear optical properties, the surface plasma absorption peak behaves blue-shift [12], [13], [14], [15] as the particle size decreases, which can be fully elucidated by the hard spherical model, which appraises the enlargement of energy splitting as the particle size decreases. The abnormal red-shift [16], [17], [18] for metallic nanoparticles embedded in a reactive matrix is retrieved due to the electron diffusion outside the surface. The electromagnetic field can be enhanced at the shallow surface of the nanoparticles, therefore it greatly pronounces the nonlinear optical susceptibility [19], [20].
The crucial factor that determines the electrical properties of small metallic particles is the filling factor f, which is defined as the volume ratio of the sum of the metallic particles to the total volume of the matrix and particles. When the filling factor is large enough for particles to aggregate into conducting chains, the sample behaves current conduction. At the condition of positive temperature coefficient of resistivity (TCR), the electron phonon interaction implies a straight linear dependence of resistivity R on temperature T. An abnormal break occurs at low temperatures on the R–T measurement [21], [22] for noble metals that can be addressed by the reduction from diamagnetic to spin glass states for nanoparticles [3].
The finely dispersed nanoparticles of noble metals behaves paramagnetic property while those of ferromagnetic materials reveal superparamagnetism [7], [23]. The ferromagnetic nanaoparticles are dispersed in a more or less crystallized diamagnetic matrix constituting a disordered super paramagnetic system. In this case, all individual spins inside a particle are coupled by exchange interaction resulting in a single domain spontaneous magnetization Ms. The particles are subjected to a magneto-crystalline anisotropy field Ha that depends on the nature of particles, and a demagnetizing field Hd that depends on the particle shape. At high measuring temperatures and small particle volumes, the two latter fields are reduced considerably by thermal fluctuation implying a symmetric and narrow magnetic resonance line. There is a trend to change from superparamagnetic to ferromagnetic behavior as the particle size increases.
Section snippets
Sample preparation and characterization
Various methods in preparation metallic nanoparticles invoke different properties with desired purposes. The widely exploited methods are as follows.
Linear optical properties
In the visible and near visible light absorption spectra of bulk metals can be satisfactorily expressed by the free electron Drude model [35], which yields high reflectivity and reveals white surface. To a dispersed metallic nanoparticles, the random distribution of normal surface direction of the particles implying the excitation of surface plasmon requiring phase matching condition [36] can be easily complied with. The surface plasmon in resonance to the discrete energy level due to the
Nonlinear optical generation
For metal particles with structure of inversion symmetry, the electric quadruple field within the selvedge region is the dominant source for the generation of second harmonic light [48], [49], [50]. The excitation of surface plasmon (SP), which couples the incident field to propagate along the surface, is thus a main strategy for the enhancement of second harmonic generation. The efficiency of generating surface plasmon depends on the momentum conservation of the electromagnetic waves, which
Electrical conduction
At low filling factor, the particles may be separated individually. The mechanism of conductivity can be extensively addressed by simple tunneling between localized states [58], [59] that imply a high resistivity at low temperatures. When the filling factor is large enough for the particles to aggregate into clusters, and at least a conducting path is connected from one side of the sample to the other, the sample begins to exhibit current conduction. The resistivity ρ(R, f, A) will be dependent
Electron spin resonance study
The spin system of the metallic particles can be elucidated more fully by the experiments of electron spin resonances. For bulk metals due to the strong spin-orbit interaction and extremely narrow skin depth, only few finely dispersed particles (~μm in size) can display detectable conduction electron spin resonance (CESR) [66], [67]. The intricate g shift as given by the average of the orbital angular momentum induced by the spin-orbit interaction at the top of the Fermi distribution which can
Summary and conclusions
In this scenario we have surveyed the fabrication, the characterization, and the electrical, optical and magnetic property studies of metallic nanoparticles especially prepared by sol gel method. The most salient features associated with the metallic nanoparticles are the demonstration of spin glass property from the diamagnetic bulk silver, the nonlinear susceptibility enhancement as the particle size reduces, and the interplay between super paramagnetic and ferromagnetic resonance with the
Acknowledgements
In preparing this review, the author has benefited from the help of his outstanding students Dr W. C. Huang, Dr K. Y. Lo, Dr S. K. Ma, Mr C. T. Hsieh and Mr W. L. Huang for preparing the experiments. This work was supported from the National Science Council of the Republic of China under the contract NSC 90-2112-M007-001 and the Ministry of Education under the contract 90-FA04-AA.
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