The chapter focuses on the most versatile method of the production of nanoparticle assemblies, that is, formation in the gas-phase and subsequent deposition onto surfaces. The various types of gas-phase cluster sources are described with their advantages and disadvantages. This section includes a discussion of techniques used to mass-select the clusters, including both charged-particle and neutral-particle methods. The chapter then goes on to discuss the deposition process itself and how to produce three-dimensional assemblies of clusters within matrices. Finally there is a brief description of the experimental methods used to form nanoscale islands on surfaces by Volmer-Weber growth on surfaces. This ties in with chapter 4 in which a comprehensive discussion of island growth on surfaces is provided.
We start this chapter with a review of Monte Carlo simulations of nucleation and growth at surfaces, and the size distributions and scale-invariance they produce. We then appraise a common theoretical approach using mean field rate equations and scaling analyses, which allow some successful confrontation with experimental and simulation data. However, the limitations of this approach highlight the need for theory to look beyond the mean field, which is done through the Capture Zone concept, and we show how this leads to a fuller understanding of the scale-invariant properties. A variant of this scheme to describe post-deposition ripening of the islands is also presented. Atomistic models, and in particular long timescale simulations, are discussed to show how the rather general theory can be applied to specific material systems. We finish with a brief discussion of other growth methods on surfaces, and their relation to the rest of the chapter.
The first part of this chapter surveys various theoretical methods for determining the structure of clusters. The methods are more accurate than the shell models discussed in chapter 2. The techniques covered include Post-Hartree-Fock Quantum Chemistry Methods, Density Functional Theory, Tight-Binding Methods and Semi-Empirical Potentials for use in Global Optimisation. The methods, which have varying degrees of accuracy, allow one to study clusters from a few atoms to tens of thousands of atoms in size. The second part of the chapter provides a compendium of results from calculations on alkali, noble, transition, divalent and trivalent metals. Predictions on the metal-insulator transition are noted and the topic is further developed in chapter 7. Large clusters exhibit transitions from icosahedral through decahedral to fcc structures as the size is increased; the calculated sizes at which these transitions occur are listed for several metals.
The first section of the chapter describes the characterisation techniques that are mostly used to determine the magnetic behaviour of nanoparticle assemblies. The simple laboratory based methods are given brief explanations but there is a detailed coverage of dichroism-based methods using central facilities since a detailed understanding of these is required to exploit their full power. The chapter goes on to describe the behaviour of very small clusters, containing less than 10 atoms, of noble metals and 3d, 4d and 5d transition metals supported on graphite, Fe, Co, Ni, Pd and Pt surfaces. The magnetic properties of larger (100s of atoms) clusters are dealt with in a subsequent section. The chapter concludes by considering the behaviour of clusters supported in various three-dimensional matrices in the dilute and the interacting regime. Finally the particularly interesting exchange-bias behaviour of core-shell particles composed of a ferromagnetic core and an antiferromagnetic shell is described.
This chapter concerns laser light as a probe of the properties of metallic nanoparticles. The first part discusses photoionisation, photoemission, the ionisation potential and electron affinity. Structure in the ionisation potential and electron affinity as a function of cluster size can be used to determine magic numbers. The distinction between the vertical and adiabatic ionisation potentials is noted. The relation between features in the photoemission spectrum and the electronic structure of the cluster is discussed, with examples drawn from transition metals and from the metal-insulator transition in divalent materials. The second part of the chapter deals with optical absorption. The Mie theory of absorption by particles smaller than the wavelength of light is outlined, and deviations from the Mie theory in actual clusters is discussed. The dependence of the optical absorption on the shape of the cluster is examined. Finally effective medium theories for dealing with composite systems are described.
The first section of the chapter provides a background description of magnetism in atoms and solids. It covers spin and orbital moments and the Stoner theory of magnetism in the transition metals. The origins of magnetic anisotropy are discussed. In the second section, two methods for studying magnetic clusters are described. The first, the chemical probe method, has been extensively used to determine the geometric structure of Ni, Co and Fe clusters. The second, the gradient-field deflection technique, is similar to the Stern-Gerlach method used in atomic physics, and is the principle method used to obtain the magnetic moments of free clusters. Finally theoretical and experimental results on the magnetism of free clusters are surveyed. The topics discussed include ferromagnetic and antiferromagnetic 3 d transition metals, non-magnetic (in bulk) 3 d and 4 d metals, the rare earths, and the magnetic ordering temperature.
Nanotechnology is science and engineering at the scale of atoms and molecules. It is the manipulation and use of materials and devices at this level, which is going to create significant impacts on our daily life ranging from fabrication of new construction materials to applications in biomedicine and pharmaceuticals. In this chapter, two areas of applications, namely biotechnology and industrial catalysis, are particularly highlighted. In the biotechnology area, magnetic nanoparticles for drug screening, contrast enhancement agents, magnetic resonance imaging, medicine and bio-separation are focused. Catalysis plays an important role in our society. Presently, over 90% of all industrial chemical productions involve the use of at least one catalytic step. Structure of catalyst used determines how atoms interact on surface. This chapter will demonstrate the use of tailored nanoparticles which offers exciting possibilities for engineering reaction selectivity of catalytic systems of interests at molecular level.
This chapter gives an overview of the electronic and geometric shell models. These models provide a useful guide to cluster sizes of high stability (magic numbers). The electronic shell model is most applicable to small (<100 atoms) clusters of simple metals. Phenomenological, self-consistent and ellipsoidal versions are discussed, together with its predictions for various metals. The geometric shell model is effective for larger clusters. The Wulff construction is outlined and the various polyhedral forms are described. The mechanisms of filling between complete shells, the phenomenon of supershells and cluster melting are also discussed.
This chapter focuses on the quasicrystals and approximants in Cd-M systems and related alloys. A diffraction pattern with icosahedral symmetry, long considered impossible in classiccrystallography, in the A1–Mn alloy prepared by a rapid cooling technique from the melt, has inspired the world of solid-state science. Traditionally, a “crystalline structure” is defined by a periodic repetition of a unit cell, and the periodicity is another definition for translational symmetry, the strictest form of long-range order. Thus, “Order in crystals is periodicity” is the knowledge of long-standing in crystallography. In addition, more than one hundred alloys are verified to be stable quasicrystalline phases, and quasicrystalline phases exist in the equilibrium phase diagrams. It has been known for a long time prior to the discovery of quasicrystals that although five-fold and icosahedral symmetry is forbidden in the presence of two- and three-dimensional translation groups, local icosahedral arrangements of atoms in crystals are possible and in fact, for complex metallic alloys fairly common.
This chapter focuses on the dynamical aspects of the phason modes in aperiodic crystals. The hyperspace description of quasiperiodic structures together with the hydrodynamic mode analysis provides a useful approach to understanding the common aspects of phason dynamics in different structural classes of aperiodic crystals. A phason mode is, in this approach, characterized by its wavevector and a polarization vector that lie in the perpendicular space. Phason dynamics has been observed in all the different classes of aperiodic crystal, and has been shown to be in agreement with the hydrodynamic theory. The chapter reviews the case of binary composites where one expects some degree of mixing between phason and acoustic degrees of freedom. It also reviews a few experimental results on the A1PdMn icosahedral phase to illustrate the concepts of quenched and thermal phasons, of phason strains, spatially homogeneous or not, of phason elasticity, and of phason-driven structural phase transitions. The chapter also highlights the future developments and remaining challenges in this exciting field.
This chapter discusses metastability and pseudomorphic growth, kinetics, strain in hetero-epitaxial growth, alloy phase diagrams and defects, metastability and Bain distortions, and interfaces in metallic multilayers—Pb–Nb and Ag–Nb. A suitable free energy, when monitored as a function of various parameters such as lattice parameters, temperature or pressure is a complicated function of many variables with possibly numerous local minima. Metastability that is discussed in this chapter has to do with the existence of such local minima. To crossover from one local minimum to a neighboring minimum, the system needs to overcome an energy barrier. Growth conditions clearly play a vital role in determining the resulting metastable phase. Transition metals provide a platform for studying metastable phases. A layer by layer, pseudomorphic growth (that is, in registry with the substrate) usually provides insight into numerous size, surface and interface dependent properties of metastable structures. Kinetic processes do not necessarily select the most stable, lowest energy state but the most easily (kinetically) accessible state. It is seen that surface diffusion and migration of atoms can play complicated roles in the kinetics of growth. Thin epitaxial films are known to exhibit a wide variety of fascinating fundamental properties because of the interplay of electronic structure, morphology, strain, and magnetism. Idealized, simple systems are highly desirable. A great deal of attention has been paid to Fe films grown on various metallic substrates, such as Fe/Cu, Fe/W and Fe/Mo. One major reason for this interest is the magnetism associated with Fe films, which may depend on the growth mechanism. There are details of Magnetic 3d metals, Epitaxially grown magnetic systems, epitaxially grown fcc Fe/Cu and Fe16 N2films— experimental background: Fe nitrides, discussion of large Fe moments, Fe/Cr, and bcc Nickel grown on Fe and GaAs.
We present a rigorous formulation of generalized Kohn-Sham density-functional theory. This provides a straightforward Kohn-Sham description of many-body systems based not only on particle-density but also on any other observable. We illustrate the formalism for the case of a particle-density based description of a nonrelativistic many-electron system. We obtain a simple diagrammatic expansion of the exchange-correlation functional in terms of Kohn-Sham single-particle orbitals and energies; develop systematic Kohn-Sham formulation for one-electron propagators and many-body excitation energies. This work is ideally suited for practical applications and provides a rigorous basis for a systematic development of the existing body of first-principles calculations in a controllable fashion.
This chapter briefly examines the concept magnetic RAM (“MRAM”) and how it is expected to compete with the current high speed data transfer techniques. Magnetic RAM (MRAM) is expected to handle various shortcomings of data transfers, providing instant boot-up capability, with high speed access, low power consumption and high density. Early MRAM devices used magnetic hysteresis to store information. A basic storage mechanism is tied to the magnetic state of a tunnel junction. MRAM does not require any refreshing to maintain the magnetic state of a given bit, once it is magnetized, and hence the memory is non-volatile. Not only does this mean that it retains its memory with the power turned off, but also that there is no need for a constant power supply. A magnetic tunnel junction (MTJ) is different from the metallic sandwich structures discussed with regard to the GMR effect. A MTJ consists of two metallic layers, sometimes called electrodes, separated by an insulating layer thin enough to allow some tunneling current. The interest in such devices is at least partly due their potential applications in magnetoresistive random access memory (MRAM) devices and sensors. The chapter discusses the theological aspects of tunneling magnetoresistance (TMR), devices with large TMR values, double barriers, vortex domain structures, spin transfer torques in metallic multilayers, ultra-fast reversal of magnetization, and transistors based on spin orientation. The chapter illustrates two key features of the scattering process. The first is that the spin current along the direction of the magnetization is con-served. The second point is that the reflected and transmitted currents have no transverse components. When combined, these two results show that the spin transfer torque is approximately given by the absorption of the transverse component of the incident spin current.
The ability to synthesize small clusters containing a few atoms provides an opportunity to examine and tune fundamental physical and chemical properties. Numerical methods can provide significant insight. This chapter describes a number of surprising and novel results obtained using a combination of exact diagonalization of a many-body Hamiltonian and statistical mechanics. Microscopic studies, which account accurately for short-range dynamical correlations in finite clusters, are relevant with regard to understanding the physics of the underdoped high Tc superconductors (HTSCs). Such studies are also relevant to nanomagnetism and spintronics applications. The geometry of an elementary building block (cluster), its orientation, shape, and position in two and three dimensions can be controlled in a multidimensional parameter space of interaction strength, magnetic field, electron concentration, and temperature. In a recent study, it was concluded that inhomogeneous clustered states should be considered as a new paradigm in condensed matter physics. The method developed in this chapter can be used to describe magnetic instabilities in the spin-tetrahedral systems in terms of weakly coupled tetrahedrons with a singlet-triplet gap and low-lying singlets. The (thermodynamical) theory presented provides a mechanism of phase separation into carrier-rich and carrier-poor regions observed in the HTSC cuprates; it has been able to predict some of the observations related to local electron pairing on the atomic scale. Although numerous properties including eigenvalues and susceptibilities, of Hubbard clusters have been calculated previously, many open questions remain with regard to microscopic origins of charge-spin separation, pseudogap behavior, and various scenarios of possible pairings at low temperature. This chapter searches for transitions, over an extended parameter space that includes variations in chemical potential, magnetic field, Coulomb repulsion, and temperature. Insight into the properties of Hubbard clusters are gained by monitoring weak singularities in susceptibilities over this extended parameter space.
This chapter explains quantum diffusion and electronic conduction properties in quasiperiodic and periodic systems. The chapter presents a detailed physical interpretation of anomalous diffusion and low frequency conductivity laws in crystals and quasicrystals. This anomalous diffusion mode has deep consequences on the conduction properties at zero and low frequency. The anomalous diffusion mode is related to a tendency to localization and to a phenomenon of backscattering, which is well known in disordered systems. The phenomenon of backscattering is the fact that an impulse of electric field creates a current density that is opposite to the electric field at large time. The physics of phonons in quasicrystals could be affected by the anomalous diffusion phenomenon. In particular it has been argued that the heat conductivity can be sensitive to this effect. The concepts developed in the chapter also open a new insight in the physics of correlated systems. In particular a transition from a metallic like regime at low temperature where scattering is weak to an insulating like regime at higher temperature with a stronger scattering is observed.
