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Springer International Publishing AG 2018

Francis Leonard Deepak (ed.) Metal Nanoparticles and Clusters

1. From Nano- to Angstrom Technology

Yolanda Pieiro 1

(1)

Applied Physics, University of Santiago de Compostela, La Corua, E-15782, Spain

(2)

Chemistry Physics Department, Campus Vida, University of Santiago de Compostela, La Corua, E-15782, Spain

Yolanda Pieiro

Email:

David Buceta

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Jos Rivas

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M. Arturo Lpez-Quintela (Corresponding author)

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Keywords

Clusters Sub-nanometer particles Atomic quantum clusters Metal clusters Quantum size effects Atomic-level semiconductors Angstrom-technology

1.1 Introduction: Industrial Revolutions, from Metals to Semiconductors and Backwards

Our technological world based on electricity and electronics began in the nineteenth century, when different electric properties of macroscopic metals were technologically combined to produce devices. This provoked the Second Industrial Revolution and the mass production of goods. However, a new technical step in the middle of the twentieth century brought us to the exploitation of semiconducting materials, which settled up the time of silicon, the pillar of the digital era and the Third Industrial Revolution.

The enabling key of these technological developments was the deepening at a fundamental level in the knowledge of the electronic structure of solid materials and the optimization of new synthetic procedures to control the size and physicochemical quality of small particles.

The idea behind this electronic revolution was the use of the energy gap between the valence and conduction bands (VB and CB) of semiconducting materials that allowed to create light-emitting devices (LEDS), electric control components (diodes, transmitters) or photovoltaic materials. However, these applications, designed in terms of the range (IR, visible or UV) and width of the band gap (few or many eV) in the energy diagram (see Fig. ), are material specific.

Fig. 1.1

Schematic comparison of band gaps predicted for some silver and copper clusters ( M N , N = number of atoms) and those of well-known semiconductors. Band gaps ( E g ) were calculated from the spherical jellium model ( E g = E F / N 1/3; E F = Fermi level), and the position of the conduction band, ECB, was estimated by the formula E CB = E F E g (Note: no allowance for the scaling change of the Fermi level with cluster size detected in very small clusters (see, e.g., Ref. []) is considered)

The need to produce new electronic implementations such us digital components for military devices, ultralow-consuming solid-state lighting LEDs, lasers, radio-frequency devices (radars) or photovoltaic materials with high yield for energy applications fostered the field to move from conventional (silicon and germanium based) to new wideband gap (carbides and nitrides) semiconductors.

Nowadays, metal AQCs with ultrasmall (between a few to less than 1 nm) and controlled sizes offer the possibility to produce a new generation of materials with a tuneable band gap and designed electronic response for highly specific biomedical applications, photovoltaic materials in the visible range or selective and sensitive catalysts. Their stability, facile synthesis by means of wet chemistry procedures, large availability of metals and physical properties make them an interesting alternative to conventional SCs.

1.2 The Basic Physics Behind Two Size Ranges: Scaling Laws of Surface Phenomena in NPs and Quantum Confinement in AQCs

AQCs, defined as particles composed of a countable number of atoms with sizes below 1 nm, show non-monotonic properties which are critically dependent on their precise number of atoms. On the other side, NPs, from 1 to 100 nm, present smooth size-dependent properties which do not change upon the addition of a single atom and tend to collapse with the bulk behaviour at large sizes.

Both length scales bridge the path between atomic and macroscopic physics but are profoundly different taking into account the effects they produce []:

• 1. The leading phenomena observable on NPs, i.e. magnetic surface effects in magnetic nanoparticles, or superparamagnetism, the size-dependent surface plasmon band resonance and melting point, are mainly scalable effects arising from the large surface-to-volume ratio of lattice ions in small particles; smoothly scaling properties which evidence the dominating role surface ions vary as the inverse of the particle size and extrapolate continuously up to the bulk values.
• 2. The discrete energy levels and shell-like physical properties, i.e. the so-called magic numbers, that appear in AQCs, are due quantum confinement effects that emerge when certain dimensions of the particle are smaller than characteristic physical lengths like the de Broglie or the Fermi wavelength.

Surface physics and quantum many-body approaches are the physical frames that capture the main mechanisms governing NPs and AQCs behaviour [].

1.2.1 Surface Phenomena in NPs

It is a well-known fact in surface sciences that on atomic length scales, interfaces are not abruptly discontinuous but corrugated, and most physical properties change continuously with superimposed oscillations along the direction perpendicular to the interface (or surface) [) experience an asymmetric interaction due to the different structural environment on both sides which translates into atoms with different coordination numbers. This physical diversity induces local relaxations in the lattice in search of a configuration that shows a balance between the different interactions on both sides.

Fig. 1.2

Different lattice sites showing different coordination numbers due to crystal imperfections, like voids, dislocations or corners, and illustrating the different structural environment of inner and outer ions

Surface reconstruction mechanisms allow for lowering the surface energy and involve, among others, different strategies like:

• Shrinking the interatomic distances of surface atoms, which therefore changes the bulk lattice structure into another one much more closely packed (i.e. bulk Au is cubic, while the surface Au is hexagonally closely packed)
• Inducing oscillating charge profile between subsequent layers, i.e. ion pair formation as in NaCl, where Na+ ions are displaced towards the bulk and Cl ions are displaced outwards to the surface
• Surface segregation mechanisms in materials composed by more than one type of atom, where one of the species diffuses from the core and accumulates on the surface, i.e. in alloys the component with the lower melting diffuses to the surface and lowers the total surface energy (i.e. in silver/gold alloys, silver accumulates on the first layer)
• Adsorption of molecules on the surface that allows for the energy lowering

Equilibrium, growth, morphology, structure and properties of NPs emerge as a complex balance between different surface equilibration mechanisms and the physical interactions arising from the bulk core.

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