Nanoclusters are atomically precise, crystalline materials most often existing on the 0–2 nanometer scale. They are often considered kinetically stable intermediates that form during the synthesis of comparatively larger materials such as semiconductor and metallic nanocrystals. The majority of research conducted to study nanoclusters has focused on characterizing their crystal structures and understanding their role in the nucleation and growth mechanisms of larger materials. Materials can be categorized into three different regimes, namely bulk, nanoparticles and nanoclusters. Bulk metals are electrical conductors and good optical reflectors and metal nanoparticles display intense colors due to surface plasmon resonance. However, when the size of metal nanoclusters is further reduced to form a nanocluster, the band structure becomes discontinuous and breaks down into discrete energy levels, somewhat similar to the energy levels of molecules. This gives nanoclusters similar qualities as a singular molecule and does not exhibit plasmonic behavior; nanoclusters are known as the bridging link between atoms and nanoparticles. Nanoclusters may also be referred to as molecular nanoparticles.
History of nanoclusters The formation of stable nanoclusters such as Buckminsterfullerene (C60) has been suggested to have occurred during the early universe. In retrospect, the first nanoclustered ions discovered were the Zintl phases, intermetallics studied in the 1930s. The first set of experiments to consciously form nanoclusters can be traced back to 1950s and 1960s. During this period, nanoclusters were produced from intense molecular beams at low temperature by supersonic expansion. The development of laser vaporization technique made it possible to create nanoclusters of a clear majority of the elements in the periodic table. Since 1980s, there has been tremendous work on nanoclusters of semiconductor elements, compound clusters and transition metal nanoclusters. Subnanometric metal clusters typically contain fewer than 10 atoms and measure less than one nanometer in size.
Size and number of atoms in metal nanoclusters According to the Japanese mathematical physicist Ryogo Kubo, the spacing of energy levels can be predicted by
δ = E F N {\displaystyle \delta ={\frac {E_{\rm {F}}}{N}}}
where EF is Fermi energy and N is the number of atoms. For quantum confinement 𝛿 can be estimated to be equal to the thermal energy (δ = kT), where k is the Boltzmann constant and T is temperature.
Stability Not all the clusters are stable. The stability of nanoclusters depends on the number of atoms in the nanocluster, valence electron counts and encapsulating scaffolds. In the 1990s, Heer and his coworkers used supersonic expansion of an atomic cluster source into a vacuum in the presence of an inert gas and produced atomic cluster beams. Heer's team and Brack et al. discovered that certain masses of formed metal nanoclusters were stable and were like magic clusters. The number of atoms or size of the core of these magic clusters corresponds to the closing of atomic shells. Certain thiolated clusters such as Au25(SR)18, Au38(SR)24, Au102(SR)44 and Au144(SR)60 also showed magic number stability. Häkkinen et al explained this stability with a theory that a nanocluster is stable if the number of valence electrons corresponds to the shell closure of atomic orbitals as (1S2, 1P6, 1D10, 2S2 1F14, 2P6 1G18, 2D10 3S2 1H22.......).
Synthesis and stabilization
Solid state medium Molecular beams can be used to create nanocluster beams of virtually any element. They can be synthesized in high vacuum by with molecular beam techniques combined with a mass spectrometer for mass selection, separation and analysis. And finally detected with detectors.
Cluster Sources Seeded supersonic nozzle Seeded supersonic nozzles are mostly used to create clusters of low-boiling-point metal. In this source method metal is vaporized in a hot oven. The metal vapor is mixed with (seeded in) inert carrier gas. The vapor mixture is ejected into a vacuum chamber via a small hole, producing a supersonic molecular beam. The expansion into vacuum proceeds adiabatically cooling the vapor. The cooled metal vapor becomes supersaturated, condensing in cluster form. Gas aggregation Gas aggregation is mostly used to synthesize large clusters of nanoparticles. Metal is vaporized and introduced in a flow of cold inert gas, which causes the vapor to become highly supersaturated. Due to the low temperature of the inert gas, cluster production proceeds primarily by successive single-atom addition. Laser vaporization Laser vaporization source can be used to create clusters of various size and polarity. Pulse laser is used to vaporize the target metal rod and the rod is moved in a spiral so that a fresh area can be evaporated every time. The evaporated metal vapor is cooled by using cold helium gas, which causes the cluster formation. Pulsed arc cluster ion This is similar to laser vaporization, but an intense electric discharge is used to evaporate the target metal. Ion sputtering Ion sputtering source produces an intense continuous beam of small singly ionized cluster of metals. Cluster ion beams are produced by bombarding the surface with high energetic inert gas (krypton and xenon) ions. The cluster production process is still not fully understood. Liquid-metal ion In liquid-metal ion source a needle is wetted with the metal to be investigated. The metal is heated above the melting point and a potential difference is applied. A very high electric field at the tip of the needle causes a spray of small droplets to be emitted from the tip. Initially very hot and often multiply ionized droplets undergo evaporative cooling and fission to smaller clusters.
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