Nanocluster

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Nanoclusters are atomically precise, crystalline materials most often existing on the 0-2 nanometer scale.^{[1] [2][3]} 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.^[4][5] These nanoclusters can be composed either of a single or of multiple elements, and exhibit interesting electronic, optical, and chemical properties compared to their larger counterparts.^[3][2][6][7]

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.^[6][7] 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.^{[6][7][8][9][10]} This gives nanoclusters similar qualities as a singular molecule^[11] and does not exhibit plasmonic behavior; nanoclusters are known as the bridging link between atoms and nanoparticles.^{[12][6][7][8][9][10][13][14][15][16][17]} Nanoclusters may also be referred to as molecular nanoparticles.^[18]

History of nanoclusters[]

The formation of stable nanoclusters such as Buckminsterfullerene (C₆₀) has been suggested to have occurred during the early universe. The first set of experiments to form nanoclusters can be traced back to 1950s and 1960s.^[19][13] 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.^[13]

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

${\displaystyle \delta ={\frac {E_{\rm {F}}}{N}}}$

where E_F is Fermi energy and N is the number of atoms. For quantum confinement