Nanocluster

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Nanoclusters are atomically precise, crystalline materials most often existing on the 0-2 nanometer scale.^{[citation needed]} They are often considered^{[by whom?]} 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,

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 𝛿 can be estimated to be equal to the thermal energy (δ = kT), where k is the Boltzmann constant and T is temperature.^[15][16]

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.^[17] 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.^[16] Heer's team and Brack et al. discovered that certain masses of formed metal nanoclusters were stable and were like magic clusters.^[18] 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.^[3] 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.......).^[19][20]

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.^[21]

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.

Mass Analyzer

Wein filter In Wien filter mass separation is done with crossed homogeneous electric and magnetic fields perpendicular to ionized cluster beam. The net force on a charged cluster with mass M, charge Q, and velocity v vanishes if E = Bv/c . The cluster ions are accelerated by a voltage V to an energy QV. Passing through the filter, clusters with M/Q = 2V/(Ec/B) are not deflected. These cluster ions that are not deflected are selected with appropriately positioned collimators.

Quadrupole mass filter The quadrupole mass filter operates on the principle that ion trajectories in a two-dimensional quadrupole field are stable if the field has an AC component superimposed on a DC component with appropriate amplitudes and frequencies. It is responsible for filtering sample ions based on their mass-to-charge ratio.

Time of flight mass spectroscopy

Molecular beam chromatography In this method, cluster ions produced in a laser vaporized cluster source are mass selected and introduced in a long inert-gas-filled drift tube with an entrance and exit aperture. Since cluster mobility depends upon the collision rate with the inert gas, they are sensitive to the cluster shape and size.

Aqueous medium

In general, metal nanoclusters in an aqueous medium are synthesized in two steps: reduction of metal ions to zero-valent state and stabilization of nanoclusters. Without stabilization, metal nanoclusters would strongly interact with each other and aggregate irreversibly to form larger particles.

Reduction

There are several methods reported to reduce silver ion into zero-valent silver atoms:

• Chemical Reduction Chemical reductants can reduce silver ions into silver nanoclusters. Some examples of chemical reductants are sodium borohydride (NaBH₄) and sodium hypophosphite (NaPO₂H₂.H₂O). For instance, Dickson and his research team have synthesized silver nanoclusters in DNA using sodium borohydride.^[10][9]
• Electrochemical Reduction Silver nanoclusters can also be reduced electrochemically using reductants in the presence of stabilizing agents such as dodecanethiol [de] and tetrabutylammonium.^[12]
• Photoreduction Silver nanoclusters can be produced using ultraviolet light, visible or infrared light. The
  photoreduction process has several advantages such as avoiding the introduction of impurities, fast synthesis, and controlled reduction. For example Diaz and his co-workers have used visible light to reduce silver ions into nanoclusters in the presence of a PMAA polymer. Kunwar et al produced silver nanoclusters using infrared light.^[22][2]
• Other reduction methods Silver nanoclusters are also formed by reducing silver ions with
  gamma rays, microwaves, or ultrasound. For example silver nanoclusters formed by gamma reduction technique in aqueous solutions that contain sodium polyacrylate or partly carboxylated polyacrylamide or glutaric acids. By irradiating microwaves Linja Li prepared fluorescent silver nanoclusters in PMAA, which typically possess a red color emission. Similarly Suslick et al. have synthesized silver nanoclusters using high ultrasound in the presence of PMAA polymer.^[2][11]

Thiols Thiol-containing small molecules are the most commonly adopted stabilizers in metal nanoparticle synthesis owing to the strong interaction between thiols and gold and silver. Glutathione has been shown to be an excellent stabilizer for synthesizing gold nanoclusters with visible luminescence by reducing Au³⁺ in the presence of glutathione with sodium borohydride (NaBH₄). Also other thiols such as tiopronin, phenylethylthiolate, thiolate α-cyclodextrin and 3-mercaptopropionic acid and bidentate dihydrolipoic acid are other thiolated compounds currently being used in the synthesis of metal nanoclusters. The size as well as the luminescence efficiency of the nanocluster depends sensitively on the thiol-to-metal molar ratio. The higher the ratio, the smaller the nanoclusters. The thiol-stabilized nanoclusters can be produced using strong as well as mild reductants. Thioled metal nanoclusters are mostly produced using the strong reductant sodium borohydride (NaBH₄). Gold nanocluster synthesis can also be achieved using a mild reducant tetrakis(hydroxymethyl)phosphonium (THPC). Here a zwitterionic thiolate ligand, D-penicillamine (DPA), is used as the stabilizer. Furthermore, nanoclusters can be produced by etching larger nanoparticles with thiols. Thiols can be used to etch larger nanoparticles stabilized by other capping agents.

Look up mercaptoundecanoic acid or microgel in Wiktionary, the free dictionary.

Dendrimers

DNA, proteins and peptides DNA oligonucleotides are good templates for synthesizing metal nanoclusters. Silver ions possess a high affinity to cytosine bases in single-stranded DNA which makes DNA a promising candidate for synthesizing small silver nanoclusters. The number of cytosines in the loop could tune the stability and fluorescence of Ag NCs. Biological macromolecules such as peptides and proteins have also been utilized as templates for synthesizing highly fluorescent metal nanoclusters. Compared with short peptides, large and complicated proteins possess abundant binding sites that can potentially bind and further reduce metal ions, thus offering better scaffolds for template-driven formation of small metal nanoclusters. Also the catalytic function of enzymes can be combined with the fluorescence property of metal nanoclusters in a single cluster to make it possible to construct multi-functional nanoprobes.^{[2][3][4][1][10]}

Inorganic scaffolds Inorganic materials like glass and zeolite are also used to synthesize the metal nanoclusters. Stabilization is mainly by immobilization of the clusters and thus preventing their tendency to aggregate to form larger nanoparticles. First metal ions doped glasses are prepared and later the metal ion doped glass is activated to form fluorescent nanoclusters by laser irradiation. In zeolites, the pores which are in the Ångström size range can be loaded with metal ions and later activated either by heat treatment, UV light excitation, or two-photon excitation. During the activation, the silver ions combine to form the nanoclusters that can grow only to oligomeric size due to the limited cage dimensions.^[2][25]

Properties

Magnetic properties

Most atoms in a nanocluster are surface atoms. Thus, it is expected that the magnetic moment of an atom in a cluster will be larger than that of one in a bulk material. Lower coordination, lower dimensionality, and increasing interatomic distance in metal clusters contribute to enhancement of the magnetic moment in nanoclusters. Metal nanoclusters also show change in magnetic properties. For example, vanadium and rhodium are paramagnetic in bulk but become ferromagnetic in nanoclusters. Also, manganese is antiferromagnetic in bulk but ferromagnetic in nanoclusters. A small nanocluster is a nanomagnet, which can be made nonmagnetic simply by changing its structure. So they can form the basis of a nanomagnetic switch.^[3][8]

Reactivity properties

Large surface-to-volume ratios and low coordination of surface atoms are primary reasons for the unique

• "Atomically Precise Clusters of Noble Metals: Emerging Link between Atoms and Nanoparticles" by Chakraborty and Pradeep [28]

Further reading (primary references)

• Gary, Dylan C.; Flowers, Sarah E.; Kaminsky, Werner; Petrone, Alessio; Li, Xiaosong; Cossairt, Brandi M. (2016-02-10). "Single-Crystal and Electronic Structure of a 1.3 nm Indium Phosphide Nanocluster". Journal of the American Chemical Society. 138 (5): 1510–1513.
  PMID 26784649
  .
• Kunwar, P; Hassinen, J; Bautista, G; Ras, R. H. A.; Toivonen, J (2016). "Sub-micron scale patterning of fluorescent silver nanoclusters using low-power laser". Scientific Reports. 6: 23998.
  PMID 27045598
  .
• Kunwar, P; Hassinen, J; Bautista, G; Ras, R. H. A.; Toivonen, J (2014). "Direct Laser Writing of Photostable Fluorescent Silver Nanoclusters in Polymer Films". ACS Nano. 8 (11): 11165–11171.
  PMID 25347726
  .
• Beecher, Alexander N.; Yang, Xiaohao; Palmer, Joshua H.; LaGrassa, Alexandra L.; Juhas, Pavol; Billinge, Simon J. L.; Owen, Jonathan S. (2014-07-30). "Atomic Structures and Gram Scale Synthesis of Three Tetrahedral Quantum Dots". Journal of the American Chemical Society. 136 (30): 10645–10653.
  PMID 25003618
  .
• Gary, Dylan C.; Terban, Maxwell W.; Billinge, Simon J. L.; Cossairt, Brandi M. (2015-02-24). "Two-Step Nucleation and Growth of InP Quantum Dots via Magic-Sized Cluster Intermediates". Chemistry of Materials. 27 (4): 1432–1441.
  ISSN 0897-4756
  .
• Tanaka S. I, Miyazaki J, Tiwari D. K., Jin T, Inouye Y. (2011). "Fluorescent Platinum Nanoclusters: Synthesis, Purification, Characterization, and Application to Bioimaging". Angewandte Chemie International Edition. 50 (2): 431–435.
  PMID 21154543.{{cite journal}}: CS1 maint: multiple names: authors list (link
  )
• Karimi N, Kunwar P, Hassinen J, Ras R. H. A, Toivonen J (2016). "Micropatterning of silver nanoclusters embedded in polyvinyl alcohol films". Optics Letters. 41 (15): 3627–3630.
  S2CID 3477288.{{cite journal}}: CS1 maint: multiple names: authors list (link
  )