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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 categorize into three different regimes, namely bulk,nanoparticles andnanoclusters.[according to whom?] Bulk metals areelectrical conductors and good optical reflectors and metalnanoparticles display intense colors due to surfaceplasmonresonance.[1] However, when the size of metal nanoclusters is further reduced to form a nanocluster bulk , theband structure becomes discontinuous and breaks down into discreteenergy levels, somewhat similar to the energy levels ofmolecules.[2][1][3][4][5] This gives nanoclusters similar qualities as a singular molecule[6] and does not exhibitplasmonic behavior; nanoclusters are known as the bridging link between atoms andnanoparticles.[7][2][1][3][4][5][8][9][10][11][12] Nanoclusters may also be referred to as molecular nanoparticles.[13]
The formation of stable nanoclusters such asBuckminsterfullerene (C60) has been suggested to have occurred during the early universe.[14][8]
In retrospect, the first nanoclustered ions discovered were theZintl phases, intermetallics studied in the 1930s.[citation needed]
The first set of experiments to consciously form nanoclusters can be traced back to 1950s and 1960s.[8] During this period, nanoclusters were produced from intense molecular beams at low temperature by supersonic expansion. The development oflaser 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 ofsemiconductor elements, compound clusters andtransition metal nanoclusters.[8]
Subnanometric metal clusters typically contain fewer than 10 atoms and measure less than one nanometer in size.[15][16][17][18][19]
According to the Japanese mathematical physicistRyogo Kubo, the spacing of energy levels can be predicted by
whereEF isFermi energy andN is the number of atoms. Forquantum confinement𝛿 can be estimated to be equal to thethermal energy (δ =kT), wherek is theBoltzmann constant andT is temperature.[20][21]
Not all the clusters are stable. The stability of nanoclusters depends on the number ofatoms in the nanocluster,valenceelectron counts and encapsulating scaffolds.[22] In the 1990s, Heer and his coworkers usedsupersonic expansion of an atomic cluster source into avacuum in the presence of aninert gas and produced atomic cluster beams.[21] Heer's team and Brack et al. discovered that certain masses of formed metal nanoclusters were stable and were like magic clusters.[23] 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 showedmagic number stability.[3] Häkkinenet al explained this stability with a theory that a nanocluster is stable if the number of valence electrons corresponds to the shell closure ofatomic orbitals as (1S2, 1P6, 1D10, 2S2 1F14, 2P6 1G18, 2D10 3S2 1H22.......).[24][25]
Molecular beams can be used to create nanocluster beams of virtually any element. They can be synthesized in highvacuum by with molecular beam techniques combined with a mass spectrometer for mass selection, separation and analysis. And finally detected with detectors.[26]
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 supersonicmolecular beam. The expansion into vacuum proceedsadiabatically cooling the vapor. The cooled metal vapor becomessupersaturated, 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 coldhelium 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 andxenon) 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.
Wein filter InWien filter mass separation is done with crossed homogeneous electric and magnetic fields perpendicular to ionized cluster beam. The net force on a charged cluster withmassM, chargeQ, andvelocityv vanishes ifE =Bv/c . The cluster ions are accelerated by avoltageV to an energyQV. Passing through the filter, clusters withM/Q = 2V/(Ec/B) are not deflected. These cluster ions that are not deflected are selected with appropriately positionedcollimators.
Quadrupole mass filter Thequadrupole mass filter operates on the principle that iontrajectories in a two-dimensional quadrupole field are stable if the field has an AC component superimposed on a DC component with appropriateamplitudes andfrequencies. It is responsible for filtering sample ions based on theirmass-to-charge ratio.
Time of flight mass spectroscopyTime-of-flight spectroscopy consists of anion gun, a field-freedrift space and an ion cluster source. The neutral clusters are ionized, typically using pulsed laser or anelectron beam. The ion gun accelerates the ions that pass through the field-free drift space (flight tube) and ultimately impinge on an ion detector. Usually anoscilloscope records the arrival time of the ions. The mass is calculated from the measuredtime of flight.
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 thecollision rate with theinert gas, they are sensitive to the cluster shape and size.
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.
There are several methods reported to reduce silver ion into zero-valent silver atoms:
Cryogenic gas molecules are used as scaffolds for nanocluster synthesis in solid state.[4] In aqueous medium there are two common methods for stabilizing nanoclusters:electrostatic (charge, or inorganic) stabilization andsteric (organic) stabilization. Electrostatic stabilization occurs by the adsorption ofions to the often-electrophilic metal surface, which creates anelectrical double layer. Thus, thisCoulomb repulsion force between individual particles will not allow them to flow freely without agglomeration. Whereas on the other hand in steric stabilization,the metal center is surrounded by layers of sterically bulk material. These largeadsorbates provide a steric barrier which prevents close contact of the metal particle centers.[2]
ThiolsThiol-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 visibleluminescence by reducing Au3+ in the presence of glutathione withsodium borohydride (NaBH4). Also other thiols such astiopronin, 2-phenylethanethiol,thiolated α-cyclodextrin and3-mercaptopropionic acid and bidentatedihydrolipoic 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-metalmolar 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 reducanttetrakis(hydroxymethyl)phosphonium (THPC). Here azwitterionic thiolateligand, 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.
DendrimersDendrimers are used as templates to synthesize nanoclusters. Gold nanoclusters embedded inpoly(amidoamine) dendrimer (PAMAM) have been successfully synthesized. PAMAM is repeatedly branched molecules with different generations. The fluorescence properties of the nanoclusters are sensitively dependent on the types of dendrimers used as template for the synthesis. Metal nanoclusters embedded in different templates show maximum emission at differentwavelengths. The change in fluorescence property is mainly due to surface modification by thecapping agents. Although gold nanoclusters embedded in PAMAM are blue-emitting thespectrum can be tuned from theultraviolet to thenear-infrared (NIR) region and the relative PAMAM/gold concentration and the dendrimer generation can be varied. The green-emitting gold nanoclusters can be synthesized by adding mercaptoundecanoic acid (MUA) into the prepared small gold nanoparticle solution. The addition of freshly reducedlipoic acid (DHLA) gold nanoclusters (AuNC@DHLA) become red-emittingfluorophores.[2][1]
PolymersPolymers with abundantcarboxylic acid groups were identified as promising templates for synthesizing highly fluorescent, water-soluble silver nanoclusters. Fluorescent silver nanoclusters have been successfully synthesized onpoly(methacrylic acid), microgels of poly(N-isopropylacrylamide-acrylic acid-2-hydroxyethyl acrylate) polyglycerol-block-poly(acrylic acid)copolymerspolyelectrolyte, poly(methacrylic acid) (PMAA) etc.[5] Gold nanoclusters have been synthesized withpolyethylenimine (PEI) andpoly(N-vinylpyrrolidone) (PVP) templates. The linearpolyacrylates, poly(methacrylic acid), act as an excellent scaffold for the preparation of silver nanoclusters in water solution byphotoreduction. Poly(methacrylic acid)-stabilized nanoclusters have an excellent highquantum yield and can be transferred to other scaffolds or solvents and can sense the local environment.[27][2][1][3][4][28][29]
DNA, proteins and peptides DNAoligonucleotides are good templates for synthesizing metal nanoclusters. Silver ions possess a high affinity tocytosine 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. Biologicalmacromolecules such aspeptides andproteins have also been utilized as templates for synthesizing highly fluorescent metal nanoclusters. Compared with shortpeptides, large and complicated proteins possess abundant binding sites that can potentially bind and further reduce metalions, thus offering better scaffolds for template-driven formation of small metal nanoclusters. Also the catalytic function ofenzymes 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 andzeolite 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 theÅngström 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][30]
Most atoms in a nanocluster are surface atoms. Thus, it is expected that themagnetic 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 andrhodium areparamagnetic in bulk but becomeferromagnetic in nanoclusters. Also,manganese is antiferromagnetic in bulk but ferromagnetic in nanoclusters. A small nanocluster is ananomagnet, which can be made nonmagnetic simply by changing its structure. So they can form the basis of a nanomagnetic switch.[3][8]
Large surface-to-volume ratios and low coordination of surface atoms are primary reasons for the uniquereactivity of nanoclusters. Thus, nanoclusters are widely used as catalysts.[11] Gold nanocluster is an excellent example of acatalyst. While bulk gold is chemicallyinert, it becomes highly reactive when scaled down to nanometer scale. One of the properties that govern cluster reactivity iselectron affinity.Chlorine has highest electron affinity of any material in theperiodic table. Clusters can have high electron affinity and nanoclusters with high electron affinity are classified as super halogens. Super halogens are metal atoms at the core surrounded byhalogen atoms.[3][8]
The optical properties of materials are determined by their electronic structure andband gap. The energy gap between the highest occupied molecular orbital and lowest unoccupied molecular orbital (HOMO/LUMO) varies with the size and composition of a nanocluster. Thus, the optical properties of nanoclusters change. Furthermore, the gaps can be modified by coating the nanoclusters with different ligands orsurfactants. It is also possible to design nanoclusters with tailored band gaps and thus tailor optical properties by simply tuning the size and coating layer of the nanocluster.[31][2][3][8]
Nanoclusters potentially have many areas of application as they have unique optical, electrical, magnetic and reactivity properties. Nanoclusters arebiocompatible, ultrasmall, and exhibit bright emission, hence promising candidates for fluorescence bio imaging or cellular labeling. Nanoclusters along with fluorophores are widely used for staining cells for study bothin vitro andin vivo. Furthermore, nanoclusters can be used for sensing and detection applications.[32] They are able to detectcopper andmercury and silions in an aqueous solution based on fluorescence quenching. Also many small molecules, biological entities such asbiomolecules, proteins,DNA, andRNA can be detected using nanoclusters. The uniquereactivity properties and the ability to control the size and number of atoms in nanoclusters have proven to be a valuable method for increasing activity and tuning the selectivity in a catalytic process. Also since nanoparticles are magnetic materials and can be embedded in glass these nanoclusters can be used in optical data storage that can be used for many years without any loss of data.[31][2][1][3][4]
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