Transcript Document
06525 lecture 3. Nanocrystals (Quantum Dots) 1. Historical Background Very many different kinds of nanoparticles are present in nature, although most nanoparticles have only been recognised as such in the last two decades. New synthetic and analytical tools and techniques had to be developed and existing techniques modified to prepare, purify and then identify the chemical composition, purity, size, shape and structure of nanoparticles, such as nancrystals, nanorods, metal clusters, etc., as they are smaller than the wavelength of light used in normal spectroscopic methods of analysis. Analytical techniques include: FTIR UV-vis NMR XRD, SAXS, WAXS, EXAFS TEM & SEM STM & AFM Colloidal Suspensions of Gold Nanoparticles ● d > 10 nm ● appear almost any colour from red or blue ● colour depends on the size of the nanocrystals ● colour due to quantum confinement ● first observed by Michael Faraday in 1858 ● used since the end of the middle ages to colour ruby glass Synthesis: Nanoparticles of gold (> 10 nm) are formed as a powder by reaction with water-soluble phosphine ligands, such as P(m-C6H4SO3Na)3, which form an organic coating on the nanoparticle surface. These powders can be dissolved in water - forming blood-red solutions - and then applied to a surface. Evaporation of the solvent on a smooth surface creates a bright film of metallic gold. Deposition of the same solution on a porous surface leads to a red colour. Stained Glass Windows ● nano-sized CdS dispersed in the transparent amorphous glass matrix ● blue-shift in the colour of some soda-lime glasses due to nanoparticles ● attributed in the twentieth century to very small polarised CdS particles ● the blue-shift actually arises from the quantum confinement Nebula Dust ● Red colour of dust clouds (nebula) in our galaxy first observed in 1980 ● red colour attributed to very small quantum-confined silicon nanocrystals 2. Quantum Confinement Reducing the size of bulk solids changes the magnitude of the physical properties of the nano-sized solid to intermediate values between those of the original bulk solid and those of individual atoms or molecules, e.g., whereas metals conduct electric charge, individual metal atoms do not. Therefore, there is a critical size of some metal clusters below which no conductivity can be observed, i.e., these metal clusters contain the last "free electrons" for electrical conductivity. Classical mechanics are no longer applicable for such small nanometer-sized particles and quantum mechanics take over. The quasi-continuous density of [electronic] states of a bulk material is gradually reduced to a limited number of discrete energy levels as the size of a particle decreases and "particle in a box" quantised energy levels are then observed at a critical nanocrystal or nanocluster diameter. Quantumconfinement in very small particles means that the colour of a nanocrystal often referred to as a quantum dot - depends as much on the size of the particle as on the nature of the material itself, e.g., the colour of small CdSe (2.3 nm) nanocrystals is turquoise, whereas that of larger CdSe (5.5 nm) nanocrystals is orange. Quantum effects in the absorption and emission of light from nanoparticles were first observed in 1967. unoccupied 5/6 nm energy Eg = band gap occupied CdSe nanocrystal atoms nanocrystals bulk semiconductor density of states Chalcogenide Nanocrystals - Synthesis a) Colloidal Dispersions Early 1980s: colloidal dispersions or suspensions of semiconductors, such as CdS, were first synthesised by mixing reagents in solvents in the as potential catalysts in photochemical reactions. There was no real attempt to make colloidal dispersions containing nanoparticles of a controlled size and shape as these are not of critical importance in catalytic applications. Disadvantages: The colloidal dispersions of semiconductor nanocrystals were often unstable the nanocrystals could not be isolated and characterised. b) Physical Confinement Solid nanocrystalline particles of good optical quality were synthesised within the nano-sized pores of solid inorganic porous media, such as zeolites, clays and glasses. Advantages: The nanocrystals are size-limited: the nanocrystals grow in the pores within the inorganic host up to, but not beyond, the maximum size of the nanometersized pores of the host material. The shape and size is determined by the nanopore. Disadvantages: The nanocrystals cannot be removed and isolated from the host nanoporous materials, as the nanoparticles and the nanoporous host are both inorganic and the nanoparticles are stuck within the nanopores. c) Polymer Coatings Nanocrystals are covered in an ionic polymer-coating by preparing them in an aqueous solution containing a sodium polyphosphate derivative e.g., sodium hexaphosphate [NaPO3]6. The cadmium ions form a complex with the polyphosphonate (PP) chains in aqueous solution: nCd2+ + PP2n- CdnPP CdnPP Advantages: The charged polymer coating prevents agglomeration of the nanocrystals due to electrostatic repulsion and steric effects. The polymer controls the size and shape of the nanocrystals [formed by adding a chalcogenide gas to the reaction, e.g., by bubbling hydrogen suphide gas through it, which provides the required sulphur S2- anions in this case]. The formation and growth of the nanocrystals can be controlled by modifying the pH of the reaction solution. Cd2+ + S2- CdS c) Polymer Coatings Nanocrystals are covered in an ionic polymer-coating by preparing them in an aqueous solution containing a sodium polyphosphate derivative e.g., sodium hexaphosphate [NaPO3]6. The cadmium ions form a complex with the polyphosphonate (PP) chains in aqueous solution: nCd2+ + PP2n- CdnPP CdnPP Advantages: The charged polymer coating prevents agglomeration of the nanocrystals due to electrostatic repulsion and steric effects. The polymer controls the size and shape of the nanocrystals [formed by adding a chalcogenide gas to the reaction, e.g., by bubbling hydrogen suphide gas through it, which provides the required sulphur S2- anions in this case]. The formation and growth of the nanocrystals can be controlled by modifying the pH of the reaction solution. Cd2+ + S2- CdS d) Monolayer Coatings Soluble semiconductor nanocrystals of a defined size and shape and with an organic monolayer coating are prepared in the aqueous droplets of reverse micelles. Advantages: The organic monolayer inhibits flocculation and aggregation of the nanocrystals. The addition of Cd2+ and S2- reagents leads to further growth of the nanoparticles. Addition of phenyl(trimethylsilyl)selenium to a CdSe nanocrystal microemulsion leads to the replacement of the surfactant coating with a thin layer of Se-Ph chemically bound to the nanocrystal surface, which passivates the reactive nanocrystal surface, which increases stability and QE. e) Controlled Precipitation Controlled precipitation of nanocrystals in colloidal solutions containing stabilisers, e.g., trioctylphosphine and trioctylphosphine oxide, allows the nanocrystals to remain in solution and grow in a controlled fashion by suppressing aggregation of individual nanocrystals. Cd(CH3)2 + (TMS)2S TOPO TOP CdS + 2CH4 Advantages: nanocrystal powders soluble in organic solvents [organic coating] Post-synthesis processing improves size distribution and quantum yield Good control over shape ands size of nanocrystals by: ● Temperature variation ● Rate of addition of reagents Disdavantages: High temperatures Toxic and explosive reagents e) Controlled Precipitation [continued] The main alternative variation of this method for the synthesis of cadmium chalcogenides involves using aliphatic thiol and aromatic thiophenol stabilisers in colloidal, often aqueous, solutions. Aliphatic stabilisers include thioethanol, thioglycerine and thioglycolic acid for the preparation of water soluble nanocrystals. Aromatic thiols can be used and a range of organically soluble cadmium chalcogenide nanocrystals have been prepared. Cd(ClO4)2 + H2S Advantages: Lower reaction temperatures Non-toxic reagents Water soluble nanocrystals Thiols CdS + 2HCl + 4O2 f) Core-Shell Nanocrystals The highly reactive surface of nanocrystals with atoms that are not completely co-ordinated allows a second layer of a different nanocrystalline material to be deposited by epitaxial growth to form core-shell nanocrystals. For example pre-formed CdSe nanocrystals in solution can be coated with CdS in situ to form stable CdSe(CdS) core/shell nanocrystals with a passivated inner interface. Advantages: Core shell nanocrystals are more stable than simple nanocrystals Higher quantum yield due to quantum confinement and no dangling bonds Disadvantages: Complex synthesis 4. Applications of Semiconductor Nanocrystals CdS nanocrystals are being investigated as photoluminescent biological tags: ● protein coated nanocrystals bind selectively to cancer cells, ● the size and position of a cancer tumor are accurately mapped [PL], ● more sophisticated treatment with less side effects is facilitated, ● nanocrystals absorb laser light and destroy tagged cancer cells selectively. Advantages: Inorganic nanocrystal labels are superior to organic dyes: ● they are brighter, ● their emission is narrower, ● their lifetime is longer, ● they are also biocompatible, ● are non-toxic and cause no adverse side-effects, ● the nanocrystals can also transferred from one cancer cell to another. Light-Emitting Diodes Cadmium chalcogenide II-VI semiconductor nanocrystals, such as CdS, CdSe and CdTe, are being investigated as electroluminescent components of hybrid inorganic/organic light-emitting diodes as a new kind of flat panel display device to compete with with LCDs. Advantages: ● wide viewing angles, ● low power consumption, ● clean colours, e.g., green light not contaminated by yellow or blue light, ● colour tuning by changing nanocrystals size. Gas Sensors Metal oxide nanocrystals, such as SnO2 and In2O3, are already being used in commercial gas sensors. Advantages: ● higher sensitivity and selectivity Plastic Solar Cells Metal oxide quantum dots, such as TiO2, are already being used in plastic solar cells. Advantages: ● high efficiency (10%) and comparable with solid-state silicon solar cells, ● light, ● robust, ● cheap to manufacture, ● produced in large-area formats. Fuel Cells Fuel cells using TiO2 nanocrystals are used to produce hydrogen photocatalytically from water. Advantages: ● water is cheap and plentiful, ● hydrogen is a clean fuel [water as a by-product], ● very environmentally friendly.