Nanoparticles and Quantum Dots.

Transcription

1 Nanoparticles and Quantum Dots. 1 Prof.P. Ravindran, Department of Physics, Central University of Tamil Nadu, India & Center for Materials Science and Nanotechnology, University of Oslo, Norway

2 Shapes Of Nanoparticles 2 Basic motifs of inorganic nanocrystals: 0D spheres, cubes, and polyhedrons; 1D rods and wires; 2D discs, prisms, and plates.

3 Top down approaches 3 Milling/Attrition Nanoparticles have a relatively broad size distribution have varied particle shape or geometry may contain a significant amount of impurities from the milling medium may contain defects resulting from milling used in the fabrication of nanocomposites and nanograined bulk materials In nanocomposites and nanograined bulk materials, defects may be annealed during sintering Repeated quenching Repeated thermal cycling may also break a bulk material into small pieces, if the material has very small thermal conductivity but a large volume change as a function of temperature. Fine particles can be produced The process is difficult to design and control so as to produce desired particle size and shape The process is limited to materials with very poor thermal conductivity but a large volume change

4 Bottom-up approach 4 Nanoparticles synthesised by homogeneous nucleation from liquid or vapour Nanoparticles synthesised by heterogeneous nucleation on substrates Nanoparticles or quantum dots prepared by phase segregation through annealing appropriately designed solid material at elevated temperatures Nanoparticles synthesized by confining chemical reactions, nucleation and growth processes in a small space such as micelles

5 Categories of synthesis methods 5 Thermodynamic equilibrium approach Kinetic approach

6 Thermodynamic approach 6 Synthesis process consists of i. generation of supersaturation ii. Nucleation, & iii. Subsequent growth

7 Kinetic approach 7 Formation of nanoparticles is achieved by either Limiting the amount of precursors available for growth e.g. molecular beam epitaxy) Confining the process in a limited space (e.g. aerosol synthesis or micelle synthesis)

8 8 Controlling Shape in the Nanoscale Time Minutes vs. hours Temperature Structure Difference Zinc Blend vs. Wurtzite Surface Energetics Different phases have different chemical potential and chemistry Capping Organic Molecules Different Molecules can attach to different phases Monomer Concentration Different Shape depending on concentrations

9 Desired Characteristics of nanoparticles 9 Identical size of all particles (monosized particles/ paricles with uniform size distribution) Identical shape or morphology Identical chemical composition and crystal structure that are desired among different particles and within individual particles, such as core and surface composition must be the same Monodispersed (individually dispersed) particles (i.e. no agglomeration). If agglomeration does occur, nanoparticles should be readily redispersible

10 Nanoparticles 10 Dimension: not > several hundred nm s Can be Single crystal (aka nanocrystal) Polycrystalline amorphous Different morphologies e.g. spheres, cubes, platelets Quantum dots The characteristic dimension of the nanoparticles is sufficiently small and quantum effects are observed

11 Formation of nanoparticles dispersed in a solvent 11 Most common approach Advantages Easiness of stabilization of nanoparticles from agglomeration extraction of nanoparticles from solvent surface modification and application processing control mass production

12 Synthesis of metallic nanoparticles (i.e. metallic colloidal dispersion) 12 General method Reduction of metal complexes in dilute solutions The formation of monosized nanoparticles is achieved in most cases by a combination of low concentration of solute and polymeric monolayer adhered onto the growth surfaces Both low concentration and a polymeric monolayer would hinder the diffusion of growth species from the surrounding solution to the growth surfaces Various types of precursors, reduction reagents and polymeric stabilizers are commonly used

13 Colloidal gold 13 Sodium citrate reduction of chlorauric acid (HAuCl 4 ) Most commonly used method Add 1 ml of 0.5 % sodium citrate into a boiling dilute aqueous solution of chlorauric acid (~2.5 x 10-4 M) Keep the mixture at 100 o C until the colour changes, while maintaining the overall volume of the solution by adding water A large number of initial nuclei formed in the nucleation stage result in a large number of nanoparticles with smaller size and narrower size distribution The colloidal sol has excellent stability has uniform particle size of ~20 nm in diameter

14 Pt nanoparticles 14 Radiolysis The gamma rays of Co-60 used to generate hydrated electrons, hydrogen atom and 1-hydroxylmethyl radicals Radicals reduce Pt 2+ in K 2 PtCl 4 to the zerovance state, which form Pt paticles (mean diameter 1.8 nm) Citrate reduction of PtCl 6 2- Pt nanoparticles (diameter ~2.5 nm) obtained by boiling (1 h) a mixture of H 2 PtCl 6 with sodium citrate Hydrogen reduction of K 2 PtCl 4 Hydrolyse precursor (K 2 PtCl 4 in dilute aq solution) to form hydroxides prior to hydrogen reduction. NaOH used as a catalyst to promote hydrolysis PVA (Poly(vinyl alcohol) used as a stabilizer

15 Ag nanoparticles 15 UV illumination of aqueous solutions containing AgClO 4, acetone and various polymer stabilizers UV illumination generates ketyl radicals via excitation of acetone and subsequent hydrogen abstraction from 2-propanol: The ketyl radical may undergo protolytic dissociation reaction: Both the ketyl radical and radical anions react with and reduce silver ions to silver atoms: Both reactions have a low reaction rate and thus favor the production of monosized Ag nanoparticles Using polyethyleneimine as polymer stabilizer: Size 7 nm, narrow size distribution

16 Ag nanoparticles 16 Sonochemical reduction of an aq silver nitrate solution at 10 o C in an atmosphere of Ar and H 2. Water decomposed into hydrogen and hydroxyl radicals by ultrasound Hydrogen radicals reduce silver ions into silver atoms, which subsequently nucleate and grow to silver nanoclusters H 2 removes hydrogen peroxides (which may oxidise silver nanoclusters to silver oxide) formed from hydroxyl radicals

17 Influence of reduction reagents 17 A strong reduction reagent promotes a fast reaction rate, and favors the formation of smaller nanoparticles A weak reduction reagent induces a slow reaction rate and favors relatively larger particles

18 Influence of polymer stabilizers 18 Polymer stabilizers are introduced primarily to form a monolayer on the surface of nanoparticles so as to prevent agglomeration of particles. A strong adsorption of polymer stabilizers would occupy the growth site and thus reduce the growth rate of nanoparticles. A full coverage of polymer stabilizer would hinder the diffusion of growth species from the surrounding solution to the growth site. Polymer stabilizers may interact with solute, catalyst, or solvent, and thus directly contribute to the reaction.

19 Preparation of nanoparticles 19 Plasma processing - Both thermal (plasma arc, plasma torch, plasma spray) and low temperature (cold) plasma (discussed) Chemical Vapor Deposition - Either on a substrate or in the gas phase (for bulk production) - Metallic oxides and carbides Electrodeposition Sol-gel processing Ball mill or grinding (old fashioned top-down approach) Key Issue: Agglomoration

20 Quantum Dots 20 Also referred to as spherical nanoparticles, nanocrystals etc. Properties based on the optical features of their absorption and emission spectra. Semiconductor nanostructure Motion of conduction band electrons confined Also refers to holes and excitons Hole-electron separation known as Exciton Bohr radius. When size of particle same size as EBR then treat En levels as discrete. En levels separated and leads to quantum confinement An artificial atom 2-10 nm (10-50 atoms) up to nm Compare with quantum wire and well (confined in two and one DIRECTION) Size controls band gap. High quantum yields

21 Synthetic droplets containing anything from a single electron to thousands of atoms but behave like a single huge atom. Size: nanometers to microns These are nanocrystals with extraordinary optical properties - The light emitted can be tuned to desired wavelength by altering the particle size - QDs absorb light and quickly re-emit but in a different color - Colors from blue to IR Common QDs: CdS, CdSe, PbS, PbSe, PbTd, CuCl Manufacturing - Wet chemistry - Template synthesis (zeolites, alumina template) 21

22 22 Synthesis of Quantum Dots Can be synthesized by single-source precursor route or conventional route Conventional route: involves the reaction of metal salts and a base, NaOH or alkoxides in the presence of stabilizing organic molecules. Single-source precursor: involves the preparation of metal complexes which in turn undergo thermal decomposition in a coordinating organic solvent.

23 Valence Band Conduction Band 23 Electronic structure of Qdots Energy levels Absorption Radiationless decay Fluorescence Band gap Small Molecules Qdots Semiconductors Source: Bala Manian, Quantum Dot Corp.

24 Size vs Bandgap in Qdot 24

25 Optical properties of nanoparticles 25 Ordinary light excites all color quantum dots. (Any light source bluer than the dot of interest works.) Quantum dots change color with size because additional energy is required to confine the semiconductor excitation to a smaller volume. Source: Bala Manian, Quantum Dot Corp.

26 Size and material dependent optical properties 26 ZnSe CdSe CdTe Normalized Intensity Emission Wavelength (nm) Excitation: 290 nm, others 365 nm Material band-gap determines the emission range; particle size tunes the emission within the range Nanocrystal quantum yields are as high as 80% Narrow, symmetric emission spectra minimize overlap of adjacent colors

27 Constants and calculated excitonic diameter for some II VI and III V materials 27 Compound Band gap/ev Effective masses structure Lattice spacings/å m e * m h * CdS 2.53(490 nm) //; 0.7 wurtzite a: c: CdSe 1.74(712 nm) //, 0.4 wurtzite a: c: Applications Photovoltaic cells Photovoltaic cells CdTe 1.50(827 nm) zinc blende Electrooptic modulators ZnS 3.8(326 nm) 0.28 >1 //, 0.5 Wurtzite a: c: Photovoltaic cells ZnSe 2.58(480 nm) 0.17 Zinc blende Infrared windows, LED ZnTe 2.28(543 nm) 0.15 Zinc blend Photovoltaic cells PbS 0.37(3351 nm) Sodium chloride PbSe 0.26(4769 nm) Sodium chloride PbTe 0.29(4275 nm) Sodium chloride IR sensors IR sensors IR sensors

28 Applications Qdots 28 LEDs, solar cells, solid state lighting Biomedical - Bioindicators - Lateral flow assays - DNA/gene identification, gene chips - Cancer diagnostics Biological Labeling Agent Broad output spectrum Sharper spectrum Fades quickly ~ 100 ps 5-40 ns Unstable Stable output over time One dye excited at a time Multicolor imaging, multiple dyes excited simultaneously