Lecture 6: Individual nanoparticles, nanocrystals and quantum dots

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Kory Chase
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1 Lecture 6: Individual nanoparticles, nanocrystals and quantum dots Definition of nanoparticle: Size definition arbitrary More interesting: definition based on change in physical properties. Size smaller than some critical length that characterize physical phenomena Examples: Electron mean free path, thermal diffusion length 1

2 Metal nanoparticles Structure: Common metals have close-packed crystal structure: fcc or hcp (some bcc) Smallest fcc crystal : one unit cell, 13 atoms 2

3 Another atom layer outside: = 55 atoms More layers: 13, 55, 147, 309, 561 atoms Structural magic numbers N = 1 ( 3 10n3 15n 2 +11n 3) n = number of layers Surface atoms: N surf =10n 2 20n +12 3

4 Geometric structure, general comments: Nanoparticle structure generally similar to bulk Small particles (< 5 nm) may have different structures Example Al 13 : theory predicts icosahedral structure Also size-dependent deviations from the ideal structure Indium nanoparticles, tetragonal c/a ratio Theoretical Al 13 clusters 4

5 Electronic magic numbers Simple model: Clusters as superatoms, with electronic shell structure Jellium model: free valence electrons in a homogeneous positive background Potential: Ur ()= U 0 [( ) +1] exp r r 0 r 0 : cluster effective radius U 0 : potential constant : steepness parameter Electronic magic numbers: 2, 18, 20, 40. corresponding to filled shells Compare to single atoms: 2, 10, 18, 36 (He, Ne, Ar, Kr) 5

Electronic structure, general Solve the Schrödinger equation for an exact atomic model, using realistic potentials Density-functional theory and molecular dynamics most commonly used Find

6 Electronic structure, general Solve the Schrödinger equation for an exact atomic model, using realistic potentials Density-functional theory and molecular dynamics most commonly used Find minimum-energy structure (depends on electronic energy levels) Typical solution has discrete energy levels, depending on cluster size (compare particle in a box ) - big difference to bulk metal. Quantum size effects when cluster size is comparable to Fermi level wavelength. Much larger size for semiconductors (μm compared to nm) 6

7 Experimental studies of nanoparticle energy levels Optical spectroscopy (absorption, fluorescence) UV photoemission STM/STS Example: combined STM/STS and photoemission study of gold clusters 7

8 Gold nanoparticles on HOPG STM STS, current images Photoemission: Gold nanoparticles gold surface 8

9 Reactivity - catalysis Depends on size and structure most (all) atoms on the surface closed shell most stable (compare noble gas atoms) strong variations with size Production of nanoparticles - large commercial activities Example: Nanostellar Inc., a Silicon Valley startup company ( 9

10 Fluctuations Small nanoparticles, many surface atoms with less movement restrictions structure changes (fluctuations) 10

11 Semiconducting nanoparticles Strong variation in optical properties compared to bulk - blue shift Bulk: excitons are important for the optical properties Nanoparticles: E n = 13.6 m* m 0 ( r ) 2 n 2 ev weak confinement: particle radius larger than exciton radius blue shift strong confinement: particle radius smaller than exciton radius no exciton, electron and hole independent. blue shift + new set of energy levels 4.19 Optical absorption in Cu 2 O 11

12 Light absorption in colloidal solution Figure 7.2. Solutions of quantum dots of varying size. Note the variation in color of each solution illustrating the particle size dependence of the optical absorption for each sample. Note that the smaller particles are in the red solution (absorbs blue), and that the larger ones are in the blue (absorbs red). 12

13 4.20 Optical absorption spectrum of CdSe nanoparticles with sizes 20 and 40 Å 13

14 Karlstad: ZnO nanocrystals on surfaces SEM images: STM images: a) 250 x 250 nm b) 50 x 50 nm c) 28 x 28 nm 14

15 Other nanoparticles Inert-gas clusters - weak van der Waals forces Magnetic nanoparticles very hot research area (chapter 7) Superfluid clusters - Bose-Einstein condensates Molecular clusters - example: water, hydrogen-bonded clusters 15

16 Synthesis of nanoparticles Laser evaporation methods: 16

17 Aerosol techniques, Lund Univ. Metal nanoparticles Semiconductor nanoparticles 17

18 Differential mobility analyzer - analysis and size selection 18

Chemical methods - colloidal growth - nanoparticles grown in solution with surfactant layer - most promising for volume production, good scalability (Example Nanostellar) - monodispersive growth

19 Chemical methods - colloidal growth - nanoparticles grown in solution with surfactant layer - most promising for volume production, good scalability (Example Nanostellar) - monodispersive growth possible (similar size of particles) - control over composition, size, shape, structure, surface properties Kinetic size control Figure 7.3. La Mer model for the growth stages of nanocrystals. 19

Colloidal growth Kinetic size control: Monodisperse colloidal nanocrystals synthesized under kinetic size control. a, Transmission electron microscopy (TEM) image of CdSe nanocrystals.

20 Colloidal growth Kinetic size control: Monodisperse colloidal nanocrystals synthesized under kinetic size control. a, Transmission electron microscopy (TEM) image of CdSe nanocrystals. b, TEM image of cobalt nanocrystals. c, TEM micrograph of an AB13 superlattice of _-Fe2O3 and PbSe nanocrystals. The precise control on the size distributions of both nanocrystals allows their selfassembly into ordered threedimensional superlattices. Scale bars, 50 nm. 20

21 Colloidal growth - shape control Shape control of colloidal nanocrystals. a, Kinetic shape control at high growth rate. The high-energy facets grow more quickly than low energy facets in a kinetic regime. b, Kinetic shape control through selective adhesion. The introduction of an organic molecule that selectively adheres to a particular crystal facet can be used to slow the growth of that side relative to others, leading to the formation of rod- or disk-shaped nanocrystals. c, More intricate shapes result from sequential elimination of a high-energy facet. The persistent growth of an intermediate-energy facet eventually eliminates the initial high-energy facet, forming complex structures such as an arrow- or zigzag-shaped nanocrystals. d, Controlled branching of nanocrystals. The existence of two or more crystal structures in different domains of the same crystal, coupled with the manipulation of surface energy at the nanoscale, can be exploited to produce branched inorganic nanostructures such as tetrapods. Inorganic dendrimers can be further prepared by creating subsequent branch points at the defined locations on the existing nanostructures. The red and green dots in a and b represent metal coordinating groups with different affinities to nanocrystal facets. 21

22 Other synthesis techniques Radiofrequency plasma methods Epitaxial growth - self-assembled quantum dots (lecture 3) Ion implantation Thermolysis - high-temperature decomposition Mechanical milling Cavitation, sonochemistry, detonation After treatments Passivation of cluster surfaces Powder consolidation Nanoparticle coatings 22

Semiconductor quantum dots

Semiconductor quantum dots Quantum dots are spherical nanocrystals of semiconducting materials constituted from a few hundreds to a few thousands atoms, characterized by the quantum confinement of the