纳米结构物理学—5.pptVIP

* 团簇与晶粒 Clusters Nano-crystallites Clusters: N ~ 10–104, configuration different from bulk Nano-crystallites: N ≥ 104, bulk-like lattice structure Free-standing vs. supported Evolution of shape, interior surface structures, thermal properties with nano-particle size Relevant in catalysis, photonics, magnetic storage, bio-object label,… Creation of free-standing clusters: Cluster generator and analyzer ? dramatic vapor temperature drop ? cluster nucleation ? ionization and acceleration ? mass-filter (TOF or QMA) ? counting, analyzing, deposition… Vapor mixed in high-pressure carrying gas (e.g., He) ? supersonic expansion through nozzle pin-hole Alternative sources: Injection of metal vapor from heated reservoir into cold carrier gas, typical cluster size range 102 - 106 Pulsed laser ablation or ion bombardment: neutral ionized Supported clusters: soft-landing of clusters, or atomic deposition and nucleation on an inert substrate, for static analyses using electron microscopy and/or spectroscopy Production of uniform size spherical QDs: Controlled nucleation and growth in supersaturated solution All clusters nucleate at basically same moment, QD size distribution 15% QDs of certain average size are obtained by removing them out of solution after a specific growth period Further size-selective processing to narrow the distribution to ? 5% Mono-dispersed nanocrystals of many semiconductors, such as CdS, CdSe, CdTe, ZnO, CuCl, and Si, are fabricated in this way Assembly of QD lattices: proper control of solvent composition and speed of separation QD color labels for bio-molecules (Bruchez, Jr. et al., Science 281, 2013 (1998)) Electronic Shell model of small clusters (N ? 1000) Magic numbers: Abundance peaks in mass spectra of free clusters Magic number series: N = 8, 20, 40, 58, 92 Appear in cluster spectra of Na, K, Au, Ag, Cu Reasons of extraordinary abundance? These clusters are truly energetically (electronic or atomic configuratio