纳米科学与技术-光学性质

• The Bohr radius determines the type of confinement
– 3-10 time Bohr radius: Weak Confinement
• ΔE ~ 1/M* • M* effective mass of exciton
– Smaller than 3 Bohr radius: Strong Confinement
Ne 2 = mε 0
•
ω particle
1 Ne 2 = 3 mε 0
Surface plasmon resonance （表面等离子体共振）
When a nanoparticle is much smaller than the wave length of light, coherent oscillation of the conduction band electrons induced by interaction with an electromagnetic field. This resonance is called Surface Plasmon Resonance (SPR).
Source:NanoscaleMaterialsinChemistry,Wiley, 2001
单个纳米颗粒荧光的闪烁行为
• single nanocrystal luminescence - blinking
晶体中的电子能级结构 Electronic band structure: energy & momentum
非直接带隙半导体 (indirect band-gap)
Electronic band structure: Silicon
直接带隙半导体 (direct band-gap)
Inter-band absorption in direct gap semiconductors
E Light can induce electronic transitions if energy and momentum are conserved: Efinal – Einitial = Ephot and Δk = hk phot ≈ 0
多孔硅、纳米硅材料：光电集成
Why use Nanocrystals?
Tunablebandgap
Why use Nanocrystals?
carriermultification
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R.D.Schaller,V.I.Klimov,PRL92,186601
Laser - light amplification by stimulated emission of
ψ k ( r ) = Ae
r r ik ⋅r
h 2 k e2 E =V + 2m
p = √(2mE) =
or
h 2 ke2 E= 2m
for V=0
hke
The photon wave vector (and corresponding momentum hk phot ) is small compared to electron wave vector (λe of the order of an atomic spacing)
radiation
布局数反转 - population inversion
激光器的种类和工作波长
Solid state laser
纳米线/点激光
Fabry–Perot optical cavities in a ZnO nanowire
Yang, PD group, Room-Temperature Ultraviolet Nanowire Nanolasers, Science 292, 1897 (2001)
Figure: Schematic of plasmon oscillation for a sphere, showing the displacement of the conduction electron charge cloud relative to the nuclei.
Sphere in a uniform static electric field
Klimov et al., “Single-exciton optical gain in semiconductor nanocrystals”, Nature, Vol. 447, 441 (2007)
Optical amplification in single NCs
QD在发光器件和光电器件中的应用
QD LED
DSSC
What is a plasmon?
“plasma-oscillation”: density fluctuation of free electrons
+
+ +
•
-wk.baidu.com
k
•
Plasmons in the bulk oscillate at ωp determined by the free electron density and effective mass drude
纳米科学与技术
第五章： 低维纳米材料的光学特性及纳米光学技术
裘晓辉 国家纳米科学中心
电话：82545583 Email: xhqiu@nanoctr.cn
基本概念
• 金属纳米颗粒的SPR • 半导纳米颗粒的荧光和尺寸关系 • 半导纳纳米线激光发射 • 光子晶体
Sunflowers - Vincent Van Gogh
Fluorescent minerals - Wikipedia
Optical transitions in a molecule
紫外可见吸收光谱和荧光光谱
Franck-Condon 效应 Stokes shift
半导体纳米颗粒的UV-Vis和PL
半导体材料的应用1
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半导体材料的应用2
Why sky is blue? Where the rainbow comes?
Why butterfly has colors? Why flowers are red?
光激发和光发射
（ optical excitation and de-excitation ）
• • • • • •
Rayleigh scattering ( ) Refraction (refractive index) Photonic crystal (photonic band gap) Surface plasmon resonance (noble metal NPs) Fluorescence (dye molecules, ion) …
Direct gap semiconductors Photons with E < Egap have insufficient energy to ‘kick a valence electron into a conduction state’ ⇒ absorption starts at Ephot = Egap These band-band absorptions have the usual implications for n and κ (recall Kramers-Kronig relations)
• ΔE ~ 1/μ* • μ* effective mass of hole and electron
激子波尔半径 - Exciton Bohr radius
表面效应: luminescence quantum yields
Surface passivation Core-Shell structure
Egap
Other possibility: momentum and energy can be conserved by photon absorption and simultaneous absorption or emission of a phonon: Indirect transitions possible with ‘assistance of a phonon’
能带中电子-空穴的库仑相互作用
室温下 kT=25meV
激子波尔半径 - Exciton Bohr radius
• Material dependent parameter
– The same size dot of different materials may not both be quantum dots
Classical relations: Energy Momentum E = ½ mv2 p = mv = √(2mE)
Quantum mechanical:
⎤ ⎡ h2 ∂2 ∂ + V ( x )⎥ Ψ (x, t ) = ih Ψ (x, t ) ⎢− 2 ∂t ⎦ ⎣ 2m ∂x
Energy Momentum
(Photon: long wavelength compared to atomic spacing ⇒ kphot « π/a )
k
Direct-gap semiconductor: highest occupied and lowest unoccupied state occur at k=0
单根纳米线的电致激光发射
单根纳米线的电致激光发射
Nanolaser:
Single photon light source Nanolithography Cell surgery Nanoscale optoelectronics
The concept：single-exciton nanocrystal lasing
Quantum size effect:
wannier exciton
硒化镉(CdSe)纳米晶体
L. E. Brus, ``Electron-electron and Electron-Hole Interactions in Small Semiconductor Crystallites: The Size Dependence of the Lowest Excited Electronic State,’’ J. Chem. Phys. 80, 4403 (1984).
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Silicon solar cells
Light emitting diodes
The Beijing Olympics at 2008
interaction of light with electronic states in semiconductors
量子限域效应 （Quantum confinement）
Inter-band absorption in indirect gap semiconductors
Direct transitions possible for Δk≈0 ⇒ strong direct interband absorption occurs at E > Egap
Egap
Indirect-gap semiconductor: highest occupied and lowest unoccupied state have Δk≠0
ωp
+
-
+
Plasmons confined to surfaces that can interact with light to form propagating “surface plasmon polaritons (SPP) (表 面等离子体激元) ” Confinement effects result in resonant SPP modes in nanoparticles drude
Nanowire Fabry–Perot optical cavities
Optically pumped CdS nanowire laser
Lieber Group, Single-nanowire electrically driven lasers, NATURE, 421, 16, 2003
particle can be considered as a dipole: in a metal cluster placed in an electric field, the negative charges are displaced from the positive ones