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7.2 Radiative Recombination and Nonradiative Recombination
7.2.1 Radiative recombination of non-equilibrium carriers
3）Recombination of donor-acceptor pairs (D-A pairs)
• The ionization energy of D-A pairs depends on the impurity atoms
and the ways of substitution. E.g., for GaP, if donor O and acceptor
C substitute P, at T=1.6K, ionization energy = 941meV. If donor O
substitutes for P and acceptor Zn substitues for Ga, at T=1.6K,
ionization energy = 956.6 meV.
• At room temperature, due to the strong
interaction with phonons, the line
spectrum of D-A pairs is hard to detect.
But at low temperatures, it is easy to
observe the line spectrum.
7.2 Radiative Recombination and Nonradiative Recombination
7.2.1 Radiative recombination of non-equilibrium carriers
4）Recombination via deep levels
• The emitted photon energy is much smaller than the
bandgap, for recombination via deep levels, and the
wavelength of emission is far from the absorption edge.
• The recombination via deep levels is useful for wide
bandgap materials, e.g., it gives red emission in GaP.
• The deep levels usually result in non-radiative
recombination, obvious in direct bandgap semiconductors.
In practice, one should try to eliminate deep levels to
enhance the fluorescence efficiency.
7.2 Radiative Recombination and Nonradiative Recombination
7.2.1 Radiative recombination of non-equilibrium carriers
5）Recombination of excitons
• If photon energy is lower than the bandgap, electrons can
be excited to the exciton levels, where electron and hole are
bound together due to Coulomb interactions. The bound
electron hole pairs, excitons, can move freely in the crystal.
• Excitons are neutral, whose movements will not give rise
to currents. But the energies of
excitons can be released in the form
of radiation or non-radiation.
7.2 Radiative Recombination and Nonradiative Recombination
7.2.1 Radiative recombination of non-equilibrium carriers
5）Recombination of excitons
• Two types of excitons:
1. Frenkel exiton or tightly-bound exciton: the distance between
electron and hole is on the order of crystal constants.
2. Wannier exciton: weekly bound excitons. The distance is much
larger than the crystal constants. Wannier excitons are
commonly present in semiconductors.
• Bound exciton: the movements of excitons are not free in
crystals. Excitons can be bound by the donors, acceptors, D-A
pairs and isoelectronic traps.
7.2 Radiative Recombination and
Non-radiative Recombination
n
Eexc
7.2.1 Radiative recombination of nonequilibrium carriers
Eg
5）Recombination of excitons
• Exciton energy: Hydrogen-like atom model
n
Eexc
Ionization energy of ground state of
1  mr*  EH
hydrogen atom.
 2  2
r  m  n
1 * 1 * 1 *
mq 4
EH  2 2  13.6(eV)
mr
mn
mp
8 0 h
Dielectric constant
of crystal
Effective mass of Effective
mass of
electrons
holes
Exciton energy is discrete.
n=1: ground state.
n=, exciton energy=0  bottom of conduction
band, electron and hole is not bound together.
7.2 Radiative Recombination and Nonradiative Recombination
7.2.1 Radiative recombination of non-equilibrium carriers
5）Recombination of excitons
• For free excitons, the recombination of electron and hole will
emit photons.
• For direct bandgap semiconductor, the photon energy is:
hv  Eg  E
n
exc
n
Eexc
Eg
• For indirect bandgap semiconductor,
the emitted photon energy is:
hv  Eg  E
n
exc
 NE p
Energy of N phonons
absorbed or released.
7.2 Radiative Recombination and Nonradiative Recombination
7.2.1 Radiative recombination of non-equilibrium carriers
5）Recombination of excitons
• For bound excitons, if the binding energy with impurity
atoms is Ebx, the peak position of the emission spectrum is:
hv  Eg  E
n
exc
 Ebx
• It was found that bound excitons are important to emissions,
leading to high emission efficiency.
• E.g., in GaP-LED, the main emissions are red emission from
Zn-O pair bounded excitons, and green emission from N
isoelectronic trap bounded excitons, which greatly enhanced
the emission efficiency of GaP.
7.2 Radiative Recombination and Nonradiative Recombination
7.2.1 Radiative recombination of non-equilibrium carriers
6）Isoelectronic trap recombination
• Isoelectronic impurity: atom that in the same group of the
matrix atom in the periodic table, which has the same
number of valence electrons as the matrix atom. E.g., N is
the isoelectronic impurity for P in GaP.
• Isoelectronic trap: bound states induced by the
isoelectronic imurities.
• The substitution of matrix atoms with the isoelectronic
impurity will not offer more free electrons and holes, but
form neutral centers.
7.2 Radiative Recombination and Nonradiative Recombination
7.2.1 Radiative recombination of non-equilibrium carriers
6）Isoelectronic trap recombination
• Reasons for “traps” or “bound states” ?
The differences in the electronnegativity and
atomic radius between the isoelectronic impurity atom
and the matrix atoms will result in distortion of
lattice potential, which could bound carriers (electrons
and holes) and form charged centers.
7.2 Radiative Recombination and Nonradiative Recombination
7.2.1 Radiative recombination of non-equilibrium carriers
6）Isoelectronic trap recombination
• How to distinguish electron bound state and hole bound
states ?
 Electronnegativity of isoelectronic impurity>
electronnegativity of matrix atom  electron bound
states, isoelectronic electron trap, isoelectronic acceptor.
 Electronnegativity of isoelectronic impurity<
electronnegativity of matrix atom  hole bound states,
isoelectronic hole trap, isoelectronic donor.
 E.g., N substitutes P in GaP: electron bound states,
isoelectronic acceptor. Bi substitutes P in GaP: hole
bound states, isoelectronic donor.
7.2 Radiative Recombination and Nonradiative Recombination
7.2.1 Radiative recombination of non-equilibrium carriers
6）Isoelectronic trap recombination
• Light emission
Isoelectronic trap becomes charged center after capturing certain type
of carriers, and captures oppositely charged carriers due to Colomb
interaction.  bounded exciton to the isoelectronic impurity. The
recombination of exciton could emit light.
E.g., in GaP:N, atom N is the isoelectronic
electron trap, capturing electron first, and
then capturing hole  bound exciton. The
recombination of exciton gives green
emission. The photon energy nearly equals
the bandgap of GaP (2.22eV)。
h=2.22-0.01-0.037=2.17 eV
7.2 Radiative Recombination and Nonradiative Recombination
7.2.1 Radiative recombination of non-equilibrium carriers
6）Isoelectronic trap recombination
• Isoelectronic trap can effectively enhance the emission efficiency of
indirect bandgap semiconductors.
E.g., GaP is a indirect bandgap semiconductor, whose emission
efficiency is very low. The doping with N atoms can greatly enhance
its emission efficiency. Possible reasons:
 The formation of isoelectronic electron trap due to the substitution
of P in GaP by N;
 Due to the short range force, the bound electrons are confined in the
vicinity of the N atoms;
 Based on Heisenberg uncertainty relation, if the electron
wavefunction is localized in the real space, it will spread in a wide
range of the momentum space. x  p  / 2
7.2 Radiative Recombination and Nonradiative Recombination
7.2.1 Radiative recombination of non-equilibrium carriers
6）Isoelectronic trap recombination
 Higher probability density at wave vector
of the top of valence band, which greatly
enhance the direct transition probability
and the emission efficiency of GaP:N.


7.2 Radiative Recombination and Nonradiative Recombination
7.2.1 Radiative recombination of non-equilibrium carriers
6）Isoelectronic trap recombination
• The transition probability is still small in GaP:N,
compared to the direct transition.
• Besides, two or more N atoms can also form isoelectronic
traps, e.g., GaAs1-xPx:NN and GaAs1-xPx:NN3
7.2 Radiative Recombination and Nonradiative Recombination
7.2.1 Radiative recombination of nonequilibrium carriers
6）Isoelectronic trap recombination
V(x)
-a 0
V0
a
• Conditions for electron or hole bound states?
The potential at the isoelectronic impurity can be regarded
as a square potential well with depth of V0 and radius of a.
Calculations show, the condition for bound states is the
following,
2 2
V0 a 
2

*
8m
Isoelectronic traps are more probable for electrons with larger
effective mass. Generally, electrons in wide bandgap
materials have larger effiective mass, so isoelectronic traps are
usually present in wide bandgap materials.
x
7.2 Radiative Recombination and Nonradiative Recombination
7.2.1 Radiative recombination of non-equilibrium carriers
6）Isoelectronic trap recombination
• How to form isoelectronic traps that can strongly bind
carriers?
If there is a large difference in the radius of
isoelectronic impurity atom and the matrix atom, large
lattice distortion will lead to strongly bound states.
7.2 Radiative Recombination and Nonradiative Recombination
7.2.2 Non-radiative recombination
• Non-radiative recombination centers are also called
killer centers (消光中心), harmful to emissions.
• The impurities, lattice defects, and the combination of
defect and impurity can become non-radiative
recombination centers.
• The following types of non-radiative recombination
can be clearly explained: multi-phonon transitions,
Auger process, and surface recombination.
7.2 Radiative Recombination and Nonradiative Recombination
7.2.2 Non-radiative recombination
1）Multi-phonon transition
• The released energy of electron-hole recombination can excite more
phonons.
• The bandgap of fluorescent semiconductors are usually larger than 1
eV, while one phonon energy is about 0.06eV. If the released energy
of electron-hole recombination completely form phonons, the
probability of generating numerous phonons is very low.
• The crystal defects and impurities introduce discrete energy levels in
the forbidden band, in favor of electron transitions to these levels,
and subsequent generation of phonons.  multi-phonon transition.
• Multi-phonon transition is a multilevel process with a low probability.
7.2 Radiative Recombination and Nonradiative Recombination
7.2.2 Non-radiative recombination
2）Auger process
N-type
• Auger recombination: The released energy from
the recombination of an electron with a hole can
excite a third carrier to higher energy levels of
the conduction or valence band. The relaxation of
the third carrier is the multi-phonon transition in
the continuous states of the band.
• Auger process includes the interactions between
two electrons (or holes) and one hole (electron). So
it is prominent when electron (or hole) density is
very high. Therefore, the doping density of p-n
junction LED should not too high.
Band-band
Auger Process
P-type
7.2 Radiative Recombination and Nonradiative Recombination
7.2.2 Non-radiative recombination
2）Auger process
• Band-impurity level Auger
process: the recombination of
electron and hole in the
conduction/valence band and
the impurity levels.
• In heavily doped wide
bandgap materials, e.g., GaAs,
band-band or band-impurity
level Auger process is the
primary non-radiative
recombination process.
N-type
P-type
7.2 Radiative Recombination and Nonradiative Recombination
7.2.2 Non-radiative recombination
2）Auger process
• The impurity level Auger
process is similar to the D-A
pair recombination, however, the
released energy is transferred to
a free carrier, but not to a
photon.
• E.g, for the red emission in GaP:
Zn-O, if the acceptor density is
too high, the emission efficiency
will drop, due to the Auger
process.
7.2 Radiative Recombination and Nonradiative Recombination
7.2.2 Non-radiative recombination
3）Surface recombination
• Dangling bonds at the crystal surface can generate high
density of deep and shallow levels to serve as
recombination centers.
• Continuous distribution of surface levels: within the
diffusion length at the surface, the recombination of
electron and hole is the continuous transitions across the
continuous surface levels, in favor of non-radiative
recombination.
• Surface treatments and protections are very important in
improving the emission efficiency of LEDs.
7.3 Basic Structure and Principle of LED
 LED is a p-n junction under forward bias.
 Fabricated by using planar technology.
Basic structure:
7.3 Basic Structure and Principle of LED
 Operational principle: e.g., interband radiative recombination.
Forward bias  minority injection  recombination of
non-equilibrium carriers  photon emission.
 Injection electroluminescence (注入式电致发光): conversion of
electrical energy to photon energy.
P
N
P
N
No Bias
Forward Bias
P-side: recombination of injected electrons and the holes in the valence band.
N-side: recombination of injected holes and the electrons in the conduction band.
The emitted phonon energy is Eg
7.4 Charactristics of LED
7.4.1 I-V Characteristics
 Similar to that of an ordinary diode.
Low cur-in voltage (开启电压)~1-2 V
working current (工作电流)~10 mA.
 LED can be very small in size. Point light source, very
useful for optical display
7.4 Charactristics of LED
7.4.2 Quantum Efficiency
 Quantum efficiency reflects the efficiency of photon
generation through the recombination the injected carriers.
 External quantum efficiency: the ratio of the number of
output photons to the number of injected carriers per unit
time.
 Internal quantum efficiency: the ratio of the number of
generated photons due to recombination to the number of
injected carriers per unit time.
 Quantum efficiency is related to the injection efficiency,
radiation efficiency, and the escape efficiency.
7.4 Charactristics of LED
7.4.2 Quantum Efficiency
 Injection efficiency  —— the ratio of the current that can
generate radiative recombination to the total current.
 Band tail in heavily doped cases, ~1018 cm-3
For n-type, Eph is slightly larger than Eg, easy to be absorbed again.
While for p-type, Eph is slightly smaller than Eg, hard to be
absorbed again. So LEDs should be designed to realize p-side
emission.
7.4 Charactristics of LED
7.4.2 Quantum Efficiency
In: the injected electron current on the p-side.
In
Injection

Ip: the injected hole current on the n-side.
efficiency:
I n  I p  I R I : recombination current in the depletion region.
R
 Ways to improve the injection efficiency:
1) Donor density in the n-side > acceptor density in the p-side
N+P junction.
2) Reduce IR: the materials and fabrication procedures should
retain crystal perfection, avoiding doping with unwanted
impurities.
3) Materials selection: electron mobility> hole mobility, e.g.,
GaAs, electron mobility/hole mobility ~30.
7.4 Charactristics of LED
7.4.2 Quantum Efficiency
 Radiation efficiency r—— On the p-side, the ratio of the
number of recombined electrons to the total number of
injected electrons.
Ur
Ur: radiative recombination rate.
r 
U r  U nr Unr: non-radiative recombination rate.
 R1, R2 : radiative recombination,
Eph ~Eg.
 R3 : radiative or non-radiative
recombination, Eph <Eg.
7.4 Charactristics of LED
7.4.2 Quantum Efficiency
 For the case of competition between R1 and R3, one way to
improve the radiation efficiency is to increase the doping level
on the p-side.
 However, higher doping level  more crystal defects  more
non-radiative recombination centers.
 Higher doping level on the p-side
lower injection efficiency.
 For GaP LED, highest efficiency
doping level Na=2.51017 cm-3
Internal quantum efficiency r=r
7.4 Charactristics of LED
7.4.2 Quantum Efficiency
 Escape efficiency o —— The ratio of the number of
escaped photons to the total number of photons due to
radiative recombination.
External quantum efficiency e=io =ro
Interface reflection/
Absorption
total reflection
1) Small depth of junction. But surface recombination can
reduce i if the depth is less than one diffusion length.
 Ways to improve o:
2) Low absorption coefficient: emitted photon energy less
than Eg.
7.4 Charactristics of LED
7.4.2 Quantum Efficiency
3) Optical window.
Interface recombination center density is greatly reduced, so the
p-n junction could be very close to the interface of AlGaAs/GaAs
7.4 Charactristics of LED
7.4.2 Quantum Efficiency
3) Optical window.
Dome-like shape can eliminate the total reflectance.
Epoxy (n=1.5) is used to make the dome – reduce interface reflectance,
low cost.
7.5 Semiconductor Laser
7.5.1 Semiconductor
Laser and its structures

Based on the operation
materials, laser can be
classified into solid laser,
liquid laser, gas laser, and
semiconductor laser.

Compared to other lasers,
semiconductor laser has
small volume, high
efficiency, simple and
substantial structure, and
can be directly modulated.
7.5 Semiconductor Laser
7.5.2 Conditions for Stimulated Emission

Stimulated emission of laser: external stimulation induced
carrier recombination. Coherent light emission: same frequency,
phase, polarization, and direction. Superior monochromaticity,
directionality, and brightness, compared to LED.

The operation of semiconductor laser depends on carrier
injection. Three basic conditions for laser generation:
1) Distribution for population inversion: number of particles at
high energy level>>number of particles at low energy level.
2) Resonance cavity: feedback effect, multiplication of simulated
emission  laser ocillation.
3) Threshold condition: to realize photon multiplication >=
photon loss.
7.5 Semiconductor Laser
7.5.2 Conditions for Stimulated Emission
1） Distribution for population inversion
 Spontaneous emission V.S.
stimulated emission
In the process of
simulated emission, the
incident light is magnified
and the output light is
coherent.
Semiconductor laser
works based on
stimulated emission.
Initial State
Final State
Spontaneous Emission
Stimulated Emission
7.5 Semiconductor Laser
7.5.2 Conditions for Stimulated Emission
1） Distribution for population inversion
 In semiconductors, light absorption, spontaneous emission,
and stimulated emission coexist.
 Conditions for light gain (光增益):
stimulated emission rate > absorption rate 
the probability of occupying a level in the conduction band by
electrons > the probability of occupying a level in the valence
band by electrons (related to radiative transition) ——
distribution for population inversion.
7.5 Semiconductor Laser
7.5.2 Conditions for Stimulated Emission
1） Distribution for population inversion
 How to realize distribution for population inversion:
EFn  EFp  Eg
Quasi-Fermi levels enter the conduction
and valence bands.
 For injection semiconductor laser, to realize the above
condition, 1) semiconductor should be heavily doped, 2)
the applied voltage should satisfy,
qV  EFn  EFp  Eg
7.5 Semiconductor Laser
7.5.2 Conditions for Stimulated Emission
1） Distribution for population inversion
Active
area
Band diagram of heavily doped GaAs p-n junction laser
7.5 Semiconductor Laser
7.5.2 Conditions for Stimulated Emission
2）Optical resonance cavity
 Spontaneous emission in the
active area  a small amount
of photons can serve as the
excitation source for
simulated emission (coherent
light).
 Optical resonance cavity:
photons can reflect back and
forth between the two parallel
interfaces, and amplified.
 Emission are confined inside
the p-n junction plane.
7.5 Semiconductor Laser
7.5.2 Conditions for Stimulated Emission
3）Threshold condition for resonance
 Laser has end loss and internal loss.
 When light gain larger than loss after light propagating back and
forth once, laser is formed.
 The gain factor (增益系数) should
reach a certain value for laser formation.
1
1
g a
ln
2 L R1 R2
Gain factor
Absorption
coefficient
g  dI g /( Idx)
Reflection of
the two end
surfaces
L
7.5 Semiconductor Laser
7.5.2 Conditions for Stimulated Emission
4）Threshold current
 For GaAs junction laser,
forward current is applied
to provide light gain.
 Threshold current: only
when gain factor g
increases to a certain values,
laser could occure.
Outline
Chap. 1 Introduction
Chap. 2 Basics of Semiconductor Physics
Chap. 3 P-N Junctions
Chap. 4 Metal-Semiconductor Junctions
Chap. 5 Semiconductor Heterojunctions
Chap. 6 Semiconductor Solar Cells & Photodiodes
Chap. 7 Light Emitting Diodes & Semiconductor Lasers
Chap. 8 Quantum Dots for Biological Fluorescent Probes
Chapter 5
Quantum Dots for
Biological Fluorescent
Probes


Optical Properties of QDs
Quantum confinement effect (QCE)—
Discrete energy, widen bandgap
Bulk
QDs with
different
sizes
Single
molecule
The variation of bandgap of
PbS QDs with their sizes
ACS Nano 3, 3023（2009）

Optical Properties of QDs
 Blue shift of the absorption edges and PL peaks.
CdS
CdSe/ZnS

Quantum Dots for Biological
Fluorescent Probes

Introduction to biological fluorescent probes

QDs V.S. traditional organic dye fluorescent probes

Labeling and detection of biomacromolecules by QDs

Imaging of biological tissues and cells by QDs

NIR fluorescent QDs for biomedical imaging in living
tissue

Quantum Dots for Biological
Fluorescent Probes

Introduction to biological fluorescent probes

QDs V.S. traditional organic dye fluorescent probes

Labeling and detection of biomacromolecules by QDs

Imaging of biological tissues and cells by QDs

NIR fluorescent QDs for biomedical imaging in living
tissue

Introduction to biological fluorescent
probes

In bioscience, fluorescence spectroscopy (荧光光谱学)
provides the structures, functions and interactions of
biomacromolecules by studying the chromophores of
biomacromolecules or the fluorescent labels and
combining relevant fluorescent techniques.
Fluorescent probes have been widely used in AIDS
virus detection, human genome sequencing, protein
solution conformations, intracellular activity and so on.


Generally, organic dye fluorescent molecules are used
as probes, i.e., rhodamine.

Introduction to biological fluorescent
probes

QDs as biological fluorescent probes
Medintz et al., Nature Materials, 4, 435 (2005)

Quantum Dots for Biological
Fluorescent Probes

Introduction to biological fluorescent probes

QDs V.S. traditional organic dye fluorescent probes

Labeling and detection of biomacromolecules by QDs

Imaging of biological tissues and cells by QDs

NIR fluorescent QDs for biomedical imaging in living
tissue
Organic
fluorescent
dyes

Narrow PL
peaks, tunable
peak position.
Fluorescent
QDs
CdSe/ZnS QDs
Medintz et al., Nature Materials, 4, 435 (2005)

QDs have much higher fluorescence intensity
than Alexa dye
（Alexa dyes are reported to be brighter than any other known organic dyes.)
X. Wu et al., Nature Biotechnology, 21,41 (2003)
Fluorescence intensity comparison between Alexa and QDs

QDs are more photostable than Alexa dye
（Alexa dyes are more photostable than other organic dyes.)
X. Wu et al., Nature Biotechnology, 21,41 (2003)
Top row: Nuclear antigens labeled with QD (red), microtubules labeled
with Alexa 488 (green).
Top row: Nuclear antigens labeled with Alexa 488 (green), microtubules
labeled with QD (red).

Advantages of QDs as fluorescent probes
Fluorescent QDs
Organic fluorescent dyes
Excitation
spectrum
Wide and continuous
Narrow
Emission
spectrum
Symmetrically distributed,
narrow and tunable
emission peaks
Wide emission peaks
Photostability
High, hard to decompose
Easy photobleacing and
photodecomposition
(products are usually
harmful to biomolecules)
Linkage to
biomolecules
Facile and easy
Specific linkage methods
for each organic florescent
dyes.

Quantum Dots for Biological
Fluorescent Probes

Introduction to biological fluorescent probes

QDs V.S. traditional organic dye fluorescent probes

Labeling and detection of biomacromolecules by QDs

Imaging of biological tissues and cells by QDs

NIR fluorescent QDs for biomedical imaging in living
tissue

Labeling and detection of biological
macromolecules by QDs

The application of QDs in biology started from
labeling and detection of simple biological
macromolecules.

The superior fluorescent properties and
appropriate dimensions of QDs make them ideal
in studying the structures, functions and
interactions of biomacromolecules, by combining
other techniques, such as fluorescence spectrum,
fluorescence polarization, energy transfer, etc.

Conjugation of DNA to silanized colloidal QDs
CdSe
core
W. J. Parak et al., Chem Mater, 14, 2113 (2002)
The conjugation of single- or double-stranded DNA to QDs had
little effect on the optical properties of the nanocrystals.
 In fluorescence in situ
hybridization (荧光原位
杂交技术), QDs
conjugated to DNA are
used as fluorophore for
hybridizaiton of human
metaphase chromosomes.。
 In immunofluorescence (荧光免
疫), S. Wang et al. conjugated
red- and green-emitting CdTe
QDs to antigen (BSA) and
antibody (IgG), respectively.
The formation of BSA-IgG
immunocomplex resulted in a
resonance energy transfer
between the two QDs.
Green
Red
Nucl. Acids Res. 32, e28 (2004)
Nano Lett. 2, 817 (2002)

Quantum Dots for Biological
Fluorescent Probes

Introduction to biological fluorescent probes

QDs V.S. traditional organic dye fluorescent probes

Labeling and detection of biomacromolecules by QDs

Imaging of biological tissues and cells by QDs

NIR fluorescent QDs for biomedical imaging in living
tissue

Imaging of biological tissues and cells
by QDs
 Cellular labelling and imaging for the observation
of cellular activity .
 Multicolor labelling with QDs for the observation
of live cell surface and the intracellular activities
and mobility, and the study of the realization of
biological functions and their interactions.

Imaging of yeast cells
As one of the natural polysaccharids, chitosan (壳聚糖)
possesses many important biological properties, such as
biocompatibility, biodegradability, and bioactivity, and thus has
found a wide application in the field of biological medicine.
CdSe/ZnS
core/shell QDs
carboxymethyl
chitosan
羧甲基壳聚糖
M. Xie et al., Chem Commun., 44, 5518 (2005)

Imaging of yeast cells
M. Xie et al., Chem Commun., 44, 5518 (2005)
CdSe/ZnS-labeled carboxymethyl chitosan as bioprobes for
live yeast cell imgaing

Imaging of organelles in one cell
H. J. Tanke et al., Curr Opin Biotech, 16, 49 (2005)
Green, microtublules; orange, Golgi; red, nuclei.
Single excitation for three different emissions.

Imaging of organelles in one cell
Medintz et al., Nature Materials, 4, 435 (2005)
Pseudocoloured image depicting five-colour QD staining of fixed human
epithelial cells. Cyan, 655-nm Qdots labelling the nucleus; magenta, 605-Qdots
labelling Ki-67 protein; orange, 525-Qdots labelling mitochondria; green, 565Qdots labelling microtubules; red, 705-Qdots labelling actin filaments.

The structure of copolymer ligands
determines the distribution of coated QDs
t=0
t=0
t=1~2h
t=1~2h
H. W. Duan et al., JACS, 129, 3333 (2007)
PEI-g-PEG4 -coated
QDs are trapped in
organelles.
PEI-g-PEG2-coated
QDs have escaped
from the endosomes
and are released into
the cytoplasm.
PEG:聚乙二醇
PEI:聚乙二胺

Quantum Dots for Biological
Fluorescent Probes

Introduction to biological fluorescent probes

QDs V.S. traditional organic dye fluorescent probes

Labeling and detection of biomacromolecules by QDs

Imaging of biological tissues and cells by QDs

NIR fluorescent QDs for biomedical imaging in living
tissue

NIR fluorescent QDs for biomedical
imaging in living tissue
• Biomedical imaging in living tissue requires low absorption
and scattering of the incident excitation light and the
emission from the QDs.
• Visible lights can penetrate tissue with a largest thickness of
mm, while for NIR light, the penetration depth reaches cm.
• NIR QDs with emissions at 700-900 nm permit imaging of
living tissue and thus allow investigations on the internal
situation of tissue, in favor of disease diagnosis.
 NIR QDs provide
the surgeon with
direct visual
guidance
throughout the
entire SLN (前哨淋
巴结) mapping
procedure, minimize
incision and
dissection
inaccuracies, and
permit real-time
confirmation of
complete resection.
SLN mapping has
already
revolutionized
cancer surgery.
Kim et al., Nat Biotechnol, 22, 93 (2004)