1. Semiconductor Quantum Dots:
CdSe, ZnSe, ZnS, ZnO
Group’ members:
Trần Phúc Thành
Cao Văn Phước
Hoàng Văn Tiến

2. Outline
• Introduction
– What is semiconductor quantum dots
– Why quantum dots
– Properties
• Synthesis
• Applications, challenges, and potentials
• Conclusions

3. Introduction:
Image courtesy of Dr. D. Talapin, University of Hamburg

4. What is Quantumdots?
• Quantum dots are semiconductor
nanocrystals.
• They are made of many of the same
materials as ordinary semiconductors (mainly
combinations of transition metals and/or
metalloids).
• Unlike ordinary bulk semiconductors, which
are generally macroscopic objects, quantum
dots are extremely small, on the order of a few
nanometers. They are very nearly zero-
dimensional.

5. Exciton Bohr Diameter
• Material Dependent Parameter
– The same size dot of different materials may not both be
quantum dots
• The Bohr Diameter determines the type of
confinement
– 3-10 time Bohr Diameter: Weak Confinement
– Smaller than 3 Bohr Diameter: Strong Confinement

6. Experimental Observation of
Confinement
• Just imaging a small dot is not enough to say it is
confined
• Optical data allows insight into confinement
– Optical Absorption
– Raman Vibration Spectroscopy
– Photoluminescence Spectroscopy

7. Optical Absorption
• Optical Absorption is a
technique that allows one to
directly probe the band gap
• The band gap edge of a
material should be blue
shifted if the material is
confined
• Bukowski et al. present the
optical absorption of Ge
quantum dots in a SiO2
matrix.
• As the dot decreases in size
there is a systematic shift of
the band gap edge toward
shorter wavelengths

8. Raman Vibrational Spectroscopy
• Raman vibrational
spectroscopy probes the
vibrational modes of a
sample using a laser
• As the nanocrystal becomes
more confined the peak will
broaden and shrink
• Here we see a peak shift
toward the laser line
• Various Ge dots of different
sizes on an Alumina film

9. Direction of Raman Shift
• Here we see the same
broadening and shrinking of
the Raman Peak
• We see a peak shift away
from the laser line
• No systematic shift of the
Raman line
– Shifts toward the laser line
are due to confinement
– Shifts away from the line
are due to lattice tension
due to film miss-match

10. Photoluminescence Spectroscopy
• Photoluminescence
spectroscopy is a
technique to probe the
quantum levels of
quantum dots
• Here we see dots of
various size in a
quantum well
– (a) is quantum well
spectrum
– (d) is smallest particles
80 nm

11. Properties of Quantum Dots Compared to
Organic Fluorphores?
High quantum yield; often 20 times brighter
 Narrower and more symmetric emission spectra
 100-1000 times more stable to photobleaching
 High resistance to photo-/chemical degradation
Tunable wave length range 400-4000 nm
CdSe
CdTe
http://www.sussex.ac.uk/Users/kaf18/QDSpectra.jpg
J. Am. Chem. Soc. 2001, 123, 183-184

12. Why

13. Excitation in a Semiconductor
The excitation of an electron from the valance band
to the conduction band creates an electron hole pair
E
ECB hυ E
=g
−
( ) + )
h↔ BhV
υ e C+ ( B
Creation of an electron hole pair
where hν is the photon energy EVB
semiconductor Band Gap
optical
detector (energy barrier)
E=hυ
exciton: bound electron and hole pair
usually associated with an electron trapped in a
localized state in the band gap

14. Recombination of Electron Hole Pairs
Recombination can happen two ways:
radiative and non-radiative
E
ECB
recombination processes
EVB
E
band-to-band recom bination
recom bination atinterband trap states
ECB (e.g. dopants, impurities)
E = hυ
radiative recombination → photon
EVB
non-radiative recombination → phonon (lattice
vibrations)
−
) + ) υ
ra d ia tive n o n -ra d ia tive
re co m b in a tio n re c o m b in a tio n e C+ ( B h
( BhV→

15. Effective Mass Model
Developed in 1985 By Louis Brus
Relates the band gap to particle size of a spherical
quantum dot
Band gap of spherical particles
The average particle size in suspension can be obtained from the absorption
onset using the effective mass model where the band gap E* (in eV) can be
approximated by:
−
π .e 1 
h2 1 1 1 03 1 1
2
8 .42e 
1
E + + 
= − −  + 
*b
u
l
k
E 2 m hπ h 2 ε 0 h
m ε ( 0m m
)
m ε 4 2e m
g
 0
2 e
e
r  0
m 04 r π
ε m0 
Egbulk - bulk band gap (eV), h - Plank’s constant (h=6.626x10-34 J·s)
r - particle radius e - charge on the electron (1.602x10-19 C)
me - electron effective mass ε - relative permittivity
mh - hole effective mass ε0 - permittivity of free space (8.854 x10-14 F cm-1)
m0 - free electron mass (9.110x10-31 kg)
Brus, L. E. J. Phys. Chem. 1986, 90, 2555

16. Term 1
The second term on the rhs is consistent with the particle in a
box quantum confinement model
Adds the quantum localization energy of effective mass me
High Electron confinement due to small size alters the effective
mass of an electron compared to a bulk material
Consider a particle of mass m confined
P oten tia l E nergy
in a potential well of length L. n = 1, 2, …
For a 3D box: n2 = nx2 + ny2 + nz2
2
π
n h2 2 n h
22
E=
n 2
= x
2 L 8 L
m m2 0 L
−
h 1 1  1e
. 2 0 2e  1 1 
2 1
8 . 44
1
E = g lk+ 2
*
Eu
b
+ − −  + 
r m 0 m 0 4 ε 0 h ( ε 0 2 e 0 m 0
8  em hm πεr 2 2ε ) m m hm
Brus, L. E. J. Phys. Chem. 1986, 90, 2555

17. Term 2
 The Coulombic attraction between electrons and holes lowers
the energy
Accounts for the interaction of a positive hole me+ and a negative
electron me-
Electrostatic force (N) between two charges (Coulomb’s Law):
qq
F= 1 2 2 Work, w = ∫F·dr
εε
4π 0r
Consider an electron (q=e-) and a hole (q=e+)
The decrease in energy on bringing a positive r
charge to distance r from a negative charge is:
2 2
e e
∆ =∫
E d=
r −
4 ε0
πε r2 4 ε0
πε r
−
h 1 1  1e
. 2 0 2e  1 1 
2 1
8 . 44
1
E = g l + 2
*
Euk
b
+ − −  + 
r m 0 m 0 4 ε 0 h ( ε 0 2 e 0 m 0
8  em hm πεr 2 2ε ) m m hm
Brus, L. E. J. Phys. Chem. 1986, 90, 2555

18. Term Influences
The last term is negligibly small
Term one, as expected, dominates as the radius is decreased
Energy (eV )
Modulus 1 term 1
term 2
term 3
0
0 5 10
d (nm)
Conclusion: Control over the
particle’s fluorescence is possible
by adjusting the radius of the
particle

19. Quantum Confinement of ZnO
ZnO has small effective masses  quantum effects can be
observed for relatively large particle sizes
Confinement effects are observed for particle sizes <~8 nm
ZnO CdSe
ZnO λbulk = 365 nm λbulk = 709 nm
ε = 8.66 ε = 10.6
Eg (eV)
4
me* = 0.24 me* = 0.13
mh* = 0.59 mh* = 0.45
3
400
λ onset (nm)
350
300
250
0 5 10
d (nm)

20. Fabrication Methods
• Goal: to engineer potential energy barriers to confine
electrons in 3 dimensions
• 3 primary methods
– Lithography
– Colloidal chemistry
– Epitaxy

21. 2. Fabrication of Quantum Dots
How to Make Quantum Dots
• There are three main ways to confine excitons in
semiconductors:
– Lithography
– Colloidal synthesis
– Epitaxy:
» Patterned Growth
» Self-Organized Growth
21

22. 2. Fabrication of Quantum Dots
Lithography
• Quantum wells are covered with a polymer mask and
exposed to an electron or ion beam.
• The surface is covered with a thin layer of metal, then
cleaned and only the exposed areas keep the metal layer.
• Pillars are etched into the entire surface.
• Multiple layers are applied this
way to build up the properties
and size wanted.
• Disadvantages: slow,
contamination, low density,
defect formation.
Resulting quantum dot etched in GaAs/AlGaAs superlattice.
22
L. Jacak, P. Hawrylak, A. Wojs. Quantum dots fig. 2.2.

23. 2. Fabrication of Quantum Dots
Colloidal Synthesis
• Immersion of semiconductor microcrystals in glass
dielectric matrices.
• Taking a silicate glass with 1% semiconducting phase
(CdS, CuCl, CdSe, or CuBr).
• Heating for several hours at high temperature.
⇒ Formation of microcrystals of nearly equal size.
• Typically group II-VI materials (e.g. CdS, CdSe)
• Size variations (“size dispersion”).
23

24. 2. Fabrication of Quantum Dots
Epitaxy: Patterned Growth
• Semiconducting compounds with a smaller bandgap
(GaAs) are grown on the surface of a compoundwith a
larger bandgap (AlGaAs).
• Growth is restricted by coating it with a masking
compound (SiO2) and etching that mask with the shape
of the required crystal cell wall shape.
• Disadvantage: density of
quantum dots limited by
mask pattern. L. Jacak,
P. Hawrylak, A.
Wojs. Quantum
dots
24 fig 2.7.

25. 2. Fabrication of Quantum Dots
Epitaxy: Self-Organized Growth
• Uses a large difference in the lattice constants of the
substrate and the crystallizing material.
• When the crystallized layer is thicker than the critical
thickness, there is a strong strain on the layers.
• The breakdown results in randomly distributed islets of
regular shape and size.
• Disadvantages: size and
shape fluctuations, ordering.
Schematic drawing of lens-shaped self-
organized quantum dot.
L. Jacak,
P. Hawrylak,
A. Wojs. Quantum
dots
25 fig 8.1.

26. Lithography
• Etch pillars in quantum well heterostructures
– Quantum well heterostructures give 1D confinement
• Mismatch of bandgaps ⇒ potential energy well
– Pillars provide confinement in the other 2 dimensions
• Electron beam lithography
• Disadvantages: Slow, contamination, low density, defect formation
A. Scherer and H.G. Craighead. Fabrication of small laterally patterned multiple quantum wells. Appl. Phys. Lett., Nov 1986.

27. Colloidal Particles
• Engineer reactions to precipitate quantum dots from solutions or a
host material (e.g. polymer)
• In some cases, need to “cap” the surface so the dot remains
chemically stable (i.e. bond other molecules on the surface)
• Can form “core-shell” structures
• Typically group II-VI materials (e.g. CdS, CdSe)
• Size variations ( “size dispersion”)
CdSe core with ZnS shell QDs
Red: bigger dots!
Blue: smaller dots!
Evident Technologies: http://www.evidenttech.com/products/core_shell_evidots/overview.php
Sample papers: Steigerwald et al. Surface derivation and isolation of semiconductor cluster molecules. J. Am. Chem. Soc., 1988.

28. Epitaxy: Patterned Growth
• Growth on patterned
substrates
– Grow QDs in pyramid-shaped
recesses
– Recesses formed by selective
ion etching
– Disadvantage: density of QDs
limited by mask pattern
T. Fukui et al. GaAs tetrahedral quantum dot structures fabricated using selective area metal
organic chemical vapor deposition. Appl. Phys. Lett. May, 1991

29. Epitaxy: Self-Organized Growth
• Self-organized QDs through epitaxial growth strains
– Stranski-Krastanov growth mode (use MBE, MOCVD)
• Islands formed on wetting layer due to lattice mismatch (size ~10s nm)
– Disadvantage: size and shape fluctuations, ordering
– Control island initiation
• Induce local strain, grow on dislocation, vary growth conditions, combine
with patterning
AFM images of islands
epitaxiall grown on GaAs
substrate.
(a) InAs islands randomly
nucleate.
(b) Random distribution of
InxGa1­xAs ring­shaped
islands.
(c) A 2D lattice of InAs islands on
a GaAs substrate.
P. Petroff, A. Lorke, and A. Imamoglu. Epitaxially self-assembled quantum dots. Physics Today, May 2001.

30. APPLICATIONS

31. Quantum computing
A quantum computer is a computer design which uses the principles of quantum physi
to increase the computational power beyond what is attainable by a traditional compute

32. The tiny seven-atom quantum dot created by scientists (left), with a close-up of
that dot forming an atomic-scale transistor (right)
 essential for the creation of semiconductor­based quantum computers,
which could be faster and provide more memory than conventional
technology.

33. Rules:
-By applying small voltages to the leads, the flow of electrons through the
quantum dot can be controlled and thereby precise measurements of the
spin and other properties therein can be made.
-With several entangled quantum dots, or qubits , plus a way of performing
operations, quantum calculations and the computers that would perform
them might be possible.
Bits :a bit can be defined as a variable or computed quantity that can have
only two possible values : 0/1 or on/off…
Qubits: -quantum mechanics allows the qubit to be in a superposition of
both states at the same time, a property which is fundamental to quantum
computing.
- a qubit can be 0, 1, or a superposition of both.

34. Quantum-dot light-emmiting
diodes ( QDLED)

35. -The spectrum of photon emission is narrow, characterized by its full
width at half the maximum value
-pure and saturated emission colors with narrow bandwidth
- emission wavelength is easily tuned by changing the size of the
quantum dots

36. • Advantages :
– Low power consumption
– Ranger and accuracy color
– Brightness: 50~100 times brighter than CRT and LCD
displays ~40,000 cd/m2
– QLED screens are said to be twice as power efficient as
OLED screens, and offer 30 to 40% improved brightness.
• Disadvantages :
– Less saturated blue
– expensive

37. Quantum dot solar cell
• Quantum dots have bandgaps that are tunable across a
wide range of energy levels by changing the quantum
dot size.

38. 5–20 layer QD-enhanced solar cells show a net increase in external quantum
efficiency (EQE) at IR wavelengths compared to the baseline GaAs

39. How Can Quantum Dots Improve
the Efficiency?
• Quantum dots can generate multiple exciton (electron-
hole pairs) after collision with one photon.
39
39

40. Spin­cast quantum dot solar cell roll-to-roll solar cell fabrication.
• Results:
– -conversion efficiencies up to a record-breaking 7 percent efficiency.
– The efficiency of solar cells could be increased to more than 60%
from the current limit of just 30%...

41. Another applications
• Photodetector devices
• Quantum dots laser
• Colloidal quantum dots
• Uses in biology:use of QDs as passive labels in sensor
applications, gene technology, pathogen and toxin
detection…

42. Challenges and potentials
• Potentials :
– The composition and very small size of quantum dots (5–8
nm) gives them unique and very stable fluorescent optical
properties that are readily tunable by changing their physical
composition or size.
– Their broad absorption spectra but very narrow emission
spectra allows multiplexing of many quantum dots of
different colors in the same sample.
– quantum dots can be made from many different elements,
and come in many different shapes and sizes,
• Challenges :
– Hard to control the side of dots.
– Cytotoxic Effect
– Environmental toxicity:
• Phytotoxicity
• Marine toxicity

43. conclusion
• Quantum dot are the new and inovative perspective on the
traditional semiconductor
• Quantum dot can be synthesized to be essentially any size,
and therefore produce essentially any wavelength of light
• Application of Quantum Dots that they are useful in many
conditions due to their physiochemical properties Quantum
Dots may used in different fields by altering their surface
property, internal structures, preparation techniques, coating
material etc.
• The future looks bright and exciting on all the possible
applications of quantum dots

44. references
1, http://en.wikipedia.org/wiki/Quantum_dot
2. http://nature.com
3. http://google.com
4. Y. Masumoto and T. Takagahara. Semiconductor Quantum Dots: Physics,
Spectroscopy, and Applications. New York: Springer-Verlag, 2002.
5. Synthesis and Applications of Nanoparticles and
Quantum Dots-Richard D. Tilley
6. http://www.nanowerk.com
7. http://led-professional.com
8. http://quantumdotstech.com/
9. http://youtube.com
………