Download Terahertz Spectroscopy of CdSe Quantum Dots

What is a quantum dot?
• Nanocrystals
• 2-10 nm diameter
• semiconductors
What is a quantum dot?
• Exciton Bohr Radius
• Discrete electron
energy levels
• Quantum
confinement
Motivation
•
Semiconducting nanocrystals
are significant due to;
strong size dependent
optical properties
(quantum confinement)
•
applications solar cells
Terahertz gap
1 THz = 300 µm = 33 cm-1 = 4.1 meV
Time domain terahertz Spectrometer
The pulse width = ΔtFWHM/√2 = 17.6±0.5 fs
(A Gaussian pulse is assumed)
Terahertz Signal
To obtain the response of the
sample to the THz radiation 2
measurements are made
Fourier Transform
•THz electric field transmitted
through the empty cell
•THz electric field transmitted
through the sample cell
Terahertz signal
Doping
• Intentionally adding impurities to change
electrical and optical properties
• Add free electrons to conduction band
or free holes in valence band
• Tin and Indium dopants
Free carrier Absorption in Quantum Dots
Purification and sample
preparation of quantum dots
Experimental procedure & Data analysis
time domain:
frequency domain:
~
E (t )
E ( )
 ~
 T ( ) exp( i ( ))
E0 (t ) E0 ( )
Power transmittance
√T(ω), Φ(ω)
Relative phase
Complex refractive index (nr(ω) + i.nim(ω))
No Kramer-Kronig analysis!!!
Changes upon charging large quantum dot:
Intrinsic Imaginary Dielectric constant
The frequency dependent complex dielectric constants determined
by experimentally obtained
• Frequency dependent absorbance and refractive index.
The complex dielectric constant = (nr(î) + ini(î))2
8
2.0
3.2 nm uncharged
3.2 nm charged
6
1.0
im()
im()
1.5
6.3 nm uncharged
6.3 nm charged
0.5
4
2
0.0
-0.5
0
2
3
4
5
6
Frequency (THz)
7
2
3
4
5
6
Frequency (THz)
•For the charged samples Frohlich Band diminishes: A broader and
weaker band appears
•The reason of this is the presence of coupled plasmon-phonon modes
Nano Lett., Vol. 7, No. 8, 2007
7
Results
2.0
1.5
Absorbance
• Surface
phonon
• Shift of
resonance of
tin doped
• Agreement
with charged
QDs
indium doped
undoped
tin doped
1.0
0.5
0.0
1
2
3
4
5
THz
6
7
8
Results
Ref index
1.34
Un
Sn
In
1.32
1.30
1.28
1.26
2
3
4
5
THz
6
7
8
Semiconductor Quantum Dots
Justin Galloway
2-26-07
Department of Materials Science & Engineering
Outline
I.
Introduction
II.
Effective Mass Model
III.
Reaction Techniques
IV.
Applications
V.
Conclusion
How
Quantum Dots
Semiconductor nanoparticles that exhibit
quantum confinement (typically less than 10 nm in
diameter)
Nanoparticle: a microscopic particle of an
inorganic material (e.g. CdSe) or organic material
(e.g. polymer, virus) with a diameter less than 100
nm
More generally, a particle with diameter less than
1000 nm
1. Gaponenko. Optical properties of
semiconductor nanocrystals
2. www.dictionary.com
Properties
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
Excitation
Excitation in a Semiconductor
The excitation of an electron from the valance band
to the conduction band creates an electron hole pair
E
ECB
h=E g
h  e (CB)  h (VB)
Creation of an electron hole pair
where h is the photon energy
EVB

optical
detector
Band Gap
(energy barrier)
semiconductor
E=h
exciton: bound electron and hole pair
usually associated with an electron trapped in a
localized state in the band gap
Release
Recombination of Electron Hole Pairs
Recombination can happen two ways:
radiative and non-radiative
E
ECB
recombination processes
EVB
E
ECB
E=h
EVB
radiative
recombination
non-radiative
recombination
band-to-band
recombination
recombination
atinterband trap s tates
(e.g. dopants, impurities)
radiative recombination  photon
non-radiative recombination  phonon (lattice
vibrations)
e (CB)  h (VB)  h
Model
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:
 2  1
1  1.8e
*
bulk


E  Eg 
2 m m  m m  4 r 
2er  e 0
h 0 
0
2
Egbulk - bulk band gap (eV),
r - particle radius
me - electron effective mass
mh - hole effective mass
cm-1)
m0 - free electron mass (9.110x10-31
1
 1
1 


2 m m  m m 
2
4 0   e 0
h 0 
0.124e 3
h - Plank’s constant (h=6.626x10-34 J·s)
e - charge on the electron (1.602x10-19 C)
 - relative permittivity
0 - permittivity of free space (8.854 x10-14 F
kg)
Brus, L. E. J. Phys. Chem. 1986, 90, 2555
Model
Term 2
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
Consider a particle of mass m confined
in a potential well of length L. n = 1, 2, …
For a 3D box: n2 = nx2 + ny2 + nz2
n2 2 2 n2h2
En 

2
2mL
8mL2
Pote ntia l Ene rgy
mass of an electron compared to a bulk material
•
0
L
x
1
2 
2
4 


h
1
1
1.8e
0.124e
1
1
E*  E gbulk  2 





2 m m
2
m
m
m
m
4
r
8r  e 0
h 20   e 0 mh m0 
h 0 
0
Brus, L. E. J. Phys. Chem. 1986, 90, 2555
Model
Term 3
 The Coulombic attraction between electrons and holes lowers
the energy
Accounts for the interaction of a positive hole me+ and a negative
electron meElectrostatic force (N) between two charges (Coulomb’s Law):
qq
F 1 2 2
Work, w = F·dr
40r
Consider an electron (q=e-) and a hole (q=e+)
The decrease in energy on bringing a positive
charge to distance
 r from a negative charge is:
r
e2
e2
E  
dr  
2
40r
40r
1
2 
2
4 


h
1
1
1.8e
0.124e
1
1
E*  E gbulk  2 





2 m m
2
m
m
m
m
4
r
8r  e 0
h 20   e 0 mh m0 
h 0 
0
Brus, L. E. J. Phys. Chem. 1986, 90, 2555
The last term is negligibly small
Term one, as expected, dominates as the radius is decreased
Energy (eV)
Modulus
Model
Term Influences
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
Model
Quantum Confinement of ZnO & TiO2
ZnO has small effective masses  quantum effects can be
observed for relatively large particle sizes
Confinement effects are observed for particle sizes <~8 nm
TiO2 has large effective masses  quantum effects are nearly
unobservable
4
TiO2
Eg (eV)
Eg (eV)
ZnO
4
3
400
 on set (nm)
 on set (nm)
3
400
350
300
350
300
250
250
0
5
d (nm)
10
0
5
d (nm)
10
The
Making
Formation of Nanoparticles
Varying methods for the synthesis of
nanoparticles
Synthesis technique is a function of
the material, desired size, quantity and
quality of dispersion
Synthesis Techniques
• Vapor phase (molecular beams, flame synthesis etc…
• Solution phase synthesis
Semiconductor Nanoparticles
•Aqueous Solution
II-VI: CdS, CdSe, PbS, ZnS
•Nonaqueous Solution
III-V: InP, InAs
MO: TiO2, ZnO, Fe2O3, PbO, Y2O3
Semiconductor Nanoparticles Synthesis:
Typically occurs by the rapid reduction
of organmetallic precusors in hot
organics with surfactants
some examples of in vitro imaging with
QDs (http://www.evidenttech.com/)
The
Nucleation and Growth
Making
Figure 1. (A) Cartoon depicting the stages of nucleation and growth for the preparation of monodisperse NCs
in the framework of the La Mer model. As NCs grow with time, a size series of NCs may be isolated by
periodically removing aliquots from the reaction vessel. (B) Representation of the simple synthetic apparatus
employed in the preparation of monodisperse NC samples.
Horizontal dashed lines represent the critical concentration for nucleation and the saturation concentration
C. B. Murray, C. R. Kagan, and M. G. Bawendi, Annu. Rev. Mater. Sci. 30, 545, 2000.
The
Making
Capping Quantum Dots
Due to the extremely high surface area of a
nanoparticle there is a high quantity of “dangling bonds”
Adding a capping agent consisting of a higher band
gap energy semiconductor (or smaller) can eliminate
dangling bonds and drastically increase Quantum Yield
With the addition of
CdS/ZnS the Quantum
Yield can be increased
from ~5% to 55%
Synthesis typically consisted
of lower concentrated of
precursors injected at lower
temperatures at slow speeds
Shinae, J. Nanotechnology. 2006, 17, 3892
The
Making
560000x
Quantum Dot Images
Quantum dot images prepared in the Searson Lab using
CdO and TOPSe with a rapid injection
770000x
455000x
Application
QD’s
Quantum Dot Ligands Provide new Insight
into erbB/HER receptor – Mediated Signal
Transduction
Used biotinylated EGF bound to commercial quantum
dots
Studied in vitro microscopy the binding of EGF to erbB1
and erbB1 interacts with erbB2 and erbB3
Conclude that QD-ligands are a vital reagent for in vivo
studies of signaling pathways – Discovered a novel
retrograde transport mechanism
Dynamics of endosomal fusion
A431 cell
expressing
erbB3-mCitrine
Nat. Biotechnol. 2004, 22; 198-203
Application
Multiplexed Toxin Analysis Using Four Colors
of Quantum Dot Fluororeagents
Demonstrated multiplexed assays for toxins in the same
well
QD’s
Four analyte detection was shown at 1000 and 30 ng/mL
for each toxin
At high concentrations all four toxins can be deciphered
and at low concentrations 3 of the 4
Fluoresence data for all 4 toxin
assays at high concentrations
Cartoon of
assay
Anal. Chem. 2004, 76; 684-688
Application
QD’s
Quantum Dot Imaging
QDs with antibodies to human prostate-specific
membrane antigen indicate murine tumors
developed from human prostate cells
15 nm CdSe/ZnS TOPO/Polymer/PEG/target
Gao et al., “In vivo cancer targeting and imaging with semiconductor quantum dots,” Nat. Biotechnol. 22, 969 (2004).
Biological
Particles
Magnetic Nanoparticles
Nano-sized magnetic particles can be
superparamagnetic
 Widely Studied – Suggested as early as the 1970’s
 Offers control/manipulation in magnetic field
 Co has higher magnetization
compared to magnetite and
maghemite
Science 291, 2001; 2115-2117.
J. Phys. D: Appl. Phys. 36, 2003; 167-181.
 An Attractive Biological Tool
Application
Magnetic Nanoparticles: Inner Ear Targeted
Molecule Delivery and Middle Ear Implant
Magnetic
Particles
SNP controlled by magnets while transporting a
payload
Studies included in vitro and in vivo on rats, guinea pigs
and human cadavers
Demonstrated magnetic gradients can enhance drug
delivery
Perilymphatic fluid from the cochlea
of magnet-exposed temporal bone
Perilymphatic fluid samples
from animals exposed to magnetic forces
Audiol Neurotol 2006; 11: 123-133
Magnetic
Quantum
Dot
Composite with A Novel Structure for Active Sensing in Living cells
① Cobalt core : active manipulation
diameter : ~10 nm
What
is
MQD ?
Co
superparamagnetic NPs
CdSe
→ manipulated or positioned by an
external field without aggregation in
the absence of an external field
ZnS
Silica
② CdSe shell : imaging with fluorescence
thickness : 3-5 nm
visible fluorescence (~450 – 700 nm)
④ Silica shell : bio-compatibility &
functionalization with specific
targeting group
thickness : ~10 nm
bio-compatible,
& non-toxic to live cell functions
ability to tune the band gap
→ by controlling the thickness, able to
tune the emission wavelength, i.e.,
emission color
③ ZnS shell : electrical passivation
thickness : 1-2 nm
stable in aqueous environment
having wider band gap (3.83 eV)
than CdSe (1.91 eV)
ability to functionalize its surface
enhancement of QY
with specific targeting group
→ CdSe (5-10%)  CdSe/ZnS (~50%)
Rap-Up
Conclusions
The effective mass model give an excellent
approximation of the size dependence of
electronic properties of a quantum dot
Recent synthesis advances have shown many
quantum dot reactions to be robust, cheap, and
safe then previously thought
Quantum dots offer wide range electronic
properties that make them an attractive tool for
biological and medical work
MQD’s improve afford in vivo manipulation
expanded the applicability of quantum dots
From an Atom to a Solid
3d
4s
Photoemission spectra of negative copper
clusters versus number of atoms in the
cluster. The highest energy peak corresponds to the lowest unoccupied energy
level of neutral Cu.
Typically, there are two regimes:
1) For < 102 atoms per cluster, the energy
levels change rapidly when adding a single
atom (e. g. due to spin pairing).
2) For > 102 atoms per cluster, the energy
levels change continuously (e. g. due to
the electric charging energy (next slide).
Energy below the Vacuum level (eV)
Energy Levels of Cu Clusters vs. Cluster Radius R
Solid
Atom
ΔE = (E- ER)  1/R (charged sphere)
The Band Gap of Silicon Nanoclusters
GaAs
Bulk Silicon
3 nm : Gap begins to change
The Band Gap of Silicon Nanoclusters
3 nm : Gap begins to change
Increase of the Band Gap in Small Nanoclusters
by Quantum Confinement
Conduction
Band
k2
Valence
Band
k1
Gap
Size Dependent Band Gap in CdSe Nanocrystals
The Band Gap
of CdSe
Size:
Nanocrystals
Photon Energy vs. Wavelength:
h (eV) = 1240 /  (nm)

Beating the size distribution of quantum dots
Quantum dots formed by thin spots in GaAs layers
Termination of nanocrystals
Critical for their electronic behavior
H-terminated Si nanocrystal:
Electrons stay inside,
passivation, long lifetime
Oxyen atom at the surface:
Electrons drawn to the oxygen
Fluorine at the surface:
Complex behavior
From Giulia Galli’s group
Single Electron Transistor
e-
edot
A single electron etunnels in two steps
from source to drain
through the dot.
The dot replaces the
channel of a normal
transistor (below).
electrons
Designs for
Single Electron
Transistors
Large (≈ m)
for operation
at liquid He
temperature
Small (10 nm)
for operation
around room
temperature
Nanoparticle attracted
electrostatically to the
gap between source
and drain electrodes.
The gate is underneath.
Quantum Dots as Artificial Atoms in Two Dimensions
*
* The elements of this Periodic Table are named after team members from NTT and Delft.
Filling electron shells in 2D
Magnetic Clusters
“Ferric Wheel”
Magnetic Nanoclusters in Biology
The Holy Grail of Catalysis: Reactions at a Specific Nanoparticle
Want this image chemically resolved.
Have chemical resolution in microspectroscopy via X-ray absorption
but insufficient spatial resolution.
Fischer-Tropsch
process converts
coal to fuel using
an iron catalyst.
Di and
Schlögl
De Smit et al.,
Nature (2008)
The Oxygen Evolving Complex
4 Mn + 1 Ca
Instead of rare metals with 5d or 4d electrons, such as Pt, Rh, Ru,
one finds plentiful 3d transition metals in bio - catalysts: Mn, Fe .
Nature does it by necessity. Can we do that in artificial photosynthesis ?
Biocatalysts = Enzymes
Most biocatalysts consist of a protein with a small metal cluster at the active site.
The active Fe6Mo center of nitrogenase,
Nature’s efficient way of fixing nitrogen.