1. What is a quantum dot?
Nanocrystals
2-10 nm diameter
semiconductors

2. What is a quantum dot? Exciton Bohr Radius
Discrete electron energy levels
Quantum confinement

3. Motivation Semiconducting nanocrystals are significant due to;
strong size dependent optical properties (quantum confinement)
applications solar cells

4. Terahertz gap
1 THz
= 300 µm = 33 cm-1 = 4.1 meV

5. Time domain terahertz Spectrometer
The pulse width = ΔtFWHM/√2 = 17.6±0.5 fs
(A Gaussian pulse is assumed)

6. Terahertz Signal
To obtain the response of the sample to the THz radiation 2 measurements are made
THz electric field transmitted through the empty cell
THz electric field transmitted through the sample cell
Fourier Transform

7. Terahertz signal

8. 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

9. Free carrier Absorption in Quantum Dots

10. Purification and sample preparation of quantum dots

11. Experimental procedure & Data analysis
time domain:
frequency domain:
Power transmittance
Relative phase
√T(ω), Φ(ω)
Complex refractive index (nr(ω) + i.nim(ω))
No Kramer-Kronig analysis!!!

12. 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
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

13. Results Surface phonon Shift of resonance of tin doped
Agreement with charged QDs

14. Results

15. Semiconductor Quantum Dots
Justin Galloway
Department of Materials Science & Engineering

16. Outline Introduction Effective Mass Model Reaction Techniques
Applications
Conclusion

17. 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.

18. Properties of Quantum Dots Compared to Organic Fluorphores? Properties
High quantum yield; often 20 times brighter
Narrower and more symmetric emission spectra
times more stable to photobleaching
High resistance to photo-/chemical degradation
Tunable wave length range nm
Properties
CdSe
CdTe
J. Am. Chem. Soc. 2001, 123,

19. Excitation in a Semiconductor Excitation
The excitation of an electron from the valance band to the conduction band creates an electron hole pair
Excitation
Creation of an electron hole pair where h is the photon energy
Band Gap
(energy barrier)
exciton: bound electron and hole pair
usually associated with an electron trapped in a localized state in the band gap

20. Release Recombination of Electron Hole Pairs
Recombination can happen two ways:
radiative and non-radiative
Release
recombination processes
radiative recombination  photon
non-radiative recombination  phonon (lattice vibrations)

21. Model Effective Mass Model Developed in 1985 By Louis Brus
Relates the band gap to particle size of a spherical quantum dot
Model
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:
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  - relative permittivity
mh - hole effective mass 0 - 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

22. 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 mass of an electron compared to a bulk material
Model
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
Brus, L. E. J. Phys. Chem. 1986, 90, 2555

23. 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 me-
Model
Electrostatic force (N) between two charges (Coulomb’s Law):
Work, w = F·dr
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:
Brus, L. E. J. Phys. Chem. 1986, 90, 2555

24. Model Modulus Term Influences
The last term is negligibly small
Term one, as expected, dominates as the radius is decreased
Model
Modulus
Conclusion: Control over the particle’s fluorescence is possible by adjusting the radius of the particle

25. 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
Model

26. 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
The
Making
Synthesis Techniques
• Vapor phase (molecular beams, flame synthesis etc…
• Solution phase synthesis
Aqueous Solution
Nonaqueous Solution
Semiconductor Nanoparticles
II-VI: CdS, CdSe, PbS, ZnS
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 (

27. The Making Nucleation and Growth
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.

28. 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
The
Making
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

29. The Making Quantum Dot Images
Quantum dot images prepared in the Searson Lab using CdO and TOPSe with a rapid injection
The
Making
770000x
455000x
560000x

30. 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
Application
QD’s
Dynamics of endosomal fusion
A431 cell expressing erbB3-mCitrine
Nat. Biotechnol. 2004, 22;

31. Fluoresence data for all 4 toxin assays at high concentrations
Multiplexed Toxin Analysis Using Four Colors of Quantum Dot Fluororeagents
Demonstrated multiplexed assays for toxins in the same well
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
Application
QD’s
Fluoresence data for all 4 toxin assays at high concentrations
Cartoon of
assay
Anal. Chem. 2004, 76;

32. 15 nm CdSe/ZnS TOPO/Polymer/PEG/target
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
Application
QD’s
Gao et al., “In vivo cancer targeting and imaging with semiconductor quantum dots,” Nat. Biotechnol. 22, 969 (2004).

33. Magnetic Nanoparticles Biological Particles
Nano-sized magnetic particles can be superparamagnetic
Widely Studied – Suggested as early as the 1970’s
Offers control/manipulation in magnetic field
Biological Particles
Co has higher magnetization compared to magnetite and maghemite
Science 291, 2001;
J. Phys. D: Appl. Phys. 36, 2003;
An Attractive Biological Tool

34. Magnetic Nanoparticles: Inner Ear Targeted Molecule Delivery and Middle Ear Implant
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
Application
Magnetic
Particles
Magnetic
Particles
Perilymphatic fluid from the cochlea
of magnet-exposed temporal bone
Perilymphatic fluid samples
from animals exposed to magnetic forces
Audiol Neurotol 2006; 11:

35. Composite with A Novel Structure for Active Sensing in Living cells
Magnetic
Quantum
Dot
Composite with A Novel Structure for Active Sensing in Living cells
Silica
ZnS
CdSe Co
① Cobalt core : active manipulation
diameter : ~10 nm
superparamagnetic NPs
→ manipulated or positioned by an external field without aggregation in the absence of an external field
What is
MQD ?
② CdSe shell : imaging with fluorescence
thickness : 3-5 nm
visible fluorescence (~450 – 700 nm)
ability to tune the band gap
→ by controlling the thickness, able to tune the emission wavelength, i.e., emission color
④ Silica shell : bio-compatibility & functionalization with specific targeting group
thickness : ~10 nm
bio-compatible,
& non-toxic to live cell functions
stable in aqueous environment
ability to functionalize its surface
with specific targeting group
③ ZnS shell : electrical passivation
thickness : 1-2 nm
having wider band gap (3.83 eV) than CdSe (1.91 eV)
enhancement of QY
→ CdSe (5-10%)  CdSe/ZnS (~50%)

36. 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
Rap-Up

37. Energy below the Vacuum level (eV)3d 4s
From an Atom to a Solid
Photoemission spectra of negative copper clusters versus number of atoms in the cluster. The highest energy peak corres-ponds 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).

38. Energy Levels of Cu Clusters vs. Cluster Radius R
Solid Atom
ΔE = (E- ER)  1/R (charged sphere)

39. The Band Gap of Silicon Nanoclusters
GaAs
The band gap between the occupied valence band and the empty conduction band of a semiconductor determines not only the color (as in the previous example), it also is crucial for the performance of a transistor. In principle, one could increase the band gap of silicon to that of gallium arsenide (a high-performance, but expensive semiconductor) by making it small enough. However, this does not happen at the dimensions of today’s silicon transistors, which are built with 40 nanometer technology. One would have to get down to 3 nanometers.
This possibility of changing the behavior of a material by just changing its size bring back the dream of the medieval alchemists: If we can make silicon to behave like GaAs, why not make lead behave like gold? In principle, this can be done by manipulating the wave function of the electron with nanotechnology, but in practice it has not been achieved yet. However, it has been possible to make gold behave like platinum the element right next to it in the Periodic Table. While gold is inert in bulk form, it becomes a catalyst like platinum when dispersed into clusters of a few atoms onto an oxide surface. It could be that the oxygen removes an electron from the gold and thereby changes the number of electrons to that of platinum, but that remains to be confirmed.
A goal of the modern alchemist is the ability to use abundant, inexpensive 3d transition metals (such as iron) as catalysts instead of expensive 5d and 4d transition metals (such as platinum, rhodium, and ruthenium). This would be a breakthrough not only for catalytic converters in cars, but also for splitting water to create fuel from solar energy or for dye-sensitized solar cells.
Bulk Silicon
3 nm : Gap begins to change

40. The Band Gap of Silicon Nanoclusters
The band gap between the occupied valence band and the empty conduction band of a semiconductor determines not only the color (as in the previous example), it also is crucial for the performance of a transistor. In principle, one could increase the band gap of silicon to that of gallium arsenide (a high-performance, but expensive semiconductor) by making it small enough. However, this does not happen at the dimensions of today’s silicon transistors, which are built with 40 nanometer technology. One would have to get down to 3 nanometers.
This possibility of changing the behavior of a material by just changing its size bring back the dream of the medieval alchemists: If we can make silicon to behave like GaAs, why not make lead behave like gold? In principle, this can be done by manipulating the wave function of the electron with nanotechnology, but in practice it has not been achieved yet. However, it has been possible to make gold behave like platinum the element right next to it in the Periodic Table. While gold is inert in bulk form, it becomes a catalyst like platinum when dispersed into clusters of a few atoms onto an oxide surface. It could be that the oxygen removes an electron from the gold and thereby changes the number of electrons to that of platinum, but that remains to be confirmed.
A goal of the modern alchemist is the ability to use abundant, inexpensive 3d transition metals (such as iron) as catalysts instead of expensive 5d and 4d transition metals (such as platinum, rhodium, and ruthenium). This would be a breakthrough not only for catalytic converters in cars, but also for splitting water to create fuel from solar energy or for dye-sensitized solar cells.
3 nm : Gap begins to change

41. Increase of the Band Gap in Small Nanoclusters by Quantum Confinement
Conduction Band
Valence Band
k k1
Gap

42. Size Dependent Band Gap in CdSe Nanocrystals
Nanocrystals demonstrate how the properties of a material change, when the electrons and their wave functions are boxed into a region comparable to their wavelength. The band gap changes, and with it the color of the emitted light. Smaller particles have a larger band gap and emit blue light, while large particles emit red light, which is characteristic of normal CdSe.

43. The Band Gap of CdSe Nanocrystals
Size:
The band gap between the occupied valence band and the empty conduction band of a semiconductor determines not only the color (as in the previous example), it also is crucial for the performance of a transistor. In principle, one could increase the band gap of silicon to that of gallium arsenide (a high-performance, but expensive semiconductor) by making it small enough. However, this does not happen at the dimensions of today’s silicon transistors, which are built with 40 nanometer technology. One would have to get down to 3 nanometers.
This possibility of changing the behavior of a material by just changing its size bring back the dream of the medieval alchemists: If we can make silicon to behave like GaAs, why not make lead behave like gold? In principle, this can be done by manipulating the wave function of the electron with nanotechnology, but in practice it has not been achieved yet. However, it has been possible to make gold behave like platinum the element right next to it in the Periodic Table. While gold is inert in bulk form, it becomes a catalyst like platinum when dispersed into clusters of a few atoms onto an oxide surface. It could be that the oxygen removes an electron from the gold and thereby changes the number of electrons to that of platinum, but that remains to be confirmed.
A goal of the modern alchemist is the ability to use abundant, inexpensive 3d transition metals (such as iron) as catalysts instead of expensive 5d and 4d transition metals (such as platinum, rhodium, and ruthenium). This would be a breakthrough not only for catalytic converters in cars, but also for splitting water to create fuel from solar energy or for dye-sensitized solar cells.
Photon Energy vs. Wavelength:
h (eV) = /  (nm) 

44. Beating the size distribution of quantum dots
Quantum dots formed by thin spots in GaAs layers

45. 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

46. Single Electron Transistor
dot e-
A single electron e- tunnels in two steps from source to drain through the dot.
The dot replaces the channel of a normal transistor (below).
electrons

47. 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.

48. * 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

49. Magnetic Clusters
“Ferric Wheel”

50. Magnetic Nanoclusters in Biology

51. The Holy Grail of Catalysis: Reactions at a Specific Nanoparticle
Want this image chemically resolved.
Have chemical resolution in micro-spectroscopy 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)

52. 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 ?

53. 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.