Terahertz Spectroscopy of CdSe Quantum Dots (PowerPoint) by yaoyufang

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									What is a quantum dot?
           • Nanocrystals
           • 2-10 nm diameter
           • semiconductors
What is a quantum dot?
            • Exciton Bohr Radius
            • Discrete electron
              energy levels
            • Quantum
•   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

                         •THz electric field transmitted
                         through the empty cell
                         •THz electric field transmitted
     Fourier Transform
                         through the sample cell
Terahertz signal

• 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         Relative phase

       √T(ω), Φ(ω)             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

            2.0                                                   8
                           3.2 nm uncharged                                6.3 nm uncharged
            1.5            3.2 nm charged                         6        6.3 nm charged



                  2       3       4    5      6      7                2   3      4    5       6   7
                        Frequency (THz)                                   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
                           indium doped                     • Surface
                           undoped                            phonon
             1.5           tin doped
                                                            • Shift of

                                                              resonance of
             1.0                                              tin doped
                                                            • Agreement
                                                              with charged
             0.5                                              QDs


                   1   2      3       4     5   6   7   8
Ref index



                   2   3    4         5   6   7   8

Semiconductor Quantum Dots

                                        Justin Galloway
            Department of Materials Science & Engineering
          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

                                  Nanoparticle: a microscopic particle of an
                                  inorganic material (e.g. CdSe) or organic material
                                  (e.g. polymer, virus) with a diameter less than 100

                                  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



                                                           J. Am. Chem. Soc. 2001, 123, 183-184
                       Excitation in a Semiconductor
                     The excitation of an electron from the valance band
                     to the conduction band creates an electron hole pair


                                                                            ECB           h=E g

         h  e (CB)  h (VB)
     Creation of an electron hole pair
     where h is the photon energy                                          EVB


                      semiconductor                                          Band Gap
          detector                                                           (energy barrier)

                                                exciton: bound electron and hole pair
                                                usually associated with an electron trapped in a
                                                localized state in the band gap
                            Recombination of Electron Hole Pairs
Release              Recombination can happen two ways:

                                 radiative and non-radiative

                            recombination processes


                                                                 band-to-band    recombination
                                                                 recombination   atinterband trap states
                                           ECB                                   (e.g. dopants, impurities)

                                                      radiative recombination  photon
                                                      non-radiative recombination  phonon (lattice
                                                              e (CB)  h (VB)  h
            radiative          non-radiative
            recombination      recombination
                      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 2
                            1      1  1.8e            0.124e 3  1       1 
   E *  E bulk
           g               
                                        
                                                                              
                                                                    2 m m  m m 
                    2er 2   me m0 m h m0  40 r     2
                                                            40   e 0    h 0 

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
m0 - free electron mass (9.110x10-31       kg)               Brus, L. E. J. Phys. Chem. 1986, 90, 2555
                     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

Consider a particle of mass m confined

                                                       Poten tia l En erg y
in a potential well of length L. n = 1, 2, …                                                   •

For a 3D box: n2 = nx2 + ny2 + nz2

      n2 2 2 n2h2
 En      2
                                                                                                     x
       2mL     8mL2                                                            0           L

            h2  1   1  1.8e2      0.124e 4  1      1 
E*  E g  2                                        
                mem0 mh m0  40r h2 20 2 mem0 mh m0 
            8r 
                                                                              Brus, L. E. J. Phys. Chem. 1986, 90, 2555
                  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-

                 Electrostatic force (N) between two charges (Coulomb’s Law):
                        F 1 2 2                   Work, w = F·dr
 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:

                                       e2              e2
                              E          2
                                              dr  
                                     40r          40r
                h2  1   1  1.8e2      0.124e 4  1      1 
    E*  E g  2                                        
                    mem0 mh m0  40r h2 20 2 mem0 mh m0 
                8r 

                                                              Brus, L. E. J. Phys. Chem. 1986, 90, 2555
          Term Influences
        The last term is negligibly small

        Term one, as expected, dominates as the radius is decreased

              Energy (eV)   1                                       term 1

                                                                    term 2

                                                                    term 3

                                0             5           10
                                            d (nm)

                                    Conclusion: Control over the
                                    particle’s fluorescence is possible
                                    by adjusting the radius of the
                               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

                                                                 Eg (eV)
    Eg (eV)


                    3                                                           400

                                                                on se t (nm)
  on se t (nm)



                                                                                      0    5          10
                        0         5               10
                                 d (nm)                                                   d (nm)
                  Formation of Nanoparticles
 The           Varying methods for the synthesis of

Making         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/)
                                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.
          Capping Quantum Dots
 The     Due to the extremely high surface area of a

Making   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
            Quantum Dot Images
 The       Quantum dot images prepared in the Searson Lab using

Making     CdO and TOPSe with a rapid injection

 560000x                         455000x
                Quantum Dot Ligands Provide new Insight
              into erbB/HER receptor – Mediated Signal
              Used biotinylated EGF bound to commercial quantum
              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

                    Nat. Biotechnol. 2004, 22; 198-203
                 Multiplexed Toxin Analysis Using Four Colors
               of Quantum Dot Fluororeagents
               Demonstrated multiplexed assays for toxins in the same
   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

 Anal. Chem. 2004, 76; 684-688
Application              Quantum Dot Imaging
                       QDs with antibodies to human prostate-specific
                       membrane antigen indicate murine tumors
                       developed from human prostate cells
   QD’s                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               Magnetic Nanoparticles
 Particles           Nano-sized magnetic particles can be

                      Widely Studied – Suggested as early as the 1970’s

                      Offers control/manipulation in magnetic field

  Co has higher magnetization
 compared to magnetite and

                                                              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
              SNP controlled by magnets while transporting a
 Magnetic     payload
 Particles    Studies included in vitro and in vivo on rats, guinea pigs
              and human cadavers
              Demonstrated magnetic gradients can enhance drug
                                        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        Composite with A Novel Structure for Active Sensing in Living cells
Dot                                                 ① Cobalt core : active manipulation

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

          Photoemission spectra of negative copper
3d   4s
          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).

             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


                                    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


               k2   k1     Gap

Size Dependent Band Gap in CdSe Nanocrystals
                 The Band Gap
                    of CdSe

            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-          e-
                 dot                 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).

                               Designs for
                             Single Electron

                                      Large (≈ m)
                                      for operation
                                      at liquid He

                           Small (10 nm)
                           for operation
                           around room

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 micro-
                                                 spectroscopy via X-ray absorption
                                                 but insufficient spatial resolution.

                                                                       process converts
                                                                       coal to fuel using
                                                                       an iron catalyst.

                                       Di and                             De Smit et al.,
                                       Schlögl                            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.

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