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
              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

                         •THz electric field transmitted
                         through the empty cell
                         •THz electric field transmitted
     Fourier Transform
                         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         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

            1.0




                                                         im()
                                                                  4
 im()




            0.5
                                                                  2
            0.0
                                                                  0
           -0.5
                  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
                                            Results
             2.0
                           indium doped                     • Surface
                           undoped                            phonon
             1.5           tin doped
                                                            • Shift of
Absorbance




                                                              resonance of
             1.0                                              tin doped
                                                            • Agreement
                                                              with charged
             0.5                                              QDs

             0.0

                   1   2      3       4     5   6   7   8
                                          THz
                       Results
            1.34
                           Un
                           Sn
                           In
            1.32
Ref index




            1.30




            1.28




            1.26
                   2   3    4         5   6   7   8

                                THz
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 in a Semiconductor
Excitation
                     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



                      semiconductor                                          Band Gap
          optical
          detector                                                           (energy barrier)
                                         E=h


                                                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
                                                          E

                                                                                             ECB
                            recombination processes


                                                                                             EVB


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




                                                      radiative recombination  photon
                                           EVB
                                                      non-radiative recombination  phonon (lattice
                                                      vibrations)
                                                              e (CB)  h (VB)  h
            radiative          non-radiative
            recombination      recombination
                      Effective Mass Model
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:
                                                                                    1
                     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
cm-1)
m0 - free electron mass (9.110x10-31       kg)               Brus, L. E. J. Phys. Chem. 1986, 90, 2555
                     Term 2
Model
                 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



                                                               1
            h2  1   1  1.8e2      0.124e 4  1      1 
       bulk
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
Model
                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
                             4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:

                                       e2              e2
                              E          2
                                              dr  
                                     40r          40r
                                                                   1
                h2  1   1  1.8e2      0.124e 4  1      1 
           bulk
    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
Model
        The last term is negligibly small

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




             Modulus
              Energy (eV)   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
                               Quantum Confinement of ZnO & TiO2
Model
                            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
                                            ZnO




                                                                 Eg (eV)
    Eg (eV)




                   4




                                                                                  3
                    3                                                           400
                  400


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




                                                                                350
                  350

                                                                                300
                  300

                                                                                250
                  250
                                                                                      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
       The
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.
          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
                                      770000x




 560000x                         455000x
                Quantum Dot Ligands Provide new Insight
Application
              into erbB/HER receptor – Mediated Signal
              Transduction
              Used biotinylated EGF bound to commercial quantum
              dots
   QD’s
              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
                 Multiplexed Toxin Analysis Using Four Colors
Application
               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              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
                     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
              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
              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        Composite with A Novel Structure for Active Sensing in Living cells
Quantum
Dot                                                 ① Cobalt core : active manipulation

                                                        diameter : ~10 nm
  What
                                                        superparamagnetic NPs
   is                           Co
  MQD ?                                               → manipulated or positioned by an
                                                       external field without aggregation in
                              CdSe                     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 &            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
             bio-compatible,
                                                         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%)
          Conclusions
Rap-Up
         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




                                    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   k1     Gap



         Valence
          Band
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-          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).




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




                                                                       Fischer-Tropsch
                                                                       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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