1. MOLECULE-LIKE CdSe NANOCLUSTERS
PASSIVATED WITH STRONGLY INTERACTING LIGANDS
ENERGY LEVEL ALIGNMENT
AND PHOTOINDUCED ULTRAFAST CHARGE TRANSFER PROCESSES
Yizhou Xie, Bill Pandit,* and Jinjun Liu
Department of Chemistry
University of Louisville (UofL)
Meghan B Teunis and Rajesh Sardar
Indiana University-Purdue University Indianapolis (IUPUI)
International Symposium on Molecular Spectroscopy
University of Illinois Urbana-Champaign
06/23/15
* Current address: Department of Chemistry, Northwestern University.

2. Outline Introduction Experimental Results Conclusions
Quantum Dots (QDs) and Semiconductor Nanoclusters (SCNCs)
Phenyldithiocarbamate (PDTC) Passivated CdSe SCNCs
Experimental
Ultrafast Transient Absorption (TA) Spectroscopy
Results
TA spectra of 1.6 nm CdSe SCNC-PDTC Conjugates
Control Experiments
Spectral Assignment and Analysis
Global Fitting of TA Spectra
CdSe SCNCs with Substituted PDTC
Conclusions

3. Quantum Dots (QDs) - + exciton Semiconductors such as CdSe
Crystalline spherical particles with diameters in the range of 1-10 nm
Quantum confinement effect
Discrete structure of energy levels
Band gap and energy level structure strongly dependent on their sizes
Brus Eq.:
* P. Kambhampati, Accounts Chem Res, 44, 1 (2011)
* D. J. Norris and M. G. Bawendi, Phys. Rev. B, 53, (1996)
* L. Brus, J. Phys. Chem. 90, (1986).

4. Applications of QDs Solar Cells Photocatalysis Biological Imaging
Light Emission Diodes
… …
P. V. Kamat, et al., ACS Nano, 2009, 3, 1467.
H. Yang, et al., Chem. Mater., 2012, 24, 1961.
J. V. Frangioni, et al., Nat. Biotechnol., 2004, 22, 93.
E. A. Weiss Group, Northwesten

5. Se+DDA+1-hexanethiol (HT)
PDTC-coated Ultrasmall CdSe Nanoclusters (NCs)
PDTC=Phenyldithiocarbamate
Se+DDA+1-hexanethiol (HT)
[(Se)m(DDA)n]
Toluene, RT, 4h
CdCl2 +
dodecylamine (DDA)
HT:
DDA:
Advantages:
Well controlled mass: (CdSe)34
Well controlled (“magic”) size: d=1.6 nm
Well engineered surface with less surface defects than QDs
Large surface-area-to-volume ratio with 80% atoms on surface
Strong coupling with PDTC ligands
LDI-TOF-MS Spectra
(CdSe)34
F-PDTC-coated (CdSe)34 NCs
DDA-coated (CdSe)34 NCs
* S Dolai, P. R. Nimmala, M Mandal, B. B. Muhoberac, K. Dria, A. Dass, and R. Sardar, Chem. Mater., 2014, 26, 1278–1285
* M. B. Teunis, S. Dolai, and R. Sardar, Langmuir 2014, 30, 7851−7858

6. Coupling between Surface Ligands and NCs
withdrawing
e- donating
X-PDTC HOMO level
Partial-in-a-Box
Model
* M. B. Teunis, S. Dolai, and R. Sardar, Langmuir 2014, 30, 7851−7858
* M. T. Frederick, V. A. Amin, N. K. Swenson, A. Y. Ho, E. A. Weiss, Nano Lett. 2013, 13, 287−292

7. Transient Absorption (TA) Spectroscopy
probe Δt
lock-in
amplifier PC
photodiode
array detector PC
time
sample
(CdSe-PDTC in DCM)
pump
I*: transmission with pump
I0: transmission without pump

8. . . . Transient Absorption (TA) Spectroscopy
Ground-state bleach/state filling
Excited-state absorption
Stimulated emission Sn . . . S1
ΔOD S0

9. Femtosecond TAPPS Apparatus
Clark-MXR
Femtosecond
Laser
Ti:Sapphire
Amplifier
l = 775 nm
Epulse ≈ 1 mJ
r.r. = 1 kHz
Δtpulse ≤ 150 fs
NOPA
and
pulse compressor
l = nm
Δtpulse ≈30 fs
pump
SHG
λ= nm
motor
motorized delay stage WL x2 x2
manual
delay stage
NOPA
and
pulse compressor
l = nm
Δtpulse ≈30 fs
sample
probe
Spectrometer +
Array Detector
manual
delay stage
Photo-
diode
r.r.=repetition rate; NOPA=Nocollinear Optical Parametric Amplifier; SHG=Second Harmonic Generator; WL= Whitelight Generator.

10. TA Spectrum of PDTC-passivated 1.6 nm CdSe NCs
UV/Vis Absorption
388 nm
UV/Vis Abs. TA
Transient Absorption (TA) Spectroscopy Δt
sample
(CdSe-PDTC in DCM)
probe
pump
Ultraviolet
time
* E. A. Weiss, et al. Nano Lett. 2013, 13, 287−292

11. TA Spectrum of PDTC-passivated 1.6 nm CdSe NCs
Sub-picosecond
Component
ΔOD(488 nm), normalized
ΔOD(517 nm), normalized
ΔΔOD=
ΔOD(488 nm)-ΔOD(517 nm)

12. Interfacial Charge Transfer
ET and HT Processes
Wavelength Domain
445 nm
488 nm
517 nm
545 nm 4
1P(e)
hot ET
Pump:
388nm
“State Filling”
1S(e)
relaxation
Interfacial Charge Transfer
State 1 2 3
ET, recombination
Time Domain
ET: electron transfer
HT: hole transfer
hot ET
relaxation
HT
1S3/2(h)
2S3/2(h)
HOMO
1S1/2(h)
HT
1P(h)
CdSe SCNC PDTC
ligand

13. TA Spectra with Electron and Hole Quenchers
1,4-benzoquinone (BQ)
Electron Quencher
Pyridine (Py)
Hole Quencher

14. Pump-wavelength Dependence
λpump=490 nm X

15. NC-size Dependence d=2.8 nm; Eg=2.15 eV (578 nm) Hot electron transfer
Hole transfer
CdSe SCNCs passivated with substituted PDTCs

16. Global Fitting of TA spectrum

17. Global Fitting of TA spectrum
Instrumental Response Function:

18. X-PTC-Passivated CdSe NCs

19. Conclusions
Ultrafast spectroscopic study of PDTC-passivated 1.6 nm diameter CdSe SCNCs unraveled excitonic dynamics of these conjugates;
Hole transfer and hot electron transfer on a sub-picosecond timescale were observed;
Energy level alignment is critical to the observed ultrafast processes as demonstrated in control experiments;
Hole transfer and hot electron transfer processes can be utilized to increase the power conversion efficiency of solar cells.
"Molecule-like CdSe nanoclusters passivated with strongly interacting ligands: energy level alignment and photoinduced ultrafast charge transfer processes", Y. Xie, M. Teunis, B. Pandit, R. Sardar,and J. Liu, J. Phys. Chem. C. 119, 2813−2821 (2015).

21. Thank you! Acknowledgements Current Group Members: Former Members:
Dr. Bill Pandit
Northwestern
Dr. Neil Reilly
UMass Boston
Collaborators:
Meghan B Teunis
Rajesh Sardar
Yizhou Xie
Funding:
Thank you!