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
Department of Chemistry
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.
Quantum Dots (QDs)
•
•
•
•
•
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.:
E
qd
g
E
bulk
g
exciton
- +
h2 1
1
1.8e 2
 2 ( *  *)
8r me mh 4 r  0 r
* P. Kambhampati, Accounts Chem Res, 44, 1 (2011)
* L. Brus, J. Phys. Chem. 90, 2555-2560 (1986).
* D. J. Norris and M. G. Bawendi, Phys. Rev. B, 53, 16338
(1996)
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
PDTC-coated Ultrasmall CdSe Nanoclusters (NCs)
Se+DDA+1-hexanethiol (HT)
CdCl2
+
dodecylamine (DDA)
PDTC=Phenyldithiocarbamate
[(Se)m(DDA)n]
Toluene, RT, 4h
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
(CdSe)34
LDI-TOF-MS Spectra
DDA-coated (CdSe)34 NCs
(CdSe)34
F-PDTC-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
TA Spectrum of PDTC-passivated 1.6 nm CdSe NCs
UV/Vis Absorption
0.4
DDA-passivated CdSe NCs
PDTC-passivated CdSe NCs
Absorbance
0.3
388 nm
0.2
0.1
effective
R  RCdSe-PDTC
 RCdSe
0.0
300
350
400
450
500
550
600
650
700
Wavelength (nm)
Transient Absorption (TA) Spectroscopy
Δt
pump
UV/Vis Abs.
probe
sample
time
(CdSe-PDTC in DCM)
 I * ( t ,  ) 
OD ( t ,  )  -log 

 I 0 ( ) 
* E. A. Weiss, et al. Nano Lett. 2013, 13, 287−292
TA
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)
Wavelength Domain
ET and HT Processes
445 nm
0.010
0.005
4
hot ET
1P(e)
0.000
-0.005
relaxation
OD
“State Filling”
1S(e)
Interfacial
Charge
Transfer
State
-0.020
-0.025
460
1
480
500
520
540
560
580
600
Wavelength (nm)
3
2
100 fs
200 fs
400 fs
800 fs
5 ps
-0.010
-0.015
ET,
recombination
Pump:
388nm
488 nm 517 nm 545 nm
ET: electron transfer
HT: hole transfer
Time Domain
hot ET
455 nm
488 nm
517 nm
545 nm
relaxation
0.005
0.000
HT
1S3/2(h)
2S3/2(h)
OD
-0.005
-0.010
HOMO
-0.015
1S1/2(h)
-0.020
1P(h)
CdSe SCNC
PDTC
ligand
-0.025
HT
0
1
2
Time (ps)
3
4
5
TA Spectra with Electron and Hole Quenchers
1,4-benzoquinone
(BQ)
Electron Quencher
Pyridine (Py)
Hole Quencher
Pump-wavelength Dependence
λpump=490 nm
X
NC-size Dependence
d=2.8 nm; Eg=2.15 eV (578 nm)
Hot electron
transfer
Hole transfer
CdSe SCNCs passivated with substituted PDTCs
Global Fitting of TA spectrum
Global Fitting of TA spectrum
Instrumental Response Function:
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 subpicosecond 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).
Acknowledgements
Current Group Members:
Former Members:
Dr. Bill Pandit
Northwestern
Funding:
Yizhou Xie
Dr. Neil Reilly
UMass Boston
Collaborators:
• Meghan B Teunis
• Rajesh Sardar