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MOLECULE-LIKE CdSe NANOCLUSTERS

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Presentation on theme: "MOLECULE-LIKE CdSe NANOCLUSTERS"— Presentation transcript:

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).

20

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!


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