Raman spectroscopy (SMSERS), the change in the interface properties of
alizarinÀTiO 2 system can be probed as a result of the externally applied electric
field [36]. The perturbation, caused by the external potential, has been observed as a
shift and splitting of the 648 cm
À1 peak, typical indicator of the strong coupling
between alizarin and TiO 2 . On the basis of the experimental results and DFT
calculations, the presence of perturbed alizarinÀTiO 2 coupling under interfacial
electric potential may lead to changes in the interfacial electron transfer dynamics.
Additionally, heterogeneously distributed dye molecules at the interface on nanometer length scale and different molecule–semiconductor binding interactions under
charge accumulation associated interfacial electric field changes create intrinsically
inhomogeneous interfacial ET dynamics associated with both static and dynamic
disorders.
2.4 In Situ Atomic Force Microscopy and Fluorescence
Combining the single-molecule fluorescence spectroscopy approach with various
other techniques such as computational studies, atomic force microscopy (AFM),
electrochemistry, and Raman spectroscopy can facilitate inspection of multiple
parameters with high chemical selectivity and wide temporal and spatial resolutions.
Significant efforts made in this direction have already indicated the importance of
various factors in determining injection dynamics and physical origins for interfacial
ET rate fluctuations.
The fundamental information related to the energy flow between molecules and
substrate surfaces as a function of surface site geometry and molecular structure is
critical for understanding interfacial ET dynamics. The inhomogeneous nanoscale
moleculeÀsurface and moleculeÀmolecule interactions are presumably the origins
of the complexity in interfacial ET dynamics; thus, identifying the environment of
molecules at nanoscale is crucial. It is challenging to correlate the observed heterogeneity of interfacial CT dynamics to the material and interfacial structures based on
fluorescence measurement alone. The AFM correlated single-molecule fluorescence
intensity/lifetime imaging microscopy (AFM-SMFLIM) is capable of identifying
and characterizing individual molecules distributed across the heterogeneous surface
at the nanometer length scale. Nanoscale morphology and interfacial ET dynamics at
a single-molecule level can be observed. Moreover, the blinking behavior and
lifetime of each molecule in combination with the topography of the environment
at nanoscale provide the location of each molecule on the surface at nanoscale and
the coupling strength of each molecule with TiO 2 nanoparticles.
Using single CdSe/CdS quantum dot (QD) functionalized atomic force microscopy (AFM) tips, the spatial dependence of photoinduced electron transfer dynamics
from the single QD to TiO 2 nanoparticles can be controlled and probed with high
spatial (sub diffraction-limited) and temporal (limited by fluorescence microscopy)
resolutions (Fig. 2.18) [37]. CdSe/CdS QDs with a CdSe core ($1.2 nm in radius)
and six monolayers of CdS (2.2 nm in thickness) were attached on the n-Si AFM tip
38
2 In Situ Characterization of Photocatalytic Activity
alizarinÀTiO 2 system can be probed as a result of the externally applied electric
field [36]. The perturbation, caused by the external potential, has been observed as a
shift and splitting of the 648 cm
À1 peak, typical indicator of the strong coupling
between alizarin and TiO 2 . On the basis of the experimental results and DFT
calculations, the presence of perturbed alizarinÀTiO 2 coupling under interfacial
electric potential may lead to changes in the interfacial electron transfer dynamics.
Additionally, heterogeneously distributed dye molecules at the interface on nanometer length scale and different molecule–semiconductor binding interactions under
charge accumulation associated interfacial electric field changes create intrinsically
inhomogeneous interfacial ET dynamics associated with both static and dynamic
disorders.
2.4 In Situ Atomic Force Microscopy and Fluorescence
Combining the single-molecule fluorescence spectroscopy approach with various
other techniques such as computational studies, atomic force microscopy (AFM),
electrochemistry, and Raman spectroscopy can facilitate inspection of multiple
parameters with high chemical selectivity and wide temporal and spatial resolutions.
Significant efforts made in this direction have already indicated the importance of
various factors in determining injection dynamics and physical origins for interfacial
ET rate fluctuations.
The fundamental information related to the energy flow between molecules and
substrate surfaces as a function of surface site geometry and molecular structure is
critical for understanding interfacial ET dynamics. The inhomogeneous nanoscale
moleculeÀsurface and moleculeÀmolecule interactions are presumably the origins
of the complexity in interfacial ET dynamics; thus, identifying the environment of
molecules at nanoscale is crucial. It is challenging to correlate the observed heterogeneity of interfacial CT dynamics to the material and interfacial structures based on
fluorescence measurement alone. The AFM correlated single-molecule fluorescence
intensity/lifetime imaging microscopy (AFM-SMFLIM) is capable of identifying
and characterizing individual molecules distributed across the heterogeneous surface
at the nanometer length scale. Nanoscale morphology and interfacial ET dynamics at
a single-molecule level can be observed. Moreover, the blinking behavior and
lifetime of each molecule in combination with the topography of the environment
at nanoscale provide the location of each molecule on the surface at nanoscale and
the coupling strength of each molecule with TiO 2 nanoparticles.
Using single CdSe/CdS quantum dot (QD) functionalized atomic force microscopy (AFM) tips, the spatial dependence of photoinduced electron transfer dynamics
from the single QD to TiO 2 nanoparticles can be controlled and probed with high
spatial (sub diffraction-limited) and temporal (limited by fluorescence microscopy)
resolutions (Fig. 2.18) [37]. CdSe/CdS QDs with a CdSe core ($1.2 nm in radius)
and six monolayers of CdS (2.2 nm in thickness) were attached on the n-Si AFM tip
38
2 In Situ Characterization of Photocatalytic Activity