10 Hot Carrier Transfer and Carrier Manipulation of Semiconductor …
177
reveal the origin of two different electron transfer rates, we examined the lateral size
dependence on the rate of electron transfer dynamics by using four kinds of CdSe
NPLs. The rates of fast and slow electron transfer processes became longer as the
lateral dimension of NPLs increased (τ fast : from 370 to 580 fs, and τ slow : from 2.4 to
6.2 ps). The electronic coupling strength,
H (E)
, between the CdSe NPLs and MV
2+
should be different at the different NPLs faces since the shape of CdSe NPLs is highly
anisotropic, with a large lateral size (short axis of 5.1–7.5 nm and long axis of 15.0–
22.8 nm) and small thickness (4 monolayers, ~1.2 nm), resulting in the difference of
wavefunction penetration depending on the face. Therefore, the fast electron transfer
is probably originated from adsorbed MV
2+ on long axes of CdSe NPLs faces, and
the slow electron transfer is due to MV
2+ adsorbed on short axes of CdSe NPLs faces,
since the wavefunction penetration of electrons along with the long axes of NPLs is
expected to be much larger than that of the short axes. If MV
2+ were adsorbed on the
plane of CdSe NPLs,
H (E)
would not change with the lateral size because all the
CdSe NPLs have the same thickness, and the electron transfer rate should be similar
irrespective of the lateral size. In addition, it is known that the faces perpendicular
to the thickness direction are well passivated by carboxylic acid groups because of
the termination with cadmium atoms [23]. Furthermore, MV
2+ tends to adsorb to the
Se-terminated anionic surface, and thus, the adsorption of MV
2+ is not so easy on
the face perpendicular to the thickness direction. By considering the wavefunction
penetration and electronic coupling,
H (E)
, the observed fast and slow electron
transfer from the CdSe NPLs to MV
2+ can be assigned to electron transfer at the
faces of the long and short axes in the lateral dimension, respectively (Fig. 10.5a).
These rate constants are plotted against the half-length of the short and long axes in
the lateral dimension of CdSe NPLs, as illustrated in Fig. 10.5b. A good correlation
was obtained between two electron transfer rates and the length of the short and long
axes, showing the well-known exponential behavior of k ET (d) = k 0 exp(−βd), where
d is the distance between donor and acceptor (half-length of the lateral dimension,
or the distance from the center of NPLs), k 0 is the preexponential factor, and β
characterizes the exponential distance dependence of the transfer (β ~0.33 nm
−1 ).
This correlation also supports the importance of wavefunction penetration and the
electronic coupling with MV
2+ . This electronic coupling of CdSe NPL-MV
2+ HNs
is relatively weak as compared with that of CdSe QD-MV
2+ HNs, since the ultrafast
electron transfer was observed from CdSe QDs to MV
2+ within 100 fs [24]. Because
of the fast intraband relaxation due to efficient phonon emission in quasi-continuous
conduction band of NPLs and the weak electronic coupling, the hot electron transfer
does not occur from CdSe NPLs to MV
2+ .
Au NPs can be used to extract electrons from CdSe NPLs. As shown in Fig. 10.6a,
small-sized Au NPs (~1.1 nm) were attached with four monolayer CdSe NPLs (lateral
size of 5.8 × 20.8 nm
2 ). Transient absorption dynamics at heavy-hole wavelength
(~510 nm) of both CdSe NPLs and CdSe NPL-Au HNs were illustrated in Fig. 10.6b.
The heavy-hole band dynamics of CdSe NPL-Au HNs could be fitted by a sum of
exponential functions with the lifetime of ~0.9 ps and ~4 ps components, which can
be assigned to the electron transfer from the band-edge state of CdSe NPLs to Au
NPs. The rates of electron transfer are slower than those for CdSe QD-Au HNs as
177
reveal the origin of two different electron transfer rates, we examined the lateral size
dependence on the rate of electron transfer dynamics by using four kinds of CdSe
NPLs. The rates of fast and slow electron transfer processes became longer as the
lateral dimension of NPLs increased (τ fast : from 370 to 580 fs, and τ slow : from 2.4 to
6.2 ps). The electronic coupling strength,
H (E)
, between the CdSe NPLs and MV
2+
should be different at the different NPLs faces since the shape of CdSe NPLs is highly
anisotropic, with a large lateral size (short axis of 5.1–7.5 nm and long axis of 15.0–
22.8 nm) and small thickness (4 monolayers, ~1.2 nm), resulting in the difference of
wavefunction penetration depending on the face. Therefore, the fast electron transfer
is probably originated from adsorbed MV
2+ on long axes of CdSe NPLs faces, and
the slow electron transfer is due to MV
2+ adsorbed on short axes of CdSe NPLs faces,
since the wavefunction penetration of electrons along with the long axes of NPLs is
expected to be much larger than that of the short axes. If MV
2+ were adsorbed on the
plane of CdSe NPLs,
H (E)
would not change with the lateral size because all the
CdSe NPLs have the same thickness, and the electron transfer rate should be similar
irrespective of the lateral size. In addition, it is known that the faces perpendicular
to the thickness direction are well passivated by carboxylic acid groups because of
the termination with cadmium atoms [23]. Furthermore, MV
2+ tends to adsorb to the
Se-terminated anionic surface, and thus, the adsorption of MV
2+ is not so easy on
the face perpendicular to the thickness direction. By considering the wavefunction
penetration and electronic coupling,
H (E)
, the observed fast and slow electron
transfer from the CdSe NPLs to MV
2+ can be assigned to electron transfer at the
faces of the long and short axes in the lateral dimension, respectively (Fig. 10.5a).
These rate constants are plotted against the half-length of the short and long axes in
the lateral dimension of CdSe NPLs, as illustrated in Fig. 10.5b. A good correlation
was obtained between two electron transfer rates and the length of the short and long
axes, showing the well-known exponential behavior of k ET (d) = k 0 exp(−βd), where
d is the distance between donor and acceptor (half-length of the lateral dimension,
or the distance from the center of NPLs), k 0 is the preexponential factor, and β
characterizes the exponential distance dependence of the transfer (β ~0.33 nm
−1 ).
This correlation also supports the importance of wavefunction penetration and the
electronic coupling with MV
2+ . This electronic coupling of CdSe NPL-MV
2+ HNs
is relatively weak as compared with that of CdSe QD-MV
2+ HNs, since the ultrafast
electron transfer was observed from CdSe QDs to MV
2+ within 100 fs [24]. Because
of the fast intraband relaxation due to efficient phonon emission in quasi-continuous
conduction band of NPLs and the weak electronic coupling, the hot electron transfer
does not occur from CdSe NPLs to MV
2+ .
Au NPs can be used to extract electrons from CdSe NPLs. As shown in Fig. 10.6a,
small-sized Au NPs (~1.1 nm) were attached with four monolayer CdSe NPLs (lateral
size of 5.8 × 20.8 nm
2 ). Transient absorption dynamics at heavy-hole wavelength
(~510 nm) of both CdSe NPLs and CdSe NPL-Au HNs were illustrated in Fig. 10.6b.
The heavy-hole band dynamics of CdSe NPL-Au HNs could be fitted by a sum of
exponential functions with the lifetime of ~0.9 ps and ~4 ps components, which can
be assigned to the electron transfer from the band-edge state of CdSe NPLs to Au
NPs. The rates of electron transfer are slower than those for CdSe QD-Au HNs as