Communications
ments. After all, reducing the particle size to 1.74 nm
enhances not only the magnetic properties, but also the
photoisomerization efficiency of AZ-Au NPs.
may either decrease or increase the molecule-to-surface
dipole moment.[26] The change in the work function is related
to the dipole moment density perpendicular to the surface
through the relationship given in Equation (1).[26]
Subsequently, we investigated the influence of photo-
illumination on the magnetic properties of AZ-Au NPs at
300 K (Figure 3b). During the UV illumination, the initial
magnetization value under an applied field of 5 T decreased
from 0.24 to 0.18 emugAuꢀ1. Even after the illumination was
stopped, the decreased magnetization value was maintained
for several hours at room temperature. The very slow
recovery of the magnetization could be due to the cis-to-
trans thermal back-isomerization of the AZ moiety. We then
mN cosq
ee0
ð1Þ
DF ¼
In other words, the magnitude of the electron transfer to
the organic layer becomes smaller for adsorbates with
electronegative end groups, while it becomes larger for
adsorbates with electropositive end groups. In fact, exper-
imental evidence for such changes in the magnitude of the
electron transfer from gold to the adsorbates has been
reported previously.[27]
In the system presented here, the values of mcosq for the
AZ ligands were calculated as ꢀ0.20 D (D = Debye) for the
trans state and 2.74 D for the cis state, meaning that the sign of
the work function change would be negative for the trans state
and positive for the cis state.[28] Hence, the magnitude of
electron transfer in the initial trans state becomes smaller in
the cis state under UV illumination, and it recovers to its
initial value under illumination with visible light. Schematic
illustrations of the changes in the work function resulting
from the photoisomerization of AZ are shown in Figure 4. As
a consequence, the magnetization values could be controlled
by employing alternating photoillumination with UV and
visible light, as caused by changes in the d-charge loss owing
to the photoisomerization of AZ ligands.
illuminated the AZ-Au NPs with visible light, and the
ꢀ1
magnetization value increased from 0.18 to 0.24 emugAu
.
After this process, the UV-light-induced decrease and the
visible-light-induced increase in magnetization were repeated
without any attenuation. The photoinduced changes in the
magnetization values were estimated to be about 27%. We
also observed such photoinduced magnetic transitions in the
AZ-Au NPs even at 5 K with almost the same efficiency of
about 25% (Figure 3c). Because the thermal back-isomer-
ization of the AZ ligands did not occur at low temperature, we
also observed significant changes in the magnetization curves
at 5 K (Figure 3d).
As mentioned above, in Au NPs, the apparent ferromag-
netism is associated with 5d localized holes in the 5d shell that
ꢀ
are generated through Au S bonding. The magnetic ordering
is not due to the large exchange interaction, but rather is due
to extremely high local magnetic anisotropy (estimated to be
as high as 1 108 ergcmꢀ3), which blocks the moments from
switching, resulting in very high TB (blocking temperature)
values in Au NPs.[17] From the saturated magnetization value
of the AZ-Au NPs under an applied field of 5 T at 5 K, an
estimation of the lower limit value of the magnetic moment of
the gold atoms is straightforward. The values of
the magnetic moment per Au atom bound to
sulfur in the trans state and the cis state are
estimated to be 0.033mB and 0.024mB, respectively.
In a recent report concerning an XMCD (X-ray
magnetic circular dichroism) study of thiolated
gold clusters, a considerable orbital magnetic
moment of the Au 5d electrons was indicated,
and the ratio of the orbital magnetic moment to
the spin magnetic moment was estimated to be
12%.[18] Therefore, the lower limit value of the d-
hole counts per Au atom bound to sulfur in the
trans state and the cis state are estimated as to be
about 0.0039 and 0.0027 eatomꢀ1, respectively.
This result clearly suggests that the d-charge loss
of about 0.0012 eatomꢀ1 was decreased in the case
of the cis state, that is, the charge transfer from Au
to S could be reversed with trans-to-cis photo-
isomerization.
In conclusion, we have designed azobenzene-passivated
ferromagnetic gold nanoparticles whose magnetic moments
are localized at the organic–inorganic interfaces. The mag-
netic properties of these compounds could be controlled by
alternating photoillumination with UVand visible light in the
Figure 4. Schematic illustrations of changes in the work function resulting from
photoisomerization of AZ; left: schematic energy-level diagrams for an untreated
interface (without surface passivation); middle: passivation of trans-AZ imposes an
interface dipole that decreases the local vacuum energy level (Evac); right: photoiso-
merization to cis-AZ imposes an interface dipole that increases the local vacuum
energy level (Evac). mper is the vertical component of the surface dipole, FM is the
These phenomena are also consistent with the
reported cooperative effect of organic molecules
in the electron transfer between a metal substrate
and organic layers. In such organic–inorganic
interfaces, charge transfer acts to reduce the
dipole–dipole interaction between molecules but metal work function, and Fe is the electron injection barrier.
162
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