Angewandte
Chemie
nanoparticle changed from a faceted structure, like in
Figure 3a, to a more spherical topology; starting from the
kinks between the facets, exfoliation and amorphization of
the outer molecular Cs2O sheets took place and advanced
progressively inwards. Careful inspection of this nanoparticle
revealed that the number of perfectly closed molecular layers
decreased from about 30 to 8, and most of the Cs2O layers (ca.
20) were only partially ruptured. The interlayer distance
increased from about 6.4 ꢀ in the pristine nanoparticle to
6.8 ꢀ (Figure 5b,c) and in some places even to 8.5 ꢀ (not
shown), likely due to intercalation of water. In general,
however, the inner layers of the IF-Cs2O nanoparticles were
found to be undamaged and exhibited smaller expansion.
metals, such as silver and bismuth, that are known to enhance
the quantum yield of photocathodes may in fact be related to
their possible catalytic effect in the synthesis of IF-Cs2O
nanoparticles. Theoretical work is underway to elucidate the
electronic structure of these nanoparticles. Previous studies
on various semiconducting nanotubes[17,18] indicated that
while quantum-size effects are of lesser importance in
closed-cage nanostructures, folding of the molecular sheets
reduces the energy gap and makes the nanotubes more
conductive, and this could favorably affect their thermionic
and photoemission.
Electron energy loss spectroscopy (EELS) and imaging Experimental Section
The 3R-Cs2O powder precursor in the present work was synthesized
with the Gatan imaging filter (GIF) revealed only Cs and O
(excess oxygen; Figure 5d), while GIF analysis of the
surrounding amorphous material (not shown), revealed Cs,
O, and C. These results, together with the above TEM
observations, imply that the amorphous and platelet materials
on the grid react vigorously with ambient CO2 to form cesium
carbonate, while the IF nanoparticles react slowly with water,
which could be possibly removed by mild heat treatment or
prolonged evacuation in the HRTEM column. In keeping
with previous observations on alkali-metal-doped WS2 and
MoS2 nanoparticles,[16] water uptake into the van der Waals
gap between the layers is common in layered compounds.
Furthermore, in analogy to intercalated IF-WS2,[16] evidence
for some time-dependent recovery of the damaged nano-
particles, accompanied by rounding of its initial polyhedral
shape, was observed after a few days in the high vacuum of the
HRTEM. The partially recovered Cs2O layers were less
faceted than the original ones. These observations indicate
that, in contrast to the presently used cesium oxide films in
photocathodes, which are irreversibly damaged under low
vacuum, the IF nanoparticles, if damaged, could possibly be
recovered by mild heating under vacuum.
The presence of the closed Cs-O-Cs hexagonal planes in
the IF-Cs2O structures clearly enhances the stability of the
material in ambient atmosphere. The inert van der Waals
surface of the closed nanoparticles presents a diffusion barrier
to the intercalation process, which leads to a sequential and
therefore slower hydrolytic attack on the layers in these
structures. In contrast, the existence of dangling bonds at the
prismatic edges on the periphery of the Cs2O quasi-two-
dimensional planar structures explains the high reactivity of
amorphous or platelet powder. Therefore, the formation of
closed-cage IF-Cs2O nanostructures induces kinetic stabiliza-
tion of the material and accounts for its intrinsically low
reactivity.
by reaction of measured amounts of cesium metal and oxygen at
1808C for 3 d.[15] The product was subsequently sealed in evacuated
quartz ampoules. The orange-tinted powder was characterized by
XRD and Raman spectroscopy.[8a,b] The Raman peak at n˜ = 103 cmÀ1
can be assigned to the A1g mode.[8a]
Cs2O powder precursor in a vacuum-sealed quartz ampoule was
ablated with a pulsed Nd:YAG laser; the beam was manually scanned
so as to impinge on a new position. Cooling with liquid N2 vapor was
performed through a copper tube in the form of a helical coil, which
wrapped part of the ampoule surface. Figure 1a shows the exper-
imental arrangement. During ablation, a deeply colored powder
accumulated on the walls of the cooled side of the quartz ampoule.
Further manipulations were performed in a specially designed
and built environmental cell attached as a matching counterpart to
the CompuStage entry of the TEM, operating under a flow of Ar of
purity > 99.99% and without any modification of the microscope or
of the sample holder; it allowed safe preparation, mounting, handling,
and introduction into the microscope of the specimen grid without
exposure to the ambient atmosphere. Figure 1b shows a schematic of
the environmental chamber; Figure 1c is an overview of the
installation of the chamber attached to the CM120 TEM column.
The quartz ampoule was broken inside the chamber, and a small
amount of the laser-ablated material transferred onto a gold grid for
mounting into the TEM. The environmental chamber was operated
with both the room-temperature and the cryo holders of the CM120
TEM. A completely different system, which only excludes air during
insertion of the grid into the microscope, has been previously
reported.[19]
The following microscopes were used: a TEM-Philips CM120
(120 kV) with EDS system (EDAX Phoenix) and a field-emission gun
high-resolution TEM (HRTEM) model FEI Tecnai F-30 (300 kV). A
Gatan imaging filter (GIF) was used for for electron energy loss
spectroscopy (EELS) and for elemental mapping. Fast Fourier
transform (FFT) of the high-resolution images was obtained by
Digital Micrograph software (Gatan). The computer simulation was
performed with the software MacTempas. The following parameters
were used for the simulation: thickness 35 nm; defocus of À30 nm.
Received: January 30, 2005
Revised: March 22, 2005
Published online: June 3, 2005
The synthesis of IF-Cs2O has led to a new member of the
fullerene-like family of otherwise extremely unstable but
important materials. The unprecedented relative stability of
Keywords: cage compounds · cesium · electron microscopy ·
the IF-Cs O nanoparticles synthesized here, both under the
.
2
nanostructures · oxides
electron beam and in air, is very encouraging and suggests
that once larger amounts of pure IF nanoparticles have been
synthesized and fully characterized, their beneficial effect on
lowering the work function and photoemission from various
surfaces could be directly evaluated and fully exploited in
NEA, photoemissive, and other devices. The action of other
[1] A. H. Sommer, Photoemissive Materials, Krieger, Huntington,
1980, p. 132 – 166, pp. 167–174.
[2] W. E. Pickett, Phys. Rev. Lett. 1994, 73, 1664 – 1667.
Angew. Chem. Int. Ed. 2005, 44, 4169 –4172
ꢀ 2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
4171