Optical-absorption spectra of inorganic fullerenelike M S
2
„M Mo, W…
G. L. Frey
Department of Materials and Interfaces, Weizmann Institute, Rehovot 76100, Israel
S. Elani
The Racah Institute of Physics, The Hebrew University, Jerusalem 91904, Israel
M. Homyonfer, Y. Feldman, and R. Tenne
Department of Materials and Interfaces, Weizmann Institute, Rehovot 76100, Israel
Received 8 May 1997; revised manuscript received 23 September 1997
Optical-absorption spectroscopy of inorganic fullerenelike MoS
2
and WS
2
IF-MoS
2
and IF-WS
2
is reported
in the range 400–800 nm, at temperatures between 4–300 K, and compared to the corresponding bulk 2H
material. A systematic study of the effect of IF size and number of atomic layers on the optical properties
shows that the semiconductivity of the layered material is preserved in the IF structures. Nevertheless, all IF
with number of layers ( n ) 6 exhibit a decrease in the A and B exciton energies. This redshift becomes larger
as additional inner layers are formed, until a saturation value is reached ( n 10). We assign this redshift to the
deformations, curvature, and discommensuration between adjacent atomic layers the structure must accommo-
date in order to form an IF structure. An increase in the exciton energies is observed in IF consisting of a few
sulfide layers ( n 5). This blueshift is attributed to a quantum confinement in the z direction. Band-structure
calculations show that an expansion of 7% along the c axis leads to a convergence of the levels K
1
and K
4
,
which is displayed in the absorption spectra of IF with 1 or 2 layers. S0163-18299803111-7
INTRODUCTION
In analogy to graphite nanoclusters, it was suggested that
the abundance of dangling bonds on the periphery of layered
metal dichalcogenides MX
2
where M =Mo, W; X =S, Se
nanoparticles destabilize their planar topology. This instabil-
ity was utilized for the synthesis of hollow cage structures of
MX
2
, including nested polyhedra, nanotubes and structures
with negative curvature, generically called inorganic
fullerenelike material
1,2
IFsee Fig. 1. Subsequently, the
large-scale synthesis of IF- M S
2
( M =Mo, W powder has
been reported.
3–5
The IF of metal dichalcogenides exhibit a
variety of sizes and shapes and have been investigated by
x-ray powder diffraction XRD,
3
scanning tunneling
microscopy
6
STM, and high-resolution transmission elec-
tron microscopy TEM.
3–5
The present work is the study of
the optical properties of these materials.
The physical and structural properties of crystalline
transition-metal dichalcogenides are reviewed by Wilson and
Yoffe.
7
Electronic band-structure calculations were per-
formed for 2H- MX
2
M =Mo, W; X =S, Se and are re-
viewed by Doni and Girlanda.
8
The present status of experi-
mental evidence appears to favor the model proposed by
Coehoorn and co-workers.
9,10
They found that the lowest al-
lowed transition is indirect and the two higher excitonic tran-
sitions occur at the K point of the Brillouin zone. The optical
transition in 2H-MoS
2
and 2H-WS
2
are summarized in Table
I. The A and B excitons are assigned to transitions at the K
point of the Brillouin zone, with K
4
and K
1
being the initial
states, and K
5
the final state.
10
The A and B splitting K
4
and
K
1
is due to interlayer interaction and spin-orbit splitting.
The splitting values are 0.18 and 0.41 eV for 2H-MoS
2
and
2H-WS
2
, respectively.
Quantum confinement of carriers was also studied in
transition-metal dichalcogenides. Their characteristic layered
structure leads to a quantization effect in the c axis perpen-
dicular to the layers in ultrathin platelets
14–16
and to a quan-
tum size effect in nanoparticles 45 Å.
17,18
This paper presents a comprehensive study of the optical
properties of IF- M S
2
materials. The optical-absorption mea-
surements show that the semiconductivity of M S
2
is pre-
served in the IF structure. Nevertheless, the position of the A
and B excitons are altered in comparison to the 2H bulk. A
systematic study of the effect of IF size and number of
atomic layers reveals that the position of the excitons is de-
pendent on the number of IF layers ( n ) rather than the par-
ticle size. All samples consisting of IF with n 6 exhibit a
redshift of the exciton energies, while all samples consisting
of IF with n 5 exhibit a blueshift of the exciton energies.
This paper contains the experimental results and a statistical
analysis in order to estimate the error introduced to the spec-
tra due to sample inhomogeneity. In the discussion we argue
the origins of the blueshift and the somewhat surprising red-
FIG. 1. TEM micrograph of a typical IF-MoS
2
particle, b
typical WS
2
nanotube. The distance between two fringes M S
2
lay-
ers is 6.2 Å.
PHYSICAL REVIEW B 15 MARCH 1998-I VOLUME 57, NUMBER 11
57 0163-1829/98/5711/66666/$15.00 6666 © 1998 The American Physical Society