NANO
LETTERS
Optical Properties of Rectangular
Cross-sectional ZnS Nanowires
2004
Vol. 4, No. 9
1663-1668
Qihua Xiong,†,‡ G. Chen,† J. D. Acord,‡ X. Liu,† J. J. Zengel,‡ H. R. Gutierrez,†
J. M. Redwing,‡ L. C. Lew Yan Voon,§ B. Lassen,§ and P. C. Eklund*,†,‡
Department of Physics, Department of Materials Science and Engineering,
The PennsylVania State UniVersity, UniVersity Park, PennsylVania 16802, and
Department of Physics, Worcester Polytechnic Institute,
Worcester, Massachusetts 01609
Received June 1, 2004; Revised Manuscript Received July 6, 2004
ABSTRACT
ZnS nanowaveguides with rectangular cross-section (∼50 × 50 nm2 ) and tens of microns in length have been synthesized by pulsed laser
vaporization of ZnS/10% Au targets in a flow of Ar/5% H2. The highly crystalline filaments exhibit the wurtzite structure, growing mainly along
the [001] or [100] directions. Photoluminescence at room temperature shows strong near-edge luminescence doublets (∼3.75 eV and 3.68 eV)
and a weak defect luminescence structure attributed to stoichiometric defects and possibly to Au impurities. Optical absorption (OA) at room
temperature shows a strong broadening of the fundamental direct absorption edge identified with stoichoimetric defects. Two peaks (3.75 and
3.85 eV) in the OA are also observed. We believe that the structure in the photoluminescence and optical absorption (3.68, 3.75, 3.85 eV) are
from direct transitions between the conduction band and the spin−orbit/crystal field splitting of the valance bands. Theoretical results are also
presented that show the size-dependence of the band gap in ZnS nanowires.
Wide band-gap semiconducting nanowires (e.g., ZnO, GaN)
have attracted considerable attention recently because of their
promising applications in optoelectronics and UV nanolasers.1-4 Optically and electrically driven lasing has been
demonstrated in ZnO2,4 and GaN nanowires,3 respectively.
One essential requirement to obtain nanowire lasers is that
photonic confinement must be achieved. This requires a
smooth surface along the wire and, ideally, cleaved end
surfaces which can work as efficient, partially transmitting
mirrors that define the optical cavity.3 It is technically
important to explore methods to grow highly crystalline
nanowires with smooth surfaces and controlled cross sections.
Yang et al. reported epitaxial growth of hexagonal cross
section ZnO nanowires that grow perpendicular to sapphire
substrates. They used a vapor transport and condensation
technique.5 Using a simple thermal evaporation method, Pan
et al. grew semiconducting nanobelts or nanoribbons (ZnO,
SnO2, In2O3, and CdO), i.e., the width-to-thickness ratio
g10.6 Most recently, Kuykendall et al. reported GaN
nanowires with a triangular cross section grown by a
metallorganic chemical vapor deposition route.7 It seems that
nanowires may be grown with a variety of cross-sectional
configurations depending on the details of the growth
conditions. Fundamental understanding of how to control
specific cross-sectional configurations is still lacking.
* Corresponding author. E-mail: pce3@psu.edu.
† Department of Physics, PSU.
‡ Department of Materials Science and Engineering, PSU.
§ Worcester Polytechnic Institute.
10.1021/nl049169r CCC: $27.50
Published on Web 07/31/2004
© 2004 American Chemical Society
ZnS is another important direct band-gap semiconductor
that has been investigated extensively because of its potential
optical applications.8 The cubic ZnS phase has Eg ) 3.68
eV.9 Considerable effort has been devoted recently into
controlling the size and shape of ZnS nanofilaments so that
size-dependent and shape-dependent properties can be studied, e.g., ZnS nanobelts with wurtzite structure were grown
by thermal evaporation methods.10,11 These nanobelts were
several tens or hundreds of nanometers in width and several
microns in length.10,11 Using the same technique,10,11 Wang
et al. were able to grow micron-long, cylindrical ZnS
nanowires with diameter d ) 30-60 nm.12 Most recently,
Zapien et al.13 demonstrated room-temperature laser action
in ZnS nanoribbons with an optical pumping threshold φ ∼
40-60 kW/cm2.
Further detailed investigations of the optical properties of
nanowires in general, and ZnS nanowires in particular, are
needed to appreciate their possible applications in nanooptoelectronics and nanolasers. For example, how do the
morphology, doping, and cross-sectional area affect the
photoluminescence, optical band gap, and the laser efficiency? In this paper, we present a route to grow highquality, rectangular cross-sectional wurtzite ZnS nanowaveguides ∼50 × 50 nm2 × 10 µm in length using pulsed
laser vaporization (PLV). Photoluminescence and optical
absorption experiments have been carried out to study the
optical properties of these “waveguides”. We also present
results of theoretical calculations on the nanowire diameter
dependence of the fundamental band gap.
Figure 1. FESEM image of ZnS nanowires dispersed on a piece
of Si wafer. The inset at the upper left corner shows a nanowire
growth tip with higher magnification. The inset to the bottom right
corner shows a thickness distribution obtained by AFM.
Figure 2. X-ray diffraction spectrum of ZnS nanowires using a
Cu KR radiation. The peaks are identified with a wurtzite 2H ZnS
structure with lattice constants a ) 3.83 Å and c ) 6.26 Å, which
are consistent with powder diffraction data (10-434, a ) 3.82 Å,
c ) 6.26 Å).
The ZnS nanowires were grown by PLV.14 A heated target
of well-mixed ZnS (Alfa Aesar) and 10 at. % Au powder
(Alfa Aesar) was ablated by a Nd:YAG laser. The nanowire
growth was found to proceed via the vapor-liquid-solid
(VLS) growth mechanism.15 A detailed description of our
apparatus is available elsewhere.16 Briefly, the target was
placed in a double-quartz tube system centered in a tube
furnace. A carrier gas of 100 sccm Ar (5%H2) was introduced
between the inner and outer quartz tubes, allowing the gas
to be preheated before entering the reaction zone (pressure
∼ 225 Torr). The target was centered in the 1-meter-long
furnace that was operated at 950 °C. The as-grown nanowires
were collected from the inner quartz tube down stream by
∼40 cm from the target. The ZnS nanowires appeared as a
white powder.
Field emission scanning electron microscopy (FE-SEM,
JEOL 6700F), X-ray diffraction (XRD, Phillips X-Pert MPD
Cu KR), transmission electron microscopy (TEM, Philips
420, JEOL 2010F), and atomic force microscopy (AFM,
Digital Instruments, Multimode) have been used to characterize the nanowires. The nanowires were first dispersed into
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Figure 3. (a) Bright-field TEM image of ZnS nanowires. Faceted
tips (solid arrows) at one end of the nanowires and abrupt ends
(hollow arrows) at the other ends are quite prominent. Inset at the
right corner is the width distribution calculated from several TEM
images, showing a most abundant population around 45 nm. (b)
Higher magnification TEM image of an individual nanowire shows
parallelepiped feature.
ethanol by ultrasonication. Then a few drops of this suspension were deposited either onto Si substrates, copper grids,
or freshly cleaved mica substrates, respectively, for optical
studies, FE-SEM, TEM, and AFM; the solvent was then
allowed to vaporize under ambient conditions. To have direct
visualization of the cross sections of our wires, we used an
ultramicrotome technique to prepare specimens for TEM
observations. Detailed information on sample preparation can
be found in ref 17.
Photoluminescence (PL) of the ZnS nanowires was taken
at room temperature and excited by a pulsed Nd:YAG 4th
harmonic (266 nm) laser (Nanolase, MicroChip NanoUV 266
nm) with the beam at 45° to the surface of the supporting
substrate. The pulse width and repetition rate are, respectively, about 0.5 ns and 10 kHz. The average power was ∼5
mW and the laser spot was ∼4 mm2. Optical absorption
experiments (Pelkin Elmer UV-vis Lambda 900) were carried
out on a thin film of ZnS nanowires deposited on sapphire
substrates. For PL experiments, Si/SiOx substrates were used.
Figure 1 shows a FE-SEM image (15 kV) of ZnS
nanowires deposited on a Si substrate. The nanowires are
flat with abrupt ends; they were found to be tens of microns
in length. At one end of some of the nanowires, a solidified
Nano Lett., Vol. 4, No. 9, 2004
Figure 4. (a-d) TEM and HRTEM images of the cross section of the ZnS nanowires. Most of the wires have rectangular or nearly square
cross sections (a and b). Some of the nanowires also have hexagonal cross sections (c and d). (b) and (d) are HRTEM images taken at
corners of (a) and (c), respectively.
metal particle was observed (inset to Figure 1, upper left
corner). We take this as proof of the VLS growth mechanism.14 The nanowires are clearly electron transparent, i.e.,
the electron beam can easily penetrate through the nanowires.
AFM height measurements of many isolated nanowires were
obtained to determine the thickness distribution. The average
thickness of the wires ht relative to the mica was found to be
ht ∼ 49 ( 13.6 nm, calculated from ∼30 separated wires;
13.6 nm is the standard deviation. The bottom right inset to
Figure 1 shows the thickness distribution of our wires. A
more detailed explanation of our sample preparation procedure for AFM can be found in ref 18. Figure 2 shows an
X-ray powder diffraction spectrum of the ZnS nanowires
using Cu KR radiation. All the peaks are identified with the
wurtzite 2H polytype of ZnS, except the three peaks from
Au. The ZnS lattice constants obtained are a ) 3.83 Å, c )
6.26 Å, in good agreement with powder XRD data for the
bulk (10-434, a ) 3.82 Å, c ) 6.26 Å).
Figure 3 shows two bright-field TEM images of our ZnS
nanowires. From several such TEM images, the lateral
dimension (width) distribution was determined as shown in
the inset to Figure 3a. We find that the most abundant
nanowires have a width of w
j ∼ 55 nm, while a few wires
are as wide as 200 nm. The good agreement between the
thickness distribution by AFM and width distribution by
TEM suggests that the majority of the filaments have nearly
square cross section. If they were ribbons we would expect
the ribbons would lie flat on mica rather than on edge. In
this case, we would expect ht , w
j . However, we observe ht
Nano Lett., Vol. 4, No. 9, 2004
∼w
j , suggesting that nearly square cross-sectional wires were
grown. This is further confirmed by cross-sectional TEM
observations (Figure 4a). Two other interesting features were
observed in TEM. First, most of the metallic particles at the
growth tip of the nanowires are observed to be polyhedrons
(solid arrows in Figure 3a), instead of the usual spheres we
have seen in other nanowires grown by PLV. Second, the
other free end of the nanowires (hollow arrows in Figure
3a) appears cleaved in many cases, and the cleaved surfaces
are almost perpendicular to the wire axis, consistent with
observations by FE-SEM (Figure 1). The stripe contrast in
the TEM image can be identified with well-known “bend
contours”.19 A magnified TEM image of a nanowire end is
shown in Figure 3b and exhibits a rectangular cross section
and a well-cleaved end. The cleavage probably occurs in
the ultrasonic processing that is used to achieve a metastable
suspension in ethanol for preparing TEM grids, etc. The wellcleaved ends of nanowires introduced by ultrasonication have
also been observed in CdS nanowires.3 From our SEM and
TEM results, we believe that the small and intermediate
cross-sectional nanowires follow the VLS growth mechanism. The larger cross-sectional wires may stem from a
thermal evaporation growth similar to earlier observations.6,10,11 However, it should be noted that the width
distribution (Figure 3a) is continuous. Therefore, we tentatively invoke a 2nd growth mechanism only because we have
not found evidence of ∼100 nm diameter metallic particles
in the TEM images of our samples. For the second growth
mechanism (thermal), we believe the furnace drives the
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Figure 6. EDX spectra from the tip (up panel) and the body
(bottom panel) of an individual nanowire. The tip consists of mainly
Au and Zn, little S was detected. So the solidified liquid droplet is
Au-Zn alloy; while the body contains mainly Zn and S, no other
impurities were detected. Copper peaks are from the grid.
Figure 5. (a) HRTEM image of a ZnS naowire growing along
[001] direction. The spacing between adjacent lattice fringes is 0.319
nm. The inset shows a selective-area electron diffraction pattern
from the same nanowire. There is no appreciable amorphous outer
coating on the nanowire. (b) Some wires growing along [100] were
also observed. The lattice spacing between fringes is 0.335 nm and
the inset shows a corresponding FFT of the image.
decomposition of the target and the carrier gas simply moves
the vapor to a cooler region in the furnace, where the growth
takes place. Figure 4 a-d shows TEM and HRTEM images
of the cross sections of our ZnS nanowires. Most of the wires
have a rectangular or nearly square cross section (Figure 4a),
while a few of the wires also have a hexagonal cross section
(Figure 4c). Figure 4b,d shows lattice images taken at the
corner of the cross sections shown in (a) and (c), respectively.
High-resolution TEM (HRTEM), selective-area diffraction
(SAD), and energy-dispersive X-ray spectroscopy (EDX)
were used to characterize the structure and composition of
our nanowires. Figure 5 shows two HRTEM images of ZnS
nanowires growing along the ⟨001⟩ (Figure 5a) or ⟨100⟩
directions (Figure 5b). Our nanowires are highly crystalline,
as indicated by the lattice fringes and SAD pattern. The
distances between lattice fringes (0.335 nm for ⟨100⟩ and
0.319 nm for ⟨001⟩ planes) were measured and they are
consistent with the d spacing values listed in the XRD
powder diffraction file (10-434). The two insets to Figures
5a and 5b show the Fourier transform of the HRTEM image
and SAD pattern from the same nanowire. Figure 6 shows
two EDX spectra from the tip (top panel) and the body
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(bottom panel) of our nanowire. The tip consists mainly of
Au and Zn. Very little S was detected in the tip, suggesting
that the solidified liquid droplet is a Au-Zn alloy. While
the nanofilament body contains only Zn and S, no other
impurities (e.g., Au) were detected in the filament by EDX.
It may be that the S diffuses out of the Au-Zn particle after
the nanowire growth stops.
Optical absorption (OA) and photoluminescence (PL) are
two closely related methods to probe the band structure (e.g.,
fundamental edges, excitons, surface, and impurity states)
of semiconductor materials.20 OA and PL analyses yield
information about how the optical properties of semiconducting nanowires can be tailored by intentional doping or by
surface modifications. Two emission bands in PL of ZnS
nanowires12 and nanobelts10 were reported in the previous
literature. However, the explanations presented for the peaks
are slightly different. On the one hand, Wang et al.12
attributed the emission band around 450 nm to surface states,
while Zhu et al.10 attributed it to defect-related emission.
Another stronger emission band was reported between 500
and 600 nm, depending on the specific impurity species, e.g.,
Au impurities form luminescence centers around 520 nm,12
while Mn2+ impurities introduce emission around 600 nm.10
A luminescence study on ZnS colloidal nanocrystals attributed visible emission (∼590 nm) to Mn2+ impurities and
the structures between 415 and 440 nm to vacancies and
interstitials.21 Tran et al.22 studied the photoluminescence of
ZnS epilayers on GaAs substrates systematically from 1.6
to 320 K. They observed free exciton photoluminescence at
room temperature. The fundamental band gap was determined to be 3.723 eV at room temperature, and the exciton
binding energy is about 40 meV.22
As shown in Figure 7a, a strong luminescence doublet
(3.68 and 3.75 eV) and weaker defect luminescence structure
Nano Lett., Vol. 4, No. 9, 2004
Figure 7. (a) PL spectrum of ZnS nanowires taken at roomtemperature excited by a 266 nm UV laser. Two strong emission
bands near band edge and four weak defect luminescence bands
were observed. Blue curves are Lorentzian line shape analyses after
a least-squares multi-Lorentzian fit (red curves). (b) Optical density
plot versus photon energy of ZnS nanowires. The inset shows a
zoomed view at the vicinity of the fundamental absorption edge.
at 2.44, 2.66, 2.86, 3.06 eV were observed for our nanowires.
The defect luminescence spectrum is multiplied by a factor
of 40 in the figure for clarity. These features were fitted with
multiple Lorentzians, as shown by the blue curves in the
figure. The 2.44 eV (∼510 nm) emission band might be due
to Au impurities,12 although no EDX evidence for Au in the
nanowire body was found. The other three weak emission
bands are probably due to stoichiometric vacancies or
interstitial impurities.21 The position of the structure (in eV)
in Figure 7a we observed is slightly different from that
reported for vacancies and interstitials in nanoparticles.21
Figure 7b shows the optical density vs photon energy for
our ZnS nanowires. The inset to Figure 7b gives a magnified
view of the optical density near the position of the band edge
in the bulk. First of all, even though the nanowire PL is in
good agreement with that of the bulk, the absorption edge
is very broad. We take this broadness to indicate strong band
tailing stemming from the stoichiometric defects. Two peaks
in the optical density are observed at 3.75 and 3.85 eV. These
positions are close in energy to the structure obtained from
our PL measurements.
Nano Lett., Vol. 4, No. 9, 2004
Wurtzite ZnS (in the bulk) is a direct gap semiconductor
with Eg ) 3.7 eV at the center of Brillouin zone.9 The
topmost valance band is split into three bands due to the
crystal field and spin-orbit coupling. Exciton states associated with electron-hole pairs in each of these valance bands
involved are usually denoted as A, B, C excitons (LandoltBornstein new series III 41B, p 1309). The spin-orbit
coupling and crystal-field splitting energy given in ref 9 are
89 and 57 meV, respectively. The three features of 3.68,
3.75, 3.85 eV observed in PL and OA are tentatively assigned
to the A, B, C excitons. The energy splittings we obtained
are approximately 100 and 70 meV. The reason we could
not observe structure at 3.68 eV in the optical absorption or
structure at 3.85 eV in PL is probably that these features are
masked by the strong band tailing, while the 3.85 eV exciton
state may decay rapidly to lower energy exciton states, i.e.,
3.75 eV and 3.68 eV. Time-resolved photoluminescence
would be important to support this assignment. Taking the
exciton binding energy of 40 meV into account, the
fundamental band gap we observed in w-ZnS nanowires is
∼3.72 eV, and the structures at 3.75 and 3.85 eV are
identified with B and C exciton states. Even though the large
exciton binding energy (40 meV) is much higher than thermal
energy at room temperature (∼26 meV), the band edge
photoluminescence of ZnS at room temperature was observed
only in very high quality single crystals.22 The observation
of strong band edge luminescence indicates that our nanowires are highly crystalline. At photon energy g 5.0 eV, we
observe a slight decrease in absorption, associated with
diminished high energy oscillator strength.20
We attempted to observe laser action in our ZnS nanowires
using a pulsed Nd:YAG laser at 266 nm. Unfortunately, we
must not have exceeded the lasing threshold in our experiments, as there were no signs of lasing as has been observed
in GaN,1 ZnO,2and CdS3 nanowires. Our approximate power
density was ∼500 mW/cm2, much lower than the threshold
reported for ZnS nanowires.13
In anticipation of the growth of nanowires with much
smaller cross sections, we have used a band-structure model
to study how the band gap changes with the linear dimension
for a square nanowire. The theory employed is the six-band
k.p model for the valence band, as originally proposed by
Mireles and Ulloa,23 and a one-band model for the conduction
band. We have adapted them for a free-standing nanowire,
i.e., we have assumed no confinement along the wire axis
and infinite confinement inside the transverse boundaries of
the nanowire. The valence-band parameters for ZnS were
obtained from Xia,24 and the band gap and conduction
effective mass from Landolt-Bornstein.9 The confinement
energies for various [100] and [001] nanowires, as a function
of the linear dimension w, are plotted in Figure 8; the band
gaps of the nanowires can then be obtained by adding the
bulk band gap to the two confinement energies. The
calculation verifies that, for the square nanowire with linear
dimension of about 50 nm, the quantum confinement energy
is negligible (less than 2 meV), and that the analysis of the
optical data in terms of the bulk band structure is appropriate.
As expected, the band gap energy increases with decreasing
1667
two exciton energy states of 3.75 and 3.85 eV are due to
deeper valence bands split by spin-orbit and crystal field
effects. Photoluminescence also shows weak defect luminescence due to stoichiometric defects and interstitial impurities. We cannot ambiguously identify Au as a luminescent
center in the wires.
Acknowledgment. This work was supported by NSFNIRT program (Nanotechnology and Interdisciplinary Research Initiative), grant DMR-0304178, Upenn MRSEC and
NSF-DMR (LYV). We thank Dr. Jinguo Wang for helpful
discussion on TEM.
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Figure 8. Variation of conduction (top, dotted curve) and valence
(bottom) confinement energies as a function of the size of square
[100] (solid curve) and [001] (dashed curve) nanowires.
lateral dimension w; the PL spectrum should therefore be
blue-shifted (e.g., by more than 100 meV for w < 10 nm).
We note, nevertheless, that the band gap increase remains
small until w e 20 nm or less. Due to the hexagonal
symmetry, nanowires with [100] and [001] growth directions
are expected to have different electronic and optical properties. The difference in the valence band energies is clear in
Figure 8. The conduction energies are the same because the
conduction effective mass is isotropic. We find that the band
gap is indeed different, but the difference is at most about
12 meV for the 5 nm wire. If such wires form a thin film of
entangled wires as for the current samples, the effect of the
different types of nanowires should be observed as a small
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We also carried out systematic room-temperature Raman
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Photoluminescence and optical absorption investigations
revealed that the fundamental energy gap of 3.72 eV and
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