Size-induced symmetric enhancement and its relevance to photoluminescence of scheelite CaWO4 nanocrystals

Article abstract of DOI:10.1063/1.2450659

This work explores size-induced lattice modification and its relevance to photoluminescence properties of scheelite nanostructures. Ca WO4 nanocrystals, a prototype scheelite compound, exhibited a lattice expansion and an increased symmetry of structural units with physical dimension reduction, which is in contradiction to the trend previously reported in bulk CaW O4 at high pressures or high temperatures. Lattice variations in CaW O4 nanocrystals are probably due to the negative pressures that originated from strong defect dipole interactions on surfaces. The increased structural symmetry along with surface citric modifications produced a significant enhancement in photoluminescence of CaW O4 nanocrystals, indicating a quantitatively structural control over the electronic properties.

Full text of DOI:10.1063/1.2450659

Size-induced symmetric enhancement and its relevance to
photoluminescence of scheelite CaWO4 nanocrystals
Liping Li, Yiguo Su, and Guangshe Li
Citation: Appl. Phys. Lett. 90, 054105 (2007); doi: 10.1063/1.2450659
View online: http://dx.doi.org/10.1063/1.2450659
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Published by the American Institute of Physics.
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APPLIED PHYSICS LETTERS 90, 054105 ͑2007͒
Size-induced symmetric enhancement and its relevance
to photoluminescence of scheelite CaWO4 nanocrystals
Liping Li, Yiguo Su, and Guangshe Lia͒
State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of Matter,
Graduate School of Chinese Academy of Sciences, Fuzhou 350002, People’s Republic of China
͑Received 21 August 2006; accepted 8 January 2007; published online 30 January 2007͒
This work explores size-induced lattice modification and its relevance to photoluminescence
properties of scheelite nanostructures. CaWO4 nanocrystals, a prototype scheelite compound,
exhibited a lattice expansion and an increased symmetry of structural units with physical dimension
reduction, which is in contradiction to the trend previously reported in bulk CaWO4 at high
pressures or high temperatures. Lattice variations in CaWO4 nanocrystals are probably due to the
“negative pressures” that originated from strong defect dipole interactions on surfaces. The
increased structural symmetry along with surface citric modifications produced a significant
enhancement in photoluminescence of CaWO4 nanocrystals, indicating a quantitatively structural
control over the electronic properties. © 2007 American Institute of Physics.
͓DOI: 10.1063/1.2450659͔
Alkaline metal tungstates of scheelite-like structures
have broad applications in fundamental and applied sciences
including photoluminescence, microwave devices, optical fi-
bers, and next generation of hybrid cryogenic phonon-
scintillation detectors.1–5 All these applications are highly de-
pendent on the lattice dimension and band structures. From
the solid state physics viewpoint, details of the band struc-
ture are primarily determined by the static potential within
unit cells, and any symmetry perturbation in unit cell can
have consequences on the electronic structures and physical
properties. Nanocrystals show properties that depend
strongly on structure. Variations in lattice dimension and
symmetry often accompany particle size reductions.5–7
Therefore, an experimental identification of the nanoscale
lattice modifications is advantageous for tuning the band
structure and moreover physical properties of nanocrystals.
Photoluminescence is sensitive to the local lattice symmetry
and could be used as an intrinsic probe to map out such
variations in scheelite nanocrystals. Sample quality and
nanoparticle sizes currently limit this approach.
For instance, particle size control of scheelite CaWO4
nanocrystals has been extensively studied,8–11 it is still very
difficult to get the diameter smaller than 10 nm for pro-
nounced size effects without loss of sample quality. Using
ethylene glycol at 180 °C, Chen et al.,8 obtained CaWO4
nanoparticles of 40 nm in diameter, which is too large to
result in apparent changes in band structures and physical
properties. Ryu et al.9 prepared a small diameter of
12–35 nm using a microwave irradiation as followed by a
heat treatment at high temperatures, while some amounts of
amorphous phases are still observed to coexist with 12 nm
CaWO4. The difficulties in diameter control of high quality
CaWO4 nanocrystals and the lack of experimental knowl-
edge about the relevant lattice dimensions impose obvious
limitations regarding the application to the interpretation of
the band structures and the entire set of physical properties.
CaWO4 represents a prototype scheelite compound that
shows a closely structural link to many materials such as
CaMoO4, PbMoO4, PbWO4, YLiF4, and high pressure
phases of TbVO4 and DyVO4. A systematic study on the
physical dimension control and size-related photolumines-
cence properties of CaWO4 nanocrystals is expected to pro-
vide information on the band-gap tailorings of a broad class
of nanoscale materials for many applications. In this work,
we report on the synthesis of CaWO4 nanocrystals at sizes
ranging from 3.4 to 31.7 nm. We investigated the size-
inducted lattice variations and their correlation to the photo-
luminescence properties of CaWO4 nanocrystals. Our highly
crystalline CaWO4 nanocrystals luminesced more intensively
than those samples prepared by other synthetic techniques.
Highly crystalline CaWO4 nanocrystals were prepared
by a hydrothermal method. 0.5M Na2WO4 solution was
mixed with 80 ml of CaCl2 and citrate acid solution at given
concentrations. A well-controlled amount of NaOH solution
was then carefully added to the clear solution until pH=8.
After aging at room temperature for 12 h, a white dispersion
͑CaWO4 seeds͒ was homogeneously formed. The mother so-
lution containing CaWO4 seeds was sealed in Teflon-lined
stainless steel autoclaves which were allowed to react at dif-
ferent temperatures for 12 h. Sample structures were charac-
terized by x-ray diffraction ͑XRD͒. Lattice parameters were
calculated using Retica Rietveld program with peak positions
that are calibrated by internal standard of nickel. Average
sizes were calculated using Scherrer formula and were fur-
ther confirmed by transmission electron microscope ͑TEM͒.
Chemical species bonded to sample surfaces were examined
by infrared spectra, while their amounts were quantitatively
determined by a combustion analysis method. Photolumines-
cence ͑PL͒ spectra of the samples were recorded using
FLS920 luminescence spectrometer under an excitation line
of 243 nm.
All samples gave diffraction peaks ͑Fig. 1͒ that are es-
sentially identical to the standard data for bulk CaWO4,12
which indicates the formation of single-phase tetragonal
scheelite structure. The internal strains were very small and
ignored here. The average size for CaWO4 seeds was calcu-
a͒
Author to whom correspondence should be addressed; FAX: ϩ86-591-
83714946; electronic mail: guangshe@fjirsm.ac.cn
0003-6951/2007/90͑5͒/054105/3/$23.00
90, 054105-1
© 2007 American Institute of Physics
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054105-2
Li, Su, and Li
Appl. Phys. Lett. 90, 054105 ͑2007͒
FIG. 1. XRD patterns for CaWO4 nanocrystals at given particle sizes. The
vertical bars represent the standard diffraction data for bulk CaWO4 from
JCPDS file ͑Ref. 12͒. The inset is a high resolution TEM image for 15.1 nm
CaWO4.
lated to be 3.4 nm from peak broadening, which seems to be
the only example of preparation of CaWO4 with dimension
smaller than 5 nm. With increasing the reaction temperatures
to 190 °C, the diffraction peaks became narrowed ͑Fig. 1͒,
and the physical dimension of CaWO4 seeds grew up to
31.7 nm. TEM observations ͑inset of Fig. 1͒ indicated that
all particles were tiny single crystals in a spindly shape with
average diameters being much closer to those calculated
from broadened diffraction line. The spacing between adja-
cent lattice fringes along the orientation direction is
0.282 nm, which is comparable with d spacing of 0.285 nm
for ͑004͒ plane as calculated for 15.1 nm CaWO4. Therefore,
CaWO4 nanocrystals were oriented along c-axis direction.
The surfaces of CaWO4 nanocrystals were confirmed by IR
spectra ͑not shown͒ to be chemically bonded with citrate
species and hydration layers. The chemical formula of
CaWO4 nanocrystals is thus described as CaWO4 xH2O.
yC6H8O7. Our careful chemical analysis of carbon and hy-
drogen clearly demonstrated that the molar ratios of citrate
species ͑y͒ and absorbed water ͑x͒ increased with particle
size reduction ͑Fig. 2͒.
FIG. 3. Particle size dependence of ͑a͒ lattice volume and ͑b͒ axis ratio of
c/a for CaWO4 nanocrystals.
expansion͒. Consequently, the lattice dimension dependence
of structural symmetry for CaWO4 nanocrystals ͓Fig. 3͑b͔͒ is
opposite to what is observed in bulk under high pressures13
or at high temperatures.14 For the latter cases, a decrease in
lattice volume gives rise to an increased symmetry ͑or c/a
axis ratio decrease͒.
The lattice expansion observed in Fig. 3͑a͒ could arise
from a number of possible sources. An unlikely source
would be surface stresses, since these give a lattice contrac-
tion in metal nanocrystals.15 We proposed that the causes of
lattice expansion in CaWO4 nanocrystals are primarily due
to the chemical bonding variations of framework ions under
“negative pressure” effects. In a tetragonal scheelite struc-
ture, one tungsten atom is coordinated with four oxygen at-
oms and formed an isolated tetrahedron.11 The bonding be-
tween Ca2+ and WO42− is mainly ionic, whereas inside WO4
polyhedron, the W–O bonds are primarily covalent with al-
most equal bond lengths. The outmost surfaces of CaWO4
nanocrystals could be terminated with a large fraction of un-
saturated calcium ions and/or tungstate groups, most likely
creating an imbalance of the charges at solid boundaries/or
surfaces. Consequently, a defect dipole layer is expected to
occur.5 Though these dipole moments on surfaces could be
partially removed by chemical bonding of citrate species and
absorbed water molecules, the repulsive interactions among
these dipoles might be strong enough to yield negative pres-
sure by varying the chemical nature of ionic Ca2+–WO42−
bonds for lattice expansion ͓Fig. 3͑a͔͒. Size-dependent
covalency/ionicity data are unavailable for the present nano-
crystals, nevertheless Boerio-Goates et al.5 have argued that
lattice expansion in ionic rutile TiO2 nanocrystals is corre-
lated with the negative pressure and the resulting covalency
enhancement.
Figure 3 illustrated the lattice parameters of CaWO4
nanocrystals as a function of particle sizes. Lattice volume
increased monotonically with particle size reduction
͓Fig. 3͑a͔͒, while the axial ratio of c/a decreased from 2.169
to 2.154 ͓Fig. 3͑b͔͒, tending to c/a=2 for an ideal structure
of scheelite phase with equal cation coordination.13 This ob-
servation clearly indicated that the symmetry of structural
units increased with physical dimension reduction ͑or lattice
With regards to the symmetry enhancement in CaWO4
nanocrystals ͓Fig. 3͑b͔͒, it has to take into account the influ-
ences of hydrated surfaces on surface energies. Just like what
is observed in many nanoscale oxides and mineral oxides,16
the surfaces of CaWO4 nanocrystals were highly hydrated
͑Fig. 2͒. According to a theoretical work on bulk CaWO4,17
surface hydration gives rise to decreased surface energies
FIG. 2. Particle size dependence of molar ratios of absorbed water mol-
ecules and citric species in CaWO4 nanocrystals. The chemical formula of
CaWO4 nanocrystals is described as CaWO4 ·xH2O·yC6H8O7.
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054105-3
Li, Su, and Li
Appl. Phys. Lett. 90, 054105 ͑2007͒
intensity can be always decreased with particle size reduc-
tion, since CaWO4 nanocrystal surfaces are also chemically
bonded with citric species ͑Fig. 2͒ which may reduce dan-
gling bond density on surfaces and hence the number of trap
sites for nonradiative recombination. For the latter case, the
quantum efficiency of photoluminescence could be signifi-
cantly enhanced,22 somewhat like what has been observed
for sulfide nanocrystals with surfaces being modified by or-
ganic materials.23 The balance of all these factors might be
the primary cause for the enhanced photoluminescence emis-
sion of CaWO4 nanocrystals even at a small size of 3.4 nm.
This work was financially supported by NSFC under the
contract ͑No. 20671092͒, a grant from Hundreds Youth Tal-
ents Program and a Directional Program of CAS ͑Li G͒, and
in part by a Science and Technology Program from Fujian
Province ͑Nos. Z0513026 and 2005HZ01-1͒.
FIG. 4. PL spectra of CaWO4 nanocrystals with diameters of ͑a͒ 31.7 nm,
͑b͒ 15.1 nm, ͑c͒ 9.3 nm, and ͑d͒ 3.4 nm.
1V. Pankratov, L. Grigorjeva, D. Millers, S. Chernov, and A. S. Voloshi-
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depending on the crystal planes. Consequently, size reduc-
tion may have impacts on the symmetry of structural units
and the resulting properties. In bulk, WO4 units are directly
aligned along a axis, whereas along c axis there is a Ca
cation located between two WO4 tetrahedra. As a result, the
variability of axis length along c and a directions is reason-
able to occur, owing to the different arrangements of hard
WO4 tetrahedra. Consistent with the oriented growth of
CaWO4 nanocrystals along the c-axis direction ͑Fig. 1͒, the
hydrated ͕001͖ plane exhibits the lowest surface energy of
0.36 J m−2 among all exposed planes, while the surface en-
ergy for hydrated ͕100͖ and ͕010͖ planes is comparatively
large, being around 1.06 J m−2.17 Therefore, a decreased c/a
axis ratio is expected since the dipole-dipole interactions on
the polar end surfaces ͑001͒ might have very limited influ-
ence on the lattice dimension, which explains the enhanced
structural symmetry in CaWO4 nanocrystals.
Photoluminescence is sensitive to the lattice variations
and surface modifications. All CaWO4 nanocrystals gave a
dominant emission centered at 421 nm ͑Fig. 4͒, which is
attributed to the intrinsic electronic transfer of WO4 ͑Ref.
18͒ or to the self-trapped centers with exciton energies being
located within the band gap.19 PL emission for the present
nanocrystals persisted smaller than 10 nm, while those for
other nanocrystals are hard to see even at particle sizes of
about 20 nm.19 To understand the nature of this apparent
inconsistency, an analysis of microstructural factors of
CaWO4 nanostructures is required. As stated above, the sym-
metry of structural units of CaWO4 nanocrystals increased
with particle size reduction, which would decrease the lumi-
nescence intensity.20 Surface hydration may weaken to cer-
tain extent the luminescence intensity because OH− groups
from the absorbed water could act as the luminescent
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