ZnO and Zn1−xCdxO nanocrystallites obtained by oxidation of precursor
Langmuir–Blodgett multilayers
Sukhvinder Singh a , R.S. Srinivasa b , S.S. Talwar a , S.S. Major a,⁎
a
b
Department of Physics, Indian Institute of Technology Bombay, Mumbai-400076, India
Department of Metallurgical Engineering and Materials Science, Indian Institute of Technology Bombay, Mumbai-400076, India
Abstract
ZnO and Zn1−xCdxO nanocrystallites were prepared by oxidation of zinc arachidate–arachidic acid and zinc arachidate–cadmium arachidate–
arachidic acid LB multilayers, respectively. The metal content of the multilayers was controlled by manipulation of subphase composition and pH.
Precursor multilayers were oxidized in the temperature range of 400 °C–700 °C. The formation of ZnO and Zn1−xCdxO was confirmed by UV–
Visible spectroscopy. Uniformly distributed, isolated and nearly mono-dispersed nanocrystallites of ZnO (11 ± 3 nm) and Zn1−xCdxO (18 ± 6 nm)
were obtained.
Keywords: ZnO; Zn1−xCdxO; Nanocrystallites; Langmuir–Blodgett
1. Introduction
Zinc oxide, a wide band gap semiconductor is an attractive
material for a variety of applications [1]. There is a growing
interest in ZnO nanostructures owing to expected improvements
in room temperature lasing due to quantum size effects [2].
Formation of ZnO has been reported in low dimensional forms,
showing interesting size dependent effects [3]. Alloying ZnO
with CdO allows the engineering of its band gap towards the
design of novel optoelectronic devices [4].
Langmuir–Blodgett (LB) multilayers of zinc arachidate
(ZnA) and cadmium arachidate (CdA) have been used to form
zinc oxide and cadmium oxide films by oxidation of the
corresponding precursor multilayers [5–7]. In the present work,
the oxidation of ZnA and mixed ZnA–CdA LB multilayers has
been carried out to form nanocrystallites of ZnO and Zn1−xCdxO
on quartz and freshly cleaved mica substrates.
2. Experimental details
ZnA and ZnA–CdA mixed multilayers were prepared using
a KSV 3000 LB instrument. A solution of arachidic acid (AA)
in chloroform (1 mg/ml) was spread on subphase containing
ZnCl2 and CdCl2 in varying proportions and a total concentration of 5 × 10− 4 M in de-ionized and ultra filtered water. The
subphase temperature was maintained at 10 ± 1 °C and subphase
pH was varied in the range 5.0–6.2. Quartz, CaF2 and freshly
cleaved mica substrates were used. The as-deposited multilayers
were heat treated for 30 minutes in a stream of oxygen at
atmospheric pressure in the temperature range of 400–700 °C.
FTIR spectra were obtained with a Perkin Elmer Spectrum1
instrument and UV–Visible spectra with a Perkin Elmer
Lambda 950 spectrophotometer. Atomic force microscopy
(AFM) studies were carried out using a Nanoscope IV
Multimode SPM.
3. Results and discussion
The zinc content of ZnA multilayers was varied by changing
the subphase pH at which the multilayers were transferred and
was monitored by studying their FTIR spectra. Typical results
are shown in Fig. 1(a). The spectrum of the multilayer
transferred at a subphase pH of 6.2 shows a strong absorption
at ∼1540 cm− 1, assigned to the asymmetric stretching of COO−
[8]. The presence of this band and the absence of C_O
stretching band of unionized carboxylic acid which is expected
at ∼ 1700 cm− 1, confirm that this multilayer consists of pure
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Fig. 1. FTIR transmission spectra of (a) ZnA and ZnA–AA composite
multilayers and (b) ZnA–CdA and ZnA–CdA–AA composite multilayers on
CaF2 substrates transferred at different subphase pH (as indicated).
ZnA. As the subphase pH is lowered to 5.8, a peak at
∼ 1700 cm− 1 appears and increases in intensity. This shows that
the ZnA content is reduced and that of AA is enhanced, forming
composite ZnA-AA multilayers. In case of multilayers
transferred at subphase pH of 5.0, the peak at ∼ 1700 cm− 1
becomes dominant, which shows that the composite multilayer
consists of primarily AA along with a small quantity of ZnA.
The spectra of the typical CdA–ZnA multilayers transferred at
subphase pH of 6.2 and 5.0 are also shown (Fig. 1(b)), which
indicate the reduction in metal content of the mixed multilayer
transferred at lower subphase pH.
The oxidation of arachidate multilayers and removal of
organic moieties has been reported earlier [7]. XRD studies
have shown that the layered structure of the multilayers
disappeared at ∼ 150 °C. The corresponding FTIR studies
have shown that all the absorption bands disappeared for
multilayers oxidized at 400 °C, indicating the removal of all
organic moieties by oxidation at 400 °C or above. Transmission
spectra of the ZnA and ZnA–AA multilayers transferred on
quartz substrates and oxidized at 400 °C are shown in Fig. 2(a).
The spectrum of oxidized ZnA multilayer clearly shows an
absorption edge at ∼ 3.2 eV, close to the bulk band gap of ZnO
which confirms the formation of ZnO. Subsequent heat
treatments up to 700 °C did not show any change in the
transmission spectra. With decrease in subphase pH, the
absorption edge of oxidized ZnA–AA multilayers becomes
less pronounced and only a small drop in transmittance on the
lower wavelength side is seen for the ZnA–AA multilayer
transferred at subphase pH of 5.0. These features are attributed
to the formation of ZnO and the decrease in absorption of
oxidized multilayers with decreasing subphase pH is attributed
to the reduction in their zinc content.
Fig. 2(b) shows the transmission spectra of the composite
ZnA–CdA multilayers transferred at subphase pH of 6.2 for
different molar concentrations of CdA (in subphase) and after
oxidation at 400 °C. The spectrum of pure CdA after oxidation
is also given as a reference. A monotonic shift in the absorption
edge towards lower energies is clearly seen with increase in the
CdA content. This confirms that the oxidation of mixed ZnA–
CdA multilayers results in the formation of Zn1−xCdxO alloy.
AFM studies were carried out in tapping mode using etched
silicon probes to investigate the surface morphology of the
oxides. Typical results are shown in Figs. 3 and 4. AFM images
of ZnO obtained from precursor multilayers consisting of 15
monolayers (ML) transferred at subphase pH of 6.2 and 5.0 and
oxidized at 400 °C are shown respectively in Fig. 3(a) and (b).
Fig. 3(a) shows the presence of a large number of ZnO particles
with a mean size of 1.1 ± 0.7 μm along with particles as large as
∼4 μm. On closer observation, the larger particles were found
to be agglomerates, as shown in the inset. Fig. 3(b), in
comparison, shows more uniformly distributed particles with
lesser spatial density and a smaller mean size of 0.8 ± 0.5 μm.
The reduction in size and spatial density is attributed to the fact
that the ZnA–AA multilayer transferred at subphase pH of 5.0
had the smallest ZnA content and thus the surface density of
zinc atoms available for oxidation was much reduced.
For further oxidation studies at higher temperatures, both
ZnA–AA and ZnA–CdA–AA multilayers transferred at a
Fig. 2. UV–Visible transmittance spectra of oxidized (a) ZnA and ZnA–AA
composite multilayers transferred at different subphase pH (as indicated) and
(b) ZnA–CdA and ZnA–CdA–AA composite multilayers transferred with
different CdA contents in the subphase (as indicated).
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(Fig. 4(c)) for a 7 ML ZnA–CdA–AA multilayer (transferred
from subphase containing 20 mol.% CdA at pH of 5.0) on mica
substrate, oxidized at 700 °C shows the presence of isolated
nanocrystallites of Zn1−xCdxO. In comparison to the ZnO
nanocrystallites, the Zn1−xCdxO nanocrystallites have a much
reduced spatial density and slightly larger mean size of 18 ± 6 nm.
Similar results were obtained on quartz substrates, but the smooth
surface of freshly cleaved mica allows smaller nanocrystallites to
be more clearly seen.
Fig. 3. AFM images of ZnO particles obtained by oxidation at 400 °C of ZnA–AA
multilayers (15 ML) on quartz, transferred at (a) subphase pH of 6.2 and
(b) subphase pH of 5.0. Inset of (a) shows a zoomed in image of an agglomerate.
subphase pH of 5.0 were taken up. The ZnA–AA multilayer
(15ML) transferred on freshly cleaved mica was oxidized at
700 °C and the corresponding AFM image is shown in Fig. 4(a).
This image shows the presence of uniformly distributed ZnO
particles of much smaller size with a high spatial density, in
contrast to the larger size ZnO particles obtained from ZnA
precursor multilayers (Fig. 3). The inset of Fig. 4(a) shows the
mean size of the ZnO nanocrystallites to be 11 ± 7 nm. No larger
particles or agglomerates were seen in this case even for large
area scans at lower magnifications. The drastic changes in the
morphology of particles are attributed to the changes in
interfacial tension between the molten organic moieties and
the substrate surface during oxidation. In order to obtain
crystallites with smaller size and density, the zinc content was
further decreased by reducing the number of monolayers in the
precursor. Fig. 4(b) shows the AFM image of a 7ML ZnA–AA
multilayer on mica, oxidized at 700 °C. The image shows
uniformly distributed isolated nanocrystallites with a much
reduced spatial density and a mean size of 11 ± 3 nm (shown as
inset). The nanocrystallites were nearly mono-dispersed in a
narrow range of 10–15 nm. Similar studies were carried out on
ZnA–CdA–AA composite multilayers. A typical AFM image
Fig. 4. AFM images of ZnO and Zn1−xCdxO nanocrystallites obtained by oxidation
of (a) 15 ML ZnA–AA, (b) 7 ML ZnA–AA and (c) 7 ML ZnA–CdA–AA
transferred on mica at subphase pH of 5.0 and oxidized at 700 °C.
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4. Conclusions
Isolated nanocrystallites of ZnO and Zn1−xCdxO with narrow
size distribution were obtained by controlling the metal content
of precursor LB multilayers through variation of the subphase
concentration and pH and the number of transferred monolayers
as well as by proper selection of oxidation temperature. A 7ML
ZnA multilayer on mica substrate, oxidized at 700 °C showed
uniformly distributed, isolated and nearly mono-dispersed ZnO
nanocrystallites of mean size ∼ 11 ± 3 nm. Zn1−xCdxO alloy
nanocrystallites formed under similar conditions, showed
reduced spatial density and slightly larger mean size of 18 ±
6 nm. Thus, the control of metal content of the precursor LB
multilayers and the oxidation temperature can be effectively
utilized to obtain isolated nanocrystallites of pure and mixed
oxides with a narrow size distribution.
Acknowledgements
The financial support received from MHRD (Govt. of India)
for this work is gratefully acknowledged. Sukhvinder Singh is
thankful to CSIR, New Delhi (India) for Senior Research
Fellowship. FIST (Physics)-IRCC Central SPM Facility of IIT
Bombay is acknowledged for providing the facilities for AFM
studies.
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