World Journal of Nano Science and Engineering, 2011, 1, 15-19
doi:10.4236/wjnse.2011.11003 Published Online March 2011 (
Copyright © 2011 SciRes. WJNSE
Modification at Lattice Scale for an Optimized Optical
Response of Alx(ZnO)1x Nanostructures
Avanish Kumar Srivastava1, Karuppanan Senthil2, Melepurath Deepa3, Ruchi Gakhar1,
Jai Shankar Tawale1
1Materials and Chemical Metrology, National Physical Laboratory, New Delhi , I ndia.
2School of Advanced Materials Science & Engineering, Sungkyunkwan University, Suwon, South Korea.
3Department of Chemistry, Indian Institute of Technology, Hyderabad, India.
Received February 7, 2011; revised March 14, 2011; accepted March 18, 2011
We report the systematic changes of nano-scaled features and optical properties in a promising transparent
oxide material, namely, Alx (x = 0, 1, 2 and 5%) doped ZnO1-x (AZO). Electron microscopy investigations
revealed the alterations at lattice scale depending on the presence of Al-content in ZnO nanostructures. Lat-
tice spacings of 0.26 and 0.28 nm oriented along the (0002) and (1010) planes, respectively, were attributed
to euhedral-and facetted-structures of hexagonal-ZnO. The AZO samples were further characterized by XRD,
SEM, UV-vis spectrophotometry, Raman spectroscopy and photoluminescence studies. It has been shown
that at a dopant concentration of 2% Al in ZnO, an optimal balance could be achieved between microstruc-
ture and optical properties.
Keywords: Electron Microscopy, Nanostructures, ZnO, Photoluminescence
1. Introduction
Oxide nanostructures of ZnO [1-3], WO3 [4] and TiO2
[5] are of great interest due to their tunable microstruc-
ture, phase transformation capability and quantum con-
finement, required for multifunctional usage. Of the lot,
ZnO is an n-type wide band gap semiconductor (Eg ~
3.3 eV) with a wurtzite hexagonal-crystal structure with
excellent chemical and thermal stability, large exciton
binding energy (60 meV), large electron mass ~ 0.3 me
(me: Bare electron mass) and a large exciton emissivity
at room temperature. Doping of ZnO by transition metal
ions such as Mn2+ [6], Co2+ [7], Ni2+ [8], V3+ [9] and
Fe3+ [2] yields materials suitable for various optical and
electromagnetic applications. Moreover various emis-
sion characteristics and shifting of luminescence bands
have been attributed to the fine nanocrystalline struc-
tures leading to a large surface area and defects [10-12].
Recently neat or undoped ZnO has also been investi-
gated by incorporating light metal ions like Al3+ for op-
tical applications [13-15]. To this end, in the present
study, we report the syntheses and characterization of Al
doped ZnO nanostructures.
2. Experimental Details
Neat ZnO was prepared by dissolution of 2 gm zinc ace-
tate dehydrate in 25 ml ethanol and 1 ml DEA (dietha-
nolamine), followed by sintering at 500oC for two hours.
For doped samples, aluminum nitrate nonahydrate was
added in terms of weight % (1, 2 and 5 wt.%) and sub-
jected to the same sintering procedure to yield Al-doped
ZnO. Hereafter, these samples are referred to as 0AZO,
1AZO, 2AZO and 5AZO for 0%Al-ZnO, 1%Al-ZnO,
2%Al-ZnO and 5%Al-ZnO, respectively. XRD patterns
were recorded on a X-ray diffractometer (Mini Flex II)
with an incident Cu-Kα radiation of = 1.5418 Å. A
scanning electron microscope (SEM, VP-EVO MA10,
Carl Zeiss) and a high resolution transmission electron
microscope (HRTEM, JEOL 2200 FS with a Cs-co-
rrector) were employed to characterize the surface and
structure of the samples.
3. Results and Discussion
A detailed structural and microstructural characteriza-
tion was carried out on all four samples (0AZO, 1AZO,
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Figure 1. XRD pattern of 0AZO sample showing the presence of important planes of ZnO-hexagonal crystal structure.
a b
c d
Figure 2. SEM micrographs of AZO samples with x = 0 (a),1 (b), 2 (c) and 5 (d) showing the fine topographical fea-
tures on the surface of these samples. Insets in individual micrographs further reveal the high magnification surface
2AZO and 5AZO). It was noted that all the four samples
exhibited the characteristic peaks of hexagonal – ZnO
(wurtzite type; a = 0.32, c = 0.52 nm, reference: JCPDS
file no. 21-1486) in X-ray diffractogram (XRD). As an
illustrative example, a XRD pattern of 0AZO sample
exhibiting the intense planes (hkil); 1010, 0002,
1011,1012, 1120, 1013, 2020, 1122, 2021, 0004
and 2022 with interplanar spacing (d); 0.28, 0.26, 0.25,
0.19, 0.16, 0.15, 0.14, 0.138, 0.136, 0.130 and 0.124,
respectively, are marked on diffraction peaks (Figure 1).
Further analyses inferred that, initially, the intensity of
the 1010 peak decreases in 1AZO, as compared to
0AZO, but ongoing to 2AZO, the intensity of this peak
is higher than that of 0AZO. But for the 5AZO sample,
the intensity of this peak falls again. The crystallite size
estimated from the Scherrer’s formula was 5.12, 5.12,
6.14 and 4.72 nm for 0, 1, 2 and 5% AZO samples, re-
The surface topography and microstructures of the
samples are analyzed by scanning electron microscopy
(Figure 2). These samples are consisted of different
morphologies depending on the dopant concentration. In
an undoped condition, normally a dense fine size parti-
cles are seen (Figure 2a). On increasing the magnifica
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Figure 3. HRTEM micrographs of AZO samples with x = 0 (a,b), 1 (c,d), 2 (e,f). (a,b) lattice scale images of nanoparticles as
shown in insets of (a) and (b). SADPs as shown in the left hand side insets of (a) and (b) correspond to the respective nanopar-
ticles. (c) bright field micrograph and (d) lattice scale image of a nanoparticle (inset of d). Inset in (c) is the corresponding
SADP. (e) bright field micrograph, (f) lattice scale image of a nanoparticle shown as an inset in (e). SADP of nanoparticle is
also shown as an inset in (e).
tion, the particles with the size between 50 to 150 nm
are resolved in the micrograph (inset of Figure 2a). On
addition of 1%Al as dopant, there is a change in mor-
phology of the nanoparticles having worm-like appear-
ance with a coarse-structure (Figure 2b) with a very
smooth surface (inset of Figure 2b). On further increase
in dopant concentration microstructural transformation
takes place from worm-like to random particles with
carrying more like individual identity (Figure 2c). At
high magnification, the surface of these individual parti-
cles have developed some cellular microstructure which
was seen on every individual nanoparticle (inset of Fig-
ure 2c). At highest dopant concentration (5% Al) the
microstructure is refined (Figure 2d) compared to pre-
vious one (2% Al, Figure 2c) and the surface micro-
structure was also grown (inset of Figure 2d). It is also
seen that on increasing the dopant concentration, the
agglomeration among nanoparticles increases and it is
significant at highest doping (Figure 2d). It is probably
due to the fact at high doping, the excess Al, precipitates
and goes into the interparticle boundaries and tries to act
as binder between the particles. Sometimes the precipi-
tation is so obvious that the excess Al starts forming its
own commonly known dendritic structure (inset as left
top corner of Figure 2d).
High resolution transmission electron microscopy was
employed to investigate the neat and Al-doped ZnO
samples even at atomic scale and in reciprocal space. In
the 0AZO sample, well facetted grains with euhedral
shapes (diameter ~ 80 nm and length ~ 180 nm) having
conical tips were observed (inset of Figure 3a). The
corresponding lattice scale image (Figure 3a) shows a
set of planes with an interplanar spacing of 0.26 nm ori-
ented along the 0002 direction. The nanoparticles have a
hexagonal structure with each facet of about 40 nm (in-
set of Figure 3b). The corresponding lattice scale image
(Figure 3b) shows a set of planes with an interplanar
spacing of 0.28 nm oriented along the 1010 reflection.
Selected area electron diffraction patterns (SADPs) re-
corded from a euhedral grain (inset of Figure 3a) ori-
ented along the [1216] zone axis (inset of Figure 3a)
and from a hexagonal crystallite (inset in Figure 3b)
oriented along the [0001] zone axis (inset of Figure 3b).
Since the lattice image in Figure 3b has been recorded
along the [0001] zone axis which is also the c-axis of
hexagonal ZnO, six planes of oriented along the {1010}
direction are clearly seen. For the 1AZO sample,
nanoparticles tend to aggregate and the diameter of these
particles varies from 15 to 30 nm (Figure 3c). A high
resolution image recorded from one such nanoparticle
(inset of Figure 3d) shows that these particles have lat-
tice scale defects as shown by the encircled region in
Figure 3d. On either side of the encircled region, the
structure is regular with an interplanar spacing of 0.26
nm that corresponds to the 0002 plane of hexagonal ZnO.
The corresponding SADP, recorded along the [0110]
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Figure 4. Lattice scale image of a nanoparticle of about 90 nm,
shown as inset. The inset on the right hand side shows a
cluster of nanoparticles.
Figure 5. (a) Raman and (b) photoluminescence spectra of
AZO samples. 1 = 0AZO, 2 = 1AZO, 3 = 2AZO and 4 = 5AZO.
zone axis is displayed as an inset of Figure 3c. For the
2AZO sample fine nanoparticles (of about 20 nm diame-
ter) embedded in larger sized particles were seen (Fig-
ure 3e). A defect free image recorded along the c-axis of
the hexagonal crystal structure (Figure 3f) from a parti-
cle of about 70 nm size (inset of Figure 3e). For the
5AZO sample, dislocations and other prominent defects
at lattice scale were observed and these have been en-
circled by dotted lines (Figure 4). These defects were
seen in high resolution image of a nanoparticle of about
90 nm diameter (inset of Figure 4), isolated from a clus-
ter of nanoparticles (inset of Figure 4). In general the
XRD analyses also corroborate the electron microscopy
Raman spectra recorded at a wavelength of 532 nm
for all AZO samples, shows that the overall features of
the spectra remain the same except for the two peaks
observed at 328 and 432 cm-1 (Figure 5a). These Raman
active modes are ascribed to the multi-phonon process
(326 cm-1) and the E22 high phonon mode (436 cm-1)
[13-15]. It is evident from Raman spectra, that the varia-
tion in the intensity of the peaks is due to the incorpora-
tion of Al3+. Initially for 1AZO sample, the intensity of
the Raman bands decreases as compared to 0AZO but
for 2AZO, the intensity increases in comparison to
0AZO (Figure 5a). For 5AZO, the intensity of Raman
peaks again decreases. However, the 1AZO sample was
characterized by peaks with least intensity (Figure 5a).
This behavior may be associated with the change the
degree of crystallinity of undoped ZnO induced by dop-
ing with Al3+. From XRD and electron microscopy ex-
periments, the size of the nanocrystallites varies in a
same manner as the intensity profile of Raman active
bands does. The wurtzite-hexagonal structure of ZnO is
non-centrosymmetric and the Zn atom (63.59 amu) is
about four times heavier than that of oxygen atom
(15.999 amu), and therefore the structure is polar. On
introducing Al3+ in the ZnO lattice, the non-polar inter-
action becomes strong. Initially the structure tries to
stabilize by filling the vacancies and defect centers at
low dopant levels (1AZO). However in 2AZO, the non-
polar phonon mode (E22 high mode at 436 cm-1) is
stronger with respect to that observed for 0AZO. The
quality of nanostructured-ZnO improves with the addi-
tion of dopant like Al3+, with 2AZO being the sample
with high crystallinity and a uniform almost defect free
The transmittance spectra recorded within the 300
700 nm wavelength range revealed that the transmit-
tance was almost same (~ 50%) for 0AZO and 2AZO
samples and this value was reduced for the other two
samples, 1AZO and 5AZO to about 35% in the same
wavelength range. Photoluminescence spectra of all the
four samples of AZO recoded at an excitation wave-
length of 320 nm are shown in Figure 5b. Four bands
[2] were seen in all spectra and these have assigned as
follows: 1) 377 nm (3.29 eV): excitonic emission due to
the recombination of excited electron with a hole to
form a pair of exciton in the valence band, 2) 425 nm
(2.91 eV): transition between conduction band (CB) and
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zinc vacancy (Vzn), 3) 455 nm (2.72 eV): A transition
between exciton level (E) and interstitial oxygen (Oi)
and 4) 525 nm (2.36 eV): a transition between VoZni and
valence band. Vo represents oxygen vacancies. Among
these bands, the emission around 455 nm (blue) was
most intense and the intensity of all the peaks was
maximum in case of 2AZO (Figure 5b). On reducing
the excitation wavelength to 250 nm (4.96 eV), the
highest intensity peak shifted from 455 nm to 417 nm.
The blue shift from 2.72 eV (blue) to 2.97 eV (violet)
when the material was excited with 4.96 eV instead of
3.87 eV radiation, shows that the emission of doped
ZnO nanostructures is tunable, which is beneficial for
practical applications of ZnO.
4. Summary
The morphology and crystallinity of the AZO samples is
dependent on dopant concentration. Raman spectros-
copy elucidates the increase in the intensity of E22 (high)
mode, which is non-polar in character and it signifies the
reduction in the polar nature of ZnO:Al with increase in
Al content. A 2% Al-ZnO sample was found to be most
suitable for optical applications.
5. Acknowledgements
The authors are grateful to the Director, NPL, New Del-
hi for his permission to publish these results. Mr. K. N.
Sood and Dr. N. Bahadur are acknowledged for discus-
sions on ZnO. AKS acknowledges INSA-KOSEF pro-
gram to carry out research at POSTECH, Korea. Profes-
sor K. Yong at POSTECH is gratefully acknowledged.
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