ZnO nanostructures with tunable visible luminescence Effects of kinetics of chemical reduction and annealing lable at ScienceDirect Journal of Science Advanced Materials and Devices 2 (2017) 51e58 Contents lists avai Journal of Science Advanced Materials and Devices journal homepage www elsevier com/locate/jsamd Original Article ZnO nanostructures with tunable visible luminescence Effects of kinetics of chemical reduction and annealing R Raji, K G Gopchandran* Department of Optoelectronics, Univ[.]
Trang 1Original Article
ZnO nanostructures with tunable visible luminescence: Effects of
kinetics of chemical reduction and annealing
Department of Optoelectronics, University of Kerala, Thiruvananthapuram 695581, India
a r t i c l e i n f o
Article history:
Received 20 December 2016
Received in revised form
6 February 2017
Accepted 7 February 2017
Available online 13 February 2017
Keywords:
Semiconductor
Zinc oxide
Defects
Raman spectroscopy
Luminescence
a b s t r a c t
Highly crystalline ZnO nanoparticles were synthesized using a co-precipitation method The morphology and optical properties of these nanoparticles are found to be highly sensitive to the growth parameters such as the concentration of reducing agent and annealing temperature Indeed, the concentration of the reducing agent can alter the morphology of nanoparticles from quasi-spherical to rod-like and then to flower-like structures Attempts were made to tune the emission wavelength over the visible region by varying the kinetics of chemical reduction and annealing The possibility of tuning the emission in a visible range from orange to red and then to green by changing the nature of defects by annealing is also reported Analysis of the Raman spectrum, with its intensity observed at 580 cm1corresponding to E1
(LO) mode, revealed that the kinetics and thermodynamics of formation and growth of these nano-particles determined the nature and density of the probable defects such as oxygen vacancies, interstitial zinc atoms and their complexes
© 2017 The Authors Publishing services by Elsevier B.V on behalf of Vietnam National University, Hanoi This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/)
1 Introduction
Over the past few decades, the scientific community has devoted
considerable attention in the design and development of
semi-conductor nanostructures, which can show enhanced optical,
elec-trical, mechanical and sensing properties owing to their quantum
size effect[1,2] Zinc oxide (ZnO), a wide band gap (3.37 eV) oxide
semiconductor gained substantial interest due to their tremendous
demand for wide range of applications in photonic crystals, light
emitting devices, photo detectors, photo diodes, solar cells,
piezo-electric transducers, gas sensors, biological and chemical sensors
etc.,[3e5] Large exciton binding energy (60 meV) of ZnO at room
temperature makes ZnO a promising material for the development
of the blue and ultra-violet lasers and LEDs[6] Due to its unique and
fascinating features, ZnO is considered as an appropriate substitute
to GaN for the next generation light emitting devices[7]
Extensive researches have been carried out to study the
lumi-nescence mechanism in ZnO nanostructures[8,9] In most cases,
luminescence spectra of ZnO nanoparticles has two emission
bands: one is the typical band edge transition or the exciton
combination and the other is the defect emission in the visible region due to trap states in ZnO[10] The origin of the defect related emission in the visible region is still a controversial question[11,12] Recently Ozgur et al reported that oxygen vacancies are respon-sible for virespon-sible luminescence from ZnO [9] The defect states arising from zinc or oxygen vacancies and the electrons or holes from the shallow trap states within the bandgap of ZnO are responsible for the broad visible luminescence in the region from
400 to 700 nm[11,13] In order to improve the luminescence ef fi-ciency and to tune the wavelength over a wide range from blue to red, size and shape controlled synthesis of ZnO nanocrystals has been widely employed[14]
Recently, nanostructured ZnO materials with tunable particle size and shape have been prepared by adopting several physical or chemical synthetic methods such as thermal evaporation[15], pulsed laser deposition[16], thermal decomposition[17], solegel technique [18], hydrothermal process[19]and co-precipitation method[20] Among the various methods, we adopt co-precipitation method because of its simplicity andflexible post synthesis process, which offers a possibility of large area yield at low cost
In this work, we studied the effect of reaction kinematics such as concentration of KOH and annealing temperature on the structural, morphological and optical properties of ZnO nanoparticles syn-thesized by simple co-precipitation method Attempts were made
to tune the emission intensity and wavelength over the entire
* Corresponding author.
E-mail address: gopchandran@yahoo.com (K.G Gopchandran).
Peer review under responsibility of Vietnam National University, Hanoi.
Contents lists available atScienceDirect Journal of Science: Advanced Materials and Devices
j o u r n a l h o m e p a g e : w w w e l s e v i e r c o m / l o c a t e / j s a m d
http://dx.doi.org/10.1016/j.jsamd.2017.02.002
2468-2179/© 2017 The Authors Publishing services by Elsevier B.V on behalf of Vietnam National University, Hanoi This is an open access article under the CC BY license
Journal of Science: Advanced Materials and Devices 2 (2017) 51e58
Trang 2visible region by varying the concentration of KOH and annealing
temperature The observed change in color and the intensity of
photoemission of ZnO nanoparticles with the size and morphology
are discussed in detail
2 Experimental
Zinc nitrate hexahydrate (Zn (NO3)2$6H2O) and potassium
hy-droxide (KOH) were procured from SigmaeAldrich All the
chem-icals were used in analytical grade and without further purification
Deionized water was used throughout the experiment
ZnO nanoparticles were prepared by co-precipitation method
A volume of 120 ml of 0.1 M zinc nitrate hexahydrate precursor
aqueous solution was prepared under vigorous stirring for
20 min 20 ml aqueous solution of 0.2 M KOH was added drop
wise to 20 ml of zinc salt precursor solution under constant
stirring and the reaction was continued for 40 min, yielding a
white precipitate Thereafter, the solution was allowed to settle
for 3 h, the particle suspension was transferred to centrifuging
tubes and subjected to centrifuging at 3000 rpm for 10 min In
each centrifugation, the reaction medium was changed using
distilled water and ethanol and is repeated thrice Then, the
precipitates were dried at room temperature for overnight The
precursor powder thus obtained were fully grounded and then
subjected to heat treatment in a muffle furnace at 300C at the
rate of 5C/minutes for 120 min In order to study the influence
of KOH on the formation of ZnO nanoparticles, concentration of
KOH was varied from 0.2 to 1.2 M, in steps of 0.2 M Also, to
investigate the effect of annealing on the luminescence properties
of ZnO nanoparticles, sample with enhanced emission was
sub-jected to annealing at various temperatures from 300 to 900C
The crystal structure and phase analysis of the nanostructures
were investigated using an X-ray diffractometer (Philips
PAN-analyticalX’Pert Pro) with CuKaradiation (l¼ 1.54056 A) in the
angular range of 2q from 20 to 80 The surface morphological
analysis of the nanoparticles was studied using FE-SEM (JEOL-JSM
5600) The Raman spectra of the samples were recorded using a
Horiba JobinYvon LABRAM-HR 800 spectrometer equipped with an
Arþion laser having 514 nm emissions Thermogravimetric analysis
(TGA) and differential thermal analysis (DTA) of the as-synthesized
samples were recorded using TA instrument (Q600 SDT and Q 20
DSC) in the temperature range room temperature 28C to 900C
UV-visible absorption and reflectance spectra of all the samples
were recorded using a UV-visible spectrophotometer (JascoV550)
A Horiba JobinYvonFluorolog (FL III) spectrofluorophotometer
modified and equipped with HeeCd laser (325 nm) and R928P
photomultiplier tube in photon counting mode as detector was
used to record photoluminescence emission spectra at room
temperature
3 Results and discussion
3.1 TGA-DTA analysis
The thermal behavior of as prepared ZnO nanocrystals has been
investigated by differential thermal analysis (DTA) and thermo
gravimetric analysis (TGA) and is shown inFig 1 Two weight losses
were observed in the TGA curve Thefirst step is in the temperature
range 30e72C indicating the evaporation of water adsorbed at the
surface The second weight loss occurs in the range 72e288C and
may be due to the decomposition of residual compounds[21] In
the TGA curve, aflat terrain is observed between 290 and 700C
indicating the formation of the ZnO nanoparticles as a
decompo-sition product
The DTA curve shows an endothermic peak at 53C and is due to the transition of ZnO nanoparticles Also, an exothermic peak was observed in the range from 55 to 296C and it appears due to thermal lattice vibrations[22] There was no further weight loss above 300C So the optimum annealing temperature taken for this study is 300C
3.2 XRD analysis
To study the effect of concentration of the reducing agent (KOH)
on the formation of ZnO nanoparticles, it was varied from 0.2 to 1.2 M by keeping the concentration of Zn (NO3)$6H2O constant The corresponding X-ray diffraction patterns are shown inFig 2 All the peaks in the diffraction patterns are indexed according to the wurtzite structure of ZnO (hexagonal phase, space group P63mc), corresponding to JCPDS Card No 79-2205 The preferential growth was found to be along (101) crystal plane The other prominent peaks were (100) and (002) No excess peaks, such as Zn (OH)2were detected, which indicated that the crystalline ZnO was formed at
300C The phase of ZnO is found to be same for all the samples i.e., concentration of KOH has not influenced the formation of crystal-lite phase of ZnO The crystalcrystal-lite size (D) of the samples is calculated from the XRD data using DebyeeScherrer formula[23]:
Fig 1 TGA and DTA curve of the as synthesized ZnO nanoparticles.
Fig 2 XRD patterns of ZnO nanoparticles, prepared with different KOH concentra-tions; (a) 0.2, (b) 0.4, (c) 0.6, (d) 0.8, (e) 1 and (f) 1.2 M [Inset: Typical HalleWilliamson
R Raji, K.G Gopchandran / Journal of Science: Advanced Materials and Devices 2 (2017) 51e58
Trang 3D¼ 0:9l
where, D represents crystallite size in nm, l is the wavelength
(0.154178 nm) of the Cu KaX-ray radiation used,qis the Bragg angle
andbis the full-width at half-maximum (FWHM) corresponding to
most prominent peak (101) measured in radians It is observed that
the crystallite size of the ZnO nanoparticles decreases from 36 to
26 nm with increase of the concentration of KOH from 0.2 to 1 M,
beyond that the size increases (Table 1) The increase in size of
particles with excess KOH may be due to higher precipitation rate
On increasing the concentration of KOH, the diffraction peaks shift
towards smaller Bragg's angles and it indicates a decrease in lattice
parameters
The lattice parameters of the prepared nanoparticles were
calculated from the equation of lattice d spacing of the (h k l) planes
for hexagonal crystal system and is given by[23]:
1
d2
hkl
¼4
3
h2þ hk þ k2
a2
þl2
where dhkl is the interplanar separation corresponding to Miller
indices h, k, l; a, b, and c are lattice parameters.Table 1 shows
calculated values of lattice parameters of all the samples
corre-sponding to (101) and (002) planes
The cell volume and the number of atoms per unit cell for the
ZnO samples with hexagonal form are estimated using equations
[23]:
V¼3
ffiffiffi
3
p
2 a
n¼4p
3V
D
2
3
(4)
where a and c are lattice parameters, D is the crystallite size (nm)
All the samples shows positive or extensive strain and may be due
to the incorporation of defects in the form of interstitial oxygen or
zinc[24]
The strain (ε) and crystallite size (D) of the samples are also
calculated by WilliamsoneHall (WH)method with the following
relation[25]:
bcosq¼Kl
where K (0.9) is crystallite shape constant
The inset ofFig 2shows the typical WeH plots, i.e., by plotting
ðbcosq=lÞ as a function of ðsinq=lÞ of samples prepared with KOH
concentrations of 1 and 1.2 M The intercept on theðbcosq=lÞ axis
gives the crystallite size corresponding to zero strain and slope of
the line gives strain
The sample with enhanced luminescence, prepared with 1 M
KOH, was selected for understanding the influence of heat
treatment on the structural and optical properties The XRD pat-terns of this sample subjected to annealing at different tempera-tures is shown in Fig 3 and the structural properties of these samples derived from XRD data is also provided inTable 2 It is found that the intensity of all the peaks in the XRD patterns and the crystallite size increases with increase of annealing temperature (Fig 4) It is also observed that there is an increase in the number of unit cell with annealing temperature The observed size increment
at high annealing temperature may be due to the migration of grain boundaries and the coalescence of grains[26] The lattice param-eters are found to vary with temperature The change in lattice parameters indicates the presence of strain in the lattice of ZnO
3.3 Micro-Raman spectroscopy Raman spectroscopy is one of the effective methods to investi-gate the phase and purity of semiconductor nanocrystals ZnO is hexagonal structured with a space group C4
vhaving two formula units per primitive cell, where all the atoms occupy the C3vsites [27] The group theoretical calculation predicts nine optical modes which are distributed as follows:
Goptical¼ A1ðR; RÞ þ 2B1þ E1ðIR; RÞ þ 2E2ðRÞ (6)
The B1are silent modes, the A1and E1mode are active in both Raman and infrared spectra and split into transverse optical (A1T&
E1T) and longitudinal optical (A1L& E1L) phonons with different frequencies due to microscopic electricfield associated with the LO phonons E2modes are non polar and Raman active only E2mode split into E2 (high) and E2(low) modes[27,28] Based on earlier Table 1
Structural and optical properties of ZnO nanoparticles prepared with different concentrations of KOH [For bulk ZnO a ¼ b ¼ 0.3250 nm, c ¼ 0.5207 nm].
Con: of KOH (M) FWHM ( ) Crystallite size D hkl (nm) Lattice parameter Strain ε Band gap (eV) No.of atoms/unit cell (n)
DebyeeScherrer WeH plot a (A ) c (A )
Fig 3 XRD patterns of ZnO nanoparticles annealed at different temperatures; (a) 300, (b) 400, (c) 500, (d) 600, (e) 700, (f) 800 and (g) 900C [Inset: Typical HalleWilliamson plot of ZnO nanoparticles at 400 and 600C ].
R Raji, K.G Gopchandran / Journal of Science: Advanced Materials and Devices 2 (2017) 51e58
Trang 4investigations, the frequencies of fundamental optical modes in
ZnO can be assigned as follows,
E2(low)¼ 100 cm1, E2(low)(TA)¼ 208 cm1, E2(high)¼ 437 cm1,
E2(high) E2(low)) ¼ 339 cm1, E1(LO) ¼ 584 cm1,
A1(TO) ¼ 388 cm1 and 2A1(LO), 2E1(LO) ¼ 1050 e 1200 cm1
[27e29]
Fig 5(a) represents micro-Raman spectra of ZnO nanoparticles
over the spectral range 50e1300 cm1 The intense peak located at
99 cm1is assigned to E2(low) mode and is due to the vibration of
Zn sub lattice The E2(high) mode observed at 438 cm1is mainly due to the vibration of oxygen sub lattice and is the characteristic peak of hexagonal wurtzite phase of ZnO The peaks at 205 and
331 cm1correspond to multi phonon process and are assigned to
2 TA (M) and 2 E2(M) or E2higheE2lowmodes The peak at 382 cm1 corresponds to A1 (TO) mode The peak with medium intensity observed at 581 cm1corresponds to E1(LO) mode and may be caused by the formation of oxygen vacancies and interstitial zinc and their complexes The broad feature between 1100 and
1200 cm1 is assigned to the two phonon modes (2LO), charac-teristic of II-IV semiconductors[27e33] The observed small shift (few cm1) in Raman modes at higher wave numbers compared to that of the bulk may be due to the strain existing in the samples and
is confirmed from X-ray diffraction patterns
All the phonon modes in the spectra of ZnO nanoparticles turn out to be stronger and sharper with increase of annealing tem-perature (Fig 5(b)), indicating the improvement of crystal quality as evident from the XRD data Also, at high annealing temperature, the
E2(high) mode is slightly red shifted indicating the optical phonon confinement At low temperature, the observed E1 (LO) mode is associated with the formation of structural defects whereas at high annealing temperature the intensity of E1(LO) mode enhances and
is related to the formation of surface defects[32,33]
3.4 FE-SEM analysis The morphology is found to be highly sensitive to the concen-tration of KOH (Fig 6) The shape of nanoparticles is found to vary first from quasi-spherical to rod like structures and then to flower-like nanostructures with increase in concentration of KOH Quasi spherical nanoparticles with a diameter of 31e35 nm were formed
Table 2
Structural and optical parameters of rod like ZnO nanoparticles annealed at different temperatures.
Temperature (C) FWHM () Crystallite size D hkl (nm) Lattice parameter Strain ε Band gap (eV) No.of atoms/unit cell (n)
DebyeeScherrer WeH plot a (A ) c (A )
Fig 4 Variation of intensity of (101) plane in XRD pattern with annealing
temperature.
Fig 5 Micro-Raman spectra of ZnO nanoparticles prepared at; (a) different concentrations of KOH (a) 0.2, (b) 0.4, (c) 0.6, (d) 0.8, (e) 1 and (f) 1.2 M; and (b) different annealing
R Raji, K.G Gopchandran / Journal of Science: Advanced Materials and Devices 2 (2017) 51e58
Trang 5when the concentration of KOH used was low (0.2 M) On
increasing the concentration, it was found that the shape of the
particles changes to elongated type with confinement along two
directions (not shown) and a progressive increase in the intensity of
photo emission was also accompanied this change as described in
Section3.6 For particles prepared with a KOH concentration of 1 M,
rod like structures with maximum emission intensity was obtained
However, further increase in concentration of KOH led to the
for-mation offlower-like structure resulting from the clustering of
highly confined two dimensional layers Hence, it is evident from
this work that KOH plays two roles viz., confinement making layers
and agglomeration in a periodic manner so that the morphology
look like that offlowers The observed decrease in luminescence
fromflower-like structures may be attributed to the lesser surface
area exposed to the radiation whenflower like nanoparticles were
formed
In order to prepare samples with tunable luminescence in the
visible region the sample with intense visible emission having a
morphology consisting of rod shaped particles was subjected to
annealing at different temperatures The morphology of the
sam-ples undergoes various transformations as shown inFig 7
Gener-ally, looking at the morphology of the samples (Figs 7 and 10)
obtained in this work, exhibiting tunable luminescence in the
visible region, they are similar to that of morphology obtained for
ZnO based phosphors reported by Hameed et al [34] At high
temperature, atoms have large activation energy for diffusion and
can stimulate the coalescence of smaller particles and may lead to
an increase in size of the particle[26,35] Thus, it is concluded that
concentration of KOH (OH ions) and annealing temperature
played essential role in the growth habits of ZnO nanostructures
3.5 UV-visible absorption spectroscopy
Fig 8(a) shows the absorption spectra of the ZnO nanoparticles
prepared by varying the concentration of KOH The peaks observed
in the absorption spectra are attributed to the transition of electrons
between the valence band, conduction band and the intrinsic defect
levels[1] The synthesized ZnO nanoparticles exhibit blue shifted
absorption peaks with respect to their bulk counterpart having the
absorption peak at 386 nm[24,36] With increase in concentration
of KOH from 0.2 to 1.2 M, the absorption peak was found to shift
from 377 to 371 nm progressively and can be attributed to the
changes in their surface morphologies and particle size
From the diffuse reflectance spectrum taken in the 220e850 nm
region, the optical band gap of ZnO nanoparticles were calculated
using KubelkaeMunk relation[37,38]
K
S¼ð1 R∞Þ2
FðR∞Þhy¼hy Eg
n
(8)
where FðR∞Þ is called remission or KubelkaeMunk function,
R∞¼ Rsample=Rstandard, K and S are absorption and scattering
co-efficients, hyis the energy of the incident photon and the exponent
n depends on the nature of the optical transition caused by the photon absorption, for direct allowed transition n¼ 1/2[37] When the material scatters in a perfectly diffuse manner, the dependence
of S on the energy becomes weak and can be assumed that FðR∞Þ is proportional to absorption coefficient The band gap energy ob-tained from KubelkaeMunk plot was found to be in the range 3.25e3.18 eV and is in accordance with the particle size variation estimated from X-ray diffraction patterns The observed red shift in the optical band gap energy of the ZnO nanoparticles with respect
to the bulk value can be due to the band bending effect caused by smaller size of the crystallites In the nanoregime, surface to volume ratio of particles is larger than that of the bulk material and can increase the effect due to band bending[24]
The absorption spectra of ZnO nanoparticles (Fig 9(a)) annealed
at different temperatures shows sharp peak for samples annealed
up to 500C, beyond that peak get broadened and may be due to large particle size distribution As the annealing temperature in-creases, the absorption peak gradually shifts from 371 to 387 nm and is attributed to increased crystallite size The optical band gap energy of the samples annealed at different temperatures obtained from KubelkaeMunk method was found to decreases with increase
in annealing temperature and are reported inTable 2
3.6 Photoluminescence studies Fig 10(a) shows the room temperature photoluminescence spectra of ZnO nanoparticles prepared with different concentra-tions of KOH measured at an excitation wavelength of 325 nm using HeeCd laser The excitation energy (3.8 eV) used is higher than the band gap energy of ZnO (3.4 eV); therefore, it is easy for an electron
in the valence band to be directly excited to the conduction band; in addition, excitation to the deep levels within the band gap was also possible [38] The PL spectra of ZnO nanoparticles exhibit two emission bands: one is in the UV region (389 nm) and the other is in the visible region (400e650 nm)
The UV emission peak at 389 nm corresponds to the near band-edge (NBE) emission of ZnO and is due to the radiative recombi-nation of free excitons [9,39] The intensity of UV emission was found to increase with concentration of KOH up to 1 M and can be related to the decrease in size of the particles, beyond which in-tensity diminishes Size reduction causes more atoms to be closer to the surface and thereby increasing the rate of trapping of photo-generated holes at the surface, which in turn enhances the emis-sion intensity[40] Photoluminescence spectra reveal that for all
R Raji, K.G Gopchandran / Journal of Science: Advanced Materials and Devices 2 (2017) 51e58
Trang 6the samples visible emission is observed at orange region
(~582 nm) except for the sample prepared with a KOH
concentra-tion of 1.2 M, which shows emission band at yellow region
(~570 nm) The radiative recombination of localized electrons with
deeply trapped holes in the oxygen interstitials (Oi) located at 2.14
and 2.2 eV below conduction band results in orange (OL) and
yel-low luminescence (YL) bands respectively[41] Furthermore, it can
be concluded that, higher concentration of reducing agent will
result in the shifting of defect levels towards higher energies
The Gaussian deconvolution of the emission spectrum of the
sample with enhanced intensity is shown inFig 10(b) Five bands
were reproduced from the spectra without deviations at 389, 422,
516, 550 and 609 nm The emission due to band gap transition is
observed at 389 nm without any deviation and can be attributed to the radiative recombination of free excitons [9,10] The violet emission (VL) band at 422 nm can be ascribed to the transition of an electron from Znilevel located at 0.46 eV below conduction band to the valence band [10,42] The green luminescence (GL) band observed at 516 nm can be related to recombination of electrons in the singly ionized oxygen vacancies with photo excited holes in the valence band[9,39e43] YL band at 550 nm can be due to recom-bination of electron with deeply trapped holes in the oxygen in-terstitials (Oi) located at ~2.2 eV below conduction band[41,44] The orange luminescence (OL) band at 609 nm can be attributed to the transition of electrons from conduction band to oxygen in-terstitials located at 1.34 eV above the valance band[12,41]
Fig 7 FE-SEM images of ZnO nanoparticles prepared at different annealing temperatures.
Fig 8 (a) Absorption spectra and (b) KubelkaeMunk plots of ZnO nanoparticles prepared with different KOH concentrations; (a) 0.2, (b) 0.4, (c) 0.6, (d) 0.8, (e) 1 and (f) 1.2 M.
Fig 9 (a) Absorption spectra and (b) KubelkaeMunk plots of ZnO nanoparticles annealed at different temperatures; (a) 300, (b) 400, (c) 500, (d) 600, (e) 700, (f) 800 and (g) 900 C.
R Raji, K.G Gopchandran / Journal of Science: Advanced Materials and Devices 2 (2017) 51e58
Trang 7Fig 11represents the room temperature PL spectra of the ZnO
nanoparticles annealed at different temperatures The intensity of
emission, irrespective of whether it is UV or visible, is found to
decrease with annealing temperature up to 600C and beyond that
it increases All the samples showed the typical band edge emission
at 389 nm It can be seen that the intensity of UV band decreases
initially with annealing temperature and may be due to the partial
dissociation of donor bound exciton, supporting the assignment of
Teke et al [39] The increase in intensity of UV band at higher
temperature can be attributed to the dissociation of donor bound
excitons into free excitons and neutral donors It can lead to an
increase in probability of recombination of free excitons and
thereby increasing the intensity of UV emission[9,44] It is of
in-terest that the visible emission band first shifted from orange
luminescence (OL) to red luminescence (RL) region upon increasing
the annealing temperature from 300 to 700C and then to green
luminescence (GL) when annealed at temperatures above 700C
These shifts of visible emission band on annealing, may be due to
changes in the local environments of the defect centers in the
samples[9,43] The origin of OL, RL and GL bands are opposite in
nature OL and RL bands are attributed to oxygen interstitials (Oi) whereas GL band is attributed to singly ionized oxygen vacancies (VO)[10,12] The nature of green luminescence in ZnO is the most controversial and many hypotheses have been proposed for this emission[9,11,41e44] Annealing from 300 to 700C reduces the oxygen vacancies while increases the amount of oxygen interstitials
in the sample[45,46] Depending on the energy levels of oxygen interstitials formed in the band gap, OL and RL emissions appears in the spectra OL band is ascribed to the transition of electron from the conduction band to oxygen interstitials located at 2.14 eV below the conduction band[9,46] RL band is attributed to the transition
of electron from conduction band to oxygen interstitials located at 1.95 eV below the conduction band At elevated temperatures, GL band is observed at around 527 nm and can be related to oxygen interstitials or oxygen antisites[47] Thus, it is evident from the emission spectra that, the annealing of ZnO nanoparticles not only purges the moisture and OHions from the material but also in-fluence various channels of optical recombination and it indicates that visible emission spread is altered by annealing treatment[48] Hence, this work also provides a method to control polychromic visible emission of ZnO nanoparticles; covering almost the whole visible region, with limitations Such adjustments in luminescent properties can pave ways open for applications of ZnO nano-particles in the fabrication of white light emitting diodes, display devices, biological labeling etc
4 Conclusion Highly crystalline ZnO nanoparticles were synthesized by co-precipitation method In this work, attempts were made to tune the visible luminescence of ZnO nanoparticles by controlling the growth parameters such as concentration of reducing agent and heat treatment The XRD and micro-Raman analysis of the samples confirmed its hexagonal wurtize phase FE-SEM micrographs showed the transformation of morphology of nanoparticles from quasi-spherical to rod-like and then toflower-like structures with increase in concentration of the reducing agent KOH from 0.2 to 1.2 M SEM images also indicated that morphology of these parti-cles is highly sensitive to annealing temperature We have attempted to correlate the observed change in color and intensity of photoemission of ZnO nanoparticles with the size and morphology
of the nanoparticles The UV emission was predominant in the emission spectra of all the samples but the visible emission was
Fig 10 (a) Photoluminescence spectra of ZnO nanoparticles prepared with different concentrations of KOH; (a) 0.2, (b) 0.4, (c) 0.6, (d) 0.8, (e) 1 and (f) 1.2 M excited atl¼ 325 nm and (b) Deconvolution bands of sample prepared with 1 M KOH.
Fig 11 Photoluminescence spectra of ZnO nanoparticles prepared at different
annealing temperatures; (a) 300, (b) 400, (c) 500, (d) 600, (e) 700, (f) 800 and (g)
R Raji, K.G Gopchandran / Journal of Science: Advanced Materials and Devices 2 (2017) 51e58
Trang 8found to vary in color with both concentration of KOH and
annealing temperature It was found that annealing temperature
alter the position of visible emission band from orange to red and
then to green luminescence region; this shift in the visible emission
band is attributed to change in the local environment of the defect
centers The present study reveals that the emission intensity can
befinely tuned by suitably selecting the amount of KOH during
synthesis and emission color can be tuned by varying the annealing
temperature Thus, this study provides a method to control visible
emission of ZnO nanoparticles covering almost the whole visible
region The results obtained are fruitful and the synthesized ZnO
nanostructures can be used for the fabrication of white light
emitting diodes, display devices, biological labeling, etc.,
Acknowledgments
One of authors Raji.R wishes to express her gratitude to KSCSTE,
Govt of Kerala, India (No (T)010-20/FSHP/10/CSTE) for providing
thefinancial support to this work
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