Facile synthesis and an effective doping method for ZnO:In3+ nanorods with improved optical properties

Article abstract of DOI:10.1016/j.jallcom.2015.08.088

The sol-gel spin-coating method is usually used for thin-film deposition rather than to grow one-dimensional nanostructures. In this study, a novel regrowth method for spin-coated ZnO:In³⁺ films is demonstrated, using vapor-confined face-to-face annealing (VC-FTFA) in which a mica sheet is inserted between the two films prior to FTFA. ZnO:In³⁺ nanorods are regrown when indium chloride is used as the solvent because ZnCl₂ and InCl₃ vapors are generated and confined between the films. The near-band-edge emission intensity of the ZnO:In³⁺ nanorods resulting from VC-FTFA at 700 °C is enhanced by a factor of 17 compared with that of ZnO:In³⁺ films annealed in open air at the same temperature. Our method offers a simple and low-cost route for the fabrication of ZnO nanorods.

Full text of DOI:10.1016/j.jallcom.2015.08.088

Journal of Alloys and Compounds 651 (2015) 1e7
Contents lists available at ScienceDirect
Journal of Alloys and Compounds
journal homepage: http://www.elsevier.com/locate/jalcom
Facile synthesis and an effective doping method for ZnO:In3þ
nanorods with improved optical properties
Giwoong Nam, Byunggu Kim, Jae-Young Leem*
Department of Nanoscience & Engineering, Inje University, 197, Inje-ro, Gimhae-si, Gyeongsangnam-do 621-749, Republic of Korea
a r t i c l e i n f o
a b s t r a c t
Article history:
The solegel spin-coating method is usually used for thin-film deposition rather than to grow one-
dimensional nanostructures. In this study, a novel regrowth method for spin-coated ZnO:In3þ films is
demonstrated, using vapor-confined face-to-face annealing (VC-FTFA) in which a mica sheet is inserted
between the two films prior to FTFA. ZnO:In3þ nanorods are regrown when indium chloride is used as
the solvent because ZnCl2 and InCl3 vapors are generated and confined between the films. The near-
band-edge emission intensity of the ZnO:In3þ nanorods resulting from VC-FTFA at 700 ꢀC is enhanced
by a factor of 17 compared with that of ZnO:In3þ films annealed in open air at the same temperature. Our
method offers a simple and low-cost route for the fabrication of ZnO nanorods.
Received 10 March 2015
Received in revised form
11 August 2015
Accepted 13 August 2015
Available online 15 August 2015
Keywords:
Oxide materials
Nanostructured materials
Sol-gel process
© 2015 Elsevier B.V. All rights reserved.
Optical properties
1. Introduction
presence changes the conductivity of the n-type material; there-
fore, ZnO is doped with In to control resistivity for electrical ap-
ZnO is one of the most promising materials for the fabrication of
optoelectronic devices operating in the blue and ultraviolet (UV)
regions [1e4], because of a wide direct band gap (Eg ¼ 3.37 eV at
room temperature) and a large exciton binding energy (60 meV),
much larger than that of GaN (25 meV), as well as a thermal energy
at room temperature (26 meV) that ensures efficient exciton
emission under ambient conditions. Moreover, one-dimensional
(1D) ZnO nanostructures, including nanorods, nanowires, and
nanotubes, have aroused considerable research interest in many
areas of nanoscience and nanotechnology. 1D nanostructures have
unusual and unique physical properties with quantum-mechanical
confinement effects (QCE) [5e7] and a high surface-to-volume ratio
making them relevant for the development of new devices and
sensors [8e10]. In particular, it is believed that QCE plays a crucial
role in improving the optical properties of materials [11,12].
plications, notably in UV photodetectors, field-effect transistors,
solar cells, and gas sensors [14e17]. To date however, the ZnO:In3þ
films or nanostructures prepared in most studies exhibit poor
photoluminescence (PL) properties [14,18e21]. Considerable efforts
have been devoted to the synthesis of 1D ZnO nanostructures, and
various approaches have been demonstrated for the fabrication of
semiconducting ZnO nanowires or nanorods.
Metal-organic chemical vapor deposition, chemical vapor
deposition, the hydrothermal method, electrochemical deposition,
and chemical bath deposition have all be used to prepare ZnO
nanorods [22e26]. Another option is the solegel spin-coating
method, which is conventionally used to deposit thin film rather
than to grow 1D nanostructures. Should it be adapted for the
growth of nanorods however, this method offers better reproduc-
ibility, lower costs, and mass production capability.
Intrinsic ZnO films are usually n-type semiconductors because
of their native point defects, namely oxygen vacancies and Zn in-
terstitials [13]. Their number can be reduced either by annealing
the ZnO films in an oxygen atmosphere or by appropriate doping.
Group III elements, such as Al, Ga, and In can be used as donor
dopants for ZnO. Indium is an important dopant in ZnO whose
A face-to-face annealing (FTFA) approach is commonly
employed for GaAs semiconductors to prevent the out-diffusion of
arsenic [27]. The GaAs wafer to be annealed is placed between a
bottom Si wafer and a top GaAs wafer with the polished surfaces
facing each other (hence the name of the method). In previous
study, we grew the ZnO nanorods by vapor-confined FTFA (VC-
FTFA) using magnesium chloride hexahydrate or zinc chloride
[28,29]. In this study, we fabricate ZnO:In3þ nanorods, using indium
chloride to regrow the nanorods. The VC-FTFA method has several
advantages over conventional FTFA methods; in particular, samples
* Corresponding author.
E-mail address: jyleem@inje.ac.kr (J.-Y. Leem).
http://dx.doi.org/10.1016/j.jallcom.2015.08.088
0925-8388/© 2015 Elsevier B.V. All rights reserved.

2
G. Nam et al. / Journal of Alloys and Compounds 651 (2015) 1e7
prepared by VC-FTFA show more intense the near-band-edge (NBE)
emission in PL spectra. In this study, we describe the mechanism for
the generation of vapor during annealing and the influence of the
confined vapor during VC-FTFA on the optical properties of the
ZnO:In3þ nanorods.
acetate dihydrate, which becomes anhydrous. Further decomposi-
tion of the anhydrous zinc acetate causes weight loss near 150 ꢀC
and the decomposition process is completed by an exothermic re-
action before 300 ꢀC. The 61.5% weight loss measured here is
greater than the 46.5% predicted theoretically. The extra 15.0%
weight loss can be attributed to sublimation of zinc acetate species
or to the formation of other volatile zinc organic compounds such
as Zn4O(CH3COO)6 [30,31]. As the temperature increases therefore,
the ZnO precursor gradually decomposes to form ZnO through the
following chemical reactions [31]:
2. Experimental procedures
2.1. Preparation of ZnO:In3þ film
The precursor solutions for the ZnO:In3þ films were prepared by
dissolving zinc acetate dihydrate (4.302 g, Zn(CH3COO)2$2H2O, ACS
Reagent, >98%, SigmaeAldrich) and indium chloride (0.088 g, InCl3,
98%, SigmaeAldrich) in 2-methoxyethanol (99.8%, SigmaeAldrich)
up to a total volume of 40 mL. The concentration of the metal
precursors was 0.5 M. Monoethanolamine (C2H7NO, MEA, ACS
Reagent, >99.0%, SigmaeAldrich) was used as a stabilizing agent to
improve the solubility of the precursor salt. The molar ratio of MEA
to metal salts was 1.0 (1.204 mL), and the ratio of In to Zn was fixed
to 2 at.%. The stabilized sol solution was stirred at 60 ꢀC for 2 h until
it became clear and homogeneous. It was subsequently cooled to
room temperature and aged for 24 h before it was used as the
coating solution to deposit the films. The p-Si substrates were ul-
trasonically cleaned in acetone and ethanol for 10 min, rinsed with
deionized water, and blow-dried with nitrogen. The precursor so-
lution was spin-coated onto a p-Si substrate at 2000 rpm for 20 s,
and the films were then dried at 200 ꢀC for 10 min in an oven. These
spin-coating and drying procedures were repeated five times.
Zn(CH3COO)2$2H2O / Zn(CH3COO)2 þ 2H2O([)
(1)
(2)
(3)
(4)
4Zn(CH3COO)2 þ 2H2O / Zn4O(CH3COO)6 þ 2CH3COOH([)
Zn4O(CH3COO)6 þ 3H2O / 4ZnO þ 6CH3COOH([)
Zn4O(CH3COO)6 / 4ZnO þ 3CH3COCH3([) þ 3CO2([)
On the other hand, the ZnO:In3þ precursor exhibits further
weight loss in the 150e260 ꢀC and 260e390 ꢀC regions, due to the
decomposition of organic compounds. Specifically, between 390
and 640 ꢀC, the ZnO:In3þ precursor slowly evaporates, generating
ZnCl2 and InCl3 vapors, as described by the following reactions:
Zn(CH3COO)2 þ 2H2O / Zn(OH)2 þ 2CH3COOH([)
InCl3 þ 3H2O / In(OH)3 þ 3HCl([)
(5)
(6)
2.2. Fabrication of ZnO:In3þ nanorods though regrowth
Zn(OH)2 þ In(OH)3 þ 5HCl / ZnCl2([) þ InCl3([) þ 5H2O([) (7)
For the ZnO and ZnO:In3þ precursors, endothermic and
exothermic peaks are observed in the heat flow analysis of Fig. 2
between 25 and 330 ꢀC and between 25 and 550 ꢀC, respectively.
These peaks are attributed to the evaporation of water and organics
from these precursors. The last exothermic peaks in the TG-DTA
curves of the zinc acetate dihydrate and zinc chloride/zinc acetate
dihydrate precursors, respectively at 370 and 580 ꢀC, result from
the crystallization of ZnO.
The basic strategy for annealing spin-coated ZnO films is illus-
trated in Fig. 1. Three samples were prepared at 700 ꢀC for 1 h for a
comparative study. Sample 1 was annealed in open air. Sample 2
was annealed using the conventional FTFA method, whereby two
films are placed together in an FTF arrangement during annealing.
For sample 3, a mica sheet was inserted between the two films,
which were then annealed using the FTF method.
The SEM image in Fig. 3(a) shows that the surface of sample 1
consists of numerous spherical ZnO:In3þ particles approximately
100 nm in diameter. There are many dangling bonds on the surface
of these samples before annealing, related to the oxygen vacancies
of the grain boundaries. When annealed in open air, adjacent grains
merge into larger particles approximately 100 nm in diameter. For
samples 2 and 3, during FTFA and VC-FTFA respectively, ZnO:In3þ
nanorods grow from spherical ZnO:In3þ seeds. ZnCl2 and InCl3
vapors are generated between 260 and 640 ꢀC, leading to the
regrowth of spherical nanoparticles. The nanorods in sample 3 are
longer than those in sample 2. As shown in Fig. 3(b), the regrowth
of the ZnO:In3þ nanorods occurs through a vapor-solid mechanism,
whereby nanorods grow via oxidation of the produced Zn vapor,
which then condenses. ZnCl2 and InCl3 vapors decompose at
390e640 ꢀC, and the Zn and In vapor absorbed on the spherical
nanoparticles with oxygen-containing organic compounds or with
oxygen present in the air. In contrast, Fig. 3(c) shows that the sur-
face morphology does not change when the ZnO:In3þ films were
prepared using an indium acetate precursor as dopant. The chem-
ical reactions of indium acetate are similar to those of zinc acetate
dihydrate but according to Equations (1e4), an acetate-based sol-
vent does not generate the vapors that facilitate regrowth. ZnO:In3þ
nanorods therefore only form by regrowth when a chloride-based
solvent is used, through the ZnCl2 and InCl3 vapors produced.
Fig. 4 shows the X-ray diffraction patterns of the three samples.
Three ZnO diffraction peaks were observed at 31.9ꢀ, 34.7ꢀ, and
2.3. Characterization
PL spectra were measured at various temperatures in a He gas
cryostat. The samples were excited using an unfocused 325 nm
20 mW HeeCd laser and the emitted photons were dispersed using
a single monochromator with a 0.75-m focal length, and counted in
a Hamamatsu R928 photomultiplier tube. The wavelength resolu-
tion of the PL system was 0.1 nm. The surface morphology of the
samples was analyzed using field-emission scanning electron mi-
croscope (FE-SEM, HITACHI, S-4800). The thermal analysis of the
zinc acetate dihydrate and zinc chloride/zinc acetate dihydrate
precursors was performed by thermogravimetric-differential
thermal analysis (TG-DTA, TA Instruments, SDT Q600) at a heat-
ing rate of 10 ꢀC$minꢁ1 in air.
3. Results and discussion
The thermal-decomposition of the ZnO (zinc acetate dihydrate)
and ZnO:In3þ (indium chloride þ zinc acetate dihydrate, In/
Zn ¼ 2 at.%) precursors was analyzed using TG-DTA to quantify
their thermal stability and decomposition temperature; the mate-
rials were heated in air from room temperature up to 800 ꢀC at
10 ꢀC$minꢁ1, as shown in Fig. 2. An initial endothermic reaction is
observed for both precursors between 27 and 150 ꢀC, amounting to
6.9% and 8.5% weight loss for ZnO and ZnO:In3þ precursors,
respectively, that arises from the thermal dehydration of zinc

G. Nam et al. / Journal of Alloys and Compounds 651 (2015) 1e7
3
Fig. 1. Schematic diagram outlining the fabrication of the three samples investigated in this study. The effects of different annealing configurations were compared, viz. In open air
(sample 1), FTFA (sample 2), and VC-FTFA for which an extra mica sheet is inserted between the two films (sample 3).
Fig. 2. TG-DTA curves obtained for the ZnO (left) and ZnO:In3þ (right) precursors used in this study, respectively showing the weight loss percentage (in black) and the difference in
temperature (in blue). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
36.5ꢀ, which correspond to the (100), (002), and (101) planes,
respectively. No traces of In metal or oxide were detected, indi-
cating that the wurtzite structure was not modified by the incor-
poration of In into the ZnO matrix. Sample 2 and 3 exhibited a
strong (002) peak, indicating that the c-axis orientation of the ZnO
grains was perpendicular to the substrate. It was also observed that
the intensity of the (002) peak in sample 3 was larger than that in
sample 2.

4
G. Nam et al. / Journal of Alloys and Compounds 651 (2015) 1e7
Fig. 3. (a) SEM images of ZnO:In3þ films prepared by annealing in open air (sample 1), FTFA (sample 2), or VC-FTFA (sample 3). (b) Schematic illustration of the regrowth
mechanism of ZnO nanorods by VC-FTFA. (c) SEM images of ZnO:In3þ films prepared using indium acetate instead of indium chloride in the precursor and annealed at 700 ꢀC in
open air (left) or using VC-FTFA (right).
NBE emission. Fig. 6(a) shows that the NBE emission intensity from
the ZnO:In3þ films deposited using indium acetate precursor as a
dopant and annealed in open air, is similar to that from films pre-
pared by VC-FTFA method. However, the deep level (DL) emission
from the latter is slightly lower than from the former. This result is
similar to that reported by Wang et al. [32], and does not arise from
regrowth. Chloride-containing metals, such as magnesium chloride
(MgCl2), can generate ZnCl2 and metal chloride vapors, and Fig. 6(b)
shows that the resulting NBE emission is more intense and blue-
shifted when annealing is performed in open air rather than by
VC-FTFA.
Fig. 7 presents the PL spectra of ZnO:In3þ films prepared by VC-
FTFA at different temperatures from 400 to 800 ꢀC. The NBE
emission intensity increases for annealing temperatures up to
700 ꢀC. The emission from the film annealed at 400 ꢀC is the
weakest, because the generation and decomposition of ZnCl2 and
InCl3 is minimal at this temperature. At 700 ꢀC on the contrary,
ZnCl2 and InCl3 vapors are optimally generated and decomposed,
Fig. 4. XRD analysis of the three samples.
resulting in active regrowth on the ZnO:In3þ films through the
confined vapors. Confinement fails however at 800 ꢀC and the va-
pors escape, leading to weaker NBE emission from the films pre-
pared at this temperature.
Fig. 5(b) shows the degree of uniformity of the NBE emission
intensities in the PL spectra of the three samples using
a
Fig. 8 shows the PL spectra recorded at 12 K for sample 3 and for
an undoped ZnO film annealed in open air at 700 ꢀC. Five distinct PL
peaks appear in the spectra of the ZnO film, at 3.376, 3.361, 3.323,
3.251, and 3.036 eV. The first three of these are attributed to the
emission of free excitons (FXs), neutral-donor-bound excitons
(D0Xs), and two-electron satellites (TESs), respectively [33]. The
peak at 3.251 eV is attributed to first-order longitudinal optical (LO)
phonon replicas of TESs. In ZnO, the latter are generally identified
based on the energy interval between the TES and the LO phonon
energy (huLO ¼ 72 meV) [34]. The DL emission at 3.036 eV is
attributed to a native zinc vacancy defect. In general, TESs are
associated with D0X transitions occurring between 3.32 and
3.34 eV [33]. Bound excitons are extrinsic transitions related to
dopants or defects, which usually create discrete electronic states in
the bandgap. In theory, excitons could be bound to neutral or
charged donors and acceptors. Shallow neutral donor-bound exci-
tons often dominate because of the presence of donors from
continuous-wave HeeCd laser as the optical excitation source, as
measured at the points marked in blue in Fig. 5(a). The NBE emis-
sion intensities vary randomly across the surface of sample 1,
whereas for sample 2 the emission is stronger towards the edges of
the sample due to insufficient confinement of the ZnCl2 and InCl3
vapors. Indeed, when these vapors remain too briefly at the center
of the surface, only relatively small spherical nanoparticles are
regrown. For sample 3 however, the NBE emission intensities are
evenly distributed across the entire surface because in VC-FTFA, the
vapors are confined by the mica sheet. We thereby attribute the
active regrowth to the confinement of these vapors. Fig. 5(c) pre-
sents the PL spectra obtained for the three samples, with a NBE
peak at 3.250 eV attributed to the ZnO:In3þ phase. The NBE emis-
sion is clearly enhanced in sample 3, prepared by VC-FTFA, with a
ZnO:In3þ peak about 17 times more intense than sample 1,
annealed in open air. These results show that VC-FTFA regrowth is
effective in improving both the intensity and the uniformity of the

G. Nam et al. / Journal of Alloys and Compounds 651 (2015) 1e7
5
Fig. 5. (a) Samples were measured at the points marked in blue to show NBE emission intensity uniformity. (b) The degree of NBE emission intensity uniformity in the PL spectra of
the three samples. (c) PL spectra obtained from the three samples at 300 K. (For interpretation of the references to colour in this figure legend, the reader is referred to the web
version of this article.)
unintentional impurities and/or shallow donor-like defects. During
recorded for sample 3 at temperatures from 12 to 300 K, as shown
in Fig. 9(a). The D0X peak is the most intense at 12 K but the FX
emission becomes dominant above 75 K, indicating that the
probability for the bound excitons to be ionized increases with the
temperature. The UV emission of a ZnO crystal is usually attributed
to the interband recombination of electrons and holes in form of
excitons. Generally, due to the temperature-induced lattice dilata-
tion and electronelattice interaction, the interband emission peak
energy follows the well-known Varshni formula [41]:
the recombination of the D0Xs, the final state of the donor can
either be 1s (resulting in a normal D0 X line) or 2s/2p (resulting in a
i
TESi line). The energy difference between the D0 X and its TESi
i
peaks therefore indicates the difference between the donor en-
ergies in the 1s and 2s/2p states. The donor excitation energy from
the ground state to the first excited state is equal to 3/4 of the donor
binding energy (ED) [33,35]. Here, the TES and D0X peak positions
indicate an ED of 50.7 meV, the same value as estimated using
Haynes' empirical rule [33]. Similarly, the binding energy of the
neutral donor-bound excitons is estimated to be 15 meV from the
energy difference between the FX and D0X emissions, which is
identical to the activation energy for thermal release of excitons
from neutral donors. For sample 3, four distinct peaks are observed
in the spectra, at 3.361, 3.356, 3.316, and 3.227 eV, which are
attributed to FXs, D0Xs, excitons bound to surface defects, and
donoreacceptor pair transitions, respectively [36e39]. The D0X
peak is blue-shifted due to the incorporation of indium [39] and the
emission at 3.316 eV is associated with the large surface-to-
volume-ratio nanorods [40].
.
EgðTÞ ¼ Egð0Þ ꢁ aT2 ðb þ TÞ;
(8)
where T is the absolute temperature, Eg(0) is the band-gap energy
at T ¼ 0, and are constants specific to ZnO, and is proportional
to the Debye temperature qD. The and values obtained from
fitting the PL peaks of FX were ¼ 880 K, as
¼ 1.1 meV/K and
shown in Fig. 9(b). These values of and well agree with those
a
b
b
a
b
a
b
a
b
reported for the bulk ZnO semiconductor materials [42,43].
The intensities of all these PL peaks decrease with increasing
temperature because of the thermally induced dissociation of
To identify the origin of these UV emissions, PL spectra were

6
G. Nam et al. / Journal of Alloys and Compounds 651 (2015) 1e7
Fig. 6. PL spectra of ZnO:In3þ films prepared using an indium acetate/zinc acetate dihydrate precursor and annealed at 700 ꢀC in open air (black line) or by VC-FTFA (red line). (b) PL
spectra of Mg0.3Zn0.7O annealed at 600 ꢀC in open air (black line) or by VC-FTFA (red line). (For interpretation of the references to colour in this figure legend, the reader is referred
to the web version of this article.)
Fig. 7. PL spectra of ZnO:In3þ nanorods prepared using VC-FTFA method at different
temperatures.
electron-hole pairs as described by Ref. [44].
Fig. 8. PL spectra obtained at 12 K from undoped ZnO films annealed in open air
(upper panel) and from sample 3 (lower panel).
I ¼ I0=½1 þ C expð ꢁ DEA=kBTÞꢂ;
(9)
where I and I0 are the integrated intensities of the emission band, C
and 62 meV for the D0X and FX intensities, respectively. These
activation energies are in agreement with the properties derived
from the PL spectrum in Fig. 8. Indeed, the D0X peaks at T < 75 K
originate from the recombination of the donor-bound excitons with
a localization energy of 15 meV, a value identical to the localization
energy obtained from Fig. 8, while the FX peaks at T > 75 K originate
from the recombination of free excitons with a binding energy of
62 meV, a value very close to the exciton binding energy of ZnO
(60 meV).
is a constant, kB is Boltzmann's constant, and EA is the activation
D
energy for thermal quenching. Equation (9) highlights the advan-
tage of a large exciton binding energy for certain applications.
Indeed, the PL intensity is maximal when the Boltzmann factor in
the denominator is zero, i.e. when the activation energy (binding or
localization) becomes much greater than the thermal energy.
Fig. 9(c) shows the PL intensity of the D0X and FX peaks as a
function of inverse temperature for sample 3, along with fits of the
experimental data to Equation (9) using activation energies of 15

G. Nam et al. / Journal of Alloys and Compounds 651 (2015) 1e7
7
Fig. 9. (a) Temperature-dependent PL spectra of NBE emissions of sample 3. (b) The bandgap energies of FX as a function of temperature. The solid line indicates fitting on the basis
of Varshni equation. (c) Integrated dependence of PL intensity on temperature. The solid lines are fits to I0/[1 þ Cexp(e EA/kBT)] (Equation (9) in the main text).
D
T. Walther, M.S. Skolnick, L.R. Wilson, Nano Lett. 13 (2013) 861e865.
[13] M.D. McCluskey, S.J. Jokela, J. Appl. Phys. 106 (2009) 071101.
[14] S.-J. Chang, B.-G. Duan, C.-H. Hsiao, S.-J. Young, B.-C. Wang, T.-H. Kao, K.-
S. Tsai, S.-L. Wu, Phot. Technol. Lett. 25 (2013) 2043e2046.
[15] D. Kilian, S. Polster, I. Vogeler, M.P.M. Jank, L. Frey, W. Peukert, ACS Appl.
Mater. Interfaces 6 (2014) 12245e12251.
[16] K. Mahmood, H.W. Kang, R. Munir, H.J. Sung, RSC Adv. 3 (2013) 25136e25144.
[17] S. Pati, P. Banerji, S.B. Majumder, Int. J. Hydrogen Energy 39 (2014)
15134e15141.
[18] G.C. Park, S.M. Hwang, J.H. Choi, Y.H. Kwon, H.K. Cho, S.-W. Kim, J.H. Lim, J. Joo,
Phys. Status Solidi A 210 (2013) 1552e1556.
[19] M. Benhaliliba, C.E. Benouis, Z. Mouffak, Y.S. Ocak, A. Tiburcio-Silver, M.S. Aida,
A.A. Garcia, A. Tavira, A.S. Juarez, Superlattice Microstruct. 63 (2013) 228e239.
[20] S. Kim, S.-h. Lee, J.S. Kim, J.S. Kim, D.Y. Kim, S.-O. Kim, J.-Y. Leem, Electron.
Mater. Lett. 9 (2013) 509e512.
4. Conclusions
In summary, ZnO:In3þ nanorods prepared using VC-FTFA exhibit
significantly enhanced PL properties compared with ZnO:In3þ and
undoped ZnO films annealed in open air. The regrowth of the
ZnO:In3þ nanorods was due to the chloride-containing metal; here
indium chloride, which generated ZnCl2 and InCl3 vapors. The
ZnO:In3þ nanorods evolve via a vapor-solid mechanism whereby
growth occurs through oxidation of Zn vapor, which then con-
denses. ZnCl2 and InCl3 vapors were generated in the temperature
region of 390e640 ꢀC, and round-shaped nanoparticles were
regrown by the ZnCl2 and InCl3 vapors. The regrowth afforded by
VC-FTFA is effective in increasing both the intensity and the uni-
formity of the NBE emission. The VC-FTFA method offers a low-coat
fabrication route for optoelectronic devices with high external
efficiency.
[21] M. Rezapour, N. Talebian, Ceram. Int. 40 (2014) 3453e3460.
[22] Z.-F. Shi, Y.-T. Zhang, X.-P. Cai, H. Wang, B. Wu, J.-X. Zhang, X.-J. Cui, X. Dong,
H.-W. Liang, B.-L. Zhang, G.-T. Du, CrystEngComm 16 (2014) 455e463.
[23] R. Zhu, W. Zhang, C. Li, R. Yang, Nano Lett. 13 (2013) 5171e5176.
[24] J.-L. Hou, S.-J. Chang, T.-J. Hsueh, C.-H. Wu, W.-Y. Weng, J.-M. Shieh, Prog.
Photovolt. Res. Appl. 21 (2013) 1645e1652.
[25] T. Shinaqawa, M. Izaki, RSC Adv. 4 (2014) 30999e31002.
[26] X. Pu, J. Liu, J. Liang, X. Yusheng, W. Feng, Y. Wang, X. Yu, RSC Adv. 4 (2014)
23149e23154.
Acknowledgments
[27] H. Kanber, R.J. Cipolli, W.B. Henderson, J.M. Whelan, J. Appl. Phys. 57 (1985)
4732e4737.
[28] G. Nam, B. Kim, Y. Park, S. Park, J. Moon, D.Y. Kim, S.-O. Kim, J.-Y. Leem,
J. Mater. Chem. C 2 (2014) 9918e9923.
[29] G. Nam, Y. Park, I. Ji, B. Kim, S.-h. Lee, D.Y. Kim, S. Kim, S.-O. Kim, J.-Y. Leem,
ACS Appl. Mater. Interfaces 7 (2015) 873e879.
This research was supported by Global PH.D Fellowship Program
through the National Research Foundation of Korea (NRF) funded
by the Ministry of Education (No. 2014H1A2A1018051).
[30] F. Paraguay, D.,W. Estrada, L.,D.R. Acosta, N.,E. Andrade, M. Miki-Yoshida, Thin
Solid Films 350 (1999) 192e200.
References
[31] C.-C. Lin, Y.-Y. Li, Mater. Chem. Phys. 113 (2009) 334e337.
[32] Y.G. Wang, S.P. Lau, X.H. Zhang, H.H. Hng, H.W. Lee, S.F. Yu, B.K. Tay, J. Cryst.
Growth 259 (2003) 335e342.
[33] Ü. Ozgür, Ya I. Alivov, C. Liu, A. Teke, M.A. Reshchikov, S. Dogan, V. Avrutin, S.-
J. Cho, H. Morkoc, J. Appl. Phys. 98 (2005) 041301.
[34] K. Johnston, M.O. Henry, D. McCabe, E. McGlynn, M. Dietrich, E. Alves, M. Xia,
Phys. Rev. B 73 (2006) 165212.
[35] H.-M. Chiu, Y.-T. Chang, W.-W. Wu, J.-M. Wu, ACS Appl. Mater. Interface 6
(2014) 5183e5191.
[36] S. Lin, Z. Ye, H. He, Y. Zeng, H. Tang, B. Zhao, L. Zhu, J. Cryst. Growth 306 (2007)
339e343.
[37] S. Lin, H. He, Z. Ye, B. Zhao, J. Huang, J. Appl. Phys. 104 (2008) 114307.
[38] C. Liu, H. He, L. Sun, Q. Yang, Z. Ye, L. Chen, Solid State Commun. 150 (2010)
2303e2305.
[39] S.P. Watkins, Z.W. Deng, D.C. Li, H. Huang, J. Appl. Phys. 110 (2011) 083506.
[40] H.P. He, H.P. Tang, Z.Z. Ye, L.P. Zhu, B.H. Zhao, L. Wang, X.H. Li, Appl. Phys. Lett.
90 (2007) 023104.
[1] H. Jeong, D.J. Park, H.S. Lee, Y.H. Ko, J.S. Yu, S.-B. Choi, D.-S. Lee, E.-K. Suh,
M.S. Jeong, Nanoscale 6 (2014) 4371e4378.
[2] T. Singh, T. Lehnen, T. Leuning, D. Sahu, S. Mathur, Appl. Surf. Sci. 289 (2014)
27e32.
[3] D. He, S. Sheng, J. Yang, L. Chen, K. Zhu, X. Feng, J. Am. Chem. Soc. 136 (2014)
16772e16775.
[4] S. Ameen, M.S. Akhtar, H.-K. Seo, M.K. Nazeeruddin, H.-S. Shin, J. Phys. Chem. C
119 (2015) 10379e10390.
[5] Y.-C. Chang, Catal. Commun. 56 (2014) 45e49.
[6] C. Sandoval, O. Marin, S. Real, D. Comedi, M. Tirado, Mater. Sci. Eng. B-Adv. 187
(2014) 21e25.
[7] S. Li, C. Zhang, F. Li, W. Ji, P. Li, M. Ren, P. Wang, M. Yuan, RSC Adv. 4 (2014)
24399e24405.
[8] A. Pescaglini, A. Martin, D. Cammi, G. Juska, C. Ronning, E. Pelucchi,
D. Lacopino, Nano Lett. 14 (2014) 6202e6209.
[9] A.M. Lord, M.B. Ward, J.E. Evans, P.R. Davies, N.A. Smith, T.G. Maffeis,
S.P. Wilks, J. Phys. Chem. C 118 (2014) 21177e21184.
[10] H. Nguyen, C.T. Quy, N.D. Hoa, N.T. Lam, N.V. Duy, V.V. Quang, N.V. Hieu, Sens.
Actuators B Chem. 193 (2014) 888e894.
€
ꢀ
[41] Y.P. Varshni, Physica 34 (1967) 149e154.
[42] N. Kumar, R. Kaur, R.M. Mehra, J. Lumin. 126 (2007) 784e788.
[43] T.-H. Fang, S.-H. Kang, J. Appl. Phys. 105 (2009) 113512.
[11] F. Qian, S. Gradecak, Y. Li, C.-Y. Wen, C.M. Lieber, Nano Lett.
5 (2005)
ꢀ
[44] F. Schuster, B. Laumer, R.R. Zamani, C. Magen, J.R. Morante, J. Arbiol,
2287e2291.
M. Stutzmann, ACS Nano 8 (2014) 4376e4384.
[12] M.N. Makhonin, A.P. Foster, A.B. Krysa, P.W. Fry, D.G. Davies, T. Grange,