materials

Article
Effect of Silver Dopants on the ZnO Thin Films
Prepared by a Radio Frequency Magnetron
Co-Sputtering System
Fang-Cheng Liu 1 , Jyun-Yong Li 1 , Tai-Hong Chen 2 , Chun-How Chang 2 , Ching-Ting Lee 3 ,
Wei-Hua Hsiao 1 and Day-Shan Liu 1, *
1 Institute of Electro-Optical and Materials Science, National Formosa University, Yunlin 63201, Taiwan;
chengliuxd@gmail.com (F.-C.L.); 10476110@gm.nfu.edu.tw (J.-Y.L.); s706802000213@gmail.com (W.-H.H.)
2 Additive Manufacturing and Laser Application, Industrial Technology Research Institute, Tainan 73445,
Taiwan; tsaicc1221@gmail.com (T.-H.C.); a0922639175@gmail.com (C.-H.C.)
3 Institute of Microelectronics, National Cheng Kung University, Tainan 70101, Taiwan; tsaicc@ee.ncku.edu.tw
* Correspondence: dsliu@sunws.nfu.edu.tw; Tel.: +886-5-6315665

Received: 29 May 2017; Accepted: 9 July 2017; Published: 14 July 2017

Abstract: Ag-ZnO co-sputtered films at various atomic ratios of Ag (Ag/(Ag + Zn) at.%) were
prepared by a radio frequency magnetron cosputtering system, using the co-sputtered targets of
Ag and ZnO. The activation of the Ag acceptors (AgZn ) and the formation of the Ag aggregations
(Ag0 ) in the ZnO matrix were investigated from XRD, Raman scattering, and XPS measurements.
The Ag-ZnO co-sputtered film behaving like a p-type conduction was achievable after annealing at
350 ◦ C under air ambient for 1 h.

Keywords: Ag-ZnO co-sputtered film; radio frequency magnetron co-sputtered system; Ag acceptors;
Ag aggregations; p-type conduction

1. Introduction
Zinc oxide (ZnO) is a multi-functional material of the II–VI group for its wide and direct band
gap (Eg ~ 3.37 eV) with a large exciton binding energy of 60 meV at room temperature. It also has
the advantages of excellent resistance to radiation damage, suitability for the wet-etching process,
physical and chemical stability, high oxidative capacity, low cost, and availability. In recent decades,
ZnO has become a very encouraging material utilized in multiple fields, such as solar cells, transparent
conductive contacts, light emitting devices, spintronic devices, laser deflectors, paints, antibacterial
agents, bio-sensors, piezoelectric transducers, and gas sensors [1–5]. Among these devices and
component applications, many efforts have been made to modify the optical and/or electrical
properties of ZnO through doping, ion irradiation, etc., so that it can be used more widely in research
fields undoped, as untreated ZnO is generally inactive to carrier transmission and ineffective for solar
energy adsorption. For instance, when the ZnO material is applied as a photocatalyst to promote
the decomposition of the organic pollutants, less than 5% of the solar spectrum at the Earth’s surface
consisting of the UV wavelengths can drive the photocatalytic process because the absorption edge is
constrained by the natural band gap of ZnO. Band gap modification of ZnO through metal doping
is one of the promising approaches to extend the absorption of light into the visible wavelengths.
In addition, these metal dopants have also demonstrated properties of electron sinks to increase the
life-span of the photo-generated electron-hole pairs [6–8]. On the other hand, when ZnO is applied to
optoelectronic device fabrication, one of the major obstacles for realizing ZnO-based optoelectronic
devices is the difficulty in achieving quality p-type ZnO because of the self-compensation effect that
originates from native defects, as well as the limited solubility and inactivation of the acceptor dopants

Materials 2017, 10, 797; doi:10.3390/ma10070797 www.mdpi.com/journal/materials

Materials 2017, 10, 797 2 of 13

in the ZnO material. Although steady progress in doping ZnO with p-type, using group-I elements for
zinc sites and/or group-V elements for oxygen sites, has been reported [9–11], the reproducibility of
the p-type ZnO still is challenging due to the group-I and group-V dopants being prone to forming
the interstitial site or antisite defects, respectively, due to the significant size-mismatch to the lattice
atoms. Recently, group-IB elements (such as Ag and Cu) with less size-mismatch and larger ionization
energy than the group-I elements were announced as an alternative dopants to achieve a quality
p-type ZnO [12–14]. In addition to realize p-type ZnO, nanoparticles- or nanorods-ZnO prepared using
group-IB elements have also been applied to enhance the performance of the resulting ZnO-based
optoelectronic devices via the localized surface plasmon [15–18]. Among these group-IB elements,
silver has excellent electrical, optical, and chemical properties for promoting the photocatalytic activity,
conductive type, and luminescence emission of the ZnO material. Accordingly, insights into the
activation of the Ag dopants in the ZnO matrix are critical.
Silver-doped ZnO (referred to as Ag-ZnO, hereafter) have been synthesized using several
technologies, such as photochemical, solvothermal, and pulse laser deposition [19–21]. It is also
acceptable to prepare using sputtering technology, which is widely used in the coating industry [22,23].
In this work, we used a radio frequency (RF) magnetron co-sputtering system, which has the advantage
of simple and in situ control on the elemental composition of the resulting film over the conventional
sputtering system, to prepare Ag-ZnO co-sputtered films at various Ag atomic ratios. Electrical, optical,
and material properties of the Ag-ZnO co-sputtered films at various theoretical Ag atomic ratios were
measured to understand the behavior of the Ag dopants in the ZnO matrix. The origin responsible for
the change in the conduction type of the Ag-ZnO co-sputtered films can be reasonably explained by
the evolutions in their crystalline structures and chemical bond configurations.

2. Experimental Procedure
The RF magnetron co-sputtering system was constructed from a dual RF power supply that
generated two different RF powers with synchronized phases. The configuration of the RF magnetron
co-sputtering chamber has been illustrated elsewhere [24]. High-purity ZnO (99.99%) and metallic
Ag (99.99%) were selected as the co-sputtering targets. Figure 1 depicts the deposition rates of the
single ZnO and metallic Ag films as functions of the RF power supplied to the ZnO and Ag targets,
respectively. To deposit Ag-ZnO films at various Ag doping levels, the RF power supplied to the
ZnO target was fixed at 250 W while that supplied to the Ag target was varied from 2 to 6.8 W.
The theoretical Ag atomic ratios [Ag/(Ag + Zn) at.%] introduced into the ZnO films could be evaluated
from the following expression similar to our previous reports [24–26]:

D1 × A × d1 D2 × A × d2
: = P : Q, (1)
M1 M2

where D1 and D2 (nm/min), respectively, are the deposition rates of the single Ag and ZnO films
prepared at specific RF powers; A (nm2 ) is defined as the cross-section area of the substrate surface;
d1 and d2 (g/cm3 ) are related to the density of the Ag (10.49 g/cm3 ) and ZnO (5.66 g/cm3 ) materials;
M1 and M2 (g/mole) are the atom and molecular weights of the Ag and ZnO materials; P and Q (mole)
are the mole ratios of Ag and Zn atoms in the co-sputtered films. According to the deposition rates of
the single ZnO and Ag films prepared at each RF power, shown in Figure 1, we controlled the Ag-ZnO
co-sputtered films at the theoretical Ag atomic ratios of 1, 3, 5, and 8 at.%. The films’ thickness was
fixed at about 200 nm. All the films were grown onto n-type Si (100) substrates at room temperature.
Moreover, in order to measure the films’ optical transmittance at visible and ultraviolet wavelengths,
one set of the films were deposited onto the glass substrates. To activate the Ag dopants and facilitate
the crystalline re-growth, all the undoped ZnO and Ag-ZnO co-sputtered film were post-annealed at
350 ◦ C for 1 h under ambient air.
Materials 2017, 10, 797 3 of 13
Materials 2017, 10, 797 3 of 12

Figure 1.
Figure 1. Deposition
Depositionrates
ratesofofthe
thesingle
singleZnO
ZnO and
and AgAg films
films as as functions
functions of the
of the RF RF power
power supplied
supplied on
on the
the ZnO
ZnO and and Ag targets,
Ag targets, respectively.
respectively.

Film thickness of these films was measured using a surface profile system (Dektak 6M, Veeco,
Film thickness of these films was measured using a surface profile system (Dektak 6M, Veeco,
New York, NY, USA). Resistivity, carrier concentration, and hall mobility were measured using the
New York, NY, USA). Resistivity, carrier concentration, and hall mobility were measured using the
van der Pauw method with a Hall measurement system (HMS-5000, Ecopia, Anyang, Korea).
van der Pauw method with a Hall measurement system (HMS-5000, Ecopia, Anyang, Korea). Optical
Optical transmittance was measured by a UV-VIS spectrophotometer (UVD-3500, Labomed, Inc.,
transmittance was measured by a UV-VIS spectrophotometer (UVD-3500, Labomed, Inc., Los Angeles,
Los Angeles, CA, USA). The surface morphologies were examined using a field emission scanning
CA, USA). The surface morphologies were examined using a field emission scanning electron
electron microscopy (FE-SEM; JSM-6700F, JEOL, Tokyo, Japan) with the accessory of the
microscopy (FE-SEM; JSM-6700F, JEOL, Tokyo, Japan) with the accessory of the energy-dispersive
energy-dispersive X-ray spectroscopy (EDS). Evidence of the dopants activation in the ZnO film
X-ray spectroscopy (EDS). Evidence of the dopants activation in the ZnO film resulted in the evolutions
resulted in the evolutions on the material properties were conducted from X-ray diffraction (XRD;
on the material properties were conducted from X-ray diffraction (XRD; D-500, Siemens, Munich,
D-500, Siemens, Munich, Germany) patterns, Raman spectra (MRI-A003, ProTrusTech, Tainan, Taiwan),
Germany) patterns, Raman spectra (MRI-A003, ProTrusTech, Tainan, Taiwan), and X-ray photoelectron
and X-ray photoelectron spectroscopy (XPS; Quantera SXM™, ULVAC-PHI, Kanagawa, Japan).
spectroscopy (XPS; Quantera SXM™, ULVAC-PHI, Kanagawa, Japan).

3. Results
3. Results and
and Discussion
Discussion
Table11summarizes
Table summarizes thethe electrical
electrical properties
properties of the of undoped
the undoped ZnO and ZnOZnO andfilms
ZnOdoped
films atdoped
various at
various theoretical Ag atomic ratios after annealing ◦ at 350 °C for 1 h
theoretical Ag atomic ratios after annealing at 350 C for 1 h under ambient air, measured using the under ambient air, measured
using
van derthe vanmethod
Pauw der Pauw method
at room at room temperature.
temperature. The resistivity Theof resistivity
the undoped of ZnO
the undoped
film was ZnO filmtowas
too high be
too high to be measured, while these Ag-ZnO films behaved in a different
measured, while these Ag-ZnO films behaved in a different conductive manner after the post-annealing conductive manner after
the post-annealing
treatment. A p-typetreatment.
conductor A p-type
with conductor
a hole concentrationwith aofhole 5.2 ×concentration
1016 cm−3 was of 5.2 × 1016 cm
obtained from was
−3
the
obtained from the ZnO film co-sputtered with a theoretical Ag concentration
ZnO film co-sputtered with a theoretical Ag concentration of 1 at.%. The hole carriers were further of 1 at.%. The hole
carriers were
increased to 5.9further
× 1017increased
cm−3 as the to 5.9
Ag ×dopants
1017 cmin −3 as the Ag dopants in the ZnO film reached a
the ZnO film reached a theoretical atomic ratio
theoretical
of atomic ratio
3 at.%. However, the of 3 at.%. However,
conductive the conductive
type converted into n-type type converted
with very high into n-type carriers
electron with very of
high electron
20 −carriers
3 of 1.9 × 10 20 cm−3 as the ZnO film co-sputtered at a theoretical Ag atom ratio of
1.9 × 10 cm as the ZnO film co-sputtered at a theoretical Ag atom ratio of 5 at.%. In addition, more
5 at.%. In
electron addition,
carriers more
as high electron
as 1.7 × 1021carriers
cm−3 were as high
measuredas 1.7from× 1021thecmZnOwere
−3
film measured
doped at a from the ZnO
theoretical Ag
film doped
atom ratio ofat8 a theoretical
at.%. Ag atom
The associated ratio transmittance
optical of 8 at.%. Thespectra associated opticalintransmittance
are shown Figure 2a. Thespectra
undoped are
shown in Figure 2a. The undoped
◦ ZnO film annealed at 350 °C for 1
ZnO film annealed at 350 C for 1 h under ambient air had a high average transmittance of about 89%h under ambient air had a high
average
at visible transmittance
wavelengths (400–700of about nm).89% Foratthe
visible
ZnO film wavelengths
co-sputtered (400–700
with the nm). For thetheZnO
Ag atoms, averagefilm
co-sputtered with
transmittance the wavelengths
at visible Ag atoms, the averageastransmittance
decreased more Ag atoms atwere
visible wavelengths
introduced into thedecreased
ZnO films, as
more Ag atoms were introduced into the ZnO films, as listed in Table
as listed in Table 1. Eventually, the annealed Ag-ZnO film at a theoretical atomic ratio of 8% became 1. Eventually, the annealed
Ag-ZnO filmwith
semi-opaque at aa low
theoretical
averageatomic ratio ofof 8%
transmittance about became semi-opaque
38%. Figure 2b shows with
the acorresponding
low average
transmittance of about 38%. Figure 2b shows the 2corresponding
optical energy band gap determined from the plot of (αhν) versus the photon energy. Compared optical energy bandto gapthe
determined from the plot of (αhν) 2 versus the photon energy. Compared to the undoped ZnO film,
undoped ZnO film, the onset of the absorption edge in the ultraviolet wavelengths for these annealed
the onset
Ag-ZnO of the absorption
co-sputtered films initiallyedge in the
shifted ultraviolet
toward the short wavelengths
wavelength, for theseinannealed
resulting a widenedAg-ZnO
optical
co-sputtered films initially shifted toward the short wavelength, resulting
energy band gap from 3.25 to 3.28 eV as the theoretical Ag dopants reached 3%. Then, a slight redshift in a widened optical
energy band gap from 3.25 to 3.28 eV as the theoretical Ag dopants reached 3%. Then, a slight
redshift on the absorption edge with the optical energy band gap narrowing was observed from the
Materials 2017, 10, 797 4 of 13

Materials 2017, 10, 797 4 of 12

on the absorption edge with the optical energy band gap narrowing was observed from the annealed
annealed co-sputtered
Ag-ZnO film at the
Ag-ZnO co-sputtered theoretical
film doping level
at the theoretical of 5%
doping and
level 8% and
of 5% (the 8%
corresponding optical
(the corresponding
energy energy
optical band gaps were
band gaps3.23 and3.23
were 3.21and
eV,3.21
respectively).
eV, respectively).

Table 1.
Table Electrical properties
1. Electrical properties of
of the
the undoped
undoped ZnO
ZnO and
and ZnO
ZnO films
films doped
doped at
at various
various theoretical
theoretical Ag
Ag
atomic ratios annealed at 350 ◦ C for 1 h under ambient air.
atomic ratios annealed at 350 °C for 1 h under ambient air.

Sample Sample n or p n(cm

or p )(cm−3 ) μ (cm
-3 2/V2 s)
µ (cm /V s) ρ (Ω
ρ (Ω cm)cm) T avg (%) Tavg (%)
UndopedUndoped
ZnO ZnO N/A N/A N/AN/A N/A N/A 89 89
Ag-ZnO (1%)
Ag-ZnO (1%) 5.2 × 10 16
5.2 × 1016 3.33.3 24.124.1 81 81
Ag-ZnO (3%)
Ag-ZnO (3%) 5.9 × 10 17 × 1017
5.9 2.92.9 3.5 3.5 76 76
Ag-ZnO (5%) −1.9 ×− 20 × 1020 1.8 × −2
Ag-ZnO (5%) 1.9
10 1.91.9 1.810× 10−2 69 69
21 − 3
Ag-ZnO (8%)
Ag-ZnO (8%) −1.7 ×−10 21 × 10
1.7 1.51.5 2.4 ×
2.410× 10−3 38
38

Figure
Figure 2.
2. (a)
(a) Optical
Optical transmittance
transmittance spectra
spectra and
and (b)
(b) the
the plot
plot of
of (αhν)
(αhν)2 versus
2
versus the
the photon
photon energy
energy of
of the
the
undoped ZnO and Ag-ZnO co-sputtered films annealed at 350 °C
◦ for 1 h under ambient
undoped ZnO and Ag-ZnO co-sputtered films annealed at 350 C for 1 h under ambient air. air.

The crystalline structures conducted from XRD measurements for the undoped ZnO and the
The crystalline structures conducted from XRD measurements for the undoped ZnO and the
Ag-ZnO co-sputtered films at the theoretical Ag atomic ratios of 1%, 3%, and 5%, respectively, after
Ag-ZnO co-sputtered films at the theoretical Ag atomic ratios of 1%, 3%, and 5%, respectively, after
annealing at 350 °C for 1 h under air ambient are shown in Figure 3. All samples behaved like
annealing at 350 ◦ C for 1 h under air ambient are shown in Figure 3. All samples behaved like
polycrystalline structures identified as the ZnO hexagonal wurtzite type, and no signal related to
polycrystalline structures identified as the ZnO hexagonal wurtzite type, and no signal related to Ag
Ag or its oxides phase was detected. The undoped ZnO film exhibiting the preferred growth
or its oxides phase was detected. The undoped ZnO film exhibiting the preferred growth orientation
orientation along the c-axis as evidence of the peak at◦ about 34.38° assigned as ZnO (002) phase
along the c-axis as evidence of the peak at about 34.38 assigned as ZnO (002) phase according to
according to JCPDS database (JCPDS card no. 36-145) was predominant throughout the XRD
JCPDS database (JCPDS card no. 36-145) was predominant throughout the XRD pattern. The ZnO
pattern. The ZnO (002) phase was also the dominant growth structure as Ag atoms was
(002) phase was also the dominant growth structure as Ag atoms was incorporated into the ZnO films
incorporated into the ZnO films at the theoretical atomic ratios of 1% and 3%. By contrast, two
at the theoretical atomic ratios of 1% and 3%. By contrast, two peaks assigned as ZnO (100) and ZnO
peaks assigned as ZnO (100) and ZnO (101) phase became the dominant signal in the XRD pattern
measured from the ZnO film co-sputtered at a theoretical Ag atomic ratio of 5%, revealing the
disappearance of the c-axis preferred growth orientation. Moreover, compared to the undoped ZnO
film, the full width at half maximum (FWHM) of the ZnO (002) peak increased as the Ag atoms
Materials 2017, 10, 797 5 of 13

(101) phase became the dominant signal in the XRD pattern measured from the ZnO film co-sputtered
at a theoretical Ag atomic ratio of 5%, revealing the disappearance of the c-axis preferred growth
orientation. Moreover, compared to the undoped ZnO film, the full width at half maximum (FWHM)
of the ZnO (002) peak increased as the Ag atoms doped into the ZnO film increased and slightly shifted
on the peak position. Table 2 summarizes the peak position and the FWHM of the ZnO (002) phase as
well as the corresponding crystalline size, D, evaluated from the FWHM of the preferred orientation
ZnO (002) according to the following Debye-Scherer formula:

kλ
D= , (2)
β cos θ

where k is a constant (k = 0.9), λ is the wavelength of the X-ray radiation, β is the FWHM in radians,
and θ is the Bragg diffraction angle. The crystalline size growing along the c-axis was found to be
suppressed as the Ag atoms doped into the ZnO film. The crystalline size apparently decreased from
about 19.1 nm to 14.9 nm as the ZnO film co-sputtered at a theoretical Ag atomic ratio of 5% due
to the degradation on the c-axis growth orientation. In terms of the peak position of the ZnO (002)
phase, a theoretical Ag atom ratio of 1% introduced into the ZnO matrix resulted in the peak shifting
toward a low 2θ value of 34.34◦ . The reason responsible for the shift of the ZnO (002) peak was likely
the activated Ag1+ ions substituted for the Zn2+ lattice sites (AgZn ) since the ionic radius of the Ag
atom (0.126 nm) was higher than that of the Zn atom (0.074 nm). In addition, the activation of the Ag
dopants also led to the Ag-ZnO film behaving as a p-type conduction. As the theoretical Ag doping
level in the ZnO films reached 3% which had a higher hole carriers, the increase in the amounts of
the activated Ag acceptors caused a further shift of the ZnO (002) peak toward a low 2θ value of
34.32◦ . In contrast, a high diffraction angle of the ZnO (002) phase at about 34.56◦ was measured from
the ZnO film doped with the Ag atoms at a theoretical atomic ratio of 5% while exhibiting n-type
degenerated conduction without c-axis growth orientation. This indicated that the activation of the
Ag acceptor was saturated and another mechanism would be induced as the theoretical Ag dopants
reached 5%. Figure 4a,b show the surface morphologies of the undoped ZnO film and the Ag-ZnO
co-sputtered film at a theoretical atomic ratio of 3% after annealing at 350 ◦ C for 1 h under air ambient
(the elemental compositions conducted from EDS measurement are also shown in the inset figures).
Textures with wedge-like grains were observed from the surface of the annealed undoped ZnO film
and only the elements of Zn, O, and Si (the signal emerging from the substrate) were measured.
By contrast, the grain size distributed over the surface of the annealed Ag-ZnO co-sputtered film was
reduced with ambiguous grain boundaries and a weak peak denoted as Ag could be found from
the corresponding EDS spectrum. The shrink in the grain size and the disappearance of the surface
textures as the silver atoms incorporated into the ZnO film also supported the degradation in the
crystalline structure as investigated from the XRD measurements. The vibration properties of the
annealed undoped ZnO and Ag-ZnO co-sputtered films investigated using micro-Raman spectroscopy
are plotted in Figure 5. The peaks at about 303, 521, and 618 cm−1 , respectively, were due to scattering
from the silicon substrate. The Raman spectrum is an essential and versatile diagnostic study on the
crystallization, structural disorder, and defects in micro- and/or nano-structures. Complying with the
Raman selection rules in wurtzite crystal structures, two specific lines corresponding to the E2 high
frequency branch and A1 longitudinal optical modes (denoted as E2 (high) and A1 (LO) in the spectrum)
at around 435 and 580 cm−1 , respectively, were observed in the Raman spectrum of the undoped ZnO
sample [27–29].
Materials 2017, 10, 797 6 of 12
Materials 2017, 10, 797 6 of 13
Materials 2017, 10, 797 6 of 12

Figure 3. XRD patterns of the undoped ZnO and the co-sputtered Ag-ZnO films at the theoretical
Figure 3.
Figure XRD
XRD patterns of
ofthe
theundoped ZnO
ZnOandandthe co-sputtered Ag-ZnO films at the theoretical Ag
Ag3.atomic patterns
ratios of 1%, 3%, undoped
and 5%, respectively, the
after co-sputtered Ag-ZnO
annealing at 350 °C for 1 films atambient
h under the theoretical
air.
atomic ratios of 1%, 3%, and 5%, respectively, after annealing at 350 ◦ C for 1 h under ambient air.
Ag atomic ratios of 1%, 3%, and 5%, respectively, after annealing at 350 °C for 1 h under ambient air.

Figure 4. Surface morphologies of the (a) undoped ZnO film and (b) Ag-ZnO co-sputtered film at a
theoretical atomic ratio of 3% after annealing at 350 °C for 1 h under ambient air (the inset figures
shows the elemental compositions conducted from EDS measurement).

Figure
Figure 4.
4. Surface
Surfacemorphologies
morphologiesofofthe
the(a)
(a)undoped
undopedZnOZnOfilm
filmand
and(b)
(b)Ag-ZnO
Ag-ZnOco-sputtered
co-sputteredfilm atat
film a
theoretical atomic ratio of 3% after annealing at 350 °C
◦ for 1 h under ambient air (the inset figures
a theoretical atomic ratio of 3% after annealing at 350 C for 1 h under ambient air (the inset figures
shows
shows the
the elemental
elemental compositions conducted from
compositions conducted from EDS
EDS measurement).
measurement).
Materials 2017, 10, 797 7 of 13
Materials 2017, 10, 797 7 of 12

Figure 5. Raman spectra of the undoped ZnO and Ag-ZnO co-sputtered films annealed at 350 °C for
Figure 5. Raman spectra of the undoped ZnO and Ag-ZnO co-sputtered films annealed at 350 ◦ C for
1 h under ambient air.
1 h under ambient air.

Table 2. Peak position and the FWHM of the ZnO (002) phase, as well as the corresponding
Table 2. Peak position and the FWHM of the ZnO (002) phase, as well as the corresponding crystalline
crystalline size, D, for the undoped ZnO and the Ag-ZnO co-sputtered films annealed at 350 °C for
size, D, for the undoped ZnO and the Ag-ZnO co-sputtered films annealed at 350 ◦ C for 1 h under
1 h under ambient air.
ambient air.
Sample 2θ (deg.) FWHM (deg.) D (nm)
Undoped ZnO Sample 34.48 2θ (deg.) FWHM (deg.)
0.43 D (nm) 19.1
Ag-ZnO (1%)Undoped ZnO34.34 34.48 0.43
0.48 19.1 17.2
Ag-ZnO (3%) Ag-ZnO (1%) 34.32 34.34 0.48
0.50 17.2 16.6
Ag-ZnO (3%) 34.32 0.50 16.6
Ag-ZnO (5%) Ag-ZnO (5%) 34.56 34.56 0.56
0.56 14.9
14.9

When the ZnO film was doped with the Ag atoms, the E2(high) mode was gradually decreased
When the as
and broadened ZnO thefilm was doped
theoretical with the increased,
Ag dopants Ag atoms,and the E 2 (high)
then this mode
mode was was gradually decreased
hardly observed in
and broadened as the theoretical Ag dopants increased, and then
the Raman spectrum of the Ag-ZnO film at an atomic ratio of 5%. As quoted from the previous this mode was hardly observed in
the Raman spectrum of the Ag-ZnO film at an atomic ratio of 5%. As
reports, the E2(high) phonon mode is related to the crystalline nature, phase orientation, and strainquoted from the previous reports,
the E2 (high)
present in thephonon
ZnO matrix mode[30–32].
is related Theto decrease
the crystalline
in thenature,
intensityphase orientation,
of the and strain
E2(high) signal when present
the Ag in
the ZnO
atoms matrix
were [30–32].into
introduced Thethedecrease in the intensity
ZnO system of the Eto
was attributed 2 (high) signal when
the disordered the Ag atoms
crystalline were
structure
of the ZnO film, as confirmed from the degradation of the c-axis growth orientation discussed in film,
introduced into the ZnO system was attributed to the disordered crystalline structure of the ZnO the
as confirmed
XRD patterns.from the degradation
Furthermore, of the c-axis
the Ag-ZnO film growth orientation
at an atomic ratiodiscussed
of 5% grew in thewithout
XRD patterns.
c-axis
Furthermore,
orientation, the Ag-ZnO
resulting in thefilm at an atomic
E2(high) mode ratiobeingofalmost
5% grew without
absent c-axis
in the orientation,
associated Ramanresulting in the
spectrum.
E (high) mode being almost absent in the associated Raman spectrum.
In2 contrast to the change of the E2(high) phonon, the signal of the A1(LO) mode became more In contrast to the change of the
E2 (high) phonon,
significant as the Ag theatoms
signaldoped A1 (LO)
of the into the mode
ZnO filmbecame more significant
increased, as the Ag
and an apparent andatoms
widedoped
peak was into
the ZnO film increased, and an apparent and wide peak was identified
identified from the Raman spectrum of the ZnO film doped at the Ag level of 5%. The enhancement from the Raman spectrum of
the ZnO film doped at the Ag level of 5%. The enhancement on the
on the A1(LO) signal implied the increase of the defects in the ZnO1 film since the A1(LO) mode is A (LO) signal implied the increase
of the defects
represented to in
thethe ZnO complexes,
defect film since the A1 (LO)
such as zincmode is represented
interstitial (ZnI) and to the defectvacancy
oxygen complexes,
(VO) suchin theas
zinc interstitial (Zn ) and oxygen
ZnO lattice [33,34]. IIn addition, another weak vacancy (V ) in the ZnO lattice [33,34]. In
O peak at around 414 cm induced by the localized
−1 addition, another weak
peak at around
vibration mode (LVM)414 cm− 1 induced by the localized vibration mode (LVM) in the ZnO film appeared in
in the ZnO film appeared in the Raman spectra of the Ag-ZnO co-sputtered
the Raman
film. spectrafrom
As referred of thethe Ag-ZnO
reports co-sputtered
[35,36], this film.
LVM As mode
referred wasfrom anthe reports [35,36],
indication of thethis LVM
dopant
mode was an indication of the dopant incorporation associated with
incorporation associated with the Ag ion in substitution for the Zn lattice site in the Zn-O bond the Ag ion in substitution for the
Zn lattice site(denoted
configuration in the Zn-O bond configuration
as LVM(Ag Zn-O)).
(denoted as LVM(AgZn -O)).
Although the activation of the Ag acceptors in
Although the activation of the Ag acceptors inthe
theAg-ZnO
Ag-ZnOco-sputtered
co-sputteredfilms filmswerewere confirmed
confirmed by
the appearance of the LVM signal, only the co-sputtered films at the
by the appearance of the LVM signal, only the co-sputtered films at the theoretical Ag atomic ratio theoretical Ag atomic ratio of 1%
and
of 1%3% andbehaved
3% behavedlike a p-type conduction.
like a p-type Thus, the
conduction. chemical
Thus, bond configurations
the chemical bond configurations conducted from the
conducted
from the XPS measurement were carried out to understand the mechanism responsible for from
XPS measurement were carried out to understand the mechanism responsible for the conversion the
conversion from p- to n-type conduction of the Ag-ZnO co-sputtered film at the theoretical Ag
Materials 2017, 10, 797 8 of 13

p- to n-type conduction of the Ag-ZnO co-sputtered film at the theoretical Ag dopants of an atomic ratio
of 5%. The XPS survey spectra taken on the surface of the undoped ZnO and Ag-ZnO co-sputtered film
at a theoretical atomic ratio of 3% annealed at 350 ◦ C for 1 h under ambient air are shown in Figure 6a,b,
respectively. Both spectra were characterized as the peaks of Zn and O elements with the appearance
of the C 1s peak at 284.5 eV for reference. Two peaks at about 368 and 374 eV assigned as the signal
related to Ag 3d5/2 and Ag 3d3/2 were observed only in the spectrum of the Ag-ZnO co-sputtered
film. Figure 7a,b show the high resolution of the O 1s spectra for further realizing the evolution of the
oxidized states when Ag was incorporated into the ZnO matrix. As can be seen in Figure 7a, the core
level of the O 1s for the undoped ZnO film exhibited a peak at 532.0 eV with asymmetric behavior,
which could be deconvoluted into three types of oxygen groups. The peaks at around 529.7 eV and
531.1 eV (denoted as OI and OII in the figure) were respectively attributed to the oxygen ions in the
fully oxidized surrounding (i.e., Zn-O bonding) and in oxygen-deficient regions (i.e., oxygen vacancy),
whereas the peak at about 532.2 eV (denoted as OIII ) was related to the hydroxyl (OH) group or the
loosely bound oxygen on the surface [21,34,37,38]. Compared to the undoped ZnO film, the O 1s peak
measured from the surface of the ZnO film co-sputtered at the Ag atomic ratio of 3% shifted to about
531.5 eV with a significant satellite peak at 529.7 eV and a tail extending to low binding energy. This
curve could be deconvoluted into the above-mentioned three oxidized states with an additional weak
peak at 528.9 eV (denoted as OIV ). As indicated in the previous reports [39–41], this oxidized state
observed only in the Ag-ZnO co-sputtered film was the contribution of the atomic oxygen with an
ionic Ag-O bond, which implied the activated Ag dopants (AgZn ) as a substitution for the lattice Zn in
the ZnO matrix. Additionally, in agreement with [21], the incorporation of the Ag atoms in the ZnO
film also led to the reduction in the oxygen vacancy-related defects as evidence of the decrease in the
ratio of the peak area (OII /(OI + OII + OIV )). Although the formation of the Ag-O chemical bond and
the suppression on the native oxygen vacancy donors confirmed from the investigation of the O 1s core
level were both helpful for realizing a p-type Ag-ZnO co-sputtered film, an n-type Ag-ZnO still was
measured when the theoretical Ag dopants reached 5%. The core level of the Ag 3d5/2 for the Ag-ZnO
co-sputtered films at the atomic ratios of 3% and 5%, shown in Figure 8a,b, respectively, are given
to elucidate the conversion of the conduction type. The peak of the Ag 3d5/2 shifted from 367.5 eV
for the Ag-ZnO (3%) co-sputtered film to 368.1 eV for the Ag-ZnO (5%) co-sputtered film. Such an
asymmetric peak could be deconvoluted into two peaks at about 367.4 and 368.2 eV, which were, in
turn, ascribed to the bonds associated with the metallic and ionic Ag (denoted as Ag0 and Ag-O in the
figure), respectively [21,40,42,43]. Clearly, the Al-ZnO (3%) co-sputtered film mainly contained the
Ag-O chemical bond, whereas the metallic Ag-Ag bond dominated over the Ag-ZnO (5%) co-sputtered
film. Combined with the electrical property, the achievement of the p-type conduction for the Ag-ZnO
(3%) co-sputtered film was attributed to the efficient activation of the Ag acceptors (AgZn ) as evidence
of most of the Ag dopants forming the Ag–O chemical bonds. However, as the theoretical Ag doping
level reached 5%, the overwhelming metallic Ag bond that was closely linked to the aggregation of the
Ag dopants led the film to perform n-type degenerated conduction. These Ag aggregations would
also constrict the growth of the ZnO matrix, thereby resulting in the decrease of the c-axis lattice
constant and a significant degradation of the crystalline structure, as shown in Figure 3. In addition,
the Ag-ZnO co-sputtered films at theoretical atomic ratios of 3% and 5 at.% were about 0.3 and 1.1 at.%,
respectively, as determined by the XPS measurements. The significant discrepancy between the actual
and theoretical values in the Ag-ZnO co-sputtered films could be attributed to the poison of the Ag
target during the co-sputtering deposition.
Materials 2017, 10, 797 9 of 13
Materials 2017, 10, 797 9 of 12

Materials 2017, 10, 797 9 of 12

Figure 6. XPS survey spectra taken on the surface of the (a) undoped ZnO film and (b) Ag-ZnO
Figure 6. XPS survey spectra taken on the surface of the (a) undoped ZnO film and (b) Ag-ZnO
co-sputtered film survey
Figure 6. XPS at a theoretical atomic
spectra taken onratio
the of 3% after
surface annealing
of the at 350ZnO
(a) undoped °C for 1 hand
film under ambient air.
(b) Ag-ZnO
co-sputtered film at a theoretical atomic ratio of 3% after annealing at 350 ◦ C for 1 h under ambient air.
co-sputtered film at a theoretical atomic ratio of 3% after annealing at 350 °C for 1 h under ambient air.

Figure 7. Core level of the O 1s spectra for the (a) undoped ZnO film and (b) Ag-ZnO co-sputtered
Figure 7. Core level of the O 1s spectra for the (a) undoped ZnO film and (b) Ag-ZnO co-sputtered
film 7.
Figure at Core
a theoretical
level ofatomic
the Oratio of 3% after
1s spectra for annealing at 350 °CZnO
the (a) undoped for 1film
h under
andambient air. co-sputtered
(b) Ag-ZnO
film at a theoretical atomic ratio of 3% after annealing at 350 ◦ C for 1 h under ambient air.
film at a theoretical atomic ratio of 3% after annealing at 350 °C for 1 h under ambient air.
Materials 2017, 10, 797 10 of 13
Materials 2017, 10, 797 10 of 12

Figure 8. Core level of the Ag 3d5/2 spectra for the Ag-ZnO co-sputtered films at the theoretical
Figure 8. Core level of the Ag 3d5/2 spectra for the Ag-ZnO co-sputtered films at the theoretical atomic
atomic ratios of (a) 3% and (b) 5%, respectively, after annealing at 350 °C for 1 h under ambient air.
ratios of (a) 3% and (b) 5%, respectively, after annealing at 350 ◦ C for 1 h under ambient air.
4. Conclusions
4. Conclusions
Various Ag atoms doped into the ZnO films were prepared by an RF magnetron co-sputtering
Varioususing
system, Ag atoms
Ag and doped
ZnO into the The
targets. ZnOconduction
films weretype prepared
of theby an RF magnetron
Ag-ZnO co-sputteredco-sputtering
film was
system, usingbyAg
controlled and ZnO
altering the Agtargets.
dopantsThe conduction
in the ZnO film with typeanofadditional
the Ag-ZnO co-sputtered
post-annealed film was
treatment
at 350 °C
controlled byfor 1 h under
altering the air
Agambient.
dopantsFor the Ag-ZnO
in the ZnO film co-sputtered films at atomic
with an additional ratios of 1%
post-annealed and
treatment
3%,◦ C
at 350 p-type
for 1conduction
h under air was linked toFor
ambient. the the
formation
Ag-ZnO of the Ag–O chemical
co-sputtered filmsbond originating
at atomic ratios from
of 1%the and
activation of Ag acceptors substituted for the Zn 2+ lattice sites (AgZn) through the XPD, Raman
3%, p-type conduction was linked to the formation of the Ag–O chemical bond originating from
scattering, and
the activation of AgXPS investigations.
acceptors However,
substituted for as
thethe
ZnAg2+ atoms
latticeintroduced
sites (AgZninto the ZnOthe
) through film reached
XPD, Raman
a theoretical atomic ratio of 5%, the conduction type converted into the generated n-type
scattering, and XPS investigations. However, as the Ag atoms introduced into the ZnO film reached a
conduction. The mechanism responsible for the conduction conversion was the large amounts of
theoretical atomic ratio of 5%, the conduction type converted into the generated n-type conduction.
the metallic Ag bond (Ag0) appearing on the ZnO matrix, which was correlated to the formation of
The the
mechanism responsible for the conduction conversion was the large amounts of the metallic Ag
Ag aggregations due to the excess incorporation of the Ag atoms. The control maintained over
bond (Ag 0 ) appearing
the conduction typeonofthe
theZnO matrix,
Ag-ZnO which
film was correlated
prepared to the formation
using RF magnetron of the Ag
co-sputtering aggregations
technology
duewas
to the
very promising for realizing a ZnO-based homojunction optoelectronic device. In addition, thetype
excess incorporation of the Ag atoms. The control maintained over the conduction
of the Ag-ZnO film
aggregation of theprepared
doping using
Ag inRF themagnetron
ZnO matrix co-sputtering technology was
might be advantageous for very promising
preventing the for
realizing a ZnO-based
recombination of thehomojunction
photogenerated optoelectronic device.
electron-hole pairs In addition, the
in photocatalytic aggregation of the doping
applications.
Ag in the ZnO matrix might be advantageous for preventing the recombination of the photogenerated
Acknowledgments: This work was supported by the National Science Council and Industrial Technology
electron-hole pairs in photocatalytic applications.
Research Institute (ITRI South) under no. A200-105BA2 and the Ministry of Science and Technology under
105-2622-E-150-004-CC2.
Acknowledgments: This work was supported by the National Science Council and Industrial Technology
Research
AuthorInstitute (ITRI Day-Shan
Contributions: South) under no. A200-105BA2
Liu organized andexperiment
and designed the the Ministry of Science
procedures; andChen
Tai-Hong Technology
and
under 105-2622-E-150-004-CC2.
Chun-Hao Chang wrote the paper; Fang-Cheng Liu and Jyun-Yong Li executed the film deposition;
Wei-Hua
Author Hsiao andDay-Shan
Contributions: Ching-Ting Lee
Liu performed
organized and and supported
designed the the thin filmprocedures;
experiment measurements and analysis.
Tai-Hong Chen and
Chun-Hao Chang
All authors wrote
read and the paper;the
approved Fang-Cheng Liu
final version ofand Jyun-Yong Li
the manuscript to executed the film deposition; Wei-Hua Hsiao
be submitted.
and Ching-Ting Lee performed and supported the thin film measurements and analysis. All authors read and
Conflicts
approved the of Interest:
final versionTheofauthors declare noto
the manuscript conflict of interest.
be submitted.
Conflicts of Interest: The authors declare no conflict of interest.
Materials 2017, 10, 797 11 of 13

References
1. Steglich, M.; Bingel, A.; Jia, G.; Falk, F. Atomic layer deposited ZnO: Al for nanostructured silicon
heterojunction solar cells. Sol. Energy Mater. Sol. Cells 2012, 103, 62–68. [CrossRef]
2. Lu, T.C.; Lai, Y.Y.; Lan, Y.P.; Huang, S.W.; Chen, J.R.; Wu, Y.C.; Hsieh, W.F.; Deng, H. Room temperature
polariton lasing vs. photon lasing in a ZnO-based hybrid microcavity. Opt. Express 2012, 20, 5530–5537.
[CrossRef] [PubMed]
3. Wang, L.; Kang, Y.; Liu, X.; Zhang, S.; Huang, W.; Wang, S. ZnO nanorod gas sensor for ethanol detection.
Sens. Actuator B Chem. 2012, 162, 237–243. [CrossRef]
4. Ho, C.C.; Lai, L.W.; Lee, C.T.; Yang, K.C.; Lai, B.T.; Liu, D.S. Transparent cosputtered ITO-ZnO electrode
ohmic contact to n-type ZnO for ZnO/GaN heterojunction light-emitting diode. J. Phys. D Appl. Phys. 2013,
46, 315102. [CrossRef]
5. Ibupoto, Z.H.; Khun, K.; Eriksson, M.; AlSalhi, M.; Atif, M.; Ansari, A.; Willander, M. Hydrothermal growth
of vertically aligned ZnO nanorods using a biocomposite seed layer of ZnO nanoparticles. Materials 2015, 6,
3584–3597. [CrossRef]
6. Jaramllo-Paez, C.; Navio, J.A.; Hidalgo, M.C.; Macias, M. High UV-photocatalytic activity of ZnO and
Ag/ZnO synthesized by a facile method. Catal. Today 2017, 284, 121–128. [CrossRef]
7. Zhang, X.J.; Chen, Y.; Zhang, S.; Qiu, C.Y. High photocatalytic perfromance of hig concentration Al-doped
ZnO nanoparticles. Sep. Purif. Technol. 2017, 172, 236–241. [CrossRef]
8. Bhatic, S.; Verma, N.; Bedi, R.K. Sn-doped ZnO nanopetal networks for efficient photocatalytic degradation
of dye and gas sensing applications. Appl. Surf. Sci. 2017, 407, 495–502.
9. Saaedi, A.; Yousefi, R.; Jamali-Sheini, F.; Cheraghizade, M.; Zak, A.K.; Huang, N.M. Optical and electrical
properties of p-type Li-doped ZnO nanowires. Superlattices Microstruct. 2013, 61, 91–96. [CrossRef]
10. Ma, Y.; Gao, Q.; Wu, G.G.; Li, W.C.; Gao, F.B.; Yin, J.Z.; Zhang, B.L.; Du, G.T. Growth and conduction
mechanism of As-doped p-type ZnO thin films deposited by MOCVD. Mater. Res. Bull. 2013, 48, 1239–1243.
[CrossRef]
11. Ren, X.L.; Zhang, X.H.; Liu, N.S.; Wen, L.; Ding, L.W.; Ma, Z.W.; Su, J.; Li, L.Y.; Han, J.B.; Gao, Y.H.
White light-emitting diode from Sb-doped p-ZnO nanowire arrays/n-GaN film. Adv. Funct. Mater. 2015, 25,
2182–2188. [CrossRef]
12. Yan, Y.; Al-Jassim, M.M.; Wei, S. Doping of ZnO by group-IB elements. Appl. Phys. Lett. 2006, 89, 181912.
[CrossRef]
13. Suja, M.; Bashar, S.B.; Morshed, M.M.; Liu, J.L. Realization of Cu-doped p-type ZnO thin films by molecular
beam epitaxy. ACS Appl. Mater. Interfaces 2015, 7, 8894–8899. [CrossRef] [PubMed]
14. Myers, M.A.; Khranovskyy, V.; Jian, J.; Lee, J.H.; Wang, H.; Wang, H. Photoluminescence study of p-type vs.
n-type Ag-doped ZnO films. J. Appl. Phys. 2015, 118, 065702. [CrossRef]
15. Liu, W.Z.; Xu, H.Y.; Wang, C.L.; Zhang, L.X.; Zhang, C.; Sun, S.Y.; Ma, J.G.; Zhang, X.T.; Wang, J.N.;
Liu, Y.C. Enhanced ultraviolet emission and improved spatial distribution uniformity of ZnO nanorod array
light-emitting diodes via Ag nanoparticles decoration. Nanoscale 2013, 5, 8634–8639. [CrossRef] [PubMed]
16. Echresh, A.; Chey, C.O.; Shoushtari, M.Z.; Nur, O.; Willander, M. Tuning the emission of ZnO nanorods
based light emitting diodes using Ag doping. J. Appl. Phys. 2014, 116, 193104. [CrossRef]
17. Zhang, K.; Wang, H.; Gan, Z.K.; Zhou, P.Q.; Mei, C.L.; Huang, X.; Xia, Y.X. Localized surface plasmon
resonances dominated giant lateral photovoltaic effect observed in ZnO/Ag/Si nanostructure. Sci. Rep.
2016, 6, 22906. [CrossRef] [PubMed]
18. Khan, R.; Yun, J.H.; Bae, K.B.; Lee, I.H. Enhanced photoluminescence of ZnO nanorods via coupling with
localized surface plasmon of Au nanoparticles. J. Alloys Compd. 2016, 682, 643–646. [CrossRef]
19. Tarwal, N.L.; Patil, P.S. Enhanced photoelectrochemical performance of Ag-ZnO thin films synthesized by
spray pyrolysis technique. Electrochim. Acta 2011, 56, 6510–6516. [CrossRef]
20. Balachandran, S.; Selvam, K.; Babu, B.; Swaminathan, M. The simple hydrothermal synthesis of
Ag-ZnO-SnO2 nanochain and its multiple applications. Dalton Trans. 2013, 42, 16365–16374. [CrossRef]
[PubMed]
Materials 2017, 10, 797 12 of 13

21. Cao, L.; Zhu, L.; Ye, Z. Enhancement of p-type conduction in Ag-doped ZnO thin films via Mg alloying:
The role of oxygen vacancy. J. Phys. Chem. Solids 2013, 74, 668–672. [CrossRef]
22. Duan, L.; Yu, X.C.; Ni, L.; Wang, Z. ZnO: Ag film growth on Si substrate with ZnO buffer layer by rf
sputtering. Appl. Surf. Sci. 2011, 257, 3463–3467. [CrossRef]
23. Sytchkova, A.; Grilli, M.L.; Rinaldi, A.; Vedraine, S.; Torchio, P.; Piegari, A.; Flory, F. Radio frequency
sputtered Al:ZnO-Ag transparent conductor: A plasmonic nanostructure with enhanced optical and electrical
properties. J. Appl. Phys. 2013, 114, 094509. [CrossRef]
24. Liu, D.S.; Wu, C.C.; Lee, C.T. A transparent and conductive oxide film prepared by RF magnetron
co-sputtering system at room temperature. Jpn. J. Appl. Phys. 2005, 44, 5119–5121. [CrossRef]
25. Liu, D.S.; Sheu, C.S.; Lee, C.T. Aluminum-nitride codoped zinc oxide films prepared by radio-frequency
magnetron cosputtering system. J. Appl. Phys. 2007, 102, 033516. [CrossRef]
26. Liu, D.S.; Tsai, F.C.; Lee, C.T.; Sheu, C.W. Properties of zinc oxide films cosputtered with various aluminum
content at room temperature. Jpn. J. Appl. Phys. 2008, 47, 3056–3062. [CrossRef]
27. Zhang, Y.; Jia, H.B.; Wang, R.M.; Chen, C.P.; Luo, X.H.; Yu, D.P. Low-temperature growth and Raman
scattering study of vertically aligned ZnO nanowires on Si substrate. Appl. Phys. Lett. 2003, 83, 4631–4633.
[CrossRef]
28. Wang, J.B.; Zhong, H.M.; Li, Z.F.; Lu, W. Raman study of N+ –implanted ZnO. Appl. Phys. Lett. 2006, 88,
101913. [CrossRef]
29. Kerr, L.L.; Li, X.; Canepa, M.; Sommer, A.J. Raman analysis of nitrogen doped ZnO. Thin Solid Films 2007,
515, 5282–5286. [CrossRef]
30. Gao, D.; Zhang, Z.; Fu, J.; Xu, Y.; Qi, J.; Xue, D. Room temperature ferromagnetism of pure ZnO nanoparticles.
J. Appl. Phys. 2009, 105, 113928. [CrossRef]
31. Xue, X.; Wang, T.; Jiang, X.; Jiang, J.; Pan, C.; Wu, Y. Interaction of hydrogen with defects in ZnO
nanoparticles—Study by positron annihilation, Raman and photoluminescence spectroscopy. CrystEngComm
2017, 16, 1207–1216. [CrossRef]
32. Satyarthi, P.; Ghosh, S.; Mishra, P.; Sekhar, B.R.; Singh, F.; Kumar, P.; Kanjilal, D.; Dhaka, R.S.; Srivastava, P.
Defects controlled ferromagnetism in xenon ion irradiated zinc oxide. J. Magn. Magn. Mater. 2015, 385,
318–325. [CrossRef]
33. Silambarasan, M.; Saravanan, S.; Soga, T. Raman and photoluminescence studies of Ag and Fe-doped ZnO
nanoparticles. Int. J. Chem. Technol. Res. 2015, 7, 1644–1650.
34. Kumar, A.; Kumar, P.; Kumar, K.; Singh, T.; Singh, R.; Asokan, K.; Kanjilal, D. Role of growth temperature
on the structural, optical and electrical properties of ZnO thin films. J. Alloys Compd. 2015, 649, 1205–1209.
[CrossRef]
35. Wang, X.B.; Song, C.; Geng, K.W.; Zeng, F.; Pan, F. Luminescence and Raman scattering properties of
Ag-doped ZnO films. J. Phys. D Appl. Phys. 2006, 39, 4992–4996. [CrossRef]
36. Li, W.J.; Kong, C.Y.; Ruan, H.B.; Qin, G.P.; Huang, G.J.; Yang, T.Y.; Liang, W.W.; Zhao, Y.H.; Meng, X.D.;
Yu, P.; et al. Electrical properties and Raman scattering investigation of Ag doped ZnO thin films.
Solid State Commun. 2012, 152, 147–150. [CrossRef]
37. Chen, M.; Wang, X.; Yu, Y.H.; Pei, Z.L.; Bai, X.D.; Sun, C.; Huang, R.F.; Wen, L.S. X-ray photoelectron
spectroscopy and auger electron spectroscopy studies of Al-doped ZnO films. Appl. Surf. Sci. 2000, 158,
134–140. [CrossRef]
38. Ogata, K.; Komuro, T.; Hama, K.; Koike, K. Control of chemical bonding of the ZnO surface grown by
molecular beam epitaxy. Appl. Surf. Sci. 2004, 237, 348–351. [CrossRef]
39. Bukhtiyarov, V.I.; Carley, A.F.; Dollard, L.A.; Roberts, M.W. XPS study of oxygen adsorption on supported
silver: Effect of particle size. Surf. Sci. 1997, 381, L605–L608. [CrossRef]
40. Boronin, A.I.; Koscheev, S.V.; Zhidomirov, G.M. XPS and UPS study of oxygen states on silver. J. Electron.
Spectrosc. Relat. Phenom. 1998, 96, 43–51. [CrossRef]
41. Kaur, A.; Ibhadon, A.O.; Kansal, S.K. Photocatalytic degradation of ketorolac tromethamine (KTC) using
Ag-doped ZnO microplates. J. Mater. Sci. 2017, 52, 5256–5267. [CrossRef]
Materials 2017, 10, 797 13 of 13

42. Sampaio, M.J.; Lima, M.J.; Baptista, D.L.; Silva, A.M.T.; Silva, C.G. Ag-loaded ZnO materials for
photocatalytic water treatment. Chem. Eng. J. 2017, 318, 95–102. [CrossRef]
43. Singh, S.K.; Singhal, R.; Kumar, V.V.S. Study on swift heavy ions induced modification of Ag-ZnO
nanocomposite thin film. Superlattices Microstruct. 2017, 103, 195–204. [CrossRef]

© 2017 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access
article distributed under the terms and conditions of the Creative Commons Attribution
(CC BY) license (http://creativecommons.org/licenses/by/4.0/).