Aluminum doped core-shell ZnO/ZnS nanowires: Doping and shell layer
induced modification on structural and photoluminescence properties
Soumen Dhara, Kenji Imakita, P. K. Giri, Minoru Mizuhata, and Minoru Fujii
Citation: J. Appl. Phys. 114, 134307 (2013); doi: 10.1063/1.4824288
View online: http://dx.doi.org/10.1063/1.4824288
View Table of Contents: http://jap.aip.org/resource/1/JAPIAU/v114/i13
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JOURNAL OF APPLIED PHYSICS 114, 134307 (2013)

Aluminum doped core-shell ZnO/ZnS nanowires: Doping and shell layer
induced modification on structural and photoluminescence properties
Soumen Dhara,1,a) Kenji Imakita,1 P. K. Giri,1,2 Minoru Mizuhata,3 and Minoru Fujii1,a)1

Department of Electrical and Electronic Engineering, Faculty of Engineering, Kobe University,
1-1 Rokkodai, Nada, Kobe 657-8501, Japan2
Department of Physics, Indian Institute of Technology Guwahati, Guwahati 781 039, India
3
Department of Chemical Science and Engineering, Faculty of Engineering, Kobe University,
1-1 Rokkodai, Nada, Kobe 657-8501, Japan(Received 20 August 2013; accepted 18 September 2013; published online 2 October 2013)
In this work, we investigated the combined effects of Al doping and surface modification on the
fabrication of a core–shell type ZnO/ZnS nanowires (NWs) and its structural, electrical, andphotoluminescence (PL) properties. A systematic investigation for different concentrations of Al
doping followed by surface modification with different thicknesses of ZnS layer was performed.
Significant changes in the nature of PL spectra and electronic conductivity are observed and insight
discussions are present. Structural characterization on the core-shell NWs reveals the successful
fabrication of Al doped highly single crystalline ZnO core and polycrystalline ZnS shell with bothZnO and ZnS are of hexagonal wurtzite structure. Compared with the bare undoped ZnO NWs, Al
doped core-shell ZnO/ZnS NWs exhibit two orders of magnitude improvement in the electronic
conductivity and fivefold enhancement in the UV PL intensity. The Al doped core-shell ZnO/ZnS
NWs shows an efficient improvement in the UV PL intensity than the undoped core-shell ZnO
NWs. The obtained improvement in the PL result is explained on the basis of interfacial transfer of
C 2013 AIP Publishing LLC.
photogenerated charge carriers and modification of defects. V[http://dx.doi.org/10.1063/1.4824288]
I. INTRODUCTION

ZnO, a versatile material with several important properties, e.g., strong room temperature UV photoluminescence(PL), high transparency in the visible region, piezoelectric,
ferromagnetic, nonlinear optical properties, etc., has
attracted the attention of the researchers worldwide. These
interesting features of ZnO enables to fabricate various nanodevices ranging from field effect transistors to biosensors
using various types of ZnO nanostructures.1–4 ZnO nanowires (NWs) are extensively studied for the ease of the devices fabrication process and have the ability to be integrated
into the mainstream Si based electronics for future largescale optoelectronic device integration.5,6 However, successful commercial application of ZnO NWs based devices
requires detail understanding and control over the growth of
the ZnO NWs, structural quality, and specific properties
related to the application purpose. Depending upon thegrowth method, the as-grown ZnO NWs invariably containseveral surface defects which have a detrimental effect on
the band edge UV emission and related applications.7 Hence,
obtaining highly efficient UV emission from the ZnO nanostructures is one of most important issues for optoelectronic
applications of ZnO. Furthermore, the electronic conductivity of the as-grown ZnO NWs is usually low for device application. The electrical, optical, and magnetic properties of
ZnO can be adjusted desirably not only by impurity doping
but also through surface modification.8–13 The modificationa)Authors to whom correspondence should be addressed. Electronic
addresses: fujii@eedept.kobe-u.ac.jp and soumen5484@yahoo.co.in

0021-8979/2013/114(13)/134307/8/$30.00

of the surface is considered as one of the advanced techniques to change the physical properties of ZnO for a betternanoscale application.14–16 In this context, researchers fabricated various types of hybrid or heterostructures with somesuitable external material. Here, the external material significantly influences the specific properties of the core material.
In our previous reports, we demonstrated the use of anthracene and copper pthalocyanine (CuPc) for the fabrication of
ZnO/organic NWs heterostructure.17,18 We observed a notable improvement in the UV PL and photoresponse properties
and it was explained on the basis of photo-induced interfacial
charge transfer process. It is also noticed that the photosensitivity of the external material and the structural quality of theinterface strongly control the properties of the heterostructures. Therefore, it is very important to devote an in-detailstudy about the structure and other properties of a NW heterostructure with a new external material for better control andunderstanding. There are few reports on the fabrication of
ZnO NW heterostructure with ZnS as a shell layer. Panda
et al.19 prepared ZnO-ZnS core-shell nanorods by partial
conversion of the ZnO nanorod surface to ZnS at 400  C
under H2S and argon gas mixture and studied the luminescent properties. Liu et al.20 reported the enhancement of photocurrent conversion efficiency by using ZnO/ZnS nanorods
as the photoanode for a photovoltaic device. Li et al.21
showed the enhancement of UV emission from ZnS-coatedZnO nanowires fabricated by self-assembling method using
sodium sulfide and zinc nitrate. Efficient conversion of mechanical energy (by deflection of the NWs) to electricalenergy (voltage generation) is also demonstrated by using
ZnO-ZnS axial heterojunction NW arrays which were

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synthesized by thermal evaporation process.22 However, thecombined effect of doping and a shell layer on the resulting
optoelectronic properties of ZnO NWs has not been studied
systematically.
In this work, we employ Al doping in ZnO NWs followed by surface modification with a ZnS shell layer toinvestigate the doping and shell layer induced modificationon the ZnO NWs. The as-prepared Al doped ZnO NWs havesufficiently higher electronic conductivity. However, the UVemission is not sufficiently strong for optoelectronic applications in large scale commercial purpose. To improve further,ZnS was chosen as a shell layer due to the higher bandgap
energy and semiconducting nature of the ZnS, it is expectedthat outer shell layer will influenced to get an improvement
in UV PL. A detailed investigations on structural and PL
properties were performed by preparing a set of samples
with different concentrations of Al doping and with various
thicknesses of outer ZnS shell layer.
II. EXPERIMENTAL

ZnO NWs were grown by an aqueous chemical growthmethod as described elsewhere.23 In brief, uniform distribution of ZnO nanocrystal seeds were prepared on a Si substrate by spin coating followed by thermal decomposition ofa zinc acetate precursor. Coating process was repeated several times for 30 s each to get sufficient thickness of the zincacetate layer. Then the coated substrate was heated at
350  C for 30 min to get ZnO seed layer. The verticallyaligned ZnO NWs were synthesized by hydrolysis of zinc nitrate (Wako Chem.) in water in presence of hexamethylenetetramine (HMT, Wako Chem.) on the above prepared
substrate. For Al doping, aluminum nitrate was used as precursor solution which was mixed properly in the reaction solvent according to the stoichiometric ratio. An equimolar(25 mM) concentration of zinc nitrate, aluminum nitrate, and
HMT in DI-water and polyethyleneimine (9 mM) weremixed properly in a Teflon made reaction flask. The above
prepared substrate was dipped in to the reaction solvent mixture and placed vertically. Then the reaction flask was sealed
inside an autoclave. The chemical reaction was carried out at
95  C for 24 h in the autoclave. The substrate was thenremoved from the solution, rinsed in DI-water and dried in
hot air at 100  C for 30 min. In this process, 0.5, 1.0, 2.0,
and 5.0 at. % of Al doped ZnO (Al:ZnO) NWs were prepared. For comparison, undoped ZnO NWs were preparedby the same process without using doping precursor solution.
For the ZnS shell preparation, the previously preparedAl(2 at. %):ZnO NWs were dipped into 10 mM aqueous solution of thioacetamide (TAA, Wako Chem.) at 60  C for 1,
2, and 4 h and then dried. In this process, we prepared coreshell type ZnO/ZnS NWs with varying thickness of ZnSshell layer. Samples are marked as CS1, CS2, and CS4 forthe sulfidization time duration of 1, 2, and 4 h, respectively.
For comparative studies, above mentioned sulfidization process was repeated for undoped ZnO NWs for 2 h. Field emission scanning electron microscopy (FESEM, JSM6335F,
JEOL) was used to confirm the formation of ZnO NWs. The
crystal structure (structural analysis) of the above prepared

J. Appl. Phys. 114, 134307 (2013)

NWs was characterized using x-ray diffraction (XRD,
Ultima IV, Rigaku) and transmission electron microscopy(TEM, JEM2010, JEOL) operated at 200 keV with selectedarea electron diffraction (SAED). To trace the elemental
composition in the as grown NWs, energy dispersive x-ray
spectroscopy (EDX) and x-ray photoelectron spectroscopy(XPS) were employed. XPS measurement was carried out
with a fully automated XPS microprobe (Phi-vtool, UlvacPhi) using Al Ka x-ray beam (1486.7 eV) with beam currentof 5 mA. Carbon 1s spectrum was used for the calibration of
the XPS spectra recorded for various samples. The room
temperature PL spectra of all the samples were recordedwith a 325 nm He–Cd laser excitation using a commercial
PL spectrometer (Flurolog-3, Horiba) equipped with a PMT(photo multiplier tube) detector. For comparative analysis,
PL measurements on all the samples were made under identical experimental conditions.
III. RESULTS AND DISCUSSION
A. Structural studies

Figure 1(a) shows the FESEM morphology image of theundoped ZnO NWs grown by an aqueous chemical methodwith the help of ZnO seed layer. As seen in the image, highdensity ZnO NWs grow vertically on the Si substrate over alarge surface area with NWs density of 3.2  1013 cm2.
Figures 1(b) and 1(c) show the tilted view of the undoped
and Al(1 at. %):ZnO NWs. The diameters of the NWs arevery small, which ranges from 35 to 60 nm for the undoped
one and from 40 to 60 nm for the Al(1 at. %):ZnO NWs andthe lengths are about a few microns. The Al:ZnO NWs withdifferent concentrations of Al show similar morphology withvarying diameter. From the morphology studies, it is seenthat degree of alignment of the NWs goes down with theincrease in Al doping concentration. During the chemical
growth process, ZnO NWs preferentially nucleate from thetip near the grain boundaries between two adjacent grains in

FIG. 1. (a) FESEM image (top view) of the chemically grown undoped ZnO
NWs. (b) and (c) Magnified tilted view image of the above ZnO NWs and
Al doped Al(1 at. %):ZnO NWs, respectively. (d) Diameter variation of thechemically grown undoped and Al doped ZnO NWs as a function of Al
concentration.

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the ZnO seed film.24 The degree of alignment in theas-grown ZnO NWs is strongly controlled by the orientationof the ZnO seed layer. During growth process, the doping of
Al atoms inside the ZnO lattice may cause lattice distortion,
which affects the initial growth orientation. Figure 1(d)shows the diameter variation of the Al:ZnO NWs as a function of Al doping concentration. The Al doping slowly
increases the diameter of the NWs and it is saturated at
higher concentration of Al (more than 5 at. %, data notshown). Whereas the Al doping has no significant influenceon the length of the NWs, as expected. It is known that thediameter of the as-grown NWs usually depends on the grain
size of the ZnO seeds and the reaction process, while thelength of the NW primarily controls by the reaction time
duration.
Figure 2(a) shows FESEM image of the bare Al(2at. %):ZnO NWs and Fig. 2(b) shows the core-shell typeAl(2 at. %):ZnO NWs with ZnS shell. The Al doping in the
as-prepared NWs was confirmed from the corresponding
EDX spectrum. The EDX spectrum of the Al(2 at. %):ZnO
NWs in Fig. 2(c) shows the presence of 1.46 at. % of Al
atoms. From the EDX elemental analysis, it is found that the
Al doping concentration is lower than those in the mixed
reaction solutions. This inadequacy increases with theincrease in Al concentration in the mixed reaction solution.
In the case of chemical doping at higher dopant concentration, the Al atoms might be nucleated with hydroxyl ionspresent in the growth solution due to the kinetic equilibrium.
Thus, it would be difficult for all of the Al3þ ions to be incorporated into ZnO lattice at higher concentration, similar tothe case of Mn doping.25 In the case of Al doping, majorityof the Al atoms occupy the ZnO lattice by substitution of Zn
atoms. However, there is also a possibility of formation ofinterstitial Al atoms in the ZnO lattice due to its smaller size
than the Zn atoms. EDX analysis further shows that the oxygen content is slightly higher than Zn content, although oxygen vacancies are usually present on the NWs surface. The

FIG. 2. (Tilted view) FESEM images of the chemically grown ZnO NWs:(a) Al(2 at. %):ZnO and (b) sample CS4. (c) EDX spectrum of the above
Al(2 at. %):ZnO NWs. Inset shows the FTIR reflectance data of the undoped
ZnO NWs. (d) EDX line scan across the diameter of a single core-shell NWof sample CS4.

J. Appl. Phys. 114, 134307 (2013)

extra oxygen in the EDX spectrum may arise from thehydroxyl radicals (-OH groups) present on the surface of theZnO NWs. The FTIR data of the undoped ZnO NWs shownas inset in the Fig. 2(c) confirm the existence of sufficientamount of hydroxyl radicals on the surface of the NWs. We
also observed stretching and bending vibration mode of
Zn-O bonds in the FTIR spectrum. Presence of hydroxyl radicals (OH groups) is further, confirms, from the XPS andPL spectroscopic data (explained later).
After the sulfidization process, a clear difference in thesurface morphology of the bare Al(2 at. %):ZnO and sample
CS4 NWs can be visualized from the images as the sidewallsof the former are smooth, whereas those of the latter are
rough and wrinkled indicating some modification has been
occurred on the surface due to ZnS layer formation. The CS1
and CS2 show similar surface morphology with relatively
smoother sidewall than the case of CS4. There is no significant change in the diameter of the core-shell NWs indicatingthat only the surface of the ZnO layer is converted to ZnS.
This is because the conversion of ZnO to ZnS is essentially asubstitution reaction,26 and only growth of the ZnS shelltakes place. To confirm the formation of outer ZnS shelllayer further, EDX elemental line scan technique isemployed across the diameter of a single core-shell NW,
which is shown in Fig. 2(d). This profile illustrated the presence of higher O concentration in the central region andhigher S concentrations at both edge regions of the NW. It
confirms the successful formation of interface between the
Al:ZnO core and ZnS shell. The estimated core diameter is41 nm and shell thickness is 14 nm for the sample CS4.
The structural analysis of the undoped, Al doped, anddoped core-shell ZnO NWs is performed from the TEM and
XRD results. Figure 3 shows the typical TEM images of theundoped, Al doped ZnO, and doped core-shell ZnO/ZnS
NWs. Diameters and lengths of the NWs measured from the
TEM images are consistent with the sizes obtained from thecorresponding FESEM images. Figure 3(a) shows the lowresolution TEM image of the undoped ZnO NWs and thecorresponding SAED pattern. The SAED pattern indicatesthe highly crystalline hexagonal structure of the undoped
ZnO NWs with zone axis along (001). Figure 3(b) shows thehigh-resolution lattice image of a single NW which further
confirms the highly crystalline structure with growth direction along h001i (c-axis). The lattice spacing is calculated tobe 2.6 Å corresponding to the (002) plane of hexagonal structure. Figure 3(c) shows the TEM image of the Al(2at. %):ZnO NW and its corresponding SAED pattern and lattice image. The doped NWs also possess high crystallinity,
as seen from the Fig. 3(c). The successful formation of coreshell structure is further confirmed from the TEM image ofthe sample CS4 as shown in Fig. 3(d). The contrast difference in the center and at the edges of the NW confirms thecore-shell structure with core diameter 33 nm. The latticespacing calculated in the centre and at the edge corresponds
to the (002) plane of ZnO and (100) plane of ZnS with bothhave hexagonal structure. It is noticed that the shell layer is
not continuously smooth rather it is a formation of polycrystalline grains of ZnS, as seen from the inset image. The
measured shell thickness is 19 nm which is larger than the134307-4

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J. Appl. Phys. 114, 134307 (2013)

FIG. 3. (a) A typical low-resolution TEM image of the undoped ZnO NWs.
(b) High resolution lattice image of the above ZnO NW. Inset in (a) shows
the corresponding SAED pattern. (c) and (d) TEM images of the Al dopedAl(2 at. %):ZnO and Al(2 at. %):ZnO/ZnS NW (sample CS4), respectively.
Inset images in (c) show the corresponding SAED pattern and highresolution lattice image of the NW. Inset images in (d) show the highresolution lattice image of the corresponding selected area in the abovecore-shell NW. Hollow arrow indicates the growth direction of the NW.

size obtained from the EDX line scan data. The smaller
estimation in the EDX data might be due to the very roughsidewall of the ZnO/ZnS NWs. Further structural characterization by XRD measurements shows characteristic peaks of
crystalline hexagonal phase of ZnO in the XRD patterns.
Comparatively, stronger intensity of the (002) peak (Fig.
4(a)) indicates that the ZnO NWs are aligned along the normal to the substrate plane. No other peaks related to the Al
or aluminum oxide are detected. The relative intensity of thestrong (002) peak goes down with the increase in Al doping
concentration. As compared with the undoped case, the intensity decreases by six fold after 5 at. % of Al doping. This
indicates a deterioration of the structural quality with theincreasing in Al doping concentration, although the Al(5at. %):ZnO NWs still have good crystalline nature. Exact
position of the (002) peak is calculated from the Lorentzian
fitting to the experimental data. It is found that the peak position is slightly shifted towards higher 2h. This up shift indicates lower lattice spacing and it implies that the Al:ZnO
NWs are weakly strained compressively. It is also noticed
that the compressive strain increases with increase in Al concentration, as expected. The calculated strain value increases
from 0.09% to 0.25% from undoped to Al(5 at. %):ZnO
NWs. Similar compressive strain has been reported earlier inthe Al doped ZnO thin films.27 As explained earlier, due tothe smaller size of the Al atom than Zn atom, incorporationof Al atoms into ZnO leads to slight decrease in lattice constants in ZnO, resulting in the compressive strain in the
Al:ZnO NWs. Figure 4(b) shows the XRD patterns of thecore-shell ZnO NWs. The relative intensities of the peaks
corresponding to ZnO further reduced due to the formation

FIG. 4. (a) X-ray diffraction patterns of the chemically prepared undoped
and Al doped ZnO NWs with different concentrations of Al doping; (b)X-ray diffraction patterns of the core-shell ZnO/ZnS NWs for varying thickness of the shell layer.

of outer shell. Additional peaks correspond to the latticeplanes of hexagonal phase of ZnS are also observed in the
XRD data. It is also noticed that the relative intensities of theZnS related peak increase for sample CS4. This indicatesthat in the sample CS4, the ZnO NW has thicker outer shellthan the NW in the sample CS2, as expected.
Further information about chemical elemental environment and surface defects of the ZnO NWs is obtained fromthe XPS analysis. Figure 5 shows the collective binding
energy spectra of the elements under analysis for the
undoped, Al:ZnO NWs, and sample CS2. Al 2p core level
spectrum of undoped ZnO NWs shown in Fig. 5(a) shows nodetectable signal in the range 68–80 eV indicating absenceof Al as impurity. On the other hand, a significant peak at
74.0 eV is detected from each of the Al:ZnO NWs (Figs.
5(b)–5(d)). The observed peak can be attributed to the binding energy of Al 2p core level doublet. This peak position of
Al corresponds to an oxidation state close to Al3þ in Al2O3(Al 2p: 75.6 eV)28 thereby giving further support for Al
residing at substitutional positions rather than at interstitialsites in the ZnO crystal. This also ruled out the possibility ofpresence of Al2O3 or segregation of metallic Al (Al 2p:
72.8 eV)29 on the surface of the Al:ZnO NWs. It is also
noticed that the integrated peak area for the Al 2p core level
gradually increases with the increase in Al doping concentration, as expected. The Al doping level in the Al:ZnO NWs isfurther quantify from the integrated peak area of Al 2p, O 1s,
and Zn 2p core level XPS peaks. The Al concentration is134307-5

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J. Appl. Phys. 114, 134307 (2013)

core levels show broad asymmetric nature indicating presence of multi-component species. The O 1s spectrum isdeconvoluted to three components centered at 530.6, 531.8,
and 532.2 eV. The first peak with lowest binding energy
2
ions
(O2
L ) of the O 1s spectrum can be attributed to the O
in the hexagonal structure of ZnO lattice surrounded by zincatoms.30 The intermediate binding energy peak (OV) is associated with O2 ions that are in oxygen deficient regions
within the matrix of ZnO.30 The higher binding energy peak(OOH) at 532.2 eV usually corresponds to absorbed hydroxyl
group (OH) on the surface (preferable at the oxygen vacancy sites) of the NWs.31 From the comparative studies, it
is found that the integrated peak area for the OV peak gradually increases with the increase in Al doping concentration.
The calculated content of OV peak in O 1s XPS spectrum
changes from 29% to 45% for the undoped to Al(5at. %):ZnO NWs. This indicates a gradual increasing of
defect concentration with the increase in Al doping concentration and it is consistent with the XRD analysis. The XPS
data for the Zn 2p3/2 core level are also deconvoluted and thepeak is best fitted with two symmetric Gaussian peaks. The
peak centered at 1021.9 eV is known as Zn 2p3/2 spectral line
for Zn2þ in ZnO lattice. Along with the Zn 2p3/2 peak,another peak at 1045.2 eV corresponds to the Zn 2p1/2
observed in the XPS spectra (data not shown). The second
peak centered at 1023.8 eV can be attributed to the zinc complex with adsorbed hydroxyl group (Zn-OH).32 From theCS2 sample (Fig. 5(e)), along with the O 1s and Zn 2p corelevel, sufficient amount of signal is obtained for the S 2p
core level in ZnS lattice. After the formation of outer ZnS
shell layer, the defect concentration significantly enhanced
to 62.8% of O 1s XPS spectrum, as expected. During sulfidization process, the S atoms in TAA solution replace the O
atoms in the ZnO lattice. Due to the uncontrolled rate of sulfidization process, oxygen vacancy concentration is likely toincrease.
B. Electrical studies

FIG. 5. (a)–(d) XPS binding energy spectra of the undoped, 1 at. %, 2 at. %,5 at. % of Al doped ZnO NWs, respectively. (e) XPS data of the core-shell
CS2 NWs sample. To find the individual components, experimental XPS
data (circle) are fitted with multi-Gaussian Peakfit. Solid red line represents
the combined fitting of sum of all the individual components (blue line).

estimated according to the standard method (calculation of
fractional peak area divided by individual sensitivity factorof each XPS peak). The calculated Al concentrations arefound to be close (1.6 at. % for 2% mixed Al in the solution)
to the doping concentration in the mixed reaction solution
and it is slightly higher than the concentration obtained fromthe EDX analysis. The XPS spectra for the O 1s and Zn 2p3/2

The electrical characterization is performed on the
undoped and Al:ZnO NWs. The resistivity of the NWs is estimated from the current-voltage data measured by standard
four-probe setup. It is found that the electronic conductivity
gradually increases with the increase in the Al concentrationin the NWs. The undoped ZnO NWs show low electronic conductivity with a resistivity of 46.8 X cm. Al doping causes
dramatic improvement in conductivity of the ZnO NWs. The
conductivity improved by two orders of magnitude for theAl(5 at. %):ZnO NWs with the lowest resistivity of 0.45 X
cm. The obtained high conductivity is the result of activationof large numbers of Al dopants in the ZnO lattice.
C. Photoluminescence studies

The room temperature PL spectra of the undoped and
Al:ZnO NWs are shown in Figs. 6(a)–6(e). The ZnO NWs
exhibit near band edge (NBE) UV emission at 380 nm anda broad visible emission band in the green–yellow region.
Deconvolution of this broad emission by Gaussian multipeak
fitting shows the existence of two emission bands, one at

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intensities significantly reduced at a doping level of 5 at. %.
The observed changes in the green emissions are consistent
with the XPS spectra, which show increment in the oxygen
vacancy content in the Al:ZnO NWs. It is known that thestructural quality and various kinds of lattice defects significantly affect the UV emission of the ZnO nanostructures. In
the present case, the gradual reduction of the UV PL intensity is caused by the deterioration of the structural quality of
the Al:ZnO NWs. Here, as the doping concentration isincreased, the substitution of dopant into the Zn lattice cancause strain and the oxygen in the ZnO host lattice can beshared by both Zn and Al. This will lead to neighboring disorder and local geometric disorientation in the ZnO
NWs.35,36 As a result, deterioration of the structural quality
and formation of various kinds of surface defects (radiativeand nonradiative) are expected. At higher doping concentration (5 at. % or more), simultaneous reduction of the UV as
well as green emissions might be due to the significant
enhancement of the nonradiative defects in the Al:ZnO
NWs. This is consistent with the XRD analysis.
Figures 7(a)–7(c) show the room temperature PL spectraof the Al doped core-shell NWs for the samples CS1, CS2,

FIG. 6. Room temperature PL spectra of the (a) undoped and Al doped ZnO
NWs for Al doping concentration of (b) 0.5 at. %; (c) 1 at. %; (d) 2 at. %,and (e) 5 at. %. (f) Variations of PL enhancement (intensity ratio) of UV andgreen emissions with respect to undoped ZnO NWs as a function of Al doping concentration. Dotted lines are shown as a viewing guidance to the data
points. To find the individual PL peak, experimental data (circle) are fittedwith multi-Gaussian Peakfit. Solid red line represents the combined fitting ofsum of all the individual components (blue line).

green region (500 nm) and another at yellow region(580 nm). The observed NBE emission is due to free excitonic recombination, while green emission arises from the33
The
oxygen vacancy (Vþo ) states present on the surface.
yellow emission is commonly observed from the oxide NWs
grown by the hydrothermal method and it is attributed thehydroxyl radicals present on the surface of the NWs.34
During chemical reaction, large amount of hydroxyl radicals
are formed in the growth solution and easily attached on thesurface of the NWs at the oxygen vacancy sites. Presence oflarge amount of hydroxyl radicals is also detected by XPS
measurements as shown earlier. The integrated peak intensity of this yellow emission for different samples is consistent with the obtained content of hydroxyl component in the
XPS spectra for the O 1s core level. Fig. 6(f) shows theenhancement in UV and green PL intensity (with respect to
PL intensity of the undoped NWs) as a function of Al doping
concentration. It is found that the PL enhancement (intensity
ratio) for the UV peak first decreases up to 1% of doping andthen increases for 2% of Al doping. For the green emission,
it is gradually increases with the increase in Al doping concentration. However, both the UV and green emission

FIG. 7. Room temperature PL spectra of the Al(2 at. %):ZnO/ZnS core-shell
NWs for different sulfidization time: (a) CS1, (b) CS2, and (c) CS4. (d) PL
spectrum of the undoped core-shell ZnO NWs sulfidized for 2 h.
(e) Variations of PL enhancement (intensity ratio) of UV and green emissions of the above core-shell NWs with respect to undoped bare ZnO NWs
with sulfidization time duration. Dotted lines are shown as a viewing guidance to the data points. (f) Schematic energy band diagram of the abovecore-shell ZnO/ZnS NW with possible interfacial photogenerated chargetransfer process.

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and CS4, respectively. The PL spectra show a strong UVemission peak at 378 nm and green emission band at 500 nm.
The observed PL spectra indicate significant changes in thenature of luminescence due to the formation of ZnS layer on
the surface. The observed broad emission in the visible
region is the superposition of two individual emissions fromthe core-shell NWs. Along with the green emission (fromZnO), additional peak in the blue region is observed at
460 nm. In the PL spectra, the hydroxyl radicals related yellow emission are completely suppressed, as expected. Afterthe ZnS shell layer formation, no shift in the peak position of
the UV emission observed. Note that previous reports
showed a blueshift in the UV peak from ZnS coated ZnOnanospheres.19 Figure 7(d) shows the PL spectrum of theundoped core-shell ZnO NWs sulfidized for 2 h. In this case,along with UV and green emissions, the blue emission isobserved at 436 nm. This blue emission at 436 nm is associated with the ZnS shell originating from elemental sulfur vacancy or interstitial states.37 In the Al doped core-shell NWs,
the observed blue emission (at 460 nm) is shifted to the lower
energy position. It is known that in case of Al doping, theimpurity energy level is situated just below the conduction
band of the host materials. Therefore, the observed redshift
in the blue emission might be due to the involvement of the
impurity (Al atoms) energy levels in the recombination process with the elemental sulfur atoms. The variations of theintensity ratio of different PL peaks (with respect to PL intensity of the undoped bare NWs) as a function of sulfidization time are shown in Fig. 7(e). The PL enhancement
factors (intensity ratios) for the UV and green emissions
gradually increase with the increase in ZnS shell thickness.
However, after the sulfidization of 4 h, the enhancement for
the green emission gets saturated and for the UV emission, it
is slightly reduced. The maximum enhancement is obtained
from the sample CS2, indicating the optimum thickness(10 nm) of the ZnS shell layer. Compared with the undoped
bare ZnO NWs, fivefold enhancement in the UV PL intensity
is obtained from the sample CS2. As compared with theundoped ZnO/ZnS core-shell NWs, Al doped core-shell
NWs show 2.8 times enhancement in UV PL intensity. Thus,
the Al doped core-shell ZnO NWs show significant improvement in the UV PL intensity than the undoped core-shell
ZnO NWs. Therefore, the fabricated Al doped ZnO/ZnS
core-shell NWs possess very high electronic conductivity as
well as sufficiently strong UV luminescence for commercial
application in optoelectronic devices.
The observed improvement in the PL spectra can be
explained taking the band alignment and surface modification into consideration. When the core-shell structure is
excited with 325 nm light, it is expected that carriers in both
the ZnO (absorption maxm  367 nm) and ZnS (absorption
maxm  340 nm) get excited and excitons are created. From
the absorption studies (data not shown), it is found that at
325 nm, the core-shell NWs have stronger absorption magnitude than the bare ZnO NWs. Therefore, photogenerated
charge carriers are expected to be higher for the core-shell
NWs than the bare NWs. It is known that the electron affinities (Ea) of ZnO and ZnS are 4.5 and 3.9 eV, respectively.38
Therefore, an upward band offset of 0.6 eV between

J. Appl. Phys. 114, 134307 (2013)

conduction bands of ZnO and ZnS exists in the energy band
diagram of the ZnO/ZnS NWs. Then, these photogenerated
energetic electrons transferred to the conduction band of
ZnO through the ZnO/ZnS interface and this is schematicallyshown in Fig. 7(f). At the same time, the outer shell layer
suppresses the tunneling of photogenerated charge carriers
from the core to the newly formed surface states. This process leads to the confinement of large number of photogenerated charge carriers at the core (ZnO) resulting in strong UV
PL. After substitution of sulfur atoms, the oxygen vacancies
on the surface of the ZnO NW significantly modify the visible emission. In the present case, the lattice mismatch
between ZnO and ZnS might be responsible for the increaseof surface states at the interface causing a strong green emission. Therefore, the interfacial transfer of photogenerated
electrons and confinement by suppression of tunneling by
the outer ZnS shell are most likely sources of the enhanced
UV PL intensity.
IV. CONCLUSIONS

We investigated the combined effects of Al doping andsurface modification by ZnS shell layer on the structural,electrical, and PL properties of the ZnO NWs. A systematicinvestigation on effect of different concentrations of Al doping followed by formation of ZnS shell layer with different
thicknesses are performed. Compared with the bare undoped
ZnO NWs, a major improvement in the electronic conductivity (by two orders of magnitude) and fivefold enhancementin the UV PL intensity is obtained from the aluminum doped
core-shell ZnO/ZnS NWs. As compared with the undoped
ZnO/ZnS core-shell NWs, Al doped core-shell NWs show
2.8 times enhancement in UV PL intensity. The Al doped
core-shell ZnO NWs show significant improvement in the
UV PL than the undoped core-shell ZnO NWs. The UV PL
enhancement factor could be improved further by controllingthe sufidization rate and shell thickness. These results indicate that Al doping followed by ZnS shell formation on theZnO NWs is very effective to improve electronic conductivity as well as the UV luminescence, which is promising forcontrolled fabrication of ZnO based optoelectronic devices.
ACKNOWLEDGMENTS

Part of the experiment was carried out under “Grant-inAid for Scientific Research” from Japan Society for the
Promotion of Science (JSPS). S.D. would like to express sincere thanks to JSPS for awarding the postdoctoral fellowshipfor foreign researchers.
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