Catalyst-free direct vapor-phase growth of Zn1−xCuxO micro-cross structures and their optical properties.

2.7 MB·English

by  Xu, Danhua

Xuetal.NanoscaleResearchLetters2013,8:46
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NANO EXPRESS Open Access
Catalyst-free direct vapor-phase growth of
Zn Cu O micro-cross structures and their
−x x
1
optical properties
Danhua Xu1,2, Donghua Fan3 and Wenzhong Shen1,2*
Abstract
We report a simple catalyst-free vapor-phase method to fabricate Zn1−xCuxO micro-cross structures. Through a
series ofcontrolled experiments bychanging the location of the substrate and reaction time, wehave realized the
continuous evolution of product morphology from nanorods intobrush-like structures and micro-cross structures at
different positions, together with the epitaxial growth of branched nanorods from the central stem withthetime
extended. The growth mechanism of the Zn1−xCuxO micro-cross structureshas been proposed to involve the
synthesis of Cu/Zn square-like core, surface oxidation, and the secondary growth ofnanorod arrays. Bythe detailed
structural analysis of theyielded Zn1−xCuxO samples atdifferent locations, wehave shown thattheCuO phases
were gradually formed inZn1−xCuxO, which is significant to induce the usual ZnO hexagonal structures changing
intofour-folded symmetrical hierarchical micro-cross structures. Furthermore, thevisible luminescencecan be
greatly enhanced by the introduction of Cu, and the observed inhomogeneous cathode luminescence in an
individual micro-cross structureis caused by thedifferent distributions of Cu.
Keywords: Cu-doped ZnO, Micro-crossstructures, Optical properties, Epitaxialgrowth,
Catalyst-free vapor-phasemethod
Background these processes often require high temperature, complex
One-dimensional (1D) ZnO nanostructures (e.g., nano- multi-step process, or introduction of impurities by the
wires, nanorods, and nanotubes) are promising with ex- templates or foreign catalysts in the reaction system.
tensiveapplicationsinnanoelectronicsandnanophotonics Therefore, it is still a challenge to find a simple and con-
due to their efficient transport of electrons and excitons trollable synthetic process to fabricate 3D hierarchical
[1]. In recent years, increasing attention has been paid to ZnOarchitectureswithnovelorpotentialapplications.
three-dimensional (3D) hierarchical ZnO architectures On the other hand, doping is a widely used method to
which derived from 1D nanostructures as building blocks improve the electrical and optical properties of semicon-
based on various novel applications [2-6]. To date, differ- ductors [11]. Copper, considered as a valuable dopant
ent kinds of hierarchical branched ZnO nanostructures, for the achievement of long-searched-for p-type ZnO
including nanobridges [7], nanoflowers [2,8], rotor-like [12], can serve not only as a luminescence activator but
structures[9], and nanotubes surrounded by well-ordered also as a compensator of ZnO [13]. In addition, Cu dop-
nanorod structures [10], have been reported by using ei- ing, leading to form donor-acceptor complexes, can in-
ther solution-phase or vapor-phase method. However, duce a polaron-type ferromagnetic order in ZnO [14,15].
Zn1−xCuxO has been previously employed as phosphor
[16], an active material in varistors [17] and spintronic
*Correspondence:[email protected]
1LaboratoryofCondensedMatterSpectroscopyandOpto-ElectronicPhysics, devices [18]. Up to now, most of the investigations in
DepartmentofPhysics,ShanghaiJiaoTongUniversity,800DongChuan the Zn1−xCuxO system have been focused on thin films
Road,Shanghai200240,China
and 1D nanostructures, such as Cu-doped ZnO nano-
2KeyLaboratoryofArtificialStructuresandQuantumControl(Ministryof
Education),DepartmentofPhysics,ShanghaiJiaoTongUniversity,800Dong wires [19], nanonails, and nanoneedles [20]. 3D hier-
ChuanRoad,Shanghai200240,China archicalZn1−xCuxO nanostructures, posing many unique
Fulllistofauthorinformationisavailableattheendofthearticle
©2013Xuetal.;licenseeSpringer.ThisisanOpenAccessarticledistributedunderthetermsoftheCreativeCommons
AttributionLicense(http://creativecommons.org/licenses/by/2.0),whichpermitsunrestricteduse,distribution,andreproduction
inanymedium,providedtheoriginalworkisproperlycited.
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properties arisen from their special geometrical shapes furnace (150 cm long). Figure 1a shows the schematic
and inherently large surface-to-volume ratios, show con- drawing of the experimental setup. Zn powders (0.80 g,
siderable promise for the development of nanodevices 99.99% purity) and Cu nanoparticle (diameter 100 to
with multiple functions (e.g., gas sensor [21] and photo- 200 nm) powders (0.32 g) were firstly mixed as the pre-
catalytic hydrogen generation [22]). However, thus far, cursor substances. Due to the size effect, the copper
there have been no reports of such Zn1−xCuxO hierarch- nanoparticles can vaporize at relatively low temperatures
icalnanostructures. (approximately 600°C), although the melting point of
Herein, we realize a simple catalyst-free vapor-phase bulk copper is higher than 1,000°C. These Cu particles
deposition method to synthesize the Zn1−xCuxO hier- were synthesized by adding Zn powders into the CuCl2
archical micro-cross structures. The branched nanorods solution via the following chemical reaction: Zn + Cu2+
are neatly aligned on four sides of the backbone prism, → Zn2+ + Cu. The mixture was loaded into an alumina
assembling the shape of crosses. The subtle variations of boat and placed at the center of a quartz tube (2 cm
environmental conditions have triggered the observed diameter, 120 cm long). N-type Si (100) wafer cleaned
continuous morphological evolution from 1D nanorod by sonication in ethanol and acetone was employed as
to 3D hierarchical micro-cross structures. A possible the substrate and was placed about a few centimeters
growth mechanism for the micro-crosses has been pro- (from 6 to 12 cm) away from the source materials to re-
posed. Detailed structural and optical studies reveal that ceive the products. As we will show later, the location of
the CuO phases are gradually formed in Zn1−xCuxO and the substrate appears to be an important factor deter-
Cu concentration can greatly influence the structural mining the morphologies and the Cu contents of the
defects. Interestingly, the Zn1−xCuxO micro-cross struc- final products. The quartz tube was evacuated to ap-
ture exhibits distinct inhomogeneous cathode lumines- proximately 10 Pa using a mechanical rotary pump to
cence (CL), which can be attributed to the different remove the residual oxygen before heating. The heated
defect concentrations induced by Cu through character- temperature of the furnace was raised to 750°C at a rate
izing the emission of defects and contents of Cu over of 20°C/min. When the temperature reached 350°C,
theindividualmicro-crossstructure. argon (99.999%, 220 sccm) was introduced, and then
oxygen (99.999%, 80 sccm) was added to the carrier gas
Methods at the desired temperature of 750°C. The duration of
Zn1−xCuxO nanostructures were prepared on Si substrate growth lasted for 5, 30, and 60 min, respectively. We fi-
by a simple vapor-phase method in a horizontal tube nally obtained a black layer on the Si substrate after the
Figure1SEMimagesoftheas-fabricatedsamplestakenatdifferentpositions.(a)Aschematicdrawingoftheexperimentalsetup.
(b)AFE-SEMimageofpureZnOnanowiresgrownwithoutCuinthesource.(c,d,e)FE-SEMimagesofZn1−xCuxOsampleslocatedatpositions
C,B,A,respectively.Insets(b’)and(c’)showthecorrespondinghigh-magnificationSEMimages.
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quartz tube was cooled to room temperature natural- definition of micro-cross comes from the geometrical
ly. For comparative studies, we have also prepared the similarity to the cross structures.
Zn1−xCuxO samples with different Cu contents as well Figure 2a presents the corresponding high-magnification
as the pure ZnO nanostructure synthesized under the image of such a single micro-cross. We can notice that
same experiment condition as the others but without themicro-crossisa3Dhierarchical structure,which con-
copper source. sistsoffour-foldedsymmetricalnanorodarraysof1μmin
The morphology and microstructure of the structures length and approximately 350 nm in diameter, together
were characterized by field-emission scanning electron withananorodonthecentralstemhavingauniformhex-
microscopy (FE-SEM; Philips XL30FEG, Portland, OR, agonal cross section. Four arrayed nanorod branches
USA) with an accelerating voltage of 5 kV, high- stand perpendicular to the side surfaces of the central
resolution transmission electron microscopy (HRTEM; stem. We have also reduced the reaction time to 30 and
JEOL JEM-2100 F, Akishima-shi, Japan), and X-ray dif- 5 min in order to observe directly the morphology evolu-
fraction (XRD; Bruker/D8 Discover diffractometer with tion withthe reactiontimeand getinformationabout the
GADDS, Madison, WI, USA) equipped with a Cu Kα growth process of the micro-cross structures. Under the
source (λ = 1.5406 Å). Energy-dispersive X-ray (EDX) heating time of 30 min (Figure 2b), the homogenous
analysis was also performed during the FE-SEM obser- cross-like structures have also been formed, growing with
vation. The bonding characteristics were analyzed by the length of the four-folded nanorods typically reduced
PHI Quantum 2000 X-ray photoelectron spectroscopy to approximately 450 nm. When the reaction time was
(XPS; Chanhassen, MN, USA). The micro-Raman in 5 min, we could only obtain one-dimensional square-like
the backscattering geometry and photoluminescence nanostructures with the edge length of about 200 to
(PL) spectra were recorded at room temperature using 300 nm (Figure 2c), which stands for the early growth
a Jobin Yvon LabRAM HR800UV micro-Raman sys- stage of the structures. The corresponding EDX analysis
tem (Kyoto, Japan) under Ar+ (514.5 nm) and He-Cd shown in Figure 2d indicates that the major components
(325.0 nm) laser excitation, respectively. The CL mea- of the as-fabricated sample are Zn and Cu, with a small
surements were carried out at room temperature using amountofoxygen.
a Gatan Mono-CL system-attached FE-SEM (Pleasan- Further morphological and structural analysis of the
ton, CA, USA) with the accelerating voltage of 10 kV. micro-crossstructurecanbecharacterizedbytheHRTEM
and selected-area electron diffraction (SAED) techniques.
Results and discussions Figure 2epresents theTEM morphology of the individual
As a reference, specimens of pure ZnO nanostructures crossstructure, whichconsistsofthe nanorod inthe cen-
were grown in the tube furnace system using Zn pow- tral stem, together with the nanorod arrays on the side
der as the only source material. We can observe that surface of the core. The central stem is too thick to be
the as-grown products always present the commonly detected from the TEM observation. The lattice fringes
reported nanowire morphology (Figure 1b). The length and the corresponding SAED pattern of the cross-like
of the undoped nanowires ranges from 4 to 8 μm, and structure are shown in Figure2f,g, respectively, which are
the diameter is about 150 nm. The high-magnification indicated in Figure 2e with a red square. The lattice spa-
SEM image is shown in Figure 1 (b’), demonstrating cingof0.52nmcorrespondstothespacingof[0001]crys-
uniform hexagonal cross sections and a smooth sur- talplanesofwurtziteZnO.
face. With the introduction of Cu in the precursor, the Theaboveexperimentalobservation reveals thatthe lo-
as-grown Zn1−xCuxO samples exhibit three different cation of the substrate and reaction time exercise great
morphologies (see in Figure 1c,d,e), which are depos- influences on the morphologies of the products. After Cu
ited on the substrates at different positions (marked as is introduced, the ordinary pure ZnO nanowires change
C, B, and A in Figure 1a, respectively). For the sample into three different morphologies with the variation of
at position C (as shown in Figure 1c), the nanorods the location, i.e., stumpy nanorods, randomly assembled
are formed, of which the lengths become shorter (ap- brushes, and well-organized micro-cross structures. It is
proximately 1.5 μm) and the diameters become big- speculated that the higher temperature (at position A,
ger (approximately 250 nm). Some Zn1−xCuxO nanor- whichisclosetothecentralzoneofthetube)ishelpfulto
ods display deformed hexagon sections (see Figure 1 formacentralcoreofthehierarchicalstructure.Wecould
(c’)), which may be induced by doping. As seen in findoutthecluefromtheoriginalsquare-likecore,which
Figure 1d, a kind of brush-like structures appears (at is shaped in the early stage of the growth process at pos-
position B). These brushes are randomly assembled by ition A (see Figure 2c). With the reaction time extended,
the nanowires. For the sample at position A, the low- branched nanorods grow epitaxially on the side face of
magnification SEM image in Figure 1e shows that a the central stem (see Figure 2a,b). Since Cu has a high-
large quantity of micro-cross structures formed. The symmetry cubic structure [23], we can assume that the
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d
0.52 nm
500 nm
Figure2SEMandTEMimagesofZn1−xCuxOsamplespreparedfordifferentreactiontimes.(a,b,c)FE-SEMimagesofZn1−xCuxO
nanostructurespreparedat750°Cfor60,30,and5min,respectively.(d)EDXofthesamplepreparedat750°Cfor5min.(e)TEMimage,
(f)HRTEMimage,and(g)SAEDofZn1−xCuxOmicro-crossstructures.
reason for growing into four-fold hierarchical cross-like Figure 3a presents the corresponding EDX spectra of
structures is because of the tetragonal-symmetry major the yielded samples at different locations, which exhibit
coreinducedbytheintroductionofabundantCu.Incom- different Cu concentrations. The undoped ZnO nanos-
bination with previous reports [24,25] and the details in tructures (noted as ‘0’ for ZnO) is used as a reference.
our experiment, we suggest the following possible growth Its EDX analysis indicates that the obtained structures
mechanismoftheZn1−xCuxOmicro-crossstructures. are composed of only Zn and O elements. After adding
At the stage of temperature rise, oxygen was still not Cu powder in the precursor, the appearance of the elem-
introduced into the tube. Zn/Cu vapor easily condensed ent Cu demonstrates that Cu is introduced successfully
into a square-like core on the substrate. When the in the as-fabricated samples. From the atomic ratio of
temperature reached up to the desired 750°C, the core Cu to Zn in the EDX spectra, we can determine the
was oxidized with the introduction of oxygen. The cubic molar ratio of Cu to (Cu + Zn) in the Zn1−xCuxO sam-
core prism could provide its four prismatic facets as ples (from positions A to C in Figure 1a) to be x = 0.33,
growth platforms for the secondary branched nanorod 0.18, and 0.07, respectively. The Cu vapor is more easily
arrays. With the successive arrival of Zn/Cu and O , the condensed on the substrate at the position closer to the
2
branched nanorods began to grow perpendicular to the centralzone.
central stem. Due to the considerable anisotropy in the The structural phase evolution of the as-fabricated pro-
speed of the crystal growth along different directions of ducts with different Cu concentrations was also investi-
ZnO, the nanorods with the right orientation, i.e., with gated by XRD, which is shown in Figure 3b. It is clear
the [0001] direction perpendicular to the surface of the that all the diffraction peaks can be indexed to the hex-
prism, could grow much faster than others. The lengths agonal wurtzite structure of ZnO (JCPDS No. 36–1451)
of the branched nanorods are increased with the growth in the undoped one. In contrast, five small new phases
time extended (see Figure 2a,b). In the whole growth emerge in the sample with the Cu content of 7%. These
process, there are no external metallic catalysts (e.g., Au new phases in the XRD spectrum correspond to CuO
and In) involved in the formation of micro-cross struc- (matched with JCPDS No. 01–1117), owing to the fact
tures. That is, the 3D hierarchical micro-cross structure that the solubility of Cu ions in ZnO is quite low [12].
is synthesized by a simple catalyst-free direct vapor- Moreover,itisnotedthatwiththeincreaseofCucontent,
phasegrowth method. these CuO diffraction peaks become more obvious and
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(a) 33% (b) ZnO CuO 33%
18% 18%
Cu
7% 7%
Zn
u.) Cu Zn 0 u.) 0
a. O a.
y ( Zn y (
nsit O Cu Zn Z n nsit
e e
Int O Zn Int
Cu Zn
O Zn
Zn
0 2 4 6 8 10 12 30 40 50 60
Energy (keV) 2 (degree)
Figure3EDXandXRDspectra.(a)EDXand(b)XRDspectraofundopedZnOandZn1−xCuxOsampleswiththeCucontentof7%,18%,and33%.
stronger. Meanwhile, the ZnO diffraction peaks remain the copper ion in ZnO. Figure 4 illustrates the high-
nearly unshifted, indicating that the added Cu elements resolution XPS spectra of Zn 2p, O 1s, and Cu 2p in the
have no effects on the crystal structure of ZnO, which is samplewiththehighestCucontentof33%(atypicalcon-
coincidentwiththeHRTEMresultsinFigure2f. centration in this work). As shown in Figure 4a, the XPS
Further evidence for the component of the as-prepared spectrumofZn2prevealsthebindingenergiesofZn2p
3/2
samplesisobtainedbyXPSmeasurement,whichisanex- at about 1,021.8 eV and Zn 2p centered at 1,045.1eV,
1/2
cellent technique for understanding the oxidation state of withoutanynoticeableshiftafterthehigh-Cudoping[26].
(a) Zn 2p (b) O 1s
Zn 2p3/2
532.4
Zn 2p1/2
530.3
)
.
u
.
a
(

y
1020 1040 528 532 536
t
i
s
(c)
n Cu 2p3/2 Cu 2p
e
t
n
Cu 2p1/2
I
sat.
930 940 950 960
Binding Energy (eV)
Figure4XPSspectra.High-resolutionXPSspectraof(a)Zn2p,(b)O1s,and(c)Cu2pinmicro-crossstructuresofZn Cu O.
0.67 0.33
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TheXPSspectrumofO1s(Figure4b)isbroadandasym- the E (high) mode confirms that they all have a typical
2
metric, indicating the presence of multi-component oxy- hexagonal wurtzite structure,which is consistent withthe
gen species. It can be resolved by using a curve fitting aboveHRTEMandXRDobservations.WhentheCucon-
procedure: one is located at 530.3 eVandthe otherone is tent is 7%, the E (high) and E (LO) modes become
2 1
locatedat532.4eV.TheformerisinherentOatomsbound broader and shift to lower frequency, as compared with
tometals(suchasCuandZn),whilethelatterisassociated theundopedcounterpart.Thismaybeduetothedecrease
with adsorbed oxygen [27]. Figure 4c shows the core-level in the binding energies of Zn-O bonds as a result of the
and shake-up satellite (sat.) lines of Cu 2p. The Cu 2p Cu doping, indicating that the long-range order of the
3/2
and 2p core levels are located at ca. 933.2 and ca. ZnOcrystalisdestroyedbyCudopants[32].
1/2
952.9eV,respectively,whichareclosetothedataforCu2p On the other hand, three additional modes at around
inCuO[28].Inoursamples,itiseasytoobservetwoshake- 290,340,and628cm−1canbeobserved.Theyareattrib-
up satellites at about 8.7 and 10.9 eVabove the main 2p uted to the A , B1, and B2 modes of CuO due to the
3/2 g g g
peak. The existence of strong satellite features for Cu 2p vibrations of oxygen atoms, respectively [33,34]. From
rulesoutthepossibilityofthepresenceofCu Ophase[29], Figure 5, it is obvious that the intensity of the CuO
2
correspondingwellwiththeXRDobservationinFigure3b. peaks enhanced while that of ZnO peaks decreases with
Figure5shows the Ramanspectraof boththeundoped theCuconcentrationincreasesupto33%.Suchbehavior
ZnO and Zn1−xCuxO nanostructures with different Cu is caused by the competition of Zn and Cu during the
contents in the range 200 to 800 cm−1 measured at room oxidization process. In the sample with the highest Cu
temperature. In the undoped ZnO sample, the peaks at content of 33%, the formation of CuO is dominant, in
331, 384, and 584 cm−1 correspond to the second-order spite of the fact that the lower melting point and higher
acoustic (2-E (M)) mode, A transverse optical (A (TO)) vapor pressure of Zn than those of Cu under the same
2 1 1
mode,andE longitudinaloptical(E (LO))mode,respect- conditions [35]. The formation of CuO is significant to
1 1
ively[30].Thesharpand strongpeakataround437cm−1 induce the usual ZnO hexagonal structures changing
can be attributed to the high-frequency branch of the E into four-folded cross-like structures, in good agreement
2
(E (high)) mode of ZnO, which is the strongest, and typ- with thegrowthmechanism we haveproposedabove.
2
ical Raman-active branch of the wurtzite crystal structure In order to investigate the effects of the different Cu
[31]. In the Zn1−xCuxO nanostructures, the presence of concentrationsontheopticalcharacteristicsintheyielded
0 33%
33% 1
9550 18%
18% 4 3
5 7%
g 7%
A
h) 0 0
u.) 1Bg (hig 2Bg u.) 380 400 52
a. E2 a. 5
( (
y O) ty 75
it L si 5
s ( n
1
n E e
e nt 4
) 0
t M ) I 6
n O
( L
I E2T P
2- A(1
×10
200 300 400 500 600 700 800
400 500 600 700
-1
Raman Shift (cm )
Wavelength (nm)
Figure5Ramanspectra.RamanspectraofundopedZnOand Figure6PLspectra.PLspectraofundopedZnOandZn1−xCuxO
Zn1−xCuxOsampleswiththeCucontentsof7%,18%,and33%. sampleswiththeCucontentsof7%,18%,and33%.
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samples,wehavecarriedoutPLspectroscopyasshownin Ascan be clearlyobservedfrom Figure 6,the undoped
Figure 6. We can see that all the samples show two ZnO possesses a strong near-band-edgeUV emissionto-
emission peaks: a sharp one appearing at approximately gether with a weak visible emission, indicating that the
377 nm in the ultraviolet (UV) region and another undoped ZnO nanostructures have a fairly high quality
broad one in the visible region. The former is ascribed with low defect concentration (its PL intensity was 10
to the near-band-edge (NBE) exciton recombination, times magnified). After Cu is introduced, the UV emis-
while the latter is quite complicated due to the native sion is rapidly suppressed while the visible luminescence
and dopant-induced defects in ZnO. The intensive PL is greatly enhanced compared with the undoped coun-
emission peak at 495 nm is suggested to be mainly terpart, suggesting the poorer crystallinity and greater
due to the presence of various point defects, which level of structural defects introduced by Cu ion incorp-
can easily form recombination centers. The peak cor- oration into ZnO. The intensity ratio of the visible band
responding to 510 nm is usually generated by the re- emission to the UV peak increases from approximately
combination of electrons in singly ionized oxygen 0.2 to approximately 150 with the Cu content change
vacancies with photogenerated holes in the valence from 0% to 33%, demonstrating that the Cu doping
band [36,37]. Apart from the strong peaks at 495 and strongly increases the concentration of defects. Never-
510 nm, the visible band consists of at least four sub- theless, the defects are believed to significantly improve
peaks at wavelengths of 530, 552, 575, and 604 nm, avarietyofsurfaceproperties,suchasheterogeneousca-
resulting from the local levels in the bandgap of ZnO. talysis, corrosion inhibition, and gas sensing, which have
The green shoulders at 530 and 552 nm are attribu- been addressed by theoretical calculation and experi-
ted to the antisite oxygen and interstitial oxygen, re- mental data [38-40]. Furthermore, we have also pre-
spectively [35]. The peak at 604 nm is possibly sented in the inset the enlarged view of the UV peak
caused by the univalent vacancies of zinc in ZnO. between 360 and 405 nm. It is obvious that the intro-
The origin of another peak at 575 nm has been rarely duction of Cu will cause a little redshift of the UV peak
mentioned and is still unclear. (34 meV under Cu contents from 0% to 33%) compared
(a) (b)
0
0
0
1
0
0
5
0
0
0
5
-
0
0
500 nm 0 500 nm
1
-
(c) 1000nm 100 (d) CL Ratio 100
750nm
Cu Content
500nm 80 80
250nm
0 60 60
40 40
20 20
0 0
400 500 600 7000 0 250 500 750 1000
Wavelength (nm) Position (nm)
Figure7SEandCLimagesofasinglemicro-crossstructurewithitscorrespondingspectra.(a)SEimageoftheZn1−xCuxOmicro-cross.
(b)CLpanchromaticimagepaddedwiththebrightnessdistributioncurvealongtheaxiallineofthesample.(c)CorrespondingCLspectraatfive
differentlocationsalongtheaxiallineofonebranchednanorod.(d)CLratioandCucontentvariationwithdifferentpositionsofthebranched
nanorod.
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with the undoped one, i.e., a reduction of ZnO bandgap Conclusions
caused bytheCudoping. In summary, we report a new and delicate cross-like
We have also employed the high-spatial resolution CL Zn1−xCuxO structure, in which four-sided branched
technique at various locations within the same cross nanorod arrays grow perpendicular to the side surfaces
structure to explore the defect distribution and the local of the central stem. This structure is formed through the
optical properties in an individual Zn1−xCuxO micro- directvapor-phasedepositionmethodbutwithoutintrodu-
cross. A typical secondary electron (SE) image of such cing any catalyst. By changing the reaction time, the pos-
an individual micro-cross is shown in Figure 7a. Clearly, sible growth mechanism of the micro-cross structures
there is a 200-nm square hole in the center of the stem, has been proposed to involve the synthesis of Cu/Zn
which confirms that the central zone is a cubic prism. core, surface oxidation, and the secondary growth of the
Figure 7b presents the corresponding panchromatic CL branchednanorods.Thelocationofthesubstrateisanim-
image at the same place. Interestingly, the cross struc- portant factor determining the morphologies (from 1D
ture exhibits inhomogeneous luminescence. The strong nanorods to 3D micro-cross structures) and Cu concen-
CL emissions are mainly focused on the middle of the trations (from 7% to 33%) of the yielded Zn1−xCuxO
four-folded branched nanorod according to the intense samples. We have employed the XRD, Raman, and PL
distribution curve obtained along the axial line (yellow spectroscopies to demonstrate that the formation of CuO-
curve). related phases and concentration of thedefectsin the pro-
Figure 7c illustrates the typical CL spectra, which are ducts have been greatly influenced by the Cu content.
acquired at the center stem (noted as ‘0’ on the axis in Moreover, inhomogeneous CL has been observed in a sin-
Figure 7b) and four different locations along one gle micro-cross structure, which is generated from struc-
branched nanorod. The spectra exhibit similar features tural defects created by the Cu incorporation into ZnO.
as the PL spectra, that is, a comparatively weak UV peak The presented method is expected to be employed in
duetothe NBEemissionandabroad,strongpeak inthe a broad range to fabricate other similar metal-doped
visible region, which is attributed to the deep-level (DL) ZnO 3D hierarchical structures for their potential device
emission affiliated with defects and impurities. To fur- applications.
ther reveal the variation of the defect concentration, the
intensity ratios of the DL emission to the NBE emission Abbreviations
(I /I ) at different locations are plotted in Figure 7d 1D:One-dimensional;2-E2(M):Second-orderacoustic;3D:Three-dimensional;
DL NBE A(TO):A transverseoptical;CL:Cathodeluminescence;DL:Deeplevel;E
(marked as ‘CL Ratio’). We can notice that the ratio of (L1O):E lo1ngitudinaloptical;E(high):High-frequencybranchoftheE; 1
1 2 2
I /I decreases from approximately 92 to approxi- EDX:Energy-dispersiveX-ray;FE-SEM:Field-emissionscanningelectron
DL NBE
microscopy;HRTEM:High-resolutiontransmissionelectronmicroscopy;
mately 5 with the location change from 0 to 1,000 nm,
NBE:Near-band-edge;PL:Photoluminescence;SAED:Selected-areaelectron
demonstrating that the concentration of defects strongly diffraction;SE:Secondaryelectron;UV:Ultraviolet;XPS:X-rayphotoelectron
depends on the location. The center part of the cross- spectroscopy;XRD:X-raydiffraction.
like structure exhibits the highest defect density. We
have also performed the EDX analysis on three different Competinginterests
Theauthorsdeclarethattheyhavenocompetinginterests.
location points along the branched nanorod to illustrate
the evolution of the Cu content (marked as ‘Cu Content’ Authors’contributions
inFigure7d).Itisclearthatthecentralzoneofthe cross DHXparticipatedinthedesignofthestudy,carriedouttheexperiments,
structure has the higher Cu concentration of approxi- andperformedthestatisticalanalysis,aswellasdraftedthemanuscript.DHF
participatedinthedesignofthestudyandprovidedtheexperimental
mately 53.6%, while the edge part of the branched
guidance.WZStookchargeofthetheoreticalguidanceandrevisedthe
nanorod has ultra-low Cu content (nearly zero). The manuscript.Allauthorsreadandapprovedthefinalmanuscript.
introduction of abundant Cu inthe corehas induced the
usual ZnO hexagonal structures changing into four- Acknowledgements
ThisworkwassupportedbytheNationalMajorBasicResearchProject
folded symmetrical micro-cross structures, which is con-
(2012CB934302)andtheNaturalScienceFoundationofChina(11174202and
sistent with the abovementioned growth mechanism and 61234005).
EDX analysis (shown in Figure 2d). The Cu contents are
Authordetails
consistently and significantly reduced from the central
1LaboratoryofCondensedMatterSpectroscopyandOpto-ElectronicPhysics,
zone to the edge part of the branched nanorod, which DepartmentofPhysics,ShanghaiJiaoTongUniversity,800DongChuan
may be caused bythe Cudiffusionat thestage ofepitax- Road,Shanghai200240,China.2KeyLaboratoryofArtificialStructuresand
QuantumControl(MinistryofEducation),DepartmentofPhysics,Shanghai
ial growth of branched nanorods from the central core.
JiaoTongUniversity,800DongChuanRoad,Shanghai200240,China.
The spatial differences of the Cu content along the 3SchoolofAppliedPhysicsandMaterials,WuyiUniversity,22DongCheng
structure would induce the variation of the defect distri- Village,Jiangmen529020,China.
bution,resultinginthedistinctinhomogeneouslumines-
Received:26December2012Accepted:15January2013
cence within onemicro-crossstructure. Published:22January2013
Xuetal.NanoscaleResearchLetters2013,8:46 Page9of9
http://www.nanoscalereslett.com/content/8/1/46
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