Nanoscale
PAPER

Cite this: Nanoscale, 2014, 6, 9166

Observation of the origin of d0 magnetism in ZnO
nanostructures using X-ray-based microscopic andspectroscopic techniques†
Shashi B. Singh,‡§a Yu-Fu Wang,‡a Yu-Cheng Shao,a Hsuan-Yu Lai,aShang-Hsien Hsieh,a Mukta V. Limaye,a Chen-Hao Chuang,a Hung-Chung Hsueh,*aHsaiotsu Wang,b Jau-Wern Chiou,*c Hung-Ming Tsai,d Chih-Wen Pao,dChia-Hao Chen,d Hong-Ji Lin,d Jyh-Fu Lee,d Chun-Te Wu,e Jih-Jen Wu,eWay-Faung Pong,*a Takuji Ohigashi,f Nobuhiro Kosugi,f Jian Wang,g Jigang Zhou,g
Tom Regierg and Tsun-Kong ShamhEﬀorts have been made to elucidate the origin of d0 magnetism in ZnO nanocactuses (NCs) and nanowires(NWs) using X-ray-based microscopic and spectroscopic techniques. The photoluminescence and O
K-edge and Zn L3,2-edge X-ray-excited optical luminescence spectra showed that ZnO NCs containmore defects than NWs do and that in ZnO NCs, more defects are present at the O sites than at the Znsites. Speciﬁcally, the results of O K-edge scanning transmission X-ray microscopy (STXM) and thecorresponding X-ray-absorption near-edge structure (XANES) spectroscopy demonstrated that the
impurity (non-stoichiometric) region in ZnO NCs contains a greater defect population than the thickregion. The intensity of O K-edge STXM-XANES in the impurity region is more predominant in ZnO NCs
than in NWs. The increase in the unoccupied (occupied) density of states at/above (at/below) the
conduction-band minimum (valence-band maximum) or the Fermi level is related to the population of
defects at the O sites, as revealed by comparing the ZnO NCs to the NWs. The results of O K-edge andReceived 11th April 2014
Accepted 25th May 2014
DOI: 10.1039/c4nr01961jwww.rsc.org/nanoscaleZn L3,2-edge X-ray magnetic circular dichroism demonstrated that the origin of magnetization is
attributable to the O 2p orbitals rather than the Zn d orbitals. Further, the local density approximation(LDA) + U veriﬁed that vacancies in the form of dangling or unpaired 2p states (due to Zn vacancies)
induced a signiﬁcant local spin moment in the nearest-neighboring O atoms to the defect center, which
was determined from the uneven local spin density by analyzing the partial density of states of O 2p in ZnO.

IntroductionaDepartment of Physics, Tamkang University, Tamsui 251, Taiwan. E-mail: hchsueh@
mail.tku.edu.tw; jwchiou@nuk.edu.tw; wfpong@mail.tku.edu.twb

Department of Physics, National Tsinghua University, Hsinchu 300, TaiwancDepartment of Applied Physics, National University of Kaohsiung, Kaohsiung 811,
TaiwandNational Synchrotron Radiation Research Center, Hsinchu 300, TaiwaneDepartment of Chemical Engineering, National Cheng Kung University, Tainan 701,
TaiwanfInstitute for Molecular Science, Okazaki 444-8585, Japang

Canadian Light Source Inc., Saskatchewan S7N 2V3, CanadahDepartment of Chemistry, University of Western Ontario, London N6A 5B7, Canada† Electronic supplementary information (ESI) available: Scanning photoelectron
microscopy (SPEM) results of ZnO NCs and NWs. Computational details andcalculated total and partial density of states (PDOS) of bulk wurtzite ZnO withoxygen anion vacancies (VO). See DOI: 10.1039/c4nr01961j‡ These authors contributed equally to this work.
§ Presently at Department of Physics, Indian Institute of Science Education andResearch, Bhopal, India.

9166 | Nanoscale, 2014, 6, 9166–9176

In the past, numerous eﬀorts have been made to develop
diluted magnetic semiconductors that can support spin transport at or above room temperature by means of the magnetic
doping of semiconductor materials.1–4 However, the role ofpossible extrinsic (magnetic atom segregation, etc.) and/or
intrinsic room-temperature ferromagnetism in these materials
is not yet fully understood.1,5 New complexity has been introduced by the observation of ferromagnetism in undoped
semiconductor thin lms and nanostructures.6 In recent years,
the d0 magnetism of nanomaterials has been considered to be
one of the most interesting and challenging phenomena thatmay provide an avenue for investigating and understanding theorigin of nanomagnetism.7–9 Venkatesan et al.6 have observed d0
magnetism in HfO2 thin lms and have triggered extensive
experimental activities in the eld of the room-temperature
ferromagnetism of many semiconductors and nanomaterials.
Magnetism has been reported in various nonmagnetic
oxides,6,10 selenides11,12 and Au-metal nanoparticles.13,14 Among

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Paperthem, magnetism in ZnO has also been studied more than thatof any other compound or semiconductor.7–9 Surface-capping
molecules,7,15 defects and/or vacancies6,10 are considered to bevery important causes for d0 magnetism in ZnO nanostructures
and thin lms. The origin of magnetization in ZnO and the roleof surface defects,16 O vacancies (VO),6,17,18 Zn vacancies (VZn),19,20
and so on in the d0 magnetism mechanism remain the topics ofcontroversy. Keavney et al.21 have used the Cu, Zn L3,2- and O Kedge X-ray magnetic circular dichroism (XMCD) techniques andfound no XMCD signal in a Cu-doped ZnO thin lm. They havealso reported the absence of any magnetic moment at either theZn or O sites and only a small paramagnetic contribution to theCu 3d orbitals, which is prevalent at the surface of the ZnO lm.
Chaboy et al.22 have veried the role of the conduction band in
the magnetism and have ruled out the possibility of a valenceband (basically, Zn 3d orbitals) in thiol-capped ZnO nanoparticles by performing a Zn K-edge XMCD measurement. They
found that the occurrence of magnetism was related to thestructural modication at the interface between the nanoparticles and the capped molecules. The intrinsic ferromagnetic
behavior at room temperature has been observed in various
complex (capped with organic molecules and core–shell-structured) ZnO nanostructures using the XMCD technique.23 The
variation of vacancies in ZnO lms with varying N2 pressure has
also been observed via positron annihilation spectroscopy.20
Nonetheless, the controversy related to anion and cation
vacancies in d0 magnetism also exists in the theoretical calculations.24–29 Specically, Wang et al.24 have utilized density
functional theory (DFT) calculations to demonstrate the role ofVZn, which induces spin polarization at the top of the valenceband in d0 magnetism. Morozovska et al.26 have suggested thatboth VO and VZn are involved in the magnetism because of their
triplet ground state and Hund's rule. On the other hand, Podila
et al.29 using the ab initio calculations highlighted the role of thesample preparation method and annealing conditions in theorigin and tuning of magnetism in ZnO. The origin of d0
magnetism in ZnO is still not clearly understood, and most of
the available explanations of d0 magnetism as induced bysurface characteristics or defects are drawn based on bulksensitivity experimental techniques [such as photoluminescence (PL), magnetic-hysteresis (M–H) curve measurements, etc.]. A ZnO nanostructure is the ideal system to study
and gain an understanding of the role of defects and/or
vacancies present in the edge or surface regions in d0 magnetism, as such a nanostructure has a large edge- or surface-tovolume ratio. However, spatially resolved and element-specicmicroscopic and spectroscopic techniques are required todiscriminate electronic structures between surface and coreeﬀects in nanostructures and also able to identify defects and/or
vacancies in a particular region and at the sites of a specicelement, thus directly providing evidence related to d0 magnetism in ZnO nanostructures.
In this work, ZnO nanocactuses (NCs) and nanowires (NWs)
are extensively studied using X-ray-based microscopic andspectroscopic techniques to probe the defects and/or vacancies
in specic regions and at specic sites in terms of their chemical states, electronic structures and magnetic moments, which

This journal is © The Royal Society of Chemistry 2014

Nanoscale

are closely related to the origin of d0 magnetism in ZnO nanostructures. The X-ray-based microscopic and spectroscopic
techniques used in this study include extended X-ray-absorption ne structure (EXAFS) spectroscopy, X-ray-excited opticalluminescence (XEOL) spectroscopy, scanning transmission
X-ray microscopy (STXM) and the corresponding X-ray-absorption near-edge structure (XANES) spectroscopy. The main
advantage of the use of STXM-XANES in this work is its ability to
map chemical states and determine spatially resolved electronic
structures in a selected area and site of interest, and these are
typically extracted from image masks applied to the STXM
data.30,31 Scanning photoelectron microscopy (SPEM) and
XMCD measurements were also performed, and they supported
the claim that defects and/or vacancies and O 2p orbitals areplaying critical roles in inducing d0 ferromagnetism in ZnO
nanostructures. These ndings provide conclusive evidence
that the strength of d0 magnetism in ZnO nanostructures isdetermined not only by VZn in the surface/edge (non-stoichiometric) regions but also by the population of defects and/or
vacancies. The local density approximation (LDA) + U calculations further conrm that dangling and/or unpaired O 2p statesthat arise because of VZn, rather the Zn 3d orbitals and VO, areprimarily responsible for d0 magnetism in ZnO nanostructures.

Experimental
Sample preparation
Aligned ZnO NW arrays were grown on seeded uorine-doped
tin oxide (FTO) substrates via chemical bath deposition (CBD)
in a 0.02 M aqueous solution of zinc acetate and hexamethylenetetramine at 95  C for 3 h. A limpid solution, which was
obtained by diluting a solution of 0.57 M ZnAc$2H2O and 5 M
NaOH by a factor of 10, was prepared for the subsequent CBD.
The ZnO NW array was immersed in the stirred limpid solution
at room temperature (RT) for 20 min. The spines were directly
formed on the ZnO NWs, and the diameters of the NWs werealso slightly increased aer RT CBD. A hierarchical ZnO array,referred to as a ZnO NC array, was then formed on an FTO
substrate. In order to fabricate ZnO NCs, the second CBDexperiment was performed on the top of ZnO NWs, during thisprocess RT CBD is performed by using ZnAc and NaOH as
precursors. The synthesis, morphology and characteristics of
the ZnO NC and NW samples are described elsewhere.32
X-ray-based microscopic and spectroscopic experiments andsample characterization
Zn K-edge EXAFS spectra, SPEM and O K-edge and Zn L3,2-edge
XMCD were collected at the Wiggler-17C, Undulator-09A andDragon-11A beamlines, respectively, at the National Synchrotron Radiation Research Center in Hsinchu, Taiwan. The Zn
K-edge EXAFS spectra were obtained in uorescence mode,
while the O K-edge and Zn L3,2-edge XMCD spectra wereobtained in surface-sensitive electron-yield mode. The angle ofincidence of the X-rays was xed at 30 from the sample normal,
and a magnetic eld of 1 tesla was applied parallel and antiparallel to the sample normal throughout the XMCD

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Nanoscalemeasurements. The resolution was set to 0.1 eV (0.2 eV) at a
photon energy of 530 eV (1020 eV) for the O K-edge (Zn L3,2edge) XANES measurements. The SPEM measurements were
performed using a spherical grating monochromator with fourinterchangeable gratings. The monochromator had a Fresnel
zone plate (ZP) with an order-selection aperture, a exurescanning stage and a hemispherical electron-analyzer system. A
hemispherical analyzer with a 16-channel multichannel
detector was used to collect photoelectrons. The valence-band
SPEM spectra were calibrated using the EF of a clean gold metal,
and the energy of the incident X-rays was xed at 380 eV with theenergy resolution set to 0.1 eV. Synchrotron-radiation-based
XEOL spectra were obtained at the undulator-based sphericalgrating monochromator beamline at the CLS, Saskatchewan,
Canada. The XEOL spectra were collected using a dispersive
spectrometer and were measured by collecting the total (zeroorder, 200–950 nm) and wavelength-selected luminescence as
the excitation-photon energy was tuned across the O K-edge andZn L3,2-edge absorption. The O K-edge STXM and the corresponding XANES spectra were also obtained at the SM beamlineof the CLS and at the 4U beamline of the UVSOR facility of theInstitute for Molecular Science, Okazaki, Japan. In the STXM
mapping measurements, the monochromatic X-ray beam was
focused using a Fresnel ZP to a 30 nm spot on the sample, andthe sample was raster-scanned with synchronized detection oftransmitted X-rays to generate a sequence of images (stacks)
over the range of photon energies of interest; the transmitted
beam intensities were measured at the image pixels. Energyscans were performed stepwise through the regions of interest
with a typical resolving power (E/DE) of 1  104. The samplesof ZnO NCs and NWs were gently scratched from the substrateand dispersed in 1 ml of methanol to form 1% suspensions,which were sonicated for 1 min. Double-sided carbon tape wasused to tape two blank Si3N4 windows onto the sample plate.
Then, a tiny drop (<1 ml) of each of the ZnO NC and NW
suspensions was placed onto each Si3N4 window. The samples
were allowed to dry in air and then transferred into the STXM
chamber for measurement. The O K-edge STXM-XANES image
stacks were normalized to an empty area of the same stacks. The
STXM data were analyzed using aXis2000 (available free for noncommercial applications at http://unicorn.mcmaster.ca/
aXis2000.html) and PCA_GUI (available free at http://
www.xray1.physics.sunysb.edu/data/soware.php). The magnetization measurements of the ZnO NCs and NWs were performed using a superconducting quantum interference device
magnetometer.

Results and discussion
Fig. 1(a) presents the Fourier-transformed (FT) k3c data fromthe Zn K-edge EXAFS of ZnO NC, NW and powder samples. The
FT spectra include two main features at 1.8 Å and 3.1 Å(without phase correction), which correspond to the nearestneighbor (NN) Zn–O and Zn–Zn bond distances in ZnO nanostructures,33 respectively. Clearly, the main features in the FT
spectra at the Zn K-edges of the NCs and NWs are similar to
those in the spectrum of the powder sample, indicating a9168 | Nanoscale, 2014, 6, 9166–9176

Papersimilarity between the local atomic structures around the Znsites in the ZnO nanostructures and the powder. The intensityof the rst main feature (Zn–O bond distance) in the FT spectrum of the ZnO NCs is almost the same as that of the NWs,
while the second main feature (Zn–Zn bond distance) is
considerably lower than that of the NWs, as shown in Fig. 1(a).
The qualitative analysis of the EXAFS data suggests that either adecient coordination number of the Zn atoms or higher disordering of the atomic structure in the second shell around theZn atoms in ZnO NCs with respect to the NW and powder
samples. The reference (ZnO powder) sample shows a highly
disordered structure compared to ZnO NCs and NWs, and thegrowth condition is possibly the reason for such a disorderedstructure of the ZnO powder. The lower inset in Fig. 1(a) presents the X-ray diﬀraction patterns of the ZnO NC, NW and
powder samples. The enhancement of the (002) peak intensity
and the drop in the (101) peak intensity of the ZnO NC and the
NW relative to the powder sample reveal the preferred orientation of the c axis in the nanostructures. The shi in the (002)
peak position of ZnO NCs and NWs compared to ZnO powder
can be related to the substrate eﬀect (lattice mismatch FTO/ZnO
1.6% of ZnO NCs and NWs).34 The upper insets in Fig. 1(a)display top-view and cross-sectional scanning electron microscopic (SEM) images of ZnO NCs and NWs. The images showthat the diameters of the NCs and NWs in ZnO are approximately 130 nm and 100 nm, and their lengths are approximately
2.8 mm and 3.1 mm, respectively. Apparently, the spine-likestructures developed from the etched pit on the ZnO NCs, which
exhibits a more defective structure than the NWs, as observed
from the FT spectra at the Zn K-edge. The details of thesynthesis, morphology and characterization of the ZnO NCs andNWs can be found elsewhere.32 The inset in Fig. 1(b) presents
the M–H curves of the ZnO NCs and NWs, measured at 300 K(the diamagnetic character of the ZnO powder sample is notshown here). A magnetic eld was applied parallel to thedirection of growth of the sample, along the c axis (schematic
inset I). The inset II in Fig. 1(b) magnies the M–H curves forclarity. This inset demonstrates that the saturation magnetization (Ms) and coercivity (Hc) of ZnO NCs exceed those of the
NWs. The values of Ms and Hc are approximately 3.0 emu cm3and 200 Oe, respectively, for ZnO NCs and 2.3 emu cm3 and100 Oe for NWs. The enhancement of the Ms of the ZnO NCs
relative to that of the NWs can be attributed to the higher
population of defects in ZnO NCs, as identied previously via PL
measurements of capped ZnO NPs.15 The low temperature CBD
method used to fabricate ZnO NCs compared to NWs was
attributed to the origin of the higher saturation magnetizationand defect concentration in NCs. The EXAFS study also
supports these arguments. The use of PL to determine the roleof defects and vacancies in magnetism has been widely
studied.35–38 Fig. 1(b) presents the PL spectra of ZnO NCs andNWs, which exhibit two features – one with a sharp peak at
380 nm and another broad feature that is centered at 580 nm
that are typically attributed to a near-band-edge transition andthe emission that is caused by defects and vacancies, respectively. Although the involvement of defects such as VO,34 VZn (ref.
37) and oxygen antisites39 in green (520 nm) luminescence inThis journal is © The Royal Society of Chemistry 2014

PaperNanoscale(a) Fourier-transformed k3c data of Zn K-edge EXAFS measurements from k ¼ 3.0 to 13.0 Å (not shown here). The insets show top-view
and cross-sectional SEM images and the XRD patterns of ZnO NC, NW and powder samples. (b) PL spectra and M–H plots of ZnO NCs and NWs.
Insets I and II present the magnetic ﬁeld applied parallel to the growth direction (c axis) and the magniﬁed M–H loops of ZnO NCs and NWs,
respectively, at 300 K.
Fig. 1

ZnO is still debated, Janotti et al.37 have reported that based on
DFT-LDA calculations, defects of the low formation energy of
VZn are the primary sources of green luminescence in ZnO.
For more element-specic PL analysis, the synchrotronradiation-based XEOL experiments were performed, in which
optical emissions were followed by the X-ray excitation of coreelectrons in a process diﬀerent from conventional PL.39–41 XEOL
is an element-specic probe and can determine direct correlations between the optical properties and electronic structures ofnanomaterials.42 Fig. 2(a) and (b) present the O K-edge XEOL
spectra of ZnO NCs and NWs, respectively, under excitationenergies in the range of 520–580 eV. The O K-edge XEOL spectrum of ZnO NCs has a sharp peak at 380 nm and a strong
broad feature that is centered at 580 nm. The spectrum of ZnO
NWs also exhibits a sharp peak at 380 nm and a weak broadfeature in the region of 580 nm. As stated above, the sharp
peak is attributed to a near-band-edge transition, while thebroad feature is related to defects and/or vacancies in ZnO
nanostructures. Fig. 2(c) and (d) present the Zn L3,2-edge XEOL
spectra of ZnO NCs and NWs, respectively, under excitationenergies in the range of 1010–1050 eV. Similarly, the Zn L3,2edge XEOL spectrum of ZnO NCs has a sharp near-band-edge
emission peak at 380 nm and a strong broad feature that iscentered at 580 nm, whereas the corresponding spectrum of
ZnO NWs exhibits a near-band-edge emission peak and a weakbroad feature that is similar to that present in the O K-edge
XEOL spectrum. Fig. 2(e) schematically shows the O K-edge andThis journal is © The Royal Society of Chemistry 2014

Zn L3,2-edge XEOL processes of ZnO nanostructures, showingthat when core electrons are excited by photons of a suitableenergy (E ¼ hn), the core electrons from both the O 1s and Zn 2pstates are primarily excited to unoccupied O 2p and Zn 4 d/sstates by O K-edge and Zn L3,2-edge absorptions, respectively.
The excited electrons in the conduction band further rst relax
non-radiatively or thermalize41 (dotted arrows) and thenundergo a near-band-edge transition, which corresponds to the
sharp emission peak at 380 nm in the O K-edge and Zn L3,2edge XEOL spectra [blue solid arrow, indicating the band gap(Eg) of 3.3 eV in ZnO], similar to that observed in theconventional PL spectra as shown in Fig. 1(b). Meanwhile, theradiative emission related to defects and/or vacancies yields thestrong (weak) broad-emission feature in the region of 520–670
nm in O K-edge and Zn L3,2-edge XEOL spectra of ZnO NCs(NWs), which are indicated by the green/yellow arrows as shownin Fig. 2(e). Clearly, the O K-edge and Zn L3,2-edge XEOL results
reveal that the population of defects and/or vacancies in ZnO
NCs is much higher than that in the NWs and is consistent withthe results of conventional PL [Fig. 1(b)]. Importantly, theemission intensity of the broad feature that is associated withdefects and/or vacancies in the O K-edge XEOL spectrum
exceeds that of the sharp peak that is associated with the nearband-edge transition in ZnO NCs. However, such a diﬀerence inemission intensity is not observed in the Zn L3,2-edge XEOL
spectrum of ZnO NCs, in which the intensity of the broadfeature is weaker than that of the sharp peak. Overall, the ratio

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Fig. 2 (a and b) O K-edge XEOL spectra of ZnO NCs and NWs, respectively, under excitation at various energies in the range of 520–580 eV. (cand d) Zn L3,2-edge XEOL spectra of ZnO NCs and NWs, respectively, under excitation at various energies in the range of 1010–1050 eV. (e)Schematic of the O K-edge and Zn L3,2-edge XEOL processes in ZnO nanostructures.

of peak intensity of 580 : 380 nm is almost 1.5 : 1 and 1 : 2 in O
K-edge and Zn L3,2-edge of ZnO NCs, respectively. The results
clearly suggest that the population of defects associated withVZn at the O sites in ZnO NCs is greater than that of the defects
associated with VO at the Zn sites. Combining the EXAFS, PL, O
K-edge and Zn L3,2-edge XEOL measurements reveals that ZnONCs contain more defects than the NWs do, further suggestingthat the number of defects of VZn at the O sites in ZnO NCs islarger than that of the defects of VO at the Zn sites.
Fig. 3(a) and (b) display the O K-edge STXM stack mappingsof randomly selected sample regions of ZnO NCs and NWs(upper le panel). The stack mappings are decomposed into
blue, green and red maps (lower le panel), which correspondto the regions that are associated with diﬀerent thicknesses andspectroscopic variations of the samples. Overall, the divisions of
the mappings into thick (blue), thin (green) and impurity (red)regions were generated via principle component analysis (PCA)
for cluster analysis based on spectroscopic diﬀerences. The PCA
spectra for those three regions are the average spectra from all
image pixels in each region. A more intense average spectrum
generally means that the sample thickness is higher, where alltypes of chemical species in that region contribute to thethickness. The PCA cluster analysis indicated that the blue andgreen regions contain pure and predominantly pure ZnO (ormore crystalline ZnO), respectively, and the red region represents the presence of signicant impurities or surface/edge
structures. However, the possibility that some thinner regions
and impurities may be folded to form thick regions in ZnO
nanostructures cannot be excluded. The background is shown

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in grey; the optical density or absorbance of the background isnearly zero, corresponding to the near-null intensity of the O Kedge and Zn L3,2-edge STXM-XANES spectra. More details
regarding STXM measurement and the analysis of data concerning various nanomaterials can be found elsewhere.30,31,43
Clearly, the stack images of the O K-edge STXM maps that
contain blue (thick), green (thin) and red (impurity) regions in
ZnO NCs are distributed more randomly than those of the NWs
because of their spine-like structures that are formed at theetched pits on the ZnO NCs, as seen from the SEM image [inset
in Fig. 1(a)]. The O K-edge STXM-XANES spectra in the righthand panels are the sum of the corresponding XANES spectrafrom the blue, green and red regions, which are presented in thelower le panels in Fig. 3(a) and (b). The general line shapes andpositions of the features in the O K-edge STXM-XANES spectra
are consistent with the features in the O K-edge XANES spectraof aligned ZnO nanorods (NR) obtained in the author's previous
studies.44,45 According to the dipole-transition selection rule, thefeatures at 535 to 550 eV, which are denoted by A1–D1 in the O
K-edge STXM-XANES spectra, are attributed to the electron
excitations from O 1s-derived states to 2ps-derived (along thebilayer) and O 2pp-derived (along the c axis) states, which areapproximately proportional to the density of the unoccupied O
2p-derived states. The enhancements of the features A1–D1
represent the enhanced local densities of states (DOSs) thatarise from the defects or dangling bonds in the tip region inaligned ZnO NRs.44 A similarly enhanced local DOS in the tip
region has also been observed in aligned carbon nanotubes(CNTs).46 However, as displayed in the right-hand panel inThis journal is © The Royal Society of Chemistry 2014

PaperNanoscaleFig. 3 (a and b) O K-edge STXM and the corresponding XANES spectra of ZnO NCs and NWs, respectively. The top left panels in (a) and (b)present stacked images of the target region with diﬀerent thickness indices (blue, green and red), which correspond to the maps of the thick, thinand impurity/edge regions in the left-hand sides of the lower panels, respectively. The right-hand sides of (a) and (b) present the sums of the
XANES spectra of the diﬀerently colored (blue, green and red) regions. (c) Comparison of the O K-edge STXM-XANES spectra of the thick, thinand impurity/edge regions of ZnO NCs with those of the NWs.

Fig. 3(a), the O K-edge STXM-XANES spectra clearly reveal thatthe intensities of features A1–D1 in the impurity region of ZnO
NCs are higher than those in the other two regions. The
enhanced intensities of the features A1–D1 in the O K-edge
STXM-XANES spectra can result in a higher population of
defects in the impurity or surface/edge region because of thenon-stoichiometric (relative concentration of Zn : O s 1 : 1)
chemical composition or dangling bonds. Accordingly, the local
DOS at the O sites in ZnO NCs is enhanced, in a manner similar
to the enhancement of the local DOS in the tip region in aligned
ZnO NRs and CNTs.44,46 Surprisingly, for ZnO NWs, the intensities of features C1 and D1 and the intensities of features A1 andB1 were found to be higher and lower, respectively, in the
impurity region than in the thick region, as illustrated in theright-hand panel in Fig. 3(b). This phenomenon may arise fromthe fact that the O K-edge STXM-XANES spectra of ZnO NWs arehighly sensitive to the orientation of the wurtzite structure of
the ZnO with respect to the angle of incidence of the photon orelectric polarization44 or from the fact that the impurity region
in NWs are chemically diﬀerent from that in NCs. To compare
the intensities of the STXM-XANES features in the regions ofdiﬀerent thicknesses in ZnO NCs with the intensities of those in
the NWs, Fig. 3(c) shows the O K-edge STXM-XANES spectra in
the thick, thin and impurity regions of ZnO NCs and NWs.
Apparently, the feature intensity in the impurity region of ZnO
NCs is much higher than in the NWs, indicating that more

This journal is © The Royal Society of Chemistry 2014

defects and/or vacancies (or dangling bonds) are present at O
sites in ZnO NCs than in the NWs. It also reveals the enhanced
local DOS of O 2p at/above the conduction-band minimum(ECBM) or the Fermi level (EF) in ZnO NCs. This nding is
consistent with the PL and O K-edge XEOL measurements
described above. Notably, the intensities and line shapes of thefeatures of the O K-edge STXM-XANES spectra in the thickregion in ZnO NCs and NWs are almost the same, exhibiting an
ideal wurtzite structure (no clear defects or dangling bonds at
the O sites) in the thick region in both ZnO NCs and NWs.
The STXM-XANES spectra that are presented in Fig. 3(a) and(b) were obtained at the Canadian Light Source (CLS) in Saskatchewan, Canada, and conrm that the enhanced local DOSsor intensities of features in the O K-edge STXM-XANES spectra
increased because of the higher population of defects and/or
vacancies at the O sites in ZnO NCs than in the NWs. The O Kedge STXM spectra and the corresponding XANES spectra of theZnO nanostructures were also obtained at the 4U beamline of
the Ultraviolet Synchrotron Orbital Radiation (UVSOR) facility
in Okazaki, Japan, verifying that the number of defects and/or
vacancies at the O sites in ZnO NCs exceeds that in the NWs andfurther supporting the claim that the population of defects and/or vacancies at the O sites in ZnO nanostructures is closely
related to the d0 magnetism. Fig. 4(a)–(d) show the O K-edge
STXM stack mappings for two randomly selected regions in ZnO
NCs (NC-1 and NC-2) and NWs (NW-1 and NW-2), respectively,

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Fig. 4 (a)–(d) O K-edge STXM images of two selected regions in ZnO NCs (NC-1 and NC-2) and NWs (NW-1 and NW-2), respectively. (e) O Kedge STXM-XANES spectra of regions bordered by yellow dashed lines in ZnO NCs and NWs. The insets show the corresponding SEM images.

obtained from UVSOR. The insets in the gures also present thecorresponding SEM images that help to identify the regions andcan be used to verify the STXM images. To verify the signal-tonoise ratio of the XANES spectra obtained from specicmapping areas in the STXM images, the O K-edge STXM-XANES
spectra in Fig. 4(e) were obtained as the sum of the XANES
spectra in the regions that are bordered by yellow dashed lines,
as shown in NC-1, NC-2, NW-1 and NW-2. The intensities of theO K-edge STXM-XANES spectra of NC-1 and NC-2 are clearlyhigher than those of NW-1 and NW-2; this result is similar tothat of the STXM-XANES spectra in Fig. 3(c), which wereobtained from the CLS; which consistently demonstrate that the
population of defects and/or vacancies at the O sites in ZnO NCs
is larger than in the NWs and conrming the enhanced DOS ofO 2p-derived states, as the population of defects and/or vacancies (or dangling bonds) at/above ECBM or EF in ZnO NCs exceedsthat in the NWs.
The defects or dangling bonds that are produced by topological defects close to the ends of aligned ZnO NRs, CNTs andgraphene nanocones have been previously suggested to be
responsible for the increase in the local DOSs at the tips or cone
apices,44–49 indicating that defects in bent vertical graphite
sheets can yield localized states in the gap that are oriented in
the direction of the eld. The density of the O 2p-derived states
are clearly enhanced in the impurity (or surface/edge) region at/above ECBM or EF, as was observed in ZnO NCs above. A similar
enhancement of the DOS at/below the valence-band maximum(EVBM) or EF in ZnO NCs relative to that in the NWs was also be
observed. Fig. S1 in the ESI† presents the valence-band DOSs at/

9172 | Nanoscale, 2014, 6, 9166–9176

below the EVBM or EF of ZnO NCs and NWs, which were obtainedusing spatially resolved SPEM.50 Because the occupied states at/
below EVBM or EF in ZnO nanostructures are dominated by
defects (or dangling bonds) and/or anion-derived p states,
which are O 2p-derived states, the strong enhancement of thefeature near/below EVBM or EF in ZnO NCs is attributed to thehigher DOS of the O 2p-derived states. Together, the results of O
K-edge STXM-XANES spectroscopy (which was used to probe thelocal DOS at/above ECBM or EF) and the valence-band SPEM
measurements (which were used to probe the local DOS at/
below EVBM or EF) strongly support the claim that the superior
d0 magnetism in ZnO NCs with respect to that in the NWs isclosely related to the enhancement of the DOS of O 2p-derived
states close to EF, because the population of defects and/or
vacancies at the O sites in the impurity region in ZnO NCs ishigher.
As direct evidence that the enhancement of the density of O
2p-derived states near EF induces magnetization in ZnO nanostructures, Fig. 5(a) and (b) present the O K-edge and Zn L3,2edge XANES spectra of the ZnO NCs and NWs that wereobtained in surface-sensitive electron-yield mode with thephoton helicity of the incident X-rays parallel (m+) and antiparallel (m) to the direction of magnetization, respectively. The
diﬀerence in m+ and m intensities can be observed in themagnied O K-edge XANES spectra of ZnO NCs and NWs, which
are presented in the lower and middle insets, respectively. The
upper inset in Fig. 5(a) displays the O K-edge XMCD spectrum,(m+  m)/(m+ + m). The weak but conrmed magnetic momentat the O sites in ZnO NCs and NWs results in the imperfect

This journal is © The Royal Society of Chemistry 2014

PaperNanoscale(a) Normalized O K-edge and (b) Zn L3,2-edge XANES spectra with the photon helicity of the incident X-rays parallel (m+) and anti-parallel(m ) to the direction of magnetization for ZnO NCs and ZnO NWs. The insets display the magniﬁed O K-edge XANES spectra and the magniﬁed O
K-edge and Zn L3,2-edge XMCD spectra of ZnO NCs and ZnO NWs, respectively.
Fig. 5alignment of the spin moments of O 2p electrons and/or
dangling p bonds in the impurity region (each XMCD spectrum
typically has been merged at least 20 scans together). The
intensity of the O K-edge XMCD spectrum of ZnO NCs clearlyexceeds that of the NWs. The maximum value of XMCD isapproximately 7% in ZnO NCs and 4% in the NWs. This result is
associated with the higher intensities of features A1–D1 in the O
K-edge STXM-XANES spectrum and feature I in the valenceband SPEM spectra that are presented in Fig. 3(c), 4(e) and S1(in the ESI†), evidently revealing that the larger magnetic
moment at the O sites in ZnO NCs exceeds that in the NWs. Thisnding is also consistent with the M–H curves that are shown in
the inset of Fig. 1(b). However, the Zn L3,2-edge XANES spectrain Fig. 5(b) probe the unoccupied Zn s- and d-derived states,
according to the dipole-transition selection rule. The Zn 3d
orbital is fully occupied, so that the lowest unoccupied orbital of
the Zn ion is Zn 4s, followed by Zn 4p and 4d. Accordingly, thefeatures in the Zn L3,2-edge XANES spectra are primarily associated with the transition of Zn 2p electrons to Zn 4 d/s-derived
states.44,45 In contrast, Fig. 5(b) reveals no clear magnetic
moment of the Zn 4d states in either ZnO NCs or NWs andexcludes the possibility of Zn d orbitals' role in d0 magnetism in
ZnO nanostructures. Therefore, combining the results of O Kedge and Zn L3,2-edge XEOL, XMCD, O K-edge STXM-XANES
and valence-band SPEM spectra reveals not only that the originof d0 magnetism in ZnO nanostructures is closely related to O
2p-derived states rather than Zn d orbitals but also that thehigher magnetic moment is associated with the higher population of defects and/or vacancies in the impurity region in
ZnO NCs compared with that in the NWs.
To verify the importance of enhanced O 2p states in the d0
magnetism of ZnO nanostructures, we performed rst-

This journal is © The Royal Society of Chemistry 2014

principles LDA + U calculations of defect- and/or vacancyinduced spontaneous spin polarization in bulk ZnO. A vacancy
concentration of 2.8% was simulated by introducing an isolated
neutral cation vacancy VZn and anion vacancy VO (Fig. S2 in the
ESI†), respectively, into a defect-free 72-atom wurtzite ZnO
supercell (Zn36O36). Details of the computations can be found in
the ESI.† To elucidate the origin of the defect- and/or vacancyinduced d0 magnetization, the total and partial DOSs (TDOSand PDOS) of the native VZn in ZnO were calculated. The spinpolarized TDOS of VZn in ZnO (Zn35O36) was compared to that in
defect-free bulk ZnO, as presented in the top panel of Fig. 6(a).
The results of the calculations indicate that the majority spin([) defect states are fully occupied, whereas the minority states(Y) are partially occupied. Clearly, a net spin moment associatedwith the diﬀerence between the majority and minority spin
densities is present in ZnO with VZn. A nearly negligible
contribution of the asymmetric majority–minority spin density
was derived from Zn 3d (t2g and eg); see the calculated PDOS ofoccupied Zn 3d orbitals in the middle panel of Fig. 6(a). Indeed,careful analysis of the calculated PDOS indicated that thedangling or unpaired O 2p orbitals of the four NN tetrahedral O
sites (ONN
2p ) around each defect/vacancy center are responsible
for most (>75%) of the VZn-induced local magnetic moment(1.65 mB), as shown in the yellow areas of the bottom panel inFig. 6(a). To visualize the eﬀects of a defect on both electronic
and local structures, the spin-density distribution of a 72-atom
ZnO with one defect (VZn) is presented in Fig. 6(b), in which the
net spin density is represented as yellow isosurfaces near thedefect and/or vacancy center, and the Zn and O atoms areshown as blue and red spheres, respectively. The removal of a
Zn atom (VZn) from the ZnO results in four dangling or unpaired
O bonds. These dangling or unpaired bonds cause a signicant

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Nanoscale

Paper

Fig. 6 (a) Calculated TDOS (top panel) and PDOSs of Zn 3d (middle panel) and O 2p (bottom panel) of wurtzite ZnO with VZn (Zn35O36). The
TDOS of defect-free bulk ZnO (Zn36O36, indicated by dotted lines) is also shown in the top panel for comparison. (b) Calculated Jahn–Teller
distortion of the native defect VZn in a 72-atom supercell of wurtzite ZnO. The net spin density (deﬁned as the diﬀerence between the majority
spin density and the minority spin density) is indicated by yellow isosurfaces near the defect center. Zn and O atoms are depicted as blue and red
spheres, respectively. EF, denoted by the dashed line, is aligned to 0 eV.

Jahn–Teller distortion (displacing four NN O atoms outward
from their original wurtzite symmetrical sites and away fromthe defect and/or vacancy center), reducing the local magnetic
moment from the ideal value of 2 mB to 1.65 mB. Our calculated
result suggests a slightly larger value of the magnetic moment
than Wang et al. predicted for VZn on the surface of ZnO thinlms (1.5 mB),25 the additional NN coordination of O atoms
around the defect center in a bulk system in comparison with asurface conguration possibly the reason. Additionally, theformation of a VZn-induced local magnetic moment can bepredicted via Stoner's band theory,51,52 in which itinerant
ferromagnetism can be stabilized when the gain in exchange
energy is greater than the loss in kinetic energy of itinerant
electrons. According to the results based on calculations of theStoner parameter and TDOS at/near EF (Fig. S3 in the ESI†),
stable spontaneous ferromagnetism due to VZn in ZnO can beobtained. This calculation is consistent with the results of the O
K-edge XMCD and STXM-XANES presented above and thevalence-band SPEM presented in the ESI.† Indeed, suchprofound cation-vacancy-induced magnetism, associatedmainly with the spatial local character of the 2p states of the NN
anion sites around the defect and/or vacancy centers, has also
been found in other rst-row d0 semiconductors.53,54 However,
the anion vacancy, VO, does not contribute signicantly to the
local spin moment. The symmetry of the diﬀerent spin states ispreserved for VO in ZnO (Fig. S2 in the ESI;† the TDOS and PDOS
plots of the native VO in ZnO are shown in detail). Accordingly,
the origin of d0 magnetization in ZnO nanostructures is related
to the presence of dangling/unpaired O 2p states. Nonetheless,the existence of defects and/or vacancies does not suﬃce toinduce macroscopic magnetization. Stated otherwise, intrinsic
defects and/or vacancies may cause spin polarization but notnecessarily macroscopic magnetization. For example, the9174 | Nanoscale, 2014, 6, 9166–9176

defects and/or vacancies at Zn sites in ZnO NCs that were also
observed in the Zn L3,2-edge XEOL spectra, which do not exhibit
macroscopic magnetization behavior or a spin moment, originate from Zn d orbitals, as demonstrated by the null Zn L3,2edge XMCD that is presented in the inset in Fig. 5(b). Thisdiﬀerence in magnetism may arise from the fact that the population of defects and/or vacancies may not be suﬃcient toallow both a ferromagnetic interaction and a lack of preservation of long-range coupling between dangling or unpaired
bonds at Zn sites in ZnO.28,29 Again, the above conclusions are
based on calculations for a bulk ZnO system. For low-dimensional systems, such as ZnO NCs and NWs, the possibility that
more complicated orbital hybridizations may contribute to nonnegligible magnetic properties cannot be excluded.

Conclusion
In summary, the presence of more defects/vacancies in ZnO
NCs than in the NWs was observed and conrmed via EXAFS, PL
and XEOL measurements. The low temperature CBD method
used to fabricate NCs compared to NWs was attributed to theorigin of the higher saturation magnetization and defectconcentration in NCs. Specically, O K-edge STXM mappings
reveal that in ZnO NCs, the defects and/or vacancies werelocated primarily in the impurity or surface/edge (non-stoichiometric) region rather than in the thick (core) region; thelocal DOS at/near EF, as determined from the O K-edge STXMXANES and valence-band SPEM spectra of ZnO NCs, wasenhanced. The increased density of O 2p-derived states in ZnO
NCs plays an important role in inducing d0 magnetism. The
experimental XMCD results presented herein also demonstratethat the magnetic moments arise from the O 2p orbitals in ZnO
nanostructures rather than from Zn d orbitals. Theoretically,

This journal is © The Royal Society of Chemistry 2014

PaperLDA + U calculations further support the conclusion that thepresence of defects and/or vacancies or dangling/unpaired 2p
bonds at O sites around cation vacancy (VZn) centers is
responsible for d0 magnetism in ZnO.

Acknowledgements
The authors H. C. H., J. W. C. and W. F. P. acknowledge the
National Science Council of Taiwan for nancial support underContract No. NSC 102-2112-M-032-001. HCH also thanks NCTS
of Taiwan for support. The Canadian Light Source was supported by the Natural Sciences and Engineering ResearchCouncil of Canada, the National Research Council Canada, theCanadian Institutes of Health Research, the Province of Saskatchewan, Western Economic Diversication Canada, and the
University of Saskatchewan.

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