Two-band luminescence from an intrinsic defect in spherical and terraced MgOnanoparticles
Peter V. Pikhitsa, Changhyuk Kim, Sukbyung Chae, Seungha Shin, Sekwon Jung, Mamoru Kitaura, Shin-ichi
Kimura, Kazutoshi Fukui, and Mansoo Choi
Citation: Applied Physics Letters 106, 183106 (2015); doi: 10.1063/1.4918804
View online: http://dx.doi.org/10.1063/1.4918804
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APPLIED PHYSICS LETTERS 106, 183106 (2015)

Two-band luminescence from an intrinsic defect in spherical and terraced
MgO nanoparticles
Peter V. Pikhitsa,1,a),b) Changhyuk Kim,1,b) Sukbyung Chae,1 Seungha Shin,1
Sekwon Jung,1 Mamoru Kitaura,2 Shin-ichi Kimura,3 Kazutoshi Fukui,4 and Mansoo Choi1,a)1
Global Frontier Center for Multiscale Energy Systems, Division of WCU Multiscale Mechanical Design,
School of Mechanical and Aerospace Engineering, Seoul National University, Seoul 151-744, South Korea
2
Deparment of Physics, Faculty of Science, Yamagata University, Kojirakawa 1-4-12, Yamagata 990-8560,
Japan
3
UVSOR Facility, Institute for Molecular Science and the Graduate University for Advanced Studies,
Okazaki 444-8585, Japan
4
Department of Electrical and Electronics Engineering, Faculty of Engineering, University of Fukui,
Fukui 910-8507, Japan(Received 28 January 2015; accepted 10 April 2015; published online 5 May 2015)
Luminescent defect centers in wide bandgap materials such as MgO are of great interest forscience and technology. Magnesium oxide nanocubes obtained by the self-combustion of Mg metal
have long exhibited only a broad 2.9 eV cathodoluminescence band owing to oxygen vacancies(F centers). However, in this work, a room-temperature ultraviolet 4.8 eV cathodoluminescenceband has been observed coincident with a 2.5 eV band of the same intensity from an unexploredintrinsic defect in MgO terraced nanocubes and nanospheres produced from Mg metal combustionin an H2/O2 flame. Synchrotron radiation excitation spectra reveal that the excitation energy at theonset of both bands is just above the bandgap energy of 7.7 eV, where electrons and holes are
generated. We determine that a defect, responsible for both emission bands, creates proximal
anion-cation vacancy pairs named P centers that may appear instead of F centers because of
C 2015 AIP Publishing LLC.
changes in the MgO nanoparticle growth conditions. V[http://dx.doi.org/10.1063/1.4918804]

It has long been known that defects determine the optical properties of bandgap crystalline materials interesting for
applications. Wide bandgap rock salt cubic crystals such as
NaCl and MgO are perfect platforms to study Schottky
defects, which are anion (cation) vacancies called F (V) centers possessing various charge states with trapped electrons(holes). In monovalent alkali halides, the color centers andthe pairs of vacancies are understood well enough that the Fcenters are being used for lasing.1 However, in divalent
MgO, there is still a controversy about these defects concerning the position of their energy states within the bandgap andtheir interaction with the excitation.2–5 Recent calculations6
have provided a description of F centers in MgO that agree
perfectly with the experimental absorption measurements ofFþ and F0 charge states. Much less is known about the close
pairs of anion-cation vacancies when the entire MgO molecule is absent from the lattice (so-called P center3), although
ab initio calculations have shown that, among complex
Schottky and Frenkel defects, the P center possesses thelowest formation energy.3,7 Its electronic state has also been
calculated3,8 and an attempt to experimentally observe thisand other color centers on the surface of MgO has beenmade.9The bulk MgO F centers are active in cathodoluminescence (CL) with the characteristic broad emission bands ofF0 (520 nm) and Fþ (390 nm). Ultraviolet (UV) CL hasa)Authors to whom correspondence should be addressed. Electronic addresses:
peter@snu.ac.kr and mchoi@snu.ac.kr.
b)
P. V. Pikhitsa and C. Kim contributed equally to this work.
0003-6951/2015/106(18)/183106/5/$30.00

been also observed, though mostly at low temperatures,
induced by aliovalent impurities such as Al3þ and Si4þ
where all of the UV bands are strictly above 5 eV.10,11 At thesame time, aliovalent impurities are known to suppress thecreation of F centers so that these observed UV impurity
bands are followed by suppressed F center bands. At an evenlower UV energy of 4.8 eV, the CL that has been claimed asoriginating from V centers has been observed in electron/
X-ray-irradiated MgO2,12,13 but has remained elusive. The
difficulty has been that only a single broad and featureless
UV band is typically observed, so the observation of morebands originating from the same defect would be of greatuse, whereupon the UV band at 4.8 eV would find its placein the line graph of Fig. 5 from Ref. 11 that contains theenergies of trapping hole centers, such as VAl, [Li]0, and[Be]þ, becoming the ending point of the line with presumably the highest thermal decay temperature that exceeds
500 K. Note that a previous study14 has reported 4.8 eV UV
CL accompanied by 2.5 eV CL from pure MgO nanoparticles, which were intended to be applied in fluorescenceenhancement layers in liquid crystal displays, although theorigin and physics of the CL spectra were not discussed
therein.
Here, we report two-band room-temperature CL andlow-temperature synchrotron radiation (SR) spectra from
MgO nanoparticles, prepared in an H2/O2 flame or underCO2 laser irradiation, and discuss the possible origin of thetwo bands. Both bands are found to arise in the SR spectra
simultaneously when the excitation energy exceeds 7.7 eV ofthe bandgap. We attribute the room-temperature UV

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(260 nm: 4.8 eV) CL band along with the concurrent 2.5 eVemission band (Fig. 1(c)) to a robust bandgap defect, which
is presumably the P center. We find that Mg-metal combustion that is modified either by an oxy-hydrogen flame15(Fig. 1(a)) or by a 10.56 lm continuous wave (cw) CO2 laser(Fig. 1(b)) creates terraced and spherical MgO nanoparticles(UV-MgO) that contain the defect responsible for both bands
but which are devoid of Fþ centers. On the contrary, MgO
nanoparticles generated in Mg-metal combustion (in a drygas atmosphere)16 or under insufficient modification of the
combustion process by an H2/O2 flame or CO2 laser irradiation, exhibit only Fþ (420 nm: 2.9 eV) CL (Fig. 1(d)) or a
SR band instead of the two-band CL/SR (non-UV-MgO).
Therefore, we link the pronounced nanoparticle morphology
change to the change of the defect that, in turn, is responsible
for the luminescence.
Let us show that both bands have the common origin ofa bulk intrinsic defect. To exclude any participation of
extrinsic defects, the possible effect of an impurity upon the
two-band emission has been thoroughly examined, because
UV CL strictly above 5 eV has been previously found for
Al-, Be-, or Si-doped MgO crystals or powders.10,11 No correlation can be found between the existence either of thetwo-band CL or the Fþ (2.9 eV) CL and the impurity levels(supplementary material,17 Table SI) in our samples, and the
X-ray diffraction (XRD) spectra also confirm the perfectcubic structure of the nanoparticles (Fig. S1). There is theseparate question of the role of hydrogen and hydroxyls in
the luminescence for the UV-MgO nanoparticles produced
inside the H2/O2 flame. Although we successfully generate
UV-MgO nanoparticles without an H2/O2 flame by modifying the Mg metal combustion in dry air or oxygen using a cw
CO2 laser, a previous study has demonstrated that anadditional weak 2.9 eV band may arise from hydrogen at

Appl. Phys. Lett. 106, 183106 (2015)

low-coordination (LC) sites of 3–10 nm MgO cubes.18
Additionally, Fourier transform infrared (FT-IR) vibrational
spectroscopy shows the presence of an OH vibrational band
located around 3500 cm1 (Fig. S2). However, this weakband is quite similar for both UV and non-UV-MgO nanoparticles and is an unlikely cause for the dramatic differencein the CL spectra. To prove conclusively that hydroxyls or
surface defects play no role in producing these bands, wesoak the samples in water for several hours along with sonication whereupon, in spite of a considerable reshaping of the
MgO nanoparticles, the results confirm that there is nochange in the shape of CL spectra for both the UV and nonUV CL (Fig. S3). Some reduction in the CL intensity arises
from the appearance of hexagonal Mg(OH)2 (brucite) that isclearly seen from XRD data as well as from transmission
electron microscopy (TEM) images of the water-etched
nanoparticles (Fig. S4). Therefore, neither the presence ofhydrogen nor hydroxyls nor the dramatic reconstruction of
the surface and its subsequent defects can modify or destroy
the robust bulk defects responsible for the UV CL fromUV-MgO.
Room-temperature CL spectra at 3–30 keV acceleration
voltages from a scanning electron microscope (SEM-CL)(Gatan MonoCL3, Japan) are given in Figs. 1(c), 1(d), and 2.
The UV-MgO obtained from the oxy-hydrogen diffusion
flame or from CO2 laser irradiation exhibit a 4.8 eV UV band
and a 2.5 eV band in CL (Fig. 1(c)), together with several
other distinct bands located at 7.7 eV arising from band-toband transitions (Fig. S5) and at 1.65 eV arising from Mn
impurities (Figs. 1 and 2). The MgO cubes (inset in
Fig. 1(d)) obtained from self-burning in air, however, do notexhibit the two 4.8 and 2.5 eV bands, but instead exhibit a
2.9 eV band from the Fþ center with the Mn impurity bandof 1.65 eV (Fig. 1(d)). For the smallest 5 nm-diameter

FIG. 1. MgO nanoparticles obtained in
Mg combustion in an H2/O2 flame orunder CO2 laser irradiation and the CL
emanating from them. Schematic of asection of the experimental set-up with(a) the oxy-hydrogen flame (inset
shows a TEM image of terraced nanoparticles) or (b) a continuous wave
CO2 laser. (c) Two-band CL fromspherical MgO nanoparticles (inset)
obtained with oxy-hydrogen flame.
(d) The CL from MgO nanocubes(inset) obtained by Mg combustion in
air. (Insets) Transmission electron
microscope images of the samples.
Scale bars represent (a) 50 nm, (c)100 nm, and (d) 200 nm.

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Appl. Phys. Lett. 106, 183106 (2015)

FIG. 2. Intensity, spatial and depth-dependent correlations of the CL from MgO nanoparticles. (a) Correlation between the integrated intensities of the 4.8 and2.5 eV CL bands. The straight line shows the direct proportionality law. (b) CL spot image at 4.8 eV. (Inset) Scanning electron microscope image of the same
spot. (c) CL spot image at 2.9 eV. (d) CL spot image at 2.5 eV. (b) and (d) show good coincidence. (e)–(g) An acceleration voltage sweep from 3 to 15 kV
supports the correlation (the intensities of 4.8 and 2.5 eV bands approach 1:1 ratio at voltages >5 kV) and bulk nature (the intensities dominating at
voltages >5 kV) of the two-band CL when compared with the tendency of the LC surface defect band to decrease in proportion. (g) Normalized spectra of (f)based upon the peak intensity of the 2.5 eV band. Notice that only at the lowest acceleration voltages of 3 and 5 keV, which mostly excite the near-surface andsurface defects, the LC band increases at the cost of the 4.8 eV UV band, thus slightly violating the law of proportionality between the 4.8 and 2.5 eV bands.

nanoparticles separated by centrifugation and sedimentation,
additional bands can be seen at around 3.2–3.3 eV (Figs. 2(f)and 2(g)), which come from abundant LC oxygen defect centers (e.g., corners of terraces and cubes)19 together withenhanced Mn impurity peaks. The increase of the LC bandin the smaller nanoparticles demonstrates that it is a surface
CL band. Meanwhile, the other bands remain unalteredbecause they are bulk in origin, excepting the Mn2þ band,
which may indicate that the impurities tend to come to thesurface. Importantly, a strong correlation between the 2.5
and 4.8 eV bands from UV-MgO is observed for all samples
obtained with different carrier gases (O2 or Ar) and for different CL spot positions (Fig. 2(a)), which indicates that the
two CL bands originate from the same defect.
Spatially resolved CL supports the division of the
MgO nanoparticles into two types. The UV-emitting nanoparticles exclusively show both 4.8 and 2.5 eV bands,
where the band intensities at 4.8 and 2.5 eV from different
UV nanoparticles statistically follow the proportionality
law I(4.8 eV) ¼ I(2.5 eV), as shown in Fig. 2(a). On theother hand, individual non-UV nanoparticles show only
the Fþ (2.9 eV) band. Spatially resolved CL images in
Figs. 2(b)–2(d) and S6(a)–S6(c) clearly show a perfectspatial correlation of the 4.8 and 2.5 eV bands. Indeed, thespot image in Fig. 2(b) and the area image in Fig. S6(a)for the UV band show an overlap with the corresponding
images for 2.5 eV in Figs. 2(d) and S6(c), while theimages for 2.9 eV do not overlap with either the UV or
2.5 eV emitting sites. These observations provide proof
that the two-band CL comes from a specific type of nanoparticle that can be created only under combustion in an
H2/O2 flame or under CO2 laser irradiation.
The SEM-CL acceleration voltage is swept from
3–15 kV to vary the penetration depth of the electron beamfrom tens of nm to a lm, and the correlation between the
two CL bands is found to be preserved, while the band intensities increase with the penetration depth (Figs. 2(e) and2(f)). At small penetration depths and at low voltages, thesurface LC defect contribution is comparable to the two-band CL (Fig. 2(e) and the normalized spectra in Fig. 2(g)with the somewhat reduced 4.8 eV band); but at higher voltages (>5 kV) with penetration into the bulk, the two correlated CL bands dominate and their ratio comes to 1:1 as
dictated by the correlation law that we established (Figs. 2(e)and 2(g)). Meanwhile, the LC defect band, which is a surface
defect band, grows much more slowly with the increase of
the acceleration voltage and penetration depth, thereby proving that the two-band CL originate from centers in the bulk
and that the surface defects can be rejected as a possible
cause of the two-band CL.
The most clarifying experiment to confirm the common
origin of the two bands and to determine why the electron
beam in CL is so efficient for their excitation is accomplished by measuring the photoluminescence (PL) under SR,
carried out at the visible–vacuum UV (VIS-VUV) luminescence beamline BL3B of UVSOR-III in Japan. Figure 3
compares the PL spectra, taken at 8 K, for photon excitationenergies swept through the forbidden band and up to 10 eVfrom the UV-MgO (Fig. 3(a)) and the non-UV-MgO cubes(Fig. 3(b)). It is clear that the two PL bands observed in CL
arise simultaneously at 7.7 eV and persist as the excitationenergy sweeps deeper into the conduction band (Fig. 3(a)).
This is a strong indication that both bands (marked by red
circles in Fig. 3(a)) have the same origin and that electrons
and holes from the conduction band are solely responsible
and necessary for the excitation of the two bands, similar to
the electron and hole “shower” in CL, when it is understoodthat a photon of 8 eV leads to the formation in MgO of a conduction electron and a hole that do not undergo transformation into a self-trapped state. Nothing comparable happens in
the non-UV-MgO cubes devoid of the defect needed for thetwo band PL excitation (Fig. 3(b)), wherein only an intracenter excitation at 6 eV manifests itself in a 420 nm emission band. The SR experiment can be compared with that
performed on doped and undoped MgO nanoparticles20
where, although 212 and 235 nm emission bands havealways been observed from doped MgO nanoparticles, noconcurrent 490 nm band has been excited above 7.7 eV,This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP:
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Appl. Phys. Lett. 106, 183106 (2015)

FIG. 3. Photoluminescence spectra vs.
excitation energy from synchrotron
radiation (intensity grows from blue tored). (a) Red circles marking correlated
emission bands at 4.8 eV (upper) and
2.5 eV (lower) from UV-emitting
MgO. The Fþ band at 420 nm is fromthe non-UV-MgO present in the powder sample. (b) Emission from nonUV-MgO cubes, where the Fþ band at
420 nm is present but the 4.8 and2.5 eV bands are absent. (c) CL spectrum for UV-MgO taken at 79 K (blue)
and the deconvoluted spectral peaks(red and green curves). The additional
two-band spectrum (green curve) is
compared with (d) the two-band synchrotron radiation spectrum (not corrected for intensity magnitude) excited
at 8 eV and 8 K taken from (a).

which indicates the similar but different nature of the defectresponsible for the two-band luminescence that is reported
here. At the same time, a 400 nm band has been observed20
but only with a sub-band excitation, as we also observe for
MgO cubes. This certainly indicates that our cubic non-UVMgO nanoparticles behave like undoped MgO crystals.20
Meanwhile, our spherical and terraced nanoparticles exhibit
the two bands above 7.7 eV, manifesting the excitation ofsome peculiar defect by the separate electrons and holes.
The phenomenon closely resembles KI crystals where theirradiation of the crystal by 7.7 or 13 eV photons, which create separate electrons and holes, causes the appearance of
two emission bands at 4.15 and 3.31 eV.21 These singlet andtriplet components of the self-trapped exciton emission arise
owing to the recombination of electrons with relaxed selftrapped holes. Excitations lower than the bandgap width produce only a single band.21
The P center concept may account for the 4.8 and2.5 eV two-band CL/SR and explain why it has not previously been detected in spite of considerable efforts to find a
UV CL from pure MgO.21,22 The two-band emission may beviewed in this way: instead of any aliovalent chargecompensating substitution impurity, the V  center neighbors with the Fþ center (thus constituting the P center) andemits the lowest energy of 4.8 eV, ending up the line ofemission band energies.11 Meanwhile, the presence of theattached Fþ center, screened by the nearby V  center,reveals itself in the 2.5 eV concurrent emission with a wavelength of 490 nm, similar to 520 nm of the neutral F0 centerin bulk MgO. However, to create the MgO P center
di-vacancy (which is a threshold phenomenon3) it is necessary to generate the MgO using an extreme method which
can produce defects quite different from the F centers that
result from self-combustion in dry air.
The discovery of two-band room-temperature CL fromspherical and terraced MgO nanoparticles may find applications in defect-center lasers based upon MgO and in spectral
transformers.21 The two-band CL may also have applicationsin the geosciences because its presence or absence should be
useful to distinguish the history of MgO formation in the
Earth’s mantle or in planetary nebula.
This work was accomplished with financial support
from the Global Frontier Center for Multiscale Energy
Systems (2011-0031561) supported by the Korean Ministryof Science and Technology. We thank Daegyu Kim forassistance with the CO2 laser experiment. Financial support
from the BK21 program and WCU (World Class University)
multiscale mechanical design program (R31-2008-00010083-0) through the Korea Research Foundation is
gratefully acknowledged. A part of this work was supportedby the use-of-UVSOR Facility Program (BL3B, Proposal
No. 25-T502) of the Institute for Molecular Science.
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