Journal of Physics and Chemistry of Solids 83 (2015) 47–51

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Journal of Physics and Chemistry of Solids
journal homepage: www.elsevier.com/locate/jpcs

Effect of microwave irradiation on the electronic structure of ZnO
Kyohei Yoshida a,n, Taro Sonobe b, Heishun Zen a, Kan Hachiya c, Kensuke Okumura c,
Kenta Mishima c, Motoharu Inukai c, Hani Negm c, Konstantin Torgasin c, Mohamed Omer c,
Ryota Kinjo d, Toshiteru Kii a, Kai Masuda a, Hideaki Ohgaki aaInstitute of Advanced Energy, Kyoto University, Gokasyo, Uji, Kyoto 611-0011, Japan
Kyoto University Research Administration Ofﬁce, Kyoto University, Yoshida-honmachi, Sakyo-ku, Kyoto 606-8501, Japanc
Graduate School of Energy Science, Kyoto University, Yoshida-honmachi, Sakyo-ku, Kyoto 606-8501, Japand
RIKEN SPring-8 Center, 1-1-1, Kouto, Sayo-Cho, Sayo-Gun, Hyogo 679-5148, Japanb

art ic l e i nf o

a b s t r a c t

Article history:
Received 11 November 2014
Received in revised form
19 February 2015
Accepted 13 March 2015
Available online 14 March 2015

Modifying the electronic structure may allow a bulk material to efﬁciently absorb radiation as well as
excite and emit atomic plasma. The interaction between microwaves and metal oxides is investigated byanalyzing the electronic structure of ZnO with and without microwave (MW) irradiation using absorption, photoluminescence (PL), and PL excitation (PLE) spectroscopies and utilizing an ultraviolet synchrotron light source. MW irradiation lowers the energy of the absorption edge of ZnO producing defects.
Additionally, MW irradiation causes a resonantly enhanced change in the intensity and peak shift of the
PL band. These phenomena indicate that the defects generated by MW irradiation change the electronic
structure of ZnO and the electron transition process.
& 2015 Elsevier Ltd. All rights reserved.

Keywords:
Electronic structure
Optical properties
Semiconductors
Inorganic compounds
Oxides

1. Introduction
Microwave (MW) heating is an advanced method for materials
processing due to its unique advantages such as rapid and selective heating compared to conventional methods [1]. To date, several approaches have been implemented. For example, MW heating in the liquid phase has enhanced the catalytic activity of ZnO
and MgO [2, 3]. It also can create ZnO architectures such as ZnO
nanorods, wire-like, or ﬂower-like structures [4, 5]. Additionally,
MW sintering has been reported to improve the electronic property of ZnO as a varistor for semiconductor application [6, 7, 8].
Sonobe et al. reported that the surface modiﬁcation of TiO2 forenhancement of catalytic activity and the direct reduction without
a reducing agent of TiO2 were available by MW treatment [9, 10].
Similarly, the enhancement of the photocatalytic activity of
PbMoO4 by MW irradiation has been reported [11]. Thus, MW
technology has been used in advanced materials processing to
improve the properties of functional materials.
MW heating of ZnO enhances the photocatalytic activity,
synthesis of ZnO architectures, improvement of the capability as avaristor or thin ﬁlm fabrication and so on [2, 6, 7, 8, 12, 13]. In
addition, the interaction between microwaves and ZnO has beennCorresponding author.
E-mail address: k-yoshi@iae.kyoto-u.ac.jp (K. Yoshida).

http://dx.doi.org/10.1016/j.jpcs.2015.03.013
0022-3697/& 2015 Elsevier Ltd. All rights reserved.

investigated [14]. It was also conﬁrmed that the MW irradiation
did not induce the change of the crystallinity of ZnO [13]. In aprevious study [14], a phenomenological model was constructedwithout exploring the detailed electronic structure in the bandgap
which was created by MW irradiation. A phenomenological modelis useful to design MW-induced Zn/O atomic plasma excitations
and emissions, or oxidation/reduction of bulk ZnO. The change inthe physical properties such as the electronic structure by MW
heating should be indispensable to elucidate this mechanism.
Moreover, the electronic structure is a signiﬁcant parameterwith respect to the capability of a functional material such as aphotocatalyst or an electronic device. Therefore, the effect of MW
irradiation on the electronic structure has been also investigated[1, 5, 15]. Previous research by our group [13] includes investigation of the changes in both the absorption and PL spectra with MW
irradiation. The observed emergence of bandgap absorption andthe change in the PL band suggest that MW-induced defects are
created. However, the correspondence between the enhanced PLband and the bandgap absorption was not conﬁrmed. Therefore,
details of the electronic structures with MW irradiation such as
the change in the electron transition process have yet to beinvestigated.
To apply MW heating technology for material processing, theobjective of this research is to clarify the effect of MW irradiationon the electronic structural change of ZnO.

K. Yoshida et al. / Journal of Physics and Chemistry of Solids 83 (2015) 47–51

This study employs absorption, photoluminescence (PL), and PL
excitation (PLE) spectroscopies to analyze the electronic structureof ZnO with MW irradiation. The absorption spectra provide details about the energy shift of the absorption edge of ZnO throughMW irradiation, while the PL spectra provide information not only
about the electronic structure of the material but also about thedefects or impurities. In addition, the PLE spectra help to understand the difference between the excitation mechanisms with andwithout MW irradiation as well as provide insight on the electronic structure and the electron transition process.

2. Material and methods
Commercially available ZnO powder (1 g) (Wako Pure Chemical
Industry,  5 μm 99.9%) was pressed into a rectangular pellet(27.0  6.7  1.5 mm3) and sintered in air by a furnace at 873 K for2 h before MW irradiation. Then the sample exposed to 500 W
MW irradiation for 10 min using reactor under vacuum conditions
with a vacuum pressure of about 1 Pa and a MW frequency of
2.45 GHz (wavelength: 124 mm). Details about MW irradiation arereported elsewhere [13].
The absorption spectra were acquired with a UV–vis–NIR
spectrometer (JASCO, V-670). BL7B beamline in UVSOR [16] wasused as the light source for PL and PLE spectroscopies. Fig. 1 depicts the experimental setup for PL and PLE spectroscopies. The
photon energy of the excitation light was varied between 2.92 eV(425 nm) and 3.54 eV (350 nm). A spectrometer (Acton Research
Corporation, Spectra Pro-300i) and a CCD cooled by liquid nitrogen(Princeton Instruments Inc, In/CCD-1340/100-EB/1) were used for
PL detection. The exposure time, measurement temperature, andenergy resolution of the excitation light were 10 s, room temperature, and 0.02 eV (FWHM) respectively.
In addition, PL spectroscopy with a He-Cd laser (wavelength:
325 nm (3.81 eV), power: 10 mW, beam diameter: 1.0 mm: Kimmon IK5451R-E) was conducted at low temperature (14 K) to investigate the band structure. The emission was observed by aspectrometer (Zolix, Omni-λ 300) with a CCD array (INTEVAC
Mosir 350) and an exposure time of 1 s.
All data shown in this paper was corrected from single sample.
To check the sample-to-sample variation of the phenomenon,
several samples were prepared and it was conﬁrmed that the same
phenomenon was observed in samples prepared in same
condition.

(α(hν)hν/S)2 [arb. unit]

48

Absorption spectrum(W/o MW irradiation)
Absorption spectrum(With MW irradiation)
Fitting line

0
3.0

3.1
3.2
3.3
Photon energy [eV]3.4

Fig. 2. (hνα /S )2 as a function of photon energy for the absorption edge of ZnO withand without MW irradiation. Lines are visual guides for hvα ∝ (hv-E’)1/2 and toevaluate the absorption threshold (E’¼ Eg for energy of the absorption edge). (Forinterpretation of the references to color in this ﬁgure legend, the reader is referredto the web version of this article).

the absorption edge. Fig. 2 shows the absorption coefﬁcient
spectra. These spectra were converted from the diffuse reﬂection
spectra using Kubelka–Munk formula [9, 17], which is expressed as

α/S = (1 − R)2 /2R.

(1)

Here, α is the absorption coefﬁcient, S is the scattering coefﬁcient, and R is the diffuse reﬂectance. S is assumed to be constant,which is usually reasonable around the absorption edge [9].
The energy of absorption edge of ZnO with or without MW
irradiation were calculated using as [18, 19](hνα)2 = C (hν − E g ).

(2)Here, hν is the photon energy (h is Planck's constant and ν isthe frequency of light), C is a constant, and Eg is the energy of theabsorption edge. The blue arrows in Fig. 2 indicate the energy of
the absorption edge determined from the absorption spectra. Theenergies of the absorption edges of ZnO without and with MW
irradiation are around 3.26 eV and 3.22 eV, respectively. The absorption edge energy of the sample with MW irradiation (3.22 eV)
is lower than that without MW irradiation (3.26 eV) (Phenomenon1), suggesting that MW irradiation reduces the bandgap energy orcreates new electronic states below the conduction band.
3.2. PL and PLE spectra at room temperature

3. Results and discussion
3.1. Absorption spectra
Absorption spectroscopy was conducted with an emphasis on

Fig. 1. Schematic of the setup for PL and PLE spectroscopies.

To conﬁrm the defect-level energy, PL spectroscopies were investigated as functions of excitation energy. Figs. 3 and 4 show thePL spectra for several excitation energies. The broad peaks between 2.70 eV and 3.50 eV are the excitation light and the sharp
peaks between 1.50 eV and 1.80 eV are inherently generated by
the second order diffraction of the excitation light on the gratingused to measure the PL spectra. The case without MW irradiation
contains broad peak at 2.20 eV and a band-edge emission at 3.20–
3.30 eV (Fig. 4). To conﬁrm that observed broad peaks are PL fromthe ZnO, a PL measurement with another material was conducted.
The measurement was conducted on commercially available TiO2
powder (Furuuchi Chemical Corporation, TIC-72208B,  300 M,
purity:99.99%) pressed into a rectangular pellet (27.0 
6.7  1.5 mm3) of the same form with ZnO pellet and sintered in
air by a furnace at 1273 K for 6 h. The crystal structure of the
prepared TiO2 pellet was rutile [20]. The TiO2 pellet was irradiatedthe photon of 350 nm with the exposure time of 10 s. The prepared
TiO2 sample is white color and barely shows the absorption invisible region [20]. As shown in Fig. 3(a), no peaks were observedin the case of TiO2 pellet. If the origin of the observed broad peak is

K. Yoshida et al. / Journal of Physics and Chemistry of Solids 83 (2015) 47–51

3.23 eV
3.35 eV

2.0

2.31 2.5
2.309

300
330
360
390
420
450
480
510
540
570
600

PL intensity[arb. unit]

600
500
400
300
1.5

49

3.0

3.1
1.5

2000
1500

3.2

3.2

2.20
2.0 2.195
2.5
Photon energy of PL [eV]

3.1
3.0

600
500
400
300

Intensity [arb.unit]

TiO2
ZnO without
MW irradiation

2500

3.229
3.23

2.31 eV

3000

3.3

3.3

2.20 eV

Photon energyof excitation light [eV]

3.351
3.35

Fig. 5. PLE spectra of ZnO sintered at 873 K with MW irradiation. (For interpretation of the references to color in this ﬁgure the reader is referred to the web versionof this article).

1000
500
1.70

1.75

1.80

1.85

Photon energy [eV]Intensity [arb. unit]

Fig. 3.
(a) PL spectra of ZnO sintered at 873 K without MW irradiation (b)The
intensity of the second order diffraction of the excitation light with 3.54 eV(350 nm) in the case of TiO2 and ZnO without MW irradiation.

1.5

Excitation energy
2.92 eV
3.01 eV
3.10 eV
3.20 eV
3.30 eV
3.42 eV
3.54 eV

2.0
2.5
3.0
Photon energy [eV]3.5

Fig. 4. PL spectra of ZnO sintered at 873 K with MW irradiation.

the measurement system such as the stray light of the excitation
light, the similar peak with higher intensity should be observed in
the case of TiO2 because the collected photon in spectrometer inthe case of TiO2 is larger than that of the case of ZnO as shown in
Fig. 3(b). These facts show that the observed broad peak is the PLfrom ZnO. As the result, the intensity of the broad peak at 2.20 eV
is nearly constant regardless of the excitation energy (Phenomenon 2). Thus, the transition process of ZnO is independent of theexcitation energy without MW irradiation.
However, in the case of the MW irradiation, the emission intensity of the broad peak around 2.40 eV strongly depends on the
excitation energy, and the highest emission intensity occurs withan excitation energy of 3.20 eV (Fig. 4). This phenomenon implies
that the electron transition process was changed when the excitation energy was changed.
PLE spectroscopy was conducted to understand the detailed
electronic structure and the electron transition process in thesample with MW irradiation. The upper panel and right panel inFig. 5 shows the horizontal cross-section in which the excitation
photons emerge at 3.23 eV and 3.35 eV, and the vertical crosssection whose photon energies in the PL are 2.31 eV and 2.20 eV in
the 2D graph, respectively. The small peak shift in the centralenergy in the broad PL peak from 2.31 eV (vertical red line) to
2.20 eV (vertical black line) occurs when the excitation energy ischanged from 3.23 eV to 3.35 eV. The broad peak (around 2.31 eV)
has the highest PL when the excitation energy is 3.23 eV. These
results show that the emission intensity of the broad peak depends on the excitation energy and that the maximum occurs at
3.23 eV (Phenomenon 3). The broad peak shifts from 2.31 eV to
2.20 eV as the excitation energy is elevated from 3.23 eV to 3.35 eV(Phenomenon 4).
The peak at 2.20–2.40 eV in ZnO is attributed to point defects
such as oxygen vacancies, zinc vacancies, zinc interstitials, oxygen
interstitials, and oxygen-antisites [22, 23, 24, 25, 26, 27, 28]. The
electron transition process changes as the excitation energy isswept and has a maximum of approximately 2.31 eV when the
excitation energy is 3.23 eV. Therefore, the electron is trappedstrongly at this energy level (3.23 eV). Because the excitation energy resolution in the PLE is sufﬁciently high (around 0.02 eV(FWHM)), the photon energy of the excitation light with a maximized PL intensity is coincident with the energy of the absorption
edge (3.22 eV) in Phenomenon 1. This fact supports the possibility
that MW irradiation creates a new energy state at 3.22 eV.
If Phenomenon 1 of the absorption spectra and Phenomenon
3 of the PLE spectra originate from the energy shift of the conduction band, the electron transition process due to relaxationfrom the conduction band would not shift. However, excitation
above 3.23 eV of the sample with MW irradiation actually shifts
the broad peak, indicating that the above assumption is incorrect.
Hence Phenomena 1, 3, and 4 imply that MW irradiation creates a
new energy state at 3.22 eV, which is below the conduction band.
3.3. PL spectra at low temperature
PL measurements using a He-Cd laser at low temperature wereconducted to understand the electronic structure near the bandgap energy. A band-edge emission with several peaks is observedin the specimen without MW irradiation (Fig. 6), and the center

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K. Yoshida et al. / Journal of Physics and Chemistry of Solids 83 (2015) 47–51

Intensity [arb. unit]

3.30 eV

3.23 eV
3.20 eV
3.16 eV
3.13 eV
3.09 eV
2.7

2.8

2.9

3.0

3.1

3.2

3.36 eV

3.3

3.4

3.5

Photon energy [eV]Fig. 6. PL spectra using a He-Cd laser (325 nm, 3.81 eV) at low temperature (14 K)
on the surface of ZnO without MW irradiation.

Intensity [arb. unit]

3.36 eV

2.7

optical phonon [29]. A phonon replica is observed in the PL
spectrum of a material with a high purity and a low defect density
because the PL spectrum of a material with low purity and highdefect density is dominated by the emission from the defect level[30]. Therefore, these results conﬁrm that the specimen without
MW irradiation has a high purity and low defect density.
On the other hand, the band-edge emission of the ZnO with
MW irradiation (Fig. 7) has two peaks with energies of 3.36 eV and
3.32 eV (Phenomenon 6), and a phonon replica is not observed incase of the PL spectrum with MW irradiation. The peak at 3.32 eV
is assigned as excitons bound to Zn vacancy [31]. Therefore, theseobservations demonstrate that the MW irradiation creates Zn-vacancy defects in ZnO.
Both specimens with and without MW irradiation show an
exciton emission at 3.36 eV (Figs. 6 and 7). Because the exciton
emission of each specimen is coincident with each other [32], MW
irradiation does not change the band structure outside of the
bandgap. Therefore, these results are also consistent with thecreation of a new energy state between the conduction and valance bands due to MW irradiation (Section 3.2).
3.4. Electronic structures of ZnO with and without MW irradiation

3.32 eV

2.8

2.9

3.0 3.1 3.2 3.3
Photon energy [eV]

3.4

3.5

Fig. 7. PL spectra using a He-Cd laser (325 nm, 3.81 eV) at low temperature (14 K)
on the surface of ZnO with MW irradiation.

energies of those peaks were 3.36 eV, 3.30 eV, 3.23 eV, 3.20 eV,
3.16 eV, 3.13 eV, and 3.09 eV (Phenomenon 5). The peaks appear inintervals of 0.07 eV (3.23 eV-3.16 eV) and about 0.10 eV(3.30 eV-3.20 eV-3.09 eV, or 3.23 eV-3.13 eV ). The peaks at
3.36 eV, 3.30 eV and 3.23 eV in Fig. 7 are assigned to the excitonbound to an ionized donor, a two-electron transition of an excitonbound to a neutral donor, and a donor to an acceptor pair transition (DAP), respectively [28]. This spectrum is a typical phonon
replica, and peaks appearing at 0.07 eV intervals have been attributed to the phonon replicas of the longitudinal optical mode,
while peaks appearing at 0.10 eV intervals have been reported as
the phonon replica induced by two phonons of the transverse

Fig. 8(a) and (b) shows the energy diagram with and without
MW irradiation at room temperature, respectively. The band at
2.20 eV in Fig. 3 reportedly originates from the defect level [22, 24,26]. In addition, the broad PL emission band, which is observed forall excitation energies (2.92–3.42 eV) is similar to the case for alarge Stokes-shift PL. Therefore, the PL band at 2.20 eV is a typicalthermal creation of the deep level with a large lattice relaxation,and the electron is trapped around 3 eV. The fact that the PL intensity is independent of the excitation energy suggests that thetrap level has a deﬁned width (Fig. 8(a)). Moreover, there is not anenergy difference in the band-edge emissions between the two
specimens at low temperature, suggesting that the bandgap energy at room temperature is 3.26 eV for both specimens.
On the other hand, for ZnO with MW irradiation, an additionalenergy state is created near the conduction band due to the defectlevel generation by MW irradiation, which may be attributed tothe plasma emission such as a zinc plasma and an oxygen plasmaby MW irradiation or to the thermal effect induced by MW heating. Because, in the previous reports [13, 14], the plasma emissionof zinc atom and oxygen atom was observed by microwave irradiation, and it shows that atoms of zinc and oxygen in ZnO areemitted from the sample. Therefore, this phenomenon shows thepossibility that defects are created by the evaporation of the zincatom and oxygen as plasma. In addition, the plasma emission

Fig. 8. Schematic of the ZnO energy diagram at room temperature: (a) without and (b) with MW irradiation.

K. Yoshida et al. / Journal of Physics and Chemistry of Solids 83 (2015) 47–51

shows that the sample was heated at high temperature over2000 °C because the temperature around 2000 °C is needed fordecomposition, which is required to observe the plasma [13]. It isreported that defects are created when the sintering temperature
is over 750 °C [33]. When the sample was partially heated over2000 °C by MW irradiation, temperature of the rest of the sample
is also elevated to enough high temperature for defect creation.
Therefore, as the second candidate of the cause of the defectcreation, it is considered that defects are created by the thermal
effect. However, additional investigations are required to clarify
the detailed mechanism of the defect generation.
Because the band of the peak at 2.31 eV in Fig 5 is also broad,the additional energy state by MW irradiation is also at a deeplevel with large lattice relaxation. In addition, the maximized intensity of the PL at a particular excitation energy (3.23 eV) and theabsorption edge at 3.22 eV suggest that a clear trap level is generated around 3.22 eV.
In the case of the sample with MW irradiation, two route of theelectron transition were considered (Fig 8(b)) In Route 1 an electron transition occurs via the trap level. In Route 2 an electrontransition occurs via a trap level at 3.22 eV. Because the peak of
the band shifts from 2.31 eV to 2.20 eV, it is possible that theprobability of the electron transition from the conduction band to
the trap level at 3.22 eV is lower than that from conduction bandto the trap level near 3 eV (i.e., once electron is excited to the
conduction band, Route 1 is favored to Route 2). These results
indicate that MW irradiation in ZnO creates a new energy stateand modiﬁes the electronic structure.

4. Conclusion
It was conﬁrmed that MW irradiation affected the bandgap
electronic structure of ZnO, through conventional absorption
spectroscopy, photoluminescence spectroscopy and photoluminescence excitation spectroscopy by an ultraviolet synchrotron light source. A new defect energy state slightly below(  0.04 eV) bandgap energy is created as a shallow energy level by
MW irradiation to ZnO. The new MW irradiation–induced defects
also alter the electron transition process. These ﬁndings indicatethat this type of bandgap should allow a higher conductivity,
which will enhance microwave-material interactions.

Acknowledgments
This work was partly supported by the “Energy Science in theAge of Global Warming” of Global Center of Excellence (G-COE)program (J-051). The PL and PLE measurements were supported by
the Use-of-UVSOR (BL7B 2012–2013) of the Institute for Molecular
Science, Japan. We would like to thank all the staff members of the
UVSOR facility. Part of this study was supported by a Grant-in-Aid

51

for Scientiﬁc Research (C) (23561030) from the Japan Society for
the Promotion of Science.

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