DSpace at VNU: Photoluminescence properties of Co-doped ZnO nanorods synthesized by hydrothermal method

The PL spectra at low temperatures exhibit a group of ultraviolet narrow lines in the near-band-edge region of 3.0–3.4 eV and a very broad band peaked at 3.20 eV.. The origin of the near

Photoluminescence properties of Co-doped ZnO nanorods synthesized by hydrothermal method

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IOP P UBLISHING J OURNAL OF P HYSICS D: A PPLIED P HYSICS

Photoluminescence properties of

Co-doped ZnO nanorods synthesized

by hydrothermal method

Trinh Thi Loan, Nguyen Ngoc Long1and Le Hong Ha

Faculty of Physics, Hanoi University of Science, 334 Nguyen Trai Road, Thanh Xuan District, Hanoi,

Vietnam

E-mail:longnn@vnu.edu.vn

Received 3 November 2008, in final form 5 January 2009

Published 26 February 2009

Online atstacks.iop.org/JPhysD/42/065412

Abstract

Cobalt doped zinc oxide nanorods Zn1−xCox O (x = 0.01, 0.10) have been synthesized by a

hydrothermal process with Zn(NO3)2, Co(NO3)2, NH4OH, CO(NH2)2and C2H5OH at 150◦C

for 1 h X-ray diffraction and scanning electron microscopy were used to characterize the

crystalline structure, size and morphology of the samples The photoluminescence (PL) and

the PL excitation spectra of the nanorods were measured in the range of temperature from 15 K

to room temperature The PL spectra at low temperatures exhibit a group of ultraviolet narrow

lines in the near-band-edge region of 3.0–3.4 eV and a very broad band peaked at 3.20 eV The

origin of the near-band-edge PL is interpreted as an emission from free excitons, neutral

donor-bound excitons, radiative transitions from a donor to the valence band and

donor–acceptor pairs In particular, a group of emission lines in the red region of 1.8–1.9 eV

have been revealed These emission lines were assigned to the radiative transitions within the

tetrahedral Co2+ions in the ZnO host crystal

(Some figures in this article are in colour only in the electronic version)

1 Introduction

One-dimensional (1D) nanostructures including nanowires,

nanorods, nanobelts and nanotubes have attracted a great deal

of interest not only because of their basic scientific richness but

also for their potential application in electronic, optoelectronic,

electrochemical and electromechanical nanodevices [1] In

the past few years, considerable effort has been devoted to

developing various 1D semiconductor nanostructures Many

nanostructures based on various metal oxides, III–V and II–VI

compound semiconductors were synthesized

Zinc oxide (ZnO) is recognized as a promising material

for photonics and optoelectronics because of its wide

band gap Eg of 3.37 eV and a large exciton binding

energy of 60 meV Furthermore, ZnO is bio-safe and

biocompatible and may be used for biomedical applications

without coating [2,3] Many methods have been used to

synthesize nanostructured ZnO: vapour-phase transport [4,5],

1 Author to whom any correspondence should be addressed.

chemical vapour deposition [6], metal-organic chemical vapour deposition [7,8] and hydrothermal methods [9,10] Recently, 3d transition-metal elements (Co, Ni, Mn and Cu) have been alloyed with ZnO and their properties have been investigated [11–15] Due to their spin-transport properties, transition-metal doped ZnO is a diluted magnetic semiconductor and has attracted much attention because of the possibility of its application in spintronic devices It

is known that knowledge of the electronic structure of the dopants may enhance the understanding of the mechanisms inducing high-temperature ferromagnetism; therefore, optical properties such as absorption and photoluminescence (PL) have been of interest to many researchers However, in the existing literature, most of the papers deal with the absorption properties of Co-doped ZnO; meanwhile, there are few papers dealing with the PL properties of this material [11,16]

In this study, we have synthesized cobalt doped zinc oxide nanorods Zn1−xCox O (x = 0.01, 0.10) by a

hydrothermal process The x-ray diffraction (XRD) and the scanning electron microscopy (SEM) techniques were used

J Phys D: Appl Phys 42 (2009) 065412 T T Loan et al

to characterize the crystalline structure, the size and the

morphology of the samples Our present studies were focused

on the PL and the PLE spectra of the Zn1−xCoxO nanorods

In the PL spectra at low temperatures apart from a group of

lines related to the near-band-edge emission, in particular, a

group of emission lines related to radiative transitions within

the tetrahedral Co2+ions in the ZnO host crystal were observed

and analysed

2 Experimental

2.1 Materials

All the chemicals used in our experiment, including zinc nitrate

Zn(NO3)2·6H2O, CO(NH2)2, cobalt nitrate Co(NO3)2, sodium

hydroxide NaOH and ethanol C2H5OH, are of analytic grade

without further purification

2.2 Synthesis of Zn1−xCo x O nanorods

The samples of Zn1−xCox O (x = 0.01, 0.10) have been

synthesized under hydrothermal conditions from Zn(NO3)2,

Co(NO3)2 with molar proportions (1− x) : x The synthesis

process of the samples is as follows: 3.00 g Zn(NO3)2· 6H2O

and 0.65 g CO(NH2)2were completely dissolved in 175 ml of

double distilled water, forming a transparent solution Then,

to this solution, 5.1 ml of 0.02 M Co(NO3)2 solution for the

sample with x = 0.01 or 5.1 ml of 0.2 M Co(NO3)2 solution

for the sample with x = 0.10 was added, followed by

steady stirring for 30 min An appropriate quantity of 10 M

solution of NaOH and then an appropriate quantity of pure

alcohol C2H5OH were added into the last solution, followed by

continuous stirring for another 30 min The above-mentioned

solution mixture was placed in a sealed Teflon-lined autoclave,

which was heated to 150◦C and maintained at that temperature

for 30 min Then the temperature of the autoclave was raised

to 200◦C and kept constant for 5 min Finally, the temperature

was reduced to 150◦C and kept constant for 25 min After

that process, a precipitated product of navy blue colour was

obtained The products with x = 0.10 are a darker blue than

those with x = 0.01.

2.3 Characterization of the samples

The crystal structure of the samples was analysed by using

an x-ray diffractometer (SIEMENS D5005, Bruker, Germany)

with Cu-Kα (λ = 0.154 056 nm) irradiation The morphology

of the samples was characterized by using a scanning electron

microscope (JSM 5410 LV, JEOL, Japan) Diffuse reflection

spectroscopy measurements were carried out on a

UV-VIS-NIR Cary 5G spectrophotometer Spectra were recorded at

room temperature Transmission spectra of the samples were

obtained from the diffuse reflectance values by using the

spectrophotometer software The PL and the PLE spectra

measured in the range of temperatures from 15 up to 300 K

were carried out on a spectrofluorometer (Fluorolog FL

3-22 Jobin Yvon Spex, USA) with a 450 W xenon lamp as an

excitation source

2 θ (º)

Figure 1 Typical XRD patterns for the samples of Zn1−xCoxO with

x = 0.01, 0.10.

3 Results and discussion

3.1 Structure characterization and morphology

Typical XRD patterns for the samples of Zn1−xCoxO with

x = 0.01 and 0.10 are shown in figure1, where the diffraction peaks corresponding to the (1 0 0), (0 0 2), (1 0 1), (1 0 2), (1 1 0), (1 0 3), (2 0 0), (1 1 2) and (2 0 1) diffraction planes can

be seen All the peaks in the XRD patterns clearly indicate that the Zn1−xCoxO samples possess hexagonal wurtzite crystal structure No other diffraction peaks are detected except for the ZnO related peaks These results are in agreement with those

of other authors [14,15], who showed that no additional phase

was observed in the samples with the doping level x below 0.10.

A small amount of the CoO phase was detected in the samples

when the doping level x reached 0.15 [15] Furthermore, from figure1, it can be noted that the positions of all the diffraction peaks of the sample of Zn1−xCox O with x = 0.10 are shifted towards those larger 2θ angle in comparison with that in the sample with x = 0.01 The lattice constants determined from the XRD patterns are a1 = 3.2476 Å, c1 = 5.2062 Å and

a2= 3.2247 Å, c2= 5.1664 Å for the samples of Zn1−xCoxO

with x = 0.01, 0.10, respectively.

The lattice constants in the ZnO bulk crystal are a =

3.2498 Å, c = 5.2066 Å [17] As compared with the lattice constants of the ZnO bulk crystal, we reveal that the lattice constants for the sample of Zn1−xCox O with x = 0.10 are decreased remarkably (a − a2 = 0.0251 Å, b − b2 =

0.0402 Å) The reason for this is explained as follows: when

ZnO crystals were doped with cobalt, the Co2+ions substituted for Zn2+ ions [13–16] The effective ionic radius of Co2+ in the tetrahedral configuration (0.58 Å) is slightly smaller than that of Zn2+ (0.60 Å) [18]; the ionic radius of Co2+ in the 12-coordinated metal configuration (1.25 Å) is smaller than that of Zn2+ (1.39 Å) [19] Therefore, when Zn2+ ions are substituted by Co2+ions, the lattice is shrunk so that the lattice constants are decreased

Figure2 shows typical SEM images of the samples of

Zn1−xCox O with x = 0.01, 0.10 For the sample with

2

J Phys D: Appl Phys 42 (2009) 065412 T T Loan et al

Figure 2 Typical SEM images of the samples of Zn1−xCoxO:

(a) flower-like nanostructures in the samples with x = 0.01 and

(b) nanorods in the samples with x = 0.10.

x = 0.01, two kinds of flower-like Zn1−xCoxO are formed

(figure2(a)) One is large scale, which is composed of

needle-like rods with average diameters of∼800 nm and lengths of

∼4 µm, and another is small scale, which is composed of

thin rods with average diameters and lengths of ∼170 nm

and ∼2 µm, respectively Meanwhile, for the sample with

x = 0.10, rods with various diameters in the range from 100

to 250 nm are arranged chaotically (figure2(b)).

3.2 Absorbance and PL properties

Figure3shows the optical transmission spectra of Zn1−xCoxO

with x = 0.001 and x = 0.01 measured at room temperature.

Unlike the samples with x = 0.001, for the samples with

x = 0.01, apart from an absorption edge (∼3.22 eV), which

corresponds to the band gap Egfor the host ZnO material, a

group of absorption bands at around 2 eV are observed The

absorption bands at about 1.907 eV, 2.035 eV and 2.189 eV

are assigned to4A2(4F)→2E(2G),4A2(4F)→4T1(4P) and

4A2(4F)→2A1(2G) transitions, respectively, in the

high-spin state of the tetrahedrally coordinated Co2+ (3d7) ions

substituting Zn2+ions [20]

The PL spectra of the Zn1−xCoxO rods were measured

in the temperature range from 15 K to room temperature

under various excitation wavelengths The PL spectra for the

Zn1−xCox O rods with x = 0.01, 0.10, basically exhibited the

same behaviour The emission spectra at low temperatures

were composed of two groups of lines, one in the ultraviolet

Figure 3 Room temperature optical transmittance spectra of the

samples of Zn1−xCox O: (a) x = 0.001 and (b) x = 0.01.

Figure 4 PL spectrum of the Zn1−xCox O nanorods with x = 0.10

at a temperature of 50 K, excited with the wavelength of 325 nm of a xenon lamp The open circles represent the experimental PL spectrum, while the solid line shows the calculated spectrum Individual contributions are represented by the dashed lines

(UV) region (3.4–3.2 eV) and the other in the red region (1.9–1.8 eV)

The group of UV lines in both the samples of Zn1−xCoxO

with x = 0.01 and x = 0.10 shows the same characteristics.

Figure 4 represents the PL spectrum of the Zn1−xCoxO

nanorods with x = 0.10 at a temperature of 50 K, excited

with the wavelength of 325 nm (3.815 eV) In figure 4 the fitted result of the PL spectrum is shown, where the open circles represent the experimental PL spectrum, while the solid line shows the calculated result The experimental PL spectrum can be analysed into several lines, whose individual contributions are represented by the dashed lines As seen from figure4, the UV emission can be assigned to the free excitons (denoted by XA) at 3.376 eV, the excitons bound

to neutral donor (D◦X) at 3.367 eV, the recombination of electrons bound on a donor with free holes in the valence band (BF) at 3.314 eV, its longitudinal optical (LO) phonon

J Phys D: Appl Phys 42 (2009) 065412 T T Loan et al

Figure 5 PL spectra of the samples of Zn1−xCox O with x = 0.10

at different temperatures in the range 15–300 K, excited with the

325 nm light of a xenon lamp

Figure 6 PL spectra of the samples of Zn1−xCox O with x = 0.10

at 50, 75, 100 and 135 K The spectra exhibit the appearance of

the shoulders due to emission of free excitons A(XA)

replica (BF-1LO) at 3.243 eV and donor–acceptor pair (DAP)

emission at 3.139 eV (DAP1) and at 3.010 eV (DAP2)

In order to investigate the origins of emission lines, we

have measured the PL spectra of the sample of Zn1−xCoxO in

the temperature range from 15 to 300 K (figures5and6) In

the spectrum at 15 K, the D◦X bound exciton line dominated,

Figure 7 The temperature dependence of the peak energies for the

XAemission (solid squares), the D◦X emission (reverse solid triangles), BF emission (solid circles) and the BF-1LO emission (solid triangles) Solid lines are calculated using Varshni’s formula

The dotted line is calculated using the formula hν = Eg− ED+ kBT

peaked at 3.370 eV and had a full width at half-maximum of

18 meV As can be seen from figures5and6, the intensity of the

D◦X line is decreased rapidly and its position is slightly shifted

to the low-energy side with increasing measuring temperature

At 100 K, this line becomes much weaker than the BF line

It is also evident from figures5and6that beginning from

50 K a shoulder with an energy value of 3.376 eV appears at the high-energy side of the D◦X line The position of the shoulder

is shifted to the low-energy side with increasing measuring temperature This shoulder is still maintained up to 250 K and

is mixed with the BF line at higher temperatures This shoulder

is interpreted to originate from radiative recombination of free excitons (XA) Indeed, as the temperature is increased, the thermal activation energy is enough for the release of excitons from the neutral donor (D◦X → D◦ + X), then radiative

transitions take place via states of the free excitons

Assuming that the peak position of the free exciton and the donor-bound exciton emission varies with temperature as the energy band gap, we tried to fit the observed temperature dependence to Varshni’s semiempirical formula [21]:

E(T ) = E(0) − αT2

T + β , where E(0), α and β are fitting parameters As can be seen in

figure7, the experimental values for the XAline and the D◦X line fit rather well to Varshni’s curve with fitting parameters:

α = −4.5 × 10−4eV K−1, β = −650 K and E(0) = 3.380 eV

and 3.370 eV for XA and D◦X lines, respectively It can

be noted that Varshni’s formula is an empirical one The

coefficients α and β are fitting parameters In some cases, β

is supposed to be related to the Debye temperature However,

in a number of cases (for example, diamond, 6H–SiC [21], ZnO [22,23]) the coefficients α and β turn out to be negative,

which in general have not found a physical interpretation While the D◦X line decreases rapidly in intensity and almost disappears at 135 K, the BF line is still maintained

up to room temperature This line is dominant and located 4

J Phys D: Appl Phys 42 (2009) 065412 T T Loan et al

at 3.243 eV at 300 K This line cannot be attributed to a LO

phonon replica of the free exciton line, on the one hand,

because while the line at 3.314 eV appeared at very low

temperatures (15 K), the free exciton line appeared only at

temperatures higher than 50 K On the other hand, the energy

distance between the free exciton line and the BF line was

decreased from 62 meV at 50 K to 49 meV at 200 K These

energy distances are smaller than the energy of an LO phonon

(72 meV) The BF line cannot be attributed to an exciton

bound to a neutral acceptor either, because of the low binding

energy for this complex In the case of the exciton bound to

a neutral acceptor, the excitons are thermally detached from

these acceptors even at low temperatures, which is contrary to

our experimental results

For the origin of the BF emission line, we believe that

this line probably corresponds to the recombination of carriers

bound on a defect with free carriers in some band In our

case, the sample is an n-type semiconductor, in which the

donor concentration is larger than the acceptor concentration,

so it is more likely that electrons bound on donors recombine

with free holes in the valence band, causing the BF line The

same BF transition was observed in n-type GaP [24] and n-type

GaN [25] In that case, the peak position of the BF emission

should vary with the temperature more slowly than the energy

band gap varies according to Varshni’s model, as may be seen

in figure7 In addition, the experimental values of the BF peak

position at various temperatures fit rather well to the formula

describing the peak position of emission due to transitions from

a donor to the valence band [24–26]:

hν = Eg− ED+ kBT ,

where Eg is the band-gap energy, ED, as mentioned above,

is the binding energy of the donor, T is the temperature and

kB is the Boltzmann constant The free-to-bound radiative

transitions have been observed in ZnO both at low temperatures

and at room temperature by other authors [27,28]

The BF-1LO line is assigned to a LO phonon replica of the

BF line, because the energy distance between them at different

temperatures is 72.4 meV on average, which is equal to the

energy of an LO phonon The very broad DAP1 band centred

at 3.139 eV at 50 K is considerably shifted to the low-energy

side with increasing temperature (see figure5) This emission

band is interpreted as a DAP emission It is known that the

DAP emission energy is described as [29]

hν = Eg− EA− ED+q

2

εr ,

where Egis the band gap, EAand EDare the acceptor and the

donor binding energy, respectively, q is the electrical charge

of the acceptor and the donor ions, ε is the dielectric constant

and r is the distance between the donor and the acceptor With

increasing temperature, carriers on DAP with a small distance

r are released into the bands, which results in extinguishing

the high-energy side of the DAP emission band, and the band

is shifted to the low-energy side as observed in our experiment

At 135 K, when the DAP1 band completely extinguishes,

a DAP2 band can be observed (figure 5) Some weak lines

Figure 8 PL spectra of the rods of Zn1−xCox O: (a) with x = 0.01 and (b) x = 0.10 at different temperatures, excited with the 565 nm

light of a xenon lamp

peaked at 3.272, 3.205 and 3.166 eV (figure4) were observed

at temperatures lower than 75 K The origins of these lines are not yet clear at the present time, but they, perhaps, can

be related to the neutral donor-bound exciton because of their simultaneous appearance at low temperatures

Currently the nature of defects related to near-band-edge emission is not clear Maybe they are some lattice defects

or uncontrollable impurities On the other hand, because the surface-to-volume ratio of the nanoscale materials is larger than that of the bulk materials, the surface states become important in the emission process

Thus, the above-mentioned group of UV lines in both the samples of Zn1−xCox O with x = 0.01 and x = 0.10

is mainly related to the near-band-edge emission of the host ZnO materials, while the group of lines in the red region (1.9– 1.8 eV) is related to the emission transitions within Co2+ions Figure 8 illustrates the PL spectra of the rods of

Zn1−xCox O with x = 0.01, 0.10 in the temperature range

from 15 K to room temperature excited with the 565 nm (2.194 eV) light of the xenon lamp From figure8 one can note that the fine structure of the spectra is influenced by changing the Co2+ content Indeed, the PL spectrum of the

J Phys D: Appl Phys 42 (2009) 065412 T T Loan et al

sample of the Zn1−xCox O with x = 0.01 at 15 K shows

two lines overlaying each other: 669.1 nm (1.853 eV) and

681.2 nm (1.820 eV) (figure8(a)), while the PL spectrum of

the sample of Zn1−xCox O with x = 0.10 at 15 K shows three

separate lines: 662.0 nm (1.873 eV), 667.3 nm (1.858 eV) and

681.2 nm (1.820 eV) and a shoulder at 672.4 nm (1.844 eV)

(figure 8(b)) In addition, as the temperature is increased,

the above-mentioned lines broaden and overlap each other,

and the fine structure of the spectra disappears The line at

1.873 eV can be interpreted as a2E(2G)→4A2(4F) transition

in the Co2+ ions with 3d7 high-spin configuration under the

tetrahedral (Td) crystal field formed by neighbouring O2−

ions [20] The energy distance between the lines at 1.873 and

1.820 eV is found to be 0.053 eV which is in good agreement

with the energy for the E2(high) mode (0.054 eV) in the Raman

spectrum of a polycrystalline ZnO sample [15] The energy

distances between the lines at 1.873 eV and 1.858 eV and the

lines at 1.858 eV and 1.844 eV are found to be 0.015 eV and

0.014 eV, respectively, which are in agreement with the energy

for the E2 (low) mode (0.013 eV) in the Raman spectrum of

a ZnO crystal [30] The participation of these phonons in the

Co2+emission transitions clearly demonstrates that the Co2+

ions are indeed incorporated in the ZnO host crystal lattice

The PLE spectra monitored at the emission lines 662.0 nm

(1.873 eV), 667.3 nm (1.858 eV), 672.4 nm (1.844 eV) and

681.2 nm were measured The results showed that the PLE

spectra for all these emission lines were the same This fact

indicates that these emission lines were generated from the

same luminescence centre Here we show only the PLE spectra

of the emission line at 1.820 eV, because in the PLE spectra

of the emission lines at the higher energy side we could not

get the excitation peak at 1.914 eV The reason for this is that

the emission photon and the excitation photon are too close to

each other in the energy position The PLE spectra monitored

at the emission line 681.2 nm (1.820 eV) for both the samples

of Zn1−xCox O with x = 0.01 and x = 0.10 were measured

at a temperature of 15 K (figure9) The PLE spectra exhibit

fine structure As seen from figure9, the Co2+ion-related PL

can be excited both at energies near the band edge of the ZnO

host (the UV region) and at energies below the band edge (the

visible region) The UV group consists of a peak at 3.341 eV

for the sample with x = 0.01 and a peak at 3.357 eV for the

sample with x = 0.10 (figure9(a)) These peaks are related to

the near-band-edge absorption of the ZnO host materials The

visible group consists of four peaks at 1.914, 2.023, 2.195 and

2.506 eV (figure9(b)), and the first three peaks among them

are very close to the absorption peaks in the above-mentioned

transmission spectrum These four peaks are attributed to the

transitions from the basic state4A2(4F) to the2E(2G),4T1(4P),

2A1(2G) and2T1(2P) excited states of tetrahedrally coordinated

Co2+ions, respectively

Figure10schematically represents the energy levels split

in the crystal-field of Co2+ ions [20,31] This figure also

shows the excitation transitions and the emission transitions

within the tetrahedral Co2+ ions Indeed, based on the PL

and the PLE spectral analysis, we reveal that visible light can

immediately excite electrons from the basic state4A2(4F) to

the 2E(2G),4T1(4P), 2A1(2G) and2T1(2P) excited states of

Figure 9 PLE spectra monitored at the emission line 681.2 nm

(1.820 eV) for the samples of Zn1−xCox O with x = 0.01 and

x = 0.10 measured at a temperature of 15 K: (a) for the UV region and (b) for the visible region.

Figure 10 The energy levels split in the crystal field of Co2+ions and the excitation and emission transitions within the tetrahedral

Co2+ions

6

J Phys D: Appl Phys 42 (2009) 065412 T T Loan et al

tetrahedrally coordinated Co2+ions The electrons relax from

the higher excited states (4T1(4P),2A1(2G) and2T1(2P)) to the

2E(2G) lowest excited state and then return to the4A2(4F) basic

state, emitting the photon of 1.873 eV In order to explain the

existence of excitation peaks observed near the band edge of

the ZnO host, an energy transfer mechanism is proposed as

follows: when the Zn1−xCoxO nanocrystals are excited by UV

light, electrons in the valence band of the ZnO host absorb the

photon energy and transfer to the conduction band, generating

free electrons in the conduction band and free holes in the

valence band These photogenerated electrons and holes then

recombine via the near-band-edge states, exhibiting band-edge

emission In this process, some of the photogenerated charge

carriers transfer their energy to the Co2+ions, exciting the Co2+

ions, resulting in red luminescence

4 Conclusions

Cobalt doped zinc oxide nanorods Zn1−xCox O with x = 0.01,

0.10 have been successfully synthesized by a hydrothermal

process with Zn(NO3)2, Co(NO3)2, NH4OH, CO(NH2)2

and C2H5OH The XRD analysis clearly indicated that the

Zn1−xCoxO samples possess a hexagonal wurtzite crystal

structure For the Zn1−xCox O samples with x = 0.10, when

Zn2+ions are substituted by Co2+ions, the lattice constants are

decreased in comparison with those of an undoped ZnO bulk

crystal

The PL spectra of the Zn1−xCoxO nanorods were

composed of two groups of emission lines, one in the UV

region (3.4–3.2 eV) and another in the red region (1.9–1.8 eV)

The group of UV lines is mainly related to the near-band-edge

emission of the host ZnO materials The free excitons (XA), the

neutral donor-bound excitons (D◦X), the bound-to-free (BF)

transitions and the DAPs were observed in the low temperature

PL spectra Cobalt doping leads to a group of emission lines

in the red region The origins of these lines can be interpreted

as a2E(G)→4A2( F) transition and its phonon replicas in the

Co2+ion with 3d7high-spin configuration under the tetrahedral

(Td) crystal field formed by neighbouring O2−ions The Co2+

related PL can be excited at energies below the band edge The

visible peaks in the PLE spectra are attributed to the transitions

from the basic state4A2(4F) to the2E (2G),4T1(4P),2A1(2G)

and the2T1(2P) excited states of the tetrahedrally coordinated

Co2+ion

Acknowledgments

The authors thank the European Commission Project

Selectnano-TTC (Contract No 516922) and the Natural

Science Council, Ministry of Science and Technology of

Vietnam (Project 4 055 06), for financial assistance They also

thank Dr Nguyen Hoang Nam (Graduate School of Science, Osaka University) for transmission measurements

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