I .INTRODUCTION M.H. Cho · Y.S. Lee ) andSrCO OpticalDetectionofMnImpurityinOxides:ACaseStudyofSr(NO

Optical Detection of Mn Impurity in Oxides:

A Case Study of Sr(NO ₃ ) ₂ and SrCO ₃

M. H. Cho · Y. S. Lee ^∗

Department of Physics, Soongsil University, Seoul 156-743, Korea (Received 11 August 2014 : revised 22 August 2014 : accepted 25 August 2014)

A minute amount of Mn-ion impurity is known to be able to alter the electric property of func- tional oxide materials significantly. Recently, we found that low-temperature photoluminescence spectroscopy was quite useful for detecting the Mn-ion impurity. As a case study, we examined the existence of Mn impurity in the widely-used commerial raw materials Sr(NO

)

and SrCO

. In addition to a broad near-band-edge emission with a strong temperature dependence, both samples showed a sharp emission near 690 nm, which corresponded to the Mn-related red emission. From a quantitative analysis, we estimated the amounts of the Mn impurty in the two samples to be below 0.001%. This case study suggests that low-temperature photoluminescence measurements may be utilized to identify Mn impurities in various oxides.

PACS numbers: 78.20.-e, 77.80.-e, 71.20.-b

Keywords: Mn impurity, Low-temperature photoluminescence, Sr(NO

)

, SrCO

I. INTRODUCTION

The Mn-ion doping into oxides has attracted much attention for a variety of useful applications. Carrier- induced ferromagnetism in Mn-based dilute magnetic semiconductors, such as SrTiO 3 :Mn, has been suggested for potential spintronic device applications [1]. The Mn ion has been a good visible emitter. Its emission prop- erty depends strongly on the position and distribution inside the host materials as well as the valency [2–4]. On the other hand, the small doping of Mn ions can affect strongly the transport property of oxide semiconductors and functional oxide materials such as ferroelectrics. For example, significant improvements in the fatigue and the retention properties for Pb(Zr,Ti)O 3 have been obtained with small amounts of Mn ²⁺ -ion doping [5]. Mn ²⁺ -doped ZnO shows good current-voltage characteristics for ZnO varistors, where Mn ions affect the defect chemistry at the grain boundaries [6]. Since even the minute amount of Mn ion impurity could alter the electric property in the materials significantly, it is pertinent to control the

∗

E-mail: [email protected]

intentional/unintentional doping of the Mn impurity in manipulating the property of oxide bulks, nanocrystals, and films.

Very recently we have studied the Mn ion doped SrZrO ₃ in order to search for high efficiency of red phos- phors [7]. While this phosphor showed very strong red emission near 690 nm, the doping concentration at what the maximum red emission occurred was found to be x

= 0.001 or below, as shown in Fig. 1(c). This indicates that the red emission near 690 nm could be a fingerprint for existence of the minute Mn ion impurites inside ma- terials.

On basis of this idea, we re-examined our previous data on bulk SrZrO ₃ and nanocrystalline SrZrO ₃ samples.

Figure 1(a) shows the photoluminescence (PL) spectra of the nanocrystalline SrZrO 3 synthesized in the comub- stion method [8]. A broad blue-violet emission near 450 nm is well known to originate from oxygen vacancy [9–

11]. While this emission is quite strong even at room temperature, rather a sharp peak is identified near 690 nm at low temperatures in addition to the violet-blue emission. In case of the bulk SrZrO ₃ synthesized in the solid state reaction method [12], as shown in Fig. 1(b), no 891

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shifts to longer wavelengths compared with the case of SrZrO ₃ nanocrystals, it should be assigned to originate from the oxygen vacancy. Interestingly the sharp struc- ture near 690 nm nearly corresponds to that in nanocrys- talline SrZrO 3 . Here, it should be noted that this 690 nm peak also corresponds to the Mn-related peak in the Mn-doped SrZrO 3 shown in Fig. 1(c). In this sense we speculate that the 690 nm peak in undoped SrZrO 3 bulk and nanocrystalline compounds can be assigned to the Mn ion related emission, and accordingly, the bulk and nanocrystalline SrZrO ₃ have a minute amount of Mn im- purity. Since we did not intend to put the Mn impu- rity to our samples, we suggest that raw materials (e.g., Sr(NO ₃ ) ₂ and SrCO ₃ ) used for synthesizing the materi- als might include the Mn impurity.

Motivated by this, as a case study, we chose and in- vestigated two commercial raw materials, Sr(NO 3 ) 2 and SrCO 3 , which are widely used for nanocrysalline sam- ples in the combustion method and the polycrystalline sample in the solid state reaction method, respectively.

We performed the low temperature PL measurement on two samples. We found that two samples show the red emission similar to that of the Mn-doped SrZrO ₃ , and estimated the amount of Mn impurities to be more or less 0.001%.

II. EXPERIMENT

We used raw materials Sr(NO 3 ) 2 (purity: 99.97%) and SrCO ₃ (purity: 99.994%) from Alfa-Aesar Co. and sin- tered them in a proper condition. For structural analysis, X-ray diffraction (XRD) patterns were measured using a Bruker-AXS Discover D8 system with a Cu target X-ray tube (λ = 1.5418 ˚ A). The X-ray beam was focused to a parallel beam by using a Gobel mirror. The PL spectra of Sr(NO 3 ) 2 and SrCO 3 were measured with excitation of a He-Cd Laser (λ = 325 nm). The light emitted from samples was measured with a grating-type monochro- mator and a photomultiplier tube detector [12,13]. For temperature dependent measurements, the sample was

Fig. 1. (Color online) PL spectra of (a) SrZrO 3 nanocrys- tal sample annealed at 600 ^◦ C, (b) SrZrO 3 bulk sample sintered at 1400 ^◦ C, (c) SrZrO 3 :Mn (0.1%).

positioned in a closed-circulating refrigerator with opti- cal windows, and the measurement temperature was con- trolled from 10 K to 300 K. To investigate the electronic structures of Sr(NO ₃ ) ₂ and SrCO ₃ , the diffusive reflec- tivity spectra of the powder samples were measured using Jasco V600 series spectrophotometer in the near-infrared to ultraviolet region (0.5 - 6.7 eV) at room temperature [14].

III. RESULT AND DISCUSSION

1. XRD pattern of Sr(NO 3 ) 2 and SrCO 3

Figure 2 shows the XRD θ-2θ scan results of Sr(NO 3 ) 2

and SrCO 3 . The observed XRD peaks of two com-

pounds are quite consistent with those of the correspond-

Fig. 2. (Color online) X-ray diffraction (XRD) θ-2θ scans of (a) Sr(NO ₃ ) ₂ and (b) SrCO ₃ . The data of JCPDS for each compound are included, represented by the bars.

ing JCPDS data (25-0746 for cubic Sr(NO 3 ) 2 and 05- 0418 for orthorhombic SrCO 3 ), while two small addi- tional peaks at 2θ = 27.1 ^◦ and 28.6 ^◦ , which originate from the SrO ₂ phase, are detected in the case of SrCO ₃ . No phase of Mn-oxides was detected in both samples. It implies that the minute amount of impurity can not be detected in normal XRD measurement.

2. Optical property of Sr(NO 3 ) 2

We first discuss the electronic structure of Sr(NO ₃ ) ₂ . We obtained the absorption spectra derived from the dif- fusive reflectance spectroscopy (inset of Fig. 3(a)). The strong peak near 4.5 eV is assigned to be a charge trans- fer excitation from N/O 2p to Sr 4s/3d bands. The bandgap of Sr(NO 3 ) 2 was estimated by using Wood and Tauc (WT) method [15],

hνα ∝ (hν − E g ) ² (1) where α, ν, and E g are absorption coefficient, frequency, and optical bandgap, respectively. In this method, the

Fig. 3. (Color online) PL spectra of (a) Sr(NO 3 ) 2 , and (b) SrCO 3 with the temperature dependence. The inset of (a) and (b) show the absorption coefficient spectra of Sr(NO ₃ ) ₂ and SrCO ₃ at room temperature, respectively.

E g of Sr(NO 3 ) 2 was estimated to be 3.88 eV (319.6 nm).

It is noted that this bandgap energy is comparable to that of the photoexcitation energy (He-Cd Laser, λ = 325 nm) for PL measurement.

In order to investigate the emission property, we mea-

sured the photoluminescence spectra of Sr(NO 3 ) 2 with

temperature variation. The results are displayed in

Fig. 3(a). We observed two dominant emission struc-

tures: a broad blue-violet emission near 350 - 620 nm

and sharp red emisson near 690 nm. Since the bandgap

of Sr(NO ₃ ) ₂ is about 319.6 nm, the violet-blue emission

can be assigned with the near-band- edge (NBE) emis-

sion. The violet-blue emission shows the strong temper-

ature dependence: As the temperature increases the in-

tensity of the structure decreases abruptly and the peak

position shifts to longer wavelengths. On the other hand,

compared with other oxides [16], the NBE peak of the

Sr(NO ₃ ) ₂ sample is rather broad, probably due to the

defect state generated inside the bandgap. The origin

of the defects could be a oxyen vacancy, forming a Sr-O

complex.

decreasing. Instead, this emission is quite similar to the Mn-impurity emission. The details will be discussed later.

3. Optical property of SrCO 3

Now we turn to the optical property of SrCO 3 . In the absorption spectra shown in the inset of Fig. 3(b) we estimated the E g of SrCO 3 to be 3.18 eV (389.9 nm) by using the WT method. A strong absorption structure near 5 eV is a charge transfer excitation from C/O 2p to Sr 4s/3d bands. It is noted that the bandgap of SrCO 3

is higher than the excitation energy of Hd-Cd laser (λ = 325 nm).

Figure 3(b) shows the temperature dependent PL spectra of SrCO 3 . First we observed a broad violet-blue emission near 390 nm. The broad violet-blue emission with strong temperature dependence is assigned to be the NBE emission [17]. The broad tail toward the longer wavelength may originate from the local defect or oxygen vacancy [18,19].

Interestingly, we observed the sharp peak near 690 nm, which is quite similar to the case of the Sr(NO 3 ) 2 sam- ple. In contrast to the difference in the NBE emission whose position is determined by the optical bandgap size, the 690 nm emissions appear to be common in both of Sr(NO 3 ) 2 and SrCO 3 samples. This indicates that the 690 nm emission may originate from the extrinsic effect such as impurity.

4. Mn-impurity in SrCO 3 and Sr(NO 3 ) 2

To get more insight into the 690 nm emission, we com- pared the PL spectra of Sr(NO 3 ) 2 and SrCO 3 with that of Mn-doped SrZrO ₃ . Figure 4 displays the magnified temperature-dependent PL spectra near 690 nm. We found that although three compounds have quite differ- ent structural/electronic properties, the 690 nm emis- sions are almost identifical. From this comparison, we conclude that the 690 nm emission in Sr(NO 3 ) 2 and

Fig. 4. (Color online) Magnified PL spectra of (a) Sr(NO ₃ ) ₂ and (b) SrCO ₃ near 690 nm. The dotted lines represent the 10 K data of SrZrO ₃ :Mn (0.1%). The in- tensity is divided by order of 4.

SrCO ₃ originate from the Mn ions, and accordingly, two raw materials include the Mn impurity. The intensity of the 690 nm emission in Sr(NO 3 ) 2 and SrCO 3 is smaller by a order of 4 than that of 0.1% Mn-doped SrZrO 3 . From simple proportional calculation, we could estimate the amount of the Mn-impurity to be below 0.001%.

This estimation appears rather reasonable in sense that the purities of Sr(NO 3 ) 2 and SrCO 3 are 99.97% and 99.994%, respectively. On basis of our finding we suggest that one should consider a small impurity of Mn ions in- cluded inevitably in Sr(NO 3 ) 2 and SrCO 3 when synthe- sizing some targets for film growth and polycrystalline samples by using these commercial items as raw materi- als. A similar study is worthwhile to be done for various raw materials used for the synthesis of compounds whose properties are sensitive to the Mn impurity.

IV. CONCLUSION

We examined the existence of Mn impurity in widely

used commerial raw materials Sr(NO ₃ ) ₂ (Alfa-Aesar, pu-

rity 99.97%) and SrCO 3 (Alfa-Aesar, purity 99.994%)

by using the low temperature photoluminescence spec- troscopy. In addition to broad NBE emission with strong temperature dependence, both samples show the sharp emission near 690 nm which corresponds to the Mn- related red emission in Mn-doped SrZrO ₃ . From the quantitative analysis we estimated the amount of Mn impurty in two samples to be below 0.001%. This case study suggests that low-temperature photoluminescence measurement may be utilized to identify Mn impurities in oxides.

ACKNOWLEDGEMENTS

This work was supported by a National Research Foundation of Korea (NRF) grant funded by the Korea government (MOE) (NRF-2013R1A1A2012281).

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