Chemical Physics Letters 460 (2008) 200–204

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Chemical Physics Letters

journal homepage: www.elsevier .com/locate /cplet t

Room temperature and long-lasting blue phosphorescence of Cr-doped a-Al2O3

nanowires

Sheng Wang a, Ming-Wang Shao a,*, Guang Shao b,*, Hong Wang a, Liang Cheng a

a Anhui Key Laboratory of Functional Molecular Solids, College of Chemistry and Materials Science, Anhui Normal University, Wuhu 241000, PR Chinab School of Chemistry and Chemical Engineering, Sun Yat-Sen University, Guangzhou 510275, PR China

a r t i c l e i n f o a b s t r a c t

Article history:Received 26 February 2008In final form 27 May 2008Available online 12 June 2008

0009-2614/$ - see front matter � 2008 Elsevier B.V. Adoi:10.1016/j.cplett.2008.05.089

* Corresponding authors. Fax: +86 553 3869303 (ME-mail addresses: mwshao@mail.ahnu.edu.cn (M.

edu.cn (G. Shao).

Aligned a-alumina nanowires were synthesized employing chromium oxide and aluminum as raw mate-rials at temperature of 1250 �C. The as-prepared products exhibited room temperature and long-lastingphosphorescence at 470 nm, in addition to the well-known 694 nm photoluminescence. A possible expla-nation was proposed.

� 2008 Elsevier B.V. All rights reserved.

1. Introduction

Due to the tunable size, morphology, phase and crystallographicorientation, one-dimensional (1D) nanostructures display quitedistinct physical and chemical performance, which has drawnmuch attention in today’s research in nanotechnology [1]. Amongthe 1D nanomaterials, corundum has been a distinguished memberof the family with its high melting point, hydrophobicity, high elas-tic modulus, high optical transparency, thermal and chemical sta-bility, and fine optical and dielectric characteristics [2,3]. In thepast few years, alumina nanostructures including nanotubes [4],nanowires [5], nanosheets [6], nanoplatelets [7] and nanoribbons[8] have been prepared with various methods. Corundum has awide range of technological applications, such as high-temperaturestructural materials, laser emitters, and electrical devices.

Alumina, an insulator with a wide band gap (9 eV) [9], providesa good matrix, which is favorable to the implantation of variousions in order to achieve emitting properties. Dopants in Al2O3 havea great influence on its properties, because mass transport proper-ties of Al2O3 are altered. Thus, Al2O3 with dopants has yielded newproperties. Titanium doped Al2O3 has been developed into a suc-cessful and momentous tunable laser material [10]. Magnesiumdoped a-Al2O3 [11] single crystals display their photochromic ef-fect and Er3+ doped alumina [12] obviously present different opti-cal properties.

One of the main applications of a-Al2O3 in optics is determinedby its excellent emitting properties when doped with Cr3+ ions. It iswell-known the principal luminescence of Cr3+ in Al2O3 consists oftwo sharp lines (R lines) at about 693 and 694 nm, which arise

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.-W. Shao).-W. Shao), shaog@mail.sysu.

from transitions from the lowest-excited state (2E) to the (4A2)ground state of Cr3+ [13]. Being the basis for the ruby laser, variousmethods of preparation Cr3+:Al2O3 have been applied, for instancecharged particle irradiation [14], chemical vapor-deposition [15],pulsed laser deposition [16], flame spray pyrolysis [17]. All thesemethods are important and valuable.

Inorganic luminescent materials are key components, whichhave been prerequisite to the functionality and success of manylighting and display systems [18,19]. Compared with phosphores-cent materials doped with rare-earth elements, those doped withtransition metal elements are much fewer in number. These lumi-nescent materials displaying persistent afterglow with decay timefrom milliseconds to hours have been reported, such as the after-glow from the europium-doped ZnS nanowires resulted from tran-sitions between atomic eigenstates of the system [20–22].Nowadays, luminescent materials can be found in wide applica-tions, for example, television tubes, computer monitor tubes, oscil-loscopes, radar screens, and displays in electron microscopes.

Although many investigations have been carried out to studyoptical properties of Cr3+:Al2O3 system, there are a few investiga-tions on the preparation of 1D nanostructure of Al2O3 doped withCr3+ [23]. In this Letter, the Cr3+:Al2O3 nanowires were fabricatedby a simple vapor-deposition method under vacuum condition.

The prominent long-lasting phosphorescence was discoveredwhich had not been reported. Persistent blue luminescence (orafterglow) at 470 nm lasting several minutes was discovered. Theluminescence mechanism was studied.

2. Experimental

The synthesis of Cr-doped Al2O3 nanowires was carried out in ahigh-temperature tube furnace. One gram of Cr2O3 (99.99%,100 mesh) was added in an alumina boat located in the centralzone of a horizontal alumina tube in the furnace and a piece of

S. Wang et al. / Chemical Physics Letters 460 (2008) 200–204 201

aluminum (99.9%, 50 � 10 � 0.3 mm3) covered the top of the boat.The distance between Cr2O3 powder and aluminum foil was 1 cm.After the furnace had been evacuated to 10 Pa, the system washeated to 800 �C at a rate of 10 �C min�1, and held for 30 min, thenheated again up to 1250 �C and maintained 2.5 h. Then, the furnacewas allowed to cool to room temperature naturally. A thick layer ofwhite and fluffy products was grown on the surface of the alumi-num foil, which was collected for characterization.

The phase and the crystallography of the products were charac-terized with a Shimadzu XRD-6000 X-ray diffractometer (XRD)equipped with Cu Ka radiation (k = 0.15406 nm); a scanning rateof 0.02� s�1 was applied to record the pattern in the 2h range of20–80�. The morphology and microstructure of the samples wereanalyzed using a field emission scanning electron microscope(FESEM) (S 4800, 5 kV), transmission electron microscope (TEM)(Hitachi H-800), and a high-resolution transmission electronmicroscope (HRTEM) (JEOL-2010, 200 kV). The room temperaturephotoluminescence (PL) and photoluminescence excitation (PLE)spectra and afterglow luminescence spectra of the products wererecorded by using a fluorescence spectrophotometer (EdinburghFLS920) with a 450 W xenon lamp as the light source. The powerof the 254 nm UV lamp was 6 W. X-ray photoelectron spectroscopy(XPS) was recorded on a VGESCALAB MK II X-ray photoelectronspectrometer, using non-monochromatized Al Ka X-ray as theexcitation source.

3. Results and discussion

The XRD pattern of the as-synthesized sample is presented inFig. 1a. All diffraction peaks may be indexed as a-Al2O3 with latticeconstants a = 0.4757 ± 0.0099 nm, c = 1.299 ± 0.004 nm, which areadjacent to the standard data (a = 0.4758 nm, c = 1.299 nm, JCPDScard No.10-0173). In addition to the diffraction peaks of a-Al2O3,there exist two low peaks, which may be indexed as Cr2O3 (JCPDScard No.38-1479). This XRD result indicates that the concentrationof Cr3+ is quite low.

The room temperature PL and PLE spectra of the sample areacquired and shown in Fig. 1b–d. In the PLE spectrum of 694 nm

Fig. 1. (a) the XRD pattern of as-prepared products; and room temperature emission spePLE spectrum of the as-prepared products with the emission of 694 nm; PL spectra exci

(Fig. 1b), two strong broad absorption bands are observed withits peak positions at ca. 404 and 565 nm, corresponding to the tran-sition 4A2 (4F) ? 4T1 (2G) and 4A2 (4F) ? 4T2 (4F) (spin-allowedtransitions) of Cr3+ ions or the octahedral sites of a-Al2O3 [23,24].PL spectra excited at 556 nm with a filter of 645 nm (Fig. 1c) andat 404 nm with a filter of 550 nm (Fig. 1d) both reveal a narrowemission centered at 694 nm. The peak at 694 nm is the well-known transition of 2E ? 4A2 [13,23,24], which is attributed tothe substitution between Cr3+ and Al3+ ions, that is, the isolatedsingle Cr3+ ions substitute for Al3+ ions on the octahedral sites ofthe spinel block. The peak at 694 nm further expatiates that Cr3+

ions have incorporated into Al2O3 and formed substituted solidsolution. Fig. 1b–d is the sufficient evidence to prove that Cr3+

had been doped into the Al2O3.Fig. 2a and b shows the aligned Cr3+:Al2O3 nanowires with diam-

eters around 30 nm and lengths greater than 150 lm. The nano-wires grew almost parallel to one another (Fig. 2b).

A TEM image, as shown in Fig. 2c, reveals the high uniformity ofthe Cr3+:Al2O3 nanowires. The selected area electron diffractionpattern (Fig. 2c, inset) suggests that the nanowire is single-crystal-line and may be indexed as (11�20) and (0009) diffractionplanes. HRTEM image (Fig. 2c, inset) shows the interplanar spac-ings are 0.24 and 1.45 nm, which corresponds to (11�20) and(0009) crystal planes. It indicates that the nanowires exhibit a pre-ferred growth orientation along the [0001] direction, and XPSindicated that the concentration of Cr was 0.36 at.%.

It was quite interesting that the as-synthesized products pre-sented an intense blue emission, which belonged to phosphores-cence. Although the optical properties of the nanometer-sizedAl2O3 materials doped with Cr3+ have been researched, those re-searches did not show long-afterglow phenomena. The obviousblue phosphorescence property of Cr3+ doped Al2O3 was displayedin the present experiment. When the 254 nm UV lamp had beenshining for 1 min and then was turned off, the phosphorescencecan be observed with the naked eyes in the darkroom clearly formore than 1 min. Fig. 3 shows the photos of photoluminescence.The phosphorescence is not homogeneous, which may due to theunequal distribution of Cr.

ctra for the Cr3+:Al2O3 nanowires measured with a xenon lamp as a light source; (b)ted (c) at 565 nm with filter of 645 nm, and (d) at 404 nm with filter of 550 nm.

Fig. 2. SEM images of Cr3+:Al2O3 nanowires reveal (a) length larger than 150 lm, and (b) aligned products with diameter around 30 nm; and (c) TEM image, selected areaelectron diffraction pattern and HRTEM image of an individual Cr3+:Al2O3 nanowire indicating the [0001] growth direction.

Fig. 3. The photos of as-prepared products taken after the 254 nm UV lamp had been shining 1 min and was turned off (a) 0 s; (b) 5 s; (c) 15 s; and (d) 1 min.

202 S. Wang et al. / Chemical Physics Letters 460 (2008) 200–204

In the PLE spectrum of 470 nm (Fig. 4a), two strong bands areobserved with their peak positions at 261 and 323 nm. The bandat 261 nm is due to the well-known two-electron transitions fromthe manifold of 4T1 states at these energies. The sharp peak at 323might be connected with the transition of 3T1g ?

2T2g [25]. PL spec-tra with a filter of 455 nm excited at 261 nm (Fig. 4b) and at323 nm (Fig. 4c) both result in strong emission centered at470 nm. Fig. 4d shows the decay curve of afterglow of the Cr3+:A-l2O3 nanowires at room temperature measured at 470 nm. Theexcitation light of the samples was blocked when the sampleshad been exposed for 30 s under the 254 nm UV lamp and the

emitted afterglow from them was recorded in the kinetic analysismode of the spectrometer system. The scan interval is set to 1 s.

The as-prepared products were further confirmed by the XPSpattern. The survey XPS spectrum is shown in Fig. 5a using C 1sas reference at 284.6 eV. No peaks of other elements except Al, O,Cr, C and N are observed in the spectrum, indicating the high purityof the product. Fig. 5b reveals the photoelectron spectrum of the Cr2p core level. The broad peak ranging from 574 to 580 eV corre-spond to several oxidation states of Cr in addition to the Cr3+.

The strong blue emission band of 470 nm can be connectedwith the transition of 4A2 (4F) ? 4T2 (2G) [23]. The cause of long-

Fig. 4. Room temperature emission spectra for the Cr3+:Al2O3 nanowires measured with a xenon lamp as a light source: (a) PLE spectrum of the as-prepared products with theemission of 470 nm; PL spectra with filter of 455 nm excited (b) at 261 nm and (c) at 323 nm; and (d) afterglow decay curve of Cr3+:Al2O3 nanowires with emission at 470 nm.

Fig. 5. The XPS spectrum of: (a) the survey, and (b) the Cr 2p core level.

S. Wang et al. / Chemical Physics Letters 460 (2008) 200–204 203

lasting phosphorescence might be the thermo-stimulated recombi-nation of holes and electrons, which was enhanced by the incorpo-ration of Cr3+ to form a highly dense trapping level. When thehighly dense trapping level is thermally released with a proper rateat room temperature, the intense blue long-lasting phosphores-cence is available. After the excitation source is removed, the elec-trons captured by traps can be released slowly. And they migrate torecombine with the excited states of Cr3+ under thermal re-excita-tion, finally returning to the ground states of Cr3+ accompanied bylight emission. The trapping–transporting–detrapping process re-sults in the long-afterglow properties of these phosphor materials.In this case, the introduction of Cr3+ ions into the Al2O3 also pro-duced a highly dense trapping level at appropriate depth and therecombination of trapped electrons and holes delayed the Cr3+

emissions, which resulted in the blue long-lasting phosphores-cence. Yersin et al. have reported that the fluorescence of Cr com-pounds may change to phosphorescence under fast thermalequilibration [26], which might help account for our work.

4. Conclusion

In summary, Cr3+:Al2O3 nanowires were obtained at high-tem-perature using Al foil and Cr2O3 powder as raw materials. Long-

lasting phosphorescence phenomenon was observed from chro-mium-doped Al2O3 phosphors. The luminescence properties,including fluorescence spectra and long-lasting phosphorescencespectra were studied. The experiment was operated tens of times;both of the nanowires’ dimensions and the products’ phosphores-cence were reproducible. The bright room temperature and long-lasting phosphorescence will find its application in various fields.

Acknowledgements

This work was granted financial support by the National Natu-ral Foundation of China (20571001 and 20675002), the EducationDepartment (No. 2006KJ006TD) of Anhui Province and Anhui Pro-vincial Natural Science Foundation (070414185).

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