Journal of Luminescence 131 (2011) 2784–2787

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Journal of Luminescence

0022-23

doi:10.1

n Corr

Georgia

E-m

journal homepage: www.elsevier.com/locate/jlumin

Long-lasting near-infrared persistent luminescence fromb-Ga2O3:Cr3þ nanowire assemblies

Yi-Ying Lu a, Feng Liu a,b, Zhanjun Gu b,c, Zhengwei Pan a,b,n

a Department of Physics and Astronomy, University of Georgia, Athens, GA 30602, USAb Faculty of Engineering, University of Georgia, Athens, GA 30602, USAc Lab for Bio-Environmental Effects of Nanomaterials and Nanosafety, Institute of High Energy Physics, Chinese Academy of Sciences, Beijing 100049, China

a r t i c l e i n f o

Article history:

Received 13 May 2011

Received in revised form

29 June 2011

Accepted 4 July 2011Available online 13 July 2011

Keywords:

b-Ga2O3

Persistent luminescence from Cr3þ

Near infrared (NIR)

Nanowires

13/$ - see front matter & 2011 Elsevier B.V. A

016/j.jlumin.2011.07.007

esponding author at: Department of Physics a

, Athens, GA 30602, USA. Tel.: þ1 706 542 46

ail address: panz@uga.edu (Z. Pan).

a b s t r a c t

Near-infrared (NIR) persistent luminescent b-Ga2O3:Cr3þ nanowire assemblies were synthesized by a

hydrothermal process followed by calcination. The phosphor exhibits more than 4 h afterglow in the

wavelength range of 650–850 nm after ceasing the ultraviolet light (280–360 nm) irradiation. The trap

structure and persistent luminescence mechanism were revealed by thermoluminescence measure-

ment. The b-Ga2O3:Cr3þ nanowire assemblies may find applications as identification taggants in

security and optical probes in bio-imaging.

& 2011 Elsevier B.V. All rights reserved.

1. Introduction

Long-persistent luminescent materials are receiving consider-able attention because of their important applications in security,solar energy utilization, and in vivo bio-imaging [1,2]. Significantprogress has been made on visible light emitting persistentphosphors [1,3–5]. The research and development in near-infra-red (NIR) light emitting persistent phosphors, in contrast, are farbehind their visible counterparts. Till now, only a few NIRpersistent phosphors were reported [2,6–8], with the afterglowtime ranging from several minutes to several hours.

In the design of NIR persistent phosphors, the emitter and hostneed to be carefully selected. For NIR photoluminescence, trivalentchromium ion (Cr3þ) is a favorable emitter because of its wide-rangeemission from 650 to 1400 nm, including the possible R-line emis-sion from the 2E level and the broadband emission from the 4T2 level,depending on the crystal field strength of the host lattices [9,10]. Inthe development of Cr3þ-activated NIR laser crystals, b-Ga2O3 andGa2O3-containing gallates were usually used as the hosts because ofthe excellent ability of Cr3þ ions to substitute for Ga3þ ions indistorted octahedral sites and due to the suitable host crystal fieldstrength around Cr3þ ions for intense NIR luminescence [11–14].However, persistent NIR luminescence from Cr3þ-gallates was

ll rights reserved.

nd Astronomy, University of

57; fax: þ1 706 542 8806.

realized only very recently in La3Ga5GeO14:Cr3þ powders synthe-sized by a solid-state reaction method [7,8].

Herein we report the persistent NIR luminescence from Cr3þ-doped b-Ga2O3 nanowire assemblies synthesized by a hydrothermalprocess followed by calcination. The b-Ga2O3:Cr3þ nanowire assem-blies exhibit persistent luminescence in the 650–850 nm wavelengthrange with an afterglow time of more than 4 h at room temperature.No persistent NIR luminescence was reported in Cr3þ-doped Ga2O3

before.

2. Experimental

The Cr3þ-doped b-Ga2O3 nanowire assemblies were synthe-sized by a hydrothermal process followed by calcination. In atypical synthesis, 7 ml of 4 M NaOH aqueous solution was addeddropwise to 11 ml of 0.2 M GaCl3 aqueous solution under rigorousstirring to form white colloidal precipitates. 0.02 ml, 0.05 MCr(NO3)3 aqueous solution and 0.14 g Ca(NO3)2 �4H2O powderwere then added to the colloidal suspension under stirring. Theresulting solution was transferred to a Teflon-lined stainless steelautoclave and heated at 180 1C in an oven for 24 h. After thereaction, the white product was separated by centrifugation,washed with deionized water and pure alcohol for several times,and dried at 80 1C in an oven for overnight. Finally, the whiteproduct was calcinated at 900 1C in air for 2 h.

The crystal structures of the as-synthesized products beforeand after calcination were analyzed by X-ray diffraction (XRD)

Y.-Y. Lu et al. / Journal of Luminescence 131 (2011) 2784–2787 2785

using Cu Ka radiation (l¼1.5406 A; PANalytical X’Pert PRO).The morphology and microstructure were measured using ascanning electron microscopy (SEM; FEI Inspect F FEG-SEM at20 kV) and a transmission electron microscopy (TEM; HitachiHF-3300 TEM-STEM at 300 kV). The compositions and valencewere measured using an energy-dispersive X-ray spectroscope(EDS) attached to the SEM and an X-ray photoelectronspectroscope (XPS).

The photoluminescence spectra and persistent luminescencedecay curves were recorded using a HORIBA Jobin Yvon Fluoro-Log-3 spectrofluorometer equipped with a 450 W xenon lamp asthe excitation source. Prior to the persistent luminescence mea-surements, the sample was sufficiently exposed to irradiations ofeither the xenon lamp or a 4-W 254 nm ultraviolet (UV) lamp for10 min. The persistent luminescence was also monitored andimaged using an ITT PVS-14 Generation III night vision mono-cular. The thermoluminescence (TL) curves were recorded usingthe spectrofluorometer and a homemade TL setup consisting of acopper sample holder, a cartridge heater, and a temperaturecontroller. In the TL measurement, the sample was heated from25 to 300 1C at a linear heating rate of 4 1C/s.

3. Results and discussion

XRD analyses showed that the as-synthesized white productsbefore calcinations are crystalline a-GaOOH with orthorhombicstructure (Fig. 1a). No peaks related to Ca-containing phases weredetected within the detection limit of the XRD. SEM (Fig. 2a) and TEM(Fig. 2b) observations showed that the a-GaOOH crystals are presentin the form of nanowires with diameters of 50–80 nm and lengths ofup to 8 mm. One unique growth phenomenon for a-GaOOH

Fig. 1. (a) XRD pattern of a-GaOOH nanowire assemblies, which is in agreement

with JCPDS 06-0180. (b) XRD pattern of b-Ga2O3 nanowire assemblies, which is in

consistence with JCPDS 41-1103.

nanowires is that tens of nanowires tend to assemble into aspindle-like morphology (Fig. 2a). This phenomenon was alsoobserved by other groups in hydrothermal synthesis of other metaloxide nanowires or nanotubes [15,16]. No NIR luminescence wasdetected from the a-GaOOH nanowire assemblies.

In order to achieve NIR photoluminescence and long-persis-tent luminescence, the a-GaOOH nanowire assemblies were thencalcinated at 900 1C in air for 2 h. After the calcination, theorthorhombic a-GaOOH nanowires were converted to monoclinicb-Ga2O3 nanowires (Fig. 1b), whilst the morphology and size ofthe nanowires and nanowire assemblies were preserved, as theimages shown in Fig. 2c and d. EDS and XPS measurements (notpresented here) confirmed that the nanowires are Ga2O3 dopedwith about 0.3 atom% Cr3þ; the Cr3þ concentration is consistentwith the amount of Cr3þ ions added during synthesis.

Fig. 3a shows the normalized excitation and emission spectraof the b-Ga2O3:Cr3þ nanowire assemblies at room temperature.The coexistence of the R-lines (at �688 and �696 nm attributedto 2E-4A2 transition of Cr3þ) and the broad emission band(at 650–850 nm attributed to 4T2-

4A2 transition of Cr3þ) in theemission spectrum indicates that the Cr3þ ions are doped intob-Ga2O3, substitute Ga3þ ions in the distorted octahedral sites ofb-Ga2O3, and experience an intermediate crystal field in the hostlattice [11]. The excitation spectrum of b-Ga2O3:Cr3þ nanowireassemblies monitored at 720 nm (i.e., the peak of the 650–850 nmbroad emission band) consists of three excitation bands withpeaks at around 300 nm (attributed to 4A2-

4T1(te2) transition),437 nm (attributed to 4A2 -4T1(t2e) transition), and 609 nm(attributed to 4A2 -4T2 transition).

Besides NIR photoluminescence, the b-Ga2O3:Cr3þ nanowireassemblies also exhibit long-lasting NIR luminescence after thestoppage of UV light irradiation. Fig. 3b shows the afterglowdecay curve of the b-Ga2O3:Cr3þ nanowire assemblies monitoredat 720 nm after irradiation by 300 nm light for 10 min. Thepersistent NIR emission lasts for more than 4 h. We also measuredthe emission spectra during the afterglow process. The insertedcurve in Fig. 3b is the afterglow spectrum recorded at 1 h after theremoval of the irradiation. The good agreement of the spectralprofiles between the photoluminescence (Fig. 3a) and afterglowspectrum (inset of Fig. 3b) indicates that the afterglow exists inthe whole spectral region of the photoluminescence emissionband. The long NIR afterglow was also monitored and imaged by aPVS-14 night vision monocular. The photos inserted in Fig. 3b arethe afterglow images of �0.1 g b-Ga2O3:Cr3þ nanowire assem-blies taken in a dark room at 15 s and 1 h after the stoppage of theillumination of a 254 nm UV lamp. The NIR afterglow is detect-able by the monocular for more than 4 h, which is consistent withthe decay measurement.

Although the NIR photoluminescence of b-Ga2O3:Cr3þ nano-wires can be effectively excited by a broad range of irradiationwavelengths (�280–650 nm; see Fig. 3a), the persistent lumines-cence can be different because of the different activation mechan-isms between them [7]. To understand the effectiveness of differentexcitation wavelengths on the persistent luminescence ofb-Ga2O3:Cr3þ nanowires, we studied the relationship betweenafterglow luminescence intensity and excitation wavelengths usingthe Fluorolog-3 spectrofluorometer. To avoid the influence of thefast early decay on the analysis of persistent luminescence, theafterglow intensity values recorded at time of 1 s after the stoppageof the irradiation were used as the comparison points, written as I1s.Three afterglow decay curves monitored at 720 nm emission, afterirradiations at 300, 370, and 410 nm, are shown in the inset of Fig. 4as examples for clear illustration. Fig. 4 shows the afterglowintensity I1s as the function of irradiation wavelengths over280–650 nm spectral range. It is clear that the NIR persistentluminescence of the b-Ga2O3:Cr3þ nanowire assemblies can be

Fig. 2. (a) SEM image of spindle-like a-GaOOH:Cr3þ nanowire assemblies. (b) TEM image of a-GaOOH:Cr3þ nanowires. (c) SEM image of spindle-like b-Ga2O3:Cr3þ

nanowire assemblies. (d) TEM image of b-Ga2O3:Cr3þ nanowires.

Y.-Y. Lu et al. / Journal of Luminescence 131 (2011) 2784–27872786

effectively activated by UV light irradiation at 280–360 nm butcannot be activated by the low-energy visible light illumination,even though the blue and red lights are effective to the NIRphotoluminescence.

In persistent phosphors, two kinds of active centers aregenerally involved: emitters and traps [1]. Emitters (Cr3þ ionsin present study) are centers capable of emitting radiation afterbeing excited. Traps usually do not emit radiation, but storeexcitation energy and release it gradually to the emitters due tothermal stimulation or other physical processes. To learn thedistribution of the traps in the b-Ga2O3:Cr3þ nanowire assem-blies, we conducted TL measurements at 25–300 1C. Fig. 5 showsthe TL curves monitored at 720 nm with delay times of 10 s and30 min after ceasing the irradiation (300 nm for 10 min). Thecurve recorded with delay time of 10 s shows an asymmetricbroad band covering from 25 to �250 1C. When the delay timeincreases to 30 min, the low-temperature band (25–100 1C) dis-appears and the high-temperature band (�50–250 1C) still exists,indicating that the shallow traps are emptied. The differencebetween the 10 s and 30 min curves, which is represented by thedashed line curve in Fig. 5, reveals the existence of at least twotraps with different trapping depths in b-Ga2O3:Cr3þ: low-tem-perature shallow trap and high-temperature deep trap [17], withthe latter one being mainly responsible for the long afterglow ofthe persistent NIR luminescence.

Based on the above results and analyses, the persistent NIRluminescence process is proposed and illustrated in the inset ofFig. 5. Initially, Cr3þ ions are excited under external irradiation.When the excitation energy reaches the photoionization thresh-old of the system, part of the excited electrons is captured by the

traps (e.g., oxygen vacancies) via the conduction band of the host.After the trapped electrons are thermally released and recombinewith the holes located at the ionized Cr3þ ions, the excited levelof the Cr3þ ions, e.g., 4T1(te2), is populated again. Subsequently,long persistent NIR emission occurs from the 2E and 4T2 emittinglevels, which correspond, respectively, to the R-line and broad-band emissions.

Finally, besides the b-Ga2O3:Cr3þ nanowires reported herein,we also fabricated bulk b-Ga2O3:Cr3þ powder using a solid-statereaction method and measured its optical properties. The bulkpowder exhibits similar photoluminescence spectrum as thenanowires, but the afterglow persistence time is shorter thanthat of the nanowires (1 h vs. 4 h). The reasons are not clear at thecurrent stage; however, it is probably because the solid-statereaction prepared b-Ga2O3:Cr3þ particles have fewer defects(traps) for persistent luminescence than the hydrothermal nano-wires. Nevertheless, the realization of persistent luminescence inb-Ga2O3:Cr3þ nanowires, bulk b-Ga2O3:Cr3þ powder, and La3Ga5

GeO14:Cr3þ powders [7,8] strongly suggest that persistent NIRluminescence from Cr3þ ions is probably a common phenomenonin most Ga2O3-containing gallates. Work is underway in our lab tofind phosphors with more intense persistent NIR luminescenceand much longer persistence time.

4. Conclusion

We reported the first NIR persistent luminescent nanowires,b-Ga2O3:Cr3þ nanowire assemblies, with persistence timeof more than 4 h. Such long afterglow time, along with the

Fig. 4. Afterglow intensity I1s (represented by red balls) monitored at 720 nm

emission as a function of irradiation wavelengths (280–650 nm) in b-Ga2O3:Cr3þ

nanowire assemblies. Insets are the decay curves after irradiations at 300, 370,

and 410 nm. The photoluminescence excitation spectrum (gray curve) monitored

at 720 nm is also displayed. (For interpretation of the references to color in this

figure legend, the reader is referred to the web version of this article.)

Fig. 5. Thermoluminescence curves of b-Ga2O3:Cr3þ nanowire assemblies mon-

itored at 720 nm with delay times of 10 s and 30 min after ceasing the irradiation

of a xenon lamp (at 300 nm for 10 min). Dashed line curve is the difference

between the 10 s and 30 min curves. Inset is the proposed schematic diagram of

the persistent NIR luminescence process.

Fig. 3. (a) Normalized excitation and emission spectra of b-Ga2O3:Cr3þ nanowire

assemblies. The emission spectrum is acquired under 300 nm light excitation and

the excitation spectrum is obtained by monitoring at 720 nm emission.

(b) Afterglow decay curve of b-Ga2O3:Cr3þ nanowire assemblies monitored at

720 nm at room temperature. The inserted curve is the afterglow luminescence

spectrum recorded at 1 h. The inserted images are the afterglow photos taken at

15 s and 1 h by a digital camera via a night vision monocular in a dark room.

Before measurement and imaging, the samples were, respectively, excited by a

xenon lamp (at 300 nm) for 10 min and a 254 nm UV lamp for 10 min.

Y.-Y. Lu et al. / Journal of Luminescence 131 (2011) 2784–2787 2787

invisible (to naked eyes) nature of the NIR emission, makes theb-Ga2O3:Cr3þ nanowire assemblies very suitable for many impor-tant applications, e.g., as identification taggants in security and asoptical probes in bio-imaging.

Acknowledgment

This work was supported by the U.S. Office of Naval Research(N00014-07-1-0060) and the ACS Petroleum Research Fund(PRF 50265-DN10).

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