Enhanced near-infrared persistent luminescence in MgGa₂O₄:Cr³⁺ through codoping

Highlights

• High efficiency near-infrared emitting persistent phosphors MgGa₂O₄:Cr³⁺.

• Enhanced emission of MgGa₂O₄:Cr³⁺ by codoping.

• More than 7 h of persistent near-infrared emission of MgGa₂O₄:Cr³⁺, Si⁴⁺.

• Formation of an additional trap through Si⁴⁺ codoping.

• High potential as an infrared emitter for medical imaging.

Abstract

Near-infrared emitting persistent luminescent materials have potential applications in night-vision surveillance and medical imaging. Rational designing of the phosphor composition is of great importance for achieving sufficiently strong and long-lasting persistent luminescence. Herein, a series of MgGa₂O₄:Cr³⁺ phosphors with variable concentrations of Cr³⁺ were prepared via solid state reaction. MgGa₂O₄:Cr³⁺ exhibits strong emission in the range from 650 to 800 nm and three excitation bands centered at 275, 420 and 576 nm. With appropriate selection of codopant M (M = Al³⁺, Si⁴⁺, Ge⁴⁺ or Sn⁴⁺), the MgGa₂O₄:Cr³⁺, M phosphor presents remarkably enhanced persistent luminescence. Moreover, the trap distributions were tuned and the number of traps was greatly increased via codoping. In particular, MgGa₂O₄:Cr³⁺, Si⁴⁺ shows more than 7 h of persistent near-infrared light emission before the intensity drops to 5 × 10⁻⁴ mWsr⁻¹m⁻². Codoping with appropriate ions provides a good example in optimization and development of persistent luminescent phosphors for potential bioimaging applications.

Introduction

Persistent luminescence is a unique optical process where the material continues to emit light appreciably for minutes or hours after the irradiation source has finished. The delayed emission originates from charge carriers (electrons or holes) stored in trapping defects within the forbidden band gap of hosts, which are generated by ultraviolet or visible light irradiation in most cases [1]. With the appropriate trap depths, charge carriers can escape from trapping levels and energy can be liberated gradually at room temperature, leading to persistent luminescence. On the other hand, phosphors with very deep traps can be used as storage phosphors for dosimetry [2]. The release rate of carriers captured in very deep traps is very low at room temperature. Additional external fields can however accelerate the detrapping process, for instance, thermal, optical or mechanical stimulations, resulting in stimulated emissions from the emitting centers in the host. According to the different types of external stimulations, this leads to thermally stimulated luminescence, optically stimulated luminescence (OSL) or mechanoluminescence (ML) [3,4]. Especially, thermally stimulated luminescence (TSL) or thermoluminescence (TL) where the energy is released by heating, has been widely used to gain information on the nature of traps [[4], [5], [6]].

Persistent luminescent phosphors and storage phosphors have been developed for a variety of applications in security signs, decorations, night-vision displays, dosimeters, and in vivo imaging [[5], [6], [7]]. Especially for the burgeoning in vivo imaging of small animals, the use of functionalized near-infrared emitting persistent luminescent nanophosphors allows for improved photon penetration through deep tissue and minimizes the auto fluorescence effect. The biomarkers are fully excited before injecting into animal tissues, largely minimizing the tissue autofluorescence caused by the excitation source, resulting in a high signal-to-noise ratio [8,9]. In view of long-term medical imaging, additional luminescence characteristics such as in vivo recharge ability, up conversion, near-infrared stimulated luminescence, and red light excitable persistent luminescence could be highly desirable.

As NIR persistent phosphors with emission in the first optical transparency window (650–950 nm), both Cr³⁺ and Mn⁴⁺ have been explored as effective dopants [10,11]. To date, the best results have been obtained using trivalent chromium ion (Cr³⁺), as an efficient NIR emitter, which can be incorporated into a variety of host lattices, with the representative ones including Zn₃Ga₂Ge₂O₁₀:Cr³⁺, ZnGa₂O₄:Cr³⁺, Y₃Al₂Ga₃O₁₂:Cr³⁺, Ca₃Ga₂Ge₃O₁₂:Cr³⁺, LiGa₅O₈:Cr³⁺, La₃Ga₅GeO₁₄:Cr³⁺, etc [[12], [13], [14], [15], [16]]. Among them, Cr³⁺ doped ZnGa₂O₄ has attracted extensive attention due to its excellent NIR persistent luminescence performance for bioimaging application [7]. The NIR persistent luminescence from MgGa₂O₄:Cr³⁺ phosphor has also been reported by Basavaraju et al. [17]. Despite its promising optical properties and the resemblance to ZnGa₂O₄:Cr³⁺, both in terms of composition and structure, the reported NIR persistent luminescence performance from MgGa₂O₄:Cr³⁺ is still inferior compared to that in ZnGa₂O₄:Cr³⁺ [18]. The optimization and improvement of MgGa₂O₄:Cr³⁺ phosphor is still needed for its practical use in long-term bioimaging [19,20].

Codoping or composition modification has been an effective strategy to regulate the surrounding environment of emitting centers or trap formation thus affecting the luminescence properties of persistent phosphors [21]. For instance, the enhancement of the persistent luminescence of ZnGa₂O₄:Cr³⁺ has been achieved by codoping various additional ions, such as Ge, Sn or Bi [12,22,23]. The improvement of luminescence properties with charge compensation in LaAlO₃:Mn⁴⁺ was investigated by codoping Ca²⁺, Na⁺ or Sr²⁺ etc. [10,24]. Qiu et al. reported a long-persistence phosphor Zn₂SnO₄:Cr³⁺ in an inverse spinel type structure. Additional Al dopants were introduced to form a Zn_2-xAl_2xSn_1-xO₄ solid solution and the local crystal field was precisely tailored [25]. Codoping with Ge, Si ions has been studied to make LiGa_5-xM_xO₈:Cr³⁺ (M = Ge, Si) with longer afterglow duration [15,26]. Therefore, the selection of the appropriate codopants is of importance for better optical properties.

In this paper, MgGa₂O₄:x%Cr³⁺ (x = 0.2, 0.4, 0.6, 0.8, 1, 2, 4) phosphors were synthesized using a high temperature solid state reaction method. The crystal structures and the morphology were studied using powder X-ray diffraction (XRD), scanning electron microscopy (SEM) as well as energy-dispersive X-ray spectroscopy (EDS) measurements. In addition, effects of dopant concentrations on photoluminescence, persistent luminescence and thermoluminescence were investigated. Persistent luminescence was recorded in absolute units to be comparable to other reports. In the optimized phosphor, 5-min activation can result in more than 7 h of persistent near-infrared light emission before the intensity drops to 5*10⁻⁴ mWsr⁻¹m⁻². Incorporation of codopants (Al³⁺, Si⁴⁺, Ge⁴⁺ or Sn⁴⁺ ions) allowed tuning the trap distributions and increasing the amount of traps. The codoping strategy with appropriate ions provides a good example in optimization and development of phosphors with persistent luminescence.