1. Introduction

Since the first discovery in 1966 [1], photon upconversion (UC), the nonlinear optical process of converting two or more low-energy photons into high-energy photons has attracted much interest, both from the fundamental and practical applications. UC luminescence is a common phenomenon in Ln³⁺-doped bulk hosts and nanomaterials, and has been extensively studied in the past several decades. Such materials have found a variety of applications in displays [2,3], photovoltaics [4], bio-imaging [5,6] and theranostics [7,8]. Transition-metal (TM) ions (specifically d-block element ions with unfulfilled d orbitals) can also produce UC emission. Unfortunately, UC behavior are seldom reported for most TM ions (except Mn²⁺ and Cr³⁺ ions) at room temperature (RT) because the non-radiative transition probability in the UC process becomes serious as the temperature increases, which results in UC luminescence quenching at RT [9]. For example, the UC emission of Ni²⁺ ions were observed in crystalline chlorides and bromides at cryogenic temperature [10–14], and only Ni²⁺-doped KZnF₃ crystal exhibited a green UC emission at RT so far [15], not to mention noncrystalline hosts. However, the great advantage of such TM ions compared to Ln³⁺ ions is that their single-band UC behavior is very sensitive to their ligand field environments, which facilitate a tuning of the UC properties of TM ions by changing the surrounding chemical variation. Therefore, it is absolutely necessary to search for appropriate hosts for such TM ions to achieve UC emission at RT for practical applications.

In the past two decades, Ni²⁺-doped transparent glass ceramics (GCs) with broad near-infrared (NIR) luminescence have received considerable attention for potential applications in tunable lasers and broad-band optical amplifiers [16–27]. Furthermore, Yb³⁺ ions as sensitizers were also introduced into Ni²⁺-doped transparent GCs and enhanced the NIR luminescent properties of Ni²⁺ ions [28,29]. Transparent GCs are usually produced by controlled crystallization, and consist of both fine-grained nanocrystals and residual glass matrix. The nanocrystals can provide the active sites for the incorporation of Ni²⁺ ions in octahedral coordination and the consequent activation of broad-band NIR luminescence of Ni²⁺ ions, meanwhile the glass matrix holds the easy fabrication and processing advantages of glasses. Although the NIR luminescent properties of Ni²⁺ in transparent GCs have been extensively studied, the room-temperature UC emission of Ni²⁺ has not been observed. In this letter, we will, for the first time, present the room-temperature UC emission of Ni²⁺ in Yb³⁺ sensitized Ni²⁺-doped transparent GCs containing wide bandgap semiconductor γ-Ga₂O₃ nanocrystals. The underlying UC mechanism is discussed by power dependence and lifetime measurement.

2. Experimental method

Glass with composition of 13Li₂O-23Ga₂O₃-64SiO₂-0.1NiO-0.75Yb₂O₃ (in mol %) as well as NiO or Yb₂O₃ single-doped and undoped glasses were prepared by a conventional melt-quenching method [17,29]. The mixtures of analytical reagents of SiO₂, Ga₂O₃, Li₂CO₃, Yb₂O₃ and high purity NiO (99.99%) were melt in a platinum crucible at 1600 °C for 2 h and then was cast onto a stainless steel plate. The glass transition temperature T_g = 630 °C and the crystallization temperature T_x = 718 °C were estimated according to differential thermal analysis. An annealing temperature of 680 °C between T_g and T_x and an annealing time of 10 h were chosen to obtain transparent GCs. The glass and GC samples were cut and polished for optical measurements.

The crystallite phase and microstructure of the GCs were examined by X-ray diffraction (XRD) using Cu/Kα₁ radiation and transmission electron microscopy (TEM, JEOL 2010) operated at 200 kV. The optical absorption spectra of the samples were measured on JASCO V-570 spectrophotometer. The visible UC luminescent spectra were obtained on ZOLIX SBP300 spectrophotometer with a 980-nm laser diode (LD) as excitation source. The lifetime decay curves were recorded using a storage digital oscilloscope (Tektronix TDS3052) by exciting the samples with a modulated 980 nm LD. All the measurements were carried out at RT.

3. Results and discussion

XRD patterns of the glass and GCs are shown in Fig. 1(a). The broad unstructured feature of the glass XRD curve indicates its amorphous nature. In contrast, after the heat treatment the sole crystallization of γ-Ga₂O₃ occurs in the GC samples by comparing the diffraction peaks with the stand γ-Ga₂O₃ crystal (JCPDF# 20-0426). It should be noted that LiGa₅O₈ (JCPDF# 76-0199) and γ-Ga₂O₃ (JCPDF# 20-0426) have similar crystal structure, but it can be identified that the nanocrystal phase in the GC is γ-Ga₂O₃ by comparing the relative diffraction intensity of the (311) and (400) peaks at 36.19° and 44.14° or the (440) and (511) peaks at 64.18° and 58.4°, respectively [19,30]. It is also found that the undoped and doped GCs have indistinguishable diffraction peak positons and widths, indicating that the sizes of γ-Ga₂O₃ in these GCs are similar. Typical TEM image in Fig. 1(b) shows that the precipitated γ-Ga₂O₃ nanocrystals with a relatively uniform size of about 10 nm are dispersed in the glass matrix. Indexed bright rings in the inserted electron diffraction pattern in Fig. 1(b) further confirm the presence of the γ-Ga₂O₃ polycrystalline phase.

 

Fig. 1 (a) XRD patterns of Ni²⁺/Yb³⁺ codoped glass and undoped, Ni²⁺-doped, Yb³⁺-doped and Ni²⁺/Yb³⁺ codoped GCs. XRD patterns of other glasses are very similar to the Ni²⁺/Yb³⁺ codoped one, and thus not repeated here. The standard JCPDF card of γ-Ga₂O₃ is also presented. (b) Typical TEM image of the GC. The inset is the selected-area electron diffraction pattern.

Download Full Size | PDF

Absorption spectra of the glass and GCs are compared in Fig. 2(a). The absorption spectra of undoped and Yb³⁺-doped glasses are indistinguishable from the corresponding GC ones, and thus not presented here. The undoped glass and GC show no observable absorption character. The absorption features of Ni²⁺ in the Ni²⁺- and Ni²⁺/Yb³⁺-doped glasses resemble those of Ni²⁺ in trigonal bipyramid fivefold and tetrahedral fourfold coordination in other glass systems, whereas the typical absorption of Ni²⁺ in octahedral sites is observed in the GC and indicates that Ni²⁺ ions has been incorporated into the γ-Ga₂O₃ nanocrystals of the GCs, similar to those reported previously [16–19,23]. The broad absorption bands of Ni²⁺ peaking at 1060, 630, and 382 nm in the GCs are assigned to the Ni²⁺ transitions from the ³A₂(³F) ground state to the ³T₂(³F), ³T₁(³F), and ³T₁(³P) excited states, respectively, and two weak absorption shoulder bands at 750 and 455 nm are attributed to the ¹E(¹D) and ¹T₂(¹D) states. It is also found that the strong and sharp absorption corresponding to ²F_7/2 → ²F_5/2 transition of Yb³⁺ is almost unchanged in the Yb³⁺- and Ni²⁺/Yb³⁺-doped glasses and GCs due to the shielding effect of the outer shell electrons of Yb³⁺, and thus it is difficult to distinguish the distribution of Yb³⁺ ions in the GCs by the absorption spectra. Recently the distribution of Yb³⁺ ions in the same γ-Ga₂O₃ GC as our case were studied by Gao et al., and it was found that portion of Yb³⁺ ions (≈70%) were incorporated into γ-Ga₂O₃ nanocrystals and substituted the octahedral Ga³⁺ sites or cation vacancies at the octahedral sites [31,32]. Therefore, Ni²⁺ and most of Yb³⁺ ions are all incorporated into the γ-Ga₂O₃ nanocrystals in the GCs. The resulting energy level diagram of the Ni²⁺/Yb³⁺ codoped GC is shown in Fig. 2(b).

 

Fig. 2 (a) Absorption spectra of undoped GC, Ni²⁺-doped glass and GC, Yb³⁺-doped GC, and Ni²⁺/Yb³⁺ codoped glass and GC. (b) Energy level diagram of Ni²⁺/Yb³⁺ codoped GC which shows energy transfer upconversion process of Ni²⁺. Solid and curly arrows represent radiative and nonradiative energy transfer processes respectively.

Download Full Size | PDF

Visible emission spectra of the glasses and GCs excited by 980 nm LD are shown in Fig. 3(a). For undoped glass and GC, any signal cannot be detected in the visible wavelength range (data not shown). For doped glasses, Ni²⁺-doped glass shows no visible emission, while Yb³⁺- and Ni²⁺/Yb³⁺-doped glasses show a blue emission at 474 nm, originating from the well-known cooperative UC process of Yb³⁺ pairs [33]. Although a broad NIR emission corresponding to the Ni^{2+ 3}T₂(³F) → ³A₂(³F) transition has been observed in Ni²⁺-doped γ-Ga₂O₃ GCs [17,19,29,32], no emission is detected for Ni²⁺-doped γ-Ga₂O₃ GCs in the visible range under the excitation of 980 nm. For Yb³⁺-doped γ-Ga₂O₃ GC, like Yb³⁺- and Ni²⁺/Yb³⁺-doped glasses, the same blue emission at 474 nm appears, and can be ascribe to cooperative UC emission of Yb³⁺ pairs from Yb³⁺ ions in both the nanocrystals and the glass matrix. And in the Ni²⁺/Yb³⁺ codoped GC a green luminescence centered at 497 nm due to Ni^{2+ 1}T₂(¹D) → ³A₂(³F) UC emission is observed, accompanied by an emission shoulder at 474 nm. The luminescent photographs of Yb³⁺- and Ni²⁺/Yb³⁺-doped GCs are shown in the inset of Fig. 3(a). It has shown that the average distance between Ni²⁺ and Yb³⁺ ions in the γ-Ga₂O₃ nanocrystals was shorter than the critical distance for energy transfer [32]. Therefore, an energy transfer involving a Ni²⁺-Yb³⁺ pair in the γ-Ga₂O₃ nanocrystals will occur, as observed in the previous studies [14,28,29,32]. The UC mechanism will be discussed later. The emission shoulder at 474 nm can be ascribed to the cooperative UC luminescence of Yb³⁺ pair from Yb³⁺ ions in the glass matrix which do not involve the energy transfer to Ni²⁺ ions. The decay curves of Yb³⁺- and Ni²⁺/Yb³⁺-doped GCs under the 980-nm excitation are exhibited in Fig. 3(b). The UC emission of Yb³⁺ pairs in Yb³⁺-doped GC shows a single exponential decay with a lifetime of about 328 μs. In contrast, the decay of Ni²⁺/Yb³⁺-doped GC can be well fitted by a two-exponential function, with a fast component τ₁ of 42 μs and a slow component τ₂ of 292 μs. τ₁ and τ₂ can be assigned to the decays of Ni²⁺ and cooperative Yb³⁺ pair UC emissions in the codoped GC, respectively, consistent well with the observed two emission centers in the emission spectrum. Furthermore, it is seen that the fitted curves for the UC emission of Yb³⁺ pairs in Yb³⁺- and Ni²⁺/Yb³⁺-doped GCs are parallel, indicating they have a similar origin, in good agreement with their measured decay lifetime.

 

Fig. 3 (a) Visible emission spectra of Ni²⁺-, Yb³⁺- and Ni²⁺/Yb³⁺-doped glasses and GCs excited at 980 nm. The inset shows the luminescent photographs of Yb³⁺- and Ni²⁺/Yb³⁺-doped GCs. (b) Decay curves of Yb³⁺- and Ni²⁺/Yb³⁺-doped GCs under 980 nm excitation. Green and blue lines represent single- and two-exponential fitting respectively.

Download Full Size | PDF

The energy level diagram in Fig. 2(a) shows the possible energy transfer upconversion (ETU) process between Ni²⁺ and Yb³⁺ in γ-Ga₂O₃ nanocrystals. By 980 nm LD excitation Ni²⁺ is excited to the intermediate ³T₂(³F) state either by direct absorption of a laser photon or by energy transfer from a neighboring Yb³⁺ ion, which subsequently serves as the initial state for a Yb^{3+ 2}F_5/2 energy transfer to be brought to the ¹T₂(¹D) upper emitting state followed by the Ni²⁺ UC emission, and at the same time the excited ³T₂(³F) state will also back to the ³A₂(³F) ground state producing a NIR luminescence. Furthermore, due to the perfect resonance between the Yb^{3+ 2}F_5/2 state and Ni^{2+ 3}T₂(³F) state, very efficient energy transfer will take place, which can be verified by emission spectrum of Ni²⁺/Yb³⁺-doped GC under the excitation at 382 nm, as shown in Fig. 4. After direct Ni^{2+ 3}T₁(³P) excitation at 382 nm the photons populated from the ground state ³A₂(³F) to the excitated state ³T₂(³P) will nonradiatively relax to the Ni^{2+ 3}T₂(³F) state, and then not only Ni²⁺ but also Yb³⁺ NIR luminescence is observed, which is the experimental evidence for an energy transfer between Ni^{2+ 3}T₂(³F) and Yb^{3+ 2}F_5/2 states. Furthermore, the emission spectrum in visible range when excited at 382 nm was also recorded, but down conversion emission from Ni²⁺ was not detected. The reason is deserved to be explored later. Combined with large Yb³⁺ absorption cross-section, the Ni^{2+ 3}T₂(³F) intermediate state will be populated with a high state density and its lifetime was also increased from 480 μs in Ni²⁺-doped GC to 920 μs in Ni²⁺/Yb³⁺-doped GC [29]. From the absorption spectrum shown in Fig. 2, it can be deduced that an excitation energy provided by Yb³⁺ is sufficient to bring the excited Ni²⁺ in the ³T₂(³F) intermediate state to the upper emitting level ¹T₂(¹D). All these will vastly facilitate the ETU of Ni²⁺ ions.

 

Fig. 4 Emission spectrum of Ni²⁺/Yb³⁺-doped GC excited at 382 nm.

Download Full Size | PDF

One feature of ETU mechanism is that there is a rise in the time-evolution of UC emission after a short pulse excitation. In an ETU process the intermediate state serves as an energy reservoir to store all the excitation during the laser pulse. ETU then starts and is followed by the decay of the upper emitting state. Thus the ETU transient exhibits a rise and a decay part. A rapid rise (about 4.5 μs) is clearly seen in Fig. 3(b), suggesting an ETU-like character. Another fingerprint of ETU mechanism can be found in the power dependence of UC luminescence. A theoretical model predicted that for a two-photon ETU process the slope of the power dependence of UC luminescence will reduce from 2 in low power limit to 1 in high power limit [34]. Figure 5 shows the power dependence of Ni²⁺ UC luminescence in Ni²⁺/Yb³⁺-doped GC, which is in good agreement with the theoretical model. Based on the above experimental evidence the mechanism behind the Ni²⁺ UC luminescence in Ni²⁺/Yb³⁺-doped GC can ascribe to ETU.

 

Fig. 5 Excitation power dependence of Ni^{2+ 1}T₂(¹D) UC luminescence at 494 nm under 980 nm excitation. Red lines are the linear fitting.

Download Full Size | PDF

The room-temperature visible UC emission of Ni²⁺ in Ni²⁺/Yb³⁺-doped γ-Ga₂O₃ GC can mainly attribute to the low thermal quenching effect of wide bandgap semiconductor γ-Ga₂O₃ nanocrystals. The mechanisms of temperature-dependent nonradiative processes are referred to as thermal quenching. Wide bandgap semiconductors are good host materials for Ln³⁺ ions because the thermal quenching effects are inversely proportional to their bandgaps [35,36]. For example, the thermal quenching of Er³⁺ photoluminescence is much less in wide bandgap semiconductor hosts than in smaller bandgap semiconductors [35], and only a small reduction of Er³⁺ photoluminescence intensity in wide bandgap semiconductor host was observed as the temperature increased from 10 K to 300K [37]. γ-Ga₂O₃ has a low photon energy compared to other oxide crystals such as MgAl₂O4, ZnAl₂O4 and Al₂O₃, and is a wide bandgap semiconductor with the bandgap of as high as ~5eV [38], and thus non-radiative transition probability during the Ni²⁺ UC process in Ni²⁺/Yb³⁺-doped γ-Ga₂O₃ GC will be reduced. Furthermore, the Ni^{2+ 3}T₂(³F) intermediate state with high population and long lifetime induced by Yb³⁺ sensitizing also facilitate the UC emission. As a result, the room-temperature visible UC emission of Ni²⁺ is observed. In contrast, we did not observe any visible Ni²⁺ UC emission in Ni²⁺/Yb³⁺ codoped oxide nanocrystal (such as MgAl₂O₄, ZnAl₂O₄) GCs.

4. Conclusions

In summary, room-temperature visible UC luminescence of Ni²⁺ in Ni²⁺/Yb³⁺-doped transparent wide bandgap semiconductor γ-Ga₂O₃ GC have been observed and characterized. Upon Yb³⁺ sensitizer excitation at 980 nm, the GC exhibit green Ni^{2+ 1}T₂(¹D) UC luminescence at RT. Lifetime measurement in combination with power dependence data distinguish the potential UC mechanism as ETU. It is believed that the low thermal quenching effect of wide bandgap semiconductor γ-Ga₂O₃ and Ni^{2+ 3}T₂(³F) intermedia state with high population and long lifetime induced by Yb³⁺ sensitization facilitate room-temperature UC luminescence of Ni²⁺. Our study may provide a new platform-wide bandgap semiconductor nanocrystal GCs to activate Ni²⁺ room-temperature UC luminescence. Experiments in other wide bandgap semiconductor nanocrystal GCs (such as TiO₂ and SnO₂ with available octahedral sites for Ni²⁺ [39,40]) are also interestingly executed to explore visible UC luminescence of Ni²⁺ for the potential applications from wavelength-tunable light sources to displays.