1. Introduction

In the past decades, lanthanide ions (Ln³⁺) doped II-VI semiconductor nanocrystals such as ZnO [1–5] and CdSe [6,7] have been the focus of research interest due to their unique optical properties and the promising applications in many technological fields as diverse as optoelectronic devices, flat plane displays and biological labels. It is anticipated that the optical properties of Ln³⁺ ions doped II-VI semiconductor nanocrystals could be tailored in the visible to infrared region by doping with different luminescent centers. ZnO as a well-known wide band gap semiconductor and environmentally friendly material is considered to be a promising host candidate for Ln³⁺ ions doping. To date, many important results for Ln³⁺ doped ZnO nano- and microstructures have been reported [1–4,8–10]. However, most research work was mainly focused on the emissions in the visible region [1–5,10]. Ln³⁺ doped ZnO nanocrystals are also expected to be one of the promising materials for the near-infrared (NIR) emissions in spectral region of 860-1600 nm. At present, the majority of attentions were paid to the ⁴I_13/2→⁴I_15/2 transition of Er³⁺ around 1.54 μm in Er³⁺ doped ZnO thin films because of the potential applications in optical communications [11,12]. Trivalent neodymium has several technically important luminescent bands including the transitions of ⁴F_3/2→⁴I_9/2 (a second lasing transition at ~900 nm), ⁴F_3/2→⁴I_11/2 (the basis of 1.06 µm Nd³⁺ lasers) and ⁴F_3/2→⁴I_13/2 (1.35 µm laser channel). The NIR emission of Nd³⁺ ions in the region of 860-1100 nm is desirable for possible applications in biological tissues, because biological tissues have the maximal light transmission in the region of 850-1100 nm. Moreover, the ~1.42 and ~1.8 μm emission bands of trivalent thulium ions are also important for expanding the transmission bandwidth of optical fibers beyond the range available from Er³⁺ doped fiber amplifiers (C-band), remote sensing and potential medical laser applications [13]. However, because of the large mismatch in ionic radius and charge imbalance between Ln³⁺ and Zn²⁺, the successful incorporation of Nd³⁺ and Tm³⁺ ions into the ZnO nanocrystals via a chemical way still remains a great challenge. There are no reports describing the preparation and photoluminescence (PL) properties of Nd³⁺ and Tm³⁺ doped ZnO nanocrystals up to now.

In this Letter, Nd³⁺ and Tm³⁺ ions are effectively incorporated into the 10-15 nm wurtzite ZnO nanocrystals by employing a facile sol-gel process, which thus result in characteristic NIR luminescence of Nd³⁺ and Tm³⁺ ions. The optical properties such as laser-excited spectra, luminescence dynamics and time-resolved spectra of Nd³⁺ and Tm³⁺ in ZnO nanocrystals were investigated in detail.

2. Experimental

The Nd³⁺ and Tm³⁺ ions doped ZnO nanocrystals, with nominal concentrations of 1.25 and 0.45 at.% respectively, were prepared by using a modified sol-gel method similar to the report elsewhere [2]. For better crystallinity of ZnO nanocrystals and enhanced NIR luminescence of Nd³⁺ and Tm³⁺ ions, the as-grown samples (dried at 60 °C for 12 h) were further annealed at 400 °C for 30 min to yield the final white products. The precise concentrations of Nd³⁺ and Tm³⁺ ions were determined to be 1.05 and 0.31 at.% by the Ultima2 ICP optical emission spectrometer, respectively. The morphology, crystallinity and phase purity of the samples were characterized by powder X-ray diffraction (XRD) and JEOL-2010 transmission electron microscope (TEM). The room temperature (RT) UV-visible diffuse reflectance spectra for Nd³⁺ and Tm³⁺ doped ZnO nanocrystals were recorded on a Perkin-Elmer Lambda 900 UV/vis/NIR spectrometer. Laser spectroscopic measurements were carried out upon excitation by a mode-locked picosecond Ti: sapphire laser (700-1000 nm, pulse width ≤ 1.5 ps, Tsunami, Spectra-Physics) in the temperature range of 4.2-300 K. For low-temperature measurements, all samples were mounted on an optical cryostat (Janis SHI-950, 4-300 K). Time-resolved spectra and luminescence lifetimes were measured with mid/near infrared steady-state and phosphorescence lifetime spectrometer (FSP920-C, Edinburgh) equipped with a digital oscilloscope (TDS3052B, Tektronix) and a tunable mid-band OPO laser as excitation source (410-2400 nm, Vibrant 355II, OPOTEK). For NIR PL, all the spectra were detected with liquid nitrogen cooled InGaAs detector.

3. Results and discussion

Figure 1(a) and 1(b) illustrated the XRD patterns of Nd³⁺ and Tm³⁺ doped ZnO nanocrystals. All the diffraction peaks can be exclusively indexed to hexagonal wurtzite ZnO (JCPDS No. 36-1451), indicating the presence of highly crystalline ZnO nanocrystals without any other impurity phases such as Nd₂O₃ or Tm₂O₃. By means of Debye-Scherrer’s formula, the mean sizes of Nd³⁺ and Tm³⁺ doped ZnO nanocrystals were estimated to be ~11 and 14 nm, respectively. The corresponding high-resolution TEM image (Fig. 1(c)) showed that the Nd³⁺ doped ZnO nanocrystals were not ideally spherical with diameters between 10 and 15 nm, which was basically consistent with the XRD estimates. Moreover, the crystalline lattice fringes of ZnO were very clear, indicative of the higher crystallinity of the annealed ZnO nanocrystals. Similar morphology was also observed for Tm³⁺ doped ZnO nanocrystals.

 

Fig. 1 XRD patterns of Nd³⁺ (a) and Tm³⁺ (b) doped ZnO nanocrystals, and (c) high-resolution TEM image of Nd³⁺ doped ZnO nanocrystals.

Download Full Size | PDF

The UV/vis diffuse reflectance spectra of Nd³⁺ and Tm³⁺ doped ZnO nanocrystals were recorded using BaSO₄ plate as a reference. As shown in Fig. 2(a) and 2(b), both samples exhibited a strong absorption onset at ~380 nm, which corresponds to the excitonic 1S_h→1S_e transition of ZnO. By virtue of the method proposed by Cao et al. [14], the band gap energies (E _g) of Nd³⁺ and Tm³⁺ doped ZnO nanocrystal were determined to be 3.30 and 3.27 eV (inset of Fig. 2), respectively. The obtained E _g of Nd³⁺ doped ZnO nanocrystals was slightly larger than that of Tm³⁺ doped ZnO nanocrystals, which might be caused by the quantum confinement effect of the smaller nanoparticles. Besides the strong absorption of ZnO nanocrystals in the UV region, some typical absorption peaks of Nd³⁺ and Tm³⁺ were also identified, which can be assigned to the transitions from the ground state of ⁴I_9/2 to ⁴F_3/2, ⁴F_5/2 + ²H_9/2, ⁴S_3/2 + ⁴F_7/2, ⁴G_5/2 + ²G_7/2, ⁴G_7/2 for Nd³⁺ ions (Fig. 2(a)) and from the ground state of ³H₆ to ³H₅, ³H₄, ³F₂ for Tm³⁺ ions (Fig. 2(b)), respectively.

 

Fig. 2 The RT diffuse reflectance spectra of Nd³⁺ (a) and Tm³⁺ (b) doped ZnO nanocrystals. The inset shows the plot of F(R)² vs photon energy of Nd³⁺ and Tm³⁺ doped ZnO nanocrystals, where F(R) = (1-R)²/2R, and R is the reflectance. Band gap energies of ZnO nanocrystals were determined by the extrapolation to F(R) = 0.

Download Full Size | PDF

To investigate NIR PL properties of Nd³⁺ ions in ZnO nanocrystals, laser-excited spectra were measured under the 811-nm excitation at RT. As shown in Fig. 3(a) , upon direct excitation from the ground state ⁴I_9/2 to the ⁴F_5/2 state of Nd³⁺ at 811 nm, three emission bands centered at 898, 1082 and 1373 nm were observed at RT, which were attributed to the radiative relaxations from ⁴F_3/2 to its low-lying multiplets of ⁴I_9/2, ⁴I_11/2 and ⁴I_13/2, respectively. The emission pattern, namely, three emission bands of Nd³⁺ ions with partially resolved crystal-field (CF) splittings and the dominant peak at 1082 nm, was totally different from that of Nd₂O₃ nanocrystals in terms of line positions and shapes [15], indicative of the incorporation of Nd³⁺ ions in ZnO nanocrystals instead of the formation of Nd₂O₃ impurity phase. To reveal the fine CF splittings experienced by Nd³⁺ ions in ZnO nanocrystals, high-resolution PL spectra were measured under the laser excitation at 811 nm at 4.2 K. Much sharper and better resolved emission lines originating from CF levels of ⁴F_3/2 to that of ⁴I_9/2, ⁴I_11/2 and ⁴I_13/2, were observed at 4.2 K (Fig. 3(b)). Similar to the PL spectra at RT, the ⁴F_3/2→⁴I_11/2 transition centered at 1082 nm dominated the whole spectrum, but with much smaller full width at half-maximum (FWHM), decreasing from ~8 nm at RT to ~1 nm at 4.2 K. The FWHM value almost kept the same in the temperature range of 4.2-50 K, as revealed in the inset of Fig. 3(b), thus confirming the well-ordered crystalline environment around Nd³⁺ ions in ZnO lattices. Compared to the RT spectra, the reduction of hot bands that originated from the upper sublevel of ⁴F_3/2 and the line narrowing at 4.2 K indicated that the upper sublevel of ⁴F_3/2 was not thermally populated at low temperature. Owing to the time-reversal (Kramers) degeneracy for the f ³ configuration (Nd³⁺), theoretically, five emission peaks for the transition from ⁴F_3/2 to ⁴I_9/2 are expected for single Nd³⁺ luminescence center in ZnO nanocrystals at 4.2 K. However, as depicted in the inset of Fig. 3(b), ten moderately resolved transition lines assigned to ⁴F_3/2→⁴I_9/2, more than the theoretical splitting number, were identified upon excitation at 811 nm, indicating that these emission lines arose from the PL superimposition of at least two Nd³⁺ sites in ZnO lattices. Because of the different chemical properties between Nd³⁺ and Zn²⁺ ions and the introduction of charge-compensating Li⁺ ions [2], multiple sites are expected and understandable in Nd³⁺ ions doped ZnO nanocrystals. From the viewpoint of spectroscopy, most of Nd³⁺ ions were very likely located at the substitutional Zn²⁺ sites as revealed in Eu³⁺ doped ZnO nanocrystals [2]. Further evidence such as the extended x-ray-absorption fine structure may help reveal the local surroundings of Nd³⁺ ions in ZnO nanocrystals. Due to very close CF levels (~13 cm⁻¹) of ⁴F_3/2 for various Nd³⁺ sites as depicted in Fig. 3(b), we are unable to select the PL from single Nd³⁺ site by site-selective spectroscopy under current experimental conditions. Moreover, it should be pointed out that no NIR luminescence of Nd³⁺ ions in the as-grown samples was detected due to the quenching of the high-energy vibrations of impurities such as H₂O adsorbed at the surface of nanoparticles, which to some extent verified that the observed NIR luminescence was ascribed to the Nd³⁺ ions in the lattice sites of ZnO nanocrystals.

 

Fig. 3 The RT (a) and 4.2 K (b) NIR luminescence of Nd³⁺ doped ZnO nanocrystals under 811-nm laser excitation. The insets show the schematic diagram of excitation and emission levels of Nd³⁺ (upper) and the normalized PL spectra at 4.2-50 K (lower).

Download Full Size | PDF

To reveal the multi-site structure of Nd³⁺ ions in ZnO nanocrystals, time-resolved emission spectra and luminescence decays of Nd³⁺ were measured at RT. The time-resolved PL spectra collected at different delay time ranging from 0 to 85 μs were shown in Fig. 4 . Different from the steady-state laser-excited spectrum presented in Fig. 3(a), two emission peaks centered at 1066 and 1082 nm (marked as peak A and B, respectively) in the region of ⁴F_3/2→⁴I_11/2 transition of Nd³⁺ ions were clearly distinguished from each other despite the low resolution (10 nm) of the time-resolved PL spectra. As shown in Fig. 4, the intensity of peak A increased dramatically with the increasing delay time, reaching the maximum at ~10 μs, and then became undetectable after a delay time of ~30 μs. In sharp contrast, a much slower time evolution process was observed for peak B at 1082 nm. The noticeable difference in the evolution of peaks A and B was mainly caused by the multiple luminescence centers of Nd³⁺ ions in the lattice sites of ZnO nanocrystals that possess various CF surroundings and thus different decay behaviors. By monitoring the emission peaks centered at 1066 and 1082 nm, the luminescence decays of Nd³⁺ ions at different sites were compared upon excitation at 811 nm, respectively. As shown in the inset of Fig. 4, the luminescence decay of peak A exhibited obviously multi-exponential nature, which could be well fitted by a three-exponential function. The fitted lifetimes were determined to be ~22 (52%), 110 (19%) and 358 µs (29%), respectively. Unlike peak A, a rising edge in the initial stage and a double-exponential decay in the tail were observed for the decay curve of peak B (inset of Fig. 4(b)). The advent of a rise time for the decay of peak B implies a much slower nonradiative relaxation rate from ⁴F_5/2 to ⁴F_3/2 (in the order of a few microsecond) than that of peak A. By fitting with a double-exponential function in the tail, the PL lifetimes of Nd³⁺ ions at 1082 nm were determined to be ~91 (18%) and 328 µs (82%), respectively. Such multi-exponential decay behaviors of Nd³⁺ ions are expected and understandable in view of the closely-spaced energy levels of Nd³⁺ ions in various sites of ZnO nanocrystals.

 

Fig. 4 Time evolutions of PL spectra of Nd³⁺ ions doped ZnO nanocrystals obtained under the 811-nm laser excitation. The inset exhibits the RT luminescence decay curves of Nd³⁺ ions in ZnO nanocrystals by monitoring the ⁴F_3/2 →⁴I_11/2 transitions at 1066 (a) and 1082 nm (b), respectively.

Download Full Size | PDF

In addition to the NIR luminescence of Nd³⁺ ions, the Tm³⁺ luminescence at ~1.49 µm in ZnO nanocrystals was also achieved at 4.2-300 K. This NIR luminescence band may be beneficial to expanding the transmission bandwidth of optical fibers beyond the range available from Er³⁺ doped fiber amplifiers (C-band). Figure 5(a) showed the emission spectrum of Tm³⁺ doped ZnO nanocrystals at RT. The diagram of the excitation and emission levels of Tm³⁺ that are of interest was schematically plotted in the inset of Fig. 5(b). Upon the direct excitation from the ground state ³H₆ to the ³H₄ state of Tm³⁺ at 801 nm, abundant emission peaks overlapping in the spectral region of 1380-1550 nm were observed, which were ascribed to the ³H₄→³F₄ transition of multiple-site Tm³⁺ ions in ZnO nanocrystals. To the best of our knowledge, PL spectrum of Tm³⁺ doped ZnO nanocrystals that exhibit NIR luminescence at ~1492 nm were not reported previously. For comparison, the emission spectrum of Tm³⁺ was also measured at 4.2 K when excited at 801 nm (Fig. 5(b)). Due to the depopulation from the upper sublevels of ³H₄ and the minimal thermal line-broadening at low temperature (4.2 K), much sharper and better resolved emission lines of the ³H₄→³F₄ transition of Tm³⁺ were observed. As a result, those hot bands, emission lines at high energies (1380-1420 nm) as denoted in Fig. 5(a) disappeared at 4.2 K. Like ZnO:Nd³⁺ nanocrystals, the decay curve of ³H₄ of Tm³⁺ under the excitation at 801 nm exhibited the multi-exponential characteristics (not shown), and the intrinsic lifetimes of ³H₄ were determined to be ~50 (70%) and 230 μs (30%) by a bi-exponential fit. Owing to the wavelength detection limit of our system, emission bands of the ³F₄→³H₆ transition of Tm³⁺ were not detected.

 

Fig. 5 The RT (a) and 4.2 K (b) NIR luminescence of Tm³⁺ doped ZnO nanocrystals under 801 nm laser excitation. The inset shows the schematic diagram of excitation and emission levels of Tm³⁺.

Download Full Size | PDF

In summary, Nd³⁺ and Tm³⁺ ions were effectively incorporated into the hexagonal wurtzite ZnO nanocrystals by using a sol-gel method, and thereby resulted in the typical NIR luminescence of Nd³⁺ and Tm³⁺ ions in the spectra region of 860-1550 nm. Intense and sharp luminescence bands with resolved CF splittings of ⁴F_3/2→⁴I_J (J = 9/2,11/2,13/2) transitions for Nd³⁺ and ³H₄→³F₄ transition for Tm³⁺ have been achieved either at low temperature or at RT. Different time evolutions and decay behaviors of ⁴F_3/2→⁴I_11/2 of Nd³⁺ ions were observed by employing time-resolved laser spectroscopy, which were attributed to the existence of multiple luminescence centers of Nd³⁺ ions with various PL lifetimes. Further studies of NIR luminescent properties of lanthanide ions in ZnO nanocrystals may have vital impact on material applications in biolabels, optical communications, remote sensing and medical laser, which is presently of great interest.