Improved NIR emission from Tb³⁺, Yb³⁺ and Nd³⁺ co-doped La₂O₃ nano-phosphor

Abstract

NIR and visible emission have been reported from Tb³⁺, Nd³⁺ and Yb³⁺ doped La₂O₃. XRD, TEM and HRTEM corroborate good crystallinity of the nano-phosphor. The average particle size is around 15 nm examined via TEM and HRTEM. The photoluminescence up-conversion spectra of Tb³⁺ and Yb³⁺ co-doped La₂O₃ exhibited characteristic emission peaks of Tb³⁺ ion at 480, 544 and 610 nm. It is possible due to the absorption of two 980 nm pump photons by Tb³⁺ ion via Yb³⁺ sensitizer. The energy migration from Tb³⁺ to Nd³⁺ takes place so that the intensity of 820 nm (⁴F_5/2, ²H_9/2 → ⁴I_9/2) is increased significantly. The energy transfer between these two ions reduces the intensity of 544 nm Tb³⁺ ion whereas the intensity of 820 nm improved. Energy transfer mechanism between Tb³⁺, Nd³⁺ and Yb³⁺ ions has been discussed in detail. It would be a promising candidate for solar cells and biomedical applications.

1 Introduction

The maximum loss in solar cells occurs due to the passage of lower energy photon through it without creating an electron–hole pair (exciton) and also due to the high energy photons which cause thermal loss at the junction [1]. Lanthanide activated phosphors garnered much attention because these convert energy from higher to lower unit and vice versa [2,3,4,5]. Solar cells are divided into various categories namely c-Si, amorphous solar cells, dye-sensitized and organic–inorganic solar cells and their band gap is found in the range 1.1–1.8 eV (~ 650 to 1100 nm). It is previously reported that Nd³⁺, Tb³⁺, Ce³⁺, Eu³⁺, Tm³⁺ co-doped with Yb³⁺ ion gives emission around ~ 800 to 1000 nm via ultra-violet or visible excitation [3,4,5,6,7]. However, recent researches are focused on the development of such type of phosphor materials which significantly absorb infrared and UV–visible radiation. These absorbed radiations are converted into the radiations comparable to the solar cell band gap. Nd³⁺ ion would absorb UV–visible as well as NIR photons and it could be sensitized through Yb³⁺ ion [4, 8]. Additionally, Yb³⁺ co-doped Tb³⁺ also gives up-conversion (UC) green emission [9]. Prorok et al. [10] reported energy migration up-conversion from Tb³⁺ to Yb³⁺ and Nd³⁺ co-doped fluoride core–shell structure. La₂O₃ is a promising host for rare-earth doping because it has low phonon energy which significantly reduces the possibility of non-radiative (NR) transitions [11]. Also, it is compatible with other fluorescent lanthanides which also reduce the possibility of non-radiative transitions. Chen et al. [12] reported LiYF₄: Yb³⁺, Er³⁺ single crystal to improve the power conversion efficiency of perovskite solar cell. Here, we report photoluminescence (PL) UC-emission by Tb³⁺, Yb³⁺ and Nd³⁺ co-doped La₂O₃. The samples were synthesized by the polyol method. The NIR emission intensity of 820 nm (Nd³⁺ ion) might be improved due to energy migration from Tb³⁺ and Yb³⁺ ions.

2 Experimental method

La₂O₃ (99.9%), Tb₂O₃ (99.98%), Yb₂O₃ (99.995%) were purchased from Alfa Aesar. Nd₂O₃ (99.9%), ethylene glycol, urea and HNO₃ (69%) were purchased from Merck. La(NO₃)₃, Tb(NO₃)₃, Nd(NO₃)₃ and Yb(NO₃)₃ were prepared by dissolving their respective oxides in de-ionized water with the addition of few drops of HNO₃. The nitrate was mixed by taking the solution of La(NO₃)_3, Tb(NO₃)₃ Yb(NO₃)₃ and Nd(NO₃)₃ in a volumetric flask. Another solution was prepared by dissolving 4 g urea in 25 ml ethylene glycol (pH around 8–9). This solution is added to the rare earth nitrate solution kept at continuous stirring at the temperature of 120 °C for 4 h to complete the precipitation. The precipitate obtained was washed with de-ionized water and methanol and then dried it at 80 °C. Finally, the sample was calcined at 800 °C for the phase formation of La₂O₃. The structural analysis was characterized by using D8 Bruker X-ray diffractometer (XRD) with Ni-filtered Cu-Kα (1.5405 Å) radiation (10° < 2θ < 80° with a step size of 0.02°). The morphology of particles was determined by using the transmission electron microscope (TECNAI G2, 300 keV). The up-conversion spectra were recorded by exciting with diode laser 980 nm (CW, 1 W) using an iHR320 Jobin–Yvon spectrometer equipped with R928 photomultiplier tube. The PL decay measurement for a 544 nm emission band was performed with a 980 nm diode source. The collected signal was interfaced to a 150 MHz digital oscilloscope (model no. HM 1507, Hameg Instruments).

3 Results and discussion

Figure 1a shows XRD pattern of La₂O₃: 1%Tb, 10%Yb, x % Nd (x = 5, 10 at%) nanoparticles. All the diffraction peaks match with JCPDS 65-3185 reflecting the cubic phase (space group Ia$\bar{3}$) of La₂O₃ (a = 11.42 Å and V = 1489.36 Å³). Absence of impurity phases corroborates successful substitution of La³⁺ with Tb³⁺, Nd³⁺ and Yb³⁺ ions. However, a diffraction peak splits into two parts in which a peak mark by * at 27.96° obtained due to the formation of La₂O₃ cubic structure and space group Im$\bar{3}$m from JCPDS card number 89-4016. The broadness of the peaks shows the formation of nano-particle with good crystallinity. The crystalline size was lies in the range 6–8 nm which was calculated by Scherrer formula. Moreover, the morphology and size of the particles is determined by SEM and TEM which is shown in Fig. 1b, c. The particles are spherical in shape, uniformly distributed and exhibit particle size of around 8–26 nm. In addition, the particle size distribution plot is shown in the inset of Fig. 1c. The average particle size estimated from Gaussian fitting is 15 nm. The corresponding high-resolution TEM (HRTEM) is given in Fig. 1d. It is indexed according to JCPDS file representing (125) and (600) planes of La₂O₃. There are no defects or absence of grain boundaries found in HRTEM.

Fig. 1

a XRD pattern of La₂O₃: 1%Tb³⁺, 10% Yb³⁺, x % Nd³⁺ (x = 0, 5, 10) with JCPDS data 65-3185, b SEM image, c TEM image (inset particle distribution plot) and d HRTEM of La₂O₃: 1%Tb³⁺, 10%Yb³⁺

PL emission spectra recorded under 980 nm excitation of Tb³⁺ and Yb³⁺ co-doped La₂O₃ shows emission peaks of Tb³⁺ at 544 (⁵D₄ → ⁷F₅), 610 nm (⁵D₄ → ⁷F₃) shown in Fig. 2 [2]. The dominant intensity was observed for 544 nm emission. Nd³⁺ co-doped La₂O₃ exhibits Tb³⁺ as well as Nd³⁺ emission peaks. The emission peaks are centered at 760 and 820 nm observed due to ⁴S_3/2 → ⁴I_9/2 and ⁴F_5/2, ²H_9/2 → ⁴I_9/2 transitions, respectively [8, 13, 14]. From PL emission spectra it is clearly shown that the intensity of 820 nm emission increases with increasing Nd³⁺ concentration. However, from the inset of Fig. 2, it can be interpreted that the intensity of 544 nm emission decreases with increasing Nd³⁺ concentration. It might be due to the energy migration from Tb³⁺ ion to Nd³⁺ ion so that the intensity of 820 nm would improve. Yb³⁺ also transfers its energy to Nd³⁺ so that UC emission is possible from the sample. However, the intensity of other peaks of Tb³⁺ ion is not much affected by Nd³⁺ doping. Tb³⁺ ion also gives emission with UV-excitation and it transfers its absorbed energy to Nd³⁺ ion. So present phosphor also able to reduce heating loss from solar cell junction because could absorb UV-radiation and convert into NIR radiation (820 nm) comparable to the solar cell band gap. Moreover, if the number of photons absorbed by solar cells increases than the efficiency of solar cells obviously increases.

Fig. 2

Up-conversion PL emission spectra of 1%Tb³⁺ and 10%Yb³⁺, x%Nd³⁺ (x = 0, 5, 10%) co-doped La₂O₃ (λ_ex = 980 nm). Inset show PL spectra in visible region

The number of photons involved in the emission process is calculated by using the relation I α Pⁿ where I is the integrated area, P is pumped laser power and n is the number of significant photons involved for single-photon emission [15]. Figure 3a, b represents power-dependent emission spectra and ln(I) versus ln (P) plot for of La₂O₃: 1% Tb³⁺, 10% Yb³⁺, 5% Nd³⁺. The slope of the logarithmic graph indicates the number of significant photons involved for 543 and 820 nm emission. It is 1.03 for Nd³⁺ ions 820 nm emission and 1.47 for Tb³⁺ ions 544 nm emission. Nearly two photons are absorbed by Tb³⁺ ion for green emission whereas only one photon is needed for 820 nm emission [16, 17]. Tb³⁺ ion gets excited via absorbing a 980 nm pump photon by excited-state absorption or energy transfer UC process through Yb³⁺ sensitizer. However, Nd³⁺ absorbs energy from Tb³⁺ ion (⁵D₄ state) and gets excited to ⁴S_3/2/⁴F_5/2/²H_9/2 state then relaxes to ground state (⁴I_9/2) by emitting visible and NIR photons.

Fig. 3

a Power dependent emission spectra of La₂O₃:1%Tb³⁺, 10%Yb³, 5%Nd³⁺ and b its logarithmic dependent pump power versus integrated intensity of 544 nm and 820 nm emission

Figure 4 illustrates schematic energy level diagram for energy transfer (ET) between Tb³⁺, Nd³⁺ and Yb³⁺ ions. When 980 nm photon incidents on the sample, Yb³⁺ absorbs that photon and get excited to ⁵F_5/2 state. Yb³⁺ absorbs another 980 nm photon in its excited state (⁵F_5/2) and get populate to its virtual excited state. The energy of Yb³⁺ ions virtual excited state is approximately equal to the energy of the ⁵D₄ state of Tb³⁺ ion. Therefore, Tb³⁺ ions electrons get excited to ⁵D₄ state by co-operative energy transfer (CET) then relaxe to the ground by emitting visible photons. Meanwhile, Tb³⁺ also transfers its energy to Nd³⁺ ion by back energy transfer (BET) which is supported from PL emission spectra given in inset of Fig. 2 [10, 18]. Moreover, some electrons excite from excited-state absorption (ESA) to ²G_9/2 state in Nd³⁺ and electrons transit from this state to ⁴F_5/2 NR relaxation. Simultaneously, energy transfer between ⁵F_5/2 (Yb³⁺) and ⁴F_5/2/⁴F_3/2 (Nd³⁺) also takes place so that more electron gets populated in ⁴F_5/2 state of Nd³⁺ ion. Then it relaxes to the ground state by an emitting photon of 820 nm radiation.

Fig. 4

Schematic UC energy level diagram at 980 nm excitation

The lifetime of the sample has been measured for 544 nm emission and 980 nm excitation wavelength. The decay profile is shown in Fig. 5 for Tb³⁺–Yb³⁺ and Nd³⁺ co-doped sample. The decay time estimated by the mono-exponential fitting equation I = I₀ e^− t/τ1, where I₀ is the initial intensity and τ is the lifetime. Here, lifetime values observed from mono-exponential fitting are 0.42, 0.34 and 0.23 ms for La₂O₃:1%Tb³⁺, 10%Yb³⁺, La₂O₃:1%Tb³⁺, 10%Yb³, 5%Nd³⁺ and La₂O₃:1%Tb³⁺, 10%Yb³, 10%Nd³⁺. The energy transfer efficiency from Tb³⁺ ion to Nd³⁺ was calculated by using the following equation:

$$\upeta = \, 1 \, - \frac{\tau }{{\tau_{0} }}$$

(1)

where η is energy transfer efficiency from Tb³⁺ ion to Nd³⁺ ion, τ is the lifetime in presence of acceptor Nd³⁺ ion, $\tau_{0}$ an average lifetime of sensitizer (Tb³⁺ ion). From this equation maximum transfer efficiency observed for La₂O₃:1%Tb³⁺, 10%Yb³, 10%Nd³⁺ is 45.6% which represents that Tb³⁺ efficiently transfer energy to Nd³⁺ ion.

Fig. 5

Decay time measurement for La₂O₃: 1%Tb³⁺, 10% Yb³⁺, x% Nd³⁺ (x = 0, 5, 10) at λ_em = 544 nm and λ_ex = 980 nm

4 Conclusions

Tb³⁺, Nd³⁺ and Yb³⁺ doped exhibiting cubic structure have been successfully synthesized. UC PL emission spectra indicate improvement in emission intensity of 820 nm of Nd³⁺ ion due to absorption of energy from both Yb³⁺ and Tb³⁺ ions. The enhancement is observed due to migration of energy between lanthanides. Moreover, reduction in intensity of 544 nm confirms energy transfer from Tb³⁺ to Nd³⁺ ion. These studied reveals that Yb³⁺ co-doping improves up-conversion performance of nano-phosphors. Tb³⁺ also transfers its energy to Nd³⁺ ion so its emission would significantly improve which is beneficial for improving solar cell efficiency.