PbS quantum dots and BaF2:Tm3+ nanocrystals co-doped glass for ultra-broadband near-infrared emission [Invited]

Wei Wang; Qinpeng Chen; Yifei Zhao; Yakun Le; Shengda Ye; Mang Wan; Xiongjian Huang; Guoping Dong

doi:10.3788/COL202220.021603

Abstract

With the rapid growth of optical communications traffic, the demand for broadband optical amplifiers continues to increase. It is necessary to develop a gain medium that covers more optical communication bands. We precipitated PbS quantum dots (QDs) and

{BaF}_{2} : {Tm}^{3 +}

nanocrystals (NCs) in the same glass to form two independent emission centers. The

{BaF}_{2}

NCs in the glass can provide a crystal field environment with low phonon energy for rare earth (RE) ions and prevent the energy transfer between RE ions and PbS QDs. By adjusting the heat treatment schedule, the emission of the two luminescence centers from PbS QDs and

{Tm}^{3 +}

ions perfectly splices and covers the ultra-broadband near-infrared emission from 1200 nm to 2000 nm with bandwidth over 430 nm. Therefore, it is expected to be a promising broadband gain medium for fiber amplifiers.

Keywords

broadband near-infrared emission nanocrystal-glass composite PbS quantum dot Tm3+

1. Introduction

With the advent of the big data age, the application of broadband fiber amplifiers has become more and more important^[1–4]. Replacing repeaters with simple, low cost, and broadband fiber amplifiers to increase the non-relay distance has become a continuous research topic in the current optical fiber communication field. At present, rare earth (RE) ion-doped fiber amplifiers are relatively mature in laser devices, such as ${Er}^{3 +}$ ^[5], ${Ho}^{3 +}$ ^[6], ${Tm}^{3 +}$ ^[7], ${Yb}^{3 +}$ ^[8], and Nd³⁺^[9] ions and their combinations^[10–12]. Due to the extranuclear electron orbit characteristics of RE ions, the emission of RE ions originates from the 4f−4f transition, which results in a fixed emission wavelength with a narrow bandwidth^[13]. Although the emission of RE ions can be enhanced and broadened by adding a sensitizer, their doping concentrations are usually no more than 2% to avoid the loss of excitation energy caused by cross relaxation^[14]. The emission bandwidths of transition metal (TM) ions can reach several hundreds of nanometers due to their d−d orbital transitions, which is a better choice for realizing broadband luminescence. However, the luminescence of TM ions is very sensitive to the environment, and the glass network with a weak crystal field leads to low luminescence efficiency of TM ions^[15–17].

In recent years, semiconductor quantum dot (QD)-doped glasses with tunable wavelength and broadband emission have attracted much attention^[18–21]. Semiconductor QDs with different bandgaps, such as PbS QDs and PbSe QDs, are obtained by nucleation and crystal-growth mechanisms in different glass matrices under thermal field treatment. In our previous work, we have studied the influence of the introduction of the PbS QDs precursor on the luminescence spectra of QD-doped glass and detected the optical amplification signals at 1330 nm and 1530 nm in the samples^[22]. Furthermore, we have successfully fabricated all-solid-state PbS QD-doped glass fibers with tunable near-infrared (NIR) emission by using the melt-in-tube method^[23]. However, the emission range (1000–1700 nm) and the bandwidths of PbS QDs-doped glass cannot be further expanded, which limits the properties of optical amplification.

RE ions and semiconductor QDs are both efficient luminescent materials. Whether the effective combination of them can produce high-efficiency luminescence or laser devices has always been a question to researchers. Meijerink et al.^[24] reported the successful coupling of CdSe QDs with the ${Yb}^{3 +}$ ions. The adsorption of ${Yb}^{3 +}$ on the surface of CdSe QDs showed energy transfer from the QD to the ${{}^{2}F}_{5 / 2}$ state of ${Yb}^{3 +}$ , creating an emission in the NIR regions. Serqueira et al.^[25] confirmed a quantitative description of the cross section of energy transfer between ${Nd}^{3 +}$ ions and QDs through the rate equations model. ${Nd}^{3 +}$ ions were embedded in a glass system with CdS QDs, which can increase the quantum efficiency of ${Nd}^{3 +}$ ions. Recently, our group added ${Tm}^{3 +}$ ions in the CdS QD-doped glass and obtained white-light emission through energy transfer between ${Tm}^{3 +}$ ions and CdS QDs^[26]. Although these works have proved that RE ions can be incorporated into QDs, energy transfer occurs between them, which makes it impossible to effectively broaden the emission band.

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In this work, ${Tm}^{3 +}$ -ion- doped ${BaF}_{2}$ nanocrystals (NCs) and PbS QDs were simultaneously precipitated in the glass through post thermal treatment. Different from previous work^[24–26], ${BaF}_{2}$ NCs can provide a crystal field with lower phonon energy, which enhances the emission intensity of ${Tm}^{3 +}$ ions and avoids energy transfer between ${Tm}^{3 +}$ ions and PbS QDs. Therefore, ultra-broadband NIR emission covering 1200–2000 nm with a full width at half-maximum (FWHM) over 430 nm was obtained by combining the emission from PbS QDs and ${BaF}_{2} : {Tm}^{3 +}$ NCs under the excitation of an 808 nm laser diode (LD).

2. Experiments

2.1. Fabrication

The glasses were prepared by the melt-quenching method with a composition of ${15 SiO}_{2} - 40 B_{2} O_{3} - 10 ZnO - 22 K_{2} O - 13 {BaF}_{2} - 1 PbO - 1 ZnS - 3 {TmF}_{3}$ (mole fraction). After being fully mixed, 30 g stoichiometric raw materials were melted at 1100°C for 30 min in a low-temperature furnace. The glass melt was quenched on a preheated stainless-steel plate to form transparent glasses and then transferred to the muffle furnace at 350°C for 3 h to eliminate the inner stress. After the glasses were completely cooled, the precursor glasses (PGs) without QDs and NCs were obtained, then cut into suitable sizes ( $1 cm \times 1 cm$ ), and underent heat treatment at 460°C–480°C for 10 h. Afterward, PbS QDs and ${BaF}_{2}$ NCs were formed in the glasses.

2.2. Characterization

The glasses were analyzed by X-ray diffraction (XRD, PANalytical X′pert PRO, Cu Kα, $λ = 1.540598 Å$ ) to determine the crystal forms in the glass. Transmission electron microscopy (TEM, Tecnai G2, FEI, Amsterdam, the Netherlands) was used to confirm the size, size distribution, and morphology of PbS QDs and ${BaF}_{2}$ NCs. A UV/visible (VIS)/NIR double beam spectrophotometer (Perkin-Elmer Lambda 900, Waltham, MA) was used to measure the absorption spectra of the glasses. An Omni k300 spectrometer (Zolix, China) was utilized to record the fluorescence spectra upon the excitation of an 808 nm LD. The lifetime decay curves of the glass samples were measured by a digital phosphor oscilloscope (TDS3012C, Tektronix, America) and a signal generator. The variable temperature spectrum test was also measured by the Omni k300 spectrometer (Zolix, China) for testing. Orient-KOJI’s TAP-02 high-temperature fluorescence accessory was used as an external device for the spectrometer, and the test range was from room temperature to 453 K.

2.3. Calculation

Theoretical simulations were carried out based on the density functional theory (DFT) and the generalized gradient approximation (GGA) Perdew–Burke–Ernzerhof (PBE) exchange-correlation functional for describing the interactions^[27,28]. The plane wave cutoff energy was set to be 400 eV. A $3 \times 3 \times 3 k$ -mesh centered at the gamma point was used for all calculations. All of the structures were allowed to relax until the energy on the atoms was less than $5.0 \times 10^{- 4} eV$ , and all of the forces on atoms are below 0.2 eV/Å.

3. Results

Figure 1(a) shows the differential scanning calorimetry (DSC) curve of the PG. According to the curve, the glass transition temperature ( $T_{g}$ ) of the PG is 402°C and the exothermic peak at 473.7°C is the crystallization peak ( $T_{p}$ ) of the PG. Thus, the heat treatment temperature range is set to 460°C–480°C. Figure 1(b) is the XRD pattern of the PG and the glass is heat treated at different temperatures for 10 h. There are only amorphous peaks in the XRD curves of the PG, indicating that there are no crystals in the PG. As the heat treatment temperature increases, sharper diffraction peaks appear at 470°C, and the intensity of the diffraction peaks becomes stronger. The three weak diffraction peaks appearing at 26°, 30°, and 43° in the sample heat treated at 480°C refer to the (111), (200), and (220) crystal planes of the cubic structure of PbS QDs. The other three diffraction peaks appearing at 25°, 29°, and 41° in the sample are ascribed to the (111), (200), and (220) crystal planes of ${BaF}_{2}$ NCs, respectively. According to the Scherrer formula calculation, the average particle size of ${BaF}_{2}$ NCs in the samples heat treated at 470°C and 480°C is about 33.41 nm and 38.59 nm, respectively. It can be observed that the diffraction peaks of ${BaF}_{2}$ NCs heat treated at 480°C shifted to larger angles compared to the standard card. This indicates that ${Tm}^{3 +}$ ions entered the ${BaF}_{2}$ NCs lattice, resulting in the change of the unit cell parameters. Through refinement of the XRD curve, as shown in Fig. 1(c), it is found that the volume of the crystal ( $a = 6.17726$ ) is reduced compared with that of the standard crystal ( $a = 6.2001$ ).

Figure 1.(a) DSC curves of PG. (b) XRD patterns of PG and glasses heat treated at different temperatures for 10 h. (c) XRD refinement patterns of glass heat treated at 480°C for 10 h.

Figure 2.Crystal structure of (a) BaF₂ and (b) PbS doped with a Tm³⁺ ion.

To find out why ${Tm}^{3 +}$ ions tend to enter the ${BaF}_{2}$ NCs instead of PbS QDs, theoretical simulations were then employed to investigate the defect formation energies of ${Tm}^{3 +}$ ions in ${BaF}_{2}$ or PbS (Fig. 2). A $2 \times 2 \times 2$ supercell was built based on the pristine lattice of PbS (Fm-3m) and ${BaF}_{2}$ (Fm-3m) before one ${Tm}^{3 +}$ atom was introduced. The structures were fully relaxed under the same criterion, and the defect formation energy was then calculated by the following equation: $E^{f} (defect) = E_{tot} (defect) - E_{tot} (perfect) - \sum_{i} n_{i} μ_{i},$ (1)where $E_{tot} (defect)$ and $E_{tot} (perfect)$ are the total energy of the cells with and without defects. $\sum_{i} n_{i} μ_{i}$ is the change in chemical potential before and after introducing the defects, in which $μ_{i}$ is the corresponding chemical potential of the elements, and $n_{i}$ is the number of atoms. $μ_{Pb}$ , $μ_{Ba}$ , and $μ_{Tm}$ were correspondingly derived from Pb (Fm-3m), Ba (Im-3m), and Tm ( $P 6_{3} / mmc$ ). The calculated results were listed in Table 1. The Tm-Ba incorporation is the most likely to take place since it has a smaller formation energy (0.91 eV), while ${Tm}^{3 +}$ ions are difficult to be introduced into the PbS lattice due to the higher formation energy of 7.03 eV. The simulation results reveal that the ${Tm}^{3 +}$ ions preferentially enter the ${BaF}_{2}$ lattice during the formation of NCs.

Model	Total Energy (eV)	$\sum_{i} n_{i} μ_{i}$ (eV)	Formation energy (eV)
Perfect ${BaF}_{2}$	$- 549.23$	0	0
${BaF}_{2} : Tm$	$- 550.59$	$- 2.26$	0.91
Perfect PbS	$- 269.93$	0	0
PbS:Tm	$- 263.98$	$- 1.08$	7.03

Table 1. Key Parameters for the Defect Formation Energy Calculation

View all Tables

To further confirm the formation and morphology of crystals in the designed glass samples, the microstructure and element distribution of the glass heat treated at 470°C were studied by TEM measurement, as shown in Fig. 3. It can be seen that two NCs with different sizes are precipitated in the glass sample under 470°C heat treatment, namely 3–4 nm and 32–35 nm, corresponding to the sizes of PbS QDs and ${BaF}_{2}$ NCs in XRD. The high-resolution TEM (HRTEM) image shows that the lattice fringe of the larger crystal is 0.31 nm, which corresponds to the (200) crystal plane of ${BaF}_{2}$ NCs [Fig. 3(b)], and the lattice fringe of the other crystal is 0.21 nm, which corresponds to the (220) crystal plane of PbS QDs [Fig. 3(c)]. To further analyze the distribution of PbS QDs, ${BaF}_{2}$ NCs, and ${Tm}^{3 +}$ ions in the glass, a two-dimensional elemental mapping analysis was performed on the sample. Due to the smaller size of PbS QDs, only ${BaF}_{2}$ NCs are observed in the strong diffraction region [Fig. 3(d)]. In the strong diffraction region, it can be observed that Ba, F, and Tm elements are more abundantly distributed in the NC region than in the glass phase, which indicates that ${Tm}^{3 +}$ ions are concentrated around ${BaF}_{2}$ NCs in the glass [Figs. 3(g)–3(i)].

Figure 3.(a) TEM image and (b), (c) HRTEM images of the glass heat treated at 470°C for 10 h. (d) HAADF-STEM image and (e)–(i) the distribution of representative S, Pb, Ba, F, and Tm elements by two-dimensional element mapping of the glass heat treated at 470°C for 10 h.

Figure 4 shows the absorption spectra of the PG and the heat-treated glass. In the PG, there are four absorption peaks at 686 nm, 794 nm, 1210 nm, and 1690 nm corresponding to the energy level transitions of ${Tm}^{3 +}$ ions from the ground state to ${{}^{3}F}_{2}$ , ${{}^{3}H}_{4}$ , ${{}^{3}H}_{5}$ , and ${{}^{3}F}_{4}$ excited states, respectively. After heat treatment, the glasses have a wider absorption band in the NIR range, especially the glass heat treated at 470°C, which is related to the formation of PbS QDs. As the heat treatment temperature increases, ${BaF}_{2}$ NCs and PbS QDs are gradually precipitated in the glass, the color of the glasses becomes darker, and the absorption rate gradually increases.

Figure 4.Absorption spectra of PG and glasses heat treated at different temperatures for 10 h.

To explore the luminescent performance of the glass samples, an 808 nm laser was used as the excitation source to measure the NIR emission [Fig. 5(a)]. The emission at 1810 nm is obtained in the PG, which is ascribed to the emission of electrons in the $3 F_{4}$ energy state of the ${Tm}^{3 +}$ metastable state and reaching the ground state $3 F_{6}$ . In the heat-treated glass, the emission peak at 1810 nm also appears, which has a significant emission enhancement compared with the PG. This is due to the precipitation of ${BaF}_{2}$ NCs in the glass, which provides a crystal field with lower phonon energy for ${Tm}^{3 +}$ ions. At the same time, another broadband emission peak can be observed, and, as the heat treatment temperature increases, its emission peak position moves from 1200 nm to 2000 nm. Since the emission peak position of ${Tm}^{3 +}$ ions is fixed and located at 1810 nm, it can be inferred that the tunable emission originates from the emission of PbS QDs. Due to the quantum confinement effect, as the heat treatment temperature increases, the size of the PbS QDs increases, and the bandgap structure becomes smaller. Thus, the corresponding PL spectrum will be red-shifted. When the glass is heat treated at 470°C, the emission of PbS QDs and ${Tm}^{3 +}$ ions can combine as an ultra-broadband NIR emission covering 1200–2000 nm with FWHM over 430 nm [Fig. 5(b)]. To further discuss the luminescence mechanism of the glass samples, the fluorescence lifetime was explored, as shown in Fig. 5(c). It can be observed that as the heat treatment temperature increases, the luminescence gradually increases, and the fluorescence lifetime at 1810 nm increases from 0.678 ms to 1.082 ms. Since ${Tm}^{3 +}$ ions are mainly confined in the ${BaF}_{2}$ NCs, which means that the distance between ${Tm}^{3 +}$ ions and PbS QDs is larger than that for energy transfer, it is difficult to transfer energy from PbS QDs to ${Tm}^{3 +}$ ions. Power-dependent photoluminescence (PL) spectra of the glass heat treated at 470°C are shown in Fig. 5(d). It can be observed that the emission of ${Tm}^{3 +}$ ions at 1810 nm increases with the increase of laser power. When the laser power reaches 2.1 W, the emission intensity from PbS QDs reaches saturation relative to that from ${Tm}^{3 +}$ ions. With further increase of power, the intensity of PL is relatively reduced, which is due to the thermal quenching of PL intensity.

Figure 5.(a) PL spectra of PG and glasses heat treated at different temperatures for 10 h excited by an 808 nm laser. (b) PL spectra of the glass heat treated at 470°C; PL1 and PL2 are the luminescence bands related to the PbS QDs and Tm³⁺ ions. (c) Lifetime decay curves of the glass samples. The inset is the enlarged curves. The excitation wavelength is 808 nm, and the emission wavelength is 1810 nm. (d) Power-dependent PL spectra of the glass heat treated at 470°C. The inset is PL intensity of the luminescence bands related to the PbS QDs and Tm³⁺ ions.

In practical applications, the influence of ambient temperature on the luminescence of PbS QDs and ${BaF}_{2} : {Tm}^{3 +}$ NCs is of great significance to the design of devices. In Fig. 6, we further explore the temperature-dependent emission of the glass. As the test temperature increases, the luminescence peak of PbS QDs gradually shifts to a longer wavelength [Fig. 6(b)]. There are two main reasons for this phenomenon: electron-phonon coupling and thermal expansion of crystals^[29]. At the same time, it is observed that the emission intensity of PbS QDs and ${Tm}^{3 +}$ ions gradually decreases with the increase of temperature, which is a typical thermal-quenching process. As the temperature increases, the vibration energy of the host lattice increases, and the non-radiative process is enhanced, which leads to the loss of luminescence. But PbS QDs and ${Tm}^{3 +}$ ions show different downward trends with the increasing temperature. Therefore, we can use their emission intensity ratio for temperature detection. Figure 6(c) shows that the fluorescence intensity ratio (FIR) emitted by ${Tm}^{3 +}$ ions to PbS QDs has an exponential relationship with temperature. The expression is as follows, and the correlation coefficient is as high as 99.5%^[30]: $FIR (I_{Tm} / I_{QDs}) = A + B \exp (- \frac{Δ E}{k_{B} T}) .$ (2)

Figure 6.(a) Temperature-dependent PL spectra of glass heat treated at 460°C. (b) Temperature dependence of the PbS QDs PL peak position. (c) The FIR of I_Tm/I_QDs as a function of temperature in the range of 310–453 K. (d) The relative sensitivity S_R and the absolute sensitivity S_A in (c).

In the above formula, $A$ , $B$ are constants, $Δ E$ is the thermal-quenching energy level difference of the non-thermal coupling system, $k_{B}$ is the Boltzmann constant, and $T$ is the absolute temperature.

To further measure its absolute temperature performance, it is necessary to evaluate the absolute sensitivity $S_{A}$ and relative sensitivity $S_{R}$ , and the expression is as follows: $S_{A} = \frac{d (FIR)}{d T} = FIR \times \frac{Δ E}{k_{B} T^{2}},$ (3) $S_{R} = \frac{1}{FIR} \frac{d (FIR)}{d T} = \frac{Δ E}{k_{B} T^{2}} .$ (4)

As shown in Fig. 6(d), it can be seen that in the temperature range of 310–453 K, both $S_{R}$ and $S_{A}$ decrease monotonously with increasing temperature. In addition, the relative sensitivity and absolute sensitivity of FIR reach the maximum values of 4.98% and 1.99% at 310 K. The above results show that it also has a good application prospect in the field of optical thermometry.

4. Conclusions

PbS QDs and ${BaF}_{2}$ NCs-doped glasses were fabricated by the melt-quenching method and subsequent heat treatment. According to the XRD and TEM test results, PbS QDs and ${BaF}_{2}$ NCs were uniformly precipitated in the glass. With the heat treatment temperature increased from 460°C to 480°C, the sizes of PbS QDs and ${BaF}_{2}$ NCs increased obviously. Under the excitation of 808 nm LD, the ${Tm}^{3 +} : {{}^{3}F}_{4} \to {{}^{3}H}_{6}$ emission at 1810 nm and the NIR tunable emission of PbS QDs appeared in the heat-treated glass simultaneously. When the emissions of PbS QDs and ${Tm}^{3 +}$ ions overlap after proper heat treatment, ultra-broadband emission in the NIR regions can be obtained, the emission range is 1200–2000 nm, and the FWHM reaches 430 nm, which shows great potential in the field of broadband fiber amplifiers.

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