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Synthesis of highly luminescent nanocomposite LaF3:Ln3+/Q-dots-CdTe system, exhibiting tunable red-to-green emission

  • Lenka Řezáčová
  • Marcin Runowski
  • Přemysl LubalEmail author
  • Andrzej Szyczewski
  • Stefan Lis
Original Paper
  • 15 Downloads

Abstract

Preparation of LaF3:Gd3+ 30%, Ce3+ 10%, Eu3+ 1% NPs conjugated with CdTe quantum dots was performed by two methods. The first method includes mixing of two products, while the second method is based on the co-precipitation approach. The mixing of individual components did not lead to the formation of a new product. On the contrary, the product obtained by co-precipitation synthesis resulted in particles having about 140 nm in diameter. This nanocomposite system exhibits tunable red (λexc = 248 nm) and green (λexc = 340 m) luminescence due to the presence of Eu3+ ion and CdTe quantum dots.

Keywords

Luminescence CdTe quantum dots Lanthanide-doped fluorides Energy transfer Nanocomposites 

Luminescent materials have been extensively studied in the past 20 years by many scientists in many fields of science (Auzel 2004; Hölsä 2009; Binnemans 2009; Li and Lin 2010; Ma et al. 2011; Guo et al. 2015; Runowski et al. 2019). Recently, much attention has been paid to luminescent nanomaterials, namely materials composed of small luminescent nanoparticles (Hsu et al. 2013; Runowski et al. 2017; Hernández-Rodríguez et al. 2018; Ling et al. 2018). There are many classes of such compounds, luminescent organic compounds, semiconductor quantum dots (Q-dots), inorganic materials based on lanthanide-doped compounds, and many others (Binnemans 2009; Tang et al. 2013; Bünzli 2015; Runowski et al. 2018a). Each class of these compounds has its advantages and disadvantages. Luminescent quantum dots are stable, exhibit bright multicolor luminescence, and can be easily obtained in the form of nanoparticles (Alivisatos 1996; Dabbousi et al. 1997; Xing et al. 2007; Hsu et al. 2013). On the other hand, some of them are cytotoxic, similarly to those commonly studied based on cadmium and tellurium components (Gagné et al. 2008; Elsaesser and Howard 2012; Modlitbova et al. 2018). Inorganic materials doped with Ln3+ ions can exhibit multicolor luminescence under UV or NIR excitation, depending on the selected dopant ions (Grzyb et al. 2014; Runowski 2017; Ye et al. 2018; Runowski et al. 2018b). They can also be obtained in the form of nanosized, crystalline materials (Grzyb et al. 2014; Runowski et al. 2016). Other advantages of Ln3+-doped nanomaterials are their high thermal and photo-stability, large Stokes shift, and long radiative lifetime (in the case of Eu3+/Tb3+ as dopant ions in the range of ms) (Kłonkowski et al. 2003; Binnemans 2009; Grzyb et al. 2014; Runowski and Lis 2016).

In this contribution, the synthesis and basic photophysical study of a new nanocomposite system consisting of LaF3 material doped with Ce3+, Gd3+, and Eu3+ ions and QD-CdTe nanoparticles is presented. This LaF3 material doped with those Ln(III) ions can be excited in the UV region, while emission of red light due to the presence of Eu(III) ions can be explained by the energy transfer (Ce(III) → Gd(III) → Eu(III)) (Runowski and Lis 2014). There are only a few papers devoted to the preparation of similar systems, i.e., Ln3+-doped NPs combined with quantum dots based on CdTe (Yao et al. 2010; Hossu et al. 2012; Ju et al. 2017), which are supposed to be suitable for detection of X-ray radiation. Comparing our unique nanocomposite system with those described in the literature, it shows dual emission in the VIS region due to the presence of two luminescence centers.

A detailed description of the synthesis and details of photophysical studies are given in supporting information.

LaF3:Gd3+ 30% Ce3+ 10% Eu3+ nanoparticles have been successfully synthesized via a co-precipitation approach according to the slightly modified procedure of (Runowski and Lis 2014). The sample showed a bright, red luminescence (Fig. 1 and S1) due to the effective energy-transfer cascade from Ce(III) to Eu(III) via Gd(III) ion as a mediator. The photophysical study (see Fig. S2) shows that several band maxima in the excitation spectra belong to the absorption bands of Ce(IIII) ion (Fig. S2—inset). The most intense excitation band at 248 nm was used to monitor emission spectra (Fig. S2), in which one can see the characteristic four sharp emission bands of Eu(III) ion at 592, 618, 650, and 700 nm.
Fig. 1

Single luminescent LaF3:Ln3+ (red emission, λexc = 248 nm) and CdTe-QD nanoparticles (green emission, λexc = 340 nm) (left) and their nanocomposite LaF3:Ln3+/QD-CdTe system obtained by the co-precipitation approach, illuminated under the same conditions as single nanoparticles (right)

CdTe:QD nanoparticles were prepared in a one-step synthesis (Duan et al. 2009), which was slightly modified by change of microwave to thermal heating (Škarková et al. 2017). According to Duan et al. 2009 (HR-TEM), their diameter should be about ≈ 2–3 nm. In fact, our NPs have the emission band centered around ≈ 525 nm (see the further discussion), which agree well with the emission maximum reported for such small QDs. The nanoparticles covered by mercaptopropionic acid (MPA) were characterized. Their diameter was estimated at around 5 nm from DLS experiments and the zeta potential was estimated at around − 45 mV, while acidification leads to agglomeration by an increase in diameter, as well as zeta potential (Škarková et al. 2017). This agglomeration effect is responsible for the formation of various sets of nanoparticles differing in diameter. It is worth noting that the hydrodynamic size of the NPs (taking into account the surface water molecules and agglomeration effect) determined based on the DLS method is typically larger than the real size of the NPs. Anyway, the luminescence of aqueous solution of CdTe-QD nanoparticles is very intense and bright (see Fig. 1).

Two approaches to get nanocomposite consisting of both materials were tested. The first method was based on simple mixing of two products, a single nanomaterial of LaF3:Gd3+ 30% Ce3+ 10% Eu3+ 1% composition and CdTe-QDs NPs covered by MPA. The first approach was unsuccessful, because the prepared product was not stable—it was observed that the material became black probably due to Te2− ions in CdTe nanoparticles which have been oxidized.

The second method utilized the co-precipitation approach, in which the pre-synthesized QDs were covered by the LaF3:Gd3+ 30% Ce3+ 10% Eu3+ 1% material prepared in situ (Fig. S3). On the contrary, the second approach showed the formation of a new nanocomposite system, which can be seen on the basis of dynamic light scattering (DLS) measurements. It is evident that the product formed by the synthesis consists of clusters of particles around ca. 140 nm (Fig. S4), while both starting nanomaterials are smaller, i.e., about ≈ 50–60 nm. The peaks observed around ≈ 500 nm are associated with larger agglomerates formed from the individual NPs. The powder XRD analysis confirmed the structure of the nanomaterial formed (see Fig. S5). All reflexes fit the reference pattern of hexagonal LaF3, confirming the successful incorporation of the Ln3+ dopant ions into the crystal lattice of the host. The reflexes are broadened due to the nanocrystallinity of the particles, and they are shifted toward higher 2θ values (smaller interplanar distances), because the dopant ions have smaller ionic radii than the La3+ ions from the host matrix. Based on the measured diffractogram and using Scherrer’s equation (Langford and Wilson 1978), the average size of the individual lanthanide NPs was estimated to about ≈ 20 nm. The reflexes from the QDs are not observed due to their very small size and the resulting low crystallinity, which is typical of such small (≈ 2–3 nm) NPs. The ratio of both nanoparticles (100 mg LaF3:Ln3+: 35 mg QD-CdTe) has been optimized to achieve the equal ratio of maximum of emission bands of both components (see Fig. 2), since the decreasing amount of QD-CdTe in the mixture leads to decrease of its emission band. It was also observed that dilution of the nanocomposite solution resulted in decrease of both emission bands.
Fig. 2

Excitation (a) and emission (b) spectra of LaF3:Ln3+/QD-CdTe nanocomposite system. Experimental details are given in text

Due to the presence of both entities exhibiting the dual luminescence, the nanocomposite system was photophysically examined in detail. First, the excitation spectra were measured at two emission wavelengths 525 nm and 592 nm for the QD-CdTe and LaF3: Ln(III) nanomaterials, respectively (see Fig. 2a). As one can see, there are two excitation bands. One narrower band (248 nm) is assigned to Ce(III) ion, while the broad excitation band with the highest value at 340 nm belongs to the QD-CdTe nanoparticles. Accordingly, the emission spectra of the nanocomposite were measured at four different excitation wavelengths, i.e., 248, 270, 290, and 340 nm (Fig. 2b). Excitation of the nanocomposite system at 248 nm leads to specific emission of Eu(III) ion, while excitation at 340 nm shows emission of CdTe-QD’s. On the contrary, excitation at 270 and 290 nm causes an increase of both components in the emission spectra.

The prepared substrate was illuminated with UV light and its red luminescence was caused by the presence of Ce(III)/Gd(III)/Eu(III) ions at excitation wavelength of 248 nm (Fig. 1 and Fig. S3). The green luminescence was due to the presence of CdTe-QD nanoparticles in the structure of nanocomposite system using a 340 nm excitation wavelength (Fig. 1 and Fig. S3). The tuning of the emission color from red–orange to green, which results from the use of different excitation wavelengths (selective excitation of the emitting species), can be clearly seen in the chromaticity diagram (derived from the emission spectra) presented in Fig. 3.
Fig. 3

Chromaticity diagram (CIE 1931) for the obtained nanocomposite LaF3:Ln3+/QD-CdTe; λexc = 248, 270, 290, and 340 nm

To explain the photophysics of this process, the luminescence decays of this nanocomposite system for long-lived Eu(III) ion were also measured at three excitation wavelengths (248, 270, and 290 nm), at λem = 592 nm (Fig. 4). The luminescence of the nanocomposite excited at 248 nm consists of the Eu(III) ion luminescence only, while the new short-decay contribution appears for the other excitation wavelengths of 270 and 290 nm. This decay (~ 0.2 ms) is more pronounced for the longer wavelength, and both components of the decay become longer (2.6 ms compared to 3.9 ms, 9.1 ms compared to 9.8 ms) (see Table 1). This phenomenon can be explained as the contribution of CdTe-QD emission center, to the overall red emission of the system.
Fig. 4

Luminescence decays of the LaF3:Ln3+/QD-CdTe nanocomposite system. The dotted blue line represents experimental data obtained for pure LaF3:Ln3+ nanomaterial. Other experimental details are given in the text

Table 1

The determined luminescence lifetimes and fitting parameters for LaF3:Ln3+ and LaF3:Ln3+/QD-CdTe samples; λem = 592 nm

Sample/λexc (nm)

Fitting parameters

LaF3:Ln3+

248 nm

A1 = 0.807(2)

τ1 = 9.35(2) ms

A2 = 0.200(2)

τ2 = 2.52(2) ms

LaF3:Ln3+/QD-CdTe

(248 nm)

A1 = 0.821(2)

τ1 = 9.12(2) ms

A2 = 0.190(2)

τ2 = 2.61(3) ms

LaF3:Ln3+/QD-CdTe

(270 nm)

A1 = 0.748(3)

τ1 = 9.18(3) ms

A2 = 0.185(3)

τ2 = 3.88 (19) ms

A3 = 0.071(1)

τ3 = 0.272 (8) ms

LaF3:Ln3+/QD-CdTe

(290 nm)

A1 = 0.365(11)

τ1 = 9.83(17) ms

A2 = 0.143(12)

τ2 = 3.88 (19) ms

A3 = 0.523

τ3 = 0.216(1) ms

The nanocomposite based on the LaF3:Ln3+ and CdTe-QDs nanomaterials was successfully synthesized via a combined co-precipitation and thermal heating approaches. The product exhibits tunable red-to-green emission, i.e., dual luminescence after irradiation with different wavelengths of excitation. The prepared material can be potentially utilized as a scintillator for RTG/VUV radiation [red Eu(III) luminescence] and/or UV radiation (green Q-dot luminescence), in bioapplications as a luminescent marker/label, forensics, as well as a new, tunable light source.

Notes

Acknowledgements

Financial support from the Ministry of Education of the Czech Republic (grant MUNI/A/1359/2018 and CEITEC LQ 1601) and EU ERASMUS programs are acknowledged.

Compliance with ethical standards

Conflict of interest

On behalf of all authors, the corresponding author states that there is no conflict of interest.

Supplementary material

11696_2019_816_MOESM1_ESM.docx (725 kb)
Supplementary material 1 (DOCX 725 kb)

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Copyright information

© Institute of Chemistry, Slovak Academy of Sciences 2019

Authors and Affiliations

  1. 1.Department of Chemistry, Faculty of ScienceMasaryk UniversityBrnoCzech Republic
  2. 2.Department of Rare Earths, Faculty of ChemistryAdam Mickiewicz UniversityPoznanPoland
  3. 3.Central European Institute of Technology (CEITEC)Masaryk UniversityBrnoCzech Republic
  4. 4.Department of Medical Physics, Faculty of PhysicsAdam Mickiewicz UniversityPoznanPoland

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