Synthesis of highly luminescent nanocomposite LaF3:Ln3+/Q-dots-CdTe system, exhibiting tunable red-to-green emission
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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.
KeywordsLuminescence 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.
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.
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 determined luminescence lifetimes and fitting parameters for LaF3:Ln3+ and LaF3:Ln3+/QD-CdTe samples; λem = 592 nm
A1 = 0.807(2)
τ1 = 9.35(2) ms
A2 = 0.200(2)
τ2 = 2.52(2) ms
A1 = 0.821(2)
τ1 = 9.12(2) ms
A2 = 0.190(2)
τ2 = 2.61(3) ms
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
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.
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.
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