Ultrafast Synthesis and Coating of High Quality β- NaYF4:Yb,Ln Short Nanorods

An ultrafast route to prepare up-converting single β-phase NaYF4:Yb3+,Ln3+ (Ln: Er, Tm, or Tb) short nanorods (UCNRs) of high quality was developed. This new procedure affords reactive-surface nanorods that are easily coated by direct injection of suitable capping ligands. Thus highly crystalline nanorods with excellent UC fluorescence and good solvent-selective dispersion are obtained, which represents a significant advance in the field and enlarges their use for biomedical and other technological applications. Unlike other methodologies, the short reaction time provides a kinetic control over crystallization processes, and the β-phase and rod morphology is preserved regardless of the optically active Ln3+ ion. The UC emission was finely tuned by using the most popular Yb3+/Tm3+ and Yb3+/Er3+ pairs. More importantly, UCNRs doped with the unusual Yb3+/Tb3+ pair, with no ladder-like energy levels, provided a nice emission upon near-infrared excitation, which constitutes the first example of phonon-assisted cooperative sensitization to date in pure β-NaYF4 nanocrystals.

Lanthanide co-doped β-NaYF4 nanocrystals (Ln 3+ : Er,Tm,Yb) are high brightness near infrared to visible light up-converters with great interest in technological applications across many fields such as photonics, security, sensors, energy and biomedicine. [1][2] Key for their effective industrial integration in commercial devices will be the prospect to fabricate monodisperse high-quality nanocrystals with nanometric size (<100 nm), well-defined shape of pure hexagonal (β) phasewhich affords the highest optical emission-through a facile, cheap and fast scalable synthetic route. Furthermore, a one-pot processing to make solvent-soluble dispersions of the nanoparticles is highly desirable. 3 Up-converting nanoparticles (UCNPs) are routinely prepared by thermal decomposition, 4-7 coprecipitation, [8][9] and hydro-or solvothermal [10][11] routes. However, harsh conditions such as high reaction temperature (>300 °C) and pressure, long reaction times (6-48h), and/or waterless oxygen-free conditions are usually required, hindering their industrial up-scaling. Furthermore, NaYF4 crystallizes first as a metastable cubic α-phase with very poor optical activity. The cubicto-hexagonal conversion needs to overcome a high free-energy barrier 12 and a subsequent fast growth of crystals occurs; thus, mixtures of both phases are usually obtained, especially in the case of small nanoparticles.
Microwave-assisted (MW) synthesis has appeared as an attractive way to prepare monodisperse colloids with complex kinetic/thermodynamic control over crystallization processes. [13][14] Indeed, this route has already been employed to prepare different fluoride materials. [15][16][17] However, the accurate attribution of the crystalline phase is difficult in small nanocrystals and even sometimes avoided by the authors, since it is a crucial issue in highly sensitive luminescent systems. Reports concerning UC NaYF4 encompass either small nanoparticles (< 8 nm) of the 10-times less efficient α-phase [18][19][20] and α/β mixtures, 21 or pure β-phase prisms 20,22 and long wires 23 of micrometric size.
Small nanorods of less than 100 nm, highly crystalline and pure β-phase by using a mild and fast microwave route have never been reported to our knowledge.
An additional advantage of our MW protocol is that a functional coating can be added upon completion of the reaction. 24 This allows great versatility in designing stable colloids in solvents of different polarity in sight of the envisaged application. For example, in biologically relevant applications such as bioimaging, drug targeting or nanothermometers, luminescent nanoparticles must be water dispersible to be compatible in physiological conditions. 25 By contrast, other technological applications need their dispersion in low boiling solvents such as cyclohexane in order to make thin films in miniaturized designs. This is the case for solar cells, optoelectronic devices or anti-counterfeiting systems. [26][27] Here, we present a fast, energy efficient and versatile microwave route to successfully prepare pure β-NaYF4:Yb 3+ ,Ln 3+ nanorods (NRs) surface coated for solvent-selective dispersions. Along with the so-characterized Er 3+ and Tm 3+ ions as Ln 3+ activators, we focused also on Tb 3+ ions since it shows an unusual up-conversion process. The distinctive energy level structure of Tb ions does not match with the 980 nm excitation. However, the long-lived excited 5 D4 level can be populated based on the energy migration mechanism 28 in which two adjacent Yb 3+ ions cooperatively sensitise one Tb 3+ ion. This process have been demonstrated in Yb,Tb co-doped single crystals of SrCl2 29 and ceramic glasses containing LiYbF4, 30 NaLuF4 31 and NaYF4 32 nanocrystals but very scarcely reported on NaGdF4 33 and -NaYF4 34 nanoparticles. This approach can also be extended to other lanthanide ions such as Eu 3+ , Dy 3+ , or Sm 3+ and afford tuneable emissions spanning from the UV to the visible spectral region. Thus, this simple and universal strategy to prepare NIRactivated fluorescent β-NaYF4 nanorods represents a significant advance in the field.  Table SI1 and Figure SI1.
The optimized synthesis employs benzyl alcohol and lanthanide acetates as solvent and Ln 3+ precursors, respectively. Benzyl alcohol and benzyl mercaptan have previously been employed to prepare metal oxides 35 and sulphides 36 by non-aqueous routes with good results in controlling particle size but never tried for the synthesis of fluoride crystals. In such studies, the alcohol and mercaptan groups promoted the metal-oxygen-metal and metal-sulphur-metal bonds. The synthetic route reported in here is not analogous since the main roles of the benzyl alcohol is to act as suitable solvent in terms of solubility in water (4 g/100 mL), thermal stability (boiling point of 205ºC) and probably as capping agent minimizing the nanocrystal aggregation. Although a detailed mechanistic study goes beyond the scope of this communication, the results will be shortly discussed.
The synthesis of the UC nanorods includes a pre-heating at 60 ºC to completely dissolve the lanthanide acetates and the sodium fluoride in the benzyl alcohol/water mixture (4/1 molar ratio).
The use of lanthanide acetates instead of stearates, which are salts commonly employed in the synthesis of lanthanide-doped fluoride nanocrystals, 37 favours the dissolution of the precursors, without significant coordinating effect of the acetate ligand. After 5 minutes at 60 ºC, the temperature is rapidly increased to 180 ºC and kept for 10 minutes. In this step, the kinetically controlled nucleation of -NaYF4 seeds and the nanocrystal growth promoting the -to- phase 38 transformation takes place. Further growth of the thermodynamically favored-NaYF4 phase is drastically reduced by a fast cooling of the reactor.
We further take advantage of the particles surface reactivity just after the MW synthesis to graft hydrophilic, such as polyvinylpyrrolidone (PVP), or hydrophobic, such as oleic acid (OA), molecules on the particle's surface. A functional coating was formed by simple injection of the capping ligands into the reaction tubes, which were kept at 80 ºC after the nanocrystals synthesis to better preserve their surface reactivity. We adopted this strategy instead of the addition of PVP or OA during the UCNRs synthesis because both molecules demonstrated to suffer oxidation or degradation processes during the microwave treatment. The formation of PVP-and OA-capped UCNRs was verified by FTIR analysis ( Figure SI2). [39][40][41] Figure 2a shows a TEM image of the bare UCNRs doped with Tb, as an example. The particles exhibit an anisotropic morphology with rather uniform transversal size (or diameter) of around 15 nm and a longitudinal size distribution ranging between 30 and 100 nm (Fig. 2e). The rods appear stacked along the longitudinal axis forming small aggregates of few NRs. In accordance to the XRD results, the diffraction rings of the SAED patterns for the UCNRs (Fig. 2d) were indexed to the β-phase. In comparison to previous reports in which MW routes are used to prepare small monodisperse -NaYF4 nanocrystals 18 , our synthesis of -NaYF4 nanorods provides larger but less uniform particle sizes. The larger sizes are beneficial for the UC photoluminescence since it is wellestablished that the particle size dramatically influences the non-radiative properties (multiphonon relaxation and energy transfer) reducing the UC efficiency drastically, from values of 13-14% to 0.001% in Er,Yb:NaYF4 particles of few microns to few nanometers (<8 nm). This effect is related to the presence of defects at the surface of the nanocrystals. 42,43 Regarding the less uniform morphology, we can find an explanation on the model of phase transition reported by M. Berry. 38 We hypothesize that the size polydispersity would be related to small differences in the duration of the stage during which the α particle ripens, before β particles begin to appear. Therefore, it would be interesting to explore in the future if mild annealing conditions for long time can be used to refine nanomaterials shape. Anyway, the described methodology provides pure hexagonal nanorods in extremely short time (15 min) with excellent up-conversion luminescence intensity.
After coating with PVP molecules (see Fig. 2b), the small aggregates disappear and the nanorods are found isolated with an average inter-rods distance of ca. 8 nm, a distance compatible with a uniform PVP coating. In the case of OA-grafted NRs, the particles also appeared less aggregated than for the bare UCNR but the inter-rod distance is not so evident (Figure 2c). Figure 2f depicts the nanoparticles stability in polar and non-polar solvents. The as-obtained rods quickly sediment in all solvents (acetone is used here to illustrate this fact). In contrast, the rods coated with PVP are colloidally stable for months when dispersed in water; similarly, the particles coated with oleic acid remain well dispersed in cyclohexane.   It is remarkable to see the intense manifold emissions from 5 D4, 5 D3 → 7 FJ (J = 6, 5, 4, 3) transitions of Tb 3+ generated by excitation of two Yb 3+ ions and simultaneous energy transfer at the ground state 7 F6, which then populates the excited 5 D4 and 5 D3 levels. 343434 To our knowledge, this is the first example of phonon assisted cooperative sensitisation in pure β-NaYF4:Yb 3+ ,Tb 3+ to date.
The visible colour emission is efficiently tuned by changing the activator ion (Tm, Tb, Er), spanning from blue to green shades, as represented in the colour coordinates (Figure 4 right).
To sum up, this work reports a ultrafast, cheap and easily scalable microwave route to prepare pure hexagonal NaYF4:Yb,Ln 3+ nanorods of small size with strong upconversion luminescence.
The process allows an easy coating with hydrophilic or hydrophobic molecules to render the rods dispersible in solvents of different nature. The crystal growth process is kinetically controlled by the short reaction time and permits the choice of multiple active ions (Er, Tm, Tb…) without strongly affecting the nanorods crystallinity, morphology or size. Thus, tuneable visible colours can be readily prepared by rational selection of the doping ions. Owing to their small size, excellent crystallinity and good dispersibility, these nanorods could be used for a wide range of application.. Therefore, it is expected that this simple and ultrafast route to prepare the so-used NaYF4 nanocrystals will be warmly received by the big research community working on up-conversion nanomaterials.