Single-band upconversion emission in lanthanide-doped KMnF3 nanocrystals.

The preparation of upconversion (UC) nanocrystals that exhibit anti-Stokes emission is important for applications in fields as diverse as photonics, photovoltaics, biological imaging, and therapeutics. In particular, there has been an increasing focus on the synthesis of nanocrystals with tunable UC emission from ultraviolet to near-infrared through doping with lanthanide ions. However, the synthesis of nanocrystals featuring single-band UC with high chromatic purity remains a formidable challenge, as lanthanide ions generally have more than one metastable excited state. In principle, these lanthanide-doped nanocrystals display multipeak emission profiles. Recently, several attempts have been made to obtain high purity of single-band UC red emission attractive for anticounterfeiting and color display applications. For example, high red-to-green (R/G) emission ratio can be achieved by increasing the concentration of Yb in Yb/Er co-doped NaYF4 nanoparticles. [8] In addition, Yb/Er co-doped MnF2 and KMnF3 nanoparticles have shown substantially enhanced R/G emission ratios because of the energy transfer between the Er and Mn. However, a general method for providing single-band red emission has not been conclusively established. Herein, we report a novel oil-based procedure for the synthesis of lanthanide-doped KMnF3 nanocrystals with only single-band UC emissions from Er, Ho, and Tm dopants, respectively. Importantly, we observe that the singleband feature is independent of dopant concentration, pump power, and temperature. We also show that these nanocrystals can serve as ideal optical biolabels for deep-tissue imaging without the constraints associated with conventional multipeak UC nanocrystals. The lanthanide-doped KMnF3 nanocrystals for the present study were prepared in three steps according to a modified literature procedure. In the first step, lanthanide and manganese precursors were first dissolved in oleic acid at 150 8C to form metal–oleate complexes. A stoichiometric amount of potassium fluoride was then added to a solution of the metal–oleate complexes at room temperature to initiate the crystal growth. Subsequently, the reaction temperature was increased to 290 8C to facilitate the growth of the nanocrystals. Figure 1a shows a typical transmission electron microscopy (TEM) image of the as-synthesized KMnF3:Yb/Er (18:2 mol%) nanocrystals with cubic morphology (Figure S1 in the Supporting Information). The high-magnification TEM mage of a single KMnF3:Yb/Er nanocube shown in Figure 1b reveals lattice fringes of the {110} with a d spacing of 0.31 nm, which is typical for cubic KMnF3 (Figure 1b). Selected-area electron diffraction patterns obtained from the Fourier transform of the high-magnification TEM image confirms single-crystalline cubic phase of the nanocube (Figure 1c). It should be noted that charge balance will be disturbed when the trivalent lanthanide ions are substituted for the Mn ions in KMnF3 nanocrystals. To maintain charge balance, either manganese or potassium vacancies are formed (Figure 1d,e). X-ray powder diffraction studies (Figure S2) show peak positions and intensities that can be well indexed in accordance with cubic KMnF3 crystals (JCPDS file no. 82-1334), which is consistent with TEM analysis of the samples.