Date Published: December 18, 2017
Publisher: John Wiley and Sons Inc.
Author(s): Bo Zhou, Long Yan, Lili Tao, Nan Song, Ming Wu, Ting Wang, Qinyuan Zhang.
Upconverting materials have achieved great progress in recent years, however, it remains challenging for the mechanistic research on new upconversion strategy of lanthanides. Here, a novel and efficient strategy to realize photon upconversion from more lanthanides and fine control of lanthanide donor–acceptor interactions through using the interfacial energy transfer (IET) is reported. Unlike conventional energy‐transfer upconversion and recently reported energy‐migration upconversion, the IET approach is capable of enabling upconversions from Er3+, Tm3+, Ho3+, Tb3+, Eu3+, Dy3+ to Sm3+ in NaYF4‐ and NaYbF4‐based core–shell nanostructures simultaneously. Applying the IET in a Nd–Yb coupled sensitizing system can also enable the 808/980 nm dual‐wavelength excited upconversion from a single particle. More importantly, the construction of IET concept allows for a fine control and manipulation of lanthanide donor–acceptor interactions and dynamics at the nanometer‐length scale by establishing a physical model upon an interlayer‐mediated nanostructure. These findings open a door for the fundamental understanding of the luminescence dynamics involving lanthanides at nanoscale, which would further help conceive new scientific concepts and control photon upconversion at a single lanthanide ion level.
Photon upconversion of lanthanides has proved to be an important experimental strategy for realizing the anti‐Stokes type emission, which has been obtained in many kinds of materials greatly promoting their diverse frontier applications in solid‐state lasers, displays, photovoltaics, biological imaging, photodynamic therapy, and nanophotonics.1 Benefitting from their unique 4f configuration, the 3+ lanthanides are rich in discrete energy levels, from which the upconverted emissions covering near‐ultraviolet, visible, and near‐infrared spectral region were already obtained in lanthanide‐doped upconversion nanocrystals and bulks.2, 3 The merits of upconversion nanoparticles including sharp emission bandwidths, large anti‐Stokes shifts, and superior photochemical stability make them to be one class of ideal candidate for the biological nanoprobe.4 In addition, their applications in many other frontier fields have further been motivated by the important progresses just recently obtained such as for super‐resolution nanoscopy,5 information security and encryption,6 and in‐depth description of basic physical phenomena like Brownian motion.7
As a proof of concept, we first investigated the photon upconversion performance from the conventionally studied lanthanide ions including Er3+, Tm3+, and Ho3+ through the IET strategy. In this scheme, Yb3+ is adopted as the energy donor because of its capability in absorption of infrared excitation at 980 nm (2F5/2 ← 2F7/2 transition).10 As shown in Figure2a, Yb3+ and luminescent activator A (A = Er, Tm, Ho) are spatially separated into different layers of the core–shell nanostructure, which were synthesized using a two‐step coprecipitation method (Figure S1a, Supporting Information). Epitaxial growth of a shell layer outside core seeds leads to an increment of resultant nanoparticles in size, which remain the hexagonal phase according to powder X‐ray diffraction (XRD) diffraction profiles (Figure S1b, Supporting Information). Moreover, the spatial separation of Yb3+ and Er3+ in respective core and shell layers is clearly observed in the element mapping images (Figure 2b). Upon a 980 nm laser irradiation, typical upconversion emission bands of Er3+, Tm3+, and Ho3+ from the NaYF4:Yb(40 mol%)@NaYF4:A (A = Er, Tm, Ho) core–shell samples were recorded (Figure 2c and Figures S2 and S3 (Supporting Information)). In contrast, without doping of Yb3+ in the core area, almost no upconversion was recorded for the control NaYF4@NaYF4:A core–shell nanoparticles because of the much low absorption at 980 nm (for Er3+) or the absence of energy levels matching with the 980 nm laser photon energy (for Tm3+ and Ho3+) as shown in Figure S4 (Supporting Information). These results have clearly evidenced the validity of Yb3+‐mediated IET strategy for realizing the upconverting emissions from conventional upconversion lanthanides.
In conclusion, we have experimentally demonstrated that interfacial energy transfer is an efficient and more general strategy for achieving the photon upconversion from a series of lanthanides. By constructing an interlayer‐thickness controllable trilayer nanostructure, we further developed a physical model for quantitatively examining the interactions involving lanthanide donor–acceptor pairs (Yb–Er/Tm/Ho, Gd–Eu/Tb, and Nd–Yb) at nanometer levels, and their separation for efficiently facilitating the energy transfer was precisely determined to be confined in a range less than 1.6–2.1 nm. These findings present an in‐depth insight into the mechanistic understanding of upconversion luminescence physics involving lanthanides at the nanometer‐length scale. More significantly, they may help to construct new scientific concepts and ingenious experimental designs for manipulating and controlling photon upconversion at a single lanthanide ion level in the near future.
Materials: The materials including yttrium(III) acetate hydrate (99.9%), gadolinium(III) acetate hydrate (99.9%), lutetium(III) acetate hydrate (99.9%), lanthanum(III) acetate hydrate (99.9%), ytterbium(III) acetate hydrate (99.99%), neodymium(III) acetate hydrate (99.9%), erbium(III) acetate hydrate (99.9%), holmium(III) acetate hydrate (99.9%), thulium(III) acetate hydrate (99.9%), europium(III) acetate hydrate (99.9%), terbium(III) acetate hydrate (99.9%), dysprosium(III) acetate hydrate (99.9%), samarium(III) acetate hydrate (99.9%), oleic acid (90%), 1‐octadecene (90%), sodium hydroxide (NaOH; >98%), and ammonium fluoride (NH4F; >98%) were all purchased from Sigma‐Aldrich, and used as received unless otherwise noted.
The authors declare no conflict of interest.