The most commonly proposed mechanisms for NIR-to-red upconversion in the well-studied material β-NaYF4:Er(3+),Yb(3+) are evaluated in order to resolve inconsistencies that persist in the literature. Each of four possible mechanisms is evaluated in terms of the direct analysis of spectroscopic data. It is shown that there are no important mechanisms that involve the first excited state of Er(3+), (4)I13/2, as an intermediate state. A large body of evidence overwhelmingly supports the proposed mechanism of Anderson et al., which suggests an intimate connection between NIR-to-red and NIR-to-blue upconversion. Namely, both red and blue upconversion are produced primarily by a three-photon excitation process that proceeds through the green emitting state to a dense manifold of states, (4)G/(2)K, above the blue emitting state, (2)H9/2. Competing relaxation mechanisms out of (4)G/(2)K determine the relative amounts of blue and red upconversion produced. Multiphonon relaxation from (4)G/(2)K results in blue upconversion, whereas back energy transfer from Er(3+)((4)G/(2)K) to Yb(3+)((2)F7/2) results in red emission.

Here, we show that the long-accepted mechanism for the production of red and blue emission through upconversion (UC) of 1 μm excitation in Yb(3+)/Er(3+)-doped materials does not apply in the popular β-NaYF4 host. We propose a new mechanism involving Yb(3+)-to-Er(3+) energy-transfer UC out of the green-emitting (2)H11/2,(4)S3/2 states that quantitatively accounts for all of the observed optical behavior. Rate constants for the relevant radiative and nonradiative processes are reported along with a prediction of the power dependence of the pulsed and continuous-wave UC quantum efficiency.

Counterfeiting costs governments and private industries billions of dollars annually due to loss of value in currency and other printed items. This research involves using lanthanide doped β-NaYF(4) nanoparticles for security printing applications. Inks comprised of Yb(3+)/Er(3+) and Yb(3+)/Tm(3+) doped β-NaYF(4) nanoparticles with oleic acid as the capping agent in toluene and methyl benzoate with poly(methyl methacrylate) (PMMA) as the binding agent were used to print quick response (QR) codes. The QR codes were made using an AutoCAD file and printed with Optomec direct-write aerosol jetting(®). The printed QR codes are invisible under ambient lighting conditions, but are readable using a near-IR laser, and were successfully scanned using a smart phone. This research demonstrates that QR codes, which have been used primarily for information sharing applications, can also be used for security purposes. Higher levels of security were achieved by printing both green and blue upconverting inks, based on combinations of Er(3+)/Yb(3+) and Tm(3+)/Yb(3+), respectively, in a single QR code. The near-infrared (NIR)-to-visible upconversion luminescence properties of the two-ink QR codes were analyzed, including the influence of NIR excitation power density on perceived color, in term of the CIE 1931 chromaticity index. It was also shown that this security ink can be optimized for line width, thickness and stability on different substrates.

Nanocrystals of β-NaYF4:Yb3+, Er3+ generally have lower NIR-to-visible upconversion (UC) internal quantum efficiency, IQE, compared to high-quality bulk materials, and exhibit more rapid UC dynamics, typical of quenching, when excited with a pulsed source near 980 nm. The addition of a protective shell increases the IQE of the nanocrystals and slows the overall excited-state dynamics. Here, we show that an extension of a recently developed model for UC in powders of micron-sized β-NaYF4:18%Yb3+, 2%Er3+ crystals correctly predicts the time-resolved luminescence curve shapes, relative intensities, and observed drop in IQE of the various emission lines for core and core–shell nanoparticles following pulsed excitation. The model clearly shows that the nanoscale effect on visible upconversion luminescence in these materials, with typical high-Yb3+ and low-Er3+ doping, is largely due to rapid energy migration among Yb3+(2F5/2) and Er3+(4I11/2) ions at the 1 μm energy level, such that an equilibrium is achieved between interior sites and rapidly relaxing surface sites. The faster kinetics observed in visible emission following pulsed NIR excitation is mainly a propagation of the effect of surface quenching of the 1 μm reservoir states and is not due to direct quenching of the visible emitting states themselves. For Er3+ ions contributing to UC emission, the relaxation rate constants for the blue (2H9/2), green (2H11/2, 4S3/2), and red (4F9/2) emitting states are essentially unchanged from their bulk values, indicating that Er3+ ions close to the nanoparticle surface are nearly silent with regard to UC. The addition of a passive β-NaYF4 shell retards the drain of the 1 μm excitation reservoir and recovers the participation of outer Er3+ sites in UC. The dependence of IQE on shell thickness is well explained in terms of a Förster-type model describing an energy donor (Er3+, Yb3+) interacting with a thin plane layer of acceptors (oleate). The UC behavior of both the core and the core–shell nanocrystals can be modeled, almost quantitatively, solely on the basis of quenching at the 1 μm level, without separate consideration of a near-surface Er3+ population. However, a two-layer model for the core nanoparticles is revealing with regard to the modest extent to which near-surface ions do participate in UC and gives a better representation of the detailed dynamics of the NIR emitting states. A method is presented for allowing investigators to estimate the IQE for any nanosample (with 18% Yb3+, 2%Er3+ doping) as a function of excitation power density (cw) or pulse-energy density based on the low pulse energy measurement of the decay constant for the 1 μm emission.

Recent advances in producing pre-defined 2D patterns of upconversion nanophosphors via photolithography and printing techniques present new opportunities for the use of these materials in security applications. Here, we demonstrate an RGB additive-color printing system that produces highlyresolved pre-defined patterns that are invisible under ambient lighting, but which are viewable as luminescent multi-color images under NIR excitation. Patterns are generated by independent deposition of three primary-color (red, green and blue) upconverting inks using an aerosol jet printer. The primarycolor inks are printed as isolated and overlapping features to produce images that simultaneously emit red, green, blue, cyan, magenta, yellow and white upconversion luminescence. The dependence of the chromaticity of certain secondary colors (cyan and magenta) and white on NIR excitation power density can be exploited as an additional authentication feature. The development of an RGB upconversion printing system paves the way for an entirely new arena in security printing.