Volume and surface effects on two-photonic and three-photonic processes in dry co-doped upconversion nanocrystals

Abstract

Despite considerable advances in synthesizing high-quality core/shell upconversion (UC) nanocrystals (NC; UCNC) and UCNC photophysics, the application of near-infrared (NIR)-excitable lanthanide-doped UCNC in the life and material sciences is still hampered by the relatively low upconversion luminescence (UCL) of UCNC of small size or thin protecting shell. To obtain deeper insights into energy transfer and surface quenching processes involving Yb3+ and Er3+ ions, we examined energy loss processes in differently sized solid core NaYF4 nanocrystals doped with either Yb3+ (YbNC; 20% Yb3+) or Er3+ (ErNC; 2% Er3+) and co-doped with Yb3+ and Er3+ (YbErNC; 20% Yb3+ and 2% Er3+) without a surface protection shell and coated with a thin and a thick NaYF4 shell in comparison to single and co-doped bulk materials. Luminescence studies at 375 nm excitation demonstrate back-energy transfer (BET) from the 4G11/2 state of Er3+ to the 2F5/2 state of Yb3+, through which the red Er3+4F9/2 state is efficiently populated. Excitation power density (P)-dependent steady state and time-resolved photoluminescence measurements at different excitation and emission wavelengths enable to separate surface-related and volume-related effects for two-photonic and three-photonic processes involved in UCL and indicate a different influence of surface passivation on the green and red Er3+ emission. The intensity and lifetime of the latter respond particularly to an increase in volume of the active UCNC core. We provide a three-dimensional random walk model to describe these effects that can be used in the future to predict the UCL behavior of UCNC.

Simultaneously Achieving Multicolor Emission of Down‐Shifting and Up‐Conversion in Yb3+, Er3+ ‐Codoped Cs2NaGdCl6 Double Perovskites

“…Upconversion (UC) luminescence materials have garnered significant attention in display and bio-sensing applications due to the high color purity, long fluorescence lifetime, and low energy excitation. [20][21][22][23][24][25][26] NaGdF 4 and NaYF 4 , known for their low phonon energy, are suitable host for rare earth doping, while the core@shell structure usually is commonly employed to enhance luminescence and modulate emission color. However, the synthesis process for these materials is complex, involving nitrogen protection and high reaction temperature.…”

Probing nearby molecular vibrations with lanthanide-doped nanocrystals

“…9,11,14–16 EVET is undesired for most applications but can be prevented by growing a protective shell of undoped material around the doped core. 2,5,6,8,11–13,17,18 Alternatively, control over EVET rates from different excited states of the lanthanide means control over their respective QYs, which is an interesting strategy to tune the spectral distribution of the PL. 6…”

Lanthanide-Doped KMgF3 Upconversion Nanoparticles for Photon Avalanche Luminescence with Giant Nonlinearities

“…In this regard, materials with low phonon energies and aliovalent Ln 3+ doping such as KMgF 3 : Ln 3+ that can minimize the nonradiative energy loss and maximize the CR rate by forming Ln 3+ clusters could be ideal candidates for achieving the efficient PA of Ln 3+ . However, UCNPs with aliovalent Ln 3+ doping are vulnerable to charged defects, especially internal hydroxyls that are readily introduced during the synthesis. − It is well recoganized that the internal OH – inside the lattice and the external OH – on the surface are two main detrimental factors affecting the upconversion efficiency of Ln 3+ -doped UCNPs. − While the influence of surface OH – can be largely mitigated through strategies such as core/shell engineering, − surface reconstruction, − and dye sensitization, the internal OH – defects, however, are difficult to remove from UCNPs, which could be the reason that pre-PA energy looping instead of PA was usually observed in previous studies of PA in Ln 3+ -doped UCNPs. − …”