Tuning Phonon Energies in Lanthanide‐doped Potassium Lead Halide Nanocrystals for Enhanced Nonlinearity and Upconversion

Abstract

Optical applications of lanthanide‐doped nanoparticles require materials with low phonon energies to minimize nonradiative relaxation and promote nonlinear processes like upconversion. Heavy halide hosts offer low phonon energies but are challenging to synthesize as nanocrystals. Here, we demonstrate the size‐controlled synthesis of low‐phonon‐energy KPb2X5 (X=Cl, Br) nanoparticles and the ability to tune nanocrystal phonon energies as low as 128 cm−1. KPb2Cl5 nanoparticles are moisture resistant and can be efficiently doped with lighter lanthanides. The low phonon energies of KPb2X5 nanoparticles promote upconversion luminescence from higher lanthanide excited states and enable highly nonlinear, avalanche‐like emission from KPb2Cl5 : Nd3+ nanoparticles. The realization of nanoparticles with tunable, ultra‐low phonon energies facilitates the discovery of nanomaterials with phonon‐dependent properties, precisely engineered for applications in nanoscale imaging, sensing, luminescence thermometry and energy conversion.

The Coming of Age of Neodymium: Redefining Its Role in Rare Earth Doped Nanoparticles

“…Most studies of RENPs are focused on ESA or ETU processes because these upconversion mechanisms can be easily induced by a low-power continuous wave (CW) diode lasers, avoiding the need for expensive high-power pulsed laser sources . ETU is the preferred and most commonly implemented option and is several orders of magnitude more efficient to generate upconversion emission than ESA or cooperative sensitization, although the first demonstration of readily available PA upconversion in RENPs has been recently showcased, and has paved the way for more of such RENPs to emerge, − including avalanche-like emission from Nd 3+ ions in low-phonon-energy RENPs.…”

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

“…In addition to the steep slope and clear threshold, another signature of PA regarding the slowdown in the excited-state population rise time was expected by measuring the time-dependent UCL of the Tm 3+ emission at 802 nm. The rise time, defined as the time needed to reach 95% of the asymptotic value, was found to be substantially increased near the PA threshold and reached the maximum of 281 ms (Figure c,d and Figure S33), which is approximately 104 times the decay time (2.7 ms) of the 3 F 4 state (Figure S34), confirming the prevailing PA mechanism in KMgF 3 : Tm 3+ UCNPs. It is worth mentioning that the rise time of 281 ms is several orders of magnitude shorter than that of traditional PA (from seconds to minutes), signifying a fast response time of PA in KMgF 3 : Tm 3+ UCNPs.…”

“…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. − …”

Laser-Induced Broadband White Emission of NdAlO3 Nanocrystals in Alcohols

“…For NdAlO 3 in vacuum, the shape of the emission can be described of a very broad band comprising the whole visible range with the presence of several well-defined bands. Two different explanations are given in the literature: Nd 3+ reabsorption ,,− and the Nd 3+ avalanche-like mechanism. − Although in methanol small drops in intensity can be seen, when immersed in the other alcohols the well-defined structure disappeared, resulting in only a broadband typical of other materials, ,,, and had a very high resemblance to the LaAlO 3 in vacuum. For this reason, it seems likely that Nd 3+ reabsorptions or avalanche-like emissions are broadened due to interactions with the solvent appearing as a single broadband.…”