Upconversion core/shell nanocrystals with different mean sizes ranging from 15 to 45 nm were prepared via a modified synthesis procedure based on anhydrous rare-earth acetates. All particles consist of a core of NaYF :Yb,Er, doped with 18 % Yb and 2 % Er , and an inert shell of NaYF , with the shell thickness being equal to the radius of the core particle. Absolute measurements of the photoluminescence quantum yield at a series of different excitation power densities show that the quantum yield of 45 nm core/shell particles is already very close to the quantum yield of microcrystalline upconversion phosphor powder. Smaller core/shell particles prepared by the same method show only a moderate decrease in quantum yield. The quantum yield of 15 nm core/shell particles, for instance, is reduced by a factor of three compared to the bulk upconversion phosphor at high power densities (100 W cm ) and by approximately a factor of 10 at low power densities (1 W cm ).

The upconversion quantum yield (ΦUC) is an essential parameter for the characterization of the optical performance of lanthanoid-doped upconverting nanoparticles (UCNPs). Despite its nonlinear dependence on excitation power density (Pexc), it is typically reported only as a single number. Here, we present the first measurement of absolute upconversion quantum yields of the individual emission bands of blue light-emitting LiYF4:Yb3+,Tm3+ UCNPs in toluene. Reporting the quantum yields for the individual emission bands is required for assessing the usability of UCNPs in various applications that require upconverted light of different wavelengths, such as bioimaging, photocatalysis and phototherapy. Here, the reliability of the ΦUC measurements is demonstrated by studying the same batch of UCNPs in three different research groups. The results show that whereas the total upconversion quantum yield of these UCNPs is quite high-typically 0.02 at a power density of 5 W cm-2-most of the upconverted photon flux is emitted in the 794 nm upconversion band, while the blue emission band at 480 nm is very weak, with a much lower quantum yield of ∼6 × 10-5 at 5 W cm-2. Overall, although the total upconversion quantum yield of LiYF4:Yb3+,Tm3+ UCNPs seems satisfying, notably for NIR bioimaging, blue-light demanding phototherapy applications will require better-performing UCNPs with higher blue light upconversion quantum yields.

A set of similarly sized (Yb3+, Nd3+, Er3+)-doped upconversion nanoparticles of different architecture were spectroscopically examined in water at broadly varied excitation power at 980 nm & 808 nm to study the sensitizer dependent penetration-depth.

arsenic precursor [2,16] besides the less reactive tris(trimethylgermyl)arsane (As(GeMe 3 ) 3 ) [17] or highly toxic arsine gas (AsH 3 ). [18] Recently, Srivastava et al. [19] and Grigel et al. [20] highlighted tris(dimethylamino)arsane (As(NMe 2 ) 3 ) as commercially available pre cursor for the facile synthesis of InAs QDs. Mechanistically, As(NMe 2 ) 3 undergoes a transamination reaction with the solvent and reagent oleylamine (OLAH) to give trioleyl arsane (As(OLA) 3 ) under the release of three equivalents of flammable dimethylamine. The reaction of As(OLA) 3 with indium(III) chloride (InCl 3 ) to InAs QDs does not proceed without the subsequent addition of a reducing agent for which typically diisobutylaluminum hydride (DIBALH) [19] and nowadays also trioleylphosphane (P(OLA) 3 ) [20] or a N,Ndimethylethylamine com plex (DMEAAlH 3 ) [21] are used. In the case of DIBALH and DMEA AlH 3 , the As-N bonds are cleaved to give in situ (OLA) x AsH 3x species which react with the indium precursor to InAs QDs. [22,23] With the applied concentration of DIBALH the size of InAs QDs can be tuned according to the method by Srivastava et al. [19] By contrast, the approach by Grigel et al. [20] applies in situ generated P(OLA) 3 as reducing agent for the InAs QD synthesis (Scheme 1). This species is known to act both, as a remarkably efficient phos phorus precursor and reducing agent for the preparation of InP QDs. [24][25][26] Grigel et al. [20] pointed out that phase pure InAs QDs are obtained since P(OLA) 3 only provides the electrons required for the reduction of As(OLA) 3 to InAs (Scheme 1).In previous work we introduced tripyrazolylphosphanes (P(pyr) 3 , e.g., pyr = 3,5dimethylpyrazole, 2) for the convenient preparation of a long term stable (>6 months) P(OLA) 3 stock solution and its application for the synthesis of highly emitting InP/ZnS QDs. [5,6] Derivatives of P(pyr) 3 are accessible on a multigram scale and represent a safe to handle, less toxic sur rogate for cancerogenic hexaalkylphosphorous triamide deriva tives such as trisdimethylaminophosphane (P(NMe 2 ) 3 ) and tris diethylaminophosphane (P(NEt 2 ) 3 ). Moreover, the release of cor rosive, flammable, and gaseous dialkylamines (Me 2 NH, Et 2 NH) during the transamination with OLAH is prevented. The released pyrazoles can easily be recovered in high yields (>94%) and reused for the precursor synthesis. The tripyrazolylarsane (As(pyr) 3 , e.g., pyr = 3,5dimethylpyrazole, 1) analogues are envisioned to react similarly, thus, representing a convenient to handle and stable arsenic precursor for the InAs QD synthesis. Tris(3,5-dimethylpyrazolyl)arsane (1) is introduced as a low-cost and convenient to handle arsenic precursor for the straight forward synthesis of InAs quantum dots (QDs). Transamination of 1 with the solvent oleylamine (OLAH) gives trioleylarsane (As(OLA) 3 ) which in the presence of the reducing agents diisobutylaluminum hydride (DIBAL-H) or trioleylphosphane (P(OLA) 3 ) yields InAsQDs via a typical hot injection approach. The size of the obtain...