Upconversion photoluminescence is a nonlinear effect where multiple lower energy excitation photons produce higher energy emission photons.

Despite recent achievements to reduce surface quenching in NaYF₄:Yb,Er nanocrystals, a complete understanding of how the nanocrystal size affects the brightness of upconversion luminescence is still incomplete. Here we investigated upconversion luminescence of Yb,Er-doped nanocrystals in a broad range of sizes from 6 nm to 45 nm (cubic or hexagonal phases), displaying an increasing red-to-green luminescence intensity ratio and reduced luminescence lifetimes with decreasing size. By analyzing the upconversion process with a set of rate equations, we found that their asymptotic analytic solutions explain lower decay rates of red compared to green upconversion luminescence. Furthermore, we quantified the effect of the surface on luminescence lifetime in a model where nanocrystal emitters are divided between the near-surface and inside regions of each nanocrystal. We clarify the influence of the four nonradiative recombination mechanisms (intrinsic phonon modes, vibration energy of surface ligands, solvent-mediated quenching, and surface defects) on the decay rates for different-size nanocrystals, and find that the defect density dominates decay rates for small (below 15 nm) nanocrystals. Our results indicate that a defect-reduction strategy is a key step in producing small upconversion nanocrystals with increased brightness for a variety of bioimaging and biosensing applications.

Measurements of piezoelectric coefficients $d_{33}$ and $d_{31}$ in wurtzite GaN and AlN using an interferometric technique are presented. We report on the clamped values, $d_{33}^c$ of these coefficients found in GaN and AlN thin films, and we derive the respective bulk values, $d_{33}^b.$ The clamped value of $d_{33}^c$ in GaN single crystal films is $\text{2.8±0.1\hspace{0.05em}}{\mathrm{pm V}}^{\mathrm{-1}}$ which is 30% higher than in polycrystalline films grown by laser assisted chemical vapor deposition. The value of $d_{33}^b$ in bulk single crystal GaN is found to be $\text{3.7±0.1\hspace{0.05em}}{\mathrm{pm V}}^{\mathrm{-1}}.$ The value of $d_{33}^c$ in plasma assisted and laser assisted chemical vapor deposited AlN films was $\text{3.2±0.3}$ and $\text{4.0±0.1\hspace{0.05em}}{\mathrm{pm V}}^{\mathrm{-1}},$ respectively. The bulk value estimate of $d_{33}^b$ in AlN of $\text{5.6±0.2\hspace{0.05em}}{\mathrm{pm V}}^{\mathrm{-1}}$ was deduced. The values of $d_{31},$ both clamped and bulk, were calculated for wurtzite GaN and AlN. We have also calculated the values of $d_{14}$ in cubic phase film and bulk GaN and AlN. Interferometric measurements of the inverse piezoelectric effect provide a simple method of identifying the positive direction of the c axis, which was found to be pointing away from the substrate for all films.

Liposomes have been well established as an effective drug delivery system, due to simplicity of their preparation and unique characteristics. However conventional liposomes are unsuitable for the on-demand content release, which limits their therapeutic utility. Here we report X-ray-triggerable liposomes incorporating gold nanoparticles and photosensitizer verteporfin. The 6 MeV X-ray radiation induces verteporfin to produce singlet oxygen, which destabilises the liposomal membrane and causes the release of cargos from the liposomal cavity. This triggering strategy is demonstrated by the efficiency of gene silencing in vitro and increased effectiveness of chemotherapy in vivo. Our work indicates the feasibility of a combinatorial treatment and possible synergistic effects in the course of standard radiotherapy combined with chemotherapy delivered via X-ray-triggered liposomes. Importantly, our X-ray-mediated liposome release strategy offers prospects for deep tissue photodynamic therapy, by removing its depth limitation.