Identification and characterization of the intermediate phase in hybrid organic-inorganic MAPbI3 perovskite.

Perovskite films were prepared using single step solution deposition at different annealing temperatures and annealing times. The crystal structure, phases and grain size were investigated with XRD, XPS and SEM/EDX. The prepared films show a typical orientation of tetragonal perovskite phase and a gradual transition at room temperature from the yellow intermediate phase to the black perovskite phase. Films with high purity were obtained by sintering at 100 °C. In addition, the chemical composition and crystal structure of intermediate phase were investigated in detail. FTIR, UV-vis and NMR spectra revealed the occurance of DMF complexes. Interestingly, the intermediate phase could be transformed to the black perovskite phase upon X-ray irradiation. In addition, the recovery of the aged perovskite films from a yellow intermediate phase back to the black perovskite was shown to be viable via heating and X-ray irradiation.

Effect of the functionalization of the axial phthalocyanine ligands on the energy transfer in QD-based donor-acceptor pairs.

This study examines the electronic coupling between quantum dots (QDs) and molecules on their surfaces as a function of the modality of their interaction. As a probe, the energy transfer (ET) between CdSe QDs and phthalocyanines (Pcs) was monitored and evaluated with regard to the functionalization of the axial phthalocyanine ligand, bulkiness of the functional group bridging the QD donor and Pc acceptor, and the number of the functionalized axial ligands. New silicon PCs and their conjugates with CdSe QDs were synthesized. The ET efficiency and kinetics were studied by steady state and femtosecond time-resolved absorption spectroscopy. We observed a decrease in ET efficiency with the increase in functional group bulkiness, which could be explained by increasing steric hindrance between the ET pair. In addition, a higher ET efficiency was observed for amino and thiol functionalized Pcs compared to Pcs without functional group on the axial alkyl chain.

Quantum dot-based energy transfer: perspectives and potential for applications in photodynamic therapy.

Quantum dots have emerged as an important class of material that offers great promise to a diverse range of applications ranging from energy conversion to biomedicine. Here, we review the potential of using quantum dots and quantum dot conjugates as sensitizers for photodynamic therapy (PDT). The photophysics of singlet oxygen generation in relation to quantum dot-based energy transfer is discussed and the possibility of using quantum dots as photosensitizer in PDT is assessed, including their current limitations to applications in biological systems. The biggest advantage of quantum dots over molecular photosensitizers that comes into perspective is their tunable optical properties and surface chemistries. Recent developments in the preparation and photophysical characterization of quantum dot energy transfer processes are also presented in this review, to provide insights on the future direction of quantum dot-based photosensitization studies from the viewpoint of our ongoing research.

Ligand effect on the growth and the digestion of Co nanocrystals.

The reaction product of cobalt carbonyl decomposition depends on the concentration of the oleic acid ligand. With a low concentration of ligand, nanocrystals nucleate and grow to large ferromagnetic particles through the process of Ostwald ripening and coalescence coarsening. With a high concentration of ligand, stable cluster complexes are formed. Addition or removal of ligand from the reaction products can interchange the formation of cluster complexes and nanocrystals.

Investigation of the crystallization process in 2 nm CdSe quantum dots.

Investigation of the growth of CdSe nanocrystals ( approximately 160 atoms) to the uniquely stable size of 2 nm allows the monitoring of the crystallization process in semiconductor quantum dots. By using a combination of optical techniques, high-resolution transmission electron microscopy (HRTEM), and powder X-ray diffractometry (XRD), new phenomena were explored during the CdSe nanocrystal growth process, which involved significant morphological reconstruction and crystallization of the initially formed amorphous nanoparticles. During the crystallization, the absorption onset of the CdSe quantum dots blue shifted toward higher energies at 3 eV (414 nm), while the photoluminescence red shifted to lower energies. Furthermore, an apparent increasing Stokes shift was observed during the formation of small CdSe nanoparticles. On the other hand, the photoluminescence excitation spectra showed constant features over the reaction time. Additionally, results from HRTEM and XRD studies show that the CdSe nanoparticles were amorphous at early reaction stages and became better crystallized after longer reaction times, while the particle size remained the same during the crystallization process. These observations demonstrate the important role of the surface on the optical properties of small CdSe quantum dots and facilitated the spectroscopic monitoring of the crystallization process in quantum dots.

Semiconductor quantum dots for photodynamic therapy.

The applicability of semiconductor QDs in photodynamic therapy (PDT) was evaluated by studying the interaction between CdSe QDs with a known silicon phthalocyanine PDT photosensitizer, Pc4. The study revealed that the QDs could be used to sensitize the PDT agent through a fluorescence resonance energy transfer (FRET) mechanism, or interact directly with molecular oxygen via a triplet energy-transfer process (TET). Both mechanisms result in the generation of reactive singlet oxygen species that can be used for PDT cancer therapy.

Fluorescence resonance energy transfer reveals a binding site of a photosensitizer for photodynamic therapy.

Phthalocyanine (Pc) 4, like many photosensitizers for photodynamic therapy (PDT), localizes to intracellular membranes, especially mitochondria. Pc 4-PDT photodamages Bcl-2 and Bcl-xL, antiapoptotic proteins interacting with the permeability transition pore complex that forms at contact sites between the inner and outer mitochondrial membranes. These complexes and the inner membrane are unique in containing the phospholipid cardiolipin. Nonyl-acridine orange (NAO) is a specific probe of cardiolipin. Here we show evidence for fluorescence resonance energy transfer from NAO to Pc 4, defining a binding site for the photosensitizer. This observation establishes an innovative tool for exploring the localization of other photosensitizers and additional fluorescent, mitochondrion-localizing drugs having appropriate spectral properties.

Experimental evidence for the photoisomerization of higher fullerenes.

The initial relaxation dynamics of the photoexcited fullerenes C60, C70, C76, C84, C86, and C90 were investigated by dispersion-free femtosecond pump-probe spectroscopy. Under identical excitation conditions, the formation of the lowest excited state slows down for the larger fullerenes. This trend in dynamics, monitored throughout the visible and NIR range, is found to correlate with the number of isomers in accordance with the isomerization mechanism suggested by Stone and Wales. The Stone-Wales isomerization was calculated as thermally inaccessible but photoinduced barrierless. The energy difference of the isomers is in the 1 meV range, and back-isomerization is observed on the picosecond time scale. The characteristic spectrally broad transient absorption of the investigated fullerenes is promising for fast optical gating applications.