The pursuit of stability in halide perovskites: the monovalent cation and the key for surface and bulk self-healing.

We find significant differences between degradation and healing at the surface or in the bulk for each of the different APbBr3 single crystals (A = CH3NH3+, methylammonium (MA); HC(NH2)2+, formamidinium (FA); and cesium, Cs+). Using 1- and 2-photon microscopy and photobleaching we conclude that kinetics dominate the surface and thermodynamics the bulk stability. Fluorescence-lifetime imaging microscopy, as well as results from several other methods, relate the (damaged) state of the halide perovskite (HaP) after photobleaching to its modified optical and electronic properties. The A cation type strongly influences both the kinetics and the thermodynamics of recovery and degradation: FA heals best the bulk material with faster self-healing; Cs+ protects the surface best, being the least volatile of the A cations and possibly through O-passivation; MA passivates defects via methylamine from photo-dissociation, which binds to Pb2+. DFT simulations provide insight into the passivating role of MA, and also indicate the importance of the Br3- defect as well as predicts its stability. The occurrence and rate of self-healing are suggested to explain the low effective defect density in the HaPs and through this, their excellent performance. These results rationalize the use of mixed A-cation materials for optimizing both solar cell stability and overall performance of HaP-based devices, and provide a basis for designing new HaP variants.

Measurements of the stabilities of isolated retinal chromophores.

The barrier energies for isomerization and fragmentation were measured for a series of retinal chromophore derivatives using a tandem ion mobility spectrometry approach. These measurements allow us to quantify the effect of charge delocalization on the rigidity of chromophores. We find that the role of the methyl group on the C13 position is pivotal regarding the ground state dynamics of the chromophore. Additionally, a correlation between quasi-equilibrium isomer distribution and fragmentation pathways is observed.

Origin and structure of polar domains in doped molecular crystals.

Doping is a primary tool for the modification of the properties of materials. Occlusion of guest molecules in crystals generally reduces their symmetry by the creation of polar domains, which engender polarization and pyroelectricity in the doped crystals. Here we describe a molecular-level determination of the structure of such polar domains, as created by low dopant concentrations (<0.5%). The approach comprises crystal engineering and pyroelectric measurements, together with dispersion-corrected density functional theory and classical molecular dynamics calculations of the doped crystals, using neutron diffraction data of the host at different temperatures. This approach is illustrated using centrosymmetric α-glycine crystals doped with minute amounts of different L-amino acids. The experimentally determined pyroelectric coefficients are explained by the structure and polarization calculations, thus providing strong support for the local and global understanding of how different dopants influence the properties of molecular crystals.

Development of latent fingerprints using a corona discharge.

A novel technique for the development of latent fingerprints is presented. It is based on placing a fingerprint-bearing object inside a corona discharge induced plasma. The development of various real and artificial fingerprints on metallic substrates under a wide range of conditions is studied. Using the results of the development experiments and the results of X-ray photoelectron spectroscopy, it is shown that the development is based on oxidation of the fingerprint background. This is achieved by strong oxidizers generated by the discharge process, while saturated fatty-acids found in sebaceous fingerprints protect the area beneath them, resulting in a visible fingerprint. The process is optimized by minimizing the electrode gap distance and maximizing the peak discharge voltage and the pulse repetition frequency.

Effects of mirror jitter and loss modulation in Q-switched lasers: a state-space analysis.

The effect of mirror instability and jitter in Q-switched lasers is investigated. Pulse shapes and timing are significantly disturbed by these effects. A simple model is presented based on a state-space approach. Several types of pulses are obtained, which are compared with experimental traces of a CO(2) rotating mirror Q-switched laser. Loss modulation as a global means of pulse shaping is also discussed.