Olga Bubnova

Stand up to stand out

Two-dimensional (2D) Ruddlesden–Popper metal halide perovskites (MHPs) offer a promising solution to the notorious moisture sensitivity of bulk perovskites. However, in devices with vertical architecture, the insulating bulky ammonium cations that separate 2D perovskites slabs impede efficient out-of-plane charge transport. Now, Chen et al. have devised a strategy to enhance optoelectronic properties of 2D MHPs by controlling the alignment of vertically oriented layers. The researchers used a two-step fabrication procedure involving solution processing and low-temperature annealing to achieve thin films with various degrees of molecular orientation. X-ray measurements reveal that nucleation and growth of 2D perovskites occur at the precursor liquid–air interface. The assumed orientation stems from the tendency of the aliphatic chain of butylammonium molecules to remain in the solution environment. By tuning the crystallization conditions, specifically by adding a pre-crystallization annealing step, Chen and co-workers were able to obtain thin films of vertically oriented 2D perovskites with near 100% degree of the out-of-plane preferential alignment. When made into a solar cell, such optimized 2D MHPs show a strong short-circuit current increase and an overall efficiency improvement in comparison with randomly oriented crystals. OB

One and the same

Scanning transmission electron microscopy (STEM) with aberration correction can map atomic order with sub-ångström resolution. Yet, many nanomaterials are sensitive to high-energy electrons, which limits the applicable electron dose. This results in images that present weak contrast, hindering an unambiguous determination of the atomic structure. Jarmo Fatermans et al. now propose an advanced fitting procedure that enables a more reliable and unbiased detection of single atoms in high-noise, lowcontrast electron microscopy images. The researchers combine a known model fitting with an advanced statistical selection method and apply this to STEM images of atomic-scale systems. The fitting approximates the contrast of atomic columns in an STEM image with Gaussian functions on a constant background. Then, they introduce an approximate analytical implementation of a probability rule, which, based on the pixel intensities, determines the probability for a specific number of atomic columns to give rise to this image. The result is a set of configurations with different numbers of atomic rows and their position together with the probability for each configuration. This method is more reliable than conventional peak-fitting methods, avoids the bias in visual inspection and provides the likeliness of one atomic configuration compared to alternative structures. BH

Memory mille feuille

It is notoriously difficult to react only one alkyne moiety of a symmetric dialkyne with an azide, as both alkynes are likely to undergo a [3+ 2] cycloaddition reaction (a click-chemistry reaction) and form a bis-triazole. As a result, lengthy chemical protection and deprotection procedures have been developed. Now, Huxley et al. have reported a direct synthetic strategy to convert a symmetric dialkyne into an alkynefunctionalized triazole by taking advantage of the steric constrains that a metal–organic framework (MOF), used as a nanovessel, imposes on the reaction pathway. The researchers first prepare a Mn-based MOF in which the Mn atoms are about 13 Å apart. They then substitute the bromine atoms, which are attached to each Mn, with an azide moiety, therefore creating an environment in which the azides are uniformly spaced within the pores of the MOF. When reacting the MOF with a Credit: American Chemical Society MeBr