Photoexcitation of PbS nanosheets leads to highly mobile charge carriers and stable excitons.

Solution-processable two-dimensional (2D) semiconductors with chemically tunable thickness and associated tunable band gaps are highly promising materials for ultrathin optoelectronics. Here, the properties of free charge carriers and excitons in 2D PbS nanosheets of different thickness are investigated by means of optical pump-terahertz probe spectroscopy. By analyzing the frequency-dependent THz response, a large quantum yield of excitons is found. The scattering time of free charge carriers increases with nanosheet thickness, which is ascribed to reduced effects of surface defects and ligands in thicker nanosheets. The data discussed provide values for the DC mobility in the range 550-1000 cm2 V-1 s-1 for PbS nanosheets with thicknesses ranging from 4 to 16 nm. Results underpin the suitability of colloidal 2D PbS nanosheets for optoelectronic applications.

The influence of coarticulatory and phonemic relations on individual compensatory formant production.

Previous auditory perturbation studies have shown that speakers are able to simultaneously use multiple compensatory strategies to produce a certain acoustic target. In the case of formant perturbation, these findings were obtained examining the compensatory production for low vowels /ɛ/ and /æ/. This raises some controversy as more recent research suggests that the contribution of the somatosensory feedback to the production of vowels might differ across phonemes. In particular, the compensatory magnitude to auditory perturbations is expected to be weaker for high vowels compared to low vowels since the former are characterized by larger linguopalatal contact. To investigate this hypothesis, this paper conducted a bidirectional auditory perturbation study in which F2 of the high central vowel /ɨ/ was perturbed in opposing directions depending on the preceding consonant (alveolar vs velar). The consonants were chosen such that speakers' usual coarticulatory patterns were either compatible or incompatible with the required compensatory strategy. The results demonstrate that speakers were able to compensate for applied perturbations even if speakers' compensatory movements resulted in unusual coarticulatory configurations. However, the results also suggest that individual compensatory patterns were influenced by additional perceptual factors attributable to the phonemic space surrounding the target vowel /ɨ/.

Micron-Size Two-Dimensional Methylammonium Lead Halide Perovskites.

Hybrid lead halide perovskites with 2D stacking structures have recently emerged as promising materials for optoelectronic applications. We report a method for growing 2D nanosheets of hybrid lead halide perovskites (I, Br and Cl), with tunable lateral sizes ranging from 0.05 to 8 µm and a structure consisting of n stacked monolayers separated by long alkylamines, tunable from bulk down to n = 1. The key to obtaining such a wide range of perovskite properties hinged on utilizing the respective lead halide nanosheets as precursors in a hot-injection synthesis that afforded careful control over all process parameters. The layered, quantum-confined ( n ≤ 4) nanosheets were comprised of major and minor fractions with differing n. Energy funneling from low to high n (high to low energy) regions within a single sheet, mediated by the length of the ligands between stacks, produced photoluminescent quantum yields as high as 49%. These large, tunable 2D nanosheets could serve as convenient platforms for future high-efficiency optoelectronic devices.

In-Plane Anisotropic Faceting of Ultralarge and Thin Single-Crystalline Colloidal SnS Nanosheets.

The colloidal synthesis of large, thin two-dimensional (2D) nanosheets is fascinating but challenging, since the growth along the lateral and vertical dimensions needs to be controlled independently. In-plane anisotropy in 2D nanosheets is attracting more attention as well. We present a new synthesis for large colloidal single-crystalline SnS nanosheets with the thicknesses down to 7 nm and lateral sizes up to 8 µm. The synthesis uses trioctylphosphine-S (TOP-S) as sulfur source and oleic acid (with or without trioctylphosphine, TOP) as ligands. Upon adjusting the capping ligand amount, the growth direction can be switched between anisotropic directions (armchair and zigzag) and isotropic directions ("ladder" directions), leading to an edge-morphology anisotropy. This is the first report on solution-phase synthesis of large thin tin(II) sulfide (SnS) nanosheets (NSs) with tunable edge faceting. Furthermore, electronic transport measurements show strong dependency on the crystallographic directions confirming structural anisotropy.

Copper sulfide nanosheets with shape-tunable plasmonic properties in the NIR region.

2D copper sulfide nanocrystals are promising building blocks of plasmonic materials in the near-infrared (NIR) spectral region. We demonstrate precise shape and size control (hexagonal/triangle) of colloidal plasmonic copper sulfide (covellite) nano-prisms simply by tuning the precursor concentration without the introduction of additional ligands. The ultra-thin 2D nanocrystals possess sizes between 13 and 100 nm and triangular or hexangular shapes. We also demonstrate CuS nanosheets (NSs) with lateral sizes up to 2 microns using a syringe pump. Based on the experimental findings and DFT simulations, we propose a qualitative and quantitative mechanism for the formation of different shapes. The analysis of the spectral features in the NIR region of the synthesized CuS nanocrystals has been performed with respect to the shape and the size of particles by the discrete dipole approximation method and the Drude-Sommerfeld theory.

Colloidal lead iodide nanorings.

Colloidal chemistry of nanomaterials experienced a tremendous development in the last decades. In the course of the journey 0D nanoparticles, 1D nanowires, and 2D nanosheets have been synthesized. They have in common to possess a simple topology. We present a colloidal synthesis strategy for lead iodide nanorings, with a non-trivial topology. First, two-dimensional structures were synthesized in nonanoic acid as the sole solvent. Subsequently, they underwent an etching process in the presence of trioctylphosphine, which determines the size of the hole in the ring structure. We propose a mechanism for the formation of lead iodide nanosheets which also explains the etching of the two-dimensional structures starting from the inside, leading to nanorings. In addition, we demonstrate a possible application of the as-prepared nanorings in photodetectors. These devices are characterized by a fast response, high gain values, and a linear relation between photocurrent and incident light power intensity over a large range. The synthesis approach allows for inexpensive large-scale production of nanorings with tunable properties.

Insights into the formation mechanism of two-dimensional lead halide nanostructures.

We present a colloidal synthesis strategy for lead halide nanosheets with a thickness of far below 100 nm. Due to the layered structure and the synthesis parameters the crystals of PbI2 are initially composed of many polytypes. We propose a mechanism which gives insight into the chemical process of the PbI2 formation. Further, we found that the crystal structure changes with increasing reaction temperature or by performing the synthesis for longer time periods changing for the final 2H structure. In addition, we demonstrate a route to prepare nanosheets of lead bromide as well as lead chloride in a similar way. Lead halides can be used as a detector material for high-energy photons including gamma and X-rays.

New ways to synthesize lead sulfide nanosheets-substituted alkanes direct the growth of 2D nanostructures.

Two-dimensional colloidal nanosheets represent very attractive optoelectronic materials. They combine good lateral conductivity with solution-processability and geometry-tunable electronic properties. In the case of PbS nanosheets, so far synthesis has been driven by the addition of chloroalkanes as coligands. Here, we demonstrate how to synthesize two-dimensional lead sulfide nanostructures using other halogen alkanes and primary amines. Further, we show that at a reaction temperature of 170 °C a coligand is not even necessary and the only ligand, oleic acid, controls the anisotropic growth of the two-dimensional structures. Also, using thiourea as a sulfide source, nanosheets with lateral dimensions of over 10 µm are possible.