Design and Processing as Ultrathin Films of a Sublimable Iron(II) Spin Crossover Material Exhibiting Efficient and Fast Light-Induced Spin Transition.

Materials based on spin crossover (SCO) molecules have centered the attention in molecular magnetism for more than 40 years as they provide unique examples of multifunctional and stimuli-responsive materials, which can be then integrated into electronic devices to exploit their molecular bistability. This process often requires the preparation of thermally stable SCO molecules that can sublime and remain intact in contact with surfaces. However, the number of robust sublimable SCO molecules is still very scarce. Here, we report a novel example of this kind. It is based on a neutral iron(II) coordination complex formulated as [Fe(neoim)2], where neoimH is the ionogenic ligand 2-(1H-imidazol-2-yl)-9-methyl-1,10-phenanthroline. In the first part, a comprehensive study, which covers the synthesis and magnetostructural characterization of the [Fe(neoim)2] complex as a bulk microcrystalline material, is reported. Then, in the second part, we investigate the suitability of this material to form thin films through high-vacuum sublimation. Finally, the retainment of all present SCO capabilities in the bulk when the material is processed is thoroughly studied by means of X-ray absorption spectroscopy. In particular, a very efficient and fast light-induced spin transition (LIESST effect) has been observed, even for ultrathin films of 15 nm.

Interplay between Optical Emission and Magnetism in the van der Waals Magnetic Semiconductor CrSBr in the Two-Dimensional Limit.

The van der Waals semiconductor metamagnet CrSBr offers an ideal platform for studying the interplay between optical and magnetic properties in the two-dimensional limit. Here, we carried out an exhaustive optical characterization of this material by means of temperature- and magnetic-field-dependent photoluminescence (PL) on flakes of different thicknesses down to the monolayer. We found a characteristic emission peak that is quenched upon switching the ferromagnetic layers from an antiparallel to a parallel configuration and exhibits a temperature dependence different from that of the peaks commonly ascribed to excitons. The contribution of this peak to the PL is boosted around 30-40 K, coinciding with the hidden order magnetic transition temperature. Our findings reveal the connection between the optical and magnetic properties via the ionization of magnetic donor vacancies. This behavior enables a useful tool for the optical reading of the magnetic states in atomically thin layers of CrSBr and shows the potential of the design of 2D heterostructures with magnetic and excitonic properties.

Photoluminescence Enhancement by Band Alignment Engineering in MoS2/FePS3 van der Waals Heterostructures.

Single-layer semiconducting transition metal dichalcogenides (2H-TMDs) display robust excitonic photoluminescence emission, which can be improved by controlled changes to the environment and the chemical potential of the material. However, a drastic emission quench has been generally observed when TMDs are stacked in van der Waals heterostructures, which often favor the nonradiative recombination of photocarriers. Herein, we achieve an enhancement of the photoluminescence of single-layer MoS2 on top of van der Waals FePS3. The optimal energy band alignment of this heterostructure preserves light emission of MoS2 against nonradiative interlayer recombination processes and favors the charge transfer from MoS2, an n-type semiconductor, to FePS3, a p-type narrow-gap semiconductor. The strong depletion of carriers in the MoS2 layer is evidenced by a dramatic increase in the spectral weight of neutral excitons, which is strongly modulated by the thickness of the FePS3 underneath, leading to the increase of photoluminescence intensity. The present results demonstrate the potential for the rational design of van der Waals heterostructures with advanced optoelectronic properties.

Mirror effect in atomic force microscopy profiles enables tip reconstruction.

In this work, the tip convolution effect in atomic force microscopy is revisited to illustrate the capabilities of cubic objects for determination of the tip shape and size. Using molecular-based cubic nanoparticles as a reference, a two-step tip reconstruction process has been developed. First, the tip-to-face angle is estimated by means of an analysis of the convolution error while the tip radius is extracted from the experimental profiles. The results obtained are in good agreement with specification of the tip supplier even though the experiments have been conducted using real distribution of nanoparticles with dispersion in size and aspect ratio. This demonstrates the reliability of our method and opens the door for a more accurate tip reconstruction by using calibration standards.