UHV Deposition and Characterization of a Mononuclear Iron(III) \u3b2-diketonate Complex on Au(111)

The adsorption of the sterically hindered \u3b2-diketonate complex Fe(dpm)3, where Hdpm = dipivaloylmethane, on Au(111) was investigated by ultraviolet photoelectron spectroscopy (UPS) and scanning tunnelling microscopy (STM). The high volatility of the molecule limited the growth of the film to a few monolayers. While UPS evidenced the presence of the \u3b2-diketonate ligands on the surface, the integrity of the molecule on the surface could not be assessed. The low temperature STM images were more informative and at submonolayer coverage they showed the presence of regular domains characterized by a flat morphology and height of 480.3 nm. Along with these domains, tetra-lobed features adsorbed on the kinks of the herringbone were also observed. DFT-simulated images of the pristine molecule and its possible decomposition products allowed to assess the partial fragmentation of Fe(dpm)3 upon adsorption on the Au(111) surface. Structural features with intact molecules were only observed for the saturation coverage. An ex situ prepared thick film of the complex was also investigated by X-ray magnetic circular dichroism (XMCD) and features typical of high-spin iron(III) in octahedral environment were observed

Magnetic fingerprint of individual Fe4 molecular magnets under compression by a scanning tunnelling microscope

Single-molecule magnets (SMMs) present a promising avenue to develop spintronic technologies. Addressing individual molecules with electrical leads in SMM-based spintronic devices remains a ubiquitous challenge: interactions with metallic electrodes can drastically modify the SMM\u2019s properties by charge transfer or through changes in the molecular structure. Here, we probe electrical transport through individual Fe4 SMMs using a scanning tunnelling microscope at 0.5 K. Correlation of topographic and spectroscopic information permits identification of the spin excitation fingerprint of intact Fe4 molecules. Building from this, we find that the exchange coupling strength within the molecule\u2019s magnetic core is significantly enhanced. First-principles calculations support the conclusion that this is the result of confinement of the molecule in the two-contact junction formed by the microscope tip and the sample surface

Anion-order driven polar interfaces at LaTiO₂N surfaces

Perovskite oxynitrides have recently attracted attention for their ability to photocatalytically split water. Compared to oxides the arrangement of anions in the material represents a further structural degree of freedom. The bulk oxynitride LaTiO2N prefers a bonding-dominated cis nitrogen arrangement, while we have previously shown that the (001) surface prefers a non-polar trans order to compensate polarity. Here we consider, using density functional theory calculations, the polar/non-polar interface that would necessarily be present between the two anion orders. We show that the Ti-terminated surface will adopt up to two trans ordered surface layers, which has a beneficial effect on the oxygen evolution efficiency. We then consider the hypothetical case of a polar cis ordered surface layer atop a non-polar trans bulk and show that similar electronic reconstructions as in the LaAlO3/SrTiO3 interface can be expected when interfaces between different anion orders are engineered in one and the same oxynitride material

Surface structure and anion order of the oxynitride LaTiO₂N

Oxynitrides are promising materials for water splitting under visible light. Members of this class of semiconductors that crystallise in the perovskite structure are often characterised by O/N disorder, while some studies observe 2D cis-chain ordering of the M–N–M bonds in the bulk. Despite the fact that the surface structure and composition is expected to have a significant influence on the surface chemistry and therefore the photocatalytic activity, little is known about the O/N arrangement at surfaces of these materials. In the present study, we investigate the surface structure of LaTiO2N, a particularly promising candidate for water splitting, using density functional theory (DFT) calculations. Based on slab calculations with different anion order we find that the N atoms prefer to form trans-chains at the (001) surface, as opposed to the bulk. This is governed by the electrostatic stability that is optimal for alternating charge-neutral (LaN)–(TiO2) atomic layers. We show that polar surfaces that do not fulfil this requirement will electronically or structurally reconstruct. Our results predict that in contact with vacuum, the LaTiO2N (001) surface will preferentially be LaN-terminated

Surface Chemistry of Perovskite Oxynitride Photocatalysts: A Computational Perspective

Perovskite oxynitrides are an established class of photocatalyst materials for water splitting. Previous computational studies have primarily focused on their bulk properties and have drawn relevant conclusions on their light absorption and charge transport properties. The actual catalytic conversions, however, occur on their surfaces and a detailed knowledge of the atomic-scale structure and processes on oxynitride surfaces is indispensable to further improve these materials. In this contribution, we summarize recent progress made in the understanding of perovskite oxynitride surfaces, highlight key processes that set these materials apart from their pure oxide counterparts and discuss challenges and possible future directions for research on oxynitrides