Search

In this study, we investigate the "Slip and Stick" mechanism governing the epitaxial growth of CsPbBr 3 on amorphous silica surfaces and its implications for silicon/perovskite tandem solar cell applications. The unique, low-energy diffusion behavior of cesium lead bromide on amorphous silica enables molecular species to traverse the surface efficiently without bond-breaking, thereby preserving structural integrity. Consequently, the chemically inert nature of amorphous silica facilitates the formation of crystalline CsPbBr 3 thin films on silicon substrates, which is essential for tandem solar cell architectures. In contrast, the reactive silicon (111) surface, that induces fragment decomposition and Br doping of Si, poses challenges to device stability due to potential disruptions in structural and electronic continuity. Our findings elucidate the observed difficulties in epitaxially growing metal halide perovskites directly on silicon surfaces and underscore the role of amorphous silica as an ideal passivation layer, promoting the precise layer-by-layer assembly necessary for high-efficiency tandem solar cells.

Perovskites exhibit outstanding performance in applications such as photocatalysis, electrochemistry, or photovoltaics, yet their practical use is hindered by the instability of these materials under operating conditions, specifically caused by the segregation of alkali cations toward the surface. The problem arises from the bulk strain related to different cation sizes, as well as the inherent electrostatic instability of perovskite surfaces. Here, we focus on atomistic details of the surface-driven process of interlayer switching of alkali atoms at the inorganic perovskite surface. We show that the (001) surface of KTaO 3 cleaved at room temperature contains equally populated TaO 2 and KO terminations, while the uncompensated polarity of these terminations promotes diffusion of KO from the subsurface toward the topmost surface layer at temperatures as low as 200 °C. This effect is directly probed at the atomic scale by Atomic Force Microscopy and the chemical properties of the resulting surfaces are investigated by the adsorption of CO and H 2 O. The experiments indicate that KO segregation is associated with the formation of K and O vacancies in the near-surface region, which is further supported by depth-dependent X-ray Photoelectron Spectroscopy measurements and Density Functional Theory calculations. Our study shows that the KO segregation influences the surface reactivity both toward CO and water, which was probed at the atomic scale.

Ferroionic materials combine ferroelectric properties and spontaneous polarization with ionic phenomena of fast charge recombination and electrodic functionalities. In this paper, we propose the concept of tunable polarization in CeO 2-δ (ceria) thin (5 nm) films induced by built-in remnant polarization of a BaTiO 3 (BTO) ferroelectric thin film interface, which is buried under the ceria layer. Upward and downward fixed polarizations at the BTO thin film (10 nm) are achieved by the lattice termination engineering of the SrO or TiO 2 terminated Nb:SrTiO 3 (NSTO or STN) substrate. We find that the ceria layer punctually replicates the polarization of the BTO interface via a dynamic reconfiguration of its intrinsic defects, i.e., oxygen vacancies and small polarons. Tunable oxidative or reducing properties (redox) also arise at the surface from the built-in polarization. Opposite polarities at the ceria termination tune the chemo-physical dynamics toward water molecule adsorbates. The inversion of the surface potential leads to a modulation of the water adsorption-desorption equilibrium and water ionization (splitting) redox overpotentials within ±400 mV at room temperature, depending on the ceria termination's charges. Such tunability opens up the perspectives of using ferroionics for wireless electrochemically enhanced catalysis.

The emergence of correlated phenomena arising from the combination of 1T and 1H van der Waals layers is the focus of intense research. Here, we synthesize a self-stacked 6R phase in NbSeTe, showing perfect alternating 1T and 1H layers that grow coherently along the c-direction, as revealed by scanning transmission electron microscopy. Angle-resolved photoemission spectroscopy shows a mixed contribution of the trigonal and octahedral Nb bands to the Fermi level. Diffuse scattering reveals temperature-independent short-range charge fluctuations with propagation vector q CO = (0.25 0), derived from the condensation of a longitudinal mode in the 1T layer, while the long-range charge density wave is quenched by ligand disorder. Magnetization measurements suggest the presence of an inhomogeneous, short-range magnetic order, further supported by the absence of a clear phase transition in the specific heat. These experimental analyses in combination with ab initio calculations indicate that the ground state of 6R-NbSeTe is described by a statistical distribution of short-range charge-modulated and spin-correlated regions driven by ligand disorder. Our results demonstrate how natural 1T-1H self-stacked bulk heterostructures can be used to engineer emergent phases of matter.

Hybrid perovskites are well-known for their optoelectronic and photovoltaic properties. Molecular dynamics simulations allow the study of these soft and ionic crystals by including dynamical effects (e.g., molecular rotations, octahedra tilting, ionic diffusion and hysteresis), yet the high computational cost restricts the use of accurate ab initio forces for bulk or small atomic systems. Hence, great interest exists in the development of classical force-fields for hybrid perovskites of low and linear scaling computational cost, via both empirical methods and machine-learning. This work aims at extending the transferability of our MYP0 model, which has been successfully tailored to methylammonium lead iodide (MAPI) and applied to the study of molecular rotations, vibrations, diffusion of defects, and many other properties. The extended model, named MYP2, improves the description of inorganic or hybrid fragments and the processes of crystal formation while preserving a good description of bulk properties. More importantly, it allows for the direct simulation of the crystal growth of cubic MAPI from deposition of PbI and MAI precursors on the surfaces. Our findings pave the way toward classical force-fields able to model the microstructure evolution of hybrid perovskites and the crystalline synthesis from deposition in vacuo .

Two-dimensional metal-organic frameworks (2D-MOFs) represent a category of atomically thin materials that combine the structural tunability of molecular systems with the crystalline structure characteristic of solids. The strong bonding between the organic linkers and transition metal centers is expected to result in delocalized electronic states. However, it remains largely unknown how the band structure in 2D-MOFs emerges through the coupling of electronic states in the building blocks. Here, we demonstrate the on-surface synthesis of a 2D-MOF exhibiting prominent π-conjugation. Through a combined experimental and theoretical approach, we provide direct evidence of band structure formation upon hierarchical self-assembly, going from metal-organic complexes to a conjugated two-dimensional framework. Additionally, we identify the robustly dispersive nature of the emerging hybrid states, irrespective of the metallic support type, highlighting the tunability of the band structure through charge transfer from the substrate. Our findings encourage the exploration of band-structure engineering in 2D-MOFs for potential applications in electronics and photonics.

The complexity of the geometric and electronic structure of boron allotropes is associated with the multicentric bonding character and the consequent B polymorphism. When growth is limited to two-dimensions (2D), the structural and electronic confinement yields the borophenes family, where the interaction with the templating substrate actually determines the stability of inequivalent boron phases. We report here a detailed study of the growth of the honeycomb AlB 2 phase on Al(111), followed by an investigation of its oxidation and reduction properties. By means of a combined experimental and theoretical approach, we show that the structure of the B/Al interface is affected by the complex interplay between B, Al, and common reactive agents like oxygen and hydrogen. While kinetic effects associated with diffusion and strain release influence the AlB 2 growth in vacuo, Al, B, O, and H chemical affinities determine its redox behavior. Reduction with atomic hydrogen involves the B layer and yields an ordered honeycomb borophane H/AlB 2 phase. Instead, oxidation takes place at the Al interface, giving origin to buried and 1D surface aluminum oxide phases.

Light is a versatile tool to remotely activate molecules adsorbed on a surface, for example, to trigger their polymerization. Here, we explore the spatial distribution of light-induced chemical reactions on a Au(111) surface. Specifically, the covalent on-surface polymerization of an anthracene derivative in the submonolayer coverage range is studied. Using scanning tunneling microscopy and X-ray photoemission spectroscopy, we observe a substantial increase of the local molecular coverage with the sample illumination time at the center of the laser spot. We find that the interplay between thermally induced diffusion and the reduced mobility of reaction products steers the accumulation of material. Moreover, the debromination of the adsorbed species never progresses to completion within the experiment time, despite a long irradiation of many hours., Competing Interests: The authors declare no competing financial interest., (© 2024 The Authors. Published by American Chemical Society.)

In recent years, the correlation between the existence of topological electronic states in materials and their catalytic activity has gained increasing attention, due to the exceptional electron conductivity and charge carrier mobility exhibited by quantum materials. However, the physicochemical mechanisms ruling catalysis with quantum materials are not fully understood. Here, we investigate the chemical reactivity, ambient stability, and catalytic activity of the topological nodal-line semimetal AuSn 4 . Our findings reveal that the surface of AuSn 4 is prone to oxidation, resulting in the formation of a nanometric SnO 2 skin. This surface oxidation significantly enhances the material's performance as a catalyst for the hydrogen evolution reaction in acidic environments. We demonstrate that the peculiar atomic structure of oxidized AuSn 4 enables the migration of hydrogen atoms through the Sn-O layer with a minimal energy barrier of only 0.19 eV. Furthermore, the Volmer step becomes exothermic in the presence of Sn vacancies or tin-oxide skin, as opposed to being hindered in the pristine sample, with energy values of -0.62 and -1.66 eV, respectively, compared to the +0.46 eV energy barrier in the pristine sample. Our model also suggests that oxidized AuSn 4 can serve as a catalyst for the hydrogen evolution reaction in alkali media. Additionally, we evaluate the material's suitability for the carbon dioxide reduction reaction, finding that the presence of topologically protected electronic states enhances the migration of hydrogen atoms adsorbed on the catalyst to carbon dioxide.

Tin diselenide (SnSe 2 ) is a van der Waals semiconductor, which spontaneously forms a subnanometric SnO 2 skin once exposed to air. Here, by means of surface-science spectroscopies and density functional theory, we have investigated the charge redistribution at the SnO 2 -SnSe 2 heterojunction in both oxidative and humid environments. Explicitly, we find that the work function of the pristine SnSe 2 surface increases by 0.23 and 0.40 eV upon exposure to O 2 and air, respectively, with a charge transfer reaching 0.56 e - /SnO 2 between the underlying SnSe 2 and the SnO 2 skin. Remarkably, both pristine SnSe 2 and defective SnSe 2 display chemical inertness toward water, in contrast to other metal chalcogenides. Conversely, the SnO 2 -SnSe 2 interface formed upon surface oxidation is highly reactive toward water, with subsequent implications for SnSe 2 -based devices working in ambient humidity, including chemical sensors. Our findings also imply that recent reports on humidity sensing with SnSe 2 should be reinterpreted, considering the pivotal role of the oxide skin in the interaction with water molecules.

We have studied the production of neutral fragments in high-Rydberg (HR) states at the C 1s and O 1s edges of the CO2 molecule by performing two kinds of experiments. First, the yields of neutral HR fragments were measured indirectly by ionizing such fragments in a static electric field and by collecting resulting singly charged positive ions as a function of the photon energy. Such measurements reveal not only excitations below the core ionization thresholds but also thresholds for single core-hole and shakeup photoionization. Second, we obtained the mass spectra of neutral HR fragments at selected photon energies by exploiting pulsed field ionization; they show atomic fragments C(HR) and O(HR). We discuss dissociation pathways leading to the production of neutral HR fragments in core excitation and ionization of CO2.

Here we report an in-depth structural characterization of transition metal-doped zinc oxide nanoparticles that have recently been used as anode materials for Li-ion batteries. Structural refinement of powder X-ray diffraction (XRD) data allowed the determination of small though reproducible changes in the unit cell dimensions of four ZnO samples (wurtzite structure) prepared with different dopants or different synthesis conditions. Moreover, large variations of the full width at half-maximum of the XRD reflections indicate that the crystallinity of the samples decreases in the order ZnO, Zn0.9Co0.1O, Zn0.9Fe0.1O/C, and Zn0.9Fe0.1O (the crystallite sizes as determined by Williamson-Hall plots are 42, 29, 15, and 13 nm, respectively). X-ray absorption spectroscopy data indicate that Co is divalent, whereas Fe is purely trivalent in Zn0.9Fe0.1O and 95% trivalent (Fe(3+)/(Fe(3+) + Fe(2+)) ratio = 0.95) in Zn0.9Fe0.1O/C. The aliovalent substitution of Fe(3+) for Zn(2+) implies the formation of local defects around Fe(3+) such as cationic vacancies or interstitial oxygen for charge balance. The EXAFS (extended X-ray absorption fine structure) data, besides providing local Fe-O and Co-O bond distances, are consistent with a large amount of charge-compensating defects. The Co-doped sample displays similar EXAFS features to those of pure ZnO, suggesting the absence of a large concentration of defects as found in the Fe-doped samples. These results are of substantial importance for understanding and elucidating the modified electrochemical lithiation mechanism by introducing transition metal dopants into the ZnO structure for the application as lithium-ion anode material.