Charge Transfer and Orbital Reconstruction at an Organic-Oxide Interface.

The two-dimensional electron system (2DES) located at the surface of strontium titanate (STO) and at several other STO-based interfaces has been an established platform for the study of novel physical phenomena since its discovery. Here we report how the interfacing of STO and tetracyanoquinodimethane (TCNQ) results in a charge transfer that depletes the number of free carriers at the STO surface, with a strong impact on its electronic structure. Our study paves the way for efficient tuning of the electronic properties, which promises novel applications in the framework of oxide/organic-based electronics.

Controlling On-Surface Photoactivity: The Impact of π-Conjugation in Anhydride-Functionalized Molecules on a Semiconductor Surface.

On-surface synthesis has become a prominent method for growing low-dimensional carbon-based nanomaterials on metal surfaces. However, the necessity of decoupling organic nanostructures from metal substrates to exploit their properties requires either transfer methods or new strategies to perform reactions directly on inert surfaces. The use of on-surface light-induced reactions directly on semiconductor/insulating surfaces represents an alternative approach to address these challenges. Here, exploring the photochemical activity of different organic molecules on a SnSe semiconductor surface under ultra-high vacuum, we present a novel on-surface light-induced reaction. The selective photodissociation of the anhydride group is observed, releasing CO and CO2. Moreover, we rationalize the relationship between the photochemical activity and the π-conjugation of the molecular core. The different experimental behaviour of two model anhydrides was elucidated by theoretical calculations, showing how the molecular structure influences the distribution of the excited states. Our findings open new pathways for on-surface synthesis directly on technologically relevant substrates.

Unraveling intrinsic correlation effects with angle-resolved photoemission spectroscopy.

Interaction effects can change materials properties in intriguing ways, and they have, in general, a huge impact on electronic spectra. In particular, satellites in photoemission spectra are pure many-body effects, and their study is of increasing interest in both experiment and theory. However, the intrinsic spectral function is only a part of a measured spectrum, and it is notoriously difficult to extract this information, even for simple metals. Our joint experimental and theoretical study of the prototypical simple metal aluminum demonstrates how intrinsic satellite spectra can be extracted from measured data using angular resolution in photoemission. A nondispersing satellite is detected and explained by electron-electron interactions and the thermal motion of the atoms. Additional nondispersing intensity comes from the inelastic scattering of the outgoing photoelectron. The ideal intrinsic spectral function, instead, has satellites that disperse both in energy and in shape. Theory and the information extracted from experiment describe these features with very good agreement.

Rotation in an Enantiospecific Self-Assembled Array of Molecular Raffle Wheels.

Tailored nano-spaces can control enantioselective adsorption and molecular motion. We report on the spontaneous assembly of a dynamic system-a rigid kagome network with each pore occupied by a guest molecule-employing solely 2,6-bis(1H-pyrazol-1-yl)pyridine-4-carboxylic acid on Ag(111). The network cavity snugly hosts the chemically modified guest, bestows enantiomorphic adsorption and allows selective rotational motions. Temperature-dependent scanning tunnelling microscopy studies revealed distinct anchoring orientations of the guest unit switching with a 0.95 eV thermal barrier. H-bonding between the guest and the host transiently stabilises the rotating guest, as the flapper on a raffle wheel. Density functional theory investigations unravel the detailed molecular pirouette of the guest and how the energy landscape is determined by H-bond formation and breakage. The origin of the guest's enantiodirected, dynamic anchoring lies in the specific interplay of the kagome network and the silver surface.

Catalyst Proximity-Induced Functionalization of h-BN with Quat Derivatives.

Inert single-layer boron nitride (h-BN) grown on a catalytic metal may be functionalized with quaternary ammonium compounds (quats) that are widely used as nonreactive electrolytes. We observe that the quat treatment, which facilitates the electrochemical transfer of two-dimensional materials, involves a decomposition of quat ions and leads to covalently bound quat derivatives on top of the 2D layer. Applying tetraoctylammonium and h-BN on rhodium, the reaction product is top-alkylized h-BN as identified with high-resolution X-ray photoelectron spectroscopy. The alkyl chains are homogeneously distributed across the surface, and the properties thereof are well-tunable by the choice of different quats. The functionalization further weakens the 2D material-substrate interaction and promotes easy transfer. Therefore, the functionalization scheme that is presented enables the design of 2D materials with tailored properties and with the freedom to position and orient them as required. The mechanism of this functionalization route is investigated with density functional theory calculations, and we identify the proximity of the catalytic metal substrate to alter the chemical reactivity of otherwise inert h-BN layers.

Investigation of the surface species during temperature dependent dehydrogenation of naphthalene on Ni(111).

The temperature dependent dehydrogenation of naphthalene on Ni(111) has been investigated using vibrational sum-frequency generation spectroscopy, X-ray photoelectron spectroscopy, scanning tunneling microscopy, and density functional theory with the aim of discerning the reaction mechanism and the intermediates on the surface. At 110 K, multiple layers of naphthalene adsorb on Ni(111); the first layer is a flat lying chemisorbed monolayer, whereas the next layer(s) consist of physisorbed naphthalene. The aromaticity of the carbon rings in the first layer is reduced due to bonding to the surface Ni-atoms. Heating at 200 K causes desorption of the multilayers. At 360 K, the chemisorbed naphthalene monolayer starts dehydrogenating and the geometry of the molecules changes as the dehydrogenated carbon atoms coordinate to the nickel surface; thus, the molecule tilts with respect to the surface, recovering some of its original aromaticity. This effect peaks at 400 K and coincides with hydrogen desorption. Increasing the temperature leads to further dehydrogenation and production of H2 gas, as well as the formation of carbidic and graphitic surface carbon.

Heteroatom-Doped Perihexacene from a Double Helicene Precursor: On-Surface Synthesis and Properties.

We report on the surface-assisted synthesis and spectroscopic characterization of the hitherto longest periacene analogue with oxygen-boron-oxygen (OBO) segments along the zigzag edges, that is, a heteroatom-doped perihexacene 1. Surface-catalyzed cyclodehydrogenation successfully transformed the double helicene precursor 2, i.e., 12a,26a-dibora-12,13,26,27-tetraoxa-benzo[1,2,3-hi:4,5,6-h'i']dihexacene, into the planar perihexacene analogue 1, which was visualized by scanning tunneling microscopy and noncontact atomic force microscopy. X-ray photoelectron spectroscopy, Raman spectroscopy, together with theoretical modeling, on both precursor 2 and product 1, provided further insights into the cyclodehydrogenation process. Moreover, the nonplanar precursor 2 underwent a conformational change upon adsorption on surfaces, and one-dimensional self-assembled superstructures were observed for both 2 and 1 due to the presence of OBO units along the zigzag edges.

Surface science at the PEARL beamline of the Swiss Light Source.

The Photo-Emission and Atomic Resolution Laboratory (PEARL) is a new soft X-ray beamline and surface science laboratory at the Swiss Light Source. PEARL is dedicated to the structural characterization of local bonding geometry at surfaces and interfaces of novel materials, in particular of molecular adsorbates, nanostructured surfaces, and surfaces of complex materials. The main experimental techniques are soft X-ray photoelectron spectroscopy, photoelectron diffraction, and scanning tunneling microscopy (STM). Photoelectron diffraction in angle-scanned mode measures bonding angles of atoms near the emitter atom, and thus allows the orientation of small molecules on a substrate to be determined. In energy scanned mode it measures the distance between the emitter and neighboring atoms; for example, between adsorbate and substrate. STM provides complementary, real-space information, and is particularly useful for comparing the sample quality with reference measurements. In this article, the key features and measured performance data of the beamline and the experimental station are presented. As scientific examples, the adsorbate-substrate distance in hexagonal boron nitride on Ni(111), surface quantum well states in a metal-organic network of dicyano-anthracene on Cu(111), and circular dichroism in the photoelectron diffraction of Cu(111) are discussed.

Surface aligned magnetic moments and hysteresis of an endohedral single-molecule magnet on a metal.

The interaction between the endohedral unit in the single-molecule magnet Dy_{2}ScN@C_{80} and a rhodium (111) substrate leads to alignment of the Dy 4f orbitals. The resulting orientation of the Dy_{2}ScN plane parallel to the surface is inferred from comparison of the angular anisotropy of x-ray absorption spectra and multiplet calculations in the corresponding ligand field. The x-ray magnetic circular dichroism is also angle dependent and signals strong magnetocrystalline anisotropy. This directly relates geometric and magnetic structure. Element specific magnetization curves from different coverages exhibit hysteresis at a sample temperature of ∼4 K. From the measured hysteresis curves, we estimate the zero field remanence lifetime during x-ray exposure of a submonolayer to be about 30 seconds.

EUV-induced hydrogen desorption as a step towards large-scale silicon quantum device patterning.

Atomically precise hydrogen desorption lithography using scanning tunnelling microscopy (STM) has enabled the development of single-atom, quantum-electronic devices on a laboratory scale. Scaling up this technology to mass-produce these devices requires bridging the gap between the precision of STM and the processes used in next-generation semiconductor manufacturing. Here, we demonstrate the ability to remove hydrogen from a monohydride Si(001):H surface using extreme ultraviolet (EUV) light. We quantify the desorption characteristics using various techniques, including STM, X-ray photoelectron spectroscopy (XPS), and photoemission electron microscopy (XPEEM). Our results show that desorption is induced by secondary electrons from valence band excitations, consistent with an exactly solvable non-linear differential equation and compatible with the current 13.5 nm (~92 eV) EUV standard for photolithography; the data imply useful exposure times of order minutes for the 300 W sources characteristic of EUV infrastructure. This is an important step towards the EUV patterning of silicon surfaces without traditional resists, by offering the possibility for parallel processing in the fabrication of classical and quantum devices through deterministic doping.

Unconventional Band Structure via Combined Molecular Orbital and Lattice Symmetries in a Surface-Confined Metallated Graphdiyne Sheet.

Graphyne (GY) and graphdiyne (GDY)-based monolayers represent the next generation 2D carbon-rich materials with tunable structures and properties surpassing those of graphene. However, the detection of band formation in atomically thin GY/GDY analogues has been challenging, as both long-range order and atomic precision have to be fulfilled in the system. The present work reports direct evidence of band formation in on-surface synthesized metallated Ag-GDY sheets with mesoscopic (≈1 µm) regularity. Employing scanning tunneling and angle-resolved photoemission spectroscopies, energy-dependent transitions of real-space electronic states above the Fermi level and formation of the valence band are respectively observed. Furthermore, density functional theory (DFT) calculations corroborate the observations and reveal that doubly degenerate frontier molecular orbitals on a honeycomb lattice give rise to flat, Dirac and Kagome bands close to the Fermi level. DFT modeling also indicates an intrinsic band gap for the pristine sheet material, which is retained for a bilayer with h-BN, whereas adsorption-induced in-gap electronic states evolve at the synthesis platform with Ag-GDY decorating the (111) facet of silver. These results illustrate the tremendous potential for engineering novel band structures via molecular orbital and lattice symmetries in atomically precise 2D carbon materials.

Ferromagnetic Order in 2D Layers of Transition Metal Dichlorides.

Magnetism in two dimensions is traditionally considered an exotic phase mediated by spin fluctuations, but far from collinearly ordered in the ground state. Recently, 2D magnetic states have been discovered in layered van der Waals compounds. Their robust and tunable magnetic state by material composition, combined with reduced dimensionality, foresee a strong potential as a key element in magnetic devices. Here, a class of 2D magnets based on metallic chlorides is presented. The magnetic order survives on top of a metallic substrate, even down to the monolayer limit, and can be switched from perpendicular to in-plane by substituting the metal ion from iron to nickel. Using functionalized STM tips as magnetic sensors, local exchange fields are identified, even in the absence of an external magnetic field. Since the compounds are processable by molecular beam epitaxy techniques, they provide a platform with large potential for incorporation into current device technologies.

Resistless EUV lithography: Photon-induced oxide patterning on silicon.

In this work, we show the feasibility of extreme ultraviolet (EUV) patterning on an HF-treated silicon (100) surface in the absence of a photoresist. EUV lithography is the leading lithography technique in semiconductor manufacturing due to its high resolution and throughput, but future progress in resolution can be hampered because of the inherent limitations of the resists. We show that EUV photons can induce surface reactions on a partially hydrogen-terminated silicon surface and assist the growth of an oxide layer, which serves as an etch mask. This mechanism is different from the hydrogen desorption in scanning tunneling microscopy-based lithography. We achieve silicon dioxide/silicon gratings with 75-nanometer half-pitch and 31-nanometer height, demonstrating the efficacy of the method and the feasibility of patterning with EUV lithography without the use of a photoresist. Further development of the resistless EUV lithography method can be a viable approach to nanometer-scale lithography by overcoming the inherent resolution and roughness limitations of photoresist materials.

An endohedral single-molecule magnet with long relaxation times: DySc2N@C80.

The magnetism of DySc(2)N@C(80) endofullerene was studied with X-ray magnetic circular dichroism (XMCD) and a magnetometer with a superconducting quantum interference device (SQUID) down to temperatures of 2 K and in fields up to 7 T. XMCD shows hysteresis of the 4f spin and orbital moment in Dy(III) ions. SQUID magnetometry indicates hysteresis below 6 K, while thermal and nonthermal relaxation is observed. Dilution of DySc(2)N@C(80) samples with C(60) increases the zero-field 4f electron relaxation time at 2 K to several hours.

Electron-momentum dependence of electron-phonon coupling underlies dramatic phonon renormalization in YNi2B2C.

Electron-phonon coupling, i.e., the scattering of lattice vibrations by electrons and vice versa, is ubiquitous in solids and can lead to emergent ground states such as superconductivity and charge-density wave order. A broad spectral phonon line shape is often interpreted as a marker of strong electron-phonon coupling associated with Fermi surface nesting, i.e., parallel sections of the Fermi surface connected by the phonon momentum. Alternatively broad phonons are known to arise from strong atomic lattice anharmonicity. Here, we show that strong phonon broadening can occur in the absence of both Fermi surface nesting and lattice anharmonicity, if electron-phonon coupling is strongly enhanced for specific values of electron-momentum, k. We use inelastic neutron scattering, soft x-ray angle-resolved photoemission spectroscopy measurements and ab-initio lattice dynamical and electronic band structure calculations to demonstrate this scenario in the highly anisotropic tetragonal electron-phonon superconductor YNi2B2C. This new scenario likely applies to a wide range of compounds.

Metamagnetic transition and a loss of magnetic hysteresis caused by electron trapping in monolayers of single-molecule magnet Tb2@C79N.

Realization of stable spin states in surface-supported magnetic molecules is crucial for their applications in molecular spintronics, memory storage or quantum information processing. In this work, we studied the surface magnetism of dimetallo-azafullerene Tb2@C79N, showing a broad magnetic hysteresis in a bulk form. Surprisingly, monolayers of Tb2@C79N exhibited a completely different behavior, with the prevalence of a ground state with antiferromagnetic coupling at low magnetic field and a metamagnetic transition in the magnetic field of 2.5-4 T. Monolayers of Tb2@C79N were deposited onto Cu(111) and Au(111) by evaporation in ultra-high vacuum conditions, and their topography and electronic structure were characterized by scanning tunneling microscopy and spectroscopy (STM/STS). X-ray photoelectron spectroscopy (XPS), in combination with DFT studies, revealed that the nitrogen atom of the azafullerene cage tends to avoid metallic surfaces. Magnetic properties of the (sub)monolayers were then studied by X-ray magnetic circular dichroism (XMCD) at the Tb-M4,5 absorption edge. While in bulk powder samples Tb2@C79N behaves as a single-molecule magnet with ferromagnetically coupled magnetic moments and blocking of magnetization at 28 K, its monolayers exhibited a different ground state with antiferromagnetic coupling of Tb magnetic moments. To understand if this unexpected behavior is caused by a strong hybridization of fullerenes with metallic substrates, XMCD measurements were also performed for Tb2@C79N adsorbed on h-BN|Rh(111) and MgO|Ag(100). The co-existence of two forms of Tb2@C79N was found on these substrates as well, but magnetization curves showed narrow magnetic hysteresis detectable up to 25 K. The non-magnetic state of Tb2@C79N in monolayers is assigned to anionic Tb2@C79N- species with doubly-occupied Tb-Tb bonding orbital and antiferromagnetic coupling of the Tb moments. A charge transfer from the substrate or trapping of secondary electrons are discussed as a plausible origin of these species.

Ferromagnetic Layers in a Topological Insulator (Bi,Sb)2Te3 Crystal Doped with Mn.

Magnetic topological insulators (MTIs) have recently become a subject of poignant interest; among them, Z2 topological insulators with magnetic moment ordering caused by embedded magnetic atoms attract special attention. In such systems, the case of magnetic anisotropy perpendicular to the surface that holds a topologically nontrivial surface state is the most intriguing one. Such materials demonstrate the quantum anomalous Hall effect, which manifests itself as chiral edge conduction channels that can be manipulated by switching the polarization of magnetic domains. In the present paper, we uncover the atomic structure of the bulk and the surface of Mn0.06Sb1.22Bi0.78Te3.06 in conjunction with its electronic and magnetic properties; this material is characterized by naturally formed ferromagnetic layers inside the insulating matrix, where the Fermi level is tuned to the bulk band gap. We found that in such mixed crystals septuple layers (SLs) of Mn(Bi,Sb)2Te4 form structures that feature three SLs, each of which is separated by two or three (Bi,Sb)2Te3 quintuple layers (QLs); such a structure possesses ferromagnetic properties. The surface obtained by cleavage includes terraces with different terminations. Manganese atoms preferentially occupy the central positions in the SLs and in a very small proportion can appear in the QLs, as indirectly indicated by a reshaped Dirac cone.

Break of symmetry at the surface of IrTe2upon phase transition measured by x-ray photoelectron diffraction.

IrTe2undergoes a series of charge-ordered phase transitions below room temperature that are characterized by the formation of stripes of Ir dimers of different periodicities. Full hemispherical x-ray photoelectron diffraction (XPD) experiments have been performed to investigate the atomic position changes undergone near the surface of 1T-IrTe2in the first-order phase transition, from the (1 × 1) phase to the (5 × 1) phase. Comparison between experiment and simulation allows us to identify the consequence of the dimerization on the Ir atoms local environment. We report that XPD permits to unveil the break of symmetry of IrTe2trigonal to a monoclinic unit cell and confirm the occurrence of the (5 × 1) reconstruction within the first few layers below the surface with a staircase-like stacking of dimers.

Order from a Mess: The Growth of 5-Armchair Graphene Nanoribbons.

The advent of on-surface chemistry under vacuum has vastly increased our capabilities to synthesize carbon nanomaterials with atomic precision. Among the types of target structures that have been synthesized by these means, graphene nanoribbons (GNRs) have probably attracted the most attention. In this context, the vast majority of GNRs have been synthesized from the same chemical reaction: Ullmann coupling followed by cyclodehydrogenation. Here, we provide a detailed study of the growth process of five-atom-wide armchair GNRs starting from dibromoperylene. Combining scanning probe microscopy with temperature-dependent XPS measurements and theoretical calculations, we show that the GNR growth departs from the conventional reaction scenario. Instead, precursor molecules couple by means of a concerted mechanism whereby two covalent bonds are formed simultaneously, along with a concomitant dehydrogenation. Indeed, this alternative reaction path is responsible for the straight GNR growth in spite of the initial mixture of reactant isomers with irregular metal-organic intermediates that we find. The provided insight will not only help understanding the reaction mechanisms of other reactants but also serve as a guide for the design of other precursor molecules.

Determination of the preferred reaction pathway of acetophenone on Si(001) using photoelectron diffraction.

The adsorption configurations of a technologically relevant model organic adsorbate on the silicon (001) surface were studied using energy scanned x-ray photoelectron diffraction (PhD). Previous work has established the existence of an interesting vertically-aligned ('flagpole') configuration, where the acetophenone attaches to Si(001) via the acetyl group carbon and oxygen atoms. Density functional theory calculations have predicted two energetically similar variants of this structure, where the phenyl ring is orientated parallel or perpendicular to the rows of silicon dimers on this reconstructed surface. However, previously published experimental measurements, including scanning tunnelling microscopy, x-ray photoelectron spectroscopy, and near-edge x-ray absorption fine structure investigations were unable to distinguish between these two configurations. Here, we apply the unique experimental capabilities of the PhD technique to this system and demonstrate that the dominant adsorption configuration has the phenyl ring parallel to the dimer rows (the end-bridge structure). This information in turn facilitates the determination of the dominant reaction pathway for acetophenone on Si(001), which has remained elusive until now. Information about subtle preferences in reaction pathways that affect the alignment and orientation of organic adsorbates such as acetophenone on technologically-relevant semiconductor surfaces such as Si(001) is critical for the fabrication of future atomically-precise atomic and molecular-scale electronic devices utilising the organic-silicon interface, and this work demonstrates the unique and complementary capabilities of PhD for providing this information.