Under Ultrahigh Vacuum Research Articles

Solution Versus On-Surface Synthesis of Peripherally Oxygen-Annulated Porphyrins through C-O Bond Formation.

This study investigates the synthesis of tetra- and octa-O-fused porphyrinoids employing an oxidative O-annulation approach through C-H activation. Despite encountering challenges such as overoxidation and instability in conventional solution protocols, successful synthesis was achieved on Au(111) surfaces under ultra-high vacuum (UHV) conditions. X-ray photoelectron spectroscopy, scanning tunneling microscopy, and non-contact atomic force microscopy elucidated the preferential formation of pyran moieties via C-O bond formation and subsequent self-assembly driven by C-H⋅⋅⋅O interactions. Furthermore, the O-annulation process was found to reduce the HOMO-LUMO gap by lifting the HOMO energy level, with the effect rising upon increasing the number of embedded O-atoms.

Exploring Order/Disorder Molecular Networks on Metal Surfaces

Materials featuring disordered structures often exhibit unique properties. Metal-organic frameworks (MOFs) are a class of hybrid materials composed of metal nodes and coordinating organic linkers[1]. Recently, there has been growing interests in MOFs with structural disorder, and the investigations of amorphous structures on surfaces[2]. Herein, we demonstrate a bottom-up method to construct molecular networks on metal surfaces with order-disorder transition by preparing two-component samples with different stoichiometric ratios of two organic molecule linkers sharing the same symmetry but differing in size[3]. We investigate the topological and electronic structures of the order/disorder molecular networks by scanning tunneling microscopy (STM) and scanning tunneling spectroscopy (STS) under ultra-high vacuum (UHV). The results may advance our understanding of the formation mechanism and electronic behaviors characteristic of monolayer molecular networks with structural disorders, holding the potential to catalyze advancements in the design and application of new amorphous MOF materials with intriguing properties.[1] L. Dong, et al., Progress in Surface Science, 91, 101–135 (2016).[2] J. Fonseca, et al., Journal of Materials Chemistry A, 9.17, 10562-10611 (2021).[3] J. Lu, et al., ACS Nano, 17, 20, 20194–20202 (2023).

Fabrication of Novel Carbon-based Polymers by On-surface Cycloaromatization of Isopropyl Substituents

Surface-catalyzed reactions have been employed for the synthesis of carbon nanomaterials featuring precisely defined atomic structures. A recent breakthrough involves the gold surface-catalyzed [3 + 3] cycloaromatization of isopropyl-substituted arenes, allowing for on-surface synthesis of novel carbon-based polymers[1-3]. In this process, the surface plays a crucial role in activating the isopropyl substituents, facilitating the formation of phenylene rings through intermolecular coupling. For instance, such investigations might open new avenues in the field of carbon magnetism, which has gained an increased attention in view of the recent progress made in the synthesis and characterization of open-shell polycyclic aromatic hydrocarbons[2]. Under this scenario, the comprehensive fabrication of magnetic polymers emerges as a highly appealing field of research.Here, we introduce exemplary approaches toward the bottom-up fabrication of different carbon-based polymers homocoupled via surface-catalyzed [3 + 3] cycloaromatization of isopropyl substituents studied on Au(111) under ultra-high vacuum (UHV) conditions. The chemical structures of the polymers have been clearly elucidated by non-contact atomic force microscopy (nc-AFM). Scanning tunneling spectroscopy (STS), complemented by computational investigations reveals their ground state. Our approach can be a highly relevant step towards the on-surface synthesis of covalently linked 1D organic polymers with prospects in sensing, catalysis and nanoscale spintronic devices.

Scanning Probe Microscopy Characterization of Biomolecules enabled by Mass-Selective, Soft-landing Electrospray Ion Beam Deposition.

Scanning probe microscopy (SPM), in particular at low temperature (LT) under ultra-high vacuum (UHV) conditions, offers the possibility of real-space imaging with resolution reaching the atomic level. However, its potential for the analysis of complex biological molecules has been hampered by requirements imposed by sample preparation. Transferring molecules onto surfaces in UHV is typically accomplished by thermal sublimation in vacuum. This approach however is limited by the thermal stability of the molecules, i. e. not possible for biological molecules with low vapour pressure. Bypassing this limitation, electrospray ionisation offers an alternative method to transfer molecules from solution to the gas-phase as intact molecular ions. In soft-landing electrospray ion beam deposition (ESIBD), these molecular ions are subsequently mass-selected and gently landed on surfaces which permits large and thermally fragile molecules to be analyzed by LT-UHV SPM. In this concept, we discuss how ESIBD+SPM prepares samples of complex biological molecules at a surface, offering controls of the molecular structural integrity, three-dimensional shape, and purity. These achievements unlock the analytical potential of SPM which is showcased by imaging proteins, peptides, DNA, glycans, and conjugates of these molecules, revealing details of their connectivity, conformation, and interaction that could not be accessed by any other technique.

Structures formed by tetrabromobiphenyl molecules with adapted orientations on the Au(111) surface

We analyzed the structures formed by 3,3’,5,5’-tetrabromobiphenyl (TBBP) on the Au(111) surface under ultra-high vacuum (UHV) conditions using scanning tunneling microscopy. TBBP, which contains two phenyls, is expected to form the same structure after combining with other molecules with different orientations. Islands composed of three types of dimers were obtained at room temperature. Annealing of the dimers at temperatures above 423 K formed a porous nanostructure with neighboring nanohole distances of 0.7 nm, consistent with our expectations. The porous nanostructure was composed of combinations of dimers with various orientations, indicating the flexibility of the precursor molecule in forming the obtained structure.

Low-Temperature Synthesis of WSe2 by the Selenization Process under Ultrahigh Vacuum for BEOL Compatible Reconfigurable Neurons.

Low-temperature large-area growth of two-dimensional (2D) transition-metal dichalcogenides (TMDs) is critical for their integration with silicon chips. Especially, if the growth temperatures can be lowered below the back-end-of-line (BEOL) processing temperatures, the Si transistors can interface with 2D devices (in the back end) to enable high-density heterogeneous circuits. Such configurations are particularly useful for neuromorphic computing applications where a dense network of neurons interacts to compute the output. In this work, we present low-temperature synthesis (400 °C) of 2D tungsten diselenide (WSe2) via the selenization of the W film under ultrahigh vacuum (UHV) conditions. This simple yet effective process yields large-area, homogeneous films of 2D TMDs, as confirmed by several characterization techniques, including reflection high-energy electron diffraction, atomic force microscopy, transmission electron microscopy, and different spectroscopy methods. Memristors fabricated using the grown WSe2 film are leveraged to realize a novel compact neuron circuit that can be reconfigured to enable homeostasis.

VerSoX B07-B: a high-throughput XPS and ambient pressure NEXAFS beamline at Diamond Light Source.

The beamline optics and endstations at branch B of the Versatile Soft X-ray (VerSoX) beamline B07 at Diamond Light Source are described. B07-B provides medium-flux X-rays in the range 45-2200 eV from a bending magnet source, giving access to local electronic structure for atoms of all elements from Li to Y. It has an endstation for high-throughput X-ray photoelectron spectroscopy (XPS) and near-edge X-ray absorption fine-structure (NEXAFS) measurements under ultrahigh-vacuum (UHV) conditions. B07-B has a second endstation dedicated to NEXAFS at pressures from UHV to ambient pressure (1 atm). The combination of these endstations permits studies of a wide range ofinterfaces and materials. The beamline and endstation designs are discussed in detail, as well as their performance and the commissioning process.

Refractory Plasmonics of Reactively Sputtered Hafnium Nitride Nanoparticles: Pushing Limits

AbstractHigh‐temperature plasmonics deals with optically active nanostructures that can withstand high temperatures. A conventional approach relying on standalone noble metal nanoparticles fails to deliver refractory plasmonic nanomaterials, and an alternative route envisions metal nitrides. The main challenge remains the development of advanced synthesis techniques and the insight into thermal stability under real‐life application conditions. Here, hafnium nitride nanoparticles (HfN NPs) can be produced by gas aggregation using reactive magnetron sputtering, a technique with a small environmental footprint are shown. As‐deposited NPs are of 10 nm mean size and consist of stoichiometric, crystalline fcc HfN. They are characterized by optical absorption below 500 nm caused by interband transitions and in the red/near‐infrared (NIR) region due to intraband transitions and localized surface plasmon resonance (LSPR). The optical response can be engineered by tuning the NP composition as predicted by finite‐difference time‐domain (FDTD) calculations. Going beyond the state‐of‐the‐art, the HfN NP thermal stability is focued under ultrahigh vacuum (UHV) and in air. During UHV annealing to 850 °C, the NPs retain their morphology, chemical and optical properties, which makes them attractive in space mission and other applications. During air annealing to 800 °C, HfN NPs remain stable until 250 °C, which sets a limit for air‐mediated use.

Growth and Properties of Ultra-Thin PTCDI-C8 Films on GaN(0001)

Ultra-thin PTCDI-C8 films are vapor-deposited under ultra-high vacuum (UHV) conditions onto surfaces of p- or n-doped GaN(0001) samples. The X-ray photoelectron spectroscopy (XPS) results reveal a lack of strong chemical interaction between the PTCDI-C8 molecule and the substrate. Changes in the electronic structure of the substrate or the adsorbed molecules due to adsorption are not noticed at the XPS spectra. Work function changes have been measured as a function of the film thickness. The position of the HOMO level for films of thicknesses 3.2–5.5 nm has been determined. Energy diagrams of the interface between p- and n-type GaN(0001) substates and the PTCDI-C8 films are proposed. The fundamental molecular building blocks of the PTCDI-C8 films on GaN(0001), assembled by self-organization, have been identified. They are rows of PTCDI-C8 molecules stacked in “stand-up” positions in reference to the substrate, supported by the π–π bonds which are formed between the molecular cores of the molecules and monomolecular layers constituted by rows which are tilted in reference to the layer plane. The layers are epitaxially oriented. The epitaxial relation between the rows and the crystallographic directions of the substrate are determined. A model of the PTCDI-C8 film’s growth on GaN(0001) substrate is proposed. The 3D islands of PTCDI-C8 molecules formed on the substrate surface during film deposition are thermodynamically unstable. The Volmer–Weber type of growth observed here is a kinetic effect. Rewetting processes are noticeable after film aging at room temperature or annealing at up to 100 °C.

Thermal Transport and Mechanical Stress Mapping of a Compression Bonded GaN/Diamond Interface for Vertical Power Devices.

Bonding diamond to the back side of gallium nitride (GaN) electronics has been shown to improve thermal management in lateral devices; however, engineering challenges remain with the bonding process and characterizing the bond quality for vertical device architectures. Here, integration of these two materials is achieved by room-temperature compression bonding centimeter-scale GaN and a diamond die via an intermetallic bonding layer of Ti/Au. Recent attempts at GaN/diamond bonding have utilized a modified surface activation bonding (SAB) method, which requires Ar fast atom bombardment immediately followed by bonding within the same tool under ultrahigh vacuum (UHV) conditions. The method presented here does not require a dedicated SAB tool yet still achieves bonding via a room-temperature metal-metal compression process. Imaging of the buried interface and the total bonding area is achieved via transmission electron microscopy (TEM) and confocal acoustic scanning microscopy (C-SAM), respectively. The thermal transport quality of the bond is extracted from spatially resolved frequency-domain thermoreflectance (FDTR) with the bonded areas boasting a thermal boundary conductance of >100 MW/m2·K. Additionally, Raman maps of GaN near the GaN-diamond interface reveal a low level of compressive stress, <80 MPa, in well-bonded regions. FDTR and Raman were coutilized to map these buried interfaces and revealed some poor thermally bonded areas bordered by high-stress regions, highlighting the importance of spatial sampling for a complete picture of bond quality. Overall, this work demonstrates a novel method for thermal management in vertical GaN devices that maintains low intrinsic stresses while boasting high thermal boundary conductances.

Iodine passivation facilitates on-surface synthesis of robust regular conjugated two-dimensional organogold networks on Au(111).

Two-dimensional conjugated organogold networks with anthra-tetrathiophene repeat units are synthesized by thermally activated debrominative coupling of 2,5,9,12-tetrabromoanthra[1,2-b:4,3-b':5,6-b'':8,7-b''']tetrathiophene (TBATT) precursor molecules on Au(111) surfaces under ultra-high vacuum (UHV) conditions. Performing the reaction on iodine-passivated Au(111) surfaces promotes formation of highly regular structures, as revealed by scanning tunneling microscopy (STM). In contrast, coupling on bare Au(111) surfaces results in less regular networks due to the simultaneous expression of competing intermolecular binding motifs in the absence of error correction. The carbon-Au-carbon bonds confer remarkable robustness to the organogold networks, as evidenced by their high thermal stability. In addition, as suggested by density functional theory (DFT) calculations and underscored by scanning tunneling spectroscopy (STS), the organogold networks exhibit a small electronic band gap in the order of 1.0 eV due to their high π-conjugation.

Same size, same support, same spectator? Selective acetylene hydrogenation on supported Pd nanoparticles.

The selective hydrogenation of acetylene catalyzed by Pd nanoparticles is industrially used to increase the purity of ethylene. Despite the implementation of Pd based catalysts on an industrial scale, little is known about metal-support interactions on a fundamental level due to the complexity of these systems. In this study, the influence of metal-support interactions between Pd nanoparticles and two electronically modified a-SiO2 thin films on acetylene hydrogenation is investigated under ultra-high vacuum (UHV) conditions. The hydrogenation is performed under isothermal reaction conditions using a pulsed molecular beam reactive scattering (pMBRS) technique. Besides the activity and selectivity of clean Pd particles also the impact of dehydrogenated species intentionally introduced a priori is elucidated, whereas the active phase of the catalyst is additionally characterized by CO infrared reflection-absorption spectroscopy (IRRAS) and post-mortem temperature-programmed reaction (TPR). Metal-support interactions are found to influence the catalytic properties of Pd particles by charge-transfer, where positive charging leads to increased activity for acetylene hydrogenation. However, the increased activity is accompanied by formation of undesired byproducts. The active sites for acetylene and ethylene hydrogenation are shown to be different as previously proposed by the A and E model. The availability of the two different active sites on the Pd nanoparticles is determined by dehydrogenated species, whose nature and stability can be tuned by metal-support interactions. Based on these findings an electronic model is proposed how selectivity for acetylene hydrogenation can be steered solely by metal-support interactions leading to blocking of unselective sites in situ.

Light-enhanced oxygen degradation of MAPbBr3 single crystal.

Organometal halide perovskites are promising materials for optoelectronic applications, whose commercial realization depends critically on their stability under multiple environmental factors. In this study, a methylammonium lead bromide (MAPbBr3) single crystal was cleaved and exposed to simultaneous oxygen and light illumination under ultrahigh vacuum (UHV). The exposure process was monitored using X-ray photoelectron spectroscopy (XPS) with precise control of the exposure time and oxygen pressure. It was found that the combination of oxygen and light accelerated the degradation of MAPbBr3, which could not be viewed as a simple addition of that by oxygen-only and light-only exposures. The XPS spectra showed significant loss of carbon, bromine, and nitrogen at an oxygen exposure of 1010 Langmuir with light illumination, approximately 17 times of the additive effects of oxygen-only and light-only exposures. It was also found that the photoluminescence (PL) emission was much weakened by oxygen and light co-exposure, while previous reports had shown that PL was substantially enhanced by oxygen-only exposure. Measurements using a scanning electron microscope (SEM) and focused ion beam (FIB) demonstrated that the crystal surface was much roughened by the co-exposure. Density functional theory (DFT) calculations revealed the formation of superoxide and oxygen induced gap state, suggesting the creation of oxygen radicals by light illumination as a possible microscopic driving force for enhanced degradation.

(First Best Paper Award) Vacuum Quality Impact on Covalent Bonding

As already described in the literature [1, 2, 3, 4], covalent bonding is based on direct bonding process. The first step of this process consists in creating dangling bonds at the wafer surface using an Ar+ ion bombardment. Then, the two activated surfaces are brought in contact, resulting in covalent bonds between them. Using silicon surfaces, no further annealing is required to enhance the bonding strength, as opposed to direct hydrophilic bonding. The adherence energy (Gc=2γc) of such Si/Si bond pairs reaches the silicon fracture energy (5J/m²). Indeed, all bonded samples break during double cantilever beam (DCB) measurements of it.Dangling bonds are highly reactive. Queue time between activation and bonding is crucial, as dangling bonds must be preserved. One way, commonly used, is to process activated materials while always staying under Ultra High Vacuum (UHV). Indeed, reactive dangling bonds should not get in contact with others reactive molecules, like H2O, O2 or N2, for instance. However, some molecules always remain, for any given vacuum value. Depending on their nature, the UHV quality for bonding purpose is affected. This paper aims at studying the impact of the UHV quality on the dangling bonds stability and bonding quality.In order to study the UHV quality, a mass spectrometer has been installed inside the bonding chamber. It has been shown that among all the reactive potential molecules, water is the most harmful one. Indeed, its removal enhances the vacuum quality. Baking vacuum systems is a well-known technic to remove adsorbed water molecules from metallic surfaces [5]. Although background water concentration in UHV is hardly detectable at room temperature, tracking its partial pressure during bakeout cycles enables to define a water threshold. Indeed, during a bakeout cycle, a water concentration peak is clearly seen by the mass spectrometer, as shown in figure 1. The impact of several system bakeouts at 100°C was also evaluated. After each bakeout, a Si/Si bonding energy was measured. After the first bakeout, the adherence energy was only around 2.5J/m². However, after the second bakeout, it reached the silicon fracture energy of 5J/m². More bakeout cycles were also tested. Adherence energy results will be discussed as well as partial pressures of other reactive molecules.The impact of queue times between wafer surfaces activations and bonding was also studied. For a standard process, this waiting time represents the handling time to activate both substrates (in a system having only one activation chamber), and to move them from the activation chamber to the bond chamber. We showed that the impact of queue time depended on the number of bakeout cycles and that the adherence energy decreasing rate was strongly affected by the UHV quality. With our best tool conditioning, an adherence energy higher than 3,4J/m² can be maintained up to 5mn added to the minimum queue time.Thanks to our EVG®Combond® system, covalent bonding in temperature can be performed using heated electrostatic chucks (ESC)[6]. However, even after two or more system bakeouts, 200°C covalent bonding failed to exhibit high adherence energy and it was not possible to reach the silicon fracture energy. Tracking the pressure level inside the bonding chamber during a high temperature bakeout of the ESC (>300°C) showed that water still desorbed during this process. Optimized tool preconditioning thus have to be implemented for such specific hot bonding, with a need for global baking (even of internal chamber pieces).In conclusion, we will describe the experimental setup and the impact of tool preconditioning (i.e. bakeout sequences) on manufacturing. Mass spectrometers and bond strength measurements will be presented, as well as curves of bond strength against the waiting time between the activation and the bonding, for bond pairs processed at room temperature or higher (200°C). Suga T, Takahashi Y. STRUCTURE OF A1-A1 A N D A1-Si3N4 INTERFACES BONDED AT ROOM TEMPERATURE BY MEANS OF THE SURFACE ACTIVATION METHOD. :5.Takagi H, Kikuchi K, Maeda R, Chung TR, Suga T. Surface activated bonding of silicon wafers at room temperature. Appl Phys Lett. 15 avr 1996;68(16):2222-4.Taniyama S, Wang YH, Fujino M, Suga T. Room temperature wafer bonding using surface activated bonding method. In: 2008 IEEE 9th VLSI Packaging Workshop of Japan. 2008. p. 141-4.Flötgen C, Razek N, Dragoi V, Wimplinger M. Novel Surface Preparation Methods for Covalent and Conductive Bonded Interfaces Fabrication. ECS Trans. 14 août 2014;64(5):103.Berman A. Water vapor in vacuum systems. Vacuum. avr 1996;47(4):327-32.Lomonaco Q, et al. Stress Engineering in Germanium-Silicon Heterostructure Using Surface Activated Hot Bonding. ECS Trans. 30 sept 2022;109(4):277-87. Figure 1

Soft Surface Activated Bonding of Hydrophobic Silicon Substrates

Surface Activated Bonding (SAB) is interesting for strong silicon to silicon bonding at room temperature without any annealing needed, afterwards (1). Although it is a well-known technique, the activation step, in particular, is scarcely documented. This paper offers insights about the impact of soft activation parameters on the amorphous region at the bonding interface. In addition, the adherence energy of hydrophobic silicon bonding with SAB is quantified to better understand bonding mechanisms.With very low dose and acceleration activation parameters, the surface preparation prior to bonding becomes of paramount importance. Indeed, the silicon native oxide is typically removed during the activation step. The thin amorphous silicon region is a side effect of this singular surface preparation(2).In order to work around this potential roadblock, we used instead hydrophobic surface preparation to remove the native oxide, before entering into the activation step. Two types of preparation were evaluated in this study. First, a standard “HF-Last” chemical treatment was used on standard silicon wafers. This treatment removed the silicon native oxide and passivated the surface with Si-H and, to a lesser extent, Si-F bonds (3). We otherwise used epitaxy-reconstructed silicon wafers with fully hydrophobic surfaces (4). Silicon native oxide was removed thanks to an ultra-pure H2 bake at 1100°C, 20 Torr for 2 minutes in an epitaxy chamber. Then, several tens of nm of Silicon were deposited at 950°C to obtain, after another H2 bake, a silicon surface fully passivated by hydrogen atoms with atomically smooth terraces and mono-atomic step edges.Our EVG®ComBond® bonding tool, operating under ultra-high vacuum (UHV), is equipped with an accelerated argon ion beam to perform the activation step. The softest functional settings, on our set up, are 50V (acceleration) and 26 mA (dose). After beam initialization, the two sets of substrates pass through the activation chamber. Activated substrates are then transferred to the bonding chamber within 5 minutes of handling.The exposure time in the activation chamber was evaluated, the aim being to remove adsorbed hydrogen atoms on the silicon surface without any amorphous silicon generation. Different characterization techniques such as transmission electron microscopy or FTIR-MIR were used to quantify the amorphous layer formation and the potential Si-H bonds remaining (after activation). The adherence energy of the bonded pair was measured by a double cantilever beam method under prescribed displacement control in anhydrous atmosphere (5).Figure 1 shows the adherence energy (Gc=2γc) in mJ/m² as a function of activation exposure time with soft activation parameters for both wafer preparations. The 0s reference bonding was conducted without passing through the activation module. We then had very low adherence energies, around 50 mJ/m², as expected for standard hydrophobic silicon wafer bonding under UHV (6). Upon Ar+ exposure, behaviors were very different depending on surface preparation. The adherence energy barely increased with the Ar+ exposure time for “HF-Last” surfaces. Meanwhile, even 1s of exposure to Ar+ had a definite impact on the adherence energy of epi-reconstructed, atomically smooth silicon surfaces, which was definitely higher. The maximum difference between both wafer preparations occurred for 30 up to 60 seconds exposure times. This indicate a change in the bonding mechanism as the comparatively high roughness of the “HF-Last” silicon wafer started to be counter-balanced by activation.The experimental set up, the manufacturing process, as well as further characterizations will be presented. Cross-sectional TEM imaging of the bonding interface, FTIR-MIR and AFM measurements after surface preparation will help us better understand the specificities of such soft activation process on the SAB of hydrophobic surfaces. The impact of the amorphous silicon layer on bonding will be discussed. Suga T et al. STRUCTURE OF A1-A1 A N D A1-Si3N4 INTERFACES BONDED AT ROOM TEMPERATURE BY MEANS OF THE SURFACE ACTIVATION METHOD. Acta Metallurgica et Materialia 1992.Takagi H et al. Surface activated bonding of silicon wafers at room temperature. Appl Phys Lett. 1996.Abbadie A et al. Low thermal budget surface preparation of Si and SiGe. Appl Surf Sci. 2004.Sordes D et al. Nanopackaging of Si(100)H Wafer for Atomic-Scale Investigations. 2017.Maszara WP et al. Bonding of silicon wafers for silicon‐on‐insulator. J Appl Phys. 15 nov 1988;64(10):4943-50.Tong QY et al. The Role of Surface Chemistry in Bonding of Standard Silicon Wafers. J Electrochem Soc. 1997. Figure 1

Stability of the Fe3O4 (√2 x √2)R 45° Surface in 0.1 M NaOH Investigated by High-Energy Surface X-Ray Diffraction

Transition metal oxides, especially Fe, Co, and Ni oxides, are of great interest as electrocatalysts for the oxygen evolution reaction (OER) in alkaline solution. However, detailed data on the atomic-scale oxide-electrolyte interface structure of these systems are rare. We present studies of a model oxide electrocatalyst, -reconstructed Fe3O4(001). This oxide surface has been extensively investigated under ultrahigh vacuum (UHV) conditions, leading to a detailed understanding of its atomic structure [2]. In previous in situ surface X-ray diffraction studies, we demonstrated that the reconstructed surface can survive transfer into electrochemical environment and determined the stability of the reconstruction as a function of potential [2]. Specifically, we showed that the ()R45° reconstructed Fe3O4(001) surface can be transferred into 0.1 M NaOH at 0.2 V vs. Ag/AgCl while maintaining surface structure and that the reconstruction is irreversibly lifted at sufficiently negative potentials.Here, we present more detailed in situ studies of this system, using the novel technique of High-Energy Surface X-Ray Diffraction (HESXRD) [3,4]. This technique enabled us to measure rapidly a huge reciprocal space volume, which allows to obtain very large sets of crystal truncation rods (CTRs) that can be analysed by surface crystallographic methods. These data provide detailed insights into the temporal stability of the surface at fixed potentials as well as potential-dependent changes in surface structure.In the experiments the Fe3O4(001) single crystal samples were prepared by sputtering and annealing in UHV, followed by sample transfer into the hanging-meniscus cell employed for the HESXRD studies. A new experimental procedure allowed to reduce the time of sample transfer and establishment of potential control to below one minute.The in situ HESXRD measurements were performed at beamline P21.2 of PETRA III at DESY. 7 symmetrically independent CTRs and 2 ()R45° fractional order rods could be measured. These data enabled detailed comparison of the oxide-electrolyte interface with the surface structure under UHV conditions [1]. The potential-dependent data indicate clear structural changes when applying more positive or more negative potentials than 0.2 V vs. Ag/AgCl, as illustrated in part A and B of Fig. 1. However, all surface structures were stable at constant potential over several hours, as indicated in part C of Fig. 1. The quality of the obtained results allows to determine quantitative analysis of the interface structure. First results of this analysis will be presented.We gratefully acknowledge financial support by the German ministry of Science and Education (BMBF) via projects 05K19FK3 and 05K22FK1.

Nanoscale friction of tetrahedral amorphous diamond-like carbon film after thermal annealing

Diamond-like carbon (DLC) coatings have demonstrated significant potential as solid lubricants for a wide range of applications. However, thermal-induced structural changes lead to alterations in its mechanical and frictional properties. In this study, we investigate the friction properties of DLC films after high-temperature annealing, by conducting reciprocating single asperity scanning experiments of self-mated hydrogen-free DLC tribopair under ultra-high vacuum (UHV) conditions. As the annealing temperature increases, the coefficients of friction (COFs) first increase owing to the desorption of water molecules; while, with the further increase of temperature, friction shows a decreasing trend because of the high-temperature-induced sp3-to-sp2 rehybridization which lowers the interfacial shear strength. The insights gained from this study provide a better understanding of the frictional properties of DLC films.

Vacuum Quality Impact on Covalent Bonding

Covalent bonding is based on a direct bonding process. The first step of this process consists in creating dangling bonds at the wafers surface. When the two activated surfaces are brought in contact, covalent bonds result between them. Dangling bonds are highly reactive. Any queue time between activation and bonding is crucial, as dangling bonds must be preserved. It can be even more difficult to preserve the dangling bonds in the case of bonding process in temperature. One way, commonly used, is to always stay under Ultra High Vacuum (UHV). However, the UHV environment may affect the bonding process. This paper aims at studying the impact of the UHV quality on the dangling bonds stability and bonding quality.

Aperiodic Modulation of Graphene Driven by Oxygen-Induced Reconstruction of Rh(110).

Artificial nanostructuring of graphene has served as a platform to induce variations in its structural and electronic properties, fostering the experimental observation of a wide and fascinating phenomenology. Here, we present an approach to graphene tuning, based on Rh(110) surface reconstruction induced by oxygen atoms intercalation. The resulting nanostructured graphene has been characterized by scanning tunneling microscopy (STM) complemented by low-energy electron microscopy (LEEM), micro low-energy electron diffraction (μ-LEED), micro angle-resolved photoemission spectroscopy (μ-ARPES), and micro X-ray photoelectron spectroscopy (μ-XPS) measurements under ultrahigh vacuum (UHV) conditions at room temperature (RT). It is found that by fine-tuning the O2 exposure amount, a mixture of missing row surface reconstructions of the metal surface below the graphene layer can be induced. This atomic rearrangement under the graphene layer results in aperiodic patterning of the two-dimensional (2D) material. The electronic structure of the resulting nanostructured graphene is dominated by a linear dispersion of the Dirac quasiparticles, characteristic of its free-standing state but with a p-doping character. The local effects of the underlying missing rows on the interfacial chemistry and on the quasiparticle scattering processes in graphene are studied using atomically resolved STM images. The possibilities offered by this nanostructuring approach, which consists in inducing surface reconstructions under graphene, could provide a novel tuning strategy for this 2D material.

(Invited) Interplay Between π–Conjugation and Exchange Magnetism in One-Dimensional Porphyrinoid Polymers

The field of carbon magnetism has gained an increased attention in view of the recent progress made in the synthesis and characterization of open-shell polycyclic aromatic hydrocarbons following a bottom-up synthetic approach[1,2]. Under this scenario, the comprehensive fabrication of magnetic porphyrinoid-based polymers emerges as a highly appealing field of research. However, the vast majority of the porphyrinoid species and polymers studied to date exhibit a closed-shell ground state[3].In this work, we introduce an exemplary approach toward the bottom-up fabrication of unprecedented magnetic porphyrinoid-based polymers homocoupled via surface-catalyzed [3 + 3] cycloaromatization of isopropyl substituents studied on Au(111) under ultra-high vacuum (UHV) conditions. The chemical structure of the polymer, formed by thermal-activated intra- and intermolecular oxidative ring closure reactions followed by controlled tip-induced hydrogen dissociation from the porphyrinoid units, have been clearly elucidated by scanning tunneling microscopy (STM) and non-contact atomic force microscopy (nc-AFM). Scanning tunneling spectroscopy (STS), complemented by computational investigations reveals the open-shell character, i.e. antiferromagnetic singlet ground state (S=0) of the formed polymers, which display singlet−triplet inelastic excitations observed between spins of adjacent porphyrinoid units only along a specific π-conjugation pathway, thus revealing the interplay between π-conjugation and magnetic exchange. We envision that our approach can be a highly relevant step towards the on-surface synthesis of covalently linked 1D magnetic organic polymers with prospects in nanoscale spintronic devices.