On-Surface Synthesis of Unsaturated Hydrocarbon Chains through C-S Activation.

We use an on-surface synthesis approach to drive the homocoupling reaction of a simple dithiophenyl-functionalized precursor on Cu(111). The C-S activation reaction is initiated at low annealing temperature and yields unsaturated hydrocarbon chains interconnected in a fully conjugated reticulated network. High-resolution atomic force microscopy imaging reveals the opening of the thiophenyl rings and the presence of trans- and cis-oligoacetylene chains as well as pentalene units. The chemical transformations were studied by C 1s and S 2p core level photoemission spectroscopy and supported by theoretical calculations. At higher annealing temperature, additional cyclization reactions take place, leading to the formation of small graphene flakes.

Modular Assembly of Vibrationally and Electronically Coupled Rhenium Bipyridine Carbonyl Complexes on Silicon.

Hybrid inorganic/organic heterointerfaces are promising systems for next-generation photocatalytic, photovoltaic, and chemical-sensing applications. Their performance relies strongly on the development of robust and reliable surface passivation and functionalization protocols with (sub)molecular control. The structure, stability, and chemistry of the semiconductor surface determine the functionality of the hybrid assembly. Generally, these modification schemes have to be laboriously developed to satisfy the specific chemical demands of the semiconductor surface. The implementation of a chemically independent, yet highly selective, standardized surface functionalization scheme, compatible with nanoelectronic device fabrication, is of utmost technological relevance. Here, we introduce a modular surface assembly (MSA) approach that allows the covalent anchoring of molecular transition-metal complexes with sub-nanometer precision on any solid material by combining atomic layer deposition (ALD) and selectively self-assembled monolayers of phosphonic acids. ALD, as an essential tool in semiconductor device fabrication, is used to grow conformal aluminum oxide activation coatings, down to sub-nanometer thicknesses, on silicon surfaces to enable a selective step-by-step layer assembly of rhenium(I) bipyridine tricarbonyl molecular complexes. The modular surface assembly of molecular complexes generates precisely structured spatial ensembles with strong intermolecular vibrational and electronic coupling, as demonstrated by infrared spectroscopy, photoluminescence, and X-ray photoelectron spectroscopy analysis. The structure of the MSA can be chosen to avoid electronic interactions with the semiconductor substrate to exclusively investigate the electronic interactions between the surface-immobilized molecular complexes.

Electronic properties of carbon nanotubes as detected by photoemission and inverse photoemission.

The relation between morphology and energy level alignment in carbon nanotubes (CNT) is a crucial information for the optimization of applications in nanoelectronics, optics, mechanics and (bio)chemistry. Here we present a study of the relation between the electronic properties and the morphology of single wall CNT (SWCNT), aligned multi wall CNT (MWCNT) and unaligned MWCNT. The CNT were synthesized via catalytic chemical vapor deposition in ultra-high vacuum conditions. Combined ultraviolet photoemission and inverse photoemission (IPES) spectra reveal a high sensitivity to the nanotube morphology. In the case of unaligned SWCNT the distinctive unoccupied Van Hove singularities (vHs) features are observed in the high resolution IPES spectra. Those features are assigned to semiconducting and metallic SWCNT states, according to calculated vHs DOS. The two MWCNT samples are similar in the diameter of the tube (about 15 nm) and present similar filled and empty electronic states, although the measured features in the aligned MWCNT are more defined. Noteworthy, interlayer states are also revealed. Their intensities are directly related to the MWCNT alignment. Focussing and geometrical effects associated to the MWCNT alignment are discussed to account the spectral differences.

Morphological characterization and electronic properties of pristine and oxygen-exposed graphene nanoribbons on Ag(110).

Graphene nanoribbons (GNRs) are at the frontier of research on graphene materials since the 1D quantum confinement of electrons allows for the opening of an energy gap. GNRs of uniform and well-defined size and shape can be grown using the bottom-up approach, i.e. by surface assisted polymerization of aromatic hydrocarbons. Since the electronic properties of the nanostructures depend on their width and on their edge states, by careful choice of the precursor molecule it is possible to design GNRs with tailored properties. A key issue for their application in nanoelectronics is their stability under operative conditions. Here, we characterize pristine and oxygen-exposed 1.0 nm wide GNRs with a well-defined mixed edge-site sequence (two zig-zag and one armchair) synthesized on Ag(110) from 1,6-dibromo-pyrene precursors. The energy gap and the presence of quantum confined states are investigated by scanning tunneling spectroscopy. The effect of oxygen exposure under ultra-high vacuum conditions is inferred from scanning tunneling microscopy images and photoemission spectra. Our results demonstrate that oxygen exposure deeply affects the overall system by interacting both with the nanoribbons and with the substrate; this factor must be considered for supported GNRs under operative conditions.

Electronic Structures of the Vanadium-Intercalated and Substitutionally Doped Transition-Metal Dichalcogenides TixVySe2.

The electronic structures of V-intercalated TiSe2 and substitutionally doped dichalcogenides Ti1-xVxSe2 have been studied using soft X-ray photoelectron, resonant photoelectron, and absorption spectroscopies. In the case of the substitution of Ti by V, the formation of coherently oriented structural fragments VSe2 and TiSe2 is observed and a small charge transfer between these fragments is found. Intercalation of the V atoms into TiSe2 leads to charge transfer from the V atoms to the Ti atoms with the formation of covalent complexes Ti-Se3-V-Se3-Ti.

Hydrogen Bonding of Ammonia with (H,OH)-Si(001) Revealed by Experimental and Ab Initio Photoelectron Spectroscopy.

Combining experimental and ab initio core-level photoelectron spectroscopy (periodic DFT and quantum chemistry calculations), we elucidated how ammonia molecules bond to the hydroxyls of the (H,OH)-Si(001) model surface at a temperature of 130 K. Indeed, theory evaluated the magnitude and direction of the N 1s (and O 1s) chemical shifts according to the nature (acceptor or donor) of the hydrogen bond and, when confronted to experiment, showed unambiguously that the probe molecule makes one acceptor and one donor bond with a pair of hydroxyls. The consistency of our approach was proved by the fact that the identified adsorption geometries are precisely those that have the largest binding strength to the surface, as calculated by periodic DFT. Real-time core-level photoemission enabled measurement of the adsorption kinetics of H-bonded ammonia and its maximum coverage (0.37 ML) under 1.5 × 10-9 mbar. Experimental desorption free energies were compared to the magnitude of the adsorption energies provided by periodic DFT calculations. Minority species were also detected on the surface. As in the case of H-bonded ammonia, DFT core-level calculations were instrumental to attribute these minority species to datively bonded ammonia molecules, associated with isolated dangling bonds remaining on the surface, and to dissociated ammonia molecules, resulting largely from beam damage.

Guest-Host Chemical Bonding and Possibility of Ordering of Intercalated Metals in Transition-Metal Dichalcogenides.

The comparison of the specifics of the guest-host chemical bonding in the materials with (Fe xTiSe2) and without (Fe xTiTe2) ordering of the iron atoms was performed. For this purpose the electronic structure of the materials were studied using X-ray photoelectron spectroscopy, X-ray absorption spectroscopy, resonant X-ray photoelectron spectroscopy, and theoretical calculations (total density of states, partial density of states, and multiplet calculations). For the iron-intercalated TiTe2 compound iron-chalcogen bonds are formed, whereas the formation of iron-iron bonds is most typical for the iron-intercalated TiSe2 compound. This leads to an increase in the lifetime of electrons on the titanium atoms and does not allow the formation of atomic chains of intercalated metal.

Synthesis of corrugated C-based nanostructures by Br-corannulene oligomerization.

The structure and electronic properties of carbon-based nanostructures obtained by metal surface assisted synthesis is highly dependent on the nature of the precursor molecule. Here, we report on a combined scanning tunneling microscopy, soft X-ray spectroscopy and density functional theory investigation on the surface assisted polymerization of Br-corannulene at Ag(110) and on the possibility of building a mesh of π-conjugated polymers starting from buckyball shaped molecules. Indeed, the corannulene units form one-molecule-wide ribbons in which the natural concavity of the precursor molecule is maintained. These C-based nanostructures are corrugated and merge into a covalent network on the surface.

Stable Fe nanomagnets encapsulated inside vertically-aligned carbon nanotubes.

Well-defined sized (5-10 nm) metallic iron nanoparticles (NPs) with body-centered cubic structure encapsulated inside the tip of millimeter-long vertically aligned carbon nanotubes (VACNTs) of uniform length have been investigated with high-resolution transmission electron microscopy and soft X-ray spectroscopy techniques. Surface-sensitive and chemically-selective measurements have been used to evaluate the magnetic properties of the encapsulated NPs. The encapsulated Fe NPs display magnetic remanence up to room temperature, low coercivity, high chemical stability and no significant anisotropy. Our surface-sensitive measurements combined with the specific morphology of the studied VACNTs allow us to pinpoint the contribution of the surface oxidized or hydroxidized iron catalysts present at the VACNT-substrate interface.

The electronic structure of Ag1-xSn1+xSe2 (x = 0.0, 0.1, 0.2, 0.25 and 1.0).

We have studied the valence electronic structure of Ag1-xSn1+xSe2 (x = 0.0, 0.1, 0.2, 0.25) and SnSe (x = 1.0) by a combined analysis of X-ray absorption spectroscopy (XAS) and X-ray photoemission spectroscopy (XPS) measurements. Both XAS and XPS reveal an increase in electron carriers in the system with x (i.e. excess Sn concentration) for 0 ≤ x ≤ 0.25. The core-level spectra (Sn 3d, Ag 3d and Se 3d) show that the charge state of Ag is almost 1+, while that of of Sn splits into Sn2+ and Sn4+ (providing clear evidence of valence skipping for the first time) with a concomitant splitting of Se into Se2- and Se2-δ states. The x dependence of the split components in Sn and Se together with the Se-K edge XAS reveals that the Se valence state may have an essential role in the transport properties of this system.

The magnetization orientation of Fe ultrathin layers in contact with graphene.

In this paper, we study the magnetic and chemical properties of Fe/graphene vertically stacked ultrathin films by means of X-ray magnetic circular dichroism and X-ray photoelectron spectroscopy. We compare two systems: an iron layer deposited directly on top of the Pt(111) surface, and an intercalated Fe film sandwiched between graphene and Pt(111). The system composed of a submonolayer Fe deposited directly on Pt(111) maintains an out-of-plane easy magnetization axis, even if it has been covered by graphene that quenches effectively the magnetic orbital moment of Fe. However, when the Fe coverage is increased above 1 ML the easy magnetization axis flips in the in-plane direction.

Near-unity quantum yields from chloride treated CdTe colloidal quantum dots.

Colloidal quantum dots (CQDs) are promising materials for novel light sources and solar energy conversion. However, trap states associated with the CQD surface can produce non-radiative charge recombination that significantly reduces device performance. Here a facile post-synthetic treatment of CdTe CQDs is demonstrated that uses chloride ions to achieve near-complete suppression of surface trapping, resulting in an increase of photoluminescence (PL) quantum yield (QY) from ca. 5% to up to 97.2 ± 2.5%. The effect of the treatment is characterised by absorption and PL spectroscopy, PL decay, scanning transmission electron microscopy, X-ray diffraction and X-ray photoelectron spectroscopy. This process also dramatically improves the air-stability of the CQDs: before treatment the PL is largely quenched after 1 hour of air-exposure, whilst the treated samples showed a PL QY of nearly 50% after more than 12 hours.

Control of the intermolecular coupling of dibromotetracene on Cu(110) by the sequential activation of C-Br and C-H bonds.

Dibromotetracene molecules are deposited on the Cu(110) surface at room temperature. The complex evolution of this system has been monitored at different temperatures (i.e., 298, 523, 673, and 723 K) by means of a variety of complementary techniques that range from STM and temperature-programmed desorption (TPD) to high-resolution X-ray spectroscopy (XPS) and near-edge X-ray absorption fine structure spectroscopy (NEXAFS). State-of-the-art density-functional calculations were used to determine the chemical processes that take place on the surface. After deposition at room temperature, the organic molecules are transformed into organometallic monomers through debromination and carbon-radical binding to copper adatoms. Organometallic dimers, trimers, or small oligomers, which present copper-bridged molecules, are formed by increasing the temperature. Surprisingly, further heating to 673 K causes the formation of elongated chains along the Cu(110) close-packed rows as a consequence of radical-site migration to the thermodynamically more stable molecule heads. Finally, massive dehydrogenation occurs at the highest temperature followed by ring condensation to nanographenic patches. This study is a paradigmatic example of how intermolecular coupling can be modulated by the stepwise control of a simple parameter, such as temperature, through a sequence of domino reactions.

Surface properties of nanocrystalline PbS films deposited at the water-oil interface: a study of atmospheric aging.

Nanocrystalline thin films of PbS are obtained in a straightforward reaction by precipitation at the interface between toluene (containing a Pb precursor) and water (containing Na2S). Lead thiobiuret [Pb(SON(CN(i)Pr2)2)2] and lead diethyldithiocarbamate [Pb(S2CNEt2)2] precursors are used. The films are characterized by X-ray diffraction and electron microscopy, revealing typical particle sizes of 10-40 nm and preferred (200) orientation. Synchrotron-excited depth-profiling X-ray photoelectron spectroscopy (XPS) is used to determine the depth-dependent chemical composition as a function of surface aging in air for periods of up to 9 months. The as-synthesized films show a 1:1 Pb/S composition. Initial degradation occurs to form lead hydroxide and small quantities of surface-adsorbed -SH species. A lead-deficient Pb1-xS phase is produced as the aging proceeds. Oxidation of the sulfur occurs later to form sulfite and sulfate products that are highly localized at the surface layers of the nanocrystals. These species show logarithmic growth kinetics, demonstrating that the sulfite/sulfate layer acts to passivate the nanocrystals. Our results demonstrate that the initial reaction of the PbS nanocrystals (forming lead hydroxide) is incongruent. The results are discussed in the context of the use of PbS nanocrystals as light-harvesting elements in next-generation solar technology.

Adsorption geometry, conformation, and electronic structure of 2H-octaethylporphyrin on Ag(111) and Fe metalation in ultra high vacuum.

Due to the growing interest in the ferromagnetic properties of Fe-octaethylporphyrins (Fe-OEP) for applications in spintronics, methods to produce stable Fe-porphyrins with no Cl atoms are highly demanded. Here, we demonstrate the formation of Fe-OEP layers on Ag(111) single crystal by the ultra high vacuum in situ metalation of the free-base 2H-2,3,7,8,12,13,17,18-octaethylporphyrin (2H-OEP) molecules. The metalation proceeds exactly as in the case of 2H-5,10,15,20-tetraphenylporphyrin (2H-TPP) on the same substrate. An extensive surface characterization by means of X-ray photoemission spectroscopy, valence band photoemission, and NEXAFS with synchrotron radiation light provides information on molecular conformation and electronic structure in the monolayer and multilayer cases. We demonstrate that the presence of the ethyl groups affects the tilt of the adsorbed molecules, the conformation of the macrocycle, and the polarization screening in multilayers, but has only a minor effect in the metalation process with respect to 2H-TPP.

Lithium-Induced Reorientation of Few-Layer MoS2 Films.

Molybdenum disulfide (MoS2) few-layer films have gained considerable attention for their possible applications in electronics and optics and also as a promising material for energy conversion and storage. Intercalating alkali metals, such as lithium, offers the opportunity to engineer the electronic properties of MoS2. However, the influence of lithium on the growth of MoS2 layers has not been fully explored. Here, we have studied how lithium affects the structural and optical properties of the MoS2 few-layer films prepared using a new method based on one-zone sulfurization with Li2S as a source of lithium. This method enables incorporation of Li into octahedral and tetrahedral sites of the already prepared MoS2 films or during MoS2 formation. Our results discover an important effect of lithium promoting the epitaxial growth and horizontal alignment of the films. Moreover, we have observed a vertical-to-horizontal reorientation in vertically aligned MoS2 films upon lithiation. The measurements show long-term stability and preserved chemical composition of the horizontally aligned Li-doped MoS2.

Physical Delithiation of Epitaxial LiCoO2 Battery Cathodes as a Platform for Surface Electronic Structure Investigation.

We report a novel delithiation process for epitaxial thin films of LiCoO2(001) cathodes using only physical methods, based on ion sputtering and annealing cycles. Preferential Li sputtering followed by annealing produces a surface layer with a Li molar fraction in the range 0.5 < x < 1, characterized by good crystalline quality. This delithiation procedure allows the unambiguous identification of the effects of Li extraction without chemical byproducts and experimental complications caused by electrolyte interaction with the LiCoO2 surface. An analysis by X-ray photoelectron spectroscopy (XPS) and X-ray absorption spectroscopy (XAS) provides a detailed description of the delithiation process and the role of O and Co atoms in charge compensation. We observe the simultaneous formation of Co4+ ions and of holes localized near O atoms upon Li removal, while the surface shows a (2 × 1) reconstruction. The delithiation method described here can be applied to other crystalline battery elements and provide information on their properties that is otherwise difficult to obtain.

A ferromagnetic Eu-Pt surface compound grown below hexagonal boron nitride.

One of the fundamental applications for monolayer-thick 2D materials is their use as protective layers of metal surfaces and in situ intercalated reactive materials in ambient conditions. Here we investigate the structural, electronic, and magnetic properties, as well as the chemical stability in air of a very reactive metal, Europium, after intercalation between a hexagonal boron nitride (hBN) layer and a Pt substrate. We demonstrate that Eu intercalation leads to a hBN-covered ferromagnetic EuPt2 surface alloy with divalent Eu2+ atoms at the interface. We expose the system to ambient conditions and find a partial conservation of the di-valent signal and hence the Eu-Pt interface. The use of a curved Pt substrate allows us to explore the changes in the Eu valence state and the ambient pressure protection at different substrate planes. The interfacial EuPt2 surface alloy formation remains the same, but the resistance of the protecting hBN layer to ambient conditions is reduced, likely due to a rougher surface and a more discontinuous hBN coating.

Energy Level Alignment at the Cobalt Phosphate/Electrolyte Interface: Intrinsic Stability vs Interfacial Chemical Reactions in 5 V Lithium Ion Batteries.

The intrinsic stability of the 5 V LiCoPO4-LiCo2P3O10 thin-film (carbon-free) cathode material coated with MoO3 thin layer is studied using a comprehensive synchrotron electron spectroscopy in situ approach combined with first-principle calculations. The atomic-molecular level study demonstrates fully reversible electronic properties of the cathode after the first electrochemical cycle. The polyanionic oxide is not involved in chemical reactions with the fluoroethylene-containing liquid electrolyte even when charged to 5.1 V vs Li+/Li. The high stability of the cathode is explained on the basis of the developed energy level model. In contrast, the chemical composition of the cathode-electrolyte interface evolves continuously by involving MoO3 in the decomposition reaction with consequent leaching of oxide from the surface. The proposed mechanisms of chemical reactions are attributed to external electrolyte oxidation via charge transfer from the relevant electron level to the MoO3 valence band state and internal electrolyte oxidation via proton transfer to the solvents. This study provides a deeper insight into the development of both a doping strategy to enhance the electronic conductivity of high-voltage cathode materials and an efficient surface coating against unfavorable interfacial chemical reactions.

Supramolecular engineering through temperature-induced chemical modification of 2H-tetraphenylporphyrin on Ag(111): flat phenyl conformation and possible dehydrogenation reactions.

Scratching the surface: Formation of a monolayer of 2H-tetraphenylporphyrins (2H-TPP) on Ag(111), either by sublimation of a multilayer in the range 525-600 K or by annealing (at the same temperature) a monolayer deposited at room temperature, induces a chemical modification of the molecules. Rotation of the phenyl rings into a flat conformation is observed and tentatively explained, by using DFT calculations, as a peculiar reaction due to molecular dehydrogenation.