Highlights

Probing photo-induced lattice distortion

We have investigated the photo-induced dynamics of a 2H-MoTe2 crystal by means of time-resolved X-ray photoemission spectroscopy. We detect two distinct phenomena: (1) structural deformations in the out-of-plane direction with lifetime in the sub-nanosecond regime and (2) a surface photovoltage shift that persists for microseconds. These results show that high-resolution time-resolved photoemission, combined with theoretical simulations, can provide valuable information not only on electronic and chemical modifications of photoexcited systems, but also on lattice distortions and phase transitions. R. Costantini et al. DOI:10.1039/D1FD00105A

Transition metal dichalcogenides (TMDs) have been widely explored for the development of innovative nanoscale devices. Molybdenum ditelluride (MoTe2) is a prototypical TMD. It presents two crystal structures at room temperature: the semiconducting hexagonal phase (2H or α-phase) and the semimetallic monoclinic phase (1T′ or β-phase). In its 2H phase, MoTe2 has attracted significant interest from a technological point of view due to its bandgap in the near infrared region (0.9 eV in the bulk and 1.1 eV in the monolayer), which makes it a potential alternative to silicon for optoelectronic applications. Very recently, carrier multiplication has been demonstrated in 2H-MoTe2, making it a promising material for the development of light harvesting devices. The technological interest in MoTe2 as a phase engineered material is related to the possibility of triggering the 2H–1T′ phase transition by optical excitation, potentially allowing for an accurate patterning of metallic areas into a semiconducting canvas via laser irradiation. In this paper, we investigate the photo-induced modifications of a bulk 2H-MoTe2 crystal by means of time-resolved X-ray photoemission spectroscopy.

We observe that in the microsecond timescale, the core levels shift to higher kinetic energies due to surface photovoltage fields, while in the sub-nanosecond range, the photoemission peaks shift in the opposite direction. With the support of DFT calculations, we ascribe the latter effect to the deformation of the lattice in the out-of-plane direction, which is along the pathway for the 2H–1T′ phase transition. Retrieve article Photo-induced lattice distortion in 2H-MoTe2 probed by time-resolved core level photoemission,  R. Costantini, F. Cilento, F. Salvador, A. Morgante, G. Giorgi, M. Palummo and  M. Dell’Angela, Faraday Discussions 236, 429-441 (2022) DOI:10.1039/D1FD00105A

Nitric Oxide at a Surface-Bound Nickel Porphyrinoid

A stable NO2 species forms at the Ni sites at room temperature upon exposure to NO of a Nickel-containing tetraphenyl porphyrins monolayer grown on Cu(100). The NO uptake was characterized by a quantitative analysis of the NO2 formation rates as a function of the NO exposure conditions, and we conclude that NO2 origins through a disproportionation mechanism. Together with the support of ab initio simulations, we propose an atomistic model of the reaction that is compatible with our observations, paving the way towards further investigations on NO disproportionation at biomimetic single-atom sites within the framework of surface science. M. Stredansky et al. DOI:10.1002/anie.202201916

In a biomimetic picture at surfaces, novel model systems for heterogeneous catalysis may be built by exploiting  a rich variety of 2-dimensional self-assembled structures formed by tetrapyrrolic compounds such as metal-containing tetraphenyl porphyrins. These supramolecular assemblies act as a network, stabilizing ordered arrays of single metal atom active sites and can be considered single-atom catalyst offering axial coordination for the anchoring of ligands and for the chemical conversion of small molecules. In the specific case of NO adsorption at 2D arrays of surface-supported porphyrins, it is commonly accepted that only a single NO molecule coordinates to the metal center under UHV conditions. We present the results obtained upon exposure of a Ni tetraphenyl porphyrin (NiTPP) film deposited on the Cu(100) surface to NO at room temperature, providing a first evidence of NO disproportionation observed at tetrapyrroles in UHV, yielding the NO2-NiTPP complex. By means of vibronic and vibrational spectroscopies and scanning tunneling microscopy, we unequivocally identify the ligand coordinated with the Ni atom of the porphyrin to be the NO2 molecule. Independent photoemission-based experiments allow to draw the same conclusions. Density Functional Theory calculations indicate that the disproportionation reaction path (3 NO→NO2+N2O) is extremely exothermic so that the formation of the NO2-NiTPP complex is energetically favorable, although kinetically hindered, being a high order reaction.

We report in detail the results obtained by means of each of the many techniques, both experimental and theoretical, that we exploited to investigate this complex system. We also discuss our observations in light of the well-known, unavoidable, and non-trivial issues associated with the role of contaminants in NO experiments, critically considering the role of pathways including: i) NO2 contamination of the NO bottles, ii) NO2 formation at the UHV chamber walls, iii) NO2 formation at the Cu substrate, iv) NO2 formation at the single-atom Ni sites. Retrieve article Disproportionation of Nitric Oxide at a Surface-Bound Nickel Porphyrinoid M. Stredansky, S. Moro, M. Corva, H. Sturmeit, V. Mischke, D. Janas, I. Cojocariu, M. Jugovac, A. Cossaro, A. Verdini, L. Floreano, Z. Feng, A. Sala, G. Comelli, A. Windischbacher, P. Puschnig, C. Hohner, M. Kettner, J. Libuda, M. Cinchetti, C. M. Schneider, V. Feyer, E. Vesselli, G. Zamborlini Anghew. Chem. Int. Ed. 60 25988-25993 (2022)  DOI:10.1002/anie.202201916

Playing with molecular LEGO: dodging the surface ligand effect for on-surface 3D self-assembly

We provided the evidence for the room temperature construction of an unprecedented out-of-plane self-assembled heterostructure guided by in-vacuum coordination chemistry. The whole structure, albeit ordered and commensurate, is settled on a pure “physical contact interface” ever realized between a molecule and a metallic surface which represents a new concept in the field of interface design and engineering. A. Orbelli Biroli et al. DOI:10.1002/adfm.202011008

Our approach demonstrates a strategy for the assembly of single molecule/electrode interface and the exploitation in true bottom-up device engineering of molecular 3D coherent growth by vapour phase deposition through coordination chemistry. Thanks to the specific and complementary expertise of the collaborating team of chemists and physicists, we demonstrated a chemical route to the in-vacuum stacking of chemically pure and spatially coherent alternating molecular layers, as driven by intermolecular metal coordination. In-vacuum deposition is exploited as the way to integrate a number of ingredients: (i) the chemical decoupling of the contact organic layer from the metal electrode, (ii) the lateral (2D) ordering of the molecules, (iii) the rigid molecule- to-molecule coupling along the normal to the surface, (iv) the sharp (chemical) termination of the layered film and the readily available and exploitable building blocks.

As a result of our LEGO construction, the topmost molecule is directly coupled to the substrate. Retrieve article Out-Of-Plane Metal Coordination for a True Solvent-Free Building with Molecular Bricks: Dodging the Surface Ligand Effect for On-Surface Vacuum Self-Assembly, A. Orbelli Biroli, A. Calloni, A. Bossi, M.S. Jagadeesh, G. Albani, L. Duò, F. Ciccacci, A. Goldoni, A. Verdini, L. Schio, L. Floreano, and G. Bussetti Adv. Funct. Mater. 31, 2011008 (2021) DOI:10.1002/adfm.202011008

Tailoring the hydration of melamine by on-surface confinement

Melamine is the building block of polymeric carbon nitrides (p-CNH), that are able to catalyze the water-splitting reaction under visible light irradiation. We reveal that, upon water adsorption, two adjacent melamine molecules concurrently work for stabilizing the H-bonded water-catalyst complex. V. Lanzilotto et al. DOI:10.1039/D0NA01034K

With the perspective to shed light on the water-splitting mechanism with p-CNH materials, we have studied the hydration properties of one monolayer (1 ML) of melamine grown on the Cu(111) surface under ultra-high-vacuum conditions. The affinity of the amino nitrogen for the copper surface drives the melamine molecules to chemisorb strongly tilted (~65° off the surface) through two partially de-hydrogenated amino groups (NH-Cu). Upon water adsorption, the free amino N of melamine molecule acts as H-donor towards it (N-H⋯OHH)  cooperatively working with an adjacent molecule that acts as a H-acceptor via the triazine-N (CN⋯HOH).

Our study has shown how hydration properties of carbon nitride-based photocatalysts strongly depend on (i) the sample morphology and (ii) the co-presence of different types of functional groups. Retrieve article Tailoring surface-supported water–melamine complexes by cooperative H-bonding interactions, V. Lanzilotto, C. Grazioli, M. Stredansky, Teng Zhang, L. Schio, A. Goldoni, L. Floreano, A. Motta, A. Cossaro, C. Puglia, Nanoscale Adv. 3, 2359 (2021) DOI:10.1039/D0NA01034K

Metal-porphyrins on ultra-thin transition metal- oxide layer

We focus on the role of an oxidized ultra-thin buffer-layer in passivating a buried metal substrate. Our evidence proves that, within the same molecule family, the topmost layer oxidation effectively decouples the electronic structure of the molecule from the metal substrate and allows one to tune the assembling properties. A. Calloni et al. DOI:10.1016/j.apsusc.2019.144213

In the last years, a new strategy has been explored. The basic idea is to interpose a single 2D atomic layer between the substrate and the molecules, with the goal of screening the former enough to avoid dramatic changes in the molecular properties. With our strategy, we have coupled a highly reactive metal substrate, such as iron, to heteroaromatic complexes, such as metal-tetra-phenyl porphyrins (MTPP), which maximize their coordination with the substrate by orienting their macrocycle parallel to the surface. On the other hand, Fe(001) surface can be confined to the topmost layer and form the Fe(001)-p(1 × 1)O surface reconstruction, with oxygen atoms filling the hollow sites of the square iron lattice. Our results definitely prove that ultra-thin metal oxide layers

help the stabilization of ordered molecular overlayers and preserve the electronic properties characteristic of quasi-free porphyrins at monolayer coverage. Retrieve article Cobalt atoms drive the anchoring of Co-TPP molecules to the oxygen-passivated Fe(0 0 1) surface, A. Calloni, M.S. Jagadeesh, G. Bussetti, G. Fratesi, S. Achilli, A. Picone, A. Lodesani, A. Brambilla, C. Goletti, F. Ciccacci, L. Duò, M. Finazzi, A. Goldoni, A. Verdini, L. Floreano, Applied Surface Science 505, 144213 (2020)   DOI:10.1016/j.apsusc.2019.144213

Elucidating the influence of anchoring geometry on the reactivity of NO 2 -functionalized N-heterocyclic carbene monolayers

We demonstrated that the chemical reactivity of surface-anchored NO2-functionalized NHCs (NO2–NHCs) can be tuned by modifying the distance between the functional group and the reactive surface, which is governed by the deposition technique. S. Dery et al., DOI:10.1021/acs.jpclett.9b01808

The exceptional chemical tunability and metal affinity of N-heterocyclic carbene molecules (NHCs) have enabled the formation of chemically-active NHC- based self-assembled monolayers (SAMs) in which the chemically-active groups reside in proximity to the metal surface. Herein, in-depth analysis of the influence of the deposition technique on the anchoring geometry, chemical reactivity, and thermal stability of the SAM was performed by using NO 2 -functionalized NHCs that were anchored on Pt (111). The properties of NO 2 -functionalized NHCs were identified by conducting X-ray photoelectron spectroscopy (XPS) and near-edge x-ray absorption fine structure (NEXAFS) measurements.

These results demonstrate the impact of the distance between functional groups and the reactive surface on the reactivity of chemically addressable SAMs. The anchoring geometry is therefore identified as a crucial factor for the design of chemically addressable SAMs. Retrieve article Elucidating the Influence of Anchoring Geometry on the Reactivity of NO2-Functionalized N-Heterocyclic Carbene Monolayers, S. Dery, S. Kim, G. Tomaschun, I. Berg, D. Feferman, A. Cossaro, A. Verdini, L. Floreano, T. Klüner, F.D. Toste, E. Gross, J. Phys. Chem. Lett. 10, 17, 5099 (2019);DOI:10.1021/acs.jpclett.9b01808

Insight into Organometallic Intermediate and Its Evolution to Covalent Bonding in Surface-Confined Ullmann Polymerization

Surface polymerization is of great interest as it enables the realization of graphene-like layers with tunable properties by simply modifying the architecture of the molecular building blocks used as precursors. We focus on some open points concerning the fundamentals of surface-catalyzed dehalogenative polymerization based on Ullmann coupling, widely used over the past decade to obtain 1D and 2D polymers on surfaces. M. Di Giovannantonio et al. ACS Nano 7 (9), 8190 (2013) http://pubs.acs.org/doi/abs/10.1021/nn4035684

1,4-dibromobenzene molecules were used as precursors, forming poly(para-phenylene) polymers by Ullmann coupling on Cu(110). Chemically sensitive techniques such as x-ray photoelectron spectroscopy (XPS) and near-edge x-ray absorption fine structure (NEXAFS) spectroscopy allow to unequivocally identify the existence of an organometallic intermediate product of reaction and the of a final extended conjugated structure. Scanning tunneling microscopy (STM), low energy electron diffraction (LEED) and first-principles calculations provide a deeper insight into the intermediate organometallic phase and on the fundamental role of the halogen in stabilizing specific structures. Fast-XPS analysis of the system during the transformation from organometallic chains to polymers unveils the exact transition temperature for this process.

Retrieve article Insight into Organometallic Intermediate and Its Evolution to Covalent Bonding in Surface-Confined Ullmann Polymerization, Marco Di Giovannantonio, Mohamed El Garah, Josh Lipton-Duffin, Vincent Meunier, Luis Cardenas, Yannick Fagot Revurat, Albano Cossaro, Alberto Verdini, Dmitrii F. Perepichka, Federico Rosei and Giorgio Contini. ACS Nano 2013, 7 (9), pp 8190-8198 DOI:10.1021/nn4035684 http://pubs.acs.org/doi/abs/10.1021/nn4035684

Trimethyltin-Mediated Covalent Gold–Carbon Bond Formation

Spectroscopic evidence of C-Au bond formation, responsible for   the “electron gateway” state, is shown in the process of TrimethylTin break-up on gold. A.Batra et al., J. Am. Chem. Soc., 2014, 136 (36), pp 12556–12559

The formation of covalent gold-carbon bonds is studied in benzyltrimethylstannane (C10H16Sn) deposited on Au in ultra high vacuum. Through X-ray Photoemission Spectroscopy and X-ray absorption measurements, we find that the molecule fragments at the Sn-Benzyl bond when exposed to gold surfaces at temperatures as low as 380K. We show that the resulting benzyl species is stabilized by the presence of Au(111), but only forms covalent Au-C bonds on more reactive Au surfaces like Au(110). In addition, we present spectroscopic proof for the existence of an electronic ‘gateway state’ localized on the Au-C bond that is responsible for its unique electronic properties. Finally, we use density functional theory based nudged elastic band calculations to elucidate the crucial role played by the undercoordinated Au surface in the formation of Au-C bonds.

Retrieve article Trimethyltin-Mediated Covalent Gold–Carbon Bond Formation, Arunabh Batra, Gregor Kladnik, Narjes Gorjizadeh, Jeffrey Meisner, Michael Steigerwald, Colin Nuckolls, Su Ying Quek, Dean Cvetko, Alberto Morgante, and Latha Venkataraman http://pubs.acs.org/doi/abs/10.1021/ja5061406