Nanosize pores can turn semimetallic graphene into a semiconductor and, from being impermeable, i... more Nanosize pores can turn semimetallic graphene into a semiconductor and, from being impermeable, into the most efficient molecular-sieve membrane. However, scaling the pores down to the nanometer, while fulfilling the tight structural constraints imposed by applications, represents an enormous challenge for present top-down strategies. Here we report a bottom-up method to synthesize nanoporous graphene comprising an ordered array of pores separated by ribbons, which can be tuned down to the 1-nanometer range. The size, density, morphology, and chemical composition of the pores are defined with atomic precision by the design of the molecular precursors. Our electronic characterization further reveals a highly anisotropic electronic structure, where orthogonal one-dimensional electronic bands with an energy gap of ∼1 electron volt coexist with confined pore states, making the nanoporous graphene a highly versatile semiconductor for simultaneous sieving and electrical sensing of molecular species.
Ullmann coupling is a widely used reaction for the on-surface growth of low-dimensional carbon na... more Ullmann coupling is a widely used reaction for the on-surface growth of low-dimensional carbon nanomaterials. The irreversible nature of this reaction prevents ‘self-healing’ of defects and detailed knowledge of its mechanism is therefore essential to enable the growth of extended ordered structures. However, the dynamics of the Ullmann polymerization remain largely unexplored as coupling events occur on a time-scale faster than conventional scanning probe microscopy imaging frequencies. Here, we reveal the dynamics of these surface events using high-speed variable temperature scanning tunneling microscopy (STM) (10 frames per second). Performing the measurements at the on-set reaction temperatures provides an unprecedented description of the evolution of organometallic (OM) and covalent surface species during Ullmann polymerization of para-dibromobenzene on Cu(110). Our results demonstrate the existence of two intermediate OM phases with Cu adatoms that inhibits the polymerization. These observations now complete the picture of the pathways of on-surface Ullmann polymerization, which include the complex interplay of the phenylene moieties and metal atoms. Our work demonstrates the unique capability of high-speed STM to capture the dynamics of molecular self-assembly and coupling.
Lateral quantum confinement is of great interest in tuning the electronic properties of graphene-... more Lateral quantum confinement is of great interest in tuning the electronic properties of graphene-based nanostructures, making them suitable for technological applications. In principle, these properties might be controlled through the edge topology: for example, zigzag nanoribbons are predicted to have spin-polarized edge states. The practical realization of these structures is of utmost importance in fully harnessing the electronic properties of graphene. Here, the formation of regular, 1.4 nm wide ribbon-like graphene structures with zigzag edges are reported, showing 1D electronic states. It is found that these "pseudo-ribbons" embedded in single-layer graphene supported on Ni(100) can spontaneously form upon carbon segregation underneath 1D graphene moiré domains, extending hundreds of nanometers in length. On the basis of both microscopy/spectroscopy/diffraction experiments and theoretical simulations, it is shown that these structures, even though seamlessly incorporated in a matrix of strongly interacting graphene, exhibit electronic properties closely resembling those of zigzag nanoribbons.
Since the invention of scanning probe microscopy, researchers have desired to use this technique ... more Since the invention of scanning probe microscopy, researchers have desired to use this technique to monitor sub-second surface dynamics with atomic spatial resolution. A recently presented addon electronics module enables the speed-up of existing, conventional scanning probe microscopes without any modification of the actual instrument. The resulting one-dimensional (1D) data stream, recorded while the tip oscillates in a sinusoidal motion, has to be reconstructed into a layered rectangular matrix in order to visualize the movie. The Python-based pyfastspm package performs this conversion, while also correcting for sample tilt, noise frequencies, piezo creep, and thermal drift. Quick automatic conversion even of considerable batches of data is achieved by efficient algorithms that bundle time-expensive steps, such as interpolation based on Delaunay triangulation.
The electronic properties of graphene can be modified by the local interaction with a selected me... more The electronic properties of graphene can be modified by the local interaction with a selected metal substrate. To probe this effect, Scanning Tunneling Microscopy is widely employed, particularly by means of local measurement via lock-in amplifier of the differential conductance and of the field emission resonance. In this article we propose an alternative, reliable method of probing the graphene/substrate interaction that is readily available to any STM apparatus. By testing the tunneling current as function of the tip/sample distance on nanostructured graphene on Ni(100) and Ir (100), we demonstrate that I(z) spectroscopy can be quantitatively compared with Density Functional Theory calculations and can be used to assess the nature of the interaction between graphene and substrate. This method can expand the capabilities of standard STM systems to study graphene/substrate complexes, complementing standard topographic probing with spectroscopic information.
We report the deposition of monolayers and multilayers of {Cr 10 (OMe) 20 (O 2 CCMe 3) 10 } wheel... more We report the deposition of monolayers and multilayers of {Cr 10 (OMe) 20 (O 2 CCMe 3) 10 } wheels, hereafter {Cr 10 }, onto Au(111) and Cu(111) single-crystal substrates, and their characterization combining scanning tunneling microscopy, X-ray photoelectron spectroscopy, X-ray absorption spectroscopy, X-ray natural linear dichroism, and X-ray magnetic circular dichroism (XMCD). {Cr 10 } in bulk shows axial magnetic anisotropy and a cluster total spin S = 9 ground state, stemming from an interaction scheme of two semi-crowns containing four Cr 3+ ions interacting ferromagnetically, separated by two Cr 3+ antiferromagnetically coupled. The one-monolayer (1ML) samples of {Cr 10 } sublimated on Ag(111) and Cu(111) show slightly different applied magnetic field dependence of XMCD signal. The field-dependence of the magnetization evolves from a lower curve for the 1ML {Cr 10 } samples to a curve resembling the bulk one as the number of layers is increased, as shown in a 14ML sample. Monte Carlo simulations allow rationalizing the magnetization curves of the 1ML samples in terms of a reduction of the cluster ground-state total spin, S = 3 or S = 6, as a consequence of variations in the intra-wheel coupling interactions induced by the on-surface deposition. The sensitivity of the magnetic configuration of {Cr 10 } to minor distortions of the intramolecular conformation might be exploited to control magnetism by external stimuli for applications.
The role of moiré graphene superstructures in favoring confined adsorption of different metal ato... more The role of moiré graphene superstructures in favoring confined adsorption of different metal atoms is an intriguing problem not yet completely solved. Graphene (G) grown on Ni(100) forms a striped moiré pattern of valleys, where G approaches the nickel substrate and interacts with it rather strongly, and ridges, where G stays far away from the substrate and acts almost free-standing. Combining density functional theory (DFT) calculations and scanning-tunneling microscopy (STM) measurements, we show that this peculiar moiré constitutes a regular nanostructured template on a 2D support, confining in 1D trails single metal atoms and few atoms clusters. DFT calculations show that the confinement is selective and highly dependent on the atomic species, with some species preferring to adsorb on ridges and the other showing preference for valleys. Co and Au adsorbates, for instance, have opposite behavior, as predicted by DFT and observed by STM. The origin of such disparate behavior is traced back to the electrostatic interaction between the charged adsorbate and the nickel surface. Moreover, the selectivity is not restricted to the adsorption process only, but persists as adsorbate starts its diffusion, resulting in unidirectional mass transport on a continuous 2D support. These findings hold great promise for exploiting tailored nanostructured templates in a wide range of potential applications involving mass transport along element-specific routes.
In this paper, we describe the data management practices and services developed for making FAIR c... more In this paper, we describe the data management practices and services developed for making FAIR compliant a scientific archive of Scanning Tunneling Microscopy (STM) images. As a first step, we extracted the instrument metadata of each image of the dataset to create a structured database. We then enriched these metadata with information on the structure and composition of the surface by means of a pipeline that leverages human annotation, machine learning techniques, and instrument metadata filtering. To visually explore both images and metadata, as well as to improve the accessibility and usability of the dataset, we developed "STM explorer" as a web service integrated within the Trieste Advanced Data services (TriDAS) website. On top of these data services and tools, we propose an implementation of the W3C PROV standard to describe provenance metadata of STM images.
To improve reactivity and achieve a higher material efficiency, catalysts are often used in the f... more To improve reactivity and achieve a higher material efficiency, catalysts are often used in the form of clusters with nanometer dimensions, down to single atoms. Since the corresponding properties are highly structure-dependent, a suitable support is thus required to ensure cluster stability during operating conditions. Herein, an efficient method to stabilize cobalt nanoclusters on graphene grown on nickel substrates, exploiting the anchoring effect of nickel atoms incorporated in the carbon network is presented. The anchored nanoclusters are studied by in situ variable temperature scanning tunneling microscopy at different temperatures and upon gas exposure. Cluster stability upon annealing up to 200 °C and upon CO exposure at least up to 1 × 10−6 mbar CO partial pressure is demonstrated. Moreover, the dimensions of the cobalt nanoclusters remain surprisingly small (<3 nm diameter) with a narrow size distribution. Density functional theory calculations demonstrate that the interplay between the low diffusion barrier on graphene on nickel and the strong anchoring effect of the nickel atoms leads to the increased stability and size selectivity of these clusters. This anchoring technique is expected to be applicable also to other cases, with clear advantages for transition metals that are usually difficult to stabilize.
One of the promising ways to functionalize graphene is incorporation of heteroatoms in carbon sp2... more One of the promising ways to functionalize graphene is incorporation of heteroatoms in carbon sp2 lattice, as it is proven to be an efficient and versatile method for controllably tuning chemistry of graphene. We present unique, contamination-free method for selectively doping graphene with B dopants, which are incorporated in layer from a reservoir created in the bulk of Ni(111) single crystal, during standard CVD growth process, leading to clean, versatile and efficient method for creating B-doped graphene. We combine experimental (STM, XPS) and theoretical (DFT, simulated STM) studies to understand structural and chemical properties of substitutional B dopants. Along with previously reported substitutional B in fcc sites, we have observed, for the first time, two more defects, namely substitutional B in top sites and interstitial B in octahedral subsurface sites. Extensive STM investigations confirm presence of low and high concentration regions of B dopants in as-prepared B-doped graphene, indicating non-uniform boron incorporation. Among two substitutional sites, no preference is observed in low-concentration B-doped regions, whereas in high B concentration regions, one of the sublattices is preferred for incorporation, along with alignment of defects. This generates an asymmetric sublattice doping in as-grown B-doped graphene, which is theoretically predicted to result in notable band gap.
Under near-ambient pressure conditions, carbon monoxide molecules intercalate underneath an epita... more Under near-ambient pressure conditions, carbon monoxide molecules intercalate underneath an epitaxial graphene monolayer grown on Ni(111), getting trapped into the confined region at the interface. On the basis of ab-initio density functional theory calculations, we provide here a full investigation of the intercalated CO pattern, highlighting the modifications induced on the graphene electronic structure. For a CO coverage as low as 0.14 monolayer (ML), the graphene layer is spatially decoupled from the metallic substrate, with a significant C 1s core level shift towards lower binding energies. The most relevant signature of the CO intercalation is a clear switching of the graphene doping state, which changes from n-type, when strongly interacting with the metal surface, to p-type. The shift of the Dirac cone linearly depends on the CO coverage, reaching about 0.9 eV for the saturation value of 0.57 ML. Theoretical predictions are compared with the results of scanning tunnelling microscopy, low-energy electron diffraction and photoemission spectroscopy experiments, which confirm the proposed scenario for the nearly saturated intercalated CO system. This result opens the way to the application of the graphene/Ni(111) interface as gas sensor to easily detect and quantify the presence of carbon monoxide.
Dynamic covalent chemistry is a powerful approach to design covalent organic frameworks, where hi... more Dynamic covalent chemistry is a powerful approach to design covalent organic frameworks, where high crystallinity is achieved through reversible bond formation. Here, we exploit near-ambient pressure X-ray photoelectron spectroscopy to elucidate the reversible formation of a two-dimensional boroxine framework. By in situ mapping the pressure-temperature parameter space, we identify the regions where the rates of the condensation and hydrolysis reactions become dominant, being the key to enable the thermodynamically controlled growth of crystalline frameworks.
Understanding the chemical processes at the interface between a metal and topological insulator (... more Understanding the chemical processes at the interface between a metal and topological insulator (TI) is important when it comes to designing devices that exploit the peculiar topological surface states or studying the properties of TI heterostructures. In this paper we show that the interface between Ti and Bi 2 Se 3 is unstable at RT and results in the formation of interfacial phases of titanium selenides and metallic Bi. The reaction has shown significant kinetics already at cryogenic temperatures, which is very surprising for a solid-state redox reaction. This can be explained with the possibility of electrons in the topological surface states playing a role in enhancing the Bi 2 Se 3 surface reactivity due to the electronbath effect. For the Ti coverage above 40 nm, the interfacial processes cause compressive stress that triggers the morphological change (buckling) of the deposited film. The observed interface reaction, with all of its consequences, has to be considered not only in the design of devices, where the Ti adhesion layer is often used for contacts, but also for possible engineering of 2D TI heterostructures.
Taming the magnetic anisotropy of lanthanides through coordination environments is crucial to tak... more Taming the magnetic anisotropy of lanthanides through coordination environments is crucial to take advantage of the lanthanides properties in thermally robust nanomaterials. In this work, the electronic and magnetic properties of Dy-carboxylate metal–organic networks on Cu(111) based on an eightfold coordination between Dy and ditopic linkers are inspected. This surface science study based on scanning probe microscopy and X-ray magnetic circular dichroism, complemented with density functional theory and multiplet calculations, reveals that the magnetic anisotropy landscape of the system is complex. Surface-supported metal–organic coordination is able to induce a change in the orientation of the easy magnetization axis of the Dy coordinative centers as compared to isolated Dy atoms and Dy clusters, and significantly increases the magnetic anisotropy. Surprisingly, Dy atoms coordinated in the metallosupramolecular networks display a nearly in-plane easy magnetization axis despite the out-of-plane symmetry axis of the coordinative molecular lattice. Multiplet calculations highlight the decisive role of the metal–organic coordination, revealing that the tilted orientation is the result of a very delicate balance between the interaction of Dy with O atoms and the precise geometry of the crystal field. This study opens new avenues to tailor the magnetic anisotropy and magnetic moments of lanthanide elements on surfaces.
Taming the magnetic anisotropy of lanthanides through coordination environments is crucial to tak... more Taming the magnetic anisotropy of lanthanides through coordination environments is crucial to take advantage of the lanthanides properties in thermally robust nanomaterials. In this work, the electronic and magnetic properties of Dy-carboxylate metal–organic networks on Cu(111) based on an eightfold coordination between Dy and ditopic linkers are inspected. This surface science study based on scanning probe microscopy and X-ray magnetic circular dichroism, complemented with density functional theory and multiplet calculations, reveals that the magnetic anisotropy landscape of the system is complex. Surface-supported metal–organic coordination is able to induce a change in the orientation of the easy magnetization axis of the Dy coordinative centers as compared to isolated Dy atoms and Dy clusters, and significantly increases the magnetic anisotropy. Surprisingly, Dy atoms coordinated in the metallosupramolecular networks display a nearly in-plane easy magnetization axis despite the out-of-plane symmetry axis of the coordinative molecular lattice. Multiplet calculations highlight the decisive role of the metal–organic coordination, revealing that the tilted orientation is the result of a very delicate balance between the interaction of Dy with O atoms and the precise geometry of the crystal field. This study opens new avenues to tailor the magnetic anisotropy and magnetic moments of lanthanide elements on surfaces.
Molecules intercalate at the graphene/metal interface even though defect-free graphene is imperme... more Molecules intercalate at the graphene/metal interface even though defect-free graphene is impermeable to any atomic and molecular species in the gas and liquid phase, except hydrogen. The mechanism of molecular intercalation is still a big open question. In this Letter, by means of a combined experimental (STM, XPS, and LEED) and theoretical (DFT) study, we present a proof of how CO molecules succeed in permeating the graphene layer and get into the confined zone between graphene and the Ni(111) surface. The presence of N-dopants in the graphene layer is found to highly facilitate the permeation process, reducing the CO threshold pressure by more than one order of magnitude, through the stabilization of multiatomic vacancy defects that are the open doors to the bidimensional nanospace, with crucial implications for the catalysis under cover and for the graphene-based electrochemistry.
The characterization of graphene by electron and optical spectroscopy is well established and has... more The characterization of graphene by electron and optical spectroscopy is well established and has led to numerous breakthroughs in material science. Yet, it is interesting to note that these characterization methods are almost never carried out on the same sample, i.e., electron spectroscopy uses epitaxial graphene while optical spectroscopy relies on cleaved graphene flakes. In order to bring coherence and convergence to this branch, a universal and easy-to-prepare substrate is needed. Here we suggest that chemical vapour deposition (CVD) grown graphene on thin monocrystalline Ir(1 1 1) films, which are grown heteroepitaxially on Si(1 1 1) wafers with an yttria stabilized zirconia (YSZ) buffer layer, perfectly meets these needs. We investigate graphene prepared in this way by low-energy electron diffraction (LEED), X-ray photoelectron spectroscopy (XPS), near edge X-ray absorption fine structure (NEXAFS) spectroscopy, angle-resolved photoemission spectroscopy (ARPES), resonance Raman spectroscopy, and scanning tunnelling microscopy (STM). Our results highlight the excellent crystalline quality of graphene, comparable to graphene prepared on Ir(1 1 1) bulk single crystals. This synthesis route allows for large-area, inexpensive growth on standardized disposable substrates, suitable for both optical and electron spectroscopic characterization, which meets the needs of many researchers in the field.
Nanosize pores can turn semimetallic graphene into a semiconductor and, from being impermeable, i... more Nanosize pores can turn semimetallic graphene into a semiconductor and, from being impermeable, into the most efficient molecular-sieve membrane. However, scaling the pores down to the nanometer, while fulfilling the tight structural constraints imposed by applications, represents an enormous challenge for present top-down strategies. Here we report a bottom-up method to synthesize nanoporous graphene comprising an ordered array of pores separated by ribbons, which can be tuned down to the 1-nanometer range. The size, density, morphology, and chemical composition of the pores are defined with atomic precision by the design of the molecular precursors. Our electronic characterization further reveals a highly anisotropic electronic structure, where orthogonal one-dimensional electronic bands with an energy gap of ∼1 electron volt coexist with confined pore states, making the nanoporous graphene a highly versatile semiconductor for simultaneous sieving and electrical sensing of molecular species.
Ullmann coupling is a widely used reaction for the on-surface growth of low-dimensional carbon na... more Ullmann coupling is a widely used reaction for the on-surface growth of low-dimensional carbon nanomaterials. The irreversible nature of this reaction prevents ‘self-healing’ of defects and detailed knowledge of its mechanism is therefore essential to enable the growth of extended ordered structures. However, the dynamics of the Ullmann polymerization remain largely unexplored as coupling events occur on a time-scale faster than conventional scanning probe microscopy imaging frequencies. Here, we reveal the dynamics of these surface events using high-speed variable temperature scanning tunneling microscopy (STM) (10 frames per second). Performing the measurements at the on-set reaction temperatures provides an unprecedented description of the evolution of organometallic (OM) and covalent surface species during Ullmann polymerization of para-dibromobenzene on Cu(110). Our results demonstrate the existence of two intermediate OM phases with Cu adatoms that inhibits the polymerization. These observations now complete the picture of the pathways of on-surface Ullmann polymerization, which include the complex interplay of the phenylene moieties and metal atoms. Our work demonstrates the unique capability of high-speed STM to capture the dynamics of molecular self-assembly and coupling.
Lateral quantum confinement is of great interest in tuning the electronic properties of graphene-... more Lateral quantum confinement is of great interest in tuning the electronic properties of graphene-based nanostructures, making them suitable for technological applications. In principle, these properties might be controlled through the edge topology: for example, zigzag nanoribbons are predicted to have spin-polarized edge states. The practical realization of these structures is of utmost importance in fully harnessing the electronic properties of graphene. Here, the formation of regular, 1.4 nm wide ribbon-like graphene structures with zigzag edges are reported, showing 1D electronic states. It is found that these "pseudo-ribbons" embedded in single-layer graphene supported on Ni(100) can spontaneously form upon carbon segregation underneath 1D graphene moiré domains, extending hundreds of nanometers in length. On the basis of both microscopy/spectroscopy/diffraction experiments and theoretical simulations, it is shown that these structures, even though seamlessly incorporated in a matrix of strongly interacting graphene, exhibit electronic properties closely resembling those of zigzag nanoribbons.
Since the invention of scanning probe microscopy, researchers have desired to use this technique ... more Since the invention of scanning probe microscopy, researchers have desired to use this technique to monitor sub-second surface dynamics with atomic spatial resolution. A recently presented addon electronics module enables the speed-up of existing, conventional scanning probe microscopes without any modification of the actual instrument. The resulting one-dimensional (1D) data stream, recorded while the tip oscillates in a sinusoidal motion, has to be reconstructed into a layered rectangular matrix in order to visualize the movie. The Python-based pyfastspm package performs this conversion, while also correcting for sample tilt, noise frequencies, piezo creep, and thermal drift. Quick automatic conversion even of considerable batches of data is achieved by efficient algorithms that bundle time-expensive steps, such as interpolation based on Delaunay triangulation.
The electronic properties of graphene can be modified by the local interaction with a selected me... more The electronic properties of graphene can be modified by the local interaction with a selected metal substrate. To probe this effect, Scanning Tunneling Microscopy is widely employed, particularly by means of local measurement via lock-in amplifier of the differential conductance and of the field emission resonance. In this article we propose an alternative, reliable method of probing the graphene/substrate interaction that is readily available to any STM apparatus. By testing the tunneling current as function of the tip/sample distance on nanostructured graphene on Ni(100) and Ir (100), we demonstrate that I(z) spectroscopy can be quantitatively compared with Density Functional Theory calculations and can be used to assess the nature of the interaction between graphene and substrate. This method can expand the capabilities of standard STM systems to study graphene/substrate complexes, complementing standard topographic probing with spectroscopic information.
We report the deposition of monolayers and multilayers of {Cr 10 (OMe) 20 (O 2 CCMe 3) 10 } wheel... more We report the deposition of monolayers and multilayers of {Cr 10 (OMe) 20 (O 2 CCMe 3) 10 } wheels, hereafter {Cr 10 }, onto Au(111) and Cu(111) single-crystal substrates, and their characterization combining scanning tunneling microscopy, X-ray photoelectron spectroscopy, X-ray absorption spectroscopy, X-ray natural linear dichroism, and X-ray magnetic circular dichroism (XMCD). {Cr 10 } in bulk shows axial magnetic anisotropy and a cluster total spin S = 9 ground state, stemming from an interaction scheme of two semi-crowns containing four Cr 3+ ions interacting ferromagnetically, separated by two Cr 3+ antiferromagnetically coupled. The one-monolayer (1ML) samples of {Cr 10 } sublimated on Ag(111) and Cu(111) show slightly different applied magnetic field dependence of XMCD signal. The field-dependence of the magnetization evolves from a lower curve for the 1ML {Cr 10 } samples to a curve resembling the bulk one as the number of layers is increased, as shown in a 14ML sample. Monte Carlo simulations allow rationalizing the magnetization curves of the 1ML samples in terms of a reduction of the cluster ground-state total spin, S = 3 or S = 6, as a consequence of variations in the intra-wheel coupling interactions induced by the on-surface deposition. The sensitivity of the magnetic configuration of {Cr 10 } to minor distortions of the intramolecular conformation might be exploited to control magnetism by external stimuli for applications.
The role of moiré graphene superstructures in favoring confined adsorption of different metal ato... more The role of moiré graphene superstructures in favoring confined adsorption of different metal atoms is an intriguing problem not yet completely solved. Graphene (G) grown on Ni(100) forms a striped moiré pattern of valleys, where G approaches the nickel substrate and interacts with it rather strongly, and ridges, where G stays far away from the substrate and acts almost free-standing. Combining density functional theory (DFT) calculations and scanning-tunneling microscopy (STM) measurements, we show that this peculiar moiré constitutes a regular nanostructured template on a 2D support, confining in 1D trails single metal atoms and few atoms clusters. DFT calculations show that the confinement is selective and highly dependent on the atomic species, with some species preferring to adsorb on ridges and the other showing preference for valleys. Co and Au adsorbates, for instance, have opposite behavior, as predicted by DFT and observed by STM. The origin of such disparate behavior is traced back to the electrostatic interaction between the charged adsorbate and the nickel surface. Moreover, the selectivity is not restricted to the adsorption process only, but persists as adsorbate starts its diffusion, resulting in unidirectional mass transport on a continuous 2D support. These findings hold great promise for exploiting tailored nanostructured templates in a wide range of potential applications involving mass transport along element-specific routes.
In this paper, we describe the data management practices and services developed for making FAIR c... more In this paper, we describe the data management practices and services developed for making FAIR compliant a scientific archive of Scanning Tunneling Microscopy (STM) images. As a first step, we extracted the instrument metadata of each image of the dataset to create a structured database. We then enriched these metadata with information on the structure and composition of the surface by means of a pipeline that leverages human annotation, machine learning techniques, and instrument metadata filtering. To visually explore both images and metadata, as well as to improve the accessibility and usability of the dataset, we developed "STM explorer" as a web service integrated within the Trieste Advanced Data services (TriDAS) website. On top of these data services and tools, we propose an implementation of the W3C PROV standard to describe provenance metadata of STM images.
To improve reactivity and achieve a higher material efficiency, catalysts are often used in the f... more To improve reactivity and achieve a higher material efficiency, catalysts are often used in the form of clusters with nanometer dimensions, down to single atoms. Since the corresponding properties are highly structure-dependent, a suitable support is thus required to ensure cluster stability during operating conditions. Herein, an efficient method to stabilize cobalt nanoclusters on graphene grown on nickel substrates, exploiting the anchoring effect of nickel atoms incorporated in the carbon network is presented. The anchored nanoclusters are studied by in situ variable temperature scanning tunneling microscopy at different temperatures and upon gas exposure. Cluster stability upon annealing up to 200 °C and upon CO exposure at least up to 1 × 10−6 mbar CO partial pressure is demonstrated. Moreover, the dimensions of the cobalt nanoclusters remain surprisingly small (<3 nm diameter) with a narrow size distribution. Density functional theory calculations demonstrate that the interplay between the low diffusion barrier on graphene on nickel and the strong anchoring effect of the nickel atoms leads to the increased stability and size selectivity of these clusters. This anchoring technique is expected to be applicable also to other cases, with clear advantages for transition metals that are usually difficult to stabilize.
One of the promising ways to functionalize graphene is incorporation of heteroatoms in carbon sp2... more One of the promising ways to functionalize graphene is incorporation of heteroatoms in carbon sp2 lattice, as it is proven to be an efficient and versatile method for controllably tuning chemistry of graphene. We present unique, contamination-free method for selectively doping graphene with B dopants, which are incorporated in layer from a reservoir created in the bulk of Ni(111) single crystal, during standard CVD growth process, leading to clean, versatile and efficient method for creating B-doped graphene. We combine experimental (STM, XPS) and theoretical (DFT, simulated STM) studies to understand structural and chemical properties of substitutional B dopants. Along with previously reported substitutional B in fcc sites, we have observed, for the first time, two more defects, namely substitutional B in top sites and interstitial B in octahedral subsurface sites. Extensive STM investigations confirm presence of low and high concentration regions of B dopants in as-prepared B-doped graphene, indicating non-uniform boron incorporation. Among two substitutional sites, no preference is observed in low-concentration B-doped regions, whereas in high B concentration regions, one of the sublattices is preferred for incorporation, along with alignment of defects. This generates an asymmetric sublattice doping in as-grown B-doped graphene, which is theoretically predicted to result in notable band gap.
Under near-ambient pressure conditions, carbon monoxide molecules intercalate underneath an epita... more Under near-ambient pressure conditions, carbon monoxide molecules intercalate underneath an epitaxial graphene monolayer grown on Ni(111), getting trapped into the confined region at the interface. On the basis of ab-initio density functional theory calculations, we provide here a full investigation of the intercalated CO pattern, highlighting the modifications induced on the graphene electronic structure. For a CO coverage as low as 0.14 monolayer (ML), the graphene layer is spatially decoupled from the metallic substrate, with a significant C 1s core level shift towards lower binding energies. The most relevant signature of the CO intercalation is a clear switching of the graphene doping state, which changes from n-type, when strongly interacting with the metal surface, to p-type. The shift of the Dirac cone linearly depends on the CO coverage, reaching about 0.9 eV for the saturation value of 0.57 ML. Theoretical predictions are compared with the results of scanning tunnelling microscopy, low-energy electron diffraction and photoemission spectroscopy experiments, which confirm the proposed scenario for the nearly saturated intercalated CO system. This result opens the way to the application of the graphene/Ni(111) interface as gas sensor to easily detect and quantify the presence of carbon monoxide.
Dynamic covalent chemistry is a powerful approach to design covalent organic frameworks, where hi... more Dynamic covalent chemistry is a powerful approach to design covalent organic frameworks, where high crystallinity is achieved through reversible bond formation. Here, we exploit near-ambient pressure X-ray photoelectron spectroscopy to elucidate the reversible formation of a two-dimensional boroxine framework. By in situ mapping the pressure-temperature parameter space, we identify the regions where the rates of the condensation and hydrolysis reactions become dominant, being the key to enable the thermodynamically controlled growth of crystalline frameworks.
Understanding the chemical processes at the interface between a metal and topological insulator (... more Understanding the chemical processes at the interface between a metal and topological insulator (TI) is important when it comes to designing devices that exploit the peculiar topological surface states or studying the properties of TI heterostructures. In this paper we show that the interface between Ti and Bi 2 Se 3 is unstable at RT and results in the formation of interfacial phases of titanium selenides and metallic Bi. The reaction has shown significant kinetics already at cryogenic temperatures, which is very surprising for a solid-state redox reaction. This can be explained with the possibility of electrons in the topological surface states playing a role in enhancing the Bi 2 Se 3 surface reactivity due to the electronbath effect. For the Ti coverage above 40 nm, the interfacial processes cause compressive stress that triggers the morphological change (buckling) of the deposited film. The observed interface reaction, with all of its consequences, has to be considered not only in the design of devices, where the Ti adhesion layer is often used for contacts, but also for possible engineering of 2D TI heterostructures.
Taming the magnetic anisotropy of lanthanides through coordination environments is crucial to tak... more Taming the magnetic anisotropy of lanthanides through coordination environments is crucial to take advantage of the lanthanides properties in thermally robust nanomaterials. In this work, the electronic and magnetic properties of Dy-carboxylate metal–organic networks on Cu(111) based on an eightfold coordination between Dy and ditopic linkers are inspected. This surface science study based on scanning probe microscopy and X-ray magnetic circular dichroism, complemented with density functional theory and multiplet calculations, reveals that the magnetic anisotropy landscape of the system is complex. Surface-supported metal–organic coordination is able to induce a change in the orientation of the easy magnetization axis of the Dy coordinative centers as compared to isolated Dy atoms and Dy clusters, and significantly increases the magnetic anisotropy. Surprisingly, Dy atoms coordinated in the metallosupramolecular networks display a nearly in-plane easy magnetization axis despite the out-of-plane symmetry axis of the coordinative molecular lattice. Multiplet calculations highlight the decisive role of the metal–organic coordination, revealing that the tilted orientation is the result of a very delicate balance between the interaction of Dy with O atoms and the precise geometry of the crystal field. This study opens new avenues to tailor the magnetic anisotropy and magnetic moments of lanthanide elements on surfaces.
Taming the magnetic anisotropy of lanthanides through coordination environments is crucial to tak... more Taming the magnetic anisotropy of lanthanides through coordination environments is crucial to take advantage of the lanthanides properties in thermally robust nanomaterials. In this work, the electronic and magnetic properties of Dy-carboxylate metal–organic networks on Cu(111) based on an eightfold coordination between Dy and ditopic linkers are inspected. This surface science study based on scanning probe microscopy and X-ray magnetic circular dichroism, complemented with density functional theory and multiplet calculations, reveals that the magnetic anisotropy landscape of the system is complex. Surface-supported metal–organic coordination is able to induce a change in the orientation of the easy magnetization axis of the Dy coordinative centers as compared to isolated Dy atoms and Dy clusters, and significantly increases the magnetic anisotropy. Surprisingly, Dy atoms coordinated in the metallosupramolecular networks display a nearly in-plane easy magnetization axis despite the out-of-plane symmetry axis of the coordinative molecular lattice. Multiplet calculations highlight the decisive role of the metal–organic coordination, revealing that the tilted orientation is the result of a very delicate balance between the interaction of Dy with O atoms and the precise geometry of the crystal field. This study opens new avenues to tailor the magnetic anisotropy and magnetic moments of lanthanide elements on surfaces.
Molecules intercalate at the graphene/metal interface even though defect-free graphene is imperme... more Molecules intercalate at the graphene/metal interface even though defect-free graphene is impermeable to any atomic and molecular species in the gas and liquid phase, except hydrogen. The mechanism of molecular intercalation is still a big open question. In this Letter, by means of a combined experimental (STM, XPS, and LEED) and theoretical (DFT) study, we present a proof of how CO molecules succeed in permeating the graphene layer and get into the confined zone between graphene and the Ni(111) surface. The presence of N-dopants in the graphene layer is found to highly facilitate the permeation process, reducing the CO threshold pressure by more than one order of magnitude, through the stabilization of multiatomic vacancy defects that are the open doors to the bidimensional nanospace, with crucial implications for the catalysis under cover and for the graphene-based electrochemistry.
The characterization of graphene by electron and optical spectroscopy is well established and has... more The characterization of graphene by electron and optical spectroscopy is well established and has led to numerous breakthroughs in material science. Yet, it is interesting to note that these characterization methods are almost never carried out on the same sample, i.e., electron spectroscopy uses epitaxial graphene while optical spectroscopy relies on cleaved graphene flakes. In order to bring coherence and convergence to this branch, a universal and easy-to-prepare substrate is needed. Here we suggest that chemical vapour deposition (CVD) grown graphene on thin monocrystalline Ir(1 1 1) films, which are grown heteroepitaxially on Si(1 1 1) wafers with an yttria stabilized zirconia (YSZ) buffer layer, perfectly meets these needs. We investigate graphene prepared in this way by low-energy electron diffraction (LEED), X-ray photoelectron spectroscopy (XPS), near edge X-ray absorption fine structure (NEXAFS) spectroscopy, angle-resolved photoemission spectroscopy (ARPES), resonance Raman spectroscopy, and scanning tunnelling microscopy (STM). Our results highlight the excellent crystalline quality of graphene, comparable to graphene prepared on Ir(1 1 1) bulk single crystals. This synthesis route allows for large-area, inexpensive growth on standardized disposable substrates, suitable for both optical and electron spectroscopic characterization, which meets the needs of many researchers in the field.
We report the deposition of monolayers and multilayers of {Cr10(OMe)20(O2CCMe3)10} wheels, hereaf... more We report the deposition of monolayers and multilayers of {Cr10(OMe)20(O2CCMe3)10} wheels, hereafter {Cr10}, onto Au(111) and Cu(111) single‐crystal substrates, and their characterization combining scanning tunneling microscopy, X‐ray photoelectron spectroscopy, X‐ray absorption spectroscopy, X‐ray natural linear dichroism, and X‐ray magnetic circular dichroism (XMCD). {Cr10} in bulk shows axial magnetic anisotropy and a cluster total spin S = 9 ground state, stemming from an interaction scheme of two semi‐crowns containing four Cr3+ ions interacting ferromagnetically, separated by two Cr3+ antiferromagnetically coupled. The one‐monolayer (1ML) samples of {Cr10} sublimated on Ag(111) and Cu(111) show slightly different applied magnetic field dependence of XMCD signal. The field‐dependence of the magnetization evolves from a lower curve for the 1ML {Cr10} samples to a curve resembling the bulk one as the number of layers is increased, as shown in a 14ML sample. Monte Carlo simulati...
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Papers by Mirco Panighel
Copyright © 2020 American Chemical Society
Copyright © 2020 American Chemical Society