Unveiling the Interlayer Interaction in a 1H/1T TaS2 van der Waals Heterostructure.

This study delves into the intriguing properties of the 1H/1T-TaS2 van der Waals heterostructure, focusing on the transparency of the 1H layer to the charge density wave of the underlying 1T layer. Despite the sizable interlayer separation and metallic nature of the 1H layer, positive bias voltages result in a pronounced superposition of the 1T charge density wave structure on the 1H layer. The conventional explanation relying on tunneling effects proves insufficient. Through a comprehensive investigation combining low-temperature scanning tunneling microscopy, scanning tunneling spectroscopy, non-contact atomic force microscopy, and first-principles calculations, we propose an alternative interpretation. The transparency effect arises from a weak yet substantial electronic coupling between the 1H and 1T layers, challenging prior understanding of the system. Our results highlight the critical role played by interlayer electronic interactions in van der Waals heterostructures to determine the final ground states of the systems.

Probing the Phase Transition to a Coherent 2D Kondo Lattice.

Kondo lattices are systems with unusual electronic properties that stem from strong electron correlation, typically studied in intermetallic 3D compounds containing lanthanides or actinides. Lowering the dimensionality of the system enhances the role of electron correlations providing a new tuning knob for the search of novel properties in strongly correlated quantum matter. The realization of a 2D Kondo lattice by stacking a single-layer Mott insulator on a metallic surface is reported. The temperature of the system is steadily lowered and by using high-resolution scanning tunneling spectroscopy, the phase transition leading to the Kondo lattice is followed. Above 27 K the interaction between the Mott insulator and the metal is negligible and both keep their original electronic properties intact. Below 27 K the Kondo screening of the localized electrons in the Mott insulator begins and below 11 K the formation of a coherent quantum electronic state extended to the entire sample, i.e., the Kondo lattice, takes place. By means of density functional theory, the electronic properties of the system and its evolution with temperature are explained. The findings contribute to the exploration of unconventional states in 2D correlated materials.

Nanotube-Like Electronic States in [5,5]-C90 Fullertube Molecules.

Fullertubes, that is, fullerenes consisting of a carbon nanotube moiety capped by hemifullerene ends, are emerging carbon nanomaterials whose properties show both fullerene and carbon nanotube (CNT) traits. Albeit it may be expected that their electronic states show a certain resemblance to those of the extended nanotube, such a correlation has not yet been found or described. Here it shows a scanning tunneling microscopy (STM) and spectroscopy (STS) characterization of the adsorption, self-assembly, and electronic structure of 2D arrays of [5,5]-C90 fullertube molecules on two different noble metal surfaces, Ag(111) and Au(111). The results demonstrate that the shape of the molecular orbitals of the adsorbed fullertubes corresponds closely to those expected for isolated species on the grounds of density functional theory calculations. Moreover, a comparison between the electronic density profiles in the bands of the extended [5,5]-CNT and in the molecules reveals that some of the frontier orbitals of the fullertube molecules can be described as the result of the quantum confinement imposed by the hemifullerene caps to the delocalized band states in the extended CNT. The results thus provide a conceptual framework for the rational design of custom fullertube molecules and can potentially become a cornerstone in the understanding of these new carbon nanoforms.

2D Co-Directed Metal-Organic Networks Featuring Strong Antiferromagnetism and Perpendicular Anisotropy.

Antiferromagnetic spintronics is a rapidly emerging field with the potential to revolutionize the way information is stored and processed. One of the key challenges in this field is the development of novel 2D antiferromagnetic materials. In this paper, the first on-surface synthesis of a Co-directed metal-organic network is reported in which the Co atoms are strongly antiferromagnetically coupled, while featuring a perpendicular magnetic anisotropy. This material is a promising candidate for future antiferromagnetic spintronic devices, as it combines the advantages of 2D and metal-organic chemistry with strong antiferromagnetic order and perpendicular magnetic anisotropy.

Experimental Demonstration of a Magnetically Induced Warping Transition in a Topological Insulator Mediated by Rare-Earth Surface Dopants.

Magnetic topological insulators constitute a novel class of materials whose topological surface states (TSSs) coexist with long-range ferromagnetic order, eventually breaking time-reversal symmetry. The subsequent bandgap opening is predicted to co-occur with a distortion of the TSS warped shape from hexagonal to trigonal. We demonstrate such a transition by means of angle-resolved photoemission spectroscopy on the magnetically rare-earth (Er and Dy) surface-doped topological insulator Bi2Se2Te. Signatures of the gap opening are also observed. Moreover, increasing the dopant coverage results in a tunable p-type doping of the TSS, thereby allowing for a gradual tuning of the Fermi level toward the magnetically induced bandgap. A theoretical model where a magnetic Zeeman out-of-plane term is introduced in the Hamiltonian governing the TSS rationalizes these experimental results. Our findings offer new strategies to control magnetic interactions with TSSs and open up viable routes for the realization of the quantum anomalous Hall effect.

On-Surface Synthesis of Non-Benzenoid Nanographenes Embedding Azulene and Stone-Wales Topologies.

The incorporation of non-benzenoid motifs in graphene nanostructures significantly impacts their properties, making them attractive for applications in carbon-based electronics. However, understanding how specific non-benzenoid structures influence their properties remains limited, and further investigations are needed to fully comprehend their implications. Here, we report an on-surface synthetic strategy toward fabricating non-benzenoid nanographenes containing different combinations of pentagonal and heptagonal rings. Their structure and electronic properties were investigated via scanning tunneling microscopy and spectroscopy, complemented by computational investigations. After thermal activation of the precursor P on the Au(111) surface, we detected two major nanographene products. Nanographene Aa-a embeds two azulene units formed through oxidative ring-closure of methyl substituents, while Aa-s contains one azulene unit and one Stone-Wales defect, formed by the combination of oxidative ring-closure and skeletal ring-rearrangement reactions. Aa-a exhibits an antiferromagnetic ground state with the highest magnetic exchange coupling reported up to date for a non-benzenoid containing nanographene, coexisting with side-products with closed shell configurations resulted from the combination of ring-closure and ring-rearragement reactions (Ba-a , Ba-s , Bs-a and Bs-s ). Our results provide insights into the single gold atom assisted synthesis of novel NGs containing non-benzenoid motifs and their tailored electronic/magnetic properties.

Steering Large Magnetic Exchange Coupling in Nanographenes near the Closed-Shell to Open-Shell Transition.

The design of open-shell carbon-based nanomaterials is at the vanguard of materials science, steered by their beneficial magnetic properties like weaker spin-orbit coupling than that of transition metal atoms and larger spin delocalization, which are of potential relevance for future spintronics and quantum technologies. A key parameter in magnetic materials is the magnetic exchange coupling (MEC) between unpaired spins, which should be large enough to allow device operation at practical temperatures. In this work, we theoretically and experimentally explore three distinct families of nanographenes (NGs) (A, B, and C) featuring majority zigzag peripheries. Through many-body calculations, we identify a transition from a closed-shell ground state to an open-shell ground state upon an increase of the molecular size. Our predictions indicate that the largest MEC for open-shell NGs occurs in proximity to the transition between closed-shell and open-shell states. Such predictions are corroborated by the on-surface syntheses and structural, electronic, and magnetic characterizations of three NGs (A[3,5], B[4,5], and C[4,3]), which are the smallest open-shell systems in their respective chemical families and are thus located the closest to the transition boundary. Notably, two of the NGs (B[4,5] and C[4,3]) feature record values of MEC (close to 200 meV) measured on the Au(111) surface. Our strategy for maximizing the MEC provides perspectives for designing carbon nanomaterials with robust magnetic ground states.

Metastable Polymorphic Phases in Monolayer TaTe2.

Polymorphic phases and collective phenomena-such as charge density waves (CDWs)-in transition metal dichalcogenides (TMDs) dictate the physical and electronic properties of the material. Most TMDs naturally occur in a single given phase, but the fine-tuning of growth conditions via methods such as molecular beam epitaxy (MBE) allows to unlock otherwise inaccessible polymorphic structures. Exploring and understanding the morphological and electronic properties of new phases of TMDs is an essential step to enable their exploitation in technological applications. Here, scanning tunneling microscopy (STM) is used to map MBE-grown monolayer (ML) TaTe2 . This work reports the first observation of the 1H polymorphic phase, coexisting with the 1T, and demonstrates that their relative coverage can be controlled by adjusting synthesis parameters. Several superperiodic structures, compatible with CDWs, are observed to coexist on the 1T phase. Finally, this work provides theoretical insight on the delicate balance between Te…Te and Ta-Ta interactions that dictates the stability of the different phases. The findings demonstrate that TaTe2 is an ideal platform to investigate competing interactions, and indicate that accurate tuning of growth conditions is key to accessing metastable states in TMDs.

Stoichiometry-Directed Two-Level Hierarchical Growth of Lanthanide-Based Supramolecular Nanoarchitectures.

The design of a well-ordered arrangement of atoms on a solid surface has long been sought due to the envisioned applications in many different fields. On-surface synthesis of metal-organic networks is one of the most promising fabrication techniques. Hierarchical growth, which involves coordinative schemes with weaker interactions, favours the formation of extended areas with the desired complex structure. However, the control of such hierarchical growth is in its infancy, particularly for lanthanide-based architectures. Here the hierarchical growth of a Dy-based supramolecular nanoarchitecture on Au(111) is described. Such an assembly is based on a first hierarchical level of metallo-supramolecular motifs, which in a second level of hierarchy self-assemble through directional hydrogen bonds, giving rise to a periodic two-dimensional supramolecular porous network. Notably, the size of the metal-organic based tecton of the first level of hierarchy can be tailored by modifying the metal-ligand stoichiometric ratio.

Selectively Addressing Plasmonic Modes and Excitonic States in a Nanocavity Hosting a Quantum Emitter.

Controlling the interaction between the excitonic states of a quantum emitter and the plasmonic modes of a nanocavity is key for the development of quantum information processing devices. In this Letter we demonstrate that the tunnel electroluminescence of electrically insulated C60 nanocrystals enclosed in the plasmonic nanocavity at the junction of a scanning tunneling microscope can be switched from a broad emission spectrum, revealing the plasmonic modes of the cavity, to a narrow band emission, displaying only the excitonic states of the C60 molecules by changing the bias voltage applied to the junction. Interestingly, excitonic emission dominates the spectra in the high-voltage region in which the simultaneously acquired inelastic rate is low, demonstrating that the excitons cannot be created by an inelastic tunnel process. These results point toward new possible mechanisms for tunnel electroluminescence of quantum emitters and offer new avenues to develop electrically tunable nanoscale light sources.

Interplay between π-Conjugation and Exchange Magnetism in One-Dimensional Porphyrinoid Polymers.

The synthesis of novel polymeric materials with porphyrinoid compounds as key components of the repeating units attracts widespread interest from several scientific fields in view of their extraordinary variety of functional properties with potential applications in a wide range of highly significant technologies. The vast majority of such polymers present a closed-shell ground state, and, only recently, as the result of improved synthetic strategies, the engineering of open-shell porphyrinoid polymers with spin delocalization along the conjugation length has been achieved. Here, we present a combined strategy toward the fabrication of one-dimensional porphyrinoid-based polymers homocoupled via surface-catalyzed [3 + 3] cycloaromatization of isopropyl substituents on Au(111). Scanning tunneling microscopy and noncontact atomic force microscopy describe the thermal-activated intra- and intermolecular oxidative ring closure reactions as well as the controlled tip-induced hydrogen dissociation from the porphyrinoid units. In addition, scanning tunneling spectroscopy measurements, complemented by computational investigations, reveal the open-shell character, that is, the antiferromagnetic singlet ground state (S = 0) of the formed polymers, characterized by singlet-triplet inelastic excitations observed between spins of adjacent porphyrinoid units. Our approach sheds light on the crucial relevance of the π-conjugation in the correlations between spins, while expanding the on-surface synthesis toolbox and opening avenues toward the synthesis of innovative functional nanomaterials with prospects in carbon-based spintronics.

On-Surface Design of a 2D Cobalt-Organic Network Preserving Large Orbital Magnetic Moment.

The design of antiferromagnetic nanomaterials preserving large orbital magnetic moments is important to protect their functionalities against magnetic perturbations. Here, we exploit an archetype H6HOTP species for conductive metal-organic frameworks to design a Co-HOTP one-atom-thick metal-organic architecture on a Au(111) surface. Our multidisciplinary scanning probe microscopy, X-ray absorption spectroscopy, X-ray linear dichroism, and X-ray magnetic circular dichroism study, combined with density functional theory simulations, reveals the formation of a unique network design based on threefold Co+2 coordination with deprotonated ligands, which displays a large orbital magnetic moment with an orbital to effective spin moment ratio of 0.8, an in-plane easy axis of magnetization, and large magnetic anisotropy. Our simulations suggest an antiferromagnetic ground state, which is compatible with the experimental findings. Such a Co-HOTP metal-organic network exemplifies how on-surface chemistry can enable the design of field-robust antiferromagnetic materials.

Electrically Tunable Reactivity of Substrate-Supported Cobalt Oxide Nanocrystals.

First-row transition metal oxides are promising materials for catalyzing the oxygen evolution reaction. Surface sensitive techniques provide a unique perspective allowing the study of the structure, adsorption sites, and reactivity of catalysts at the atomic scale, which furnishes rationalization and improves the design of highly efficient catalytic materials. Here, a scanning probe microscopy study complemented by density functional theory on the structural and electronic properties of CoO nanoislands grown on Au(111) is reported. Two distinct phases are observed: The most extended displays a Moiré pattern (α-region), while the less abundant is 1Co:1Au coincidental (ß-region). As a result of the surface registry, in the ß-region the oxide adlayer is compressed by 9%, increasing the unoccupied local density of states and enhancing the selective water adsorption at low temperature through a cobalt inversion mechanism. Tip-induced voltage pulses irreversibly transform α- into ß-regions, thus opening avenues to modify the structure and reactivity of transition metal oxides by external stimuli like electric fields.

Engineering Periodic Dinuclear Lanthanide-Directed Networks Featuring Tunable Energy Level Alignment and Magnetic Anisotropy by Metal Exchange.

The design of lanthanide multinuclear networks is an emerging field of research due to the potential of such materials for nanomagnetism, spintronics, and quantum information. Therefore, controlling their electronic and magnetic properties is of paramount importance to tailor the envisioned functionalities. In this work, a multidisciplinary study is presented combining scanning tunneling microscopy, scanning tunneling spectroscopy, X-ray absorption spectroscopy, X-ray linear dichroism, X-ray magnetic circular dichroism, density functional theory, and multiplet calculations, about the supramolecular assembly, electronic and magnetic properties of periodic dinuclear 2D networks based on lanthanide-pyridyl interactions on Au(111). Er- and Dy-directed assemblies feature identical structural architectures stabilized by metal-organic coordination. Notably, despite exhibiting the same +3 oxidation state, there is a shift of the energy level alignment of the unoccupied molecular orbitals between Er- and Dy-directed networks. In addition, there is a reorientation of the easy axis of magnetization and an increment of the magnetic anisotropy when the metallic center is changed from Er to Dy. Thus, the results show that it is feasible to tune the energy level alignment and magnetic anisotropy of a lanthanide-based metal-organic architecture by metal exchange, while preserving the network design.

Setting the limit for the lateral thermal expansion of layered crystals via helium atom scattering.

The knowledge of the thermal expansion coefficient is of crucial importance to prevent the poor performance of devices, especially when these are made up of several layers of different materials, as in the case of 2D heterostructures. Helium atom scattering is a suitable tool for the direct measurement of the surface thermal expansion coefficient of materials. This information can be obtained directly from the position of the helium diffraction peaks, which allows determining the surface lattice constant at different temperatures by merely applying Bragg's law. We present new data for PdTe2 which confirm a trend observed for several 2D dichalcogenides, namely, that the in-plane lattice constant remains unchanged (within experimental error) in the temperature range of interest for applications, which enables setting an upper limit for the lateral thermal expansion coefficients of these materials.

Methylene Blue functionalized carbon nanodots combined with different shape gold nanostructures for sensitive and selective SARS-CoV-2 sensing.

The development of DNA-sensing platforms based on new synthetized Methylene Blue functionalized carbon nanodots combined with different shape gold nanostructures (AuNs), as a new pathway to develop a selective and sensitive methodology for SARS-CoV-2 detection is presented. A mixture of gold nanoparticles and gold nanotriangles have been synthetized to modify disposable electrodes that act as an enhanced nanostructured electrochemical surface for DNA probe immobilization. On the other hand, modified carbon nanodots prepared a la carte to contain Methylene Blue (MB-CDs) are used as electrochemical indicators of the hybridization event. These MB-CDs, due to their structure, are able to interact differently with double and single-stranded DNA molecules. Based on this strategy, target sequences of the SARS-CoV-2 virus have been detected in a straightforward way and rapidly with a detection limit of 2.00 aM. Moreover, this platform allows the detection of the SARS-CoV-2 sequence in the presence of other viruses, and also a single nucleotide polymorphism (SNPs). The developed approach has been tested directly on RNA obtained from nasopharyngeal samples from COVID-19 patients, avoiding any amplification process. The results agree well with those obtained by RT-qPCR or reverse transcription quantitative polymerase chain reaction technique.

Amplification-free detection of SARS-CoV-2 using gold nanotriangles functionalized with oligonucleotides.

Gold nanotriangles (AuNTs) functionalized with dithiolated oligonucleotides have been employed to develop an amplification-free electrochemical biosensor for SARS-CoV-2 in patient samples. Gold nanotriangles, prepared through a seed-mediated growth method and exhaustively characterized by different techniques, serve as an improved electrochemical platform and for DNA probe immobilization. Azure A is used as an electrochemical indicator of the hybridization event. The biosensor detects either single stranded DNA or RNA sequences of SARS-CoV-2 of different lengths, with a low detection limit of 22.2 fM. In addition, it allows to detect point mutations in SARS-CoV-2 genome with the aim to detect more infective SARS-CoV-2 variants such as Alpha, Beta, Gamma, Delta, and Omicron. Results obtained with the biosensor in nasopharyngeal swab samples from COVID-19 patients show the possibility to clearly discriminate between non-infected and infected patient samples as well as patient samples with different viral load. Furthermore, the results correlate well with those obtained by the gold standard technique RT-qPCR, with the advantage of avoiding the amplification process and the need of sophisticated equipment.

Electronic Temperature and Two-Electron Processes in Overbias Plasmonic Emission from Tunnel Junctions.

The accurate determination of electronic temperatures in metallic nanostructures is essential for many technological applications, like plasmon-enhanced catalysis or lithographic nanofabrication procedures. In this Letter, we demonstrate that the electronic temperature can be accurately measured by the shape of the tunnel electroluminescence emission edge in tunnel plasmonic nanocavities, which follows a universal thermal distribution with the bias voltage as the chemical potential of the photon population. A significant deviation between electronic and lattice temperatures is found below 30 K for tunnel currents larger than 15 nA. This deviation is rationalized as the result of a two-electron process in which the second electron excites plasmon modes with an energy distribution that reflects the higher temperature following the first tunneling event. These results dispel a long-standing controversy on the nature of overbias emission in tunnel junctions and adds a new method for the determination of electronic temperatures and quasiparticle dynamics.

Synthesis and Characterization of peri-Heptacene on a Metallic Surface.

The synthesis of long n-peri-acenes (n-PAs) is challenging as a result of their inherent open-shell radical character, which arises from the presence of parallel zigzag edges beyond a certain n value. They are considered as π-electron model systems to study magnetism in graphene nanostructures; being potential candidates in the fabrication of optoelectronic and spintronic devices. Here, we report the on-surface formation of the largest pristine member of the n-PA family, i.e. peri-heptacene (n=7, 7-PA), obtained on an Au(111) substrate under ultra-high vacuum conditions. Our high-resolution scanning tunneling microscopy investigations, complemented by theoretical simulations, provide insight into the chemical structure of this previously elusive compound. In addition, scanning tunneling spectroscopy reveals the antiferromagnetic open-shell singlet ground state of 7-PA, exhibiting singlet-triplet spin-flip inelastic excitations with an effective exchange coupling (Jeff ) of 49 meV.

Tuning the Magnetic Anisotropy of Lanthanides on a Metal Substrate by Metal-Organic Coordination.

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.