Single Atom Level Research Articles

Quantum superpositions of current states in Rydberg-atom networks

Quantum simulation of many-body quantum systems using Rydberg-atom platforms has become of extreme interest in the last years. The possibility to realize spin Hamiltonians and the accurate control at the single atom level paved the way for the study of quantum phases of matter and dynamics. Here, we propose a quantum optimal control protocol to engineer current states: quantum states characterized by Rydberg excitations propagating in a given spatially closed tweezer networks. Indeed, current states with different winding numbers can be generated on demand. Besides those ones with single winding number, superposition of quantum current states characterized by more winding numbers can be obtained. The single current states are eigenstates of the current operator that therefore can define an observable that remains persistent at any time. In particular, the features of the excitations dynamics reflects the nature of current states, a fact that in principle can be used to characterize the nature of the flow experimentally without the need of accessing high order correlators. Published by the American Physical Society 2024

Coherent spin dynamics between electron and nucleus within a single atom

The nuclear spin, being much more isolated from the environment than its electronic counterpart, presents opportunities for quantum experiments with prolonged coherence times. Electron spin resonance (ESR) combined with scanning tunnelling microscopy (STM) provides a bottom-up platform to study the fundamental properties of nuclear spins of single atoms on a surface. However, access to the time evolution of nuclear spins remained a challenge. Here, we present an experiment resolving the nanosecond coherent dynamics of a hyperfine-driven flip-flop interaction between the spin of an individual nucleus and that of an orbiting electron. We use the unique local controllability of the magnetic field emanating from the STM probe tip to bring the electron and nuclear spins in tune, as evidenced by a set of avoided level crossings in ESR-STM. Subsequently, we polarize both spins through scattering of tunnelling electrons and measure the resulting free evolution of the coupled spin system using a DC pump-probe scheme. The latter reveals a complex pattern of multiple interfering coherent oscillations, providing unique insight into hyperfine physics on a single atom level.

Low-voltage single-atom electron microscopy with carbon-based nanomaterials

The properties of materials are strongly correlated with their atomic scale structures. Achieving a comprehensive understanding of the atomic-scale structure-property relationship requires advancements of imaging and spectroscopy techniques. Aberration-corrected scanning transmission electron microscopy (STEM) has seen rapid development over the past decades and is now routinely employed for atomic-scale characterization. However, quantitative STEM imaging and spectroscopy analysis at the single-atom level is challenging due to the extremely weak signals generated from individual atom, thus imposing stringent requirements for analysis sensitivity. This review discusses the development and application of low-voltage STEM techniques with single-atom sensitivity, primarily based on recent research presented on an invited talk at the 5th 2D23 SALVE Symposium, including annular dark-field (ADF) imaging, functional imaging and electron energy-loss spectroscopy (EELS) analysis. Carbon-based nanomaterials were chosen as model systems for demonstrating the capabilities of single-atom STEM imaging and EELS analysis, due to their structural stability under low accelerating voltages and their rich physical and chemical properties. Moreover, this review summarizes recent advancements and applications of low-voltage single-atom STEM imaging and spectroscopy in the study of functional materials and discusses prospects for future developments.

Isolated attosecond pulse generation in a semi-infinite gas cell driven by time-gated phase matching

Isolated attosecond pulse (IAP) generation usually involves the use of short-medium gas cells operated at high pressures. In contrast, long-medium schemes at low pressures are commonly perceived as inherently unsuitable for IAP generation due to the nonlinear phenomena that challenge favourable phase-matching conditions. Here we provide clear experimental evidence on the generation of isolated extreme-ultraviolet attosecond pulses in a semi-infinite gas cell, demonstrating the use of extended-medium geometries for effective production of IAPs. To gain a deeper understanding we develop a simulation method for high-order harmonic generation (HHG), which combines nonlinear propagation with macroscopic HHG solving the 3D time-dependent Schrödinger equation at the single-atom level. Our simulations reveal that the nonlinear spatio-temporal reshaping of the driving field, observed in the experiment as a bright plasma channel, acts as a self-regulating mechanism boosting the phase-matching conditions for the generation of IAPs.

Benchmark comparison study on metal single atom versus metal nanoparticle in photoredox catalysis: Which is better?

Reducing the size of metal nanoparticle (NP) cocatalyst down to single-atom level to improve photocatalytic efficiency is inevitably accompanied by the changes of its coordination environment, geometric configuration, electronic structure and active site. Thus, the construction of single metal atom (SA) photocatalyst is not necessarily a panacea for activity improvement toward target catalytic reactions. Herein, we report a critical and benchmark comparison in a reasonable framework of ZnIn2S4/Pt NP (ZIS/Pt NP) and ZnIn2S4/Pt SA (ZIS/Pt SA) towards photocatalytic hydrogen (H2) evolution, aiming to demonstrate which is better between Pt NP and Pt SA as cocatalyst in boosting photoredox catalysis. Mechanism study proves that the higher charge separation/transfer and weaker H* adsorption strength over ZIS/Pt NP than ZIS/Pt SA promote the more effective reduction of protons to H2, leading to the higher activity of ZIS/Pt NP than ZIS/Pt SA. Our work is expected to timely inspire the critical and rational thinking on the function and intrinsic mechanism of SA and NP cocatalysts in enhancing the photoredox catalysis performance.

Induced Manipulation of Atomically Dispersed Cobalt through S Vacancy for Photocatalytic Water Splitting: Asymmetric Coordination and Dynamic Evolution.

It is still a challenge to construct single-atom level reduction and oxidation sites in single-component photocatalyst by manipulating coordination configuration for photocatalytic water splitting. Herein, the atomically dispersed asymmetric configuration of six-coordinated Co-S2O4 (two exposed S atoms, two OH groups, and two Co─O─Zn bonds) suspending on ZnIn2S4 nanosheets verified by combining experimental analysis with theoretical calculation, is applied into photocatalytic water splitting. The Co-S2O4 site immobilized by Vs acts as oxidation sites to guide electrons transferring to neighboring independent S atom, achieving efficient separation of reduction and oxidation sites. It is worth mentioning that stabilized Co-S2O4 configuration show dynamic structure evolution to highly active Co-S1O4 configuration (one exposed S atom, one OH group, and three Co─O─Zn bonds) in reaction, which lowers energy barrier of transition state for H2O activization. Ultimately, the optimized photocatalyst exhibits excellent photocatalytic activity for water splitting (H2: 80.13 µmol g-1 h-1, O2: 37.81 µmol g-1 h-1) and outstanding stability than that of multicomponent photocatalysts due to dynamic and reversible evolution between stable Co-S2O4 configuration and active Co-S1O4 configuration. This work demonstrates new cognitions on immobilized strategy through vacancy inducing, manipulating coordination configuration, and dynamic evolution mechanism of single-atom level catalytic site in photocatalytic water splitting.

Electronic interaction between sub-nanometric Ru entity and TiO2 support regulates the hydrogenation chemistry for selective C-O bond cleavage

Selective C-O bond cleavage without ring saturation is the pillar pursuit in the hydrotreating process for lignin valorization/depolymerization since it avoids the consumption of excessive hydrogen. In this work, the sub-nanometric cluster and nanometric particle of Ru were supported on TiO2 for the hydrogenolysis of C-O bond in aromatic ether. The experimental and theoretical results reveal that, in the catalyst of larger Ru nanoparticles, less electron transfer occurs from individual Ru atom to TiO2, making the Ru-related domains retain the nature of bulky Ru for deep hydrogenation. Sub-nanometric Ru cluster shows more intense electron transfer for individual Ru atom, leading to a stronger modification to Ru domain by TiO2. Such electronic hybridization makes Ru gain the catalytic property of TiO2 to attenuate the activation of aromatic ring, inhibiting ring saturation with maintained C-O cleavage activity. This work implies that further decrease of Ru entity size to single-atom level may achieve the complete inhibition of ring saturation.

Synthetic Electronics

Building electronic circuitry with atomic precision is beyond the capability of current, as well as in the foreseeable future, top-down lithographic techniques. In recent years, the US Office of Naval Research (ONR) has championed several research initiatives that sought to address this key challenge. One of the most exciting development and initial success that resulted from these initiatives is a bottom-up chemical synthesis technique that has proven highly effective at assembling two-dimensional (2D) graphene nanostructures with spatial resolution down to single atom and single bond level. In this presentation I will briefly review, from a government program manager perspective, the current status of this exciting new research field which I call Synthetic Electronics. I will highlight some recent technical and scientific advances, such as various on-surface synthesis techniques, that enable and help define future directions of synthetic electronics. I will share some of my own personal perspectives and speculate on possible future directions, and also draw attention to some of the major challenges that are facing synthetic electronics. The motivation behind the discussion is to socialize the basic ideas and capabilities with, and seek feedback from, the relevant research communities, in order to inform and further refine research agenda in the future.

Hidden Impurities Generate False Positives in Single Atom Catalyst Imaging.

Single-atom catalysts (SACs) are an emerging class of materials, leveraging maximum atom utilization and distinctive structural and electronic properties to bridge heterogeneous and homogeneous catalysis. Direct imaging methods, such as aberration-corrected high-angle annular dark-field scanning transmission electron microscopy, are commonly applied to confirm the atomic dispersion of active sites. However, interpretations of data from these techniques can be challenging due to simultaneous contributions to intensity from impurities introduced during synthesis processes, as well as any variation in position relative to the focal plane of the electron beam. To address this matter, this paper presents a comprehensive study on two representative SACs containing isolated nickel or copper atoms. Spectroscopic techniques, including X-ray absorption spectroscopy, were employed to prove the high metal dispersion of the catalytic atoms. Employing scanning transmission electron microscopy imaging combined with single-atom-sensitive electron energy loss spectroscopy, we scrutinized thin specimens of the catalysts to provide an unambiguous chemical identification of the observed single-atom species and thereby distinguish impurities from active sites at the single-atom level. Overall, the study underscores the complexity of SACs characterization and establishes the importance of the use of spectroscopy in tandem with imaging at atomic resolution to fully and reliably characterize single-atom catalysts.

Ph3PCN2: A stable reagent for carbon-atom transfer.

Precise modification of a chemical site in a molecule at the single-atom level is one of the most elegant yet difficult transformations in chemistry. A reagent specifically designed for chemoselective introduction of monoatomic carbon is a particularly formidable challenge. Here, we report a straightforward, azide-free synthesis of a crystalline and isolable diazophosphorus ylide, Ph3PCN2, a stable compound with a carbon atom bonded to two chemically labile groups, triphenylphosphine (PPh3) and dinitrogen (N2). Without any additives, the diazophosphorus ylide serves as a highly selective transfer reagent for fragments, including Ph3PC, to deliver phosphorus ylide-terminated heterocumulenes and CN2 to produce multisubstituted pyrazoles. Ultimately, even exclusive carbon-atom transfer is possible. In reactions with aldehydes and acyclic and cyclic ketones (R2C=O), the carbon-atom substitution forms a vinylidene (R2C=C:) en route to alkynes or butatrienes.

A Strontium Quantum-Gas Microscope

The development of quantum-gas microscopes has brought novel ways of probing quantum degenerate many-body systems at the single-atom level. Until now, most of these setups have focused on alkali atoms. Expanding quantum-gas microscopy to alkaline-earth elements will provide new tools, such as SU(N)-symmetric fermionic isotopes or ultranarrow optical transitions, to the field of quantum simulation. Here we demonstrate the site-resolved imaging of a 84Sr bosonic quantum gas in a Hubbard-regime optical lattice. The quantum gas is confined by a two-dimensional in-plane lattice and a light-sheet potential, which operate at the strontium clock-magic wavelength of 813.4 nm. We realize fluorescence imaging using the broad 461-nm transition, which provides high spatial resolution. Simultaneously, we perform attractive Sisyphus cooling with the narrow 689-nm intercombination line. We reconstruct the atomic occupation from the fluorescence images, obtaining imaging fidelities above 94%. Finally, we realize a 84Sr superfluid in the Bose-Hubbard regime. We observe its interference pattern upon expansion, a probe of phase coherence, with single-atom resolution. Our strontium quantum-gas microscope provides a new platform to study dissipative Hubbard models, quantum optics in atomic arrays, and SU(N) fermions at the microscopic level. Published by the American Physical Society 2024

Precision Engineering of Nanorobots: Toward Single Atom Decoration and Defect Control for Enhanced Microplastic Capture

AbstractNanorobots are being received with a great attention for their move‐sense‐and‐act capabilities that often originate from catalytic decomposition of fuels. In the past decade, single‐atom engineering has demonstrated exceptional efficiency in catalysis, energy‐related technologies, and medicine. Here, a novel approach involving point defect engineering and the incorporation of platinum (Pt) single atoms and atomic level species onto the surface of titanium dioxide nanotubes (TiO2‐NT)‐based nanorobots is presented and its impact on the propulsion capabilities of the resulting nanorobots is investigated. The achievement of point defect engineering is realized through the annealing of TiO2‐NT in a hydrogen atmosphere yielding to the point‐defect decorated nanotube (TiO2‐HNT) nanorobots. Subsequently, the atomic level Pt species decorated TiO2 nanotube (TiO2‐SA‐NT) nanorobots are achieved through a wet‐chemical deposition process. Whereas TiO2‐SA‐NT nanorobots showed the highest negative photogravitaxis when irradiated with ultraviolet (UV) light, TiO2‐HNT nanorobots reached the highest velocity calculated in 2D. Both TiO2‐HNT and TiO2‐SA‐NT nanorobots demonstrated a pronounced affinity for microplastics, exhibiting the capability to irreversibly capture them. This pioneering approach utilizing point‐defect and atomic level Pt species nanorobotics is anticipated to pave the way for highly efficient solutions in the remediation of nano‐ and microplastics and related environmental technologies.

Atomic-Scale Time-Resolved Imaging of Krypton Dimers, Chains and Transition to a One-Dimensional Gas.

Single-atom dynamics of noble-gas elements have been investigated using time-resolved transmission electron microscopy (TEM), with direct observation providing for a deeper understanding of chemical bonding, reactivity, and states of matter at the nanoscale. We report on a nanoscale system consisting of endohedral fullerenes encapsulated within single-walled carbon nanotubes ((Kr@C60)@SWCNT), capable of the delivery and release of krypton atoms on-demand, via coalescence of host fullerene cages under the action of the electron beam (in situ) or heat (ex situ). The state and dynamics of Kr atoms were investigated by energy dispersive X-ray spectroscopy (EDS), electron energy loss spectroscopy (EELS), and X-ray photoelectron spectroscopy (XPS). Kr atom positions were measured precisely using aberration-corrected high-resolution TEM (AC-HRTEM), aberration-corrected scanning TEM (AC-STEM), and single-atom spectroscopic imaging (STEM-EELS). The electron beam drove the formation of 2Kr@C120 capsules, in which van der Waals Kr2 and transient covalent [Kr2]+ bonding states were identified. Thermal coalescence led to the formation of longer coalesced nested nanotubes containing more loosely bound Krn chains (n = 3-6). In some instances, delocalization of Kr atomic positions was confirmed by STEM analysis as the transition to a one-dimensional (1D) gas, as Kr atoms were constrained to only one degree of translational freedom within long, well-annealed, nested nanotubes. Such nested nanotube structures were investigated by Raman spectroscopy. This material represents a highly compressed and dimensionally constrained 1D gas stable under ambient conditions. Direct atomic-scale imaging has revealed elusive bonding states and a previously unseen 1D gaseous state of matter of this noble gas element, demonstrating TEM to be a powerful tool in the discovery of chemistry at the single-atom level.

Copper Dispersed Covalent Organic Framework for Azide-Alkyne Cycloaddition and Fast Synthesis of Rufinamide in Water.

A crystalline porous bipyridine-based Bpy-COF with a high BET surface area (1864m2g-1) and uniform mesopore (4.0nm) is successfully synthesized from 1,3,5-tris-(4'-formyl-biphenyl-4-yl)triazine and 5,5'-diamino-2,2'-bipyridine via a solvothermal method. After Cu(I)-loading, the resultant Cu(I)-Bpy-COF remained the ordered porous structure with evenly distributed Cu(I) ions at a single-atom level. Using Cu(I)-Bpy-COF as a heterogeneous catalyst, high conversions for cycloaddition reactions are achieved within a short time (40min) at 25°C in water medium. Moreover, Cu(I)-Bpy-COF proves to be applicable for aromatic and aliphatic azides and alkynes bearing various substituents such as ester, hydroxyl, amido, pyridyl, thienyl, bulky triphenylamine, fluorine, and trifluoromethyl groups. The high conversions remain almost constant after five cycles. Additionally, the antiepileptic drug (rufinamide) is successfully prepared by a simple one-step reaction using Cu(I)-Bpy-COF, proving its practical feasibility for pharmaceutical synthesis.

Atomic-scale identification of defects in alite

Crystallographic defects play a crucial role in cement hydration, with initial dissolution being dominated by the formation of etch pits that rely on the intersection of defects on surfaces. However, the defects present in cement particles have remained a mystery due to the lack of detailed direct observation at the atomic scale. In this study, we used scanning transmission electron microscopy to unravel the defects in alite particles at the single-atom level. Our observations identified different types of defects, including vacancies, doping, dislocations, rough surfaces, and grain boundaries. Atomic ordering in the alite crystal was further examined based on our single-atom recognition method. Our findings indicate that defects in cement particles may serve as reactive sites during the early hydration stage, facilitating the initial dissolution and providing nucleation sites for hydration products. This work provides insights into cement defect formation at the single-atom level and offers new opportunities in tuning the hydration process through defect engineering of cement particles.

Watching atoms at work during reactions

Abstract The development of new technologies for the current energy and environmental challenges requires the acquisition of a very fundamental knowledge about the structure and activity of catalytic materials at the nanometric scale. As a consequence, in situ and operando methodologies are blossoming, but only a fraction of them really aims at a local vision that would allow watching atoms at work during reactions. In this short report, we want to outline the merits of a new technique based on scanning tunnelling microscopy (Current-roughness electrochemical scanning tunneling microscopy, cr-EC-STM) which can visualize electrocatalytic reactions down at the single atom level. Results of two case studies in the field of hydrogen evolution reaction (HER) are briefly summarized, witnessing the capability of cr-EC-STM to provide critical information about the structure and catalytic performance of the active sites with atomic resolution.

Imaging surfaces at the space–time limit: New perspectives of time-resolved scanning tunneling microscopy for ultrafast surface science

Many fundamental processes in nature occur on ultrashort time scales within picoseconds to attoseconds, and on intrinsic length scales from nanometers to picometers. The structure of crystalline solids is dictated by long range order and the periodic arrangement of atoms, but the elementary excitations that define its interaction with the environment may vary locally at the atomic scale. Multiple domains and phases can coexist on length scales down to a few nanometer, and impurities and defects can influence the collective many-body response of solids at the single-atom level. Ultrafast pump–probe techniques provide valuable information about fundamental many-body interactions in solids and at surfaces, but spatially average over macroscopic spot sizes such that the influence of local order or disorder at angstrom scales is not directly accessible. Therefore, real-space observation of ultrafast dynamics with atomic spatial resolution is highly desirable, and motivates the development of time-resolved ultrafast scanning tunneling microscopy (USTM) since the early 1990’s. Tremendous progress has been made in this field in the past decade, and a number of breakthrough achievements have significantly advanced our possibilities to add ultrafast time resolution to the angstrom spatial resolution of STM. This article reviews new technical approaches and developments in the field of USTM. A particular focus will be the classification of light-matter interaction in tunnel junctions, based on the criteria for adiabatic tunneling from Keldysh's theory of strong-field ionization and a tunneling time as defined by Büttiker and Landauer, and on Tucker's definition of quantum detection in a tunnel junction mixer. Moreover, various mechanisms to generate an ultrafast tunneling current in USTM are discussed and are to some extent related to those from other techniques such as optical spectroscopy or photoemission spectroscopy. The resulting new possibilities for imaging the ultrafast dynamics of electronic and vibrational excitations at surfaces with USTM will be highlighted. Finally, the article outlines possible future directions of USTM for studying ultrafast processes and light-induced phenomena at surfaces and in quantum materials.

Formation and Growth of Atomic Scale Seeds of Au Nanoparticle in the Nanospace of an Organic Cage Molecule.

Seed-mediated growth has been widely used to synthesize noble metal nanoparticles with controlled size and shape. Although it is becoming possible to directly observe the nucleation process of metal atoms at the single atom level by using transmission electron microscopy (TEM), it is challenging to control the formation and growth of seeds with only a few metal atoms in homogeneous solution systems. Herein, we report site-selective formation and growth of atomic scale seeds of the Au nanoparticle in a nanospace of an organic cage molecule. We synthesized a cage molecule with amines and phenols, which were found to both capture and reduce Au(III) ions to spontaneously form the atomic scale seeds containing Au(0) in the nanospace. The growth reaction of the atomic scale seeds afforded Au nanoparticles with an average diameter of 2.0 ± 0.2 nm, which is in good agreement with the inner diameter of the cage molecule.

Probing the atomically diffuse interfaces in Pd@Pt core-shell nanoparticles in three dimensions

Deciphering the three-dimensional atomic structure of solid-solid interfaces in core-shell nanomaterials is the key to understand their catalytical, optical and electronic properties. Here, we probe the three-dimensional atomic structures of palladium-platinum core-shell nanoparticles at the single-atom level using atomic resolution electron tomography. We quantify the rich structural variety of core-shell nanoparticles with heteroepitaxy in 3D at atomic resolution. Instead of forming an atomically-sharp boundary, the core-shell interface is found to be atomically diffuse with an average thickness of 4.2 Å, irrespective of the particle’s morphology or crystallographic texture. The high concentration of Pd in the diffusive interface is highly related to the free Pd atoms dissolved from the Pd seeds, which is confirmed by atomic images of Pd and Pt single atoms and sub-nanometer clusters using cryogenic electron microscopy. These results advance our understanding of core-shell structures at the fundamental level, providing potential strategies into precise nanomaterial manipulation and chemical property regulation.

Rydberg Macrodimers: Diatomic Molecules on the Micrometer Scale.

Controlling molecular binding at the level of single atoms is one of the holy grails of quantum chemistry. Rydberg macrodimers─bound states between highly excited Rydberg atoms─provide a novel perspective in this direction. Resulting from binding potentials formed by the strong, long-range interactions of Rydberg states, Rydberg macrodimers feature bond lengths in the micrometer regime, exceeding those of conventional molecules by orders of magnitude. Using single-atom control in quantum gas microscopes, the unique properties of these exotic states can be studied with unprecedented control, including the response to magnetic fields or the polarization of light in their photoassociation. The high accuracy achieved in spectroscopic studies of macrodimers makes them an ideal testbed to benchmark Rydberg interactions, with direct relevance to quantum computing and information protocols where these are employed. This review provides a historic overview and summarizes the recent findings in the field of Rydberg macrodimers. Furthermore, it presents new data on interactions between macrodimers, leading to a phenomenon analogous to Rydberg blockade at the level of molecules, opening the path toward studying many-body systems of ultralong-range Rydberg molecules.