Scanning Probe Techniques Research Articles

Scanning tunneling microscopy of ultrathin indium intercalated between graphene and SiC using confinement heteroepitaxy

Large-scale and air-stable two-dimensional metal layers intercalated at the interface between epitaxial graphene and SiC offer an appealing material for quantum technology. The atomic and electronic details, as well as the control of the intercalated metals within the interface, however, remain very limited. In this Letter, we explored ultrathin indium confined between graphene and SiC using cryogenic scanning tunneling microscopy, complemented by first-principle density functional theory. Bias-dependent imaging and tunneling spectroscopy visualize a triangular superstructure with a periodicity of 14.7 ± 3 Å and an occupied state at about −1.6 eV, indicating proof of highly crystalline indium. The scanning tunneling microscopy tip was used to manipulate the number of indium layers below graphene, allowing to identify three monatomic In layers and to tune their corresponding electronic properties with atomic precision. This further allows us to attribute the observed triangular superstructure to be solely emerging from the In trilayer, tentatively explained by the lattice mismatch induced by lattice relaxation in the topmost In layer. Our findings provide a microscopic insight into the structure and electronic properties of intercalated metals within the graphene/SiC interface and a unique possibility to manipulate them with atomic precision using the scanning probe technique.

In air STM observation of Au(111) surface disturbance including Au magic fingers as modified by solvent choice

A widely studied surface phenomena on Au(111) is the formation of Au magic fingers, which were first discovered nearly 20 years ago. A variety of experimental conditions have been used to observe the formation of Au magic fingers with a slight preference to ultra-high vacuum and low temperature studies. With the advances in scanning probe techniques, it is possible to study these unique structures under more relevant conditions including in air and at room temperature. After exposure to a 0.1 M solvent solution, Au(111) displayed three types of surface disturbances, including the formation of Au magic fingers, based on the identity of the solvent. The type of disturbance was dependent on the solvent molecule's characteristics, specifically its total charge and its electrolytic behavior in aqueous environments. The mechanism of disturbance relied on a strong tip-surface interaction and the mass transport of Au atoms, which was modified by the solvent selected. Overall, the ability to form organized nanostructures, like Au magic fingers, in a repeated way in environments outside of UHV and without a protective liquid layer increases the utility of these structures into a wider array of fields and applied areas.

A General Method for Calibration of Active Scanning Thermal Probes

Scanning thermal microscopy (SThM) is a scanning probe technique aimed at quantitative characterization of local thermal properties at the length scale down to tens of nanometers. With many probe designs and approaches to interpretation of probe responses, there is a need for a universal framework, which would allow probe calibration and comparison of probe performance. Herein, a calibration framework based on an abstracted, formal, probe model for active SThM probes is developed. The calibration can be accomplished through measurements with two or three calibration samples. Requirements for calibration samples are described with examples of structures of suitable samples identified in published literature. A link to a published experimental work indirectly verifying the proposed procedure is provided. The calibration does not require knowledge of internal probe properties and yields a small and universal set of parameters that can be used to quantify thermal resistance presented to the probe by samples as well as to characterize active‐mode SThM probes of any type and at any measurement frequency. How the probe calibration parameters can be used to guide probe design is illustrated. When the calibration approach can be used directly to measure the thermal conductivity of unknown samples is also analyzed.

Advancing scanning probe microscopy simulations: A decade of development in probe-particle models

The Probe-Particle Model combine theories designed for the simulation of scanning probe microscopy experiments, employing non-reactive, flexible tip apices to achieve sub-molecular resolution. In the article we present the latest version of the Probe-Particle Model implemented in the open-source ppafm package, highlighting substantial advancements in accuracy, computational performance, and user-friendliness. To demonstrate this we provide a comprehensive review of approaches for simulating non-contact Atomic Force Microscopy. They vary in complexity from simple Lennard-Jones potential to the latest full density-based model. We compared those approaches with ab initio calculated references, showcasing their respective merits. All parts of the ppafm package have undergone acceleration by 1-2 orders of magnitude using OpenMP and OpenCL technologies. The updated package includes an interactive graphical user interface and seamless integration into the Python ecosystem via pip, facilitating advanced scripting and interoperability with other software. This adaptability positions ppafm as an ideal tool for high-throughput applications, including the training of machine learning models for the automatic recovery of atomic structures from nc-AFM measurements. We envision significant potential for this application in future single-molecule analysis, synthesis, and advancements in surface science in general. Additionally, we discuss simulations of other sub-molecular scanning-probe imaging techniques, such as bond-resolved scanning tunneling microscopy and kelvin probe force microscopy, all built on the robust foundation of the Probe-Particle Model. Altogether this demonstrates the broad impact of the model across diverse domains of on-surface science and molecular chemistry.

Engineering Topological States in Nanographenes

Our research focuses on the rational design, deterministic assembly, and detailed investigation of the physical phenomena emerging from quantum confinement effects in graphene nanomaterials. We pursue a highly integrated multidisciplinary program, founded on synthetic bottom-up approaches toward functional materials with precisely defined structure. We control their assembly into hierarchically ordered architectures and evaluate inherent physical properties using modern scanning probe techniques cross multiple length, time, and energy scales. We recently demonstrated the rational design and experimental realization of a graphene nanoribbon (GNR) superlattices that hosts a 1D array of symmetry-protected topological states, thus generating otherwise inaccessible electronic structure.1 This new class of materials can be thought of as an extended form of poly-acetylene wherein highly localized half-filled topological states replace the familiar single occupied p-orbitals along a conjugated π-system. Experimental results and first-principles calculations reveal that the frontier band structure of these GNR superlattices is defined purely by the coupling between adjacent topological interface states and can be tuned all the way from a semiconductor and a metal.2,3 This novel manifestation of 1D topological phases presents an entirely new route to band engineering in 1D materials based on precise control of their electronic topology and is a promising new platform for future studies of 1D quantum spin physics and metallicity in low dimensional carbon nanomaterials. 1. Topological Band Engineering of Graphene Nanoribbons. Rizzo, D. J.; Veber, G.; Cao, T.; Bronner, C.; Chen, T.; Zhao, F.; Rodriguez, H.; Louie, S. G.; Crommie, M. F.; Fischer, F. R. Nature 2018, 560, 204-206. 2. Inducing Metallicity in Graphene Nanoribbons via Zero-Mode Superlattices. Rizzo, D. J.; Veber, G.; Jiang, J.; McCurdy, R.; Cao, T.; Bronner, C.; Chen, T.; Louie, S. G.; Fischer, F. R.; Crommie, M. F. Science 2020, 369, 1597-1603. 3. Robust Metallic Zero-Mode States in Olympicene Graphene Nanoribbons. McCurdy, R. D.; Delgado, A.; Jiang, J.; Zhu, J.; Wen, E. C. H.; Blackwell, R. E.; Veber, G. C.; Wang, S.; Louie, S. G.; Fischer, F. R. J. Am. Chem. Soc. 2023, 145, 15162-15170.

Synchrotron Infrared Nano-Spectroscopy and Imaging of Lithium Fluoride in the Solid Electrolyte Interphase on Li-Ion Anodes

The solid electrolyte interphase layer that forms on the surface of the negative electrode in Li-ion batteries is crucial for battery cycle/calendar life, rate performance and safety. This 10's of nanometers layer is formed from the decomposition of the liquid electrolyte and its passivation properties are very sensitive to its formation and aging conditions. Despite this critical role, there is still a lack of fundamental understanding of how its structure and composition relate to its stability and passivation performance.Amongst the SEI products that form during the decomposition of liquid electrolyte, lithium fluoride (LiF) is a ubiquitous component and many attribute its presence to be a determining factor of a well passivated interface. Despite the importance, LiF’s nanostructure and distribution within the SEI layer is rarely investigated due to the lack of suitable characterization techniques. X-ray photoelectron spectroscopy (XPS) is the main tool to identify LiF in the SEI layer, but its micron scale lateral resolution precludes any nanometer scale mapping of LiF’s distribution which limits its ability to identify the role LiF plays in the SEI layer.Recently our group has been characterizing the SEI layer of Li-ion negative electrodes with nano-Fourier transform infrared spectroscopy (nano-FTIR).1,2 Nano-FTIR is a scanning probe technique based in a typical atomic force microscope (AFM) in which a broadband laser illuminates a metallic tip/sample interface. The metallic tip acts as an antenna, focusing the infrared light adjacent to the interface and creating a near-field interaction. By operating in constant tapping mode and due to the nonlinearity of the near-field signal on the tip/sample distance, the corresponding backscattered light from the near-field interaction with the sample can be separated from the background with lock-in amplification. The result is IR reflection and absorption spectra with lateral resolution close to the tip radius (ca. 20 nm). This far exceeds the diffraction limited resolution of typical IR microscopy (~10’s µm) creating new opportunities for IR characterization on nanometer length scales.In this talk, we’ll discuss our recent work related to utilizing synchrotron-based nano-FTIR to characterize and image LiF in the SEI layer of Li-ion based negative electrodes. The synchrotron infrared light source allows the detection of photons out to the far-IR (322 cm-1 limit) which enables the detection of the vibrational modes of Li containing inorganic phases such as LiF, Li2O, and LiH. By comparing model thin film LiF nano-FTIR spectra with experimental spectra take from the surfaces of Cu, Si thin film, and a novel metallic glass anode, we can gauge the heterogeneity of the LiF in the SEI layer and then suggest different LiF formation mechanisms. Based on the results, we believe that nano-FTIR with a synchrotron based light source will be a key tool to unravelling the nanoscale structure of the SEI layer due to its nanoscale chemical sensitivity and nondestructive nature.This research was supported by the US Department of Energy (DOE)’s Vehicle Technologies Office under the Silicon Consortium Project directed by Brian Cunningham and managed by Anthony Burrell. This research used resources of the Advanced Light Source from beamline 2.4, which is a DOE Office of Science User Facility under contract no. DE-AC02-05CH11231. References (1) Dopilka, A.; Gu, Y.; Larson, J. M.; Zorba, V.; Kostecki, R. Nano-FTIR Spectroscopy of the Solid Electrolyte Interphase Layer on a Thin-Film Silicon Li-Ion Anode. ACS Appl. Mater. Interfaces 2023, 15 (5), 6755–6767.(2) He, X.; Larson, J. M.; Bechtel, H. A.; Kostecki, R. In Situ Infrared Nanospectroscopy of the Local Processes at the Li/Polymer Electrolyte Interface. Nature Communications 2022, 13 (1), 1398.

Investigating mass transport in Li-ion battery electrodes using SECM and SICM

Lithium-ion batteries (LIBs) are indispensable as global energy production transitions to sustainable production. Nevertheless, the use of LIBs in renewable energy storage applications is challenging due to their limited power densities. To comprehend the origin of this limitation, it is crucial to investigate the effect of electrode architecture on the Li+ ion transport within their pores (solution-phase). In this work, the solution phase transport in various porous Li4Ti5O12 (LTO) films was investigated using scanning ion conductance microscopy (SICM) and scanning electrochemical microscopy (SECM). When the porosity of LTO film increases, SECM and SICM approach curves show an increase in current. This is attributed to the ion transport through the film pores. The 2D topographical mapping using both techniques shows their ability to detect the LTO film's heterogeneity. Most importantly, this work gives insight into the complementary nature of the two scanning probe techniques as demonstrated by the comparable MacMullin numbers.

Charge induced frustration in C60 monolayers on h-BN/Ni(111)

Frustration in a system occurs when all interacting components cannot reach their minimum energy simultaneously. We report on the development of a frustrated adsorption configuration of a C60 monolayer on epitaxial hexagonal boron nitride (h-BN) grown on Ni(111). In the low-temperature adsorption configuration, we discover non-periodic patterns with striped and zigzag elements, which are characteristic of frustrated systems, formed by molceules with different orientational alignement. Using scanning probe techniques, we investigate these complex patterns' structural and electronic properties. We find two distinct C60 species, one of which is neutral, and the other is charged due to a locally preferred energy level alignment that favors an electron transfer from the Ni(111). Discharging single or multiple intrinsically charged C60 molecules creates local changes in the electrostatic potential, shifting the energy of surrounding molecular orbitals. This allows us to measure the inter-fullerene Coulomb interactions, which plays a crucial role in the formation of the frustrated structure. Monte Carlo simulations derived from the antiferromagnetic Ising model replicate the experimentally observed frustration pattern by considering competing attractive and repulsive intermolecular interactions. As such, the C60 h-BN/Ni(111) system opens a window for essential insights into charge-induced frustration and intermolecular interactions.

Johnson-noise-limited cancellation-free microwave impedance microscopy with monolithic silicon cantilever probes

Microwave impedance microscopy (MIM) is an emerging scanning probe technique for nanoscale complex permittivity mapping and has made significant impacts in diverse fields. To date, the most significant hurdles that limit its widespread use are the requirements of specialized microwave probes and high-precision cancellation circuits. Here, we show that forgoing both elements not only is feasible but also enhances performance. Using monolithic silicon cantilever probes and a cancellation-free architecture, we demonstrate Johnson-noise-limited, drift-free MIM operation with 15 nm spatial resolution, minimal topography crosstalk, and an unprecedented sensitivity of 0.26 zF/√Hz. We accomplish this by taking advantage of the high mechanical resonant frequency and spatial resolution of silicon probes, the inherent common-mode phase noise rejection of self-referenced homodyne detection, and the exceptional stability of the streamlined architecture. Our approach makes MIM drastically more accessible and paves the way for advanced operation modes as well as integration with complementary techniques.

Spatially Resolved High Voltage Kelvin Probe Force Microscopy: A Novel Avenue for Examining Electrical Phenomena at Nanoscale

AbstractKelvin probe force microscopy (KPFM) is a well‐established scanning probe technique, used to measure surface potential accurately; it has found extensive use in the study of a range of materials phenomena. In its conventional form, KPFM frustratingly precludes imaging samples or scenarios where large surface potential or surface potential gradients exist outside the typical ±10 V window. If the potential regime measurable via KPFM can be expanded, to enable precise and reliable metrology, through a high voltage KPFM (HV‐KPFM) adaptation, it can open up pathways toward a range of novel experiments, where the detection limit of regular KPFM has so far prevented the use of the technique. In this work, HV‐KPFM is realized and shown to be capable of measuring large surface potential and potential gradients with accuracy and precision. The technique is employed to study a range of materials (positive temperature coefficient of resistivity ceramics, charge storage fluoropolymers, and pyroelectrics) where accurate, spatially resolved mapping of surface potential within high voltage regime facilitates novel physical insight. The results demonstrate that HV‐KPFM can be used as an effective tool to fill in existing gaps in surface potential measurements while also opening routes for novel studies in materials physics.

Membrane fluctuations of human red blood cells investigated by the current signal noise in scanning ion conductance microscopy

Human red blood cells show submicron membrane fluctuations (CMFs) that have been mainly studied with optical microscopies. Although the functional role of this phenomenon is still uncertain, the amplitude of membrane fluctuations is considered as an indicator of mechanical resilience to the stress encountered in the capillary beds. We investigate here the membrane fluctuations in red blood cells using the scanning ion conductance microscopy (SICM), a scanning probe technique that avoids the probe-sample contact. The ion current noise was recorded at a fixed distance from the cell and converted in terms of membrane fluctuation amplitude using as a converting factor the slope of the current-distance curve. We found that CMF amplitude was irreversibly reduced by membrane cross-link. Both whole cell and local increase of membrane tension induced a reduction of CMF amplitude. As for the biochemical regulation of membrane dynamics, we observed that the activation of the noradrenergic transduction pathway, via β-receptors, increased the CMF amplitude. We conclude that the CMFs recorded by SICM and those optically recorded on red blood cells share the main features. In addition SICM provides high spatial and temporal resolution as well as the possibility to apply through the glass pipette acting as probe chemical or physical stimuli to the membrane area where the CMFs are recorded.

In Operando Study of Charge Modulation in MoS2 Transistors by Excitonic Reflection Microscopy.

Monolayers of transition metal dichalcogenides (2D TMDs) experience strong modulation of their optical properties when the charge density is varied. Indeed, the transition from carriers composed mostly of excitons at low electron density to a situation in which trions dominate at high density is accompanied by a significant evolution of both the refractive index and the extinction coefficient. Using optical interference reflection microscopy at the excitonic wavelength, this (n, κ)-q relationship can be exploited to directly image the electron density in operating TMD devices. In this work, we show how this technique, which we call XRM (excitonic reflection microscopy), can be used to study charge distribution in MoS2 field-effect transistors with subsecond throughput, in wide-field mode. Complete maps of the charge distribution in the transistor channel at any drain and gate bias polarization point (VDS, VGS) are obtained, at ∼3 orders of magnitude faster than with scanning probe techniques such as KPFM. We notably show how the advantages of XRM enable real-time mapping of bias-dependent charge inhomogeneities, the study of resistive delays in 2D polycrystalline networks, and the evaluation of the VDS vs VGS competition to control the charge distribution in active devices.

Simultaneous Scanning Ion Conductance and Electrochemical Microscopy in Lithium‐Ion Battery Research

AbstractInvited for this issue's Front Cover is the group of Prof. F.‐M. Matysik. The cover picture shows a typical situation in the life of a scientist focusing on scanning probe techniques. A decision needs to be made, which technique to employ. But what if more than one technique could be employed. In the context of lithium‐ion battery research, several scanning probe microscopies are currently utilized. Scanning ion conductance microscopy and scanning electrochemical microscopy were simultaneously applied in the investigation of a commercial graphite electrode. Read the full text of the Research Article at 10.1002/celc.202300577.

Torsional force microscopy of van der Waals moirés and atomic lattices

In a stack of atomically thin van der Waals layers, introducing interlayer twist creates a moiré superlattice whose period is a function of twist angle. Changes in that twist angle of even hundredths of a degree can dramatically transform the system's electronic properties. Setting a precise and uniform twist angle for a stack remains difficult; hence, determining that twist angle and mapping its spatial variation is very important. Techniques have emerged to do this by imaging the moiré, but most of these require sophisticated infrastructure, time-consuming sample preparation beyond stack synthesis, or both. In this work, we show that torsional force microscopy (TFM), a scanning probe technique sensitive to dynamic friction, can reveal surface and shallow subsurface structure of van der Waals stacks on multiple length scales: the moirés formed between bi-layers of graphene and between graphene and hexagonal boron nitride (hBN) and also the atomic crystal lattices of graphene and hBN. In TFM, torsional motion of an Atomic Force Microscope (AFM) cantilever is monitored as it is actively driven at a torsional resonance while a feedback loop maintains contact at a set force with the sample surface. TFM works at room temperature in air, with no need for an electrical bias between the tip and the sample, making it applicable to a wide array of samples. It should enable determination of precise structural information including twist angles and strain in moiré superlattices and crystallographic orientation of van der Waals flakes to support predictable moiré heterostructure fabrication.

Single-emitter super-resolved imaging of radiative decay rate enhancement in dielectric gap nanoantennas

High refractive index dielectric nanoantennas strongly modify the decay rate via the Purcell effect through the design of radiative channels. Due to their dielectric nature, the field is mainly confined inside the nanostructure and in the gap, which is hard to probe with scanning probe techniques. Here we use single-molecule fluorescence lifetime imaging microscopy (smFLIM) to map the decay rate enhancement in dielectric GaP nanoantenna dimers with a median localization precision of 14 nm. We measure, in the gap of the nanoantenna, decay rates that are almost 30 times larger than on a glass substrate. By comparing experimental results with numerical simulations we show that this large enhancement is essentially radiative, contrary to the case of plasmonic nanoantennas, and therefore has great potential for applications such as quantum optics and biosensing.

Piezoelectricity in wide bandgap semiconductor 2D crystal GaN nanosheets.

Gallium nitride (GaN) exhibits various potential applications in optics and optoelectronics due to its outstanding physical characteristics, including a wide direct bandgap, strong deep-ultraviolet emission, and excellent electron transport properties. However, research on the piezoelectric and related properties of GaN nanosheets are scarce, as previous small-scale GaN investigations have mainly concentrated on nanowires and nanotubes. Here, we report a strategy for growing 2D GaN nanosheets using chemical vapor deposition on Ga/W liquid-phase substrates. Additionally, utilizing scanning probe techniques, it has been observed that 700 nm-thick GaN nanosheets demonstrate a piezoelectric constant of deff33 = 1.53 ± 0.21 pm V-1 and possess the capability to effectively modulate the Schottky barrier. The piezoelectric characteristics of 2D GaN are offering new options for innovative applications in various fields, including energy harvesting, electronics, sensing, and communications.

Applications of scanning probe microscopy in neuroscience research

Abstract Scanning probe microscopy techniques allow for label-free high-resolution imaging of cells, tissues, and biomolecules in physiologically relevant conditions. These techniques include atomic force microscopy (AFM), atomic force spectroscopy, and Kelvin probe force microscopy, which enable high resolution imaging, nanomanipulation and measurement of the mechanoelastic properties of neuronal cells, as well as scanning ion conductance microscopy, which combines electrophysiology and imaging in living cells. The combination of scanning probe techniques with optical spectroscopy, such as with AFM-IR and tip-enhanced Raman spectroscopy, allows for the measurement of topographical maps along with chemical identity, enabled by spectroscopy. In this work, we review applications of these techniques to neuroscience research, where they have been used to study the morphology and mechanoelastic properties of neuronal cells and brain tissues, and to study changes in these as a result of chemical or physical stimuli. Cellular membrane models are widely used to investigate the interaction of the neuronal cell membrane with proteins associated with various neurological disorders, where scanning probe microscopy and associated techniques provide significant improvement in the understanding of these processes on a cellular and molecular level.

Electrochemical Nucleation and Growth: A Multimicroscopy Approach at the Same Scale

Electrodeposition is a critical electrochemical reaction with broad applications in technology. This demands a comprehensive understanding of the electrochemical nucleation and growth process (EN&G), and the different factors driving the formation of a new phase on a given substrate. Our understanding of the role of the substrate and the real nature of active sites still remains elusive due to the challenge of probing specific sites and bringing a statistical interpretation of the nucleation process. Moreover, conventional macroscopic approaches often record electrochemical magnitudes over large areas (cm2) which are then correlated with surface characterizations obtained from a vastly different scale (~μm2), correlations, which are not necessarily representative of the macroscopic surface. In order to establish meaningful and unambiguous correlations, it becomes necessary to employ techniques capable of surveying the surface with a high spatial-temporal resolution, such as scanning probe techniques, capable of recovering information in a scale closer to the growth of the new phase, i.e., local scale1. To address these challenges, we employed Scanning Electrochemical Cell Microscopy (SECCM), to study the EN&G of Cu on a glassy carbon substrate at a scale closer to the initial stages of the electrodeposition2. SECCM can be combined with other imaging methods such as scanning electron microscopy (SEM) to recover information from the deposited nanostructures at the same scale as the electrochemical response3. This so-called multimicroscopy approach allows us establishing direct correlations between the EN&G and the morphological, structural, and compositional features of the substrate and deposit. As a result, new insights that are not available with conventional macro-electrochemical analysis can be gained, such as the time-, potential-, and site-dependent nature of the EN&G process with a statistical perspective. This approach provides a new avenue for developing novel analysis strategies to understand the real nature of active sites for nucleation.References M. Bernal, D. Torres, S. S. Parapari, M. Čeh, K. Ž. Rožman, S. Šturm, and J. Ustarroz, Electrochim. Acta, 445, 142023 (2023).D. Torres, M. Bernal, A. Demaude, S. Hussain, L. Bar, P. Losada-Pérez, F. Reniers, and J. Ustarroz, J. Electrochem. Soc., 169, 102513 (2022).D. Torres, M. Bernal Lopez, and J. Ustarroz, Electrochemical nucleation and growth: A multimicroscopy Approach at the same scale, In preparation. (2023). Figure 1

Unidirectional Nano-modulated Binding and Electron Scattering in Epitaxial Borophene.

A complex interplay between the crystal structure and the electron behavior within borophene renders this material an intriguing 2D system, with many of its electronic properties still undiscovered. Experimental insight into those properties is additionally hampered by the limited capabilities of the established synthesis methods, which, in turn, inhibits the realization of potential borophene applications. In this multimethod study, photoemission spectroscopies and scanning probe techniques complemented by theoretical calculations have been used to investigate the electronic characteristics of a high-coverage, single-layer borophene on the Ir(111) substrate. Our results show that the binding of borophene to Ir(111) exhibits pronounced one-dimensional modulation and transforms borophene into a nanograting. The scattering of photoelectrons from this structural grating gives rise to the replication of the electronic bands. In addition, the binding modulation is reflected in the chemical reactivity of borophene and gives rise to its inhomogeneous aging effect. Such aging is easily reset by dissolving boron atoms in iridium at high temperature, followed by their reassembly into a fresh atomically thin borophene mesh. Besides proving electron-grating capabilities of the boron monolayer, our data provide comprehensive insight into the electronic properties of epitaxial borophene which is vital for further examination of other boron systems of reduced dimensionality.

Length-Dependent Magnetic Evolution of Anthenes on Au(111).

Nanographenes with zigzag edges, for example, anthenes, exhibit a unique nonbonding π-electron state, which can be described as a spin-polarized edge state that yields specific magnetic ground state. However, prior researches on the magnetism of anthenes with varying lengths on a surface is lacking. This study systematically fabricated anthenes with inherent zigzag carbon atoms of different lengths ranging from bisanthene to hexanthene. Their magnetic evolution on the Au(111) surface was analyzed through bond-resolved scanning probe techniques and density functional theory calculations. The analyses revealed a transition in magnetic properties associated with the length of the anthenes, arising from the imbalance between hybridization energy and the Coulomb repulsion between valence electrons. With the increasing length of the anthenes, the ground state transforms gradually from a closed-shell to an antiferromagnetic open-shell singlet, exhibiting a weak exchange coupling of 4 meV and a charge transfer-induced doublet. Therefore, this study formulated a chemically tunable platform to explore size-dependent π magnetism at the atomic scale, providing a framework for research in organic spintronics.