Controlling whether a molecular radical retains its spin on a metal surface is a key prerequisite for building switchable, atomically precise carbon-based spin architectures. Here, we use 3,6-bis(4-bromophenyl)-9H-fluorene to synthesize covalently linked fluorene trimers and oligomeric chains on Au(111) via Ullmann coupling and then generate strongly localized fluorenyl-type radical centers by site-selective tip-induced dehydrogenation. Combining the bond-resolved nc-AFM with scanning tunneling spectroscopy, we identify two interconvertible adsorption configurations: a non-bonded radical state that displays a pronounced zero-bias Kondo resonance and a chemisorbed state in which a local C-Au bond is formed at the radical site, accompanied by a characteristic geometric relaxation of the five-membered ring and complete quenching of the Kondo resonance. In both the macrocycles and chains, the distribution of the Kondo-active sites depends on metastable global adsorption geometries and can be reversibly reconfigured by tip perturbation. These results establish a structure-resolved chemisorption versus physisorption switch as a practical design rule for stabilizing and toggling spins in multi-radical rings and chains directly on metallic substrates, opening opportunities for programmable quantum spin functionalities in surface-supported π-systems.

In recent years,continuous advances in on-surface synthesis have led to the emergence of carbon-based quantum materials with rich structures and precisely tunable properties.The novel physical behaviors exhibited by carbon-based quantum materials—such as nontrivial topology,quantum magnetism,and strong electronic correlations—provide abundant material platforms for future spintronics,quantum computing,and quantum information science.Because these materials possess atomic-level precision as well as structurally complex and variable motifs,and because their electronic and spin states are easily influenced by the surrounding environment,multi-domain detection and global analysis of their properties become particularly important.This paper focuses on cutting-edge characterization techniques for on-surface synthesized carbon-based quantum materials and reviews advances in the ex-situ macroscopic characterizations and the in-situ microscopic characterizations.Emphasis is placed on new spectroscopic and spectrographic methods integrated with scanning probe microscopy to explore,across spatial,energetic,and temporal dimensions,the chemical structure,electronic states,vibrational modes,spin states,and their dynamics in carbon-based materials.We also point out future trends in constructing carbon-based spin qubits,integrating comprehensive measurement platforms,and ensuring device environmental compatibility,thereby further expanding the application prospects of carbon-based quantum materials.

The design of three-dimensional (3D) superlattices from known two-dimensional (2D) and 3D topological units provides a versatile platform for engineering spin-dependent electronic states. While most previous studies have focused on topological surface states (TSS), other prominent spectral features have received less attention. In the prototypical nature superlattice compound Bi1Se1 from the [Bi2]x[(Bi, Sb)2(Te, Se)3]y family, angle-resolved photoemission spectroscopy (ARPES) reveals a dominant Dirac-like band whose spin texture has remained unresolved. Here, using low-temperature cleavage, scanning tunneling microscopy (STM), circular dichroism ARPES (CD-ARPES), and ab initio calculations, we achieve uniformly terminated Bi1Se1(0001) surfaces, enabling unambiguous identification of its spin polarization. We find that this Dirac-like state exhibits a reversed in-plane helical spin texture inherited from the residual electronic remnant of TSS of the Bi2Se3 subunits, persisting on the "topologically" dark (0001) surface despite the absence of global protection. A conduction band minimum showing dichroism consistent with Rashba-type splitting is also observed, similar to the surface state on bulk Bi2Se3. The close correspondence between our findings and those reported in identically terminated Bi1Te1 suggests that such residual-TSS derived Dirac states are a general feature of [Bi2]x[(Bi, Sb)2(Te, Se)3]y superlattices. The experimental-theoretical strategy demonstrated here offers a framework for exploring spin-textured states in engineered topological heterostructures.

Understanding and controlling matter at atomic scales is critical for materials science and condensed matter physics, as many macroscopic properties stem from phenomena and mechanisms at sub-nanometre dimensions. Although optical spectroscopy remains a cornerstone of materials characterization, scanning tunnelling microscopy (STM) has become an essential tool because of its atomic-level spatial resolution. Terahertz (THz) STM brings together these two approaches by introducing picosecond THz pulses into the STM junction. This enables the exploration and manipulation of electron dynamics, molecular motions and many-body states with both atomic spatial and sub-picosecond temporal resolution. Here, we review the principles, methodologies and applications of THz-STM, highlighting its unique ability to simultaneously access temporal, spatial and energy domains to provide insight into ultrafast nanoscale phenomena and driving advances in next-generation technologies. We project future opportunities for THz-STM in quantum materials, including measuring non-equilibrium quantum topology that may feature Floquet and non-Hermitian physics as well as exploring optical control of superconductivity and light-induced Cooper pairing. Terahertz scanning tunnelling microscopy integrates picosecond pulses with atomic resolution, enabling detailed analysis of electron dynamics and molecular motions. This Technical Review outlines methods and applications, emphasizing insights into ultrafast phenomena and potential advances in quantum materials and technologies.

Topological solitons can act as mobile domain walls between topologically non-trivial and trivial phases, merging hybrid zero-mode properties from both solitonic and symmetry-protected boundary states, and providing both fundamental insights and unprecedented opportunities for quantum technologies. However, their experimental realization is challenging. Here, we demonstrate on-surface engineering of topological structures and introduction of topological solitons in π-conjugated pentacene polymers through end-group modification on Au(111), using combined multiple techniques including scanning tunneling microscopy, non-contact atomic force microscopy and tip-enhanced Raman spectroscopy, along with density functional theory and tight-binding calculations. We fabricate cumulene-bridged pentacene oligomers and polymers with nearly length independence by anchoring both their termini to the surface. By converting a near-end segment into the trivial phase through its end-group modification, we realize the interpolation of topological solitons as domain walls between non-trivial and trivial phases, which are well supported by observations of the solitonic zero-energy peaks across the domain walls, the band reverse between the separated regions, and the distinct region-dependent vibration modes, as well as theoretical calculations. The realization of topological solitons as domain walls between non-trivial and trivial phases offers a rich platform for fundamental research, and illustrates potential applications of π-conjugated polymers in quantum devices.

The self-assembly and multiple tessellation structures of organic molecules on the metal surfaces are of great significance for the design of functional materials. Some examples of the manipulation of supramolecular tessellations and the exploration of the impact arising from deliberate modifications of the underlying substrate have been demonstrated on various noble metal substrates. However, due to the effects of varied substrates and molecule coverages, the patterns of molecular tessellations have not been precisely predictable. Here, we utilize scanning tunneling microscopy (STM) and density functional theory (DFT) to study various molecular tessellations on M(111) (M=Ag, Au, Cu) surface through the deliberate modulation of substrate activity, surface coverage, substrate temperature, and substrate lattice to change the vertex symmetry. Using 1,2-di(4-pyridyl) ethylene (BPE) molecules, we obtained seven molecular tessellations, where the intermolecular interaction conversion from the weak hydrogen bond (C-H & ctdot;N) to the possible metal-organic coordination bond has been analyzed. We identified that the hydrogen bonding through C-H & ctdot;N plays the predominant role in the BPE patterns on Au(111) and Ag(111), while the participation of Cu adatoms plays an important role for the intermolecular interactions through Cu-N interaction in the BPE patterns on Cu(111).

Semiconductor oxide-based heterogeneous catalysis and/or photocatalysis provide pivotal solutions to global energy and environmental crises, yet a fundamental understanding of their atomic-level mechanisms lags far behind the practical applications. A typical example is the degradation of formaldehyde (FA) on ZnO surfaces. Although ZnO is extensively employed in sensing and removing FA from the atmosphere, the atomic-level reaction mechanisms remain unclear. Here, we combine scanning tunneling microscopy (STM), temperature-programmed desorption (TPD) measurements, and density functional theory (DFT) calculations to reveal the reaction process of FA on a ZnO(1010) single-crystalline surface. We directly visualize two competing pathways: dissociation versus dimerization, with dimerization getting significantly accelerated upon ultraviolet (UV) irradiation at room temperature (RT). Furthermore, at elevated temperatures, intermediates react aggressively with surface lattice oxygen, generating abundant oxygen vacancies-a discovery that fundamentally advances our understanding of the Mars-van Krevelen mechanism for FA and similar organics on ZnO. These atomic-level insights open new avenues for designing highly efficient, environment friendly photocatalytic systems based on the semiconducting oxide materials.

Structural and chemical identification of individual molecules with chemical-bond precision has been achieved by integrating scanning tunneling microscopy with tip-enhanced Raman spectroscopy (STM-TERS), but practical implementation remains limited by the manual, operator-dependent correlation between scanning tunneling microscopy (STM) topography and TERS spectral interpretation. Here, we present a proof-of-principle AI-enabled STM-TERS platform that integrates deep learning with instrument control to achieve autonomous imaging, molecular targeting, tip navigation, spectral acquisition, and supervised chemical/isotopic assignment within a predefined molecular label space. A Faster R-CNN model localizes molecules in STM images and generates navigation coordinates with coarse topological labels, while a one-dimensional ResNet-18 classifier assigns single-molecule TERS spectra to distinct predefined chemical/isotopic classes. Using pentacene/Ag(110) and thermally dehydrogenated derivatives as a model system, the platform achieves similar to 1.1 & Aring; localization precision, >90% detection accuracy, and 98.04% spectral classification accuracy. This proof-of-principle architecture demonstrates a closed-loop STM-TERS workflow for autonomous acquisition and supervised classification on a well-defined model system.

Harnessing plasmonic energy to drive selective chemical transformations is central to photocatalysis, solar energy conversion, and molecular optoelectronics. Plasmon-driven chemistry proceeds through nonresonant plasmonic hot electron transfer or more efficient resonant charge transfer, yet cleanly disentangling these channels has remained a long-standing challenge, especially in synthetic nanoparticle systems with heterogeneous molecular environments. Here, using the nanocavity plasmon generated between an Ag tip and individual pentacene molecules on Ag(110) in a scanning tunneling microscopy (STM) junction, we directly unveil both channels with submolecular resolution. Combining chopper-modulated laser excitation with lock-in detection enables plasmon-induced current mapping at 0.3 nm resolution, allowing real-space correlation with frontier molecular orbitals. By tuning photon energy and bias voltage, we identify nonresonant hot-electron transfer via the lowest unoccupied molecular orbital (LUMO) and resonant charge transfer via LUMO + 1, where the resonant pathway markedly accelerates C-H bond breaking.

We report the development of a cryogen-free, low-temperature optical-coupled scanning probe microscope (LT-OC-SPM) designed for high-resolution multimodal imaging and spectroscopy. To mitigate the mechanical vibrations from the cryocooler cold head, we implement a remote liquefaction scheme that effectively decouples the noise from the tunneling junction. The system achieves a stable base operation temperature below 3K and a tunneling current noise level under 20 fA/Hz1/2, comparable to the performance of the conventional SPM systems employing bath cryostats. Moreover, our system features a customized rigid scanner integrated with in-vacuo piezo-driven high-numerical aperture lenses, facilitating simultaneous topographic and spectroscopic measurements. Multimodal characterization of silver phthalocyanine (AgPc) molecules on Ag(110) demonstrates ångström scale probing of intramolecular structure and localized vibrational modes, highlighting the versatility of our system in high-resolution surface characterization. Consequently, this remote liquefaction architecture provides a sustainable, high-performance cryogen-free platform for ångström-resolved spectroscopic imaging, establishing a robust foundation for future multifunctional near-field optical spectroscopy.

On-surface chemistry provides an efficient approach to construction of diverse covalent architectures with atomic precision, ranging from one-dimensional chains and ribbons to two-dimensional covalent organic frameworks (COFs) and metal-organic frameworks (MOFs) on coinage metal substrates. This study explores a distinct on-surface pyrolysis approach to MOFs derived from a crown ether molecular precursor on Au(111) and Ag(111) surfaces. Utilizing scanning tunneling microscopy (STM) and non-contact atomic force microscopy (nc-AFM) combined with density functional theory (DFT) calculations, we elucidate the adsorption behavior and the characteristic macrocyclic configuration of the crown ether on Au(111). Subsequent surface-catalyzed Ullmann coupling reactions at an annealing temperature of 470 K lead to highly disordered COFs with the formation of four-membered and six-membered rings through dimerization and trimerization. For the Ag(111) surface, further annealing at 520 K initiates a unique dehydrogenative reaction within the macrocyclic rings, resulting in the loss of six hydrogen atoms. At an elevated temperature of 720 K, breaking of the second C−O bonds yields a long-range ordered triphenylene-based MOF structure. Electronic characterizations reveal the presence of both regular and diatomic kagome lattices, together with distinct quantum-dot states emerging in the pore regions. Additionally, we investigate the selective encapsulation of single guest picenes within the MOF structure, emphasizing the potential of triphenylene-based frameworks for advanced applications in sensing and molecular filtering. Our findings provide a comprehensive insight into the chemical reactivity of crown ethers on metal substrates and demonstrate a novel pathway to designing MOFs through an on-surface pyrolysis process.

The multiplicity of orbitals in quantum systems significantly influences the competition between Kondo screening and local spin magnetization. The identification of orbital-specific processes is essential for advancing spintronic devices, as well as for enhancing the understanding of many-body quantum phenomena, but it remains a great challenge. Here, we use a combination of scanning tunneling microscopy/spectroscopy and electron spin resonance (ESR) spectroscopy to investigate single iron phthalocyanine (FePc) molecules on MgO/Ag(100). We observe the coexistence of ESR and Kondo resonance from the Fe(II) ion center at a temperature well below the Kondo temperature T_{K}, where the magnetic moments are determined to be 1 Bohr magneton (1μ_{B}), corresponding to a spin S=1/2 state in either of the ESR and Kondo resonance channels. On the basis of the observed in-plane twofold symmetry in the spatial distribution of ESR linewidths, in combination with density functional theory calculations, we attribute one spin S=1/2 state in the degeneracy-lifted d_{π} orbital to the ESR channel, and the other in the highly screenable d_{z^{2}} orbital to the Kondo resonance, respectively, within the framework of the S=1 two-channel Kondo model of an ordinary Fermi liquid.

Surface with well-defined components and structures possesses unique electronic, magnetic, optical and chemical properties. As a result, surface chemistry research plays a crucial role in various fields such as catalysis, energy, materials, quantum, and microelectronics. Surface science mainly investigates the correspondence between surface property and functionality. Scanning probe microscopy (SPM) techniques are important tools to characterize surface properties because of the capability of atomic-scale imaging, spectroscopy and manipulation at the single-atom level. In this review, we summarize recent advances in surface electronic, magnetic and optical properties characterized mainly by SPM-based methods. We focus on elucidating the π-magnetism in graphene-based nanostructures, construction of spin qubits on surfaces, topology properties of surface organic structures, STM-based light emission, tip-enhanced Raman spectroscopy and integration of machine learning in SPM studies.

The universality of critical phenomena and finite-size scaling are effective methods for measuring critical exponents in experiments and inferring the intrinsic interactions within materials. Here, we establish the finite-size scaling form of the Landau–Ginzburg model for fractal time processes and quantitatively calculate the critical exponents at the upper critical dimension. Interestingly, contrary to the traditional conception that critical exponents are independent of dynamic processes and proportional to correlation length, we find that fractal time processes can not only change critical exponents but also yield a scaling form of size dependent on fractional order and spatial dimension. These theoretical results provide a reasonable method to determine and measure the existence of fractal time processes and their associated critical exponents. The simulations of the Landau–Ginzburg model with fractional temporal derivatives and the Ising model with long-range temporal interactions not only reveal critical exponents distinct from those of standard models but also exhibit unique size effects characteristic of fractal time processes. These results validate the emergence of a new universality class and confirm the predictions of the finite-size scaling theory for fractal time processes.

As conventional electronic materials approach their physical limits, the application of ultrafast optical fields to access transient states of matter captures imagination. The inversion symmetry governs the optical parity selection rule, differentiating between accessible and inaccessible states of matter. To circumvent parity-forbidden transitions, the common practice is to break the inversion symmetry by material design or external fields. Here we report how the application of femtosecond ultraviolet pulses can energize a parity-forbidden dark exciton state in black phosphorus while maintaining its intrinsic material symmetry. Unlike its conventional bandgap absorption in visible-to-infrared, femtosecond ultraviolet excitation turns on efficient Coulomb scattering, promoting carrier multiplication and electronic heating to ~3000 K, and consequently populating its parity-forbidden states. Interferometric time- and angle-resolved two-photon photoemission spectroscopy reveals dark exciton dynamics of black phosphorus on ~100 fs time scale and its anisotropic wavefunctions in energy-momentum space, illuminating its potential applications in optoelectronics and photochemistry under ultraviolet optical excitation.

Light is a preeminent spectroscopic tool for investigating the electronic structure of surfaces. Time-resolved photoelectron spectroscopy has mainly been developed in the last 30 years. It is therefore not surprising that the topic was hardly mentioned in the issue on "The first thirty years"of surface science. In the second thirty years, however, we have seen tremendous progress in the development of time-resolved photoelectron spectroscopy on surfaces. Femtosecond light pulses and advanced photoelectron detection schemes are increasingly being used to study the electronic structure and dynamics of occupied and unoccupied electronic states and dynamic processes such as the energy and momentum relaxation of electrons, charge transfer at interfaces and collective processes such as plasmonic excitation and optical field screening. Using spin- and time-resolved photoelectron spectroscopy, we were able to study ultrafast spin dynamics, electron-magnon scattering and spin structures in magnetic and topological materials. Light also provides photon energy as well as electric and magnetic fields that can influence molecular surface processes to steer surface photochemistry and hot-electron-driven catalysis. In addition, we can consider light as a chemical reagent that can alter the properties of matter by creating non-equilibrium states and ultrafast phase transitions in correlated materials through the coupling of electrons, phonons and spins. Electric fields have also been used to temporarily change the electronic structure. This opened up new methods and areas such as high harmonic generation, light wave electronics and attosecond physics. This overview certainly cannot cover all these interesting topics. But also as a testimony to the cohesion and constructive exchange in our ultrafast community, a number of colleagues have come together to share their expertise and views on the very vital field of dynamics at surfaces. Following the introduction, the interested reader will find a list of contributions and a brief summary in Section 1.3.

Molecular materials offer a boundless design palette for light absorption and charge transport in both natural photosynthesis and engineered photovoltaics. They function in combination as chromophores, donors, conductors, and acceptors, enabling the excitation and charge carrier transport through space and wire-like intramolecular pathways. Although quantum coherence is believed to enhance photoexcitation and photoinduced charge transfer, fluctuating and inhomogeneous environments accelerate decoherence. Here, we assemble a nanoporous medium consisting of a templated bipyridyl ethylene (BPE) molecule array on a Ag(111) surface that functions as an exceptional intermolecular nonnuclear quantum well conductor of coherent electron waves spanning over 20 Å length. Time-periodic driving of the Ag/BPE interface by femtosecond pulses promotes electrons into a ladder of Floquet quasi-energy donor states, where intermolecular quantum well states act as a resonant doorway for coherent electron transport into BPE/vacuum image potential acceptor states. The bifurcation of electron passage between the Floquet donor ladder and the charge transfer acceptor channel is recorded by projecting the active electrons into the photoemission continuum in an interferometric time- and angle-resolved multiphoton photoemission experiment. We find that exceptional decoupling of electrons from the metal substrate by the molecule-dressed vacuum preserves the coherence on the ∼150 fs time scale. This offers a new paradigm for quantum state design where a molecule-dressed vacuum mediates coherent electron transport in nanoporous molecular architectures.

On-surface synthesis via metal-surface-catalyzed C-C bond formation presents unique advantages for the design of graphitic nanomaterials with atomic precision. Following this approach, the coimplantation of nontrivial topology and flatband structures in graphene nanoribbons (GNRs) has emerged as a compelling pursuit, serving as platforms for realizing exotic quantum phases of matter through the interplay of topological states and strong correlations. However, the exploration of these intriguing properties has been largely constrained by the limited known on-surface reactions capable of creating topological flatbands in GNRs. In this work, we promote the intermolecular oxidative coupling of concealed non-Kekuléan nanographenes to construct topological flatband GNRs and GNR heterojunctions on the Au(111) surface. Utilizing Clar's goblet as a proof of concept, we demonstrate repetitive intermolecular cyclodehydrogenation with high regioselectivity to form pentagon-embedded GNRs. The coupling of the zero modes in Clar's goblets generates extended electronic states with evident nodes between them, arising from inherent topological frustration, thus resulting in topological flatbands close to the Fermi level and topologically protected end states. Our atomically resolved measurements obtained using scanning tunneling microscopy and noncontact atomic force microscopy, complemented by density functional theory and tight-binding model calculations, illustrate the on-surface reaction cascade and the electronic properties of the designed products. These findings open significant opportunities for the on-surface construction of low-dimensional carbon-based quantum materials.