High-resolution Scanning Probe Microscopy Research Articles

Computational Characterization of the On-Surface Synthesis of Low-Dimensional Nanostructures

On-surface chemistry represents a fast-growing field allowing to synthesize molecular structures not available by traditional wet chemistry. In this context, high-resolution scanning probe microscopy techniques are able to provide unprecedented spatial resolution, allowing to precisely identify individual products of the reactions. Nevertheless, a deep understanding of the reaction mechanism under the conditions imposed by the substrate remains unknown.Nowadays, various computational strategies are employed to characterize the optimal reaction pathways of the chemical processes that take place on the surfaces of solids. For example, one of the most popular approaches is the nudged elastic band (NEB) as well as different transition state theory methods beyond it. These approaches had demonstrated their capability to characterize the potential energy landscape of the reaction pathways [1-2]. Another approach that had attracted the interest of the on-surface synthesis community is the application of molecular dynamics (MD) methods, within the canonical ensemble, and based on the QM/MM (quantum mechanics/molecular mechanics) of the interatomic forces [3-4]. This approach, which combines Density functional theory (DFT) and empirical force fields to describe the interaction forces, allows to simulate very large systems, at a very low computational cost while still accounting for the crucial quantum mechanics interactions. Also, such temperature-dependent simulations include the effect of entropy, vibrations modes, concerted motion, etc.I will present the experimental and computational results of different projects in which we have made use of the aforementioned computational approaches to achieve a better understanding of the on-surface synthesis experiments performed by the group. For example, the rationalization of the selective cyclodehydrogenation of non-benzenoid nanographenes deposited on a Au(111) substrate by means of temperature-dependent MD calculations and the analysis of the obtained vibrational modes (See Figure 1a). The group had also studied the stepwise transformation of coronene-based organometallic networks into two-dimensional covalent patches through intermolecular homocoupling (See Figure 1b). We had as well investigated the transformation of indenyl precursors, through a triple C-H cleavage followed by a peripheral directed homocoupling, to form a polyacetylene backboned polymer that features a parity dependent and highly delocalized soliton in-gap state (See Figure 1c).

On-Surface Synthesis of Macrocycles, Non-Planar Carbon Ribbons and Heteroatom-Doped Nanographenes

On-surface synthesis via covalent coupling of adsorbed molecules on metal surfaces has attracted significant attention recently due to its potential to fabricate low-dimensional carbon materials with atomic precision. The bottom-up, atomically precise synthesis of carbon nanostructures enables the tailoring of their electronic properties at a molecular level. To understand and control the surface-chemistry-driven synthesis, many efforts have been made to design innovative precursors, explore novel reaction schemes, and utilize templating effects from the substrate.My presentation focuses on high-resolution scanning probe microscopy experiments combined with density functional theory to demonstrate recent highlights on the assembly of surface-supported low-dimensional carbon structures on metal surfaces. First, the assembly and electronic structure of planar π-extended cycloparaphenylene macrocycles, representing the first nanographene with an all-armchair edge topology, will be presented [1]. The second part will discuss the bottom-up synthesis of covalently-linked non-planar carbon ribbons and their electronic properties depending on their adsorption geometry. Finally, I will conclude with the on-surface cyclomerization of oxygen heterocycles to understand the on-surface synthesis of furan and pyran derivatives from ketone-functionalized precursors on metal surfaces.[1] Xiang, et al. Nature Chem., 2022 14, 871–876.

Syntheses of Heteroatom-Substituted and Three-Dimensional Nanocarbon Materials on Surfaces

I will present our recent studies on on-surface chemistry with high-resolution scanning probe microscopy operating at low temperature under ultra-high vacuum. We synthesized three-dimensional organometallic compound and graphene nanoribbon by coupling hexabromo substituted-propellane molecules on Au(111) and Ag(111).[1] In the structure, the C-Br bonds distant from the surface remained intact even after the reaction. The radical species were formed by tip-induced debromination and were also stabilized by either tip-manipulated Br atom or fullerene molecule. We also demonstrate systematic tip-induced isomerization via embedding a text of “NIMS” by following binary and ternary ascii cords, in which two chiral dehydroazulene and diradical units were used.[2] Among the three states, the diradical unit showed the spin coupling of 90 meV. For heteroatom substitution, we developed an on-surface reaction, which can synthesize graphene nanoribbon and covalent organic frameworks with silabenzene units by coupling Si atom and Br-substituted molecule.[3] The heavier congeners of cyclic aromatic compounds have been studied as an elusive target product for organic synthesis due to their high reactivity at ambient temperature and difficult isolation. Thus, this result shows the advantage of on-surface synthesis.Reference[1] S. Kawai, et al Sci. Adv. 6, eaay8913 (2020).[2] S. Kawai, et al Nat. Commun. 14, 7741 (2023).[3] K. Sun et al, Nat. Chem. 15, 136 (2023).

Spin-Stabilization by Coulomb Blockade in a Vanadium Dimer in WSe2.

Charged dopants in 2D transition metal dichalcogenides (TMDs) have been associated with the formation of hydrogenic bound states, defect-bound trions, and gate-controlled magnetism. Charge-transfer at the TMD-substrate interface and the proximity to other charged defects can be used to regulate the occupation of the dopant's energy levels. In this study, we examine vanadium-doped WSe2 monolayers on quasi-freestanding epitaxial graphene, by high-resolution scanning probe microscopy and ab initio calculations. Vanadium atoms substitute W atoms and adopt a negative charge state through charge donation from the graphene substrate. VW-1 dopants exhibit a series of occupied p-type defect states, accompanied by an intriguing electronic fine-structure that we attribute to hydrogenic states bound to the charged impurity. We systematically studied the hybridization in V dimers with different separations. For large dimer separations, the 2e- charge state prevails, and the magnetic moment is quenched. However, the Coulomb blockade in the nearest-neighbor dimer configuration stabilizes a 1e- charge state. The nearest-neighbor V-dimer exhibits an open-shell character for the frontier defect orbital, giving rise to a paramagnetic ground state. Our findings provide microscopic insights into the charge stabilization and many-body effects of single dopants and dopant pairs in a TMD host material.

Robust Switchable Polarization and Coupled Electronic Characteristics of Magnesium-Doped Zinc Oxide.

Ferroelectrics possess a spontaneous polarization that is switchable by an electric field and is critical for the development of low-energy nanoelectronics and neuromorphic applications. However, apart from a few recent developments, the realization of switchable polarization in metal oxides with simpler structures has been a major challenge. Here, we demonstrate the presence of robust switchable polarization at the level of a single nanocrystallite in magnesium-doped zinc oxide thin films with polar wurtzite crystal structures. Using a combination of high-resolution scanning probe microscopy and spectroscopic techniques, voltage control of the polarization and the coupled electronic transport behavior revealing a giant resistance change of approximately 10000% is unveiled. Time- and frequency-resolved nanoscale measurements provide key insights into the polarization phenomenon and a 9-fold increase in the effective longitudinal piezoelectric coefficient. Our work thus constitutes a crucial step toward validating nanoscale ferroelectricity in polar wurtzites for use in advanced nanoelectronics and memory applications.

Collective Quantum Magnetism in Nitrogen-Doped Nanographenes.

The emergence of quantum magnetism in nanographenes provides ample opportunities to fabricate purely organic devices for spintronics and quantum information. Although heteroatom doping is a viable way to engineer the electronic properties of nanographenes, the synthesis of doped nanographenes with collective quantum magnetism remains elusive. Here, a set of nitrogen-doped nanographenes (N-NGs) with atomic precision are fabricated on Au(111) through a combination of imidazole [2+2+2]-cyclotrimerization and cyclodehydrogenation reactions. High-resolution scanning probe microscopy measurements reveal the presence of collective quantum magnetism for nanographenes with three radicals, with spectroscopic features which cannot be captured by mean-field density functional theory calculations but can be well reproduced by Heisenberg spin model calculations. In addition, the mechanism of magnetic exchange interaction of N-NGs has been revealed and compared with their counterparts with pure hydrocarbons. Our findings demonstrate the bottom-up synthesis of atomically precise N-NGs which can be utilized to fabricate low-dimensional extended graphene nanostructures for realizing ordered quantum phases.

Novel type of whisker-tip cantilever based on GaN microrods for atomic force microscopy

High-resolution scanning probe microscopy (SPM) is a fundamental and efficient technology for surface characterization of modern materials at the subnanometre scale. The bottleneck of SPM is the probe and scanning tip. Materials with stable electrical, thermal, and mechanical properties for high-aspect-ratio (AR) tips are continuously being developed to improve their accuracy. Among these, GaN is emerging as a significant contender that serves as a replacement for standard Si probes. In this paper, for the first time, we present an approach that demonstrates the application of GaN microrods (MRs) as high-AR SPM probes. GaN MRs were grown using molecular beam epitaxy, transferred and mounted on a cantilever using focused electron beam-induced deposition and milled in a whisker tip using a focused ion beam in a scanning electron/ion microscope. The presence of a native oxide layer covering the GaN MR surface was confirmed by X-ray photoelectron spectroscopy. Current-voltage map measurements are also presented to indicate the elimination of the native oxide layer from the tip surface. The utility of the designed probes was tested using conductive atomic force microscopy and a 24-hour durability test in contact mode atomic force microscopy. Subsequently, the graphene stacks were imaged.

On-surface synthesis of enetriynes

Belonging to the enyne family, enetriynes comprise a distinct electron-rich all-carbon bonding scheme. However, the lack of convenient synthesis protocols limits the associated application potential within, e.g., biochemistry and materials science. Herein we introduce a pathway for highly selective enetriyne formation via tetramerization of terminal alkynes on a Ag(100) surface. Taking advantage of a directing hydroxyl group, we steer molecular assembly and reaction processes on square lattices. Induced by O2 exposure the terminal alkyne moieties deprotonate and organometallic bis-acetylide dimer arrays evolve. Upon subsequent thermal annealing tetrameric enetriyne-bridged compounds are generated in high yield, readily self-assembling into regular networks. We combine high-resolution scanning probe microscopy, X-ray photoelectron spectroscopy and density functional theory calculations to examine the structural features, bonding characteristics and the underlying reaction mechanism. Our study introduces an integrated strategy for the precise fabrication of functional enetriyne species, thus providing access to a distinct class of highly conjugated π-system compounds.

Influencing mechanism of fiber deformation on mesh pattern noise in inverting-image fiber-optic arrays

Inverting-image fiber-optic array (IIFOA) is an important optical element for transferring the inverted images at 180° with high resolution and contrast. Mesh pattern noise (MPN) generated at the boundary of the multi-multi fibers in IIFOA significantly deteriorates the imaging quality and detection accuracy. The unexpected deformation of the optical fibers at the boundary of multi-fiber is one of the main factors causing MPN. In this study, the brightness distribution of the optical fibers and the corresponding microstructure were characterized by high-resolution Lambert-light microscopy and scanning probe microscopy with laser scanning, respectively. The relative transmittance of the individual optical fibers located in the MPN zone was normalized and calculated, and the cross-sectional deformation coefficient (De) of optical fiber in the twisted zone was proposed. Furthermore, the relationship among De, the relative transmittance of optical fibers, and MPN of IIFOA was established. The results show that the relative transmittance of optical fibers indicates an initial decrease and a subsequent increase with the decrease of De in the MPN generated zone. When De of the optical fibers is within the range of [0.3497, 0.9225], the relative transmittance is no more than 96.74%. The aggregation and distribution of these optical fibers at the adjacent side of the multi-multi fiber boundary causes black mesh pattern noise. When De of the optical fibers is less than 0.3228, the relative transmittance is consistently higher than 3.26% of the non-deformed fibers owing to the enhanced crosstalk of the adjacent optical fibers and even the new imaging unit formed by bonding some core fibers. The aggregation and distribution of such optical fibers at the boundary causes white mesh pattern noise. When De is between 0.3228 and 0.3497, there is no MPN, but the resolution is significantly reduced. The research results provide data support and theoretical guidance for controlling defects such as MPN and transmittance inhomogeneity in IIFOA, thus enabling the improvement of the image quality of IIFOA.

High-Throughput Preparation of Supramolecular Nanostructures on Metal Surfaces.

One of the contemporary challenges in materials science lies in the rapid materials screening and discovery. Experimental sample libraries can be generated by high-throughput parallel synthesis to map the composition space for rapid material discoveries. Molecular self-assembly on surfaces has proved a useful way to construct nanostructures with interesting topologies or properties. Despite the strong dependence of molecular stoichiometry on the structures, high-throughput preparations of supramolecular surface nanostructures have been far less explored. Here, by integrating a physical mask into the standard ultra-high-vacuum (UHV) molecular preparation system we show a high-throughput approach for preparing supramolecular nanostructures of continuous composition spreads on metal surfaces. The spatially addressable sample libraries of supramolecular self-assemblies are characterized by high-resolution scanning probe microscopy. We could explore different binary nanostructures of varying molecular ratios on one single substrate. Moreover, we use the minimum spanning tree approach to qualitatively and quantitatively study the structural properties of the formed nanostructures. This high-throughput approach may accelerate the screening and exploration of surface-supported, low-dimensional nanostructures not limited to supramolecular interactions.

Topological Defects Induced High-Spin Quartet State in Truxene-Based Molecular Graphenoids

Topological Defects Induced High-Spin Quartet State in Truxene-Based Molecular Graphenoids

Deep Bayesian local crystallography

The advent of high-resolution electron and scanning probe microscopy imaging has opened the floodgates for acquiring atomically resolved images of bulk materials, 2D materials, and surfaces. This plethora of data contains an immense volume of information on materials structures, structural distortions, and physical functionalities. Harnessing this knowledge regarding local physical phenomena necessitates the development of the mathematical frameworks for extraction of relevant information. However, the analysis of atomically resolved images is often based on the adaptation of concepts from macroscopic physics, notably translational and point group symmetries and symmetry lowering phenomena. Here, we explore the bottom-up definition of structural units and symmetry in atomically resolved data using a Bayesian framework. We demonstrate the need for a Bayesian definition of symmetry using a simple toy model and demonstrate how this definition can be extended to the experimental data using deep learning networks in a Bayesian setting, namely rotationally invariant variational autoencoders.

Double sample holder for efficient high-resolution studies of an insulator and a metal surface.

A double sample holder supporting both a metal sample and an insulator crystal for high-resolution scanning probe microscopy experiments is described. The metal sample serves as a substrate for tip preparation and tip functionalization to efficiently and reliably enable high-resolution studies of the adjacent insulator surface. Imaging of Ag(111)/mica, Au(111)/mica, CaF2(111), and calcite(104) surfaces is demonstrated at 5K, including images on calcite(104) produced with a CO terminated tip, which was prepared on the adjacent metal sample.

Characterization of carbon materials using high-resolution scanning probe microscopy

Characterization of carbon materials using high-resolution scanning probe microscopy

Applications of high-resolution scanning probe microscopy in hydroprocessing catalysis studies

Scanning probe microscopy offers real space atomic and nanoscale imaging of surfaces and supported nanoparticles, and these techniques are used in a planar catalyst model system context to analyze important atomic-scale aspects of nanoparticle structure, particle edges, defects, vacancies and to image the resulting interaction of individual reactants with the active surfaces. Here we review applications of high-resolution scanning probe microscopy to hydroprocessing catalysis, to demonstrate how these atom-resolved imaging techniques have recently advanced to become tools that elucidate the catalytically active edge structures of metal sulfide hydrotreating catalyst, resolve the intramolecular structure of complex hydrocarbons, and provide atomic-scale information on the mechanisms in which heteroatom-bearing hydrocarbons interact with a catalyst particle. This mini-review addresses the advantages of scanning tunneling microscopy for studies of planar model systems relevant for hydroprocessing catalysis, including the emerging use of near-ambient pressure (operando) scanning tunneling microscopy together with examples of the use of low-temperature molecular imaging with non-contact atomic force microscopy.

Study on a Rapid Imaging Method for Scanning Ion Conductance Microscopy Using a Double-Barreled Theta Pipette.

Scanning ion conductance microscopy (SICM) is a new noncontact, high-resolution scanning probe microscopy technique, which has become increasingly popular in recent years. The hopping mode-currently the most widely used scanning mode-can be used for imaging samples with complicated surface topographies. However, its slow scanning rate seriously restricts its broader application. This paper proposes a fast imaging control mode using a double-barreled theta pipette as the probe, which effectively increases the imaging rate. In this mode, sample surface height information is obtained when the double-barreled theta pipette approaches the sample in a two-step downward process. The ion current sum of two barrels and ion current of one barrel are used as feedback signals to approach the sample until the feedback signals decrease to the set threshold, respectively, thereby obtaining the height of the imaging point. First, this work used COMSOL to establish an SICM model and perform simulation analysis. The simulation results verified the proposed method's feasibility. Second, a scanning time mathematical model was established. The results revealed that the new method is superior to the traditional method in terms of imaging rate. Finally, experiments were performed on poly(dimethylsiloxane) (PDMS) samples using the two imaging modes described above. The results demonstrated that the new scanning mode could significantly improve the imaging rate of SICM without a loss in imaging quality and stability.

On-Surface Synthesis of Unsaturated Carbon Nanostructures with Regularly Fused Pentagon-Heptagon Pairs.

Multiple fused pentagon–heptagon pairs are frequently found as defects at the grain boundaries of the hexagonal graphene lattice and are suggested to have a fundamental influence on graphene-related materials. However, the construction of sp2-carbon skeletons with multiple regularly fused pentagon–heptagon pairs is challenging. In this work, we found that the pentagon–heptagon skeleton of azulene was rearranged during the thermal reaction of an azulene-incorporated organometallic polymer on Au(111). The resulting sp2-carbon frameworks were characterized by high-resolution scanning probe microscopy techniques and feature novel polycyclic architectures composed of multiple regularly fused pentagon–heptagon pairs. Moreover, the calculated analysis of its aromaticity revealed a peculiar polar electronic structure.

Intentional carbon doping reveals CH as an abundant charged impurity in nominally undoped synthetic WS2 and WSe2

Understanding the physical properties and controlling the generation of intrinsic and extrinsic defects is central to the technological adoption of 2D materials in devices. Here we identify a charged carbon-hydrogen complex at a chalcogen site (CHX) as a common, charged impurity in synthetically grown transition metal dichalcogenides (TMDs). This conclusion is drawn by comparing high resolution scanning probe microscopy measurements of nominally undoped and intentionally carbon doped TMD samples. While CH impurity densities in undoped CVD-grown WS2 and MOCVD-grown WSe2 can range anywhere from parts per million to parts per thousand, CH densities in the percentage levels were selectively generated by a post-synthetic methane plasma treatment. Our study indicates that methane plasma treatment is a selective and clean method for the controlled introduction of a charged carbon-hydrogen complex at a surface chalcogen site, a defect that is commonly present in synthetic TMDs.

Tailoring molecular island shapes: influence of microscopic interaction on mesostructure

Controlling the structure formation of molecules on surfaces is fundamental for creating molecular nanostructures with tailored properties and functionalities and relies on tuning the subtle balance between intermolecular and molecule-surface interactions. So far, however, reliable rules of design are largely lacking, preventing the controlled fabrication of self-assembled functional structures on surfaces. In addition, while so far many studies focused on varying the molecular building blocks, the impact of systematically adjusting the underlying substrate has been less frequently addressed. Here, we elucidate the potential of tailoring the mesoscopic island shape by tuning the interactions at the molecular level. As a model system, we have selected the molecule dimolybdenum tetraacetate on three prototypical surfaces, Cu(111), Au(111) and CaF2(111). While providing the same hexagonal geometry, compared to Cu(111), the lattice constants of Au(111) and CaF2(111) differ by a factor of 1.1 and 1.5, respectively. Our high-resolution scanning probe microscopy images reveal molecular-level information on the resulting islands and elucidate the molecular-level design principles for the observed mesoscopic island shapes. Our study demonstrates the capability to tailor the mesoscopic island shape by exclusively tuning the substrate lattice constant, in spite of the very different electronic structure of the substrates involved. This work provides insights for developing general design strategies for controlling molecular mesostructures on surfaces.

Giant thermal expansion of a two-dimensional supramolecular network triggered by alkyl chain motion

Thermal expansion, the response in shape, area or volume of a solid with heat, is usually large in molecular materials compared to their inorganic counterparts. Resulting from the intrinsic molecule flexibility, conformational changes or variable intermolecular interactions, the exact interplay between these mechanisms is however poorly understood down to the molecular level. Here, we investigate the structural variations of a two-dimensional supramolecular network on Au(111) consisting of shape persistent polyphenylene molecules equipped with peripheral dodecyl chains. By comparing high-resolution scanning probe microscopy and molecular dynamics simulations obtained at 5 and 300 K, we determine the thermal expansion coefficient of the assembly of 980 ± 110 × 10−6 K−1, twice larger than other molecular systems hitherto reported in the literature, and two orders of magnitude larger than conventional materials. This giant positive expansion originates from the increased mobility of the dodecyl chains with temperature that determine the intermolecular interactions and the network spacing.