Noncontact Atomic Force Microscopy Research Articles

Stiffness calibration of qPlus sensors at low temperature through thermal noise measurements

Non-contact atomic force microscopy (nc-AFM) offers a unique experimental framework for topographical imaging of surfaces with atomic and/or sub-molecular resolution. The technique also permits to perform frequency shift spectroscopy to quantitatively evaluate the tip–sample interaction forces and potentials above individual atoms or molecules. The stiffness of the probe, k, is then required to perform the frequency shift-to-force conversion. However, this quantity is generally known with little precision. An accurate stiffness calibration is therefore mandatory if accurate force measurements are targeted. In nc-AFM, the probe may either be a silicon cantilever, a quartz tuning fork (QTF), or a length extensional resonator (LER). When used in ultrahigh vacuum (UHV) and at low temperature, the technique mostly employs QTFs, based on the so-called qPlus design, which actually covers different types of sensors in terms of size and design of the electrodes. They all have in common a QTF featuring a metallic tip glued at the free end of one of its prongs. In this study, we report the stiffness calibration of a particular type of qPlus sensor in UHV and at 9.8 K by means of thermal noise measurements. The stiffness calibration of such high-k sensors, featuring high quality factors (Q) as well, requires to master both the acquisition parameters and the data post-processing. Our approach relies both on numerical simulations and experimental results. A thorough analysis of the thermal noise power spectral density of the qPlus fluctuations leads to an estimated stiffness of the first flexural eigenmode of ≃2000 N/m, with a maximum uncertainty of 10%, whereas the static stiffness of the sensor without tip is expected to be ≃3300 N/m. The former value must not be considered as being representative of a generic value for any qPlus, as our study stresses the influence of the tip on the estimated stiffness and points towards the need for the individual calibration of these probes. Although the framework focuses on a particular kind of sensor, it may be adapted to any high-k, high-Q nc-AFM probe used under similar conditions, such as silicon cantilevers and LERs.

N-Heterocyclic Carbene-Derived 1,3,5-Trimethylenebenzene: On-Surface Synthesis and Electronic Structure.

1,3,5-Trimethylenebenzene (1,3,5-TMB), a 3-fold-symmetric triradical with a high-spin ground state, is an attractive platform for investigating the unique spin properties of π-conjugated triangular triradicals. Here, we report the on-surface synthesis of N-heterocyclic carbene (NHC)-derived 1,3,5-TMB (N-TMB) via surface-assisted C-C and C-N coupling reactions on Au(111). The chemical and electronic structures of N-TMB on the Au(111) surface are revealed with atomic precision using scanning tunneling microscopy and noncontact atomic force microscopy, combined with density functional theory (DFT) calculations. It is demonstrated that there is substantial charge transfer between N-TMB and the substrate, resulting in a positively charged N-TMB on Au(111). DFT calculations at the UB3LYP/def2-TZVP level of theory and multireference method, e.g., CASSCF/NEVPT2, indicate that N-TMB possesses a doublet ground state with reduced Cs symmetry in the gas phase, contrasting the quartet ground state of 1,3,5-TMB with D3h symmetry, and exhibits a doublet-quartet energy gap of -0.80 eV. The incorporation of NHC structures and the extended π-conjugation promote the spin-orbital overlaps in N-TMB, leading to Jahn-Teller distortion and the formation of a robust doublet state. Our results not only demonstrate the fabrication of polyradicals based on NHC but also shed light on the effect of NHC and π-conjugation on the electronic structure and spin coupling, which opens up new possibilities for precisely regulating the spin-spin exchange coupling of organic polyradicals.

NH3 adsorption and competition with H2O on a hydroxylated aluminosilicate surface.

The interaction between ammonia (NH3) and (alumino)silicates is of fundamental and applied importance, yet the specifics of NH3 adsorption on silicate surfaces remain largely unexplored, mainly because of experimental challenges related to their electrically insulating nature. An example of this knowledge gap is evident in the context of ice nucleation on silicate dust, wherein the role of NH3 for ice nucleation remains debated. This study explores the fundamentals of the interaction between NH3 and microcline feldspar (KAlSi3O8), a common aluminosilicate with outstanding ice nucleation abilities. Atomically resolved non-contact atomic force microscopy, x-ray photoelectron spectroscopy, and density functional theory-based calculations elucidate the adsorption geometry of NH3 on the lowest-energy surface of microcline, the (001) facet, and its interplay with surface hydroxyls and molecular water. NH3 and H2O are found to adsorb molecularly in the same adsorption sites, creating H-bonds with the proximate surface silanol (Si-OH) and aluminol (Al-OH) groups. Despite the closely matched adsorption energies of the two molecules, NH3 readily yields to replacement by H2O, challenging the notion that ice nucleation on microcline proceeds via the creation of an ordered H2O layer atop pre-adsorbed NH3 molecules.

Clarification of Au dimer adsorption sites on Si (111)-7 × 7 surface by AFM/KPFM

Atomic-scale surface adsorption has been a significant research topic in recent years, with a particular emphasis on the adsorption properties of Au/Si(111)-7 × 7, which are vitally important for pioneering future novel semiconductor devices. Here, we investigated the adsorption of Au dimers on the Si(111)-7 × 7 surface with atomic resolution using non-contact atomic force microscopy and Kelvin probe force microscopy at room temperature. Our results show that the Au dimer adsorbs in the vicinity of the Si rest atoms, exhibiting a distinct localized electron distribution. In density functional theory calculations, three candidate Au dimer adsorption sites have been identified, and the most stable site of Au dimer adsorption aligns with experimental findings. Furthermore, the local electron transfer of Au dimer adsorption has been analyzed, confirming the distribution of electrons around the Au dimer adsorption site. This research reveals that the structure and charge transfer of adsorbed Au dimers on Si(111)-7 × 7 provide insight into the mechanism of the metal-semiconductor system.

Automated Structure Discovery for Scanning Tunneling Microscopy.

Scanning tunneling microscopy (STM) with a functionalized tip apex reveals the geometric and electronic structures of a sample within the same experiment. However, the complex nature of the signal makes images difficult to interpret and has so far limited most research to planar samples with a known chemical composition. Here, we present automated structure discovery for STM (ASD-STM), a machine learning tool for predicting the atomic structure directly from an STM image, by building upon successful methods for structure discovery in noncontact atomic force microscopy (nc-AFM). We apply the method on various organic molecules and achieve good accuracy on structure predictions and chemical identification on a qualitative level while highlighting future development requirements for ASD-STM. This method is directly applicable to experimental STM images of organic molecules, making structure discovery available for a wider scanning probe microscopy audience outside of nc-AFM. This work also allows more advanced machine learning methods to be developed for STM structure discovery.

Scanning Tunneling Spectroscopy Investigation of Au-bis-acetylide Networks on Au(111): The Influence of Metal-Organic Hybridization.

Graphdiyne (GDY) is an appealing two-dimensional carbon material, but the on-surface synthesis of a single layer remains challenging. Demetalation of well-crystalline metal acetylide networks, though in its infancy, provides a new avenue to on-surface synthesized GDY substructures. In spite of the synthetic efforts and theoretical concerns, there are few reports steeped in elaborate characterization of the electronic influence of metalation. In this context, we focused on the surface supported Au-bis-acetylide network, which underwent demetalation after further annealing to form hydrogen-substituted GDY. We made a comprehensive study on the geometric structure and electronic structure and the corresponding demetalized structure on Au(111) through STM, noncontact atomic force microscopy (nc-AFM), scanning tunneling spectroscopy (STS), and density functional theory (DFT) simulations. The bandgap of the Au-bis-acetylide network on Au(111) is measured to be 2.7 eV, while the bandgap of a fully demetalized Au-bis-acetylide network is estimated to be about 4.1 eV. Our findings reveal that the intercalated Au adatoms are positioned closer to the metal surface compared with the organic skeletons, facilitating electronic hybridization between the surface state and unoccupied frontier molecular orbitals of organic components. This leads to an extended conjugation through Au-bis-acetylene bonds, resulting in a reduced bandgap.

Topology selectivity of a conformationally flexible precursor through selenium doping

Conformational arrangements within nanostructures play a crucial role in shaping the overall configuration and determining the properties, for example in covalent/metal organic frameworks. In on-surface synthesis, conformational diversity often leads to uncontrollable or disordered structures. Therefore, the exploration of controlling and directing the conformational arrangements is significant in achieving desired nanoarchitectures. Herein, a conformationally flexible precursor 2,4,6-tris(3-bromophenyl)−1,3,5-triazine is employed, and a random phase consisting of C3h and Cs conformers is firstly obtained after deposition of the precursor on Cu(111) at room temperature to 365 K. At low coverage (0.01 ML) selenium doping, we achieve the selectivity of the C3h conformer and improve the nanopore structural homogeneity. The ordered two-dimensional metal organic nanostructure can be fulfilled by selenium doping from room temperature to 365 K. The formation of the conformationally flexible precursor on Cu(111) is explored through the combination of high-resolution scanning tunneling microscopy and non-contact atomic force microscopy. The regulation of energy diagrams in the absence or presence of the Se atom is revealed by density functional theory calculations. These results can enrich the on-surface synthesis toolbox of conformationally flexible precursors, for the design of complex nanoarchitectures, and for future development of engineered nanomaterials.

Unraveling Enyne Bonding via Dehydrogenation–Hydrogenation Processes in On‐Surface Synthesis with Terminal Alkynes

AbstractOn‐surface reactions of terminal alkynes in ultrahigh vacuum have attracted widespread attention due to their high technological promise. However, employing different precursors and substrate materials often intricate reaction schemes appear far from being well‐understood. Thus, recent investigations of alkyne coupling on noble metal surfaces suggest non‐dehydrogenative scenarios, contradicting earlier reports. Herein, the study employs noncontact atomic force microscopy (nc‐AFM) with high spatial resolution to conclusively characterize exemplary alkyne coupling products. Contrary to initial interpretations proposing dehydrogenative homocoupling on Ag(111), bond‐resolved AFM imaging reveals the expression of enyne motifs. Based on complementary, extensive density functional theory calculations, the pertaining reaction mechanisms are explored. It is proposed that enyne formation initiates with a direct carbon–carbon coupling between two alkyne groups, followed by surface‐assisted dehydrogenation‐hydrogenation processes. Thereby consecutive steps of atomic hydrogen cleavage, surface migration and recombination to a different carbon atom enable bridging via carbon–carbon double bonding. The new results shed light on subtle, but crucial surface‐mediated hydrogen transfer processes involved in the chemical bond formation, which are suggested to be of general relevance in on‐surface synthesis.

Investigation of microstructural, micromorphology, and surface plasmon resonance characteristics in Ni/Al, Ni/Cu, and Ni/SS thin films.

As the first boundary between the environment and the material, the surface plays an important role in their interaction with each other, therefore, the use of appropriate tools and analysis to examine the mechanical properties and morphology of surfaces has particular importance in industry and research. In this research, a thin film of nickel was deposited on metal substrates made of aluminum, copper, and steel by using the RF magnetic cathode. Then, using a non-contact atomic force microscope, the morphological properties of the nickel film with static parameters, Minkowski functionals (MF's), fractal, and multifractal were extracted to be analyzed and studied. After that, using parameters such as root mean square (RMS) roughness, skewness, and kurtosis, it was determined how the surface roughness, distribution, and probability density of particles on the film surface alters with the change of the substrate. Next, by examining and analyzing the Δα and Δf parameters obtained from the multifractal section, the morphology of the produced film on the metal substrates was investigated. Then, the change in the surface plasmon resonance (SPR) peak position is changed for the prepared film in the range of the absorption spectrum due to the substrate effect and the microstructural properties of the formed film. HIGHLIGHTS: Ni film has been deposited by Rf magnetron sputtering. The effect of metal substrates on the topography, fractality, and optical properties was studied. Minkowski functionals were used to investigate the surface morphology of the samples. Substrate's material and the topography of the formed film can changed the surface plasmon resonance position.

Artificial kagome lattices of Shockley surface states patterned by halogen hydrogen-bonded organic frameworks

Artificial electronic kagome lattices may emerge from electronic potential landscapes using customized structures with exotic supersymmetries, benefiting from the confinement of Shockley surface-state electrons on coinage metals, which offers a flexible approach to realizing intriguing quantum phases of matter that are highly desired but scarce in available kagome materials. Here, we devise a general strategy to construct varieties of electronic kagome lattices by utilizing the on-surface synthesis of halogen hydrogen-bonded organic frameworks (XHOFs). As a proof of concept, we demonstrate three XHOFs on Ag(111) and Au(111) surfaces, which correspondingly deliver regular, breathing, and chiral breathing diatomic-kagome lattices with patterned potential landscapes, showing evident topological edge states at the interfaces. The combination of scanning tunnelling microscopy and noncontact atomic force microscopy, complemented by density functional theory and tight-binding calculations, directly substantiates our method as a reliable and effective way to achieve electronic kagome lattices for engineering quantum states.

Charge state-dependent symmetry breaking of atomic defects in transition metal dichalcogenides

The functionality of atomic quantum emitters is intrinsically linked to their host lattice coordination. Structural distortions that spontaneously break the lattice symmetry strongly impact their optical emission properties and spin-photon interface. Here we report on the direct imaging of charge state-dependent symmetry breaking of two prototypical atomic quantum emitters in mono- and bilayer MoS2 by scanning tunneling microscopy (STM) and non-contact atomic force microscopy (nc-AFM). By changing the built-in substrate chemical potential, different charge states of sulfur vacancies (VacS) and substitutional rhenium dopants (ReMo) can be stabilized. VacS−1\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$${}_{{{{{{{{\\rm{S}}}}}}}}}^{-1}$$\\end{document} as well as ReMo0\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$${}_{{{{{{{{\\rm{Mo}}}}}}}}}^{0}$$\\end{document} and ReMo−1\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$${}_{{{{{{{{\\rm{Mo}}}}}}}}}^{-1}$$\\end{document} exhibit local lattice distortions and symmetry-broken defect orbitals attributed to a Jahn-Teller effect (JTE) and pseudo-JTE, respectively. By mapping the electronic and geometric structure of single point defects, we disentangle the effects of spatial averaging, charge multistability, configurational dynamics, and external perturbations that often mask the presence of local symmetry breaking.

Topological Structure Realized in Cove-Edged Graphene Nanoribbons via Incorporation of Periodic Pentagon Rings.

Cove-edged zigzag graphene nanoribbons are predicted to show metallic, topological, or trivial semiconducting band structures, which are precisely determined by their cove offset positions at both edges as well as the ribbon width. However, due to the challenge of introducing coves into zigzag-edged graphene nanoribbons, only a few cove-edged graphene nanoribbons with trivial semiconducting bandgaps have been realized experimentally. Here, we report that the topological band structure can be realized in cove-edged graphene nanoribbons by embedding periodic pentagon rings on the cove edges through on-surface synthesis. Upon noncontact atomic force microscopy and scanning tunneling spectroscopy measurements, the chemical and electronic structures of cove-edged graphene nanoribbons with periodic pentagon rings have been characterized for different lengths. Combined with theoretical calculations, we find that upon inducing periodic pentagon rings the cove-edged graphene nanoribbons exhibit nontrivial topological structures. Our results provide insights for the design and understanding of the topological character in cove-edged graphene nanoribbons.

Phase-selective in-plane heteroepitaxial growth of H-phase CrSe2

Phase engineering of two-dimensional transition metal dichalcogenides (2D-TMDs) offers opportunities for exploring unique phase-specific properties and achieving new desired functionalities. Here, we report a phase-selective in-plane heteroepitaxial method to grow semiconducting H-phase CrSe2. The lattice-matched MoSe2 nanoribbons are utilized as the in-plane heteroepitaxial template to seed the growth of H-phase CrSe2 with the formation of MoSe2-CrSe2 heterostructures. Scanning tunneling microscopy and non-contact atomic force microscopy studies reveal the atomically sharp heterostructure interfaces and the characteristic defects of mirror twin boundaries emerging in the H-phase CrSe2 monolayers. The type-I straddling band alignments with band bending at the heterostructure interfaces are directly visualized with atomic precision. The mirror twin boundaries in the H-phase CrSe2 exhibit the Tomonaga-Luttinger liquid behavior in the confined one-dimensional electronic system. Our work provides a promising strategy for phase engineering of 2D TMDs, thereby promoting the property research and device applications of specific phases.

On-Surface Synthesis of Anthracene-Fused Zigzag Graphene Nanoribbons from 2,7-Dibromo-9,9'-bianthryl Reveals Unexpected Ring Rearrangements.

On-surface synthesis has emerged as a powerful strategy to fabricate unprecedented forms of atomically precise graphene nanoribbons (GNRs). However, the on-surface synthesis of zigzag GNRs (ZGNR) has met with only limited success. Herein, we report the synthesis and on-surface reactions of 2,7-dibromo-9,9'-bianthryl as the precursor toward π-extended ZGNRs. Characterization by scanning tunneling microscopy and high-resolution noncontact atomic force microscopy clearly demonstrated the formation of anthracene-fused ZGNRs. Unique skeletal rearrangements were also observed, which could be explained by intramolecular Diels-Alder cycloaddition. Theoretical calculations of the electronic properties of the anthracene-fused ZGNRs revealed spin-polarized edge-states and a narrow bandgap of 0.20 eV.

Determination of in-plane surface directions in scanning probe microscopy images.

We describe an approach to determine the in-plane crystallographic surface directions in scanning probe microscopy (SPM) images. This method is based on a one-time characterization of the SPM instrument with an appropriate test sample and is exemplified by the analysis of non-contact atomic force microscopy (NC-AFM) images on surfaces whose natural cleavage occurs along {111} planes. We introduce a two-dimensional rotation matrix relating the crystallographic surface directions known from an analysis of the macroscopic crystal to the directions in the NC-AFM images. The procedure takes into account rotations and mirror axes resulting from sample mounting, the SPM scanner rotation, the choice of scan direction, as well as data processing, storage, and display. We demonstrate the practicability of the approach by determining the [112̄] direction in topographic images of a CeO2(111) film grown on a Si(111) wafer and atomic resolution images of CaF2(111) with an instrument based on the beetle-type scanner.

How Precisely Can Individual Molecules Be Analyzed? A Case Study on Locally Quantifying Forces and Energies Using Scanning Probe Microscopy.

Recent advances in scanning probe microscopy methodology have enabled the measurement of tip-sample interactions with picometer accuracy in all three spatial dimensions, thereby providing a detailed site-specific and distance-dependent picture of the related properties. This paper explores the degree of detail and accuracy that can be achieved in locally quantifying probe-molecule interaction forces and energies for adsorbed molecules. Toward this end, cobalt phthalocyanine (CoPc), a promising CO2 reduction catalyst, was studied on Ag(111) as a model system using low-temperature, ultrahigh vacuum noncontact atomic force microscopy. Data were recorded as a function of distance from the surface, from which detailed three-dimensional maps of the molecule's interaction with the tip for normal and lateral forces as well as the tip-molecule interaction potential were constructed. The data were collected with a CO molecule at the tip apex, which enabled a detailed visualization of the atomic structure. Determination of the tip-substrate interaction as a function of distance allowed isolation of the molecule-tip interactions; when analyzing these in terms of a Lennard-Jones-type potential, the atomically resolved equilibrium interaction energies between the CO tethered to the tip and the CoPc molecule could be recovered. Interaction energies peaked at less than 160 meV, indicating a physisorption interaction. As expected, the interaction was weakest at the aromatic hydrogens around the periphery of the molecule and strongest surrounding the metal center. The interaction, however, did not peak directly above the Co atom but rather in pockets surrounding it.

Dramatic Acceleration of the Hopf Cyclization on Gold(111): From Enediynes to Peri-Fused Diindenochrysene Graphene Nanoribbons.

Hopf et al. reported the high-temperature 6π-electrocyclization of cis-hexa-1,3-diene-5-yne to benzene in 1969. Subsequent studies using this cyclization have been limited by its very high reaction barrier. Here, we show that the reaction barrier for two model systems, (E)-1,3,4,6-tetraphenyl-3-hexene-1,5-diyne (1a) and (E)-3,4-bis(4-iodophenyl)-1,6-diphenyl-3-hexene-1,5-diyne (1b), is decreased by nearly half on a Au(111) surface. We have used scanning tunneling microscopy (STM) and noncontact atomic force microscopy (nc-AFM) to monitor the Hopf cyclization of enediynes 1a,b on Au(111). Enediyne 1a undergoes two sequential, quantitative Hopf cyclizations, first to naphthalene derivative 2, and finally to chrysene 3. Density functional theory (DFT) calculations reveal that a gold atom from the Au(111) surface is involved in all steps of this reaction and that it is crucial to lowering the reaction barrier. Our findings have important implications for the synthesis of novel graphene nanoribbons. Ullmann-like coupling of enediyne 1b at 20 °C on Au(111), followed by a series of Hopf cyclizations and aromatization reactions at higher temperatures, produces nanoribbons 12 and 13. These results show for the first time that graphene nanoribbons can be synthesized on a Au(111) surface using the Hopf cyclization mechanism.

Precise determination of molecular adsorption geometries by room temperature non-contact atomic force microscopy

High resolution force measurements of molecules on surfaces, in non-contact atomic force microscopy, are often only performed at cryogenic temperatures, due to needing a highly stable system, and a passivated probe tip (typically via CO-functionalisation). Here we show a reliable protocol for acquiring three-dimensional force map data over both single organic molecules and assembled islands of molecules, at room temperature. Isolated cobalt phthalocyanine and islands of C60 are characterised with submolecular resolution, on a passivated silicon substrate (B:Si(111)-(sqrt{3}times sqrt{3})R3{0}^{circ }). Geometries of cobalt phthalocyanine are determined to a ~ 10 pm accuracy. For the C60, the protocol is sufficiently robust that areas spanning 10 nm × 10 nm are mapped, despite the difficulties of room temperature operation. These results provide a proof-of-concept for gathering high-resolution three-dimensional force maps of networks of complex, non-planar molecules on surfaces, in conditions more analogous to real-world application.

Water adsorption lifts the (2 × 1) reconstruction of calcite(104).

The adsorption of water on calcite(104) is investigated in ultra-high vacuum by density functional theory (DFT) and non-contact atomic force microscopy (NC-AFM) in the coverage regime of up to one monolayer (ML). DFT calculations reveal a clear preference for water to adsorb on the bulk-like carbonate group rows of the (2 × 1) reconstructed surface. Additionally, an apparent water attraction due to carbonate group reorientation suggest island formation for water adsorbed on the reconstructed carbonate group rows. Experimentally, water is found to exclusively occupy specific positions within the (2 × 1) unit cell up to 0.5 ML, to form islands at coverage between 0.5 and 1 ML, and to express a (1 × 1) structure at coverage of a full monolayer.

On surface synthesis of an eleven-ring sulfur-doped nonacene.

Dithienoacenes with a heptacene core, heptaceno[2,3-b:11,12-b']bis[1]benzothiophene, have been synthesized through the combination of solution and surface assisted chemistry. The atomic composition, structural arrangement and electronic properties of the molecules on the Au(111) surface have been deeply explored by non-contact atomic force microscopy (nc-AFM), bond-resolved scanning tunnelling microscopy (BR-STM) and scanning tunneling spectroscopy (STS) corroborated by density functional theory (DFT) calculations. Our combined experiments reveal modifications induced by sulfur substitution.