Selective Formation of Homochiral Dimers by Intermolecular Charge Transfer on a hBN Nanomesh

In this study, a comprehensive characterization was conducted on a chiral starburst molecule (C57H48N4, SBM) using scanning tunneling microscopy. When adsorbed onto the hBN/Rh(111) nanomesh, these molecules demonstrate homochiral recognition, leading to a selective formation of homochiral dimers. Further tip manipulation experiments reveal that the chiral dimers are stable and primarily controlled by strong intermolecular interactions. Density functional theory (DFT) calculations supported that the chiral recognition of SBM molecules is governed by the intermolecular charge transfer mechanism, different from the common steric hindrance effect. This study emphasizes the importance of intermolecular charge transfer interactions, offering valuable insights into the chiral recognition of a simple bimolecular system. These findings hold significance for the future advancement in chirality-based electronic sensors and pharmaceuticals, where the chirality of molecules can impact their properties.

Stripe-Like hBN Monolayer Template for Self-Assembly and Alignment of Pentacene Molecules

Metallic surfaces with unidirectional anisotropy are often used to guide the self-assembly of organic molecules along a particular direction. Such supports thus offer an avenue for the fabrication of hybrid organic-metal interfaces with tailored morphology and precise elemental composition. Nonetheless, such control often comes at the expense of detrimental interfacial interactions that might quench the pristine properties of molecules. Here, hexagonal boron nitride grown on Ir(100) is introduced as a robust platform with several coexisting 1D stripe-like moiré superstructures that effectively guide unidirectional self-assemblies of pentacene molecules, concomitantly preserving their pristine electronic properties. In particular, highly-aligned longitudinal arrays of equally-oriented molecules are formed along two perpendicular directions, as demonstrated by comprehensive scanning tunneling microscopy and photoemission characterization performed at the local and non-local scale, respectively. The functionality of the template is demonstrated by photoemission tomography, a surface-averaging technique requiring a high degree of orientational order of the probed molecules. The successful identification of pentacene's pristine frontier orbitals underlines that the template induces excellent long-range molecular ordering via weak interactions, preventing charge transfer.

Spectroscopic Characterization of Thiacarbocyanine Dye Molecules Adsorbed on Hexagonal Boron Nitride: a Time-Resolved Study

Physisorption on hexagonal boron nitride (hBN) gained interest over the years thanks to its properties (chemically and thermally stable, insulating properties, etc.) and similarities to the well-known graphene. A recent study showed flat-on adsorption of several cationic thiacarbocyanine dyes on hBN with a tendency to form weakly coupled H- or I-type aggregates, while a zwitterionic thiacarbocyanine dye rather led to a tilted adsorption. With this in-depth time-resolved study using the TC-SPC technique, we confirm the results proven by adsorption isotherms, atomic force microscopy, and stationary state spectroscopy combined with molecular mechanics simulations and estimation of the corresponding exciton interaction. The absence of a systematic trend for the dependence of the decay times, normalized amplitudes of the decay components, and contribution of different components to the stationary emission spectra upon the emission wavelength observed for all studied dyes and coverages suggests the occurrence of a single emitting species. At low coverage levels, the non-mono-exponential character of the decays was attributed to adsorption on different sites characterized by different intramolecular rotational freedom or energy transfer to nonfluorescent traps or a combination of both. The difference between the decay rates of the four dyes reflects a different density of the nonfluorescent traps. Although the decay time of the unquenched dyes was in the order of magnitude of that of dye monomers in a rigid environment, it is also compatible with weakly coupled aggregates such as proposed earlier based on the stationary spectra. Hence, the adsorption leads to a rigid environment of the dyes, blocking internal conversion. Increasing the concentration of the dye solution from which the adsorption on hBN occurs increases not only the coverage of the hBN surface but also the extent of energy transfer to nonfluorescent traps. For TDC (5,5-dichloro-3-3'-diethyl-9-ethyl-thiacarbocyanine) and TD2 (3-3'-diethyl-9-ethyl-thiacarbocyanine), besides direct energy transfer to traps, exciton hopping between dye dimers followed by energy transfer to these traps occurs, which resulted in a decreasing decay time of the longest decaying component. For all dyes, it was also possible to analyze the fluorescence decays as a stretched exponential as would be expected for energy transfer to randomly distributed traps in a two-dimensional (2D) geometry. This analysis yielded a fluorescence decay time of the unquenched dyes similar to the longest decay time obtained by analysis of the fluorescence decays as a sum of three of four exponentials.

Formation of an Extended Quantum Dot Array Driven and Autoprotected by an Atom-Thick h-BN Layer

Engineering quantum phenomena of two-dimensional nearly free electron states has been at the forefront of nanoscience studies ever since the first creation of a quantum corral. Common strategies to fabricate confining nanoarchitectures rely on manipulation or on applying supramolecular chemistry principles. The resulting nanostructures do not protect the engineered electronic states against external influences, hampering the potential for future applications. These restrictions could be overcome by passivating the nanostructures with a chemically inert layer. To this end we report a scalable segregation-based growth approach forming extended quasi-hexagonal nanoporous CuS networks on Cu(111) whose assembly is driven by an autoprotecting h-BN overlayer. We further demonstrate that by this architecture both the Cu(111) surface state and image potential states of the h-BN/CuS heterostructure are confined within the nanopores, effectively forming an extended array of quantum dots. Semiempirical electron-plane-wave-expansion simulations shed light on the scattering potential landscape responsible for the modulation of the electronic properties. The protective properties of the h-BN capping are tested under various conditions, representing an important step toward the realization of robust surface state based electronic devices.

Ullmann-Like Covalent Bond Coupling without Participation of Metal Atoms

Ullmann-like on-surface synthesis is one of the most appropriate approaches for the bottom-up fabrication of covalent organic nanostructures and many successes have been achieved. The Ullmann reaction requires the oxidative addition of a catalyst (a metal atom in most cases): the metal atom will insert into a carbon-halogen bond, forming organometallic intermediates, which are then reductively eliminated and form C-C covalent bonds. As a result, traditional Ullmann coupling involves reactions of multiple steps, making it difficult to control the final product. Moreover, forming the organometallic intermediates will potentially poison the metal surface catalytic reactivity. In the study, we used the 2D hBN, an atomically thin sp²-hybridized sheet with a large band gap, to protect the Rh(111) metal surface. It is an ideal 2D platform to decouple the molecular precursor from the Rh(111) surface while maintaining the reactivity of Rh(111). We realize an Ullmann-like coupling of a planar biphenylene-based molecule, i.e., 1,8-dibromobiphenylene (BPBr₂), on an hBN/Rh(111) surface with an ultrahigh selectivity of the biphenylene dimer product, containing 4-, 6-, and 8-membered rings. The reaction mechanism, including electron wave penetration and the template effect of the hBN, is elucidated by combining low-temperature scanning tunneling microscopy and density functional theory calculations. Our findings are expected to play an essential role regarding the high-yield fabrication of functional nanostructures for future information devices.

Role of Interfacial Water in the Tribological Behavior of Graphene in an Electric Field

Friction properties in the electric field are important for the application of graphene as a solid lubricant in graphene-based micro/nanoelectromechanical systems. The studies based on conductive atomic force microscopy show that interfacial water between graphene and the SiO₂/Si substrate affects the friction of graphene in the electric field. Friction without applying voltage remains low because the interfacial water retains a stable ice-like network. However, friction after applying voltage increases because the polar water molecules are attracted by the electric field and gather around the tip. The gathered interfacial water not only increases the deformation of graphene but is also pushed by the tip during frictional sliding, which results in the increased friction. These studies provide beneficial guidelines for the applications of graphene as a solid lubricant in the electric field.

Two-Dimensional Ultrathin Silica Films

Two-dimensional (2D) ultrathin silica films have the potential to reach technological importance in electronics and catalysis. Several well-defined 2D-silica structures have been synthesized so far. The silica bilayer represents a 2D material with SiO₂ stoichiometry. It consists of precisely two layers of tetrahedral [SiO₄] building blocks, corner connected via oxygen bridges, thus forming a self-saturated silicon dioxide sheet with a thickness of ∼0.5 nm. Inspired by recent successful preparations and characterizations of these 2D-silica model systems, scientists now can forge novel concepts for realistic systems, particularly by atomic-scale studies with the most powerful and advanced surface science techniques and density functional theory calculations. This Review provides a solid introduction to these recent developments, breakthroughs, and implications on ultrathin 2D-silica films, including their atomic/electronic structures, chemical modifications, atom/molecule adsorptions, and catalytic reactivity properties, which can help to stimulate further investigations and understandings of these fundamentally important 2D materials.

Metal dichalcogenide nanomeshes: structural, electronic and magnetic properties

Motivated by the successful preparation of two-dimensional transition metal dichalcogenide (2D-TMD) nanomeshes in the last three years, we use density functional theory (DFT) to study the structural stability, mechanical, magnetic, and electronic properties of porous 2H-MoX₂ (X = S, Se and Te) without and with pore passivation. We consider structures with multiple, systematically created pores. The molecular dynamics simulations and cohesive energy calculations showed the stability of the 2D-TMD nanomeshes, with larger stability for those with smaller pores. The lattice undergoes some deformations to accommodate the pore energetically, and as the pore size increases Young's modulus decreases. In most cases, the missing metal atoms disrupt the spin states' even population, resulting in some nanomeshes becoming magnetic. The electronic gaps of the MoX₂ nanomesh systems are diminished because of the emergence of pore-edge localized mid-gap metal 4d states in the spin-polarized spectrum, making some systems half-metallic. The oxygen passivation of the pore edges of 2D-TMD nanomeshes restores the even population of spin states, and makes those systems metallic. Our results can be used in different applications such as spintronics, ion chelation, and molecular sensing applications.

Epitaxial Growth of Two-Dimensional Insulator Monolayer Honeycomb BeO

The emergence of two-dimensional (2D) materials launched a fascinating frontier of flatland electronics. Most crystalline atomic layer materials are based on layered van der Waals materials with weak interlayer bonding, which naturally leads to thermodynamically stable monolayers. We report the synthesis of a 2D insulator composed of a single atomic sheet of honeycomb structure BeO (h-BeO), although its bulk counterpart has a wurtzite structure. The h-BeO is grown by molecular beam epitaxy (MBE) on Ag(111) thin films that are also epitaxially grown on Si(111) wafers. Using scanning tunneling microscopy and spectroscopy (STM/S), the honeycomb BeO lattice constant is determined to be 2.65 Å with an insulating band gap of 6 eV. Our low-energy electron diffraction measurements indicate that the h-BeO forms a continuous layer with good crystallinity at the millimeter scale. Moiré pattern analysis shows the BeO honeycomb structure maintains long-range phase coherence in atomic registry even across Ag steps. We find that the interaction between the h-BeO layer and the Ag(111) substrate is weak by using STS and complementary density functional theory calculations. We not only demonstrate the feasibility of growing h-BeO monolayers by MBE, but also illustrate that the large-scale growth, weak substrate interactions, and long-range crystallinity make h-BeO an attractive candidate for future technological applications. More significantly, the ability to create a stable single-crystalline atomic sheet without a bulk layered counterpart is an intriguing approach to tailoring 2D electronic materials.

High-Quality Hexagonal Boron Nitride from 2D Distillation

The production of high-quality two-dimensional (2D) materials is essential for the ultimate performance of single layers and their hybrids. Hexagonal boron nitride (h-BN) is foreseen to become the key 2D hybrid and packaging material since it is insulating, impermeable, flat, transparent, and chemically inert, though it is difficult to attain in ultimate quality. Here, a scheme is reported for producing single layer h-BN that shows higher quality in view of mosaicity and strain variations than material from chemical vapor deposition (CVD). We delaminate CVD h-BN from Rh(111) and transfer it to a clean metal surface. The twisting angle between BN and the second substrate yields metastable moiré structures. Annealing above 1000 K leads to 2D distillation, i.e., catalyst-assisted BN sublimation from the edges of the transferred layer and subsequent condensation into superior quality h-BN. This provides a way for 2D material production remote from CVD instrumentation.

Adsorption of Azobenzene on Hexagonal Boron Nitride Nanomesh Supported by Rh(111)

Adsorption properties of azobenzene, the prototypical molecular switch, were investigated on a hexagonal boron nitride (h-BN) monolayer (“nanomesh”) prepared on Rh(111). The h-BN layer was produced by decomposing borazine (B₃N₃H₆) at 1000–1050 K. Temperature-programmed desorption (TPD) studies revealed that azobenzene molecules adsorbed on the “wire” and “pore” regions desorb at slightly different temperatures. Angle-resolved high-resolution electron energy loss spectroscopy (HREELS) measurements demonstrated that the first molecular layer is characterized predominantly by an adsorption geometry with the molecular plane parallel to the surface. Scanning tunneling microscopy (STM) indicated a clear preference for adsorption in the pores, manifesting a templating effect, but in some cases one-dimensional molecular stripes also form, implying attractive molecule–molecule interaction. Density functional theory (DFT) calculations provided further details regarding the adsorption energetics and bonding and confirmed the experimental findings that the molecules adsorb with the phenyl rings parallel to the surface, preferentially in the pores, and indicated also the presence of an attractive molecule–molecule interaction.

Artificial relativistic molecules

We fabricate artificial molecules composed of heavy atom lead on a van der Waals crystal. Pb atoms templated on a honeycomb charge-order superstructure of IrTe₂ form clusters ranging from dimers to heptamers including benzene-shaped ring hexamers. Tunneling spectroscopy and electronic structure calculations reveal the formation of unusual relativistic molecular orbitals within the clusters. The spin–orbit coupling is essential both in forming such Dirac electronic states and stabilizing the artificial molecules by reducing the adatom–substrate interaction. Lead atoms are found to be ideally suited for a maximized relativistic effect. This work initiates the use of novel two-dimensional orderings to guide the fabrication of artificial molecules of unprecedented properties.

Artificial molecules supported on templated surfaces attract enormous interest due to their tunable electronic properties. Here the authors use STM experiments and DFT calculations to show the formation of Pb artificial clusters on a IrTe₂ honeycomb template that are maximally stabilized by relativistic effects.

Imaging and Dynamics of Water Hexamer Confined in Nanopores

Epitaxial two-dimensional (2D) nanostructures with regular patterns show great promise as templates for adsorbate confinement. Prospectively, employing 2D semiconductors with reduced density of states leads to a long excited-state lifetime that allows us to directly image the dynamics of the adsorbate. We show that epitaxial blue phosphorene (blueP) on Au(111) provides such a platform to trap water molecules in the periodic nanopores without formation of strong bonds. The trapped water aggregate is tentatively assigned to a hexamer based on our scanning tunneling microscopy studies and first-principles calculations. Real-space observation of conformational switching of the hexamer induced by inelastic electrons is achieved by using low-temperature scanning tunneling microscopy with molecular resolution. We found a localized interfacial charge rearrangement between the water hexamer and P atoms underneath that is responsible for the reversible desorption and adsorption of water molecules by changing the sample bias polarity from positive to negative, offering a promising strategy for engineering the electronic properties of blueP.

Pore-ridge nanostructures on the surface of trichoid sensilla of the male silkmoth Bombyx mori: Aerodynamic trapping and transporting of the pheromone molecules

This paper tries to reveal the mechanism of the high-efficient adsorption of the sex pheromone by the trichoid sensilla of the male silk moth Bombyx mori. Scanning electron microscopy (SEM) and atomic force microscopy (AFM) were used to acquire the topographies and nanostructures of the surfaces of the trichoid sensilla. SEM and AFM images present mostly regular pore-ridge nanostructures on the sensilla, and all the pores are located at or near the feet of the ridges. AFM phase-shift images demonstrate that the variation of phase-shift, which appears along the ridge cannot simply be attributed to heterogeneity in surface lipid properties, for the phase-shift was present in the same region with the sudden difference in height. Simulations of computational fluid dynamics were applied to investigate the effects on the airflow velocity field and streamlines by the pore-ridge nanostructures and the antenna vibration. Simulation results indicate that the airflow vortexes that form on the sensillum surface are generated by the combined effect of ambient airflow and pore-ridge structure as well as spontaneous vibration of the antenna. We suggest that the vortex intercepts and traps the pheromone molecules passing nearby, and transports them through its periodical movement to the pore. We speculate that the vortex is the aerodynamic factor benefitting the highly efficient adsorption of pheromone molecules.

Watching nanostructure growth: kinetically controlled diffusion and condensation of Xe in a surface metal organic network

Diffusion, nucleation and growth provide the fundamental access to control nanostructure growth. In this study, the temperature activated diffusion of Xe at and between different compartments of an on-surface metal organic coordination network on Cu(111) has been visualized in real space. Xe atoms adsorbed at lower energy sites become mobile with increased temperature and gradually populate energetically more favourable binding sites or remain in a delocalized 'fluid' form confined to diffusion along a topological subset of the on-surface network. These diffusion pathways can be studied individually under kinetic control via the chosen thermal energy kT of the sample and are determined by the network and sample architecture. The spatial distribution of Xe in its different modes of mobility and the time scales of the motion is revealed by Scanning Tunneling Microscopy (STM) at variable temperatures up to 40 K and subsequent cooling to 4 K. The system provides insight into the diffusion of a van der Waals gas on a complex structured surface and its nucleation and coarsening/growth into larger condensates at elevated temperature under thermodynamic conditions.

Tailoring Intercalant Assemblies at the Graphene-Metal Interface

The influence of graphene on the assembly of intercalated material is studied using low-temperature scanning tunneling microscopy. Intercalation of Pt under monolayer graphene on Pt(111) induces a substrate reconstruction that is qualitatively different from the lattice rearrangement induced by metal deposition on Pt(111) and, specifically, the homoepitaxy of Pt. Alkali metals Cs and Li are used as intercalants for monolayer and bilayer graphene on Ru(0001). Atomically resolved topographic data reveal that at elevated alkali metal coverage (2 × 2)Cs and (1 × 1)Li intercalant structures form with respect to the graphene lattice.

Phase Transitions in Confinements: Controlling Solid to Fluid Transitions of Xenon Atoms in an On-Surface Network

This study reports on "phase" transitions of Xe condensates in on-surface confinements induced by temperature changes and local probe excitation. The pores of a metal-organic network occupied with 1 up to 9 Xe atoms are investigated in their propensity to undergo "condensed solid" to "confined fluid" transitions. Different transition temperatures are identified, which depend on the number of Xe atoms in the condensate and relate to the stability of the Xe clustering in the condensed "phase." This work reveals the feature-rich behavior of transitions of confined planar condensates, which provide a showcase toward future "phase-transition" storage media patterned by self-assembly. This work is also of fundamental interest as it paves the way to real space investigations of reversible solid to fluid transitions of magic cluster condensates in an array of extremely well-defined quantum confinements.

Tuning Band Gap and Work Function Modulations in Monolayer hBN/Cu(111) Heterostructures with Moiré Patterns

The moiré pattern formed between a two-dimensional (2D) material and the substrate has played a crucial role in tuning the electronic structure of the 2D material. Here, by using scanning tunneling microscopy and spectroscopy, we found a moiré-pattern-dependent band gap and work function modulation in hexagonal boron nitride (hBN)/Cu(111) heterostructures, whose amplitudes increase with the moiré pattern wavelength. Moreover, the work function modulation shifts agree well with the conduction band edge shifts, indicating a spatially constant electron affinity for the hBN layer. Density functional theory calculations showed that these observations in hBN/Cu(111) heterostructures mainly originated from the hybridization of the N 3p _z orbital and Cu 4s orbital in different atomic configurations. Our results show that the twist-angle dependence of moiré patterns in hBN/Cu(111) heterostructures can be used to tailor the electronic properties including band gap and work function.

Supramolecular Assemblies on Surfaces: Nanopatterning, Functionality, and Reactivity

Understanding how molecules interact to form large-scale hierarchical structures on surfaces holds promise for building designer nanoscale constructs with defined chemical and physical properties. Here, we describe early advances in this field and highlight upcoming opportunities and challenges. Both direct intermolecular interactions and those that are mediated by coordinated metal centers or substrates are discussed. These interactions can be additive, but they can also interfere with each other, leading to new assemblies in which electrical potentials vary at distances much larger than those of typical chemical interactions. Earlier spectroscopic and surface measurements have provided partial information on such interfacial effects. In the interim, scanning probe microscopies have assumed defining roles in the field of molecular organization on surfaces, delivering deeper understanding of interactions, structures, and local potentials. Self-assembly is a key strategy to form extended structures on surfaces, advancing nanolithography into the chemical dimension and providing simultaneous control at multiple scales. In parallel, the emergence of graphene and the resulting impetus to explore 2D materials have broadened the field, as surface-confined reactions of molecular building blocks provide access to such materials as 2D polymers and graphene nanoribbons. In this Review, we describe recent advances and point out promising directions that will lead to even greater and more robust capabilities to exploit designer surfaces.

Guided Molecular Assembly on a Locally Reactive 2D Material

Atomically precise engineering of the position of molecular adsorbates on surfaces of 2D materials is key to their development in applications ranging from catalysis to single-molecule spintronics. Here, stable room-temperature templating of individual molecules with localized electronic states on the surface of a locally reactive 2D material, silicene grown on ZrB₂ , is demonstrated. Using a combination of scanning tunneling microscopy and density functional theory, it is shown that the binding of iron phthalocyanine (FePc) molecules is mediated via the strong chemisorption of the central Fe atom to the sp³ -like dangling bond of Si atoms in the linear silicene domain boundaries. Since the planar Pc ligand couples to the Fe atom mostly through the in-plane d orbitals, localized electronic states resembling those of the free molecule can be resolved. Furthermore, rotation of the molecule is restrained because of charge rearrangement induced by the bonding. These results highlight how nanoscale changes can induce reactivity in 2D materials, which can provide unique surface interactions for enabling novel forms of guided molecular assembly.

Immobilization of single argon atoms in nano-cages of two-dimensional zeolite model systems

The confinement of noble gases on nanostructured surfaces, in contrast to bulk materials, at non-cryogenic temperatures represents a formidable challenge. In this work, individual Ar atoms are trapped at 300 K in nano-cages consisting of (alumino)silicate hexagonal prisms forming a two-dimensional array on a planar surface. The trapping of Ar atoms is detected in situ using synchrotron-based ambient pressure X-ray photoelectron spectroscopy. The atoms remain in the cages upon heating to 400 K. The trapping and release of Ar is studied combining surface science methods and density functional theory calculations. While the frameworks stay intact with the inclusion of Ar atoms, the permeability of gasses (for example, CO) through them is significantly affected, making these structures also interesting candidates for tunable atomic and molecular sieves. These findings enable the study of individually confined noble gas atoms using surface science methods, opening up new opportunities for fundamental research.

While noble gases can be trapped in 3D porous structures, immobilizing them on 2D surfaces represents a formidable challenge. Here, the authors cage individual argon atoms in 2D model zeolite frameworks at room temperature, providing exciting opportunities for the fundamental study of isolated noble gas atoms using surface science methods.

Template Effect of the Graphene Moiré Lattice on Phthalocyanine Assembly

Superstructures of metal-free phthalocyanine (2H-Pc) molecules on graphene-covered Ir(111) have been explored by scanning tunnelling microscopy. Depending on the sub-monolayer coverage different molecular assemblies form at the surface. They reflect the transition from a graphene template effect on the 2H-Pc arrangement to molecular superstructures that are mainly governed by the intermolecular coupling.

Molecular assembly on two-dimensional materials

Molecular self-assembly is a well-known technique to create highly functional nanostructures on surfaces. Self-assembly on two-dimensional (2D) materials is a developing field driven by the interest in functionalization of 2D materials in order to tune their electronic properties. This has resulted in the discovery of several rich and interesting phenomena. Here, we review this progress with an emphasis on the electronic properties of the adsorbates and the substrate in well-defined systems, as unveiled by scanning tunneling microscopy. The review covers three aspects of the self-assembly. The first one focuses on non-covalent self-assembly dealing with site-selectivity due to inherent moiré pattern present on 2D materials grown on substrates. We also see that modification of intermolecular interactions and molecule-substrate interactions influences the assembly drastically and that 2D materials can also be used as a platform to carry out covalent and metal-coordinated assembly. The second part deals with the electronic properties of molecules adsorbed on 2D materials. By virtue of being inert and possessing low density of states near the Fermi level, 2D materials decouple molecules electronically from the underlying metal substrate and allow high-resolution spectroscopy and imaging of molecular orbitals. The moiré pattern on the 2D materials causes site-selective gating and charging of molecules in some cases. The last section covers the effects of self-assembled, acceptor and donor type, organic molecules on the electronic properties of graphene as revealed by spectroscopy and electrical transport measurements. Non-covalent functionalization of 2D materials has already been applied for their application as catalysts and sensors. With the current surge of activity on building van der Waals heterostructures from atomically thin crystals, molecular self-assembly has the potential to add an extra level of flexibility and functionality for applications ranging from flexible electronics and OLEDs to novel electronic devices and spintronics.

Adsorption and electronic properties of pentacene on thin dielectric decoupling layers

With the increasing use of thin dielectric decoupling layers to study the electronic properties of organic molecules on metal surfaces, comparative studies are needed in order to generalize findings and formulate practical rules. In this paper we study the adsorption and electronic properties of pentacene deposited onto h-BN/Rh(111) and compare them with those of pentacene deposited onto KCl on various metal surfaces. When deposited onto KCl, the HOMO and LUMO energies of the pentacene molecules scale with the work functions of the combined KCl/metal surface. The magnitude of the variation between the respective KCl/metal systems indicates the degree of interaction of the frontier orbitals with the underlying metal. The results confirm that the so-called IDIS model developed by Willenbockel et al. applies not only to molecular layers on bare metal surfaces, but also to individual molecules on thin electronically decoupling layers. Depositing pentacene onto h-BN/Rh(111) results in significantly different adsorption characteristics, due to the topographic corrugation of the surface as well as the lateral electric fields it presents. These properties are reflected in the divergence from the aforementioned trend for the orbital energies of pentacene deposited onto h-BN/Rh(111), as well as in the different adsorption geometry. Thus, the highly desirable capacity of h-BN to trap molecules comes at the price of enhanced metal-molecule interaction, which decreases the HOMO-LUMO gap of the molecules. In spite of the enhanced interaction, the molecular orbitals are evident in scanning tunnelling spectroscopy (STS) and their shapes can be resolved by spectroscopic mapping.

Bandgap renormalization and work function tuning in MoSe2/hBN/Ru(0001) heterostructures

The van der Waals interaction in vertical heterostructures made of two-dimensional (2D) materials relaxes the requirement of lattice matching, therefore enabling great design flexibility to tailor novel 2D electronic systems. Here we report the successful growth of MoSe2 on single-layer hexagonal boron nitride (hBN) on the Ru(0001) substrate using molecular beam epitaxy. Using scanning tunnelling microscopy and spectroscopy, we found that the quasi-particle bandgap of MoSe2 on hBN/Ru is about 0.25 eV smaller than those on graphene or graphite substrates. We attribute this result to the strong interaction between hBN/Ru, which causes residual metallic screening from the substrate. In addition, the electronic structure and the work function of MoSe2 are modulated electrostatically with an amplitude of ∼0.13 eV. Most interestingly, this electrostatic modulation is spatially in phase with the Moiré pattern of hBN on Ru(0001) whose surface also exhibits a work function modulation of the same amplitude.

Mass Spectrometry as a Preparative Tool for the Surface Science of Large Molecules

Measuring and understanding the complexity that arises when nanostructures interact with their environment are one of the major current challenges of nanoscale science and technology. High-resolution microscopy methods such as scanning probe microscopy have the capacity to investigate nanoscale systems with ultimate precision, for which, however, atomic scale precise preparation methods of surface science are a necessity. Preparative mass spectrometry (pMS), defined as the controlled deposition of m/z filtered ion beams, with soft ionization sources links the world of large, biological molecules and surface science, enabling atomic scale chemical control of molecular deposition in ultrahigh vacuum (UHV). Here we explore the application of high-resolution scanning probe microscopy and spectroscopy to the characterization of structure and properties of large molecules. We introduce the fundamental principles of the combined experiments electrospray ion beam deposition and scanning tunneling microscopy. Examples for the deposition and investigation of single particles, for layer and film growth, and for the investigation of electronic properties of individual nonvolatile molecules show that state-of-the-art pMS technology provides a platform analog to thermal evaporation in conventional molecular beam epitaxy. Additionally, it offers additional, unique features due to the use of charged polyatomic particles. This new field is an enormous sandbox for novel molecular materials research and demands the development of advanced molecular ion beam technology.

Prospects of Spin Catalysis on Spin-Polarized Graphene Heterostructures

Extreme points on potential energy surfaces of Ni adatom on free-standing graphene and top:fcc and hcp:fcc graphene/Ni(111) heterostructures in different spin states were studied using periodic boundary conditions density functional theory approach. It was found that the spin states of the substrates strongly influence the energy of the Ni adatom extreme points on potential energy surface by decreasing (top:fcc heterostructure) or increasing (hcp:fcc heterostructure) the total energies of η1, η1′, and η2 Ni adatom coordinations on graphene. This phenomenon offers unique possibilities to control the potential energy surfaces of transition metal adatoms and promote surface chemical reactions using induced spin polarization of graphene substrates.

Cyclotetrahalo-p-phenylenes: simulations of halogen substituted cycloparaphenylenes and their interaction with C60

Halogen substitution of cycloparaphenylenes (‘nanohoops’) shifts the HOMO and LUMO, for example eliminating redox coupling in the complex C₆₀@[10]CPP-F.

Cu- and Pd-catalyzed Ullmann reaction on a hexagonal boron nitride layer

This study demonstrates that Cu and Pd can efficiently activate Ullmann reactions on inert h-BN with two distinctive reaction paths.

Interplay between Energy-Level Position and Charging Effect of Manganese Phthalocyanines on an Atomically Thin Insulator

Understanding the energy-level alignment and charge transfer of organic molecules at large bandgap semiconductors is of crucial importance to optimize device performance in organic electronics. We have studied submonolayer coverage of manganese phthalocyanine (MnPc) on hexagonal boron nitride (h-BN) on Rh(111) as a model system by low-temperature scanning tunneling microscopy (STM) and spectroscopy (STS). The adsorbed molecules show three distinctly different bias-dependent topographic signatures, which depend on their adsorption positions on the h-BN. Among these three types of MnPc, one shows pronounced charging because of the proximity of the highest occupied molecular orbital (HOMO) to the Fermi level on the decoupling h-BN substrate. The charging of the MnPc from its neutral to the MnPc(+) state leads to a down shift of the Mn 3d-related orbital by 840 meV as determined from the difference in energy position between high- and low-bias charging. We find that the charging field is linearly related to the HOMO position with respect to the Fermi level, with a clear correlation to the adsorption orientations of the MnPc. Our results show how critically energy level alignment and field-induced charge transfer process can depend on adsorption configurations, even on an apparently low-interacting substrate like metal supported monolayer h-BN.

Wetting of water on hexagonal boron nitride@Rh(111): a QM/MM model based on atomic charges derived for nano-structured substrates

The wetting of water on corrugated and flat hexagonal boron nitride (h-BN) monolayers on Rh(111) is studied within a hybrid quantum mechanics/molecular mechanics (QM/MM) approach. Water is treated by QM methods, whereas the interactions between liquid and substrate are described at the MM level. The electrostatic properties of the substrate are reproduced by assigning specifically generated partial charges to each MM atom. We propose a method to determine restrained electrostatic potential (RESP) charges that can be applied to periodic systems. Our approach is based on the Gaussian and plane waves algorithm and allows an easy tuning of charges for nano-structured substrates. We have successfully applied it to reproduce the electrostatic potential of the corrugated and flat h-BN/Rh(111) known as nanomesh. Molecular dynamics simulations of water films in contact with these substrates are performed and structural and dynamic properties of the interfaces are analyzed. Based on this analysis and on the interaction energies between water film and substrate, we found that the corrugated nanomesh is slightly more hydrophilic. On a macroscopic scale, we expect a smaller contact angle for a droplet on the corrugated surface.

Highly Ordered Surface Self-Assembly of Fe₄ Single Molecule Magnets

Single molecule magnets (SMMs) have attracted considerable attention due to low-temperature magnetic hysteresis and fascinating quantum effects. The investigation of these properties requires the possibility to deposit well-defined monolayers or spatially isolated molecules within a well-controlled adsorption geometry. Here we present a successful fabrication of self-organized arrays of Fe4 SMMs on hexagonal boron nitride (h-BN) on Rh(111) as template. Using a rational design of the ligand shell optimized for surface assembly and electrospray as a gentle deposition method, we demonstrate how to obtain ordered arrays of molecules forming perfect hexagonal superlattices of tunable size, from small islands to an almost perfect monolayer. High-resolution low temperature scanning tunneling microscopy (STM) reveals that the Fe4 molecule adsorbs on the substrate in a flat geometry, meaning that its magnetic easy axis is perpendicular to the surface. By scanning tunneling spectroscopy (STS) and density functional theory (DFT) calculations, we infer that the majority- and minority-spin components of the spin-split lowest unoccupied molecular orbital (LUMO) can be addressed separately on a submolecular level.

Interplay of weak interactions in the atom-by-atom condensation of xenon within quantum boxes

Condensation processes are of key importance in nature and play a fundamental role in chemistry and physics. Owing to size effects at the nanoscale, it is conceptually desired to experimentally probe the dependence of condensate structure on the number of constituents one by one. Here we present an approach to study a condensation process atom-by-atom with the scanning tunnelling microscope, which provides a direct real-space access with atomic precision to the aggregates formed in atomically defined ‘quantum boxes’. Our analysis reveals the subtle interplay of competing directional and nondirectional interactions in the emergence of structure and provides unprecedented input for the structural comparison with quantum mechanical models. This approach focuses on—but is not limited to—the model case of xenon condensation and goes significantly beyond the well-established statistical size analysis of clusters in atomic or molecular beams by mass spectrometry.

Condensation in the regime of weakly interactions is of fundamental importance. Here, the authors study the condensation process one atom at a time, showing the forces driving the behaviour of xenon atoms as they condense into aggregate structures in nanoscale pores.

Boron nitride coated rhodium black for stable production of syngas

A blanket of boron nitride grown by CVD stablizes rhodium black for syngas production in methane oxidation and avoid agglomeration of metal particle by carbon deposition.

Resonant-enhanced spectroscopy of molecular rotations with a scanning tunneling microscope

We use rotational excitation spectroscopy with a scanning tunneling microscope to investigate the rotational properties of molecular hydrogen and its isotopes physisorbed on the surfaces of graphene and hexagonal boron nitride (h-BN), grown on Ni(111), Ru(0001), and Rh(111). The rotational excitation energies are in good agreement with ΔJ = 2 transitions of freely spinning p-H2 and o-D2 molecules. The variations of the spectral line shapes for H2 among the different surfaces can be traced back to a molecular resonance-mediated tunneling mechanism. Our data for H2/h-BN/Rh(111) suggest a local intrinsic gating on this surface due to lateral static dipoles. Spectra on a mixed monolayer of H2, HD, and D2 display all three J = 0 → 2 rotational transitions, irrespective of tip position, thus pointing to a multimolecule excitation, or molecular mobility in the physisorbed close-packed layer.

Two-nanometer voids in single-layer hexagonal boron nitride: formation via the "can-opener" effect and annihilation by self-healing

The exposure of hexagonal boron nitride single layers to low energy ions leads to the formation of vacancy defects that are mobile at elevated temperatures. For the case of h-BN on rhodium, a superhoneycomb surface with 3 nm lattice constant (nanomesh), a concerted self-assembly of these defects is observed, where the "can-opener" effect leads to the cut-out of 2 nm "lids" and stable voids in the h-BN layer. These clean-cut voids repel each other, which enables the formation of arrays with a nearest neighbor distance down to about 8 nm. The density of voids depends on the Ar ion dose, and can reach 10(12) cm(-2). If the structures are annealed above 1000 K, the voids disappear and pristine h-BN nanomesh with larger holes is recovered. The results are obtained by scanning tunneling microscopy and density functional theory calculations.

Dehalogenation and coupling of a polycyclic hydrocarbon on an atomically thin insulator

Catalytic activity is of pivotal relevance in enabling efficient and selective synthesis processes. Recently, covalent coupling reactions catalyzed by solid metal surfaces opened the rapidly evolving field of on-surface chemical synthesis. Tailored molecular precursors in conjunction with the catalytic activity of the metal substrate allow the synthesis of novel, technologically highly relevant materials such as atomically precise graphene nanoribbons. However, the reaction path on the metal substrate remains unclear in most cases, and the intriguing question is how a specific atomic configuration between reactant and catalyst controls the reaction processes. In this study, we cover the metal substrate with a monolayer of hexagonal boron nitride (h-BN), reducing the reactivity of the metal, and gain unique access to atomistic details during the activation of a polyphenylene precursor by sequential dehalogenation and the subsequent coupling to extended oligomers. We use scanning tunneling microscopy and density functional theory to reveal a reaction site anisotropy, induced by the registry mismatch between the precursor and the nanostructured h-BN monolayer.

Lateral and vertical stiffness of the epitaxial h-BN monolayer on Rh(111)

The response to strain in covalently bound single layers has a large impact on the growth and properties. We investigate the quasi-two-dimensional hexagonal boron nitride on Rh(111), which is interesting due to its high intrinsic corrugation. We use combined atomic force and scanning tunneling microscopy to measure the response of this monolayer to probing forces. Three-dimensional force maps and the atomic resolution of the layer enable us to determine lateral and vertical stiffness of this prototypical system with unprecedented spatial resolution. Extremely low stiffnesses ≈1 N/m are derived. Our experiments give insights into the mechanical properties of corrugated incommensurate layers that buckle into the third dimension to relieve strain.

High quality single atomic layer deposition of hexagonal boron nitride on single crystalline Rh(111) four-inch wafers

The setup of an apparatus for chemical vapor deposition (CVD) of hexagonal boron nitride (h-BN) and its characterization on four-inch wafers in ultra high vacuum (UHV) environment is reported. It provides well-controlled preparation conditions, such as oxygen and argon plasma assisted cleaning and high temperature annealing. In situ characterization of a wafer is accomplished with target current spectroscopy. A piezo motor driven x-y stage allows measurements with a step size of 1 nm on the complete wafer. To benchmark the system performance, we investigated the growth of single layer h-BN on epitaxial Rh(111) thin films. A thorough analysis of the wafer was performed after cutting in atmosphere by low energy electron diffraction, scanning tunneling microscopy, and ultraviolet and X-ray photoelectron spectroscopies. The apparatus is located in a clean room environment and delivers high quality single layers of h-BN and thus grants access to large area UHV processed surfaces, which had been hitherto restricted to expensive, small area single crystal substrates. The facility is versatile enough for customization to other UHV-CVD processes, e.g., graphene on four-inch wafers.

Implantation length and thermal stability of interstitial ar atoms in boron nitride nanotents

Hyperthermal atoms may be implanted beneath single layers of graphene or hexagonal boron nitride (h-BN) on a substrate. For the case of h-BN on rhodium, which is a corrugated honeycomb superstructure with a periodicity of 3.2 nm, Ar atoms are implanted at distinct interstitial sites within the supercell, where the h-BN is weakly bound to the substrate. These peculiar structures are reminiscent of "nanotents" with an ultimately thin "rainfly". Here we explore the implantation length (i.e., the distance the atoms move before they come to rest as interstitial defects) and the thermal stability of these atomic agglomerates above room temperature. The results are obtained by variable-temperature scanning tunneling microscopy and density functional theory calculations.

Control of molecular organization and energy level alignment by an electronically nanopatterned boron nitride template

Suitable templates to steer the formation of nanostructure arrays on surfaces are indispensable in nanoscience. Recently, atomically thin sp(2)-bonded layers such as graphene or boron nitride (BN) grown on metal supports have attracted considerable interest due to their potential geometric corrugation guiding the positioning of atoms, metallic clusters or molecules. Here, we demonstrate three specific functions of a geometrically smooth, but electronically corrugated, sp(2)/metal interface, namely, BN/Cu(111), qualifying it as a unique nanoscale template. As functional adsorbates we employed free-base porphine (2H-P), a prototype tetrapyrrole compound, and tetracyanoquinodimethane (TCNQ), a well-known electron acceptor. (i) The electronic moirons of the BN/Cu(111) interface trap both 2H-P and TCNQ, steering self-organized growth of arrays with extended molecular assemblies. (ii) We report an effective decoupling of the trapped molecules from the underlying metal support by the BN, which allows for a direct visualization of frontier orbitals by scanning tunneling microscopy (STM). (iii) The lateral molecular positioning in the superstructured surface determines the energetic level alignment; i.e., the energy of the frontier orbitals, and the electronic gap are tunable.

Site-selective adsorption of phthalocyanine on h-BN/Rh(111) nanomesh

STM and DFT study of site selectivity of h-BN/Rh(111) (nanomesh) for the adsorption phthalocyanine, showing impressive agreement between experiment and theory.

Tailoring molecular self-assembly of magnetic phthalocyanine molecules on Fe- and Co-intercalated graphene

We investigate molecule-molecule, as well as molecule-substrate, interactions of phthalocyanine molecules deposited on graphene. In particular, we show how to tune the self-assembly of molecular lattices in two dimensions by intercalation of transition metals between graphene and Ir(111): modifying the surface potential of the graphene layer via intercalation leads to the formation of square, honeycomb, or Kagome lattices. Finally, we demonstrate that such surface induced molecular lattices are stable even at room temperature.

Templated self-assembly and local doping of molecules on epitaxial hexagonal boron nitride

Using low-temperature scanning tunneling microscopy, we show that monolayer hexagonal boron nitride (h-BN) on Ir(111) acts as ultrathin insulating layer for organic molecules, while simultaneously templating their self-assembly. Tunneling spectroscopy experiments on cobalt phthalocyanine (CoPC) reveal narrow molecular resonances and indicate that the charge state of CoPC is periodically modulated by the h-BN moiré superstructure. Molecules in the second layer show site-selective adsorption behavior, allowing the synthesis of molecular dimers that are spatially ordered and inaccessible by usual chemical means.