Molecular Nanocrystals on Ultrathin NaCl Films on Au(111)

Construction and Structural Transformation of Metal-Organic Nanostructures Induced by Alkali Metals and Alkali Metal Salts

Metal-organic nanostructures are attractive in a variety of scientific fields, such as biomedicine, energy harvesting, and catalysis. Alkali-based metal-organic nanostructures have been extensively fabricated on surfaces based on pure alkali metals and alkali metal salts. However, their differences in the construction of alkali-based metal-organic nanostructures have been less discussed, and the influence on structural diversity remains elusive. In this work, from the interplay of scanning tunneling microscopy imaging and density functional theory calculations, we constructed Na-based metal-organic nanostructures by applying Na and NaCl as sources of alkali metals and visualized the structural transformations in real space. Moreover, a reverse structural transformation was achieved by dosing iodine into the Na-based metal-organic nanostructures, revealing the connections and differences between NaCl and Na in the structural evolutions, which provided fundamental insights into the evolution of electrostatic ionic interactions and the precise fabrication of alkali-based metal-organic nanostructures.

Adsorption properties of a paracyclophane molecule on NaCl/Au surfaces: a first-principles study

Ultrathin insulating layers are commonly applied in scanning tunneling microscope (STM) measurements on molecular systems to preserve the intrinsic properties of a sample. We examine in the present work the adsorption properties of a double-decker 3,3-paracyclophane (PCP) molecule supported on Au surfaces with thin NaCl monolayers (MLs) as the decoupling spacer by using first-principles calculations. The interactions between the adsorbed molecule and the substrate were analyzed in terms of the adsorption energy, dispersion interactions, charge transfer, and molecular structure changes. The simulation results show that the presence of NaCl can significantly reduce the adsorption energy as well as the charge transfer between the molecule and the substrate. Detailed analysis of the differential charge density and partial charge density of states indicates that three MLs of NaCl are sufficient to decouple the molecule from the Au substrate with no significant changes in the adsorption properties of the PCP with the further increase of the thickness of the NaCl spacer. These results could be helpful for the application of the interesting double-decker molecules as functional single-molecule devices where the intrinsic molecular properties need to be preserved.

Dissimilar Decoupling Behavior of Two-Dimensional Materials on Metal Surfaces

The efficiency of hexagonal boron nitride and graphene to separate the hydrocarbon molecule C₆₄H₃₆ from Ru(0001) and Pt(111) surfaces is explored in low-temperature scanning tunneling microscopy and spectroscopy experiments. Both 2D materials enable the observation of the Franck-Condon effect in both frontier orbitals. On hexagonal boron nitride, vibronic progression with two vibrational energies gives rise to sharp orbital sidebands that are clearly visible up to the second order of the vibrational quantum number with different Huang-Rhys factors. In contrast, on graphene, orbital and vibronic spectroscopic signatures exhibit broad line shapes, with the second-order progression being hardly discriminable. Only a single vibrational quantum energy leaves its fingerprint in the Franck-Condon spectrum.

Structural, Electronic, and Magnetic Properties of Cobalt Tetrakis (Thiadiazole) Porphyrazine Molecule Films on Au(111)

We investigated the structural and electronic/spin configurations of a film of the Co tetrakis(1,2,5-thiadiazole) porphyrazine (CoTTDPz) molecule adsorbed on the Au(111) surface by a scanning tunneling microscope (STM). CoTTDPz has a structure similar to that of the Co phthalocyanine molecule, but the benzene ring of the isoindole of the phthalocyanine molecule is replaced by the pentagon moiety of 1,2,5-thiadiazoles that has an S atom at the apex. We find an ordered molecular lattice with a threefold symmetry where a nearest-neighbor distance of 1.30 nm was measured, which is significantly smaller than that observed for the metal Pc molecule. The unit cell of the lattice contains two molecules that are rotated by 60° relative to each other. With the configuration achieved by this rotation, the neighboring molecules can form a stronger interaction through bonding between the S atom at the apex of one molecule and the N atom of the other (the N atom that is bridging the thiadiazoles). The strong interaction between the molecule and the substrate appears in the spin state examined by the detection of the Kondo resonance, which is formed by the screening of an isolated spin by the conduction electron. Even though the existence of the spin was confirmed for the bulk and thick films of this molecule, no Kondo features are detected for the molecules in the first, second, and third layers of the films. However, the isolated molecule in the third layer showed an intriguing combination of the Kondo feature and an inelastic excitation feature caused by a spin-flip process.

Fraternal twins: distinction between PbPc and SnPc by their switching behaviour in a scanning tunnelling microscope

In this contribution, we compare the optical absorbance behaviour and the structural properties of lead(II)-phthalocyanine (PbPc) and tin(II)-phthalocyanine (SnPc) thin films. To this end, we employ a Ag(1 1 1) substrate terminated with a monolayer of 3,4,9,10-perylene tetracarboxylic dianhydride constituting an internal interface whose main effect is an electronic decoupling of the phthalocyanine adlayer from the metal surface. As deduced from low-energy electron diffraction and scanning tunnelling microscopy (STM) measurements, the epitaxial relations and unit cell compositions of the prevailing PbPc monolayer and multilayer domains are confusingly similar to those of SnPc on PTCDA/Ag(1 1 1). However, SnPc and PbPc can be readily distinguished by their STM-induced switching behaviours: while the former is capable of reversible configurational changes, no effect on the latter could be achieved by us under comparable conditions. This corroborates earlier theoretical predictions and even renders the chemical identification of individual shuttlecock-shaped metal-phthalocyanines feasible.

Exciting vibrons in both frontier orbitals of a single hydrocarbon molecule on graphene

Vibronic excitations in molecules are key to the fundamental understanding of the interaction between vibrational and electronic degrees of freedom. In order to probe the genuine vibronic properties of a molecule even after its adsorption on a surface appropriate buffer layers are of paramount importance. Here, vibrational progression in both molecular frontier orbitals is observed with submolecular resolution on a graphene-covered metal surface using scanning tunnelling spectroscopy. Accompanying calculations demonstrate that the vibrational modes that cause the orbital replica in the progression share the same symmetry as the electronic states they couple to. In addition, the vibrational progression is more pronounced for separated molecules than for molecules embedded in molecular assemblies. The entire vibronic spectra of these molecular species are moreover rigidly shifted with respect to each other. This work unravels intramolecular changes in the vibronic and electronic structure owing to the efficient reduction of the molecule-metal hybridization by graphene.

Electronic effects and fundamental physics studied in molecular interfaces

Scanning probe instruments in conjunction with a very low temperature environment have revolutionized the ability of building, functionalizing, and analysing two dimensional interfaces in the last twenty years. In addition, the availability of fast, reliable, and increasingly sophisticated methods to simulate the structure and dynamics of these interfaces allow us to capture even very small effects at the atomic and molecular level. In this review we shall focus largely on metal surfaces and organic molecular compounds and show that building systems from the bottom up and controlling the physical properties of such systems is no longer within the realm of the desirable, but has become day to day reality in our best laboratories.

Off-Center Rotation of CuPc Molecular Rotor on a Bi(111) Surface and the Chiral Feature

Molecular rotors with an off-center axis and the chiral feature of achiral CuPc molecules on a semi-metallic Bi(111) surface have been investigated by means of a scanning tunneling microscopy (STM) at liquid nitrogen (LN₂) temperature. The rotation axis of each CuPc molecular rotor is located at the end of a phthalocyanine group. As molecular coverage increases, the CuPc molecules are self-assembled into various nanoclusters and finally into two-dimensional (2D) domains, in which each CuPc molecule exhibits an apparent chiral feature. Such chiral features of the CuPc molecules can be attributed to the combined effect of asymmetric charge transfer between the CuPc and Bi(111) substrate, and the intermolecular van der Waals interactions.

Organic Multilayer Films Studied by Scanning Tunneling Microscopy

This Minireview focuses exclusively on work with scanning tunneling microscopy to study the self-assembled multilayer films (SAMTs) of organic molecules. The π-conjugated organic molecules form different structures within different monolayers on various substrates. The interplay between molecule-substrate and intermolecular interactions plays a key role in determining the stacking mode of organic multilayer films. Different substrates strongly influence the organic-film growth and electronic properties of the organic molecules. Geometric and electronic structures of SAMTs are important factors that may determine device performance. In addition to the inorganic interface, this Minireview addresses the organic-organic interface. Homo- and hetero-SAMTs of organic molecules are also considered. The subtle interplay between structural and electronic characteristics, on one hand, and functionality and reactivity, on the other hand, are highlighted.

Vacuum synthesis of magnetic aluminum phthalocyanine on Au(111)

Air-unstable magnetic aluminum phthalocyanine (AlPc) molecules are prepared by an on-surface metalation reaction of phthalocyanine with aluminum (Al) atoms on Au(111) in ultrahigh vacuum. Experiments and density functional theory calculations show that an unpaired spin is located on the conjugated isoindole lobes of the molecule rather than at the Al position.

Superstructures and Electronic Properties of Manganese-Phthalocyanine Molecules on Au(110) from Submonolayer Coverage to Ultrathin Molecular Films

The adsorption of manganese-phthalocyanine molecules on Au(110) was investigated using a low-temperature scanning tunneling microscope. A rich variety of commensurate superstructures was observed upon increasing the molecule coverage from submonolayers to ultrathin films. All structures were associated with reconstructions of the Au(110) substrate. Molecules adsorbed in the second molecular layer exhibited negative differential conductance occurring symmetrically around zero bias voltage. A double-barrier tunneling model rationalized this observation in terms of a peaked molecular resonance at the Fermi energy together with a voltage drop across the molecular film.

Depopulation of Single-Phthalocyanine Molecular Orbitals upon Pyrrolic-Hydrogen Abstraction on Graphene

Single-molecule chemistry with a scanning tunneling microscope has preponderantly been performed on metal surfaces. The molecule-metal hybridization, however, is often detrimental to genuine molecular properties and obscures their changes upon chemical reactions. We used graphene on Ir(111) to reduce the coupling between Ir(111) and adsorbed phthalocyanine molecules. By local electron injection from the tip of a scanning tunneling microscope the two pyrrolic H atoms were removed from single phthalocyanines. The detachment of the H atom pair induced a strong modification of the molecular electronic structure, albeit with no change in the adsorption geometry. Spectra and maps of the differential conductance combined with density functional calculations unveiled the entire depopulation of the highest occupied molecular orbital upon H abstraction. Occupied π states of intact molecules are proposed to be emptied via intramolecular electron transfer to dangling σ states of H-free N atoms.

Growth of Metal Phthalocyanine on Deactivated Semiconducting Surfaces Steered by Selective Orbital Coupling

Using scanning tunneling microscopy and density functional theory, we show that the molecular ordering and orientation of metal phthalocyanine molecules on the deactivated Si surface display a strong dependency on the central transition-metal ion, driven by the degree of orbital hybridization at the heterointerface via selective p-d orbital coupling. This Letter identifies a selective mechanism for modifying the molecule-substrate interaction which impacts the growth behavior of transition-metal-incorporated organic molecules on a technologically relevant substrate for silicon-based devices.

Low-temperature scanning tunneling microscopy study on the electronic properties of a double-decker DyPc2 molecule at the surface

Inaccessibility of the Dy 4f states in the double-decker DyPc₂ molecule, as evidenced by dI/dV mapping in the constant height mode.

Nanoelectrical analysis of single molecules and atomic-scale materials at the solid/liquid interface

Evaluating the built-in functionality of nanomaterials under practical conditions is central for their proposed integration as active components in next-generation electronics. Low-dimensional materials from single atoms to molecules have been consistently resolved and manipulated under ultrahigh vacuum at low temperatures. At room temperature, atomic-scale imaging has also been performed by probing materials at the solid/liquid interface. We exploit this electrical interface to develop a robust electronic decoupling platform that provides precise information on molecular energy levels recorded using in situ scanning tunnelling microscopy/spectroscopy with high spatial and energy resolution in a high-density liquid environment. Our experimental findings, supported by ab initio electronic structure calculations and atomic-scale molecular dynamics simulations, reveal direct mapping of single-molecule structure and resonance states at the solid/liquid interface. We further extend this approach to resolve the electronic structure of graphene monolayers at atomic length scales under standard room-temperature operating conditions.

Variation of Kondo temperature induced by molecule-substrate decoupling in film formation of bis(phthalocyaninato)terbium(III) molecules on Au(111)

We demonstrate that the lattice formation of an adsorbed molecule decouples the molecule-substrate interaction to change the Kondo resonance, which occurs due to interactions between conduction electrons and the molecule's unpaired spin. The double-decker bis(phthalocyaninato)terbium(III) complex, which is single-molecule magnet and forms a Kondo resonance on a Au(111) surface through an unpaired π-radical spin, is studied using scanning tunneling microscopy/spectroscopy (STM/STS). In the STS spectra, an unusual sharp, strong peak (peak A) is found only for the molecule in a film. The peak position of peak A (εA) cyclically shifts by several hundred millivolts as the STS tip position shifts along the outer circle of the molecule, reflecting the tilting of the upper phthalocyanine (Pc) ligand from the flat-lying lower Pc ligand. The Kondo resonance, which is detected as a sharp peak at the Fermi level, also shows cyclic variations of the peak width and intensity. As εA approaches EF, the Kondo temperature (TK) increases. We propose a model that peak A originates from the singly occupied molecular orbital state whose energy is shifted by an unscreened final state effect due to a decrease in the molecule-substrate chemisorptive interaction. We further examine this model using density functional theory calculations, confirming a decreased molecule-substrate interaction for molecules in the film compared to that of isolated molecules. Further calculations of a tilted upper Pc ligand configuration show a site-dependent, cyclic variation of the molecule-substrate interaction within a molecule.

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.

STM tip-assisted engineering of molecular nanostructures: PTCDA islands on Ge(001):H surfaces

Islands composed of perylene-3,4,9,10-tetracarboxylic dianhydride (PTCDA) molecules are grown on a hydrogen passivated Ge(001):H surface. The islands are studied with room temperature scanning tunneling microscopy and spectroscopy. The spontaneous and tip-induced formation of the top-most layer of the island is presented. Assistance of the scanning probe seems to be one of the factors that facilitate and speed the process of formation of the top-most layer.

Nanoscale periodic modulations on sodium chloride surface revealed by tuning fork atomic force microscopy

The sodium chloride surface is one of the most common platforms for the study of catalysts, thin film growth, and atmospheric aerosols. Here we report a nanoscale periodic modulation pattern on the surface of a cleaved NaCl single crystal, revealed by non-contact atomic force microscopy with a tuning fork sensor. The surface pattern shows two orthogonal domains, extending over the entire cleavage surface. The spatial modulations exhibit a characteristic period of 5.4 nm, along <110> crystallographic directions of the NaCl. The modulations are robust in vacuum, not affected by the tip-induced electric field or gentle annealing (<300 °C); however, they are eliminated after exposure to water and an atomically flat surface can be recovered by subsequent thermal annealing after water exposure. A strong electrostatic charging is revealed on the cleavage surface which may facilitate the formation of the observed metastable surface reconstruction.