Self-similar fractal structures are of fundamental importance in science, mathematics, and aesthetics. A series of molecular defect-free Sierpiński triangle fractals have been constructed on surfaces recently. However, the highest order of the fractals is only 4 because of the limitation of kinetic growth. Here complete fifth-order Sierpiński triangles with a lateral length of 0.05 μm were successfully prepared in ultrahigh vacuum by a combination of templating and coassembly methods. Fe atoms, 4,4″-dicyano-1,1′:3′,1″-terphenyl, and 1,3-bis(4-pyridyl)benzene molecules were used to build fractals on the reconstructed Au(100)-(hex) surface. The new strategy may be applied to construct various Sierpiński triangles of higher orders.

We report the on-surface formation and characterization of [30]-honeycombene, a cyclotriacontaphenylene, which consists of 30 phenyl rings (C180H120) and has a diameter of 4.0 nm. This shape-persistent, conjugated, and unsubstituted hexagonal hydrocarbon macrocycle was obtained by solvent-free synthesis on a silver (111) single-crystal surface, making solubility-enhancing alkyl side groups unnecessary. Side products include strained macrocycles with square, pentagonal, and heptagonal shape. The molecules were characterized by scanning tunneling microscopy and density functional theory (DFT) calculations. On the Ag(111) surface, the macrocycles act as molecular quantum corrals and lead to the confinement of surface-state electrons inside the central cavity. The energy of the confined surface state correlates with the size of the macrocycle and is well described by a particle-in-the-box model. Tunneling spectroscopy suggests conjugation within the planar rings and reveals influences of self-assembly on the electronic structure. While the adsorbed molecules appear to be approximately planar, the free molecules have nonplanar conformation, according to DFT.Keywords: conjugation; electron confinement; macrocycle; on-surface synthesis; quantum corral; scanning tunneling microscopy; Ullmann reaction;

The self-assembly behavior of a V-shaped bispyridine, 1,3-bi(4-pyridyl)benzene (BPyB), was studied by scanning tunneling microscopy on the (111) surfaces of Cu, Ag, and Au. BPyB molecules coordinately bonded with active Cu adatoms on Cu(111) in the form of complete polygonal rings at low coverages. On Ag(111), BPyB molecules aggregated into two-dimensional islands by relatively weak intermolecular hydrogen bonds. The coexistence of hydrogen bonds and coordination interaction was observed on the BPyB-covered Au(111) substrate. Density functional theory calculations of the metal–molecule binding energy and Monte Carlo simulations were performed to help understand the forming mechanism of molecular superstructures on the surfaces. In particular, the comprehensive orbital composition analysis interprets the observed metal–organic complexes and reveals the importance of relativistic effects for the extraordinary activity of gold adatoms. The relativistic effects cause the energy stability of the Au 6s atomic orbital and decrease the energy separation between the Au 6s and 5d orbitals. The enhanced sd hybridization strengthens the N–Au–N bond in BPyB–Au–BPyB complexes.Keywords: coordination chemistry; relativistic effects; self-assembly; STM; surface chemistry;

Hierarchical self-assembly of 13-cis-retinoic acid on Au(111) and Ag(111) was investigated using low-temperature scanning tunnelling microscopy. On both surfaces, molecules form dimers by hydrogen bonds and the dimers arrange into ordered two-dimensional arrays through van der Waals forces. Three packing modes are observed on Au(111) and only one on Ag(111). We tentatively attribute the different patterns on the two surfaces to a stronger molecule–substrate interaction on Ag(111) and site-dependent molecular adsorption on different atomic lattices. In addition, 13-cis-ReA on Au(111) can be made to carry a localized spin.

Kinetically controlled hierarchical self-assemblies of all-trans-retinoic acid on Au(111) were investigated via low-temperature scanning tunneling microscopy in ultra-high vacuum. The dominant molecular hierarchical superstructure could be selectively controlled to dimer, tetramer, or pentamer patterns, which were stabilized by hydrogen bonds and van der Waals interactions.

One of the most striking features of organic semiconductors compared with their corresponding inorganic counterparts is their molecular diversity. The major challenge in organic semiconductor material technology is creating molecular structural motifs to develop multifunctional materials in order to achieve the desired functionalities yet to optimize the specific device performance. Azo-compounds, because of their special photoresponsive property, have attracted extensive interest in photonic and optoelectronic applications; if incorporated wisely in the organic semiconductor groups, they can be innovatively utilized in advanced smart electronic applications, where thermal and photo modulation is applied to tune the electronic properties. On the basis of this aspiration, a novel azo-functionalized liquid crystal semiconductor material, (E)-1-(4-(anthracen-2-yl)phenyl)-2-(4-(decyloxy)phenyl)diazene (APDPD), is designed and synthesized for application in organic thin-film transistors (OTFTs). The UV–vis spectra of APDPD exhibit reversible photoisomerizaton upon photoexcitation, and the thin films of APDPD show a long-range orientational order based on its liquid crystal phase. The performance of OTFTs based on this material as well as the effects of thermal treatment and UV-irradiation on mobility are investigated. The molecular structure, stability of the material, and morphology of the thin films are characterized by thermal gravimetric analysis (TGA), polarizing optical microscopy (POM), (differential scanning calorimetry (DSC), UV–vis spectroscopy, atomic force microscopy (AFM), and scanning tunneling microscopy (STM). This study reveals that our new material has the potential to be applied in optical sensors, memories, logic circuits, and functional switches.Keywords: anthracene; azo-compound; liquid crystal; organic thin-film transistors; photoresponsive;

Crystalline structures with Sierpiński triangles as building blocks were constructed via a templating method in an ultra-high vacuum and studied by low-temperature scanning tunneling microscopy. Cobalt atoms and 4,4′′-dicyano-1,1′:3′,1′′-terphenyl molecules were used to build the Sierpiński triangles. The Au(100)-(hex), Au(111)-22 × , and stepped Au(111) surfaces were used as templates to guide the packing of Sierpiński triangles. One-dimensional double chains and two-dimensional small domains of Sierpiński triangles were successfully prepared using the method in experiments.

AbstractAtomic-scale mechanochemistry is realized from force exerted by a C60-functionalized scanning tunneling microscope tip. Two conformers of tin phthalocyanine can be prepared on coinage-metal surfaces. A transition between these conformers is induced on Cu(111) and Ag(100). Density-functional calculations reveal details of this reaction. Because of the large energy barrier of the reaction and the strong interaction of SnPc with Cu(111), the process cannot be achieved by electrical means.

AbstractAtomic-scale mechanochemistry is realized from force exerted by a C60-functionalized scanning tunneling microscope tip. Two conformers of tin phthalocyanine can be prepared on coinage-metal surfaces. A transition between these conformers is induced on Cu(111) and Ag(100). Density-functional calculations reveal details of this reaction. Because of the large energy barrier of the reaction and the strong interaction of SnPc with Cu(111), the process cannot be achieved by electrical means.

An intermediate shuttling structure of a chloroaluminum phthalocyanine(ClAlPc)-based molecular switch is transiently created and analyzed by combined scanning tunneling microcopy/spectroscopy and density-functional theory calculations, which suggests that the Cl atom is squeezed into the space between the central Al atom and the inner N-containing ring in ClAlPc.

Sierpiński triangles up to the fourth order were successfully prepared using 4,4′′-dicyano-1,1′:3′,1′′-terphenyl molecules and Fe or Co atoms with the coexistence of the third molecules (C60 or MnPc) on a reconstructed Au(111) substrate. Sierpiński triangles could be disturbed and restored by C60 molecules after annealing at 400 and 360 K, respectively. The dispersed MnPc molecules did not affect the formation of Sierpiński triangles when annealed at high temperatures. The selective adsorption of MnPc molecules at certain holes of Sierpiński triangles was observed.

Sierpiński triangle fractals were constructed on both Ag(111) and symmetry-mismatched fourfold Ag(100) surfaces through chemical reaction between H3PH molecules and Fe atoms under vacuum. Density functional theory calculations revealed that the fractals were stabilized by the strong coordination interaction between Fe and O atoms. In comparison, pure H3PH molecules formed fractals via moderately strong hydrogen bonds only on Ag(111), not on Ag(100).

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.

Growth of covalently bonded Sierpiński triangles (CB-STs) on metal surfaces was investigated by scanning tunneling microscopy (STM). Three synthetic strategies (namely, dehydration condensation, cyclotrimerization coupling and Schiff-base reactions) were used to fabricate CB-STs. Second-generation CB-STs were obtained at the solid–vacuum interface utilizing the Schiff-base reaction between 4,4′′-dialdehyde-1,1′:3′,1′′-terphenyl (TPDAL) and 1,3,5-tris(4-aminophenyl)benzene (TAPB) on Au(111). The CB-ST patterns persist at annealing temperatures as high as 500 K. Homotactic three-fold motifs, insufficient migration and irreversible covalent reaction are the main limitations for growing higher-generation STs. The present results provide new insights on the growth of STs on metal surfaces.

To date, supramolecular chemistry is an ever growing research field owing to its crucial role in molecular catalysis, recognition, medicine, data storage and processing as well as artificial photosynthetic devices. Different isolated supramolecules were prepared by molecular self-assembly on surfaces. This review mainly focuses on supramolecular aggregations on noble metal surfaces studied by scanning tunneling microscopy, including dimers, trimers, tetramers, pentamers, wire-like assemblies and Sierpiński triangular fractals. The variety of self-assembled structures reflects the subtle balance between intermolecular and molecule–substrate interactions, which to some extent may be controlled by molecules, substrates and the molecular coverage. The comparative study of different architectures helps identifying the operative mechanisms that lead to the structural motifs. The application of these mechanisms may lead to novel assemblies with tailored physicochemical properties.This review compares a variety of isolated supramolecular architectures on noble metal surfaces studied by scanning tunneling microscopy. Assemblies as diverse as dimers, trimers, tetramers, pentamers, as well as wire-like structures and Sierpiński triangular fractals on Au(111) and Ag(111) are presented.

To fully achieve potential applications of the double-decker molecules containing rare earth elements as single-molecule magnets in molecular spintronics, it is crucial to understand the 4f states of the rare earth atoms sandwiched in the double-decker molecules by metal electrodes. In this study, low-temperature scanning tunneling microscopy and spectroscopy were employed to investigate the isolated double-decker DyPc2 molecule adsorbed on Au(111) via its differential conductance measurements. The experimental results revealed that the differential conductance maps acquired at a constant height mode simply depicted the authentic molecular orbitals; moreover, the differential conductance maps achieved at a constant current mode could not directly probe the 4f states of the sandwiched Dy atom. This was consistent with the spectra obtained over the molecule center around the Fermi level, indicative of no Kondo feature. Upon decreasing the tip-molecule distance, the CH-mode images presented high-resolution structure but no information of the 4f states. All results indicated that the Dy atom barely contributed to the tunneling current because of the absence of coupling with the microscope tip, echoing the inaccessibility of the Dy 4f states in the double-decker DyPc2 molecule.

Self-similar fractals are of importance in both science and engineering. Metal-organic Sierpiński triangles are particularly attractive for applications in gas separation, catalysis and sensing. Such fractals are constructed in this study by using 120° V-shaped 4,4″-dicyano-1,1′:3′,1″-terphenyl molecules and Fe atoms on Au(1 1 1), and studied in detail by low-temperature scanning tunneling microscopy. Density functional theory calculations are employed to rationalize the invisible Fe atoms in STM images. Monte Carlo simulations are performed to understand the formation mechanism of the surface-supported fractal crystals.Self-similar fractals are of importance in both science and engineering. Such fractals are constructed in this study by using 120° V-shaped 4,4″-dicyano-1,1′:3′,1″-terphenyl molecules and Fe atoms on Au(1 1 1), and studied in detail by low-temperature scanning tunneling microscopy.

Recent studies demonstrate that simple functional molecules, which usually form two-dimensional (2D) crystal structures when adsorbed on solid substrates, are also able to self-assemble into ordered openwork fractal aggregates. To direct and control the growth of such fractal supramolecules, it is necessary to explore the conditions under which both fractal and crystalline patterns develop and coexist. In this contribution, we study the coexistence of Sierpiński triangle (ST) fractals and 2D molecular crystals that were formed by 4,4″-dihydroxy-1,1′:3′,1″-terphenyl molecules on Au(111) in ultrahigh vacuum. Growth competition between the STs and 2D crystals was realized by tuning substrate and molecular surface coverage and changing the functional groups of the molecular building block. Density functional theory calculations and Monte Carlo simulations are used to characterize the process. Both experimental and theoretical results demonstrate the possibility of steering the surface self-assembly to generate fractal and nonfractal structures made up of the same molecular building block.Keywords: fractal; hydrogen bond; molecular crystal; scanning tunneling microscopy; self-assebly; Sierpiński triangle;

Surface reactions of 2,5-diethynyl-1,4-bis(phenylethynyl)benzene on Ag(111), Ag(110), and Ag(100) were systematically explored and scrutinized by scanning tunneling microscopy, molecular mechanics simulations, and density functional theory calculations. On Ag(111), Glaser coupling reaction became dominant, yielding one-dimensional molecular wires formed by covalent bonds. On Ag(110) and Ag(100), however, the terminal alkynes reacted with surface metal atoms, leading to one-dimensional organometallic nanostructures. Detailed experimental and theoretical analyses revealed that such a lattice dependence of the terminal alkyne reaction at surfaces originated from the matching degree between the periodicities of the produced molecular wires and the substrate lattice structures.Keywords: Ag single crystals; lattice match; molecular wires; surface reaction; terminal alkynes;

Chiral pentamers of all-trans-retinoic acid molecules have been prepared on Au(111) surfaces and on a molecular monolayer. Over a range of coverages, pentamers are the building blocks of larger arrays that become increasingly enantiopure. The stability of pentamers is analyzed from experiments on an isomer and a more reactive substrate as well as from density functional theory. The linear shape of the molecule and suitable densities are crucial for the formation of pentamers, driven by cyclic hydrogen bonding between carboxylic acid moieties.

Sierpiński triangle fractals were constructed on both Ag(111) and symmetry-mismatched fourfold Ag(100) surfaces through chemical reaction between H3PH molecules and Fe atoms under vacuum. Density functional theory calculations revealed that the fractals were stabilized by the strong coordination interaction between Fe and O atoms. In comparison, pure H3PH molecules formed fractals via moderately strong hydrogen bonds only on Ag(111), not on Ag(100).

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.

Kinetically controlled hierarchical self-assemblies of all-trans-retinoic acid on Au(111) were investigated via low-temperature scanning tunneling microscopy in ultra-high vacuum. The dominant molecular hierarchical superstructure could be selectively controlled to dimer, tetramer, or pentamer patterns, which were stabilized by hydrogen bonds and van der Waals interactions.

Crystalline structures with Sierpiński triangles as building blocks were constructed via a templating method in an ultra-high vacuum and studied by low-temperature scanning tunneling microscopy. Cobalt atoms and 4,4′′-dicyano-1,1′:3′,1′′-terphenyl molecules were used to build the Sierpiński triangles. The Au(100)-(hex), Au(111)-22 × , and stepped Au(111) surfaces were used as templates to guide the packing of Sierpiński triangles. One-dimensional double chains and two-dimensional small domains of Sierpiński triangles were successfully prepared using the method in experiments.

Hierarchical self-assembly of 13-cis-retinoic acid on Au(111) and Ag(111) was investigated using low-temperature scanning tunnelling microscopy. On both surfaces, molecules form dimers by hydrogen bonds and the dimers arrange into ordered two-dimensional arrays through van der Waals forces. Three packing modes are observed on Au(111) and only one on Ag(111). We tentatively attribute the different patterns on the two surfaces to a stronger molecule–substrate interaction on Ag(111) and site-dependent molecular adsorption on different atomic lattices. In addition, 13-cis-ReA on Au(111) can be made to carry a localized spin.

To fully achieve potential applications of the double-decker molecules containing rare earth elements as single-molecule magnets in molecular spintronics, it is crucial to understand the 4f states of the rare earth atoms sandwiched in the double-decker molecules by metal electrodes. In this study, low-temperature scanning tunneling microscopy and spectroscopy were employed to investigate the isolated double-decker DyPc2 molecule adsorbed on Au(111) via its differential conductance measurements. The experimental results revealed that the differential conductance maps acquired at a constant height mode simply depicted the authentic molecular orbitals; moreover, the differential conductance maps achieved at a constant current mode could not directly probe the 4f states of the sandwiched Dy atom. This was consistent with the spectra obtained over the molecule center around the Fermi level, indicative of no Kondo feature. Upon decreasing the tip-molecule distance, the CH-mode images presented high-resolution structure but no information of the 4f states. All results indicated that the Dy atom barely contributed to the tunneling current because of the absence of coupling with the microscope tip, echoing the inaccessibility of the Dy 4f states in the double-decker DyPc2 molecule.