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Abstract
olecular self-assembly is an efficient approach to fabricate supramolecular nanostructures on well-defined surfaces. The nanostructures can be regulated through functionalizing the molecular precursors with different functional groups. Here, from an interplay of high-resolution scanning tunneling microscopy imaging and density functional theory calculations, we have at the atomic scale investigated the influence of tert-butyl groups on the on-surface self-assembled behaviors of the organic molecules where intermolecular interactions mainly originate from relatively weak van der Waals interactions. Our results demonstrate that the tert-butyl groups can not only affect the adsorption geometry but also change the self-assembled properties of organic molecules on surfaces due to the enhanced intermolecular interactions.
... Fig. 7.7 b) shows a highly-resolved zoom into an area with a HB and a P grain on the left and right side, respectively, which reveals that both arrangements consist of the identical two building blocks (encircled). Both building blocks consist of a group of four small features in the center and two large elongated features at the ends, which can be readily attributed to the phenyl groups with the attached tert-butyl groups [198,261,264,251] and the perylene-derived core of ID28, respectively. Therefore, both building blocks can be identified as consisting of a pair of ID28 molecules that have their long molecular axis ca. ...
... 7.8 i) and j) and Fig. 7.11 d)]. This particular arrangement might be favored by (at least) three possible contributions: Firstly, intermolecular attraction mediated by tert-butyl-groups, as was recently shown for di(tert-butyl)terphenyl/Au(111) [264], secondly, a dense packing facilitated by the interdigitation of the tert-butyl-groups, similar to what was observed for hexa-tert-butylhexabenzocoronene/Cu(110) [251], and thirdly, the formation of an anti-ferroelectric phase, resulting from the antiparallel arrangement. Notably, an enantiopure pair deviates significantly from the side-by-side antiparallel arrangement that would be expected when simply assuming two ideal dipoles with the electrostatic potential of ID28, which is also shown in Fig. 7.15 a) for comparison. ...
Diese Arbeit behandelt elektronische und strukturelle Eigenschaften dünner Schichten aus konjugierten organischen Molekülen (COMs), aufgebracht auf Metalloberflächen per Vakuum-Sublimation. Diese Eigenschaften sind essenziell für Funktionsrealisierung und -optimierung organischer Elektronikbauteile. Teil 1 diskutiert zwei Ansätze zur Energieniveauanpassung (ELA) an Organik-Metall-Grenzflächen zur Einstellung der dortigen Löcherinjektionsbarrieren (HIBs) durch (Über-)Kompensation des abträglichen "Push-back"-Effekts: - Ausnutzung der besonderen ELA bei Chalkogen-Metall-Bindungen, hier gezeigt mit Hilfe von Röntgen- und Ultraviolettphotoelektronenspektroskopie (UPS/XPS) für ein Seleno-funktionalisiertes COM - Einfügen von COMs mit ausgeprägtem Elektronen-Akzeptorcharakter vor dem Aufbringen der aktiven Schicht. UPS-Messungen zeigen, dass beide Ansätze HIBs von ca. 0.3 eV ermöglichen. Teil 2 untersucht ausgewählte organische Heterostrukturen auf Metallen. Die Untersuchungen identifizieren einen Ladungstransfer vom Metall zur Überschicht (MOCT) als verantwortlich dafür, das System bei Ferminiveau-Pinning in den Gleichgewichtszustand zu überführen. Detaillierte Untersuchungen gestatten die Identifikaton von ganzzahligem Ladungstransfer zu einem Teil der Moleküle in der ersten Überschichtlage und den Einfluss der Dipol-Abstoßung in der Überschicht. In Teil 3 dienen Metalloberflächen als Auflage für supramolekulare Architekturen mit dipolaren Bausteinen. Rastertunnelmikroskopie (STM) an einer Serie von teils partiell fluorierten, stäbchenförmigen COMs mit unterschiedlich großen Dipolmomenten ermöglicht die Entflechtung von Dipol-Dipol- und konkurierenden Wechselwirkungen physisorbierter Submonolagen auf Ag(111). Ein anderes, stark dipolares COM bildet bei Monolagenbedeckung auf Au(111) sechs Phasen, alle mit antiferroelektrischer Einheitszellen. UPS-Messungen ergeben eine bevorzugte Ausrichtung der Moleküle in Multilagen.
... In previous work, we and other researchers also found that the tert-butyl substituent on a molecule could influence the molecular adsorption conformation, diffusion, and assembly behavior. 16,20,21 Therefore, it is of particular interest to explore the possibility of constructing on-surface nanostructures by modifying the organic molecules with tert-butyl substituents for more general cases. ...
... In conclusion, from a combination of high-resolution STM imaging and DFT modeling, we have investigated the role of tert-butyl substituent in constructing molecular nanostructures on substrates. As combined with our previous studied system, 20 it was further indicated that the tert-butyl substituent could lead to the changing of the molecule adsorption geometry on surfaces and vary the intermolecular interactions. Such a method of tert-butyl groups modified on organic molecule may provide a relatively general and simple route for building desired functional network structures on surfaces. ...
On-surface formation of two-dimensional supramolecular network structures using molecular self-assembly has been an efficient and the most widely employed method. Through delicate modifying the molecular candidates with specific substituents, it is possible to build on-surface nanostructures according to one’s will. Here, from the interplay of high-resolution STM imaging and DFT calculations we have investigated role of the tert-butyl substituent in the formation of self-assembled network structures by planar aromatic molecules on metal surfaces. Our results demonstrate that the tert-butyl groups can change adsorption geometry of molecule from entirely flat-lying to a bit of upright-standing and vary the intermolecular interactions, which resulted in the formation of two-dimensional supramolecular network structures on both Au(111) and Ag(110). These findings further present that tert-butyl substituent could be a good candidate for fabricating on-surface self-assembled nanostructures from more general aromatic molecules.
... Especially, the tert-butylphenyl groups present in the compound contribute toward the better self-assembly in the thin film. [37] This uniform coverage of the thin films can ensure excellent charge transfer across the surface leading to better performance of these thin films showing improved memory characteristics. Single crystal analysis is performed for compounds 9 a-c, which were concocted by the slow evaporation of chloroform: toluene mixture. ...
Donor‐Acceptor systems are highly appreciated in the field of organic memory devices due to their efficient charge transport within the systems. In this work, we have designed and synthesized a D−π−A system constituting ester‐flanked quinolines and functionalized triarylamines (TAA) through a single‐step cross‐coupling reaction to fabricate memory devices via Write‐Once Read‐Many times (WORM) non‐volatile memory. Structure‐property relationships are reconnoitered for these conjugated D−π−A systems through a series of UV, fluorescence, XRD, DFT, and memory characterizations. The UV and CV data show efficient charge transfer with intramolecular charge transfer occurring at 407–417 nm and a short band gap of 2.56–2.65 eV. An enhancement in the resistive switching behavior of the memory devices is observed for the compounds with simple TAA‐quinoline and tert‐butylphenyl substituted TAA and fluorophenyl substituted quinoline due to balanced charge distribution in the compounds. This enhanced switching induces an on/off ratio of 10³ by generating a highly ordered arrangement in the thin films. The HOMO, LUMO levels, and the ESP images together estimate a charge transfer and charge trapping as the plausible mechanism for the solution‐processable WORM memory devices. The longer retention time (10³ s) and lower threshold voltages (−1.21–−2.12 V) of the devices makes them intriguing compounds for memory applications.
... 5−7 The intermolecular interactions are central to the self-assembly, especially on relatively inert surfaces. Inspired by solution chemistry, intermolecular interactions including van der Waals (vdW) force, 8,9 hydrogen bonding, 10−12 dipole−dipole interaction, 13,14 electrostatic interaction, 15−17 and coordination interaction 18,19 have been successfully introduced on surfaces to construct various nanoarchitectures. Generally, the molecular precursors are functionalized with one kind of functional group for directing the self-assembly. ...
On-surface self-assembly from molecular building blocks directed by supramolecular interactions has been widely reckoned as an efficient method for controllable construction of low-dimensional nanostructures and nanomaterials. Numerous efforts have been devoted to exploring the self-assembled behaviors of molecular precursors on different surfaces and unravelling the underlying mechanism. Generally, the molecular precursors are functionalized with one kind of functional groups for directing the self-assembly. In this study, by combining real-space direct visualization and DFT calculations, we have investigated the self-assembly behaviors of an organic molecule functionalized by two different functional groups: terminal alkyne and aldehyde groups on Au(111). An ordered racemic island nanostructure is formed on Au(111), which is resulted from the hybrid interactions between the two functional groups. Detailed DFT calculations have been performed to compare the different binding ways and binding strengths between the organic molecules.
Tert-butyl functional groups can modulate the self-assembly behavior of organic molecules on surfaces. However, the precise construction of supramolecular architectures through their controlled thermal removal remains a challenge. Herein, we precisely controlled the removal amount of tert-butyl groups in tetraazaperopyrene derivatives by stepwise annealing on Ag(111). The evolution of 4tBu-TAPP supramolecular self-assembly from the grid-like structure composed of 3tBu-TAPP through the honeycomb network formed by 2tBu-TAPP to the one-dimensional chain co-assembled by tBu-TAPP and TAPP was successfully realized. This series of supramolecular nanostructures were directly visualized by high resolution scanning tunneling microscopy. Tip manipulation and density functional theory calculations show that the formation of honeycomb network structure can be attributed to the van der Waals interactions, N–Ag–N coordination bonds, and weak C–H⋯N hydrogen bonds. Further addition of two tert-butyl groups (6tBu-TAPP) leads to a completely different assembly evolution, due to the fact that the additional tert-butyl groups affect the molecular adsorption behavior and ultimately induce desorption. This work can possibly be exploited in constructing stable and long-range ordered nanostructures in surface-assisted systems, which can also promote the development of nanostructures in functional molecular devices.
Iodine adsorption on metal surfaces has been extensively studied, but iodine’s effect on modulating the self-assembly structures of organic molecules has been less discussed. Here, we used low-temperature scanning tunneling microscopy and density functional theory to systematically study the self-assembly structure transformation of organic molecules affected by iodine dosing on the Ag(111) surface. We chose 2,6-naphthalenedicarboxylic acid and persilylated hexaethynylbenzene molecules and their reaction products as models to explore the changes in the molecular self-assembly structures on the Ag(111) surface induced by iodine deposition. Some new self-assembly structures are obtained by depositing iodine onto the molecules’ co-covered surface at different stages. These findings show that iodine can modulate the self-assembly structure of organic molecules with a weak intermolecular interaction, while those with a stronger intermolecular hydrogen bonding are not affected. The trend is verified with the density functional theory calculations.
A series of phenanthroline functionalized triarylamines (TAA) has been designed and synthesised to evaluate their OFET characteristics. Solution processed OFET devices have exhibited p-channel/ambipolar behaviour with respect to the substituents, in particular methoxyphenyl substitution resulted with highest mobility (m h) up to 1.1 cm 2 V À1 s À1 with good I on/off (10 6) ratio. These compounds can be potentially utilized for the fabrication of electronic devices.
We studied the self-assembly of melem on the Au(111) and Ag(111) surface. By scanning tunneling microscopy imaging, we have observed two different STM appearances of the melem molecule within the self-assembled nanostructure on Au(111), which are resulted from the different intermolecular bonding configurations. Moreover, further DFT details including the intermolecular charge density difference and bonding energy have also been performed to compare the different nature between intermolecular bonding configurations.
Two-dimensional self-assemblies of four partially fluorinated molecules, 1,4-bis(2,6-difluoropyridin-4-yl)benzene, 4,4'-bis(2,6-difluoropyridin-4-yl)-1,1'-biphenyl, 4,4'-bis(2,6-difluoropyridin-4-yl)-1,1':4',1”-terphenyl and 4,4'-bis(2,6-difluoropyridin-3-yl)-1,1'-biphenyl, involving weak intermolecular C-H…F and C-H…N hydrogen bonds were systematically investigated on Au(111) with low-temperature scanning tunneling microscopy. The inter-molecular connecting modes and binding sites were closely related to the backbones of the building blocks, i.e., the molecule length determines its binding sites with neighboring molecules in the assemblies while the attaching positions of the N and F atoms dictate its approaching and docking angles. The experimental results demonstrate that multiple weak hydrogen bonds such as C-H…F and C-H…N can be efficiently applied to tune the molecular orientations and the self-assembly structures accordingly.
The design of future materials for biotechnological applications via deposition of molecules on surfaces will require not only exquisite control of the deposition procedure, but of equal importance will be our ability to predict the shapes and stability of individual molecules on various surfaces. Furthermore, one will need to be able to predict the structure patterns generated during the self-organization of whole layers of (bio)molecules on the surface. In this review, we present an overview over the current state of the art regarding the prediction and clarification of structures of biomolecules on surfaces using theoretical and computational methods.
9-(bis-p-tert-octylphenyl)-amino-perylene-3, 4-dicarboxy anhydride (BOPA-PDCA) is a strongly dipolar molecule representing a group of asymmetrically substituted perylenes that are employed in dye-sensitized solar cells and hold great promise for discotic liquid crystal applications. Thin BOPA-PDCA Films with orientated dipole moments can potentially be used to tune the energy-level alignment in electronic devices and store information. To help assessing these prospects, we here elucidate the molecular self-assembly and electronic structure of BOPA-PCDA employing room temperature scanning tunneling microscopy and spectroscopy in combination with ultraviolet and X-ray photoelectron spectroscopies. BOPA-PCDA monolayers on Au(111) exclusively form in-plane antiferroelectric phases. The molecular arrangements, the increase of the average number of molecules per unit cell via ripening, and the rearrangement upon manipulation with the STM tip indicate an influence of the dipole moment on the molecular assembly and the rearrangement. A slightly preferred out-of-plane orientation of the molecules in the multilayer induces a surface potential of 1.2 eV. This resembles the giant surface potential effect that was reported for vacuum-deposited tris(8-hydroxyquinoline)aluminum and deemed applicable for data storage. Notably, the surface potential in the case of BOPA-PDCA can in part be reversibly removed by visible light irradiation.
The construction of ordered on-surface nanostructures from molecular self-assembly has attracted wide attention due to their potential applications in nanoscience and nanotechnology. To date, main efforts have been concentrated on delicately functionalizing the molecular precursors with certain functional groups to induce specific intermolecular interactions such as hydrogen bonding, van der Waals force, metal-ligand ionic bonding and metal-ligand coordination bonding. Here, from high-resolution UHV-STM imaging and DFT calculations we have demonstrated a novel method to construct well-ordered molecular nanostructures of an unfunctionalized aromatic molecule (4Ph) on both Ag(110) and Cu(110) by introducing oxygen molecules. The dissociated oxygen atoms are found to locate in-between 4Ph molecules to facilitate the formation of ordered molecular arrangements by electrostatic interactions. In addition, single Ag-O/Cu-O chains could also be fabricated by deposition of oxygen molecules on 4Ph-molecule-covered surfaces, and meanwhile guide the alignment of 4Ph molecules along the chains. These findings exhibit an alternative route for constructing ordered molecular nanostructures on surfaces, especially for unfunctionalized aromatic molecules
We have performed the systematic studies on three structurally similar aromatic molecules with different functional groups on a Cu(110) surface and investigated their on-surface molecular diffusion behaviors by the interplay of scanning tunneling microscopy imaging and density functional theory calculations. We have found that the tert-butyl groups could significantly affect the molecular adsorption geometries and moreover the mobility of the molecules on the surface. These findings could give further insights into the understanding of diffusion behaviors of organic molecules specifically with tert-butyl groups on surfaces.
We present the design and performance of a high-pressure scanning tunneling microscope (HP-STM), which allows atom-resolved imaging of metal surfaces at pressures ranging from ultrahigh vacuum (UHV) to atmospheric pressures (1×10-10-1000 mbar) on a routine basis. The HP-STM is integrated in a gold-plated high-pressure cell with a volume of only ~0.5 l, which is attached directly to an UHV preparation/analysis chamber. The latter facilitates quick sample transfer between the UHV chamber and the high-pressure cell, and allows for in situ chemical and structural analysis by a number of analytical UHV techniques incorporated in the UHV chamber. Reactant gases are admitted to the high-pressure cell via a dedicated gas handling system, which includes several stages of gas purification. The use of ultrapure gasses is essential when working at high pressures in order to achieve well-defined experimental conditions. The latter is demonstrated in the case of H/Cu(110) at atmospheric H2 pressures where impurity-related structures were observed.
The native copper adatoms get trapped in a self-assembled molecular nanostructure which is mainly formed by the intermolecular van der Waals interactions, and two dominating specific binding modes between the adatoms and the molecules are revealed at the atomic scale by high-resolution STM imaging.
The adsorption of 2,6-naphthalenedicarboxylic acid (NDCA) molecules on the Ag(110), Cu(110), and Ag(111) surfaces at room
temperature has been studied by means of scanning tunnelling microscopy (STM). Further supporting results were obtained using
X-ray photoelectron spectroscopy (XPS) and soft X-ray absorption spectroscopy (XAS). On the Ag(110) support, which had an
average terrace width of only 15 nm, the NDCA molecules form extended one-dimensional (1-D) assemblies, which are oriented
perpendicular to the step edges and have lengths of several hundred nanometres. This shows that the assemblies have a large
tolerance to monatomic surface steps on the Ag(110) surface. The observed behaviour is explained in terms of strong intermolecular
hydrogen bonding and a strong surface-mediated directionality, assisted by a sufficient degree of molecular backbone flexibility.
In contrast, the same kind of step-edge crossing is not observed when the molecules are adsorbed on the isotropic Ag(111)
or more reactive Cu(110) surfaces. On Ag(111), similar 1-D assemblies are formed to those on Ag(110), but they are oriented
along the step edges. On Cu(110), the carboxylic groups of NDCA are deprotonated and form covalent bonds to the surface, a
situation which is also achieved on Ag(110) by annealing to 200 °C. These results show that the formation of particular self-assembled
molecular nanostructures depends significantly on a subtle balance between the adsorbate-adsorbate and adsorbate-substrate
interactions and that kinetic factors play an important role.
KeywordsMolecular self-assembly-hydrogen bonding-scanning tunnelling microscopy-X-ray photoelectron spectroscopy
We present an efficient scheme for calculating the Kohn-Sham ground state of metallic systems using pseudopotentials and a plane-wave basis set. In the first part the application of Pulay's DIIS method (direct inversion in the iterative subspace) to the iterative diagonalization of large matrices will be discussed. Our approach is stable, reliable, and minimizes the number of order N-atoms(3) operations. In the second part, we will discuss an efficient mixing scheme also based on Pulay's scheme. A special ''metric'' and a special ''preconditioning'' optimized for a plane-wave basis set will be introduced. Scaling of the method will be discussed in detail for non-self-consistent calculations. It will be shown that the number of iterations required to obtain a specific precision is almost independent of the system size. Altogether an order N-atoms(2) scaling is found for systems up to 100 electrons. If we take into account that the number of k points can be implemented these algorithms within a powerful package called VASP (Vienna ab initio simulation package). The program and the techniques have been used successfully for a large number of different systems (liquid and amorphous semiconductors, liquid simple and transition metals, metallic and semiconducting surfaces, phonons in simple metals, transition metals, and semiconductors) and turned out to be very reliable.
A low-temperature scanning tunneling microscopy (LT-STM) study of four slightly different hydrocarbons based on hexaphenylbenzene (HPB) and hexa-peri-hexabenzocoronene (HBC) on the Cu(111) surface at submonolayer coverage is presented. All molecules show a commensurate monolayer structure, with significant differences in structure and growth mode. We find that the long range order and the size of defect free domains increase when the intermolecular interaction becomes stronger with respect to the molecule-substrate interaction. Moreover, we can assign adsorption and self-ordering properties to the different chemical groups within the molecules.
The formal relationship between ultrasoft (US) Vanderbilt-type pseudopotentials and Blöchl's projector augmented wave (PAW) method is derived. It is shown that the total energy functional for US pseudopotentials can be obtained by linearization of two terms in a slightly modified PAW total energy functional. The Hamilton operator, the forces, and the stress tensor are derived for this modified PAW functional. A simple way to implement the PAW method in existing plane-wave codes supporting US pseudopotentials is pointed out. In addition, critical tests are presented to compare the accuracy and efficiency of the PAW and the US pseudopotential method with relaxed core all electron methods. These tests include small molecules (H2, H2O, Li2, N2, F2, BF3, SiF4) and several bulk systems (diamond, Si, V, Li, Ca, CaF2, Fe, Co, Ni). Particular attention is paid to the bulk properties and magnetic energies of Fe, Co, and Ni.
Supramolecular chemistry has a very large impact on chemistry of current interest and the use of non-covalent but directional forces is appealing for the construction of 'supramolecular architectures'. The invention of scanning probe microscopy techniques has opened new doorways to study these concepts on surfaces. This review deals with recent progress in the study of two-dimensional supramolecular self-assembly on surfaces probed by scanning tunneling microscopy, with a special emphasis on structure, dynamics and reactivity of hydrogen bonded systems.
The fabrication methods of the microelectronics industry have been refined to produce ever smaller devices, but will soon reach their fundamental limits. A promising alternative route to even smaller functional systems with nanometre dimensions is the autonomous ordering and assembly of atoms and molecules on atomically well-defined surfaces. This approach combines ease of fabrication with exquisite control over the shape, composition and mesoscale organization of the surface structures formed. Once the mechanisms controlling the self-ordering phenomena are fully understood, the self-assembly and growth processes can be steered to create a wide range of surface nanostructures from metallic, semiconducting and molecular materials.
The DNA origami method, in which long, single-stranded DNA segments are folded into shapes by short staple segments, was used to create nucleic acid probe tiles that are molecular analogs of macroscopic DNA chips. One hundred trillion probe tiles were fabricated in one step and bear pairs of 20-nucleotide-long single-stranded DNA segments that act as probe sequences. These tiles can hybridize to their targets in solution and, after adsorption onto mica surfaces, can be examined by atomic force microscopy in order to quantify binding events, because the probe segments greatly increase in stiffness upon hybridization. The nucleic acid probe tiles have been used to study position-dependent hybridization on the nanoscale and have also been used for label-free detection of RNA.
A monoalkylated tetrathiafulvalene derivative forms multilayer structures at the solid-liquid interface, with high density of cross junctions, which are interesting for molecular electronic circuit self-assembly.
The fabrication methods of the microelectronics industry have been refined to produce ever smaller devices, but will soon reach their fundamental limits. A promising alternative route to even smaller functional systems with nanometre dimensions is the autonomous ordering and assembly of atoms and molecules on atomically well-defined surfaces. This approach combines ease of fabrication with exquisite control over the shape, composition and mesoscale organization of the surface structures formed. Once the mechanisms controlling the self-ordering phenomena are fully understood, the self-assembly and growth processes can be steered to create a wide range of surface nanostructures from metallic, semiconducting and molecular materials. © 2010 Nature Publishing Group, a division of Macmillan Publishers Limited and published by World Scientific Publishing Co. under licence. All Rights Reserved.
We report controlled self-assemblies of two molecules as crystalline arrays on gold substrates at room temperature. The size, shape, orientation, and ordered assemblies of molecular wires can be engineered through a delicate interplay of the intermolecular pi-pi stacking and chemisorptive substrate-linker interactions. In particular, the molecule based on a fused aromatic, anthracene, forms an ordered 2D stacked array. Through scanning tunneling spectroscopy, we demonstrate changes in the electronic behavior of single molecules that form into superlattices. The ability to assemble predictable 2D crystals and understand the order-related electronic behavior of single molecular components could allow for the future design of nanopatterned arrays as a controlled platform toward further miniaturization of electronic devices.
On-surface fabrication of covalently interlinked conjugated nanostructures has attracted significant attentions mainly due to their high stability and efficient electron transportability. Here, from the interplay of scanning tunneling microscopy imaging and density functional theory calculations we have for the first time reported on-surface formation of one-dimensional polyphenylene chains through Bergman cyclization followed by radical polymerization on Cu(110). The formed surface nanostructures are further corroborated by the results of the ex-situ synthesized molecular product after Bergman cyclization. These findings are of particular interest and importance to the construction of molecular electronic nanodevices on surfaces.
The realization of molecule-based miniature devices with advanced functions requires the development of new and efficient approaches for combining molecular building blocks into desired functional structures, ideally with these structures supported on suitable substrates. Supramolecular aggregation occurs spontaneously and can lead to controlled structures if selective and directional non-covalent interactions are exploited. But such selective supramolecular assembly has yielded almost exclusively crystals or dissolved structures; the self-assembly of absorbed molecules into larger structures, in contrast, has not yet been directed by controlling selective intermolecular interactions. Here we report the formation of surface-supported supramolecular structures whose size and aggregation pattern are rationally controlled by tuning the non-covalent interactions between individual absorbed molecules. Using low-temperature scanning tunnelling microscopy, we show that substituted porphyrin molecules adsorbed on a gold surface form monomers, trimers, tetramers or extended wire-like structures. We find that each structure corresponds in a predictable fashion to the geometric and chemical nature of the porphyrin substituents that mediate the interactions between individual adsorbed molecules. Our findings suggest that careful placement of functional groups that are able to participate in directed noncovalent interactions will allow the rational design and construction of a wide range of supramolecular architectures absorbed to surfaces.
An approach for electronic structure calculations is described that generalizes both the pseudopotential method and the linear augmented-plane-wave (LAPW) method in a natural way. The method allows high-quality first-principles molecular-dynamics calculations to be performed using the original fictitious Lagrangian approach of Car and Parrinello. Like the LAPW method it can be used to treat first-row and transition-metal elements with affordable effort and provides access to the full wave function. The augmentation procedure is generalized in that partial-wave expansions are not determined by the value and the derivative of the envelope function at some muffin-tin radius, but rather by the overlap with localized projector functions. The pseudopotential approach based on generalized separable pseudopotentials can be regained by a simple approximation.
Scanning tunnelling microscopy (STM) has proved to be a fascinating and powerful technique in the field of surface science. The fact that sets the STM apart from most other surface sensitive techniques is its ability to resolve the structure of surfaces on an atomic scale, that is atom-by-atom, and furthermore its ability to study the dynamics of surface processes. This article presents a survey of recent STM studies of well characterized single crystal metal surfaces under ultra-high vacuum conditions. It particularly addresses STM investigations of clean metal surfaces, adsorbates on metal surfaces, adsorbate-induced restructuring of metal surfaces, chemical reactions on metal surfaces, metal-on-metal growth and finally studies of electron confinement and quantum size effects on metal surfaces.
Enantioselective heterogeneous catalysis is an important and rapidly expanding research area. The two most heavily researched examples of this type of catalysis are the enantioselective hydrogenation of -ketoesters over Pt-based catalysts and the enantioselective hydrogenation of -ketoesters over Ni-based catalysts. These systems share one extremely important common feature—the enantioselective surface reaction is controlled by the presence of adsorbed chiral molecules (modifiers) on the surface of the metal component of the catalyst. In each system, a number of models have been proposed to explain the enantioselective behavior in the light of catalytic experiments. In recent years, surface science has begun to address the issues relevant to this branch of catalysis. This article reviews to what extent surface science has enabled the verification of the proposed models and, in addition, what new light surface science has shed on the possible mechanisms of enantioselective heterogeneous catalysis.
The products of photoreactions of conjugated organic molecules may be allowed by selection rules but not observed in solution
reactions because of unfavorable reaction geometries. We have used defect sites in self-assembled alkanethiolate monolayers
on gold surfaces to direct geometrically unfavorable photochemical reactions between individual organic molecules. High conductivity
and stochastic switching of anthracene-terminated phenylethynylthiolates within alkanethiolate monolayers, as well as in situ
photochemical transformations, have been observed and distinguished with the scanning tunneling microscope (STM). Ultraviolet
light absorbed during imaging increases the apparent heights of excited molecules in STM images, a direct manifestation of
probing electronically excited states.
The design of networks of organic molecules at metal surfaces, highly attractive for a variety of applications ranging from molecular electronics to gas sensors to protective coatings, has matured to a degree that patterns with multinanometre unit cells and almost any arbitrary geometry can be fabricated. This Review provides an overview of vacuum-deposited organic networks at metal surfaces, using intermolecular hydrogen bonding, metal-atom coordination and in situ polymerization. Recent progress in these areas highlights how the design of surface patterns can benefit from the wealth of information available from solution- and bulk-phase chemistry, while at the same time providing novel insights into the nature of such bonds through the applicability of direct scanning probe imaging at metal surfaces.
Supramolecular self-assembly on surfaces, guided by hydrogen bonding interactions, has been widely studied, most often involving planar compounds confined directly onto surfaces in a planar two-dimensional (2-D) geometry and equipped with structurally rigid chemical functionalities to direct the self-assembly. In contrast, so-called molecular Landers are a class of compounds that exhibit a pronounced three-dimensional (3-D) structure once adsorbed on surfaces, arising from a molecular backboard equipped with bulky groups which act as spacer legs. Here we demonstrate the first examples of extended, hydrogen-bonded surface architectures formed from molecular Landers. Using high-resolution scanning tunnelling microscopy (STM) under well controlled ultrahigh vacuum conditions we characterize both one-dimensional (1-D) chains as well as five distinct long-range ordered 2-D supramolecular networks formed on a Au(111) surface from a specially designed Lander molecule equipped with dual diamino-triazine (DAT) functional moieties, enabling complementary NH...N hydrogen bonding. Most interestingly, comparison of experimental results to STM image calculations and molecular mechanics structural modeling demonstrates that the observed molecular Lander-DAT structures can be rationalized through characteristic intermolecular hydrogen bonding coupling motifs which would not have been possible in purely planar 2-D surface assembly because they involve pronounced 3-D optimization of the bonding configurations. The described 1-D and 2-D patterns of Lander-DAT molecules may potentially be used as extended molecular molds for the nucleation and growth of complex metallic nanostructures.
We employed de novo synthesized porphyrin modules to construct discrete cyclic supramolecular architectures supported on a copper surface. The programmed geometry and functionality of the molecular modules together with their conformational flexibility and substrate interaction yields symmetric discrete assemblies, including dimers and chains as well as three- to six-membered cyclic structures. The area of the molecular cavities is extended by creating bicomponent structures combining building blocks with different symmetry.
Roberto Otero is acknowledged for stimulating discussion. We acknowledge the financial support from the Danish Ministry for Science, Technology, and Innovation for the iNANO Center, from the Danish Research Councils, and from the Carlsberg Foundation. H.G acknowledges the Marie Curie-Intra-European Fellowship (MEIF-CT-2004-010038). We would also like to acknowledge the computer time on the HPCx supercomputer provided via the Materials Chemistry Consortium. R.E.A.K. is also grateful to the EPSRC for financial support (grant GR/P01427/01).
We show that quantum-mechanical molecular-dynamics simulations in a finite-temperature local-density approximation based on the calculation of the electronic ground state and of the Hellmann-Feynman forces after each time step are feasible for liquid noble and transition metals. This is possible with the use of Vanderbilt-type ``ultrasoft'' pseudopotentials and efficient conjugate-gradient techniques for the determination of the electronic ground state. Results for liquid copper and vanadium are presented.
Generalized gradient approximations (GGA{close_quote}s) for the exchange-correlation energy improve upon the local spin density (LSD) description of atoms, molecules, and solids. We present a simple derivation of a simple GGA, in which all parameters (other than those in LSD) are fundamental constants. Only general features of the detailed construction underlying the Perdew-Wang 1991 (PW91) GGA are invoked. Improvements over PW91 include an accurate description of the linear response of the uniform electron gas, correct behavior under uniform scaling, and a smoother potential. {copyright} {ital 1996 The American Physical Society.}
From the interplay of scanning tunneling microscopy and theoretical calculations, we study the chiral self-assembly of achiral HtB-HBC molecules upon adsorption on the Cu(110) surface. We find that chirality is expressed at two different levels: a +/-5 degrees rotation of the molecular axis with respect to the close-packed direction of the Cu(110) substrate and a chiral close-packed arrangement expected for star-shaped molecules in 2D. Out of the four possible chiral expressions, only two are found to exist due the effect of van der Waals (vdW) interactions forcing the molecules to simultaneously adjust to the atomic template of the substrate geometry and self-assemble in a close-packed geometry.
A new density functional (DF) of the generalized gradient approximation (GGA) type for general chemistry applications termed B97-D is proposed. It is based on Becke's power-series ansatz from 1997 and is explicitly parameterized by including damped atom-pairwise dispersion corrections of the form C(6) x R(-6). A general computational scheme for the parameters used in this correction has been established and parameters for elements up to xenon and a scaling factor for the dispersion part for several common density functionals (BLYP, PBE, TPSS, B3LYP) are reported. The new functional is tested in comparison with other GGAs and the B3LYP hybrid functional on standard thermochemical benchmark sets, for 40 noncovalently bound complexes, including large stacked aromatic molecules and group II element clusters, and for the computation of molecular geometries. Further cross-validation tests were performed for organometallic reactions and other difficult problems for standard functionals. In summary, it is found that B97-D belongs to one of the most accurate general purpose GGAs, reaching, for example for the G97/2 set of heat of formations, a mean absolute deviation of only 3.8 kcal mol(-1). The performance for noncovalently bound systems including many pure van der Waals complexes is exceptionally good, reaching on the average CCSD(T) accuracy. The basic strategy in the development to restrict the density functional description to shorter electron correlation lengths scales and to describe situations with medium to large interatomic distances by damped C(6) x R(-6) terms seems to be very successful, as demonstrated for some notoriously difficult reactions. As an example, for the isomerization of larger branched to linear alkanes, B97-D is the only DF available that yields the right sign for the energy difference. From a practical point of view, the new functional seems to be quite robust and it is thus suggested as an efficient and accurate quantum chemical method for large systems where dispersion forces are of general importance.
The engineering of highly organized systems from instructed molecular building blocks opens up new vistas for the control of matter and the exploration of nanodevice concepts. Recent investigations demonstrate that well-defined surfaces provide versatile platforms for steering and monitoring the assembly of molecular nanoarchitectures in exquisite detail. This review delineates the principles of noncovalent synthesis on metal substrates under ultrahigh vacuum conditions and briefly assesses the pertaining terminology-self-assembly, self-organization, and self-organized growth. It presents exemplary scanning-tunneling-microscopy observations, providing atomistic insight into the self-assembly of organic clusters, chains, and superlattices, and the metal-directed assembly of low-dimensional coordination architectures. This review also describes hierarchic-assembly protocols leading to intricate multilevel order. Molecular architectonic on metal surfaces represents a versatile rationale to realize structurally complex nanosystems with specific shape, composition, and functional properties, which bear promise for technological applications.
Bei zunehmender Bedeckung geht die Selbstorganisation des Fullerenderivats Phenyl-C61-buttersäuremethylester (PCBM) auf Au(111) von substratgesteuert zu wasserstoffbrückengesteuert über. Bei niedriger Bedeckung lagert sich PCBM nur auf den fcc-Bereichen der „Fischgräten“-Rekonstruktion ab (linkes Bild). Bei höherer Bedeckung bilden sich Doppelreihen aus PCBM-Molekülen, die durch H-Brücken verbunden sind (rechtes Bild). ML: Monoschicht.
A study was conducted, using high-resolution scanning tunneling microscopy (STM) imaging and manipulation, to investigate the spontaneous assembly and STM-induced disassembly of one of the DNA base molecules, thymine (T), deposited on the gold (Au) surface under ultrahigh vacuum (UHV) conditions. The Au (111) substrate was chosen as a noble inert substrate, to minimize molecule-substrate interactions, allowing the self-assembly process to be dominated by intermolecular interactions. The STM images revealed a two-step self-assembly process, in which hydrogen bonds influenced the growth of one dimensional (ID) filaments of T molecules that reassemble into 2D T islands the weaker van der Waals (vdW) interactions. It was also demonstrated that the hierarchy bond strengths involved in the surface self-assembly of T molecules, with highly anisotropic interactions, can be directly probed using STM.
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