Synthesis of mesoscale ordered two-dimensional π-conjugated polymers with semiconducting properties

Two-dimensional materials with high charge carrier mobility and tunable band gaps have attracted intense research effort for their potential use in nanoelectronics. Two-dimensional π-conjugated polymers constitute a promising subclass because the band structure can be manipulated by varying the molecular building blocks while preserving key features such as Dirac cones and high charge mobility. The major barriers to the application of two-dimensional π-conjugated polymers have been the small domain size and high defect density attained in the syntheses explored so far. Here, we demonstrate the fabrication of mesoscale ordered two-dimensional π-conjugated polymer kagome lattices with semiconducting properties, Dirac cone structures and flat bands on Au(111). This material has been obtained by combining a rigid azatriangulene precursor and a hot dosing approach, which favours molecular diffusion and eliminates voids in the network. These results open opportunities for the synthesis of two-dimensional π-conjugated polymer Dirac cone materials and their integration into devices.

Temperature-induced molecular reorganization on Au(111) driven by oligomeric defects

The formation of ordered molecular structures on surfaces is determined by the balance between molecule-molecule and molecule-substrate interactions. Whether the aggregation process is guided by non-covalent forces or on-surface reactions, a deeper understanding of these interactions is pivotal to formulating a priori predictions of the final structural features and the development of bottom-up fabrication protocols. Theoretical models of molecular systems corroborate the information gathered through experimental observations and help explain the thermodynamic factors that underpin on-surface phase transitions. Here, we report a scanning tunneling microscopy investigation of a tribromo-substituted heterotriangulene on the Au(111) surface, which initially forms an extended close-packed ordered structure stabilized by BrBr halogen bonds when deposited at room temperature. X-ray photoelectron spectroscopy reveals that annealing the self-assembled layer induces a fraction of the molecular precursors to partially dehalogenate that in turn leads to the formation of a less stable BrO non-covalent network which coexists with the short oligomers. Density functional theory (DFT) and Monte Carlo (MC) simulations illustrate how dimer moieties act as defects whose steric hindrance prevents the retention of the more stable configuration. A small number of dimers is sufficient to drive the molecular reorganization into a lower cohesive energy phase. Our study shows the importance of a combined DFT - MC approach to understand the evolution of molecular systems on substrates.

Surface-mediated assembly, polymerization and degradation of thiophene-based monomers

Ullmann coupling of halogenated aromatics is widely used in on-surface synthesis of two-dimensional (2D) polymers and graphene nanoribbons. It stands out among other reactions for regioselectively connecting aromatic monomers into 1D and 2D π-conjugated polymers, whose final structure and properties are determined by the initial building blocks. Thanks to their exceptional electronic properties, thiophene-containing monomers are frequently used for the synthesis of various conjugated materials. On the other hand, their use in on-surface polymerization is hampered by the possibility of ring opening when adsorbed on metal surfaces. In the present work, we mapped the temperature regime for these two competing reactions by investigating the adsorption of a thiophene-based prochiral molecule using scanning tunneling microscopy, X-ray photoelectron spectroscopy and density functional theory calculations. We followed the formation of organometallic (OM) networks, their evolution into covalent structures and the competition between C-C coupling and thiophene ring opening. The effect of surface reactivity was explored by comparing the adsorption on three (111) coinage metal substrates, namely Au, Ag and Cu. While outlining strategies to minimize the ring opening reaction, we found that the surface temperature during deposition is of paramount importance for the preparation of 2D OM networks, greatly enhancing the overall ordering of the product by depositing on hot Ag surface. Notably, the same protocol permits the creation of OM structures on the air-stable Au surface, thereby allowing the synthesis and application of 2D OM networks outside the ultra-high vacuum environment.

Self-assembly of 5,6-dihydroxyindole-2-carboxylic acid: polymorphism of a eumelanin building block on Au(111)

Investigating two-dimensional (2D) self-assembled structures of biological monomers governed by intermolecular interactions is a prerequisite to understand the self-assembly of more complex biomolecular systems. 5,6-Dihydroxyindole carboxylic acid (DHICA) is one of the building blocks of eumelanin - an irregular heteropolymer and the most common form of melanin which has potential applications in organic electronics and bioelectronics. By means of scanning tunneling microscopy, density functional theory and Monte Carlo calculations, we investigate DHICA molecular configurations and interactions underlying the multiple 2D patterns formed on Au(111). While DHICA self-assembled molecular networks (SAMNs) are dominated by the hydrogen bonding of carboxylic acid dimers, a variety of 2D architectures are formed due to the multiple weak interactions of the catechol group. The hydroxyl group also allows for redox reactions, caused by oxidation via O2 exposure, resulting in molecular rearrangement. The susceptibility of the molecules to oxidation is affected by their SAMNs architectures, giving insights on the reactivity of indoles as well as highlighting non-covalent assembly as an approach to guide selective oxidation reactions.

Room-temperature surface-assisted reactivity of a melanin precursor: silver metal-organic coordination versus covalent dimerization on gold

The ability of catecholamines to undergo oxidative self-polymerization provides an attractive route for preparation of coatings for biotechnology and biomedicine applications. However, efforts toward developing a complete understanding of the mechanism that underpins polymerization have been hindered by the multiple catechol crosslinking reaction pathways that occur during the reaction. Scanning tunneling microscopy allows the investigation of small molecules in a reduced-complexity environment, providing important insight into how the intermolecular forces drive the formation of supramolecular assemblies in a controlled setting. Capitalizing on this approach, we studied the self-assembly of 5,6-dihydroxy-indole (DHI) on Au(111) and Ag(111) to investigate the interactions that affect the two-dimensional growth mechanism and to elucidate the behavior of the catechol group on these two surfaces. X-ray photoelectron spectroscopy, together with density functional theory and Monte Carlo modeling, helps unravel the differences between the two systems. The molecules form large ordered domains, yet with completely different architectures. Our data reveal that some of the DHI molecules deposited on Ag are in a modified redox state, with their catechol group oxidized into quinone. On Ag(111), the molecules are deposited in long-range lamellar patterns stabilized by metal-organic coordination, while covalent dimer pairs are observed on Au(111). We also show that the oxidation susceptibility is affected by the substrate, with the DHI/Au remaining inert even after being exposed to O2 gas.

Enhanced conversion efficiency in Si solar cells employing photoluminescent down-shifting CdSe/CdS core/shell quantum dots

Silicon solar cells have captured a large portion of the total market of photovoltaic devices mostly due to their relatively high efficiency. However, Silicon exhibits limitations in ultraviolet absorption because high-energy photons are absorbed at the surface of the solar cell, in the heavily doped region, and the photo-generated electron-hole pairs need to diffuse into the junction region, resulting in significant carrier recombination. One of the alternatives to improve the absorption range involves the use of down-shifting nano-structures able to interact with the aforementioned high energy photons. Here, as a proof of concept, we use downshifting CdSe/CdS quantum dots to improve the performance of a silicon solar cell. The incorporation of these nanostructures triggered improvements in the short circuit current density (Jsc, from 32.5 to 37.0 mA/cm2). This improvement led to a ∼13% increase in the power conversion efficiency (PCE), from 12.0 to 13.5%. Our results demonstrate that the application of down-shifting materials is a viable strategy to improve the efficiency of Silicon solar cells with mass-compatible techniques that could serve to promote their widespread utilization.

Nanoelectromagnetic of the N-doped single wall carbon nanotube in the extremely high frequency band

Materials offering excellent mechanical flexibility, high electrical conductivity and electromagnetic interference (EMI) attenuation with minimal thickness are in high demand, particularly if they can be easily processed into films. Carbon nanotube films deposited on a PDMS substrate combine these requirements. In this work, the potential of single wall carbon nanotubes (SWCNT) deposited on flexible polydimethylsiloxane (PDMS) polymer substrates for EMI attenuation is demonstrated. A 6-micrometer-thick SWCNT film exhibits EMI shielding effectiveness of 24.5 decibels in the extreme high frequency band (EHF), reaching 40 decibels when the SWCNTs are N-doped, which is one of the highest specific EMI attenuation performances optimized with film thickness realized to date. This performance stems from the good electrical conductivity of N-SWCNT films (150 Siemens per centimeter) and possible internal multireflections within the SWCNTs network. The excellent mechanical flexibility and easy coating processing enable them to sheathe complex shaped surfaces while providing high electromagnetic interference attenuation efficiency.

Combined magnetron sputtering and pulsed laser deposition of TiO 2 and BFCO thin films

We report the successful demonstration of a hybrid system that combines pulsed laser deposition (PLD) and magnetron sputtering (MS) to deposit high quality thin films. The PLD and MS simultaneously use the same target, leading to an enhanced deposition rate. The performance of this technique is demonstrated through the deposition of titanium dioxide and bismuth-based perovskite oxide Bi₂FeCrO₆ (BFCO) thin films on Si(100) and LaAlO₃ (LAO) (100). These specific oxides were chosen due to their functionalities, such as multiferroic and photovoltaic properties (BFCO) and photocatalysis (TiO₂). We compare films deposited by conventional PLD, MS and PLD combined with MS, and show that under all conditions the latter technique offers an increased deposition rate (+50%) and produces films denser (+20%) than those produced by MS or PLD alone, and without the large clusters found in the PLD-deposited films. Under optimized conditions, the hybrid technique produces films that are two times smoother than either technique alone.

2D Supramolecular networks of dibenzonitrilediacetylene on Ag(111) stabilized by intermolecular hydrogen bonding

The two-dimensional (2D) surface-directed self-assembly of dibenzonitrile diacetylene (DBDA) on Ag(111) under ultrahigh vacuum (UHV) conditions was investigated by combining scanning tunneling microscopy (STM), X-ray photoelectron spectroscopy (XPS) and theoretical simulations based on density functional theory (DFT) calculations. The molecule consists of two benzonitrile groups (-C₆H₄-C[triple bond, length as m-dash]N) on each side of a diacetylene (-C[triple bond, length as m-dash]C-C[triple bond, length as m-dash]C-) backbone. The terminating nitrile (-C[triple bond, length as m-dash]N) groups at the meta position of the phenyl rings lead to cis and trans stereoisomers. The trans isomer is prochiral and can adsorb in the R or S configuration, leading to the formation of enantiomeric self-assembled networks on the surface. We identify two simultaneously present supramolecular networks, termed parallel and chevron phases, as well as a less frequently observed butterfly phase. These networks are formed from pure R (or S) domains, racemic mixtures (RS), and cis isomers, respectively. Our complementary data illustrates that the formation of the 2D supramolecular networks is driven by intermolecular hydrogen bonding between nitrile and phenyl groups (-C[triple bond, length as m-dash]NH-C₆H₃). This study illustrates that the molecular arrangement of each network depends on the geometry of the isomers. The orientation of the nitrile group controls the formation of the most energetically stable network via intermolecular hydrogen bonding.

Selective binding in different adsorption sites of a 2D covalent organic framework

Monolayer covalent organic frameworks selectively bind fullerenes and trichlorobenzene at different sites via different types of interactions.

Photovoltaic properties of Bi2FeCrO6 films epitaxially grown on (100)-oriented silicon substrates

We demonstrate the promising potential of using perovskite Bi2FeCrO6 (BFCO) for niche applications in photovoltaics (PV) (e.g. self-powered sensors that simultaneously exploit PV conversion and multiferroic properties) or as a complement to mature PV technologies like silicon. BFCO thin films were epitaxially grown on silicon substrates using an MgO buffer layer. Piezoresponse force microscopy measurements revealed that the tensile strained BFCO phase exhibits a polarization predominantly oriented through the in-plane direction. The semiconducting bandgap of the ordered BFCO phase combined with ferroelectric properties, opens the possibility of a ferroelectric PV efficiency above 2% in a thin film device and the use of ferroelectric materials simultaneously as solar absorber layers and carrier separators in PV devices. A large short circuit photocurrent density of 13.8 mA cm(-2) and a photovoltage output of 0.5 V are typically obtained at FF of 38% for BFCO devices fabricated on silicon. We believe that the reduced photovoltage is due to the low diffusion length of photogenerated charge carriers in the BFCO material where the ferroelectric domains are predominately oriented in-plane and thus do not contribute efficiently to the photocharge separation process.

Antibacterial Coatings: Challenges, Perspectives, and Opportunities

Antibacterial coatings are rapidly emerging as a primary component of the global mitigation strategy of bacterial pathogens. Thanks to recent concurrent advances in materials science and biotechnology methodologies, and a growing understanding of environmental microbiology, an extensive variety of options are now available to design surfaces with antibacterial properties. However, progress towards a more widespread use in clinical settings crucially depends on addressing the key outstanding issues. We review release-based antibacterial coatings and focus on the challenges and opportunities presented by the latest generation of these materials. In particular, we highlight recent approaches aimed at controlling the release of antibacterial agents, imparting multi-functionality, and enhancing long-term stability.

Substrate Effects in the Supramolecular Assembly of 1,3,5-Benzene Tricarboxylic Acid on Graphite and Graphene

The behavior of small molecules on a surface depends critically on both molecule-substrate and intermolecular interactions. We present here a detailed comparative investigation of 1,3,5-benzene tricarboxylic acid (trimesic acid, TMA) on two different surfaces: highly oriented pyrolytic graphite (HOPG) and single-layer graphene (SLG) grown on a polycrystalline Cu foil. On the basis of high-resolution scanning tunnelling microscopy (STM) images, we show that the epitaxy matrix for the hexagonal TMA chicken wire phase is identical on these two surfaces, and, using density functional theory (DFT) with a non-local van der Waals correlation contribution, we identify the most energetically favorable adsorption geometries. Simulated STM images based on these calculations suggest that the TMA lattice can stably adsorb on sites other than those identified to maximize binding interactions with the substrate. This is consistent with our net energy calculations that suggest that intermolecular interactions (TMA-TMA dimer bonding) are dominant over TMA-substrate interactions in stabilizing the system. STM images demonstrate the robustness of the TMA films on SLG, where the molecular network extends across the variable topography of the SLG substrates and remains intact after rinsing and drying the films. These results help to elucidate molecular behavior on SLG and suggest significant similarities between adsorption on HOPG and SLG.

Pentacene on Ni(111): room-temperature molecular packing and temperature-activated conversion to graphene

We investigate, using scanning tunnelling microscopy, the adsorption of pentacene on Ni(111) at room temperature and the behaviour of these monolayer films with annealing up to 700 °C. We observe the conversion of pentacene into graphene, which begins from as low as 220 °C with the coalescence of pentacene molecules into large planar aggregates. Then, by annealing at 350 °C for 20 minutes, these aggregates expand into irregular domains of graphene tens of nanometers in size. On surfaces where graphene and nickel carbide coexist, pentacene shows preferential adsorption on the nickel carbide phase. The same pentacene to graphene transformation was also achieved on Cu(111), but at a higher activation temperature, producing large graphene domains that exhibit a range of moiré superlattice periodicities.

Reaction pathways for pyridine adsorption on silicon (0 0 1)

Density functional theory is used to describe the reactions of chemisorption of pyridine on the silicon (0 0 1) surface. Adsorption energies of six relevant structures, and the activation energies between them are reported. We consider in detail the dative to tight-bridge transition for which conflicting results have been reported in the literature, and provide a description of the formation of inter-row chains observed in high-coverage experiments. We demonstrate that the choice of DFT functional has a considerable effect on the relative energetics and of the four DFT functionals considered, we find that the range-separated hybrid ωB97X-D functional with empirical dispersion provides the most consistent description of the experiment data.

Solution and air stable host/guest architectures from a single layer covalent organic framework

Single-layer COF-1 hosts fullerene guests, which can be introduced by a brief immersion in fullerene solution.

Reduced graphene oxide growth on 316L stainless steel for medical applications

We report a new method for the growth of reduced graphene oxide (rGO) on the 316L alloy of stainless steel (SS) and its relevance for biomedical applications. We demonstrate that electrochemical etching increases the concentration of metallic species on the surface and enables the growth of rGO. This result is supported through a combination of Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), atomic force microscopy (AFM), scanning electron microscopy (SEM), density functional theory (DFT) calculations and static water contact angle measurements. Raman spectroscopy identifies the G and D bands for oxidized species of graphene at 1595 cm(-1) and 1350 cm(-1), respectively, and gives an ID/IG ratio of 1.2, indicating a moderate degree of oxidation. XPS shows -OH and -COOH groups in the rGO stoichiometry and static contact angle measurements confirm the wettability of rGO. SEM and AFM measurements were performed on different substrates before and after coronene treatment to confirm rGO growth. Cell viability studies reveal that these rGO coatings do not have toxic effects on mammalian cells, making this material suitable for biomedical and biotechnological applications.

Tip-induced C-H activation and oligomerization of thienoanthracenes

The tip of a scanning tunneling microscope (STM) can be used to dehydrogenate freely-diffusing tetrathienoanthracene (TTA) molecules on Cu(111), trapping the molecules into metal-coordinated oligomeric structures. The process proceeds at bias voltages above ~3 V and produces organometallic structures identical to those resulting from the thermally-activated cross-coupling of a halogenated analogue. The process appears to be substrate dependent: no oligomerization was observed on Ag(111) or HOPG. This approach demonstrates the possibility of controlled synthesis and nanoscale patterning of 2D oligomer structures on selected surfaces.

Controlling photoinduced electron transfer from PbS@CdS core@shell quantum dots to metal oxide nanostructured thin films

N-type metal oxide solar cells sensitized by infrared absorbing PbS quantum dots (QDs) represent a promising alternative to traditional photovoltaic devices. However, colloidal PbS QDs capped with pure organic ligand shells suffer from surface oxidation that affects the long term stability of the cells. Application of a passivating CdS shell guarantees the increased long term stability of PbS QDs, but can negatively affect photoinduced charge transfer from the QD to the oxide and the resulting photoconversion efficiency (PCE). For this reason, the characterization of electron injection rates in these systems is very important, yet has never been reported. Here we investigate the photoelectron transfer rate from PbS@CdS core@shell QDs to wide bandgap semiconducting mesoporous films using photoluminescence (PL) lifetime spectroscopy. The different electron affinity of the oxides (SiO2, TiO2 and SnO2), the core size and the shell thickness allow us to fine tune the electron injection rate by determining the width and height of the energy barrier for tunneling from the core to the oxide. Theoretical modeling using the semi-classical approximation provides an estimate for the escape time of an electron from the QD 1S state, in good agreement with experiments. The results demonstrate the possibility of obtaining fast charge injection in near infrared (NIR) QDs stabilized by an external shell (injection rates in the range of 110-250 ns for TiO2 films and in the range of 100-170 ns for SnO2 films for PbS cores with diameters in the 3-4.2 nm range and shell thickness around 0.3 nm), with the aim of providing viable solutions to the stability issues typical of NIR QDs capped with pure organic ligand shells.

Molecular self-assembly on graphene

The formation of ordered arrays of molecules via self-assembly is a rapid, scalable route towards the realization of nanoscale architectures with tailored properties. In recent years, graphene has emerged as an appealing substrate for molecular self-assembly in two dimensions. Here, the first five years of progress in supramolecular organization on graphene are reviewed. The self-assembly process can vary depending on the type of graphene employed: epitaxial graphene, grown in situ on a metal surface, and non-epitaxial graphene, transferred onto an arbitrary substrate, can have different effects on the final structure. On epitaxial graphene, the process is sensitive to the interaction between the graphene and the substrate on which it is grown. In the case of graphene that strongly interacts with its substrate, such as graphene/Ru(0001), the inhomogeneous adsorption landscape of the graphene moiré superlattice provides a unique opportunity for guiding molecular organization, since molecules experience spatially constrained diffusion and adsorption. On weaker-interacting epitaxial graphene films, and on non-epitaxial graphene transferred onto a host substrate, self-assembly leads to films similar to those obtained on graphite surfaces. The efficacy of a graphene layer for facilitating planar adsorption of aromatic molecules has been repeatedly demonstrated, indicating that it can be used to direct molecular adsorption, and therefore carrier transport, in a certain orientation, and suggesting that the use of transferred graphene may allow for predictible molecular self-assembly on a wide range of surfaces.

Photovoltaic effect in multiphase Bi-Mn-O thin films

We report an external solar power conversion efficiency of ~0.1% in Bi-Mn-O thin films grown onto (111) oriented Niobium doped SrTiO3 (STO) single crystal substrate by pulse laser deposition (PLD). The films contain BiMnO3 (BMO) and Mn3O4 (MO) phases, which both grow epitaxially. The growth conditions were tailored to obtain films with different Bi/Mn ratios. The films were subsequently illuminated under a sun simulator (AM 1.5 G). We find that the Bi/Mn ratio in the film affects the magnitude of the photo induced voltage and photocurrent and therefore the photovoltaic conversion efficiency. Specifically, a higher Bi/Mn ratio (towards unity) in the film increases the power conversion efficiency. This effect is described in terms of a more favorable energy band alignment of the film/substrate hetero-structure junction, which controls photo carrier separation.

Surface structure of Pd(111) with less than half a monolayer of Zn

We have characterized the structural properties of submonolayer amounts of Zn on Pd(111) using scanning tunneling microscopy (STM) and spot-profile analysis low energy electron diffraction (SPA-LEED). Following room temperature deposition of ≈0.06 monolayers (ML) Zn onto Pd(111), we observe the substitution of Zn for Pd in the surface layer. At ≈0.20 ML of deposited Zn, STM reveals a locally ordered phase with a (2/√3 × 2/√3)R30° unit cell located near Zn substitutions; SPA-LEED patterns reveal the same periodicity. We attribute this phase to the metastable bonding of atoms or clusters predominantly in hollow sites surrounding Zn substitutions in the surface layer. At ≈0.4 ML, STM images reveal local (√3 × √3)R30° and (2 × 1) ordering on surfaces annealed to 350 K. At coverages near 0.5 ML, both STM and SPA-LEED show the onset of the formation of the (2 × 1) ordering associated with the Zn : Pd 1 : 1 alloy phase. At all coverages, the surface is dominated by island growth; the islands' size and density is shown to depend critically on annealing at temperatures as low as 350 K. These results provide insight into the structural features of a Zn/Pd(111) coverage regime that has been much debated in recent years.

Mechanical and electrical properties of epitaxial Si nanowires grown by pulsed laser deposition

We report on the elastic and piezoresistive properties of individual epitaxial Si-NWs grown on n-doped Si(111) by pulsed laser deposition. Using scanning probe microscopy, we obtained a Young's modulus between 82 and 900 GPa for the nanowires, unaffected by the nanowire shape. A relative resistivity change is observed in the prestrained (curved) Si-NWs, which we attribute to a large piezoresistance coefficient in the NW along its axis. Assuming that for the bent NWs the effect of longitudinal stress on resistivity is compensated, the piezoresistance coefficient originating in the shear strain alone, we found a piezoresistance gauge factor (GF) of 600, which is close to the values reported in literature for Si-NWs.

Binding geometry of hydrogen-bonded chain motif in self-assembled gratings and layers on Ag(111)

Upon adsorption on the (111) facet of Ag, 4-[trans-2-(pyrid-4-yl-vinyl)] benzoic acid (PVBA) self-assembles into a highly ordered, chiral twin chain structure at submonolayer coverages with domains that can extend for micrometers in one dimension. Using polarization-dependent measurements of C and N K-shell excitations in near-edge X-ray absorption fine structure (NEXAFS) spectra, we determine the binding geometry of single PVBA molecules within this unique ensemble for both low and high coverage regimes. At submonolayer coverage, the molecule is twisted to facilitate the formation of hydrogen bonds. The gas-phase planarity is gradually recovered as the coverage is increased, with complete planarity coinciding with loss of order in the overlayer. Thermal treatment of the PVBA film results in deprotonation of the carboxyl tail of the molecule, but despite the suppression of the stabilizing hydrogen-bonds, the overlayer remains ordered.

Multiferroic nanoscale Bi2FeCrO6 material for spintronic-related applications

We report the control of the growth mode of Bi(2)FeCrO(6) thin and ultrathin films by either tuning the pulsed laser deposition parameters or by using a buffer layer. The films are epitaxial and the heterostructures exhibit very smooth interfaces, thus eliminating the main obstacle in the realization of tunnel junctions. By characterizing the functional properties of thin films we find that Bi(2)FeCrO(6) retains its room temperature multiferroic character even at the nanoscale. The coexistence of these properties in ultra-thin Bi(2)FeCrO(6) films will pave the way to design multifunctional devices for applications in spintronics and electronics, such as ferroelectric tunnel junctions or magnetic tunnel junctions with ferroelectric barriers.

Coexistence of antiferromagnetic and ferromagnetic orders at remanent state in epitaxial multiferroic Bi2FeCrO6 nanostructures

We report the local electronic and magnetic properties of Bi(2)FeCrO(6) nanostructures by element-specific polarized x-ray techniques. Sizable magnetic ordering in the remanent state is observed at room temperature for both Fe and Cr ions. The Bi(2)FeCrO(6) system offers an example of d(5)-d(3) magnetic superexchange interaction with a magnetic order for both Fe and Cr, which are both formally in the  + 3 valence state. The results suggest a coexistence of antiferromagnetic and ferromagnetic superexchange interaction between Fe and Cr spins in the nanostructures at the remanent state and at room temperature.

Modified Stranski-Krastanov growth in Ge/Si heterostructures via nanostenciled pulsed laser deposition

The combination of nanostenciling with pulsed laser deposition (PLD) provides a flexible, fast approach for patterning the growth of Ge on Si. Within each stencilled site, the morphological evolution of the Ge structures with deposition follows a modified Stranski-Krastanov (SK) growth mode. By systematically varying the PLD parameters (laser repetition rate and number of pulses) on two different substrate orientations (111 and 100), we have observed corresponding changes in growth morphology, strain and elemental composition using scanning electron microscopy, atomic force microscopy and μ-Raman spectroscopy. The growth behaviour is well predicted within a classical SK scheme, although the Si(100) growth exhibits significant relaxation and ripening with increasing coverage. Other novel aspects of the growth include the increased thickness of the wetting layer and the kinetic control of Si/Ge intermixing via the PLD repetition rate.

Synthesis of polyphenylene molecular wires by surface-confined polymerization

The surface-mediated synthesis of epitaxially aligned and separated polyphenylene lines on Cu(110) by exploiting the Ullmann dehalogenation reaction is reported. Scanning tunneling microscopy (STM) and X-ray photoelectron spectroscopy (XPS) show that the C-I bonds of 1,4-diiodobenzene and 1,3-diiodobenzene (C(6)H(4)I(2)) are catalytically cleaved when dosed onto the surface. Subsequent annealing transforms the copper-bound phenylene intermediates into covalent conjugated structures: linear chains of poly(p-phenylene) for 1,4-diiodobenzene and zigzag chains of poly(m-phenylene) as well as macrocyclic oligomers in the case of 1,3-diiodobenzene. The chains are strongly bound to the surface (likely through C--Cu bonds at the chain-ends) while the macrocycles are very mobile and can only be imaged by STM at low temperature. The detached halogens adsorb on the surface and separate the polymer chains from each other.

Self-assembly of rubrene on Cu(111)

We performed an ultra-high vacuum scanning tunneling microscopy (STM) investigation of the self-assembly of rubrene at room temperature on Cu(111), a metal surface with threefold symmetry. Rubrene self-assembles into two different structures called row and trimer. Both are different than the structures already observed on Cu(110) and Cu(100). Row and trimer structures have comparable molecular packing densities and are equally distributed across the surface. In the row structure the molecules are oriented with their backbone along the same high symmetry directions of the surface: [[Formula: see text]], [[Formula: see text]] or [[Formula: see text]]. The trimer structure is composed of units of three rubrene molecules, oriented along the high symmetry surface directions. These units are chiral, as revealed by height profile measurements by STM, and self-assemble in domains containing only one type of enantiomer.

Stabilization of exotic minority phases in a multicomponent self-assembled molecular network

Trimesic acid (TMA) and alcohols were recently shown to self-assemble into a stable, two-component linear pattern at the solution/highly oriented pyrolytic graphite (HOPG) interface. Away from equilibrium, the TMA/alcohol self-assembled molecular network (SAMN) can coexist with pure-TMA networks. Here, we report on some novel characteristics of these non-equilibrium TMA structures, investigated by scanning tunneling microscopy (STM). We observe that both the chicken-wire and flower-structure TMA phases can host 'guest' C(60) molecules within their pores, whereas the TMA/alcohol SAMN does not offer any stable adsorption sites for the C(60) molecules. The presence of the C(60) molecules at the solution/solid interface was found to improve the STM image quality. We have taken advantage of the high-quality imaging conditions to observe unusual TMA bonding geometries at domain boundaries in the TMA/alcohol SAMN. Boundaries between aligned TMA/alcohol domains can give rise to doubled TMA dimer rows in two different configurations, as well as a tripled-TMA row. The boundaries created between non-aligned domains can create geometries that stabilize TMA bonding configurations not observed on surfaces without TMA/alcohol SAMNs, including small regions of the previously predicted 'super flower' TMA bonding geometry and a tertiary structure related to the known TMA phases. These structures are identified as part of a homologic class of TMA bonding motifs, and we explore some of the reasons for the stabilization of these phases in our multicomponent system.

Diffusion dynamics during the nucleation and growth of Ge/Si nanostructures on Si(111)

We report a low energy electron microscopy study of the relation between self-organized Ge/Si(111)nanostructures and their local environment. By comparison with Monte Carlo simulations, three-dimensional islands are shown to display a substantial tendency towards self-ordering. This tendency may result from the diffusive nature of the nucleation processes. The size of individual nanostructures does not significantly correlate with the distance between neighboring islands. Thus energetic factors are thought to govern the competition among coexisting nanostructures to capture the deposited mass.

Long jumps in the surface diffusion of large molecules

We have studied the diffusion of the two organic molecules DC and HtBDC on the Cu(110) surface by scanning tunneling microscopy. Surprisingly, we find that long jumps, spanning multiple lattice spacings, play a dominating role in the diffusion of these molecules--the root-mean-square jump lengths are as large as 3.9 and 6.8 lattice spacings, respectively. The presence of long jumps is revealed by a new and simple method of analysis, which is tested by kinetic Monte Carlo simulations.

Organic molecules acting as templates on metal surfaces

The electronic connection of single molecules to nanoelectrodes on a surface is a basic, unsolved problem in the emerging field of molecular nanoelectronics. By means of variable temperature scanning tunneling microscopy, we show that an organic molecule (C90H98), known as the Lander, can cause the rearrangement of atoms on a Cu(110) surface. These molecules act as templates accommodating metal atoms at the step edges of the copper substrate, forming metallic nanostructures (0.75 nanometers wide and 1.85 nanometers long) that are adapted to the dimensions of the molecule.