Electrical monitoring of organic crystal phase transition using MoS2 field effect transistor.

Hybrid van der Waals heterostructures made of 2D materials and organic molecules exploit the high sensitivity of 2D materials to all interfacial modifications and the inherent versatility of the organic compounds. In this study, we are interested in the quinoidal zwitterion/MoS2 hybrid system in which organic crystals are grown by epitaxy on the MoS2 surface and reorganize in another polymorph after thermal annealing. By means of field-effect transistor measurements recorded in situ all along the process, atomic force microscopy and density functional theory calculations we demonstrate that the charge transfer between quinoidal zwitterions and MoS2 strongly depends on the conformation of the molecular film. Remarkably, both the field effect mobility and the current modulation depth of the transistors remain unchanged which opens up promising prospects for efficient devices based on this hybrid system. We also show that MoS2 transistors enable fast and accurate detection of structural modifications that occur during phases transitions of the organic layer. This work highlights that MoS2 transistors are remarkable tools for on-chip detection of molecular events occurring at the nanoscale, which paves the way for the investigation of other dynamical systems.

A one-dimensional high-order commensurate phase of tilted molecules.

We report on the formation of a high-order commensurate (HOC) structure of 5,14-dihydro-5,7,12,14-tetraazapentacene (DHTAP) molecules on the highly corrugated Cu(110)-(2 × 1)O surface. Scanning tunnelling microscopy shows that the DHTAP molecules form a periodic uniaxial arrangement in which groups of seven molecules are distributed over exactly nine substrate lattice spacings along the [1̄10] direction. DFT-calculations reveal that this peculiar arrangement is associated with different tilting of the seven DHTAP molecules within the quasi one-dimensional HOC unit cell. The orientational degree of freedom thus adds a new parameter, which can efficiently stabilize complex molecular structures on corrugated surfaces.

On-surface chemistry using local high electric fields.

Dihydrotetraazapentacene (DHTAP) molecules can be dehydrogenated on the surface to form tetraazapentacene (TAP), by applying a high electric field between the tip of a scanning tunnelling microscope (STM) and a metallic substrate in the zero-current limit. The method can be applied either to single molecules or more extended layers by successively scanning a selected area using an STM tip.

Optical and morphological properties of thin films of bis-pyrenyl π-conjugated molecules.

1,4-Di-n-octyloxy-2,5-bis(pyren-1-ylethenyl)benzene (bis-pyrene) has been studied by the means of surface cavity ring-down (s-CRD) spectroscopy on an amorphous BK7 glass substrate and scanning tunnelling microscopy (STM) on Au(111). Absorption spectra show a modification of the optical properties as a function of coverage, i.e. appearance of a shoulder around 505 nm followed by a saturation of the intensity of this signal observed at higher coverages. We attribute this shoulder to the change of the molecular orientation between the first and the second monolayer and thus to an interfacial effect. These results are confirmed by scanning tunnelling microscopy (STM) measurements where the bis-pyrene molecules have been deposited on Au(111) at room temperature (RT) and onto a cold substrate. Independently of the temperature in the range from 210 K to RT, the first monolayer is always highly organized. At low temperature bis-pyrene molecules constituting the second monolayer are randomly distributed, suggesting that self-organisation is kinetically hindered. Deposited at room temperature, the molecular diffusion is enhanced and the formation of an organized second layer takes place after storing the sample for 150 minutes at room temperature. A HOMO-LUMO gap of 2.85 eV has been determined by scanning tunnelling spectroscopy, which is in very good agreement with the observed optical transition at 434 nm (2.86 eV) in s-CRD spectroscopy.

Distribution of Pd clusters on ultrathin, epitaxial TiO x films on Pt3Ti(111).

Scanning tunnelling microscopy (STM) was used to investigate the nucleation and growth of palladium clusters on two different, ultrathin, epitaxial, titania films grown on a Pt3Ti(111) surface. The first oxide phase, z'-TiO x , is anisotropic and consists of parallel stripes separated by trenches. Defects (i.e., oxygen vacancies) in this structure are confined to these trenches and act as nucleation sites. Therefore, the Pd clusters are mostly arranged in unidirectional rows along the trenches, creating a template effect. The second phase, w'-TiO x , exhibits a hexagonal, long range, (7 × 7)R21.8°, Moiré-type superstructure with fewer and shallower defects, making the template effect less discernible.

Highly ordered molecular films on Au(111): the N-heteroacene approach.

This study presents an innovative synthesis of dihydrotetraazapentacene (DHTAP) and the scanning tunneling microscopy (STM) investigation of the initial stages of its growth on Au(111). We were able to demonstrate that, up to the fourth monolayer, the DHTAP films show a high structural order and growths in perfect epitaxy. This behavior can be unequivocally attributed to the stabilizing effect of intralayer hydrogen bonding interactions.

Atomic structures of silicene layers grown on Ag(111): scanning tunneling microscopy and noncontact atomic force microscopy observations.

Silicene, the considered equivalent of graphene for silicon, has been recently synthesized on Ag(111) surfaces. Following the tremendous success of graphene, silicene might further widen the horizon of two-dimensional materials with new allotropes artificially created. Due to stronger spin-orbit coupling, lower group symmetry and different chemistry compared to graphene, silicene presents many new interesting features. Here, we focus on very important aspects of silicene layers on Ag(111): First, we present scanning tunneling microscopy (STM) and non-contact Atomic Force Microscopy (nc-AFM) observations of the major structures of single layer and bi-layer silicene in epitaxy with Ag(111). For the (3 × 3) reconstructed first silicene layer nc-AFM represents the same lateral arrangement of silicene atoms as STM and therefore provides a timely experimental confirmation of the current picture of the atomic silicene structure. Furthermore, both nc-AFM and STM give a unifying interpretation of the second layer (√3 × âˆš3)R ± 30° structure. Finally, we give support to the conjectured possible existence of less stable, ~2% stressed, (√7 × âˆš7)R ± 19.1° rotated silicene domains in the first layer.

Structural and compositional characterization of ultrathin titanium oxide films grown on Pt3Ti(111).

We have investigated the growth of ultrathin titanium oxide (TiO(x)) films on a Pt(3)Ti(111) single crystal surface as a function of oxidation temperature (300-1000 K) and oxygen exposure (up to 4500 l) by means of Auger electron spectroscopy, low-energy electron diffraction, ultraviolet photoelectron spectroscopy and high-resolution electron energy loss spectroscopy (HREELS). Both the surface composition and the surface structure of the resulting TiO(x) films exhibit a strong dependence on the preparation conditions. Loss of the chemical order and Ti segregation are observed at the Pt(3)Ti(111) surface upon oxygen exposures of more than 135 l at 1000 K. Increasing oxygen exposure enhances Ti segregation and oxide growth. At a threshold of ≈220 l (at 1000 K) a transition in the oxide structure occurs, namely from a (6 × 3√3) rectangular structure (a = 16.6 Å, b = 14.4 Å) below 220 l to a (7 × 7)R21.8° hexagonal structure (a = b = 19.3 Å) above 220 l. Two additional incommensurate rectangular metastable structures are observed for the highest oxygen exposures (above 900 l) at intermediate oxidation temperatures (800-900 K). In all cases the changes in the valence band spectra and the work function with respect to the clean Pt(3)Ti(111) surface are independent of the chosen oxidation parameters. Based on their HREELS spectra we identify the (6 × 3√3) and (7 × 7)R21.8° structures grown at 1000 K with a stoichiometric TiO phase, while the other and less stable oxide phases grown at 800-900 K exhibit more complex phonon structures that could not simply be associated with any of the stoichiometric phases TiO, Ti(2)O(3) or TiO(2). Our results are rather similar to those found by Granozzi et al for the deposition of Ti onto a Pt(111) surface in an oxygen atmosphere, except a few interesting deviations as a consequence of the different preparation conditions.

Surfaces: two-dimensional templates.

Producing nanoscale structures on solid surfaces in a controlled way is a technologicalchallenge that has an important impact on a variety of fields such as microelectronics, magnetic storagetechnology, and heterogeneous catalysis. Currently most processes are based on a top-down approach,which relies on an active patterning of a surface by, for example, lithography or imprinting. As thedesired structures become smaller these top-down processes will reach the physical limit of their resolution.A bottom-up approach, which is based on a template-controlled growth of nanostructures on a prestructuredsubstrate, can provide access to structural dimensions of only a few nanometers. The challenge forthe growth of such nanostructures on surfaces is to identify and design suitable surfaces, which act astemplates due to their intrinsic physical and chemical properties. These 2-dimensional (2D) templates canthen be utilized to produce nanostructures of metals, semiconductors, or organic compounds.

Disorder or complexity? Understanding a nanoscale template structure on alumina.

One strategy in creating functional nanostructures is templating where active nanoparticles are arranged on a regular nanoscale array of anchor sites on an inert substrate. An extraordinarily well ordered substrate with a 4.2 nm template periodicity is an alumina (aluminum oxide) film grown on a Ni3Al(111) metallic alloy support. Templating on the alumina film is facilitated by a dot and a network superstructure that can readily be prepared but has not yet been understood at the atomic scale. By imaging the alumina surface with dynamic scanning force microscopy (SFM) operated in the noncontact mode (NC-AFM), we reveal that the main structural element of the oxide film is a lattice of hexagons with a 0.29 nm side length that is pinned to the 0.51 nm periodicity of the substrate. The surface unit cell is defined by distinguished sites forming the dot structure. Pinning the oxide film to the substrate furthermore results in a honeycomb-like topographic modulation referred to as the network structure. These findings demonstrate how long range order is generated by the superposition of complex structures that locally exhibit apparent atomic disorder.

Surface structure of an ultrathin alumina film on Ni3Al(111): a dynamic scanning force microscopy study.

The surface structure of an ultrathin alumina film on a Ni3Al(111) substrate has been studied by dynamic scanning force microscopy. The alumina film exhibits a hexagonal superstructure with a lattice parameter of 4.14 nm and a (1/sqrt[3] x 1/sqrt[3])R30 degrees substructure. Two domains rotated by 24 degrees are present. The film is terminated by a hexagonal lattice of oxygen ions with a lattice parameter of 0.293 nm, which is rotated by 30 degrees with respect to the substrate lattice. The nodes of the 4.14 nm superstructure and the 2.39 nm substructure are pinned on points of the substrate lattice, where the surface atomic lattice is almost commensurable. The oxygen lattice is perfectly hexagonal close to these nodes and disordered in the surrounding regions.

Thin alumina films on Ni3Al(111): a template for nanostructured Pd cluster growth.

The growth of vapor deposited palladium on a well-ordered thin alumina film grown on a Ni3Al(111) surface was studied as a function of the sample temperature during deposition and the palladium flux. The superstructure of the oxide film turns out to be an excellent template for the growth of nanostructured palladium cluster arrays. By taking advantage of the growth steering properties of the alumina film we were able to prepare nearly perfectly ordered hexagonal arrays of palladium clusters with a uniform distance of 4.5 nm between the particles. Furthermore the dependence of the cluster height and diameter on the sample voltage as measured by STM has been investigated. In the range between 0.7 V and 2.0 V the measured cluster heights are nearly independent of the sample voltage whereas at voltages higher than 2 V they are continuously decreasing.