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.

CO and O2 Adsorption and CO Oxidation on Pt Nanoparticles by Indirect Nanoplasmonic Sensing.

We used indirect nanoplasmonic sensing (INPS) coupled with mass spectrometry to study CO and oxygen adsorption as well as CO oxidation, on Pt nanoparticles, in the Torr pressure range. Due to an optimization of the physical parameters of our plasmonic sample, we obtain a highly sensitive probe that can detect gas adsorption of a few hundredths of a monolayer, even with a very low number density of Pt particles. Moreover and for the first time, a similarity is observed between the sign and the evolution of the localized surface plasmon resonance (LSPR) peak shift and the work function measurements for CO and oxygen chemisorption. Controlling the size, shape, and surface density of Pt particles, the turnover frequency (TOF) has also been accurately determined. For similar experimental conditions, the TOF is close to those measured on Pt/oxide powder catalysts and Pt(100) single crystals.

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.

Water Adsorption by a Sensitive Calibrated Gold Plasmonic Nanosensor.

We demonstrate in this work that using nanoplasmonic sensing it is possible to follow the adsorption/desorption of water molecules on gold nanodisks nanofabricated by electron beam lithography. This quantitative method is highly sensitive allowing the detection of a few hundredths of adsorbed monolayer. Disk parameters (height, diameter, and interdisk distance) have been optimized after finite-difference time-domain (FDTD) simulations in order to obtain the best localized surface plasmon resonance (LSPR) signal-to-noise ratio. Finally, we have precisely measured the adsorption kinetics of water on gold as a function of the relative humidity of the surrounding medium.

N-Substituted Azacalixphyrins: Synthesis, Properties, and Self-Assembly.

Pre- and postintroduction of substituents with respect to the macrocyclization step leads to previously unknown N-substituted azacalixphyrins. The stepwise synthetic approach has been studied in detail to highlight the key role of the N-substituents of the precursors and/or intermediates in terms of reactivity. Based on a combined experimental and theoretical investigation, the relationship between the properties of the macrocycles and their degree of substitution is rationalized. Depending on the nature of the N-substituents, the formation of supramolecular ribbon-like structures could also be observed, as demonstrated by combined TEM, SEM, AFM, and FTIR experiments.

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.

Magnetic properties of self-organized Co dimer nanolines on Si/Ag(110).

We demonstrate the kinetically controlled growth of one-dimensional Co nanomagnets with a high lateral order on a nanopatterned Ag(110) surface. First, self-organized Si nanoribbons are formed upon submonolayer condensation of Si on the anisotropic Ag(110) surface. Depending on the growth temperature, individual or regular arrays (with a pitch of 2 nm) of Si nanoribbons can be grown. Next, the Si/Ag(110) system is used as a novel one-dimensional Si template to guide the growth of Co dimer nanolines on top of the Si nanoribbons, taking advantage of the fact that the thermally activated process of Co diffusion into the Si layer is efficiently hindered at 220 K. Magnetic characterization of the Co nanolines using X-ray magnetic circular dichroism reveals that the first atomic Co layer directly adsorbed onto the Si nanoribbons presents a weak magnetic response. However, the second Co layer exhibits an enhanced magnetization, strongly suggesting a ferromagnetic ordering with an in-plane easy axis of magnetization, which is perpendicular to the Co nanolines.

Indirect Nanoplasmonic Sensing to Probe with a High Sensitivity the Interaction of Water Vapor with Soot Aerosols.

We demonstrate in this work that the indirect nanoplasmonic sensing lets us follow the adsorption/desorption of water molecules on soot particles that are a major contributor of the global warming. Increasing the relative humidity of the surrounding medium we measure a shift in wavelength of the localized surface plasmon resonance response of gold nanodisks on which soot particles are deposited. We show a singular and reversible blue shift with hydrophilic aircraft soot particles interpreted from a basic model as a reversible morphological change of the soot aggregates. This new method is highly sensitive and interesting to follow the change of optical properties of aerosols during their aging in the atmosphere, where they can adsorb and react with different gas molecules.

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.

Are conductance plateaus independent events in atomic point contact measurements? A statistical approach.

Conductance-elongation curves of gold atomic wires are measured using a scanning tunneling microscope break junction technique at room temperature. Landauer's conductance plateaus are individually identified and statistically analyzed. Both the probabilities to observe and the lengths of the two last plateaus (at conductance values close to 2e(2)/h and 4e(2)/h) are studied. All results converge to show that the occurrences of these two conductance plateaus on a conductance-elongation curve are statistically independent events.