Paolo Samorì

Light‐Powered Electrical Switch Based on Cargo‐Lifting Azobenzene Monolayers

Inspired by the complex molecular machines found in nature, chemists have developed much simpler molecular motors. Among them, several systems incorporating azobenzene have been proposed, which exploit the reversible trans–cis isomerization triggered by light or an electric field for applications such as optical data-storage devices, switchable supramolecular cavities, and sensors. Recently, it has been demonstrated that the photoisomerization process of individual polymer chains incorporating azobenzenes can express mechanical work. In light of these findings, one can foresee self-assembled monolayers (SAMs) of aromatic azobenzenes as molecular systems able to express forces of unprecedented magnitude by exploiting a collective subnanometer structural change. We recently designed a rigid and fully conjugated azobenzene exposing a thiol anchoring group, which was able to form a tightly packed SAM on Au(111) (SAMAZO). Scanning tunneling microscopy (STM) studies revealed that upon light irradiation of the chemisorbed SAMs, a collective isomerization of entire molecular-crystalline domains occurred with an outstandingly high directionality. Based on these results, a cooperative nature of the isomerization of adjacent AZO molecules has been proposed. Furthermore, the joint action of the molecules in the SAM provides an ideal system as a potential “cargo” lifter. Herein, we show that, upon irradiation, azobenzene SAMs incorporated in a junction between an Au(111) surface and a mercury drop are able to 1) lift the “heavy” Hg drop, and 2) reversibly photoswitch the current flowing through the junction (Figure 1). Current–voltage (I–V) characteristics averaged over more than 30 junctions incorporating AZO SAMs in the trans and the cis conformations are shown in Figure 2a. The SAMAZO in the cis conformation was obtained with extremely high yield (98%) upon irradiation by UV light of the SAMAZO initially formed by the trans conformer. The difference in the measured currents, which amounts to about 1.4 orders of magnitude, is in agreement with a through-bond tunneling mechanism described by Equation (1).

Self-Assembly of a Conjugated Polymer: From Molecular Rods to a Nanoribbon Architecture with Molecular Dimensions

Candidates for molecular nanowires for the interconnection of gold nanoelectrodes in a molecular-scale electronic device are to be found in end-functionalised poly(para-phenyleneethynylene)s. The self-assembly of these polymers was studied on atomically flat solid substrates. On graphite, sub-molecularly resolved imaging with scanning tunneling microscopy revealed nanorods in a 2 D nematic-like texture, while on mica, scanning force microscopy (see picture) shows that the nanorods can self-assemble into highly oriented micrometer-long nanoribbons with a molecular cross-section.

Charge transport and mobility engineering in two-dimensional transition metal chalcogenide semiconductors

Two-dimensional (2D) van der Waals semiconductors represent the thinnest, air stable semiconducting materials known. Their unique optical, electronic and mechanical properties hold great potential for harnessing them as key components in novel applications for electronics and optoelectronics. However, the charge transport behavior in 2D semiconductors is more susceptible to external surroundings (e.g. gaseous adsorbates from air and trapped charges in substrates) and their electronic performance is generally lower than corresponding bulk materials due to the fact that the surface and bulk coincide. In this article, we review recent progress on the charge transport properties and carrier mobility engineering of 2D transition metal chalcogenides, with a particular focus on the markedly high dependence of carrier mobility on thickness. We unveil the origin of this unique thickness dependence and elaborate the devised strategies to master it for carrier mobility optimization. Specifically, physical and chemical methods towards the optimization of the major factors influencing the extrinsic transport such as electrode/semiconductor contacts, interfacial Coulomb impurities and atomic defects are discussed. In particular, the use of ad hoc molecules makes it possible to engineer the interface with the dielectric and heal the vacancies in such materials. By casting fresh light on the theoretical and experimental studies, we provide a guide for improving the electronic performance of 2D semiconductors, with the ultimate goal of achieving technologically viable atomically thin (opto)electronics.

The Self‐Assembly of Lipophilic Guanosine Derivatives in Solution and on Solid Surfaces

The self-assembly of lipophilic deoxyguanosine derivatives 1 and 2 has been studied in solution by NMR spectroscopy and ESI-MS (electrospray ionization mass spectrometry). NMR data show the existence of two types of self-assembled, ribbonlike structures (A and B), which are connected at the guanine moieties through two different H-bonded networks. The first species (A), which is stable in the solid state and characterised by cyclic NH(2)-O(6) and NH(1)-N(7) hydrogen bonds, is detected soon after dissolving the polycrystalline powder in rigorously anhydrous CDCl3. In solution it slowly undergoes a structural transition towards a thermodynamically stable ribbon characterised by NH(1)-O(6) and NH(2)-N(3) cyclic hydrogen bonds (B). On the other hand, at surfaces, self-assembled ribbon nanostructures have been grown from solutions of derivative 1 both on mica and at the graphite-solution interface. They have been investigated by means of tapping mode scanning force microscopy (SFM) and scanning tunnelling microscopy (STM), respectively. SFM revealed dry, micrometer-long nanoribbons with a molecular cross-section. while STM imaging at submolecular resolution indicates a molecular packing of type A, like the one detected in the solid state. This indicates that, upon adsorption at the solid-liquid interface, the guanosine moieties undergo a structural rearrangement from a B-type to an A-type ribbon.

Poly-para-phenylene-ethynylene assemblies for a potential molecular nanowire: an SFM study

The self-assembly of a poly-para-phenylene-ethynylene (PPE) derivative cast onto muscovite mica is investigated by Tapping Mode Scanning Force Microscopy. The film morphology depends on the concentration of the PPE in the casting solution. At high concentration a grain morphology with some linear aggregates is observed. At low concentration PPE self-assembles into well defined and stable needles with a cross section with molecular dimensions. A model for the packing of the molecules in these supramolecular structures is suggested. Properly functionalised needles exhibit the molecular architecture of a molecular nanowire ready to be interfaced by metallic nanoelectrodes.

Extended triphenylenes: synthesis, mesomorphic properties and molecularly resolved scanning tunneling microscopy images of hexakis(dialkoxyphenyl)triphenylenes and dodeca(alkoxy)tris(triphenylenylene)s

Palladium-catalyzed cross-coupling between 3,4-dialkoxyphenylboronic acids (1a–d) and 2,3,6,7,10,11-hexabromotriphenylene (2) provided 2,3,6,7,10,11-hexakis[3,4-bis(alkoxy)phenyl]triphenylenes, C18H6[C6H3(OCnH2n + 1)2]6 where n = 6, 8, 10, and 12 (3a–d). Cyclodehydrogenation of the aryl-substituted triphenylenes 3a–d using ferric chloride oxidation followed by methanol reduction produced 6,6′,6″,7,7′,7″,10,10′,10″,11,11′,11″-dodecaalkoxy-2,3′:3,2″:2′,3″-tris(triphenylenylene)s, C54H18(OCnH2n + 1)12 where n = 6, 8, 10, and 12 (4a–d). The mesomorphic properties of the compounds 3a–d and 4a–d were investigated by differential scanning calorimetry (DSC) measurements, polarizing microscopy, and wide angle X-ray diffraction (WAXD). The triphenylenes 3a–d exhibited a columnar mesophase in the range of 111–126, 85–104, 74–103, and 47–101 °C, respectively. Upon oxidation of the moiety, the columnar mesophases shift to higher temperatures and exist in a much broader range of temperatures: for the tris(triphenylenylene)s 4a–d, they have been observed in the range of 180–430, 150–370, 120–322, and 104–306 °C, respectively. Finally, the self-assembly at the interface between a solution of 4c and a graphite substrate has been studied by scanning tunneling microscopy. Molecularly resolved imaging revealed a highly ordered monolayer exhibiting a two-dimensional hexagonal lattice.

STM Insight into Hydrogen-Bonded Bicomponent 1 D Supramolecular Polymers with Controlled Geometries at the Liquid–Solid Interface

Bicomponent supramolecular polymers, consisting of two alternating molecules bridged through six H-bonds, are observed by STM at the solid-liquid interface. Control of the geometry of the 1D architecture was obtained by using two different connecting molecules with different conformational rigidity, affording either linear (see picture, left) or zigzag (right) motifs.

Temperature-Enhanced Solvent Vapor Annealing of a C-3 Symmetric Hexa-peri-Hexabenzocoronene: Controlling the Self-Assembly from Nano- to Macroscale

Temperature-enhanced solvent vapor annealing (TESVA) is used to self-assemble functionalized polycyclic aromatic hydrocarbon molecules into ordered macroscopic layers and crystals on solid surfaces. A novel C3 symmetric hexa-peri-hexabenzocoronene functionalized with alternating hydrophilic and hydrophobic side chains is used as a model system since its multivalent character can be expected to offer unique self-assembly properties and behavior in different solvents. TESVA promotes the molecules long-range mobility, as proven by their diffusion on a Si/SiO(x) surface on a scale of hundreds of micrometers. This leads to self-assembly into large, ordered crystals featuring an edge-on columnar type of arrangement, which differs from the morphologies obtained using conventional solution-processing methods such as spin-coating or drop-casting. The temperature modulation in the TESVA makes it possible to achieve an additional control over the role of hydrodynamic forces in the self-assembly at surfaces, leading to a macroscopic self-healing within the adsorbed film notably improved as compared to conventional solvent vapor annealing. This surface re-organization can be monitored in real time by optical and atomic force microscopy.

Self-assembly of discotic molecules into mesoscopic crystals by solvent-vapour annealing

Solvent vapour annealing (SVA) is used to control the reorganization of ultrathin films of three different polycyclic aromatic hydrocarbons self-assembled on solid surfaces. To this end, two perylene-bis(dicarboximide) (PDI) derivatives exposing branched side alkyl chains with different length and a dodecyl substituted hexa-peri-hexabenzocoronene (HBC) have been used, in view of their n- and p-type semiconducting nature. For all the three molecules, nanoscopic crystals grown from solution by spin-coating and drop-casting undergo reorganization into sub-millimetric fibers, domes and needles, proving the general applicability of the SVA method. Moreover, such an approach exhibits a mass transport of the molecules on surfaces over hundreds of microns. The self-healing of the films through SVA treatment leads to a decrease of the structural defects and an increase in the lateral size of the self-assembled domains, ultimately providing an improved 3D conjugation. Therefore the SVA can be considered an important strategy with potential to enhance the performance of macroscopic organic electronic devices.

Nanoribbons from conjugated macromolecules on amorphous substrates observed by SFM and TEM

Micrometre long nanoribbons have been grown from solutions of functionalized poly( para-phenyleneethynylene)s (PPE)s on noncrystalline insulating substrates including glass and carbon coated copper grids. Tapping mode scanning force microscopy (SFM) and transmission electron microscopy (TEM) revealed that these nanostructures possess a molecular cross section with a typical thickness of 2-3 molecular layers and a width which reflects the distribution of macromolecular lengths. The ribbons are therefore quite similar to the ones found on the crystalline mica substrate except that they are not oriented within the surface plane. This indicates that the growth of these architectures from solution is mainly governed by intermolecular interactions between the π-conjugated macromolecules. The possibility to self-assemble these nanoribbons also on amorphous silica opens a prospect for their application as molecular nanowires bridging gold nanoelectrodes grown on oxidized silicon wafers.