Thin layers of diarylethene oligomers (oligo­(DAE)) were deposited by electrochemical reduction of a diazonium salt on glassy carbon and gold electrodes. The layers were fully characterized using electrochemistry, XPS, and AFM, and switching between open and closed forms using light was evidenced. Solid-state molecular junctions (MJs), in which a C-AFM tip is used as the top contact, were fabricated with total layer thicknesses fixed at 2–3 nm and 8–9 nm, i.e. below and above the direct tunneling limit. DAE was then photoswitched between its open and closed forms. Oligo­(DAE) MJs using the open form of DAE are highly resistive while those with DAE in the closed form are more conductive. ON/OFF ratios of 2–3 and 200–400 were obtained for 3-nm- and 9-nm-thick DAE MJs, respectively.

Correction for ‘Multi-functional switches of ditopic ligands with azobenzene central bridges at a molecular scale’ by Imen Hnid et al., Nanoscale, 2019, 11, 23042–23048.

Thin films of photochromic coordination polymers deposited by layer-by-layer technique, based on iron­(II) bis-terpyridine-appended dithienylethene blocks, were used to create light-switchable molecular junctions with high rectification ratios. These multilayer films show a modulation of the attenuation factor when exposed to light irradiation.

Semiconducting monolayer of 2D material are able to concatenate multiple interesting properties into a single component. Here, by combining opto-mechanical and electronic measurements, we demonstrate the presence of a partial 2H-1T' phase transition in a suspended 2D monolayer membrane of MoS2. Electronic transport shows unexpected memristive properties in the MoS2 membrane, in the absence of any external dopants. A strong mechanical softening of the membrane is measured concurrently and may only be related to the phase 2H-1T' phase transition which imposes a 3% directional elongation of the topological 1T' phase with respect to the semiconducting 2H. We note that only a few percent 2H-1T'phase switching is sufficient to observe measurable memristive effects. Our experimental results combined with First-principles total energy calculations indicate that sulfur vacancy diffusion playsa key role in the initial nucleation of the phase transition. Our study clearly shows that nanomechanics represents an ultrasensitive technique to probe the crystal phase transition in 2D materials or thin membranes. Finally, a better control of the microscopic mechanisms responsible for the observed memristive effect in MoS2is important for the implementation of future devices.

Photoactive molecular junctions, based on 4 nm-thick diarylethene (DAE) and 5 nm-thick bisthienylbenzene (BTB) layers, were fabricated by electrochemical deposition.Total thickness was around 9 nm, i.e. above the direct tunneling limit and in the hopping regime. The DAE units were switched between their open and closed forms. The DAE/BTB bilayer structure exhibits new electronic functions combining photoswitching and photorectification. The open form of DAE/BTB shows low conductance and asymmetric IV curves while the closed form shows symmetric IV curves and high conductance. More importantly, unprecedented ON/OFF current ratios of over 10 000 at 1 volts were reproducibly measured. KEYWORDS. photoswitch; electroswitch; diarylethenes; modified surfaces; molecular junctions, MAIN TEXT. Photochromic molecules, i.e. molecules with two different forms(open/non-conjugated and closed/conjugated) which can be interconverted by light irradiation, have been proposed as building blocks for photoresistive switches in molecular electronics. 1,2 Most studies devoted to the transport characteristics in such systems, have been limited to monolayer-based or single-molecule-based molecular junctions (MJs) where the dominant transport mechanism is direct tunneling. [3][4][5][6] In this regime, photoswitching shows poor reversibility 7,8 and small ON/OFF ratios (less than two orders of magnitude) due to strong molecule-electrode coupling which induces quenching effects, 9-11 and it is only recently that a high ON/OFF ratio (around 100) with good reversibility was reported, using a strategy that decouples the active unit from the electrode. 12 When the thickness of the layer incorporated in an MJ is above 5 nm the main transport mechanism is hopping, which has a stronger molecular signature than tunneling. 13,14 MJs incorporating photochrome-based layers above 5 nm thick have been studied [15][16][17][18] while two recent studies 19,20 have compared the performances of photoactive devices in both regimes. In the first, 19 junctions showed ON/OFF ratios of 10 and were not significantly affected by the length of the wire. In contrast, we recently demonstrated that ON/OFF ratios increase from 2-3 to 200-400 for 3 nm and 9 nm-thick diarylethene-based layers, respectively. This improvement was explained by the OFF state of the thicker devices being much less conductive than that of the thinner ones, while the conductance of the ON states is similar.We have now prepared 9 nm-thick MJs using a bilayer system. The first layer (5 nm) is based on bisthienylbenzene oligomers, (BTB), which are known to be good molecular wires and show activation-less transport and low attenuation factors in the hopping regime. 21 The second layer (around 4 nm) is based on photochromic diarylethene oligomers, (DAE).

Diarylethene is a prototypical molecular switch that can be reversibly photoisomerized between its open and closed forms. Ligands bpy-DAE-bpy, consisting of a phenyl-diarylethene-phenyl (DAE) central core and bipyridine (bpy) terminal substituents, are able to self-organize. They are investigated by scanning tunneling microscopy at the solid–liquid interface. Upon light irradiation, cooperative photochromic switching of the ligands is recognized down to the submolecular level. The closed isomers show different electron density of states (DOS) contrasts, attributed to the HOMO or LUMO molecular orbitals observed. More importantly, the LUMO images show remarkable differences between the open and closed isomers, attributed to combined topographic and electronic contrasts mainly on the DAE moieties. The electronic contrasts from multiple HOMO or LUMO distributions, combined with topographic distortion of the open or closed DAE, are interpreted by density functional theory (DFT) calculations.

This paper reports multi-functional switches from a ditopic ligand bpy-azo-bpy. The molecule can adopt a cis-to-trans isomerization at the bipyridine terminals and can switch between their TRANS and CIS configurations at the central azobenzene unit by protonation or by light irradiation.

A series of ligands, which terminate with ditopic bipyridines connected by a variable central bridge, fluorene biEDOT or azo-benzene, have been investigated by high-resolution scanning tunneling microscopy (STM). The bipyridines can form different hydrogen bonds or can coordinate with transition metal ions at the solid/liquid interface. Their interactions allow such molecules to self-assemble on HOPG. The bridges provide redox-active, photoactive, or fluorescent functionalities. Both the bipyridine terminals and the bridges are units able to switch the molecules in terms of structure organization and electronic properties. Multi-functional switches or transitions are thus generated, for example (i) protonation generates a molecular cis-like to trans-like isomerization and switches the nanoscale organization of the molecule on the surface, (ii) Co(II) cations react with the pyridines to generate on-surface coordination complexes, and (iii) changing the central bridge induces different self-organized structures on the surface. A series of bpy-X-bpy molecules showing different molecular isomers, structural phase transitions and coordination configurations have thus been revealed by means of high-resolution STM.