Metal/organic monolayer/GaAs junctions, prepared by adsorbing a set of dicarboxylic ligands, with systematic change of ligand substituents, on GaAs, are measured and characterized electrically. The molecules are chemically bound to the semiconductor surface under ambient conditions and form roughly a monolayer (MoL), with average order in the direction perpendicular to the semiconductor surface. This suffices to yield systematic changes in electron affinity and work function of the modified GaAs. Junctions are made by a soft metal deposition method, used here for Au and Al. Experimentally, we find strong molecular effects, reaching differences in current at a given voltage of up to 6 orders of magnitude, depending on the substituent on the molecules making up the monolayer. These and the changes in the effective barrier height of the metal/MoL/ GaAs junctions, extracted by analyses of their current-voltage characteristics, can be explained by electrostatic effects of the molecular layer, rather than by electrodynamic ones (current flow through the molecular film). This can be understood by realizing that the samples are relatively large area devices with extremely narrow (∼1 nm) films of organic molecules, showing only average order, which makes dominance of tunneling effects very unlikely. We show that not only the molecule's electronic and electrical properties but also the way the metals contact the molecules, as well as the doping type of the semiconductor, can determine the direction of the molecular effect. Also the type of metal governs the effect that we identify as being due to interfacial dipoles formed as a result of triple metal/organic molecule/semiconductor interaction.

We show that electron beam evaporation of metal onto a monolayer of organic molecules can yield reproducible electrical contacts, if evaporation is indirect and the sample is on a cooled substrate. The metal contact forms without damaging even the molecules' outermost groups. In contrast, direct evaporation seriously damages the molecules. By comparing molecular effects on metal/molecular layer/GaAs junctions, prepared by indirect evaporation and by other soft contacting methods, we confirm experimentally that Au is not an optimal choice as an evaporated contact metal. We ascribe this to the ease by which Au can diffuse between molecules, something that can, apart from direct contact-substrate connections, lead to undesired and uncontrollable interfacial interactions. Such phenomena are largely absent with Pd as evaporated contact.

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We describe a method for the preparation of novel long (hundreds of nanometers), uniform, inter-molecular G4-DNA molecules composed of four parallel G-strands. The only long continuous G4-DNA reported so far are intra-molecular structures made of a single G-strand. To enable a tetra-molecular assembly of the G-strands we developed a novel approach based on avidin–biotin biological recognition. The steps of the G4-DNA production include: (i) Enzymatic synthesis of long poly(dG)-poly(dC) molecules with biotinylated poly(dG)-strand; (ii) Formation of a complex between avidin-tetramer and four biotinylated poly(dG)-poly(dC) molecules; (iii) Separation of the poly(dC) strands from the poly(dG)-strands, which are connected to the avidin; (iv) Assembly of the four G-strands attached to the avidin into tetra-molecular G4-DNA. The average contour length of the formed structures, as measured by AFM, is equal to that of the initial poly(dG)-poly(dC) molecules, suggesting a tetra-molecular mechanism of the G-strands assembly. The height of tetra-molecular G4-nanostructures is larger than that of mono-molecular G4-DNA molecules having similar contour length. The CD spectra of the tetra- and mono-molecular G4-DNA are markedly different, suggesting different structural organization of these two types of molecules. The tetra-molecular G4-DNA nanostructures showed clear electrical polarizability. This suggests that they may be useful for molecular electronics.

Reproducible electrical contacts to organic molecules are created non-destructively by indirect electron beam evaporation of Pd onto molecular films on cooled substrates. In contrast, directly evaporated contacts damage the molecules seriously. Our conclusions are based on correlating trends in properties of a series of molecules with systematically varying, exposed functional groups, with trends in the electrical behaviour of Pd/molecule/GaAs junctions, where these same molecules are part of the junctions