Molecule/Electrode Interface Energetics in Molecular Junction: A “Transition Voltage Spectroscopy” Study

We assess the performances of the transition voltage spectroscopy (TVS) method to determine the energies of the molecular orbitals involved in the electronic transport through molecular junctions. A large number of various molecular junctions made with alkyl chains but with different chemical structure of the electrode/molecule interfaces are studied. In the case of molecular junctions with “clean, unoxidized” electrode/molecule interfaces, that is, alkylthiols and alkenes directly grafted on Au and hydrogenated Si, respectively, we measure transition voltages in the range 0.9–1.4 V. We conclude that the TVS method allows estimating the onset of the tail of the LUMO density of states, at energy located 1.0–1.2 eV above the electrode Fermi energy. For “oxidized” interfaces (e.g., the same monolayer measured with Hg or eGaIn drops, or monolayers formed on a slightly oxidized silicon substrate), lower transition voltages (0.1–0.6 V) are systematically measured. These values are explained by the presence of oxide-related density of states at energies lower than the HOMO/LUMO of the molecules. As such, the TVS method is a useful technique to assess the quality of the molecule/electrode interfaces in molecular junctions

Conductance Statistics from a Large Array of Sub-10 nm Molecular Junctions

Devices made of few molecules constitute the miniaturization limit that both inorganic and organic-based electronics aspire to reach. However, integration of millions of molecular junctions with less than 100 molecules each has been a long technological challenge requiring well controlled nanometric electrodes. Here we report molecular junctions fabricated on a large array of sub-10 nm single crystal Au nanodots electrodes, a new approach that allows us to measure the conductance of up to a million of junctions in a single conducting atomic force microscope (C-AFM) image. We observe two peaks of conductance for alkylthiol molecules. Tunneling decay constant (β) for alkanethiols, is in the same range as previous studies. Energy position of molecular orbitals, obtained by transient voltage spectroscopy, varies from peak to peak, in correlation with conductance values

Conductance Statistics from a Large Array of Sub-10 nm Molecular Junctions

Devices made of few molecules constitute the miniaturization limit that both inorganic and organic-based electronics aspire to reach. However, integration of millions of molecular junctions with less than 100 molecules each has been a long technological challenge requiring well controlled nanometric electrodes. Here we report molecular junctions fabricated on a large array of sub-10 nm single crystal Au nanodots electrodes, a new approach that allows us to measure the conductance of up to a million of junctions in a single conducting atomic force microscope (C-AFM) image. We observe two peaks of conductance for alkylthiol molecules. Tunneling decay constant (β) for alkanethiols, is in the same range as previous studies. Energy position of molecular orbitals, obtained by transient voltage spectroscopy, varies from peak to peak, in correlation with conductance values

Role of Hydration on the Electronic Transport through Molecular Junctions on Silicon

Molecular electronics is a fascinating area of research with the ability to tune device properties by a chemical tailoring of organic molecules. However, molecular electronics devices often suffer from dispersion and lack of reproducibility of their electrical performances. Here, we show that water molecules introduced during the fabrication process or coming from the environment can strongly modify the electrical transport properties of molecular junctions made on hydrogen-terminated silicon. We report an increase in conductance by up to 3 orders of magnitude, as well as an induced asymmetry in the current–voltage curves. These observations are correlated with a specific signature of the dielectric response of the monolayer at low frequency. In addition, a random telegraph signal is observed for these junctions with macroscopic area. Electrochemical charge transfer reaction between the semiconductor channel and H⁺/H₂ redox couple is proposed as the underlying phenomenon. Annealing the samples at 150 °C is an efficient way to suppress these water-related effects. This study paves the way to a better control of molecular devices and has potential implications when these monolayers are used as hydrophobic layers or incorporated in chemical sensors

Establishment of a Derivatization Method To Quantify Thiol Function in Sulfur-Containing Plasma Polymer Films

Thiol-supported surfaces draw more and more interest in numerous fields of applications from biotechnology to catalysis. Among the various strategies to generate such surfaces, the plasma polymerization of a thiol-containing molecule appears to be one of the ideal candidates. Nevertheless, considering such an approach, a careful characterization of the material surface chemistry is necessary. In this work, an original chemical derivatization method aiming to quantitatively probe the −SH functions in plasma polymers was established using <i>N</i>-ethylmaleimide as a labeling molecule. The method was qualitatively and quantitatively validated on self-assembled monolayers of 3-mercaptopropyltrimethoxysilane exhibiting a −SH-terminated group used as “model” surface. For a quantitative determination of the −SH content in propanethiol plasma polymers, the kinetics of the reaction was investigated. The latter is described as a two-step mechanism, namely a fast surface reaction followed by a diffusion-limited one. The density of −SH groups deduced from the derivatization method (∼4%) is in good agreement with typical values measured in some other plasma polymer families. The whole set of our data opens up new possibilities for optimizing the −SH content in thiol-based plasma polymer films

Langmuir–Blodgett Films of Amphiphilic Thieno[3,4‑<i>c</i>]pyrrole-4,6-dione-Based Alternating Copolymers

The synthesis of four amphiphilic thieno­[3,4-<i>c</i>]­pyrrole-4,6-dione (TPD)-based alternating copolymers and their behavior at the air–water interface are reported. Homogeneous and stable monolayers of TPD-based copolymers were prepared. Brewster angle microscopy (BAM) was utilized to characterize the morphology and topography of these Langmuir films. UV–vis absorption spectroscopy as well as atomic force microscopy has revealed a regular transfer of some copolymers on glass substrates. It was possible to obtain homogeneous Langmuir–Blodgett films of up to 30 layers. Infrared dichroic measurements revealed an edge-on orientation. These Langmuir–Blodgett films made of conjugated polymers are therefore good candidates for organic field-effect transistors (OFETs)

Probing Frontier Orbital Energies of {Co₉(P₂W₁₅)₃} Polyoxometalate Clusters at Molecule–Metal and Molecule–Water Interfaces

Functionalization of polyoxotungstates with organoarsonate coligands enabling surface decoration was explored for the triangular cluster architectures of the composition [Co^II₉(H₂O)₆(OH)₃(<i>p</i>-RC₆H₄As^VO₃)₂­(α-P^V₂W^VI₁₅O₅₆)₃]^25– ({Co₉(P₂W₁₅)₃}, R = H or NH₂), isolated as Na₂₅[Co₉(OH)₃(H₂O)₆­(C₆H₅AsO₃)₂­(P₂W₁₅O₅₆)₃]·86H₂O (<b>Na-1</b>; triclinic, <i>P</i>1̅, <i>a</i> = 25.8088(3) Å, <i>b</i> = 25.8336(3) Å, <i>c</i> = 27.1598(3) Å, α = 78.1282(11)°, β = 61.7276(14)°, γ = 60.6220(14)°, <i>V</i> = 13888.9(3) ų, <i>Z</i> = 2) and Na₂₅[Co₉(OH)₃­(H₂O)₆­(H₂NC₆H₄AsO₃)₂­(P₂W₁₅O₅₆)₃]·86H₂O (<b>Na-2</b>; triclinic, <i>P</i>1̅, <i>a</i> = 14.2262(2) Å, <i>b</i> = 24.8597(4) Å, <i>c</i> = 37.9388(4) Å, α = 81.9672(10)°, β = 87.8161(10)°, γ = 76.5409(12)°, <i>V</i> = 12920.6(3) ų, <i>Z</i> = 2). The axially oriented <i>para</i>-aminophenyl groups in <b>2</b> facilitate the formation of self-assembled monolayers on gold surfaces and thus provide a viable molecular platform for charge transport studies of magnetically functionalized polyoxometalates. The title systems were isolated and characterized in the solid state, in aqueous solutions, and on metal surfaces. Using conducting tip atomic force microscopy, the energies of {Co₉(P₂W₁₅)₃} frontier molecular orbitals in the surface-bound state were found to directly correlate with cyclic voltammetry data in aqueous solution