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

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

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

Field and Thermal Emission Limited Charge Injection in Au–C60–Graphene van der Waals Vertical Heterostructures for Organic Electronics

Among the family of 2D materials, graphene is the ideal candidate as top or interlayer electrode for hybrid van der Waals heterostructures made of organic thin films and 2D materials due to its high conductivity and mobility and its inherent ability of forming neat interfaces without diffusing in the adjacent organic layer. Understanding the charge injection mechanism at graphene/organic semiconductor interfaces is therefore crucial to develop organic electronic devices. In particular, Gr/C60 interfaces are promising building blocks for future n-type vertical organic transistors exploiting graphene as tunneling base electrode in a two back-to-back Gr/C60 Schottky diode configuration. This work delves into the charge transport mechanism across Au/C60/Gr vertical heterostructures fabricated on Si/SiO2 using a combination of techniques commonly used in the semiconductor industry, where a resist-free CVD graphene layer functions as a top electrode. Temperature-dependent electrical measurements show that the transport mechanism is injection limited and occurs via Fowler–Nordheim tunneling at low temperature, while it is dominated by a nonideal thermionic emission at room and high temperatures, with energy barriers at room temperature of ca. 0.58 and 0.65 eV at the Gr/C60 and Au/C60 interfaces, respectively. Impedance spectroscopy confirms that the organic semiconductor is depleted, and the energy band diagram results in two electron blocking interfaces. The resulting rectifying nature of the Gr/C60 interface could be exploited in organic hot electron transistors and vertical organic permeable-base transistors

Charge Transport Across Au–P3HT–Graphene van der Waals Vertical Heterostructures

Hybrid van der Waals heterostructures based on 2D materials and/or organic thin films are being evaluated as potential functional devices for a variety of applications. In this context, the graphene/organic semiconductor (Gr/OSC) heterostructure could represent the core element to build future vertical organic transistors based on two back-to-back Gr/OSC diodes sharing a common graphene sheet, which functions as the base electrode. However, the assessment of the Gr/OSC potential still requires a deeper understanding of the charge carrier transport across the interface as well as the development of wafer-scale fabrication methods. This work investigates the charge injection and transport across Au/OSC/Gr vertical heterostructures, focusing on poly(3-hexylthiophen-2,5-diyl) as the OSC, where the PMMA-free graphene layer functions as the top electrode. The structures are fabricated using a combination of processes widely exploited in semiconductor manufacturing and therefore are suited for industrial upscaling. Temperature-dependent current–voltage measurements and impedance spectroscopy show that the charge transport across both device interfaces is injection-limited by thermionic emission at high bias, while it is space charge limited at low bias, and that the P3HT can be assumed fully depleted in the high bias regime. From the space charge limited model, the out-of-plane charge carrier mobility in P3HT is found to be equal to μ ≈ 2.8 × 10–4 cm2 V–1 s–1, similar to the in-plane mobility reported in previous works, while the charge carrier density is N0 ≈ 1.16 × 1015 cm–3, also in agreement with previously reported values. From the thermionic emission model, the energy barriers at the Gr/P3HT and Au/P3HT interfaces result in 0.30 eV and 0.25 eV, respectively. Based on the measured barriers heights, the energy band diagram of the vertical heterostructure is proposed under the hypothesis that P3HT is fully depleted

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)

High On−Off Conductance Switching Ratio in Optically-Driven Self-Assembled Conjugated Molecular Systems

A new azobenzene−thiophene molecular switch is designed, synthesized, and used to form self-assembled monolayers (SAM) on gold. An “on/off” conductance ratio up to 7 × 103 (with an average value of 1.5 × 103) is reported. The “on” conductance state is clearly identified to the cis isomer of the azobenzene moiety. The high on/off ratio is explained in terms of photoinduced, configuration-related changes in the electrode−molecule interface energetics (changes in the energy position of the molecular orbitals with respect to the Fermi energy of electrodes) in addition to changes in the tunnel barrier length (length of the molecules). First principles density functional calculations demonstrate a better delocalization of the frontier orbitals as well as a stronger electronic coupling between the azobenzene moiety and the electrode for the cis configuration over the trans one. Measured photoionization cross sections for the molecules in the SAM are close to the known values for azobenzene derivatives in solution

Molecular Junctions for Terahertz Switches and Detectors

Molecular electronics targets tiny devices exploiting the electronic properties of the molecular orbitals, which can be tailored and controlled by the chemical structure and configuration of the molecules. Many functional devices have been experimentally demonstrated; however, these devices were operated in the low-frequency domain (mainly dc to MHz). This represents a serious limitation for electronic applications, although molecular devices working in the THz regime have been theoretically predicted. Here, we experimentally demonstrate molecular THz switches at room temperature. The devices consist of self-assembled monolayers of molecules bearing two conjugated moieties coupled through a nonconjugated linker. These devices exhibit clear negative differential conductance behaviors (peaks in the current–voltage curves), as confirmed by ab initio simulations, which were reversibly suppressed under illumination with a 30 THz wave. We analyze how the THz switching behavior depends on the THz wave properties (power and frequency), and we benchmark that these molecular devices would outperform actual THz detectors