Investigating Molecular Junctions Based on Mixed Self-Assembled Monolayers to Understand the Impact of Intermolecular Interactions on Transport.

To interrogate the importance of intermolecular interactions on charge transport at the nanoscale, we investigate molecular tunnel junctions based on mixed self-assembled monolayers (SAMs) of 1-alkyl (CnT) thiols and their fluorinated counterparts (F-CnT) that have substantially different tunneling conductances. Experiments on mixed CnT:F-CnT SAMs between Au contacts reveal a strongly nonlinear (exponential) dependence of the tunneling conductance on composition , a behavior that is tempting to assign to the strong impact of intra-SAM intermolecular interactions. However, analysis suggests that the exponential dependence of on does not arise from intra-SAM intermolecular interactions, but instead emerges from the work function modification of the Au electrode which varies linearly with .

Room Temperature Crystallized Phase-Pure α-FAPbI Perovskite with In-Situ Grain-Boundary Passivation.

Energy loss in perovskite grain boundaries (GBs) is a primary limitation toward high-efficiency perovskite solar cells (PSCs). Two critical strategies to address this issue are high-quality crystallization and passivation of GBs. However, the established methods are generally carried out discretely due to the complicated mechanisms of grain growth and defect formation.

Breakdown of Ohm's Law in Molecular Junctions with Electrodes of Single-Layer Graphene.

For sufficiently low biases, Ohm's law, the cornerstone of electricity, stating that current and voltage are proportional, is satisfied at low biases for all known systems ranging from macroscopic conductors to nanojunctions. In this study, we predict theoretically and demonstrate experimentally that in single-molecule junctions fabricated with single-layer graphene as electrodes the current at low scales as the cube of , thereby invalidating Ohm's law. The absence of the ohmic regime is a direct consequence of the unique band structure of the single-layer graphene, whose vanishing density of states at the Dirac points precludes electron transfer from and to the electrodes at low biases.

Quantifying Image Charge Effects in Molecular Tunnel Junctions Based on Self-Assembled Monolayers of Substituted Oligophenylene Ethynylene Dithiols.

A number of factors contribute to orbital energy alignment with respect to the Fermi level in molecular tunnel junctions. Here, we report a combined experimental and theoretical effort to quantify the effect of metal image potentials on the highest occupied molecular orbital to Fermi level offset, ε, for molecular junctions based on self-assembled monolayers (SAMs) of oligophenylene ethynylene dithiols (OPX) on Au. Our experimental approach involves the use of both transport and photoelectron spectroscopy to extract the offsets, ε and ε, respectively.

Quantitative analysis of weak current rectification in molecular tunnel junctions subject to mechanical deformation reveals two different rectification mechanisms for oligophenylene thiols alkane thiols.

Metal-molecule-metal junctions based on alkane thiol (CT) and oligophenylene thiol (OPT) self-assembled monolayers (SAMs) and Au electrodes are expected to exhibit similar electrical asymmetry, as both junctions have one chemisorbed Au-S contact and one physisorbed, van der Waals contact. Asymmetry is quantified by the current rectification ratio RR apparent in the current-voltage (-) characteristics. Here we show that RR 1 for CT and RR 1 for OPT junctions, in contrast to expectation, and further, that RR behaves very differently for CT and OPT junctions under mechanical extension using the conducting probe atomic force microscopy (CP-AFM) testbed.

Strain-Work Function Relationship in Single-Crystal Tetracene.

Understanding the impact of strain on organic semiconductors is important for the development of electronic devices and sensors that are subject to environmental changes and mechanical stimuli; it is also important for understanding the fundamental mechanisms of charge trapping. Following our previous study on the strain effects in rubrene, we present here only the second example of the strain-work function relationship in an organic semiconductor; in this case, the benchmark material tetracene. Thin, platelike single crystals of tetracene with large (001) facets were laminated onto silicon and rubber substrates having significantly different coefficients of thermal expansion; mechanical strain in tetracene was subsequently induced by varying the temperature of the assembly.

Energy Level Alignment in Molecular Tunnel Junctions by Transport and Spectroscopy: Self-Consistency for the Case of Alkyl Thiols and Dithiols on Ag, Au, and Pt Electrodes.

We report here an extensive study of transport and electronic structure of molecular junctions based on alkyl thiols (CT; = 7, 8, 9, 10, 12) and dithiols (CDT; = 8, 9, 10) with various lengths contacted with different metal electrodes (Ag, Au, Pt). The dependence of the low-bias resistance () on contact work function indicates that transport is HOMO-assisted (p-type transport). Analysis of the current-voltage () characteristics for CT and CDT tunnel junctions with the analytical single-level model (SLM) provides both the HOMO-Fermi energy offset ε and the average molecule-electrode coupling (Γ) as a function of molecular length (), electrode work function (Φ), and the number of chemical contacts (one or two).

Gate-Tuned Insulator-Metal Transition in Electrolyte-Gated Transistors Based on Tellurene.

Tellurene is a recently discovered 2D material with high hole mobility and air stability, rendering it a good candidate for future applications in electronics, optoelectronics, and energy devices. However, the physical properties of tellurene remain poorly understood. In this paper, we report on the fabrication and characterization of high-performance electrolyte-gated transistors (EGTs) based on solution-grown tellurene flakes <30 nm in thickness.

Determination of Energy-Level Alignment in Molecular Tunnel Junctions by Transport and Spectroscopy: Self-Consistency for the Case of Oligophenylene Thiols and Dithiols on Ag, Au, and Pt Electrodes.

We report detailed measurements of transport and electronic properties of molecular tunnel junctions based on self-assembled monolayers (SAMs) of oligophenylene monothiols (OPT n, n = 1-3) and dithiols (OPD n, n = 1-3) on Ag, Au, and Pt electrodes. The junctions were fabricated with the conducting probe atomic force microscope (CP-AFM) platform. Fitting of the current-voltage ( I-V) characteristics for OPT n and OPD n junctions to the analytical single-level tunneling model allows extraction of both the HOMO-to-Fermi-level offset (ε) and the average molecule-electrode coupling (Γ) as a function of molecular length ( n) and electrode work function (Φ).

Mechanical Deformation Distinguishes Tunneling Pathways in Molecular Junctions.

Developing a clearer understanding of electron tunneling through molecules is a central challenge in molecular electronics. Here we demonstrate the use of mechanical stretching to distinguish orbital pathways that facilitate tunneling in molecular junctions. Our experiments employ junctions based on self-assembled monolayers (SAMs) of homologous alkanethiols (C nT) and oligophenylene thiols (OPT n), which serve as prototypical examples of σ-bonded and π-bonded backbones, respectively.

Why one can expect large rectification in molecular junctions based on alkane monothiols and why rectification is so modest.

Many attempts to obtain high current rectification ratios (RRs) in molecular electronics are triggered by a potentiometer rule argument, which predicts that a strongly asymmetric location of the dominant molecular orbital yields large RR-values. Invoking this argument, molecular junctions based on alkane monothiols (CnT) can be expected to exhibit high RRs; the HOMO of these molecules is localized on the thiol terminal group bonded to one electrode. The extensive current-voltage (-) results for CP-AFM (conducting probe atomic force microscope) CnT junctions of various molecular lengths ( = 7, 8, 9, 10, and 12) and different metallic contacts (Ag, Au, and Pt) are consistent with conduction dominated by the HOMO, but the measured RR ∼ 1.

Work function and temperature dependence of electron tunneling through an N-type perylene diimide molecular junction with isocyanide surface linkers.

Conducting probe atomic force microscopy (CP-AFM) was employed to examine electron tunneling in self-assembled monolayer (SAM) junctions. A 2.3 nm long perylene tetracarboxylic acid diimide (PDI) acceptor molecule equipped with isocyanide linker groups was synthesized, adsorbed onto Ag, Au and Pt substrates, and the current-voltage (I-V) properties were measured by CP-AFM.

Exceptionally Small Statistical Variations in the Transport Properties of Metal-Molecule-Metal Junctions Composed of 80 Oligophenylene Dithiol Molecules.

Strong stochastic fluctuations witnessed as very broad resistance (R) histograms with widths comparable to or even larger than the most probable values characterize many measurements in the field of molecular electronics, particularly those measurements based on single molecule junctions at room temperature. Here we show that molecular junctions containing 80 oligophenylene dithiol molecules (OPDn, 1 ≤ n ≤ 4) connected in parallel display small relative statistical deviations-δR/R ≈ 25% after only ∼200 independent measurements-and we analyze the sources of these deviations quantitatively. The junctions are made by conducting probe atomic force microscopy (CP-AFM) in which an Au-coated tip contacts a self-assembled monolayer (SAM) of OPDs on Au.

Effect of Heteroatom Substitution on Transport in Alkanedithiol-Based Molecular Tunnel Junctions: Evidence for Universal Behavior.

The transport properties of molecular junctions based on alkanedithiols with three different methylene chain lengths were compared with junctions based on similar chains wherein every third -CH- was replaced with O or S, that is, following the general formula HS(CHCHX)CHCHSH, where X = CH, O, or S and n = 1, 2, or 3. Conducting probe atomic force microscopy revealed that the low bias resistance of the chains increased upon substitution in the order CH < O < S. This change in resistance is ascribed to the observed identical trend in contact resistance, R, whereas the exponential prefactor β (length sensitivity) was essentially the same for all chains.

Large Magnetoresistance at Room Temperature in Organic Molecular Tunnel Junctions with Nonmagnetic Electrodes.

We report room-temperature resistance changes of up to 30% under weak magnetic fields (0.1 T) for molecular tunnel junctions composed of oligophenylene thiol molecules, 1-2 nm in length, sandwiched between gold contacts. The magnetoresistance (MR) is independent of field orientation and the length of the molecule; it appears to be an interface effect.

Experimental and Theoretical Analysis of Nanotransport in Oligophenylene Dithiol Junctions as a Function of Molecular Length and Contact Work Function.

We report the results of an extensive investigation of metal-molecule-metal tunnel junctions based on oligophenylene dithiols (OPDs) bound to several types of electrodes (M1-S-(C6H4)n-S-M2, with 1 ≤ n ≤ 4 and M1,2 = Ag, Au, Pt) to examine the impact of molecular length (n) and metal work function (Φ) on junction properties. Our investigation includes (1) measurements by scanning Kelvin probe microscopy of electrode work function changes (ΔΦ = ΦSAM - Φ) caused by chemisorption of OPD self-assembled monolayers (SAMs), (2) measurements of junction current-voltage (I-V) characteristics by conducting probe atomic force microscopy in the linear and nonlinear bias ranges, and (3) direct quantitative analysis of the full I-V curves. Further, we employ transition voltage spectroscopy (TVS) to estimate the energetic alignment εh = EF - EHOMO of the dominant molecular orbital (HOMO) relative to the Fermi energy EF of the junction.

Uncovering a law of corresponding states for electron tunneling in molecular junctions.

Laws of corresponding states known so far demonstrate that certain macroscopic systems can be described in a universal manner in terms of reduced quantities, which eliminate specific substance properties. To quantitatively describe real systems, all these laws of corresponding states contain numerical factors adjusted empirically. Here, we report a law of corresponding states deduced analytically for charge transport via tunneling in molecular junctions, which we validate against current-voltage measurements for conducting probe atomic force microscope junctions based on benchmark molecular series (oligophenylenedithiols and alkanedithiols) and electrodes (silver, gold, and platinum), as well as against transport data for scanning tunneling microscope junctions.

Spin specific electron conduction through DNA oligomers.

Spin-based properties, applications, and devices are commonly related to magnetic effects and to magnetic materials. Most of the development in spintronics is currently based on inorganic materials. Despite the fact that the magnetoresistance effect has been observed in organic materials, until now spin selectivity of organic based spintronics devices originated from an inorganic ferromagnetic electrode and was not determined by the organic molecules themselves.

How isolated are the electronic states of the core in core/shell nanoparticles?

We investigated how isolated are the electronic states of the core in a core-shell (c/s) nanoparticles (NPs) from the surface, when the particles are self-assembled on Au substrates via a dithiol (DT) organic linker. Applying photoemission spectroscopy the electronic states of CdSe core only and CdSe/ZnS c/s NPs were compared. The results indicate that in the c/s NPs the HOMO interacts strongly with electronic states in the Au substrate and is pinned at the same energies, relative to the Fermi level, as the core only NPs.