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)<i>_n</i>–S–M₂, with 1 ≤ <i>n</i> ≤ 4 and M_1,2 = Ag, Au, Pt) to examine the impact of molecular length (<i>n</i>) and metal work function (Φ) on junction properties. Our investigation includes (1) measurements by scanning Kelvin probe microscopy of electrode work function changes (ΔΦ = Φ_SAM – Φ) caused by chemisorption of OPD self-assembled monolayers (SAMs), (2) measurements of junction current–voltage (<i>I</i>–<i>V</i>) characteristics by conducting probe atomic force microscopy in the linear and nonlinear bias ranges, and (3) direct quantitative analysis of the full <i>I</i>–<i>V</i> curves. Further, we employ transition voltage spectroscopy (TVS) to estimate the energetic alignment ε_h = <i>E</i>_F – <i>E</i>_HOMO of the dominant molecular orbital (HOMO) relative to the Fermi energy <i>E</i>_F of the junction. Where photoelectron spectroscopy data are available, the ε_h values agree very well with those determined by TVS. Using a single-level model, which we justify <i>via ab initio</i> quantum chemical calculations at post-density functional theory level and additional UV–visible absorption measurements, we are able to quantitatively reproduce the <i>I</i>–<i>V</i> measurements in the whole bias range investigated (∼1.0–1.5 V) and to understand the behavior of ε_h and Γ (contact coupling strength) extracted from experiment. We find that Fermi level pinning induced by the strong dipole of the metal–S bond causes a significant shift of the HOMO energy of an adsorbed molecule, resulting in ε_h exhibiting a weak dependence with the work function Φ. Both of these parameters play a key role in determining the tunneling attenuation factor (β) and junction resistance (<i>R</i>). Correlation among Φ, ΔΦ, <i>R</i>, transition voltage (<i>V</i>_t), and ε_h and accurate simulation provide a remarkably complete picture of tunneling transport in these prototypical molecular junctions

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 −CH₂– was replaced with O or S, that is, following the general formula HS­(CH₂CH₂X)_<i>n</i>CH₂CH₂SH, where X = CH₂, O, or S and <i>n</i> = 1, 2, or 3. Conducting probe atomic force microscopy revealed that the low bias resistance of the chains increased upon substitution in the order CH₂ < O < S. This change in resistance is ascribed to the observed identical trend in contact resistance, <i>R</i>_c, whereas the exponential prefactor β (length sensitivity) was essentially the same for all chains. Using an established, analytical single-level model, we computed the effective energy offset ε_h (<i>i.e.</i>, Fermi level relative to the effective HOMO level) and the electronic coupling strength Γ from the current–voltage (<i>I–V</i>) data. The ε_h values were only weakly affected by heteroatom substitution, whereas the interface coupling strength Γ varied by over an order of magnitude. Consequently, we ascribe the strong variation in <i>R</i>_c to the systematic change in Γ. Quantum chemical calculations reveal that the HOMO density shifts from the terminal SH groups for the alkanedithiols to the heteroatoms in the substituted chains, which provides a plausible explanation for the marked decrease in Γ for the dithiols with electron-rich heteroatoms. The results indicate that the electronic coupling and thus the resistance of alkanedithiols can be tuned by substitution of even a single atom in the middle of the molecule. Importantly, when appropriately normalized, the experimental <i>I–V</i> curves were accurately simulated over the full bias range (±1.5 V) using the single-level model with no adjustable parameters. The data could be collapsed to a single universal curve predicted by the model, providing clear evidence that the essential physics is captured by this analytical approach and supporting its utility for molecular electronics

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. Theoretical analysis suggests that the source of the MR is a two-carrier (two-hole) interaction at the interface, resulting in spin coupling between the tunneling hole and a localized hole at the Au/molecule contact. Such coupling leads to significantly different singlet and triplet transmission barriers at the interface. Even weak magnetic fields impede spin relaxation processes and thus modify the ratio of holes tunneling <i>via</i> the singlet state versus the triplet state, which leads to the large MR. Overall, the experiments and analysis suggest significant opportunities to explore large MR effects in molecular tunnel junctions based on widely available molecules

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 (<i>R</i>) 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 ≤ <i>n</i> ≤ 4) connected in parallel display small relative statistical deviationsδ<i>R</i>/<i>R</i> ≈ 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. Using contact mechanics and direct measurements of the molecular surface coverage, the tip radius, tip-SAM adhesion force (<i>F</i>), and sample elastic modulus (<i>E</i>), we find that the tip-SAM contact area is approximately 25 nm², corresponding to about 80 molecules in the junction. Supplementing this information with <i>I–V</i> data and an analytic transport model, we are able to quantitatively describe the sources of deviations <i>δR</i> in <i>R</i>: namely, <i>δN</i> (deviations in the number of molecules in the junction), <i>δε</i> (deviations in energetic position of the dominant molecular orbital), and <i>δΓ</i> (deviations in molecule-electrode coupling). Our main results are (1) direct determination of <i>N</i>; (2) demonstration that <i>δN</i>/<i>N</i> for CP-AFM junctions is remarkably small (≤2%) and that the largest contributions to <i>δR</i> are <i>δε</i> and <i>δΓ</i>; (3) demonstration that δ<i>R</i>/<i>R</i> after only ∼200 measurements is substantially smaller than most reports based on >1000 measurements for single molecule break junctions. Overall, these results highlight the excellent reproducibility of junctions composed of tens of parallel molecules, which may be important for continued efforts to build robust molecular devices

Determination of the Electronic Energetics of CdTe Nanoparticle Assemblies on Au Electrodes by Photoemission, Electrochemical, and Photocurrent Studies

This work explores the electronic states of CdTe semiconductor nanoparticles (NPs) that are immobilized on a polycrystalline Au film through an organic linker (dithiol). The HOMO and LUMO energies of the CdTe NPs were determined by using photoelectron spectroscopy and cyclic voltammetry. The results from these measurements show that the HOMO energy is independent of the nanoparticle size and is pinned to the Fermi level, whereas the LUMO energy changes systematically with the size of the NP. Studies with different capping ligands imply that the dithiol ligand removes surface states and enhances the optoelectronic properties of the NPs