6 research outputs found
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 (M<sub>1</sub>–S–(C<sub>6</sub>H<sub>4</sub>)<i><sub>n</sub></i>–S–M<sub>2</sub>, with 1 ≤ <i>n</i> ≤ 4 and M<sub>1,2</sub> = 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<sub>SAM</sub> – 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<sub>h</sub> = <i>E</i><sub>F</sub> – <i>E</i><sub>HOMO</sub> of the dominant molecular orbital (HOMO) relative to the Fermi energy <i>E</i><sub>F</sub> of the junction. Where photoelectron spectroscopy data are available, the ε<sub>h</sub> 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<sub>h</sub> 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<sub>h</sub> 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><sub>t</sub>), and ε<sub>h</sub> 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 −CH<sub>2</sub>– was replaced with O or S, that is, following the general
formula HSÂ(CH<sub>2</sub>CH<sub>2</sub>X)<sub><i>n</i></sub>CH<sub>2</sub>CH<sub>2</sub>SH, where X = CH<sub>2</sub>, 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 CH<sub>2</sub> < O < S. This
change in resistance is ascribed to the observed identical trend in
contact resistance, <i>R</i><sub>c</sub>, 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<sub>h</sub> (<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<sub>h</sub> 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><sub>c</sub> 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
Exceptionally Small Statistical Variations in the Transport Properties of Metal–Molecule–Metal Junctions Composed of 80 OligoÂphenylene 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 measurementsî—¸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 nm<sup>2</sup>, 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
Table S4 from Snake fungal disease: an emerging threat to wild snakes
Additional samples analyzed by fungal culture to assess the known host range and geographic distribution of Ophidiomyces. Location data is displayed only to the county level due to concerns with disclosing specific locations of rare or sensitive snake populations. The type of growth medium upon which the fungus culture was performed is listed in the last column (DTM = dermatophyte test medium; IMA = inhibitory mold agar; PFA = potato flake agar; SD = Sabouraud's dextrose agar
Table S2 from Snake fungal disease: an emerging threat to wild snakes
Fungal operational taxonomic units (OTUs) recovered from the skin of snakes. Each unique internal transcribed spacer (ITS) region DNA sequence variant (i.e., 100% identity) was assigned a numerical code and a presumptive taxon identification. The NWHC case number (see table S1) of the host(s) from which each variant was recovered is specified. A representative DNA sequence for each variant has been deposited in GenBank; bolded case numbers depict the snakes from which these deposited fungal DNA sequences originated. The assignment of each ITS variant to an OTU based on cut-offs of 99.5%, 99%, 98%, and 97% sequence identities is shown, with each OTU given an alpha-numeric cod
Table S1 from Snake fungal disease: an emerging threat to wild snakes
Samples used to determine the types of fungi associated with dermatitis in wild snakes. Location data are displayed only to the county (or sometimes state) level due to concerns with disclosing specific locations of rare or sensitive snake populations. Fungal infection was assessed by examining histologic sections of skin lesions. The number of gross lesions consistent with dermatitis were categorized as "none" (no gross skin lesions observed), "single" (one lesion), "multiple" (more than one discrete lesion), or "not assessed" (no information was available on the number of skin lesions present). The type of fungal growth medium upon which samples were cultured is listed as "DTM" (dermatophyte test medium) or "SD" (Sabouraud dextrose medium containing chloramphenicol and gentamycin). The number of unique internal transcribed spacer region DNA sequences identified per snake (or operational taxonomic units [OTUs] at 100% sequence identity) is listed. Samples originating from snakes cited in previous literature are specifie