Conductance of Alkanediisothiocyanates:  Effect of Headgroup−Electrode Contacts

Abstract

The realization of molecular electronics requires comprehension of single-molecule I-V characteristics. Aside from the electron-transport properties of the molecular framework, the molecule-electrode binding contributes significantly to the contact resistance, R n)0 , and thus to the values of single-molecule resistance. Isothiocyanate (-NCS), a versatile ligand for organometallics, can bind to a metal substrate to complete a metal-moleculemetal configuration for external measurements. Isothiocyanate has the advantage of being a π-conjugated moiety that presumably exhibits a relatively smaller impedance than the commonly used methylene thiol headgroup (-CH 2 SH) in many molecular wires. For example, this study shows that the single-molecule conductance of n-butanediisothiocyanate is an order of magnitude better than that of n-octanedithiol even though they both contain 10 atoms counted from sulfur to sulfur. For a homologous series of molecules, R n)0 can be extrapolated from the intercept of the resistance obtained by the repeated formation of molecular junctions using scanning tunneling microscopy. To isolate the contact effect of the -NCS-Au electrode from other factors, alkanediisothiocyanates were studied because the large HOMO-LUMO gap of alkyl chains is not sensitive to the number of methylene units. The results show two sets of R n)0 values, with the smaller set being 128 kΩ, about 1/12 the other value. A detailed examination of the results suggests that the preferential adsorption site for isothiocyanate on gold is the atop site rather than the 3-fold-hollow sites of thiol on gold.

Length‐Dependent Conductance of Molecular Wires and Contact Resistance in Metal–Molecule–Metal Junctions

“…[32] Recent work performed on alkanedithiol by using STM break junctions also demonstrated that the contact resistance is about 22 and 130 KW for the high-and low-conductance steps, respectively. [23] However, on a SiA C H T U N G T R E N N U N G (111) surface, the interfacial resistance of an alkyl monolayer increases to 84 AE 15 KW. [12] In spite of the big span of experimental results, the contact resistance predicted by the theoretical methods is smaller than all of them.…”

“…[26] We obtained the b values of (0.88 AE 0.10) and 1.48 À1 for alkyl monolayers on gold [11] and silicon, [12] respectively, by using C-AFM. Although these results depend on the particular exper- [28] electrochemical 0.85 [2] Ag-SAM//SAM-Hg junction 0.87 AE 0.10 [31] electrochemical 1.0 [3] metal-capped nanopores 0.79 AE 0.10 [81] electrochemical 0.91, 1.31 [5] DFT/NEGF 0.69, 0.66 [26] C-AFM 0.63 AE 0.10 [10] DFT/NEGF 0.74 [61] C-AFM 0.86 AE 0.10 [11] DFT/NEGF 0.97 [65] C-AFM 1.48 [12] ab initio % 0.5-0.9 [82] C-AFM 0.94 AE 0.06 [13] DFT 0.70 [83] C-AFM 1.13 [14] Polyphenyl C-AFM 1.10 [15] AC voltammetry 0.36 [4] C-AFM 0.88 [16] Ag-SAM/SAM-Hg junction 0.61 AE 0.10 [31] STM 0.41 AE 0.05 [7] C-AFM 0.35-0.50 [9] STM break junction 0.78 AE 0.10 [20] C-AFM 0.42 AE 0.07 [13] STM break junction 0.84 [21] DFT/Green function 0.16 (planar) 0.26 (twisted) [59] STM break junction 0.79, 0.77 [23] DFT/NEGF 0.40 (planar) [60] 0.51 (twisted) STM break junction 0.75, 0.73, 0.35 [26] Oligoacene SPM 1.37 AE 0.03 [80] C-AFM 0.51 [1] [a] Some attenuation-factor values were converted to À1 (from the unit "per methylene"). The unit length of the alkane chain is 1.28 .…”

Anchoring Molecular Chromophores to Colloidal Gold Nanocrystals: Surface-Enhanced Raman Evidence for Strong Electronic Coupling and Irreversible Structural Locking

“…10–12 Thiolated linkers with conjugated π-electrons are especially interesting because they could allow more efficient electron transfer and molecular orbital overlapping than saturated alkanethiols. 13 In addition, conjugated groups such as isothiocyanate (ITC) (-N=C=S, a reactive group for covalent conjugation of organic dyes with biomolecules) have enabled the stable anchoring of reporter molecules to atomically smooth gold surfaces for tip-enhanced Raman scattering (TERS) 14,15 and to colloidal gold nanoparticles for surface-enhanced Raman scattering (SERS) 16,17 . Recent work by Halas and coworkers 18 has further shown that the electronic conductance and surface-enhanced Raman spectra of conjugated thiol molecules can be measured simultaneously from nano-sized junctions.…”

Formation Mechanism of Metal–Molecule–Metal Junctions: Molecule-Assisted Migration on Metal Defects

“…101 The π-conjugated isothiocyanate end group also provides smaller impedance than methylene thiolate. 102 We use atomic scale calculations to study the formation of thiocyanate-linked citrateprotected gold nanoparticles (n-mer clusters). Following a similar protocol as described for the migration calculations above, three levels of atomic calculations are used to describe the nanoparticlemolecule-nanoparticle nanostructure, namely two types of electronic structure calculations followed by 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 ...…”