Tuning the Effective Work Function of Gold and Silver Using ω-Functionalized Alkanethiols: Varying Surface Composition through Dilution and Choice of Terminal Groups

Abstract

Thiol-based self-assembled monolayers (SAMs) have been used to tune the effective work function of gold over a range of ca. 1.8 eV via two strategies: (i) the use of ω-functionalized alkanethiols where the tail groups have widely varying electronegativity or (ii) by the creation of two-component SAMs from selected mixtures of methyl-terminated alkanethiols (C16) and alkanethiols fluorinated at the two terminal carbon atoms (C16F2). UV-photoelectron spectroscopy (UPS) was used to monitor changes in effective work function, using shifts in the low kinetic energy edge of these photoemission spectra to quantify the shift in local vacuum level resulting from the interface dipole effect created by the surface modifier. Tail groups on alkanethiol chains varied from −CH3, to −phenyl, −Cl, −Br, and −CF3 or −CF2CF3, which provided a shift in local vacuum level that varied linearly with the calculated molecular dipole moment of the individual modifiers, as observed previously for a more limited range of alkanethiols (J. Phys. Chem. B 2003, 107, 11690). The studies presented here confirm that the intrinsic dipole in the gold−thiolate bond is small (less than 100 meV), whereas the silver−thiolate bond appears to have a strongly polar character, in the direction Ag+−S− (ca. 900 meV). The use of a simple point dipole model to rationalize these apparent shifts in vacuum level was further explored using SAMs derived from various mixtures of C16 and C16F2. The low kinetic energy edge in the UV-photoemission spectra and the effective work function are observed to increase monotonically in energy with increasing C16F2 coverage, confirming that little surface segregation occurs in these self-assembled monolayers over a wide concentration range.

Fluorinated alkyne-derived monolayers on oxide-free silicon nanowires via one-step hydrosilylation

“…In addition, this peak shifts ~ 3 cm -1 towards lower wavenumber compared to corresponding flat F17-coated Si surfaces. [33] This suggests less short-range ordering of the alkyne chains, which can be attributed to the curvature of the Si NWs surface [47,48] (more details in Figure S6). Figure S7), confirming the proper monolayer formation.…”

“…In this regard, the use of fluorinated alkynes would provide an easily accessible additional tool, namely the stepwise variation of the work function of the underlying Si NWs surface. [32][33][34] In this study, we investigated the formation of five different 1-hexadecyne-derived monolayers with a varying number of fluorine atoms (HCC-(CH 2 ) 6 C 8 H 17-x F x ; x = 0, 1, 3, 9, 17) onto oxidefree hydrogen-terminated Si NWs by a one-step hydrosilylation using 1-alkynes F0 to F17 (Scheme 1). These monolayers were characterized in detail by X-ray photoelectron spectroscopy (XPS), infrared spectroscopy (IR), static contact angle measurements, scanning electron microscopy (SEM) and transmission electron microscopy (TEM …”

Short chain molecular junctions: Charge transport versus dipole moment

“…The presence of molecular dipole layer on a surface imparts a significant electrostatic effect due to high static electric field associated with dipole layer, in case of silicon the presence of organic molecules change the surface potential by more than 1 eV [23]. Recently, we [15,16] and others [24] are actively engaged in the investigating the effect associated with the orientation and magnitude of molecular dipole moment on molecular junctions. It is observed that the charge transport curves of molecular junctions can be systematically controlled by adjusting these parameters.…”

Tuning the Work Functions of 2D Silver Nanoparticle Sheets Using Local Oxidation Nanolithography

“…[ 17 ] The 1-dodecanethiol SAM utilized in this study is expected to decrease the effective work function of the gold electrode, typically by 1.0 eV in vacuum, due to the positive surface dipoles provided by both the thiol headgroups and the alkyl chains. [ 18 ] If the 1-dodecanethiol SAM on the silver substrate is considered to be identical to that on the gold substrate, the work functions on the SAM-modifi ed gold and silver substrates should be 4.31 eV and 3.74 eV, respectively. [ 19 , 20 ] The CPD values in Table 1 can be converted to work functions by taking the work function of the Au+SAM substrate to be 4.31 eV.…”