Organic Monolayers onto Oxide-Free Silicon with Improved Surface Coverage: Alkynes versus Alkenes

Abstract

On H-Si(111), monolayer assembly with 1-alkenes results in alkyl monolayers with a Si-C-C linkage to the silicon substrate, while 1-alkynes yield alkenyl monolayers with a Si-C=C linkage. To investigate the influence of the different linkage groups on the final monolayer structure, organic monolayers were prepared from 1-alkenes and 1-alkynes with chain lengths from C(12) to C(18), and the final monolayer structures were studied in detail by static water contact angles measurements, ellipsometry, attenuated total reflectance infrared (ATR-IR) spectroscopy, and X-ray photoelectron spectroscopy (XPS). The thicknesses, tilt angles, and packing densities of the alkyl monolayers are in good agreement with literature values, whereas increased thicknesses, reduced tilt angles, and improved packing densities were observed for the alkenyl monolayers. Finally, the surface coverages for alkyl monolayers were determined to be 50-55% (in line with literature values), while those for the alkenyl monolayers increased with the chain length from 55% for C(12) to as high as 65% for C(18)! The latter value is very close to the theoretical maximum of 69% obtainable on H-Si(111). Such enhanced monolayer quality and increased surface coverage of the alkenyl monolayers, in combination with the oxidation-inhibiting nature of the Si-C=C linkage, significantly increases the chance of successful implementation of organic monolayers on oxide-free silicon in molecular electronic and biosensor devices, especially in view of the importance of a defect-free monolayer structure and the corresponding stability of the monolayer-silicon interface.

Barrier height distribution and dipolar relaxation in metal-insulator-semiconductor junctions with molecular insulator: Ageing effects

“…10 We will show that, although the molecular coverage is not optimized, 71 dc electrical characteristics are fully consistent with devicequality junctions reported in previous work. We have chosen a device representative of the average of the conductance distribution in order to observe chemical and electrical changes brought by long term (18 months) ageing in ambient air.…”

“…[1][2][3][4][5][6] In the last two decades, important progress has been made in the controlled covalent grafting of molecular monolayers on semiconductors. [7][8][9][10] The organization of linear molecules in quasi-2D assemblies is the result of a delicate interplay between substrateadsorbate interactions and lateral intermolecular (electrostatic, van der Waals) interactions. 11,12 Strong covalent grafting produces robust assemblies, however irreversible binding to the semiconductor substrate may lead to substantial fluctuations in molecular packing density and formation of domains with variable average orientation.…”

Organic Monolayers by B(C6F5)3-Catalyzed Siloxanation of Oxidized Silicon Surfaces

“…This is attributed to the presence of Si-bound methyl groups. 54 The hydrosilane-modified silicon surfaces also showed a clear CH 3 stretching vibration at 2965 cm –1 .…”

Temperature dependence of current density and admittance in metal-insulator-semiconductor junctions with molecular insulator

“…Due to sterical constraints, the density of surface sites which are not grafted with C 12 H 25 molecules is larger than 4 Â 10 14 cm À2 (%50% of surface Si atom density) even in the case of a closely packed molecular layer 54 ; the low density of electrically active interface states, D S < 10 11 eV À1 cm À2 , found in this study thus requires that (1) the Si dangling bonds are fully passivated by the hydrogenation process and (2) that the Si-H bonding is not disrupted by the grafting process of 1-alkene molecules. However, ageing in air affects the midgap density which typically rises to the high 10 11 eV À1 .cm À2 range; future work might address the role of molecular coverage in the kinetics of this oxidation reaction.…”