Structure-Building Forces in Biphenyl-Substituted Alkanethiolate Self-Assembled Monolayers on GaAs(001): The Effect of the Bending Potential

Abstract

Molecular assembly on a technologically relevant GaAs substrate is an important and application-related issue. In this context, self-assembled monolayers (SAMs) formed from a series of ω-(4′-methylbiphenyl-4-yl)alkanethiols, CH 3 (C 6 H 4 ) 2 (CH 2 ) n SH (BPn, n = 1−6), were prepared on GaAs(001) and characterized in detail by highresolution X-ray photoelectron spectroscopy and near-edge X-ray absorption fine structure spectroscopy. The resulting films exhibited pronounced, "odd−even" variation in molecular orientation and packing density with the number (n) of methylene groups in the alkyl linker; viz., smaller molecular inclination, associated with a higher packing density, was observed for an odd n, while the opposite was the case for an even n. Such an odd−even behavior confirms once again the existence of a bending potential for GaAs(001), which is equivalent to the analogous potential for the Au(111) substrate, where similar odd−even behavior has been observed. This potential plays an important role in the balance of the structure-building interactions, predefining the orientation of the alkyl linker, transferred in an odd−even fashion, depending on the parity of n, to the adjacent biphenyl spacer. The above effects were found to be superimposed onto pronounced dependence of the film quality on the length of the BPn precursor. This occurred due to proneness of the GaAs substrate to oxidation, hindering an efficient self-assembly, as well as due to a limited ability of the shortchain monolayers to protect this sensitive substrate from the postpreparation oxide regrowth. A proper selection of the parameters is, thus, very important for the design of functional monomolecular films on GaAs. ■ INTRODUCTIONSelf-assembled monolayers (SAMs) of organic molecules on solid substrates attract significant attention due to their technological relevance and as important test systems for fundamental research. 1 The lateral density and structure of these films stem from a complex interplay of several factors, viz., intermolecular interaction, the anchor group−substrate interaction, corrugation of the binding energy hypersurface, and mismatch between the optimal molecular lattice and the structural template provided by the substrate. 2 At a given mismatch and the corrugation of the binding energy hypersurface, the intermolecular interaction is frequently believed to play a dominant role as compared to the anchor group− substrate interaction, as was in particular assumed for SAMs of nonsubstituted alkanethiolates (ATs) on coinage metal substrates. These films exhibit intermolecular lattice spacing of ∼5 and ∼4.67−4.77 Å on Au(111) and Ag(111), respectively, accompanied by a molecular tilt of ∼27−35°on Au(111) and ∼10−12°on Ag(111). 1−9 The tilt can then be considered as the only means to achieve the optimal interchain spacing (∼4.4 Å), characteristic of the respective bulk materials, whereas the substrate−S−C moiety is assumed to behave as a "free joint", adapting to an optimal geometrical configuration and enabling the nece...

Charge transport at a molecular GaAs nanoscale junction

“…Furthermore, extensive literature shows that high-quality self-assembled monolayers (SAMs) can be prepared at a GaAs surface, [9][10][11][12] mainly through formation of As-S bonds. 13 The organic monolayer provides strong and efficient molecule-semiconductor electrical contacts and passivates the surface against oxidation to Ga 2 O 3 and As 2 O 3 , with samples stable for days with minimal oxidation as inferred from XPS spectra. 14 Multi-molecule, large area (>1 mm) metal-molecules-GaAs devices have been extensively studied in the literature, [15][16][17] using the determination of I-V characteristics to study the charge transport mechanism and overall device behaviour.…”

Microstructuring GaAs Using Reverse-Patterning Lithography: Implications for Transistors and Solar Cells

“…To confirm the presence of the monolayer, the GaAs substrates were subjected to elemental analysis using X-ray photoelectron spectroscopy (XPS). Figure shows the XPS spectra of the Ga 3d, As 3d, C 1s, F 1s, S 2p, and O 1s binding regions of the bare GaAs and PFPDT-printed GaAs after 30 min of ambient exposure, and Table lists the assigned peaks along with their binding energies; peak assignments were based on examples found in the literature. − The Ga 3d binding energy region of both substrates exhibits a peak at 19.3 eV (Figure A). Because of a spin–orbit splitting of only 0.43 eV, the Ga 3d 3/2 and Ga 3d 5/2 peaks are indistinct and appear as a single peak.…”