Advanced scanning probe lithography

The nanoscale control afforded by scanning probe microscopes has prompted the development of a wide variety of scanning-probe-based patterning methods. Some of these methods have demonstrated a high degree of robustness and patterning capabilities that are unmatched by other lithographic techniques. However, the limited throughput of scanning probe lithography has prevented its exploitation in technological applications. Here, we review the fundamentals of scanning probe lithography and its use in materials science and nanotechnology. We focus on robust methods, such as those based on thermal effects, chemical reactions and voltage-induced processes, that demonstrate a potential for applications.

Electroless patterned assembly of metal nanoparticles on hydrogen-terminated silicon surfaces for applications in photoelectrocatalysis

The deposition of gold and platinum nanoparticles (NPs) on hydrogen-terminated Si(100) (Si(100)-H) surfaces has been performed by galvanic displacement using fluoride-free sub-millimolar metallic salt solutions. The scanning electron microscopy (SEM) images showed the formation of oblate hemispherical NPs, with an average diameter of ca. 40 nm and an average height of 20 ± 10 and 10 ± 5 nm for Au and Pt, respectively. Furthermore, the calculated number density was (6.0 ± 0.8) × 10(9) Au NPs cm(-2) and (6.6 ± 1.3) × 10(9) Pt NPs cm(-2) with a larger size distribution measured for Au NPs. The Au 4f and Pt 4f X-ray photoelectron spectra of the metallized surfaces were characterized by a principal component corresponding to either the metallic gold or platinum. However, two other components located at higher binding energies were also visible and ascribed to gold or platinum silicides. Using this fluoride-free deposition process and a "reagentless" UV photolithography technique, we have also demonstrated that it was possible to prepare metallic NP micropatterns. Following this approach, single metal (Au) and two metals (Au and Pt) patterns have been produced and characterized by energy-dispersive X-ray spectroscopy (EDS) which revealed the presence of the expected metal(s). Such metallic NP micropatterned surfaces were used as photocathodes for H(2) evolution from water as a proof-of-concept experiment. These electrodes exhibited much higher electrocatalytic performance than that of nonmetallized Si(100)-H, both in the absence of light and under illumination. The overpotential for hydrogen evolution was significantly decreased by ca. 450 mV with respect to Si(100)-H (measured for a current density of 0.1 mA cm(-2)) under identical illumination conditions.

Selective surface modification of lithographic silicon oxide nanostructures by organofunctional silanes

This study investigates the controlled chemical functionalization of silicon oxide nanostructures prepared by AFM-anodization lithography of alkyl-terminated silicon. Different conditions for the growth of covalently bound mono-, multi- or submonolayers of distinctively functional silane molecules on nanostructures have been identified by AFM-height investigations. Routes for the preparation of methyl- or amino-terminated structures or silicon surfaces are presented and discussed. The formation of silane monolayers on nanoscopic silicon oxide nanostructures was found to be much more sensitive towards ambient humidity than, e.g., the silanization of larger OH-terminated silica surfaces. Amino-functionalized nanostructures have been successfully modified by the covalent binding of functional fluorescein dye molecules. Upon excitation, the dye-functionalized structures show only weak fluorescence, which may be an indication of a relatively low surface coverage of the dye molecules on length scale that is not accessible by standard AFM measurements.