The rate of mechanical energy dissipation in 300-nm-thick, megahertz-range micromechanical silicon resonators is sensitive to single monolayer changes in surface chemistry. Resonators terminated with a single monolayer of methyl groups have significantly higher quality factors (Q's), and thus lower mechanical energy dissipation, than comparable resonators terminated with either long-chain alkyl monolayers (C2H2n+1, n = 2-18) or monolayers of hydrogen atoms. This effect cannot be attributed to mechanical energy dissipation within the alkyl monolayer, as a 9-fold increase in alkyl chain length does not lead to an observable increase in dissipation. Similarly, this effect is not correlated with the chemical structure of the silicon-monolayer interface (e.g., the density of Si-H vs Si-C bonds.) Instead, the chemical trends in resonator quality and stability are consistent with a dissipation mechanism involving the coupling of long-range strain fields to localized, electronically active defects in the monolayer coatings.

The quality factor and long-term stability of megahertz-range micromechanical silicon resonators can be significantly improved by a methyl monolayer directly bonded to the silicon surface. Mechanical energy dissipation in functionalized resonators is shown to be a sensitive function of surface chemistry. At least 18% and 41% of the dissipation in H-terminated and long-chain alkyl-terminated resonators, respectively, is surface related. Surface-induced dissipation is poorly correlated with the mechanical properties of the terminating layer, but may be related to the surface defect density.

Micro-fabricated channel with ultra-thin yet ultra-strong windows enables electron microscopy under 4-bar pressure Applied Physics Letters 100, 081903 (2012);

The rate of energy dissipation in megahertz-range micromechanical silicon resonators is unaffected by the controlled oxidation of one-half monolayer of surface sites, thereby proving that silicon oxidation does not inherently decrease the quality factor (Q). Homogeneously mono-oxidized surfaces were prepared by the controlled reaction of dodecyl aldehyde with H-terminated silicon surfaces to form dodecoxy-terminated surfaces (C 12 H 25 O-Si). The existence of an approximately half monolayer of Si-O-R species (the maximum allowed by steric constraints) was confirmed by infrared spectroscopy. As a control, dodecyl-terminated surfaces (C 12 H 25 -Si) were prepared by the reaction of 1-dodecene with H-terminated surfaces. Infrared spectroscopy showed that the density and ordering of the alkyl chains on the dodecoxy-and dodecyl-terminated surfaces were nearly identical, and no subsurface oxidation was detected on either surface. Dodecoxy-and dodecylterminated resonators displayed similar quality factors both immediately after functionalization and after extended exposure to vacuum and H 2 O-saturated air. Although the dodecoxy-terminated resonators appeared to adsorb more mass (as evidenced by frequency shifts), the increased adsorption did not have a deleterious effect on resonator quality. The relatively high qualities of mono-oxidized resonators stand in stark contrast to the low qualities displayed by resonators terminated with native or chemical oxides. This result suggests that oxide defect species may play a role in mechanical energy dissipation at surfaces.

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