Gold nanoparticles ranging in diameter from 1 to 8 nanometers were prepared on top of silicon wafers in order to study the size dependence of their oxidation behavior when exposed to atomic oxygen. X-ray photoelectron spectroscopy showed a maximum oxidation resistance for "magic-number" clusters containing 55 gold atoms. This inertness is not related to electron confinement leading to a size-induced metal-to-insulator transition, but rather seems to be linked to the closed-shell structure of such magic clusters. The result additionally suggests that gold-55 clusters may act as especially effective oxidation catalysts, such as for oxidizing carbon monoxide.

KEYWORDSNitrogen-vacancy center, diamond, spin-mechanical interaction, nanomechancial sensing, NEMS 2 ABSTRACT Nanomechanical sensors and quantum nanosensors are two rapidly developing technologies that have diverse interdisciplinary applications in biological and chemical analysis and microscopy.For example, nanomechanical sensors based upon nanoelectromechanical systems (NEMS) have demonstrated chip-scale mass spectrometry capable of detecting single macromolecules, such as proteins. Quantum nanosensors based upon electron spins of negatively-charged nitrogen-vacancy (NV) centers in diamond have demonstrated diverse modes of nanometrology, including single molecule magnetic resonance spectroscopy. Here, we report the first step towards combining these two complementary technologies in the form of diamond nanomechanical structures containing NV centers. We establish the principles for nanomechanical sensing using such nano-spinmechanical sensors (NSMS) and assess their potential for mass spectrometry and force microscopy. We predict that NSMS are able to provide unprecedented AC force images of cellular biomechanics and to, not only detect the mass of a single macromolecule, but also image its distribution. When combined with the other nanometrology modes of the NV center, NSMS potentially offer unparalleled analytical power at the nanoscale. TEXTNanomechanical sensors based upon nanoelectromechanical systems (NEMS) are a burgeoning nanotechnology with diverse microscopy and analytical applications in biology and chemistry. Two applications with particular promise are force microscopy in geometries that transcend the constraints of conventional atomic force microscopy (AFM) 1-3 and on-chip mass spectrometry with single molecule sensitivity. [4][5][6] Another burgeoning nanotechnology is quantum nanosensors based upon the electron spin of the NV center in diamond. The NV center has been 3 used to locate single elementary charges, 7 to perform thermometry within living cells 8 and to realize nanoscale MRI in ambient conditions. 9-11 However, there is yet to be a nanosensing application of the NV center that exploits its susceptibility to local mechanical stress/ strain.Here, we propose that the mechanical susceptibility of the NV center's electron spin can be exploited together with the extreme mechanical properties of diamond nanomechanical structures to realize nano-spin-mechanical sensors (NSMS) that outperform the best available technology. Such NSMS will be capable of both high-sensitivity nanomechanical sensing and the quantum nanosensing of electric fields, magnetic fields and temperature. NSMS will thereby constitute the unification of two burgeoning nanotechnologies and have the potential to perform such unparalleled analytical feats as mass-spectrometry and MRI of single molecules. As the first step towards realizing NSMS, we report the complete characterization of the spin-mechanical interaction of the NV center. We use this to establish the design and operating principles of diamond NSMS and to perfor...

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