Reflectance anisotropy spectroscopy is applied to submonolayer growth of In on the vicinal silicon (111) surface. Deposition in the region of 1 monolayer onto a clean stepped Si(111) surface at elevated temperature produces a single-domain In-induced (4×1) superstructure consisting of quasi-one-dimensional chains aligned parallel to the vicinal surface step edges. A significant optical anisotropy (1.65%), uncharacteristic of semiconductor systems, develops in the region of 1.9 eV which saturates upon completion of the (4×1) superstructure. We relate this feature to an optical transition involving a flat, highly populated filled surface state observed previously. We argue that the intensity and direction of this peak are indicative of electronic confinement within this system perpendicular to the In-induced chain length.

The high sensitivity of silicon microcantilever sensors has expanded their use in areas ranging from gas sensing to bio-medical applications. Photochromic molecules also represent promising candidates for a large variety of sensing applications. In this work, the operating principles of these two sensing methods are combined in order to detect the reversible conformational change of a molecular switch, spiropyran. Thus, arrays of silicon microcantilever sensors were functionalized with spiropyran on the gold covered side and used as test microcantilevers. The microcantilever deflection response was observed, in five sequential cycles, as the transition from the spiropyran (SP) (CLOSED) to the merocyanine (MC) (OPEN) state and vice-versa when induced by UV and white light LED sources, respectively, proving the reversibility capabilities of this type of sensor. The microcantilever deflection direction was observed to be in one direction when changing to the MC state and in the opposite direction when changing back to the SP state. A tensile stress was induced in the microcantilever when the SP to MC transition took place, while a compressive stress was observed for the reverse transition. These different type of stresses are believed to be related to the spatial conformational changes induced in the photochromic molecule upon photo-isomerisation.Sensors 2020, 20, 854 2 of 12 areas ranging from gas, humidity and thermal sensing to novel applications in microbiology, genomics and cancer detection [9][10][11][12][13][14][15]. Innovative microcantilever coatings include polymer brushes-based on phenylboronic acid which have been used to detect glucose binding events [15] and graphene oxide (GO) thin films for high-sensitivity humidity sensing [16]. Other examples involve the use of microcantilever sensors for detection of various diseases through, for example, antibody-antigen interactions for the detection of disease-related C-reactive proteins or for the screening of heart-related diseases by using cardiomyocytes functionalized microcantilevers [17][18][19]. Different fabrication methods for microcantilevers as well as the range of available modification methods in the substrate and sensing layer allow for the realization of different types of cantilever-based sensors [20,21]. An improvement in microcantilevers sensor efficiency of the n-type over p-type silicon cantilevers was demonstrated, and this effect was explained by their greater piezoresistive coefficient [20]. Examples include micromachined silicon cantilever paddle sensors which can be used in high flow rate gas sensing [20,21], and resonant cantilevers for pressure sensing [22].Spiropyrans (SP) are one of the most popular families of photochromic molecules [23,24]. Upon irradiation with UV light, the orthogonal SP isomer converts to the planar merocyanine (MC) form due to the photo-cleavage of the C spiro -O bond. The MC isomer shows a strong absorption band in the visible region due to its conjugation. When the MC is exposed to visible light, the structu...

A vicinal W(110) substrate, offcut by 1.4° in the [1 10] direction, has been used as a template to grow Fe nanostructures under UHV conditions. For sub-monolayer coverages, deposition of Fe on the substrate at room temperature, followed by annealing, forms continuous Fe stripes aligned with the step edges, which run along [001]. In situ reflection anisotropy spectroscopy (RAS) between 1.5 eV and 5.0 eV shows the damping of the W(110) surface state with increased Fe coverage. Broad and featureless step and Fe nanostructure contributions are identified in the 4 eV to 5 eV spectral region, but no strong signature of the aligned nanostructures or steps is found, in contrast to recent RAS studies of vicinal Cu(111). Capping by Ag for ex situ characterization was investigated by using an eclipsing mask to produce a checkerboard pattern of 0 to 3 ML Fe coverage in 0.25 ML steps, followed by deposition of 18 ML of Ag at room temperature. RAS of the capped structure is dominated by the response of the Ag interfaces. The W(110)/Ag interface shows a distinctive feature at 3.4 eV, which diminishes rapidly with increasing Fe coverage and may be related to the sharp peak observed at 3.9 eV on clean Ag(110) surfaces. The absence of other significant RAS features in this spectral region that can be clearly associated with the aligned Fe nanostructures indicates that characterization of capped structures by other epioptic techniques, such as optical second-harmonic generation, needs to be explored.

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