Two switchable, palindromically constituted bistable [3]rotaxanes have been designed and synthesized with a pair of mechanically mobile rings encircling a single dumbbell. These designs are reminiscent of a "molecular muscle" for the purposes of amplifying and harnessing molecular mechanical motions. The location of the two cyclobis(paraquat-p-phenylene) (CBPQT 4+ ) rings can be controlled to be on either tetrathiafulvalene (TTF) or naphthalene (NP) stations, either chemically ( 1 H NMR spectroscopy) or electrochemically (cyclic voltammetry), such that switching of inter-ring distances from 4.2 to 1.4 nm mimics the contraction and extension of skeletal muscle, albeit on a shorter length scale. Fast scan-rate cyclic voltammetry at low temperatures reveals stepwise oxidations and movements of one-half of the [3]rotaxane and then of the other, a process that appears to be concerted at room temperature. The active form of the bistable [3]rotaxane bears disulfide tethers attached covalently to both of the CBPQT 4+ ring components for the purpose of its self-assembly onto a gold surface. An array of flexible microcantilever beams, each coated on one side with a monolayer of 6 billion of the active bistable [3]rotaxane molecules, undergoes controllable and reversible bending up and down when it is exposed to the synchronous addition of aqueous chemical oxidants and reductants. The beam bending is correlated with flexing of the surfacebound molecular muscles, whereas a monolayer of the dumbbell alone is inactive under the same conditions. This observation supports the hypothesis that the cumulative nanoscale movements within surface-bound "molecular muscles" can be harnessed to perform larger-scale mechanical work.

We determined the electromechanical properties of a suspended graphene layer by scanning tunneling microscopy (STM) and scanning tunneling spectroscopy (STS) measurements, as well as computational simulations of the graphene-membrane mechanics and morphology. A graphene membrane was continuously deformed by controlling the competing interactions with a STM probe tip and the electric field from a back-gate electrode. The probe tip-induced deformation created a localized strain field in the graphene lattice. STS measurements on the deformed suspended graphene display an electronic spectrum completely different from that of graphene supported by a substrate. The spectrum indicates the formation of a spatially confined quantum dot, in agreement with recent predictions of confinement by strain-induced pseudomagnetic fields.

Low-temperature scanning tunneling microscopy (STM) has been used to image CH 3 -terminated Si(111) surfaces that were prepared through a chlorination/alkylation procedure. The STM data revealed a wellordered structure commensurate with the atop sites of an unreconstructed 1 × 1 overlayer on the silicon (111) surface. Images collected at 4.7 K revealed bright spots, separated by 0.18 ( 0.01 nm, which are assigned to adjacent H atoms on the same methyl group. The C-H bonds in each methyl group were observed to be rotated by 7 ( 3°away from the center of an adjacent methyl group and toward an underlying Si atom. Hence, the predominant interaction that determines the surface structure arises from repulsions between hydrogen atoms on neighboring methyl groups, and secondary interactions unique to the surface are also evident.Hydrogen-terminated (111)-oriented Si surfaces are well documented to have a low number of structural and electrically active defect sites. 1,2 However, these surfaces degrade rapidly in air and in other oxidizing environments. 3,4 Consequently, several wet chemical methods have been developed for the functionalization of both crystalline and porous Si surfaces. [5][6][7][8][9][10] These chemical methods offer molecular-level control over the interfacial chemistry of Si surfaces, attracting attention for applications in molecular electronics, 11 sensing, 12-14 photoelectrochemistry, 4 chemical and electrical surface passivation, 8,15 porous Si photoluminescence, 9 and control of photopatterning. 6 Molecular modeling indicates that methyl groups are the only saturated hydrocarbon moiety that can terminate every Si atop site on the unreconstructed Si(111) surface. 8,[16][17] Such complete chemical termination is expected to offer the most robust passivation of surface defects and to provide the best resistance to oxidation of the resulting Si surfaces. Prior workers have hypothesized that functionalization with longer alkyl chains yields incomplete coverage of the Si(111) surface, 18 with the remainder of the sites being terminated by either -OH, -H, or other unidentified surface species. 16,19 In this work, we report low-temperature STM studies that have revealed the structure of the fully methyl-terminated Si(111) surface prepared by wet chemical methods.Silicon surfaces were functionalized using a two-step chlorination/alkylation procedure. 8 The samples were obtained from (111)-oriented, Sb-doped, 0.005-0.02 Ω cm resistivity, n-type Si wafers having a miscut error of (0.5°. The samples were cleaned and oxidized for 5 min at 80°C in a solution of 1:1:5 (vol) 30% H 2 O 2 /30% NH 3 /H 2 O and were then terminated with Si-H bonds by etching for 15 min in 40% NH 4 F(aq). This etching method has been demonstrated to produce large atomically flat terraces. 20 Chlorination was performed by exposing the samples to a solution of PCl 5 in chlorobenzene. 8 A small amount of benzoyl peroxide was added to initiate a radical reaction, and the samples were heated to 90-100°C for 45 min. The surfaces were remo...

In integrated photonics, specific wavelengths are preferred such as 1550 nm due to low-loss transmission and the availability of optical gain in this spectral region. For chip-based photodetectors, layered two-dimensional (2D) materials bear scientific and technologicallyrelevant properties such as electrostatic tunability and strong light-matter interactions. However, no efficient photodetector in the telecommunication C-band has been realized with 2D transition metal dichalcogenide (TMDCs) materials due to their large optical bandgap. Here, we demonstrate a MoTe2-based photodetector featuring strong photoresponse (responsivity = 0.5 A/W) operating at 1550 nm on silicon micro ring resonator enabled by strain engineering of the transition-metal-dichalcogenide film. We show that an induced tensile strain of ~4% reduces the bandgap of MoTe2, resulting in large photo-response in the telecommunication wavelength, in otherwise photo-inactive medium when unstrained. Unlike Graphene-based photodetectors relying on a gapless band structure, this semiconductor-2D material detector shows a ~100X improved dark current enabling an efficient noise-equivalent power of just 90 pW/Hz 0.5 . Such strain-engineered integrated photodetector provides new opportunities for integrated optoelectronic systems.

Characterization of subsurface morphology and mechanical properties with nanoscale resolution and depth control is of significant interest in soft matter fields like biology, polymer science, and even in future applications like nanomanufacturing, where buried structural and compositional features are important to the functionality of the system. However, controllably "feeling" the subsurface is a challenging task for which the available imaging tools are relatively limited. In this paper, we propose a trimodal atomic force microscopy (AFM) imaging scheme, whereby three eigenmodes of the microcantilever probe are used as separate control "knobs" to simultaneously measure the topography, modulate sample indentation by the tip during tip-sample impact, and map compositional contrast, respectively. We illustrate this multifrequency imaging approach through computational simulation and experiments conducted on ultrathin polymer films with embedded glass nanoparticles in ambient air. By actively increasing the tip-sample indentation using a higher eigenmode of the cantilever, we are able to gradually and controllably reveal glass nanoparticles which are buried tens of nanometers deep under the surface, while still being able to refocus on the surface.