Self-Assembled Nanoparticle Drumhead Resonators

The self-assembly of nanoscale structures from functional nanoparticles has provided a powerful path to developing devices with emergent properties from the bottomup. Here we demonstrate that freestanding sheets selfassembled from various nanoparticles form versatile nanomechanical resonators in the megahertz frequency range. Using spatially resolved laser-interferometry to measure thermal vibrational spectra and image vibration modes, we show that their dynamic behavior is in excellent agreement with linear elastic response for prestressed drumheads of negligible bending stiffness. Fabricated in a simple one-step drying-mediated process, these resonators are highly robust and their inorganic−organic hybrid nature offers an extremely low mass, low stiffness, and the potential to couple the intrinsic functionality of the nanoparticle building blocks to nanomechanical motion

Self-Assembled Nanoparticle Drumhead Resonators

The self-assembly of nanoscale structures from functional nanoparticles has provided a powerful path to developing devices with emergent properties from the bottom-up. Here we demonstrate that freestanding sheets self-assembled from various nanoparticles form versatile nanomechanical resonators in the megahertz frequency range. Using spatially resolved laser-interferometry to measure thermal vibrational spectra and image vibration modes, we show that their dynamic behavior is in excellent agreement with linear elastic response for prestressed drumheads of negligible bending stiffness. Fabricated in a simple one-step drying-mediated process, these resonators are highly robust and their inorganic–organic hybrid nature offers an extremely low mass, low stiffness, and the potential to couple the intrinsic functionality of the nanoparticle building blocks to nanomechanical motion

Frustrated Etching during H/Si(111) Methoxylation Produces Fissured Fluorinated Surfaces, Whereas Direct Fluorination Preserves the Atomically Flat Morphology

Two solution-based strategies for the preparation of partially fluorinated Si(111) surfaces from H/Si(111) were investigated using a combination of scanning tunneling microscopy, X-ray photoemission spectroscopy, infrared spectroscopy, and kinetic Monte Carlo simulations. Direct fluorination of H/Si(111) with HF (aq) produced atomically flat surfaces with 11% fluorination. A two-step reaction that first methoxylated the surface by reaction in methanol and then converted the methoxy termination to F termination by reaction in HF (aq) produced atomically rough, fissured surfaces with 24% fluorination. The atomic-scale roughness was induced by the methoxylation reaction. Methanol was shown to react with H/Si(111) surfaces through two parallel mechanisms: an etching reaction and a methoxylation reaction. The methoxylation reaction locally inhibited or “frustrated” the etching reaction, leading to the development of a characteristic fissured morphology. The H and F atoms on the fluorinated surface were imaged with atomic resolution, and no evidence of the previously proposed nanopatterning mechanism was observed

Size-Dependent Energy Levels of InSb Quantum Dots Measured by Scanning Tunneling Spectroscopy

The electronic structure of single InSb quantum dots (QDs) with diameters between 3 and 7 nm was investigated using atomic force microscopy (AFM) and scanning tunneling spectroscopy (STS). In this size regime, InSb QDs show strong quantum confinement effects which lead to discrete energy levels on both valence and conduction band states. Decrease of the QD size increases the measured band gap and the spacing between energy levels. Multiplets of equally spaced resonance peaks are observed in the tunneling spectra. There, multiplets originate from degeneracy lifting induced by QD charging. The tunneling spectra of InSb QDs are qualitatively different from those observed in the STS of other III–V materials, for example, InAs QDs, with similar band gap energy. Theoretical calculations suggest the electron tunneling occurs through the states connected with <i>L</i>-valley of InSb QDs rather than through states of the Γ-valley. This observation calls for better understanding of the role of indirect valleys in strongly quantum-confined III–V nanomaterials

Low-Temperature Synthesis of a TiO₂/Si Heterojunction

The classical SiO₂/Si interface, which is the basis of integrated circuit technology, is prepared by thermal oxidation followed by high temperature (>800 °C) annealing. Here we show that an interface synthesized between titanium dioxide (TiO₂) and hydrogen-terminated silicon (H:Si) is a highly efficient solar cell heterojunction that can be prepared under typical laboratory conditions from a simple organometallic precursor. A thin film of TiO₂ is grown on the surface of H:Si through a sequence of vapor deposition of titanium tetra­(<i>tert</i>-butoxide) (<b>1</b>) and heating to 100 °C. The TiO₂ film serves as a hole-blocking layer in a TiO₂/Si heterojunction solar cell. Further heating to 250 °C and then treating with a dilute solution of <b>1</b> yields a hole surface recombination velocity of 16 cm/s, which is comparable to the best values reported for the classical SiO₂/Si interface. The outstanding performance of this heterojunction is attributed to Si–O–Ti bonding at the TiO₂/Si interface, which was probed by angle-resolved X-ray photoelectron spectroscopy. Attenuated total reflectance Fourier transform infrared spectroscopy (ATR-FTIR) showed that Si–H bonds remain even after annealing at 250 °C. The ease and scalability of the synthetic route employed and the quality of the interface it provides suggest that this surface chemistry has the potential to enable fundamentally new, efficient silicon solar cell devices