Elasticity of Cross-Linked Titania Nanocrystal Assemblies Probed by AFM-Bulge Tests.

In order to enable advanced technological applications of nanocrystal composites, e.g., as functional coatings and layers in flexible optics and electronics, it is necessary to understand and control their mechanical properties. The objective of this study was to show how the elasticity of such composites depends on the nanocrystals' dimensionality. To this end, thin films of titania nanodots (TNDs; diameter: ~3-7 nm), nanorods (TNRs; diameter: ~3.4 nm; length: ~29 nm), and nanoplates (TNPs; thickness: ~6 nm; edge length: ~34 nm) were assembled via layer-by-layer spin-coating. 1,12-dodecanedioic acid (12DAC) was added to cross-link the nanocrystals and to enable regular film deposition. The optical attenuation coefficients of the films were determined by ultraviolet/visible (UV/vis) absorbance measurements, revealing much lower values than those known for titania films prepared via chemical vapor deposition (CVD). Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) images showed a homogeneous coverage of the substrates on the µm-scale but a highly disordered arrangement of nanocrystals on the nm-scale. X-ray photoelectron spectroscopy (XPS) analyses confirmed the presence of the 12DAC cross-linker after film fabrication. After transferring the films onto silicon substrates featuring circular apertures (diameter: 32-111 µm), freestanding membranes (thickness: 20-42 nm) were obtained and subjected to atomic force microscopy bulge tests (AFM-bulge tests). These measurements revealed increasing elastic moduli with increasing dimensionality of the nanocrystals, i.e., 2.57 ± 0.18 GPa for the TND films, 5.22 ± 0.39 GPa for the TNR films, and 7.21 ± 1.04 GPa for the TNP films.

Cross-Linked Gold-Nanoparticle Membrane Resonators as Microelectromechanical Vapor Sensors.

We report a novel approach for the detection of volatile compounds employing electrostatically driven drumhead resonators as sensing elements. The resonators are based on freestanding membranes of alkanedithiol cross-linked gold nanoparticles (GNPs), which are able to sorb analytes from the gas phase. Under reduced pressure, the fundamental resonance frequency of a resonator is continuously monitored while the device is exposed to varying partial pressures of toluene, 4-methylpentan-2-one, 1-propanol, and water. The measurements reveal a strong, reversible frequency shift of up to ∼10 kHz, i.e., ∼5% of the fundamental resonance frequency, when exposing the sensor to toluene vapor with a partial pressure of ∼20 Pa. As this strong shift cannot be explained exclusively by the mass uptake in the membrane, our results suggest a significant impact of analyte sorption on the pre-stress of the freestanding GNP membrane. Thus, our findings point to the possibility of designing highly sensitive resonators, which utilize sorption induced changes in the membrane's pre-stress as primary transduction mechanism.

Electrostatically driven drumhead resonators based on freestanding membranes of cross-linked gold nanoparticles.

Freestanding, nanometer-thin membranes of alkanedithiol cross-linked gold nanoparticles represent elastic, mechanically robust and electrically conductive materials, which are interesting for the fabrication of novel nano- and microelectromechanical devices. In this work we present the first electrostatically driven drumhead resonators based on such nanoparticle membranes. These circular membranes have a thickness of 33 to 52 nm, a diameter of either 50 µm or 100 µm, and are equally spaced from their back electrode by ∼10 µm. Using an interferometric nanovibration analyzer various vibrational modes with resonance amplitudes of up to several 100 nm could be detected when the membranes are excited by applying AC voltages (<30 V) with drive frequencies of up to 2 MHz. Further, spatial amplitude distributions of different vibrational modes could be imaged. The devices showed fundamental resonance frequencies in the high kHz range and quality factors Q up to ∼2000. Finally, vibrational spectra and observed mode patterns could be well interpreted using the theory for a clamped circular membrane with negligible bending stiffness. Our findings mark an important step towards the integration of freestanding gold nanoparticle composite membranes into electromechanical devices with various applications, such as novel types of pressure or mass sensors.

Freestanding membranes of cross-linked gold nanoparticles: novel functional materials for electrostatic actuators.

Their tunable electrical, optical, and mechanical properties make freestanding membranes of organically cross-linked gold nanoparticles (GNPs) interesting materials for applications in micro- and nanoelectromechanical systems. Here, we demonstrate the application of α,ω-alkanedithiol-cross-linked GNP membranes as electrostatically driven actuators. The devices were fabricated by depositing these membranes (thickness 29-45 nm) onto cylindrical cavities (diameter ∼200 µm; depth ∼8-15 µm), which were lithographically patterned in a SU-8 resist. Applying voltages of up to ±40 V across the membrane and the silicon substrate deflected the membranes by several hundreds of nanometers, as measured by atomic force microscopy, confocal microscopy, and interferometry. A simple electrostatic model, which takes into account the membranes' mechanical properties, was used to interpret the experimental data.