Transfer Printing of Freestanding Nanoassemblies: A Route to Membrane Resonators with Adjustable Prestress.

Freestanding nanoassemblies represent a new class of functional materials with highly responsive optical, electrical, and mechanical properties. Hence, they are well-suited for applications in advanced sensor devices. Here, it is shown that transfer printing enables the well-controlled fabrication of freestanding membranes from layered nanoassemblies: Using a polydimethylsiloxane (PDMS) stamp, thin films (thickness: ∼45 to ∼51 nm) of 1,6-hexanedithiol cross-linked gold nanoparticles (diameter: ∼3.9 ± 0.8 nm) were transferred onto surface-oxidized silicon substrates featuring square microcavities with edge lengths of ∼78 µm. After adjusting the contact pressure to 1.8 bar, intact membranes were printed in yields of ∼70%. The prestress of printed membranes was determined by measuring their resonance frequencies under electrostatic actuation. In general, the prestress values were in the ∼10 MPa range with standard deviations below 10% for parallel printed resonators. The deviations in average prestress between resonators printed onto different substrates were 21% or less. By increasing the temperature during the final transfer step from 5 to 48 °C, it was possible to tune the average prestress from ∼14 to ∼28 MPa. This effect was attributed to the pronounced thermal expansion of the PDMS stamp. Finally, by transfer printing layered films of graphene oxide/silk fibroin (GO/SF), it is shown that the approach can be adapted for the fabrication of freestanding membranes from very different nanomaterials.

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.