Electro-mechanical sensing in freestanding monolayered gold nanoparticle membranes

Electron Transport in Discontinuous Metal Thin Films

The structure and basic experimental electrical properties of vacuum evaporated discontinuous (island) metal thin films of discrete metal nanoparticles on insulating substrates are briefly reviewed. Then the widely accepted Neugebauer and Webb (N&W) electrostatically activated electron tunneling conduction model is covered (with enhancements) before the numerous discrepancies between this model and experimental observations are identified, e.g. minimal substrate bias effect, non-linear field distribution, anomalous AC effects, asymmetrical contact effects, and switching. A modified model, based on contact electron injection and extraction, and computer simulations are introduced which explain these discrepancies at a qualitative level. However, quantitative experimental verification of the model is not possible without stable, reproducible films of known structures. The paper concludes with a review of possible preparation techniques which could yield satisfactory samples, especially self-assembly of organically protected metal nanoparticles. One of these has already demonstrated electrostatically activated conduction.

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.

Recent Progress of Layer-by-layer Assembly, Free-Standing Film and Hydrogel Based on Polyelectrolytes

Nowadays, polyelectrolytes play an essential role in the development of new materials. Their use allows creating new properties of materials and surfaces and vary them in a wide range. Basically, modern methods are divided into three areas-the process of layer-by-layer deposition, free-standing films, and hydrogels based on polyelectrolytes. Layer-by-layer assembly of polyelectrolytes on various surfaces is a powerful technique. It allows giving surfaces new properties, for example, protect them from corrosion. Free-standing films are essential tools for the design of membranes and sensors. Hydrogels based on polyelectrolytes have recently shown their applicability in electrical and materials science. The creation of new materials and components with controlled properties can be achieved using polyelectrolytes. This review focuses on new technologies that have been developed with polyelectrolytes over the last five years.

Towards wireless highly sensitive capacitive strain sensors based on gold colloidal nanoparticles

We designed, produced and characterized new capacitive strain sensors based on colloidal gold nanoparticles. The active area of these sensors, made up of a 1 mm2 close-packed assembly of gold nanoparticles between interdigitated electrodes, was designed to achieve measurable capacitance (>∼1 pF) and overcome parasitic capacitances. Electro-mechanical experiments revealed that the sensitivity of such capacitive sensors increases in relation to the size of the nanoparticles. In the case of 14 nm gold NPs, such sensors present a relative capacitance variation of -5.2% for a strain of 1.5%, which is more than 5 times higher than that observed for conventional capacitive strain gauges. The existence of two domains (pure capacitive domain and mixed capacitive-resistance domain) as a function of the frequency measurement allows for the adaptation of sensitivity of these capacitive sensors. A simple low-cost circuit based on a microcontroller board was finally developed to detect the capacitance variations of such NP based strain sensors. This low-cost equipment paves the way for the development of an entirely wireless application set-up.

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.

Robust Gold Nanoparticle Sheets by Ligand Cross-Linking at the Air-Water Interface

We report the results of cross-linking of two-dimensional gold nanoparticle (Au-NP) assemblies at the air-water interface in situ. We introduce an aqueous soluble ruthenium benzylidene catalyst into the water subphase to generate a robust, elastic two-dimensional network of nanoparticles containing cyclic olefins in their ligand framework. The most striking feature of the cross-linked Au-NP assemblies is that the extended connectivity of the nanoparticles enables the film to preserve much of its integrity under compression and expansion, features that are absent in its non-cross-linked counterparts. The cross-linking process appears to "stitch" the nanoparticle crystalline domains together, allowing the cross-linked monolayers to behave like a piece of fabric under lateral compression.

Plasmonic photo-current in freestanding monolayered gold nanoparticle membranes

We report on photo-current generation in freestanding monolayered gold nanoparticle membranes excited by using a focused laser beam. The absence of a substrate leads to a 50% increase of the photo-current at the surface plasmon resonance. This current is attributed to a combination of trap state dynamics and bolometric effects in a nanocomposite medium yielding a temperature rise of 40 K.