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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 mm. 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.

Due to their tunneling-based charge transport mechanism, gold nanoparticle (GNP) composites attract significant interest for applications as resistive sensors for strain and volatile organic compounds. Here, we present a facile method for patterning and re-functionalization of alkanedithiol cross-linked GNP thin films based on deep ultraviolet photolithography. A lift-off process using a sacrificial layer of polymethyl methacrylate (PMMA) was used in combination with a rapid, spin-coating-based GNP film deposition procedure to fabricate patterned GNP films. Furthermore, PMMA mask layers were utilized for selective molecular doping of individual GNP film chemiresistors in a chemiresistor array with different ligand molecules, leading to altered chemical selectivities. The methods presented in this work are suitable for patterning organically cross-linked GNP films for various sensing applications as well as the fabrication of electronic noses based on GNP film chemiresistor arrays. Due to the photolithographic approach, the deposition and re-functionalization methods outlined in this paper are easily scalable, in contrast to serial techniques such as ink-jet printing or microdispensing.

Composite materials of organically stabilized or cross-linked metal nanoparticles represent a versatile material class with manifold potential applications. Numerous studies explored their tunable optical and charge transport properties. However, due to challenging experimental requirements, only a few studies addressed their mechanical properties. Here, we report the first investigation on the tunability of the elastic properties of cross-linked gold nanoparticle (GNP) composites. Thin films consisting of GNPs (diameter 3-4 nm) cross-linked with a,o-alkanedithiols of different chain length, as well as 1,4-benzenedithiol, were fabricated by spin-coating and transferred onto circular apertures with diameters of ∼100 mm. The mechanical properties of thus-prepared freestanding membranes with thicknesses between 21 and 51 nm were probed using bulge tests with atomic force microscopy (AFM) based deflection readout. We demonstrate that, along with their optical and charge transport characteristics, the elastic modulus of these GNP composites can be adjusted in a range from ∼3.6 to ∼10 GPa by shortening the a,o-alkanedithiol chain length from 10 to 3 methylene units. These variations in elasticity are attributed to the varying fraction of soft organic matter and to structural differences within the composites. Our results provide a basis for further experimental and theoretical studies, as well as for applications of cross-linked nanoparticle composites in future micro- and nanoelectromechanical (MEMS/NEMS) devices, their design, and modeling.

This study presents a method for printing flexible chemiresistors comprising thin film transducers based on cross-linked gold nanoparticles (GNPs). First, interdigitated silver paste electrodes are printed onto polyimide (PI) foil via dispenser printing. Second, coatings of GNPs and dithiol/monothiol blends are inkjet-printed onto these electrode structures. 1,9-Nonanedithiol (9DT) is used as cross-linking agent and a variety of monothiols are added to tune the sensors&rsquo, chemical selectivity. When dosing these sensors with different analyte vapors (n-octane, toluene, 4-methyl-2-pentanone, 1-butanol, 1-propanol, ethanol, water, concentration range: 25&ndash, 2000 ppm) they show fully reversible responses with short response and recovery times. The response isotherms follow a first-order Langmuir model, and their initial slopes reveal sensitivities of up to 4.5 &times, &nbsp, 10&minus, 5&nbsp, ppm&minus, 1. Finally, it is demonstrated that arrays of printed sensors can be used to clearly discern analytes of different polarity.

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.

A worldwide unique CMOS based chemical sensor device comprising an array of 8 microhotplates (µhps) for a total of 16 chemical sensors has been fabricated (Figure 1). [...]

We present novel microelectromechanical chemical sensors for the detection of volatile organic compounds based on electrostatically actuated, nanometer-thin, freestanding membranes of organically cross-linked gold nanoparticles. The sensors employ quasi-static deflection amplitudes or resonance frequency shifts of the membranes as highly sensitive sensing signals and are capable of detecting analytes at concentrations down to the low ppm range. We show that the devices can further be utilized for analyte discrimination. Additionally, we demonstrate that freestanding GNP membranes can be used for the fabrication of highly sensitive resistive pressure sensors.

In this study, we developed a novel and efficient process for the fabrication of resistive strain gauges for healthcare-related applications. First, 1,9-nonanedithiol cross-linked gold nanoparticle (GNP) films were prepared via layer-by-layer (LbL) spin-coating and subsequently transferred onto flexible polyimide foil by contact printing. Four-point bending tests revealed linear response characteristics with gauge factors of ∼14 for 4 nm GNPs and ∼26 for 7 nm GNPs. This dependency of strain sensitivity is attributed to the perturbation of charge carrier tunneling between neighboring GNPs, which becomes more efficient with increasing particle size. Fatigue tests revealed that the strain-resistance performance remained nearly the same after 10.000 strain/relaxation cycles. We demonstrate that these sensors are well suited to monitor muscle movements. Furthermore, we fabricated all-printed strain sensors by directly transferring cross-linked GNP films onto soft PDMS sheets equipped with interdigitated electrodes. Due to the low elastic modulus of poly(dimethylsiloxane) (PDMS), these sensors are easily deformed and, therefore, they respond sensitively to faint forces. When taped onto the skin above the radial artery, they enable the well-resolved and robust recording of pulse waves with diagnostically relevant details.

Nanoparticle composites are promising regarding their applications in novel electronic devices. These materials generally comprise nanoparticles embedded in matrices that consist e.g. of short molecules or polymers. Their optical, electronic, mechanical and sorption properties are manifold, widely tunable, and depend on the particle material as well as their matrix. We demonstrate the syntheses of different tailor-made nanoparticles and their integration into prototypical devices. On the one hand, composite materials of cross-linked gold nanoparticles (GNPs) are well-known for their chemiresistive properties. Here, we show a novel sensing method, employing these materials as micro-/nanoelectromechanical (MEMS/NEMS) chemical sensors. Nanometer-thin GNP membranes are fabricated and electrostatically actuated. The resulting static deflections and resonant vibrations are strongly influenced by the adsorption of volatile organic compounds, and can hence be employed as sensing signals. In combination with their chemiresistive properties such composites can thus act as multivariate sensing platforms. On the other hand, we present semiconductor nanoparticles synthesized following highly reproducible, high-throughput, continuous-flow syntheses. Due to the quantum-size effect, they offer absorption and emission features extending over the visible (CdSe/CdS) and infrared (PbS) range. Herein, we demonstrate the integration of such particles as emitters in QLEDs and highlight their potential as absorber materials in IR photodetectors.

We report the application of freestanding membranes of cross-linked gold nanoparticles (GNPs) as functional materials in novel MEMS/NEMS sensors. GNP composite films are fabricated using a facile ink-based process and transferred onto lithograpically fabricated 3D microstructures, suitable to support and electronically address the resulting freestanding membranes. We show that 1,6-hexanedithiol cross-linked GNP membranes can be utilized both, as diaphragm and strain-sensitive transducer in highly sensitive gauge pressure sensors. Aside from this application, we demonstrate electrostatic actuation of such composite membranes and the fabrication of electrostatically driven GNP membrane drumhead resonators with resonance frequencies in the high kilohertz to low mega-hertz range. Further, these GNP membrane resonators are successfully employed as electromechanical chemical sensors for the detection of vapors and gases, using sorption-induced shifts of their fundamental resonance frequency as sensing signal.

We report the preparation of freestanding membranes of cross-linked gold nanoparticles (GNPs) and demonstrate their application as electromechanical sensors for volatile organic compounds (VOCs). First, we show that the fundamental vibrational mode frequency of electrostatically excited GNP-membranes shifts significantly when exposing them to solvent vapors. We attribute this effect mainly to the reduction of the membranes’ pre-stress. Second, the relief in pre-stress upon analyte sorption can also be detected via quasi-static actuation of the membranes. In this case, the increase of the deflection amplitudes at constant bias voltages can be measured as the sensor signal and correlated to the analyte’s concentration. Additionally, we propose a facile route to the fabrication of such hybrid MEMS/NEMS sensors using layer-by-layer spin-coating and contact printing.

Conductance switching through chemical modification of a molecular bridge is a major goal in molecular electronics, with the potential to lead to molecule-based functional devices. In terms of switching speed, mechanisms that rely on only minor rearrangements of molecular structures are particularly promising. We demonstrate, based on density functional theory calculations combined with a coherent tunneling approach, how protonation and deprotonation of amine-substituted or amine-bridged model molecular wires can switch off and on π-sites and thus: a) remove or introduce interference features in the electron transmission, and b) decrease or increase coupling along a chain. This mechanism may also be relevant for interactions between molecular bridges and metal cations, for example, in sensor applications.

To enable applications of nanoparticle films in flexible electronics, actuators, and sensors, their mechanical properties are of critical concern. Here, we demonstrate that the elastic and viscoelastic properties of covalently cross-linked gold nanoparticles (GNPs) can be probed using AFM bulge tests. For this purpose 30–60 nm thick films consisting of 1,9-nonanedithiol (NDT) cross-linked GNPs (3.8 nm core diameter) were transferred onto substrates with ∼100 μm circular apertures. The resulting freestanding membranes were bulged by applying pressure differences of up to 10 kPa, and the deflection was measured by intermittent contact atomic force microscopy (AFM). Analyzing the pressure-deflection data using the spherical cap model, either by taking into account the peak deflection values or the measured arc profiles of the bulge, yielded 2.3 ± 0.3 and 2.7 ± 0.4 GPa for Young’s modulus, respectively. When cycling the stress–strain measurements at overpressures up to 2.4 kPa, hysteresis was observed and ass...

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 (

In this communication the application of gold nanoparticle membranes as ambient pressure sensors with electromechanical signal transduction is demonstrated. The devices were fabricated by sealing microstructured cavities with membranes of 1,6-hexanedithiol cross-linked gold nanoparticles, which were electrically contacted by metal electrodes deposited on both sides of the cavities. Variations of the external pressure resulted in a deflection of the membranes and, thus, increased the average interparticle distances. Therefore, the pressure change could easily be detected by simply monitoring the resistance of the membranes.

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.

A new, extremely efficient method for the fabrication of films comprised of gold nanoparticles (GNPs) crosslinked by organic dithiols is presented in this paper. The method is based on layer-by-layer spin-coating of both components, GNPs and crosslinker, and enables the deposition of films several tens of nanometers in thickness within a few minutes. X-ray diffraction and conductance measurements reveal the proper adjustment concentration of the crosslinker solution of the critical is in order to prevent the destabilization and coalescence of particles. UV/vis spectroscopy, atomic force microscopy, and conductivity measurements indicate that films prepared via layer-by-layer spin-coating are of comparable quality to coatings prepared via laborious layer-by-layer self-assembly using immersion baths. Because spin-coated films are not bound chemically to the substrate, they can be lifted-off by alkaline underetching and transferred onto 3d-electrodes to produce electrically addressable, freely suspended films. Comparative measurements of the sheet resistances indicate that the transfer process does not compromise the film quality.