Self-assembled nanoparticle drumhead resonators

Gold Nanodrum Resonators

Nanodrum resonators have been fabricated using nanometer-thick gold films as the drumheads. The fabrication method is favorable for large-area array manufacture of arbitrary shapes. The drum resonators exhibit fundamental mode vibration frequencies in the MHz regime. We use the stretched-plate model to describe the natural vibrations of the drum. The Q factor of the fundamental mode increases as the thickness of the drum increases and decreases as the drum diameter goes up. The highest Q factor of the fundamental mode reaches 290 at room temperature and atmospheric pressure. Based on the deduced material properties we estimate that the resonator has a mass sensitivity of 1.11 × 10-22 g/Hz.

Free-standing two-dimensional sheets of polymer-linked nanoparticles

Here, we report a simple and general approach to fabricate free-standing two-dimensional (2D) sheets of nanoparticles by the simultaneous self-assembly of hydrophobic nanoparticles and hydrophilic polymers at the liquid-liquid interface. The nanoparticle-polymer interaction at the interface generates well-defined 2D sheets of densely packed nanoparticles with a lateral dimension of tens of micrometers. The nanosheets transferred in water are stable over months without any additional cross-linking step. The method is applicable for a broad range of nanoparticles including oxide, semiconductor, and metal nanoparticles as well as functional polymers.

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.

Mutually Reinforced Polymer-Graphene Bilayer Membranes for Energy-Efficient Acoustic Transduction

Graphene holds promise for thin, ultralightweight, and high-performance nanoelectromechanical transducers. However, graphene-only devices are limited in size due to fatigue and fracture of suspended graphene membranes. Here, a lightweight, flexible, transparent, and conductive bilayer composite of polyetherimide and single-layer graphene is prepared and suspended on the centimeter scale with an unprecedentedly high aspect ratio of 10⁵ . The coupling of the two components leads to mutual reinforcement and creates an ultrastrong membrane that supports 30 000 times its own weight. Upon electromechanical actuation, the membrane pushes a massive amount of air and generates high-quality acoustic sound. The energy efficiency is ≈10-100 times better than state-of-the-art electrodynamic speakers. The bilayer membrane's combined properties of electrical conductivity, mechanical strength, optical transparency, thermal stability, and chemical resistance will promote applications in electronics, mechanics, and optics.

Thermal stability and dynamics of soft nanoparticle membranes: role of entropy, enthalpy and membrane compressibility

Nanoparticle based ultra-thin membranes have been shown to have remarkable mechanical properties while also possessing novel electrical, optical or magnetic properties, which could be controlled by tailoring properties at the level of individual nanoparticles. Since in most cases the ultra-thin membranes are coupled to some substrates, the role of membrane-substrate interactions, apart from nanoparticle-nanoparticle interactions become very crucial in understanding their mechanical and thermal stability, as well as their plethora of applications. However, systematic studies in this direction have been conspicuously absent. Here we report thermal stability and the corresponding microscopic dynamics of polymer supported ultra-thin membranes comprising of self-assembled, ordered grains of polymer grafted nanoparticles having tunable mechanical properties. The initially ordered membranes show distinct pathways for temperature induced disordering depending on membrane flexibility as well as on interfacial entropic and enthalpic interactions with the underlying polymer thin film. We also observe contrasting temperature dependence of microscopic dynamics of these membranes depending on whether the graft polymer-substrate polymer interactions are predominantly entropic or enthalpic in nature. Our results suggest that apart from their varied applications, the soft nanoparticle-polymer hybrid membranes are a playground for rich physics involving subtle entropic and enthalpic effects along with the nanoparticles softness, which eventually determine their thermo-mechanical stability.

Covalent-Cross-Linked Plasmene Nanosheets

Thiol-polystyrene (SH-PS)-capped plasmonic nanoparticles can be fabricated into free-standing, one-nanoparticle-thick superlattice sheets (termed plasmene) based on physical entanglement between ligands, which, however, suffer from irreversible dissociation in organic solvents. To address this issue, we introduce coumarin-based photo-cross-linkable moieties to the SH-PS ligands to stabilize gold nanoparticles. Once cross-linked, the obtained plasmene nanosheets consisting of chemically locked nanoparticles can well maintain structural integrity in organic solvents. Particularly, arising from ligand-swelling-induced enlargement of the interparticle spacing, these plasmene nanosheets show significant optical responses to various solvents in a specific as well as reversible manner, which may offer an excellent material for solvent sensing and dynamic plasmonic display.

Thermodynamic Model for Quantum Dot Assemblies Formed Because of Charge Transfer

Two initially neutral semiconductor quantum dots with appropriate band offsets can participate in a ground state charge transfer process. The charge transfer manifests itself in the form of bleaching of optical transitions and also causes the quantum dots to precipitate from solution, giving rise to assemblies with unusual properties. As this represents a postsynthetic modification of the electronic structure of quantum dots, it holds tremendous potential for improving the characteristics of quantum dot devices. Here, we study the dependencies of the properties of these assemblies on the structure of the participating quantum dots. In particular, we find that for assemblies formed out of Cu:CdS and ZnTe/CdS quantum dots, the composition of the assembly varies from 1:1.26 to 1:0.23 ZnTe/CdS to Cu:CdS as the shell thickness of CdS in ZnTe/CdS is increased. In contrast, the composition changes from 1:1.1 to 1:15 for PbSe/CdSe and Cu:CdS quantum dots, as the size of the PbSe core is increased. These observations are explained on the basis of a phenomenological thermodynamic model. The applicability of thermodynamics to this example of self-assembly is verified empirically.

Thermomechanical Response of Self-Assembled Nanoparticle Membranes

Monolayers composed of colloidal nanoparticles, with a thickness of less than 10 nm, have remarkable mechanical moduli and can suspend over micrometer-sized holes to form free-standing membranes. In this paper, we discuss experiments and coarse-grained molecular dynamics simulations characterizing the thermomechanical properties of these self-assembled nanoparticle membranes. These membranes remain strong and resilient up to temperatures much higher than previous simulation predictions and exhibit an unexpected hysteretic behavior during the first heating-cooling cycle. We show this hysteretic behavior can be explained by an asymmetric ligand configuration from the self-assembly process and can be controlled by changing the ligand coverage or cross-linking the ligand molecules. Finally, we show the screening effect of water molecules on the ligand interactions can strongly affect the moduli and thermomechanical behavior.

The role of ligands in the mechanical properties of Langmuir nanoparticle films

Langmuir monolayers of ligand-capped inorganic nanoparticles exhibit rich morphologies under lateral compression such as wrinkling, folding, and multilayer nucleation. We demonstrate that the ligands play a crucial role in the mechanical properties of nanoparticle films by probing the morphology and anisotropic stress response during lateral compression of films with systematically varied ligand concentrations. Increasing the ligand concentration of the films past a threshold value inhibits monolayer wrinkling and folding in favor of multilayer formation, and sharply reduces the compressive and shear moduli. We attribute these drastic mechanical effects to modifications to the ligand interactions between adjacent particles as well as to two-dimensional crystalline structure changes occurring on the scale of tens of particles, as determined by transmission electron microscopy.

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.

Tolerance to structural disorder and tunable mechanical behavior in self-assembled superlattices of polymer-grafted nanocrystals

Large, freestanding membranes with remarkably high elastic modulus (>10 GPa) have been fabricated through the self-assembly of ligand-stabilized inorganic nanocrystals, even though these nanocrystals are connected only by soft organic ligands (e.g., dodecanethiol or DNA) that are not cross-linked or entangled. Recent developments in the synthesis of polymer-grafted nanocrystals have greatly expanded the library of accessible superlattice architectures, which allows superlattice mechanical behavior to be linked to specific structural features. Here, colloidal self-assembly is used to organize polystyrene-grafted Au nanocrystals at a fluid interface to form ordered solids with sub-10-nm periodic features. Thin-film buckling and nanoindentation are used to evaluate the mechanical behavior of polymer-grafted nanocrystal superlattices while exploring the role of polymer structural conformation, nanocrystal packing, and superlattice dimensions. Superlattices containing 3-20 vol % Au are found to have an elastic modulus of ∼6-19 GPa, and hardness of ∼120-170 MPa. We find that rapidly self-assembled superlattices have the highest elastic modulus, despite containing significant structural defects. Polymer extension, interdigitation, and grafting density are determined to be critical parameters that govern superlattice elastic and plastic deformation.

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.

Directed self-assembly of inorganic nanoparticles at air/liquid interfaces

Inorganic nanoparticles (NPs) appear as the forefront functional structure in nanotechnology. The preparation of functional materials based on inorganic NPs requires their assembly onto well-defined structures. Within this context, self-assembly at air-liquid interfaces is probably the best candidate for a universal procedure for active materials composed of assembled NPs. The detailed in situ mechanism of the lateral self-assembly and vertical organization of NPs at air-liquid interfaces is still unknown despite its extended use. The most common and promising methods for addressing this open issue are reviewed herein. The self-assembled films can be used in situ or further be transferred to solid substrates as the main constituents of novel functional materials. Plasmonic NPs at interfaces are highly interesting, given the broad range of applications of the plasmonic field, and will be discussed more in detail.

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.

Self-Assembly of Colloidal Nanocrystals: From Intricate Structures to Functional Materials

Chemical methods developed over the past two decades enable preparation of colloidal nanocrystals with uniform size and shape. These Brownian objects readily order into superlattices. Recently, the range of accessible inorganic cores and tunable surface chemistries dramatically increased, expanding the set of nanocrystal arrangements experimentally attainable. In this review, we discuss efforts to create next-generation materials via bottom-up organization of nanocrystals with preprogrammed functionality and self-assembly instructions. This process is often driven by both interparticle interactions and the influence of the assembly environment. The introduction provides the reader with a practical overview of nanocrystal synthesis, self-assembly, and superlattice characterization. We then summarize the theory of nanocrystal interactions and examine fundamental principles governing nanocrystal self-assembly from hard and soft particle perspectives borrowed from the comparatively established fields of micrometer colloids and block copolymer assembly. We outline the extensive catalog of superlattices prepared to date using hydrocarbon-capped nanocrystals with spherical, polyhedral, rod, plate, and branched inorganic core shapes, as well as those obtained by mixing combinations thereof. We also provide an overview of structural defects in nanocrystal superlattices. We then explore the unique possibilities offered by leveraging nontraditional surface chemistries and assembly environments to control superlattice structure and produce nonbulk assemblies. We end with a discussion of the unique optical, magnetic, electronic, and catalytic properties of ordered nanocrystal superlattices, and the coming advances required to make use of this new class of solids.

Electro-mechanical sensing in freestanding monolayered gold nanoparticle membranes

The electro-mechanical sensing properties of freestanding monolayered membranes of dodecanethiol coated 7 nm gold nanoparticles (NPs) are investigated using AFM force spectroscopy and conductive AFM simultaneously. The electrical resistance of the NP membranes increases sensitively with the point-load force applied in the center of the membranes using an AFM tip. Numerical simulations of electronic conduction in a hexagonally close-packed two-dimensional (2D) array of NPs under point load-deformation are carried out on the basis of electronic transport measurements at low temperatures and strain modeling of the NP membranes by finite element analysis. These simulations, supporting AFM-based electro-mechanical measurements, attribute the high strain sensitivity of the monolayered NP membranes to the exponential dependence of the tunnel electron transport in 2D NP arrays on the strain-induced length variation of the interparticle junctions. This work thus evidences a new class of highly sensitive nano-electro-mechanical systems based on freestanding monolayered gold NP membranes.

Resistive pressure sensors based on freestanding membranes of gold nanoparticles

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.

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.

Fabrication of freestanding nanoparticle membranes over wells

Freestanding nanoparticle membranes over circular wells are prepared by utilizing surface engineering. The crucial step of this method is the hydrophobic treatment of the substrate surface, which causes the water droplet to be suspended over wells during drying. Consequently, the nanoparticle monolayer self-assembled at the surface of the water droplet would drape itself over wells instead of being dragged into wells and ruptured into patches after the evaporation of water. This scenario was confirmed by the results of control experiments with changes in the hydrophobicity of the surface and the depth of wells. Moreover, the NaCl crystallization experiment provides additional evidence for the dynamic process of drying. Freestanding nanoparticle membranes with different nanoparticle core sizes and different lengths of ligands have been successfully prepared using the same route. The Young's modulus of one typical kind of prepared freestanding nanoparticle membrane was measured with force microscopy.

Mechanical properties of self-assembled nanoparticle membranes: stretching and bending

Monolayers composed of colloidal nanoparticles, with a thickness of less than ten nanometers, have remarkable mechanical strength and can suspend over micron-sized holes to form free-standing membranes. We discuss experiments probing the tensile strength and bending stiffness of these self-assembled nanoparticle sheets. The fracture behavior of monolayers and multilayers is investigated by attaching them to elastomer substrates which are then stretched. For different applied strain, the fracture patterns are imaged down to the scale of single particles. The resulting detailed information about the crack width distribution allows us to relate the measured overall tensile strength to the distribution of local bond strengths within a layer. We then introduce two methods by which freestanding nanoparticle monolayers can be rolled up into hollow, tubular “nano-scrolls”, either by electron beam irradiation during imaging with a scanning electron microscope or by spontaneous self-rolling. Indentation measurements on the nano-scrolls yield values for the bending stiffness that are significantly larger than expected from the response to stretching. The ability to stretch, bend, and roll up nanoparticle sheets offers new possibilities for a variety of applications, including sensors and mechanical transducers.

Ordered nanoparticle arrays interconnected by molecular linkers: electronic and optoelectronic properties

Arrays of metal nanoparticles in an organic matrix have attracted a lot of interest due to their diverse electronic and optoelectronic properties.

Giant plasmene nanosheets, nanoribbons, and origami

We introduce Plasmene- in analogy to graphene-as free-standing, one-particle-thick, superlattice sheets of nanoparticles ("meta-atoms") from the "plasmonic periodic table", which has implications in many important research disciplines. Here, we report on a general bottom-up self-assembly approach to fabricate giant plasmene nanosheets (i.e., plasmene with nanoscale thickness but with macroscopic lateral dimensions) as thin as ∼40 nm and as wide as ∼3 mm, corresponding to an aspect ratio of ∼75,000. In conjunction with top-down lithography, such robust giant nanosheets could be milled into one-dimensional nanoribbons and folded into three-dimensional origami. Both experimental and theoretical studies reveal that our giant plasmene nanosheets are analogues of graphene from the plasmonic nanoparticle family, simultaneously possessing unique structural features and plasmon propagation functionalities.

Comparison of the mechanical properties of self-assembled Langmuir monolayers of nanoparticles and phospholipids

Nanoparticles with hydrophobic capping ligands and amphiphilic phospholipids are both found to self-assemble into monolayer films when deposited on the air/water interface. By separately measuring the anisotropic stress response of these films under uniaxial compression, we obtain both the 2D compressive and shear moduli of a range of different thin nanoparticle and phospholipid films. The compressive moduli of both nanoparticle and lipid films in the solid phase are on the same order of magnitude, whereas the shear moduli of the lipid films are found to be significantly lower. Additionally, the moduli of the nanoparticle films depended substantially on the polydispersity of the constituent particles-broader size distribution lowered the stiffness of the nanoparticle film.