Evolution of nanomechanical properties and crystallinity of individual titanium dioxide nanotube resonators.

Herein a complete characterization of single TiO2 nanotube resonator was reported for the first time. The modal vibration response analysis allows a non-invasive indirect evaluation of the mechanical properties of the TiO2 nanotube. The effect of post-grown thermal treatments on nanotube mechanical properties was investigated and carefully correlated to the chemico-physical parameters evolution. The Young's modulus of TiO2 nanotube rises linearly from 57 GPa up to 105 GPa for annealing at 600 °C depending on the compositional and crystallographic evolution of the nanostructure. Considering the growing interest in single nanostructure devices, the reported findings allow a deeper understanding of the properties of individual titanium dioxide nanotubes extrapolated from their standard arrayed architecture.

Functionalized ZnO nanowires for microcantilever biosensors with enhanced binding capability.

An efficient way to increase the binding capability of microcantilever biosensors is here demonstrated by growing zinc oxide nanowires (ZnO NWs) on their active surface. A comprehensive evaluation of the chemical compatibility of ZnO NWs brought to the definition of an innovative functionalization method able to guarantee the proper immobilization of biomolecules on the nanostructured surface. A noteworthy higher amount of grafted molecules was evidenced with colorimetric assays on ZnO NWs-coated devices, in comparison with functionalized and activated silicon flat samples. ZnO NWs grown on silicon microcantilever arrays and activated with the proposed immobilization strategy enhanced the sensor binding capability (and thus the dynamic range) of nearly 1 order of magnitude, with respect to the commonly employed flat functionalized silicon devices. Graphical Abstract An efficient way to increase the binding capability of microcantilever biosensors is represented by growing zinc oxide nanowires (ZnO NWs) on their active surface. ZnO NWs grown on silicon microcantilever arrays and activated with an innovative immobilization strategy enhanced the sensor binding capability of nearly 1 order of magnitude, with respect to the commonly employed flat functionalized silicon devices.

Flexible tactile sensing based on piezoresistive composites: a review.

The large expansion of the robotic field in the last decades has created a growing interest in the research and development of tactile sensing solutions for robot hand and body integration. Piezoresistive composites are one of the most widely employed materials for this purpose, combining simple and low cost preparation with high flexibility and conformability to surfaces, low power consumption, and the use of simple read-out electronics. This work provides a review on the different type of composite materials, classified according to the conduction mechanism and analyzing the physics behind it. In particular piezoresistors, strain gauges, percolative and quantum tunnelling devices are reviewed here, with a perspective overview on the most used filler types and polymeric matrices. A description of the state-of-the-art of the tactile sensor solutions from the point of view of the architecture, the design and the performance is also reviewed, with a perspective outlook on the main promising applications.

Length-dependent charge generation from vertical arrays of high-aspect-ratio ZnO nanowires.

Aqueous chemical growth of zinc oxide nanowires is a flexible and effective approach to obtain dense arrays of vertically oriented nanostructures with high aspect ratio. Herein we present a systematic study of the different synthesis parameters that influence the ZnO seed layer and thus the resulting morphological features of the free-standing vertically oriented ZnO nanowires. We obtained a homogeneous coverage of transparent conductive substrates with high-aspect-ratio nanowire arrays (length/diameter ratio of up to 52). Such nanostructured vertical arrays were examined to assess their electric and piezoelectric properties, and showed an electric charge generation upon mechanical compressive stress. The principle of energy harvesting with these nanostructured ZnO arrays was demonstrated by connecting them to an electronic charge amplifier and storing the generated charge in a series of capacitors. We found that the generated charge and the electrical behavior of the ZnO nanowires are strictly dependent on the nanowire length. We have shown the importance of controlling the morphological properties of such ZnO nanostructures for optimizing a nanogenerator device.

Silicon and Silicon Carbide Recrystallization by Laser Annealing: A Review.

Modifying material properties within a specific spatial region is a pivotal stage in the fabrication of microelectronic devices. Laser annealing emerges as a compelling technology, offering precise control over the crystalline structure of semiconductor materials and facilitating the activation of doping ions in localized regions. This obviates the necessity for annealing the entire wafer or device. The objective of this review is to comprehensively investigate laser annealing processes specifically targeting the crystallization of amorphous silicon (Si) and silicon carbide (SiC) samples. Silicon finds extensive use in diverse applications, including microelectronics and solar cells, while SiC serves as a crucial material for developing components designed to operate in challenging environments or high-power integrated devices. The review commences with an exploration of the underlying theory and fundamentals of laser annealing techniques. It then delves into an analysis of the most pertinent studies focused on the crystallization of these two semiconductor materials.

Three-Dimensional Printing of Triboelectric Nanogenerators by Digital Light Processing Technique for Mechanical Energy Harvesting.

Triboelectric nanogenerators (TENGs) represent intriguing technology to harvest human mechanical movements for powering wearable and portable electronics. Differently, compared to conventional fabrication approaches, additive manufacturing can allow the fabrication of TENGs with good dimensional resolution, high reproducibility, and quick production processes and, in particular, the obtainment of complex and customized structures. Among 3D printing technologies, digital light processing (DLP) is well-known for being the most flexible to produce functional devices by controlling both the geometry and the different ingredients of printable resins. On the other hand, DLP was not exploited for TENG fabrication, and consequently, the knowledge of the performance of 3D printable materials as charge accumulators upon friction is limited. Here, the application of the DLP technique to the 3D printing of triboelectric nanogenerators is studied. First, several printable materials have been tested as triboelectric layers to define a triboelectric series of DLP 3D printable materials. Then, TENG devices with increased geometrical complexity were printed, showcasing the ability to harvest energy from human movement. The method presented in this work illustrates how the DLP may represent a valuable and flexible solution to fabricate triboelectric nanogenerators, also providing a triboelectric classification of the most common photocurable resins.

Reaching silicon-based NEMS performances with 3D printed nanomechanical resonators.

The extreme miniaturization in NEMS resonators offers the possibility to reach an unprecedented resolution in high-performance mass sensing. These very low limits of detection are related to the combination of two factors: a small resonator mass and a high quality factor. The main drawback of NEMS is represented by the highly complex, multi-steps, and expensive fabrication processes. Several alternatives fabrication processes have been exploited, but they are still limited to MEMS range and very low-quality factor. Here we report the fabrication of rigid NEMS resonators with high-quality factors by a 3D printing approach. After a thermal step, we reach complex geometry printed devices composed of ceramic structures with high Young's modulus and low damping showing performances in line with silicon-based NEMS resonators ones. We demonstrate the possibility of rapid fabrication of NEMS devices that present an effective alternative to semiconducting resonators as highly sensitive mass and force sensors.

Coffee-ring effect-based three dimensional patterning of micro/nanoparticle assembly with a single droplet.

We develop a novel patterning technique to create 3D patterns of micro and nanoparticle assembly via evaporative self-assembly based on the coffee-ring effect of an evaporating suspension. The principle of the technique is analyzed theoretically by the scaling analysis of main parameters of the process and the scaling effect, the effect of the volume, the concentration of the suspension, and the effect of surface treatment on the patterning are studied. On the basis of the presented technique, we demonstrate that the patterns of 3D assembly of various sizes of microparticles (Silica), metal oxide nanoparticles (TiO(2), ZnO), and metallic nanoparticles (Ag) can be successfully generated by low-concentrated particle suspension (1.25-5 wt %) without additional sintering steps, and we also show the geometries of the patterns can be finely controlled by adjusting the parameters of the process.

Large-scale parallelization of nanomechanical mass spectrometry with weakly-coupled resonators.

Nanomechanical mass spectrometry is a recent technological breakthrough that enables the real-time analysis of single molecules. In contraposition to its extreme mass sensitivity is a limited capture cross-section that can hinder measurements in a practical setting. Here we show that weak-coupling between devices in resonator arrays can be used in nanomechanical mass spectrometry to parallelize the measurement. This coupling gives rise to asymmetric amplitude peaks in the vibrational response of a single nanomechanical resonator of the array, which coincide with the natural frequencies of all other resonators in the same array. A rigorous theoretical model is derived that explains the physical mechanisms and describes the practical features of this parallelization. We demonstrate the significance of this parallelization through inertial imaging of analytes adsorbed to all resonators of an array, with the possibility of simultaneously detecting resonators placed at distances a hundred times larger than their own physical size.

PDMS/Polyimide Composite as an Elastomeric Substrate for Multifunctional Laser-Induced Graphene Electrodes.

Laser-induced graphene (LIG) emerged as one of the most promising materials for flexible functional devices. However, the attempts to obtain LIG onto elastomeric substrates never succeed, hindering its full exploitation for stretchable electronics. Herein, a novel polymeric composite is reported as a starting material for the fabrication of graphene-based electrodes by direct laser writing. A polyimide (PI) powder is dispersed into the poly(dimethylsiloxane) (PDMS) matrix to achieve an easily processable and functional elastomeric substrate, allowing the conversion of the polymeric surface into laser-induced graphene (LIG). The mechanical and electrical properties of the proposed material can be easily tuned by acting on the polyimide powder concentration. The reported procedure takes advantage from the simple casting process, typical of silicone elastomer, allowing to produce electrodes conformable to any kind of shape and surface as well as complex three-dimensional structures. Electrochemical capacitors and strain gauges are selected as flexible prototypes to demonstrate the multifunctional properties of the obtained LIG on the PDMS/PI composite substrate.

Nanomechanical DNA resonators for sensing and structural analysis of DNA-ligand complexes.

The effect of direct or indirect binding of intercalant molecules on DNA structure is of fundamental importance in understanding the biological functioning of DNA. Here we report on self-suspended DNA nanobundles as ultrasensitive nanomechanical resonators for structural studies of DNA-ligand complexes. Such vibrating nanostructures represent the smallest mechanical resonator entirely composed of DNA. A correlative analysis between the mechanical and structural properties is exploited to study the intrinsic changes of double strand DNA, when interacting with different intercalant molecules (YOYO-1 and GelRed) and a chemotherapeutic drug (Cisplatin), at different concentrations. Possible implications of our findings are related to the study of interaction mechanism of a wide category of molecules with DNA, and to further applications in medicine, such as optimal titration of chemotherapeutic drugs and environmental studies for the detection of heavy metals in human serum.

Crystallization of TiO2 Nanotubes by In Situ Heating TEM.

The thermally-induced crystallization of anodically grown TiO2 amorphous nanotubes has been studied so far under ambient pressure conditions by techniques such as differential scanning calorimetry and in situ X-ray diffraction, then looking at the overall response of several thousands of nanotubes in a carpet arrangement. Here we report a study of this phenomenon based on an in situ transmission electron microscopy approach that uses a twofold strategy. First, a group of some tens of TiO2 amorphous nanotubes was heated looking at their electron diffraction pattern change versus temperature, in order to determine both the initial temperature of crystallization and the corresponding crystalline phases. Second, the experiment was repeated on groups of few nanotubes, imaging their structural evolution in the direct space by spherical aberration-corrected high resolution transmission electron microscopy. These studies showed that, differently from what happens under ambient pressure conditions, under the microscope's high vacuum (p < 10-5 Pa) the crystallization of TiO2 amorphous nanotubes starts from local small seeds of rutile and brookite, which then grow up with the increasing temperature. Besides, the crystallization started at different temperatures, namely 450 and 380 °C, when the in situ heating was performed irradiating the sample with electron beam energy of 120 or 300 keV, respectively. This difference is due to atomic knock-on effects induced by the electron beam with diverse energy.

Experimental evidence of Fano resonances in nanomechanical resonators.

Fano resonance refers to an interference between localized and continuum states that was firstly reported for atomic physics and solid-state quantum devices. In recent years, Fano interference gained more and more attention for its importance in metamaterials, nanoscale photonic devices, plasmonic nanoclusters and surface-enhanced Raman scattering (SERS). Despite such interest in nano-optics, no experimental evidence of Fano interference was reported up to now for purely nanomechanical resonators, even if classical mechanical analogies were referred from a theoretical point of view. Here we demonstrate for the first time that harmonic nanomechanical resonators with relatively high quality factors, such as cantilevers vibrating in vacuum, can show characteristic Fano asymmetric curves when coupled in arrays. The reported findings open new perspectives in fundamental aspects of classical nanomechanical resonators and pave the way to a new generation of chemical and biological nanoresonator sensors with higher parallelization capability.

Polymeric 3D Printed Functional Microcantilevers for Biosensing Applications.

In this study, we show for the first time the production of mass-sensitive polymeric biosensors by 3D printing technology with intrinsic functionalities. We also demonstrate the feasibility of mass-sensitive biosensors in the form of microcantilever in a one-step printing process, using acrylic acid as functional comonomer for introducing a controlled amount of functional groups that can covalently immobilize the biomolecules onto the polymer. The effectiveness of the application of 3D printed microcantilevers as biosensors is then demonstrated with their implementation in a standard immunoassay protocol. This study shows how 3D microfabrication techniques, material characterization, and biosensor development could be combined to obtain an engineered polymeric microcantilever with intrinsic functionalities. The possibility of tuning the composition of the starting photocurable resin with the addition of functional agents, and consequently controlling the functionalities of the 3D printed devices, paves the way to a new class of mass-sensing microelectromechanical system devices with intrinsic properties.

Nanoconfinement: an effective way to enhance PVDF piezoelectric properties.

The dimensional confinement and oriented crystallization are both key factors in determining the piezoelectric properties of a polymeric nanostructured material. Here we prepare arrays of one-dimensional polymeric nanowires showing piezoelectric features by template-wetting two distinct polymers into anodic porous alumina (APA) membranes. In particular, poly(vinylidene fluoride), PVDF, and its copolymer poly(vinylidene fluoride-trifluoroethylene), PVTF, are obtained in commercially available APA, showing a final diameter of about 200 nm and several micrometers in length, reflecting the templating matrix features. We show that the crystallization of both polymers into a ferroelectric phase is directed by the nanotemplate confinement. Interestingly, the PVDF nanowires mainly crystallize into the ß-phase in the nanoporous matrix, whereas the reference thin film of PVDF crystallizes in the α nonpolar phase. In the case of the PVTF nanowires, needle-like crystals oriented perpendicularly to the APA channel walls are observed, giving insight on the molecular orientation of the polymer within the nanowire structure. A remarkable piezoelectric behavior of both 1-D polymeric nanowires is observed, upon recording ferroelectric polarization, hysteresis, and displacement loops. In particular, an outstanding piezoelectric effect is observed for the PVDF nanowires with respect to the polymeric thin film, considering that no poling was carried out. Current versus voltage (I-V) characteristics showed a consistent switching behavior of the ferroelectric polar domains, thus revealing the importance of the confined and oriented crystallization of the polymer in monodimensional nanoarchitectures.

Evaluation of different conductive nanostructured particles as filler in smart piezoresistive composites.

This work presents a comparison between three piezoresistive composite materials based on nanostructured conductive fillers in a polydimethylsiloxane insulating elastomeric matrix for sensing applications. Without any mechanical deformation upon an applied bias, the prepared composites present an insulating electric behavior, while, when subjected to mechanical load, the electric resistance is reduced exponentially. Three different metal fillers were tested: commercial nickel and copper spiky-particles and synthesized highly-pointed gold nanostars. These particles were chosen because of their high electrical conductivity and especially for the presence of nanosized sharp tips on their surface. These features generate an enhancement of the local electric field increasing the tunneling probability between the particles. Different figures of merit concerning the morphology of the fillers were evaluated and correlated with the corresponding functional response of the composite.