Three-dimensional nanoprinting via charged aerosol jets.

Three-dimensional (3D) printing1-9 has revolutionized manufacturing processes for electronics10-12, optics13-15, energy16,17, robotics18, bioengineering19-21 and sensing22. Downscaling 3D printing23 will enable applications that take advantage of the properties of micro- and nanostructures24,25. However, existing techniques for 3D nanoprinting of metals require a polymer-metal mixture, metallic salts or rheological inks, limiting the choice of material and the purity of the resulting structures. Aerosol lithography has previously been used to assemble arrays of high-purity 3D metal nanostructures on a prepatterned substrate26,27, but in limited geometries26-30. Here we introduce a technique for direct 3D printing of arrays of metal nanostructures with flexible geometry and feature sizes down to hundreds of nanometres, using various materials. The printing process occurs in a dry atmosphere, without the need for polymers or inks. Instead, ions and charged aerosol particles are directed onto a dielectric mask containing an array of holes that floats over a biased silicon substrate. The ions accumulate around each hole, generating electrostatic lenses that focus the charged aerosol particles into nanoscale jets. These jets are guided by converged electric-field lines that form under the hole-containing mask, which acts similarly to the nozzle of a conventional 3D printer, enabling 3D printing of aerosol particles onto the silicon substrate. By moving the substrate during printing, we successfully print various 3D structures, including helices, overhanging nanopillars, rings and letters. In addition, to demonstrate the potential applications of our technique, we printed an array of vertical split-ring resonator structures. In combination with other 3D-printing methods, we expect our 3D-nanoprinting technique to enable substantial advances in nanofabrication.

Ultrasensitive mechanical crack-based sensor inspired by the spider sensory system.

Recently developed flexible mechanosensors based on inorganic silicon, organic semiconductors, carbon nanotubes, graphene platelets, pressure-sensitive rubber and self-powered devices are highly sensitive and can be applied to human skin. However, the development of a multifunctional sensor satisfying the requirements of ultrahigh mechanosensitivity, flexibility and durability remains a challenge. In nature, spiders sense extremely small variations in mechanical stress using crack-shaped slit organs near their leg joints. Here we demonstrate that sensors based on nanoscale crack junctions and inspired by the geometry of a spider's slit organ can attain ultrahigh sensitivity and serve multiple purposes. The sensors are sensitive to strain (with a gauge factor of over 2,000 in the 0-2 per cent strain range) and vibration (with the ability to detect amplitudes of approximately 10 nanometres). The device is reversible, reproducible, durable and mechanically flexible, and can thus be easily mounted on human skin as an electronic multipixel array. The ultrahigh mechanosensitivity is attributed to the disconnection-reconnection process undergone by the zip-like nanoscale crack junctions under strain or vibration. The proposed theoretical model is consistent with experimental data that we report here. We also demonstrate that sensors based on nanoscale crack junctions are applicable to highly selective speech pattern recognition and the detection of physiological signals. The nanoscale crack junction-based sensory system could be useful in diverse applications requiring ultrahigh displacement sensitivity.

Effect of Metal Thickness on the Sensitivity of Crack-Based Sensors.

Among many attempts to make a decent human motion detector in various engineering fields, a mechanical crack-based sensor that deliberately generates and uses nano-scale cracks on a metal deposited thin film is gaining attention for its high sensitivity. While the metal layer of the sensor must be responsible for its high performance, its effects have not received much academic interest. In this paper, we studied the relationship between the thickness of the metal layer and the characteristics of the sensor by depositing a few nanometers of chromium (Cr) and gold (Au) on the PET film. We found that the sensitivity of the crack sensor improves/increases under the following conditions: (1) when Au is thin and Cr is thick; and (2) when the ratio of Au is lower than that of Cr, which also increases the transmittance of the sensor, along with its sensitivity. As we only need a small amount of Au to achieve high sensitivity of the sensor, we have suggested more efficient and economical fabrication methods. With this crack-based sensor, we were able to successfully detect finger motions and to distinguish various signs of American Sign Language (ASL).

Three-dimensional assembly of nanoparticles from charged aerosols.

The capability of assembling nanoparticles into a desired ordered pattern is a key to realize novel devices which are based not only on the unique properties of nanoparticles but also on the arrangements of nanoparticles. While two-dimensional arrays of nanoparticles have been successfully demonstrated by various techniques, a controlled way of building ordered arrays of three-dimensional (3D) nanoparticle structures remains challenging. We report that a variety of 3D nanoparticle structures can be formed in a controlled way based on the ion-induced focusing, electrical scaffold, and antenna effects from charged aerosols. Particle trajectory calculations successfully predict the whole process of 3D assembly. New surface enhanced Raman scattering substrates based on our 3D assembly were constructed as an example showing the viability of the present approach. This report extends the current capability of positioning nanoparticles on surface to another spatial dimension, which can serve as the foundation of future optical, magnetic, and electronic devices taking the advantage of multidimensions.

Reversible Wrinkling Surfaces for Enhanced Grip on Wet/Dry Conditions.

Friction is important in material design for robotic systems that need to perform tasks regardless of environmental changes. Generally, robotic systems lose their friction in wet environments and fail to accomplish their tasks. Despite the significance of maintaining friction in dry and wet environments, it is still challenging. Here, we report a smart switching surface, which helps to complete missions in both wet and dry environments. Inspired by the reversible wrinkling mechanism of a human finger, the surface reversibly generates and removes wrinkles to adapt to both environments using volume-changing characteristics of the Nafion film. The switchable surfaces with manipulated wrinkle morphologies via patterns of diverse densities, sizes, and shapes induce a relationship between the wrinkle morphologies and friction: wrinkles on denser and smaller hexagonal patterns generate six times more friction than non-switching flat surfaces in wet environments and a similar amount of friction to the flat surfaces in dry environments. In addition, the wrinkle morphologies according to the patterns are predicted through numerical simulation, which is in good agreement with experimental results. This work presents potential applications in robotic systems that are required to perform in and out of water and paves the way for further understanding of wrinkling dynamics, manipulation, and evolutionary function in skin.

Room temperature CO and H2 sensing with carbon nanoparticles.

We report on a shell-shaped carbon nanoparticle (SCNP)-based gas sensor that reversibly detects reducing gas molecules such as CO and H(2) at room temperature both in air and inert atmosphere. Crystalline SCNPs were synthesized by laser-assisted reactions in pure acetylene gas flow, chemically treated to obtain well-dispersed SCNPs and then patterned on a substrate by the ion-induced focusing method. Our chemically functionalized SCNP-based gas sensor works for low concentrations of CO and H(2) at room temperature even without Pd or Pt catalysts commonly used for splitting H(2) molecules into reactive H atoms, while metal oxide gas sensors and bare carbon-nanotube-based gas sensors for sensing CO and H(2) molecules can operate only at elevated temperatures. A pristine SCNP-based gas sensor was also examined to prove the role of functional groups formed on the surface of functionalized SCNPs. A pristine SCNP gas sensor showed no response to reducing gases at room temperature but a significant response at elevated temperature, indicating a different sensing mechanism from a chemically functionalized SCNP sensor.

Symmetry, topology and the maximum number of mutually pairwise-touching infinite cylinders: configuration classification.

We provide a complete classification of possible configurations of mutually pairwise-touching infinite cylinders in Euclidian three-dimensional space. It turns out that there is a maximum number of such cylinders possible in three dimensions independently of the shape of the cylinder cross-sections. We give the explanation of the uniqueness of the non-trivial configuration of seven equal mutually touching round infinite cylinders found earlier. Some results obtained for the chirality matrix, which is equivalent to the Seidel adjacency matrix, may be found useful for the theory of graphs.

Multifurcation Assembly of Charged Aerosols and Its Application to 3D Structured Gas Sensors.

Multifurcated assemblies composed of charged nanoparticles (NPs) are fabricated by using spark discharge and manipulating the electric field. The multifurcated structure of the assembly of NPs and spontaneous interconnections between the near structures are described. The gas sensor with the tetrafurcated-NP-assembled structure demonstrates ≈200% enhanced response to 100 ppm CO at 300 °C.

Ultra-sensitive Pressure sensor based on guided straight mechanical cracks.

Recently, a mechanical crack-based strain sensor with high sensitivity was proposed by producing free cracks via bending metal coated film with a known curvature. To further enhance sensitivity and controllability, a guided crack formation is needed. Herein, we demonstrate such a ultra-sensitive sensor based on the guided formation of straight mechanical cracks. The sensor has patterned holes on the surface of the device, which concentrate the stress near patterned holes leading to generate uniform cracks connecting the holes throughout the surface. We found that such a guided straight crack formation resulted in an exponential dependence of the resistance against the strain, overriding known linear or power law dependences. Consequently, the sensors are highly sensitive to pressure (with a sensitivity of over 1 × 105 at pressures of 8-9.5 kPa range) as well as strain (with a gauge factor of over 2 × 106 at strains of 0-10% range). A new theoretical model for the guided crack system has been suggested to be in a good agreement with experiments. Durability and reproducibility have been also confirmed.

Studies on mechanism of single-walled carbon nanotube formation.

We presented detailed studies of the formation of single-walled carbon nanotubes by an aerosol method based on the introduction of pre-formed catalyst particles into conditions leading to carbon nanotube synthesis. Carbon monoxide and iron nanoparticles were used as a carbon source and a catalyst, respectively. The vital role of etching agents such as CO2 and H2O in CNT formation was demonstrated on the basis of on-line Fourier-transform infrared spectroscopy measurements. Hydrogen was shown to participate in the reaction of carbon release and to prevent the oxidation of the catalyst particles and the hot wire. The addition of H2 and small amounts of CO2 and H2O led to an increase in the carbon nanotube lengths. The catalyst particle evaporation process inside the reactor was found to become significant at temperatures higher than 1100 degrees C. The carbon nanotube growth was found to occur at a temperature of around 900 degrees C in the heating section of the reactor by in situ sampling and the growth rate was calculated to exceed 1.1 microm/s. A detailed analysis of possible processes during carbon nanotube formation revealed heptagon transformation as a limiting stage. A mechanism for carbon nanotube formation was proposed.

Trapped charge-driven degradation of perovskite solar cells.

Perovskite solar cells have shown unprecedent performance increase up to 22% efficiency. However, their photovoltaic performance has shown fast deterioration under light illumination in the presence of humid air even with encapulation. The stability of perovskite materials has been unsolved and its mechanism has been elusive. Here we uncover a mechanism for irreversible degradation of perovskite materials in which trapped charges, regardless of the polarity, play a decisive role. An experimental setup using different polarity ions revealed that the moisture-induced irreversible dissociation of perovskite materials is triggered by charges trapped along grain boundaries. We also identified the synergetic effect of oxygen on the process of moisture-induced degradation. The deprotonation of organic cations by trapped charge-induced local electric field would be attributed to the initiation of irreversible decomposition.

Hotspot-engineered 3D multipetal flower assemblies for surface-enhanced Raman spectroscopy.

Novel 3D metallic structures composed of multipetal flowers consisting of nanoparticles are presented. The control of surface plasmon hotspots is demonstrated in terms of location and intensity as a function of petal number for uniform and reproducible surfaceenhanced Raman spectroscopy (SERS) with high field enhancement.

A novel hybrid carbon material.

Both fullerenes and single-walled carbon nanotubes (SWNTs) exhibit many advantageous properties. Despite the similarities between these two forms of carbon, there have been very few attempts to physically merge them. We have discovered a novel hybrid material that combines fullerenes and SWNTs into a single structure in which the fullerenes are covalently bonded to the outer surface of the SWNTs. These fullerene-functionalized SWNTs, which we have termed NanoBuds, were selectively synthesized in two different one-step continuous methods, during which fullerenes were formed on iron-catalyst particles together with SWNTs during CO disproportionation. The field-emission characteristics of NanoBuds suggest that they may possess advantageous properties compared with single-walled nanotubes or fullerenes alone, or in their non-bonded configurations.

Parallel patterning of nanoparticles via electrodynamic focusing of charged aerosols.

The development of nanodevices that exploit the unique properties of nanoparticles will require high-speed methods for patterning surfaces with nanoparticles over large areas and with high resolution. Moreover, the technique will need to work with both conducting and non-conducting surfaces. Here we report an ion-induced parallel-focusing approach that satisfies all requirements. Charged monodisperse aerosol nanoparticles are deposited onto a surface patterned with a photoresist while ions of the same polarity are introduced into the deposition chamber in the presence of an applied electric field. The ions accumulate on the photoresist, modifying the applied field to produce nanoscopic electrostatic lenses that focus the nanoparticles onto the exposed parts of the surface. We have demonstrated that the technique could produce high-resolution patterns at high speed on both conducting (p-type silicon) and non-conducting (silica) surfaces. Moreover, the feature sizes in the nanoparticle patterns were significantly smaller than those in the original photoresist pattern.