Additive printing of pure nanocrystalline nickel thin films using room environment electroplating.

Given its high temperature stability, oxidation-, corrosion- and wear-resistance, and ferromagnetic properties, Nickel (Ni) is one of the most technologically important metals. This article reports that pure and nanocrystalline (Ni) films with excellent mechanical and magnetic properties can be additively printed at room environment without any high-temperature post-processing. The printing process is based on a nozzle-based electrochemical deposition from the classical Watt's bath. The printed Ni film showed a preferred (220) and (111) texture based on x-ray diffraction spectra. The printed Ni film had close to bulk electrical conductivity; its indentation elastic modulus and hardness was measured to be 203 ± 6.7 GPa and 6.27 ± 0.34 GPa, respectively. Magnetoresistance, magnetic hysteresis loop, and magnetic domain imaging showed promising results of the printed Ni for functional applications.

Localized Pulsed Electrodeposition Process for Three-Dimensional Printing of Nanotwinned Metallic Nanostructures.

Nanotwinned-metals (nt-metals) offer superior mechanical (high ductility and strength) and electrical (low electromigration) properties compared to their nanocrystalline (nc) counterparts. These properties are advantageous in particular for applications in nanoscale devices. However, fabrication of nt-metals has been limited to films (two-dimensional) or template-based (one-dimensional) geometries, using various chemical and physical processes. In this Letter, we demonstrate the ambient environment localized pulsed electrodeposition process for direct printing of three-dimensional (3D) freestanding nanotwinned-Copper (nt-Cu) nanostructures. 3D nt-Cu structures were additively manufactured using pulsed electrodeposition at the tip of an electrolyte-containing nozzle. Focused ion beam (FIB) and transmission electron microscopy (TEM) analysis revealed that the printed metal was fully dense, and was mostly devoid of impurities and microstructural defects. FIB and TEM images also revealed nanocrystalline-nanotwinned-microstructure (nc-nt-microstructure), and confirmed the formation of coherent twin boundaries in the 3D-printed Cu. Mechanical properties of the 3D-printed nc-nt-Cu were characterized by direct printing (FIB-less) of micropillars for in situ SEM microcompression experiments. The 3D-printed nc-nt-Cu exhibited a flow stress of over 960 MPa, among the highest ever reported, which is remarkable for a 3D-printed material. The microstructure and mechanical properties of the nc-nt-Cu were compared to those of nc-Cu printed using the same process under direct current (DC) voltage.

Dynamics of the nanoneedle probe in trolling mode AFM.

Atomic force microscopy (AFM), as an indispensable tool for nanoscale characterization, presents major drawbacks for operation in a liquid environment arising from the large hydrodynamic drag on the vibrating cantilever. The newly introduced 'Trolling mode' (TR-mode) AFM resolves this complication by using a specialized nanoneedle cantilever that keeps the cantilever outside of the liquid. Herein, a mechanical model with a lumped mass was developed to capture the dynamics of such a cantilever with a nanoneedle tip. This new developed model was applied to investigate the effects of the needle-liquid interface on the performance of the AFM, including the imaging capability in liquid.

Nanofountain probe electroporation (NFP-E) of single cells.

The ability to precisely deliver molecules into single cells is of great interest to biotechnology researchers for advancing applications in therapeutics, diagnostics, and drug delivery toward the promise of personalized medicine. The use of bulk electroporation techniques for cell transfection has increased significantly in the past decade, but the technique is nonspecific and requires high voltage, resulting in variable efficiency and low cell viability. We have developed a new tool for electroporation using nanofountain probe (NFP) technology, which can deliver molecules into cells in a manner that is highly efficient and gentler to cells than bulk electroporation or microinjection. Here we demonstrate NFP electroporation (NFP-E) of single HeLa cells within a population by transfecting them with fluorescently labeled dextran and imaging the cells to evaluate the transfection efficiency and cell viability. Our theoretical analysis of the mechanism of NFP-E reveals that application of the voltage creates a localized electric field between the NFP cantilever tip and the region of the cell membrane in contact with the tip. Therefore, NFP-E can deliver molecules to a target cell with minimal effect of the electric potential on the cell. Our experiments on HeLa cells confirm that NFP-E offers single cell selectivity, high transfection efficiency (>95%), qualitative dosage control, and very high viability (92%) of transfected cells.

Individual GaN nanowires exhibit strong piezoelectricity in 3D.

Semiconductor GaN NWs are promising components in next generation nano- and optoelectronic systems. In addition to their direct band gap, they exhibit piezoelectricity, which renders them particularly attractive in energy harvesting applications for self-powered devices. Nanowires are often considered as one-dimensional nanostructures; however, the electromechanical coupling leads to a third rank tensor that for wurtzite crystals (GaN NWs) possesses three independent coefficients, d(33), d(13), and d(15). Therefore, the full piezoelectric characterization of individual GaN NWs requires application of electric fields in different directions and measurements of associated displacements on the order of several picometers. In this Letter, we present an experimental approach based on scanning probe microscopy to directly quantify the three-dimensional piezoelectric response of individual GaN NWs. Experimental results reveal that GaN NWs exhibit strong piezoelectricity in three dimensions, with up to six times the effect in bulk. Based on finite element modeling, this finding has major implication on the design of energy harvesting systems exhibiting unprecedented levels of power density production. The presented method is applicable to other piezoelectric NW materials as well as wires manufactured along different crystallographic orientations.

Intrinsically high-Q dynamic AFM imaging in liquid with a significantly extended needle tip.

Atomic force microscope (AFM) probe with a long and rigid needle tip was fabricated and studied for high Q factor dynamic (tapping mode) AFM imaging of samples submersed in liquid. The extended needle tip over a regular commercially available tapping-mode AFM cantilever was sufficiently long to keep the AFM cantilever from submersed in liquid, which significantly minimized the hydrodynamic damping involved in dynamic AFM imaging of samples in liquid. Dynamic AFM imaging of samples in liquid at an intrinsic Q factor of over 100 and an operational frequency of over 200 kHz was demonstrated. The method has the potential to be extended to acquire viscoelastic material properties and provide truly gentle imaging of soft biological samples in physiological environments.

Additive-Free and Support-Free 3D Printing of Thermosetting Polymers with Isotropic Mechanical Properties.

The democratization of thermoplastic 3D printing is rooted in the ease of processing enabled by economical melting and shaping. Thermosetting polymers, on the other hand, have not enjoyed this advantage given that thermosetting resins cannot hold their shape without cross-linking or excessive fillers, and once cross-linked, they cannot be extruded for printing. Due to this formidable challenge, thus far, 3D printing of thermosetting polymers has been limited to the photopolymerization of specialized photosensitive resins or extrusion of resins loaded with large fractions (as high as 20 wt %) of rheology modifiers. Here, we report a rheology-modifier- and photoinitiator-free process for the 3D printing of a pure commercial epoxy polymer, without any resin modification and using a conventional 3D printer. A low-cost non-Newtonian support material that switches between solid-fluid states under a nozzle shear stress enables the printing of complex 3D structures and the subsequent and ″one-step″ curing. Our results show that the one-step curing eliminates the often-compromised interlayer adhesion common in layer-by-layer 3D printing processes and results in unprecedented isotropic mechanical properties (strength, elastic modulus, tensile toughness, and strain to failure). This in-bath print and cure (IBPC) 3D printing process for thermosetting polymers is low-cost, scalable, high-speed (nozzle speeds exceeding 720 cm/min), and high-resolution (down to 220 µm filament size). We demonstrate potential applications for hobbyists, structural and aerospace components, and fiber-reinforced composites, among others.

Interconnect Fabrication by Electroless Plating on 3D-Printed Electroplated Patterns.

The metallic interconnects are essential components of energy devices such as fuel cells and electrolysis cells, batteries, as well as electronics and optoelectronic devices. In recent years, 3D printing processes have offered complementary routes to the conventional photolithography- and vacuum-based processes for interconnect fabrication. Among these methods, the confined electrodeposition (CED) process has enabled a great control over the microstructure of the printed metal, direct printing of high electrical conductivity (close to the bulk values) metals on flexible substrates without a need to sintering, printing alloys with controlled composition, printing functional metals for various applications including magnetic applications, and for in situ scanning electron microscope (SEM) nanomechanical experiments. However, the metal deposition rate (or the overall printing speed) of this process is reasonably slow because of the chemical nature of the process. Here, we propose using the CED process to print a single layer of a metallic trace as the seed layer for the subsequent selected-area electroless plating. By controlling the activation sites through printing by the CED process, we control, where the metal grows by electroless plating, and demonstrate the fabrication of complex thin-film patterns. Our results show that this combined process improves the processing time by more than 2 orders of magnitude compared to the layer-by-layer printing process by CED. Additionally, we obtained Cu and Ni films with an electrical resistivity as low as ∼1.3 and ∼2 times of the bulk Cu and Ni, respectively, without any thermal annealing. Furthermore, our quantitative experiments show that the obtained films exhibit mechanical properties close to the bulk metals with an excellent adhesion to the substrate. We demonstrate potential applications for radio frequency identification (RFID) tags, for complex printed circuit board patterns, and resistive sensors in a Petri dish for potential biological applications.

Computational Nanomechanics of Noncollagenous Interfibrillar Interface in Bone.

The noncollagenous interfibrillar interface in bone provides the critical function of transferring loads among collagen fibrils and their bundles, with adhesive mechanisms at this site thus significantly contributing to the mechanical properties of bone. Motivated by the experimental observations and hypotheses, a computational study is presented to elucidate the critical roles of two major proteins at the nanoscale interfibrillar interface, that is, osteopontin (OPN) and osteocalcin (OC) in bone. This study reveals the extremely high interfacial toughness of the OPN/OC composite. The previously proposed hypothesis of sacrificial bonds in the extracellular organic matrix is tested, and the remarkable mechanical properties of the nanoscale bone interface are attributed to the collaborative interactions between the OPN and OC proteins.

Direct-Write Printing Copper-Nickel (Cu/Ni) Alloy with Controlled Composition from a Single Electrolyte Using Co-Electrodeposition.

Although various processes for metal printing at the micro- and mesoscale have been demonstrated, printing functional devices such as thermocouples, thermopiles, and heat flux sensors that function based on interfaces between an alloy and another alloy/metal demands processes for printing alloys. Furthermore, a high-quality and crystalline alloy is required for acceptable function of these devices. This article reports for the first time co-electrodeposition-based printing of single-phase solid solution nanocrystalline copper/nickel (Cu/Ni) alloy with various controllable compositions (Cu100Ni0 to Cu19Ni81) from a single electrolyte. The printed alloy is nanocrystalline (<35 nm), continuous, and dense with no apparent porosity, with remarkable mechanical and magnetic properties, without any postprocessing annealing such as heat treatment. In addition, a functional thermocouple fabricated using this process is demonstrated. Such a process can not only be used for fabrication of functional devices, it may also facilitate fundamental studies on alloys by printing a continuous library of alloy composition for material characterization.

Three-Dimensional Printing of Ceramics through "Carving" a Gel and "Filling in" the Precursor Polymer.

Achieving a viable process for three-dimensional (3D) printing of ceramics is a sought-after goal in a wide range of fields including electronics and sensors for harsh environments, microelectromechanical devices, energy storage materials, and structural materials, among others. Low laser absorption of ceramic powders renders available additive manufacturing (AM) technologies for metals not suitable for ceramics. Polymer solutions that can be converted to ceramics (preceramic polymers) offer a unique opportunity to 3D-print ceramics; however, due to the low viscosity of these polymers, so far, their 3D printing has only been possible by combining them with specialized light-sensitive agents and subsequently cross-linking them layer by layer by rastering an optical beam. The slow rate, lack of scalability to large specimens, and specialized chemistry requirements of this optical process are fundamental limitations. Here, we demonstrate 3D printing of ceramics enabled by dispensing the preceramic polymer at the tip of a moving nozzle into a gel that can reversibly switch between fluid and solid states, and subsequently thermally cross-linking the entire printed part "at-once" while still inside the same gel. The solid gel, which is composed of mineral oil and silica nanoparticles, converts to fluid at the tip of the moving nozzle, allows the polymer solution to be dispensed, and quickly returns to a solid state to maintain the geometry of the printed polymer both during printing and the subsequent high-temperature (160 °C) cross-linking. We retrieve the cross-linked part from the gel and convert it to ceramic by high-temperature pyrolysis. This scalable process opens up new opportunities for low-cost and high-speed production of complex three-dimensional ceramic parts and will be widely used for high temperature and corrosive environment applications, including electronics and sensors, microelectromechanical systems, energy and structural applications.

Nanomechanical heterogeneity in the gap and overlap regions of type I collagen fibrils with implications for bone heterogeneity.

The microstructure of type I collagen, consisting of alternating gap and overlap regions with a characteristic D period of approximately 67 nm, enables multifunctionalities of collagen fibrils in different tissues. Implementing near-surface dynamic and static nanoindentation techniques with atomic force microscope, we reveal mechanical heterogeneity along the axial direction of a single isolated collagen fibril from tendon and show that, within the D period, the gap and overlap regions have significantly different elastic and energy dissipation properties, correlating the significantly different molecular structures in these two regions. We further show that such subfibrillar heterogeneity holds in collagen fibrils inside bone and might be intrinsically related to the excellent energy dissipation performance of bone.

Nanoscale characterization of isolated individual type I collagen fibrils: polarization and piezoelectricity.

Piezoresponse force microscopy was applied to directly study individual type I collagen fibrils with diameters of approximately 100 nm isolated from bovine Achilles tendon. It was revealed that single collagen fibrils behave predominantly as shear piezoelectric materials with a piezoelectric coefficient on the order of 1 pm V(-1), and have unipolar axial polarization throughout their entire length. It was estimated that, under reasonable shear load conditions, the fibrils were capable of generating an electric potential up to tens of millivolts. The result substantiates the nanoscale origin of piezoelectricity in bone and tendons, and implies also the potential importance of the shear load-transfer mechanism, which has been the principle basis of the nanoscale mechanics model of collagen, in mechanoelectric transduction in bone.

Low-Cost Manufacturing of Metal-Ceramic Composites through Electrodeposition of Metal into Ceramic Scaffold.

Infiltration of a molten metal phase into a ceramic scaffold to manufacture metal-ceramic composites often involves high temperature, high pressure, and expensive processes. Low-cost processes for fabrication of metal-ceramic composites can substantially increase their applications in various industries. In this article, electroplating (electrodeposition) as a low-cost, room-temperature process is demonstrated for infiltration of metal (copper) into a lamellar ceramic (alumina) scaffold. Estimation shows that this is a low energy consumption process. Characterization of mechanical properties showed that metal infiltration enhanced the flexural modulus and strength by more than 50% and 140%, respectively, compared to the pure lamellar ceramic. More importantly, metal infiltration remarkably enhanced the crack initiation and crack growth resistance by more than 230% and 510% compared to the lamellar ceramic. The electrodeposition process for development of metal-ceramic composites can be extended to other metals and alloys that can be electrochemically deposited, as a low-cost and versatile process.

Deformation Mechanisms of "Two-Part" Natural Adhesive in Bone Interfibrillar Nano-Interfaces.

Noncollagenous proteins at nanoscale interfaces in bone are less than 2-3% of bone content by weight, while they contribute more than 30% to fracture toughness. Major gaps in quantitative understanding of noncollagenous proteins' role in the interfibrillar interfaces, largely because of the limitation of probing their nanoscale dimension, have resulted in ongoing controversies and several outstanding hypotheses on their role and function, arguably going back to centuries ago to the original work from Galileo. Our results from the first detailed computational model of the nano-interface in the bone reveal "synergistic" deformation mechanism of a "double-part" natural glue, that is, noncollagenous osteopontin and osteocalcin at the interfibrillar interface. Specifically, through strong anchoring and formation of dynamic binding sites on mineral nanoplatelets, the nano-interface can sustain a large nonlinear deformation with ductility approaching 5000%. This large deformation results in an outstanding specific energy to failure exceeding ∼350 J/g, which is larger than the most known tough materials (such as Kevlar, spider silk, and so forth.).

A Hybrid Process for Printing Pure and High Conductivity Nanocrystalline Copper and Nickel on Flexible Polymeric Substrates.

Printing functional devices on flexible substrates requires printing of high conductivity metallic patterns. To prevent deformation and damage of the polymeric substrate, the processing (printing) and post-processing (annealing) temperature of the metal patterns must be lower than the glass transition temperature of the substrate. Here, a hybrid process including deposition of a sacrificial blanket thin film, followed by room environment nozzle-based electrodeposition, and subsequent etching of the blanket film is demonstrated to print pure and nanocrystalline metallic (Ni and Cu) patterns on flexible substrates (PI and PET). Microscopy and spectroscopy showed that the printed metal is nanocrystalline, solid with no porosity and with low impurities. Electrical resistivity close to the bulk (~2-time) was obtained without any thermal annealing. Mechanical characterization confirmed excellent cyclic strength of the deposited metal, with limited degradation under high cyclic flexure. Several devices including radio frequency identification (RFID) tag, heater, strain gauge, and temperature sensor are demonstrated.

Enhancement of the Electrical Properties of DNA Molecular Wires through Incorporation of Perylenediimide DNA Base Surrogates.

DNA has long been viewed as a promising material for nanoscale electronics, in part due to its well-ordered arrangement of stacked, pi-conjugated base pairs. Within this context, a number of studies have investigated how structural changes, backbone modifications, or artificial base substitutions affect the conductivity of DNA. Herein, we present a comparative study of the electrical properties of both well-matched and perylene-3,4,9,10-tetracarboxylic diimide (PTCDI)-containing DNA molecular wires that bridge nanoscale gold electrodes. By performing current-voltage measurements for such devices, we find that the incorporation of PTCDI DNA base surrogates within our macromolecular constructs leads to an approximately 6-fold enhancement in the observed current levels. Together, these findings suggest that PTCDI DNA base surrogates may enable the preparation of designer DNA-based nanoscale electronic components.

An improved in situ measurement of offset phase shift towards quantitative damping-measurement with AFM.

An improved approach is introduced in damping measurement with atomic force microscope (AFM) for the in situ measurement of the offset phase shift needed for determining the intrinsic mechanical damping in nanoscale materials. The offset phase shift is defined and measured at a point of zero contact force according to the deflection part of the AFM force plot. It is shown that such defined offset phase shift is independent of the type of sample material, varied from hard to relatively soft materials in this study. This improved approach allows the self-calibrated and quantitative damping measurement with AFM. The ability of dynamic mechanical analysis for the measurement of damping in isolated one-dimensional nanostructures, e.g. individual multiwalled carbon nanotubes, was demonstrated.

Microscale 3D Printing of Nanotwinned Copper.

Nanotwinned (nt)-metals exhibit superior mechanical and electrical properties compared to their coarse-grained and nanograined counterparts. nt-metals in film and bulk forms are obtained using physical and chemical processes including pulsed electrodeposition (PED), plastic deformation, recrystallization, phase transformation, and sputter deposition. However, currently, there is no process for 3D printing (additive manufacturing) of nt-metals. Microscale 3D printing of nt-Cu is demonstrated with high density of coherent twin boundaries using a new room temperature process based on localized PED (L-PED). The 3D printed nt-Cu is fully dense, with low to none impurities, and low microstructural defects, and without obvious interface between printed layers, which overall result in good mechanical and electrical properties, without any postprocessing steps. The L-PED process enables direct 3D printing of layer-by-layer and complex 3D microscale nt-Cu structures, which may find applications for fabrication of metamaterials, sensors, plasmonics, and micro/nanoelectromechanical systems.

Scalable, hydrophobic and highly-stretchable poly(isocyanurate-urethane) aerogels.

Scalable, low-density and flexible aerogels offer a unique combination of excellent mechanical properties and scalable manufacturability. Herein, we report the fabrication of a family of low-density, ambient-dried and hydrophobic poly(isocyanurate-urethane) aerogels derived from a triisocyanate precursor. The bulk densities ranged from 0.28 to 0.37 g cm-3 with porosities above 70% v/v. The aerogels exhibit a highly stretchable behavior with a rapid increase in the Young's modulus with bulk density (slope of log-log plot > 6.0). In addition, the aerogels are very compressible (more than 80% compressive strain) with high shape recovery rate (more than 80% recovery in 30 s). Under tension even at high strains (e.g., more than 100% tensile strain), the aerogels at lower densities do not display a significant lateral contraction and have a Poisson's ratio of only 0.22. Under dynamic conditions, the properties (e.g., complex moduli and dynamic stress-strain curves) are highly frequency- and rate-dependent, particularly in the Hopkinson pressure bar experiment where in comparison with quasi-static compression results, the properties such as mechanical strength were three orders of magnitude stiffer. The attained outcome of this work supports a basis on the understanding of the fundamental mechanical behavior of a scalable organic aerogel with potential in engineering applications including damping, energy absorption, and substrates for flexible devices.