Investigating the Electromechanical Sensitivity of Carbon-Nanotube-Coated Microfibers.

The piezoresistance of carbon nanotube (CNT)-coated microfibers is examined using diametric compression. Diverse CNT forest morphologies were studied by changing the CNT length, diameter, and areal density via synthesis time and fiber surface treatment prior to CNT synthesis. Large-diameter (30-60 nm) and relatively low-density CNTs were synthesized on as-received glass fibers. Small-diameter (5-30 nm) and-high density CNTs were synthesized on glass fibers coated with 10 nm of alumina. The CNT length was controlled by adjusting synthesis time. Electromechanical compression was performed by measuring the electrical resistance in the axial direction during diametric compression. Gauge factors exceeding three were measured for small-diameter (<25 µm) coated fibers, corresponding to as much as 35% resistance change per micrometer of compression. The gauge factor for high-density, small-diameter CNT forests was generally greater than those for low-density, large-diameter forests. A finite element simulation shows that the piezoresistive response originates from both the contact resistance and intrinsic resistance of the forest itself. The change in contact and intrinsic resistance are balanced for relatively short CNT forests, while the response is dominated by CNT electrode contact resistance for taller CNT forests. These results are expected to guide the design of piezoresistive flow and tactile sensors.

Surface Plasmon Enhanced Fluorescence Temperature Mapping of Aluminum Nanoparticle Heated by Laser.

Partially aggregated Rhodamine 6G (R6G) dye is used as a lights-on temperature sensor to analyze the spatiotemporal heating of aluminum nanoparticles (Al NPs) embedded within a tetrafluoroethylene, hexafluoropropylene, and vinylidene fluoride (THV) fluoropolymer matrix. The embedded Al NPs were photothermally heated using an IR laser, and the fluorescent intensity of the embedded dye was monitored in real time using an optical microscope. A plasmonic grating substrate enhanced the florescence intensity of the dye while increasing the optical resolution and heating rate of Al NPs. The fluorescence intensity was converted to temperature maps via controlled calibration. The experimental temperature profiles were used to determine the Al NP heat generation rate. Partially aggregated R6G dyes, combined with the optical benefits of a plasmonic grating, offered robust temperature sensing with sub-micron spatial resolution and temperature resolution on the order of 0.2 °C.

Nanoscale surface reactions by laser irradiation of Al nanoparticles on MoO3 flakes.

Surface reactions between heated aluminum nanoparticles (Al NPs) and thin α-MoO3 sheets are investigated. Localized photothermal heating on Al NP clusters is provided by a Raman spectrometer laser, while enhanced heating rates and imaging resolution are enabled by the use of a plasmonic grating substrate. Prominent linear reaction zones extending from Al NPs in the 〈001〉 crystal direction are observed on the surface of the host MoO3 sheets after heating. Raman spectroscopy and x-ray diffraction indicate that α-Al2O3 is generated within these extended reacted regions, while AFM and SEM indicate that the topology of the reaction regions are indistinguishable from the MoO3 host. We hypothesize that these Al2O3 zones are formed by surface diffusion and subsequent sub-surface adsorption of heated Al adatoms along the low-energy 〈001〉 MoO3 direction. Understanding and controlling these reaction mechanisms could lead to enhanced combustion of Al/MoO3 nanothermite systems.

Enhanced fluorescence for in situ temperature mapping of photothermally heated aluminum nanoparticles enabled by a plasmonic grating substrate.

In situ dynamic temperature mapping of photothermally heated aluminum nanoparticles (Al NPs) embedded in a fluoropolymer (THV) is achieved using fluorescent dye (rhodamine 6G). A plasmonic grating substrate enhances the dye fluorescence intensity by a factor of seven over a glass substrate, to enable image capture rates of 500 frames per second. Further, the fluorescence intensity is linearly related to temperature and reversible. Photothermal heating of embedded Al NPs using a 2380 W cm-2 incident flux produced an Al NP heating rate of 1.2 × 104 °C s-1. Localized Al NP motion was also observed and attributed to thermal expansion and melting of the polymer. Multiphysics simulation provided agreement with experimental observations, bolstering confidence in the technique. The plasmonic grating platforms were shown to significantly improve both fluorescence intensity and the photothermal heating of Al compared to glass substrates, opening a new path for fast and high-resolution in situ temperature mapping.

Mechanics of capillary forming of aligned carbon nanotube assemblies.

Elastocapillary self-assembly is emerging as a versatile technique to manufacture three-dimensional (3D) microstructures and complex surface textures from arrangements of micro- and nanoscale filaments. Understanding the mechanics of capillary self-assembly is essential to engineering of properties such as shape-directed actuation, anisotropic wetting and adhesion, and mechanical energy transfer and dissipation. We study elastocapillary self-assembly (herein called "capillary forming") of carbon nanotube (CNT) microstructures, combining in situ optical imaging, micromechanical testing, and finite element modeling. By imaging, we identify sequential stages of liquid infiltration, evaporation, and solid shrinkage, whose kinetics relate to the size and shape of the CNT microstructure. We couple these observations with measurements of the orthotropic elastic moduli of CNT forests to understand how the dynamic of shrinkage of the vapor-liquid interface is coupled to the compression of the forest. We compare the kinetics of shrinkage to the rate of evporation from liquid droplets having the same size and geometry. Moreover, we show that the amount of shrinkage during evaporation is governed by the ability of the CNTs to slip against one another, which can be manipulated by the deposition of thin conformal coatings on the CNTs by atomic layer deposition (ALD). This insight is confirmed by finite element modeling of pairs of CNTs as corrugated beams in contact and highlights the coupled role of elasticity and friction in shrinkage and stability of nanoporous solids. Overall, this study shows that nanoscale porosity can be tailored via the filament density and adhesion at contact points, which is important to the development of lightweight multifunctional materials.

Deformation response of conformally coated carbon nanotube forest.

The deformation mechanism and mechanical properties of carbon nanotube (CNT) forests conformally coated with alumina using atomic layer deposition (ALD) are investigated using in situ and ex situ micro-indentation. While micro-indentation of a CNT forest coated with a thin discontinuous layer using 20 ALD cycles results in a deformation response similar to the response of uncoated CNT forests, a similar test on a CNT forest coated with a sufficiently thick and continuous layer using 100 ALD cycles causes fracture of both the alumina coatings and the core CNTs. With a 10 nm coating, 4-fold and 14-fold stiffness increases are measured using a flat punch and a Berkovich tip, respectively. Indentation testing with the Berkovich tip also reveals increased recoverability at relatively low strains. The results show that ALD coated CNT forests could be useful for applications that require higher stiffness or recoverability. Also, fracturing of the nanotubes shows that upper limits exist in the loading of conformally coated CNT forests.

Spallation of Isolated Aluminum Nanoparticles by Rapid Photothermal Heating.

The spallation of isolated aluminum (Al) nanoparticles (NPs) is initiated using rapid photothermal heating. The Al NPs exhibited a nominal diameter of 120 nm, with an average oxide shell thickness of 3.8 nm. Photothermal heating was achieved by coupling a focused laser (446 nm wavelength) to an optical grating substrate and to the plasmonic resonance of the Al NPs themselves. These factors enhanced the absorption cross section by a factor of 8-18 compared to no substrate and generated an Al NP heating rate on the order of 107-108 K/s. Observations indicate that molten Al is ejected from the heated NP, indicating that melting of the Al core is required for spallation. A graphene layer atop the grating substrate encouraged the formation of discrete particles of ejected Al, while irregular elongated filament products were observed without the graphene layer. Numerical simulations indicate that laser-heated Al NPs reach temperatures between approximately 1000 and 1500 K. These observations and experimental conditions are consistent with those anticipated for the melt dispersion mechanism, a thermomechanical reaction mechanism that has not previously been clearly demonstrated. Activating and controlling this mechanism is anticipated to enhance applications ranging from biological phototherapy to energetic materials.

Growth and Mechanics of Heterogeneous, 3D Carbon Nanotube Forest Microstructures Formed by Sequential Selective-Area Synthesis.

Three-dimensional carbon nanotube (CNT) forest microstructures are synthesized using sequenced, site-specific synthesis techniques. Thin-film layers of Al2O3 and Al2O3/Fe are patterned to support film-catalyst and floating-catalyst chemical vapor deposition (CVD) in specific areas. Al2O3 regions support only floating-catalyst CVD, whereas regions of layered Al2O3/Fe support both film- and floating-catalyst CNT growth. Sequenced application of the two CVD methods produced heterogeneous 3D CNT forest microstructures, including regions of only film-catalyst CNTs, only floating-catalyst CNTs, and vertically stacked layers of each. The compressive mechanical behavior of the heterogeneous CNT forests was evaluated, with the stacked layers exhibiting two distinct buckling plateaus. Finite element simulation of the stacked layers demonstrated that the relatively soft film-catalyst CNT forests were nearly fully buckled prior to large-scale deformation of the bottom floating-catalyst CNT forests.

Delamination Mechanics of Carbon Nanotube Micropillars.

The adhesion of carbon nanotube (CNT) forests to their growth substrate is a critical concern for many applications. Here, we measured the delamination force of CNT forest micropillars using in situ scanning electron microscopy (SEM) tensile testing. A flat tip with epoxy adhesive first established contact with the top surface of freestanding CNT pillars and then pulled the pillars in displacement-controlled tension until delamination was observed. An average delamination stress of 6.1 MPa was measured, based on the full pillar cross-sectional area, and detachment was observed to occur between catalyst particles and the growth substrate. Finite element simulations of CNT forest delamination show that force and strain are heterogeneously distributed among CNTs during tensile loading and that CNTs progressively lose adhesion with increased displacement. Based on combined experiments and simulations, an adhesion strength of approximately 350 MPa was estimated between each CNT and the substrate. These findings provide important insight into CNT applications such as thermal interfaces, mechanical sensors, and structural composites while also suggesting a potential upper limit of tensile forces allowed during CNT forest synthesis.

Hunter-Gatherers Harvested and Heated Microbial Biogenic Iron Oxides to Produce Rock Art Pigment.

Red mineral pigment use is recognized as a fundamental component of a series of traits associated with human evolutionary development, social interaction, and behavioral complexity. Iron-enriched mineral deposits have been collected and prepared as pigment for use in rock art, personal adornment, and mortuary practices for millennia, yet little is known about early developments in mineral processing techniques in North America. Microanalysis of rock art pigments from the North American Pacific Northwest reveals a sophisticated use of iron oxide produced by the biomineralizing bacterium Leptothrix ochracea; a keystone species of chemolithotroph recognized in recent advances in the development of thermostable, colorfast biomaterial pigments. Here we show evidence for human engagement with this bacterium, including nanostructural and magnetic properties evident of thermal enhancement, indicating that controlled use of pyrotechnology was a key feature of how biogenic iron oxides were prepared into paint. Our results demonstrate that hunter-gatherers in this area of study prepared pigments by harvesting aquatic microbial iron mats dominated by iron-oxidizing bacteria, which were subsequently heated in large open hearths at a controlled range of 750 °C to 850 °C. This technical gesture was performed to enhance color properties, and increase colorfastness and resistance to degradation. This skilled production of highly thermostable and long-lasting rock art paint represents a specialized technological innovation. Our results contribute to a growing body of knowledge on historical-ecological resource use practices in the Pacific Northwest during the Late Holocene.Figshare link to figures: https://figshare.com/s/9392a0081632c20e9484.

Isolating the Roles of Hydrogen Exposure and Trace Carbon Contamination on the Formation of Active Catalyst Populations for Carbon Nanotube Growth.

Limited understanding of the factors influencing the yield of carbon nanotubes (CNTs) relative to the number of catalyst particles remains an important barrier to their large-scale production with high quality, and to tailoring CNT properties for applications. This lack of understanding is evident in the frequent use of Edisonian approaches to give high-yield CNT growth, and in the sometimes-confusing influence of trace residues on the reactor walls. In order to create conditions wherein CNT yield is reproducible and to enable large-scale and reliable CNT synthesis, it is imperative to understand-fundamentally-how these common practices impact catalytic activity and thus CNT number density. Herein, we use ambient pressure-X-ray photoelectron spectroscopy (AP-XPS) to reveal the influence of carbon and hydrogen on the coupling between catalyst reduction and CNT nucleation, from an iron catalyst film. We observe a positive correlation between the degree of catalyst reduction and the density of vertically aligned CNTs (forests), verifying that effective catalyst reduction is critical to CNT nucleation and to the resulting CNT growth yield. We demonstrate that the extent of catalyst reduction is the reason for low CNT number density and for lack of self-organization, lift-off, and growth of CNT forests. We also show that hydrocarbon byproducts from consecutive growths can facilitate catalyst reduction and increase CNT number density significantly. These findings suggest that common practices used in the field-such as reactor preconditioning-aid in the reduction of the catalyst population, thus improving CNT number density and enabling the growth of dense forests. Our results also motivate future work using AP-XPS and complementary metrology tools to optimize CNT growth conditions according to the catalyst chemical state.

In Situ Characterization of Photothermal Nanoenergetic Combustion on a Plasmonic Microchip.

Plasmonic gratings facilitate a robust in situ diagnostic platform for photothermal combustion of nanoenergetic composite thin films using an optical microscope and a high-speed camera. Aluminum nanoparticles (Al NPs) embedded in a fluoropolymer oxidizer are cast onto a plasmonic grating microchip and ignited using a low-power laser. The plasmonic grating enhances both spatial resolution and sufficient photothermal coupling to combust small Al NP clusters, initiating localized flames as small as 600 nm in size. Two-color pyrometry obtained from a high-speed color camera indicates an average flame temperature of 3900 K. Scattering measurements using polarized light microscopy enabled precise identification of individual Al NPs over a large field of view, leading to 3D reconstruction of combustion events.

Assemblies of carbon nanotubes and unencapsulated sub-10-nm gold nanoparticles.

The development of assemblies consisting of unencapsulated, sub-10-nm gold particles attached to individual carbon nanotubes (CNTs) with diameters of 2 nm is described. The assemblies are formed on the surface of a porous anodic alumina (PAA) template on which the CNTs (single- or double-walled) are grown by plasma-enhanced chemical vapor deposition. The Au nanoparticles are formed through an indirect evaporation technique using a silicon nitride membrane mask, and diffuse along the PAA surface into the regions containing CNTs. The nanoparticles bind relatively strongly to the CNTs, as indicated by observations of nanoparticles that are suspended over pores or that move along with the CNTs. This approach may provide a new method to functionalize CNTs for chemical or biological sensing and fundamental studies of nanoscale contacts to CNTs.

Precision Milling of Carbon Nanotube Forests Using Low Pressure Scanning Electron Microscopy.

A nanoscale fabrication technique appropriate for milling carbon nanotube (CNT) forests is described. The technique utilizes an environmental scanning electron microscope (ESEM) operating with a low pressure water vapor ambient. In this technique, a portion of the electron beam interacts with the water vapor in the vicinity of the CNT sample, dissociating the water molecules into hydroxyl radicals and other species by radiolysis. The remainder of the electron beam interacts with the CNT forest sample, making it susceptible to oxidation from the chemical products of radiolysis. This technique may be used to trim a selected region of an individual CNT, or it may be used to remove hundreds of cubic microns of material by adjusting ESEM parameters. The machining resolution is similar to the imaging resolution of the ESEM itself. The technique produces only small quantities of carbon residue along the boundaries of the cutting zone, with minimal effect on the native structural morphology of the CNT forest.

A high areal capacity lithium-sulfur battery cathode prepared by site-selective vapor infiltration of hierarchical carbon nanotube arrays.

The widespread use of melt infiltration has to date restricted sulfur-carbon cathode architectures to only host materials processed as bulk powders with no site control of sulfur deposits. Here, we combine structurally designed hierarchical carbon nanotube (CNT) arrays with site-selective vapor phase sulfur infiltration to produce thick electrodes with controlled sulfur loading and high areal performance. Our results illustrate the critical role structural hierarchy plays in sustaining electrical connectivity to enable high utilization of the sulfur embedded in thick electrodes with high gravimetric loading. Here, a primary large-diameter CNT population provides robust conductive trunks that branch into a secondary small-diameter and high-surface-area CNT population capable of giving rapid electrical access to coated sulfur. Site-selective vapor phase sulfur infiltration, based on the capillary effect, controllably targets loading of one or both of the CNT populations to facilitate gravimetric loading from 60 wt% to 70 wt% sulfur. With the high areal loading of 6 mg cm-2, we demonstrate 1092 mA h gS-1 and 6.5 mA h cm-2 and excellent rate performance with >60% capacity retained at 10 times the discharge rate. Overall, our work leverages site control of sulfur incorporation into a host cathode enabled by controlled CNT growth techniques to emphasize the important principle of "quality over quantity" in designing high areal loading strategies with high areal performance and good sulfur utilization.

Bioinspired Programmable Polymer Gel Controlled by Swellable Guest Medium.

Responsive materials with functions of forming three-dimensional (3D) origami and/or kirigami structures have a broad range of applications in bioelectronics, metamaterials, microrobotics, and microelectromechanical (MEMS) systems. To realize such functions, building blocks of actuating components usually possess localized inhomogeneity so that they respond differently to external stimuli. Previous fabrication strategies lie in localizing nonswellable or less-swellable guest components in their swellable host polymers to reduce swelling ability. Herein, inspired by ice plant seed capsules, we report an opposite strategy of implanting swellable guest medium inside nonswellable host polymers to locally enhance the swelling inhomogeneity. Specifically, we adopted a skinning effect induced surface polymerization combined with direct laser writing to control gradient of swellable cyclopentanone (CP) in both vertical and lateral directions of the nonswellable SU-8. For the first time, the laser direct writing was used as a novel strategy for patterning programmable polymer gel films. Upon stimulation of organic solvents, the dual-gradient gel films designed by origami or kirigami principles exhibit reversible 3D shape transformation. Molecular dynamics (MD) simulation illustrates that CP greatly enhances diffusion rates of stimulus solvent molecules in the SU-8 matrix, which offers the driving force for the programmable response. Furthermore, this bioinspired strategy offers unique capabilities in fabricating responsive devices such as a soft gripper and a locomotive robot, paving new routes to many other responsive polymers.

Dendrimer-templated Fe nanoparticles for the growth of single-wall carbon nanotubes by plasma-enhanced CVD.

A fourth-generation (G4) poly(amidoamine) (PAMAM) dendrimer (G4-NH2) has been used as a template to deliver nearly monodispersed catalyst nanoparticles to SiO2/Si, Ti/Si, sapphire, and porous anodic alumina (PAA) substrates. Fe2O3 nanoparticles obtained after calcination of the immobilized Fe3+/G4-NH2 composite served as catalytic "seeds" for the growth of single-wall carbon nanotubes (SWNTs) by microwave plasma-enhanced CVD (PECVD). To surmount the difficulty associated with SWNT growth via PECVD, reaction conditions that promote the stabilization of Fe nanoparticles, resulting in enhanced SWNT selectivity and quality, have been identified. In particular, in situ annealing of Fe catalyst in an N2 atmosphere was found to improve SWNT selectivity and quality. H2 prereduction at 900 degrees C for 5 min was also found to enhance SWNT selectivity and quality for SiO2/Si supported catalyst, albeit of lower quality for sapphire supported catalyst. The application of positive dc bias voltage (+200 V) during SWNT growth was shown to be very effective in removing amorphous carbon impurities while enhancing graphitization, SWNT selectivity, and vertical alignment. The results of this study should promote the use of exposed Fe nanoparticles supported on different substrates for the growth of high-quality SWNTs by PECVD.

Dendrimer-assisted low-temperature growth of carbon nanotubes by plasma-enhanced chemical vapor deposition.

Using a shielded growth approach and N2-annealed, nearly monodispersed Fe2O3 nanoparticles synthesized by interdendritic stabilization of Fe3+ species within fourth-generation poly(amidoamine) dendrimers, carbon nanotubes and nanofibers were successfully grown at low substrate temperatures (200-400 degrees C) by microwave plasma-enhanced chemical vapor deposition.

Electrically Conductive Hierarchical Carbon Nanotube Networks with Tunable Mechanical Response.

Small diameter carbon nanotube (CNTs) are synthesized directly from a parent CNT forest using a floating catalyst chemical vapor deposition (CVD) method. To support a new CNT generation from an existing forest, an alumina coating was applied to the CNT forest using atomic layer deposition (ALD). The new generation of small diameter CNTs (8 nm average) surround the first generation, filling the interstitial regions. The hierarchical forests exhibit a 5-10-fold increase in stiffness, and the two generations are electrically addressable in spite of the interfacial alumina layer between them. This work enables the design of complex CNT architectures with hierarchical features that bring tailored properties such as high specific surface area and robust mechanical properties that can benefit a range of applications.

Bioinspired carbon nanotube fuzzy fiber hair sensor for air-flow detection.

Artificial hair sensors consisting of a piezoresistive carbon-nanotube-coated glass fiber embedded in a microcapillary are assembled and characterized. Individual sensors resemble a hair plug that may be integrated in a wide range of host materials. The sensors demonstrate an air-flow detection threshold of less than 1 m/s with a piezoresistive sensitivity of 1.3% per m/s air-flow change.