Uniform-thickness electrospun nanofiber mat production system based on real-time thickness measurement.

Electrospinning is a simple versatile process used to produce nanofibers and collect them as a nanofiber mat. However, due to bending instability, electrospinning often produces a nanofiber mat with non-uniform mat thickness. In this study, we developed a uniform-thickness electrospun nanofiber mat (UTEN) production system with a movable collector based on real-time thickness measurement and thickness feedback control. This system is compatible with a collector with void regions such as a mesh-type collector, two-parallel-metal-plate collector, and ring-type collector, which facilitates the measurement of light transmittance across the produced nanofiber mat during electrospinning. A real-time measurement system was developed to measure light transmittance and convert it to the thickness of the nanofiber mat in real time using the Beer-Lambert law. Thickness feedback control was achieved by repeating the following sequences: (1) finding an optimal position of the movable collector based on the measured thickness of the nanofiber mat, (2) shifting the collector to an optimal position, and (3) performing electrospinning for a given time step. We found that the suggested thickness feedback control algorithm could significantly decrease the non-uniformity of the nanofiber mat by reducing the standard deviation by more than 8 and 3 times for the numerical simulation and experiments, respectively, when compared with the conventional electrospinning. As a pioneering research, this study will contribute to the development of an electrospinning system to produce robust and reliable nanofiber mats in many research and industrial fields such as biomedicine, environment, and energy.

High-Resolution Three-Dimensional Sculpting of Two-Dimensional Graphene Oxide by E-Beam Direct Write.

On-demand switchable "additive/subtractive" patterning of two-dimensional (2D) nanomaterials is an essential capability for developing new concepts of functional nanomaterials and their device realizations. Traditionally, this is performed via a multistep process using photoresist coating and patterning by conventional photo or electron beam lithography, which is followed by bulk dry/wet etching or deposition. This limits the range of functionalities and structural topologies that can be achieved as well as increases the complexity, cost, and possibility of contamination, which are significant barriers to device fabrication from highly sensitive 2D materials. Focused electron beam-induced processing (FEBIP) enables a material chemistry/site-specific, high-resolution multimode atomic scale processing and provides unprecedented opportunities for "direct-write", single-step surface patterning of 2D nanomaterials with an in situ imaging capability. It allows for realizing a rapid multiscale/multimode approach, ranging from an atomic scale manipulation (e.g., via targeted defect introduction as an active site) to a large-area surface modification on nano- and microscales, including patterned doping and material removal/deposition with 2D (in-plane)/three-dimensional (3D) (out-of-plane) control. In this work, we report on a new capability of FEBIP for nanoscale patterning of graphene oxide via removal of oxygenated carbon moieties with no use of reactive gas required for etching complemented by carbon atom deposition using a focused electron beam. The mechanism of experimentally observed phenomena is explored using the density functional theory (DFT) calculations, revealing that interactions of e-beam that liberated reactive oxygen radicals with carbon atoms on the graphene basal plane lead to the creation of atomic vacancies in the material. The reaction byproducts are volatile carbon dioxides, which are dissociated and volatilized from the graphene oxide surface functional groups by interactions with an energetic focused electron beam. Along with selective subtractive patterning of graphene oxide, the same electron beam with increased irradiation doses can deposit out-of-plane 3D carbon nanostructures on top of or around the 2D etched pattern, thus forming a hybrid 2D/3D nanocomposite with a feature control down to a few nanometers. This in operando dual nanofabrication capability of FEBIP is unmatched by any other nanopatterning techniques and opens a new design window for forming 2D/3D complex nanostructures and functional nanodevices.

Direct Write of 3D Nanoscale Mesh Objects with Platinum Precursor via Focused Helium Ion Beam Induced Deposition.

The next generation optical, electronic, biological, and sensing devices as well as platforms will inevitably extend their architecture into the 3rd dimension to enhance functionality. In focused ion beam induced deposition (FIBID), a helium gas field ion source can be used with an organometallic precursor gas to fabricate nanoscale structures in 3D with high-precision and smaller critical dimensions than focused electron beam induced deposition (FEBID), traditional liquid metal source FIBID, or other additive manufacturing technology. In this work, we report the effect of beam current, dwell time, and pixel pitch on the resultant segment and angle growth for nanoscale 3D mesh objects. We note subtle beam heating effects, which impact the segment angle and the feature size. Additionally, we investigate the competition of material deposition and sputtering during the 3D FIBID process, with helium ion microscopy experiments and Monte Carlo simulations. Our results show complex 3D mesh structures measuring ~300 nm in the largest dimension, with individual features as small as 16 nm at full width half maximum (FWHM). These assemblies can be completed in minutes, with the underlying fabrication technology compatible with existing lithographic techniques, suggesting a higher-throughput pathway to integrating FIBID with established nanofabrication techniques.

Direct matter disassembly via electron beam control: electron-beam-mediated catalytic etching of graphene by nanoparticles.

We report electron-beam activated motion of a catalytic nanoparticle along a graphene step edge and associated etching of the edge. The catalytic hydrogenation process was observed to be activated by a combination of elevated temperature and electron beam irradiation. Reduction of beam fluence on the particle was sufficient to stop the process, leading to the ability to switch on and off the etching. Such an approach enables the targeting of individual nanoparticles to induce motion and beam-controlled etching of matter through activated electrocatalytic processes. The applications of electron-beam control as a paradigm for molecular-scale robotics are discussed.

Publisher Correction: Reply to: On the ferroelectricity of CH3NH3PbI3 perovskites.

An amendment to this paper has been published and can be accessed via a link at the top of the paper.

High Resolution Multimodal Chemical Imaging Platform for Organics and Inorganics.

Chemical analysis at the nanoscale is critical to advance our understanding of materials and systems from medicine and biology to material science and computing. Macroscale-observed phenomena in these systems are in the large part driven by processes that take place at the nanoscale and are highly heterogeneous. Therefore, there is a clear need to develop a new technology that enables correlative imaging of material functionalities with nanoscale spatial and chemical resolutions that will enable us to untangle the structure-function relationship of functional materials. Therefore, here, we report on the analytical figures of merit of the newly developed correlative chemical imaging technique of helium ion microscopy coupled with secondary ion mass spectrometry (HIM-SIMS) that enables multimodal topographical/chemical imaging of organic and inorganic materials at the nanoscale. In HIM-SIMS, a focused ion beam acts as a sputtering and ionization source for chemical analysis along with simultaneous high-resolution surface imaging, providing an unprecedented level of spatial resolution for gathering chemical information on organic and inorganic materials. In this work, we demonstrate HIM-SIMS as a platform for a next-generation tool for an in situ material design and analysis capable of down to 8 nm spatial resolution chemical imaging, layered metal structure imaging in depth profiling, single graphene layer detection, and spectral analysis of metals, metal oxides, and polymers.

Non-equilibrium adatom thermal state enables rapid additive nanomanufacturing.

A new state of radical thermal non-equilibrium in surface adsorbed molecules is discovered that enables rapid surface diffusion of energized adatoms with a negligible effect on the substrate surface temperature. Due to enhanced surface diffusion, growth rates can be achieved that improve the feasibility of many nanofabrication techniques. Since the adatom temperature cannot be directly measured without disturbing its thermodynamic state, the first principle hard-cube model is used to predict both the adatom effective temperature and the surface temperature in response to gaseous particle impingement in a vacuum. The validity of the approach is supported by local, spatially-resolved surface temperature measurements of the thermal response to supersonic microjet gas impingement. The ability to determine and control the adatom effective temperature, and therefore the surface diffusion rate, opens new degrees of freedom in controlling a wide range of nanofabrication processes that critically depend on surface diffusion of precursor molecules. This fundamental understanding has the potential to accelerate research into nanoscale fabrication and to yield the new materials with unique properties that are only accessible with nanoscale features.

Chemical nature of ferroelastic twin domains in CH3NH3PbI3 perovskite.

The extraordinary optoelectronic performance of hybrid organic-inorganic perovskites has resulted in extensive efforts to unravel their properties. Recently, observations of ferroic twin domains in methylammonium lead triiodide drew significant attention as a possible explanation for the current-voltage hysteretic behaviour in these materials. However, the properties of the twin domains, their local chemistry and the chemical impact on optoelectronic performance remain unclear. Here, using multimodal chemical and functional imaging methods, we unveil the mechanical origin of the twin domain contrast observed with piezoresponse force microscopy in methylammonium lead triiodide. By combining experimental results with first principles simulations we reveal an inherent coupling between ferroelastic twin domains and chemical segregation. These results reveal an interplay of ferroic properties and chemical segregation on the optoelectronic performance of hybrid organic-inorganic perovskites, and offer an exploratory path to improving functional devices.

Building Structures Atom by Atom via Electron Beam Manipulation.

Building materials from the atom up is the pinnacle of materials fabrication. Until recently the only platform that offered single-atom manipulation was scanning tunneling microscopy. Here controlled manipulation and assembly of a few atom structures are demonstrated by bringing together single atoms using a scanning transmission electron microscope. An atomically focused electron beam is used to introduce Si substitutional defects and defect clusters in graphene with spatial control of a few nanometers and enable controlled motion of Si atoms. The Si substitutional defects are then further manipulated to form dimers, trimers, and more complex structures. The dynamics of a beam-induced atomic-scale chemical process is captured in a time-series of images at atomic resolution. These studies suggest that control of the e-beam-induced local processes offers the next step toward atom-by-atom nanofabrication, providing an enabling tool for the study of atomic-scale chemistry in 2D materials and fabrication of predefined structures and defects with atomic specificity.

Engineering the thermal conductivity along an individual silicon nanowire by selective helium ion irradiation.

The ability to engineer the thermal conductivity of materials allows us to control the flow of heat and derive novel functionalities such as thermal rectification, thermal switching and thermal cloaking. While this could be achieved by making use of composites and metamaterials at bulk length-scales, engineering the thermal conductivity at micro- and nano-scale dimensions is considerably more challenging. In this work, we show that the local thermal conductivity along a single Si nanowire can be tuned to a desired value (between crystalline and amorphous limits) with high spatial resolution through selective helium ion irradiation with a well-controlled dose. The underlying mechanism is understood through molecular dynamics simulations and quantitative phonon-defect scattering rate analysis, where the behaviour of thermal conductivity with dose is attributed to the accumulation and agglomeration of scattering centres at lower doses. Beyond a threshold dose, a crystalline-amorphous transition was observed.

Activating "Invisible" Glue: Using Electron Beam for Enhancement of Interfacial Properties of Graphene-Metal Contact.

Interfacial contact of two-dimensional graphene with three-dimensional metal electrodes is crucial to engineering high-performance graphene-based nanodevices with superior performance. Here, we report on the development of a rapid "nanowelding" method for enhancing properties of interface to graphene buried under metal electrodes using a focused electron beam induced deposition (FEBID). High energy electron irradiation activates two-dimensional graphene structure by generation of structural defects at the interface to metal contacts with subsequent strong bonding via FEBID of an atomically thin graphitic interlayer formed by low energy secondary electron-assisted dissociation of entrapped hydrocarbon contaminants. Comprehensive investigation is conducted to demonstrate formation of the FEBID graphitic interlayer and its impact on contact properties of graphene devices achieved via strong electromechanical coupling at graphene-metal interfaces. Reduction of the device electrical resistance by ∼50% at a Dirac point and by ∼30% at the gate voltage far from the Dirac point is obtained with concurrent improvement in thermomechanical reliability of the contact interface. Importantly, the process is rapid and has an excellent insertion potential into a conventional fabrication workflow of graphene-based nanodevices through single-step postprocessing modification of interfacial properties at the buried heterogeneous contact.

Rapid Electron Beam Writing of Topologically Complex 3D Nanostructures Using Liquid Phase Precursor.

Advancement of focused electron beam-induced deposition (FEBID) as a versatile direct-write additive nanoscale fabrication technique has been inhibited by poor throughput, limited choice of precursors, and restrictions on possible 3D topologies. Here, we demonstrate FEBID using nanoelectrospray liquid precursor injection to grow carbon and pure metal nanostructures via direct decomposition and electrochemical reduction of the relevant precursors, achieving growth rates 10(5) times greater than those observed in standard gas-phase FEBID. Initiating growth at the free surface of a liquid pool enables fabrication of complex 3D carbon nanostructures with strong adhesion to the substrate. Deposition of silver microstructures at similar growth rates is also demonstrated as a promising avenue for future development of the technique.

Controlling the physicochemical state of carbon on graphene using focused electron-beam-induced deposition.

Focused electron-beam-induced deposition (FEBID) is a promising nanolithography technique using "direct-write" patterning by carbon line and dot deposits on graphene. Understanding interactions between deposited carbon molecules and graphene enables highly localized modification of graphene properties, which is foundational to the FEBID utility as a nanopatterning tool. In this study, we demonstrate a unique possibility to induce dramatically different adsorption states of FEBID-produced carbon deposits on graphene, through density functional theory calculations and complementary Raman experiments. Specifically, an amorphous carbon deposit formed by direct irradiation of high energy primary electrons exhibits unusually strong interactions with graphene via covalent bonding, whereas the FEBID carbon formed due to low-energy secondary electrons is only weakly interacting with graphene via physisorption. These observations not only are of fundamental importance to basic physical chemistry of FEBID carbon-graphene interactions but also enable the use of selective laser-assisted postdeposition ablation to effectively remove the parasitically deposited, physisorbed carbon films for improving FEBID patterning resolution.

Chemical reduction of individual graphene oxide sheets as revealed by electrostatic force microscopy.

We report continuous monitoring of heterogeneously distributed oxygenated functionalities on the entire surface of the individual graphene oxide flake during the chemical reduction process. The charge densities over the surface with mixed oxidized and graphitic domains were observed for the same flake after a step-by-step chemical reduction process using electrostatic force microscopy. Quantitative analysis revealed heavily oxidized nanoscale domains (50-100 nm across) on the graphene oxide surface and a complex reduction mechanism involving leaching of sharp oxidized asperities from the surface followed by gradual thinning and formation of uniformly mixed oxidized and graphitic domains across the entire flake.

Thermally induced transformations of amorphous carbon nanostructures fabricated by electron beam induced deposition.

We studied the thermally induced phase transformations of electron-beam-induced deposited (EBID) amorphous carbon nanostructures by correlating the changes in its morphology with internal microstructure by using combined atomic force microscopy (AFM) and high resolution confocal Raman microscopy. These carbon deposits can be used to create heterogeneous junctions in electronic devices commonly known as carbon-metal interconnects. We compared two basic shapes of EBID deposits: dots/pillars with widths from 50 to 600 nm and heights from 50 to 500 nm and lines with variable heights from 10 to 150 nm but having a constant length of 6 µm. We observed that during thermal annealing, the nanoscale amorphous deposits go through multistage transformation including dehydration and stress-relaxation around 150 °C, dehydrogenation within 150-300 °C, followed by graphitization (>350 °C) and formation of nanocrystalline, highly densified graphitic deposits around 450 °C. The later stage of transformation occurs well below commonly observed graphitization for bulk carbon (600-800 °C). It was observed that the shape of the deposits contribute significantly to the phase transformations. We suggested that this difference is controlled by different contributions from interfacial footprints area. Moreover, the rate of graphitization was different for deposits of different shapes with the lines showing a much stronger dependence of its structure on the density than the dots.

Maskless and resist-free rapid prototyping of three-dimensional structures through electron beam induced deposition (EBID) of carbon in combination with metal-assisted chemical etching (MaCE) of silicon.

In this work, we introduce a maskless, resist-free rapid prototyping method to fabricate three-dimensional structures using electron beam induced deposition (EBID) of amorphous carbon (aC) from a residual hydrocarbon precursor in combination with metal-assisted chemical etching (MaCE) of silicon. We demonstrate that EBID-made patterned aC coating, with thickness of even a few nanometers, acts as a negative "mask" for the etching process and is sufficient for localized termination of the MaCE of silicon. Optimal aC deposition settings and gold film thickness for fabrication of high-aspect-ratio nanoscale 3D silicon structures are determined. The speed necessary for optimal aC feature deposition is found to be comparable to the writing speed of standard Electron Beam Lithography and the MaCE etching rate is found to be comparable to standard deep reactive ion etching (DRIE) rate.

Three-dimensional off-lattice Monte Carlo simulations on a direct relation between experimental process parameters and fractal dimension of colloidal aggregates.

It has been a big challenge to explore a direct relation of experimental parameters such as pH, electrolyte concentration, particle size, and temperature with the final structures of aggregates, because Monte Carlo simulations have been performed on the basis of arbitrarily chosen sticking probability. We attempted to incorporate colloidal theory to Monte Carlo simulations for two model systems of CuO- and SiO(2)-water systems, so as to resolve this difficulty. Conducting three-dimensional off-lattice MC simulations at various pHs for both systems, we investigated effects of pH on fractal structures of aggregates, encompassing the whole aggregation regime from diffusion-limited cluster-cluster aggregation to reaction-limited cluster-cluster aggregation. Moreover, developing a functional analysis, we found an explicit correlation between experimental parameters, sticking probability, and the fractal dimension of aggregates for both systems.

The effect of the geometry and material properties of a carbon joint produced by electron beam induced deposition on the electrical resistance of a multiwalled carbon nanotube-to-metal contact interface.

Multiwall carbon nanotubes (MWNTs) are promising candidates for yielding next generation electrical and electronic devices such as interconnects and tips for conductive force microscopy. One of the main challenges in MWNT implementation in such devices is the high contact resistance of the MWNT-metal electrode interface. Electron beam induced deposition (EBID) of an amorphous carbon interface has previously been demonstrated to simultaneously lower the electrical contact resistance and improve the mechanical characteristics of the MWNT-electrode connection. In this work, we investigate the influence of process parameters, such as the electron beam energy, current, geometry, and deposition time, on the EBID-made carbon joint geometry and electrical contact resistance. The influence of the composition of the deposited material on its resistivity is also investigated. The relative importance of each component of the contact resistance and the limiting factor of the overall electrical resistance of a MWNT-based interconnect is determined through a combination of a model analysis and comprehensive experiments.