Dependence of Single-Wall Carbon Nanotube Alignment on the Filter Membrane Interface in Slow Vacuum Filtration.

The recent introduction of slow vacuum filtration (SVF) technology has shown great promise for reproducibly creating high-quality, large-area aligned films of single-wall carbon nanotubes (SWCNTs) from solution-based dispersions. Despite clear advantages over other SWCNT alignment techniques, SVF remains in the developmental stages due to a lack of an agreed-upon alignment mechanism, a hurdle which hinders SVF optimization. In this work, the filter membrane surface is modified to show how the resulting SWCNT nematic order can be significantly enhanced. It is observed that directional mechanical grooving on filter membranes does not play a significant role in SWCNT alignment, despite the tendency for nanotubes to follow the groove direction. Chemical treatments to the filter membrane are shown to increase SWCNT alignment by nearly 1/3. These findings suggest that membrane surface structure acts to create a directional flow along the filter membrane surface that can produce global SWCNT alignment during SVF, rather serving as an alignment template.

Comparative Operando XPS and SEM Spatiotemporal Potential Mapping of Ionic Liquid Polarization in a Coplanar Electrochemical Device.

The polarization response of a coplanar electrochemical capacitor covered with an ionic liquid as the electrolyte has been examined using a combination of two powerful analytic techniques, X-ray photoelectron spectroscopy (XPS) and scanning electron microcopy (SEM). Spatiotemporal distribution of the ionic liquid surface potential, upon DC or AC (square wave) biasing, has been monitored via chemical element binding energy shifts using XPS and secondary electron intensity variations using SEM. SEM's high spatial resolution and speedy imaging together with application of a data mining algorithm made mapping of the surface potential distribution across the capacitor possible. Interestingly, despite the differences in the detection principles, both techniques yield similar polarization relaxation time constants. The results demonstrate the power of a synergistic combination of the two techniques with complementary capabilities and pave the way to a deeper understanding of liquid/solid interfaces and for performance evaluation and diagnostics of electrochemical devices.

Nanoscale Mapping of the Double Layer Potential at the Graphene-Electrolyte Interface.

The electrical double layer (EDL) governs the operation of multiple electrochemical devices, determines reaction potentials, and conditions ion transport through cellular membranes in living organisms. The few existing methods of EDL probing have low spatial resolution, usually only providing spatially averaged information. On the other hand, traditional Kelvin probe force microscopy (KPFM) is capable of mapping potential with nanoscale lateral resolution but cannot be used in electrolytes with concentrations higher than several mmol/L. Here, we resolve this experimental impediment by combining KPFM with graphene-capped electrolytic cells to quantitatively measure the potential drop across the EDL in aqueous electrolytes of decimolar and molar concentrations with a high lateral resolution. The surface potential of graphene in contact with deionized water and 0.1 mol/L solutions of CuSO4 and MgSO4 as a function of counter electrode voltage is reported. The measurements are supported by numerical modeling to reveal the role of the graphene membrane in potential screening and to determine the EDL potential drop. The proposed approach proves to be especially useful for imaging spatially inhomogeneous systems, such as nanoparticles submerged in an electrolyte solution. It could be suitable for in operando and in vivo measurements of the potential drop in the EDL on the surfaces of nanocatalysts and biological cells in equilibrium with liquid solutions.

From Microparticles to Nanowires and Back: Radical Transformations in Plated Li Metal Morphology Revealed via in Situ Scanning Electron Microscopy.

Li metal is the preferred anode material for all-solid-state Li batteries. However, a stable plating and stripping of Li metal at the anode-solid electrolyte interface remains a significant challenge particularly at practically feasible current densities. This problem usually relates to high and/or inhomogeneous Li-electrode-electrolyte interfacial impedance and formation and growth of high-aspect-ratio dendritic Li deposits at the electrode-electrolyte interface, which eventually shunt the battery. To better understand details of Li metal plating, we use operando electron microscopy and Auger spectroscopy to probe nucleation, growth, and stripping of Li metal during cycling of a model solid-state Li battery as a function of current density and oxygen pressure. We find a linear correlation between the nucleation density of Li clusters and the charging rate in an ultrahigh vacuum, which agrees with a classical nucleation and growth model. Moreover, the trace amount of oxidizing gas (≈10-6 Pa of O2) promotes the Li growth in a form of nanowires due to a fine balance between the ion current density and a growth rate of a thin lithium-oxide shell on the surface of the metallic Li. Interestingly, increasing the partial pressure of O2 to 10-5 Pa resumes Li plating in a form of 3D particles. Our results demonstrate the importance of trace amounts of preexisting or ambient oxidizing species on lithiation processes in solid-state batteries.

Enabling Photoemission Electron Microscopy in Liquids via Graphene-Capped Microchannel Arrays.

Photoelectron emission microscopy (PEEM) is a powerful tool to spectroscopically image dynamic surface processes at the nanoscale, but it is traditionally limited to ultrahigh or moderate vacuum conditions. Here, we develop a novel graphene-capped multichannel array sample platform that extends the capabilities of photoelectron spectromicroscopy to routine liquid and atmospheric pressure studies with standard PEEM setups. Using this platform, we show that graphene has only a minor influence on the electronic structure of water in the first few layers and thus will allow for the examination of minimally perturbed aqueous-phase interfacial dynamics. Analogous to microarray screening technology in biomedical research, our platform is highly suitable for applications in tandem with large-scale data mining, pattern recognition, and combinatorial methods for spectro-temporal and spatiotemporal analyses at solid-liquid interfaces. Applying Bayesian linear unmixing algorithm to X-ray induced water radiolysis process, we were able to discriminate between different radiolysis scenarios and observe a metastable "wetting" intermediate water layer during the late stages of bubble formation.

Interfacial Electrochemistry in Liquids Probed with Photoemission Electron Microscopy.

Studies of the electrified solid-liquid interfaces are crucial for understanding biological and electrochemical systems. Until recently, use of photoemission electron microscopy (PEEM) for such purposes has been hampered by incompatibility of the liquid samples with ultrahigh vacuum environment of the electron optics and detector. Here we demonstrate that the use of ultrathin electron transparent graphene membranes, which can sustain large pressure differentials and act as a working electrode, makes it possible to probe electrochemical reactions in operando in liquid environments with PEEM.

Imaging and Analysis of Encapsulated Objects through Self-Assembled Electron and Optically Transparent Graphene Oxide Membranes.

We demonstrate a technique for facile encapsulation and adhesion of micro- and nano objects on arbitrary substrates, stencils, and micro structured surfaces by ultrathin graphene oxide membranes via a simple drop casting of graphene oxide solution. A self-assembled encapsulating membrane forms during the drying process at the liquid-air and liquid-solid interfaces and consists of a water-permeable quasi-2D network of overlapping graphene oxide flakes. Upon drying and interlocking between the flakes, the encapsulating coating around the object becomes mechanically robust, chemically protective, and yet highly transparent to electrons and photons in a wide energy range, enabling microscopic and spectroscopic access to encapsulated objects. The characteristic encapsulation scenarios were demonstrated on a set of representative inorganic and organic micro and nano-objects and microstructured surfaces. Different coating regimes can be achieved by controlling the pH of the supporting solution, and the hydrophobicity and morphology of interfaces. Several specific phenomena such as compression of encased objects by contracting membranes as well as hierarchical encapsulations were observed. Finally, electron as well as optical microscopy and analysis of encapsulated objects along with the membrane effect on the image contrast formation, and signal attenuation are discussed.

Design and Application of Variable Temperature Setup for Scanning Electron Microscopy in Gases and Liquids at Ambient Conditions.

Scanning electron microscopy (SEM) of nanoscale objects in dry and fully hydrated conditions at different temperatures is of critical importance in revealing details of their interactions with an ambient environment. Currently available WETSEM capsules are equipped with thin electron-transparent membranes and allow imaging of samples at atmospheric pressure, but do not provide temperature control over the sample. Here, we developed and tested a thermoelectric cooling/heating setup for WETSEM capsules to allow ambient pressure in situ SEM studies with a temperature range between -15 and 100°C in gaseous, liquid, and frozen conditions. The design of the setup also allows for correlation of the SEM with optical microscopy and spectroscopy. As a demonstration of the possibilities of the developed approach, we performed real-time in situ microscopy studies of water condensation on a surface of Morpho sulkowskyi butterfly wing scales. We observed that initial water nucleation takes place on top of the scale ridges. These results confirmed earlier discovery of a preexisting polarity gradient of the ridges of Morpho butterflies. Our developed thermoelectric cooling/heating setup for environmental capsules meets the diverse needs for in situ nanocharacterization in material science, catalysis, microelectronics, chemistry, and biology.

Heat dissipation from suspended self-heated nanowires: gas sensor prospective.

The strong temperature dependence of the electrical conductivity in semiconductors was employed for gas and pressure sensing with a self-heated Si nanowire resistor. The electrical conductivity in such a device depends on heat dissipation and partitioning inside the device and was studied comparatively for suspended and supported device architectures. The appearance of the exhaustion region in the temperature-dependent resistivity of a Joule-heated nanowire was used as a temperature marker for implementation of the quasi-constant temperature operation mode. At low pressures, the sensor is idle due to dominant heat dissipation from the nanowire to the substrate and/or electrodes. Above ca. 10 Torr the sensitivity to gases has a strong dependence on pressure as well as on the type of gas and is determined by conductive heat transfer between the nanowire surface and ambient. This implies that, in contrast to macroscopic devices, the heat dissipation via the convection mechanism does not contribute significantly to the heat transfer from the self-heated nanowire. The thermal sensitivity of the sensor to reactive gases depends on the effectiveness of the particular endothermic/exothermic reaction at the surface of the nanowire and was explored for the case of acetone-air mixture. The strong coupling of the electrical and thermal properties in the individual Joule-heated semiconducting nanowire allows fabrication of power-efficient multi-parametric nanoscopic gas/pressure sensors that are analogs of Pirani and pellistor type detectors.

Doping-based stabilization of the M2 phase in free-standing VO2 nanostructures at room temperature.

A new high-yield method of doping VO(2) nanostructures with aluminum is proposed, which renders possible stabilization of the monoclinic M2 phase in free-standing nanoplatelets in ambient conditions and opens an opportunity for realization of a purely electronic Mott transition field-effect transistor without an accompanying structural transition. The synthesized free-standing M2-phase nanostructures are shown to have very high crystallinity and an extremely sharp temperature-driven metal-insulator transition. A combination of X-ray microdiffraction, micro-Raman spectroscopy, energy-dispersive X-ray spectroscopy, and four-probe electrical measurements allowed thorough characterization of the doped nanostructures. Light is shed onto some aspects of the nanostructure growth, and the temperature-doping level phase diagram is established.

Electron transparent graphene windows for environmental scanning electron microscopy in liquids and dense gases.

Due to its ultrahigh electron transmissivity in a wide electron energy range, molecular impermeability, high electrical conductivity and excellent mechanical stiffness, suspended graphene membranes appear to be a nearly ideal window material for in situ (in vivo) environmental electron microscopy of nano- and mesoscopic objects (including bio-medical samples) immersed in liquids and/or in dense gaseous media. In this paper, taking advantage of a small modification of the graphene transfer protocol onto metallic and SiN supporting orifices, reusable environmental cells with exchangeable graphene windows have been designed. Using colloidal gold nanoparticles (50 nm) dispersed in water as model objects for scanning electron microscopy in liquids as proof of concept, different conditions for imaging through the graphene membrane were tested. Limiting factors for electron microscopy in liquids, such as electron beam induced water radiolysis and damage of the graphene membrane at high electron doses, are discussed.

Electromechanical actuation and current-induced metastable states in suspended single-crystalline VO2 nanoplatelets.

Current-induced electromechanical actuation enabled by the metal-insulator transition in VO(2) nanoplatelets is demonstrated. The Joule heating by a sufficient current flowing through suspended nanoplatelets results in formation of heterophase domain patterns and is accompanied by nanoplatelet deformation. The actuation action can be achieved in a wide temperature range below the bulk phase transition temperature (68 °C). The observed current-sustained heterophase domain structures should be interpreted as distinct metastable states in free-standing and end-clamped VO(2) samples. We analyze the main prerequisites for the realization of a current-controlled actuator based on the proposed concept.

Interplay between ferroelastic and metal-insulator phase transitions in strained quasi-two-dimensional VO2 nanoplatelets.

Formation of ferroelastic twin domains in vanadium dioxide (VO(2)) nanosystems can strongly affect local strain distributions, and hence couple to the strain-controlled metal-insulator transition. Here we report polarized-light optical and scanning microwave microscopy studies of interrelated ferroelastic and metal-insulator transitions in single-crystalline VO(2) quasi-two-dimensional (quasi-2D) nanoplatelets (NPls). In contrast to quasi-1D single-crystalline nanobeams, the 2D geometric frustration results in emergence of several possible families of ferroelastic domains in NPls, thus allowing systematic studies of strain-controlled transitions in the presence of geometrical frustration. We demonstrate the possibility of controlling the ferroelastic domain population by the strength of the NPl-substrate interaction, mechanical stress, and by the NPl lateral size. Ferroelastic domain species and domain walls are identified based on standard group-theoretical considerations. Using variable temperature microscopy, we imaged the development of domains of metallic and semiconducting phases during the metal-insulator phase transition and nontrivial strain-driven reentrant domain formation. A long-range reconstruction of ferroelastic structures accommodating metal-insulator domain formation has been observed. These studies illustrate that a complete picture of the phase transitions in single-crystalline and disordered VO(2) structures can be drawn only if both ferroelastic and metal-insulator strain effects are taken into consideration and understood.

Addressable graphene encapsulation of wet specimens on a chip for optical, electron, infrared and X-ray based spectromicroscopy studies.

Label-free spectromicroscopy methods offer the capability to examine complex cellular phenomena. Electron and X-ray based spectromicroscopy methods, though powerful, have been hard to implement with hydrated objects due to the vacuum incompatibility of the samples and due to the parasitic signals from (or drastic attenuation by) the liquid matrix surrounding the biological object of interest. Similarly, for many techniques that operate at ambient pressure, such as Fourier transform infrared spectromicroscopy (FTIRM), the aqueous environment imposes severe limitations due to the strong absorption of liquid water in the infrared regime. Here we propose a microfabricated multi-compartmental and reusable hydrated sample platform suitable for use with several analytical techniques, which employs the conformal encapsulation of biological specimens by a few layers of atomically thin graphene. Such an electron, X-ray, and infrared transparent, molecularly impermeable and mechanically robust enclosure preserves the hydrated environment around the object for a sufficient time to allow in situ examination of hydrated bio-objects with techniques operating under both ambient and high vacuum conditions. An additional hydration source, provided by hydrogel pads lithographically patterned in the liquid state near/around the specimen and co-encapsulated, has been added to further extend the hydration lifetime. Note that the in-liquid lithographic electron beam-induced gelation procedure allows for addressable capture and immobilization of the biological cells from the solution. Scanning electron microscopy and optical fluorescence microscopy, as well as synchrotron radiation based FTIR and X-ray fluorescence microscopy, have been used to test the applicability of the platform and for its validation with yeast, A549 human carcinoma lung cells and micropatterned gels as biological object phantoms.

Combining secondary ion mass spectrometry image depth profiling and single particle inductively coupled plasma mass spectrometry to investigate the uptake and biodistribution of gold nanoparticles in Caenorhabditis elegans.

Analytical techniques capable of determining the spatial distribution and quantity (mass and/or particle number) of engineered nanomaterials in organisms are essential for characterizing nano-bio interactions and for nanomaterial risk assessments. Here, we combine the use of dynamic secondary ion mass spectrometry (dynamic SIMS) and single particle inductively coupled mass spectrometry (spICP-MS) techniques to determine the biodistribution and quantity of gold nanoparticles (AuNPs) ingested by Caenorhabditis elegans. We report the application of SIMS in image depth profiling mode for visualizing, identifying, and characterizing the biodistribution of AuNPs ingested by nematodes in both the lateral and z (depth) dimensions. In parallel, conventional- and sp-ICP-MS quantified the mean number of AuNPs within the nematode, ranging from 2 to 36 NPs depending on the size of AuNP. The complementary data from both SIMS image depth profiling and spICP-MS provides a complete view of the uptake, translocation, and size distribution of ingested NPs within Caenorhabditis elegans.

Gas sensor based on metal-insulator transition in VO2 nanowire thermistor.

Using temperature driven sharp metal-insulator phase transition in single crystal VO(2) nanowires, the realization of a novel gas sensing concept has been tested. Varying the temperature of the nanowire close to the transition edge, the conductance of the nanowire becomes extremely responsive to the tiny changes in molecular composition, pressure, and temperature of the ambient gas environment. This gas sensing analog of the transition edge sensor radiometry used in astrophysics opens new opportunities in gas sensorics.

Probing Electrified Liquid-Solid Interfaces with Scanning Electron Microscopy.

Electrical double layers play a key role in a variety of electrochemical systems. The mean free path of secondary electrons in aqueous solutions is on the order of a nanometer, making them suitable for probing ultrathin electrical double layers at solid-liquid electrolyte interfaces. Employing graphene as an electron-transparent electrode in a two-electrode electrochemical system, we show that the secondary electron yield of the graphene-liquid interface depends on the ionic strength and concentration of the electrolyte and the applied bias at the remote counter electrode. These observations have been related to polarization-induced changes in the potential distribution within the electrical double layer and demonstrate the feasibility of using scanning electron microscopy to examine and map electrified liquid-solid interfaces.

Electron and X-ray Focused Beam-Induced Cross-Linking in Liquids: Toward Rapid Continuous 3D Nanoprinting and Interfacing using Soft Materials.

Multiphoton polymer cross-linking evolves as the core process behind high-resolution additive microfabrication with soft materials for implantable/wearable electronics, tissue engineering, microrobotics, biosensing, drug delivery, etc. Electrons and soft X-rays, in principle, can offer even higher resolution and printing rates. However, these powerful lithographic tools are difficult to apply to vacuum incompatible liquid precursor solutions used in continuous additive fabrication. In this work, using biocompatible hydrogel as a model soft material, we demonstrate high-resolution in-liquid polymer cross-linking using scanning electron and X-ray microscopes. The approach augments the existing solid-state electron/X-ray lithography and beam-induced deposition techniques with a wider class of possible chemical reactions, precursors, and functionalities. We discuss the focused beam cross-linking mechanism, the factors affecting the ultimate feature size, and layer-by-layer printing possibilities. The potential of this technology is demonstrated on a few practically important applications such as in-liquid encapsulation of nanoparticles for plasmonic sensing and interfacing of viable cells with hydrogel electrodes.

Functionalizing nanowires with catalytic nanoparticles for gas sensing application.

Metal oxide semiconducting nanowires are among the most promising materials systems for use as conductometric gas sensors. These systems function by converting surface chemical processes, often catalytic processes, into observable conductance variations in the nanowire. The surface properties, and hence the sensing properties of these devices can be altered dramatically improving the sensitivity and selectivity, by the deposition of catalytic metal nanoparticles on the nanowire's surface. This leads not only to promising sensor strategies but to a route for understanding some of the fundamental science occurring on these nanoparticles and at the metal/nanowire junction. In particular studying these systems can lead to a better understanding of the influence of the catalyst particle on the electronic structure of the nanowire and its electron transport. This report surveys results obtained so far in this area. In particular, the comparative sensing performance of single quasi-1D chemiresistors (i.e., nanowires or nanobelts) before and after surface decoration with noble metal catalyst particles show significant improvement in sensitivity toward oxidizing and reducing gases. Moreover, one finds that the sensing mechanism can depend dramatically on the degree of metal coverage of the nanowire.

In-situ Near-Field Probe Microscopy of Plasma Processing.

There exists a great necessity for in situ nanoscale characterization of surfaces and thin films during plasma treatments. To address this need, the current approaches rely on either 'post mortem' sample microscopy, or in situ optical methods. The latter, however, lack the required nanoscale spatial resolution. In this paper, we propose scanning near-field microwave microscopy to monitor plasma-assisted processes with a submicron spatial resolution. In our approach, a plasma environment with an object of interest is separated from the near-field probe and the rest of the microscope by a SiN membrane of a few-10s nm thickness, and the imaging is performed through this membrane. As a proof of concept, we were able to monitor gradual transformations of carbon nanotube films upon plasma-induced oxidation by a low-pressure air plasma. In the implemented approach with the near-field probe in contact with the membrane, the plasma processing should be interrupted during imaging to preserve the membrane integrity. Possible solutions to achieve in situ real-time imaging during plasma conditions are discussed.