Diagnosing Nanoelectronic Components Using Coherent Electrons

We present the direct observation, using off-axis electron holography (EH), of the electric potential distribution in the vicinity of a single carbon nanotube electrically biased by two closely spaced contacts. When our results are combined with finite element modeling, we demonstrate the ability to separately observe the electrostatic potential drops across the metal contacts at the interface with the nanotube and along the length of the nanotube itself. We demonstrate that the uneven resistivity of different contacts can cause an asymmetric EH phase shift, which can readily be identified and quantified. EH thus offers a unique and precise approach for in-depth understanding and quick diagnosis of many similar nanoscale electronic devices

Highly Transparent and Flexible Nanopaper Transistors

Renewable and clean “green” electronics based on paper substrates is an emerging field with intensifying research and commercial interests, as the technology combines the unique properties of flexibility, cost efficiency, recyclability, and renewability with the lightweight nature of paper. Because of its excellent optical transmittance and low surface roughness, nanopaper can host many types of electronics that are not possible on regular paper. However, there can be tremendous challenges with integrating devices on nanopaper due to its shape stability during processing. Here we demonstrate for the first time that flexible organic field-effect transistors (OFETs) with high transparency can be fabricated on tailored nanopapers. Useful electrical characteristics and an excellent mechanical flexibility were observed. It is believed that the large binding energy between polymer dielectric and cellulose nanopaper, and the effective stress release from the fibrous substrate promote these beneficial properties. Only a 10% decrease in mobility was observed when the nanopaper transistors were bent and folded. The nanopaper transistor also showed excellent optical transmittance up to 83.5%. The device configuration can transform many semiconductor materials for use in flexible green electronics

Ammonium Laurate Surfactant for Cleaner Deposition of Carbon Nanotubes

Experiments probing the properties of individual carbon nanotubes (CNTs) and those measuring bulk composites show vastly different results. One major issue limiting the results is that the procedures required to separate and test CNTs introduce contamination that changes the properties of the CNT. These contamination residues often come from the resist used in lithographic processing and the surfactant used to suspend and deposit the CNTs, commonly sodium dodecyl sulfate (SDS). Here we present ammonium laurate (AL), a surfactant that has previously not been used for this application, which differs from SDS only by substitution of ionic constituents but shows vastly cleaner depositions. In addition, we show that compared to SDS, AL-suspended CNTs have greater shelf stability and more selective dispersion. These results are verified using transmission electron microscopy, atomic force microscopy, ζ-potential measurements, and Raman and absorption optical spectroscopy. This surfactant is simple to prepare, and the nanotube solutions require minimal sonication and centrifugation in order to outperform SDS

Hoop-Strong Nanotubes for Battery Electrodes

The engineering of hollow nanostructures is a promising approach to addressing instabilities in silicon-based electrodes for lithium-ion batteries. Previous studies showed that a SiO_<i>x</i> coating on silicon nanotubes (SiNTs) could function as a constraining layer and enhance capacity retention in electrodes with low mass loading, but we show here that similarly produced electrodes having negligible SiO_<i>x</i> coating and significantly higher mass loading show relatively low capacity retention, fading quickly within the early cycles. We find that the SiNT performance can still be enhanced, even in electrodes with high mass loading, by the use of Ni functional coatings on the outer surface, leading to greatly enhanced capacity retention in a manner that could scale better to industrially relevant battery capacities. <i>In situ</i> transmission electron microscopy studies reveal that the Ni coatings suppress the Si wall from expanding outward, instead carrying the large hoop stress and forcing the Si to expand inward toward the hollow inner core. Evidence shows that these controlled volume changes in Ni-coated SiNTs, accompanied by the electrochemically inert nature of Ni coatings, unlike SiO_<i>x</i>, may enhance the stability of the electrolyte at the outer surface against forming a thick solid electrolyte interphase (SEI) layer. These results provide useful guidelines for designing nanostructured silicon electrodes for viable lithium-ion battery applications

Hoop-Strong Nanotubes for Battery Electrodes

The engineering of hollow nanostructures is a promising approach to addressing instabilities in silicon-based electrodes for lithium-ion batteries. Previous studies showed that a SiO_<i>x</i> coating on silicon nanotubes (SiNTs) could function as a constraining layer and enhance capacity retention in electrodes with low mass loading, but we show here that similarly produced electrodes having negligible SiO_<i>x</i> coating and significantly higher mass loading show relatively low capacity retention, fading quickly within the early cycles. We find that the SiNT performance can still be enhanced, even in electrodes with high mass loading, by the use of Ni functional coatings on the outer surface, leading to greatly enhanced capacity retention in a manner that could scale better to industrially relevant battery capacities. <i>In situ</i> transmission electron microscopy studies reveal that the Ni coatings suppress the Si wall from expanding outward, instead carrying the large hoop stress and forcing the Si to expand inward toward the hollow inner core. Evidence shows that these controlled volume changes in Ni-coated SiNTs, accompanied by the electrochemically inert nature of Ni coatings, unlike SiO_<i>x</i>, may enhance the stability of the electrolyte at the outer surface against forming a thick solid electrolyte interphase (SEI) layer. These results provide useful guidelines for designing nanostructured silicon electrodes for viable lithium-ion battery applications

Lithium-Assisted Electrochemical Welding in Silicon Nanowire Battery Electrodes

From in situ transmission electron microscopy (TEM) observations, we present direct evidence of lithium-assisted welding between physically contacted silicon nanowires (SiNWs) induced by electrochemical lithiation and delithiation. This electrochemical weld between two SiNWs demonstrates facile transport of lithium ions and electrons across the interface. From our in situ observations, we estimate the shear strength of the welded region after delithiation to be approximately 200 MPa, indicating that a strong bond is formed at the junction of two SiNWs. This welding phenomenon could help address the issue of capacity fade in nanostructured silicon battery electrodes, which is typically caused by fracture and detachment of active materials from the current collector. The process could provide for more robust battery performance either through self-healing of fractured components that remain in contact or through the formation of a multiconnected network architecture

A Beaded-String Silicon Anode

Interfacial instability is a fundamental issue in heterostructures ranging from biomaterials to joint replacement and electronic packaging. This challenge is particularly intriguing for lithium ion battery anodes comprising silicon as the ion storage material, where ultrahigh capacity is accompanied by vast mechanical stress that threatens delamination of silicon from the current collectors at the other side of the interface. Here, we describe Si-beaded carbon nanotube (CNT) strings whose interface is controlled by chemical functionalization, producing separated amorphous Si beads threaded along mechanically robust and electrically conductive CNT. <i>In situ</i> transmission electron microscopy combined with atomic and continuum modeling reveal that the chemically tailored Si–C interface plays important roles in constraining the Si beads, such that they exhibit a symmetric “radial breathing” around the CNT string, remaining crack-free and electrically connected throughout lithiation–delithiation cycling. These findings provide fundamental insights in controlling nanostructured interfaces to effectively respond to demanding environments such as lithium batteries

Electrolyte Stability Determines Scaling Limits for Solid-State 3D Li Ion Batteries

Rechargeable, all-solid-state Li ion batteries (LIBs) with high specific capacity and small footprint are highly desirable to power an emerging class of miniature, autonomous microsystems that operate without a hardwire for power or communications. A variety of three-dimensional (3D) LIB architectures that maximize areal energy density has been proposed to address this need. The success of all of these designs depends on an ultrathin, conformal electrolyte layer to electrically isolate the anode and cathode while allowing Li ions to pass through. However, we find that a substantial reduction in the electrolyte thickness, into the nanometer regime, can lead to rapid self-discharge of the battery even when the electrolyte layer is conformal and pinhole free. We demonstrate this by fabricating individual, solid-state nanowire core–multishell LIBs (NWLIBs) and cycling these inside a transmission electron microscope. For nanobatteries with the thinnest electrolyte, ≈110 nm, we observe rapid self-discharge, along with void formation at the electrode/electrolyte interface, indicating electrical and chemical breakdown. With electrolyte thickness increased to 180 nm, the self-discharge rate is reduced substantially, and the NWLIBs maintain a potential above 2 V for over 2 h. Analysis of the nanobatteries’ electrical characteristics reveals space-charge limited electronic conduction, which effectively shorts the anode and cathode electrodes directly through the electrolyte. Our study illustrates that, at these nanoscale dimensions, the increased electric field can lead to large electronic current in the electrolyte, effectively shorting the battery. The scaling of this phenomenon provides useful guidelines for the future design of 3D LIBs