William F. Stickle

Pyrolysis of cellulose under ammonia leads to nitrogen-doped nanoporous carbon generated through methane formation.

Here, we present a simple one-step fabrication methodology for nitrogen-doped (N-doped) nanoporous carbon membranes via annealing cellulose filter paper under NH3. We found that nitrogen doping (up to 10.3 at %) occurs during cellulose pyrolysis under NH3 at as low as 550 °C. At 700 °C or above, N-doped carbon further reacts with NH3, resulting in a large surface area (up to 1973.3 m(2)/g). We discovered that the doped nitrogen, in fact, plays an important role in the reaction, leading to carbon gasification. CH4 was experimentally detected by mass spectrometry as a product in the reaction between N-doped carbon and NH3. When compared to conventional activated carbon (1533.6 m(2)/g), the N-doped nanoporous carbon (1326.5 m(2)/g) exhibits more than double the unit area capacitance (90 vs 41 mF/m(2)).

Superior Cathode of Sodium-Ion Batteries: Orthorhombic V2O5 Nanoparticles Generated in Nanoporous Carbon by Ambient Hydrolysis Deposition

For the first time, we demonstrate that orthorhombic V2O5 can exhibit superior electrochemical performance in sodium ion batteries when uniformly coated inside nanoporous carbon. The encapsulated V2O5 shows a specific capacity as high as 276 mAh/g, while the whole nanocomposite exhibits a capacity of 170 mAh/g. The V2O5/C composite was fabricated by a novel ambient hydrolysis deposition that features sequential water vapor adsorption in nanoporous carbon, followed by a hydrolysis reaction, exclusively inside the nanopores. The unique structure of the nanocomposite significantly enhances the capacity as well as the rate performance of orthorhombic V2O5 where the composite retains a capacity of over 90 mAh/g at a current rate of 640 mA/g. Furthermore, by calculating, we also revealed that a large portion of the sodium-ion storage, particularly at high current rates, is due to the V2O5 pseudocapacitance.

Ionic/Electronic Hybrid Materials Integrated in a Synaptic Transistor with Signal Processing and Learning Functions

2010 WILEY-VCH Verlag Gmb Signal processing, memory, and learning functions are established in the human brain by modifying ionic fluxes in neurons and synapses. Through a synapse, a potential spike signal in a presynaptic neuron can trigger an ionic excitatory postsynaptic current (EPSC) or inhibitory postsynaptic current (IPSC) that temporally lasts for 1–10ms in a postsynaptic neuron. This enables the postsynaptic neuron to collectively process the EPSC or IPSC through 10–10 synapses to establish spatial and temporal correlated functions. The synaptic transmission efficacy can be modified by temporally correlated preand post-synaptic spikes via spike-timing-dependent plasticity (STDP). For example, if a postsynaptic spike is triggered momentarily after a presynaptic spike by a few milliseconds, the synaptic efficacy will be increased, resulting in long-term potentiation (LTP), but if the temporal order is reversed, the synaptic efficacy will be decreased, resulting in long-term depression (LTD). The synaptic efficacy can also be modified with reversed polarities in STDP in different types of synapses. STDP is essential to modify synapses in a neural network for learning and memory functions of the brain. Electronic materials, devices, and circuits have been explored extensively to emulate synapses, but to date they have not been able to match the synaptic functions in the brain. Synaptic transistors with nonvolatile analog memory were fabricated by integrating a charge-storage or ferroelectric materials onto the gate structure of Si metal-oxide-semiconductor (MOS) transistors, but these devices cannot emulate the essential synaptic dynamic functions such as EPSC/IPSC or STDP. Electronic neuromorphic circuits have been designed and fabricated to supply EPSC/IPSC and STDP, but these nonlinear dynamic analog circuits require many transistors and several capacitors to emulate a single synapse. The large capacitor size, complex architecture, and energy consumption of these synaptic circuits limited the number of synapses that could be integrated onto a single chip to about 10–10. The lack of a small, cheap device with the essential synaptic dynamic properties for signal processing, learning, and memory prohibits the circuits from approaching the scale and functions of the human brain that contains 10 synapses. We have designed and fabricated a synaptic transistor based on ionic/electronic hybrid materials by integrating a layer of ionic conductor and a layer of ion-doped conjugated polymer, onto the gate of a Si-based transistor. In analogy to the synapse, a potential spike can trigger ionic fluxes with a temporal lapse of a few milliseconds in the polymer, which in turn spontaneously generates EPSC in the Si layer. Temporally correlated preand post-synaptic spikes can modify ions stored in the polymer, resulting in a nonvolatile strengthening or weakening of the device transmission efficacy with STDP. A single hybrid transistor can replace presently utilized complex and energyconsuming electronic circuits to emulate the synapse for spike signal processing, learning, and memory, which could provide a new pathway to construct neuromorphic circuits approaching the scale and functions of the brain. The synaptic transistor has a Si n-p-n source-channel-drain structure of a conventional MOS transistor, with the Si channel covered by a 3-nm-thick SiO2 insulating layer (Fig. 1a). A 70-nm-thick conjugated polymer layer of poly[2-methoxy-5(20-ethylhexyloxy)-p-phenylene vinylene] (MEH-PPV) and a 70-nm-thick ionic conductive layer of RbAg4I5 were sandwiched between the gate SiO2 insulator and an Al/Ti electrode. To emulate synaptic functions, presynaptic spikes were applied to the transistor gate, and postsynaptic currents, I, were measured from the source. Postsynaptic spikes were also applied to the source. A spike was composed of a 1ms-wide positive voltage pulse with an amplitude Vþ1⁄4 3–5V immediately followed by a 1 ms-wide negative voltage pulse with an amplitude V 1⁄4 3 to 5V (Fig. 1a, Inset). After the spike, the transistor was operated at its rest state under a subthreshold condition by setting the gate voltage Vg1⁄4 0 V. A drain voltage Vd1⁄4 0.1 V was applied continuously. When a presynaptic spike with amplitudes of Vþ/V 1⁄4 4V/ 5V was applied to the transistor gate, the typical I is

Diffusion of Adhesion Layer Metals Controls Nanoscale Memristive Switching

First prominent more than 40 years ago, [ 1 ] electrical resistance switching in conductor/insulator/conductor structures has regained signifi cant attention in the last decade, [ 2–16 ] motivated by the search for alternatives to conventional semiconductor electronics. [ 17 ] Recent results have shown promising device behaviors, such as reversible, non-volatile, fast ( < 10 ns), lowpower ( ∼ 1 pJ/operation) and multiple-state switching, [ 18–26 ]

Comparison of the sputter rates of oxide films relative to the sputter rate of SiO2

There is a growing interest in knowing the sputter rates for a wide variety of oxides because of their increasing technological importance in many different applications. To support the needs of users of the Environmental Molecular Sciences Laboratory, a national scientific user facility, as well as our research programs, the authors made a series of measurements of the sputter rates from oxide films that have been grown by oxygen plasma-assisted molecular beam epitaxy, pulsed laser deposition, atomic layer deposition, electrochemical oxidation, or sputter deposition. The sputter rates for these oxide films were determined in comparison with those from thermally grown SiO2, a common reference material for sputter rate determination. The film thicknesses and densities for most of these oxide films were measured using x-ray reflectivity. These oxide films were mounted in an x-ray photoelectron or Auger electron spectrometer for sputter rate measurements using argon ion sputtering. Although the primary objec...

Chemical and Structural Investigation of High-Resolution Patterning with HafSOx

High-resolution transmission electron microscopy (TEM) imaging and energy-dispersive X-ray spectroscopy (EDS) chemical mapping have been used to examine key processing steps that enable sub-20-nm lithographic patterning of the material Hf(OH)4-2x-2y(O2)x(SO4)y·qH2O (HafSOx). Results reveal that blanket films are smooth and chemically homogeneous. Upon exposure with an electron beam, the films become insoluble in aqueous tetramethylammonium hydroxide [TMAH(aq)]. The mobility of sulfate in the exposed films, however, remains high, because it is readily exchanged with hydroxide from the TMAH(aq) solution. Annealing the films after soaking in TMAH(aq) results in the formation of a dense hafnium hydroxide oxide material that can be converted to crystalline HfO2 with a high electron-beam dose. A series of 9 nm lines is written with variable spacing to investigate the cross-sectional shape of the patterned lines and the residual material found between them.

Functional Porous Tin Oxide Thin Films Fabricated by Inkjet Printing Process

The highly transparent SnO 2 thin films were deposited through solution-based inkjet printing using a simple precursor solution. We were able to fabricate porous tin oxide thin film that has a thin nanoporous layer on top and a thicker meso- and macroporous layer beneath the top layer. The thin-film transmittance is over 98% in the visible wavelength range. A mechanism based on gas evolution was proposed to explain the formation of porous structure. A depletion-mode thin-film transistor using the porous tin oxide channel layer was fabricated with a field-effect mobility of 3.62 cm 2 /V s.

Analog memory capacitor based on field-configurable ion-doped polymers

A memory capacitor based on a field-configurable ion-doped polymer is reported. The device can be dynamically and reversibly programed to analog capacitances with low-voltage (<5V) pulses. After the device is programed to a specific value, its capacitance remains nonvolatile. The field-configurable capacitance is attributed to the modification of ionic dopant concentrations in the polymer. The memory capacitors might be used for analog memory, nonlinear analog, and neuromorphic circuits.

Glucose Sensing Using Functionalized Amorphous In–Ga–Zn–O Field-Effect Transistors

Recent advances in glucose sensing have focused on the integration of sensors into contact lenses to allow noninvasive continuous glucose monitoring. Current technologies focus primarily on enzyme-based electrochemical sensing which requires multiple nontransparent electrodes to be integrated. Herein, we leverage amorphous indium gallium zinc oxide (IGZO) field-effect transistors (FETs), which have found use in a wide range of display applications and can be made fully transparent. Bottom-gated IGZO-FETs can have significant changes in electrical characteristics when the back-channel is exposed to different environments. We have functionalized the back-channel of IGZO-FETs with aminosilane groups that are cross-linked to glucose oxidase and have demonstrated that these devices have high sensitivity to changes in glucose concentrations. Glucose sensing occurs through the decrease in pH during glucose oxidation, which modulates the positive charge of the aminosilane groups attached to the IGZO surface. The change in charge affects the number of acceptor-like surface states which can deplete electron density in the n-type IGZO semiconductor. Increasing glucose concentrations leads to an increase in acceptor states and a decrease in drain-source conductance due to a positive shift in the turn-on voltage. The functionalized IGZO-FET devices are effective in minimizing detection of interfering compounds including acetaminophen and ascorbic acid. These studies suggest that IGZO FETs can be effective for monitoring glucose concentrations in a variety of environments, including those where fully transparent sensing elements may be of interest.

Ambient hydrolysis deposition of TiO2 in nanoporous carbon and the converted TiN–carbon capacitive electrode

Despite the considerable advances of deposition technologies, formation of conformal deposition on the surface of nanoporous carbons remains a significant challenge. Here, we introduce a new ambient hydrolysis deposition method that employs and controls pre-adsorbed water vapor on nanoporous carbons to define the deposition of TiO2. We converted the deposited TiO2 into TiN via a nitridation process. The metallic-TiN-coated porous carbon exhibits superior kinetic performance as an electrode in electrical double layer capacitors. The novel deposition method provides a general solution for surface engineering on nanostructured carbons, which may result in a strong impact on the fields of energy storage and other disciplines.