Low Electronic Resistance Research Articles

Rapid synthesis of Cu2O/CeO2 p–n heterojunctions with enhanced photocatalytic properties by a sonochemical method

AbstractThe construction of p–n heterojunction is critical to reduce the electron–hole recombination rate and broaden the light absorption edge of photocatalysts. In this study, we successfully synthesize the Cu2O/CeO2 p–n heterojunctions through a rapid, simple, and environmentally friendly sonochemical method without any additional oxidants or elevation of the temperature and pressure. The prepared Cu2O/CeO2 heterojunction shows excellent adsorption of light, low electronic transfer resistance, and higher electron transfer efficiency. Under the irradiation of visible light, the as‐prepared Cu2O/CeO2 heterojunction exhibits exceptional capability for the degradation of methylene blue as well as outstanding reusability. The photo‐generated h+ is found to have a significant influence on the photocatalytic antibacterial of the as‐prepared Cu2O/CuO2 heterojunction under visible‐light irradiation and the sterilization rate toward Staphylococcus aureus and Escherichia coli of our samples can achieve up to 100%. Our work provides a green and rapid way to produce Cu2O/CeO2 p–n heterojunctions with enhanced photocatalysis.

Investigation into Oxidation Width of AlGaOx Cladding Layer for Circular Defect in 2D Photonic Crystal Laser

A circular defect in 2D photonic crystal (CirD) lasers is developed, which has the potential to achieve a target communication density of 10 Pbps cm−2 for intrachip optical communications by vertical current injection. The AlGaOx cladding layer's oxidation width strongly influences the device's light confinement and electronic resistance. Herein, samples with different oxidation widths are fabricated and the relationship between threshold power and oxidation width through photoexcitation is revealed. The optimal range of the oxidation width is found to be 349–439 nm, which can ensure low electronic resistance and low threshold power.

Nanosized LiFePO4 Reveals the Electrochemical Kinetics of Electrolytes in Porous Li-Ion Battery Electrodes

Electrified transportation is critical for a decarbonized future, but high cost and range anxiety hamper the adoption of electric vehicles (EV). For low-cost EVs, extreme fast charging of lithium-ion batteries (LIB) is an ideal solution to range anxiety, but this requires significant improvements to rate performance. The measurement of electrochemical kinetics in LIBs is central to understanding and optimizing their rate performance. However, while LIB rate performance is typically limited by the electrolyte, the behaviors of electrolytes during high-rate (dis)charge remain elusive to electrochemical characterization. It is commonly assumed that the effects of ion transport in the electrolyte are negligible in sufficiently thin electrodes (e.g., ≤ 5 μm), allowing measurement of the intrinsic electrochemical kinetics of LIB active materials (AM). On the contrary, we suggest that when the AM is not rate limiting (i.e., with nanosized AMs), electrochemical characterization of LIB electrodes reveals only the kinetics of the electrolyte, making the AM kinetics impossible to measure. In this work, we use LiFePO4 (LFP) as a model system to investigate the fundamental electrochemical kinetics of porous battery electrodes in which the electrolyte is the sole rate limiting component.We first synthesize a nanosized, carbon-coated LFP active material (LFP-C) and fabricate electrodes with both a practical composition (95% AM) and extremely low electronic resistance. Electrochemical impedance spectroscopy shows that charge-transfer resistance is immeasurably small compared to pore resistance, while cyclic voltammetry proves that semi-infinite Li+ diffusion in the electrolyte is the rate limiting process. We develop a novel pseudo-steady-state extrapolation (S3E) technique for quantification of charge-transfer kinetics in porous battery electrodes, which shows that LFP obeys Butler-Volmer kinetics, even at a current density of > 100 mA cm-2. Unexpectedly, we find that the apparent transfer coefficient of LFP-C is ~ 1.5, rather than the commonly assumed value of 0.5. Furthermore, the remarkably low electrode-level exchange current (< 1 mA cm-2) suggests that only a small fraction of particles (~ 1%) are actively reacting, even at extreme rates. These results show that a moving reaction front resulting from rapid interfacial kinetics is the root cause of the widely observed electrode-level reaction heterogeneity of phase-separating AMs. Using LFP three-electrode cells, we conducted rate performance testing on the electrolyte, comparing the conventional LP57 electrolyte to highly conductive 2M LiFSI in acetonitrile (LiFSI-AN). LiFSI-AN achieves up to 70% charge capacity at a 600C rate and 35% discharge capacity at a 3000C rate, exceeding all other reports of LIB rate performance. A novel resistance vs. capacity analysis of the rate performance data enables direct measurement of electrolyte resistance growth, revealing how the complex interaction between a moving reaction front (i.e., increase in conduction path length) and concentration polarization (i.e., decrease in electrolyte conductivity) controls electrolyte rate performance in LFP electrodes.Overall, this work shows that the electrochemical kinetics of nanosized AMs in porous battery electrodes can only be understood by explicitly accounting for the behaviors of the electrolyte. Consequently, the common a priori assumption that the effects of the electrolyte are negligible for electrochemical kinetic measurements in thin electrodes is shown to be, in many cases, fundamentally misleading. Nanosized AMs are, therefore, a powerful tool to investigate the electrochemical kinetics of electrolytes in porous battery electrodes. Particularly, all-LFP three-electrode cells were developed as an ideal platform for rate performance testing of electrolytes, enabling direct measurement of electrolyte resistance growth during high-rate battery operation.

CoP Nanoparticles Fabricated Through the Nanoscale Kirkendall Effect Immobilized in 3D Hollow Carbon Frameworks for Oxygen Evolution Reaction

Transition metal phosphides are promising oxygen evolution reaction (OER) catalysts due to their earth-abundant and cost-effective features. Here, nanostructured CoP nanoparticles locked in hollow nitrogen doped carbon frameworks (CoP@HNC) were successfully designed and characterized for their morphology, composition, and electrochemistry. In a typical low-temperature phosphorization process, the Co species in carbonized poly-dopamine (PDA) coated ZIF-67 are converted to either hollow CoP or small-sized solid CoP nanoparticles governed by the nanoscale Kirkendall effect. The PDA layers derived nitrogen-doped carbon components feature a hollow polyhedral structure, with CoP nanoparticles imbedded in the shell. CoP@HNC demonstrates a low overpotential of 327 mV for 10 mA cm−2 and a good operational stability (72 h) for alkaline OER. The HNC encapsulation affords the low electronic resistance between CoP nanoparticles, as well as the mechanical and chemical stability of composites by preventing the aggregation of CoP nanoparticles during the OER process.

Two Competing Reactions of Sulfurized Polyacrylonitrile Produce High-Performance Lithium-Sulfur Batteries.

Sulfurized polyacrylonitrile (SPAN) is an attractive cathode candidate for the advanced lithium-sulfur (Li-S) batteries owing to its outstanding cyclic stability. Nevertheless, SPAN suffers from inadequate initial Coulombic efficiency (CE) induced by the sluggish reaction kinetics, which is primarily ascribed to the low Li-ion diffusivity and high electronic resistivity of the Li2S product. In this work, an optimal trace amount of soluble lithium polysulfide of Li2S8 is introduced as a redox mediator for a freestanding fibrous SPAN cathode to enhance the reversible oxidation efficiency of Li2S. During the delithiation process, the chemical interactions between Li2S and Li2S8 additive facilitate the electrochemical oxidation of Li2S, resulting in the transformation of not only C-S/S-S bonds in SPAN but also elemental sulfur. Benefiting from the synergistic effect of the two competing reactions, a high initial CE of 82.9% could be achieved at a current density of 200 mA g-1. Moreover, a superior capacity retention along with a high capacity of 1170 mAh g-1 up to the 400th cycle is available at 1000 mA g-1. The study offers a feasible approach for Li-S batteries toward the practical applications of SPAN.

In-situ formation of Ni (oxy)hydroxide on Ni foam as an efficient electrocatalyst for oxygen evolution reaction

A binder-free Ni (oxy)hydroxide on Ni foam was prepared through an in-situ electrochemical activation method. Ni (oxy)hydroxide is active for the oxygen evolution reaction. The Ni (oxy)hydroxide directly formed on the surface of Ni foam as a binder-free catalyst not only exhibited large electrochemically active area, but also displayed low interfacial electronic resistance and low charge transfer resistance. Therefore, the optimized Ni (oxy)hydroxide exhibits an overpotential of 288 and 370 mV at 10 and 500 mA cm−2, respectively, in 1.0 M KOH for the oxygen evolution reaction, as well as favorable during 240 h at 100 mA cm−2.

Fabrication of g-C3N4 Nanomesh-Anchored Amorphous NiCoP2O7: Tuned Cycling Life and the Dynamic Behavior of a Hybrid Capacitor

Developing a novel electrode material with better electrochemical behavior and extended cyclability is a major issue in the field of hybrid capacitors. In this work, we propose a novel strategy for the facile synthesis of nickel–cobalt pyrophosphate nanoparticles anchored on graphitic carbon nitride (NiCoP2O7/g-C3N4) via the simple solvothermal method. Field emission scanning electron microscopy and transmission electron microscopy analysis revealed the uniform anchoring of NiCoP2O7 nanocomposite on g-C3N4 nanosheets. Benefitting from the effect of amorphous nature and a conductive matrix of the NiCoP2O7/g-C3N4 (NP3) composite, the material achieves a specific capacitance of 342 F g–1 at a scan rate of 5 mV s–1. Impressively, the electrode shows long-term cycling stability with 100% capacitance retention over 5000 cycles. Employing activated carbon as an anode and as-prepared NP3 as a cathode, the assembled asymmetric hybrid cell exhibits high-energy density and exceptional cyclability (specific capacitance retention over 10 000 cycles). The outstanding electrochemical and cyclic stability is attributed to the shortest electron-ion pathway with effective interfacial interaction. The low electronic resistance of the NiCoP2O7/g-C3N4 nanocomposite is revealed by varying the bias voltage variation in the electrochemical impedance spectroscopy. Our results promise better utilization of the bimetallic pyrophosphate-anchored g-C3N4 matrix as a potential electrode for high-performance energy storage devices.

Electronic and Ionic Conduction Mechanism in Intercalated MOF As Negative Electrode Materials for Energy Storage Devices

A growing interest in electrochemical behavior of metal-organic frameworks (MOFs) has emerged in past years due to their potential candidate in electrode materials for energy storage devices.1 In this presentation, an intercalated MOFs (iMOFs) consisting of p-conjugated dicarboxylate salts, which shows an electrochemically reversible intercalation reaction, is introduced as a novel electrode material for lithium ion batteries2, 3 or lithium ion based hybrid capacitors.4 As shown in Fig. 1a and b, the proposed iMOF electrode material, 2,6-naphthalene dicarboxylate dilithium (2,6-Naph(COOLi)2) as a representative iMOF, has a characteristic organic-inorganic layered crystalline structure based on π-stacked naphthalene packing layers and tetrahedral LiO4 units, respectively, and undergoes a two-electron-transfer Li intercalation reaction (220 mAh g−1) with a flat plateau at a potential of 0.8 V (vs. Li/Li+), which is between the potentials of graphite carbon (0.05 V) and Li4Ti5O12 (1.55 V). This intermediate operating potential in iMOF electrodes can be expect to overcome both the internal short-circuit risk due to Li dendrite in graphite and the low cell voltage in Li4Ti5O12. In addition, the 2,6-Naph(COOLi)2 electrode undergoes a very small change in unit-cell volume (0.33%) while maintaining the framework during Li intercalation.2 We have proposed that the iMOF negative electrodes bring Li-based energy storage devices with new design concepts such as high-voltage bipolar batteries2 and high-energy-density hybrid capacitors.4 Although these materials have been considered as having poor electronic conductivity, the essential mechanisms of electronic conduction responsible for the electrochemical reversibility remain to be elucidated. Here we will introduce the detailed electronic and ionic conduction mechanism leading to such reversible electrochemical properties of the proposed iMOF electrode materials from theoretical and experimental evidence.5 Band structure calculation indicates that electronic conduction occurs in the 2D π-stacking naphthalene layers when the band gap is decreased to 0.99 eV by Li intercalation into the 2,6-Naph(COOLi)2 framework. And electron hopping conduction mechanism determined on the basis of the Marcus theory shows the formation of an anisotropic electron hopping conduction pathway at Li intercalation state. In the I-V response (Fig. 1c) and frequency characteristics of electronic resistances (Fig. 1d) for Li-intercalated 2,6-Naph(COOLi)2 (Li2-2,6-Naph(COOLi)2) prepared using a chemically reductive lithiation agent exhibits current flow and low electronic resistance, respectively,with sufficiently high electronic conductivity, even though it displays insulating characteristics in the pristine state. From temperature dependence of in situ powder XRD patterns of Li2-2,6-Naph(COOLi)2 for temperatures of up to 400°C, the structure exhibiting electronic conductivity remains stable up to 200°C and reverts to an insulating structure, without collapsing at 400°C, offering the potential for a shutdown switch for battery safety during thermal runaway. References (1) Z. Liang, C. Qu, W. Guo, R. Zou and Q. Xu, Adv. Mater., 1702891 (2017). (2) N. Ogihara, T. Yasuda, Y. Kishida, T. Ohsuna, K. Miyamoto and N. Ohba, Angew. Chem. Int. Ed. Engl., 53, 11467 (2014). (3) T. Yasuda and N. Ogihara, Chem. Commun., 50, 11565 (2014). (4) N. Ogihara, Y. Ozawa and O. Hiruta, J. Mater. Chem. A, 4, 3398 (2016). (5) N. Ogihara, N. Ohba and Y. Kishida, Sci. Adv., 3, e1603103 (2017). Figure 1

A comparative study of activated carbon aerogel and commercial activated carbons as electrode materials for organic electric double-layer capacitors

In this study, we thoroughly compared the electrochemical performances of activated carbon aerogel (ACA) and commercial activated carbons (CACs) as electrode materials for organic electric double-layer capacitors (EDLCs) to investigate the feasibility of ACA for commercial applications. The capacitive behavior of carbon materials as EDLC electrodes is strongly influenced by their specific surface areas and electronic resistances, which are determined by several preparation factors, including raw materials and activation methods. Unlike CACs, which have only micropores, ACA has both abundant micropores and mesopores, leading to a high surface area and low density. ACA also shows relatively low electronic resistance because of its three-dimensional structural features. Accordingly, ACA exhibits considerable capacitance at a low charge–discharge rate and excellent performance even at a high charge–discharge rate, indicating that ACA is a promising carbon material for organic EDLC electrodes. Furthermore, this comparison study clearly demonstrates that newly developed carbon materials for commercially advanced EDLC electrodes should exhibit both high specific surface area and low electronic resistance.

Insight into the electronic and mechanical properties of novel TMCrSi ternary silicides from first-principles calculations.

Transition metal silicides (TMSis) are attractive advanced functional materials due to their low electronic resistivity, high melting-point, excellent mechanical properties and thermal stability. However, the overall performances of binary silicides are not satisfactory enough to meet the requirements of many commercial applications. To overcome this problem, utilizing ternary silicide is a good path to adjust the balance between the overall performances because metallic bond plays a key role in electronic properties. The TM-Si bond enhances the strength, while the alloying element (Cr) can effectively improve the oxidation and corrosion resistances. Therefore, we report the results of electronic, mechanical and thermodynamic properties of stable TMCrSi ternary silicides by using first-principles calculations. First, we found that TMCrSi ternary silicides are dynamically stable based on the phonon dispersion curves. Second, the constructed ternary silicides exhibit better electronic properties because of the formation of TM-Cr metallic bond. Importantly, these ternary silicides not only show high strength, but also have better ductility. Additionally, the Debye temperature and heat capacity of TMCrSi ternary silicides are discussed. Finally, through our study, we propose that TMCrSi ternary silicides are promising functional materials with potential applications in aerospace, microelectronic and energy storage systems.

A self-healing conductive and stretchable aligned carbon nanotube/hydrogel composite with a sandwich structure.

Self-healing conductive elastomers have emerged as a class of novel materials that are important for fabricating human-motion sensors, soft robots and healthcare monitoring systems. Herein, we report on a hydrogel of modified poly(γ-glutamic) acid polymer chains crosslinked by coordination complexes, which exhibits good stretchability (1375%), long-term stability (more than 40 days), and self-healing ability (99.0 ± 1.5% in 3 h). Furthermore, a "sandwich" structure composite was fabricated, which is composed of self-healing hydrogels and Au nanograin-decorated aligned multiwall carbon nanotube sheets. It possesses fast self-healing ability, a low stable electronic resistance of 10 ± 1 Ω sq-1, in the temperature range of -40-90 °C, the humidity range of 10-90%, and a stretching range up to 200%.

Resistivity Characterization of Solid Electrolyte Interphase on Lithium Ion Battery Silicon Anodes

Lithium-ion battery (LIB) technology plays a critical role the clean energy sector, with applications in electronic devices, electric and hybrid vehicles, and stationary energy storage. In order to improve battery efficiency and performance, novel materials must be developed and studied. Battery anode materials are of particular interest, and silicon has been selected for materials research due to its high theoretical storage capacity as well as the extensive existing knowledge regarding silicon material processing and fabrication. The initial cycling of a LIB, regardless of anode material, results in the formation of a solid electrolyte interphase (SEI) layer between the electrolyte and the anode. This passivating layer, consisting of decomposition products from the electrolyte solution, is critical to the reliability and performance of the battery. The SEI must be both electronically insulating and ionically conductive: permeable to lithium ions yet resistant to electron flow. The resistance can be inhomogeneous along the anode plane or perpendicular to it. To our knowledge, no characterization technique has been capable of measuring spatial variation in SEI resistivity. In order to measure SEI resistivity with nanometer-scale resolution, our group has utilized scanning spreading resistance microscopy (SSRM) with a specialized atomic force microscopy (AFM) system installed in an argon glove box to minimize sample exposure to oxygen and water. The setup employs a doped diamond coated silicon probe that exhibits high wear resistance and low electronic resistance. Through application of a sample-probe bias voltage while utilizing variable forces on the probe in AFM contact mode, the technique allows for the measurement of SEI resistivity laterally and vertically. The vertical depth of measurement is controllable by the applied probe force (as well as raster scan line density) and is thus capable of milling away SEI material to measure resistivity at defined depths. Subsequent AFM height measurement allows for measured resistance to be associated with the depth of measurement. The electronic system attached to the system measures current through the probe-sample with a logarithm-scale amplifier, which can be directly converted to resistance or resistivity. Our measurements of resistance vs. depth for SEIs formed on single-crystal silicon wafers [001] after a single cycle of lithiation/delithiation demonstrate strong trends for decrease in resistance as the probe penetrates deeper levels of the SEI. Such plots provide an interesting basis for the study of SEIs formed under different cycling conditions with distinct electrolyte solutions. Figure: (a) Schematic diagram of electronic setup of the described system, (b) resistance map at a depth of 43 nm on a SEI formed on a single-crystal silicon wafer [001], with a single cycle of lithiation (0.01 V for 5 hours) and delithiation (1.0 V for 5 hours) in a battery cell containing ethylene carbonate (EC) and diethyl carbonate(DEC) electrolytes, (c) topography map displaying the SEI depth of resistance mapping. Figure 1

Energy-Efficient and Environmentally Friendly Solid Oxide Membrane Electrolysis Process for Magnesium Oxide Reduction: Experiment and Modeling

This paper reports a solid oxide membrane (SOM) electrolysis experiment using an LSM(La0.8Sr0.2MnO3-δ)-Inconel inert anode current collector for production of magnesium and oxygen directly from magnesium oxide at 1423 K (1150 °C). The electrochemical performance of the SOM cell was evaluated by means of various electrochemical techniques including electrochemical impedance spectroscopy, potentiodynamic scan, and electrolysis. Electronic transference numbers of the flux were measured to assess the magnesium dissolution in the flux during SOM electrolysis. The effects of magnesium solubility in the flux on the current efficiency and the SOM stability during electrolysis are discussed. An inverse correlation between the electronic transference number of the flux and the current efficiency of the SOM electrolysis was observed. Based on the experimental results, a new equivalent circuit of the SOM electrolysis process is presented. A general electrochemical polarization model of SOM process for magnesium and oxygen gas production is developed, and the maximum allowable applied potential to avoid zirconia dissociation is calculated as well. The modeling results suggest that a high electronic resistance of the flux and a relatively low electronic resistance of SOM are required to achieve membrane stability, high current efficiency, and high production rates of magnesium and oxygen.

Fabrication of gas diffusion layer (GDL) containing microporous layer using flourinated ethylene prophylene (FEP) for proton exchange membrane fuel cell (PEMFC)

Proton exchange membrane fuel cell (PEMFC), also known as polymer electrolyte membrane fuel cell, is the type of fuel cell being developed for transport applications as well as for stationary and portable applications. Typical PEMFC consists of several essential components, and gas diffusion layer is considered to be one of the most important components. Gas diffusion layer (GDL) in PEMFC works as a protective layer for the delicate catalyst structure, provides good mechanical strength and easy gas access to the catalyst, and improves the electrical conductivity. Consequently, GDL of PEMFC should have low electronic resistivity, specific surface to enhance good electronic contact and proper hydrophobicity for each application. The properties of the conventional GDL’s can be improved by implementing microporous layer instead of common thick porous support layer. Microporous layer (MPL) reduces the contact resistance between catalyst layer and macroporous substrate. The main purpose of using MPL is the water management, as they provide effective wicking of water from the cathode catalyst layer into the diffusion media, thus improving the overall performance of the PEMFC. In this study, the fabrication of GDL containing microporous layer (MPL) made from the slurry of FEP (Flourinated Ethylene Prophylene) mixed with carbon black was investigated in detail. The properties of FEP, such as low friction and nonreactivity, are same as those of PTFE; however FEP has few advantageous points than PTFE. FEP can be formed into desired shapes more easily; its softer than PTFE (melting temperature of FEP is 260°C); it is highly transparent and resistant to sunlight. The microstructure of the developed GDL containing MPL was analyzed by SEM/EDX by observing the surface and cross section of the specimen. The properties such as viscosity, electric conductivity, density, porosity were measured. The amount of carbon loading to get optimum thickness was defined, and the performance of the fuel cell using developed GDL with MPL was evaluated.

High performance of catalysts supported by directly grown PTFE-free micro-porous CNT layer in a proton exchange membrane fuel cell

A proton exchange membrane fuel cell (PEMFC) uses a solid polymer electrolyte, viz. Nafion®, sandwiched between the two electrodes. Nafion® not only plays the role as an electronic insulator and gas barrier but also allows rapid proton transport and supports high current densities. In order to maintain the high proton conductivity of Nafion®, humidified H2 and O2 are passed through the two electrodes. However, water gets easily condensed in the electrodes. This process, called water-flooding, degrades the performance of PEMFC. Hence, a hydrophobic agent, viz. polytetrafluoroethylene (PTFE), is normally incorporated into the electrodes to prevent this phenomenon. Since it is electrically insulating, the incorporation of PTFE increases the internal resistance of the fuel cell. In this study, we successfully demonstrate a PEMFC with catalyst layer comprising of low loading of platinum nanoparticles (0.05 mg cm−2) supported by a directly grown micro-porous carbon nanotube (CNT) layer without incorporation of PTFE, (Pt/MPL-CNT). This cell performs well without exhibiting water-flooding. A commercial electrode, the catalyst layer of which was supported by a conventional micro-porous layer of carbon black mixed with 30 w.t.% PTFE, was used as a reference (Pt/PTFE-MPL-CB). In the single cell tests, PEMFCs with 0.05 mg cm−2Pt/MPL-CNT and 0.25 mg cm−2Pt/PTFE-MPL-CB were used at the cathodes. These cells yielded maximum power densities of 902 mW cm−2 and 824 mW cm−2, respectively, at 70 °C when operated with H2/O2. Notably, the Pt-loading of Pt/MPL-CNT cell is one-fifth of that of Pt/PTFE-MPL-CB, but the former still outperforms the latter. It is shown that the directly grown micro-porous CNT layer has low electronic resistance and is intrinsically hydrophobic, which are the properties responsible for the high performance obtained here.

Multilayered Nanoarchitecture of Graphene Nanosheets and Polypyrrole Nanowires for High Performance Supercapacitor Electrodes

A novel nanoarchitecture is developed by combining the nanostructured conductive polymer polypyrrole with highly electrically conductive graphene nanosheets in a multilayered configuration to achieve high specific capacitance and low electronic resistance for supercapacitor electrode applications. The fibrous network of polypyrrole nanowires with high electrolyte ionic accessibility was interspersed with electrically conductive monolayers of highly aligned large sized graphene nanosheets as a series of current collectors within the macroscopic configuration for enhanced electronic charge transport inside the electrode. A fabrication method relying on capillary force driven self-assembly coupled with the strong van der Waals attraction between highly aromatic graphene basal plane and π conjugated conductive polymer chains was employed to create a 100% binder free multilayered composite structure of these two distinct nanoscale elements to construct the electrode. This multilayer composite electrode exhibit...

Screening-theory-based description of the metallic behavior inSi∕SiGetwo-dimensional electron systems

We calculate the temperature, density, and parallel magnetic field dependence of low temperature electronic resistivity in 2D high-mobility Si/SiGe quantum structures, assuming the conductivity limiting mechanism to be carrier scattering by screened random charged Coulombic impurity centers. We obtain comprehensive agreement with existing experimental transport data, compellingly establishing that the observed 2D metallic behavior in low-density Si/SiGe systems arises from the peculiar nature of 2D screening of long-range impurity disorder. In particular, our theory correctly predicts the experimentally observed metallic temperature dependence of 2D resistivity in the fully spin-polarized system.

Synthesis and properties of cross-linkable macromers from the selective substitution of poly(oxyalkylene)-amines and cyanuric chloride

Reactive macromers, consisting of hydrophilic poly(oxyethylene) and hydrophobic poly(oxypropylene) blocks and alternating chloro-triazine linking groups, were prepared from the tri-functional cyanuric chloride and poly(oxyalkylene)-diamines (POA-amines) in a selective manner. Macromers with multiple reactive chlorides could be structurally tailored by varying the ratios of amine/cyanuric chloride, types of the starting POA-amines and temperature (lower than 30 °C). At an elevated temperature (>140 °C), the macromers could be self-cured or cured with additional amine to form polymer films. The films exhibited high thermal stability, solvent-adsorption, and low surface electronic resistivity. The facile synthesis of cross-linkable macromers provides a new synthetic approach for preparing hydrophilic antistatic films with high electrostatic dissipating (<10 5 Ω/sq) property.

Charged Impurity-Scattering-Limited Low-Temperature Resistivity of Low-Density Silicon Inversion Layers

We calculate within the Boltzmann equation approach the charged impurity scattering limited low temperature electronic resistivity of low density $n$-type inversion layers in Si MOSFET structures. We find a rather sharp quantum to classical crossover in the transport behavior in the $0 - 5$K temperature range, with the low density, low temperature mobility showing a strikingly strong non-monotonic temperature dependence, which may qualitatively explain the recently observed anomalously strong temperature dependent resistivity in low-density, high-mobility MOSFETs.

Multifunctional transition metal complexes: Information transfer at the molecular level

Recent investigations from our laboratory have described compelling experimental evidence to the effect that polyacetylenes operate as extremely effective molecular-scale wires for conducting electronic charge between redox-active terminals. The unusually low electronic resistivity of polyacetylenic bridges is derived from their relatively accessible HOMOs and LUMOs, which facilitate electron and hole tunnelling over long distances, and because of the excellent electronic coupling that occurs between adjacent carbon atoms, these being in very close proximity. In order to prevent direct participation of the acetylenic bridge in triplet energy-transfer processes or in light-induced electron-transfer reactions, it is prudent to restrict the conjugation length of the bridge to less than five ethynylene groups. We now consider various synthetic strategies for the engineering of such molecular systems that retain the favorable electronic properties of a polyacetylenic bridge but that include a relay or insulator in the bridging moiety. A convenient way to construct such systems is to use a Pt II bis-acetylide as the spacer that separates terminal metal oligopyridine complexes. In this case, the central Pt II complex becomes an insulator. By careful design of the system, this insulatory behavior can be exploited as a means by which to introduce directionality and selectivity into the system, and we demonstrate such effects by using polycyclic hydrocarbons and metalloporphyrins as the photoactive terminals. Similar effects can be obtained with polycyclic hydrocarbons built into the acetylenic wire and, in such cases, the energetics of the bridge can be tuned over an inordinately wide range by varying the extent of conjugation inherent to the aromatic nucleus. A special case is identified in which the polycycle itself possesses vacant coordination sites since the energy of the bridge can be further tuned by external complexation of adventitious cations. In order to provide for an energy gradient along the molecular axis, we have devised a versatile synthetic strategy for attaching different types of ligand to the terminals. This approach also facilitates both extension of the molecular axis and alteration of the molecular shape. The photophysical and electrochemical properties have been recorded for all the molecular systems reported herein and used as a simple experimental means by which to quantify the extent of electronic communication along the molecular axis. For mixed-metal or mixed-ligand systems, rates of intramolecular energy or electron transfer have been measured. In most cases, these rates are extremely fast and testify to the remarkable electronic coupling properties of this family of compounds. Finally, some consideration is given to the preparation of third-generation systems.