The formation of the Schottky barrier height (SBH) is a complex problem because of the dependence of the SBH on the atomic structure of the metal-semiconductor (MS) interface. Existing models of the SBH are too simple to realistically treat the chemistry exhibited at MS interfaces. This article points out, through examination of available experimental and theoretical results, that a comprehensive, quantum-mechanics-based picture of SBH formation can already be constructed, although no simple equations can emerge, which are applicable for all MS interfaces. Important concepts and principles in physics and chemistry that govern the formation of the SBH are described in detail, from which the experimental and theoretical results for individual MS interfaces can be understood. Strategies used and results obtained from recent investigations to systematically modify the SBH are also examined from the perspective of the physical and chemical principles of the MS interface.

The physics and chemistry of the Schottky barrier height

Conjugated polymers combine the interesting optical and electrical properties of metals with the processing advantages and mechanical properties of traditional synthetic polymers. With clever use of a variety of synthetic tools, researchers have prepared highly pure polymers with optimized physical properties during the past 30 years. For example, the synthesis of well-defined polyacetylenes, polyphenylenes, polythiophenes, polyfluorenes, and other conjugated polymers have significantly improved the performance of these polymeric materials. However, one important class of conjugated polymers was missing from this chemical inventory: easy access to well-defined poly(2,7-carbazole)s and related polymers. This Account highlights advances in the synthesis of poly(2,7-carbazole) derivatives since they were first reported in 2001. Starting from 2-nitro-biphenyl derivatives, 2,7-functionalized carbazoles are typically obtained from Cadogan ring-closure reactions. In a second step, Yamamoto, Stille, Suzuki, or Ho...

Poly(2,7-carbazole)s: structure-property relationships.

Lithium-ion batteries have witnessed meteoric advancement the last two decades. The anode area has seen unprecedented research activity on Si and Sn, the two anode alternatives to currently used carbon following the initial seminal work by Fuji on tin oxide nanocomposites. Recent reports on silicon nanowires, porous Si, and amorphous Si coatings on graphite nanofibers (GNF) have been very encouraging. High capacity and long cycle life anodes are still, however, elusive and much needed to meet the ever increasing energy storage demands of modern society. Herein, we report for the first time the synthesis of novel 1D heterostructures comprising vertically aligned multiwall CNTs (VACNTs) containing nanoscale amorphous/nanocrystalline Si droplets deposited directly on VACNTs with clearly defined spacing using a simple two-step liquid injection CVD process. A hallmark of these single reactor derived heterostructures is an interfacial amorphous carbon layer anchoring the nanoscale Si clusters directly to the VACNTs. The defined spacing of nanoscale Si combined with their tethered CNT architecture allow for the silicon to undergo reversible electrochemical alloying and dealloying with Li with minimal loss of contact with the underlying CNTs. The novel heterostructures thus exhibit impressive reversible stable capacities approximately 2050 mAh/g with very good rate capability and an acceptable first cycle irreversible loss approximately 20% comparable to graphitic anodes indicating their promise as high capacity Li-ion anodes. Although warranting further research, particularly with regard to long-term cycling, it can be envisaged that optimization of this simple approach could lead to reversible high capacity next generation Li-ion anodes.

http://ma.ecsdl.org/content/MA2010-01/5/352.full.pdf

Nanostructured Hybrid Silicon/Carbon Nanotube Heterostructures: Novel Reversible High-Capacity Lithium-Ion Anodes

The existence of reversible resistance switching (RS) behaviors induced by electric stimulus has been known for some time, and these intriguing physical phenomena have been observed in numerous materials, including oxides. As conventional charge-based random access memory is expected to face a size limit in the near future, a surge of renewed interest has been developed in RS phenomena for possible applications in small nonvolatile memory devices called resistance random access memory (RRAM). Of particular interest is unipolar RS, which shows the RS at two values of applied voltage of the same polarity. The unipolar RS exhibits a much larger resistance change than other RS phenomena, and this greatly simplifies the process of reading the memory state. When fabricated with oxide p-n diodes, memory cells using unipolar RS can be stacked vertically, which has the potential for dramatically increasing memory density. Therefore, unipolar RRAM may be a good candidate for multi-stacked, high density, nonvolatile memory. The most important scientific and technical issues concerning unipolar RS are how it works and the identification of its controlling parameters. Some studies have reported that unipolar RS comes from a homogeneous/inhomogeneous transition of current distribution, while others maintain that it comes from the formation and rupture of conducting filaments. Even with recent extensive studies on unipolar RS, its basic origin is still far from being understood. In addition, no model exists that actually explains how the reversible switching can occur at two values of applied voltage. This lack of a quantitative model poses a major barrier for unipolar RRAM applications. In this study, we describe RS behavior in polycrystalline TiO2 film. To explain the basic mechanism of unipolar RS behavior, we propose a new percolation model based on a network of ‘‘circuit breakers’’ with two switchable metastable states. The random circuit breaker (RCB) network model can explain the long-standing material issue of how unipolar RS occurs. This simple percolation model is different from the conventional percolation models, which have dealt only with static or irreversible dynamic processes. In addition, the RCB network model provides an indication of how to overcome the substantial distribution of switching voltages, which is currently considered the most serious obstacle to practical unipolar RRAM applications. The unipolar RS phenomenon can be explained by the current (I)-voltage (V) curves in Figure 1a, which are derived from measurements of our polycrystalline TiO2 thin capacitors. At the pristine state (green dot), they are in an insulating state. As the external voltage Vext increases from zero and reaches a threshold voltage Vforming, a sudden increase occurs in the current. If the current is not limited to a certain value, here called the compliance current Icomp, the TiO2 capacitor would experience a dielectric breakdown and be destroyed. However, [*] Prof. T. W. Noh, S. C. Chae, S. B. Lee, S. H Chang, Dr. C. Liu ReCOE & FPRD, Department of Physics and Astronomy Seoul National University Seoul 151-747 (Korea) E-mail: twnoh@snu.ac.kr

Random circuit breaker network model for unipolar resistance switching (Advanced Materials (2008) 20 (1154))

Phospholes are known to exhibit characteristic optical and electrochemical properties derived from the phosphorus-bridged 1,3-dienic π system. Particular interest has recently been paid to their π-conjugated derivatives, such as non-fused phospholes, dibenzo[b,d]phospholes, benzo[b]phospholes, benzo[c]phospholes and related compounds. This perspective focuses on recent advances in the synthesis of these phosphole-based π systems with representative examples that would give valuable guidelines for designing phosphole-containing opto-electrochemical materials.

Design and synthesis of phosphole-based pi systems for novel organic materials.

A General Strategy for the Facile Synthesis of 2,7-Dibromo-9-heterofluorenes

Diastereoconvergent Synthesis of trans-5-Hydroxy-6-Substituted-2-Piperidinones by Addition–Cyclization–Deprotection Process

The asymmetric total synthesis of lagunamide A (3.0%, 20 steps longest linear sequence) and its five analogues, including the structure dehydrated at the C37 position, are detailed in this report. The key feature in this diverse synthesis includes the elaboration of four consecutive chiral centers at C37–40 and the final macrocyclization. Starting from chiral aldehyde 10, we synthesized both 1,3-anti and 1,3-syn homoallylic alcohols 20a and 20b through asymmetric aldol condensation and stereoselective allylation. The following esterification to introduce the l-N-Me-Ala unit resulted in significant epimerization. This problem was finally overcome by coupling the alcohols with the corresponding acid chloride of the l-alanine derivative. The key α,β-unsaturated carboxylic acid unit was produced by cross-metathesis (CM) of methacrylaldehyde and related olefins. Interestingly, we found that the C7 configuration dramatically affected the ring closure. Natural lagunamide A (1a), its 39-epimer (1c), and its 2-epi...

Wei Huang

Papers

A General Strategy for the Facile Synthesis of 2,7-Dibromo-9-heterofluorenes

Diastereoconvergent Synthesis of trans-5-Hydroxy-6-Substituted-2-Piperidinones by Addition–Cyclization–Deprotection Process

Diverse Synthesis of Marine Cyclic Depsipeptide Lagunamide A and Its Analogues

Tailoring the interfaces of silicon/carbon nanotube for high rate lithium-ion battery anodes

Confining invasion directions of Li+ to achieve efficient Si anode material for lithium-ion batteries