Showing papers in "Advanced Materials in 2010"

Graphene and Graphene Oxide: Synthesis, Properties, and Applications

Abstract: There is intense interest in graphene in fields such as physics, chemistry, and materials science, among others. Interest in graphene's exceptional physical properties, chemical tunability, and potential for applications has generated thousands of publications and an accelerating pace of research, making review of such research timely. Here is an overview of the synthesis, properties, and applications of graphene and related materials (primarily, graphite oxide and its colloidal suspensions and materials made from them), from a materials science perspective.

Advanced Materials for Energy Storage

Abstract: [Liu, Chang; Li, Feng; Ma, Lai-Peng; Cheng, Hui-Ming] Chinese Acad Sci, Inst Met Res, Shenyang Natl Lab Mat Sci, Shenyang 110016, Peoples R China.;Cheng, HM (reprint author), Chinese Acad Sci, Inst Met Res, Shenyang Natl Lab Mat Sci, 72 Wenhua Rd, Shenyang 110016, Peoples R China;cheng@imr.ac.cn

For the Bright Future—Bulk Heterojunction Polymer Solar Cells with Power Conversion Efficiency of 7.4%

Abstract: Sun is the largest carbon-neutral energy source that has not been fully utilized. Although there are solar cell devices based on inorganic semiconductor to efficiently harvest solar energy, the cost of these conventional devices is too high to be economically viable. This is the major motivation for the development of organic photovoltaic (OPV) materials and devices, which are envisioned to exhibit advantages such as low cost, flexibility, and abundant availability. [1] The past success in organic light-emitting diodes provides scientists with confidence that organic photovoltaic devices will be a vital alternate to the inorganic counterpart. At the heart of the OPV technology advantage is the easiness of the fabrication, which holds the promise of very low-cost manufacturing process. A simple, yet successful technique is the solution-processed bulk heterojunction (BHJ) solar cell composed of electron-donating semiconducting polymers and electron-withdrawing fullerides as active layers. [2] The composite active layer can be prepared as a large area in a single step by using techniques such as spin-coating, inkjet-printing, spraycoating, gravure-coating, roller-casting etc. [3] In the last fifteen years, a significant progress has been made on the improvement of the power-conversion efficiency (PCE) of polymer BHJ solar cells, and the achieved efficiencies have evolved from less than 1% in the poly(phenylene vinylene) (PPV) system in 1995, [2] to 4‐5% in the poly(3-hexylthiphene) (P3HT) system in 2005, [4] to around 6%, as reported recently. [5] However, the efficiency of polymer solar cells is still significantly lower than their inorganic counterparts, such as silicon, CdTe and CIGS, which prevents practical applications in large scale.

Hydrothermal route for cutting graphene sheets into blue-luminescent graphene quantum dots

Abstract: 2010 WILEY-VCH Verlag Gm Graphene-based materials are promising building blocks for future nanodevices owing to their superior electronic, thermal, and mechanical properties as well as their chemical stability. However, currently available graphene-based materials produced by typical physical and chemical routes, including micromechanical cleavage, reduction of exfoliated graphene oxide (GO), and solvothermal synthesis, are generally micrometer-sized graphene sheets (GSs), which limits their direct application in nanodevices. In this context, it has become urgent to develop effective routes for cutting large GSs into nanometer-sized pieces with a well-confined shape, such as graphene nanoribbons (GNRs) and graphene quantum dots (GQDs). Theoretical and experimental studies have shown that narrow GNRs (width less than ca. 10 nm) exhibit substantial quantum confinement and edge effects that render GNRs semiconducting. By comparison, GQDs possess strong quantum confinement and edge effects when their sizes are down to 100 nm. If their sizes are reduced to ca. 10 nm, comparable with the widths of semiconducting GNRs, the two effects will become more pronounced and, hence, induce new physical properties. Up to now, nearly all experimental work on GNRs and GQDs has focused on their electron transportation properties. Little work has been done on the optical properties that are directly associated with the quantum confinement and/or edge effects. Most GNRand GQD-based electronic devices have been fabricated by lithography techniques, which can realize widths and diameters down to ca. 20 nm. This physical approach, however, is limited by the need for expensive equipment and especially by difficulties in obtaining smooth edges. Alternative chemical routes can overcome these drawbacks. Moreover, surface functionalization can be realized easily. Li et al. first reported a chemical route to functionalized and ultrasmooth GNRs with widths ranging from 50 nm to sub-10 nm. Very recently, Kosynkin et al. reported a simple solution-based oxidative process for producing GNRs by lengthwise cutting and unraveling of multiwalled carbon nanotube (CNT) side walls. Yet, no chemical routes have been reported so far for preparing functionalized GQDs with sub-10 nm sizes. Here, we report on a novel and simple hydrothermal approach for the cutting of GSs into surface-functionalized GQDs (ca. 9.6-nm average diameter). The functionalized GQDs were found to exhibit bright blue photoluminescence (PL), which has never been observed in GSs and GNRs owing to their large lateral sizes. The blue luminescence and new UV–vis absorption bands are directly induced by the large edge effect shown in the ultrafine GQDs. The starting material was micrometer-sized rippled GSs obtained by thermal reduction of GO sheets. Figure 1a shows a typical transmission electron microscopy (TEM) image of the pristine GSs. Their (002) interlayer spacing is 3.64 A (Fig. 1c), larger than that of bulk graphite (3.34 A). Before the hydrothermal treatment, the GSs were oxidized in concentrated H2SO4 and HNO3. After the oxidization treatment the GSs became slightly smaller (50 nm–2mm) and the (002) spacing slightly increased to 3.85 A (Fig. 1c). During the oxidation, oxygen-containing functional groups, including C1⁄4O/COOH, OH, and C O C, were introduced at the edge and on the basal plane, as shown in the Fourier transform infrared (FTIR) spectrum (Fig. 1d). The presence of these groups makes the GSs soluble in water. A series of more marked changes took place after the hydrothermal treatment of the oxidized GSs at 200 8C. First, the (002) spacing was reduced to 3.43 A (Fig. 1c), very close to that of bulk graphite, indicating that deoxidization occurs during the hydrothermal process. The deoxidization is further confirmed by the changes in the FTIR and C 1s X-ray photoelectron spectroscopy (XPS) spectra. After the hydrothermal treatment, the strongest vibrational absorption band of C1⁄4O/COOH at 1720 cm 1 became very weak and the vibration band of epoxy groups at 1052 cm 1 disappeared (Fig. 1d). In the XPS C 1s spectra of the oxidized and hydrothermally reduced GSs (Fig. 2a), the signal at 289 eV assigned to carboxyl groups became weak after the hydrothermal treatment, whereas the sp carbon peak at 284.4 eV was almost unchanged. Figure 2b shows the Raman spectrum of the reduced GSs. A G band at 1590 cm 1 and a D band at 1325 cm 1 were observed with a large intensity ratio ID/IG of 1.26. Second, the size of the GSs decreased dramatically and ultrafine GQDswere isolated by a dialysis process. Figure 3 shows typical TEM and atomic force microscopy (AFM) images of the GQDs. Their diameters are mainly distributed in the range of 5–13 nm (9.6 nm average diameter). Their topographic heights are mostly between 1 and 2 nm, similar to those observed in functionalized GNRs with 1–3 layers. More than 85% of the GQDs consist of 1–3 layers.

Beyond Intercalation-Based Li-Ion Batteries: The State of the Art and Challenges of Electrode Materials Reacting Through Conversion Reactions

Abstract: Despite the imminent commercial introduction of Li-ion batteries in electric drive vehicles and their proposed use as enablers of smart grids based on renewable energy technologies, an intensive quest for new electrode materials that bring about improvements in energy density, cycle life, cost, and safety is still underway. This Progress Report highlights the recent developments and the future prospects of the use of phases that react through conversion reactions as both positive and negative electrode materials in Li-ion batteries. By moving beyond classical intercalation reactions, a variety of low cost compounds with gravimetric specific capacities that are two-to-five times larger than those attained with currently used materials, such as graphite and LiCoO(2), can be achieved. Nonetheless, several factors currently handicap the applicability of electrode materials entailing conversion reactions. These factors, together with the scientific breakthroughs that are necessary to fully assess the practicality of this concept, are reviewed in this report.

Chemically derived graphene oxide: towards large-area thin-film electronics and optoelectronics.

Abstract: Chemically derived graphene oxide (GO) possesses a unique set of properties arising from oxygen functional groups that are introduced during chemical exfoliation of graphite. Large-area thin-film deposition of GO, enabled by its solubility in a variety of solvents, offers a route towards GO-based thin-film electronics and optoelectronics. The electrical and optical properties of GO are strongly dependent on its chemical and atomic structure and are tunable over a wide range via chemical engineering. In this Review, the fundamental structure and properties of GO-based thin films are discussed in relation to their potential applications in electronics and optoelectronics.

Inkjet Printing—Process and Its Applications

Abstract: In this Progress Report we provide an update on recent developments in inkjet printing technology and its applications, which include organic thin-film transistors, light-emitting diodes, solar cells, conductive structures, memory devices, sensors, and biological/pharmaceutical tasks. Various classes of materials and device types are in turn examined and an opinion is offered about the nature of the progress that has been achieved.

Graphene oxide: intrinsic peroxidase catalytic activity and its application to glucose detection.

Abstract: Carboxyl-modified graphene oxide (GO-COOH) possesses intrinsic peroxidase-like activity that can catalyze the reaction of peroxidase substrate 3,3,5,5-tetramethyl-benzidine (TMB) in the presence of H2O2 to produce a blue color reaction. A simple, cheap, and highly sensitive and selective colorimetric method for glucose detection has been developed and will facilitate the utilization of GO-COOH intrinsic peroxidase activity in medical diagnostics and biotechnology.

Ultralow-Fouling, Functionalizable, and Hydrolyzable Zwitterionic Materials and Their Derivatives for Biological Applications

Abstract: In recent years, zwitterionic materials such as poly(carboxybetaine) (pCB) and poly(sulfobetaine) (pSB) have been applied to a broad range of biomedical and engineering materials. Due to electrostatically induced hydration, surfaces coated with zwitterionic groups are highly resistant to nonspecific protein adsorption, bacterial adhesion, and biofilm formation. Among zwitterionic materials, pCB is unique due to its abundant functional groups for the convenient immobilization of biomolecules. pCB can also be prepared in a hydrolyzable form as cationic pCB esters, which can kill bacteria or condense DNA. The hydrolysis of cationic pCB esters into nonfouling zwitterionic groups will lead to the release of killed microbes or the irreversible unpackaging of DNA. Furthermore, mixed-charge materials have been shown to be equivalent to zwitterionic materials in resisting nonspecific protein adsorption when they are uniformly mixed at the molecular scale.

Blue photoluminescence from chemically derived graphene oxide

Abstract: Blue photoluminescence from chemically derived graphene oxide Goki Eda, Yun-Yue Lin, Cecilia Mattevi, Hisato Yamaguchi, Hsin-An Chen, I-Sheng Chen, Chun-Wei Chen, and Manish Chhowalla 1 Department of Materials, Imperial College, Exhibition Road, London SW7 2AZ, UK. 2 Department of Materials Science and Engineering, Rutgers University 607 Taylor Road, Piscataway, NJ 08854, USA. 3 Department of Materials Science and Engineering, National Taiwan University No. 1, Sec. 4, Roosevelt Road, Taipei 10617, Taiwan.

Applications of Ultrasound to the Synthesis of Nanostructured Materials

Abstract: Recent advances in nanostructured materials have been led by the development of new synthetic methods that provide control over size, morphology, and nano/microstructure. The utilization of high intensity ultrasound offers a facile, versatile synthetic tool for nanostructured materials that are often unavailable by conventional methods. The primary physical phenomena associated with ultrasound that are relevant to materials synthesis are cavitation and nebulization. Acoustic cavitation (the formation, growth, and implosive collapse of bubbles in a liquid) creates extreme conditions inside the collapsing bubble and serves as the origin of most sonochemical phenomena in liquids or liquid-solid slurries. Nebulization (the creation of mist from ultrasound passing through a liquid and impinging on a liquid-gas interface) is the basis for ultrasonic spray pyrolysis (USP) with subsequent reactions occurring in the heated droplets of the mist. In both cases, we have examples of phase-separated attoliter microreactors: for sonochemistry, it is a hot gas inside bubbles isolated from one another in a liquid, while for USP it is hot droplets isolated from one another in a gas. Cavitation-induced sonochemistry provides a unique interaction between energy and matter, with hot spots inside the bubbles of approximately 5000 K, pressures of approximately 1000 bar, heating and cooling rates of >10(10) K s(-1); these extraordinary conditions permit access to a range of chemical reaction space normally not accessible, which allows for the synthesis of a wide variety of unusual nanostructured materials. Complementary to cavitational chemistry, the microdroplet reactors created by USP facilitate the formation of a wide range of nanocomposites. In this review, we summarize the fundamental principles of both synthetic methods and recent development in the applications of ultrasound in nanostructured materials synthesis.

Engineering Carbon Materials from the Hydrothermal Carbonization Process of Biomass

Abstract: Energy shortage, environmental crisis, and developing customer demands have driven people to find facile, low-cost, environmentally friendly, and nontoxic routes to produce novel functional materials that can be commercialized in the near future. Amongst various techniques, the hydrothermal carbonization (HTC) process of biomass (either of isolated carbohydrates or crude plants) is a promising candidate for the synthesis of novel carbon-based materials with a wide variety of potential applications. In this Review, we will discuss various synthetic routes towards such novel carbon-based materials or composites via the HTC process of biomass. Furthermore, factors that influence the carbonization process will be analyzed and the special chemical/physical properties of the final products will be discussed. Despite the lack of a clear mechanism, these novel carbonaceous materials have already shown promising applications in many fields such as carbon fixation, water purification, fuel cell catalysis, energy storage, CO(2) sequestration, bioimaging, drug delivery, and gas sensors. Some of the most promising examples will also be discussed here, demonstrating that the HTC process can rationally design a rich family of carbonaceous and hybrid functional carbon materials with important applications in a sustainable fashion.

Nanostructured Thermoelectrics: Big Efficiency Gains from Small Features

Abstract: The field of thermoelectrics has progressed enormously and is now growing steadily because of recently demonstrated advances and strong global demand for cost-effective, pollution-free forms of energy conversion. Rapid growth and exciting innovative breakthroughs in the field over the last 10-15 years have occurred in large part due to a new fundamental focus on nanostructured materials. As a result of the greatly increased research activity in this field, a substantial amount of new data--especially related to materials--have been generated. Although this has led to stronger insight and understanding of thermoelectric principles, it has also resulted in misconceptions and misunderstanding about some fundamental issues. This article sets out to summarize and clarify the current understanding in this field; explain the underpinnings of breakthroughs reported in the past decade; and provide a critical review of various concepts and experimental results related to nanostructured thermoelectrics. We believe recent achievements in the field augur great possibilities for thermoelectric power generation and cooling, and discuss future paths forward that build on these exciting nanostructuring concepts.

Synthesis, Functionalization, and Biomedical Applications of Multifunctional Magnetic Nanoparticles

Abstract: Synthesis of multifunctional magnetic nanoparticles (MFMNPs) is one of the most active research areas in advanced materials. MFMNPs that have magnetic properties and other functionalities have been demonstrated to show great promise as multimodality imaging probes. Their multifunctional surfaces also allow rational conjugations of biological and drug molecules, making it possible to achieve target-specific diagnostics and therapeutics. This review first outlines the synthesis of MNPs of metal oxides and alloys and then focuses on recent developments in the fabrication of MFMNPs of core/shell, dumbbell, and composite hybrid type. It also summarizes the general strategies applied for NP surface functionalization. The review further highlights some exciting examples of these MFMNPs for multimodality imaging and for target-specific drug/gene delivery applications.

A three-dimensional carbon nanotube/graphene sandwich and its application as electrode in supercapacitors.

Abstract: Owing to its unique electrical, thermal, and mechanical properties, graphene has attracted great attention in various application areas, such as energy-storage materials, [ 1–3 ] free-standing paper-like materials, [ 4–6 ] polymer composites, [ 7–9 ] liquid crystal devices, [ 10 ] and mechanical resonators. [ 11 , 12 ] Approaches for preparing graphene include micromechanical cleavage, [ 11 , 13 , 14 ]

High‐Efficiency Solar Cell with Earth‐Abundant Liquid‐Processed Absorber

Abstract: 2010 WILEY-VCH Verlag Gmb Chalcogenide-based solar cells provide a critical pathway to cost parity between photovoltaic (PV) and conventional energy sources. Currently, only Cu(In,Ga)(S,Se)2 (CIGS) and CdTe technologies have reached commercial module production with stable power conversion efficiencies of over 9 percent. Despite the promise of these technologies, restrictions on heavy metal usage for Cd and limitations in supply for In and Te are projected to restrict the production capacity of the existing chalcogen-based technologies to <100GWp per year, a small fraction of our growing energy needs, which are expected to double to 27 TW by 2050. Earth-abundant copper-zinc-tin-chalcogenide kesterites, Cu2ZnSnS4 and Cu2ZnSnSe4, have been examined as potential alternatives for the two leading technologies, reaching promising but not yet marketable efficiencies of 6.7% and 3.2%, respectively, by multilayer vacuum deposition. Here we show a non-vacuum, slurry-based coating method that combines advantages of both solution processing and particlebased deposition, enabling fabrication of Cu2ZnSn(Se,S)4 devices with over 9.6% efficiency—a factor of five performance improvement relative to previous attempts to use highthroughput ink-based approaches and >40% higher than previous record devices prepared using vacuum-based methods. To address the issue of cost, non-vacuum ‘‘ink’’-based approaches—both from solutions and suspensions—are being developed for chalcogenide-based absorber layer deposition to replace potentially more expensive vacuum-based techniques. True solutions allow intermixing of the constituents at a molecular level and the formation of smooth homogeneous films, as demonstrated with spin-coated CIGS absorber layers from hydrazine (N2H4) solutions. [11–13] The chemically reducing character of hydrazine stabilizes solutions of anions with direct metal-chalcogen bonding for select elements (e.g. Cu, In, Ga, Sn), without the necessity to introduce typical impurities (e.g., C, O, Cl). Suspension approaches employ solid particles designed to be deposited on a substrate and reacted or fused with each other, to form a desired crystalline phase and grain structure. Normally insoluble components can be deposited by this approach using typical liquid-based deposition (e.g., printing, spin coating, slit casting, spraying). Although high-quality large-grained absorber layers can be formed for selected systems using either solutionor particlebased deposition, numerous challenges confront each approach for more general deposition needs. Solution processing is limited by the solubility of many materials of interest (e.g., ZnSe1–xSx in hydrazine solvents—relevant for the deposition of Cu2ZnSnS4 or Cu2ZnSnSe4). In addition, volume contraction upon drying of solution-deposited layers creates stress in the film that may cause crack formation in thicker films. In suspension approaches, a common difficulty is achieving single-phase crystallization among the solid particles. Particle-based approaches (as well as some solution methods) typically require the addition of organic agents to improve wetting and particle dispersion, and to avoid film cracks and delamination. Most of these non-volatile organic additives introduce carbon contamination in the final layer. Because of these challenges, vacuum-based techniques have historically shown superior performance to liquid coating. In the case of the earth-abundant Cu2ZnSn(S,Se)4 materials, ink-based approaches have to date yielded at most <1.6% efficiency devices. Here we demonstrate an hybrid solution-particle approach, using the earth-abundant Cu2ZnSn(S,Se)4 system as an example, which enables fabrication of PV devices with over 9.6% power conversion efficiency. The slurry (or ink) employed for deposition comprises a Cu–Sn chalcogenide (S or S–Se) solution in hydrazine (see Experimental section), with the in situ formation of readily dispersible particle-based Zn-chalcogenide precursors, ZnSe(N2H4) (Figure 1a,d) or ZnS(N2H4) (Figure 1b). Thermogravimetric analysis (TGA) of the isolated selenide particle precursor shows decomposition at approximately 200 8C, with mass loss of about 20%, close to the theoretical value expected upon transition to pure ZnSe (Figure 1c,d). Deposition using this hybrid slurry successfully combines the advantages of solution and suspension deposition routes by use of solutions containing solid particles, wherein both components (i.e., solution and particle) contain metal and chalcogen elements that integrate into the final film. Using the hybrid slurry method (i) solubility limitations are resolved, as virtually any materials system can be constituted by a combination of solid and dissolved components; (ii) the dissolved components can be engineered as an efficient binding media for the particles, eliminating the need of separate organic binders; (iii) solid particles act as stress-relief and crack-deflection centers allowing the deposition of thicker layers than pure solution processes; and (iv) the intimate contact between the two phases allows rapid reaction and homogeneous phase formation. Complete conversion of all constituents of the spin-coated hybrid precursor films into a single-phase, highly crystalline Cu2ZnSn(S,Se)4 is achieved by annealing at 540 8C on a hot plate. Three main types of samples were targeted – high selenium content (A), intermediate sulfoselenide (B) and pure sulfide (C) –

A Cost-Effective Supercapacitor Material of Ultrahigh Specific Capacitances: Spinel Nickel Cobaltite Aerogels from an Epoxide-Driven Sol–Gel Process

Abstract: Adv. Mater. 2010, 22, 347–351 2010 WILEY-VCH Verlag Gm The ever worsening energy depletion and global warming issues call for not only urgent development of clean alternative energies and emission control of global warming gases, but also more advanced energy storage and management devices. Supercapacitors, offering transient but extremely high powers, are probably the most important next generation energy storage device. To boost the specific capacitance of supercapacitors, the specific surface area of the electrode materials needs to be as high as possible to promote the electric double-layer capacitances and to accommodate a large amount of superficial electroactive species to participate in faradaic redox reactions. In addition, suitable pore sizes, 2–5 nm, of the porous electrode materials are critical to ease the mass transfer of electrolytes within the pores for fast redox reactions and double-layer charging/discharging. Aerogels are a class of mesoporous materials possessing highly specific surface areas and porosities, from which promising applications in a wide range of areas have been investigated. They are composed of 3D networks of nanoparticles with an average pore size of several nanometers, adjustably falling within the optimal pore sizes of 2–5 nm. Consequently, aerogels are a promising candidate for supercapacitor applications. As to the electrode material, electroactive materials possessing multiple oxidation states/structures that enable rich redox reactions for pseudocapacitance generation are desirable for supercapacitors. Transition metal oxides are such a class of materials that have drawn extensive and intensive research attention in recent years. Among them, RuO2 is themost prominent one with a specific capacitance as high as 1580F g , probably the highest ever reported. The commercialization of RuO2 based supercapacitors, however, is not promising because of the high cost and rareness of Ru. Spinel nickel cobaltite (NiCo2O4) is a low-cost, environmentally friendly transition metal oxide, which has been employed in electrocatalytic water splitting (oxygen evolution) and lithium ion batteries. Its application in supercapacitors, however, received much less attention. Nickel cobaltite has been reported to possess a much better electronic conductivity, at least two orders of magnitude higher, and higher electrochemical activity than those of nickel oxides and cobalt oxides. It is expected to offer richer redox reactions, including contributions from both nickel and cobalt ions, than the two corresponding single component oxides and is a potential cost-effective alternative for RuO2. Based on the above considerations, one would expect nickel cobaltite aerogels, with anticipated good electronic conductivity, low diffusion resistance to protons/cations, easy electrolyte penetration, and high electroactive areas to be a promising candidate for the construction of next-generation, ultrahighperformance supercapacitors. Traditionally, aerogels are prepared with sol–gel processes by taking corresponding alkoxides as the precursors. Alkoxides are generally expensive and sensitive to moisture and heat, requiring careful handling. Recently, to tackle these drawbacks, the epoxide synthetic route, enabling the use of low-cost and stable metal salts as the precursors, was successfully developed to prepare metal oxide aerogels. In this work, we reported the first successful preparation of nickel cobaltite aerogels with the epoxide-driven sol–gel process. The effects of the post-gel-drying calcination temperature on the critical properties of the product aerogels were investigated. At a starting Ni/Co ratio of 0.5 and a post-gel-drying calcination temperature of 200 8C, an optimal combination of composition, crystallinity, specific surface area, pore volume, and pore size was achieved to afford the nickel cobaltite aerogels that showed an extremely high-specific capacitance of 1400 F g 1 under a mass loading of 0.4 mg cm 2 at a sweep rate of 25mV s 1 within a potential window of 0.04 to 0.52V in a 1 M NaOH solution. The excellent reversibility and cycle stability of the product aerogels were also demonstrated. A stoichiometric mixture of nickel and cobalt chlorides was used as the precursor for the preparation of the nickel cobaltite aerogels. After the gel is dried in supercritical carbon dioxide, a post-gel-drying calcination is generally required to acquire preferred composition and/or better crystallinity of the products. The post-gel-drying calcination temperature is thus an important processing parameter to be studied. For referring convenience, we term the product aerogels as Ni–Co–O–T, with Tdenoting the calcination temperature. The T block is omitted for as-prepared samples. Also, for comparison purposes, NiO and Co3O4 aerogels were prepared, termed as Ni–O–T and Co–O–T, respectively. Figure 1a shows the X-ray diffraction (XRD) patterns of the as-prepared product aerogels and those samples calcined at 200 and 300 8C. Surprisingly, nickel cobaltite was formed even at the as-prepared condition. The diffraction peak located at the 2u value

Graphene‐On‐Silicon Schottky Junction Solar Cells

Abstract: www.MaterialsViews.com C O M Graphene-On-Silicon Schottky Junction Solar Cells M U N I By Xinming Li , Hongwei Zhu , * Kunlin Wang , * Anyuan Cao , Jinquan Wei , Chunyan Li , Yi Jia , Zhen Li , Xiao Li , and Dehai Wu C A IO N Graphene, a single atomic layer of carbon hexagons, has stimulated a lot of research interest owing to its unique structure and fascinating properties. [ 1 ] Graphene has been produced in the form of ultrathin sheets consisting of one or a few atomic layers by chemical vapor deposition (CVD) [ 2–4 ] or solution processing [ 5 , 6 ] and can be transferred to various substrates. The two-dimensionality and structural fl atness make graphene sheets ideal candidates for thin-fi lm devices and combination with other semiconductor materials such as silicon. These fi lms typically show sheet resistances on the order of several hundred ohm per square at about 80% optical transparency. [ 7 ] With modifi cation on the electronic properties and improvement of processing techniques, graphene fi lms show potential for use in conductive, fl exible electrodes, as an alternative for indium tin oxide (ITO). Graphene applications are just starting, and current investigations are on a number of areas such as fi llers for composites, nanoelectronics, and transparent electrodes. [ 8 ] For applications related to solar cells, graphene microsheets were dispersed into conjugated polymers to improve exciton dissociation and charge transport. [ 9–11 ] Also, solution-processed thin fi lms were used as conductive and transparent electrodes for organic [ 12 ] and dyesensitized [ 13 ] solar cells, although the cell effi ciency is still lower than those with ITO and fl uorine tin oxide (FTO) electrodes. Compared with carbon nanotube fi lms that have been extensively studied, graphene fi lms may have several advantages. A continuous single-layer graphene fi lm could retain high conductivity at very low (atomic) thickness, and avoid contact resistance that occurs in a carbon nanotube fi lm between interconnected nanotube bundles. In addition, graphene fi lms have minimum porosity and, in small areas, can provide an extremely fl at surface for molecule assembly and device integration. There are many opportunities in utilizing distinct properties of graphene and exploring novel applications. Bulk heterojunction structures based on carbon materials have attracted a great deal of interest for both scientifi c fundamentals and potential applications in various new optoelectronic devices,

n-Type organic semiconductors in organic electronics.

Abstract: Organic semiconductors have been the subject of intensive academic and commercial interest over the past two decades, and successful commercial devices incorporating them are slowly beginning to enter the market. Much of the focus has been on the development of hole transporting, or p-type, semiconductors that have seen a dramatic rise in performance over the last decade. Much less attention has been devoted to electron transporting, or so called n-type, materials, and in this paper we focus upon recent developments in several classes of n-type materials and the design guidelines used to develop them.

Determination of the Local Chemical Structure of Graphene Oxide and Reduced Graphene Oxide

Abstract: www.MaterialsViews.com C O M M Determination of the Local Chemical Structure of Graphene Oxide and Reduced Graphene Oxide U N IC A By Kris Erickson , Rolf Erni , Zonghoon Lee , Nasim Alem , Will Gannett , and Alex Zettl * IO N Although the unique electronic and mechanical properties of graphene suggest numerous intriguing applications, the requisite large-scale direct synthesis and solution-based handling have proven diffi cult. [ 1 , 2 ] It has been suggested that a functionalized form of graphene, graphene oxide (GO), could provide a solution-friendly route to facile, high-throughput graphene manipulation. [ 2 ] For such a route to be viable, however, GO must be convertible back to graphene, ostensibly via chemical reduction and thermal annealing. Unfortunately, transport measurements indicate that the reconstituted material, reduced and annealed graphene oxide (raGO), has electrical conductivity orders of magnitude lower than that of graphene. [ 2 , 3 ] This raises the question: can oxidized graphene be effectively converted back to graphene, and if not, what can it be converted to? Central to this question are the detailed atomic structures of GO and raGO, which, despite their importance, remain largely unknown. [ 4 ] We present here ultra-high-resolution transmission electron microscopy (TEM) images and dynamics studies of suspended sheets of graphene, GO, and raGO, obtained using aberration-corrected instrumentation. It should be noted that both the label GO and raGO (also referred to as “chemically converted graphene”) [ 5 ] refer to a wide variety of materials with the properties of each material being largely dependent upon the particular synthetic route employed. This study presents one particular synthetic method for GO and raGO. Among the various methods possible for synthesizing GO and raGO, we followed methods which have yielded the highest reported fi nal conductivities, as this material would be most suitable as a potential graphene alternative. [ 2 , 6–11 ] The local and global structure and stability of GO and raGO are revealed. We fi nd that the raGO material under study is greatly structurally dissimilar to graphene, being unstable under signifi cant electron beam

Recent advances in white organic light-emitting materials and devices (WOLEDs).

Abstract: WOLEDs offer new design opportunities in practical solid-state lighting and could play a significant role in reducing global energy consumption. Obtaining white light from organic LEDs is a considerable challenge. Alongside the development of new materials with improved color stability and balanced charge transport properties, major issues involve the fabrication of large-area devices and the development of low-cost manufacturing technology. This Review will describe the types of materials (small molecules and polymers) that have been used to fabricate WOLEDs. A range of device architectures are presented and appraised.

White‐Light‐Emitting Diodes with Quantum Dot Color Converters for Display Backlights

Abstract: Quantum dots (QDs) have attracted great attention as good candidate for the next generation displays due to their narrow emission, and high luminescence efficiency, and tunable emission covering all visible range. However, QDs easily lost their initial optical properties during the process for a device fabrication and practical operation. We synthesized well passivated green and red light emitting QDs that show almost 100% of QE. When the highly luminescent green and red light emitting QDs were applied as color converters in InGaN blue LEDs, resulting cool white QD-LEDs showed 41lm/W and more than 100% of color reproducibility compared to NTSC standard in CIE1931 and maintained their optical properties for a long time operation. We also demonstrated a 46-inch LCD panel using the white QDLED backlight was successfully demonstrated (Figure).

Properties and Applications of Colloidal Nonspherical Noble Metal Nanoparticles

Abstract: Nanoparticles of noble metals belong to the most extensively studied colloidal systems in the field of nanoscience and nanotechnology. Due to continuing progress in the synthesis of nanoparticles with controlled morphologies, the exploration of unique morphology-dependent properties has gained momentum. Anisotropic features in nonspherical nanoparticles make them ideal candidates for enhanced chemical, catalytic, and local field related applications. Nonspherical plasmon resonant nanoparticles offer favorable properties for their use as analytical tools, or as diagnostic and therapeutic agents. This Review highlights morphology-dependent properties of nonspherical noble metal nanoparticles with a focus on localized surface plasmon resonance and local field enhancement, as well as their applications in various fields including Raman spectroscopy, fluorescence enhancement, analytics and sensing, photothermal therapy, (bio-)diagnostics, and imaging.

6.5% Efficiency of polymer solar cells based on poly(3-hexylthiophene) and indene-C(60) bisadduct by device optimization.

Abstract: A power conversion efficiency of 6.48% was achieved for polymer solar cells based on poly(3-hexylthiophene) (P3HT) as donor and indene-C₆₀ bisadduct (ICBA) as acceptor with an open-circuit voltage of 0.84 V, a short-circuit current of 10.61 mA/cm², and a fill factor of 72.7% under irradiation at AM1.5G, 100 mW/cm² at the optimized conditions of P3HT:ICBA = 1:1 (w/w), solvent annealing and pre-thermal annealing at 150 °C for 10 minutes.

Liquid Crystalline Elastomers as Actuators and Sensors

Abstract: This review collects recent developments in the field of liquid crystalline elastomers (LCEs) with an emphasis on their use for actuator and sensor applications. Several synthetic pathways leading to crosslinked liquid crystalline polymers are discussed and how these materials can be oriented into liquid crystalline monodomains are described. By comparing the actuating properties of different systems, general structure-property relationships for LCEs are obtained. In the final section, how these materials can be turned into usable devices using different interdisciplinary techniques are described.

Nanosheets of oxides and hydroxides: Ultimate 2D charge-bearing functional crystallites.

Abstract: A wide variety of cation-exchangeable layered transition metal oxides and their relatively rare counterparts, anion-exchangeable layered hydroxides, have been exfoliated into individual host layers, i.e., nanosheets. Exfoliation is generally achieved via a high degree of swelling, typically driven either by intercalation of bulky organic ions (quaternary ammonium cations, propylammonium cations, etc.) for the layered oxides or by solvation with organic solvents (formamide, butanol, etc.) for the hydroxides. Ultimate two-dimensional (2D) anisotropy for the nanosheets, with thickness of around one nanometer versus lateral size ranging from submicrometer to several tens of micrometers, allows them to serve either as an ideal quantum system for fundamental study or as a basic building block for functional assembly. The charge-bearing inorganic macromolecule-like nanosheets can be assembled or organized through various solution-based processing techniques (e.g., flocculation, electrostatic sequential deposition, or the Langmuir-Blodgett method) to produce a range of nanocomposites, multilayer nanofilms, and core-shell nanoarchitectures, which have great potential for electronic, magnetic, optical, photochemical, and catalytic applications.

Nitrogen-containing hydrothermal carbons with superior performance in supercapacitors.

Abstract: [ ∗] L. Zhao , Prof. M. Antonietti , Dr. M.-M. Titirici Colloid Chemistry Department Max-Planck Institute for Colloids and Interfaces Am Muehlenberg 1, 14424 Potsdam (Germany) E-mail: Magdalena.Titirici@mpikg.mpg.de Prof. L.-Z. an , F M.-Q. Zhou , H. Guan , Qiao S. . YSchool of Materials Science and Engineering University of Science and Technology Beijing 100083 Beijing (China) E-mail: fanlizhen@ustb.edu.cn L. Zhao Institute of Coal Chemistry Chinese Academy of Sciences 27th Taoyuan South Road, 030001 Taiyuan (China)

Processing of Bulk Metallic Glass

Abstract: Bulk metallic glass (BMG) formers are multicomponent alloys that vitrify with remarkable ease during solidification. Technological interest in these materials has been generated by their unique properties, which often surpass those of conventional structural materials. The metastable nature of BMGs, however, has imposed a barrier to broad commercial adoption, particularly where the processing requirements of these alloys conflict with conventional metal processing methods. Research on the crystallization of BMG formers has uncovered novel thermoplastic forming (TPF)-based processing opportunities. Unique among metal processing methods, TPF utilizes the dramatic softening exhibited by a BMG as it approaches its glass-transition temperature and decouples the rapid cooling required to form a glass from the forming step. This article reviews crystallization processes in BMG former and summarizes and compares TPF-based processing methods. Finally, an assessment of scientific and technological advancements required for broader commercial utilization of BMGs will be made.

Photoswitches: From Molecules to Materials

Abstract: Small organic molecules, capable of undergoing efficient and reversible photochemical reactions to switch them between (at least) two (meta)stable isomers associated with markedly different properties, continue to impact the materials world. Such photoswitches are being implemented in a variety of materials for applications ranging from optical devices to "smart" polymers. All approaches exploit the photoswitching molecular entities as gates, which translate an incoming light stimulus to trigger macroscopic property changes of the materials. In this progress report, the most promising recent examples in this field are highlighted and put in perspective. Moving from supramolecular systems in solution to surfaces and finally to bulk materials, important design concepts are discussed, emphasizing both the challenges as well as the great promise of such truly advanced materials.