Showing papers by "Shuit-Tong Lee published in 2017"

C3N—A 2D Crystalline, Hole-Free, Tunable-Narrow-Bandgap Semiconductor with Ferromagnetic Properties

Abstract: Graphene has initiated intensive research efforts on 2D crystalline materials due to its extraordinary set of properties and the resulting host of possible applications. Here the authors report on the controllable large-scale synthesis of C3 N, a 2D crystalline, hole-free extension of graphene, its structural characterization, and some of its unique properties. C3 N is fabricated by polymerization of 2,3-diaminophenazine. It consists of a 2D honeycomb lattice with a homogeneous distribution of nitrogen atoms, where both N and C atoms show a D6h -symmetry. C3 N is a semiconductor with an indirect bandgap of 0.39 eV that can be tuned to cover the entire visible range by fabrication of quantum dots with different diameters. Back-gated field-effect transistors made of single-layer C3 N display an on-off current ratio reaching 5.5 × 1010 . Surprisingly, C3 N exhibits a ferromagnetic order at low temperatures (<96 K) when doped with hydrogen. This new member of the graphene family opens the door for both fundamental basic research and possible future applications.

Ligand Mediated Transformation of Cesium Lead Bromide Perovskite Nanocrystals to Lead Depleted Cs4PbBr6 Nanocrystals.

Abstract: Lead halide perovskite nanocrystals (NCs) have emerged as attractive nanomaterials owing to their excellent optical and optoelectronic properties. Their intrinsic instability and soft nature enable a post-synthetic controlled chemical transformation. We studied a ligand mediated transformation of presynthesized CsPbBr3 NCs to a new type of lead–halide depleted perovskite derivative nanocrystal, namely Cs4PbBr6. The transformation is initiated by amine addition, and the use of alkyl-thiol ligands greatly improves the size uniformity and chemical stability of the derived NCs. The thermodynamically driven transformation is governed by a two-step dissolution–recrystallization mechanism, which is monitored optically. Our results not only shed light on a decomposition pathway of CsPbBr3 NCs but also present a method to synthesize uniform colloidal Cs4PbBr6 NCs, which may actually be a common product of perovskite NCs degradation.

Over 10% EQE Near-Infrared Electroluminescence Based on a Thermally Activated Delayed Fluorescence Emitter

Abstract: Significant effort has been made to develop novel material systems to improve the efficiency of near-infrared organic light-emitting diodes (NIR OLEDs). Of those, fluorescent chromophores are mostly studied because of their advantages in cost and tunability. However, it is still rare for fluorescent NIR emitters to present good color purities in the NIR range and to have high external quantum efficiency (EQE). Here, a wedge-shaped D-π-A-π-D emitter APDC-DTPA with thermally activated delayed fluorescence property and a small single-triplet splitting (ΔEst) of 0.14 eV is presented. The non-doped NIR device exhibits excellent performance with a maximum EQE of 2.19% and a peak wavelength of 777 nm. Remarkably, when 10 wt% of APDC-DTPA is doped in 1,3,5-tris(1-phenyl-1H-benzimidazol-2-yl)benzene host, an extremely high EQE of 10.19% with an emission peak of 693 nm is achieved. All these values represent the best result for NIR OLEDs based on a pure organic fluorescent emitter with similar device structure and color gamut.

Ultrahigh-Responsivity Photodetectors from Perovskite Nanowire Arrays for Sequentially Tunable Spectral Measurement

Abstract: Compared with polycrystalline films, single-crystalline methylammonium lead halide (MAPbX3, X = halogen) perovskite nanowires (NWs) with well-defined structure possess superior optoelectronic properties for optoelectronic applications. However, most of the prepared perovskite NWs exhibit properties below expectations due to poor crystalline quality and rough surfaces. It also remains a challenge to achieve aligned growth of single-crystalline perovskite NWs for integrated device applications. Here, we report a facile fluid-guided antisolvent vapor-assisted crystallization (FGAVC) method for large-scale fabrication of high-quality single-crystalline MAPb(I1–xBrx)3 (x = 0, 0.1, 0.2, 0.3, 0.4) NW arrays. The resultant perovskite NWs showed smooth surfaces due to slow crystallization process and moisture-isolated growth environment. Significantly, photodetectors made from the NW arrays exhibited outstanding performance in respect of ultrahigh responsivity of 12 500 A W–1, broad linear dynamic rang (LDR) of 15...

Carbon Dots as Fillers Inducing Healing/Self-Healing and Anticorrosion Properties in Polymers

Abstract: Self-healing is the way by which nature repairs damage and prolongs the life of bio entities. A variety of practical applications require self-healing materials in general and self-healing polymers in particular. Different (complex) methods provide the rebonding of broken bonds, suppressing crack, or local damage propagation. Here, a simple, versatile, and cost-effective methodology is reported for initiating healing in bulk polymers and self-healing and anticorrosion properties in polymer coatings: introduction of carbon dots (CDs), 5 nm sized carbon nanocrystallites, into the polymer matrix forming a composite. The CDs are blended into polymethacrylate, polyurethane, and other common polymers. The healing/self-healing process is initiated by interfacial bonding (covalent, hydrogen, and van der Waals bonding) between the CDs and the polymer matrix and can be optimized by modifying the functional groups which terminate the CDs. The healing properties of the bulk polymer-CD composites are evaluated by comparing the tensile strength of pristine (bulk and coatings) composites to those of fractured composites that are healed and by following the self-healing of scratches intentionally introduced to polymer-CD composite coatings. The composite coatings not only possess self-healing properties but also have superior anticorrosion properties compared to those of the pure polymer coatings.

Silicon Nanowire/Polymer Hybrid Solar Cell-Supercapacitor: A Self-Charging Power Unit with a Total Efficiency of 10.5.

Abstract: An integrated self-charging power unit, combining a hybrid silicon nanowire/polymer heterojunction solar cell with a polypyrrole-based supercapacitor, has been demonstrated to simultaneously harvest solar energy and store it. By efficiency enhancement of the hybrid nanowire solar cells and a dual-functional titanium film serving as conjunct electrode of the solar cell and supercapacitor, the integrated system is able to yield a total photoelectric conversion to storage efficiency of 10.5%, which is the record value in all the integrated solar energy conversion and storage system. This system may not only serve as a buffer that diminishes the solar power fluctuations from light intensity, but also pave its way toward cost-effective high efficiency self-charging power unit. Finally, an integrated device based on ultrathin Si substrate is demonstrated to expand its feasibility and potential application in flexible energy conversion and storage devices.

A Co 3 O 4 -CDots-C 3 N 4 three component electrocatalyst design concept for efficient and tunable CO 2 reduction to syngas

Abstract: Syngas, a CO and H2 mixture mostly generated from non-renewable fossil fuels, is an essential feedstock for production of liquid fuels. Electrochemical reduction of CO2 and H+/H2O is an alternative renewable route to produce syngas. Here we introduce the concept of coupling a hydrogen evolution reaction (HER) catalyst with a CDots/C3N4 composite (a CO2 reduction catalyst) to achieve a cheap, stable, selective and efficient route for tunable syngas production. Co3O4, MoS2, Au and Pt serve as the HER component. The Co3O4-CDots-C3N4 electrocatalyst is found to be the most efficient among the combinations studied. The H2/CO ratio of the produced syngas is tunable from 0.07:1 to 4:1 by controlling the potential. This catalyst is highly stable for syngas generation (over 100 h) with no other products besides CO and H2. Insight into the mechanisms balancing between CO2 reduction and H2 evolution when applying the HER-CDots-C3N4 catalyst concept is provided.

Atomistic Origins of Surface Defects in CH3NH3PbBr3 Perovskite and Their Electronic Structures

Abstract: The inherent instability of CH3NH3PbX3 remains a major technical barrier for the industrial applications of perovskite materials. Recently, the most stable surface structures of CH3NH3PbX3 have been successfully characterized by using density functional theory (DFT) calculations together with the high-resolution scanning tunneling microscopy (STM) results. The two coexisting phases of the perovskite surfaces have been ascribed to the alternate orientation of the methylammonium (MA) cations. Notably, similar surface defect images (a dark depression at the sites of X atoms) have been observed on surfaces produced with various experimental methods. As such, these defects are expected to be intrinsic to the perovskite crystals and may play an important role in the structural decomposition of perovskite materials. Understanding the nature of such defects should provide some useful information toward understanding the instability of perovskite materials. Thus, we investigate the chemical identity of the surface...

Solution‐Processed Extremely Efficient Multicolor Perovskite Light‐Emitting Diodes Utilizing Doped Electron Transport Layer

Abstract: A specially designed n-type semiconductor consisting of Ca-doped ZnO (CZO) nanoparticles is used as the electron transport layer (ETL) in high-performance multicolor perovskite light-emitting diodes (PeLEDs) fabricated using an all-solution process. The band structure of the ZnO is tailored via Ca doping to create a cascade of conduction energy levels from the cathode to the perovskite. This energy band alignment significantly enhances conductivity and carrier mobility in the CZO ETL and enables controlled electron injection, giving rise to sub-bandgap turn-on voltages of 1.65 V for red emission, 1.8 V for yellow, and 2.2 V for green. The devices exhibit significantly improved luminance yields and external quantum efficiencies of, respectively, 19 cd A−1 and 5.8% for red emission, 16 cd A−1 and 4.2% for yellow, and 21 cd A−1 and 6.2% for green. The power efficiencies of these multicolor devices demonstrated in this study, 30 lm W−1 for green light-emitting PeLED, 28 lm W−1 for yellow, and 36 lm W−1 for red are the highest to date reported. In addition, the perovskite layers are fabricated using a two-step hot-casting technique that affords highly continuous (>95% coverage) and pinhole-free thin films. By virtue of the efficiency of the ETL and the uniformity of the perovskite film, high brightnesses of 10 100, 4200, and 16,060 cd m−2 are demonstrated for red, yellow, and green PeLEDs, respectively. The strategy of using a tunable ETL in combination with a solution process pushes perovskite-based materials a step closer to practical application in multicolor light-emitting devices.

Hydroxyl-Group-Dominated Graphite Dots Reshape Laser Desorption/Ionization Mass Spectrometry for Small Biomolecular Analysis and Imaging

Abstract: Small molecules play critical roles in life science, yet their facile detection and imaging in physiological or pathological settings remain a challenge. Matrix-assisted laser desorption ionization mass spectrometry (MALDI MS) is a powerful tool for molecular analysis. However, conventional organic matrices (CHCA, DHB, etc.) used in assisting analyte ionization suffer from intensive background noise in the mass region below m/z 700, which hinders MALDI MS applications for small-molecule detection. Here, we report that a hydroxyl-group-dominated graphite dot (GD) matrix overcomes limitations of conventional matrices and allows MALDI MS to be used in fast and high-throughput analysis of small biomolecules. GDs exhibit extremely low background noise and ultrahigh sensitivity (with limit of detection <1 fmol) in MALDI MS. This approach allows identification of complex oligosaccharides, detection of low-molecular-weight components in traditional Chinese herbs, and facile analysis of puerarin and its metabolite...

Naphthalene Diimide-Based n-Type Polymers: Efficient Rear Interlayers for High-Performance Silicon–Organic Heterojunction Solar Cells

Abstract: Silicon-organic heterojunction solar cells suffer from a noticeable weakness of inefficient rear contact. To improve this rear contact quality, here, two solution-processed organic n-type donor-acceptor naphthalene diimide (NDI)-based conjugated polymers of N2200 and fluorinated analogue F-N2200 are explored to reduce the contact resistance as well as to passivate the Si surface. Both N2200 and F-N2200 exhibit high electron mobility due to their planar structure and strong intermolecular stacking, thus allowing them to act as excellent transporting layers. Preferential orientation of the polymers leads to reduce contact resistance between Si and cathode aluminum, which can enhance electron extraction. More importantly, the substitution of fluorine atoms for hydrogen atoms within the conjugated polymer can strengthen the intermolecular stacking and improve the polymer-Si electronic contact due to the existence of F···H interactions. The power conversion efficiencies of Si-PEDOT:PSS solar cells increased from 12.6 to 14.5% as a consequence of incorporating the F-N2200 polymer interlayers. Subsequently, in-depth density functional theory simulations confirm that the polymer orientation plays a critical role on the polymer-Si contact quality. The success of NDI-based polymers indicates that planar conjugated polymer with a preferred orientation could be useful in developing high-performance solution-processed Si-organic heterojunction photovoltaic devices.

Photovoltaic Devices Based on Colloidal PbX Quantum Dots: Progress and Prospects

Abstract: A certified power conversion efficiency (PCE) of 12.0% and an outstanding air stability has been achieved for PbX quantum dots (QDs) solar cells, indicating strong potential for next generation low-cost solution-processed photovoltaics. Similar progress has been made in several other solar cell architectures employing PbX QD absorbers. This article aims to review the recent progress in understanding the photovoltaic-relevant properties of PbX QDs and highlight their application in various types of photovoltaic devices. In doing so, we hope that the unique properties of PbX QDs can be better understood in a broader context, and their potential can be fully realized with the aiding of other photovoltaic materials and novel device structures.

Facet-Selective Growth of Organic Heterostructured Architectures via Sequential Crystallization of Structurally Complementary π-Conjugated Molecules.

Abstract: In contrast to those for their polymeric counterparts, the controlled construction of organic heterostructured architectures derived from π-conjugated organic molecules has been rare and remains a great challenge. Herein, we develop a simple single-step solution strategy for the realization of organic heterostructures comprising coronene and perylene. Under a sequential crystallization process, an efficient doping step for coronene and perylene domains enables their perfect lattice matching, which facilitates facet-selective epitaxial growth of perylene domains on both the tips and the side surfaces of the preformed seed microwires by manipulating the growth pathways of the two pairs of materials. The present synthetic route provides a promising platform to investigate the detailed formation mechanism of complex organic heterostructures with specific topological configurations, further directing the construction of more functional heterostructured materials.

Centimeter-Long Single-Crystalline Si Nanowires

Abstract: The elongation of free-standing one-dimensional (1D) functional nanostructures into lengths above the millimeter range has brought new practical applications as they combine the remarkable properties of nanostructured materials with macroscopic lengths. However, it remains a big challenge to prepare 1D silicon nanostructures, one of the most important 1D nanostructures, with lengths above the millimeter range. Here we report the unprecedented preparation of ultralong single-crystalline Si nanowires with length up to 2 cm, which can function as the smallest active material to facilitate the miniaturization of macroscopic devices. These ultralong Si nanowires with augmented flexibility are of emerging interest for flexible electronics. We also demonstrate the first single-nanowire-based wearable joint motion sensor with superior performance to reported systems, which just represents one example of novel devices that can be built from these nanowires. The preparation of ultralong Si nanowires will stimulate ...

Black phosphorus induced photo-doping for high-performance organic-silicon heterojunction photovoltaics

Abstract: In conventional crystalline silicon (Si) homojunction solar cells, a strategy of doping by transporting phosphorus or boron impurities into Si is commonly used to build Ohmic contacts at rear electrodes. However, this technique involves an energy intensive, high temperature (∼800 °C) process and toxic doping materials. Black phosphorus (BP) is a two-dimensional, narrow bandgap semiconductor with high carrier mobility that exhibits broad light harvesting properties. Here, we place BP:zinc oxide (ZnO) composite films between Si and aluminum (Al) to improve their contact. Once the BP harvests photons with energies below 1.1 eV from the crystalline Si, the ZnO carrier concentration increases dramatically due to charge injection. This photo-induced doping results in a high carrier concentration in the ZnO film, mimicking the modulated doping technique used in semiconductor heterojunctions. We show that photo-induced carriers dramatically increase the conductivities of the BP-modified ZnO films, thus reducing the contact resistance between Si and Al. A photovoltaic power conversion efficiency of 15.2% is achieved in organic-Si heterojunction solar cells that use a ZnO:BP layer. These findings demonstrate an effective way of improving Si/metal contact via a simple, low temperature process.

High-Performance Ultrathin Organic–Inorganic Hybrid Silicon Solar Cells via Solution-Processed Interface Modification

Abstract: Organic–inorganic hybrid solar cells based on n-type crystalline silicon and poly(3,4-ethylenedioxythiophene)–poly(styrenesulfonate) exhibited promising efficiency along with a low-cost fabrication process. In this work, ultrathin flexible silicon substrates, with a thickness as low as tens of micrometers, were employed to fabricate hybrid solar cells to reduce the use of silicon materials. To improve the light-trapping ability, nanostructures were built on the thin silicon substrates by a metal-assisted chemical etching method (MACE). However, nanostructured silicon resulted in a large amount of surface-defect states, causing detrimental charge recombination. Here, the surface was smoothed by solution-processed chemical treatment to reduce the surface/volume ratio of nanostructured silicon. Surface-charge recombination was dramatically suppressed after surface modification with a chemical, associated with improved minority charge-carrier lifetime. As a result, a power conversion efficiency of 9.1% was ac...

Solid Confinement of Quantum Dots in ZIF-8 for Efficient and Stable Color-Conversion White LEDs.

Abstract: The powder form and low photoluminescence quantum yield (PLQY) of fluorescent metal-organic frameworks (MOFs) present a serious obstacle to fabricating high-efficiency film-like lighting devices. Here, we present a facile way to produce thin films of CdSex S1-x /ZnS quantum dots (QDs)@ZIF-8 with high PLQY by encapsulating red, green, and blue CdSex S1-x /ZnS QDs in ZIF-8 through a one-pot solid-confinement conversion process. The QDs@ZIF-8 thin film emits warm white light with good color quality and presents good thermal stability and long-term durability.

Connecting Together Nanocenters around the World.

Abstract: World Last month at ChinaNano in Beijing, we once again celebrated our ACS Nano award lectures, this year given by Profs. Hui-Ming Cheng of Shenyang National Laboratory for Materials Science, David Norris of ETH Zürich, and Teri Odom of Northwestern University. We also celebrated the 10th anniversary of our first issue in Washington, DC and Suzhou and Beijing, China. For the third time, at ChinaNano, we gathered together nanocenter directors from around the world to share our experiences and to discuss where we might work together to move our field forward. Whereas our previous two directors’ meetings questioned whether “nano” would continue as a field and whether funding initiatives around the world would be extended, this year, we explored more deeply how the field was progressing and how the nanocenters had special roles to play in these developments. These roles include enabling and accelerating significant advances in fundamental science, opening up unexplored worlds to study through the identification of problems and opportunities, developing new enabling tools, identifying and implementing applications, and initiating and nurturing technology spin-outs. They also include engaging the public, legislators, thought leaders, and others to share what it is we do and why, including targeting problems faced by the world in health, security, energy, water, air, food, and advancing technologies in many. We discussed developing nanomaterials that were safe by design, tested in ways that engaged the public, and were potentially reusable and/or recyclable. We shared our experiences and advances in education in nanoscience and nanotechnology. We also discussed the increasingly prevalent theme of how the nanoscience and nanotechnology communities have learned to communicate across fields, leaving us in an optimal position to promote and even to lead advances beyond our own areas. We discussed the importance of not overselling the prospects of our work to the public and to fundersas the nanocenters are the focal points for much publicity, we have the opportunity to be leaders in this regard.

Photoelectroreduction of Building-Block Chemicals

Abstract: Conventional photoelectrochemical cells utilize solar energy to drive the chemical conversion of water or CO2 into useful chemical fuels. Such processes are confronted with general challenges, including the low intrinsic activities and inconvenient storage and transportation of their gaseous products. A photoelectrochemical approach is proposed to drive the reductive production of industrial building-block chemicals and demonstrate that succinic acid and glyoxylic acid can be readily synthesized on Si nanowire array photocathodes free of any cocatalyst and at room temperature. These photocathodes exhibit a positive onset potential, large saturation photocurrent density, high reaction selectivity, and excellent operation durability. They capitalize on the large photovoltage generated from the semiconductor/electrolyte junction to partially offset the required external bias, and thereby make this photoelectrosynthetic approach significantly more sustainable compared to traditional electrosynthesis.

Carbon Dots for Bioimaging and Biosensing Applications

Abstract: Carbon dots (CDs, sp2 hybrid carbon particles) as a new type of biocompatible nanomaterials have received much attention because of their low toxicity, good water dispersability, ease of fabrication and functionalization, and outstanding photostability. Recently, they have been explored intensively to serve as biosensors and bioimaging agents for various bio-applications. In this chapter, we will introduce the fundamental properties of CDs and focus on their recent applications in biosensing and bioimaging. We will also summarize the recent progress in their fabrication and application in biomedical imaging, interactions with biomolecules, electrochemical biosensors. In addition, the remaining challenges and perspectives for future developments are also briefly discussed. We hope this chapter will provide some critical insights to inspire more exciting work on CDs for biological applications (sensing and imaging) in the near future.

Nanoscience and Nanotechnology Cross Borders

Abstract: Author(s): Khademhosseini, Ali; Chan, Warren WC; Chhowalla, Manish; Glotzer, Sharon C; Gogotsi, Yury; Hafner, Jason H; Hammond, Paula T; Hersam, Mark C; Javey, Ali; Kagan, Cherie R; Kotov, Nicholas A; Lee, Shuit-Tong; Li, Yan; Mohwald, Helmuth; Mulvaney, Paul A; Nel, Andre E; Parak, Wolfgang J; Penner, Reginald M; Rogach, Andrey L; Schaak, Raymond E; Stevens, Molly M; Wee, Andrew TS; Brinker, Jeffrey; Chen, Xiaoyuan; Chi, Lifeng; Crommie, Michael; Dekker, Cees; Farokhzad, Omid; Gerber, Christoph; Ginger, David S; Irvine, Darrell J; Kiessling, Laura L; Kostarelos, Kostas; Landes, Christy; Lee, Takhee; Leggett, Graham J; Liang, Xing-Jie; Liz-Marzan, Luis; Millstone, Jill; Odom, Teri W; Ozcan, Aydogan; Prato, Maurizio; Rao, CNR; Sailor, Michael J; Weiss, Emily; Weiss, Paul S

Photoelectroreduction toward Chemical Building Blocks

Abstract: Conventional photoelectrochemical cells utilize solar energy to drive the chemical conversion of water or CO2 into useful chemical fuels. Such processes are confronted with general challenges, including the low intrinsic activities and inconvenient storage and transportation of their gaseous products. A photoelectrochemical approach is proposed to drive the reductive production of industrial building-block chemicals and demonstrate that succinic acid and glyoxylic acid can be readily synthesized on Si nanowire array photocathodes free of any cocatalyst and at room temperature. These photocathodes exhibit a positive onset potential, large saturation photocurrent density, high reaction selectivity, and excellent operation durability. They capitalize on the large photovoltage generated from the semiconductor/electrolyte junction to partially offset the required external bias, and thereby make this photoelectrosynthetic approach significantly more sustainable compared to traditional electrosynthesis.

A Big Year Ahead for Nano in 2018

Abstract: In addition to publishing top work in nanoscience and nanotechnology from around the world, our goals for ACS Nano include: accelerating advances in the field by identifying and elaborating opportunities; leveraging nanoscience and nanotechnology strategies, techniques, and methods to impact other fields; being a public face for our fields; and linking together global science, engineering, and medicine. We have a number of strategies for achieving these goals and more. Among them is having our editors using our positions as active researchers to connect with you (and each other) around the world. This year, between our editors, we gave hundreds of talks in more than 35 countries, at universities, research institutions, and conferences, as well as to the public. We see the effects of our personal interactions during these visits in new submissions and our expanding readership. In the next year and beyond, we will have a sustained effort to engage even more at a number of special events.

Our First and Next Decades at ACS Nano

Abstract: Ten years ago this month, we published our first issue of ACS Nano. We had a grand vision, which we have kept to this day, that we would accelerate advances in the field by laying out the possibilities and implications for nanoscience and nanotechnology, in addition to identifying and reporting the required steps along the way.We felt that it was critical to provide comprehensive, high-profile descriptions of novel work on which others could build. We saw that previously missing venue as an important step for our field, and 10 years on, we can now see how far we have come as a result.