Highly emissive blue graphene quantum dots with excitation-independent emission via ultrafast liquid-phase photoreduction.

Graphene oxide quantum dots (GOQDs) are promising candidates for biomedical applications since they have lower toxicity and higher biocompatibility than traditional semiconductor quantum dots. However, oxygen functional groups such as epoxy and hydroxyl groups usually induce nonradiative relaxation, which leads to GOQDs exhibiting nonemissive properties. For the enhancement of the emission efficiency of GOQDs, the number of nonradiative relaxation sites should be reduced. This paper reports the synthesis of highly luminescent reduced GOQDs prepared by liquid-phase photoreduction (LPP-rGOQDs). First, GOQDs was fabricated from single-walled carbon nanotubes through chlorate-based oxidation and separation after acoustic cavitation. Subsequently, LPP-rGOQDs were obtained by liquid-phase photoreduction of the GOQD suspension under intense pulsed light irradiation. Liquid-phase photoreduction selectively reduced epoxy groups present on the basal plane of GOQDs, and hydrogenated the basal plane without removal of carbonyl and carboxyl groups at the edges of the GOQDs. Such selective removal of oxidative functional groups was used to control the reduction degree of GOQDs, closely related to their optical properties. The optimized LPP-rGOQDs were bright blue in color and showed quantum yields up to about 19.7%, which was 10 times the quantum yield of GOQDs. Furthermore, the LPP-rGOQDs were utilized to image a human embryonic kidney (HEK293A), and a low cytotoxicity level and satisfactory cell imaging performance were observed.

Achieving Both Ultrahigh Electrical Conductivity and Mechanical Modulus of Carbon Films: Templating-Coalescing Behavior of Single-Walled Carbon Nanotube in Polyacrylonitrile.

Promoting the feasibility of carbon films as electrode applications requires sufficient performances in view of both electrical and mechanical properties. Herein, carbon films with ultrahigh electrical conductivity and mechanical modulus are prepared by high temperature carbonization of polyacrylonitrile (PAN)/single-walled carbon nanotube (SWNT) nanocomposites. Achieving both performances is ascribed to remarkable graphitic crystallinity, resulting from the sequential templating-coalescing behavior of concentrated SWNT bundles (B-CNTs). While well-dispersed SWNTs (WD-CNTs) facilitate radial templating according to their tubular geometry, flattened B-CNTs sandwiched between carbonized PAN matrices induce vertical templating, where the former and latter produce concentric and planar crystallizations of the graphitic structure, respectively. After carbonization at 2500 °C with the remaining WD-CNTs as microstructural defects, the flattened B-CNTs coalesce into graphitic crystals by zipping the surrounding matrix, resulting in high crystallinity with the crystal thicknesses of 27.4 and 39.4 nm for the (002) and (10) planes, respectively. For comparison, the graphene oxide (GO) containing carbon films produce a less-ordered graphitic phase owing to irregular templating, despite the geometrical consistency. Consequently, PAN/B-CNT carbon films exhibit exceptional electrical conductivity (40.7 × 104 S m-1 ) and mechanical modulus (38.2 ± 6.4 GPa). Thus, controlling the templating-coalescing behavior of SWNTs is the key for improving final performances of carbon films.

Exploring the Chemical Structures and Phototochemical Properties of Graphene Oxide Derivatives by Laser Desorption/Ionization Time-of-Flight Mass Spectrometry to Develop an Efficient Platform for Mass Spectrometric Analysis.

Laser desorption/ionization time-of-flight mass spectrometry (LDI-TOF-MS) analysis is harnessed to investigate the chemical structure and photochemical properties of two distinct graphene oxide (GO) derivatives simultaneously. The GO derivatives are synthesized with modified Brodie's method (BGO) and Hummers' method (HGO) and characterized by LDI-TOF-MS as well as conventional tools. A series of LDI-TOF-MS analyses reveal that BGO provides higher laser energy absorption, photochemical stability, and photothermal conversion property than HGO based on their fragmentation patterns and laser desorption/ionization behavior of a thermometer molecule. Based on these characteristics, BGO exhibits higher efficiency in the LDI-TOF-MS analysis of various small molecules and synthetic polymers than HGO. These different photochemical properties of BGO are derived from its large sp2 carbon domains compared to HGO. Based on our findings, the analytical potential of LDI-TOF-MS for GO derivatives is clearly demonstrated, which can be an efficient and unique characterization tool to explore both chemical structures and photochemical properties of various carbon materials.

Electronic Textiles Based on Highly Conducting Poly(vinyl alcohol)/Carbon Nanotube/Silver Nanobelt Hybrid Fibers.

Highly stable conducting fibers have attracted significant attention in electronic textile (e-textile) applications. Here, we fabricate highly conducting poly(vinyl alcohol) (PVA) nanocomposite fibers with high thermal and chemical stability based on silver nanobelt (AgNB)/multiwalled carbon nanotube (MWCNT) hybrid materials as conducting fillers. At 20 vol % AgNB/MWCNT, the electrical conductivity of the fiber dramatically increased (∼533 times) from 3 up to 1600 S/cm after thermal treatment at 300 °C for 5 min. Moreover, PVA/AgNB/MWCNT fiber resists the harsh conditions of good solvents for PVA as well as high temperatures over the melting point of PVA, whereas pure PVA fiber is unstable in these environments. The significantly enhanced electrical conductivity and chemical stability can be realized through the post-thermal curing process, which is attributed to the coalescence between adjacent AgNBs and additional intensive cross-linking of PVA. These remarkable characteristics make our conducting fibers suitable for applications in e-textiles such as water leakage detectors and wearable heaters. In particular, heating behavior of e-textiles by Joule heating can accelerate the desorption of physically trapped moisture from the fiber surface, resulting in the fully reversible operation of water leakage monitoring. This smart e-textile sensor based on highly stable and conductive composite fibers will pave the way for diverse e-textile applications.

Synthesis of silver nanoparticles embedded with single-walled carbon nanotubes for printable elastic electrodes and sensors with high stability.

Soft electronic devices that are bendable and stretchable require stretchable electric or electronic components. Nanostructured conducting materials or soft conducting polymers are one of the most promising fillers to achieve high performance and durability. Here, we report silver nanoparticles (AgNPs) embedded with single-walled carbon nanotubes (SWCNTs) synthesized in aqueous solutions at room temperature, using NaBH4 as a reducing agent in the presence of highly oxidized SWCNTs as efficient nucleation agents. Elastic composite films composed of the AgNPs-embedded SWCNTs, Ag flake, and polydimethylsiloxane are irradiated with radiation from a Xenon flash lamp within a time interval of one second for efficient sintering of conductive fillers. Under high irradiation energy, the stretchable electrodes are created with a maximum conductivity of 4,907 S cm-1 and a highly stretchable stability of over 10,000 cycles under a 20% strain. Moreover, under a low irradiation energy, strain sensors with a gauge factor of 76 under a 20% strain and 5.4 under a 5% strain are fabricated. For practical demonstration, the fabricated stretchable electrode and strain sensor are attached to a human finger for detecting the motions of the finger.

Extraordinary thermal behavior of graphene oxide in air for electrode applications.

The thermal stability of solution-exfoliated graphene oxide (GO) in air is one of the most important physical properties influencing its potential applications. To date, the majority of the GO prepared by the KMnO4-based oxidation of graphite is thermally unstable in air due to the presence of highly oxidative functional groups, such as carboxyl and lactol groups that possess defective basal plane structures. Here, we demonstrate that less defective and metal ion-free GO nanosheets including those with a high oxidation level can remain very stable even above 300 °C under ambient conditions. These GO nanosheets were produced by the exfoliation of graphite oxide fabricated by the modified Brodie method in NH4OH solution, effectively excluding metal ions that can promote the thermal decomposition of GO in air at elevated temperatures. The deoxygenation of ammonia-assisted GO (AGO) was initiated at temperatures above 200 °C, while GO exfoliated in the KOH solution (KGO) decomposed, even at 180 °C. Notably, AGO was exceptionally resistant at 400 °C, even at a very slow heating rate of 2 °C min-1. Conversely, KGO was significantly oxidized, even at 250 °C. The superior thermal stability of AGO is favorable for the fabrication of conductive surface graphene films and conductive fibers by low-temperature annealing.

3D-Printed Sugar Scaffold for High-Precision and Highly Sensitive Active and Passive Wearable Sensors.

In this study, a pairing of a previously unidentified 3D printing technique and soft materials is introduced in order to achieve not only high-resolution printed features and flexibility of the 3D-printed materials, but also its light-weight and electrical conductivity. Using the developed technique and materials, high-precision and highly sensitive patient-specific wearable active or passive devices are fabricated for personalized health monitoring. The fabricated biosensors show low density and substantial flexibility because of 3D microcellular network-type interconnected conductive materials that are readily printed using an inkjet head. Using high-resolution 3D scanned body-shape data, on-demand personalized wearable sensors made of the 3D-printed soft and conductive materials are fabricated. These sensors successfully detect both actively changing body strain signals and passively changing signals such as electromyography (EMG), electrodermal activity (EDA), and electroencephalogram EEG. The accurately tailored subject-specific shape of the developed sensors exhibits higher sensitivity and faster real-time sensing performances in the monitoring of rapidly changing human body signals. The newly developed 3D printing technique and materials can be widely applied to various types of wearable, flexible, and light-weight biosensors for use in a variety of inexpensive on-demand and personalized point-of-care diagnostics.

Highly Exfoliated and Functionalized Single-Walled Carbon Nanotubes as Fast-Charging, High-Capacity Cathodes for Rechargeable Lithium-Ion Batteries.

Compared with traditional metal-oxide lithium-ion battery (LIB) cathodes, nanocarbon-based cathode materials have received much attention for potential application in LIBs because of their superior power density and long-term cyclability. However, their lithium-ion storage capacity needs further improvement for practical applications, and the trade-off between capacity and conductivity, when oxygen functional groups as lithium-ion storage sites are introduced to the nanocarbon materials, needs to be addressed. Here, we report a sequential oxidation-reduction process for the synthesis of single-walled carbon nanotubes (SWCNTs) for LIB cathodes with fast charging, long-term cyclability, and high gravimetric capacity. A LIB cathode based on highly exfoliated (dbundle < 10 nm) and oxygen-functionalized single-walled carbon nanotubes is obtained via the modified Brodie's method using fuming nitric acid and a mild oxidant (B-SWCNTs). Post treatment including horn sonication and hydrogen thermal reduction developed surface defects and removed the unnecessary C-O groups, resulting in an increase in the Li-ion storage capacity. The B-SWCNTs exhibit a high reversible gravimetric capacity of 344 mA h g-1 at 0.1 A g-1 without noticeable capacity fading after 1000 cycles. Furthermore, it delivers a high gravimetric energy density of 797 W h kgelectrode-1 at a low gravimetric power density of 300 W kgelectrode-1 and retains its high gravimetric energy density of ∼100 W h kgelectrode-1 at a high gravimetric power of 105 W kgelectrode-1. These results suggest that the highly exfoliated, oxygen-functionalized single-walled carbon nanotubes can be applied to LIBs designed for high-rate operations and long cycling.

Improved Charge Transport and Reduced Non-Geminate Recombination in Organic Solar Cells by Adding Size-Selected Graphene Oxide Nanosheets.

Size-selected graphene oxide (GO) nanosheets were used to modify the bulk heterojunction (BHJ) morphology and electrical properties of organic photovoltaic (OPV) devices. The GO nanosheets were prepared with sizes ranging from several hundreds of nanometers to micrometers by using a physical sonication process and were then incorporated into PTB7:PC71BM photoactive layers. Different GO sizes provide varied portions of the basal plane where aromatic sp2-hybridized regions are dominant and edges where oxygenated functional groups are located; thus, GO size distributions affect the GO dispersion stability and morphological aggregation of the BHJ layer. Electron delocalization by sp2-hybridization and the electron-withdrawing characteristics of functional groups p-dope the photoactive layer, giving rise to increasing carrier mobilities. Hole and electron mobilities are maximized at GO sizes of several hundreds of nanometers. Consequently, non-geminate recombination is significantly reduced by these facilitated hole and electron transports. The addition of GO nanosheets decreases the recombination order of non-geminate recombination and increases the generated carrier density. This reduction in the non-geminate recombination contributes to an increased power conversion efficiency of PTB7:PC71BM OPV devices as high as 9.21%, particularly, by increasing the fill factor to 70.5% in normal devices and 69.4% in inverted devices.

High-performance flexible transparent nanomesh electrodes.

A cost-effective process for producing high-performance Ag-paste-based flexible transparent nanomesh electrodes (FTNEs) was developed by optimizing their linewidth, pitch, and height. These nanomesh electrodes, with a linewidth of several hundred nanometers and a pitch of 10-200 µm on a PET substrate, achieved wide ranges of transmittance (83.1%-98.8%) and sheet resistance (1.2-30.9 Ω/sq) and a figure of merit (992-1619) superior to those of indium tin oxide and silver nanowire (AgNW) electrodes. Our evaluation of their flexibility (testing up to 50 000 cycles) and their electromagnetic interference shielding effectiveness verifies the applicability of these FTNEs to various flexible optoelectronic devices.

Publisher Correction: Synthesis of nanobelt-like 1-dimensional silver/nanocarbon hybrid materials for flexible and wearable electronics.

A correction to this article has been published and is linked from the HTML and PDF versions of this paper. The error has been fixed in the paper.

Pressure/Temperature Sensing Bimodal Electronic Skin with Stimulus Discriminability and Linear Sensitivity.

Human skin imperfectly discriminates between pressure and temperature stimuli under mixed stimulation, and exhibits nonlinear sensitivity to each stimulus. Despite great advances in the field of electronic skin (E-skin), the limitations of human skin have not previously been overcome. For the first time, the development of a stimulus-discriminating and linearly sensitive bimodal E-skin that can simultaneously detect and discriminate pressure and temperature stimuli in real time is reported. By introducing a novel device design and using a temperature-independent material, near-perfect stimulus discriminability is realized. In addition, the hierarchical contact behavior of the surface-wrinkled microstructure and the optimally reduced graphene oxide in the E-skin contribute to linear sensitivity to applied pressure/temperature stimuli over wide intensity range. The E-skin exhibits a linear and high pressure sensitivity of 0.7 kPa-1 up to 25 kPa. Its operation is also robust and exhibits fast response to pressure stimulus within 50 ms. In the case of temperature stimulus, the E-skin shows a linear and reproducible temperature coefficient of resistance of 0.83% K-1 in the temperature range 22-70 °C and fast response to temperature change within 100 ms. In addition, two types of stimuli are simultaneously detected and discriminated in real time by only impedance measurements.

Controllable Bipolar Doping of Graphene with 2D Molecular Dopants.

The fine control of graphene doping levels over a wide energy range remains a challenging issue for the electronic applications of graphene. Here, the controllable doping of chemical vapor deposited graphene, which provides a wide range of energy levels (shifts up to ± 0.5 eV), is demonstrated through physical contact with chemically versatile graphene oxide (GO) sheets, a 2D dopant that can be solution-processed. GO sheets are a p-type dopant due to their abundance of electron-withdrawing functional groups. To expand the energy window of GO-doped graphene, the GO surface is chemically modified with electron-donating ethylene diamine molecules. The amine-functionalized GO sheets exhibit strong n-type doping behaviors. In addition, the particular physicochemical characteristics of the GO sheets, namely their sheet sizes, number of layers, and degree of oxidation and amine functionality, are systematically varied to finely tune their energy levels. Finally, the tailor-made GO sheet dopants are applied into graphene-based electronic devices, which are found to exhibit improved device performances. These results demonstrate the potential of GO sheet dopants in many graphene-based electronics applications.

Synthesis of nanobelt-like 1-dimensional silver/nanocarbon hybrid materials for flexible and wearable electroncs.

Most synthetic processes of metallic nanostructures were assisted by organic/inorganic or polymeric materials to control their shapes to one-dimension or two-dimension. However, these additives have to be removed after synthesis of metal nanostructures for applications. Here we report a straightforward method for the low-temperature and additive-free synthesis of nanobelt-like silver nanostructures templated by nanocarbon (NC) materials via bio-inspired shape control by introducing supramolecular 2-ureido-4[1H]pyrimidinone (UPy) groups into the NC surface. The growth of the Ag nanobelt structure was found to be induced by these UPy groups through observation of the selective formation of Ag nanobelts on UPy-modified carbon nanotubes and graphene surfaces. The synthesized NC/Ag nanobelt hybrid materials were subsequently used to fabricate the highly conductive fibres (>1000S/cm) that can function as a conformable electrode and highly tolerant strain sensor, as well as a highly conductive and robust paper (>10000S/cm after thermal treatment).

Multi-functionalized herringbone carbon nanofiber for anodes of lithium ion batteries.

Herringbone carbon nanofibers (HCNFs) are prepared for use as anode materials in lithium-ion batteries (LIBs). HCNFs are prepared using a Ni-Fe catalyst and subsequently multi-functionalized with oxygen using the Hummers' method, and then with both oxygen and nitrogen-containing 2-ureido-4[1H]pyrimidinone (UHP) moieties, which endow the HCNFs with the ability to form quadruple hydrogen bonds (QHBs). The as-prepared HCNFs are, on average, 13 µm in length and 100 nm in diameter, with a highly graphitic structure. The oxidized HCNFs (Ox-HCNFs) obtained by Hummers' method are partially exfoliated, having double-bladed saw-like structures that extend in the direction of the graphite planes. QHBs are formed between the HCNFs after functionalization with the UHP moieties. The final surface-modified HCNFs (N-Ox-HCNFs) have more electrochemical sites, shorter Li+ diffusion lengths, and additional electron pathways compared with the as-prepared HCNF and Ox-HCNF. The introduction of oxygen- and nitrogen-containing functional groups improves the performance of LIBs: a high charge capacity of 763 mA h g-1 at 0.1 A g-1, excellent rate capability (a capacity of 402 mA h g-1 at 3 A g-1), and near 100% capacity retention after 300 cycles are reported.

Bioinspired Multifunctional Superhydrophobic Surfaces with Carbon-Nanotube-Based Conducting Pastes by Facile and Scalable Printing.

Directly printed superhydrophobic surfaces containing conducting nanomaterials can be used for a wide range of applications in terms of nonwetting, anisotropic wetting, and electrical conductivity. Here, we demonstrated that direct-printable and flexible superhydrophobic surfaces were fabricated on flexible substrates via with an ultrafacile and scalable screen printing with carbon nanotube (CNT)-based conducting pastes. A polydimethylsiloxane (PDMS)-polyethylene glycol (PEG) copolymer was used as an additive for conducting pastes to realize the printability of the conducting paste as well as the hydrophobicity of the printed surface. The screen-printed conducting surfaces showed a high water contact angle (WCA) (>150°) and low contact angle hysteresis (WCA < 5°) at 25 wt % PDMS-PEG copolymer in the paste, and they have an electrical conductivity of over 1000 S m-1. Patterned superhydrophobic surfaces also showed sticky superhydrophobic characteristics and were used to transport water droplets. Moreover, fabricated films on metal meshes were used for an oil/water separation filter, and liquid evaporation behavior was investigated on the superhydrophobic and conductive thin-film heaters by applying direct current voltage to the film.

Ultrafast Heating for Intrinsic Properties of Atomically Thin Two-Dimensional Materials on Plastic Substrates.

Despite recent progress in producing flexible and stretchable electronics based on two-dimensional (2D) nanosheets, their intrinsic properties are often degraded by the presence of polymeric residues that remain attached to the 2D nanosheet surfaces following fabrication. Further breakthroughs are therefore keenly awaited to obtain clean surfaces compatible with flexible applications. Here, we report a method that allows the 2D nanosheets to be intrinsically integrated onto flexible substrates. The method involves thermal decomposition of polymeric residues by microwave-induced ultrafast heating of the surface without affecting the underlying flexible substrate. Mapping the C═O stretching mode by Fourier-transform infrared spectroscopy in combination with atomic force microscopy confirms elimination of the polymeric residues from the 2D nanosheet surface. Flexible devices prepared using microwave-cleaned 2D nanosheets show enhanced electrical, optical, and electrothermal performances. This simple technique is applicable to a wide range of 2D nanomaterials and represents an important advance in the field of flexible devices.

Enhanced transparent conducting networks on plastic substrates modified with highly oxidized graphene oxide nanosheets.

Atomically thin and two-dimensional graphene oxide (GO) is a very fascinating material because of its functional groups, high transparency, and solution processability. Here we show that highly oxidized GO (HOGO) nanosheets serve as an effective interfacial modifier of transparent conducting films with one-dimensional (1D) silver nanowires (AgNWs) and single-walled carbon nanotubes (SWCNTs). Optically transparent and small-sized GO nanosheets, with minimal sp(2) domains, were successfully fabricated by step-wise oxidation and exfoliation of graphite. We demonstrated that under-coated HOGO further decreases the sheet resistance of the SWCNT film top-coated with HOGO by increasing the contact area between the SWCNTs and HOGO nanosheets by generating hole carriers in the SWCNT as a result of charge transfer. Moreover, HOGO nanosheets with AgNWs contribute to the efficient thermal joining of AgNW networks on plastic substrates by limiting the thermal embedding of AgNWs into the plastic surface, resulting in efficient decrease of the sheet resistance. Furthermore, flexible organic photovoltaic cells with GO-modified AgNW anodes on a flexible substrate were successfully demonstrated.

Sub-second carbon-nanotube-mediated microwave sintering for high-conductivity silver patterns on plastic substrates.

A method of microwave sintering that is mediated by carbon nanotubes (CNTs) has been developed to obtain high-conductivity Ag patterns on the top of heat-sensitive plastic substrates within a short time. The Ag patterns are printed on CNTs formed on plastic substrates and rapidly heated to a great extent by the heat transferred from the microwave-heated CNTs. The conductivity of the microwave-sintered Ag patterns reaches ∼39% that of bulk Ag within 1 s without substrate deformation. Furthermore, microwave sintering enhances the adhesion of Ag patterns to the thermoplastic substrates because the sintering causes interfacial fusion between the Ag patterns and the substrates, and CNTs physically connect the patterns with the substrates.

Stretchable and Multimodal All Graphene Electronic Skin.

A transparent and stretchable all-graphene multifunctional electronic-skin sensor matrix is developed. Three different functional sensors are included in this matrix: humidity, thermal, and pressure sensors. These are judiciously integrated into a layer-by-layer geometry through a simple lamination process.