Controlling Graphene Wrinkles through the Phase Transition of a Polymer with a Low Critical Solution Temperature.

A novel method for controlling reduced graphene oxide (rGO) wrinkles through a phase transition in a solution using a low critical solution temperature (LCST) polymer dispersant has been developed. The polymer dispersant is designed by control of architecture and composition using reversible addition-fragmentation chain transfer polymerization. Synthesized poly(2-(dimethylaminoethyl) methacrylate-block-styrene) (PDbS) can be successfully functionalized on the rGO surface via noncovalent functionalization. PDbS-functionalized rGO (PDbS-rGO) exhibits good dispersibility in an aqueous phase at room temperature and forms wrinkles on the PDbS-rGO surface because of phase transition at the LCST of the polymer dispersant. The formation of PDbS-rGO wrinkles is controlled by varying the aggregation number of the polymer dispersant on the PDbS-rGO surface that strongly depends on temperature. This is confirmed by transmission electron microscopy, scanning electron microscopy, and Raman spectroscopy (ID' /IG ratios are 0.560, 0.579, and 0.684, which correspond to 45, 70, and 95 °C, respectively). In addition, the mechanism of wrinkle control is proved by gold nanoparticles that are grown in polymer dispersant on the PDbS-rGO surface.

Tube-rolling and formation of mechanically robust micro-tubes in graphene oxide aqueous dispersions during shear flow.

We showed that GO domains at low pH are under a tube-rolling motion with a vorticity alignment at low shear rates. Mechanically robust micro-tubes were formed during tube-rolling. The micro-tubes were highly bendable and exhibited excellent elastic recovery. There was no restacking of GO sheets to graphitic structures for the GO micro-tube wall in a wet state.

Monolithic Graphene Trees as Anode Material for Lithium Ion Batteries with High C-Rates.

Monolithically structured reduced graphene oxide (rGO), prepared from a highly concentrated and conductive rGO paste, is introduced as an anode material for lithium ion batteries with high rate capacities. This is achieved by a mixture of rGO paste and the water-soluble polymer sodium carboxymethylcellulose (SCMC) with freeze drying. Unlike previous 3D graphene porous structures, the monolithic graphene resembles densely branched pine trees and has high mechanical stability with strong adhesion to the metal electrodes. The structures contain numerous large surface area open pores that facilitate lithium ion diffusion, while the strong hydrogen bonding between the graphene layers and SCMC provides high conductivity and reduces the volume changes that occur during cycling. Ultrafast charge/discharge rates are obtained with outstanding cycling stability and the capacities are higher than those reported for other anode materials. The fabrication process is simple and straightforward to adjust and is therefore suitable for mass production of anode electrodes for commercial applications.

One-step transfer and integration of multifunctionality in CVD graphene by TiO2/graphene oxide hybrid layer.

We present a straightforward method for simultaneously enhancing the electrical conductivity, environmental stability, and photocatalytic properties of graphene films through one-step transfer of CVD graphene and integration by introducing TiO2/graphene oxide layer. A highly durable and flexible TiO2 layer is successfully used as a supporting layer for graphene transfer instead of the commonly used PMMA. Transferred graphene/TiO2 film is directly used for measuring the carrier transport and optoelectronic properties without an extra TiO2 removal and following deposition steps for multifunctional integration into devices because the thin TiO2 layer is optically transparent and electrically semiconducting. Moreover, the TiO2 layer induces charge screening by electrostatically interacting with the residual oxygen moieties on graphene, which are charge scattering centers, resulting in a reduced current hysteresis. Adsorption of water and other chemical molecules onto the graphene surface is also prevented by the passivating TiO2 layer, resulting in the long term environmental stability of the graphene under high temperature and humidity. In addition, the graphene/TiO2 film shows effectively enhanced photocatalytic properties because of the increase in the transport efficiency of the photogenerated electrons due to the decrease in the injection barrier formed at the interface between the F-doped tin oxide and TiO2 layers.

Ultra-Mild Fabrication of Highly Concentrated SWCNT Dispersion Using Spontaneous Charging in Solvated Electron System.

The efficient dispersion of single-walled carbon nanotubes (SWCNTs) has been the subject of extensive research over the past decade. Despite these efforts, achieving individually dispersed SWCNTs at high concentrations remains challenging. In this study, we address the limitations associated with conventional methods, such as defect formation, excessive surfactant use, and the use of corrosive solvents. Our novel dispersion method utilizes the spontaneous charging of SWCNTs in a solvated electron system created by dissolving potassium in hexamethyl phosphoramide (HMPA). The resulting charged SWCNTs (c-SWCNTs) can be directly dispersed in the charging medium using only magnetic stirring, leading to defect-free c-SWCNT dispersions with high concentrations of up to 20 mg/mL. The successful dispersion of individual c-SWCNT strands is confirmed by their liquid-crystalline behavior. Importantly, the dispersion medium for c-SWCNTs exhibits no reactivity with metals, polymers, or other organic solvents. This versatility enables a wide range of applications, including electrically conductive free-standing films produced via conventional blade coating, wet-spun fibers, membrane electrodes, thermal composites, and core-shell hybrid microparticles.

Self-organized graphene nanosheets with corrugated, ordered tip structures for high-performance flexible field emission.

Patterned reduced graphene oxide (rGO) films with vertically aligned tip structures are fabricated by a straightforward self-assembly method. The size, uniformity of the patterns, and alignment of the tips are successfully controlled according to the concentration of a GO/octadecylamine (ODA)-dispersed solution. The surface energy difference between the GO/ODA solution and a self-assembled water droplet is a critical parameter for determining the pattern structure. Numerous rGO nanosheets are formed so as to be vertically aligned with respect to the substrate during film fabrication at GO concentrations below 2.0 g/L. These samples provide high field-emission characteristics. The patterned rGO arrays are highly flexible with preservation of the field emission properties, even at large bending angles. This is attributed to the high crystallinity, emitter density, and good chemical stability of the rGO arrays, as well as the strong interactions between the rGO arrays and the substrate.

Flexible field emission from thermally welded chemically doped graphene thin films.

Flexible field-emission devices (FEDs) based on reduced graphene oxide (RGO) emitters are fabricated by the thermal welding of RGO thin films onto a polymeric substrate. The RGO edges are vertically aligned relative to the substrate as a result of cohesive failure in the RGO layer after thermal welding. Even at large bending angles, excellent electron emission properties, such as low turn-on and threshold fields, a high emission current density, a high field enhancement factor, and long-term stability of the emission properties of RGO emitters, arise from the uniform distribution and high density of the extremely sharp RGO edges, as well as the high interfacial strength between the RGO emitters and the substrate. Al- and Au-doped RGO emitters are fabricated by introducing a dopant solution to the RGO emitters, and the resulting field-emission characteristics are discussed. The proposed approach is straightforward and enables the practical use of high-performance RGO flexible FEDs.

D-(+)-galactose-conjugated single-walled carbon nanotubes as new chemical probes for electrochemical biosensors for the cancer marker galectin-3.

D-(+)-Galactose-conjugated single-walled carbon nanotubes (SWCNTs) were synthesized for use as biosensors to detect the cancer marker galectin-3. To investigate the binding of galectin-3 to the d-(+)-galactose-conjugated SWCNTs, an electrochemical biosensor was fabricated by using molybdenum electrodes. The binding affinities of the conjugated SWCNTs to galectin-3 were quantified using electrochemical sensitivity measurements based on the differences in resistance together with typical I-V characterization. The electrochemical sensitivity measurements of the d-(+)-galactose-conjugated SWCNTs differed significantly between the samples with and without galectin-3. This indicates that d-(+)-galactose-conjugated SWCNTs are potentially useful electrochemical biosensors for the detection of cancer marker galectin-3.

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.

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.

Efficient synthesis of individual single-walled carbon nanotube by water-based catalyst with poly(vinylpyrrolidone).

Individual single-walled carbon nanotubes (SWCNTs) were synthesized on the patterned water-soluble catalyst by thermal chemical vapor deposition. The individual SWCNTs were obtained by introducing polyvinylpyrrolidone (PVP) as a dispersant. The number of SWCNTs between two electrodes were approximately 1-2 with an average diameter of about 1.7 nm and a yield of forming electrodes of nearly 70%. The PVP played an important role in dispersing catalysts and suppressing the active sites to limit the number of SWCNTs during synthesis, which is a critical condition for fabrication of field effect transistors (FETs). The measured I-V characteristics of the over layer-deposited electrodes revealed a clear gating effect in large portion, in good agreement with Raman observations in several excitation energies. The patterning procedure, catalyst preparation, and growth condition for fabrication of the SWCNT-FET were further discussed.

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.

Autophagic flux induced by graphene oxide has a neuroprotective effect against human prion protein fragments.

Graphene oxide (GO) is a nanomaterial with newly developing biological applications. Autophagy is an intracellular degradation system that has been associated with the progression of neurodegenerative disorders. Although induction of autophagic flux by GO has been reported, the underlying signaling pathway in neurodegenerative disorders and how this is involved in neuroprotection remain obscure. We show that GO itself activates autophagic flux in neuronal cells and confers a neuroprotective effect against prion protein (PrP) (106-126)-mediated neurotoxicity. GO can be detected in SK-N-SH neuronal cells, where it triggers autophagic flux signaling. GO-induced autophagic flux prevented PrP (106-126)-induced neurotoxicity in SK-N-SH cells. Moreover, inactivation of autophagic flux blocked GO-induced neuroprotection against prion-mediated mitochondrial neurotoxicity. This is the first study to demonstrate that GO regulates autophagic flux in neuronal cells, and that activation of autophagic flux signals, induced by GO, plays a neuroprotective role against prion-mediated mitochondrial neurotoxicity. These results suggest that the nanomaterial GO may be used to activate autophagic flux and could be used in neuroprotective strategies for treatment of neurodegenerative disorders, including prion diseases.

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.

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).

Modulating Electronic Properties of Monolayer MoS2 via Electron-Withdrawing Functional Groups of Graphene Oxide.

Modulation of the carrier concentration and electronic type of monolayer (1L) MoS2 is highly important for applications in logic circuits, solar cells, and light-emitting diodes. Here, we demonstrate the tuning of the electronic properties of large-area 1L-MoS2 using graphene oxide (GO). GO sheets are well-known as hole injection layers since they contain electron-withdrawing groups such as carboxyl, hydroxyl, and epoxy. The optical and electronic properties of GO-treated 1L-MoS2 are dramatically changed. The photoluminescence intensity of GO-treated 1L-MoS2 is increases by more than 470% compared to the pristine sample because of the increase in neutral exciton contribution. In addition, the A1g peak in Raman spectra shifts considerably, revealing that GO treatment led to the formation of p-type doped 1L-MoS2. Moreover, the current vs voltage (I-V) curves of GO-coated 1L-MoS2 field effect transistors show that the electron concentration of 1L-MoS2 is significantly lower in comparison with pristine 1L-MoS2. Current rectification is also observed from the I-V curve of the lateral diode structure with 1L-MoS2 and 1L-MoS2/GO, indicating that the electronic structure of MoS2 is significantly modulated by the electron-withdrawing functional group of GO.

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.

Intercalant-induced superbundle formation of single-wall carbon nanotubes.

The formation of a massive quantity of single-wall carbon nanotube (SWCNT) superbundles has been introduced through sonicating SWCNTs in tetramethylene sulfone/chloroform solution in which nitronium hexafluoroantimonate (NHFA) is dissolved. Most SWCNT bundles with the NHFA treatment are enlarged by about 10 times compared with those of the pristine sample. It is proposed that the formation of SWCNTs can occur in solution by formation of an SWCNT-intercalant charge complex. The specific surface area of the superbundle is almost doubled, while its micropore surface area is amplified by about 7 times. This development of microporosity results from the enhanced interstitial sites in the SWCNT superbundles.

Nanomanipulator-assisted fabrication and characterization of carbon nanotubes inside scanning electron microscope.

We have installed two nanomanipulators, which can travel about 20mm with a minimum increment of 1 nm, for manipulation of nanostructured materials inside field-emission scanning electron microscope (FE-SEM). Both manipulators render motions in x, y, and z directions, providing various manipulation freedoms such as moving, bending, cutting, and biasing. In addition, we have conducted in situ characterization of the electrical breakdown of multi-walled carbon nanotubes (MWCNTs). Our results demonstrate the possibility that MWCNTs can be used as a gas sensor.

Chemically doped three-dimensional porous graphene monoliths for high-performance flexible field emitters.

Despite the recent progress in the fabrication of field emitters based on graphene nanosheets, their morphological and electrical properties, which affect their degree of field enhancement as well as the electron tunnelling barrier height, should be controlled to allow for better field-emission properties. Here we report a method that allows the synthesis of graphene-based emitters with a high field-enhancement factor and a low work function. The method involves forming monolithic three-dimensional (3D) graphene structures by freeze-drying of a highly concentrated graphene paste and subsequent work-function engineering by chemical doping. Graphene structures with vertically aligned edges were successfully fabricated by the freeze-drying process. Furthermore, their number density could be controlled by varying the composition of the graphene paste. Al- and Au-doped 3D graphene emitters were fabricated by introducing the corresponding dopant solutions into the graphene sheets. The resulting field-emission characteristics of the resulting emitters are discussed. The synthesized 3D graphene emitters were highly flexible, maintaining their field-emission properties even when bent at large angles. This is attributed to the high crystallinity and emitter density and good chemical stability of the 3D graphene emitters, as well as to the strong interactions between the 3D graphene emitters and the substrate.