Wei-Hung Chiang

Linking catalyst composition to chirality distributions of as-grown single-walled carbon nanotubes by tuning Ni x Fe 1− x nanoparticles

Chirally pure single-walled carbon nanotubes (SWCNTs) are required for various applications ranging from nanoelectronics to nanomedicine. Although significant efforts have been directed towards separation of SWCNT mixtures, including density-gradient ultracentrifugation , chromatography and electrophoresis, the initial chirality distribution is determined during growth and must be controlled for non-destructive, scalable and economical production. Here, we show that the chirality distribution of as-grown SWCNTs can be altered by varying the composition of Ni(x)Fe(1-x) nanocatalysts. Precise tuning of the nanocatalyst composition at constant size is achieved by a new gas-phase synthesis route based on an atmospheric-pressure microplasma. The link between the composition-dependent crystal structure of the nanocatalysts and the resulting nanotube chirality supports epitaxial models and is a step towards chiral-selective growth of SWCNTs.

Continuous-flow, atmospheric-pressure microplasmas: a versatile source for metal nanoparticle synthesis in the gas or liquid phase

Continuous-flow, atmospheric-pressure microplamas are a unique class of plasmas that are highly suitable for emerging nanomaterials applications. Here, we present two schemes for the preparation of metal nanoparticles based on these plasma sources. Nanoparticles are synthesized in the gas phase by non-thermal dissociation of vapor precursors in a microplasma reactor. Monometallic Ni and Fe nanoparticles, as well as compositionally controlled NiFe bimetallic nanoparticles, can be grown with tunable mean diameters between 1 and 5 nm and narrow size distributions. Alternatively, colloidal metal nanoparticles are produced directly in aqueous solutions. Metal cations generated from anodic dissolution of a bulk metal or present in the form of metal salt are reduced by the microplasma to form nanoparticles and capped by a stabilizer. Both approaches are low cost, scalable and general and should allow a wide range of nanoparticle materials to be synthesized in the gas or liquid phase.

Nanoengineering Ni(x)Fe(1-x) catalysts for gas-phase, selective synthesis of semiconducting single-walled carbon nanotubes.

The inhomogeneity of as-grown single-walled carbon nanotubes (SWCNTs), in terms of chiral structure, is a major obstacle to integration of these novel materials in advanced electronics. While separation methods have circumvented this problem, current synthesis approaches must be refined for large-scale production of SWCNTs with uniform properties. In addition, it is highly desirable to alter the initial chirality distribution which constrains fundamental study and applications. Here, we demonstrate that semiconducting SWCNTs are selectively produced in the gas phase by engineering catalysts at the nanoscale with precise size and composition. The semiconducting content in as-grown mixtures of SWCNTs is assessed by UV-visible-NIR absorbance and micro-Raman spectroscopy and reaches a maximum purity of 90% for samples catalyzed by Ni(0.27)Fe(0.73) nanoparticles (2.0 nm mean diameter). Electrical studies are performed on thin film transistors (TFTs) fabricated from as-grown SWCNTs and reveal high on/off current ratios of 10(3).

Intercalation-assisted longitudinal unzipping of carbon nanotubes for green and scalable synthesis of graphene nanoribbons

We have demonstrated an effective intercalation of multi-walled carbon nanotubes (MWCNTs) for the green and scalable synthesis of graphene nanoribbons (GNRs) using an intercalation-assisted longitudinal unzipping of MWCNTs. The key step is to introduce an intercalation treatment of raw MWCNTs with KNO3 and H2SO4, making it promising to decrease the strong van der Waals attractions in the MWCNTs bundles and between the coaxial graphene walls of CNTs. Systematic micro Raman, X-ray photoelectron spectroscopy (XPS), and X-ray diffraction (XRD) characterizations suggest that potassium, nitrate, and sulfate ions play an important role in the CNT intertube and intratube intercalations during the pretreatment. Detailed scanning electron microscopy (SEM), transmission electron microscopy, XRD, and micro Raman characterizations indicate that the developed methodology possesses the ability to synthesis GNRs effectively with an improved CNT concentration in H2SO4 of 10 mg/ml at 70 °C, which is amenable to industrial-scale production because of the decreased amount of strong acid. Our work provides a scientific understanding how to enhance the GNR formation by accelerating the CNT longitudinal unzipping via suitable molecular intercalation.

Controllable Tailoring Graphene Nanoribbons with Tunable Surface Functionalities: An Effective Strategy toward High-Performance Lithium-Ion Batteries.

An effective, large-scale synthesis strategy for producing graphene nanoribbons (GNRs) with a nearly 100% yield has been proposed using a stepwise, solution-based, lengthwise unzipping carbon nanotube (CNT) method. Detailed Raman and X-ray photoelectron spectroscopy (XPS) analysis suggest that GNRs with tunable density of oxygen-containing functional groups on the GNR surfaces can be synthesized by adjusting the oxidant concentration during the CNT unzipping. The electrochemical characterization reveals that the as-produced GNRs with 42.91 atomic percent (atom %) oxygen-containing functional groups deliver a capacity of 437 mAh g(-1) after 100 cycles at 0.33C, while the as-produced GNRs with higher oxygen-containing functional groups only present a capacity of 225 mAh g(-1). On the basis of the electrochemical assessment and XPS analysis, the funtionals groups (epoxy-, carbonyl-, and carboxyl groups) in GNRs could be the effective contributor for the high-performance Li-ion batteries with appropriate adjustment.

High-Responsivity and High-Sensitivity Graphene Dots/a-IGZO Thin-Film Phototransistor

An a-IGZO thin-film phototransistor incorporating graphene absorption layer was proposed to enhance the responsivity and sensitivity simultaneously for photodetection from ultraviolet to visible regime. The spin-coated graphene dots absorb incident light, transferring electrons to the underlying a-IGZO to establish a photochannel. The 5 A/W responsivity and 1000 photo-to-dark current ratio were achieved for graphene phototransistor at 500 nm. As compared with <;1% absorption, the graphene phototransistor indicates a >2700 transistor gain. The highest responsivity and photo-to-dark current ratio is 897 A/W and 106, respectively, under 340-nm light illumination.

In situ growth of porphyrinic metal–organic framework nanocrystals on graphene nanoribbons for the electrocatalytic oxidation of nitrite

Graphene nanoribbons (GNRs) are incorporated with the nanocrystals of a porphyrinic metal–organic framework, MOF-525, by solvothermally growing MOF-525 in a suspension of well-dispersed GNRs. A nanocomposite, which is composed of the MOF-525 nanocrystals interconnected by numerous one-dimensional GNRs, is successfully synthesized. Due to the excellent dispersity, uniform thin films of the MOF-525/GNR nanocomposite can be simply deposited on conducting glass substrates by using drop casting. The obtained thin film of the MOF-525/GNR nanocomposite is applied for electrochemical nitrite sensors. The MOF-525 nanocrystals serve as a high-surface-area electrocatalyst toward nitrite and the interconnected GNRs act as conductive bridges to provide facile charge transport. The thin film of the MOF-525/GNR nanocomposite thus exhibits a much better electrocatalytic activity for the oxidation of nitrite compared to the pristine MOF-525 thin film.

Synergistic acceleration in the osteogenic and angiogenic differentiation of human mesenchymal stem cells by calcium silicate-graphene composites

Recent exciting findings of the biological interactions of graphene materials have shed light on potential biomedical applications of graphene-containing composites. Owing to the superior mechanical properties and low coefficient of thermal expansion, graphene has been widely used in the reinforcement of biocomposites. In the present study, various ratios of graphene (0.25wt%, 0.5wt% and 1.0wt%) were reinforced into calcium silicate (CS) for bone graft application. Results show that the graphene was embedded in the composites homogeneously. Adding 1wt% graphene into CS increased the youngs modulus by ~47.1%. The formation of bone-like apatite on a range of composites with graphene weight percentages ranging from 0 to 1 has been investigated in simulated body fluid. The presence of a bone-like apatite layer on the composites surface after immersion in simulated body fluid was considered by scanning electron microscopy. In vitro cytocompatibility of the graphene-contained CS composites was evaluated using human marrow stem cells (hMSCs). The proliferation and alkaline phosphatase, osteopontin and osteocalcin osteogenesis-related protein expression of the hMSCs on the 1wt% graphene-contained specimens showed better results than on the pure CS. In addition, the angiogenesis-related protein (vWF and ang-1) secretion of cells was significantly stimulated when the graphene concentration in the composites was increased. These results suggest that graphene-contained CS bone graft are promising materials for bone tissue engineering applications.

Nonlithographic fabrication of surface-enhanced Raman scattering substrates using a rastered atmospheric-pressure microplasma source

A nonlithographic patterning technique based on a numerically controlled atmospheric-pressure microplasma source has been developed to fabricate surface-enhanced Raman scattering (SERS) substrates. Microstructures in silver (Ag) films on glass are created by localized physical sputtering of Ag atoms using an argon (Ar) microplasma horizontally scanned across the glass substrate. Detection of crystal violet on patterned substrates shows an enhancement of the Raman scattering signal intensity by eight to ten orders of magnitude higher than bare Ag/glass substrates. The SERS enhancement depends on the pattern geometry showing that the mechanism is related to surface irregularities in the sputtered holes.

A high sensitivity field effect transistor biosensor for methylene blue detection utilize graphene oxide nanoribbon

In this work, we developed a field effect transistor (FET) biosensor utilizing solution-processed graphene oxide nanoribbon (GONR) for methylene blue (MB) sensing. MB is a unique material; one of its crucial applications is as a marker in the detection of biomaterials. Therefore, a highly sensitive biosensor with a low detection limit that can be fabricated simply in a noncomplex detection scheme is desirable. GONR is made by unzipping multiwall carbon nanotubes, which can be mass-produced at low temperature. The GONR-FET biosensor demonstrated a sensitivity of 12.5μA/mM (determined according to the drain current difference caused by the MB concentration change). The Raman spectra indicate that the materials quality of the GONR and the domain size for the C=C sp2 bonding were both improved after MB detection. X-ray photoelectron spectroscopy revealed that the hydroxyl groups on the GONR were removed by the reductive MB. According to XPS and Raman, the positive charge is proposed to transfer from MB to GONR during sensing. This transfer causes charge in-neutrality in the GONR which is compensated by releasing •OH functional groups. With high sensitivity, a low detection limit, and a simple device structure, the GONR-FET sensor is suitable for sensing biomaterials.