arXiv subjects

Hongjie Dai

Publications and source records attributed to Hongjie Dai.

At least 37 records · Page 2Linked to original sources

arXiv2011

Graphene-Wrapped Sulfur Particles as a Rechargeable Lithium-Sulfur-Battery Cathode Material with High Capacity and Cycling Stability

We report the synthesis of a graphene-sulfur composite material by wrapping polyethyleneglycol (PEG) coated submicron sulfur particles with mildly oxidized graphene oxide sheets decorated by carbon black nanoparticles. The PEG and graphene coating layers are important to accommodating volume expansion of the coated sulfur particles during discharge, trapping soluble polysulfide intermediates and rendering the sulfur particles electrically conducting. The resulting graphene-sulfur composite showed high and stable specific capacities up to ~600mAh/g over more than 100 cycles, representing a promising cathode material for rechargeable lithium batteries with high energy density.

cond-mat.mtrl-sci↗

arXiv2011

Thermally-Limited Current Carrying Ability of Graphene Nanoribbons

We investigate high-field transport in graphene nanoribbons (GNRs) on SiO2, up to breakdown. The maximum current density is limited by self-heating, but can reach >3 mA/um for GNRs ~15 nm wide. Comparison with larger, micron-sized graphene devices reveals that narrow GNRs benefit from 3D heat spreading into the SiO2, which enables their higher current density. GNRs also benefit from lateral heat flow to the contacts in short devices (< ~0.3 um), which allows extraction of a median GNR thermal conductivity (TC), ~80 W/m/K at 20 C across our samples, dominated by phonons. The TC of GNRs is an order of magnitude lower than that of micron-sized graphene on SiO2, suggesting strong roles of edge and defect scattering, and the importance of thermal dissipation in small GNR devices.

cond-mat.mes-hall↗

arXiv2011

Graphene nanoribbons from unzipped carbon nanotubes: atomic structures, Raman spectroscopy and electrical properties

We investigated the atomic structures, Raman spectroscopic and electrical transport properties of individual graphene nanoribbons (GNRs, widths ~10-30 nm) derived from sonochemical unzipping of multi-walled carbon nanotubes (MWNTs). Aberration-corrected transmission electron microscopy (TEM) revealed a high percentage of two-layer (2L) GNRs and some single layer ribbons. The layer-layer stacking angles ranged from 0o to 30o including average chiral angles near 30o (armchair orientation) or 0o (zigzag orientation). A large fraction of GNRs with bent and smooth edges was observed, while the rest showing flat and less smooth edges (roughness \leq 1 nm). Polarized Raman spectroscopy probed individual GNRs to reveal D/G ratios and ratios of D band intensities at parallel and perpendicular laser excitation polarization (D///D\bot). The observed spectroscopic trends were used to infer the average chiral angles and edge smoothness of GNRs. Electrical transport and Raman measurements were carried out for individual ribbons to correlate spectroscopic and electrical properties of GNRs.

cond-mat.mtrl-sci↗

arXiv2011

Deep-Tissue Anatomical Imaging of Mice Using Carbon Nanotube Fluorophores in the Second Near Infrared Window

Fluorescent imaging in the second near infrared window (NIR II, 1-1.4 μm) holds much promise due to minimal autofluorescence and tissue scattering. Here, using well functionalized biocompatible single-walled carbon nanotubes (SWNTs) as NIR II fluorescent imaging agents, we performed high frame rate video imaging of mice during intravenous injection of SWNTs and investigated the path of SWNTs through the mouse anatomy. We observed in real-time SWNT circulation through the lungs and kidneys several seconds post-injection, and spleen and liver at slightly later time points. Dynamic contrast enhanced imaging through principal component analysis (PCA) was performed and found to greatly increase the anatomical resolution of organs as a function of time post-injection. Importantly, PCA was able to discriminate organs such as the pancreas which could not be resolved from real-time raw images. Tissue phantom studies were performed to compare imaging in the NIR II region to the traditional NIR I biological transparency window (700- 900 nm). Examination of the feature sizes of a common NIR I dye (indocyanine green, ICG) showed a more rapid loss of feature contrast and integrity with increasing feature depth as compared to SWNTs in the NIR II region. The effects of increased scattering in the NIR I versus NIR II region were confirmed by Monte Carlo simulation. In vivo fluorescence imaging in the NIR II region combined with PCA analysis may represent a powerful approach to high resolution optical imaging through deep tissues, useful for a wide range of applications from biomedical research to disease diagnostics.

physics.bio-ph↗

arXiv2011

Advanced Asymmetrical Supercapacitors Based on Graphene Hybrid Materials

Supercapacitors operating in aqueous solutions are low cost energy storage devices with high cycling stability and fast charging and discharging capabilities, but have suffered from low energy densities. Here, we grow Ni(OH)2 nanoplates and RuO2 nanoparticles on high quality graphene sheets to maximize the specific capacitances of these materials. We then pair up a Ni(OH)2/graphene electrode with a RuO2/graphene electrode to afford a high performance asymmetrical supercapacitor with high energy and power densities operating in aqueous solutions at a voltage of ~1.5V. The asymmetrical supercapacitor exhibits significantly higher energy densities than symmetrical RuO2-RuO2 supercapacitors and asymmetrical supercapacitors based on either RuO2-carbon or Ni(OH)2-carbon electrode pairs. A high energy density of ~48Wh/kg at a power density of ~0.23kW/kg, and a high power density of ~21kW/kg at an energy density of ~14Wh/kg have been achieved with our Ni(OH)2/graphene and RuO2/graphene asymmetrical supercapacitor. Thus, pairing up metal-oxide/graphene and metal-hydroxide/graphene hybrid materials for asymmetrical supercapacitors represents a new approach to high performance energy storage.

cond-mat.mtrl-sci↗

arXiv2011

MoS2 Nanoparticles Grown on Graphene: An Advanced Catalyst for Hydrogen Evolution Reaction

Advanced materials for electrocatalytic and photoelectrochemical water splitting are central to the area of renewable energy. Here, we developed a solvothermal synthesis of MoS2 nanoparticles selectively on reduced graphene oxide (RGO) sheets suspended in solution. The resulting MoS2/RGO hybrid material possessed nanoscopic few-layer MoS2 structures with abundant exposed edges stacked onto graphene, in strong contrast to large aggregated MoS2 particles grown freely in solution without GO. The MoS2/RGO hybrid exhibited superior electrocatalytic activity in the hydrogen evolution reaction (HER) to other MoS2 catalysts. A Tafel slope of ~ 41 mV/decade was measured for MoS2 catalysts in HER for the first time, far exceeding the activity of previous MoS2 owing to the abundant catalytic edge sites of MoS2 nanoparticles and excellent electrical coupling to the underlying graphene network. The ~ 41 mV/decade Tafel slope suggested the Volmer-Heyrovsky mechanism for MoS2 catalyzed HER, with electrochemical desorption of hydrogen as the rate-limiting step.

cond-mat.mtrl-sci↗

arXiv2011

Near-Infrared Fluorescence Enhanced (NIR-FE) Molecular Imaging of Live Cells on Gold Substrates

Low quantum yields of near infrared (NIR) fluorophores have limited their capabilities as imaging probes in a transparent, low background imaging window. Here for the first time we reported near-infrared fluorescence enhance (NIR-FE) cell imaging using nanostructured Au substrate, which was employed as a general platform for both single-walled carbon nanotubes (SWNTs) and organic fluorescent labels in the NIR region. Fluorescence intensity, as well as cell targeting specificity, was greatly improved by this novel imaging technique. With NIR-FE imaging, we were able to image SWNT-stained cells at short exposure time of 300ms, and push the detectable limit of SWNT staining of cells down to an ultralow concentration of ~50 pM. Further, different degrees of fluorescence enhancement for endocytosed, intracellular SWNTs vs. nanotubes on the cell membrane at the cell/gold interface were observed, suggesting the possibility of using this technique to track the transmembrane behavior of NIR fluorophores.

physics.bio-ph↗

arXiv2011

Spatially Resolving Spin-split Edge States of Chiral Graphene Nanoribbons

A central question in the field of graphene-related research is how graphene behaves when it is patterned at the nanometer scale with different edge geometries. Perhaps the most fundamental shape relevant to this question is the graphene nanoribbon (GNR), a narrow strip of graphene that can have different chirality depending on the angle at which it is cut. Such GNRs have been predicted to exhibit a wide range of behaviour (depending on their chirality and width) that includes tunable energy gaps and the presence of unique one-dimensional (1D) edge states with unusual magnetic structure. Most GNRs explored experimentally up to now have been characterized via electrical conductivity, leaving the critical relationship between electronic structure and local atomic geometry unclear (especially at edges). Here we present a sub-nm-resolved scanning tunnelling microscopy (STM) and spectroscopy (STS) study of GNRs that allows us to examine how GNR electronic structure depends on the chirality of atomically well-defined GNR edges. The GNRs used here were chemically synthesized via carbon nanotube (CNT) unzipping methods that allow flexible variation of GNR width, length, chirality, and substrate. Our STS measurements reveal the presence of 1D GNR edge states whose spatial characteristics closely match theoretical expectations for GNR's of similar width and chirality. We observe width-dependent splitting in the GNR edge state energy bands, providing compelling evidence of their magnetic nature. These results confirm the novel electronic behaviour predicted for GNRs with atomically clean edges, and thus open the door to a whole new area of applications exploiting the unique magnetoelectronic properties of chiral GNRs.

cond-mat.mes-hall↗

arXiv2010

High Performance In Vivo Near-IR (>1 μm) Imaging and Photothermal Cancer Therapy with Carbon Nanotubes

Short single-walled carbon nanotubes (SWNTs) functionalized by PEGylated phospholipids are biologically non-toxic and long-circulating nanomaterials with intrinsic near infrared photoluminescence (NIR PL), characteristic Raman spectra, and strong optical absorbance in the near infrared (NIR). This work demonstrates the first dual application of intravenously injected SWNTs as photoluminescent agents for in vivo tumor imaging in the 1.0-1.4 μm emission region and as NIR absorbers and heaters at 808 nm for photothermal tumor elimination at the lowest injected dose (70 μg of SWNT/mouse, equivalent to 3.6 mg/kg) and laser irradiation power (0.6 W/cm2) reported to date. Ex vivo resonance Raman imaging revealed the SWNT distribution within tumors at a high spatial resolution. Complete tumor elimination was achieved for large numbers of photothermally treated mice without any toxic side effects after more than six months post-treatment. Further, side-by-side experiments were carried out to compare the performance of SWNTs and gold nanorods (AuNRs) at an injected dose of 700 μg of AuNR/mouse (equivalent to 35 mg/kg) in NIR photothermal ablation of tumors in vivo. Highly effective tumor elimination with SWNTs was achieved at 10 times lower injected doses and lower irradiation powers than for AuNRs. These results suggest there are significant benefits of utilizing the intrinsic properties of biocompatible SWNTs for combined cancer imaging and therapy.

physics.bio-ph↗

arXiv2010

Metal-Enhanced Fluorescence of Carbon Nanotubes

The photoluminescence (PL) quantum yield of single-walled carbon nanotubes (SWNTs) is relatively low, with various quenching effects by metallic species reported in the literature. Here, we report the first case of metal enhanced fluorescence (MEF) of surfactant-coated carbon nanotubes on nanostructured gold substrates. The photoluminescence quantum yield of SWNTs is observed to be enhanced more than 10-fold. The dependence of fluorescence enhancement on metal-nanotube distance and on surface plasmon resonance (SPR) of the gold substrate for various SWNT chiralities is measured to reveal the mechanism of enhancement. Surfactant-coated SWNTs in direct contact with metal exhibit strong MEF without quenching, suggesting a small quenching distance for SWNTs on the order of the van der Waals distance, beyond which the intrinsically fast non-radiative decay rate in nanotubes is little enhanced by metal. The metal enhanced fluorescence of SWNTs is attributed to radiative lifetime shortening through resonance coupling of SWNT emission to the re-radiating dipolar plasmonic modes in the metal.

cond-mat.mtrl-sci↗

arXiv2010

Mn3O4-Graphene Hybrid as a High Capacity Anode Material for Lithium Ion Batteries

We developed two-step solution-phase reactions to form hybrid materials of Mn3O4 nanoparticles on reduced graphene oxide (RGO) sheets for lithium ion battery applications. Mn3O4 nanoparticles grown selectively on RGO sheets over free particle growth in solution allowed for the electrically insulating Mn3O4 nanoparticles wired up to a current collector through the underlying conducting graphene network. The Mn3O4 nanoparticles formed on RGO show a high specific capacity up to ~900mAh/g near its theoretical capacity with good rate capability and cycling stability, owing to the intimate interactions between the graphene substrates and the Mn3O4 nanoparticles grown atop. The Mn3O4/RGO hybrid could be a promising candidate material for high-capacity, low-cost, and environmentally friendly anode for lithium ion batteries. Our growth-on-graphene approach should offer a new technique for design and synthesis of battery electrodes based on highly insulating materials.

cond-mat.mtrl-sci↗

arXiv2010

TiO2 Nanocrystals Grown on Graphene as Advanced Photocatalytic Hybrid Materials

Graphene/TiO2 nanocrystals hybrid is successfully prepared by directly growing TiO2 nanocrystals on graphene oxide (GO) sheets. The direct growth of nanocrystals on GO sheets was achieved by a two-step method, in which TiO2 was coated on GO sheets by hydrolysis first and crystallized into anatase nanocrystals by hydrothermal treatment in second step. Slow hydrolysis reaction through the use of EtOH/H2O mixed solvents and addition of H2SO4 allows the selectively growing TiO2 on GO and suppressing free growth in solution. The method offers easy access to the GO/TiO2 nanocrystals hybrid with well controlled coating and strong interactions between TiO2 and the underlying GO sheets. The strong coupling could lead to advanced hybrid materials for various applications including photocatalysis. The prepared graphene/TiO2 nanocrystals hybrid has demonstrated superior photocatalytic activity in degradation of rhodamine B over other TiO2 materials, showing an impressive 3-fold photocatalytic enhancement over P25. It is expected that the hybrid material could also be promising for various other applications including lithium ion battery where strong electrical coupling to TiO2 nanoparticles is essential.

cond-mat.mtrl-sci↗

arXiv2010

Etching and Narrowing of Graphene from the Edges

Large scale graphene electronics desires lithographic patterning of narrow graphene nanoribbons (GNRs) for device integration. However, conventional lithography can only reliably pattern ~20nm wide GNR arrays limited by lithography resolution, while sub-5nm GNRs are desirable for high on/off ratio field-effect transistors (FETs) at room temperature. Here, we devised a gas phase chemical approach to etch graphene from the edges without damaging its basal plane. The reaction involved high temperature oxidation of graphene in a slightly reducing environment to afford controlled etch rate (\leq ~1nm/min). We fabricated ~20-30nm wide GNR arrays lithographically, and used the gas phase etching chemistry to narrow the ribbons down to <10nm. For the first time, high on/off ratio up to ~10^4 was achieved at room temperature for FETs built with sub-5nm wide GNR semiconductors derived from lithographic patterning and narrowing. Our controlled etching method opens up a chemical way to control the size of various graphene nano-structures beyond the capability of top-down lithography.

cond-mat.mtrl-sci↗

arXiv2010

Ni(OH)2 Nanoplates Grown on Graphene as Advanced Electrochemical Pseudocapacitor Materials

Ni(OH)2 nanocrystals grown on graphene sheets with various degrees of oxidation are investigated as electrochemical pseudocapacitor materials for potential energy storage applications. Single-crystalline Ni(OH)2 hexagonal nanoplates directly grown on lightly-oxidized, electrically-conducting graphene sheets (GS) exhibit a high specific capacitance of ~1335F/g at a charge and discharge current density of 2.8A/g and ~953F/g at 45.7A/g with excellent cycling ability. The high specific capacitance and remarkable rate capability are promising for applications in supercapacitors with both high energy and power densities. Simple physical mixture of pre-synthesized Ni(OH)2 nanoplates and graphene sheets show lower specific capacitance, highlighting the importance of direct growth of nanomaterials on graphene to impart intimate interactions and efficient charge transport between the active nanomaterials and the conducting graphene network. Single-crystalline Ni(OH)2 nanoplates directly grown on graphene sheets also significantly outperform small Ni(OH)2 nanoparticles grown on heavily-oxidized, electrically-insulating graphite oxide (GO), suggesting that the electrochemical performance of these composites are dependent on the quality of graphene substrates and the morphology and crystallinity of the nanomaterials grown on top. These results suggest the importance of rational design and synthesis of graphene-based nanocomposite materials for high-performance energy applications.

cond-mat.mtrl-sci↗

arXiv2010

Aligned Graphene Nanoribbons and Crossbars from Unzipped Carbon Nanotubes

Aligned graphene nanoribbon (GNR) arrays were made by unzipping of aligned single-walled and few-walled carbon nanotube (CNT) arrays. Nanotube unzipping was achieved by a polymer-protected Ar plasma etching method, and the resulting nanoribbon array was transferred onto any substrates. Atomic force microscope (AFM) imaging and Raman mapping on the same CNTs before and after unzipping confirmed that ~80% of CNTs were opened up to form single layer sub-10 nm GNRs. Electrical devices made from the GNRs (after annealing in H2 at high temperature) showed on/off current (Ion/Ioff) ratios up to 103 at room temperature, suggesting semiconducting nature of the narrow GNRs. Novel GNR-GNR and GNR-CNT crossbars were fabricated by transferring GNR arrays across GNR and CNT arrays, respectively. The production of ordered graphene nanoribbon architectures may allow for large scale integration of GNRs into nanoelectronics or optoelectronics.

cond-mat.mtrl-sci↗

arXiv2010

Facile Synthesis of High Quality Graphene Nanoribbons

Graphene nanoribbons have attracted attention for their novel electronic and spin transport properties1-6, and because nanoribbons less than 10 nm wide have a band gap that can be used to make field effect transistors. However, producing nanoribbons of very high quality, or in high volumes, remains a challenge. Here, we show that pristine few-layer nanoribbons can be produced by unzipping mildly gas-phase oxidized multiwalled carbon nanotube using mechanical sonication in an organic solvent. The nanoribbons exhibit very high quality, with smooth edges (as seen by high-resolution transmission electron microscopy), low ratios of disorder to graphitic Raman bands, and the highest electrical conductance and mobility reported to date (up to 5e2/h and 1500 cm2/Vs for ribbons 10-20 nm in width). Further, at low temperature, the nanoribbons exhibit phase coherent transport and Fabry-Perot interference, suggesting minimal defects and edge roughness. The yield of nanoribbons was ~2% of the starting raw nanotube soot material, which was significantly higher than previous methods capable of producing high quality narrow nanoribbons1. The relatively high yield synthesis of pristine graphene nanoribbons will make these materials easily accessible for a wide range of fundamental and practical applications.

cond-mat.mtrl-sci↗

arXiv2010

Multiplexed five-color molecular imaging of cancer cells and tumor tissues with carbon nanotube Raman tags in the near-infrared

Single-walled carbon nanotubes (SWNTs) with five different C13/C12 isotope compositions and well-separated Raman peaks have been synthesized and conjugated to five targeting ligands in order to impart molecular specificity. Multiplexed Raman imaging of live cells has been carried out by highly specific staining of cells with a five-color mixture of SWNTs. Ex vivo multiplexed Raman imaging of tumor samples uncovers a surprising up-regulation of epidermal growth factor receptor (EGFR) on LS174T colon cancer cells from cell culture to in vivo tumor growth. This is the first time five-color multiplexed molecular imaging has been performed in the near-infrared (NIR) region under a single laser excitation. Near zero interfering background of imaging is achieved due to the sharp Raman peaks unique to nanotubes over the low, smooth autofluorescence background of biological species.

cond-mat.mtrl-sci↗

arXiv2010

Edge Magneto-Fingerprints in Disordered Graphene Nanoribbons

We report on (magneto)-transport experiments in chemically derived narrow graphene nanoribbons under high magnetic fields (up to 60 Tesla). Evidences of field-dependent electronic confinement features are given, and allow estimating the possible ribbon edge symmetry. Besides, the measured large positive magnetoconductance indicates a strong suppression of backscattering induced by the magnetic field. Such scenario is supported by quantum simulations which consider different types of underlying disorders (smooth edge disorder and long range Coulomb scatters).

cond-mat.mes-hall↗