Ultra-low-edge-defect graphene nanoribbons patterned by neutral beam

Top-down process, comprising lithography and plasma etching is widely used in very-large-scale integration due to its scalability, has the greatest potential to fabricate graphene nanoribbon based nanoelectronic devices for large-scale intergraded circuits. However, conventional plasma etching inevitably introduces plenty of damage or defects to the etched materials, which drastically degrades the performance of nano materials. In this study, extremely low-damage neutral beam etching (NBE) is applied to fabricate ultra-low-defect graphene nanoribbon array (GNR). The ultra-low-edge-defect GNRs are fabricated by E-beam lithography followed by oxygen NBE from large-scale chemical-vapor-deposition-grown graphene. AFM images clearly shows the GNRs patterned by NBE and E-beam lithography, and Raman spectroscopy exhibits extremely low I_D/I_G of GNRs, which indicate that high-quality GNRs can be successfully fabricated by neutral beam. We also demonstrated bottom-gated field-effect transistor with the high-quality GNR and observed a high carrier mobility (>200 cm² V⁻¹ s⁻¹) at room temperature.     

The structural and electrical evolution of chemical vapor deposition grown graphene by electron beam irradiation induced disorder

The defect formation mechanism in chemical vapor deposition grown single layer graphene devices has been investigated by increasing electron beam (e-beam) irradiation doses gradually up to 750 e⁻/nm². The evolution of D peaks in Raman spectra provides an evidence of strong lattice disorder due to e-beam irradiation. Particularly, the trajectory of D and G peak intensities ratio (I_D/I_G) suggests that the transformation of graphene from crystalline to the nanocrystalline and then towards amorphous form with increasing irradiation dose. The defect parameters were calculated by phenomenological model of amorphization trajectory for graphitic materials. The mobility decreasing gradually from ∼1200 to ∼80 cm²/V s with gradual increase of irradiation dose, which implies the formation of localized states in e-beam irradiated graphene. The Dirac point is shifted towards negative gate voltage which indicates the n-doping in graphene with increasing e-beam irradiation dose.     

Visualization of defect densities in reduced graphene oxide

Efficiently reducible graphene oxide (GO) was obtained, even if a high degree of functionalization is present. Graphite with few defects was used as starting material and oxidized according to Hummer’s method. An extremely high I_D/I_G ratio for rGO of 2.8 (532 nm) was observed in the Raman spectrum as a consequence of the lower defect density in GO. It was also possible to demonstrate the impact of local defects on the structure in rGO by local laser exposure experiments on single graphene oxide flakes. Raman spectroscopy can visualize the laser impact by I_D/I_G ratio measurements.     

The production of amorphous regions in carbon nanotubes by 140 keV He ion irradiation

The production of amorphous regions in carbon nanotubes irradiated with 140 keV He ions was studied using Raman spectroscopy and transmission electron microscopy (TEM). Intensity ratios of Raman D to G bands (I_D/I_G) initially increase and then decrease as a function of ion fluence at all investigated irradiation temperatures (room temperature, 200 and 400 °C). The critical ion fluences corresponding to the maximum in I_D/I_G ratios increase with increasing irradiation temperature because of the enhanced defect annealing. The displacement per atom (dpa) values, consistent with a maximum in I_D/I_G ratios, are determined to be 0.15 dpa at room temperature and 200 °C, and 0.3 dpa at 400 °C. TEM examination of all irradiated specimens supports Raman results indicating that the maximum in I_D/I_G correlates to the formation of amorphous regions. The study shows that after formation of amorphous regions at high fluences, I_D/I_G ratio can be no longer used to measure amorphous/graphitic content in CNTs.     

Enhancement of memory margins for stable organic bistable devices based on graphene-oxide layers due to embedded CuInS2 quantum dots

Current–voltage (I–V) curves for Al/CuInS₂ (CIS) quantum dots (QDs) embedded in graphene-oxide layer/indium-tin-oxide devices at 300 K showed a current bistability with a maximum high conductivity (ON)/low conductivity (OFF) ratio of 1 × 10⁴, which was 100 times larger than the ON/OFF ratio of the device without CIS QDs. I–V curves and write-read-erase-read voltage cycles demonstrated the rewritable nonvolatile memory properties of the organic bistable devices (OBDs) with ON and OFF current states at the same voltage. The retention time was above 1 × 10⁵ s, indicative of the memory stability of the OBDs. I–V curve at lower voltages up to 0.05 V was attributed to the thermionic emission mechanism, and the curve in the applied voltage range from 0.06 to 0.17 V was related to an ohmic mechanism. The I–V characteristics in the applied voltage above 0.18 V dominantly followed the space-charge-limited-current behaviors.     

Scalable and high-yield production of exfoliated graphene sheets in water and its application to an all-solid-state supercapacitor

One of the most critical issues in graphite exfoliation is realizing efficient, low-cost, eco-friendly, and scalable production of graphene for energy storage applications. The most promising strategies for exfoliating graphite to single-layer graphene sheets in scalable quantities with nearly non-oxidized content is the exfoliation of graphite by using an environmentally friendly solution. Herein, we demonstrate a universal exfoliation principle that uses imidazole, which has an anionic nature and π-conjugated heterocyclic coplanar structure, as the exfoliant for the successful production of large quantities of graphene suspensions in water. The exfoliant interacts with both surfaces of the exfoliated graphene sheets and the aqueous solution, significantly improving graphene dispersion (1 mg/mL) in water. Exfoliation in aqueous solutions produced graphene in high yield (>90%, ⩽3 layers), with a large lateral size (μm) and high quality (I_D/I_G ratio ≈ 0.1). The electrical conductivity of the graphene paper is 131.7 S/cm, which is superior to the values reported from exfoliated graphene prepared using solution processes. An all-solid-state supercapacitor with a new design fabricated using the atomically thin and flat conductive exfoliated graphene sheets delivered an ultrahigh area capacitance (∼71.9 mF/cm²). Therefore, imidazole -assisted exfoliation has great potential for scalable preparation of graphene suspensions for supercapacitor applications.     

Fabrication of high-quality graphene by hot-filament thermal chemical vapor deposition

To the best of our knowledge, the previously reported graphene fabricated using catalytic chemical vapor deposition techniques contained a high defect density, which will hinder its opto-electronic properties. In this work, the effects of two crucial parameters, namely deposition time and hydrogen flow rate on the growth of graphene using a hot-filament thermal chemical vapor deposition technique were systematically studied. Fabrications were conducted at substrate and filament temperatures of 1000 °C and 1750 °C, respectively. Very low I_D/I_G ratios (≪0.1) were obtained for all the samples, which reflected the formation of high-quality graphene deposited on Cu foils. A quasi-static equilibrium copper vapor inside an alumina tube was found to be an important factor to obtain a low defect density graphene. A growth mechanism was then proposed, where the cuprous oxide (Cu₂O) acted as a nucleation site for graphene growth.     

Duality of the interfacial thermal conductance in graphene-based nanocomposites

The thermal conductance of graphene–matrix interfaces plays a key role in controlling the thermal properties of graphene-based nanocomposites. Using atomistic simulations, we found that the interfacial thermal conductance depends strongly on the mode of heat transfer at graphene–matrix interfaces: if heat enters graphene from one side of its basal plane and immediately leaves it through the other side, the corresponding interfacial thermal conductance, G_across, is large; if heat enters graphene from both sides of its basal plane and leaves it at a position far away on its basal plane, the corresponding interfacial thermal conductance, G_non-across, is small. For a single-layer graphene immersed in liquid octane, G_across is ∼150 MW/m²K while G_non-across is ∼5 MW/m²K. G_across decreases with increasing multi-layer graphene thickness (i.e., number of layers in graphene) and approaches an asymptotic value of 100 MW/m²K for 7-layer graphenes. G_non-across increases only marginally as the graphene sheet thickness increases. Such a duality of the interface thermal conductance for different probing methods and its dependence on graphene sheet thickness can be traced ultimately to the unique physical and chemical structure of graphene materials. The ramifications of these results in areas such as the optimal design of graphene-based thermal nanocomposites are discussed.     

Observation of different charge transport regimes and large magnetoresistance in graphene oxide layers

We report a systematic study on charge transport properties of thermally reduced graphene oxide (rGO) layers, from room temperature to 2 K and in presence of magnetic fields up to 7 T. The most conductive rGO sheets follow different transport regimes: at room temperature they show an Arrhenius-like behavior. At lower temperature they exhibits a thermally activated behavior with resistance R following a R = R₀exp(T₀/T)^p law with p = 1/3, consistently with 2D Mott Variable Range Hopping (VRH) transport mechanism. Below a given temperature T_c, we observe a crossover from VHR to another regime, probably due to a shortening of the characteristic lengths of the disordered 2D system. The temperature T_c depends on the reduction grade of the rGO. Magnetoresistance ΔR/R of our rGO films shows as well a crossover between positive and negative and below liquid He temperature ΔR/R reaches values larger than ∼–60%, surprisingly high for a – nominally – non magnetic material.     

Exfoliated graphene-supported Pt and Pt-based alloys as electrocatalysts for direct methanol fuel cells

To greatly improve the electrocatalytic activity for methanol oxidation, high-quality exfoliated graphene decorated with uniform Pt nanocrystals (NCs) (3 nm) have been prepared by a very simple, low-cost and environmentally benign process. During the entire process, no surfactant and no halide ions were involved, which not only enabled very clean surface of Pt/graphene leading to excellent conductivity, but also greatly improved the electrocatalyst tolerance to carbon monoxide poisoning (Pt/graphene, I_f/I_b = 1.197), compared to commercial Pt/C (I_f/I_b = 0.893) catalysts. To maximize the electrocatalytic performance and minimize the amount of precious Pt, Pt–M/graphene (M = Pd, Co) hybrids have also been prepared, and these hybrids have much larger electrochemically active surface areas (ECSA), which are 4 (PtPd/graphene) and 3.3 (PtCo/graphene) times those of commercial Pt/C. The PtPd/graphene and PtCo/graphene hybrids also have remarkably increased activity toward methanol oxidation (I_f/I_b = 1.218 and 1.558). Furthermore, density functional theory (DFT) simulations demonstrate that an electronic interaction occurred between Pt atoms and graphene, indicating that graphene substrate plays a crucial role in regulating the electron structure of attached Pt atom, which confirmed that the increased efficiency of methanol oxidation was due to the synergetic effects of the hybrid structure.     

Tunable transport characteristics of p-type graphene field-effect transistors by poly(ethylene imine) overlayer

Tunable electrical transport properties of graphene field-effect transistors (GFETs) are achieved by spin-coating a poly(ethylene imine) (PEI) layer on graphene surface. The initially p-doped graphene recovers the ambipolar characteristics with the PEI overlayer. When increasing the PEI concentration in a methanol solvent, a systematic evolution of the transport properties of GFETs is observed. The carrier mobility of graphene is greatly improved by several tens of times and the hole/electron conductivity saturation is shown. The voltage of the neutrality point V_Dirac gets closer to 0 V and the plateau width around the neutrality point ΔV_Dirac becomes much smaller. It is proposed that the long-range Coulomb scattering in graphene is suppressed due to the screening effect of PEI and the performances of GFETs are consequently improved. The hysteretic behaviors of the transfer characteristic curves of GFETs are also influenced by the PEI coating. The gradual reversion of the hysteresis direction is observed when increasing the concentration of PEI, which is probably due to the two competing mechanisms between the charge trapping at the graphene/SiO₂ interface and the capacitive coupling of graphene and the PEI overlayer.     

Low threshold field emission from nitrogen-incorporated carbon nanowalls

Field emission performance of nitrogen-incorporated vertically-aligned graphene layers, so called “carbon nanowalls (CNWs)”, is found to depend upon their electrical conduction properties. The CNW samples with n-type conduction exhibit near-ideal Ohmic contacts with various metals such as Cu, Ti, and Au, regardless of the work function of the metals. The high resistivity CNWs for lower deposition temperatures (T_D) show semiconducting behavior in the Arrhenius plots, while the low resistivity CNWs for higher T_D show semi-metallic behavior. The emission turn-on field versus T_D has a very similar trend to the bulk resistivity versus T_D, and is reduced down to about 3 V/μm when the resistivity reaches a minimum (3.0 × 10^? 3 Ω cm). The lower turn-on field for semi-metallic CNWs is attributed to an upward shift of the emission level, which is responsible for a decrease in the surface potential barrier height for emission.     

Anelastic spectroscopy for studying O vacancies in perovskites

We present the results of anelastic relaxation experiments (2–30 kHz) on ceramic SrTiO₃ subjected to reduction in H₂ atmosphere, which yield a balance between V_O and (OH)⁻ defects. The resulting anelastic spectrum contains, besides the well-known structural transformation near 110 K, several thermally activated relaxation processes between 200 and 700 K. The two main elastic energy loss peaks are proposed to provide the first measurement of the hopping rate of V_O –(OH)⁻ defects in pure SrTiO₃.     

An ultra clean self-aligned process for high maximum oscillation frequency graphene transistors

Owing to its ultra high carrier mobility, graphene transistor shows great application potential as high-frequency electronics. Intrinsic cutoff frequency (f_T) of 427 GHz has been reported. But the maximum oscillation frequency (f_max) remains low, limiting its use in practical radio-frequency (RF) circuits. Here, we report an ultra clean self-aligned graphene transistors fabrication by pre-deposition of gold film on graphene as protection layer. This improved self-aligned fabrication keeps graphene away from any possible contamination, which makes our graphene transistors show good gate coupling and less parasitics, thus good dc and RF performances. The 100 nm gate-length graphene transistor exhibits a f_max of 105 GHz. Our study shows a pathway to fabrication of high-performance graphene transistors for future application in RF circuits.     

Synthesis of bimetallic PtPd nanocubes on graphene with N,N-dimethylformamide and their direct use for methanol electrocatalytic oxidation

Bimetallic PtPd nanocubes supported on graphene nanosheets (PtPdNCs/GNs) were prepared by a rapid, one-pot and surfactant-free method, in which N,N-dimethylformamide (DMF) was used as a bi-functional solvent for the reduction of both metal precursors and graphene oxide (GO) and for the surface confining growth of PtPdNCs. The morphology, structure and composition of the thus-prepared PtPdNCs/GNs were characterized by transmission electron microscopy (TEM), high resolution TEM, energy-dispersive X-ray spectroscopy and X-ray photoelectron spectroscopy. Because no surfactant or halide ions were involved in the proposed synthesis, the prepared PtPdNCs/GNs were directly modified onto a glassy carbon electrode and showed high electrocatalytic activity for methanol oxidation in cyclic voltammetry without any pretreatments. Moreover, with the synergetic effects of Pt and Pd and the enhanced electron transfer by graphene, the PtPdNCs/GNs composites exhibited higher electrocatalytic activity (j_p = 0.48 A mg⁻¹) and better tolerance to carbon monoxide poisoning (I_f/I_b = 1.27) compared with PtPd nanoparticles supported on carbon black (PtPdNPs/C) (j_p = 0.28 A mg⁻¹; I_f/I_b = 1.01) and PtNPs/GNs (j_p = 0.33 A mg⁻¹; I_f/I_b = 0.95). This approach demonstrates that the use of DMF as a solvent with heating is really useful for reducing GO and metal precursors concurrently for preparing clean metal–graphene composites.     

Electrical properties of a-C: Mo films produced by dual-cathode filtered cathodic arc plasma deposition

Molybdenum-containing amorphous carbon (a-C:Mo) thin films were prepared using a dual-cathode filtered cathodic arc plasma source with a molybdenum and a carbon (graphite) cathode. The Mo content in the films was controlled by varying the deposition pulse ratio of Mo and C. Film sheet resistance was measured in situ at process temperature, which was close to room temperature, as well as ex situ as a function of temperature (300–515 K) in ambient air. Film resistivity and electrical activation energy were derived for different Mo and C ratios and substrate bias. Film thickness was in the range 8–28 nm. Film resistivity varied from 3.55 × 10^− 4 Ω m to 2.27 × 10^− 6 Ω m when the Mo/C pulse ratio was increased from 0.05 to 0.4, with no substrate bias applied. With carbon-selective bias, the film resistivity was in the range of 4.59 × 10^− 2 and 4.05 Ω m at a Mo/C pulse ratio of 0.05. The electrical activation energy decreased from 3.80 × 10^− 2 to 3.36 × 10^− 4 eV when the Mo/C pulse ratio was increased in the absence of bias, and from 0.19 to 0.14 eV for carbon-selective bias conditions. The resistivity of the film shifts systematically with the amounts of Mo and upon application of substrate bias voltage. The intensity ratio of the Raman D-peak and G-peak (I_D/I_G) correlated with the pre-exponential factor (σ₀) which included charge carrier density and density of states.     

Electrical and chemical stability engineering of solution-processed indium zinc oxide thin film transistors via a synergistic approach of annealing duration and self-combustion process

The electrical and chemical stability of solution-processed indium zinc oxide (IZO) channel thin-film transistors (TFTs) were engineered via a synergistic approach of annealing duration and self-combustion process. In particular, the amorphous IZO TFTs that were thermally treated at 400 °C for 3 h using the specific precursor combination to generate internal self-combustion energy showed the best electrical performance [high saturation mobility (μ_SAT)=2.7 cm²/V s] and stability [low threshold voltage shift (ΔV_TH) under positive bias stress of 10.5 V] owing to the formation of oxide films with excellent metal–oxide–metal (M–O–M) bonds, fewer impurities, and an amorphous phase compared to IZO TFTs using other precursor formulas and annealing times. Longer annealing times led to a saturated M–O bond ratio and crystallization via extreme thermal annealing, which induced electrical degradation (low μ_SAT and high ΔV_TH) of IZO TFTs. In the wet chemical patterning of electrodes, conventional acidic and basic wet etchants cause severe damage to the surfaces of the IZO channels; thus, insufficiently annealed IZO TFTs exhibited considerable degradation in terms of their on-current level and mobility. Alternatively, the TFTs subjected to an excessively long-term thermal annealing showed only a moderate decrease in mobility with the formation of small nanocrystals.     

The effect of graphene dispersion on the mechanical properties of graphene/epoxy composites

The effect of dispersion state of graphene on mechanical properties of graphene/epoxy composites was investigated. The graphene sheets were exfoliated from graphite oxide (GO) via thermal reduction (thermally reduced GO, RGO). Different dispersions of RGO sheets were prepared with and without ball mill mixing. It was found that the composites with highly dispersed RGO showed higher glass transition temperature (T_g) and strength than those with poorly dispersed RGO, although no significant differences in both the tensile and flexural moduli are caused by the different dispersion levels. In particular, the T_g was increased by nearly 11 °C with the addition of 0.2 wt.% well dispersed RGO to epoxy. As expected, the highly dispersed RGO also produced one or two orders of magnitude higher electrical conductivity than the corresponding poorly dispersed RGO. Furthermore, an improved quasi-static fracture toughness (K_IC) was measured in the case of good dispersion. The poorly and highly dispersed RGO at 0.2 wt.% loading resulted in about 24% and 52% improvement in K_IC of cured epoxy thermosets, respectively. RGO sheets were observed to bridge the micro-crack and debond/delaminate during fracture process due to the poor filler/matrix and filler/filler interface, which should be the key elements of the toughening effect.     

Electrochemical properties of lithium nickel oxide synthesized by the combustion method in an O2 stream

An appropriate mole ratio of urea/nitrate for preheating to synthesize LiNiO₂ was examined by varying the ratio from 1.2 to 9.6. The chemical equation of the combustion reaction was deduced from the XRD analysis result of the mixture after preheating. The XRD pattern of the LiNiO₂ sample calcined at 800 °C for 24 h, after preheating at the mole ratio of urea/nitrate of 3.6 at 400 °C, shows clear split of the 1 0 8 and 1 1 0 peaks, and the largest value of I₀₀₃/I₁₀₄. The sample calcined at 800 °C for 24 h has a relatively high first discharge capacity (164.2 mAh g^?1) and a good cycling performance. Derivative ?dx/|dV| vs. V curve of the LiNiO₂ sample at the voltage range of 2.7–4.4 V for the first cycle exhibits four peaks for charging and discharging, showing that this sample goes through four phase transitions.     

Low temperature growth of complete monolayer graphene films on Ni-doped copper and gold catalysts by a self-limiting surface reaction

We report on the fabrication of completely uniform monolayer graphene on a metal thin film over a 150 mm Si substrate at a low temperature of 600 °C by inductively coupled plasma-enhanced chemical vapor deposition (ICPCVD). Through novel use of bimetallic catalyst such as CuNi and AuNi alloys we were able to control catalytic reaction at the metal surface and grow complete monolayer graphene with a Ni content less than 20 at.%. We also found that the 2D/G intensity ratio in the Raman spectra was almost invariant with growth time and the C 1s peak in the XPS spectra was observed only at the metal surface. This implies that monolayer graphene was possibly grown on these Ni-doped copper and gold catalysts by a self-limiting surface reaction under our CVD condition. From DFT calculations, it was shown that the catalytic activity of normally inactive Cu and Au could be enhanced through the addition of Ni atoms at surface sites, providing graphene growth at lower temperatures than pure Cu or Au. The carrier mobility of graphene films grown on these CuNi and AuNi alloy catalyst was measured to be over 9000 cm² V⁻¹ s⁻¹ at room temperature, which is comparable to that of CVD graphene film grown on Cu foil. Therefore, we suggest an efficient way in growing a complete monolayer graphene on thin films at low temperatures, which could be a key issue in the application of graphene devices.