Effect of Graphite Nanoplatelet Size and Dispersion on the Thermal and Mechanical Properties of Epoxy-Based Nanocomposites.

This study investigated the effect of graphite nanoplatelet (GNP) size and dispersion on the thermal conductivities and tensile strengths of epoxy-based composites. GNPs of four different platelet sizes, ranging from 1.6 to 3 µm, were derived by mechanically exfoliating and breaking expanded graphite (EG) particles using high-energy bead milling and sonication. The GNPs were used as fillers at loadings of 0-10 wt%. As the GNP size and loading amount increased, the thermal conductivities of the GNP/epoxy composites increased, but their tensile strengths decreased. However, interestingly, the tensile strength reached a maximum value at the low GNP content of 0.3% and thereafter decreased, irrespective of the GNP size. Our observations of the morphologies and dispersions of the GNPs in the composites indicated that the thermal conductivity was more likely related to the size and loading number of fillers, whereas the tensile strength was more influenced by the dispersion of fillers in the matrix.

Application of Titanium-Carbide MXene-Based Transparent Conducting Electrodes in Flexible Smart Windows.

Among various available materials used in transparent and flexible devices, MXenes are attracting attention as a brand-new candidate in this category. Ti3C2Tx MXene as a 2D material has exceptional properties, making it a potential material having numerous applications in different areas. Because of its high conductivity, it can be used in transparent conducting electrodes (TCEs). In this study, the MXenes etched by highly concentrated acid at 50 °C,were spin-coated on polyethylene terephthalate (PET) film and annealed at moderate temperatures up to 170 °C. The adhesion of MXene to PET was found to be remarkably improved by annealing. These TCEs exhibited a sheet resistance of ∼424 Ω/sq. and transmittance of ∼87%. The aging stability of MXene-coated PET films against oxidation under ambient conditions was studied up to 28 days and resistance change was found ∼30% during this period. The flexibility test showed low bending resistance change (∼1.5%) at 1000th cycle and cumulative resistance change of ∼20% at a bending radius of ∼3.9 mm after 1000 cycles. These transparent, flexible, and conducting electrodes were used to fabricate polymer dispersed liquid crystal (PDLC)-based flexible smart windows. The smart windows fabricated by curing PDLC mixture sandwiched between the MXene electrodes were also found flexible in ON/OFF states. The MXene-based flexible smart windows resulted in good opacity in the OFF state and high transparency in the ON state, exhibiting low threshold voltage <10 V and high transmittance ∼80% at 60 V. The flexible smart windows operated normally even at ∼4 mm bending radius.

Optimized Aluminum Reflector for Enhancement of UVC Cathodoluminescence Based-AlGaN Materials with Carbon Nanotube Field Emitters.

The far ultraviolet C (UVC) light sources based on carbon nanotube (CNT) field emitters as excitation sources have become promising light sources for sterilization, disinfection, and water purification. However, the low light extraction efficiency of UVC-CNT light sources still hinders the practical application of these structures. Herein, we report an optimized aluminum (Al) reflector to enhance the light extraction efficiency of UVC-CNT light sources. Optical analysis of UVC-CNT light sources covered by the Al reflectors with various thicknesses ranging from 30 to 150 nm was performed to realize the optimized reflector. The UVC-CNT light sources exhibit the highest light extraction efficiency when the Al reflector layer has an optimized thickness of 100 nm. For comparison, the cathodoluminescence (CL) spectra were recorded for UVC-CNT light sources with and without the optimized Al reflector. The measured light output power and the estimated power efficiency of the UVC-CNT light-source-tube with Al reflector were enhanced by about 27 times over the reference. This enhancement is mainly attributed to the outstanding reflection effect of the Al reflector.

Effect of Ti3C2T x MXenes etched at elevated temperatures using concentrated acid on binder-free supercapacitors.

The effect of Ti3C2T x MXene etched at different temperatures (25 °C, 50 °C, and 80 °C) on the capacitance of supercapacitors without the use of conducting carbon-black or a binder was studied. The MXene etched using concentrated HCl acid (12 M)/LiF was used as an active electrode and Ni-foil as a current collector. It was observed that the elevated etching temperature facilitates the etching of the MAX phase and the exfoliation of MXene layers. However, this led to the formation of additional functional groups at the MXene surface as the temperature was increased to 80 °C. The specific capacitance of Ti3C2T x -based supercapacitors increased from 581 F g-1 for MXene etched at 25 °C to 657 F g-1 for those etched at 50 °C at the scan rate of 2 mV s-1. However, the specific capacitance reduced to 421 F g-1 as the etching temperature was increased to 80 °C at the same scan rate. The supercapacitors based on MXenes etched at the intermediate temperature (50 °C) exhibited higher specific capacitance in a wide range of scan rate, symmetry in charge/discharge curves, high cyclic stability at a scan rate of 1000 mV s-1 for up to 3000 cycles. The electrochemical impedance spectroscopy studies indicated low series resistance, reduced charge-transfer resistance, and decreased Warburg impedance for the supercapacitor based on the MXene etched at the intermediate temperature.

Energy harvesting efficiency of piezoelectric polymer film with graphene and metal electrodes.

In this study, we investigated an energy harvesting effect of tensile stress using piezoelectric polymers and flexible electrodes. A chemical-vapor-deposition grown graphene film was transferred onto both sides of the PVDF and P(VDF-TrFE) films simultaneously by means of a conventional wet chemical method. Output voltage induced by sound waves was measured and analyzed when a mechanical tension was applied to the device. Another energy harvester was made with a metallic electrode, where Al and Ag were deposited by using an electron-beam evaporator. When acoustic vibrations (105 dB) were applied to the graphene/PVDF/graphene device, an induced voltage of 7.6 Vpp was measured with a tensile stress of 1.75 MPa, and this was increased up to 9.1 Vpp with a stress of 2.18 MPa for the metal/P(VDF-TrFE)/metal device. The 9 metal/PVDF/metal layers were stacked as an energy harvester, and tension was applied by using springs. Also, we fabricated a full-wave rectifying circuit to store the electrical energy in a 100 µF capacitor, and external vibration generated the electrical charges. As a result, the stored voltage at the capacitor, obtained from the harvester via a bridge diode rectifier, was saturated to ~7.04 V after 180 s charging time.

Study of Grains and Boundaries of Molybdenum Diselenide and Tungsten Diselenide Using Liquid Crystal.

Direct observation of grains and boundaries is a vital factor in altering the electrical and optoelectronic properties of transition metal dichalcogenides (TMDs), that is, MoSe2 and WSe2. Here, we report visualization of grains and boundaries of chemical vapor deposition grown MoSe2 and WSe2 on silicon, using optical birefringence of two-dimensional layer covered with nematic liquid crystal (LC). An in-depth study was performed to determine the alignment orientation of LC molecules and their correlation with other grains. Interestingly, we found that alignment of liquid crystal has discrete preferential orientations. From computational simulations, higher adsorption energy for the armchair direction was found to force LC molecules to align on it, compared to that of the zigzag direction. We believe that these TMDs with three-fold symmetric alignment could be utilized for display applications.

Experimental data of inorganic gel based smart window using silica sol-gel process.

In this article experimental data are presented for inorganic gel based smart window fabricated using silica sol-gel process. Parallel beam transmittances were measured as functions of voltages for samples fabricated with different concentrations of nitric acid. Spectroscopic transmittance data at different driving voltages for samples fabricated with different LC concentrations are shown. Transmittance spectra of the Si-Ti based gel-based-liquid-crystal (GDLC) device measured as different driving voltages were compared with those of PDLC. GDLC showed much lower operating voltages, 10-15 V, for on-state. Formation of the LC droplet in gelation process is illustrated. The methyl organic group surrounds LC droplets. Demonstration of GDLC based smart window showed the successful operation with low driving voltages. GDLC window shows clear color, even at off-state, compared with PDLC.

Selective growth of graphene in layer-by-layer via chemical vapor deposition.

Selective and precise control of the layer number of graphene remains a critical issue for the practical applications of graphene. First, it is highly challenging to grow a continuous and uniform few-layer graphene since once the monolayer graphene fully covers a copper (Cu) surface, the growth of the second layer stops, resulting in mostly nonhomogeneous films. Second, from the selective adlayer growth point of view, there is no clear pathway for achieving this. We have developed the selective growth of a graphene adlayer in layer-by-layer via chemical vapor deposition (CVD) which makes it possible to stack graphene on a specific position. The key idea is to deposit a thin Cu layer (∼40 nm thick) on pre-grown monolayer graphene and to apply additional growth. The thin Cu atop the graphene/Cu substrate acts as a catalyst to decompose methane (CH4) gas during the additional growth. The adlayer is grown selectively on the pre-grown graphene, and the thin Cu is removed through evaporation during CVD, eventually forming large-area and uniform double layer graphene. With this technology, highly uniform graphene films with precise thicknesses of 1 to 5 layers and graphene check patterns with 1 to 3 layers were successfully demonstrated. This method provides precise LBL growth for a uniform graphene film and a technique for the design of new graphene devices.

Characterization of Graphene-based FET Fabricated using a Shadow Mask.

To pattern electrical metal contacts, electron beam lithography or photolithography are commonly utilized, and these processes require polymer resists with solvents. During the patterning process the graphene surface is exposed to chemicals, and the residue on the graphene surface was unable to be completely removed by any method, causing the graphene layer to be contaminated. A lithography free method can overcome these residue problems. In this study, we use a micro-grid as a shadow mask to fabricate a graphene based field-effect-transistor (FET). Electrical measurements of the graphene based FET samples are carried out in air and vacuum. It is found that the Dirac peaks of the graphene devices on SiO2 or on hexagonal boron nitride (hBN) shift from a positive gate voltage region to a negative region as air pressure decreases. In particular, the Dirac peaks shift very rapidly when the pressure decreases from ~2 × 10(-3) Torr to ~5 × 10(-5) Torr within 5 minutes. These Dirac peak shifts are known as adsorption and desorption of environmental gases, but the shift amounts are considerably different depending on the fabrication process. The high gas sensitivity of the device fabricated by shadow mask is attributed to adsorption on the clean graphene surface.

Study of Graphene-based 2D-Heterostructure Device Fabricated by All-Dry Transfer Process.

We developed a technique for transferring graphene and hexagonal boron nitride (hBN) in dry conditions for fabrication of van der Waals heterostructures. The graphene layer was encapsulated between two hBN layers so that it was kept intact during fabrication of the device. For comparison, we also fabricated the devices containing graphene on SiO2/Si wafer and graphene on hBN. Electrical properties of the devices were investigated at room temperature. The mobility of the graphene on SiO2 devices and graphene on hBN devices were 15,000 and 37,000 cm(2) V(-1) s(-1), respectively, while the mobility of the sandwich structure device reached the highest value of ∼100,000 cm(2) V(-1) s(-1), at room temperature. The electrical measurements of the samples were carried out in air and vacuum environments. We found that the electrical properties of the encapsulated graphene devices remained at a similar level both in a vacuum and in air, whereas the properties of the graphene without encapsulation were influenced by the external environment.

Simple and Precise Quantification of Iron Catalyst Content in Carbon Nanotubes Using UV/Visible Spectroscopy.

Iron catalysts have been used widely for the mass production of carbon nanotubes (CNTs) with high yield. In this study, UV/visible spectroscopy was used to determine the Fe catalyst content in CNTs using a colorimetric technique. Fe ions in solution form red-orange complexes with 1,10-phenanthroline, producing an absorption peak at λ=510 nm, the intensity of which is proportional to the solution Fe concentration. A series of standard Fe solutions were formulated to establish the relationship between optical absorbance and Fe concentration. Many Fe catalysts were microscopically observed to be encased by graphitic layers, thus preventing their extraction. Fe catalyst dissolution from CNTs was investigated with various single and mixed acids, and Fe concentration was found to be highest with CNTs being held at reflux in HClO4/HNO3 and H2SO4/HNO3 mixtures. This novel colorimetric method to measure Fe concentrations by UV/Vis spectroscopy was validated by inductively coupled plasma optical emission spectroscopy, indicating its reliability and applicability to asses Fe content in CNTs.

Mechanically stable tuning fork sensor with high quality factor for the atomic force microscope.

A quartz tuning fork was used instead of cantilever as a force sensor for the atomic force microscope. A tungsten tip was made by electrochemical etching from a wire of 50 µm diameter. In order to have mechanical stability of the tuning fork, it was attached on an alumina plate. The tungsten tip was attached on the inside end of a prong of a tuning fork. The phase shift was used as a feedback signal to control the distance between the tip and sample, and the amplitude was kept constant using a lock-in amplifier and a homemade automatic gain controller. Due to the mechanical stability, the sensor shows a high quality factor (∼10(3)), and the image quality obtained with this sensor was equivalent to that of the cantilever-based AFM.

Visible cathodoluminescence of quantum dot films by direct irradiation of electron beam and its materialization as a field emission device.

The field emission (FE) device based on quantum dot (QD) films as a cathodoluminescent (CL) material has not emerged yet due to the relatively low quantum efficiency and weak photostability of nanocrystals (NCs). Here we improve film stability and luminescence yields by preparing neat films of well-packed core-multishell QDs using spray coating method and then using low-temperature atomic layer deposition (ALD) to infill the pores of these films with metal oxides to produce inorganic nanocomposites. The ALD coatings to protect oxidation and degradation by electrons prevent internal atomic and molecular diffusion and decrease surface trap densities of QD films. Furthermore, the CL of the core-multishell QD films is 2.4 times higher than before ALD infilling. We fabricate the FE device by combining cathode structure with carbon nanotube (CNT) emitters and anode plates with QD thin film and successfully can get brilliant images of the light-emitting FE device. Our research opens a way for developing new quantum optoelectronics with high-performance.

Effect of cutting and functionalization of single-walled carbon nanotubes on their dispersion behaviors.

Cutting of single-walled carbon nanotubes (SWCNTs) and their modification with poly (ethylene glycol) and polystyrene were successfully performed through a treatment of the SWCNTs with piranha solutions and monoamine terminated polymers. The shortening and functionalization of SWCNTs were confirmed through size exclusion chromatography and Raman spectroscopy observations. The shortened SWCNTs exhibited more aggregated morphology than as-prepared SWCNTs in scanning electron microscope and atomic force microscope observations. Cutting and functionalization of SWCNTs with different polymer chains effectively tailored their dispersion behaviors in solvents. Polystyrene composite films exhibited improved transparency employing shortened SWCNTs, suggesting that the size of SWCNTs are critical factors in controlling the transparency of polymer composite films.

Comparison of frictional forces on graphene and graphite.

We report on the frictional force between an SiN tip and graphene/graphite surfaces using lateral force microscopy. The cantilever we have used was made of an SiN membrane and has a low stiffness of 0.006 N m(-1). We prepared graphene flakes on a Si wafer covered with silicon oxides. The frictional force on graphene was smaller than that on the Si oxide and larger than that on graphite (multilayer of graphene). Force spectroscopy was also employed to study the van der Waals force between the graphene and the tip. Judging that the van der Waals force was also in graphite-graphene-silicon oxide order, the friction is suspected to be related to the van der Waals interactions. As the normal force acting on the surface was much weaker than the attractive force, such as the van der Waals force, the friction was independent of the normal force strength. The velocity dependency of the friction showed a logarithmic behavior which was attributed to the thermally activated stick-slip effect.