Hafnium diselenide (HfSe2) has a high theoretical carrier mobility but is among the most reactive transition-metal dichalcogenides (TMDs). Herein, we have investigated the air stability of 2H polytype HfSe2 single-crystal thin films by spectroscopic and microscopic techniques. Raman spectroscopy measurements in conjunction with atomic force microscopy reveal the formation of selenium-rich blisters on the surface of the crystals upon air exposure. Transmission electron microscopy analysis indicates that 2H-HfSe2 undergoes a spontaneous phase change to 1T-HfSe2. These results offer Raman spectroscopy as a fast, convenient, non-destructive technique to reliably monitor the surface degradation of TMDs and present an opportunity for further study of phase changes in this material.

Increasing the operation voltage of LiCoO2 (LCO) is a direct way to enhance the energy density of the Li-ion batteries. However, at high voltages, the cycling stability degrades very fast due to the irreversible changes in the electrode structure, and formation of an unstable solid electrolyte interface layer. In this work, Ag thin film was prepared on commercial LCO cathode by using magnetron sputtering technique. Ag coated electrode enabled an improved electrochemical performance with a better cycling capability. After 100 cycles, Ag coated LCO delivers a discharge capacity of 106.3 mAh g-1 within 3 - 4.5 V at C/5, which is increased by 45 % compared to that of the uncoated LCO. Coating the electrode surface with Ag thin film also delivered an improved Coulombic efficiency, which is believed to be an indication of suppressed parasitic reactions at the electrode interface. This work may lead to new methods on surface modifications of LCO and other cathode materials to achieve high-capacity Li-ion batteries for high-voltage operations.

Chemical vapor deposition is the most proficient method for growing graphene on copper foils due to its scalability, repeatability, and uniformity, etc. Herein, we systematically study the effect of oxygen (O2) exposure on graphene growth. We introduced O2 before and during the growth, and then studied its effects on the morphology, crystallinity, and nucleation density of graphene. We observe that introducing O2 during growth significantly improves the graphene crystallinity while pre-dosing O2 before growth reduces the graphene nucleation density. These studies suggest that intermittent O2 exposure play a significant role in graphene growth, enabling scalable production of high-quality graphene.

An in-depth look at diffusion mechanics within lithium-ion electrodes under fast charging conditions is presented. Electrochemical impedance spectroscopy is used as the primary technique to investigate lithium diffusion within electrode material and in electrolyte near the electrode-electrolyte interface. Half-cells of silicon are charged under varying galvanostatic rates while obtaining impedance data. Collected data is analyzed with the help of an electrical equivalent circuit model that provides mechanical and electrochemical parameters for each instance. The novelty of this equivalent circuit partly lies in its ability to resolve between solid-phase diffusion and liquid-phase diffusion, both of which occur during cycling of a lithium-ion electrode. Observed patterns in the parameters of this circuit provide insight into impact of fast charging on mechanics of lithium diffusion, both inside the electrode matrix and within electrolyte.

Herein, commercial Ni foam coated with self-assembled and linearly-aligned Ni wires is utilized as a cost-effective current collector for application in Li-O2 battery. The Ni wires are furthered deposited with graphene layers (g-Ni wire) to improve electrical conductivity. Multivalent Mn oxides consisting of Mn3O4, Mn2O3 and MnCO3 are used as effective oxygen reduction (ORR) and evolution reaction (OER) catalysts deposited on g-Ni wire current collectors. Specific capacities are respective ∼100 and ∼170 mAh g-1 without or with O2 introduction into the cell. The relative facile synthesis process requiring merely solution-based synthesis at ambient pressure, low temperature and short process time renders the Mn oxides/g-Ni wire electrode promising for Li-O2 battery application.

Herein, silicon nanoparticles (SiNPs) are coated with conducting hydrogel and wrapped with reduced graphene oxide (rGO) sheets via a facile and scalable solution-based sol-gel process. The in-situ polymerized polypyrrole (PPy) hydrogel forms an interconnected three-dimensional (3D) fiber matrix. Amine and hydroxyl groups from the hydrogel assist the encapsulation of the SiNPs through hydrogen bonding. The electro-conductive PPy fiber network and the wrapping of rGO offer efficient electron and ion transport pathways. The PPy/SiNPs/rGO electrodes can produce highly reversible capacities of 1312, 1285 and 1066 mAh g-1 at 100, 250 and 500 cycles at a current density of 2.1 A g-1, respectively.

Three-dimensional (3D) chrysanthemum-like carbon nanofiber (CCNF) foam architectures were synthesized on highly porous nickel foam via a one-step ambient pressure chemical vapor deposition process by introducing a mixture of precursor gases (H2 and C2H2). The as-synthesized 3D foam architectures were characterized by scanning electron microscopy and transmission electron microscopy, which demonstrate high porosity and a densely packed nature of the hierarchical carbon nanostructures. Symmetrical electrochemical double-layer capacitors were fabricated using electrodes based on the CCNF foam architectures. Cyclic voltammetry, charge–discharge measurements, and electrochemical impedance spectroscopy were conducted to determine the performance metrics. The supercapacitors (SCs) demonstrate a high areal capacitance of 1.37 F/cm2 (gravimetric specific capacitance: 23.83 F/g), which leads to superior values for per area energy density (0.19 Wh/cm2) and power density (141.77 W/cm2). In addition, capacitance retention of ∼100% over 13,000 charge–discharge cycles demonstrates the high electrochemical stability of this type of carbon nanostructure foam for high areal capacitance SCs.

We report on the successful synthesis of a graphene–carbon nanotube (CNT) hybrid architecture by a parallel chemical vapor deposition (CVD) of the two carbon allotropes. The carbon hybrid is a three-dimensional (3D) nanostructure with tuneable architecture comprising vertically grown CNTs as pillars and a large-area graphene plane as the floor. The formation of CNTs and graphene occurs simultaneously in a single CVD growth that we describe as a synchronous synthesis method. Unique nature of the fabrication approach contributes significantly to the quality and composure of final nanohybrid. Detailed characterization elucidates the cohesive structure and robust contact between the graphene floor and the CNTs in the hybrid structure. The functionality of the synthesized graphene hybrid structure has been demonstrated by its incorporation into a supercapacitor cell. Our fabrication approach provides an attractive pathway for the fabrication of novel 3D hybrid nanostructures and efficient device integration.

We describe the synthesis and fabrication of a graphene oxide (GO) and single-walled carbon nanotube (SWCNT) composite ink (GO–SWCNT ink) for electrochemically stable supercapacitors. Atomic force microscopy and scanning electron microscopy studies demonstrate that the obtained GO flakes are single layer with size distribution from 100 nm to 20 μm. SWCNTs are dispersed using a GO aqueous solution (2 mg/mL) with sonication support to achieve a SWCNT concentration of 12 mg/mL, the highest reported value so far without surfactant assistance. Raman spectroscopy studies indicate that the full-width at half-maximum of the G band increases with the mixing of SWCNT and GO indicating that electronic structure changes via π–π interactions of GO sheets and SWCNTs. Paper-based electrodes of supercapacitor were conveniently fabricated with GO–SWCNT composite ink via a dip casting method. By using different concentrations of SWCNT in the ink, the paper electrodes provide different capacitance values. The highest value of specific capacitance reaches 295 F/g at a current density of 0.5 A/g with a GO/SWCNT weight ratio of 1:5. The cycling stability for the GO–SWCNT paper electrode supercapacitors indicates capacitance retention of 85% over 60,000 cycles.

The unique structure and properties of graphene initiated broad fundamental and technological research, and highlighted graphene as a new candidate for various applications such as energy storage, solar cells and electronic devices. Chemical vapor deposition (CVD) has been utilized for industrial large-scale synthesis of graphene. Regardless of the synthesis process, graphene should be transferred to arbitrary substrates for different applications. The transfer processes, introduce defects such as wrinkles and cracks in graphene which compromise the properties and applications. In recent years, fundamental research has been focused on characterization of graphene to develop new techniques for large-scale, high-resolution graphene metrology. Herein, a complementary high throughput metrology technique using fluorescent quenching is further investigated for different fluorescent dyes to characterize CVD synthesized graphene.

Supercapacitors are promising candidates for alternative energy storage applications since they can store and deliver energy at relatively high rates. In this work, we integrated large area chemical vapor deposition (CVD) grown three dimensional graphene heterostructures with high capacitance metal oxides (MnO2) to fabricate highly conductive, large surface-area composite thin films. Uniform, large area 3D graphene heterostructures layers were produced by a one-step CVD on nickel foams. MnO2 nanowires were deposited on the as-obtained 3D graphene heterostructures film by a simple chemical bath depostion process. The oxide loading of the 3D graphene/MWNTs/MnO2 nanowires (GMM) composite films can be simply controlled by deposition time and nanowire solution concentration. The surface morphology was investigated by scanning electron microscopy (SEM) and scanning transmission electron microscopy (STEM), and Energy-dispersive X-ray spectroscopy (EDS) was performed to characterize the MnO2on the surface of the film. By introducing the fast surface redox reactions into the graphene heterostructures film via integrating pseudocapacitive material like MnO2, the capacitive ability of the system enhanced dramatically. Supercapacitor was fabricated based on the 3D graphene heterostructures /MnO2 hybrid film electrodes; the measurements of cyclic voltammetry, and electrochemical impedance spectroscopy (EIS) are conducted to determine its performance for the electrodes of supercapacitors.

Fluorescence Quenching Microscopy has been shown to be an effective means of characterizing graphene on the macroscale. Centimeter-scale CVD-grown pristine and doped graphene were manufactured in a high temperature (1000°C) furnace on pristine copper substrates. The copper was then etched away in a FeCl3solution and the graphene was coated with DCM-based fluorescent dye before being imaged in a fluorescence microscope. The fluorescence image was then image-processed using modified Matlab software. The resulting image showed clear contrast between the pristine graphene sheet and defects on the graphene surface, which revealed that fluorescence microscopy could determine the quality of a large region of graphene. Also, significant contrast was identified between single-layer and multi-layer regions, showing that this technique is also effective at determining the degree of uniformity within a graphene sample. Lastly, the fluorescence images showed contrast between doped and undoped regions of graphene.

Graphene, with unique electrical, optical and mechanical properties is a promising material in industrial applications, such as batteries, supercapacitors, transistors and semiconductor devices. These potential applications of graphene have motivated the development of large-scale synthesis of graphene on copper substrates by chemical vapor deposition (CVD). To enable practical applications of large-area, high quality graphene layers at the centimeter and wafer scales, process control needs to be implemented for optimizing the morphology and electrical properties and enable repeatable growth-cycle of graphene layers for process-line implementation. Here we investigate the effects of process quartz-tube position on the structural properties of graphene. Furthermore, we describe a procedure for process optimization of the growth parameters. Graphene is grown on copper foils by CVD, and transferred to the SiO2/Si and glass substrates. The detailed characterization of the graphene layers are conducted using Raman spectroscopy, optical microscopy (OM), scanning electron microscopy (SEM) and UV-vis spectroscopy. The experimental results show that the position of copper foil into the quartz tube plays a significant role in the Raman features of the graphene, and influences the optical, morphology and surface properties of graphene layers. We believe that these results will be useful for determining the optimum processing conditions of high quality graphene layers at the centimeter and wafer scales.

Pulsed Electron Deposition (PED) is an attractive alternative to Pulsed Laser Deposition (PLD) for growing high temperature superconductor thin films because of its relatively low cost. In this study, YBa2Cu3O7-δ(YBCO) thin film has been fabricated on silicon substrates by Pulsed Electron Deposition technique. SrTiO3 (STO) as a buffer layer has been grown between Si substrate and YBCO superconducting layer. The crystalline structures of STO/Si and YBCO/STO/Si films have been investigated by x-ray diffraction (XRD). The surface morphology and microstructure of YBCO/STO/Si thin film have been characterized with atomic force microscope (AFM) and scanning electron microscope (SEM). From the θ-2θ XRD analysis of YBCO thin films, (00l) diffraction peaks are obtained indicating they have a poor c-axis oriented structure. SEM analysis shows that the surfaces of films are crack-free, but they have some particulates. On AFM images, the droplets are clearly observed leading to a roughly surface.

In this work, we demonstrated the growth of three dimensional graphene/carbon nanotubes hybrid carbon nanostructures on metal foam through a one-step chemical vapor deposition (CVD). The as-grown three dimensional carbon nanostructure foams can be potentially used as the electrodes of energy storage devices such as supercapacitors and batteries. During the CVD process, the carbon nanostructures are grown on highly porous nickel foam to form a high surface area 3-D carbon nanostructure by introducing a mixture precursor gases (H2, C2H2). The surface morphology was investigated by scanning electron microscopy (SEM) and the results demonstrated relatively homogeneous and densely packed 3-D carbon nanostructure. The quality was characterized by Raman spectroscopy. To further increase the capacitive capability the supercapacitors were fabricated based on the electrodes of carbon nanostructure foam and cyclic voltammetry, charge-discharge, and electrochemical impedance spectroscopy (EIS) were conducted to determine their performance.

Sustainable energy is currently limited by the ability of materials to store energy and deliver it on demand. Allotropes of carbon are attractive for their potential for use in energy storage due to low weight, high chemical stability and low production cost. Carbon nanotubes and graphene can be combined to provide an effective three-dimensional material with high conductivity and high surface area. We demonstrate the use of block copolymers to obtain patterned arrays of iron nanoparticles which give rise to ordered carbon nanotubes with good size distribution. A one-step chemical vapor deposition process for large-area fabrication of the graphene and carbon nanotube hybrid structure is described. Following chemical vapor deposition the hybrid material is demonstrated in a supercapacitor device. The fabricated supercapacitor exhibits high electrical conductivity, and has potential for extremely high energy storage capability.

Using chemical vapor deposition technique, a novel 3D carbon nano-architecture called a pillared graphene nanostructure (PGN) is in situ synthesized. The fabricated novel carbon nanostructure consists of CNT pillars of variable length grown vertically from large-area graphene planes. The formation of CNTs and graphene occurs simultaneously in one CVD growth treatment. The detailed characterization of synthesized pillared graphene shows the cohesive structure and seamless contact between graphene and CNTs in the hybrid structure. The synthesized graphene-CNT hybrid has a tunable architecture and attractive material properties, as it is solely built from sp2 hybridized carbon atoms in form of graphene and CNT. Our methodology provides a pathway for fabricating novel 3D nanostructures which are envisioned for applications in hydrogen storage, nanoelectronics, and supercapacitors.

In this letter, single stranded Deoxyribonucleic Acids (ssDNA) are found to act as negative potential gating agents that increase the hole density in single layer graphene (SLG). Current-voltage measurement of the hybrid ssDNA/graphene system indicates a shift in the Dirac point and “intrinsic” conductance after ssDNA is patterned. The effect of ssDNA is to increase the hole density in the graphene layer, which is calculated to be on the order of 1.8×1012 cm-2. This increased density is consistent with the Raman frequency shifts in the G-peak and 2D band positions and the corresponding changes in the G-peak full-width half maximum. This patterning of DNA on graphene layers could provide new avenues to modulate their electrical properties and for novel electronic devices.

In this study, we investigated the effects of DNA/Pt-DNA strands as hole collecting layers in polymer heterojunction solar cells based on ITO/PEDOT:PSS/P3HT:PCBM/LiF/Al structure. We demonstrated that by introducing DNA or Pt-DNA layers between the polymer electrode (PEDOT:PSS) and the active layer (P3HT:PCBM), lead to an improvement in the hole collection efficiency and power conversion efficiency and the absorbance spectra of the devices indicate that Pt particles work as surface plasmons and increase the absorbance of the devices.