Zaka Ullah

Wide-Range Strain Sensors Based on Highly Transparent and Supremely Stretchable Graphene/Ag-Nanowires Hybrid Structures

The increasing demand of electronic devices for physical motion detection has encouraged the development of highly elastic strain sensors. Especially, to capture wide-range physical movements, supremely stretchable and wide-range strain sensors are required. Here, a novel transparent, bendable, stretchable, and wide-range strain sensor based on a sandwich-like stacked graphene and Ag-nanowires hybrid structures is reported. The hybrid structures on 200% pre-stretched polyacrylate (PAC) are patterned which possess good bendability up to 2 mm radius, impressive stretchability up to 200% and comparatively low sheet resistance ≈200 Ω sq-1 with transparency 85%. Pre-stretched PAC technique enables the sensor to work well at extremely high strains and to sense the multidirectional strains efficiently. The Ag-nanowires pattern on PAC is fabricated via the bubble-template method, by which a uniform distribution of Ag-nanowires is achieved with significant connectivity throughout the surface. This not only decreases the power consumption but also enhances the sensitivity of the strain sensor. The demonstrated strain sensor is capable to sense strains between 5% and 200%, and the response time for this sensation is <1 ms.

Roles of Oxygen and Hydrogen in Crystal Orientation Transition of Copper Foils for High-Quality Graphene Growth

The high-quality graphene film can be grown on single-crystal Cu substrate by seamlessly stitching the aligned graphene domains. The roles of O2 and H2 have been intensively studied in the graphene growth kinetics, including lowering the nucleation sites and tailoring the domain structures. However, how the O2 and H2 influence Cu orientations during recrystallization prior to growing graphene, still remains unclear. Here we report that the oxidation of Cu surface tends to stabilize the Cu(001) orientation while impedes the evolution of Cu(111) single domain during annealing process. The crystal orientation-controlled synthesis of aligned graphene seeds is further realized on the long-range ordered Cu(111) substrate. With decreasing the thickness of oxide layer on Cu surface by introducing H2, the Cu(001) orientation changes into Cu(111) orientation. Meanwhile, the average domain size of Cu foils is increased from 50 μm to larger than 1000 μm. The density functional theory calculations reveal that the oxygen increases the energy barrier for Cu(111) surface and makes O/Cu(001) more stable than O/Cu(111) structure. Our work can be helpful for revealing the roles of O2 and H2 in controlling the formation of Cu single-crystal substrate as well as in growing high-quality graphene films.

Fast Batch Production of High-Quality Graphene Films in a Sealed Thermal Molecular Movement System

Chemical vapor deposition (CVD) growth of high-quality graphene has emerged as the most promising technique in terms of its integrated manufacturing. However, there lacks a controllable growth method for producing high-quality and a large-quantity graphene films, simultaneously, at a fast growth rate, regardless of roll-to-roll (R2R) or batch-to-batch (B2B) methods. Here, a stationary-atmospheric-pressure CVD (SAPCVD) system based on thermal molecular movement, which enables fast B2B growth of continuous and uniform graphene films on tens of stacked Cu(111) foils, with a growth rate of 1.5 µm s-1 , is demonstrated. The monolayer graphene of batch production is found to nucleate from arrays of well-aligned domains, and the films possess few defects and exhibit high carrier mobility up to 6944 cm2 V-1 s-1 at room temperature. The results indicate that the SAPCVD system combined with single-domain Cu(111) substrates makes it possible to realize fast batch-growth of high-quality graphene films, which opens up enormous opportunities to use this unique 2D material for industrial device applications.

Ultra-broadband graphene-InSb heterojunction photodetector

We demonstrate a room temperature ultra-broadband graphene-InSb heterostructure photodetector. By introducing a thin oxide layer between the P-type graphene film and N-type InSb, the dark current is suppressed sharply. The device can detect light from the visible to far infrared region, exhibiting a high responsivity of ∼70 mA W−1 at a typical wavelength of 1.7 μm. It is worth mentioning that the photodetector has delivered a mid-infrared (MIR) photoresponsivity of ∼42 mA W−1, which also opens a way for MIR communication technology.

Graphene/Ag-NWs Electrodes for Highly Transparent and Extremely Stretchable Supercapacitor

A multiaxial pre-stretched PAC technique is used which endows 200 % stretch-ability to flexible and transparent electrodes based on graphene and Ag-NWs hybrid structures. A novel bubble template method is used for uniform distribution of Ag-NWs which decrease the resistivity of electrodes remarkably and enhance the connectivity between graphene segments when graphene gets partially divided into small segments upon appliance of strains >150 %. A highly transparent and stretchable supercapacitor is fabricated using these electrodes which works well under stretches and shows good cycling stability.

Self-assembly of urchin-like porphyrin/graphene microspheres for artificial photosynthetic production of formic acid from CO2

Life runs on energy and natural resources of energy are being used quickly. Artificial photosynthesis, in which sunlight is used to produce valuable chemicals from abundant natural resources, is considered as one of the auspicious ways to meet the current energy requirements. Here we report a 5,10,15,20-tetrakis(4-aminophenyl)porphyrin/graphene (TkisAPP/G) based photocatalyst with a significant and sensible design and structure for the production of formic acid from CO2. The urchin-like TkisAPP/G microspheres show remarkable enhancement in sunlight absorption and improve the light-harvesting efficiency. An effective approach has been adopted for electronic coupling to develop covalent bonding between porphyrin and graphene which increases the rate of the photoinduced charge transfer. For the fabricated photocatalyst, a 0.5% solar-to-formic acid conversion efficiency has been attained which is greater than that of natural photosynthesis.

Significant enhancement of metal heat dissipation from mechanically exfoliated graphene nanosheets through thermal radiation effect

We demonstrate a facile approach to significantly enhance the heat dissipation potential of conventional aluminum (Al) heat sinks by mechanically coating graphene nanosheets. For Al and graphene-coated Al heat sinks, the change in temperature with change in coating coverage, coating thickness and heat flux are studied. It is found that with the increase in coating coverage from 0 to 100%, the steady-state temperature is decreased by 5 °C at a heat flux of 1.8 W cm-1. By increasing the average thickness of graphene coating from 480 nm to 1900 nm, a remarkable temperature reduction up to 7 °C can be observed. Moreover, with the increase in heat flux from 1.2 W cm-1 to 2.4 W cm-1, the temperature difference between uncoated and graphene-coated samples increases from 1 °C to 6 °C. The thermal analysis and finite element simulation reveal that the thermal radiation plays a key role in enhancing the heat dissipation performance. The effect of heat convection remains weak owing to the low air velocity at surface...

INFLUENCE OF ION BEAM IRRADIATION ON STRUCTURAL, MAGNETIC AND ELECTRICAL CHARACTERISTICS OF Ho-DOPED AlN THIN FILMS

Holmium (Ho)-doped AlN thin films of thicknesses 60, 90 and 300 nm were grown in pure nitrogen atmosphere via RF magnetron sputtering. The deposited thin films were irradiated with protons at a dose of 5×1014 ions/cm2 and the effects of irradiation on structural, magnetic and electrical characteristics of thin films were investigated. Rutherford backscattering spectroscopy (RBS) confirmed the presence of Al, N and Ho in prepared samples. X-ray diffraction analysis showed that crystallinity of the thin films was enhanced after irradiation and thicker films were more crystalline. Atomic force microscopy (AFM) revealed that the surface roughness and porosity of the thin films were increased after irradiation. Magnetic measurements showed that diamagnetic AlN:Ho thin films can be transformed into paramagnetic and ferromagnetic ones via suitable irradiation. The increase in carrier concentrations after irradiation was responsible for tuning the electrical and magnetic characteristics of thin films for applications in various high voltage microelectronic and magnetic devices.

Inducing the magnetic character in reduced graphene oxide through incorporation of Fe2O3 nanoparticles

A chemical two-step approach based on solvothermal technique has been adopted to synthesize the reduced graphene oxide (rGO)/Fe2O3 hybrid materials. The rGO was prepurified by acidic treatment, followed by sensitization to attach the desired functional groups. The structural, compositional, morphological and magnetic analyzes of the prepared samples were carried out using various characterization techniques. The fabricated rGO/Fe2O3 heterostructures were confirmed by X-ray diffraction analysis and Fourier transform infrared spectroscopy. Raman spectroscopy evidenced the fabrication of multilayer graphene and scanning electron microscopy was carried out to study the morphology of the prepared samples. The average particle size of Fe2O3 nanoparticles (NPs) loaded on rGO was found to be ∼20 nm, as was observed during transmission electron microscopy. Thermogravimetric analysis of rGO/Fe2O3 hybrid structures was performed to investigate their thermal behaviors. It was evidenced that the incorporation of Fe2O3 NPs into rGO enhanced its thermal stability. Vibrating sample magnetometry showed that ferromagnetic character was induced in rGO due to involvement of Fe2O3 NPs. The rGO/Fe2O3 hybrid structures can be considered as a competent material for fabrication of various magnetic devices.

Low resistivity of graphene nanoribbons with zigzag-dominated edge fabricated by hydrogen plasma etching combined with Zn/HCl pretreatment

Graphene nanoribbons (GNRs) have attracted intensive research interest owing to their potential applications in high performance graphene-based electronics. However, the deterioration of electrical performance caused by edge disorder is still an important obstacle to the applications. Here, we report the fabrication of low resistivity GNRs with a zigzag-dominated edge through hydrogen plasma etching combined with the Zn/HCl pretreatment method. This method is based on the anisotropic etching properties of hydrogen plasma in the vicinity of defects created by sputtering zinc (Zn) onto planar graphene. The polarized Raman spectra measurement of GNRs exhibits highly polarization dependence, which reveals the appearance of the zigzag-dominated edge. The as-prepared GNRs exhibit high carrier mobility (∼1332.4 cm2 v−1 s−1) and low resistivity (∼0.7 kΩ) at room temperature. Particularly, the GNRs can carry large current density (5.02 × 108 A cm−2) at high voltage (20.0 V) in the air atmosphere. Our study develop...