Probing the mechanical properties of vertically-stacked ultrathin graphene/Al2O3 heterostructures.

The superior intrinsic mechanical properties of graphene have been widely studied and utilized to enhance the mechanical properties of various composite materials. However, it is still unclear how heterostructures incorporating graphene behave, and to what extent graphene influences their mechanical response. In this work, a series of graphene/Al2O3 composite films were fabricated via atomic layer deposition of Al2O3 on graphene, and their mechanical behavior was studied using an experimental-computational approach. The inclusion of monolayer chemical vapor deposited graphene between ultrathin Al2O3 films (1.5-4.5 nm thickness) was found to enhance the overall stiffness by as much as 70% compared to a pure Al2O3 film of similar thickness (∼150 GPa to ∼250 GPa). Here, for the first time, the combination of graphene and Al2O3 in vertically-stacked heterostructures results in advanced hybrid films of unprecedented mechanical stiffness that also possess qualities desirable for graphene-based transistors and flexible electronics.

Toward Highly Sensitive and Energy Efficient Ammonia Gas Detection with Modified Single-Walled Carbon Nanotubes at Room Temperature.

Fabrication and comparative analysis of the gas sensing devices based on individualized single-walled carbon nanotubes of four different types (pristine, boron doped, nitrogen doped, and semiconducting ones) for detection of low concentrations of ammonia is presented. The comparison of the detection performance of different devices, in terms of resistance change under exposure to ammonia at low concentrations combined with the detailed analysis of chemical bonding of dopant atoms to nanotube walls sheds light on the interaction of NH3 with carbon nanotubes. Furthermore, chemoresistive measurements showed that the use of semiconducting nanotubes as conducting channels leads to the highest sensitivity of devices compared to the other materials. Electrical characterization and analysis of the structure of fabricated devices showed a close relation between amount and quality of the distribution of deposited nanotubes and their sensing properties. All measurements were performed at room temperature, and the power consumption of gas sensing devices was as low as 0.6 µW. Finally, the route toward an optimal fabrication of nanotube-based sensors for the reliable, energy-efficient sub-ppm ammonia detection is proposed, which matches the pave of advent of future applications.

Graphene integrated circuits: new prospects towards receiver realisation.

This work demonstrates a design approach which enables the fabrication of fully integrated radio frequency (RF) and millimetre-wave frequency direct-conversion graphene receivers by adapting the frontend architecture to exploit the state-of-the-art performance of the recently reported wafer-scale CVD metal-insulator-graphene (MIG) diodes. As a proof-of-concept, we built a fully integrated microwave receiver in the frequency range 2.1-2.7 GHz employing the strong nonlinearity and the high responsivity of MIG diodes to successfully receive and demodulate complex, digitally modulated communication signals at 2.45 GHz. In addition, the fabricated receiver uses zero-biased MIG diodes and consumes zero dc power. With the flexibility to be fabricated on different substrates, the prototype receiver frontend is fabricated on a low-cost, glass substrate utilising a custom-developed MMIC process backend which enables the high performance of passive components. The measured performance of the prototype makes it suitable for Internet-of-Things (IoT) and Radio Frequency Identification (RFID) systems for medical and communication applications.

High performance metal-insulator-graphene diodes for radio frequency power detection application.

Vertical metal-insulator-graphene (MIG) diodes for radio frequency (RF) power detection are realized using a scalable approach based on graphene grown by chemical vapor deposition and TiO2 as barrier material. The temperature dependent current flow through the diode can be described by thermionic emission theory taking into account a bias induced barrier lowering at the graphene TiO2 interface. The diodes show excellent figures of merit for static operation, including high on-current density of up to 28 A cm-2, high asymmetry of up to 520, strong maximum nonlinearity of up to 15, and large maximum responsivity of up to 26 V-1, outperforming state-of-the-art metal-insulator-metal and MIG diodes. RF power detection based on MIG diodes is demonstrated, showing a responsivity of 2.8 V W-1 at 2.4 GHz and 1.1 V W-1 at 49.4 GHz.

Enhancing the stiffness of vertical graphene sheets through ion beam irradiation and fluorination.

Many applications of graphene can benefit from the enhanced mechanical robustness of graphene-based components. We report how the stiffness of vertical graphene (VG) sheets is affected by the introduction of defects and fluorination, both separately and combined. The defects were created using a high-energy ion beam while fluorination was performed in a XeF2 etching system. After ion bombardment alone, the average effective reduced modulus (E r), equal to ∼4.9 MPa for the as-grown VG sheets, approximately doubled to ∼10.0 MPa, while fluorination alone almost quadrupled it to ∼18.4 MPa. The maximum average E r of ∼32.4 MPa was achieved by repeatedly applying fluorination and ion bombardment. This increase can be explained by the formation of covalent bonds between the VG sheets due to ion bombardment, as well as the conversion from sp2 to sp3 and increased corrugation due to fluorination.

Flexible Hall sensors based on graphene.

The excellent electronic and mechanical properties of graphene provide a perfect basis for high performance flexible electronic and sensor devices. Here, we present the fabrication and characterization of flexible graphene based Hall sensors. The Hall sensors are fabricated on 50 µm thick flexible Kapton foil using large scale graphene grown by chemical vapor deposition technique on copper foil. Voltage and current normalized sensitivities of up to 0.096 V VT(-1) and 79 V AT(-1) were measured, respectively. These values are comparable to the sensitivity of rigid silicon based Hall sensors and are the highest values reported so far for any flexible Hall sensor devices. The sensitivity of the Hall sensor shows no degradation after being bent to a minimum radius of 4 mm, which corresponds to a tensile strain of 0.6%, and after 1000 bending cycles to a radius of 5 mm.

Tuning the mechanical properties of vertical graphene sheets through atomic layer deposition.

We report the fabrication and characterization of graphene nanostructures with mechanical properties that are tuned by conformal deposition of alumina. Vertical graphene (VG) sheets, also called carbon nanowalls (CNWs), were grown on copper foil substrates using a radio-frequency plasma-enhanced chemical vapor deposition (RF-PECVD) technique and conformally coated with different thicknesses of alumina (Al2O3) using atomic layer deposition (ALD). Nanoindentation was used to characterize the mechanical properties of pristine and alumina-coated VG sheets. Results show a significant increase in the effective Young's modulus of the VG sheets with increasing thickness of deposited alumina. Deposition of only a 5 nm thick alumina layer on the VG sheets nearly triples the effective Young's modulus of the VG structures. Both energy absorption and strain recovery were lower in VG sheets coated with alumina than in pure VG sheets (for the same peak force). This may be attributed to the increase in bending stiffness of the VG sheets and the creation of connections between the sheets after ALD deposition. These results demonstrate that the mechanical properties of VG sheets can be tuned over a wide range through conformal atomic layer deposition, facilitating the use of VG sheets in applications where specific mechanical properties are needed.

Single-crystalline CdTe nanowire field effect transistors as nanowire-based photodetector.

The electronic and photoconductive characteristics of CdTe nanowire-based field effect transistors were studied systematically. The electrical characterization of a single CdTe nanowire FET verifies p-type behavior. The CdTe NW FETs respond to visible-near infrared (400-800 nm) incident light with a fast, reversible and stable response characterized by a high responsivity (81 A W(-1)), photoconductive gain (∼2.5 × 10(4)%) and reasonable response and decay times (0.7 s and 1 s, respectively). These results substantiate the potential of CdTe nanowire-based photodetectors in optoelectronic applications.

A growth mechanism for free-standing vertical graphene.

We propose a detailed mechanism for the growth of vertical graphene by plasma-enhanced vapor deposition. Different steps during growth including nucleation, growth, and completion of the free-standing two-dimensional structures are characterized and analyzed by transmission electron microscopy. The nucleation of vertical graphene growth is either from the buffer layer or from the surface of carbon onions. A continuum model based on the surface diffusion and moving boundary (mass flow) is developed to describe the intermediate states of the steps and the edges of graphene. The experimentally observed convergence tendency of the steps near the top edge can be explained by this model. We also observed the closure of the top edges that can possibly stop the growth. This two-dimensional vertical growth follows a self-nucleated, step-flow mode, explained for the first time.

Bandgap engineering of Cd(x)Zn(1-x)Te nanowires.

Bandgap engineering of single-crystalline alloy Cd(x)Zn(1-x)Te (0 ≤ x ≤ 1) nanowires is achieved successfully through control of growth temperature and a two zone source system in a vapor-liquid-solid process. Extensive characterization using electron microscopy, Raman spectroscopy and photoluminescence shows highly crystalline alloy nanowires with precise tuning of the bandgap. It is well known that bulk Cd(x)Zn(1-x)Te is popular for construction of radiation detectors and availability of a nanowire form of this material would help to improve detection sensitivity and miniaturization. This is a step forward towards the accomplishment of tunable and predetermined bandgap emissions for various applications.