Quantum Nature of Charge Transport in Inkjet-Printed Graphene Revealed in High Magnetic Fields up to 60T.

Inkjet-printing of graphene, iGr, provides an alternative route for the fabrication of highly conductive and flexible graphene films for use in devices. However, the contribution of quantum phenomena associated with 2D single layer graphene, SLG, to the charge transport in iGr is yet to be explored. Here, the first magneto-transport study of iGr in high magnetic fields up to 60 T is presented. The observed quantum phenomena, such as weak localization and negative magnetoresistance, are strongly affected by the thickness of the iGr film and can be explained by a combination of intra- and inter-flake classical and quantum charge transport. The quantum nature of carrier transport in iGr is revealed using temperature, electric field, and magnetic field dependences of the iGr conductivity. These results are relevant for the exploitation of inkjet deposition of graphene, which is of particular interest for additive manufacturing and 3D printing of flexible and wearable electronics. It is shown that printed nanostructures enable ensemble averaging of quantum interference phenomena within a single device, thereby facilitating comparison between experiment and underlying statistical models of electron transport.

Graphene FETs with high and low mobilities have universal temperature-dependent properties.

We use phenomenological modelling and detailed experimental studies of charge carrier transport to investigate the dependence of the electrical resistivity,ρ, on gate voltage,Vg, for a series of monolayer graphene field effect transistors with mobilities,µ, ranging between 5000 and 250 000 cm2V-1s-1at low-temperature. Our measurements over a wide range of temperatures from 4 to 400 K can be fitted by the universal relationµ=4/eδnmaxfor all devices, whereρmaxis the resistivity maximum at the neutrality point andδnis an 'uncertainty' in the bipolar carrier density, given by the full width at half maximum of the resistivity peak expressed in terms of carrier density,n. This relation is consistent with thermal broadening of the carrier distribution and the presence of the disordered potential landscape consisting of so-called electron-hole puddles near the Dirac point. To demonstrate its utility, we combine this relation with temperature-dependent linearised Boltzmann transport calculations that include the effect of optical phonon scattering. This approach demonstrates the similarity in the temperature-dependent behaviour of carriers in different types of single layer graphene transistors with widely differing carrier mobilities. It can also account for the relative stability, over a wide temperature range, of the measured carrier mobility of each device.

The Interaction of Hydrogen with the van der Waals Crystal γ-InSe.

The emergence of the hydrogen economy requires development in the storage, generation and sensing of hydrogen. The indium selenide ( γ -InSe) van der Waals (vdW) crystal shows promise for technologies in all three of these areas. For these applications to be realised, the fundamental interactions of InSe with hydrogen must be understood. Here, we present a comprehensive experimental and theoretical study on the interaction of γ -InSe with hydrogen. It is shown that hydrogenation of γ -InSe by a Kaufman ion source results in a marked quenching of the room temperature photoluminescence signal and a modification of the vibrational modes of γ -InSe, which are modelled by density functional theory simulations. Our experimental and theoretical studies indicate that hydrogen is incorporated into the crystal preferentially in its atomic form. This behaviour is qualitatively different from that observed in other vdW crystals, such as transition metal dichalcogenides, where molecular hydrogen is intercalated in the vdW gaps of the crystal, leading to the formation of "bubbles" for hydrogen storage.

Photosensitisation of inkjet printed graphene with stable all-inorganic perovskite nanocrystals.

All-inorganic perovskite nanocrystals (NCs) with enhanced environmental stability are of particular interest for optoelectronic applications. Here we report on the formulation of CsPbX3 (X is Br or I) inks for inkjet deposition and utilise these NCs as photosensitive layers in graphene photodetectors, including those based on single layer graphene (SLG) as well as inkjet-printed graphene (iGr) devices. The performance of these photodetectors strongly depends on the device structure, geometry and the fabrication process. We achieve a high photoresponsivity, R > 106 A W-1 in the visible wavelength range and a spectral response controlled by the halide content of the perovskite NC ink. By utilising perovskite NCs, iGr and gold nanoparticle inks, we demonstrate a fully inkjet-printed photodetector with R ≈ 20 A W-1, which is the highest value reported to date for this type of device. The performance of the perovskite/graphene photodetectors is explained by transfer of photo-generated charge carriers from the perovskite NCs into graphene and charge transport through the iGr network. The perovskite ink developed here enabled realisation of stable and sensitive graphene-based photon detectors. Compatibility of inkjet deposition with conventional Si-technologies and with flexible substrates combined with high degree of design freedom provided by inkjet deposition offers opportunities for partially and fully printed optoelectronic devices for applications ranging from electronics to environmental sciences.

Room Temperature Uniaxial Magnetic Anisotropy Induced By Fe-Islands in the InSe Semiconductor Van Der Waals Crystal.

The controlled manipulation of the spin and charge of electrons in a semiconductor has the potential to create new routes to digital electronics beyond Moore's law, spintronics, and quantum detection and imaging for sensing applications. These technologies require a shift from traditional semiconducting and magnetic nanostructured materials. Here, a new material system is reported, which comprises the InSe semiconductor van der Waals crystal that embeds ferromagnetic Fe-islands. In contrast to many traditional semiconductors, the electronic properties of InSe are largely preserved after the incorporation of Fe. Also, this system exhibits ferromagnetic resonances and a large uniaxial magnetic anisotropy at room temperature, offering opportunities for the development of functional devices that integrate magnetic and semiconducting properties within the same material system.

High broad-band photoresponsivity of mechanically formed InSe-graphene van der Waals heterostructures.

High broad-band photoresponsivity of mechanically formed InSe-graphene van der Waals heterostructures is achieved by exploiting the broad-band transparency of graphene, the direct bandgap of InSe, and the favorable band line up of InSe with graphene. The photoresponsivity exceeds that for other van der Waals heterostructures and the spectral response extends from the near-infrared to the visible spectrum.

Tuning the bandgap of exfoliated InSe nanosheets by quantum confinement.

Strong quantization effects and tuneable near-infrared photoluminescence emission are reported in mechanically exfoliated crystals of γ-rhombohedral semiconducting InSe. The optical properties of InSe nanosheets differ qualitatively from those reported recently for exfoliated transition metal dichalcogenides and indicate a crossover from a direct to an indirect band gap semiconductor when the InSe flake thickness is reduced to a few nanometers.

Vertical field-effect transistor based on graphene-WS2 heterostructures for flexible and transparent electronics.

The celebrated electronic properties of graphene have opened the way for materials just one atom thick to be used in the post-silicon electronic era. An important milestone was the creation of heterostructures based on graphene and other two-dimensional crystals, which can be assembled into three-dimensional stacks with atomic layer precision. Such layered structures have already demonstrated a range of fascinating physical phenomena, and have also been used in demonstrating a prototype field-effect tunnelling transistor, which is regarded to be a candidate for post-CMOS (complementary metal-oxide semiconductor) technology. The range of possible materials that could be incorporated into such stacks is very large. Indeed, there are many other materials with layers linked by weak van der Waals forces that can be exfoliated and combined together to create novel highly tailored heterostructures. Here, we describe a new generation of field-effect vertical tunnelling transistors where two-dimensional tungsten disulphide serves as an atomically thin barrier between two layers of either mechanically exfoliated or chemical vapour deposition-grown graphene. The combination of tunnelling (under the barrier) and thermionic (over the barrier) transport allows for unprecedented current modulation exceeding 1 × 10(6) at room temperature and very high ON current. These devices can also operate on transparent and flexible substrates.