Amaia Pesquera

Photoexcitation cascade and multiple hot carrier generation in graphene

For many optoelectronic applications, such as photodetection and light harvesting, it is highly desirable to identify materials in which an absorbed photon is efficiently converted to electronic excitations. The unique properties of graphene, such as its gapless band structure, flat absorption spectrum and strong electron-electron interactions, make it a highly promising material for efficient broadband photon-electron conversion [1]. Indeed, recent theoretical work has anticipated that in graphene multiple electron-hole pairs can be created from a single absorbed photon during energy relaxation of the primary photoexcited e-h pair [2]. A photoexcited carrier relaxes initially trough two competing pathways: carrier-carrier scattering and optical phonon emission. In the former process the energy of photoexcited carriers remains in the electron system, being transferred to secondary electrons that gain energy (become hot), whereas in the phonon emission process the energy is lost to the lattice as heat. While recent experiments have shown that photoexcitation of graphene can generate hot carriers [3], it remains unknown how efficient this process is with respect to optical phonon emission.

Controlling graphene plasmons with resonant metal antennas and spatial conductivity patterns.

A controlled launch for plasmons To create nanophotonic devices, engineers must combine large-scale optics with tiny nanoelectronics. Plasmons, the collective light-induced excitations of electrons at a metals surface, can bridge that difference in size scales. Alonso-Gonzalez et al. placed structured gold “antennas” on top of a graphene layer to launch and propagate plasmonic excitations into the graphene. By carefully designing the antennas, the researchers could engineer the wavefronts of the plasmons and control the direction of propagation. This approach illustrates a versatile approach for the development of nanophotonics. Science, this issue p. 1369 Structured gold antennas are used to launch plasmons into graphene, engineer their wavefronts, and control their propagation. Graphene plasmons promise unique possibilities for controlling light in nanoscale devices and for merging optics with electronics. We developed a versatile platform technology based on resonant optical antennas and conductivity patterns for launching and control of propagating graphene plasmons, an essential step for the development of graphene plasmonic circuits. We launched and focused infrared graphene plasmons with geometrically tailored antennas and observed how they refracted when passing through a two-dimensional conductivity pattern, here a prism-shaped bilayer. To that end, we directly mapped the graphene plasmon wavefronts by means of an imaging method that will be useful in testing future design concepts for nanoscale graphene plasmonic circuits and devices.

Metal oxide induced charge transfer doping and band alignment of graphene electrodes for efficient organic light emitting diodes.

The interface structure of graphene with thermally evaporated metal oxide layers, in particular molybdenum trioxide (MoO3), is studied combining photoemission spectroscopy, sheet resistance measurements and organic light emitting diode (OLED) characterization. Thin (<5 nm) MoO3 layers give rise to an 1.9 eV large interface dipole and a downwards bending of the MoO3 conduction band towards the Fermi level of graphene, leading to a near ideal alignment of the transport levels. The surface charge transfer manifests itself also as strong and stable p-type doping of the graphene layers, with the Fermi level downshifted by 0.25 eV and sheet resistance values consistently below 50 Ω/sq for few-layer graphene films. The combination of stable doping and highly efficient charge extraction/injection allows the demonstration of simplified graphene-based OLED device stacks with efficiencies exceeding those of standard ITO reference devices.

Highly sensitive detection of DNA hybridization on commercialized graphene-coated surface plasmon resonance interfaces.

Strategies employed to interface biomolecules with nanomaterials have considerably advanced in recent years and found practical applications in many different research fields. The construction of nucleic acid modified interfaces together with the label-free detection of hybridization events has been one of the major research focuses in science and technology. In this paper, we demonstrate the high interest of graphene-on-metal surface plasmon resonance (SPR) interfaces for the detection of DNA hybridization events in the attomolar concentration range. The strategy consists on the noncovalent functionalization of graphene-coated SPR interfaces with gold nanostars carrying single-stranded DNA (ssDNA). Upon hybridization with its complementary DNA, desorption of the nanostructures takes place and thus enables the sensitive detection of the DNA hybridization event. The DNA sensor exhibits a detection limit of ≈500 aM for complementary DNA with a linear dynamic range up to 10(-8) M. This label-free DNA detection platform should spur off new interest toward the use of commercially available graphene-coated SPR interfaces.

Direct Observation of a Long-Lived Single-Atom Catalyst Chiseling Atomic Structures in Graphene

Fabricating stable functional devices at the atomic scale is an ultimate goal of nanotechnology. In biological processes, such high-precision operations are accomplished by enzymes. A counterpart molecular catalyst that binds to a solid-state substrate would be highly desirable. Here, we report the direct observation of single Si adatoms catalyzing the dissociation of carbon atoms from graphene in an aberration-corrected high-resolution transmission electron microscope (HRTEM). The single Si atom provides a catalytic wedge for energetic electrons to chisel off the graphene lattice, atom by atom, while the Si atom itself is not consumed. The products of the chiseling process are atomic-scale features including graphene pores and clean edges. Our experimental observations and first-principles calculations demonstrated the dynamics, stability, and selectivity of such a single-atom chisel, which opens up the possibility of fabricating certain stable molecular devices by precise modification of materials at the atomic scale.

Electrical control of optical emitter relaxation pathways enabled by graphene

The relaxation processes of light-emitting excited ions are tunable, but electrical control is challenging. It is now shown that graphene can be used to manipulate the optical emission and relaxation of erbium near-infrared emitters electrically.

Plasmonic photothermal destruction of uropathogenic E. coli with reduced graphene oxide and core/shell nanocomposites of gold nanorods/reduced graphene oxide

The development of non-antibiotic based treatments against bacterial infections by Gram-negative uropathogenic E. coli is a complex task. New strategies to treat such infections are thus urgently needed. This report illustrates the development of pegylated reduced graphene oxide nanoparticles (rGO-PEG) and gold nanorods (Au NRs) coated with rGO-PEG (rGO-PEG-Au NRs) for the selective killing of uropathogenic E. coli UTI89. We took advantage of the excellent light absorption properties of rGO-PEG and Au NR particles in the near-infrared (NIR) region to photothermally kill Gram-negative pathogens up to 99% in 10 min by illumination of solutions containing the bacteria. The rGO-PEG-Au NRs demonstrated better photothermal efficiency towards E. coli than rGO-PEG. Targeted killing of E. coli UTI89 could be achieved with rGO-PEG-Au NRs functionalized with multimeric heptyl α-d-mannoside probes. This currently offers a unique biocompatible method for the ablation of pathogens with the opening of probably a new possibility for clinical treatments of patients with urinary infections.

Broadband image sensor array based on graphene–CMOS integration

Integrated circuits based on complementary metal-oxide–semiconductors (CMOS) are at the heart of the technological revolution of the past 40 years, enabling compact and low-cost microelectronic circuits and imaging systems. However, the diversification of this platform into applications other than microcircuits and visible-light cameras has been impeded by the difficulty to combine semiconductors other than silicon with CMOS. Here, we report the monolithic integration of a CMOS integrated circuit with graphene, operating as a high-mobility phototransistor. We demonstrate a high-resolution, broadband image sensor and operate it as a digital camera that is sensitive to ultraviolet, visible and infrared light (300–2,000 nm). The demonstrated graphene–CMOS integration is pivotal for incorporating 2D materials into the next-generation microelectronics, sensor arrays, low-power integrated photonics and CMOS imaging systems covering visible, infrared and terahertz frequencies. Graphene–quantum dots on CMOS sensor offers broadband imaging.

Radio-frequency plasma-excited molecular beam epitaxy growth of GaN on graphene/Si(100) substrates

Strong c-axis-oriented hexagonal (0001) GaN was grown on graphene/Si(100) substrates by radio-frequency plasma-excited molecular beam epitaxy. The hexagonal symmetry of graphene transferred onto the Si(100) surface enabled the growth of a highly c-axis-oriented GaN film. The GaN showed a full width at half maximum of 11.3 arcmin for a (0002) rocking curve measured by X-ray diffraction. Strong luminescence at 3.4 eV was also observed by cathodoluminescence with a luminescence at 3.2 eV, which originated from a cubic-phase inclusion. A microstructural study using transmission electron microscopy also confirmed the growth of hexagonal (0001) GaN on a graphene/Si(100) substrate.

Up-Scaling Graphene Electronics by Reproducible Metal-Graphene Contacts

Chemical vapor deposition (CVD) of graphene on top of metallic foils is a technologically viable method of graphene production. Fabrication of microelectronic devices with CVD grown graphene is commonly done by using photolithography and deposition of metal contacts on top of the transferred graphene layer. This processing is potentially invasive for graphene, yields large spread in device parameters, and can inhibit up-scaling. Here we demonstrate an alternative process technology in which both lithography and contact deposition on top of graphene are prevented. First a prepatterned substrate is fabricated that contains all the device layouts, electrodes and interconnects. Then CVD graphene is transferred on top. Processing parameters are adjusted to yield a graphene layer that adopts the topography of the prepatterned substrate. The metal-graphene contact shows low contact resistances below 1 kΩ μm for CVD graphene devices. The conformal transfer technique is scaled-up to 150 mm wafers with statistically similar devices and with a device yield close to unity.