Mirco Cantoro

Bandgap opening in oxygen plasma-treated graphene

We report a change in the semimetallic nature of single-layer graphene after exposure to oxygen plasma. The resulting transition from semimetallic to semiconducting behavior appears to depend on the duration of the exposure to the plasma treatment. The observation is confirmed by electrical, photoluminescence and Raman spectroscopy measurements. We explain the opening of a bandgap in graphene in terms of functionalization of its pristine lattice with oxygen atoms. Ab initio calculations show more details about the interaction between carbon and oxygen atoms and the consequences on the optoelectronic properties, that is, on the extent of the bandgap opening upon increased functionalisation density.

Toward tunable doping in graphene FETs by molecular self-assembled monolayers

In this paper, we report the formation of self-assembled monolayers (SAMs) of oleylamine (OA) on highly oriented pyrolytic graphite (HOPG) and graphene surfaces and demonstrate the potential of using such organic SAMs to tailor the electronic properties of graphene. Molecular resolution Atomic Force Microscopy (AFM) and Scanning Tunneling Microscopy (STM) images reveal the detailed molecular ordering. The electrical measurements show that OA strongly interacts with graphene leading to n-doping effects in graphene devices. The doping levels are tunable by varying the OA deposition conditions. Importantly, neither hole nor electron mobilities are decreased by the OA modification. As a benefit from this noncovalent modification strategy, the pristine characteristics of the device are recoverable upon OA removal. From this study, one can envision the possibility to correlate the graphene-based device performance with the molecular structure and supramolecular ordering of the organic dopant.

Modified, semiconducting graphene in contact with a metal: Characterization of the Schottky diode

In this paper, we report the fabrication and characterization of Schottky rectifying junctions between semiconducting, modified single-layer graphene and a metal. The pristine, semimetallic behavior of graphene is altered by controlled exposure to an oxygen plasma, resulting in the opening of an optical band gap as shown by photoluminescence spectroscopy. The occurrence of a Schottky barrier between semiconducting graphene and metals with different work functions (Al, Cr, Pd, and Yb) is investigated by electrically characterizing the as-fabricated junctions. The rectifying properties of our Schottky diodes show the potential of semiconducting, modified graphene as building block of elementary logic circuits.

Controlled III/V Nanowire Growth by Selective-Area Vapor-Phase Epitaxy

We report on the growth of surface-bound, vertically oriented one-dimensional III/V nanostructures, specifically GaAs and InAs nanowires, on lattice-matched and -mismatched substrates by selective-area vapor-phase epitaxy. Control of nanowire features and growth directions is achieved by tuning the growth conditions. Grown nanostructures are characterized by scanning and transmission electron microscopy, X-ray diffraction, and Raman spectroscopy.

Switching mechanism in two-terminal vanadium dioxide devices

Two-terminal thin film VO2 devices show an abrupt decrease of resistance when the current or voltage applied exceeds a threshold value. This phenomenon is often described as a field-induced metal-insulator transition. We fabricate nano-scale devices with different electrode separations down to 100 nm and study how the dc switching voltage and current depend on device size and temperature. Our observations are consistent with a Joule heating mechanism governing the switching. Pulsed measurements show a switching time to the high resistance state of the order of one hundred nanoseconds, consistent with heat dissipation time. In spite of the Joule heating mechanism which is expected to induce device degradation, devices can be switched for more than 10(10) cycles making VO2 a promising material for nanoelectronic applications.

Graphene as anode electrode for colloidal quantum dots based light emitting diodes

Graphene films demonstrating low sheet resistance and high transparency in the visible light range are promising to be used as electrodes for light-emitting applications. In this work, we report the implementation of single layer graphene as hole injecting electrode for CdSe/ZnS quantum dot-light emitting diodes (QD-LED). We compare graphene vs. indium-tin-oxide (ITO)-based anode junctions by electroluminescence intensity performance of QD-LEDs. Our results demonstrate better hole injection efficiency for the graphene-based electrode at technologically relevant current densities J < 0.4 A/cm2, therefore, recommending single layer graphene as a valuable alternative to replace ITO in QD-LED technology.

Towards the Monolithic Integration of III-V Compound Semiconductors on Si: Selective Area Growth in High Aspect Ratio Structures vs. Strain Relaxed Buffer-Mediated Epitaxy

We report two approaches to integrate high quality III-V templates by epitaxial growth with low defectivity on Si wafers. The first approach is based on blanket, InGaAs-based Strain Relaxed Buffers grown by MOVPE on 200mm Si, and the second on the selective area MOVPE of InP in Shallow Trench Isolation structures patterned on 300mm Si. Both structures are characterized structurally and show the efficient trapping and annihilation of defects propagation from the Si/III-V interface. We believe these two approaches represent viable alternatives towards the realization of CMOS-compatible III-V templates and stacks for high-performance devices monolithically integrated on Si.

Charge transfer effects in graphene-CdSe/ZnS quantum dots composites

Graphene possesses unique physical properties, due to its specific energy bands configuration, substantially different from that of materials traditionally employed in solid-state optoelectronics. Among the variety of remarkable properties, strong field effect, high transparency in the visible-light range and low resistivity of graphene sheets are the most attractive ones for optoelectronic applications. Zero-dimensional colloidal semiconductor nanocrystals, known as quantum dots (QDs), attract immense attention in the field of photonics due to their size-dependent tunable optical properties. By combining these two types of nanomaterials together, we demonstrate the role of graphene as an efficient charge transfer medium from- and to II-VI quantum dots. The optical excitation of II-VI quantum dots dispersed on single layer graphene results in an electron transfer from the nanocrystals to graphene. This is evidenced from photoluminescence imaging and confirmed by the electrical measurements on QDs-decorated single layer graphene field effect transistors (SLG-FET). In the second part of this paper we demonstrate an efficient hole injection from graphene into QDs-layered nanocrystalline structures and the operation of the corresponding graphene-based quantum dot light emitting diodes (QD-LED). We also benchmark graphene vs. indium-tin-oxide (ITO) based QD-LEDs in terms of device electroluminescence intensity performance. Our experimental results show better hole injection efficiency for graphenebased electrode at current densities as high as 200 mA/cm2 and suggest single layer graphene as a strong candidate to replace ITO in QD-LED technology.

Shaping the future of nanoelectronics beyond the Si roadmap with new materials and devices

The use of high mobility channel materials such as Ge and III/V compounds for CMOS applications is being explored. The introduction of these new materials also opens the path towards the introduction of novel device structures which can be used to lower the supply voltage and reduce the power consumption. The results illustrate the possibilities that are created by the combination of new materials and devices to allow scaling of nanoelectronics beyond the Si roadmap.

Chemically enhanced double-gate bilayer graphene field-effect transistor with neutral channel for logic applications

In this article, we present the simulation, fabrication, and characterization of a novel bilayer graphene field-effect transistor exhibiting electron mobility up to ~1600 cm(2) V(-1) s(-1), a room temperature I on/I off ≈ 60, and the lowest total charge (~10(11) cm(-2)) reported to date. This is achieved by combined electrostatic and chemical doping of bilayer graphene, which enables one to switch off the device at zero top-gate voltage. Using density functional theory and atomistic simulations, we obtain physical insight into the impact of chemical and electrostatic doping on bandgap opening of bilayer graphene and the effect of metal contacts on the operation of the device. Our results represent a step forward in the use of bilayer graphene for high-performance logic devices in the beyond-complementary metal-oxide-semiconductor (CMOS) technology paradigm.