From diffusive to ballistic transport in etched graphene constrictions and nanoribbons

Graphene nanoribbons and constrictions are envisaged as fundamental components of future carbon-based nanoelectronic and spintronic devices. At nanoscale, electronic effects in these devices depend heavily on the dimensions of the active channel and the nature of edges. Hence, controlling both these parameters is crucial to understand the physics in such systems. This review is about the recent progress in the fabrication of graphene nanoribbons and constrictions in terms of low temperature quantum transport. In particular, recent advancements using encapsulated graphene allowing for quantized conductance and future experiments towards exploring spin effects in these devices are presented. The influence of charge carrier inhomogeneity and the important length scales which play a crucial role for transport in high quality samples are also discussed.Comment: 32 pages, 6 figures. Will appear in Annalen der Physi

Low B Field Magneto-Phonon Resonances in Single-Layer and Bilayer Graphene

Many-body effects resulting from strong electron-electron and electron-phonon interactions play a significant role in graphene physics. We report on their manifestation in low B field magneto-phonon resonances in high quality exfoliated single-layer and bilayer graphene encapsulated in hexagonal boron nitride. These resonances allow us to extract characteristic effective Fermi velocities, as high as $1.20 \times 10^6$ m/s, for the observed "dressed" Landau level transitions, as well as the broadening of the resonances, which increases with Landau level index

Transparent, flexible electrodes and sensors based on carbon nanotube thin films

Hem obtingut capes primes de Nanotubs de Carboni d'una sola paret (CNT) sobre un substrat amb un mètode molt simple, que poden ser emprades com elèctrodes flexibles i transparents en dispositius electrònics. Per tal d'obtenir dispositius reproduïbles amb propietats similars, en particular amb similar impedància Z(ω), és important relacionar les propietats elèctriques amb la quantitat de CNTs presents en la capa. Per això, hem realitzat capes primes de CNTs sobre substrats flexibles i transparents (PPC, policarbonat de propilè) amb diferents densitats de CNT. A partir d'un mètode iteratiu matemàtic i de l'Anàlisi Tèrmogravimètric (TGA) de cada mostra, hem pogut determinar la quantitat de CNTs presents en cada mostra. També n'hem fet una estimació a partir de l'espectroscòpia d'absorció òptica. Hem vist que els dos mètodes donen resultats coherents. Hem analitzat les diferents mostres mesurant la impedància elèctrica a diferents frequències, fins 110 MHz. Les capes primes amb poca densitat de CNTs són semiconductores, en canvi les denses són metàl·liques, i prou conductores per ser utilitzades com elèctrode de treball en un procés electroquímic. Podem obtenir així composites CNT-polímer conductor o CNT-metall, electroquímicament. Amb l'objectiu de les aplicacions per a sensors, utilitzant les capes primes de CNT com elèctrode de treball hem obtingut composites CNT-polímer conductor, depositant-hi electroquímicament un polímer conductor, polipirrol o polianilina. Hem analitzat les propietats del dispositiu com a sensor electroquímic, observant la seva resposta en funció del pH, mesurant el potencial en circuit obert en funció del pH de la solució, entre 1 i 13. Els resultats mostren una bona sensibilitat, linearitat i estabilitat. Per això, els dispositius CNT/polipirrol i CNT/polianilina poden tenir aplicacions com a sensors o biosensors en estat sòlid, depositats sobre qualsevol superfície de forma arbitrària, que pot ser transparent i flexible.We obtained thin films of single-walled carbon nanotubes (CNTs), which may be used as transparent, flexible electrodes in electronic devices, on a substrate using a very simple method. In order to construct reproducible devices with similar properties, in particular with similar impedance Z(ω), it is important to associate the electrical properties with the number of CNTs in a network. We prepared thin CNT networks on transparent, flexible substrates (PPC, polypropylene carbonate) with different CNT densities. The number of CNTs was estimated using a mathematical method based on the data obtained from thermo-gravimetric analysis (TGA). We were able to estimate the relative number of CNTs using optical absorption spectroscopy. These two methods are in good agreement. We also analysed the various samples using electrical impedance measurements at frequencies of up to 110 MHz. Low-density networks are semiconductors, whilst high-density networks behave like metals and are sufficiently good conductors to be used as working electrodes in electrochemical processes. It is thus possible to obtain CNT polymer and metal composite conductors electrochemically. With sensor applications in mind, we used CNT thin films as a working electrode to obtain a composite CNT-conducting polymer. This was performed by electrochemically depositing a conducting polymer ? polypyrrole or polyaniline ? on the electrode. The pH dependence of the device was measured by recording its open circuit potential in various buffer solutions. This enabled us to analyse the properties of the device as an electrochemical sensor. The results showed a good sensitivity, linearity and stability in both cases. Thus, the CNT/polypyrrole and CNT/polyaniline devices could have applications as solidstate gas sensors or biosensors when they are deposited on transparent and flexible surfaces of any shape

Tunable capacitive inter-dot coupling in a bilayer graphene double quantum dot

We report on a double quantum dot which is formed in a width-modulated etched bilayer graphene nanoribbon. A number of lateral graphene gates enable us to tune the quantum dot energy levels and the tunneling barriers of the device over a wide energy range. Charge stability diagrams and in particular individual triple point pairs allow to study the tunable capacitive inter-dot coupling energy as well as the spectrum of the electronic excited states on a number of individual triple points. We extract a mutual capacitive inter-dot coupling in the range of 2 - 6 meV and an inter-dot tunnel coupling on the order of 1.5 {\mu}eV.Comment: 6 pages, 4 figure

Fast and sensitive terahertz detection using an antenna-integrated graphene pn-junction

Although the detection of light at terahertz (THz) frequencies is important for a large range of applications, current detectors typically have several disadvantages in terms of sensitivity, speed, operating temperature, and spectral range. Here, we use graphene as photoactive material to overcome all of these limitations in one device. We introduce a novel detector for terahertz radiation that exploits the photo-thermoelectric effect, based on a design that employs a dual-gated, dipolar antenna with a gap of ~100 nm. This narrow-gap antenna simultaneously creates a pn-junction in a graphene channel located above the antenna, and strongly concentrates the incoming radiation at this pn-junction, where the photoresponse is created. We demonstrate that this novel detector has excellent sensitivity, with a noise-equivalent power of 80 pW/√Hz at room temperature, a response time below 30 ns (setup-limited), a high dynamic range (linear power dependence over more than 3 orders of magnitude) and broadband operation (measured range 1.8 - 4.2 THz, antenna-limited), which fulfils a combination that is currently missing in the state of the art. Importantly, based on the agreement we obtain between experiment, analytical model, and numerical simulations, we have reached a solid understanding of how the PTE eect gives rise to a THz-induced photoresponse, which is very valuable for further detector optimization.Peer ReviewedPostprint (author's final draft

Near-unity light absorption in a monolayer WS2 van der Waals heterostructure cavity

Excitons in monolayer transition-metal-dichalcogenides (TMDs) dominate their optical response and exhibit strong light-matter interactions with lifetime-limited emission. While various approaches have been applied to enhance light-exciton interactions in TMDs, the achieved strength have been far below unity, and a complete picture of its underlying physical mechanisms and fundamental limits has not been provided. Here, we introduce a TMD-based van der Waals heterostructure cavity that provides near-unity excitonic absorption, and emission of excitonic complexes that are observed at ultralow excitation powers. Our results are in full agreement with a quantum theoretical framework introduced to describe the light-exciton-cavity interaction. We find that the subtle interplay between the radiative, nonradiative and dephasing decay rates plays a crucial role, and unveil a universal absorption law for excitons in 2D systems. This enhanced light-exciton interaction provides a platform for studying excitonic phase-transitions and quantum nonlinearities and enables new possibilities for 2D semiconductor-based optoelectronic devices.The authors thank Mr. David Alcaraz Iranzo, Dr. Fabien Vialla, and Dr. Antoine Reserbat-Plantey for fruitful discussions. F.H.L.K. acknowledges financial support from the Spanish Ministry of Economy and Competitiveness through the "Severo Ochoa" Programme for Centres of Excellence in R and D (SEV-2015-0522), support by Fundacio Cellex Barcelona, Generalitat de Catalunya through the CERCA program, and the Mineco grants Ramon y Cajal (RYC-201212281, Plan Nacional (FIS2013-47161-P and FIS2014-59639JIN), and the Agency for Management of University and Research Grants (AGAUR) 2017 SGR 1656. Furthermore, the research leading to these results has received funding from the European Union Seventh Framework Programme under grant agreement numbers 785219 and 881603 Graphene Flagship. This work was supported by the ERC TOPONANOP under grant agreement number 726001 and the MINECO Plan Nacional Grant 2D-NANOTOP under reference number FIS2016-81044-P. S.T. acknowledges support from NSF DMR-1552220 and DMR-1838443. N.M.R.P acknowledges financing from European Commission through the project "Graphene-Driven Revolutions in ICT and Beyond" (ref. no. 785219) and from FEDER and the Portuguese Foundation for Science and Technology (FCT) through project POCI-010145-FEDER-028114. H.G. and B.F. acknowledge support from ERC advanced grant COMPLEXPLAS. J.H. and D.R. acknowledge the funding support by the NSF MRSEC program through Columbia in the Center for Precision Assembly of Superstratic and Superatomic Solids (DMR1420634)

Nanosecond spin lifetimes in single- and few-layer graphene-hBN heterostructures at room temperature

We present a new fabrication method of graphene spin-valve devices which yields enhanced spin and charge transport properties by improving both the electrode-to-graphene and graphene-to-substrate interface. First, we prepare Co/MgO spin injection electrodes onto Si$^{++}$/SiO$_2$. Thereafter, we mechanically transfer a graphene-hBN heterostructure onto the prepatterned electrodes. We show that room temperature spin transport in single-, bi- and trilayer graphene devices exhibit nanosecond spin lifetimes with spin diffusion lengths reaching 10$\mu$m combined with carrier mobilities exceeding 20,000 cm$^2$/Vs.Comment: 15 pages, 5 figure

Ultrafast, Zero-Bias, Graphene Photodetectors with Polymeric Gate Dielectric on Passive Photonic Waveguides.

We report compact, scalable, high-performance, waveguide integrated graphene-based photodetectors (GPDs) for telecom and datacom applications, not affected by dark current. To exploit the photothermoelectric (PTE) effect, our devices rely on a graphene/polymer/graphene stack with static top split gates. The polymeric dielectric, poly(vinyl alcohol) (PVA), allows us to preserve graphene quality and to generate a controllable p-n junction. Both graphene layers are fabricated using aligned single-crystal graphene arrays grown by chemical vapor deposition. The use of PVA yields a low charge inhomogeneity ∼8 × 1010 cm-2 at the charge neutrality point, and a large Seebeck coefficient ∼140 μV K-1, enhancing the PTE effect. Our devices are the fastest GPDs operating with zero dark current, showing a flat frequency response up to 67 GHz without roll-off. This performance is achieved on a passive, low-cost, photonic platform, and does not rely on nanoscale plasmonic structures. This, combined with scalability and ease of integration, makes our GPDs a promising building block for next-generation optical communication devices

Ballistic transport in graphene nanostructures

This work aims to contribute to the progress and understanding of the sources of disorder in nano-structured graphene devices. The first part of the thesis starts with the introduction of disordered two-terminal graphene nanoribbons of different aspect ratio, in order to unveil and characterize the amount of potential fluctuations on silicon dioxide ($SiO_2$) substrates. The experimental results reveal the diffusive nature of the transport behavior and a Coulomb blockade dominated transport regime close to the charge neutrality point. Besides its disordered nature, results appoint very short graphene constrictions, with levels of conductance close to $\sim \!0.1\,e^2/h$, as prime candidates for exploring Fano resonances in graphene nano-structures. In an attempt to reduce the contributions of the potential fluctuations to transport, we initially identify the different sources of disorder, with bulk and edges arising as the major contributors in nano-structured devices. The strong influence from the bulk is characterized via the tunneling processes through magnetically confined quantum dots arising from the aforementioned bulk disorder. First evidences of an edge induced disorder are treated in the following section, where we investigate the crystal structure of the nanoribbon's edges by means of Raman spectroscopy experiments. Results on lithography-free graphene nanoribbons, shaped by the exfoliation process itself, are compared to traditional plasma etched graphene ribbons. In these pristine ribbons, the correlation length $\xi$, figure of merit to characterize the edges, is one order of magnitude higher than on plasma etched structures. Results highlight the strong edge-induced disorder present in traditionally plasma-etched graphene devices. With the edge-induced disorder identified via Raman spectroscopy measurements, we implement in the next section an electrostatic approach to reduce its effects. Short and relatively narrow graphene constrictions are side-gated by graphene gate electrodes. We demonstrate the reduction in disorder by transport and bias spectroscopy measurements. Results are further supported by the formation of a quasi-1D channel upon application of a lateral electrostatic potential. The 1D-like nature of the electronic path is justified by its Fano-like interference with a 0D-like charged puddle located at the interface with the leads. Results represent the very first reported indications of Fano interference phenomena in graphene. To reduce bulk disorder, we implement a dry transfer technique for the fabrication of encapsulated graphene devices in between a top- and a bottom-layer of hexagonal boron nitride (hBN). Mobility values approaching $200\,000\,cm^2/(V\,s)$ confirm the high quality achieved with our fabrication technique. The residual disorder is characterized via the temperature dependence of the symmetry broken states in the quantum Hall regime, in a hBN/graphene/hBN Hall bar device. The values of localization length found in the variable-range-hopping (VRH) regime exceed $1\mu m$, one order of magnitude higher than the reported values for graphene on $SiO_2$ substrates. In the second part of the thesis, we demonstrate ballistic transport and quantized conductance of size-confined Dirac fermions in lithographically-defined graphene quantum point contacts (QPCs). Close to the charge neutrality point, bias voltage spectroscopy measurements reveal a renormalized Fermi velocity ($v_F\!\approx\!1.5\times10^6\,m/s$) in our graphene constrictions. Moreover, at low carrier densities, transport measurements allow probing the density of localized states at the edges, thus offering a unique handle on edge physics in graphene devices. Direct comparison between successive cool-downs of a same QPC device reveal the lifting of the four-fold degenerate subbands. Results are supported by bias voltage and magnetic field dependent measurements. The amount of dopands/contaminants collected by the edges during the successive cool-downs is appointed as the source of this degeneracy breaking process. Quantum Hall measurements are used to spatially resolve the change in capacitance profile, supporting this change in dopands/contaminants at the edges

Authors' reply to Devitt

We discuss a fabrication process for making graphene devices based on encapsulated graphene for reducing contaminations during individual processing steps. A 3–5 nm alumina layer is deposited directly after exfoliating graphene, protecting it during the entire processing. We show that the visibility of the encapsulated graphene is sufficient to identify graphene flakes and Raman spectra exhibit the characteristic finger print. We perform transport measurements to study the sample quality and compare the results with graphene samples processed without an alumina layer. In particular we observe a higher yield and significantly reduced contact resistances for devices fabricated with the here presented method