We introduce a novel two-dimensional electronic system with ultrastrong interlayer interactions, namely, twisted bilayer graphene with a large twist angle, as an ideal ground for realizing ...interlayer-coherent excitonic condensates. In these systems, sub-nanometer atomic separation between the layers allows significant interlayer interactions, while interlayer electron tunneling is geometrically suppressed due to the large twist angle. By fully exploiting these two features we demonstrate that a sequence of odd-integer quantum Hall states with interlayer coherence appears at the second Landau level (N = 1). Notably the energy gaps for these states are of order 1 K, which is several orders of magnitude greater than those in GaAs. Furthermore, a variety of quantum Hall phase transitions are observed experimentally. All the experimental observations are largely consistent with our phenomenological model calculations. Hence, we establish that a large twist angle system is an excellent platform for high-temperature excitonic condensation.
An ambipolar dual‐channel field‐effect transistor (FET) with a WSe2/MoS2 heterostructure formed by separately controlled individual channel layers is demonstrated. The FET shows a switchable ...ambipolar behavior with independent carrier transport of electrons and holes in the individual layers of MoS2 and WSe2, respectively. Moreover, the photoresponse is studied at the heterointerface of the WSe2/MoS2 dual‐channel FET.
Reduced graphene oxide (rGO) gas sensors functionalized with platinum (Pt) nanoparticles were fabricated. An alternating current dielectrophoresis technique was used for the precise alignment of the ...Pt-GO nanohybrid between microgap electrodes, proceeded by the mid-temperature thermal annealing. The gas sensing response was determined for the assembled rGO nanostructure-based devices with and without Pt decoration at various ambient temperature and gas concentrations. The tested device exhibited sensitivities of 14% (7%), 8% (5%), and 10% (8%), for 1000ppm hydrogen, ammonia, and nitric oxide gases, respectively with (without) Pt nanoparticles, at room temperature. The Pt-decorated samples show an improvement of 100%, 60% and 25% to hydrogen, ammonia, and nitric oxide gases, respectively, over without Pt decorated sensors. Besides, improving the sensitivity, the dielectrophoresis assembled rGO-Pt nanohybrid sensors have been demonstrated as a viable material for multiple gas sensors.
Electrons in graphene can travel for several microns without scattering at low temperatures, and their motion becomes ballistic, following classical trajectories. When a magnetic field B is applied ...perpendicular to the plane, electrons follow cyclotron orbits. Magnetic focusing occurs when electrons injected from one narrow contact focus onto a second contact located an integer number of cyclotron diameters away. By tuning the magnetic field B and electron density n in the graphene layer, we observe magnetic focusing peaks. We use a cooled scanning gate microscope to image cyclotron trajectories in graphene at 4.2 K. The tip creates a local change in density that casts a shadow by deflecting electrons flowing nearby; an image of flow can be obtained by measuring the transmission between contacts as the tip is raster scanned across the sample. On the first magnetic focusing peak, we image a cyclotron orbit that extends from one contact to the other. In addition, we study the geometry of orbits deflected into the second point contact by the tip.
A layered two-dimensional superconducting material 2H-NbSe2 is used to build a van der Waals heterostructure, where a proximity-coupled superconducting order can be induced in the interfacing ...materials. Vertically stacked NbSe2–graphene–NbSe2 is fabricated using van der Waals interlayer coupling, producing defect-free contacts with a high interfacial transparency. The atomically thin graphene layer allows the formation of a highly coherent proximity Josephson coupling between the two NbSe2 flakes. The temperature dependence of the junction critical current (I c) reveals short and ballistic Josephson coupling characteristics that agree with theoretical prediction. The strong Josephson coupling is confirmed by a large junction critical current density of 1.6 × 104 A/cm2, multiple Andreev reflections in the subgap structure of the differential conductance, and a magnetic-field modulation of I c. This is the first demonstration of strongly proximity-coupled Josephson junctions with extremely clean interfaces in a dry-transfer-stacked van der Waals heterostructure.
Transition metal dichalcogenides (TMDCs) encapsulation is an essential technology for improving electron transport and preventing external contamination in practical applications. Nevertheless, Ohmic ...contacts in TMDCs continue to pose many problems. A laser was irradiated along with MoTe2 encapsulated in h-BN and edge contact electrodes. We studied the properties of the edge contact resistance, in which the crystal structure of MoTe2 changes from a semiconductor hexagonal phase (2H) to a metallic monoclinic phase (1T′). The contact between TMDCs and the metal electrode, Fermi-level pinning, contact resistance, and Schottky barrier height (SBH) can be calculated. Laser irradiation of the edge contact confirmed that, due to the change in the crystal structure of MoTe2, a reduction in contact resistance by over a factor of three resulted in the development of the electrical properties of the device. Field-effect transistors (FETs) with indium (In) edge contact exhibit high performance, with the highest electron mobility reaching 7.9 cm2V−1s−1 at 300 K. Furthermore, the barrier heights for In with a MoTe2 junction were 10.3 meV after laser irradiation, which is more than ten times the low SBH. This study confirms the improved electrical properties of the two-dimensional material and metal were confirmed using a laser.
Coherent charge transport along ballistic paths can be introduced into graphene by Andreev reflection, for which an electron reflects from a superconducting contact as a hole, while a Cooper pair is ...transmitted. We use liquid-helium cooled scanning gate microscopy (SGM) to image Andreev reflection in graphene in the magnetic focusing regime, where carriers move along cyclotron orbits between contacts. Images of flow are obtained by deflecting carrier paths and displaying the resulting change in conductance. When electrons enter the superconductor, Andreev-reflected holes leave for the collecting contact. To test the results, we destroy Andreev reflection with a large current and by heating above the critical temperature. In both cases, the reflected carriers change from holes to electrons.
The design and synthesis of two-dimensional (2D) polymers is a challenging task, hitherto achieved in solution only through the aid of a solid surface “template” or preorganization of the building ...blocks in a 2D confined space. We present a novel approach for synthesizing free-standing, covalently bonded, single-monomer-thick 2D polymers in solution without any preorganization of building blocks on solid surfaces or interfaces by employing shape-directed covalent self-assembly of rigid, disk-shaped building blocks having laterally predisposed reactive groups on their periphery. We demonstrate our strategy through a thiol–ene “click” reaction between (allyloxy)12CB6, a cucurbit6uril (CB6) derivative with 12 laterally predisposed reactive alkene groups, and 1,2-ethanedithiol to synthesize a robust and readily transferable 2D polymer. We can take advantage of the high binding affinity of fully protonated spermine (positive charges on both ends) to CB6 to keep each individual polymer sheet separated from one another by electrostatic repulsion during synthesis, obtaining, for the first-time ever, a single-monomer-thick 2D polymer in solution. The arrangement of CB6 repeating units in the resulting 2D polymer has been characterized using gold nanoparticle labeling and scanning transmission electron microscopy. Furthermore, we have confirmed the generality of our synthetic approach by applying it to different monomers to generate 2D polymers. Novel 2D polymers, such as our CB6 derived polymer, may be useful in selective transport, controlled drug delivery, and chemical sensing and may even serve as well-defined 2D scaffolds for ordered functionalization and platforms for bottom-up 3D construction.
Hydrogen gas sensors based on graphene oxide (GO) nanostructures have been fabricated using ac dielectrophoresis (DEP) process. The GO nanostructures synthesized by an improved Hummer's method were ...first characterized by atomic force microscopy, X-ray diffraction, X-ray photoelectron spectroscopy, and Raman spectroscopy. GO nanostructures were assembled into gold electrodes using DEP process by varying parameters such as frequency, peak-to-peak voltage (Vpp), and processing time (t). The devices were investigated by scanning electron microscopy, current-voltage measurement, and hydrogen sensing experiment at room temperature. It was found that the optimum DEP parameters that manipulates GO nanostructures in precise manner for hydrogen gas sensing were Vpp=10V, frequency=500kHz, and t=30s. The optimized device was proved to be an effective and better hydrogen gas sensor over a typical drop-dried device with a good sensing response of 5%, fast response time (<90s), and fast recovery time (<60s) for 100ppm hydrogen gas concentration at room temperature.
Josephson junction infrared single-photon detector Walsh, Evan D; Jung, Woochan; Lee, Gil-Ho ...
Science (American Association for the Advancement of Science),
04/2021, Volume:
372, Issue:
6540
Journal Article
Peer reviewed
Open access
Josephson junctions are superconducting devices used as high-sensitivity magnetometers and voltage amplifiers as well as the basis of high-performance cryogenic computers and superconducting quantum ...computers. Although device performance can be degraded by the generation of quasiparticles formed from broken Cooper pairs, this phenomenon also opens opportunities to sensitively detect electromagnetic radiation. We demonstrate single near-infrared photon detection by coupling photons to the localized surface plasmons of a graphene-based Josephson junction. Using the photon-induced switching statistics of the current-biased device, we reveal the critical role of quasiparticles generated by the absorbed photon in the detection mechanism. The photon sensitivity will enable a high-speed, low-power optical interconnect for future superconducting computing architectures.
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