Large-Area Single-Crystal Graphene via Self-Organization at the Macroscale.

In 1665 Christiaan Huygens first noticed how two pendulums, regardless of their initial state, would synchronize.  It is now known that the universe is full of complex self-organizing systems, from neural networks to correlated materials. Here, graphene flakes, nucleated over a polycrystalline graphene film, synchronize during growth so as to ultimately yield a common crystal orientation at the macroscale.

Visualizing Poiseuille flow of hydrodynamic electrons.

Hydrodynamics, which generally describes the flow of a fluid, is expected to hold even for fundamental particles such as electrons when inter-particle interactions dominate. Although various aspects of electron hydrodynamics have been revealed in recent experiments, the fundamental spatial structure of hydrodynamic electrons-the Poiseuille flow profile-has remained elusive. Here we provide direct imaging of the Poiseuille flow of an electronic fluid, as well as a visualization of its evolution from ballistic flow.

Simultaneous voltage and current density imaging of flowing electrons in two dimensions.

A variety of physical phenomena associated with nanoscale electron transport often results in non-trivial spatial voltage and current patterns, particularly in nonlocal transport regimes. While numerous techniques have been devised to image electron flows, the need remains for a nanoscale probe capable of simultaneously imaging current and voltage distributions with high sensitivity and minimal invasiveness, in a magnetic field, across a broad range of temperatures and beneath an insulating surface. Here we present a technique for spatially mapping electron flows based on a nanotube single-electron transistor, which achieves high sensitivity for both voltage and current imaging.

Determining the Fermi level by absorption quenching of monolayer graphene by charge transfer doping.

While optical properties of graphene in the visible region are solely defined by the frequency-independent fine structure constant, an onset of absorption has been observed in the infrared region due to Pauli blocking of interband transitions. Here, we report a complete absorption quenching in the infrared region by coating graphene with bis(trifluoromethanesulfonyl)amine (TFSA), an optically transparent p-type chemical dopant. The Fermi level downshift due to TFSA doping results in enhanced transmission in the infrared region proportional to the doping concentration.

Stranski-Krastanov and Volmer-Weber CVD Growth Regimes To Control the Stacking Order in Bilayer Graphene.

Aside from unusual properties of monolayer graphene, bilayer has been shown to have even more interesting physics, in particular allowing bandgap opening with dual gating for proper interlayer symmetry. Such properties, promising for device applications, ignited significant interest in understanding and controlling the growth of bilayer graphene. Here we systematically investigate a broad set of flow rates and relative gas ratio of CH to H in atmospheric pressure chemical vapor deposition of multilayered graphene.

Wafer-Scale Single-Crystalline AB-Stacked Bilayer Graphene.

Single-crystalline artificial AB-stacked bilayer graphene is formed by aligned transfer of two single-crystalline monolayers on a wafer-scale. The obtained bilayer has a well-defined interface and is electronically equivalent to exfoliated or direct-grown AB-stacked bilayers.

Misorientation-angle-dependent electrical transport across molybdenum disulfide grain boundaries.

Grain boundaries in monolayer transition metal dichalcogenides have unique atomic defect structures and band dispersion relations that depend on the inter-domain misorientation angle. Here, we explore misorientation angle-dependent electrical transport at grain boundaries in monolayer MoS2 by correlating the atomic defect structures of measured devices analysed with transmission electron microscopy and first-principles calculations. Transmission electron microscopy indicates that grain boundaries are primarily composed of 5-7 dislocation cores with periodicity and additional complex defects formed at high angles, obeying the classical low-angle theory for angles <22°.

Oxidation Effect in Octahedral Hafnium Disulfide Thin Film.

Atomically smooth van der Waals materials are structurally stable in a monolayer and a few layers but are susceptible to oxygen-rich environments. In particular, recently emerging materials such as black phosphorus and perovskite have revealed stronger environmental sensitivity than other two-dimensional layered materials, often obscuring the interesting intrinsic electronic and optical properties. Unleashing the true potential of these materials requires oxidation-free sample preparation that protects thin flakes from air exposure.

Room Temperature Semiconductor-Metal Transition of MoTe2 Thin Films Engineered by Strain.

We demonstrate a room temperature semiconductor-metal transition in thin film MoTe2 engineered by strain. Reduction of the 2H-1T' phase transition temperature of MoTe2 to room temperature was realized by introducing a tensile strain of 0.2%.

High-performance n-type black phosphorus transistors with type control via thickness and contact-metal engineering.

Recent work has demonstrated excellent p-type field-effect switching in exfoliated black phosphorus, but type control has remained elusive. Here, we report unipolar n-type black phosphorus transistors with switching polarity control via contact-metal engineering and flake thickness, combined with oxygen and moisture-free fabrication. With aluminium contacts to black phosphorus, a unipolar to ambipolar transition occurs as flake thickness increases from 3 to 13 nm.

Seamless stitching of graphene domains on polished copper (111) foil.

Seamless stitching of graphene domains on polished copper (111) is proved clearly not only at atomic scale by scanning tunnelling microscopy (STM) and transmission electron micoscopy (TEM), but also at the macroscale by optical microscopy after UV-treatment. Using this concept of seamless stitching, synthesis of 6 cm × 3 cm monocrystalline graphene without grain boundaries on polished copper (111) foil is possible, which is only limited by the chamber size.

Observing grain boundaries in CVD-grown monolayer transition metal dichalcogenides.

Two-dimensional monolayer transition metal dichalcogenides (TMdCs), driven by graphene science, revisit optical and electronic properties, which are markedly different from bulk characteristics. These properties are easily modified due to accessibility of all the atoms viable to ambient gases, and therefore, there is no guarantee that impurities and defects such as vacancies, grain boundaries, and wrinkles behave as those of ideal bulk. On the other hand, this could be advantageous in engineering such defects.

Transferred wrinkled Al2O3 for highly stretchable and transparent graphene-carbon nanotube transistors.

Despite recent progress in producing transparent and bendable thin-film transistors using graphene and carbon nanotubes, the development of stretchable devices remains limited either by fragile inorganic oxides or polymer dielectrics with high leakage current. Here we report the fabrication of highly stretchable and transparent field-effect transistors combining graphene/single-walled carbon nanotube (SWCNT) electrodes and a SWCNT-network channel with a geometrically wrinkled inorganic dielectric layer. The wrinkled Al2O3 layer contained effective built-in air gaps with a small gate leakage current of 10(-13) A.

Integrating metal-oxide-decorated CNT networks with a CMOS readout in a gas sensor.

We have implemented a tin-oxide-decorated carbon nanotube (CNT) network gas sensor system on a single die. We have also demonstrated the deposition of metallic tin on the CNT network, its subsequent oxidation in air, and the improvement of the lifetime of the sensors. The fabricated array of CNT sensors contains 128 sensor cells for added redundancy and increased accuracy.

Small hysteresis nanocarbon-based integrated circuits on flexible and transparent plastic substrate.

We report small hysteresis integrated circuits by introducing monolayer graphene for the electrodes and a single-walled carbon nanotube network for the channel. Small hysteresis of the device originates from a defect-free graphene surface, where hysteresis was modulated by oxidation. This uniquely combined nanocarbon material device with transparent and flexible properties shows remarkable device performance; subthreshold voltage of 220 mV decade(-1), operation voltage of less than 5 V, on/off ratio of approximately 10(4), mobility of 81 cm(2) V(-1) s(-1), transparency of 83.

Thermionic field emission transport in carbon nanotube transistors.

With experimental and analytical analysis, we demonstrate a relationship between the metal contact work function and the electrical transport properties saturation current (Isat) and differential conductance (σsd=∂Isd/∂Vsd) in ambient exposed carbon nanotubes (CNT). A single chemical vapor deposition (CVD) grown 6 mm long semiconducting single-walled CNT is electrically contacted with a statistically significant number of Hf, Cr, Ti, Pd, and Au electrodes, respectively. The observed exponentially increasing relationship of Isat and σsd with metal contact work function is explained by a theoretical model derived from thermionic field emission.

Synthesis of edge-closed graphene ribbons with enhanced conductivity.

Edge-closed and edge-opened graphene ribbons were synthesized on Pd nanowire templates using plasma-enhanced chemical vapor deposition (PECVD). After metal nanowire etching, the tubular shaped thin graphene layers were collapsed to edge-closed graphene ribbon. In order to make edge-opened graphene ribbons, the graphene layers on the top part of the metal nanowire were selectively etched by O(2) plasma.

Anomalous Schottky barriers and contact band-to-band tunneling in carbon nanotube transistors.

Devices incorporating nanoscale materials, particularly carbon nanotubes (CNTs), offer exceptional electrical performance. Absent, however, is an experimentally backed model explaining contact-metal work function, device layout, and environment effects. To fill the void, this report introduces a surface-inversion channel model based on low temperature and electrical measurements of a distinct single-walled semiconducting CNT contacted by Hf, Cr, Ti, and Pd electrodes.