Asymmetrically Engineered Nanoscale Transistors for On-Demand Sourcing of Terahertz Plasmons.

Terahertz (THz) plasma oscillations represent a potential path to implement ultrafast electronic devices and circuits. Here, we present an approach to generate on-chip THz signals that relies on plasma-wave stabilization in nanoscale transistors with specific structural asymmetry. A hydrodynamic treatment shows how the transistor asymmetry supports plasma-wave amplification, giving rise to pronounced negative differential conductance (NDC).

Origin of pseudo-dispersion in non-dispersive media by terahertz time-domain spectroscopy.

We report the false appearance of dispersion in non-dispersive materials when measured by terahertz time-domain spectroscopy. This occurs when the material is measured in reflection geometry and has a bulk metal interface opposite to the incident interface, for example, when a substrate is supported by a metal stage with the THz light incident on it from above. We explain this effect in terms of the frequency-dependent response of the material when it is represented by a shorted transmission line model.

Decoupling substrate thickness and refractive index measurement in THz time-domain spectroscopy.

Terahertz time-domain spectroscopy (THz-TDS) relies heavily on knowing precisely the thickness or refractive index of a material. In practice, one of these values is assumed to be known, or their product is numerically optimized to converge on suitable values. Both approaches are prone to errors and may mask some real features or properties of the material being studied.

Approaching completely continuous centimeter-scale graphene by copolymer-assisted transfer.

Transferring graphene from copper foil to a target substrate should ideally be a nondestructive process, but cracks, holes, and wrinkles have proved difficult to prevent. Here we report a method in which we use a commercially available copolymer in addition to poly(methylmethacrylate) (PMMA) to obtain 99.8% continuous centimeter-scale transferred graphene.

Highly Stable and Tunable n-Type Graphene Field-Effect Transistors with Poly(vinyl alcohol) Films.

The intrinsic p-type behavior of graphene field-effect transistors (FETs) under ambient conditions poses a fundamental challenge for the assembly of complex electronic devices, such as integrated circuits. In this work, we present a protocol for tunable n-type doping of graphene FETs via poly(vinyl alcohol) (PVA) coating. Using graphene grown by alcohol catalytic chemical vapor deposition, functionalization of the surface by this hydroxyl anion-rich polymer results in an evolution of the FETs from p-type to ambipolar or n-type even under ambient air conditions.

Equilibrium chemical vapor deposition growth of Bernal-stacked bilayer graphene.

Using ethanol as the carbon source, self-limiting growth of AB-stacked bilayer graphene (BLG) has been achieved on Cu via an equilibrium chemical vapor deposition (CVD) process. We found that during this alcohol catalytic CVD (ACCVD) a source-gas pressure range exists to break the self-limitation of monolayer graphene on Cu, and at a certain equilibrium state it prefers to form uniform BLG with a high surface coverage of ∼94% and AB-stacking ratio of nearly 100%. More importantly, once the BLG is completed, this growth shows a self-limiting manner, and an extended ethanol flow time does not result in additional layers.

From isotope labeled CH₃CN to N₂ inside single-walled carbon nanotubes.

The observation of one-dimensional N₂ inside single-walled carbon nanotubes raises the questions, how are the N₂ molecules formed and how do they manage to make their way to this peculiar place? We have used N(15) and C(13) isotope labeled acetonitrile during the synthesis of single-walled carbon nanotubes to investigate this process. The isotope shifts of phonons and vibrons are observed by Raman spectroscopy and X-ray absorption. We identify the catalytic decomposition of acetonitrile as the initial step in the reaction pathway to single-walled carbon nanotubes containing encapsulated N₂.

Investigation of non-segregation graphene growth on Ni via isotope-labeled alcohol catalytic chemical vapor deposition.

Here we present CVD growth of graphene on Ni and investigate the growth mechanism using isotopically labeled (13)C-ethanol as the precursor. Results show that during low-pressure alcohol catalytic CVD (LP-ACCVD), a growth time of less than 30 s yields graphene films with high surface coverage (>80%). Moreover, when isotopically labeled ethanol precursors were sequentially introduced, Raman mapping revealed that both (12)C and (13)C graphene flakes exist.

Reversible diameter modulation of single-walled carbon nanotubes by acetonitrile-containing feedstock.

Changing the carbon feedstock from pure ethanol to a 5 vol % mixture of acetonitrile in ethanol during the growth of vertically aligned single-walled carbon nanotubes (SWNTs) reduces the mean diameter of the emerging SWNTs from approximately 2 to 1 nm. We show this feedstock-dependent change is reversible and repeatable, as demonstrated by multilayered vertically aligned SWNT structures. The reversibility of this process and lack of necessity for catalyst modification provides insight into the role of nitrogen in reducing the SWNT diameter.

Carbon atoms in ethanol do not contribute equally to formation of single-walled carbon nanotubes.

We propose a unique experimental technique in which isotopically labeled ethanol, e.g., 12CH3-13CH2-OH, is used to trace the carbon atoms during the formation of single-walled carbon nanotubes (SWNTs) by chemical vapor deposition (CVD).

Diameter modulation of vertically aligned single-walled carbon nanotubes.

We demonstrate wide-range diameter modulation of vertically aligned single-walled carbon nanotubes (SWNTs) using a wet chemistry prepared catalyst. In order to ensure compatibility to electronic applications, the current minimum mean diameter of 2 nm for vertically aligned SWNTs is challenged. The mean diameter is decreased to about 1.

Diameter controlled chemical vapor deposition synthesis of single-walled carbon nanotubes.

In this study, we systematically investigated the influence of catalyst preparation procedures on the mean diameter of single-walled carbon nanotubes (SWNTs) synthesized by the alcohol catalytic chemical vapor deposition (ACCVD) process. It was found that the SWNT diameter is dependent upon both reduction temperature and time, with lower reduction temperature and/or shorter reduction time resulting in smaller diameter SWNTs. The morphology of the SWNTs also changed from vertically aligned to randomly oriented when the reduction temperature was below 500 degrees C.

Parametric study of alcohol catalytic chemical vapor deposition for controlled synthesis of vertically aligned single-walled carbon nanotubes.

In this study we examine catalyst preparation and chemical vapor deposition (CVD) parameters related to synthesis of single-walled carbon nanotubes (SWNTs) by alcohol catalytic CVD. We show that modifying the catalyst recipe considerably changes the average SWNT diameter, and vertically aligned arrays with an average diameter of 1.5 nm were obtained.

High-precision selective deposition of catalyst for facile localized growth of single-walled carbon nanotubes.

In the liquid-based dip-coating, the hydrophilicity of a Si/SiO(2) substrate is found to be critical for the successful deposition of catalyst and hence the growth of single-walled carbon nanotubes (SWNTs). When the surface is functionalized by a self-assembled monolayer (SAM) and becomes hydrophobic, no catalyst remains and no SWNT grows. This concept can be utilized to localize the growth of SWNTs at designated regions where SAMs were selectively removed by, e.

Growth mechanism and internal structure of vertically aligned single-walled carbon nanotubes.

An in situ optical absorbance technique was used to monitor the growth of vertically aligned single-walled carbon nanotubes (VA-SWNTs) at various temperatures and pressures. The effects of the growth temperature and ethanol pressure on the initial growth rate and catalyst lifetime were investigated. It was found that the ideal pressure for VA-SWNT synthesis changes with the growth temperature, shifting toward higher pressure as the growth temperature increases.

All-fiber pulsed lasers passively mode locked by transferable vertically aligned carbon nanotube film.

An all-fiber passive laser mode locking is realized with a vertically aligned single-walled carbon nanotube film that can be transferred onto an arbitrary substrate using only hot water. A D-shaped fiber is employed as the substrate for the evanescent field interaction of propagating light with the nanotubes. The scheme highlights the efficient interaction achieved by the nanotube alignment as well as the dramatically simplified device preparation process.

Third and fourth optical transitions in semiconducting carbon nanotubes.

We have studied the optical transition energies of single-wall carbon nanotubes over broad diameter (0.7-2.3 nm) and energy (1.

Polarization dependence of the optical absorption of single-walled carbon nanotubes.

Anisotropic optical absorption properties of single-walled carbon nanotubes (SWNTs) are determined from a vertically aligned SWNT film for 0.5-6 eV. Absorption peaks at 4.