High-Performance Air-Stable n-Type Carbon Nanotube Transistors with Erbium Contacts

O ver the past few decades, the continued down-scaling of the physical dimensions of silicon field-effect transistors (FETs) has been the main drive for achieving higher device density while improving the transistor performance in complementary metalÀoxideÀ semiconductor (CMOS) circuits. One of the principle benefits of the conventional scaling trend, namely, reducing the power consumption per computation, has diminished in recent years. In particular, power management is increasingly becoming a major challenge because of the inability to further decrease the operating voltage without compromising the performance of silicon FETs. Incorporation of alternative channel materials with superior carrier transport properties, as presently conceived, is a favorable strategy for the semiconductor industry to complement or replace silicon FETs. Among the promising candidates, carbon nanotubes (CNTs) are predicted to offer the most energy-efficient solution for computation compared with other channel materials, 1 owing to their unique properties such as ultrathin body and ballistic carrier transport in the channel. ABSTRACT So far, realization of reproducible n-type carbon nanotube (CNT) transistors suitable for integrated digital applications has been a difficult task. In this work, hundreds of n-type CNT transistors from three different low work function metals ; erbium, lanthanum, and yttrium ; are studied and benchmarked against p-type devices with palladium contacts. The crucial role of metal type and deposition conditions is elucidated with respect to overall yield and performance of the n-type devices. It is found that high oxidation rates and sensitivity to deposition conditions are the major causes for the lower yield and large variation in performance of n-type CNT devices with low work function metal contacts. Considerable improvement in device yield is attained using erbium contacts evaporated at high deposition rates. Furthermore, the air-stability of our n-type transistors is studied in light of the extreme sensitivity of these metals to oxidation

Manipulating infrared photons using plasmons in transparent graphene superlattices

Superlattices are artificial periodic nanostructures which can control the flow of electrons. Their operation typically relies on the periodic modulation of the electric potential in the direction of electron wave propagation. Here we demonstrate transparent graphene superlattices which can manipulate infrared photons utilizing the collective oscillations of carriers, i.e., plasmons of the ensemble of multiple graphene layers. The superlattice is formed by depositing alternating wafer-scale graphene sheets and thin insulating layers, followed by patterning them all together into 3-dimensional photonic-crystal-like structures. We demonstrate experimentally that the collective oscillation of Dirac fermions in such graphene superlattices is unambiguously nonclassical: compared to doping single layer graphene, distributing carriers into multiple graphene layers strongly enhances the plasmonic resonance frequency and magnitude, which is fundamentally different from that in a conventional semiconductor superlattice. This property allows us to construct widely tunable far-infrared notch filters with 8.2 dB rejection ratio and terahertz linear polarizers with 9.5 dB extinction ratio, using a superlattice with merely five graphene atomic layers. Moreover, an unpatterned superlattice shields up to 97.5% of the electromagnetic radiations below 1.2 terahertz. This demonstration also opens an avenue for the realization of other transparent mid- and far-infrared photonic devices such as detectors, modulators, and 3-dimensional meta-material systems.Comment: under revie

High Purity Isolation and Quantification of Semiconducting Carbon Nanotubes <i>via</i> Column Chromatography

The isolation of semiconducting carbon nanotubes (CNTs) to ultrahigh (ppb) purity is a prerequisite for their integration into high-performance electronic devices. Here, a method employing column chromatography is used to isolate semiconducting nanotubes to 99.9% purity. The study finds that by modifying the solution preparation step, both the metallic and semiconducting fraction are resolved and elute using a single surfactant system, allowing for multiple iterations. Iterative processing enables a far more rapid path to achieving the level of purities needed for high performance computing. After a single iteration, the metallic peak in the absorption spectra is completely attenuated. Although absorption spectroscopy is typically used to characterize CNT purity, it is found to be insufficient in quantifying solutions of high purity (>98 to 99%) due to low signal-to-noise in the metallic region of ultrahigh purity solutions. Therefore, a high throughput electrical testing method was developed to quantify the degree of separation by characterizing ∼4000 field-effect transistors fabricated from the separated nanotubes after multiple iterations of the process. The separation and characterization methods described here provide a path to produce the ultrahigh purity semiconducting CNT solutions needed for high performance electronics

Highly Efficient Fluorescence Quenching with Graphene

Fluorescence quenching is a powerful technique used to obtain information about the dynamic changes of proteins in complex macromolecular systems. In this work, graphene is shown to be a very efficient quencher of fluorescence molecules where the quenching effect was one order of magnitude higher than that of gold. The fluorescence intensity was distance-dependent where increasing the distance between the fluorescence molecule and the graphene surface from 4 to 7 nm increased the fluorescence intensity by a factor of 7.5. This type of distance dependence suggests a nonradiative nature in the energy transfer between the graphene and the fluorophore due to the excitation of an exciton

Detection of Biomolecules via Benign Surface Modification of Graphene

Detection of Biomolecules via Benign Surface Modification of Graphen

UV-Sensitive Self-Assembled Monolayer Photoresist for the Selective Deposition of Carbon Nanotubes

The use of a UV-sensitive self-assembled monolayer photoresist to selectively deposit single-walled carbon nanotubes from solution using heterogeneous surface wettability is reported. This process combines ubiquitous photopatterning techniques with simple solution processing to yield highly selective and densely packed carbon nanotube patterns. The key mechanism is the change in surface chemistry caused by the UV-induced monolayer reaction. Selective deposition of carbon nanotubes was achieved by drop-casting the CNT solution onto the photopatterned substrates where the CNT solution only wets the exposed areas. Several compounds were employed, each with unique end groups that yield a range of surface energies. The selectivity is dependent on the contrast in the surface wettability. Increased selectivity was achieved by tuning the surface energy with addition of alcohol or a nonionic surfactant