Susmit Singha Roy

Highly Stretchable Carbon Nanotube Transistors with Ion Gel Gate Dielectrics

Field-effect transistors (FETs) that are stretchable up to 50% without appreciable degradation in performance are demonstrated. The FETs are based on buckled thin films of polyfluorene-wrapped semiconducting single-walled carbon nanotubes (CNTs) as the channel, a flexible ion gel as the dielectric, and buckled metal films as electrodes. The buckling of the CNT film enables the high degree of stretchability while the flexible nature of the ion gel allows it to maintain a high quality interface with the CNTs during stretching. An excellent on/off ratio of >10(4), a field-effect mobility of 10 cm(2) · V(-1) · s(-1), and a low operating voltage of <2 V are achieved over repeated mechanical cycling, with further strain accommodation possible. Deformable FETs are expected to facilitate new technologies like stretchable displays, conformal devices, and electronic skins.

Direct oriented growth of armchair graphene nanoribbons on germanium

Graphene can be transformed from a semimetal into a semiconductor if it is confined into nanoribbons narrower than 10 nm with controlled crystallographic orientation and well-defined armchair edges. However, the scalable synthesis of nanoribbons with this precision directly on insulating or semiconducting substrates has not been possible. Here we demonstrate the synthesis of graphene nanoribbons on Ge(001) via chemical vapour deposition. The nanoribbons are self-aligning 3° from the Ge〈110〉 directions, are self-defining with predominantly smooth armchair edges, and have tunable width to <10 nm and aspect ratio to >70. In order to realize highly anisotropic ribbons, it is critical to operate in a regime in which the growth rate in the width direction is especially slow, <5 nm h−1. This directional and anisotropic growth enables nanoribbon fabrication directly on conventional semiconductor wafer platforms and, therefore, promises to allow the integration of nanoribbons into future hybrid integrated circuits.

High performance transistors via aligned polyfluorene-sorted carbon nanotubes

We evaluate the performance of exceptionally electronic-type sorted, semiconducting, aligned single-walled carbon nanotubes (s-SWCNTs) in field effect transistors (FETs). High on-conductance and high on/off conductance modulation are simultaneously achieved at channel lengths which are both shorter and longer than individual s-SWCNTs. The s-SWCNTs are isolated from heterogeneous mixtures using a polyfluorene-derivative as a selective agent and aligned on substrates via dose-controlled, floating evaporative self-assembly at densities of ∼50 s-SWCNTs μm−1. At a channel length of 9 μm the s-SWCNTs percolate to span the FET channel, and the on/off ratio and charge transport mobility are 2.2 × 107 and 46 cm2 V−1 s−1, respectively. At a channel length of 400 nm, a large fraction of the s-SWCNTs directly span the channel, and the on-conductance per width is 61 μS μm−1 and the on/off ratio is 4 × 105. These results are considerably better than previous solution-processed FETs, which have suffered from poor on/off...

Semiconducting carbon nanotube aerogel bulk heterojunction solar cells.

Using a novel two-step fabrication scheme, we create highly semiconducting-enriched single-walled carbon nanotube (sSWNT) bulk heterojunctions (BHJs) by first creating highly porous interconnected sSWNT aerogels (sSWNT-AEROs), followed by back-filling the pores with [6,6]-phenyl-C(71)-butyric acid methyl ester (PC(71)BM). We demonstrate sSWNT-AERO structures with density as low as 2.5 mg cm(-3), porosity as high as 99.8%, and diameter of sSWNT fibers ≤ 10 nm. Upon spin coating with PC(71)BM, the resulting sSWNT-AERO-PC(71)BM nanocomposites exhibit highly quenched sSWNT photoluminescence, which is attributed to the large interfacial area between the sSWNT and PC(71)BM phases, and an appropriate sSWNT fiber diameter that matches the inter-sSWNT exciton migration length. Employing the sSWNT-AERO-PC(71)BM BHJ structure, we report optimized solar cells with a power conversion efficiency of 1.7%, which is exceptional among polymer-like solar cells in which sSWNTs are designed to replace either the polymer or fullerene component. A fairly balanced photocurrent is achieved with 36% peak external quantum efficiency (EQE) in the visible and 19% peak EQE in the near-infrared where sSWNTs serve as electron donors and photoabsorbers. Our results prove the effectiveness of this new method in controlling the sSWNT morphology in BHJ structures, suggesting a promising route towards highly efficient sSWNT photoabsorbing solar cells.

Vertical and Lateral Copper Transport through Graphene Layers

A different mechanism was found for Cu transport through multi-transferred single-layer graphene serving as diffusion barriers on the basis of time-dependent dielectric breakdown tests. Vertical and lateral transport of Cu dominates at different stress electric field regimes. The classic E-model was modified to project quantitatively the effectiveness of the graphene Cu diffusion barrier at low electric field based on high-field accelerated stress data. The results are compared to industry-standard Cu diffusion barrier material TaN. 3.5 Å single-layer graphene shows the mean time-to-fail comparable to 4 nm TaN, while two-time and three-time transferred single-layer graphene stacks give 2× and 3× improvements, respectively, compared to single-layer graphene at a 0.5 MV/cm electric field. The influences of graphene grain boundaries on Cu vertical transport through the graphene layers are explored, revealing that large-grain (10-15 μm) single-layer graphene gives a 2 orders of magnitude longer lifetime than small-grain (2-3 μm) graphene. As a result, it is more effective to further enhance graphene barrier reliability by improving single-layer graphene quality through increasing grain sizes or using single-crystalline graphene than just by increasing thickness through multi-transfer. These results may also be applied for graphene as barriers for other metals.

Observing Electron Extraction by Monolayer Graphene Using Time-Resolved Surface Photoresponse Measurements

Graphene is considered a next-generation electrode for indium tin oxide (ITO)-free organic photovoltaic devices (OPVs). However, to date, limited numbers of OPVs containing surface-modified graphene electrodes perform as well as ITO-based counterparts, and no devices containing a bare graphene electrode have been reported to yield satisfactory rectification characteristics. In this report, we provide experimental data to learn why. Time-resolved surface photoresponse measurements on templated pentacene-on-graphene films directly reveal that p-doped monolayer graphene efficiently extracts electrons, not holes, from photoexcited pentacene. Accordingly, a graphene/pentacene/MoO3 heterojunction displays a large surface photoresponse and, by inference, efficient dissociation of photogenerated excitons, with graphene serving as an electron extraction layer and MoO3 as a hole extraction layer. In contrast, a graphene/pentacene/C60 heterojunction yields a comparatively insignificant surface photoresponse because both graphene and C60 act as competing electron extraction layers. The data presented herein provide experimental insight for future endeavors involving bare graphene as an electrode for organic photovoltaic devices and strongly suggest that p-doped graphene is best considered a cathode for OPVs.

Cu diffusion barrier: Graphene benchmarked to TaN for ultimate interconnect scaling

The advantages of graphene diffusion barrier are studied and benchmarked to the industry-standard barrier material TaN for the first time. Even when the wire width is scaled to 10 nm, the effective resistivity of the Cu interconnect is maintained near the intrinsic value of Cu using a 3 Å single layer graphene (SLG) barrier. In the time dependent dielectric breakdown (TDDB) test, 4 nm multi-layer graphene (MLG) gives 6.5X shorter mean time to fail (MTTF) than 4 nm TaN. However, when the barrier thickness is reduced, 3 Å single-layer graphene (SLG) gives 3.3X longer MTTF than 2 nm TaN, showing that SLG has better scaling potential. The influences of SLG grain size and various transfer methods are presented for further improving the SLG barrier performance.

Passivation of Germanium by Graphene

The oxidation of Ge covered with graphene that is either grown on or transferred to the surface is investigated by X-ray photoelectron spectroscopy, Raman spectroscopy, and transmission electron microscopy. Graphene properly grown by chemical vapor deposition on Ge(100), (111), or (110) effectively inhibits room-temperature oxidation of the surface. When graphene is transferred to the Ge surface, oxidation is reduced relative to that on uncovered Ge but has the same power law dependence. We conclude that access to the graphene/Ge interface must occur via defects in the graphene. The excellent passivation provided by graphene grown on Ge should enhance applications of Ge in the electronic-device industry.