Nearly ballistic carbon nanotube array transistors are realized with current densities outmatching conventional semiconductors.

Laminates made of graphene oxide nanosheets have been shown to exhibit high water permeance and salt rejection and, therefore, have generated immense interest from the scientific community due to their potential in separation applications. However, there is no clear consensus on the water-transport pathways through such laminates. In this study, we synthesized chemically identical graphene oxide nanosheets with 2 orders of magnitude difference in lateral sizes and measured water permeance through laminates of different thicknesses fabricated by pressure-assisted deposition of these nanosheets. Our results reveal that water permeance through these laminates is nearly the same despite such massive difference in lateral sheet size. Furthermore, we simulated fluid flow through laminates using an interconnected nanochannel network model for comparison with experiments. The simulations in combination with the experimental data show that it is unlikely that the dominant fluid transport pathway is a circuitous, lateral pathway around individual sheets, as has been proposed in some studies. Rather, nonideal factors including trans-sheet flow through pinhole defects in sheet interiors and/or flow-through regions arising from imperfect stacking in the laminates can significantly affect the fluid transport pathways. The presence of such nonidealities is also supported by thickness- and time-dependent measurements of permeance and by infrared spectroscopy, which indicates that water predominantly adopts a bulk-like structure in the laminates. These analyses are significant steps toward understanding water transport through graphene oxide laminates and provide further insight toward the structure of water inside these materials, which could have immense potential in next-generation separation applications.

It was recently discovered that the chemical vapor deposition (CVD) of CH on Ge(001) can directly yield long, narrow, semiconducting nanoribbons of graphene with smooth armchair edges. These nanoribbons have exceptional charge transport properties compared with nanoribbons grown by other methods. However, the nanoribbons nucleate at random locations and at random times, problematically giving rise to width and bandgap polydispersity, and the mechanisms that drive the anisotropic crystal growth that produces the nanoribbons are not understood. Here, we study and engineer the seed-initiated growth of graphene nanoribbons on Ge(001). The use of seeds decouples nucleation and growth, controls where growth occurs, and allows graphene to grow with lattice orientations that do not spontaneously form without seeds. We discover that when the armchair direction (i.e., parallel to C-C bonds) of the seeds is aligned with the Ge⟨110⟩ family of directions, the growth anisotropy is maximized, resulting in the formation of nanoribbons with high-aspect ratios. In contrast, increasing misorientation from Ge⟨110⟩ yields decreasingly anisotropic crystals. Measured growth rate data are used to generate a construction analogous to a kinetic Wulff plot that quantitatively predicts the shape of graphene crystals on Ge(001). This knowledge is employed to fabricate regularly spaced, unidirectional arrays of nanoribbons and to significantly improve their uniformity. These results show that seed-initiated graphene synthesis on Ge(001) will be a viable route for creating wafer-scale arrays of narrow, semiconducting, armchair nanoribbons with rationally controlled placement and alignment for a wide range of semiconductor electronics technologies, provided that dense arrays of sub-10 nm seeds can be uniformly fabricated in the future.

Semiconducting single-walled carbon nanotubes (s-SWCNTs) have attracted significant attention as a photoactive component in thin film photovoltaic solar cells and photodetectors due to their strong optical absorptivity and high charge transport mobility. However, the external quantum efficiency (QE) of s-SWCNT/acceptor heterojunction solar cells has been limited by poor exciton harvesting efficiency. Exciton trapping and quenching at defects are a suspected source of loss. Here, we study the influence of defects on bilayer s-SWCNT/C 60 planar heterojunction photovoltaic devices via both experiment and modeling. First, diazonium chemistry is used to introduce covalent sp 3 sidewall defects to s-SWCNTs at various densities that are estimated using Raman and transient absorption spectroscopy. s-SWCNT/C 60 heterojunction photovoltaic cells are then fabricated that show a significant decrease in peak external QE (e.g., from 40% to 8%) with increasing defect density. Second, a diffusion-limited contact quenching Monte Carlo model is developed to assess the contributions of exciton quenching defects on exciton migration in bilayer s-SWCNT/C 60 heterojunction devices. The model indicates that current state-of-the-art s-SWCNT-based devices are defect limited and suggests that significant gains in exciton harvesting efficiency can be realized if more pristine, longer s-SWCNTs are utilized.

Graphene grown directly on Ge via chemical vapor deposition (CVD) can passivate the underlying Ge surface, preventing its oxidation in ambient air for at least months. However, the factors that govern oxidation of Ge coated with graphene have not been elucidated. We investigate the effect of graphene synthesis parameters and Ge surface orientation on passivation of Ge and correlate these data with the density and type of defects in graphene. Oxidation of Ge can be reduced by increasing the H2:CH4 flux ratio or decreasing the growth rate, which decrease the density of atomic-scale defects, such as point defects and grain boundaries, in graphene. Oxidation of graphene is concomitant with oxidation of Ge and occurs more readily when the density of atomic-scale defects is relatively high. Passivation of Ge, however, depends more strongly on Ge surface orientation, as Ge(110) oxidizes significantly less than Ge(001) or Ge(111), even at the same graphene defect density. These results provide a pathway for engineering high-quality graphene films on Ge, which may enable improved passivation of Ge and direct integration of graphene-based or hybrid graphene/Ge heterostructure devices on conventional semiconductor platforms.