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Low-temperature scanning tunneling microscopy measurements and first-principles calculations are employed to characterize edge structures observed for graphene nanoislands grown on the Co(0001) surface. Images of these nanostructures reveal straight well-ordered edges with zigzag orientation, which are characterized by a distinct peak at low bias in tunneling spectra. Density functional theory based calculations are used to discriminate between candidate edge structures. Several zigzag-oriented edge structures have lower formation energy than armchair-oriented edges. Of these, the lowest formation energy configurations are a zigzag and a Klein edge structure, each with the final carbon atom over the hollow site in the Co(0001) surface. In the absence of hydrogen, the interaction with the Co(0001) substrate plays a key role in stabilizing these edge structures and determines their local conformation and electronic properties. The calculated electronic properties for the low-energy edge structures are consistent with the measured scanning tunneling images.

Atomically thin forms of layered materials, such as conducting graphene, insulating hexagonal boron nitride (hBN), and semiconducting molybdenum disulfide (MoS2), have generated great interests recently due to the possibility of combining diverse atomic layers by mechanical "stacking" to create novel materials and devices. In this work, we demonstrate field-effect transistors (FETs) with MoS2 channels, hBN dielectric, and graphene gate electrodes. These devices show field-effect mobilities of up to 45 cm(2)/Vs and operating gate voltage below 10 V, with greatly reduced hysteresis. Taking advantage of the mechanical strength and flexibility of these materials, we demonstrate integration onto a polymer substrate to create flexible and transparent FETs that show unchanged performance up to 1.5% strain. These heterostructure devices consisting of ultrathin two-dimensional (2D) materials open up a new route toward high-performance flexible and transparent electronics.

The structuring in organic electrically active thin films critically influences the performance of devices comprising them. Controlling film structure, however, remains challenging and generally requires stringent deposition conditions or modification of the substrate. To this end, we have developed post-deposition processing methods that are decoupled from the initial deposition conditions to induce different out-of-plane molecular orientations in contorted hexabenzocoronene (HBC) thin films. As-deposited HBC thin films lack any long-range order; subjecting them to post-deposition processing, such as hexanes-vapor annealing, thermal annealing, and physical contact with elastomeric poly(dimethyl siloxane), induces crystallization with increasing extents of preferential edge-on orientation, corresponding to greater degrees of in-plane π-stacking. Accordingly, transistors comprising HBC thin films that have been processed under these conditions exhibit field-effect mobilities that increase by as much as 2 orders of magnitude with increasing extents of molecular orientation. The ability to decouple HBC deposition from its subsequent structuring through post-deposition processing affords us the unique opportunity to tune competing molecule-molecule and molecule-solvent interactions, which ultimately leads to control over the structure and electrical function of HBC films.

For conjugated polymer materials, there is currently a major gap in understanding between the fundamental properties observed in single molecule measurements and the bulk electronic properties extracted from measurements of highly heterogeneous thin films. New materials and methodologies are needed to follow the evolution from single chain to bulk film properties as multiple chains begin to interact. In this work, we used a controlled solvent vapor annealing process to assemble single chains of phenylene-vinylene conjugated polymers into aggregates that can be individually spectroscopically interrogated. This approach allowed us to probe the effects of interchain coupling in isolated conjugated polymer nanodomains of controlled size. By assembling these aggregates from building blocks of both pristine MEH-PPV and MEH-PPV derivatives containing structure-directing ortho- or para-terphenyl inclusions, we were able to control the ordering of these nanodomains as measured by single aggregate polarization anisotropy measurments. Depending on the individual chain constituents, these aggregates varied from highly anisotropic to nearly isotropic, respectively facilitating or inhibiting interchain coupling. From the single chain fluorescence lifetimes, we demonstrated that these structure directing inclusions effectively break the phenylene-vinylene conjugation, allowing us to differentiate interchain electronic effects from those due to hyper-extended conjugation. We observed well-defined bathochromic shifts in the fluorescence spectra of the aggregates containing extensive interchain interactions, indicating that low-energy exciton traps in MEH-PPV are the result of coupling interactions between neighboring chain segments. These results demonstrate the power of the synthetic inclusion approach to control properties at not just the single chain level, but as a comprehensive approach toward ground-up design of bulk electronic properties., (© 2011 American Chemical Society)

We report the optical reflectivity and Raman scattering of few layer (L) graphene exposed to K and Rb vapors. Samples many tens of layers thick show the reflectivity and Raman spectra of the stage 1 bulk alkali intercalation compounds (GICs) KC(8) and RbC(8). However, these bulk optical and Raman properties only begin to appear in samples more than about 15 graphene layers thick. The 1 L to 4 L alkali exposed graphene Raman spectra are profoundly different than the Breit-Wigner-Fano (BWF) spectra of the bulk stage 1 compounds. Samples less than 10 layers thick show Drude-like plasma edge reflectivity dip in the visible; alkali exposed few layer graphenes are significantly more transparent than intrinsic graphene. Simulations show the in-plane free electron density is lower than in the bulk stage 1 GICs. In few layer graphenes, alkalis both intercalate between layers and adsorb on the graphene surfaces. Charge transfer electrically dopes the graphene sheets to densities near and above 10(+14) electrons/cm(2). New intrinsic Raman modes at 1128 and 1264 cm(-1) are activated by in-plane graphene zone folding caused by strongly interacting, locally crystalline alkali adlayers. The K Raman spectra are independent of thickness for L = 1-4, indicating that charge transfer from adsorbed and intercalated K layers are similar. The Raman G mode is downshifted and significantly broadened from intrinsic graphene. In contrast, the Rb spectra vary strongly with L and show increased doping by intercalated alkali as L increases. Rb adlayers appear to be disordered liquids, while intercalated layers are locally crystalline solids. A significant intramolecular G mode electronic resonance Raman enhancement is observed in K exposed graphene, as compared with intrinsic graphene.

We have constructed devices in which the interior of a single-walled carbon nanotube (SWCNT) field-effect transistor acts as a nanofluidic channel that connects two fluid reservoirs, permitting measurement of the electronic properties of the SWCNT as it is wetted by an analyte. Wetting of the inside of the SWCNT by water turns the transistor on, while wetting of the outside has little effect. These observations are consistent with theoretical simulations that show that internal water both generates a large dipole electric field, causing charge polarization of the tube and metal electrodes, and shifts the valence band of the SWCNT, while external water has little effect. This finding may provide a new method to investigate water behavior at nanoscale. This also opens a new avenue for building sensors in which the SWCNT simultaneously functions as a concentrator, nanopore, and extremely sensitive electronic detector, exploiting the enhanced sensitivity of the interior surface.

Energy transfer from photoexcited zero-dimensional systems to metallic systems plays a prominent role in modern day materials science. A situation of particular interest concerns the interaction between a photoexcited dipole and an atomically thin metal. The recent discovery of graphene layers permits investigation of this phenomenon. Here we report a study of fluorescence from individual CdSe/ZnS nanocrystals in contact with single- and few-layer graphene sheets. The rate of energy transfer is determined from the strong quenching of the nanocrystal fluorescence. For single-layer graphene, we find a rate of approximately 4 ns(-1), in agreement with a model based on the dipole approximation and a tight-binding description of graphene. This rate increases significantly with the number of graphene layers, before approaching the bulk limit. Our study quantifies energy transfer to and fluorescence quenching by graphene, critical properties for novel applications in photovoltaic devices and as a molecular ruler.