Mathias J. Boland

Increased power factors of organic–inorganic nanocomposite thermoelectric materials and the role of energy filtering

Significant enhancements in the performance of organic–inorganic nanocomposite thermoelectrics may be obtained through appropriately adjusting energetics at the organic–inorganic interfaces. Through altering these interfacial energetics, the energy dependence of the electrical conductivity, and therefore the Seebeck coefficient, can in principle be readily manipulated through energy filtering. In this work, we controllably vary the energetic barrier between transport states in the conjugated polymer poly(3-hexylthiophene) and tellurium nanowires. The energetic barrier is adjusted from 0.08 to 0.88 eV by altering the concentration of the p-type dopant (FeCl3) present in the polymer phase. We show that the maximum power factors in these composites are increased beyond that of either the pure polymer or pure nanowires for barriers of both 0.08 and 0.88 eV. With both doping concentrations the Seebeck coefficient increases with the concentration of tellurium nanowires. Through comparison of the experimentally measured Seebeck coefficients with models for parallel and series connected composites, we determine that the enhanced Seebeck coefficients and power factors do not likely arise from energy filtering. Furthermore, we find that the electrical conductivity of the 5% FeCl3 doped blend can exceed that of either of the pure components by nearly an order of magnitude.

Striped nanoscale friction and edge rigidity of MoS2 layers

Lateral force microscopy (LFM) is used to probe the nanoscale elastic and frictional characteristics of molybdenum disulfide (MoS2). We find that MoS2 edges are effectively flexed over a region of about 10 nm when scanned with sharp single asperity LFM probes, with energies consistent with out-of-plane bending and being slightly stiffer than those of graphene. Additionally, we report the first observation of a striped nanoscale frictional phase on the surface of MoS2. This frictional phase is fixed to the underlying MoS2 with a modulation length of ∼4 nm that is insensitive to scan parameters and has domain sizes that exceed 100 nm. The amplitude of these features is found to be relatively independent of the geometry of the tip asperity and the applied load within the ranges we investigate. Experimental results suggest this periodic friction can be explained by variations in the local strain in the underlying MoS2. These results could have general applicability to understanding the nanomechanical properties of the growing array of laminar materials that are of potential use as atomically-thin coatings to future nanoscale machines.

Integrated Nanotubes, Etch Tracks, and Nanoribbons in Crystallographic Alignment to a Graphene Lattice

Carbon nanotubes, few-layer graphene, and etch tracks exposing insulating SiO2 regions are integrated into nanoscale systems with precise crystallographic orientations. These integrated systems consist of nanotubes grown across nanogap etch tracks and nanoribbons formed within the few-layer graphene films. This work is relevant to the integration of semiconducting, conducting, and insulating nanomaterials together into precise intricate systems.

Electrostatic force microscopy and electrical isolation of etched few-layer graphene nano-domains

Nanostructured bi-layer graphene samples formed through catalytic etching are investigated with electrostatic force microscopy. The measurements and supporting computations show a variation in the microscopy signal for different nano-domains that are indicative of changes in capacitive coupling related to their small sizes. Abrupt capacitance variations detected across etch tracks indicates that the nano-domains have strong electrical isolation between them. Comparison of the measurements to a resistor-capacitor model indicates that the resistance between two bi-layer graphene regions separated by an approximately 10 nm wide etch track is greater than about 1×1012 Ω with a corresponding gap resistivity greater than about 3×1014 Ω⋅nm. This extremely large gap resistivity suggests that catalytic etch tracks within few-layer graphene samples are sufficient for providing electrical isolation between separate nano-domains that could permit their use in constructing atomically thin nanogap electrodes, interconn...

Doping and hysteretic switching of polymer-encapsulated graphene field effect devices

The effects of encapsulating graphene with poly(methyl methacrylate) (PMMA) polymer are determined through in situ electrical transport measurements. After regenerating graphene devices in dry-nitrogen environments, PMMA is applied to their surfaces. Low-temperature annealing decreases the overall doping level, suggesting that residual solvent plays an important role in the doping. For few-layer graphene devices, we even observe stable n-doping through annealing. Application of solvent onto encapsulated devices demonstrates enhanced hysteric switching between p and n-doped states. The stability and ubiquitous use of PMMA in nanolithography make this polymer a potentially useful localized doping agent for graphene and other two-dimensional materials.

Parallel boron nitride nanoribbons and etch tracks formed through catalytic etching

One-dimensional (1D) catalytic etching was investigated in few-layer hexagonal boron nitride (hBN) films. Etching of hBN was shown to share a number of similarities with that of graphitic films. As in graphitic films, etch tracks in hBN commenced at film edges and occurred predominantly along certain crystal directions of its lattice, though it was shown that the tracks were generally narrower than those of few-layer graphene under similar processing conditions. It was also shown that catalytic hydrogenation can occur completely through a few-layer hBN film, demonstrating that this process can be used in the formation of isolated low-dimensional nanoscale structures from other layered 2D materials beyond graphene. This ability for thin hBN films to be etched completely through allowed for a crystalline substrate to guide the etching process, which was demonstrated with the successful etch track formation of few-layer hBN on single-crystalline sapphire substrates. The substrate-guided etching resulted in parallel few-layer hBN nanoribbons having an average width of 32 nm and spacing of 13 nm.

Nonlinear Ballistic Transport in an Atomically Thin Material.

Ultrashort devices that incorporate atomically thin components have the potential to be the smallest electronics. Such extremely scaled atomically thin devices are expected to show ballistic nonlinear behavior that could make them tremendously useful for ultrafast applications. While nonlinear diffusive electron transport has been widely reported, clear evidence for intrinsic nonlinear ballistic transport in the growing array of atomically thin conductors has so far been elusive. Here we report nonlinear electron transport of an ultrashort single-layer graphene channel that shows quantitative agreement with intrinsic ballistic transport. This behavior is shown to be distinctly different than that observed in similarly prepared ultrashort devices consisting, instead, of bilayer graphene channels. These results suggest that the addition of only one extra layer of an atomically thin material can make a significant impact on the nonlinear ballistic behavior of ultrashort devices, which is possibly due to the very different chiral tunneling of their charge carriers. The fact that we observe the nonlinear ballistic response at room temperature, with zero applied magnetic field, in non-ultrahigh vacuum conditions and directly on a readily accessible oxide substrate makes the nanogap technology we utilize of great potential for achieving extremely scaled high-speed atomically thin devices.