It is commonly assumed that charge-carrier transport in doped π-conjugated polymers is dominated by one type of charge carrier, either holes or electrons, as determined by the chemistry of the dopant. Here, through Seebeck coefficient and Hall effect measurements, we show that mobile electrons contribute substantially to charge-carrier transport in π-conjugated polymers that are heavily p-doped with strong electron acceptors. Specifically, the Seebeck coefficient of several p-doped polymers changes sign from positive to negative as the concentration of the oxidizing agents FeCl3 or NOBF4 increase, and Hall effect measurements for the same p-doped polymers reveal that electrons become the dominant delocalized charge carriers. Ultraviolet and inverse photoelectron spectroscopy measurements show that doping with oxidizing agents results in elimination of the transport gap at high doping concentrations. This approach of heavy p-type doping is demonstrated to provide a promising route to high-performance n-type organic thermoelectric materials.

Variable-pressure electron-beam lithography (VP-EBL) employs an ambient gas at subatmospheric pressure to reduce charging of insulating films and substrates during electron exposure. In this work, VP-EBL proves to be an efficient method for patterning a widely used, but challenging to process, fluoropolymer, Teflon AF. However, rather than solely mitigating charging, the ambient gas is found to alter the radiation chemistry of the exposure process. Specifically, irradiating Teflon AF under water vapor increases the dissolution rate of the exposed regions in non-fluorinated solvents and enables complete patterning in a positive tone process. When compared to conventional e-beam resists, the contrast (≈4), clearing dose (<700 μC cm-2), and resolution (≈40 nm half-pitch) of Teflon AF are adequate. However, these figures of merit are quite remarkable when the process is considered as a means for directly patterning a functional material with extremely low surface energy, dielectric constant, and refractive index. Intriguingly, VP-EBL of Teflon AF under water vapor also exhibits non-reciprocity, through dose-rate dependence, and exhibits anomalous proximity effects. Thus, the influence of the ambient gas on radiation chemistry must be considered for VP-EBL, and some of the resulting effects may offer significant benefits for patterning both functional and lithographic materials.

1D carbon nanotubes (CNTs) are grown on hexagonal boron nitride (hBN) surfaces. The nanotubes show clear preference to align to specific crystal directions of the hBN substrate. Raman spectra confirm that the nanotubes consist of sp(2) carbon, while nanomanipulation shows that they are van der Waals coupled to the underlying hBN substrate. Scanning conductance and electric force microscopy show that the CNTs have significantly greater electrical conductance compared to the hBN. This integrated aligned growth of materials with similar lattices, yet having dissimilar dimensionality and electrical conducting properties, provides a promising route to achieving intricate nanoscale electrical circuits on high-quality insulating substrates.

Crystallographically-oriented nanotubes are grown on highly-etched graphite surfaces. The nanotubes are found to align along preferred crystal directions of the graphite surface along the same directions as etch tracks. This indicates that the nanotubes adhere to the etch tracks, which are themselves oriented along the three zigzag directions of the graphite surface. This nanotube adhesion to etch tracks is supported by nanotubes that are observed to grow on top of and adjacent to the etch tracks. The adhesion of nanotubes adjacent to etch tracks suggests that strain modes at the vicinity of exposed graphene edges may allow for a lower free energy for nanotubes to be located at these locations. Direct evidence of this adhesion of nanotubes to the tops of graphene step edges is observed through their nanomanipulation. These results demonstrate that a greater net load is required to dislodge a nanotube from an edge than is needed to drag the same tube to that edge.

A lateral force microscopy (LFM) calibration technique utilizing a random low-profile surface is proposed that is successfully employed in the low-load non-linear frictional regime using a single layer of graphene on a supporting oxide substrate. This calibration at low loads and on low friction surfaces like graphene has the benefit of helping to limit the wear of the LFM tip during the calibration procedure. Moreover, the low-profiles of the calibration surface characteristic of these layered 2D materials, on standard polished oxide substrates, result in a nearly constant frictional, adhesive, and elastic response as the tip slides over the surface, making the determination of the calibration coefficient robust. Through a detailed calibration analysis that takes into account non-linear frictional response, it is found that the adhesion is best described by a nearly constant vertical orientation, rather than the more commonly encountered normally directed adhesion, as the single asperity passes over the low-profile graphene-coated oxide surface.

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.

Lateral force microscopy (LFM) is used to probe the nanoscale elastic and frictional characteristics of molybdenum disulfide (MoS2).

We studied the single-layered iridate Sr2IrO4 with a scanning tunneling microscope. The finite low temperature conductance enables the electronic structure of this antiferromagnetic Mott insulator to be measured by tunneling spectroscopy. We imaged the topography of freshly cleaved surfaces and measured differential tunneling conductance at cryogenic temperatures. We found the Mott gap in the tunneling density of states to be 2 Delta = 615 meV. Within the Mott gap, additional shoulders are observed which are interpreted as inelastic loss features due to magnons.