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Graphene has a great potential to replace silicon in prospective semiconductor industries due to its outstanding electronic and transport properties; nonetheless, its lack of energy bandgap is a substantial limitation for practical applications. To date, straining graphene to break its lattice symmetry is perhaps the most efficient approach toward realizing bandgap tunability in graphene. However, due to the weak lattice deformation induced by uniaxial or in-plane shear strain, most strained graphene studies have yielded bandgaps

We examine the formation and properties of InGaN quantum dots (QDs) on free-standing GaN and GaN/sapphire templates, with and without buried InGaN/GaN QD superlattices (SLs). We use scanning tunneling microscopy (STM) and scanning tunneling spectroscopy to image the QDs and measure their electronic states. As the number of layers preceding the QDs increases (i.e., increasing substrate complexity), the total QD density increases. For free-standing GaN, STM reveals a mono-modal QD-size-distribution, consistent with a limited density of substrate threading dislocations serving as heterogeneous nucleation sites. For GaN/sapphire templates, STM reveals a bimodal QD-size-distribution, presumably due to the nucleation of additional ultra-small InN-rich QDs near threading dislocations. For multi-period QD SLs on GaN/sapphire templates, an ultra-high density of QDs, with a mono-modal size distribution is apparent, suggesting that QD nucleation is enhanced by preferential nucleation at strain energy minima directly above buried QDs. We discuss the relative influences of strain fields associated with threading dislocations and buried QD SLs on the formation of InGaN QDs and their effective bandgaps.

In this research, we report the enhanced thermoelectric power factor in topologically insulating thin films of Bi0.64Sb1.36Te3 with a thickness of 6–200 nm. Measurements of scanning tunneling spectroscopy and electronic transport show that the Fermi level lies close to the valence band edge, and that the topological surface state (TSS) is electron dominated. We find that the Seebeck coefficient of the 6 nm and 15 nm thick films is dominated by the valence band, while the TSS chiefly contributes to the electrical conductivity. In contrast, the electronic transport of the reference 200 nm thick film behaves similar to bulk thermoelectric materials with low carrier concentration, implying the effect of the TSS on the electronic transport is merely prominent in the thin region. The conductivity of the 6 nm and 15 nm thick film is obviously higher than that in the 200 nm thick film owing to the highly mobile TSS conduction channel. As a consequence of the enhanced electrical conductivity and the suppressed bip...

We profile the local carrier concentration, n, across epitaxial InAs/GaAs quantum dots (QDs) consisting of 3D islands on top of a 2D alloy layer. We use scanning thermoelectric microscopy to measure a profile of the temperature gradient-induced voltage, which is converted to a profile of the local Seebeck coefficient, S. The S profile is then converted to a conduction band-edge profile and compared with Poisson-Schrodinger band-edge simulations. Our combined computational-experimental approach suggests a reduced carrier concentration in the QD center in comparison to that of the 2D alloy layer. The relative roles of free carrier trapping and/or dopant expulsion are discussed.

Nanowires are of intense interest on account of their ability to confine electronic and phononic excitations in narrow channels, leading to unique vibronic and optoelectronic properties. Most systems reported to date exhibit nanowire axes perpendicular to the substrate surface, while for many applications (e.g., photodetectors and sensors), a parallel orientation may be advantageous. Here, we report the formation of in-plane Sb2Te3 nanowires using femtosecond laser irradiation. High-resolution scanning transmission electron microscopy imaging and element mapping reveal that an interesting laser-driven anion exchange mechanism is responsible for the nanowire formation. This development points the way to the scalable production of a distinct class of nanowire materials with in-plane geometry.

We quantify the local Seebeck coefficient with scanning thermoelectric microscopy, using a direct approach to convert temperature gradient-induced voltages (V) to Seebeck coefficients (S). We use a quasi-3D conversion matrix that considers both the sample geometry and the temperature profile. For a GaAs p-n junction, the resulting S-profile is consistent with that computed using the free carrier concentration profile. This combined computational-experimental approach is expected to enable nanoscale measurements of S across a wide variety of heterostructure interfaces.