The heterogeneity of as-synthesized single-walled carbon nanotubes (SWNTs) precludes their widespread application in electronics, optics and sensing. We report on the sorting of carbon nanotubes by diameter, bandgap and electronic type using structure-discriminating surfactants to engineer subtle differences in their buoyant densities. Using the scalable technique of density-gradient ultracentrifugation, we have isolated narrow distributions of SWNTs in which >97% are within a 0.02-nm-diameter range. Furthermore, using competing mixtures of surfactants, we have produced bulk quantities of SWNTs of predominantly a single electronic type. These materials were used to fabricate thin-film electrical devices of networked SWNTs characterized by either metallic or semiconducting behaviour.

At the atomic-cluster scale, pure boron is markedly similar to carbon, forming simple planar molecules and cage-like fullerenes.Theoretical studies predict that two-dimensional (2D) boron sheets will adopt an atomic configuration similar to that of boron atomic clusters. We synthesized atomically thin, crystalline 2D boron sheets (i.e., borophene) on silver surfaces under ultrahigh-vacuum conditions. Atomic-scale characterization, supported by theoretical calculations, revealed structures reminiscent of fused boron clusters with multiple scales of anisotropic, out-of-plane buckling. Unlike bulk boron allotropes, borophene shows metallic characteristics that are consistent with predictions of a highly anisotropic, 2D metal.

With advances in exfoliation and synthetic techniques, atomically thin films of semiconducting transition metal dichalcogenides have recently been isolated and characterized. Their two-dimensional structure, coupled with a direct band gap in the visible portion of the electromagnetic spectrum, suggests suitability for digital electronics and optoelectronics. Toward that end, several classes of high-performance devices have been reported along with significant progress in understanding their physical properties. Here, we present a review of the architecture, operating principles, and physics of electronic and optoelectronic devices based on ultrathin transition metal dichalcogenide semiconductors. By critically assessing and comparing the performance of these devices with competing technologies, the merits and shortcomings of this emerging class of electronic materials are identified, thereby providing a roadmap for future development.

Unencapsulated, exfoliated black phosphorus (BP) flakes are found to chemically degrade upon exposure to ambient conditions. Atomic force microscopy, electrostatic force microscopy, transmission electron microscopy, X-ray photoelectron spectroscopy, and Fourier transform infrared spectroscopy are employed to characterize the structure and chemistry of the degradation process, suggesting that O2 saturated H2O irreversibly reacts with BP to form oxidized phosphorus species. This interpretation is further supported by the observation that BP degradation occurs more rapidly on hydrophobic octadecyltrichlorosilane self-assembled monolayers and on HSi(111), versus hydrophilic SiO2. For unencapsulated BP field-effect transistors, the ambient degradation causes large increases in threshold voltage after 6 hours in ambient, followed by a ~10 3 decrease in FET current on/off ratio and mobility after 48 hours. Atomic layer deposited AlOx overlayers effectively suppress ambient degradation, allowing encapsulated BP FETs to maintain high on/off ratios of ~10 3 and mobilities of ~100 cm 2 V -1 s -1 for over two weeks in ambient. This work shows that the ambient degradation of BP can be managed effectively when the flakes are sufficiently passivated. In turn, our strategy for enhancing BP environmental stability will accelerate efforts to implement BP in electronic and optoelectronic applications. On increased ambient exposure, the bubble density eventually decreases, evolving into wider and taller bubbles. These bubbles occur in BP, regardless of flake thickness (Fig. S2). In Fig. 2, we therefore use X-ray photoelectron spectroscopy (XPS) and Fourier transform infrared (FTIR) spectroscopy to assess whether chemical modifications, such as the formation of additional chemical bonds or a change in oxidation state, occur in BP upon ambient exposure. Fig. 2A shows P 2p core level XPS spectra of as-exfoliated BP flakes on SiO2 for 0 hrs, 13 hrs, 1, day, 2 days, and 3 days, respectively, of ambient exposure. All spectra are calibrated to the binding energy of adventitious carbon (284.8 eV), and electrostatic charging is compensated using an Ar + flood gun (see Supporting Information for details). At 0 hrs of ambient exposure (black spectrum in Fig. 2A), the exfoliated BP exhibits a single spin-orbit split doublet at ~130 eV, consistent with previous XPS measurements on BP bulk crystals. 27, 28 Note that these spectra do not match those for red phosphorus (~129.8 eV), white phosphorus, or amorphous P-H. 27 A broad, s photoelectronSi satellite from the substrate 300 nm SiO2 appears at ~126.5 eV. After 13 hrs of ambient exposure (maroon spectrum), the full-width at half-maximum (FWHM) for the BP increases, characteristic of some loss of long range order. After 1, 2, and 3 days in ambient (green, navy, and gray spectra, respectively), an additional doublet appears at ~134 eV. This feature is best assigned to phosphate species, 9, 29 although many oxidized phosphorus compounds exhibit peaks near ~134-135 eV. 30, 31 The la...

The isolation of a growing number of two-dimensional (2D) materials has inspired worldwide efforts to integrate distinct 2D materials into van der Waals (vdW) heterostructures. Given that any passivated, dangling bond-free surface will interact with another via vdW forces, the vdW heterostructure concept can be extended to include the integration of 2D materials with non-2D materials that adhere primarily through noncovalent interactions. In this review, we present a succinct and critical survey of emerging mixed-dimensional (2D + nD, where n is 0, 1 or 3) heterostructure devices. By comparing and contrasting with all-2D vdW heterostructures as well as with competing conventional technologies, the challenges and opportunities for mixeddimensional vdW heterostructures are highlighted.