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Scattering-type scanning near-field optical microscopy is becoming a premier method for the nanoscale optical investigation of materials well beyond the diffraction limit. A number of popular numerical methods exist to predict the near-field contrast for axisymmetric configurations of scatterers on a surface in the quasi-electrostatic approximation. Here, a fully electrodynamic approach is given for the calculation of near-field contrast of several scatterers in arbitrary configuration, based on the generalized Mie scattering method. Examples for the potential of this new approach are given by showing the coupling of hyperbolic phonon polaritons in hexagonal boron nitride layers and showing enhanced scattering in core-shell systems. In general, this method enables the numerical calculation of the near-field contrast in a variety of strongly resonant scatterers and is able to accurately recreate spatial near-field maps.

Graphene nanoribbons are prepared inside boron nitride nanotubes by liquid phase encapsulation and subsequent annealing of 1,2,4-trichlorobenzene. The product is imaged with high resolution transmission electron microscopy, and characterized by optical absorption and Raman spectroscopy. Carbon-containing material is detected inside the boron nitride nanotubes with energy-dispersive x-ray spectroscopy (EDS) and scanning transmission electron microscopy (STEM). The observed structures twist under the electron beam and the characteristic features of nanoribbons appear in the Raman spectra., Comment: 8 pages, 4 figures

Growing graphene nanoribbons from small organic molecules encapsulated in carbon nanotubes can result in products with uniform width and chirality. We propose a method based on encapsulation of 1,2,4-trichlorobenzene from the liquid phase and subsequent annealing. This procedure results in graphene nanoribbons several tens of nanometers long. The presence of nanoribbons was proven by Raman spectra both on macroscopic samples and on the nanoscale by tip-enhanced Raman scattering and high-resolution transmission electron microscopic images., Comment: 20 pages, 5 figures

The behavior of single layer van der Waals (vdW) materials is profoundly influenced by the immediate atomic environment at their surface, a prime example being the myriad of emergent properties in artificial heterostructures. Equally significant are adsorbates deposited onto their surface from ambient. While vdW interfaces are well understood, our knowledge regarding atmospheric contamination is severely limited. Here we show that the common ambient contamination on the surface of: graphene, graphite, hBN and MoS2 is composed of a self-organized molecular layer, which forms during a few days of ambient exposure. Using low-temperature STM measurements we image the atomic structure of this adlayer and in combination with infrared spectroscopy identify the contaminant molecules as normal alkanes with lengths of 20-26 carbon atoms. Through its ability to self-organize, the alkane layer displaces the manifold other airborne contaminant species, capping the surface of vdW materials and possibly dominating their interaction with the environment.

Near-field spectroscopy has been extensively applied recently to analyze collective optical properties of materials at the nanoscale. However, vibrations of small molecules were only recognizable in close proximity to a metallic resonator. We show that encapsulation in boron nitride nanotubes (BNNT) enhances the near-field vibrational spectra of C$_{60}$ fullerene, reaching a sensitivity limit of a few hundred molecules. Furthermore, products of chemical reactions inside the tubes can be identified, so long as their vibrational signatures lie in the polariton gap of the BNNT., Comment: 7 pages, 4 figures

We used scattering-type scanning near-field optical microscopy (s-SNOM) to characterize nickel nanoclusters grown inside single-walled carbon nanotubes (SWCNT). The nanotubes were filled with Ni(II) acetylacetonate and the molecules were transformed into nickel clusters via annealing. The metal clusters give high local contrast enhancement in near-field phase maps caused by the excitation of free charge carriers. The near-field contrast was simulated using the finite dipole model, approximating the clusters with elliptical nanoparticles. Compared to magnetic force microscopy, s-SNOM appears much more sensitive to localize metal clusters inside carbon nanotubes. We estimate the detection threshold to be ~600 Ni atoms., Comment: 8 pages, 7 figures

Reductive hydrogenation was applied to two types of single-walled carbon nanotubes with different diameter range. Alkali metal intercalation, followed by reaction with methanol, led to hydrogenated products. Both yield and selectivity of this reaction showed strong dependence on diameter, contrary to expectation based on simple curvature effects. The observed yield, as detected by thermogravimetry-mass spectroscopy and 1H-NMR, is drastically reduced in small-diameter tubes where the alkali dopant does not reach the inside of the bundles. Wide range optical transmission measurements were employed to determine the selectivity and indicate that besides higher yield, lower diameter selectivity occurs above a critical diameter., Comment: 14 pages, 4 figures

We present wide-range (3 meV - 6 eV) optical studies on freestanding transparent carbon nanotube films, made from nanotubes with different diameter distributions. In the far-infrared region, we found a low-energy gap in all samples investigated. By a detailed analysis we determined the average diameters of both the semiconducting and metallic species from the near infrared/visible features of the spectra. Having thus established the dependence of the gap value on the mean diameter, we find that the frequency of the low energy gap is increasing with increasing curvature. Our results strongly support the explanation of the low-frequency feature as arising from a curvature-induced gap instead of effective medium effects. Comparing our results with other theoretical and experimental low-energy gap values, we find that optical measurements yield a systematically lower gap than tunneling spectroscopy and DFT calculations, the difference increasing with decreasing diameter. This difference can be assigned to electron-hole interactions., Comment: 9 pages, 8 figures, to be published in Physical Review B, supplemental material attached v2: Figures 1, 7 and 8 replaced, minor changes to text; v3: Figures 3, 4 and 5 replaced, minor changes to text

We suggest a wavelength-dependent figure of merit for transparent conducting nanotube networks, composed of the sheet resistance and the optical density. We argue that this would be more useful than other suggestions prevailing in the literature, because it relies on more realistic assumptions regarding the optical parameters of real nanotubes: it takes into account the fact that the dc resistivity depends on the concentration of free carriers, while the visible absorption is caused by bound carriers. Based on sheet resistance measurements and wide-range transmission spectra, we compare several commercial nanotube types and find correlation between metal enrichment and figure of merit. A simple graphical approach is suggested to determine if the required optical and transport properties can be achieved by varying the thickness of the nanotube layer or a more aggressive treatment is needed. The procedure can be extended to oxide coatings as well., Comment: 17 pages, 7 figures