Anisotropic materials are characterized by a unique optical response, which is highly polarization-dependent. Of particular interest are layered materials formed by the stacking of two-dimensional (2D) crystals that are naturally anisotropic in the direction perpendicular to the 2D planes. Black phosphorus (BP) is a stack of 2D phosphorene crystals and a highly anisotropic semiconductor with a direct band gap. We show that the angular dependence of polarized Raman spectra of BP is rather unusual and can be explained only by considering complex values for the Raman tensor elements. This result can be traced back to the electron-photon and electron-phonon interactions in this material.

The method for quantifying the amount of each carbon nanotube specie, as defined by its diameter and chiral angle, as well as the semiconducting-to-metallic ratio in any type of carbon nanotube sample is discussed. Single-wall carbon nanotubes grown by the cobalt-molybdenum catalyst based ͑CoMoCAT͒ process are characterized. The semiconducting-to-metallic ratio is found to be 11:1. A single semiconducting specie, named the ͑6,5͒ nanotube represents 2 / 5 of the sample, while the most abundant metallic nanotube is the ͑7,4͒, which exhibits a diameter similar to the ͑6,5͒.

We present direct experimental observation of exciton-phonon bound states in the photoluminescence excitation spectra of isolated single-walled carbon nanotubes (SWNT) in aqueous suspension. The photoluminescence excitation spectra from several distinct SWNTs show the presence of at least one sideband related to the tangential modes, lying 0.2 eV above the main absorption or emission peak. Both the energy position and line shapes of the sidebands are in excellent agreement with recent calculations [Phys. Rev. Lett. 94, 027402 (2005)] that predict the existence of exciton-phonon bound states, a sizable spectral weight transfer to these exciton-phonon complexes, and that the amount of this transfer depends on the specific nanotube structure and diameter.

By using a sample of DNA-wrapped single-wall carbon nanotubes strongly enriched in the (6,5) nanotube, photoluminescence emissions observed at special excitation energy values were identified with specific mechanisms of phonon-assisted excitonic absorption and recombination processes associated with (6,5) nanotubes, including one-phonon, two-phonon, and some continuous-luminescence processes. Such detailed processes are not separately identified in three-dimensional semiconducting materials. A general theoretical framework is presented to interpret the experimentally observed phonon-assisted processes in terms of excitonic states.

In this article, we review the current status of Raman spectroscopy in orthorhombic black phosphorus (BP) (for simplicity, henceforth, referred to as BP). BP is a layered semiconductor crystal that recently regained interest because it can be exfoliated down to the single layer, thus exhibiting 2-D properties. First, we briefly review the crystalline structure and the phonon dispersion relations in BP. Then, the symmetries of the Raman-active modes are discussed in the light of group theory, and the scattering configurations for observing the different phonon modes are presented. Polarized Raman spectroscopy results are discussed and reveal unusual angular dependence features, which can be ascribed to the linear dichroism and to the complex nature of the electron-phonon matrix elements. Edge phonon effects originated from rearrangements of the atomic terminations are also discussed. Subsequently, Raman modes that emerge from interlayer interaction and that are only visible in the few-layer regime are presented and discussed. Finally, we outline new perspectives to BP Raman spectroscopy in directions that remain partially or totally unexplored and that can provide potentially important outcomes to problems such as defects, oxidation, doping, strain, stacking order and other BP-like 2-D materials.