Search

Photoluminescence from bulk HfS2 grown by the chemical vapor transport method is reported. A series of emission lines is apparent at low temperature in the energy range of 1.4–1.5 eV. Two groups of the observed excitonic transitions followed by their replicas involving acoustic and optical phonons are distinguished using classical intensity correlation analysis. The emission is attributed to the recombination of excitons bound to iodine (I2) molecules intercalated between layers of HfS2. The I2 molecules are introduced to the crystal during the growth as halogen transport agents in the growth process. Their presence in the crystal is confirmed by secondary ion mass spectroscopy. [ABSTRACT FROM AUTHOR]

Graphene/1T-TaS2 hybrid structures were measured using the Raman spectroscopy at room temperature. Spatially resolved Raman maps show that the region for which an effective graphene G band shift was observed correlates well with the tantalum disulfide flake, suggesting a change of the carrier concentration in graphene induced by 1T-TaS2. This effect was observed only for the sample exfoliated in vacuum, which signifies the necessity of preventing tantalum disulfide from oxidation during sample preparation. [ABSTRACT FROM AUTHOR]

Raman spectroscopy is a powerful analytical method widely used in many fields of science and applications. However, one of the inherent issues of this method is a low signal-to-noise ratio for ultrathin and two-dimensional (2D) materials. To overcome this problem, techniques like surface-enhanced Raman spectroscopy (SERS) that rely on nanometer scale metallic particles are commonly employed. Here, we demonstrate a different approach that is based on a microcavity structure consisting of a hexagonal boron nitride (h-BN) membrane spanning over an air-filled trench in germanium. In this structure, the h-BN membrane is an integral part of the cavity and, at the same time, shows an about 10-fold, polarisation-dependent h-BN Raman signal enhancement. With h-BN being transparent, flat, and chemically robust, it provides an excellent interface between the cavity and adjacent materials. We show that the Raman enhancement is also present for graphene layers transferred on top of the h-BN membrane, which proves that our approach can be extended to van der Waals heterostructures. The observed polarisation and position-dependent enhancements are in very good agreement with numerical simulations of the electric field intensity of the cavity. These results, together with the presented facile h-BN membrane fabrication process, which does not require any lithographic methods, open up new possibilities for enhancing Raman signals of 2D crystals without the need for metal particles.

Strain built-in electronic and optoelectronic devices can influence their properties and lifetime. This effect is particularly significant at the interface between two-dimensional materials and substrates. One such material is epitaxial hexagonal boron nitride (h-BN), which is grown at temperatures often exceeding 1000 °C. Due to the high growth temperature, h-BN based devices operating at room temperature can be strongly affected by strain generated during cooling due to the differences in lattice thermal expansion of h-BN and the substrate. Here, we present results of temperature-dependent Raman studies of the in-plane E 2g high phonon mode in the temperature range of 300-1100 K measured for h-BN grown by metalorganic vapor phase epitaxy. We observe a change, by an order of magnitude, in the rate of the temperature-induced frequency shift for temperatures below 900 K, indicating a strong reduction of the effective h-BN/substrate interaction. We attribute this behavior to the creation of h-BN wrinkles which results in strain relaxation. This interpretation is supported by the observation that no change of layer/substrate interaction and no wrinkles are observed for delaminated h-BN films transferred onto silicon. Our findings demonstrate that wrinkle formation is an inherent process for two-dimensional materials on foreign substrates that has to be understood to allow for the successful engineering of devices based on epitaxially grown van der Waals heterostructures., (Creative Commons Attribution license.)

Distributed Bragg Reflectors (DBR) are well-established photonic structures that are used in many photonic applications. However, most of the DBRs are based on different materials or require post-process etching which can hinder integration with other components in the final photonic structure. Here, we demonstrate the fabrication of DBR structures consisting only of undoped boron nitride (BN) layers with high refractive index contrast by using metal-organic chemical vapor deposition (MOCVD). This has been achieved in a single process, without the need for any post-process etching. The difference in the refractive index of the component BN layers stems from different degrees of porosity of the individual BN layers, which is a direct result of a different growth temperature. The fabricated DBR structures consist of 15.5 pairs of BN layers and exhibit a reflectance of 87 ± 1% at the maximum. The wavelength of maximum reflectance can be tuned from 500 nm up to the infrared region (IR), by simply adjusting the growth periods of subsequent BN layers. We also demonstrate that the fabricated structures can be used to create an optical microcavity. The fabricated DBRs are very promising candidates for future applications, for example in combination with single-photon emitters in h-BN, which could allow the building of a cavity-based all-BN single-photon source., (Creative Commons Attribution license.)