Optical contrast and refractive index of natural van der Waals heterostructure nanosheets of franckeite

Refractiveindex.info database of optical constants

We introduce the refractiveindex.info database, a comprehensive open-source repository containing optical constants for a wide array of materials, and describe in detail the underlying dataset. This collection, derived from a meticulous compilation of data sourced from peer-reviewed publications, manufacturers' datasheets, and authoritative texts, aims to advance research in optics and photonics. The data is stored using a YAML-based format, ensuring integrity, consistency, and ease of access. Each record is accompanied by detailed metadata, facilitating a comprehensive understanding and efficient utilization of the data. In this descriptor, we outline the data curation protocols and the file format used for data records, and briefly demonstrate how the data can be organized in a user-friendly fashion akin to the books in a traditional library.

Atomic scale chemical ordering in franckeite-a natural van der Waals superlattice

The mineral franckeite is a naturally occurring van der Waals superlattice which has recently attracted attention for future applications in optoelectronics, biosensors and beyond. Furthermore, its stacking of incommensurately modulated 2D layers, the pseudo tetragonal Q-layer and the pseudo hexagonal H-layer, is an experimentally accessible prototype for the development of synthetic van der Waals materials and of advanced characterization methods to reveal new insights in their structure and chemistry at the atomic scale that is crucial for deep understanding of its properties. While some experimental studies have been undertaken in the past, much is still unknown on the correlation between local atomic structure and chemical composition within the layers. Here we present an investigation of the atomic structure of franckeite using state-of-the-art high-angle annular dark-field (HAADF) scanning transmission electron microscopy (STEM) and atom probe tomography (APT). With atomic-number image contrast in HAADF STEM direct information about both the geometric structure and its chemistry is provided. By imaging samples under different zone axes within the van der Waals plane, we propose refinements to the structure of the Q-layer and H-layer, including several chemical ordering effects that are expected to impact electronic structure calculations. Additionally, we observe and characterize stacking faults which are possible sources of differences between experimentally determined properties and calculations. Furthermore, we demonstrate advantages and discuss current limitations and perspectives of combining TEM and APT for the atomic scale characterization of incommensurately modulated von der Waals materials.

Franckeite as an Exfoliable Naturally Occurring Topological Insulator

Franckeite is a natural superlattice composed of two alternating layers of different composition which has shown potential for optoelectronic applications. In part, the interest in franckeite lies in its layered nature which makes it easy to exfoliate into very thin heterostructures. Not surprisingly, its chemical composition and lattice structure are so complex that franckeite has escaped screening protocols and high-throughput searches of materials with nontrivial topological properties. On the basis of density functional theory calculations, we predict a quantum phase transition originating from stoichiometric changes in one of franckeite composing layers (the quasihexagonal one). While for a large concentration of Sb, franckeite is a sequence of type-II semiconductor heterojunctions, for a large concentration of Sn, these turn into type-III, much alike InAs/GaSb artificial heterojunctions, and franckeite becomes a strong topological insulator. Transmission electron microscopy observations confirm that such a phase transition may actually occur in nature.

Naturally occurring layered mineral franckeite with anisotropic Raman scattering and third-harmonic generation responses

Vertically stacked van der Waals (vdW) heterostructures have introduced a unique way to engineer optical and electronic responses in multifunctional photonic and quantum devices. However, the technical challenges associated with the artificially fabricated vertical heterostructures have emerged as a bottleneck to restrict their proficient utilization, which emphasizes the necessity of exploring naturally occurring vdW heterostructures. As one type of naturally occurring vdW heterostructures, franckeite has recently attracted significant interest in optoelectronic applications, but the understanding of light–matter interactions in such layered mineral is still very limited especially in the nonlinear optical regime. Herein, the anisotropic Raman scattering and third-harmonic generation (THG) from mechanically exfoliated franckeite thin flakes are investigated. The observed highly anisotropic Raman modes and THG emission patterns originate from the low-symmetry crystal structure of franckeite induced by the lattice incommensurability between two constituent stacked layers. The thickness-dependent anisotropic THG response is further analyzed to retrieve the third-order nonlinear susceptibility for franckeite crystal. The results discussed herein not only provide new insights in engineering the nonlinear light–matter interactions in natural vdW heterostructures, but also develop a testbed for designing future miniaturized quantum photonics devices and circuits based on such heterostructures.

Vertically aligned carbon nanotubes, MoS2-rGo based optoelectronic hybrids for NO2 gas sensing

A simple method is developed through drop-casting techniques to assemble a molybdenum disulfide (MoS2)-reduced graphene oxide (rGO) hybrid on vertically aligned carbon nanotubes (VACNTs) to perform as an optoelectronic device for nitrogen dioxide (NO2) gas sensing at room temperature. The VACNT not only forms an ohmic contact with the hybrid material, but also yields a weak charge impurity scattering in the rGo layers across the channel. These features dramatically affect the optical response of the device to the light through which improve the photoresponsivity by up to 236% and the response time by up to 40% compared to the Au contacted device. Next, the fabricated MoS2-rGo/VACNTs device is employed as a resistor gas sensor for NO2 under in situ exposure to the light at room temperature. Under laser illumination, the sensor demonstrates a high sensitivity of ~ 41% at an inlet NO2 concentration of 100 ppm with a complete recovery time of ~ 150 s which shows comparable improvements relative to the sensor performance in dark condition.

Optical-Based Thickness Measurement of MoO3 Nanosheets

Considering that two-dimensional (2D) molybdenum trioxide has acquired more attention in the last few years, it is relevant to speed up thickness identification of this material. We provide two fast and non-destructive methods to evaluate the thickness of MoO₃ flakes on SiO₂/Si substrates. First, by means of quantitative analysis of the apparent color of the flakes in optical microscopy images, one can make a first approximation of the thickness with an uncertainty of ±3 nm. The second method is based on the fit of optical contrast spectra, acquired with micro-reflectance measurements, to a Fresnel law-based model that provides an accurate measurement of the flake thickness with ±2 nm of uncertainty.

Achieving extremely high optical contrast of atomically-thin MoS2

Extraordinarily high optical contrast is instrumental to research and applications of two-dimensional materials, such as, for rapid identification of thickness, characterisation of optical properties, and quality assessment. With optimal designs of substrate structures and light illumination conditions, unprecedented optical contrast of MoS₂ on Au surfaces exceeding 430% for monolayer and over 2600% for bilayer is achieved. This is realised on custom-designed substrates of near-zero reflectance near the normal incidence. In particular, by using an aperture stop to restrict the angle of incidence, high-magnification objectives can be made to achieve extraordinarily high optical contrast in a similar way as the low-magnification objectives, but still retaining the high spatial resolution capability. The technique will allow small flakes of micrometre size to be located easily and identified with great accuracy, which will have significant implications in many applications.

Symmetry Breakdown in Franckeite: Spontaneous Strain, Rippling, and Interlayer Moiré

Franckeite is a naturally occurring layered mineral with a structure composed of alternating stacks of SnS₂-like and PbS-like layers. Although this superlattice is composed of a sequence of isotropic two-dimensional layers, it exhibits a spontaneous rippling that makes the material structurally anisotropic. We demonstrate that this rippling comes hand in hand with an inhomogeneous in-plane strain profile and anisotropic electrical, vibrational, and optical properties. We argue that this symmetry breakdown results from a spatial modulation of the van der Waals interaction between layers due to the SnS₂-like and PbS-like lattices incommensurability.

Controlled Plasma Thinning of Bulk MoS2 Flakes for Photodetector Fabrication

The electronic properties of layered materials are directly determined based on their thicknesses. Remarkable progress has been carried out on synthesis of wafer-scale atomically molybdenum disulfide (MoS2) layers as a two-dimensional material in the past few years in order to transform them into commercial products. Although chemical/mechanical exfoliation techniques are used to obtain a high-quality monolayer of MoS2, the lack of suitable control in the thickness and the lateral size of the flakes restrict their benefits. As a result, a straightforward, effective, and reliable approach is widely demanded to achieve a large-area MoS2 flake with control in its thickness for optoelectronic applications. In this study, thick MoS2 flakes are obtained by a short-time bath sonication in dimethylformamide solvent, which are thinned with the aid of a sequential plasma etching process using H2, O2, and SF6 plasma. A comprehensive study has been carried out on MoS2 flakes based on scanning electron microscopy, atomic force microscopy, Raman, transmission electron microscopy, and X-ray photoelectron microscopy measurements, which ultimately leads to a two-cycle plasma thinning method. In this approach, H2 is used in the passivation step in the first subcycle, and O2/SF6 plasma acts as an etching step for removing the MoS2 layers in the second subcycle. Finally, we show that this technique can be enthusiastically used to fabricate MoS2-based photodetectors with a considerable photoresponsivity of 1.39 A/W and a response time of 0.45 s under laser excitation of 532 nm.

Large birefringence and linear dichroism in TiS3 nanosheets

TiS3 nanosheets have proven to be promising candidates for ultrathin optoelectronic devices due to their direct narrow band-gap and the strong light-matter interaction. In addition, the marked in-plane anisotropy of TiS3 is appealing for the fabrication of polarization sensitive optoelectronic devices. Herein, we study the optical contrast of TiS3 nanosheets of variable thickness on SiO2/Si substrates, from which we obtain the complex refractive index in the visible spectrum. We find that TiS3 exhibits very large birefringence, larger than that of well-known strong birefringent materials like TiO2 or calcite, and linear dichroism. These findings are in qualitative agreement with ab initio calculations that suggest an excitonic origin for the birefringence and linear dichroism of the material.