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A special issue of Nanomaterials (ISSN 2079-4991). This special issue belongs to the section "Nanophotonics Materials and Devices".

Deadline for manuscript submissions: closed (26 January 2024) | Viewed by 11095

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Special Issue Editors

Dr. Giancarlo Soavi

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Guest Editor

Institute of Solid State Physics, Friedrich Schiller University, Jena, Germany
Interests: nonlinear optics; ultrafast spectroscopy; 2D materials

Dr. Ioannis Paradisanos

E-Mail Website
Guest Editor

Institute of Electronic Structure and Laser, Foundation for Research and Technology Hellas, Heraklion, Greece
Interests: optical spectroscopy; 2D semiconductors; excitons

Special Issue Information

Dear Colleagues,

After almost two decades since the first pioneering works on single-layer graphene, layered materials remain at the forefront of scientific research. While our understanding of these ideal two-dimensional systems has improved enormously, new findings maintain the interest in this field at the highest level. The recent discovery of layered ferromagnets, moiré excitons, and the emerging fields of twistronics, valleytronics, and strong correlations are just a few examples in this context. All these new emerging topics rely on our capability to control light-matter interactions at the nanoscale, a detail that has always defined optical spectroscopy as one of the most powerful tools in our hands: (i) Raman is routinely used to determine the number of layers, defects density, doping, strain and more in almost any 2D material; (ii) photoluminescence probes the cross-over from indirect to direct gap in atomically thin semiconductors, and remains the most advanced tool to investigate the valley degree of freedom, excitonic interactions, and localization of indirect excitons in moiré potentials; (iii) ultrafast spectroscopy provides unique insights into the excited state dynamics of free electrons, excitons, and phonons; (iv) nonlinear optics is an established characterization method to study strain, exciton resonances and hybridization of states, as well as the valley degree of freedom and the twist angle in layered samples.

It is challenging to provide any thorough list when it comes to optical spectroscopy of layered materials, the number, and quality of papers in the field are overwhelming. However, although a lot has been done in the past 20 years, the topic keeps flourishing and has the potential for a bright future for many years to come. Only a few dozen of layered materials have been successfully synthesized or exfoliated, however thousands are predicted to be exfoliable with unique vibrational, electronic, magnetic and topological properties. With this Special Issue, we aim to gather some of the most recent and exciting results from experts in the field of optical spectroscopy of layered materials and to present new ideas for future research directions.

Dr. Giancarlo Soavi
Dr. Ioannis Paradisanos
Guest Editors

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Keywords

optics
spectroscopy
light-matter interactions
2D materials
layered heterostructures
excitons
ultrafast spectroscopy
nonlinear optics
photoluminescence
Raman

Published Papers (7 papers)

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Research

10 pages, 1060 KiB

Open AccessArticle

Third Harmonic Generation in Thin NbOI₂ and TaOI₂

by Tianhong Tang, Deng Hu, Di Lin, Liu Yang, Ziling Shen, Wenchen Yang, Haiyang Liu, Hanting Li, Xiaoyue Fan, Zhiwei Wang and Gang Wang

Nanomaterials 2024, 14(5), 412; https://doi.org/10.3390/nano14050412 - 23 Feb 2024

Viewed by 899

Abstract

The niobium oxide dihalides have recently been identified as a new class of van der Waals materials exhibiting exceptionally large second-order nonlinear optical responses and robust in-plane ferroelectricity. In contrast to second-order nonlinear processes, third-order optical nonlinearities can arise irrespective of whether a crystal lattice is centrosymmetric. Here, we report third harmonic generation (THG) in two-dimensional (2D) transition metal oxide iodides, namely NbOI₂ and TaOI₂. We observe a comparable THG intensity from both materials. By benchmarking against THG from monolayer WS₂, we deduce that the third-order susceptibility is approximately on the same order. THG resonances are revealed at different excitation wavelengths, likely due to enhancement by excitonic states and band edge resonances. The THG intensity increases for material thicknesses up to 30 nm, owing to weak interlayer coupling. After this threshold, it shows saturation or a decrease, due to optical interference effects. Our results establish niobium and tantalum oxide iodides as promising 2D materials for third-order nonlinear optics, with intrinsic in-plane ferroelectricity and thickness-tunable nonlinear efficiency. Full article

(This article belongs to the Special Issue Recent Advances in Optical Spectroscopy of Layered Materials)

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13 pages, 7490 KiB

Open AccessArticle

Brightening and Directionality Control of Dark Excitons through Quasi-Bound States in the Continuum

by Sebastian Klimmer, Giancarlo Soavi, Isabelle Staude and Ángela Barreda

Nanomaterials 2023, 13(23), 3028; https://doi.org/10.3390/nano13233028 - 27 Nov 2023

Cited by 1 | Viewed by 1098

Abstract

Thanks to their long lifetime, spin-forbidden dark excitons in transition metal dichalcogenides are promising candidates for storage applications in opto-electronics and valleytronics. To date, their study has been hindered by inefficient generation mechanisms and the necessity for elaborate detection schemes. In this work, we propose a new hybrid platform that simultaneously addresses both challenges. We study an all-dielectric metasurface with two symmetrically protected quasi-bound states in the continuum to enhance both the excitation and emission of dark excitons in a tungsten diselenide monolayer under normal light incidence. Our simulations show a giant photoluminescence signal enhancement (∼520) along with directional emission, thus offering distinct advantages for opto-electronic and valleytronic devices. Full article

(This article belongs to the Special Issue Recent Advances in Optical Spectroscopy of Layered Materials)

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14 pages, 2542 KiB

Open AccessArticle

Tuning Interlayer Exciton Emission with TMD Alloys in van der Waals Heterobilayers of Mo_0.5W_0.5Se₂ and Its Binary Counterparts

by Mohammed Adel Aly, Emmanuel Oghenevo Enakerakpor, Martin Koch and Hilary Masenda

Nanomaterials 2023, 13(20), 2769; https://doi.org/10.3390/nano13202769 - 16 Oct 2023

Viewed by 1520

Abstract

Semiconductor heterostructures have been the backbone of developments in electronic and optoelectronic devices. One class of structures of interest is the so-called type II band alignment, in which optically excited electrons and holes relax into different material layers. The unique properties observed in two-dimensional transition metal dichalcogenides and the possibility to engineer van der Waals heterostructures make them candidates for future high-tech devices. In these structures, electronic, optical, and magnetic properties can be tuned through the interlayer coupling, thereby opening avenues for developing new functional materials. We report the possibility of explicitly tuning the emission of interlayer exciton energies in the binary–ternary heterobilayer of Mo

_{0.5}

_{0.5}

_{2}

with MoSe

_{2}

and WSe

_{2}

. The respective interlayer energies of 1.516 eV and 1.490 eV were observed from low-temperature photoluminescence measurements for the MoSe

_{2}

– and WSe

_{2}

– based heterostructures, respectively. These interlayer emission energies are above those reported for MoSe

_{2}

/WSe

_{2}

(≃1.30–1.45 eV). Consequently, binary–ternary heterostructure systems offer an extended energy range and tailored emission energies not accessible with the binary counterparts. Moreover, even though Mo

_{0.5}

_{0.5}

_{2}

and MoSe

_{2}

have almost similar optical gaps, their band offsets are different, resulting in charge transfer between the monolayers following the optical excitation. Thus, confirming TMDs alloys can be used to tune the band-offsets, which adds another design parameter for application-specific optoelectronic devices. Full article

(This article belongs to the Special Issue Recent Advances in Optical Spectroscopy of Layered Materials)

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15 pages, 2428 KiB

Open AccessArticle

Strained Monolayer MoTe₂ as a Photon Absorber in the Telecom Range

by Muhammad Sufyan Ramzan and Caterina Cocchi

Nanomaterials 2023, 13(20), 2740; https://doi.org/10.3390/nano13202740 - 10 Oct 2023

Cited by 2 | Viewed by 1336

Abstract

To achieve the atomistic control of two-dimensional materials for emerging technological applications, such as valleytronics, spintronics, and single-photon emission, it is of paramount importance to gain an in-depth understanding of their structure–property relationships. In this work, we present a systematic analysis, carried out in the framework of density-functional theory, on the influence of uniaxial strain on the electronic and optical properties of monolayer MoTe₂. By spanning a ±10% range of deformation along the armchair and zigzag direction of the two-dimensional sheet, we inspect how the fundamental gap, the dispersion of the bands, the frontier states, and the charge distribution are affected by strain. Under tensile strain, the system remains a semiconductor but a direct-to-indirect band gap transition occurs above 7%. Compressive strain, instead, is highly direction-selective. When it is applied along the armchair edge, the material remains a semiconductor, while along the zigzag direction a semiconductor-to-metal transition happens above 8%. The characteristics of the fundamental gap and wave function distribution are also largely dependent on the strain direction, as demonstrated by a thorough analysis of the band structure and of the charge density. Additional ab initio calculations based on many-body perturbation theory confirm the ability of strained MoTe₂ to absorb radiation in the telecom range, thus suggesting the application of this material as a photon absorber upon suitable strain modulation. Full article

(This article belongs to the Special Issue Recent Advances in Optical Spectroscopy of Layered Materials)

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14 pages, 4944 KiB

Open AccessArticle

Non-Additive Optical Response in Transition Metal Dichalcogenides Heterostructures

by Marwa A. El-Sayed, Andrey P. Tselin, Georgy A. Ermolaev, Mikhail K. Tatmyshevskiy, Aleksandr S. Slavich, Dmitry I. Yakubovsky, Sergey M. Novikov, Andrey A. Vyshnevyy, Aleksey V. Arsenin and Valentyn S. Volkov

Nanomaterials 2022, 12(24), 4436; https://doi.org/10.3390/nano12244436 - 13 Dec 2022

Cited by 4 | Viewed by 1521

Abstract

Van der Waals (vdW) heterostructures pave the way to achieve the desired material properties for a variety of applications. In this way, new scientific and industrial challenges and fundamental questions arise. One of them is whether vdW materials preserve their original optical response when assembled in a heterostructure. Here, we resolve this issue for four exemplary monolayer heterostructures: MoS₂/Gr, MoS₂/hBN, WS₂/Gr, and WS₂/hBN. Through joint Raman, ellipsometry, and reflectance spectroscopies, we discovered that heterostructures alter MoS₂ and WS₂ optical constants. Furthermore, despite the similarity of MoS₂ and WS₂ monolayers, their behavior in heterostructures is markedly different. While MoS₂ has large changes, particularly above 3 eV, WS₂ experiences modest changes in optical constants. We also detected a transformation from dark into bright exciton for MoS₂/Gr heterostructure. In summary, our findings provide clear evidence that the optical response of heterostructures is not the sum of optical properties of its constituents. Full article

(This article belongs to the Special Issue Recent Advances in Optical Spectroscopy of Layered Materials)

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15 pages, 3754 KiB

Open AccessEditor’s ChoiceArticle

Electrostatic and Environmental Control of the Trion Fine Structure in Transition Metal Dichalcogenide Monolayers

by Yaroslav V. Zhumagulov, Alexei Vagov, Dmitry R. Gulevich and Vasili Perebeinos

Nanomaterials 2022, 12(21), 3728; https://doi.org/10.3390/nano12213728 - 24 Oct 2022

Cited by 2 | Viewed by 1588

Abstract

Charged excitons or trions are essential for optical spectra in low-dimensional doped monolayers (ML) of transitional metal dichalcogenides (TMDC). Using a direct diagonalization of the three-body Hamiltonian, we calculate the low-lying trion states in four types of TMDC MLs as a function of doping and dielectric environment. We show that the fine structure of the trion is the result of the interplay between the spin-valley fine structure of the single-particle bands and the exchange interaction. We demonstrate that by variations of the doping and dielectric environment, the fine structure of the trion energy can be tuned, leading to anticrossing of the bright and dark states, with substantial implications for the optical spectra of the TMDC ML. Full article

(This article belongs to the Special Issue Recent Advances in Optical Spectroscopy of Layered Materials)

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15 pages, 5072 KiB

Open AccessEditor’s ChoiceArticle

Exciton Manifolds in Highly Ambipolar Doped WS₂

by David Otto Tiede, Nihit Saigal, Hossein Ostovar, Vera Döring, Hendrik Lambers and Ursula Wurstbauer

Nanomaterials 2022, 12(18), 3255; https://doi.org/10.3390/nano12183255 - 19 Sep 2022

Viewed by 1861

Abstract

The disentanglement of single and many particle properties in 2D semiconductors and their dependencies on high carrier concentration is challenging to experimentally study by pure optical means. We establish an electrolyte gated WS

_{2}

monolayer field-effect structure capable of shifting the Fermi level [...] Read more.

_{2}

monolayer field-effect structure capable of shifting the Fermi level from the valence into the conduction band that is suitable to optically trace exciton binding as well as the single-particle band gap energies in the weakly doped regime. Combined spectroscopic imaging ellipsometry and photoluminescence spectroscopies spanning large n- and p-type doping with charge carrier densities up to 10

^{14}

^{- 2}

enable to study screening phenomena and doping dependent evolution of the rich exciton manifold whose origin is controversially discussed in literature. We show that the two most prominent emission bands in photoluminescence experiments are due to the recombination of spin-forbidden and momentum-forbidden charge neutral excitons activated by phonons. The observed interband transitions are redshifted and drastically weakened under electron or hole doping. This field-effect platform is not only suitable for studying exciton manifold but is also suitable for combined optical and transport measurements on degenerately doped atomically thin quantum materials at cryogenic temperatures. Full article

(This article belongs to the Special Issue Recent Advances in Optical Spectroscopy of Layered Materials)

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