Nanomaterials

Research

13 pages, 3601 KiB

Open AccessEditor’s ChoiceArticle

Texture-Induced Strain in a WS₂ Single Layer to Monitor Spin–Valley Polarization

by George Kourmoulakis, Antonios Michail, Dimitris Anestopoulos, Joseph A. Christodoulides, Manoj Tripathi, Alan Β. Dalton, John Parthenios, Konstantinos Papagelis, Emmanuel Stratakis and George Kioseoglou

Nanomaterials 2024, 14(17), 1437; https://doi.org/10.3390/nano14171437 - 3 Sep 2024

Viewed by 995

Abstract

Nanoscale-engineered surfaces induce regulated strain in atomic layers of 2D materials that could be useful for unprecedented photonics applications and for storing and processing quantum information. Nevertheless, these strained structures need to be investigated extensively. Here, we present texture-induced strain distribution in single-layer [...] Read more.

Nanoscale-engineered surfaces induce regulated strain in atomic layers of 2D materials that could be useful for unprecedented photonics applications and for storing and processing quantum information. Nevertheless, these strained structures need to be investigated extensively. Here, we present texture-induced strain distribution in single-layer WS₂ (1L-WS₂) transferred over Si/SiO₂ (285 nm) substrate. The detailed nanoscale landscapes and their optical detection are carried out through Atomic Force Microscopy, Scanning Electron Microscopy, and optical spectroscopy. Remarkable differences have been observed in the WS₂ sheet localized in the confined well and at the periphery of the cylindrical geometry of the capped engineered surface. Raman spectroscopy independently maps the whole landscape of the samples, and temperature-dependent helicity-resolved photoluminescence (PL) experiments (off-resonance excitation) show that suspended areas sustain circular polarization from 150 K up to 300 K, in contrast to supported (on un-patterned area of Si/SiO₂) and strained 1L-WS₂. Our study highlights the impact of the dielectric environment on the optical properties of two-dimensional (2D) materials, providing valuable insights into the selection of appropriate substrates for implementing atomically thin materials in advanced optoelectronic devices. Full article

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

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10 pages, 1060 KiB

Open AccessEditor’s ChoiceArticle

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

Cited by 2 | Viewed by 1787

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 [...] Read more.

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 2 | Viewed by 1704

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, [...] Read more.

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 2068

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 [...] Read more.

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}

W

_{0.5}

Se

_{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}

W

_{0.5}

Se

_{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 4 | Viewed by 1792

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 [...] Read more.

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 1864

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 [...] Read more.

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 2014

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 [...] Read more.

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 2198

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.

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 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}

cm

^{- 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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