Session H37: 2D Materials - Optics and Excitons IV

Second Harmonic Generation of Transition Metal Dichalcogenide and BNC Alloys

The advances in growth methods is allowing the possibility of synthesizing alloys of 2-D materials: To shed light on their nonlinear optical properties, first principles calculations have been used to calculate the second harmonic generation (SHG) of alloys of transition metal dichalcogenides (TMDS) and BNC layered systems. The results obtained indicate that chalcogen alloys of the type MoSSe or WSSe are more suitable for SHG applications than alloys of the type MoWS₄ or MoWSe₄. Additionally, zigzag nanotubes made from the above mentioned alloys have been considered and it is found that as their diameter gets smaller the intensity of the SHG is higher. Surpisingly, the intensity of the SHG in TMDs Haeckelites of the type NbSSe and Nb_0.5Ta_0.5S₂ exhibit the highest intensity for the SHG. The study was extended to BNC alloy systems of the type BNC₂, B_xN_xC_y Haeckelites and porous BN (BN Schwarzites). It is found that BNC₂ possesses much higher SHG than h-BN, making this a suitable material for non-linear optical properties. BN Schwarzites can be compared with the highest SHG 3-D materials found so far.   

Nano-optical activation of tightly localized bound excitons in monolayer WSe2 at room temperature

Bound excitons confined to nanobubbles in monolayer (ML) WSe₂, are efficient quantum light sources at cryogenic temperatures. However, when temperature is increased the bound excitons are suppressed, posing a significant challenge to realizing such single-photon emitters in practical devices. Using a model plasmonic-2D semiconductor architecture, we demonstrate that a nano-optical antenna activates optical emission from bound excitons in nanobubbles of ML-WSe₂ at room-temperature. The activation is attributed to Purcell enhancement, potential charge transfer, and local strain and exemplifies how the combination of a nano-optical antenna with a 2D semiconductors can enable novel phenomena. Further, bound exciton emission from single nanobubbles is imaged with sub-diffraction resolution, and multiple distinct states that are separated by distances as small as 50 nm are resolved. Analysis that extracts the strain profile from topography reveals a strong correlation of the bound excitons with local strain extrema within the nanobubble, directly visualizing how strain controls excitonic phenomena on nanoscale dimensions. Utilizing nano-optics to enable room-temperature emission of localized excitons is a critical step towards realizing room-temperature quantum emitters in ML-WSe₂.   

Solving the Mystery of B-Peak Emission in Monolayer Transition Metal Dichalcogenides

We survey a large number of monolayer TMDs to better understand the conditions responsible for various emission characteristics that have been reported in literature. We find that the intensities for both A- and B-peak emission vary widely as a result of sample-to-sample variations. However, a measurable B-peak intensity is evident in all samples. There is a clear linear relationship between the two peak intensities. The emission from the dominant A-peak is commonly several orders of magnitude higher than B-peak emission, resulting in B/A-intensity ratios well below 1%. Yet, as the A-peak intensity decreases, the ratio of B/A monotonically increases, and we observe a B/A ratio up to 30% in monolayer MoS₂. The A-excitonic emission is further quenched when MoS₂ is incorporated into an MoS₂/MoSe₂ heterostructure, where we observe comparable A- and B-peak intensities. We attribute these variations to differences in exciton recombination times, allowing us to clarify contradictory reports regarding the feasibility and significance of B-peak emission.   

Nonlinear optics for characterization of 2D materials

In this talk, I will discuss our recent results on nonlinear optics with two-dimensional layered (e.g., graphene [1-2], transition metal dichalcogenides [3-4], and black phosphorus [5-6]) materials and their hybrid structures (e.g, heterostructures[7], plasmonic[8] and silicon/fibre structures[1]) for material characterization applications.
[1] Z. Sun et al.,Nat. Photon.10,227(2016); A. Martinez et al., Nat. Photon.7,842(2013).
[2] F. Bonaccorso et al.,Nat. Photon.4,611(2010); F. Bonaccorso, Z. Sun, Opt. Mater. Exp.4,63(2014).
[3] A. Säynätjoki et al.,Nat. Commun.8,893(2017).
[4] L. Karvonen et.al.,Nat. Comm.8,15714(2017).
[5] D. Li, et al.,Sci. Rep. 5, 15899 (2015); Appl. Mater. Tod. 4,17(2016).
[6] A. Autere et.al.,J. Phys. Chem. Lett. 8, 1343(2017); Nat. Commun. 8, 278 (2017); ACS Photonics, DOI:10.1021/acsphotonics.7b00507(2017)
[7] H. Hu et.al.,Nat. Comm. 7, 12334 (2016); Adv. Mater. 28, 2931(2016).
[8] H. Jussia et al., Optica 3, 151 (2016).   

Double Indirect Interlayer Exciton in a MoSe2/WSe2 van der Waals Heterostructure

An emerging class of heterostructures involves monolayer semiconductors such as many of the transition metal dichalcogenides (TMDs) which can be combined to form van der Waals heterostructures (vdWHs). One unique new heterostructure property is an interlayer exciton (ILE), a spatially indirect, electron-hole pair with the electron in one TMD layer and the hole in the other. Here, we use state-of-the-art preparation techniques to create MoSe₂/WSe₂ heterostructures encapsulated in hBN. We observe ILE emission around 1.35 eV at room temperature and resolve this emission into two distinct peaks (ILE1 and ILE2) separated by 24 meV at zero field at 5 K. Furthermore, we demonstrate that the two emission peaks have opposite circular polarizations with up to +20% for the ILE1 and -35% for ILE2 when excited by circularly polarized light. Ab initio calculations provide an explanation of this unique and potentially useful property and indicate that it is a result of the indirect character of both electronic transitions. These peaks are double indirect excitons. i.e. indirect in both real and reciprocal space, split by relativistic effects.   

The origin of the optical side band in the spectrum of tungsten-based transition metal dichalcogenide monolayers.

Recent experiments have shown that a new dominant peak emerges in the low-energy side of the optical spectrum of monolayer WSe2 and WS2 at high electron density or at strong excitation [1]. This peak is absent in the spectrum of molybdenum-based monolayers or in the hole-doped case [2]. In this talk, we will show that the band structure of monolayer transition-metal dichalcogenides gives rise to an intriguing mechanism through which shortwave plasmons are paired up with excitons. The coupling elucidates the origin for the optical side band that is observed repeatedly in monolayers of WSe2 and WS2 but not understood. The theory makes it clear why exciton-plasmon coupling has the right conditions to manifest itself distinctly only in the optical spectra of electron-doped tungsten-based monolayers [3].

[1] A. M. Jones et. al., Nature Nano. 8, 634 (2013); J. Shang et. al., ACS Nano 9, 647 (2015); G. Plechinger et. al., Phys. Stat. Sol. RRL 9, 457 (2015); Y. You et. al., Nature Phys. 11, 477 (2015).
[2] Z. Wang et. al., Nano Lett. 17, 740 (2017).
[3] Dinh Van Tuan, Benedikt Scharf, Igor Zutic, and Hanan Dery, arXiv:1704.01981, Phys. Rev. X (in press)

  

Photoluminescence properties of monolayer MoS2 in an aqueous solution

Physical and chemical properties of transition-metal dichalcogenides (TMDCs) are expected to be strongly affected by various surface interactions because of their extremely large specific surface area. In particular, variety of phenomena occurring at the interfaces of water and atomically thin TMDCs may be critical for their applications. However, knowledge on the optical properties of these materials in water has still been limited. Here we report photoluminescence (PL) properties of mechanically exfoliated monolayer (1L) MoS₂ in aqueous solutions with various pH. After the immersion into distilled water, considerable modifications in the PL intensity and spectral line shape were observed compared to those in ambient air condition. The results are mainly attributed to the electrochemical reactions at the interfaces between the 1L-MoS₂ and the aqueous solution that cause electron extraction from or injection into the 1L-MoS₂ depending on the redox potentials in the O₂/H₂O redox system that are determined by the solution's pH.   

Polarized Raman Spectra of Layered CrPS4

Chromium thiophosphate (CrPS₄), a monoclinic crystal of C₂ space group, is a ternary layered semiconductor with an optical bandgap of 1.4 eV and exhibits antiferromagnetism below 36 K. Despite its potential in optoelectronic and magnetic applications, its lattice vibrations have not been studied. In this work, we performed a polarized Raman spectroscopy study of bulk CrPS₄. High quality crystals grown by the chemical vapor transport method were mechanically exfoliated into thin crystals for cleavage-plane excitation and mechanically polished for edge-plane excitation. Polarized Raman spectra were obtained in parallel and cross configurations, where the polarization of the Raman signal was parallel and perpendicular to that of the excitation beam, respectively. Among 33 Raman active modes (16 A and 17 B) at the Brillouin zone center, 14 (8) peaks observed in the parallel (cross) configuration were assigned as A (B) modes. To determine the Raman tensor values, the angular dependence of A and B modes were systematically investigated by rotating crystals with respect to the excitation polarization. The results can be used in determining crystallographic orientation unequivocally.   

Raman studies of anisotropic properties of ReSe2

ReSe₂ is a van der Waals layered semiconductor with an indirect bandgap of ~ 1.3 eV which is smaller than most transition metal dichalcogenides (TMDCs). ReSe₂ has a distorted octahedral structure (1T') with much lower symmetry and in-plane anisotropy compared to other hexagonal TMDCs such as MoS₂ and WS₂ [1]. Due to the structural in-plane anisotropy resulting in an optical and electrical anisotropy, ReSe₂ can be used as polarization sensitive optoelectronic devices [2]. We studied polarization dependence of Raman response with different excitation energies. The polarization dependence patterns vary dramatically with different excitation energies. In particular, we determined the direction of the rhenium chains with a mode at ~ 160 cm^-1 by using the 1.96 eV-excitation and confirmed it with HRSTEM measurements. In addition, we developed a method to determine front and back sides of a sample.

[1] J. A. Wilson et al., Advances in Physics 18, 193 (1969).
[2] Shengxue Yang et al., Scientific Reports 4, 5442 (2014).   

The Coulomb interaction in two-dimensional semiconductors

We use a general Coulomb potential form in two-dimensional semiconductor heterostructures and compare the resulting binding energies of neutral and charged excitons in monolayer transition-metal dichalcogenides with those calculated from the Keldysh potential. The results are similar when the dielectric constant of the monolayer is much larger than that of the surrounding layers. The general Coulomb potential form, however, can be used in a wider range of applications in which the Keldysh potential is not a good description (e.g., semiconductors whose thickness is comparable or larger than their Bohr radius), or inadequate (e.g., exciton bound to charged impurities at the substrate). We point to the shortcomings of both potential forms, emphasizing experimental aspects that cannot be reproduced in monolayer transition-metal dichalcogenides. We also discuss an exciton breakup mechanism in which charged impurities at the vicinity of the monolayer dissociate the exciton leading to reduced luminescence efficiency.   

High-Pressure Vibrational and Optoelectronic Properties of Mono- and Multi-Layer Rhenium Disulfide

Rhenium disulfide (ReS₂) is a layered semiconductor of the transition metal dichalcogenide family, which are notable for their promising applications in two-dimensional electronics. ReS₂ possesses in-plane anisotropy and comparatively weak interlayer bonding that can be exploited for a range of device applications, including highly sensitive polarized light detectors and strain-robust electronic devices. The low-symmetry (triclinic) crystal structure also allows for examination of the material’s polarization-dependent lattice dynamics, which may offer insight into its anisotropic transport properties – an important consideration in ReS₂-based devices. In this study, we conduct angle-resolved polarized Raman spectroscopy under application of pressures up to 13 GPa in a diamond anvil cell to evaluate the strain response of the lattice dynamics in ReS₂. Additionally, we report the pressure dependence of the optical bandgap of both mono- and multi-layer samples using photoluminescence spectroscopy. Our results highlight the unique optoelectronic and vibrational strain response of mono- and multi-layer ReS₂ under pressure, and provide context for how ReS₂ could be implemented in next-generation electronic devices.