Session N01: Optical Properties of 2D Materials

2D Semiconductor Optoelectronics: Advances, Challenges and Opportunities

In this talk, I will present recent advancements on understanding and controlling the radiative and non-radiative recombination rates in various 2D semiconductor systems. I will discuss the mechanisms by which non-radiative recombination can be fully suppressed in TMDC monolayers, resulting in near-unity photoluminescence quantum yield at room temperature despite the presence of large defect densities.  I will discuss an AC carrier injection mechanism to enable bright light emitting devices using monolayers, overcoming the problem of Schottky contacts. Finally, I will discuss potential applications for black phosphorous (BP) thin films for midwave-IR photo detection and emission. Specifically, the BP based devices are shown to exhibit higher detectivity and luminescence efficiencies over state-of-the-art III-V and II-VI devices in mid-IR, owing to the lower Auger recombination rates and surface recombination velocity.   

Photoluminescence Quenching in N-Heterocyclic Carbene Functionalized Tungsten Dichalcogenide Mixed-Dimensional Heterostructures

Two-dimensional (2D) transition metal dichalcogenides (TMDs) are promising candidates for quantum optoelectronic devices due to their pronounced valley physics and excitonic properties. One approach to modulate these properties is to modify 2D TMDs with carbon-based ligands.¹ Among carbon-based ligands, the strong σ-donor character, stability, and structural tunability of N-heterocyclic carbenes (NHCs) has prompted investigations of NHC coordination to various atomically flat metal surfaces and metal nanoparticles. In this talk, we discuss the optical properties of WS₂ and WSe₂ monolayers functionalized with NHCs via a solvent-free route and an air-stable precursor. The deposition of NHCs on the 2D TMDs results in a significant room-temperature photoluminescence (PL) quenching. Moreover, low-temperature PL measurements (T = 3.9 K) show quenching of the WS₂ and WSe₂ excitonic emission and a shift of the defect bands to lower energies. The extent of both effects is tunable by altering the N-substituents of the NHCs as well the thickness of the deposited organic layer. The resulting mixed-dimensional heterostructures are further characterized by X-ray photoelectron spectroscopy, atomic force microscopy, and time-of-flight secondary ion mass spectrometry to elucidate the nature of the functionalization process. The observed PL quenching is consistent with a charge transfer process from defect states introduced by chalcogen vacancies to midgap states introduced by the carbene layer, as predicted from first-principles density functional theory calculations. Overall, these results demonstrate that carbene functionalization is an effective pathway for modifying the optical properties of 2D TMDs.

References:

 

(1)        M. I. B. Utama, H. Zeng, T. Sadhukhan, A. Dasgupta, et al., Nature Communications 2023, 14 (2193).   

Generating Narrow Defect Emission in Monolayer Tungsten Disulfide through Chemical Functionalization

Monolayer transition metal dichalcogenides (TMDs) have valuable excitonic properties for quantum optoelectronic applications, especially as platforms for single photon emission (SPE). Most studies of TMD quantum emission have focused on tungsten diselenide (WSe₂), leveraging intrinsic defects and strain engineering to localize excitons for SPE. While creating quantum emission from WSe₂ is reliable, the properties of that emission are inconsistent. Rather than relying on unpredictable intrinsic defects, this work introduces controlled extrinsic defects to tungsten disulfide (WS₂) using chemical functionalization methods built on those recently used with WSe₂ [1]. By treating the TMD surface with specific molecules, we engineer new defect sites. Specifically, treating unstrained monolayer WS₂ with maleimide molecules results in narrow, localized photoluminescence peaks at cryogenic temperatures. We apply additional chemical treatments to suppress native WS₂ emission to isolate the narrow peaks. These findings demonstrate that innovative methods such as chemical functionalization are powerful tools for enhancing the suitability of TMD defect emission for quantum optoelectronics. 

 

[1] Utama, M.I.B., Zeng, H., et al. “Chemomechanical Modification of Quantum Emission in Monolayer WSe₂.” Nat Comm 14, 2193 (2023).   

Bright and Dark Quadrupolar Excitons in WSe2/MoSe2/WSe2 Heterotrilayer

Transition metal dichalcogenide heterostructures have been extensively studied as a platform for investigating exciton physics. While heterobilayers , such as WSe2/MoSe2 , have received significant attention, there has been comparatively less research on heterotrilayers, which may offer new excitonic species and phases, as well as unique physical properties. In this letter, we
present theoretical and experimental investigations on the emission properties of quadrupolar excitons (QXs), a newly redicted type of exciton, in a WSe2/MoSe2/WSe2 heterotrilayer device. Our findings reveal that the optical brightness or darkness of QXs is determined by horizontal mirror symmetry and valley and spin selection rules. Additionally, the emission intensity and energy of both bright and dark QXs can be adjusted by applying an out-of-plane electric field, due to changes in hole distribution and the Stark effect. These results not only provide experimental evidence for the existence of QXs in heterotrilayers but also uncover their novel properties, which have the potential to drive the development of new exciton-based applications.   

Nonlinear nano-imaging of interlayer coupling in graphene-WSe2 heterostructures

Two-dimensional heterostructures of graphene and transition metal dichalcogenides exhibit enhanced photonic, electronic, spin, and other novel quantum properties. These emergent phenomena are controlled by the underlying interlayer coupling and associated charge and energy transfer and their dynamics. However, these processes are sensitive to interlayer distance and relative crystallographic orientation, which are in turn affected by defects, grain boundaries, bubbles, local strain, and other nanoscale heterogeneities. This obfuscates the distinction between interlayer charge and energy transfer and their competition with other relaxation processes, further amplified by spatial averaging across sample heterogeneities in conventional spectroscopy techniques. Here we combine nanoscale imaging in coherent four-wave mixing (FWM) and incoherent two-photon photoluminescence (2PPL) with nano-cavity clock spectroscopy to resolve the influence of structural heterogeneities in mono- to multi-layer graphene/WSe2 heterostructures. With selective insertion of hBN spacer layers, we show that energy as opposed to charge transfer dominates the interlayer coupled optical response. From the distinct nano-FWM and -2PPL tip-sample distance-dependent modification of interlayer and intralayer relaxation by nano-cavity enhancement and quenching, we derive an interlayer energy transfer time of τ_ET ∼ 0.35 ps consistent with recent reports. As a local probe technique, our approach highlights the ability for both nano-scale imaging and control of interlayer coupling to engineer new nonlinear nano-optical devices.   

Deterministically-Writing Quantum Emitters in Multi-Layered hBN at Room Temperature

Atomically-thin van der Waals hexagonal boron nitride (hBN) has emerged as a promising material candidate for hosting room temperature single-photon emitters (SPEs) for next-generation quantum technologies. Precise and robust control of creation, location and repeatability of the SPEs independent of the substrate are important advances. Here, we present a fabrication method for creating and deterministically placing quantum emitters in multi-layered hBN via combination of mechanical stress and thermal activation. Upon application of mechanical stress using an atomic force microscope tip, the hBN deforms while placed on a polymer film, resulting in tears on the edge of the indent. These tear sites are then activated through a thermal annealing process, giving rise to room temperature SPEs in the hBN. These findings provide a general methodology for activating SPEs in two-dimensional materials with on-demand precision and providing a platform for realizing physical phenomena through defect engineering.

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Different Types of Quantum Emitters Created by Thermal Annealing in CVD Grown Hexagonal Boron Nitride

Solid-state single photon emitters (SPEs) play an important role in photonic quantum technologies. Recently, SPEs based on defects in hexagonal boron nitride (hBN) have been demonstrated, exhibiting outstanding optical properties at room temperature. In this work, we created SPEs in chemical vapor deposition (CVD) grown 2.5-nm-thick hBN films by high temperature annealing in Ar gas flow. The thermal annealing created defects in the CVD grown hBN films can be classified into two types. The photoluminescence (PL) spectra of type-I defects show a broadened zero phonon line (ZPL) near 575 nm with a strong phonon sideband (PSB). The ZPL of type-II defects is distributed in 650-750 nm, exhibiting a relatively narrower ZPL linewidth with a much weaker PSB. Temperature dependent PL measurements show that the ZPL linewidth of type-II defects exhibits a weak temperature dependence, indicating less phonon coupling. Polarization dependent measurement and Fourier plane imaging have also been performed to identify the dipole orientation of the two defect types. Our results provide insight into different types of defects thermally created in CVD-grown hBN films.   

Scalable site-controlled activation of emitters in locally strained few-layer hexagonal boron nitride

Point defects in hexagonal boron nitride (hBN) offer promising routes toward engineering single photon emitters crucial for the next-generation photonic technologies, including quantum communication, optical quantum computing, and quantum sensing. However, such integrated quantum architectures face challenges in realizing scalable approaches for the deterministic positioning of hBN emitter arrays. Though progress has been made in such on-demand generation, their low quantum efficiency in terms of yield, photoluminescence, and compromised coherence hinders their practical applications. Here, we demonstrate a large-scale site-controlled activation of hBN emitters with a near-unity emitter creation probability by utilizing a plasmonic nanopillar architecture with the highest aspect ratio reported to date to induce a conformal nanoscale-strain perturbation that locally modifies the electronic band structure of multilayered hBN leading to the generation of optically active defect centers. Confocal PL mapping at room temperature shows, for the first time in hBN, near 100% site-controlled activation of emitters with an ultrasharp emission peak around 695 nm having an FWHM of ~7 meV along with 52-fold radiative enhancement compared to non-PNP emitters. Our results establish a scalable and high throughput process with the potential for engineering an optoelectronic heterogeneous integration platform for their applications in on-chip quantum technologies.   

Rational design of point defects with small electron-phonon coupling in 2D materials

Point defects in semiconductors have emerged as an attractive candidate for applications in quantum information science. Due to their ability to create well-localized states within the band gap, point defects can serve as effectively isolated atoms that can be utilized as single photon emitters (SPEs) and qubits.

Small Electron-phonon coupling is a key factor in producing higher photon indistinguishability which determines the suitability of defect systems as SPEs. Huang-Rhys (HR) factors are commonly calculated to measure the degree of electron-phonon coupling in a given system. However, not only is the computation of HR factors a complex task, but once determined, they are often used solely as a numerical value without any effort to establish a meaningful connection between HR factors and the physical defect system. Establishing such correlation would enable the development of a rational design principle for defects, with the goal of minimizing HR factors.

 

We propose that small HR factors are linked to the preservation of bonding character between the initial (occupied) and final (unoccupied) states involved in a specific transition. We demonstrate this principle for realistic SPEs candidates in hBN defect systems. HR factors are first calculated through first-principles employing the one-dimensional configuration coordinate diagram (1DCCD) approximation, then compared with full calculations involving phonon spectra, where we carefully extrapolate spectral functions towards the dilute defect limit.  These extrapolations are carried out through an embedding method that depends on the limited range of interatomic force constants within covalent semiconductors.  Calculated HR factors are then related to the degree of bonding-character similarity between the excited and ground states.   

Vacancy creation in graphene-based materials explored with scanning tunnelling microscopy

Ion bombardment is a powerful tool for generating point defects in two-dimensional materials. Particularly interesting are single atom vacancies which, as is the case in graphene, carry a charge that can become super-critical and exhibit atomic collapse. Furthermore, vacancies in graphene carry a local magnetic moment which, in the presence of local curvature, can be Kondo screened by the conduction electrons. Here we study the nature of vacancies and the electronic response to their presence in a variety of 2D samples including graphene-based materials, by varying ion species, ion energy and exposure times. We use scanning tunneling microscopy, scanning tunneling spectroscopy, and Landau Level spectroscopy in high magnetic field to characterize the effects of ion bombardment on the interlayer coupling and the electronic and magnetic properties of the vacancies.   

Exfoliated Two-Dimensional Tetracene Molecular Crystals Studied with Polarized Reflectance and Photoluminescence Spectroscopy

2D molecular crystals grown by physical vapor deposition using inorganic 2D crystals as assembly templates have revealed intriguing novel photophysical [1, 2] and electrical [3] properties. In this work [4], we report on a mechanical exfoliation, a top-down approach, to form 2D crystals of various polyaromatic molecules. This approach, compared to the conventional bottom-up method, can offer advantages in structural quality and thickness control besides less limitation on supporting substrates. We evaluated both methods by quantifying the crystallinity of 2D Tc crystals using wide-field photoluminescence (PL) imaging. Polarimetric analysis, based on two orthogonal polarization components, revealed long-range order that spanned more than several microns. Polarized absorption and PL measurements also showed thickness-dependent Davydov splitting, which can be attributed to varying crystalline structures and mixing of Frenkel and charge transfer exciton states [5]. This study demonstrates that mechanical exfoliation can be applied to various molecular systems to generate their 2D crystal forms, potentially leading to new findings and applications.   

van der Waals Heterostructures with InSe

The group-III chalcogenides are an emerging class of van der Waals stacked semiconductors with game-changing potential in the realms of electronics, optoelectronics, and photonics. These materials undergo dramatic transformations in their work function and band gap as they approach the 2D limit. Join us as we unveil experimental studies delving into the temperature-dependent optical properties of van der Waals heterostructures containing monolayer TMDs and the layered group-III chalcogenide, InSe. We analyze the effect of charge transfer within the structure as a function of temperature and layer thickness while comparing performance with the ubiquitous TMD-based heterostructure devices; garnering a deeper understanding of electronic and excitonic physics in van der Waals heterostructures.   

Effects of Uniaxial Strain on the Electronic Structure of Transition-Metal Dichalcogenides (TMD)

Using first-principles calculations, we investigate the effects of uniaxial strain on the electronic structure of monolayer MX₂ transition-metal dichalcogenide (TMD) semiconductors with M= (Mo, W) and X= (S, Se, Te). Under equilibrium conditions, the fundamental gaps are direct at K-K with the indirect gap K-Q being slightly higher for some of the compounds. Under uniaxial strain, along the zig-zag or armchair directions, the perfect hexagonal symmetry is broken, and the honeycomb Brillouin zone becomes distorted, leading to shifts in the maxima of the valence band and minima in the conduction band at K, Γ, and Q high-symmetry points. In the range of 0-5% uniaxial tensile strain, the TMDs remain direct band-gap semiconductors, while the difference between the indirect and direct gap increases with tensile strain. We also observe that the valence-band and conduction-band edges are slightly displaced from the K point for strain greater than 2%, changing the degeneracy of the electron and hole valleys, and likely affecting transport and optical properties. Our study emphasizes the effects of spin-orbit coupling and the use of hybrid functionals to correctly predict fundamental band gaps, the splitting of valence-band maximum, and the results of breaking the symmetry under uniaxial strain in TMDs.