In bulk and quantum-confined semiconductors, magneto-optical studies have historically played an essential role in determining the fundamental parameters of excitons (size, binding energy, spin, ...dimensionality and so on). Here we report low-temperature polarized reflection spectroscopy of atomically thin WS2 and MoS2 in high magnetic fields to 65 T. Both the A and B excitons exhibit similar Zeeman splittings of approximately -230 μeV T(-1) (g-factor ≃-4), thereby quantifying the valley Zeeman effect in monolayer transition-metal disulphides. Crucially, these large fields also allow observation of the small quadratic diamagnetic shifts of both A and B excitons in monolayer WS2, from which radii of ∼1.53 and ∼1.16 nm are calculated. Further, when analysed within a model of non-local dielectric screening, these diamagnetic shifts also constrain estimates of the A and B exciton binding energies (410 and 470 meV, respectively, using a reduced A exciton mass of 0.16 times the free electron mass). These results highlight the utility of high magnetic fields for understanding new two-dimensional materials.
Van der Waals layered materials, such as transition metal dichalcogenides (TMDs), are an exciting class of materials with weak interlayer bonding, which enables one to create so-called van der Waals ...heterostructures (vdWH). One promising attribute of vdWH is the ability to rotate the layers at arbitrary azimuthal angles relative to one another. Recent work has shown that control of the twist angle between layers can have a dramatic effect on TMD vdWH properties, but the twist angle has been treated solely through the use of rigid-lattice moiré patterns. No atomic reconstruction, that is, any rearrangement of atoms within the individual layers, has been reported experimentally to date. Here, we demonstrate that vdWH of MoSe2/WSe2 and MoS2/WS2 at twist angles ≤1° undergo significant atomic level reconstruction leading to discrete commensurate domains divided by narrow domain walls, rather than a smoothly varying rigid-lattice moiré pattern as has been assumed in prior experimental work. Using conductive atomic force microscopy (CAFM), we show that TMD vdWH at small twist angles exhibit large domains of constant conductivity. The domains in samples with R-type stacking are triangular, whereas the domains in samples with H-type stacking are hexagonal. Transmission electron microscopy provides additional evidence of atomic reconstruction in MoSe2/WSe2 structures and demonstrates the transition between a rigid-lattice moiré pattern for large angles and atomic reconstruction for small angles. We use density functional theory to calculate the band structures of the commensurate reconstructed domains and find that the modulation of the relative electronic band edges is consistent with the CAFM results and photoluminescence spectra. The presence of atomic reconstruction in TMD heterostructures and the observed impact on nanometer-scale electronic properties provide fundamental insight into the behavior of this important class of heterostructures.
Strong Coulomb attraction in monolayer transition metal dichalcogenides gives rise to tightly bound excitons and many-body interactions that dominate their optoelectronic properties. However, this ...Coulomb interaction can be screened through control of the surrounding dielectric environment as well as through applied voltage, which provides a potential means of tuning the bandgap, exciton binding energy, and emission wavelength. Here, we directly show that the bandgap and exciton binding energy can be optically tuned by means of the intensity of the incident light. Using transient absorption spectroscopy, we identify a sub-picosecond decay component in the excited-state dynamics of WS2 that emerges for incident photon energies above the A-exciton resonance, which originates from a nonequilibrium population of charge carriers that form excitons as they cool. The generation of this charge-carrier population exhibits two distinct energy thresholds. The higher threshold is coincident with the onset of continuum states and therefore provides a direct optical means of determining both the bandgap and exciton binding energy. Using this technique, we observe a reduction in the exciton binding energy from 310 ± 30 to 220 ± 20 meV as the excitation density is increased from 3 × 1011 to 1.2 × 1012 photons/cm2. This reduction is due to dynamic dipolar screening of Coulomb interactions by excitons, which is the underlying physical process that initiates bandgap renormalization and leads to the insulator–metal transition in monolayer transition metal dichalcogenides.
Hydrogen adatoms are shown to generate magnetic moments inside single layer graphene. Spin transport measurements on graphene spin valves exhibit a dip in the nonlocal spin signal as a function of ...the applied magnetic field, which is due to scattering (relaxation) of pure spin currents by exchange coupling to the magnetic moments. Furthermore, Hanle spin precession measurements indicate the presence of an exchange field generated by the magnetic moments. The entire experiment including spin transport is performed in an ultrahigh vacuum chamber, and the characteristic signatures of magnetic moment formation appear only after hydrogen adatoms are introduced. Lattice vacancies also demonstrate similar behavior indicating that the magnetic moment formation originates from p(z)-orbital defects.
An emerging class of semiconductor heterostructures involves stacking discrete monolayers such as transition metal dichalcogenides (TMDs) to form van der Waals heterostructures. In these structures, ...it is possible to create interlayer excitons (ILEs), spatially indirect, bound electron–hole pairs with the electron in one TMD layer and the hole in an adjacent layer. We are able to clearly resolve two distinct emission peaks separated by 24 meV from an ILE in a MoSe2/WSe2 heterostructure fabricated using state-of-the-art preparation techniques. These peaks have nearly equal intensity, indicating they are of common character, and have opposite circular polarizations when excited with circularly polarized light. Ab initio calculations successfully account for these observations: they show that both emission features originate from excitonic transitions that are indirect in momentum space and are split by spin–orbit coupling. Also, the electron is strongly hybridized between both the MoSe2 and WSe2 layers, with significant weight in both layers, contrary to the commonly assumed model. Thus, the transitions are not purely interlayer in character. This work represents a significant advance in our understanding of the static and dynamic properties of TMD heterostructures.
Reduced dimensionality and strong Coulombic interactions in monolayer semiconductors lead to enhanced many-body interactions. Here, we report Auger recombination, i.e., exciton–exciton annihilation, ...in large-area chemical vapor deposition-grown monolayer WS2. Using ultrafast spectroscopy, we experimentally determine the Auger rate to be 0.089 ± 0.001 cm2/s at room temperature, which is an order of magnitude greater than the bulk value. This nonradiative recombination pathway dominates, regardless of excitation energy, for exciton densities greater than 8.0 ± 0.6 × 1010 cm–2 and below the Mott density. Higher-energy excitation above the A exciton resonance may initially produce a hot electron–hole gas that precedes exciton formation. Therefore, we use resonant excitation of the A exciton to ensure accuracy and avoid artifacts associated with other photogenerated species.
Two-dimensional (2D) materials exhibit many exciting phenomena that make them promising as materials for future electronic, optoelectronic, and mechanical devices. Because of their atomic thinness, ...interfaces play a dominant role in determining material behavior. In order to observe and exploit the unique properties of these materials, it is therefore vital to obtain clean and repeatable interfaces. However, the conventional mechanical stacking of atomically thin layers typically leads to trapped contaminants and spatially inhomogeneous interfaces, which obscure the true intrinsic behavior. This work presents a simple and generic approach to create clean 2D material interfaces in mechanically stacked structures. The operating principle is to use an AFM tip to controllably squeeze contaminants out from between 2D layers and their substrates, similar to a “squeegee”. This approach leads to drastically improved homogeneity and consistency of 2D material interfaces, as demonstrated by AFM topography and significant reduction of photoluminescence line widths. Also, this approach enables emission from interlayer excitons, demonstrating that the technique enhances interlayer coupling in van der Waals heterostructures. The technique enables repeatable observation of intrinsic 2D material properties, which is crucial for the continued development of these promising materials.
Monolayer transition metal dichalcogenides are promising materials for valleytronic operations. They exhibit two inequivalent valleys in the Brillouin zone, and the valley populations can be directly ...controlled and determined using circularly polarized optical excitation and emission. The photoluminescence polarization reflects the ratio of the two valley populations. A wide range of values for the degree of circularly polarized emission, P circ, has been reported for monolayer WS2, although the reasons for the disparity are unclear. Here, we optically populate one valley and measure P circ to explore the valley population dynamics at room temperature in a large number of monolayer WS2 samples synthesized via chemical vapor deposition. Under resonant excitation, P circ ranges from 2 to 32%, and we observe a pronounced inverse relationship between photoluminescence (PL) intensity and P circ. High-quality samples exhibiting strong PL and long exciton relaxation time exhibit a low degree of valley polarization, and vice versa. This behavior is also demonstrated in monolayer WSe2 samples and transferred WS2, indicating that this correlation may be more generally observed and account for the wide variations reported for P circ. Time-resolved PL provides insight into the role of radiative and nonradiative contributions to the observed polarization. Short nonradiative lifetimes result in a higher measured polarization by limiting opportunity for depolarizing scattering events.
There is keen interest in monolayer transition metal dichalcogenide films for a variety of optoelectronic applications due to their direct band gap and fast carrier dynamics. However, the mechanisms ...dominating their carrier dynamics are poorly understood. By combining time-resolved terahertz (THz) spectroscopy and transient absorption, we are able to shed light on the optoelectronic properties of large area CVD grown mono- and multilayer MoS2 films and determine the origins of the characteristic two-component excited state dynamics. The photoinduced conductivity shows that charge carriers, and not excitons, are responsible for the subpicosecond dynamics. Identical dynamics resulting from sub-optical gap excitation suggest that charge carriers are rapidly trapped by midgap states within 600 fs. This process complicates the excited state spectrum with rapid changes in line-width broadening in addition to a red-shift due to band gap renormalization and simple state-filling effects. These dynamics are insensitive to film thickness, temperature, or choice of substrate, which suggests that carrier trapping occurs at surface defects or grain boundaries. The slow dynamics are associated with exciton recombination and lengthen from 50 ps for monolayer films to 150 ps for multilayer films indicating that surface recombination dominates their lifetime. We see no signatures of trions in these MoS2 films. Our results imply that CVD grown films of MoS2 hold potential for high-speed optoelectronics and provide an explanation for the absence of trions in some CVD grown MoS2 films.
Resonant optical Stark effect in monolayer WS2 Cunningham, Paul D.; Hanbicki, Aubrey T.; Reinecke, Thomas L. ...
Nature communications,
12/2019, Letnik:
10, Številka:
1
Journal Article
Recenzirano
Odprti dostop
Abstract
Breaking the valley degeneracy in monolayer transition metal dichalcogenides through the valley-selective optical Stark effect (OSE) can be exploited for classical and quantum valleytronic ...operations such as coherent manipulation of valley superposition states. The strong light-matter interactions responsible for the OSE have historically been described by a two-level dressed-atom model, which assumes noninteracting particles. Here we experimentally show that this model, which works well in semiconductors far from resonance, does not apply for excitation near the exciton resonance in monolayer WS
2
. Instead, we show that an excitonic model of the OSE, which includes many-body Coulomb interactions, is required. We confirm the prediction from this theory that many-body effects between virtual excitons produce a dominant blue-shift for photoexcitation detuned from resonance by less than the exciton binding energy. As such, we suggest that our findings are general to low-dimensional semiconductors that support bound excitons and other many-body Coulomb interactions.
PDF
