Exciton

About: Exciton is a research topic. Over the lifetime, 31603 publications have been published within this topic receiving 810642 citations.

Fine structure of neutral and charged excitons in self-assembled In(Ga)As/(Al)GaAs quantum dots

Abstract: The fine structure of excitons is studied by magnetophotoluminescence spectroscopy of single self-assembled In(Ga)As/(Al)GaAs quantum dots. Both strength and orientation of the magnetic field are varied. In a combination with a detailed theoretical analysis, these studies allow us to develop a comprehensive picture of the exciton fine structure. Symmetry of the dot structures as well as its breaking cause characteristic features in the optical spectra, which are determined by the electron-hole exchange and the Zeeman interaction of the carriers. The symmetry breaking is either inherent to the dot due to geometry asymmetries, or it can be obtained by applying a magnetic field with an orientation different from the dot symmetry axis. From data on spin splitting and on polarization of the emission we can identify neutral as well as charged exciton complexes. For dots with weakly broken symmetry, the angular momentum of the neutral exciton is no longer a good quantum number and the exchange interaction lifts degeneracies within the fine-structure manifold. The symmetry can be restored by a magnetic field due to the comparatively strong Zeeman interactions of electron and hole. For dots with a strongly broken symmetry, bright and dark excitons undergo a strong hybridization, as evidenced by pronounced anticrossings when states within the manifold are brought into resonance. The fine structure can no longer be described within the frame developed for structures of higher dimensionality. In particular, the hybridization cannot be broken magnetically. For charged excitons, the exchange interaction vanishes, demonstrating that the exchange splitting of a neutral exciton can be switched off by injecting an additional carrier.

Defects activated photoluminescence in two-dimensional semiconductors: interplay between bound, charged, and free excitons

Abstract: Point defects in semiconductors can trap free charge carriers and localize excitons. The interaction between these defects and charge carriers becomes stronger at reduced dimensionalities, and is expected to greatly influence physical properties of the hosting material. We investigated effects of anion vacancies in monolayer transition metal dichalcogenides as two-dimensional (2D) semiconductors where the vacancies density is controlled bya-particle irradiation or thermal-annealing. We found a new, sub-bandgap emission peak as well as increase in overall photoluminescence intensity as a result of the vacancy generation. Interestingly, these effects are absent when measured in vacuum. We conclude that in opposite to conventional wisdom, optical quality at room temperature cannot be used as criteria to assess crystal quality of the 2D semiconductors. Our results not only shed light on defect and exciton physics of 2D semiconductors, but also offer a new route toward tailoring optical properties of 2D semiconductors by defect engineering.

Computational 2D Materials Database: Electronic Structure of Transition-Metal Dichalcogenides and Oxides

Abstract: We present a comprehensive first-principles study of the electronic structure of 51 semiconducting monolayer transition-metal dichalcogenides and -oxides in the 2H and 1T hexagonal phases. The quasiparticle (QP) band structures with spin–orbit coupling are calculated in the G0W0 approximation, and comparison is made with different density functional theory descriptions. Pitfalls related to the convergence of GW calculations for two-dimensional (2D) materials are discussed together with possible solutions. The monolayer band edge positions relative to vacuum are used to estimate the band alignment at various heterostructure interfaces. The sensitivity of the band structures to the in-plane lattice constant is analyzed and rationalized in terms of the electronic structure. Finally, the q-dependent dielectric functions and effective electron and hole masses are obtained from the QP band structure and used as input to a 2D hydrogenic model to estimate exciton binding energies. Throughout the paper we focus on...

Excitation-power dependence of the near-band-edge photoluminescence of semiconductors.

Abstract: We present a model calculation for the dependence of the near-band-edge photoluminescence (NBEPL) on the power of the exciting laser light. Our model explains all features of the NBEPL power dependence that were previously observed in experiment: (i) the variation of the excitonic photoluminescence intensity I with ${\mathit{L}}^{\mathit{k}}$, where L is the excitation power and k is an exponent between 1 and 2, (ii) deviations from the I\ensuremath{\sim}${\mathit{L}}^{\mathit{k}}$ law as L is varied by more than two orders of magnitude, and (iii) the variation of k for exciton emission lines when the wavelength of the exciting laser radiation is varied. Furthermore, our model relates the k values of the free exciton, bound exciton, and the free-to-bound transitions. The results are in excellent agreement with experimental data.

Probing excitonic dark states in single-layer tungsten disulphide

Abstract: A series of long-lived excitons in a monolayer of tungsten disulphide are found to have strong binding energy and an energy dependence on orbital momentum that significantly deviates from conventional, three-dimensional, behaviour. The emergence of graphene optoelectronics has stimulated the development of near-transparent two-dimensional semiconductor materials. Much attention is focusing on the potentially extremely versatile transition metal dichalcogenides, such as molybdenum disulphide and tungsten disulphide, as components for ultrathin electronic devices. The physical origins of the unusually strong light–matter interactions in these materials remain unclear. An active topic in this area is how excitons (electron-hole pairs generated by light) behave in these low-dimensional systems. Here Xiang Zhang and colleagues report the discovery of a series of two-dimensional excitonic dark states in monolayer tungsten disulphide that have strong binding energy and an energy dependence on orbital momentum that significantly deviates from conventional (3D) behaviour. The findings open new avenues for fundamental research and opportunities to design devices such as photodetectors and photovoltaic cells. Transition metal dichalcogenide (TMDC) monolayers have recently emerged as an important class of two-dimensional semiconductors with potential for electronic and optoelectronic devices1,2. Unlike semi-metallic graphene, layered TMDCs have a sizeable bandgap3. More interestingly, when thinned down to a monolayer, TMDCs transform from indirect-bandgap to direct-bandgap semiconductors4,5, exhibiting a number of intriguing optical phenomena such as valley-selective circular dichroism6,7,8, doping-dependent charged excitons9,10 and strong photocurrent responses11. However, the fundamental mechanism underlying such a strong light–matter interaction is still under intensive investigation. First-principles calculations have predicted a quasiparticle bandgap much larger than the measured optical gap, and an optical response dominated by excitonic effects12,13,14. In particular, a recent study based on a GW plus Bethe–Salpeter equation (GW-BSE) approach, which employed many-body Green’s-function methodology to address electron–electron and electron–hole interactions, theoretically predicted a diversity of strongly bound excitons14. Here we report experimental evidence of a series of excitonic dark states in single-layer WS2 using two-photon excitation spectroscopy. In combination with GW-BSE theory, we prove that the excitons are of Wannier type, meaning that each exciton wavefunction extends over multiple unit cells, but with extraordinarily large binding energy (∼0.7 electronvolts), leading to a quasiparticle bandgap of 2.7 electronvolts. These strongly bound exciton states are observed to be stable even at room temperature. We reveal an exciton series that deviates substantially from hydrogen models, with a novel energy dependence on the orbital angular momentum. These excitonic energy levels are experimentally found to be robust against environmental perturbations. The discovery of excitonic dark states and exceptionally large binding energy not only sheds light on the importance of many-electron effects in this two-dimensional gapped system, but also holds potential for the device application of TMDC monolayers and their heterostructures15 in computing, communication and bio-sensing.