A new collaborative study using electron spectroscopy has measured the charge transfer process of a hole in an exciton. Researchers from Cambridge University, Marburg University, and Göttingen University published this breakthrough.
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The experiment utilized atomically thin semiconductors, which have gained attention as a platform for studying quantum many-body phenomena and unlocking new possibilities in optoelectronics.
Materials like graphene consist of single molecules with their chemical bonds aligned to form a sheet. These frequently possess unique electrical and optical characteristics, facilitating the production of devices with features only a few atoms thick. These 2D semiconductors support the production of excitons.
What is an Exciton?
In semiconductors, bound electron and hole pairs combine to form excitons, which are quasi-particles. The properties of excitons are extensively investigated for engineering optoelectronic devices, including photovoltaics, light-emitting diodes, and lasers.
When an electron in a semiconductor is excited into a higher energy level, either by photon absorption or by another excitation technique, it forms a positively charged "hole" in the lower energy level. This results in the creation of an electron-hole pair. Occasionally, when these two particles are in a bonded state, an exciton can be formed. Coulombic interactions link the electron and hole within an exciton, and the exciton binding energy measures the strength of this coupling.
The energy needed to split an exciton into its component electron and hole charge carriers is termed exciton binding energy. A higher exciton binding energy indicates an exciton with a longer lifetime and greater stability. Various factors influence this binding energy, including the material’s dielectric constant, the effective masses of the electron-hole pair, and the dimensional confinement of excitons within the material. In most semiconductors, the other electrons in the valence band screen out the Coulombic interaction. As a result, the exciton binding energy is generally low.
The excitation and recombination of electrons and holes are the fundamental concepts in semiconductor physics. Instead of excitons, free electrons and hole carriers recombine to produce processes like electroluminescence and fluorescence. However, when excitons do recombine, the energy of the resulting photon is less than the band gap of the material. The energy needed to break the excitonic connection equals the energy difference.
Measuring Charge Transfer Dynamics of Holes
Traditionally, photoemission spectroscopy has been used to study how charge transfer processes in quantum materials are triggered by light absorption. Thus far, the focus has been on the electrons that comprise the electron-hole pair, which are quantified using an electron analyzer. However, very little spectroscopic data concerning the hole component of the exciton has been recorded up to this point.
Until recently, a direct method of measuring the ‘holes’ dynamics in the electron-hole pair has remained elusive. New research has employed time- and angle-resolved photoemission spectroscopy (trARPES) to characterize the hole of the exciton hole and its electron charge transfer process. By tracing the evolution of electronic band structure through simultaneous observation of spectral and dynamic information, trARPES directly resolves elementary scattering processes.
The study utilized an atomically thin 2D material composed of a twisted Tungsten diselenide (WSe2) and Molybdenum disulfide (MoS2) heterostructure.
The experimental setup integrated a flexible pump beamline and a tabletop High Harmonic Generation (HHG) beamline powered by a high-intensity fiber laser system, all operating at a repetition rate of 1 MHz. A momentum microscope endstation was used to simultaneously detect energy- and in-plane-momentum-resolved photoemission data. A time-resolved optical pump-probe system was used to accurately monitor the femtosecond temporal evolution of the charge carrier and band renormalization dynamics.
During the experiment, energy loss in the electron was observed due to the breaking up of an exciton. The hole of the exciton passes from one semiconductor layer to the other. The ultrafast hole-transfer mechanism is facilitated by the Coulomb interaction between the electron- and the hole component of the excitons. The photoelectron energy of the exciton was measured during the hole-transfer process across the contact.
Future Outlooks
This study underscores the effectiveness of femtosecond electron spectroscopy in investigating hole-electron correlated interactions in twisted semiconductor heterostructures. The results show that information on the hole state is contained in the photoelectron of the correlated two-particle exciton. In conjunction with material-specific and microscopic theory, these findings enable the direct tracking of an otherwise elusive ultrafast hole-transfer process.
This research opens new avenues for future investigation of the structural properties of matter in two-dimensional quantum materials. It also emphasizes the potential of time-resolved momentum microscopy to explore optically inaccessible coupled excitonic and electronic states of matter. Specifically, it offers microscopic insights into the ultrafast hole-transfer mechanism.
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References and Further Reading
Bange, JP., et al. (2024). Probing electron-hole Coulomb correlations in the exciton landscape of a twisted semiconductor heterostructure. Science Advances. DOI: 10.1126/sciadv.adi1323
Keunecke, M., et al. (2020). Time-resolved momentum microscopy with a 1 MHz high-harmonic extreme ultraviolet beamline. Rev. Sci. Instrum. DOI: 10.1063/5.0006531
O’Kane, M. (no date). Exciton: An Introduction. [Online] Ossila. Available at: https://www.ossila.com/pages/what-is-an-exciton
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