Ultrafast Photophysics of 2D Semiconductors and Related Heterostructures

Strong light–matter interaction and excitonic effects make atomically thin single-layer transition-metal dichalcogenides (1L-TMDs) extremely appealing for optoelectronic and photonic devices. The relaxation dynamics of different photoexcited species like carriers and excitons determines the materials’ functional properties. Hence, its understanding is of great importance for developing devices based on 1L-TMDs. Optical pump–probe spectroscopy is an ideal tool to study light–matter interaction in the nonequilibrium regime over different timescales. Excitons in 1L-TMDs possess an additional degree of freedom, the valley index, which can be written and read out by circularly polarized light, paving the way to valleytronics. Combining two or more 1L-TMDs into heterostructures opens unprecedented functionalities and tuning knobs beyond those of the 1L and is instrumental for the charge separation required in light-harvesting applications. Atomically thin transition-metal dichalcogenides (TMDs) exhibit strong light–matter interaction and pronounced excitonic behavior. Understanding the nonequilibrium dynamical processes of photoexcited carriers in such materials is a key step in view of their technological applications. Here, we review the ultrafast photophysics of 2D TMDs and related heterostructures (HSs) measured by femtosecond optical spectroscopy. First, we provide a general introduction on the physics of the materials and a brief explanation of the ultrafast optical techniques. Then we discuss the physical processes governing their nonequilibrium optical response, with a particular emphasis on the intervalley scattering dynamics, and the charge-transfer processes in 2D HSs. We conclude by discussing open issues and perspectives for future experiments. Atomically thin transition-metal dichalcogenides (TMDs) exhibit strong light–matter interaction and pronounced excitonic behavior. Understanding the nonequilibrium dynamical processes of photoexcited carriers in such materials is a key step in view of their technological applications. Here, we review the ultrafast photophysics of 2D TMDs and related heterostructures (HSs) measured by femtosecond optical spectroscopy. First, we provide a general introduction on the physics of the materials and a brief explanation of the ultrafast optical techniques. Then we discuss the physical processes governing their nonequilibrium optical response, with a particular emphasis on the intervalley scattering dynamics, and the charge-transfer processes in 2D HSs. We conclude by discussing open issues and perspectives for future experiments. three-particle process where an electron in the CB recombines with a hole in the VB and transfers the energy to a third charge carrier rather than emitting it as a photon. The third carrier is either an electron in the CB that is excited to a higher level or a hole in the VB that is excited to a lower level. If all three particles are free carriers, the rate of the process is proportional to the cube of the carrier density. When they are bound into excitons, exciton–exciton annihilation (see below) is the more appropriate description. However, sometimes the two terms are used as synonyms. the bandgap is the energy difference between the energy maximum of the VB and the energy minimum of the CB (see band structure). For deviations from the equilibrium occupation of all electron levels (e.g., via doping, charge injection, or photoexcitation) the bands may shift by different amounts, thus changing the bandgap energy. representation of the electron energy as a function of the wavevector k, which is related to the electron momentum via k = p/ħ. The projection of the lines in the band structure onto the energy axis yields bands of allowed electron energies, which may overlap or be separated by forbidden-energy regions. In semiconductors there is a forbidden region called the bandgap between the highest (almost) fully occupied band (the VB) and the lowest (almost) fully empty band (the CB). the unit cell of a crystal in k space (momentum space). It delimits the possible wavevectors (momentum vectors) for an electron in the crystal. Its high symmetry points are typically labelled Γ (for the center; i.e., k = 0), K, M, Λ,…, for points at the edges. as charges originating from charge injection, doping, or photoexcitation accumulate at the CB minimum and/or VB maximum, they block the electronic transitions involving these states (see Pauli blocking), thus increasing the minimum transition energy, resulting in a blue shift of the absorption and emission spectra. the transfer of an exciton to another valley or an adjacent layer in a HS viewed as two charge-transfer mechanisms (electron and hole transfer) occurring at the same time. region of the band structure of graphene around the K point (see Brillouin zone), where both the VB and the CB have a conical shape and touch each other at the K point, making graphene a zero-gap semiconductor. Since the electron and hole effective masses are related to the curvature in the band structure, in the cones, where there is zero curvature, their effective mass is zero, leading to relativistic effects. an electron in the CB and a hole in the VB bound together by their electric (Coulomb) interaction, analogous to the electron and the proton in a hydrogen atom but with lower binding energy due to the lower effective mass of the hole compared with the proton and due to the screening by the high-dielectric-constant semiconductor. transfer of the excitation energy of one exciton to another via processes like Förster transfer or Dexter transfer, de-exciting one of the excitons towards the ground state and exciting the other to a higher-energy excited state. The rate of this process is proportional to the product of the concentrations of the two reaction partners (i.e., to the square of the exciton density), resulting in a TA signal decay component that becomes faster with increasing pump fluence. the distribution of electron energies in thermal equilibrium, based on the occupation probabilities derived from Fermi statistics. the resonant transfer of an exciton to an adjacent layer via electromagnetic interaction (emission and absorption of a virtual photon; coupling between the emission and absorption dipole of the donor and acceptor, respectively). spectroscopy techniques that are sensitive to the left/right circular polarization of the light – usually both the light used for excitation (the pump) and the detected PL or TA. a bound state of an electron and a hole that are spatially separated in two different layers of a 2D HS. since photons carry very little momentum compared with electrons, electronic transitions are almost vertical in the band structure unless they involve the creation or annihilation of a phonon. Hence, an electron in the CB can recombine with a given hole in the VB only if it is within a narrow vertical cone in the band structure. a transition between a metallic and an insulating state. In the case of TMDs, photoexcitation creates mostly excitons, which do not contribute to the photoconductivity. At increasing excitation density, the exciton-binding energy decreases due to screening and above a certain threshold density no more bound excitons exist, resulting in a conductive electron–hole plasma. due to the Pauli exclusion principle, each energy level can at most be occupied by two electrons with opposite spin. Hence, if some of the final states for an electronic transition underlying a spectral feature are occupied, the transition will appear weaker. quantum mechanical description of the lattice vibration. The interaction of carriers with the lattice is described as electron–phonon interaction/coupling/scattering. The scattering process creates or annihilates a phonon and changes the energy and momentum of the electron according to the respective conservation laws. the energetic shift of bands due to an electric field (applied through an appropriate device structure or local fields from carriers). If the two bands involved in an optical transition shift by different amounts, the transition shifts in energy. Disorder causes inhomogeneous shifts leading to broadening of spectral features. a three-particle bound excited state analogous to an exciton comprising either one hole and two electrons or two holes and one electron. Trions are excited states with positive or negative elementary charge and can arise from either the binding of an exciton to a charge or the creation of an exciton via photoexcitation in immediate proximity to a charge (within the size of a trion, usually a few nanometers in TMDs). by analogy to electronics and photonics, where information is encoded in the transport of charge or light, respectively, other degrees of freedom, such as electron spin, have been suggested as carriers of information. Monolayer TMDs exhibit two VB maxima and CB minima called valleys, which have the same energy but are not equivalent with respect to electron spin, suggesting the valley polarization as a binary unit of information.