Theory of hot-electron energy loss in polar semiconductors: Role of plasmon-phonon coupling

Observation of photo-induced plasmon-phonon coupling in PbTe via ultrafast x-ray scattering

We report the observation of photo-induced plasmon-phonon coupled modes in the group IV-VI semiconductor PbTe using ultrafast x-ray diffuse scattering at the Linac Coherent Light Source. We measure the near-zone-center excited-state dispersion of the heavily screened longitudinal optical (LO) phonon branch as extracted from differential changes in x-ray diffuse scattering intensity following above bandgap photoexcitation. We suggest that upon photoexcitation, the LO phonon-plasmon coupled (LOPC) modes themselves become coupled to longitudinal acoustic modes that drive electron band shifts via acoustic deformation potentials and possibly to low-energy single-particle excitations within the plasma and that these couplings give rise to displacement-correlations that oscillate in time with a period given effectively by the heavily screened LOPC frequency.

Hot electron cooling in Dirac semimetal Cd3As2 due to polar optical phonons

A theory of hot electron cooling power due to polar optical phonons P _op is developed in 3D Dirac semimetal (3DDS) Cd₃As₂ taking account of hot phonon effect. Hot phonon distribution N _q and P _op are investigated as a function of electron temperature T _e, electron density n _e, and phonon relaxation time [Formula: see text]. It is found that P _op increases rapidly (slowly) with T _e at lower (higher) temperature regime. Whereas, P _op is weakly decreasing with increasing n _e. The results are compared with those for three-dimensional electron gas (3DEG) in Cd₃As₂ semiconductor. Hot phonon effect is found to reduce P _op considerably and it is stronger in 3DDS Cd₃As₂ than in Cd₃As₂ semiconductor. P _op is also compared with the hot electron cooling power due to acoustic phonons P _ac. We find that a crossover takes place from P _ac dominated cooling at low T _e to P _op dominated cooling at higher T _e. The temperature at which this crossover occurs shifts towards higher values with the increase of n _e. Also, hot electron energy relaxation time [Formula: see text] is discussed. It is suggested that [Formula: see text] can be tuned to achieve faster or slower energy loss for suitable applications of Cd₃As₂.

Ultrafast carrier dynamics in bimetallic nanostructure-enhanced methylammonium lead bromide perovskites

In this work, we examine the impact of hybrid bimetallic Au/Ag core/shell nanostructures on the carrier dynamics of methylammonium lead tribromide (MAPbBr₃) mesoporous perovskite solar cells (PSCs). Plasmon-enhanced PSCs incorporated with Au/Ag nanostructures demonstrated improved light harvesting and increased power conversion efficiency by 26% relative to reference devices. Two complementary spectral techniques, transient absorption spectroscopy (TAS) and time-resolved photoluminescence (trPL), were employed to gain a mechanistic understanding of plasmonic enhancement processes. TAS revealed a decrease in the photobleach formation time, which suggests that the nanostructures improve hot carrier thermalization to an equilibrium distribution, relieving hot phonon bottleneck in MAPbBr₃ perovskites. TAS also showed a decrease in carrier decay lifetimes, indicating that nanostructures enhance photoinduced carrier generation and promote efficient electron injection into TiO₂ prior to bulk recombination. Furthermore, nanostructure-incorporated perovskite films demonstrated quenching in steady-state PL and decreases in trPL carrier lifetimes, providing further evidence of improved carrier injection in plasmon-enhanced mesoporous PSCs.

Electron-phonon relaxation and excited electron distribution in zinc oxide and anatase

We propose a first-principles method for evaluations of the time-dependent electron distribution function of excited electrons in the conduction band of semiconductors. The method takes into account the excitations of electrons by an external source and the relaxation to the bottom of the conduction band via electron-phonon coupling. The methods permit calculations of the non-equilibrium electron distribution function, the quasi-stationary distribution function with a steady-in-time source of light, the time of setting of the quasi-stationary distribution and the time of energy loss via relaxation to the bottom of the conduction band. The actual calculations have been performed for titanium dioxide in the anatase structure and zinc oxide in the wurtzite structure. We find that the quasi-stationary electron distribution function has a peak near the bottom of the conduction band and a tail whose maximum energy rises linearly with increasing energy of excitation. The calculations demonstrate that the relaxation of excited electrons and the setting of the quasi-stationary distribution occur within a time of no more than 500 fs for ZnO and 100 fs for anatase. We also discuss the applicability of the effective phonon model to energy-independent electron-phonon transition probability. We find that the model only reproduces the trends in the change of the characteristic times whereas the precision of such calculations is not high. The rate of energy transfer to phonons at the quasi-stationary electron distribution also have been evaluated and the effect of this transfer on the photocatalysis has been discussed. We found that for ZnO this rate is about five times less than in anatase.

Disorder-assisted electron-phonon scattering and cooling pathways in graphene

We predict that graphene is a unique system where disorder-assisted scattering (supercollisions) dominates electron-lattice cooling over a wide range of temperatures, up to room temperature. This is so because for momentum-conserving electron-phonon scattering the energy transfer per collision is severely constrained due to a small Fermi surface size. The characteristic T(3) temperature dependence and power-law cooling dynamics provide clear experimental signatures of this new cooling mechanism. The cooling rate can be changed by orders of magnitude by varying the amount of disorder providing means for a variety of new applications that rely on hot-carrier transport.