Ultrafast Carrier and Lattice Dynamics in Plasmonic Nanocrystalline Copper Sulfide Films

Abstract

Excited carrier dynamics in plasmonic nanostructures determines many important optical properties such as nonlinear optical response and photocatalytic activity. Here it is shown that mesoscopic plasmonic covellite nanocrystals with low free-carrier concentration exhibit a much faster carrier relaxation than in traditional plasmonic materials. A nonequilibrium hot-carrier population thermalizes within first 20 fs after photoexcitation. A decreased thermalization time in nanocrystals compared to a bulk covellite is consistent with the reduced Coulomb screening in ultrathin films. The subsequent relaxation of thermalized, equilibrium electron gas is faster than in traditional plasmonic metals due to the lower carrier concentration and agrees well with that in a bulk covellite showing no evidence of quantum confinement or hot-hole trapping at the surface states. The excitation of coherent optical phonon modes in a covellite is also demonstrated, revealing coherent lattice dynamics in plasmonic materials, which until now was mainly limited to dielectrics, semiconductors, and semimetals. These findings show advantages of this new mesoscopic plasmonic material for active control of optical processes.

Keywords: coherent optical phonons; copper sulfide; nonequilibirium processes; plasmonics; ultrafast hot‐carrier dynamics.

Figure 1

a) Transient reflection dynamics, b) pump‐power dependences of the amplitude of the induced reflection, and c) comparison of the hot‐hole relaxation for the bulk covellite and the mesoscopic CuS film.

Figure 2

a) Comparison of the transient reflection risetime in the bulk covellite and the mesoscopic CuS film. Inset shows the free‐carrier density profile in a thin (4 nm) CuS slab normalized to the bulk carrier density. b) Hot‐carrier lifetimes as a function of energy for CuS calculated with the GPAW code (See Experimental Section).

Figure 3

The transient reflection dynamics due to the excitation of coherent optical phonons in a) the mesoscopic CuS film and b) the bulk covellite: (blue) experimental data, (orange) biharmonic fit using the phonon frequencies in (c). c) The spectrum of optical phonons obtained by the fast Fourier transform of the time dependence in (a), revealing two modes. Inset shows the crystal structure of covellite. d) The spectrum of optical phonons obtained by the fast Fourier transform of the time dependence in (b), revealing mainly the low frequency phonon mode. A much weaker peak is visible around 475 cm⁻¹. Inset shows the dependence of the amplitude of the lowest energy phonon mode (measured in a bulk covellite) on the angle between polarizations of the pump and probe beams.

Figure 4

a) Spectra of Ψ and Δ obtained from the spectroscopic ellipsometric measurements of the CuS nanocrystalline film. b) In‐plane dielectric constant of CuS: (circles) obtained from the simulations in ref. [41] and (lines) the fit with the Drude–Lorenz model. Highlighted region corresponds to the spectral width of the broadband fs pulses used in the pump‐probe experiments. Please note that the connection between the dielectric constant of the bulk CuS and the nanocrystalline film require the development of the effective medium model which is beyond the scope of this work.

Figure 5

a) TEM image of the as‐prepared CuS nanocrystals; scale bar is 100 nm. b) X‐ray diffraction pattern of the CuS nanoparticles. c) IR absorption spectrum of CuS nanocrystals in chloroform, demonstrating a localized surface plasmon resonance at a wavelength of approximately 1.3 µm. d) AFM phase image of a densely‐packed monolayer of CuS nanoparticles; scale bar is 500 nm.

Figure 6

a) Schematics of the optical set‐up for degenerate pump‐probe measurements. b) Spectrum of the sub‐10 fs optical pulse used in the measurements. Inset shows the temporal shape of a compressed pulse.