Emergent properties from CuPd alloy films under near-infrared excitation

Abstract

Noble-transition metal alloys offer emergent optical and electronic properties for near-infrared (NIR) optoelectronic devices. We investigate the optical and electronic properties of CuxPd1-x alloy thin films and their ultrafast electron dynamics under NIR excitation. Ultraviolet photoelectron spectroscopy measurements supported by density functional theory calculations show strong d-band hybridization between the Cu 3 d and Pd 4 d bands. These hybridization effects result in emergent optical properties, most apparent in the dilute Pd case. Time-resolved terahertz spectroscopy (TRTS) with NIR (e.g., 1550 nm) excitation displays composition tunable electron dynamics. We posit that the negative peak in the normalized increment of transmissivity (ΔT/T) below 2 ps from dilute Pd alloys is due to non-thermalized hot-carrier generation. On the other hand, Pd-rich alloys exhibit an increase in ΔT/T due to thermalization effects upon ultrafast NIR photoexcitation. CuxPd1-x alloys in the dilute Pd regime may be a promising material for future ultrafast NIR optoelectronic devices.

Trends in hot carrier distribution for disordered noble-transition metal alloys

“…Transition metals that have almost fully filled d-orbitals are well-suited for efficient hot carrier generation [1,10,14]. Optical transition in an isolated atom usually require parity change up to leading order due to electric dipole term, ∝ ⟨ψ f |r|ψ i ⟩, thus d → d orbital transitions are forbidden.…”

“…Unlike Au, Cu does not have a strong spinorbit interaction, however, our previous study did find common band behavior between the Cu 3-d and Pd 4-d states. This phenomenon is attributable to robust d-band hybridization as indicated by photoelectron spectroscopy and density functional theory calculations [1,14]. Furthermore, ultrafast spectroscopy demonstrates composition dependent post-excitation dynamics, where dilute Pd alloys exhibit non-thermalized hot carriers right after excitation.…”

“…Previously, Stofela et al [1] and Manoukian et al [14] have synthesized and characterized Au 50 Pd 50 and Cu 1−x Pd x nobletransition metal alloy films. When the Au 50 Pd 50 system was excited at 1550 nm, it displayed a 20-fold increase in hot holes compared to Au films and three-times longer lifetimes than pure Pd.…”