Ultrafast Photoinduced Charge Separation in Metal–Semiconductor Nanohybrids

Abstract

Hybrid nano-objects formed by two or more disparate materials are among the most promising and versatile nanosystems. A key parameter in their properties is interaction between their components. In this context we have investigated ultrafast charge separation in semiconductor-metal nanohybrids using a model system of gold-tipped CdS nanorods in a matchstick architecture. Experiments are performed using an optical time-resolved pump-probe technique, exciting either the semiconductor or the metal component of the particles, and probing the light-induced change of their optical response. Electron-hole pairs photoexcited in the semiconductor part of the nanohybrids are shown to undergo rapid charge separation with the electron transferred to the metal part on a sub-20 fs time scale. This ultrafast gold charging leads to a transient red-shift and broadening of the metal surface plasmon resonance, in agreement with results for free clusters but in contrast to observation for static charging of gold nanoparticles in liquid environments. Quantitative comparison with a theoretical model is in excellent agreement with the experimental results, confirming photoexcitation of one electron-hole pair per nanohybrid followed by ultrafast charge separation. The results also point to the utilization of such metal-semiconductor nanohybrids in light-harvesting applications and in photocatalysis.

Effect of Surface Coating on the Photocatalytic Function of Hybrid CdS-Au Nanorods

“…In light of the above discussion, an additional possible effect of the surface coatings on the photocatalytic activity, is the degree of chemical and electronic surface passivation. Electron‐hole charge separation across the semiconductor‐metal interface competes effectively with the direct electron‐hole recombination as evidenced by the fluorescence quenching in the HNPs and by time resolved study . An additional competing non‐radiative route is trapping of the charge carriers in sub‐gap energy levels, which exist due to defect sites, typically present on the semiconductor rod surface.…”

Enhanced Lifetime of Excitons in Nonepitaxial Au/CdS Core/Shell Nanocrystals

“…Some evidence of the photoinduced absorption along with the reduced amplitude of the plasmon bleach was also seen in a recent TA investigation of Au/PbS nanoparticles. 46 However, in another TA-based study 49 of DDAB-reduced Au/CdS matchstick heterostructures, both plasmon (Au) and exciton (CdS) features were detectable, but very short-lived (τ exciton ≈ 20 fs). These results were corroborated by a few other reports of TA dynamics in M/S heterostructures 45,47 which reported some level of interfacial coupling effect on the ultrafast carrier decay but have not seen any definitive signature of the plasmon suppression.…”

“…As a result, the rate of carrier trapping on defect sites was found to be substantially reduced (lifetime, τ ≈ 300 ps) in comparison with that of Au/CdS heterostructures grown via a conventional, Au-salt reduction approach (τ < 0.02 ps). 49 It is hypothesized that unusually long lifetimes of excitons in nonepitaxial Au/CdS NCs can be Depending on the specifics of the synthesis, the resulting composites may acquire a large density of interfacial defects, which leads to the suppression of exciton and plasmon features 44 (as indicated in ultrafast transient absorption spectra). The right panel shows the Au/CdS core/shell morphology fabricated using the cation exchange strategy.…”

Manipulating Electron Transfer in Hybrid ZnO–Au Nanostructures: Size of Gold Matters

“…66,67 As a result, there is an increase in excitation of electrons to the conduction band in the ZnO part of the ZnO−Au NSs; the enhancement of electron density in the conduction band, further, increases the photocatalytic activity. 3 Therefore, a plausible mechanism of the photocatalysis by ZnO−Au NSs for the degradation of Evans blue under visible light irradiation could be enunciated as shown in Scheme 2.…”