Enhanced charge separation in an epitaxial metal-semiconductor nanohybrid material anchored with an organic molecule

Abstract

Heterostructure (HSs) composed of a Au metal nanoparticle and a CdSe semiconductor quantum dot (QD) have been synthesized and characterized by high-resolution TEM (HR-TEM) steady-state absorption and luminescence spectroscopic measurements. Time-resolved emission and femtosecond transient absorption (TA) spectrosopic measurements have been employed to study ultrafast charge-transfer dynamics after sensitizing CdSe{Au} HS with the bromopyrogallol red (Br-PGR) molecule. Charge-transfer (CT) complex formation between CdSe{Au} HS and Br-PGR was confirmed by steady-state absorption spectrocopy, making {Au}CdSe/Br-PGR a tricomposite system. Electron transfer from photoexcited CdSe QD to the Au NP was confirmed by both steady state and time-resolved emission studies and also by ultrafast TA measurements. Charge separation in the CdSe{Au}/Br-PGR tricomposite system took place in multiple ways: electron transfer from the conduction band (CB) of CdSe QD to Au NP, hole transfer from the valence band of CdSe QD to Br-PGR, and electron injection from photoexcited Br-PGR to the CB of CdSe QD and finally to the Fermi level of Au NP. The electron-transfer time from photoexcited CdSe to Au NP was found to be ∼270 fs; however, the hole-transfer time from photoexcited CdSe to Br-PGR was measured to be ∼500 fs. Spectroscopic investigation suggests that in the CdSe{Au}/Br-PGR tricomposite system, upon photoexcitation, all the photoexcited electrons are localized in Au NP and all the holes are localized in Br-PGR, confirming spatial charge separation in the tricomposite system. The grand charge separation process implies that the CdSe{Au}/Br-PGR tricomposite system can be emoloyed as an advanced material for quantum dot solar cells (QDSCs).