CdSe quantum dots have been encapped with aromatic ligands: α-toluenethiol, thiophenol, and p-hydroxythiophenol to enhance the photoluminescence (PL) quenching and photoelectric properties of the quantum dots. The aromatic ligand capped CdSe quantum dots are prepared through ligand exchange with trioctylphosphine oxide (TOPO) capped CdSe quantum dots. The XPS surface chemistry analysis and elemental analysis has confirmed the success of ligand exchange from TOPO to aromatic ligands. Both XRD and HRTEM-SAED studies indicate the crystalline structure of CdSe quantum dots not only remains but is also improved by the ligand exchange of TOPO with thiol molecules. Time resolved PL decay measurements indicate thiophenol and p-hydroxythiophenol ligands effectively quench the emission and have much shorter PL lifetimes than that of TOPO and that of α-toluenethiol. Thus, both thiophenol and p-hydroxythiophenol can act as an effective acceptor for photogenerated holes through aromatic π-electrons. Thiophenol also exhibits good charge transport behavior showing a 10-fold increase in short circuit current density (I_sc) as compared with TOPO in the photocurrent study of fabricated photovoltaic devices.

Huge photocurrent gains with a value of $10000$ detected in air and $100000$ detected in vacuum have been obtained from ${\text{SnO}}_2$ nanowires. Unlike previous reports that emphasized on carrier trapping by charge oxygen molecules in metal oxide nanostructures, we point out that spatial separation of photoexcited electrons and holes due to built-in electric field induced by surface charge defects does plays a significant role especially for nanostructured materials with a large surface-to-volume ratio. The study shown here provides a useful guideline to achieve photodetectors based on nanostructured materials with high sensitivity.