The study on the quantum dot quantum well (QDQW) microstructure modified by choosing different ligands containing a sulfhydryl group is of significance because it enables one to regulate photoexcited free charge carriers' (FCCs') transport behaviours in high-quality CdTe/ligand QDs via a self-assembled way. The photoelectron characteristics of ligand-capped CdTe nanoparticles were probed by a combination of surface photovoltaic (SPV) and photoacoustic technologies, supplemented by a computer simulation method of the CASTEP module. The experiment reveals that the D-value ΔEWi obtained by the associated two parameters of the SPV spectroscopy was closely related to the quantum confinement energy in the self-assembled CdTe/CdS/ligand core-shell system. In the paper the D-value was termed the depth of QWs, which were buried in the space charge regions located in the graded-band-gap and on either side of the shell-CdS. Obvious resonance quantum tunnelling may occur in the energy band structure with deep QWs on using certain ligands, resulting in an extended diffusion length of the FCCs on illumination of the photon energy hν ≥ Eg, core-CdTe, and in a strong SPV response at a specific wavelength region. In addition, the carrier-longitudinal optical phonon interaction is the reciprocal of the carriers' lifetime. The d-frontier orbital in the graded-band-gap plays an important role in both the microstructure and the resonance quantum tunnelling of the QDQW system according to the CASTEP calculations.

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Cu-doped ZnSe/ZnS/L-Cys core–shell QDs are prepared by both nucleation doping and growth doping in an aqueous synthesis method. Transport of photogenerated free charge carriers (FCCs) in these Cu-doped QDs is probed via a combination of surface photovoltaic (SPV), photoacoustic (PA), and electric-field-induced SPV techniques, supplemented by the UV–VIS absorption spectrum and Raman spectrum. The results confirm that the two doping mechanisms result in different doping locations and microelectronic structures of the Cu-doped QDs. The distinctive microelectronic structure of the QDs prepared by nucleation doping, as compared with those prepared by growth doping, results in a number of favorable SPV characteristics. For example, the QDs prepared by nucleation doping exhibit a higher SPV response intensity at 600 nm because of a higher concentration of photogenerated FCCs. The ratio of the strongest SPV response and the strongest PA signal of the QDs prepared by nucleation doping is up to 2.41 times greater than those of the QDs prepared by growth doping. This is because the greater numbers of photogenerated FCCs in the QDs prepared by nucleation doping generate the PV effect rather than the PA effect that is caused by a nonradiative de-excitation process. The position of the shoulder peak of the SPV response at a long wavelength of the QDs prepared by nucleation doping is significantly red-shifted compared with that of the QDs prepared by growth doping, leading to a broader SPV response range in the visible region. The QDs prepared by nucleation doping have a more obvious donor feature than those prepared by growth doping.

The photoelectron characteristics and nano-doping mechanism of Cu-doped ZnSe/ZnS/L-cys self-assembled core-shell quantum dots (QDs) are studied by surface photovoltaic (SPV) and photoacoustic (PA) techniques, XRD, HRTEM, FT-IR, UV-VIS adsorption, and Laser Raman spectra. The results suggest that the doped copper element prefers to locate at the Zn atom-vacancy of the (111) face of the QDs in the Cu2+ ion form. The defect-state levels are referred to the shallow accepter levels, leading to an obvious quantum confinement effect and a weakened n-type surface photovoltaic characteristic in the Cu-doped QDs. The quantum confinement effect strongly depends on the depth of the quantum well that is buried in the space charge region located in the graded-band-gap and at the side of the core-ZnSe. These electron structures are responsible for the increased lifetime and diffusion length of photogenerated free charge carriers, which significantly enhance the intensity of SPV response, enlarge the range of SPV response, and weaken the PA signals that are closely related to non-radiation deexcitation processes.

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