[Show abstract][Hide abstract] ABSTRACT: The charge dynamics at the surface of the transparent conducting oxide and photoanode
material ZnO are investigated, in the presence and absence of light-harvesting colloidal
quantum dots (QDs). The time-resolved change in the surface potential upon photoexcitation
has been measured in m-plane ZnO (101�0) using a laser pump-synchrotron X-ray probe
methodology. By varying the oxygen annealing conditions, and hence the oxygen vacancy
concentration of the sample, we find that dark carrier lifetimes at the ZnO surface vary from
hundreds of μs to ms timescales, i.e. a persistent photoconductivity (PPC) is observed. The
highly-controlled nature of our experiments under ultra-high vacuum (UHV), and the use of
band-gap and sub-band-gap photoexcitation, allow us to demonstrate that defect states ca. 340
meV above the valence band edge are directly associated with the PPC, and that PPC mediated
by these defects dominates over the oxygen photodesorption mechanism. These observations
are consistent with the hypothesis that ionized oxygen vacancy states are responsible for PPC
in ZnO. The effect of chemically linking two colloidal QD systems (type I PbS and type II
CdS/ZnSe) to the surface has also been investigated. Upon deposition of QDs onto the surface,
the dark carrier lifetime and the surface photovoltage are reduced, suggesting direct injection
of charge carriers into the ZnO conduction band. The results are discussed in the context of the
development of next-generation solar cells.
[Show abstract][Hide abstract] ABSTRACT: The time-resolved change in the surface potential upon photoexcitation has been measured in two n-type photovoltaics, Si (111) 7×7 and ZnO (101[over ¯]0), using two different laser pump-synchrotron x-ray probe methodologies. Taken together, these experiments allow the dynamics of the surface photovoltage (SPV) to be monitored over timescales of subnanoseconds to milliseconds. The timescales for the photoinduced change in the SPV are dramatically different in the two samples, with measured SPV decay time constants of 6.6 μs for Si and up to 1.2 ms (dependent on surface oxygen concentration) for ZnO. The carrier dynamics at the Si (111) 7×7 surface are well modeled by a self-decelerating relaxation model involving the recombination of carriers by thermionic emission across the surface depletion layer on nanosecond timescales. In the case of ZnO (101[over ¯]0), a persistent photoconductivity (PPC) is observed, which is influenced by oxygen annealing conditions during sample preparation. Persistent photoconductivity is also observed when the surface is illuminated with subband-gap (405 nm) radiation, revealing that defect states approximately 340 meV above the valence band edge are directly associated with the PPC. We demonstrate that, under the conditions of our experiment, PPC mediated by these defects dominates over the oxygen photodesorption mechanism. These observations are consistent with the hypothesis that ionized oxygen vacancy states are responsible for PPC in ZnO.
Physical Review B 11/2013; 88(19). · 3.66 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Ultrafast transient absorption measurements have been used to study multiple exciton generation in solutions of PbS nanoparticles vigorously stirred to avoid the effects of photocharging. The threshold and slope efficiency of multiple exciton generation are found to be 2.5 ± 0.2 ×E(g) and 0.34 ± 0.08, respectively. Photoemission measurements as a function of nanoparticle size and ageing show that the position of the valence band maximum is pinned by surface effects, and that a thick layer of surface oxide is rapidly formed at the nanoparticle surfaces on exposure to air.
Physical Chemistry Chemical Physics 12/2011; 13(45):20275-83. · 4.20 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Truly alloyed PbS(x)Se(1-x) (x = 0-1) nanocrystals (∼5 nm in size) have been prepared, and their resulting optical properties are red-shifted systematically as the sulfur content of the materials increases. Their optical properties are discussed using a modified Vegard's approach and the bowing parameter for these nanoalloys is reported for the first time. The alloyed structure of the nanocrystals is supported by the energy-filtered transmission electron microscope images of the samples, which show a homogeneous distribution of sulfur and selenium within the nanocrystals. X-ray photoelectron spectroscopy studies on ligand-exchanged nanocrystals confirmed the expected stoichiometry and various oxidized species.
Journal of the American Chemical Society 03/2011; 133(14):5602-9. · 11.44 Impact Factor