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 surface potential upon photoexcitation has been measured in the m-plane ZnO (101combining macron0) 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 the 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 the 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 the QDs onto the surface, the dark carrier lifetime and the surface photovoltage are reduced, suggesting a 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.
The time-resolved change in the surface potential upon photoexcitation has been measured in two n-type photovoltaics, Si (111) 7x7 and ZnO (1010), 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 ps for Si and up to 1.2 ms (dependent on surface oxygen concentration) for ZnO. The carrier dynamics at the Si (111) 7x7 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 (1010), 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.
The kinetics of formation of α,α-dicyano-p-toluoylcyanide anion (DCTC-) by reaction of TCNQ0 and its radical anion salts with nitrite ion have been investigated spectroscopically; the reaction of ...TCNQ0 with excess nitrite is first order, whereas oxidation of TCNQ•- appears to proceed via TCNQ0 as an intermediate. The first X-ray structural study of a simple DCTC- salt, K+2.2.2 DCTC-, is also reported.
We describe a new experimental pump-probe methodology where a 2D delay-line detector enables fast (ns) monitoring of a narrow XPS spectrum in combination with a continuous pump laser. This has been ...developed at the TEMPO beamline at Synchrotron SOLEIL to enable the study of systems with intrinsically slow electron dynamics, and to complement faster measurements that use a fs laser as the pump. We demonstrate its use in a time-resolved study of the surface photovoltage of the m-plane ZnO (101¯0) surface which shows persistent photoconductivity, requiring monitoring periods on ms timescales and longer. We make measurements from this surface in the presence and absence of chemically-linked quantum dots (QDs), using type I PbS and type II CdSe/ZnSe (core/shell) QDs as examples. We monitor signals from both the ZnO substrate and the bound QDs during photoexcitation, yielding evidence for charge injection from the QDs into the ZnO. The chemical specificity of the technique allows us to observe differences in the extent to which the QD systems are influenced by the field of the surface depletion layer at the ZnO surface, which we attribute to differences in the band structure at the interface.
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•Time-resolved surface photovoltage spectroscopy measures carrier dynamics in ZnO.•A new experimental technique at SOLEIL combines fast XPS with a modulated CW laser.•Quantum dots are chemically linked to ZnO, a model for next-generation solar cells.•Evidence of injection of charge carriers from quantum dots to the substrate
We have measured the degree of helicity of fluorescent radiation from Ar+3p43P4p 2P(o)1/2 formed by circularly polarized synchrotron radiation in the region of double excitations converging to Ar+3p4 ...nl satellite states. Angular momentum coupling allows the partitioning of the one unit of angular momentum brought into the system to be demarcated. We obtain a nonvanishing expectation value of the total spin of the residual ion-photoelectron system indicating significant relativistic interactions during the photoionization process.
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