Abstract
Zinc oxide, a wide-band-gap semiconductor, shows intriguing optoelectronic properties when coupled with Ag. Specifically, an absorbance band in the visible range that is not apparent in the separated materials emerges when the interface is formed. Interestingly, photoexcitation of this “interface band” or band-to-band results in a counterintuitive photovoltaic response when a supra/sub-band-gap light is shone. To investigate the origin of this absorbance band and photovoltaic response, we studied in detail the energy-band alignment of ultrathin layers of ZnO (3–60 nm) with Ag. Our analysis indicated that an ‘electrostatic potential cliff’ is formed within the first 1–2 nm of ZnO. In addition, oxygen vacancies, presumably generated by AgxO–Zn bonds, form mid-gap acceptor states within these first few nm. Both effects facilitate a valence band-to-defect state optical transition that is confined to the interface region. The second type of defects—hole-trap states associated with zinc hydroxide—are spread throughout the ZnO layer and dominate the supra-band-gap photovoltaic response. These findings have potential implications in emerging technologies such as photocatalytic Ag/ZnO heterostructures that will utilize the long-lived charges for chemical work or other optoelectronic applications.
Highlights
Metal/metal-oxide interfaces are prevalent in modern devices such as diodes, sensors, photovoltaic devices, and emerging technologies such as photocatalysis and metal-oxide electronics.[1−13] These interfaces often cause beneficial or deleterious effects, such as hot electron transfer or charge trapping at interface defects.[14−16] Zinc oxide has superior electronic qualities, and it is used in numerous applications.[17,18]
Band “A” at ∼362 nm stems from the valence to conduction band electronic absorption in ZnO, and band “B” at ∼390−414 nm stems from the interaction of Ag and ZnO and is not apparent in the individual materials (Figure S1)
We investigated the optoelectronic properties and chemical structure of the Ag/ZnO heterostructure and its interface in particular
Summary
Metal/metal-oxide interfaces are prevalent in modern devices such as diodes, sensors, photovoltaic devices, and emerging technologies such as photocatalysis and metal-oxide electronics.[1−13] These interfaces often cause beneficial or deleterious effects, such as hot electron transfer or charge trapping at interface defects.[14−16] Zinc oxide has superior electronic qualities, and it is used in numerous applications.[17,18] The interface of Ag and ZnO shows intriguing optoelectronic properties[9,19−21] but employing them requires better understanding of the interface structure and function. Surface plasmon resonance (SPR)[22−24] and surface plasmon polariton (SPP)[25−27] coupled with electron transfer[28,29] and trapping at interface defect states[30−33] take place at the Ag/ ZnO interface. ZnO/Ag interfaces appear to have an interface index close to one.[36,38,39] Yet, ultrathin layers of AgxO can form near the interface as a metal oxide upon reaction with lattice oxygen, which would lead to oxygen vacancies in the metal oxide, and mid-gap states and energy-level pinning
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