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Photoinduced oxygen release and persistent photoconductivity in ZnO nanowires.

Bao J, Shalish I, Su Z, Gurwitz R, Capasso F, Wang X, Ren Z - Nanoscale Res Lett (2011)

Bottom Line: The observed photoresponse is much greater in vacuum and proceeds beyond the air photoresponse at a much slower rate of increase.After reaching a maximum, it typically persists indefinitely, as long as good vacuum is maintained.The extra photoconductivity in vacuum is explained by desorption of adsorbed surface oxygen which is readily pumped out, followed by a further slower desorption of lattice oxygen, resulting in a Zn-rich surface of increased conductivity.

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Affiliation: School of Engineering and Applied Sciences, Harvard University, Cambridge, MA 02138, USA. capasso@seas.harvard.edu.

ABSTRACT
Photoconductivity is studied in individual ZnO nanowires. Under ultraviolet (UV) illumination, the induced photocurrents are observed to persist both in air and in vacuum. Their dependence on UV intensity in air is explained by means of photoinduced surface depletion depth decrease caused by oxygen desorption induced by photogenerated holes. The observed photoresponse is much greater in vacuum and proceeds beyond the air photoresponse at a much slower rate of increase. After reaching a maximum, it typically persists indefinitely, as long as good vacuum is maintained. Once vacuum is broken and air is let in, the photocurrent quickly decays down to the typical air-photoresponse values. The extra photoconductivity in vacuum is explained by desorption of adsorbed surface oxygen which is readily pumped out, followed by a further slower desorption of lattice oxygen, resulting in a Zn-rich surface of increased conductivity. The adsorption-desorption balance is fully recovered after the ZnO surface is exposed to air, which suggests that under UV illumination, the ZnO surface is actively "breathing" oxygen, a process that is further enhanced in nanowires by their high surface to volume ratio.

No MeSH data available.


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Schematic of the depletion region in the dark (A) and under UV illumination (B). Photogenerated holes accumulate at the nanowire surface, partly neutralizing negatively charged absorbed oxygen species, which reduces the surface potential, leading to a reduction of the depletion width and increased photocurrent.
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Figure 5: Schematic of the depletion region in the dark (A) and under UV illumination (B). Photogenerated holes accumulate at the nanowire surface, partly neutralizing negatively charged absorbed oxygen species, which reduces the surface potential, leading to a reduction of the depletion width and increased photocurrent.

Mentions: Figure 5 schematically illustrates the depletion layer profile in a ZnO nanowire in the dark and under illumination. As we observe, a rather low dark conductivity, compared with the photoconductivity under UV illumination, we assume that the wire is almost entirely depleted in the dark (Figure 5A). Later on, we shall justify this assumption quantitatively. The observed green subbandgap luminescence, centered at approximately 2.15 eV, has been previously suggested to be the result of surface Fermi level pinning at approximately 1.15 eV below the conduction band which implies a band bending potential, Φ ≈ 1.15 V [30,31]. Once UV light is turned on, oxygen molecules are desorbed, as photoexcited holes become available, thereby reducing the surface potential Φ and the corresponding depletion width until a steady state is reached. Photoconductivity reflects the formation of a non-depleted core at the center of the wire, where the electron density is given by the doping level n. The changes of surface potential and the corresponding depletion width are determined by the interplay between oxygen adsorption and the net desorption rate which is a function of UV light intensity [32]. However, because of the relatively high oxygen partial pressure in air, total elimination of the depletion region and of the corresponding band bending would require extremely high illumination intensity. The maximum achievable photoconductivity should correspond to the native electron density n [33]. This explains why the saturation value of the photocurrent increases sublinearly with illumination intensity (Figure 2) [32,33].


Photoinduced oxygen release and persistent photoconductivity in ZnO nanowires.

Bao J, Shalish I, Su Z, Gurwitz R, Capasso F, Wang X, Ren Z - Nanoscale Res Lett (2011)

Schematic of the depletion region in the dark (A) and under UV illumination (B). Photogenerated holes accumulate at the nanowire surface, partly neutralizing negatively charged absorbed oxygen species, which reduces the surface potential, leading to a reduction of the depletion width and increased photocurrent.
© Copyright Policy - open-access
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC3211499&req=5

Figure 5: Schematic of the depletion region in the dark (A) and under UV illumination (B). Photogenerated holes accumulate at the nanowire surface, partly neutralizing negatively charged absorbed oxygen species, which reduces the surface potential, leading to a reduction of the depletion width and increased photocurrent.
Mentions: Figure 5 schematically illustrates the depletion layer profile in a ZnO nanowire in the dark and under illumination. As we observe, a rather low dark conductivity, compared with the photoconductivity under UV illumination, we assume that the wire is almost entirely depleted in the dark (Figure 5A). Later on, we shall justify this assumption quantitatively. The observed green subbandgap luminescence, centered at approximately 2.15 eV, has been previously suggested to be the result of surface Fermi level pinning at approximately 1.15 eV below the conduction band which implies a band bending potential, Φ ≈ 1.15 V [30,31]. Once UV light is turned on, oxygen molecules are desorbed, as photoexcited holes become available, thereby reducing the surface potential Φ and the corresponding depletion width until a steady state is reached. Photoconductivity reflects the formation of a non-depleted core at the center of the wire, where the electron density is given by the doping level n. The changes of surface potential and the corresponding depletion width are determined by the interplay between oxygen adsorption and the net desorption rate which is a function of UV light intensity [32]. However, because of the relatively high oxygen partial pressure in air, total elimination of the depletion region and of the corresponding band bending would require extremely high illumination intensity. The maximum achievable photoconductivity should correspond to the native electron density n [33]. This explains why the saturation value of the photocurrent increases sublinearly with illumination intensity (Figure 2) [32,33].

Bottom Line: The observed photoresponse is much greater in vacuum and proceeds beyond the air photoresponse at a much slower rate of increase.After reaching a maximum, it typically persists indefinitely, as long as good vacuum is maintained.The extra photoconductivity in vacuum is explained by desorption of adsorbed surface oxygen which is readily pumped out, followed by a further slower desorption of lattice oxygen, resulting in a Zn-rich surface of increased conductivity.

View Article: PubMed Central - HTML - PubMed

Affiliation: School of Engineering and Applied Sciences, Harvard University, Cambridge, MA 02138, USA. capasso@seas.harvard.edu.

ABSTRACT
Photoconductivity is studied in individual ZnO nanowires. Under ultraviolet (UV) illumination, the induced photocurrents are observed to persist both in air and in vacuum. Their dependence on UV intensity in air is explained by means of photoinduced surface depletion depth decrease caused by oxygen desorption induced by photogenerated holes. The observed photoresponse is much greater in vacuum and proceeds beyond the air photoresponse at a much slower rate of increase. After reaching a maximum, it typically persists indefinitely, as long as good vacuum is maintained. Once vacuum is broken and air is let in, the photocurrent quickly decays down to the typical air-photoresponse values. The extra photoconductivity in vacuum is explained by desorption of adsorbed surface oxygen which is readily pumped out, followed by a further slower desorption of lattice oxygen, resulting in a Zn-rich surface of increased conductivity. The adsorption-desorption balance is fully recovered after the ZnO surface is exposed to air, which suggests that under UV illumination, the ZnO surface is actively "breathing" oxygen, a process that is further enhanced in nanowires by their high surface to volume ratio.

No MeSH data available.


Related in: MedlinePlus