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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.

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.


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Dark current versus voltage of the ZnO nanowire. In air (unfilled squares) and in vacuum (filled squares). The measurement was performed after the device was kept in the dark for several days. Inset: SEM image of the device. The diameter of the wire is approximately 110 nm, and the gap between the electrodes in test is approximately1.8 μm.
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Figure 1: Dark current versus voltage of the ZnO nanowire. In air (unfilled squares) and in vacuum (filled squares). The measurement was performed after the device was kept in the dark for several days. Inset: SEM image of the device. The diameter of the wire is approximately 110 nm, and the gap between the electrodes in test is approximately1.8 μm.

Mentions: A scanning electron microscopy (SEM) image of a typical ZnO nanowire device is shown in the inset of Figure 1. Seventeen of such devices were fabricated, and they showed similar performances. All data shown in this paper are from the same representative device. Under weak illumination (UV intensity <1 W/cm2), photoluminescence was found to be dominated by green emission, centered at approximately 2.15 eV. This luminescence is a ubiquitous feature of fine structured ZnO and has been recently suggested to originate at the ZnO surface [5]. Figure 1 shows the current-voltage (I-V) characteristics in the dark. The linear I-V relations indicate the desired Ohmic behavior of the contacts. The observed dark currents were low both in air and in vacuum, with a slightly greater value in vacuum.


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)

Dark current versus voltage of the ZnO nanowire. In air (unfilled squares) and in vacuum (filled squares). The measurement was performed after the device was kept in the dark for several days. Inset: SEM image of the device. The diameter of the wire is approximately 110 nm, and the gap between the electrodes in test is approximately1.8 μm.
© Copyright Policy - open-access
Related In: Results  -  Collection

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

Figure 1: Dark current versus voltage of the ZnO nanowire. In air (unfilled squares) and in vacuum (filled squares). The measurement was performed after the device was kept in the dark for several days. Inset: SEM image of the device. The diameter of the wire is approximately 110 nm, and the gap between the electrodes in test is approximately1.8 μm.
Mentions: A scanning electron microscopy (SEM) image of a typical ZnO nanowire device is shown in the inset of Figure 1. Seventeen of such devices were fabricated, and they showed similar performances. All data shown in this paper are from the same representative device. Under weak illumination (UV intensity <1 W/cm2), photoluminescence was found to be dominated by green emission, centered at approximately 2.15 eV. This luminescence is a ubiquitous feature of fine structured ZnO and has been recently suggested to originate at the ZnO surface [5]. Figure 1 shows the current-voltage (I-V) characteristics in the dark. The linear I-V relations indicate the desired Ohmic behavior of the contacts. The observed dark currents were low both in air and in vacuum, with a slightly greater value in vacuum.

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