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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.


Related in: MedlinePlus

Photoconductivity of the ZnO nanowire in vacuum when illuminated with multi-line UV light. Light intensity is approximately 30 mW/cm2. The bias voltage is 0.3 V.
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Figure 4: Photoconductivity of the ZnO nanowire in vacuum when illuminated with multi-line UV light. Light intensity is approximately 30 mW/cm2. The bias voltage is 0.3 V.

Mentions: To obtain the maximum steady-state photocurrent in vacuum, we used the entire spectrum of the mercury lamp by removing the 313-nm bandpass filter. The total UV intensity above the ZnO bandgap was about 30 mW/cm2. The corresponding time response of the photocurrent is shown in Figure 4. A steady-state current of about 8.5 μA is reached after approximately 8 h of illumination. This current is not much larger than the currents in Figure 3, although the incident light intensity has been increased by an order of magnitude. As in Figure 3, the current follows a very slow decay pattern in vacuum, after the light is turned off, falling about 5% in the first day.


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)

Photoconductivity of the ZnO nanowire in vacuum when illuminated with multi-line UV light. Light intensity is approximately 30 mW/cm2. The bias voltage is 0.3 V.
© Copyright Policy - open-access
Related In: Results  -  Collection

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

Figure 4: Photoconductivity of the ZnO nanowire in vacuum when illuminated with multi-line UV light. Light intensity is approximately 30 mW/cm2. The bias voltage is 0.3 V.
Mentions: To obtain the maximum steady-state photocurrent in vacuum, we used the entire spectrum of the mercury lamp by removing the 313-nm bandpass filter. The total UV intensity above the ZnO bandgap was about 30 mW/cm2. The corresponding time response of the photocurrent is shown in Figure 4. A steady-state current of about 8.5 μA is reached after approximately 8 h of illumination. This current is not much larger than the currents in Figure 3, although the incident light intensity has been increased by an order of magnitude. As in Figure 3, the current follows a very slow decay pattern in vacuum, after the light is turned off, falling about 5% in the first day.

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