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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

Transient photocurrent of the ZnO nanowire in air under UV illumination. The intensity of the λ = 313 nm light is approximately 1.3 mW/cm2. Inset: steady-state photocurrent versus light intensity in air. The bias voltage is 0.3 V and is the same for all other photocurrent measurements.
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Figure 2: Transient photocurrent of the ZnO nanowire in air under UV illumination. The intensity of the λ = 313 nm light is approximately 1.3 mW/cm2. Inset: steady-state photocurrent versus light intensity in air. The bias voltage is 0.3 V and is the same for all other photocurrent measurements.

Mentions: Figure 2 shows the time response of the photocurrent in air. Upon exposure to UV light, the photocurrent rises rapidly, reaching a steady-state value in several minutes. However, when the UV light is turned off, the current decays slowly following a short rapid decay. The overall decay is not exponential and slows down further over time. The current takes more than 10 h to return to the original dark value. The inset in Figure 2 shows the steady-state photocurrent as a function of light intensity. The current is clearly not a linear function of the intensity.


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)

Transient photocurrent of the ZnO nanowire in air under UV illumination. The intensity of the λ = 313 nm light is approximately 1.3 mW/cm2. Inset: steady-state photocurrent versus light intensity in air. The bias voltage is 0.3 V and is the same for all other photocurrent measurements.
© Copyright Policy - open-access
Related In: Results  -  Collection

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

Figure 2: Transient photocurrent of the ZnO nanowire in air under UV illumination. The intensity of the λ = 313 nm light is approximately 1.3 mW/cm2. Inset: steady-state photocurrent versus light intensity in air. The bias voltage is 0.3 V and is the same for all other photocurrent measurements.
Mentions: Figure 2 shows the time response of the photocurrent in air. Upon exposure to UV light, the photocurrent rises rapidly, reaching a steady-state value in several minutes. However, when the UV light is turned off, the current decays slowly following a short rapid decay. The overall decay is not exponential and slows down further over time. The current takes more than 10 h to return to the original dark value. The inset in Figure 2 shows the steady-state photocurrent as a function of light intensity. The current is clearly not a linear function of the intensity.

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