Limits...
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 at three UV intensities in vacuum. Same bias voltage and UV wavelength as in Figure 2. The steady-state currents have not been reached after about 5 h. The wire is kept in vacuum until air is let in after about 12 h (marked by a vertical arrow). The current at t = 0 is higher than that in Figures 1 and 2 because the UV was turned on before the dark current had reached its minimum.
© Copyright Policy - open-access
Related In: Results  -  Collection

License
getmorefigures.php?uid=PMC3211499&req=5

Figure 3: Photoconductivity at three UV intensities in vacuum. Same bias voltage and UV wavelength as in Figure 2. The steady-state currents have not been reached after about 5 h. The wire is kept in vacuum until air is let in after about 12 h (marked by a vertical arrow). The current at t = 0 is higher than that in Figures 1 and 2 because the UV was turned on before the dark current had reached its minimum.

Mentions: A very different photoresponse is observed in vacuum. Figure 3 shows the photocurrent at three different UV intensities. Upon exposure to UV illumination, a short rapid photocurrent increase is observed for all the three intensities, followed by a slow increase. Steady state is not reached even after 5 h, although the photocurrents are already 20 times as large as those observed in air for the same intensity. When the light is turned off, the current shows only a small decay.


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 at three UV intensities in vacuum. Same bias voltage and UV wavelength as in Figure 2. The steady-state currents have not been reached after about 5 h. The wire is kept in vacuum until air is let in after about 12 h (marked by a vertical arrow). The current at t = 0 is higher than that in Figures 1 and 2 because the UV was turned on before the dark current had reached its minimum.
© Copyright Policy - open-access
Related In: Results  -  Collection

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

Figure 3: Photoconductivity at three UV intensities in vacuum. Same bias voltage and UV wavelength as in Figure 2. The steady-state currents have not been reached after about 5 h. The wire is kept in vacuum until air is let in after about 12 h (marked by a vertical arrow). The current at t = 0 is higher than that in Figures 1 and 2 because the UV was turned on before the dark current had reached its minimum.
Mentions: A very different photoresponse is observed in vacuum. Figure 3 shows the photocurrent at three different UV intensities. Upon exposure to UV illumination, a short rapid photocurrent increase is observed for all the three intensities, followed by a slow increase. Steady state is not reached even after 5 h, although the photocurrents are already 20 times as large as those observed in air for the same intensity. When the light is turned off, the current shows only a small decay.

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