Photoelectronic transport imaging of individual semiconducting carbon nanotubes
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Abstract:
Photoconductivity in individual semiconducting single-wall carbon nanotubes was investigated using a confocal scanning optical microscope. The magnitude of the photocurrent was found to increase linearly with the laser intensity, and to be maximum for parallel orientation between the light polarization and the tube axis. Larger currents were obtained upon illuminating the tubes at 514.5 nm in comparison to those at 647.1 nm, consistent with the semiconducting tubes having a resonant absorption energy at the former wavelength. Moreover, the determination of the photoresponse as a function of position along single nanotubes has proven to be a useful tool to monitor local electronic structure effects.Keywords:
Photoconductivity
Photocurrent
This chapter contains sections titled: Photocurrent Generation and Switching in Neat Semiconductors Photocurrent Switching in MIM Organic Devices Photocurrent Switching in Semiconducting Composites Photocurrent Switching in Surface-Modified Semiconductors References
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Ultra-long AlN nanowire arrays are prepared by chemical vapor deposition, and the photoconductive performances of individual nanowires are investigated in our self-built measurement system. Individual ultra-long AlN nanowire (UAN) exhibits a clear photoconductive effect under different excited lights. We attribute the positive photocurrent response of individual UAN to the dominant molecular sensitization effect. It is found that they have a much faster response speed (a rise and decay time of about 1 ms), higher photocurrent response (2.7×106), and more reproductive working performance (the photocurrent fluctuation is lower than 2%) in the air environment. Their better photoconductive performances are comparable to many nanostructures, which are suggested to be a candidate for building promising photosensitive nanodevices in the future.
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The primary photocurrent of a‐Si:H prepared by glow discharge of SiH4 was investigated with VIDICON and ELECTROPHOTOGRAPHIC techniques. The photoresponse current depends greatly upon the device structures used and an excellent photoconductivity gain of unity is attainable if the leakage current in the dark is excluded. All results in this study can be interpreted in terms of field‐independent efficiency for carrier‐generation in a‐Si:H.
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The collected photocurrent imaging technique measures the total amount of photocurrent that can be collected from every region of a solar cell. The technique is based on the recently introduced differential photocurrent collection efficiency imaging method, which uses luminescence images taken under varying bias conditions and a constant illumination. The differential photocurrent collection efficiency provides information about how the cell or module current reacts to local changes of the generated photocurrent. We explain in detail how certain assumptions and measurements of the photocurrent collection efficiency under different illumination conditions can extend the differential information we obtain from the differential photocurrent collection efficiency to get information about the total amount of photocurrent collected from each specific region of a solar cell. The method was demonstrated on a thin film Cu(In, Ga)Se 2 cell, showing quantitatively how series resistance reduce the collection of photocurrent.
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WO3 photoanodes with remarkable photocurrent densities are presented. These photoanodes were prepared from three different commercially available WO3 nanopowders. Doctor blading of the nanopowders followed by a short annealing in air led to nanostructured films. The best photoanode showed a photocurrent density of 3.5 mA cm-2 at 1.23 V vs. RHE in 1 m CH3 SO3 H under AM 1.5 G illumination (100 mW cm-2 ), surpassing values reported so far for bare WO3 photoanodes. The study also showed that the photocurrent was strongly dependent on the electrolyte, indicating oxidation of the electrolyte rather than of water. Oxygen evolution measurements performed in different electrolytes revealed that the amounts of oxygen were highly dependent on the electrolyte. By comparing the photocurrent values in the different electrolytes with the amount of evolving oxygen, it was found that the electrolyte producing the highest photocurrent was the electrolyte with the lowest oxygen evolution. Stability measurements showed that the more oxygen is produced, the less stable is the photoanode. These results clearly underline the difficulty to correlate the photocurrent values with oxygen evolution, drawing the attention to one of the major limitations of photoelectrochemical water splitting.
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We investigated the photoconductivity effect observed in a p-type SnTe quantum well in the temperature range of 1.9–100 K. The negative photoconductivity effect is observed for temperatures below 4 K, and it is strongly dependent on the light wavelength. A systematic analysis of the photoconductivity indicates that the origin of the negative photoconductivity is not related to the topological surface states but rather to the reduction of carrier mobility when the SnTe quantum well is illuminated with energies above 2 eV.
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Photoconductivity in nanocrystal films has been previously characterized, but memory effects have received little attention despite their importance for device applications. We show that the magnitude and temperature dependence of the photocurrent in CdSe/ZnS core-shell nanocrystal arrays depends on the illumination and electric field history. Changes in photoconductivity occur on a few-hour timescale, and subband gap illumination of nanocrystals prior to measurements modifies the photocurrent more than band gap illumination. The observed effects can be explained by charge traps within the band gap that are filled or emptied, which may alter nonradiative recombination processes and affect photocurrent.
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The persistent-photocurrent effect in bulk, semi-insulating GaAs has been studied as a function of temperature. The persistent photocurrent is activated by illumination with 1.13-eV light and is preceded by an enhanced photocurrent during long-term illumination after the initial photocurrent has decayed in a quenching process similar to that of the deep donor EL2. Both the magnitude of the persistent photocurrent, and its decay rate are strongly temperature dependent and show significant reduction above 40 K. These results indicate that while the metastable transformation of EL2 is required for the activation of the persistent photocurrent, another unidentified metastable process after this is also required.
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The photoelectrochemistry of α Fe 2O 3 nanocrystalline thin films with different thickness in KCl solution was studied. It is found that the photocurrent response was decreased with the film thinkness. The decrease of photocurrent responses of thicker films is explained in terms of the electric resistance effect and recombination effect. For thinner α Fe 2O 3 film (50 μm) no difference was found for the photocurrent response under frontwall (FW) and backwall (BW) illuminations, while for thicker α Fe 2O 3 film (100 μm) the photocurrent response was larger under BW illumination than that under FW illumination.
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