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What happens to photogenerated charges in semiconductor nanowires?

Author(s) with affiliations: 

Students: Mizrain Ramirez, Kleyser Agueda Lopez, Malik Ko

Faculty Advisor:  M. Wu 

How? We use optical microscopy to determine the dynamics of photo-generated charges in individual semiconductor nanowire hetero-structures. 

Ultrafast transient absorption and photoluminescence microscopy maps were obtained from an individual CdSxSe1-x nanowire heterostructure produced through sequential source physical vapor deposition.  Photoluminescence mapping revealed shifts in the peak emission wavelength as a function of position, indicating axial compositional variation, and significant variations in emission intensity.  Time dependent transient absorption spectra exhibited features attributed to bandgap normalization and state filling effects.  The state filling feature is well fit by a bi-exponential function with lifetimes of ~ 10 ps and ~ 200 ps, values typically reported for charge trapping at surface or defects within the nanowire.  These measurements clearly illustrate the spatial inhomogeneity of nanowire properties, which may have implications for performance of devices featuring these materials.

Light emission studies show variations in peak wave-length, emission intensity along nanowire.


Ultrafast transient absorption data show rapid (< 10 -10 s)  relaxation of electrons from excited states

Charges are rapidly trapped at defect sites, defect densities vary significantly along the nanowire