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Type of Document Dissertation Author Protasenko, Vladimir Author's Email Address vprotase@nd.edu URN etd-07092008-145131 Title Electro-optical properties of CdSe nanowires Degree Doctor of Philosophy Department Chemistry and Biochemistry Advisory Committee
Advisor Name Title Mark Alber Committee Chair Dani Meisel Committee Member Gregory Hartland Committee Member Masaru Kuno Committee Member Steven Corcelli Committee Member Keywords
- intermittency
- blinking
- emission
- nanowires
- CdSe
Date of Defense 2008-07-08 Availability unrestricted Abstract This Thesis describes the results of electro-optical experiments performed onsolution grown CdSe nanowires (NWs). TEM images reveal that such NWs have
diameters between 6-40 nm, are highly crystalline and exhibit large aspect ratios (>1000,
length/diameter). The morphologies of these NWs range from straight to hyper-branched.
Absorption cross-sections determine how efficiently a material absorbs light. In
this Thesis we calculate single NW absorption cross-sections based on UV-VIS linear
extinction, TEM, and inductively coupled plasma atomic emission spectroscopy
experiments. Obtained numbers compare well with theoretical estimates, having order of
magnitude values of 10-11 cm2 per 1 μm length of a 10 nm diameter NW.
Synthesized CdSe NWs are emissive and are easily detectable at the single wire
level. A surprising observation from these experiments is the modulation of the NW emission intensity by applied electrical fields. Specifically, the part of the wire closest to
the positive electrode exhibits up to a 10x increase in intensity. Simultaneous quenching
of identical magnitude is detected on the other side of the wire. Our current working
hypothesis is that mobile electrons driven by the external electrical field passivate
emission quenching centers resulting in local emission enhancement. Similarly, the
smaller density of electrons on the other side of the wire yields emission quenching.
To confirm the existence of these mobile carriers we perform electrophoresis
measurements on NWs. Observed single wire translational and rotational dynamics can
be explained by mobile carriers residing on or within the NWs. A lower limit for the
carrier density of NWs in oleic acid is estimated to be ~1 charge/μm.
Since light absorption results in both NW emission as well as the photogeneration
of carriers, photoconductivity measurements are also possible. While doing such
measurements, we unexpectedly discovered that randomly oriented NW networks could
exhibit a significant photocurrent polarization anisotropy with values of ρ=0.25 (σ=0.04)
under excitation with linearly polarized light. The remarkable conclusion from this result
is that polarization sensitive devices can be built from random NW networks without the
need to align component wires. To explain these results a simple geometric model has
also been developed.
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