Electrical Contacts to Individual Colloidal Semiconductor Nanorods Page: 4 of 21
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observed with optical spectroscopies.s'7 However, individual nanocrystal electrical
behavior is sensitive to local charge environment,8' and two terminal measurements must
be interpreted with care if comparison to an absolute energy scale is desired. It is more
challenging to reconcile the complex charging energy patterns obtained when
nanocrystals are instead addressed by lithographically deposited electrodes with the band
gaps and level structures obtained by these other methods.10
To explore the mechanism behind these differences, we present a systematic study of the
electrical properties of single semiconductor nanocrystals addressed by lithographically
defined electrodes in a three-terminal geometry. Specifically we track variations in
nanorod charging energy as a function of electrode spacing for both bare and insulated
nanocrystals.
We prepared CdTe nanocrystals as reported previously" and deposited them from
toluene solutions onto test chips. Nanocrystals were located with respect to predefined
alignment markers and we used electron beam lithography to create source and drain
contacts (5 nm Cr / 45 nm Pd ). An Au film separated from the device by 10 nm of SiN
served as a back gate. A schematic of a single nanocrystal device is shown in Figure la.
Electrode separation varied from 30 to 100 nm as measured by scanning electron
microscopy (SEM) (Figure lb). All results reported here were measured at 5 K in a Janis
(STVP-100) He4 flow cryostat.
We measured source-drain current as a function of source-drain voltage, as well as gate
voltage. To visualize the results, we plotted the differential conductance (dIsd/dVsd) as a
function of both Vsd and Vg (Fig ic). This highlights regions of zero conductance, which
represent stable single electron charge states on the nanocrystal.2 The energy to charge an
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Trudeau, Paul-Emile; Sheldon, Matt; Altoe, Virginia & Alivisatos, A. Paul. Electrical Contacts to Individual Colloidal Semiconductor Nanorods, article, April 1, 2008; Berkeley, California. (https://digital.library.unt.edu/ark:/67531/metadc900982/m1/4/: accessed April 19, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.