Mechanical and Electrical Properties of CdTe Tetrapods Studied byAtomic Force Microscopy Page: 2 of 13
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CdSe and CdTe nanocrystals possess interesting photo-electronic properties (1) that make
them interesting materials for solar cell applications (2). Branched tetrapods with a CdSe core
and terminal CdTe branches are also important due to unusual charge-separation properties (3-5).
In addition to electronic properties, the peculiar shape of the nanocrystals might confer to these
materials interesting mechanical properties, such as large compliance and toughness, with
potential applications as shock absorbers.
In this paper we present a study of the mechanical properties of individual CdTe
tetrapods, including adhesion, compliance and resistance to fracture. We will also present some
results on the relationship between mechanical and electronic properties.
CdTe tetrapods were synthesized using colloid-chemistry (2). The crystals were suspended
in toluene with a concentration of 1 M. The solution was used without further purification. For
measurements of mechanical properties the samples were spin-casted from the toluene suspension
onto a silicon wafer at a speed of 8000 RPM, followed by washing with pure methanol. For
electrical measurements the tetrapods were deposited on a (111) oriented gold film on a glass
substrate following a similar spin casting procedure. The films were cleaned with solvents and
annealed with a butane flame prior to deposition.
To study the mechanical properties of the tetrapods the force-volume (FV) technique was
used (6-8). In this technique force-distance curves are acquired at each pixel of a selected area
imaged by the AFM. From these curves adhesion and elasticity maps can be produced. The
measurements were carried out with an Asylum Research MFP-3D AFM. The piezo scanner was
calibrated in x, y, and z directions using NIST certified calibration gratings (MikroMasch). The
spring constants of rectangular Silicon cantilevers (Olympus) were determined from the thermal
noise spectrum and found to range from 1.2 to 1.7 N/m. All experiments were performed under
ambient conditions (21 10C, 30% relative humidity). A loading speed of 200 nm/s was used in
all force-distance measurements.
The electrical measurements were carried out in an RHK AFM operating in ultra-high
vacuum using cantilevers with a conductive TiN coating (9). The spring constant of these
cantilevers was measured to be 2.5 N/m using the method of Sader et al. (10).
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Fang, Liang; Park, Jeong Young; Cui, Yi; Alivisatos, Paul; Shcrier, Joshua; Lee, Byounghak et al. Mechanical and Electrical Properties of CdTe Tetrapods Studied byAtomic Force Microscopy, article, August 30, 2007; Berkeley, California. (digital.library.unt.edu/ark:/67531/metadc897391/m1/2/: accessed January 20, 2019), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.