Force Microscopy

We study mesoscopic and nanoscale structures with atomic- and electric-force microscopy. In our lab, we have a Veeco Enviroscope, which allows to carry out measurements in vacuum or in inert atmosphere (argon, nitrogen), as well as at temperatures between room temperature and 500K. We are currently installing a cooling setup to extend this temperature range.

Correlation of nanoscale patterns and local charge transport in PbSe nanocrystal arrays

Two-dimensional PbSe nanocrystal arrays on silicon nitride membranes were investigated using electrostatic force microscopy (EFM) and transmission electron microscopy (TEM). Changes in lattice and transport properties upon annealing in a vacuum were revealed. Local charge transport behavior was directly imaged by EFM and correlated to nanopatterns observed with TEM. Charge transport through nanochannels in complex two-dimensional nanocrystal networks was identified. Our results demonstrate the importance of measurements of local transport details complementary to the conventional current-voltage (I-V) measurements.

Monolayer Suppression of Transport Imaged in Annealed PbSe Nanocrystal Arrays

We use correlated electrostatic force, transmission electron, and atomic force microscopy (EFM, TEM, and AFM) to visualize charge transport in monolayers and up to five layers of PbSe nanocrystal arrays drop-cast on electrode devices. Charge imaging reveals that current paths are dependent on the locally varying thickness and continuity of an array. Nanocrystal monolayers show suppressed conduction compared to bilayers and other multilayers, suggesting a departure from linear scaling of conductivity with array thickness. Moreover, multilayer regions appear electrically isolated if connected solely by a monolayer. Partial suppression is also observed within multilayer regions that contain narrow junctions only several nanocrystals wide. High-resolution TEM structural imaging of the measured devices reveals a larger reduction of inter-nanocrystal spacing in multilayers compared to monolayers upon vacuum-annealing, offering a likely explanation for the difference in conductivity between these two cases.

For more details on correlated measurements of PbSe nanocrystal arrays with AFM, EFM and TEM:

Zonghai Hu, Michael D. Fischbein, and Marija Drndic
“Local charge transport in two-dimensional PbSe nanocrystal arrays studied by electrostatic force microscopy”
Nano Letters 5 (7), 1463-1468, 2005.

Michael Fischbein, Matthew Puster, Marija Drndic
“Monolayer suppression of transport imaged in annealed PbSe nanocrystal arrays”
Nano Letters, 10 (6), pp 2155-2161, 2010.