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IRG-III Nugget

Imaging of charge motion in nano-crystalline arrays

Figure: (a) Charging: top image shows the charge just after the voltage is applied to the top electrode and bottom image 40 minutes later; (b) Discharging: top image is after charging for one hour and the bottom image is ten minutes after the voltage is set to zero.

Electrically insulating materials such as polymers, glasses and ceramics are a critically enabling element of nearly all complex electronic devices and components. Insulating films are characterized by a high resistance to the flow of electricity that is often very hard to measure. A new material system under investigation in IRG-III — nanocrystal arrays that consist of nanometer size semiconducting particles — has been found to exhibit unusually high electrical resistance, but microscopic properties quite unlike traditional insulators. In fact, they have such high electrical resistance that they cannot be measured by usual methods. Thus, progress in understanding the origin of these unusual properties requires the development of completely new measurement techniques.

Members of IRG-III have developed a new method for measuring highly resistive films and for following the flow of electrical charge through these materials. The colors in the Figure above show the charge in such a film imaged with an Electric Force Microscope (EFM) near two metal electrodes (outlined in black at top and bottom). Figure (a) shows the charge just after the voltage is applied to the top electrode (top) and 40 minutes later (bottom). Figure (b) shows what is observed after charging for one hour (top) and ten minutes after the voltage is set to zero (bottom). Using these data in conjunction with a suitable diffusion model, the film resistance of this nanoparticle array was determined to be 3x1018 ohms/square, a gigantic value! This work is the result of collaboration between the Bawendi and Kastner groups of IRG- III and M. Tinkham of the Harvard NSEC.

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