Tunneling mechanism and contact mechanics of colloidal nanoparticle assemblies.

Nanoparticle assemblies with thiol-terminated alkyl chains are studied by conducting atomic force microscopy (c-AFM) regarding their use as strain gauges for touch-sensitive panels. Current-force spectroscopy is used as a characterization tool complementary to the macroscopic setup since it allows a bias to be applied to a limited number of junctions, overcoming the Coulomb blockade energy and focusing on the contact electromechanics and the transport mechanism across the ligand. First, transition voltage spectroscopy is applied with varying force to target the underlying tunneling mechanism by observing whether the transition between the ohmic and exponential current-voltage behavior is force-dependent. Secondly, current-force spectroscopy in the ohmic range below the transition voltage is performed. The current-force behavior of the AFM probe in contact with a nanoparticle multilayer is associated with the spread of force and current within the nanoparticle lattice and at the level of adjacent particles by detailed contact mechanics treatment. The result is twofold: concerning the architecture of sensors, this work is a sample case of contact electromechanics at scales ranging from the device scale down to the individual ligand molecule. Regarding transport across the molecule, the vacuum tunneling mechanism is favored over the conduction by coherent molecular states, which is a decision-making aid for the choice of ligand in applications.

Charge Blinking Statistics of Semiconductor Nanocrystals Revealed by Carbon Nanotube Single Charge Sensors.

We demonstrate the relation between the optical blinking of colloidal semiconductor nanocrystals (NCs) and their electrical charge blinking for which we provide the first experimental observation of power-law statistics. To show this, we harness the performance of CdSe/ZnS NCs coupled with carbon nanotube field-effect transistors (CNTFETs), which act as single charge-sensitive electrometers with submillisecond time resolution, at room temperature. A random telegraph signal (RTS) associated with the NC single-trap charging is observed and exhibits power-law temporal statistics (τ(-α), with α in the range of ∼1-3), and a Lorentzian current noise power spectrum with a well-defined 1/f(2) corner. The spectroscopic analysis of the NC-CNTFET devices is consistent with the charging of NC defect states with a charging energy of Ec ≥ 200 meV. These results pave the way for a deeper understanding of the physics and technology of nanocrystal-based optoelectronic devices.