Charge Transport-Induced Recoil and Dissociation in Double Quantum Dots Roni Pozner, †,§,∥ Efrat Lifshitz, †,§,∥ and Uri Peskin* ,†,‡,∥ † Schulich Faculty of Chemistry, ‡ the Lise Meitner Center for Computational Quantum Chemistry, § Solid State Institute, and ∥ Russell Berrie Nanotechnology Institute, Technion-Israel Institute of Technology, Haifa 32000, Israel * S Supporting Information ABSTRACT: Colloidal quantum dots (CQDs) are free-standing nanostructures with chemically tunable electronic properties. This combination of properties offers intriguing new possibilities for nanoelectromechanical devices that were not explored yet. In this work, we consider a new scanning tunneling microscopy setup for measuring ligand-mediated effective interdot forces and for inducing motion of individual CQDs within an array. Theoretical analysis of a double quantum dot structure within this setup reveals for the first time voltage-induced interdot recoil and dissociation with pronounced changes in the current. Considering realistic microscopic parameters, our approach enables correlating the onset of mechanical motion under bias voltage with the effective ligand-mediated binding forces. KEYWORDS: Colloidal quantum dots, double quantum dots, electromechanical response, charge transport, scanning tunneling microscopy D uring past years, interest in colloidal quantum dots (CQDs) 1,2 has increased dramatically, as they offer new propositions to a variety of applications such as electronic and light emitting devices, 3,4 photovoltaic cells, 5−7 and biological labeling. 8,9 Unlike rigid structures made by lithography techniques, 10,11 CQDs offer an intriguing possibility of inducing mechanical motion of the dots themselves on the nanoscale. This possibility was not yet explored because the mechanical forces between CQDs and between CQDs to surfaces are not easy to characterize, owing to the organic ligand capping that controls the interdot interactions. Yet, the ability to manipulate the ligands using “wet chemistry” techniques 1 suggests new possibilities for electromechanical devices that exploit the unique properties of capped CQDs. Recent studies suggest that charge transport in granular materials, two-dimensional arrays, and three-dimensional assemblies 12 depends on the single CQDs properties as well as on their chemical, electronic, or magnetic coupling. 13 Hardly anything is known about the mechanical coupling between CQDs. Ligands should play a key role in this context, but only a few studies account for their structure at the atomistic level, 14−16 and their effect on the mechanical forces between dots was not yet considered. In this work, we propose a new setup for inducing mechanical motion of CQDs and for characterizing the effective forces that control their mechanical response. The motion is induced and simultaneously evaluated by applying bias voltage and measuring the currents through coupled CQDs in a scanning tunneling microscopy (STM) tip−dot−substrate architecture. Charge transport through a single quantum dot has already been characterized using scanning tunneling spectroscopy, revealing the discrete electronic levels struc- ture 17−19 and electron−phonon coupling. 20 Transport meas- urements through large quantum dots arrays 21−25 revealed the importance of interdot interactions and order/disorder on the transport properties of such arrays. The intermediate regime of several interacting CQDs in which specific interdot interactions could be manifested in transport measurements was studied much less. As a prototype system, we consider double quantum dot (DQD) 26−28 structures in an STM tip−DQD−substrate architecture (see Figure 1). The model introduced below accounts explicitly for the dependence of electronic tunneling matrix elements and electronic correlation terms (Coulomb and exchange) on the distance between the dots and therefore elucidates the relation between electronic transport and mechanical motion within the DQD for parameters chosen in consistency with typical dimensions of CQDs structures. Using a mixed quantum-classical approach to the coupled electro- mechanical dynamics, we demonstrate correlation between the measured current and the mechanical motion, which enables to estimate the effective ligand-mediated force between the dots. For a given force, the applied voltage controls the mechanical response, which varies from voltage-induced recoil to DQD dissociation. Received: July 8, 2014 Revised: September 22, 2014 Published: September 26, 2014 Letter pubs.acs.org/NanoLett © 2014 American Chemical Society 6244 dx.doi.org/10.1021/nl502562g | Nano Lett. 2014, 14, 6244−6249