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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. 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 structure 17−19 and electron−phonon coupling. 20 Transport measurements 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.

Colloidal quantum dots are free-standing nanostruc-tures with chemically tunable electronic properties. In this work, we consider a new STM tip−double quantum dot (DQD)−surface setup with a unique connectivity, in which the tip is coupled to a single dot and the coupling to the surface is shared by both dots. Our theoretical analysis reveals a unique negative differential resistance (NDR) effect attributed to destructive interference during charge transfer from the DQD to the surface. This NDR can be used as a sensitive probe for interdot interactions in DQD arrays. T he field of electronic transport through different quantum dots arrays 1 has been given much attention in recent years both experimentally and theoretically in search of different nonlinear effects. The focus has shifted between several key phenomena, such as coulomb blockade, 2−5 spin blockade, 6−9 Franck−Condon blockade, 10,11 and negative differential resistance (NDR). 12,13 The observation of NDR, where the current decreases with increasing bias potential, has found various rationalizations and was studied in single quantum dots (SQDs), double quantum dots (DQDs), and triple quantum dots (TQDs) systems. A comprehensive review by Hettler et al. 14 elucidates extensively the main mechanisms leading to NDR within SQD and DQD systems. In the extreme case, NDR can lead to vanishingly small currents. This is the case when dark states are involved, that is, when quantum mechanical superposition leads to decoupling between potentially conducting states and the leads. The effect of dark states, initially researched for the case of photon excitations, 15−18 was studied at length in recent years for the analogue electronic cases of TQD 19−24 and molecular junctions , 25−27 where internal transport pathways interfere destructively within the device. In colloidal SQD or DQD, the internal structure does not support destructively interfering pathways. However, destructive interference of different pathways at the DQD−surface interface can give rise to a yet unexplored dark state, as discussed below. Colloidal quantum dots (CQDs) 28,29 with their chemically tunable electronic properties are of special interest for numerous electronic and optoelectronics applications. Ongoing research deals with the extent to which single dot properties 30,31 and the interactions between dots in an array 1,32 are of significance in relation to the observed transport properties and nonlinear effects in particular. Yet, the interdot interactions are difficult to assess, specifically when CQDs are concerned, because the interaction between CQDs is often controlled by their surface chemistry and by the organic ligands that link between the dots. In this work, we study nonlinear effects in transport through arrays of CQDs. We address an extension of the well-established experimental studies of transport through SQDs 33−39 to the case of coupled DQDs, as proposed in ref 40. The new setup of STM tip−DQD−surface 40 is shown to give rise to a new NDR effect, attributed to destructive interference during charge transport through the DQD. Our theoretical analysis below demonstrates how a unique dark state and the related NDR phenomenon lead to an appreciable nonlinear feature in the current through the DQD, which reveals the magnitude of interdot electronic interactions. The model for the STM tip−DQD−surface configuration was recently introduced in ref 40 and is reviewed here for clarity. Each dot is represented by a single localized spin orbital, given the following assumptions: (i) Degeneracies of the neutral quantum dot orbitals are removed upon charging by an extra electron, 41,42 (ii) multiple charging of each dot is excluded due to intradot Coulomb interaction, and (iii) spin is conserved during transport, and a single spin model is sufficient. The single dot orbitals are modeled here as 3D Gaussians, χ A (r) = (2σ 2 π) −3/4 e −(r−R A) 2 /(4σ 2) and χ B (r) = (2σ 2 π) −3/4 e −(r−R B) 2 /(4σ 2) , where R A and R B are the dots center of mass coordinates and r is the electronic coordinate. The dots dimensions are captured in σ, the standard deviation of the respective probability distributions, |χ A/B (r)| 2. Setting R A ≡ (0,0,q/2) and R B ≡ (0,0,−q/2), the interdot distance, |R A − R B | = q, defines the overlap between these two localized orbitals, s(q) = ∫ dr χ A (r)χ B (r) = e −q 2 /(8σ 2) , and the effective single-electron Hamiltonian matrix in the basis of χ A/B (r) is assumed to be of a generic form = = γ γ − −

High concentration PV systems usually prefer tandem III-V cells to Si cells, due to the much lower conversion efficiency of the latter. We re-examine the efficiency achievable with Si Vertical Multi-Junction (VMJ) cells consisting of series-connected vertical p-n junctions within a single cell. A comprehensive 2D numerical analysis of a Si vertical junction has been performed, over a wide range of design parameters and concentration levels. The results show outstanding performance potential under high concentration of 1,000 suns and higher, with efficiencies above 29% under realistic (non-ideal) assumptions. This compares to reported efficiencies of about 20% for both real cells and previous simulations using realistic assumptions. This difference may be attributed to two effects: a better representation of the active layer photoconductivity, which lowers drastically the cell's series resistance under high concentration; and optimization of the junction dimensions without restri...

Colloidal quantum dots (CQDs) are free-standing nano-structures with chemically tunable electronic properties. This tunability offers intriguing possibilities for nano-electromechanical devices. In this work, we consider a nano-electromechanical nonvolatile memory (NVM) device incorporating a triple quantum dot (TQD) cluster. The device operation is based on a bias induced motion of a floating quantum dot (FQD) located between two bound quantum dots (BQDs). The mechanical motion is used for switching between two stable states, “ON” and “OFF” states, where ligand-mediated effective interdot forces between the BQDs and the FQD serve to hold the FQD in each stable position under zero bias. Considering realistic microscopic parameters, our quantum-classical theoretical treatment of the TQD reveals the characteristics of the NVM.