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1.
Nano Lett ; 2020 Jun 23.
Artigo em Inglês | MEDLINE | ID: mdl-32538634

RESUMO

External driving of the Fermion reservoirs interacting with a nanoscale charge-conductor is shown to enhance its mechanical stability during resonant tunneling. This counterintuitive cooling effect is predicted despite the net energy flow into the device. Field-induced plasmon oscillations stir the energy distribution of charge carriers near the reservoir's chemical potentials into a nonequilibrium state with favored transport of low-energy electrons. Consequently, excess heating of mechanical degrees of freedom in the conductor is suppressed. We demonstrate and analyze this effect for a generic model of mechanical instability in nanoelectronic devices, covering a broad range of parameters. Plasmon-induced stabilization is suggested as a feasible strategy to confront a major problem of current-induced heating and breakdown of nanoscale systems operating far from equilibrium.

2.
J Chem Phys ; 152(18): 184112, 2020 May 14.
Artigo em Inglês | MEDLINE | ID: mdl-32414262

RESUMO

Quantum furling and unfurling are inelastic transitions between localized and delocalized electronic states. We predict scenarios where these processes govern charge transport through donor-bridge-acceptor molecular junctions. Like in the case of ballistic transport, the resulting currents are nearly independent of the molecular bridge length. However, currents involving quantum furling and unfurling processes can be controlled by the coupling to vibrations in the intra-molecular and the extra-molecular environment, which can be experimentally tuned. Our study is based on rate equations for exchange of energy (bosons) and particles (fermions) between the molecular bridge and its environment. An efficient algorithm is introduced for a compact representation of the relevant rate equations, which utilizes the redundancies in the rate matrix and the sparsity of the creation and annihilation operators in the molecular Fock space.

3.
Nanotechnology ; 31(31): 314002, 2020 Jul 31.
Artigo em Inglês | MEDLINE | ID: mdl-32259806

RESUMO

Biological electron transfer (ET) is one of the most studied biochemical processes due to its immense importance in biology. For many years, biological ET was explained using the classical incoherent transport mechanism, i.e. sequential hopping. One of the relatively recent major observations in this field is long-range extracellular ET (EET), where some bacteria were shown to mediate electrons for extremely long distances on the micrometer length scales across individual nanowires. This fascinating finding has resulted in several suggested mechanisms that might explain this intriguing EET. More recently, the structure of a conductive G. sulfurreducens nanowire has been solved, which showed a highly ordered quasi-1D wire of a hexaheme cytochrome protein, named OmcS. Based on this new structure, we suggest here several electron diffusion models for EET, involving either purely hopping or several degrees of mixed hopping and coherent transport, in which the coherent part is due to a local rigidification of the protein structure, associated with a decrease in the local reorganization energy. The effect is demonstrated for two closely packed heme sites as well as for longer chains containing up to several dozens porphyrins. We show that the pure hopping model probably cannot explain the reported conductivity values of the G. sulfurreducens nanowire using conventional values of reorganization energy and electronic coupling. On the other hand, we show that for a wide range of the latter energy values, the mixed hopping-coherent model results in superior electron diffusion compared to the pure hopping model, and especially for long-range coherent transport, involving multiple porphyrin sites.

4.
J Chem Phys ; 151(2): 024108, 2019 Jul 14.
Artigo em Inglês | MEDLINE | ID: mdl-31301704

RESUMO

We explore the transport of fermions through a quantum conductor in the presence of contact vibrations. The latter are coupled to charge transfer between the fermion reservoirs and the conductor but remain inert to the charging state of the conductor itself. We derive explicit expressions for charge transfer rates into and out of the conductor which extend the scope of rate theories of inelastic transport to the presence of contact vibrations. Implementing the theory to a simple model with a uniform vibronic coupling at different contact orbitals, we demonstrate and analyze the effect of such vibrations on the charge current. Asymmetry between contact vibrations at the two reservoirs is shown to induce a pronounced current rectification, especially in the limit of floppy (low frequencies) contacts. At high frequencies, vibrational quantization is shown to suppress the effect, in accord with the increasing contact rigidity. This quantum result requires corrections beyond the classical theory of charge hopping.

5.
Nano Lett ; 19(4): 2555-2561, 2019 04 10.
Artigo em Inglês | MEDLINE | ID: mdl-30821465

RESUMO

The promise of the field of single-molecule electronics is to reveal a new class of quantum devices that leverages the strong electronic interactions inherent to subnanometer scale systems. Here, we form Au-molecule-Au junctions using a custom scanning tunneling microscope and explore charge transport through current-voltage measurements. We focus on the resonant tunneling regime of two molecules, one that is primarily an electron conductor and one that conducts primarily holes. We find that in the high bias regime, junctions that do not rupture demonstrate reproducible and pronounced negative differential resistance (NDR)-like features followed by hysteresis with peak-to-valley ratios exceeding 100 in some cases. Furthermore, we show that both junction rupture and NDR are induced by charging of the molecular orbital dominating transport and find that the charging is reversible at lower bias and with time with kinetic time scales on the order of hundreds of milliseconds. We argue that these results cannot be explained by existing models of charge transport and likely require theoretical advances describing the transition from coherent to sequential tunneling. Our work also suggests new rules for operating single-molecule devices at high bias to obtain highly nonlinear behavior.

6.
J Phys Chem Lett ; 9(15): 4139-4145, 2018 Aug 02.
Artigo em Inglês | MEDLINE | ID: mdl-29961322

RESUMO

Weak fluctuations about the rigid equilibrium structure of ordered molecular bridges drive charge transfer in donor-bridge-acceptor systems via quantum unfurling, which differs from both hopping and ballistic transfer, yet static disorder (low frequency motions) in the bridge is shown to induce a change of mechanism from unfurling to hopping when local fluctuations along the molecular bridge are uncorrelated. Remarkably, these two different transport mechanisms manifest in similar charge-transfer rates, which are nearly independent of the molecular bridge length. We propose an experimental test for distinguishing unfurling from hopping in DNA models with different helix directionality. A unified formulation explains the apparent similarity in the length dependence of the transfer rate despite the difference in the underlying transport mechanisms.

7.
Nano Lett ; 18(8): 4727-4733, 2018 08 08.
Artigo em Inglês | MEDLINE | ID: mdl-29923410

RESUMO

Resonant tunneling is an efficient mechanism for charge transport through nanoscale conductance junctions due to the relatively high currents involved. However, continuous charging and discharging cycles of the nanoconductor during resonant tunneling often lead to mechanical instability. The realization of efficient nanoscale electronic components therefore depends to a large extent on the ability to mechanically stabilize them during resonant transport. In this work, we focus on single-molecule junctions, demonstrating that their mechanical stability during resonant transport can be increased by increasing the bias voltage. This counter-intuitive effect is attributed to the energy dependence of the molecule-lead coupling densities, which promote the rate of transport-induced cooling of molecular vibrations at higher voltages. The required energy dependence is characteristic of realistic electrodes (such as graphene), which cannot be modeled within the commonly invoked wide-band approximation. Our research provides new guidelines for the design of mechanically stable molecular devices operating in the regime of resonant charge transport and demonstrates these guidelines while considering realistic features of single-molecule junctions.

8.
ACS Omega ; 3(6): 6224-6229, 2018 Jun 30.
Artigo em Inglês | MEDLINE | ID: mdl-31458804

RESUMO

The semiconductor device industry is constantly challenged by the demands of miniaturization. Therefore, the use of nanomaterials, such as quantum dots (QDs), is expected. At these scales, quantum effects are anticipated under industrial working conditions. Here, we present a simple fabrication method for integrating colloidal coupled QDs as components in a vertical device. Characterization of the fundamental properties of QDs as an ensemble of isolated particles and as layered QD hybrid structures is demonstrated. For the case of layered QD hybrid structures, coupling between dots is on average stronger with typical energy band gaps reduced by more than 200 meV. The shown device offers a straightforward method to measure and establish a strong coupling transport system under ambient conditions.

9.
Chem Sci ; 7(2): 1535-1542, 2016 Feb 01.
Artigo em Inglês | MEDLINE | ID: mdl-28808530

RESUMO

Experiments on hole transfer in DNA between donor and acceptor moieties revealed transfer rates which are independent of the molecular bridge length (within experimental error). However, the physical origin of this intriguing observation is still unclear. The hopping model implies that the hole propagates in multiple steps along the bridge from one localized state to another, and therefore the longer the bridge, the slower the transfer. This can explain weak length-dependence but not a length-independent transfer rate. We show that the rigid molecular structure of a poly-A bridge supports single step transitions from a localized hole state to delocalized states, spread over the entire bridge. Since propagation to the bridge end is a single step process (termed quantum unfurling) the transfer rate becomes independent of the bridge length. This explanation is consistent with experimental results, and emphasizes the importance of structural order in charge transfer through bio-molecular systems.

10.
J Phys Chem Lett ; 6(9): 1521-8, 2015 May 07.
Artigo em Inglês | MEDLINE | ID: mdl-26263306

RESUMO

Colloidal quantum dots are free-standing nanostructures 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.

11.
J Phys Chem Lett ; 6(3): 470-6, 2015 Feb 05.
Artigo em Inglês | MEDLINE | ID: mdl-26261965

RESUMO

The slow response of electronic components in junctions limits the direct applicability of pump-probe type spectroscopy in assessing the intramolecular dynamics. Recently the possibility of getting information on a sub-picosecond time scale from dc current measurements was proposed. We revisit the idea of picosecond resolution by pump-probe spectroscopy from dc measurements and show that any intramolecular dynamics not directly related to charge transfer in the current direction is missed by current measurements. We propose a pump-probe dc shot noise spectroscopy as a suitable alternative. Numerical examples of time-dependent and average responses of junctions are presented for generic models.

12.
Nano Lett ; 14(11): 6244-9, 2014 Nov 12.
Artigo em Inglês | MEDLINE | ID: mdl-25259800

RESUMO

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.

13.
Phys Chem Chem Phys ; 14(40): 13835-40, 2012 Oct 28.
Artigo em Inglês | MEDLINE | ID: mdl-22825482

RESUMO

The question whether dissipative bio-molecular systems can support efficient coherent (phase-conserving) charge transport is raised again following recent experiments on electron-energy transfer in bio-molecules. In this work we formulate conditions under which the current due to coherent ballistic resonant charge transport through DNA molecular junctions can be measured in spite of coupling to the dissipative environment.


Assuntos
DNA/química , Elétrons , Sequência de Bases , Transporte de Elétrons
14.
J Chem Phys ; 136(4): 044107, 2012 Jan 28.
Artigo em Inglês | MEDLINE | ID: mdl-22299861

RESUMO

Within a generic model, we discuss the possibility of coherent control of charge fluxes in unbiased molecular junctions. The control is induced by resonances between the Rabi frequency due to a pumping laser field and internal characteristic frequencies of pre-designed molecular donor-bridge-acceptor complexes. Two models are considered: a coherently controlled molecular charge pump and a molecular switch. The study generalizes previous consideration of light induced current [M. Galperin and A. Nitzan, Phys. Rev. Lett. 95, 206802 (2005)] and of a molecular electron pump [R. Volkovich and U. Peskin, Phys. Rev. B 83, 033403 (2011)] and accounts for the coherently driven charge transport in an unbiased molecular junction with symmetric coupling to leads. Numerical examples demonstrate the feasibility of the control mechanism for realistic junctions parameters.

15.
Phys Chem Chem Phys ; 13(32): 14333-49, 2011 Aug 28.
Artigo em Inglês | MEDLINE | ID: mdl-21776449

RESUMO

We show that individual vibrational modes in single-molecule junctions with asymmetric molecule-lead coupling can be selectively excited by applying an external bias voltage. Thereby, a non-statistical distribution of vibrational energy can be generated, that is, a mode with a higher frequency can be stronger excited than a mode with a lower frequency. This is of particular interest in the context of mode-selective chemistry, where one aims to break specific (not necessarily the weakest) chemical bond in a molecule. Such mode-selective vibrational excitation is demonstrated for two generic model systems representing asymmetric molecular junctions and/or scanning tunneling microscopy experiments. To this end, we employ two complementary theoretical approaches, a nonequilibrium Green's function approach and a master equation approach. The comparison of both methods reveals good agreement in describing resonant electron transport through a single-molecule contact, where differences between the approaches highlight the role of non-resonant transport processes, in particular co-tunneling and off-resonant electron-hole pair creation processes.

16.
J Chem Phys ; 133(8): 081102, 2010 Aug 28.
Artigo em Inglês | MEDLINE | ID: mdl-20815551

RESUMO

In a nanoscale molecular junction at finite bias voltage, the intramolecular distribution of vibrational energy can strongly deviate from the thermal equilibrium distribution and specific vibrational modes can be selectively excited in a controllable way, regardless of the corresponding mode frequency. This is demonstrated for generic models of asymmetric molecular junctions with localized electronic states, employing a master equation as well as a nonequilibrium Green's function approach. It is shown that the applied bias voltage controls the excitation of specific vibrational modes by tuning the efficiency of vibrational cooling processes due to energy exchange with the leads.

17.
J Chem Phys ; 132(12): 124106, 2010 Mar 28.
Artigo em Inglês | MEDLINE | ID: mdl-20370113

RESUMO

A quantum sampling algorithm for the interpolation of diabatic potential energy matrices by the Grow method is introduced. The new procedure benefits from penetration of the wave packet into classically forbidden regions, and the accurate quantum mechanical description of nonadiabatic transitions. The increased complexity associated with running quantum dynamics is reduced by using approximate low order expansions of the nuclear wave function within a Multi-configuration time-dependent Hartree scheme during the Grow process. The sampling algorithm is formulated and applied for three representative test cases, demonstrating the recovery of analytic potentials by the interpolated ones, and the convergence of a dynamic observable.

18.
J Chem Phys ; 132(11): 114116, 2010 Mar 21.
Artigo em Inglês | MEDLINE | ID: mdl-20331290

RESUMO

Defining the conditions for coherent site-directed transport from an electron donor to a specific acceptor through tunneling barriers in a network of multiple donor/acceptors sites is an important step toward controlling electronic processes in molecular networks. The required analysis is most challenging since the entire network in essentially involved in coherent transport. In this work we introduce an efficient approach for formulating an effective donor/acceptor coupling in terms of the entire network parameters. The approach is based on implementation of Feshbach projection operators to map the entire network Hamiltonian onto a subspace defined by two specific donor and acceptor sites. This nonperturbative approach enables to define regimes of network parameters in which the effective donor-acceptor coupling is optimal. This is demonstrated numerically for simple models of molecular networks.

19.
J Chem Phys ; 129(3): 034501, 2008 Jul 21.
Artigo em Inglês | MEDLINE | ID: mdl-18647025

RESUMO

The ability to control electronic tunneling in complex molecular networks of multiple donor/acceptor sites is studied theoretically. Our past analysis, demonstrating the phenomenon of site-directed transport, was limited to the coherent tunneling regime. In this work we consider electronic coupling to a dissipative molecular environment including the effect of decoherence. The nuclear modes are classified into two categories. The first kind corresponds to the internal molecular modes, which are coupled to the electronic propagation along the molecular bridges. The second kind corresponds to the external solvent modes, which are coupled to the electronic transport between different segments of the molecular network. The electronic dynamics is simulated within the effective single electron picture in the framework of the tight binding approximation. The nuclear degrees of freedom are represented as harmonic modes and the electronic-nuclear coupling is treated within the time-dependent Redfield approximation. Our results demonstrate that site-directed tunneling prevails in the presence of dissipation, provided that the decoherence time is longer than the time period for tunneling oscillations (e.g., at low temperatures). Moreover, it is demonstrated that the strength of electronic coupling to the external nuclear modes (the solvent reorganization energy) controls the coherent intramolecular tunneling dynamics at short times and may be utilized for the experimental control of site-directed tunneling in a complex network.


Assuntos
Elétrons , Modelos Moleculares , Simulação por Computador , Transporte de Elétrons , Estudos de Viabilidade , Temperatura , Fatores de Tempo
20.
J Chem Phys ; 129(24): 244709, 2008 Dec 28.
Artigo em Inglês | MEDLINE | ID: mdl-19123528

RESUMO

Possible mechanisms for charge-transport-induced dissociation in donor-bridge-acceptor complexes are studied. Two mechanisms for dissociation at the molecular bridge are captured within a simple model of an anharmonic bridge vibration coupled nonlinearly to an electronic degree of freedom. A direct mechanism is associated with vibronic excitations to the nuclear continuum and an alternative dissociation mechanism involves intermediate quasibound vibrational states (Feshbach resonances). The two different mechanisms of charge-transport-induced dissociation are analyzed and their interplay as a function of the system parameters is examined. A parameter regime is suggested where the phenomenon should be experimentally accessible.

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