Investigation of elastic and inelastic electronic transport through single molecules on metal and semiconductor surfaces using scanning tunnelling microscopy.
Prof. Dr. Jose Ignacio Pascual, Freie Universität Berlin
This project aims at bridging the research on single molecule tunnelling spectroscopy with research on electron transport through single-molecule junctions by using a low temperature scanning tunnelling microscope (STM) tip to approach to contact a single molecule, investigating the steps leading to the formation of a leadmolecule-lead junction. We will investigate stable olecule-electrode contacts formed by dative bonds between lone-pair electrons of amino, pyridine and nitrile molecular end-groups with oxygen functionalised copper surfaces and with atomic dangling bonds of silicon surfaces. We study i) the tunnel regime, using tunnel spectroscopy (elastic and inelastic) to reveal structure, electronic configuration and phonon structure, ii) the high conductance regime, to follow the transition to contact and the evolution of molecular resonances toward conducting bands, and iii) the contact regime, to identify transport mechanisms (elastic vs. inelastic, coherent vs. sequential) and their relation with molecular and contact structure. The goal is to provide reproducible transport (conductance-voltage) characteristics of well characterised systems, which can be then analysed in collaboration with theoretical partners in the SPP 1243 to answer the following questions: Which is the role of the leads electronic configuration in the charge transport? How do molecular orbitals evolve from local resonances and vibrations to transport bands and phonons? Can we see an electronic blockade effect in molecular junctions? How is energy dissipated in the molecule?
The main goal of our subproject in the SPP 1243 is to provide a reproducible spectral fingerprint for the transport through one single molecule contacting two leads, which can be unequivocally related to the molecular structure and to the nature of its contacts with atomically controlled electrodes.
To provide answers to the question how molecular electronic and vibrational states evolve towards quasi-one-dimensional transport bands as a single-molecule junction is formed, we use low temperature STM techniques to support experimental data we expect to compare and to place into context with the research results of the other experimental and theoretical projects under the SPP 1243. A special link with the theory group of the University of Paderborn (UPB) has been specially constructed for the preparation of this research proposal, which will be extensible to other collaborators.
Molecule-surface complexes with optimal adsorption properties
It is important to find a model system with a chemically stable configuration of molecular anchor groups and surfaces. Requirements are that the molecule should have a fixed local bond with surface atoms, and that the molecular species remains fairly unperturbed, avoiding in any case a flat-lying adsorption geometry, which is not ideal for transport measurements. To proceed with the project, we will investigate molecular species with various molecular end-groups, as model systems for a stable molecule-surface contact. Amongst others, copper and oxygen functionalised copper surfaces will be used.
Role of the substrate’s band structure and electron density
The surface is not only a support but also a source/drain of charge. Its band structure and charge/phonon density affect the transport behaviour. A consistent picture of transport should include these parameters. On semiconductors we expect that the lifetime of molecular excitations is increased, and therefore, the role of inelastic transport channels during transport becomes more evident. To solve this question, we will thus compare our results on metal surfaces (copper) with that on semiconductor surfaces, paying special attention to the type and magnitude of inelastic scattering processes. Further on precise transport mechanisms need to be modelled in detailed simulation by our collaborators, including energy dissipation at the single-molecule junction due to the existence of less effective energy decay channels into the leads.
Single electron tunnelling processes on a molecular adsorbate
Under normal transport measurements with STM, the electron transfer rate (tunnelling current) is usually much lower than the excitation/quenching rate of molecular excitations. By increasing the tunnelling rate and decreasing the excitation/quenching rate (working on semiconductor surfaces, or using decoupling anchor groups), we reach a different transport regime. Investigation of transition from tunnelling (i.e. coherent) to sequential (i.e. incoherent) implies to extend our tunnel measurements to tunnelling rates in the sub-picosecond time scale (i.e. tunnelling current larger than 100 nA), thus approaching the time scale of nuclei motion of the molecular adsorbate, and of life time of electronic excitations.
From single- to two-contact mode
STM provides a well controlled experimental workbench for transport experiments, but molecular spectra are usually measured on single contact configuration, i.e. in tunnel regime. Other techniques like MCBJ of lithographically patterned contacts provide a two contact picture of transport. We plan to explore the tunnelling spectroscopy down to the very low resistance regime, close to the 13 KΩ range. The answer to the question, how resonances of a molecule, whose arrangement is changed from a single towards a double electrode contact evolve will be provided through the analysis of STS spectra as the tunnel barrier collapses and a new chemical bond is formed. Investigating the evolution of the molecular resonances’ shape and position in the spectra, we will learn about the transport mechanism, and about the existence of potential barriers at the contact with the leads. In a symmetric configuration, knowledge of the potential profile along the electrode-molecule-electrode junction is important in order to understand the symmetry in the I-V characteristics [Datta97]. With the aid of DFT simulation of our partners, we will draw a model where the energy alignment of the molecular states, and their displacement with tip-sample position, is used to characterise the coupling of molecular states with the leads, thus revealing their role in the transport. We will also characterise the behaviour of molecular states as the molecule is further compressed between the STM tip and sample.
Inelastic scattering of electrons and excitation of vibrations
A further aim is to illuminate answers to the questions, how the gradual collapse of the tunnel barrier, and consequently the enabling of a backscattering electron channel, affect the inelastic scattering of electrons, how inelastic processes of a molecular wire are related to inelastic excitations of vibrations in a single contact picture (i.e. IETS-STS) and how much power is dissipated during charge transfer through a single molecule and how much is needed to dissociate the molecule. IETS-STS succeeded to resolve vibrations on single adsorbed molecules (Ho), while in the contact regime MCBJ measured vibrations of a H2 molecule. In one case, the activation of an inelastic channel causes an increase of the conductance, while in the other a decrease. How do these two processes relate one to the other? Should we expect the same vibrational states to be excited in both configurations? Here, we expect strong differences since in a double contact configuration a larger mixing of vibrational states should take place. We are concerned also here with dissipation of power. Our collaborators at the UPB found that the power dissipated during charge transport differs in up to three orders of magnitude around the nW scale for benzene-di-thiol with respect to alkanedithiol [Pecchia04]. The inelastic fingerprint in this contact regime thus pretends to provide model data to understand such differences.
[Datta97] Current-voltage characteristics of self-assembled monolayers by scanning tunneling microscopy, S. Datta, W. D. Tian, S. H. Hong, R. Reifenberger, J. I. Henderson, and C. P. Kubiak, Physical Review Letters 79, 2530 (1997).
[Pecchia04] Incoherent electron-phonon scattering in octanethiols, A. Pecchia and A. Di Carlo, Nano Letters 4, 2109 (2004).
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