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Electronic Response of Single Inorganic Nanowires

Final Report Summary - ERESIN (Electronic Response of Single Inorganic Nanowires)

Conducting Mo6S9-xIx nanowires (MoSI NWs) show particularly low interaction with each other and their environment and are hence an excellent model system for one-dimensional (1D) materials. They grow in bundles which can easily be dispersed in a large number of polar solvents without any functionalisation or surfactants. In low concentrations they can even debundle into individual NWs, although small bundles (~3nm diameter) are more typical. When codissolved with gold nanoparticles (GNPs) they form networks with the GNPs acting as junctions between 2-4 MoSI NWs. This is an excellent scope for assembly of two-and threedimensional structures, where MoSI NWs serve as electric connectors between active components.

To date, the application of MoSI NWs in nanoelectronics faces two major challenges: variations in the energy of the electronic levels as seen e. g. in absorption spectra, and limited conductivity. The ERESIN project aims to identify the origins of both the energy variations and the limited conductivity, to find out their role in the non-equilibrium electron dynamics, and to improve both aspects for applications.

The original objectives of the project were:
• Origin of the energy disorder. Since it is not a bulk effect, there may either be different types of NWs, or the NWs may even change their electronic structure as one moves along the axis.
• Identify the defects that limit the conductivity. Electron traps may be directly related to the energy disorder.
• Find ways to reduce the disorder and number of defects by tuning the synthesis or reduce their effect by changing the energy landscape by doping or functionalisation
• Find out the effects on the relaxation behaviour. The behaviour so far studied on bulk samples may vary within the energy distribution or change upon improving of their synthesis.

In the course of this project we found out that the absorption peak position correlates with the diameter of MoSI NWs. Hence centrifugation can fractionate them by energy level spacing. We show that this is due to electronic doping by excess Mo in the synthesis, which promotes aggregation into thick bundles. The thin, undoped bundles behave as semiconductors, while the thicker bundles are conductive due to doping.

This work is published in D. Vengust, C. Gadermaier, and D. Mihailovic, Synth. Met. 160, 2389 (2010).

Having met our first two objectives and found the other two obsolete, we changed our strategy and objectives: Rather than concentrating on single NWs, we optimised the separation procedure and compared the femtosecond non-equilibrium electron behaviour in doped and undoped samples in solution. Both studies are now in preparation for publication
Since the change of focus of the MoSI research required a lot less spectroscopy, the fellow's work force was freed for other projects in our notoriously understaffed femtosecond lab, which allowed him to integrate deeper into our spectroscopy activity.

The revised objectives for the second half were:
• Influence of doping on the relaxation behaviour
• Further study of the semiconductor behaviour (fluorescence, photovoltaics, etc.)
The first objective has been met, but still needs publication, the second has been taken over by other group members. Therefore the fellow was free to pursue other goals:
• Determine the strength of electron-phonon interactions in complex materials
• Systematically relate the emergent properties to the strength of electron-phonon interaction

Standard methods for determining the strength of electron-phonon interaction (EPI) experimentally from phonon linewidths in Raman or neutron scattering are often biased by selection rules and inhomogeneous broadening, and have given controversial results in the past. Since scattering from phonons is one of the main relaxation processes for electrons, the EPI strength can be accurately extracted from the electron-phonon relaxation time, provided that: (i) the experiment affords adequate time resolution and (ii) an appropriate model connecting the EPI strength and the relaxation time is used. We show that for materials with strong EPI, to satisfy both conditions, we need to go beyond current approaches. Here, by using optical spectroscopy with ultrahigh time-resolution (< 20 fs instrument response) and a new, more appropriate model, we obtain the EPI values for two high-critical temperature cuprate superconductors, which allows us to assess the role of the EPI in the superconducting mechanism in these materials.

This work is published in C. Gadermaier et al., Phys. Rev. Lett. 105, 257001 (2010).
Having shown – by measurements carried out at our partners in Milano – how to measure the EPI strength in cuprates, we purpose built a set-up with the best available time resolution for the low excitation densities we need to avoid sample heating and non-linear relaxation processes, to do a more systematic study comprising different cuprate, pnictide and bismuthate superconductors. This means that for the first time, the superconducting transition temperature Tc is related to a parameter that is both experimentally observable and directly linked to an interaction that should be responsible for the superconducting mechansism. From the unambiguous correlation between Tc and the EPI relaxation time we conclude that EPI plays a key role in high-temperature superconductivity. This work is of general interest because is touches on many different types of materials and has recently been submitted for publication.