Synthesis And Electronic Properties Of Nanowires Of Charge Transfer Complexes

Abstract

In case of charge-transfer complex of TTF:TCNQ lot of work had previously been done on single crystals and thin films to study various interesting properties including phase transitions which were attributed to Peierls instability. But as seen from the review of molecular wires it is clear that apart from synthesis of TTF:TCNQ in molecular wire form, not much was known about the behavior of these wires at low temperatures. There were some open questions listed below, which we tried to address in the thesis Can nanowires of TTF:TCNQ be grown across prefabricated electrodes which are separated by gaps < 1 μm. Can the nanowires grown in such smaller gaps, show Peierls transition, which is the signature of quasi one dimensional conduction. As the size and length of the grown wires are small it was expected that they will have less staking disorder as compared to the thin films. What will be conduction mechanism at low temperatures in such single/few nanowire samples. If the nanowires show Peierls transition and CDW formation at low temperatures, can nonlinear conduction be seen due to motion of CDW, if so how well do they compare with the reported results for TTF:TCNQ single crystals. In case of Cu:TCNQ it can be noted from the above review that even though much advances have been made on synthesizing good quality Cu:TCNQ films and incorporating them in novel device structures, there has been much controversy regarding conduction mechanism. There were many conflicting results in literature regarding switching in these devices. In this thesis work we wanted to address the feasibility of switching in Cu:TCNQ under reduced size of top electrodes and also address few other issues like To grow Cu:TCNQ nanowires by using vapor phase evaporation method Can resistive switching be induced in Cu:TCNQ by using a local probe STM tip (Pt-Rh) operated in high vacuum. Since the measurement will be done in high vacuum what will be the effect of environment (absence of oxygen, water vapor) on reproducibility of resistive switching. Will localized switching depend on the top electrode material. This has been probed by coating different metals on the C-AFM tip and using them as top electrode in conducting mode. With what contact force will we get reproducible resistive switching. Can a device structure be made with an array of top electrode in the form of metal dots (< 10 μm) and study switching using C-AFM.

This thesis is divided into seven main chapters and two appendix chapters, which are listed below: In the present chapter 1, a detailed overview and literature survey of charge-transfer complexes TTF:TCNQ and Cu:TCNQ which were relevant to our present study was presented. This was followed by our motivation in undertaking the present work. In chapter 2 the various experimental techniques developed during the course of the thesis work such as e-beam lithography, design of the vacuum chamber for deposition of organic molecules, design of ultra high vacuum scanning tunneling microscope (UHV-STM chamber along with the STM head, modification of conducting AFM for obtaining the switching data have been described. In chapter 3 we describe the preparation of TTF:TCNQ molecular wires across prefabricated electrodes and different measurements done on the samples. In particular the observation Peierls transition in the grown nanowires of TTF:TCNQ and the nonlinear conduction mechanism involved at low temperatures will be discussed in detail. In chapter 4 we describe the preparation of Cu:TCNQ nanowires on Cu substrate using vapor phase technique. Resistive switching measurements done on the Cu:TCNQ nanowires in high vacuum with Pt-Rh tip as top electrode will be discussed in detail. In chapter 5 we describe the resistive switching measurements performed on Cu:TCNQ nanowires with different metal coated C-AFM tips as well as FIB deposited platinum dots as top electrodes. In chapter 6 we make a few comments about possible switching mechanism involved, when STM tip, C-AFM induced as well as platinum coated dots were used as top electrodes. In chapter 7 we conclude this thesis by summarizing the main results. Also we point out the scope for future work that can be based upon the results presented in this work. In Appendix A a brief review of self assembled monolayer (SAM) of alkane thiols is presented followed by details about experiments done for insitu study of growth of SAMs of decanethiol and octadecanethiol on silver substrates using ellipsometry and force-displacement spectroscopy. In Appendix B a brief description of work done to grow isolated nanowires of Cu:TCNQ, between two metal electrodes in planar geometry and in anodic alumina membranes is given.