Scanning Tunneling Microscopy Studies of Single Molecule for Nano Technology Applications


Student thesis: Doctoral Thesis

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Award date4 Jun 2018


The atomistic world in real space can be realized by utilizing scanning probe microscopy (SPM) methods among which scanning tunnelling microscopy (STM) can explore the electronic structure of the sample and its geometry at low temperatures in ultrahigh vacuum. This SMP method is also considerably promising for the manipulation of matter on nanometer scale. STM can be used to image and move a single molecule, a single atom and a cluster of atoms, to deposit and position single molecule and to conform an molecular switch.

This thesis studies molecular self-assemblies of fullerene (C60),molecular hover board and 5-, 10-, 15-, and 20-tetrakis-(4-bromophenyl)-porphyrin-Co (TBrPP-Co) via low temperature ultrahigh vacuum STM.

The reconstructed Au (111) surface with defect areas (steps) has been used as a template to assemble the highly ordered C60 nano array at low coverage. These nano arrays are studied with STM in conjunction with density functional theory (DFT). The interaction between the substrate and C60 nano arrays is strong enough to change the geometrical shape of C60. As a result of strong interaction, the C60 molecule appears to be deformed into ellipsoidal shape which causes the reduction of C60 nano arrays length on step sites of Au (111).

The inelastic electron tunnelling (IET)-STM manipulation switching scheme is used to explore the switching of a single molecule of TBrPPCo. It was performed by locating the STM tip on one of the protruding pyrroline subunits of TBrPP-Co molecule. The tunnelling current was recorded for a few ms to seconds while the voltage and current are at fixed values. Abrupt change in the recorded characteristics curve was observed as the indication of a change to the molecule. Tunnelling electrons can induce the switching between tilt-up and tilt-down position of these pyrroline subunits by increasing the bias voltage between tip and sample. The switching of these units gives rise to the conformational changes of TBrPP-Co between two states. Energy from tunnelling electron can be transferred to the molecules on the substrate, as large as several electron volts. Some of the energy of tunnelling electrons injected from the tip of STM convert to vibrational energy of the molecules which cause the conformational switching of the molecule. The success of conformational switching from one state to another was realized by imaging the same place of the surface after manipulation.

Furthermore, a comprehensive study of molecular hover board is done on different substrates. However, single molecular nano hoverboard is only drivable on gold surface. In order to realize it, the electric field induced manipulation using an STM manipulation scheme is conducted on molecular hover board on gold surface. The molecular hover board is driven across the herringbone reconstruction of gold surface by using both positive and negative electric field of STM in controlled manner. Detail analysis reveals threshold energy required to roll the wheel of nano hoverboard on gold by both schemes.

The success in this study demonstrates the power of the scanning tunnelling probe technique to obtain deep insight into the basis, scope and application of nanomachining.