NOVEL SURFACE CHEMISTRY OF SINGLE MOLECULES AND SELF-ASSEMBLEDSTRUCTURES BY SCANNING TUNNELING MICROSCOPY

This thesis demonstrates the richness of Scanning Tunneling Microscopy (STM) as amethod to understand surface chemistry and physics and to explore the new frontiers in singlemoleculesurface reactions and molecular self-assembly. Organosulfur molecules on the Au(111)surface were studied to address unresolved and controversial issues about self-assembledmonolayers of alkanethiol molecules on gold surfaces. The key new finding is that the thermalsurface chemistry of alkanethiol molecules occurs in a dynamic chemical environment thatinvolves reactive gold adatoms to which the alkanethiol molecules chemically bond. Theproblem of alkanethiol self-assembly is thus transformed from the realm of adsorption on asurface toward organometallic surface chemistry, which is anticipated to have broad implicationsfor the field. Molecules containing a disulfide (S-S) bond were also found to be a spectacularmodel system for exploring electron-induced surface chemistry. In particular, the atomicallylocalizedinjection of electrons from the metal tip of the tunneling microscope is capable ofproducing highly delocalized chemical reactions by means of surface current of hot-electrons.Chemical reactions can therefore be a unique approach to the measurement of the local transportof hot-electrons on metal surfaces. Finally the concepts of self-assembly and electron-inducedchemistry are combined through an observation of an unusual process that flips the chirality ofmolecules self-assembled on the surface by a radical-like chain reaction. This experimentdemonstrates how self-assembly enables a new reaction coordinate by optimizing the stericfactor of the chemical reaction

Electronic switching by metastable polarization states in BiFeO3 thin films

We present an approach to control resistive switching in metal-ferroelectric contacts using a radially symmetric electric field. In ferroelectrics with significant polarization along the corresponding field lines, the field above a critical threshold will induce polarization discontinuity, a corresponding nanoscale volume of space charge, and a conducting junction. We demonstrate this principle using nanoscale polarization switching of a conventional (001)-oriented thin film of BiFeO3. Without any optimization, the conducting state created in this regime of resistive switching exhibits local currents of ∼1–10nA, approaching the ∼100nA threshold required for device implementation [J. Jiang et al., Nat. Mater. 17, 49 (2018)]. The corresponding electronic function is that of a volatile resistive switch, which is directly compatible with neuristor functionality that encodes the functioning basis of an axon [M. D. Pickett et al., Nat. Mater. 12, 114 (2013)]. Phase-field modeling further reveals that in the strongly charged local configuration, BiFeO3 locally undergoes a rhombohedral-tetragonal (R-T) phase transition, in part due to substantial piezoelectric expansion of the lattice. The estimated local charge density can be as high as ∼1021cm−3, which would be extremely difficult to achieve by conventional doping approaches without altering other material properties. Therefore, this method for creating stable and reproducible strongly charged ferroelectric junctions enables more systematic studies of their physical properties, such as the possibility of structural and electronic phase transitions, and it can lead to new ferroelectric devices for advanced information functions

Nanoscale imaging of He-ion irradiation effects on amorphous TaOx_x toward electroforming-free neuromorphic functions

Resistive switching in thin films has been widely studied in a broad range of materials. Yet the mechanisms behind electroresistive switching have been persistently difficult to decipher and control, in part due to their non-equilibrium nature. Here, we demonstrate new experimental approaches that can probe resistive switching phenomena, utilizing amorphous TaO$_x$ as a model material system. Specifically, we apply Scanning Microwave Impedance Microscopy (sMIM) and cathodoluminescence (CL) microscopy as direct probes of conductance and electronic structure, respectively. These methods provide direct evidence of the electronic state of TaO$_x$ despite its amorphous nature. For example CL identifies characteristic impurity levels in TaO$_x$, in agreement with first principles calculations. We applied these methods to investigate He-ion-beam irradiation as a path to activate conductivity of materials and enable electroforming-free control over resistive switching. However, we find that even though He-ions begin to modify the nature of bonds even at the lowest doses, the films conductive properties exhibit remarkable stability with large displacement damage and they are driven to metallic states only at the limit of structural decomposition. Finally, we show that electroforming in a nanoscale junction can be carried out with a dissipated power of < 20 nW, a much smaller value compared to earlier studies and one that minimizes irreversible structural modifications of the films. The multimodal approach described here provides a new framework toward the theory/experiment guided design and optimization of electroresistive materials