Exploring Molecule-Metal and Molecule-Topological Insulator Interface at Atomic Scale

Abstract

The silicon revolution in 1970s and 1980s made technology accessible to everyone in the form of personal computers. It also propelled the information revolution that connected the world with rapid advances in internet and wireless communication, also known today as the internet of things (IoT). A natural progression in this course was the miniaturization of devices aiming for high speed and low power consumption in all designs. In 1965, Gordon Moore, the co-founder of Intel, said that the number of transistors would double every 18 months, an observation referred to as the “Moore’s law”. The fabrication and miniaturization in silicon devices has reached the spatial limits calling out for paradigm shifts in designing new technologies. One such candidate on this frontier has been organic molecules. Using them for designing electronic devices, a pathbreaking idea first proposed by Aviram and Ratner in 1974, sowed the seeds of ‘molecular electronics’. Since then there have been efforts worldwide to harness the electronic properties of molecules towards this goal. The advantage of using molecules is their stability, small size, and extensive structural and electronic tunability for desired functionalities. Self-assembled molecules (SAM) supported by a substrate are an excellent platform to explore device functionalities in molecules. However, before converging upon a realistic application, fundamental questions like how do single molecules behave? how do their energy levels align with the substrate, what happens at the interface, how do molecules selfassemble? how does electron transport occur between the molecules and substrate? and so on need, to be addressed. This thesis has attempted to answer some of these questions using Scanning Tunneling Microscopy (STM) and Scanning Tunneling Spectroscopy (STS) at Low Temperature (LT) of 77 K and in Ultra High Vacuum (UHV). The spatial high resolution of STM gives a direct, real space information of the molecules, and the high energy resolution of STS offers a direct probe for their local electronic properties – for their measurement and for their manipulation at the molecular scale. Metal Phthalocyanine (MPc) molecules have been explored here over noble metal and topological insulator substrates. MPcs are planar -conjugated systems capable. of self-assembly and can act as versatile, robust, and tunable templates for surface functionalization. Their vast tunability engineered by substituting the central metal ion and the functional groups at the periphery makes them model systems to study the evolution of self-assembly, growth of self-assembled layers, and interface properties over a variety of surfaces. In the first part of this thesis, adsorption characteristics of copper phthalocyanine (CuPc) and copperoctacyano phthalocyanine (CuPc(CN)8) have been investigated on Au(111) using low-temperature scanning tunneling microscopy (STM) and density functional theory (DFT) calculations. At very low coverage, the adsorption of CuPc and CuPc(CN)8 leads to the formation of one-dimensional chains along the mono atomic (MA) step edge. At higher coverage, both CuPc and CuPc(CN)8, guided by tetramer unit cell formation self-assemble on flat terraces and cross over the step edge of Au(111). CuPc adsorption along theMAstep edge shows only one geometric configuration, whereas two different geometric configurations occur for CuPc(CN)8. The spectroscopic signature of these two configurations, probed using STS, manifests in a shift of the peak position of the highest occupied molecular orbital(HOMO) for the CuPc(CN)8 molecule at the MA step edge with respect to the molecule over the flat terrace of Au(111). STM imaging on a flat terrace reveals a tetramer unit cell to be the hallmark of each assembly. The periodicity of herringbone reconstruction of Au(111) is unchanged upon CuPc(CN)8 adsorption, whereas for CuPc adsorption this periodicity changes. STM imaging shows adsorption-induced organizational chirality for both assemblies. STS measurements show an increment in the highest occupied–lowest unoccupied molecular orbital (HOMO–LUMO) gap from CuPc to CuPc(CN)8. For CuPc(CN)8 at LUMO energy, the individual molecule exhibits an orbital-energy-dependent chirality on top of the existing organizational chirality. It remains achiral at HOMO energy and within the HOMO–LUMO gap. No such peculiarity is seen in the CuPc assembly. This energy-selective chiral picture of CuPc(CN)8 is ascribed to the cyano groups that participate in antiparallel dipolar coupling, thereby enhancing intermolecular interaction in the CuPc(CN)8 assembly. The density of state calculations for different molecular orbitals and the STM images simulated with DFT calculations for specific configurations agree with the experimental results. These findings advance our understanding of the role played by the pendant groups of the Pc molecules in step-edge and flat terrace and demonstrate that pendant group substitution is an effective strategy for tweaking intermolecular interactions and for surface functionalization. In the second part of this thesis, different MPcs (M=Fe,Co,Cu) were investigated on Bi2Se3, a topological insulator(TI) substrate, to explore the molecule-TI interface properties using STM and STS. TIs are a new class of materials, insulating in the bulk but conducting on the surface. The TI surface states exhibit Dirac cone-like dispersion that is protected by time-reversal symmetry due to the strong spin-orbit coupling in these materials. Bare Bi2Se3 substrate was well characterized by STM and STS. The MPcs were chosen based on the aim of each investigation and vapor-deposited on Bi2Se3. Each MPc resulted in a distinctive finding. For FePc no self-assembled layers were seen on the Bi2Se3 substrate. The molecules exhibited two distinct sitespecific configurations. In STS measurements there was no shift of the Dirac point either, indicating a weak molecule-substrate interaction. For CuPc no self-assembly could form at a lower coverage. However STS data revealed a change in the Dirac point indicating a charge transfer from CuPc molecule to Bi2Se3 substrate. In case of F16CoPc the self-assembly was characterized by nanometer sized rotational domains in a complete monolayer coverage. The highlight of F16CoPc investigation was the STS measurement unveiling a robust signature of negative differential resistance (NDR). F16CoPc deposited on the noble metal Ag(111) substrate showed no spectroscopic signature of NDR, thereby making F16CoPc/Bi2Se3 interface to be an emergent molecule-TI hybrid interface property. Thus, depending on the central metal ion, the self-assembly, and the local electronic properties at the molecule-TI interface, interface properties take shape and open up avenues for molecular engineering. To summarize, this thesis has examined some MPcs on different substrates using STM and STS at UHV and LT focusing in detail on their topography, spectroscopy, and local electronic behavior at a single molecule level. The new findings have led to a better molecular scale picture of the influence of pendant groups and of different metals on the adsorption, self-assembly, and electronic properties along with emergent properties specific to molecule-TI hybrid interfaces, thus furthering a microscopic understanding of systems that is indispensable for technological advances.