University of Groningen
Self-assembled nanostructures on metal surfaces and graphene
Schmidt, Nico Daniel Robert
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Publication date: 2019
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Schmidt, N. D. R. (2019). Self-assembled nanostructures on metal surfaces and graphene. University of Groningen.
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1 Introduction
1.1 Motivation
Data revolution1,2 or 4th industrial revolution3 are only two out of many buzz words coined to indicate the chances and challenges of a socio economic development we all experience whenever we reach into our pocket or for our nightstands – big data is firmly woven into the very fabric of our societies. We use data to reconnect with lost friends around the globe or navigate unfamiliar cities with ease, but also in efforts to influence public opinion4,5 or rate citizens.6,7 Independent of the way we collect and use data, one thing is clear – we generate and rely on an ever-increasing amount of data. So how do we deal with them?
Let us constrain ourselves purely to the hardware for handling data. The transistors providing the computational power to process data are traditionally manufactured using photolithography, i.e., a top-down process.8 Remarkable effort in industry and science over the last decades allowed us to decrease the size of transistors and “cram”, as Moore has called it in 1965,9 an ever increasing number of them onto integrated circuits. For example, Intel’s 10 nm fabrication process has a transistor density of 100.8 million transistors per mm2.10 However, processors based on this process only see limited introduction into the consumer market at the time of this writing11 as technical difficulties have repeatedly pushed a high-volume manufacturing from 2016 to 2019.12 These delays are symptomatic of an industry that has extended the limits of photolithography by enhancements such as phase-shift masks, multiple
1.1 Motivation patterning, or immersion lithography. For next-generation lithography alternative light sources, e.g., extreme ultraviolet, will need to be utilized.13
Feynman envisioned a fundamentally different approach in 1959, when he asked: “What would happen if we could arrange the atoms one by one the way we want them”.14 The idea to manipulate smallest units such as atoms and arrange them into the desired objects is the very definition of the bottom-up approach. Since atoms adhere to the rules of quantum mechanics, “[…] we can expect different things”, but “[…] in principle, that can be done”.14
In order to reach the nanoscale Feynman was referring to, suitable experimental methods are needed. Historically, optical microscopes offered a direct way to examine samples in real space. However, their resolution is limited by approximately half of the wavelength used in the experiment. Near-field scanning optical microscopy15 can overcome this limit, but only reaches resolutions of 20 nm16 – far from the 3 Å needed to resolve individual atoms on a metallic surface. Diffraction experiments such as low-energy electron diffraction (LEED) can give insight, inter alia, into the symmetry and lattice parameters of surfaces, but require ordered samples and depict only the reciprocal space.17 The first spatial observation of an atom was reported by Müller and Bahadur in 195618 using field ion microscopy19 invented by Müller in 1951.20 Unfortunately, this technique is limited to study atoms of a sharp tip and cannot be used to manipulate individual atoms on surfaces. Preluded by the invention of the topografiner by Young, Ward et al. in 197221, Binnig, Rohrer et al. presented the scanning tunneling microscope in 1982.22 By utilizing the tunneling of
electrons between a tip and a sample, the authors were able to observe the 7 x 7 reconstruction of Si(111) in real space.23 Their invention, recognized with the Nobel Prize in Physics 1986,24 pioneered modern day surface science and heralded the realization of Feynman’s vision. Indeed, it took only a few more years until Eigler and Schweizer used a scanning tunneling microscope in 1990 to position xenon atoms on a Ni(110) surface in a way that spelled IBM.25 Shortly after, Crommie, Lutz et al. arranged Fe atoms on a Cu(111) surface into a quantum corral and observed local modifications of the electronic properties of the Cu(111) surface.26
Manipulating individual atoms with a scanning tunneling microscope is a tedious task and scaling this approach up to manufacturing quantities is rather futile. Molecular self-assembly constitutes an alternative bottom-up approach. It is part of supramolecular chemistry and has been defined as “the spontaneous association of molecules under equilibrium conditions into stable, structurally well-defined aggregates joined by non-covalent bonds”.27 The scientific relevance of self-assembly has been acknowledged with the Nobel prize in Chemistry 198728 and it is employed far beyond the field of surface science.29,30 Compared to manipulating individual atoms, self-assembly has several advantages: (i) As molecules assemble autonomously under a driving force, large-scale self-assembled structures can be built very quickly. (ii) The non-covalent nature of the bonding allows for a self-correction and thus the resulting self-assembled structure can exhibit a high degree of perfection. (iii) By altering the symmetry, size, shape, and recognition sites of molecular building blocks,
1.1 Motivation scientists have been able to create a large range of self-assembled nanostructures on surfaces.31–36
One surface that experienced an increasing interest from the scientific community is graphene. The word graphene describes a monolayer of carbon atoms packed into a flat, 2D lattice. Stacking graphene yields graphite, while rolling up graphene results in fullerenes, such as C60 or carbon nanotubes. Theoretically, graphene has been studied as early as 1947,37 albeit as a first step towards the description of graphite. Graphene continued to be a topic of interest for theoretical researchers in the 1980s as a condensed matter analogue of (2 + 1)D quantum electrodynamics.38–40 Graphene was assumed to be unstable in its free-standing form. In hindsight, experimental indication of the existence of graphene can be dated back as early as 1962.41–43 However, in 2004 Novoselov, Geim et al. produced graphene by mechanical exfoliation from graphite44 and thus made it accessible to a wide range of experiments. For their groundbreaking experiments Geim and Novoselov were awarded the Nobel Prize in Physics 2010.45 Graphene has since been found to exhibit good thermal conductivity,46 high intrinsic stiffness,47 and exceptional electronic properties,48–52 making it a strong candidate for a wide variety of future applications.53
In summary, rising difficulties in conventional lithography enable the emergence of fundamentally different approaches to building electronic devices. Utilizing the self-assembly of molecular building blocks on promising materials such as graphene constitutes such novel approach, while scanning tunneling microscopy is highly suitable to study it.
1.2 Thesis Outline
In this thesis, we studied the self-assembly of organic molecules on metal surfaces and on graphene. The motivation was twofold. On the one hand, we studied the fundamental driving mechanisms of self-assembly (Chapter 4) and the subtle role of the graphene substrate on 2D molecular self-assembly (Chapter 5). On the other hand, we investigated model systems for electronic applications by studying the charge transfer between molecules (Chapter 6) and the band structure of graphene after adsorption of organic molecules (Chapter 7). We used scanning tunneling microscopy (STM) to study the self-assembled structure on the nanometer scale. LEED gave complementary structural information on the large scale. X-ray photoelectron spectroscopy (XPS) was used to probe changes of the chemical environment of the adsorbed molecules. By means of scanning tunneling spectroscopy (STS) we studied the electronic properties of our samples on the nanometer scale, while ultraviolet photoelectron spectroscopy (UPS) and angle-resolved photoelectron spectroscopy (ARPES) revealed changes to the electronic structures on the large scale.
Chapter 2 gives an overview of the above-mentioned experimental techniques deployed during the course of this thesis. We present the functional principle as well as a theoretical description of each technique. We also give a short reasoning as to why all of our experiments were carried out in ultra-high vacuum (UHV).
Chapter 3 starts with a short introduction of the basic principles of molecular self-assembly on surfaces. We then discuss the fundamentals of graphene, especially the theoretical description of its electronic structure.
1.2 Thesis Outline We conclude the chapter with a review of up-to-date research of molecular self-assembly on graphene.
Chapter 4 reports on the self-assembly of a conformational flexible compound on Au(111) using STM and LEED. Upon adsorption, we observed one self-assembled arrangement in which different conformations of our compound coexisted. Annealing this sample left the conformational diversity intact while increasing the long-range order of the arrangement. Increasing the lateral pressure on the self-assembly by means of molecular coverage resulted in the emergence of a second, coexisting arrangement. We therefore established a coverage-controlled transition from a monomorphic system with only one molecular arrangement into a polymorphic system with two coexisting arrangements.
Chapter 5 focusses on the self-assembly of a linear molecule on graphite and graphene on Cu(111). We studied the structural and electronic properties using STM, STS, and LEED. The molecules assembled into a close-packed structure with a peculiar feature – a shift of every fourth or fifth molecule. This shift was not reported for the same molecule on metal substrates or for comparable molecules in the crystal. This indicates that the observed shift is per se a unique feature of this molecule on graphitic substrates.
Chapter 6 discusses the interaction of an electron-donating and an electron-accepting molecule on Ag(111). The molecules were studied using STM, STS, LEED, XPS, UPS, and ARPES. We observed well-ordered structures in the homomolecular layer. In the mixed layer, the complementary nature of the two molecules also facilitated a well-ordered
structure with a 1:1 ratio of electron donating and accepting molecule. Probing the electronic states, we found clear changes upon intermixing the two species. Most notably, we observed a hybridization leading to an unoccupied state that showed homogeneous spatial distribution across both molecules. Our system represents a compelling candidate for organic electronics based on self-assembly of charge-transfer-complexes.
Chapter 7 presents the results of two similar molecules on graphene on Ir(111). We studied the coverage-dependent evolution of the supramolecular structure using STM and LEED. When probing the electronic structure of graphene using ARPES, we found a shift of the Dirac point towards higher binding energies. Additionally, the adsorption of one of the molecules also induced a significant band gap opening – a crucial prerequisite for a potential utilization of graphene in field-effect transistors. Our system hence suggests the feasibility of graphene based organic electronic devices.
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