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Contents
Introduction 4
Chapter 1. Scanning Tunneling and Atomic Force Microscopy
Scanning Tunneling Microscopy
1.1. Introduction 12
1.2. Principle of Operation 16
1.2.1. Metal- Metal Tunnel Contact 17
1.2.2. Metal-Semiconductor Tunnel Contact 18
1.2.3. Tunneling Current 18
1.3. Morphology and Atomic Structure Studies 19
1.4. The Current-Voltage (I-V) Characteristics 23
1.5. Local Density of States (LDOS) Studies 25
Atomic Force Microscopy
1.6 Introduction ~ 29
1.7. Principle of Operation ’ / 30
1.7.1 Force/Distance Relationship 30
1.7.2. Measuring Forces 31
1.7.3. Non-Modulated Methods 31
1.7.4. Modulation Techniques 31
1.7.5. Feedback Controls 32
1.8. Nature of Sample and Sample Surface Contamination 32
1.9. Cantilever and Tip 33
1.10. AFM Imaging Modes: (a).Contact Mode, (b) Non-Contact Mode.
(c) Tapping Mode. 35
Conclusions 39
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Chapter 2. Atomic Hydrogen Interaction with Highly Oriented
Pyrolitical Graphite
2.1. Background of Hydrogen-Graphite Interaction Studies and Motivation of our
Work 41
2.2. Samples Preparation 44
2.3. STM and AFM Morphological Studies of Atomic Hydrogen Intcracted-HOPG
Surface 48
2.4. STS Studies of Atomic 1 lydrogcn-Interactcd HOPG Surface 63
Conclusions 69
Chapter 3. Graphite layers on Ir and Re Surfaces
3.1. Graphite layers on Metals: Background and Motivation of our Work 70
3.2. Samples Preparation 72
3.2.1. Preparation of Graphite Layers on Ir(l 11) and Re(1010) 73
3.2.2. Intercalation of Cs Between Graphite Layer and Ir Substrate 73
3.3. Surface Morphology of Graphite Layer on Ir(l 11) and Re(1010) 74
3.4. Electronic Surface Superstructure on Graphite Layer on Ir(l 11) 80
Conclusions 82
Chapter 4. Thin Carbon Films
4.1. Carbon Films: Background and Motivation for our Work 83
4.2. Samples Preparation 84
4.3. Surface Morphology and Emission Properties of the Carbon Films 87
4.4. Effect of Treatments of Substrates on Characteristics of Grown Carbon Films 91
Conclusions 96
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Brief Summary of the Presented Work (Conclusions) 98
References 101
Acknowledgements 107
Introduction
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Introduction
Importance of this Research Topic
Scanning probe microscope (SPM) studies of atomic hydrogen-interacted highly oriented pyrolitical graphite (HOPG) and carbon-based thin films is a very interesting research topic because of enormous applications lor conventional, new energy sources, and electronics device technology. The SPM is a versatile technique, which in microscopy mode reveals the morphology of surfaces with resolutions up to atomic levels and in spectroscopy mode gives the occupation of electron density of states of the surfaces. Such key information is the base for further research and practical applications. Carbon-based materials play an important role in our everyday life. From burning of raw coal in power plants up to the core of tiny electronic devices, all make use of carbon-contained substances. The interaction of HOPG with atomic-hydrogen is important because of the graphite use as l sl shielding sheet in nuclear reactors and fusion devices. This interaction helps to understand changes taking place on plasma (hydrogen-contained) facing graphite plane The hydrogen storage in graphite materials has recently emerged a new burning issue and is being searched as a safe and compatible energy source for electronic devices and small power plant applications. Thin solid films on metals is a very hot research issue and it has attracted attention of a large number of investigators because of technological importance of these films in applied fields like modern communication and semiconductor electronic device manufacturing technology. Some of these forms have such unusual physical and chemical properties that can drastically change adsorption, catalytic and emission properties of the substrate materials. During some past years, carbon films having different phases like amorphous carbon, diamond-like carbon and chemical vapor deposition diamond have emerged as promising materials for cold cathodes. The electron field emission from such films has been widely studied and such films are being considered future candidates for microelectronic field emission devices. Their unique properties such as high hardness, chemical inertness and optical transparency are very attractive. These properties make these films preferable over other types of materials used for optical and electrical applications.
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Goal of these Investigations
For hydrogen-graphite system, taking into account the fact that atomic hydrogen-graphite surface interaction earlier received very small attention and hydrogen storage in graphite materials has recently emerged as a new research activity, we decided to perform detailed studies on atomic hydrogen-HOPG system. The main target was to study atomic hydrogen-HOPG interaction specially concentrating on the surface morphological changes taking place on the exposed HOPG surface. The study of the modified surface local density of states of atomic hydrogen interacted-HOPG surface was also aimed. The surface morphology study of the graphite monolayer on Ir and Re sufaces was the 2nd part of this w'ork. The surface morphology study of graphite monolayer on Ir and Re revealing atomic resolution provided direct information on structural and intercalation properties of monolayer graphite coverage on metals. In the 3rd part, we performed surface morphology studies of thin carbon films grown on Si and ceramic substrates. We studied effect of change of the films deposition parameters and effect of performing different pregrowth treatments of substrates on characteristics of the grown carbon films.
Novelty of the Work
Regarding atomic hydrogen-HOPG interaction studies, we observed a new phenomenon of hydrogen accumulation between graphene layers. The hydrogen accumulation was found stable over periods of several months. By performing thermal desorption of hydrogen, the HOPG surface was found atomically flat and covered with 1-2 layers deep, nearly circular etch-pits. At edges of the etch-pits w'e revealed localized electronic surface states by performing scanning tunneling spectroscopy.
The surface morphology studies of graphite monolayer on Ir and Re permitted to characterize the morphology of polycrystalline Ir and Re metal ribbons and directly revealed on their surfaces graphite monolayer growTi in the procedure of benzene decomposition over heated metal surface.
The surface morphology and electron emission characteristics of thin carbon films
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deposited on Si and ceramic substrates were studied in detail and effect of deposition parameters and pre-growth treatments of substrates on characteristics of grown carbon films was observed.
The Main Findings of this Work
1. It is shown that hydrogen atoms may penetrate in perfect graphite layers and accumulate in graphite in the spaces between graphite layers transforming them in the net of densely spaced blisters. This wras established in ambient AFM and UHV STM studies of graphite surface morphology before and after exposure of the graphite surface to hydrogen atoms. At\er exposure to hydrogen atoms, initially atomically Hat graphite surface demonstrated appearance of bumps with the heights upto 5 nm.
Hydrogen thermodcsorbtion from graphite makes the graphite surface again atomically flat but covered with many nearly circular etch-pits with depth 1-2 graphite monolayers. On repetition of hydrogen sorbtion-desorbtion cycles the already created etch-pits grow in sizes and new etch-pits are created in the next graphite planes. Thus gradual erosion of graphite surface takes place on repeating the hydrogen sorption-desorption cycles, resulting in layer-by-layer removal of the graphite sheet.
The scanning tunneling spectroscopy investigations manifested presence of localized electronic surface states at the edges of the circular pits. The localized edge surface state revealed by tunneling spectroscopy appears as maximum of the local density of states in the energy range of 90 - 250 meV above the Fermi level.
2. The graphite monolayers coverage grown in the procedure of benzene decomposition over heated metal surfaces was observed with atomic resolution on Ir( 111) and Re(JOlO) metal surfaces. On the flat terraces of Ir( 111) surface graphite monolayer was continues, growing in crystalline-geometrical correlation with Ir(lll) metal lattice. There were also found few local defects (dislocations in plane) in graphite monolayerlayer. In general, high crystalline perfection of the graphite monolayer on the Ir(lll) surface wras demonstrated.
CsL ions intercalation beneath the graphite monolayer on Ir( 111) surface changes initially flat morphology of graphite layer to a bump like structure which presents new
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possibilities for the intercalation phenomena studies with nanometer lateral resolution and it causes a better atomic resolution of the graphite monolayer due to possible electron exchange from the Cs.
Surface morphology and electron surface emission of carbon films on Si and ceramic substrates prepared by low temperature high frequency (VHF) plasma deposition were studied. Some correlation between morphology and emission characteristics have been found: in general, films providing superior emissivity were characterized by lower roughness and grain size. We have also found influence of pre-growth treatments of substrates on surface morphology and emission characteristics of the deposited carbon films.
The results of this work were presented at the follow ing international conferences:
1. International Conference Probe Microscopy-98, 2-5 March (1998), Nizhny Novogord, Russia.
2. Materials Research Society (MRS) Spring Meeting, (1998), San Francisco, USA.
3. The 14th International Vacuum Congress (IVC-14) and 10th International Conference of Solid Surfaces (ICSS-10), 31 Aug.-4 Sep. (1998), Birmingham, UK.
4. 10th International Conference on Radiation Effects in Insulators, 18-23 July, (1999), Jena, Germany.
5. International Conference Probe Microscopy-99, 10-13 March (1999), Nizhny Novogord, Russia.
6. International Conference on Vacuum Microelectronics (IVMC-99) 6-9 July (1999), Darmstadt, Germany.
7. 10th European Conference on Diamond, Diamond like Materials, Nitrides and Silicon Carbide, 12-17 September (1999), Prague, Czech Republic.
8. 4th International Symposium on Diamond Films and Related Materials (ISDF-4), 20.-22. Sep. (1999), Krakow, Poland.
9. 18th European Conference on Surface Science (ECOSS-18), 21 -24 September (1999), Vienna, Austria.
10. The Electrochemical Society 197th Annual Meeting, 14-18 May, (2000),
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