**1. Introduction**

Carbon materials have been widely used to promote technological advances in various applications such as energy storage, catalysis, and sensors. These materials which are mainly composed of carbon atoms exhibit diverse structures and properties depending on the valence bond hybridization of carbon. As a new member of carbon materials, graphene exhibits extraordinary physical and chemical properties. Graphene is a zero-bandgap semiconductor and exhibits room temperature electron mobility, which is much higher than those of silicon or other semiconductors and weakly affected by temperature and doping effect [1]. These properties impart the rapid development of graphene-based transistors or integrated circuits which are considered as a promising alternative to silicon electronics [1, 2]. The first graphenebased circuit was developed by Lin *et al.* in 2011 [3]. The graphene integrated circuit operates as a broadband radio-frequency mixer at frequencies up to 10 gigahertz and exhibits outstanding thermal stability. Graphene is highly optically transparent and absorbs only 2.3% of the incident white light [4]. Graphene also displays the highest thermal conductivity compared to other carbon materials [5]. Thus, graphene has applications for transparent touchscreens, organic light-emitting diodes as well

as solar cells. Large-area, continuous, transparent, and highly conductive graphene, which can be produced by chemical vapor deposition (CVD) method, is used as an anode in photovoltaic devices with a power conversion efficiency of up to 1.71% [6]. The large specific surface area, flexibility, and high electrical conductivity of graphene make it an ideal material for sensors [7]. In particular, the graphene sensor for COVID-19 detection can detect viruses faster and more accurate [7]. For example, a graphene-based field-effect transistor biosensing device was created to detect spike protein on COVID-19 with a limit of detection of 1 fg mL−1 [8]. Graphene has a unique two-dimensional (2D) single-sheet structure and is considered a building block of other carbon allotropes. It can be wrapped up into zero-dimensional (0D) fullerene, rolled into one-dimensional (1D) carbon nanotube, or stacked into three-dimensional (3D) graphite [1]. In the graphite structure, each carbon atom is sp2 hybridized. The highly oriented pyrolytic graphite (HOPG) is a highly-ordered and high-purity form of synthetic graphite. A new HOPG surface can be generated *via* simple tape cleavage. It is an ideal model to be used as a calibration standard for microscopic imaging [9, 10], and a substrate for chemical reactions [11] and surface modifications by laser irradiation and ion beam bombardment [12].

It is well known that the surface physical and chemical properties of materials are quite different from those of their bulks. Many critical physical and chemical reactions occur on surfaces of materials, such as oxidation, contamination, corrosion, and adsorption. As surface properties are governed by the atomic structure and composition of the outermost layer of the surface of a material, it is necessary to use appropriate surface analysis techniques for its characterization. X-ray photoelectron spectroscopy (XPS) is a widely used surface analysis technique with a sampling depth of 2–10 nm because of its simplicity in use and straightforward in data interpretation [13, 14]. It provides nondestructive quantitative information with an accuracy of up to 0.1 at% on the elemental composition and the chemical state of the elements present on the surface of a material. When high-sensitivity elemental analysis and spatial distribution information of chemical species on a surface are needed, an extremely surface-sensitive technique with a sampling depth of approximately 1 nm, the spatial resolution of about 0.1 μm, and low detection limit up to ppm called time-of-flight secondary ion mass spectrometry (ToF-SIMS) is applied [13, 14]. The secondary fragments generated from the surface of a material are related to its surface chemical structure, such as defects and the functional groups at its edges. However, quantitative information can hardly be deduced from the ion intensities and data interpretation is more complicated. Therefore, a combination of XPS and ToF-SIMS can provide complementary details about the chemical information of the surface of a material.

The objective of this chapter is to present an overview of the applications of XPS and ToF-SIMS in the surface characterization of graphene and graphite, including their preparation processes, impurities, surface defects as well as their surface reactions. We hope this chapter will drive a deeper understanding of graphene and graphite surfaces for faster development and wider applications of these types of materials.
