Comprehensive characterization of different metallic thin films on highly oriented pyrolytic graphite substrate

Abstract

Abstract Thin metallic films supported on highly oriented pyrolytic graphite (HOPG) with low surface roughness are needed to improve the performance of different industrial applications and devices. As a result, an experimental investigation of electron beam physical vapour deposition of M (= Ti, Au, and Ag) metals on HOPG substrate to form HOPG-M thin films with a nominal thickness of 10 nm is reported. The comprehensive characterizations of the films were performed using different characterization techniques. The findings show that the films formed had different morphologies and topographies where Ti film was continuous while Au and Ag films were made up of islands with some discontinuities. The surface chemistry shows that the Ti film existed in both metallic and oxidized states while the Au and Ag films existed in metallic states. Raman analysis shows that there were some variations in the molecular structures after deposition while FTIR analysis shows that different functional groups existed on the thin films. The surface wettability behaviour of the Ag film based on water contact angles was hydrophobic while the other samples were hydrophilic. This work provides an opportunity for further examination of the HOPG as a substrate for the preparation of different M thin films.

Introduction Carbon is currently widely regarded as one of the most important materials in nanoscale applications [[1], [2], [3]]. It has a variety of chemical, physical, and electrical properties that make them appealing for a variety of advanced technological applications such as energy storage systems [1,4]. Graphite, an allotrope of carbon, is a low-cost material with excellent corrosion resistance, and thermal and electrical conductivities. The graphite has a structure that is composed of hexagonal layered carbon (graphene) planes situated in ABAB positions [5]. Graphite has strong sp2 intralayer bonds in its hexagonal structure and very weak van der Waals bonds of the delocalized electron orbitals between its layers [2,5].

Highly oriented pyrolytic graphite (HOPG) is an exceedingly uncoated and ordered form of synthetic graphite and has a low mosaic spread angle [6]. In HOPG, the hexagonal layered carbon planes are situated in ABAB positions [[6], [7], [8]]. HOPG are particularly compatible with the materials used in fabrication equipment, making them ideal for integration into existing planar processing infrastructure. For instance, thin metallic films can be deposited on the HOPG substrate and processed by conventional means. Carbon-supported metallic thin films have played significant roles in industrial applications such as catalytic processes [[9], [10], [11]], energy harvesting [3], optoelectronics [12], and gas sensors applications [13]. When used as a substrate, carbon has a significant specific surface area, which is advantageous for producing tiny, well-dispersed, and stable metal particles with precise atomic and electronic structures [9]. The graphite surface's atomic smoothness and inertness make it a suitable template for the formation of nanostructures [13,14]. Additionally, because HOPG possesses metallic qualities, surface charging issues cannot arise when using it in electron spectroscopy and microscopy experiments [13,15]. Furthermore, HOPG's well-known cleaving method may easily introduce decorative step edges to the substrate, providing adjustable defect traps for directed nanostructure growth or creation [14]. HOPG also has a very small amount of oxygen bonds at the surface (signals in the O 1s region as measured by X-ray photoelectron spectroscopy (XPS) [15].

The fabrication of various metallic nanostructures on graphite and more specifically HOPG substrates through physical vapour deposition has received a lot of attention in recent years. D'Addato et al. [11] reported on an experimental investigation of Ag and Ni NPs deposited on HOPG through magnetron sputtering gas aggregation (MSGA). The study focuses on NP/surface morphology to understand the interaction of preformed NPs with the HOPG surface. Kaspar et al. [3] investigated the formation of HOPG-Fe2O3 thin film after electron beam physical vapour deposition (EBPVD). The findings demonstrate that a stable and ordered surface formed following the deposition of Fe on HOPG. Motin et al. [9] carried out an experiment to deposit platinum (Pt) on sputtered HOPG by physical vapour deposition (PVD) process under an ultrahigh vacuum (UHV) environment. The results show that it is possible to construct a viable Pt/HOPG model using Pt NPs of various sizes which can be used for the catalysts system. There are other studies where different metals have been deposited on HOPG substrate under different conditions [10,14,[16], [17], [18]]. In the current study, the EBPVD technique will be used for the deposition process because of its advantages such as high deposition purity, large coating area, low deposition rate, material utilization efficiency, precise film thickness, in-situ growth monitoring, and morphological control [19].

Despite the deposition of the target materials on graphite substrates receiving attention recently, it is not yet a well-understood area of research and the HOPG-metallic thin films have not been comparatively and comprehensively characterized. Because of their significance, HOPG-supported thin metallic films with a nanoscale roughness and a high degree of thickness uniformity need to be fabricated and subjected to comprehensive examination and characterization to understand them more. The current study aims to fabricate the HOPG-metallic film systems through the EBPVD technique and characterized them comparatively and comprehensively using different characterization techniques. The study will provide more information about HOPG as the substrate for the deposition of different metallic nanomaterials for the future fabrication of devices.

Section snippets Experimental materials In this research, HOPG material was used as the substrate for the deposition process. The HOPG was a single-sided ZYD grade with a mosaic spread of 1.7 ± 0.4° and was supplied by NT-MDT Spectrum Instruments. The samples had dimensions measuring approximately 5 × 5 × 1 mm. The samples received were treated by cleaving with adhesive tape once along the basal plane to remove surface impurities [8] near the evaporator in a clean room at the Central European Institute of Technology (CEITEC) Nano

Results and discussion During the deposition process in a vacuum environment, the kinematically energised gaseous atoms are deposited on the surface of the HOPG substrate (at a lower temperature) after condensation [51]. The deposited metallic films form a bond with the substrate and the bond developed depends on intrinsic properties such as energetics between gaseous metal atoms and the substrate surface and extrinsic properties such as the presence of defects, surface functional groups (e.g. oxygen, hydroxyls) of

Conclusion, recommendations and future work In this study, the formations of the thin metallic films on the HOPG substrate through the electron beam PVD method under specific deposition rates have been demonstrated. The SEM images demonstrated that pure HOPG was clean and free from defects while deposited Ti film form a uniform and continuous film on the HOPG substrate. The Au and Ag films form thin films with some discontinuities due to their growth mechanism. The topographical analysis shows that pure HOPG had a smooth surface with a

CRediT authorship contribution statement Kipkurui Ronoh: Writing – original draft, Visualization, Methodology, Investigation, Formal analysis, Data curation, Conceptualization. Saleh H. Fawaeer: Writing – original draft, Methodology, Investigation. Vladimír Holcman: Resources, Writing – review & editing. Alexandr Knápek: Writing – review & editing, Supervision. Dinara Sobola: Writing – review & editing, Supervision, Resources, Funding acquisition, Conceptualization.

Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgement We acknowledge CzechNanoLab Research Infrastructure supported by MEYS CR (LM2018110). The research described in the paper was financially supported by the Internal Grant Agency of the Brno University of Technology, grant No. FEKT-S-20-6352 and the GACR 23-07384S. We also acknowledge the Czech Academy of Sciences (RVO:68081731).

References (120) P. Kaspar Characterization of Fe2O3 thin film on highly oriented pyrolytic graphite by AFM, Ellipsometry and XPS Appl. Surf. Sci. (Nov. 2019) H. Zhang et al. Graphite as anode materials: fundamental mechanism, recent progress and advances Energy Storage Mater. (Apr. 2021) A.M. Motin et al. Surface science approach to Pt/carbon model catalysts: XPS, STM and microreactor studies Appl. Surf. Sci. (2018) H. Hao et al. Epitaxial growth of Ag-Cu bimetallic nanoparticles via thermal evaporation deposition Appl. Surf. Sci. (2020) R.J. Isaifan et al. Particle size effect on catalytic activity of carbon-supported Pt nanoparticles for complete ethylene oxidation Appl. Catal. Gen. (Aug. 2013) A.V. Bukhtiyarov et al. XPS/STM study of model bimetallic Pd–Au/Hopg catalysts Appl. Surf. Sci. (Mar. 2016) N. Fairley Systematic and collaborative approach to problem solving using X-ray photoelectron spectroscopy Appl. Surf. Sci. Adv. (Sep. 2021) M.C. Biesinger Accessing the robustness of adventitious carbon for charge referencing (correction) purposes in XPS analysis: insights from a multi-user facility data review Appl. Surf. Sci. (Sep. 2022) G. Greczynski et al. X-ray photoelectron spectroscopy: towards reliable binding energy referencing Prog. Mater. Sci. (Jan. 2020) G. Greczynski et al. Reliable determination of chemical state in x-ray photoelectron spectroscopy based on sample-work-function referencing to adventitious carbon: resolving the myth of apparent constant binding energy of the C 1s peak Appl. Surf. Sci. (2018)