Interactions of Ti and its oxides with selected surfaces: Si(100), HOPG(0001) and graphene/4H-SiC(0001)
Introduction
Since many decades titanium and its compounds have been deposited on different substrates and have exhibited unique properties like optical, photovoltaic and photocatalytic [1,2]. Among these compounds titanium oxides, and especially TiO2, are the most common systems that are used in many applications, including solar cells fabrication [3,4] photocatalytic water purification [5], improvement of optical properties [6]. More recently Ti compounds have been also deposited on graphene as being very promising two dimensional material for future nanocomposite technologies [[7], [8], [9]]. It was found that such TiO2/graphene hybridized interface can have much better photocatalytic properties that TiO2 itself [10,11].
Titanium compounds can be deposited on substrates in many ways [12], including chemical methods like: chemical vapour deposition (CVD) [13], sol-gel process [14], and physical approaches including: pulsed laser deposition (PLD) [15], electron beam (EB) induced deposition [16] and sputtering techniques like: direct current (DC) sputtering, alternating current/radio frequency (AC/RF) sputtering [17] and reactive sputtering [18]. Physical methods have some advantages like formation of more dense films and better control of possible contaminations. One of the advantages is also the ease of physical deposition and it is the best demonstrated in the case of DC sputtering.
Titanium itself is an reactive element. Thus, apart from natural oxides, Ti can form many compounds on selected surfaces like for example: silicides and silicates on Si-rich substrates [19,20], carbides [21,22] and carbonates [23] on C-rich substrates depending on preparation method. Single crystal surfaces of pure Si, pure C (i.e. highly-oriented pyrolytic graphite, HOPG) and their stoichiometric intermixture in the form of silicon carbide (SiC) are used for deposition of Ti and its compounds [[24], [25], [26]]. Si and SiC are also the substrates that are often used for studies of high-quality graphene [27,28]. In turn, HOPG substrate is often considered as reference bulk system to compare with graphene. Also all the three substrates in combination with Ti and its oxides exhibit many potential applications, especially in the field of photocatalysis [8,[29], [30], [31]]. However, it seems that there is no systematic comparison how Ti/Ti-oxides overlayer, deposited at the same conditions, behaves on S-rich substrates, C-rich substrates and an intermixture (e.g. graphene/SiC(0001)).
The aim of the work was to compare the interaction of Ti and its oxides with selected substrates and check the stability of the graphene under sputtering Ti deposition and thermal annealing in vacuum. Three the most typical substrates were used: Si(100), HOPG (i.e. (0001) oriented) and graphene/4H-SiC(0001). The intention of the studies was to use the substrates with changing Si and C content to check the differences in Ti reactivity and formation of various compounds (e.g. silicides, silicates, carbides and carbonates). Also a single layer graphene was involved to find out its interaction with deposited material and it was also compared with HOPG substrate which can be considered as a kind of multilayer graphene. The DC sputtering method was selected as being one of the easiest but giving an opportunity to control precisely the amount of the deposited Ti. The 3 nm thickness was chosen as allowing the samples (especially graphene/SiC(0001)) to be at least partially transparent which makes them useful later in OLED and photovoltaic technologies. Also 3 nm thickness was important in the context of X-ray photoelectron spectroscopy (XPS) analysis where too thick coating would not allow to get the signal from the interface between deposited material and the substrate because of the mean free path of electrons scattering (~2 nm). The chemical composition of the as obtained interfaces and thermally treated in UHV was explored using the mentioned XPS technique. This gave also an opportunity to identify reactions and compounds forming and evolving in the fabricated interfaces. Additionally, the morphology of the deposited overlayers before and after thermal treatment in ultra-high vacuum (UHV) conditions was also monitored using atomic force microscopy (AFM) and compared in the context of roughness and nanostructures formation. To check the influence of the deposited overlayer on the graphene itself the Raman spectroscopy before and after Ti deposition was also performed.
Section snippets
Experimental
In the performed research single crystalline surfaces: (i) one-side polished Si(100) (ITME, Poland), (ii) highly oriented pyrolytic graphite – HOPG(0001) (NT-MDT, Russia) and (iii) monolayer graphene on 4H-SiC(0001) (ITME, Poland) were used. Thin titanium films of 3 nm thickness were deposited using Ar+ sputtering technique with MED 020 (Bal-Tec, Germany) high vacuum coating system (base pressure: 1 × 10−3 Pa (1 × 10−5 mbar) equipped with turbomolecular pump and mechanical pre-pump) from
Interaction of Ti/Ti-oxides with Si(100) substrate
The surface characterization of Ti oxides on different substrates was performed with the use of XPS method and Ti 2p, Si 2p, O 1s and C 1s lines have been taken into account for quantitative estimation of surface composition at different temperatures for the research of Ti and its compounds on Si(100).
After deposition of 3 nm of Ti on Si(100) and transporting of the sample through the air atmosphere into UHV system, the XPS results showed the concentration of titanium at the level of 18 at.%
Conclusions
The influence of the thin layer of Ti/Ti-oxides, deposited by means of ion sputtering technique, was investigated using for comparison: Si(100), HOPG(0001) and graphene/4H-SiC(0001) substrates. The interaction of the deposited material with the selected substrates was studied together with gradual increase of the temperature and prolonged time of annealing in UHV. From the obtained XPS results the most interesting observation for the Si(100) substrate, was the presence and evolution with
CRediT authorship contribution statement
K. Pabianek: Investigation, Formal analysis, Methodology, Visualization, Writing - original draft. P. Krukowski: Investigation. K. Polański: Resources. P. Ciepielewski: Investigation. J.M. Baranowski: Resources. M. Rogala: Investigation. W. Kozłowski: Resources. A. Busiakiewicz: Writing - review & editing, Supervision.
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.
Acknowledgements
The work was co-financed by the National Science Centre, Poland, under the grant 2016/21/B/ST5/00984 and partially by The European Funds within Smart Growth Operational Programme under the project of The National Centre for Research and Development, Poland (POIR.04.01.02-00-0046/16). The authors kindly thank I. Lutsyk, D. A. Kowalczyk, E. Frątczak, M. Piskorski, P. Dąbrowski and P. J. Kowalczyk for technical support and discussions.
Dedication
The paper is dedicated to Professor Zbigniew Klusek who passed away suddenly on December 18, 2019.
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