XPS Depth Profiling Analysis of Polymer Brushes Using an Argon Gas Cluster Ion Source

Abstract

Polymer thin film materials play a crucial role in various applications, including drug delivery, medical diagnosis, biomineralization, nanolithography, and catalysis. The structure of these films often consists of multiple components or undergoes modifications specific to the film's surface. Characterizing the chemical composition and structure of polymer films as a function of depth presents technical challenges. The term "structure" in this context refers to the arrangement and organization of polymer molecules within the film, encompassing factors such as spatial distribution, orientation, crystallinity, cross-linking, and morphology. Therefore, characterizing the structure involves understanding how these factors vary throughout the film's thickness. Measurement techniques such as depth profiling offer insights into changes in chemical composition and structural features as a function of depth within polymer films. Depth profiling typically involves sequentially removing thin layers of the film and analysing the surface at each step. This allows for the investigation of variations in elemental composition, chemical states, and structural characteristics from the film's surface to its bulk. By examining the depth-dependent information obtained through techniques like X-ray Photoelectron Spectroscopy (XPS) or Secondary Ion Mass Spectrometry (SIMS), researchers can gain a comprehensive understanding of the film's chemical composition and structural variations. Therefore, characterizing polymer film materials as a function of depth involves the analysis of both the chemical composition (including changes in elemental content and chemical states) and the structural features (such as arrangement, organization, and

morphology). By studying these aspects, researchers can obtain valuable insights into the properties and behaviour of polymer films, aiding in the optimization and design of films for specific applications. However, traditional ion beam depth profiling techniques pose challenges when applied to polymers due to their high susceptibility to sample damage, issues with charging, and limitations of angle-resolved measurements in terms of time consumption and depth sensitivity. Polymers, particularly insulating ones, are prone to sample damage during ion beam bombardment. The energetic ions can cause molecular fragmentation and rearrangement within the polymer structure, leading to inaccurate depth profiling results. Furthermore, the charging phenomenon mentioned earlier can significantly affect the accuracy of depth profiling measurements. In addition to sample damage and charging issues, angle-resolved measurements of composition in polymers have certain limitations. These measurements involve tilting the sample at different angles to examine variations in composition as a function of depth. However, this approach is time-consuming, as acquiring data at multiple angles requires considerable measurement time. Moreover, the depth sensitivity of angle resolved measurements is limited, making it challenging to obtain detailed compositional information throughout the entire depth of the polymer film. In recent years, significant advancements have been made in the development of giant gas cluster sources, which offer a gentle approach for depth profiling of molecular materials. These breakthroughs hold the potential to revolutionize the characterization of polymer film materials. The primary focus of this thesis is to explore the practicality and effectiveness of giant gas cluster sources for conducting depth profiling of polymer brushes.

Polymer brushes are a highly important category of polymers that are grafted onto surfaces. They find extensive applications across various fields, including antifouling coatings, catalysis, drug delivery, optoelectronics, and lithium-ion batteries. Compared to thermoplastics, polymer brushes exhibit distinct characteristics such as mechanical softness and lower density. Additionally, their structures undergo changes in response to external stimuli. As a result, the analysis of polymer brushes presents even greater challenges compared to thermoplastic film materials. The term "giant" in the context of gas cluster sources refers to the formation of clusters comprising a large number of gas atoms or molecules. These clusters consist of thousands to millions of gas species. The utilization of such large clusters enables a gentler ionization and sputtering process during depth profiling, minimizing the risk of sample damage and enhancing the accuracy of characterization. The use of giant gas cluster sources lies in their ability to perform controlled and gentle material removal from the surface, allowing for precise analysis of composition and structure as a function of depth. This capability is particularly advantageous when investigating the intricate and responsive nature of polymer brushes. External stimuli encompass various factors, including temperature, light, humidity, pH, and electric fields. Polymer brushes exhibit structural changes in response to these stimuli, which in turn influence their properties and functionalities. Therefore, the characterization of depth-dependent structural variations in polymer brushes under different external stimuli is essential for comprehending their behaviour and optimizing their performance in specific applications. Overall, the examination of giant gas cluster sources for depth profiling of polymer brushes in this thesis represents an exciting opportunity to advance our understanding

of these materials and unleash their full potential in a wide range of technological applications. Polymer brush systems are used as model materials. The following two polymers have been selected for these fundamental investigations: PCysMA and POEGMA. PCysMA brushes were grafted via SI-ATRP and ARGET-ATRP on Si and glass. This was in order to decide which XPS-depth profiling needs to be studied, and the need to choose the best analytical method. Thicker films are produced by PCysMA ARGET-ATRP; however, have a rougher surface. Although ARGET produces slightly thicker brushes, the greater roughness is expected to lead to increased uncertainty when determining the etch profile by XPS depth profiling, making ATRP the ideal method given that PCysMA brushes are thinner than POEGMA brushes. For XPS depth profiling to be successful, polymerization must be tightly controlled. Allowing the study sought well-defined interfaces with minimal surface roughness and the ability to form layers of known, well-defined thickness in a controllable fashion. Because one of the most important factors that could affect the success of the XPS depth profiling process is the high density of controlled polymer brushes grafted by conventional ATRP; this was confirmed by very well-controlled growth kinetics of POEGMA brushes. By controlling the solvation state of the polymer, and hence its swelling chemical reactions will be carried out in polymer brushes. A range of modification types will be achieved and characterised using X-ray photoelectron spectroscopy (XPS) depth profiling by using Ar gas cluster ion beam sources to study the elemental composition, chemical states, and bonding. Other complementary analytical methods used include atomic force microscopy (AFM) to make nanomechanical measurements roughness (Rq) ellipsometry to measure the thickness of polymer brushes and contact angle to study wettability. The analysis of 3D polymer materials is challenging due to their insulating nature, which hinders the

flow of electrical current, and their susceptibility to degradation when exposed to particle beams. These insulating properties pose significant difficulties in their characterization and depth profiling. However, the use of giant argon gas cluster sources for depth-profiling in X-ray photoelectron spectroscopy (XPS) enable the examination of surface reactions in polymer brushes such as PCysMA reacted with TFAA or Glutaraldehyde-NB. Also, depth profiling of modified brush structures such as QPDMA indicate a uniform change through the whole brush chains not only at the surface.