AVS2010 Session TF-WeA: Thin Films: Growth and Characterization

Ti and its alloys are commonly used as biomaterials due to their unique mechanical properties and good corrosion resistance. Still, Ti implants can induce the formation of a fibrous layer that can compromise bonding at the interface with the living tissue. The lack of proper integration of Ti with tissue can lead to implant failure. Since the biological response to implanted biomaterials is initiated at their surfaces, the performance of Ti and Ti alloys can be improved by modifying their surfaces. A promising approach for surface bio-modification is grafting a bioactive polymer onto Ti implant surfaces. For optimal grafting, it is important to fully understand the nature of the bio-modified surfaces since it has a pivotal role in the biomaterial performance. The main thrust of this work is to graft bioactive sodium styrene sulfonate (NaSS) onto Ti surfaces to control and improve their response in biological environment.

Smooth Ti films evaporated onto silicon wafers were used as substrates. XPS showed that the surfaces of these films are covered with a layer of TiO₂. The roughness of these surfaces, as measured by AFM, was 0.7nm. Methacryloxypropyltrimethoxysilane (MPS) was used as a cross linker between the Ti and the NaSS; the substrates were soaked in a solution of MPS in chloroform (5%v/v) for 1 hr at room temperature and then removed from the solution and heated at 140°C for 4 hrs. After attaching the MPS molecules, XPS surface composition and high resolution XPS data suggested that the Ti substrates were covered with a uniform thin film. Additional evidence for the MPS attachment to the Ti surfaces was the appearance of the C_xH_yO_z fragments from the methacrylate group along with the decease of the Ti and TiO_x fragments in the ToF-SIMS data. The NaSS grafting was then done at 90°C in an oxygen free environment for 15hrs using a 0.7M solution of NaSS monomer in dimethyl sulfoxide (DMSO). After NaSS grafting the XPS composition showed an increase of the C/Ti ratio and an appearance of sulfur and sodium. ToF-SIMS successfully detected the sulfonate group, C₈H₇SO₃, and a decrease of the Ti containing fragments. Fourier transform infrared spectroscopy (FTIR) and near edge absorption fine structure (NEXAFS) indicated an ordered array of the grafted NaSS layer on the Ti surfaces and AFM showed a uniform coverage with a roughness of 1.11nm.

Due to the ease of preparation and their relatively high stability, self-assembled monolayers (SAMs) are very promising candidates to be used in the development of micro- and nano-structured materials. With numerous detailed studies available nowadays for SAMs, the identification of SAMs adsorption geometry and stability of molecule-substrate interface still remains controversial and rather difficult to access experimentally. In this presentation we report experiments on ion-induced desorption and resonance enhanced ionization mass spectrometry of SAMs on Au(111) substrate.¹ Althrough ion-induced desorption is commonly considered as a very invasive process when used for probing monomolecular films, our experiments demonstrate that this method can be successfully applied to monitor fine changes in the molecule-substrate interface stability of model SAMs systems based on thiols ( CH₃‑C₆H₄‑C₆H₄-(CH₂)_n-S-Au(111), n = 2-6) and selenols (BPnSe, CH₃‑C₆H₄‑C₆H₄-(CH₂)_n-Se-Au(111), n = 2-6) . Current desorption experiments will be discussed together with recent microscopic² and spectroscopic³ analysis of the molecular structure and stability of these SAMs. We demonstrate that lower or higher ion-induced bond scission efficiency can be correlated with, respectively, higher or lower chemical stability of particular chemical bonds. Thus, a new method for probing the stability of the substrate-SAM interface can be proposed.

References

(1) S. Wyczawska, F. Vervaecke, et al. in preparation.

(2) P. Cyganik, K. Szelagowska-Kunstman, et al. J. Phys. Chem. C 2008, 112, 15466.

(3) K. Szelagowska-Kunstman, P. Cyganik, et al. Phys. Chem. Chem. Phys. 2010, 12, 4400.

Non-dissociative deposition of gas-phase species onto surfaces of alkanethiol self-assembled monolayers (SAMs) allows creation of new types of multi-component nanoscale materials. Systems such as these have garnered much attention due to their central role as model systems for studies of orientation-controlled adsorption and non-dissociative attachment of functional molecules on organic surfaces of technological importance, including molecular electronics.

Benzene (C₆H­₆), perdeuterobenzene (C₆D₆) or a 50:50 mixture of these two isotopologues were deposited on SAM surfaces using a supersonic molecular beam. Supersonic molecular beam techniques permitted precise control over the dynamics of the deposition process by changes in the incident reagent’s translational energy (E_trans) and incident angle. The results presented here highlight the role these dynamical variables play in the adsorption, desorption and conformation of the resultant multilayer molecular film. A combination of in-situ infrared reflection absorption spectroscopy (FT-IRAS) and mass spectrometry was used to determine the surface coverage, molecular orientation and the sticking coefficient as a function of the surface coverage of the benzene molecules deposited on SAM surfaces. The interaction between adsorbates and the SAM substrate was also investigated by varying the SAM chain length and whether the SAM contains an odd or even number of carbon atoms. These results were compared to analogous results from adsorption on clean Au surfaces.

The results of these experiments uncovered the details of the adsorption process. The effects of E_trans and substrate temperature on sticking show the central role dynamics plays in the physisorption of molecules on surfaces. Most significantly, the sticking of gas-phase benzene was found to have a novel dependence on surface coverage, and non-Langmuirian uptake was observed.

In order to form stable self-assembling organic monolayers on numerous substrates, dipping deposition from organic solvents is a widespread technique. However, it is rarely investigated what is the influence of the solvent on the substrate and the molecules which are supposed to be deposited. In this study we present the self assembly of phosphonic acids from an ethanolic solution on aqueous based pretreated aluminium oxides. Following the deposition behavior with XPS and AFM throughout deposition time, it was concluded that the nature of the deposition is random and fluctuating. In order to understand better what is going on, the deposition was followed in situ with odd random phase multisine impedance spectroscopy. This technique gives the opportunity to follow the behavior of the organic molecules as well as the behavior of the buried substrate. It was observed that ethanol adheres on the surfaces, changing the water based chemistry which was obtained through the pretreatment. Furthermore, a competition between the adsorption of the phosphonic acids and ethanol was seen, explaining the non-stable behavior previously analysed with XPS and AFM.

This statement was proven by characterizing ethanol-stabilised aluminium oxide samples during different immersion times. Although here no monolayer was formed, the trend observed corresponded with continuous organic layer growth.