Theoretical Investigation of W(CO)6 Adsorption on SiO2 Surfaces – Insights to Electron Beam Induced Deposition K. Muthukumar, I. Opahle, J. Shen, H. O. Jeschke, R. Valentí Institut fϋr Theoretische Physik, Goethe-Universität, Frankfurt am Main, Germany, Email: muthukumar@itp.uni-frankfurt.de
Introduction
Method
Results
Conventional Fabrication Techniques:
Periodic Density Functional Theory (DFT), PAW Pseudo potential – GGA,
Interaction of W(CO)6 with the fully hydroxylated SiO2 substrate.
Vienna Ab-Initio Simulation Package (VASP),
Corresponds to the substrate prepared under
1. Chemical Vapor Decomposition 2. Arc Discharge Method 3. Hydrothermal Techniques etc.,
Fabrication of self-standing nanostructures at selected position on selected substrate and position controllability is still a challenge
Amorphous SiO2 (a-SiO2) used in experiments – Difficult to model amorphous SiO2 (c)
Novel Method
wet chemical conditions in the absence of an electron or ion beam. Three different orientations - Prefers bonding through its multiple CO ligands a)
Electron Beam Induced Deposition (EBID)
b)
Interaction of W(CO)6 with the partially hydroxylated SiO2 substrate. Partial de-hydroxylation of the surface of SiO2 substrate - expected to occur under irradiation with an electron or ion beam or at elevated temperatures – Direct bonding of substrate Si atoms with the precursor. Chemisorption - As seen from Fig 3. d & f, spontaneous dissociation of CO ligands were observed. Minor structural changes - upon incorporating dispersion corrections in the calculations.
Conclusion: FIG. 2: a) Schematic view of three different configurations of W(CO)6 (Config.(1-3)) are shown. b) Variation of Adsorption energy (ΔE) for different orientation on the fully and partially hydroxylated surfaces with and without inclusion of dispersion corrections are shown.
Due to the controllability of the electron beam nanometer sized structures such as nanodots, nanowires, and deposits with desired patterns have been successfully fabricated.
Aim of the study EBID with W(CO)6 has been extensively used in recent years – Many unanswered preliminary questions. 1) The nature of chemical interaction between W(CO)6 with the surface of SiO2? 2) Structure – (Deposit) Composition Relationship? 3) Growth Mechanism of Nanoparticles during EBID? Three main interactions have to be understood Substrate-precursor interaction, Electron-substrate interaction, Electron-precursor interaction This study focus on understanding Substrate (SiO2) – Precursor (W(CO)6) interaction
FIG. 1: (a) Structure of bulk -cristobalite SiO2 and (b) side view of the slab geometry with a (111) surface used for this study.(c) Comparison of bulk DOS with Slab
Weak adsorption (Physisorption) on fully hydroxylated Surfaces Fig. 2b - Minor structural changes observed both in the surface and the precursor molecules.
β- Cristobalite has many similarities in physical properties as a-SiO2 and hence used as a representative structure.
Dispersion corrections by Grimme et al implemented in VASP 5.2 have been considered.
- Activation and spontaneous dissociation on the partial hydroxylated surface. Studies are being extended to explain the growth mechanism of W nanoparticles from W(CO)6 on the surface during EBID.
1) J. Wnuk et al, Surf. Sci., 2011, 605, 257. 2) I. Utke et al, J. Vac. Sci. Tech. B, 2008, 26, 1197.
Acknowledgments
1 to 5 Layers of (111) surface cluster models were considered – Structural and electronic properties converge for 4L.
W(CO)6 Molecule – Geometry optimized – Oh geometry.
- Tendency of a precursor molecule to role on the fully hydroxylated surface
References:
The Bravais-Donnay-Friedel- Harker (BDFH) predicts (111) face of β- Cristobalite are dominant.
Two different surfaces: Fully and Partially hydroxylated surfaces corresponding to surfaces before and after irradiation of electron beam were considered.
Theoretical Investigation on the Interaction between the precursor molecule illustrates
Financial support by the Beilstein-Institute, within the research collaboration NanoBiC. Generous allotment of computer time by CSC and Loewe - CSC in Frankfurt.
FIG. 3 (a-f) Figure illustrating the role of vdW correction in determining the bonding of W(CO)6 to the fully and partially hydroxylated surfaces.
Dispersion corrections play a crucial role in stabilizing precursor molecules (cf. 3 c & e) on the fully hydroxylated surfaces.