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nanotechnologyTRANSCRIPT
Acknowledgments
Financial support by the Beilstein-Institute,
within the research collaboration NanoBiC.
Generous allotment of computer time by CSC
and Loewe - CSC in Frankfurt.
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?
Introduction
Conventional Fabrication Techniques:
Novel Method
Electron Beam Induced Deposition (EBID)
Method
Periodic Density Functional Theory (DFT),
PAW Pseudo potential – GGA,
Vienna Ab-Initio Simulation Package (VASP),
Amorphous SiO2 (a-SiO2) used in experiments
– Difficult to model amorphous SiO2
β- Cristobalite has many similarities in physical
properties as a-SiO2 and hence used as a
representative structure.
The Bravais-Donnay-Friedel- Harker (BDFH)
predicts (111) face of β- Cristobalite are
dominant.
1 to 5 Layers of (111) surface cluster models
were considered – Structural and electronic
properties converge for 4L.
Two different surfaces: Fully and Partially
hydroxylated surfaces corresponding to surfaces
before and after irradiation of electron beam were
considered.
W(CO)6 Molecule – Geometry optimized – Oh
geometry.
Dispersion corrections by Grimme et al
implemented in VASP 5.2 have been considered.
Results
Interaction of W(CO)6 with the fully
hydroxylated SiO2 substrate.
Corresponds to the substrate prepared under
wet chemical conditions in the absence of an
electron or ion beam.
Three different orientations - Prefers bonding
through its multiple CO ligands
a) b)
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.
Interaction of W(CO)6 with the
partially hydroxylated SiO2 substrate.
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
Due to the controllability of the electron beam
nanometer sized structures such as nanodots,
nanowires, and deposits with desired patterns
have been successfully fabricated.
Substrate-precursor interaction,
Electron-substrate interaction,
Electron-precursor interaction
This study focus on understanding
Substrate (SiO2) – Precursor (W(CO)6)
interaction
Three main interactions have to be understood
Theoretical Investigation on the Interaction
between the precursor molecule illustrates
- Tendency of a precursor molecule to role
on the fully hydroxylated surface
- 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.
Institut fϋr Theoretische Physik, Goethe-Universität, Frankfurt am Main, Germany,
Email: [email protected]
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
Weak adsorption (Physisorption) on fully
hydroxylated Surfaces Fig. 2b - Minor structural
changes observed both in the surface and the
precursor molecules.
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.
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:
References:
1) J. Wnuk et al, Surf. Sci., 2011, 605, 257.
2) I. Utke et al, J. Vac. Sci. Tech. B, 2008, 26, 1197.
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
(c)