Scaling Dependencies in Irradiation-Driven Molecular Dynamics Simulations: Case Study of W(CO)$_6$ Fragmentation

Abstract

Irradiation-driven fragmentation and chemical transformations of organometallic molecules play a central role in nanofabrication techniques based on the use of focused charged-particle beams. In this paper, the electron irradiation-induced fragmentation dynamics of W(CO)$_6$, a commonly used precursor for focused electron beam-induced deposition (FEBID), is investigated using the irradiation-driven molecular dynamics (IDMD) method. Simulations are performed for gas-phase systems with different precursor densities and under different irradiation conditions. The results reveal progressive fragmentation of W(CO)$_6$ molecules into W(CO)$_n$ species, accompanied by the formation of W-rich molecular clusters. The evolution of fragment abundances shows a strong dependence on both precursor density and electron fluence. Higher densities and larger fluences result in more extensive fragmentation and promote the aggregation of tungsten atoms into small metal clusters. Under certain irradiation conditions, the studied molecular systems evolve towards a steady state characterised by slightly varying fragment abundances. The obtained scaling relations between irradiation parameters and fragment distributions provide guidance for selecting simulation parameters in IDMD simulations of the FEBID process, ensuring a quantitative description of precursor fragmentation dynamics.

Figures (9)

• Figure 1: Change of atom types upon the cleavage of a C--O bond in the parent W(CO)$_6$ molecule (left panel) and the formation of a WC(CO)$_5$ fragment (right panel). The atom types for all the fragments considered in this study are summarised in Table \ref{['table:rCHARMM_param_parent']}.
• Figure 2: Absolute partial ionisation cross section (PICS) data for singly-charged cationic fragments W(CO)$_{6-n}^+$ ($n=0-6$) and WC(CO)$_{6-m}^+$ ($m=3-6$). The data have been obtained in the electron energy range from the ionisation threshold of the parent molecule ($\sim$8.5 eV) up to 5000 eV.
• Figure 3: The relative abundance of different W(CO)$_{6-n}$ ($n = 0-6$) molecular fragments at different simulation parameters for the low-density target comprising 23 parent molecules. The electron beam current (top x-axis) and simulation time (bottom x-axis) were rescaled to maintain a constant total electron fluence during each simulation. The error bars show the standard error for the respective number of independent simulations, as listed in Table \ref{['table: details of simulation']}.
• Figure 4: The relative abundance of different W(CO)$_{6-n}$ ($n = 0-6$) molecular fragments at different simulation parameters for the case of the high-density target comprising 207 parent molecules. The electron beam current (top x-axis) and simulation time (bottom x-axis) were rescaled to maintain a constant total electron fluence during each simulation. 10 independent simulations were performed for each simulation time; the error bars show the corresponding standard error.
• Figure 5: The relative abundance of different W(CO)$_{6-n}$ ($n = 0-6$) molecular fragments at different simulation parameters for the case of three molecular densities, corresponding to 23 (red symbols), 107 (green symbols), and 207 (blue symbols) W(CO)$_6$ molecules placed in the simulation box with a side length of 20 nm. The electron fluence has been kept constant throughout all simulations.