Unraveling Photodimerization of Cyclohexasilane from Molecular

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Spectroscopy and Photochemistry; General Theory

Unraveling Photodimerization of Cyclohexasilane from Molecular Dynamics Studies Yulun Han, Kenneth J. Anderson, Erik K. Hobbie, Philip Boudjouk, and Dmitri S. Kilin J. Phys. Chem. Lett., Just Accepted Manuscript • DOI: 10.1021/acs.jpclett.8b01691 • Publication Date (Web): 13 Jul 2018 Downloaded from http://pubs.acs.org on July 13, 2018

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The Journal of Physical Chemistry Letters

Unraveling Photodimerization of Cyclohexasilane from Molecular Dynamics Studies Yulun Han, Kenneth Anderson, Erik K. Hobbie, Philip Boudjouk, Dmitri S. Kilin* Department of Chemistry and Biochemistry, North Dakota State University, Fargo, North Dakota 58102

ABSTRACT:

Photoinduced reactions of a pair of cyclohexasilane (CHS) monomers are explored by timedependent excited-state molecular dynamics (TDESMD) calculations. In TDESMD trajectories, one observes vivid reaction events including dimerization and fragmentation. A general reaction pathway is identified as (i) ring-opening formation of a dimer, (ii) rearrangement induced by bond breaking, and (iii) decomposition through the elimination of small fragments. The identified pathway supports the chemistry proposed for the fabrication of silicon-based materials using CHS as a precursor. In addition, we find dimers have smaller HOMO-LUMO gaps and exhibit a red-shift and linewidth broadening in the computed photoluminescence spectra compared to a pair of CHS monomers.

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Silicon is the base material for the microelectronics and photovoltaic industries.1-2 Cyclohexasilane (Si6H12, CHS) is a clear and colorless liquid at ambient conditions that has attracted considerable attention as a silicon source for liquid-based fabrication methods.3-5 For example, Schulz et al. prepared amorphous silicon nanowires (a-SiNWs) by electrospinning a liquid precursor consisting of a polymer and CHS in toluene.3 Lu et al. obtained crystalline silicon nanorods (SiNRs) using a tin-seeded solution-liquid-solid (SLS) method with different silane reactants and found that CHS required a minimum growth temperature.4 Iyer et al. have synthesized microcrystalline silicon thin films through spin-coating liquid CHS coupled with thermal and laser annealing.5 In addition, it has been shown that CHS could be a promising alternative to traditional monosilane (SiH4) in chemical vapor deposition techniques.6-7 The preparation of Si-containing materials from CHS was proposed to follow a general chemistry, which has also been suggested for other classes of cyclic silanes such as cyclopentasilane (CPS).8-13 Namely, cyclosilanes transform into hydrogenated polysilanes, −(SiH2)n−, through thermal or photo induced ring-opening polymerization. Upon further thermal treatment, these polysilanes undergo several bond breaking events, such as the breaking of Si-Si and Si-H bonds to form amorphous silicon. Even though the ring-opening polymerization process has been manifested in experiments, e.g. by gel permeation chromatography (GPC),13-14 corresponding theoreti-

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cal studies are scarce. To better understand this photochemistry and develop efficient fabrication techniques, it is thus necessary to perform theoretical studies of CHS. The computational description of photoinduced processes is challenging because the BornOppenheimer approximation, which is the foundation of most electronic structure theories, breaks down. Various first-principle approaches beyond the Born-Oppenheimer approximation have been used to study photophysical processes for silicon nanostructures.15-27 The theoretical studies facilitate the analysis of nonradiative relaxation pathways and shed light on the optical properties of silicon semiconductors. The investigation of photochemistry requires additional resources due to the large size of the model and the necessity to perform quantum dynamics simulations with a small time step. Several methods, including nonadiabatic molecular dynamics (NAMD), have been used to study photochemical processes, such as photoisomerization,28-32 photodimerization,33 photocycloaddition,34-35 and photodissociation36-40 in a wide range of systems. In our previous study, density functional theory (DFT) based time-dependent excited-state molecular dynamics (TDESMD) was used to investigate the photochemistry of small organic molecules41-42 and large metal-organic complexes.43-46 The atomic models were isolated in the gas phase such that the photochemistry should be interpreted as unimolecular reactions. The photochemical processes were mainly characterized as photodissociation, even though minor photoisomerization events were observed for certain fragments in simulations based on organic molecules.41-42 In this work, we perform TDESMD calculations on a pair of CHS monomers in the gas phase to simulate photoinduced reactions. The study of this simple system which, to the best of our knowledge, represents the first ab initio molecular dynamics (AIMD) study of CHS, is of great importance for elucidating the chemistry involved in the more complicated case of CHS in the liquid phase. Specifically, we seek to establish a fundamental understanding of ring-opening reactions and the properties of the CHS oligomer, which will be critical for the preparation of silicon 3 ACS Paragon Plus Environment


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materials with desirable characteristics from CHS and its derivatives. The application of TDESMD to a broad range of phenomena will require systematic testing and validation for several classes of reactions. The work presented here is a test and justification of this applicability for the formation and polymerization class of reactions. In addition, for the first time we analyze intermediates of the reaction dynamics trajectory through optical characterization. The change in the photoluminescence (PL) spectra along the reaction trajectory shows a clear qualitative trend, which agrees with experiment. The additional technical novelty is an ensemble average performed in the course of PL calculation to characterize thermal broadening of the PL linewidth. We consider two placement configurations of CHS monomer pairs: face-to-face and side-by-side. All CHS molecules are prepared in “chair” conformations. We do not consider “boat” or “twist” conformations, as gas-phase CHS exists predominantly -- if not exclusively -- in the “chair” conformation.47-48 Optimized geometries of atomic models with different configurations are shown in Figure 1. The intermolecular (ring center to ring center) distances between CHS molecules are 4.9 and 8.9 Å for face-to-face (structure i) and side-by-side (structure iv) configurations, respectively. Experimentally, both excimer lasers with a wavelength of 308 nm and HgXe lamps with UV irradiation have been used for the initiation of CHS photopolymerization.5, 49 The initial excitation energies in the TDESMD calculations, chosen based on the oscillator strengths of the optical transitions, are in the range of 5.5-5.8 eV, with a corresponding wavelength of 225-214 nm. The initial conditions in the simulation are in qualitative agreement with experimental processing conditions, where excitation is initiated slightly above the bandgap. The formalism underlying TDESMD calculations has been described in detail elsewhere.43 Here, the technical details of this work are available in the supporting information. During the first 1500 fs of the TDESMD trajectory for electrons hopping between the orbital pair (HOMO-5, LUMO+5) with structure i as the starting point, the kinetic energy accumulated by the 4 ACS Paragon Plus Environment

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system is relatively small. One observes limited reaction chemistry, such as hydrogen ejection and re-adsorption. With the passing of time, one begins to observe rich reaction events, such as ring opening and hydrogen migration, because of the increasing kinetic energy. These events lead to the formation of a dimer (structure ii) at 1626 fs (Figure 1b). The dimerization reaction can be identified as 2 Si6H12 → SiH3SiH=SiH(SiH2)8SiH3. In the following trajectory, the dimer undergoes cracking, e.g. through the elimination of Si2H4. The ejected groups are later recaptured, leading to the rearrangement of the dimer. At 1980 fs, one finds the dehydrogenation product of the dimer (structure iii, Figure 1c). Note that some Si atoms exhibit three or four Si-Si coordination numbers, which are typical in amorphous silicon. Reaction events are also rare for the first half of the TDESMD trajectory with electrons hopping between the orbital pair (HOMO-2, LUMO+4) when structure iv is the starting point. During this period of time, reaction products consist of a pair of intact monomers. Subsequently, one CHS molecule undergoes ring opening and dehydrogenation, while the other CHS molecule acts as a spectator. The ejected H2 drifts away from the dehydrogenated daughter. Thereafter, the spectator undergoes ring opening and hydrogen migration. The dimerization is observed at 1567 fs, where the reaction product (structure v) consists of H2 and a dimer with the chemical formula SiH3(SiH2)6(Si4H4)SiH3 (Figure 1e). Here, we find that the dimer contains an unsaturated Si4 ring, and we note that molecular silicon compounds with similar unsaturated Si4 rings have been reported by others.50-51 Later in the trajectory, one observes the dehydrogenation of the dimer. At 1732 fs, the reaction product (structure vi) is composed of two H2 molecules and the dehydrogenated dimer SiH3(SiH2)2(Si3H4)SiH=SiH(Si3H4)SiH3 (Figure 1f). It is interesting to note that there are two saturated Si3 rings in the silicon compound, and one would expect significant ring strain in the three-membered ring. However, in this case the three-membered ring is stabilized by bulky substituents.52 In fact, both saturated and unsaturated Si3 ring structures have been reported.53-56 5 ACS Paragon Plus Environment


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In the final stages of the trajectory, the system accumulates excessive kinetic energy and the silicon compound undergoes further cracking. Figure 2 depicts the computed density of states (DOS) for structures i–vi. The HOMO–LUMO gaps for structures i and iv are 5.0 eV, indicating that there is no absorption in the visible region, which is in qualitative agreement with experimental observation of CHS as a transparent liquid. Along TDESMD trajectories, one observes the decrease of HOMO–LUMO gaps for reaction products. Namely, HOMO–LUMO gaps for structures ii and v are 2.2 and 2.3 eV, respectively. The decrease of bandgaps in these structures compared to corresponding monomers is due to the formation of a double bond upon dimerization. The introduction of double bond requires transitions from sp3 to sp2 hybridization for Si atoms through hydrogen motion, which creates trap states for both electrons and holes. The HOMO–LUMO gaps for structures iii and vi are 2.1 and 1.8 eV, respectively, and the continuous decrease of bandgap is attributed to the increased number of double bonds for the former and to the increased number of ring structures for the latter. The partial charge densities of HOMO and LUMO orbitals for structures i–vi can be found in the Supporting Information (Figure S1 and S2). As expected, charge densities for unsaturated reaction products are mainly found near Si atoms that form double bonds. There is also a minor contribution from three-membered Si3 rings to the charge density distribution for the frontier orbitals of structure vi. Figure 3 shows computed PL spectra from adiabatic AIMD calculations at 300 K for structures i–vi. Equation S1 for the calculation of PL spectra enables us to account for fluctuations of KohnSham (KS) orbital energies associated with the thermal motion of nuclei.23, 57 By using this method, we assume there is an instantaneous relaxation of any excitation to the lowest excited state, in accordance with Kasha’s rule.58 The average emission 〈〉 (Equation S2) and linewidth 〈∆〉 (Equation S3) of computed PL spectra for structures i and iv are ~ 280 nm and ~ 20 nm, respec6 ACS Paragon Plus Environment

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tively. Along TDESMD trajectories, one observes a red-shift of the average emission and linewidth broadening in computed PL spectra for the reaction products. The red–shift of average emission is a result of the decreased HOMO-LUMO bandgaps, while the linewidth broadening is due to the increased number of silyl (–SiH3) or trisilyl (–Si3H7) groups attached to Si atoms of double bonds, which exhibit significant thermal motion. We explore three TDESMD trajectories for each model. It is found that for the face-to-face model, one trajectory shows the dimerization, while for the side-by-side model, dimerization events are observed in two trajectories. Here we compute simulated mass spectra in the basis of reaction products from trajectories in which dimerization reactions are available (Figure 4). It should be noted that the dominant peak is found at m = 180.61, corresponding to the CHS monomer, even though we only include results from the second half of trajectories. The reaction products of the first half and the early stages of the second half, are characterized by two isolated non-interacting CHS molecules. If results from the first half were included, then one would expect the continued increase of the peak intensity corresponding to the monomer. The peak corresponding to Si12H22 or Si12H20 is relatively abundant in all mass spectra. In Figure 4a the peak corresponding to Si12H24 is quite obvious. Such a peak is missing in Figure 4b or 4c, which is not surprising. In the TDESMD trajectory starting with the face-to-face configuration, dimerization precedes dehydrogenation, while the opposite trend is observed for trajectories starting with the side-by-side arrangement. In photopolymerization experiments of liquid CHS, however, it is expected that both pathways are available. In addition to monomer (Si6) and dimer (Si12) features, we also find several other features in the mass spectra. For example, there are twelve features (Si1–Si12) in Figure 4a. Within each feature, there are several peaks corresponding to species with the same number of silicon atoms but differing numbers of hydrogen atoms. The presence of Si1–Si5 and Si7–Si11 features is mainly due to cracking of the dimer during the final stages of the trajectory. 7 ACS Paragon Plus Environment


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In summary, TDESMD calculations are performed to model the photoinduced reactions of CHS molecules. In the TDESMD trajectories, a set of nuclear configurations is generated at subsequent instants of time, from which several reaction events are observed, including the dimerization of CHS. A postprocessing technique after the completion of TDESMD calculations is adopted to eliminate structural ambiguity and to facilitate the property analysis of the reaction products. A general scenario identified through multiple reactions can be summarized as follows: (i) ring-opening formation of a dimer, which has a stoichiometry the same or different from the pair of monomers, (ii) rearrangement induced by bond breaking, and (iii) decomposition through the elimination of small fragments. Characterization of the reaction products shows that dimers have smaller HOMO-LUMO gaps than monomer pairs. Reaction products also serve as starting points for AIMD calculations of PL spectra. We find that the PL spectra of dimers exhibit a red shift and linewidth broadening compared to those of monomer pairs. Simulated mass spectra capture possible fragments formed in postprocessed trajectories. This study suggests that TDESMD techniques have the potential to describe photoreactions beyond photodissociation. Our first-principles calculations focusing on the photodimerization of CHS take the first step towards achieving the longterm goal of better understanding the photopolymerization of CHS.

ASSOCIATED CONTENT Supporting Information. Theoretical methods. Partial charge densities of HOMO and LUMO orbitals for structures i–vi. Energy diagrams of reaction products along TDESMD trajectories. Experimental absorption spectra of CHS upon UV irradiation. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION 8 ACS Paragon Plus Environment

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Corresponding Author *E-mail: [email protected] Funding Sources This research has been supported by DOE grant number DE-SC0001717. Notes The authors declare no competing financial interest. ACKNOWLEDGMENT This research has been supported by NSF CHE-1413614 for methods development. Authors thank DOE BES NERSC facility for computational resources, allocation award #31857, “Computational Modeling of Photo-catalysis and Photo-induced Charge Transfer Dynamics on Surfaces” supported by the Office of Science of the DOE under contract no. DE-AC02-05CH11231. The authors gratefully acknowledge use of resources at the Center for Computationally Assisted Science and Technology (CCAST) at North Dakota State University. Support for that facility was provided by the State of North Dakota and DOE grant number DE-SC0001717. The authors would like to thank Douglas Jennewein for supporting and maintaining the High-Performance Computing system at the University of South Dakota. Gratefulness is also extended to Svetlana Kilina, Bakhtiyor Rasulev, Aaron Forde, Jon Vogel, Daniel Ramirez, and Braden Weight for collective discussion and editing. DSK thanks Sergei Tretiak and Center for Integrated Nanotechnologies (LANL) for discussions and hospitality during manuscript preparation. REFERENCES 1. Reed, G. T. The Optical Age of Silicon. Nature 2004, 427, 595-596. 2. Sun, K.; Shen, S.; Liang, Y.; Burrows, P. E.; Mao, S. S.; Wang, D. Enabling Silicon for SolarFuel Production. Chem. Rev. 2014, 114, 8662-8719. 3. Schulz, D. L.; Hoey, J.; Smith, J.; Elangovan, A.; Wu, X.; Akhatov, I.; Payne, S.; Moore, J.; Boudjouk, P.; Pederson, L.; et al. Si6H12/Polymer Inks for Electrospinning a-Si Nanowire Lithium Ion Battery Anodes. Electrochem. Solid-State Lett. 2010, 13, A143-A145. 4. Lu, X.; Anderson, K. J.; Boudjouk, P.; Korgel, B. A. Low Temperature Colloidal Synthesis of Silicon Nanorods from Isotetrasilane, Neopentasilane, and Cyclohexasilane. Chem. Mater. 2015, 27, 6053-6058. 5. Iyer, G. R. S.; Hobbie, E. K.; Guruvenket, S.; Hoey, J. M.; Anderson, K. J.; Lovaasen, J.; Gette, C.; Schulz, D. L.; Swenson, O. F.; Elangovan, A.; et al. Solution-Based Synthesis of Crystalline Silicon 9 ACS Paragon Plus Environment


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Figures

Figure 1. Geometries of (a, d) optimized monomer pairs prior to Rabi oscillations and (b, c, e, f) reaction products along TDESMD trajectories. White and yellow spheres represent H and Si atoms, respectively. Panels (b, c) are based on TDESMD calculations with electrons hopping between the orbital pair (HOMO-5, LUMO+5) using the atomic model of two Si6H12 molecules in a face-to-face configuration (structure i) as the starting point. Panels (e, f) are based on TDESMD calculations with electrons hopping between the orbital pair (HOMO-2, LUMO+4) using the atomic model of two Si6H12 molecules in a side-by-side configuration (structure iv) as the starting point. Orbitals are labeled based on HOMO−LUMO notation. LUMO+n means the nth orbital above LUMO and HOMO−n means the nth orbital below HOMO.



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Figure 2. DOS of (a, d) optimized monomer pairs prior to Rabi oscillations and (b, c, e, f) reaction products along TDESMD trajectories. The filled area represents occupied states, whereas the unfilled area represents unoccupied states. Panels (b, c) are based on TDESMD calculations with electrons hopping between the orbital pair (HOMO-5, LUMO+5) using the atomic model of two Si6H12 molecules in a face-to-face configuration (structure i) as the starting point. Panels (e, f) are based on TDESMD calculations with electrons hopping between the orbital pair (HOMO-2, LUMO+4) using the atomic model of two Si6H12 molecules in a side-by-side configuration (structure iv) as the starting point.


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Figure 3. PL spectra from adiabatic AIMD calculations at 300 K for (a, d) optimized monomer pairs prior to Rabi oscillations and (b, c, e, f) reaction products along TDESMD trajectories. Panels (b, c) are based on TDESMD calculations with electrons hopping between the orbital pair (HOMO-5, LUMO+5) using the atomic model of two Si6H12 molecules in a face-to-face configuration (structure i) as the starting point. Panels (e, f) are based on TDESMD calculations with electrons hopping between the orbital pair (HOMO-2, LUMO+4) using the atomic model of two Si6H12 molecules in a side-by-side configuration (structure iv) as the starting point.



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Figure 4. Simulated mass spectra obtained by postprocessing the final 1 ps of TDESMD trajectories with electrons hopping between orbital pairs (a) (HOMO-5, LUMO+5) using the face-to-face model (structure i) as the starting point, (b) (HOMO-4, LUMO+5), and (c) (HOMO-2, LUMO+4) using the side-by-side model (structure iv) as the starting point. Simulated mass spectra are expanded to show species with low intensities. The molecular weight of possible species is used for the generation of simulated mass spectra.


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