Computational Materials Science 49 (2010) S21–S24 Contents lists available at ScienceDirect Computational Materials Science journal homepage: www.elsevier.com/locate/commatsci First-principles study of the thermally induced polymerization of cyclopentasilane Phan Viet Dung a, Pham Tien Lam a, Nguyen Dinh Duc b, Ayumu Sugiyama a, Tatsuya Shimoda a,c, Dam Hieu Chi a,b,c,* a Japan Advanced Institute of Science and Technology, 1-1 Asahidai, Nomi, Ishikawa 923-1292, Japan Vietnam National University, 144 Xuan Thuy, Cau Giay, Hanoi, Viet Nam c ERATO, Shimoda Nano-Liquid Process Project, Japan Science and Technology Agency, 2-5-3 Asahidai, Nomi, Ishikawa 923-1211, Japan b a r t i c l e i n f o Article history: Received 11 August 2009 Accepted February 2010 Available online 24 April 2010 Keywords: First-principle calculation Cyclopentasilane Polymerization a b s t r a c t The molecular structures and vibration modes of cyclopentasilane (Si5H10) have been examined by employing ab initio and density-functional methods Three different structures of Si5H10 with different symmetries are analyzed, and the results show that the envelope (Cs) and the twist (C2) forms have similar energies and that the planar form (D5h) is about 50 meV less stable than the Cs and C2 forms The ringopen reaction of Si5H10 is investigated in detail by using the first-principles molecular-dynamics simulation for screening the reaction pathways The formation of Si–H–Si is found to play an important role in the ring-open reaction Ó 2010 Elsevier B.V All rights reserved Introduction Organic semiconductors are cost-effective and compatible with flexible substrates, and owing to these advantages, studies investigating the feasibility of using a variety of organic semiconductors in electronic devices have been conducted extensively The motilities of organic semiconductors are comparable to those of amorphous silicon; however, improving the reliability of the former is still a challenging problem The production of silicon electronic devices using conventional vacuum processes and vapor phase deposition requires complex manufacturing processes, which involve the use of a number of materials and high cost [11,8,10,9,14,15,4] However, the devices can be fabricated by employing a printing technique involves the use of silicon materials in the liquid phase The use of this technique will significantly decrease the complexity of required equipment, reduce the cost involved, produce large-scale electronic circuits, and open up new applications that are prohibitively expensive when current techniques are used [13] Among silicon materials, cyclopentasilane is extremely useful for the fabrication of devices because of its ability to transform to high purity silicon and its relative stability In addition, the cyclopentasilane can act as a solvent to dissolve the polymer formed for applying to the inkjet printing technique [1,12] However, owing to the difficulties faced in the synthesis of cyclopent- * Corresponding author at: Japan Advanced Institute of Science and Technology, 1-1 Asahidai, Nomi, Ishikawa 923-1292, Japan Tel.: +81 76 151 1584; fax: +81 76 151 1535 E-mail address: dam@jaist.ac.jp (D.H Chi) 0927-0256/$ - see front matter Ó 2010 Elsevier B.V All rights reserved doi:10.1016/j.commatsci.2010.02.042 asilane and instability of the compound, very few studies on cyclopentasilane have been reported so far when compared to those on its counterpart, cyclopentane Further, most researches focus either the geometrical structures or the anion of cyclopentasilane [7,6] The ring-opening and polymerization reactions of cyclopentasilane which are manifested in the experiment have not received any attention from theoretical approach Further studies on the properties and polymerization of cyclopentasilane are required to develope more efficient fabrication techniques involving liquid silicon In this paper, we report on our computational study on cyclopentasilane carried out by using ab initio and density-functional methods Three different structures of Si5H10 with different symmetries are analyzed, and the results show that the envelope (Cs) and twist (C2) forms have similar energies and the planar form (D5h) is about 50 meV less stable than the Cs and twist C2 forms The ring-open reaction is investigated in detail by using the firstprinciples molecular-dynamics simulation for screening the reaction pathways The formation of Si–H–Si is found to be essential in the ring-open reaction of Si5H10 Computational method The calculations in this study were based on the HF theory, MP2 theory, and the density-functional theory (DFT), which are formulated using the cc-pVTZ basis set with the Gaussian 03 program [5] The ‘‘frozen core approximation” was applied for MP2 computations For the DFT calculations, Becke’s three parameter exchange functional together with the correlation functional developed by Lee, Yang, and Parr (B3LYP) and the correlation functional devel- S22 P.V Dung et al / Computational Materials Science 49 (2010) S21–S24 oped by Perdew, Burke, and Ernzerhof (PBE) were used as implemented in Gaussian 03 along with the cc-pVTZ basis set The geometrical structures of cyclopentasilane were optimized within the given symmetry point group Vibrational frequencies were evaluated analytically at the B3LYP/cc-pVTZ level Calculations based on the DFT using the PBE and BLYP functionals and the TNP basis set were also carried out with the Dmol3 [2,3] package for comparison The first-principles molecular-dynamics simulation based on the DFT with the BLYP functional is carried out by using the Dmol3 package for screening the reaction pathways of the ring-open reaction Fig Significant atomic vibrations with the lowest frequencies for the twist and envelope CPS calculated by B3LYP/cc-pVTZ Results and discussion 3.1 Geometrical structures of cyclopentasilane Fig shows the three structures of cyclopentasilane determined by our calculations, namely, twist (C2), envelope (Cs), and planar (D5h) Results of our calculations show that the twist structure is the most stable structure for cyclopentasilane The relative energies of each structure are summarized in Table Results of our calculation confirm that two structures of cyclopentasilane, twist (C2) and envelope (Cs), have similar energies at all the theoretical levels employed (the difference in their energies energy is less than 0.03 meV) These results are in good agreement with the results of previous theoretical studies [7,6] The twist (C2) structure can be considered as a distorted envelope, and only a slight distortion is needed for a transformation between the two structures The four silicon atoms that are present in a single plane in the envelope structure deviate from planarity by approximately 12.1° The energy barrier for this structural transformation is evaluated to be less than 0.1 meV at all the theoretical levels employed We also calculated the vibrational frequencies for the two most stable structures (Fig 2) Calculation by B3LYP/ccpVTZ shows that the twist and envelope structures have the lowest vibrational frequencies, at 2.6 and 3.8 cmÀ1, respectively It is apparent that these vibrations correspond to the structural transformation between the two structures This result confirms that the structural transformation can be achieved easily Fig Infrared spectra calculated by B3LYP/cc-pVTZ Blue and red solid lines indicate the infrared spectra of the twist and envelope structures, respectively The green dotted lines indicate the infrared spectrum of the intermediate structures after the formation of the Si–H–Si bridge bond (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.) The planar (D5h) structure was characterized as a second-order stationary point by using the HF, B3LYP, and MP2 theory with the cc-pVTZ basis set, and it is 60.6 meV higher in energy (by the MP2/ cc-pVTZ) than the twist and envelope structures In contrast, calculation by the PBE/cc-pVTZ method characterized the planar (D5h) structure as a minimum structure having no imaginary frequency This result suggests that the electron correlation treating method can affect strongly to the obtained results in studies of silicon materials 3.2 Vibrational spectra Fig Structures of cyclopentasilane with geometries optimized by B3LYP/ccpVTZ: twist (C2), envelope (Cs), and planar (D5h) Table Comparison of energies of the structures of CPS and the twist structure The vibrational spectra of the two most stable structures of cyclopentasilane are evaluated analytically at the B3LYP/cc-pVTZ level (Fig 3) The vibrational frequencies corresponding to the Si–H bond are found in the region from 2100 to 2200 cmÀ1 The vibrational frequencies corresponding to the scissoring and rocking vibrations of the H–Si–H groups are found in the region from 850 to 950 cmÀ1 and from 300 to 400 cmÀ1, respectively The vibrational frequencies corresponding to the wagging vibrations of the H–Si–H groups are located at around 725 cmÀ1, and the breathing frequency of the pentagonal ring is found at 344.8 cmÀ1 Further, it should be noted that no vibrational frequency is found in the region from 1000 to 2100 cmÀ1 Structure HF/ccpVTZ PBE/ccpVTZ B3LYP/ccpVTZ MP2/ccpVTZ 3.3 Thermally induced ring-open reaction Twist (eV) Envelope (meV) Planar (meV) 0.03 0.01 0.03 0.03 0.79 61.20 53.65 60.58 In the next step of our study, for screening the reaction pathways of the ring-open reaction, we carry out the first-principles molecular-dynamics simulation based on the DFT using BLYP with the Dmol3 package The simulation is performed by using an S23 P.V Dung et al / Computational Materials Science 49 (2010) S21–S24 Fig Three possible intermediate structures in the ring-open reaction, predicted by B3LYP/cc-pVTZ Table Comparison of energies of the intermediate structures ant the twist structure Intermediate structure SiH2–Si3H6–SiH2 SiH–Si3H6–SiH3 Si–H–Si bridge bond HF/cc-pVTZ PBE/cc-pVTZ B3LYP/cc-pVTZ Singlet Triplet (eV) Singlet Triplet (eV) Singlet Triplet Non-stable 1.979 eV 1.938 eV 2.100 1.906 1.926 Non-stable Non-stable Non-stable 2.896 2.758 2.888 Non-stable 2.052 eV 1.682 eV 2.802 eV 2.677 2.683 Fig Significant atomic vibration modes corresponding to the Si–H–Si bridge bond, calculated by B3LYP/cc-pVTZ Similar to the frequency region of the twist and envelope structures, the results of our calculation for the structure comprising the Si–H–Si bridge bond show the Si–H stretching vibrational frequencies at greater than 1950 cmÀ1 and H–Si–H twisting and rocking vibrational modes, at frequencies less than 950 cmÀ1 We also reveal frequencies corresponding to the Si–H–Si bridge bond as shown in Fig The results of our calculation for the intermediate structure which surrounded by some CPS molecules shows that the frequencies of the Si–H–Si bridge bond are influenced by the presence of other CPS molecules in the surrounding environment Furthermore, the other cyclopentasilane molecules also contribute to a decrease in the difference in energies between the Si5H10 molecule and the intermediate structure comprising the Si–H–Si bridge bond Conclusion appropriate periodic supercell including four cyclopentasilane molecules A canonical (NVT) ensemble is employed at 800 °C Our simulation results suggest three possible intermediate structures in the thermally induced ring-open reaction, namely, SiH2–Si3H6–SiH2, SiH–Si3H6–SiH3, and Si–H–Si bridge bond structures (Fig 4) On the basis of the results of the first-principles molecular-dynamics simulation, we carefully evaluate the stability of the three intermediate structures by HF, PBE, and B3LYP theory with the cc-pVTZ basis set The results obtained are summarized in Table The results of our calculation show that the first intermediate structure is unstable in the singlet state but stable in the triplet state, at all the theoretical levels employed In contrast, the second and third intermediate structures are found to be stable in both the singlet and triplet states, at all the theoretical levels employed except DFT/PBE It is important to note that the third intermediate structure, in which the Si–H–Si bridge bond is formed, is found to be more stable than the second intermediate structure in the singlet state These results strongly suggest that the formation of Si–H–Si plays an important role in the ring-open reaction of Si5H10 The vibrational spectra of the intermediate structure after the formation of the Si–H–Si bridge bond are also evaluated (Fig 3) A computational study on cyclopentasilane is carried out by using ab initio and density-functional methods Three different structures of Si5H10 with different symmetries are analyzed, and the results show that the envelope (Cs) and the twist (C2) structures have similar energies that are also the lowest The planar structure (D5h) is about 50 meV less stable than the Cs and C2 structures Vibrational analysis is carried out and the thermally induced ring-open reaction is investigated in detail by using the first-principles molecular-dynamics simulation for screening the reaction pathways The formation of Si–H–Si is found to play an important role in the ring-open reaction of Si5H10 Acknowledgements This research was partly supported by the Special Coordination Funds for Promoting Science and Technology commissioned by the MEXT, Japan, by the Special project QGTD.08.09 commissioned by the Vietnam National University, Hanoi, and by the Fundamental Research Project 103.01.77.09 commissioned by NAFOSTED, Vietnam S24 P.V Dung et al / Computational Materials Science 49 (2010) S21–S24 The computations presented in this study were performed at the Center for 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On the basis of the results of the first-principles molecular-dynamics simulation, we carefully evaluate the stability of the three intermediate structures by HF, PBE, and B3LYP theory with the