Three different structures of Si5H10with different symmetries are analyzed, and the results show that the envelope Cs and the twist C2 forms have sim-ilar energies and that the planar fo
Trang 1First-principles study of the thermally induced polymerization of cyclopentasilane
Dam Hieu Chia,b,c,*
a
Japan Advanced Institute of Science and Technology, 1-1 Asahidai, Nomi, Ishikawa 923-1292, Japan
b
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
a r t i c l e i n f o
Article history:
Received 11 August 2009
Accepted 2 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 Si5H10with different symmetries are analyzed, and the results show that the envelope (Cs) and the twist (C2) forms have sim-ilar energies and that the planar form (D5h) is about 50 meV less stable than the Csand C2forms The ring-open reaction of Si5H10is investigated in detail by using the first-principles molecular-dynamics simula-tion for screening the reacsimula-tion pathways The formasimula-tion of Si–H–Si is found to play an important role in the ring-open reaction
Ó 2010 Elsevier B.V All rights reserved
1 Introduction
Organic semiconductors are cost-effective and compatible with
flexible substrates, and owing to these advantages, studies
investi-gating the feasibility of using a variety of organic semiconductors
in electronic devices have been conducted extensively The
motil-ities of organic semiconductors are comparable to those of
amor-phous 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
tech-nique will significantly decrease the complexity of required
equip-ment, 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]
How-ever, owing to the difficulties faced in the synthesis of
cyclopent-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 cyclopent-asilane [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 stud-ies on the propertstud-ies and polymerization of cyclopentasilane are required to develope more efficient fabrication techniques involv-ing liquid silicon
In this paper, we report on our computational study on cyclo-pentasilane carried out by using ab initio and density-functional methods Three different structures of Si5H10with different sym-metries 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 Csand twist C2forms The ring-open reaction is investigated in detail by using the first-principles molecular-dynamics simulation for screening the reac-tion pathways The formareac-tion of Si–H–Si is found to be essential
in the ring-open reaction of Si5H10
2 Computational method The calculations in this study were based on the HF theory, MP2 theory, and the density-functional theory (DFT), which are formu-lated using the cc-pVTZ basis set with the Gaussian 03 program[5] The ‘‘frozen core approximation” was applied for MP2 computa-tions 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-0927-0256/$ - see front matter Ó 2010 Elsevier B.V All rights reserved.
* 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).
Contents lists available atScienceDirect Computational Materials Science
j o u r n a l h o m e p a g e : w w w e l s e v i e r c o m / l o c a t e / c o m m a t s c i
Trang 2oped by Perdew, Burke, and Ernzerhof (PBE) were used as
imple-mented in Gaussian 03 along with the cc-pVTZ basis set The
geo-metrical structures of cyclopentasilane were optimized within the
given symmetry point group Vibrational frequencies were
evalu-ated 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
compar-ison 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
3 Results and discussion
3.1 Geometrical structures of cyclopentasilane
Fig 1 shows the three structures of cyclopentasilane
deter-mined by our calculations, namely, twist (C2), envelope (Cs), and
planar (D5h) Results of our calculations show that the twist
struc-ture is the most stable strucstruc-ture for cyclopentasilane The relative
energies of each structure are summarized inTable 1
Results of our calculation confirm that two structures of
cyclo-pentasilane, 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
enve-lope, and only a slight distortion is needed for a transformation
be-tween 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
trans-formation 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/cc-pVTZ shows that the twist and envelope structures have the lowest
vibrational frequencies, at 2.6 and 3.8 cm 1, respectively It is
apparent that these vibrations correspond to the structural
trans-formation between the two structures This result confirms that
the structural transformation can be achieved easily
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, calcu-lation 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 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 cm 1 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 cm 1and from 300 to 400 cm 1, respectively
The vibrational frequencies corresponding to the wagging vibrations of the H–Si–H groups are located at around 725 cm 1, and the breathing frequency of the pentagonal ring is found at 344.8 cm 1 Further, it should be noted that no vibrational fre-quency is found in the region from 1000 to 2100 cm 1
3.3 Thermally induced ring-open reaction
In the next step of our study, for screening the reaction path-ways 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
Fig 1 Structures of cyclopentasilane with geometries optimized by
B3LYP/cc-pVTZ: twist (C 2 ), envelope (C s ), and planar (D 5h ).
Table 1
Comparison of energies of the structures of CPS and the twist structure.
Structure
HF/cc-pVTZ
PBE/cc-pVTZ
B3LYP/cc-pVTZ
MP2/cc-pVTZ
Envelope
(meV)
Fig 2 Significant atomic vibrations with the lowest frequencies for the twist and envelope CPS calculated by B3LYP/cc-pVTZ.
Fig 3 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.)
Trang 3appropriate 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
struc-tures (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 2
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
ex-cept 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)
Similar to the frequency region of the twist and envelope struc-tures, the results of our calculation for the structure comprising the Si–H–Si bridge bond show the Si–H stretching vibrational fre-quencies at greater than 1950 cm 1and H–Si–H twisting and rock-ing vibrational modes, at frequencies less than 950 cm 1 We also reveal frequencies corresponding to the Si–H–Si bridge bond as shown inFig 5 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 pres-ence of other CPS molecules in the surrounding environment Fur-thermore, the other cyclopentasilane molecules also contribute to
a decrease in the difference in energies between the Si5H10 mole-cule and the intermediate structure comprising the Si–H–Si bridge bond
4 Conclusion
A computational study on cyclopentasilane is carried out by using ab initio and density-functional methods Three different structures of Si5H10with different symmetries are analyzed, and the results show that the envelope (Cs) and the twist (C2) struc-tures have similar energies that are also the lowest The planar structure (D5h) is about 50 meV less stable than the Csand C2 struc-tures Vibrational analysis is carried out and the thermally induced ring-open reaction is investigated in detail by using the first-prin-ciples 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
Fig 4 Three possible intermediate structures in the ring-open reaction, predicted by B3LYP/cc-pVTZ.
Table 2
Comparison of energies of the intermediate structures ant the twist structure.
Fig 5 Significant atomic vibration modes corresponding to the Si–H–Si bridge
bond, calculated by B3LYP/cc-pVTZ.
Trang 4The computations presented in this study were performed at
the Center for Information Science, Japan Advanced Institute of
Sci-ence and Technology
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