Mechanisms of Ion-Beam Modification of Terthiophene Oligomers from

ARTICLE pubs.acs.org/JPCC

Mechanisms of Ion-Beam Modification of Terthiophene Oligomers from Atomistic Simulations Travis W. Kemper† and Susan B. Sinnott*,†,‡ †

Department of Materials Science and Engineering and ‡Quantum Theory Project, University of Florida, Gainesville, Florida 32611-6400, United States ABSTRACT: Ion-beam deposition on organic surfaces is a common approach to induce surface modification. Here, the difference in argon and polyatomic thiophene hyperthermal deposition on terthiophene oligomers is explored in classical molecular dynamics simulations. The forces on the atoms are determined using the second-generation reactive empirical bond order potential for hydrocarbons that is modified to include sulfur. Details of the potential fit and parametrization are provided. The simulations predict that the thiophene induces fracture of the terthiophene rings while largely retaining the chemical structure of terthiophene. Conversely, argon is predicted to alter the number of carbons within the terthiophene during modification.

’ INTRODUCTION Sulfur interactions with hydrocarbons play a crucial role in processes including the vulcanization of rubber1 and more relevantly its reclamation,2 oil processing,3 gas sensors,4,5 and batteries.6,7 In addition, thiophene-based oligomers and polymers are widely used in organic electronics.8,9 Alternatives to traditional solutionbased processing techniques10 are emerging to produce conducting organic films for use in electronic devices.11,12 Currently, there are many efforts being made using a combination of methods to produce materials for the next generation of electronics.13,14 One method that combines ion-beam and thermal deposition principles to produce stable conducting organic films is surface polymerization by ion-assisted deposition (SPIAD).15
19 This method employs the codeposition of thermal α-terthiophene and hyperthermal thiophene cations to induce polymerization at the gas
solid interface. The production of higher molecular weight species during the polymerization process is beneficial for organic photovoltaics in that the films show an increase in stability when compared to the precursors.15 Also the increase in electron conjugation length produces a red shift in the UV/vis absorption spectra,16 which allows for absorption of visible light at longer wavelengths. One of the main advantages of deposition treatments over wet chemical processes is the lack of solvents that can affect device performance.20 As with other plasma and ion-beam based processing methods, further understanding of the complex chemical processes occurring during SPIAD is expected to aid in optimizing industrial applications.21,22 To achieve this goal, classical atomicscale molecular dynamics (MD) simulations are employed here to illuminate the chemical mechanisms involved in certain stages of the process. In particular, these simulations offer a way to model multifaceted chemical reactions while precisely monitoring atomic and molecular compositions. The forces on the atoms r 2011 American Chemical Society

are determined using the second-generation reactive empirical bond (REBO) potential, which is able to capture bond breakage and formation by dynamically evaluating the bonding of atomic pairs depending on the immediate environment.23,24 This potential has been successfully applied to the study of the mechanical properties of graphene,25 tribology,26,27 and ion-beam modification of polymers.21,28
30 To model the SPIAD process for thiophene-based systems, the REBO potential is extended in this work to include sulfur. We thus build on previous efforts to extend the potential to include fluorine,28 oxygen,31 graphene oxide,32 and molydisulfide.33 Other modifications include the addition of long-range interactions. Stuart et al. coupled the first generation REBO potential to a Lennard-Jones (LJ) potential34 using a switching function to create the adaptive intermolecular REBO (AIREBO) potential.35 This switching function interpolates between the two potentials for each pair interaction based on the bond length, bond strength, and coordination. The AIREBO potential has been successfully applied to model diamond,35 graphite,35 graphene,36,37 and the abolition of solid surfaces.38
40 Another method to extend the REBO potential used a screening function,41 and has been successfully used to model crack formation in diamond and the breaking of carbon nanotubes. An additional recent method is long-range carbon bond order potential II (LCBOPII),42 which includes the long-range interactions in the form of a modified Morse potential.43 However, these complex inclusions come with added computational costs. For example, the AIREBO potential was found to be an order of magnitude slower than the first-generation REBO potential.44 Received: September 21, 2011 Revised: October 25, 2011 Published: October 26, 2011 23936

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As the primary motivation of this work involves changes to the intramolecular structure and the formation of intermolecular, covalent bonds, the long-range effects are included by coupling the second-generation REBO potential to a LJ potential using a bicubic spline. This has been found to accurately capture dispersion in oligomer systems with a minimal increase in computational costs.45 The rest of this paper is organized as follows: first the potential formalism used to incorporate sulfur into the second-generation REBO potential is discussed. Then the method used to establish parameters for the new formalism is examined. The potential is then evaluated for a set of small molecules, after which simulations of Ar and thiophene deposition on a thin film of terthiophene are investigated. The results are analyzed according to the yields of molecular species. Comparisons to previously published ab initio calculations are then made.

’ COMPUTATIONAL DETAILS Functional Form of the REBO Potential. Under the second-

generation REBO potential formalism, the binding energy Eb of a system is a sum of nearest-neighbor pair potential energies modulated by a bond order term Eb ¼

∑i jð∑> iÞ½V R ðrijÞ
bij V R ðrijÞ

The torsion and conjugation terms for the C
C bond are unchanged from the second-generation REBO potential for hydrocarbons24 and are not applied to the newly parametrized S
C, S
S, or S
H bonds. The bijσ
π term captures the bonding energy’s environmental dependence on angle and coordination 2 3
1=2 ε ε 1 þ ∑ fC ðrij ÞGðθjik Þe½λjik ½ðrij
Rij Þ
ðrik
Rik Þ 6 7 k6¼ i, j bijσ
π ¼ 4 5 C H S þ Pij ðNi , Ni , Ni Þ ð6Þ where k is a nearest neighbor of atom i not equal to atom j. The angular function G(θjik) is dependent on the angle between atoms j, i, and k, and for sulfur centered bonds it is described by a sixth-order polynomial of cosine θ as follows Gij ðθÞ ¼

Nielm ¼

elm

∑ fC ðrik Þ

V ðrij Þ ¼ fC ðrij Þ

∑ Bne n ¼ 1, 2, 3

ð
βn rij Þ

where elm is C, H, and S. The term that captures radicals and conjugation is a tricubic spline conj

ΠRC ¼ Fij ðNit , Njt , Nij Þ

ð4Þ The bond order term (bij) in eq 1 encompasses all the multibody effects to modulate the covalent bonding between atoms. It is composed of terms capturing the bond energy’s dependence on bond angle (G), coordination (Pij), torsion (ΠDH), and conjugation (ΠRC) 1 bij ¼ ½bσij
π þ bσji
π  þ bDH þ ΠRC ij 2

ð9Þ

which is dependent on the summation over all elements (C, H, S) of the number of neighbors (Nielm) of atoms i and j (Nit) and (Njt), respectively, and the number of conjugate neighbors Nijconj. The number of conjugate neighbors (Nijconj) is dependent on the number of carbon neighbors of atoms i and j conj

Nij

ð3Þ

Here, Q, A, α, Bn, and βn are the pair parameters that depend on bond type. The function fc smoothly reduces the potential energy to zero between minimum and maximum cutoff values Dmin and Dmax, as follows 8 > 1 rij < Dmin " !# > > min > > : rij < Dmax 0

ð7Þ

ð8Þ

k6¼ i, j

and A

an cosn
1 θ

The term λjik describes the three-body transition states of C and H and is 0 for interactions involving sulfur. The coordination function Pij in eq 6 is a three-dimensional cubic spline dependent on the number of carbon, hydrogen, and sulfur neighbors as defined as

ð1Þ

where rij is the scalar distance between atoms i and j and bij is the bond order term. The repulsive pair term (VR) is representative of the Pauli repulsion between electron clouds, and the attraction term (VA) captures the atomic attraction due to valence electrons forming covalent bonds. The formalism of the pair terms is unaltered from the second-generation REBO potential for hydrocarbons24 and is as follows ! Q R ð2Þ Aeð
αrij Þ V ðrij Þ ¼ fC ðrij Þ 1
rij

∑

n ¼ 1, 7

carbon

¼1 þ ½ þ ½

∑

k6¼ i, j

fC ðrik ÞFðxik Þ2

carbon

fC ðrjl ÞFðxjl Þ2 ∑ l6¼ i, j

Here the function F(xik) is 8 > 1 xik < 2 > > : xik > 3 0

ð10Þ

ð11Þ

and the function xik is xik ¼ Nit
fC ðrik Þ

ð12Þ

To capture the rotation of groups around a carbon
carbon bond a dihedral function is used with the form conj

¼ Tij ðNit , Njt , Nij Þ bDH ij carbon carbon

½

ð5Þ 23937

∑ ∑ ð1
cos2 Θkijl ÞfC ðrjlÞfC ðrik Þ k6¼ i, j l6¼ i, j

ð13Þ

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Table 1. Pair Parameters

A (eV)

S
C

S
H

S
S 4489.11

755.86

1033.73

α

1.90

0.81

1.79

Q (Å)

0.95

0.09

0.16 4906.07

B1 (eV)

1423.74

1167.23

β1

1.94

0.85

1.78

Dmin (Å)

2.10

1.50

2.30

Dmax (Å)

2.40

1.80

2.60

via a bicubic spline 2 !12 σ ij LJ 4 þ Vij ¼ 4ε rij

σij rij

!6 3 5

ð15Þ

The interpolation region of the bicubic spline is from the maximum REBO cutoff of Dmax, eq 4, to 0.95σij, and the LJ maximum cutoff is set to 2.5σij. The parameters εij and σij are calculated based on the element-dependent parameters using mixing rules pffiffiffiffiffiffiffi ð16Þ εij ¼ εi εj and σ ij ¼

Figure 1. Bond stretching curves for S
C, S
H, and S
S bonds.

where θkijl is the torsional angle between atoms k, i, j, and l; fC(rik) is the cutoff function; and Tij is a tricubic spline. The cos θkijl function is found using the vectors B rik, B rij, and B rjl in the following way " # ðB r ik  B r ij Þ 3 ðr F r jl Þ ij  B ð14Þ cos Θkijl ¼ rik rjl rij2 sin θjik sin θijl The long-range interactions are captured by a LJ potential that is smoothly interpolated with the second-generation REBO potential

1 ðσi þ σ j Þ 2

ð17Þ

Parameterization for C, H, S Systems. The parameter fitting of the sulfur hydrocarbon interactions is conducted in an analogous approach to that of oxygen.31 In particular, sulfur interactions with carbon and hydrogen are added while leaving the original hydrocarbon interaction the same as the second-generation form of the REBO potential.24 The data set for the C
H
S interactions consists of the properties of solid and molecular systems. The properties of the solid systems are determined using density functional theory (DFT) within the Vienna Ab-initio Simulation Package (VASP) software package.46
49 The electron wave functions are calculated using projected augmented wave (PAW) pseudopotentials50,51 and the generalized gradient approximation (GGA)52,53 (PBE)54,55 method. An energy cutoff of 600 eV, a Monkhorst
Pack56 k-point mesh of 6  6  6, and a force convergence criterion of 0.05 eV/Å are used. The properties of molecular systems are calculated with GAUSSIAN03.57 Molecular relaxed geometries and bond stretching and bending curves are determined at the B3LYP58
61/6-31G* level, while atomization and dissociation energies are calculated with the G3 approach.62 Nonzero point-corrected energies are used. While the change in energy due to bond stretching and bending is calculated at the B3LYP level, curves are shifted in energy to have minima that correspond to G3 values. The two-body related parameters, eq 2 and eq 3, for the sulfur
carbon, sulfur
hydrogen, and sulfur
sulfur interactions are fit prior to the fitting of the multibody parameters. The pair parameters are fit to molecular bond stretching curves and solid strain curve that are illustrated in Figure 1. During the fitting of the multibody term bij is left as a fitted parameter and given a range 0 to 1, thus establishing a set of pair parameters capable of capturing a range of values of the bond order term bij. Molecular species are weighted more heavily than solid phases due to the 23938

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Table 3. Tricubic Spline Values for the Coordination Function Pij for Carbon Centered Bonds NC

NH

NS

0

0

0

0.0000

0.0000

0.3412

0 0

0 0

1 2


0.0128 0.1210

0.9398 0.4464

0.4869 0.4738

0

0

3


0.0168


0.2290

0.3476

0

1

0

0.0000

0.2093

0.3797

0

1

1

0.0671

0.2899

0.4090

0

1

2


0.0770


0.4419

0.3480

0

2

0

0.0079

-0.0644

0.3389

0

2

1

0.0126


0.2852

0.4293

0 1

3 0

0 0

0.0161 0.0000

-0.3039 0.0100

0.4405 0.5200

Figure 2. Bond energy as a function of bond angle for fit of G(θ) function coefficients.

Table 2. Angular Parameters

a1 a2

PCC

PCH

PCS

1

0

1

0.0950

0.4279

0.4288

1

0

2


0.1650


0.3032

0.3394

1

1

0

0.0030

-0.1251

0.3142

X
S
C

X
S
H

X
S
S

1

1

1


0.1282


0.3864

0.3380

6.97  10
2 5.80  10
2

2.95  10
3 1.60  10
3

2.07  10
2 6.88  10
2

1

2

0

0.0063

-0.2989

0.4099

2

0

0

0.0000

-0.1220

0.3175


1


2


2

2 2

0 1

1 0


0.0927 0.0032


0.2156 -0.3005

0.3848 0.4000

3

0

0

0.0000

-0.3076

0.3966

a3

2.11  10

1.83  10

a4

1.38  10
1

2.74  10
2


7.74  10
2

a5


6.63  10
2

1.14  10
2


5.37  10
2


1


2


2

a6


1.31  10

a7


2.01  10
3

8.05  10


1.98  10

2.62  10


1.20  10
2

1.41  10
2

largely molecular nature of most sulfur hydrocarbon systems. The fitted pair parameters are given in Table 1. Potential cutoff distances Dmin and Dmax are chosen to be between the first and second nearest neighbor distances within the molecular data set. Once the minimum error for the data set is established, the angular term is then fit. A sixth-order polynomial, eq 7, is used for the angular function G(θjik) in congruency with the hydrocarbon angular function. This function also possesses the flexibility to capture the potential energy response to changes in angle while maintaining reasonable values at angles less than 60°. Parameters for each sulfur centered angle type Xk
Si
Sj, Xk
Si
Cj, and Xk
Si
Hj, where Xk is C, H, or S, are fit to the bond bending curves of representative molecules (Figure 2). For the Xk
Si
Sj parameters, the S3 molecule and the S8 ring’s bond bending curves are included in the fitting database. The Xk
Si
Cj parameters are fitted to thiobismethane, and the Xk
Si
Hj parameters are fit to hydrogen sulfide. At low bond angles, bond energies are fit to 1.0 eV at 0° for the sulfur
sulfur and sulfur
hydrogen terms and 0.5 eV for the sulfur
carbon term, to prevent the unphysical stabilization of overcoordinated structures. The resulting angular function parameters are displayed in Table 2. A tricubic spline is used for the coordination function Pij(NiC, H Ni ,NiS) for each bond type: Ci
Cj, Ci
Hj, Ci
Sj, Si
Cj, and Si
Hj. The value of the coordination function for each integer value of nearest neighbors is given in Table 3 and Table 4 . The values of Ci
Hj and Ci
Cj coordination functions, where NiS are zero, are the same as the bicubic spline values for the second-generation REBO potential and are given in italics in Table 3. For hydrocarbon bonds with sulfur neighbors and sulfur hydrocarbon bonds, spline values are fitted to dissociation and atomization energies of small

Table 4. Tricubic Spline Values for the Coordination Function Pij for Sulfur Centered Bonds NC

NH

NS

PSC

PSH

PSS

0

0

0


0.2315


0.0018

0.0007

0

0

1


0.2844


0.0052

0.0141

0 0

0 1

2 0

0.0000
0.2138

0.0000
0.0063

0.1089 0.0111

0

1

1

0.0000

0.0000

0.1028

0

2

0

0.0000

0.0000

0.1054

1

0

0


0.2765


0.0107

0.0070

1

0

1

0.0000

0.0000

0.1063

1

1

0

0.0000

0.0000

0.1051

2

0

0

0.0000

0.0000

0.0494

molecules. It is found that overcoordination of sulfur occurs in some cases; therefore, the coordination function for sulfur is expanded to penalize over coordination as indicated in Table 4. Atomization energies are included to establish the magnitude of the total cohesive energy of the molecule. This ensures reasonable changes in energy while exploring the potential energy space of a system during a MD simulation. Dissociation energies are also included in the fitting database to ensure the best possible description of the change in energy during chemical reactions. The results of the fitted dissociation energies are shown in Figure 3. The average deviation from G3 values is 13.7% Validation and Testing. To test the performance of the potential under various bonding environments, atomization energies of a set of representative molecules are calculated with the modified second-generation REBO potential and compared to G3 values. Characteristic bonding environments include elemental sulfur molecules, hydrocarbon molecules with S
S bonds, thials, thiols, sulfides, and ring structures including sulfurs. The test set 23939

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The Journal of Physical Chemistry C includes 102 molecules, 60 of which are not included in the fitting database. The average error in atomization is found to be 3.3% for the entire set and 4.2% for species not included in the fitting database. The differences in atomization energy between the

Figure 3. Dissociation energies of sulfur data set calculated with G3 plotted as a function of values calculated with REBO.

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newly developed sulfur REBO potential and G3 for molecules containing characteristic bonds are illustrated in Figure 4. The ground state of elemental sulfur is cyclooctasulfur, which is an eight-membered crown-shaped ring.63 The bond length, bond angles, and torsion angles of S8 are predicted to within 1.0% of corresponding B3LYP geometries. The atomization energy of the S8 ring is predicted within 5.0% of the G3 energy. Other pure sulfur compounds, including the sulfur dimer and S3 and S4 rings, are found to have an average deviation from G3 energies of 4.3%. However, these small sulfur molecules are included in the fitting data set. Compounds not in the fitting data set, including the S5 and S6 rings, are found to have atomization energies within 10% of G3 energies. The potential captured the atomization energies of molecules containing thial and thiol groups well, with average errors of approximately 3.0% for each group. Bond lengths are also well captured, considering the prototypical molecules propanethione and methanethiol have deviations in C
S bond lengths of 2.7% and 0.4%, respectively, when compared to B3LYP geometries. The methanethiol C
S
H bond angle is also correctly predicated within 0.4% of the B3LYP geometry. For molecules containing multiple thiol groups, the hydrogen
hydrogen repulsion from the Xk
Hi
Hj angular function from the secondgeneration REBO potential,24 where Xk is S or C, is found to be

Figure 4. Atomization energies of a test set of molecules. Energies boxed in red are included in the fitting database. 23940

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too strong for the Sk
Hi
Hj case. Specifically, it causes dissociation of the molecule during close contact between thiol groups. Therefore, the Sk
Hi
Hj angular function is set to zero to mitigate this repulsion. Sulfur bonded to two carbons is considered in the form of sulfide bonds and within ring bonds. Dimethylsulfide is fit to have a bond length within 5.0% of B3LYP and a bond angle within 1.7%. The atomization energy for dimethylsulfide is within 2.7% of G3 values, while the average error of the test set is 4.2%. The ring molecules containing sulfur atoms are predicted with the least accuracy, with an average error in atomization energy of 5.5%. This is due to overbinding of conjugate C
C bonds since the coordination function for this bonding environment is fit to C
C double bonds.

’ SIMULATION DETAILS Given the complex nature of the SPIAD process, MD simulations are conducted of hyperthermal argon and thiophene interactions with a terthiophene (3T) oligomer surface. Ar is included via the LJ component of the potential only, as it is not chemically reactive. Charge is not explicitly accounted for within the REBO potential formalism. An initial thin film of 3T is constructed by periodic replications of crystalline α-terthiophene,64 as illustrated in Figure 5. Films are created in a 5 nm  5 nm simulation cell with periodic boundaries perpendicular to the plane of the film. The films are approximately 4 nm thick. The substrate is represented by a layer of diamond one unit cell thick, which is hydrogen terminated to limit any substrate
oligomer chemical interactions. The bottommost atomic layer of the diamond is held rigid to prevent any rippling of the substrate. Thermostats are applied to the rest of the hydrogen-terminated diamond using the Langevin method65 throughout the simulation. Parallel and perpendicular orientations of the terthiophene backbone to the substrate are considered, as illustrated in Figure 6. In particular, films are generated with the α-axis and the c-axis normal to the substrate surface. The orientation of 3T to the substrate is unknown for SPIAD deposited films. However, both orientations are observed in other work66,67 and are found to be correlated with conductivity of the substrate and deposition conditions.68 Thermostats are applied to the 3T films to achieve a temperature of 300 K. Thermostats are then lifted, and the 3T film is allowed to freely evolve as “active atoms”. During the relaxation process, the temperature of the 3T film is seen to deviate from the initial value. Therefore, thermostats are reapplied and then lifted periodically until the desired temperature of 300 K is achieved. ’ RESULTS To investigate the differences between hyperthermal Ar and thiophene interactions with a 3T surface, 20 single trajectory depositions are conducted for each with 50 and 100 eV of incident kinetic energy on the perpendicular and parallel orientations of 3T molecules, which corresponds to a total of 160 individual trajectories. Experimental results show a difference in yield between 100 eV Ar and thiophene, with thiophene producing higher molecular weight species.19 Experimentally, thiophene ions and 3T neutrals are deposited simultaneously. However, given an ion flux of 5  1013 ions/s 3 cm2 and a neutral flux of 5  1015 ions/ s 3 cm2 for the 1:100 ion to neutral ratio16 and the surface of the simulation cell of 25 nm2, this corresponds to 1 neutral per 0.8 ms

Figure 5. Terthiophene unit cell.64

Figure 6. Perpendicular (left) and parallel (right) configurations of the 3T films on a hydrogen-terminated substrate.

and 1 ion per 80 ms. Therefore, single trajectories of hyperthermal ions on 3T films are most comparable to the experimental conditions and should provide insight into the SPIAD process. The simulations have a duration of 40 ps, which is found to be the necessary time for changes in hybridization to stabilize. The molecular mass distribution of the resulting films are analyzed by grouping carbon and sulfur atoms within the minimum REBO potential cutoff Dmin. Molecules with a center of mass greater than 1 nm from the initial height of the 3T surface are considered to be removed into the vacuum. Initial concentrations of each molecular species are subtracted from the finial concentrations to determine the yield of each species. Yields are given in molecules produced per deposited ion. Deposition at 100. To facilitate a direct comparison to thiophene depositions, argon is deposited at the same energies on the same two surface orientations as thiophene. In Figure 7 the resulting 23941

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Figure 7. Yield at 100 eV thiophene and Ar on the surface with 3T in the parallel configuration. The total yield for each species of a given mass is shown in red, while the yield that remains in the substrate is shown in black.

molecular weight distribution for 20 deposition events of argon and thiophene on the parallel surface configuration is shown. The primary products are atomic hydrogen, CH, C2H2, C2H2S, and [3T]H; similar molecules with a variance in hydrogen content are also found. The lower molecular weight products are formed by fracturing of the rings of 3T, which is predicted to be consumed at an average rate of 5.5 per deposition for argon and 4.4 per deposition for thiophene. For the argon depositions, the fusing of dissociated species is predicted to occur in the production of some higher molecular weight species, including C25H15S6. While their yield is found to be low, their existence is investigated due to their relation to a possible polymerization process. Upon visual analysis of the process, it is found that the creation of C11H7S3 and C25H15S6 is correlated. In a typical reaction, it is seen that as the deposited argon interacts with the stack of 3T molecules the removal of a single carbon from a 3T molecule occurs. This ejected carbon atom can be absorbed by an underlying thiophene molecule, which can further interact with another thiophene molecule to create a bridge between two 3T molecules (Figure 8). This seems to be a direct result of the narrow interaction range of a deposited atom and the stacking of the 3T in the parallel configuration. For the thiophene interacting with the parallel configured 3T surface, the yields of the primary products of atomic hydrogen, C2H2 and C2HS, are predicted to be similar to those produced by argon deposited at the same energy, as indicated in Figure 7. The yield of atomic hydrogen and C2H2 is predicted to increase from 4.6 and 4.2, respectively, for Ar to 5.1 and 4.7, respectively, for thiophene. The yield of atomic hydrogen and C2H2 molecules that contain atoms from the deposited species is predicted to be about 1.0 for each molecule. In contrast, C2HS molecules, which contain deposited atoms, are predicted to form with a yield of 0.25. Therefore, the production of atomic hydrogen and C2H2 is related to the fragmentation of the deposited thiophene, while the production of C2HS is mostly due the fragmentation of 3T molecules. In the argon case of perpendicular alignment of 3T to the substrate, the depositions produce fewer low molecular weight species, as illustrated in Figure 9. The yield of species with masses

Figure 8. Two-step process of the removal of a single atom from one 3T (1) and donating it to the set of 3T molecules below it (2), where the argon atom is removed for clarity.

Figure 9. Yield at 100 eV thiophene and Ar on the surface with 3T in the perpendicular configuration.

less than that of the thiophene (84AMU) is found to decrease from 16.1 for the parallel orientation to 11.6 for the perpendicular orientation. Yields of the primary products of hydrogen, C2H2, and C2HS are reduced by 20% to 30%. Furthermore, the formation of C11H7S3 and C25H15S6 is not predicted, due to the different configuration of the 3T with respect to the incident atom. For the perpendicularly aligned 3T, the trends predicted in the argon depositions are also predicted to occur for thiophene deposition. Yields of the primary products of atomic hydrogen, C2H2, and C2HS are predicted to decrease by about 50% for all molecules. The yield of molecules with a mass less than the mass of thiophene is predicted to drop as the oligomer orientations change. In particular, the yield varies from 17.9 for thiophene molecules depositions on the parallel 3T oligomers to 8.9 for thiophene depositions on the perpendicular 3T oligomers. Correspondingly, the number average molecular weight of the molecules 23942

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The Journal of Physical Chemistry C containing deposited species is predicted to increase from 72.9AMU to 117.6AMU. This indicates an increase in interaction between the deposited species and the modified 3T molecules for the perpendicular case. In fact, the ratio of molecules containing deposited particles in the film and in the vacuum is predicted to be 2:1 for the perpendicular case and 4:1 for the parallel case. This is due to the dissociation products, including the deposited species, being more confined due to the nature of the perpendicular film. Depositions at 50 eV. As discussed above, there is considerable fragmentation of the deposited thiophene during the 100 eV depositions. Therefore, lower energy depositions are also considered. Incident energies are reduced to 50 eV, and the differences in the products are investigated. For the parallel configuration, yields of over 1.0 are not predicted for argon deposition, as indicated in Figure 10. The primary result of argon deposition at 100 eV of H, C2H2, and C2HS is observed with limited yields; however, [3T]H is not observed as one of the primary products as is the case for the 100 eV depositions. Carbon reduced products, including C2H2, C7H5S2, and C11H7S3 and carbon increased products such as [3T]CH are produced with yields of less than 0.4. For thiophene deposited at 50 eV on the parallel configuration, there is a marked difference in products when compared to the argon modification at 50 eV, the results for which are given in Figure 11. First, the fragmentation of the thiophene molecule is predicted to occur even at half the experimental energy. The primary products are again found to be atomic hydrogen, C2H2, C2HS, and [3T]H, which are predicted to contain deposited species with respective molecular yields of 0.6, 0.5, 0.3, and 0.15, respectively. Additional products containing deposited species vary in molecular weight from single carbon and sulfur atoms to modified 3T, with an average molecular weight of 67.3AMU. Similar to the 100 eV depositions, the ratio of altered molecules that are sputtered to those that remain in the substrate is found to be 3.5:1. Furthermore, the carbon reduced species are not found in substantial yields in the thiophene modified films. The primary modification of intact 3T is the addition of hydrogen. For the 50 eV argon modification of the perpendicular oriented 3T films, an increase in C2H2 production is predicted, when compared to the parallel case for the same energy. However, the other 3T fragments are predicted to be better dispersed, as illustrated in Figure 11. This coincides with the deposited argon impacting the end of 3T molecule in the perpendicular case and produces C2H2 rather than a more random distribution of carbon containing compounds, which is seen in the parallel case. As with the parallel case, negligible quantities of [3T]H are observed. In the thiophene modified films with the 3T orientated perpendicular to the substrate, a decrease in molecular yield is predicted when compared to the parallel case. However, similar products are seen to form, as shown in Figure 11. As with the parallel 3T oligomer case, a notable product is the bonding of hydrogen to the 3T oligomer. This could be the initialization of a polymerization process. Furthermore, argon bombardment at 50 eV does not produce the same [3T]H found during the thiophene depositions. Visual analysis of the produced films indicates that no conjugated higher molecular weight species are formed during the simulation time of 40 ps. The majority of augmented products is predicted to be linear chains of varying molecular mass.

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Figure 10. Molecular analysis of films resulting from 50 eV argon and thiophene deposition on 3T in the parallel configuration.

Figure 11. Molecular analysis of films resulting from 50 eV argon and thiophene deposition on 3T in the perpendicular configuration.

’ DISCUSSION While the direct observation of the formation of conjugated products is not predicted to occur during the simulations, numerous facets of the interaction of oligomer films with energetic particles are revealed. At 100 eV both thiophene and argon are predicted to produce similar products via the dissociation of 3T oligomers. Hydrogen, C2H2, and C2HS are produced in the most abundant quantities for both 3T orientations. Furthermore, [3T]H is also found to be a probable product of bombardment. Upon visual inspection, [3T]H is seen to result from 3T bonding to an extra hydrogen atom from the environment, as indicated in Figure 12. Interestingly, [3T]H is predicted to form more readily in the case of thiophene deposition at 50 eV, when compared to argon at the same incident energy. This is in part due to the increased yield of atomic hydrogen during the thiophene depositions. Furthermore, dissociated protons are proposed to be a possible initiator of the polymerization process.18 In contrast, argon is found to principally change the number of carbon atoms in the 3T molecule. This differs from the modification by thiophene at the same energy in that a majority of the 23943

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’ REFERENCES

Figure 12. [3T]H produced by 50 eV thiophene on parallel 3T.

modification did not result in changes in the carbon content of the 3T. On the basis of previously conducted quantum chemical calculations,69 the barrier between the thiophene radical and C2H2 of 0.87 eV indicates that one of the primary products of the MD simulations may not play an active role in polymerization processes despite the product [C4H4S]C2H being more stable than the reactants ([C4H3S] + C2H2) and the significant quantities of C2H2 being produced during the simulation. Further ab inito calculations involving fragmented rings, as well as proton interactions with the thiophene ring, need to be conducted for full validation. However, previous ab initio calculations have shown that the bonding of a proton to the conjugate carbons of polystyrene is barrierless,22 which is consistent with what is predicted in these simulations.

’ CONCLUSIONS A new parametrization for the second-generation REBO potential is provided to enable the classical simulation of hydrocarbon
sulfur interactions. It is used to model the deposition of argon and thiophene on a thin film of 3T molecules with two different orientations and at two incident kinetic energies. According to the simulations of thiophene and argon at 100 eV, similar products are predicted for both species. The primary products of hyperthermal interactions with a 3T surface are found to be atomic hydrogen, C2H2, and C2HS. While the fusion of dissociated 3T molecules is predicted for argon bombardment, no clear differences between argon and thiophene are predicted from the simulations. However, at the lower energy of 50 eV, differences in the produced molecules are observed. In particular, a greater yield of [3T]H is seen for thiophene deposition on parallel terthiophene than argon. This is thought to be due to the presence of available atomic hydrogen in the system. Consequently, the effective polymerization of hyperthermal thiophene during the SPIAD process, as opposed to other ions like argon, is due to its ability to produce a sufficient amount of atomic hydrogen that can act as a possible polymerization initiator at relatively lower energies. ’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected]fl.edu.

’ ACKNOWLEDGMENT This work was funded through a grant from the National Science Foundation (CHE-0809376).

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