Vibrational Spectra and Structures of Neutral SimCn Clusters (m + n

Article pubs.acs.org/JPCA

Vibrational Spectra and Structures of Neutral SimCn Clusters (m + n = 6): Sequential Doping of Silicon Clusters with Carbon Atoms Marco Savoca,† Anita Lagutschenkov,† Judith Langer,† Dan J. Harding,‡ André Fielicke,*,‡ and Otto Dopfer*,† †

Institut für Optik und Atomare Physik, Technische Universität Berlin, Hardenbergstraße 36, 10623 Berlin, Germany Fritz-Haber-Institut der Max-Planck-Gesellschaft, Faradayweg 4-6, 14195 Berlin, Germany

‡

S Supporting Information *

ABSTRACT: Vibrational spectra of mixed silicon carbide clusters SimCn with m + n = 6 in the gas phase are obtained by resonant infrared−vacuum-ultraviolet two-color ionization (IR−UV2CI for n ≤ 2) and density functional theory (DFT) calculations. SimCn clusters are produced in a laser vaporization source, in which the silicon plasma reacts with methane. Subsequently, they are irradiated with tunable IR light from an IR free electron laser before they are ionized with UV photons from an F2 laser. Resonant absorption of one or more IR photons leads to an enhanced ionization efficiency for SimCn and provides the size-specific IR spectra. IR spectra measured for Si6, Si5C, and Si4C2 are assigned to their most stable isomers by comparison with calculated linear absorption spectra. The preferred SimCn structures with m + n = 6 illustrate the systematic transition from chain-like geometries for bare C6 to three-dimensional structures for bare Si6. In contrast to bulk SiC, carbon atom segregation is observed already for the smallest n (n = 2).

1. INTRODUCTION Although carbon and silicon are both group IV elements, their chemical bonding characteristics differ substantially. While silicon prefers formation of single bonds with sp3 hybridization, carbon is more flexible and exhibits different types of spn hybridization (n = 1−3). The latter leads to either single or multiple bonding and thus a variety of different modifications, ranging from linear (e.g., polyacetylene) to two-dimensional (e.g., fullerenes, graphene, graphite) and three-dimensional structures (e.g., diamond). In the binary bulk compound, silicon carbide (SiC), both elements show diamond-like tetrahedral coordination of all Si and C atoms, which gives rise to its technologically important high thermal and mechanical stability. On the subnanometer scale, the structural differences between C and Si are retained. For example, small carbon clusters (Cn) tend to form linear or monocyclic structures.1,2 In contrast, silicon clusters (Sim) favor threedimensional structures with distorted tetrahedral coordination in order to avoid unsaturated (dangling) bonds.3 Interestingly, theory suggests the segregation of C atoms even at C/Si ratios much below 1:1 for small SimCn clusters.4−6 In contrast, for bulk SiC, such a formation of C2 or larger Cn interstitials in the regular SiC lattice has recently been predicted only for excess C in SiC.7 Because silicon carbide appears to be a promising new material for high-power, high-field, and high-temperature electronics,8−10 the structure and stability of nanoscale SiC are of high current interest, in particular, in the context of investigating the potential of SimCn nanostructures as building blocks of novel materials. In addition to technological © 2012 American Chemical Society

applications, a variety of C-rich SimCn cluster species (e.g., SiC, SiC2, SiC4) were observed in circumstellar and interstellar space11−14 and in presolar grains,15 making SimCn clusters highly relevant for astrochemistry.16 While pure Sim and Cn clusters have extensively been studied by both experiment and theory,1,3 much less information is available for their binary clusters. For example, with the notable exception of a single photoelectron spectroscopic study on anions for larger sizes up to Si7Cn−,17 experimental spectroscopic characterization of SimCn has so far been limited to clusters containing only a few Si atoms (m ≤ 3), with the main focus on chain-like SiCn.18−32 In contrast, numerous quantum chemical calculations for small SimCn clusters were reported,4−6,33−46 and especially relevant for this work are those for the six-atom Si6−nCn clusters,4−6,17,47,48 which will be compared to the current experiments and calculations. The major prediction is that the chain-like (linear and monocyclic) structures of C6 become rapidly less stable upon sequential doping with Si, whereas the three-dimensional bipyramidal structures of Si6 become less favorable upon successive C doping.4,5 This trend in structural rearrangements within Si6−nCn as a function of n was rationalized by the strongly decreasing bond strength along the series C−C > C−Si > Si−Si Special Issue: Peter B. Armentrout Festschrift Received: May 25, 2012 Revised: August 15, 2012 Published: September 6, 2012 1158

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intensities depend strongly on the cluster size and their corresponding IEs. The observed intensity ratios for Si6−nCn in the mass spectra decrease strongly with n (500:150:20:10:1 for n = 0−4), indicating that the low CH4 concentration favors the generation of C-poor SimCn clusters. For clusters with an IE close to the photon energy of the ionizing laser, prior resonant excitation with IR photons from a single macropulse of FELIX may enhance the ionization efficiency. For details of this IR−UV ionization mechanism, the reader is referred to refs 49 and 50. The reported IR spectra are obtained by the relative ionization enhancements determined by the difference of the ion IRon and IRoff signals normalized by the IRoff signal and the IR photon flux. The observed widths of the bands of 15−45 cm−1 arise from a combination of unresolved rotational structure, sequence hot band transitions involving lowfrequency modes, the FELIX bandwidth specified as ∼0.5% of the central wavelength, and possibly the multiple photon absorption process. DFT calculations are performed at the TPSS/def2-TZVP level with the resolution of the identity approximation as implemented in TURBOMOLE 6.1.52 This computational approach is justified by the satisfactory agreement with previous MP2 calculations, as illustrated in Table S2 in the Supporting Information (SI).6,47 The search for minima was guided by literature reports and the application of basin hopping procedures.53 If not stated otherwise, all isomers shown in this work are true minima. All relative energies include zeropoint vibrational energy corrections except for the vertical IE (VIE) values. Harmonic IR frequencies are scaled by a factor of 1.01 obtained by adjusting the agreement between experimental and calculated frequencies of Si4C2, and theoretical IR stick absorption spectra are convoluted with Gaussian line profiles using a full width at half-maximum of 20 cm−1 to facilitate convenient comparison with the experimental spectra.

and the high ability of Si for multicenter bonding. Despite this general trend, the energetic order of individual SimCn isomers often depends strongly on the theoretical level (e.g., for the D2d and C2v isomers of Si4C2),5,6,41 which calls for experimental data for evaluating the various theoretical approaches. Here, we systematically investigate the vibrational spectra and structures of Si6−nCn clusters to follow the effects of sequential substitution of Si by C atoms on their geometric and electronic structures. These six-atom clusters belong to the smallest size range to systematically probe the structural transition induced by C doping of Sim clusters. IR spectra of Si6−nCn clusters with n ≤ 2 are obtained in the gas phase by resonant IR−UV two-color ionization (IR−UV2CI).49,50 To this end, SimCn clusters generated in a molecular beam are exposed to tunable IR light from the Free Electron Laser for Infrared eXperiments (FELIX)51 before they are ionized with vacuum-UV photons from an F2 laser with 7.87 eV. Resonant absorption of one or more IR photons enhances the ionization efficiency and thus provides the size-specific IR absorption signature for SimCn. Such an enhancement is particularly efficient for clusters with ionization energies (IEs) close to the ionizing photon energy. IR spectra are measured in the 350− 1250 cm−1 range to cover the majority of Si−Si and Si−C modes. The measured IR spectra are assigned to structural isomers by comparison with density functional theory (DFT) calculations at the TPSS/def2-TZVP level.

2. EXPERIMENTAL AND COMPUTATIONAL TECHNIQUES The experimental setup used for IR−UV2CI spectroscopy has been described previously.49,50 Briefly, silicon atoms (and clusters) are produced by laser ablation of a silicon rod. Si-rich SimCn clusters are generated within a pulsed flow of helium gas (∼7 bar) containing 1% CH4 and thermalized in a liquidnitrogen-cooled expansion channel to ∼100 K. Neutral SimCn clusters pass through a skimmer and are ionized by single photons from an unfocused F2 laser (7.87 eV) in the extraction region of a reflectron time-of-flight mass spectrometer. A typical mass spectrum is shown in Figure 1. Ion signal

3. RESULTS AND DISCUSSION The IR spectrum of Si6 in Figure 2 has been reported before49,54 and is included for comparison. It shows only a single peak at 464 ± 1 cm−1, which is consistent with an assignment to the eu mode of the highly symmetric D4h structure predicted at 454 cm−1 (MP2), although the very similar C2v structure could not be excluded.49,54 Si6 is a

Figure 1. Typical mass spectrum of the cluster source. Neutral SimCn clusters are generated by laser desorption of a silicon rod and subsequent reaction with a gas mixture of 1% CH4 in He. Ionization of the neutral clusters is achieved by photoionization with 7.87 eV photons from an F2 laser. The arrows indicate SimCn clusters, for which IR−UV2CI spectra are obtained (Figure 2).

Figure 2. IR spectra of (a) Si6, (b) Si5C, (c) Si4C2, and (d) Si3C3 measured by IR−UV2CI spectroscopy (Table 1). The spectra for Si6 were recorded for two different settings of FELIX. 1159

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challenge for theory due to difficulties arising from the suspected pseudo-Jahn−Teller effect. Thus, different structures are predicted at different theoretical levels (D4h at MP2, C2v at B3LYP and BP-86).49,54,55 At the TPSS/def2-TZVP level, the Si6 structures with C2v, D4h, and Oh symmetry all yield IR spectra compatible with the measured one, as shown in the SI. Replacing a single Si atom by C changes the IR spectrum completely (Figure 2), mostly due to symmetry reduction and substantial changes in the force field. The IR−UV2CI spectrum measured for Si5C reveals four prominent peaks denoted A−D (Figure 3, Table 1). In accordance with previous MP2

Table 1. Comparison of Observed Vibrational Frequencies in the IR Spectra of Si6−nCn (n ≤ 2) with Values Calculated at the TPSS/def2-TZVP Level cluster Si6 (D4h)b Si5C (C4v)

Si4C2 (C2v)

νexp (cm−1)a

νcalc (cm−1)b

464 868 785 573 427 − − − 950 770 604 488

454 (eu)c d 795 (54, e) 584 (24, a1) 429 (31, a1) 385 (4, e) 358 (1, a1) 1604 (2, a1) e 768 (94, b2) 608 (21, a1) 479 (38, b2) 418 (0, a1) 412 (13, b1) 371 (5, a1) 368 (21, b2) 190 (2, a1) 155 (4, b1)

(23) (D, 28) (C, 46) (B, 19) (A, 16)

(F, 28) (E, 38) (D, 21) (C, 16)

402 (B, 15) − 362 (A, 15) − − a

Widths of the transitions are listed in parentheses. bIR intensities (km/mol) and symmetry species are listed in parentheses. Only IR-active vibrations are listed. cMP2 value scaled by 0.96.49 dAssigned to an overtone of the 429 cm−1 mode. eAssigned to an overtone of the 479 cm−1 mode.

the vibrational frequency shifts (see Table S3 in the SI). Significantly, all three assigned fundamentals of Si5C (A−C) involve substantial motion of the C atom (Figure 4), providing detailed information about the Si−C bond strength. Their frequencies are much higher than those in Si6 due to the lighter mass of C and the stiffer Si−C bond as compared to Si−Si. The spectrum of Si4C2 in Figure 2 is even richer in structure than the one of Si5C and exhibits six distinct bands (A−F) in the examined frequency range (Table 1). This result is indicative of further symmetry reduction upon substitution of the second Si atom by C. Indeed, in line with previous MP25,6,17 but in contrast to recent QCISD(T)41 calculations, the TPSS level predicts a three-dimensional structure with C2v symmetry in a 1A1 electronic state to be lowest in energy (Figure 5). Moreover, its predicted IR spectrum agrees well with the measured one. Five (A−E) out of the six transitions observed match the calculated frequencies better than ∼10 cm−1, strongly supporting the isomer assignment. The corresponding normal modes are shown in Figure 4. Band F at 950 cm−1 cannot arise from a fundamental mode of the C2v symmetric minimum but is currently attributed to the first overtone of the intense 488 cm−1 mode (band C), like the highest-frequency band observed for Si5C. This Si4C2 isomer contains a C−C unit with a bond order between 2 and 3 (RCC = 1.31 Å).5 The corresponding C−C stretching mode is predicted to occur at 1604 cm−1, that is, significantly above the experimentally investigated frequency range. Moreover, its IR intensity is very low (2 km/mol), providing a challenge for its IR detection using the current approach. Similar to Si5C, the experimental Si4C2 spectrum can mainly be explained by a single structure and does not provide any definitive evidence for further geometrical or electronic isomers (Figure 5 and the SI). The two intense IR features predicted for the low-energy D2d isomer with ΔE ≈ 15 kJ/mol at 516 and 959 cm−1 may

Figure 3. Comparison of the experimental IR−UV2CI spectrum of Si5C (a) with linear IR absorption spectra of the two lowest-energy isomers calculated at the TPSS/def2-TZVP level (b,c). All other isomers found are at least ΔE = 130 kJ/mol above the most stable one (see the SI).

calculations,6,17 the TPSS level finds the C4v symmetric isomer in a 1A1 electronic state to be by far the lowest in energy (Figure 3). Its calculated linear absorption spectrum agrees well with the experimental spectrum (Figure 3). The transition frequencies match for three (A−C) out of the four observed bands to within 10 cm−1 (Table 1), which is comparable to the spectral resolution of the experimental approach. Band D at 868 cm−1 does not arise from a fundamental mode of the C4v structure and is currently attributed to the first overtone of the 427 cm−1 mode. Such overtones and combination bands are not included in the calculated spectra but have previously been observed with similar high intensity in IR multiphoton excitation spectra of related clusters.56,57 Significantly, none of the higher-energy isomers can account for the measured spectrum (Figure 3 and the SI). This result is expected because these isomers are predicted to be at least ∼50 kJ/mol above the C4v structure for both singlet and triplet electronic states. The geometries and vibrational data of the two lowest-energy isomers shown in Figure 3 and a variety of higher-energy isomers are given in the SI. D4h → C4v symmetry reduction upon Si → C substitution in Si6 has a drastic effect on the geometries of the two square-pyramidal units. In Si6, these are characterized by a square with RSiSi = 2.36 Å, an edge of RSiSi = 2.51 Å, and an angle of 56°. The corresponding values in the most stable isomer of Si5C are 2.54 Å, 2.44 Å, and 63° for the Si4Si pyramid and 2.54 Å, 1.90 Å, and 84° for the Si4C pyramid. These structural and force field changes translate directly into 1160

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Figure 4. Normal modes and wavenumbers (in cm−1, TPSS/def2-TZVP, Table 1) of Si5C (C4v) and Si4C2 (C2v) assigned in the experimental IR−UV2CI spectra in Figures 3 (A−C) and 5 (A−E), respectively.

those in the D2d and C1 isomers at ΔE ≈ 15 and 49 kJ/mol (1.84 and 1.48 Å). The geometries of all isomers shown in Figure 4 are given in the SI. The measured IR−UV2CI trace of Si3C3 in Figure 2 does not show any resonant enhancement, although there is a small signal in the mass spectrum (Figure 1). This might be explained by the relatively high IE of the lowest-energy isomer of Si3C3. Similar to previous MP25,6,17,47,48 and other DFT calculations,58,59 the TPSS level yields a three-dimensional global minimum structure (Cs, 1A′), whose vertical IE value of VIE = 9.12 eV (Table S1 in the SI) substantially exceeds the photon energy of 7.87 eV used for ionization. As a consequence, resonant IR heating of this Si3C3 isomer, if produced in the cluster source, is insufficient to induce a detectable enhancement of the ionization efficiency. The residual ionization signal may be due to the hot fraction of clusters. This observation is in contrast to the most stable isomers of Si6, Si5C, and Si4C2, which have lower calculated IE values of VIE = 7.05, 8.22, and 7.98 eV, respectively. The Cs global minimum of Si3C3 has a pyramid-like geometry with a central asymmetric C3 unit. The latter features one short and two substantially longer C−C bonds (RCC = 1.33 and 1.75 Å) with double- and single-bond character, respectively. So far, no experimental spectrum is available to confirm the predicted minimum structure of Si3C3. Si2C4 is the first cluster in the Si6−nCn series with a linear global minimum structure of the type SiC4Si in a 3Σg− ground electronic state.24 In contrast to Si3C3, the low VIE of 7.37 eV calculated for SiC4Si should allow for its efficient ionization. As almost no Si2C4+ signal has been detected in the mass spectrum, this C-rich species is probably not generated in sufficient abundance in the cluster source due to the low CH4 concentration employed in the expansion gas. Our TPSS calculations yield CC bond lengths of 1.28−1.30 Å for this predominantly cumulenic species and an IR spectrum in good agreement with that measured in an Ar matrix.24 For example, the frequencies predicted for the IR-active CC and Si−C stretching modes of 1845 and 715 cm−1 are consistent with the measured values of 1807.4 and 719.1 cm−1, respectively. Similar to Si4C2, no experimental IR−UV2CI spectrum was obtained for the remaining C-rich Si6−nCn species, SiC5 and C6. The microwave spectrum of SiC5 is consistent with a linear cumulenic structure terminated by Si in a 3Σ ground state,30 with RCC = 1.28−1.30 Å.41 For C6, the linear structure (3Σ−g) is nearly isoenergetic with the monocyclic ring isomer (1Ag). The linear form appears to be entropically favored and readily

Figure 5. Comparison of the experimental IR−UV2CI spectrum of Si4C2 (a) with linear IR absorption spectra of the three lowest-energy isomers calculated at the TPSS/def2-TZVP level (b−d). All other isomers found are at least ΔE = 200 kJ/mol above the most stable one (see the SI).

contribute to bands C and F. However, the good match between the relative intensities of bands A−C with those predicted for the most stable C2v isomer suggests that the D2d isomer can provide at most a minor contribution to the experimental spectrum. This observation seems to be largely incompatible with the recent QCISD(T) calculations, which predict the D2d isomer to be 167 kJ/mol below the C2v structure.41 Nevertheless, the predicted IE of the D2d isomer is significantly higher than that of the C2v isomer (8.87 versus 7.98 eV), which may hamper its detection via IR−UV2CI. The D2d isomer actually results from relaxing a D4h symmetric transition state, which is initially obtained upon the second Si → C substitution of Si6 with D4h at the top of the second pyramid.5 Interestingly, all three low-lying isomers predicted for Si4C2 feature a C2 unit (Figure 5), although the C−C distance in the most stable C2v structure (1.31 Å) is much shorter than 1161

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formed in the gas phase and cryogenic matrixes.1 A rotationally resolved IR spectrum is available for linear C6,60 whereas the cyclic isomer was characterized in rare gas matrixes.1

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4. CONCLUSIONS In summary, the combined IR−UV2CI spectroscopic and theoretical approach enables us to follow the evolution of the geometrical structures of neutral Si6−nCn clusters. The experimental IR−UV2CI spectra obtained for the Si-rich species (n ≤ 2) are in good agreement with the IR spectra of the lowest-energy isomers predicted by our TPSS and previous MP2 calculations. These structures follow the predicted preference for three-dimensional structures for Si-rich SimC6−m species (m ≥ 3). Already for the smallest possible number of carbon atoms (n = 2), segregation into a C2 unit occurs. This segregation has been rationalized by chemical bonding arguments.4,5 It would be interesting to study larger mixed SimCn clusters to detect the transition to bulk-like SiC clusters containing isolated carbon atoms as found in silicon carbide.

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ASSOCIATED CONTENT

S Supporting Information *

Geometries, relative energies, ionization energies, and vibrational data for the most relevant SinCm isomers. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: fi[email protected] (A.F.); [email protected] (O.D.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the TU Berlin and the DFG within the research unit FOR 1282 (DO 729/5, FI 893/4). We gratefully acknowledge the support of the Stichting voor Fundamenteel Onderzoek der Materie (FOM) in providing beam time of FELIX and the FELIX staff for their skillful assistance, in particular, B. Redlich and A. F. G. van der Meer. The research leading to these results has received funding from the European Community’s Seventh Framework Programme (FP7/2007-2013) under Grant Agreement No. 226716. D.J.H. thanks the Alexander-von-Humboldt-Stiftung for support. We thank Gerard Meijer for his continued support and Karsten Reuter for valuable discussions about basin hopping procedures.

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REFERENCES

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