Formation of Indium Carbide Cluster Ions - American Chemical Society

Jun 14, 2013 - Jonathan Bernstein, Eran Armon, Erez Zemel, and Eli Kolodney*. Schulich Faculty of Chemistry, Technion, Haifa 32000, Israel. •S Suppo...
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Formation of Indium Carbide Cluster Ions: Experimental and Computational Study Jonathan Bernstein, Eran Armon, Erez Zemel, and Eli Kolodney* Schulich Faculty of Chemistry, Technion, Haifa 32000, Israel S Supporting Information *

ABSTRACT: We report the observation and structural analysis of novel indium carbide gas phase cluster ions generated by bombardment of a clean indium surface by keV C60− ions. Positive InmCn+ (m = 1−21, 1 ≤ n ≤ 9) ions were ejected off the surface and analyzed mass spectrometrically. C60− ion beam irradiation is shown to be an efficient way of producing new kinds of gas phase carbide ions with relatively balanced stoichiometries. The rise kinetics of the ion signal (immediate jump within the beam opening time to a plateau value) indicates that the formation/ejection of the carbide ions constitute a single impact event. In3C2+ was found to be the most abundant carbide cluster ion. Optimal geometries of the different clusters were derived via density functional theory calculations. The acetylenic/cumulenic nature of the impact emitted cluster ions is manifested by the high abundance of In2C2+, In3C2+, and the calculated structures for InmCn+ (m = 3−4, n = 2−8). Odd/even intensity alternations in the In3Cn+ (n = 1−8) and In4Cn+ (n = 1−9) abundances are observed and rationalized by the calculations.

I. INTRODUCTION

Various methods have been used for producing gas phase metal carbide clusters. Castleman and co-workers used laser induced plasma reactor where different carbidic species were generated following gas phase reactions between laser vaporized metals and hydrocarbon molecules in the plasma.2,10−13 The method was mostly applied to the main group transition metals. Another approach used by the same group was based on a thermal gas aggregation source where neutral metallic clusters reacted with thermally decomposed hydrocarbons (e.g., filament heated acetylene for generating copper carbide clusters14). Yamada et al.15 used a similar gas aggregation method for generating bismuth carbide cluster ions (BinC2n+ with n = 3−11). Duncan and co-workers8,9 used pulsed laser vaporization of thin metal films on a rotating graphite rod and reported the formation of various novel or rare carbidic species (of Ni, Co, Cu, Bi, Sb, and Au). The same group has also used a somewhat different approach based on laser vaporization of C60 layers deposited on metallic substrates, but the main focus was on the formation of the exohedral metallofullerenes.16,17 Another approach is based on the formation of metallic gas phase clusters via laser ablation of appropriate target followed by the independent reaction of these clusters with hydrocarbon molecules. For example, Gibson et el. used laser ablation to produce Aun+ clusters in

Studies concerned with the structure, binding interactions, and formation mechanisms of gas phase metal−carbon species (carbide clusters MmCn) have constituted an important part of cluster science since the early days of the field. Sometimes, exceptionally stable and unique geometries can be discovered as was indeed the case for the so-called metcar M8C12 (M = Ti, Zr, Hf, V, Nb, Ta, and Mo) configuration.1−4 The study of novel carbide clusters can therefore potentially reveal new types of bonding, not observed before, possibly leading to the formation of new materials. While early transition metal carbides are stable also in the solid phase and can be easily prepared by simple reactions, some late transition metals (e.g., Au, Ag, Cd, Hg, etc.) are much less reactive with carbon, and their carbides are highly unstable (although some were observed as isolated gas phase species5−9). This tendency toward instability is even more pronounced for the group 13 post-transition metals (Ga, In, and Tl), which, to the best of our knowledge, do not form solid phase carbides and also their gas phase clusters, neutrals, or ions, were not yet observed experimentally or even theoretically studied. It is clearly of interest to develop efficient new approaches for the synthesis of these rare carbide clusters and explore their properties and bonding interactions. One can also speculate that if one finds a way of generating these new carbide clusters in high yield over a large range of M/C stoichiometries, it may eventually be possible also to define some specific conditions under which solid phase structures can be stabilized, at least at the nanoscale. © XXXX American Chemical Society

Special Issue: Curt Wittig Festschrift Received: March 27, 2013 Revised: June 14, 2013

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They observed oscillations in ion intensities as a function of mass and have associated it with calculated oscillations in the vertical ionization energies of AlmCn clusters. All their calculated lowest energy structures were based on a carbon backbone chain and attached peripheral aluminum atoms. In sharp contrast to the numerous studies reported on aluminum carbide clusters, we are not aware of any reliable evidence or report on gallium, indium, or thallium carbide clusters. The recent intensive study of the aluminum carbide clusters is related also with the availability of methods for producing these clusters in the gas phase. The Ga, In, and Tl trio is interesting especially due to the weak binding tendency toward carbon and the complete immiscibility of carbon in the solid phase of these three elements.35−37 Indium is chemically intermediate between gallium and thallium and can therefore be considered as a representative of this yet unexplored elements, regarding optimal geometries of their carbide cluster ions and bonding interactions. Weak binding interaction between carbon and metals in which the carbon is poorly miscible (e.g., Cu) or completely immiscible (e.g., Ga, In) has recently gained importance in relation with the growth of graphene or carbon nanotubes (CNTs) on such metallic substrates. High quality hydrocarbon-based chemical vapor deposition (CVD) growth of large graphene sheets on copper38 and gallium39 was demonstrated, and CNTs were similarly grown on liquid gallium35,36,39 and liquid indium.35 The good deposition and growth quality is associated with the unique combination of very low carbon solubility in these metals, while still maintaining weak metal−carbon binding interactions. A deeper understanding of the nature of this interaction is clearly of interest. In the present work, indium carbide clusters are studied both experimentally and theoretically for the first time. Gas phase indium carbide cluster ions, InmCn+ (m = 1−21, 1 ≤ n ≤ 9), were generated by impacting 14 keV C60− ions with a clean indium target maintained under ultrahigh-vacuum (UHV) conditions and then analyzed mass spectrometrically. It should be emphasized that (in this study) the impact interaction of the C60− ions with the target is mainly used as a tool for producing novel gas phase carbidic compounds. We are not concerned here with the general problem of cluster emission mechanisms via keV ion impact (atomic or polyatomic) or the specific mechanisms of reactive collisions of C60− with a target. Regarding the structural analysis of the different species, we have found several interesting motifs and trends that can probably be extended for the carbide clusters of the other group 13 elements.

the gas phase, which then reacted with hydrocarbons or halocarbons resulting in the formation of gold carbide cluster ions.7 A simultaneous two beam laser ablation of adjacent metal (silver or copper) and graphite targets was used by Vala and coworkers while trapping possible products in a solid argon matrix. Mainly low stoichiometry species were observed (MC3 with M = Ag, Cu).6,18 It should be noted that when reacting hydrocarbons with pre-existing metal clusters, there is a possibility that under conditions of high activation barrier combined with low vibrational excitation, the structure of the product will be dictated by that of the (metallic clusters) reagents, thus not enabling the formation of the lowest energy carbidic species. Energetic ion beam irradiation induces efficient mixing between target and projectile atoms even though the two materials can be completely immiscible (at all compositions) in the bulk. This can allow for the formation of unique nanostructure or compounds not accessible when the two elements of the binary system are under thermal or near thermal equilibrium conditions. In energetic impact of C60 ions with metal, the fullerene projectile is disintegrated to carbon atoms and small fragments. Because of the shallow penetration of the constituent small carbon fragments, there is an efficient mixing with target atoms, within the first few layers. This approach can thus serve as a general and effective method for producing novel carbidic species, which usually do not exist as a stable bulk form. Earlier C60 impact induced reactions were studied mainly for keV C60 collisions with silicon, which readily produced the stable bulk carbide form of silicon.19−23 The strong silicon carbon binding interactions in the bulk resulted in efficient carbon trapping and implantation, growth of silicon carbide, and emission of silicon carbide clusters. Recently, we have shown that impact of fullerene (C60−) ions on gold and silver metallic targets (which do not form stable carbides) at kinetic energies of several keV constitutes an efficient way of generating the rarely observed gold and silver carbide cluster ions. The single collision event results in highly efficient elemental mixing (between carbon atoms of the shattered C60− projectile and the metallic target atoms) and extreme high temperatures within the impact nanovolume (thermal-spike). We have observed both positively and negatively charged gold and silver carbide cluster ions over a large range of stoichiometries. The different species were analyzed using density functional theory (DFT) calculations. The most abundant species was found to be M3C2+ (M = Au, Ag) whose structure was calculated as a π-complex of the type M(π−M−C≡C−M) +. Out of the four post-transition metals in group 13 (Al, Ga, In, Tl), only aluminum exists as a bulk carbide. These elements have ns2p1 (n = 3−6) outer shell configuration with a fully occupied (n − 1)d shell. Therefore, the carbon−metal bonding mainly involves contribution due to p−p orbital overlap. The aluminum carbide has the formula Al4C3 and belongs to the methanides class of carbides. Its gas phase clusters have recently attracted attention and were explored in detail, both experimentally24−32 and theoretically.24,25,27−30,33,34 Using photoelectron spectoscopy and ab initio approach, the stuctures, chemical bonding, and electronic properties of various aluminum−carbon clusters were studied. Laser vaporization of a graphite/aluminum mixture in helium carrier gas was used to generate the clusters.25,27,28,30,31 Dong et al.24 also generated aluminum carbide clusters by laser ablation of mixed aluminum/carbon targets and studied their structural motifs.

II. EXPERIMENTAL AND COMPUTATIONAL METHODS A clean indium sample was irradiated by 14 keV C60− ions at an incidence angle of 45° (with respect to the surface normal), under UHV conditions. Secondary ions ejected from the surface were collected along the normal into a quadrupole mass filter (QMS, Extrel MEXM-4000) via ion transfer optics. The negative fullerene ion source was described before,5 but relevant features will be mentioned here. A highly mass pure (better than 99.9%) C60− ion beam is generated by passing heated C60 vapors through a high temperature narrow ceramic capillary. Beam stability is within 2% drift per hour, and energy spread is below 1 eV (thus minimizing chromatic aberration). Beam currents approaching 100 nA and submicronic focusing could be achieved. Some typical performance and characteristics (mass purity, stability, and quietness, beam current) of the B

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C60− source are shown in the supplementary part of ref 5. For the present experiments (and in order to avoid saturation of the secondary electron multiplier), the 14 keV beam was mildly focused at the target plane to a spot diameter of about 30 μm with reduced current of 5−7 nA. It was then rastered over a target area of 0.5 × 0.5 mm2. An indium polycrystalline sample (99.998% purity, 10 × 10 mm2, 1.0 mm thick, Alfa Aesar) was mounted inside the UHV chamber after cleaning and removing of its thin native oxide layer with mild hydrochloric acid. Some residual oxide, which either survived the chemical treatment or managed to regrow, was mostly removed by the C60− beam itself within the first 10−20 s of bombardment. Usually, the measurements of the mass spectrum started only after five minutes of C60− exposure thus assuring complete and gentle removal of any surface adsorbates and leaving only minute amount of the oxide (see also section III.1). These procedures resulted in a stable secondary ion current signal over the whole period of the measurements, with no noticeable change in the mass spectral abundances of the different ions. Alternatively, immediately after mounting the chemically cleaned sample on the manipulator, the surface was sputter-cleaned with Ar+ ions (typically 30 min per each exposure zone, 0.3 μA, 5 kV). Following the Ar+ bombardment, the complete mass spectrum showed up instantaneously within the C60− beam opening time. The mass spectra after either the Ar+ sputtering or the initial C60− exposure were practically identical. The Ar+ bombardment was used mainly for surface preparation before the signal kinetics measurements, in order to ensure complete absence of any surface or subsurface carbon deposits and complete removal of the residual oxide. The C60− ion source also produces an effusive (neutral) C60 flux, which in principle could be partially deposited on the target. However, the ion optical column is highly apertured (solid angle of 2.9 × 10−4 sr), and the C60− beam was tilted by deflection plates such that practically no C60 was deposited, and indeed, none was identified in the emitted ions mass spectra. Within our S/N, this implies a C60+ signal level that is below 10−5 of the In+ signal. By following the signal kinetics for both pure indium cluster ions (In3+, In4+) and indium carbide ions (In3C2+, In4C2+), we have found that the rise of the signal intensity of these ions was practically instantaneous with the beam “on” time (about one second) followed by a constant plateau value. This behavior clearly indicates an impulsive formation (within a single impact) of the indium carbide ions without any effect related with predeposition or a gradual growth of some carbonaceous layer. Geometry optimization calculations were performed using density functional theory (DFT) in the Gaussian03 (G03) suite of programs.40 Calculations were made using the B3P86 method with the SDD (the Stuttgart/Dresden relativistic CP + triple-ζ) basis set for the indium atoms and 6-311++G(3df) basis set for carbon atoms to locate a range of geometric isomers for each neutral cluster species at the two lowest spin multiplicities (i.e., singlet and triplet or doublet and quartet) using a range of initial geometries. All isomers were optimized without any geometrical constraints. All the functionals and basis sets employed are available in G03 as standard. Finally, as a test for the reliability of our computations at the given level of theory employed, we have compared calculated and experimental values for the indium dimer In2. Experimental bond length is reported as 2.83 Å,41 while the calculated value is 2.835 Å. Experimental dissociation energy (De) is reported as

0.97242 and 1.002 eV,43 while the calculated value is 1.024 eV. The experimental ionization energy (appearance energy) is reported as 5.8 ± 0.3 eV,42 while our calculated adiabatic ionization energy is 6.5 eV. It seems that regarding bond length and dissociation energy there is a good agreement between our calculated values and the measured ones, but there is an overestimation of ionization energy by about 0.7 eV. Additional tests are required in order to decide whether there is indeed a systematic overestimation of ionization energies for this system. However, in this study we are mainly interested in relative variations of ionization energies (with number n of C atoms) rather than their absolute values.

III. RESULTS AND DISCUSSION 1. Experimental Results: Secondary Ions Mass Spectrum and Signal Kinetics. A mass spectrum of positive secondary ions, emitted off the indium surface following a C60− ion impact at 14 keV kinetic energy is shown in Figures 1 and 2.

Figure 1. Positive secondary ions mass spectrum (over the 330−580 amu range) following impact of C60− ions on an indium target at 14 keV kinetic energy. Shown are the In3Cn+ (n = 0−8) and In4Cn+ (n = 0−9) series. Note the alternation in abundances of cluster ions with odd/even number of carbon atoms for both series and the high abundance of In3C2+.

In Figure 1 (low mass range, 330−580 amu), we present the two main series of indium carbide ions, In3Cn+ (n = 1−8) and In4Cn+ (n = 1−9), which will be compared with the computational results. We choose to focus on these two series because of their relatively balanced stoichiometries and calculated structural motifs. In Figure 2, we present the mass spectrum in the high mass range of 540−2500 amu. The carbide clusters in this size range (m = 5−21, n = 0−7) are not structurally analyzed but still present some interesting aspects. All mass peaks appearing in Figures 1 and 2 are assignable to either pure indium clusters or indium carbide clusters, except for negligibly small In3O+ and In3O2+ signals (see Figure 1 at 358 and 374 amu, respectively). Longer C60− irradiation time (higher dose) led to complete disappearance of these residual oxide mass peaks. We have observed also the smaller indium carbide clusters: InCn+ (n = 1−2) and In2Cn+ (n = 1−3) and have carried out DFT calculations to obtain the most stable geometries of these carbide clusters as well. In2C3+, for example, is made of linear C

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formation of mixed clusters can be meaningfully reduced as compared with the initial (first 1 ps) phase. Since the main species to be analyzed computationally are those appearing in Figure 1, we would like to discuss some of the features in this mass range. The highest InmCn+ abundance is clearly that of In3C2+. The In3Cn+ (n = 1−8) series show alternation in abundances with odd/even number of carbon atoms n. These alternations can be rationalized either by alternations in ionization energies of the neutrals or dissociation energies of the ions, emitted off the surface. The In4Cn+ (n = 1−9) series exhibit a somewhat different behavior than that of the In3Cn+ (n = 1−8) series. While In4C2+ is the most abundant carbide ion (similarly to In3C2+), the odd/ even alternations in the m = 4 series (for n > 3) are opposite and much more pronounced. The even In4Cn+ (n = 4, 6, 8) were not observed. We later associate this behavior with the relatively low dissociation energies of the n = 4, 6, 8 ions toward the emission of atomic indium (especially pronounced for n = 4). For the oxide-free surface, the complete mass spectrum showed up immediately within the beam opening time (about one second) and did not change, in either intensity or relative abundances, during the full measurement time. This clearly demonstrates that the formation/emission process can be described as a single impact event. Any accumulation of some form of carbonaceous surface deposit with an effect on the formation probability of the carbide would be reflected by a slow and gradual time evolution of the mass spectral appearance. In order to test this issue in a more controlled manner, we have followed the signal rise kinetics (on a fresh, unexposed sample) for In3+, In4+, and their most intense carbide ions In3C2+ and In4C2+, from the instance of the C60− beam opening. In order to improve sensitivity to any gradual buildup of surface deposits, even in the deep submonolayer region, we have reduced the C60− beam flux by about an order of magnitude to cover the range from 0.1 to 10 monolayer equivalent dose (over the C60− beam exposure time). Before each measurement, a fresh, unexposed sample (just introduced into the vacuum chamber) was sputter-cleaned by Ar+ ions (see also the experimental section II). As shown in Figure 3, starting with the beam opening time, there is an instantaneous intensity jump to a plateau value with less than 3% decrease over an exposure time of 10 min (noise level

Figure 2. Positive secondary ions mass spectrum (over the 540−2500 amu range) following impact of C60− ions on an indium target at 14 keV kinetic energy. Note the transition from relatively balanced stoichiometries (up to In7Cn+) to nearly pure indium clusters.

C3 unit end-capped by In atoms on both sides. However, since these two series of clusters were calculated to have simple linear geometries similar to those found previously for gold and silver carbides with the same M/C stoichiometries,5 we did not discuss them in detail in the present work (for calculated optimal structures see Figure 1 in Supporting Information). Regarding the high mass range of 540−2500 amu (Figure 2), the main features observed are the relatively balanced stoichiometries up to m = 7−8 and the transition to nearly pure indium clusters (m = 9−21). Another intriguing feature is the fact that for mixed clusters in the m = 9−21 size range only InmC+ and InmC3+ are observed, while InmC2+ are absent (only InmC+ for m = 20−21). Also, the abundance of InmC+ relative to Inm+ is roughly kept constant. It is tempting to interpret the (relatively sharp) transition from the high In/C stoichiometry compounds to the nearly pure indium clusters in terms of a transition to the bulk immiscibility of carbon in indium. However, our lack of present knowledge with regard to the impact formation mechanism of the large indium clusters, do not allow us to extend this idea beyond a speculation stage. It is possible for example, that the large indium clusters are formed away from the center of the impact zone, where In/C mixing is less efficient. Another possibility is that the large clusters are emitted mainly at later phases of the life cycle of the thermal spike induced at the target by the impacting C60− projectile. At this later stage (e.g., several ps after impact), probability for

Figure 3. Time dependence of the In3+, In3C2+, In4+, and In4C2+ secondary ion signal as a function of C60− exposure time. D

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before beam opening is below 0.01 cps). An example for the case of slow rise kinetics of the scattered ion signal can be found in ref 44 where hyperthermal neutral C60 was scattered off nickel, and the gradual buildup of a graphene layer led to slow appearance of both the scattered neutral C60 and the negative C60− ion. The instantaneous signal rise, as observed here, does not preclude shallow implantation of carbon (which probably does occur to some extent) during bombardment. It merely implies that, under our irradiation conditions, it is decoupled from and does not have an effect on the formation/ ejection mechanism of the pure indium and indium carbide clusters. Because of the complete immiscibility in bulk indium, residually (sub) implanted carbon will segregate and rapidly aggregate by diffusion to small carbon nanograins driven by the reduction of surface energy. These nanograins probably slowly accumulate at deeper layers and away (laterally) from the impact zone. In contrast, the prerequisite for the formation of the indium−carbon compounds is the efficient atomic level mixing available only at the early phase of the single impact event and at the core of the impact zone. 2. DFT Calculations. In order to analyze the structures of the carbide clusters observed in this work and rationalize the different abundances, the odd/even intensity alternations in the number n of carbon atoms and the high abundance of In3C2+ species, we have carried out DFT calculation. Here, we present the most stable structures found for both neutral and positively charged indium carbides (Figures 4 and 5). Briefly, the skeletal structures are quite similar when comparing neutrals and cations in the series In3Cn+ (n = 1−7) and In4Cn+ (n = 1−8), in the sense of a carbon backbone chain with peripheral indium atoms. The main differences are related with the linear vs bent nature of the carbon backbone chain and the position of the third and fourth indium atoms (in addition to the two terminally bound indium atoms). Geometry optimizations of structures in the In3Cn series found the doublet and singlet electronic states to be the lowest in energy for neutrals and cations, respectively. For the In4Cn series, singlet (neutrals) and doublet (cations) were found to be the lowest electronic states of the structures. In order to demonstrate and discuss the relative importance of indium−indium bonding contributions, we present in Figure 6 some representative neutral carbide structures with their relevant molecular orbitals. Also, while the most stable structures calculated here contain a central (nearly linear) carbon chain to which the indium atoms are either bonded at both ends or π-coordinated above or below, it is also of interest to examine alternative ring structures (triangles, squares, etc.) with alternating indium and C2 units. Such structures were proposed for bismuth carbide cluster ions.15 In Figure 7, we present the calculated structures and their excess energy (ΔE) as compared with the most stable geometries (ΔE = 0) presented in Figures 4 and 5. Furthermore, we have also considered a structure composed of a small Inm cluster (nondissociated) attached to a Cn chain and calculated it for neutral In4C8. An In4 tetramer weakly πcoordinated to the C8 chain was found to be substantially higher in energy (ΔE = +84.2 kcal/mol, structure not shown) with respect to the optimal geometry (Figure 7). Clearly, In−C bonding is the major source of stability for the indium−carbon clusters studied here. In Figures 8 and 9, we present calculated adiabatic and vertical ionization energies (AIE and VIE, correspondingly) for the neutrals and dissociation energies for the different dissociation channels of the cations. This will

Figure 4. Calculated optimal geometries of the ground state structures for the neutral In3Cn (n = 1−7) series and In3Cn+ (n = 1−7) cluster ions. All neutral structures (n = 2−7) are acetylenic/cumulenic in nature with two terminal indium atoms, while the third one is either πcoordinated to carbon−carbon bond or in a terminal position. For n = 4, 6, 7, the geometry of the positive ion is meaningfully different than that of the neutral, and both structures are shown.

provide the basis for comparison with the experimentally observed mass spectral features (mainly odd/even intensity alternations) and discussion of their origin. In Figure 2 of the Supporting Information, we present also the calculated dissociation energies (for the same dissociation channels) for neutral In3Cn (n = 1−7) and In4Cn (n = 1−7). The vertical ionization energy is calculated as the energy difference between the neutral ground state and the ion at the geometry of the neutral. The adiabatic ionization energy is calculated as the energy difference between the neutral ground state and the ion at its relaxed geometry. While the ionizing electronic transition is vertical in principle, for the extremely hot InmCn neutrals exiting the surface, it is actually not clear as to what extent the distinction (and resulting energy difference) between VIA and AIE is relevant, and we therefore present both. The indium− carbon clusters as formed by the keV C60− impact originate from a collisional thermal spike zone. Their vibrational temperature can be estimated to be several thousand degrees. E

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Figure 6. Selected neutral carbide clusters showing molecular orbitals responsible for bonding between indium atoms. Bond distances (Å) and bond angles are given.

interactions with an extremely hot (and ill-defined) environment. In light of all these considerations, we note that the relevance of the calculated AIE and VIE values to the measured mass spectra rests mainly in their relative variations. A. Structures. In3C has a planar geometry similar to that previously predicted for Al3C.34 The carbon atom has three bonds, one to each of its indium atom neighbors. In3C+ has exactly the same geometry as the neutral cluster. In3C2 has a planar, acetylidic structure with an additional πcoordinated indium atom. A similar structure was also calculated for other acetylides such as those of Al, Au, and Ag.5,24 Despite the similarity in the general structure, the terminal indium−carbon bonds in In3C2 are slightly bent toward the additional π-coordinated indium atom. This can be rationalized by the fact that the singly occupied molecular orbital (SOMO) (Figure 6, a1) of the In3C2 cluster is a combination of C−C antibonding π orbital and bonding C−In and In−In orbitals. Furthermore, additional σ bond is formed between all three indium atoms (Figure 6, a2) resulting in further attraction and bending. The three indium σ bonds also interact with one of the C−C σ bonds as well. The terminal indium atoms in In3C2+ are bent in the opposite direction as compared with the neutral cluster. Since the SOMO of In3C2 becomes unoccupied when ionized, there is no contribution of this orbital to the all-metal bonding as seen in Figure 6 a1. This leads to a new bonding interaction between the two terminal indium atoms (and the bending toward each other). The same bending direction was previously seen for Au3C2+ and Ag3C2+ acetylidic carbides where some overlap between d orbitals in the same plane led to the repulsion between the metal atoms.5 In3Cn (n = 3−7) carbide clusters have basically the same structural motif: planar geometries, a carbon backbone chain with terminal indium atoms and the third indium atom πcoordinated to a carbon−carbon bond. Unlike the linear carbon chains calculated for clusters with even number of carbon atoms, clusters with odd number of carbon atoms have an angular distortion associated with the carbon−carbon bonds, which are interacting with the additional indium atom. This distortion was seen previously in Al3C3.24 The calculated geometries for In3C3+ and In3C5+ and their neutral counterparts

Figure 5. Calculated optimal geometries of the ground state structures for the neutral In4Cn (n = 1−7) series and In4Cn+ (n = 1−7) cluster ions. All neutral structures except for n = 1, 4 are acetylenic/cumulenic in nature with two terminal indium atoms, while the other two are either π-coordinated to carbon−carbon bonds or in a terminal position. In4C4 can be viewed as a dimer of two acetylides, which are π-coordinated via two opposite indium atoms. Cations with n = 1−3 have the same structures as the neutrals, while those with n = 5−7 are different than the neutral ones in terms of the position of the two additional In atoms and the linearity of the carbon chain. In4C4+ shows different In−C bond angles and a slightly more asymmetrical structure.

For a given cluster, the difference in geometry between the neutral and ion is mainly in the shape (curvature) of the Cn chain and the position of the third (for In3Cn) or third and fourth (for In4Cn) indium atoms, with respect to the terminal C atoms. For In3C6, for example (see Figure 4), the calculated energy difference between the neutral at its optimal geometry (one In atom π-coordinated) and the neutral at the optimal geometry of the ion (all three In atoms bound to terminal C atoms) is ∼0.5 eV. This energy difference is comparable with the average vibrational excitation per mode (in the equipartion limit) of the hot neutral. Ionization will therefore take place from a broad distribution of transient geometries and high vibrational levels of the neutral. Additional uncertainty regarding the relevant ionization energies is related with the nature of the actual ionization mechanism. Ionization can take place either at the surface or on-flight via thermionic emission. Ionization at the surface (assuming resonance charge exchange) implies up-shifting of the ionization level due to image charge F

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Figure 8. Calculated adiabatic ionization energies (AIE, solid line) and vertical ionization energies (VIE, dashed line) of (optimized geometries) neutral clusters In3Cn (n = 1−7) and dissociation energies of the cations In3Cn+ (n = 1−7) for different dissociation channels. (a) Ionization energy. (b) Emission of a C atom. (c) Emission of a C2 unit. (d) Emission of an In atom. (e) Emission of an InC unit. An alternation in both IE (AIE and VIE) and dissociation energies is found for odd/even number of carbon atoms. The alternations observed for nearly all channels (except for C2 emission) are in correspondence with the intensity alternations in the mass spectrum (Figure 1). Note the different energy scales for the different channels, especially the high values (ΔE ≈ 8−9 eV) for C2 emission and low values (ΔE ≈ 3−4 eV) for the emission of an indium atom.

cation. Other linear structures (in the n = 3−7 series), with a different position of the π-coordinated indium atom, were found to be at least 10 kcal/mol higher in energy (e.g., the neutral In3C6 in Figure 7). The corresponding cation converged to the same geometry as that in Figure 4. In4C and In4C+ have a similar tetrahedral structure: a single carbon atom surrounded by four indium atoms. In4C2 has a similar structure to that previously calculated for Al4C2.24,33 It has a planar geometry with acetylidic structure with two indium atoms attached to the C−C bond on each side. In Figure 6b, an overlap between the metallic py and pz orbitals and C−C antibonding π orbital can be clearley seen in the highest occupied molecular orbital (HOMO). In4C2+ has the same geometry as the neutral cluster. In4C3 is composed of a three carbon atom chain with two pairs of indium atoms terminally attached on both sides. These two pairs of indium atoms form two In−C−In planes, which are penpedicular to each other. In4C3+ has the same structure as In4C3. In4C4 has an interesting structure, which can be described as two single In2C2 acetylides interconnected via π-interactions between the carbon−carbon bond of each acetylide and the indium atom of the second acetylide. In addition, similar allmetal bonding orbital (Figure 6c) is observed as previously shown also for In3C2. The cationic cluster, In4C4+, exhibits a

Figure 7. Calculated ground state isomers for both neutral and positively charged In3C6, In4C4, In4C5, and In4C8. ΔE represents the excess energy relative to the most stable structures of these isomers as given in Figures 4 and 5. The ΔE values given for In4C4 and In4C8 at the bottom (neutral and cation structures) are with respect to the corresponding optimal (ΔE = 0) structures.

are very similar with only minor differences in the bond angles of the terminal In−C bonds. A more meaningful difference is found for the larger clusters in the series. In3C4+, In3C6+, and In3C7+ exhibit similar behavior when comparing structures of the ionic species and the neutrals. It seems that when the neutral clusters within this group lose an electron, the third (non terminal) indium atom moves toward the terminal indium atom thus forming In−C−In structure. We have also performed geometry optimization on several other structures including a triangle-like In3C6+ geometry where each vertex indium atom is bridged to each of the other indium atoms via a C2 unit (Figure 7). This geometry was previously argued to be the most stable structure for another post-transition metal carbide, Bi3C6+.15 However, in our case, this geometry was found to be 52 kcal/mol higher in energy than the linear In3C6 (neutral) and 138 kcal/mol higher than the linear In3C6+ G

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other one below, while the two additional terminal indium atoms are capping the carbon chain on both ends. This In4C5 isomer is stabilized by the interaction between three In atoms, which impose the bending on the carbon chain. Similarly bent In4C6 and In4C7 structures will be less stable due to an increased strain for the case of the longer carbon chains. In4C8 has probably many nearly iso-energetic isomers, due to the increased number of conjugated C−C bonds available for the coordination of indium atoms with respect to the previously shown clusters (n = 1−7). Hence, we have calculated only a few isomers with characteristic trends. An isomer composed of a carbon chain end-capped by two indium atoms while two additional indium atoms are π-coordinated to the carbon chain was found to be the most stable one (Figure 7). The position and the orientation of the π-coordinated indium atoms may probably vary. It was found that an initial square geometry for In4C8 with indium atoms at the four vertices, each two connected via a bridging C2 unit (as previously calculated to be the most stable one for Bi4C8+),15 converged to an acetylidic structure (similar to that of In4C4) with additional two C2 units π-coordinated to each terminal indium atoms (see Figure 7, bottom right). This structure can be considered as a distorted square geometry where two of the initial indium−carbon bonds were replaced by four π-coordinated indium−C2 bonds (two for each indium atom). This configuration is still considerably higher in energy (81 and 132 kcal/mol for the neutral and cation, correspondingly) than the linear chain structure. We do not show the ionization and dissociation energies of this cluster due to the fact that there are too many isomers possible (and relatively close in energy) for In4C8. B. Ionization and Dissociation Energies: In3Cn and In3Cn+ (n = 1−7). The computational approach is commonly used to study distinct features in the mass spectra of various carbide cluster ions.5,24 High ionic abundances can be due to low ionization energies of the parent neutral or stability of the ion toward specific dissociation processes. The stability of the ion will be discussed only in terms of its dissociation energy as kinetic considerations (activation barriers) are not taken into account. We have therefore calculated both AIE and VIE values of the neutrals and energies for different dissociation channels of the indium carbide cluster ions as shown in Figures 8 and 9. The dissociation energies, ΔE = Eproducts − Ereactants (e.g., ΔE = E(In3Cn−1+ + C) − E(In3Cn+) for C atom emission), are calculated using optimal geometries of both products and reactants. Since the emitted species are highly vibrationally excited, one can assume that kinetic restrictions can be neglected for most processes. Figure 8 shows the calculated AIE and VIE values for the neutral In3Cn (n = 1−7) and ΔE values for the C (In3Cn+ → In3Cn−1+ + C), C2 (In3Cn+ → In3Cn−2+ + C2), In (In3Cn+ → In2Cn+ + In), and InC (In3Cn+ → In2Cn−1+ + InC) emission processes. Both ionization and dissociation energies (for C, In, and InC emissions) are in agreement with the measured intensity alternations for the In3Cn+ ions. The VIE values for n = 2−7 are (on the average) about 0.5 eV higher than the AIE values. The In3C2+ ion, which is the most abundant carbide ion in the mass spectrum, is indeed the most stable one toward fragmentation for all emission channels. The In atom emission channel (Figure 8d) is the most probable one due to its low ΔE values (2.5−4 eV), while the C atom emission is less probable due to much higher ΔE values (5.0−9.5 eV) and assumingly high activation barriers and configurational constraints. The

Figure 9. Calculated adiabatic ionization energies (AIE, solid line) and vertical ionization energies (VIE, dashed line) of (optimized geometries) neutral clusters In4Cn (n = 1−7) and dissociation energies of the cations In4Cn+ (n = 1−7) for different dissociation channels. (a) Ionization energy. (b) Emission of a C atom. (c) Emission of a C2 unit. (d) Emission of an In atom. (e) Emission of an InC unit. An alternation in both the IE (AIE and VIE) and the dissociation energies is found for odd/even number of carbon atoms. Clear odd/even alternations are observed only for In and InC emissions. Note the different energy scales for the different channels, especially the low values (ΔE ≈ 0−3 eV) for the emission of an indium atom.

noticeable change in bond angles. The indium atoms move further apart from each other thus eliminating the all-metal bonding contribution present in the neutral. It should be noted that unlike structures with n ≥ 5 (where the linear carbon chain based structure is the most stable), the alternative In4C4 geometry with a linear carbon chain terminated by two pairs of indium atoms is 16 kcal/mol higher in energy (neutral) and 22 kcal/mol higher for the cation. In4Cn (n = 5−7) have a carbon backbone chain based structure, with somewhat distorted carbon−carbon bonds due to the interaction with additional two indium atoms as seen previously for the In3Cn (n = 3−5) series. While In4C6 is planar, for In4C5 and In4C7 the two π-coordinated indium atoms are located out of plane (as defined by the carbon chain and the two terminal indium atoms), each on a different side of this plane. In comparison, In4C5+, has a fully planar structure. For both In4C6+ and In4C7+ cations, the carbon chain is long enough so that the indium atoms move further apart from each other toward the terminal indium atoms (with respect to their positions in the neutral clusters), forming planar In−C−In structures. The displacement of indium atoms, as mentioned above, results in In−In bonding to compensate for the partially positive charge on the indium atoms. In Figure 7, we present the only additional geometry of In4C5, which is found to be as stable as the one discussed above. This In4C5 structure is characterized by roughly a 90° bend in the middle of the carbon chain; one indium atom is bonded above the chain and the H

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that depositing these carbide species on a surface will enable their stabilization at the nanoscale and allow further study of their properties.

gradual loss in correlation between calculation and measurement for n > 5 can be rationalized based on an increase in the number of low energy structural isomers, which possibly contribute to the measured ion intensities. C. Ionization and Dissociation Energies: In4Cn and In4Cn+ (n = 1−7). Figure 9 shows the calculated AIE and VIE values for the neutral In4Cn (n = 1−7) and ΔE values for the C (In4Cn+ → In4Cn−1+ + C), C2 (In4Cn+ → In4Cn−2+ + C2), In (In4Cn+ → In3Cn+ + In), and InC (In4Cn+ → In3Cn−1+ + InC) emission processes. For n = 2, 3, the AIE and VIE values are very close (as expected, due to the very similar geometry between neutral and ion). The energy separation gradually grows up to about 1 eV for In4C6 and In4C7 where the change in geometry in going from the neutral to the ion is quite meaningful. The main features in the In4Cn+ (n = 1−9) mass spectrum are the high In4C2+ intensity and absence of the even (n = 4, 6, 8) peaks. Unlike the case of the In3Cn+ series, the correspondence between calculated ionization and dissociation energies and the measured ion intensities is only partial. Specifically, no satisfactory explanation was found for the absence of the n = 6, 8 peaks. The ionization energy for In4C4 is indeed the highest one but that of the In4C6 is found to be the lowest. Emission of a single C atom can explain the stability of In4C2+ and intensity alternations to some extent, but as explained before, this channel is probably of a rather low probability. On the basis of the low dissociation energies involved, the emission of a single In atom is probably the most relevant process. Especially we note the very low (ΔE = 0.2 eV) energy for the In4C4+ → In3C4+ + In channel, accompanied also by a low ΔE = 2 eV for In4C6+. The calculated structure for In4C4+ (Figure 5) as a dimer composed from two opposing interconnected In2C2 units leads us to consider the In4C4+ → In2C2 + In2C2+ channel. The calculated ΔE = 2.7 eV for this process is rather low and supports the additional contribution of this channel to the overall instability of the In4C4+ ion. The exceptional stability of In4C2+ can be rationalized based on the high dissociation energy toward InC emission.

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ASSOCIATED CONTENT

S Supporting Information *

(Figure 1) Calculated optimal geometries of the ground state structures for the InmCn+ (m = 1−2, n = 1−3) cluster ions and (Figure 2) calculated dissociation energies for neutral In3Cn (n = 1−7) and In4Cn (n = 1−7). The DFT method used and different dissociation channels are the same as given in the text for the cationic species. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*(E.K.) E-mail: [email protected]. Tel: (+972) 048292639. Fax: (+972) 04-8295703. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank Dr. Anatoly Bekkerman for technical assistance and Dr. Victor Bernstein for assistance with the DFT analysis. The research was supported by the James−Franck program.

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REFERENCES

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