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Sep 19, 2017 - Argon (Wright Brothers) was used as the matrix gas in all experiments, without further purification. To understand the underlying mecha...
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Matrix Isolation Studies of Novel Intermediates in the Reaction of Trimethylaluminum with Ozone Published as part of The Journal of Physical Chemistry virtual special issue “W. Lester S. Andrews Festschrift”. H. Dushanee M. Sriyarathne and Bruce S. Ault* Department of Chemistry, University of Cincinnati, P.O. Box 210172, Cincinnati, Ohio 45221-0172, United States S Supporting Information *

ABSTRACT: The thermal reaction of ozone with trimethyl aluminum was explored using twin jet, concentric jet, and merged jet deposition into cryogenic matrixes. Infrared spectroscopy and density functional theory calculations were employed to identify and characterize the products formed in each case. Together, these deposition techniques provide information over the essentially full course of the gas-phase reaction. At short times with twin jet deposition, the primary product is the O atom insertion product (CH3)2AlOCH3. With merged jet deposition and longer gas-phase mixing times, the methyl peroxy radical H3COO· was seen in good yield along with final stable products H2CO, H3COH, and C2H6. Production of Al2O3 and its deposition onto the walls of the reaction tube as a powdery film was noted as well. All of these outcomes were combined to propose a reaction mechanism for this system. Of particular note, the observation of H3COO· provides clear evidence for a free radical component to the overall mechanism.



INTRODUCTION The semiconductor industry is interested in deposition of metal oxide semiconducting thin films ranging from nanoscale to microscale, as these films have been utilized in wide range of applications. The method of fabricating the thin films will be determined by the nature of the application and the desired quality of the thin film. One application of these films is as a protective coatings and are typically on the order of 1−10 μm in thickness.1 Synthetic techniques such as chemical vapor deposition (CVD) or plasma-enhanced chemical vapor deposition (PECVD) have been used to deliver a high deposition rate while employing high substrate temperature to form the crystalline phases needed for maximum hardness in protective coatings.2 Atomic layer deposition (ALD) is another technique involving a sequential, self-limiting, vapor phase nanocoating process. ALD is compatible with many oxides and is ideal for the deposition of ultrathin films with high conformality and precise thickness control.3 ALD is used for the deposition of nanoscale thin films in high aspect ratio structures, such as to create gas diffusion barriers on flexible polymers,4,5 in organic light-emitting displays and solar energy conversion cells.6,7 Aluminum oxide (Al2O3) is a semiconductor having major technological interest due to its versatile physical and chemical attributes. Al2O3 semiconducting thin films are important as insulating and passivating layers in many different applications including in protective coatings, alternative dielectrics,8,9 and moisture permeation barriers.10,11 A well-known aluminum source to fabricate Al2O3 thin films is trimethylaluminum (TMAl or (CH3)3Al), as it reacts easily with water to produce alumina.12,13 Further TMAl is a thermally stable liquid at room temperature14 with high vapor pressure, which is known to be an equilibrium mixture of the monomeric and dimeric species. © 2017 American Chemical Society

The most widely studied process to form aluminum oxide is the reaction between water and Al(CH3)3.12,13,15 However, using water as the oxidizer tends to create several problems.16 As water desorbs slowly from substrates, removal out of the reactor during the purge step becomes challenging, and longer purge times are needed under low-temperature and high-pressure conditions. Further, water tends to leave unreacted hydroxyl groups in the Al2O3 films, which may affect dielectric and material properties of the thin films.17,18 Researchers have been interested in growing Al2O3 films with ozone instead of water as the oxygen source.19 Previous research conducted on Al2O3 with TMAl and O3 has shown many advantages compared to TMAl/H2O system. Faster completion of the gas purging cycle, less incorporation of hydroxyl groups,20 higher degree of crystallinity, better smoothness, less oxygen deficiency, lower leakage currents, and better chemical inertness are a few of those reported advantages of O3 as oxidizer.18 However, though trimethylaluminum and ozone have been used to grow Al2O3 thin films with promising properties, the mechanism of the TMAl + O3 process is poorly understood compared to that of TMAl + H2O. Surprisingly, no report has been found concerning the gas-phase reaction mechanism of trimethylaluminum and ozone. In this study, we explore the reaction intermediates forming upon very brief-to-moderate mixing times of trimethylaluminum with ozone at low temperature and room temperature, using the matrix isolation technique.21,22 This matrix isolation approach has been successfully used to isolate and characterize Received: August 3, 2017 Revised: September 14, 2017 Published: September 19, 2017 7335

DOI: 10.1021/acs.jpca.7b07723 J. Phys. Chem. A 2017, 121, 7335−7342

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The Journal of Physical Chemistry A

Figure 1. Infrared spectra over selected regions, arising from the twin jet deposition of samples of Ar/(CH3)3Al with Ar/O3 after annealing to 34 K (blue) compared to the same matrix before annealing (red) and to a blank experiment of Ar/(CH3)3Al after annealing (green). Trace (a) shows 1070−1230 cm−1, while trace (b) shows 1580−1780 cm−1.

Brothers) in a discharge tube cooled to 77 K. Residual O2 was pumped off before the ozone was warmed to room temperature. Isotopically labeled 18O3 was produced in the same manner from 18O-labeled O2 (94%, Cambridge Isotope Laboratories). Inside the mixing reservoir a known amount of reactant vapor was diluted with argon to the desired ratio. Argon (Wright Brothers) was used as the matrix gas in all experiments, without further purification. To understand the underlying mechanisms of reaction between trimethylaluminum with ozone, matrix samples were deposited in twin jet, merged jet, and concentric jet deposition arrangements that have been described previously.24,25 With this strategy, different time scales of the reaction could be explored, and different reaction intermediates were observed. In twin jet deposition, two gas reactants were deposited onto the CsI window from two separate nozzles simultaneously, allowing only a very short mixing time prior to matrix condensation. In

a wide variety of reactive intermediates. Combined with this technique, theoretical calculations were performed using density functional theory (DFT) with the B3LYP functional and the 6-311++G(d,2p) basis set. While reaction of O3 with both monomeric and dimeric TMAl was possible, conditions were generally chosen to minimize the presence of the dimer.



EXPERIMENTAL SECTION All experiments were conducted using a standard matrix isolation system that has been previously described.23 Trimethylaluminum (Strem Chemical) was introduced into the vacuum system from a lecture bottle and was subjected to several freeze−pump−thaw cycles at 77 K prior to sample preparation. Methane was a ubiquitous impurity with trimethylaluminum and could not be removed completely. Ozone was produced by the Tesla coil discharge of O2 (Wright 7336

DOI: 10.1021/acs.jpca.7b07723 J. Phys. Chem. A 2017, 121, 7335−7342

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The Journal of Physical Chemistry A

Table 1. Experimental and Calculated Band Positionsa and Assignments for Products Formed in the Twin Jet Deposition of Ar/ (CH3)3Al with Ar/O3 and Ar/18O3 expt product bands 16

O

714b 1135 1691 1701 1742 2835 a

18

O

714 1105 1182 1659 1667 1708 2835

shift

shift

calc 16O

calculated product bands calc 18O

I (calc)

(expt)

(calc)

assignment

719 1153 1205 1691 1691

719 1122 1193 1652 1652

129b 231 108 90 90

0 30

0 31 12 39 39

2977

2977

96

(CH3)2AlOCH3 (CH3)2AlOCH3 (CH3)2AlOCH3 (CH3)2AlCHO (CH3)2AlCHO CH2Oc (CH3)2AlOCH3

32 34 34 0

0

Band positions in inverse centimeters. bIntensities in kilometers per mole. cAssignment based on ref 35.

Ar/(CH3)3Al (1000−2000) were used, combined with more concentrated ozone samples.

merged jet deposition, a 20 cm merged jet reaction region (reaction zone) was used. For most merged jet experiments, a 1/4 in. stainless steel Ultratorr tee was used to combine flows of the two reagents, each diluted in argon. In one experiment, a 1/4 in. Teflon tee was used. Samples were deposited on the window for ∼20 h before a final spectrum was recorded. Further, in both twin jet and merged jet deposition the matrices were annealed to 35 K to permit limited diffusion and recooled to 14 K, and additional spectra were recorded. Then, the matrix was irradiated with either a red-emitting diode or a Hg lamp, and additional spectra recorded. To probe the intermediate time scale between twin jet and merged jet, a concentric jet device was employed.26,27 This setup was developed by inserting 1/8 in. o.d. Teflon fluorinated ethylene propylene (FEP) inside of a larger, 1/4 in. o.d. Teflon FEP tube. The length of the 1/8 in. tube could be adjusted to be shorter, longer, or the same as the outer tube. In particular, this allows the time scale available for reaction to be adjusted from nearly that of merged jet to nearly that of twin jet. The distance between the outlet ends of the two tubes is referred to as Δd. Thus, Δd = (position of inner tube) − (position of outer tube). In this manner, d > 0, d < 0, or d = 0 indicates that the inner tube extends beyond the outer tube, inner tube is shorter than the outer tube (more like merged jet), and the ends of the two tubes are at the same distance from the cold window, respectively. All the spectra were recorded on a PerkinElmer Spectrum One Fourier transform infrared spectrometer from 400 to 4000 cm−1 at 1 cm−1 resolution. Theoretical calculations were performed on likely intermediates in this study as well as certain transition states in the proposed mechanism, using the Gaussian 09 and 09W suite of programs.28 Density functional theory (DFT) using the hybrid functional B3LYP29,30 along with 6-311++G(d,2p) basis set was used for geometry optimization, to locate energy minima and calculate vibrational spectra.



TWIN JET When a sample of Ar/(CH3)3Al = 1000 was deposited with a sample of Ar/O3 = 500 in the twin jet mode, six relatively weak product bands were observed at 714, 1135, 1691, 1701, 1742, and 2835 cm−1. Of these, the 1135 cm−1 was the most intense with an absorbance near 0.2. While the 2835 cm−1 band overlapped a parent band, its behavior upon annealing (see below) demonstrated that it was a product band as well. When this matrix was annealed to 35 K, the bands at 714, 1135, and 2835 cm−1 grew significantly, by ∼300%. The bands at 1691 and 1701 cm−1 grew even more dramatically, by ∼400%. In contrast, the weak band at 1742 cm−1 did not grow and perhaps was very slightly reduced. These results indicate that at least three species are forming under these conditions. Figure 1 shows representative spectra over selected regions. Figure 1S shows spectra over additional regions. This initial experiment was repeated multiple times using Ar/ (CH3)3Al ratios ranging from 1000 to 2000 and Ar/O3 ratios from 250 to 500, all with annealing to 35 K. All six of the observed product bands persisted over this range of concentrations, with intensities that correlated with the sample concentrations. Of particular note, at the highest Ar/(CH3)3Al ratios almost no dimer of (CH3)3Al was detected, yet the product bands persisted in the same ratio. This strongly indicates that the reactions that occurred are between O3 and monomeric (CH3)3Al and that there is no reaction between O3 and dimeric trimethyl aluminum [(CH3)3Al]2 under these conditions. Since most of the initial products that might be envisioned contained one or more O atoms, a similar set of twin jet experiments were conducted with samples of Ar/(CH3)3Al and Ar/18O3. In these experiments, a total of seven product bands were observed. Six of these correlated well with the six product bands seen with 16O with respect to position and annealing behavior. The seventh band, at 1182 cm−1, did not have an 16O counterpart. However, there is an intense parent peak of the (CH3)3Al at somewhat higher energy that may have obscured the 16O counterpart. Moreover, the 1182 cm−1 band grew by ∼300% upon annealing, indicating that it is likely assigned to the same product that is responsible for the 714, 1135, and 2835 cm−1 bands in the 16O experiments and the 714, 1105, and 2835 cm−1 bands in the 18O experiments. The higher rate of growth of the 1691 and 1701 cm−1 bands indicates that they are due to a second species, while the very different annealing behavior of the 1742 cm−1 and its 1708 cm−1 18O counterpart clearly shows that this band is due to a third species. The



RESULTS AND DISCUSSION Separate experiments were conducted on each of the reagents alone in argon prior to any codeposition experiments. These experimentally obtained blank spectra were in good agreement with literature spectra and with blanks run previously in this laboratory.31,32 Traces of CH4 were observed as an ubiquitous impurity in the (CH3)3Al blank spectrum. However, CH4 does not react with ozone under these conditions and did not interfere with the reaction of interest. In addition, with the higher sample concentration of trimethylaluminum, the dimer of (CH3)3Al was observed as reported previously.33,34 To minimize interference from the dimer, quite dilute samples of 7337

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The Journal of Physical Chemistry A observed product bands and their 18O counterparts are listed in Table 1; Figure 2S shows the spectral region showing the 1105 and 1182 cm−1 product bands. The above experiments demonstrate that three products are formed during twin jet deposition, to be called species A, B, and C. The bands due to species A and B grew by 300 and 400%, respectively, upon annealing to 35 K. On the one hand, this indicates that the reactions forming these two species have nearly zero activation barrier, since very little thermal energy is available for reaction at 35 K. On the other hand, species C decreased slightly (or perhaps broadened slightly) upon annealing, suggesting that it was formed by a higher-energy pathway available only during the deposition process before the matrix was fully formed. This is consistent with the quite low yield of this product. Species C certainly contains a CO bond, and comparison to the literature35 makes identification of this product straightforward, namely, CH2O. Species C is also formed in higher yield in merged jet deposition (see below) and has been seen in the reaction of O3 with other group IIIA trimethyl metal compounds. An addition of O3 to TMAl to form a molecular complex is a potential initial reaction for this system, and our theoretical calculations show that this is a barrierless reaction. These calculations also show that the 1:1 complex is stable by ∼10 kcal/mol relative to the separated parent species. However, the calculated product bands and isotopic shifts for the 1:1 complex are not consistent with the product bands observed for either species A or B. Consequently, further reactions need to be considered. From an initial 1:1 complex, O atom transfer and insertion into either an Al−C bond or a C−H bond is a possibility to be considered. Since both insertion reactions were observed for the (CH3)3Ga + O3 reaction, potential products from these two possible reactions were considered. Insertion into the Al−C bond would form (CH3)2AlOCH3 [+ O2], while insertion into the C−H bond would lead to (CH3)2AlCH2OH [+O2] or, as has been seen previously, (CH3)2AlCHO (+ H2). Theoretical calculations were then performed on these potential products as well as a number of additional products. As seen in Table 1, the bands attributed to species A match very well with a number of the most intense bands calculated for (CH3)2AlOCH3, and the calculated 18O isotopic shifts match closely as well. In addition, there were additional bands of this product that were calculated to be intense. However, these bands were calculated to directly overlap much more intense bands of parent TMAl and could not be observed. Energetically, this insertion reaction is very favorable, and the exothermicity released by formation of the initial 1:1 complex is apparently sufficient to overcome any activation barrier to the insertion. Thus, species A is identified as the oxygen atom insertion product (CH3)2AlOCH3. The computed structure of this intermediate is shown in Figure 2, while the structural parameters are given in Table 1S. While formation of this species was anticipated, its significant (300%) growth upon annealing to 35 K demonstrates that the barrier to insertion is considerably lower than for (CH3)3Ga24 or (CH3)3In.36 The remaining pair of product bands at 1691 and 1701 cm−1 are attributed to species B. These two bands are most likely either a single vibrational mode of B trapped in two different matrix sites or two slightly different conformers of the same species. Thus, the pair will be treated as a single product band. This band is clearly in a region often attributed to carbonyl (CO) vibrations. Moreover, the 18O isotopic shift of this pair, to 1659 and 1667 cm−1, respectively, is precisely the shift

Figure 2. Computed structure for the initial intermediate (CH3)2AlOCH3 observed in cryogenic matrices from the twin jet codeposition of samples of Ar/ (CH3)3Al and Ar/O3.

expected for a CO oscillator. Very similar results were observed in previous studies of the reaction of O3 with (CH3)3Ga24 and (CH3)2Cd.26 In both of these systems, assignment was made to metal formyl species (CH3)2GaCHO and (CH3)CdCHO, respectively. Formyl derivatives of transition metals are known intermediates,37 and an analogous assignment here is likely. In addition, DFT calculations demonstrate that (CH3)2AlCHO is a stable species (local minimum on the global potential surface). Moreover, these calculations predict a carbonyl vibrational frequency (1691 cm−1) that is nearly identical to the experimental value and an 18 O shift that is very close to experiment. Thus, assignment the dimethylaluminum formyl species is made.



MERGED JET A number of experiments were conducted using merged jet deposition, allowing for a longer gas-phase mixing time and further reaction than in twin jet. With a 20 cm merged or reaction region and samples having a concentration of Ar/ Al(CH3)3/O3 = 2000/4/3, a large number of product bands were observed along with a marked diminution of the parent bands. These product bands are listed in Table 2. Moreover, only one of the product bands observed in twin jet deposition was seen in these merged jet experiments, the band at 1742 cm−1. This band was very weak in the twin jet experiments and very intense in the merged jet experiment. This result demonstrates that the two deposition methods are probing different portions of the reaction mechanism (earlier and later, for twin and merged jet, respectively) with only a slight overlap. Additional experiments using different sample concentrations and different lengths of the reaction region led to the same product bands, with appropriate changes to the band intensities. In particular, some product bands were relatively more intense with a longer (30 cm) merged region, while others were relatively more intense with a shorter (18 cm) merged region. Also, in one experiment the stainless steel tee at the beginning of the merged region was replaced by a Teflon tee. A merged jet experiment was then performed. The number, positions, and intensities of the product bands did not change with the Teflon tee. To explore whether additional intermediate products were formed in the reaction time region between those times associated with twin jet and merged jet, a concentric jet apparatus was employed. This was done by varying the distance between the tip of the inner deposition line and the outer tubing or deposition line. The reaction of Al(CH3)3 and O3 was explored in several concentric jet experiments with a concentration of 2000/3/4, while the distance between the outlet ends of the two tubes (Δd) ranged from −5 to −1 cm. In an experiment with Δd = −1 cm, both the bands associated 7338

DOI: 10.1021/acs.jpca.7b07723 J. Phys. Chem. A 2017, 121, 7335−7342

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The Journal of Physical Chemistry A Table 2. Band Positionsa and Assignmentsb for Products Formed in the Merged Jet Deposition of Ar/(CH3)3Al with Ar/O3 and Ar/18O3

experiments using different deposition techniques appear to be spanning the full time scale of the reaction. In addition, to assist the product identification an equivalent set of merged jet experiments was conducted with samples of Ar/Al(CH3)3 and Ar/18O3 made from 18O2. A large number of product bands were again observed, quite similar to the bands with 16O3. A number of these were shifted substantially relative to the 16O3 experiments as a result of 18O substitution, while several of the bands were not shifted. These distinct shifts with 18 O substitution indicates the presence of one or more O atoms in the absorbing species. However, for the bands that shifted, weak bands were noted at the position of the 16O product. For example, the very intense initial band at 1742 cm−1 shifted to 1708 cm−1, a 34 cm−1 18O shift. Because of the residual 16O in the 18O sample, a weak band was nonetheless observed at 1742 cm−1. These isotopic product band positions in the 18O3 experiments are also listed in Table 2. A number of the product bands in the merged jet experiments are readily assigned to three known, stable products based on comparison to literature spectra. These products are formaldehyde, CH2O, ethane, C2H6, and methanol, CH3OH. Band positions match very well with the literature,35,38,39 as do the 18O shifts (or lack thereof in the case of C2H6). Moreover, these are expected products based on previous studies of the merged jet reaction of ozone with methyl metal compounds. These product bands were relatively more intense with the longer (30 cm) merged region, indicating that the reaction is going further toward completion with the longer mixing time. Table 2 identifies and assigns these product bands. Unlike previous studies, eight additional product bands were observed that are not assignable to known, stable products. These were located at 491, 902, 1110, 1168, 1410, 1435, 1448, and 2962 cm−1 and were relatively more intense with the shorter (18 cm) merged region. This indicates that they may be attributed to a reaction intermediate rather than final product. Over the range of experiments where these product bands were observed, they maintained a constant intensity ratio with respect to one another. This suggests that they are likely due to a single product species. 18O isotopic labeling indicated that five of the bands had zero isotopic shift, while three bands had substantial isotopic shifts. This argues for the presence of an alkyl (likely methyl) portion of the molecule and a portion containing one or more oxygen atoms. One of the most intense

band position, cm−1 16

O

3642 2980 2962 2951 2921 2891 2864 2834 2798 1742 1498 1474 1466 1448 1435 1410 1426 1375 1168 1110 1033 (sh) 902 822 491

18

O

3630 2980 2962 2951 2921 2891 2849 2834 2798 1708 1489 1474 1466 1448 1435 1410 1426 1375 1166 1150 1056 1008 879 822 475

assignment CH3OH C2H6 CH3OO C2H6 C2H6 C2H6 H2CO C2H6 H2CO H2CO H2CO CH3OH C2H6 CH3OO CH3OO CH3OO CH3OO C2H6 CH2O CH3OO CH3OO CH3OH CH3OO C2H6 CH3OO

a Band positions in inverse centimeters. bAssignments based on refs 35, 38, 39, and42 for CH2O, C2H6, CH3OH, and CH3OO·, respectively.

with twin jet and merged jet depositions were seen very clearly, with the bands of the twin jet products being of similar intensity to those observed in twin jet deposition. No additional product bands were observed. When the experiment was conducted with Δd = −5 cm, the bands due to the twin jet products were observed with decreased intensities, while the bands due to the merged jet products grew substantially. Again, no additional new bands were observed. Figure 3 shows representative spectra of a concentric jet experiment compared to blank spectra of the parent compounds. Thus, these

Figure 3. Infrared spectra from 3200 to 400 cm−1 of matrices formed by the concentric jet codeposition of samples of Ar/Al(CH3)3 and Ar/O3 (green) is compared to blank spectrum of Ar/Al(CH3)3 (blue) and to a blank spectrum of Ar/O3 (red). 7339

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The Journal of Physical Chemistry A bands was at 902 cm−1 with an 18O counterpart at 879 cm−1, in a region often associated with an O−O single bond stretch. Together, these clues suggested possible formation of the methylperoxy radical H3COO·. Fortunately, this important radical has been observed in matrixes by several groups,40−42 the most comprehensive study of which was done by the Ellison group42 in 2002. Comparison with this study shows that most all of our observed product bands and 18O shifts match nearly exactly with the Ellison group spectra and thus can confidently be assigned to H3COO·. For example, they observed ν7, the O−O stretch, at 902 cm−1 with an 18O shift to 880 cm−1, nearly identical to our product bands. Similar comparisons hold for most of the bands, although there is slight shifting for some. This is probably due to the overall environment in the matrix, since the Ellison group used a very different process to generate the methylperoxy radical. Nonetheless, the observation of H3COO· in our experiments is one of the first observations of a radical intermediate in the reaction of ozone with trimethylmetal compounds and demonstrates conclusively that there is a f ree radical component to the mechanism. It is noteworthy that the methyl radical43 itself, CH3·, was not observed, suggesting either that it reacts very rapidly with available O2 or that H3COO· is not formed through the direct oxidation of CH3·. There are several possible sources of O2 in these experiments. First, O2 is produced when O3 has transferred an O atom to TMAl, yielding the insertion product observed in twin jet. Second, after ozone is produced in an allmetal system it slowly decomposed over time per 2O3→3O2, so that some level of O2 is always present. Possible reactions to form H3COO· not directly involving H3C· are presented below in the section on mechanism. Finally, it is known that O2 does not react directly with TMAl under these experimental conditions, so the products seen here are the result of ozone chemistry. A final question concerns the fate of the Al in this reaction since no Al-containing products were spectroscopically observed in matrixes formed by merged jet and concentric deposition. However, we noted the formation of a white solid film on the walls of the merged jet region prior to entering the cold cell. This white solid was scraped out of the tube at the conclusion of the experiment. Subsequent analysis of the white solid by infrared analysis and scanning electron microscopy (SEM) indicated that it was, as might be anticipated, Al2O3. Thus, the reaction is going effectively to conclusion during the short time of mixing and passage through the merged jet region.

this excess energy leads to the decomposition of the primary product via (CH3)2AlOCH3* → (CH3)2AlCHO + H2, yielding the second observed product. In contrast, in the merged jet deposition (and to some degree concentric jet) experiments, rapid gas-phase decomposition of (CH3)2AlOCH3* occurs (species in bold were experimentally observed): (CH3)2 AlOCH3* → H3CO·+Al(CH3)2

Then, potential gas-phase steps to form the observed H3COO· and H3C· are O2 + Al(CH3)2 · → Al(CH3) + H3COO·

and O2 + Al(CH3) → H3COO·+Al

Additional and/or subsequent reactions then may include H3CO·+CH3O· → H 2CO + CH3OH

and H3CO·+CH3· → H 2CO + CH4

and CH3·+O2 → H3COO·

and CH3·+CH3·+Ar → C2H 6

An alternative possibility for methyl radical formation involves adsorption of TMAl on a surface in the deposition, leading to TMAl decomposition, as has been postulated in ALD and CVD reactors: (CH3)3 Al + wall surface → 3CH3(ad) + Al(ad)

CH3(ad) → CH3·(g)

or 2CH3(ad) → C2H6(ad) → C2H 6(g)

and 2Al + O3 → Al 2O3

The primary surface material in the deposition system is Teflon, which is not known to have surface catalytic properties. In addition, the tee connecting the two input flows to the merged region is made of stainless steel and could be the source of surface adsorption. To test this possibility, the stainless steel tee was replaced by a Teflon tee in one merged jet experiment. As noted above, the number, positions, and intensities of the product bands did not change with the Teflon tee. This argues strongly that the stainless steel tee is not involved in surface adsorption, and given that the remainder the merged jet region is Teflon it is very likely that the chemical reactions reported here are, in fact, gas-phase reactions. The gas-phase mechanism above accounts for all of the observed products and is consistent with the mechanism proposed by Zare44 for the gas-phase reaction of O3 with (CH3)2Zn. There may, of course, be additional side reactions and intermediates in the mechanism that were not detected under the present experimental conditions. Lastly, it also predicts the formation of CH4. However, the ubiquitous presence of CH4 in samples of parent Al(CH3)3 makes it



REACTION MECHANISM From the species identified in the twin jet, concentric jet and merged jet experiments, some details of the reaction mechanism may be derived. The initial step is the insertion of an O atom from ozone into one of the Al−C bands of trimethylaluminum to form (CH3)2AlOCH3*, where this species is formed with a DFT-calculated 110 kcal/mol excess energy: (CH3)3 Al + O3 → (CH3)2 AlOCH3* + O2

ΔE = − 110 kcal/mol

In the twin jet deposition experiments, the rapid deposition of the cryogenic matrix leads to deactivation of the excess energy, stabilizing (CH3)2AlOCH3 as the primary isolated product. The barrier to this reaction must be very low, inasmuch as it also occurs upon annealing the matrix to 35 K. In addition, some of 7340

DOI: 10.1021/acs.jpca.7b07723 J. Phys. Chem. A 2017, 121, 7335−7342

Article

The Journal of Physical Chemistry A

(6) Poodt, P.; Lankhorst, A.; Roozeboom, F.; Spee, K.; Maas, D.; Vermeer, A. High-speed spatial atomic-layer deposition of aluminum oxide layers for solar cell passivation. Adv. Mater. 2010, 22 (32), 3564−3567. (7) Levy, D. H.; Nelson, S. F.; Freeman, D. Oxide electronics by spatial atomic layer deposition. J. Disp. Technol. 2009, 5 (12), 484− 494. (8) Klein, T. M.; Niu, D.; Epling, W. S.; Li, W.; Maher, D. M.; Hobbs, C. C.; Hegde, R. I.; Baumvol, I. J. R.; Parsons, G. N. Evidence of aluminum silicate formation during chemical vapor deposition of amorphous Al2O3 thin films on Si(100). Appl. Phys. Lett. 1999, 75 (25), 4001−4003. (9) Gusev, E. P.; Copel, M.; Cartier, E.; Baumvol, I. J. R.; Krug, C.; Gribelyuk, M. A. High-resolution depth profiling in ultrathin Al2O3 films on Si. Appl. Phys. Lett. 2000, 76 (2), 176−178. (10) Carcia, P. F.; McLean, R. S.; Reilly, M. H.; Groner, M. D.; George, S. M. Ca test of Al2O3 gas diffusion barriers grown by atomic layer deposition on polymers. Appl. Phys. Lett. 2006, 89 (3), 031915. (11) Zhu, M.; Zhang, Z.; Miao, W. Intense photoluminescence from amorphous tantalum oxide films. Appl. Phys. Lett. 2006, 89 (2), 021915. (12) Sneh, O.; Wise, M. L.; Ott, A. W.; Okada, L. A.; George, S. M. Atomic layer growth of SiO2 on Si(100) using SiCl4 and H2O in a binary reaction sequence. Surf. Sci. 1995, 334 (1), 135−152. (13) Leskelä, M.; Ritala, M. Atomic layer deposition chemistry: recent developments and future challenges. Angew. Chem., Int. Ed. 2003, 42 (45), 5548−5554. (14) Nguyen, H. M. T.; Tang, H.-Y.; Huang, W.-F.; Lin, M. C. Mechanisms for reactions of trimethylaluminum with molecular oxygen and water. Comput. Theor. Chem. 2014, 1035, 39−43. (15) Dillon, A. C.; Ott, A. W.; Way, J. D.; George, S. M. Surface chemistry of Al2O3 deposition using Al(CH3)3 and H2O in a binary reaction sequence. Surf. Sci. 1995, 322 (1), 230−242. (16) Groner, M. D.; Fabreguette, F. H.; Elam, J. W.; George, S. M. Low-temperature Al2O3 atomic layer deposition. Chem. Mater. 2004, 16 (4), 639−645. (17) Burrows, P. E.; Bulovic, V.; Forrest, S. R.; Sapochak, L. S.; McCarty, D. M.; Thompson, M. E. Reliability and degradation of organic light emitting devices. Appl. Phys. Lett. 1994, 65 (23), 2922− 2924. (18) Kim, J. B.; Kwon, D. R.; Chakrabarti, K.; Chongmu, L.; Oh, K. Y.; Lee, J. H. Improvement in Al2O3 dielectric behavior by using ozone as an oxidant for the atomic layer deposition technique. J. Appl. Phys. 2002, 92 (11), 6739−6742. (19) Goldstein, D. N.; McCormick, J. A.; George, S. M. Al2O3 atomic layer leposition with trimethylaluminum and ozone studied by in situ transmission FTIR spectroscopy and quadrupole mass spectrometry. J. Phys. Chem. C 2008, 112 (49), 19530−19539. (20) Kim, S. K.; Lee, S. W.; Hwang, C. S.; Min, Y.-S.; Won, J. Y.; Jeong, J. Low Temperature (< 100 °C) Deposition of Aluminum Oxide Thin Films by ALD with O3 as Oxidant. J. Electrochem. Soc. 2006, 153 (5), F69−F76. (21) Craddock, S.; Hinchliffe, A. J. Matrix Isolation; Cambridge University Press: Cambridge, U.K., 1975. (22) Whittle, E.; Dows, D. A.; Pimentel, G. C. Matrix isolation method for the experimental study of unstable species. J. Chem. Phys. 1954, 22 (11), 1943−1943. (23) Ault, B. S. Infrared spectra of argon matrix-isolated alkali halide salt/water complexes. J. Am. Chem. Soc. 1978, 100 (8), 2426−2433. (24) Sriyarathne, H. D. M.; Gudmundsdottir, A. D.; Ault, B. S. Infrared matrix isolation study of the thermal and photochemical reactions of ozone with trimethylgallium. J. Phys. Chem. A 2015, 119 (12), 2834−2844. (25) Carpenter, J. D.; Ault, B. S. Infrared matrix isolation characterization of aminoborane and related compounds. J. Phys. Chem. 1991, 95 (9), 3502−3506. (26) McNally, D.; Ault, B. S. Infrared matrix isolation study of the thermal and photochemical reactions of ozone with dimethylcadmium. J. Phys. Chem. A 2012, 116 (8), 1914−1922.

difficult to determine whether or not CH4 is formed in the reaction.



CONCLUSIONS Twin jet deposition of (CH3)3Al and O3 into argon matrices led to the observation of the O atom insertion product (CH3)2AlOCH3 along with a small amount of (CH3)2AlCHO. Product bands grew by 300−400% upon annealing to 35 K, indicating a very low barrier to reaction. Infrared spectroscopic, isotopic labeling, and DFT calculations were used to confirm the product identification. Merged jet deposition led to formation of the methyl peroxy radical, H3COO·, providing firm evidence for a radical pathway in the reaction mechanism. With longer merged jet lengths, the final stable products H2CO, H3COH, and C2H6 were seen in increasing yield. Concentric jet deposition spanned the twin jet and merged jet time frames, covering the full time scale of the reaction. Finally, evidence strongly suggests that ozone is reacting with the (CH3)3Al monomer, not the dimer.



ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpca.7b07723. The full citation for ref 28, additional infrared spectra from twin jet deposition, spectra from 18O twin jet experiments, and a table of computed structural parameters for (CH3)2AlOCH3 (PDF)



AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Bruce S. Ault: 0000-0003-3355-1960 Notes

The authors declare no competing financial interest.



ACKNOWLEDGMENTS The National Science Foundation is gratefully acknowledged for their support of this research through Grant No. CHE1110026. We are also thankful to the Ohio Supercomputer Center for computer time. H. Phan is acknowledged for her work in the early stages of this research.



REFERENCES

(1) Kyrylov, O.; Cremer, R.; Neuschutz, D. Deposition of alumina hard coatings by bipolar pulsed PECVD. Surf. Coat. Technol. 2003, 163−164, 203−207. (2) Gleizes, A. N.; Vahlas, C.; Sovar, M. M.; Samélor, D.; Lafont, M. C. CVD-fabricated aluminum oxide coatings from aluminum tri-isopropoxide: Correlation Between Processing Conditions and Composition. Chem. Vap. Deposition 2007, 13 (1), 23−29. (3) Riikka, L. P. Surface chemistry of atomic layer deposition: a case study for the trimethylaluminum/water process. J. Appl. Phys. 2005, 97 (12), 121301. (4) Kaariainen, T. O.; Maydannik, P.; Cameron, D. C.; Lahtinen, K.; Johansson, P.; Kuusipalo, J. Atomic layer deposition on polymer based flexible packaging materials: growth characteristics and diffusion barrier properties. Thin Solid Films 2011, 519 (10), 3146−3154. (5) Dickey, E.; Barrow, W. A. High rate roll to roll atomic layer deposition, and its application to moisture barriers on polymer films. J. Vac. Sci. Technol., A 2012, 30 (2), 021502. 7341

DOI: 10.1021/acs.jpca.7b07723 J. Phys. Chem. A 2017, 121, 7335−7342

Article

The Journal of Physical Chemistry A (27) Varma, P.; Ault, B. S. Infrared matrix isolation study of the thermal and photochemical reactions of ozone with dimethylzinc. J. Phys. Chem. A 2008, 112 (25), 5613−5620. (28) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Scalmani, G.; Barone, V.; Mennucci, B.; Petersson, G. A.; et al. Gaussian 09, revision C.01; Gaussian, Inc.: Wallingford, CT, 2009. (29) Becke, A. D. Density-functional thermochemistry. III. The role of exact exchange. J. Chem. Phys. 1993, 98 (7), 5648−5652. (30) Lee, C.; Yang, W.; Parr, R. G. Development of the Colle-Salvetti correlation-energy formula into a functional of the electron density. Phys. Rev. B: Condens. Matter Mater. Phys. 1988, 37 (2), 785−789. (31) Andrews, L.; Spiker, R. C. Argon matrix Raman and infrared spectra and vibrational analysis of ozone and the oxygen-18 substituted ozone molecules. J. Phys. Chem. 1972, 76 (22), 3208−3213. (32) Ault, B. S. Matrix isolation investigation of the reaction of (CH3)3Al with O2. J. Organomet. Chem. 1999, 572 (2), 169−175. (33) Kvisle, S.; Rytter, E. Infrared matrix isolation spectroscopy of trimethylgallium, trimethylaluminium and triethylaluminium. Spectrochim. Acta, Part A 1984, 40 (10), 939−951. (34) Kvisle, S.; Rytter, E. Isolation of monomeric (CH3)3Al in Ar matrices. J. Mol. Struct. 1984, 117 (1), 51−57. (35) Diem, M.; Lee, E. K. C. Photooxidation of formaldehyde in solid oxygen and argon/oxygen matrixes at 12 K. J. Phys. Chem. 1982, 86 (23), 4507−4512. (36) Locy, A.; Ault, B. S. Matrix isolation study of the thermal and photochemical reaction of ozone with trimethyl indium. Chem. Phys. 2012, 392 (1), 192−197. (37) Elowe, P. R.; West, N. M.; Labinger, J. A.; Bercaw, J. E. Transformations of group carbonyl complexes: possible intermediates in a homogeneous syngas conversion scheme. Organometallics 2009, 28, 6218−6227. (38) Schriver, A.; Schriver-Mazzuoli, L.; Ehrenfreund, P.; d’Hendecourt, L. One possible origin of ethanol in interstellar medium: photochemistry of mixed CO2-C2H6 films at 11 K. A FTIR study. Chem. Phys. 2007, 334 (1−3), 128−137. (39) Barnes, A. J.; Hallam, H. E. Infrared cryogenic studies. 4. Isotopically substituted methanols in argon matrices. Trans. Faraday Soc. 1970, 66, 1920−1931. (40) Morrison, A. M.; Agarwal, J.; Schaefer, H. F.; Douberly, G. E. Infrared laser spectroscopy of the CH3OO radical formed from the reaction of CH3 and O2 within a helium nanodroplet. J. Phys. Chem. A 2012, 116 (22), 5299−5304. (41) Hsu, K.-H.; Huang, Y.-H.; Lee, Y.-P.; Huang, M.; Miller, T. A.; McCoy, A. B. Manifestations of torsion-CH stretch coupling in the infrared spectrum of CH3OO. J. Phys. Chem. A 2016, 120 (27), 4827− 4837. (42) Nandi, S.; Blanksby, S. J.; Zhang, X.; Nimlos, M. R.; Dayton, D. C.; Ellison, G. B. Polarized infrared absorption spectrum of matrixisolated methylperoxyl radicals, CH3OO X 2A. J. Phys. Chem. A 2002, 106 (33), 7547−7556. (43) Jacox, M. E. Matrix isolation study of the infrared spectrum and structure of the methyl free radical. J. Mol. Spectrosc. 1977, 66 (2), 272−87. (44) Lee, H. U.; Zare, R. N. Flame emission studies of ozone with metal alkyls: Zn (CH3)2 and Zn (C2H5)2. Combust. Flame 1975, 24 (0), 27−34.

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DOI: 10.1021/acs.jpca.7b07723 J. Phys. Chem. A 2017, 121, 7335−7342