Density Functional Theory Study of the Surface Adsorption and

Article Cite This: J. Phys. Chem. C XXXX, XXX, XXX−XXX

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Density Functional Theory Study of the Surface Adsorption and Dissociation of Copper(I) Acetamidinates on Cu(110) Surfaces J. Guerrero-Sań chez,† Noboru Takeuchi,†,‡ and Francisco Zaera*,‡ †

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Centro de Nanociencias y Nanotecnología, Universidad Nacional Autónoma de México, Apartado Postal 14, Ensenada, Baja California 22800, Mexico ‡ Department of Chemistry, University of California Riverside, Riverside, California 92521, United States ABSTRACT: Metal amidinates are common compounds with many applications, and are of particular value as precursors for the chemical deposition of thin metal films on solid surfaces. In order to better understand those processes, the surface chemistry of copper(I)−N,N′dimethylacetamidinate on Cu(110) single-crystal surfaces has been studied using first-principles quantum-mechanics calculations. Most metal amidinates exist as dimers (or tetramers) in the gas phase. Here, it was found that the initial steps of the adsorption and dissociation of those dimers on metal surfaces depend on their surface coverage. At low coverages, it was found that the copper(I)−N,N′-dimethylacetamidinate dimer initially binds to the Cu surface by occupying two bridge sites, with the four N atoms on top of adjacent surface Cu atoms. This configuration is, however, not stable, so the adsorbed dimer undergoes dissociation soon after via the shedding of one of the ligands; in this more stable configuration, both Cu atoms from the inorganic precursor occupy hollow sites, and one of the ligand remains coordinated on top of them whereas the other breaks away and binds directly to the surface via its N atoms. At high coverages, the dimer dissociates partially as well, but one of its ligands remains partially attached. It is speculated here, on the basis of the energetics of the different adsorbed species, that molecular desorption in this system may occur with the copper(I)−N,N′-dimethylacetamidinate as either a dimer or a monomer, a conclusion consistent with experimental observations. An analysis of the charge distributions in the adsorbed species shows a reduction of the Cu atoms of the dimer until reaching a metallic state once the ligands are all removed.

1. INTRODUCTION The growth of copper thin films is an important step in the fabrication of micro- and nanoelectronic devices. Because of their low resistivity and resistance to electromigration, copper contacts are preferred in ultra-large-scale integrated circuits.1 Given the increasing demand for the deposition of such copper films, different physical and chemical growth methods have been studied in recent years. One promising technique for fabricating good-quality Cu thin films is atomic layer deposition (ALD).2,3 ALD processes use two or more chemicals, typically called precursors.4−6 These precursors participate in two or more separate, self-limiting, and complementary reactions and need to be thermally stable, volatile, and reactive. These criteria are particularly critical for the precursor that provides the main element of the film to be grown, typically a metalorganic complex that delivers the metallic element. Ideally, that precursor should adsorb on the surface of the substrate until saturation of a monolayer, preferably retaining the structure of its ligands so that the second reactant can remove them cleanly to activate the surface for the next cycle without leaving any byproducts behind.7,8 Unfortunately, many of the metalorganic compounds available for these applications contain large ligands with reactive moieties that can undergo © XXXX American Chemical Society

secondary reactions on surfaces. In many cases, fundamental surface-science studies have identified such pathways and have provided insights into the source of impurities in films grown by ALD.4,8−12 A promising family of metal precursors for ALD is that based on amidinates. Amidinates and related complexes (iminopyrrolidinates, guanidinates) have been long known to be stable,13−15 and their introduction for chemical vapor deposition (CVD) and ALD uses has also been assessed in years past.2,16−20 In particular, a number of Cu amidinates have been tested for the deposition of Cu-containing films,3,21−28 and we29−36 and others24,26,37,38 have looked in detail at their surface chemistry as well. For instance, temperatureprogrammed desorption (TPD) and X-ray photoelectron spectroscopy (XPS) experiments to characterize the thermal chemistry of copper(I)−N,N′-di-sec-butylacetamidinate adsorbed on Ni(110)31 and Cu(110)32 single-crystal surfaces have shown a complex chemistry over a wide range of temperatures. On Ni(110), the decomposition starts via an Received: December 17, 2018 Revised: January 25, 2019

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DOI: 10.1021/acs.jpcc.8b12131 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C

expanded in plane waves with an energy cutoff of 35 and 280 Ry for the charge density. To model the surface, a cell consisting of four 4 × 5 Cu-atom layers and an empty space of ∼18 Å was used, the latter added to avoid self-interactions between the surface and its image generated by the periodic conditions imposed on the system. In each simulation, the bottom layer was fixed to the bulk positions, whereas the remaining three were left free to move. Convergence in energy was set to 1 × 10−4 Ry and in force to 1 × 10−3 Ry/au to find the minimum energy of each system under study. In order to study reaction pathways and activation energies, the nudged elastic band (NEB) method47,48 was used within the climbing image scheme, as implemented in the Quantum ESPRESSO package.43 To calculate adsorption energies, the sum of the energies of the relaxed Cu(110) surface plus that of the dimer in gas phase was used as reference. Accordingly, the adsorption energy for a dimer can be written as

initial breaking of a C−N bond in the acetamidinate ligand at ∼200 K to form 2-butene and an N-sec-butylacetamidinate adsorbed intermediate. Some of the latter is then hydrogenated around 300 K and released into the gas phase as N-secbutylacetamidine, whereas the remaining adsorbed species dissociate further around 400 K. On Cu(110) similar chemistry is observed, albeit delayed in temperature, and also leads to the production of N-sec-butylacetamidine and butene as well as to the desorption of a small amount ofbutane. One particularly interesting aspect of the surface chemistry of metal amidinates, an issue that may also concern other CVD and ALD precursors, is that those compounds exist as dimers or tetramers in their solid state.16,39,40 We have recently shown that the dimeric or tetrameric structures are particularly stable and survive intact upon vaporization into the gas phase.36 This fact complicates our understanding of the chemistry of ALD processes, as it points to the possibility that, because of the bulkiness of the surrounding ligands, these large dimer/ tetramer structures may have their metal centers shielded and inaccessible for bonding to the surface where they are being deposited.41 Indeed, we have recently reported that, on silicon oxide surfaces, the initial interaction of copper amidinates involves a nitrogen rather than a copper atom.36 That change in bonding is likely to affect the subsequent activation of the CVD/ALD precursors. Our previous studies with the copper(I)−N,N′-di-secbutylacetamidinate precursor indicated that, although on Ni(110) full dissociation of the dimer is eventually reached,31 on Cu(110), only approximately half of the adsorbed molecules desorb molecularly, as dimers, around 300 K in TPD experiments,32 suggesting that this dissociation may be partially reversible. Moreover, the reactivity of the copper acetamidinate precursor on Cu(110) appears to depend on its surface coverage: higher coverages favor molecular desorption, whereas lower coverages favor dissociation. It was also established by XPS that, during the adsorption and dissociation process, the copper atoms of the precursors are reduced to a metallic state. The role of the dimeric structure in the subsequent thermal chemistry of these amidinate precursors on metal substrates is still unresolved. In this study, we have attempted to shed further light on the chemical processes associated with the adsorption and dissociation of these compounds by performing quantum mechanics calculations on one of the smallest theoretically possible Cu acetamidinates, copper(I)−N,N′-dimethylacetamidinate, on Cu(110) surfaces. Below we report the main results obtained from such calculations and discuss the implications to the surface chemistry of these compounds and to ALD processes.

Eads = Edimer@surface − Esurface − Edimer

where Edimer@surface and Esurface are the total energies of the slab with and without the dimer and Edimer is the total energy of the dimer in the gas phase. For the adsorption of a monomer, the corresponding equation is Eads = 2Emonomer@surface − 2Esurface − Edimer

where Emonomer@surface is the total energy of the slab with a monomer.

3. RESULTS 3.1. Gas-Phase Precursor. It is known that metal amidinates tend to dimerize in solid state,15,18,39 and it has also been established that they retain the dimeric structure in the gas phase.36 Therefore, in this study we started by optimizing the atomic structure of both monomer and dimer forms of our copper amidinate in the gas phase in order to be able to compare their total energies. Figure 1 shows schematic

Figure 1. Schematic view of the monomer (left) and the dimer (right) of copper(I)−N,N′-dimethylacetamidinate in the gas phase, showing the optimized structures obtained from our DFT calculations. Green, gray, yellow, and blue spheres represent Cu, N, C, and H atoms, respectively.

2. METHOD Spin-unrestricted first-principles total-energy calculations were performed to investigate the energetics and geometrical and electronic details of the adsorption of copper(I)−N,N′dimethylacetamidinate on Cu(110) surfaces. Periodic density functional theory (DFT)42 was used, as implemented in the PWscf code of the Quantum ESPRESSO package.43 In this work, the exchange−correlation energies were treated within the generalized gradient approximation, employing the Perdew−Burke−Ernzerhof gradient-corrected functional.44 van der Waals interactions were considered using the Grimme D2 method.45 The electron−ion interactions were treated using ultrasoft pseudopotentials.46 Electron states were

views of the resulting monomer and dimer optimized structures. It can be seen there that in the monomer the Cu atom forms two Cu−N bonds, with bond lengths of 2.01 and 2.02 Å and an N−Cu−N angle ∠(NCuN) = 69°. On the other hand, in the dimer, the two cooper atoms each form two Cu− N bonds, all 1.87 Å in length. These structures are consistent with previous crystallographic reports and with what is expected based on general inorganic chemistry intuition. As it has been previously reported with other amidinates, we here B


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although the other N atom remains bonded to the Cu atom from the metalorganic precursor, it shares that bonding with another Cu surface atom. The adsorption energy of configuration M2 was estimated at Eads = −3.83 eV, much lower than the energy for configuration M1. Complete breaking of the ligand from the Cu adatom was considered as well, as shown in Figure 2e,f. In that case, the ligand diffuses to the next row of Cu atoms on the surface. This configuration (configuration M3) is less stable than configuration M2; its adsorption energy is Eads = −3.49 eV. 3.3. Adsorption of the Dimer on Cu(110). In connection with the adsorption of the Cu acetamidinate dimers on Cu(110), several possible adsorption sites and configurations were considered as well; the most relevant structures are provided in Figure 3, and the adsorption

found that copper(I)−N,N′-dimethylacetamidinate is more stable as a dimer than as a monomer, in this case by ∼5.02 eV. 3.2. Adsorption of the Monomer on Cu(110). The adsorption and dissociation of the copper dimethylacetamidinate monomer on Cu(110) was studied next. The structures determined after energy optimization are depicted in Figure 2,

Figure 2. Top and side views of the different configurations calculated for the adsorption of the monomer of copper(I)−N,N′-dimethylacetamidinate on Cu(110). (a,b) Configuration M1, for the molecular adsorption of the monomer; (c,d) configuration M2, where the monomer is partly dissociated; and (e,f) configuration M3, with the amidinate ligand completely separated from the original Cu atom. Gray, yellow, and blue spheres represent N, C, and H atoms, respectively. The surface Cu atoms are shown as brown, whereas the Cu atom of the monomer is shown as green. The Cu atoms in the second layer are depicted as smaller spheres than those of the first layer in the top views (a,c,e) for clarity.

Figure 3. Top and side views of the different configurations calculated for the adsorption of the dimer of copper(I)−N,N′-dimethylacetamidinate on Cu(110). (a,b) Configuration D1, for the molecular adsorption of the dimer; (c,d) configuration D2, where the dimer is dissociated and both ligands are close to each other; (e,f) configuration D3, in which the dimer is partially dissociated and one of the ligands is on top of the two Cu atoms of the dimer but the other is displaced and bonded to atop sites on the Cu(110) surface; and (g,h) configuration D4, where the dimer is fully dissociated and both ligands are bonded to atop surface sites. Gray, yellow, and blue spheres represent N, C, and H atoms, respectively. The surface Cu atoms are shown as brown, whereas the Cu atom of the dimer is shown as green. The Cu atoms in the second layer are depicted as smaller than those of the first layer in the top views (a,c,e,g) for clarity.

and the resulting adsorption energies and key structural parameters are summarized in Table 1. It was found that the Table 1. Adsorption Energies and Structural Parameters of Several Configurations of Monomers and Dimers of Copper(I)−N,N′-Dimethylacetamidinate on Cu(110) configuration

Eads/eV

M1 M2 M3 D1 D2 D3 D4

−1.45 −3.83 −3.49 −3.64 −3.83 −3.98 −3.77

dN−Cu(molecule)/Å 2.04, 2.02 2.03, 3.05 2.01, 2.03, 2.02, 2.02 1.96, 1.93, 2.22, 3.12 1.93, 1.92

dN−Cu(surface)/Å 1.98, 1.94, 2.22, 2.00, 1.94, 1.94,

2.11 1.94 2.23, 2.22, 2.21 1.99 1.94 1.94, 1.94, 1.93

energies and structural parameters in Table 1. It was found that in the most stable molecular geometry, the two Cu atoms of the dimer occupy bridge sites along the (100) (i.e., long) direction, whereas the four N atoms end up on top sites above surface Cu atoms (configuration D1). Short-bridge sites (along the (11̅0) direction) were estimated not to be stable for the adsorption of Cu dimers (the adsorption energy is 0.48 eV higher than that of the most stable configuration), and the stability of configuration D1 was found to be due to the additional bonds formed between the nitrogen atoms and the surface Cu atoms. As shown in Figure 3a,b, the dimer also undergoes considerable distortion from its gas-phase planar structure to favor adsorption. The four Cu−N bonds that are formed between the surface Cu and the N atoms of the dimer exhibit bond lengths of 2.22, 2.23, 2.22, and 2.21 Å; a slight asymmetry is introduced in the dimer structure upon adsorption. Also, the long Cu−N bonds seen here within the molecule itself, ranging from 2.01 to 2.03 Å (Table 1), contrast with the Cu−N bond lengths of 1.87 Å in the gas phase and indicates a weakening of the Cu−N bond strength. The calculated adsorption energy of this configuration is Eads =

monomer adsorbs with its Cu atom occupying a hollow site on the surface and with the N,N′-dimethylacetamidinate ligand in a vertical position, that is, with its plane perpendicular to the surface, as shown in Figure 2a,b (configuration M1). The Cu− N bonds expand slightly upon bonding to the surface, to 2.04 and 2.02 Å, and the ∠(NCuN) reaches a value of 67°. The calculated adsorption energy for this configuration was estimated at Eads = −1.45 eV. It was determined that this adsorption mode is not thermodynamically stable against its dissociation. The most stable configuration (M2) is a dissociated adsorbed complex where the amidinate ligand is partially displaced and bonds directly on the surface, as shown in Figures 2c,d. In that new configuration, one of the original Cu−N bonds is broken and the respective N atom bonds to a surface Cu atom. Also, C


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The Journal of Physical Chemistry C −3.64 eV, reflecting higher stability than that of any of the monomeric configurations. Next, we considered a number of configurations on the Cu(110) surface involving the dissociation of the dimer. In configuration D2, shown in Figure 3c,d, the dimer is shifted in the (11̅0) direction in such a way that the Cu atoms from the complex now occupy stable hollow positions. One of its N,N′dimethylacetamidinate ligands is still bonded to the now Cu ad-dimer (and not to any Cu surface atom), whereas the other is partially separated from the Cu ad-dimer, with both N atoms bonded to surface Cu atoms but with one of them still retaining some interaction with one of the original Cu atoms of the precursor. The latter ligand is shifted one lattice constant with respect to configuration D1. The adsorption energy for configuration D2 was calculated at Eads = −3.83 eV, 0.19 eV lower than in configuration D1. Next, the shifting of the ligand already detached in D2 further in the (1̅00) direction was considered, to reduce steric effects. In this new configuration, configuration D3 (shown in Figure 3e,f), both ligands are oriented perpendicularly to the plane of the surface and are separated by 1.5 lattice constants in the (100) direction. This is the most stable configuration identified by our calculations, with an adsorption energy of Eads = −3.98 eV. Finally, the possibility of having both ligands displaced and bound to surface atoms was tested, as shown in Figure 3g,h. This configuration, configuration D4, still shows both ligands in vertical positions, but is less stable than configuration D3, with an adsorption energy of Eads = −3.77 eV. 3.4. Dissociation Pathway for the Adsorbed Dimer. After finding the different adsorption geometries for the adsorption of the dimer, the minimum energy path for its dissociation on the Cu(110) surface was determined using the climbing image-NEB (CI-NEB) method. The results, depicted as a plot of energy versus extent of reaction (reported as image numbers in the x axis in Figure 4), show two components separated by an intermediate metastable state D2 (a local minimum in the potential energy surface). The reaction pathway was calculated by performing two separate NEB

calculations, each involving 11 intermediate positions in the potential energy curve (images), one from the initial (configuration D1) to the metastable (configuration D2) state and the other from the metastable (configuration D2) to the final (configuration D3) state. The reference energy state chosen to report the energetics for the conversion of the adsorbed dimer is the energy of the clean Cu(110) surface plus that of a dimer in the gas phase. As seen in Figure 4, both the adsorption and the dissociation of the dimer are exothermic processes; the calculated reaction energy for the entire process, starting from the gas phase, is ΔEreact ≈ 4.0 eV. After molecular adsorption of the dimer in configuration D1, which releases Eads = 3.64 eV (as mentioned before), the energy barrier for going from configuration D1 to configuration D2 is small, ΔETS1−D1 = 0.12 eV. At the transition state TS1, the copper atoms from the dimer are shifted half-way between bridge and hollow positions along the (11̅0) direction, and the left ligand is displaced accordingly, whereas the right ligand remains virtually unchanged at its position in configuration D1. The transition from configuration D2 to configuration D3 exhibits a larger energy barrier, ΔETS2−D2 = 0.43 eV. At the transition state TS2, the N atoms of the left ligand are shifted half a lattice constant along the (1̅00) direction and occupy bridge positions on the Cu second layer row. 3.5. Evolution of the Electronic Charge Distribution. A Bader’s charge population analysis was carried out on some key structures of the copper acetamidinate adsorbates to better understand the driving force for the observed conversions. The data shown in Figure 5 suggest that the adsorption of the

Figure 5. Evolution of the charge distributions in the copper(I)− N,N′-dimethylacetamidinate dimer during adsorption and dissociation. The images correspond to the gas-phase dimer (a) and to configurations D1 (b), D2 (c), D3 (d), and D4 (e). (e) Shows only one of two equivalent ligands. Black and red numbers correspond to the charges of the Cu centers and the ligands, respectively. The color code for the atoms is the same as in Figures 2 and 3.

copper(I)−N,N′-dimethylacetamidinate dimer and its further dissociation into Cu adatoms and N,N′-dimethylacetamidinate ligands lead to the reduction of the Cu centers. Indeed, the values of the Cu charges start at +0.56 and +0.60 in the gas phase but are reduced to +0.47 and +0.44 in configuration D1, to +0.29 and 0.27 in configuration D2, to +0.22 and +0.25 in configuration D3, and finally to −0.04 and −0.04 in configuration D4. These results show that the reduction of the Cu centers is predicted to take place in a stepwise fashion, and that full reduction to a metallic state is reached only after

Figure 4. Top: Minimum energy path for the dissociation of the copper(I)−N,N′-dimethylacetamidinate dimer after molecular adsorption on a Cu(110) surface. Configurations D1, D2, and D3 correspond to minima in the potential energy surface, whereas TS1 and TS2 are transition states. Bottom: Structural details of the key adsorbates along the reaction coordinate. The color codes are the same as in Figures 2 and 3. D


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−3.98 eV = −384 kJ/mol). Note that there is an additional 44 kJ/mol gain in energy from adsorption in this partially dissociated dimeric form relative to the most stable arrangement involving monomers. It appears that coordination of the ligands to the two Cu atoms of the initial precursor still provides additional stability even after adsorption and partial dissociation. The formation of the surface species depicted in the most stable configuration D3 involves a multistep transformation starting from the intact dimer in the gas phase. The dynamics of this adsorption process was mapped out by performing calculations on the minimum energy path along the potential energy surface. It was found that, after the dimer is initially adsorbed in configuration D1, there is a small energy barrier of ΔETS1−D1 = 0.12 eV (12 kJ/mol) for one of the ligands to partially separate from the dimer (configuration D2). For this ligand to diffuse and completely separate from the remaining fragment of the original dimer (configuration D3), a larger energy barrier of ΔETS2−D2 = 0.43 eV (41 kJ/mol) needs to be surpassed. Configuration D3 is clearly the most stable of all of the structures calculated and identified in this study, and it is therefore more difficult to go back to the original dimer configuration once this new species has formed (ΔETS2−D3 = 0.58 eV = 56 kJ/mol). Overall, it is interesting to note that all of these energy barriers and differences in adsorption energies are small enough to justify a possible interchange among the three dimer configurations along the adsorption/dissociation reaction coordinate before further decomposition takes place. This would explain the detection of the protonated ligand (Nsec-butylacetamidine) as a desorbing product reported with copper(I)−N,N′-di-sec-butylacetamidinate on Cu(110)32 and suggests that molecular desorption of monomers and dimers, the latter with one or two coordinated ligands, is feasible in certain cases. This is a possibility that is being explored in more detail in our laboratory at present. One important general conclusion from the calculations reported here is that the copper acetamidinates appear to bind to metal surfaces via their copper atoms regardless of the species being attached, a monomer or a dimer. This contrasts with our previous results for adsorption on silicon oxide, where the copper atoms of the dimer were found to be inaccessible to the surface, and where bonding through one of the N atoms was identified instead.36 A couple of differences may be highlighted between the two systems that may explain this difference. First, the silicon oxide surfaces exhibit terminal silanol (Si−OH) groups, and the protonic nature of the hydroxo hydrogen atoms in those moieties makes them particularly prone to form hydrogen bonds with electronegative atoms such as the N atoms in amidinates; no equivalent preferential bonding is available on metals. In addition, it is worth noticing that bonding of the copper amidinate to the Cu(110) surface is stabilized by the formation of additional Cu−N bonds on the surface, either shared with those within the original metalorganic complex or after the ligand is displaced and binds to the surface. Covalent solids such as silicon oxide do not provide the same degree of bonding flexibility. In any case, it seems clear from the structures of all of the possible stable configurations identified by our DFT calculations, both for the monomers and for the dimers, that steric hindrance is not an important factor in defining adsorption modes in these systems. Note in particular that for the initial bonding of the dimer to the Cu(110) surface in the case of the dimer, the copper amidinate loses its

detachment of both ligands. It should be indicated that the −0.04 charge reported for the Cu adatoms in configuration D4 is due to a calculation artifact, because the Bader’s charge is only an approximation to the total electronic charge of an atom.49 By contrast, the total charge on the ligands appears to change little when the dimer is adsorbed: they evolve from values of −0.61 and −0.56 in the gas phase to −0.60 and −0.58 in configuration D1. It would appear that the positive charge removed from the copper atoms of the ligands is taken and dissipated by the Cu(110) substrate. Some oxidation is seen afterward, when the dimer is broken via partial or total ligand detachment; the ligand charges transition to values of −0.51 and −0.50 in configuration D2, to −0.53 and −0.48 in configuration D3, and finally to −0.53 and −0.53 in configuration D4. On the other hand, these numbers indicate that only in the first step, going from configuration D1 to configuration D2, there is a significant charge rearrangement in the ligand. Overall, these calculations on charge distribution show that the Cu metallic substrate is the main element responsible for the reduction of the Cu centers of the amidinate complex.

4. DISCUSSION In this section, we discuss our results, described in Section 3, and try to connect them with previous theoretical calculations and experiments on the adsorption of Cu acetamidinates on various surfaces. First of all, our conclusion that in gas phase the dimers of these copper acetamidinates are more stable than the two separate corresponding monomers is in agreement with our previous findings using mass spectrometry and with our published ab initio calculations on another precursors.36 In particular, our calculated difference between the energies of formation of the dimer versus the two monomers, ΔE dim er− 2m ono mers = −485 kJ/mol, is close to the ΔEdimer−2monomers = −390 and −364 kJ/mol values reported for Cu(I)−sec-butyl-iminopyrrolidinate and Cu(I)−tert-butyldimethyliminopyrrolidinate, respectively. The larger energy difference in our present case may originate from differences in the side substituents in the three molecules, but may also be due to the fact that the new calculations include long-range van der Waals interactions, a factor neglected before. With respect to the adsorption of the copper(I)−N,N′dimethylacetamidinate monomer on Cu(110), we have considered bonding to the surface in three modalities, molecularly and after partial and total ligand removal. In the case of the dimer, we explored adsorption without dissociation, with one ligand partially or completely removed from the original complex, and after the two ligands are partially or completely displaced from the molecule to the surface. It was found that, with the monomer, partial dissociation is more favorable; additional stability is obtained by extra bonding of the N atoms to the Cu surface atoms adjacent to those where the molecular Cu atoms bind (configuration M2, Eads = −3.83 eV = −340 kJ/mol; recall that this is the energy released after converting a gas-phase dimer into two adsorbed monomers; see Section 2). Nevertheless, the added stabilization was determined not to be sufficient to make the adsorbed monomer more stable than the species originated from the uptake of the dimer. In that case, the most stable configuration was found to be one in which one ligand migrates completely to surface sites whereas the second is retained by the molecular Cu pair, which binds to hollow sites (configuration D3, Eads = E


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issues raised may apply to many other systems; consider, for instance, the possible relevance of dimerization to the wellestablished chemical deposition of aluminum using trialkylaluminum precursors.50 The formation of dimers and higher oligomers can enclose the metal ions in metalorganic complexes, limiting their access for bonding directly to surface atoms. Although this was found here not to be a factor for Cu acetamidinates on metal surfaces, it proved to be critical for their adsorption on silicon oxides.36 A second factor to consider is the changes in the energetics of the activation of CVD/ALD precursors introduced by the fact that they may gain additional stability in the gas phase upon dimerization. This may make them more thermally stable, a desirable result in terms of their delivery in gas-phasebased processes, but also more difficult to adsorb and activate on solid surfaces. To this thermodynamic argument we need to add the kinetic consideration of possible monomer−dimer and related interconversions such as those illustrated in Figure 4, which may, among other things, reduce the effective sticking coefficient of the precursor, making the CVD/ALD processes less effective. The good news is that this type of reaction network also provides alternative pathways for the clean removal of ligands from metalorganic complexes upon adsorption, affording their possible protonation/hydrogenation by reducing agents such as hydrogen or ammonia in the second stage of ALD processes. Overall, because dimer/ oligomer formation can affect film depositions in both positive and negative ways, it is not easy to predict what may occur when metalorganic precursors that can form such paired structures are used. Until more is known about these effects and trends are identified across families of precursors, it will be necessary to carry out more detailed studies on individual cases to answer those questions.

planarity and becomes distorted in order to better expose its Cu atoms (configuration D1, Figure 3). The adsorption and dissociation process discussed so far corresponds to a low-coverage regime. To see the effect that the surface coverage of the adsorbates may have on this energetics, we performed a few additional calculations using a smaller 2 × 3 unit cell. It was found that the adsorption energy of configuration D2 is Eads = −3.81 eV, almost the same as that estimated at the lower coverage (larger unit cell). On the other hand, configurations D1 and D3 were found not to be stable in the small cell. The most likely explanation for this behavior is that there is not enough room in the small surface to accommodate the displaced ligands. Because the dimer cannot completely dissociate at such high coverages, molecular desorption as a dimer, as a monomer, or after losing some of the ligands may be more favorable than complete dissociation under such regime. This can help explain some of the observations deriving from our past TPD and XPS study of the thermal chemistry of copper(I)−N,N′-di-sec-butylacetamidinate on Cu(110) single-crystal surfaces.32 Finally, our calculated evolution of the charges in the copper atoms of the complex during the adsorption and dissociation processes shines additional light on the details of the ensuing thermal chemistry on the surface. It is interesting to note that, according to our DFT calculations, the positive charge of the copper ions in the metalorganic compound is partially shielded upon adsorption and evolves in a stepwise manner as the ligands in the adsorbate rearrange and/or migrate to the surface. Particularly curious is the fact that the major changes are seen not upon molecular adsorption, which leads to a charge change from +0.56 and +0.60 (gas phase) to +0.44 and +0.47 (configuration D1). Rather, the first big change is seen in going from configuration D1 to configuration D2, the latter with charges of +0.27 and +0.29. This change is associated with the breaking of the first Cu−N bond and the partial bonding of one of the ligands to the surface. Also, no significant charge redistribution takes place during the conversion of configuration D2 to configuration D3 (charges = +0.22 and +0.25), as the first ligand leaves completely the complex structure; only after the elimination of the second ligand, in configuration D4, do the Cu atoms become fully reduced to their metallic state (charges = −0.04 and −0.04). Given that the XPS data obtained for all copper amidinates and iminopyrrolidinates studied so far have identified only two oxidation states for the Cu atoms in the adsorbed species (oxidized and metallic),30−32,35 it would appear that the D1 ⇔ D2 ⇔ D3 ⇔ D4 interconversion of configurations discussed here is transient, occurring only in a narrow range of temperatures during the TPD experiments; once complete dissociation to configuration D4 takes place, that is irreversible and can only be followed by further decomposition of the adsorbed ligands. It may also be that it is only the conversion of configuration D2 to configuration D3, the transition with the highest activation barrier, that is relevant to the surface species detected experimentally by XPS. We end with a brief discussion of the implications of the new chemistry identified by our calculations to practical processes that rely on the adsorption of large metalorganic complexes on metal surfaces such as the chemical deposition of thin films. The first realization is that the formation of dimers or other polymers associated with many metalorganic precursors adds to the complexity of the surface chemistry involved. We have here discussed the case of copper amidinates, but the ideas and

5. CONCLUSIONS In summary, the adsorption and dissociation of copper(I)− N,N′-dimethylacetamidinate on Cu(110) surfaces have been studied using DFT. Particular emphasis was placed on understanding the initial reactions that may occur on the solid surface in connection with the existence of the precursor as a dimer in the gas phase. The dimers were found to indeed be more stable than the monomers in the gas phase, as already reported and as has been known from crystallographic studies in the solid state. Initial adsorption was found to preserve the dimeric form, but what happens afterward depends on surface coverage. At low coverages, the copper(I)−N,N′-dimethylacetamidinate dimer initially absorbs with its Cu atoms occupying bridge sites and the four N atoms on top of surface Cu atoms. However, in its more stable configuration, one of the ligands migrates to Cu(110) sites whereas the other remains on top of the Cu ad-dimer but reorients to adopt a perpendicular configuration. Once this initial breaking of the dimer has occurred, the ligands probably desorb as a whole unit, after protonation, or dissociate further. A partially dissociated intermediate, and also a less stable end point with both ligands removed from the complex and bonded to surface sites, were identified as well, and relatively low activation barriers were estimated from dynamic potential energy calculations for the interconversion among all of these surface species. At high coverages, on the other hand, the most stable configuration corresponds to a partially dissociated dimer, with only one bond of one of its ligands partially separated from the original metalorganic structure. Upon adsorption, Bader’s charge F


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analysis shows that the reduction of the Cu atoms of the dimer takes place in a stepwise fashion, until reaching a metallic state once both ligands are removed from the complex and displaced to the surface. Our results are in agreement with and help explain experimental results reported in the past for analogue Cu acetamidinates.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Francisco Zaera: 0000-0002-0128-7221 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS F.Z. acknowledges financial support for this project provided by a grant from the U.S. Department of Energy, Office of Science, Basic Energy Sciences, Materials Sciences and Engineering (MSE) Division, under award no. DESC0001839. N.T. and J.G.-S. thank DGAPA-UNAM project IN101019, and Conacyt grant A1-S-9070 of the Call of Proposals for Basic Scientific Research 2017−2018 for partial financial support. N.T. thanks DGAPA-UNAM for a scholarship to spend time at the University of California, Riverside. Calculations were performed in the DGCTIC-UNAM Supercomputing Center, project LANCAD-UNAM-DGTIC-051.

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Density Functional Theory Study of the Surface Adsorption and

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