Asymmetrically Substituted Tetrahedral Cobalt NHC Complexes and

Article Cite This: Organometallics XXXX, XXX, XXX−XXX

Asymmetrically Substituted Tetrahedral Cobalt NHC Complexes and Their Use as ALD as well as Low-Temperature CVD Precursors Katharina Lubitz,† Varun Sharma,‡ Shashank Shukla,‡ Johannes H. J. Berthel,† Heidi Schneider,† Christoph Hoßbach,*,‡ and Udo Radius*,† †

Institut für Anorganische Chemie, Julius-Maximilians-Universität Würzburg, Am Hubland, 97074 Würzburg, Germany Institut für Halbleiter- und Mikrosystemtechnik, Technische Universität Dresden, Helmholtzstrasse 10, 01069 Dresden, Germany

‡

S Supporting Information *

ABSTRACT: The synthesis of novel asymmetrically substituted cobalt complexes of the type [Co(CO)(NO)(NHC)(PR3)] (NHC = iPr2Im, PR3 = PMe3 (1), PEt3(2), PHiPr2 (3); PR3 = PMe3;, NHC = Me2ImMe (4), MeiPrIm (5), MetBuIm (6), iPr2ImMe (7); R2Im = 1,3-dialkylimidazolin-2-ylidene) is reported. These complexes are stabilized by N-heterocyclic carbene (NHC), phosphine, carbonyl, and nitrosyl ligands and have been synthesized from the reaction of a NHC-substituted precursor of the type [Co(CO)2(NO)(NHC)] and the corresponding phosphine. The synthesis of [Co(CO)(NO)(MetBuIm)(PMe3)] (6) and [Co(CO)(NO)(iPr2ImMe)(PMe3)] (7) proceeds in a thermal reaction even at room temperature by quantitative replacement of one carbonyl with a phosphine ligand. All of the other complexes were synthesized using photochemical conditions. Complexes 1−6 have been characterized by elemental analysis, IR spectroscopy, and multinuclear NMR spectroscopy and in some cases by X-ray crystallography. All complexes are volatile, are stable upon sublimation, and decompose readily in a stepwise manner at elevated temperature. The complex [Co(CO)(NO)(iPr2Im)(PMe3)] (1) as well as cobalt complexes that were reported earlier, i.e. [Co(CO)(NO)(iPr2Im)2], [Co(CO)(NO)(MetBuIm)2], and [Co(CO)2(NO)(iPr2Im)], are evaluated and have been successfully applied in the deposition of cobalt-based thin films.

■

and 3D surfaces.4 In both techniques, CVD and ALD, suitable precursors require certain physical and chemical properties including volatility, thermal stability, and well-defined chemical reactivity for film deposition. A sufficiently large temperature “window” between evaporation and decomposition is an essential criterion of the deployed precursors.5 Hence, the development and design of suitable molecular precursors, especially for ALD, is an important part of ongoing research.3 The deposition of highly pure metal films, e.g. elemental cobalt, has become a significant aspect in the area of electrical engineering, especially regarding its application as an electrode material. Recently, many new applications have been identified that exploit thin metallic cobalt films; examples of such applications are in the fabrication of spintronic devices for nonvolatile magnetic random access memories (MRAM) and diluted magnetic semiconductors (DMS).6 Deposition of pure cobalt films by CVD has already been accomplished using carbonyl-substituted cobalt complexes such as [Co2(CO)8] and [Co(η3-C3H5)(CO)3].3 Some of these precursors have also found application in ALD techniques, but the quest to develop new organometallic cobalt species, especially for ALD, is of great interest.

INTRODUCTION The technique of chemical vapor deposition (CVD) has been known since the late 1950s as an efficient method for the deposition of thin films. As a typical feature of CVD, the film is formed by a thermal decomposition of one or more volatile precursors upon contact with a heated surface.1 The segregated film and the chemical reaction on the surface are quite variable and heavily depend on the precursors applied.2 As a further development of this technique, atomic layer deposition (ALD), elaborated in the late 1970s, offers a promising alternative and novel benefits in comparison to CVD and is currently already used for the fabrication of microelectronic devices.3 In ALD, a reactive surface is sequentially exposed to two or more gaseous precursors and each precursor pulse is separated by an inert gas purge step. Ideally, each precursor reacts with the adsorbed species at the surface in a self-limiting process and unreacted precursor molecules and byproducts are purged away during the inert gas purge steps. Thus, undesirable byproducts and incorporation of contaminants can be avoided by deploying ALD as a technique for the deposition of thin films. Furthermore, the desired film thickness can be accurately controlled by repetition of the sequences (precursor pulse, purge step, coreactant pulse, purge step). In fact, ALD represents an excellent method for the deposition of highly conformal thin films on a nanometer scale even on complex © XXXX American Chemical Society

Received: January 30, 2018

A

DOI: 10.1021/acs.organomet.8b00060 Organometallics XXXX, XXX, XXX−XXX

Article

Organometallics

[Co(CO)(NO)(iPr2Im)(PMe3)] (1), [Co(CO)(NO)(iPr2Im)(PEt3)] (2), and [Co(CO)(NO)(iPr2Im)(PHiPr2)] (3) were isolated in 70%, 55%, and 69% yields after workup, respectively. Complexes 1−3 are chiral at the metal center, but we were not interested in separating the isomers and isolated the compounds as racemic mixtures. Since NHCs offer a variety of stereoelectronic properties and molecular weights, we were especially interested in the synthesis of complexes of the basic small phosphine PMe3 and different low-molecular-weight NHCs. Therefore, since complex 1 was easily accessibile, the synthesis of further PMe3substituted complexes was investigated. Accordingly, the reaction of PMe3 with a slightly substoichiometric amount of various mono-NHC-substituted complexes [Co(CO)2(NO)(NHC)] leads to the desired complexes [Co(CO)(NO)(NHC)(PMe3)] (NHC = Me2ImMe (4), MeiPrIm (5), MetBuIm (6), iPr2ImMe (7)) in good to fair yield after workup. Interestingly, the formation of 6 and 7 occurs even at room temperature without the need for irradiation (Scheme 2).

Over the past few years, we have reported on many nickel complexes of small, alkyl-substituted N-heterocyclic carbenes (NHCs),7 which are volatile and probably suitable for deposition of elemental nickel, nickel carbides, or nickel oxide. Previously, we reported the synthesis of a variety of NHC-stabilized cobalt half-sandwich complexes and only recently cobalt carbonyl nitrosyl complexes of the type [Co(CO)(NO)(NHC) 2 ] and [Co(CO) 2 (NO)(NHC)] (NHC = iPr2Im, nPr2Im, Cy2Im, Me2Im, iPr2ImMe, Me2ImMe, MeiPrIm, MetBuIm; R2Im = 1,3-dialkylimidazolin-2-ylidene).8 Tetrahedral nitrosyl carbonyl complexes have chemical and physical properties which make them suitable as precursors for any type of vapor deposition (see below). In general, lowweight molecules typically reveal high volatility in many cases, which is an important feature for a CVD or ALD precursor. In addition, asymmetric molecules often possess low sublimation and melting temperatures, because of their high vapor pressure. Thus, to bring more asymmetry into these systems we were interested in complexes of the type [Co(CO)(NO)(NHC)(PR3)], especially with NHCs and phosphines of low molecular weight. Herein, we report the synthesis, characterization, and thermal properties of such phosphine carbonyl cobalt complexes of the type [Co(CO)(NO)(NHC)(PR3)] as well as the first vapor-phase deposition experiments on [Co(CO)(NO)(iPr2Im)(PMe3)] (1) and some cobalt complexes that were reported earlier:8c [Co(CO)(NO)(iPr2Im)2], [Co(CO)(NO)(MetBuIm)2], and [Co(CO)2(NO)(iPr2Im)].

Scheme 2. Reaction of Various NHC-Substituted Complexes [Co(CO)2(NO)(NHC)] with 1.1 equiv of PMe3 To Afford the Complexes [Co(CO)(NO)(NHC)(PMe3)] (NHC = Me2ImMe (4), MeiPrIm (5), MetBuIm (6), iPr2ImMe (7))

■

RESULTS AND DISCUSSION Synthesis and Characterization of Complexes. The complexes [Co(CO)(NO)(NHC)(PR3)] (NHC = iPr2Im, PR3 = PMe3 (1), PEt3(2), PHiPr2 (3); PR3 = PMe3, NHC = Me2ImMe (4), MeiPrIm (5), MetBuIm (6), iPr2ImMe (7); R2Im = 1,3-dialkylimidazolin-2-ylidene) were synthesized by starting from the NHC-substituted complexes [Co(CO) 2(NO)(NHC)] and phosphines on the replacement of any of the CO ligands by a PR3 ligand in a selective reaction. We have found no evidence for NO or NHC replacement for one of the reactions investigated. However, to accomplish quantitative substitution of one CO ligand, in some cases photochemical conditions were employed.9 [Co(CO)2(NO)(iPr2Im)], for example, reacts upon irradiation (broad-band UV, Hg-vapor lamp) with a slight excess of various alkyl-substituted phosphines to afford the complexes [Co(CO)(NO)(iPr2Im)(PR3)] (PR3: PMe3 (1), PEt3 (2), PHiPr2 (3)) (Scheme 1). For the phosphines PMe3, PEt3, and PHiPr2 a highly chemoselective and quantitative conversion was observed in the NMR tube. The synthesis of the PMe3-substituted complex 1 was optimized in a high-yielding and simple one-pot synthesis, on the basis of [Co(CO)3(NO)]. The complexes

Complexes 1−6 have been isolated and characterized by elemental analyses, IR spectroscopy, and NMR spectroscopy. The 1H NMR spectra of the complexes [Co(CO)(NO)(NHC)(PR3)] (1−7) reveal one set of signals for each NHC and each phosphine ligand, as expected. For complexes 1−3, the resonances of the methyl groups of the isopropyl unit split into two doublets, due to the hindered rotation along the C−N axis of the nitrogen and isopropyl group. The resonances of the methyl groups of the PHiPr2-substituted complex 3 split into four individual doublets of doublets, caused by additional 2JPH coupling (2JPH = 14.3, 16.2 Hz). For all of the complexes, the 31 P NMR resonances of the coordinated phosphine ligands are broadened due to the cobalt nuclear spin of I = 7/2, as well as the large quadrupole moment of the 59Co isotope. Hence, the detection of the resonances of the carbon atoms coordinated to cobalt, i.e. carbene as well as carbonyl carbon atoms, in the 13 C{1H} NMR spectra was also affected and these resonances were usually detected as broadened singlets. In some cases the resonances of cobalt-bonded carbon atoms could not be observed at all in the 13C{1H} NMR spectra of these complexes. Some relief to this circumstance came from 13 C{1H} HMBC experiments, where the NHC carbon atom resonances were detected as the cross peaks between the carbene carbon atom and the backbone protons. The IR spectra of all complexes reveal one intense signal for the CO stretching mode (1886−1892 cm−1) and one intense signal for the NO stretching mode (1643−1656 cm−1), and for [Co(CO)(NO)(iPr2Im)(PHiPr2)] (3) the characteristic PH

Scheme 1. Reaction of [Co(CO)2(NO)(iPr2Im)] with PR3: Synthesis of [Co(CO)(NO)(iPr2Im)(PR3)] (PR3 = PMe3 (1), PEt3 (2), PHiPr2 (3))

B


Article

Organometallics Table 1. Characteristic Spectroscopic Parameters for Complexes of the Type [Co(CO)(NO)(NHC)(PR3)] (1−6) 1 2 3 4 5 6

NHC

PR3

νCO (cm−1)

νNO (cm−1)

δ(NCN) (ppm)

δ(31P) (ppm)

Pr2Im Pr2Im i Pr2Im Me2ImMe MeiPrIm MetBuIm

PMe3 PEt3 HPiPr2 PMe3 PMe3 PMe3

1886 1886 1896 1888 1892 1886

1643 1643 1654 1643 1653 1656

197.1 187.2 195.8 186.8 198.6 /

7.65 41.9 55.0 9.27 22.2 5.28

i i

stretching mode was also observed at 2306 cm−1. Important characteristic spectroscopic features of all isolated compounds 1−6 are summarized in Table 1. Single crystals suitable for X-ray diffraction of the complexes [Co(CO)(NO)(iPr2Im)(PHiPr2)] (3) and [Co(CO)(NO)(MetBuIm)(PMe3)] (6) were obtained from cooling roomtemperature saturated solutions of the complexes in n-pentane to −30 °C. Figure 1 depicts the molecular structures of the compounds in the solid state.

2. The DTA graph of each complex reveals one significant endothermic signal, associated with the melting point of the compound, and one intense exothermic signal for the decomposition of the complex at higher temperature. All of the TG curves of the compounds describe a stepwise process of the decomposition of the complexes, with a gradual decrease in the mass. [Co(CO)(NO)(iPr2Im)(PMe3)] (1), for example, has a melting point of 111 °C. The exothermic decay starts with an onset temperature of 223 °C, associated with a mass loss of 42%, which can be correlated to the simultaneous loss of the phosphine, carbonyl, and nitrosyl ligands, in the first step. Complexes 1−6 all undergo sublimation without any decomposition, which was proven by NMR spectroscopy, and show a high thermal stability below the decomposition points. Furthermore, these complexes possess the desired large window between melting/sublimation and decomposition. The melting points determined for 1−6, which lie in the range between 79 and 130 °C, are quite distinct from the decomposition points of the complexes, which lie in the range between 207 and 248 °C. Melting and decomposition points are dependent on the molecular weight of the complexes, as expected, and the use of asymmetrically substituted complexes with low-weight ligands leads to a significant improvement in the thermal properties.8c Hence, the lowest melting point was observed for [Co(CO)(NO)(MeiPrIm)(PMe3)] (5) (onset temperature of 79 °C). Because of the additional high thermal stability (onset temperature of 221 °C) at elevated temperatures, complex 5 shows the largest temperature range between melting and decomposition. The correlation of the melting points, as well as the sublimation temperatures of the complexes [Co(CO)(NO)(NHC)(PR3)] (1−6) to their molecular weight is illustrated in Figure 3. The thermal properties of the complexes [Co(CO)2(NO)(NHC)] and [Co(CO)(NO)(NHC)2], which were reported before,8c are also included in Figure 3. In general, the mono-NHC-substituted complexes [Co(CO)2(NO)(NHC)] are very volatile but decompose at rather low temperatures. The bis-NHC-substituted complexes [Co(CO)(NO)(NHC)2], in contrast, are thermally more stable but have rather high melting points. The replacement of one CO ligand in complexes [Co(CO)2(NO)(NHC)] by a phosphine ligand to give complexes of the type [Co(CO)(NO)(NHC)(PR3)] leads, by and large, to a higher volatility in comparison to [Co(CO)(NO)(NHC)2], as well as to a higher thermal stability in comparison to [Co(CO)2(NO)(NHC)], and thus eliminates the drawbacks of these NHC-based systems. Thereby, the use of asymmetric substituted ligands, in general, leads to a significant effect on the melting points, as well as the sublimation temperatures, as a result of their high vapor pressure. The complex [Co(CO)(NO)(iPr2Im)(PHiPr2)] (3) has a significantly lower melting point in comparison to [Co(CO)(NO)(iPr2Im)(PEt3)] (2), despite their equal molecular mass (387.3 g mol−1). The same pattern could be observed

Figure 1. Molecular structures of [Co(CO)(NO)(iPr2Im)(PHiPr2)] (3, left) and [Co(CO)(NO)(MetBuIm)(PMe3)] (6, right) in the solid state (ellipsoids set at the 50% probability level). Hydrogen atoms have been omitted for clarity. Selected bond lengths (Å) and angles (deg) for 3: Co−C1 1.9756(16), Co−C2 1.7334(15), Co−P 2.2089(5), Co−N2 1.6681(14), C2−O2 1.1569(19), N2−O1 1.833(18); C1−Co−P 95.92(5), C1−Co−C2 106.06, C1−Co−N2 117.39(6), C2−Co−P 106.91(5), C2−Co−N2 116.93(7), P−Co−N2 111.23(5). (plane C1−Co−P)−(plane C2−Co−N2) 87.828(9). Selected bond lengtsh (Å) and angles (deg) for 6: Co−C1 2.0013(15), Co−C2 1.7240(15), Co−P1 2.2165(4), Co−N2 1.6821(14), C2−O2 1.1640(15), N2−O1 1.1781(18); C1−Co−C2 108.57(6), C1−Co−N2 119.18(6), C1−Co−P1 96.05(4), C2−Co− N2 115.08(7), C2−Co−P1 106.94(5), P1−Co−N2 108.75(5); (plane C1−Co−P)−(plane C2−Co−N2) 87.474(40).

Both complexes adopt a slightly distorted tetrahedral geometry around the chiral cobalt atoms, which is formed by the four different ligands (NHC, carbonyl, nitrosyl, and phosphine). The angles between the planes C1−Co−P and C2−Co−N2 are 87.828(9)° (3) and 87.474(49)° (6). In both cases, the interatomic distances Co−C2 and Co−N2 are much shorter than the Co−C1 distances to the NHC ligand. This can be attributed on one hand to the higher steric demand of NHC ligands and on the other hand to the stronger π-acceptor abilities of NO and CO ligands in comparison to simple NHCs. To gain more information about the thermal stability and decomposition of the synthesized complexes 1−6 at elevated temperatures as a function of (i) the phosphine ligand used and (ii) the NHC used, thermogravimetric studies (DTA/TG) were performed on all complexes. By way of example, the DTA/TG graphs of [Co(CO)(NO)(iPr2Im)(PMe3)] (1) and [Co(CO)(NO)(MeiPrIm)(PMe3)] (5) are provided in Figure 2. The characteristic thermal parameters for all the complexes [Co(CO)(NO)(NHC)(PR3)] (1−6) are summarized in Table C


Article

Organometallics

Figure 2. DTA/TG graphs of [Co(CO)(NO)(iPr2Im)(PMe3)] (1, left) and [Co(CO)(NO)(MeiPrIm)(PMe3)] (5, right). Important parameters for 1: melting point 111 °C, onset of decomposition 223 °C, amount of Co in the complex 17.1%, residual weight at 600 °C 21.6%, residual weight at 875 °C 20.3%. Important parameters for 5: melting point 79 °C, onset of decomposition 221 °C, amount of Co in the complex 18.5%, residual weight at 600 °C 26%, residual weight at 875 °C 24%.

Table 2. Characteristic Thermal Parameters for the Complexes of the Type [Co(CO)(NO)(NHC)(PR3)] (1−6) resid mass (%) 1 2 3 4 5 6

NHC

PR3

mass (g mol−1)

mp (°C)

Texo (°C)

Tsubl (°C)

amt of Co (%)

600 °C

875 °C

Pr2Im Pr2Im i Pr2Im Me2ImMe MeiPrIm MetBuIm

PMe3 PEt3 HPiPr2 PMe3 PMe3 PMe3

345.3 387.3 387.3 317.2 317.2 331.0

111 130 92 124 79 112

223 248 214 207 221 225

56 50 45 48 40 52

17.1 15.2 15.2 18.6 18.5 17.8

21.6 19.3 16.8 28 26 23.1

20.3 17.9 15.2 27 24 23.3

i i

Figure 3. Correlation of the melting points (left) and of the sublimation temperatures (right) of the complexes [Co(CO)(NO)(NHC)(PR3)] (1− 6) to their molecular mass.

for the complexes [Co(CO)(NO)(Me2ImMe)(PMe3)] (4) and [Co(CO)(NO)(MeiPrIm)(PMe3)] (5). The use of the asymmetrically substituted, low-weight ligand MeiPrIm in complex 5 leads to the largest range of temperature between evaporation and decomposition. Vapor Deposition Experiments. The vapor-phase deposition tests were performed to evaluate the suitability of [Co(CO)(NO)(iPr2Im)(PMe3)] (1) and some cobalt complexes reported earlier,8c i.e. [Co(CO)(NO)(iPr2Im)2], [Co(CO)(NO)(MetBuIm)2], and [Co(CO)2(NO)(iPr2Im)], for

ALD and/or CVD precursors. Approximately 2 g of each precursor were therefore synthesized for vapor phase deposition tests. Deposition Tests for [Co(CO)2(NO)(iPr2Im)]. As mentioned earlier,8c the melting of [Co(CO) 2 (NO)( iPr2 Im)] was observed at a sublimation temperature (Tsubl) of 45 °C, similar to what was determined by thermogravimetrical studies. For all deposition experiments Tsubl was set to 45 °C, unless otherwise mentioned. The main advantage of this precursor is that it is liquid and stable at its low sublimation temperature. D


Article

Organometallics

At deposition temperatures above 90 °C, the FDPC attains similar levels when either an NH3/H2 mixture or no reactant was used and therefore CVD effects are prominent. At a deposition temperature (Tdepo) of 70 °C, the CVD component is less in comparison to the FDPC levels when either NH3/H2 or air as a coreactant was used. This may imply that the precursor [Co(CO)2(NO)(iPr2Im)] shows less CVD component (less self-decomposition) at temperatures below 70 °C and higher decomposition on the QCM surface at deposition temperatures above 90 °C was obtained. Therefore, [Co(CO)2(NO)(iPr2Im)] can be used in low-temperature CVD of cobalt-based films. Assuming a density of about 4 g/cm3, the total of 1000 cycles with FDPC of about 0.8 Hz can be roughly converted to approximately 20 nm of film. Figure 5 shows the SEM images of an ∼19 nm rough cobalt oxide (most probably oxidized in the ambient air) film

For the deposition of cobalt-based films, air, H2, and an NH3/H2 (1/1 with 100 sccm each) mixture were used as second reactants, but also direct deposition without any reactant but with inert argon gas instead was investigated. The aim of testing [Co(CO)2(NO)(iPr2Im)] without any coreactant was to observe any thermal, catalytic, or surface decomposition which could be attributed to CVD type growth. For ALD or CVD experiments, the substrate temperature was varied from 70 to 175 °C. The substrate surface was sequentially exposed to 5 s of the cobalt complex (see Figures S33 and S34 in the Supporting Information) and 30 s of argon purge followed by 30 s of one of the coreactants and 30 s of a second argon purge. The whole cycle of sequential exposure of both precursors, each separated by inert gas purges, is defined as one ALD cycle, and many ALD cycles were repeated to deposit certain thicknesses of cobalt-based films. For the pulsed CVD experiments, the substrate surface was sequentially exposed to [Co(CO)2(NO)(iPr2Im)] followed by 90 s of total argon gas purge without use of any coreactant and this is defined as one cycle of pulsed CVD. In pulsed CVD experiments, 90 s of total argon gas purge was used to minimize temperature effects on the QCM (quartz crystal microbalance) crystal and obtain FDPC (frequency drop per cycle) results comparable to the FDPC results obtained in an ALD type fashion. In Figure 4, the FDPC is plotted at different deposition temperatures for the various aforementioned coreactants. The

Figure 5. SEM picture of ∼19 nm rough cobalt oxide (most probably oxidized in ambient air) film deposited at a Tdepo value of 130 °C on HF-dipped c-Si substrate. For a significant growth rate Tsubl was set to 65 °C. A total of 1000 cycles was performed with 5 s of [Co(CO)2(NO)(iPr2Im)] exposure time, 30 s of each argon purge step, and 30 s of H2 exposure time.

Figure 4. Effect of deposition temperature on the FDPC for different coreactants. A total of 50 deposition cycles per data point was performed with 5 s of [Co(CO)2(NO)(iPr2Im)] exposure time and 30 s of both purges as well as 30 s of coreactant exposure time.

deposited [Co(CO)2(NO)(iPr2Im)] and H2. The deposited film is observed to have some pinholes on the surface and might not be continuous. The chemical composition of the sample shown in Figure 5 was analyzed with the help of an XPS depth profile and is shown in Figure 6. XPS showed a surface carbon amount of about 43 atom %. After 60 s of sputtering, the surface carbon contamination was removed and at this stage about 5 atom % Si signal (binding energy of ∼99 eV) from the c-Si substrate could be already seen. The measured Si signal could be due to either a

overall linear increase in the FDPC with deposition temperature indicates the increase in deposition rate of cobalt-based films, which may be attributed to a CVD type of growth. In the deposition temperature regime of 70−175 °C, this precursor did not show any self-limiting growth behavior (Figure S33 in the Supporting Information) and therefore cannot be used as a pure ALD precursor. The figure may also indicate that the reactivity of the adsorbed [Co(CO)2(NO)(iPr2Im)] precursor on the QCM surface depends on the coreactant, especially when air was used. This implies that, under similar process conditions, the CoOx films grown with air as coreactant are thicker than the cobalt-based films deposited from other coreactants. At a Tdepo value of 140 °C and for air as a coreactant, two FDPC values have been plotted that include as well as exclude the oxidation effect. It was found that exposure to air leads to alone resulted in an irreversible decrease in QCM frequency by a value of about 10 Hz at a Tdepo value of 140 °C.

Figure 6. XPS depth profile of the sample shown in Figure 5. The sputter time of 30 s was a surface-cleaning step and the rest of the etching steps were performed by sputtering the sample surface with 4 keV of Ar+ beam energy. E


Article

Organometallics noncontinuous film (a similar observation is seen from the SEM picture in Figure 5) or the remaining

Asymmetrically Substituted Tetrahedral Cobalt NHC Complexes and

Recommend Documents