Synthesis and Characterization of Liquid MOCVD Precursors for Thin

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Synthesis and Characterization of Liquid MOCVD Precursors for Thin Films of Cadmium Oxide Antonino Gulino,*,† Paolo Dapporto,‡ Patrizia Rossi,‡ and Ignazio Fragala`*,† Dipartimento di Scienze Chimiche, Universita` di Catania and I.N.S.T.M. UdR of Catania, V.le A. Doria 6, 95125 Catania, Italy, and Dipartimento di Energetica, Universita` di Firenze, V. Santa Marta 3, 50139 Firenze, Italy Received April 5, 2002. Revised Manuscript Received July 15, 2002

Four novel Cd(C5F6HO2)2‚polyether adducts were prepared through simple procedures with stoichiometric quantities of cadmium oxide, hexafluoroacetylacetone C5F6H2O2, and different polyethers. The products were characterized by elemental analysis, X-ray single-crystal analysis, mass spectra, NMR spectra, thermal measurements, and infrared transmittance spectroscopy. X-ray single-crystal data of the Cd(C5F6HO2)2‚polyether adducts showed that all the oxygen atoms of the polyether molecules coordinate the cadmium cation. Very mild heating (44-74 °C) resulted in thermal stable, liquid compounds which, in turn, can be easily evaporated. Gas-phase deposition experiments, in a low-pressure horizontal hot-wall reactor, on SiO2 substrates, resulted in CdO films. XRD measurements provided evidence that they consist of cubic, (100)-oriented, crystals. UV-vis spectra showed that the transmittance of as-deposited films in the visible region is about 90%.

Introduction CdO exhibits interesting electronic and optical properties that have been thoroughly studied in a scientific perspective and for industrial and technological applications.1 Among the post-transition metal oxides, CdO is the third member of the series following SnO2 and In2O3. Surprisingly, the trend of increasing band gap between SnO2 (Eg ) 3.62 eV) and In2O3 (Eg ) 3.75 eV) is reversed for CdO, which has a narrower direct gap of 2.27 eV between the O 2p based valence band and the Cd 5s based conduction band minimum.2,3 Moreover, CdO adopts the centrosymmetric rock-salt structure (cubic-face-centered system). Because of the mixing between the O 2p states at the top of the valence band and the shallow core Cd 4d states, only allowed away from the zone center of the rock-salt structure, the roomtemperature gap of CdO results in further narrowing, reaching 0.55 eV.2,3 Many of the properties of CdO are originated by its nonstoichiometric composition that, in turn, strongly depends on the synthetic procedure adopted. In fact, the presence of cadmium interstitials, Cd+ ions, or oxygen vacancies gives rise to donor states whose carrier concentration ranges from semiconductors to degenerate metallic conductors.1-4 In addition, CdO represents a material with a large linear refractive index (n0 ) 2.49). This fact, associated with a narrow band gap, in turn, causes a large thirdorder optical nonlinearity in the nonresonant region.5 * To whom correspondence should be addressed. E-mail: agulino@ dipchi.unict.it. † Universita ` di Catania and I.N.S.T.M. UdR of Catania. ‡ Universita ` di Firenze. (1) Ginley, D. S., Bright, C., Eds. MRS Bull. 2000, 25. (2) Dou, Y.; Egdell, R. G.; Walker, T.; Law, D. S. L.; Beamson, G. Surf. Sci. 1998, 398, 241. (3) Jaffe, J. E.; Pandey, R.; Kunz, A. B. Phys. Rev. B 1991, 43, 14030. (4) Gulino, A.; Fragala, I. J. Mater. Chem. 1999, 9, 2837.

As the particle size decreases down to the nanometer scale, its nonlinear optical response is further enhanced due to the quantum size effect.5 Many studies have been reported for preparation of thin films of CdO,1,6-10 but few of them involve the metal organic chemical vapor deposition (MOCVD)11-14 technique. In this context, interesting results concerning the MOCVD of CdO using the novel Cd(hfa)2(TMEDA) precursor have been reported by Marks and co-workers.12 Obviously, MOCVD from liquid precursors certainly represents an issue of considerable relevance because of the accurate reproducibility associated with constant evaporation (hence constant mass-transport) rates for given source temperatures. In this regard, recently, we reported preliminary results on MOCVD of CdO using the novel, low-melting (72-73 °C), Cd(C5F6HO2)2‚CH3OCH2CH2OCH3, cadmium hexafluoroacetylacetonate dimethoxyethane complex, (hexafluoroacetylacetonate ) 1,1,1,5,5,5,-hexafluoro-2,4-pentanedionate) as the precursor.14 To our knowledge, to date, this is a unique (5) Xiaochun, W.; Rongyao, W.; Bingsuo, Z.; Li, W.; Shaomei, L.; Jiren, X. J. Mater. Res. 1998, 13, 604. (6) Ferro, R.; Rodrı`guez, J. A. Thin Solid Films 1999, 347, 295. (7) Gurumurugan K.; Mangalaraj, D.; Narayandass, Sa. K.; Nakanishi, Y.; Hatanaka, Y. Appl. Surf. Sci. 1997, 113/114, 422. (8) Meinhold, R. H. J. Phys. Chem. Solids 1987, 48, 927. (9) Ale´tru, C.; Greaves, G. N.; Sankar, G. J. Phys. Chem. B 1999, 103, 4147. (10) Dragon R.; Wacke, S.; Go`recki, T. J. Appl. Electrochem. 1995, 25, 699. (11) Coutts, T. J.; Young, D. L.; Li, X.; Mulligan, W. P.; Wu, X. J. Vac. Sci. Technol., A 2000, 18, 2646. (12) Babcock, J. R.; Wang, A.; Metz, A. W.; Edleman, N. L.; Metz, M. V.; Lane, M. A.; Kannewurf, C. R.; Marks, T. J. Chem. Vap. Deposition 2001, 7, 239. (13) (a) Gulino, A.; Castelli, F.; Dapporto, P.; Rossi, P.; Fragala`, I. Chem. Mater. 2002, 14, 704. (b) Chattoraj, S. C.; Cupka, A. G.; Sievers, R. E. J. Inorg. Nucl. Chem. 1966, 28, 1937. (14) Gulino, A.; Dapporto, P.; Rossi, P.; Fragala`, I. Chem. Mater. 2002, 14, 1441.

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example of MOCVD of CdO using a liquid (at MOCVD conditions) precursor. In this perspective, there is enough motivation to further investigate the synthesis of novel liquid precursors better suited for MOCVD of CdO thin films. Therefore, in the present investigation we report an extensive study concerning four novel liquid adducts that, in addition, have proven to be well-suited precursors for MOCVD of CdO films. Experimental Details Cadmium-containing compounds are exceedingly toxic; therefore, care was taken during all sample manipulations. Both synthesis and characterization of the simple Cd(C5F6HO2)2‚2H2O (1) were already reported.13 The Cd(C5F6HO2)2‚CH3OCH2CH2OCH3 (2), Cd(C5F6HO2)2‚ CH3(OCH2CH2)2OCH3 (3), Cd(C5F6HO2)2‚CH3(OCH2CH2)3OCH3 (4), and Cd(C5F6HO2)2‚CH3(OCH2CH2)4OCH3 (5) adducts (hereafter called Cd(hfa)2‚monoglyme, Cd(hfa)2‚diglyme, Cd(hfa)2‚triglyme, and Cd(hfa)2‚tetraglyme, respectively; monoglyme ) dimethoxyethane, diglyme ) bis(2-methoxyethyl)ether, triglyme ) 2,5,8,11-tetraoxadodecane, tetraglyme ) 2,5,8,11,14-pentaoxapentadecane, and C5F6HO2 ) hfa) were synthesized from stoichiometric quantities of CdO, C5F6H2O2 (hereafter called H-hfa), and the appropriate polyether. Aldrich grade reagents were used throughout all present syntheses. The elemental analyses were performed using a Carlo Erba Elemental Analyzer EA 1108. Fast atom bombardment mass spectra (FAB-MS) were obtained using a Kratos MS 50 spectrometer, using 3-nitrobenzyl alcohol (O2NC6H4CH2OH ) 3NBA) as a matrix and cesium as bombarding atoms (35 kV). Electron impact mass spectra (EI-MS) were obtained using a 70-eV electron beam. 1H NMR spectra were recorded using a Varian 500-MHz spectrometer. The thermal behavior of the Cd(hfa)2‚polyether adducts was investigated by thermal (TGA), differential gravimetric analysis (DTG) and differential scanning calorimetry (DSC), at a pressure of 1 atm of prepurified nitrogen, using a 5 °C/min heating rate. A Mettler TA 4000 system equipped with a DSC30 cell, a TG 50 thermobalance, and a TC 11 processor was used;15 6-8 mg of samples were accurately weighed and examined in the 20-400 °C range. Indium was employed to calibrate the transitional enthalpies.15b Enthalpies were evaluated from the peak areas using the integration program of the TC11 processor.15b Integration errors lie within (5%. Infrared transmittance spectra of samples in Nujol mull were recorded using a Jasco FT/IR-430 spectrometer. The instrumental resolution was 4 cm-1. Good single crystals of 2, 3, and 4 for X-ray analysis were obtained from hexane solutions. Crystal structure determination: cell parameters and intensity data for compounds 2, 3, and 4 were obtained on a Nonius CAD4 diffractometer, using graphite monochromatized Mo KR radiation (λ ) 0.71069 Å). Cell parameters were determined by least-squares fitting of 25 centered reflections. Intensity data were corrected for Lorentz and polarization effects. An absorption correction was applied once the structures were solved by using the Walker and Stuart method.16 The structures were solved using the SIR-9717 program and subsequently refined by the full-matrix least-squares program SHELX-97.18 The hydrogen atoms of the hfa anions were (15) (a) Gulino, A.; Castelli, F.; Dapporto, P.; Rossi, P.; Fragala`, I. Chem. Mater. 2000, 12, 548. (b) Castelli, F.; Caruso, S.; Giuffrida, N. Thermochim. Acta 1999, 327, 125. (16) Walker, N.; Stuart, D. D. Acta Crystallogr., Sect. A 1983, 39, 158. (17) Altomare, A.; Cascarano, G.; Giacovazzo, C.; Guagliardi, A.; Burla, M. C.; Polidori, G.; Camalli, M. J. Appl. Crystallogr. 1994, 27, 435. (18) Sheldrick, G. M. SHELXL-97; University of Go¨ttingen: Go¨ttingen, Germany, 1997.

Figure 1. ORTEP drawing of the two molecules present in the unit cell of Cd(hfa)2‚diglyme. introduced in calculated position and their coordinates refined in agreement with those of the linked atoms. All the nonhydrogen atoms were refined anisotropically. Atomic scattering factors and anomalous dispersion corrections for all the atoms were taken from ref 19. Geometrical calculations were performed by PARST97.20 The molecular plots were produced by the ORTEP-3 program,21 and Figures 1 and 2 show ORTEP views of compound 3 and 4. Crystal and structure refinement data for 3 and 4 are reported in Table 1. Selected bond distances are reported in Table 2. X-ray diffraction (XRD) film data were recorded on a Bruker D-5005 diffractometer operating in a θ-2θ geometry (Cu KR radiation, 30 mA, and 40 kV). X-ray photoelectron spectra (XPS) were made with a PHI 5600 Multi Technique System (base pressure of the main chamber 3 × 10-10 Torr). Resolution, correction for satellite contributions ,and background removal have been described elsewhere.15 MOCVD experiments were performed using a horizontal hot-wall reactor,13-15 under reduced pressure. The reactor system mainly consists of a gas-handling facility, a tubular furnace, a quartz reactor tube (total length ) 80 cm and i.d. ) 2.4 cm), two separate parallel quartz inlet tubes for Ar and O2, and a vacuum system. The precursors were contained in alumina boats and maintained at temperatures of 100-110 °C (Table 3). All the present, as-synthesized, Cd(hfa)2‚polyether (19) Press: (20) (21)

International Tables for X-ray Crystallography; Kynoch Birmingham, UK, 1974; Vol. 4. Nardelli, M. Comput. Chem. 1983, 7, 95. Farrugia, L. J. J. Appl. Crystallogr. 1997, 30, 565.

Liquid MOCVD Precursors for Thin Films of CdO

Figure 2. ORTEP drawing of the Cd(hfa)2‚triglyme. adducts were used. Fused SiO2 (quartz), kept in the center of the heated zone, was used as the substrate after cleaning in an ultrasonic bath with isopropyl alcohol. Pure Ar (100 sccm) carrier gas was used to transport the precursor to the substrate. The reactant gas consisted of water-saturated oxygen (100-200-300-400 sccm) introduced directly into the reactor close to the substrate (Table 3). The total pressure, kept in the 2-6 Torr range, was measured using a MKS Baratron 122AAX system. Flow rates were controlled within (2 sccm using MKS flow controllers and a MKS 147 Multigas Controller. Temperatures of the evaporator and reaction zone were controlled by EUROTHERMS controls. Scanning electron microscopy (SEM) analysis was performed with a LEO 1400 Microscope equipped with energy-dispersive X-ray fluorescence (EDX) microanalysis. The film thickness was estimated from UV-visible data and SEM cross sections. Synthesis of Cd(hfa)2‚Polyether: General Remarks. Almost no reactions took place, after several hours, when the reaction mixtures were refluxed at 40 °C or higher temperatures in a CH2Cl2, in an ethanol, and in a basic aqueous medium. Therefore, the title compounds were synthesized in two different ways: (a) stirring for a few minutes a CH2Cl2 suspension of stoichiometric quantities of CdO, H-hfa, and the appropriate polyether at room temperature (T e 25 °C); (b) mixing stoichiometric quantities of CdO, H-hfa, and the appropriate polyether at room temperature without any solvent. In both cases exothermic immediate reactions were observed (∆H(react) < 0). Similar behavior was already observed by Chattoraj et al. during the preparation of 1.13b In fact, the reaction mixture was cooled during their, solventfree, synthesis and, after cooling, further vigorous reaction with evolution of heat again took place.13b Therefore, within procedure (a), after a few minutes of stirring, the suspensions became clear. No excess of CdO was filtered off. Colorless oils were obtained after evaporation of the CH2Cl2 solvent. White powders were obtained after the addition of the oils to 30 mL of pentane. Colorless, transparent crystals resulted by dissolving the oils into 90 mL of hexane and leaving the solutions to concentrate to room temperature. The synthetic procedure (b), carried out without any solvent, gave identical white powders with less yield. Therefore, only results of procedure (a) will be discussed hereafter. Details of the synthesis of Cd(hfa)2‚monoglyme (2) have already been reported.14 Synthesis of Cd(hfa)2‚Diglyme (3). CdO (0.642 g, 0.005 mol), 1.42 mL (0.01 mol) of H-hfa, and 0.72 mL of diglyme (0.005 mol) were stirred for a few minutes with 40 mL of CH2Cl2. Yield: 99%. Melting point (mp) of the crude product: 4446 °C. Elemental analysis for CdC16H16F12O7 (molar mass 660.64): calcd, C 29.07, H 2.42; found, C 28.91, H 2.16%. MS (EI+, 70 eV, m/z fragments; M ) Cd(hfa)2‚diglyme): 643 (M F)+, 603 (M - CH3OCH2CH2)+, 566 (M - diglyme + 2F)+, 552

Chem. Mater., Vol. 14, No. 12, 2002 4957 (M - CF3COCH)+, 528 (M - diglyme)+, 493 (M - CH2CH2OCH3 - COCHCF3)+, 459 (M - diglyme - CF3)+, 321 (M hfa - diglyme)+. IR (Nujol; ν/cm-1): 1652 (s), 1600 (w), 1553 (m), 1527 (m), 1465 (m), 1345 (w), 1256 (s), 1199 (s), 1153 (s), 1086 (m), 1064 (m), 1013 (w), 987 (w), 944 (w), 870 (m), 840 (m), 793 (s), 764 (w), 741 (w), 724 (m), 659 (s). 1H NMR (CDCl3): δ 5.97 (s, 2H), 3.84-3.82 (quartet 4H), 3.74-3.72 (quartet 4H), 3.41 (s, 6 H). Synthesis of Cd(hfa)2‚Triglyme (4). CdO (0.642 g, 0.005 mol), 1.42 mL (0.01 mol) of H-hfa, and 0.9 mL of triglyme (0.005 mol) were stirred for a few minutes with 40 mL of CH2Cl2. Yield: 99%. Melting point of the crude product: 65-67 °C. Elemental analysis for CdC18H20F12O8 (molar mass 704.74): calcd, C 30.66, H 2.84; found, C 30.67, H 2.54%. FABMS (m/z fragments; M ) Cd(hfa)2‚triglyme): 493 (M - CH2CH2OCH2CH2OCH3 - COCHCF3)+, 311 (M - triglyme - CF3 - COCHCF3 - 2F)+, 292 (M - hfa - CF3 - F - CH3OCH2CH2OCH2CH2O)+, 233 (M - triglyme - hfa - CF3 - F)+. IR (Nujol; ν/cm-1): 3557 (b), 3298 (b), 1650 (s), 1615 (vw), 1587 (vw), 1552 (s), 1530 (s), 1502 (m), 1366 (vw), 1351 (vw), 1253 (s), 1196 (s), 1145 (s), 1088 (s), 1023 (w), 994 (vw), 943 (m), 867 (m), 853 (m), 797 (s), 763 (m), 740 (m), 671 (s). 1H NMR (CDCl3): δ 5.95 (s, 2H), 3.87 (s, 4H), 3.74-3.72 (quartet 4H), 3.60-3.58 (quartet 4H), 3.33 (s, 6 H). Synthesis of Cd(hfa)2‚Tetraglyme (5). CdO (0.642 g, 0.005 mol), 1.42 mL (0.01 mol) of H-hfa, and 1.1 mL of tetraglyme (0.005 mol) were stirred for a few minutes with 40 mL of CH2Cl2. Yield: 99%. Melting point of the crude products: 47-49 °C. Elemental analysis for CdC20H24F12O9 (molar mass 748.79): calcd, C 32.07, H 3.21; found, C 32.41, H, 3.41%. FAB-MS (m/z fragments; M ) Cd(hfa)2‚tetraglyme): 531 (M - CH2CH2OCH2CH2OCH2CH2OCH3 - COCHCF3 + 2F)+, 353 (M - tetraglyme - 2 COCF3 + F)+, 233 (M - tetraglyme hfa - CF3 - F)+. IR (Nujol; ν/cm-1): 3557 (b), 3463 (b), 3261 (b), 1647 (s), 1610 (vw), 1591 (vw), 1549 (sh), 1535 (s), 1502 (m), 1253 (s), 1196 (s), 1140 (s), 1093 (m), 1027 (m), 990 (sh), 943 (m), 862 (vw), 849 (m), 762 (s), 735 (w), 660 (s). 1H NMR (CDCl3): δ 5.95 (s, 2H), 3.82-3.80 (quint. 4H), 3.77-3.75 (quint. 4H), 3.70-3.68 (quint. 4H), 3.59-3.58 (quint. 4H), 3.36 (s, 6 H).

Results and Discussion A schematic drawing of the homologous compounds 2,14 3, and 4 is given in Scheme 1 and compared to Cd(hfa)2‚2H2O (1).13 In the asymmetric unit of 3 two independent molecules, 3a and 3b, of the complex Cd(hfa)2‚diglyme are present. Such molecules are quite similar and show a root-mean-square (RMS) value, calculated using all the non-hydrogen atoms except fluorines, of 0.210 Å. The only relevant difference between 3a and 3b is related to the torsional angle around O11-C25 (3a) and O14C31 (3b) in the diglyme molecules. In fact, this dihedral angle is 105(1)° in 3a while it is 137(1)° in 3b. In compounds 2, 3, and 4 the disposition of the polyether molecules shows, as expected,14 a trans-gauche-trans arrangement of the torsion angles around the coordinating oxygen atoms. The C-C distances in the hfa anions are comparable in all four structures. For example, the C1-C2 (C6C7) and C2-C3 (C7-C8) (the distances of the hfa anion for both independent molecules of compound 3 are considered) are shorter than the C1-C4 (C6-C9) and C3-C5 (C8-C10) due to the π resonance involving the hfa rings. Moreover, for compounds 1 and 2 there is a gauche disposition of the two planes containing two hfa anions (the angle between such planes is 69.6(2)° and 47.9(2)°, for 1 and 2, respectively). In compounds 3 and 4 the two planes are quite perpendicular (the angle is

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Table 1. Crystal Data and Structure Refinement for Compounds 2, 3, and 4 empirical formula formula weight temperature (K) wavelength (Å) crystal system space group unit cell dimensions (Å, deg) volume (Å3) Z, d (calc., Mg/m3) µ (mm-1) reflections collected/unique data/parameters final R indices [I > 2σ(I)] R indices (all data)

2

3

4

C14H12CdF12O6 616.64 293 0.71069 monoclinic, P21/n a ) 12.118(7) b ) 13.066(5), β ) 100.92(7) c ) 14.453(7) 2247.0(19) 4, 1.823 0.9002 2307/2307 2307/339 R1 ) 0.0812, wR2 ) 0.1989 R1 ) 0.1048, wR2 ) 0.2222

C16H16CdF12O7 660.69 293 0.71069 triclinic, P1 h a ) 3.230(4), R ) 77.02(2) b ) 13.350(3), β ) 78.99(3) c ) 14.904(5), γ ) 80.47(2) 2497.1(13) 4, 1.757 0.996 8941/8586 8586/655 R1 ) 0.0673, wR2 ) 0.1962 R1 ) 0.0921, wR2 ) 0.2182

C18H20CdF12O8 704.74 293 0.71069 orthorhombic, Pc21/n a ) 11.694(4) b ) 14.972(6) c ) 15.561(5) 2724.5(17) 4, 1.718 0.922 2614/2487 2487/356 R1 ) 0.0421, wR2 ) 0.1224 R1 ) 0.0587, wR2 ) 0.1399

Table 2. Selected Bond Distances (Å) for 1,13 2,14 3, and 4 1 Cd-Ohfa

Cd1-O1 Cd1-O3

2 2.233 2.253

Cd-Opolyether

Cd-Owater

Cd1-O2

3

Cd1-O1 Cd1-O2 Cd1-O3 Cd1-O4

2.19 2.23 2.24 2.20

Cd1-O5 Cd1-O6

2.34 2.34

Cd1-O1 Cd1-O2 Cd1-O3 Cd1-O4 Cd2-O5 Cd2-O6 Cd2-O7 Cd2-O8 Cd1-O9 Cd1-O10 Cd1-O11 Cd2-O12 Cd2-O13 Cd2-O14

4 2.281(6) 2.238(5) 2.279(5) 2.220(5) 2.286(5) 2.239(5) 2.317(5) 2.233(5 2.456(6) 2.430(5) 2.492(6) 2.458(5) 2.384(5) 2.493(6)

Cd1-O1 Cd1-O2 Cd1-O3 Cd1-O4

2.283(5) 2.252(5) 2.27(2) 2.32(1)

Cd1-O5 Cd1-O6 Cd1-O7 Cd1-O8

2.65(2) 2.41(2) 2.46(2) 2.70(2)

2.293

Scheme 1. Schematic Drawing of Compounds 1 and 2

In compounds 2, 3, and 4 all the oxygen atoms of the ether molecules coordinate the cadmium cation and, consequently, the cadmium results in hexa-, hepta-, and octacoordination, respectively. Therefore, the coordination polyhedron is an octahedron for both 113 and 2,14 a pentagonal bipyramid for 3, and a bicapped trigonal prism22 for 4. In compound 3 the hfa oxygen atoms O2 and O4 (O6 and O8 in 3b) occupy the apical positions. In compound 4 the two triglyme oxygen atoms O5 and O8 cap the two rectangular faces (O1O2O3O6) and (O1O2O4O7), respectively. The oxygen atoms O1, O6, O7 and O2, O3, O4 define the two triangular faces of the prism. In compounds 2, 3, and 4 the Cd-O distances may be grouped into two different sets: Cd-Ohfa and Cd-Opolyether. Inspection of Table 2 reveals that CdOhfa bond lengths are shorter than those of the CdOpolyether analogues. All these distances are in agreement with those found for analogous systems.13-15,23,24 Information about the cadmium coordination geometry was retrieved from the Cambridge Structural

89.9(2)°, 87.5(2)°, and 88.9(5)°, for 3a, 3b, and 4, respectively). The fluorine atoms in all present compounds show rather large anisotropic factors, as observed for other similar metal complexes.13-15

(22) Guggenberger, L. J.; Muetterties, E. L. J. Am. Chem. Soc. 1976, 10, 7221. (23) (a) Maslen, E. N.; Greaney, T. M.; Raston, C. L.; White, A. H. J. Chem. Soc., Dalton Trans. 1975, 400. (b) Bustos, L.; Green, J. H.; Hencher, J. L.; Khan, M. A.; Tuck, D. G. Can. J. Chem., 1983, 61, 2141. (c) McSharry, W. O.; Cefola, M.; White, J. G. Inorg. Chim. Acta 1980, 38, 160. (d) Greaney, T. M.; Raston, C. L.; White, A. H.; Maslen, E. N. J. Chem. Soc., Dalton Trans. 1975, 876. (e) Casabo, J.; Colomer, J.; Llobet, A.; Teixidor, F.; Molins, E.; Miravitlles, C. Polyhedron 1989, 8, 2743. (24) (a) Clegg, W.; Wheatley, P. J. J. Chem. Soc., Dalton Trans. 1974, 424. (b) Lei, X.; Shang, M.; Fehlner, T. P. Polyhedron 1997, 16, 1803. (c) Borras-Almenar, J. J.; Coronado, E.; Gomez-Garcia, C. J.; Ouahab, L. Angew. Chem., Int. Ed. Engl. 1993, 32, 561. (d) Iwamoto, R.; Wakano, H. J. Am. Chem. Soc. 1976, 98, 3764. (e) Fuhr, O.; Fenske, D. Z. Anorg. Allg. Chem. 2000, 626, 1822. (f) Rogers, R. D.; Bond, A. H.; Aguinaga, S.; Reyes, A. Inorg. Chim. Acta 1993, 212, 225.

Table 3. Optimized MOCVD Conditions depositotal tion substrate O2 flow Ar flow source rate rate sublimat. pressure time temp. (sccm) (sccm) temp. (°C) (Torr) (min) film precursor (°C) A B C D E F G H K I J L M N O P

2 2 2 2 3 3 3 3 4 4 4 4 5 5 5 5

400 400 400 400 400 400 400 400 400 400 400 400 400 400 400 400

100 200 300 400 100 200 300 400 100 200 300 400 100 200 300 400

100 100 100 100 100 100 100 100 100 100 100 100 100 100 100 100

110 110 110 110 100 100 100 100 110 110 110 110 100 100 100 100

2 3 4 6 2 3 5 6 2 3 4 6 3 4 6 6

120 120 120 120 120 120 120 120 120 120 120 120 120 120 120 120

Liquid MOCVD Precursors for Thin Films of CdO

Database (CSD v.5.21).25 In particular, the bipyramidal geometry has been found quite usual in heptacoordinated cadmium complexes (of 145 cadmium complexes retrived, 93 show a bipydramidal coordination).26 In contrast, only two (over 45) octacoordinated cadmium complexes show a bicapped trigonal prism as the coordination polyhedron.27 As far as β-diketonate metal complexes saturated with ancillary ligands similar to present polydentate glymes are concerned, only one complex with an O-donor28 and eight with N-donors28,29 have been found. The most common coordination geometry, concerning the octacoordinated complexes, is the distorted square antiprism shown by five complexes28,29a,b,d,e while, only two complexes [Sr(tdh)2‚ triglyme (29a) and Ba(tdh)2‚triglyme (29d), tdh ) 1,1,1,6,6,6-hexamethylheptane-2,4-dione] show, like compound 4, a bicapped trigonal prism coordination geometry. The only found eptacoordinated complex [Ba(tdh)2‚ pmdt, pmdt ) pentamethyldiethylentriamine] shows a coordination pattern not referable to a regular polyhedron.28 Finally, we can notice that in compounds 2, 3, and 4 intermolecular contacts are not present while in compound 1, due to the presence of crystallization water molecules, there is the formation of a hydrogen-bonded chain.13 The mass spectra of the Cd(hfa)2‚polyether adducts do not show the molecular ion peaks. The observed peaks always show the characteristic isotope pattern of Cd and are due to the loss of hfa and polyether fragments. The COCHCF3, CF3, and F groups are the most common observed fragments and have already been observed in mass spectra of similar complexes.13-15 In addition, there is evidence of fluorine group transfer processes similar to that observed in closely related adducts.15 The most intense peak shown by 2, 3, and 4 at 493 m/z (100%) corresponds to the fragment [Cd(hfa)COCH3‚OCH2CH2OCH3]+. At lower mass, the most significant peaks are at 311 m/z (10-20%) and 233 m/z (5-10%) and can be associated with the [M-polyether-CF3-COCHCF3-2F]+ and [M-polyether-hfa-CF3-F]+ fragments, respectively. The 1H NMR spectra of the Cd(hfa)2‚polyether adducts always show a singlet at δ ) 6.04-5.95, whose integration accounts for the two protons of the hfa ring ligands.13,14 In addition, multiplets at δ ) 3.87-3.58 represent resonances of methylenic protons of poly(25) Allen, F. H.; Kennard, O. Cambridge Structural Database. Chem. Soc. Perkin Trans. 2 1989, 1131. (26) (a) Karunakaran, C.; Thomas, K. R. J.; Shunmugasundaram, A.; Murugesan, R. J. Inclusion Phenom. Macrocyclic Chem. 2000, 38, 233. (b) Marsh, R. E. Acta Crystallogr., Sect. B: Struct. Sci. 1995, 51, 897. (c) Carlucci, L.; Ciani, G.; Proserpio, D. M. J. Chem. Soc., Dalton Trans. 1999, 1799. (d) Witherby, M. A.; Blake, A. J.; Champness, N. R.; Cooke, P. A.; Hubberstey, P.; Li, W.; Schrodeer, M. Inorg. Chem. 1999, 38, 2259. (e) Pettinari, C.; Marchetti, F.; Cingolani, A.; Troyanov, S. I.; Doznov, A. Polyhedron 1998, 17, 1677. (27) (a) Chung, K. H.; Hong, E.; Moon, C. H.; Chem. Commun. 1995, 2333. (b) Skoulika, S.; Michaelides, A.; Aubry, A. Acta Crystallogr., Sect. C: Cryst. Struct. Commun. 1988, 44, 931. (28) Gardiner, R. A.; Gordon, D. C.; Stauf, G. T.; Vaartstra, B. A.; Ostrander, R. L.; Rheingold, A. L. Chem. Mater. 1994, 6, 1967. (29) (a) Drake, S. R.; Hursthouse, M. B.; Malik, K. M. A.; Miller, S. A. S.; Chem. Commun. 1993, 478. (b) Drake, S. R.; Hursthouse, M. B.; Malik, K. M. A.; Miller, S. A. A. Inorg. Chem. 1993, 32, 4464. (c) Pollard, K. D.; Vittal, J. J.; Yap, G. P. A.; Puddephatt, R. J. J. Chem. Soc., Dalton Trans. 1998, 1265. (d) Drake, S. R.; Piller, S. A. S.; Williams, D. J. Inorg. Chem. 1993, 32, 3227. (e) Arunasalam, V.-C.; Baxter, I.; Drake, S. R.; Hursthouse, M. B.; Malik, K. M. A.; Miller, S. A. S.; Mingos, D. M. P.; Otway, D. J. J. Chem. Soc., Dalton Trans. 1997, 1331.

Chem. Mater., Vol. 14, No. 12, 2002 4959

Figure 3. DTG (a) and TG (b) of Cd(hfa)2‚polyether 3, 4, and 5 adducts.

Figure 4. DSC curve for Cd(hfa)2‚diglyme.

ethers while the singlet at δ ) 3.33-3.51 is consistent with the six protons of the two methyl groups of the same ligands. Present resonances are in agreement with those already observed for similar Zn systems.15 The thermal behavior of Cd(hfa)2‚monoglyme adduct (2) was already reported.14 TGA and DTG analyses of the Cd(hfa)2‚polyether adducts 3, 4, and 5 all show a quantitative mass loss process with peak temperatures at 192, 205, and 225 °C, respectively (Figure 3), consistent with the evaporation of the adducts. No residue (