Synthesis and Characterization of Liquid MOCVDPrecursors 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 withstoichiometric quantities of cadmium oxide, hexafluoroacetylacetone C5F6H2O2, and differentpolyethers. The products were characterized by elemental analysis, X-ray single-crystalanalysis, mass spectra, NMR spectra, thermal measurements, and infrared transmittancespectroscopy. X-ray single-crystal data of the Cd(C5F6HO2)2‚polyether adducts showed thatall the oxygen atoms of the polyether molecules coordinate the cadmium cation. Very mildheating (44-74 °C) resulted in thermal stable, liquid compounds which, in turn, can beeasily evaporated. Gas-phase deposition experiments, in a low-pressure horizontal hot-wallreactor, on SiO2 substrates, resulted in CdO films. XRD measurements provided evidencethat they consist of cubic, (100)-oriented, crystals. UV-vis spectra showed that thetransmittance of as-deposited films in the visible region is about 90%.

Introduction

CdO exhibits interesting electronic and optical prop-erties that have been thoroughly studied in a scientificperspective and for industrial and technological applica-tions.1 Among the post-transition metal oxides, CdO isthe third member of the series following SnO2 andIn2O3. Surprisingly, the trend of increasing band gapbetween SnO2 (Eg ) 3.62 eV) and In2O3 (Eg ) 3.75 eV)is reversed for CdO, which has a narrower direct gap of2.27 eV between the O 2p based valence band and theCd 5s based conduction band minimum.2,3 Moreover,CdO adopts the centrosymmetric rock-salt structure(cubic-face-centered system). Because of the mixingbetween the O 2p states at the top of the valence bandand the shallow core Cd 4d states, only allowed awayfrom the zone center of the rock-salt structure, the room-temperature gap of CdO results in further narrowing,reaching 0.55 eV.2,3 Many of the properties of CdO areoriginated by its nonstoichiometric composition that, inturn, strongly depends on the synthetic procedureadopted. In fact, the presence of cadmium interstitials,Cd+ ions, or oxygen vacancies gives rise to donor stateswhose carrier concentration ranges from semiconductorsto degenerate metallic conductors.1-4

In addition, CdO represents a material with a largelinear refractive index (n0 ) 2.49). This fact, associatedwith a narrow band gap, in turn, causes a large third-order optical nonlinearity in the nonresonant region.5

As the particle size decreases down to the nanometerscale, its nonlinear optical response is further enhanceddue to the quantum size effect.5

Many studies have been reported for preparation ofthin films of CdO,1,6-10 but few of them involve the metalorganic chemical vapor deposition (MOCVD)11-14 tech-nique. In this context, interesting results concerning theMOCVD of CdO using the novel Cd(hfa)2(TMEDA)precursor have been reported by Marks and co-work-ers.12

Obviously, MOCVD from liquid precursors certainlyrepresents an issue of considerable relevance becauseof the accurate reproducibility associated with constantevaporation (hence constant mass-transport) rates forgiven source temperatures. In this regard, recently, wereported preliminary results on MOCVD of CdO usingthe novel, low-melting (72-73 °C), Cd(C5F6HO2)2‚CH3-OCH2CH2OCH3, cadmium hexafluoroacetylacetonatedimethoxyethane complex, (hexafluoroacetylacetonate) 1,1,1,5,5,5,-hexafluoro-2,4-pentanedionate) as theprecursor.14 To our knowledge, to date, this is a unique

* 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.

(5) Xiaochun, W.; Rongyao, W.; Bingsuo, Z.; Li, W.; Shaomei, L.;Jiren, X. J. Mater. Res. 1998, 13, 604.

(6) Ferro, R.; Rodrıguez, J. A. Thin Solid Films 1999, 347, 295.(7) Gurumurugan K.; Mangalaraj, D.; Narayandass, Sa. K.; Na-

kanishi, Y.; Hatanaka, Y. Appl. Surf. Sci. 1997, 113/114, 422.(8) Meinhold, R. H. J. Phys. Chem. Solids 1987, 48, 927.(9) Aletru, C.; Greaves, G. N.; Sankar, G. J. Phys. Chem. B 1999,

103, 4147.(10) Dragon R.; Wacke, S.; Gorecki, 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.

4955Chem. Mater. 2002, 14, 4955-4962

10.1021/cm021183m CCC: $22.00 © 2002 American Chemical SocietyPublished on Web 11/27/2002

example of MOCVD of CdO using a liquid (at MOCVDconditions) precursor.

In this perspective, there is enough motivation tofurther investigate the synthesis of novel liquid precur-sors better suited for MOCVD of CdO thin films.Therefore, in the present investigation we report anextensive study concerning four novel liquid adductsthat, in addition, have proven to be well-suited precur-sors for MOCVD of CdO films.

Experimental Details

Cadmium-containing compounds are exceedingly toxic; there-fore, care was taken during all sample manipulations.

Both synthesis and characterization of the simple Cd(C5F6-HO2)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)3-OCH3 (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; monogly-me ) dimethoxyethane, diglyme ) bis(2-methoxyethyl)ether,triglyme ) 2,5,8,11-tetraoxadodecane, tetraglyme ) 2,5,8,11,-14-pentaoxapentadecane, and C5F6HO2 ) hfa) were synthe-sized from stoichiometric quantities of CdO, C5F6H2O2 (here-after called H-hfa), and the appropriate polyether. Aldrichgrade reagents were used throughout all present syntheses.

The elemental analyses were performed using a Carlo ErbaElemental Analyzer EA 1108.

Fast atom bombardment mass spectra (FAB-MS) wereobtained using a Kratos MS 50 spectrometer, using 3-ni-trobenzyl alcohol (O2NC6H4CH2OH ) 3NBA) as a matrix andcesium as bombarding atoms (35 kV). Electron impact massspectra (EI-MS) were obtained using a 70-eV electron beam.

1H NMR spectra were recorded using a Varian 500-MHzspectrometer.

The thermal behavior of the Cd(hfa)2‚polyether adducts wasinvestigated by thermal (TGA), differential gravimetric analy-sis (DTG) and differential scanning calorimetry (DSC), at apressure of 1 atm of prepurified nitrogen, using a 5 °C/minheating rate. A Mettler TA 4000 system equipped with a DSC-30 cell, a TG 50 thermobalance, and a TC 11 processor wasused;15 6-8 mg of samples were accurately weighed andexamined in the 20-400 °C range. Indium was employed tocalibrate the transitional enthalpies.15b Enthalpies were evalu-ated from the peak areas using the integration program of theTC11 processor.15b Integration errors lie within (5%.

Infrared transmittance spectra of samples in Nujol mullwere recorded using a Jasco FT/IR-430 spectrometer. Theinstrumental resolution was 4 cm-1.

Good single crystals of 2, 3, and 4 for X-ray analysis wereobtained from hexane solutions.

Crystal structure determination: cell parameters and in-tensity data for compounds 2, 3, and 4 were obtained on aNonius CAD4 diffractometer, using graphite monochromatizedMo KR radiation (λ ) 0.71069 Å). Cell parameters weredetermined by least-squares fitting of 25 centered reflections.Intensity data were corrected for Lorentz and polarizationeffects. An absorption correction was applied once the struc-tures were solved by using the Walker and Stuart method.16

The structures were solved using the SIR-9717 program andsubsequently refined by the full-matrix least-squares programSHELX-97.18 The hydrogen atoms of the hfa anions were

introduced in calculated position and their coordinates refinedin agreement with those of the linked atoms. All the non-hydrogen atoms were refined anisotropically. Atomic scatteringfactors and anomalous dispersion corrections for all the atomswere taken from ref 19. Geometrical calculations were per-formed by PARST97.20 The molecular plots were produced bythe ORTEP-3 program,21 and Figures 1 and 2 show ORTEPviews of compound 3 and 4. Crystal and structure refinementdata for 3 and 4 are reported in Table 1. Selected bonddistances are reported in Table 2.

X-ray diffraction (XRD) film data were recorded on a BrukerD-5005 diffractometer operating in a θ-2θ geometry (Cu KRradiation, 30 mA, and 40 kV).

X-ray photoelectron spectra (XPS) were made with a PHI5600 Multi Technique System (base pressure of the mainchamber 3 × 10-10 Torr). Resolution, correction for satellitecontributions ,and background removal have been describedelsewhere.15

MOCVD experiments were performed using a horizontalhot-wall reactor,13-15 under reduced pressure. The reactorsystem mainly consists of a gas-handling facility, a tubularfurnace, a quartz reactor tube (total length ) 80 cm and i.d.) 2.4 cm), two separate parallel quartz inlet tubes for Ar andO2, and a vacuum system. The precursors were contained inalumina boats and maintained at temperatures of 100-110°C (Table 3). All the present, as-synthesized, Cd(hfa)2‚polyether

(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 Gottingen: Got-tingen, Germany, 1997.

(19) International Tables for X-ray Crystallography; KynochPress: Birmingham, UK, 1974; Vol. 4.

(20) Nardelli, M. Comput. Chem. 1983, 7, 95.(21) Farrugia, L. J. J. Appl. Crystallogr. 1997, 30, 565.

Figure 1. ORTEP drawing of the two molecules present inthe unit cell of Cd(hfa)2‚diglyme.

4956 Chem. Mater., Vol. 14, No. 12, 2002 Gulino et al.

adducts were used. Fused SiO2 (quartz), kept in the center ofthe heated zone, was used as the substrate after cleaning inan ultrasonic bath with isopropyl alcohol. Pure Ar (100 sccm)carrier gas was used to transport the precursor to thesubstrate. The reactant gas consisted of water-saturatedoxygen (100-200-300-400 sccm) introduced directly into thereactor close to the substrate (Table 3). The total pressure,kept in the 2-6 Torr range, was measured using a MKSBaratron 122AAX system. Flow rates were controlled within(2 sccm using MKS flow controllers and a MKS 147 MultigasController. Temperatures of the evaporator and reaction zonewere controlled by EUROTHERMS controls.

Scanning electron microscopy (SEM) analysis was performedwith a LEO 1400 Microscope equipped with energy-dispersiveX-ray fluorescence (EDX) microanalysis. The film thicknesswas 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 thereaction mixtures were refluxed at 40 °C or higher tempera-tures in a CH2Cl2, in an ethanol, and in a basic aqueousmedium. Therefore, the title compounds were synthesized intwo different ways: (a) stirring for a few minutes a CH2Cl2

suspension of stoichiometric quantities of CdO, H-hfa, and theappropriate polyether at room temperature (T e 25 °C); (b)mixing stoichiometric quantities of CdO, H-hfa, and theappropriate polyether at room temperature without anysolvent. In both cases exothermic immediate reactions wereobserved (∆H(react) < 0). Similar behavior was alreadyobserved by Chattoraj et al. during the preparation of 1.13b Infact, the reaction mixture was cooled during their, solvent-free, synthesis and, after cooling, further vigorous reactionwith evolution of heat again took place.13b Therefore, withinprocedure (a), after a few minutes of stirring, the suspensionsbecame clear. No excess of CdO was filtered off. Colorless oilswere obtained after evaporation of the CH2Cl2 solvent. Whitepowders were obtained after the addition of the oils to 30 mLof pentane. Colorless, transparent crystals resulted by dis-solving the oils into 90 mL of hexane and leaving the solutionsto concentrate to room temperature. The synthetic procedure(b), carried out without any solvent, gave identical whitepowders 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, 0.005mol), 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 CH2-Cl2. Yield: 99%. Melting point (mp) of the crude product: 44-46 °C. Elemental analysis for CdC16H16F12O7 (molar mass660.64): 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

(M - CF3COCH)+, 528 (M - diglyme)+, 493 (M - CH2CH2-OCH3 - 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, 0.005mol), 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 CH2-Cl2. Yield: 99%. Melting point of the crude product: 65-67°C. Elemental analysis for CdC18H20F12O8 (molar mass704.74): calcd, C 30.66, H 2.84; found, C 30.67, H 2.54%. FAB-MS (m/z fragments; M ) Cd(hfa)2‚triglyme): 493 (M - CH2-CH2OCH2CH2OCH3 - COCHCF3)+, 311 (M - triglyme - CF3

- COCHCF3 - 2F)+, 292 (M - hfa - CF3 - F - CH3OCH2-CH2OCH2CH2O)+, 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 oftetraglyme (0.005 mol) were stirred for a few minutes with 40mL of CH2Cl2. Yield: 99%. Melting point of the crude prod-ucts: 47-49 °C. Elemental analysis for CdC20H24F12O9 (molarmass 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 compounds2,14 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 mol-ecules, 3a and 3b, of the complex Cd(hfa)2‚diglyme arepresent. Such molecules are quite similar and show aroot-mean-square (RMS) value, calculated using all thenon-hydrogen atoms except fluorines, of 0.210 Å. Theonly relevant difference between 3a and 3b is relatedto the torsional angle around O11-C25 (3a) and O14-C31 (3b) in the diglyme molecules. In fact, this dihedralangle is 105(1)° in 3a while it is 137(1)° in 3b. Incompounds 2, 3, and 4 the disposition of the polyethermolecules shows, as expected,14 a trans-gauche-transarrangement of the torsion angles around the coordinat-ing oxygen atoms.

The C-C distances in the hfa anions are comparablein all four structures. For example, the C1-C2 (C6-C7) and C2-C3 (C7-C8) (the distances of the hfa anionfor both independent molecules of compound 3 areconsidered) are shorter than the C1-C4 (C6-C9) andC3-C5 (C8-C10) due to the π resonance involving thehfa rings. Moreover, for compounds 1 and 2 there is agauche disposition of the two planes containing two hfaanions (the angle between such planes is 69.6(2)° and47.9(2)°, for 1 and 2, respectively). In compounds 3 and4 the two planes are quite perpendicular (the angle is

Figure 2. ORTEP drawing of the Cd(hfa)2‚triglyme.

Liquid MOCVD Precursors for Thin Films of CdO Chem. Mater., Vol. 14, No. 12, 2002 4957

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

In compounds 2, 3, and 4 all the oxygen atoms of theether molecules coordinate the cadmium cation and,consequently, the cadmium results in hexa-, hepta-, andoctacoordination, respectively. Therefore, the coordina-tion polyhedron is an octahedron for both 113 and 2,14 apentagonal bipyramid for 3, and a bicapped trigonalprism22 for 4. In compound 3 the hfa oxygen atoms O2and O4 (O6 and O8 in 3b) occupy the apical positions.In compound 4 the two triglyme oxygen atoms O5 andO8 cap the two rectangular faces (O1O2O3O6) and(O1O2O4O7), respectively. The oxygen atoms O1, O6,O7 and O2, O3, O4 define the two triangular faces ofthe prism. In compounds 2, 3, and 4 the Cd-O distancesmay be grouped into two different sets: Cd-Ohfa andCd-Opolyether. Inspection of Table 2 reveals that Cd-Ohfa bond lengths are shorter than those of the Cd-Opolyether analogues. All these distances are in agreementwith those found for analogous systems.13-15,23,24

Information about the cadmium coordination geom-etry was retrieved from the Cambridge Structural

(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 1. Crystal Data and Structure Refinement for Compounds 2, 3, and 4

2 3 4

empirical formula C14H12CdF12O6 C16H16CdF12O7 C18H20CdF12O8formula weight 616.64 660.69 704.74temperature (K) 293 293 293wavelength (Å) 0.71069 0.71069 0.71069crystal system space group monoclinic, P21/n triclinic, P1h orthorhombic, Pc21/nunit cell dimensions (Å, deg) a ) 12.118(7) a ) 3.230(4), R ) 77.02(2) a ) 11.694(4)

b ) 13.066(5), â ) 100.92(7) b ) 13.350(3), â ) 78.99(3) b ) 14.972(6)c ) 14.453(7) c ) 14.904(5), γ ) 80.47(2) c ) 15.561(5)

volume (Å3) 2247.0(19) 2497.1(13) 2724.5(17)Z, d (calc., Mg/m3) 4, 1.823 4, 1.757 4, 1.718µ (mm-1) 0.9002 0.996 0.922reflections collected/unique 2307/2307 8941/8586 2614/2487data/parameters 2307/339 8586/655 2487/356final R indices [I > 2σ(I)] R1 ) 0.0812, wR2 ) 0.1989 R1 ) 0.0673, wR2 ) 0.1962 R1 ) 0.0421, wR2 ) 0.1224R indices (all data) R1 ) 0.1048, wR2 ) 0.2222 R1 ) 0.0921, wR2 ) 0.2182 R1 ) 0.0587, wR2 ) 0.1399

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

1 2 3 4

Cd-Ohfa Cd1-O1 2.233 Cd1-O1 2.19 Cd1-O1 2.281(6) Cd1-O1 2.283(5)Cd1-O3 2.253 Cd1-O2 2.23 Cd1-O2 2.238(5) Cd1-O2 2.252(5)

Cd1-O3 2.24 Cd1-O3 2.279(5) Cd1-O3 2.27(2)Cd1-O4 2.20 Cd1-O4 2.220(5) Cd1-O4 2.32(1)

Cd2-O5 2.286(5)Cd2-O6 2.239(5)Cd2-O7 2.317(5)Cd2-O8 2.233(5

Cd-Opolyether Cd1-O5 2.34 Cd1-O9 2.456(6) Cd1-O5 2.65(2)Cd1-O6 2.34 Cd1-O10 2.430(5) Cd1-O6 2.41(2)

Cd1-O11 2.492(6) Cd1-O7 2.46(2)Cd2-O12 2.458(5) Cd1-O8 2.70(2)Cd2-O13 2.384(5)Cd2-O14 2.493(6)

Cd-Owater Cd1-O2 2.293

Table 3. Optimized MOCVD Conditions

film precursor

substratetemp.(°C)

O2 flowrate

(sccm)

Ar flowrate

(sccm)

sourcesublimat.temp. (°C)

totalpressure

(Torr)

deposi-tiontime(min)

A 2 400 100 100 110 2 120B 2 400 200 100 110 3 120C 2 400 300 100 110 4 120D 2 400 400 100 110 6 120E 3 400 100 100 100 2 120F 3 400 200 100 100 3 120G 3 400 300 100 100 5 120H 3 400 400 100 100 6 120K 4 400 100 100 110 2 120I 4 400 200 100 110 3 120J 4 400 300 100 110 4 120L 4 400 400 100 110 6 120M 5 400 100 100 100 3 120N 5 400 200 100 100 4 120O 5 400 300 100 100 6 120P 5 400 400 100 100 6 120

Scheme 1. Schematic Drawing of Compounds 1and 2

4958 Chem. Mater., Vol. 14, No. 12, 2002 Gulino et al.

Database (CSD v.5.21).25 In particular, the bipyramidalgeometry has been found quite usual in heptacoordi-nated cadmium complexes (of 145 cadmium complexesretrived, 93 show a bipydramidal coordination).26 Incontrast, only two (over 45) octacoordinated cadmiumcomplexes show a bicapped trigonal prism as thecoordination polyhedron.27 As far as â-diketonate metalcomplexes saturated with ancillary ligands similar topresent polydentate glymes are concerned, only onecomplex with an O-donor28 and eight with N-donors28,29

have been found. The most common coordination ge-ometry, concerning the octacoordinated complexes, isthe distorted square antiprism shown by five com-plexes28,29a,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 com-pound 4, a bicapped trigonal prism coordination geom-etry. The only found eptacoordinated complex [Ba(tdh)2‚pmdt, pmdt ) pentamethyldiethylentriamine] shows acoordination pattern not referable to a regular polyhe-dron.28 Finally, we can notice that in compounds 2, 3,and 4 intermolecular contacts are not present while incompound 1, due to the presence of crystallization watermolecules, there is the formation of a hydrogen-bondedchain.13

The mass spectra of the Cd(hfa)2‚polyether adductsdo not show the molecular ion peaks. The observedpeaks always show the characteristic isotope pattern ofCd and are due to the loss of hfa and polyetherfragments. The COCHCF3, CF3, and F groups are themost common observed fragments and have alreadybeen observed in mass spectra of similar complexes.13-15

In addition, there is evidence of fluorine group transferprocesses similar to that observed in closely relatedadducts.15 The most intense peak shown by 2, 3, and 4at 493 m/z (100%) corresponds to the fragment [Cd-(hfa)COCH3‚OCH2CH2OCH3]+. At lower mass, themost significant peaks are at 311 m/z (10-20%) and233 m/z (5-10%) and can be associated with the[M-polyether-CF3-COCHCF3-2F]+ and [M-poly-ether-hfa-CF3-F]+ fragments, respectively.

The 1H NMR spectra of the Cd(hfa)2‚polyether ad-ducts always show a singlet at δ ) 6.04-5.95, whoseintegration accounts for the two protons of the hfa ringligands.13,14 In addition, multiplets at δ ) 3.87-3.58represent resonances of methylenic protons of poly-

ethers while the singlet at δ ) 3.33-3.51 is consistentwith the six protons of the two methyl groups of thesame ligands. Present resonances are in agreement withthose 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 ofthe Cd(hfa)2‚polyether adducts 3, 4, and 5 all show aquantitative mass loss process with peak temperaturesat 192, 205, and 225 °C, respectively (Figure 3), consis-tent with the evaporation of the adducts. No residue(<1%) is left in all cases. Figure 4 shows the DSCanalysis of the prototypical Cd(hfa)2‚diglyme adduct (3).Two distinct endothermic peaks are observed at 45 °C(-8.6 kcal‚mol-1) and 224 °C (-30.8 kcal/mol). Theyaccount for melting and evaporation from melt, respec-tively. The melting and evaporation enthalpies of com-pound 2 are -11.8 and -45.9 kcal‚mol-1, respectively,and those of compound 4 are -15.4 and -42.1 kcal‚mol-1,respectively. All these values are entirely consistentwith the heats of fusion and vaporization alreadyreported for others â-diketonate complexes.30 For ex-amples, the heats of fusion already reported for Gd(hfa)-monoglyme and Gd(thd)3 are 8.0430b and 18.630c kcal‚mol-1, respectively.

The FT-IR transmittance spectra of solids 2, 3, 4, and5, in Nujol mull, show bands characteristic of the Cd-(hfa)2 system.13 Moreover, bands at 1018-1027, 942-

(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., DaltonTrans. 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.

(30) (a) Malandrino, G.; Fragala, I. L.; Aime, S.; Dastru, W.; Gobetto,R.; Benelli, C. J. Chem. Soc., Dalton Trans. 1998, 1509. (b) Malandrino,G.; Incontro, O.; Castelli, F.; Fragala, I. L.; Benelli, C. Chem. Mater.1996, 8, 1292. (c) Mehrotra, R. C.; Bohra, R.; Gaur, D. P. Metalâ-Diketonates and Allied Derivatives; Academic Press: London, 1978.

Figure 3. DTG (a) and TG (b) of Cd(hfa)2‚polyether 3, 4, and5 adducts.

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

Liquid MOCVD Precursors for Thin Films of CdO Chem. Mater., Vol. 14, No. 12, 2002 4959

947, and 872-864 cm-1 can be safely associated withpolyether modes.15,31

MOCVD experiments have been carried out using thecrude Cd(hfa)2‚polyether adducts. Evaporation rates,suitable for MOCVD experiments (4.0-4.5 mg/min),have been found in the 100-110 °C range sourcetemperature. The substrate temperature was 400 °C.The deposition time was 120 min (Table 3).

XRD measurements (Figure 5) of as-deposited filmsprovide evidence of cubic CdO crystallites. Only the(200) reflection (θ ) 38.32) has been observed, thuspointing to highly textured CdO.32 This result has beenfound highly reproducible using the deposition param-eter reported in Table 3. Finally, the mean crystallitesize evaluated from the XRD line broadening33,34 (sub-strate temperature ) 400 °C) is equal to 48 ( 5 nm. Intextured materials, the grain shape is usually aniso-tropic. In this context, the presence of the (200) reflec-tion only implies that the width of the dominant Braggpeak mainly depends on the grain thickness in the (h00)direction. Therefore the present value is to be consideredonly qualitative. No differences were observed in theXRD patterns of films A-P depending on the differentMOCVD conditions.

EDX results confirm the absence of any carbon orfluorine contamination in the bulk of the films.

The deposited films are yellow and transparent. TheirUV-visible spectra (Figure 6) find counterparts inpreviously reported data.11,13-14,35-40 In particular, thetransmittance minimum (absorption edge) ranges from

λ ) 376 nm (films deposited using O2 ) 400 sccm) to446 nm (films deposited using O2 ) 100 sccm) and, inthe former case, reaches a 90% value in the visible andnear-infrared range. This transmittance value is similarto that recently observed by Marks and co-workers.12

Moreover, the moving of the absorption edge, dependingon the different oxygen flow rate during MOCVD ofCdO, has already been observed also for films depositedby dc magnetron reactive sputtering,41 by activatedreactive evaporation at different substrate tempera-tures,39 and by MOCVD using Cd(hfa)2‚2H2O as aprecursor.14 This fact is probably due to the decreasingdensity of defect centers with increasing oxygen partialpressure.

The film thickness d has been evaluated from UV-visible data (uncertainties lie within (5-10%) using theclassical equation42

where n1 and n2 are the refractive indices at twoadjacent maxima or minima at λ1 and λ2 wavelengths.Assuming n1 ) n2 ) 2.49 for cubic CdO films,43 thecalculated d values are 380 nm (O2 ) 400 sccm) and2808 nm (O2 ) 100 sccm) for 120-min experiments. Thefilm cross sections, obtained by SEM analysis, confirmedthese results. In particular, thicknesses of 2500 ( 100nm were obtained for (O2 ) 100 sccm) 120-min experi-ments (Figure 7). Therefore, the resulting growth rateof the CdO films significantly varies depending on thereactive gas flow rates and ranges from 32 to 234 Å/min.No relevant differences were observed depending on theparticular precursor used during the MOCVD. Similartransmittance and growth rate behaviors, with respectto the oxygen partial pressure during deposition, havealready been reported for CdO films deposited by dc

(31) Iwamoto, R. Spectrochim. Acta, Part A 1971, 27, 2385.(32) Powder Diffraction Files; Joint Committee on Powder Diffrac-

tion Standards, American Society for Testing and Material: Philadel-phia, PA, 1971, #5-0640.

(33) Gulino, A.; La Delfa, S.; Fragala, I.; Egdell, R. G. Chem. Mater.1996, 8, 1287.

(34) Jiang, H. G.; Ruhle, M.; Lavernia, E. J. J. Mater. Res. 1999,14, 549.

(35) Benramdane, N.; Murad, W. A.; Misho, R. H.; Ziane, M.;Kebbab, Z. Mater. Chem. Phys. 1997, 48, 119.

(36) Wu, X.; Coutts, T. J.; Mulligan, W. P. J. Vac. Sci. Technol., A1997, 15, 1057.

(37) Narushima, S.; Hosono, H.; Jisun, J.; Yoko, T.; Shimakawa,K. J. Non-Cryst. Solids 2000, 274, 313.

(38) Ortega, M.; Santana, G.; Morales-Acevedo, A. Solid-StateElectron. 2000, 44, 1765.

(39) Reddy, K. T. R.; Sravani, C.; Miles, R. W. J. Cryst. Growth1998, 184/185, 1031.

(40) Miyata, N.; Miyake, K.; Fukushima, T. J. Electrochem. Soc.:Solid-State Sci. Technol. 1980, 127, 918.

(41) Subramanyam, T. K.; Uthanna, S.; Srinivasulu Naidu, B.Mater. Lett. 1998, 35, 214.

(42) Swanepoel, R. J. Phys. E: Sci. Instrum. 1983, 16, 1214.(43) Handbook of Chemistry and Physics, 65th ed.; Weast, R. C.,

Ed.; CRC Press: Boca Raton, FL, 1985.

Figure 5. X-ray diffraction pattern, over a 30° < 2θ < 70°angular range, for SiO2-supported, CdO as-deposited thinfilms.

Figure 6. UV-visible transmission spectra for two repre-sentative CdO thin films on a SiO2 substrate: (a) refers to filmsobtained using 100 sccm O2 flow rate; (b) refers to filmsobtained using 400 sccm O2 flow rate.

d )λ1λ2

2(λ1n2 - λ2n1)

4960 Chem. Mater., Vol. 14, No. 12, 2002 Gulino et al.

magnetron reactive sputtering41 and by MOCVD usingCd(hfa)2‚2H2O as a precursor.14 In these case, thetransmittance behavior was interpreted on the basis ofthe nonstoichiometry of the CdO films obtained at lowoxygen partial pressure.14,41

Figure 8 shows the Al KR XPS of a representative as-deposited CdO film, obtained using a 400 sccm O2 gasflow rate, in the Cd 3d, O 1s binding energy (B.E.)regions. The Cd 3d features consist of the main 3d5/2,3d3/2 spin-orbit components at 404.5 and 411.5 eV,respectively.2,13,14 Two O 1s peaks are evident at 529.4eV and at 531.8 eV. These features are identical to thosealready reported for related CdO systems and are dueto the presence of hydroxide species on the surfacewhich are ubiquitous in air-exposed CdO materi-

als.13,14,44-48 In fact, it has been reported that atomicallyclean CdO surfaces give the characteristic double XPSO 1s peak after a few minutes air exposure.47

Photoemission spectra show, in addition, weak fea-tures (at 285.1 and 289.3 eV) due to surface carboncontamination (2-3 at. %) which is almost ubiquitousin similarly MOCVD-fabricated materials (Figure 9).13-15

No fluorine signal was detectable. Depth profiles (Figure

(44) Niles, D. W.; Waters, D.; Rose, D. Appl. Surf. Sci. 1998, 136,221.

(45) Nefedov, V. I.; Firsov, M. N.; Shaplygin, I. S. J. Electron.Spectrosc. Relat. Phenom. 1982, 26, 65.

(46) Gaarenstroom, S. W.; Winograd, N. J. Chem. Phys. 1977, 67,3500.

(47) Hammond, J. S.; Gaarenstroom, S. W.; Winograd, N. Anal-.Chem. 1975, 47, 2193.

(48) Nefedov, V. I.; Gati, D.; Dzhurinskii, B. F. Russ. J. Inorg. Chem.1975, 20, 2307.

Figure 7. SEM cross section of a representative CdO film obtained for 120-min experiments (O2 ) 100 sccm).

Figure 8. Al KR excited XPS of a representative CdO thinfilm measured in the (a) Cd 3d and (b) O 1s energy regions.Solid lines refer to films before sputtering; dotted lines referto films after 1 min of sputtering. Structures due to satelliteradiation have been subtracted from the spectra.

Figure 9. Al KR excited XPS of a representative CdO thinfilm measured in the (a) F 1s and (b) C1s energy regions.Structures due to satellite radiation have been subtracted fromthe spectra.

Liquid MOCVD Precursors for Thin Films of CdO Chem. Mater., Vol. 14, No. 12, 2002 4961

10), obtained by alternating XPS core level measure-ments with Ar+-ion sputter etching (4 kV, beam current1.0 µA) every 60 s, show the absence of any bulk carbonand fluorine contamination, in accordance with EDXresults. Moreover, after 1 min of sputtering, only the529.4-eV O 1s component remains evident (Figure 8),due to removal of hydroxide surface species. No signifi-cant binding energy variation was observed for Cd 3dstates (Figure 8).

Figure 11 shows a plot of the film resistivities, as afunction of the oxygen flow rate, observed for the CdOfilms as deposited from the compound 3. The resultingvalues are in the (6.8 × 10-4)-(4.2 × 10-5) Ω‚cm rangeand points to conducting films. In particular, higherresistivity values were observed for films depositedusing higher oxygen flow rates (Figure 11). In CdO theelectronic conduction is promoted by the presence of

cadmium interstitials or oxygen vacancies which act asn-type defects and produce donor states in the bulk bandgap. Therefore, synthesis under more oxidizing condi-tions yields less defective and thus lower conductingcompounds. Similar resistivity values have already beenreported for other CdO and CdO-based films.12-14,36,39-41

Conclusions

Four novel, thermally stable, Cd(hfa)2‚polyether ad-ducts have been synthesized with simple procedures.X-ray single-crystal data of the Cd(hfa)2‚polyether ad-ducts show that in compounds 2, 3, and 4 all the oxygenatoms of the ether molecules coordinate the cadmiumcation and, consequently, the cadmium results in hexa-,hepta-, and octacoordination, respectively. All presentadducts melt upon mild heating, thus allowing MOCVDfrom a liquid source. Deposition experiments, in a low-pressure horizontal hot-wall reactor on fused SiO2substrates, result in CdO thin films whose XRD spectraprovide evidence that they are cubic and consist of (100)-oriented crystals. Optical spectra show that the trans-mittance of films is around 90% in the visible range.This value is better than that observed for CdO filmshaving similar thickness (340 nm) obtained using thesimple Cd(hfa)2‚2H2O precursor.14 Resistivity valuesalso are 1 order of magnitude lower than those observedfor CdO films obtained using the simple Cd(hfa)2‚2H2Oprecursor.14

Finally, the major difference between the Cd(hfa)2‚polyether adducts and that reported in ref 12 is due tothe different donor ligand attached to the Cd center.Nevertheless, present low-melting (melting point of Cd-(hfa)2‚diglyme ) 44 °C) adducts allow MOCVD of CdOfrom liquid precursors, which certainly represents aconsiderable improvement due to the accurate repro-ducibility associated with constant evaporation rates forgiven source temperatures.

Acknowledgment. The authors thanks the Con-siglio Nazionale delle Ricerche (CNR, Roma, ProgettoOssidi Come Materiali Funzionali) and the MinisteroIstruzione Universita e Ricerca (MIUR, Roma) forfinancial support. Profs. F. Castelli, S. Giuffrida, G.Condorelli, and E. Amato are gratefully acknowledgedfor thermal, UV-vis, SEM, and NMR facilities, respec-tively.

Supporting Information Available: Tables of crystaldata, structure solution and refinement, atomic coordinates,bond lengths and angles, and anisotropic thermal parametersfor 2, 3, and 4 (PDF). This material is available free of chargevia the Internet at http://pubs.acs.org.

CM021183M

Figure 10. XPS depth profile of a representative CdO thinfilm.

Figure 11. Film resistivities, as a function of the O2 flow rate,for films E-H.

4962 Chem. Mater., Vol. 14, No. 12, 2002 Gulino et al.