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Functional Nanostructured Materials (including low-D carbon) x

Growth of WO from Tungsten (VI) Oxo-Fluoroalkoxide Complexes with Partially Fluorinated #-diketonate/#-ketoesterate Ligands: Comparison of Chemical Vapor Deposition to Aerosol-Assisted CVD Nathan C Ou, Duane C Bock, Xiaoming Su, Doina Craciun, Valentin Craciun, and Lisa McElwee-White ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b08830 • Publication Date (Web): 17 Jul 2019 Downloaded from pubs.acs.org on July 18, 2019

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ACS Applied Materials & Interfaces

Growth of WOx from Tungsten (VI) OxoFluoroalkoxide Complexes with Partially Fluorinated β-diketonate/β-ketoesterate Ligands: Comparison of Chemical Vapor Deposition to Aerosol-Assisted CVD. Nathan C. Ou,a Duane C. Bock,a Xiaoming Su,a Doina Craciun,b Valentin Craciunb,c and Lisa McElwee-Whitea,*

aDepartment

bNational

of Chemistry, University of Florida, Gainesville, Florida, 32611-7299 USA.

Institute for Laser, Plasma, and Radiation Physics, Bucharest-Magurele,

Romania

cExtreme

Light Infrastructure for Nuclear Physics, Bucharest-Magurele, Romania

ACS Paragon Plus Environment

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ACS Applied Materials & Interfaces 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Abstract.

Page 2 of 59

Tungsten (VI) oxo complexes of the type WO(OR)3L [R = C(CH3)2CF3,

C(CF3)2CH3,

CH(CF3)2,

L

=

hexafluoroacetylacetonate

(hfac),

ethyl

trifluoroacetoacetonate (etfac), acetylacetonate (acac)] bearing partially fluorinated alkoxide and/or chelating ligands were synthesized. The thermal decomposition behavior and mass spectrometry fragmentation patterns of selected examples were studied. Thermolysis products of WO(OC(CF3)2CH3)3(hfac) were characterized by NMR and GCMS. Studies of the sublimation behavior of the complexes demonstrated that the volatility of the complex depends on the degree of fluorination.

Comparative studies of the

deposition of tungsten oxide by chemical vapor deposition (CVD) and aerosol-assisted (AA)CVD were carried out using WO(OC(CF3)2CH3)3(hfac) as a single source precursor. WOx materials were successfully deposited by both deposition methods, but the deposits differed in morphology, structure and crystallinity.

Keywords chemical vapor deposition, tungsten oxide, nanostructures, volatility, thermolysis

Introduction


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Tungsten oxides (WOx) are well known for their electrochromic properties which have found widespread applications in optoelectronic devices and “smart-window” technology.1,2

Tungsten oxide has also been used in other applications such as

photovoltaics,3 catalysis4 and gas sensors.5

The combination of numerous

stoichiometries and morphologies broadens the range of applications of the material. Tungsten oxide materials can exist in substoichiometric compositions such as WO2.72,6 WO2.66,7 and WO2.8,8 as well as the stoichiometric WO3.9 Different morphologies of WOx, such as nanotubes, nanobelts and nanorods have been obtained by varying deposition temperature, and precursor choice.10-13 The properties of these materials depend on the deposition technique and conditions.14,15 Chemical vapor deposition (CVD) is an attractive deposition technique due to its ability to cover large growth areas with high film conformality.16 Previous reports of CVD of WOx have demonstrated control of composition and morphology to produce both amorphous thin films and nanostructures.17,18 An important step in the CVD process is the volatilization of the precursor, which in traditional CVD involves evaporation of a liquid precursor or sublimation of a solid in the presence of a carrier gas. Therefore, this


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technique requires precursors with significant vapor pressure that are also sufficiently stable to survive the volatilization process. However, the volatility requirement can be circumvented through a variant known as Aerosol-Assisted (AA)CVD. In contrast to traditional CVD, AACVD volatilizes the precursor by nebulization of a solution, so that the carrier gas transports an aerosol of the precursor and solvent to the substrate.19 Once the solvent evaporates, the growth mechanism is understood to be the same as CVD.19 Therefore, the precursor choice becomes dependent on solubility instead of volatility, permitting a wider scope of precursor chemistry to be explored.20 Two precursors commonly used in CVD of tungsten oxide are the homoleptic complexes WCl621 and W(CO)6.22 Although these precursors demonstrate good transport properties, the CVD process produces undesirable, hazardous byproducts such as HCl and CO. They also require a coreactant for oxygen incorporation, making prevention of premature reaction between the coreactants and control of the stoichiometry in the film more challenging.23 As an alternative, the use of single-source precursors facilitates control of film stoichiometry and precursor decomposition through consideration of ligand choice.24 Single-source precursors for WOx CVD have included tungsten alkoxides, such


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as W(OEt)6, but their manipulation is difficult due to their sensitivity to moisture.25,26 This sensitivity can be lessened through use of chelating ligands. Preparation of WOx material through AACVD has been successfully demonstrated by using precursors that contain both alkoxide and chelating ligands.27,28 Fluorination of the ligands can improve the volatility of these complexes due to the repulsive nature of the fluorine atoms.29 By employing this strategy in precursor design, we previously reported the preparation of a series of oxo alkoxide complexes WO(OR)3L where L is a chelating β-diketonate or βketoesterate ligand and the alkoxide ligands -OR are partially fluorinated.30 In this work, we explore the properties of WO(OR)3L complexes where both the alkoxide ligands and the chelating ligand are partially fluorinated. Extending the fluorination to the chelating ligand resulted in a significant increase in precursor stability and volatility with respect to complexes where the fluorination was only on the alkoxide. The increased volatility was sufficient in these complexes for direct volatilization in a conventional CVD process. It is generally understood that AACVD and CVD should involve the same mechanistic steps (after solvent evaporation from the aerosol of the solution)19 and give the same material. Experimental comparisons of material grown by AACVD and CVD


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from the same precursor remain rare. There are isolated examples, such as deposition of WOx materials from W(OEt)5 in the presence of O2 by AACVD31 and atmospheric pressure CVD17 but the reactors and deposition conditions are so different that it is difficult to make a comparison of the two methods. There is a report of growth of fluorine-doped tin dioxide films deposited using dual-source precursors, monobutyl tin trichloride and trifluoroacetic acid, by AACVD and atmospheric pressure CVD (APCVD).32

These

materials grown with different volatilization techniques in similar reactors showed differences in substrate coverage and morphology. However, to our knowledge, a similar comparison of growth of a metal oxide from the same single-source precursor under CVD and AACVD conditions has not been reported. We now report deposition of WOx using WO(OC(CF3)2CH3)3(hfac) as a precursor under both CVD and AACVD conditions. Differences in film composition, morphology and structure were evident, which suggests that solvent plays a critical role during the deposition process.

Experimental Section


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Representative Synthesis:

WO(OC(CF3)2CH3)3(hfac) (5).

In a Schlenk flask,

WO(OC(CF3)2CH3)4 (0.693 mmol, 0.491 g) was dissolved in diethyl ether (40 mL) and chilled in an ice bath for 30 min. Hhfac (0.843 mmol, 0.175 g), dissolved in diethyl ether (10 mL), was added dropwise to the WO(OC(CF3)2CH3)4 solution. Then the reaction was warmed to room temperature and allowed to stir for 15 h. Volatiles were removed in

vacuo to afford a yellowish solid. The solid was sublimed under vacuum (30 mTorr) at 36 °C for 15 h to yield a solid yellowish-white sublimate. Yield: 0.45 g (68 %). 1H NMR (300 MHz, CDCl3, 25 °C): δ 6.49 (s, 1H, OC(CF3)CH), 1.88 (s, 6H, OC(CF3)2CH3), 1.85 (s, 3H, OC(CF3)2CH3).

13C{1H}

NMR (126 MHz, CDCl3, 25 °C): δ 182.5 (q, OC(CF3)CH, JC-F =

39.1 Hz), 174.0 (q, OC(CF3)CH, JC-F = 37.8 Hz), 122.9 (q, OC(CF3)2CH3, JC-F = 288.5 Hz), 122.8 (q, OC(CF3)2CH3, JC-F = 286.0 Hz), 122.3 (q, OC(CF3)2CH3, JC-F = 288.5 Hz), 117.8 (q, OC(CF3)CH, JC-F = 286.0 Hz), 116.2 (q, OC(CF3)CH, JC-F = 282.2 Hz), 96.6 (s, OC(CF3)CH), 88.6 (sp, OC(CF3)2CH3, JC-F = 31.5 Hz), 86.2 (sp, OC(CF3)2CH3, JC-F = 31.5 Hz), 15.9 (s, OC(CF3)2CH3), 15.6 (s, OC(CF3)2CH3).

19F{13C}

NMR (282 MHz, CDCl3, 25

°C): δ -74.71 (s, 3F, OC(CF3)CH), -76.72 (s, 6F, OC(CF3)2CH3), -77.10 (s, 3F,


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OC(CF3)CH), -77.29 (m, 6F, OC(CF3)2CH3), -77.47 (m, 6F, OC(CF3)2CH3). Anal. Calc’d for WO6C17H10F24: C: 21.49; H: 1.06%. Found: C: 21.66; H: 0.82%. Thermolysis of 5: NMR analysis. A sample of 5 (0.10 g, 0.11 mmol) was added to a 10 mL Schlenk tube in a nitrogen-filled glove box and capped with a septum. The arm of the Schlenk tube was connected to a U-tube via Tygon tubing. A flow of nitrogen gas was introduced into the Schlenk tube via needle on the Schlenk line. The gas flow rate was measured with a bubbler attached to the other end of the U-tube. A sand bath was pre-heated to 250 °C. Then the Schlenk tube was submerged into the sand bath and the U-tube was placed in a liquid nitrogen bath to collect the thermolysis products for 60 min. After 60 min, the U-tube was removed from the liquid nitrogen bath and allowed to warm to room temperature. CDCl3 (0.50 mL) was used to dissolve the thermolysis products, which were characterized by 1H and 19F NMR. Thermolysis of 5: GC-MS analysis. A sample of 5 (0.10 g, 0.11 mmol) was added to a 10 mL screw-top headspace vial in a nitrogen-filled glove box and capped with a septum lid. The vial was placed into an oven pre-heated to 250 °C for 45 min. A sample


8

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of the headspace gas was collected using a 10 μL glass syringe and needle and characterized by GC-MS. Volatility Analysis. Sublimation experiments were performed on a Schlenk line equipped with MKS pressure controlling equipment (throttling valve 253A-1-40-2, pressure controller 651CD2S1N, and absolute pressure transductor 121AA-01000B). The active pressure set point during sublimation was 1300 ± 1 mTorr. In a nitrogen-filled glovebox, 100-200 mg of the compound to be studied (1-9 sequentially in individual experiments) was loaded into a 10 mL Schlenk tube equipped with a drip tip cold finger. Prior to heating, the Schlenk tube was placed under a flow of N2 until the cold finger was cooled to 10 °C. The Schlenk tube was placed in a silicone oil heating bath during sublimation. The temperature of the hot plate was ramped from room temperature by 5 °C every 15 minutes until sublimate was detected as the appearance of a white-yellowish coating for 1-6, a yellow coating for 7 and green droplets for 8-9. The reported sublimation temperature was the temperature of the silicone oil bath at the onset of sublimation. AACVD of WOx from 5. Native silicon dioxide (Si/SiOx, n-type, ) and bare glass substrates were prepared by cutting squares of approximately 1 cm2.

The


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substrates were cleaned in boiling acetone, methanol, and deionized water for 3 min each. The substrates were placed on a SiC coated graphite susceptor heated by a radio frequency (RF) induction heat generator within a custom AACVD reactor and held under vacuum for 18 h.

The down-flow vertical impinging jet reactor has been detailed

previously.33,34 Compound 5 (0.912 g, 0.960 mmol) was dissolved in 12 mL of anhydrous diglyme and loaded into a Hamilton 10 mL gastight syringe and pumped into the nebulizer, vibrating at 1.44 MHz, at a rate of 4 mL h-1. High purity nitrogen (99.999 % purity, 1000 sccm) was used as a carrier gas and the reactor pressure was maintained at 650 Torr with a reaction time of 150 min. Conventional CVD of WOx from 5. Conventional depositions were performed on the same reactor as the AACVD depositions. In a nitrogen-filled glovebox, compound 5 (0.50 g) was loaded into a stainless-steel bubbler. High purity nitrogen (99.999 % purity, 1000 sccm) was used as a carrier gas and the reactor pressure was maintained at 650 Torr with a reaction time of 210 min. During deposition, the bubbler was heated in a 90 °C sand bath and the transfer line connecting the outlet of the bubbler to the reaction chamber was heated to 80 °C.


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Materials Characterization. The chemical composition of the deposited materials was studied using X-ray Photoelectron Spectroscopy (XPS) with an ESCALAB 250Xi instrument (Thermo Fisher Scientific, Pittsburgh, PA) working with a monochromatic aluminum anode (hν = 1486.6 eV) as the X-ray source. Survey scans were initially acquired from the as-prepared surface of the deposited film with an electron pass energy of 100 eV and step size of 0.5 eV. High resolution scans for detailed peak analysis were performed at an electron pass energy of 20 eV and an energy step size of 0.1 eV. The binding energies were referenced to the adventitious C 1s peak located at 284.6 eV. New spectra were also acquired after gentle sputtering with 500 eV Ar ions rastered on a 2 mm2 area at various time intervals (30 s to several min). This low energy sputtering was employed to remove surface contaminants while simultaneously minimizing the reduction of tungsten oxide towards metallic W.35

XPS Advantage software was used to

deconvolute the acquired XPS core-level peaks. A Shirley type background line was subtracted before peak fitting. The XPS spectra of the W 4f core level were fitted into peak doublets with parameters of spin-orbit separation ΔEP (4f5/2–4f7/2) = 2.18 eV. The intensity ratio of the W 4f7/2 and W 4f5/2 peak doublet was set to 4:3.48 The O1s peak for


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WO3 films also exhibits components due to oxyhydrates, C-O-C and C-O-H species, as well as adsorbate H2O.48 The crystalline structure of the deposited films and structures was investigated with grazing incidence X-ray diffraction (GIXRD) (Empyrean, Panalytical) working with a Cu anode (45 kV and 40 mA power settings). The instrument was set in a parallel beam geometry with an X-ray mirror and a (1/8)° slit in the incident optics side and a 0.27° parallel beam collimator in front of a detector on the diffraction side. Scans were recorded from 20° to 80° 2θ angles with a 0.03º step size and a 3 s time per step. The incidence angle of X-ray on the samples surface was adjusted within the 1° to 4° range, depending on the film thickness, in order to maximize the diffraction signal from the deposits while reducing the contribution from the Si substrate. The acquired diffraction patterns were fitted using the Panalytical HighscorePlus software to identify the crystalline phases using the ICCD 2018 data base. The grain size was estimated from a line profile analysis using several of the most intense diffraction lines. The morphologies of the deposits were measured by field emission scanning electron microscopy (FESEM, FEI Nova NanoSEM 430).

Results and Discussion

Precursor Design. Single-source precursors bearing terminal oxo, alkoxide, and β-diketonate/β-ketoesterate ligands have been previously reported for the deposition of


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metal oxides.24,36,37 Terminal oxo ligands, either present in the precursor or generated from the alkoxide ligands during decomposition, provide the source of oxygen in the film deposits, eliminating the need for secondary oxygen sources.37 Chelating ligands can improve the stability of the complexes to air and moisture by occupying an empty coordination site, if one is present on the metal. Also, chelating ligands have been proven to be effective for obtaining monomeric complexes, should the alkoxide ligands lack sufficient steric bulk to prevent oligomerization.38 The physical properties necessary for AACVD precursors are different from those for conventional CVD. For conventional CVD, in which neat precursor is volatilized in a bubbler, a moderately high vapor pressure is critical.39 In contrast, the AACVD process involves nebulization of a solution of the precursor, which is then delivered to the substrate as aerosol droplets.

Therefore, AACVD precursors must have sufficient

solubility in the solvent used to generate the aerosol but their volatility requirements are not as stringent. Because AACVD widens the scope of available precursors due to the decreased need for precursor volatility, AACVD can be used to test ligand types to develop sets of precursors without having to deal with the volatility constraints of CVD.


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Then modification of the ligands to increase the precursor volatility would allow use of the complexes in conventional bubbler systems as well. Ideally, newly developed precursors would be suitable for use in both deposition methods. A common approach for increasing precursor volatility is use of sterically demanding ligands, which diminish intermolecular interactions.37

Incorporation of

fluoroalkyl substituents on the ligands has also been shown to increase volatility by the introduction of repulsive interactions between non-bonding p electrons on the fluorines.40 We previously reported the series of complexes WO(OR)3L [R = C(CH3)2CF3 and C(CF3)2CH3, L = acetylacetonate (acac), dipivaloylmethanate (dpm), ethyl acetoacetate (etac), and tert-butyl pivaloyl acetate (tbac)], where fluorination was present only on the alkoxide.30 Our interest in testing the effects of fluorination of the chelating ligands suggested complexes 1-9 as target precursors. Synthesis of Precursors.

The oxo-tetraalkoxide complexes WO(OR)4 [R =

C(CH3)2CF3, C(CF3)2CH3, CH(CF3)2] react with one equivalent of the β-diketone or βketoester [HL = Hacac, Hhfac, Hetfac] upon dropwise addition at 0 °C followed by warming to room temperature and stirring for 15 h to form the WO(OR)3L complexes 1-9


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as shown in Scheme 1. The solvent and volatile byproducts are removed in vacuo and 1-9 are purified via sublimation or distillation.

Scheme 1. Synthesis of compounds 1-9.

O

O

R1O W OR1 R1O OR1

O

Et2O

+

0 °C, 15 h

R3

R2

R 1O 1

R O

O W O

OR1 O R3

R2

WO(OR)3L

R1

R2

R3

1

C(CH3)2CF3

CH3

CH3

2

C(CH3)2CF3

CF3

CF3

3

C(CH3)2CF3

OCH2CH3

CF3

4

C(CF3)2CH3

CH3

CH3

5

C(CF3)2CH3

CF3

CF3

6

C(CF3)2CH3

OCH2CH3

CF3

7

CH(CF3)2

CH3

CH3

8

CH(CF3)2

CF3

CF3

9

CH(CF3)2

OCH2CH3

CF3

NMR Characterization. The splitting patterns and intensities of the signals in the 1H, 13C,

19F

and 19F NMR spectra of 1-9 are consistent with their structural assignments. The

NMR spectra for complexes bearing trifluoro-tert-butoxide ligands, 1-3, show two

symmetry inequivalent signals in a 2:1 ratio, corresponding to the CF3 groups. The same pattern is observed in the 1H NMR spectra for the methyl groups on the hexafluoro-tert-


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butoxide ligands of 4-6 and for the methine protons on the hexafluoro-isopropoxide ligands of 7-9. In the 13C NMR spectra, the tertiary carbons on the alkoxide ligands for 16, and the secondary carbons for 7-9, also display two signals in a 2:1 ratio which are split by coupling to fluorines two bonds away (2JC-F ≈ 30 Hz).

In the 1H NMR spectra for 1-3, there are three distinct signals attributed to the methyl groups on the alkoxide ligands. Two of the signals arise from two alkoxide ligands that are trans to one another. The methyl groups on these alkoxides are inequivalent with respect to each other but each methyl group has an enantiotopic group located on the other alkoxide. This results in two sets of enantiotopic methyl groups, consistent with the

mer isomer. The last signal is from the third alkoxide bearing equivalent CH3 groups. The analogous enantiotopic groups are also observed for the CF3 groups for compounds 4-6. The integrations in the 1H NMR and 19F NMR spectra for 1-9 confirm that only one β-diketonate or β-ketoesterate is coordinated to the complex. Furthermore, the


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substituents on the β-diketonate ligand display symmetry inequivalence with respect to each other which is further consistent with the mer isomer. Mass Spectrometry. Investigating decomposition mechanisms of CVD precursors enables improvement in precursor design by understanding how these complexes fragment during the CVD process.

One strategy is analyzing mass spectrometry

fragmentation patterns as they are known to model the gas-phase portions of precursor decomposition during CVD.41 However, over-interpretation should be avoided as mass spectrometry produces charged fragments whereas CVD produces neutral fragments.42 Mass spectra were obtained for compounds 2, 3 and 5 using electrospray ionization (ESI) (Figure S32-34, Supporting Information). This softer ionization technique results in less fragmentation. However, the lack of fragmentation can be compensated by coupling ESIMS with tandem mass spectrometry (MS/MS), which allows for further fragmentation of a selected peak using additional collision energy. The observed molecular ion was a sodium adduct of the complex, [M + Na]+. This fragment was further fragmented by MS/MS. Table 1 lists selected ions of interest.


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Table 1. Selected observed ions for ESI MS/MS for compounds 2, 3, and 5 in positive mode from [M+Na]+ WO(OR)3L

WO(OC(CH3)2CF3)3(hfac)

[M+Na]+

[M-

[M-C4H5F3-

[M-C4H5F3-

[M-HL+H]+

m/z (%)b

HOR+H]+

HL+H]+

HOR+H]+

m/z (%)

m/z (%)

m/z (%)

m/z (%)

n. o.

471.02 (30)

n. o.

811.04 (6)

581.06

(2) WO(OC(CH3)2CF3)3(etfac)

(100) 787.08 (39)

n. o.

471.02 (83)

527.01 (23)

581.06

(3) WO(OC(CF3)2CH3)3(hfac) (5) an.o.

(100) n.o.a

768.95

n. o.

n. o.

n. o.

(100)

= not observed, bRelative abundances were calculated by normalizing peak

intensities relative to the highest peak at 100 %. The ion that results from loss of C4H3F3, an alkene fragment derived from an alkoxide, and the protonated β-diketonate/β-ketoesterate ligand HL (471 amu) is observed for compounds 2 and 3. This ion corresponds to pathways that involve W-O bond cleavage on the chelate through proton transfer from the alkyl group on a neighboring alkoxide ligand (Scheme 2a). This fragment was also previously observed for 1.30 This common fragment observed for complexes that bear trifluorinated-tertbutoxide ligands (1, 2, and 3) suggests they share similar decomposition pathways. Complex 3 also produced a fragment (527 amu) formed from the loss of C4H5F3 and the fluoroalcohol HOR, both derived from alkoxide ligands. The formation of this ion occurs


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through a proton transfer of a methyl hydrogen on one alkoxide ligand to a neighboring alkoxide (Scheme 2b), which is consistent with the behavior of related alkoxide complexes.43,44 For complex 5, the base peak (768 amu) observed was loss of alcohol by protonation. Fragmentation patterns from the mass spectra of compounds 6-9 were obtained but the fragments could not be identified. Scheme 2. a) Proposed decomposition mechanism to produce the observed ions [MC4H5F3-L+H]+ from 2 and 3, b) Proposed decomposition mechanism to produce the observed ion [M-C4H5F3-HOR+H]+ from 3.


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Thermolysis of 5.

Page 20 of 59

Thermolysis of 5 was performed to obtain mechanistic

information from conditions that more closely resemble CVD. Mass spectrometry detects ionized species whereas both CVD and thermolysis are more likely to produce neutral fragments. A neat sample of 5 was heated above 250 °C and the volatile products were characterized by NMR and GC-MS (Scheme 3). In the 1H and

19F

NMR spectra of the

thermolysis products, the only identifiable signals were from hexafluoroacetylacetone (HL), and HOC(CF3)2CH3 (HOR) (Figures S25-26, Supporting Information). Analyzing the headspace gas by GC-MS after thermolysis resulted in detection of the fluorinated alkene, (CF3)2C=CH2 (164 amu), and HL (208 amu). Although the molecular ion of HOR (182 amu) was not observed directly, ions produced from its fragmentation were observed (Figure S27-28, Supporting Information). All observed spectral fragmentation patterns were compared with NIST library spectra to confirm their identities. The presence of these thermolysis products provides further support that hexafluorinated-tert-butoxide complexes share similar decomposition pathways with their trifluorinated analogues.


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Scheme 3. Products of the thermolysis of 5.

CF3 F 3C

O

F 3C

O CF3

CF3 O W O

O

CF3

O CF3

O



+

> 250 °C

F 3C

CF3

F 3C

CF3

O + HO CF3

CF3

F 3C 5

Thermolysis of 8. Thermolysis of 8 was also performed in the same manner as for 5. A neat sample of 8 was heated above 250 °C for 45 min (Scheme 4). Analysis of the headspace gas by GC-MS resulted in the detection of hexafluoroacetone (166 amu), hexafluoroacetylacetone (208 amu), and hexafluoroisopropanol (99 amu) (Figures S2931, Supporting Information).

The formation of these products is consistent with

intramolecular proton transfer as proposed above for 5.

The formation of

hexafluoroacetone and hexafluoroacetylacetone is attributed to a deprotonation of a methine proton on an alkoxide ligand by the chelating ligand (Scheme 5). The proposed route to hexafluoroacetone and hexafluoroisopropanol involves deprotonation of a methine proton on an alkoxide by a neighboring alkoxide, shown in Scheme 6. Formation of the ketone contrasts with compounds bearing tert-butoxide ligands, which have no


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methine hydrogen. In the partially fluorinated tert-butoxide cases, deprotonation of a methyl group results in the cleavage of the W-O bond.

Scheme 4. Products of the thermolysis of 8.

CF3

CF3

F 3C

O

F 3C

O CF3

O

O

W

CF3

O

 F 3C

> 250 °C

CF3

CF3

CF3

O + HO CF3

+

O

O

O F 3C

CF3

F 3C 8

Scheme 5.

Proposed decomposition mechanism to produce the products

hexafluoroacetylacetone and hexafluoroacetone from 8.

O CF3

CF3

F 3C

O

F 3C

O CF3

O W O

O O

H

F 3C

O

O

+ CF3

F 3C

CF3

CF3 WO(OR)2 CF3

F 3C 8


22

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Scheme 6.

Proposed decomposition mechanism to produce the products

hexafluoroisopropanol and hexafluoroacetylacetone from 8.

CF3 HO CF3 F 3C F 3C

H

CF3

O O

CF3

O W O

O

O +

CF3

CF3

F 3C

CF3

WO(OR)L

O CF3

F 3C 8

Thermogravimetric Analysis.

Thermogravimetric analysis (TGA) was used to

obtain information about the volatilization temperatures and decomposition of 1-9 (Figures 1a-b).

Compounds 2 and 8 undergo decomposition concomitant with

sublimation leaving residual masses of 16.3% and 17.6% respectively. Decomposition occurring simultaneously with sublimation for compounds 2 and 8 suggests compounds that contain the hfac ligand are inherently less stable. However, when there is greater steric bulk and more fluorine on the alkoxide ligands, the compound exhibits greater volatility, allowing clean sublimation as is the case for compound 5. All other compounds


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undergo complete sublimation or evaporation without any decomposition at atmospheric pressure, indicating adequate volatility and thermal stability for transport during the CVD process. Complexes with fluorinated β-diketonates (2, 5, and 8) undergo initial mass loss at much lower temperatures in contrast to their non-fluorinated analogues 1, 4 and 7. Similarly, 3 and 6 exhibit evaporation temperatures 20-30 °C lower than their counterparts WO(OC(CH3)2CF3)3(etac) and WO(OC(CF3)2CH3)3(etac), respectively, which are not fluorinated on the chelating ligand.30

This augmented volatility is attributed to the

repulsive nature of the fluorine lone pairs, which lessens intermolecular interactions.45 Once the alkoxide ligand has a CF3 group, increased fluorination on the alkoxide has a negligible effect on volatility as 1 and 4 have similar sublimation temperatures despite 4 exhibiting

greater

fluorine

content

on

the

alkoxide.

Complexes

in

the

hexafluoroisopropoxide series, 7-9, overall exhibit lower sublimation temperatures than 1-4.


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Figure 1. TGA plots of compounds a) 1-5 and b) 6-9.

Volatility Analysis.

Determining the sublimation temperature of a precursor can

be used as a surrogate for measuring its vapor pressure.46 The volatility of 1-9 was thus assessed by sublimation on a Schlenk line at 1300 mTorr (Table 2). This pressure was held constant to within ± 1 mTorr using a throttle valve and digital pressure controller. The onset of sublimation (Tsub) was determined by the oil bath temperature at which sublimate could be detected on a cold finger cooled to 10 °C. The TGA curves for 1-9 (Figures 1a-b) demonstrate that these compounds remain stable when heated to Tsub at


25


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atmospheric pressure. Therefore, it was assumed that no decomposition occurred during sublimation. For compounds that bear tert-butoxide ligands (1-5) there is little change to Tsub when an additional CF3 group is added only on the alkoxide. However, when additional CF3 groups are added onto the chelate, the decrease in Tsub is more pronounced. Fluorination on the chelate thus has a greater effect on volatility than additional fluorination on the alkoxide. Compounds bearing hexafluoro-isoproxide ligands (7-9) exhibited lower sublimation temperatures than the analogous compounds with tertbutoxide ligands. Lastly, the sufficient volatility and thermal stability demonstrated for these compounds permits volatilization in a bubbler, thus enabling a comparison between materials grown by CVD and AACVD. Table 2. Temperatures for the onset of sublimation for 1-9.

WO(OR)3L

Tsub (°C) at 1300 ±1 mTorr

WO(OC(CH3)2CF3)3(acac) (1)

48

WO(OC(CH3)2CF3)3(etfac) (3)

47a

WO(OC(CH3)2CF3)3(hfac) (2)

35


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WO(OC(CF3)2CH3)3(acac) (4)

47

WO(OC(CF3)2CH3)3(etfac) (6)

43a

WO(OC(CF3)2CH3)3(hfac) (5)

32

WO(OCH(CF3)2)3(acac) (7)

40

WO(OCH(CF3)2)3(etfac) (9)

38a

WO(OCH(CF3)2)3(hfac) (8)

34a

aEvaporation

of liquid precursor, not sublimation

Growth of WOx from AACVD and Conventional CVD. Compound 5 exhibited the lowest onset sublimation temperature of 32 °C and was therefore chosen as the precursor for deposition studies. For AACVD, precursor 5 was volatilized from a 0.080 M solution in diglyme. For CVD, 0.50 g of neat precursor was loaded into a stainless-steel bubbler for direct volatilization.

Growth of tungsten oxide on silicon substrates was

achieved at deposition temperatures of 350, 400, 450 and 500 °C. A blue color was observed for the deposited films and became more apparent with increasing deposition temperature.

Scanning Electron Microscopy. Scanning electron microscopy (SEM) was used to investigate the morphology of the deposits. As the deposition temperature decreased,


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the coverage of the film also decreased and areas of the substrate could be observed. In AACVD (Figure 2a), deposits grown at 400 and 450 °C displayed a platelet-like morphology with more dense growth observed at 450 °C. Grain sizes for deposits grown at 350, 400, and 450 °C appear to be 140 – 160 nm (Figure S44). Nanorods or nanowires were observed in samples grown at 500 °C with diameters measuring at 250 – 350 nm. Bundled nanorods have been previously observed for deposits grown from similar tungsten oxo-alkoxide complexes at higher temperatures.47

In CVD (Figure 2b),

nanocubes were deposited at temperatures above 350 °C. Nanocube sizes remained about the same for deposits grown at 350 and 400 °C (60 – 80 nm) and increased at 500 °C (100 – 250 nm) (Figure S45). Complete coverage of the substrate was achieved only at 500 °C. An amorphous deposit, as determined by GIXRD, was obtained at 350 °C.


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Figure 2. Plane-view SEM images of deposits grown on Si at 500, 450, 400 and 350 °C from precursor 5 by a) AACVD and b) CVD.

Grazing Incidence X-Ray Diffraction. Grazing incidence X-ray diffraction (GIXRD) was used to determine the crystallinity of the deposited material (Figure 3a-b). The deposits grown by CVD at 350 °C were found to be almost amorphous. At higher growth temperatures, strong diffraction peaks were observed, indicative of highly crystalline material. In the CVD deposits, the most intense peaks at around 23° 2θ were assigned to the (002), (020), and (200) planes of monoclinic WO3 (ICDD 04-007-1277). Other monoclinic and orthorhombic WO3 phases were also matched to the recorded patterns


29


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with similar matching scores. The diffraction patterns for the AACVD deposits grown at 450, 400 and 350 °C also match well with the same crystal structure as the CVD deposits. However, at 500 °C, the diffraction peaks were consistent with monoclinic WO2.83 phase (ICDD 00-036-0103) or orthorhombic WO2.63 phase (ICCD 04-005-5745). Cubic WO3 phases (ICCD 00-046-1096 and 00-041-0905) could also match the strong diffraction lines near 23° 2θ, but the primary crystal structure should either be a monoclinic or orthorhombic phase in order to account for all of the observed peaks. The broadening of the diffraction peaks could be due to the presence of multiple phases of monoclinic, orthorhombic and cubic WO3, with overlapping peaks.

Grain sizes were estimated from line profile analysis to be 12 – 83 nm. These grain sizes are smaller in comparison to the measurements obtained from SEM images. The larger grain sizes from SEM images are most likely due to clusters of several grains which have slightly different crystalline orientations.


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Figure 3. Grazing incidence X-ray diffraction patterns for deposits grown from precursor 5 by a) CVD and b) AACVD with labeled diffraction peaks corresponding to monoclinic WO3 (violet), monoclinic WO2.83 (green), and orthorhombic WO2.63 (blue).

X-Ray Photoelectron Spectroscopy. X-ray photoelectron spectroscopy (XPS) was used to analyze the elemental composition and the chemical bonding of the deposited films. A survey of the as-deposited films revealed that fluorine was not present, indicating that the fluorinated groups dissociate cleanly from the precursor (Figure S36, Supporting Information). The core-level spectra for as-deposited W 4f and O 1s for both CVD and AACVD are shown in Figures 4a-b and 5a-b. The deconvoluted W 4f spectrum showed


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the presence of primarily W6+ with minor sub-stoichiometric W5+ (Figure S41, Supporting Information). The W 4f7/2 binding energies (BE) for W6+ (35.5 eV) and W5+ (34.0 eV) were similar among the films deposited by both methods across all temperatures. Gentle sputtering was used to remove any surface-bound intermediates. Adventitious carbon, with a BE of 284.6 eV, was observed on the material surface and its amount was reduced by sputtering (Figures S37-38, Supporting Information).

As expected, continued

sputtering of the sample preferentially removes oxygen atoms from the material, reducing the W and producing additional W 4f7/2 peaks with BE of 33.3 eV for W4+ and 31.6 eV for metallic W (Figure S42, Supporting Information). The preferential sputtering of O in tungsten oxide has been previously reported in the literature.35 As a result, accurate O:W ratios cannot be obtained from either as-deposited or sputtered samples. However, as determined by GIXRD, the crystalline material deposited was WO3 or very close to this stoichiometry. The presence of WO3 was also confirmed by Rutherford backscattering investigations for deposits that presented a smooth surface morphology (Figure S43, Supporting Information).


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The O 1s peak structure was rather complex. In addition to O2- in the WO3 lattice (530.6 eV), there was also the presence of hydroxides, WOxHy (531.2 eV), C-O-C, C-OH compounds (~532.5 eV) and adsorbed H2O (533.1 eV).48 The relative fractions of these components depended on the surface structure and morphology. After gentle sputtering, the peaks corresponding to water, hydroxides and carbonites decreased in intensity while the peak corresponding to WO3 increased. Continued sputtering resulted in further broadening of the O 1s peak, indicative of the formation of several new types of tungsten oxides due to the reduction of WO3 from Ar ion sputtering (Figure S39-S40, Supporting Information).35 For films deposited at lower substrate temperatures, Si from the substrate appeared in the survey spectra, which corroborated the SEM images that indicated that the deposits do not completely cover the substrate (Figure 2).


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Figure 4. W 4f7/2 and W f5/2 core-level spectra for a) AACVD and b) CVD deposits grown from 5 at 500, 450, 400, and 350 °C.

Figure 5. O 1s core-level spectra for a) AACVD and b) CVD deposits grown from 5 at 500, 450, 400, and 350 °C.


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Conclusions

Tungsten oxo complexes bearing partially fluorinated tert-butoxide and βdiketonate/ketoesterate ligands were synthesized as precursors for CVD and AACVD of tungsten oxide.

Assessment of their thermal properties was performed by

thermogravimetric analysis and determination of sublimation temperatures. Complexes 1, 3-5, 6-7, and 9 sublimed cleanly without any decomposition while 2 and 8 exhibited decomposition concomitant with sublimation.

Mass spectrometry of 3 showed

decomposition pathways that lead to the loss of fluoroalcohol (HOC4H7F3) and trifluoroisobutene (C4H5F3) or loss of β-diketonate (Hhfac) and trifluoroisobutene, with both pathways occurring through proton transfer and cleavage of a C-O bond. Compound 5 was selected as the precursor for AACVD and CVD deposition studies and tungsten oxide was deposited at 500, 450, 400 and 350 °C. Although fully stoichiometric WO3 was deposited by both deposition methods (as determined by GIXRD and XPS), there were some notable differences among the deposits. GIXRD showed that all crystalline samples were monoclinic WO3, except for the sample grown at 500 °C


35


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by AACVD which was assigned to a mixture of orthorhombic-monoclinic phases. Scanning electron microscopy images showed differences in morphology between materials grown by the two deposition methods. In AACVD deposits, nanowires were grown at 500 °C and nanoplates were grown at 450 and 400 °C whereas in CVD deposits, nanocubes were grown at those same temperatures. This difference in morphology suggests the solvent used in AACVD plays a role during film growth. Having the ability to deposit material by both deposition methods from the same precursor allows for not only flexibility in choice of delivery method, but also in the ability to obtain a broadened range of film properties. Further studies probing differences between CVD and AACVD are underway.

ASSOCIATED CONTENT Supporting Information. Experimental procedures, NMR spectra for 2-9, mass spectra for 2-9, thermolysis analysis for 5 and 8, particle size distribution histograms for CVD and AACVD deposits, RBS data for WOx deposition of 5, XPS of WOx deposition of 5.

AUTHOR INFORMATION


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Corresponding Author

*Email: [email protected]

ACKNOWLEDGMENTS

We thank the National Science Foundation for partial support of this work under grant number CHE-1213965 and thank the Laplas Nucleu project for funding.

Mass

spectrometry was performed at the Mass Spectrometry Research and Education Center at University of Florida, supported by grant NIH S10 OD021758-01A1. We also thank Dr. Dan Pantelica and Mrs. Diana Maria Dracea (IFIN-HH, Romania) for the RBS analysis.


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Scheme 1. Synthesis of compounds 1-9. 161x155mm (300 x 300 DPI)


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Scheme 2. a) Proposed decomposition mechanism to produce the observed ions [M-C4H5F3-L+H]+ from 2 and 3, b) Proposed decomposition mechanism to produce the observed ion [M-C4H5F3-HOR+H]+ from 3. 236x151mm (150 x 150 DPI)



Scheme 3. Products of the thermolysis of 5. 200x52mm (300 x 300 DPI)


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Scheme 4. Products of the thermolysis of 8. 208x52mm (300 x 300 DPI)



Scheme 5. Proposed decomposition mechanism to produce the products hexafluoroacetylacetone and hexafluoroacetone from 8. 163x61mm (300 x 300 DPI)


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Scheme 6. Proposed decomposition mechanism to produce the products hexafluoroisopropanol and hexafluoroacetylacetone from 8. 161x64mm (300 x 300 DPI)



Figure 1. TGA plots of compounds a) 1-5 and b) 6-9. 356x151mm (150 x 150 DPI)


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Figure 2. Plane-view SEM images of deposits grown on Si at 500, 450, 400 and 350 °C from precursor 5 by a) AACVD and b) CVD. 356x151mm (150 x 150 DPI)



Grazing incidence X-ray diffraction patterns for deposits grown from precursor 5 by a) CVD and b) AACVD with labeled diffraction peaks corresponding to monoclinic WO3 (violet), monoclinic WO2.83 (green), and orthorhombic WO2.63 (blue).


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Figure 4. W 4f7/2 and W f5/2 core-level spectra for a) AACVD and b) CVD deposits grown from 5 at 500, 450, 400, and 350 °C. 710x315mm (72 x 72 DPI)



Figure 5. O 1s core-level spectra for a) AACVD and b) CVD deposits grown from 5 at 500, 450, 400, and 350 °C.


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TOC graphics 264x264mm (96 x 96 DPI)


Oxo-Fluoroalkoxide Complexes with Partially ... - ACS Publications

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