Low-Temperature Photochemical Conversion of Organometallic

Article pubs.acs.org/cm

Low-Temperature Photochemical Conversion of Organometallic Precursor Layers to Titanium(IV) Oxide Thin Films Patrick C. With,† Ulrike Helmstedt,† Sergej Naumov,† Axel Sobottka,† Andrea Prager,† Ulrich Decker,† Roswitha Heller,† Bernd Abel,†,‡ and Lutz Prager*,† †

Leibniz-Institut für Oberflächenmodifizierung e.V. (IOM), Permoserstrasse 15, D-04318 Leipzig, Germany Universität Leipzig, Wilhelm-Ostwald-Institut für Physikalische und Theoretische Chemie, Linnéstrasse 2, D-04102 Leipzig, Germany

‡

S Supporting Information *

ABSTRACT: Thin layers of titanium(IV) ethoxide [Ti(OEt)4] as a metal−organic precursor were spin-coated onto silicon wafers under inert conditions and subsequently photochemically converted to thin titanium(IV) oxide (TiOx) films employing vacuum ultraviolet (VUV) radiation from a xenon excimer lamp. The photochemical conversion was performed below 35 °C and at ambient pressure in a nitrogen atmosphere with an optimized content of oxygen. Ti(OEt)4 decomposition and its kinetics were monitored and analyzed by gas chromatography and infrared spectroscopy. Precursor layers with a thickness between 270 and 1060 nm could be converted into much thinner TiOx films (40−165 nm). The decrease in thin film thickness was found to coincide with the removal of organic side chains and densification to a compact oxide network. For precursor layers with a thickness of up to 550 nm, VUV irradiation with a moderate radiant exposure (He) of 2.3 J cm−2 led to almost carbon-free amorphous layers with a composition close to stoichiometric titanium dioxide (TiO2) having a density of ∼2.95 g cm−3 determined by X-ray photoelectron spectroscopy and X-ray reflectometry, respectively. In turn, crack-free thin films exhibiting high UV−visible transparency and smooth surface topography were obtained. The highlighted example of Ti(OEt)4 shows that photochemically initiated decomposition of a metal alkoxide is a powerful approach for the generation of thin metal oxide layers at normal pressure and near ambient temperatures. alternative, especially for thin film deposition on flexible and thermally sensitive polymer substrates, such as polyethylene terephthalate (PET), polyethylene naphthalate (PEN), and polycarbonate (PC), because it can be conducted close to ambient conditions.16−18 However, this process needs to be developed with regard to accessible inorganic materials and energy efficient precursor conversion to allow for roll-to-roll processes, e.g., for printed electronics. It was shown by Kim et al.19 that annealing temperatures can be reduced from 350 to 150 °C by applying a photochemically assisted annealing of sol−gel thin films using vacuum ultraviolet (VUV) and far-UV (FUV) irradiation [applying a low-pressure mercury lamp with photon energies (EPh) for VUV and FUV of ∼6.7 eV (185 nm) and ∼4.9 eV (254 nm), respectively], for fabrication of amorphous indium gallium zinc oxide (IGZO) layers as transparent electron conductors. Besides the complete removal of “organics” from the oxide precursors after irradiation for ∼30 min [radiant exposures (He) of 135−201 J cm−2], it was shown that resulting key properties, e.g., density, are comparable to those of high-temperature annealed materials; therefore, comparable electric characteristics were achieved. Advantageously, structuring of solution-processed

1. INTRODUCTION The demand for large-area and flexible metal oxide thin films for applications in low-cost electronics is steadily increasing.1,2 Titanium oxide (TiOx) is a very attractive candidate especially for low-cost and disposable electronics because it is available from quite abundant and inexpensive resources.2 Its importance in heterogeneous catalysis, especially for preparation of photocatalytic active surfaces, is widely known.3 However, a key requirement is the presence of crystalline photocatalytic active TiO2 phases. An application of titanium oxide or Ticontaining multinary oxides in electronics comprises usage in thin film transistors,4,5 components in resistive random access memories (ReRam),6,7 and high-k dielectrics for flexible thin film transistors (TFTs).8,9 Moreover, flexible and optically transparent TiOx thin films, alone or in combination with other metal oxides within mixed compounds or multilayers, can also be applied as layers for gas barriers to protect sensitive electronics from an environmental impact.10 Common approaches to depositing inorganic thin films include chemical or physical vapor deposition methods (CVD or PVD, respectively),11,12 reactive sputtering,7,10 atomic layer deposition (ALD),13,14 and sol−gel techniques.15 However, either high processing temperatures of >350 °C, e.g., in sol−gel methods and atmospheric-pressure CVD, or low pressures, e.g., in most CVD, ALD, and PVD methods, are required. A photochemical conversion approach represents an attractive © 2016 American Chemical Society

Received: July 6, 2016 Revised: October 10, 2016 Published: October 10, 2016 7715

DOI: 10.1021/acs.chemmater.6b02757 Chem. Mater. 2016, 28, 7715−7724

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Chemistry of Materials

dibutyl ether (ACROS organics, >99 wt %, water content of EPh > 6.36 eV; 614 kJ mol−1 < ΔE < 748 kJ mol−1) with the maximum (λmax) at 172 nm (EPh = 7.21 eV; 696 kJ mol−1). At a distance of 3 cm from the lamp surface, the average irradiance was 5.1 mW cm−2 [average radiant exposure per minute (He/t) = 0.31 J cm−2 min−1]. The irradiation chamber was flushed with nitrogen (1.2 m3 h−1) containing 0.25 vol % oxygen (ultra pure plus, Air Products GmbH) to minimize VUV absorption by oxygen molecules and ozone formation on one hand27 and to maintain oxidative conditions on the other, similar to the conditions for the conversion of PHPS to silica published previously.23 Fourier transform infrared (FTIR) spectra of pristine and irradiated Ti(OEt)4 thin films were recorded under a flowing nitrogen atmosphere using a Varian 670 spectrometer (Agilent Technologies, Inc.) in absorbance mode equipped with a variable grazing angle VariGATR unit (Harrick Scientific Products, Inc.) at incident angles of 50−65°. During irradiation, the temperature at the sample surface increased from 25 to 34 °C after 20 min (see Figure S1). Quantitative determination of the degree of Ti(OEt) 4 decomposition is based on the ATR-FTIR spectra obtained after different VUV irradiation times. For this, the IR spectrum of the pristine wafer (background) was subtracted from the IR spectra of spin-coated and irradiated Ti(OEt)4 thin films, followed by a baseline correction and peak fit as shown in Figure S2. Gaseous reaction products were enriched on a solid phase microextraction fiber assembly (Aldrich, DVB/CAR/PDMS polymer fiber coating) and analyzed using a model 6890N gas chromatography (GC) system equipped with an HP-5MS capillary column (Agilent technologies, Waldbronn, Germany; 30 m × 0.25 mm, He carrier gas at 1 mL min−1, splitless pulsed injection for 1 min, isotherm conditions at 250 °C) and coupled with a model 5973N mass selective detector (Agilent technologies, mass detection in the range of m/z 30−300). VUV irradiation of Ti(OEt)4 was performed in a gastight UV cell (Suprasil, 10 mm × 10 mm × 35 mm) with a radiant exposure (He) of 4.1 J cm−2. 2.3. Thin Film Characterization. Scanning electron microscopy (SEM) images of samples were recorded using the ULTRA 55 SEM system (Carl Zeiss SMT, Oberkochen, Germany) at an acceleration voltage of 1 kV. X-ray photoelectron spectroscopy (XPS) was conducted with an AXIS Ultra Probe (KRATOS Analytical Ltd., Manchester, U.K.), equipped with a monochromatized Al Kα X-ray excitation source (150 W, 15 kV/10 mA, spot size of 700 μm × 300 μm). Survey spectra were recorded in the range of 0−1200 eV with a resolution of 1.0 eV. For high-resolution scans in the vicinity of 532 eV (O 1s), 284 eV (C 1s), 100 eV (Si 2p), and 74 eV (Al 2p), the nominal resolution was 0.1 eV with a pass energy of 40 eV. Depth profiles were produced by the removal of material via sputtering with argon ions (0.5 kV, sputter area of 2 mm × 2 mm). Multiple analysis cycles were executed with alternating steps of sputtering (120/150 s per step), which was sufficient for successive removal of the top layer down to the surface of the silicon substrate wafer, and XPS measurements. The thin films structure was investigated using X-ray diffraction (XRD) in Bragg−Brentano geometry with Cu Kα1 radiation (λ = 0.15406 nm; Seifert XRD 3003 TT, Seifert GmbH, Ahrensburg, Germany). Measurements were taken in the 2θ range of 20−70° with a 2θ step width of 0.05° and an accumulation time of 35 s. For

IGZO can be achieved directly on the substrate by light patterning.20 In further studies, the impact of the atmosphere (nitrogen, oxygen-enriched nitrogen, and ozone) during irradiation of sol−gel thin films was investigated.21 Ozone treatment alone without any irradiation results in decomposition of organics. Moreover, a dramatic increase in decomposition rate was observed under VUV irradiation in an oxygen-enriched nitrogen atmosphere. In a recent publication, Park et al.22 reported in more detail about the VUV- and FUV-triggered sol−gel thin film conversion. In this case, the sol−gel precursor film contained water and numerous organic compounds. Overall, the complex precursor mixture makes it difficult to understand the mechanism of dense oxide film formation using VUV- and FUV-assisted processes. In particular, the required radiant exposure for full precursor conversion could not be clarified, which is necessary for evaluation of the technological potential of approaches using UV irradiation, e.g., for roll-to-roll (R2R) printing processes.1 Prager et al.23−25 have shown that chemically pure silica thin films can be obtained from polysilazanes, e.g., perhydropolysilazane (PHPS) as a precursor, using different commercially available VUV and FUV lamps (EPh values of ∼7.2, ∼6.7, and ∼5.6 eV). This photochemically triggered process proceeds at temperatures below 80 °C. For this approach, further studies showed that R2R processes became technically feasible because of the required low-radiant exposures of ∼0.5 J cm−2. In this manner, 100 nm thick PHPS layers can be converted to SiOx (x ≈ 1.8−1.95) with high densities of 2.1 g cm−3 directly on thermally sensitive polymer substrates (PET and PEN) to produce transparent and flexible gas barriers.16 Besides polysilazanes, further studies were performed using a polymeric metal organic aluminum hexanoato complex as a precursor to obtain AlxOy photochemically.26 However, the necessary radiant exposure for complete mineralization was 2 orders of magnitude higher than for PHPS and reaches values as high as 36 J cm−2, which is not beneficial for technical applications such as R2R coating. Thus, a contemporary challenge is the development of this technology with the aim of minimizing the needed radiant exposure for decomposition and mineralization of appropriate organometallic precursors, especially at low processing temperatures. Here we highlight the photochemical decomposition of titanium(IV) ethoxide [Ti(OEt)4] to titanium(IV) oxide using energy rich VUV light (EPh ≈ 7.2 eV). For this, Ti(OEt)4 thin films were spin-coated on silicon wafers, subsequently irradiated under a defined nitrogen/oxygen atmosphere, and studied in turn with regard to their decomposition kinetics by applying variable grazing angle attenuated total reflection Fourier transform infrared spectroscopy (VariGATR-FTIR). The degree of precursor decomposition was correlated with the radiant exposure of a commercially available Xe2* excimer irradiation source. Furthermore, calculations applying density functional theory (DFT) were performed to provide insight into the molecular decomposition reaction path. Prepared TiOx thin films were characterized with regard to chemical composition, thin film thickness, crystallinity, and density by means of X-ray photoelectron spectroscopy (XPS), white light interferometry and/or spectroscopic ellipsometry, X-ray diffraction (XRD), and X-ray reflectivity, respectively.

2. EXPERIMENTAL SECTION 2.1. Thin Film Preparation. Under inert conditions inside a glovebox, a 0.75 mol L−1 solution of Ti(OEt)4 (Merck, 95 wt %) in n7716


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Chemistry of Materials calculation of the average crystallite size (dcrystal), the Scherrer equation (eq 1) was applied. dcrystal =

Kλ β cos(θ)

(1)

X-ray reflectivity (XRR) measurements were performed using a combined high-resolution reflectometer/diffractometer setup (Seifert XRD 3003 PTS, Seifert GmbH) with parallel-beam geometry. X-ray radiation was collimated by a multilayer-gradient X-ray mirror. Cu Kα1 radiation (λ = 0.15406 nm) was exclusively obtained by passing a twobounce Ge(220) crystal monochromator. Measurements were taken in the range between 0° and 1.25° of incidence angles with a step width of 0.002°. The intensity of the radiation that was reflected from the sample was determined with a scintillation detector. UV−vis transmittance and absorbance measurements were taken using a VARIAN Cary 5000 UV−vis−NIR spectrophotometer in the wavelength range of 200−900 nm. The Ti(OEt)4 absorbance spectra were recorded from the liquid precursor pressed between two quartz plates under a nitrogen flow. The optical band gap was determined according to the method of Tauc et al.28 Thin film thicknesses were determined by white light interferometry (WLI) with an F20 instrument (Filmetrics, San Diego, CA). For this, a perpendicular geometry was applied with light in the wavelength range of 380−1050 nm suitable for measurements in the thickness (d) range of 15 nm to 70 μm. Beyond that, thicknesses as well as refractive indices of the spin-coated and VUV-irradiated samples have been measured by spectroscopic ellipsometry (SE) in the spectral range from 400 to 1700 nm at three angles of incidence (65°, 70°, and 75°) for 30 s each, using a model RC2-DI ellipsometer (J. A. Woollam Co., Inc., Lincoln, NE). Water vapor transmission rates of the TiOx thin film-covered PET foils were determined using a PERMATRAN-W 3/61 device (MOCON, Minneapolis, MN) at 38 °C and 90% relative humidity. 2.4. Quantum Chemical Approach. Quantum chemical calculations employing density functional theory (DFT, B3LYP;29,30 Jaguar program package31) helped us to understand the underlying molecular mechanisms. The structures and energies of possible transformations of the molecules under investigation were calculated at the B3LYP/LACVP* level of theory. The LACVP* basis set uses the standard 6-31G(d) basis set for light elements and the LAC pseudopotential32 for heavier elements, such as titanium in this case. Frequency analysis was performed at the same level of theory to characterize the stationary points on the potential surface and to obtain the total enthalpy (H) and Gibbs free energy (G) at the standard temperature of 298.15 K using unscaled vibrations. The reaction enthalpies (ΔH) and Gibbs free energies of reaction (ΔG) were calculated as the difference in the calculated H and G between reactants and products, respectively. Additionally, to improve our understanding of observed experimental IR spectra, frequency analysis was conducted on possible structures, which could be built during transformations of studied geometries after excitation by UV photons (EPh = 7.2 eV). The electronic excitation energies were calculated using the Time Dependend4 (TD) DFT method33 at the B3LYP/ LACVP* level of theory.

Figure 1. Proposed reaction scheme for the photochemical conversion of Ti(OEt)4 precursor thin films to titanium(IV) oxide using vacuum UV irradiation (hv).

the formation of Ti−OCO-ring structures as intermediates on the other. Further irradiation causes the nearly complete removal of carbon and the formation of Ti−O−Ti bonds comparable to the outcome of a high-temperature thermal treatment (film densification). The reaction pathway will be described in detail below. 3.1. Photochemical Conversion. Obtained IR spectra for the spin-coated precursor layers showed all characteristics of Cbased vibrations as found for liquid Ti(OEt)4 (see Figure S3). IR bands in the spectrum of Ti(OEt)4 in the 3000−800 cm−1 region could be assigned according to the method of Finnie et al.34 (Table 1). Spin-coated precursor thin films were found to Table 1. Band Positions and Assignments for IR Spectra of the Ti(OEt)4 Precursor34 wavenumber (cm−1) 3000−2800 1480−1300 1080−1200 1067 and 995 1044 1013 919 and 910

assignment ν(C−H) stretch of CHx δ(C−H) deformation of CHx ρ(CH3) in- and out-of-plane rocking modes of both terminal and bridging ethoxide ν(C−O) stretch of terminal ethoxide ν(C−O) stretch of bridging ethoxide ν(C−O) stretch of both terminal and bridging ethoxide ν(C−C) stretch of terminal ethoxide

be stable under a flowing nitrogen/oxygen atmosphere for up to 30 min but in the presence of traces of water readily hydrolyze within a few minutes (see Figures S4 and S5). Therefore, IR spectra were recorded under a flowing nitrogen atmosphere. The Xe2* excimer lamp generates photons with energies of ∼7.21 eV (696 kJ mol−1), enough for photoinitiated radical bond cleavage reactions of C−C (348 kJ mol−1), C−O (358 kJ mol−1), and C−H (413 kJ mol−1) bonds in the organic ligands of Ti(OEt)4. Furthermore, 0.25 vol % oxygen was added to the nitrogen flow during irradiation, with the aim of finding the optimum between the establishment of oxidative conditions on one hand and minimization of absorption on the optical pathway above the sample surface on the other.23 Obtained IR spectra for irradiated Ti(OEt)4 thin films with thicknesses (d) of 270 ± 60 nm (Figure 2) show that all CHx- and C−O-based vibrations vanished within 240 s; i.e., organics were mostly removed because of VUV irradiation. Here it should be noted that under a pure nitrogen atmosphere even after VUV irradiation for 450 s no significant conversion was observed (see Figures S6 and S7).

3. RESULTS AND DISCUSSION To explain the formation of dense TiOx thin films by VUV irradiation, we propose the reaction pathway illustrated in Figure 1, which is based on experimental results from ATRFTIR spectroscopy, X-ray photoelectron spectroscopy, ellipsometry, and scanning electron microscopy and on theoretical calculations using DFT. VUV irradiation of thin Ti(OEt)4 layers using a Xe2* excimer lamp leads to a complete decomposition of the organometallic precursor and conversion to amorphous inorganic titanium(IV) oxide thin films. The presence of additional oxygen during irradiation under a nitrogen atmosphere was found to dramatically increase the reaction kinetics. The high photon energy of the Xe2* excimer radiation allows the cleavage of alkoxy groups on one hand and 7717


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where A is the absorption, I0 is the intensity of the incident light at the sample surface, and I is the intensity of the light at the position of measurement. Furthermore, a much longer irradiation time of 3600 s [radiant exposure (He) of 18.5 J cm−2] is needed for the decomposition of IR active CHx and C−O vibrations (see Figure 3 and Figure S9). Moreover, the disappearance of these vibrations and, thereby, the completeness of the thin film conversion suggest the penetration of the whole thin film by the 172 nm VUV photons. The measured data of the C−O bond scission initiated by VUV photons can be fit by a first-order law (correlation r2 > 0.98) with apparent rate constants of 2.1 × 10−2, 1.5 × 10−2, and 0.9 × 10−2 s−1 (at He/t = 0.31 J cm−2 min−1) for precursor film thicknesses of 270, 550, and 1060 nm, respectively. According to the Lambert−Beer law, the absorbance of light within the absorbing layer decreases following a logarithmic function. Therefore, the logarithmic apparent rate constant, ln(kapp), shows a linear dependence with an increase in the Ti(OEt)4 film thickness, as shown in the inset of Figure 3. Because the precursor Ti(OEt)4 was found to form trimeric and tetrameric configurations,35−38 terminal and bridged ethoxy groups are present. In addition, it was reported by Biechel et al.39 that both functionalities possess a different reactivity, with a more basic character of bridging OR groups. A discrimination of these ethoxy groups by IR vibrations at 1067 cm−1 (terminal ethoxy) and 1044 cm−1 (bridging ethoxy) allows us to compare separately their degrees of photochemical decomposition. Interestingly, the different reactivities of both bridging and terminal ethoxy groups seem to have no significant influence on the decomposition kinetics (see Figure S10). Besides the disappearance of C−O vibrations, new vibrations appear in the IR spectra for irradiated and only partially converted precursor thin films in the range of 1670−1220 cm−1, with two most intensive peaks centered at 1550 and 1446 cm−1, respectively (Figure 2). Therefore, an intermediate product has been characterized by asymmetric and symmetric ν(OCO) stretching vibrations in a comparable wavenumber range as found typically for Ti-based carboxylates,40,41 most probably Ti−OCO-ring structures. Moreover, their peak intensity was found to reach a maximum after irradiation for 15 s (for 270 nm precursor thin films) when 40% of the precursor was converted. This peak disappears after longer exposure times until complete removal after 240 s, together with all stretching vibrations ν(CO) at 1200−980 cm−1 and ν(CHx) at 2800−3100 cm−1. Formation of Ti−OCO-ring structures was further supported by analysis of gaseous reaction products via gas chromatography−mass spectrometry measurements (see Figure S11) that gave as main photochemical final products 2-butanone, methoxyacetone, and ethyl acetate, besides ethanol. Here it should be noted that irradiation under a pure nitrogen atmosphere also results in the formation of the Ti−OCO-ring structures mentioned above. This suggests that Ti−OCO-ring structures can be formed by excitation of the Ti(OEt)4 tetramer. However, the presence of additional oxygen is needed to fully decompose the Ti(OEt)4 and Ti− OCO-ring structures within short irradiation times. Note that in a pure oxygen atmosphere Ti(OEt)4 also decomposed (see Figure S12). However, in this case, the apparent rate constant was found to be 1 order of magnitude smaller (0.14 s−1 vs 1.5 × 10−2 s−1), which can be attributed to a nearly complete VUV light absorption by oxygen above the sample.27,42

Figure 2. ATR-IR spectra for VUV-irradiated 270 ± 60 nm Ti(OEt)4 precursor thin films [average radiant exposure per minute (He/t) of 0.31 J cm−2 min−1].

Ti(OEt)4 decomposition kinetics was obtained assuming that the peak area of C−O-based vibrations between 1200 and 980 cm−1 represents the remaining precursor content. To minimize the influence of different film thicknesses and positions on the ATR stage, C−O peak areas were normalized to their initial peak areas before irradiation. Quantitative evaluation of 270 ± 60 nm thick Ti(OEt)4 precursor films revealed that during a short irradiation time of 60 s, corresponding to a radiant exposure (He) of 319 mJ cm−2, more than 92% precursor decomposition could be achieved (Figure 3). As a consequence

Figure 3. Area of normalized IR peaks for C−O-based vibrations in the range of 1200−980 cm−1 (AIR 1200−980 cm−1) after VUV irradiation (tirradiation) and different radiant exposures (He) and a logarithmic plot of the initial reaction rates (kapp) for different precursor thin film thicknesses [dTi(OEt)4] (inset) [average radiant exposure per minute (He/t) of 0.31 J cm−2 min−1].

of increasing the precursor film thickness to 1060 ± 80 nm, the conversion at 60 s was halved down to ∼48% (see Figures S8 and S9). This can best be understood as a consequence of the decreasing VUV light intensity within the precursor film (top to bottom within the layer), according to the Lambert−Beer law (eq 2) A = log

I0 I

(2) 7718


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Figure 4. Energetically favorable reaction pathways for the formation of Ti−OCO-ring structures. ΔH and ΔG are given in kilocalories per mole.

Table 2. Band Positions and Assignments of the IR Vibrations from DFT Calculations and from FTIR Spectra of Irradiated Ti(OEt)4 Thin Films wavenumber (cm−1) calculated

experimentally observed

geometry

ν(OCO)symmetric

ν(OCO)asymmetric

Δν

ν(OCO)symmetric

ν(OCO)asymmetric

Δν

D1T D1• D2T D2•

1460 1460 1513 1513

1628 1624 1582 1586

168 164 69 73

1446

1550

104

3.2. Insight into the Reaction Pathway. To support the idea of intermediate Ti−OCO-ring structures, vibration frequencies of possible intermediate geometries were investigated with DFT. Various structural discussions in the literature seem to indicate that the amorphous thin film consists of a mixture of oligomeric structures of two to four monomers.38,43,44 This discussion is shown for the tetrameric structure as one representative example. The bond lengths and angles of the optimized tetrameric ground state structure of the starting material used in this theoretical study agree well with the experimental data observed from single-crystal X-ray structure determinations of titanium(IV) alkoxides given in the literature.37 Energetically favorable reaction pathways for formation of Ti−OCO-ring structures are shown in Figure 4. Because of interaction of the tetramer in the ground state with UV light, the excited singlet AS* state will be formed. After excitation and after the lowest excited state S1, which was calculated to be E(S1) = 4.02 eV (309 nm) above the ground state energy, had

been reached, it can undergo intersystem crossing (ISC) into triplet state AT with an E(T1) of 2.68 eV. With this reactive state as a starting point, two energetically favorable reaction pathways, namely, bimolecular and monomolecular reactions leading to structures B• and BT, have been considered. Radical B• can be formed through hydrogen abstraction by one tetramer in triplet state AT from a second tetramer in its ground state, AS. On the other hand, BT can be built in the triplet state through an intramolecular H-shift from one ligand (R) to an oxygen atom from another ligand. Both structures B• and BT can conceivably yield Ti−OCO-ring structures as shown in Figure 4. The best correlations of theoretically predicted IR vibrations that belong to Ti−OCO-ring structures with experimentally observed absorptions in the infrared wavelength region were obtained for structures D1• and D2• as well as D1T and D2T. The D1 geometries dominate bicyclic structures containing a [-Ti-OCO-Ti-O-] six-ring motif (Figure 4, red) formed by intramolecular reaction of ligand C-centered radicals with neighboring ligands at a different titanium atom. D2 7719


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photochemical precursor conversion rather than a VUVinduced dehydroxylation of titanium(IV) oxide obtained from hydrolysis. In contrast, Kim et al.19 reported that hydroxyl groups in IGZO sol−gel precursor coatings can be condensed with photons from FUV irradiation, having even less energy than VUV irradiation applied herein. However, this could be observed only by a combination of irradiation and elevated reaction temperatures of >150 °C, which should be avoided. Moreover, the TiOx thin film density determined using XRR reached values as high as 2.95 g cm−3 for photochemically obtained layers (see Figure S15), which is much higher than for only hydrolyzed ones, ∼2.55 g cm−3. However, these densities are lower than in the case of a crystalline TiO2 anatase phase at ∼3.8 g cm−3. Despite the decrease in film thickness due to densification, the SEM pictures showed a smooth and crackfree surface topography for obtained TiOx thin films (Figure 6).

geometries result from reaction of these radicals with neighboring ligands at the same titanium center and are characterized by four-ring [-Ti-OCO-Ti-] structures (Figure 4, blue). All multistep reaction pathways leading to ring structures D1• and D2• as well as D1T and D2T are calculated to be exergonic and should be energetically favorable. It appears that calculated IR vibrations that are characteristic for Ti−OCO-ring structures compare well to the experimentally observed vibrations in the IR spectra (see Table 2). Calculated vibrational frequencies depend strongly on the structural motif formed (four- or six-ring structure) but only slightly on the electronic state of the molecule (triplet state or radical). Thus, Δν values (difference in symmetric and asymmetric stretching vibration frequencies) of the calculated geometries are even comparable to known values from ground state IR spectra obtained from experiments.40 3.3. TiOx Thin Film Characterization. After photochemical conversion of Ti(OEt)4, the obtained TiOx thin film thicknesses were determined using white light interferometry and ellipsometry. Complete photochemical conversion of ∼550 ± 60 nm thick precursor thin films resulted in densification to 75 ± 5 nm thick TiOx layers (refractive index n = 1.91). The increasing level of densification can be followed for stepwise converted precursor layers as shown in Figure 5 (corresponding

Figure 6. SEM surface images with different magnifications of a photochemically obtained TiOx thin film surface on (a and b) a silicon wafer and (c) a thin film cross section.

This is in agreement with XRR measurements giving very small values for surface roughness of ∼0.4 nm, i.e., 1 order of magnitude lower than for amorphous TiO2 thin films obtained by magnetron sputtering.10 The photochemically obtained TiOx thin films with thicknesses between 100 and 160 nm are shown to be noncrystalline (Figure 7). As a reference, hydrolyzed layers subsequently calcined at 600 °C possess reflexes for the crystalline anatase phase (the average crystallite size was found to be ∼30 nm). Besides the complete photochemical precursor conversion and thin film densification, the degree of carbon removal and thin film homogeneity are important features for the suitability of TiOx layers as transparent gas barriers or for semiconductors in optoelectronics. Chemical states of Ti(2p), O(1s), and C(1s) at the surface of TiOx thin films were examined using XPS. Because oxidative conditions were applied for the photochemical conversion of Ti(OEt)4, titanium is present in its Ti4+ oxidation state with a binding energy of 459 eV, without any indication of VUV-induced reduction to metallic Ti (binding energy of 454 eV) (Figure 8a).46 Furthermore, XPS spectra showed similar O(1s) binding states for both photochemically and thermally obtained TiOx thin films (Figure 8b), with two predominant peaks at 531 eV (O2− in

Figure 5. Plot of overall TiOx thin film thicknesses (dTiOx) vs applied radiant exposure (He). Thicknesses were determined using ellipsometry for Ti(OEt)4 thin films irradiated for different periods of time followed by subsequent hydrolysis of unconverted organics (squares) and for hydrolyzed precursor thin films that were irradiated afterward (circles). Hydrolysis was conducted overnight under ambient conditions.

ATR-IR spectra can be found in Figures S13 and S14). Note that before film thickness measurements were taken under ambient conditions, the remaining nonconverted precursor was hydrolyzed to form titanium(IV) oxide.45 Therefore, the thin film is composed of portions of TiOx from both photochemical conversion and hydrolysis. In addition, the overall TiOx thickness decrease proceeds exclusively during the disappearance of C−O-based vibrations observed in the IR spectra (see Figures 3 and 5). To understand this behavior, further experiments were conducted using hydrolyzed Ti(OEt)4 thin films. Hydrolyzed but nonirradiated thin films showed a thickness of ∼95 ± 5 nm (n = 1.73). Even after a subsequent VUV irradiation up to a radiant exposure (He) of 9.3 J cm−2, the overall film thickness remained unchanged (Figure 5). This suggests that thin film densification is a direct result of 7720


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285 eV, C−O−C of 286.5 eV, and O−CO of 289 eV, which are typical for adsorbed organic carbon contaminants on top of the thin film,47 most probably because of the transfer of the sample from the glovebox to the XPS facility. However, depth profiling of the elemental composition using stepwise Ar+ sputtering and XPS measurement showed that the carbon concentrations within the thin films are much lower, even 80%, the crack-free smooth surface and the low reaction temperature are advantageous for an application of TiOx thin films on polymers as gas barrier films. TiO2 is known to be a dielectric material itself with a dielectric constant between 60 and 80 (high-k dielectric), depending on crystal structure and preparation conditions.50,51 Besides the mostly studied polycrystalline materials, amorphous (glassy) TiO2 is increasingly attractive because grain boundaries and therefore high leakage currents can be avoided. In addition to the low content of remaining carbon and the Ti/O stoichiometry of 1/2 (indicating fewer oxygen defects that could result in carrier traps and leakage paths), photochemically derived TiOx is an attractive candidate for applications in ReRams6,7 or TFTs.4,5,8,9 Furthermore, the low processing temperature is a key factor for deposition of TiOx on thermally sensitive substrates, e.g., polymers.

Figure 9. UV−vis transmittance (T) spectra for TiOx obtained from hydrolysis (dTiOx = 92 ± 5 nm) as well as from photochemical conversion (He = 3.6 J cm−2; dTiOx = 64 ± 5 nm) of a Ti(OEt)4 thin film and the corresponding Tauc plot (inset).

photochemically prepared TiOx exhibits a high transmittance, especially at short wavelengths in the range of 350−400 nm. Considering the shifted absorption edge, the determined optical band gap (Figure 9) of amorphous TiO2 is much higher than for its crystalline phase analogue (3.55 eV vs 3.2 eV for the anatase phase). 3.4. Assessment of Technological Potential. A challenging task is the comparison of the photochemical Ti(OEt)4 conversion highlighted here with other photochemical methods and precursors in the literature, because information about reaction conditions is scarce, especially details regarding photon energy, radiant exposure of the samples, reaction/process temperature, and atmosphere composition. However, conversion of Ti(OEt)4 can be compared to that of PHPS23,48 and that of a polymeric hexanoato aluminum complex26 with regard to an applied photon energy of 7.2 eV (Figure 10). For these, the reaction temperatures were close to ambient conditions. Here it should be noted that because of different lamp powers and distances

4. CONCLUSIONS AND OUTLOOK In summary, we have shown that amorphous titanium(IV) oxide thin films can be photochemically prepared at ambient temperature and pressure. In the photoconversion process, Ti(OEt)4 as the metal−organic precursor was converted into thin films by employing high-energy vacuum UV irradiation (7.2 eV) in an oxygen-enriched nitrogen atmosphere. The precursor decomposition kinetics, which depends on the thin film thickness, was investigated in detail. Besides ethanol as the main organic precursor decomposition product, formation of intermediate Ti−OCO structures was indicated by IR spectroscopy and by formation of alkyl acetates found in the gas phase. In addition, accompanying DFT calculations suggested that four- and six-ring structures are included in energetically favored photochemical reaction paths of Ti(OEt)4. Overall, precursor thin films with thicknesses ranging from 270 to 1060 nm could be fully converted in a few minutes to much denser 40−165 nm thick TiO2 thin films already at low radiant exposures (He) of 1−5 J cm−2. Advantageously, the decrease in thickness results in smooth and crack-free TiO2 surface topographies together with bulk densities of up to 2.95 g

Figure 10. Comparison of VUV-induced (using Xe2* excimer irradiation; EPh ≈ 7.2 eV) decomposition of different metal−organic precursor (XPrecursor) thin films depending on the applied radiant exposure (He) close to ambient temperature and pressure in an oxygen-enriched nitrogen atmosphere [dTi(OEt)4 = 550 ± 60 nm; d[H2SiNH]n = 120 ± 20 nm; d[Al(OH){O2C(CH2)4CH3}]n = 79 ± 0.2 nm; 0.25 vol % O2]. 7722


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Chemistry of Materials cm−3, much higher than for hydrolyzed thin films. Thin film composition was found to be homogeneous and nearly carbonfree throughout the layer with a stoichiometry close to TiO2, which is comparable to titanium dioxide obtained from thermal treatment at 600 °C. Further studies should be devoted to studying the influence of different precursor ligands as well as acid and base catalysts in further enhancing reaction kinetics, resulting in lower radiant exposures and, therefore, allowing energy efficient and fast rollto-roll processes. It can be assumed that the low-temperature photochemical conversion of organometallics to TiOx thin films shown here can be transferred to the generation of further metal oxide and mixed metal oxide thin films, as well, leading to an interesting alternative method for applications in electronics and as gas barrier layers on thermally sensitive polymer substrates.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.chemmater.6b02757. Further ATR-FTIR data for irradiated thin films, a gas chromatography−mass spectrometry profile for gaseous reaction products, a precursor UV−vis spectrum, WVTR measurements, and X-ray reflectometry measurements for TiOx thin films (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The work leading to these results has received funding from the European Union Seventh Framework Programme (FP7/20072013) under Grant Agreement n° 604000. The authors thank Michael Mensing for conducting the XRR measurements, Nadja Schönherr for conducting the UV−vis measurements, and Carsten Bundesmann for discussions about spectroscopic ellipsometry measurements.

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