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Reactive Chemical Vapor Deposition Method as New Approach for Obtaining Electroluminescent Thin Film Materials
The new reactive chemical vapor deposition (RCVD) method has been proposed for thin film deposition of luminescent nonvolatile lanthanide aromatic carboxylates. This method is based on metathesis reaction between the vapors of volatile lanthanide dipivaloylmethanate (Ln(dpm)3) and carboxylic acid (HCarb orH2Carb′) and was successfully used in case of HCarb. Advantages of the method were demonstrated on example of terbium benzoate (Tb(bz)3) and o-phenoxybenzoate thin films, and Tb(bz)3 thin films were successfully examined in the OLED with the following structure glass/ITO/PEDOT:PSS/TPD/Tb(bz)3/Ca/Al. Electroluminescence spectra of Tb(bz)3 showed only typical luminescent bands, originated from transitions of the terbium ion. Method peculiarities for deposition of compounds of dibasic acids H2Carb′ are established on example of terbium and europium terephtalates and europium 2,6-naphtalenedicarboxylate.
A search for new electroluminescent (EL) materials for organic light-emitting diodes (OLEDs) is still an actual challenge for chemists. The solution of it means not only finding a compound which fits the EL material requirements but also the right choice or creation of a technique providing the transformation of such precursors into the thin films of high smoothness and low thickness. This can be demonstrated on examples of lanthanide coordination compounds which are well-known potential luminescent materials [1–5]. However, it is not that simple to find among them the luminescent lanthanide coordination compound, which demonstrates simultaneously high thermal and UV stability, bright luminescence, while being volatile or possesses high solubility to deposit thin film of high quality by physical methods from gas phase or solutions. In case of highly volatile β-diketonates the obvious physical deposition technique allows to obtain thin films of high quality, and their luminescence efficiency can be increased by variation of the substituents in ligands. However these compounds do not exhibit required UV stability. A different situation arises from lanthanide aromatic carboxylates which show high UV, thermal, and chemical stability, as well as high luminescence efficiency, which can reach 100% in case of Tb(bz)3 [6–9], but for these compounds the choice for the film deposition technique among known methods appears rather impossible. The aromatic carboxylates form extremely rigid 3D networks [10–12], which make them nonvolatile and poorly soluble in organic solvents that complicate deposition of their thin films from gas phase or solution and cause the necessity for search or development of the new methods. Thus, we have proposed the reactive chemical vapor deposition (RCVD) technique for thin films of nonvolatile luminescent lanthanide aromatic carboxylates (HCarb—aromatic monocarboxylic acid) [13, 14].
New RCVD method is based on the gas phase reaction between volatile Ln(dik)3 (Hdik = β-diketone) and HCarb (reaction (1)):
The development of RCVD method involved two main steps: (1) study of general principles and (2) elicitation of reaction (1) parameters influencing the quality of thin films deposited (transparency, roughness, thickness, and coating continuity). Lanthanide dipivaloylmethanates, Ln(dpm)3, were selected as precursors because they are known as the most volatile and thermally stable nonfluorinated Ln(dik)3; their vapor consists of mononuclear species only, and temperature dependencies of the saturated vapor pressure have been determined for compounds of all rare earth elements (REES) [15, 16]. Among monocarboxylic aromatic acids the series of HCarb was first selected to study the gas phase reaction (1), namely, benzoic acid (Hbz) and its o-substituted derivatives: salicylic acid (HSal), anthranilic acid (Habz), N-phenylanthranilic acid (HPA), and o-phenoxybenzoic acid (Hpobz), which differ by chemical stability and volatility . These experiments were run within a horizontal quartz glass reactor with two temperature zones: for more volatile HCarb evaporation and for less volatile Ln(dpm)3 evaporation and film deposition (Figure 1).
Variation of both and values allowed us to control the reagents partial vapor pressure ratio during the gas phase reaction. The vapors of HCarb and Ln(dpm)3 converged in the zone , their interaction occurred, and nonvolatile products deposited on substrate and reactor walls in this hot zone. The composition of products deposited was evaluated by elemental, IR, 1H NMR and luminescent spectroscopy analyses. The following main features of the proposed RCVD method were established: (1) gas phase ligand exchange reaction (1) takes place with a formation of Ln(Carb)3 product; however there is a risk of its incompleteness, which causes product contamination by the Ln(Carb)3-x(dpm)x impurities; (2) to exclude the formation of these impurities the ratio between the partial vapor pressures of HCarb and Ln(dpm)3 should be >3 : 1; (3) the temperature of the reaction/deposition area should be sufficient to overcome the activation barrier of the reaction but it should not be higher than the temperatures of the decomposition of reactants and product; (4) Ln impurities can be removed by reaction (2) under thermal treatment of the gas phase reaction product in the range of Ln(Carb)3 stability :
In this reactor the evaporation zones (, ) and film deposition zone () were separated, and reagents were transported separately into the deposition zone by a stream of carrier gas. The deposition parameters of transparent thin films with uniform composition, morphology, and thickness in the range of 50–100 nm were found for Tb(bz)3 and Tb(pobz)3. In order to analyze the composition of the thin films the special analytical approach, which corresponded to the combination of luminescent and Raman spectroscopy, was suggested [14, 18]. It worth noting that the temperatures (, , ) within the RCVD reactor play a crucial role in the composition, transparency, and morphology of Tb(Carb)3 thin films. Therefore, transparent Tb(Carb)3 thin films can be obtained under excessive pressures of HCarb (p(HCarb) : p(Tb(dpm)3) 3), which is set by ratio, and value must be enough for activation of the reaction (2). The variation of Tb(Carb)3 thin film thickness from 200 nm to 40 nm is possible by simply decreasing the amount of the reagents within the evaporation zones. Once these principles were fulfilled, it allowed us to deposit high-quality thin films of Tb(bz)3 and Tb(pobz)3 .
Here we present new results on EL test of Tb(bz)3 thin films within OLED heterostructure and estimation of proposed RCVD method applicability for deposition of terbium and europium carboxylate thin films derivatives of dibasic acids H2Carb′ such as terephtalic (H2tph) and 2,6-naphtalenedicarboxylic (H2nda) acid.
2. Results and Discussion
2.1. Tb(bz)3 as EL Layer within OLED
Thin films of Tb(bz)3 deposited using RCVD method in three temperature zone reactor (Figure 2) were successfully used in the OLED structure such as glass/ITO/PEDOT:PSS/TPD/Tb(bz)3/Ca/Al. EL spectra of Tb(bz)3 showed only typical luminescent bands, originated from transitions of the terbium ion 5D4 → 7 (), while no emission of hole transporting layers in the blue region was observed (Figure 3).
Thus the efficiency of the RCVD technique was demonstrated for deposition of the thin films for nonvolatile Tb(Carb)3 complexes using highly volatile monobasic aromatic carboxylic acids (HCarb) as a precursors. The Tb(bz)3 films deposited were of such high quality that their use in the OLED structure led to the bright Tb-centered electroluminescence without showing any sublayers luminescence.
2.2. RCVD Method Applicability for Thin Film Deposition of Tb and Eu Complexes with H2Carb′
Some lanthanide compounds with aromatic dibasic acids H2Carb′ could be prospective candidates for EL materials due to their bright luminescence and extremely high thermal stability [6, 9]. Terbium and europium derivatives of terephtalic (H2tph) and 2,6-naphtalenedicarboxylic (H2nda) acids are among the most promising candidates among these compounds; thus they were selected for study of the RCVD applicability for thin film deposition of nonvolatile lanthanide complexes with dibasic acids. The solid state carboxylates Eu2(tph)3(H2O)4 (I), Tb2(tph)3(H2O)4 (II), Eu2(nda)3(H2O)6 (III), and Tb2(nda)3(H2O)6 (IV) were synthesized according to the traditional method , and their luminescent properties and thermal stability were characterized to use them as standards for identification of the gas phase reaction products.
2.2.1. Luminescent Properties and Thermal Stability of the Ln2(Carb′)3(H2O)n Solid Samples (I–IV)
Luminescent properties of the compounds I–IV were studied in comparison with those of H2tph and H2nda. Luminescence spectra of I and II (Figure 4) show only typical ion luminescent bands for both complexes, originated from the electronic transitions of the terbium or europium ions. No emission in the blue region was observed, which is the result of complete energy transfer from the organic part to the central ion. Besides a typical Stark splitting of the terbium emission bands is observed in the spectrum of II.
In case of Ln2(nda)3(H2O)6 only europium complex (III) shows ion luminescence, while in spectrum of terbium complex (IV) there are no terbium luminescence bands present, and a broad band in blue region, originated from the ligand emission, is observed. The hypsochromic shift and narrowing of this band in comparison with H2nda luminescence band originate from the acid deprotonation and complexation with terbium ion.
The energy of nda2− triplet level was estimated using the gadolinium compound Gd2(nda)3(H2O)6 phosphorescence spectrum. Unlike europium and terbium complexes, whose luminescence can be sensitized by coordinated ligands, gadolinium compounds do not emit visible light. However, the phosphorescence spectra of these complexes show signals resulting from triplet to singlet transitions and are thus reflecting the triplet state energies of the coordinated ligands . According to the luminescence spectrum of Gd2(nda)3(H2O)6 (Figure S2) (see supplementary material available online at http://dx.doi.org/10.1155/2012/809028) excited state level energies of nda2− are S1(nda2−) = 24200 cm−1, (nda2−) = 18400 cm−1.
The simplified energy diagram (Figure 5) show that energy of the nda2− triplet level is high enough only for energy transfer according to the scheme (nda2−)→5D0(Eu3+), while no energy transfer to 5D4(Tb3+) can occur.
In the excitation spectrum of H2tph two bands are observed (~280 and ~330 nm) (Figure 6), which also presented in the spectra of I and II. Besides in the spectra of I and II the low intensity direct ion excitation bands at 360–420 nm and 350–370 nm, correspondingly, are observed. In the excitation spectrum of H2nda there are three bands (~280 nm, ~370 nm, and ~425 nm). First two bands are also present in the spectrum of III, while the band at 425 nm is absent, which results from the absence of the energy transfer via the corresponding triplet state. The excitation spectrum of compound IV was recorded in the range of 200–350 nm since the maximum of its luminescence is 390 nm, and thus only the band at ~280 nm could be measured.
Thermogravimetric (TG) curves of I–IV were recorded in argon atmosphere and contain two steps of weight loss in all cases: the first step of water molecules elimination and the second step of Ln2(Carb′)3 decomposition (Figure 7).
In case of I and II (Carb′= tph) the residues contained lanthanide oxides Eu2O3 and Tb4O7, while III and IV (Carb′ = nda) could not decompose completely in the inert atmosphere, and the residue most probably is contaminated with carbon.
2.2.2. Gas Phase Synthesis
The reaction providing the gas phase synthesis of Ln2(Carb′)3 should take place according to (3): Brightly luminescent compounds I, II, and III were selected for these syntheses with the aim of their luminescent thin films deposition, but initially reactions (3) were performed in the reactor with two temperature zones (Figure 1).
A choice of the temperature regime providing course of the reaction (3) was based on analysis of the relations between volatility and stability of the starting reagents and products. The stability of Tb2Carb′3 against thermal decomposition, determined from thermal analysis data, is rather high: I, II are stable until 400°C in inert atmosphere, III—until 600°C (Figure 7). H2tph is stable at least until >400°C even in air . This data is not available for H2nda; however it can be purified by sublimation at 370°C in vacuum (0.01 mmHg) without decomposition, and thus it can be concluded that it is stable at least until this temperature. The data on the dependence between the vapor pressure and temperature for H2tph and H2nda is absent in the literature, and therefore volatility of these acids was estimated using thermal analysis in inert atmosphere in comparison with Hbz (Figure 8). For Hbz saturated vapor pressure temperature dependence is known , and empirical relation between the temperature range of its evaporation in gas phase reaction (1) and half weight loss temperature () in TG curve was established.
Thus, the thermogravimetric curves (Figure 8) indicate that the values increase in the row Hbz (180°C) < H2tph (350°C) < H2nda (390°C), which reflects the volatility row. For Hbz it was shown [14, 23] that temperature of the acid evaporation zone should lie in the range of ()°C, and it means that for H2tph and H2nda the values of evaporation temperature could not be less than 250 and 300°C, respectively. According to the mass spectrometry data, Ln(dpm)3 molecules are stable in gas phase only up to ~290°C, though the Ln-containing species with low partial pressures are present in the gas phase until at least 540°C . Thus, Ln(dpm)3 complexes are the least thermal stable reagents that can prevent carrying out reaction (3).
The diprotic nature of the H2Carb′ acids can also hamper reaction (3). Earlier we have already studied the gas phase synthesis of the lanthanide complexes with diprotic ligands on example of yttrium complex with a Schiff base H2Salen (from ethylenediamine and salicylic aldehyde) . It was shown that only a mixed-ligand complex of the composition Y(dpm)(Salen) was formed in both gas phase and solution interactions of Y(dpm)3 and H2Salen even in the excess presence of the last one. We proposed that it resulted from the weak acidity of H2Salen (pKa~11-12, ) contrary to HCarb ( for Hbz) and the formation of two six-membered chelate rings with yttrium ion.
The dibasic acids selected in this work are strong ( and for H2tph; and for H2nda), and thus we expect that Ln2Carb′3 as a product of the reaction (3) should form.
For the gas phase reaction less volatile H2Carb′ were placed in the hotter zone and more volatile Ln(dpm)3—in the colder zone . For gas phase synthesis of I the following initial conditions were selected based on both thermal analysis for H2tph (Figure 8) and vapor pressure temperature dependences for Eu(dpm)3 data : (Eu(dpm)3) = 160°C and (H2tph) = 250°C. Probably the value of was too low for sufficient vapor pressure of H2tph, and thus the product was not formed. The repeated experiment with (Eu(dpm)3) = 160°C and (H2tph) = 300°C led to a product formation, but its quantity was extremely low, and it was formed as a thin film (i film) on the surface of glass substrate and reactor walls. The experiment on terbium terephthalate deposition at the same values of and led to ii film formation.
The emission spectra of both i and ii films show a broad fluorescence signal with the maximum at ~360 nm besides the ion luminescence of Eu3+ and Tb3+, which is illustrated in Figure 9 on example of ii film. At the same time excitation spectra demonstrate blue shift of the band edges in comparison with the spectra of I and II concomitantly with the absence of the band at >350 nm, corresponding to the dpm−-ligand (Figure 9) and thus revealing that the most probable composition of thin films corresponds to the Ln2Carb3(H2Carb)x. The product of the same composition was also formed by the reaction (3) in solution (see Supplementary Information), which annealing at 360°C led to pure Ln2Carb3 formation. However, during gas phase synthesis it was not possible to increase the in order to exclude the acid codeposition into the film, since this temperature is limited by the Ln(dpm)3 stability in the gas phase. Therefore the annealing procedure was used to remove the H2tph impurity from i and ii films. Films annealed at 360°C for 1 hour did not contain the acid impurities according to both excitation and emission spectra, and their composition corresponded to Ln2(tph)3.
The explanation of the blue shift of the excitation spectra band edges for the thin films containing H2tph can be found from the estimation of the singlet and triplet energy levels of terephtalic acid from emission spectra of H2tph (Figure S3). The obtained value of the triplet energy level (Figure 10) shows that in case of europium ion the ligands triplet states and europium resonance levels lie in the row (tph2−) > (H2tph) > 5D1(Eu3+) (Figure 10), which means that presence of terephtalic acid should simplify the energy transfer from tph2− to europium ion. However according to the recorded spectra H2tph presence is most likely preventing the energy transfer to Eu3+; thus we can conclude that H2tph triplet state does not at all participate in the energy transfer processes, which could be described according to the scheme, presented in Figure 10.
In the case of terbium containing ii film two possible mechanisms of H2tph impact on the terbium ion luminescence can be proposed: (1) either H2tph triplet level can compete with terbium resonance level 5D4 during the energy transfer processes or (2) the mechanism can be the same as in case of i film. Since the observed profiles of excitation spectra of i and ii films are the same (Figure 11), we can suggest that the scheme of H2tph participation in the energy transfer in case of terbium compound is similar to that one for europium compound (Figure 10). Elimination of H2tph after annealing results in the band edge for thin films being identical to the reference solid state samples I or II (Figure 9) and proves the formation of the pure Ln2(tph)3 films.
Since volatility of H2nda is lower than of H2tph, the gas phase reaction (3) with H2nda was performed at higher temperature (H2nda) = 370°C and the same temperature (Eu(dpm)3) = 160°C, which led to the iii film deposition. According to the luminescence spectra (Figure 12) this film was also contaminated with codeposited acid H2nda, which was removed after annealing at 370°C for 1 hour.
Thus the metathesis reaction (3) between Ln(dpm)3 and dibasic acids H2Carb′ takes place in the gas phase, which was shown on example of H2Carb′ = H2tph, H2nda. However, due to the low volatility of these acids it is necessary to significantly increase the temperature of their evaporation in order to assure the vapor pressure. Simultaneously the raise of this temperature leads to the risk of Ln(dpm)3 thermal decomposition in hot zone of the reactor. This mismatch of the temperature ranges of Ln(dpm)3 stability in gas phase and H2Carb′ volatility results in the low yield of the product and the unsteadiness of the gas phase reaction, which cannot be excluded even by using the three-zone reactor. Thus gas phase reaction (3) takes place but is significantly hampered by the low volatility of H2Carb′.
In conclusion we presented here the reactive chemical vapor deposition (RCVD) method for thin film deposition of nonvolatile aromatic carboxylates, based on metathesis reaction between volatile Ln(dpm)3 and aromatic carboxylic acids. The advantages of this method for deposition of thin films of lanthanide derivatives of the highly volatile monobasic acids were demonstrated on examples of terbium benzoate and o-phenoxybenzoate. The thin films of high quality were deposited which allowed us to use them for OLED manufacturing on the example of Tb(bz)3-based OLED with the structure glass/ITO/PEDOT:PSS/TPD/Tb(bz)3/Ca/Al. The RCVD method was examined for deposition of the thin films of luminescent lanthanide derivatives of low-volatile dibasic carboxylic acids H2tph and H2nda. Here, the use of this method is complicated by the low volatility of the selected acids due to the mismatch of the temperature ranges of Ln(dpm)3 stability in gas phase and H2Carb′ volatility. Thus RCVD method is perspective for highly volatile acids derivatives deposition and is significantly hampered in case of low volatile acids.
4. Experimental Part
Commercial dipivaloylmethane (Hdpm, Fluka) and pure for analysis grade terbium, europium, and gadolinium nitrate hexahydrates, sodium hydroxide (NaOH), and terephtalic acid (H2tph, Acros organics) were used as received. Naphthalenedicarboxylic acid (H2nda) and benzoic acid (Hbz) were purified by vacuum sublimation (0.01 mmHg) at 370 and 80°C, respectively.
The elemental analysis of complexes was carried out on a C,H,N-analyzer of the Organic Chemistry Division, Chemical Department, Lomonosov Moscow State University. Thermal analysis was carried out on a thermoanalyzer STA 409 PC Luxx (NETZSCH, Germany) in the temperature range of 20–1000°C in argon atmosphere, heating rate 10°/min. Photoluminescence and excitation spectra were measured on Perkin-Elmer LS-55 luminescence spectrometer utilizing with a xenon lamp with a tunable wavelength as an excitation source at 25°C as well as multichannel spectrometer S2000 (Ocean Optics) with a nitrogen laser LGI-21 ( nm) as an excitation source at 77 K. All emission and excitation spectra were corrected for the instrumental functions. The triplet state energy was estimated from maximum of the broad not resolved phosphorescence band in Gd2(nda)3(H2O)6 and H2tph luminescence spectra at 77 K, distinguished from the second short-wave band, attributed to fluorescence (Figures S2 and S3).
The complexes Ln(dpm)3·2H2O (Ln = Eu, Tb) were synthesized and characterized by a standard procedure [15, 16]. Anhydrous Ln(dpm)3 were obtained by sublimation of the hydrated complexes in vacuum (0.01 mmHg) at 120–150°C.
OLED manufacturing took place in a clean room class 10000 (Lebedev Physical Institute, Moscow, Russia) in a glovebox with argon atmosphere. Thin films of PEDOT:PSS with thickness ~30 nm were obtained by spin-coating. TPD layer was thermally evaporated (Univex-300, Leybord-Heraeus) under a pressure below 10−6 mmHg. The thickness was ~40 nm controlled by quartz indicator. Emitting layer (Tb(bz)3) was deposited using RCVD technique at (Tb(dpm)3) = 125°C, (Hbz) = 140°C, = 250°C. The contacts were attached to the electrodes, and the device was sealed with epoxy resin. Electroluminescence spectra were measured on aPicoQuant time-correlated single photon counting system used as a conventional spectrofluorimeter. Spectral resolution was 4 nm.
4.1. Synthesis of Ln2(Carb′)3 (Ln = Eu, Tb; H2Carb′ = H2tph, H2nda)
Ln2(Carb′)3·nH2O (n = 4, 6) were synthesized by a standard procedure  by the reaction of water solutions of Tb(NO3)3·6H2O (1 mmol) and of K2(Carb′) (1.5 mmol, from KOH and H2Carb′).
Eu2(tph)3(H2O)4(I) : for Eu2C24H20O16 (M = 868) anal. calcd. (%): C, 33.2; H, 2.3. Found (%): C, 30.7; H, 2.5.
IR spectrum, cm−1 : 3200–3600 ((O–H)), 3065 ((C–H)), 2982 ((C–H)), 2900 ((C–H)), 1590 ((C–C)), 1541 ( (COO)), 1505, 1410, 1420.
Tb2(tph)3(H2O)4(II): for Tb2C24H20O16 (M = 882) anal. calcd. (%): C, 32.7; H, 2.4. Found (%): C, 32.7; H, 2.6.
Raman spectrum, cm−1: 3067 (C–H), 1612 s. (C–C arom.), 1529 (), 1456 s. (), 1310, 1145, 869 s., 629.
IR spectrum, cm−1: 3200–3600 ((O–H)), 3063 ((C–H)), 2983 ((C–H)), 2903 ((C–H)), 1587 ((C–C)), 1541 ((COO)), 1504, 1402, 1424.
Eu2(nda)3(H2O)6(III): for Eu2C36H30O18 (M = 1054) anal. calcd. (%): C, 40.8; H, 2.9. Found (%): C, 40.8; H, 2.8.
Raman spectrum, cm−1: 1632 (C–C arom.), 1487 (COO-), 1439 (COO-), 1394, 1122, 784, 523.
IR spectrum, cm−1: 3200–3600 ((O–H)), 3059 ((C–H)), 2970 ((C–H)), 2913 ((C–H)), 1605 ((C–C)), 1540 ((COO)), 1490, 1409, 1358.
Gd2(nda)3(H2O)6: for Gd2C36H30O18 (M = 1064) anal. calcd. (%): C, 40.0; H, 2.8. Found (%): C, 40.8; H, 2.8.
IR spectrum, cm−1: 3200–3600 ((O–H)), 3060 ((C–H)), 2973 ((C–H)), 2910 ((C–H)), 1607 ((C–C)), 1544 ((COO)), 1493, 1410, 1353.
Tb2(nda)3(H2O)6(IV) : for Tb2C36H30O18 (M = 1068) anal. calcd. (%): C, 39.1; H, 2.8. Found (%): C, 40.5; H, 2.9.
IR spectrum, cm−1: 3200–3600 ((O–H)), 3073 ((C–H)), 2887 ((C–H)), 2829 ((C–H)), 1602 ((C–C)), 1544 ((COO)), 1490, 1410, 1360.
4.2. Gas Phase Reaction of Ln(dpm)3 and H2Carb′ (H2Carb′= H2tph, H2nda; Ln = Eu, Tb)
The gas phase reaction of Ln(dpm)3 with appropriate H2Carb′ was carried out under reduced pressure (0.01 mm Hg) in a horizontal glass reactor with two temperature zones ( and ) (Figure 1). The unreacted volatile reactants were collected in the zone maintained at room temperature. The nonvolatile reaction products condensed in the reactor zone with the temperature .
The time of experiments was 60 min. The values of temperatures of the reaction zones and masses of the reagents are summarized in Table 1.
The authors thank the Ministry of Education and Science of Russian Federation (contract no. 16.516.11.6071) for the financial support.
Supplementary Materials contain the details of the solution interaction of Tb(dpm)3 and H2tph and the details of the nda2- and H2tph triplet levels estimation.
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