Article pubs.acs.org/IC
Molecular Precursors for ZnO Nanoparticles: Field-Assisted Synthesis, Electrophoretic Deposition, and Field-Effect Transistor Device Performance Rudolf C. Hoffmann, Shawn Sanctis, and Jörg J. Schneider* Fachbereich Chemie, Eduard-Zintl-Institut, Fachgebiet Anorganische Chemie, Technische Universität Darmstadt, Alarich-Weiss-Straße 12, 64287 Darmstadt, Germany S Supporting Information *
ABSTRACT: Zinc complexes with multidentate Schiff base ligands are suitable precursors for ZnO in microwave-assisted transformation reactions. [Bis(acetylacetonato)ethylenediimine]zinc(II) and [bis(methylacetoacetato)ethylenediimine]zinc(II) have been synthesized with high purity and good yield from the direct reaction of the respective diimine ligand with diethylzinc in tetrahydrofuran. The thermal decay is studied by thermogravimetry coupled with online infrared spectroscopy. The ceramization reaction in ethoxyethanol yields stable dispersions of spherical ZnO nanoparticles with very small particle sizes (around 5−6 nm), which can be employed for coating and thin-film deposition processes. Field-effect transistors (FETs) composed of thin films fabricated from these semiconducting ZnO particles possess charge-carrier mobilities of 6.0 × 10−3 and 5.4 × 10−2 cm2/(V s) after processing at 350 and 450 °C, respectively. Electrophoretic deposition affords dense film coatings composed of these ZnO nanoparticles with thicknesses of 30−90 nm on ITO (indium tin oxide) glass-electrodes. The positive ζ-potentials of the ZnO nanoparticles in these dispersions are in agreement with the electrocoating process at the cathode.
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are ideal for depositing the functional layers of field-effect transistors (FETs) or for producing dense coatings by electrophoretic deposition (EPD). In another investigation on EPD, Bai and co-workers examined zinc oxide nanoparticles on indium tin oxide (ITO) coated glass for use as an intermediate layer for charge injection in organic light-emitting diodes.12 EPD has also been suggested for the deposition of Co-doped ZnO on fluorine tin oxide as the active layer in an electrocatalytic device.13 In the present work, we investigated the suitability of zinc complexes containing multidentate chelating Schiff base ligands such as [bis(acetylacetonato)ethylenediimine]zinc (1) or [bis(methyl-acetoacetato)ethylenediimine]zinc (2) as precursors for ZnO. As far as we know, these zinc compounds have not been investigated in microwave-assisted decomposition reactions before. The semiconducting properties of the resulting ZnO particles were determined and tested in a functional field-effect transistor (FET) device. Further experiments concerned the behavior in EPD and characterization of the coatings obtained therefrom. Possible side reactions such as the formation of metallic zinc or corruption of the ITO electrode material were studied and could be excluded by X-ray
INTRODUCTION The synthesis of dispersions of metal oxides with particle sizes in the nanometer regime can be significantly facilitated by employing microwave-heating.1 Thereby, the energy input is introduced directly in the reaction medium without a pronounced heating gradient from the walls to the interior of the reaction vessel.2 This technique allows a high degree of control over the particle formation process and can be further improved by employing molecular precursors for the synthesis of metal oxides. The oxidic oxygen is supplied either by the ligand framework of the precursor itself (single source precursor concept)3 or by the solvent.4 The addition of further reactive compounds, such as surface capping agents providing steric, electrostatic, or electrosteric stabilization, to inhibit hard or soft agglomerates is not necessarily required, as residues from the ligand and the solvent will remain on the particle surface after the ceramization which can fulfill this purpose.5,6 The ligand is thus a valuable means of control for regulating and adjusting the primary crystallite size,7 agglomerate formation,8 as well as morphology9 and even structure10 of the resulting oxide particles. Therefore, thermodynamically and kinetically stable chelate complexes attract special interest. Recently, we reported about the use of zinc compounds with 1,3-diketonates for the synthesis of very small (i.e., with a diameter below 10 nm) zinc oxide nanoparticles.11 Such small particles exhibit a high packing density in film formation and © 2017 American Chemical Society
Received: May 2, 2017 Published: June 9, 2017 7550
DOI: 10.1021/acs.inorgchem.7b01088 Inorg. Chem. 2017, 56, 7550−7557
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Inorganic Chemistry
FET Measurements. The design of the FET substrates (Fraunhofer IWS-Dresden, Germany) and details concerning data collection and evaluation (HP 4155A semiconductor parameter analyzer) were published earlier.11 Slides (1.5 × 1.5 cm2) consisted of n-doped silicon with SiO2 dielectric (90 nm). Gold electrodes (40 nm) had an interdigital structure (channel width W 10 mm, channel length L 10 μm) to avoid fringing current artifacts. Dispersions of ZnO nanoparticles were prepared in the aforementioned microwave reaction from 0.50 and 0.66 wt % solutions of precursor (2) and by then evaporating 2/3 of the solvent with a cold trap. ZnO films were deposited by spin-coating (20 s at 2000 rpm) and annealing on a hot plate (250−450 °C) in air. This process was repeated 5 times to obtain a sufficient thickness. The calcination was carried out over a period of 4 min, except for the last annealing step which lasted 10 min. Electrophoretic Deposition. Electrodes (1.5 × 6 cm2) consisted of a layer of ITO (140 nm) on glass (0.4 mm). ZnO dispersions were synthesized by microwave reaction from 0.33, 0.50, and 0.66 wt % suspensions of 2. The electrodes had a distance of 1 cm and were immersed (∼4 cm) into 40 mL of the ZnO dispersion in a glass vessel. EPD was carried out at different deposition times (30−120 min) and voltages (20−40 V). The ZnO coating formed on the negative electrode (cathodic deposition), whereas no obvious deposition was observed on the counter electrode.
photoelectron spectroscopy (XPS) as well as Auger depth profiling.
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EXPERIMENTAL SECTION
Precursor Synthesis. Synthesis of N,N′-bis(acetylacetone)ethylenediimine was as follows: The reaction was carried out following the procedure of Chimpalee et al.14 Instead of using ethanol, the product was recrystallized twice from methanol and washed with methyl butyl ether. Found: C, 64.13; H, 10.03; N, 12.40%. Anal. Calcd for C12H24N2O2: C, 63.12; H, 10.59; N, 12.27%. 1H NMR (500 MHz, [D6]dimethyl sulfoxide, 25 °C): δ 1.90 (COCH3), 1.93 (NC CH3), 3.40 (NCH2), 5.07 (CH) ppm. 13C{1H} NMR (500 MHz, [D6]dimethyl sulfoxide, 25 °C): δ 18.82 (COCH3), 28.56 (NC CH3), 42.87 (NCH2), 95.08 [NCC(COO)], 162.90 (NC), 193.19 (CO) ppm. Synthesis of N,N′-bis(methylacetoacetate)ethylenediimine was as follows: The reaction was carried out following the procedure of Hofmann et al.15 Found: C, 55.94; H, 9.10; N, 10.73%. Anal. Calcd for C12H24N2O4: C, 55.36; H, 9.28; N, 10.76%. 1H NMR (500 MHz, [D6]dimethyl sulfoxide, 25 °C): δ 1.91 (CH3), 3.19 (NCH2), 3.50 (OCH3), 4.50 (CH) ppm. 13C{1H} NMR (500 MHz, [D6]dimethyl sulfoxide, 25 °C): δ 18.90 (NCCH3), 43.12 (NCH2), 49.37 (OCH3), 81.55 [NCC(COO)], 162.04 (NC), 169.54 (CO) ppm. Synthesis of [bis(acetylacetonato)ethylenediimine]zinc (1) was as follows: Bis(acetylacetone)ethylenediimine (2.28 g, 10 mmol) was suspended in 80 mL of tetrahydrofuran. (Caution! The escape of pyrophoric gaseous diethylzinc f rom the reaction vessel should be avoided by working in a f ume hood.) Diethylzinc (1 M in hexane, 10 mL, 10 mmol) was added carefully. The evolution of gas started immediately and continued for about 2 h. The mixture was stirred overnight, and the product was collected by filtration and dried under vacuum. Yield 2.78 g (95.3%). Found: C, 49.43; H, 7.82; N, 9.22%. Anal. Calcd for ZnC12H22N2O2: C, 49.41; H, 7.60; N, 9.60%. Mass spectrometry m/z: 111 (100%), 243 (74), 286 (73), 292 (M+). (The complete mass spectrum is shown in Figure S1 in the Supporting Information.) Synthesis of [bis(methylacetoacetato)ethylenediimine]zinc (2) was as follows: Bis(methylaceto-acetate)ethylenediimine (3.90 g, 15 mmol) was suspended in 80 mL of tetrahydrofuran. (Caution! The escape of pyrophoric gaseous diethylzinc f rom the reaction vessel should be avoided by working in a f ume hood.) Diethylzinc (1 M in hexane, 10 mL, 10 mmol) was added carefully. The mixture became clear after 2 h, and the product started to precipitate after another 4 h. The suspension was stirred overnight. The product was collected by filtration and kept in a cabinet dryer at 75 °C for 12 h. Drying at slightly elevated temperatures was necessary to remove THF, which was otherwise incorporated in the form of a solvate giving products of indefinite composition. Yield 3.90 g (80.3%). Found: C, 45.11; H, 6.37; N, 8.44%. Anal. Calcd for ZnC12H22N2O4: C, 44.53; H, 6.85; N, 8.65%. 1H NMR (500 MHz, [D3]acetonitrile, 25 °C): δ 1.86 (CH3), 3.20 (NCH2), 3.47 (OCH3), 4.50 (CH) ppm. 13C{1H} NMR (500 MHz, [D3]acetonitrile, 25 °C): δ 22.06 (NCCH3), 43.02 (N CH2), 50.08 (OCH3), 78.57 [NCC(COO)], 170.94 (NC), 174.58 (CO) ppm. Mass spectrometry m/z: 96 (79%), 127 (100), 191 (48), 288 (32), 318 (78), 324 (M+). (The complete mass spectrum is shown in Figure S2.) Microwave Reaction. The synthesis of the dispersions was carried out in a microwave reactor (Discover; CEM, Germany) in 35 mL glass vials. Compound 2 was suspended in ethoxyethanol (0.05, 0.075, and 0.10 g per 15 g of the solvent, representing 0.33, 0.50, and 0.66 wt % solutions) and the mixture heated to 75 °C whereby a clear solution formed after 1 min. The vial was further heated to 205 °C and this temperature maintained for 1 min. The total process required about 8 min. For the isolation of solid ZnO particles, the solvent was removed in a rotary evaporator, which yielded a brownish residue. After this was dissolved in tetrahydrofuran and precipitated with pentane, an offwhite powder formed, which was separated by centrifugation and dried at 100 °C for about 3 h.
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RESULTS AND DISCUSSION
Complexes of metals with the ligands N,N′-bis(acetylacetone)ethylenediimine and N,N′-bis(methylacetoacetate)ethylenediimine have been known for decades.16 The structures of these complexes exhibit great variety depending on the nature of the central metal atom. Nickel(II)17,18 and copper(II)19 form mononuclear compounds [Ni−L] or [Cu−L], in which the metal atom possesses a square planar coordination. Further molecules or anions can attach to the central atom in mononuclear complexes with iron(III)20 or cobalt(III)21 and thus lead to higher coordination numbers. In contrast, oligomers were obtained with zinc, whereby dimers (Zn− L)222 and tetramers (Zn−L)423 were characterized by the structure determination of single crystals. The zinc atoms show tetrahedral coordination by the [N2O2] ligands, which causes the formation of polynuclear entities. The synthesis of nickel and copper complexes could be carried out by the addition of metal salts (preferably acetates) to the Schiff base adduct. Surprisingly, the analogous reaction with zinc salts is not possible with many Schiff base ligands and leads to an incomplete conversion with products containing larger amount of the educts. As an alternative, the use of diethylzinc was suggested for the reaction with multidentate Schiff base ligands.24 In the present work (Scheme 1), this procedure was employed successfully and further optimized.25 The precursors 1 and 2 were obtained with high purity and good yields. A complete thermal analysis using TG/MS−IR (Figure 1a,b) was performed in order to study the ceramization of the precursors 1 and 2. Both compounds showed a gradual decay in oxygen which started above 200 °C and ended at about 450 °C. The onset of the decomposition was slightly lower in the case of 2 and was accompanied by a sharper step. The courses of the mass-loss curves of 1 and 2 appeared very similar at first glance. An analysis of the reaction gases with IR spectroscopy (Figure 2 as well as Figures S3 and S4), though, revealed slightly different gaseous reaction products. In all cases, the volatile decomposition products comprised a mixture of compounds and were very difficult to analyze. The composition of the gases corresponding to the first (I in Figure 1a) and second (II in Figure 1a) maxima of the Gram−Schmidt curve seemed similar 7551
DOI: 10.1021/acs.inorgchem.7b01088 Inorg. Chem. 2017, 56, 7550−7557
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Inorganic Chemistry Scheme 1. Reaction Scheme for the Synthesis of Precursors 1 and 2a
a
Oligomers are represented as multiples of monomeric units for simplification.
Figure 2. IR spectra of the evolved gases at the denoted maxima of the Gram−Schmidt signals in Figure 1: (a) I, (b) III, (c) IVa, and (d) VI. The spectra of IVa and IVb are almost identical.
moiety remains unclear, however, as only ammonia was detected. In a related study of the volatile decomposition, products by MS fragments with an intact diamine unit were proposed. This attribution could, however, be ambiguous as a plethora of products are formed.35 Surprisingly, a detailed investigation does not seem to be available to date. In any case, it became clear that Schiff base condensation is not simply reversed, but that both 1 and 2 undergo a complex decomposition, in which the ligand framework converts into a number of smaller fragments. The composition of the solid residues in air was analyzed by means of XRD (Figure S5). In all cases, only reflections from wurtzite were observed (JCPDS 36-1451). The simple thermal decomposition in air of metal complexes with multidentate Schiff base ligands was already reported in previous works, but did not seem to offer advantages with respect to other precursors as hard agglomerates were formed.36,37 In contrast, the thermolysis of 1 and 2 in solution in microwave-assisted reactions, which are the central part of this work, offers for the first time a process which benefits from the inherent structural and thus tunable ligand situation of this type of molecule and yields dispersions of ZnO nanoparticles, which are stable against flocculation or sedimentation. The central aim in the synthesis of nanoparticle dispersions was to obtain a high content of ZnO and at the same time avoid the formation of larger agglomerates. Both aspects are required to employ the dispersions in the fabrication of uniform coatings. We found that following this paradigm led to a very narrow window in which stable dispersions could be obtained. The solubility of 1 in many organic solvents such as ethoxyethanol was drastically lower in comparison to that of 2. Thus, no sufficiently high loadings of ZnO were to be expected from this precursor, and no further investigations
Figure 1. Mass-loss curves () and Gram−Schmidt signals (---) for the decomposition in oxygen of (a) 1 and (b) 2.
for 1. Apart from carbon dioxide (670 and 2730 cm−1)26 and ammonia (1097 cm−1),27 only acetone (530, 1215, 1435, 1731, and 2972 cm−1)28 could be identified. Further contributions exist between 1700 and 1400 cm−1 which hint at further products, but could not be attributed. In the case of 2 (corresponding to IVa,b and V in Figure 1b), dimethyl carbonate (1290, 1432, and 1780 cm−1) and methanol (1030 cm−1)29,30 were present (Figure S4a−d). At higher temperatures (i.e., III in Figure 1a and VI in Figure 1b), only carbon dioxide and carbon monoxide (2220 cm−1),31 as well as methane (3010 cm−1)32 and ammonia, could be detected for both 1 and 2 (Figure 2b,d). These findings are in accordance with earlier investigations by other groups which suggested that acetone is formed during the decomposition of the nickel complex of 2.33 In another study, rather unspecific fragments such as methane could also be detected.34 The mechanism of the decay of the enamine 7552
DOI: 10.1021/acs.inorgchem.7b01088 Inorg. Chem. 2017, 56, 7550−7557
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Inorganic Chemistry were carried out in this work. In the case of 2, dissolving the precursor at an intermediate temperature first and then carrying out the decomposition to zinc oxide in a second step at high temperature were attempted (Figure S6). Reaction temperatures lower than 200 °C did not lead to the formation of zinc oxide, whereas temperatures higher than 210 °C resulted in very turbid dispersions. The best results were obtained by dissolving 0.33−0.66 wt % of precursor 2 in ethoxyethanol at 75 °C and observing decomposition at 205 °C. Following this protocol, we obtained clear or only slightly turbid dispersions with slightly yellow color. All dispersions discussed in this work were fabricated in this way. Concentrations of the precursor higher than 0.66 wt % lead again to turbidity or sedimentation. The size distributions of the ZnO nanoparticles in the dispersions were determined by DLS. In all cases, the dispersions contained a larger fraction in the range 7−12 nm and a minor fraction of particles with a larger size in the range 15−30 nm (Figure 3). The ζ-potential was slightly positive for
Figure 4. HRTEM image showing agglomerates of nanocrystalline zinc oxide obtained from microwave processing of precursor 2 with a concentration of 0.66 wt %. The inset images show the corresponding SAED pattern (upper left) and isolated nanoparticles (upper right, scale bar 10 nm).
Figure 5. IR spectra of (a) the solid residues from the microwaveassisted decomposition of 2 with a concentration of 0.66 wt %, (b) a reference sample of precursor 2 itself, and (c) ethoxyethanol.
Figure 3. (a) Size distribution as determined by DLS (dynamic light scattering) of dispersions from the microwave-assisted decomposition of 2 with different starting concentrations of the precursor. (b) ζpotential as a function of starting concentration of the precursor.
functional groups, however, were not related to the ligand framework of either the precursor or the solvent. Field-Effect Transistors. The dispersions of the ZnO nanoparticles showed good film-forming properties when employed in spin-coating. This property was used to build FETs, in which the ZnO served as a semiconductor. The overall device setup was a bottom gate/bottom electrode arrangement (Figure S9) with highly doped silicon as gate and support, silicon dioxide acting as dielectric, and gold electrodes. The ZnO film was calcined at temperatures of 250, 350, and 450 °C, sequentially, after spin-coating. This additional annealing step was necessary to achieve a reasonable performance. A summary of the performance parameters (charge-carrier mobility μSAT, on/off ratio Ion/off, and threshold voltage Vth) determined from the measurements is depicted in Figure 6a. Higher calcination temperatures led to an increase of the charge-carrier mobility, while the threshold voltage decreased. The on/off ratio reached
all precursor concentrations. A higher amount of organic residues seemed to afford higher absolute values. Solid residues could be obtained from the dispersions by removal of the solvent under rotary evaporation of the solvent. XRD revealed that nanocrystalline zinc oxide (Figure S7) was formed in all cases. The crystallite sizes as determined from Scherrer’s equation were in the range 5−6 nm. No other crystalline phases could be detected. HRTEM images showed aggregates of nanoscale crystallites (Figure 4), which is in accordance with the good film-forming properties of the dispersions. Individual crystallites became evident at higher magnifications confirming the size determination of about 6 nm from the XRD (Figure S8). IR spectra (Figure 5) of ZnO particles, which were not subjected to additional annealing, exhibited signals which can mainly be attributed to the solvent (i.e., ethoxyethanol). Other 7553
DOI: 10.1021/acs.inorgchem.7b01088 Inorg. Chem. 2017, 56, 7550−7557
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an attractive way to better FET performance. Accordingly, the combination of very small ZnO particles (∼3−4 nm) and additional unreacted precursor in methanol resulted in ZnO films with superior charge-carrier mobilities up to 1.75 cm2/(V s).42 In a similar approach, we reported the synthesis of ZnO particles with a diameter of ∼5−6 nm in acetonitrile, which showed excellent charge-carrier mobilities of 0.32 cm2/(V s) in FETs.11 The performance in the current example (Figure 6b,c) was determined as μ 0.006 cm2/(V s) with Ion/off 140 000, and μ 0.054 cm2/(V s) with Ion/off 95 000 after processing at 350 and 450 °C, respectively. As discussed in earlier publications, the influence of the additional annealing after the spin-coating is manifold. Apart from the removal of organics and residual solvent, changes in the defect chemistry occur.11,43 Sintering cannot be completely excluded, but is highly unlikely at such low temperatures.39 The employment of lower-boiling-point solvents for the deposition of semiconductors is considered advantageous.43 Since higher calcination temperatures did not show any significant effects on the performance, we conclude that grafting of the surface with organic functionalities is predominant. This finding was reported by various groups and in this work verified by means of IR spectroscopy and photoluminescence.4,9,11,39 Electrophoretic Deposition. The nanoparticle dispersions were also applicable for the formation of coatings using the EPD process (Figure 7a). In a series of experiments, the best
Figure 6. (a) Performance parameters (charge-carrier mobility, blue ◆; threshold voltage, green ▲; and on/off ratio, red ●) of FETs obtained from the spin-coating of ZnO particles synthesized by the microwave-assisted reaction of precursor 2 (0.50 wt % in ethoxyethanol) and annealing at various temperatures. Output and transfer characteristic refer to the sample with a calcination temperature of 350 °C. (b) Output characteristics obtained from variation of the drain-source voltage from 0 to 30 V, for gate-source voltages from 0 to 30 V, in 5 V steps. Data were acquired for increasing as well as decreasing drain-source voltages. (c) Transfer characteristics for a constant drain-source voltage of 30 V (μ 5.98 × 10−3 cm2/(V s), Vth +10.1 V, Ion/off ∼140 000).
a maximum value after the species was annealed at 350 °C, but declined drastically after being annealed at 450 °C. An assessment of these results with those of previous works needs to consider that, apart from the manufacturing process, setup and geometry of the FET also have considerable effects on the electronic performance. Thus, sol−gel procedures which require much higher annealing temperatures (above 500 °C) yield charge-carrier mobilities as high as 5−6 cm2/(V s).38 Several examples for the manufacturing of ZnO thin films from the deposition of particles were reported in earlier works. Generally, larger particles proved disadvantageous. Aggregates (of about 50 nm diameter) from the microwave-assisted decomposition of a precursor showed a charge-carrier mobility μ of 0.045 cm2/(V s),39 and ZnO rods (65 nm length) of comparable size yielded 0.023 cm2/(V s).40 Notably, a postdeposition treatment which filled larger voids raised the mobility to 0.65 cm2/(V s).40,41 This strategy seemed to offer
Figure 7. (a) Schematic depiction of the coating formation by EPD. (b) Film thickness determined by means of ellipsometry for coatings deposited by EPD at different voltages and deposition times.
conditions for obtaining uniform films were examined. The cathodic formation of films of ZnO, using dispersions from the microwave reaction of 0.50 wt % 2 in ethoxyethanol, occurred at dc voltages of 20−40 V and with deposition times of up to 120 min. Such films adhered well to the substrate and exhibited interference colors. Longer deposition times and higher voltages did not lead to satisfactory results, as films from 7554
DOI: 10.1021/acs.inorgchem.7b01088 Inorg. Chem. 2017, 56, 7550−7557
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Inorganic Chemistry
attributed to the exciton emission, further broader signals were present in the visible regime, which comprised at least two contributions with maxima at ∼525 and ∼685 nm.44−46 These findings were in accordance with those of films consisting of nanocrystalline ZnO. No major contributions from the organic surface capping of the nanoparticles could be detected. Finally an Auger depth profile (Figure 9) was recorded to characterize the chemical composition of a cross section of the
such experiments were rather thick and peeled off of the substrate after drying. The thinner adherent coatings were investigated by ellipsometry (Figure 7b), which revealed that films of up to 100 nm thickness had formed. Further characterization by SEM (Figure 8 and Figure S10) showed
Figure 9. Auger depth profile of ZnO films deposited by EPD at 40 V for 120 min from dispersions obtained from the microwave-assisted reaction with precursor concentrations of 0.50 wt %. The glasssubstrate was not reached.
sample. The part of the layer deposited by EPD, which was closest to the surface, contained a larger amount of carbon. Within the layer, still, a significant amount of carbon could be detected. This is in agreement with the earlier finding that the particles synthesized in the microwave reaction possess a surface capping.9 Apparently, these organic residues are included in the film during the EPD process. The interface between the EPD-ZnO layer and the ITO is well-defined and becomes visible in the sigmoidal shape of the curves. This indicates that the ITO layer is not corrupted in the EPD process. Investigations by X-ray photoelectron spectroscopy, XPS, gave further proof for the deposition of ZnO particles by EPD (Figure S12). The measured Zn 2p3/2 spectrum could be accurately fitted using two different peak components (Figure S12a). The contribution with the lower binding energy of 2021.8 eV corresponds to Zn in crystalline ZnO. The other component at 2023.3 eV could be identified as Zn in a mixed environment of oxide and carboxylate anions.45 Correspondingly, the O 1s signal (Figure S12b) consists of three contributions related to oxygen in the form of oxide (531.1 eV), hydroxide (532.54 eV), and presumably carboxylate (533.61).47 A very weak N 1s peak (Figure S12d) hints at further residues originating from the ligand framework. This is in accordance with the aforementioned IR investigations of ZnO (Figure 5) which gave evidence for a surface functionalization of the ZnO particles synthesized in the microwave-assisted decomposition reaction of the precursor as well as with the Auger studies which also support these findings.
Figure 8. SEM top view of coatings obtained after 120 min with a deposition voltage of (a) 20, (b) 30, and (c) 40 V.
crack-free and homogeneous surface coverage. No larger agglomerates or other defects in the as-deposited layers were observed. Films obtained by employing higher voltages and longer deposition times appeared to be porous, though. This is in accordance with the fitting procedure of the thickness determination by ellipsometry which indicated a density between 20% and 25% of the theoretical value. The optical properties of the ZnO films from EPD were investigated by photoluminescence spectroscopy. Hereby, spectra of films obtained from dispersions with different precursor concentrations were compared (Figure S11). In addition to a sharp signal in the ultraviolet range, which was
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CONCLUSIONS Coordination compounds of zinc with multidentate Schiff base ligands are suitable molecular precursors for zinc oxide nanoparticles. The ceramization can be initiated by microwave-heating in protic, polar solvents such as ethoxyethanol. The particles, which were synthesized in this work, exhibit 7555
DOI: 10.1021/acs.inorgchem.7b01088 Inorg. Chem. 2017, 56, 7550−7557
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Inorganic Chemistry
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diameters of 5−6 nm and a monomodal size distribution. The dispersions are very stable against aging and sedimentation and thus suitable for fabricating coatings by electrophoretic deposition or spin-coating. An investigation of the semiconducting properties by fabrication of field-effect transistors showed, however, that high-boiling-point solvents and organic residues are detrimental to the electronic performance. The thermodynamically and kinetically stable complexes introduced allow a very high degree of control over the conversion to zinc oxide as well as to ZnO particle formation. The employment of zinc(II) compounds with less strongly coordinating ligands such as acetates or oximates, which have been investigated thoroughly in previous works, does not inhibit the formation of hard agglomerates from the primarily formed zinc oxide single crystals. Thus, precursors with multidentate ligands such as the ones introduced herein clearly offer advantages for nanoparticle synthesis in comparison to multicomponent educt mixtures with sterically demanding additives and an intricate choice of solvents.48,49
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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.inorgchem.7b01088. Additional XRD, TG/IR, TEM, XPS, and SEM data; descriptions of analytical instruments (PDF)
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Jörg J. Schneider: 0000-0002-8153-9491 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS Auger and XPS measurements were performed at the Karlsruhe Nano Micro Facility (KNMF proposal 2016-015-010549) at Karlsruhe Institute of Technology (KIT). The support of Dr. Michael Bruns (XPS) and Tobias Weingärtner (Auger) is gratefully acknowledged. TEM investigations were performed at ERC Jülich under contract ERC-TUD1. We acknowledge the assistance of Dr. Jörg Engstler (TUDa). We acknowledge the help of Elizabeth White (University of Toronto) who participated in the project during the summer internship “International Research Experience Program” (IREP) at TUDa.
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DEDICATION In memory of Prof. Kenneth J. Klabunde, Department of Chemistry, Kansas State University, Manhattan, Kansas, United States.
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