Electrochemical Synthesis of Flat-[Ga13-xInx(μ3-OH)6(μ- OH)18(H2O)24(NO3)15] Clusters as Aqueous Precursors for Solution- Processed Semiconductors

Flat-[Ga13(μ3-OH)6(μ-OH)18(H2O)24](NO3)15 (Ga13) and heterometallic [Ga13xInx(μ3-OH)6(μ-OH)18(H2O)24](NO3)15 (x = 5, 4) clusters were synthesized by the electrolysis of metal nitrate salt solutions to directly form, without purification, aqueous precursor inks for InxGa13-xOy semiconducting films in < 2 hr. Raman spectroscopy and 1 H-NMR spectroscopy confirm the presence of [Ga13-xInx(μ3-OH)6(μ-OH)18(H2O)24(NO3)15]. Bottom-gated thin-film transistors were fabricated using ~16 nm-thick Ga13-xInxOy films as an active channel layer, displaying turn-on voltages of -2 V, and on/off current ratios greater than 10 6 . The average channel mobility of the transistors fabricated from the cluster solutions generated by electrolysis was ~ 5 cm -2 V -1 s -1 which was more than twice that of transistors fabricated from control solutions with the simple nitrate salt precursors of ~ 2 cm -2 V -1 s -1 . Electrochemical cluster synthesis thus provides a simple and direct route to aqueous precursors for solution-processed inorganic electronics. Page 2 of 17 Journal of Materials Chemistry C Jo ur na lo fM at er ia ls C he m is tr y C A cc ep te d M an us cr ip t Thin film deposition using aqueous inorganic-cluster precursors provides an alternative to traditional vacuum processing techniques for thin-film deposition. 1,2 As one example, “flat” Group 13 [M13(μ3-OH)6(μ-OH)18(H2O)24(NO3)15], homoand heterometallic clusters (Figure 1) have been used to deposit high-performance semiconductor 3 and dielectric films 4 . Because of this, significant effort has been aimed at improving Group-13 cluster synthesis. Early syntheses took two weeks and used dibutylnitrosamine (DBNA), a known carcinogen. 3,5 Wang et al. showed that the addition of Zn powder to acidic Al(NO3)3 solutions results in condensation of [Al13(μ3-OH)6(μ-OH)18(H2O)24(NO3)15] (Al13) clusters via a gradual pH increase of the solution through nitrate reduction. The reaction is complete in approximately two days and the carcinogenic DBNA is no longer needed. 6 A disadvantage to this method is that extensive purification is required to remove Zn 2+ from the precursor solution. The preferential solubility of zinc nitrate in alcohol is used to purify the clusters, as M13 clusters are negligibly soluble in many organic solvents. In contrast, electrochemistry provides a direct mechanism to drive reduction reactions without the use of chemical reagents that must be later removed. Recently, both flat 7 and Keggin 8 Al13 clusters have been electrochemically synthesized. Page 3 of 17 Journal of Materials Chemistry C Jo ur na lo fM at er ia ls C he m is tr y C A cc ep te d M an us cr ip t

Thin film deposition using aqueous inorganic-cluster precursors provides an alternative to traditional vacuum processing techniques for thin-film deposition. 1,2As one example, "flat" Group 13 [M 13 (µ 3 -OH) 6 (µ-OH) 18 (H 2 O) 24 (NO 3 ) 15 ], homo-and heterometallic clusters (Figure 1) have been used to deposit high-performance semiconductor 3 and dielectric films 4 .Because of this, significant effort has been aimed at improving Group-13 cluster synthesis.Early syntheses took two weeks and used dibutylnitrosamine (DBNA), a known carcinogen. 3,5 Wang et al.   showed that the addition of Zn powder to acidic Al(NO 3 ) 3 solutions results in condensation of [Al 13 (µ 3 -OH) 6 (µ-OH) 18 (H 2 O) 24 (NO 3 ) 15 ] (Al 13 ) clusters via a gradual pH increase of the solution through nitrate reduction.The reaction is complete in approximately two days and the carcinogenic DBNA is no longer needed. 6A disadvantage to this method is that extensive purification is required to remove Zn 2+ from the precursor solution.The preferential solubility of zinc nitrate in alcohol is used to purify the clusters, as M 13 clusters are negligibly soluble in many organic solvents.In contrast, electrochemistry provides a direct mechanism to drive reduction reactions without the use of chemical reagents that must be later removed.Recently, both flat 7 and Keggin 8 Al 13 clusters have been electrochemically synthesized.Here we report the electrochemical synthesis of [Ga 13-x In x (µ 3 -OH) 6 (µ-OH) 18 (H 2 O) 24 ] 15+ (x = 0, 4, 5) clusters and show that the aq.solutions obtained can be used, without purification, to deposit Ga-In-O channel layers with good thin-film transistor (TFT) performance.The elimination of secondary reagents and purification steps is beneficial for mass production, sustainability, and cost.Films can be cast directly from the modified salt solutions, making this a direct method for obtaining various homo-and heterometallic Group 13 oxide thin films with a variety of applications.
The synthesis is performed in a two-compartment electrochemical cell comprising 1) a beaker housing the Pt working electrode, a Ag/AgCl reference electrode, and a pH probe and 2) a medium fritted tube, inside the beaker, containing a Pt counter electrode (Figure S1).
Experimental details are provided in the supporting information.The applied working electrode potentials were chosen to be slightly negative of the reduction potential of the metal cations at the pH of interest as described by their Pourbaix diagrams. 9Potentials of -1.00 V vs. Ag/AgCl

Journal of Materials Chemistry C Accepted Manuscript
for Ga and -0.49V vs. Ag/AgCl for Ga-In mixtures were used to generate the desired products with the given apparatus.The voltage of -1.00 V for aq.solutions of gallium nitrate caused a change in the luster of the Pt surface which could be seen by eye. 10 Yields of washed product show this plating results in a relatively small amount of Ga loss overall (< 2%).
The primary mechanism of this reaction appears to be the removal of nitrate from the solution via its reduction to ammonium, NO x , and other species.The removal of nitrate counter anions from the solution raises the pH of the solution by consuming protons as in eqn.(1) and thus drives the formation of the cluster via LeChatelier's Principle as it acts on the reaction as given in eqn.(2).
Analysis of an air-dried aliquot of the crude reaction by 1 H-NMR shows a prominent triplet peak with equal peak heights corresponding to the 1 H-14 N coupling of ammonium ions centered at 7.1 ppm 11 (Figure S3).This indicates that nitrate is reduced to ammonium as a part of one pathway in which counterions are removed from solution and the pH is raised.Although the presence of ammonium ions indicates that nitrate reduction is involved in raising the pH of the cluster solution and forcing olation of the metal aqua species, it does not rule out other contributing mechanisms.We find that electrolysis at sufficiently high current results in evolution of a brown gas.This is likely due to the reduction of NO 3 -to NO x gases. 12We performed the electrochemical synthesis of Ga 13 and Ga 13-x In x mixed clusters at a constant applied voltage which was high enough to reduce small amounts of metal but low enough to prevent large losses of material to plating.We believe that some metal plating onto the electrode is important to  We find evidence for M 13 species forming with fewer reducing equivalents than that reported for the Zn-based synthesis of [Al 13 (µ 3 -OH) 6 (µ-OH) 18 (H 2 O) 24 (NO 3 ) 15 ].6a Ga 13 clusters are observed after passing a cathodic charge of 0.7-0.8electrons per Ga, and 0.4-0.5 electrons per metal in the case of the Ga 13-x In x clusters.The Zn-based synthesis of Al 13 used 1.0 reducing equivalents per Al (1:2 Zn:Al as Zn is a 2e -reductant).The synthesis of a related Sc 2 cluster used 0.75 reducing equivalents per Sc.6b Our hypothesis to explain such behavior is that if hydroxyl-bridged metal cluster formation is under equilibrium control, not all of the excess nitrate counterions need to be consumed for clusters to form.Our analysis does not however exclude the possibility that the reaction does not go to completion under the conditions used.
Nitrate ions can also be effectively removed from association with the growing clusters by

Journal of Materials Chemistry C Accepted Manuscript
counterbalancing the positive charge associated with newly formed ammonium ions, leaving this new ammonium nitrate salt in solution but allowing ions to diffuse away from clustering species.
Raman spectroscopy is also useful for identifying M 13 clusters. 17The Raman spectra of aliquots from the electrochemical synthesis agree with previous reports of Ga 13 clusters, highlighted by the v 1 Ga-O symmetric stretch, or breathing mode at 464 ± 1 cm -1 (Figure 3). 17e Raman spectra of the structurally analogous Ga 13-x In x cluster reveal similar vibrational features to those observed in Ga 13 clusters, with the v 1 breathing mode slightly red-shifted to 449 ± 1 cm -1 .This shift is consistent with the substitution of the larger In for Ga, and with the observed difference between the vibrational modes of In and Ga hexa-aqua salts (Figure 3).samples.Spectra for cluster compounds were collected on a single crystal using a Raman microscope and are largely free of metal nitrate impurities.Note the red-shift in the v 1 breathing mode center for the In-substituted cluster (449 ± 1 cm -1 ) when compared to that for the Ga cluster (464 ± 1 cm -1 ).The uncertainties given are associated with the error in fitting the peak center.

Journal of Materials Chemistry C Accepted Manuscript
nitrates are consumed electrochemically during the cluster synthesis.This decrease in nitrate concentration drives olation and preorganization of the metal hydroxides into clusters. 19Because nitrates must be removed during the annealing step to give an oxide thin film, we attribute the enhanced performance of the electrolyzed solution to reduced porosity in the final semiconductor channel that would be caused by decomposing counter ions.
Although the goal of this work is to show the new electrochemical synthesis route yields cluster precursors whose TFT performance is similar to clusters made by conventional methods, it is also useful to compare the performance to other solution-derived oxide thin films.Kim et.al.
reported the use of "combustion processing" to deposit related In-Zn-O films at temperatures as low as 200 °C from methoxyethanol solutions.20a Composition-optimized In 0.7 Zn 0.3 O 1.35 devices fabricated with a SiO 2 gate dielectric (as is done here) had saturation mobilities (µ sat ) of 10 cm V -1 s -1 after annealing at 400 °C.Hwang et.al. reported µ sat of 8 cm 2 V -1 s -1 for In 0.7 Zn 0.3 O 1.35 after annealing at 300 °C when Zn(NO 3 ) 2 and In(NO 3 ) 3 were deposited from an aqueous solution.20b The In 0.46 Ga 0.53 O 1.5 studied here had average channel mobilities of 5 cm 2 V -1 s -1 .
Studies of vapor-deposited films show that mobility increases sharply with higher In concentration.20c Increasing the In:Ga ratio in the clusters would be expected to further increase TFT performance.Alternative gate dielectrics (e.g.amorphous alumina 20a ), and surface/interface passivation layers, 21 also dramatically improve the TFT performance of films made from other solution precursors.These strategies can directly be used to improve the performance of the cluster precursors reported here.In summary, an alternate synthetic method is reported for the synthesis of flat homo-and heterometallic Group 13 cluster precursor solutions that can be directly used in the fabrication of thin-film transistors.This new method reduces the processing time to generate M 13 cluster solutions from two days to two hours.The synthesis is carried out electrochemically so as to reduce protons and nitrate ions in a controlled fashion.Heterometallic clusters synthesized using this method are functionally similar in transistor applications to previously synthesized and characterized clusters.(16) Kamunde-Devonish, M. K.; Jackson, Jr., M. N.; Mensinger, Z. L.; Zakharov, L. N.;
Figure 2. 1 H-NMR (d 6 -dmso) spectra of washed and unwashed precipitated cluster products from DBNA and electrochemical syntheses.Based on comparison to the DBNA-derived control samples, the unwashed electrochemical product is assigned the composition Ga 9 In 4 , while the washed electrochemical product is assigned the composition Ga 8 In 5 .

Figure 3 .
Figure 3. Solid-state Raman spectra of nitrate salts and electrochemically generated cluster

Figure 4 .
Figure 4. (a) Transmission electron microscopy image demonstrating the uniform morphology of thin films processed from the electrochemically-synthesized precursor.(b) Average transfer curve compiled from five bottom-gate TFTs processed using the electrochemically synthesized Ga 13-x In x heterometallic clusters to generate channel layers.(c) Representative transfer plots for 550 ˚C air-annealed In-Ga-O films created using the electrochemically synthesized Ga 13-x In x heterometallic cluster and starting salt solution precursors.(d) Average channel mobility determined at V GS = 40 V for films made at various electrolyzed time intervals (and thus different average numbers of electrons passed into the solution per metal ion).Device performance is increased with longer electrolysis, consistent with removal of nitrate and formation of clusters.The devices consist of the following structures: Al/Si (p+)/SiO 2 (100 nm)/In-Ga-O(15 nm)/Al, length = 150 µm, width = 1000 µm, and V DS = 0.1 V (V DS drain source voltage; V GS = gate source voltage; I D = drain current).

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Samples are prepared for NMR by dissolving the sample in d 6 -DMSO and allowing them to set for 24 hr.Before that time the spectra is too complicated to analyze.Slow exchange with the solvent with the external waters of the cluster eventually results in a more simplified spectrum with a distinct fingerprint region.Figure S4 in the supporting information shows how this fingerprint region changes as a function of In substitution within crystallographically confirmed Ga 13-x In x clusters.(15) Oliveri, A. F.; Carnes, M. E.; Baseman, M. M.; Richman, E. K.; Hutchison, J. E.; Johnson, D. W. Angew.Chem.Int.Ed. 2012, 51, 10992-10996.
condition the Pt toward nitrate reduction.Nitrate can undergo a number of reduction processes to form species including N 2 O 4 , HNO 2, NO, and NH 4 + .The standard reduction potentials are similar, between +0.8-1.0V vs. NHE, 13a and all much more positive than the hydrogen reduction potential.At a clean Pt electrode, however, H 2 generation might be expected to dominate given the fast kinetics relative to nitrate reduction.We did not observe significant bubbles (that would be associated with H 2 formation) on the Pt electrode surface.After Pt is modified by Ga/In plating it likely becomes poisoned for hydrogen evolution and thus kinetically favors the nitrate reduction reaction.13b These data support the hypothesis that nitrate reduction is the predominant electrochemical reaction.Regardless of the cathode reaction, charge balance requires additional 3+) to migrate from the counter electrode compartment into the working electrode compartment or negatively charged species (e.g.
3These films are capable of being spin-cast directly from unpurified