## Review on CSD and possible optimization Optimizing shrinkage for nanoimprintings. --- ### Brief introduction on chemical solution deposition CSD is a technique developed at mid-20th century and applied on oxide thin films from 1980s. During this period, sol-gel route and MOD route are developed for lead zirconate titanate (PZT) thin films. Later this method was soon developed for other perovskites and even more complex compounds. The typical procedure of CSD include: ![[Drawing 2025-02-07 13.47.53.excalidraw.svg]] The imprinting is essentially the same, like the following figure, ![[Drawing 2025-02-07 14.05.10.excalidraw.svg]] Nowadays, CSD has three main routes for layer deposition, although other possibilities exist (like Penchini routes). They are classical sol-gel, metallo-organic (MOD), and hybrid routes. Our interests mainly focus on barium titanate (BTO). [[Chemical solution deposition of electronic oxide films|1]] --- ### Precursors Typical precursors include **Metal source**: 1. Alkoxide $\rm M(OR)_{n}$: titanium isopropoxide $\rm Ti(OiPr)_{4}$, titanium butoxide $\rm Ti(OBu)_{4}$, etc. Highly reactive and extremely easy to hydrolysis. 2. Carboxylates $\rm (R-COO)_{n}M$: barium acetate $\rm Ba(CH3COO)_{2}$, barium citrate $\rm Ba_{3}(C_{6}H_{5}O_{7})_{2}$, barium trifluoroacetate [[Interfacial reactions and microstructure of BaTiO3 films prepared using fluoride precursor method|3]], etc. 3. beta-diketonates: Titanium acetylacetonate $\rm Ti(acac)_{4}$ 4. mixed ligand precursors and mixed metal precursors **Solvent and stabilizer**: 1. Alcohols: isopropanol, methanol, 2-methoxyethanol, etc 2. carboxylic acid: acetic acid, citrate acid, etc. 3. beta-diketone: $\rm acac$. 4. water (in aqueous systems) or other organic solvent (e.g., xylene for MOD). Organic solvents for rheology or processing purposes are also applied. --- ### Chemical solution deposition: classical sol-gel - The sol-gel reaction for metal alkoxide in system with water or alcohol is described as follow. [[Chemical Solution Deposition of Functional Oxide Thin Films|2, chapter 1]] 1. Hydrolysis $\ce{M(OR)_{n} + mH2O -> M(OR)_{n-m}(OH)_{m} + mROH}$ 2. Condensation (water/alcohol elimination) $\ce{2M(OR)_{n-m}(OH)_{m} -> (OR)_{n-m}M-O-M(OR)_{n-m} + H2O}$ or $\ce{(OR)_{n-1}M-(OR) + (HO)-M(OR)_{n-1} -> (OR)_{n-1}M-O-M(OR)_{n-1} + ROH}$ 3. Crosslinking $\ce{(OR)_{n-m}M-O-M(OR)_{n-m} -> [3D cross-linked network]}$ During these reactions, we form metal-oxygen-metal bonds. Only at the end of these chains we have organic groups. For BTO, the metal-oxygen-metal chain is $\rm Ti-O-Ti$. During the final anneal, barium ions will react with such chains to form BTO. Ba ions are introduced by their carboxylate compound, the common one is, barium acetate. The chain length and structures (crosslinkings) could be modified by accurately controlling of hydrolysis. Shorter chain, less hydrolysis and crosslinking could create oligomers, denser structures and less surface area. More hydrolysis would create greater surface area and pore volume. [[Chemical solution deposition of electronic oxide films|1]] This could be done by introducing coordinating ligand (β-Diketones, e.g. $\rm AcAc$), carboxylic acid or alcohol $\rm R'OH$ to have the Alcohol exchange reaction to decrease the reactivity of precursors. [[Chemical Solution Deposition of Functional Oxide Thin Films|2, chapter 1.4.2, Chemical modifications]] It is not desirable to have a highly densified titanium network, which could make the diffusion length for barium ions becomes very large and failed to form barium titanate finally (macroscopic phase separation). From this p --- ### Chemical solution deposition: MOD Metallo-organic decomposition, or MOD, has historically used large carboxylate (e.g., barium acetate) and/or beta-diketonate compounds (e.g., titanium di-methoxy-di-neodecanoate) as precursors. [[Raman, FT-IR and dielectric studies of PZT 4060 films deposited by MOD technology|4]] The reaction does not form metal-oxygen-metal chains or oligomers or crosslinking structures, but relies on the direct decomposition of precursors. Such direct decomposition ensures the composition of the system. Macroscopic phase separation is eliminated. However, the large organic ligands may cause cracking due to pyrolysis process when organic groups and solvents decompose. [[Chemical solution deposition of electronic oxide films|1]] --- ### Chemical solution deposition: hybrid route This is the reaction route we are currently using. It can be considered as a combination of previous two method. The precursors are carboxylates and alkoxides in carboxylic acid system. Alkoxide does not react with water or alcohol, but with carboxylic acid. Take acetic acid and titanium alkoxide as an example, we first have acetolysis: $\ce{nTi(OR)_{4} +2nCH_{3}COOH ->R-(O-Ti(CH_{3}COO)_{2}-O)-R + R-OH}$ The reactivity with acetic acid is less compared to water, this tend to form oligomers or small polymers. [[Chemical solution deposition of electronic oxide films|1]] Acetic acid here also acts as the solvent for barium acetate. After pyrolysis, the solvent first evaporates, then $\ce{-CH3COOH}$ decompose at higher temperature. --- ### Source of shrinkage: pyrolysis and annealing Although it is difficult to know what is exactly happening inside the layer and imprinted structures during pyrolysis, but techniques like thermal gravimetric analysis (TGA) had been applied to see the weight loss during the pyrolysis process. An example is shown in [[978-3-211-99311-8.pdf#page=357&selection=51,0,52,0|Fig. 15.2]]. [[978-3-211-99311-8.pdf#page=356&selection=55,0,56,0|Fig 15.1]] in [[Chemical Solution Deposition of Functional Oxide Thin Films]] shows the corresponding temperature for processes involving densification (i.e., shrinkage). They are, 1. *Capillary contraction*. Solvent and reaction byproducts evaporate from the system during heating. Such capillary contraction creates pressure and collapse the amorphous network. 2. *Continued densification reaction*. Alkoxy, hydroxyl or carboxylic groups at the end and middle of the chain decompose. 3. *Backbone densification and structural relaxation*. The backbone structure ($\ce {O-M-O}$) could be characterized as the metastable liquid and free energy decrease. 4. *Crystallization and viscous flow*. The shrinkage at first two steps are difficult to optimize. Higher concentration probably would decrease the capillary contraction (to a limited extend though), but all reaction route has the problem of densification during functional group removal. Decomposition temperature for barium titanate system is: - Titanium precursor decomposes between 300-350 degrees. - Pure barium carboxylate decompose between 400-500 degrees. - Intermediate phase formed during pyrolysis depends. [[Chemical solution deposition of electronic oxide films|1]] --- ### Beyond current CSD: Alternative method creating BTO structures #### Adding extra layers The bottom edges tend to have larger edge energy and have stronger attraction to deposited layers. This cause the widening at the bottom. #### Chemical bath deposition: creating crystalized structures at low temperature Although the procedure seems easy, but we do not know the exact proportion of stabilizer and solvent. The methods seems not well verified. The layer and coating properties are also unknown. [[Improved osteointegration response using high strength perovskite BaTiO3 coatings prepared by chemical bath deposition]] [[Investigation and optimization of In-Vitro behaviour of Perovskite barium titanate as a scaffold and protective coatings]] #### Low temperature crystallization techniques If we could achieve dense amorphous or even crystalline structures at low temperature with the PDMS mold together, we would automatically eliminate severe shrinkage. Some early works suggest that placing the layer in 200 degree autoclaves with barium rich environment could form crystallized structures. This is called sol-gel-hydrothermal method. Putting PMDS molds into an autoclaves seems not possible though. [[Structure and dielectric nonlinear characteristics of BaTiO3 thin films prepared by low temperature process]] One possibility is low temperature crystallization with special atmosphere. The precursors are barium acetate and titanium isopropoxide, stabilizer and solvent being methanol and 2-methoxyethanol. The author use a sealed reaction cell to provide saturated mist of water and alcohol for low temperature condensation and crystallization. The environment is set to be 50 degrees Celsius. Film quality seems not perfect but okay. Shrinkage still exist for such low temperature. [[Room-temperature synthesis of crystalline barium titanate thin films by high-concentration sol–gel method]] --- ### Outline >[!Note] >1. Intro to CSD >2. Precursor systems >3. Typical CSD routes > 1. solgel > 2. MOD > 3. hybrid >4. Source of shrinkage >5. Alternatives, adding layer or CBD > 1. Depositing extra layers > 2. CBD > 3. temperature crystallization --- Ref 1. [[Chemical solution deposition of electronic oxide films]] 2. [[Chemical Solution Deposition of Functional Oxide Thin Films]] 3. [[Interfacial reactions and microstructure of BaTiO3 films prepared using fluoride precursor method]] 4. [[Raman, FT-IR and dielectric studies of PZT 4060 films deposited by MOD technology]] 5. [[Improved osteointegration response using high strength perovskite BaTiO3 coatings prepared by chemical bath deposition]] 6. [[Investigation and optimization of In-Vitro behaviour of Perovskite barium titanate as a scaffold and protective coatings]] 7. [[Structure and dielectric nonlinear characteristics of BaTiO3 thin films prepared by low temperature process]] 8. [[Room-temperature synthesis of crystalline barium titanate thin films by high-concentration sol–gel method]]