Formation Of Precursor Film Research Articles

Key Sputtering Parameters for Precursor In2O3 Films to Achieve High Carrier Mobility.

Owing to their extremely high carrier mobility (μ) of >100 cm2/(V s) and suitable low carrier concentrations, transparent conducting films of solid-phase crystallized H-doped In2O3 (spc-IO:H) exhibit high conductivity with high optical transparency over a broad frequency range. These properties can be attributed to solid-phase crystallization of the amorphous precursor film. Therefore, the development of high-quality spc-IO:H films requires the deposition conditions of the precursor films to be optimized. This study systematically investigates the effects of three key sputtering parameters, namely, water vapor partial pressure (PH2O), radio frequency magnetron sputtering power (PRF), and flow ratio of O2 to total sputter gas (fO2) on the crystallographic texture evolution of spc-IO:H films during solid-phase crystallization. In addition, the carrier transport in the resulting films is examined. PH2O, PRF, and fO2 are found to be indispensable for producing high-mobility (>100 cm2/(V s)) spc-IO:H films. Furthermore, it is found that introducing a small amount of PH2O during deposition, a lower PRF, and a suitable fO2 facilitates the formation of precursor films having a lower crystallite density. Moreover, after annealing at a temperature of 200 °C, the IO:H precursor films with a lower crystallite density are found to have larger crystal grains. However, the μ values of the postannealed IO:H films are mainly correlated with the stoichiometric deviation.

Studying the effect of ambient temperature variations on perovskite precursor film formation using different antisolvents

This study investigates the sensitivity of two antisolvents to ambient temperature fluctuations during the fabrication of triple-cation perovskite thin films. Chlorobenzene (CB) and ethyl acetate (EA) were employed as antisolvents in the preparation of perovskite solar cells (PSCs) under identical ambient temperature variation conditions. The experimental results reveal that the quality of the perovskite films does not exhibit a consistent trend in response to ambient temperature changes depending on the antisolvent used. When CB was used as the antisolvent, the highest-performing device, produced at an ambient temperature of 18 °C, achieved a power conversion efficiency (PCE) of 19.9 %. In contrast, when EA was employed as the antisolvent, the highest-performing device was fabricated at 25 °C, reaching a PCE of 20.5 %. Furthermore, when the ambient temperature exceeded 30 °C, all films prepared with either antisolvent exhibited significant cracking, indicating that elevated temperatures adversely affect perovskite film formation. To further investigate the mechanism through which ambient temperature affects the film-forming process, antisolvents at varying temperatures were used during perovskite film preparation to simulate transient temperature increases during formation. This study reveals that the impact of ambient temperature variations on perovskite film formation does not exhibit a uniform trend but is significantly influenced by the choice of antisolvent. Moreover, this work offers valuable insights for optimizing the fabrication of high-quality perovskite thin films.

Dynamic wetting of a CO2-H2O-montmorillonite system using molecular dynamics

Injecting carbon dioxide into shale reservoirs achieves geological carbon sequestration (GCS), effectively reducing atmospheric carbon dioxide levels. Rock wetting characterizes the strength of the interaction between fluids and rocks, thereby determining fluid distribution, which is crucial for GCS trapping efficiency. In this study, the dynamic wetting of H2O-montmorillonite and CO2-H2O-montmorillonite systems was studied using molecular dynamics (MD) simulations. Montmorillonites with different lattice substitution arrangements based on pyrophyllite are strongly water-wet, and a precursor film forms at the leading edge of the bulk water. Na+ controls precursor film formation, which is rapidly spreading forward at a constant speed along the lattice substitution site lines based on Na+ distribution. The lower the proportion of Al3+/Si4+ lattice substitution in the Si-O tetrahedral layer on the fluid side, the higher the degree of water-wetness. The competitive adsorption of carbon dioxide and water molecules reduces the degree of spreading, resulting in a decrease in the degree of water-wetness. However, compared to the H2O-montmorillonite system, the spreading behavior remained unchanged. High carbon dioxide pressure caused the wetting to change from strongly water-wet to weakly water-wet, however no wetting reversal was exhibited. The wetting changes were particularly prominent near the supercritical carbon dioxide pressure, with a limit of 18 MPa. For the first time, this study distinguished the dynamic behaviors of bulk water and precursor films, revealed the formation mechanism of precursor films, and revealed the control mechanism of carbon dioxide pressure on dynamic wetting from a molecular perspective. The quantitative relationship between the contact angle and flow velocity at different carbon dioxide pressures presented in this study could effectively guide the establishment of a fluid force equation for infiltrating flow, providing theoretical support for GCS numerical simulation research.

Molecular Origin of Wetting Characteristics on Mineral Surfaces

Accurate determination of the wetting characteristics on mineral surfaces is critical for many natural processes and industrial applications where multiphase flow in porous media is involved. The wetting behaviors on mineral surfaces are controlled by water-mineral interactions, giving rise to various wetting characteristics, including contact line advancement, formation of precursor films, etc. However, a fundamental understanding of wetting characteristics on different mineral surfaces is still lacking at the molecular level. Here, utilizing a comprehensive set of molecular dynamics simulations, we investigate the wetting characteristics of water on various mineral surfaces and obtain the corresponding water-mineral interaction properties (including the areal density of water-mineral interaction energy and the work of adhesion of the water-mineral interface), mineral wettability, and structural and diffusion properties of water molecules near the surface. We show that the diffusion properties of water molecules on mineral surfaces play an important role in wetting characteristics. We find that the contact line tends to advance forward in the jumping mode or the rolling mode during the wetting process, which depends on the diffusion capacity of the water molecules on mineral surfaces. The corresponding evolution of the solid-liquid friction coefficient during dynamic spreading is also analyzed. We further demonstrate the strong impact of isomorphic substitution and charge-balancing counterions on wetting characteristics on the surfaces of clay minerals. It is shown that the introduction of charge-balancing counterions can shift the mineral surface from strongly hydrophilic to strongly hydrophobic and lead to completely different wetting characteristics. Our results provide a clearer picture of the molecular underpinnings in mineral wetting phenomena and deepen the understanding of the control of water-mineral interactions on the wetting properties.

The formation mechanism of the precursor film in high temperature molten metal systems: insight into structural disjoining pressure.

A precursor film is a unique microfluidic entity that arises at the liquid/solid interface. The formation mechanism of this entity in high-temperature systems is yet to be explained, mainly due to the limitations posed by the increased reaction at the solid/liquid interface. In this study, we investigate the formation process of the precursor film in high-temperature molten metal systems (Ag/Ni, Au/Ni, and Cu/Ni) using molecular dynamics simulations. The alloying energies for different alloying pairs were determined to extract the excess energy, which was found to be distributed from the interface to the upper liquid. The pattern of this energy distribution determines the shape of the near-surface liquid, including the precursor film. This relationship is further reflected by the structural disjoining pressure, which is the excess pressure exerted by the ordered microstructures within the wedge-shaped area of the droplet. Strong nonlinearity has been found in the structural disjoining pressure of Ag/Ni and Au/Ni systems, which is considered to be the main reason for the formation of the precursor film. The fluctuation of the dissolution rate is also reflected in the disjoining pressure, and the inhibition of dissolution on the precursor film formation is phenomenally clarified.

Precursor film formation on catalyst–electrolyte–gas boundaries during CO2 electroreduction with gas diffusion electrodes

Thin and long layers of catholyte precursor films spread near triple-phase boundaries on composite catalysts containing hydrophobic materials. Dissolved CO2 molecules in the precursor films reduce on the composite catalyst surface without depletion.

Wettability and spreading behavior of Sn–Cr alloys on SiC

Knowing of the wetting of filler alloy on substrate is significant for brazing. Here, the isothermal wetting behavior of Sn-xCr alloys (x = 1, 2, 3, 4 and 5 wt%) on SiC substrates was studied by a modified sessile drop method in the temperature range of 1173 K–1323 K. The interfacial microstructures were characterized by scanning electron microscope (SEM) and X-ray diffraction (XRD) and its evolution was analyzed thermodynamically. Also, the spreading kinetics is calculated and the formation of precursor film is discussed. The spreading of Sn melt on SiC consists of rapid-spreading and sluggish-spreading stages. A better wettability is obtained at higher Cr concentration and isothermal temperature. The direct reaction between Cr and SiC forming metallics Cr–C compounds (Cr3C2, Cr7C3 and Cr23C6) at the interface accounts for the improving wettability of Sn melt on SiC. More metallic Cr–C compound occupies a higher proportion of interfacial layer inducing a lower final contact angle. Temperature has a violent influence on the spreading kinetic constant Kr. The spreading activity energy is calculated to be 312 kJ/mol.

Perovskite Crystallization Dynamics during Spin-Casting: An In Situ Wide-Angle X-ray Scattering Study.

In situ wide-angle X-ray scattering (WAXS) has been measured during the spin coating process used to make the precursor films required for the formation of thin films of perovskite. A customized hollow axis spin coater was developed to permit the scattered X-rays to be collected in transmission geometry during the deposition process. Spin coating is the technique most commonly used in laboratories to make thin perovskite films. The dynamics of spin-casting MAPbI3–xClx and FAPbI3–xClx films have been investigated and compared to investigate the differences between the dynamics of MAPbI3–xClx and FAPbI3–xClx film formation. In particular, we focus on the crystallization dynamics of the precursor film formation. When casting MAPbI3–xClx, we observed relatively fast 1D crystallization of the intermediate product MA2PbI3Cl. There was an absence of the desired perovskite phase formed directly; it only appeared after an annealing step that converted the MA2PbI3Cl to MAPbI3. In contrast, slower crystallization via a 3D precursor was observed for FAPbI3–xClx film formation compared to MAPbI3–xClx. Another important finding was that some FAPbI3–xClx perovskite was generated directly during spin-casting before annealing. These findings indicate that there are significant differences between the crystallization pathways for these two perovskite materials. These are likely to explain the differences in the lifetimes of the resulting perovskite solar cell devices produced using FA and MA cations.

Reactive wetting of Sn-V solder alloys on polycrystalline CVD diamond

In this paper, the reactive wetting of polycrystalline chemical vapour deposition (CVD) diamond by a novel Sn-V solder alloy was critically examined using the sessile drop method. It has been found the Sn-V solder alloys start to spread on the CVD diamond at around 600 °C. Moreover, 0.5 wt% addition of V is adequate to induce an outstanding spreading of Sn-V active solder alloys on CVD diamond; while the original V content in Sn-V alloys has a negligible influence on final contact angles after wetting at 950 °C. The spreading kinetics of Sn-3V alloy on CVD diamond can be described using the classical chemical reaction-limited model at 700–850 °C. However, ultrafast adsorption of V element to the fresh diamond surface was evidenced by the formation of precursor film on the periphery of solidified Sn-V droplets. Hence, both interface reaction and adsorption of active element are accountable for the reactive spreading of Sn-V alloy on CVD diamond, especially during the early stage of spreading at relatively low temperatures. Results obtained in this study are therefore significant for not only the development of diamond bonding technique, but also the understanding of reactive wetting of metals on diamond and other carbonous materials.

Nondestructive high-throughput screening of nanopore geometry in porous membranes by imbibition

A fluid dynamic model for imbibition into closed-end, axisymmetric pores having diameters that change as a function of the pore depth is presented. Despite the fact that liquid invasion into nonbranched closed-end pores is characterized by a wealth of different transient and/or metastable nonequilibrium stages related to precursor film formation, we show that a simple hydraulic model accounting for geometry- and air compression-induced deviations from classical Lucas-Washburn dynamics precisely describes the imbibition dynamics except at the late stage. The model was validated by laser interferometry experiments with submillisecond temporal resolution. Imbibition of three simple liquids (isopropanol, ethanol, and hexane) into self-ordered anodic alumina membranes containing arrays of parallel closed-end nanopores characterized by slight conicity was studied. The model provides an improved description of nanoscale fluid dynamics and allows geometric characterization of nanoporous membranes by their imbibition kinetics accounting for the back pressure of the compressed gas. Thus, a precise calibration of porous membranes with simple liquids becomes possible, and changes in the mean pore diameter as a function of the pore depth can be assessed.

Droplet Shape and Wetting Behavior under the Influence of Cyclically Changing Humidity.

Relative humidity (RH) plays a crucial role in wetting and spreading phenomena by affecting the evaporation rate, evaporation modes, and spreading dynamics via precursor film formation, surface modification, and surface tension alteration. We examined the effect of the periodically varied relative humidity (RH) between low (20%) and high (85%) levels on the wetting of the droplet of nonhygroscopic (pure surfactants) and hygroscopic (ethylene glycol, glycerol) liquids on a hydrophobic surface. It was revealed that the changing RH induces two modes of transition between the wetting states of the droplet: with hysteresis and without hysteresis. Droplets of both nonhygroscopic and hygroscopic liquids exhibit shape hysteresis during the first cycle: (i) droplets of surfactants irreversibly spread saving an initial volume; and (ii) ethylene glycol and glycerol droplets irreversibly absorb the moisture, increasing the volume and the base diameter. Further, cyclically changing the RH results in the droplet breathing effect, i.e., the nonhysteresis transition of the droplet shape between two wetting states corresponding to the minimum and maximum RH levels. In the case of the glycerol droplet for three cycles of the RH variation, the volume hysteresis (the droplet volume increases in each cycle) was observed. This is determined by the moisture absorption due to high hygroscopicity of glycerol. We also revealed that for all liquids studied, the droplet spreading at each increase in RH started at reaching the RH threshold level.

The wetting phenomenon and precursor film characteristics of Sn-37Pb/Cu under ultrasonic fields

The wetting phenomenon of Sn-37Pb alloy solder spreading on Cu substrate under different ultrasonic fields was investigated. Sn-37Pb/Cu wetting systems showed significant precursor film characteristics with the assist of ultrasonic energy. A short-time ultrasonic field evidently enhanced the wettability of Sn-37Pb/Cu compare to a common wetting field which had no introduced ultrasonic. At the temperature of 213 °C, spreading area of a Sn-37Pb solder drop in a 20 kHz, 1000 W ultrasonic field reached maximum at 2 s, and showed significant precursor film characteristics. Atomistic dynamic models showed the relevance between wettability and precursor film formation. The observed dynamic results agreed with the experimental results.

Effect of micro alumina particles additions on the interfacial behavior and mechanical properties of Sn-9Zn-1Al2O3 nanoparticles on low temperature wetting and soldering of 6061 aluminum alloys

The effects of micro Al2O3 particles additions on microstructures and thermal and mechanical properties of the Sn-9Zn-1Al2O3 nanoparticles on low temperature wetting and soldering of 6061 aluminum alloys are investigated in this study. The results show that the melting temperature of the alloys firstly slightly decreases and then slightly increases with the increase in the micro Al2O3 particles concentration, while the pasty ranges of the alloys are simultaneously increased. The spreading coefficient of Sn-9Zn-1Al2O3-0.25microAl2O3 is about 13.75%, and is increased to about 41.5% of Sn-9Zn-1Al2O3-0.75microAl2O3. Both the surface diffusion, rapid adsorption and phase flow contribute to the formation of precursor film. When micro Al2O3 is added to the composite solder alloys, acicular Sn-Zn eutectic structures are smaller in size from acicular to acicular or circular spot. The strengthening mechanisms with bimodal size Al2O3 particles are on account of grain refinement and load transfer effect of bimodal sized Al2O3. The strength induced by particles is a very important part, which it can account for 21% of the total tensile strength of the solder.

Reactive wetting of Ti-6Al-4V alloy by molten Al 4043 and 6061 alloys at 600–700°C

Abstract Wetting of Ti-6Al-4V alloy by two industrial grade Al alloys (i. e., Al 6061 and 4043 alloys) was studied using the sessile drop method at 600–700 °C under high vacuum. Al/Ti-6Al-4V is a typical reactive wetting system with good final wettability accompanied by the formation of precursor film which is actually an extended reaction layer. The formation mechanism for the precursor film is “subcutaneous infiltration”. The small amount of alloying element Si in the alloys can cause significant segregation at the liquid/solid interface which satisfies the thermodynamic condition. The wetting behavior can be described by the classic reaction product control models, and Ti7Al5Si12 decomposition and Al3Ti formation correspond to the two spreading stages. The small difference in alloying elements in Al 6061 and 4043 resulted in distinctly different interface structures, formation of precursor film and spreading dynamics, especially for the Si segregation at the interface.

(Invited) Controlling Solution Chemistry from Lab-Scale Spin Coating to Scalable Deposition for High-Performance Perovskite Solar Cells

Organic-Inorganic hybrid halide perovskites have recently emerged as a new class of light absorbers that have demonstrated a rapid progress and impressive efficiencies (>22%) for solar conversion applications. Despite the rapid progress demonstrated by these light absorbers, there is still a lack of understanding of some fundamental material/physical/chemical properties of these materials. There is also a significant processing gap between the lab-scale spin coating and scalable deposition methods toward future roll-to-roll manufacturing. The challenge results from the sensitivity of perovskite crystallization and film formation to the processing conditions, which depend strongly on the specific deposition method. In this presentation, I will present our recent studies toward a better understanding and control of perovskite nucleation, grain growth, and microstructure evolution using solution processing. The precursor chemistry and growth conditions are found to affect significantly the structural and electro-optical properties of perovskite thin films. We also find that the precursor ink chemistry (solvent, coordination chemistry) of perovskite is critical for scalable deposition; but this has largely been underexplored. We present a rational design of perovskite precursor film formation to achieve highly specular films using scalable deposition methods. Using these high quality perovskite films, we have achieved device efficiencies approaching the values obtained on small scale by spin coating. These findings make a significant advance towards commercialization of the perovskite photovoltaic technology. These results and others will be discussed.

Observation of Ultrathin Precursor Film Formation during Ge-Si Liquid-Phase Epitaxy from an Undersaturated Solution.

Our in situ X-ray study shows that a silicon substrate in contact with an undersaturated In(Ge) solution is wetted by an approximately 1 nm thin germanium film, which does not grow any thicker. The results can be understood by the use of thickness-dependent correlated interfacial energies. This near-equilibrium heterogeneous interface structure marks the initial stage of crystal growth before the formation of bulk material, which can only form under conditions of supersaturation. This finding uncovers a fundamental aspect of the thermodynamics at solid-liquid interfaces relevant for understanding the transition from equilibrium to supersaturation and is of importance for nanoscale solution growth methods.

Humidity-accelerated spreading of ionic liquids on a mica surface

The role of relative humidity (RH) on the wetting behavior of droplets of two [Rmim][NTf2] ionic liquids (ILs) on a mica surface was investigated and water vapor adsorption was found to enhance the ILs precursor film formation and droplet spreading.

Reactive wetting of low carbon steel by Al 4043 and 6061 alloys at 600–750 °C

Abstract Wetting of low carbon steel by two industrial grades Al (4063 and 6061) alloys was investigated by the modified sessile drop method at temperatures between 600 and 750 °C under high vacuum. The formation of intermetallics which would lead to the replacement of the oxidized surface by a clean surface, and further induces the spreading. The formation of precursor film in the wetting of Al 6061/steel was caused by subcutaneous infiltration mechanism. The different chemical compositions in Al alloys also lead to the different interfacial structures and spreading dynamics. The Mg in Al 6061 alloy reduces the oxide film and then make molten Al is easier to infiltrate into the steel substrate; the high affinity of the Si in Al 4043 alloy at Al/steel interface which induced the segregation of Si. The reduction reaction by the Mg in Al 6061 and the reaction between Al and Fe caused the apparent activation energy for spreading is almost an order of magnitude larger than Al 4043.

Impact of Nanoscale Surface Heterogeneity on Precursor Film Growth and Macroscopic Spreading of [Rmim][NTf2] Ionic Liquids on Mica

The connection between the interfacial properties of ionic liquids and their wetting behavior has been studied very little to date and not at all on heterogeneous surfaces. Therefore, we have investigated the static and dynamic wetting for a family of ionic liquids, 1-alkyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide, [Rmim][NTf2], on mica, where R represents an ethyl, butyl, or hexyl alkyl chain on the imidazolium ring. Spreading is impacted greatly by a precursor film that forms on both homogeneous and heterogeneous mica surfaces. Macroscopically, the initial viscous spreading of the ionic liquid droplet on bare mica occurs within seconds but is then followed by a very slow relaxation that can be closely correlated with the typical time-scales of the precursor film growth. The contact angle for [emim][NTf2] and [bmim][NTf2] relaxes from about 40° to 23° over 30 and 90 min, respectively. For [hmim][NTf2], the process takes approximately 24 h and approaches complete wetting. The thickness of the precursor films for [emim][NTf2], [bmim][NTf2], and [hmim][NTf2] were 0.53, 0.65, and 1.0 nm, respectively, according to atomic force microscopy (AFM). These values are consistent with a monolayer of ionic liquid cations on mica, rather than ion pairs. A monolayer of octadecylphosphonic acid (OPA) on mica prevents both the formation of a precursor film and the relaxation of the contact angle. However, only a partial surface coverage of ~60% OPA is required to have the same effect. Quenching of precursor film formation (and associated contact angle relaxation) is due to an increasingly connected network of OPA regions that closes the nanoscale paths of bare mica on which the precursor film can develop via surface diffusion.

Formation, morphology and internal structure of one-dimensional nanostructures of the ferroelectric polymer P(VDF-TrFE)

We present an investigation of the formation, morphology, thermodynamic properties and crystal texture of P(VDF-TrFE) nanostructures obtained by wetting of porous anodic aluminum oxide templates, using electron microscopy, DSC and X-ray scattering. Wetting occurs initially via precursor film formation producing thin walled nanotubes. Formation of nanorods happens by subsequent thickening of the tube walls with thickness undulations occurring as a transient state. While crystallization within the one-dimensional nanostructures during cooling leads to the formation of highly oriented crystals in the high temperature paraelectric phase, the subsequent transition to the ferroelectric phase goes along with partial loss of orientation. Nevertheless, at room temperature the samples show a dominant orientation of the chains perpendicular to the long axis of the nanostructures. For narrow pores the size of crystalline domains is restricted by the strong confinement.