Self-assembled Patterns Research Articles

Self-assembled micro-patterns in uphill-diffusion solution system.

In this work we present self-organized regular patterns in a solution system through uphill-diffusion. Micrometer thick organic semiconductor solution is sandwiched between a substrate and cover-plate. Self-assembled regular patterns can be observed on the substrate after solvent evaporation. Different micro-patterns and pattern defects were displayed and analyzed. Mechanisms of defect formation, mode selection process during patten generation, and pattern sedimentation onto substrate from solution were proposed. Organic thin film transistors were fabricated with the assembled line patterns which demonstrate a promising way to produce patterned micro/nano materials.

New Insight into Quantity-Dependent Self-Assembly Pattern of Light Levitating Droplet Clusters.

It has been reported that the self-assembly pattern of light levitating droplet clusters above the hot gas-liquid interface is dependent on the quantity of droplets. However, the already-reported theoretical explanation of the quantity-dependent self-assembly pattern cannot work well when the quantity of the light levitating droplet exceeds 15. Herein, we propose a new theoretical perspective to understand the self-assembly of a light levitating droplet cluster by referring to the classical densest packing problem of identical rigid circles in a larger circle with the introduction of the minimum total potential energy principle. Amazingly, the theoretical results obtained by this new approach agree well with experimental results, even though the quantity of the light levitating droplet is up to 142. This study deepens our understanding of the quantity-dependent self-assembly pattern of the light levitating droplet clusters and provides significant inspiration for other analogous self-assembly phenomena.

Supramolecular J- and H-Aggregation of Naphthalimide-Conjugated Dipeptide in Mixed-Solvent Systems and its Application in Cell Imaging.

In this work, a core-substituted NMI-conjugated dipeptide (4MNLV) was extensively studied in mixed solvent systems to explore the polarity effect on the self-assembly pattern and their photophysical property. 4MNLV adopted J- or H- type aggregation pattern depending upon the polarity index of the solvent system chosen. The self-assembly process was achieved through the anti-solvent effect. UV-vis study suggested that if the stock solution of 4MNLV was diluted with a relatively more polar solvent (compared to the stock solvent), then the system acquired J- type of aggregation pattern by showing a red-shift in their absorption maxima (λmax). Conversely, when the stock was diluted by a relatively less polar solvent, H-type of aggregation was observed, where blue shift of λmax was noticed. The emission spectra and the lifetime of the self-assembled materials were also influenced by the chosen solvent system. The chirotopic behaviour of these self-assembled materials was studied through CD spectroscopy. Morphological study indicated the formation of helical nanofibrillar structures. The bright green fluorescence of these highly biocompatible naphthalimide-peptide conjugate was used for cell imaging application, indicating its futuristic scope.

Harnessing Polar Interactions Tunes the Stability of Ultrathin Polymer Solution Films.

The stability of ultrathin (<100 nm) polymer films is essential in applications like protective coatings. On the contrary, their instability may actually be desirable for the emergence of self-assembled nanoscale patterns utilized in the fabrication of functional devices. Polymer solution films exhibit two distinct kinds of instabilities, viz., dewetting (long-wave) and decomposition (short-wave). Dewetting refers to the rupture of the continuous film to form isolated domains, while decomposition leads to phase separation within the polymer solution. The focus of this work is on leveraging polar interactions between the solute and solvent molecules to tune the stability of the film. A gradient dynamics-based thin film model is developed to investigate pattern formation in a thin polar polymer solution film. The Flory-Huggins theory is suitably modified by introducing a polar interaction parameter that depends upon the concentration of the polymer and the dipole moments of monomer (μ1) and solvent molecules (μ0). A linear stability analysis is performed to determine the characteristic length scale and growth rate of the instabilities. It is shown that the range of concentration space for the occurrence of the decomposition mode is directly affected by the Flory interaction parameter (χ0), μ0, and μ1, thereby serving as control parameters to tune the width of the concentration range. It is further shown that ignoring polar interactions may lead to incorrect predictions of the instability mode, including a complete loss of the decomposition mode. In addition, the long-wave dewetting length scale is found to decrease due to bulk dipolar interactions at higher polymer concentrations. Finally, numerical simulations are carried out to track the nonlinear evolution of the interface and concentration field for both the decomposition and dewetting modes of instability.

Tailored Physicochemical Cues Direct Human Mesenchymal Stem Cell Differentiation through Epigenetic Regulation Using Colloidal Self-Assembled Patterns.

The extracellular matrix (ECM) shapes the stem cell fate during differentiation by exerting relevant biophysical cues. However, the mechanism of stem cell fate decisions in response to ECM-backed complex biophysical cues has not been fully understood due to the lack of versatile ECMs. Here, we designed two versatile ECMs using colloidal self-assembly technology to probe the mechanisms of their effects on mechanotransduction and stem cell fate regulation. Binary colloidal crystals (BCC) with a hexagonally close-packed structure, composed of silica (5 μm) and polystyrene (0.4 μm) particles as well as a polydimethylsiloxane-embedded BCC (BCCP), were fabricated. They have defined surface chemistry, roughness, stiffness, ion release, and protein adsorption properties, which can modulate the cell adhesion, proliferation, and differentiation of human adipose-derived stem cells (hASCs). On the BCC, hASCs preferred osteogenesis at an early stage but showed a higher tendency toward adipogenesis at later stages. In contrast, the results of BCCP diverged from those of BCC, suggesting a unique regulation of ECM-dependent mechanotransduction. The BCC-mediated cell adhesion reduced the size of the focal adhesion complex, accompanying an ordered spatial organization and cytoskeletal rearrangement. This morphological restriction led to the modulation of mechanosensitive transcription factors, such as c-FOS, the enrichment of transcripts in specific signaling pathways such as PI3K/AKT, and the activation of the Hippo signaling pathway. Epigenetic analyses showed changes in histone modifications across different substrates, suggesting that chromatin remodeling participated in BCC-mediated mechanotransduction. This study demonstrates that BCCs are versatile artificial ECMs that can regulate human stem cells' fate through unique biological signaling, which is beneficial in biomaterial design and stem cell engineering.

Investigating ripple pattern formation and damage profiles in Si and Ge induced by 100 keV Ar+ ion beam: a comparative study.

Desired modifications of surfaces at the nanoscale may be achieved using energetic ion beams. In the present work, a complete study of self-assembled ripple pattern fabrication on Si and Ge by 100 keV Ar+ ion beam bombardment is discussed. The irradiation was performed in the ion fluence range of ≈3 × 1017 to 9 × 1017 ions/cm2 and at an incident angle of θ ≈ 60° with respect to the surface normal. The investigation focuses on topographical studies of pattern formation using atomic force microscopy, and induced damage profiles inside Si and Ge by Rutherford backscattering spectrometry and transmission electron microscopy. The ripple wavelength was found to scale with ion fluence, and energetic ions created more defects inside Si as compared to that of Ge. Although earlier reports suggested that Ge is resistant to structural changes upon Ar+ ion irradiation, in the present case, a ripple pattern is observed on both Si and Ge. The irradiated Si and Ge targets clearly show visible damage peaks between channel numbers (1000-1100) for Si and (1500-1600) for Ge. The clustering of defects leads to a subsequent increase of the damage peak in irradiated samples (for an ion fluence of ≈9 × 1017 ions/cm2) compared to that in unirradiated samples.

Formation and Application of Polymer Spherulite-like Patterns of Halloysite Nanotubes by Evaporation-Induced Self-Assembly.

Halloysite nanotubes (HNTs) are one-dimensional clay nanomaterials featuring distinct tubular structures and unique surface charges. HNTs can readily form ordered assembly structures under specific conditions, which shows significant potential applications in optical and biological fields. In this study, sodium hexametaphosphate (SHMP) was employed as a stabilizer to prepare polymer spherulite-like patterns via the evaporation-induced self-assembly (EISA) technique. The incorporation of SHMP enhanced the repulsion force among the nanotubes and the surface potential, which facilitated the orderly deposition of HNTs. The influence of HNT concentration, SHMP concentration, drying temperature, and substrate on the polymer spherulites-like pattern has been investigated in detail. The optimal conditions were 10 wt % HNT dispersion, 0.6 wt % SHMP concentration, 30 °C as drying temperature, and glass substrates. In addition, by changing the droplet volume and shape of the three-phase contact line, patterns of different sizes and shapes can be achieved. Bovine serum albumin or metal salt compounds were incorporated into the dispersion of SHMP-modified HNTs, which altered the charge and the self-assembled patterns with different area ratios. Thus, this technology can be utilized for the analysis and comparison of protein and metal ion concentration accurately. This study creates the correlation between the structural parameters and the preparation process involved in creating polymer spherulite-like patterns of modified HNTs and offers fresh insights into potential applications for the self-assembly of HNT droplets in the realms of anticounterfeiting and solution concentration analysis.

Investigation of Orderliness of Breath Figures on Polydimethylsiloxane.

Breath figures, the self-assembled water droplet patterns formed on cold surfaces, are ideally hexagonal. A deviation from the ideal honeycomb pattern can occur due to variation of roughness of the substrate, change in vapor from water to other liquids, etc. The thermodynamics of breath figure formation is complex, and any deviation from ideality is even more difficult to understand. In the absence of a unified theory to understand such patterns and experimental difficulties in monitoring all aspects of formation of breath figures, the patterns formed are studied in terms of their orderliness by determining their Voronoi entropy. We report here the Voronoi entropy calculations of the breath figure fabricated over the smooth and constrained surfaces using polydimethylsiloxane (PDMS) of molecular weight 235 g/mol in two different environments: (a) water and (b) binary mixture of ethanol-propanol over the entire concentration range. Ordered honeycomb patterns are seen on the smooth surfaces, and disordered patterns are seen on constrained surfaces when imaged using confocal microscopy. The latter is attributed to the depinning of the triple-phase contact line, implying that the underlying constraints influence the pore morphology. Contact angle studies of water over the breath figure patterned surfaces indicate the hydrophobic nature of the patterned surfaces.

Co-precipitation vs. Green Synthesis: A Comparative Analysis of Structure, Morphology, Optical Properties, and Antibacterial Activity of Ag/ZnO Nanocomposite

Ag/ZnO nanocomposites were successfully prepared through co-precipitation (CS) and a green synthesis (GS) route mediated by Moringa Oleifera. The confirmation of their formation was evident from the simultaneous presence of Ag and ZnO reflections in powder X-ray diffraction. Field emission scanning electron microscopy (FESEM) analysis disclosed an average particle size of approximately 22 nm and 146 nm for CS and GS samples, respectively. The CS sample exhibited a disstinctive flower-like morphology, indicating a unique self-assembly and growth pattern, while the GS sample displayed an almost rectangular shape. Structural composition and functional groups in the nanocomposites were analyzed using FT-IR spectroscopy. The direct band gaps were determined as 3.12 eV for CS and 3.10 eV for GS samples using a Tauc plot derived from UV-Vis NIR spectrometry. Notably, the antibacterial evaluation of the green-synthesized Ag/ZnO exhibited a pronounced zone of inhibition against both gram-positive (S. aureus) and gram-negative (E. coli) bacteria, demonstrating superior efficacy compared to chemically prepared Ag/ZnO. Overall, the study highlights the successful synthesis, structural characterization, and enhanced antibacterial properties of Ag/ZnO nanocomposites via a green route mediated by Moringa Oleifera. GRAPHICAL ABSTRACT

Exploring the self-assembly dynamics of novel steroid-coumarin conjugates: a comprehensive spectroscopic and solid-state investigation.

The design, synthesis, and characterization of seven novel steroid-coumarin conjugates with diverse steroidal nuclei as lipophilic fluorescent materials for bioimaging applications are presented. The conjugates were synthesized through amidation, characterized using spectroscopic and spectrometric methods, and their main photophysical properties were determined. Dioxane : water titration experiments revealed their ability to self-assemble, forming J-aggregates as evidenced by new spectral bands at higher wavelengths. Monocrystal X-ray diffraction analysis disclosed distinctive aggregation patterns exhibiting J- or H-aggregates for selected compounds. Bioimaging studies demonstrated cell membrane localization for most conjugates, with some of them displaying an interesting selectivity for lipid droplets. Notably, the presence of the steroid fragments significantly influenced both the self-assembly patterns and the cellular localization of the fluorescent probes.

Incisive analysis of hydrogen-bonded supramolecular architectures in designer polycyclitols: observation of some interesting self-assembly patterns

This article examines C–H⋯O and O–H⋯O hydrogen-bonded patterns in polyoxygenated decalins, showcasing varied supramolecular architectures influenced by oxyfunctionalisation and surrogate carbonate groups.

Unveiling the secrets of unusual long-pitch periodic assembly of poly(ʟ-Lactide) ring-banded spherulites

Although polymer physicists have conducted extensive research over the decades, the molecular assembly in lamellae of ring-banded spherulites (RBS) remains a puzzle that has yet to be solved. Nonetheless, a breakthrough has occurred in this field where long-pitch banded rings made of a model poly(ʟ-lactide) (PLLA) were examined using advanced synchrotron X-ray micro-focused diffraction techniques. Through the microbeam diffraction mapping and microscopy 3D dissection morphology, the periodic banded structure of RBS is composed of lamellae that self-assemble as structural gratings. Furthermore, a comparison of optical micrograph image with the mapping results revealed the sequential arrangement and orientation of lamellae in these gratings. These extensive experimental findings now provide a comprehensive and novel explanation for the formation of long-pitch lamellar assembly in RBS. This new methodology is adaptable to a wide variety of polymers and allows for the analysis of spherulites displaying periodically self-assembled patterns. The novel methodology employed here has a versatile applicability for a diverse range of polymers. This technique can be extended to analyze spherulites showcasing periodically self-assembled patterns. Such revelation provides valuable understanding into the complex structures and behaviors of polymers, and this discovery opens up new frontiers in materials science research and its practical applications.

Polarity-Induced Morphological Transformation with Tunable Optical Output of Terpyridine-Phenanthro[9,10-d]imidazole-Based Ligand and Its Zn(II) Complexes with I-V Characteristics.

Self-assembled nanostructures obtained from various functional π-conjugated organic molecules have been able to draw substantial interest due to their inherent optical properties, which are imperative for developing optoelectronic devices, multiple-color-emitting devices with color-tunable displays, and optical sensors. These π-conjugated molecules have proven their potential employment in various organic electronic applications. Therefore, the stimuli-responsive fabrication of these π-conjugated systems into a well-ordered assembly is extremely crucial to tuning their inherent optical properties for improved performance in organic electronic applications. To this end, herein, we have designed and synthesized a functional π-conjugated molecule (TP) having phenanthro[9,10-d]imidazole with terpyridine substitution at the 2 position and its corresponding metal complexes (TPZn and (TP)2Zn). By varying the polarity of the self-assembly medium, TP, TPZn, and (TP)2Zn are fabricated into well-ordered superstructures with morphological individualities. However, this medium polarity-induced self-assembly can tune the inherent optical properties of TP, TPZn, and (TP)2Zn and generate multiple fluorescence colors. Particularly, this property makes them useful for organic electronic applications, which require adjustable luminescence output. More importantly, in 10% aqueous-THF medium, TPZn exhibited H-type aggregation-induced white light emission and behaved as a single-component white light emitter. The experimentally obtained results of the solvent polarity-induced variation in optical properties as well as self-assembly patterns were further confirmed by theoretical investigation using density functional theory calculations. Furthermore, we investigated the I-V characteristics, both vertical and horizontal, using ITO and glass surfaces coated with TP, TPZn, and (TP)2Zn, respectively, and displayed maximum current density for the TPZn-coated surface with the order of measured current density TPZn > TP > (TP)2Zn. This observed order of current density measurements was also supported by a direct band gap calculation associated with the frontier molecular orbitals using the Tauc plot. Hence, solvent polarity-induced self-assembly behavior with adjustable luminescence output and superior I-V characteristics of TPZn make it an exceptional candidate for organic electronic applications and electronic device fabrication.

Self-Assembled Monolayers of N-Heterocyclic Olefins on Au(111).

Self-assembled monolayers (SAMs) of N-heterocyclic olefins (NHOs) have been prepared on Au(111) and their thermal stability, adsorption geometry, and molecular order were characterized by X-ray photoelectron spectroscopy, polarized X-ray absorption spectroscopy, scanning tunneling microscopy (STM), and density functional theory (DFT) calculations. The strong σ-bond character of NHO anchoring to Au induced high geometrical flexibility that enabled a flat-lying adsorption geometry via coordination to a gold adatom. The flat-lying adsorption geometry was utilized to further increase the surface interaction of the NHO monolayer by backbone functionalization with methyl groups that induced high thermal stability and a large impact on work-function values, which outperformed that of N-heterocyclic carbenes. STM measurements, supported by DFT modeling, identified that the NHOs were self-assembled in dimers, trimers, and tetramers constructed of two, three, and four complexes of NHO-Au-adatom. This self-assembly pattern was correlated to strong NHO-Au interactions and steric hindrance between adsorbates, demonstrating the crucial influence of the carbon-metal σ-bond on monolayer properties.

Charge-transfer-driven enhanced room-temperature ferromagnetism in BiFeO3/Ag nanocomposite

We report observation of more than an order of magnitude jump in saturation magnetization in BiFeO3/Ag nanocomposite at room temperature compared to what is observed in bare BiFeO3 nanoparticles. Using transmission electron microscopy together with energy dispersive x-ray spectra (which maps the element concentration across the BiFeO3/Ag interface) and x-ray photoelectron spectroscopy, we show that both the observed specific self-assembly pattern of BiFeO3 and Ag nanoparticles and the charge transfer between Ag and O are responsible for such an enormous rise in room-temperature magnetization. The BiFeO3/Ag nanocomposites, therefore, could prove to be extremely useful for a variety of applications including biomedical.

Experimental and numerical studies of droplet self-assembly behaviors on hydrophilic/hydrophobic patterned surface

Self-assembled and well-defined fluid patterns of ink droplet deposition are building blocks for printed electronics and next generation of OLED displays. Using surface energy differences created by hydrophobic/hydrophilic patterned (Alt-Wet) surface to propel/retain liquid during the droplet deposition has renewed interests for high resolution printing. In this study, the droplet deposition and split dynamics on the patterned surface have been visualized experimentally. The numerical simulations have revealed that the contact angle contrast (ΔΘ) of the Alt-Wet surface can be as small as 25° to successfully split a droplet by the hydrophobic stripe, providing large enough Reynolds number (Re) for kinetic energy against droplet internal viscous bonding energy holding droplet together. In addition, the droplet splitting, satelliting, and splashing modes are highly influenced by the Alt-Wet surface pattern geometry that governs the stability of liquid diffusion and pressure distribution during the droplet deposition. The derived energy conservation model has theoretically confirmed the experimental and numerical findings that the droplet deposition dynamics are the function of fluid properties and surface wetting characteristics when given Alt-Wet unit. This systematic study on droplet deposition and self-assemble dynamics can provide general design guidance for high resolution ink patterning for printed flexible electronics and micro-fluidics applications.

Temperature-Dependent Sheet and Contact Resistivity Measurements on Ag and Ag-Ni Circuit Pastes

The metal circuits and brazes used in electronic, energy conversion, and/or energy storage devices often have difficulty wetting and adhering to ceramic and/or ceramic-passivated metal substrates. Here, a novel Particle Interlayer Directed Wetting and Spreading (PIDWAS) technique is demonstrated that utilizes screen printing, the low wetting angle of silver on nickel, and the high work of adhesion between nickel and various ceramics to produce well-adhered, self-assembled silver patterns on ceramic and/or stainless-steel substrates not normally wet by silver. The resulting Ag-Ni circuits have higher sapphire adhesion strengths (up to 30 MPa on sapphire), higher densities (>97% on sapphire), and similar high-temperature electronic resistivities to those made using commercially-available Heraeus C8710 or DAD-87 silver circuit pastes. Similarly, Ag-Ni brazes between a variety of ceramic and/or stainless-steel substrates have better microstructural stability with rapid thermal cycling, reduction-oxidation cycling, and dual atmosphere isothermal aging than conventional Ag-CuO brazes. In addition, the residual Ni in these Ag-Ni circuits and brazes can be used to chemically getter surface segregated Al, ensuring low contact resistances on a variety of chromia- and/or alumina-passivated stainless steels.Reference:[1] Hu G, Zhou Q, Bhatlawande A, Park J, Termuhlen R, Ma Y, Bieler TR, Yu HC, Qi Y, Hogan T & Nicholas JD. Patterned Nickel Interlayers for Enhanced Silver Wetting, Spreading and Adhesion on Ceramic Substrates. Scripta Materialia, 2021; 196, 113767. 10.1016/j.scriptamat.2021.113767[2] Park J, Phongpreecha T, Nicholas JD & Qi Y. Enhanced Liquid Metal Wetting on Oxide Surfaces via Patterned Particles. Acta Materialia, 2020; 199, 551-560. 10.1016/j.actamat.2020.08.037

Direct Printing of Ultrathin Block Copolymer Film with Nano-in-Micro Pattern Structures.

Nanotransfer printing (nTP) is one of the most promising nanopatterning methods given that it can be used to produce nano-to-micro patterns effectively with functionalities for electronic device applications. However, the nTP process is hindered by several critical obstacles, such as sub-20nm mold technology, reliable large-area replication, and uniform transfer-printing of functional materials. Here, for the first time, a dual nanopatterning process is demonstrated that creates periodic sub-20nm structures on the eight-inch wafer by the transfer-printing of patterned ultra-thin (<50nm) block copolymer (BCP) film onto desired substrates. This study shows how to transfer self-assembled BCP patterns from the Si mold onto rigid and/or flexible substrates through a nanopatterning method of thermally assisted nTP (T-nTP) and directed self-assembly (DSA) of Si-containing BCPs. In particular, the successful microscale patternization of well-ordered sub-20nm SiOx patterns is systematically presented by controlling the self-assembly conditions of BCP and printing temperature. In addition, various complex pattern geometries of nano-in-micro structures are displayed over a large patterning area by T-nTP, such as angular line, wave line, ring, dot-in-hole, and dot-in-honeycomb structures. This advanced BCP-replicated nanopatterning technology is expected to be widely applicable to nanofabrication of nano-to-micro electronic devices with complex circuits.

AIE gels based on Schiff base with stimuli-responsive fluorescence switching behaviors as chemosensors

Based on Schiff base unit with/without a perfluoroalkyl group, novel multi-functional compounds 1 and 2 were proven to be able to form gels with different solvents. The difference in the molecular structure of compounds 1 and 2 was the presence of a perfluoroalkyl chain. The self-assembly pattern was studied by the employment of rheological analysis, UV–vis absorption, fluorescence spectrum, field emission scanning electron microscopy (FESEM), fourier transforms infrared (FTIR) spectra, and X-ray powder diffraction (XRD). The gelation behaviors showed a sensitive and selective “turn-off” fluorescence response with Cu2+. Due to the protonation or deprotonation of compounds 1 and 2 with trifluoroacetic acid (TFA) or triethylamine (TEA), the xerogel films which were prepared from DMSO:H2O (9:1, v/v) at 5 wt% presented the emission quenching and recovery upon TFA and TEA vapors. In this sense, the work will open up a novel pathway to detect Cu2+, TFA, and TEA in the environment, using stimuli-responsive fluorescent materials with aggregation-induced emission (AIE) effect.

Defined Surface Physicochemical Cues Inhibit M1 Polarization of Human Macrophages Using Colloidal Self-Assembled Patterns.

Biophysical and biochemical cues modulate mammalian cell behavior and phenotype simultaneously. Macrophages, indispensable cells in the innate immune system, respond to external threats such as bacterial infections and implanted devices, undergoing the classical M1 polarization to become a pro-inflammatory phenotype. In the study, lipopolysaccharide (LPS)-induced M1 polarization was examined using RAW264.7, THP-1, and primary human PBMCs on a family of artificial extracellular matrix (ECM), named colloidal self-assembled patterns (cSAPs). The results showed that cSAPs were biocompatible, which cannot induce M1 or M2 polarization. Interestingly, specific cSAPs (e.g., cSAP3) suppress the level of M1 polarization (i.e., reduced nitric oxide production, down-regulated gene expression of iNOS, IL-6, TNF-α, IL-1β, and TLR4, and reduced proportion of CD11b+CD86+ cells). Transcriptome analysis showed that cell adhesion and cell-ECM interaction participated in the M1 polarization, and the mechano-sensitive genes such as PIEZO1 were down-regulated on the cSAP3. More interestingly, these genes were also down-regulated under LPS stimulation, indicating that cells became insensitive to the LPS. The abovementioned results indicate that the defined physicochemical cues can govern macrophage polarization. This study illustrates a potential surface design at biointerface, which is critical in tissue engineering and materiobiology. The outcome is also inspiring in ECM-mediated immune responses.