Packing Morphology Research Articles

Aligned carbon nanotube array stiffness from stochastic three-dimensional morphology.

The landmark theoretical properties of low dimensional materials have driven more than a decade of research on carbon nanotubes (CNTs) and related nanostructures. While studies on isolated CNTs report behavior that aligns closely with theoretical predictions, studies on cm-scale aligned CNT arrays (>10(10) CNTs) oftentimes report properties that are orders of magnitude below those predicted by theory. Using simulated arrays comprised of up to 10(5) CNTs with realistic stochastic morphologies, we show that the CNT waviness, quantified via the waviness ratio (w), is responsible for more than three orders of magnitude reduction in the effective CNT stiffness. Also, by including information on the volume fraction scaling of the CNT waviness, the simulation shows that the observed non-linear enhancement of the array stiffness as a function of the CNT close packing originates from the shear and torsion deformation mechanisms that are governed by the low shear modulus (∼1 GPa) of the CNTs.

Air-liquid interfacial self-assembly of conjugated block copolymers into ordered nanowire arrays.

The ability to control the molecular packing and nanoscale morphology of conjugated polymers is important for many of their applications. Here, we report the fabrication of well-ordered nanoarrays of conjugated polymers, based on the self-assembly of conjugated block copolymers at the air-liquid interface. We demonstrate that the self-assembly of poly(3-hexylthiophene)-block-poly(ethylene glycol) (P3HT-b-PEG) at the air-water interface leads to large-area free-standing films of well-aligned P3HT nanowires. Block copolymers with high P3HT contents (82-91%) formed well-ordered nanoarrays at the interface. The fluidic nature of the interface, block copolymer architecture, and rigid nature of P3HT were necessary for the formation of well-ordered nanostructures. The free-standing films formed at the interface can be readily transferred to arbitrary solid substrates. The P3HT-b-PEG films are integrated in field-effect transistors and show orders of magnitude higher charge carrier mobility than spin-cast films, demonstrating that the air-liquid interfacial self-assembly is an effective thin film fabrication tool for conjugated block copolymers.

Bulk charge carrier transport in push-pull type organic semiconductor.

Operation of organic electronic and optoelectronic devices relies on charge transport properties of active layer materials. The magnitude of charge carrier mobility, a key efficiency metrics of charge transport properties, is determined by the chemical structure of molecular units and their crystallographic packing motifs, as well as strongly depends on the film fabrication approaches that produce films with different degrees of anisotropy and structural order. Probed by the time-of-flight and grazing incidence X-ray diffraction techniques, bulk charge carrier transport, molecular packing, and film morphology in different structural phases of push-pull type organic semiconductor, 7,7'-(4,4-bis(2-ethylhexyl)-4H-silolo[3,2-b:4,5-b']dithiophene-2,6-diyl)bis(6-fluoro-4-(5'-hexyl-[2,2'-bithiophen]-5yl)benzo[c][1,2,5] thiadiazole), one of the most efficient small-molecule photovoltaic materials to-date, are described herein. In the isotropic phase, the material is ambipolar with high mobilities for a fluid state. The electron and hole mobilities at the phase onset at 210.78 °C are 1.0 × 10(-3) cm(2)/(V s) and 6.5 × 10(-4) cm(2)/(V s), respectively. Analysis of the temperature and electric field dependences of the mobilities in the framework of Gaussian disorder formalism suggests larger energetic and positional disorder for electron transport sites. Below 210 °C, crystallization into a polycrystalline film with a triclinic unit cell symmetry and high degree of anisotropy leads to a 10-fold increase of hole mobility. The mobility is limited by the charge transfer along the direction of branched alkyl side chains. Below 90 °C, faster cooling rates produce even higher hole mobilities up to 2 × 10(-2) cm(2)/(V s) at 25 °C because of the more isotropic orientations of crystalline domains. These properties facilitate in understanding efficient material performance in photovoltaic devices and will guide further development of materials and devices.

Optimization of Solubility, Film Morphology and Photodetector Performance by Molecular Side‐Chain Engineering of Low‐Bandgap Thienothiadiazole‐Based Polymers

A series of donor–acceptor (D‐A) type low‐bandgap polymers containing the terthiophene and thieno[3,4‐b]thiadiazole units in the main chain but different numbers of identical side chains are designed and synthesized in order to study the effect of side chain on the polymer properties and optimize the performance of polymer photodetectors. Variation in the side chain content can influence the polymer solubility, molecular packing, and film morphology, which in turn affects the photodetector performance, particularly with regard to the photoresponsivity and dark current. X‐ray diffraction patterns indicate that molecular ordering increases with more side chains. Atomic force microscopy shows that appropriate morphology of the active layer in the polymer photodetector is necessary for high photocurrent and low dark current. Using BCP as a hole blocking layer (10 nm), the photodetector based on P4 exhibits the optimized performance with specific detectivity of 1.4 × 1012 Jones at 800 nm, which is among the best reported values for polymer photodetectors and even comparable to that of a silicon photodetector.

Scalable synthesis of 5,11-diethynylated indeno[1,2-b]fluorene-6,12-diones and exploration of their solid state packing.

We report a new synthetic route to 5,11-disubstituted indeno[1,2-b]fluorene-6,12-diones that is amenable to larger scale reactions, allowing for the preparation of gram amounts of material. With this new methodology, we explored the effects on crystal packing morphology for the indeno[1,2-b]fluorene-6,12-diones by varying the substituents on the silylethynyl groups.

Impact of microstructure on the effective diffusivity in random packings of hard spheres

We present results of computer simulations of the effective diffusion coefficient in bulk random packings of hard monosized spheres with solid volume fraction between 0.54 (random-loose packing) and 0.634 (maximally random jammed). Six types of sphere packings were generated with different protocols and parameters resulting in a systematically varied degree of microstructural heterogeneity. The packing morphology is qualitatively characterized by statistical analyses of Voronoi cells obtained from spatial tessellation of the packing space. Diffusive transport of point-like tracers in the pore space of the packings was simulated with a random-walking particle-tracking technique. Our results indicate that the effective transport characteristics of the random sphere packings are not fully defined from the solid volume fraction but also depend on the packing microstructure. For the first time, we compared (i) the values of the effective diffusion coefficient Deff simulated in packings with different morphologies, and (ii) the corresponding values of Deff obtained from an approximate analytical formula involving the three-point microstructural parameter ζ2. This analysis reveals that this approximation involving ζ2 clearly reflects key morphological specificity of individual sphere packings and provides a sufficiently accurate estimate of the effective diffusion coefficient.

3D printed porous media columns with fine control of column packing morphology

In this paper we demonstrate, for the first time, the use of 3D printing (also known as additive manufacturing or rapid prototyping) to create porous media with precisely defined packing morphologies, directly from computer aided design (CAD) models. We used CAD to design perfectly ordered beds with octahedral beads (115μm apothem) packed in a simple cubic configuration and monoliths with hexagonal channels (150μm apothem) in parallel and herringbone arrangements. The models were then printed by UV curing of acrylonitrile-butadiene-styrene powder layers. Each porous bed was printed at 1.0, 1.5 and 2.0mL volumes, within a complete column, including internal flow distributors and threaded 10–32 flow connectors. Close replication of CAD models was achieved. The resultant individual octahedral beads were highly uniform in size, with apothems of 113.6±1.9μm, while the monolith hexagonal cross-section channels had apothems of 148.2±2.0μm. Residence time distribution measurements show that the beds largely behaved as expected from their design void volumes. Radial and fractal flow distributor designs were also tested. The former displayed poor flow distribution in parallel and herringbone pore columns, while the fractal distributors provided uniform flow distribution over the entire cross section. The results show that 3D printing is a feasible method for producing precisely controlled porous media. We expect our approach to revolutionize not only fundamental studies of flow in porous media but methods of chromatography column production.

Panchromatic small molecules for UV-Vis-NIR photodetectors with high detectivity

Two donor–acceptor–donor (D–A–D) type low-bandgap small molecules, M1 and M2, with bis(2-thienyl)-N-alkylpyrrole (TPT) as the donor and thieno[3,4-b]thiadiazole (TT) as the acceptor were designed and synthesized. The absorption, transmission, electrochemical, thermal and film properties were studied. The compounds showed panchromatic absorption in the spectral range of 300–1000 nm. Moreover, they also exhibited semi-transparent property in the visible region (400–700 nm). Small molecule photodetectors (SMPDs) based on M1 and M2 were fabricated and studied. For the SMPD with BCP as the hole blocking layer (HBL), a detectivity of 5.0 × 1011 Jones at 800 nm at −0.1 V was obtained, which is among the highest detectivities reported for NIR SMPDs. With a sufficiently thin silver electrode, visibly transparent PDs with an average transmittance of 45% in the visible region were obtained for the first time. The transparent PDs exhibited fairly constant and high detectivity between 1011 and 1012 Jones over a broad spectral range of 300–900 nm. In addition, side chains of the compounds exhibited a great influence on the device performance, which could be assigned to their film absorption coefficient, molecular packing and active layer morphology.

Molecular stacking structure and field-effect transistor characteristics of crystalline poly(3-hexylthiophene)-block-syndiotactic polypropylene through solvent selectivity

The effect of both the solvent selectivity and rod/coil ratio on the molecular packing structures, morphologies and field-effect characteristics of the crystalline–crystalline P3HT-b-sPP diblock copolymers are reported.

Membrane Insertion Potential of Synthetic Cell Penetrating Peptides

Cell penetrating peptides (CPPs) have been established as excellent candidates for mediating drug delivery into cells. When designing synthetic CPPs for drug delivery applications, it is important to understand their ability to penetrate the cell membrane. In this paper, anionic or zwitterionic phospholipid monolayers at the air-water interface are used as model cell membranes to monitor the membrane insertion potential of synthetic CPPs. The insertion potential of CPPs having different cationic and hydrophobic amino acids were recorded using a Langmuir monolayer approach that records peptide adsorption to model membranes. Fluorescence microscopy was used to visualize alterations in phospholipid packing due to peptide insertion. All CPPs had the highest penetration potential in the presence of anionic phospholipids. In addition, two of three amphiphilic CPPs inserted into zwitterionic phospholipids, but none of the hydrophilic CPPs did. All the CPPs studied induced disruptions in phospholipid packing and domain morphology, which were most pronounced for amphiphilic CPPs. Overall, small changes to amino acids and peptide sequences resulted in dramatically different insertion potentials and membrane reorganization. Designers of synthetic CPPs for efficient intracellular drug delivery should consider small nuances in CPP electrostatic and hydrophobic properties.

Dynamic measurements of membrane insertion potential of synthetic cell penetrating peptides.

Cell penetrating peptides (CPPs) have been established as excellent candidates for mediating drug delivery into cells. When designing synthetic CPPs for drug delivery applications, it is important to understand their ability to penetrate the cell membrane. In this paper, anionic or zwitterionic phospholipid monolayers at the air-water interface are used as model cell membranes to monitor the membrane insertion potential of synthetic CPPs. The insertion potential of CPPs having different cationic and hydrophobic amino acids were recorded using a Langmuir monolayer approach that records peptide adsorption to model membranes. Fluorescence microscopy was used to visualize alterations in phospholipid packing due to peptide insertion. All CPPs had the highest penetration potential in the presence of anionic phospholipids. In addition, two of three amphiphilic CPPs inserted into zwitterionic phospholipids, but none of the hydrophilic CPPs did. All the CPPs studied induced disruptions in phospholipid packing and domain morphology, which were most pronounced for amphiphilic CPPs. Overall, small changes to amino acids and peptide sequences resulted in dramatically different insertion potentials and membrane reorganization. Designers of synthetic CPPs for efficient intracellular drug delivery should consider small nuances in CPP electrostatic and hydrophobic properties.

Control of self-assembled morphology and molecular packing of asymmetric glycolipids by association/dissociation with poly(thiopheneboronic acid).

The molecular packing and self-assembled morphologies of asymmetric bolaamphiphiles, N-(2-aminoethyl)-N'-(β-d-glucopyranosyl)alkanediamide [1(n), n = 12, 14, 16, 17, 18, and 20], were precisely controlled by association/dissociation with poly(thiopheneboronic acid) (PTB). Differential scanning calorimetry, X-ray diffraction, and infrared spectroscopy revealed that the starting film of 1(n) associated with 1 equiv of the boronic acid moiety of PTB, (Film-1(n)PTB), had antiparallel molecular packing of 1(n) moiety within the monolayer membranes. However, the molecular packing of the starting film that contained 0.5 equiv of the boronic acid moiety of PTB (Film-2eq1(n)PTB) was parallel. The dispersion of Film-1(n)PTB in water gave only nanotapes, whereas that of Film-2eq1(n)PTB in water selectively formed nanotubes, through a dissociation reaction of PTB based on the hydrolysis of the boronate esters in the complexes. The nanotapes and nanotubes memorized the antiparallel and parallel molecular packing of the starting films, respectively. Changes in the length of the oligomethylene spacer of 1(n) never affected the molecular packing or self-assembled morphologies. However, the inner diameters of the nanotubes increased irregularly in the range of 67.9-79.6 nm as the length of the oligomethylene spacer of 1(n) increased from n = 12 to n = 18.

Coexistence of Helical Morphologies in Columnar Stacks of Star-Shaped Discotic Hydrazones

Discotic hydrazone molecules are of particular interest as they form discotic phases where the discs are rigidified by intramolecular hydrogen bonds. Here, we investigate the thermotropic behavior and solid-state organizations of three discotic hydrazone derivatives with dendritic groups attached to their outer peripheries, containing six, eight, and ten carbons of linear alkoxy chains. On the basis of two-dimensional wide angle X-ray scattering (2DWAXS), the elevated temperature liquid crystalline (LC) phases were assigned to a hexagonal columnar (Colh) organization with nontilted hydrazone discs for all three compounds. With WAXS, advanced solid-state nuclear magnetic resonance (SSNMR) techniques, and ab initio computations, the compounds with six and ten carbons of achiral alkoxy side chains were further subjected to studies at 25 °C, revealing complex crystalline phases with rigid columns and flexible side chains. This combined approach led to models of coexisting helical columnar stacking morphologies for both systems with two different tilt/pitch angles between successive hydrazone molecules. The differences in tilt/pitch angles between the two compounds illustrate that the columns with short alkoxy chains (six carbons) are more influenced by the presence of other stacks in their vicinity, while those with long side chains are less tilted due to a larger alkoxy (ten carbons) buffer zone. The formation of different packing morphologies in the crystalline phase of a columnar LC has rarely been reported so far, which suggests the possibility of complex stacking structures of similar organic LC systems, utilizing small molecules as potential materials for applications in organic electronics.

Morphology and chain aggregation dependence of optical gain in thermally annealed films of the conjugated polymer poly[2-methoxy-5-(2′-ethylhexyloxy)-p-phenylene vinylene

Aggregate formation in conjugated polymer films is one of the most important phenomena thought to influence the photophysical properties of optical devices based on these materials. In the current work, we report the results of a detailed investigation on the morphology and chain aggregation dependence of optical gain in spin-coated thin films of the conjugated polymer poly[2-methoxy-5-(2′-ethylhexyloxy)-p-phenylene vinylene] (MEH-PPV). Extensive gain measurements are performed using the variable stripe length technique with picosecond pulse excitation. The polymer morphology and extent of aggregate formation in the films are controlled by thermal annealing, which is relevant to the fabrication and optimization of conjugated polymer-based optical devices. The aggregation state of the polymer chains increases with the annealing temperature, which results in a decrease in luminescence efficiency at low excitation density (≤1018 cm−3). However, the increase in aggregate formation with increasing annealing temperature does not significantly alter the optical gain; very large gain coefficients are still achieved in films containing a relatively large fraction of aggregates. Although the largest gain coefficients, 450 cm−1, are observed for as-cast (non-annealed) MEH-PPV films, very large gain coefficients of 315 and 365 cm−1 are also demonstrated for MEH-PPV films annealed at 60 and 80 °C, respectively, in spite of the enhanced packing morphology and conformational order of the polymer chains. These results are contrary to the commonly held view that aggregate formation has a detrimental effect on the amplified spontaneous emission behavior of polymer-based devices operating in the stimulated emission regime, as would be characteristic of lasers and optical amplifiers. Moreover, because aggregates promote favorable charge transport properties, our data have important implications for future development of electrically driven polymer lasers; improving carrier mobility through controlled increases in chain aggregation should provide a viable path for enhancing injection efficiency without significantly degrading optical gain.

Linear Long Alkyl Chain Substituted POSS Cages: The Effect of Alkyl Chain Length on the Self-Assembled Packing Morphology

The self-assembled long-range ordering, packing morphology and thermal behavior of a series of polyhedral oligomeric silsesquioxanes (POSS) T8 cage systems with long n-alkyl chain pendant groups is presented. State-of-the-art time-resolved small- and wide-angle X-ray scattering (SAXS/WAXS), thermal and optical techniques were used to elucidate their morphology. X-ray scattering showed long-range crystalline ordering where the alkyl chains align in an axial disposition from the POSS core giving a “rod-like” self-assembled packing morphology with a length-scale correlating to the overall molecular dimensions. Phase transitions observed by thermal methods were comparable to those seen in the crystallization of bulk n-alkanes. Optical measurements and crystallization kinetics confirmed a lamellar macromorphology again being similar to the crystal structure of bulk n-alkanes. This study reveals for the first time, the direct effect that the alkyl chain length has on the self-assembled crystalline packing morphology of this novel and potentially commercially relevant, series of POSS systems.

Study on preparation method for short-chain alkylsiloxane self-assembled monolayers and the diffusion behavior of copper on silica surfaces

The properties of self-assembled alkylsiloxane on hydroxylized silica surfaces were studied to explore the preparation method of the short-chain self-assembled monolayers (SAMs), and the diffusion behavior of copper on the SAMs was also investigated using molecular dynamics (MD) simulation method. Six models were built on four kinds of silica crystalline planes to form different interface bonding modes. The influences of bonding modes, chain lengths and terminal groups on the thickness, packing, morphologies and roughness of the SAMs were studied to analyze the relationships between these factors respectively and the stability of the SAMs. The results show that the thickness values of the SAMs with cross-linking mode are similar to the experimental values, and are much greater than those of the single-linking SAMs. For single-linking systems, long-chain SAMs are more ordered than short-chain SAMs, while for cross-linking systems, short-chain SAMs are more ordered. Terminal groups are of little influence on packing and surface morphology. In the systems studied, 2-methylene SAMs could perfectly be formed on α-quartz (100) surface in a cross-linking mode. The copper diffusion behavior on this surface was also studied for the ordered short-chain SAMs might be promising diffusion barriers for copper connections in the ultra-large-scale integrated circuit fabrication in the future.

Structural modification of thieno[3,4-c]pyrrole-4,6-dione: structure–property relationships and application in solution-processed small-molecule organic solar cells

We present a method for the modification of the structure of thieno[3,4-c]pyrrole-4,6-dione (TPD) to gain two novel acceptors TPN and TIN. And a series of donor–acceptor–donor-type oligomers 3TTPD3T, 3TTPN3T, and 3TTIN3T, based on TPD, TPN and TIN, have been designed and synthesized for solution-processed small-molecule bulk-heterojunction (BHJ) solar cells. The impact of these different central acceptor moieties on their optical, electrochemical properties, morphology, and solar cell performance were studied. Optical and electrochemical data showed that structural modification of TPD exhibited a slight impact on the band gap and HOMO/LUMO energy levels. Atomic force microscopy and grazing incidence wide-angle X-ray scattering studies revealed that the central acceptors of these oligomers played an important role in determine the morphology of solid-state packing. Oligomer 3TTPD3T showed weak crystallinity in a thin film. The larger conjugation plane counterpart 3TTPN3T showed an improved structure order. Surprisingly, the structurally twisted compound 3TTIN3T showed a strong crystalline nature with large crystalline domains of several hundred nanometers. The photovoltaic properties of these oligomers were evaluated by fabricating BHJ devices with PC71BM. 3TTPD3T/PC71BM blends with a weight ratio of 2 : 1 showed a best power conversion efficiency of 1.87%, with a Jsc of 4.93 mA cm−2, a Voc of 0.87 V, and a fill factor of 44%.

1,5-, 2,6- and 9,10-distyrylanthracenes as luminescent organic semiconductors

We report the synthesis, molecular and crystal structures, optoelectronic properties and organic field-effect transistor (OFET) studies of three isomeric distyrylanthracene derivatives 2,6-DPSAnt, 1,5-DPSAnt and 9,10-DPSAnt. The analysis of the structure–property relationships reveals that π-conjugation in this series is defined by two competing factors: the co-planarity of the styryl substituents (highest in 2,6-DPSAnt) and the electronic communication through the anthracene core (strongest in 9,10-DPSAnt). The charge-transport properties of the compounds, on the other hand, are primarily controlled by the solid state packing which is more efficient for the linear 2,6-DPSAnt derivative. Accordingly, the highest field-effect mobility (up to 0.75 cm2 V−1 s−1) was measured for 2,6-DPSAnt. Unexpectedly high charge mobility of 1,5-DPSAnt (up to 0.15 cm2 V−1 s−1) was attributed to excellent thin-film morphology (nearly layer-by-layer growth) and favorable molecular packing in the films, different from that found in single crystals. Unfavorable molecular packing and poor film morphology observed for 9,10-DPSAnt predictably led to no transistor properties being observed for this isomer. Both 2,6-DPSAnt and 1,5-DPSAnt afforded light-emitting transistors with green light emission area localized near an electron-injecting electrode.

Coordination number model to quantify packing morphology of aligned nanowire arrays

The average inter-wire spacing in aligned nanowire systems strongly influences both the physical and transport properties of the bulk material. Because most studies assume that the nanowire coordination is constant, a model that provides an analytical relationship between the average inter-wire spacings and measurable physical properties, such as nanowire volume fraction, is necessary. Here we report a continuous coordination number model with an analytical relationship between the average nanowire coordination, diameter, and volume fraction. The model is applied to vertically aligned carbon nanotube (VACNT) and nanofiber (VACNF) arrays, and the effective nanowire coordination number is established from easily accessible measures, such as the nanowire spacing and diameter. VACNT analysis shows that the coordination number increases with increasing nanowire volume fraction, leading the measured inter-CNT spacing values to deviate by as much as 13% from the spacing values predicted by the typically assumed hexagonal packing. VACNF analysis suggests that, by predicting an inter-fiber spacing that is within 6% of the reported value, the continuous coordination model outperforms both square and hexagonal packing in real nanowire arrays. Using this model, the average inter-wire spacing of nanowire arrays can be predicted, thus allowing more precise morphology descriptions, and thereby supporting the development of more accurate structure-property models of bulk materials comprised of aligned nanowires.

Properties and self-assembled packing morphology of long alkyl-chained substituted polyhedral oligomeric silsesquioxanes (POSS) cages

Polyhedral oligomeric silsesquioxane (POSS) cubic cage systems (octa-n-octadecyloctasilsesquioxane, (T8C18) and octakis(n-octadecyldimethylsiloxy)octasilsesquioxane, (Q8C18)) were synthesised with eight long n-alkyl chain (R = C18H37) substituent arms, as model nano-functionalized compounds. The crystalline packing morphology of the cages was studied using time-resolved Small- and Wide-angle X-ray scattering (SAXS/WAXS), thermal and optical techniques. From thermal analysis the melting and crystallization temperatures of the Q8 cage were significantly less than those for the T8 cage. X-ray scattering showed that both cage systems have long-range crystalline ordering where the alkyl chains align in a parallel axial disposition from the POSS core giving a 'rod-like' self-assembled packing morphology. The packing length-scale can be directly related to the overall dimensions of the POSS molecules. Compared to the T8 cages, the Q8 cages pack more efficiently allowing the interdigitation of the alkyl chain arms. Different packing modes and thermal behaviour observed for the T8 and Q8 cages is directly attributed to their structural chemistry. For the Q8 cage, the presence of the OSiMe2 spacer groups which tether the alkyl chain arms to the cage (absent in the T8 cages) allows greater flexibility of the arms letting them interdigitate with each other when packing which is not observed for the analogous T8 cages.