A Low-Symmetry Cubic Mesophase of Dendronized CdS Nanoparticles and Their Structure-Dependent Photoluminescence

Deep insight into regulation mechanism of band distribution in phase junction CdS for enhanced photocatalytic H2 production

An in-depth understanding of structure-activity relationship between the phase constitution and solar-to-hydrogen (STH) conversion efficiency is conducive to guiding the optimization route of targeted photocatalyst candidates, further establishing advanced photocatalytic systems. Herein, based on the concept of phase engineering, we encompassed the crystalline phase of CdS and achieved precise regulation of phase proportion as well as phase boundary width in the phase junction for the first time. The above cooperative effect not only modifies energy band distribution for sufficient redox potentials, but also guarantees the reverse migration orientation of photogenerated carriers in phase junction, thereby endowing photocarriers with a prolonged lifetime. Compared to pure cubic or hexagonal phase (72.6 or 101.1 μmol h-1 g-1), this CdS system with optimized phase junction demonstrates an improved photocatalytic hydrogen evolution activity of 1.04 mmol h-1 g-1 and favorable stability without cocatalyst assistance, which mainly stems from an efficient protons reduction process interacting with long-lived photogenerated electrons. This research explores the mechanism behind phase regulation and its relationship with junction capability, providing a powerful strategy to manipulate crystal phase distribution and paving a feasible avenue for other phase-dependent photocatalysts towards rational design of heterostructures based on different phases in solar energy conversion field.

One-Dimensionally Arranged Quantum-Dot Superstructures Guided by a Supramolecular Polymer Template

Colloidal quantum dots (QDs) exhibit important photophysical properties, such as long-range energy diffusion, miniband formation, and collective photoluminescence, when aggregated into well-defined superstructures, such as three-dimensional (3D) and two-dimensional (2D) superlattices. However, the construction of one-dimensional (1D) QD superstructures, which have a simpler arrangement, is challenging; therefore, the photophysical properties of 1D-arranged QDs have not been studied previously. Herein, we report a versatile strategy to obtain 1D-arranged QDs using a supramolecular polymer (SP) template. The SP is composed of self-assembling cholesterol derivatives containing two amide groups for hydrogen bonding and a carboxyl group as an adhesion moiety on the QDs. Upon mixing the SP and dispersed QDs in low-polarity solvents, the QDs self-adhered to the SP and self-arranged into 1D superstructures through van der Waals interactions between the surface organic ligands of the QDs, as confirmed by transmission electron microscopy. Furthermore, we revealed efficient photoinduced fluorescence resonance energy transfer between the 1D-arranged QDs by an in-depth analysis of the emission spectra and decay curves.

The Effect of Mesogenic Coronas on the Type and Anisotropy of Gold Nanoparticle Superlattices: When Can the Tail Wag the Dog?

The correlation between the size of nanoparticles, the structure and shape of mesogenic ligands and the ensuing assembly behaviour is not really understood. Closer inspection shows very surprising features. Here, 2- and 4-nm gold nanoparticles (NPs) were synthesized, and grafted with a forked ligand containing two rod-like mesogens in its branches: one cholesterol, the other with azobenzene. The 4-nm NPs also contained n-hexylthiol as co-ligand. They were found to form a FCC cubic superlattice, whereas the 2-nm NPs form hexagonal HCP with weak birefringence, hence with partially oriented ligands. The structures were compared with those of related systems containing a range of different azobenzene-to-cholesterol ratios, all giving body-centred tetragonal superlattices with various degrees of anisotropy. Geometric analysis is presented in terms of the asphericity of the NPs' surroundings, requirement for space-filling and structural anisotropy. Some general rules are derived to help design the soft corona around the NPs in order to obtain superlattices with the desired structure and anisotropy.

Self-assembly of gold nanoparticles into an adjustable plasmonic 3D lattice using Janus-type forked mesogenic ligands

We report the formation of a 3D body-centred self-assembled superlattice of gold nanoparticles whose interparticle gap, and hence its plasmonic properties, are adjustable exclusively in the xy -plane. Thus, even though the particles are spherical, their anisotropic packing generates tailorable plasmonic dichroism. The gold nanoparticles are coated with forked ligands containing two mesogens: either two cholesterols ("twin"), one cholesterol and one azobenzene ("Janus"), or a mixture of the two. Beside the body-centered arrangement of gold nanoparticles, the structure also contains unusual two-dimensionally modulated smectic-like layers of mesogens in an egg-box geometry. Moreover, the presence of azobenzene mesogens allows the superlattice to be melted through UV-induced photo-isomerization; the process is reversible displaying low fatigue on repeated cycling.

An Accelerated Modular-Orthogonal Ni-Catalyzed Methodology to Symmetric and Nonsymmetric Constitutional Isomeric AB2 to AB9 Dendrons Exhibiting Unprecedented Self-Organizing Principles

Five libraries of natural and synthetic phenolic acids containing five AB₃, ten constitutional isomeric AB₂, one AB₄, and one AB₅ were previously synthesized and reported by our laboratory in 5 to 11 steps. They were employed to construct seven libraries of self-assembling dendrons, by divergent generational, deconstruction, and combined approaches, enabling the discovery of a diversity of supramolecular assemblies including Frank-Kasper phases, soft quasicrystals, and complex helical organizations, some undergoing deracemization in the crystal state. However, higher substitution patterns within a single dendron were not accessible. Here we report three libraries consisting of 30 symmetric and nonsymmetric constitutional isomeric phenolic acids with unprecedented sequenced patterns, including two AB₂, three AB₃, eight AB₄, five AB₅, six AB₆, three AB₇, two AB₈, and one AB₉ synthesized by accelerated modular-orthogonal Ni-catalyzed borylation and cross-coupling. A single etherification step with 4-(n-dodecyloxy)benzyl chloride transformed all these phenolic acids, of interest also for other applications, into self-assembling dendrons. Despite this synthetic simplicity, they led to a diversity of unprecedented self-organizing principles: lamellar structures of interest for biological membrane mimics, helical columnar assemblies from rigid-solid angle dendrons forming Tobacco Mosaic Virus-like assemblies, columnar organizations from adaptable-solid angle dendrons forming disordered micellar-like nonhelical columns, columns from supramolecular spheres, five body-centered cubic phases displaying supramolecular orientational memory, rarely encountered in previous libraries forming predominantly Frank-Kasper phases, and two Frank-Kasper phases. Lessons from these self-organizing principles, discovered within a single generation of self-assembling dendrons, may help elaborate design principles for complex helical and nonhelical organizations of synthetic and biological matter.

Thermomechanically controlled fluorescence anisotropy in thin films of InP/ZnS quantum dots

Macroscopic scale sources of polarized light play a fundamental role in designing light-emitting devices. In this communication we report the formation of nano- and macro-scale ordered, layered assemblies of InP/ZnS quantum dots (QDs) exhibiting fluorescence anisotropy (FA), as well as thermo- and mechano-responsive properties. The long-range organization of small, quasi-isotropic nanoparticles was achieved by introducing liquid crystal molecules to the surface of QDs, without the need to use an organic matrix. Melting/crystallization of the ligand at 95 deg. C translated to a reversible reconfiguration of QDs thin film between 2D layered and body-centered cubic structures, characteristic for a temperature range below and above the melting point, respectively. The low-temperature, layered structure exhibited mechano-responsiveness which was key to introduce and control the sample alignment. Interestingly, transverse and parallel alignment modes of QDs layers were achieved, depending on the temperature of mechanical shearing. As prepared QD samples exhibited fluorescence anisotropy strongly correlated to the macroscopic orientation of the layers. Correlated small-angle X-ray diffraction (SAXRD) and fluorescence spectroscopy studies confirmed the mm-scale alignment of the thin films of QDs. Such films may be advantageous for developing efficient, densely packed, and uniform macro-scale FA sources.

A Highly Ordered Quantum Dot Supramolecular Assembly Exhibiting Photoinduced Emission Enhancement

Multicomponent supramolecular assembly systems enable the generation of materials with outstanding properties, not obtained from single-component systems, via a synergetic effect. Herein, we demonstrate a novel supramolecular coassembly system rendering highly ordered quantum dot (QD) arrangement structures formed via the self-assembly of azobenzene derivatives, where the photocontrollable photoluminescence (PL) properties of the QDs are realized based on photoisomerization. Upon mixing the assembled azobenzene derivatives and QDs in apolar media, a time-evolution coaggregation into hierarchical nanosheets with a highly ordered QD arrangement structure occurs. Upon photoirradiation, the nanosheets transform into ill-defined aggregates without arranged QDs together with enhancing the PL intensity. In days, the photoirradiated coaggregates undergo recovery of the PL properties corresponding to the arranged QDs through thermal isomerization.

Frustrated Layered Self-Assembly Induced Superlattice from Two-Dimensional Nanosheets

Here we reported a hierarchical self-assembly approach toward well-defined superlattices in supramolecular liquid crystals by fullerene-based sphere-cone block molecules. The fullerenes crystallize to form monolayer nanosheets intercalated by the attached soft hydrocarbon cones. The frustration caused by cross-sectional area mismatch between the spheres and the somewhat oversize cones leads to a unique lamellar superlattice whereby each stack of six pairs of alternating sphere-cone sublayers is followed by a cone double layer. While such areal mismatch problems in soft matter are usually solved by interface curvature, the lamellar superlattice solution is best suited to systems with rigid layers. Meanwhile, formation of the superlattice significantly improves the material's transient electron conductivity, with the maximum value being among the highest for π-conjugated organic materials. The design principle of solving steric frustration by forming a superlattice opens a new avenue toward self-assembled optoelectronic materials.

Self-Assembly of Semiconductor Quantum Dots using Organic Templates

Colloidal semiconductor nanocrystals, known as quantum dots (QDs), are regarded as brightly photoluminescent nanomaterials possessing outstanding photophysical properties, such as high photodurability and tunable absorption and emission wavelengths. Therefore, QDs have great potential for a wide range of applications, such as in photoluminescent materials, biosensors and photovoltaic devices. Since the development of synthetic methods for accessing high-quality QDs with uniform morphology and size, various types of QDs have been designed and synthesized, and their photophysical properties dispersed in solutions and at the single QD level have been reported in detail. In contrast to dispersed QDs, the photophysical properties of assembled QDs have not been revealed, although the structures of the self-assemblies are closely related to the device performance of the solid-state QDs. Therefore, creating and controlling the self-assembly of QDs into well-defined nanostructures is crucial but remains challenging. In this Minireview, we discuss the notable examples of assembled QDs such as dimers, trimers and extended QD assemblies achieved using organic templates. This Minireview should facilitate future advancements in materials science related to the assembled QDs.

Magnetic field induced uniaxial alignment of the lyotropic liquid-crystalline PMMA-grafted Fe3O4 nanoplates with controllable interparticle interaction

Magnetite (Fe₃O₄) nanoplates with a hexagonal platelet shape were synthesized by two steps: hydrothermal synthesis of iron(iii) oxide (α-Fe₂O₃) nanoplates followed by wet chemical reduction of the α-Fe₂O₃ nanoplates. Then, poly(methyl methacrylate) (PMMA) chains were grafted onto the surface of the hexagonal Fe₃O₄ nanoplates (F) via surface-initiated atom transfer radical polymerization (SI-ATRP), which ensures dispersion stability in organic solvents and ionic liquids. After mixing with 1-ethyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide ([Emim⁺][NTf₂ ^-]), a representative ionic liquid, the resulting PMMA-modified F were found to show good lyotropic liquid-crystalline (LC) behaviour in [Emim⁺][NTf₂ ^-] and to exhibit a fast response to the application of an external magnetic field. Ultrasmall-angle synchrotron X-ray scattering (USAXS) measurements verified that the PMMA chain length, the weight ratio of the ionic liquid and the external magnetic field could significantly influence the interparticle distance (I _D) of the PMMA-modified F in [Emim⁺][NTf₂ ^-]. In particular, the lyotropic LC phase could be assigned as a nematic phase with a columnar alignment. In addition, the PMMA-modified F maintained a uniaxially aligned nematic columnar structure along the magnetic field direction. Our study also determined the mechanism for the special alignment of the PMMA-modified F under an external magnetic field by analysing the growth axis, the easy magnetic axes, and the interparticle distance of F. The results suggested that the special alignment of the PMMA-modified F was affected by the interparticle interaction caused by the PMMA long chains on F under the magnetic field. Furthermore, the present study revealed that PMMA-modified F exhibited a new magnetic field responsive behaviour that led not only to the formation of a uniaxial alignment structure but also to control of I _D with the help of the PMMA soft corona under the application of a magnetic field. These features could prove to be a promising advance towards novel applications of magnetic nanoparticles (NPs), such as functional magnetic fluids, rewritable magnetic switching devices, and smart magneto-electrochemical nanosensors.

Size-Dependent Thermo- and Photoresponsive Plasmonic Properties of Liquid Crystalline Gold Nanoparticles

Achieving remotely controlled, reversibly reconfigurable assemblies of plasmonic nanoparticles is a prerequisite for the development of future photonic technologies. Here, we obtained a series of gold-nanoparticle-based materials which exhibit long-range order, and which are controlled with light or thermal stimuli. The influence of the metallic core size and organic shell composition on the switchability is considered, with emphasis on achieving light-responsive behavior at room temperature and high yield production of nanoparticles. The latter translates to a wide size distribution of metallic cores but does not prevent their assembly into various, switchable 3D and 2D long-range ordered structures. These results provide clear guidelines as to the impact of size, size distribution, and organic shell composition on self-assembly, thus enhancing the smart design process of multi-responsive nanomaterials in a condensed state, hardly attainable by other self-assembly methods which usually require solvents.

Self-assembled structure of dendronized CdS nanoparticles

Self-assembled dendronized CdS nanoparticles have been attracting considerable attention because of their photoluminescence properties depending on annealing treatments. In this study, their annealing-induced self-assembled structure was investigated via scanning transmission electron microscopy; thin foil specimens of self-assembled dendronized CdS nanoparticles were prepared by ultramicrotomy and the STEM images revealed their ordered structure and the effect of the annealing treatment. In addition, a structural order belonging to the P213 space group was identified via an autocorrelation analysis. The results indicated that this structural order could be achieved only over a few tens of nanometers.

Tackling the Scalability Challenge in Plasmonics by Wrinkle-Assisted Colloidal Self-Assembly

Electromagnetic radiation of a certain frequency can excite the collective oscillation of the free electrons in metallic nanostructures using localized surface plasmon resonances (LSPRs), and this phenomenon can be used for a variety of optical and electronic functionalities. However, nanostructure design over a large area using controlled LSPR features is challenging and requires high accuracy. In this article, we offer an overview of the efforts made by our group to implement a wrinkle-assisted colloidal particle assembly method to approach this challenge from a different angle. First, we introduce the controlled wrinkling process and discuss the underlying theoretical framework. We then set out how the wrinkled surfaces are utilized to guide the self-assembly of colloidal nanoparticles of various surface chemistry, size, and shape. Subsequently, template-assisted colloidal self-assembly mechanisms and a general guide for particle assembly beyond plasmonics will be presented. In addition, we also discuss the collective plasmonic behavior in depth, including strong plasmonic coupling due to nanoscale gap size as well as magnetic mode excitation and demonstrate the potential applications of wrinkle-assisted colloidal particle assembly method in the field of mechanoresponsive metasurfaces and surface-enhanced spectroscopy. Lastly, a general perspective in the field of template-assisted colloidal assembly with regard to potential applications in plasmonic photocatalysis, solar cells, optoelectronics, and sensing devices is provided.

Molecular ejection transition in liquid crystal columns self-assembled from wedge-shaped minidendrons

Fan-shaped molecules with aromatic head-groups and two or more flexible pendant chains often self-assemble into columns that form columnar liquid crystals by packing on a 2d lattice. Such dendrons or minidendrons are essential building blocks in a large number of synthetic self-assembled systems and organic device materials. Here we report a new type of phase transition that occurs between two hexagonal columnar phases, Colh1 and Colh2, of Na-salt of 3,4,5-tris-dodecyloxy benzoic acid. Interestingly, the transition does not change the symmetry, which is p6mm in both phases, but on heating it involves a quantised drop in the number of molecules n in the cross-section of a column. The drop is from 4 to 3.5, with a further continuous decrease toward n = 3 as temperature increases further above Tc. The finding is based on evidence from X-ray diffraction. Using a transfer matrix formulation for the interactions within a column, with small additional mean field terms, we describe quantitatively the observed changes in terms of intermolecular forces responsible for the formation of supramolecular columns. The driving force behind temperature-induced molecular ejection from the columns is the increase in conformational disorder and the consequent lateral expansion of the alkyl chains. The asymmetry of the transition is due to the local order between 4-molecule discs giving extra stability to purely n = 4 columns.

Modifying Thermal Switchability of Liquid Crystalline Nanoparticles by Alkyl Ligands Variation

By coating plasmonic nanoparticles (NPs) with thermally responsive liquid crystals (LCs) it is possible to prepare reversibly reconfigurable plasmonic nanomaterials with prospective applications in optoelectronic devices. However, simple and versatile methods to precisely tailor properties of liquid-crystalline nanoparticles (LC NPs) are still required. Here, we report a new method for tuning structural properties of assemblies of nanoparticles grafted with a mixture of promesogenic and alkyl thiols, by varying design of the latter. As a model system, we used Ag and Au nanoparticles that were coated with three-ring promesogenic molecules and dodecanethiol ligand. These LC NPs self-assemble into switchable lamellar (Ag NPs) or tetragonal (Au NPs) aggregates, as determined with small angle X-ray diffraction and transmission electron microscopy. Reconfigurable assemblies of Au NPs with different unit cell symmetry (orthorombic) are formed if hexadecanethiol and 1H,1H,2H,2H-perfluorodecanethiol were used in the place of dodecanethiol; in the case of Ag NPs the use of 11-hydroxyundecanethiol promotes formation of a lamellar structure as in the reference system, although with substantially broader range of thermal stability (140 vs. 90 °C). Our results underline the importance of alkyl ligand functionalities in determining structural properties of liquid-crystalline nanoparticles, and, more generally, broaden the scope of synthetic tools available for tailoring properties of reversibly reconfigurable plasmonic nanomaterials.