Curvature and self-assembly of semi-conducting nanoplatelets

Growth Pathway of CdS Nanoplatelets Investigated by In Situ X-ray Scattering and Optical Spectroscopy

Two-dimensional (2D) CdS nanoplatelets (NPLs) have attracted much attention due to their sharp absorption peaks and unique optical properties, but their formation mechanism has not been clearly explained. In this study, the whole growth process of 2D CdS NPLs from the original precursor compounds to the final products was investigated by in situ small-angle X-ray scattering (SAXS), ultraviolet-visible (UV-vis), and transmission electron microscopy (TEM). It reveals that the growth of CdS NPLs can be divided into three stages, i.e. dissolution of the precursors Cd(OA)x(OAc)2-x and sulfur powder accompanied by the formation of CdS precursor compounds (PCs) from 30 to 120 °C (stage I), formation of the CdS magic-sized clusters (MSCs) and their growth into NPLs from 120 to 240 °C (stage II), and curling and uncurling growth of the NPLs at 240 °C (stage III). Dramatically, the shape of the NPLs changes from flat to curly when the temperature reaches 240 °C, but very interestingly, they become flat again after a prolonged stay at 240 °C. The curling and uncurling of NPLs is proposed to be driven by their surface stress imbalance induced by ligands.

Siamese neural network improves the performance of a convolutional neural network in colloidal self-assembly state classification

Identifying the state of the colloidal self-assembly process is critical to monitoring and controlling the system into desired configurations. Recent application of convolutional neural networks with unsupervised clustering has shown a comparable performance to conventional approaches, in representing and classifying the states of a simulated 2D colloidal batch assembly system. Despite the early success, capturing the subtle differences among similar configurations still presents a challenge. To address this issue, we leverage a Siamese neural network to improve the accuracy of the state classification. Results from a Brownian dynamics-simulated electric field-mediated colloidal self-assembly system and a magnetic field-mediated colloidal self-assembly system demonstrate significant improvement from the original convolutional neural network-based approach. We anticipate the proposed improvement to further pave the way for automated monitoring and control of colloidal self-assembly processes in real time and real space.

Self-Assembly of Chiral Ligands on 2D Semiconductor Nanoplatelets for High Circular Dichroism

Group II-VI semiconductor nanoplatelets (NPLs) with atomically defined thicknesses and extended atomically flat (001) facets are used for ligand binding and chiro-optical effects. In this study, we demonstrate that tartrate ligands, anchored by two carboxylate groups, chelate the (001) facets of NPLs at an average ratio of one tartrate molecule to two cadmium (Cd) surface atoms. This assembly of chiral molecules on inorganic nanocrystals generates a circular dichroism g-factor as high as 1.3 × 10-2 at the first excitonic transition wavelength of NPLs. Tartrate ligands induce an orthorhombic distortion of the initially "cubic" crystal structure, classifying the NPLs within the 222-point group. Unlike spherical nanocrystals, where it is difficult to discern whether chiral ligands affect only the surface atoms or the entire crystal structure, our findings unequivocally show that the crystal structure of NPLs is modified due to their thinness and atomically precise thickness. The in-plane lattice parameters experience compressive and tensile stresses, significantly splitting the heavy-hole and light-hole bands. Additionally, tartrate ligands adopt different conformations on the NPL surface over time, resulting in dynamic changes in the circular dichroism signal, including an inversion of its sign.

Ligand Tail Controls the Conformation of Indium Sulfide Ultrathin Nanoribbons

We report the conformational control of 2D ultrathin indium sulfide nanoribbons by tuning their amine ligands' alkyl chain. The initial orthorhombic InS nanoribbons bare n-octylamine ligands and display a highly curved geometry with a characteristic figure of eight shapes. Exchanging the native ligand by oleylamine induces their complete unfolding to yield flat board-shaped nanoribbons. Significant strain variations in the InS crystal structure accompany this shape-shifting. By tuning the linear alkyl chain length from 4 to 18 carbon atoms, we show using small-angle X-ray scattering in solution and transmission electron microscopy that the curvature of the nanoribbon subtly depends on the ligand-ligand interactions at the nanoribbon's surface. The curvature decreases gradually as the chain length increases, while carbon unsaturation has an unexpectedly significant effect at constant chain length. These experiments shed light on the critical role of the ligand monolayer on the curvature of ultrathin 2D crystalline nanosheets and demonstrate that weak supramolecular forces within the organic part of colloidal nanocrystals can dramatically impact their shape. This transduction mechanism, in which changes in the organic monolayer impact the shape of a nanocrystal, will help to devise new strategies to design stimuli-responsive systems that take advantage of both the flexibility of organic moieties and the physical properties of the inorganic core.

Orientation-Dependent Photoconductivity of Quasi-2D Nanocrystal Self-Assemblies: Face-Down, Edge-Up Versus Randomly Oriented Quantum Wells

Here, strongly orientation-dependent lateral photoconductivity of a CdSe monolayer colloidal quantum wells (CQWs) possessing short-chain ligands is reported. A controlled liquid-air self-assembly technique is utilized to deliberately engineer the alignments of CQWs into either face-down (FO) or edge-up (EO) orientation on the substrate as opposed to randomly oriented (RO) CQWs prepared by spin-coating. Adapting planar configuration metal-semiconductor-metal (MSM) photodetectors, it is found that lateral conductivity spans ≈2 orders of magnitude depending on the orientation of CQWs in the film in the case of utilizing short ligands. The long native ligands of oleic acid (OA) are exchanged with short-chain ligands of 2-ethylhexane-1-thiol (EHT) to reduce the inter-platelet distance, which significantly improved the photoresponsivity from 4.16, 0.58, and 4.79 mA W-1 to 528.7, 6.17, and 94.2 mA W-1, for the MSM devices prepared with RO, FO, and EO, before and after ligands exchange, respectively. Such CQW orientation control profoundly impacts the photodetector performance also in terms of the detection speed (0.061 s/0.074 s for the FO, 0.048 s/0.060 s for the EO compared to 0.10 s/0.16 s for the RO, for the rise and decay time constants, respectively) and the detectivity (1.7 × 1010, 2.3 × 1011, and 7.5 × 1011 Jones for the FO, EO, and RO devices, respectively) which can be further tailored for the desired optoelectronic device applications. Attributed to charge transportation in colloidal films being proportional to the number of hopping steps, these findings indicate that the solution-processed orientation of CQWs provides the ability to tune the photoconductivity of CQWs with short ligands as another degree of freedom to exploit and engineer their absorptive devices.

Highly Transparent, Yet Photoluminescent: 2D CdSe/CdS Nanoplatelet-Zeolitic Imidazolate Framework Composites Sensitive to Gas Adsorption

In this work, thin composite films of zeolitic imidazolate frameworks (ZIFs) and colloidal two-dimensional (2D) core-crown CdSe/CdS nanoplatelet (NPL) emitters with minimal scattering are formed by a cycled growth method and yield highly transparent coatings with strong and narrow photoluminescence of the NPLs at 546 nm (FWHM: 25 nm) in a solid-state composite structure. The porous ZIF matrix acts as functional encapsulation for the emitters and enables the adsorption of the guest molecules water and ethanol. The adsorption and desorption of the guest molecules is then characterized by a reversable photoluminescence change of the embedded NPLs. The transmittance of the composite films exceeds the values of uncoated glass at visible wavelengths where the NPL emitters show no absorption (>540 nm) and renders them anti-reflective coatings. At NPL absorption wavelengths (440-540 nm), the transmittance of the thin composite film-coated glass lies close to the transmittance of uncoated glass. The fast formation of innovative, smooth NPL/ZIF composite films without pre-polymerizing the colloidal 2D nanostructures first provides a powerful tool toward application-oriented photoluminescence-based gas sensing.

Ligand-induced incompatible curvatures control ultrathin nanoplatelet polymorphism and chirality

The ability of thin materials to shape-shift is a common occurrence that leads to dynamic pattern formation and function in natural and man-made structures. However, harnessing this concept to rationally design inorganic structures at the nanoscale has remained far from reach due to a lack of fundamental understanding of the essential physical components. Here, we show that the interaction between organic ligands and the nanocrystal surface is responsible for the full range of chiral shapes seen in colloidal nanoplatelets. The adsorption of ligands results in incompatible curvatures on the top and bottom surfaces of the NPL, causing them to deform into helicoïds, helical ribbons, or tubes depending on the lateral dimensions and crystallographic orientation of the NPL. We demonstrate that nanoplatelets belong to the broad class of geometrically frustrated assemblies and exhibit one of their hallmark features: a transition between helicoïds and helical ribbons at a critical width. The effective curvature [Formula: see text] is the single aggregate parameter that encodes the details of the ligand/surface interaction, determining the nanoplatelets' geometry for a given width and crystallographic orientation. The conceptual framework described here will aid the rational design of dynamic, chiral nanostructures with high fundamental and practical relevance.

Cation Exchange and Spontaneous Crystal Repair Resulting in Ultrathin, Planar CdS Nanosheets

Cation exchange has become a major postsynthetic tool to obtain nanocrystals with a combination of stoichiometry, size, and shape that is challenging to achieve by direct wet-chemical synthesis. Here, we report on the transformation of highly anisotropic, ultrathin, and planar PbS nanosheets into CdS nanosheets of the same dimensions. We monitor the evolution of the Cd-for-Pb exchange by ex-situ TEM, HAADF-STEM, and EDX. We observe that in the early stages of the exchange the sheets show large in-sheet voids that repair spontaneously upon further exchange and annealing, resulting in ultrathin, planar, and crystalline CdS nanosheets. After cation exchange, the nanosheets show broad sub-band gap luminescence, as often observed in CdS nanocrystals. The photoluminescence excitation spectrum reveals the heavy- and light-hole exciton features, with very strong quantum confinement and large electron-hole Coulomb energy, typical for 2D ultrathin Cd-chalcogenide nanosheets.

Controlled Assembly of CdSe Nanoplatelet Thin Films and Nanowires

We assemble semiconductor CdSe nanoplatelets (NPs) at the air/liquid interface into 2D monolayers several micrometers wide, distinctly displaying nematic order. We show that this configuration is the most favorable energetically and that the edge-to-edge distance between neighboring NPs can be tuned by ligand exchange without disrupting film topology and nanoparticle orientation. We explore the rich assembly phase space by using depletion interactions to direct the formation of 1D nanowires from stacks of NPs. The improved control and understanding of the assembly of semiconductor NPs offers opportunities for the development of cheaper optoelectronic devices that rely on 1D or 2D charge delocalization throughout the assembled monolayers and nanowires.

NbSe2 Nanosheets/Nanorolls Obtained via Fast and Direct Aqueous Electrochemical Exfoliation for High-Capacity Lithium Storage

Layered transition-metal dichalcogenides (LTMDs) in two-dimensional (2D) form are attractive for electrochemical energy storage, but research efforts in this realm have so far largely focused on the best-known members of such a family of materials, mainly MoS₂, MoSe₂, and WS₂. To exploit the potential of further, currently less-studied 2D LTMDs, targeted methods for their production, preferably by cost-effective and sustainable means, as well as control over their nanomorphology, are highly desirable. Here, we report a quick and straightforward route for the preparation of 2D NbSe₂ and other metallic 2D LTMDs that relies on delaminating their bulk parent solid under aqueous cathodic conditions. Unlike typical electrochemical exfoliation methods for 2D materials, which generally require an additional processing step (e.g., sonication) to complete delamination, the present electrolytic strategy yielded directly exfoliated nano-objects in a very short time (1-2 min) and with significant yields (∼16 wt %). Moreover, the dominant morphology of the exfoliated 2D NbSe₂ products could be tuned between rolled-up nanosheets (nanorolls) and unfolded nanosheets, depending on the solvent where the nano-objects were dispersed (water or isopropanol). This rather unusual delamination behavior of NbSe₂ was explored and concluded to occur via a redox mechanism that involves some degree of hydrolytic oxidation of the material triggered by the cathodic treatment. The delamination strategy could be extended to other metallic LTMDs, such as NbS₂ and VSe₂. When tested toward electrochemical lithium storage, electrodes based on the exfoliated NbSe₂ products delivered very high capacity values, up to 750-800 mA h g^-1 at 0.5 A g^-1, where the positive effect of the nanoroll morphology, associated to increased accessibility of the lithium storage sites, was made apparent. Overall, these results are expected to expand the availability of fit-for-purpose 2D LTMDs by resorting to simple and expeditious production strategies of low environmental impact.

High External Quantum Efficiency Light-Emitting Diodes Enabled by Advanced Heterostructures of Type-II Nanoplatelets

Colloidal quantum wells (CQWs), also known as nanoplatelets (NPLs), are exciting material systems for numerous photonic applications, including lasers and light-emitting diodes (LEDs). Although many successful type-I NPL-LEDs with high device performance have been demonstrated, type-II NPLs are not fully exploited for LED applications, even with alloyed type-II NPLs with enhanced optical properties. Here, we present the development of CdSe/CdTe/CdSe core/crown/crown (multi-crowned) type-II NPLs and systematic investigation of their optical properties, including their comparison with the traditional core/crown counterparts. Unlike traditional type-II NPLs such as CdSe/CdTe, CdTe/CdSe, and CdSe/CdSe_xTe_1-x core/crown heterostructures, here the proposed advanced heterostructure reaps the benefits of having two type-II transition channels, resulting in a high quantum yield (QY) of 83% and a long fluorescence lifetime of 73.3 ns. These type-II transitions were confirmed experimentally by optical measurements and theoretically using electron and hole wave function modeling. Computational study shows that the multi-crowned NPLs provide a better-distributed hole wave function along the CdTe crown, while the electron wave function is delocalized in the CdSe core and CdSe crown layers. As a proof-of-concept demonstration, NPL-LEDs based on these multi-crowned NPLs were designed and fabricated with a record high external quantum efficiency (EQE) of 7.83% among type-II NPL-LEDs. These findings are expected to induce advanced designs of NPL heterostructures to reach a fascinating level of performance, especially in LEDs and lasers.

2D II-VI Semiconductor Nanoplatelets: From Material Synthesis to Optoelectronic Integration

The field of colloidal synthesis of semiconductors emerged 40 years ago and has reached a certain level of maturity thanks to the use of nanocrystals as phosphors in commercial displays. In particular, II-VI semiconductors based on cadmium, zinc, or mercury chalcogenides can now be synthesized with tailored shapes, composition by alloying, and even as nanocrystal heterostructures. Fifteen years ago, II-VI semiconductor nanoplatelets injected new ideas into this field. Indeed, despite the emergence of other promising semiconductors such as halide perovskites or 2D transition metal dichalcogenides, colloidal II-VI semiconductor nanoplatelets remain among the narrowest room-temperature emitters that can be synthesized over a wide spectral range, and they exhibit good material stability over time. Such nanoplatelets are scientifically and technologically interesting because they exhibit optical features and production advantages at the intersection of those expected from colloidal quantum dots and epitaxial quantum wells. In organic solvents, gram-scale syntheses can produce nanoparticles with the same thicknesses and optical properties without inhomogeneous broadening. In such nanoplatelets, quantum confinement is limited to one dimension, defined at the atomic scale, which allows them to be treated as quantum wells. In this review, we discuss the synthetic developments, spectroscopic properties, and applications of such nanoplatelets. Covering growth mechanisms, we explain how a thorough understanding of nanoplatelet growth has enabled the development of nanoplatelets and heterostructured nanoplatelets with multiple emission colors, spatially localized excitations, narrow emission, and high quantum yields over a wide spectral range. Moreover, nanoplatelets, with their large lateral extension and their thin short axis and low dielectric surroundings, can support one or several electron-hole pairs with large exciton binding energies. Thus, we also discuss how the relaxation processes and lifetime of the carriers and excitons are modified in nanoplatelets compared to both spherical quantum dots and epitaxial quantum wells. Finally, we explore how nanoplatelets, with their strong and narrow emission, can be considered as ideal candidates for pure-color light emitting diodes (LEDs), strong gain media for lasers, or for use in luminescent light concentrators.

The Crucial Role of Solvation Forces in the Steric Stabilization of Nanoplatelets

The steric stability of inorganic colloidal particles in an apolar solvent is usually described in terms of the balance between three contributions: the van der Waals attraction, the free energy of mixing, and the ligand compression. However, in the case of nanoparticles, the discrete nature of the ligand shell and the solvent has to be taken into account. Cadmium selenide nanoplatelets are a special case. They combine a weak van der Waals attraction and a large facet to particle size ratio. We use coarse grained molecular dynamics simulations of nanoplatelets in octane to demonstrate that solvation forces are strong enough to induce the formation of nanoplatelet stacks and by that have a crucial impact on the steric stability. In particular, we demonstrate that for sufficiently large nanoplatelets, solvation forces are proportional to the interacting facet area, and their strength is intrinsically tied to the softness of the ligand shell.

Layer-by-Layer Deposition of 2D CdSe/CdS Nanoplatelets and Polymers for Photoluminescent Composite Materials

Two-dimensional (2D) semiconductor nanoplatelets (NPLs) are strongly photoluminescent materials with interesting properties for optoelectronics. Especially their narrow photoluminescence paired with a high quantum yield is promising for light emission applications with high color purity. However, retaining these features in solid-state thin films together with an efficient encapsulation of the NPLs is a challenge, especially when trying to achieve high-quality films with a defined optical density and low surface roughness. Here, we show photoluminescent polymer-encapsulated inorganic-organic nanocomposite coatings of 2D CdSe/CdS NPLs in poly(diallyldimethylammonium chloride) (PDDA) and poly(ethylenimine) (PEI), which are prepared by sequential layer-by-layer (LbL) deposition. The electrostatic interaction between the positively charged polyelectrolytes and aqueous phase-transferred NPLs with negatively charged surface ligands is used as a driving force to achieve self-assembled nanocomposite coatings with a well-controlled layer thickness and surface roughness. Increasing the repulsive forces between the NPLs by increasing the pH value of the dispersion leads to the formation of nanocomposites with all NPLs arranging flat on the substrate, while the surface roughness of the 165 nm (50 bilayers) thick coating decreases to R_a = 14 nm. The photoluminescence properties of the nanocomposites are determined by the atomic layer thickness of the NPLs and the 11-mercaptoundecanoic acid ligand used for their phase transfer. Both the full width at half-maximum (20.5 nm) and the position (548 nm) of the nanocomposite photoluminescence are retained in comparison to the colloidal CdSe/CdS NPLs in aqueous dispersion, while the measured photoluminescence quantum yield of 5% is competitive to state-of-the-art nanomaterial coatings. Our approach yields stable polymer-encapsulated CdSe/CdS NPLs in smooth coatings with controllable film thickness, rendering the LbL deposition technique a powerful tool for the fabrication of solid-state photoluminescent nanocomposites.

A Sensitizer of Purpose: Generating Triplet Excitons with Semiconductor Nanocrystals

The process of photon upconversion promises importance for many optoelectronic applications, as it can result in higher efficiencies and more effective photon management. Upconversion via triplet-triplet annihilation (TTA) occurs at low incident powers and at high efficiencies, requirements for integration into existing optoelectronic devices. Semiconductor nanocrystals are a diverse class of triplet sensitizers with advantages over traditional molecular sensitizers such as energetic tunability and minimal energy loss during the triplet sensitization process. In this Perspective, we review current progress in semiconductor nanocrystal triplet sensitization, specifically focusing on the nanocrystal, the ligand shell which surrounds the nanocrystal, and progress in solid-state sensitization. Finally, we discuss potential areas of improvement which could result in more efficient upconversion systems sensitized by semiconductor nanocrystals. Specifically, we focus on the development of solid-state TTA upconversion systems, elucidation of the energy transfer mechanisms from nanocrystal to transmitter ligand which underpin the upconversion process and propose novel configurations of nanocrystal-sensitized systems.

Hybrid Macrocyclic Polymers: Self-Assembly Containing Cucurbit[m]uril-pillar[n]arene

Supramolecular self-assembly by hybrid macrocycles containing both cucurbit[m]uril (CB[m]) and pillar[n]arene was discussed and summarized in this review. Due to different solubility, diverse-sized cavities, and various driving forces in recognizing guests, the role of CB[m] and pillar[n]arene in such hybrid macrocyclic systems could switch between competitor in capturing specialized guests, and cooperator for building advanced hybridized macrocycles, by controlling their characteristics in host-guest inclusions. Furthermore, both CB[m] and pillar[n]arene were employed for fabricating advanced supramolecular self-assemblies such as mechanically interlocked molecules and supramolecular polymers. In those self-assemblies, CB[m] and pillar[n]arene played significant roles in, e.g., microreactor for catalyzing particular reactions to bridge different small pieces together, molecular "joint" to connect different monomers into larger assemblies, and "stabilizer" in accommodating the guest molecules to adopt a favorite structure geometry ready for assembling.