Toward Surface Chemistry of Semiconductor Nanocrystals at an Atomic-Molecular Level.

ConspectusProperties of colloidal semiconductor nanocrystals with a single-crystalline structure are largely dominated by their surface structure at an atomic-molecular level, which is not well understood and controlled, due to a lack of experimental tools. However, if viewing the nanocrystal surface as three relatively independent spatial zones (i.e., crystal facets, inorganic-ligands interface, and ligands monolayer), we may approach an atomic-molecular level by coupling advanced experimental techniques and theoretical calculations.Semiconductor nanocrystals of interest are mainly based on compound semiconductors and mostly in two (or related) crystal structures, namely zinc-blende and wurtzite, which results in a small group of common low-index crystal facets. These low-index facets, from a surface-chemistry perspective, can be further classified into polar and nonpolar ones. Albeit far from being successful, the controlled formation of either polar or nonpolar facets is available for cadmium chalcogenide nanocrystals. Such facet-controlled systems offer a reliable basis for studying the inorganic-ligands interface. For convenience, here facet-controlled nanocrystals refer to a special class of shape-controlled ones, in which shape control is at an atomic level, instead of those with poorly defined facets (e.g., typical spheroids, nanorods, etc).Experimental and theoretical results reveal that type and bonding mode of surface ligands on nanocrystals is facet-specific and often beyond Green's classification (X-type, Z-type, and L-type). For instance, alkylamines bond strongly to the anion-terminated (0001) wurtzite facet in the form of ammonium ions, with three hydrogens of an ammonium ion bonding to three adjacent surface anion sites. With theoretically assessable experimental data, facet-ligands pairing can be identified using density functional theory (DFT) calculations. To make the pairing meaningful, possible forms of all potential ligands in the system need to be examined systematically, revealing the advantage of simple solution systems.Unlike the other two spatial zones, the ligands monolayer is disordered and dynamic at an atomic level. Thus, an understanding of the ligands monolayer on a molecular scale is sufficient for many cases. For colloidal nanocrystals stably coordinated with surface ligands, their solution properties are dictated by the ligands monolayer. Experimental and theoretical results reveal that solubility of a nanocrystal-ligands complex is an interplay between the intramolecular entropy of the ligands monolayer and intermolecular interactions of the ligands/nanocrystals. By introducing entropic ligands, solubility of nanocrystal-ligands complexes can be universally boosted by several orders of magnitude, i.e., up to >1 g/mL in typical organic solvents. Molecular environment in the pseudophase surrounding each nanocrystal plays a critical role in its chemical, photochemical, and photophysical properties.For some cases, such as the synthesis of high-quality nanocrystals, all three spatial zones of the nanocrystal surface must be taken into account. By optimizing nanocrystal surface at an atomic-molecular level, semiconductor nanocrystals with monodisperse size and facet structure become available recently through either direct synthesis or afterward facet reconstruction, implying full realization of their size-dependent properties.

Reversible Facet Reconstruction of CdSe/CdS Core/Shell Nanocrystals by Facet-Ligand Pairing.

Synthesis of colloidal semiconductor nanocrystals with defined facet structures is challenging, though such nanocrystals are essential for fully realizing their size-dependent optical and optoelectronic properties. Here, for the mostly developed colloidal wurtzite CdSe/CdS core/shell nanocrystals, facet reconstruction is investigated under typical synthetic conditions, excluding nucleation, growth, and interparticle ripening. Within the reaction time window, two reproducible sets of facets─each with a specific group of low-index facets─can be reversibly reconstructed by switching the ligand system, indicating thermodynamic stability of each set. With a unique <0001> axis, atomic structures of the low-index facets of wurtzite nanocrystals are diverse. Experimental and theoretical studies reveal that each facet in a given set is paired with a common ligand in the solution, namely, either fatty amine and/or cadmium alkanoate. The robust bonding modes of ligands are found to be strongly facet-dependent and often unconventional, instead of following Green's classification. Results suggest that facet-controlled nanocrystals can be synthesized by optimal facet-ligand pairing either in synthesis or after-synthesis reconstruction, implying semiconductor nanocrystal formation with size-dependent properties down to an atomic level.

Epitaxial Integration of Multiple CdSe Quantum Dots in a Colloidal CdS Nanoplatelet.

Presynthesized CdSe/CdS core/shell quantum dots (QDs) are two-dimensionally (2D) and epitaxially fused in solution to form a CdS nanoplatelet with multiple epitaxially embedded CdSe QDs (CdSe@CdS coupled-dots@platelet). In addition to providing spatial confinement for the excitonic states of multiple CdSe QDs in a CdS nanoplatelet, the continuous and single-crystalline nanoplatelet with controlled thickness enables quantum coupling between the CdSe QDs, resulting in inhomogeneous-free optical properties for the colloidal CdSe@CdS coupled-dots@nanoplatelets with bright photoluminescence. The results here suggest that solution synthesis can offer a simple means to obtain semiconductor nanocrystals for realizing unique yet complex excitonic properties that are otherwise difficult to achieve.

Self-Assembly of Amphiphilic BODIPY Derivatives on Micropatterned Ionic Liquid Surfaces for Fluorescent Films with Excellent Stability and Sensing Performance.

Fluorescent films have been widely recognized as one of the most powerful tools for trace analyte detection. However, their use has been limited due to the poor photochemical stability of fluorophores at a gas-solid interface and inefficient film mass transfer. Herein, novel fluorescent films were developed through self-assembly of amphiphilic BODIPY derivatives on micropatterned ionic liquid surfaces. Unlike solid-state films, the obtained monolayer films exhibit excellent photochemical stability, similar to that of a solution. Moreover, the interfacial assembly of amphiphilic fluorophores can avoid gas diffusion inside the microdroplets, significantly improving the sensing performance. The 1/1-butyl-3-methylimidazolium tetrafluoroborate ([BMIM]BF4) monolayer exhibits high sensitivity, high selectivity, and a fast response to detect diethylchlorophosphate (DCP) vapor. The detection limit was 226 ppt, with a response time to DCP of 2.0 s. Importantly, the 1/[BMIM]BF4 monolayer can be reused for at least 50 cycles with no obvious signal fading. This study is expected to benefit the development of new strategies for designing fluorescence sensing films and lay a solid foundation for the fabrication of multifunctional sensing devices with excellent photochemical stability and sensing performance.

Observation of photon antibunching with only one standard single-photon detector.

The second-order photon correlation function g2(τ) is of great importance in quantum optics. g2(τ) is typically measured with the Hanbury Brown and Twiss (HBT) interferometer, which employs a pair of single-photon detectors and a dual-channel time acquisition module. Here, we demonstrate a new method to measure and extract g2(τ) with a standard single-photon avalanche photodiode (dead-time = 22 ns) and a single-channel time acquisition module. This is realized by shifting the coincidence counts of interest to a time window not affected by the dead-time and after-pulse of the detection system using a fiber-based delay line. The new scheme is verified by measuring g2(τ) from a single colloidal nanocrystal. Photon antibunching is unambiguously observed and agrees well with the result measured using the standard HBT setup. Our scheme simplifies the higher-order correlation technique and might be favored in cost-sensitive circumstances.

Delocalized Surface Electronic States on Polar Facets of Semiconductor Nanocrystals.

Wurtzite CdSe@CdS dot@platelet nanocrystals with (001) and (00-1) polar facets as the basal planes and (100) family of nonpolar facets as the side planes are applied for studying surface defects on semiconductor nanocrystals. When they are terminated with cadmium ions coordinated with carboxylate ligands, a single set of absorption features and band-edge photoluminescence (PL) with near unity PL quantum yield and monoexponential PL decay dynamics (lifetime ∼28 ns) are observed. In addition to these spectral signatures, when the surface is converted to sulfur-terminated, a second set of sharp absorption features with decent extinction coefficients and a secondary band-edge PL with low PL quantum yield and long-lifetime (>78 ns) PL decay dynamics are reproducibly recorded. Photochemical analysis confirms that the secondary UV-vis and PL spectral features are quantitatively correlated with each other. Chemical analysis and X-ray photoelectron spectroscopy measurements confirm that such secondary spectral features are well correlated with the sulfide (such as -SH) and disulfide (such as -S-S-) surface sites of a basal plane, which likely form surface hole electronic states delocalized on the entire basal plane. Results suggest that, for studying surface defects on semiconductor nanocrystals, it is essential to prepare a nearly monodisperse surface structure in terms of facets and surface chemical bonding.

CdSe@CdS Dot@Platelet Nanocrystals: Controlled Epitaxy, Monoexponential Decay of Two-Dimensional Exciton, and Nonblinking Photoluminescence of Single Nanocrystal.

Wurtzite CdSe@CdS dot@platelet nanocrystals, dot-shaped CdSe nanocrystals encased within epitaxially grown CdS nanoplatelets, are controllably synthesized with nearly monodisperse size and shape distribution and outstanding photoluminescence (PL) properties. The excellent size and shape control with their lateral to thickness dimension ratio up to 3:1 is achieved by systematically studying the synthetic parameters, which results in a simple, tunable, yet reproducible epitaxy scheme. These special types of core/shell nanocrystals possess two-dimensional emission dipoles with the ab plane of the wurtzite structure. While their near-unity PL quantum yield and monoexponential PL decay dynamics are at the same level of the state-of-art CdSe/CdS core/shell nanocrystals in dot shape, CdSe@CdS dot@platelet nanocrystals possess ∼2 orders of magnitude lower probability for initiating PL blinking at the single-nanocrystal level than the dot-shaped counterparts do.

Magnetic Field-Assisted Perovskite Film Preparation for Enhanced Performance of Solar Cells.

Perovskite solar cells (PSCs) are promising low-cost photovoltaic technologies with high power conversion efficiency (PCE). The crystalline quality of perovskite materials is crucial to the photovoltaic performance of the PSCs. Herein, a simple approach is introduced to prepare high-quality CH3NH3PbI3 perovskite films with larger crystalline grains and longer carriers lifetime by using magnetic field to control the nucleation and crystal growth. The fabricated planar CH3NH3PbI3 solar cells have an average PCE of 17.84% and the highest PCE of 18.56% using an optimized magnetic field at 80 mT. In contrast, the PSCs fabricated without the magnetic field give an average PCE of 15.52% and the highest PCE of 16.72%. The magnetic field action produces an ordered arrangement of the perovskite ions, improving the crystallinity of the perovskite films and resulting in a higher PCE.

Luminescent Helical Nanofiber Self-Assembled from a Cholesterol-Based Metalloamphiphile and Its Application in DNA Conformation Recognition.

Compared to pure organic amphiphiles, metalloamphiphiles display distinctive features, including luminescence, magnetism and catalytic properties. However, the self-organization of metalloamphiphiles is commonly driven by solvophobic effects. Alkyl chains and oligomeric ethylene glycol moieties are thus the most frequently used aggregation units to drive the self-assembly of metalloamphiphiles. We expect novel metallo-supramolecular structures with exciting functions to be created if additional noncovalent interaction modes are incorporated. In this work, a new type of metalloamphiphile, consisting of a Tb(III) complex head and a cholesteryl unit (TbL3+(I)), was designed and synthesized. TbL3+(I) spontaneously self-assembles into helical nanofibers (d = 6 nm) in water. This synthetic multivalent nanoscale binding array displays powerful capability for the recognition of DNA conformations through a turn-on luminescence sensing mechanism. ssDNA-kit1 triggered a 26-fold increase in the luminescence intensity of TbL3+(I). Its corresponding G-quadruplex structure (G-quadruplex-kit1), however, induced a 6.6-fold enhancement under the same conditions. Consequently, TbL3+(I) nanofibers can monitor DNA folding. In contrast, neither ssDNA-kit1 nor G-quadruplex-kit1 markedly promoted the luminescence of molecularly dispersed TbL3+(II), illustrating that the multivalent electrostatic interactions between the phosphate groups on the backbone of DNA and TbL3+(I) self-assembled into nanofibers could greatly improve the efficiency of the energy transfer between the guanine units and the organized TbL3+(I). The TbL3+(I) nanofibers could bind and distinguish not only the kit1-ssDNA/G-quadruplex but also the conformations of other G-rich DNA, such as spb1, htelo, and intermolec-htelo. The self-assembly of luminescent metalloamphiphiles thus provides a general and convenient strategy for the efficient recognition and conversion of molecular information.

Hierarchical Self-Assembly of Luminescent Tartrate-Bridged Chiral Binuclear Tb(III) Complexes in Ethanol.

A new family of supramolecular metalloamphiphiles carrying two metal centers is developed. They are formed by bridging two coordinatively unsaturated lipophilic Tb3+ complexes (TbL+) with chiral dicarboxylate anions. The formation of bridging coordination bonds is confirmed using UV spectroscopy, induced circular dichroism (ICD), increased luminescence intensity of TbL+, and electrospray ionization mass spectrometry (ESIMS) analysis. These supramolecular metalloamphiphiles hierarchically self-assemble in ethanol to give luminescent nanospheres, as observed using transmission electron microscopy (TEM) and scanning electron microscopy (SEM). The two hydroxyl groups introduced in the bridging ligands of [TbL]2(d-/l-tartrate) significantly promote self-assembly by increasing coherent forces via intermolecular hydrogen bonding. The observed self-assembly in ethanol also merits mention because such polar alcoholic media have been unfavorable for conventional molecular self-assemblies. The present approach offers a new molecular design strategy for composable metalloamphiphiles.

Ferrocene-containing thixotropic molecular gels: creation and a novel strategy for water purification.

Low-molecular-mass gelators and relevant molecular gels have been employed for water purification owing to their convenience and efficiency, but the process is time consuming due to low extraction efficiency originated from limited contact of the two phases. In this work, two novel di-cholesterol-based gelators, 1 and 2, with a ferrocenyl unit were synthesized and the gels based on 2 possess a smart thixotropic property. In particular, 2/heptane gel, the shear force induced phase transition is fast (within seconds) and fully reversible without the need of heating-cooling cycle. Based upon the thixotropic molecular gel, a novel separation strategy, which combines the great efficiency of liquid-liquid extraction and the convenience of liquid-solid separation, has been successfully conducted for removing iodine from wastewater. It was demonstrated that iodine was removed within several minutes and the extraction efficiency (72%) was the same with the one using corresponding liquid. Furthermore, 2/heptane gel is also responsive to chemical oxidation and variation in temperature. FTIR, NMR, CD and XRD studies revealed that helical fibers were formed via intermolecular hydrogen bonding and van der Waals interaction. It is believed that the results presented in this work are of importance for extending real-life applications of molecular gels.

Luminescent vesicular nanointerface: a highly selective and sensitive "turn-on" sensor for guanosine triphosphate.

A novel amphiphilic Tb(3+) complex (TbL(3+)(I)) consisting of a +3 charged head and a hydrophobic alkyl chain has been developed. It spontaneously self-assembles in water and forms stable vesicles at neutral pH. TbL(3+)(I) has no aromatic groups (functioning as an antenna), and its intrinsic luminescence is thus minimized. These features lead to the self-assembling TbL(3+)(I) receptor molecules demonstrating an increased luminescence intensity upon binding of nucleotides. Upon addition of guanosine triphosphate (GTP), the luminescence from Tb(3+) was notably promoted (127-fold), as the light energy absorbed by the guanine group of GTP was efficiently transferred to the Tb(3+) center. In the case of guanosine diphosphate (GDP) and guanosine monophosphate (GMP), respectively, 78-fold and 43-fold increases in luminescence intensity were observed. This enhancement was less significant than that observed for GTP, due to fewer negative charges on GDP and GMP. No other nucleotides or the tested nonphosphorylated nucleosides affected the luminescence intensity to any notable extent. In marked contrast, all tested nucleotides, including guanine nucleotides, barely promoted the luminescence of molecularly dispersed receptors, TbL(3+)(II), indicating that the confinement and organization of molecules in a nanointerface play vital roles in improving the performance of a sensing system. This Tb(3+) complex nanointerface is successfully used for monitoring the GTP-to-GDP conversion.