Synthesis Of Semiconductor Nanocrystals Research Articles

The 2023 Nobel Prize in Chemistry: Quantum dots

The 2023 Nobel Prize in Chemistry was awarded to Aleksey I. Ekimov (prize share 1/3), Louis E. Brus (prize share 1/3), and Moungi G. Bawendi (prize share 1/3) for groundbreaking inventions in the field of nanotechnology, i.e., for the discovery and synthesis of semiconductor nanocrystals, also termed quantum dots, that exhibit size-dependent physicochemical properties enabled by quantum size effects. This feature article summarizes the main milestones of the discoveries and developments of quantum dots that paved the road to their versatile applications in solid-state lighting, display technology, energy conversion, medical diagnostics, bioimaging, and image-guided surgery.

Engineering of Exciton Spatial Distribution in CdS Nanoplatelets.

Zinc-blende CdS nanoplatelets with atomically flat and very large {100} basal planes terminated solely by one type of element (either Cd or S atoms) are synthesized. Optical spectroscopy, X-ray diffraction, X-ray absorption, and transmission electron microscopy confirm that the surface structures of newly developed S-terminated CdS nanoplatelets are at least as well-defined as the original Cd-terminated nanoplatelets. Band gaps of the nanoplatelets are found to depend on not only the quantum-confined dimension (thickness) but also the nature of the surface termination. The facet structure dictates the packing of the ligands (carboxylate for Cd-terminated nanoplatelets and alkyl for S-terminated nanoplatelets), which causes a difference in the lattice strain and significantly affects the optical spectral width. Experimental and theoretical results reveal that engineering the exciton spatial distribution by the tailored synthesis of semiconductor nanocrystals with a precisely controlled surface structure is fully possible, which should open a new door for delivering the long-promised potential of semiconductor nanocrystals.

Extended Nucleation and Superfocusing in Colloidal Semiconductor Nanocrystal Synthesis.

Hot-injection synthesis is renowned for producing semiconductor nanocolloids with superb size dispersions. Burst nucleation and diffusion-controlled size focusing during growth have been invoked to rationalize this characteristic yet experimental evidence supporting the pertinence of these concepts is scant. By monitoring a CdSe synthesis in-situ with X-ray scattering, we find that nucleation is an extended event that coincides with growth during 15-20% of the reaction time. Moreover, we show that size focusing outpaces predictions of diffusion-limited growth. This observation indicates that nanocrystal growth is dictated by the surface reactivity, which drops sharply for larger nanocrystals. Kinetic reaction simulations confirm that this so-called superfocusing can lengthen the nucleation period and promote size focusing. The finding that narrow size dispersions can emerge from the counteracting effects of extended nucleation and reaction-limited size focusing ushers in an evidence-based perspective that turns hot injection into a rational scheme to produce monodisperse semiconductor nanocolloids.

DBU-Catalyzed One-Pot Synthesis of Nearly Any Metal Salt of Fatty Acid (M-FA): A Library of Metal Precursors to Semiconductor Nanocrystal Synthesis.

The metal salts of fatty acid (M-FA) are the most widely used metal precursors to colloidal semiconductor nanocrystals (NCs). They play a key role in controlling the composition, shape, and size of semiconductor NCs, and their purity is essential for attaining impeccable batch-to-batch reproducibility in the optical and electrical properties of the NCs. Herein, we report a novel, one-pot synthesis of a library of highly pure M-FAs at near-quantitative yields (up to 91%) using 1,8-diazabicyclo[5.4.0]undec-7-ene or the related nonionic/noncoordinating base as an inexpensive and ecofriendly catalyst in a green solvent medium. The method is highly general and scalable with vast academic and industrial potential. As a practical application, we also demonstrate the use of these high-quality M-FAs in the synthesis of the spectrum of colloidal semiconductor NCs (III–V, II–VI, IV–VI, I–VI, I–III–VI, and perovskite) having absorption/emission in visible to the near-infrared region.

Tailoring the Dopant Distribution in ZnO:Mn Nanocrystals

The synthesis of semiconductor nanocrystals with controlled doping is highly challenging, as often a significant part of the doping ions are found segregated at nanocrystals surface, even forming secondary phases, rather than incorporated in the core. We have investigated the dopant distribution dynamics under slight changes in the preparation procedure of nanocrystalline ZnO doped with manganese in low concentration by electron paramagnetic resonance spectroscopy, paying attention to the formation of transient secondary phases and their transformation into doped ZnO. The acidification of the starting solution in the co-precipitation synthesis from nitrate precursors lead to the decrease of the Mn2+ ions concentration in the core of the ZnO nanocrystals and their accumulation in minority phases, until ~79% of the Mn2+ ions were localized in a thin disordered shell of zinc hydroxynitrate (ZHN). A lower synthesis temperature resulted in polycrystalline Mn-doped ZHN. Under isochronal annealing up to 250 °C the bulk ZHN and the minority phases from the ZnO samples decomposed into ZnO. The Mn2+ ions distribution in the annealed nanocrystals was significantly altered, varying from a uniform volume distribution to a preferential localization in the outer layers of the nanocrystals. Our results provide a synthesis strategy for tailoring the dopant distribution in ZnO nanocrystals for applications ranging from surface based to ones involving core properties.

Contact and Noncontact Measurement of Electronic Transport in Individual 2D SnS Colloidal Semiconductor Nanocrystals.

Colloidal-based solution syntheses offer a scalable and cost-efficient means of producing 2D nanomaterials in high yield. While much progress has been made toward the controlled and tailorable synthesis of semiconductor nanocrystals in solution, it remains a substantial challenge to fully characterize the products' inherent electronic transport properties. This is often due to their irregular morphology or small dimensions, which demand the formation of colloidal assemblies or films as a prerequisite to performing electrical measurements. Here, we report the synthesis of nearly monodisperse 2D colloidal nanocrystals of semiconductor SnS and a thorough investigation of the intrinsic electronic transport properties of single crystals. We utilize a combination of multipoint contact probe measurements and ultrafast terahertz spectroscopy to determine the carrier concentration, carrier mobility, conductivity/resistivity, and majority carrier type of individual colloidal semiconductor nanocrystals. Employing this metrological approach, we compare the electronic properties extracted for distinct morphologies of 2D SnS and relate them to literature values. Our results indicate that the electronic transport of colloidal semiconductors may be tuned through prudent selection of the synthetic conditions. We find that these properties compare favorably to SnS grown using vapor deposition techniques, illustrating that colloidal solution synthesis is a promising route to scalable production of nanoscale 2D materials.

Nanocluster-Mediated Synthesis of Diverse ZnTe Nanostructures: from Nanocrystals to 1D Nanobelts.

Liquid phase one-pot synthesis of semiconductor nanocrystals, by direct nucleation-growth crystallization, is unsuccessful for synthesis of some kinds of semiconductors. Using ZnTe as an example here, highly disperse ZnTe nanoclusters with diameters of 2-3 nm were first synthesized by a facile solvothermal method. Then the ZnTe nanoclusters were chosen as starting crystallization seeds to mediate the synthesis of flexible semiconductor nanostructures. Three-dimensional (3D) oriented assembly of ZnTe nanoclusters to monodisperse dendrimer-like nanocrystals (DLNCs), and one-dimensional (1D) ZnTe nanobelts with cubic phase, have been achieved successfully. Supported by TEM characterization of time-dependent morphology evolution, the oriented attachment assisted seed growth, based on ZnTe nanoclusters, enabled the 1D flexible ZnTe nanobelts formation, which could reach to ≈10 micrometers length.

Shape Control of Ternary Sulfide Nanocrystals

Synthesis of semiconductor nanocrystals with a definite shape is the foundation of their anisotropy properties investigation; however, it is more challenging in ternary metal sulfides than that of noble metal and binary sulfides. In this paper, we report a solvent polarity control strategy to prepare a family of ternary sulfide (Ag3SbS3) semiconductor nanocrystals with tunable polyhedral shapes. The crystal growth speed along different directions was confined by the capping effect of the polarity of solvents that was defined by reaction temperature. Crystal shape of Ag3SbS3 nanocrystals could be tailored as a sphere, hexagonal plate, and prism. A shape-controllable growth mechanism was analyzed based on the Bravais–Friedel–Donnay–Harker theory by taking crystal structure characteristics and the polarity of solvents into consideration. The semiconductor nanocrystals show a near value of the band gaps for different shaped samples and facet-dependent photocatalytic water-splitting activities, which may resul...

Tunable Room-Temperature Synthesis of Coinage Metal Chalcogenide Nanocrystals from N-Heterocyclic Carbene Synthons

We present a new toolset of precursors for semiconductor nanocrystal synthesis, N-heterocyclic carbene (NHC)-metal halide complexes, which enables a tunable molecular platform for the preparation of coinage metal chalcogenide quantum dots (QDs). Phase-pure and highly monodisperse coinage metal chalcogenide (Ag2E, Cu2–xE; E = S, Se) QDs are readily synthesized from the direct reaction of an NHC-MBr synthon (where M = Ag, Cu) with alkylsilyl chalcogenide reagents at room temperature. We demonstrate that the size of the resulting QDs is well-tailored by the electron-donating ability of the L-type NHC ligands, which are further confirmed to be the only organic capping ligands on the QD surface, imparting excellent colloidal stability. Local superstructures of the NHC-capped Ag2S QDs are observed by TEM, further demonstrating their potential for synthesizing monodisperse ensembles and mediating self-assembly.

Continuous synthesis of CuInS2 quantum dots

The impact of reactor type on synthesis parameters and disperse properties.

Insight into Cd0.9Zn0.1S solid-solution nanotetrapods: Growth mechanism and their application for photocatalytic hydrogen production

Shape-controlled synthesis of semiconductor nanocrystals enables fine manipulation over their physicochemical properties. In this paper, unique Cd0.9Zn0.1S tetrapod nanostructures consisting of two different polymorphisms, i.e., wurtzite and zinc blende, were successfully synthesized via a simple ethylenediamine-assisted solvothermal route. It is found that the temperature determines the formation of the zinc blende-structured seeds, while the ethylenediamine facilitates the subsequent preferential growth of the wurtzite phase arms. Furthermore, the tetrapods, characterized by a type-II staggered band alignment, have shown enhanced photocatalytic performance for H2 generation from an aqueous solution containing electron donors under visible light irradiation, in comparison to the nanocrystals with distinct morphologies. Our results indicate that coupling different polymorphisms in a nanocrystal may be advantageous for photocatalytic proton reduction over single-crystal photocatalyst.

Facile synthesis and optical properties of zincblende and wurtzite Cu3ZnInSnSe6 nanocrystals

Controllable synthesis of semiconductor nanocrystals is an attractive issue yet still remains challenging. Here, we describe our efforts toward this direction by a simple colloidal method to engineer quinary Cu3ZnInSnSe6 (CZITSe) nanocrystals with different crystal phases and shapes. In this work, wurtzite and zincblende CZITSe nanocrystals with different morphology could be selectively synthesized by tuning the addition order of the Se precursor to metal reactants solution. Furthermore, the optical and electrical behavior of the resultant nanocrystals exhibits obvious structure-dependent properties. This work will open new exciting avenues to explore the controllable synthesis of multicomponent semiconductor nanocrystals with metastable structures.

Synthesis of Semiconductor Nanocrystals, Focusing on Nontoxic and Earth-Abundant Materials.

We review the synthesis of semiconductor nanocrystals/colloidal quantum dots in organic solvents with special emphasis on earth-abundant and toxic heavy metal free compounds. Following the Introduction, section 2 defines the terms related to the toxicity of nanocrystals and gives a comprehensive overview on toxicity studies concerning all types of quantum dots. Section 3 aims at providing the reader with the basic concepts of nanocrystal synthesis. It starts with the concepts currently used to describe the nucleation and growth of monodisperse particles and next takes a closer look at the chemistry of the inorganic core and its interactions with surface ligands. Section 4 reviews in more detail the synthesis of different families of semiconductor nanocrystals, namely elemental group IV compounds (carbon nanodots, Si, Ge), III-V compounds (e.g., InP, InAs), and binary and multinary metal chalcogenides. Finally, the authors' view on the perspectives in this field is given.

Symmetry breaking in semiconductor nanocrystals via kinetic-controlled surface diffusion: a strategy for manipulating the junction structure.

The synthesis of semiconductor nanocrystals is usually limited to high-level symmetry, as constrained by the inherent, for example, face-centered cubic or hexagonal close-packed lattices of the crystals. Herein, we report a robust approach for breaking the symmetry of the CdS lattice and obtaining high-quality CdS ultrathin monopods, bipods, tripods, and tetrapods. The success relies on manipulating reaction kinetics by dropwise addition of a precursor solution, which permits deterministic control over the number of CdS monomers in the reaction solution. With rapid monomer supply by fast precursor injection, growth was restricted to only one {111} facet of the nascent CdS tetrahedron to produce an asymmetric ultrathin monopod (a zinc-blende tip with a wurtzite arm). Otherwise, growth monomers could access adjacent {111} facets through surface diffusion and thus lead to the switch of the growth pattern from asymmetric to symmetric to generate an ultrathin multipod (a zinc-blende tip/core with multi-wurtzite arms). These symmetry-controlled photocatalysts were characterized by a fine-tuned zinc blende-wurtzite intergrowth type-II homojunction. After evaluating their structure-dependent solar-hydrogen-production properties, the CdS ultrathin monopod with an appropriate length for controllable charge transportation showed the highest photocatalytic activity.

The flow reactor system for in-line synthesis of semiconductor nanoparticle

A flow reactor nanoparticle synthesis technique is proposed as replacement for «hot injection» synthesis of semiconductor nanocrystals in a glass flask. The main advantages are possibility of continuous nanoparticles production, technology flexibility and lower cost of the final products in comparison with currently applied methods.

The Role of Halogens in the Synthesis of Semiconductor Nanocrystals

Abstract The presence of halogen compounds in the synthesis of semiconductor nanocrystals (NCs) produced by hot injection drastically influence their capping ligand sphere and/or chemical reactivity, allowing for the growth of hybrid systems, including composites combining NCs decorating carbon sp2 surfaces. As a result, the presence of halide anions on the surface of some NCs has proven to have a positive impact on the optical and electrical properties. In this work the effects that halogen co-solvents (including alkylchlorides, bromides, and iodides) induce in the synthesis of rod-like Wurtzite CdSe NCs generated by hot-injection will be reviewed. The proposed mechanism of the reaction as well as a detailed characterization of the capping ligand sphere by Nuclear Magnetic Resonance (NMR) and X-Ray photoelectron Spectroscopy (XPS) will be discussed. Correlated cyclic voltammetry (CV) and XPS studies have been performed to address the effect of the halide anions on the NCs surface. Finally an example of the effect of the addition of chlorine-containing co-solvents on the synthesis of PbS rock-salt NCs will be shown.

In situ synthesis of semiconductor nanocrystals at the surface of tubular J-aggregates

Hybrid nanostructures consisting of dye molecules and semiconducting nanocrystals are currently much investigated because they can benefit from the strong absorption of organic dyes and the chemical stability of inorganic components, a combination of properties that is promising for various optoelectronic applications. Here, we report an in situ synthesis method to design hybrid nanotubes by growing metal sulfide (MeS) nanocrystals at the surface of tubular J-aggregates of amphiphilic carbocyanine dyes. In this method, metal cations (Me2+ = Cd2+, Zn2+, and Ni2+) are enriched at the negatively charged surface of J-aggregates via electrostatic attraction, giving rise to a diffuse Helmholtz layer. Subsequent addition of thioacetamide and ammonium hydroxide leads to the formation of a dense layer of MeS nanocrystals exclusively close to or at the surface of J-aggregates. Typically, a shell of CdS nanocrystals (size ∼ 3 nm) is formed at the surface of tubular J-aggregates without significantly affecting the aggregate morphology. The crystal structure is analyzed by electron diffraction revealing a sphalerite structure. The coverage of CdS nanocrystals on J-aggregates can be tuned by varying the concentration of the sulphur source. Efficient energy transfer from CdS nanocrystals to J-aggregates results in the quenching of the CdS photoluminescence. Structurally and morphologically similar hybrid systems of tubular J-aggregates and semiconductor nanocrystals (e.g., ZnS and NiS) are synthesized by analog chemical reactions.

无机半导体纳米晶光学性质的设计、调控及其生物医学应用

Semiconductor nanocrystals have attracted extensive attention in photoelectrical devices and biomedical applications over the past decades due to their plentiful band gaps and optical properties. Therefore, rational design and precise modulation of their optical properties are of great importance. This review briefly summarized our recent progress in controlled synthesis of semiconductor nanocrystals with different bandgaps, and the self-assembly of nanostructures with novel optical properties assembled using DNA and protein as templates. In the end, we summarized the novel optical properties and biomedical applications of the obtained semiconductor nanocrystallines and nanostructures.

Shape-controlled synthesis of PbS nanostructures from −20 to 240 °C: the competitive process between growth kinetics and thermodynamics

The traditional concept of the synthesis of semiconductor nanocrystals (NCs) by solvent routes usually performed under high temperatures, causes the semiconductor materials to nucleate and grow into various shaped NCs in solution. Therefore, these methods are named as “solvent-thermal approachs”. In this work, we describe a simple and reproducible strategy for the synthesis of PbS NCs at temperatures even as low as −20 °C by using frozen and solidified precursors. With the aid of alkylamines, nano-sized PbS could also nucleate and grow at such low temperatures within a short time (a few seconds). The experimental results not only break people's traditional thinking but also provide a significant and novel direction in the engineering of the synthesis of NCs. In addition, we further systematically investigated the effect of two types of temperatures (the mixing temperature of the precursors and the ripening temperature of the PbS NCs). Combining this with different alkylamines, we found an obvious competition between a growth kinetic process caused by the alkylamines and a thermodynamic process induced by the temperature, which formed variously shaped monodispersed PbS NCs, including flower-, star-, sphere-, truncated octahedron-, cuboctahedron-, quasi cube-, cube-shaped and some hollow PbS NCs. Furthermore, this competition process could also provide a facile and cost-effective route to synthesize size-tunable but shape-permanent PbS NCs and their self-assembly superlattices in the same reaction systems, which is still a major challenge at present. Afterward, both the formation mechanisms of the PbS nanostructures synthesized below room temperature and the shape transformation depending on two types of temperature and alkylamines are systematically discussed.

DNA‐based programing of quantum dot properties

Nucleic acid molecules can serve as robust ligands for aqueous synthesis of semiconductor nanocrystals or quantum dots (QDs). QD properties including size, morphology, dispersity, emission maximum, and quantum yield are highly dependent on the sequences and structures of nucleic acids used for the synthesis. This synthetic strategy provides a novel facile means of constructing compact, stable, and biofunctionalized QDs in one step, which is of particular interest for a variety of applications such as biosensing, bioimaging, and self-assembly. This article summarizes recent advances in nucleic acid-templated QD synthesis with an emphasis on the nucleic acids-based programing of quantum dots properties. A variety of applications based on DNA-passivated QDs are also discussed.