A thermoelectric figure of merit that is unprecedentedly high has been observed in SnSe crystals. This finding suggests that bulk materials with a layered structure and highly anharmonic lattice ...vibrations can feature an intrinsically low thermal conductivity and high thermoelectric efficiency. Together with other recent findings in this field, it urges us to revisit basic design principles for thermoelectric materials.
We propose a general strategy to synthesize largely unexplored soluble chalcogenidometallates of cadmium, lead, and bismuth. These compounds can be used as "solders" for semiconductors widely used in ...photovoltaics and thermoelectrics. The addition of solder helped to bond crystal surfaces and link nano- or mesoscale particles together. For example, CdSe nanocrystals with Na2Cd2Se3 solder was used as a soluble precursor for CdSe films with electron mobilities exceeding 300 square centimeters per volt-second. CdTe, PbTe, and Bi2Te3 powders were molded into various shapes in the presence of a small additive of composition-matched chalcogenidometallate or chalcogel, thus opening new design spaces for semiconductor technologies.
Here, quasi-two-dimensional semiconductor nanoplatelets (NPLs) have garnered widespread interest because of their uniquely narrow emission spectra and favorable characteristics for optical gain and ...lasing.(1–5) Strongly quantum confined along thickness, NPLs exhibit the electronic structure of quantum wells determined by their thickness, which can be controlled with atomic precision.(6–10) Among all semiconductor NPLs, CdSe NPLs are probably the most studied. So far, four kinds of CdSe NPLs, emitting at around 396 nm, 463 nm, 513m, and 553 nm, have been intensely investigated for almost a decade.(11–18) Despite some early confusions in identifying actual thicknesses of those NPLs, they have been successfully assigned to be 2-monolayer (2 ML) to 5-monolayer (5 ML) thickness by high-resolution TEM.(19) Here, nML thickness corresponds to n chalcogen layers sandwiched between n + 1 metal layers. Another kind of NPLs with 6MLs of CdSe, once briefly mentioned in 2013,(20) however, was not explored, likely because its synthesis proved hard to reproduce. The 6 ML CdSe NPLs can be prepared by the colloidal atomic layer deposition (c-ALD) method,(21) but there have been no reports on direct synthesis of 6 ML CdSe NPLs, and their direct synthesis has been in doubt as being difficult by traditional approaches.(22,23)
Single-crystal PbSe nanowires are synthesized in solution through oriented attachment of nanocrystal building blocks. Reaction temperatures of 190−250 °C and multicomponent surfactant mixtures result ...in a nearly defect-free crystal lattice and high uniformity of nanowire diameter along the entire length. The wires' dimensions are tuned by tailoring reaction conditions in a range from ∼4 to ∼20 nm in diameter with wire lengths up to ∼30 μm. PbSe nanocrystals bind to each other on either {100}, {110}, or {111} faces, depending on the surfactant molecules present in the reaction solution. While PbSe nanocrystals have the centrosymmetric rocksalt lattice, they can lack central symmetry due to a noncentrosymmetric arrangement of Pb- and Se-terminated {111} facets and possess dipole driving one-dimensional oriented attachment of nanocrystals to form nanowires. In addition to straight nanowires, zigzag, helical, branched, and tapered nanowires as well as single-crystal nanorings can be controllably prepared in one-pot reactions by careful adjustment of the reaction conditions.
The self-assembly of two sizes of spherical nanocrystals has revealed a surprisingly diverse library of structures. To date, at least 15 distinct binary nanocrystal superlattice (BNSL) structures ...have been identified. The stability of these binary phases cannot be fully explained using the traditional conceptual framework treating the assembly process as entropy-driven crystallization of rigid spherical particles. Such deviation from hard sphere behavior may be explained by the soft and deformable layer of ligands that envelops the nanocrystals, which contributes significantly to the overall size and shape of assembling particles. In this work, we describe a set of experiments designed to elucidate the role of the ligand corona in shaping the thermodynamics and kinetics of BNSL assembly. Using hydrocarbon-capped Au and PbS nanocrystals as a model binary system, we systematically tuned the core radius (R) and ligand chain length (L) of particles and subsequently assembled them into binary superlattices. The resulting database of binary structures enabled a detailed analysis of the role of effective nanocrystal size ratio, as well as softness expressed as L/R, in directing the assembly of binary structures. This catalog of superlattices allowed us to not only study the frequency of different phases but to also systematically measure the geometric parameters of the BNSLs. This analysis allowed us to evaluate new theoretical models treating the cocrystallization of deformable spheres and to formulate new hypotheses about the factors affecting the nucleation and growth of the binary superlattices. Among other insights, our results suggest that the relative abundance of the binary phases observed may be explained not only by considerations of thermodynamic stability, but also by a postulated preordering of the binary fluid into local structures with icosahedral or polytetrahedral symmetry prior to nucleation.
Stable colloids in molten inorganic salts Zhang, Hao; Dasbiswas, Kinjal; Ludwig, Nicholas B ...
Nature (London),
02/2017, Letnik:
542, Številka:
7641
Journal Article
Recenzirano
A colloidal solution is a homogeneous dispersion of particles or droplets of one phase (solute) in a second, typically liquid, phase (solvent). Colloids are ubiquitous in biological, chemical and ...technological processes, homogenizing highly dissimilar constituents. To stabilize a colloidal system against coalescence and aggregation, the surface of each solute particle is engineered to impose repulsive forces strong enough to overpower van der Waals attraction and keep the particles separated from each other. Electrostatic stabilization of charged solutes works well in solvents with high dielectric constants, such as water (dielectric constant of 80). In contrast, colloidal stabilization in solvents with low polarity, such as hexane (dielectric constant of about 2), can be achieved by decorating the surface of each particle of the solute with molecules (surfactants) containing flexible, brush-like chains. Here we report a class of colloidal systems in which solute particles (including metals, semiconductors and magnetic materials) form stable colloids in various molten inorganic salts. The stability of such colloids cannot be explained by traditional electrostatic and steric mechanisms. Screening of many solute-solvent combinations shows that colloidal stability can be traced to the strength of chemical bonding at the solute-solvent interface. Theoretical analysis and molecular dynamics modelling suggest that a layer of surface-bound solvent ions produces long-ranged charge-density oscillations in the molten salt around solute particles, preventing their aggregation. Colloids composed of inorganic particles in inorganic melts offer opportunities for introducing colloidal techniques to solid-state science and engineering applications.
Colloidal quantum wells are two-dimensional materials grown with atomically-precise thickness that dictates their electronic structure. Although intersubband absorption in epitaxial quantum wells is ...well-known, analogous observations in non-epitaxial two-dimensional materials are sparse. Here we show that CdSe nanoplatelet quantum wells have narrow (30-200 meV), polarized intersubband absorption features when photoexcited or under applied bias, which can be tuned by thickness across the near-infrared (NIR) spectral window (900-1600 nm) inclusive of important telecommunications wavelengths. By examination of the optical absorption and polarization-resolved measurements, the NIR absorptions are assigned to electron intersubband transitions. Under photoexcitation, the intersubband features display hot carrier and Auger recombination effects similar to excitonic absorptions. Sequenced two-color photoexcitation permits the sub-picosecond modulation of the carrier temperature in such colloidal quantum wells. This work suggests that colloidal quantum wells may be promising building blocks for NIR technologies.
Colloidal CdSe Quantum Rings Fedin, Igor; Talapin, Dmitri V
Journal of the American Chemical Society,
08/2016, Letnik:
138, Številka:
31
Journal Article
Recenzirano
Semiconductor quantum rings are of great fundamental interest because their non-trivial topology creates novel physical properties. At the same time, toroidal topology is difficult to achieve for ...colloidal nanocrystals and epitaxially grown semiconductor nanostructures. In this work, we introduce the synthesis of luminescent colloidal CdSe nanorings and nanostructures with double and triple toroidal topology. The nanorings form during controlled etching and rearrangement of two-dimensional nanoplatelets. We discuss a possible mechanism of the transformation of nanoplatelets into nanorings and potential utility of colloidal nanorings for magneto-optical (e.g., Aharonov–Bohm effect) and other applications.
Group III nitrides are of great technological importance for electronic devices. These materials have been widely manufactured via high-temperature methods such as physical vapor transport (PVT), ...chemical vapor deposition (CVD), and hydride vapor phase epitaxy (HVPE). The preparation of group III nitrides by colloidal synthesis methods would provide significant advantages in the form of optical tunability via size and shape control and enable cost reductions through scalable solution-based device integration. Solution syntheses of III-nitride nanocrystals, however, have been scarce, and the quality of the synthesized products has been unsatisfactory for practical use. Here, we report that incorporating a molten salt phase in solution synthesis can provide a viable option for producing crystalline III-nitride nanomaterials. Crystalline GaN and AlN nanomaterials can be grown in a biphasic molten-salt/organic-solvent mixture under an ammonia atmosphere at moderate temperatures (less than 300 °C) and stabilized under ambient conditions by postsynthetic treatment with organic surface ligands. We suggest that microscopic reversibility of monomer attachment, which is essential for crystalline growth, can be achieved in molten salt during the nucleation and the growth of the III-nitride nanocrystals. We also show that increased ammonia pressure increases the size of the GaN nanocrystals produced. This work demonstrates that use of molten salt and high-pressure reactants significantly expands the chemical scope of solution synthesis of inorganic nanomaterials.
In this work, we synthesized InP and InAs nanocrystals (NCs) capped with different inorganic ligands, including various molecular metal chalcogenide complexes (MCCs) and chalcogenide ions. We found ...that MCCs and chalcogenide ions can quantitatively displace organic ligands from the surface of III–V NCs and serve as the inorganic capping groups for III–V NC surfaces. These inorganic ligands stabilize colloidal solutions of InP and InAs NCs in polar solvents and greatly facilitate charge transport between individual NCs. Charge transport studies revealed high electron mobility in the films of MCC-capped InP and InAs NCs. For example, we found that bridging InAs NCs with Cu7S4 – MCC ligands can lead to very high electron mobility exceeding 15 cm2/(V s). In addition, we observed unprecedented ambipolar (positive/negative) photoresponse of MCC-capped InAs NC solids that changed sign depending on the ligand chemistry, illumination wavelength, and doping of the NC solid. For example, the sign of photoconductance of InAs NCs capped with Cu7S4 – or Sn2S6 4– ions converted from positive at 0.80 and 0.95 eV to negative at 1.27 and 1.91 eV. We propose an explanation of this unusually complex photoconductivity of InAs NC solids.
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