Phosphorus-Doped Silicon Nanocrystals Exhibiting Mid-Infrared Localized Surface Plasmon Resonance

Localized surface plasmon resonances (LSPRs) enable tailoring of the optical response of nanomaterials through their free carrier concentration, morphology, and dielectric environment. Recent efforts to expand the spectral range of usable LSPR frequencies into the infrared successfully demonstrated LSPRs in doped semiconductor nanocrystals. Despite silicon’s importance for electronic and photonic applications, no LSPRs have been reported for doped silicon nanocrystals. Here we demonstrate doped silicon nanocrystals synthesized via a nonthermal plasma technique that exhibits tunable LSPRs in the energy range of 0.07–0.3 eV or mid-infrared wavenumbers of 600–2500 cm^–1

Direct Synthesis of 7 nm-Thick Zinc(II)–Benzimidazole–Acetate Metal–Organic Framework Nanosheets

Direct Synthesis of 7 nm-Thick Zinc(II)–Benzimidazole–Acetate Metal–Organic Framework Nanosheet

Nonthermal Plasma Synthesis of Core/Shell Quantum Dots: Strained Ge/Si Nanocrystals

In this work, we present an all-gas-phase approach for the synthesis of quantum-confined core/shell nanocrystals (NCs) as a promising alternative to traditional solution-based methods. Spherical quantum dots (QDs) are grown using a single-stage flow-through nonthermal plasma, yielding monodisperse NCs, with a concentric core/shell structure confirmed by electron microscopy. The in-flight negative charging of the NCs by plasma electrons keeps the NC cores separated during shell growth. The success of this gas-phase approach is demonstrated here through the study of Ge/Si core/shell QDs. We find that the epitaxial growth of a Si shell on the Ge QD core compressively strains the Ge lattice and affords the ability to manipulate the Ge band structure by modulation of the core and shell dimensions. This all-gas-phase approach to core/shell QD synthesis offers an effective method to produce high-quality heterostructured NCs with control over the core and shell dimensions

Observation of Electrically-Inactive Interstitials in Nb-Doped SrTiO₃

Despite rapid recent progress, controlled dopant incorporation and attainment of high mobility in thin films of the prototypical complex oxide semiconductor SrTiO₃ remain problematic. Here, analytical scanning transmission electron microscopy is used to study the local atomic and electronic structure of Nb-doped SrTiO₃ both in ideally substitutionally doped bulk single crystals and epitaxial thin films. The films are deposited under conditions that would yield highly stoichiometric <i>undoped</i> SrTiO₃, but are nevertheless insulating. The Nb incorporation in such films was found to be highly inhomogeneous on nanoscopic length-scales, with large quantities of what we deduce to be interstitial Nb. Electron energy loss spectroscopy reveals changes in the electronic density of states in Nb-doped SrTiO₃ films compared to undoped SrTiO₃, but without the clear shift in the Fermi edge seen in bulk single crystal Nb-doped SrTiO₃. Analysis of atomic-resolution annular dark-field images allows us to conclude that the interstitial Nb is in the Nb⁰ state, confirming that it is electrically inactive. We argue that this approach should enable future work establishing the vitally needed relationships between synthesis/processing conditions and electronic properties of Nb-doped SrTiO₃ thin films

Nonthermal Plasma Synthesis of Titanium Nitride Nanocrystals with Plasmon Resonances at Near-Infrared Wavelengths Relevant to Photothermal Therapy

Titanium nitride has attracted attention for its plasmonic properties as a thermally stable, biocompatible, and cost-effective alternative to gold. In this work, we synthesized titanium nitride nanocrystals in a nonthermal plasma using tetrakis (dimethylamino) titanium (TDMAT) and ammonia as the titanium and nitrogen precursors. Extinction measurements of as-produced 6–8 nm titanium nitride nanocrystals exhibit a broad plasmon resonance peaking near 800 nm, possibly suitable for photothermal therapy treatments. Ammonia flow rate and plasma power were found to affect nanocrystal morphology and chemical composition, and therefore significantly impact the plasmonic properties. A moderate ammonia flow rate of 1.2 sccm and relatively high nominal plasma power of 100 W produced samples with the best plasmon resonances, narrower than those previously reported for plasma-synthesized titanium nitride nanocrystals

A New Line Defect in NdTiO₃ Perovskite

Perovskite oxides form an eclectic class of materials owing to their structural flexibility in accommodating cations of different sizes and valences. They host well-known point and planar defects, but so far no line defect has been identified other than dislocations. Using analytical scanning transmission electron microscopy (STEM) and ab initio calculations, we have detected and characterized the atomic and electronic structures of a novel line defect in NdTiO₃ perovskite. It appears in STEM images as a perovskite cell rotated by 45°. It consists of self-organized Ti–O vacancy lines replaced by Nd columns surrounding a central Ti–O octahedral chain containing Ti⁴⁺ ions, as opposed to Ti³⁺ in the host. The distinct Ti valence in this line defect introduces the possibility of engineering exotic conducting properties in a single preferred direction and tailoring novel desirable functionalities in this Mott insulator

A New Line Defect in NdTiO₃ Perovskite

Perovskite oxides form an eclectic class of materials owing to their structural flexibility in accommodating cations of different sizes and valences. They host well-known point and planar defects, but so far no line defect has been identified other than dislocations. Using analytical scanning transmission electron microscopy (STEM) and ab initio calculations, we have detected and characterized the atomic and electronic structures of a novel line defect in NdTiO₃ perovskite. It appears in STEM images as a perovskite cell rotated by 45°. It consists of self-organized Ti–O vacancy lines replaced by Nd columns surrounding a central Ti–O octahedral chain containing Ti⁴⁺ ions, as opposed to Ti³⁺ in the host. The distinct Ti valence in this line defect introduces the possibility of engineering exotic conducting properties in a single preferred direction and tailoring novel desirable functionalities in this Mott insulator

Plasmonic Interactions through Chemical Bonds of Surface Ligands on PbSe Nanocrystals

When functional films are cast from colloidal dispersions of semiconductor nanocrystals, the length and structure of the ligands capping their surfaces determine the electronic coupling between the nanocrystals. Long chain oleic acid ligands on the surface of IV–VI semiconductor nanocrystals such as PbSe are typically considered to be insulating. Consequently, these ligands are either removed or replaced with short ones to bring the nanocrystals closer to each other for increased electronic coupling. Herein, using high-angle annular dark-field scanning transmission electron microscopy imaging combined with electron energy loss spectroscopy, we show that partial oxidation of PbSe nanocrystals forms conjugated double bonds within the oleic ligands, which then facilitates enhanced plasmonic interaction among the nanocrystals. The changes in the geometric configurations of the ligands are imaged directly and correlated with the changes in the surface plasmon intensities as they oxidize and undergo structural modifications

Propagating Nanocavity-Enhanced Rapid Crystallization of Silicon Thin Films

We demonstrate a mechanism of solid-phase crystallization (SPC) enabled by nanoscale cavities formed at the interface between an hydrogenated amorphous silicon film and embedded 30 to 40 nm Si nanocrystals. The nanocavities, 10 to 25 nm across, have the unique property of an internal surface that is part amorphous and part crystalline, enabling capillarity-driven diffusion from the amorphous to the crystalline domain. The nanocavities propagate rapidly through the amorphous phase, up to five times faster than the SPC growth rate, while “pulling behind” a crystalline tail. Using transmission electron microscopy it is shown that twin boundaries exposed on the crystalline surface accelerate crystal growth and influence the direction of nanocavity propagation

Propagating Nanocavity-Enhanced Rapid Crystallization of Silicon Thin Films

We demonstrate a mechanism of solid-phase crystallization (SPC) enabled by nanoscale cavities formed at the interface between an hydrogenated amorphous silicon film and embedded 30 to 40 nm Si nanocrystals. The nanocavities, 10 to 25 nm across, have the unique property of an internal surface that is part amorphous and part crystalline, enabling capillarity-driven diffusion from the amorphous to the crystalline domain. The nanocavities propagate rapidly through the amorphous phase, up to five times faster than the SPC growth rate, while “pulling behind” a crystalline tail. Using transmission electron microscopy it is shown that twin boundaries exposed on the crystalline surface accelerate crystal growth and influence the direction of nanocavity propagation