Jong Seok Jeong

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 silicons 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).

Synthesis of diamond-shape titanate molecular sheets with different sizes and realization of quantum confinement effect during dimensionality reduction from two to zero

Synthesis of semiconductor nanoparticles with uniform shapes, sizes, and compositions in series with a gradual size reduction has not been achieved for two-dimensional molecular sheets. We report a large-scale (>2.6 g) synthesis of 0.75-nm-thick diamond-shape lepidocrocite-type titanate molecular sheets with the sizes decreasing from (27.3, 19.1) to (7.7, 5.5), where the numbers in parentheses represent the long and short diagonal lengths, respectively, in nm. This is the first example of synthesizing semiconductor nanoparticles in series with the dimensionality reduction from two to zero, without coating the surfaces with surface-passivating ligands. The titanate molecular sheets showed three exciton-absorption bands in the 4.0-6.5 eV region, the absorption energies of which increased with decreasing the area. Contrary to the common belief, the per-unit cell oscillator strengths gradually increased with increasing area and the per-particle oscillator strengths increased in proportion to the area. The average reduced exciton masses along the two diagonal axes were 0.10 and 0.11 m e, respectively, which were much smaller than those of bulk titanates (by 60-130 times). The estimated average Bohr radii along the two-diagonal axes were 4.8 and 4.3 nm, respectively.

Disproportionation of (mg,fe)sio3 perovskite in earth’s deep lower mantle

The mineralogical constitution of the Earth’s mantle dictates the geophysical and geochemical properties of this region. Previous models of a perovskite-dominant lower mantle have been built on the assumption that the entire lower mantle down to the top of the D″ layer contains ferromagnesian silicate [(Mg,Fe)SiO3] with nominally 10 mole percent Fe. On the basis of experiments in laser-heated diamond anvil cells, at pressures of 95 to 101 gigapascals and temperatures of 2200 to 2400 kelvin, we found that such perovskite is unstable; it loses its Fe and disproportionates to a nearly Fe-free MgSiO3 perovskite phase and an Fe-rich phase with a hexagonal structure. This observation has implications for enigmatic seismic features beyond ~2000 kilometers depth and suggests that the lower mantle may contain previously unidentified major phases. A presumed dominant mineral in Earth’s interior can disassociate into two phases at lower mantle conditions [Also see Perspective by Williams] Delving deeper into the lower mantle Earths lower mantle is an enigmatic region, a transition zone between slowly churning solids and a liquid outer core. Large seismic structures and discontinuities in this region are probably due to sharp gradients in temperature, composition, or mineralogy. Teasing apart the precise effects of these factors requires experiments at lower mantle temperatures and pressures (see the Perspective by Williams). Zhang et al. found that the major mineral phase of the lower mantle decomposes into two minerals. Andrault et al. show how the melting of subducted basalt from the oceanic crust will form pile-like structures on top of the core/mantle boundary. Science, this issue p. 877, p. 892; see also p. 800.

Structure and transport in high pressure oxygen sputter-deposited BaSnO3−δ

BaSnO3 has recently been identified as a high mobility wide gap semiconductor with significant potential for room temperature oxide electronics. Here, a detailed study of the high pressure oxygen sputter-deposition, microstructure, morphology, and stoichiometry of epitaxial BaSnO3 on SrTiO3(001) and MgO(001) is reported, optimized conditions resulting in single-phase, relaxed, close to stoichiometric films. Most significantly, vacuum annealing is established as a facile route to n-doped BaSnO3−δ, leading to electron densities above 1019 cm−3, 5 mΩ cm resistivities, and room temperature mobility of 20 cm2 V−1 s−1 in 300-A-thick films on MgO(001). Mobility limiting factors, and the substantial scope for their improvement, are discussed.

Growth behaviour of well-aligned ZnO nanowires on a Si substrate at low temperature and their optical properties

Well-aligned ZnO nanowires were successfully synthesized on a silicon substrate at the low temperature of 550 degrees C by catalyst-free vapour phase deposition. The ZnO nanowires had diameters in the range of 70-100 nm and lengths over several tens of micrometres. The synthesized ZnO nanowires, which had a single-crystalline wurtzite structure, showed a uniform morphology and faceted planes at the tips of the nanowires. The photoluminescence of the ZnO nanowires showed a strong UV band at 3.28 eV and a broad green band at 2.29 and at 2.53 eV at room temperature. A detailed discussion regarding the growth behaviour and the growth mechanism of the ZnO nanowires on the silicon substrate is presented in this work.

Atomic and electronic structure of exfoliated black phosphorus

Black phosphorus, a layered two-dimensional crystal with tunable electronic properties and high hole mobility, is quickly emerging as a promising candidate for future electronic and photonic devices. Although theoretical studies using ab initio calculations have tried to predict its atomic and electronic structure, uncertainty in its fundamental properties due to a lack of clear experimental evidence continues to stymie our full understanding and application of this novel material. In this work, aberration-corrected scanning transmission electron microscopy and ab initio calculations are used to study the crystal structure of few-layer black phosphorus. Directly interpretable annular dark-field images provide a three-dimensional atomic-resolution view of this layered material in which its stacking order and all three lattice parameters can be unambiguously identified. In addition, electron energy-loss spectroscopy (EELS) is used to measure the conduction band density of states of black phosphorus, which agrees well with the results of density functional theory calculations performed for the experimentally determined crystal. Furthermore, experimental EELS measurements of interband transitions and surface plasmon excitations are also consistent with simulated results. Finally, the effects of oxidation on both the atomic and electronic structure of black phosphorus are analyzed to explain observed device degradation. The transformation of black phosphorus into amorphous PO3 or H3PO3 during oxidation may ultimately be responsible for the degradation of devices exposed to atmosphere over time.

Dry thermal oxidation of a graded SiGe layer

A method for the dry thermal oxidation of a strained SiGe layer is proposed. By oxidation of a graded Si1−xGex layer, the effect of Ge pileup was significantly reduced and the undesirable strain relaxation by defect formation is prohibited. After oxidation, the oxidized SiGe layer was homogenized by postannealing process, and thereby a SiO2/SiGe interface with good structural properties was obtained. During postannealing, the homogenization was significantly enhanced by strain-induced diffusion, and it was clearly proved by the uphill diffusion. This result can propose an alternative oxidation method of strained SiGe/Si heterostructures.

Stoichiometry-driven metal-to-insulator transition in NdTiO3/SrTiO3 heterostructures

By controlling stoichiometry via a hybrid molecular beam epitaxy approach, we report on the study of thin film growth and the electronic transport properties of phase-pure, epitaxial NdTiO3/SrTiO3 heterostructures grown on (001) (La0.3Sr0.7)(Al0.65Ta0.35)O3 (LSAT) substrates as a function of cation stoichiometry in NdTiO3. Despite the symmetry mismatch between bulk NdTiO3 and the substrate, NdTiO3 films grew in an atomic layer-by-layer fashion over a range of cation stoichiometry; however amorphous films resulted in cases of extreme cation non-stoichiometry. Temperature-dependent sheet resistance measurements were consistent with Fermi-liquid metallic behavior over a wide temperature range, but revealed a remarkable crossover from metal-to-insulator (M-I) type behavior at low temperatures for all compositions. A direct correlation between cation stoichiometry, transport behavior, and the temperature of M-I transition is established.

Investigation of initial growth of ZnO nanowires and their growth mechanism

ZnO nanowires were synthesized on Si substrates by a simple metal vapor deposition method without any catalysts. The initial growth and the growth mechanism of the ZnO nanowires were studied using scanning and transmission electron microscopy. We found that the ZnO nanowires grew on the Si substrate via a self-seeding vapor-solid mechanism. The growth process of the ZnO nanowires consisted of four steps: self-seeding, one-dimensional epitaxial growth of the nanowires on the seeds by a base-growth mode, further acceleration of nanowire growth with additional seeding, and active formation of the nanowires.

Hybrid molecular beam epitaxy for the growth of stoichiometric BaSnO3

Owing to its high room-temperature electron mobility and wide bandgap, BaSnO3 has recently become of significant interest for potential room-temperature oxide electronics. A hybrid molecular beam epitaxy (MBE) approach for the growth of high-quality BaSnO3 films is developed in this work. This approach employs hexamethylditin as a chemical precursor for tin, an effusion cell for barium, and a radio frequency plasma source for oxygen. BaSnO3 films were thus grown on SrTiO3 (001) and LaAlO3 (001) substrates. Growth conditions for stoichiometric BaSnO3 were identified. Reflection high-energy electron diffraction (RHEED) intensity oscillations, characteristic of a layer-by-layer growth mode were observed. A critical thickness of ∼1 nm for strain relaxation was determined for films grown on SrTiO3 using in situ RHEED. Scanning transmission electron microscopy combined with electron energy-loss spectroscopy and energy dispersive x-ray spectroscopy confirmed the cube-on-cube epitaxy and composition. The importan...