Formation of dopant-pair defects and doping efficiency in B- and P-doped silicon nanowires

Recent Progress on Manganese-Based Nanostructures as Responsive MRI Contrast Agents

Manganese-based nanostructured contrast agents (CAs) entered the field of medical diagnosis through magnetic resonance imaging (MRI) some years ago. Although some of these Mn-based CAs behave as classic T₁ contrast enhancers in the same way as clinical Gd-based molecules do, a new type of Mn nanomaterials have been developed to improve MRI sensitivity and potentially gather new functional information from tissues by using traditional T₁ contrast enhanced MRI. These nanomaterials have been designed to respond to biological environments, mainly to pH and redox potential variations. In many cases, the differences in signal generation in these responsive Mn-based nanostructures come from intrinsic changes in the magnetic properties of Mn cations depending on their oxidation state. In other cases, no changes in the nature of Mn take place, but rather the nanomaterial as a whole responds to the change in the environment through different mechanisms, including changes in integrity and hydration state. This review focusses on the chemistry and MR performance of these responsive Mn-based nanomaterials.

Controllable Carrier Type in Boron Phosphide Nanowires Toward Homostructural Optoelectronic Devices

The p-n junction is one important and fundamental building block of the optoelectronic age. However, electrons and holes will be severely scattered in heterostructures led by the grain boundary at the alloy interface between two dissimilar semiconductors. In this work, we present boron phosphide (BP) nanowires with artificially controllable carrier type for the fabrication of homojunctions via adjusting borane/phosphine ratio during the deposition process, both prove high crystallization with fewer impurities. The homojunctions that consist of  n-type and p-type BP nanowires show apparent photovoltaic effect [external quantum efficiency ≈ 10% under a ∼0.4 pW light @ 600 nm] and the quenched photoluminescence within the junction area, which indicates the effective separation and transfer of photogenerated charge carriers at the interface. The achievement of controllable carrier type implemented in the same material ushers in a frontier for the design of nanoscale homojunctions toward advanced optoelectronic devices.

Enhancing Thermoelectric Performances of Bismuth Antimony Telluride via Synergistic Combination of Multiscale Structuring and Band Alignment by FeTe2 Incorporation

It has been a difficulty to form well-distributed nano- and mesosized inclusions in a Bi₂Te₃-based matrix and thereby realizing no degradation of carrier mobility at interfaces between matrix and inclusions for high thermoelectric performances. Herein, we successfully synthesize multistructured thermoelectric Bi_0.4Sb_1.6Te₃ materials with Fe-rich nanoprecipitates and sub-micron FeTe₂ inclusions by a conventional solid-state reaction followed by melt-spinning and spark plasma sintering that could be a facile preparation method for scale-up production. This study presents a bismuth antimony telluride based thermoelectric material with a multiscale structure whose lattice thermal conductivity is drastically reduced with minimal degradation on its carrier mobility. This is possible because a carefully chosen FeTe₂ incorporated in the matrix allows its interfacial valence band with the matrix to be aligned, leading to a significantly improved p-type thermoelectric power factor. Consequently, an impressively high thermoelectric figure of merit ZT of 1.52 is achieved at 396 K for p-type Bi_0.4Sb_1.6Te₃-8 mol % FeTe₂, which is a 43% enhancement in ZT compared to the pristine Bi_0.4Sb_1.6Te₃. This work demonstrates not only the effectiveness of multiscale structuring for lowering lattice thermal conductivities, but also the importance of interfacial band alignment between matrix and inclusions for maintaining high carrier mobilities when designing high-performance thermoelectric materials.

Ab initio study of hydrogenic effective mass impurities in Si nanowires

The effect of B and P dopants on the band structure of Si nanowires is studied using electronic structure calculations based on density functional theory. At low concentrations a dispersionless band is formed, clearly distinguishable from the valence and conduction bands. Although this band is evidently induced by the dopant impurity, it turns out to have purely Si character. These results can be rigorously analyzed in the framework of effective mass theory. In the process we resolve some common misconceptions about the physics of hydrogenic shallow impurities, which can be more clearly elucidated in the case of nanowires than would be possible for bulk Si. We also show the importance of correctly describing the effect of dielectric confinement, which is not included in traditional electronic structure calculations, by comparing the obtained results with those of G₀W₀ calculations.

Retarded dopant diffusion by moderated dopant–dopant interactions in Si nanowires

The mechanical softening and quantum confinement found in nanostructures are the physical origin of the suppressed dopant diffusion.

Chemically doped radial junction characteristics in silicon nanowires

We evaluate the boron (B) and phosphorus (P) core-surface codoped radial p-n junction characteristics in silicon nanowires (SiNWs) using density functional theory calculations. We find that the formation of radial p-n junction is energetically favorable. The stability depends on the diameter of SiNWs and the dopant concentration. Generally, a higher concentration of B-P pair dopants results in a more stable nanowire. More importantly, we predict that the radial p-n junction can evolve into a Schottky-like junction in relatively highly doped SiNWs when the diameter increases, attributing to the change of the core p-doping characteristic, that is, the core p-junction becomes metallic, while the n-junction near the surface remains semiconducting. The interfacial contact between the junctions is found to be the key for such change. Our calculated results support an experimental observation in SiNW solar cells.

Stability and segregation of B and P dopants in Si/SiO2 core-shell nanowires

Using molecular dynamics simulations, we generate realistic atomic models for oxidized Si nanowires which consist of a crystalline Si core and an amorphous SiO(2) shell. The amorphous characteristics of SiO(2) are well reproduced, as compared to those for bulk amorphous silica. Based on first-principles density functional calculations, we investigate the stability and segregation of B and P dopants near the radial interface between Si and SiO(2). Although substitutional B atoms are more stable in the core than in the oxide, B dopants can segregate to the oxide with the aid of Si self-interstitials which are generated during thermal oxidation. The segregation of B dopants occurs in the form of B interstitials in the oxide, leaving the self-interstitials in the Si core. In the case of P dopants, dopant segregation to the oxide is unfavorable even in the presence of self-interstitials. Instead, we find that P dopants tend to aggregate in the Si region near the interface and may form nearest-neighbor donor pairs, which are energetically more stable than isolated P dopants.

Site-selected doping in silicon nanowires by an external electric field

The properties of dopant-related defects in silicon nanowires are key characteristics in semiconductive devices. Our first-principles calculations predicted that the preferred doping sites of B and P atoms in hydrogen-passivated silicon nanowires have opposite distribution behavior under electric field, suggesting a steady intrinsic p-n junction can be spontaneously formed in (B and P) codoped silicon nanowires.

First-principles study of the structural, electronic, and optical properties of oxide-sheathed silicon nanowires

Using a density functional theory approach, we examine the dielectric function (ε(ω)) optical spectra and electronic structure of various silicon nanowire (SiNW) orientations (<100>, <110>, <111>, and <112>) with amorphous oxide sheaths (-a-SiOx) and compare the results against H-terminated reference SiNWs. We extend the same methods to investigate the effects of surface passivation on <111> SiNW properties using functional group termination (-H, -OH, and -F) and three different thicknesses of oxide sheath passivation. Oxide layer growth is evidenced in the spectra by concomitant appearance of tail oxide character with signatures of increased Si disorder. Suboxide contributions and increased Si disorder from oxidation average out the band structure dispersion observed in the reference SiNWs. Furthermore, we plot average Seraphin coefficients for <111> passivations that clearly distinguish functional group termination from surface oxidation and discuss the suboxide and disorder contributions on the characteristic intersection of these coefficients. The substantial difference in properties observed between <111>-OH and <111>-a-SiOx SiNWs emphasizes the importance of using realistic oxidation models to improve understanding of SiNW properties.

Asymmetric doping in silicon nanostructures: the impact of surface dangling bonds

We investigate peculiar dopant deactivation behaviors of Si nanostrucures with first principle calculations and reveal that surface dangling bonds (SDBs) on Si nanostructures could be fundamental obstacles in nanoscale doping. In contrast to bulk Si, as the size of Si becomes smaller, SDBs on Si nanostructures prefer to be charged and asymmetrically deactivate n- and p-type doping. The asymmetric dopant deactivation in Si nanostructures is ascribed to the preference for negatively charged SDBs as a result of a larger quantum confinement effect on the conduction band. On the basis of our results, we show that the control of the growth direction of silicon nanowire as well as surface passivation is very important in preventing dopant deactivation.