Xiao-Bao Yang

Water‐Soluble Fluorescent Carbon Quantum Dots and Photocatalyst Design

Carbon nanostructures are attracting intense interest because of their many unique and novel properties. The strong and tunable luminescence of carbon materials further enhances their versatile properties; in particular, the quantum effect in carbon is extremely important both fundamentally and technologically. Recently, photoluminescent carbonbased nanoparticles have received much attention. They are usually prepared by laser ablation of graphite, electrochemical oxidation of graphite, electrochemical soaking of carbon nanotubes, thermal oxidation of suitable molecular precursors, vapor deposition of soot, proton-beam irradiation of nanodiamonds, microwave synthesis, and bottom-up methods. Although small (ca. 2 nm) graphite nanoparticles show strong blue photoluminescence (PL), definitive experimental evidence for luminescence of carbon structure arising from quantum-confinement effects and size-dependent optical properties of carbon quantum dots (CQDs) remains scarce. Herein, we report the facile one-step alkali-assisted electrochemical fabrication of CQDs with sizes of 1.2– 3.8 nm which possess size-dependent photoluminescence (PL) and excellent upconversion luminescence properties. Significantly, we demonstrate the design of photocatalysts (TiO2/CQDs and SiO2/CQDs complex system) to harness the use of the full spectrum of sunlight (based on the upconversion luminescence properties of CQDs). It can be imagined that judicious cutting of a graphite honeycomb layer into ultrasmall particles can lead to tiny fragments of graphite, yielding CQDs, which may offer a straightforward and facile strategy to prepare high-quality CQDs. Using graphite rods as both anode and cathode, and NaOH/EtOH as electrolyte, we synthesized CQDs with a current intensity of 10–200 mAcm . As a reference, a series of control experiments using acids (e.g. H2SO4/EtOH) as electrolyte yielded no formation of CQDs. This result indicates that alkaline environment is the key factor, and OH group is essential for the formation of CQDs by the electrochemical oxidation process. Figure 1a shows a trans-

Photo and pH stable, highly-luminescent silicon nanospheres and their bioconjugates for immunofluorescent cell imaging.

We report a novel kind of oxidized silicon nanospheres (O-SiNSs), which simultaneously possess excellent aqueous dispersibility, high photoluminescent quantum yield (PLQY), ultra photostability, wide pH stability, and favorable biocompatibility. Significantly, the PLQY of the O-SiNSs is as high as 25%, and is stable under intense UV irradiation and in acidic-to-basic environments covering the pH range 2-12. To our best knowledge, it is the first example of water-dispersed silicon nanoparticles which possess both high PLQY and robust pH stability suitable for broad utility in bioapplications. Furthermore, the O-SiNSs are readily conjugated with antibody, and the resultant O-SiNSs/antibody bioconjugates are successfully applied in immunofluorescent cell imaging. The results show that the highly luminescent and stable O-SiNSs/antibody bioconjugates are promising fluorescent probes for wide-ranging bioapplications, such as long-term and real-time cellular labeling.

Surface Passivation and Transfer Doping of Silicon Nanowires

One-dimensional nanomaterials are expected to play a key role in future nanotechnology, in addition to providing model systems to demonstrate the unique characteristics of nanoscale effects. Silicon nanowires (SiNWs) in particular are potentially very attractive, given the central role of Si in the semiconductor industry, and are being extensively studied. A SiNW for use in nanodevices is composed of three sections: SiNW core, surface passivant, and adsorbates or interface compounds. A unique way to modulate the transport properties of SiNWs could depend on the individual sections. Volume doping is a conventional method to control conductivity. In volume doping, impurity atoms are introduced into the crystal lattice in the SiNW core by an in situ process during growth, 5] ion implantation, and related methods. However, volume doping for SiNWs has inherent disadvantages, such as poor controllability and destructive processing. Interestingly, the conductivity of amorphous Si films was found to be sensitive to adsorbates, which indicates the importance of the surface of low-dimensional systems in determining the electrical properties of materials. The large surface-to-volume ratio of SiNWs could potentially be important in influencing their transport properties. Its effect could be exploited through SiNW functionalization. Indeed, recent studies of SiNW-based chemical sensors 10] find strong conductivity responses of SiNWs to environmental conditions. Other relevant observations include conductivity modification by adsorbents in the hydrogen-terminated (H-terminated) surfaces of diamond crystals, conductivity determination by surface states in nanoscale thin silicon-on-insulator (SOI) systems, and conductivity enhancement of hydrogenated SiNWs in air and recovery through vacuum or gas purging. Thus, the possibility to modulate the conductivity of SiNWs using surface effects is promising. The ease of such an approach, economically and nondestructively, would offer a unique advantage for use of SiNWs in device fabrication. However, the success of this approach will depend on its controllability and repeatability, and most importantly on the understanding of the mechanisms of the surface effect on SiNWs. Herein, we present a new doping approach, namely surface passivation doping, built on the known surface transfer doping and based on extensive first-principles theoretical investigations and systematic experiments on the surface effects of SiNWs. We also elucidate the involved mechanism and provide better understanding to predetermine the electrical properties of nanomaterials. Surface hydrogen termination is a natural consequence of the hydrogen fluoride treatment of SiNWs. To reveal the role of hydrogen termination in conductivity, we first performed first-principles calculations based on density functional theory (DFT) with an efficient SIESTA code. 15] We adopted popularly used basis sets with double zeta and polarization functions and the Lee–Yang–Parr functional of generalized gradient approximation. We collected atomic charges from a Mulliken population analysis based on DFT calculation, which gave a reasonable charge distribution, as verified using a water molecule ( 0.46 j e j charges on the oxygen atom and 0.23 j e j charges on each hydrogen atom). Interestingly, we obtained extra charges of 0.06 j e j on average on each surface hydrogen atom of the H-terminated SiNWs (H-SiNWs). Clearly, the partial negative charge on the hydrogen atom is due to the higher electronegativity of the hydrogen atom compared to that of the silicon atom (2.2 vs. 1.9). This partial electron transfer from the silicon core to the surface hydrogen is negligible for bulk silicon but is significant for surface-dominated SiNWs whose carrier concentration could be considerably modified, as is estimated below. Assuming each surface silicon atom is terminated by two hydrogen atoms on average, we can calculate the total number of electrons trapped on the terminating hydrogen atoms using Equation (1):

The nucleation and growth of borophene on the Ag (111) surface

B sheets have been intently studied, and various candidates with vacancies have been reported in theoretical investigations, including their possible growth on metal surfaces. However, a recent experiment reported that the borophene formed on a Ag (111) surface consisted of a buckled triangular lattice without vacancies. Our calculations propose a novel nucleation mechanism of B clusters and emphasize the B–Ag interaction in the growth process of borophene, demonstrating the structural evolution of triangular fragments with various profiles and vacancy distributions. Compared with the triangular lattice without vacancies, we have confirmed that the sheet energetically favored during the nucleation and growth is that containing 1/6 vacancies in a stripe pattern, whose scanning tunneling microscopy image is in better agreement with the experimental observation.Boron (B) sheet has been intently studied and various candidates with vacancies have been proposed by theoretical investigations, including the possible growth on metal surface. However, a recent experiment (Science 350, 1513, 2015) reported that the sheet formed on the Ag(111) surface was a buckled triangular lattice without vacancy. Our calculations combined with High-Throughput screening and the first-principles method demonstrate a novel growth mechanism of boron sheet from clusters, ribbons, to monolayers, where the B-Ag interaction is dominant in the nucleation of boron nanostructures. We have found that the simulated STM image of the sheet with 1/6 vacancies in a stripe pattern is in better agreement with the experimental observation, which is energetically favored during the nucleation and growth.

Plasma-assisted growth and nitrogen doping of graphene films

Microwave plasmas were employed to synthesize single- or double-layer graphene sheets on copper foils using a solid carbon source, polymethylmetacrylate. The utilization of reactive plasmas enables the graphene growth at reduced temperatures as compared to conventional thermal chemical vapor deposition processes. The effects of substrate temperature on graphene quality were studied based on Raman analysis, and a reduction of defects at elevated temperature was observed. Moreover, a facile approach to incorporate nitrogen into graphene by plasma treatment in a nitrogen/hydrogen gas mixture was demonstrated, and most of the nitrogen atoms were verified to be pyridinelike in carbon network.

Indirect-to-direct band gap transitions in phosphorus adsorbed ⟨112⟩ silicon nanowires

Using first-principles calculations, we investigated the modification of the band structures of ⟨112⟩ silicon nanowires (SiNWs) that were adsorbed with phosphorus atoms. We found that the phosphorus atom adsorption on the (110) and (111) facets causes considerable modifications in the conduction bands. Interestingly, the modifications result in the indirect band gap characteristic enhancement for the adsorption on the (110) facet and induce an indirect-to-direct band gap transition for the adsorption on the (111) facet due to the distribution of the local density of states that are parallel to the (110) facet. The finding has significant implications for SiNWs in optoelectronic applications.

Understanding the stable boron clusters: A bond model and first-principles calculations based on high-throughput screening.

The unique electronic property induced diversified structure of boron (B) cluster has attracted much interest from experimentalists and theorists. B30-40 were reported to be planar fragments of triangular lattice with proper concentrations of vacancies recently. Here, we have performed high-throughput screening for possible B clusters through the first-principles calculations, including various shapes and distributions of vacancies. As a result, we have determined the structures of Bn clusters with n = 30-51 and found a stable planar cluster of B49 with a double-hexagon vacancy. Considering the 8-electron rule and the electron delocalization, a concise model for the distribution of the 2c-2e and 3c-2e bonds has been proposed to explain the stability of B planar clusters, as well as the reported B cages.

Theoretical search for half-Heusler topological insulators

We have performed ab-initio band structure calculations on more than two thousand half-Heusler compounds in order to search for new candidates for topological insulators. Herein, LiAuS and NaAuS are found to be the strongest topological insulators with the bulk band gap of 0.20 and 0.19 eV, respectively, different from the zero band gap feature reported in other Heusler topological insulators. Due to the inversion asymmetry of the Heusler structure, their topological surface states on the top and bottom surfaces exhibit p-type and n-type carriers, respectively. Thus, these materials may serve as an ideal platform for the realization of topological magneto-electric effects as polar topological insulators. Moreover, these topological surface states exhibit the right-hand spin-texture in the upper Dirac cone, which distinguish them from currently known topological insulator materials. Their topological nontrivial character remains robust against in-plane strains, which makes them suitable for epitaxial growth of films.

An energetic stability predictor of hydrogen-terminated Si nanostructures

We present a linear relationship between the cohesive energies and the H/Si ratio for hydrogen-terminated Si semiconductor nanostructures based on our model analysis and first-principles calculations. The H/Si ratio is shown to be a universal predictor of the nanostructure’s energetic stability and allows easily searching of magic numbers in Si quantum dots. Our findings substantially improve the understanding of nanostructure stability and make practical the prediction of structural properties.

A systematic first-principles study of surface energies, surface relaxation and Friedel oscillation of magnesium surfaces

We systematically study the surface energies and surface relaxations of various low-index and high-index Mg surfaces. It is found that low-index surfaces are not necessarily stable as Mg(1 0 ¯ 0) is the most unstable surface in the series of Mg(1 0 ¯ 1 n) (n = 0–9). A surface-energy predicting model based on the bond cutting is proposed to explain the relative surface stabilities. The local relaxations of the low-index surfaces could be explained by the Friedel oscillation. For the high-index surfaces, the combination of charge smoothing effect and dramatic charge depletion influences the relaxations, which show a big difference from the low-index ones. Our findings provide theoretical data for considerable insights into the surface energies of hexagonal close-packed metals.