Search

Fracture mechanism of cover glasses on mobile devices in a drop event was investigated through high speed camera observation and conventional fracture surface analysis. Drop tests were performed for various chemically strengthened glasses with different mechanical properties and stress profiles. These were dropped on sandpapers with grit size of 180. Based on the observations, we confirmed that the fracture of cover glasses in the drop tests is caused by two consecutive steps. The first one is nucleation of a crack due to the contact with the protrusions of the sandpaper, which causes a strong tensile stress localized around the protrusions. The second one is the propagation of the crack due to a stress caused by bending of the smart phone during the rebound process after the first contact with the sandpaper. According to these investigations, we established a prediction formula for the “fracture drop height” for the sandpaper drop tests. It enables us to understand key parameters to improve the fracture drop height. For example, in addition to the stress induced by chemical strengthening, the mechanical properties of the cover glass and the body of the smartphone are key factors to improve the fracture drop height. We believe the knowledge in this study is important for improving the strength of the cover glasses in the future.

Silicon (Si) nanocrystals (NCs) with high boron (B) and phosphorus (P) concentration shells are dispersible in polar solvents without organic ligands. In order to understand the mechanism of the solution dispersibility, the surface structure is studied by infrared absorption spectroscopy. It is shown that water molecules are adsorbed at the B–oxygen (O) bond sites on NC surface with high hydrogen bond strength, and thus B and P co-doped Si-NCs are a kind of hydrate containing large amounts of water molecules (Si-NC·xH2O). The current transport properties of Si-NC films made from the solutions are studied. It is found that the conductivity is very sensitive to the amount of adsorbed water molecules and changes by 8 orders of magnitude. The high affinity of the NC surface with water molecules is considered to be the origin of the high sensitivity.

A composite nanoparticle (NP) consisting of a gold nanoparticle (Au-NP) core and a thick shell of silicon quantum dot (Si-QD) agglomerates was developed. In the composite NPs with an optimized amount of Si-QDs per a Au-NP, photoluminescence from Si-QDs was enhanced, when it was excited in the wavelength range of the localized surface plasmon resonances (LSPRs) of Au-NPs. The experimental results could be well-explained by a simple model consisting of a spherical Au-NP and a spherical shell made from Si-QD aggregates. It was found that the enhancement is due to increased excitation efficiency of Si-QDs via the LSPR of Au-NPs.

The upconversion property of an individual composite nanoparticle consisting of a metal (Ag) nanocap and a rare-earth doped upconversion nanoparticle (Er- and Yb-doped Y2O3nanoparticle) was studied. The structural parameters of the composite nanoparticle were chosen so that the resonant wavelengths of the electric dipole and magnetic dipole surface plasmon modes of a nanocap coincide with the upconversion luminescence peaks of Er3+. Strong modification of the upconversion spectrum was observed by the formation of a Ag nanocap. Upon excitation at 980 nm, the green (∼550 nm) and red (∼670 nm) peaks were on average 23 and 48-fold, respectively, enhanced. The strong modification of the spectral shape, i.e., the intensity ratio of the green to red luminescence, suggests that the enhancement of radiative decay rates by the two surface plasmon modes is mainly responsible for the upconversion enhancement.

We report on the lattice strain dependence of the nonlinear optical (NLO) parameters of strained Si nanoparticles (NPs), which are prepared in a controlled way by a mechanical ball milling process. X-ray diffraction analysis shows that the nature of strain is compressive and is primarily caused by milling-induced lattice dislocations, which is further supported by high-resolution transmission electron microscopy imaging. It is found that the nonlinear refractive index (n_2) and nonlinear absorption coefficient (β) are strongly influenced by the associated lattice strain present in Si NPs. With the increase of lattice strain, the β gradually decreases while n_2 increases slowly. The strain-dependent observed changes in the NLO parameters of Si NPs are found to be advantageous for application purpose, and it is explained on the basis of strain-induced modification in the electronic structure of the highest occupied molecular orbital and lowest unoccupied molecular orbital states of Si NPs. These results demonstrate the potential of strain-dependent enhancement of nonlinearities for silicon photonics applications.

We demonstrate graphene-assisted controlled fabrication of various ZnO 1D nanostructures on the SiO2/graphene substrate at a low temperature (540 °C) and elucidate the growth mechanism. Monolayer and a few layer graphene prepared by chemical vapor deposition (CVD) and subsequently coated with a thin Au layer followed by rapid thermal annealing is shown to result in highly aligned wurtzite ZnO nanorods (NRs) with clear hexagonal facets. On the other hand, direct growth on CVD graphene without a Au catalyst layer resulted in a randomly oriented growth of dense ZnO nanoribbons (NRBs). The role of in-plane defects and preferential clustering of Au atoms on the defect sites of graphene on the growth of highly aligned ZnO NRs/nanowires (NWs) on graphene was established from micro-Raman and high-resolution transmission electron microscopy analyses. Further, we demonstrate strong UV and visible photoluminescence (PL) from the as-grown and post-growth annealed ZnO NRs, NWs, and NRBs, and the origin of the PL emission is correlated well with the X-ray photoelectron spectroscopy analysis. Our results hint toward an epitaxial growth of aligned ZnO NRs on graphene by a vapor–liquid–solid mechanism and establish the importance of defect engineering in graphene for controlled fabrication of graphene–semiconductor NW hybrids with improved optoelectronic functionalities.

A terahertz (THz) wire-grid polarizer is fabricated by imprinting porous Si followed by oblique evaporation of Ag. We demonstrate that it works in a wide frequency region covering from 5 to 18 THz with the extinction ratio of 10 dB. The frequency region is much wider than that of THz wire-grid polarizers fabricated by conventional imprint lithography using organic materials. The result suggests that imprinting of porous Si is a promising fabrication technique to realize low-cost wire-grid polarizers working in the THz region.

Despite decades of research on the role of intrinsic defects in enhancing the performance of reduced TiO2based materials, unambiguous identification of defects responsible for visible light absorption, and near-infrared (NIR) photoluminescence from undoped TiO2has remained challenging. Herein, through in situ photoluminescence (PL) studies under a controlled environment, we investigated the origin of an extended visible absorption, visible and NIR PL emission from undoped TiO2nanoribbons grown by a solvothermal route. Our studies reveal that oxygen vacancies, Ti3+, and F+center in TiO2are responsible for absorption in the violet and blue-green region and the PL emission in the visible region. On the other hand, absorption in the yellow-red to NIR region and PL emission in the NIR region at 1.47 and ∼1.30 eV are due to Ti4+and Ti3+interstitials, respectively, near the surface identified for the first time. The above conclusions are supported by electron paramagnetic resonance and X-ray photoelectron spectroscopy analyses. The development of such nanoporous undoped TiO2nanoribbons with strong visible absorption and optical identification of Ti interstitial induced band gap states serves as an important milestone toward realizing improved visible light photocatalytic and photovoltaic applications of this novel material.

Bi-doped siliconoxynitride (SiON:Bi) thin films were prepared by a sputtering method and the photoluminescence (PL) properties were studied. Without any thermal treatments, broad Bi-related luminescence was observed in the near-infrared (NIR) range. The luminescence efficiency depended strongly on the film composition. It was found that N atoms play a crucial role for the formation of Bi NIR luminescence centers. The effect of annealing on the luminescence efficiency was also studied. The optimum annealing temperature to have the largest number of Bi NIR luminescence centers depended strongly on the film composition and it was lower for films with lower N concentration. The PL excitation spectra revealed that two different Bi NIR luminescence centers exist in the films.

We present a novel synthesis of ligand-free colloidal silicon nanocrystals (Si-NCs) that exhibits efficient photoluminescence (PL) in a wide energy range (0.85–1.8 eV) overcoming the bulk Si band gap limitation (1.12 eV). The key technology to achieve the wide-range controllable PL is the formation of donor and acceptor states in the band gap of Si-NCs by simultaneous doping of n- and p-type impurities. The colloidal Si-NCs are very stable in an ordinary laboratory atmosphere for more than a year. Furthermore, the PL spectra are very stable and are not at all affected even when the colloids are drop-cast on a substrate and dried in air. The engineering of the all-inorganic colloidal Si-NC and its optical data reported here are important steps for Si-based optoelectronic and biological applications.

The surface structure of P and B codoped colloidal Si-NCs are studied by photoluminescence (PL) in hydrofluoric acid (HF) solution and X-ray photoelectron spectroscopy (XPS). We find that codoped Si-NCs are much more stable in HF solution than undoped, P-doped, and B-doped Si-NCs. The PL study combined with XPS results reveal that a high B concentration layer is formed on the surface of codoped Si-NCs and the layer acts as a kind of inorganic atomic ligands for Si-NCs. The high B concentration layer makes Si-NCs hydrophilic and dispersible in polar liquids. Furthermore, the layer effectively protects Si-NCs from oxidation in solution and in air.

Upconversion photoluminescence (PL) of a composite nanoparticle consisting of an Er and Yb codoped NaYF4core and a Au shell is studied theoretically and experimentally. We first investigate the effects of a Au shell on the radiative and nonradiative emission rates of a dipole placed in a core, the absorption and scattering cross sections of a composite nanoparticle, and the electric field within a core at the excitation wavelength. We then synthesize the composite nanoparticle and study the PL properties. From the analyses of the PL data in combination with the data obtained by theoretical calculations, the mechanism of the enhancement and quenching of upconversion PL by the formation of a Au shell is studied.

We demonstrate the formation of a new type of surfactant-free colloidal silicon nanocrystal (Si-NC). The characteristic structural feature of the Si-NCs is simultaneous doping of phosphorus (P) and boron (B) in and on the surface of Si-NCs. The codoped Si-NCs are stable in methanol for more than a year and exhibit luminescence in the near-infrared range. We perform comprehensive studies on the structure of codoped colloidal Si-NCs and discuss the mechanism of the high solution dispersibility.

Nonlinear optical responses of boron (B)-doped silicon nanocrystals (Si-ncs) embedded in borosilicate glass were studied by z-scan and optical Kerr gate methods under femtosecond excitation at 780 nm. The nonlinear refractive index (n_2) and the two photon absorption coefficients (β) of B-doped Si-ncs were found to be 3 times enhanced, compared to those of intrinsic Si-ncs. The response time was faster than 100 fs even at 5 K. The origin of the large nonlinear optical response was discussed, based on the experimental data of n_2, electron spin resonance spectra, and linear absorption spectra.

Developing Si compatible optical sources has attracted a great deal of attention owing to the potential for forming inexpensive, monolithic Si-based integrated devices. In this Letter, we show that ultra broadband near-IR (NIR) luminescence in the optical telecommunication window of silica optical fibers was obtained for Bi-doped silicon-rich silica films prepared by a co-sputtering method. Without excess Si, i.e., Bi-doped pure silica films, no luminescence was observed in the NIR range. A broad Bi-related NIR photoluminescence appears when excess Si was doped in the Bi-doped silica. The luminescence properties depended strongly on the amount of excess Si and the annealing temperature. Photoluminescence results suggest that excess Si acts as an agent to activate Bi NIR luminescence centers and also as an energy donor to transfer excitation energy to the centers. It is believed that this peculiar structure might find some important applications in Si photonics.

Si nanocrystals (Si-NCs) dispersible in polar liquid without surface functionalization by organic molecules have been realized by simultaneously doping n and p type impurities. We show that the codoped Si-NCs are stable in methanol for more than five months, while intrinsic Si-NCs prepared by the same procedure form large agglomerates. The different behavior of the intrinsic and codoped Si-NCs in solutions suggests that doped impurities exist on the surface of Si-NCs and the surface potential is large enough to prevent the agglomeration. The colloidal solution of codoped Si-NCs exhibits broad photoluminescence with the maximum in the near infrared range (1.1–1.3?eV).