We developed a broadband dielectric–metal hybrid nanogap resonator composed of a silicon nanoparticle (Si NP) and gold (Au) flat surface. We fabricate the nanogap resonator by depositing a monolayer ...of colloidal quantum dots (QDs) (∼2.8 nm in diameter) on a Au surface followed by dropping a diluted colloidal solution of Si NPs (∼150 nm in diameter). The QD monolayer acts as a precisely length-controlled nanogap as well as a light emitter to monitor the radiative properties of the nanogap resonator. We investigate the light-scattering properties of single-nanogap resonators experimentally and theoretically and found that the coupling of the Mie resonance of Si NPs with a Au surface effectively confines the electromagnetic field into the nanogap in a wider wavelength range than an all-metal nanogap resonator with a comparable size. Furthermore, we show that the resonance wavelength of the hybrid nanogap resonator is less sensitive to the gap length than that of the all-metal one. We demonstrate that the broadband hybrid nanogap resonator enhances photoluminescence of a QD monolayer integrated in the nanogap by a factor of 786.
An ink of silicon nanoparticles (Si NPs) having the lowest-order Mie resonance in the visible range can generate noniridescent and nonfading structural colors in a wide area through a painting ...process. However, the strong wavelength dependence of the radiation pattern and the extinction coefficient make the multiple reflection behavior very complicated, and thus, a reliable tool is necessary to predict the hue, saturation, and brightness of the reflection color. In this work, a Monte Carlo simulation to predict the reflection color of Si NP inks is first developed. The simulation takes into account the scattering and absorption cross-sections, a radiation pattern of an individual NP, and multiple scattering in NP dispersion. The simulation shows that the reflection color of a Si NP ink depends strongly on the concentration because of the wavelength dependence of the multiple scattering behavior. To extend the controllable range of the hue, saturation, and brightness of Si NP inks, a mixture ink with light-absorbing carbon black (CB) NPs is developed. It is experimentally demonstrated that the combination of the Kerker-type back scattering of a Si NP and a broad absorption by a CB NP allows us to control the hue, saturation, and brightness in a wide range and to realize vivid reflection colors under room light.
This work reports the development of an agglomeration‐free colloidal solution of silicon (Si) spheres exhibiting the electric and magnetic dipole Mie resonances in the visible region as an ...alternative to the optical nanoantenna based on plasmonic nanoparticles. Size‐controlled crystalline Si spheres with 20–250 nm in diameters are grown by a bottom‐up process. The Si spheres have heavily boron (B) and phosphorus (P) codoped surface layers, which induce negative surface potential and make the spheres dispersible in alcohol almost perfectly due to the electrostatic repulsions. Formation of agglomeration‐free colloidal dispersion allows to deposit Si spheres on an arbitrary substrate and to produce a dielectric nanoantenna for tailored wavelengths. This study demonstrates that, thanks to the almost perfect spherical shape of the developed Si spheres, the forward and backward scattering spectra of single Si spheres can be well‐explained by the Mie resonance in a wide size range. It is demonstrated that the Si spheres work as nanoantennas for fluorescence enhancement and a single Si sphere can enhance dye fluorescence at maximum 200‐fold.
Colloidal silicon (Si) spheres exhibiting electric and magnetic dipole Mie resonances in the visible region are developed. The colloids are highly stable and the crystalline Si spheres exhibit size‐tunable resonant scattering. The Si spheres can be deposited on arbitrary substrates and work as antennas for fluorescence enhancement of dyes by a factor of 200.
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.
Size dependence of the boron (B) acceptor and phosphorus (P) donor levels of silicon (Si) nanocrystals (NCs) measured from the vacuum level was obtained in a very wide size range from 1 to 9 nm in ...diameter by photoemission yield spectroscopy and photoluminescence spectroscopy for B and P codoped Si-NCs. In relatively large Si-NCs, both levels are within the bulk Si band gap. The levels exhibited much smaller size dependence compared to the valence band and conduction band edges. The Fermi level of B and P codoped Si-NCs was also studied. It was found that the Fermi level of relatively large codoped Si-NCs is close to the valence band and it approaches the middle of the band gap with decreasing the size. The results suggest that below a certain size perfectly compensated Si-NCs, that is, Si-NCs with exactly the same number of active B and P, are preferentially grown, irrespective of average B and P concentrations in samples.
The behavior of the rarefied gas in the thermal transpiration pump with the porous material is investigated numerically by the direct simulation Monte Carlo method. The mass flux achieved by the pump ...is analyzed for a wide range of Knudsen numbers and ratios of the pore length to the pore diameter. The results show that the thermal edge flows around the ends of the pore play an essential role in determining the maximum performance. The effect of the thermal edge flow leads to a qualitative difference in the driving mechanism from a similar thermal transpiration pump by Knudsen. The mass flow takes a maximum value at a considerably large Knudsen number when the pore length is much larger than the pore diameter. The numerical tests show that a larger mass flux is possible when the edge flow is suppressed. The mass flux is investigated for several values of accommodation coefficient and complex pore geometries. The present results show that only the latter leads to the reduction in the mass flux. The compression ratio, including the performance curve of the pump, is also analyzed for several cases. The results show that the small accommodation coefficient decreases the compression ratio of the pump.
The use of a cobalt porphyrin ((TPP)CoCl, 1) in combination with dimethylaminopyridine (DMAP) for the alternating copolymerization of CO2 and epoxide is described. The (TPP)CoCl (1)−DMAP system ...quantitatively produced the alternating copolymer from CO2 and cyclohexene oxide (CHO) under optimized conditions (50 atm, 80 °C). This calatyst system also worked satisfactorily for the alternating copolymerization of CO2 and a terminal epoxide, e.g., propylene oxide (PO), without formation of cyclic carbonate to give the polycarbonate. The alternating copolymerization of CO2 and epoxide (CHO, PO) was achieved under very mild conditions, such as at ambient temperature and under CO2 at 1 atm, by using the 1−DMAP catalyst system.
Efficient excitation of a triplet (T1) state of a molecule has far‐reaching effects on photochemical reaction and energy conversion systems. Because the optical transition from a ground singlet (S0) ...to a T1 state is spin‐forbidden, a T1 state is generated via intersystem crossing (ISC) from an excited singlet (S1) state. Although the excitation efficiency of a T1 state can be increased by enhancing ISC utilizing a heavy atom effect, energy loss during S1→T1 relaxation is inevitable. Here, a general approach to directly excite a T1 state from a ground S0 state via magnetic dipole transition, which is boosted by enhanced magnetic field induced by a dielectric metasurface, is proposed. As a dielectric metasurface, a hexagonal array of silicon (Si) nanodisks is employed; the nanodisk array induces a strongly enhanced magnetic field on the surface due to the toroidal dipole (TD) resonance. A proof‐of‐concept experiment is performed using ruthenium (Ru) complexes placed on a metasurface and demonstrates that the phosphorescence is 35‐fold enhanced on a metasurface when the TD resonance is tuned to the wavelength of the direct S0→T1 transition. These results indicate that photon energy necessary to excite the T1 state can be reduced by more than 400 meV compared to the process involving the ISC. By combining optical measurements with numerical simulations, the mechanism of the phosphorescence enhancement is quantitatively discussed.
A general approach to directly excite a T1 state from a ground S0 state in a molecule via magnetic dipole transition under an intense optical magnetic field is proposed. Phosphorescence excitation spectroscopy of molecules placed on a dielectric metasurface reveals that the magnetic S0‐T1 transition is boosted by an enhanced magnetic field induced by the metasurface.
Shallow impurity doping is an efficient route to tailor optical and electronic features of semiconductor quantum dots (QDs). However, the effect of doping is often smeared by the size, shape, and ...composition inhomogeneities. In this paper, we study optical properties of almost monodispersed spherical silicon (Si) QDs that are heavily doped with boron (B) and phosphorus (P). The narrow size distribution achieved by a size-separation process enables us to extract doping-induced phenomena clearly. The degree of doping-induced shrinkage of the optical band gap is obtained in a wide size range. Comparison of the optical band gap with theoretical calculations allow us to estimate the number of active donor–acceptor pairs in a QD. Furthermore, we found that the size and detection energy dependence of the luminescence decay rate is significantly modified below a critical diameter, that is ∼5.5 nm. In the diameter range above 5.5 nm, the luminescence decay rate is distributed in a wide range depending on the detection energy even in size-purified Si QDs. The distribution may arise from that of donor–acceptor distances. On the other hand, in the diameter range below 5.5 nm the detection energy dependence of the decay rate almost disappears. In this size range, which is smaller than twice of the effective Bohr radius of B and P in bulk Si crystal, the donor–acceptor distance is not a crucial factor to determine the recombination rate.
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