비만아 부모의 돌봄 경험: q 방법론

Physical Review B

Large amounts of anthropogenic CO2 emissions associated with increased fossil fuel consumption have led to global warming and an energy crisis. The photocatalytic reduction of CO2 into solar fuels such as methane or methanol is believed to be one of the best methods to address these two problems. In addition to light harvesting and charge separation, the adsorption/activation and reduction of CO2 on the surface of heterogeneous catalysts remain a scientifically critical challenge, which greatly limits the overall photoconversion efficiency and selectivity of CO2 reduction. This review describes recent advances in the fundamental understanding of CO2 photoreduction on the surface of heterogeneous catalysts and particularly provides an overview of enhancing the adsorption/activation of CO2 molecules. The reaction mechanism and pathways of CO2 reduction as well as their dependent factors are also analyzed and discussed, which is expected to enable an increase in the overall efficiency of CO2 reduction through minimizing the reaction barriers and controlling the selectivity towards the desired products. The challenges and perspectives of CO2 photoreduction over heterogeneous catalysts are presented as well.

CO2 photo-reduction: insights into CO2 activation and reaction on surfaces of photocatalysts

Photovoltaics based on nanowire arrays could reduce cost and materials consumption compared with planar devices but have exhibited low efficiency of light absorption and carrier collection. We fabricated a variety of millimeter-sized arrays of p-type/intrinsic/n-type (p-i-n) doped InP nanowires and found that the nanowire diameter and the length of the top n-segment were critical for cell performance. Efficiencies up to 13.8% (comparable to the record planar InP cell) were achieved by using resonant light trapping in 180-nanometer-diameter nanowires that only covered 12% of the surface. The share of sunlight converted into photocurrent (71%) was six times the limit in a simple ray optics description. Furthermore, the highest open-circuit voltage of 0.906 volt exceeds that of its planar counterpart, despite about 30 times higher surface-to-volume ratio of the nanowire cell.

https://science.sciencemag.org/content/sci/339/6123/1057.full.pdf

InP Nanowire Array Solar Cells Achieving 13.8% Efficiency by Exceeding the Ray Optics Limit

We analyze theoretically the optical properties of ideal semiconductor crystallites so small that they show quantum confinement in all three dimensions [quantum dots (QD's)]. In the limit of a QD much smaller than the bulk exciton size, the linear spectrum will be a series of lines, and we consider the phonon broadening of these lines. The lowest interband transition will saturate like a two-level system, without exchange and Coulomb screening. Depending on the broadening, the absorption and the changes in absorption and refractive index resulting from saturation can become very large, and the local-field effects can become so strong as to give optical bistability without external feedback. The small QD limit is more readily achieved with narrow-band-gap semiconductors.

Theory of the linear and nonlinear optical properties of semiconductor microcrystallites

We report all-optical switching due to state-filling in quantum dots (QDs) within a Mach-Zehnder Interferometric (MZI) switch. The MZI was fabricated using InGaAsP/InP waveguides containing a single layer of InAs/InP QDs. A 1530-1570 nm probe beam is switched by optical excitation of one MZI-arm from the top. By exciting below the InGaAsP bandgap, we prove that the refractive index nonlinearity is only due to the QDs. The switching efficiency is 2 rad/(microW absorbed power). Probe wavelength insensitivity was obtained using a broad distribution of QDs.

/pdf/all-optical-switching-due-to-state-filling-in-quantum-dots-j50kr8vxz4.pdf

All-optical switching due to state-filling in quantum dots

In a light-induced drift experiment the drift velocity is dependent on the change of the diffusion cross section of a particle upon optical excitation. The authors discuss an approximate method for deriving diffusion cross sections from interaction potentials. The method is used to calculate the diffusion cross section of all alkali-noble gas pairs, the alkali being in the ground state or in the first excited state. Implications for light-induced drift in alkali-noble gas mixtures are discussed.

http://iopscience.iop.org/article/10.1088/0022-3700/19/24/014/pdf

Calculation of alkali-noble gas diffusion cross sections relevant to light-induced drift

We present calculations of the carrier capture efficiency into various types of quantum well lasers. The carrier capture into a quantum well can be due to either optical phonon emission or carrier‐carrier scattering. Both capture mechanisms have been calculated and show oscillations as a function of the quantum well thickness. By optimizing the carrier capture efficiency the carrier accumulation in the barrier layers can be reduced, resulting in an improved modulation response and threshold current.

Carrier‐carrier scattering induced capture in quantum well lasers

We investigated the optical properties of wurtzite (WZ) GaP nanowires by performing photoluminescence (PL) and time-resolved PL measurements in the temperature range from 4 K to 300 K, together with atom probe tomography to identify residual impurities in the nanowires. At low temperature, the WZ GaP luminescence shows donor-acceptor pair emission at 2.115 eV and 2.088 eV, and Burstein-Moss band-filling continuum between 2.180 and 2.253 eV, resulting in a direct band gap above 2.170 eV. Sharp exciton α-β-γ lines are observed at 2.140–2.164–2.252 eV, respectively, showing clear differences in lifetime, presence of phonon replicas, and temperature-dependence. The excitonic nature of those peaks is critically discussed, leading to a direct band gap of ∼2.190 eV and to a resonant state associated with the γ-line ∼80 meV above the Γ8C conduction band edge.

/pdf/optical-study-of-the-band-structure-of-wurtzite-gap-3izxeyott6.pdf

Jos E. M. Haverkort

Papers

All-optical switching due to state-filling in quantum dots

Calculation of alkali-noble gas diffusion cross sections relevant to light-induced drift

Carrier‐carrier scattering induced capture in quantum well lasers

Optical study of the band structure of wurtzite GaP nanowires

Collision kernels and transport coefficients.