Cynthia S. Lo

Enhanced thermoelectric efficiency via orthogonal electrical and thermal conductances in phosphorene.

Thermoelectric devices that utilize the Seebeck effect convert heat flow into electrical energy and are highly desirable for the development of portable, solid state, passively powered electronic systems. The conversion efficiencies of such devices are quantified by the dimensionless thermoelectric figure of merit (ZT), which is proportional to the ratio of a devices electrical conductance to its thermal conductance. In this paper, a recently fabricated two-dimensional (2D) semiconductor called phosphorene (monolayer black phosphorus) is assessed for its thermoelectric capabilities. First-principles and model calculations reveal not only that phosphorene possesses a spatially anisotropic electrical conductance, but that its lattice thermal conductance exhibits a pronounced spatial-anisotropy as well. The prominent electrical and thermal conducting directions are orthogonal to one another, enhancing the ratio of these conductances. As a result, ZT may reach the criterion for commercial deployment along the armchair direction of phosphorene at T = 500 K and is close to 1 even at room temperature given moderate doping (∼2 × 10(16) m(-2) or 2 × 10(12) cm(-2)). Ultimately, phosphorene hopefully stands out as an environmentally sound thermoelectric material with unprecedented qualities. Intrinsically, it is a mechanically flexible material that converts heat energy with high efficiency at low temperatures (∼300 K), one whose performance does not require any sophisticated engineering techniques.

Carbon dioxide activation and dissociation on ceria (110): A density functional theory study

Ceria (CeO(2)) is a promising catalyst for the reduction of carbon dioxide (CO(2)) to liquid fuels and commodity chemicals, in part because of its high oxygen storage capacity, yet the fundamentals of CO(2) adsorption, activation, and reduction on ceria surfaces remain largely unknown. We use density functional theory, corrected for onsite Coulombic interactions (GGA+U), to explore various adsorption sites and configurations for CO(2) on stoichiometric and reduced ceria (110), the latter with either an in-plane oxygen vacancy or a split oxygen vacancy. We find that CO(2) adsorption on both reduced ceria (110) surfaces is thermodynamically favored over the corresponding adsorption on stoichiometric ceria (110), but the most stable adsorption configuration consists of CO(2) adsorbed parallel to the reduced ceria (110) surface at a split oxygen vacancy. Structural changes in the CO(2) molecule are also observed upon adsorption. At the split vacancy, the molecule bends out of plane to form a unidentate carbonate with the remaining oxygen anion at the surface; this is in stark contrast to the bridged carbonate observed for CO(2) adsorption at the in-plane vacancy. Also, we analyze the pathways for CO(2) conversion to CO on reduced ceria (110). The subtle difference in the energies of activation for the elementary steps suggest that CO(2) dissociation is favored on the split vacancy, while the reverse process of CO oxidation may favor the formation of the in-plane vacancy. We thus show how the structure and properties of the ceria catalyst govern the mechanism of CO(2) activation and reduction.

Methanol coupling in the zeolite chabazite studied via Car–Parrinello molecular dynamics

We have used Car–Parrinello constrained molecular dynamics to study the coupling of two molecules of methanol in the zeolite chabazite to form ethanol and water. We have chosen to study this reaction because it represents the formation of the first C–C bond, which is thought to be the rate limiting step for the MTO and MTG processes. We have elucidated a new mechanism for this reaction that does not require the prior formation of surface methoxy groups or dimethyl ether intermediates. The mechanism involves stable intermediates of methane and protonated formaldehyde. We have also calculated an upper bound of the free energy barrier for the overall reaction, and found that it compares favourably with the rough experimental measurements available. Finally, we consider what are the natural reaction coordinates for the methanol–methanol coupling process.

Characterization of the peridinin–chlorophyll a-protein complex in the dinoflagellate Symbiodinium

The water-soluble peridinin-chlorophyll a-proteins (PCPs) are one of the major light harvesting complexes in photosynthetic dinoflagellates. PCP contains the carotenoid peridinin as its primary pigment. In this study, we identified and characterized the PCP protein and the PCP gene organization in Symbiodinium sp. CS-156. The protein molecular mass is 32.7kDa, revealing that the PCP is of the monomeric form. The intronless PCP genes are organized in tandem arrays. The PCP gene cassette is composed of 1095-bp coding regions and spacers in between. Despite the heterogeneity of PCP gene tandem repeats, we identified a single form of PCP, the sequence of which exactly matches the deduced sequence of PCP gene clone 7 (JQ395030) by LC-MS/MS analysis of tryptic digested PCP, revealing the mature PCP apoprotein is 312 amino acids in length. Pigment analysis showed a peridinin-to-Chl a ratio of 4. The peridinin-to-Chl a Q(y) energy transfer efficiency is 95% in this complex.

Non-radiative relaxation of photoexcited chlorophylls: theoretical and experimental study.

Nonradiative relaxation of high-energy excited states to the lowest excited state in chlorophylls marks the first step in the process of photosynthesis. We perform ultrafast transient absorption spectroscopy measurements, that reveal this internal conversion dynamics to be slightly slower in chlorophyll B than in chlorophyll A. Modeling this process with non-adiabatic excited state molecular dynamics simulations uncovers a critical role played by the different side groups in the two molecules in governing the intramolecular redistribution of excited state wavefunction, leading, in turn, to different time-scales. Even given smaller electron-vibrational couplings compared to common organic conjugated chromophores, these molecules are able to efficiently dissipate about 1 eV of electronic energy into heat on the timescale of around 200 fs. This is achieved via selective participation of specific atomic groups and complex global migration of the wavefunction from the outer to inner ring, which may have important implications for biological light-harvesting function.

Low-Temperature Spectroscopic Properties of the Peridinin–Chlorophyll a–Protein (PCP) Complex from the Coral Symbiotic Dinoflagellate Symbiodinium

The spectroscopic properties of the peridinin-chlorophyll a-protein (PCP) from the coral symbiotic dinoflagellate Symbiodinium have been characterized by application of various ultrafast optical spectroscopies including femto- and nanosecond time-resolved absorption and picosecond time-resolved fluorescence (TRF) at 77 K. Excited state properties of peridinin and Chl a and their intramolecular interaction characteristics have been obtained from global fitting analysis and directed kinetic modeling of the data sets and compared to their counterparts known for the PCP from Amphidinium carterae. The lifetimes of the excited state of peridinin show close agreement with those known for the counterpart PCP, demonstrating that molecular interactions have the same characteristics in both complexes. More variances have been recorded for the excited state properties of Chl a including elongation of both the intramolecular energy transfer time between Chls within the pair in the protein monomer and the excited state lifetime of the long wavelength form of Chl a (terminal acceptor). Kinetic modeling of formation of the peridinin triplet state has shown that the PCP is protected from potential photodamage due to an extremely fast peridinin triplet state formation of kTT = (14.4 ± 2.3) × 10(9) s(-1) ((70 ± 12)(-1) (ps)(-1)) that guarantees instantaneous depletion of Chl a triplets and prevents formation of harmful singlet oxygen ((1)ΔgO2).

Water quality modeling in the dead end sections of drinking water distribution networks.

Dead-end sections of drinking water distribution networks are known to be problematic zones in terms of water quality degradation. Extended residence time due to water stagnation leads to rapid reduction of disinfectant residuals allowing the regrowth of microbial pathogens. Water quality models developed so far apply spatial aggregation and temporal averaging techniques for hydraulic parameters by assigning hourly averaged water demands to the main nodes of the network. Although this practice has generally resulted in minimal loss of accuracy for the predicted disinfectant concentrations in main water transmission lines, this is not the case for the peripheries of the distribution network. This study proposes a new approach for simulating disinfectant residuals in dead end pipes while accounting for both spatial and temporal variability in hydraulic and transport parameters. A stochastic demand generator was developed to represent residential water pulses based on a non-homogenous Poisson process. Dispersive solute transport was considered using highly dynamic dispersion rates. A genetic algorithm was used to calibrate the axial hydraulic profile of the dead-end pipe based on the different demand shares of the withdrawal nodes. A parametric sensitivity analysis was done to assess the model performance under variation of different simulation parameters. A group of Monte-Carlo ensembles was carried out to investigate the influence of spatial and temporal variations in flow demands on the simulation accuracy. A set of three correction factors were analytically derived to adjust residence time, dispersion rate and wall demand to overcome simulation error caused by spatial aggregation approximation. The current model results show better agreement with field-measured concentrations of conservative fluoride tracer and free chlorine disinfectant than the simulations of recent advection dispersion reaction models published in the literature. Accuracy of the simulated concentration profiles showed significant dependence on the spatial distribution of the flow demands compared to temporal variation.

Ab initio electronic transport model with explicit solution to the linearized Boltzmann transport equation

Author(s): Faghaninia, A; Ager, JW; Lo, CS | Abstract:

Excitation energy transfer in the peridinin-chlorophyll a-protein complex modeled using configuration interaction.

We modeled excitation energy transfer (EET) in the peridinin-chlorophyll a-protein (PCP) complex of dinoflagellate Amphidinium carterae to determine which pathways contribute dominantly to the high efficiency of this process. We used complete active space configuration interaction (CAS-CI) to calculate electronic structure properties of the peridinin (PID) and chlorophyll a (CLA) pigments in PCP and the transition density cube (TDC) method to calculate Coulombic couplings between energy transfer donors and acceptors. Our calculations show that the S1 → Qy EET pathway from peridinin to chlorophyll a is the dominant energy transfer pathway in PCP, with two sets of interactions-between PID612 and CLA601 and between PID622 and CLA602-contributing most strongly. EET lifetimes for these two interactions were calculated to be 2.66 and 2.90, with quantum efficiencies of 85.75 and 84.65%, respectively. The calculated Coulombic couplings for EET between two peridinin molecules in the strongly allowed S2 excited states are extremely large and suggest excitonic coupling between pairs of peridinin S2 states. This methodology is also broadly applicable to the study of EET in other photosynthetic complexes and/or organic photovoltaics, where both single and double excitations are present and donor and acceptor molecules are tightly packed.

First principles study of defect formation in thermoelectric zinc antimonide, β-Zn4Sb3

Understanding the formation of various point defects in the promising thermoelectric material, β-Zn(4)Sb(3), is crucial for theoretical determination of the origins of its p-type behavior and considerations of potential n-type dopability. While n-type conductivity has been fleetingly observed in Te:ZnSb, there have been no reports, to the best of our knowledge, of stable n-type behavior in β-Zn(4)Sb(3). To understand the origin of this difficulty, we investigated the formation of intrinsic point defects in β-Zn(4)Sb(3) density functional theory calculations. We found that a negatively charged zinc vacancy is the dominant defect in β-Zn(4)Sb(3), as it is also in ZnSb. This explains the unintentional p-type behavior of the material and makes n-doping very difficult since the formation of the defect becomes more favorable at higher Fermi levels, near the conduction band minimum (CBM). We also calculated the formation energy of the cation dopants: Li, Na, B, Al, Ga, In, Tl; of these, only Li and Na are thermodynamically favorable compared to the acceptor Zn vacancy over a range of Fermi levels along the band gap. Further analysis of the band structure shows that Li:Zn(4)Sb(3) has a partially occupied topmost valence band, making this defect an acceptor so that Li:Zn(4)Sb(3) is indeed a p-type thermoelectric material. The introduction of Li, however, creates a more orderly and symmetric configuration, which stabilizes the host structure. Furthermore, Li reduces the concentration of holes and increases the Seebeck coefficient; hence, Li:Zn(4)Sb(3) is more stable and better performing as a thermoelectric material than undoped β-Zn(4)Sb(3).