SEARCH · Engineering Papers
Results for “tuning”
Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.
Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.
Tuning Excited-State Charge Transfer Character in Cofacial Core-Substituted Perylenediimide Dimers
Understanding the interplay between excimer formation and symmetry-breaking charge separation is important for optimizing charge separation in organic photovoltaic materials. To explore this connection, we synthesized four 1,6,7,12-tetrakis( p -X-phenoxy)perylene-(3,4:9,10)-bisdicarboximide cofacially stacked dimers, where X = MeO, tert -butyl, Br, and CF 3 . Steady-state spectroscopy reveals H-type aggregation and excimer formation in all four dimers, while transient absorption spectroscopy shows relatively small changes in their excited-state absorptions. However, time-resolved fluorescence (TRF) spectroscopy shows that relaxation occurs from an initial Frenkel exciton-dominated excimer state to one in which charge transfer (CT) character contributes. Relaxation to the lower-lying state with CT character is attributed to a combination of structural and charge distribution changes elicited by varying the substituents. Finally, this study illustrates how subtle changes in charge distribution and structure can combine to influence the excited state dynamics that influence charge separation in molecular dimers.
Tuning the Electronic Properties of Graphane via Hydroxylation: An Ab Initio Study
The thermodynamic stability of hydroxylated graphane, that is, fully sp 3 graphene derivatives coordinated with -H and -OH groups, has been recently demonstrated by ab initio calculations. Within the density functional theory approach, we investigate the electronic property modifications of graphane by progressive hydroxylation, that is, by progressively substituting -H with -OH groups. When 50% of graphane is hydroxylated, the energy bandgap reaches its largest value of 6.68 eV. The electronic affinity of 0.8 eV for graphane can widely change in the 0.28–1.60 eV range depending on the geometric configuration. Hydroxylated graphane has two interfaces with vacuum, hence its electron affinity can be different on each interface with the formation of an intrinsic dipole perpendicular to the monolayer. We envisage the possibility of using hydroxylated graphane allotropes with tunable electronic affinity to serve as interfacial layers in 2D material-based heterojunctions.
Water Adsorption on Mica Surfaces with Hydrophilicity Tuned by Counterion Types (Na, K, and Cs) and Structural Fluorination
Not Available
Tuning the Microenvironment of Water Confined in Ti 3 C 2 T x MXene by Cation Intercalation
Not Available
Strategic Lifetime Tuning of Visible-Light Absorbing Two-Coordinate Metal Complexes
This paper highlights how the singlet and triplet amide ligand centered ( 1,3 LC) and interligand charge transfer ( 1,3 ICT) states in (carbene)M(amide), M = Cu, Au (cMa) complexes influence the excited state properties when the triplet states are close in energy. To that end we prepared a set of five cMa complexes, in which the amide (i.e., 5H-benzo[b]carbazole, H-BnCz) was kept constant giving a 3 LC energy of ca. 2.15 eV. Four different carbene ligands were selected to develop M BnCz Carbene complexes which have energies for the ICT state that vary from being either markedly higher, lower, or close to the energy of the 3 LC state on the amide ligand. Steady-state and time-resolved spectroscopic studies show that the emission spectrum of the cMa complex mirrors the phosphorescence of the parent H-BnCz amide and has a luminescence decay lifetime in the millisecond regime when the lowest energy excited state is 3 LC. When the lowest energy states are 1,3 ICT in nature, the emission band is broad and featureless, giving a lifetime in the nanosecond regime. As the 3 LC and 1,3 ICT states have comparable energies, dynamic equilibrium between the states is observed with the luminescence consisting of a mixture of 3 LC and 1,3 ICT transitions. The measured lifetime of this equilibrating system is between 45 and 350 μs depending on the solvent, for both copper- and gold-based cMa complexes. Furthermore, these complexes demonstrate the ability to “park” the excited state population in the 3 LC state, allowing it to act as a reservoir for thermally activated emission, while still maintaining a very rapid equilibrium between LC and ICT states.
Tuning Proton Transfer Thermodynamics in SARS-CoV-2 Main Protease: Implications for Catalysis and Inhibitor Design
The catalytic reaction in SARS-CoV-2 main protease is activated by a proton transfer (PT) from Cys145 to His41. The same PT is likely also required for the covalent binding of some inhibitors. Here we use a multiscale computational approach to investigate the PT thermodynamics in the apo enzyme and in complex with two potent inhibitors, N3 and the α-ketoamide 13b. We show that with the inhibitors the free energy cost to reach the charge-separated state of the active-site dyad is lower, with N3 inducing the most significant reduction. We also show that a few key sites (including specific water molecules) significantly enhance or reduce the thermodynamic feasibility of the PT reaction, with selective desolvation of the active site playing a crucial role. The approach presented is a cost-effective procedure to identify the enzyme regions that control the activation of the catalytic reaction and is thus also useful to guide the design of inhibitors.
Spatial patterns of light-harvesting antenna complex arrangements tune the transfer-to-trap efficiency of excitons in purple bacteria.
In photosynthesis, the efficiency with which a photogenerated exciton reaches the reaction center is dictated by chromophore energies and the arrangement of chromophores in the supercomplex. Here, we explore the interplay between the arrangement of light-harvesting antennae and the efficiency of exciton transport in purple bacterial photosynthesis. Using a Miller-Abrahams-based exciton hopping model, we compare different arrangements of light-harvesting proteins on the intracytoplasmic membrane. We find that arrangements with aggregated LH1s have a higher efficiency than arrangements with randomly distributed LH1s in a wide range of physiological light fluences. This effect is robust to the introduction of defects on the intracytoplasmic membrane. Our result explains the absence of species with aggregated LH1 arrangements in low-light niches and the large increase seen in the expression of LH1 dimer complexes in high fluences. We suggest that the effect seen in our study is an adaptive strategy toward solar light fluence across different purple bacterial species.
Tuning Electronic Properties of 2D Materials Using Metal Adsorbates: Cu at WTe 2 Edges
Two-dimensional materials exhibit properties promising for novel applications. Topologically protected states at their edges can be harnessed for use in quantum devices. We use ab initio simulations to examine properties of edges in 1T'-WTe 2 monolayers, known to exhibit topological order, and their interactions with Cu atoms. Comparison of (010)-oriented edges that have the same composition but different terminations shows that, as the number of Cu atoms increases, their thermodynamically-preferred arrangement depends on the details of the edge structure. Cu atoms aggregate into a cluster at the most stable edge; while the cluster is non-magnetic, it spin-polarizes the W atoms along the edge, which removes the topological protection. At the metastable edge, Cu atoms form a chain incorporated into the WTe 2 lattice; the topological state is preserved in spite of the dramatic edge restructuring. This suggests that exploiting interactions of metal species with metastable edge terminations can provide a path towards non-invasive interfaces.
Role of Polycyclic Aromatic Alkylammonium Cations in Tuning the Electronic Properties and Band Alignment of Two-Dimensional Hybrid Perovskite Semiconductors
Two-dimensional hybrid organic–inorganic perovskites (HOIPs) have recently drawn intense attention as potential photovoltaic materials. However, n = 1 two-dimensional (2D) HOIPs face the challenge of low conductivity between the inorganic layers, leading to unsatisfactory device performance. Interestingly, 2D HOIPs employing π-conjugated molecules as organic moieties show energy and charge transfers between organic and inorganic layers, indicating potentially efficient carrier transport for photovoltaic applications. Nevertheless, the development of 2D HOIP-based solar cells especially utilizing polycyclic aromatic alkylammonium as cations is in its infancy. Herein, we investigated the electronic structure and band alignment of a series of n = 1 2D Ruddlesden–Popper (RP) phase HOIPs containing different polycyclic aromatic groups and alkyl chains, based on density functional theory calculations. We find that the polycyclic aromatic group plays an important role in controlling the functionality of 2D HOIPs by directly modifying band-edge states, and the band alignment at the organic–inorganic interface can be designed to promote either exciton trapping or dissociation for light-emitting or photovoltaic applications, respectively.
Tuning Photoexcited Charge Transfer in Imine-Linked Two-Dimensional Covalent Organic Frameworks
The generation of a long-lived charge-separated state in versatile π-conjugated two-dimensional covalent organic frameworks (2D COFs), a process essential to extending their great potentials in advanced semiconducting applications, is yet fully elucidated. Herein, we report a systematic investigation of the photophysical properties of three highly crystalline imine-linked 2D COFs using steady-state and transient absorption spectroscopy accompanied by time-dependent density functional theory (TDDFT) calculations. The different electron affinity between 5,5',5''-(1,3,5-benzenetriyl)-tris(2-pyridinecarboxaldehyde) (BTPA) and three tunable electron-donating/accepting triamine monomers dominated the formation of the excited state, charge transfer direction and lifetime. A prominent charge transfer from electron-rich 4,4',4''-triaminotriphenylamine (TAPA) to BTPA in COFTAPA-BTPA led to the long-lived charge-separated state, which was attributed to a greater degree of delocalization compared to the two other COFs. Furthermore, these results provide fundamental insight into the importance of structure-property correlation for designing advanced photoactive 2D COF materials with the efficient charge transfer and long-lived charge-separated state.
Tuning Selectivity in the Direct Conversion of Methane to Methanol: Bimetallic Synergistic Effects on the Cleavage of C–H and O–H Bonds over NiCu/CeO 2 Catalysts
Not Available
Tuning Metal–Dihydrogen Interaction in Metal–Organic Frameworks for Hydrogen Storage
Not provided.