Singlet Fission, Polaron Generation and Intersystem Crossing in Hexaphenyl Film.

The ultrafast dynamics of triplet excitons and polarons in hexaphenyl film was investigated by time-resolved fluorescence and femtosecond transient absorption techniques under various excitation photon energies. Two distinct pathways of triplet formation were clearly observed. Long-lived triplet states are populated within 4.5 ps via singlet fission-intersystem crossing, while the short-lived triplet states (1.5 ns) are generated via singlet fission from vibrational electronic states. In the meantime, polarons were formed from hot excitons on a timescale of <30 fs and recombined in ultrafast lifetime (0.37 ps). In addition, the characterization of hexaphenyl film suggests the morphologies of crystal and aggregate to wide applications in organic electronic devices. The present study provides a universally applicable film fabrication in hexaphenyl system towards future singlet fission-based solar cells.

Tuning the π-π overlap and charge transport in single crystals of an organic semiconductor via solvation and polymorphism.

Polymorphism is a central phenomenon in materials science that often results in important differences of the electronic properties of organic crystals due to slight variations in intermolecular distances and positions. Although a large number of π-conjugated organic compounds can grow as polymorphs, it is necessary to have at disposal a series of several polymorphs of the same molecule to establish clear and predictive structure-property relationships. We report here on the occurrence of two solvates and three polymorphs in single crystalline form of the organic p-type semiconductor 2,2',6,6'-tetraphenyldipyranylidene (DIPO). When grown from chlorobenzene or toluene, the DIPO crystals spontaneously capture solvent molecules to form two pseudopolymorphic 1 : 1 binary solvates. Independently, three solvent-free DIPO polymorphs are obtained either from the vapor phase or from acetonitrile and benzene. Surprisingly, single crystal field-effect transistors (SC-FETs) reveal that the DIPO 1 : 1 binary solvate grown from chlorobenzene possesses a higher hole mobility (1.1 cm2 V-1 s-1) than the three solvent-free polymorphs (0.02-0.64 cm2 V-1 s-1). A refined crystallographic analysis combined with a theoretical transport model clearly shows that the higher mobility of the solvate results from an improved π-π overlap. Our observations demonstrate that solvation allows to tune the π-π overlap and transport properties of organic semiconductors by selecting appropriate solvents.

Trisulfide-Bond Acenes for Organic Batteries.

The molecular design of organic battery electrodes is a big challenge. Here, we synthesize two metal-free organosulfur acenes and shed insight into battery properties using first-principles calculations. A new zone-melting chemical-vapor-transport (ZM-CVT) apparatus was fabricated to provide a simple, solvent-free, and continuous synthetic protocol, and produce single crystals of tetrathiotetracene (TTT) and hexathiapentacene (HTP) at a large scale. Single crystals of HTP showed better Li-ion battery performance and higher cycling stability than those of TTT. A two-step, three-electron lithiation mechanism instead of the commonly depicted two-electron mechanism is proposed for the HTP Li-ion battery. The superior performance of HTP is linked to unique trisulfide bonding scenarios, which are also responsible for the formation of empty channels along the stacking direction. In-depth theoretical analysis suggests that organosulfur acenes are potential prototypes for organic battery materials with tunable properties, and that the tuning of sulfur bonds is critical in designing these new materials.

Preparation of 1T'-Phase ReS2 xSe2(1- x) ( x = 0-1) Nanodots for Highly Efficient Electrocatalytic Hydrogen Evolution Reaction.

As a source of clean energy, a reliable hydrogen evolution reaction (HER) requires robust and highly efficient catalysts. Here, by combining chemical vapor transport and Li-intercalation, we have prepared a series of 1T'-phase ReS2 xSe2(1- x) ( x = 0-1) nanodots to achieve high-performance HER in acid medium. Among them, the 1T'-phase ReSSe nanodot exhibits the highest hydrogen evolution activity, with a Tafel slope of 50.1 mV dec-1 and a low overpotential of 84 mV at current density of 10 mA cm-2. The excellent hydrogen evolution activity is attributed to the optimal hydrogen absorption energy of the active site induced by the asymmetric S vacancy in the highly asymmetric 1T' crystal structure.

Hole Mobility Modulation in Single-Crystal Metal Phthalocyanines by Changing the Metal-π/π-π Interactions.

Weak intermolecular interactions in organic semiconducting molecular crystals play an important role in determining molecular packing and electronic properties. Single crystals of metal-free and metal phthalocyanines were synthesized to investigate how the coordination of the central metal atom affects their molecular packing and resultant electronic properties. Single-crystal field-effect transistors were made and showed a hole mobility order of ZnPc>MnPc>FePc>CoPc>CuPc>H2 Pc>NiPc. Density functional theory (DFT) and 1D polaron transport theory reach a good agreement in reproducing the experimentally measured trend for hole mobility. Additional detail analysis at the DFT level suggests the metal atom coordination into H2 Pc planes can tune the hole mobility via adjusting the intermolecular distances along the shortest axis with closest parallel π stackings.

From Linear to Angular Isomers: Achieving Tunable Charge Transport in Single-Crystal Indolocarbazoles Through Delicate Synergetic CH/NH⋠⋠⋠π Interactions.

Weak intermolecular interaction in organic semiconducting molecular crystals plays an important role in molecular packing and electronic properties. Here, four five-ring-fused isomers were rationally designed and synthesized to investigate the isomeric influence of linear and angular shapes in affecting their molecular packing and resultant electronic properties. Single-crystal field-effect transistors showed mobility order of 5,7-ICZ (3.61 cm2 V-1 s-1 ) >5,11-ICZ (0.55 cm2 V-1 s-1 ) >11,12-ICZ (ca. 10-5  cm2 V-1 s-1 ) and 5,12-ICZ (ca. 10-6  cm2 V-1 s-1 ). Theoretical calculations based on density functional theory (DFT) and polaron transport model revealed that 5,7-ICZ can reach higher mobilities than the others thanks to relatively higher hole transfer integral that links to stronger intermolecular interaction due to the presence of multiple NH⋅⋅⋅π and CH⋅⋅⋅π(py) interactions with energy close to common NH⋅⋅⋅N hydrogen bonds, as well as overall lower hole-vibrational coupling owing to the absence of coupling of holes to low frequency modes due to better π conjugation.

Correlation of Local Structure and Diffusion Pathways in the Modulated Anisotropic Oxide Ion Conductor CeNbO(4.25).

CeNbO4.25 is reported to exhibit fast oxygen ion diffusion at moderate temperatures, making this the prototype of a new class of ion conductor with applications in a range of energy generation and storage devices. To date, the mechanism by which this ion transport is achieved has remained obscure, in part due to the long-range commensurately modulated structural motif. Here we show that CeNbO4.25 forms with a unit cell ∼12 times larger than the stoichiometric tetragonal parent phase of CeNbO4 as a result of the helical ordering of Ce(3+) and Ce(4+) ions along z. Interstitial oxygen ion incorporation leads to a cooperative displacement of the surrounding oxygen species, creating interlayer "NbO6" connectivity by extending the oxygen coordination number to 7 and 8. Molecular dynamic simulations suggest that fast ion migration occurs predominantly within the xz plane. It is concluded that the oxide ion diffuses anisotropically, with the major migration mechanism being intralayer; however, when obstructed, oxygen can readily move to an adjacent layer along y via alternate lower energy barrier pathways.

Preparation of Single-Layer MoS(2x)Se2(1-x) and Mo(x)W(1-x)S2 Nanosheets with High-Concentration Metallic 1T Phase.

The high-yield and scalable production of single-layer ternary transition metal dichalcogenide nanosheets with ≈66% of metallic 1T phase, including MoS(2x)Se2(1-x) and Mo(x)W(1-x)S2 is achieved via electrochemical Li-intercalation and the exfoliation method. Thin film MoS(2x)Se2(1- x) nanosheets drop-cast on a fluorine-doped tin oxide substrate are used as an efficient electrocatalyst on the counter electrode for the tri-iodide reduction in a dye-sensitized solar cell.

Channels of oxygen diffusion in single crystal rubrene revealed.

Electronic devices made from organic materials have the potential to support a more ecologically friendly and affordable future. However, the ability to fabricate devices with well-defined and reproducible electrical and optical properties is hindered by the sensitivity to the presence of chemical impurities. Oxygen in particular is an impurity that can trap electrons and modify conductive properties of some organic materials. Until now the 3-dimensional profiling of oxygen species in organic semiconductors has been elusive and the effect of oxygen remains disputed. In this study we map out high-spatial resolution 3-dimensional distributions of oxygen inclusions near the surface of single crystal rubrene, using Time of Flight Secondary Ion Mass Spectroscopy (ToF-SIMS). Channels of diffused oxygen are found extending from uniform oxygen inclusion layers at the surface. These channels extend to depths in excess of 1.8 µm and act as an entry point for oxygen to diffuse along the ab-plane of the crystal with at least some of the diffused oxygen molecularly binding to rubrene. Our investigation of surfaces at different stages of evolution reveals the extent of oxygen inclusion, which affects rubrene's optical and transport properties, and is consequently of importance for the reliability and longevity of devices.

Excited-state dynamics in an α-perylene single crystal: two-photon- and consecutive two-quantum-induced singlet fission.

Excited-state dynamics in α-perylene single crystals is studied by time-resolved fluorescence and transient absorption techniques under different excitation conditions. The ultrafast lifetimes of the "hot" excimer (Y) state were resolved. Three competing excited-states decay channels are observed: excimer formation, dimer cation generation, and singlet fission. The singlet fission induced by two-photon and consecutive two-quantum absorption is reported for the first time in an α-perylene crystal.

Origin of indirect optical transitions in few-layer MoS2, WS2, and WSe2.

It has been well-established that single layer MX2 (M = Mo, W and X = S, Se) are direct gap semiconductors with band edges coinciding at the K point in contrast to their indirect gap multilayer counterparts. In few-layer MX2, there are two valleys along the Γ-K line with similar energy. There is little understanding on which of the two valleys forms the conduction band minimum (CBM) in this thickness regime. We investigate the conduction band valley structure in few-layer MX2 by examining the temperature-dependent shift of indirect exciton photoluminescence peak. Highly anisotropic thermal expansion of the lattice and the corresponding evolution of the band structure result in a distinct peak shift for indirect transitions involving the K and Λ (midpoint along Γ-K) valleys. We identify the origin of the indirect emission and concurrently determine the relative energy of these valleys.

Atomically flat, large-sized, two-dimensional organic nanocrystals.

Large-sized, 2D single crystals of perylene are grown by both solution-cast and physical vapor transport methods. The crystals have a atomically flat parallelogram morphology and the aspect ratios of the lateral extension compared to the thickness are up to 10(3) . The atomically flat feature leads to good interface contact, making a single-crystal field-effect transistor with higher mobility. The mobility of atomically flat crystals can be 10(3) -10(4) times higher than rough crystals.

Photoluminescence emission and Raman response of monolayer MoS2, MoSe2, and WSe2.

We mechanically exfoliate mono- and few-layers of the transition metal dichalcogenides molybdenum disulfide, molybdenum diselenide, and tungsten diselenide. The exact number of layers is unambiguously determined by atomic force microscopy and high-resolution Raman spectroscopy. Strong photoluminescence emission is caused by the transition from an indirect band gap semiconductor of bulk material to a direct band gap semiconductor in atomically thin form.

Revealing surface oxidation on the organic semi-conducting single crystal rubrene with time of flight secondary ion mass spectroscopy.

To address the question of surface oxidation in organic electronics the chemical composition at the surface of single crystalline rubrene is spatially profiled and analyzed using Time of Flight - Secondary Ion Mass Spectroscopy (ToF-SIMS). It is seen that a uniform oxide (C42H28O) covers the surface while there is an increased concentration of peroxide (C42H28O2) located at crystallographic defects. By analyzing the effects of different primary ions, temperature and sputtering agents the technique of ToF-SIMS is developed as a valuable tool for the study of chemical composition variance both at and below the surface of organic single crystals. The primary ion beams C60(3+) and Bi3(+) are found to be most appropriate for mass spectroscopy and spatial profiling respectively. Depth profiling of the material is successfully undertaken maintaining the molecular integrity to a depth of ~5 µm using an Ar cluster ion source as the sputtering agent.

Two-photon-induced singlet fission in rubrene single crystal.

The two-photon-induced singlet fission was observed in rubrene single crystal and studied by use of femtosecond pump-probe spectroscopy. The location of two-photon excited states was obtained from the nondegenerate two-photon absorption (TPA) spectrum. Time evolution of the two-photon-induced transient absorption spectra reveals the direct singlet fission from the two-photon excited states. The TPA absorption coefficient of rubrene single crystal is 52 cm∕GW at 740 nm, as obtained from Z-scan measurements. Quantum chemical calculations based on time-dependent density functional theory support our experimental data.

Crystal chemistry of melilite [CaLa]2[Ga]2[Ga2O7]2: a five dimensional solid electrolyte.

Melilite-type [A(2)](2)[B(I)](2)[B(II)(2)O(7)](2) gallates are promising ion conducting electrolytes for deployment in solid oxide fuel cells. Single crystals of [CaLa](2)[Ga](2)[Ga(2)O(7)](2), grown in an optical floating zone furnace, were investigated using a combination of transmission electron microscopy and single crystal X-ray diffraction. Strong anisotropic displacements of oxygen arise from the structural misfit between the interlayer Ca/La cations and the [Ga]-[Ga(2)O(7)] tetrahedral layers. A model employing two-dimensional modulation achieves bond lengths and bond angles that preserve satisfactory bond valence sums throughout the structure. The melilite belongs to the tetragonal superspace group P42(1)m(α, α, 0)00s(α, α, 0)000, α = 0.2160(5), with a subcell metric of a = 7.9383(2) Å, c = 5.2641(3) Å, onto which modulation vectors are superimposed: q(1) = α (a* + b*), q(2) = α (-a* + b*). Both displacive (cation and anion) and occupational (cation) modulations contribute to incommensuration. The analysis of structural adjustments that accompany changes in temperature and composition provides assurance that the crystal chemical model is correct. By better understanding the flexibility of this modulated structure a rational approach toward crystallochemical optimization of electrolyte performance by enhancing oxygen mobility becomes feasible.

Synthesis, crystal structure, and optical properties of a three-dimensional quaternary Hg-In-S-Cl chalcohalide: Hg7InS6Cl5.

A crystalline three-dimensional (3D) quaternary chalcohalide, Hg(7)InS(6)Cl(5) (1), has been synthesized through a solid-state reaction under medium temperature. It is the first example in the family of the Hg-IIIA-Q-X (Q = S, Se, Te; X = F, Cl, Br, I) systems. Compound 1 features a 3D network and has an optical band gap of 2.54 eV.

Singlet fission in rubrene single crystal: direct observation by femtosecond pump-probe spectroscopy.

The excited state dynamics of rubrene in solution and in the single crystal were studied by femtosecond pump-probe spectroscopy under various excitation conditions. Singlet fission was demonstrated to play a predominant role in the excited state relaxation of the rubrene crystal in contrast to rubrene in solution. Upon 500 nm excitation, triplet excitons form on the picosecond time scale via fission from the lowest excited singlet state. Upon 250 nm excitation, fission from upper excited singlet states is observed within 200 fs.

Growth direction dependent separate-channel charge transport in the organic weak charge-transfer co-crystal of anthracene-DTTCNQ.

Co-crystallization is an efficient way of molecular crystal engineering to tune the electronic properties of organic semiconductors. In this work, we synthesized anthracene-4,8-bis(dicyanomethylene)4,8-dihydrobenzo[1,2-b:4,5-b']-dithiophene (DTTCNQ) single crystals as a template to study the crystal growth direction dependent charge transport properties and attempted to elucidate the mechanism by proposing a separate-channel charge transport model. Single-crystal anthracene-DTTCNQ field-effect transistors showed that ambipolar transport properties could be observed in all crystal growth directions. Furthermore, upon changing the measured crystal directions, the electronic properties experienced a weak change from n-type dominated ambipolar, balanced ambipolar, to p-type dominated ambipolar properties. The theoretical calculations at density functional theory (DFT) and higher theory levels suggested that the anthracene-DTTCNQ co-crystal motif was a weak charge-transfer complex, in line with the experiment. Furthermore, the detailed theoretical analysis also indicated that electron or hole transport properties originated from separated channels formed by DTTCNQ or anthracene molecules. We thus proposed a novel separate-channel transport mechanism to support additional theoretical analysis and calculations. The joint experimental and theoretical efforts in this work suggest that the engineering of co-crystallization of weak charge-transfer complexes can be a practical approach for achieving tuneable ambipolar charge transport properties by the rational choice of co-crystal formers.

Five-dimensional incommensurate structure of the melilite electrolyte [CaNd]2[Ga]2[Ga2O7]2.

Melilite-type gallium oxides are potential intermediate temperature electrolytes for solid oxide fuel cells. Single crystals of [CaNd](2)[Ga](2)[Ga(2)O(7)](2) grown using an optical floating zone furnace have been investigated using transmission electron microscopy and powder and single-crystal X-ray diffraction. The anion array topologically conforms to a [(3.5.4.5)(2), 3.5.3.5] network that contains distorted pentagonal tunnels. The distortion is necessary to achieve space filling and accommodate structural misfit between the layers. Satisfactory bond lengths and angles are obtained through two-dimensional modulation in the tetragonal based plane, leading to five-dimensional symmetry in the superspace group P(4⁻)2(1)m(α,α,0)00s((a⁻)a,0)000, α = 0.2319(2), with modulation vectors q(1) = α(a* + b*) and q(2) = α(-a* + b*). Both displacive and occupational modulations are found. Through this mechanism, melilites are primed to accommodate mobile oxygen interstitials, suggesting a rational approach to crystallochemical tailoring that will enhance ionic diffusion and optimize electrolyte performance.