Conical Intersections at Interfaces Revealed by Phase-Cycling Interface-Specific Two-Dimensional Electronic Spectroscopy (i2D-ES).

Conical intersections (CIs) hold significant stake in manipulating and controlling photochemical reaction pathways of molecules at interfaces and surfaces by affecting molecular dynamics therein. Currently, there is no tool for characterizing CIs at interfaces and surfaces. To this end, we have developed phase-cycling interface-specific two-dimensional electronic spectroscopy (i2D-ES) and combined it with advanced computational modeling to explore nonadiabatic CI dynamics of molecules at the air/water interface. Specifically, we integrated the phase locked pump pulse pair with an interface-specific electronic probe to obtain the two-dimensional interface-specific responses. We demonstrate that the nonadiabatic transitions of an interface-active azo dye molecule that occur through the CIs at the interface have different kinetic pathways from those in the bulk water. Upon photoexcitation, two CIs are present: one from an intersection of an optically active S2 state with a dark S1 state and the other from the intersection of the progressed S1 with the ground state S0. We find that the molecular conformations in the ground state are different for interfacial molecules. The interfacial molecules are intimately correlated with the locally populated excited state S2 being farther away from the CI region. This leads to slower nonadiabatic dynamics at the interface than in bulk water. Moreover, we show that the nonadiabatic transition from the S1 dark state to the ground state is significantly longer at the interface than that in the bulk, which is likely due to the orientationally restricted configuration of the excited state at the interface. Our findings suggest that orientational configurations of molecules manipulate reaction pathways at interfaces and surfaces.

Symmetry-Breaking Enhanced Herzberg-Teller Effect with Brominated Polyacenes.

Molecular symmetry is vital to the selection rule of vibrationally resolved electronic transition, particularly when the nuclear dependence of electronic wave function is explicitly treated by including Franck-Condon (FC) factor, Franck-Condon/Herzberg-Teller (FC/HT) interference, and Herzberg-Teller (HT) coupling. Our present study investigated the light absorption spectra of highly symmetric tetracene, pentacene, and hexacene molecules of point-group D2h, as well as their monobrominated derivatives with a lower Cs symmetry. It was found that the symmetry-breaking monobromination allows more vibrational normal modes and their pairs to contribute to FC/HT interference and HT coupling, respectively. Through a projection of a molecule's vibrational normal modes to its irreducible representations, a linear relationship between the FC/HT intensity to the polyacene's size was deduced alongside a quadratic dependence of the HT intensity. Both theoretically derived correlations were well justified by our numerical simulations, which also demonstrated an approximately 20% improvement on the agreement with experimental line shape if the HT theory is adopted to replace the FC approximation. Moreover, for these low-symmetry monobrominated polyacenes, the FC intensity was even weaker than its FC/HT and HT counterparts at some excitation energies, making the HT theory imperative to decipher vibronic coupling, a fundamental driving force behind numerous chemical, biological, and photophysical processes.

Herzberg-Teller Effect on the Vibrationally Resolved Absorption Spectra of Single-Crystalline Pentacene at Finite Temperatures.

The line shape of an electronic spectrum conveys the coupling between electronic and vibrational degrees of freedom. In the present study, the light absorption spectra of single-crystalline pentacene were measured by polarized UV-vis microscopy at 77, 185, and 293 K. The vibronic coupling encoded in each spectrum was resolved by the Herzberg-Teller theory that considers the contributions from the Franck-Condon (FC) factor, Franck-Condo/Herzberg-Teller (FC/HT) interference, and Herzberg-Teller (HT) coupling. Specifically, excitation energies, electronic transition dipole moments, and their nuclear gradients were evaluated by the GW method to ensure numerical accuracy, while the computationally efficient density function theory was employed to determine the optimized structures and vibrational normal modes. For every pair of electronic transition and normal mode that gives rise to a strong vibronic transition intensity, we examined their spatial characteristics by projecting them onto the three crystal axes. It was found that all normal modes strongly coupled to the lowest-lying a-polarized electronic transitions oscillate along axis a, whereas none of their counterparts for the lowest-lying b-polarized electronic transitions is predominantly along axis b. This notable difference on the alignment between the electronic transition and molecular vibration could help the directional control of charge dissociation and/or spin separation. Moreover, a significant variance of the destructive FC/HT interference was discovered with increasing temperatures that can well explain the a-polarized fading tableland near 650 nm. Finally, the importance of HT coupling was corroborated by comparing its intensity with those of FC factor and FC/HT interference. Taken all together, the vibrational dependence of the electronic wave function is critical to resolve the light absorption spectra of single-crystalline pentacene and its temperature effects, facilitating the systematic design of functional optical materials based on pentacene and its derivatives.

The Critical Role of Electrolyte Gating on the Hydrogen Evolution Performance of Monolayer MoS2.

According to density functional theory, monolayer (ML) MoS2 is predicted to possess electrocatalytic activity for the hydrogen evolution reaction (HER) that approaches that of platinum. However, its observed HER activity is much lower, which is widely believed to result from a large Schottky barrier between ML MoS2 and its electrical contact. In order to better understand the role of contact resistance in limiting the performance of ML MoS2 HER electrocatalysts, this study has employed well-defined test platforms that allow for the simultaneous measurement of contact resistance and electrocatalytic activity toward the HER during electrochemical testing. At open circuit potential, these measurements reveal that a 0.5 M H2SO4 electrolyte can act as a strong p-dopant that depletes free electrons in MoS2 and leads to extremely high contact resistance, even if the contact resistance of the as-made device in air is originally very low. However, under applied negative potentials this doping is mitigated by a strong electrolyte-mediated gating effect which can reduce the contact and sheet resistances of properly configured ML MoS2 electrocatalysts by more than 5 orders of magnitude. At potentials relevant to HER, the contact resistance becomes negligible and the performance of MoS2 electrodes is limited by HER kinetics. These findings have important implications for the design of low-dimensional semiconducting electrocatalysts and photocatalysts.

Ultrasensitive gas detection of large-area boron-doped graphene.

Heteroatom doping is an efficient way to modify the chemical and electronic properties of graphene. In particular, boron doping is expected to induce a p-type (boron)-conducting behavior to pristine (nondoped) graphene, which could lead to diverse applications. However, the experimental progress on atomic scale visualization and sensing properties of large-area boron-doped graphene (BG) sheets is still very scarce. This work describes the controlled growth of centimeter size, high-crystallinity BG sheets. Scanning tunneling microscopy and spectroscopy are used to visualize the atomic structure and the local density of states around boron dopants. It is confirmed that BG behaves as a p-type conductor and a unique croissant-like feature is frequently observed within the BG lattice, which is caused by the presence of boron-carbon trimers embedded within the hexagonal lattice. More interestingly, it is demonstrated for the first time that BG exhibits unique sensing capabilities when detecting toxic gases, such as NO2 and NH3, being able to detect extremely low concentrations (e.g., parts per trillion, parts per billion). This work envisions that other attractive applications could now be explored based on as-synthesized BG.

Observation of rapid exciton-exciton annihilation in monolayer molybdenum disulfide.

Monolayer MoS2 is a direct-gap two-dimensional semiconductor that exhibits strong electron-hole interactions, leading to the formation of stable excitons and trions. Here we report the existence of efficient exciton-exciton annihilation, a four-body interaction, in this material. Exciton-exciton annihilation was identified experimentally in ultrafast transient absorption measurements through the emergence of a decay channel varying quadratically with exciton density. The rate of exciton-exciton annihilation was determined to be (4.3 ± 1.1) × 10(-2) cm(2)/s at room temperature.

Ultra-Thin Lithium Silicide Interlayer for Solid-State Lithium-Metal Batteries.

All-solid-state batteries with metallic lithium (LiBCC ) anode and solid electrolyte (SE) are under active development. However, an unstable SE/LiBCC interface due to electrochemical and mechanical instabilities hinders their operation. Herein, an ultra-thin nanoporous mixed ionic and electronic conductor (MIEC) interlayer (≈3.25 µm), which regulates LiBCC deposition and stripping, serving as a 3D scaffold for Li0 ad-atom formation, LiBCC nucleation, and long-range transport of ions and electrons at SE/LiBCC interface is demonstrated. Consisting of lithium silicide and carbon nanotubes, the MIEC interlayer is thermodynamically stable against LiBCC and highly lithiophilic. Moreover, its nanopores (<100 nm) confine the deposited LiBCC to the size regime where LiBCC exhibits "smaller is much softer" size-dependent plasticity governed by diffusive deformation mechanisms. The LiBCC thus remains soft enough not to mechanically penetrate SE in contact. Upon further plating, LiBCC grows in between the current collector and the MIEC interlayer, not directly contacting the SE. As a result, a full-cell having Li3.75 Si-CNT/LiBCC foil as an anode and LiNi0.8 Co0.1 Mn0.1 O2 as a cathode displays a high specific capacity of 207.8 mAh g-1 , 92.0% initial Coulombic efficiency, 88.9% capacity retention after 200 cycles (Coulombic efficiency reaches 99.9% after tens of cycles), and excellent rate capability (76% at 5 C).

Pulsed Light Synthesis of High Entropy Nanocatalysts with Enhanced Catalytic Activity and Prolonged Stability for Oxygen Evolution Reaction.

The ability to synthesize compositionally complex nanostructures rapidly is a key to high-throughput functional materials discovery. In addition to being time-consuming, a majority of conventional materials synthesis processes closely follow thermodynamics equilibria, which limit the discovery of new classes of metastable phases such as high entropy oxides (HEO). Herein, a photonic flash synthesis of HEO nanoparticles at timescales of milliseconds is demonstrated. By leveraging the abrupt heating and cooling cycles induced by a high-power-density xenon pulsed light, mixed transition metal salt precursors undergo rapid chemical transformations. Hence, nanoparticles form within milliseconds with a strong affinity to bind to the carbon substrate. Oxygen evolution reaction (OER) activity measurements of the synthesized nanoparticles demonstrate two orders of magnitude prolonged stability at high current densities, without noticeable decay in performance, compared to commercial IrO2 catalyst. This superior catalytic activity originates from the synergistic effect of different alloying elements mixed at a high entropic state. It is found that Cr addition influences surface activity the most by promoting higher oxidation states, favoring optimal interaction with OER intermediates. The proposed high-throughput method opens new pathways toward developing next-generation functional materials for various electronics, sensing, and environmental applications, in addition to renewable energy conversion.

Dislocation theory of chirality-controlled nanotube growth.

The periodic makeup of carbon nanotubes suggests that their formation should obey the principles established for crystals. Nevertheless, this important connection remained elusive for decades and no theoretical regularities in the rates and product type distribution have been found. Here we contend that any nanotube can be viewed as having a screw dislocation along the axis. Consequently, its growth rate is shown to be proportional to the Burgers vector of such dislocation and therefore to the chiral angle of the tube. This is corroborated by the ab initio energy calculations, and agrees surprisingly well with diverse experimental measurements, which shows that the revealed kinetic mechanism and the deduced predictions are remarkably robust across the broad base of factual data.

Singlet Fission Driven by Anisotropic Vibronic Coupling in Single-Crystalline Pentacene.

Vibronic coupling is believed to play an important role in siglet fission, wherein a photoexcited singlet exciton is converted into two triplet excitons. In the present study, we examine the role of vibronic coupling in singlet fission using polarized transient absorption microscopy and ab initio simulations on single-crystalline pentacene. It was found that singlet fission in pentacene is greatly facilitated by the vibrational coherence of a 35.0 cm-1 phonon, where anisotropic coherence persists extensively for a few picoseconds. This coherence-preserving phonon that drives the anisotropic singlet fission is made possible by a unique cross-axial charge-transfer intermediate state. In the same fashion, this phonon was also found to predominantly drive the quantum decohence of a correlated triplet pair to form a decoupled triplet dimer. Moreover, our transient kinetic experimental data illustrates notable directional anisotropicity of the singlet fission rate in single-crystalline pentacene.

Nickel particle-enabled width-controlled growth of bilayer molybdenum disulfide nanoribbons.

Transition metal dichalcogenides exhibit a variety of electronic behaviors depending on the number of layers and width. Therefore, developing facile methods for their controllable synthesis is of central importance. We found that nickel nanoparticles promote both heterogeneous nucleation of the first layer of molybdenum disulfide and simultaneously catalyzes homoepitaxial tip growth of a second layer via a vapor-liquid-solid (VLS) mechanism, resulting in bilayer nanoribbons with width controlled by the nanoparticle diameter. Simulations further confirm the VLS growth mechanism toward nanoribbons and its orders of magnitude higher growth speed compared to the conventional noncatalytic growth of flakes. Width-dependent Coulomb blockade oscillation observed in the transfer characteristics of the nanoribbons at temperatures up to 60 K evidences the value of this proposed synthesis strategy for future nanoelectronics.

Quantifying the semiconducting fraction in single-walled carbon nanotube samples through comparative atomic force and photoluminescence microscopies.

A new method was used to measure the fraction of semiconducting nanotubes in various as-grown or processed single-walled carbon nanotube (SWCNT) samples. SWCNT number densities were compared in images from near-IR photoluminescence (semiconducting species) and AFM (all species) to compute the semiconducting fraction. The results show large variations among growth methods and effective sorting by density gradient ultracentrifugation. This counting-based method provides important information about SWCNT sample compositions that can guide controlled growth methods and help calibrate bulk characterization techniques.

Anisotropic Geminate and Non-Geminate Recombination of Triplet Excitons in Singlet Fission of Single Crystalline Hexacene.

Singlet fission is believed to improve the efficiency of solar energy conversion by breaking up the Shockley-Queisser thermodynamic limit. Understanding of triplet excitons generated by singlet fission is essential for solar energy exploitation. Here we employed transient absorption microscopy to examine dynamical behaviors of triplet excitons. We observed anisotropic recombination of triplet excitons in hexacene single crystals. The triplet exciton relaxations from singlet fission proceed in both geminate and non-geminate recombination. For the geminate recombination, the different rates were attributed to the significant difference in their related energy change based on the Redfield quantum dissipation theory. The process is mainly governed by the electron-phonon interaction in hexacene. On the other hand, the non-geminate recombination is of bimolecular origin through energy transfer. In the triplet-triplet bimolecular process, the rates along the two different optical axes in the a-b crystalline plane differ by a factor of 4. This anisotropy in the triplet-triplet recombination rates was attributed to the interference in the coupling probability of dipole-dipole interactions in the different geometric configurations of hexacene single crystals. Our experimental findings provide new insight into future design of singlet fission materials with desirable triplet exciton exploitations.

Surfactant-Mediated Growth and Patterning of Atomically Thin Transition Metal Dichalcogenides.

The role of additives in facilitating the growth of conventional semiconducting thin films is well-established. Apparently, their presence is also decisive in the growth of two-dimensional transition metal dichalcogenides (TMDs), yet their role remains ambiguous. In this work, we show that the use of sodium bromide enables synthesis of TMD monolayers via a surfactant-mediated growth mechanism, without introducing liquefaction of metal oxide precursors. We discovered that sodium ions provided by sodium bromide chemically passivate edges of growing molybdenum disulfide crystals, relaxing in-plane strains to suppress 3D islanding and promote monolayer growth. To exploit this growth model, molybdenum disulfide monolayers were directly grown into desired patterns using predeposited sodium bromide as a removable template. The surfactant-mediated growth not only extends the families of metal oxide precursors but also offers a way for lithography-free patterning of TMD monolayers on various surfaces to facilitate fabrication of atomically thin electronic devices.

The catalyst for growing single-walled carbon nanotubes by catalytic chemical vapor deposition method.

The complexity of the catalyst's actual role and properties, along with the numerous synthesis parameters for the growth of single-walled carbon nanotubes (SWCNTs), has hindered the efforts to understand the formation of this fascinating material. In this manuscript, we review and discuss the data regarding the properties and peculiarities of a wide variety of catalyst nanoparticles available in the scientific literature, in order to reveal common features that are favorable for SWCNTs growth. A special effort is made to show the influence of the support material on the thermodynamic properties of the catalyst and, thereby, on the growth path of the nanotubes. This review is an attempt to help guide the catalyst search for SWCNT growth.

Anisotropic Singlet Fission in Single Crystalline Hexacene.

Singlet fission is known to improve solar energy utilization by circumventing the Shockley-Queisser limit. The two essential steps of singlet fission are the formation of a correlated triplet pair and its subsequent quantum decoherence. However, the mechanisms of the triplet pair formation and decoherence still remain elusive. Here we examined both essential steps in single crystalline hexacene and discovered remarkable anisotropy of the overall singlet fission rate along different crystal axes. Since the triplet pair formation emerges on the same timescale along both crystal axes, the quantum decoherence is likely responsible for the directional anisotropy. The distinct quantum decoherence rates are ascribed to the notable difference on their associated energy loss according to the Redfield quantum dissipation theory. Our hybrid experimental/theoretical framework will not only further our understanding of singlet fission, but also shed light on the systematic design of new materials for the third-generation solar cells.

Low-temperature single-wall carbon nanotubes synthesis: feedstock decomposition limited growth.

We report on the lowest temperature of SWCNT growth using endothermic decomposition of CH4 gas on a specially activated alumina-supported Fe:Mo catalyst. However, the observed lowest growth temperature (560 degrees C) is higher than that reported previously for exothermic feedstock type. Our observation indicates that the decomposition threshold temperature of the feedstock limits the SWCNT growth. This study also suggests that, by using more active carbon feedstock or somehow facilitating its decomposition, one could enable the synthesis of SWCNT at an even lower temperature.

Computational studies of small carbon and iron-carbon systems relevant to carbon nanotube growth.

Density functional theory (DFT) calculations show that dimers and longer carbon strings are more stable than individual atoms on Fe(111) surfaces. It is therefore necessary to consider the formation of these species on the metal surfaces and their effect on the mechanism of single-walled nanotube (SWNT) growth. The good agreement between the trends (energies and structures) obtained using DFT and those based on the Brenner and AIREBO models indicate that these analytic models provide adequate descriptions of the supported carbon systems needed for valid molecular dynamics simulations of SWNT growth. In contrast, the AIREBO model provides a better description of the relative energies for isolated carbon species, and this model is preferred over the Brenner potential when simulating SWNT growth in the absence of metal particles. However, the PM3 semiempirical model appears to provide an even better description for these systems and, given sufficient computer resources, direct dynamics methods based on this model may be preferred.

Exfoliation of a non-van der Waals material from iron ore hematite.

With the advent of graphene, the most studied of all two-dimensional materials, many inorganic analogues have been synthesized and are being exploited for novel applications. Several approaches have been used to obtain large-grain, high-quality materials. Naturally occurring ores, for example, are the best precursors for obtaining highly ordered and large-grain atomic layers by exfoliation. Here, we demonstrate a new two-dimensional material 'hematene' obtained from natural iron ore hematite (α-Fe2O3), which is isolated by means of liquid exfoliation. The two-dimensional morphology of hematene is confirmed by transmission electron microscopy. Magnetic measurements together with density functional theory calculations confirm the ferromagnetic order in hematene while its parent form exhibits antiferromagnetic order. When loaded on titania nanotube arrays, hematene exhibits enhanced visible light photocatalytic activity. Our study indicates that photogenerated electrons can be transferred from hematene to titania despite a band alignment unfavourable for charge transfer.