Organic radical emitters: nature of doublet excitons in emissive layers.

Organic radical emitters have received significant attention as a new route to efficient organic light-emitting diodes (OLEDs). The electronic structure of radical emitters allows bypassing the triplet harvesting issue in current OLED devices. However, the nature of doublet excited states remains elusive due to the complex nature of emissive layers. To date, the computational efforts have treated radical carrying materials as isolated entities in the gas phase. However, OLED materials are applied as thin solid films where intermolecular interactions significantly impact optoelectronic properties of the devices. Here, we combine molecular dynamics simulations and quantum chemical calculations to evaluate the effect of emitter-host interactions on the performance of radical-based emissive layers. Results demonstrate that intermolecular interactions remarkably modulate the electronic properties of the radicals in the thin solid films. The doublet excitons of isolated emitters demonstrate a hybrid character of charge-transfer (CT) and local-excitation (LE), while the emitter-host clusters present a significant CT character. Further, the impact of static and dynamic disorders on the hole-electron recombination is studied. Although the host-emitter interactions simultaneously decrease radiative rates and increase non-radiative rates, the latter rates are 100 times smaller than the former rates, allowing internal quantum efficiency to reach 100% for the doublet-based emission process. The results of this study highlight the significant impact of host-emitter interactions on radiative and non-radiative recombination processes and offer guidelines to tune these interactions for advancing radical-based OLEDs.

Local Structure of Sulfur Vacancies on the Basal Plane of Monolayer MoS2.

The nature of the S-vacancy is central to controlling the electronic properties of monolayer MoS2. Understanding the geometric and electronic structures of the S-vacancy on the basal plane of monolayer MoS2 remains elusive. Here, operando S K-edge X-ray absorption spectroscopy shows the formation of clustered S-vacancies on the basal plane of monolayer MoS2 under reaction conditions (H2 atmosphere, 100-600 °C). First-principles calculations predict spectral fingerprints consistent with the experimental results. The Mo K-edge extended X-ray absorption fine structure shows the local structure as coordinatively unsaturated Mo with 4.1 ± 0.4 S atoms as nearest neighbors (above 400 °C in an H2 atmosphere). Conversely, the 6-fold Mo-Mo coordination in the crystal remains unchanged. Electrochemistry confirms similar active sites for hydrogen evolution. The identity of the S-vacancy defect on the basal plane of monolayer MoS2 is herein elucidated for applications in optoelectronics and catalysis.

Mass Spectrometry of Au10(4-tert-butylbenzenethiolate)10 Nanoclusters Using Superconducting Tunnel Junction Cryodetection Reveals Distinct Metastable Fragmentation.

Cryodetection mass spectrometry (MS) was used to study the Au10(TBBT)10 (TBBT = 4-tert-butylbenzenethiolate) catenane nanocluster. The matrix-assisted laser desorption ionization (MALDI) process generates distinct fragments that can be arranged into two distinct regimes: (i) in-source fragmentation, which occurs rapidly in a relatively short (<170 ns) time frame, and (ii) metastable fragmentation, which occurs postacceleration during a time-of-flight (TOF) mass analysis over a longer time frame (>170 ns-250 µs). Using MALDI-TOF MS with superconducting tunnel junction (STJ) cryodetection, distinct metastable nanocluster fragments were resolved at lower energies deposited into the detector. The results also demonstrated that STJ cryodetection MS can be used to acquire multiple (>10), simultaneous tandem mass spectra in a single experiment. Simulated fragmentation of the Au10 nanocluster using ab initio molecular dynamics (AIMD) revealed the different fragmentation processes and confirmed the MS results. Using both the empirical MS data and AIMD calculations, fragmentation pathways are proposed for Au10(TBBT)10, which terminate with two small, stable ringed species.

Operando Study of Thermal Oxidation of Monolayer MoS2.

Monolayer MoS2 is a promising semiconductor to overcome the physical dimension limits of microelectronic devices. Understanding the thermochemical stability of MoS2 is essential since these devices generate heat and are susceptible to oxidative environments. Herein, the promoting effect of molybdenum oxides (MoO x ) particles on the thermal oxidation of MoS2 monolayers is shown by employing operando X-ray absorption spectroscopy, ex situ scanning electron microscopy and X-ray photoelectron spectroscopy. The study demonstrates that chemical vapor deposition-grown MoS2 monolayers contain intrinsic MoO x and are quickly oxidized at 100 °C (3 vol% O2/He), in contrast to previously reported oxidation thresholds (e.g., 250 °C, t ≤ 1 h in the air). Otherwise, removing MoO x increases the thermal oxidation onset temperature of monolayer MoS2 to 300 °C. These results indicate that MoO x promote oxidation. An oxide-free lattice is critical to the long-term stability of monolayer MoS2 in state-of-the-art 2D electronic, optical, and catalytic applications.

Design of Organic Electronic Materials With a Goal-Directed Generative Model Powered by Deep Neural Networks and High-Throughput Molecular Simulations.

In recent years, generative machine learning approaches have attracted significant attention as an enabling approach for designing novel molecular materials with minimal design bias and thereby realizing more directed design for a specific materials property space. Further, data-driven approaches have emerged as a new tool to accelerate the development of novel organic electronic materials for organic light-emitting diode (OLED) applications. We demonstrate and validate a goal-directed generative machine learning framework based on a recurrent neural network (RNN) deep reinforcement learning approach for the design of hole transporting OLED materials. These large-scale molecular simulations also demonstrate a rapid, cost-effective method to identify new materials in OLEDs while also enabling expansion into many other verticals such as catalyst design, aerospace, life science, and petrochemicals.

Active Learning Accelerates Design and Optimization of Hole-Transporting Materials for Organic Electronics.

Data-driven methods are receiving increasing attention to accelerate materials design and discovery for organic light-emitting diodes (OLEDs). Machine learning (ML) has enabled high-throughput screening of materials properties to suggest new candidates for organic electronics. However, building reliable predictive ML models requires creating and managing a high volume of data that adequately address the complexity of materials' chemical space. In this regard, active learning (AL) has emerged as a powerful strategy to efficiently navigate the search space by prioritizing the decision-making process for unexplored data. This approach allows a more systematic mechanism to identify promising candidates by minimizing the number of computations required to explore an extensive materials library with diverse variables and parameters. In this paper, we applied a workflow of AL that accounts for multiple optoelectronic parameters to identify materials candidates for hole-transport layers (HTL) in OLEDs. Results of this work pave the way for efficient screening of materials for organic electronics with superior efficiencies before laborious simulations, synthesis, and device fabrication.

Atomically precise nanoclusters with reversible isomeric transformation for rotary nanomotors.

Thermal-stimuli responsive nanomaterials hold great promise in designing multifunctional intelligent devices for a wide range of applications. In this work, a reversible isomeric transformation in an atomically precise nanocluster is reported. We show that biicosahedral [Au13Ag12(PPh3)10Cl8]SbF6 nanoclusters composed of two icosahedral Au7Ag6 units by sharing one common Au vertex can produce two temperature-responsive conformational isomers with complete reversibility, which forms the basis of a rotary nanomotor driven by temperature. Differential scanning calorimetry analysis on the reversible isomeric transformation demonstrates that the Gibbs free energy is the driving force for the transformation. This work offers a strategy for rational design and development of atomically precise nanomaterials via ligand tailoring and alloy engineering for a reversible stimuli-response behavior required for intelligent devices. The two temperature-driven, mutually convertible isomers of the nanoclusters open up an avenue to employ ultra-small nanoclusters (1 nm) for the design of thermal sensors and intelligent catalysts.

Free Valence Electron Centralization Strategy for Preparing Ultrastable Nanoclusters and Their Catalytic Application.

Metal nanoclusters have attracted extensive interests owing to their atomically precise structures as well as intriguing properties. However, silver nanoclusters are not as stable as their gold counterparts, impeding the practical applications of Ag nanoclusters. In this work, a strategy of free valence electron centralization was exploited to render parent Ag nanoclusters highly stable. The stability of Ag29(SSR)12(PPh3)4 (SSR: benzene-1,3-dithiol) was controllably enhanced by stepwisely alloying the Ag29 nanocluster to Ag17Cu12(SSR)12(PPh3)4 and Au1Ag16Cu12(SSR)12(PPh3)4. Specifically, the trimetallic Au1Ag16Cu12 is ultrastable even at 175 °C, which is close to the nanocluster decomposition temperature. The structures of Ag17Cu12 and Au1Ag16Cu12 nanoclusters are determined by single-crystal X-ray diffraction. Furthermore, a combination of X-ray photoelectron spectroscopy measurements and density functional theory calculations demonstrates that the enhanced stability is induced by the centralization of the free valence electrons to the interior of the nanocluster. More importantly, the Au1Ag16Cu12 enables the multicomponent A3 coupling reaction at high temperatures, which remarkably shortens the catalytic reaction time from ∼5 h to 3 min. Overall, this work presents a strategy for enhancing the thermal stability of nanoclusters via centralizing the free valence electrons to the nanocluster kernels.

Gold-Palladium Nanoalloys Supported by Graphene Oxide and Lamellar TiO2 for Direct Synthesis of Hydrogen Peroxide.

Hybrid catalysts composed of gold-palladium nanoalloys that are sandwiched between layers of graphene oxide (GO) and lamellar TiO2 are synthesized via the deposition-reduction method. The resulting AuPd catalysts with different compositions of metal and support are fully characterized by a series of techniques, including X-ray diffraction, scanning transmission electron microscopy, X-ray photoelectron spectroscopy, and inductively coupled plasma mass spectrometry. The catalysts are also optimized against Au, Pd, GO, and TiO2 contents and employed in the direct synthesis of hydrogen peroxide (DSHP) from H2 and O2. The sandwich-like AuPd nanoalloy comprising 1 wt % nanoparticle of an equimolar mixture of Au and Pd with 6 wt % GO and 93 wt % TiO2 supports shows a promising catalytic performance toward the DSHP reaction with H2O2 productivity and selectivity of 5.50 mol H2O2 gmetal-1 h-1 and 64%, respectively. The catalyst is found to be considerably more active than those reported in the literature. Furthermore, the H2O2 selectivity of the catalyst is found to improve considerably to 88% when the TiO2 support is pretreated by HNO3. It is found that the perimeter sites of the interface of AuPd alloy and TiO2 are deemed as catalytically active sites for the DSHP reactions and the acidic property of TiO2 can retard the other overreactions and the decomposition of yielded H2O2. Results of the present study may provide a design strategy for partially covered catalysts that are confined by 2D materials for selective reactions.

Nature of Lone-Pair-Surface Bonds and Their Scaling Relations.

We investigate the (surface) bonding of a class of industrially and biologically important molecules in which the chemically active orbital is a 2 p electron lone pair located on an N or O atom bound via single bonds to H or alkyl groups. This class includes water, ammonia, alcohols, ethers, and amines. Using extensive density functional theory (DFT) calculations, we discover scaling relations (correlations) among molecular binding energies of different members of this class: the bonding energetics of a single member can be used as a descriptor for other members. We investigate the bonding mechanism for a representative (H2O) and find the most important physical surface properties that dictate the strength and nature of the bonding through a combination of covalent and noncovalent electrostatic effects. We describe the importance of surface intrinsic electrostatic, geometric, and mechanical properties in determining the extent of the lone-pair-surface interactions. We study systems including ionic materials in which the surface positive and negative centers create strong local surface electric fields, which polarize the dangling lone pair and lead to a strong "electrostatically driven bond". We emphasize the importance of noncovalent electrostatic effects and discuss why a fully covalent picture, common in the current first-principles literature on surface bonding of these molecules, is not adequate to correctly describe the bonding mechanism and energy trends. By pointing out a completely different mechanism (charge transfer) as the major factor for binding N- and O-containing unsaturated (radical) adsorbates, we explain why their binding energies can be tuned independently from those of the aforementioned species, having potential implications in scaling-driven catalyst discovery.

Dual effects of water vapor on ceria-supported gold clusters.

Atomically precise nanocatalysts are currently being intensely pursued in catalysis research. Such nanocatalysts can serve as model catalysts for gaining fundamental insights into catalytic processes. In this work we report a discovery that water vapor provokes the mild removal of surface long-chain ligands on 25-atom Au25(SC12H25)18 nanoclusters in a controlled manner. Using the resultant Au25(SC12H25)18-x/CeO2 catalyst and CO oxidation as a probe reaction, we found that the catalytic activity of cluster/CeO2 is enhanced from nearly zero conversion of CO (in the absence of water) to 96.2% (in the presence of 2.3 vol% H2O) at the same temperature (100 °C). The cluster catalysts exhibit high stability during the CO oxidation process under moisture conditions (up to 20 vol% water vapor). Water vapor plays a dual role in gold cluster-catalyzed CO oxidation. FT-IR and XPS analyses in combination with density functional theory (DFT) simulations suggest that the "-SC12H25" ligands are easier to be removed under a water vapor atmosphere, thus generating highly active sites. Moreover, the O22- peroxide species constitutes the active oxygen species in CO oxidation, evidenced by Raman spectroscopy analysis and isotope experiments on the CeO2 and cluster/CeO2. The results also indicate the perimeter sites of the interface of Au25(SC12H25)18-x/CeO2 to be active sites for catalytic CO oxidation. The controlled exposure of active sites under mild conditions is of critical importance for the utilization of clusters in catalysis.

Chiral Ag23 nanocluster with open shell electronic structure and helical face-centered cubic framework.

We report the synthesis and crystal structure of a nanocluster composed of 23 silver atoms capped by 8 phosphine and 18 phenylethanethiolate ligands. X-ray crystallographic analysis reveals that the kernel of the Ag nanocluster adopts a helical face-centered cubic structure with C2 symmetry. The thiolate ligands show two binding patterns with the surface Ag atoms: tri- and tetra-podal types. The tetra-coordination mode of thiolate has not been found in previous Ag nanoclusters. No counter ion (e.g., Na+ and NO3-) is found in the single-crystal and the absence of such ions is also confirmed by X-ray photoelectron spectroscopy analysis, indicating electrical neutrality of the nanocluster. Interestingly, the nanocluster has an open shell electronic structure (i.e., 23(Ag 5s1)-18(SR) = 5e), as confirmed by electron paramagnetic resonance spectroscopy. Time-dependent density functional theory calculations are performed to correlate the structure and optical absorption/emission spectra of the Ag nanocluster.

Shuttling single metal atom into and out of a metal nanoparticle.

It has long been a challenge to dope metal nanoparticles with a specific number of heterometal atoms at specific positions. This becomes even more challenging if the heterometal belongs to the same group as the host metal because of the high tendency of forming a distribution of alloy nanoparticles with different numbers of dopants due to the similarities of metals in outmost electron configuration. Herein we report a new strategy for shuttling a single Ag or Cu atom into a centrally hollow, rod-shaped Au24 nanoparticle, forming AgAu24 and CuAu24 nanoparticles in a highly controllable manner. Through a combined approach of experiment and theory, we explain the shuttling pathways of single dopants into and out of the nanoparticles. This study shows that the single dopant is shuttled into the hollow Au24 nanoparticle either through the apex or side entry, while shuttling a metal atom out of the Au25 to form the Au24 nanoparticle occurs mainly through the side entry.Doping a metal nanocluster with heteroatoms dramatically changes its properties, but it remains difficult to dope with single-atom control. Here, the authors devise a strategy to dope single atoms of Ag or Cu into hollow Au nanoclusters, creating precise alloy nanoparticles atom-by-atom.

Gold Nanoclusters Promote Electrocatalytic Water Oxidation at the Nanocluster/CoSe2 Interface.

Electrocatalytic water splitting to produce hydrogen comprises the hydrogen and oxygen evolution half reactions (HER and OER), with the latter as the bottleneck process. Thus, enhancing the OER performance and understanding the mechanism are critically important. Herein, we report a strategy for OER enhancement by utilizing gold nanoclusters to form cluster/CoSe2 composites; the latter exhibit largely enhanced OER activity in alkaline solutions. The Au25/CoSe2 composite affords a current density of 10 mA cm-2 at small overpotential of ∼0.43 V (cf. CoSe2: ∼0.52 V). The ligand and gold cluster size can also tune the catalytic performance of the composites. Based upon XPS analysis and DFT simulations, we attribute the activity enhancement to electronic interactions between nanocluster and CoSe2, which favors the formation of the important intermediate (OOH) as well as the desorption of oxygen molecules over Aun/CoSe2 composites in the process of water oxidation. Such an atomic level understanding may provide some guidelines for design of OER catalysts.

Surface modification of adamantane-terminated gold nanoclusters using cyclodextrins.

The surface functionality of Au38S2(SAdm)20 nanoclusters (-SAdm = adamantanethiolate) in the presence of α-, ß-, and γ-cyclodextrins (CDs) is studied. The supramolecular chemistry and host-guest interactions of CDs and the protecting ligands of nanoclusters are investigated using UV-vis and NMR spectroscopies, MALDI mass spectrometry, and molecular dynamics simulations. In contrast to α- and γ-CDs, the results show that ß-CDs are capable of efficiently chemisorbing onto the Au38S2(SAdm)20 nanoclusters to yield Au38S2(SAdm)20-(ß-CD)2 conjugates. MD simulations revealed that two -SAdm ligands of the nanoparticle with the least steric hindrance are capable to selectively be accommodated into hydrophobic cavity of ß-CDs, as furthermore confirmed by NMR spectroscopy. The conjugates largely improve the stability of the nanoclusters in the presence of strong oxidants (e.g., TBHP). Further, the electrochemical properties of Au38S2(SAdm)20 nanoclusters and Au38S2(SAdm)20-(ß-CD)2 conjugates are compared. The charge transfer to the redox probe molecules (e.g., K3Fe(CN)6) in solution was monitored by cyclic voltammetry. It is found that ß-CDs act as an umbrella to cover the fragile metal cores of the nanoclusters, thereby blocking direct interaction with destabilizing agents and hence quenching the charge transfer process.

Silicon Nanoparticles with Surface Nitrogen: 90% Quantum Yield with Narrow Luminescence Bandwidth and the Ligand Structure Based Energy Law.

Silicon nanoparticles (NPs) have been widely accepted as an alternative material for typical quantum dots and commercial organic dyes in light-emitting and bioimaging applications owing to silicon's intrinsic merits of least toxicity, low cost, and high abundance. However, to date, how to improve Si nanoparticle photoluminescence (PL) performance (such as ultrahigh quantum yield, sharp emission peak, high stability) is still a major issue. Herein, we report surface nitrogen-capped Si NPs with PL quantum yield up to 90% and narrow PL bandwidth (full width at half-maximum (fwhm) ≈ 40 nm), which can compete with commercial dyes and typical quantum dots. Comprehensive studies have been conducted to unveil the influence of particle size, structure, and amount of surface ligand on the PL of Si NPs. Especially, a general ligand-structure-based PL energy law for surface nitrogen-capped Si NPs is identified in both experimental and theoretical analyses, and the underlying PL mechanisms are further discussed.

Tailoring the Electronic and Catalytic Properties of Au25 Nanoclusters via Ligand Engineering.

To explore the electronic and catalytic properties of nanoclusters, here we report an aromatic-thiolate-protected gold nanocluster, [Au25(SNap)18](-) [TOA](+), where SNap = 1-naphthalenethiolate and TOA = tetraoctylammonium. It exhibits distinct differences in electronic and catalytic properties in comparison with the previously reported [Au25(SCH2CH2Ph)18](-), albeit their skeletons (i.e., Au25S18 framework) are similar. A red shift by ∼10 nm in the HOMO-LUMO electronic absorption peak wavelength is observed for the aromatic-thiolate-protected nanocluster, which is attributed to its dilated Au13 kernel. The unsupported [Au25(SNap)18](-) nanoclusters show high thermal and antioxidation stabilities (e.g., at 80 °C in the present of O2, excess H2O2, or TBHP) due to the effects of aromatic ligands on stabilization of the nanocluster's frontier orbitals (HOMO and LUMO). Furthermore, the catalytic activity of the supported Au25(SR)18/CeO2 (R = Nap, Ph, CH2CH2Ph, and n-C6H13) is examined in the Ullmann heterocoupling reaction between 4-methyl-iodobenzene and 4-nitro-iodobenzene. Results show that the activity and selectivity of the catalysts are largely influenced by the chemical nature of the protecting thiolate ligands. This study highlights that the aromatic ligands not only lead to a higher conversion in catalytic reaction but also markedly increase the yield of the heterocoupling product (4-methyl-4'-nitro-1,1'-biphenyl). Through a combined approach of experiment and theory, this study sheds light on the structure-activity relationships of the Au25 nanoclusters and also offers guidelines for tailoring nanocluster properties by ligand engineering for specific applications.

Theoretical study of interactions of a Li(+)(CF3SO2)2N(-) ion pair with CR3(OCR2CR2)nOCR3 (R = H or F).

Interactions of a lithium bis(trifluoromethane sulfonyl)imide (Li(+)Tf2N(-)) ion pair with oligoethers are investigated via density functional theory (DFT). As a model for polymer electrolytes polyethyleneoxide (PEO) and perfluoropolyether (PFPE), CR3(OCR2CR2)n=1-5OCR3 (R = H or F) is considered. Topographical analysis of the molecular electrostatic potential (MESP) is performed to determine preferential binding sites of Li(+). Our study shows that the MESP value near the oxygen sites of the polymer backbone is more negative for PEO than for PFPE. This result indicates that substitution of hydrogen by fluorine in polyethers leads to reduction in Li(+)-polymer interactions, in concert with the experimental ionic conductivity results. S-O stretching vibrations of Tf2N(-) are calculated for the lithium salt in the presence and absence of electrolytes. The blue and red shifts predicted for S-O stretching are further explained by natural bond orbital analysis and molecular electron density topography. The S-O stretching vibrations can be used as a useful tool to understand the ion pair interactions and thus ion transport phenomena in polymer electrolytes.

Experimental and Mechanistic Understanding of Aldehyde Hydrogenation Using Au25 Nanoclusters with Lewis Acids: Unique Sites for Catalytic Reactions.

The catalytic activity of Au25(SR)18 nanoclusters (R = C2H4Ph) for the aldehyde hydrogenation reaction in the presence of a base, e.g., ammonia or pyridine, and transition-metal ions M(z+), such as Cu(+), Cu(2+), Ni(2+) and Co(2+), as a Lewis acid is studied. The addition of a Lewis acid is found to significantly promote the catalytic activity of Au25(SR)18/CeO2 in the hydrogenation of benzaldehyde and a number of its derivatives. Matrix-assisted laser desorption ionization (MALDI) and electrospray ionization (ESI) mass spectrometry in conjunction with UV-vis spectroscopy confirm the generation of new species, Au25-n(SR)18-n (n = 1-4), in the presence of a Lewis acid. The pathways for the speciation of Au24(SR)17 from its parent Au25(SR)18 nanocluster as well as its structure are investigated via the density functional theory (DFT) method. The adsorption of M(z+) onto a thiolate ligand "-SR-" of Au25(SR)18, followed by a stepwise detachment of "-SR-" and a gold atom bonded to "-SR-" (thus an "Au-SR" unit) is found to be the most likely mechanism for the Au24(SR)17 generation. This in turn exposes the Au13-core of Au24(SR)17 to reactants, providing an active site for the catalytic hydrogenation. DFT calculations indicate that M(z+) is also capable of adsorbing onto the Au13-core surface, producing a possible active metal site of a different kind to catalyze the aldehyde hydrogenation reaction. This study suggests, for the first time, that species with an open metal site like adducts [nanoparticle-M]((z-1)+) or fragments Au25-n(SR)18-n function as the catalysts rather than the intact Au25(SR)18.

On the structural stability of ionic liquid-IRMOF composites: a computational study.

The structural stability of isoreticular metal organic frameworks, IRMOF-1 and IRMOF-10, confining ionic liquids (ILs) inside their nano-porous cavities is studied via molecular dynamics (MD) simulations. Imidazolium- and pyridinium-based ILs, including BMI(+)PF6(-), BMI(+)Br(-), BMI(+)Tf2N(-), BMI(+)DCA(-), and BuPy(+)Tf2N(-) (BMI(+) = 1-butyl-3-methylimidazolium, PF6(-) = hexafluorophosphate, Br(-) = bromide, Tf2N(-) = bis(trifluoromethylsulfonyl)imide, DCA(-) = dicyanamide, and BuPy(+) = N-butylpyridinium), at different loadings are considered. It is found that both IRMOFs are structurally unstable and deform dramatically from their crystal structure in the presence of ILs. The interactions between the metallic parts of IRMOFs and IL anions play a major role in structural disruption and collapse of these MOFs. Thus elongated anions such as Tf2N(-) and DCA(-) that can interact with two different metal sites tend to lower IRMOF stability compared to spherical anions such as Br(-) and PF6(-). A further analysis via density functional theory (DFT) and ab initio molecular dynamics (AIMD) simulations lends support to the MD results regarding structural instability of IRMOFs in the presence of ILs.