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The development of electrically conductive and high surface area carbon is important for the improvement of supercapacitor energy densities. Yttrium hydroxide microspindles were prepared and shown ...

Identifying the nature and reactivity of surface intermediate species is critical to understanding the fundamental reaction pathways of NO oxidation on mullite-type oxide catalysts. Using in situ F...

Topological transitions between considerably different phases typically require harsh conditions to collectively break chemical bonds and overcome the stress caused to the original structure by altering its correlated bond environment. In this work we present a case system that can achieve rapid rearrangement of the whole lattice of a metal–organic framework through a domino alteration of the bond connectivity under mild conditions. The system transforms from a disordered metal–organic framework with low porosity to a highly porous and crystalline isomer within 40 s following activation (solvent exchange and desolvation), resulting in a substantial increase in surface area from 725 to 2,749 m2 g–1. Spectroscopic measurements show that this counter-intuitive lattice rearrangement involves a metastable intermediate that results from solvent removal on coordinatively unsaturated metal sites. This disordered–crystalline switch between two topological distinct metal–organic frameworks is shown to be reversible over four cycles through activation and reimmersion in polar solvents. A disordered metal–organic framework converts into a more porous, crystalline phase within 40 s following solvent exchange and desolvation. The rapid domino rearrangement of the whole lattice, which involves carboxylate migration on coordinatively unsaturated metal sites, is accompanied by a substantial increase in surface area.

We investigate the much debated mechanism that leads to an enhancement of the photoluminescence upon small aromatic molecule adsorption in the Zn-based metal–organic framework (MOF) material Zn2(bd...

By combining experimental and theoretical approaches, we investigate the quantitative relationship between molecular desorption temperature and binding energy on d and f metal oxide surfaces. We demonstrate how temperature-programmed desorption can be used to quantitatively correlate the theoretical surface chemistry of metal oxides (via on-site Hubbard U correction) to gas surface interactions for catalytic reactions. For this purpose, both CO and NO oxidation mechanisms are studied in a step-by-step reaction process for perovskite and mullite-type oxides, respectively. Additionally, we show solutions for over-binding issues found in COx, NOx, SOx, and other covalently bonded molecules that must be considered during surface reaction modeling. This work shows the high reliability of using TPD and density functional theory in conjunction to create accurate surface chemistry information for a variety of correlated metal oxide materials.

The correlation between lattice oxygen (O) binding energy and O oxidation activity imposes a fundamental limit in developing oxide catalysts, simultaneously meeting the stringent thermal stability ...

We have synthesized a stable luminescent metal–organic framework (LMOF) through modification of an established Zr-based structure. The three-dimensional porous network of LMOF-321 represents a step forward in the development of robust, dual-ligand Zr-MOFs. This material is based on Zr6-nodes, which underlie chemically and thermally stable frameworks. LMOF-321 exhibits notable durability in diverse types of water samples (deionized, acidic/basic, seawater). The porosity, luminescence, and specific functionality from LMOF-321 establishes itself as a fluorescent chemical sensor and adsorbent for aqueous analytes. Studies have been implemented to analyze interactions of LMOF-321 with Li+ and other metals commonly found in water. The fluorescence intensity from LMOF-321 is responsive to Li+ at a parts per billion level (3.3 ppb) and demonstrates high selectivity for Li+ over other light metals, with detection ratios of 6.2, 14.3, and 44.9 for Li+/Na+, Li+/Ca2+, and Li+/Mg2+, respectively. These performances we...

By combining low energy ion scattering spectroscopy and density functional theory calculation, we study the surface composition and surface formation energy of AMn2O5 (A = Sm, Bi) mullite-type oxides synthesized by different methods and their effects on NO catalytic performance. It is well-known that hydrothermal (HT) synthesis at low temperatures produces materials with higher specific surface areas (SSAs) compared with those synthesized by coprecipitation (CP) and high-temperature calcination; however, it is less clear how synthesis methods affect surface chemistry. We find that the NO oxidation performance of SmMn2O5–HT does not match the SSA increase when compared to the lower SSA SmMn2O5–CP. Combined experimental and theoretical investigation reveals that SmMn2O5–HT includes a higher fraction of inactive Sm-terminated surfaces, which explains its lower than expected activity. However, the surface chemistry change depends strongly on the A-site element. The exposed surfaces of BiMn2O5–CP are predomina...

International audience; The preparation of a triptych nanomaterial made of TiO2 nanoparticles as semiconductor, Ag plasmonic nanoparticles and a carbo-benzene macrocyclic molecule as photosensitizer is described, and used to produce hydrogen by photo-reduction of pure deionized water under 2.2 bar argon pressure without any electrical input. Silver nanoparticles (~5 nm) are grafted onto the surface of commercial TiO2 nanoparticles (~23 nm) by a photo-deposition process using an original silver amidinate precursor. The thickness of the photosensitive layer (2 nm), which completes the assembly, plays a crucial role in the efficiency and robustness of the triptych nanocatalyst. Thanks to the organic layer reorganization during the first ~24 h of irradiation, it leads to an enhancement of the hydrogen production rate up to 5 times. The amount of silver and carbo-benzene are optimized, along with the mass concentration of nanocatalyst in water and the pH of the aqueous medium, to allow reaching a hydrogen production rate of 22.1 μmol·h−1·gphotocatalyst−1.

We uncover the exact mechanisms that lead to a quenching of the photoluminescence in a Zn-based metal organic framework (MOF) material upon nitroaromatic molecule adsorption. We present evidence based on ab initio simulations, coupled with in situ IR spectroscopy and photoluminescence measurements, showing that quenching occurs as the result of a shift of the lowest unoccupied orbital from the MOF host to the guest molecule. Our results provide a mechanistic understanding of quenching of photoluminescence in MOFs and are of importance for chemical sensing applications, where they aid the design of novel MOFs with increased sensing selectivity.

By studying their surface chemistry, metal–oxygen bond strength, and critical energy barrier heights, we elucidate the differences in the NO oxidation catalytic performance of PrMn2O5 and SmMn2O5 mullite-type oxides. The 50% conversion temperature is lower (230 °C vs. 275 °C) and the maximum conversion efficiency is higher (81% at 282 °C vs. 68% at 314 °C) for PrMn2O5 compared to SmMn2O5, despite having a ∼15% lower specific surface area. Furthermore, PrMn2O5 exhibits higher maximum efficiency compared to Pt/Al2O3. Combined experimental and theoretical findings indicate that the superior catalytic performance of PrMn2O5 at low temperatures arises from the presence of more labile and reactive surface lattice oxygen due to weaker Mn–O bond strength and lower thermal stability of surface NOx ad-species.

Hollow carbon microsphere flowers were nanocast from glucose, acrylamide, and acetylene sources. Carbon growth was catalyzed by a lanthanum graft copolymer template using wet acetylene. The resulting spherical shape is beneficial for 3-D porosity, and has a highly graphitic content as indicated by raman spectroscopy (ID:IG = 0.99). The carbon has a high surface area of 1000 m2/g, as well as strong π-π stacking of aromatic carbons.

Managing deposition of multilayered nanocrystal quantum dot (NQD) thin films is crucial for future photonic devices to maximize solar energy extraction efficiency. Solution-based NQD deposition methods require additional protection to achieve a discrete layered structure and to prevent optical degradation during processing. An attractive method to passivate and protect NQD films is overcoating with metal oxides, usually grown using atomic layer deposition (ALD). However, a significant quenching of NQD photoluminescence (PL) is typically observed after encapsulation, hindering performance and applicability. Here, we demonstrate a modified gas-phase deposition technique that fully passivates NQD assemblies and, in contrast to standard ALD, maintains PL properties. Combined in situ FTIR and ex situ XPS measurements reveal that upon Al2O3 deposition by ALD, the metal precursor trimethylaluminum (TMA) interacts with oleic acid-capped CdSe–CdS–ZnS core–shell-shell NQDs by reorganizing the ligands and replacing ...

The transformation of an oxide substrate by its reaction with a chemical precursor during atomic layer deposition (ALD) has not attracted much attention, as films are typically deposited on top of the oxide substrate. However, any modification to the substrate surface can impact the electrical and optical properties of the device. We demonstrate herein the ability of a precursor to react deep within an oxide substrate to form an interfacial layer that is distinct from both the substrate and deposited film. This phenomenon is studied using a scandium precursor, Sc(MeCp)2(Me2pz) (1, MeCp = methylcyclopentadienyl, Me2pz = 3,5-dimethylpyrazolate), and five oxide substrates (SiO2, ZnO, Al2O3, TiO2, and HfO2). In situ Fourier transform infrared (FTIR) spectroscopy shows that at moderate temperatures (∼150 °C) the pyrazolate group of 1 reacts with the surface hydroxyl groups of OH-terminated SiO2 substrates. However, at slightly higher temperatures (≥225 °C) typically used for the ALD of Sc2O3, there is a direct...

ZnO is a high-band gap semiconductor material important for microelectronic and catalytic applications, such as water splitting among others. Although the nonpolar face of ZnO has been well-studied...

The effects of complex-oxide crystalline phase on catalytic activity and the pathways of NO oxidation are investigated by comparing SmMn2O5 mullite and SmMnO3 perovskite crystals. Synthesized under the same conditions, SmMn2O5 shows activity at lower temperature (200 °C) with a higher maximum conversion efficiency compared to SmMnO3 (52% vs. 36%), inspite of similar active site density. Fourier transform infrared spectroscopy (FTIR) and X-ray photoelectron spectroscopy (XPS) results show nitrates as the primary surface reaction species. FTIR, XPS, and temperature programmed desorption (TPD) measurements all indicate that nitrate species are less stable on SmMn2O5 ( 300 °C). Results from density functional theory calculations show that the barrier for nitrate dissociation is much higher on the Sm-terminated (001) surface of SmMnO3. Combined experimental and theoretical findings suggest that the superior catalytic performance of SmMn2O5 mullite arises from the ability to regenerate its active sites with nitrate dissociation at lower temperatures for subsequent NO oxidation reactions.

Monolayer doping (MLD) is an attractive method to precisely tailor dopant profiles for nanoelectronic semiconductor devices. The approach has been demonstrated for a number of different dopant/subs...

The need for the conformal deposition of TiO2 thin films in device fabrication has motivated a search for thermally robust titania precursors with noncorrosive byproducts. Alkylamido-cyclopentadienyl precursors are attractive because they are readily oxidized, yet stable, and afford environmentally mild byproducts. We have explored the deposition of TiO2 films on OH-terminated SiO2 surfaces by in situ Fourier transform infrared spectroscopy using a novel titanium precursor [(EtCp)Ti(NMe2)3 (1), Et = CH2CH3] with either ozone or water. This precursor initially reacts with surface hydroxyl groups at ≥150 °C through the loss of its NMe2 groups. However, once the precursor is chemisorbed, its subsequent reactivities toward ozone and water are very different. There is a clear reaction with ozone, characterized by the formation of monodentate formate and/or chelate bidentate carbonate surface species; in contrast, there is no detectable reaction with water. For the ozone-based ALD process, the surface formate/c...

Single charge electronics offer a way for disruptive technology in nanoelectronics. Coulomb blockade is a realistic way for controlling the electric current through a device with the accuracy of one electron. In such devices the current exhibits a step-like increase upon bias which reflects the discrete nature of the fundamental charge. We have assembled a double tunnel junction on an oxide-free silicon substrate that exhibits Coulomb staircase characteristics using gold nanoparticles (AuNPs) as Coulomb islands. The first tunnel junction is an insulating layer made of a grafted organic monolayer (GOM) developed for this purpose. The GOM also serves for attaching AuNPs covalently. The second tunnel junction is made by the tip of an STM. We show that this device exhibits reproducible Coulomb blockade I-V curves at 40 K in vacuum. We also show that depending on the doping of the silicon substrate, the whole Coulomb staircase can be adjusted. We have developed a simulation approach based on the orthodox theory that was completed by calculating the bias dependent tunnel barriers and by including an accurate calculation of the band bending. This model accounts for the experimental data and the doping dependence of Coulomb oscillations. This study opens new perspectives toward designing new kind of single electron transistors (SET) based on this dependence of the Coulomb staircase with the charge carrier concentration.

International audience; Al/CuO energetic structure are attractive materials due to their high thermal output and propensity to produce gas. They are widely used to bond components or as next generation of MEMS igniters. In such systems, the reaction process is largely dominated by the outward migration of oxygen atoms from the CuO matrix toward the aluminum layers, and many recent studies have already demonstrated that the interfacial nanolayer between the two reactive layers plays a major role in the material properties. Here we demonstrate that the ALD deposition of a thin ZnO layer on the CuO prior to Al deposition (by sputtering) leads to a substantial increase in the efficiency of the overall reaction. The CuO/ZnO/Al foils generate 98% of their theoretical enthalpy within a single reaction at 900 °C, whereas conventional ZnO-free CuO/Al foils produce only 78% of their theoretical enthalpy, distributed over two distinct reaction steps at 550 °C and 850 °C. Combining high-resolution transmission electron microscopy, X-ray diffraction, and differential scanning calorimetry, we characterized the successive formation of a thin zinc aluminate (ZnAl2O4) and zinc oxide interfacial layers, which act as an effective barrier layer against oxygen diffusion at low temperature.

Effective capture of radioactive organic iodides from nuclear waste remains a significant challenge due to the drawbacks of current adsorbents such as low uptake capacity, high cost, and non-recyclability. We report here a general approach to overcome this challenge by creating radioactive organic iodide molecular traps through functionalization of metal-organic framework materials with tertiary amine-binding sites. The molecular trap exhibits a high CH3I saturation uptake capacity of 71 wt% at 150 °C, which is more than 340% higher than the industrial adsorbent Ag0@MOR under identical conditions. These functionalized metal-organic frameworks also serve as good adsorbents at low temperatures. Furthermore, the resulting adsorbent can be recycled multiple times without loss of capacity, making recyclability a reality. In combination with its chemical and thermal stability, high capture efficiency and low cost, the adsorbent demonstrates promise for industrial radioactive organic iodides capture from nuclear waste. The capture mechanism was investigated by experimental and theoretical methods., Capturing radioactive organic iodides from nuclear waste is important for safe nuclear energy usage, but remains a significant challenge. Here, Li and co-workers fabricate a stable metal–organic framework functionalized with tertiary amine groups that exhibits high capacities for radioactive organic iodides uptake.

Toward a truly photostable PbSe quantum dot (QD), we apply the thick-shell or "giant" QD structural motif to this notoriously environmentally sensitive nanocrystal system. Namely, using a sequential application of two shell-growth techniques-partial-cation exchange and successive ionic layer adsorption and reaction (SILAR)-we are able to overcoat the PbSe QDs with sufficiently thick CdSe shells to impart new single-QD-level photostability, as evidenced by suppression of both photobleaching and blinking behavior. We further reveal that the crystal structure of the CdSe shell (cubic zinc-blende or hexagonal wurtzite) plays a key role in determining the photoluminescence properties of these giant QDs, with only cubic nanocrystals sufficiently bright and stable to be observed as single emitters. Moreover, we demonstrate that crystal structure and particle shape (cubic, spherical, or tetrapodal) and, thereby, emission properties can be synthetically tuned by either withholding or including the coordinating ligand, trioctylphosphine, in the SILAR component of the shell-growth process.

Silicon-rich silicon nitride thin films were deposited by co-sputtering silicon and silicon nitride and annealed at high temperatures to form precipitates of silicon nanocrystals within the nitride matrix. This process was chosen to replicate the typical fabrication used for silicon nitride based quantum dots. Photoluminescence (PL) spectra were observed from the silicon nitride samples after a high temperature anneal and the PL peak shifted with increasing Si concentration. However, the time resolved PL spectra exhibited a fast decay in the nanosecond range, indicating that the PL does not originate from a radiative transition due to quantum confinement in the silicon nanocrystals. The chemical composition and the structural properties of the thin films were studied using X-ray photoelectron spectroscopy (XPS), Fourier transform infrared spectroscopy (FTIR), X-ray diffraction, and Raman spectroscopy. XPS and FTIR demonstrated that there was some oxygen incorporation in the as-deposited films, forming silicon oxynitride, which increased after high temperature annealing in N 2 ambient. It is proposed that the PL originates from defect states related to the increased film oxidation after high temperature annealing and the shift in PL peak energies are due to transitions between the defect levels and the band edges of the nitride matrix.

Uniform and conformal deposition of tin oxide thin films is important for several applications in electronics, gas sensing, and transparent conducting electrodes. Thermal atomic layer deposition (ALD) is often best suited for these applications, but its implementation requires a mechanistic understanding of the initial nucleation and subsequent ALD processes. To this end, in situ FTIR and ex situ XPS have been used to explore the ALD of tin oxide films using tributyltin ethoxide and ozone on an OH-terminated, SiO2-passivated Si(111) substrate. Direct chemisorption of tributyltin ethoxide on surface OH groups and clear evidence that subsequent ligand exchange are obtained, providing mechanistic insight. Upon ozone pulse, the butyl groups react with ozone, forming surface carbonate and formate. The subsequent tributyltin ethoxide pulse removes the carbonate and formate features with the appearance of the bands for CH stretching and bending modes of the precursor butyl ligands. This ligand-exchange behavior ...

Deposition of Al on ZnO is used for a number of electronic and catalytic devices as well as for nanoenergetic materials. The interface structure and chemical composition often control the performance of devices. In this study, in situ infrared spectroscopy, X-ray photoemission spectroscopy, and low energy ion scattering are combined to investigate the initial stage of interface formation between Al and ZnO. We find that (a) the interface is highly inhomogeneous with discontinuous Al patches, leaving ∼10% of the ZnO surface uncovered even after deposition of an equivalent of 11 nm-thick Al film; (b) upon Al deposition, Al reduces ZnO by forming Al2O3 and releasing Zn to the surface, and this process continues as more Al is deposited; (c) the reduced surface Zn atoms readily desorb at 150 °C; and (d) at higher temperature (>600 °C) all Al is oxidized as a result of mass transport. Deposition of a thin Al2O3 layer on ZnO prior to Al deposition effectively prevents Al penetration and Zn release, requiring hig...

In situ infrared spectroscopy and ab initio density functional theory (DFT) calculations are combined to study the interaction of the corrosive gases SO2 and NO2 with metal organic frameworks M-MOF-74 (M = Zn, Mg, Ni, Co). We find that NO2 dissociatively adsorbs into MOF-74 compounds, forming NO and NO3–. The mechanism is unraveled by considering the Zn-MOF-74 system, for which DFT calculations show that a strong NO2–Zn bonding interaction induces a significant weakening of the N–O bond, facilitating the decomposition of the NO2 molecules. In contrast, SO2 is only molecularly adsorbed into MOF-74 with high binding energy (>90 kJ/mol for Mg-MOF-74 and >70 for Zn-MOF-74). This work gives insight into poisoning issues by minor components of flue gases in metal organic frameworks materials.

The Y-doped and pure CeO2 nanorods were tested as dehydrogenation catalysts in the oxidative dehydrogenation of cyclohexene and cyclohexane. The dehydrogenation process involves a redox process and the increased amount of oxygen vacancies improved the catalytic activity. At a temperature as low as 150 °C, compared to the temperature of ~600 °C required for vanadium catalysts, the Y-doped CeO2 nanorods exhibits a cyclohexane conversion of ~20% with a product distribution for 38% cyclohexene, 11% benzene, 1% 1,3-cyclohexanediene and 50% combustion products.

Section 26.2 discusses the essential concepts of infrared (IR) spectroscopy as it pertains to studies of surfaces and interfaces, focusing on different potential measurement geometries and the type of information that can be extracted from an IR absorption measurement. Section 26.3 provides a brief overview of low-energy ion scattering (LEIS)-based quantification of surface composition and thin film structure. The fundamentals of x-ray photoemission spectroscopy (XPS) have already been covered in a previous chapter and are therefore not addressed here. Section 26.4 presents examples from a variety of different fields to illustrate how the combination of these three techniques has yielded quantitative information on adsorption sites, bonding configurations, surface reactions, and mass transport. These examples include ultrashallow monolayer doping of semiconductors, growth of ultrathin metal oxide layers, characterization of catalyst model surface structure, reactivity of transition-metal surfaces with nitrogen and hydrogen plasmas, dielectric surface etching and functionalization, and gas storage in nanoporous materials.

International audience; This work proposes a new efficient, long-lasting scalable and low-cost triptych photocatalyst by assembling a semiconductor thin film (planar anatase TiO2), a photosensitive molecule of the carbo-benzene (Cbz) family and plasmonic Ag nanoparticles with exquisite degree of intimacy with the semiconductor. Under simulated sunlight conditions over 48 h, the triptych TiO2/Ag/Cbz photocatalyst allows a hydrogen production rate of 0.18 mmol gphotocatalyst−1 h−1 in conditions of applicative pressure (2.2 bars) and temperature (ambient) suitable for commercial applications. A ternary synergy (~33%) for hydrogen production is clearly evidenced with the triptych material in comparison with the diptych counterpart.The role of each component (TiO2, Ag and Cbz) on the H2 production is investigated systematically by discriminating the light absorption from the different materials and interfaces. We show how to achieve an efficient vertical Schottky junction between Ag nanoparticles and the TiO2 substrate that is demonstrated to be of crucial importance in the water-splitting process.

While a large variety of organic and molecular materials have been found to exhibit charge memory effects, the underlying mechanism is not well-understood, which hinders rational device design. Here, we study the charge retention mechanism of a nanoscale memory system, an organic monolayer on a silicon substrate, with Au nanoparticles on top serving as the electrical contact. Combining scanning probe imaging/manipulation and density functional simulations, we observe stable charge retention effects in the system and attributed it to polaron effects at the amine functional groups. Our findings can pave the way for applications in nonvolatile memory, neuromorphic computing, and high dielectric breakdown devices. U.S. Department of Energy DE-AC02-05CH11231 U.S. National Science Foundation CHE-1300180 Marie Curie FP7 ILSES 612620 Nanotwinning FP7 NN294952

International audience; Gold nanoparticles (NPs) exhibit optical, catalytic, and physical properties that are scientifically fascinating and essential for many applications. However, the challenge is to synthesize and disperse ultra-small and highly stable NPs. We demonstrate here that isolated and ultra-small (~1 nm) Au NPs can be synthesized by photo-reduction of HAuCl4 inside thiol-functionalized MOFs, including MIL-101 (Cr) and a novel class of MOFs, MOF-808-SH. A combination of physical, imaging, spectroscopic measurements, and ab initio calculations confirms that the Au NPs are dispersed inside the bulk of the MOFs and demonstrate that the thiol group is critical to stabilize the Au NPs deeply inside individual pores which overcomes the major issue of external nucleation on the outer surfaces. The thiol-functionalized MOFs thus fulfill a dual purpose: they foster the nucleation of the Au NP and also provide confinement and a framework that keeps the NPs separate. Moreover, the stability of NPs is tested by a series of wet chemical processes (transmetalation) and photocatalytic water reduction. Spectroscopic and TEM studies of the system after these chemical tests show that the Au NPs are remarkably stable (

The correlation between lattice oxygen (O) binding energy and O oxidation activity imposes a fundamental limit in developing oxide catalysts, simultaneously meeting the stringent thermal stability and catalytic activity standards for complete oxidation reactions under harsh conditions. Typically, strong O binding indicates a stable surface structure, but low O oxidation activity, and

Nitrate is a frequent water pollutant that results from human activities such as fertilizer over-application and agricultural runoff and improper disposal of human and animals waste. Excess levels of nitrate in watersheds can trigger harmful algal blooms (HABs) and biodiversity loss with consequences that affect the economy and pose a threat to human health. Municipal drinking water and wastewater treatment plants are therefore required to control nitrogen levels to ensure the safety of drinking water and the proper discharge of effluent. Nitrate exhibits distinct absorption bands in the infrared spectral range. While infrared radiation is strongly attenuated in water, implementation of fiber optic evanescent wave spectroscopy (FEWS) enables monitoring of water contaminants in real-time with high sensitivity. This work outlines the development of a non-dispersive infrared (NDIR) detector for the real-time monitoring of nitrate, nitrite and ammonia concentrations targeting implementation at municipal wastewater treatment plants (WWTPs) and onsite wastewater treatment systems (OWTS).

Observation of energy transfer (ET) from multiexcitonic (MX) complexes in nanocrystal quantum dots (NQDs) has been severely restricted due to efficient nonradiative Auger recombination leading to very low MX emission quantum yields. Here we employed "giant" CdSe/CdS NQDs with suppressed Auger recombination to study ET of biexcitons (BX) and charged excitons (trions) into Si substrate. Photoluminescence (PL) measurements of (sub)monolayers of gNQDs controllably assembled on various interacting surfaces and augmented by single gNQD's imaging reveal appearance of BX spectral signatures and progressive acceleration of PL lifetimes of all excitonic species on Si substrates. From statistical analysis of a large number of PL lifetime traces, representative exciton, trion and BX ET efficiencies are measured as ∼75%, 55% and 45% respectively. Detailed analysis of the MX's radiative rates demonstrate the crucial role of the radiative (waveguide) ET in maintaining high overall transfer efficiency despite the prevalent Auger recombination. Our observations point towards practical utilization of MX-bearing nanocrystals in future optoelectronics architectures.

Integration of colloidal nanocrystal quantum dots (NQDs) with strongly absorbing semiconductors offers the possibility of developing optoelectronic and photonic devices with new functionalities. We examine the process of energy transfer (ET) from photoactive CdSe/ZnS core/shell NQDs into lead-halide perovskite polycrystalline films as a function of distance from the perovskite surface using time-resolved photoluminescence (TRPL) spectroscopy. We demonstrate near-field electromagnetic coupling between vastly dissimilar excitation in two materials that can reach an efficiency of 99% at room temperature. Our experimental results, combined with electrodynamics modeling, reveal the leading role of non-radiative ET at close distances, augmented by the waveguide emission coupling and light reabsorption at separations >10 nm. These results open the way to combining materials with different dimensionalities to achieve novel nanoscale architectures with improved photovoltaic and light emitting functionalities.

Carbon nanofibers with high surface area have become promising electrode materials for supercapacitors because of their importance in increasing energy density. In this study, a high free volume polymer, 6FDA-DAM:DABA (6FDD) was blended with polybenzimidazole (PBI) in different ratios to obtain different compositions of PBI/6FDD immiscible polymer blends. Freestanding nanofiber mats were obtained via electrospinning using blend precursors dissolved in N,N-dimethylacetamide (DMAc). Subsequently, carbonization, followed by CO2 activation at 1000 °C was applied to convert the fiber mats into porous carbon nanofibers (CNFs). The addition of 6FDD shows significant effects on the microstructure and enhancement of the surface area of the CNFs. The obtained CNFs show specific surface area as high as 3010 m2 g−1 with pore sizes comparable to those of the electrolyte ions (PYR14TFSI). This provides good electrolyte accessibility to the pore of the carbon materials resulting in enhanced energy density compared to the CNFs obtained from pure PBI. Electrodes derived from PBI:6FDD (70 : 30) exhibited outstanding supercapacitor performance in coin cells with a specific capacitance of 142 F g−1 at the scan rate of 10 mV s−1 and energy density of 67.5 W h kg−1 at 1 A g−1 (58 W h kg−1 at 10 A g−1) thus demonstrating promising electrochemical performance for high performance energy storage system.

Bio-inspired non-precious-metal catalysts based on iron and cobalt porphyrins are promising alternatives to replace costly platinum-based catalysts for oxygen reduction reaction (ORR) in fuel cells. However, the exact nature of the active sites is still not clearly understood, and further optimization design is needed for practical applications. Here, we report a rational catalyst design process by combining density functional theory (DFT) calculations and experimental validations. Two sets of square-planar (MNxC4−x) and square-pyramid (MNxC5−x) active centers (M=Mn, Fe, Co, Ni) incorporated in graphene were examined using DFT. Fe-N5 and Co-N4 sites were identified theoretically to have the best performance in fuel cells, while Ni-NxC4−x sites catalyze the most H2O2 byproduct. Graphene samples with well-dispersed incorporations of metals were synthesized, and the following electrochemical measurements show an excellent agreement with the theoretical predictions, indicating that a successful design framework and systematic understanding toward the catalytic nature of these materials are established.

The main challenge for gas storage and separation in nanoporous materials is that many molecules of interest adsorb too weakly to be effectively retained. Instead of synthetically modifying the internal surface structure of the entire bulk—as is typically done to enhance adsorption—here we show that post exposure of a prototypical porous metal-organic framework to ethylenediamine can effectively retain a variety of weakly adsorbing molecules (for example, CO, CO2, SO2, C2H4, NO) inside the materials by forming a monolayer-thick cap at the external surface of microcrystals. Furthermore, this capping mechanism, based on hydrogen bonding as explained by ab initio modelling, opens the door for potential selectivity. For example, water molecules are shown to disrupt the hydrogen-bonded amine network and diffuse through the cap without hindrance and fully displace/release the retained small molecules out of the metal-organic framework at room temperature. These findings may provide alternative strategies for gas storage, delivery and separation., Metal-organic frameworks are extensively studied for gas storage applications, but one potential limitation is their relatively weak adsorption of gases. Here, the authors report that the exposure of metal-organic frameworks to ethylenediamine forms a monolayer thick cap which improves gas molecule retention.

In an effort to grow metal oxide films (e.g., MoO3) at low temperatures, a novel molybdenum precursor, Si(CH3)3CpMo(CO)2(η3-2-methylallyl) or MOTSMA, is used with ozone as the coreactant. As is often observed in atomic layer deposition (ALD) processes, the deposition of molybdenum trioxide displays an incubation period (∼15 cycles at 250 °C). In situ FTIR spectroscopy reveals that ligand exchange reactions can be activated at 300 °C, leading to a shorter incubation periods (e.g., ∼ 9 cycles). Specifically, the reaction of MOTSMA with OH-terminated silicon oxide surfaces appears to be the rate limiting step, requiring a higher temperature activation (350 °C) than the subsequent ALD process itself, for which 250 °C is adequate. Therefore, in order to overcome the nucleation delay, the MOTSMA precursor is initially grafted at 350 °C, with spectroscopic evidence of surface reaction, and the substrate temperature then lowered to 250 or 300 °C for the rest of the ALD process. After this initial activation, a st...

Aqueous-processed aluminum oxide phosphate (AlPO) dielectric films were studied to determine how water desorbs and absorbs on heating and cooling, respectively. In-situ Fourier transform infrared spectroscopy showed a distinct, reversible mono- to bidentate phosphate structural change associated with water loss and uptake. Temperature programmed desorption measurements on a 1-μm thick AlPO film revealed water sorption was inhibited by an aqueous-processed HfO2 capping film only 11-nm thick. The HfO2 capping film prevents water resorption, thereby preserving the exceptional performance of AlPO as a thin-film dielectric.

The selectivity of clean Si(100)-(2 × 1) surfaces fully reacted with H2O- and hydrogen-passivated Si(100)-(2 × 1) surfaces is investigated for atomic layer deposition (ALD) of TiO2, Al2O3, and HfO2 using TiCl4, TMA, or TDMA-Hf precursors with H2O, respectively, in an ultra-high-vacuum (UHV) environment. The initial reaction probability is estimated by determining the minimum exposure necessary for complete reaction of the metal precursors on both H2O-reacted and H-passivated Si(100)-(2 × 1) surfaces and examining the first full cycle of the ALD process for each oxide. Under these UHV conditions, the first cycle selectivity is 17:1 for TiO2, 37:1 for Al2O3, and only 4:3 for HfO2. Additionally, TMA is found to react with approximately half of the Si-H sites in addition to all the Si-OH sites, while TiCl4 and TDMA-Hf gases are found to react principally with the surface −OH on H2O-reacted Si(100) surfaces with no reaction with the −H sites.

International audience; The DNA-directed assembly of nano-objects has been the subject of many recent studies as a means to construct advanced nanomaterial architectures. Although much experimental in silico work has been presented and discussed, there has been no in-depth consideration of the proper design of single-strand sticky termination of DNA sequences, noted as ssST, which is important in avoiding self-folding within one DNA strand, unwanted strand-to-strand interaction, and mismatching. In this work, a new comprehensive and computationally efficient optimization algorithm is presented for the construction of all possible DNA sequences that specifically prevents these issues. This optimization procedure is also effective when a spacer section is used, typically repeated sequences of thymine or adenine placed between the ssST and the nano-object, to address the most conventional experimental protocols. We systematically discuss the fundamental statistics of DNA sequences considering complementarities limited to two (or three) adjacent pairs to avoid self-folding and hybridization of identical strands due to unwanted complements and mismatching. The optimized DNA sequences can reach maximum lengths of 9 to 34 bases depending on the level of applied constraints. The thermodynamic properties of the allowed sequences are used to develop a ranking for each design. For instance, we show that the maximum melting temperature saturates with 14 bases under typical solvation and concentration conditions. Thus, DNA ssST with optimized sequences are developed for segments ranging from 4 to 40 bases, providing a very useful guide for all technological protocols. An experimental test is presented and discussed using the aggregation of Al and CuO nanoparticles and is shown to validate and illustrate the importance of the proposed DNA coding sequence optimization. ■ INTRODUCTION The interest in DNA nanotechnology to program the assembly of nanoparticles into macroscopic nanocomposites emerged in the 1990s. 1,2 Undoubtedly, the controlled interplay of DNA complementary and noncomplementary strands made DNA nanotechnologies one of the most powerful bottom-up approaches to building hierarchical architectures of nano-objects (noble metals, semiconductors, oxides, and polymers) leading to an almost infinite variety of high-performance programmable DNA/nanoparticles hybrid materials. One mainstream DNA-based assembly approach consists of directing the assembly of colloids of interest, mostly gold nanoparticles, by taking advantage of the thiol/metal chemistry 3 to covalently attach DNA strands to nanoparticle surfaces. Other chemical alternatives have also been investigated , such as antigen/antibody-like binding. 4,5 Since the seminal work by Alavisatos and Mirkin 1,2 on gold nanoparticles, many DNA/nanoparticle assembly processes have been reported, notably by varying the DNA length and processing conditions and with a consideration of other materials for applications in catalytics, 6 spectroscopy, 7−10 optical devices

Amination of surfaces is useful in a variety of fields, ranging from device manufacturing to biological applications. Previous studies of ammonia reaction on silicon surfaces have concentrated on vapor phase rather than wet chemical processes, and mostly on clean Si surfaces. In this work, the interaction of liquid and vapor-phase ammonia is examined on three types of oxide-free surfaces – passivated by hydrogen, fluorine (1/3 monolayer) or chlorine – combining infrared absorption spectroscopy, X-ray photoelectron spectroscopy, and first-principles calculations. The resulting chemical composition highly depends on the starting surface; there is a stronger reaction on both F- and Cl-terminated than on the H-terminated Si surfaces, as evidenced by the formation of Si-NH2. Side reactions can also occur, such as solvent reaction with surfaces, formation of ammonium salt by-products (in the case of 0.2 M ammonia in dioxane solution), and nitridation of silicon (in the case of neat and gas-phase ammonia reactions for instance). Unexpectedly, there is formation of Si-H bonds on hydrogen-free Cl-terminated Si(111) surfaces in all cases, whether vapor phase of neat liquid ammonia is used. The first-principles modeling of this complex system suggests that step-edge surface defects may play a key role in enabling the reaction under certain circumstances, despite the endothermic nature for Si-H bond formation.