A New Mechanism for Metal-Catalyzed Stannane Dehydrocoupling Based on α-H-Elimination in a Hafnium Hydrostannyl Complex

The hafnium hydrostannyl complex CpCp*Hf(SnHMes2)Cl (Cp* = η5-C5Me5) has been synthesized from the reaction of CpCp*Hf(H)Cl with Mes2SnH2. This compound has been identified as an intermediate in the metal-catalyzed dehydrocoupling of Mes2SnH2 to the distannane Mes2HSnSnHMes2 (Mes = 2,4,6-trimethylphenyl). The dehydrocoupling in this system appears to occur by elimination of :SnMes2 from CpCp*Hf(SnHMes2)Cl, with Sn−Sn bond formation proceeding via insertion of the stannylene into a Sn−H bond

All First Row Transition Metal Oxide Photoanode for Water Splitting Based on Cu₃V₂O₈

Identification of viable photoanode candidates for use in a tandem photoelectrochemical water splitting system remains a significant challenge to the realization of efficient solar-driven hydrogen production. Herein, copper vanadate (Cu₃V₂O₈) is introduced as a new, all first row transition metal oxide with a band gap of near 2 eV that makes it suitable as a photoanode candidate in such a solar water splitting system. In this work, many of the key physical and photoelectrochemical properties of Cu₃V₂O₈ are established including band gap, doping type, ability to extrinsically dope, flat-band potential, band positions, electron diffusion length, chemical stability, and O₂ evolution faradaic efficiency. This study provides a key initial step in identifying the features that can lead to a complete understanding of this new ternary metal oxide and motivate discovery of related photoanodes comprised of multicomponent oxides

A New Mechanism for Metal-Catalyzed Stannane Dehydrocoupling Based on α-H-Elimination in a Hafnium Hydrostannyl Complex

The hafnium hydrostannyl complex CpCp*Hf(SnHMes2)Cl (Cp* = η5-C5Me5) has been synthesized from the reaction of CpCp*Hf(H)Cl with Mes2SnH2. This compound has been identified as an intermediate in the metal-catalyzed dehydrocoupling of Mes2SnH2 to the distannane Mes2HSnSnHMes2 (Mes = 2,4,6-trimethylphenyl). The dehydrocoupling in this system appears to occur by elimination of :SnMes2 from CpCp*Hf(SnHMes2)Cl, with Sn−Sn bond formation proceeding via insertion of the stannylene into a Sn−H bond

Synthesis, Structure, and α-Elimination Chemistry of Hafnocene Triarylstannyl Complexes

New hafnocene triarylstannyl complexes were prepared and were shown to undergo clean thermal decompositions via α-aryl-elimination to produce the corresponding stannylene and a hafnocene aryl complex. The rate of the decomposition is highly dependent on the nature of the ancillary ligand, with the stabilities of the CpCp*Hf(SnPh3)X compounds following the order X = NMe2 > Np (α-agostic) > OMe > Cl > Me. Mechanistic information suggests that α-aryl-elimination may be viewed as a concerted process involving nucleophilic attack of the migrating aryl group onto the electrophilic metal center

Nonthermal Plasma-Synthesized Phosphorus–Boron co-Doped Si Nanocrystals: A New Approach to Nontoxic NIR-Emitters

We report on the successful creation of nonthermal plasma-synthesized phorphorus and boron co-doped Si nanocrystals (PB:Si NCs) with diameters (DNC) ranging from 2.9 to 7.3 nm. Peak photoluminescence (PL) emission energies for all PB:Si NC diameters are ca. 400–500 meV lower than the excitonic emission values in intrinsic Si NCs, which can be attributed to prevalent donor–acceptor (D–A) transitions within the co-doped system. This D–A transition model is further evidenced by PL lifetimes within the range of 40–80 μs, faster than what is observed for intrinsic Si NCs. By reducing the level of confinement within PB:Si NCs (i.e., DNC > 4 nm), we are able to red-shift the near-infrared (NIR)-emitting D–A transitions to below the band gap of bulk Si (1.12 eV). We quantify the PL quantum yield (PLQY) for a range of DNC and show that the plasma method can achieve reasonably high PLQY values (12% for DNC = 2.9 nm), even without any optimization of the synthesis or surface chemistry. We posit that perfect charge compensation cannot explain these results and propose a model in which dominant n-type doping accounts for the observations. Ultimately, these results demonstrate that nonthermal plasma synthesis is a viable pathway for preparing PB:Si NCs featuring NIR sub-band gap D–A transitions with relatively high quantum yields. More generally, this study provides insight into how doping affects energy and charge transfer within quantum-confined systems

Tuning Confinement in Colloidal Silicon Nanocrystals with Saturated Surface Ligands

The optical properties of silicon nanocrystals (Si NCs) are a subject of intense study and continued debate. In particular, Si NC photoluminescence (PL) properties are known to depend strongly on the surface chemistry, resulting in electron–hole recombination pathways derived from the Si NC band-edge, surface-state defects, or combined NC-conjugated ligand hybrid states. In this Letter, we perform a comparison of three different <i>saturated</i> surface functional groupsalkyls, amides, and alkoxideson nonthermal plasma-synthesized Si NCs. We find a systematic and <i>size-dependent</i> high-energy (blue) shift in the PL spectrum of Si NCs with amide and alkoxy functionalization relative to alkyl. Time-resolved photoluminescence and transient absorption spectroscopies reveal no change in the excited-state dynamics between Si NCs functionalized with alkyl, amide, or alkoxide ligands, showing for the first time that saturated ligandsnot only surface-derived charge-transfer states or hybridization between NC and low-lying ligand orbitalsare responsible for tuning the Si NC optical properties. To explain these PL shifts we propose that the atom bound to the Si NC surface strongly interacts with the Si NC electronic wave function and modulates the Si NC quantum confinement. These results reveal a potentially broadly applicable correlation between the optoelectronic properties of Si NCs and related quantum-confined structures based on the interaction between NC surfaces and the ligand binding group

Negligible Electronic Interaction between Photoexcited Electron–Hole Pairs and Free Electrons in Phosphorus–Boron Co-Doped Silicon Nanocrystals

Phosphorus (P) and boron (B) co-doped Si nanocrystals (NCs) have raised interest in the optoelectronic industry due to their electronic tunability, optimal carrier multiplication properties, and straightforward dispersibility in polar solvents. Yet a basic understanding of the interaction of photoexcited electron–hole (e–h) pairs with new physical features that are introduced by the co-doping process (free carriers, defect states, and surface chemistry) is missing. Here, we present the first study of the ultrafast carrier dynamics in SiO₂-embedded P–B co-doped Si NC ensembles using induced absorption spectroscopy through a two-step approach. First, the induced absorption data show that the large fraction of the dopants residing on the NC surface slows down carrier relaxation dynamics within the first 20 ps relative to intrinsic (undoped) Si NCs, which we interpret as enhanced surface passivation. On longer time-scales (picosecond to nanosecond regime), we observe a speeding up of the carrier relaxation dynamics and ascribe it to doping-induced trap states. This argument is deduced from the second part of the study, where we investigate multiexciton interactions. From a stochastic modeling approach we show that localized carriers, which are introduced by the P or B dopants, have minor electronic interactions with the photoexcited e–h pairs. This is understood in light of the strong localization of the introduced carriers on their original P- or B-dopant atoms, due to the strong quantum confinement regime in these relatively small NCs (<6 nm)

Size-Dependent Asymmetric Auger Interactions in Plasma-Produced n- and p‑Type-Doped Silicon Nanocrystals

Nonradiative Auger recombination (AR) tends to dominate carrier dynamics in charged, quantum-confined structures. Consequently, it complicates the practical realization of many semiconductor nanocrystal (NC)-based devices such as light-emitting diodes, photovoltaic cells, and single-photon emitters, in which charged exciton states often occur. To this end, extensive experimental studies on direct band gap NCs have investigated the trion components (both positive and negative) that construct the total AR rate. However, such an analysis has remained elusive for indirect band gap Si NCs. In this study, we investigate AR decay of non-thermal plasma-produced n- and p-type-doped Si NCs. We expand the study over a large NC size range (DNC ≈ 3–8 nm), in which n- and p-type doping is achieved by either a substitutional or surface doping effect, respectively. First, we monitor the AR of charge-neutral multiexcitons by measuring the biexciton lifetime (τXX) as a function of the NC size and doping configuration. We show that this method can be used to determine the presence of free carriers for any doped NC system, regardless of the presence/absence of defect channels in the carrier dynamics. Second, we develop a photophysical fitting model to determine the Auger lifetime of the simplest charged states in Si NCs: the negative (τX–) and positive (τX+) trions. Trion lifetimes shorten with increasing quantum confinement, as expected from (1) closer spatial proximity of the interacting charges and (2) increased relaxation of the momentum conservation rule. While both τX– and τX+ are in the nanosecond time regime (and both therefore completely dominate the carrier dynamics), AR with excess holes is faster. This asymmetry is explained by a higher density of valence band states in comparison to the conduction band states, due to effective mass differences between electrons and holes

Control of Plasmonic and Interband Transitions in Colloidal Indium Nitride Nanocrystals

We have developed a colloidal synthesis of 4–10 nm diameter indium nitride (InN) nanocrystals that exhibit both a visible absorption onset (∼1.8 eV) and a strong localized surface plasmon resonance absorption in the mid-infrared (∼3000 nm). Chemical oxidation and reduction reversibly modulate both the position and intensity of this plasmon feature as well as the band-to-band absorption onset. Chemical oxidation of InN nanocrystals with NOBF₄ is found to red-shift the absorption onset to ∼1.3 eV and reduce the plasmon absorption energy (to 3550 nm) and intensity (by an order of magnitude at 2600 nm). Reduction of these oxidized species with Bu₄NBH₄ fully recovers the original optical properties. Calculations suggest that the carrier density in these InN nanocrystals decreases upon oxidation from 2.89 × 10²⁰ cm^–3 to 2.51 × 10²⁰ cm^–3, consistent with the removal of ∼4 electrons per nanocrystal. This study provides a unique example of the ability to tune the optical properties of colloidal nanomaterials, and in particular the LSPR absorption, with reversible redox reactions that do not affect the semiconductor chemical composition or phase

Constructing Ordered Sensitized Heterojunctions: Bottom-Up Electrochemical Synthesis of p-Type Semiconductors in Oriented n-TiO₂ Nanotube Arrays

Fabrication of efficient semiconductor-sensitized bulk heterojunction solar cells requires the complete filling of the pore system of one semiconductor (host) material with nanoscale dimensions (2 in nanoporous anatase n-TiO2 oriented nanotube arrays and nanoparticle films. We show that by controlling the ambipolar diffusion length the p-type semiconductors can be deposited from the bottom-up, resulting in complete pore filling