Aminoalkanoic Acids as Alternatives to Mercaptoalkanoic Acids for the Linker-Assisted Attachment of Quantum Dots to TiO₂

Linear aminoalkanoic acids (AAAs) and mercaptoalkanoic acids (MAAs) were characterized as bifunctional ligands to tether CdSe QDs to nanocrystalline TiO₂ thin films and to mediate excited-state electron transfer (ET) from the QDs to TiO₂ nanoparticles. The adsorption of 12-aminododecanoic acid (ADA) and 12-mercaptododecanoic acid (ADA) to TiO₂ followed the Langmuir adsorption isotherm. Surface adduct formation constants (<i>K</i>_ad) were ∼10⁴ M^–1; saturation amounts of the ligands per projected surface area of TiO₂ (Γ₀) were ∼10^–7 mol cm^–2. Both <i>K</i>_ad and Γ₀ differed by 20% or less for the two linkers. CdSe QDs adhered to ADA- and MDA-functionalized TiO₂ films; data were well modeled by the Langmuir adsorption isotherm and Langmuir kinetics. For ADA- and MDA-mediated assembly values of <i>K</i>_ad were (1.8 ± 0.4) × 10⁶ and (2.4 ± 0.4) × 10⁶ M^–1, values of Γ₀ were (1.6 ± 0.3) × 10^–9 and (1.2 ± 0.1) × 10^–9 mol cm^–2, and rate constants were (14 ± 5) and (60 ± 20) M^–1 s^–1, respectively. Thus, the thermodynamics and kinetics of linker-assisted assembly were slightly more favorable for MDA than for ADA. Steady-state and time-resolved emission spectroscopy revealed that electrons were transferred from both band-edge and surface states of CdSe QDs to TiO₂ with rate constants (<i>k</i>_et) of ∼10⁷ s^–1. ET was approximately twice as fast through thiol-bearing linker 4-mercaptobutyric acid (MBA) as through amine-bearing linker 4-aminobutyric acid (ABA). Photoexcited QDs transferred holes to adsorbed MBA. In contrast, ABA did not scavenge photogenerated holes from CdSe QDs, which maximized the separation of charges following ET. Additionally, ABA shifted electron-trapping surface states to higher energies, minimizing the loss of potential energy of electrons prior to ET. These trade-offs involving the kinetics and thermodynamics of linker-assisted assembly; the driving force, rate constant, and efficiency of ET; and the extent of photoinduced charge separation can inform the selection bifunctional ligands to tether QDs to surfaces

Excited-State Charge Transfer within Covalently Linked Quantum Dot Heterostructures

We synthesized quantum dot (QD) heterostructures via the <i>N</i>,<i>N</i>′-dicyclohexylcarbodiimide-mediated formation of amide bonds between capping ligands on CdS QDs and CdSe QDs. Products of ligand-exchange and coupling reactions were characterized by FTIR, ¹H NMR, transmission electron micrscopy, and electronic absorption and emission spectroscopy. This cross-linking strategy yields exclusively heterostructures and prohibits the undesired formation of homostructures consisting of a single type of QD. The ground-state absorption spectra of the presynthesized colloidal QDs were unperturbed upon incorporation into heterostructures. Photoexcited CdS QDs transferred holes to molecularly tethered CdSe QDs, as evidenced by significant dynamic quenching of the trap-state emission from CdS QDs and the rapid (<10^–8 s) growth of a broad and long-lived (>10^–5 s) transient absorption band in the visible region. These spectral signatures were absent for mixed dispersions of noninteracting CdS and CdSe QDs. Our results reveal that carbodiimide coupling chemistry can be used to tether colloidal QDs selectively and covalently to each other and that the resulting heterostructures can undergo efficient photoinduced interfacial charge transfer

Integrating β‑Pb_0.33V₂O₅ Nanowires with CdSe Quantum Dots: Toward Nanoscale Heterostructures with Tunable Interfacial Energetic Offsets for Charge Transfer

Achieving directional charge transfer across semiconductor interfaces requires careful consideration of relative band alignments. Here, we demonstrate a promising tunable platform for light harvesting and excited-state charge transfer based on interfacing β-Pb_<i>x</i>V₂O₅ nanowires with CdSe quantum dots. Two distinct routes are developed for assembling the heterostructures: linker-assisted assembly mediated by a bifunctional ligand and successive ionic layer adsorption and reaction (SILAR). In the former case, the thiol end of a molecular linker is found to bind to the quantum dot surfaces, whereas a protonated amine moiety interacts electrostatically with the negatively charged nanowire surfaces. In the alternative SILAR route, the surface coverage of CdSe nanostructures on the β-Pb_<i>x</i>V₂O₅ nanowires is tuned by varying the number of successive precipitation cycles. High-energy valence band X-ray photoelectron spectroscopy measurements indicate that “mid-gap” states of the β-Pb_<i>x</i>V₂O₅ nanowires derived from the stereoactive lone pairs on the intercalated lead cations are closely overlapped in energy with the valence band edges of CdSe quantum dots that are primarily Se 4p in origin. Both the midgap states and the valence-band levels are in principle tunable by variation of cation stoichiometry and particle size, respectively, providing a means to modulate the thermodynamic driving force for charge transfer. Steady-state and time-resolved photoluminescence measurements reveal dynamic quenching of the trap-state emission of CdSe quantum dots upon exposure to β-Pb_<i>x</i>V₂O₅ nanowires. This result is consistent with a mechanism involving the transfer of photogenerated holes from CdSe quantum dots to the midgap states of β-Pb_<i>x</i>V₂O₅ nanowires

Programming Interfacial Energetic Offsets and Charge Transfer in β‑Pb_0.33V₂O₅/Quantum-Dot Heterostructures: Tuning Valence-Band Edges to Overlap with Midgap States

Semiconductor heterostructures for solar energy conversion interface light-harvesting semiconductor nanoparticles with wide-band-gap semiconductors that serve as charge acceptors. In such heterostructures, the kinetics of charge separation depend on the thermodynamic driving force, which is dictated by energetic offsets across the interface. A recently developed promising platform interfaces semiconductor quantum dots (QDs) with ternary vanadium oxides that have characteristic midgap states situated between the valence and conduction bands. In this work, we have prepared CdS/β-Pb_0.33V₂O₅ heterostructures by both linker-assisted assembly and surface precipitation and contrasted these materials with CdSe/β-Pb_0.33V₂O₅ heterostructures prepared by the same methods. Increased valence-band (VB) edge onsets in X-ray photoelectron spectra for CdS/β-Pb_0.33V₂O₅ heterostructures relative to CdSe/β-Pb_0.33V₂O₅ heterostructures suggest a positive shift in the VB edge potential and, therefore, an increased driving force for the photoinduced transfer of holes to the midgap state of β-Pb_0.33V₂O₅. This approach facilitates a ca. 0.40 eV decrease in the thermodynamic barrier for hole injection from the VB edge of QDs suggesting an important design parameter. Transient absorption spectroscopy experiments provide direct evidence of hole transfer from photoexcited CdS QDs to the midgap states of β-Pb_0.33V₂O₅ NWs, along with electron transfer into the conduction band of the β-Pb_0.33V₂O₅ NWs. Hole transfer is substantially faster and occurs at <1-ps time scales, whereas completion of electron transfer requires 530 ps depending on the nature of the interface. The differentiated time scales of electron and hole transfer, which are furthermore tunable as a function of the mode of attachment of QDs to NWs, provide a vital design tool for designing architectures for solar energy conversion. More generally, the approach developed here suggests that interfacing semiconductor QDs with transition-metal oxide NWs exhibiting intercalative midgap states yields a versatile platform wherein the thermodynamics and kinetics of charge transfer can be systematically modulated to improve the efficiency of charge separation across interfaces