Synthesis of Hafnium Oxide-Gold Core–Shell Nanoparticles

Developing cheap composite nanoparticle systems that combines a high dielectric constant with good conductivity is important for the future of the electronic industry. In this study, two different sizes, 7.3 ± 2.2 and 5.6 ± 1.9 nm, of HfO₂@Au core–shell nanoparticles are prepared by using a high-temperature reduction method. The core–shell nanoparticles are characterized by powder X-ray diffraction, high-resolution transmission electron microscopy (HRTEM), energy dispersive X-ray analysis (EDX), and UV–visible absorption spectroscopy. HfO₂ exhibits no absorption in the visible region, but the HfO₂@Au core–shell nanoparticles show a plasmon absorption band at 555 nm that is 25 nm red-shifted as compared to pure gold nanoparticles. According to transmission electron microscopy and energy dispersive X-ray analysis, the HfO₂ particles are coated with approximately three atomic layers of gold

Fe and Ni Dopants Facilitating Ammonia Synthesis on Mn₄N and Mechanistic Insights from First-Principles Methods

Cyclic step-catalysis enables intermittent, atmospheric ammonia production, and can be integrated with sustainable and renewable energy sources. By employing metal (e.g., Mn) nitride, a nitrogen carrier, the rate-limiting N₂ activation step is bypassed. In this work, molecular-level pathways, describing the reduction of Mn₄N by dissociatively adsorbed hydrogen, were investigated using periodic density functional theory (DFT). The established mechanism confirmed that Fe and Ni doped in the nitride sublayer and top layer can disturb local electronic structures and be exploited to tune the ammonia production activity. The strength of N–M (M = Mn, Fe, Ni) and H–M bonds both determine the overall reduction thermochemistry. DFT-based modeling further showed that the low concentration of Fe or Ni in the Mn₄N sublayer facilitates N diffusion by lowering the diffusion energy barrier. Also, these heteroatom dopant species, particularly Ni, decrease the reduction endergonicity, thanks to the strong hydrogen binding with the surface Ni dopant. The Brønsted–Evans–Polanyi relationship and linear scaling relationships have been developed to reveal ammonia evolution kinetic and energetic trends for a series of idealized Fe- and Ni-doped Mn₄N. Deviations from the linear scaling relationship have been observed for certain doped systems, indicating potentially more complex behaviors of metal nitrides and intriguing promises for greater ammonia synthesis materials design opportunities

Rapid Induction and Microwave Heat-Up Syntheses of CdSe Quantum Dots

The production of nanoparticles on an industrial scale requires an approach other than the widely used hot-injection method. In this work, two heat-up methods are applied to nanoparticle synthesis. The induction heating method produces CdSe quantum dots with ultrasmall properties in seconds. Initial flow-through experiments demonstrate that induction heating continuously produces quantum dots. These results are compared with those from microwave synthesis, which produces quantum dots on a longer timescale but provides fast, continuous heating. Both methods can produce quantum dots within seconds because of rapid heating. In addition, different precursors, single source and separate source, give different results, ultimately providing a handle to control quantum dot properties

Investigation of Charge Transfer Interactions in CdSe Nanorod P3HT/PMMA Blends by Optical Microscopy

CdSe nanorods and dilute poly(3-hexylthiophene) (P3HT) dispersed in poly(methyl methacrylate) (PMMA) films are investigated by wide field fluorescence microscopy and by analysis of single-point time transients. The data depict enhanced band-edge luminescence from nanorod/P3HT films, consistent with filling of CdSe surface traps by static charge transfer from P3HT. Band-edge luminescence from the nanorods is also shown to be enhanced by photoactivated (e.g., dynamic) charge transfer from P3HT to surface trap states on the CdSe nanorods. The role played by charge transfer in enhanced CdSe luminescence is further demonstrated by differences observed in the CdSe nanorod blinking behavior in P3HT and PMMA films

Investigation of Fluorescence Emission from CdSe Nanorods in PMMA and P3HT/PMMA Films

Complementary fluorescence microscopy and ultrafast transient absorption spectroscopy measurements spanning a range of time scales from seconds to femtoseconds probe the interfacial dynamics of charge carriers in CdSe nanorod/polymer blends. Together, these very different techniques provide new information about the origin and dynamics of below-band-edge emission from CdSe nanorods in CdSe/PMMA and CdSe/P3HT/PMMA films [PMMA = poly­(methyl methacrylate); P3HT = poly­(3-hexylthiophene)]. Emission below the band edge of the CdSe nanorods is associated with surface defects (traps) at the nanoparticle/polymer interface, where conduction band electrons radiatively relax to the intraband defect sites. The fluorescence microscopy experiments simultaneously monitor both the trap emission and the band edge emission from single nanoparticles, and reveal that the two emission channels are distinct. Transitions between the two emissive states occur on time scales longer than ∼20 ms, and always involve an intermediate dark state in which no emission is observed. The presence of P3HT increases the relative band edge emission intensity and reduces the fluorescence intermittency (blinking) of both emissive states. The ultrafast transient absorption experiments monitor the evolution of a stimulated emission band below the CdSe band edge following excitation of P3HT. The measurements reveal ultrafast electron transfer from photoexcited P3HT to the CdSe nanorods within the instrument response time of approximately 140 fs, and confirm that there is strong coupling between the nanorods and P3HT in these dilute blends. Analysis of separate CdSe nanorod etching experiments suggests that the trap states are formed by the removal of atoms from the ends of the nanorods in the presence of chloroform. Mechanisms for charge trapping at the nanoparticle/polymer interface are discussed

Investigation of Fluorescence Emission from CdSe Nanorods in PMMA and P3HT/PMMA Films

Complementary fluorescence microscopy and ultrafast transient absorption spectroscopy measurements spanning a range of time scales from seconds to femtoseconds probe the interfacial dynamics of charge carriers in CdSe nanorod/polymer blends. Together, these very different techniques provide new information about the origin and dynamics of below-band-edge emission from CdSe nanorods in CdSe/PMMA and CdSe/P3HT/PMMA films [PMMA = poly­(methyl methacrylate); P3HT = poly­(3-hexylthiophene)]. Emission below the band edge of the CdSe nanorods is associated with surface defects (traps) at the nanoparticle/polymer interface, where conduction band electrons radiatively relax to the intraband defect sites. The fluorescence microscopy experiments simultaneously monitor both the trap emission and the band edge emission from single nanoparticles, and reveal that the two emission channels are distinct. Transitions between the two emissive states occur on time scales longer than ∼20 ms, and always involve an intermediate dark state in which no emission is observed. The presence of P3HT increases the relative band edge emission intensity and reduces the fluorescence intermittency (blinking) of both emissive states. The ultrafast transient absorption experiments monitor the evolution of a stimulated emission band below the CdSe band edge following excitation of P3HT. The measurements reveal ultrafast electron transfer from photoexcited P3HT to the CdSe nanorods within the instrument response time of approximately 140 fs, and confirm that there is strong coupling between the nanorods and P3HT in these dilute blends. Analysis of separate CdSe nanorod etching experiments suggests that the trap states are formed by the removal of atoms from the ends of the nanorods in the presence of chloroform. Mechanisms for charge trapping at the nanoparticle/polymer interface are discussed

Investigation of Fluorescence Emission from CdSe Nanorods in PMMA and P3HT/PMMA Films

Complementary fluorescence microscopy and ultrafast transient absorption spectroscopy measurements spanning a range of time scales from seconds to femtoseconds probe the interfacial dynamics of charge carriers in CdSe nanorod/polymer blends. Together, these very different techniques provide new information about the origin and dynamics of below-band-edge emission from CdSe nanorods in CdSe/PMMA and CdSe/P3HT/PMMA films [PMMA = poly­(methyl methacrylate); P3HT = poly­(3-hexylthiophene)]. Emission below the band edge of the CdSe nanorods is associated with surface defects (traps) at the nanoparticle/polymer interface, where conduction band electrons radiatively relax to the intraband defect sites. The fluorescence microscopy experiments simultaneously monitor both the trap emission and the band edge emission from single nanoparticles, and reveal that the two emission channels are distinct. Transitions between the two emissive states occur on time scales longer than ∼20 ms, and always involve an intermediate dark state in which no emission is observed. The presence of P3HT increases the relative band edge emission intensity and reduces the fluorescence intermittency (blinking) of both emissive states. The ultrafast transient absorption experiments monitor the evolution of a stimulated emission band below the CdSe band edge following excitation of P3HT. The measurements reveal ultrafast electron transfer from photoexcited P3HT to the CdSe nanorods within the instrument response time of approximately 140 fs, and confirm that there is strong coupling between the nanorods and P3HT in these dilute blends. Analysis of separate CdSe nanorod etching experiments suggests that the trap states are formed by the removal of atoms from the ends of the nanorods in the presence of chloroform. Mechanisms for charge trapping at the nanoparticle/polymer interface are discussed

Exciton Dynamics in MoS₂‑Pentacene and WSe₂‑Pentacene Heterojunctions

We measured the exciton dynamics in van der Waals heterojunctions of transition metal dichalcogenides (TMDCs) and organic semiconductors (OSs). TMDCs and OSs are semiconducting materials with rich and highly diverse optical and electronic properties. Their heterostructures, exhibiting van der Waals bonding at their interfaces, can be utilized in the field of optoelectronics and photovoltaics. Two types of heterojunctions, MoS2-pentacene and WSe2-pentacene, were prepared by layer transfer of 20 nm pentacene thin films as well as MoS2 and WSe2 monolayer crystals onto Au surfaces. The samples were studied by means of transient absorption spectroscopy in the reflectance mode. We found that A-exciton decay by hole transfer from MoS2 to pentacene occurs with a characteristic time of 21 ± 3 ps. This is slow compared to previously reported hole transfer times of 6.7 ps in MoS2-pentacene junctions formed by vapor deposition of pentacene molecules onto MoS2 on SiO2. The B-exciton decay in WSe2 shows faster hole transfer rates for WSe2-pentacene heterojunctions, with a characteristic time of 7 ± 1 ps. The A-exciton in WSe2 also decays faster due to the presence of a pentacene overlayer; however, fitting the decay traces did not allow for the unambiguous assignment of the associated decay time. Our work provides important insights into excitonic dynamics in the growing field of TMDC-OS heterojunctions