A critical step involved in many applications of one-dimensional seeded CdSe@CdS nanorods, such as luminescent solar concentrators, optical gains, and photocatalysis, is the localization of excitons from the light-harvesting CdS nanorod antenna into the light-emitting CdSe quantum dot seed. We report that the rod-to-seed exciton localization efficiency decreases with the rod length but is independent of band alignment between the CdSe seed and CdS rod. This universal dependence can be well modeled by the competition between exciton one-dimensional diffusion to the CdSe seed and trapping on the CdS rod. This finding provides a rational approach for optimizing these materials for their various device applications.

Atomic layer deposition (ALD) is a powerful tool for achieving atomic level control in the deposition of thin films. However, several physical and chemical phenomena can occur which cause deviation from “ideal” film growth during ALD. Understanding the underlying mechanisms that cause these deviations is important to achieving even better control over the growth of the deposited material. Herein, we review several precursor chemisorption mechanisms and the effect of chemisorption on ALD growth. We then follow with a discussion on diffusion and its impact on film growth during ALD. Together, these two fundamental processes of chemisorption and diffusion underlie the majority of mechanisms which contribute to material growth during a given ALD process, and the recognition of their role allows for more rational design of ALD parameters.

A methodology providing access to dumbbell-tipped, metal-semiconductor and metal oxide-semiconductor heterostructured nanorods has been developed. The synthesis and characterization of CdSe@CdS nanorods incorporating ferromagnetic cobalt nanoinclusions at both nanorod termini (i.e., dumbbell morphology) are presented. The key step in the synthesis of these heterostructured nanorods was the decoration of CdSe@CdS nanorods with platinum nanoparticle tips, which promoted the deposition of metallic CoNPs onto Pt-tipped CdSe@CdS nanorods. Cobalt nanoparticle tips were then selectively oxidized to afford CdSe@CdS nanorods with cobalt oxide domains at both termini. In the case of longer cobalt-tipped nanorods, heterostructured nanorods were observed to self-organize into complex dipolar assemblies, which formed as a consequence of magnetic associations of terminal CoNP tips. Colloidal polymerization of these cobalt-tipped nanorods afforded fused nanorod assemblies from the oxidation of cobalt nanoparticle tips at the ends of nanorods via the nanoscale Kirkendall effect. Wurtzite CdS nanorods survived both the deposition of metallic CoNP tips and conversion into cobalt oxide phases, as confirmed by both XRD and HRTEM analysis. A series of CdSe@CdS nanorods of four different lengths ranging from 40 to 174 nm and comparable diameters (6-7 nm) were prepared and modified with both cobalt and cobalt oxide tips. The total synthesis of these heterostructured nanorods required five steps from commercially available reagents. Key synthetic considerations are discussed, with particular emphasis on reporting isolated yields of all intermediates and products from scale up of intermediate precursors.

Thermal atomic layer deposition (ALD) of metals on metal oxide surfaces typically suffers from nucleation delays that result in poor-quality films. The poor nucleation may be caused by a lack of suitable chemisorption sites on the oxide surface, which are needed for metal nucleation to occur. In this work, we demonstrate that prefunctionalizing the surface with a single monolayer of small organometallic molecules from the vapor phase can lead to a significant increase in surface coverage of the metal deposited by ALD. This process is demonstrated for Pt ALD from (methylcyclopentadienyl)­trimethylplatinum (MeCpPtMe3) and O2, with nucleation enhanced almost 3-fold at 100 ALD cycles after the pretreatment. We hypothesize that the high coverage of the organometallic molecule provides an alternative chemisorption mechanism for the platinum precursor and thus leads to an increase in its uptake. The proposed chemisorption mechanism is robust across several organometallic molecule pretreatments and could potentially be exploited for other organometallic-based metal ALD processes. This chemisorption mechanism was probed using in situ quadrupole mass spectrometry (QMS). The growth of the platinum deposits was investigated in depth through scanning electron microscopy (SEM) and grazing incidence small-angle X-ray scattering (GISAXS). These studies show that the pretreatment also results in the improved wettability of Pt nanoparticles (NPs). The improved wettability is likely to affect the Pt diffusion properties, further contributing to the enhancement observed on the treated substrates. In addition, GISAXS and SEM studies indicate the growth of larger, denser, and more highly ordered Pt NPs at early cycle numbers, which subsequently coalesce into continuous and pinhole-free films. Surface pretreatment by organometallic molecules therefore introduces a potential route to achieve improved nucleation and growth of ultrathin films.

Area-selective atomic layer deposition (AS-ALD) is a highly sought-after strategy for the fabrication of next-generation electronics. This work reveals how key precursor design parameters strongly influence the efficacy of AS-ALD by comparing a series of precursors possessing the same metal center but different ligands. When the number of methyl and chloride groups in Al­(CH3) x Cl3–x (x = 0, 2, and 3) and the chain length of alkyl ligands in AlC y H2y+1 (y = 1 and 2) are changed, the effect of precursor chemistry (reactivity and molecular size) on the selectivity is elucidated. The results show that optimized parameters for the Al2O3 ALD processes on a self-assembled monolayer (SAM)-terminated substrate, which serves as the nongrowth surface, differ significantly from those on a Si substrate. Chlorine-containing precursors need a much longer purging time on the SAMs because of a stronger Lewis acidity compared to that of alkyl precursors. With reoptimized conditions, the ALD of Al2O3 using the Al­(C2H5)3 precursor is blocked most effectively by SAM inhibitors, whereas the widely employed Al­(CH3)3 precursor is blocked least effectively among the precursors tested. Finally, we show that a selectivity exceeding 0.98 is achieved for up to 75 ALD cycles with Al­(C2H5)3, for which 6 nm of Al2O3 film grows selectively on SiO2-covered Si. Quantum chemical calculations show significant differences in the energetics of dimer formation across the Al precursors, with only ∼1% of AlCl3 and Al­(CH3)2Cl precursors but 99% of the alkyl precursors, Al­(CH3)3 and Al­(C2H5)3, existing as monomers at 200 °C. We propose that a combination of precursor reactivity and effective molecular size affects the blocking of the different precursors, explaining why Al­(C2H5)3, with weaker Lewis acidity and relatively large size, exhibits the best blocking results.