Variation of Energy Density of States in Quantum Dot Arrays due to Interparticle Electronic Coupling

Subnanometer-resolved local electron energy structure was measured in PbS quantum dot superlattice arrays using valence electron energy loss spectroscopy with scanning transmission electron microscopy. We found smaller values of the lowest available transition energies and an increased density of electronic states in the space between quantum dots with shorter interparticle spacing, indicating extension of carrier wave functions as a result of interparticle electronic coupling. A quantum simulation verified both trends and illustrated the wave function extension effect

Methodology for Studying Surface Chemistry and Evolution during the Nucleation Phase of Atomic Layer Deposition Using Scanning Tunneling Microscopy

We study the nucleation stage and growth of atomic layer deposition (ALD) on hydrogen terminated silicon (Si:H) by in situ and ex situ scanning tunneling microscopy (STM). STM allows the in-depth study of surface chemistry and evolution during the ALD nucleation phase. Here, the ALD systems studied to demonstrate this technique are ZnO via diethyl zinc (DEZ) and TiO₂ via titanium tetrachloride (TiCl₄). In-situ STM revealed that DEZ does not discriminate between different surface sites, in contrast to TiCl₄ which shows a strong preference toward dangling or OH bonds. Continued deposition showed distinct island growth for TiO₂ deposition on Si:H, versus homogeneous growth for DEZ. ZnO ALD exhibited a delay of approximately 5 ALD cycles in transitioning from lateral to vertical growth and nominal physical film closure occurred after approximately 12–15 cycles. STM observations of these ALD chemistries demonstrated the strength of this technique in quantifying film closure and the effects of surface termination and defects on ALD growth mode. This technique can be applied to the study of a broad variety of ALD systems

Atomically Flat Silicon Oxide Monolayer Generated by Remote Plasma

We demonstrate stable, atomically smooth monolayer oxidation of Si(111) using a remote plasma. Scanning tunneling microscopy (STM) confirms the atomically flat nature of the oxidized surface, while cross-sectional transmission electron microscopy (TEM) proves the monolayer to bilayer oxide thickness. Fourier transform infrared spectroscopy (FTIR) and atomic layer deposition (ALD) indicate oxygen is incorporated onto the silicon surface in the form of Si–O–Si and Si–OH bonds. The incorporation of Si–OH bonds is inferred by using TiCl₄, a highly specific ALD precursor, for TiO₂ ALD. This plasma technique provides precise control of the surface chemistry and yields abrupt yet stable SiO/Si interfaces. It enables production of atomically flat, ALD-active silicon surfaces that could serve as a well-defined platform for investigation of various surface chemistries via STM. Using this substrate, we present the first ever STM observations of ALD TiO₂ on silicon oxide

Two new megastigmanes from Chinese traditional medicinal plant <i>Sedum sarmentosum</i>

<p>To discover new bioactive compounds from nature plants, a primary screening of traditional Chinese medicines had been taken. The screening results showed that a EtOAc extract of <i>Sedum sarmentosum</i> displayed a certain degree of cytotoxic activity and bioassay-directed isolation of EtOAc extract gave two new megastigmanes, <i>(6S,9R)</i>-2-hydroxy-4-(2,6,6-trimethyl-4-oxo-cyclohex-2-enyl)-butyric acid (<b>1</b>) and <i>(6S,9R)</i>-2-hydroxy-4-(2,6,6-trimethyl-4-oxo-cyclohex-2-enyl)-butyric acid methyl ester (<b>2</b>) together with seven known flavonoids. The chemical structures of <b>1</b> and <b>2</b> were elucidated on the basis of detailed 1D, 2D NMR and MS data. When tested against HepG2 and Hep3B hepatocellular carcinoma cell lines, compounds <b>1</b>–<b>9</b> showed weak anti-HCC activity. In addition, <i>in vitro</i> antioxidant activities of <b>1</b>–<b>9</b> were evaluated by ABTS radical cation-scavenging assay. <b>1</b> and <b>2</b> exhibited weak activity with per micromoles equivalent to 0.039 and 0.042 μM of Trolox, respectively. The flavonoid component, quercetin (<b>9</b>) showed the highest antioxidant activities with per micromoles equivalent 0.67 μM of Trolox.</p

Quantifying Geometric Strain at the PbS QD-TiO₂ Anode Interface and Its Effect on Electronic Structures

Quantum dots (QDs) show promise as the absorber in nanostructured thin film solar cells, but achieving high device efficiencies requires surface treatments to minimize interfacial recombination. In this work, lead sulfide (PbS) QDs are grown on a mesoporous TiO₂ film with a crystalline TiO₂ surface, versus one coated with an amorphous TiO₂ layer by atomic layer deposition (ALD). These mesoporous TiO₂ films sensitized with PbS QDs are characterized by X-ray and electron diffraction, as well as X-ray absorption spectroscopy (XAS) in order to link XAS features with structural distortions in the PbS QDs. The XAS features are further analyzed with quantum simulations to probe the geometric and electronic structure of the PbS QD-TiO₂ interface. We show that the anatase TiO₂ surface structure induces PbS bond angle distortions, which increases the energy gap of the PbS QDs at the interface