Discriminating Crystal Binding from the Atomic Trapping of a Core Electron at Energy Levels Shifted by Surface Relaxation or Nanosolid Formation

Cluster beam deposition of lead sulfide nanocrystals into organic matrices

Lead sulfide nanocrystals (PbS NCs) were codeposited into two organic films, titanyl phthalocyanine (TiOPc) and alpha-sexithiophene, using cluster beam deposition (CBD). NCs of average diameters of approximately 3-4 nm were evenly distributed in these organic films with average particle spacings of approximately 4 nm, as determined by transmission electron microscopy. The film composition and NC surface chemistry were monitored by X-ray photoelectron spectroscopy (XPS) and other methods. Pb:S stoichiometry in the NC/TiOPc film was determined by XPS to correspond to the PbS cubic rock salt structure. Soft-XPS using 200 eV energy photons determined the NC-organic surface chemistry by resolving the S 2p core level into four distinct components for sulfur. The soft-XPS results found that the PbS NC surface chemistry could be tuned by varying the H(2)S/Ar gas ratio within the CBD source.

Silver nanocubes formed on ATP-mediated nafion film and a visual method for formaldehyde

The ability to construct size- and shape-controllable architectures is essential for the exploration of nanoparticle-structured properties, and it is a good strategy of employing metal nanoparticles embedded in a polymer matrix in order to prepare new materials with particular properties. Herein, we found that adenosine-5'-triphosphate (ATP) could be applied to adjust and control the formation of silver nanocubes (Ag-NCs) on Nafion film. Nafion could be saturated with [Ag(NH3)2]+ when incubated in silver ammonia solution, and it was found that the Nafion film saturated with [Ag(NH3)2]+ becomes yellow after immerged in a mixture containing NaOH, ATP, and formaldehyde, resulting in monodisperse Ag-NCs on the film. Thus, ATP as a molecular mediator and Nafion film as a polymer matrix are employed toward the preparation of size-controllable and monodisperse Ag-NCs, and a novel visual method for formaldehyde is further developed on the basis of the color change of the Nafion film, which gives sensitive detection of formaldehyde with a limit of determination of 60 ppb (3sigma).

Depth Profiling of Charging Effect of Si Nanocrystals Embedded in SiO2: A Study of Charge Diffusion among Si Nanocrystals

Si nanocrystal (nc-Si) embedded in SiO2 thin film is synthesized with low-energy Si ion implantation. Depth profiling of the charging effect of the nc-Si is determined from X-ray photoemission measurement. It is observed that there is a strong correlation between the depth profile of the charging effect and the nc-Si depth distribution. The charging effect is found to decrease with the increase of nc-Si concentration and to vanish when a densely stacked nanocrystal layer is formed. The phenomenon is attributed to the charge diffusion among the nanocrystals. The charge diffusion in the nanocrystal layer may have an important implication for nanocrystal flash memory. When such a layer is used as the charge-storage layer in the memory cells, the stored charges could be lost due to the rapid charge diffusion among the nc-Si if a single defect exists in the tunneling oxide, causing a reliability problem in data retention.

X-ray Photoelectron Spectroscopic Analysis of Si Nanoclusters in SiO2 Matrix

We investigated silicon nanoclusters Si(nc) in a SiO2 matrix prepared by the plasma-enhanced chemical vapor deposition technique, using X-ray photoelectron spectroscopy (XPS) with external voltage stimuli in both static and pulsed modes. This method enables us to induce an additional charging shift of 0.8 eV between the Si2p peaks of the oxide and the underlying silicon, both in static and time-resolved modes, for a silicon sample containing a 6 nm oxide layer. In the case of the sample containing silicon nanoclusters, both Si2p peaks of Si(nc) and host SiO2 undergo a charging shift that is 1 order of magnitude larger (>15 eV), with no measurable difference between them (i.e., no differential charging between the silicon nanoclusters and the oxide matrix could be detected). By use of a measured Auger parameter, we estimate the relaxation energy of the Si(nc) in the SiO2 matrix as -0.4 eV, which yields a -0.6 eV shift in the binding energy of the Si(nc) with respect to that of bulk Si in the opposite direction of the expected quantum size effect. This must be related to the residual differential charging between the silicon nanoclusters and the oxide host. Therefore, differential charging is still the biggest obstacle for extracting size-dependent binding energy shifts with XPS when one uses the oxide peak as the reference.