Observations Of Absorption Features Research Articles

Control of size of PbI 2 nanocrystals using Langmuir–Blodgett films of n-octadecyl succinic acid

Lead iodide (PbI 2) particulate films, composed of highly oriented nanocrystals have been generated in situ using an oriented inorganic extended solid within a deposited Langmuir–Blodgett film (LB film) of octadecyl lead succinate (ODSAPb). Solid (PbI 2) was formed by the reaction of gaseous HI with ODSAPb LB film, which acts as the template for building the inorganic solid. Mixed LB systems with 1:1 mol ratio of ODSAPb with octadecyl amine (ODAM) were also used to form the PbI 2. The films were characterized using attenuated total reflection FTIR (ATR-FTIR) to check whether HI had completely protonated the films. The observation of absorption features between 550 and 300 nm region in the UV-Visible spectra of these films strongly suggest sizes of PbI 2 ranging between 2.0 and 3.0 nm. The optical band gap estimated from LB films of these two systems showed a marked increase from 0.36 eV for the pure ODSAPb template to 0.47 eV for the ODSAPb+ODAM mixed film. The LB films were examined by small angle XRD and atomic force microscopy (AFM) and indicated that there is a variation in the structural organization in the monolayers and in the size of PbI 2 in the pure ODSAPb LB films as compared with the mixed films with ODAM. The average contraction in a axis and elongation in the c axis of the unit cell ( a= b=3.9 Å and c=7.0 Å) for PbI 2 in the template, compared with the values for PbI 2 in bulk confirm the presence of the nanocrystals in the LB films. The increase in the band gap is also reflected in the increase in the c value of the unit cell parameter that changes from 6.3 to 7.0 Å in going from the pure ODSAPb matrix to mixed film of ODSAPb and ODAM.

Chemical and Size Characterization of Layered Lead Iodide Quantum Dots via Optical Spectroscopy and Atomic Force Microscopy

Lead iodide (PbI2) clusters were synthesized from the chemical reaction of NaI (or KI) with Pb(NO3)2 in H2O, D2O, CH3OH, and C3H7OH solvents. The observation of absorption features between the 550 and 350 nm region obtained with an integrating sphere strongly suggests PbI2 quantum dot formation in solution. Comparison of spectra of PbI2 clusters in solution with PbI2 clusters formed by impregnation of PbI2 in four different pore-sized porous silica substrates indicates that the PbI2 cluster size in solution is less than 2.5 nm in the lateral dimension. Atomic force microscopy (AFM) measurements of PbI2 solutions deposited on mica and highly oriented pyrolytic graphite surfaces indicate that the clusters are single layered. The measured height is 1.0 ± 0.1 nm, which is ∼0.3 nm larger than the layer thickness observed for the bulk materials. The swollen layer thickness can be attributed to the intralayer contraction from the strong lateral interaction among PbI2 molecules, which is supported by ab initio calculations. Raman scattering measurements of the LO and TO modes of PbI2 in bulk and in the confined state were also conducted in 50−150 cm-1 region. Three bands observed at 74, 96, and 116 cm-1 for the confined materials are assigned to the TO2, LO2, and LO1 modes, respectively. The relatively small red shift in the LO modes for PbI2 in the porous hosts may be caused by the surface phonon of PbI2 nanoparticles confined in the porous silica.

Highly ionized atoms toward HD 93521

Results are reported from the HST High Resolution Spectrograph observations of absorption features of C IV and Si IV in the spectrum of the high-latitude O star HD 93521 (l = 183 deg; b = 62 deg). A comparison of Si IV and C IV profiles showed that the FWHM of both features is about 50 km/sec, in contrast to the 7 km/sec found for one of the several S II features. The line centers for C IV and Si IV are at v = -67 km/sec and -60 km/sec, respectively. As the interval velocity decreases from 90 to 50 km/sec, the Si IV/C IV ratio of the column density per unit velocity interval increases from about 0.2 to 0.4. The result is qualitatively consistent with a fountain model of Shapiro and Benjamin (1991) if the slower gas has cooled and recombined more than the faster gas.