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Very small ZnS and CdS crystallites are made and stabilized in aqueous and methanolic media without organic surfactants. Low temperature (−77 °C) synthesis in methanol produces the smallest crystallites, ≈30 Å diameter cubic CdS and <20 Å diameter cubic ZnS. The crystallites are characterized by transmission electron microscopy and in situ optical spectroscopy (λ≳200 nm). The crystallites are too small to exhibit bulk band gaps in their optical spectra. In the band gap region, the small crystallites show a higher energy absorption threshold with a resolved spectral feature (quantum size exciton peak), not present in the spectra of larger crystals. The far ultraviolet spectra are unaffected by size at present resolution. These results can be understood in terms of the crystallite molecular orbitals, and an elementary confined electron and hole model.

Articles you may be interested inElectron ground state energy level determination of ZnSe self-organized quantum dots embedded in ZnS Metal selenide clusters have been made and characterized, using the arrested precipitation colloidal technique. A comparison of sulfide and selenide spectra enables observation of the effect of changes in the highest occupied molecular orbitals upon cluster electronic properties. The first and second excited electronic states are both observed as a function of size in ZnSe clusters. The systematic dependence of the spectra lead to assignment of the higher state to a IS-type hole based upon the split-off valence band. It is shown that the energy spectrum of discrete hole states is controlled by the spin-orbit energy and the isotropic hole mass in small, highly symmetrical clusters. This result contrasts with the heavy hole and light hole states observed for planar confinement. In ~ 20 A diameter ZnS clusters, there is a strong vibronic temperature dependence in the excited state spectra, while in clusters of smaller gap materials such vibronic effects are very minor. We conjecture that lifetime broadening is severe in clusters of small gap materials.

The luminescence of CdS Clusters prepared by arrested precipitation has been studied as function of time, temp., wavelength, and physical size.

Tiny single PbS crystals of ∼25 Å diameter are synthesized and studied optically in low-temperature colloidal solutions. Electron microscopic examination shows a simple cubic rock salt structure with a lattice constant unchanged, within experimental error, from the bulk value. These crystallites lack the near infrared electronic absorption characteristic of bulk PbS. The small crystallite absorbance in the visible rises more steeply than does the bulk absorbance. These results reflect electron and hole localization if one considers the variation in effective mass across the band structure. A simple discussion of localization anywhere in the Brillouin zone is given. For the first time, crystallite syntheses are carried out in solvent mixtures that form transparent glasses upon cooling. The PbS spectra are independent of temperature (at current experimental resolution) down to 130 K, in contrast to earlier results for quantum size exciton peaks in ∼20 Å ZnS crystallites. Previously published observations of size dependence in the excited state electronic properties of AgI and AgBr are explained as consequences of electron and hole localization in the small crystallites. AgBr appears to be the first indirect gap semiconductor to be examined in the regime where bulk properties are not fully formed.

Laser‐induced breakdown spectroscopy (LIBS) is an elemental analysis technique that is based on the measurement of atomic emissions generated on a sample surface by a laser‐induced microplasma. Although often recognized in the literature as a well‐established analytical technique, LIBS remains untested relative to the quantitative analysis of elements in chemically complex matrices, such as soils. The objective of this study was to evaluate the capabilities of LIBS relative to the elemental characterization of surface soils. Approximately 65 surface soil samples from the Pond Creek watershed in east Tennessee were collected and subjected to total dissolution and elemental analysis by inductively coupled argon plasma‐optical emission spectroscopy (ICP–OES). The samples were analyzed by LIBS using a Nd:YAG laser at 532 nm, with a beam energy of 25 mJ per pulse, a pulse width of 5 ns, and a repetition rate of 10 Hz. The wavelength range for the LIBS spectra collection was 200 to 600 nm, with a resolution of 0.03 nm. Elemental spectral lines were identified through the analysis of analytical reagent‐grade chemicals and the NIST and Kurucz spectral databases. The elements that dominated the LIBS spectra were Al, Ca, Fe, and Mg. In addition, emission lines for Ti, Ba, Na, Cu, and Mn were isolated. The emission lines of Cr, Ni, and Zn, which were >100 mg kg−1 in numerous soil samples, were not detected. Further, spectral emission lines for P and K are >600 nm, eliminating them from LIBS analysis. The integrated peak areas of interference‐free elemental emission lines were determined, then normalized to the area of the 288.16 nm Si(I) emission (internal standard) to reduce the variability between replicate analyses. The normalized spectral areas, coupled with linear regression (standard curves for single wavelength response) and multivariate techniques (chemometrics and multiple wavelengths), were used to predict ICP–OES elemental data. In general, the quantitative capabilities of LIBS proved disappointing. Detection and quantitation were generally restricted to those elements with concentrations > 0.5 g kg−1 The correlation between LIBS response and elemental content was poor (r < 0.98). Further, the relative errors of prediction for the LIBS‐detected elements were less than acceptable for an analytical technique (<20%), ranging from ∼20 to ∼40% using linear regression analysis, and from 18 to 48% using partial least squares analysis. Based on these findings, the analytical capability of the LIBS method for soil metals analysis should be considered questionable.

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Production data analysis and reservoir simulation of the Eagle Ford shale are very challenging due to the complex characteristics of the reservoir and the fluids. Eagle Ford reservoir complexity is expressed in the enormous vertical and horizontal petro-physical heterogeneity, stress-sensitive permeability, and existence of multi-scale natural fracture and fault systems. This complexity makes the prediction of the geometry and conductivity of the hydraulic fracture resulting from the stimulation process rather challenging. On the other hand, reservoir fluid complexity is demonstrated in multi-phase flow, liquid loading in the wellbore, condensate banking, etc. Based on this complexity, 3D reservoir modeling and numerical simulation have the relative advantage of addressing irregular fracture geometry, variable SRV, and multi-phase flow aspects. The South Texas Asset Team at Pioneer Natural Resources is establishing a workflow for dynamic reservoir modeling that can integrate all reservoir/wellbore parameters (formation evaluation, drilling, completion, stimulation, pre-/post-fracture surveillance, and well performance data) in order to address key questions relating to field development; such as depletion efficiency, drainage area, wells interference, and condensate banking effects. In this paper, a case study is presented to demonstrate the integration of various measurements and surveillance data to build a variable SRV reservoir model. The variable SRV model described here has the following building blocks: 1) Formation evaluation: included all the reservoir characterization data derived from logs and 3D seismic inversions and structural attributes. 2) Surveillance data integration: microseismic data (backbone for this work) are integrated with chemical and radioactive tracer logs. 3) Well performance data integration: Production data is used to determine different flow regimes during the well history and to set bounds for stimulation parameters, such as fracture half-length and permeability ( √ ). 4) Numerical simulation: Micro-seismic attributes (density and magnitude) are converted to a permeability model after being calibrated with tracer logs and production flow regime parameters ( √ ). PVT data is matched against an Equation of State (EOS) and input into the model. Production data history matching, sensitivity and forecasting indicate the following: a) The SRV created by fracture stimulation has permeability fading away from the wellbore; b) Fracture geometry is variable and results in an irregular drainage area along the lateral; C) Onset of condensate banking near wellbore and along the fracture(s) can occur within the first year of production if draw down is not managed properly.