Ensemble Effects in the Temperature-Dependent Photoluminescence of Silicon Nanocrystals

In this work the temperature-dependent photoluminescence of alkyl-capped silicon nanocrystals with mean diameters of between 3 and 9 nm has been investigated. The nanocrystals were characterized extensively by FTIR, TEM, powder XRD, and X-ray photoelectron spectroscopy prior to low-temperature and time-resolved photoluminescence spectroscopy experiments. The photoluminescence (PL) properties were evaluated in the temperature range of 41–300 K. We found that the well-known temperature-dependent blueshift of the PL maximum decreases with increasing nanocrystal diameter and eventually becomes a redshift for nanocrystal diameters larger than 6 nm. This implies that the observed shifts cannot be explained solely by band-gap widening, as is commonly assumed. We propose that the luminescence of drop-cast silicon nanocrystals is affected by particle ensemble effects, which can explain the otherwise surprising temperature dependence of the luminescence peak.     

The budget constrained r-interdiction median problem with capacity expansion

In this article, we elaborate on a budget constrained extension of the r-interdiction median problem with fortification (RIMF). The objective in the RIMF is to find the optimal allocation of protection resources to a given service system consisting of p facilities so that the disruptive effects of r possible attacks to the system are minimized. The defender of the system needs to fortify q facilities of the present system to offset the worst-case loss of r non-fortified facilities due to an interdiction in which the attacker’s objective is to cause the maximum possible disruption in the service level of the system. The defender-attacker relationship fits a bilevel integer programming (BIP) formulation where the defender and attacker take on the respective roles of the leader and the follower. We adopt this BIP formulation and augment it with a budget constraint instead of a predetermined number of facilities to be fortified. In addition, we also assume that each facility has a flexible service capacity, which can be expanded at a unit cost to accommodate the demand of customers who were serviced by some other interdicted facility before the attack. First, we provide a discrete optimization model for this new facility protection planning scenario with a novel set of closest assignment constraints. Then, to tackle this BIP problem we use an implicit enumeration algorithm performed on a binary tree. For each node representing a different fortification scheme, the attacker’s problem is solved to optimality using Cplex 11. We report computational results obtained on a test bed of 96 randomly generated instances. The article concludes with suggestions for future research.     

Some metal chelates with Schiff base ligand: synthesis,structure elucidation,thermal behavior,XRD evaluation,antioxidant activity,enzyme inhibition,and molecular docking studies

Molecular Diversity - Schiff bases are well-known compounds for having significant biological properties. In this study, a new Schiff base ligand and its metal complexes were synthesized, and their...     

Chemical Reactivity on Gas‐Phase Metal Clusters Driven by Blackbody Infrared Radiation

We report the observation of chemical reactions in gas‐phase Rh_n(N₂O)_m⁺ complexes driven by absorption of blackbody radiation. The experiments are performed under collision‐free conditions in a Fourier transform ion cyclotron resonance mass spectrometer. Mid‐infrared absorption by the molecularly adsorbed N₂O moieties promotes a small fraction of the cluster distribution sufficiently to drive the N₂O decomposition reaction, leading to the production of cluster oxides and the release of molecular nitrogen. N₂O decomposition competes with molecular desorption and the branching ratios for the two processes show marked size effects, reflecting variations in the relative barriers. The rate of decay is shown to scale approximately linearly with the number of infrared chromophores. The experimental findings are interpreted in terms of calculated infrared absorption rates assuming a sudden‐death limit.     

Multielement Analysis of Iliac Crest Bone by Neutron Activation

The k ₁-method for standardization in INAA specifically tackles the problem of the interpretation of gamma-ray spectra as obtained with highly efficient detectors, as opposed to the k ₀-method. In this paper, results obtained from three NIST reference materials, measured after neutron activation with a gold-lined well-type detector, are presented. It is concluded that the accuracy of the method is better than 1%.     

Open vehicle routing problem with driver nodes and time deadlines

In this paper, we consider a variant of the open vehicle routing problem in which vehicles depart from the depot, visit a set of customers, and end their routes at special nodes called driver nodes. A driver node can be the home of the driver or a parking lot where the vehicle will stay overnight. The resulting problem is referred to as the open vehicle routing problem with driver nodes (OVRP-d). We consider three classes of OVRP-d: with no time constraints, with a maximum route duration, and with both a maximum route duration as well as time deadlines for visiting customers. For the solution of these problems, which are not addressed previously in the literature, we develop a new tabu search heuristic. Computational results on randomly generated instances indicate that the new heuristic exhibits a good performance both in terms of the solution quality and computation time.     

Radiotracer method to study the transport of mercury(II)chloride from water to sediment and air

The fate of dissolved Hg(II) in surface waters is an important component of the Hg cycle. A simple experimental methodology was used to understand and measure the transport of Hg(II) from water to air and sediment. The use of radioactive dissolved Hg tracer for the determination of evasion and deposition is found to be a very useful technique. The evasion of mercury was investigated during a 140-hour period. It was observed that about a quarter of mercury chloride remained in the water phase, the other quarter was emitted via the evasion process and half of it deposited in sediment. This revised version was published online in August 2006 with corrections to the Cover Date.     

Trace elements in the atmosphere of Ankara

Atmospheric particulate material collected in Ankara was analyzed by instrumental neutron activation analysis. The results are interpreted with respect to trace element concentrations of the earth's crust. Volatile elements such as As, Sb, Zn, Br and Hg are highly enriched suggesting noncrustal origin. Enrichment factors increase with decreasing particle size.Supported in part by the Scientific and Technical Research Council (TÜBITAK) of Turkey.     

Variable neighbourhood search heuristics for the probabilistic multi-source Weber problem

The Multi-source Weber Problem (MWP) is concerned with locating m facilities in the Euclidean plane, and allocating these facilities to n customers at minimum total cost. The deterministic version of the problem, which assumes that customer locations and demands are known with certainty, is a non-convex optimization problem and difficult to solve. In this work, we focus on a probabilistic extension and consider the situation where customer locations are randomly distributed according to a bivariate distribution. We first present a mathematical programming formulation for the probabilistic MWP called the PMWP. For its solution, we propose two heuristics based on variable neighbourhood search (VNS). Computational results obtained on a number of test instances show that the VNS heuristics improve the performance of a probabilistic alternate location-allocation heuristic referred to as PALA. In its original form, the applicability of the new heuristics depends on the existence of a closed-form expression for the expected distances between facilities and customers. Unfortunately, such an expression exists only for a few distance function and probability distribution combinations. We therefore use two approximation methods for the expected distances, which make the VNS heuristics applicable for any distance function and bivariate distribution of customer locations.