S. G. Lambrakos

Bright triplet excitons in caesium lead halide perovskites

Nanostructured semiconductors emit light from electronic states known as excitons. For organic materials, Hund’s rules state that the lowest-energy exciton is a poorly emitting triplet state. For inorganic semiconductors, similar rules predict an analogue of this triplet state known as the ‘dark exciton’. Because dark excitons release photons slowly, hindering emission from inorganic nanostructures, materials that disobey these rules have been sought. However, despite considerable experimental and theoretical efforts, no inorganic semiconductors have been identified in which the lowest exciton is bright. Here we show that the lowest exciton in caesium lead halide perovskites (CsPbX3, with X = Cl, Br or I) involves a highly emissive triplet state. We first use an effective-mass model and group theory to demonstrate the possibility of such a state existing, which can occur when the strong spin–orbit coupling in the conduction band of a perovskite is combined with the Rashba effect. We then apply our model to CsPbX3 nanocrystals, and measure size- and composition-dependent fluorescence at the single-nanocrystal level. The bright triplet character of the lowest exciton explains the anomalous photon-emission rates of these materials, which emit about 20 and 1,000 times faster than any other semiconductor nanocrystal at room and cryogenic temperatures, respectively. The existence of this bright triplet exciton is further confirmed by analysis of the fine structure in low-temperature fluorescence spectra. For semiconductor nanocrystals, which are already used in lighting, lasers and displays, these excitons could lead to materials with brighter emission. More generally, our results provide criteria for identifying other semiconductors that exhibit bright excitons, with potential implications for optoelectronic devices.

Data-Driven Design Optimization for Composite Material Characterization

The main goal of the present paper is to demonstrate the value of design optimization beyond its use for structural shape determination in the realm of the constitutive characterization of anisotropic material systems such as polymer matrix composites with or without damage. The approaches discussed are based on the availability of massive experimental data representing the excitation and response behavior of specimens tested by automated mechatronic material testing systems capable of applying multiaxial loading. Material constitutive characterization is achieved by minimizing the difference between experimentally measured and analytically computed system responses as described by surface strain and strain energy density fields. Small and large strain formulations based on additive strain energy density decompositions are introduced and utilized for constructing the necessary objective functions and their subsequent minimization. Numerical examples based on both synthetic (for one-dimensional systems) and actual data (for realistic 3D material systems) demonstrate the successful application of design optimization for constitutive characterization.

A general framework for numerical simulation of improvised explosive device (IED)-detection scenarios using density functional theory (DFT) and terahertz (THz) spectra.

We have developed a general framework for numerical simulation of various types of scenarios that can occur for the detection of improvised explosive devices (IEDs) through the use of excitation using incident electromagnetic waves. A central component model of this framework is an S-matrix representation of a multilayered composite material system. Each layer of the system is characterized by an average thickness and an effective electric permittivity function. The outputs of this component are the reflectivity and the transmissivity as functions of frequency and angle of the incident electromagnetic wave. The input of the component is a parameterized analytic-function representation of the electric permittivity as a function of frequency, which is provided by another component model of the framework. The permittivity function is constructed by fitting response spectra calculated using density functional theory (DFT) and parameter adjustment according to any additional information that may be available, e.g., experimentally measured spectra or theory-based assumptions concerning spectral features. A prototype simulation is described that considers response characteristics for THz excitation of the high explosive β-HMX. This prototype simulation includes a description of a procedure for calculating response spectra using DFT as input to the S-matrix model. For this purpose, the DFT software NRLMOL was adopted.

Purification and defect elimination of single-walled carbon nanotubes by the thermal reduction technique

A thermal reduction (TR) technique is used to purify single-walled carbon nanotubes (SWNTs) and to reduce chemical defects from their lattice structure. This technique is mainly comprised of two processes, high-temperature reactions of raw SWNTs in a pressurized hydrogen chamber followed by a slow annealing under vacuum. Analyses by TEM, TGA, XRF, and XRD of SWNT samples before and after the purification reveal that over 90% of the carbonaceous impurities formed during the syntheses of SWNTs (by carbon arc, laser ablation, and HiPCO) are eliminated. Post-purification IR spectroscopic analyses show no evidence of hydrogen in the SWNT samples and AFM studies suggest that the samples contain very few chemical defects. In addition, analyses by TEM and Raman spectroscopy do not reveal any significant structural damage of the SWNTs purified by the TR technique.

Dielectric Response of High Explosives at THz Frequencies Calculated Using Density Functional Theory

We present in this study calculations of the ground-state resonance structures associated with the high explosives β-HMX, PETN, RDX, TNT1, and TNT2 using density functional theory (DFT). Our objective is the construction of parameterized dielectric-response functions for excitation by electromagnetic waves at compatible frequencies. These dielectric-response functions provide the basis for analyses pertaining to the dielectric properties of explosives. In particular, these dielectric-response functions provide quantitative initial estimates of spectral-response features for subsequent adjustment with knowledge of additional information, such as laboratory measurements and other types of theory-based calculations. With respect to qualitative analyses, these spectra provide for the molecular-level interpretation of response structure. The DFT software GAUSSIAN was used for the calculations of the ground-state resonance structures presented here.

Simulation of deep penetration welding of stainless steel using geometric constraints based on experimental information

Results of a numerical simulation of deep penetration welding of 304 stainless steel are presented. This numerical model calculates the temperature and fluid velocity fields in a three-dimensional workpiece undergoing deep-penetration electron beam welding. The deposition of power from the beam and energy outflow at the model-system boundaries is effected by means of time-dependent boundary conditions on the equations of energy and momentum transfer. The vapor-liquid interface defining the keyhole is represented by a surface whose temperature is that of vaporization for the steel. On this surface, are specified boundary conditions for the momentum transfer equations such that the component of the velocity normal to the keyhole vapor-liquid interface is zero. In addition, this study introduces two new numerical procedures. These procedures are based on the inclusion of experimental information concerning beam spot size and weld pool geometry into the model system via constraints and the deduction of effective keyhole shape via an inverse mapping scheme.

A numerical model for deep penetration welding processes

The general features of a numerical model, and of its extensions, for calculating the temperature and fluid velocity field in a three-dimensional workpiece undergoing deep penetration laser beam welding are described. In the current model, the deposition of power from the beam is represented by time-dependent boundary conditions on the equations of energy and momentum transfer. These boundary conditions are specified at each timestep on a surface whose configuration can change with time and upon which energy is deposited according to a specified power distribution. This model also includes the effects of the buoy-ancy force on the melt pool and of the surface tension gradient on the surface of the fluid. The coupled equations of energy, momentum transfer, and continuity combined with the time-dependent boundary conditions representing the keyhole and the moving boundaries of the workpiece are solved by using a specific implementation of the SIMPLE algorithm. The important features of the numerical methods used in the model are discussed. Isotherms and convection patterns calculated using the current model are presented, and their significance for predicting weldment properties is discussed. A significant result of the simulations is that they demonstrate the overwhelming influence of the keyhole vapor/liquid inter-face on fluid convection and conduction in deep penetration welding.

Inverse Thermal Analysis of 304L Stainless Steel Laser Welds

An inverse thermal analysis of 304L stainless steel laser welds is presented. This analysis employs a methodology that is in terms of analytical basis functions. The results of this analysis provide parametric representations of weld temperature histories that can be adopted as input data to various types of computational procedures, such as those for prediction of solid-state phase transformations. In addition, these temperature histories can be used to construct parametric-function representations for inverse thermal analysis of welds corresponding to other process parameters or welding processes whose process conditions are within similar regimes. Specific aspects of the inverse-analysis methodology employed that relevant to its understanding and further development are examined.

Inverse Thermal Analysis of Welds Using Multiple Constraints and Relaxed Parameter Optimization

Aspects of a methodology for inverse thermal analysis of welds are examined that provide for relaxed model-parameter optimization. These aspects are associated with the inherent insensitivity of temperature fields, obtained by inverse analysis, to local shape variations of constrained boundaries within these fields. The inverse analysis methodology is in terms of numerical-analytical basis functions for construction parametric temperature histories, which can be adopted as input data to computational procedures for further analysis. In addition, these parametric temperature histories can be used for inverse thermal analysis of welds corresponding to other process parameters or welding processes whose process conditions are within similar regimes. The inverse thermal analysis procedure provides for the inclusion of volumetric constraint conditions whose two-dimensional projections are mappings onto transverse cross sections of experimentally measured boundary conditions, such as solidification and transformation boundaries, and isothermal surfaces associated with thermocouple measurements. Issues concerning relaxed parameter optimization are discussed with respect to inverse thermal analysis of Ti-6Al-4V pulsed-mode laser welds using multiple constraint conditions.

A numerical model and scaling relationship for energetic electron beams propagating in air

Scaling relationships for energy loss and scattering are combined with a particle code to construct a purely algebraic expression for the energy deposited by an energetic electron beam injected into field-free homogeneous air. An algebraic formulation is possible because the mean free paths for the major collisional processes depend similarly on density and energy above 1 keV. Accordingly, the spatial behavior of an initially cold pencil beam is approximately self-similar when expressed in terms of the nominal beam range, provided the beam energy at injection exceeds several keV. Since a warm and broad beam can always be decomposed into a series of cold pencil beams, the total energy deposited can be obtained through a simple sum. With such a model, the ionization and excitation generated by a beam can be computed quickly and easily at every point in space. Similar formulations can be developed for other media using particle codes or experimental data. In liquids or solids, the energy deposited is quickly...