The use of single wall carbon nanotubes (SWCNTs) in current and future applications depends on the ability to process SWCNTs in a solvent to yield high-quality dispersions characterized by individual SWCNTs and possessing a minimum of SWCNT bundles. Many approaches for the dispersion of SWCNTs have been reported. However, there is no general assessment which compares the relative quality and dispersion efficiency of the respective methods. Herein we report a quantitative comparison of the relative ability of "wrapping polymers" including oligonucleotides, peptides, lignin, chitosan, and cellulose and surfactants such as cholates, ionic liquids, and organosulfates to disperse SWCNTs in water. Optical absorption and fluorescence spectroscopy provide quantitative characterization (amount of SWCNTs that can be suspended by a given surfactant and its ability to debundle SWCNTs) of these suspensions. Sodium deoxy cholate (SDOCO), oligonucleotides (GT)(15), (GT)(10), (AC)(15), (AC)(10), C(10-30), and carboxymethylcellulose (CBMC-250K) exhibited the highest quality suspensions of the various systems studied in this work. The information presented here provides a good framework for further study of SWCNT purification and applications.

Single-walled carbon nanotubes (SWNTs) have unique photophysical properties but low fluorescence efficiency. We have found significant increases in the fluorescence efficiency of individual DNA-wrapped SWNTs upon addition of reducing agents, including dithiothreitol, Trolox, and β-mercaptoethanol. Brightening was reversible upon removal of the reducing molecules, suggesting that a transient reduction of defect sites on the SWNT sidewall causes the effect. These results imply that SWNTs are intrinsically bright emitters and that their poor emission arises from defective nanotubes.

The luminescence efficiency of individual single-walled carbon nanotubes was determined by comparing the fluorescence from individual nanotubes to single CdTe/ZnS quantum dots with a well-defined fluorescence quantum yield (QY). The single carbon nanotube QY was determined to be 3 +/- 1%, nearly 100 times greater than values previously reported for ensembles. The intrinsic nanotube QY is potentially much higher than previously believed and appears lower in ensembles due to defective nanotubes and residual bundles.

Single-walled carbon nanotubes (SWNTs) are cylindrical graphitic molecules that have remained at the forefront of nanomaterials research since 1991, largely due to their exceptional and unusual mechanical, electrical, and optical properties. The motivation for understanding how nanotubes interact with light (i.e., SWNT photophysics) is both fundamental and applied. Individual nanotubes may someday be used as superior near-infrared fluorophores, biological tags and sensors, and components for ultrahigh-speed optical communications systems. Establishing an understanding of basic nanotube photophysics is intrinsically significant and should enable the rapid development of such innovations. Unlike conventional molecules, carbon nanotubes are synthesized as heterogeneous samples, composed of molecules with different diameters, chiralities, and lengths. Because a nanotube can be either metallic or semiconducting depending on its particular molecular structure, SWNT samples are also mixtures of conductors and semiconductors. Early progress in understanding the optical characteristics of SWNTs was limited because nanotubes aggregate when synthesized, causing a mixing of the energy states of different nanotube structures. Recently, significant improvements in sample preparation have made it possible to isolate individual nanotubes, enabling many advances in characterizing their optical properties. In this Account, single-molecule confocal microscopy and spectroscopy were implemented to study the fluorescence from individual nanotubes. Single-molecule measurements naturally circumvent the difficulties associated with SWNT sample inhomogeneities. Intrinsic SWNT photoluminescence has a simple narrow Lorentzian line shape and a polarization dependence, as expected for a one-dimensional system. Although the local environment heavily influences the optical transition wavelength and intensity, single nanotubes are exceptionally photostable. In fact, they have the unique characteristic that their single molecule fluorescence intensity remains constant over time; SWNTs do not “blink” or photobleach under ambient conditions. In addition, transient absorption spectroscopy was used to examine the relaxation dynamics of photoexcited nanotubes and to elucidate the nature of the SWNT excited state. For metallic SWNTs, very fast initial recovery times (300–500 fs) corresponded to excited-state relaxation. For semiconducting SWNTs, an additional slower decay component was observed (50–100 ps) that corresponded to electron–hole recombination. As the excitation intensity was increased, multiple electron–hole pairs were generated in the SWNT; however, these e−h pairs annihilated each other completely in under 3 ps. Studying the dynamics of this annihilation process revealed the lifetimes for one, two, and three e−h pairs, which further confirmed that the photoexcitation of SWNTs produces not free electrons but rather one-dimensional bound electron–hole pairs (i.e., excitons). In summary, nanotube photophysics is a rapidly developing area o...

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Escalation processes are found in many types of international conflict. However, a great deal of the theoretical and empirical literature on escalation is context specific and concentrates on explaining the outcomes of an escalation process. This approach has generated numerous insights; however, our understanding of escalation processes, in general, remains partial and incomplete. In this article, the author develops a two-sided incomplete information model to identify the kinds of escalation strategies states are likely to adopt in conflict. The model produces several hypotheses, one of which is tested empirically in the context of militarized interstate disputes. The hypothesis states that as the disparity between the players' cost tolerances increases, the lower cost tolerant actor is more likely to escalate to the maximum of his or her ability on the first move in the conflict. The results of the test confirm the theory's expectations of an inverse relationship between cost tolerance and an actor's escalation behavior. The article concludes by noting implications for future research on escalation processes.

The poliheuristic theory of foreign policy decision making posits a two-stage process wherein the decision maker first employs a noncompensatory decision rule to eliminate politically unacceptable alternatives and then employs a (perhaps) traditional decision procedure to select from the remaining set of acceptable alternatives. A general decision analysis is used to provide a structured account of the elimination process of the first stage of the poliheuristic theory by displaying a noncompensatory decision rule for eliminating unacceptable policy alternatives. The results show how general decision analysis can be used to specify when an alternative is unacceptable to a political decision maker who is sensitive to public opinion.Thepoliheuristictheoryofforeign policy decision making is characterized by a twostage process, described by Mintz and Geva (1997, 82-83) as follows:Foreign policy decision making often entails a two-stage process in which the first stage involves the elimination of certain alternatives from the choice set, and the second stage consists of an analytic process of choosing an alternative that minimizes risk and guarantees rewards. The first phase in the decision process typically involves a nonholistic (nonexhaustive) search, to select a subset of alternatives using a simplifying process. . . . The second phase typically involves a maximization or lexicographic decision rule for selecting an alternative from the subset of surviving alternatives.The purpose of this study is to employ a general decision analysis to provide a structured account of the elimination process of the first stage of the poliheuristic theory by displaying a noncompensatory decision rule for eliminating unacceptable, as well as retaining acceptable, policy alternatives. 38AUTHORS' NOTE: We wish to acknowledge Alex Mintz and the participants in the

The model developed in this paper provides a formal prospect theory account of Challenger's behavior in the traditional deterrence game played under sequential decision analysis. The model is used to analyze two basic claims commonly made in the international relations literature regarding the importation of prospect theory into the analysis of crisis games. These claims pertain to Challenger's behavior when the valuation of the status quo is positive and Challenger's behavior when the valuation of the status quo is negative.We model the traditional deterrence game between Challenger and Defender as a one-sided incomplete information game where Challenger is uncertain about Defender's preference ordering. We examine the behavior of Challenger under the condition that the status quo is declining, using a von Neumann-Morgenstern decision rule as specified by expected utility theory, and a Kahneman-Tverksy decision rule as specified by prospect theory.The formal results show that these claims do not hold unconditionally and must be stated more precisely. Furthermore, there exist conditions under which the claims are false. We show that in addition to specifying the valuation of the status quo and the value of the probability of loss, as per the claims, a fuller understanding of Challenger's behavior also requires a specification of Challenger's valuation of the status quo vis-à-vis the valuations of other major payoffs in the game. In so doing, we capture many of the details relevant to a more complete analysis of Challenger's behavior in the deterrence game.