Jeffrey R. Simpson
National Institute of Standards and Technology
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Featured researches published by Jeffrey R. Simpson.
ACS Nano | 2014
Rusen Yan; Jeffrey R. Simpson; Simone Bertolazzi; Jacopo Brivio; Michael Watson; Xufei Wu; Andras Kis; Tengfei Luo; Angela R. Hight Walker; Huili Grace Xing
Atomically thin molybdenum disulfide (MoS2) offers potential for advanced devices and an alternative to graphene due to its unique electronic and optical properties. The temperature-dependent Raman spectra of exfoliated, monolayer MoS2 in the range of 100-320 K are reported and analyzed. The linear temperature coefficients of the in-plane E2g 1 and the out-of-plane A1g modes for both suspended and substrate-supported monolayer MoS2 are measured. These data, when combined with the first-order coefficients from laser power-dependent studies, enable the thermal conductivity to be extracted. The resulting thermal conductivity κ = (34.5(4) W/mK at room temperature agrees well with the first principles lattice dynamics simulations. However, this value is significantly lower than that of graphene. The results from this work provide important input for the design of MoS2-based devices where thermal management is critical.
ACS Nano | 2013
Jeffrey A. Fagan; Ming Zheng; Vinayak Rastogi; Jeffrey R. Simpson; Constantine Y. Khripin; Carlos A. Silvera Batista; Angela R. Hight Walker
The structure and density of the bound interfacial surfactant layer and associated hydration shell were investigated using analytical ultracentrifugation for length and chirality purified (6,5) single-wall carbon nanotubes (SWCNTs) in three different bile salt surfactant solutions. The differences in the chemical structures of the surfactants significantly affect the size and density of the bound surfactant layers. As probed by exchange of a common parent nanotube population into sodium deoxycholate, sodium cholate, or sodium taurodeoxycholate solutions, the anhydrous density of the nanotubes was least for the sodium taurodeoxycholate surfactant, and the absolute sedimentation velocities greatest for the sodium cholate and sodium taurodeoxycholate surfactants. These results suggest that the thickest interfacial layer is formed by the deoxycholate, and that the taurodeoxycholate packs more densely than either sodium cholate or deoxycholate. These structural differences correlate well to an observed 25% increase in fluorescence intensity relative to the cholate surfactant for deoxycholate and taurodeoxycholate dispersed SWCNTs displaying equivalent absorbance spectra. Separate sedimentation velocity experiments including the density modifying agent iodixanol were used to establish the buoyant density of the (6,5) SWCNT in each of the bile salt surfactants; from the difference in the buoyant and anhydrous densities, the largest hydrated diameter is observed for sodium deoxycholate. Understanding the effects of dispersant choice and the methodology for measurement of the interfacial density and hydrated diameter is critical for rationally advancing separation strategies and applications of nanotubes.
ACS Nano | 2015
Jeffrey A. Fagan; Erik Haroz; Rachelle Ihly; Hui Gui; Jeffrey L. Blackburn; Jeffrey R. Simpson; Stephanie Lam; Angela R. Hight Walker; Stephen K. Doorn; Ming Zheng
In this contribution we demonstrate the effective separation of single-wall carbon nanotube (SWCNT) species with diameters larger than 1 nm through multistage aqueous two-phase extraction (ATPE), including isolation at the near-monochiral species level up to at least the diameter range of SWCNTs synthesized by electric arc synthesis (1.3-1.6 nm). We also demonstrate that refined species are readily obtained from both the metallic and semiconducting subpopulations of SWCNTs and that this methodology is effective for multiple SWCNT raw materials. Using these data, we report an empirical function for the necessary surfactant concentrations in the ATPE method for separating different SWCNTs into either the lower or upper phase as a function of SWCNT diameter. This empirical correlation enables predictive separation design and identifies a subset of SWCNTs that behave unusually as compared to other species. These results not only dramatically increase the range of SWCNT diameters to which species selective separation can be achieved but also demonstrate that aqueous two-phase separations can be designed across experimentally accessible ranges of surfactant concentrations to controllably separate SWCNT populations of very small (∼0.62 nm) to very large diameters (>1.7 nm). Together, the results reported here indicate that total separation of all SWCNT species is likely feasible by the ATPE method, especially given future development of multistage automated extraction techniques.
Advanced Materials | 2011
Jeffrey A. Fagan; Barry J. Bauer; Erik K. Hobbie; Matthew L. Becker; Angela R. Hight Walker; Jeffrey R. Simpson; Jaehun Chun; Jan Obrzut; Vardhan Bajpai; Fred Phelan; Daneesh O. Simien; Ji Yeon Huh; Kalman B. Migler
Advanced technological uses of single-walled carbon nanotubes (SWCNTs) rely on the production of single length and chirality populations that are currently only available through liquid-phase post processing. The foundation of all of these processing steps is the attainment of individualized nanotube dispersions in solution. An understanding of the colloidal properties of the dispersed SWCNTs can then be used to design appropriate conditions for separations. In many instances nanotube size, particularly length, is especially active in determining the properties achievable in a given population, and, thus, there is a critical need for measurement technologies for both length distribution and effective separation techniques. In this Progress Report, the current state of the art for measuring dispersion and length populations, including separations, is documented, and examples are used to demonstrate the desirability of addressing these parameters.
ACS Nano | 2011
Jeffrey A. Fagan; Ji Yeon Huh; Jeffrey R. Simpson; Jeffrey L. Blackburn; Josh Holt; Brian A. Larsen; Angela R. Hight Walker
The separation of empty and water-filled laser ablation and electric arc synthesized nanotubes is reported. Centrifugation of these large-diameter nanotubes dispersed with sodium deoxycholate using specific conditions produces isolated bands of empty and water-filled nanotubes without significant diameter selection. This separation is shown to be consistent across multiple nanotube populations dispersed from different source soots. Detailed spectroscopic characterization of the resulting empty and filled fractions reveals that water filling leads to systematic changes to the optical and vibrational properties. Furthermore, sequential separation of the resolved fractions using cosurfactants and density gradient ultracentrifugation reveals that water filling strongly influences the optimal conditions for metallic and semiconducting separation.
ACS Nano | 2009
Hyeonggon Kang; Matthew L. Clarke; Jianyong Tang; John T. Woodward; Shin G. Chou; Zhenping Zhou; Jeffrey R. Simpson; Angela R. Hight Walker; Tinh Nguyen; Jeeseong Hwang
A multimodality imaging technique integrating atomic force, polarized Raman, and fluorescence lifetime microscopies, together with 2D autocorrelation image analysis is applied to the study of a mesoscopic heterostructure of nanoscale materials. This approach enables simultaneous measurement of fluorescence emission and Raman shifts from a quantum dot (QD)-single-wall carbon nanotube (SWCNT) complex. Nanoscale physical and optoelectronic characteristics are observed including local QD concentrations, orientation-dependent polarization anisotropy of the SWCNT Raman intensities, and charge transfer from photoexcited QDs to covalently conjugated SWCNTs. Our measurement approach bridges the properties observed in bulk and single nanotube studies. This methodology provides fundamental understanding of the charge and energy transfer between nanoscale materials in an assembly.
Nano Letters | 2017
Amber McCreary; Jeffrey R. Simpson; Yuanxi Wang; Daniel Rhodes; Kazunori Fujisawa; L. Balicas; Madan Dubey; Vincent H. Crespi; Mauricio Terrones; Angela R. Hight Walker
The strong in-plane anisotropy of rhenium disulfide (ReS2) offers an additional physical parameter that can be tuned for advanced applications such as logic circuits, thin-film polarizers, and polarization-sensitive photodetectors. ReS2 also presents advantages for optoelectronics, as it is both a direct-gap semiconductor for few-layer thicknesses (unlike MoS2 or WS2) and stable in air (unlike black phosphorus). Raman spectroscopy is one of the most powerful characterization techniques to nondestructively and sensitively probe the fundamental photophysics of a 2D material. Here, we perform a thorough study of the resonant Raman response of the 18 first-order phonons in ReS2 at various layer thicknesses and crystal orientations. Remarkably, we discover that, as opposed to a general increase in intensity of all of the Raman modes at excitonic transitions, each of the 18 modes behave differently relative to each other as a function of laser excitation, layer thickness, and orientation in a manner that highlights the importance of electron-phonon coupling in ReS2. In addition, we correct an unrecognized error in the calculation of the optical interference enhancement of the Raman signal of transition metal dichalcogenides on SiO2/Si substrates that has propagated through various reports. For ReS2, this correction is critical to properly assessing the resonant Raman behavior. We also implemented a perturbation approach to calculate frequency-dependent Raman intensities based on first-principles and demonstrate that, despite the neglect of excitonic effects, useful trends in the Raman intensities of monolayer and bulk ReS2 at different laser energies can be accurately captured. Finally, the phonon dispersion calculated from first-principles is used to address the possible origins of unexplained peaks observed in the Raman spectra, such as infrared-active modes, defects, and second-order processes.
Nature Communications | 2018
Jeffrey R. Simpson; Oleksiy Roslyak; Juan G. Duque; Erik H. Hároz; Jared Crochet; Hagen Telg; Andrei Piryatinski; Angela R. Hight Walker; Stephen K. Doorn
Electronic interactions in low-dimensional nanomaterial heterostructures can lead to novel optical responses arising from exciton delocalization over the constituent materials. Similar phenomena have been suggested to arise between closely interacting semiconducting carbon nanotubes of identical structure. Such behavior in carbon nanotubes has potential to generate new exciton physics, impact exciton transport mechanisms in nanotube networks, and place nanotubes as one-dimensional models for such behaviors in systems of higher dimensionality. Here we use resonance Raman spectroscopy to probe intertube interactions in (6,5) chirality-enriched bundles. Raman excitation profiles for the radial breathing mode and G-mode display a previously unobserved sharp resonance feature. We show the feature is evidence for creation of intertube excitons and is identified as a Fano resonance arising from the interaction between intratube and intertube excitons. The universality of the model suggests that similar Raman excitation profile features may be observed for interlayer exciton resonances in 2D multilayered systems.Bundles of single-wall carbon nanotubes with enriched chirality can be used as model systems for exploring exciton physics in low-dimensional nanostructures. Here, the authors use resonant Raman spectroscopy to probe intertube interactions in bundles of (6,5)-enriched carbon nanotubes, and observe a Fano resonance arising from coupling between intertube and intratube excitons.
Journal of the American Chemical Society | 2007
Jeffrey A. Fagan; Jeffrey R. Simpson; Barry J. Bauer; Silvia H. De Paoli Lacerda; Matthew L. Becker; Jaehun Chun; Kalman B. Migler; and Angela R. Hight Walker; Erik K. Hobbie
Physical Review B | 2008
Wei Zhou; Hui Wu; Tanner Yildirim; Jeffrey R. Simpson; Angela R. Hight Walker