Samuel Graham

Bio: Samuel Graham is an academic researcher from Georgia Institute of Technology. The author has contributed to research in topics: Thermal conductivity & Thermal resistance. The author has an hindex of 48, co-authored 347 publications receiving 9774 citations. Previous affiliations of Samuel Graham include Merck & Co. & United States Military Academy.

Experimental Observation of Localized Interfacial Phonon Modes

Abstract: Interfaces impede heat flow in micro/nanostructured systems. Conventional theories for interfacial thermal transport were derived based on bulk phonon properties of the materials making up the interface without explicitly considering the atomistic interfacial details, which are found critical to correctly describing thermal boundary conductance (TBC). Recent theoretical studies predicted the existence of localized phonon modes at the interface which can play an important role in understanding interfacial thermal transport. However, experimental validation is still lacking. Through a combination of Raman spectroscopy and high-energy resolution electron energy-loss spectroscopy (EELS) in a scanning transmission electron microscope, we report the first experimental observation of localized interfacial phonon modes at ~12 THz at a high-quality epitaxial Si-Ge interface. These modes are further confirmed using molecular dynamics simulations with a high-fidelity neural network interatomic potential, which also yield TBC agreeing well with that measured from time-domain thermoreflectance (TDTR) experiments. Simulations find that the interfacial phonon modes have obvious contribution to the total TBC. Our findings may significantly contribute to the understanding of interfacial thermal transport physics and have impact on engineering TBC at interfaces in applications such as electronics thermal management and thermoelectric energy conversion.

Transparent Electrodes From Graphene/Single Wall Carbon Nanotube Composites

Abstract: A transparent conductive electrode comprised of alternating layers of graphene grown by chemical vapor deposition (CVD) and metallic single wall nanotubes (M-SWNTs) is presented. It was found that the addition of two single-layer graphene sheets enhances the conduction pathways in the M-SWNT film, yielding up to a 75% decrease in the sheet resistance with little sacrifice in the optical transmittance. Enhancements in the electrical properties of the films were made through a heat treatment process followed by nitric acid and thionyl chloride doping, yielding a sheet resistance of 70 Ω/sq with a transmittance of 78% at 550 nm. Composite films having undergone an annealing step were found to have stable electrical properties upon exposure to atmospheric conditions while doped films demonstrated limited stability.Copyright © 2013 by ASME

Quantifying the Effects of Inhomogeneity and Doping on the Electronic Contribution to Thermal Conductivity in Semiconducting Polymers

Abstract: Quantifying contributions to thermal conductivity from electrons and atomic vibrations in doped semiconducting polymers is important for heat transfer. Several studies report Lorenz numbers (L) that are larger than the Sommerfeld limit (L0), counterintuitively implying that charge carriers in semiconducting polymers carry more heat than those in metals. Alternatively, this phenomenon can be explained by recognizing that semiconducting polymers often contain insulating and conducting domains. Microstructures can lead to misinterpretation of the effective Lorenz number (Leff) observed macroscopically. Herein, effective medium theory (EMT) shows that inhomogeneity can result in macroscopic measurements where Leff ≠ L0, even when each component exhibits L0 at the microscopic level. The authors then extend the semi‐localized transport (SLoT) model to explain the origins of the large Leff values, validating with the prototypical poly(3,4‐ethylenedioxythiophene) system. This electro‐thermal extension of the SLoT model (ET‐SLoT) improves the ability to engineer the electronic contribution to thermal conductivity of semiconducting polymers.

Electrothermal Modeling and Analysis of Gallium Oxide Power Switching Devices

Abstract: Gallium oxide is an emerging wide band-gap material that has the potential to penetrate the power electronics market in the near future. In this paper, a finite-element gallium oxide semiconductor model is presented that can predict the electrical and thermal characteristics of the device. The finite element model of the two-dimensional device architecture is developed inside the Sentaurus environment. A vertical FinFET device architecture is employed to assess the device’s behavior and its static and dynamic characteristics. Enhancement-mode device operation is realized with this type of device architecture without the need for any selective area doping. The dynamic thermal behavior of the device is characterized through its short-circuit behavior. Based on the device static and dynamic behavior, it is envisioned that reliable vertical transistors can be fabricated for the power electronics applications.

I and i

Abstract: There is, I think, something ethereal about i —the square root of minus one. I remember first hearing about it at school. It seemed an odd beast at that time—an intruder hovering on the edge of reality. Usually familiarity dulls this sense of the bizarre, but in the case of i it was the reverse: over the years the sense of its surreal nature intensified. It seemed that it was impossible to write mathematics that described the real world in …

Fast parallel algorithms for short-range molecular dynamics

Abstract: Three parallel algorithms for classical molecular dynamics are presented. The first assigns each processor a fixed subset of atoms; the second assigns each a fixed subset of inter-atomic forces to compute; the third assigns each a fixed spatial region. The algorithms are suitable for molecular dynamics models which can be difficult to parallelize efficiently—those with short-range forces where the neighbors of each atom change rapidly. They can be implemented on any distributed-memory parallel machine which allows for message-passing of data between independently executing processors. The algorithms are tested on a standard Lennard-Jones benchmark problem for system sizes ranging from 500 to 100,000,000 atoms on several parallel supercomputers--the nCUBE 2, Intel iPSC/860 and Paragon, and Cray T3D. Comparing the results to the fastest reported vectorized Cray Y-MP and C90 algorithm shows that the current generation of parallel machines is competitive with conventional vector supercomputers even for small problems. For large problems, the spatial algorithm achieves parallel efficiencies of 90% and a 1840-node Intel Paragon performs up to 165 faster than a single Cray C9O processor. Trade-offs between the three algorithms and guidelines for adapting them to more complex molecular dynamics simulations are also discussed.

Interface engineering of highly efficient perovskite solar cells

Abstract: Advancing perovskite solar cell technologies toward their theoretical power conversion efficiency (PCE) requires delicate control over the carrier dynamics throughout the entire device. By controlling the formation of the perovskite layer and careful choices of other materials, we suppressed carrier recombination in the absorber, facilitated carrier injection into the carrier transport layers, and maintained good carrier extraction at the electrodes. When measured via reverse bias scan, cell PCE is typically boosted to 16.6% on average, with the highest efficiency of ~19.3% in a planar geometry without antireflective coating. The fabrication of our perovskite solar cells was conducted in air and from solution at low temperatures, which should simplify manufacturing of large-area perovskite devices that are inexpensive and perform at high levels.

An electron acceptor challenging fullerenes for efficient polymer solar cells.

Abstract: A novel non-fullerene electron acceptor (ITIC) that overcomes some of the shortcomings of fullerene acceptors, for example, weak absorption in the visible spectral region and limited energy-level variability, is designed and synthesized. Fullerene-free polymer solar cells (PSCs) based on the ITIC acceptor are demonstrated to exhibit power conversion effi ciencies of up to 6.8%, a record for fullerene-free PSCs.