Featured

Understanding the Link Between Photonic and Electronic Performance of 2D Semiconducting Layers

  • By Burcu Ozden
  • 20 December 2017

Susan Fullerton and her colleagues wrote a scientific report on deconvoluting the photonic and electronic response of two-dimensional (2D) materials for the case of molybdenum disulfide (MoS2). What are the main criteria which provide evidence that the material is “high quality”? Are the photonic properties or electronic performance? Susan Fullerton and her colleagues have studied the MoS2 materials and their devices to answer this question and to find the correlation between electronic and optical properties in 2D materials. In their study, they used Raman, photoluminescence (PL), time-resolved photoluminescence (TRPL), high-resolution scanning transmission electron microscopy (HR-STEM), X-ray photoelectron spectroscopy (XPS), field effect transistors (FET) fabrication electrolyte gate application methods to characterize MoS2.

Discovery of a Mechanism for Dislocation Nucleation and Migration Driven by Surface Segregation

  • By Burcu Ozden
  • 6 December 2017

Judith C. Yang and her colleagues answered the question of how dislocations nucleate and migrate at heterointerfaces in dissimilar-material systems on their recently published article on Nature Materials. n this study, Judith Yang and her colleagues showed that atomic segregation acts as a source for generating dislocations for the first time. They have used Cu–Au alloy system for studying surface segregation. Real-time transmission electron microscopy (TEM) was used to both spatially and temporally resolve the transition of the coherent, dislocation free interface between a Cu3Au-segregated surface and a Cu(Au) crystal substrate into a semi-coherent structure through the nucleation and subsequent migration of misfit accommodating dislocations. They combined their experimental study with the teory by using density functional theory (DFT) and molecular dynamics (MD) simulations. They discovered a mechanism for dislocation nucleation and migration driven by surface segregation of solute atoms in a solid solution. Their results show that the surface-segregation-induced composition variations act as the source of strain/stress that drives the nucleation and migration of misfit dislocations, and demonstrate how the surface segregation phenomenon of an alloy constituent can be employed for developing atomistic insight into understanding the formation processes of misfit-accommodating dislocations.

Where are hot carriers created in plasmonically enhanced semiconductor substrates?

  • By Burcu Ozden
  • 5 December 2017

PQI members Hrvoje Petek, Jin Zhao and their colleagues investigated a less known fact about the microscopic details of how the combined optical, electronic and chemical properties of metal/semiconductor interfaces define the coupling of light into the electronic reagents on their recent paper published in Nature Photonics. In this study, they investigated the coherence and hot electron dynamics in a prototypical Ag nanocluster/TiO2 heterojunction via ultrafast two-photon photoemission (2PP) spectroscopy, scanning tunneling microscopy (STM) and density functional theory (DFT).  The silver nanoclustors used in this study were grown via e-beam evaporation of Ag on top of TiO2 surface.They have shown that the plasmon excitation, dephasing and hot electron processes that are related to plasmonically enhanced photocatalysis involve complex physical and chemical interactions, with strong interfacial character involving the chemical and plasmonic coupling of Ag nanoclusters and the TiO2 substrate that cannot be predicted by the properties of the component materials, but rather require an understanding of their interactions. They found that the dephasing of the perpendicular and parallel plasmons by the dielectric screening response of the TiO2 substrate generates hot electrons with anisotropic and non-thermal distributions.

Prof. Lillian Chong has created the Creative Science Summer Writing Program for Undergraduates

  • By Leena Aggarwal
  • 5 December 2017

This past summer, Prof. Lillian Chong started a creative science writing program to help undergraduates develop skills for communicating science to non-scientists. The pilot group consisted of three highly motivated chemistry majors who pursued various types of creative writing, including poetry and narrative nonfiction. 

The development of effective writing skills in the sciences has become increasingly more important given the critical roles that science plays in society. To help undergraduates at the University of Pittsburgh develop such highly valuable skills, the Creative Science Writing Summer Program is intended to foster undergraduate writing projects that are focused on communicating science in a compelling, accessible manner to non-scientists. This Program is available to undergraduates in the Departments of Chemistry, Biological Sciences, Physics & Astronomy, Neuroscience, History & Philosophy of Science, and/or English. Each of six selected participants will be awarded a prize of $250 to pursue creative writing involving scientific journalism, poetry, and other works of nonfiction during the summer.

 

From Quantum Mechanics to Force Fields

  • By Burcu Ozden
  • 22 November 2017

Ken Jordan and his colleague are invited to write a special topic issue in the journal of chemical physics (JCP). This work is dedicated to the ongoing efforts of the theoretical chemistry community to develop a new generation of accurate force fields based on data from high-level electronic structure calculations and to develop faster electronic structure methods for testing and designing force fields as well as for carrying out simulations. 

What molecular properties give rise to a strong piezoelectric response?

  • By Burcu Ozden
  • 22 November 2017

In this study Geoffrey R. Hutchison and his colleagues tried to answer the question of " What molecular properties give rise to a strong piezoelectric response?"  To do so, they systematically probe the interplay among peptide chemical structure, folding propensity, and piezoelectric properties, uncovering in the process new insights into the origin of peptide electromechanical response. They have designed variety of peptides and peptoids and test the effect of molecular properties on piezoelectric response via serious measurements including ircular dichroism (CD), Polarization-modulated infrared reflection−absorption spectroscopy (PM-IRRAS), tomic force microscopy (AFM), piezo-force microscopy (PFM), and X-ray photoelectron spectroscopy (XPS) measurements. They showed backbone rigidity is an important determinant in peptide electromechanical responsiveness. 

New Era in Thermal Scanning Probe Lithography

  • By Burcu Ozden
  • 15 November 2017

Tevis Jacobs and his collaborators from IBM and SwissLitho were achieved sub-10 nanometer feature size in Silicon using thermal scanning probe lithography. In this work, they  the t-SPL parameters that influence high-resolution patterning on the transfer stack and demonstrate that sub-15 nm half-pitch resolution patterning and transfer by t-SPL are feasible. They found that the resolution in t-SPL is limited by the extent of the plastic zone in thermo-mechanical indentation on the pattern transfer stack because, at temperatures approaching the resist’s decomposition temperature, the line shape widens, reducing the achievable resolution. They achieved reliable transfer of patterned dense lines down to 14 nm half-pitch and in the best case 11 nm half-pitch. Furthermore, evidently they showed that an enhanced resolution below 10 nm half-pitch might be possible on a mechanically different transfer stack.

Quantum materials: Where many paths meet

  • By Leena Aggarwal
  • 27 October 2017

In Nature, there exist materials with exotic properties that cannot be understood in the framework of classical theories. Such properties, however, are beautifully described by more sophisticated theoretical tools involving quantum mechanics.  Such materials are now known as the “quantum materials”. The range of exotic properties exhibited by the quantum materials is extremely broad and includes superconductivity, superfluidity, ferromagnetism, quantum hall effect, spin-liquidity, topological insulation, to name a few.

Superconductors, discovered by Kammerlingh Onnes, 1911, were first to emerge as quantum materials. In normal metals, the resistance arises due to inelastic scattering between the charge carriers (electrons) and defects in the periodic crystal lattice. The defects or scattering centres can be any distortion to the periodicity of the lattice like those due to presence of impurity or the thermal vibration of the lattice points. In superconductors, surprisingly, the resistance becomes zero despite the presence of a large number of impurities and at high temperatures where the lattice points can undergo vigorous thermal vibration. The question that how the charge carriers remained insensitive to such strong scattering centres could not be answered within any classical picture. A microscopic understanding of superconductivity was first provided by Bardeen, Cooper and Schrieffer (BCS) in 1951, only after substantial development of quantum mechanics and quantum field theories – the theories where quantum mechanics is combined with Einstein’s theory of relativity.

New Technique for Measuring the Layer-Resolved Charge Density

  • By Burcu Ozden
  • 25 October 2017

Recently Benjamin M. Hunt and his colleagues developed a new technique for measuring the layer-resolved charge density, from which they can map layer polarization of the valley or spin quantum numbers in bilayer graphine and other two dimensional materials. In this study, they demonstrated direct measurement of valley and orbital levels in bilayer graphite. They have detected that the four valley and orbital components have different weights on the two layers of the bilayer. By using Hunt’s technique one can probe layer, valley, and spin polarization quantitatively in other atomic layered materials, including twisted bilayer graphene and both homobilayer and heterobilayer of transition metal dichalcogenide

 

The new era of Polariton condensates

  • By Leena Aggarwal
  • 17 October 2017

In a recently published paper in Physics Today, David Snoke, a professor of physics and astronomy at the University of Pittsburgh and Jonathan Keeling who is a reader in theoretical condensed-matter physics at the University of St. Andrews in Scotland, have shown the superfluidity of light where photon treats as a gas of interacting bosonic atoms. They have demonstrated that how to engineer a Bose–Einstein condensation from light.

 

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