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News Article | May 2, 2017
Site: www.eurekalert.org

Thinning a material down to a single-atom thickness can dramatically change that material's physical properties. For example, graphene, the best-known 2D material, has unparalleled strength and electrical conductivity, unlike its bulk form, graphite. Researchers have begun to study hundreds of other 2D materials for the purposes of electronics, sensing, early cancer diagnosis, water desalination and a host of other applications. Now, a team of Penn State researchers in the Department of Physics and the Center for Two-Dimensional and Layered Materials (2DLM) has developed a fast, nondestructive optical method for analyzing defects in 2D materials. "In the semiconductor industry, for example, defects are important because you can control properties through defects," said Mauricio Terrones, professor of physics, materials science and engineering and chemistry. "This is known as defect engineering. Industry knows how to control defects and which types are good for devices." To really understand what is going on in a 2D material like tungsten disulfide, which has a single atom-thick layer of tungsten sandwiched between two atomic layers of sulfur, would require a high-power electron microscope capable of seeing individual atoms and the holes, called vacancies, where the atoms are missing. "The benefit of transmission electron microscopy (TEM) is that you get an image and you can see directly what is going on -- you get direct evidence," said Bernd Kabius, staff scientist at Penn State's Materials Research Institute, an expert in TEM and a coauthor on the paper, which appeared recently in Science Advances. The downsides, according to Kabius, are an increased possibility of damage to the delicate 2D material, the complex preparation required of the sample, and the time involved -- an entire day of instrument time to image a single sample and a week or more to interpret the results. For those reasons, and others, researchers would like to combine TEM with another method of looking at the sample that is simpler and faster. The technique developed by Terrones and his team uses an optical method, fluorescent microscopy, in which a laser of a specific wavelength is shone on a sample. The excited electrons, pushed to a higher energy level, each emit a photon of a longer wavelength when they drop down to a lower energy level. The longer wavelength can be measured by spectroscopy and gives information about the defect type and location on the sample. The team can then correlate the results with visual confirmation under the TEM. Theoretical calculations also helped to validate the optical results. The sample must be placed in a temperature-controlled specimen holder and the temperature lowered to 77 Kelvin, almost 200 degrees Celsius below zero. At this temperature, the electron-hole pairs that produce the fluorescence are bound to the defect -- in the case of this work a group of sulfur vacancies in the top layer of the sandwich -- and emit a signal stronger than the pristine areas of the material. "For the first time, we have established a direct relationship between the optical response and the amount of atomic defects in two-dimensional materials," said Victor Carozo, former postdoctoral scholar in Terrones' lab and first author of the work. Terrones added, "For the semiconductor industry, this is a quick measurement, an optical nondestructive method to evaluate defects in 2D systems. The important thing is that we were able to correlate our optical method with TEM and also with atomistic simulations. I think this method can be very helpful in establishing a protocol for characterization of 2D crystalline materials." In this context, co-author Yuanxi Wang, a postdoc in the 2DLM and a theorist, added, "Our calculations show that electrons trapped by vacancies emit light at wavelengths different than the emission from defect-free regions. Regions emitting light at these wavelengths can easily identify vacancies within samples." Vincent Crespi, distinguished professor of physics, materials science and engineering and chemistry, Penn State, said "We can establish not just an empirical correlation between the presence of certain defects and modified light emission, but also identify the reason for that correlation through first-principles calculations." Device applications that could be enhanced by this work include membranes with selective pore sizes for removing salt from water or for DNA sequencing, gas sensing when gas molecules bind to specific vacancies and the doping of 2D materials, which is the addition of foreign atoms to enhance properties. Other authors on the Science Advances paper, "Optical Identification of Sulfur Vacancies: Bound Excitons at the Edges of Monolayer Tungsten Disulfide," are postdoctoral scholars Kazunori Fujisawa, Bruno Carvalho and Amber McCreary; doctoral students Simin Feng, Zhong Lin and Chanjing Zhou; and research associates Nestor Perea-Lopez and Ana Laura Elias. The National Science Foundation and the U.S. Army Research Office supported this work.


News Article | May 1, 2017
Site: phys.org

Molecular model of a WS2 triangular monolayer targeted with a green laser (hv’). Red light (hv) is emitted from the edges where defects consisting of sulfur vacancies are located. Electron-hole pairs are bound at the vacancy site (see inset). Credit: Yuanxi Wang, Penn State By now, it is well understood that thinning a material down to a single atom thickness can dramatically change that material's physical properties. Graphene, the best known 2-D material, has unparalleled strength and electrical conductivity, unlike its bulk form as graphite. Researchers have begun to study hundreds of other 2-D materials for the purposes of electronics, sensing, early cancer diagnosis, water desalination and a host of other applications. Now, a team of Penn State researchers in the Department of Physics and the Center for Two-Dimensional and Layered Materials (2DLM) has developed a fast, nondestructive optical method for analyzing defects in two-dimensional materials. "In the semiconductor industry, for example, defects are important because you can control properties through defects," said Mauricio Terrones, professor of physics, materials science and engineering and chemistry, Penn State. "This is known as defect engineering. Industry knows how to control defects and which types are good for devices." To really understand what is going on in a 2-D material like tungsten disulfide, which has a single atom-thick layer of tungsten sandwiched between two atomic layers of sulfur, would require a high-power electron microscope capable of seeing individual atoms and the holes, called vacancies, where the atoms are missing. "The benefit of transmission electron microscopy (TEM) is that you get an image and you can see directly what is going on – you get direct evidence," said Bernd Kabius, staff scientist at Penn State's Materials Research Institute, an expert in TEM and a coauthor on the paper appearing April 28 in the online journal Science Advances. The downsides, according to Kabius, are an increased possibility of damage to the delicate 2-D material, the complex preparation required of the sample and the time involved - an entire day of instrument time to image a single sample and a week or more to interpret the results. For those reasons, and others, researchers would like to combine TEM with another method of looking at the sample that is simpler and faster. The technique developed by Terrones and his team uses an optical method, fluorescent microscopy, in which a laser of a specific wavelength is shone on a sample and the excited electrons, pushed to a higher energy level, each emit a photon of a longer wavelength when the electron drops down to a lower energy level. The wavelength, or color of light, can be measured by spectroscopy and gives information about the defect type and location on the sample. This data shows up as peaks on a graph, which the team then correlated to visual confirmation under the TEM. Theoretical calculations also helped to validate the optical results. A necessary step in the process requires placing the sample in a temperature-controlled specimen holder, or stage, and lowering the temperature to 77 kelvin, almost 200 degrees C below zero. At this temperature, the electron-hole pairs that produce the fluorescence are bound to the defect - in the case of this work a group of sulfur vacancies in the top layer of the sandwich - and emit a signal stronger than the pristine areas of the material. "For the first time, we have established a direct relationship between the optical response and the amount of atomic defects in two-dimensional materials," said Victor Carozo, former postdoctoral scholar in Terrones' lab and first author of the work. Terrones added, "For the semiconductor industry, this is a quick measurement, an optical nondestructive method to evaluate defects in 2-D systems. The important thing is that we were able to correlate our optical method with TEM and also with atomistic simulations. I think this method can be very helpful in establishing a protocol for characterization of 2-D crystalline materials." In this context, co-author Yuanxi Wang, a postdoctoral researcher in the 2DLM and a theorist, added, "Our calculations show that electrons trapped by vacancies emit light at wavelengths different than the emission from defect-free regions. Regions emitting light at these wavelengths can easily identify vacancies within samples." And Vincent Crespi, Distinguished Professor of Physics, Materials Science and Engineering and Chemistry, Penn State, said "We can establish not just an empirical correlation between the presence of certain defects and modified light emission, but also identify the reason for that correlation through first-principles calculations." Device applications that could be enhanced by this work include membranes with selective pore sizes for removing salt from water or for DNA sequencing, gas sensing when gas molecules bind to specific vacancies and the doping of 2-D materials, which is the addition of foreign atoms to enhance properties. Explore further: Defects in 2D semiconductors could lead to multi-colored light-emitting devices More information: Victor Carozo et al. Optical identification of sulfur vacancies: Bound excitons at the edges of monolayer tungsten disulfide, Science Advances (2017). DOI: 10.1126/sciadv.1602813


News Article | May 18, 2017
Site: www.materialstoday.com

A team of researchers in the Department of Physics and the Center for Two-Dimensional and Layered Materials (2DLM) at Penn State has developed a fast, non-destructive optical method for analyzing defects in 2D materials. They report this novel method in a paper in Science Advances. "In the semiconductor industry, for example, defects are important because you can control properties through defects," said Mauricio Terrones, professor of physics, materials science and engineering and chemistry. "This is known as defect engineering. Industry knows how to control defects and which types are good for devices." To really understand what is going on in a 2D material like tungsten disulfide, which comprises a single atom-thick layer of tungsten sandwiched between two atomic layers of sulfur, requires a high-power electron microscope capable of seeing individual atoms and the holes, called vacancies, where the atoms are missing. "The benefit of transmission electron microscopy (TEM) is that you get an image and you can see directly what is going on – you get direct evidence," said Bernd Kabius, staff scientist at Penn State's Materials Research Institute, an expert in TEM and a co-author of the paper. The downsides to TEM, according to Kabius, are an increased possibility of damaging the delicate 2D material, complex sample preparation processes, and the time involved – an entire day of instrument time to image a single sample and a week or more to interpret the results. For those reasons, and others, researchers would like to combine TEM with another method of looking at the sample that is simpler and faster. The technique developed by Terrones and his team employs fluorescent microscopy, which involves shining laser light at a specific wavelength on a sample. In this novel technique, electrons excited by the laser light are pushed to a higher energy level, and then each emit a photon of a longer wavelength when they subsequently drop back down to a lower energy level. The longer wavelength can be measured by spectroscopy to provide information on the type and location of defects in the sample. The team can then correlate the results with visual images produced by the TEM; theoretical calculations can also help to validate the optical results. The sample must be placed in a temperature-controlled specimen holder and the temperature lowered to 77K, almost 200°C below zero. At this temperature, the electron-hole pairs that produce the fluorescence are bound to the defect – in this case, a group of sulfur vacancies in the top layer of the sandwich – and emit a signal stronger than the pristine areas of the material. "For the first time, we have established a direct relationship between the optical response and the amount of atomic defects in two-dimensional materials," said Victor Carozo, former postdoctoral scholar in Terrones' lab and first author of the work. "For the semiconductor industry, this is a quick measurement, an optical non-destructive method to evaluate defects in 2D systems," added Terrones. "The important thing is that we were able to correlate our optical method with TEM and also with atomistic simulations. I think this method can be very helpful in establishing a protocol for characterization of 2D crystalline materials." "Our calculations show that electrons trapped by vacancies emit light at wavelengths different than the emission from defect-free regions," said Yuanxi Wang, a postdoc in the 2DLM and a theorist. "Regions emitting light at these wavelengths can easily identify vacancies within samples." "We can establish not just an empirical correlation between the presence of certain defects and modified light emission, but also identify the reason for that correlation through first-principles calculations," said Vincent Crespi, professor of physics, materials science and engineering and chemistry. This novel analytical technique could lead to advances in various technologies. These include membranes with selective pore sizes for removing salt from water or for DNA sequencing, gas sensing when gas molecules bind to specific vacancies and the doping of 2D materials. This story is adapted from material from Penn State, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.


Penn State researchers have developed a new method for sintering, a widely used manufacturing process for powdered materials. Using far less time and energy than the standard approach, the new method could have global implications on manufacturing and energy savings and pave the way for new discoveries. Cold sintering, a process devised by a team led by Clive Randall, professor of materials science and engineering and director of Penn State's Materials Research Institute, is a new take on sintering, a process through which powder-form materials are densified—compressed—using heat and pressure. Sintering is used to manufacture many materials including glass, metals, bricks and plastics. Randall's approach uses liquid to complete the sintering process at times and temperatures that are a fraction of current methods. Because the process is completed in minutes instead of hours, time and energy savings could result in huge productivity and cost gains for the manufacturing sector and could lead to far fewer greenhouse gas emissions from manufacturing. "What we're doing is using a liquid in a dissolution process. It then works by an evaporation process," he said. "That's been done before but usually with phases that aren't transient. What's really important about this process is that this liquid is there and then it's gone, and in the process of being there and gone it's capturing all the exchange and diffusional and growth processes that you need to drive the sintering." Because traditional sintering occurs over many hours at temperatures around 1,000 degrees Celsius, and cold sintering takes place at temperatures from room temperature to 200 degrees Celsius, the process has opened the door for novel manufacturing materials that can't sustain the higher temperatures of traditional sintering. "The ability to incorporate new materials into that whole process and make new types of functionality and then finally to have a system where it's basically densified in 20 minutes means that your through-put and your manufacturing yields could go up enormously," Randall said. "This is great for manufacturing, it's great for energy savings, it's great for the environment and it's now permitting new intellectual endeavors in making materials." Explore further: Cold sintering of ceramics instead of high-temperature firing


Lee C.M.,Materials Research Institute, LLC | Chen X.,Pennsylvania State University | Weiss P.A.,Pennsylvania State University | Jensen L.,Pennsylvania State University | Kim S.H.,Materials Research Institute, LLC
Journal of Physical Chemistry Letters | Year: 2017

Vibrational sum-frequency-generation (SFG) spectroscopy is capable of selectively detecting crystalline biopolymers interspersed in amorphous polymer matrices. However, the spectral interpretation is difficult due to the lack of knowledge on how spatial arrangements of crystalline segments influence SFG spectra features. Here we report time-dependent density functional theory (TD-DFT) calculations of cellulose crystallites in intimate contact with two different polarities: parallel versus antiparallel. TD-DFT calculations reveal that the CH/OH intensity ratio is very sensitive to the polarity of the crystallite packing. Theoretical calculations of hyperpolarizability tensors (βabc) clearly show the dependence of SFG intensities on the polarity of crystallite packing within the SFG coherence length, which provides the basis for interpretation of the empirically observed SFG features of native cellulose in biological systems. © 2016 American Chemical Society.


Chiadini F.,University of Salerno | Fiumara V.,University of Basilicata | Scaglione A.,University of Salerno | Pulsifer D.P.,Pennsylvania State University | And 4 more authors.
Optics and Photonics News | Year: 2011

Researchers are looking to the compound eyes of insects as a model for developing their unique approach to harvesting sunlight. Each compound eye comprises several cylindrical eyelets called ommatidia that are arrayed on a curved surface. Light propagating along the axis of an ommatidium is collected to form an image, but light propagating in other directions and reaching an ommatidium is absorbed by its dark side wall. The first phase requires the numerical simulation of light interacting with the air-silicon interface. A simplified two-dimensional bioinspired texturing of the exposed face was considered as the first step of this phase. Results indicated that the bioinspired textured solar cell exhibits light-coupling efficiency. A Nano4 technique has been developed to manufacture multiple high-fidelity replicas of a single biotemplate. The technique can produce multiple replicas simultaneously of multiple biotemplates.


Sun K.G.,Materials Research Institute, LLC | Sun K.G.,Pennsylvania State University | Nelson S.F.,Eastman Kodak Co. | Jackson T.N.,Materials Research Institute, LLC | Jackson T.N.,Pennsylvania State University
Device Research Conference - Conference Digest, DRC | Year: 2015

Vertical thin film transistors (VTFTs) achieve sub-micron channel length without expensive high-resolution photolithography by taking advantage of a three-dimensional device structure. Recently, ZnO VTFTs with active layers deposited by spatial atomic layer deposition (SALD) were demonstrated with large current density (10 mA/mm), high mobility (>14 cm2/Vs) and large on-off ratio (>107) [1]. Asymmetric saturation-region current-voltage characteristics were also obtained when the transistor source and drain electrodes were interchanged. Using the Synopsys Sentaurus drift-diffusion simulator we developed a physics-based two-dimensional model for SALD ZnO VTFTs. Using the model, we are able to reproduce the electrical behavior of the ZnO VTFTs and understand the role of nanometer-scale features in the device structure. © 2015 IEEE.


Zhang Y.,University of Texas at Arlington | Tran R.T.,Materials Research Institute, LLC | Qattan I.S.,University of Texas at Arlington | Tsai Y.-T.,University of Texas at Arlington | And 3 more authors.
Biomaterials | Year: 2013

The field of tissue engineering and drug delivery calls for new measurement tools, non-invasive real-time assays, and design methods for the next wave of innovations. Based on our recent progress in developing intrinsically biodegradable photoluminescent polymers (BPLPs) without conjugating organic dyes or quantum dots, in this paper, we developed a new type urethane-doped biodegradable photoluminescent polymers (UBPLPs) that could potentially serve as a new tool to respond the above call for innovations. Inherited from BPLPs, UBPLPs demonstrated strong inherent photoluminescence and excellent cytocompatibility in vitro. Crosslinked UBPLPs (CUBPLPs) showed soft, elastic, but strong mechanical properties with a tensile strength as high as 49.41 ± 6.17 MPa and a corresponding elongation at break of 334.87 ± 26.31%. Porous triphasic CUBPLP vascular scaffolds showed a burst pressure of 769.33 ± 70.88 mmHg and a suture retention strength of 1.79 ± 0.11 N. Stable but photoluminescent nanoparticles with average size of 103 nm were also obtained by nanoprecipitation. High loading efficiency (91.84%) and sustained release of 5-fluorouracil (up to 120 h) were achieved from UBPLP nanoparticles. With a quantum yield as high as 38.65%, both triphasic scaffold and nanoparticle solutions could be non-invasively detected in vivo. UBPLPs represent an innovation in fluorescent biomaterial design and may offer great potential in advancing the field of tissue engineering and drug delivery where bioimaging has gained increasing interest. © 2013 Elsevier Ltd.


Mor G.K.,Materials Research Institute, LLC | Basham J.,Materials Research Institute, LLC | Paulose M.,Materials Research Institute, LLC | Kim S.,Pennsylvania State University | And 4 more authors.
Nano Letters | Year: 2010

Solid-state dye-sensitized solar cells (SS-DSCs) offer the potential to make low cost solar power a reality, however their photoconversion efficiency must first be increased. The dyes used are commonly narrow band with high absorption coefficients, while conventional photovoltaic operation requires proper band edge alignment significantly limiting the dyes and charge transporting materials that can be used in combination. We demonstrate a significant enhancement in the light harvesting and photocurrent generation of SS-DSCs due to Förster resonance energy transfer (FRET). TiO2 nanotube array films are sensitized with red/near IR absorbing SQ-1 acceptor dye, subsequently intercalated with Spiro-OMeTAD blended with a visible light absorbing DCM-pyran donor dye. The calculated Förster radius is 6.1 nm. The donor molecules contribute a FRET-based maximum IPCE of 25% with a corresponding excitation transfer efficiency of approximately 67.5%. © 2010 American Chemical Society.


Grant
Agency: Department of Defense | Branch: Air Force | Program: SBIR | Phase: Phase I | Award Amount: 149.98K | Year: 2013

ABSTRACT: This SBIR Phase I program is to develop and transition a lightweight shielding material technology for wideband electromagnetic shielding and high power microwave hardening of aircraft signal and power cables. Materials Research Institute proposes to use metalized nanometric and micrometric materials as conductive fillers to formulate the lightweight shielding material. Model simulation will be employed to identify and verify the most promising conductive filler and optimize the structure and composition of the shielding material. Prototype shielding materials will be fabricated for shielding effectiveness characterization. Experimental data and simulation results will be reviewed to refine the shielding material with a goal leading to a successful demonstration of the material concept feasibility on 6-ft long shielded cables. BENEFIT: The proposed lightweight shielding material is expected to have advantages over cable shields made of metal wires or metal particulates in weight savings, flexibility, mechanical durability, higher shielding capability, and lower manufacturing cost. It is also expected to have advantages over shielding materials made of carbon nanotubes or carbon nanofibers in lower cost, greater mechanical durability, and higher shielding performance.

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