- 2015-10-07 16:45:00
- Article ID: 641064
Laser Spectroscopy of Ultrathin Semiconductor Reveals Rise of ‘Trion’ Quasiparticles
Insight into dynamics may spur improved materials for solar energy, quantum computing
-
Credit: Image credit - Oak Ridge National Laboratory, U.S. Dept. of Energy
In a pump-probe experiment, the pump laser pulse first excites the 2D material, and later, at controllable time-delays, the probe laser pulse returns to the energy-pumped site to provide information about the evolution of the pump’s effect on the material. In the ORNL experiment, absorption of pumped energy first generated two excitons, X1 and X2. Dissociation of these excitons through hole trapping at the substrate freed their electrons. Then the arriving probe pulse generated new electron–hole pairs, which joined the remaining free electrons to form trions T1 and T2.
Insight into dynamics may spur improved materials for solar energy, quantum computing
“We observed negative trions in a two-dimensional tungsten disulfide monolayer excited by a laser beam,” said ultrafast laser spectroscopist Abdelaziz Boulesbaa, who co-led the study with theorist Bing Huang and consulted with laser spectroscopy expert Alex Puretzky. “This discovery may open new opportunities to optoelectronic applications, including information technology, as well as fundamental research in the physics of low-dimensional materials.”
When a semiconductor absorbs light, electrons can be knocked loose and can participate in an electrical current. However, typically two charges form—one negative (an electron) and one positive (a hole)—and are bound to each other for a short time, traveling through the crystal as a quasiparticle called an “exciton.” When an exciton binds to an additional electron, the complex formed is a negative trion, or if it binds to an additional hole, the resulting quasiparticle is a positive trion.
Quasiparticles like excitons may sound exotic, but getting electrons and holes together is the basis for everyday light-emitting diodes (LEDs). When an electron and hole recombine in an LED, a photon is emitted. That’s the light we see in applications from traffic lights and electronic signage to camera flashes and vehicle headlights.
Whereas LEDs emit light, solar cells absorb light and convert its energy into electricity. To make solar cells work, scientists try to separate the electrons from the holes and collect those charges before they have a chance to recombine. Future materials may make use of negative trions to improve charge collection in solar cells, according to Boulesbaa.
Pump–probe experiment
To harness negative trions for improving solar cells and other optoelectronic technologies, scientists need answers to basic questions: How do negative trions form? How long do they live? Why do they form so efficiently in an ultrathin semiconductor?
To answer these questions, the ORNL scientists needed a “camera” of sorts that could make a super-slow-motion movie to reveal quasiparticle dynamics, akin to the camera techniques photographers employ to capture speeding bullets obliterating apples—only a billion times faster. A split laser beam created that camera.
Employing half the laser beam, they fired laser pulses lasting a mere 40 femtoseconds (million-billionths of a second) to excite an ultrathin crystal of tungsten disulfide. Then, for their super slow-motion movie, they fashioned a strobe using the other half of the laser beam—an ultrafast flash of white light—and passed it through the crystal at different delayed times. By measuring the photon energy wavelengths (colors) the crystals absorbed at each time, the scientists built, frame by frame, a slow-motion “movie” of how trions form and fade. They probably skipped the popcorn, as their movie lasted only a nanosecond (one billionth of a second).
Their movie revealed trions form only after electron–hole pairs form. Then the holes get trapped, most likely by the substrate in contact with the crystal, leaving extra electrons.
These extra electrons allow the crystal to absorb another photon to form a negative trion. Because the ultrathin crystals are all “surface,” they have a lot of opportunity to interact with surroundings and to separate charges that are created, making them great trion generators.
Because the researchers used white light, a mixture of all frequencies of light in the visible spectrum, their observation of light of different colors revealed that two different trions had formed, which had not been seen previously.
Next the scientists will study the role of the substrate in defining optical and electrical properties of 2D semiconducting materials. Some substrates trap electrons, leaving excess holes to carry charges, whereas others trap holes, leaving excess electrons to carry charges. Furthermore, the researchers will isolate the 2D semiconductor from the substrate by introducing, in between, an insulator to prevent holes and electrons from reaching the substrate, allowing excitons to live longer and emit light for a greater duration.
The title of the paper is “Observation of two distinct negative trions in tungsten disulfide monolayers.” This research was conducted at the Center for Nanophase Materials Sciences at ORNL. Computations were performed at the National Energy Research Scientific Computing Center at Lawrence Berkeley National Laboratory. Both are DOE Office of Science User Facilities.
UT-Battelle manages ORNL for DOE’s Office of Science. The single largest supporter of basic research in the physical sciences in the United States, the Office of Science is working to address some of the most pressing challenges of our time. For more information, please visit http://science.energy.gov/.—by Dawn Levy
MORE NEWS FROM
Oak Ridge National Laboratory
comments 
commenting policy
Participating Labs
- DOE Office of Science
- Argonne National Laboratory
- Oak Ridge National Laboratory
- Pacific Northwest National Laboratory
- Iowa State University, Ames Laboratory
- Brookhaven National Laboratory
- Princeton Plasma Physics Laboratory
- Lawrence Berkeley National Laboratory
- Thomas Jefferson National Accelerator Facility
- Fermi National Accelerator Laboratory (Fermilab)
- SLAC National Accelerator Laboratory
A New Parallel Strategy for Tackling Turbulence on Summit
A team at Georgia Tech created a new turbulence algorithm optimized for the Summit supercomputer. It reached a performance of less than 15 seconds of wall-clock time per time step for more than 6 trillion grid points--a new world record surpassing the prior state of the art in the field for the size of the problem.
Modeling Every Building in America Starts with Chattanooga
An ORNL team used the Titan supercomputer to model every building serviced by the Electric Power Board of Chattanooga--all 178,368 of them--and discovered that EPB could potentially save $11-$35 million per year by adjusting electricity usage during peak critical times.
Climate Change Expected to Shift Location of East Asian Monsoons
More than a billion people in Asia depend on seasonal monsoons for their water needs. The Asian monsoon is closely linked to a planetary-scale tropical air flow which, according to a new study by Lawrence Berkeley National Laboratory, will most likely shift geographically as the climate continues to warm, resulting in less rainfall in certain regions.
Nuclear warheads? This robot can find them
PPPL and Princeton University are developing a unique neutron-detector robot for arms control and nuclear security purposes. The robot recently passed a key neutron-detection test.
Deep neural networks speed up weather and climate models
A team of environmental and computation scientists at the U.S. Department of Energy's (DOE) Argonne National Laboratory are collaborating to use deep neural networks, a type of machine learning, to replace the parameterizations of certain physical schemes in the Weather Research and Forecasting Model, an extremely comprehensive model that simulates the evolution of many aspects of the physical world around us.
New AI Model Tries to Synthesize Patient Data Like Doctors Do
A new approach developed by PNNL scientists improves the accuracy of patient diagnosis up to 20 percent when compared to other embedding approaches.
Scientists Explore Egyptian Mummy Bones With X-Rays and Infrared Light to Gain New Insight on Ancient Life
Experiments at Berkeley Lab are casting a new light on Egyptian soil and ancient mummified bone samples that could provide a richer understanding of daily life and environmental conditions thousands of years ago. In a two-monthslong research effort that concluded in late August, two researchers from Cairo University in Egypt brought 32 bone samples and two soil samples to study using X-ray and infrared light-based techniques at the Lab's Advanced Light Source.
Etalumis 'Reverses' Simulations to Reveal New Science
A multinational collaboration using computing resources at the National Energy Research Scientific Computing Center has developed the first probabilistic programming framework capable of controlling existing simulators and running at large-scale on HPC platforms.
Deep Learning Expands Study of Nuclear Waste Remediation
A research collaboration between Berkeley Lab, Pacific Northwest National Laboratory, Brown University, and NVIDIA has achieved exaflop performance with a deep learning application used to model subsurface flow in the study of nuclear waste remediation
Biofuel producers make significant gains in efficiency, productivity and conservation, Argonne survey shows
The nation's biofuel producers have made significant gains in both energy efficiency and water conservation in recent years, according to a comprehensive survey conducted by Argonne National Laboratory.
PPPL wins $70,000 in project funding from DOE for entrepreneurship
The Princeton Plasma Physics Laboratory receives funding from the U.S. Department of Energy for two projects to encourage entrepreneurship and mentor and encourage potential entrepreneurs.
Brookhaven-Commonwealth Fusion Energy Project Wins DOE Funding
Brookhaven's Superconducting Magnet Division will partner with industry to develop and characterize superconducting power cables.
U.S. Department of Energy to Hold Fifth CyberForce Competition(tm)
Going on its fourth year, DOE's CyberForce Competition(tm) on Nov. 15-16 will give teams of cybersecurity students and professionals the opportunity to compete and refine their skills in real-time at 10 national laboratories across the U.S.
Daniel Gruen awarded 2019 Panofsky Fellowship at SLAC
Daniel Gruen's work on how massive objects bend light from distant galaxies is aimed at unraveling some of the greatest mysteries of modern physics: What is dark matter? What is dark energy, and how is it accelerating the expansion of the universe?
DOE Announces FY 2020 Small Business Innovation Research Funding Opportunity
The Department of Energy (DOE) Small Business Innovation Research (SBIR) and Small Business Technology Transfer (STTR) programs issued its FY 2020 Phase II Release 1 Funding Opportunity Announcement (FOA) with approximately $97 million in available funding.
Research effort by Argonne National Laboratory and the University of Chicago results in R&D 100 Award
A joint effort by the U.S. Department of Energy's Argonne National Laboratory and the University of Chicago has led to a prestigious R&D 100 Award and is expected to bring an innovation closer to market so it ultimately can be used in many industrial applications.
Department of Energy Awards Fermilab Funding for Next-Generation Dark Matter Research
The U.S. Department of Energy announced that it has awarded scientists at its Fermi National Accelerator Laboratory funding to boost research on dark matter, the mysterious substance that makes up an astounding 85% of the matter in the universe.
Fermilab Scientist Xingchen Xu Receives Prestigious DOE Award to Develop Superconductors
Fermilab scientist Xingchen Xu has received the prestigious $2.5 million Department of Energy Early Career Research Award to fund his five-year mission: advancing two technologies that will improve the performance niobium-tin superconductor by 50% or more, allowing for smaller coils, stronger magnetic fields and lower costs.
ORNL to take on nine power grid modernization projects as part of DOE award
Oak Ridge National Laboratory researchers will lead two new projects and support seven more to enhance the reliability and resilience of the nation's power grid as part of the U.S. Department of Energy's 2019 Grid Modernization Lab Call.
Berkeley Lab Innovations Recognized With 3 R&D 100 Awards
Cutting-edge technologies from Lawrence Berkeley National Laboratory (Berkeley Lab) to detect radiation, make buildings more energy efficient, and accelerate neuroscience research were honored with R&D 100 Awards by R&D World magazine.
Harvesting Energy from Light using Bio-inspired Artificial Cells
Scientists designed and connected two different artificial cells to each other to produce molecules called ATP (adenosine triphosphate).
Engineering Living Scaffolds for Building Materials
Bone and mollusk shells are composite systems that combine living cells and inorganic components. This allows them to regenerate and change structure while also being very strong and durable. Borrowing from this amazing complexity, researchers have been exploring a new class of materials called engineered living materials (ELMs).
Excavating Quantum Information Buried in Noise
Researchers developed two new methods to assess and remove error in how scientists measure quantum systems. By reducing quantum "noise" - uncertainty inherent to quantum processes - these new methods improve accuracy and precision.
How Electrons Move in a Catastrophe
Lanthanum strontium manganite (LSMO) is a widely applicable material, from magnetic tunnel junctions to solid oxide fuel cells. However, when it gets thin, its behavior changes for the worse. The reason why was not known. Now, using two theoretical methods, a team determined what happens.
When Ions and Molecules Cluster
How an ion behaves when isolated within an analytical instrument can differ from how it behaves in the environment. Now, Xue-Bin Wang at Pacific Northwest National Laboratory devised a way to bring ions and molecules together in clusters to better discover their properties and predict their behavior.
Tune in to Tetrahedral Superstructures
Shape affects how the particles fit together and, in turn, the resulting material. For the first time, a team observed the self-assembly of nanoparticles with tetrahedral shapes.
Tracing Interstellar Dust Back to the Solar System's Formation
This study is the first to confirm dust particles pre-dating the formation of our solar system. Further study of these materials will enable a deeper understanding of the processes that formed and have since altered them.
Investigating Materials that Can Go the Distance in Fusion Reactors
Future fusion reactors will require materials that can withstand extreme operating conditions, including being bombarded by high-energy neutrons at high temperatures. Scientists recently irradiated titanium diboride (TiB2) in the High Flux Isotope Reactor (HFIR) to better understand the effects of fusion neutrons on performance.
Better 3-D Imaging of Tumors in the Breast with Less Radiation
In breast cancer screening, an imaging technique based on nuclear medicine is currently being used as a successful secondary screening tool alongside mammography to improve the accuracy of the diagnosis. Now, a team is hoping to improve this imaging technique.
Microbes are Metabolic Specialists
Scientists can use genetic information to measure if microbes in the environment can perform specific ecological roles. Researchers recently analyzed the genomes of over 6,000 microbial species.
Spotlight
Barbara Garcia: A first-generation college student spends summer doing research at PPPL
Princeton Plasma Physics Laboratory
Argonne organization's scholarship fund blazes STEM pathway
Argonne National Laboratory
Brookhaven Lab, Suffolk Girl Scouts Launch Patch Program
Brookhaven National Laboratory
From an acoustic levitator to a "Neutron Bloodhound" robot, hands-on research inspires PPPL's summer interns
Princeton Plasma Physics Laboratory
Brookhaven Lab Celebrates the Bright Future of its 2019 Interns
Brookhaven National Laboratory
PPPL apprenticeship program offers young people chance to earn while they learn high-tech careers
Princeton Plasma Physics Laboratory
JSA Awards Graduate Fellowships for Research at Jefferson Lab
Thomas Jefferson National Accelerator Facility
ILSAMP Symposium showcases benefits for diverse students, STEM pipeline
Argonne National Laboratory
Integrating Scientific Computing into Science Curricula
Brookhaven National Laboratory
Students from Minnesota and Massachusetts Win DOE's 29th National Science Bowl(r)
Department of Energy, Office of Science
DOE's Science Graduate Student Research Program Selects 70 Students to Pursue Research at DOE Laboratories
Department of Energy, Office of Science
Young Women's Conference in STEM seeks to change the statistics one girl at a time
Princeton Plasma Physics Laboratory
Students team with Argonne scientists and engineers to learn about STEM careers
Argonne National Laboratory
Lynbrook High wins 2019 SLAC Regional Science Bowl competition
SLAC National Accelerator Laboratory
Equipping the next generation for a technological revolution
Argonne National Laboratory
Chemistry intern inspired by Argonne's real-world science
Argonne National Laboratory
Argonne intern streamlines the beamline
Argonne National Laboratory
Research on Light-Matter Interaction Could Lead to Improved Electronic and Optoelectronic Devices
Rensselaer Polytechnic Institute (RPI)
Innovating Our Energy Future
Oregon State University, College of Engineering
Physics graduate student takes her thesis research to a Department of Energy national lab
University of Alabama at Birmingham
"Model" students enjoy Argonne campus life
Argonne National Laboratory
Writing Code for a More Skilled and Diverse STEM Workforce
Brookhaven National Laboratory
New graduate student summer school launches at Princeton Plasma Physics Laboratory
Princeton Plasma Physics Laboratory
The Gridlock State
California State University (CSU) Chancellor's Office
Meet Jasmine Hatcher and Trishelle Copeland-Johnson
Brookhaven National Laboratory
Argonne hosts Modeling, Experimentation and Validation Summer School
Argonne National Laboratory
Undergraduate Students Extoll Benefits of National Laboratory Research Internships in Fusion and Plasma Science
Princeton Plasma Physics Laboratory
Students affected by Hurricane Maria bring their research to SLAC
SLAC National Accelerator Laboratory
Brookhaven Lab Pays Tribute to 2018 Summer Interns
Brookhaven National Laboratory
CSUMB Selected to Host Architecture at Zero Competition in 2019
California State University, Monterey Bay
From Hurricane Katrina Victim to Presidential Awardee: A SUNO Professor's Award-Winning Mentoring Efforts
Brookhaven National Laboratory
Department of Energy Invests $64 Million in Advanced Nuclear Technology
Rensselaer Polytechnic Institute (RPI)
Professor Miao Yu Named the Priti and Mukesh Chatter '82 Career Development Professor
Rensselaer Polytechnic Institute (RPI)
2018 RHIC & AGS Annual Users' Meeting: 'Illuminating the QCD Landscape'
Brookhaven National Laboratory
High-School Students Studying Carbon-Based Nanomaterials for Cancer Drug Delivery Visit Brookhaven Lab's Nanocenter
Brookhaven National Laboratory
Argonne welcomes The Martian author Andy Weir
Argonne National Laboratory
UW Professor and Clean Energy Institute Director Daniel Schwartz Wins Highest U.S. Award for STEM Mentors
University of Washington
Creating STEM Knowledge and Innovations to Solve Global Issues Like Water, Food, and Energy
Illinois Mathematics and Science Academy (IMSA)
Professor Emily Liu Receives $1.8 Million DoE Award for Solar Power Systems Research
Rensselaer Polytechnic Institute (RPI)
Celebrating 40 years of empowerment in science
Argonne National Laboratory
Introducing Graduate Students Across the Globe to Photon Science
Brookhaven National Laboratory
Students from Massachusetts and Washington Win DOE's 28th National Science Bowl(r)
Department of Energy, Office of Science
Q&A: Al Ashley Reflects on His Efforts to Diversify SLAC and Beyond
SLAC National Accelerator Laboratory
Insights on Innovation in Energy, Humanitarian Aid Highlight UVA Darden's Net Impact Week
University of Virginia Darden School of Business
Ivy League Graduate, Writer and Activist with Dyslexia Visits CSUCI to Reframe the Concept of Learning Disabilities
California State University, Channel Islands
Photographer Adam Nadel Selected as Fermilab's New Artist-in-Residence for 2018
Fermi National Accelerator Laboratory (Fermilab)
Showing results
0-4 Of 2215