Building a better semiconductor

 

(l-r) Faran Zhou, physics and astronomy doctoral student; Terry Han, who just earned his Ph.D. in physics and astronomy; and Chong-Yu Ruan, associate professor of physics and astronomy. They are part of a team that developed an ultrafast microscope that allows researchers to view changes in materials caused by laser pulses. Credit: Harley Seeley Research led by Michigan State University could someday lead to the development of new and improved semiconductors. In a paper published in the journal Science Advances, the scientists detailed how they developed a method to change the electronic properties of materials in a way that will more easily allow an electrical current to pass through. The electrical properties of semiconductors depend on the nature of trace impurities, known as dopants, which when added appropriately to the material will allow for the designing of more efficient solid-state electronics. The MSU researchers found that by shooting an ultrafast laser pulse into the material, its properties would change as if it had been chemically "doped." This process is known as "photo-doping." "The material we studied is an unconventional semiconductor made of alternating atomically thin layers of metals and insulators," said Chong-Yu Ruan, an associate professor of physics and astronomy who led the research effort at MSU. "This combination allows many unusual properties, including highly resistive and also superconducting behaviors to emerge, especially when 'doped.'" An ultrafast electron-based imaging technique developed by Ruan and his team at MSU allowed the group to observe the changes in the materials. By varying the wavelengths and intensities of the laser pulses, the researchers were able to observe phases with different properties that are captured on the femtosecond timescale. A femtosecond is 1 quadrillionth, or 1 millionth of 1 billionth, of a second. "The laser pulses act like dopants that temporarily weaken the glue that binds charges and ions together in the materials at a speed that is ultrafast and allow new electronic phases to spontaneously form to engineer new properties," Ruan said. "Capturing these processes in the act allows us to understand the physical nature of transformations at the most fundamental level." Philip Duxbury, a team member and chairperson of the department of physics and astronomy, said ultrafast photo-doping "has potential applications that could lead to the development of next-generation electronic materials and possibly optically controlled switching devices employing undoped semiconductor materials." A semiconductor is a substance that conducts electricity under some conditions but not others, making it a good medium for the control of electrical current. They are used in any number of electronics, including computers. Story Source: The above post is reprinted from materials provided by Michigan State University. Note: Materials may be edited for content and length. Journal Reference: Chong-Yu Ruan et al. Exploration of metastability and hidden phases in correlated electron crystals visualized by femtosecond optical doping and electron crystallography. Science Advances, June 2015 DOI: 10.1126/sciadv.1400173

Mastering extraordinary properties of an emerging semiconductor

 

Wed, 06/03/2015 - 10:58am École Polytechnique de Montréal   Chemical analysis by hyperspectral TEM-EELS spectroscopy of a multilayer 2D-phosphane exfoliated under ambient light in air.A team of researchers from Université de Montréal, Polytechnique Montréal and the Centre national de la recherche scientifique (CNRS) in France is the first to succeed in preventing two-dimensional layers of black phosphorus from oxidating. In so doing, they have opened the doors to exploiting their striking properties in a number of electronic and optoelectronic devices. The study's results were published in the journal Nature Materials.  Black phosphorus: future key player in new technologies Black phosphorus, a stable allotrope of phosphorus that presents a lamellar structure similar to that of graphite, has recently begun to capture the attention of physicists and materials researchers. It is possible to obtain single atomic layers from it, which researchers call 2D phosphane. A cousin of the widely publicized graphene, 2D phosphane brings together two very sought-after properties for device design.  First, 2D phosphane is a semiconductor material that provides the necessary characteristics for making transistors and processors. With its high-mobility, it is estimated that 2D phosphane could form the basis for electronics that is both high-performance and low-cost.  Furthermore, this new material features a second, even more distinctive, characteristic: its interaction with light depends on the number of atomic layers used. One monolayer will emit red light, whereas a thicker sample will emit into the infrared. This variation makes it possible to manufacture a wide range of optoelectronic devices, such as lasers or detectors, in a strategic fraction of the electromagnetic spectrum.  A scientific first: preserving single-atom layers of 2D phosphane from degrading Until now, the study of 2D phosphane's properties was slowed by a major problem: in ambient  conditions, very thin layers of the material would degrade, to the point of compromising its future in the industry despite its promising potential.  As such, the research team has made a major step forward by succeeding in determining the physical mechanisms at play in this degradation, and in identifying the key elements that lead to the layers' oxidation.  “We have demonstrated that 2D phosphane undergoes oxidation under ambient conditions, caused jointly by the presence of oxygen, water and light. We have also characterized the phenomenon's evolution over time by using electron beam spectroscopy and Raman spectroscopy,” reports Professor Richard Martel of Université de Montréal's Department of Chemistry. Next, the researchers developed an efficient procedure for producing these very fragile single-atom layers and keeping them intact. “We were able to study the vibration modes of the atoms in this new material. Since earlier studies had been carried out on heavily degraded materials, we revealed the as-yet-unsuspected effects of quantum confinement on atoms' vibration modes,” notes Professor Sébastien Francoeur of Polytechnique's Department of Engineering Physics. The study's results will help the world scientific community develop 2D phosphane's very special properties with the aim of developing new nanotechnologies that could give rise to high-performance microprocessors, lasers, solar cells and more. This work is financially supported by the Natural Sciences and Engineering Research Council of Canada, the Canada Foundation for Innovation, and Fonds de Recherche du Québec-Nature et technologie. SOURCE: École Polytechnique de Montréal

New way to move atomically thin semiconductors for use in flexible devices

 

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Researchers from North Carolina State University have developed a new way to transfer thin semiconductor films, which are only one atom thick, onto arbitrary substrates, paving the way for flexible computing or photonic devices. The technique is much faster than existing methods and can perfectly transfer the atomic scale thin films from one substrate to others, without causing any cracks.

At issue are molybdenum sulfide (MoS₂) thin films that are only one atom thick, first developed by Dr. Linyou Cao, an assistant professor of materials science and engineering at NC State. MoS₂ is an inexpensive semiconductor material with electronic and optical properties similar to materials already used in the semiconductor industry.

"The ultimate goal is to use these atomic-layer semiconducting thin films to create devices that are extremely flexible, but to do that we need to transfer the thin films from the substrate we used to make it to a flexible substrate," says Cao, who is senior author of a paper on the new transfer technique. "You can't make the thin film on a flexible substrate because flexible substrates can't withstand the high temperatures you need to make the thin film."

Cao's team makes MoS₂ films that are an atom thick and up to 5 centimeters in diameter. The researchers needed to find a way to move that thin film without wrinkling or cracking it, which is challenging due to the film's extreme delicacy.

"To put that challenge in perspective, an atom-thick thin film that is 5 centimeters wide is equivalent to a piece of paper that is as wide as a large city," Cao said. "Our goal is to transfer that big, thin paper from one city to another without causing any damage or wrinkles."

Existing techniques for transferring such thin films from a substrate rely on a process called chemical etching, but the chemicals involved in that process can damage or contaminate the film. Cao's team has developed a technique that takes advantage of the MoS₂'s physical properties to transfer the thin film using only room-temperature water, a tissue and a pair of tweezers.

MoS₂ is hydrophobic -- it repels water. But the sapphire substrate the thin film is grown on is hydrophilic -- it attracts water. Cao's new transfer technique works by applying a drop of water to the thin film and then poking the edge of the film with tweezers or a scalpel so that the water can begin to penetrate between the MoS₂ and the sapphire. Once it has begun to penetrate, the water pushes into the gap, floating the thin film on top. The researchers use a tissue to soak up the water and then lift the thin film with tweezers and place it on a flexible substrate. The whole process takes a couple of minutes. Chemical etching takes hours.

"The water breaks the adhesion between the substrate and the thin film -- but it's important to remove the water before moving the film," Cao says. "Otherwise, capillary action would case the film to buckle or fold when you pick it up.

"This new transfer technique gets us one step closer to using MoS₂ to create flexible computers," Cao adds. "We are currently in the process of developing devices that use this technology."

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Story Source:

The above story is based on materials provided by North Carolina State University. Note: Materials may be edited for content and length.

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Journal Reference:

1. Alper Gurarslan, Yifei Yu, Liqin Su, Yiling Yu, Francisco Suarez, Shanshan Yao, Yong Zhu, Mehmet Ozturk, Yong Zhang, Linyou Cao. Surface-Energy-Assisted Perfect Transfer of Centimeter-Scale Monolayer and Few-Layer MoS2Films onto Arbitrary Substrates. ACS Nano, 2014; 141106093600005 DOI: 10.1021/nn5057673

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