Los Alamos National Laboratory

Los Alamos National Laboratory

Delivering science and technology to protect our nation and promote world stability

Research

The Light to Energy (L2E) team expertise comprises photophysics of organic and nano-sized materials.

 

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Nano-sized materials

Synthesis of graphene-based materials

Our team has expertise in the synthesis of graphene-based nano-sized materials. There are three synthesis methods we are capable of:

  • chemical exfoliation
  • chemical vapor deposition (CVD)
  • mechanical exfoliation

Chemical Exfoliation (Solution processing)

For chemical exfoliation, we use oxidizing agents and acids to chemically oxidize initial graphite powders into graphite oxide using well known Hummer’s method (originally developed in 1960s). The oxidization act as an intercalation of graphite layers with the attached oxygen functional groups, thus graphene oxide (GO, a single layer graphene sheet with oxygen functional groups attached to both sides of the basal plane) can readily be obtained by simply immersing graphite oxide into water or with assist of mild ultrasonication (mechanical force). Produced GO is scalable (can be produced in tons), inexpensive, and solution processable (even in water), and thus is attractive for applications, which need low cost graphene in large quantity such as catalysts and electrodes for electrochemical devices

Chemical Vapor Deposition (Vacuum process)

Copper (Cu) and Nickel (Ni) films are the substrates usually used for CVD. With a presence of inert gas (N2 or Ar), hydrogen gas (H2), and carbon source gas [methane (CH4)] flow, Cu or Ni substrates are placed in a quartz tube, which can be heated up to around 1,000 oC. Grown single and multiple layer CVD graphene can be transferred onto arbitrary substrates using established polymer-supported method (PMMA transfer). CVD graphene generally has many fewer atomic structural defects compared to GO, and roll-to-roll deposition has already been demonstrated (over 100 meters long) thus it is suitable for applications that require large area high quality single to few layer graphene films such as transparent conductors for touch panel and displays, and possibly interconnects for integrated circuit (IC) devices.

Mechanical Exfoliation (Scotch tape method)

In mechanical exfoliation, also known as scotch tape method, single to few layers of graphene can be exfoliated from graphite single crystal using adhesive tape such as scotch tape. Inventors of this method were awarded Nobel Prize in Physics in 2010 (Sir Andre Geim and Sir Konstantin Novoselov). Out of the three methods mentioned here, this method is capable of producing graphene with highest atomic structural quality (minimal defects) and thus suitable for fundamental physical experiments. In fact, majority of new physical phenomena on graphene were discovered using mechanically exfoliated graphene. The drawback is that this method is not scalable, yield is low, and the lateral dimension of graphene produced is generally in the range of few tens to few hundreds of micrometers (10-6 m).

Optical and optoelectronic characterizations

One of the strengths at Los Alamos National Laboratory lies in our advanced spectroscopy capabilities. We are interested in investigating intrinsic and novel optical and optoelectronic properties of nano-sized materials/organic polymers by utilizing full strength of these capabilities. We correlate obtained optical and optoelectronic properties with various other characterization techniques to reveal fundamental photophysical phenomena to gain detailed understanding of nature & dynamics of excitons in nano-sized materials/organic polymers as well as charge and energy transfer at interfaces of nano-sized materials/organic polymers at the core of high performance light to energy (L2E) conversion devices.

Electrical properties of nano-sized materials

2n

Graphene analogues, layered transition metal dichalcogenides (TMDs) for example, have recently generated interests to researchers owing to their unique materials properties including electrical transport. Though graphene transistors have exceptionally high mobility values, the lack of band-gap yields a very low ON/OFF ratio, a factor that is detrimental for logic devices. In sharp contrast to graphene, TMDs possess intrinsic band-gaps which lead to high ON/OFF ratios. Furthermore, TMD transistors have respectable mobilities and sharp turn-ons which are represented by their low sub-threshold swing. Our interest is making top and bottom gated transistors by using different TMDs (MoS2, WS2, etc.) as transport layers, with an aim to improve the performance of transistors by solving important problems such as high contact resistance and lack of ambipolar behavior. Reduction of contact resistant allows access to intrinsic transport properties, and achieving ambipolar behavior in TMD transistors will pave a pathway toward their use in CMOS technology and logic operations.

Hydrogen production using catalytic nano-sized materials

LTMDS

An important portion of our future energy is expected to be defined by the production of hydrogen via renewable, efficient water splitting using non-toxic, earth abundant catalysts. Our team at Los Alamos is interested in using single layer transition metal dichalcogenides (TMDs) such as MoSand WS2 for novel device integration and materials processing to achieve enhanced catalytic activities of Hydrogen Evolution Reaction (HER) for water splitting.

Sulfide

Although the catalytic activity of bulk TMDs such as MoSand WS2 is well known and has been used industrially, an exceptionally high catalytic activity unique to their atomically-thin form for energy application was not known until very recently. A finding in 2013 has revealed that chemically exfoliated single layer transition metal dichalcogenide shows HER approaching to that of state-of-the-art Platinum (Pt) catalyst. The impact of this finding was the demonstration of earth abundant and low cost TMDs as a potential replacement candidate for currently used precious metal Pt catalyst for the future hydrogen economy.

Sulfide

In addition to chemically exfoliated TMDs, we are also interested in using CVD-grown TMDs core-shell nanowires for which the catalytic properties are being optimized using crystallographic changes, as well as various n and p type dopants.

With expertise in device manufacturing and electrochemistry the capability of measuring HER catalysis on a micron scale, i.e., a single flake of TMD sheet, we are exploring new ways of understanding the factors that impact hydrogen catalysis.

Organic polymers

Interface engineering for improved photovoltaic

Improved photovoltaicOne of our interests in organic polymer related research is gaining fundamental understandings of the charge transfer behavior at the donor-acceptor interface of organic optoelectronic devices. To suppress the electron-hole recombination or facilitate the charge separation and achieve improvement on power conversion efficiency, modification of the Donor/Acceptor interface with functionalized materials could be an advantageous strategy. Our focus is in exploring the physical mechanism for different functional materials with different physical properties. Those mechanisms are expected to provide insights into a design of novel device structure and conjugated block-copolymers for high efficiency optoelectronic devices.

Interface engineered polymers for organic photovoltaics

Engineered Polyme The electron-hole pair dissociation process at the interface of an electron-donor and an electron-acceptor in organic photovoltaics (OPVs) plays an important role in charge generation efficiency upon absorbing sunlight. In contrast to the kinetically trapped structures in bulk heterojunction OPVs, all-conjugated block copolymers can address tasks at interface control of donor-acceptor materials and improve performance of OPVs. This improvement may be due to the stable nanostructures with well-defined interfaces from all-conjugated block copolymers. With development of various chemically engineered all-conjugated block copolymers, we are able to search the key to improve electron-hole pair dissociation in OPVs.


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