PORTLAND, Ore. — Graphene as the successor to silicon is the subject of intense research efforts in the US, Japan, Europe, and even China where it has become a government-backed priority.
However, because of graphene's lack of a natural backgap, researchers at the Massachusetts Institute of Technology (MIT) and Harvard University, both in Cambridge, Mass., claim everybody may be barking up the wrong tree. Instead, they should be looking for compounds with the same good properties as graphene, but which also possess a natural bandgap. One such family of materials is exemplified by nickel hexa-imino-triphenylene (NiHITP).
MIT professor of chemistry Mircea Dinca told EE Times in an interview:
Just like graphite can be exfoliated into graphene (which is nothing other than single sheets of graphite), our material can also be exfoliated into single sheets. As opposed to the graphene layers, our layers should be tunable by controlled chemical modification, which will alter their electronic properties in a systematic manner.
Officially, the new material is called Ni3(HITP)2, since it bonds three atoms of nickel with two organic molecules of HITP, but NiHITP is just the first of a family of materials with natural bandgaps that can have their electronic functions tuned on the atomic scale for specific applications.
The molecular structure of the graphene-analog material naturally forms a hexagonal lattice structure so that the openings in the hexagons are all perfectly aligned.
Dinca told EE Times:
We have not yet measured its bandgap, but it is larger than 0. Graphene's bandgap is 0, which makes it not suitable, as is, as a semiconductor. One of the things that we are working on right now is to make large single sheets of the material to firstly measure its bandgap and better characterize its electronic properties, and secondly to make devices with it. So those are our immediate challenges. And because there really is much room for tunability, I really don't see limitations as far as producing more materials. We already have several other related two-dimensional materials that should all have slightly different electronic properties, which is precisely what we were going after -- a tunable metal-organic graphene analog.
Instead of introducing impurities or defects into NiHITP by doping, as is done with traditional semiconductors, Dinca's family of new semiconductors will be carefully constructed at the atomic level -- from the bottom up‚ in the process tuning its capabilities to favor specific electronic and possibly optical properties as well. Dinca continued:
Our material is well-defined down to the atomic level. That's the point, instead of relying on defects, which is not well understood and you can't really control very well, we are creating a material whose properties are intrinsically modulated through its molecular components. We assemble this from the "bottom-up," molecule-by-molecule, if you will.
NIHITP, like graphene, self-assembles into perfect hexagonal honeycombs that stack into multiple layers with their hexagonal openings precisely aligned leaving two nanometers holes through the stack.
SEM (scanning electron microscope) shows nano particles that are formed from as collections of two-dimensional flakes.
Next, the researchers want to form these new materials into monolayers in order to precisely measure their bandgap as well as to build and characterize devices. The group hopes to create a family of related materials using slightly different formulations of the basic elements whose properties are tuned to specific applications including solar cells that capture different wavelengths of light, supercapacitors with unprecedented storage densities as well as more exotic materials such as topological insulators
and quantum Hall Effect
All the details of Dinca's new material, created with the help of seven co-authors, can be found in the Journal of the American Chemical Society at High Electrical Conductivity in Ni3(hexaiminotriphenylene)2, a Semiconducting Metal-Organic Graphene Analogue.
Funding was provided by the US Department of Energy and the Center for Excitonics at MIT.
— R. Colin Johnson, Advanced Technology Editor, EE Times