What are Armchair Graphene Nanoribbons?
Gary Kardys | May 04, 2017Graphene is a supermaterial because the material has superlative properties such as:
- Strongest material
- Thinnest material (0.345 nanometers thick)
- Best thermal conductor
- Best transparent, electrical conductor
- Very high specific surface area
- Unique electronic properties from Dirac cones
What characteristics cause the outstanding properties of graphene?
Intrinsically, graphene does not have a band gap like conventional insulators or semiconductors, or a partially filled band like metals rather properties like a tunable conductivity. Graphene has unique electronic properties resulting from the bonding states between carbon atoms. The carbon atoms in graphene are sp2 hybridized, resulting in covalent or sigma bonds between atoms within the graphene plane. Carbon atoms with sp3 hybridization occur in diamond, another supermaterial (super for superhardness and thermal conductivity). The atoms in the graphene plane are arranged in a hexagonal pattern, meaning each carbon atom forms three bonds to other carbon atoms. The strong carbon bonds within the graphene nanoribbons or nanosheets give rise to graphene’s 130 GPa ultimate tensile strength, which is orders of magnitude higher than the strength of A36 structural steel (0.4 GPa).
Carbon has a valence of four, so the fourth bond or remaining p or Pi (π) orbital becomes a delocalized electron cloud on the surface. In some ways, this is not unlike metals, where a sea of electrons are shared by all of the metal atoms. Graphene is often described as a zero-overlap semi-metal. The shared sea of electrons gives rise to high conductivity in metals. Electrons in metal are still slowed down by localized fields from impurities and defects when traveling through a metallic crystal. The electrons on the surface of the graphene are not encumbered by crystal lattice variations. The Pi orbital electrons are tied to the bonding and anti-bonding (the valance and conduction bands), which gives rise to the “topological insulator” or “Dirac cone” electronic nature of graphene. At the Dirac point, the electrons act like they have zero mass akin to photons. The cones help give graphene massless fermions or charge carriers, which lead to various quantum Hall effects and enormous carrier mobility.
What forms of graphene are available?
In conventional semiconductors, conductivity is controlled by doping to narrow the gap and by an applied voltage, which allow electrons to tunnel through or bridge the gap. With a Dirac cone, electrons behave more like light in a relativistic fashion and like light moving at high speeds. An applied voltage can be used to control the number and nature of the graphene charge carriers, electrons or holes. Conductivity can be adjusted by doping to alter the Fermi level.
Conventional materials are available in a variety of forms such as plate, sheet, foil, strip, wire, block, square bar or rod, which are defined by the three dimensions of the stock shape. Graphene is a two dimensional material and the thinnest known material. The two dimensional nature limits the form available to monolayer sheets, bilayer sheets, superlattices, quantum dots, fibers, nanocoils, honeycombs, aerogel foams and nanoribbons. Doping, nanotube reinforcement, oxidation and other modifications are used to tailor graphene’s properties for specific applications. When graphene is in the form of narrow sheet less than 50 nanometers wide, it's called a graphene strip, a nanoribbon or nanostripe.
How do zig-zag and armchair graphene nanoribbons differ?
Graphene nanoribbons are available in two forms or orientations: “zig-zag” and “armchair” edge shapes. Zig-zag graphene nanostripes form at low temperatures and have spin-polarized metallic edge currents. Graphene zig-zag nanoribbons hold promise for spintronics applications. Graphene "armchair" nanoribbon edges are particularly interesting because they behave like semiconductors. Research has shown nanomaterials provide continued miniaturization and performance enhancement in microelectronics device and manufacturing. Some believe the limits of conventional semiconductor materials will end Moore’s Law. Armchair graphene nanoribbons might provide the next leap in semiconductor technology. The chemical nature or bonding energies between zig-zag and armchair graphene nanoribbons differs, which can impact functionalization or forming chemical modifications such as fluorographene, chlorographene, graphene oxide and other functionalized graphene nanoribbon hybrids.
Fig. 1 Two types of edges in graphene nanoribbons: (a) zigzag edge and (b) armchair edge. The edge is indicated by bold line. The light and dark circles denote the A and B-site carbon atoms, respectively.
Graphene nanoribbons also have interesting physical and chemical properties for other applications. The carbon atoms in graphene are bonded to only three other atoms, so the fourth atom on the surface is available for bonding to other chemicals, coupling agents or binder resins in composites. Forming a good interface in a composite is a critical in fully utilizing a high strength reinforcement and transferring the structural load from the resin matrix to nanoribbon. Composite applications look promising for graphene nanoribbon when the capability for bonding is combined with ultrahigh tensile strength and the high specific surface area.
Interesting graphic about the properties and potentiality of uses in electronics.
It is also known that GPO (or GO?) graphene oxide is a nearly perfect proton conduit when this material is in contact with water solutions. Question is this: If a proton enters a graphitic structure as a result of diffusion or charge induced transport through the defect opening of GPO, how hard would it be (not hard at all IMHO) for the proton to become immobilized between sheets of graphene, and held there in the lattice interstitial space, almost as if there were a nano-scale pair of tweezers, so tightly that the Heisenberg uncertainty in momentum becomes wildly large. Large enough to allow capture of an electron from the SPP cloud of graphite? I am referring to nuclear capture, not electronic capture.
In reply to #1
It would probably be easiet to start with He3 with the aim of converting to H3.
In reply to #2
Are you sure that is energetically favorable? I don't happen to have any 3He in my pocket, anyway.
In reply to #3
Relatively more favorable than going from 1H to n.
Consider the half lives of a thermal neutron as compared to 3He. Minutes compared to years.
In reply to #4
At the heart of the Heisenberg trap, is the idea that the trapped entity may only acquire sufficient energy to undergo the reaction with the electron (1H+e- → n(cold)), the nascent neutron is considered to be at 0 °K at the instant it is formed.
Granted that thermalization of said neutron would be fairly rapid in condensed media (I think we are talking 500 femtoseconds to 1 nanosecond), however one also has to consider the absorption cross-section for cold neutrons is usually >1000X that for a thermal one (for the same target isotope as the absorber). For example 235U has a thermal neutron σ of some 200-300 barns, but the cold neutron σ is ~106 barns.
I should normally expect more 13C as 2H (or 3H or the mysterious 4H→4He+β-), since the greater number of near neighbors might easily be carbon rather than protons, or water molecules. The only thing going for absorption of the cold neutron by proton is the idea that the cold neutron would begin to thermalize and move out of location very quickly, and might have a probability of "seeing" a nearby trapped proton in the interstices between sheets of graphene.
In reply to #5
I really don't know anything about cold neutrons.
Anything you would suggest I read?
In reply to #6
https://en.wikipedia
Of note are several classifications. Do you agree that for Cadmium to act as a neutron absorber, the neutrons have to be above "thermal" kinetic energy?
Instead of "cold" neutrons, I should have referred to my supposed nascent neutrons as UCN (ultra-cold neutrons). These can be held in traps for some amount of time.
https://en.wikipedia
Information about UCN (ultra-cold neutrons).
https://arxiv.org/pd
Paper on the determination of the asymmetry parameter A (apparently has to do with calculation of theoretical lifetime of UCN.
I think what I have been leaving out of any supposed or pre-supposed hypothesis about UCN formation from Heisenberg trapped protons (with energy arriving from SPP electron cloud of graphene) is the necessary extra mass of the neutrino (or exceedingly high SPP energy contribution). I believe now the limiting factor might well be the neutrino flux density, along with the neutrino cross-section for a cold proton, which might be
practicallynearly vanishing. Typically neutrino experiments are known for exceedingly large tanks of water lined with event detectors.In reply to #7
Thank you.
.
No, I would not agree that for cadmium to act as a poison the neutrons need to have greater than thermal energy.
In reply to #8
Explain, please, in terms of the energy thresholds listed, as I apparently also failed to grasp that.
In reply to #9
Cadmium is a strong absorber of neutrons in the thermal and epithermal regions. It is used as a burnable poison, an emergency shutdown poison, and in control rods for this very reason.
Most isotopes of cadmium have cross section for absorption that is roughly inversely proportional to neutron velocity, except through the epithermal region, in which resonances an create peaks with very large cross sections. By the time neutrons are above epithermal, the resonances are gone and the absorption, still roughly following the inverse of velocity is pretty low.
There is one standout isotope is Cd-113. It makes up 12+% of natural cadmium, yet it dominates neutron absorption as its cross sections are much higher especially in the thermal range...and also with much less adherence to the inverse velocity proportionality.
In reply to #10
Thank you for the teaching lesson, and it was well explained, so even I could understand it.