Present research at Toyota Riken“Field emission from nano-carbon and quest of its
functional properties”
Yahachi Saito
Research Objects
Electronic states and atomic structures of nanocarbon surfaces and edges, and molecules
adsorbed on nanocarbon field emitters are studied by using field emission microscopy (FEM). The
obtained insights for nanocarbon field emitters will be exploited to develop a highly coherent, lowenergy projection resolution microscope. We also try to verify the theoretically predicted electronspin ordering at the graphene edge by using field emission (FE) from an edge of graphene.
Furthermore, exotic carbon substances called carbyne (sp-hybridized carbon), which is formed in
CNT cathode films after FE experiment.

We will clarify and explore the novel and functional

properties of nanocarbon, which possesses a wide variety of structures and properties, aiming todevelop nanoscience and technology related with nanocarbon materials.

Research Methods
1. Observation of electron orbitals and spin polarization at graphene edges by fieldemissionIn the field emission from CNT and graphene, π-electron orbitals existing at their surface andedge prevail tunneling of electrons, being in contrast to conventional metal emitters where thetunneling are governed by free electrons.

Depending the structure of carbon, spatial distribution

and symmetry of the electron orbitals change greatly.

For

example, at the closed tip of CNT, the π-orbital spreads over thesurface of a carbon hexagon net, whereas at the end of a broken
CNT and/or the edge of graphene it extends perpendicular to thehexagon net surface.

Therefore, it is expected that spatial

distributions and natures of electron orbitals such as the symmetrycan be obtained by observing FEM images (Fig. 1) fromFig. 1. FEM image of agraphene edge.

nanocarbon substances having different geometric structures.
Furthermore, at the edge of graphene, it is theoretically predicted thatelectron spins are ferromagnetically ordered (Fig. 2).

In thisresearch, the spin state is studied by directly measuring the spinpolarization of electrons field-emitted from the edge.

Fig. 2. Spin ordering atgraphene edges.

2. Development of atomic resolution FEM and low-energy projection microscopy using
CNT field emitters
It is well-known that FEM images of organic molecules such as copper-phthalocyanine adsorbed
on a tungsten (W) needle show a four-leaf pattern, which was first observed by Müller who invented
field ion microscopy.

The FEM pattern has attracted much attention since then because it wasconsidered to show directly the shape of the planar molecule with four-fold symmetry.

However,even other organic molecules that do not have the four-fold symmetry give similar four-fold patterns,
and phthalocyanine molecules sometimes show two-leaf and/or circular patterns.

It has beenargued that the FEM patterns from organic molecules do not necessarily reflect the shapes ofmolecules. The origin of the FEM image called the cloverleaf pattern and the relation with themolecular shape is still unclear and a mystery.

We are trying to solve the mystery of the FEM imageof the organic molecules by using advantages of the CNT emitter which is expected to have atomicresolution.
A highly coherent electron beam can be generated from the CNT tip because the electron
emission originates from a coherently spreading electronic state.

In fact, interference fringes areobserved in the FEM images from 5-member rings located at the CNT tip.

This indicates thatelectrons emitted from different emission sites (pentagons) interfere with each other.

Such a highly

coherent phenomenon does not occur in the traditional metallic field emitters like W needles.
Using this highly coherent CNT electron sources, aprojection type electron microscope will be developed(Fig.3).

Since the electron emission from the CNT tip

occurs under low applied voltages, an electron beam withlow energy on the order of 100 eV can be obtained. Thislow-energy electron projection microscopy has a possibility
to obtain high magnification images of very thin samples

Fig. 3. Low energy projection

comprised of light elements such as carbon, nitrogen andelectron microscope with a CNT

oxygen (e.g., organic and biomaterials).

field emitter.

3. Creation of a new substance of carbon
Carbon exists in a variety allotropes, e.g., diamond (sp3 hybridization), graphite (sp2), fullerene(sp2+ α ), and CNT (sp2+ α ).

In addition to these well-known allotropes, sp hybridized one-dimensional carbon, carbyne, has been attracted much interest with significant controversy since thelate 1960’s. Recently, linear carbon-chains (LCCs) confined inside CNTs have been reportedly
found in high-temperature annealed double-wall CNTs (DWCNTs) and scarcely in multiwall CNTs
produced by arc discharge.

With a method different from these, we unexpectedly found thepresence of bulk amount of LCCs in CNT films after an electric discharge of the films which wereused as FE cathodes.

LCCs were confined inside single-wall CNTs (Fig. 4) as well as DWCNTs.

Our method is highlighted by the first formation of single-wall CNTs encapsulating long LCCs (> 9nm in length), which could not be synthesized so far by theconventional thermal annealing of CNTs.

Formationmechanism of LCCs encapsulated in CNT as well as theirstructural and electronic properties will be elucidated.

Expected Effects
Nanocarbon electron emitters that possess unusual
properties, e.g., generation of a highly coherent electron
beam, manifestation of electron orbitals participating
electron emission, unique edge states with polarized spins

Fig. 4. TEM image of a LCC
inside CNT.

and so on, have potential abilities to provide new insights
to the understanding of electronic properties of nanocarbon materials and to develop a novel
electron microscope. In addition, FEM with nanocarbon emitters would enable to clarify themysteries of FEM images of organic molecules that are unsolved 80 years since the Müller’s firstreport on four- and two-leaf patterns of organic molecules. Emergence of graphene based
spintronics and creation of a new carbon allotrope, e.g., carbyne are also expected.