Zhibing Li

Field Electron Emission Characteristics and Physical Mechanism of Individual Single-Layer Graphene

Due to its difficulty, experimental measurement of field emission from a single-layer graphene has not been reported, although field emission from a two-dimensional (2D) regime has been an attractive topic. The open surface and sharp edge of graphene are beneficial for field electron emission. A 2D geometrical effect, such as massless Dirac fermion, can lead to new mechanisms in field emission. Here, we report our findings from in situ field electron emission characterization on an individual singe-layer graphene and the understanding of the related mechanism. The measurement of field emission from the edges was done using a microanode probe equipped in a scanning electron microscope. We show that repeatable stable field emission current can be obtained after a careful conditioning process. This enables us to examine experimentally the typical features of the field emission from a 2D regime. We plot current versus applied field data, respectively, in ln(I/E(3/2)) ∼ 1/E and ln(I/E(3)) ∼ 1/E(2) coordinates, which have recently been proposed for field emission from graphene in high- and low-field regimes. It is observed that the plots all exhibit an upward bending feature, revealing that the field emission processes undergo from a low- to high-field transition. We discuss with theoretical analysis the physical mechanism responsible for the new phenomena.

Oxygen density dependent band gap of reduced graphene oxide

We investigated the stability of reduced graphene oxide for oxygen density ranging from 6.25% to 50% with the density functional theory and found the most, the second most, and the third most stable oxygen configurations. The effect of relaxation of lattice on the electronic properties is found to be negligible for low O coverage and crucial for higher O coverage, respectively. The densities of states and the band gaps were calculated. The bandgap is found to be a non-monotonic function of oxygen density, with minima at O/Cu2009=u200911.1% and 25%.

Potential barrier of graphene edges

We calculated row resolved density of states, charge distribution and work function of graphene’s zigzag and armchair edge (either clean or terminated alternatively with H, O, or OH group). The zigzag edge saturated via OH group has the lowest work function of 3.76 eV, while the zigzag edge terminated via O has the highest work function of 7.74 eV. The angle-dependent potential barrier on the edge is fitted to a multipole model and is explained by the charge distribution.

Analytical treatment of cold field electron emission from a nanowall emitter, including quantum confinement effects

An elementary approximate analytical treatment of cold field electron emission (CFE) from a classical nanowall (i.e. a blade-like conducting structure on a flat surface) is presented. This paper first discusses basic CFE theory for situations where quantum confinement occurs transverse to the emitting direction. It develops an abstract CFE equation more general than Fowler–Nordheim type (FN-type) equations, and then applies this to classical nanowalls. With sharp emitters, the field in the tunnelling barrier may diminish rapidly with distance; an expression for the on-axis transmission coefficient for nanowalls is derived by conformal transformation. These two effects interact to generate complex emission physics, and lead to regime-dependent equations different from FN-type equations. Thus: (i) the zero-field barrier height HR for the highest occupied state at 0u2009K is not equal to the local thermodynamic work-function φ, and HR rather than φ appears in equations; (ii) in the exponent, the power dependence on macroscopic field FM can be F−2M rather than F−1M; (iii) in the pre-exponential, explicit power dependences on FM and HR differ from FN-type equations. Departures of this general kind are expected when nanoscale quantum confinement occurs. FN-type equations are the equations that apply when no quantum confinement occurs.

Field electron emission characteristic of graphene

The field electron emission current from graphene is calculated analytically on a semiclassical model. The unique electronic energy band structure of graphene and the field penetration in the edge from which electrons emit have been taken into account. The relation between the effective vacuum barrier height and the applied field is obtained. The calculated slope of the Fowler-Nordheim plot of the current-field characteristic is in agreement with existing experiments.

The roles of apex dipoles and field penetration in the physics of charged, field emitting, single-walled carbon nanotubes

A 1u2002μm long, field emitting, (5, 5) single-walled carbon nanotube (SWCNT) closed with a fullerene cap, and a similar open nanotube with hydrogen-atom termination, have been simulated using the modified neglect of diatomic overlap quantum-mechanical method. Both contain about 80u2009000 atoms. It is found that field penetration and band bending, and various forms of chemically and electrically induced apex dipole play roles. Field penetration may help explain electroluminescence associated with field emitting CNTs. Charge-density oscillations, induced by the hydrogen adsorption, are also found. Many of the effects can be related to known effects that occur with metallic or semiconductor field emitters; this helps both to explain the effects and to unify our knowledge about FE emitters. However, it is currently unclear how best to treat correlation-and-exchange effects when defining the CNT emission barrier. A new form of definition for the field enhancement factor (FEF) is used. Predicted FEF values for these ...

Image potentials of single-walled carbon nanotubes in the field emission condition

We calculated the image potentials of single-walled carbon nanotubes of various structures with a quantum chemistry method. The image potentials of the single-walled carbon nanotubes can be well fitted with the image potential of an ideal metal sphere of a size comparable to an atom. The image potentials are not sensitive to the applied fields and the structures of the tubes. When the image potentials are included, the emission current increases by one order.

Electronic and magnetic properties of oxygen patterned graphene superlattice

Graphene superlattices formed by adsorbing oxygen lines on graphene are studied using spin-polarized density functional theory calculations. It is found that separating zigzag chains (armchair dimer lines) with one oxygen adsorbate on each periodic segment of the chain (line) are sufficient to divide the graphene sheet into series of graphene nanoribbons. The energy band gap of the graphene with armchair (zigzag) oxidation lines (chains) is modulated with line (chain) separation as the width modulation of the freestanding graphene ribbon. The magnetic properties of superlattices with different oxidized line width for both zigzag and armchair oxidized lines are also investigated. Magnetism is found in oxygen-patterned graphene superlattice with zigzag interface and is enhanced with the increasing width of separating oxidation chains.

Field-dependent electron emission patterns from individual SWCNTs simulated with a multi-scale algorithm

The electron distribution of open-ended single-walled carbon nanotubes (CNTs) with chirality indexes (7,0) and (5,5) in field emission conditions was calculated via a multi-scaled algorithm. The field emission images were produced numerically. It was found that the emission patterns change with the applied macroscopic field. Especially, the symmetry of the emission pattern of the (7,0) carbon nanotube is breaking in the lower field but the breaking is less obvious in the higher field. The magnification factor increases with the applied macroscopic field.The electron distribution of open-ended single-walled carbon nanotubes with chirality indexes (7,0) and (5,5) in the field emission conditions was calculated via a multi-scaled algorithm. The field emission images were produced numerically. It was found that the emission patterns change with the applied macroscopic field. Especially, the symmetry of the emission pattern of the (7,0) carbon nanotube is breaking in the lower field but the breaking is less obvious in the higher field. The enlargement factor increases with the applied macroscopic field.

Image potential of C60: A density functional theory calculation

The authors investigated the image potential of C60 with a composite quantum chemistry method. The image potential is found to be almost isotropic. The image potential of isolated C60 is similar to that of an isolated ideal metal sphere with the radius of C60, while the image potential of grounded C60 is significantly different from that of a grounded ideal metal sphere. At a distance of 1.6 nm from the grounded C60, the image electric force exhibits a knee point. This phenomenon is attributed to the correlation between the testing electron and electronic structure of C60 and to the electron confinement in small systems.