Atomic Structural Response to External Strain for AGNRs

Wenfu Liao & Guanghui ZhouWenfu Liao & Guanghui Zhou

KITPC Program—Molecular Junctions

Supported by NSFC under Grant No. 10974052

CONTETCONTET

I. Background

II. Bond Variation for AGNRs

under Uniaxial Strain

III. Summary

I. Backgroud

Gapless Zero band mass Electron-hole symmetry Pair creation Chiral (Pseudospin) Berry phase No back-scattering

tight-binding electron energy dispersion of graphene

Material for Novel Devices? 1. Typical speed 2. Huge current density 3. Large mean free path (high conductivity) 4. Large phase coherence lengths (coherent electronic circuits) 5. Easily cutting the sheet into nanoribbons (nanoscaled molecular electronic devices) 6. Strong field effect (metallic FET) 7. Ballistic transport up to room temperature 8. High-strength composites 9. Spin-valve, spin-qubit and hydrogen storage

610 /FV m s

600el nm

28 /10 cmAI

ml 1

Open and/or tune an energy gap ?!— gap engineering (manipulation)

1. Finite size graphene nanoribbons—GNRs 1. Finite size graphene nanoribbons—GNRs i. quasi-1D nature (a new type of quantum i. quasi-1D nature (a new type of quantum

wires) wires) ii. similar to carbon nanotubes (CNTs)ii. similar to carbon nanotubes (CNTs) iii. building blocks for nanoelectronic devices iii. building blocks for nanoelectronic devices

2. Disorders (defects, impurity, …)2. Disorders (defects, impurity, …)

3. External fields (EM-field, etc.)3. External fields (EM-field, etc.)

4. Multi-layers4. Multi-layers

5. Mechanically 5. Mechanically !?— “strain engineering ”

Strain, even if it does not generate gaps, can also introduce strong anisotropies in the atomic structure and charge transport that can be used for applications !

Among all these methods, strain may be one of the most competitive candidates to exercise due to its continuous tunability and easiness performance even at nano-scale.

(1) Single-walled CNTs under strain(1) Single-walled CNTs under strain

Small band-gap semiconducting (or quasimetallic) nanotubes exhibit the largest resistance changes and piezoresistive gauge factors under axial strains.

Maki et al, Nano Lett. 7, 890 (2007)Maki et al, Nano Lett. 7, 890 (2007)

Photoluminescence MeasurementPhotoluminescence Measurement

(2) Graphene under strain(2) Graphene under strain

Nano. Lett. 10, 3486 (2010)

Appl. Phys. Lett. 98, 023112 (2011)

Band gap as a function of strain for AGNR with different width

Band gap as a function of strain for ZGNR with different width

Questions:1. Variation of atomic structure, bond length and angle?2. What is the distribution of the applied strain? Which part of

bonds afford the force mostly?3. Nanomechanical detector (sensor) design?

AC-strain

ZZ-strain

II. Bond variation for AGNRs under a strain

Band distribution for supercells of asymmetric 6- and 8-AGNRBand distribution for supercells of asymmetric 6- and 8-AGNR

Band distribution for supercell of symmetric 7-AGNRBand distribution for supercell of symmetric 7-AGNR

Table of bond lengths for 6-, 7- and 8-AGNRTable of bond lengths for 6-, 7- and 8-AGNR

1. AC-strain is mostly afforded by the central region bonds while ZZ-strain is afforded by the edge region ones.

2. AC-strain elongates all bond while ZZ-strain only elongates most bond but a small part of bond lengths are compressed.

Isosurface charge density for optimized supercellsIsosurface charge density for optimized supercells

Percentage of varied bonds for N-AGNRs under a strainPercentage of varied bonds for N-AGNRs under a strain

1. Asymmetric 2n-AGNRs show 2n types of bonds, while symmetric (4n+1)/(4n+3)-AGNRs present only (3n+1)/(3n+2) types of bonds.

2. (4n+1)/(4n+3)-AGNRs trend to be more stable/unstable against strain as n increases, among which the narrowest 7-AGNR is the

most stable one against external strain.

N-AGNRs can be classified into 3 types according to their structural response to a strain: symmetric 2n-, asymmetric (4n+1)- and (4n+3)-AGNRs. After doing a large amount of calculations for many AGNRs we conclude a general rule.

Symmetric AGNRs are better building block for electroniccircuits and devices for stability consideration, while asymmetricones may be useful in electromechanical nanodevices, such as force sensor , etc.

1. Strained GNRs — detailed relation between atomic and electronic structures?

2. Electron level explain for bond variation .3. Predicted atomic and electronic structures

can be observed experimentally? 4. Strained GNRs can used to design the nano-

electromechanical devices and opto-electronic devices?

III. Summary

Thanks for your Thanks for your attention !!!attention !!!

Thanks for your Thanks for your attention !!!attention !!!