Graphene Nanoribbons: A Route to Atomically Precise Nanoelectronics

Mike Crommie

Dept. of Physics, UC Berkeley and

Materials Science Division, LBNL Berkeley, CA

Outline

1) Graphene Graphene Nanoribbon (GNR) behavior

2) Potential applications for GNRs.

3) How do we make GNRs?

4) New developments in bottom-up fabrication Molecular bandgap engineering New bottom-up strategies

Graphene Nanoribbon

Electronic Structure of Graphene

zigzag direction

K

M

armchair direction

kx

ky

K

M

Reciprocal Space

kx

ky

Ene

rgy

Conduction band

Valence band Dirac pt.

Graphene Nanoribbon Electronic Structure

∆(e

V)

Armchair

∆(e

V)

Zigzag

Spin-polarized

M. Fujita, et al., J. Phys. Soc. Jpn 65, 1920 (1996); K. Nakada, et al., PRB 54, 17954 (1996); J. Heyd et al, J. Chem. Phys. 118, 8207 (2003); Y.-W. Son, et al, PRL 97, 216803 (2006)

E

k

0

0

Reciprocal Space

No Edge State

Reciprocal Space

Edge StateE

k 0

0

……

Armchairw

… …

Zigzag

Potential GNR Device Advantages

Energy gap, capacitance Bandgap Engineering

GNRs solve the nanotube metallicity problem

ID – VG for nanotubes

H. Park, et al., Nat. Nanotech. 7, 787 (2012)

“on”

“off”

Big gap Small gap

Width

Energy

Length

Uniquely Efficient Tunneling

Fast onset

Length

VSD

VG

k

E

µ

GNR GNR

tube tube

Smaller capacitance Faster

Potential GNR Device Advantages

GNR GNR

tube tube

Smaller capacitance Faster “on”

“off”

Big gap Small gap

Width

Energy

Length

Uniquely Efficient Tunneling

Fast onset

Bandgap Engineering Energy gap, capacitance

GNRs solve the nanotube metallicity problem

ID – VG for nanotubes

H. Park, et al., Nat. Nanotech. 7, 787 (2012)

VSD

VG

k

E

µ

High-current low-dissipation switching: HFET

J. Bokor

New proposed GNR implementation

Requires GNR doping and bandgap/width variation

p n

Large gap

Small gap

Large gap

S D

How Do We Make High Quality GNR Devices?

M. Y. Han et al., PRL 98, 206805 (2007)

Y. Kobayash et al., PRB 71, 193406 (2005) Ritter and Lyding, Nat. Mat. 8, 235 (2009)

Rough Edges are a Problem Graphene platelet on Si(100)

Top-down Lithography

Unzipping Nanotubes: A Better Edge

Au(111)

(8, 1) GNR

0.0 Å

5.5 Å

48 Å × 48 Å

width = 20 nm

C. Tao, et al., Nat. Phys. 7, 616 (2011)

Unzipped GNRs Smooth Edges

X. Wang, et al, Nat. Nanotechnol 6, 563 (2011)

Unzipped GNR FETs High Mobility

Jiao, et al, Nat. Nanotechnol 5, 321 (2010) Kosynkin, et al, Nature 458, 872 (2009)

A problem

Controlling width and chirality

A New Idea

Bottom-up Fabrication: + + + Molecular precursor Final assembled GNR

Bottom-up Fabrication: + + + Molecular precursor Final assembled GNR

+ + + Width

+ + + Chirality

+ + + Edge functionalization

Bottom-up Heterostructures:

n p p ∆1 ∆2 ∆1

p-n junctions bandgap engineering

A New Idea

GNR Bottom-up Synthesis Breakthrough: n=7 AGNRs

.

. 200 °C 400 °C

Metallic surfaces

200 °C n = 7 AGNR

(7 atoms across)

Precursor molecule

STM Image: AGNRs / Au(111)

Fasel, Muellen, & co-workers

J. Cai et al, Nature 466, 470 (2010)

Width = 0.7 nm

STM Allows Measurement of Local Electronic Structure

STM Spectroscopy

EF

EF

Ñwω

tip sample

LDOS(E)

0

V tip

sample EF

M. Koch, et al., Nat. Nanotech. 7, 713 (2012)

P. Ruffieux, et al., ACS Nano 6, 6930 (2012)

∆ = 2.5 eV

GNR Electronic Structure

Au reference

GNR

STM

Using STM to Measure GNR Energy Gap

LUMO dI/dV map

Bandedge Electrons Show Higher Density at GNR Edges

Can We Tune the Energy Gap?

?

N = 7 N > 7

∆(e

V)

Armchair

0

2

4 N=7 Tune?

Must synthesize new precursor molecules

A New Precursor Molecule to Tune GNR Bandgap: N=13

Yen-Chia Chen, et al., ACS Nano 7, 6123 (2013)

.

. 200 °C 400 °C

Metallic surface

200 °C

New Precursor

n = 13

F. Fischer & Crommie

width = 1.4 nm STM image: Polymer stage Fully cyclized (after annealing)

STM Spectroscopy of N=13 AGNR

dI/dV maps

HOMO LUMO

GNR

Au

Y.-C. Chen et al., ACS Nano 7, 6123 (2013)

6.0 7

13 =∆∆

=

=

N

N

2 7

13 ==

=

N

N

widthwidth

+

Molecular Bandgap Engineering (B.E.)

Previous Mesoscale B.E. :

GaAs GaAs AlGaAs

x

E

Sevinçli, et al, PRB 78, 245402 (2008)

Variable-Width Heterostructures

5-9 Junction

New Molecule-scale B.E. : (theory, DFT)

Ec

Ev

Molecular Bandgap Engineering: 7-13 Junctions n=7

n=13

7-13 Junction

Fabricating 7-13 Molecular Junctions on Au(111)

Molecular Bandgap Engineering: 7-13 Junctions n=7

n=13

7-13 Junction

3.8 nm 2 nm

Topograph

++

+

N=7

N=13

3 4

STM Spectroscopy of 7-13 Junction

Topograph

++

++

N=7

N=13

3 4

1 2

STM Spectroscopy of 7-13 Junction: Interface States

Topograph

++

++

N=7

N=13

3 4

1 2

STM Spectroscopy of 7-13 Junction: Interface States

Unit Cell

Assume Periodic Structure: Perform DFT Calculation

Theoretically Modeling the 7-13 Molecular Junction

Theory: Ting Cao, Steven Louie

0 0.5 -1.0-0.5-1.0Energy (eV)

D.O

.S.

Electronic Structure of 7-13 Molecular Junction

n = 130.9 eV

n = 71.7 eV

n=13 LUMO

-1.0

-0.5

0.0

0.5

1.0

S. G. Louie, T. Cao

Calculate LDOS distribution for these states, compare to experiment

1 4 2 3

Theoretical LDOS

Topograph

++

++

34

1 2

Comparing Theoretical Wave-function Maps

to Experiment

Experimental LDOS

How Can Devices be Made From Bottom-up GNRs?

Must transfer GNRs to insulator:

Fischer, Crommie, Bokor

Device Layout for 7-AGNRs

A

26nm gap

PMMA GNRs

Au Mica

Mica

SiO2

Bottom-up GNR Device Results

Schottky barrier behavior: Bottom-up N=7 GNR FET

P. B. Bennett et al., APL 103, 253114 (2013)

n-type

k

E

µe Φ’

metal contact GNR

metal contact

E

x

Challenges: • Improve contacts • Improve transfer to insulator • New GNR heterostructures • Grow directly on insulator?

New Chemistry: New Opportunities

Bergman Cyclization of Enediynes:

Polymerization: . . Q. Sun, et al., JACS 135, 8448 (2013)

A. Riss, et al., Nano Lett. 14, 2251 (2014)

Flexible coupling: + .. Radical step growth

Alkyne coupling

Au(111) SiO2

Difficult

Currently requires metal substrate

Model System for Surface Chemistry

Enediyne Fragment:

Expected Reaction Path:

Felix Fischer (UC Berkeley)

But what really happens?

heat

STM

Imag

es

Imaging Enediyene Cyclization on Ag(100)

heat

STM

Imag

es

Imaging Enediyene Cyclization on Ag(100)

F. J. Giessibl, Appl. Phys. Lett. 76, 1470 (2000)

L. Gross, F. Mohn, N. Moll, P. Liljeroth, G. Meyer, Science 325, 1110 (2009)

tip

Qplus nc-AFM

G. Meyer & co-workers (2009)

Imaging Enediyene Cyclization on Ag(100)

heat

STM

Imag

es

nc-A

FM Im

ages

D. Oteyza, et al., Science, 10.1126/science.1238187 (2013)

Modifying Enediyne Molecules to Induce Coupling

Enediynes on Ag(100)

Alkyne Coupling

heat

heat

Modifying Enediyne Molecules to Induce Coupling

Radical Polymerization

Enediynes on Au(111)

D. de Oteyza, et al., Science 340, 1434 (2013)

Energy Landscape and Reaction Pathway

Theory: A. Rubio & co-workers

D. de Oteyza, et al., Science 340, 1434 (2013)

Energy Landscape and Reaction Pathway

Theory: A. Rubio & co-workers

Improved Structural Control at the Nanoscale

Conclusions 1) GNRs novel electronic properties.

2) Bottom-up synthesis molecular bandgap engineering.

3) New chemistries new nanostructures.

Future 1) Incorporate bottom-up heterojunctions into devices.

2) New bottom-up GNR structures, improved control.

3) Grow GNRs directly on insulators.

Nano-Bio Spect. Gp., ETSF Sci. Dev. Center, UPV, San Sebastian, Spain: Angel Rubio, Duncan J. Mowbray, Alejandro Perez UC Berkeley / LBL: M. F. Crommie (Physics) Dimas G.de Oteyza (now at Centro de Fisica, San Seb., Spain) Felix Fischer (Chemistry) Alexander Riss (now at Inst. of App. Phys., TU Wien) Steven Louie (Physics) Sebastian Wickenberg Jeff Bokor (EECS) Hsinzon Tsai Marvin Cohen (Physics) Patrick Bennett Alex Zettl (Physics) Miguel Moreno-Ugeda Zahra Pedramrazi Chen Chen Aaron Bradley Danny Haberer Grisha Etkin Patrick Gorman Liang Z. Tan Ivan Pechenezhskiy Yenchia Chen

Collaborators / Funding

THE END