A PeakForce KPFM study in a controlled

Mapping Graphene’s surface potential with <20nm resolution: A PeakForce KPFM study in a controlled <1ppm water and oxygen environment Greg Andreev and Aravind Vijayaraghavan 9/27/13

Webinar Outline

• Introduction to Graphene by Aravind • Introduction to PF-KPFM • Results on Graphene in Air and Glovebox • Questions

10/7/2013 2 Bruker Nano Surfaces

Potential

250nm -100mV

100mV

An introduction to graphene

and the importance of environment

Dr. Aravind Vijayaraghavan

The University of Manchester

Bruker Webinar "Mapping Graphene’s Surface

Potential with less than 20nm Resolution"

WHAT IS GRAPHENE

Graphene is

imensional

Buckyballs Carbon Nanotubes Graphite A. K. Geim, K. S. Novoselov

Nat Mater 2007, 6, 183.

HOW TO MAKE GRAPHENE

1. Micromechanical cleavage of Graphite

(a) Attach a piece of graphite

to sticky-tape (Cellotape)

(b) Use the sticky tape to thin

out the graphite

(c) Place the thin graphite on

a Silicon wafer, with a

surface layer of Silicon

Dioxide

(d) Remove most layers of

graphite leaving behind

graphene.

HOW TO MAKE GRAPHENE

2. Chemical Vapour deposition(a)Carbon atoms are

deposited on the surface

of a metal

(b)At high temperature this

forms graphene.

(c) A layer of polymer is

deposited on top of the

graphene.

(d)The polymer is removed

and the graphene with it.

(e)The polymer is placed on

a suitable substrate

(f) The polymer is dissolved

away leaving the

graphene behind.

HOW TO FIND GRAPHENESeeing is believing!

P. Blake, et al., Appl. Phys. Lett. 2007, 91, 063124.

ELECTRONIC STRUCTURE

Monolayer Graphene

Electronically, monolayer, bilayers and trilayer graphene are electronically

distinct materials. Beyond three layers, graphene’s electronic properties

tends towards that of bulk graphite.

T. Ohta, et al., Science 2006, 313, 951.

EFFECT OF ENVIRONMENT

Ponomarenko, L. A.; et. al. (2009). Physical Review Letters 102(20): 206603.

EFFECT OF DOPING

Changes in resistivity, at zero B

caused by graphene’s exposure to

various gases diluted in

concentration to 1 p.p.m. The

positive (negative) sign of changes

is chosen here to indicate electron

(hole) doping.

Region I: the device is in vacuum

before its exposure;

II: exposure to a 5 l volume of a

diluted chemical;

III: evacuation of the experimental

set-up; and

IV: annealing at 150 C.

Schedin, F., et al (2007). Nature Materials, 6(9), 652-655.

3

2nd trace1st trace

Lift Height

Topography

Magnetic field source

LiftMode: Separates topography from the electric/magnetic info: long range vs. short range forces, on active or passive devices. Surface Potential: measure the ‘surface potential’ together with topography.

Before getting into KPFM: Lift mode

10/7/2013 Bruker Nano Surfaces

KPFM basic principle

AM Amplitude-Modulation

FM Frequency-Modulation Better spatial resolution Better accuracy

Physical Review B 2005, 71(12) 125424

KPFM measures the work function difference of tip/sample.

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Many Ways of Doing KPFM FM and PeakForce scaling do not compete

AFM KPFM Tapping PeakForce

AM TP-AM PeakForce KPFM-AM

FM TP-FM PeakForce KPFM

Bot

h A

M &

FM

KP

FM

impr

oves

with

low

er k

Tapping is limited to high k levers due to adhesive forces. PeakForce Tapping k is not.

*Except TP-FM, all are done in lift-mode. 10/7/2013 Bruker Nano Surfaces 5

PeakForce KPFM combines FM with PFT Allows for higher sensitivity

• But Tapping Mode Requires : • k to be not too small • Q not to be too big

Tapping and KPFM scaling in conflict.

• Peak Force Tapping Mode Allows Freedom to use: • Smaller k (10x or more) • Big Q (10x or more)

PeakForce Tapping and KPFM scaling aligned.


𝐾𝐾𝐾𝐾 𝑆𝑆𝑆𝑆𝑆𝑆𝑆𝑆𝑆𝑆𝑆 ∝ 𝑄𝑘

KPFM: nanoscale measurement of Work Function, Fermi Energy & Carrier Density

• Previous work shows Work Function relationship to Efermi for Graphene

Bilayer

Single layer

KPFM can be used as a nanoscale measurement of Fermi energy

Y. Yu et. al. NanoLett. (9) 10 – 2009

10/7/2013 Bruker Nano Surfaces 7

Energy diagram of our KPFM experiment on Graphene


𝑊smp 𝑊elec

𝑆∆𝑉𝑉𝐾𝑉

𝑊tip

What KPFM measures

Vacuum level

𝑊0gr ~ 4.6eV

𝑊tip ~ 4.5eV

𝑊elec ~ 4.9eV

𝑊a ~ 0.05eV

𝐸F ~±0.3eV

Approx. Values

𝐸F

𝑊0gr

𝑊𝑎

Energy band diagram of 1L Graphene (hole-doped)


Optical Microscope (Blue filter)

Our sample: Single layer Graphene on a Silicon Nitride window

1L Graphene

1um

SiN window

1L Graphene

Au 0nm

14.5nm 25um AFM

Why this sample? Largely insulating substrate – KPFM should probe only Graphene, little crosstalk

Roughness may create interesting inhomogeneities

SiN window


AFM Potential

Results (in Air): PF-KPFM on Graphene shows incredible contrast

AFM Potential

Linear debris does not show up!

<40nm gap b/n layers clearly visible

<40nm gap b/n layers barely visible in Height

Linear debris clearly visible

-200mV

500mV 1um

Hole doping of Graphene confirmed, Wg ~ 4.85V Signs of incredible nanoscale contrast, is it real?

Lift height dependence: a way to check for topographic artificats in KPFM


Example

From Bruker App. note #140

Proof of real contrast: evolution of PF-KPFM resolution w/ lift height


50nm 40nm 28nm 75nm

No sudden loss of resolution = contrast not due to tip sticking

1um

Lift Height

Let’s zoom in on an interesting region..


0nm

14.5nm 1um

Zooming in: how homogeneous is Graphene?

Bruker Nano Surfaces

Height

0nm

11nm Adhesion

6nN

11nN

Substrate residue easily picked up by Adhesion channel

Will it have some effect on Graphene’s work function?

250nm

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PF-KPFM of zoomed-in region: <20nm resolution reveals inhomogeneities


15

Potential, z = 28nm

250nm -100mV

+100mV

0mV

100mV

50mV

Similar result to previous work:

J. Martin et. al. Nature 2007

20nm wide “hump” resolved!

4.85V

Bruker PF-KPFM: inhomogeneities correlate with high potential


Adhesion

6nN

11nN Adhesion+High Potential

6nN

11nN

PF-KPFM adhesion channel makes it easy to see the inhomogeneities!

Is the doping mechanism here also related to

Water + Oxygen as in previous work?

scale bar = 250nm

What is the effect of the environment on Graphene doping?

• Water? • Oxygen? • Is it reversible?

An environmentally controlled measurement is needed..

Cryogenic UHV KPFM would be great, but difficult/costly

Is there an easier way to study Graphene in a H2O+O2 free environment?


Unique Bruker technology: Glovebox Integrated System

Antechamber for introducing samples, pressure below 1E-6mbar

Argon Atmosphere, <1ppm H2O, 1ppm O2


Physical picture: what we expect to happen


Ambient GloveBox

𝐸F

Graphene is hole doped by Water and Oxygen

Water and Oxygen removed, Graphene nearly intrinsic

Potential for studying nearly-intrinsic Graphene without

Cryogenics or UHV

𝐸F

𝑊0gr 𝑊smp

𝑊𝑎

𝑊elec 𝑆∆𝑉𝑉𝐾𝑉

𝑊tip

Vacuum level

Physical picture confirmed: PF-KPFM in GloveBox shows lower contrast


Ambient GloveBox

Contrast between Graphene and SiN greatly reduced

Spatial inhomogeneities in Graphene also reduced

-200mV

500mV

-150mV

150mV

Physical picture confirmed: PF-KPFM in GloveBox shows lower contrast


Ambient GloveBox (same scale)

-200mV

500mV

-200mV

500mV

Graphene signal is nearly that of an insulator

Inhomogeneities greatly reduced in Glovebox


Potential

Glovebox

AFM

Air

250nm

-100mV

+100mV

-100mV

+100mV

4.85V ~4.6V

Another sample: larger KPFM scan


SiN Graphene

Au

Au

Graphene SiN

250 - 470mV 93mV

Three vastly different potentials for Graphene?

Potential in Air

KPFM Potential, in Glovebox before vacuum

SiN Graphene

Au

KPFM Potential, in Glovebox after vacuum


Au

Graphene SiN/chuck

284mV 193mV

334mV 133mV

Glovebox environment reduced Graphene workfunction by ~100mV

Wg ~ 4.7eV near Au contact, smaller farther away

Conclusion

• PF-KPFM’s spatial resolution is great: 20nm resolution images are achievable on Graphene

• PF-KPFM is sensitive: ~10meV sensitivity allows to easily distinguish inhomogeneities

• Glovebox Integrated System is a convenient promising environment for reducing Graphene’s work function to nearly intrinsic levels.


Backup slides


Tip Cone Contribution in KPFM FM gradient detection isolates contribution from tip

• FM-KPFM:

• The foremost 0.3% of the tip cone accounts for half of the signal in.

• FM can achieve a lateral resolution better than 50nm.

• AM-KPFM

• The contribution from the tip cone never reaches 50%.

• Its lateral resolution is dictated by the um-scale lever.

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0%

10%

20%

30%

40%

50%

60%

70%

80%

90%

100%

0% 0% 1% 10% 100%

Cone

Con

trib

utio

n%

Height Inclusion (h/H)%

FM z=10 nm FM z=50 nm

AM z=10 nm

AM z=50 nm

Based on SCM-PIT Geometry: W=30um, L=225um, H=10um, Cone Angle=45

Bruker Nano Surfaces

A PeakForce KPFM study in a controlled

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