Investigating 2D Materials with AFM: Five recent Nature Publications Bede Pittenger, Senior Applications Scientist

Enabling New Research on 2D materials Correlate chemistry and defects with topography, mechanical properties and electronic structure

7/30/2015 2 Bruker

Nanoscale deformation & other mechanical properties

-PeakForce QNM

SPIR illuminates defect rich area

-PeakForce IR at 1730cm-1

Maps of workfunction show layers and charges in substrate

-PeakForce KPFM

AFM Height:

Shows Graphene layer structure and points to

defect rich area

1 2

3

4

Graphene plasmons

-PeakForce IR at 870cm-1

Lattice orientation

-Contact Mode

Why AFM for 2d Materials?

• Atomic resolution:

• Z: easily identify monatomic steps and ripples

• XY: Lattice resolution for orientation, observation of defects

• Controlled force (some AFM modes)

• Preserves AFM tip and fragile samples

• Allows local stress to be applied to sample

• More than just topography

• surface potential, modulus, adhesion, conductivity, IR absorption & reflection, etc.

• Works under liquids as well as in ambient conditions

7/30/2015 3 Bruker

water

AFM Imaging Technology A Brief Review

• Mapping topography -> More information

• Contact mode & LFM (1986)

• Tapping Mode (1992)

• Force-Volume Mapping (~1992)

• PeakForce QNM (2010)

• PeakForce TUNA (2011)

• PeakForce KPFM (2012)

• PeakForce IR (2014)

• PeakForce XYZ (…)

7/30/2015 4 Bruker

Rich property mapping made possible by Bruker’s PeakForce Tapping

(ii)

The complete force curve from every interaction between tip and sample is analyzed in real-time, allowing:

• Feedback based on the peak force, protecting the tip and sample.

• Mechanical properties mapped simultaneously with topography.

• Individual curves can be examined and analyzed offline

• Off-resonance: allows combination with other techniques

7/30/2015 5 Bruker

AFM application of local stress Investigating a potential MoS2 strain sensor

• Piezoelectric effect observed in single layer MoS2 devices

• AFM and Raman used to confirm single layer

• Potential strain sensor application

• Found to be very high performance & expected to be suitable for multidirectional nanoscale sensors

7/30/2015 6 Bruker

Qi, J. et al., 2015. Nat. Commun., doi:10.1038/ncomms8430.

AFM application of local stress Inducing piezoelectric effect in MoS2 monolayer

• PeakForce QNM measured deformation under varying load

• Saturation observed due to substrate

• AFM Force Spectroscopy used to apply load near center of device

• IV relations show decreasing current with increasing load

• Different S-D pairs provide very similar results

7/30/2015 7 Bruker

Qi, J. et al., 2015. Nat. Commun., doi:10.1038/ncomms8430.

S S D

D

AFM application of local stress Piezoelectric effect explains device characteristics

• AFM tip loads device at different locations

• Results in either compressive or tensile strain

• Piezoelectric polarization changes Schottky barriers

• Causes different I-V curves

• Consistent with classic thermionic emission-diffusion theory

7/30/2015 8 Bruker

Qi, J. et al., 2015. Nat. Commun., doi:10.1038/ncomms8430.

Compressive Tensile

+ + - -

Mechanical properties of 2d materials Suspended graphene

• Suspend graphene using SiN membrane = direct observation of deformation under load

• TappingMode AFM fails over the unsupported region.

• PeakForce Tapping allows imaging with controlled force

• Provides force curves at each point

• Fitting curves gives modulus ~350N/m

7/30/2015 9 Bruker

Clark, N. et al., 2013. Phys. Status Solidi B, doi:10.1002/pssb.201300137.

Mechanical properties of 2d materials Embedded Graphene

• Embedding the graphene in a polymer matrix allows use of a four point bending setup for more controlled strain

• Raman shift allows characterization of critical axial compressive strain ~-0.6% (or -6GPa)

• AFM enabled direct observation of wrinkling that occurs during compressive failure

7/30/2015 10 Bruker

PeakForce Tapping Height

Androulidakis, C. et al., 2014. Sci. Rep., doi:10.1038/srep05271.

Visualizing 2d structures in liquid Ordered gas molecules at solid-liquid interfaces

• Ordered gases at interfaces influence friction and biological processes involving collection of dissolved gases

• Interfacial Nanobubbles often form at solid-liquid interfaces, but sometimes other structures form as well…

• Why do they last so long? Young-Laplace equation predicts they should dissolve within a few milliseconds

7/30/2015 11 Bruker

Water supersaturated with O2 Water supersaturated with N2

Lu, Y.-H. et al., 2014. Sci. Rep., doi:10.1038/srep07189.

Visualizing 2d structures in liquid Epitaxial O2 molecules at HOPG-water interface

• AFM lattice resolution allows comparison of epitaxial O2 layers to underlying HOPG lattice

• Two types of epitaxial O2

structures reflecting HOPG symmetry directions were observed

• Both had spacing ~4-5nm

7/30/2015 12 Bruker

Lu, Y.-H. et al., 2014. Sci. Rep., doi:10.1038/srep07189.

Visualizing 2d structures in liquid Dissecting O2 structures with PeakForce Tapping

• TappingMode (FM) shows nanobubble on disordered micropancake

• PeakForce Tapping at different forces allows the penetration of the tip into the surface to be controlled

• Allows observation of epitaxial layers beneath micropancake

• Micropancakes only form on epitaxial layers – need crystalline, hydrophobic substrate

7/30/2015 13 Bruker

Lu, Y.-H. et al., 2014. Sci. Rep., doi:10.1038/srep07189.

Visualizing 2d structures in liquid Dissecting N2 structures with PeakForce Tapping

• N2 also forms 2d structures

• Epitaxial rows only along the HOPG zig-zag direction

• Micropancakes not observed

• PeakForce Tapping at different forces shows

• Nanobubbles are disordered (Adhesion map)

• Nanobubbles form directly on the HOPG, not on an epitaxial overlayer

• Epitaxial domains form the perimeter of the bubble

• Interfacial water may be stabilizing the structures

7/30/2015 14 Bruker

150pN: Height

1500pN 2250pN

150pN: Adhesion

Lu, Y.-H. et al., 2014. Sci. Rep., doi:10.1038/srep07189.

A nanoscale pressure reactor Graphene on diamond (100)

• When graphene is placed on diamond in the presence of water, interfacial energy drives water from some areas while accumulating it in others

• Annealing to ~1275K causes graphene to bond to diamond, trapping accumulated water into nanobubbles

• A mat of nanoscale hydrothermal anvils will be formed containing any molecules present

• Raman and FTIR can study the contents of the nanobubbles

7/30/2015 15 Bruker

Lim, C. H. Y. X. et al., 2013. Nature Communications, doi:10.1038/ncomms2579.

A nanoscale pressure reactor Observation of the nanobubbles (GNB)

• AFM imaging allows direct observation and measurement of the GNB

• Dimensions can be used to obtain a measure of the strain in the graphene (~6%)

• Also allow estimation of the pressure within a GNB

• Combined AFM-Raman allows selection of areas without GNB for control measurements

• PeakForce QNM modulus mapping shows the GNB to be ~10x softer than the area where the graphene is bonded to the diamond

7/30/2015 16 Bruker

Lim, C. H. Y. X. et al., 2013. Nature Communications, doi:10.1038/ncomms2579.

A nanoscale pressure reactor High-pressure chemistry

• If temperature increased >1500K, GNBs rupture

• AFM and SEM images show square shaped voids (100) diamond planes

• At 1375K and 1GPa, the water in the GNB is superheated

• Capable of etching the diamond

• Concave graphene is less reactive than diamond

• More recently, similar methods were used to study polymerization of C60

7/30/2015 17 Bruker

Lim, C. H. Y. X. et al., 2013. Nature Communications, doi:10.1038/ncomms2579. Lim, C. H. Y. X. et al., 2014. Angew. Chemie, doi: 10.1002/anie.201308682.

Strain in 2d heterostructures Moire patterns and commensurate domains

• Lattice mismatch between graphene and hBN causes a moiré pattern that affects electrical, optical properties

• Periodicity depends on rotation of lattices

• When lattices are nearly aligned, there is a transition to a state with commensurate domains where graphene deforms to match the hBN lattice

• Domains are separated by a domain wall where strain is accumulated

7/30/2015 18 Bruker

Woods, C R et al., 2014. Nature Physics doi:10.1038/nphys2954.

Aligned Misaligned ~3deg

Strain in 2d heterostructures PeakForce QNM ‘Modulus’ shows clear transition

• Different periodicities of moiré visible in CAFM

• ‘Modulus’ cross section changes when lattices are aligned

• Misaligned lattices: ~sinusoidal modulus

• Aligned lattices: narrow domain walls separating commensurate regions

• PeakForce QNM shows clear transition when lattices are aligned (period>~10nm)

• Variation in strain at domain walls causes contrast in modulus

7/30/2015 19 Bruker

Woods, C R et al., 2014. Nature Physics doi:10.1038/nphys2954.

CA

FM

8nm 14nm

‘Modulus’

Strain in 2d heterostructures STM confirms source of ‘modulus’ contrast

• STM confirms that the source of the modulus contrast is strain in the graphene

• Lattice constant in domain wall ~2% smaller than within domain

• Transition appears to be responsible for discrepancies in earlier transport measurements of graphene/hBN devices

7/30/2015 20 Bruker

Woods, C R et al., 2014. Nature Physics doi:10.1038/nphys2954.

• AFM with PeakForce Tapping enabled…

• Stimulation of device at precise location with controlled force

• Observation of deformation of graphene under tension and compression

• Dissection of 2d structures at solid-liquid interfaces with controlled force

• Quantification of pressure in nanoanvils by measurement of nanobubbles

• Observation of strain in graphene on BN through high resolution mapping of ‘modulus’

Showcasing the range of AFM enabled research on 2d materials

7/30/2015 21 Bruker

© Copyright Bruker Corporation. All rights reserved.

www.bruker.com

contact me here:

Bede Pittenger, PhD

bede.pittenger@bruker.com

www.bruker.com/service/education-training/webinars/afm.html

Also check out Nanoscale world community

and SPM Digest Forum:

nanoscaleworld.bruker-axs.com/

www.bruker.com/service/education-training/training-courses/afm-optical-training-courses.html