Integration Challenges and Opportunities
for Two-Dimensional Materials
Professor Mark C. Hersam
Bette and Neison Harris Chair in Teaching Excellence
Director, Materials Research Center
Northwestern University
http://www.hersam-group.northwestern.edu/
NSF US-EU Workshop on 2D
Layered Materials and Devices
Arlington, Virginia
April 23, 2015
Hersam Group
Lessons from One-Dimensional Materials
http://www.nanointegris.com/
Transitioning from the research
laboratory to the marketplace for
carbon nanotubes required:
•  Focus on homogeneity,
reproducibility, and reliability (as
opposed to chasing exceptional,
champion performance)
•  Alternative manufacturing and
fabrication approaches (e.g.,
printing to avoid competition with
established methods)
•  NanoIntegris founded in 2007
•  ~700 customers in 40+ countries
•  Acquired by Raymor in 2012
•  Unique applications (as opposed
to attempting to supplant an
incumbent technology)
Hersam Group
Outline
Review Articles:
Chem. Soc. Rev., 42, 2824 (2013).
ACS Nano, 8, 1102 (2014).
•  Monodisperse materials
•  Additive manufacturing
•  New device concepts
Hersam Group
Outline
Review Articles:
Chem. Soc. Rev., 42, 2824 (2013).
ACS Nano, 8, 1102 (2014).
•  Monodisperse materials
•  Additive manufacturing
•  New device concepts
Hersam Group
Density Gradient Ultracentrifugation of Graphene
Nano Letters, 9, 4931 (2009).
•  Exfoliate graphite powder via sonication in aqueous solution with
the planar surfactant sodium cholate.
•  DGU enables sorting by the number of graphene layers.
Hersam Group
DGU of Transition Metal Dichalcogenides
Nature Communications, 5, 5478 (2014).
DGU enables sorting of transition metal dichalcogendies by
thickness including MoS2, MoSe2, WS2, and WSe2.
Hersam Group
Solution-Processed Black Phosphorus (BP)
ACS Nano, in press, DOI: 10.1021/acsnano.5b01143 (2015).
Solution-processed BP is comparable to mechanically exfoliated BP
in field-effect transistors (mobility ~50 cm2/Vs; on/off ratio ~104)
Hersam Group
2D Black Phosphorus in Ambient on SiO2
Collaboration with Lincoln Lauhon and Tobin Marks (Northwestern University)
Nano Letters, 14, 6964 (2014).
Au
D
C
•  AFM images of unencapsulated
black phosphorus (BP) in
ambient conditions
Ti
or
Ni
Au
1 day
0 days
E
•  Samples are stored in the dark
in ambient (relative humidity:
30-40%) between AFM images
F
•  Apparent bubble or droplet
formation and coalescence with
increasing ambient exposure
•  Scale bars = 1 µm
3 days
2 days
0 nm
20 nm
Hersam Group
s
Atomic Layer Deposition AlOx Passivation of BP
Collaboration with Lincoln Lauhon and Tobin Marks (Northwestern University)
I
y
s
Nano Letters,
7 days 14, 6964 (2014).
1 day
J
2 days
1 day
K
L
3 days
0 nm
7 days
10 nm
1 µm
34 days
AlOx
0 nm
20 nm
AlOx passivated surface shows no BP degradation, even after
34 days (now up to ~6 months) in ambient conditions
Hersam Group
Improved BP FET Stability After Encapsulation
Collaboration with Lincoln Lauhon and Tobin Marks (Northwestern University)
Nano Letters, 14, 6964 (2014).
On/Off Ratio
10
102
101
100
0
50
100
Time (hrs)
150
102
Mobilty (cm2/Vs)
Encapsulated
Unencapsulated
3
Encapsulated
Unencapsulated
101
100
10-1
10-2
10-3
0
50
100
150
Time (hrs)
•  Unencapsulated devices rapidly degrade in ambient conditions
•  ION/IOFF ratios and mobilities nearly constant for encapsulated devices
Hersam Group
Outline
Review Articles:
Chem. Soc. Rev., 42, 2824 (2013).
ACS Nano, 8, 1102 (2014).
•  Monodisperse materials
•  Additive manufacturing
•  New device concepts
Hersam Group
Highly Concentrated Graphene in Benign Solvents
Journal of the American Chemical Society, 132, 17661 (2010).
•  Use of a stabilizing polymer (ethyl cellulose) increases the concentration of
graphene in ethanol by ~100-fold.
•  Iterative solvent exchange with terpineol and water increases the graphene
concentration by another factor of 10 (~1 mg/mL) without centrifugation.
Hersam Group
Inkjet Printable Graphene for Flexible Interconnects
Journal of Physical Chemistry Letters, 4, 1347 (2013).
Available from Sigma-Aldrich: Catalog # 793663
400 µm
•  Inkjet printable graphene based on ethyl cellulose stabilizer in terpineol.
•  Low resistivity of 4 mΩ-cm maintained following repeated flexing and
even folding.
Hersam Group
Large-Area Gravure Printable Graphene
Advanced Materials, 26, 4533 (2014).
Collaboration with Lorraine Francis and Dan Frisbie (University of Minnesota)
Ethyl cellulose stabilizer allows viscosity tuning over multiple orders of
magnitude, enabling compatibility with a diverse range of printing methods
Hersam Group
Screen Printable Graphene for Flexible Electronics
Advanced Materials, 27, 109 (2015).
Collaboration with Lorraine Francis and Dan Frisbie (University of Minnesota)
Screen printable graphene is compatible with other materials that are
commonly employed in printed/flexible electronics Hersam Group
3D Printable Graphene as Conductive Bioscaffolds
ACS Nano, in press, DOI: 10.1021/acsnano.5b01179 (2015).
Collaboration with Ramille Shah (Northwestern University Medical School)
High-content (60 vol%) graphene inks can be 3D printed into selfsupporting, electrically conductive, and mechanically resilient structures
(e.g., implantable tubular nerve conduits)
Hersam Group
Outline
Review Articles:
Chem. Soc. Rev., 42, 2824 (2013).
ACS Nano, 8, 1102 (2014).
•  Monodisperse materials
•  Additive manufacturing
•  New device concepts
Hersam Group
SWCNT/MoS2 p-n Heterojunction Diode
Collaboration with Lincoln Lauhon and Tobin Marks (Northwestern University)
Proceedings of the National Academy of Sciences USA, 110, 18076 (2013).
10 µm
2.5 µm
Hersam Group
Gate-Tunable Diode Characteristics
Collaboration with Lincoln Lauhon and Tobin Marks (Northwestern University)
Proceedings of the National Academy of Sciences USA, 110, 18076 (2013).
•  Gate bias tunes the diode rectification ratio by 5 orders of magnitude.
•  As a three-terminal device, it shows anti-ambipolar transfer curves.
Hersam Group
Wafer-Scale SWCNT/IGZO p-n Heterojunctions
Collaboration with Lincoln Lauhon and Tobin Marks (Northwestern University)
Nano Letters, 15, 416 (2015).
•  Wafer-scale p-n heterojunctions can be fabricated via photolithography
using p-type SWCNTs and n-type indium gallium zinc oxide (IGZO).
•  ALD deposited 15 nm thick hafnia enables low voltage operation.
Hersam Group
Anti-Ambipolar SWCNT/IGZO Heterojunctions
Collaboration with Lincoln Lauhon and Tobin Marks (Northwestern University)
Nano Letters, 15, 416 (2015).
10
-­‐4
C N T
IG Z O C N T -­‐IG Z O ju n c tio n
-­‐6
-­‐1
0
1
2
V G (V )
3
4
0
10
-­‐2
10
-­‐4
V D = 2V
1.5
1.0
-­‐2
10
V D = 2 V
0
10
10
I D ( µA )
10
2
I D ( µA )
10
0.5
0.0
10
-­‐6
-­‐2
0
V G (V )
2
4
•  Anti-ambipolarity results from p-type SWCNT and n-type IGZO
being fully depleted at positive and negative VG, respectively.
•  Low voltage operation with on/off ratio in excess of 104.
Hersam Group
Anti-Ambipolar Phase/Frequency Shift Keying
Collaboration with Chris Kim (University of Minnesota)
Nano Letters, 15, 416 (2015).
à Anti-ambipolarity
enables efficient
realization of
communications
circuits such as binary
phase and frequency
shift keying
à Anti-ambipolar
heterojunctions
present opportunities
for next-generation
WiFi technology
Hersam Group
CVD MoS2 Grain Boundary Memristors
Collaboration with Lincoln Lauhon and Tobin Marks (Northwestern University)
Nature Nanotechnology, in press, DOI: 10.1038/nnano.2015.56 (2015).
1
Monolayer
MoS2
Au
2
SiO2
Si
Grain boundary
3
4
4 µm
Ø  Hysteretic I-V curve with low and high
resistance states à memristor
Ø  Switching ratio (ON/OFF) ~ 103
Ø  Observed in devices with grain boundaries
Hersam Group
Gate Tunability in MoS2 Memristors
Collaboration with Lincoln Lauhon and Tobin Marks (Northwestern University)
Nature Nanotechnology, in press, DOI: 10.1038/nnano.2015.56 (2015).
Pt:Ir
Vtip
MoS2
ground
Vdrain
Si, gate
49
source
drain
44
Ø  90% of the EFM phase shift at GB
5V
0V
Ø  GB dominates charge transport
Ø  Gate-tunable set voltage in memristor
Hersam Group
Summary
•  Centrifugal solution processing allows
the scalable production of highly
monodisperse 2D materials
•  Tunable solution rheology enables a
suite of additive manufacturing options
for 2D materials
•  Atomically thin materials can serve as
the basis of new device concepts:
•  Anti-ambipolar heterojunctions for
communications circuits
•  Gate-tunable memristors for nonvolatile memory and/or
neuromorphic computing
Hersam Group
Acknowledgments: Research Group and Funding
Postdocs/Scholars
Kan-Sheng Chen
V. Demers-Carpentier
Sunghwan Jin
Junmo Kang
Jae-Hyeok Lee
Krishna Matte
Karl Putz
Vinod Sangwan
Josh Wood
Jian Zhu
PhD Graduate Students
Heather Arnold
Kyle Luck
Jade Balla
Andy Mannix
Megan Beck
Julian McMorrow
Hadallia Bergeron Niki Mansukhani
Sarah Clark
Eric Pozzi
Matt Duch
Chris Ryder
Michael Geier
Ethan Secor
Linda Guiney
Ted Seo
Laila Jaber-Ansari
Hong Sham
Deep Jariwala
Tejas Shastry
Joohoon Kang
Yujin Shin
Hunter Karmel
Amanda Walker
Brian Kiraly
Shay Wallace
Xiaolong Liu
Spencer Wells
Undergraduate/MS
Luqman Azhari
Theo Gao
Jianting He
Peter Kim
Norman Luu
Greg Mulderink
Laura Pettersen
Andrew Rowberg
Yichao Zhao
Funding provided by:
Office of Naval Research MURI, DURIP, SBIR
National Science Foundation MRSEC, CEIN, EFRI
National Institutes of Health; NIST CHiMaD
Department of Energy EFRC, SISGR
NSF, NDSEG, NASA Fellowships
W. M. Keck Foundation, MacArthur Foundation
Hersam Group