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Intrinsic roughness in suspended van der Waals heterostructures
Thomsen, Joachim Dahl; Bøggild, Peter; Booth, Tim
Publication date:
2016
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Thomsen, J. D., Bøggild, P., & Booth, T. (2016). Intrinsic roughness in suspended van der Waals
heterostructures. Poster session presented at Carbonhagen 2016, Copenhagen, Denmark.
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Intrinsic roughness in suspended van der Waals heterostructures
Joachim Dahl Thomsen, Peter Bøggild, Timothy J. Booth
DTU Nanotech, Technical University of Denmark, 2800 Kgs. Lyngby, Denmark
Introduction
Intrinsic roughness/ripples
• Ripples in graphene are ubiquitous – seen for both for graphene on hBN,
SiO2 [1] and suspended graphene [2]
• Ripples may be a large factor limiting electron mobility [3] [4]
Fig. by
Jannick C.
Meyer
Encapsulated graphene
• Making hBN-G-hBN heterostructures is a well known strategy for
obtaining high mobility grapene samples
• Measurements of supported graphene on hBN have shown root mean
square (RMS) roughnesses around 1.4 Å similar to the roughness of
suspended graphene [1]
hBN-G-hBN heterostructure
• Here we show that the roughness can
Graphene
hBN
be reduced further by suspending the
Figure taken
from ref. [6]
heterostructure.
Roughness
of
heterostructures
Roughness and normal angle spread
The roughness of two BN/G heterostructures and three suspended
graphene samples were measured in a TEM.
a
On one
heterostructure
sample the
roughnes of the
suspended
graphene part was
also measured
(Fig. 4)
b
c
Graphene /hBN
Graphene
Fig. 4 – Optical image (a) and optical dark field image (b) showing a G/BN heterostructure (sample 2) on silicon dioxide, (c) TEM image of the same sample.
Methods – In-situ TEM and Fabrication
Plots of ln(A) vs. G2 for the samples are shown in Fig. 5 and the
corresponding measured roughnesses are listed in the legend.
Reciprocal space of rough graphene
The full 3D Fourier transform of rough
graphene in reciprocal space (u,v,w) consists of
a set of cones (Fig. 1)
hBN/G
0
If the graphene is rough the diffraction spots
become diffuse when tilting the sample
Graphene 0o tilt
2G (0o)
Fig. 1 – the 3D Fourier transform of rippled graphene
consists of cones. The green plane represents the Ewald
sphere intersecting the density distribution to form
diffraction patterns. Figure taken from Ref. [5]
Graphene 18o tilt
2G (20
(18oo))
2G
Graphene 36o tilt
2G (36o)
ln(A) (arb. u)
For G/hBN samples the spots remain the same
Sample
-1
RMS roughness (Å)
1 - G/BN
0.23 ± 0.01
2 – G/BN
0.19 ± 0.04
2 – G only 1.22 ± 0.03
3 – G only 1.14 ± 0.02
4 – G only 1.19 ± 0.02
-2
-3
5 – CVD G 1.56 ± 0.05
Suspended
graphene
-4
20
22
24
26
2
28
30
32
-2
G (nm )
Graphene/hBN 0o tilt
Graphene/hBN 18o tilt
Graphene/hBN 36o tilt
Fig. 2 – diffration patterns of graphene and graphene/hBN at 0o, 18o and 36o tilt.
Fig. 5 – plots of the logarithm of the diffraction spot amplitude as a function of the square of the distance from the zero order diffraction spot to the spot center. The slope
of the linear fits is proportional to the RMS roughness of the graphene.
Roughness
The spot intensity of rough graphene
varies as 𝐴𝐴 ∝ exp − 2𝜋𝜋𝐺𝐺 2 ℎ2 , where
ℎ2 is the RMS roughness [5]. Hence
ℎ2
1 𝑑𝑑ln A
=
2𝜋𝜋
𝑑𝑑𝐺𝐺 2
Sample fabrication
The samples were made using the hot
pick-up method (Fig. 3) [6]
Fig. 3 – Hot pick-up
procedure, (i) pick-up of
hBN, (ii) drop down on
graphene, (iii) release of
polymer stack. Fig. taken
from ref. [6]
The work leading to these results has received
funding from the European Union Seventh Framework Programme
(FP7/2007-2013) under grant agreement n° FP7-604000
Conclusion
• Roughness is possibly the main factor limiting mobility in suspended
samples
• Even graphene encapsulated in hBN shows a RMS roughness around 1.4 Å
• By suspending the heterostructures we have measured a significant
decrease in RMS roughness – around 0.2 Å.
[1] – M. Yamamoto et.al, ACS Nano 6, no. 9 (2012) ,p. 8335-8341
[2] – J.C. Meyer et. Al, Nature 446, p. 60-63 (2007)
[3] – M.I. Katsnelson and A.K. Geim, Phil. Trans R. Soc. 336, p. 195-204 (2008)
[4] – E.V. Castro et. al, Phys. Rev. Lett. 105, p. 266601 (2010)
[2] - N. D. Mermin, Phys. Rev. 176, p. 250 (1968)
[5] – D.A. Kirilenko et. al, Phys. Rev. B 84, 235417 (2011)
[6] – F. Pizzocchero et. al, Nat. Comm. 7, 11894 (2016)