This document describes techniques for exfoliating and synthesizing few-layered molybdenum ditelluride (MoTe2) semiconductor flakes. MoTe2 exhibits a transition from indirect to direct bandgap as it is exfoliated into monolayers. The authors adapt graphene exfoliation methods to produce MoTe2 flakes using mechanical exfoliation with tape and cleaving with a razor blade. Future work will characterize the material properties using Raman spectroscopy, atomic force microscopy, and optical imaging to inform experiments on novel optoelectronic properties of MoTe2.

1. Exfoliation and Synthesis of Few-Layered MoTe2 2-D Semiconductors
2D Semiconductor
Exfoliation Basics
Mechanical Exfoliation Methodology
Future
Considerations
Joshua M. Woods1,2, Dennis Pleskot1,2, Nathaniel Gabor2,3
University of California, Riverside
1Department of Materials Science and Engineering,
2Quantum Materials Optoelectronics Lab,
3Department of Physics and Astronomy
Characteristics of MoTe2
Discussion
AuthorContact:
jwood020@ucr.edu
It has now been well established that single layer molybdenum
ditelluride (MoTe2) exhibits a band gap in the near infrared range
(~1.0 eV) comparable to silicon, a more conventional
semiconducting material. MoTe2 exhibits a transition from
indirect band gap to direct band gap as the material reaches the
monolayer limit. In this work, we are developing techniques to
produce functional MoTe2-based layered heterostructures by
mechanical exfoliation and semi-dry contact alignment transfer.
Methods to exfoliate and transfer graphene—an atomically thin
semi-metallic electronic material—have been adapted to the
exfoliation and transfer of MoTe2. In order to prepare atomically
thin flakes of MoTe2, substrates are cleaned by wet chemical
processing and radio frequency (RF) oxygen plasma. MoTe2
flakes are then mechanically exfoliated onto target substrates,
followed by careful classification of the flake quality and
thickness under an inspection microscope. Our future efforts
will assess the material’s properties using Raman spectroscopy,
atomic force microscopy, and optical imaging.
5 microns
1 micron
[A] Graphene 60x [B] Graphene 20x [C] Graphene 60x
MoTe2 Exfoliation Method
A bulk flake of MoTe2 was exfoliated three
times carefully onto separate pieces of tape
before final exfoliation onto SiO2. This method
typically yields samples with large surface
area, but few-layer samples exhibit low yield.
Razor Cleaving Method
A millimeter-sized crystal flake attached to a
carbon-taped Allen wrench bottom is stamped
onto tape. Separated layers are then taken from
the tape, contacted onto SiO2 wafers, and then
very gently “cleaved” with a razorblade.
Material characterization in the QMO lab encompasses the testing, analysis, and
observation of the properties of numerous atomic layer materials. Without first
analyzing material properties, we are not able to correctly apply the material in
question to different applications. Some testing has been shown to strain the
material quality to the point of severe sample degradation. The Gabor group
currently characterizes by atomic force microscopy, Raman spectroscopy, and
photoluminescence spectroscopy. These characterization techniques will ultimately
inform experiments that probe novel optoelectronics properties of MoTe2.
Structure Synthesis via Transfer Process
The Gabor group currently fabricates MoTe2-based heterostructures which are
characterized by photoluminescence spectroscopy. Current heterostructure layers
are comprised of graphene, kish graphite, MoTe2, and boron nitride in various
stacking sequences. In order to properly stack and contact these layers, a precision
dry transfer microscope was custom-built. This transfer microscope takes two
different layers of material and contacts them together via a multi-step polymer
adhesion process. We are currently developing various heterostructures based on
transition metal dichalcogenides such as MoTe2.
Due to its ability to produce ultrathin molybdenum
ditelluride (MoTe2) flakes, we conduct mechanical
exfoliation of MoTe2 with the use of Scotch® Magic™
Tape. MoTe2 is exfoliated onto silicon dioxide (SiO2)
substrates and then examined under an inspection
microscope. Improper cleaning techniques—such as
the use of methanol in place of isopropyl alcohol
(IPA)—of the SiO2 substrates will result in a loss of
MoTe2 flakes. The extra method of cleaning in
addition to sonication is performed in a cleanroom
using radio frequency oxygen plasma to create a
more pure sample. After cleaning, the pristine
substrates are hand-cut, etched, and patterned using
a custom photolithography mask. This mask creates
patterned marks on the substrate in order to more
easily locate and size specimens under microscope
observation. After exfoliation, substrates are kept in
vacuum until needed in transfer process.
Ideal MoTe2 flakes exhibit uniform surface area. As MoTe2 layers
near the monolayer limit, they begin to develop a tunable, direct
band gap for use in optoelectronics.
A
C
B
BN
MoTe2
Graphite
Thick BN
The typical graphene exfoliation method
results in a high surface area of bulk crystal on
the tape. When exfoliating on the sample,
speckled results occur. Thin layering does take
place, but crystal domain sizes are smaller
than required for further fabrication.
Graphene Exfoliation Method
Characterization
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(2014)
• (2dsemiconductors.com, 2015)
Wavelength and Band Gap of Various Materials
Te
Mo
Crystal Structure of MoTe2
Bulk Crystal
MoTe2
Near IR Range
~10 μm ~20 μm
~10 μm
~5 μm
~15 μm
~10 μm
~10 μm
~ 3 μm
~5 μm
~ 4 mm