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2D Mater. 3(2016)041003 doi:10.1088/2053-1583/3/4/041003
LETTER
Bottom-up synthesis of vertically oriented two-dimensional
materials
R A Vilá
1,2
, K Momeni
1,3
, Q Wang
4
, B M Bersch
1,2
,NLu
4
, M J Kim
4
, L Q Chen
1
and J A Robinson
1,2
1
Department of Materials Science and Engineering, The Pennsylvania State University, University Park, PA 16802, USA
2
The Center for 2-Dimensional and Layered Materials, The Pennsylvania State University, University Park, PA 16802, USA
3
Department of Mechanical Engineering, Louisiana Tech University, Ruston, LA 71272, USA
4
Department of Materials Science and Engineering, The University of Texas at Dallas, Richardson, TX 75080, USA
E-mail: jrobinson@psu.edu
Keywords: MoS
2
, two-dimensional materials, transition-metal dichalcogenide, vertical, synthesis, transmission electron microscopy,
modeling
Supplementary material for this article is available online
Abstract
Understanding nucleation and growth of two-dimensional (2D)and layered materials is a challenging
topic due to the complex van der Waals interactions between layers and substrate. The morphology of
2D materials is known vary depending on experimental conditions. For the case of MoS
2
, the
morphology has been shown to vary from rounded (molybdenum rich)domains to equilateral
triangular (sulfur rich)domains. These different morphologies can result in drastically different
properties, which can be exploited for applications in catalytic reactions, digital electronics,
optoelectronics, and energy storage. Powder vaporization (PV)synthesis of molybdenum disulfide
(MoS
2
)can yield vertical domains, however, these domains are often ignored when the morphology
evolution of MoS
2
is discussed, thereby completely omitting a major part of the impact of the Mo:S
ratio to the growth mode of MoS
2
during PV. Combining experimental and numerical simulation
methods, we reveal a vertical-to-horizontal growth mode transition for MoS
2
that occurs in the
presence of a molybdenum oxide partial pressure gradient. Transmission electron microscopy
reveals that the growth of vertical MoS
2
results from initial seeding of single crystalline molybdenum
dioxide, followed by sulfurization from the substrate upward to form vertically oriented MoS
2
domains.
Introduction
Semiconducting two-dimensional (2D)materials such
as molybdenum disulfide (MoS
2
)are of interest for
their potential applications in digital electronics,
catalytic reactions, optoelectronics, and energy storage
[1–4]. The ‘traditional’route to synthesizing such
materials is powder vaporization (PV)(often referred
to as chemical vapor deposition)[4–9], where the
equilibrium morphology is a strong function of the
metal:chalcogen ratio (Mo:S for MoS
2
)[9,10].PV
synthesis of MoS
2
can yield vertical MoS
2
structures
[11,12], however, these structures are often
ignored when the morphology evolution of MoS
2
is
discussed [9,10], thereby completely omitting a major
part of the Mo:S ratio impact on morphology in this
deposition technique. Vertically grown MoS
2
exhibits
enhanced catalytic activity at exposed edge sites
[13–19]; therefore, understanding and controlling the
synthesis of vertical MoS
2
is crucial. Furthermore, a
fundamental understanding of the thermodynamics
and kinetics governing the reaction, nucleation, and
growth of MoS
2
from horizontal to vertical is vital
for engineering layered material systems for the
entire spectrum of applications. In this article, we
elucidate the nucleation and growth mechanism of
MoS
2
from vertical to horizontal using experimental
techniques and numerical simulations. We reveal that
the partial pressure ratio of molybdenum oxide
to sulfur, and its distribution, governs the growth
mode of MoS
2
between vertical and horizontal
configurations.
RECEIVED
29 June 2016
REVISED
18 August 2016
ACCEPTED FOR PUBLICATION
15 September 2016
PUBLISHED
7 October 2016
© 2016 IOP Publishing Ltd