While layered materials are increasingly investigated for their potential in nanoelectronics, their functionality and efficiency depend on charge injection into the materials via metallic contacts.This work explores the characteristics of different metals (aluminium, tungsten, gold, and platinum) deposited on to nanostructured thin films made of two-dimensional (2D) MoS2 flakes. Metals are chosen based on their work functions relative to the electron affinity of MoS2. It is observed, and analytically verified that lower work functions of the contact metals lead to smaller Schottky barrier heights and consequently higher charge carrier injection through the contact

Ab Kadir, R

Atkin, P

Balendhran, S

Bhaskaran, M

Kalantar Zadeh, K

Ou, J

Sriram, S

Walia, S

Wang, Y

Zoolfakar, A

Sumeet Walia

Sivacarendran Balendhran

Yichao Wang

Rosmalini Ab Kadir

Ahmad Sabirin Zoolfakar

Paul Atkin

Jian Zhen Ou

Sharath Sriram

Kourosh Kalantar-zadeh

Madhu Bhaskaran

Crossref

Applied Physics Letters

Characterization of metal contacts for two-dimensional MoS<sub>2</sub> nanoflakes

While layered materials are increasingly investigated for their potential in nanoelectronics, their functionality and efficiency depend on charge injection into the materials via metallic contacts. This work explores the characteristics of different metals (aluminium, tungsten, gold, and platinum) deposited on to nanostructured thin films made of two-dimensional (2D) MoS 2 flakes. Metals are chosen based on their work functions relative to the electron affinity of MoS2. It is observed, and analytically verified that lower work functions of the contact metals lead to smaller Schottky barrier heights and consequently higher charge carrier injection through the contacts

Walia, Sumeet

Balendhran, Sivacarendran

Wang, Yichao (R21062)

Kadir, Rosmalini A.

Zoolfakar, Ahmad S.

Atkin, Paul

Ou, Jian Zhen

Sriram, Sharath

Kalantar-Zadeh, Kourosh

Bhaskaran, Madhu

Western Sydney ResearchDirect

Characterization of metal contacts for two-dimensional MoS2 nanoflakes

RMIT Research Repository

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Walia, S, Balendhran, S, Wang, Y, Ab Kadir, R, Zoolfakar, A, Atkin, P, Ou, J, Sriram, S,
Kalantar Zadeh, K and Bhaskaran, M 2013, 'Characterization of metal contacts for
two-dimensional MoS2 nanoflakes', Applied Physics Letters, vol. 103, pp.
232105-1-232102-4.
http://researchbank.rmit.edu.au/view/rmit:22637
Accepted Manuscript
2013 AIP Publishing LLC
http://dx.doi.org/10.1063/1.4840317
1 
 
Characterization of metal contacts for two-dimensional MoS2 
nanoflakes 
Sumeet Walia,
1,2,a)
 Sivacarendran Balendhran,
1,2
 Yichao Wang,
2
 Rosmalini Ab Kadir,
2
 
Ahmad Sabirin Zoolfakar,
2
 Paul Atkin,
2
 Jian Zhen Ou,
2
 Sharath Sriram,
1,2
 Kourosh Kalantar-
zadeh
2,a) 
 and Madhu Bhaskaran
1,2,a)
   
1 Functional Materials and Microsystems Research Group, RMIT University, Melbourne, Victoria 
3000, Australia 
2 School of Electrical and Computer Engineering, RMIT University, Melbourne, Victoria 3000, 
Australia 
 
ABSTRACT  
While layered materials are increasingly investigated for their potential in nanoelectronics, 
their functionality and efficiency depend on charge injection into the materials via metallic 
contacts. This work explores the characteristics of different metals (aluminium, tungsten, 
gold and platinum) deposited on to nanostructured thin films made of two-dimensional (2D) 
MoS2 flakes. Metals are chosen based on their work functions relative to the electron affinity 
of MoS2. It is observed, and analytically verified, that lower work functions of the contact 
metals lead to smaller Schottky barrier heights and consequently higher charge carrier 
injection through the contacts.  
 
KEYWORDS 
Contacts, work function, electron affinity, nanoflakes, MoS2 
  
2 
 
Two-dimensional (2D) materials are gaining significant attention due to their unique 
electrical, mechanical and optical properties, which are different than those of their bulk 
counterparts.
1,2
 In particular, 2D transition metal dichalcogenides are attracting extensive 
research interest due to possibilities of electronic band structure manipulations.
1,2
 Amongst 
them, molybdenum disulphide (MoS2) which is a n-type semiconductor, has been the most 
popular material owing to its relatively small bandgap (1.2 eV for multi-layer and 1.8 eV for 
monolayer), modest electron mobilities and transition from indirect to direct bandgap when it 
is exfoliated to monolayer thickness.
1,2
 Amidst the three known polymorphs of MoS2 (1T, 2H 
and 3R), 2H-MoS2 is the naturally occurring and thermodynamically stable phase.
1,2
 Bulk 
2H-MoS2 is composed of atomically thin planes comprising of covalently bonded S–Mo–S 
structures across the planes and weak van der Waals forces between the planes.
3
 Such a 
crystal structure enables the realisation of atomically thin stable 2D sheets of MoS2 via 
mechanical and liquid exfoliation techniques.
1,2
 
2D MoS2 has been applied in the development of a variety of devices including field 
effect transistors, electrically driven optical devices and solar cells, as well as electrochemical 
devices.
4,5,6
 All these devices rely on electrical contacts for their operation. As such, 
understanding metal/2D MoS2 interface is crucial in such devices and the nature of the 
metal/2D MoS2 depends on the electronic band structures of both the metal and 2D MoS2.  
In this letter, we provide a thorough experimental analysis of metal/nanostructured thin 
films made of 2D MoS2 nanoflakes contacts for selected metals. The choice of metal contacts 
is based on their work functions (ΦM) relative to the electron affinity (χ) of MoS2. Relatively 
low work function metals, aluminium (Al) and tungsten (W), are studied along with relatively 
high work function metals, gold (Au) and platinum (Pt), in order to provide a wide-ranging 
picture of the behaviour of the metal/2D MoS2 contacts.  
3 
 
In this study, a grinding-sonication liquid exfoliation technique, adapted from the work 
of Yao et al.,
7
 is employed as a high yield method for producing suspensions of 2D MoS2 
nanoflakes.
8
 Atomic force microscopy (AFM) was employed to assess the thickness of the 
MoS2 nanoflakes. It can be seen from Fig. 1(a) that typical MoS2 nanoflakes comprise of 6 to 
8 fundamental layers (1 layer = 0.65 nm)
4
. Measurements using a large number of AFM 
images shows that the majority of MoS2 nanoflakes exhibit thicknesses in the range of 2-7 
monolayers.
9
 Dynamic light scattering (DLS) analysis was conducted to assess the average 
lateral dimensions of the exfoliated nanoflakes (Fig. 1(b)) to be approximately 100 nm. The 
nanoflake suspension was drop-casted onto electrically conductive fluorine-doped tin oxide 
(FTO) substrates and left to dry under ambient conditions, resulting in thin films of ~500 nm 
thickness.  
Raman spectroscopy was carried out to further characterise the MoS2 thin films and 
estimate the number of layers they are comprised of. Figure 2(a) shows the Raman spectra of 
the bulk and the nanostructured MoS2 thin film. The two dominant modes  (  and ) 
reflect the vibrations of the Mo–S bond along the horizontal and vertical planes, 
respectively.
10
 For the MoS2 nanoflakes, the  mode showed no significant shift, while the 
 mode was observed to have red shifted in comparison to their bulk counterpart. This 
indicates a dominant presence of few-layered (<7 layers) MoS2 nanoflakes in the thin film,
9,11
 
which is also in agreement to the AFM analysis presented earlier.  
X-ray diffraction (XRD) was utilized to determine the crystallinity of the drop-casted 
2D MoS2 nanostructured thin films. It can be seen from Fig. 2(b) that the dominant peak 
reflecting the (002) plane (ICDD Card no: 77-1716) was observed at 14.4°. In contrast to the 
XRD pattern of the bulk MoS2, the nanostructured MoS2 thin film did not reveal the presence 
1
2gE 1gA
1gA
1
2gE
4 
 
of (100), (106), (006), (105) and (008) planes. This reflects the nature of the thin films which 
consist of 2D MoS2 nanoflakes composed of mostly (002) planes.  
High resolution transmission electron microscopy (HRTEM) was conducted in order to 
verify the crystal structure of the 2D MoS2 nanoflakes. The HRTEM image of the edges of 
the nanoflakes (Fig. 2(c)) confirms the presence of 2D structures with only several 
fundamental layers. The inset of Fig. 2(c) shows that the parallel lattice fringes of the MoS2 
nanoflakes are 0.27 nm apart, which is in agreement with previously reported values for 2H-
MoS2.
1
  
In order to investigate the properties of metals when contacted with nanostructured 
films made of 2D MoS2 nanoflakes, four different metals of Al, W, Au and Pt with varying 
work functions were considered. Al (ΦAl = 4.54 eV)
12
 and W (ΦW = 5.09 eV)
12
 were chosen 
as the low work function metals, while Au (ΦAu = 5.40 eV)
12
 and Pt (ΦPt = 5.70 eV)
12
 were 
chosen as the high work function metals, relative to the electron affinity of MoS2 
(χ = 4.0 eV).6,13 A band diagram illustration of all the aforementioned metals relative to 
MoS2, prior to contact is shown in Fig. 3(a). Figure 3(b) shows the current-voltage (I-V) 
characteristics acquired for the different metal/MoS2 contacts. It can be seen that Al/MoS2 
interface results in the highest current (~90 µA), while the Pt/MoS2 interface shows the least 
current (~10 µA), under identical bias conditions.   
When the metal and the MoS2 are brought in a contact, the bands of MoS2 bend such 
that the Fermi levels equilibrate. Figure 4(a) schematically illustrates the respective electronic 
band alignment of the metals for the case of ΦM > χ (in our case, the ΦM of Al, W, Au and Pt 
all are larger than χ), before the metal and MoS2 and brought into contact. On making 
contact, the conduction and valence bands of the MoS2 bend in order to achieve equilibrium 
between the metal Fermi level and the MoS2 chemical potential (4.36 eV).
14
 The degree and 
5 
 
the direction of band bending depend on the work function of the metal relative to the Fermi 
level of the semiconductor (MoS2 in this case). With MoS2 as the n-type semiconductor and 
for the case where the metal work function is larger than the electron affinity of MoS2, the 
depiction shown in Fig. 4(b) arises after the metal and semiconductor are brought into 
contact. In this case, the electrons diffuse from the semiconductor to the lower energy states 
of the metal in order to achieve a constant Fermi level throughout the system in thermal 
equilibrium. This results in the upward band bending within MoS2 at the interface. 
Consequently, the electrons in the metal see a potential barrier against their migration into the 
semiconductor. This barrier is known as the Schottky barrier (ΦB) which can be estimated by 
the Schottky-Mott rule (ΦB = ΦM – χ), where χ is the electron affinity of the semiconductor). 
A higher ΦM thus results in a higher ΦB. Hence, a larger forward bias is required in order to 
overcome this Schottky barrier.  
This is confirmed from the I-V characteristics (Fig. 3(b)), in which it is seen that the 
current is progressively higher for lower work function metals such as Al and W under 
identical bias conditions. Table I summarises the experimental results obtained using 
different metal contacts with varying work functions, relative to the electron affinity of MoS2. 
This shows that Al (the lowest work function metal) results in the highest bias current, 
indicating its superior room temperature charge injection capabilities for an interface with 
MoS2, compared to commonly used contact metals such as Au and Pt. The estimated 
Schottky barrier heights using the Schottky-Mott relationship are shown in Table I. We also 
calculated the Schottky barrier heights from the IV curves. The calculated ΦB for Al 
(0.55 eV) corresponds well to the theoretically estimated value. However, an accurate 
calculation for the other metals could not be obtained, as their relatively large ΦB required the 
application of higher bias voltages, causing the junction to break down. 
 
6 
 
In summary, we investigated the properties of the metal/2D MoS2 contacts using metals 
of different work functions and demonstrated that for the chosen metals that have the work 
functions between 4.5 to 5.7 eV, always Schottky behaviours were observed. While, there are 
reports in literature that have claimed the Au/2D MoS2 contact to be Ohmic, our study clearly 
shows the formation of a barrier height and therefore Schottky behaviour for all of the metals 
under investigation. This study showed that the choice of the metal contacts is crucial in 
adjusting the barrier heights at their interface with 2D MoS2, which is fundamental 
knowledge in designing electronic and optoelectronic devices based on 2D MoS2.  
 
ACKNOWLEDGMENTS 
The authors acknowledge support from the Australian Research Council (ARC) with 
partial project support through Discovery Project DP130100062; Australian Post-Doctoral 
Fellowships through Discovery Projects DP1092717 (MB) and DP110100262 (SS); and 
equipment funding through the Linkage, Infrastructure, Equipment, and Facilities Grant 
LE100100215 (SS and KKZ). SB acknowledges a publication grant from the School of 
Graduate Research at RMIT University. 
 
NOTES 
a) Corresponding authors. E-mail: madhu.bhaskaran@rmit.edu.au, kourosh.kalantar@rmit.edu.au and  
sumeet.walia@rmit.edu.au   
7 
 
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TABLE I:  Performance summary of different metal/2D MoS2 contacts 
Metal Work function, 
ΦM (eV) 
Schottky barrier height, 
ΦB=ΦM – χ (eV) 
Current at +2V bias 
(µA) 
Al 4.5 0.5 91.9 
W 5.1 1.1 38.7 
Au 5.4 1.4 22.5 
Pt 5.7 1.7  17.3 
 
FIGURE CAPTIONS 
FIG. 1. (a) AFM image of typical exfoliated MoS2 nanoflakes with its corresponding cross-
sectional thickness profile. (b) DLS analysis of the lateral size distribution of the MoS2 
nanoflakes.  
FIG. 2. (a) Raman spectra comparison of bulk MoS2 and a nanostructured 2D MoS2 thin film. 
(b) X-ray diffractograms comparing bulk MoS2 powder, a nanostructured thin film made of 
2D MoS2 nanoflakes, with the FTO substrate spectrum provided for reference (* denotes the 
peaks corresponding to the FTO substrate). (c) Transmission electron micrograph of a liquid 
exfoliated 2D MoS2 nanoflake with the inset corresponding to its high resolution transmission 
electron micrograph. 
FIG. 3. (a) Band diagram illustrations of a metal and MoS2. χ is the MoS2 electron affinity, EF 
is the Fermi level of MoS2, EVAC is the reference vacuum level, EV and EC are the valence and 
9 
 
the conduction band levels respectively. (b) I-V characteristics for different metal/MoS2 
contacts under identical bias conditions.  
FIG. 4. Band alignments of a metal and MoS2 for ΦM > χ (a) before establishing a contact and 
(b) their corresponding band bending after establishing a contact.  
 
 
 
ab
ab
c
ba
ab


While layered materials are increasingly investigated for their potential in nanoelectronics, their functionality and efficiency depend on charge injection into the materials via metallic contacts. This work explores the characteristics of different metals (aluminium, tungsten, gold, and platinum) deposited on to nanostructured thin films made of two-dimensional (2D) MoS 2 flakes. Metals are chosen based on their work functions relative to the electron affinity of MoS 2 . It is observed, and analytically verified that lower work functions of the contact metals lead to smaller Schottky barrier heights and consequently higher charge carrier injection through the contacts

Characterization of metal contacts for two-dimensional MoS2 nanoflakes

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