highly sensitive detection of mercury (ii) ions with few ... · mercury ion and the sulfur sites on...
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Nano Res
1
Highly sensitive detection of mercury (II) ions with
few-layer molybdenum disulfide
Shan Jiang1, Rui Cheng2, Rita Ng1, Yu Huang2,3 Xiangfeng Duan1,3 ()
Nano Res., Just Accepted Manuscript • DOI: 10.1007/s12274-014-0658-x
http://www.thenanoresearch.com on November 28 2014
© Tsinghua University Press 2014
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Nano Research
DOI 10.1007/s12274-014-0658-x
TABLE OF CONTENTS (TOC)
Highly sensitive detection of mercury (II) ions with
few-layer molybdenum disulfide
Shan Jiang1, Rui Cheng2, Rita Ng1, Yu Huang2,3,
Xiangfeng Duan*1,3
1Department of Chemistry and Biochemistry, 2Department
of Materials Science and Engineering, 3California
Nanosystems Institute, University of California, Los
Angeles, California 90095, USA
Here we investigate the effects of mercury (II) ion on electronic
transport of few-layer molybdenum disulfide, and explore
MoS2 FETs for highly sensitive detection of mercury (II).
Provide the authors’ webside if possible.
Xiangfeng Duan, http://xduan.chem.ucla.edu
Highly sensitive detection of mercury (II) ions with
few-layer molybdenum disulfide
Shan Jiang1, Rui Cheng2, Rita Ng1, Yu Huang2,3 Xiangfeng Duan1,3 ()
Received: day month year
Revised: day month year
Accepted: day month year
(automatically inserted by
the publisher)
© Tsinghua University Press
and Springer-Verlag Berlin
Heidelberg 2014
KEYWORDS
Molybdenum disulfide,
2D layered materials,
mercury, doping effect,
sensors
ABSTRACT
The two-dimensional (2D) layered transition metal dichalcogenide (TMD)
materials (e.g., MoS2) have attracted considerable interest due to their
atomically thin geometry and semiconducting electronic properties. With
ultrahigh surface to volume ratio, the electronic properties of these atomically
thin semiconductors can be readily modulated by their environment. Here we
report an investigation on the effects of mercury (II) (Hg2+) ions on electrical
transport properties of few-layer molybdenum disulfide (MoS2). The interaction
between Hg2+ ions with few-layer MoS2 was studied by field-effect transistor
measurements and photoluminescence. Due to a high binding affinity between
mercury ion and the sulfur sites on the surface of MoS2 layers, Hg2+ ions can
strongly bind to MoS2. We show that the binding of Hg2+ can produce a p-type
doping effect to reduce the electron concentration in n-type few-layer MoS2. It
can thus effectively modulate the electron transport and photoluminescence
properties in few-layer MoS2. By monitor the conductance change of few-layer
MoS2 in varying concentration Hg2+ solutions, we further show that few-layer
MoS2 transistors can function as highly sensitive sensors for rapid electrical
detection of Hg2+ ion with a detection limit of 30 pM.
1 Introduction
Transition metal dichalcogenides (TMDs) are
emerging as an exciting material system for a new
generation of atomically thin electronics due to
their unique electronic and chemical properties
[1-16]. With the ultra-thin geometry, surface
chemical doping can significantly modulate the
electronic properties of single- or few-layer TMDs
[17-22]. Because of the high binding affinity
between mercury and sulfur, Hg2+ ion can strongly
bind to sulfur on the surface of MoS2 and thus affect
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2 Nano Res.
the electrical transport properties of the few-layer
MoS2 devices. Here we report the investigation into
the effects of mercury (II) ions on electronic
transport properties of few-layer MoS2. The
interaction between Hg2+ ion and few-layer MoS2
was studied by field-effect transistor (FET)
measurement and photoluminescence. We show
that the electrical transport properties of MoS2 FET
devices can be significantly modulated by Hg2+ ions.
These atomically thin MoS2 FET devices can thus be
configured as effective sensors for real-time
electrical detection of Hg2+ ions.
Mercury contamination is an important
environmental problem originating from
anthropogenic sources such as coal and fuel
combustion [23]. According to the World Health
Organization (WHO)'s standards, mercury
concentration should be less than 0.001 mg/L (5 nM)
in drinking water [24]. It is therefore essential to
develop highly sensitive mercury detection
techniques to determine very low concentrations of
mercury for reliable risk assessment [25-31]. Here,
we show that the MoS2 FET device can selectively
respond to Hg2+ ion in real-time with a detection
limit of 30 pM. The MoS2 FET device promises
significant potential for highly sensitive, low-cost
and rapid detection of mercury ions in aquatic
environment.
2 Experimental
To fabricate the MoS2 EFT, few-layer MoS2
flakes were mechanically exfoliated onto 300 nm
Si/SiO2 substrate. Electron-beam lithography and
electron-beam evaporation were used to define the
contact electrodes. A thin Ni/Au film (5nm/50 nm)
was used as the electrode to form Ohmic contact
with minimized contact resistance and potential
barrier [32]. The MoS2 device is then coupled with a
polydimethylsiloxane (PDMS) micro-fluidic
channel for Hg2+ solution delivery.
3 Results and discussion
We have first characterized the electrical
transport properties of MoS2 to ensure Ohmic
contacts were achieved. To this end, the MoS2 FETs
were fabricated on Si/SiO2 substrate, with Ni/Au
thin film as the source-drain contacts, and the
silicon substrate as the back gate electrode (Figure
1a). The Ids-Vds characteristics at varying back gate
voltage (Figure 2a) and the Ids-Vbg characteristics
(Figure 2b) for MoS2 were measured. Importantly, a
linear Ids-Vds relationship is clearly observed,
indicating Ohmic contacts are achieved.
Furthermore, Ids-Vbg plot shows that the current
increases with increasing positive gate voltages for
MoS2, consistent with the expected n-type
semiconductor behavior. The logarithmic plot of
Ids-Vbg curve shows that the device exhibits an on-off
ratio up to106. Additionally, the carrier mobility of
the MoS2 device can also be derived to be 87 cm2/Vs
(see Electronic Supplementary Material for the
details), comparable to the that of the best MoS2
devices, demonstrating the high quality of our
device.
We next investigated the interaction between
MoS2 and Hg2+ ion by FET measurement. To this
end, the experiments were performed using a
micro-fluidic system with a PDMS channel
integrated on top of the MoS2 device (Figure 1b),
with which the Hg2+ ion solutions of increasing
concentrations were introduced into the PDMS
channel through a syringe pump during the
electrical measurement. The Hg2+ ion solutions were
made by dissolving Hg(ClO4)2 into de-ionized water.
To show the effect of Hg2+ ion on the electrical
properties of the MoS2 device, the conductance of
the device was plotted against solution gate voltage
under different Hg2+ ion concentrations (Figure 2c).
Importantly, the MoS2 devices show a consistent
positive shift of the threshold voltage with
increasing Hg2+ ion concentration from 0 to 1 M
(Figure S1). This positive shift of the threshold
voltage suggests a p-doping effect of the Hg2+ ion to
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3 Nano Res.
the MoS2 channel. It is well known Hg2+ ion has a
high binding affinity toward sulfur, with a stability
constant of 2.5×1052 for Hg2+ ion coordination with
an S2- ligand. In this case, the coordination between
Hg2+ ions and sulfur on the surface of MoS2 can
cause a partial electron transfer from MoS2 to Hg2+,
resulting in a p-type doping effect. It is also noted
that the slope in Ids-Vg plot in Figure 2c decreases
slightly with increasing Hg2+ ion concentrations
(Figure S1), suggesting a decreasing
transconductance and decreasing mobility, which
may be attributed to charge impurity (bound Hg2+
ion) induced scattering effect.
To further understand the doping effect from
Hg2+ ion to MoS2, photoluminescence (PL)
spectroscopy was also used to characterize the MoS2
device in the aqueous solution of Hg2+ ion. In
general, the PL emission peak of MoS2 exhibits an
excitonic A (1.85 eV) peak and B (2.05 eV) peak,
which are associated with the direct gap transition
at K (K' ) point, and the energy difference between
the A and B peaks is caused by the valence-band
splitting due to the strong spin−orbital interaction
[33-35] . With the presence of Hg2+ ion in the
aqueous solution, the peak energy of excitonic A
peak is blue-shifted as compared to that in
deionized water (Figure 3a). The spectral shape of
excitonic A peak is also clearly sharper in Hg2+ ion
solution. The excitonic A peak can be further
decomposed into the exciton (X; ~1.88 eV; red line)
peak and the negative trion (X−; ~1.84 eV; blue line)
using Lorentzian fitting functions (Figure 3b, c) [35].
Compared that in dionized water (Figure 3b), the
exciton peak X becomes more dominant in Hg2+ ion
solution (Figure 3c). In as prepared MoS2, trion (X−)
recombination is dominant because its intrinsic
n-type behavior [36]. With increasing p-type doping
by Hg2+ ion, the excess number of electrons is
decreased, exciton peak X becomes more dominant
because the excitons can recombine without
forming trions [37]. It is also evident that a broad
shoulder peak (L; ~1.75 eV; yellow line) becomes
much stronger in Hg2+ ion solution. L is assigned as
a bound exciton peak [38]. It is attributed to neutral
excitons bound to defects. The increase in peak
height of L indicates there are more binding sites or
defects available for bound excitons [39], which can
be attributed to Hg-S binding. This is also consistent
with reducing carrier mobility with increasing
bound Hg2+ ions as the scattering center.
The above studies clearly demonstrate the
binding of Hg2+ on MoS2 surface can effectively
modulate the electron concentration in MoS2 and
therefore its conductance. It is therefore possible to
use the ultra-thin MoS2 FET to construct electronic
sensor for the detection of Hg2+ in aqueous solution.
To this end, we have monitored the conductance of
MoS2 FET in response to Hg2+ solutions of
increasing concentrations. Figure 4a shows a
real-time electrical readout of different
concentrations of Hg2+ ion from 0 to 1 M. The
conductance of MoS2 FET shows a clear step-wise
decrease as Hg2+ ion concentration is increased. We
have further plotted the amount of the conductance
change against the Hg2+ ion concentration to obtain
the calibration curve for Hg2+ ion detection (Figure.
4b), which can be fitted with a logarithmic plot.
Based on this plot, a lowest absolute detection limit
of 30 pM can be achieved with a signal to noise ratio
of 3. We have conducted similar measurement on 10
devices. A similar detection limit of 30-120 pM is
achieved.
To explore the application of MoS2 devices as
Hg2+ ion sensors, we have further investigated the
selectivity and specificity of the MoS2 devices.
Using a similar protocol, we have tested a series of
potentially interfering chemicals including sodium
(I), potassium (I), magnesium (II), calcium (II),
manganese (II), iron (II), iron (III), cobalt (II), nickel
(II), tin (II), lead (II), zinc (II), cadmium (II), silver (I)
and copper (II). At the same concentration of 1 nM,
the device responds to mercury with the strongest
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4 Nano Res.
signal (Figure. 4c), demonstrating the potential
application of the MoS2 devices for mercury
detection in aqueous solution.
4 Conclusions
In brief, we have reported an investigation into
the interaction between MoS2 and Hg2+ ion using
FET measurement and photoluminescence. The
conductance change of MoS2 FET with Hg2+ ion
concentration was observed, which can be
attributed to the p-doping effect and increased
scattering center caused by Hg2+ ion binding to
MoS2. The photoluminescence studies show a
similar consistent trend. The doping mechanism is
based on the strong binding affinity between Hg2+
ions and sulfur on the surface of MoS2. Our study
shows that the MoS2 FET holds significant potential
application for highly sensitive, low-cost, and fast
detection of mercury ions in aquatic environment.
Acknowledgements
We acknowledge the Nanoelectronics Research
Facility (NRF) and Center for High Frequency
Electronics (CHFE) at UCLA for technical support.
X.D. acknowledges support by NSF CAREER award
0956171. Y.H. acknowledges the NIH Director's New
Innovator Award Program 1DP2OD007279.
Electronic Supplementary Material: Supplementary
material (Figure 2c with liner fits and calculation of
mobility) is available in the online version of this
article at http://dx.doi.org/10.1007/s12274-***-****-*
(automatically inserted by the publisher). References
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6 Nano Res.
FIGURES
Figure 1 Schematic illustration of a MoS2 FET device as
mercury (II) sensor. (a) Schematic illustration a MoS2 device
with the green dots representing Hg2+ ion that could bind to
surface sulfur atoms. (b) Schematic of microfluidic Hg2+
solution delivery system integrated on the MoS2 device. The
sizes of source and drain electrodes are 2 5 μm. (All the
electrodes described here are the ends near the MoS2 in the
figure. The other ends have 100 100 μm pads for wire
bonding). The size of gate electrode is 50 50 μm. The PDMS
channel is 1-mm in width, 0.5-mm in height, 1-cm in length.
The source-drain electrodes are passivated with PMMA layer.
Figure 2 Electrical transport properties of a MoS2 FET. (a) Ids-Vds characteristics at varying back gate voltage under ambient
conditions. (b) Linear (red) and logarithmic plot (black) of Ids-Vbg characteristics under ambient conditions at Vds=0.1V. (c) Solution
gate dependent measurement in varying concentrations of Hg2+ ion in aqueous solution, with 10 mM KClO4 as electrolyte.
Figure 3 Photoluminescence spectroscopy of MoS2 in Hg2+ ion solution. (a) PL spectroscopy of few-layer MoS2 in water and in Hg2+
ion solution. (b) Analysis of the PL spectroscopy for few-layered MoS2. The A peak was decomposed into two peaks with Lorentzian
functions, corresponding to the trion (X−) (blue) and the exciton (X) (red) peaks. To additional peaks was assigned as L peak (yellow)
and B peak(green). (c) Analysis of the PL spectroscopy for few-layered MoS2 in Hg2+ ion solution. The PL peaks are decomposed in
a similar way to that in b.
Figure 4 Real-time electrical readout of Hg2+ ion signal by MoS2 sensor. (a) Real-time electrical measurement at different
concentrations of Hg2+ ions. (b) Calibration curve: conductance change versus Hg2+ ion concentration. The red line is the fitted curve
in natural log scale. (c) Selectivity of MoS2 Mercury (II) sensor. Concentrations of Mercury (II) and all the interferences are 1 nM.