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-***-****-*

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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.