Nano Res

1

Enhanced photocurrent and photoluminescence

spectra in MoS2 under ionic liquid gating Zhen Li1, Shun-Wen Chang2, Chun-Chung Chen1, and Stephen B. Cronin1,2 ()

Nano Res., Just Accepted Manuscript • DOI: 10.1007/s12274-014-0459-2

http://www.thenanoresearch.com on March 27, 2014

© Tsinghua University Press 2014

Just Accepted

This is a “Just Accepted” manuscript, which has been examined by the peer-review process and has been

accepted for publication. A “Just Accepted” manuscript is published online shortly after its acceptance,

which is prior to technical editing and formatting and author proofing. Tsinghua University Press (TUP)

provides “Just Accepted” as an optional and free service which allows authors to make their results available

to the research community as soon as possible after acceptance. After a manuscript has been technically

edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP

article. Please note that technical editing may introduce minor changes to the manuscript text and/or

graphics which may affect the content, and all legal disclaimers that apply to the journal pertain. In no event

shall TUP be held responsible for errors or consequences arising from the use of any information contained

in these “Just Accepted” manuscripts. To cite this manuscript please use its Digital Object Identifier (DOI®),

which is identical for all formats of publication.

Nano Research

DOI 10.1007/s12274-014-0459-2

1

Table of Contents Graphic

This article constitutes the first experimental measurement of photocurrent spectra from monolayer MoS2. We report substantial improvements (2- to 3-fold) and modulation in the photoluminescence efficiency and photocurrent of monolayer MoS2 flakes under both ionic liquid and electrostatic gating. This improvement arises from the passivation of surface states and trapped charge of the material.

1.7 1.8 1.9 2.0 2.1 2.2

In ionic liquid In air

Ph

oto

lum

ines

cen

ce

Energy (eV)

1.6 1.8 2.0 2.2 2.4

0

200

400

600

800 w/ ionic liquid w/o ionic liquid Fitted curve

Ph

oto

curr

ent

(pA

)

Energy (eV)

2

Enhanced Photocurrent and Photoluminescence Spectra in MoS2 under Ionic Liquid Gating

Zhen Li1, Shun-Wen Chang2, Chun-Chung Chen1 and Stephen B. Cronin1,2 1 Department of Electrical Engineering, 2Department of Physics, University of Southern

California, 3737 Watt Way, PHE 624 Los Angeles, CA 90089

Phone: 213-740-8787 Fax: 213-740-8677 Email: scronin@usc.edu

Abstract:

We report substantial improvements and modulation in the photocurrent (PC) and

photoluminescence (PL) spectra of monolayer MoS2 taken under electrostatic and ionic liquid

gating conditions. The photocurrent and photoluminescence spectra show good agreement with a

dominant peak at 1.85eV. The magnitude of the photoluminescence can be increased 300% by

ionic liquid gating due to the passivation of surface states and trapped charges that act as

recombination centers. The photocurrent also doubles when passivated by the ionic liquid.

Interestingly, a significant shift of the PL peak position is observed under electrostatic (14meV)

and ionic liquid (30meV) gating, as a result of passivation. The ionic liquid provides significant

screening without any externally applied voltage, indicating that these surface recombination

centers have net charge. The acute sensitivity of monolayer MoS2 to ionic liquid gating and

passivation arises because of its high surface-to-volume ratio, which makes it especially sensitive

to trapped charge and surface states. These results reveal that, in order for efficient

optoelectronic devices to be made from monolayer MoS2, some passivation strategy must be

employed to mitigate the issues associated with surface recombination.

Keywords: ionic liquid, dichalcogenide, MoS2, photoluminescence, photocurrent, passivation

3

Introduction:

Graphene’s lack of an intrinsic band gap presents a challenge for the use of graphene in

electronic and energy conversion devices. As a result, other 2D layered materials with finite band

gap energies have begun to receive increasing attention. Of particular interest are the layered

transition metal dichalcogenides, such as MoS2 and WSe2, which show spectacular

optoelectronic properties.[1-3] While bulk MoS2 is an indirect band gap semiconductor with a

band gap of 1.3eV, quantum confinement converts monolayer MoS2 into a direct band gap

material with a band gap of 1.85eV[3]. This band structure transition has been confirmed by

optical absorption, photoluminescence, and electroluminescence spectroscopy[4]. This property

of MoS2 makes it a good candidate for photovoltaic and photocatalytic applications, due to its

strong absorption in the solar spectral range. While several research groups have reported strong

photoluminescence from monolayer MoS2[1, 5, 6], measurements of photocurrent in this

material are limited. In previous work, the photoconductivity of multilayer MoS2 from the

ultraviolet to the infrared wavelength range was reported[7]. Also, current generated by

photothermoelectric effects (PTE) were studied by scanning photocurrent microscopy, where a

photothermal voltage is created across the junction between the MoS2 and metal electrode by a

photo-induced temperature gradient.[8] More recently, Wu et al. reported scanned photocurrent

microscopy of 4-layer MoS2 field effect transistors (FETs), which verified that the photoresponse

was due to band bending-assisted separation of photo-excited carriers at the MoS2/Au Schottky

interface.[9]

For many years, ionic liquids have been studied for applications in catalysis and energy

storage.[10, 11] More recently, ionic liquids have been used in high-performance organic

electronics[12], field-induced electronic phase transitions[13], and inducing superconductivity in

4

layered materials.[14] Ionic liquid gating has also been used to circumvent surface depletion in

III-V semiconductors.[15] The intimate contact made by the ions at the liquid/solid interface

provides strong gating with a small gate capacitance. This type of electrochemical gating has

been used in other 2D materials to shift the Fermi energy of graphene by as much as ±0.85eV

from its charge neutrality point[16] and to realize ambipolar conduction in thin flakes of tungsten

disulfide (WS2).[17]

Here, we study the effects of electrostatic and ionic liquid gating on the optoelectronic

properties of monolayer MoS2. Electron transport measurements (i.e., I-V characteristics)

measured over the same range of electrostatic and ionic liquid gating are used to correlate the

strong modulation observed in the PL and PC spectra with doping. The quantum efficiency of

these devices with and without ionic liquid are obtained from the photocurrent spectra based on

the incident photon flux.

1. Experimental Section:

In this work, MoS2 flakes are exfoliated on p-doped silicon substrates with 300nm thick

SiO2 by the “Scotch tape” method, as shown in Figure 1a.[18, 19] As with graphene[20], we can

distinguish monolayer MoS2 flakes by their contrast under an optical microscope. Raman and

photoluminescence spectroscopy is used to confirm whether a given flake is in fact a

monolayer.[21] Photoluminescence spectra are taken on a Renishaw InVia spectrometer using a

532nm laser (0.1mW) focused through a 50X objective lens. Once a monolayer flake is

identified, metal electrodes are fabricated using electron-beam lithography, followed by 5nm Ti

and 50nm gold deposition. To improve the contacts, samples are annealed at 200oC in Ar.[22]

5

Before depositing the ionic liquid 1-Ethyl-3-methylimidazolium

bis(trifluoromethylsulfonyl)imide ([EMIM]-[TFSI]), it is baked in a vacuum oven at 120oC for

24 hours to remove water. Immediately prior to the measurements, a drop of ionic liquid is

deposited on the sample to serve as the electrochemical gate, as illustrated schematically in

Figure 3a.[15] Photocurrent spectra are collected using a Fianium supercontinuum white light

laser source in conjunction with a Princeton Instruments double grating monochromator to

provide monochromatic light over the 450-1000 nm wavelength range. The laser power incident

on the device is 0.05mW after passing through neutral density filters and a 50X objective lens

with a numerical aperture of 0.42.

2. Results and Discussion:

Figure 1 shows an optical microscope image of monolayer and few layer MoS2 flakes

deposited on a Si/SiO2 substrate, together with their corresponding Raman spectra (Figure 1b)

and photoluminescence spectra (Figure 1c). The photoluminescence of monolayer MoS2 is more

than 10,000 times more intense than the bulk sample and 4X more intense than the few layer

flake, reflecting the direct band gap nature of monolayer MoS2.[1] The PL spectrum of

monolayer MoS2 exhibits one dominant peak at 1.85eV (0.11eV FWHM), corresponding to band

gap emission. There is another peak around 2.0eV due to spin-orbit coupling.[23, 24] At room

temperature, we do not observe the trion peak in our spectra.[24]

Figures 2a and Figures 2b (inset) show a schematic diagram and an optical microscope

image of a monolayer MoS2 flake with two metal electrodes serving as the source and drain in a

field effect transistor geometry with the underlying silicon substrate serving as the gate electrode.

The I-V characteristics of the device are shown in Figures 2b and Figures 2d, which reveal that

the MoS2 transistor is n-doped, and can only be electrostatically gated p-type very weakly.

6

Figure 2c shows the photocurrent spectra taken from the device under various electrostatic gating

conditions with no bias voltage applied. Although the device is very resistive (~Gat Vg=0),

free carriers will be generated by photons when the device is illuminated. The Schottky junction

formed at the metal electrode-MoS2 interface provides a built-in potential that is able to separate

photoexcited electron-hole pairs, thus producing a net photocurrent.[7, 25] While an applied bias

voltage can help extract electron-hole pairs and increase the photocurrent, this results in a photo-

induced conductance change dominates over the photocurrent.

The photocurrent spectra show two dominant peaks at 1.85eV and 2.0eV, as in the PL

spectra, corresponding to the two direct optical transitions at the K-point. The higher energy peak

is more pronounced in the photocurrent spectra than the photoluminescence spectra. The higher

energy peak is more pronounced in the photocurrent spectra than the photoluminescence spectra.

Here, photocurrent is generated at the Schottky junction, which creates a built-in potential that is able

to separate photo-generated charge. Electron-hole pairs created at both optical transition energies can,

therefore, result in a net photocurrent. In the PL process, on the other hand, light is mostly generated

in the MoS2 flake away from the Schottky junction, and thus higher energy electron-hole pairs are

able to relax to the band edge before emitting light. It should be noted that the PC and PL spectra are

collected from different parts of the MoS2 flake, one at the Au/MoS2 junction and the other in the

center of the flake.

The intensity of these peaks in the spectra can be modulated by varying the electrostatic

gate voltage. The integrated intensity of these peaks in the PC spectra is plotted in the inset of

Figure 2c as a function of the applied gate voltage, showing a more than 3-fold enhancement in

the photocurrent under applied gating. Here, there is a tradeoff between the size of the depletion

width and the series resistance, both of which decrease with doping. At -40V and 0V, the

7

photocurrent is small due to the large series resistance corresponding to the insulting phase of

MoS2 (see Figure 2b). The anomalously high photocurrent observed at -20V arises from an

increased depletion width at the charge neutrality point.

Figure 3 shows the gate voltage dependence of the photoluminescence spectra taken from

another monolayer MoS2 FET device shown in Figure 2. While there is no gate dependence of

the PL intensity, a slight blue shift is observed in the PL peak position under positive applied

gate voltages, as shown in Figure 3b. A total blue shift of 14meV is observed between 0V and

+40V. This blue shift cannot be attributed to Pauli exclusion of optical transitions at band edge,

by the following argument. If the Fermi energy is 14meV above the conduction band edge, the

density of free carriers will be over 1015cm-2, assuming * 00.45m m . However, the carrier

density n can be estimated from either the gate capacitance 2300nm SiO gn C V , where

2

2300 =11.6nF/cmnm SiOC , or from the conductivity by the relation, n

e

, where is the

conductance and is the mobility of monolayer MoS2[26]. Both approaches predict a carrier

density less than 1011cm-2, even at Vg=+40V. It is known that sub-band gap states can cause a

redshift of the PL peak position.[27] Previous work has shown that the PL peak of MoS2 on-

substrate is always lower in energy, around 1.85eV[1], than that of suspended MoS2, which is

approximately 1.90eV[3]. Therefore, we believe that the surface states and trapped charges at the

MoS2/SiO2 interface cause a redshift of the PL observed of MoS2 in air. Positive electrostatic

gate voltages can reduce the redshift through passivation of surface states. However, the PL

intensity is mostly determined by the minority carrier lifetime. With both positive and negative

trapped charges acting as recombination centers, electrostatic doping can only partially passivate

these centers. Thus, the electrostatic gating does not have a strong effect on the PL intensity. If

8

we compare the relative intensity between the 1.85eV PL peak and the 2.0eV peak, a decrease of the

higher energy PL peak is observed when the PL peak blueshifts. This indicates that electrostatic

doping helps to increase the carrier lifetime, making it easier to decay to the lower energy states. On

the other hand, the PC spectra shown in Figure 2c do not exhibit a spectral shift over the entire

gate voltage range from -40V to 40V. This is most likely due to pinning of the Fermi level at the

MoS2/Au interface, where most of the photocurrent is generated.[4]

Since silicon back gating can only provide a limited range of doping, we have also

explored ionic liquid gating to further compensate for surface states and surface doping. Figure

4a shows a schematic diagram of the photoluminescence measurement of the monolayer MoS2

device under ionic liquid gating conditions. Figure 4b shows the I-V characteristics of the same

monolayer MoS2 FET shown in Figure 2 under ionic liquid gating, plotted together with the

Si/SiO2 back gating data (inset). Here, the ionic liquid gate is able to tune the MoS2 flake all the

way from n-type to p-type, indicating ambipolar conduction, and is able to screen the surface

charge more effectively than the electrostatic gate, even at zero applied voltage. As a result, the

conductance of the device increases by a factor of 100 after depositing the ionic liquid. In fact, Ids

increases from 0.02nA in air to 3nA in ionic liquid at zero applied gate voltage, as shown in the

Supplementary Information Figure S1. We can estimate the mobility change due to the ionic

liquid, based on the capacitance and transconductance of the device. The capacitance of the ionic

liquid is about 40µF/cm[28], and the quantum capacitance of MoS2 is 2

2

*30q

e mC

µF/cm2,

yielding 17q gtotal

q g

C CC

C C

µF/cm2. The transconductance of the device with ionic liquid

gating is about 1.3µS/V. On the other hand, transconductance of the device with silicon back

gating is 0.45nS/V. This reduction in transconductance is largely caused by the small capacitance

9

of the silicon back gate, which is 11.6nF/cm2. We can compare the electron mobilities with and

without ionic liquid gating by comparing their transconductance/capacitance ratios:

1.3 S/V 0.45nS/ 2

17 F/V 11.6nF

. Therefore, the mobility of the MoS2 increases 2-fold after depositing

ionic liquid, which is consistent with previous observations of few layer MoS2 devices[29].

Figure 4c shows a comparison of the photocurrent spectra of monolayer MoS2 taken with

and without ionic liquid. Here, a large enhancement in the photocurrent (more than 2X) is

observed in the presence of the ionic liquid, even without an applied voltage. Again, this

enhancement is due to the passivation of surface states that would otherwise cause carrier

recombination, thus lowering the photovoltaic performance of this optoelectronic device. Based

the 10% optical absorption of monolayer MoS2[30] and the incident photon flux, we estimate the

quantum efficiency of this device to be approximately 5 x 10-4 with and 2.5 x 10-4 without ionic

liquid. However, it should be noted that the effective area of the charge separation region is

considerably smaller than the focused laser spot, and thus we expect this device to have larger

actual quantum efficiency. Unfortunately, the relatively high leakage current (a few nA when the

laser is on) through the ionic liquid prevented us from exploring PC spectra under different gate

voltages applied to the ionic liquid. The ionic liquid gate dependence of the photoluminescence

spectra, however, could be measured, as shown in Figure 4d. The spectra taken in ionic liquid are

2-3X higher in intensity (and significantly narrower in linewidth) than those taken in air, as

shown in Figure 4d. Even at zero applied gate voltage, the ionic liquid produces a substantial

enhancement of both PC and PL intensities (2X) indicating significant passivation of the both

positive and negative surface charges simultaneously, that would otherwise cause non-radiative

recombination. Upon application of an applied gate voltage, the PL intensity can be further

10

increased, indicating more thorough passivation of these surface states and increased minority

carrier lifetimes, as shown in Figure S2 of the Supplementary Information. The slight decrease in

the photoluminescence intensity observed under high positive gate voltages could be due to

Auger recombination. In addition, the PL peak position changes from 1.85eV (in air) to 1.88eV

(in ionic liquid); a 30meV shift. The PL peak position of MoS2 in ionic liquid is very close to the

peak position of suspended MoS2, and no further shift of the peak position is observed upon the

application of gate voltage to the ionic liquid, as shown in Figure S2 of the Supplementary

Information. These results further confirm that the spectral shifts observed in Figures 3b and

Figures 4c are not due to Pauli exclusion, and most likely arise from passivation of the surface

charge/states, which result in the initial redshift of the PL.

While both the photocurrent and photoluminescence increase by a factor of 2-3 when

immersed in the ionic liquid, subtle differences arise in their spectral response. Most notably, the

relative intensity of the higher energy peak in the photocurrent spectrum is reduced in the ionic

liquid, whereas it is enhanced in the PL spectra. This difference can also be understood in terms

of an increase in the minority carrier lifetime, which enables more radiative recombination in the

PL process. In the photocurrent process, however, there is a tradeoff between charge separation

and recombination, which includes radiative recombination after decaying from the higher

energy excitation. As such, there are different recombination rates for the lower and higher

energy excitations.

3. Conclusion:

In conclusion, photocurrent and photoluminescence spectroscopy of monolayer MoS2

exhibit two predominant peaks in their spectra corresponding to the optical transitions at the K-

11

point in the Brillouin zone. Enhanced photocurrent and photoluminescence efficiency in

monolayer MoS2 flakes are achieved through the passivation of surface states via both ionic

liquid and electrostatic gating. Ionic liquid and electrostatic gating also reduce the initial

redshifts in PL spectra caused by surface states and trapped charges. The ionic liquid enables

ambipolar doping of the MoS2 flake and improves the conductance and mobility of the device

substantially. This general approach of ionic liquid gating/passivation can be applied to enhance

a wide range of other nanomaterials and devices with high surface-to-volume ratios, which are

inherently sensitive to the effects of trapped charge and surface states.

Our results suggest that trapped charges and surface states play a particularly important

role in the optoelectronic properties of monolayer MoS2, causing unintentional doping and

increased surface recombination of photo-generated electron-hole pairs. We should note that the

issue of surface states is a universal problem for most nanostructure devices with high surface to

volume ratios, including nanowires or nanoparticles. These results reveal that, in order for

efficient optoelectronic devices or electronic devices to be made from monolayer MoS2, some

form of mitigation of the surface states will be required. The need to passivate surface states in

III-V semiconductors[15, 31] is a known problem dating back several decades[32-34]. While

much is known about the nature of these states in the III-V materials, little is known about the

surface states in the transition metal dichalcogenides. Although in principle, there are no

dangling bonds for the material, as in the case of bulk material surfaces, the nature of atomic thin

monolayer film of transition metal dichalcogenides is expected to be quite different from bulk

material systems, and will require further studies.

12

Supporting Information: Supporting Information is available from the Wiley Online

Library or from the author.

Acknowledgements: The authors would like to thank Prof. Li Shi and Dr. Insun Jo for

helpful discussions. This work was supported by Department of Energy Award No. DE-FG02-

07ER46376.

13

Figure 1. (a) Optical microscope image of MoS2 flakes exfoliated on a Si/SiO2 substrate. (b)

Comparison of Raman spectra of monolayer, few-layer, and bulk MoS2 (normalized by the peak

value). The difference between the two Raman peaks of monolayer MoS2 is 18cm-1.[21] (c) A

comparison of photoluminescence spectra taken from monolayer, few-layer, and bulk MoS2. The

dashed line indicates a fitting of the monolayer PL peak corresponding to a band gap of 1.85eV.

370 380 390 400 410 420 430

Monolayer Few layers Bulk

Raman Shift (cm-1)

R

ama

n C

ou

nts

(n

orm

aliz

ed)

(a) (b)

A1g

E12g

(c)

1.6 1.7 1.8 1.9 2.0 2.10

2000

4000

6000

8000

PL

In

ten

sity

Monolayer Few layers <5 FittedCurves

Energy / eV

Bulk

14

-60 -40 -20 0 20 40 600.0

0.2

0.4

0.6

0.8

1.0

C

urr

ent

(nA

)Gate Voltage (V)

Vds

= 0.1V

-40 -20 0 20 40

1

2

3

4

5

6

7

Inte

rgat

ed

P

ho

toc

urr

ent

(A.U

.)

Gate Voltage (V)

Figure 2. (a) Schematic diagram and (b) I-V curve a monolayer MoS2 field effect transistor.

Inset shows an optical microscope image the MoS2 device. (c) Photocurrent spectra of the

monolayer MoS2 device under various electrostatic gate voltages from -40V to 40V. Inset:

Integrated total area of the photocurrent peaks plotted as a function of gate voltage. (d) The I-V

curve plotted on a log scale shows p-type behavior in the device below -20V. The n-type region

exhibits an on/off ratio of 104.

-60 -40 -20 0 20 40 601E-5

1E-4

1E-3

0.01

0.1

1

C

urr

ent

(nA

)

Gate Voltage (V)

Vds

= 0.1V

1.6 1.8 2.0 2.2 2.40

100

200

300

400 Vg=-40V

Vg=-20V

Vg=0V

Vg=20V

Vg=40V

Fitted curve of V

g=40V

Ph

oto

curr

ent

(pA

)

Energy (eV)

(a) (b)

(c) (d)

15

Figure 3. (a) Normalized photoluminescence spectra taken under various applied gate voltages

from 40V to -40V. (b) Gate dependence of the photoluminescence peak position, showing a blue

shift of 14meV under positive applied gate voltages.

1.7 1.8 1.9 2.0 2.1

No

mai

lize

d P

L

40V 30V 20V 10V 0V -10V -20V -30V -40V

-40 -20 0 20 401.85

1.86

1.87

1.95

2.00

2.05 Peak around 1.85eV Peak around 2.00eV

PL

Pea

k C

ente

r

Gate Voltage (V)

(a)

Energy (eV)

(b)

16

-40 -20 0 20 400.0

0.2

0.4

0.6

0.8

1.0

Cu

rren

t (n

A)

Si/SiO2 Gate Voltage (V)

Figure 4. (a) Schematic diagram of MoS2 FET measured in ionic liquid. (b) The I-V curve

measured in a monolayer MoS2 FET under ionic liquid gating with a bias voltage of 0.1V. The

inset shows the same device measured in air with SiO2 back gating. (c) Comparison of

photocurrent spectra of monolayer MoS2 flake with and without ionic liquid, showing that the

ionic liquid increases the photocurrent, doubling the dominant peak, while keeping the shoulder

peak relatively low. (d) Comparison of photoluminescence spectra of the monolayer MoS2 flake

with and without ionic liquid gating, showing a 30meV blue shift after depositing the ionic liquid.

1.6 1.8 2.0 2.2 2.4

0

100

200

300

400

500

600

700

800 w/ ionic liquid w/o ionic liquid Fitted curve

Ph

oto

curr

ent

(pA

)

Energy (eV)

(a) (b)

(c) (d)

Ionic Liquid Gate Voltage (V)

FWHM w/o liquid ~0.11eVw/ liquid ~0.06eV

1.6 1.7 1.8 1.9 2.0 2.1 2.2

PL

In

ten

sity

-1.5V -1V -0.5V 0V 0.5V 1V 1.5V

Energy (eV)

without ionic liquid

with ionic liquid (different voltage)

17

References:

[1] Splendiani, A.;Sun, L.;Zhang, Y.;Li, T.;Kim, J.;Chim, C.-Y.;Galli, G.; Wang, F. Emerging photoluminescence in monolayer MoS2. Nano Letters 2010, 10, 1271-1275.

[2] Zhao, W.;Ghorannevis, Z.;Chu, L.;Toh, M.;Kloc, C.;Tan, P.-H.; Eda, G. Evolution of electronic structure in atomically thin sheets of WS2 and WSe2. ACS nano 2012, 7, 791-797.

[3] Mak, K. F.;Lee, C.;Hone, J.;Shan, J.; Heinz, T. F. Atomically Thin MoS2: A New Direct-Gap Semiconductor. Physical Review Letters 2010, 105, 136805.

[4] Sundaram, R.;Engel, M.;Lombardo, A.;Krupke, R.;Ferrari, A.;Avouris, P.; Steiner, M. Electroluminescence in Single Layer MoS2. Nano Letters 2013, 13, 1416-1421.

[5] Tongay, S.;Suh, J.;Ataca, C.;Fan, W.;Luce, A.;Kang, J. S.;Liu, J.;Ko, C.;Raghunathanan, R.; Zhou, J. Defects activated photoluminescence in two-dimensional semiconductors: interplay between bound, charged, and free excitons. Scientific reports 2013, 3.

[6] Yin, Z.;Li, H.;Li, H.;Jiang, L.;Shi, Y.;Sun, Y.;Lu, G.;Zhang, Q.;Chen, X.; Zhang, H. Single-layer MoS2 phototransistors. ACS nano 2011, 6, 74-80.

[7] Choi, W.;Cho, M. Y.;Konar, A.;Lee, J. H.;Cha, G. B.;Hong, S. C.;Kim, S.;Kim, J.;Jena, D.; Joo, J. High‐Detectivity Multilayer MoS2 Phototransistors with Spectral Response from Ultraviolet to Infrared. Advanced Materials 2012, 24, 5832-5836.

[8] Buscema, M.;Barkelid, M.;Zwiller, V.;van der Zant, H. S.;Steele, G. A.; Castellanos-Gomez, A. Large and Tunable Photothermoelectric Effect in Single-Layer MoS2. Nano Letters 2013, 13, 358-363.

[9] Wu, C.-C.;Jariwala, D.;Sangwan, V. K.;Marks, T. J.;Hersam, M. C.; Lauhon, L. J. Elucidating the photoresponse of ultrathin MoS2 field-effect transistors by scanning photocurrent microscopy. The Journal of Physical Chemistry Letters 2013, 4, 2508-2513.

[10] Simon, P.; Gogotsi, Y. Materials for electrochemical capacitors. Nature Materials 2008, 7, 845-854.

[11] Arico, A. S.;Bruce, P.;Scrosati, B.;Tarascon, J. M.; Van Schalkwijk, W. Nanostructured materials for advanced energy conversion and storage devices. Nature Materials 2005, 4, 366-377.

[12] Sandberg, H. G. O.;Backlund, T. G.;Osterbacka, R.; Stubb, H. High-performance all-polymer transistor utilizing a hygroscopic insulator. Advanced Materials 2004, 16, 1112.

[13] Dhoot, A. S.;Yuen, J. D.;Heeney, M.;McCulloch, I.;Moses, D.; Heeger, A. J. Beyond the metal-insulator transition in polymer electrolyte gated polymer field-effect transistors. Proceedings of the National Academy of Sciences of the United States of America 2006, 103, 11834-11837.

[14] Ye, J. T.;Inoue, S.;Kobayashi, K.;Kasahara, Y.;Yuan, H. T.;Shimotani, H.; Iwasa, Y. Liquid-gated interface superconductivity on an atomically flat film. Nature Materials 2010, 9, 125-128.

[15] Alarcon-Llado, E.;Mayer, M.;Boudouris, B.;Segalman, R.;Miller, N.;Yamaguchi, T.;Wang, K.;Nanishi, Y.;Haller, E.; Ager, J. PN junction rectification in electrolyte gated Mg-doped InN. Applied Physics Letters 2011, 99, 102106-102103.

[16] Chen, C.-C.;Chang, C.-C.;Li, Z.;Levi, A.; Cronin, S. B. Gate tunable graphene-silicon Ohmic/Schottky contacts. Applied Physics Letters 2012, 101, 223113-223114.

18

[17] Braga, D.;Lezama, I. G.;Berger, H.; Morpurgo, A. F. Quantitative Determination of the Band Gap of WS2 with Ambipolar Ionic Liquid-Gated Transistors. Nano Letters 2012, 12, 5218-5223.

[18] Novoselov, K. S.;Geim, A. K.;Morozov, S.;Jiang, D.;Zhang, Y.;Dubonos, S.;Grigorieva, I.; Firsov, A. Electric field effect in atomically thin carbon films. Science 2004, 306, 666-669.

[19] Radisavljevic, B.;Radenovic, A.;Brivio, J.;Giacometti, V.; Kis, A. Single-layer MoS2 transistors. Nature nanotechnology 2011, 6, 147-150.

[20] Blake, P.;Hill, E.;Neto, A. C.;Novoselov, K.;Jiang, D.;Yang, R.;Booth, T.; Geim, A. Making graphene visible. Applied Physics Letters 2007, 91, 063124.

[21] Li, H.;Zhang, Q.;Yap, C. C. R.;Tay, B. K.;Edwin, T. H. T.;Olivier, A.; Baillargeat, D. From bulk to monolayer MoS2: evolution of Raman scattering. Advanced Functional Materials 2012, 22, 1385-1390.

[22] Ishigami, M.;Chen, J.;Cullen, W.;Fuhrer, M.; Williams, E. Atomic structure of graphene on SiO2. Nano Letters 2007, 7, 1643-1648.

[23] Wang, Q. H.;Kalantar-Zadeh, K.;Kis, A.;Coleman, J. N.; Strano, M. S. Electronics and optoelectronics of two-dimensional transition metal dichalcogenides. Nature nanotechnology 2012, 7, 699-712.

[24] Mak, K. F.;He, K.;Lee, C.;Lee, G. H.;Hone, J.;Heinz, T. F.; Shan, J. Tightly bound trions in monolayer MoS2. Nature materials 2013, 12, 207-211.

[25] Lopez-Sanchez, O.;Lembke, D.;Kayci, M.;Radenovic, A.; Kis, A. Ultrasensitive photodetectors based on monolayer MoS2. Nature nanotechnology 2013.

[26] Fuhrer, M. S.; Hone, J. Measurement of mobility in dual-gated MoS2 transistors. Nature nanotechnology 2013, 8, 146-147.

[27] Dhaka, V.;Oksanen, J.;Jiang, H.;Haggren, T.;Nyka�nen, A.;Sanatinia, R.;Kakko, J.-P.;Huhtio, T.;Mattila, M.; Ruokolainen, J. Aluminum-Induced Photoluminescence Red Shifts in Core–Shell GaAs/Al x Ga1–x As Nanowires. Nano Letters 2013, 13, 3581-3588.

[28] Cho, J. H.;Lee, J.;He, Y.;Kim, B.;Lodge, T. P.; Frisbie, C. D. High‐Capacitance Ion Gel Gate Dielectrics with Faster Polarization Response Times for Organic Thin Film Transistors. Advanced Materials 2008, 20, 686-690.

[29] Perera, M. M.;Lin, M.-W.;Chuang, H.-J.;Chamlagain, B. P.;Wang, C.;Tan, X.;Cheng, M. M.-C.;Tománek, D.; Zhou, Z. Improved Carrier Mobility in Few-Layer MoS2 Field-Effect Transistors with Ionic-Liquid Gating. ACS nano 2013.

[30] Mak, K. F.;Lee, C.;Hone, J.;Shan, J.; Heinz, T. F. Atomically Thin MoS2: A New Direct-Gap Semiconductor. Physical Review Letters 2010, 105.

[31] Chang, C.-C.;Chi, C.-Y.;Yao, M.;Huang, N.;Chen, C.-C.;Theiss, J.;Bushmaker, A. W.;LaLumondiere, S.;Yeh, T.-W.;Povinelli, M. L.;Zhou, C.;Dapkus, P. D.; Cronin, S. B. Electrical and Optical Characterization of Surface Passivation in GaAs Nanowires. Nano Letters 2012, 12, 4484-4489.

[32] Offsey, S. D.;Woodall, J. M.;Warren, A. C.;Kirchner, P. D.;Chappell, T. I.; Pettit, G. D. UNPINNED (100) GAAS-SURFACES IN AIR USING PHOTOCHEMISTRY. Applied Physics Letters 1986, 48, 475-477.

[33] Yablonovitch, E.;Sandroff, C. J.;Bhat, R.; Gmitter, T. NEARLY IDEAL ELECTRONIC-PROPERTIES OF SULFIDE COATED GAAS-SURFACES. Applied Physics Letters 1987, 51, 439-441.

19

[34] Wilmsen, C. W.;Geib, K. M.;Shin, J.;Iyer, R.;Lile, D. L.; Pouch, J. J. THE SULFURIZED INP SURFACE. Journal of Vacuum Science & Technology B 1989, 7, 851-853.