Supplementary information

High-performance photocurrent generation from two-dimensional WS2

field effect transistors

Seung Hwan Lee,1,2,a) Daeyeong Lee,1,2,a) Wan Sik Hwang,3,b) Euyheon Hwang,1 Debdeep Jena,4 and Won Jong Yoo1,2,b)

1Department of Nano Science and Technology, SKKU Advanced Institute of Nano-Technology (SAINT),

Sungkyunkwan University (SKU), 2066 Seobu-ro, Suwon-si, Gyeonggi-do, 440-746, Korea

2Samsung-SKKU Graphene Center (SSGC), 2066 Seobu-ro, Jangan-gu, Suwon-si, Gyeonggi-do, 440-746,

Korea

3Department of Materials Engineering, Korea Aerospace University, 76 Hanggongdaehang-ro, Deogyang-gu,

Goyang-si, Gyeonggi-do, 412-791, Korea

4Department of Electrical Engineering, University of Notre Dame, Notre Dame, Indiana, 46556, USA

_____________________________

a) S. H. Lee and D. Lee contributed equally to this work.

b) Electronic mails of corresponding authors: whwang@kau.ac.kr (W. S. Hwang); yoowj@skku.edu (W. J. Yoo).

Gate bias and wavelength dependent transfer curve and photocurrent

At negative gate bias region (blue arrow in Fig. S1), the photogain is relatively higher than that of

positive gate bias region (red arrow in Fig. S1) and vice versa for the photocurrent. It reveals clearly by

comparing linear (main panel in Fig. S1) and log scale (inset in Fig. S1) graphs.

-6 -4 -2 0 2 4 6

10-12

10-10

10-8

Light off 700 nm 630 530 450 300

I D (A

)

VG (V)

-6 -4 -2 0 2 4 6

0

10

20

30

40

I D (n

A)

VG (V)

-6 -4 -2 0 2 4 610-14

10-12

10-10

10-8

Phot

ocur

rent

(A)

VG (V)

-6 -4 -2 0 2 4 60

3

6

9

Phot

ocur

rent

(nA)

VG (V)

(b)(a)

FIG S1. The log scale transfer (a) and calculated photocurrent (b) curve depending on incident light

energy. Insets are corresponding linear scale graph. The amount of photocurrent is larger in the positive

gate bias region (red arrow), while on/off ratio is larger in the negative gate bias region (blue arrow).

Measurement was done in VD = 20 mV. The legend in (a) applies to all graph in this figure.

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Photoresponse measurement system setup

The Fig. S2 shows schematic diagram of the photo-response measurement system used in this study.

The xenon arc lamp (component 1) is used as a light source of the monochromator. The light from the

xenon arc lamp is collimated and focused to the entrance slit of the monochromator (light path from 2 to

4). The light from the entrance slit is collimated again by the collimating mirror (component 5) and

dispersed by the grating (component 6). The dispersed light spectrum is focused by the focusing mirror

(component 7) and the monochromatic light is selected by the out-slit (component 8). The

monochromatic light from the out-slit runs through the optical fiber and it is collimated at the output of

the optical fiber (light path from 9 to 10). Finally, the focused light is illuminated on the sample after the

collimated light run through the optic system of the optical microscope (light path from 11 to sample)

VG

VD

1234

5

67

8

910

11

FIG S2. The schematic diagram of the photo-response measurement system used in this study. 1: Xenon

arc lamp, 2: collimating lens, 3: focusing lens, 4: entrance slit, 5: collimating mirror, 6: grating for

dispersing light, 8: out-slit for monochromatic light selection, 9: optical, 10: collimating lens, and 11:

optic system of the optical microscope.

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Calculation of photocurrent density and optical power density

The photocurrent density, Jph, is calculated by subtracting the drain current without light illumination

from the drain current with light illumination and dividing it by the area of the channel. The channel

dimensions of width and length are 4 and 1 μm, respectively. The optical power density, Popt, is

calculated by dividing optical power into the area of projected light on the sample. The shape of the

projected light is circle and its diameter is ~58 μm. The optical power is measured where samples are

placed using optical power meter.

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Comparison of the figures of merit for photodetectors prepared using different TMDCs

The figures of merit are summarized and compared with those measured using other TMDC

phototransistors in Table SI. The applied electrical field and optical power are listed together to enable a

fair comparison. Our WS2 phototransistor shows comparable Iillum/Idark and higher photoresponsivity

compared to the bulk MoS2 phototransistor1, even upon application of a 15-fold smaller drain electrical

field. Furthermore, the performance of our WS2 device is comparable to that of other TMCD

phototransistors, even with a relatively small electrical field and optical power (Table SI).

TABLE SI. Comparison of the figures of merit for photodetectors prepared using different TMDCs.

Device structureElectrical bias Photo-

responsivity (A/W)

Iillum/Idark

/ Popt (W/cm2)EQE (%) RefED

(mV/nm)EG

(mV/nm)d

~10La WS2 0.60 0 (on) 92 μ - 20ma 2b

~30L WS20.02 –22 (off) 0.27 102 – 103 / 0.25 8 This work1 38 (on) 36 - 7k

1L MoS2 8.00 –259 (off) 880 30a / 2.4 190ka 31L MoS2 0.48 167 (on) 7.5m - 2a 43L MoS2 1.25 0 (on) 1.04 24 / 2 242a 5c

~50La MoS2 0.31 –158 (off) 0.12 102 – 103 / 0.05 24a 1aObtained from calculations or estimates using the information in the reference.bWS2 was synthesized using the chemical vapor deposition method. Other TMDCs were exfoliated from

the bulk crystal.cThe device in this ref had an interdigitated source and drain structure, whereas the device described in

the other ref had a one-fingered source and drain structure.dNo (no gating), off (transistor-off gate bias was applied), on (transistor-on gate bias was applied). All

values considered the effective oxide thickness.

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Photoresponsivity calculation

The photoresponsivity of WS2 device is calculated from the maximum slope of the linearly plotted

photocurrent graph as a function of illuminating light power density (see Fig. S3). The photoresponsivity

from 630 nm wavelength of light is relatively high than those from the other wavelength of light.

0.0 0.1 0.2

0

1

2

3

4

5Ph

otoc

urre

nt (m

A/cm

2 )

Light power (W/cm2)

630 nm

Fig. S3. The linearly plotted photocurrent density graph as a function of illuminating light power

density. The blue line is the guild line for the maximum slop of this graph. Measurements were

performed at VD = 20 mV and VG = –2 V.

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