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Supplementary Information

Engineering carbon quantum dots for enhancing broadband

photoresponse in silicon process line compatible photodetector

K. Sarkar1, Pooja Devi2, A. Lata1, R. Ghosh1 and Praveen Kumar1*

School of Materials Science, Indian Association for the Cultivation of Science, Kolkata 700032

Central Scientific Instruments Organization, Sector-30C, Chandigarh- India-160030, India

Figure S1. Process steps for synthesizing AgNPs

Electronic Supplementary Material (ESI) for Journal of Materials Chemistry C.This journal is © The Royal Society of Chemistry 2019

2

0 2 4 6 8 10 12 14 16 18Co

unts

(a.u

.)Diameter (nm)

(a) (b)

Figure S2. (a) Transmission electron microscope image of AgNPs. (b) Histogram showing size distribution with an average diameter at 9 nm.

Figure S3. Selected area electron diffraction due to AgNPs reveals polycrystalline structure.

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(a) (b)

Figure S4. Scanning electron microscope image of (i) rGO-CQD composite, (ii) rGO-CQD-AgNP composite

Figure S5. (a) Atomic force microscopy of S3 sample displays layers of rGO with randomly distributed nanostructures on it. (b) AFM image of CQDs coated on Si shows regular distribution over the scanned area.

Figure S6. UV-Vis absorption spectroscopy of AgNPs displays localized surface plasmon resonance at 420 nm.

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0.40 0.42 0.44 0.46 0.48

Abso

rban

ce (a

.u.)

Photon energy (eV)

Eg = 0.42 eV

Figure S7: Optical bandgap of rGO estimated from absorption spectroscopy

500 1000 1500 2000 25000

20

40

60

80

100

% T

(nm)

Figure S8: Broadband optical transmission from rGO (from UV to IR)

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400 450 500 550 600 650 700 750

Inte

nsity

(a.u

.)

Wavelength (nm)

(b)

3.2 3.4 3.6 3.8 4.0

0.0

0.2

0.4

0.6

0.8

1.0F(

h)2

Energy (eV)

(a)

Figure S9. (a) Optical bandgap of CQD coated on Si substrate estimated by applying Kubelka-Munk theory. (b) Photoluminescence spectra of the CQDs exhibiting emission around 450 nm.

0.0 0.4 0.8 1.2 1.60.0

0.2

0.4

0.6

0.8

I (A

)

V (Volt)

S1

Dark

0.0 0.4 0.8 1.2 1.60

10

20

30

40

50

60

I (nA

)

V (Volt)

S2

Dark

0.0 0.4 0.8 1.2 1.60

2

4

6

8I (

nA

)

V (Volt)

S3

Dark(a) (b) (c)

Figure S10. Dark current increases exponentially with a forward bias for all three sets of devices. S3 exhibits minimum dark current out of all sets of devices.

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0.6 0.8 1.0 1.2 1.4 1.6-24

-23

-22

-21

-20

-19

-18

-22

-21

-20

-19

-18

lnI

V (volt)

ln(I/V

)S3

0.6 0.8 1.0 1.2 1.4 1.6-19

-18

-17

-16

-15

-14

-13

0.6 0.8 1.0 1.2 1.4 1.6-19

-18

-17

-16

-15

-14

lnI

V (Volt)

S1

ln(

I/V)

0.6 0.8 1.0 1.2 1.4 1.6-20

-19

-18

-17

-16

0.6 0.8 1.0 1.2 1.4 1.6-20

-19

-18

-17

lnI

V (Volt)

ln(I/V

)

S2

(a)

(b)

(c)

Figure S11: Semi-logarithmic plot of dark current with respect to forward bias voltage for three sets of devices. ln(I/V) as a function of forward bias for three sets of devices.

ESI Note: Figure S11 displays the variation of as a function of forward bias for S1, S2 𝑙𝑛( 𝐼

𝑉) (𝑉)

and S3 respectively in (a), (b) and (c). Sample S1 exhibits a continuously varying slope with

increasing forward bias which signifies the gradual saturation of defect states and interfacial trap

states through complex recombination process. In Sample S2 the distinct difference in the

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variation of the slope is observed compared to S1 which can be ascribed to the diminished trap

states due to reduced graphene oxide. Whereas in S3, almost constant slope is observed in 𝑙𝑛( 𝐼

𝑉)vs plot which signifies considerable decrease in unsaturated trap states due to highly reduced 𝑉

graphene oxide. Therefore, Sample S3 exhibits lowest value of ideality factor and highest value

of on/off ratio under present experimental conditions.

230 239 240

4

8

12

I (A

)

Time (S)

S1360 nm

820 ms 345 ms

250 251 252 260 261 2622

3

4

5

I (A

)

Time (s)

S1

250 ms

510 ms

550 nm

69 70 79 800.1

0.2

0.3

0.4

0.5

I (A

)

Time (Sec)

S2

360 nm

413 ms113 ms

64 65 66 74 75 76

0.3

0.4

0.5

0.6

I (A

)

Time (S)

550 nmS2

882 ms 499 ms

468 470 480 482

3

4

5

6

7

8

I (A

)

Time (S)

S3 360 nm

485 ms

810 nm

211 212 219 220 221

0.8

1.0

1.2

1.4

I (A

)

Time (S)

S3 550 nm

556 ms

526 ms

(a) (b)

(c) (d)

(e) (f)

Figure S12. Calculation of rise time and fall time for three sets of samples under UV and Vis excitation.

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Table T1: Estimated rise time and fall time

UV VisSample

tr tf tr tf

S1 820 ms 345 ms 250 ms 500 ms

S2 413 ms 113 ms 882 ms 499 ms

S3 810 ms 485 ms 526 ms 562 ms

10 100

0.01

0.1

1

I (A

)

Intensity (W/cm2)

1.5 V 1 V 0.5 V

Si/CQD

550 nm

1 10 100 1000

0.1

1

I (A

)

Intensity (A/cm2)

1.5 V 1V 0.5 V

S3

360 nm

10 100

0.1

1

I (A

)

Intensity (A/cm2)

1.5 V 1V 0.5 V

S3

550 nm

1 10 100 1000

0.01

0.1I (A

)

Intensity (W/cm2)

1.5 V 1 V 0.5 V

360 nm

S2(a) (b)

(c) (d)

Figure S13. Photocurrent as a function of optical intensity

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Table T2: Fitting parameters for excitation power dependence of three sets of devices

0.5 V 1 V 1.5 VSample Fitting

function a b c a b c a b c

S1 𝑎 + 𝑏𝑥𝑐 0.06 0.002 0.56 0.4 0.01 0.61 1.18 0.09 0.43

S2 𝑎 + 𝑏𝑥𝑐 4.4×10-9 10-13 1.6 3×10-8 1.25×10-10 0.88 6.2×10-8 1.06×10-9 0.78

S3 𝑏𝑥𝑐 -- 0.03 0.04 -- 0.2 0.06 -- 1.49 0.13