Supplementary Information

Electrochemical half-reaction assisted sub-bandgap photon sensing

in graphene hybrid photodetectorBy Ze Xiong1,§, Jiawei Chen1,§, Jizhuang Wang1, Yu Cai1, Xiang Liu2, Zhicheng Su2,

Shijie Xu2, Arshad Khan3, Wendi Li3, Juncao Bian4, Gaomin Li,5 Mingyuan Huang5 &

Jinyao Tang1,*

1.Department of Chemistry, The University of Hong Kong,Hong Kong 999077, China.* Correspondence and requests for materials should be addressed to J.Y.T. (Email: jinyao@hku.hk). § These authors contributed equally to this work

2.Department of Physics, The University of Hong Kong,Hong Kong 999077, China.

3.Department of Mechanical Engineering, The University of Hong Kong,Hong Kong 999077, China.

4.Department of Physics and Materials Science, City University of Hong Kong,Hong Kong 999077, China.

5.Department of Physics,South University of Science and Technology of China,Shenzhen, China.

Supplementary Figure 1. Process flow of photodetector fabrication. (a) After

transferred to a substrate (Si or quartz), the CVD grown graphene was patterned into

ribbon by ebeam lithography and oxygen plasma etching. (b) 0.5 nm Ag was

deposited to the central area of the graphene ribbon defined by ebeam lithography

with thermal deposition. (c) PMMA removed by soaking in a mixture solution of

dichloroethene and acetone (1:5, v/v). (d) Converting Ag into AgCl by reaction 2Ag +

Cl2 = 2AgCl. (e) Coating the sample with 5 nm Al2O3 using ALD. (f) After removing

the Al2O3 on the contact area by dipping into 5:1 buffered HF solution (BHF), 100 nm

nickel (Ni) was sputtered as the electrodes. (g) Lift-off in a mixture of dichloroethane

and acetone (1:5, v/v).

Supplementary Figure 2. XPS spectra of AgCl nanoparticles synthesised by gas

phase reaction and aqueous solution reaction. (a) Ag 3d of AgCl synthesised by

gas phase reaction, (b) Ag 3d of AgCl synthesised by solution reaction, (c) Cl 2p of

AgCl synthesised by gas phase reaction and (d) Cl 2p of AgCl synthesised by solution

reaction. The fitted peaks at 367.2 and 373.2 eV can be attributed to Ag+, whereas the

peaks at 368.2 and 374.0 eV can be ascribed to trace amount of Ag0. AgCl

nanoparticles prepared in aqueous solution are synthesised via mixing 0.005 M

AgNO3 and 0.1 M KCl aqueous solution to ensure that there is no Ag metal particles

in as synthesized AgCl. The trivial Ag0 signal in both samples could be caused by the

X-ray induced partial reduction of Ag+ during XPS measurement.

Supplementary Figure 3. TEM sample preparation. (a) SPI G200HS fine square

mesh copper grid (without the supporting film) was coated with 5 nm Al2O3 by ALD

at 250 C to prevent the copper from the subsequent reaction. (b) The PMMA

supported graphene was scooped by the ALD coated TEM grid and dried in air. (c)

0.5 nm Ag was deposited through the mesh of the grid by thermal evaporation. (d)

After converting Ag into AgCl by reaction 2Ag + Cl2 = 2AgCl, 5 nm Al2O3 was

coated by ALD at 70 C.

Supplementary Figure 4. TEM characterization of AgX morphology. (a) TEM

images of (a) Ag, (b) AgCl, (c) AgBr and (d) AgI nanoparticles on the graphene

surface (Scale bars: 100 nm). The inset plot shows the diameter distribution of AgCl

nanoparticles with the most frequently occurring particle diameter of ~ 18 nm. The

binary star-like morphology could be caused by the electron beam induced partial

reduction of AgBr and AgI into Ag during TEM imaging process.

Supplementary Figure 5. The temporal photoresponse of Ag-graphene (Ag-G)

hybrid photodetectors. 0.5 nm Ag nanoparticles were deposited on graphene without

reaction with halide gas as a control photodetector. The visible light illumination

power is ~ 54.9 nW and the source-drain voltage VSD = 1 V.

Supplementary Figure 6. The normalised absorbance spectrum of AgCl

nanoparticles. AgCl was synthesised by mixing 0.005 M AgNO3 and 0.1 M KCl

aqueous solution.

Supplementary Figure 7. The temporal photoelectrochemical response of AgCl

nanoparticles. 0.5 nm AgCl nanoparticles were prepared on a Pt foil with small

exposure area (1 1 mm2) defined by PMMA sealing, using gas phase reaction as in

photodetector fabrication. Pt wire was used as the counter electrode at 0 V bias under

chopped green light illumination (520 nm wavelength).

Supplementary Figure 8. Relative spectral radiance of KPFM measurement.

Light source data was extracted from the EKE lamp spectrum provided by Dolan-

Jenner Industries. A 450 nm longpass filter was used to block UV light.

Supplementary Figure 9. Structure and surface potential map of bare graphene.

(a) AFM and (b) the corresponding KPFM images taken on the surface of bare

graphene (G, left) and gold electrode (Au, right), under the alternating “dark”, “light”

and “dark” illumination states at VG = 0 V. Scale bar: 2 m. (c) Height and surface

potential profiles along the green dashed lines on the graphene surface. (d) Height and

surface potential profiles along the blue dotted lines on the Au surface. There is no

substantial potential difference with and without visible light illumination due to the

fast recombination of photoexcited hot carriers in graphene without the AgCl/Ag0

redox couple.

Supplementary Figure 10. The photoresponsivity of AgCl-graphene

photodetector. The magnitude of the responsivity as a function of VSD under 400 nm

illumination of different powers at VG = 0 V.

Supplementary Figure 11. Gate tunable photoresponse under UV excitation. Photocurrent of AgCl-graphene as a function of the VG under 385 nm light illumination with the power of ~ 11.35 nW at VSD = 1 V.

Supplementary Figure 12. The power absorption intensity map of AgCl-

graphene hybrid by FDTD simulation. (a) The map of the power absorption

intensity for AgCl-graphene device on 470 nm SiO2 at 400 nm incident wavelength.

The ratio between the power absorbed by graphene (Pgraphene) and the power absorbed

by AgCl (PAgCl) is 126 : 1. (b) The map of the power absorption intensity for AgCl-

graphene device on 470 nm SiO2 at 500 nm incident wavelength. The ratio between

Pgraphene and PAgCl is 246 : 1.

Supplementary Figure 13. Temporal response of AgCl-G photodetector with

higher operation speed. The lifetime of electrons residing in AgCl nanoparticles can

be reduced to 2.3 s by applying a voltage pulse to back-gate to lower the barrier

height at AgCl-graphene junction and speed up the photocarrier recombination. The

visible light illumination power is ~ 54.9 nW and the source-drain voltage VSD = 1 V.

Supplementary Figure 14. The Analysis of noise spectral density of the graphene

photodetector. (a) The dark current of the AgCl-graphene photodetector at source-

drain voltage VSD = 1 V. (b) Noise spectral density of the graphene photodetector

based on the dark current in (a).

Supplementary NoteTo evaluate the noise characteristics of our photodetector, noise-equivalent-power

(NEP), as the optical power that yields a unity signal-to-noise ratio for a given

bandwidth, is estimated by analysing the noise in the dark current without intentional

shielding. The dark current was recorded with a sampling rate of 10 kHz at source-

drain voltage VSD = 1 V. As shown in Supplementary Fig. 13b, the noise spectral

density is given by S (f n )=1

√F s N √ ⟨|I ( f n)|2 ⟩, where I ( f n ) is discrete Fourier transform

of dark current I (t n); Fs is the sampling rate; N is the number of sampling points. The

unit of S (f n ) is A/Hz1/2. Dividing noise spectral density by the responsivity (~ 7.3

104 AW-1 at 500 nm, Fig. 3b), a NEP of ~ 10-14 W/Hz1/2 can be achieved at a frequency

of 1 Hz.