nfrared Photodetectors Based on CVD-Grown Graphene I

and PbS Quantum Dots with Ultrahigh Responsivity

Zhenhua Sun ,Zhike Liu ,Jinhua Li ,Guo-an Tai ,Shu-Ping Lau ,and Feng Yan *

Infrared (IR) photodetectors are highly desired for various demanding applications, e.g., telecommunication, thermal

[1imaging, biological imaging, and remote sensing. , 2 ] Recently, [2IR photodetectors based on quantum walls, ] quantum dots

[1–8][13,14](QDs), graphene, [9–12 ] organic semiconductors and

[15]carbon nanotubes have been extensively studied. The advan-tages of using colloidal QDs in IR sensors include solution

processability, low cost, tunable wavelength and high respon-[1sivity. ] It is notable that the record responsivity of IR sensors based on PbS QDs is about 2700A/W, which is comparable to

[4that of photoconductors made of single crystalline silicon. ]

The two main types of IR photodetectors are photodiodes

[1,16]and photoconductors. IR photodiodes are fast responding

but have low gains (quantum ef ciencies) that are normally

[2,16]less than 1. The conductance of photoconductors can be

[4modulated by light illumination. ] A eld-effect phototran-[16–19]sistor also can be regarded as a type of photoconductor.

A photoconductor can have a gain greater than one because carriers can circulate in the conductor between two electrodes

[1,16]many times before they recombine with opposite charges.

Therefore, the gain of an IR photoconductor based on QDs can

3 due to the long lifetime of carriers in the QDs. [4be up to 10 ]

The photocurrent I lm photoconductor is given p across a thin by Ip = en μ EW , where e is the electronic charge, n is the den-sity of photo-induced carriers per unit area, μ is the carrier mobility, E is the applied electric eld and W is the width of

[16]the device. Consequently, the gain and the responsivity of

the photoconductor are proportional to the carrier mobility μ .However, the carrier mobility in QDs is lower than the one of silicon, carbon nanotubes, graphene, ZnO and some organic

[15]semiconductors. , 20–23

Graphene, which structure is two-dimensional sheets of

carbon atoms packed in a hexagonal honeycomb lattice, has aroused considerable interest because of the behavior of Dirac Fermions and the extremely high carrier mobilities up

2V to 200000 cm 1s 1.[21 ] Graphene transistors have been used

as ultrafast IR photodetectors for high-speed optical com-munications because of their high carrier mobility and their

[9light absorption in a broad wavelength range. , 24 ] However,

1)the responsivity of the IR detectors is very low ( ≤ 6.1 mAW

because of the low light absorbance of graphene ( ～ 2% for single

layer graphene). Therefore, they can only be used for detecting

[24]

highly intense IR light. Moreover, the hot carrier effect instead of the photovoltaic effect was found to be one reason

[25,26]for the photoresponse of graphene. Recently, Chitara et al.

reported IR photoconductors based on reduced graphene oxide or graphene nanoribbons, which showed responsivities up to

1[12]1 AW . Therefore, graphene-based IR detectors have much lower responsivities than photoconductors based on QDs.

It is reasonable to consider that the responsivity of a

graphene-based IR detector can be improved substantially by modifying the graphene lm with QDs, which can absorb IR light more ef ciently. On the other hand, if the carriers gener-ated by IR light can transfer to the graphene lm, their mobility will be much higher and thus the responsivity will be dramati-cally improved for a QD-based IR detector. Moreover, an array of graphene devices can be easily patterned by state-of-the-art techniques and this could possibly lead to elimination of the crosstalk between neighboring pixels that occur in silicon

[4devices. , 27 ]

Recently, phototransistors with ultrahigh responsivity up to 710 AW 1 based on mechanically exfoliated single or bilayer graphene akes and PbS QDs modi ed with ethanedithiol were

[28]reported. However, the fabrication of large areas is not pos-sible with exfoliated graphene. In this regard, devices based on

chemical vapor deposition (CVD) grown graphene are more suitable for practical applications. In this paper, we report on near infrared (NIR) photoconductors made of CVD-grown single-layer graphene coated with PbS QDs and prepared by solution processing. The devices were fabricated on various substrates, including exible ones, by solution processing and

7AW 1 showed high responsivities up to 10 , which are much

higher than the responsivities of the visible-light detector based on CVD grown graphene and PbS thin lm prepared by elec-[29]tron beam deposition. The sensing mechanism is attributed to the charges that are generated in PbS QDs under NIR light, and which can modulate the Fermi level and thus the conduc-tivity of the underlying graphene lm. In addition, the ligand capping the surface of the QDs was found to be crucial to the photo responsivity of the device as the charge transfer from the QDs to the graphene lm was dominated by the ligand layer. Single-layer graphene was prepared on copper foils by the

CVD method and transferred to the substrates with the state-[30,31]of-the-art technique. The single-layer graphene was char-acterized by Raman spectroscopy to show the monolayer

[30]property (see supporting Information, Figure S1). A single-layer graphene on a Si/SiO 2 substrate before and after being

coated with PbS QDs capped with pyridine was observed under atomic force microscopy (AFM) (see Supporting Information, Figure S2). The roughness (r.m.s.) of the pristine graphene lm

Z. H. Sun, Z. K. Liu, J. H. Li, G. Tai, Prof. S. P. Lau,

Prof. F. Yan

Department of Applied Physics and Materials Research Centre

The Hong Kong Polytechnic UniversityHong Kong, China

E-mail: apafyan@polyu.edu.hk DOI: 10.1002/adma.201202220 5878

http://www.wendangwang.com

© 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Adv. Mater. 2012, 24, 5878–5883