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. 2015 Oct 8:6:8589.
doi: 10.1038/ncomms9589.

Planar carbon nanotube-graphene hybrid films for high-performance broadband photodetectors

Yuanda Liu  1 , Fengqiu Wang  1 , Xiaomu Wang  2 , Xizhang Wang  3 , Emmanuel Flahaut  4   5 , Xiaolong Liu  1 , Yao Li  1 , Xinran Wang  1 , Yongbing Xu  1 , Yi Shi  1 , Rong Zhang  1
Affiliations

Planar carbon nanotube-graphene hybrid films for high-performance broadband photodetectors

Yuanda Liu et al. Nat Commun. .

Abstract

Graphene has emerged as a promising material for photonic applications fuelled by its superior electronic and optical properties. However, the photoresponsivity is limited by the low absorption cross-section and ultrafast recombination rates of photoexcited carriers. Here we demonstrate a photoconductive gain of ∼10(5) electrons per photon in a carbon nanotube-graphene hybrid due to efficient photocarriers generation and transport within the nanostructure. A broadband photodetector (covering 400-1,550 nm) based on such hybrid films is fabricated with a high photoresponsivity of >100 A W(-1) and a fast response time of ∼100 μs. The combination of ultra-broad bandwidth, high responsivities and fast operating speeds affords new opportunities for facile and scalable fabrication of all-carbon optoelectronic devices.

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Conflict of interest statement

F.W., Y.D.L., Y.L, Y.X. and R.Z have filed a Chinese patent based on this work (application no. 201410265599.1).

Figures

Figure 1
Figure 1. Planar SWNT–graphene hybrid film and photodetector.
(a) Schematic of the phototransistor. (b) AFM image of the hybrid film on the SiO2/Si substrate (Scale bar, 200 nm). Black arrows are used to pinpoint individual SWNT. (c) AFM image of one individual SWNT partially covered by graphene (Scale bar, 20 nm). SWNT was uncovered by mechanically exfoliating the top graphene using a tape stripe. The black dashes show the edge of the graphene (left: SiO2; right: graphene). (d) Height profile along the red line in b. The blue dot in b marks the zero point in d. (e) Comparison of the respective height profiles of the graphene-covered portion and the uncovered portion of SWNT shown in panel c. (f) Ultraviolet–visible-infrared absorbance curves of graphene and SWNT–graphene hybrid film on quartz. (g) Transfer characteristics of the graphene, SWNTs and SWNT–graphene transistors without light. Comparing with the graphene transistor, the Dirac point of the SWNT–graphene transistor shifted from 2 to 17 V, indicating p-type doping in the graphene sheet induced by SWNTs. The electron (hole) mobility decreases from 6,142 cm2 V−1 s−1 (7,146 cm2 V−1 s−1) to 3,771 cm2 V−1 s−1 (4,666 cm2 V−1 s−1). The inset exhibits that the source-drain current of the SWNTs transistor is <10−11 A, indicating disconnected electric pathways for the pristine SWNTs channel. (h) The energy band diagram at the junction formed by graphene and semiconducting SWNTs. Photogenerated electrons in SWNTs are transferred to graphene due to the built-in field at the junction.
Figure 2
Figure 2. Photoresponse performance under 650 nm illumination.
(a) Source-drain current (ISD) as a function of back-gate voltage (VG) for the SWNT–graphene device with increasing 650 nm illumination intensities. VSD=0.5 V. Increasing of the illumination leads to a photogating effect that shift the Dirac point to lower VG, indicating electron doping of the graphene sheet. The inset is the optical micrograph of the fabricated device (Scale bar, 10 μm). The gold areas indicate the metal electrodes. The orange area and LT orange areas are graphene channel and SiO2/Si substrate, respectively. (b) Responsivity (black dots) and shift of Dirac point (red cross) as a function of the 650 nm illumination. (c) The magnitude of the photocurrent increases linearly with source-drain bias voltage (VSD) for different optical powers (VBG=0). Red lines are linear fits. (d) Energy-band diagram of the SWNT–graphene phototransistors at different back-gate voltage. The dash lines correspond to the Fermi level of graphene at different gate voltages. The blue and red lines schematically illustrate the gate voltage dependence of the SWNT energy levels.
Figure 3
Figure 3. Temporal characteristics and ultra-broadband photoresponse.
(a,b) Temporal photocurrent response of the SWNT–graphene hybrid photodetector, indicating a rise time and a fall time on the order of ∼100 μs. The illumination power is 440 μW and the laser wavelength is 650 nm. (c) External quantum efficiency as a function of illumination power at 650 nm. (d) Responsivities as a function of the optical power for different illumination wavelengths (405, 532, 650, 980 and 1,550 nm). (e) Comparison of responsivities measured from devices using purified metallic- and (6,5) chirality-enriched SWNTs. It is shown that the responsivity of the metallic tube-based devices is ∼5% of that of (6,5) chirality tube-based devices.
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