High-Performance Photodetectors Based on Semiconducting Graphene Nanoribbons

The inherent zero-band gap nature of graphene and its fast photocarrier recombination rate result in poor optical gain and responsivity when graphene is used as the light absorption medium in photodetectors. Here, semiconducting graphene nanoribbons with a direct bandgap of 1.8 eV are synthesized and employed to construct a vertical heterojunction photodetector. At a bias voltage of −5 V, the photodetector exhibits a responsivity of 1052 A/W, outperforming previous graphene-based heterojunction photodetectors by several orders of magnitude. The achieved detectivity of 3.13 × 1013 Jones and response time of 310 μs are also among the best values for graphene-based heterojunction photodetectors reported until date. Furthermore, even under zero bias, the photodetector demonstrates a high responsivity and detectivity of 1.04 A/W and 2.45 × 1012 Jones, respectively. The work shows a great potential of graphene nanoribbon-based photodetection technology.


Materials and Methods
Fabrication of GNRs 50 mg of single-walled carbon nanotubes (SWCNTs, Sigma Aldrich) were first annealed at 300 ℃ for 30 minutes under air condition and then immersed in 50 mL concentrated sulfuric acid (Sinopharm) for 2 hours.Next, 25 mg of KMnO 4 (Sigma Aldrich) was added into the mixture and then stirred for 30 minutes at 45 ℃ until KMnO 4 was fully consumed.
The solution mixture was then poured into 200 mL of ice deionized water, then the mixture was filtered by a 0.45 μm PTFE filtration to separate reacted SWCNTs.The reacted SWCNTs were washed by a copious water and then dried in air.To obtain the GNR solution, 20 mg of the as-fabricated reacted SWCNTs was dispersed in 200 mL of 1% aq sodium dodecyl benzene sulfonate (Sigma Aldrich) solution and then sonicated for 60 minutes.

Fabrication of the GNR/Al 2 O 3 /Si heterojunctions
The process flow of GNR-based heterojunction photodetectors is presented in Figure 1(a).First, the n-type Si wafers were cleaned with acetone, ethanol, water and dried by N 2 .
Then the Si wafers were immersed in a 1% HF solution for 10s to eliminate the thin selfoxidizing layer on the surface.A 10 nm-thick Al 2 O 3 film was deposited on the cleaned Si wafer by an atomic layer deposition (ALD).Then a 10 μL GNR aqueous solution was dropcasted on the surface of Al 2 O 3 and dried for overnight.Prior to the deposition of electrodes, the sample was annealed in Ar atmosphere at 400 ℃ for 2 hours.Then 50 nm Au were deposited as the top electrodes on the surface of the fabricated GNR film.The active area of the device is 0.1 mm 2 .

Characterization of GNRs and the fabricated photodetectors
Raman spectra and photoluminescence spectra of SWCNTs and GNRs are characterized by the Raman Spectrometer (RENISHAW, invia) with a special excitation wavelength of 523 nm.An atomic force microscope (AFM, Benyuan CSPM5500) is used to characterize the morphology of SWCNTs and GNRs.The cross-section image is characterized by a transmission electron microscope (TEM, FEI Talos F200X).The current-voltage characteristics of devices is measured by an Agilent 2902A under air and dark condition.
The optoelectrical performance of devices is measured under a laser illumination with a wavelength of 635 nm.The laser power density is obtained by an optical power meter (Ophir Nova II).An oscilloscope (Keysight MSOX6004A) is used to evaluate the response speed of the photodetector.

Effect of Al 2 O 3 thickness gradient on device performance
We have investigated the performance of devices with different thickness of Al 2 O 3 layers.S2 and Fig. 2, the 10 nm Al 2 O 3device exhibits higher photocurrent and lowest dark current in the reverse bias region under the same power of light stimulation.The current response show that at 0 V bias, the 10nm Al 2 O 3 -device shows high and stable photocurrent.Under -5 V bias, the 10 nm Al 2 O 3 -device also shows higher photocurrent and lower dark current.

Background introduction of GNRs
Graphene has experienced great progress in recent years, owing to its myriad of advantages, including ultra-high carrier mobility and exceptional mechanical properties and so on 1-3 .However, graphene develops slowly in the field of photoelectric devices, especially when graphene is used as the light absorption medium.This is primarily attributable to graphene's intrinsic zero-bandgap characteristic, which results in a low optical gain 4 .Hence, graphene nanoribbons (GNRs) attract much attention due to their opening bandgap which induced by strong lateral quantum confinement 5 .Previous work predicted that GNRs are direct bandgap semiconductors [6][7][8] , which gives it unlimited potential to replace graphene as a light absorption medium for fabricating high-performance photodetectors.However, theoretical research shows that the width of graphene nanoribbons must be less than 3 nm to produce a considerable band gap (>0.7 eV) to produce a sufficiently strong quantum confinement effect 9 .Therefore, it is very difficult to produce such a narrow GNR with high-precision lithography technology.The method of obtaining GNR by unzipping SWCNTs stands out among many methods of synthesizing GNR because of its solution synthesis and no need of vacuum.Meanwhile, the solution process gives the potential to prepare large-area GNR films, which is very important, because it is difficult for traditional two-dimensional materials to fabricate large-area films.As shown in Figure S5, the absorption intensity of the planar Si and GNR film is both lower than that of the GNR/Al 2 O 3 /Si heterostructure.This can be attributed to the enhancement of the photogenerated carrier separation efficiency by the built-in electric field in the heterojunction leading to an increase in the apparent absorption intensity.

Figure
Figure S2 shows device performance with Al 2 O 3 thickness of 2.5 nm and 5 nm.The reverse Figure S1.AFM images and height distributions of a SWCNT(a) and GNR (b).

Figure
Figure S3.(a) Current-voltage (I-V) curves of Au-Al 2 O 3 -Si device without GNR layer

Figure S4 .
Figure S4.Dark currents of the GNR/Al 2 O 3 /Si device measured at the bias of (a) 0 V and