Supporting Information. Highly efficient and air stable infrared photodetector based. on 2D layered graphene-black phosphorus heterostructure

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1 Supporting Information Highly efficient and air stable infrared photodetector based on 2D layered graphene-black phosphorus heterostructure Yan Liu 1,, Bannur Nanjunda Shivananju 1,3,, Yusheng Wang 1, Yupeng Zhang 2,3, Wenzhi Yu 1, Si Xiao 4, Tian Sun 1, Weiliang Ma 1, Haoran Mu 1, Shenghuang Lin 1, Han Zhang 2, Yuerui Lu 5, Cheng-Wei Qiu 6, Shaojuan Li 1,* 1, 3,*, Qiaoliang Bao 1 Institute of Functional Nano and Soft Materials (FUNSOM), Jiangsu Key Laboratory for Carbon-Based Functional Materials and Devices, and Collaborative Innovation Center of Suzhou Nano Science and Technology, Soochow University, Suzhou , P. R. China. 2 SZU-NUS Collaborative Innovation Centre for Optoelectronic Science & Technology, and Key Laboratory of Optoelectronic Devices and Systems of Ministry of Education and Guangdong Province, School of Electronic Science and Technology, and College of Optoelectronics Engineering, Shenzhen University, Shenzhen, China. 3 Department of Materials Science and Engineering, and ARC Centre of Excellence in Future Low-Energy Electronics Technologies (FLEET), Monash University, Clayton, Victoria 3800, Australia. 4 Hunan Key Laboratory for Super-Microstructure and Ultrafast Process Institute of Super- Microstructure and Ultrafast Process in Advanced Materials, School of Physics and Electronics, Central South University, China. 5 College of Engineering and Computer Science, Australian National University, Australia. 6 Department of Electrical and Computer Engineering, National University of Singapore, 4 Engineering Drive 3, Singapore , Singapore Corresponding Authors * qiaoliang.bao@monash.edu; and sjli@suda.edu.cn S-1

2 KEYWORDS: infrared photodetector, black phosphorus, graphene, heterostructure, responsivity, gain, stability. 1. Raman spectrum and AFM image of the graphene-bp heterostructure. Figure S1. (a) Raman spectrum of the graphene-bp heterostructure. (b) AFM (Atomic force microscope) image of the graphene-bp heterostructure. (c) Height profile along the white dashed line, showing that the thickness of the Graphene-BP is 35 nm, corresponding to 70 monolayers. To further confirm the graphene-bp heterostructure, Raman spectroscopy was taken as shown in Supplementary Figure S1. The gaphene-bp heterostructure is excited with 514 nm laser, where the Raman peaks of thin film BP can be identified at the low frequency region at around 365, 440 and 470 cm -1, which are corresponding to the A 1 g, B 2g and A 2 g, vibration modes of BP crystal lattice, and the Raman peaks of graphene film are observed at higher frequency region at 1580 cm -1 and 2700 cm -1, which are corresponding to the G band and 2D band of S-2

3 monolayer graphene, respectively. 2. Photoresponse of graphene-bp heterostructure at 1550 nm and 635 nm wavelengths. (a) (b) Figure S2. (a) The photocurrents (I light -I dark ) of the graphene-bp, pure BP and pure graphene photodetectors under illumination from a NIR light source (1550 nm). (b) Photocurrents of the graphene-bp heterostructure, pure graphene and pure BP photodetectors under light illumination of 635 nm wavelength. S-3

4 3. Hall measurements to calculate the hole concentration in graphene and BP. In order to determine the sheet carrier density n s, Van der Pauw technique is used, which is widely used in the semiconductor industry to determine the electrical properties of uniform samples. We used a thin-plate graphene containing four very small ohmic contacts placed on the periphery (in the corners) of the plate. Schematic of Van der Pauw configuration is shown below: Contact 1 Contact 4 sample Contact 2 Contact 3 Figure S3. Van der Pauw configuration in our work. The electrical properties of the samples were measured by Hall measurements at room temperature (300 K). The sheet carrier density is given by n s IB = q V H where I is the current, B is the magnetic field, q ( C) is the elementary charge, and V H is the Hall voltage. To measure the Hall voltage V H, a current I is forced through the opposing pair of contacts 1 and 3 in a constant magnetic field B applied perpendicular to the plane of the sample, and the voltage V 24 is measured across the remaining pair of contacts 2 and 4. The magnitude of V H is equal to (V 24P - V 24n ) + (V 42P - V 42n ) + (V 13P - V 13n ) + (V 31P - V 31n ), where the subscript p represents positive magnetic field and n represents negative magnetic field. In our case, for the Hall measurements of monolayer graphene sample, I = A, B= 6750 Gauss, and the Hall voltage measurements results are listed below: S-4

5 I 13 V 24p I 31 V 42p I 42 V 13p I 24 V 31p A 2.00E E E E-6 V E E E E-6 I 13 V 24n I 31 V 42n I 42 V 13n I 24 V 31n A 2.00E E E E-6 V E E E E-6 Table S1. Hall voltage measurements results for monolayer graphene. Therefore, the calculated value for the hole concentration is cm -2 for monolayer graphene. For the Hall measurements of graphene-bp heterostructure sample, I = A, B= 6750 Gauss, and the Hall voltage measurements results are listed below: I 13 V 24p I 31 V 42p I 42 V 13p I 24 V 31p A 4.00E E E E-6 V E E E E-3 I 13 V 24n I 31 V 42n I 42 V 13n I 24 V 31n A 4.00E E E E-6 V -3.14E E E E-6 Table S2. Hall voltage measurements results for monolayer graphene with a BP flake embedded beneath it. Therefore, the calculated value for the hole concentration is cm -2 for graphene with a BP flake embedded beneath it. S-5

6 4. The effect of light illumination on the gate response of the graphene-bp heterostructure photodetector at 1550 nm. Figure S4. Transfer curve of the graphene-bp photodetector with and without light. The Y axis represents logarithmic scale. Inset: schematic band diagram of the graphene-bp heterostructure with photoexcited hot carrier transport process under light illumination. The dot line represents the fermi level (E f ), and V sd = 1V. Incident laser power: 211 nw. As shown in Figure S4, photo-excited holes in BP film can increase the hole current in the p- doping graphene (left inset in Figure S4) and have an opposite function as the Coulomb traps scatter (or recombine) the electrons in the n-doping graphene (right inset in Figure S4). When V g was larger than -5 V, where the Fermi level of graphene is shifted to above the Dirac point, recombination takes place between photogenerated holes transferred from BP to graphene and electrons induced by the back gate, but does not change the polarity of photocurrent. As V g further increases to V g = ~20 V, the electrons from the back gate voltage are just completely recombined by the photo-excited holes, as a result, the photocurrent falls to zero at this point and the current with light on is the same as the current with light off. As V g further increases to more positive direction, more electrons are induced by the back gate, so a reversible current is observed in the same device under the same irradiation condition. S-6

7 5. The dependence of the photocurrent on the excitation wavelength (from 900 nm to 1600 nm) for devices on polyethylene naphthalate (PEN) substrate. BP: ~30 nm Figure S5. The dependence of the photocurrent on the excitation wavelength (from 900 nm to 1600 nm) for BP photodetector device on soft polyethylene naphthalate (PEN) substrate. BP thickness: ~30 nm. S-7

8 6. Temporal photoresponse and photoresponsivity of the graphene-bp heterostructure (a) photodetector at 635 nm. (b) Figure S6. (a) Temporal photoresponse of the graphene-bp heterostructure photodetector. The illumination power is 500 nw and the laser wavelength is 635 nm. (b) Responsivity of the graphene-bp heterostructure photodetector with respect to incident power at 635 nm wavelength, V sd = 1V. S-8

9 7. Rise time and responsivity of graphene-bp heterostructure photodetector. Figure S7. The rise time of graphene-bp heterostructure photodetector with different thickness measured at 1550 nm wavelength. Table S3. Response time and responsivity of graphene-bp photodetectors in which BP has different thicknesses. S-9

10 8. Power dependent photoresponsivity of the graphene-bp heterostructure photodetector at 980 nm wavelength. Figure S8. Power dependent photoresponsivity of the graphene-bp heterostructure photodetector at 980 nm wavelength. S-10

11 9. Thickness dependent photoresponse of the graphene-bp heterostructure photodetectors at 1550 nm. Figure S9. (a) Atomic force microscope (AFM) images of the graphene-bp heterostructures. The thickness of the graphene-bp heterostructure was measured by scanning the AFM tip along the white dot line. The BP thickness varies from 15 nm to 60 nm. (b) Height profile, showing the thickness of the Graphene-BP is 60 nm, which is responsible for maximum photocurrent and photoresponsivity. (c) Thickness dependent photocurrent with different incident power. S-11

12 10. Calculation of responsivity and gain of graphene-bp photodetector. We calculated responsivity using a most commonly used method, which is reported in reference. 1 The responsivity (R) is calculated by dividing the photocurrent (I ph = I laser,on I laser,off ) by the incident power on the device area (P d = P in (A device /A spot ), where P in is the total optical power, A device is the area of the device, and A spot is the area of the laser spot. In order to calculate the photoconductive gain, we calculate the field effect carrier mobility of graphene-bp heterostructure from the device transfer curve. The mobility can be estimated by = Where, W is the channel width, L is the channel length, d is the thickness of SiO 2 (d = 300 nm), and ε is the dielectric constant of SiO 2, ε = ε ε 0 = F/m. Hence we have the carrier (hole) mobility µ = 6813 cm 2 V -1 s -1. The carrier transit time is obtained from 2 = = And it is found that t L = s. As shown below, the photocurrent decay lifetime measurement was performed to calculate the lifetime of photo-excited carriers. The device dynamic characteristics under laser illumination could be described by = + S-12

13 where I 0 is the dark current, A is a coefficient and t 1 is a constant which is considered as the lifetime of the carriers. From the curve fitting, the lifetime of carriers is estimated to be about 0.23 s. Using the formula 2 = Finally we can get a high photoconductive gain of Figure S10. The time-resolved photocurrent decay for the graphene-bp heterostructure photodetector excited by 1550 nm laser. S-13

14 11. Pump-probe measurements. Figure S11. Measured transmittivity transients of (a) graphene-bp heterostructure at 1550 nm with an average pump power of 800 µw, (b) pure graphene and (c) pure BP. The pump-probe experiment was performed using a femtosecond pulse laser system (Spectra-Physics, Spitfire ACE-35F-2KXP, Maitai SP and Empower 30) with a pump and probe laser at 1550 nm with a pump power of 800 µw. Femtosecond pulses at 1550 nm was generated by a regenerative optical parametric amplifier (TOPAS, USF-UV2) seeded by a mode-locked oscillator. The femtosecond pulses (at a repetition rate of 2 khz and pulse duration of ~ 35 fs) S-14

15 were split into two parts by a beam splitter and used as the pump and probe beams. The pump and probe beams were focused by a same lens at the graphene-bp heterostructure sample. A femtosecond pulse excites the graphene-bp heterostructure, and the photo-induced changes in the transmission spectrum ( T/T) of probe light were probed by a high-sensitivity photomultiplier detector after controlled time delays. The probe beam time delay was controlled by a motorized delay stage and the pump-probe signal was recorded using lock-in detection with a chopping frequency of 333 Hz. Figure S11 shows the relaxation dynamics of photocarriers in pure graphene, pure BP and graphene-bp heterostructure. It is found that these materials have dynamic relaxation process exhibiting exponential decay. About the falling process of transmittance before hot carriers were excited in Figure S11, the problem of Gaussian beam might induce this abnormal phenomenon. The measured transmittivity transients can be fitted by exponential decay function of = + ( )/, where τ represents the fast relaxation time during dynamics relaxation. It is revealed that both graphene and BP have very fast carrier dynamics, i.e., τ = 45 fs for graphene and τ = 23 fs for BP. The fast decay time (τ) is correlated to the intra-band optical relaxation. While BP and graphene form a heterostructure, it is remarkable to find that the relaxation dynamics is yet very fast with the similar time scale (τ = 41 fs, see Figure S11a). This suggests an ultrafast charge transfer in graphene-bp heterostructure, i.e., upon the photoexcitation the electron-hole pairs are generated in the graphene-bp heterostructure and the holes are quickly transferred from BP to graphene due to intra-band relaxation, while electrons will remain in the BP layer and cause photogating effect. S-15

16 12. Comparision of figures-of-merit of photodetectors based on different materials. Materials V ds (V) V g (V) Thicknes Spectral Response Responsivity Gain Ref. s range time ( ms) ( A/W ) Graphene/ BP nm 1550 nm This work Graphene nm L MoS nm >1L MoS nm 630 nm > BP 0 PN 6 nm 532 nm BP nm 640 nm BP nm 1550 nm f 3dB = GHz BP/MoS nm/ 1550 nm nm Ge nm GeSn nm InGaAs nm Graphene ribbons nm Table S4. Comparison of response time, responsivity, gain in photodetectors based on different materials. S-16

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