Supporting Information

Transcription

1 Photothermal Effect Induced Negative Photoconductivity and High Responsivity in Flexible Black Phosphorus Transistors Jinshui Miao,, Bo Song,, Qing Li, Le Cai, Suoming Zhang, Weida Hu, Lixin Dong, Chuan Wang,* Electrical and Computer Engineering, Michigan State University, East Lansing, Michigan 48824, United States National Lab for Infrared Physics, Shanghai Institute of Technical Physics, Chinese Academy of Sciences, 500 Yutian Road, Shanghai , China * Corresponding author: cwang@msu.edu ( equal contribution) Supporting Information S1

2 S1. Optical microscope images of BP flakes on SiO 2 /Si and polyimide substrates before and after Raman measurements After the Raman measurements, the BP flakes on SiO 2 /Si substrate typically do not show noticeable change. In contrast, the BP flakes on polyimide film can sometimes get burned easily after Raman measurements. Because the thermal conductivity of polyimide film (~ 0.2 Wm -1 K -1 ) is significantly smaller than silicon (~ 150 Wm -1 K -1 ), the heat induced by the laser in Raman system cannot be effectively dissipated, resulting in burning of BP flakes. As shown in Figure S1e, the maximum laser intensity in our Raman system (~ 258 W/cm 2 ) is significantly higher than the near-infrared laser (~ 16.5 W/cm 2 ) used for photocurrent measurements. In order to collect Raman spectra from BP flakes on polyimide substrate, we have to reduce the laser intensity (normally using D 0.3 or D 0.6 filters) to avoid BP flake from being burned. Figure S1. Optical microscope images of BP flake on SiO 2 /Si substrate before (a) and after (b) Raman measurement. Optical microscope images of BP flake on polyimide film before (c) and after (d) Raman measurement. The scale bar is 5 µm. (e) Laser intensity used in the Raman system with various filter values (wavelength: 532 nm). S2

3 S2. Photoresponse of BP device fabricated on SiO 2 /Si substrate The photoresponse in BP transistors fabricated directly on SiO 2 /Si substrate were systematically studied. It can be seen that the device shows monotonic current increase under illumination. No noticeable current decrease was observed as the gate voltage was swept from -45 to 45 V. Such photoresponse is obviously different from devices built on polyimide (Figure 2 and 3 of the main paper) and can be attributed to the larger thermal conductivity of the silicon substrate (~ 150 vs. ~ 0.2 Wm -1 K -1 ), allowing the heat generated by the incident laser to be quickly dissipated. Therefore, there is no photothermal effect induced conductance decrease in such BP device. The device also exhibits decent responsivity of ~ 1.3 A/W. Figure S2. (a) Transfer characteristics of BP transistors fabricated on SiO 2 /Si substrate measured under near-infrared laser illumination. (b) Photo-switching characteristics of the same device plotted as a function of time. (c, d) Device photocurrent and photoconductive gain (c) and responsivity (d) plotted as a function of laser intensity. S3

4 S3. Gate bias-dependent photocurrent and responsivity Photocurrent is normally defined as the current under illumination minus the dark current. In this work, the device photocurrent (value and sign) largely depends on the gate bias used. Figure S3 shows the gate bias-dependent photocurrent and responsivity of our flexible BP photodetector on freestanding polyimide film, which are extracted from Figure 3a of the main paper. From the data, one can see that the flexible BP transistor exhibits a significant negative photocurrent when it is biased in the on-state. On the other hand, when the device is biased in its off-state, it exhibits a small positive photocurrent. The device responsivity follows similar trend with very large responsivity in the on-state and small responsivity in the off-state. Figure S3. Linear (a) and semi-logarithmic (b) scale plots of photocurrent in the flexible BP transistor as a function of gate bias. Linear (c) and semi-logarithmic (d) scale plots showing the gate bias-dependent responsivity of flexible BP transistor. S4

5 S4. Relative change in device conductance ( G/G 0 ) induced by incident laser or substrate heating The relative conductance change G/G 0 (where G = G G 0, G is the instantaneous conductance of the device and the G 0 is the conductance of the device at room temperature or dark condition) plotted as a function of laser intensity or substrate temperature is shown in Figure S4. As can be seen, laser illumination and temperature rise exert almost the same effect on the device conductance. Figure S4. (a) Relative conductance change plotted as a function of laser intensity after the BP device is delaminated from the SiO 2 /Si handling wafer. (b) Relative conductance change of the same BP device plotted as a function of substrate temperature when it was still attached on SiO 2 /Si handling wafer. A Peltier heater was used the heat up the device from 293 K to 373 K. S5

6 S5. Bending test of flexible BP transistors Figure S5 shows the systematic bending tests conducted on flexible BP devices. As can be seen in Figure S5a and b, the device transfer and output characteristics show negligible variations even when bent down to a very small curvature radius of 5.9 mm. Additionally, the device performance remains essentially unchanged throughout the process of up to 1000 bending cycles with a curvature radius of ~ 6 mm. Figure S5. (a, b) Transfer (a) and output (b) characteristics of a flexible BP transistor measured at various curvature radii. (c, d) Transfer (c) and output (d) characteristics of another flexible BP transistor measured after various bending cycles. S6