Highly Stable Persistent Photoconductivity with Suspended Graphene Nanoribbons

Abstract

Graphene nanoribbon (GNR), also known as 1-dimensional graphene, with a non-zero band gap has a huge potential for various electrical and optoelectrical applications because of its high transparency, flexibility, controllable band gap, and unique edge states. Recent advances in the synthesis of GNR enable us to show the possibility of GNRs as future high performance electrical devices. However, the applicability of GNRs to optoelectrical devices is unclear. Here we report that suspended GNR devices can show persistent photoconductivity (PPC) with long decay time (over 72 h) and adequate environmental stability. Repeated non-volatile memory operation is also demonstrated with an integrated PPC device using GNRs. This very stable PPC device can be applied to a wide variety of fields such as ultra-low-power non-volatile memory, nanoscale imaging, and biological sensors. Our results have opened the door to advance the study of GNRs in novel directions such as optoelectrical applications.

Figure 1

(a) Schematic illustration of GNRs FET. (b) Typical I_DS − V_G curve of a GNRs-array FET under V_DS = 1 V. SEM images of GNRs-array FET under (c) low and (d) high magnification. (e) Definition for I_dark, I_on, I_off, I_per, and I_temp. Top and bottom show example of the case of positive and negative ΔI_DS, respectively. (f) Typical photoresponse properties of fresh (black) and old (purple) GNR samples.

Figure 2

(a–c) Photoresponse of ΔI_DS with different total mild O₂ plasma treatment times (t_total) (t_total = (a) 0, (b) 60, (c) 570 sec). (d) Plot of I_per as a function of t_total. (e) Continuously measured time profile of I_DS for an old GNR sample up to ~4000 sec. The inset shows a plot of ΔI_DS measured for a total of 3 days. All of the data in this figure were taken under V_DS = 1 V, V_G = −60 V.

Figure 3

(a,b) (a) Optical microscope image and (b) Schematic illustration of PPC measurement in water. (c) Typical time profile of I_DS for a GNR measured in water.

Figure 4

(a) Typical time sequence of wiring and erasing operation. Definition of dark current after erasing (I′_dark) and erasing current (I_eras) are shown as arrows. (b) Writing and erasing operation process with different values of the erasing bias voltage (V_eras). (c) The dependence of the recovery rate (R_e) on V_eras. (d) Repeated operation (8 cycles) of optical memory, timing of photo illumination (green) and application of V_eras (orange) is shown in the upper graph. (e) Low and high magnification optical microscope images and SEM image of the integrated GNR memory (9 cells). Each cell includes 440 GNRs. (f) Plot of the resistance change before and after photo-irradiation for pristine (green) and mild O₂-plasma-treated 9-cell GNR devices (yellow). (g,h) (g) Schematic illustration and (h) typical SEM image of a single-GNR device. (i) Comparison of device specifications between our GNR device and other memory devices. The arrow denotes the future possibilities of our GNR device.

Figure 5

(a,b) Time resolved photoresponse of ΔI_DS under (a) V_G = −60 V, (b) V_G = 0 V under V_DS = 1 V. (c) V_G dependency of temporary photocurrent (I_temp), persistent photocurrent (I_per), and I_dark under V_DS = 1 V. (d,e) Light power (P) dependence of (d) charge neutral point (V_CNP) and (e) transconductance of electrons (g_m^e) (blue triangle) and holes (g_m^h) (red square) normalized by the dark value under V_DS = 1 V. (f) Schematic illustration of V_G − I_DS curve under dark (green) and (yellow) photo-irradiation.

Figure 6

(a–c) Schematic illustration and (d–f) charge-trapping mechanism of GNRs-Ni(OH)₂ system for (a,d) before, (b,e) during photo-irradiation and (c,f) after the application of V_eras.