Direct-writing on monolayer GO with Pt-free AFM tips in the

Transcription

1 Supplementary Figure S1 Direct-writing on monolayer GO with Pt-free AFM tips in the presence of hydrogen. We replaced the Pt-coated tip with a gold-coated tip or an untreated fresh silicon tip, and kept the other conditions unchanged. The images of a monolayer GO sheet on sapphire before (a) and after (b) writing with a gold-coated tip; and those before (c) and after (d) writing with an unmodified silicon tip. GO reduction is obviously not catalyzed by these Pt-free tips. The scale bars are all 500 nm.

2 Supplementary Figure S2 Direct-writing on monolayer GO at a low vacuum condition free of hydrogen gas. The topographic images before (a) and after (c) the writing, and the corresponding friction images (b) and (d), respectively. It can be seen that the writing was not successful. The scale bars are all 500 nm.

3 Supplementary Figure S3 C1s XPS spectra of the GO films under different treatments. The Pt nanoparticles (NPs) were formed on the GO surface through sputtering a very thin sub-monolayer Pt film. (1) The pristine GO (black curve) shows two typical peaks, the low energy one at ~285 ev is attributed to sp 2 carbon, and the high energy one at ~287 ev corresponds to the oxidation species. (2) For the GO film after one-hour annealing at 100 C in hydrogen gases (red curve), both peaks show slight changes, indicating that the hydrogen has some effect on the oxidation species, but not much. (3) For the GO film coated with Pt NPs (blue curve), in comparison with that of pristine GO, the intensity change of each peak is hardly observable. (4) For the GO film coated with Pt NPs, after one-hour annealing at 100 C in vacuum without hydrogen, the intensity of the high energy peak decreases slightly (cyan curve). The results of (3) and (4) demonstrate that the Pt coating alone does not have any catalytic activity for GO s reduction at RT but can reduce GO indistinctively at high temperature. (5) For the GO film coated with Pt NPs, after one-hour annealing at 100 C in hydrogen gases, the high energy peak is almost completely disappeared (magenta curve). This strongly suggests that the catalytic reaction can take place only when both the Pt and the hydrogen are presented.

4 Supplementary Figure S4 Direct-writing of rgo nanoribbons on monolayer GO with Pt-coated tips at different load forces and scanning speeds. (a) The topographic and (b) the friction images of the rgo nanoribbons under different load forces, 1, 5, and 10 nn (from the left to the right), respectively, while the scanning speed is kept at 2 nm/s, together with their corresponding profiles in (c) and (d). The full width at half maximum (FWHM) of the rgo nanoribbon is 62, 67, and 69 nm, respectively. The corresponding depth of the groove is 4.3, 5.7, and 6.1 Å with an experimental uncertainty of 1 Å. The reduced distance between the tip and the GO can cause more GO structures to be involved in the reduction process, resulting in a wider and deeper groove. (e) The topographic and (f) the friction images of the rgo nanoribbons under different scanning speeds, 10, 5, and 2 nm/s (from the left to the right), respectively, while the load force is kept at 5 nn, together with their corresponding profiles in (g) and (h). The FWHM of the rgo nanoribbon is measured to be 54, 61, and 74 nm, respectively, while the corresponding depth is 3.3, 5.3, and 5.5 Å. The longer the AFM tip stays at each position, the more GO structures can be reduced. The scale bars in (a), (b, (e) and (f) are all 200 nm.

5 Supplementary Figure S5 Direct-writing of square patterns on monolayer GO with Pt-coated tips at different temperatures. The applied temperatures are 20, 50, 70 and 100 C, respectively. The corresponding results are marked by dashed squares and labeled by 1, 2, 3 and 4. The scale bar is 500 nm. It can be seen that the reduction of GO takes place only when the temperature is higher than 50 C. The further increase in the temperature can result in more efficient reduction process. When the temperature is raised to 100 C, the GO within the square becomes well reduced, and the height for the whole area in the square is entirely decreased.

6 Supplementary Figure S6 Characterization of various rgo nanoribbon FETs. (a1-a3) The device produced at 100 C. a1 is the topographic image of the rgo nanoribbon bridged between two electrodes (the white areas). a2 and a3 are the topographic and the corresponding current images of the marked area in a1. (b1-b3) and (c1-c3) are the devices fabricated at 105 and 110 C, respectively. The black scale bars are 200 nm and the white scale bars are 100 nm for all above images. (d) I-V curves of the rgo nanoribbons produced at the different temperatures. (e) (Left axis) The conductivities of GO (shadowed squares) and rgo nanoribbons produced at the different temperatures. The conductivity of the rgo nanoribbon reported in Ref. 17 is also shown (pentagram) for comparison. (Right axis) The hole mobilities of the rgo nanoribbon FETs produced at the different temperatures.

7 Supplementary Figure S7 Characterization of rgo nanoribbon FETs fabricated beyond 115 C. (a1-a3) and (b1-b3) are the topographic and the corresponding current images of the rgo nanoribbons produced at 120 and 125 C, respectively. The black scale bars are 200 nm and the white scale bars are 100 nm. Note that beyond 115 C, the quality of the nanoribbon decreases with the increase of the temperature.

8 Supplementary Methods Control experiments to examine the roles of the Pt-coated tip and the surrounding gas environments on the cspl. Firstly, we replaced the Pt-coated tip with a gold-coated tip or an untreated fresh silicon tip, and kept the other conditions unchanged (Supplementary Figure S1). In the second control experiment, we put the Pt-coated tip in low vacuum to avoid the presence of any hydrogen gas (Supplementary Figure S2). It is clear that the combination of the Pt-coated tip and the hydrogen gas is the key for the catalytic reduction reaction of GO. With current technologies, at least with the ones that we have access to, it is impossible to monitor the reaction process in situ and to calibrate the written area due to its small size. We have instead carried out a series of control experiments using XPS to verify that the reaction does take place when the Pt nanoparticles (NPs) and hydrogen gas coexist. In the experiment, the Pt NPs were formed on the GO surface through sputtering a very thin sub-monolayer Pt film. Five XPS spectra for the carbon K edge of GO with different treatments were recorded: (1) pristine GO film (black curve in Supplementary Figure S3), (2) GO film after one-hour annealing at 100 C in hydrogen gases (red curve), (3) GO film coated with Pt NPs (blue curve), (4) GO film coated with Pt NPs, after one-hour annealing at 100 C in vacuum without hydrogen (cyan curve), (5) GO film coated with Pt NPs, after one-hour annealing at 100 C in hydrogen gases (magenta curve). The result strongly suggests that the catalytic reaction can take place only when both the Pt and the hydrogen are presented.

9 Control experiments to examine the roles of the load force and the scanning speed of the tip as well as the substrate temperature on the cspl. The reduction degree of GO, well reflected from its electrical characteristic, can be controlled by experimental conditions. The load force and the scanning speed are two of the decisive parameters that can be adjusted experimentally. The load force determines the distance between an AFM tip and a GO sample, while the scanning speed determines the time that an AFM tip could stay at one particular position in a scanning process. It is anticipated that, the reduced distance between the tip and the GO can cause more GO structures to be involved in the reduction process, resulting in a wider and deeper groove (as shown in Supplementary Figure S4a-d); while the longer the AFM tip stays at each position, the more GO structures can be reduced (as shown in Supplementary Figure S4e-h). The direct-writing process is also affected by the substrate temperature. To demonstrate this effect, square patterns have been written on the same sample (monolayer) at a series of temperatures ranging from 20 to 100 C, the topographic images are given in Supplementary Figure S5. The effect of the reduction temperature on the conductivity and mobility of the rgo nanoribbons. Through direct writing rgo nanoribbon FETs at a series of working temperatures (i.e., 100, 105, 110, 115, 120 and 125 C, respectively), we have carefully examined the effect of the reduction temperature on the conductivity, mobility and current contrast (with

10 respect to the surrounding unwritten areas) of the written rgo nanoribbons. The results are shown in Supplementary Figures S6 and S7.