Supporting Online Material for

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1 Supporting Online Material for N-Doping of Graphene Through Electrothermal Reactions with Ammonia Xinran Wang, Xiaolin Li, Li Zhang, Youngki Yoon, Peter K. Weber, Hailiang Wang, Jing Guo, and Hongjie Dai* This PDF file includes: *To whom correspondence should be addressed. Materials and Methods SOM Text Figs. S1 to S6 References Published 8 May 2009, Science 324, 768 (2009) DOI: /science.[insert ms no.]

2 N-Doping of Graphene Through Electro-Thermal Reactions with Ammonia Xinran Wang, 1 Xiaolin Li, 1 Li Zhang, 1 Youngki Yoon, 2 Peter K. Weber, 3 Hailiang Wang, 1 Jing Guo, 2 and Hongjie Dai 1,* 1 Department of Chemistry and Laboratory for Advanced Materials, Stanford University, Stanford, CA 94305, USA 2 Department of Electrical and Computer Engineering, University of Florida, Gainesville, FL, 32611, USA 3 Chemical Sciences Division, Lawrence Livermore National Laboratory, Livermore, CA 94550, USA Supporting Online Materials: 1. Materials and methods: Graphene nanoribbon (GNR) devices fabrication, scanning electron microscopy (SEM) and atomic force microscopy (AFM) imaging of chemically derived GNRs; electrical measurement details; Raman measurement details; Graphene sheets (GS) film preparation, XPS and nanosims spectroscopy on GS; Band structure calculation of GNRs terminated by oxygen and nitrogen groups. 2. Detailed e-annealing process and Dirac point shifts in vacuum. 3. Detailed e-annealing sequences, control experiments and Dirac point shifts in NH E-annealing of lithographically patterned GNRs from pristine peel off graphene in NH Raman measurements on GNRs with high power input and temperature estimate. 6. E-annealing of sub-10nm GNRFETs.

3 1. Materials and methods Graphene nanoribbon (GNR) devices fabrication, scanning electron microscopy (SEM) and atomic force microscopy (AFM) imaging of chemically derived GNRs We obtained the solution containing GNRs as described in Ref. 1, soaked the 300nm SiO 2 /p ++ Si substrate with pre-patterned metal markers in the solution for ~1hr, rinsed with isopropanol and blew dry. We then used SEM with 1kV acceleration voltage to look for GNRs, which was shown to deliver good positive contrast on isolated carbon nanotubes (2). Under 1kV, GNRs showed bright positive contrast on the edges. Due to limited spatial resolution, sub-10nm GNRs appeared as single bright lines under SEM (Fig. S1c, d). Such bright contrast made the SEM approach much more efficient than previous AFM approach (1) to find and locate GNRs. We also tried higher acceleration voltages and found that the contrast turned to negative above ~2kV, making it very difficult to find GNRs on substrate. The chips were then calcined at 300 C for 10mins to remove amorphous carbon deposited during SEM, followed by AFM measurements of width and height of GNRs. We used electron beam lithography to pattern source/drain, thermally evaporated 20nm Pd and did metal lift-off. The devices were finally annealed in Ar at 200 C for ~15mins to improve the contacts. Electrical measurement details The electrical data of GNR devices were taken by a standard semiconductor analyzer (Agilent 4156C) inside a Lakeshore table-top cryogenic vacuum probe station connected to a turbo pump. The base pressure of the system was ~10-6 Torr. All the vacuum data were taken under base pressure after more than ~8hrs pumping. During NH 3 e-annealing, we connected ultra high purity 10% NH 3 in Ar (Praxair Inc.) to the probe station and maintained ~ 1Torr pressure in the chamber.

4 Raman measurement details We used a Horiba Jobin Yvon LabRAM HR Raman microscope with a 633nm He-Ne laser to probe the temperature of GNRs under high input power. The measurements were done in air with Ar flow protection. We wirebonded the GNR devices to a chip carrier clamped on the stage, and connected the chip carrier to a semiconductor analyzer to apply a constant V ds while taking measurements. We located the GNR by high resolution mapping near the device area. Then we went to the spot with highest Raman signal and took Raman data. First we took the data with V ds =0, corresponding to room temperature, and then increased V ds until the devices broke down in air. All the Raman data were collected under the same condition except V ds, with only a few minutes between each measurement. The accumulation time was usually ~5mins to reduce noise level. Graphene sheets (GS) film preparation, XPS and nanosims spectroscopy on GS GS were made following Ref. 3. After centrifugation, the GS were collected from the aggregate, which were sonicated and washed again in fresh N,N-dimethylformamide and collected by centrifugation. This procedure was repeated for 3-5 times to remove most surfactants. Then we transferred the GS to 1,2-dichloroethane (DCE) and did the centrifugation-resuspension in DCE for several more times. Then the GS were deposited on silicon substrate by drying the DCE suspension. Thermal annealing was done in a vacuum CVD furnace. The pressure of the furnace was ~1Torr and ~800mTorr during Ar and NH 3 thermal annealing, respectively. To observe more obvious effects, we annealed the GS films at 1100 C in NH 3, although the GNRs electrically annealed was likely to be heated to significantly lower temperatures (hundreds of degree C). The control sample was annealed in Ar to 800 C. XPS survey scan measurements were carried out using an SSI S-Probe monochromatized XPS spectrometer, which used Al (Ka) radiation as a probe. The

5 analysis spot size was μm. NanoSIMS was performed using the Lawrence Livermore National Laboratory NanoSIMS 50 (Cameca, Gennevilliers, France). The analyses were made with a ~1 pa, 16 kev 133 Cs + primary ion beam focused into a ~100 nm diameter spot, rastered over a sample area of 20 x 20 μm 2. Each analysis consisted of 10 replicate scans of pixels with a dwell time of 1ms/pixel. Secondary ion intensities for 16 O -, 12 C - 2, 12 C 1 2 H -, 12 C 14 N - and 32 S - were collected simultaneously in multi-collection mode. A mass resolving power of ~6000 was used to separate isobaric interferences from the isotopes of interest, e.g., 12 C 2 1 H - from 13 C 12 C at mass C 2 - and 12 C 2 1 H - were selected for the analysis because the yields of these C 2 species are ~3x higher than the yields of 12 C - and 12 C 1 H - species. The use of 12 C 14 N - is standard because the yield of N as a monomer is more than 10x lower. The data for each analysis area was subdivided into twenty-five 40x40 pixel regions of interest to calculate ion intensities normalized to 12 C - 2 against analysis time and the standard deviations. The data are presented as means +/- standard error. Because of the high sensitivity of nanosims, CN - is detected on most sample surfaces, and therefore the 12 C 14 N - / 12 C 2 data for the sample annealed in NH 3 must be compared to the data for the control sample to assess the incorporation of N into the GS. For both the control and the NH 3 -annealed sample, the 12 C 14 N - / 12 C 2 ratio drops off from the surface, and at depth (~10x deeper than these analyses), the 12 C 14 N - / 12 C 2 ratios for the two samples are within error of each other. For these analyses, the 12 C 14 N - / 12 C 2 ratio in the NH 3 -annealed sample is ~3x higher than the 12 C 14 N - / 12 C 2 ratio in the control sample. The 12 C 14 N - / 12 C 2 data for the first 7 planes are reported in Fig. 4 in main text because a single layer graphene is largely consumed after that number of scans (depth ~1nm), and therefore these data are taken to best reflect the GNRs exposed to the NH 3 gas. Band structure calculation of GNRs terminated by oxygen and nitrogen groups

6 To understand the edge doping effect of narrow GNRs used as the channel of the GNRFETs, electronic structures of 21-armchair GNRs (w~2.5nm) terminated by oxygen and nitrogen groups (Fig. 3C and 3D in main text) were calculated by the ab initio density-functional theory (DFT) within the local density approximation (LDA) (4). The structures (insets of Fig. 3C and 3D in main text) were fully relaxed until the force is reached to 0.01 ev/å. An energy cutoff of 400 Ry and a double-ζ polarization basis set were used. To understand the edge doping effects for wider GNRs explored in the experiment, we also modeled w~40nm GNRs with nitrogen atoms on the edge sites (Fig. 2D in main text). We used the extended Hückel theory (EHT) to calculate the E-k relation, DOS, and the position of the Fermi level. We chose the EHT over the ab initio DFT method because in this case the large size of the problem makes the ab initio simulations computationally impractical. The simulated width of w~40nm is still narrower than the experimental value of w~125nm due to computational constrains, but it is already sufficiently wide for the DOS of the GNR to approach that of a 2D graphene. The EHT has a good quantitative agreement with the ab initio calculation for carbon structures (5). To further validate the EHT, we have also simulated and compared the edge chemistry effect on narrower GNRs by using both the ab initio method and the EHT. We confirmed that the EHT offers a qualitative agreement with the ab initio DFT result for N-doped GNRs. 2. Detailed e-annealing process and Dirac point shifts in vacuum (Fig. S2). After GNR device fabrication, we first probed the devices in air and always observed p-doping with the Dirac point beyond V gs =40V in I ds -V gs curves (Fig. S3a). Then we pumped the devices to ~10-6 Torr for overnight and recorded I ds -V gs curves in vacuum, followed by e-annealing in vacuum. Fig. S2 shows the e-annealing process in vacuum for a w~67nm GNR and the I ds -V gs curves after each V ds double sweep. The Dirac point gradually moved towards zero V gs after each higher bias sweep until all

7 the physisorbed p-doping sources removed, which was indicated by overlapping back and forth V ds double sweep. Further increase of V ds often resulted in device breakdown due to high temperature of the GNRs. 3. Detailed e-annealing sequences, control experiments and Dirac point shifts in NH 3 (Fig. S3, the same device as Fig. 2 in main text). After the vacuum e-annealing discussed in the above section, we exposed the devices in air (the devices were then p-doped by physisorbed oxygen in air, Fig. S3a dashed red line, Fig. S3b solid red line) and pumped to ~10-6 Torr (Fig. S3a dashed orange line, Fig. S3b solid orange line). Compared to as-made GNRs, the devices after e-annealing in vacuum were slightly less p-doped due to removal of PmPV coatings, yet the Dirac point remained beyond V gs =40V. The devices were e-annealed in high vacuum again to remove the physisorbed oxygen molecules (after which I ds -V gs curve was very close to that after 1 st vacuum annealing, Fig. S3b solid green line) before exposed and e-annealed in ~1Torr NH 3. Upon exposure to NH 3, the Dirac point of the devices moved slightly towards negative V gs (Fig. S3b solid light blue line), due to the n-doping effect of physisorbed NH 3 molecules on the plane of GNRs (6). Then we employed similar e-annealing cycles on the devices in NH 3. During e-annealing in NH 3, the Dirac point gradually moved towards negative V gs until no hysteresis between back and forth V ds sweeps. After the annealing process, the Dirac point usually moved further towards negative V gs for ~ -20V (Fig. S3b solid dark blue line), indicating much higher n-doping concentration than exposed to NH 3 alone. Then we pumped the chamber back to ~10-6 Torr for overnight (~8 hours) and observed the Dirac point moving slightly to positive V gs due to the desorption of NH 3 molecules on GNR plane (Fig. S3b solid black line) (6). The I ds -V gs curve shown in Fig. 2C of main text was recorded at this stage. In a control experiment where we exposed the device in NH 3 without e-annealing and pumped back to ~10-6 Torr for overnight, we observed the complete removal of physisorbed NH 3 which was indicated by almost identical I ds -V gs curves before and after exposure to NH 3 (Fig. S3c). Therefore, the ~-20V shift after e-annealing in NH 3 was not due to

8 physisorption of NH 3 on GNRs. Such shift remained as we re-exposed the devices in air (Fig. S3b dashed red line) and re-pumped to vacuum (Fig. S3b dashed orange line), where we observed the Dirac point at V gs <40V. This stable n-doping of GNRs was attributed to edge functionalization by nitrogen species as discussed in main text. 4. E-annealing of lithographically patterned GNRs from pristine peel off graphene in NH 3 (Fig. S4). We fabricated lithographically patterned GNR devices using similar methods described elsewhere (7, 8). Fig. S4a inset shows a typical device with w~90nm and channel length L~3µm. We recorded the I ds -V gs curves in high vacuum before e-annealing in vacuum. Then we exposed the devices in NH 3 and e-annealed them. Fig. S4a shows the I ds -V gs curves after each step, and fig. S4b shows the gradual shift of Dirac point after each higher V ds sweep during e-annealing in NH 3. Due to different GNR synthesis process (e.g. no PmPV was coated on the lithographically patterned GNRs) and device geometries (e.g. different channel lengths), the absolute positions of Dirac point of the lithographically patterned GNRs were not the same as chemically derived GNRs. But we observed similar n-doping effects after e-annealing in NH 3. Since peel-off graphene had very low defect density in the plane, this result suggested that edge functionalization by nitrogen species was most likely responsible for the n-doping. 5. Raman measurements on GNRs with high power input and temperature estimate We utilized the red shift of graphene Raman peaks at high temperature to estimate the GNRs temperature during e-annealing (9). Fig. S5b shows such Raman data of a typical GNR. At room temperature, the G peak of this GNR was centered at ~1587.5cm -1. Under V ds =2V (356µW input power, Fig. S4a symbol), the G peak red-shifted by ~4.4cm -1, corresponding to ~320 C (This particular GNR is double-layer according to AFM and Raman, the temperature coefficient is cm -1 / C) (9). Under V ds =3V (660µW input power, Fig. S4a symbol), the device

9 broke down after ~10seconds. Assuming a constant thermal coupling between the GNR and environments, we estimated the breakdown temperature of this GNR to be ~570 C, similar to the breakdown temperature of carbon nanotubes in ambient (10). The same GNR was e-annealed in NH 3 up to V ds =4.5V (1170µW input power, Fig. S4a line), corresponding to a temperature of >600 C. Since the Raman laser spot was ~1µm in diameter, much larger than the device geometry, the temperature estimate from Raman data was the average temperature along the channel. Some other factors such as doping (11) and strain (12) could also affect the position of G peak. However, since the V gs =0V for both measurements under V ds =0V and 2V, the peak shift was not likely due to doping change. The strain was not likely responsible either because the GNR was confined by two metal leads on a hard SiO 2 surface. 6. E-annealing of sub-10nm GNRFETs (Fig. S6) Fig. S6 shows the evolution of I ds -V gs curves of the same GNRFET shown in Fig. 3 in main text. As made device was p-type with suppressed n-channel conductance. After e-annealing in vacuum (up to V ds =5V), the device became ambipolar, with threshold voltage V th shifted slightly towards negative V gs. After we exposed the device in NH 3, V th moved further towards negative V gs, p- (n-) channel conductance decreased (increased) slightly, indicating small n-doping from physisorbed NH 3 (6). We then e-annealed the device in NH 3 (up to V ds =5V, the same as vacuum e-annealing) and observed that the I ds -V gs curve changed dramatically to n-type transistor behavior, with V th shifted to V gs =-30V. The n-gnrfet showed similar on state current, on/off ratio and subthreshold slope as the original p-type transistor.

10 Supplementary figures and captions Figure S1. 1kV SEM images (a) (d) and the corresponding AFM images (e) (f) of GNRs on SiO 2 /Si substrate. GNRs in (c), (d), (g) and (f) are sub-10nm in width. Figure S2. E-annealing of a typical w~67nm GNR device in vacuum. (a) The e-annealing process consisted of several double I ds -V ds sweeps (direction pointed by the arrows) with gradually increasing V ds (V ds =2V, 2.5V, 2.8V and 3V from left to right). Upper and lower insets show AFM image of the same device as-made and after e-annealing in vacuum, respectively. The height was reduced from ~1.8nm to ~1.2nm due to removal of PmPV coatings by e-annealing (see inset images). (b) I gs -V ds curves recorded after each V ds sweep. The Dirac point moved gradually from beyond 40 to

11 ~14V and stayed. V ds =1V for all the curves. Figure S3. Complete sequences and control experiments on the GNR device shown in Fig. 2 of main text. V ds =1mV for all the I ds -V gs curves. (a) E-annealing in vacuum. The I ds -V gs curves were recorded as made in air (solid red line), as made in vacuum (solid orange line, same as the red curve in Fig. 2b in main text), after e-annealing in vacuum (solid blue line, same as the blue curve in Fig. 2b in main text), exposed in air after e-annealing in vacuum (dashed red line) and re-pumped to vacuum (dashed orange line). (b) E-annealing in NH 3. The I ds -V gs curves were recorded in air (solid red line, same as the dashed red line in (a)), in vacuum (solid orange line, same as the dashed orange line in (a) and the red line in Fig. 2c in main text), after 2 nd e-annealing in vacuum (solid green line, the I ds -V gs curve was similar to that after 1 st e-annealing in vacuum), exposed in NH 3 (solid light blue line), after e-annealing in NH 3 (solid dark blue line), after pumping to high vacuum for overnight to remove physisorbed NH 3 (solid black line, same as the blue curve in Fig. 2c in main text), after re-exposed to air (dashed red line) and after re-pumped to high vacuum for overnight (dashed orange line, we observed the Dirac point within V gs =40V). (c) Control experiment showing that physisorbed NH 3 could be removed by pumping. The I ds -V gs curves were recorded in vacuum (red), after exposed to NH 3 (green) and after pumping to high vacuum for overnight (blue). The red and blue curves almost overlapped, indicating complete removal of physisorbed NH 3 by pumping.

12 Figure S4. E-annealing of lithographically patterned GNRs from peel-off graphene. V ds =10mV for all the I ds -V gs curves. (a) I ds -V gs curves recorded in vacuum (red), after e-annealing in vacuum (orange), after exposed in NH 3 (green) and after e-annealing in NH 3 (blue). This device was less p-doped than chemically derived GNRs because of no PmPV doping and less contact doping (longer channel length). Inset shows the AFM image of the device. (b) I ds -V gs curves recorded in NH 3 and after each V ds sweep during e-annealing in NH 3 (V ds =7V, 8V, 8.3V, 8.5V and 8.8V). The Dirac point moved gradually towards negative V gs. Because of much longer L, we had to apply much higher V ds in order to achieve similar GNR temperature (10). Figure S5. Raman measurements on GNRs with high power input. (a) Symbol: current recorded during Raman measurements. The device broke down under V ds =3V and I ds =220µA. Line: e-annealing curve up to V ds =4.5V (1170µW peak power) in NH 3.

13 Inset shows the AFM image of this device. (b) Raman G band of the GNR under V ds =0V (blue) and V ds =2V (red). The shift was 4.4cm -1. Figure S6. E-annealing of sub-10nm GNRFETs. The device is the same as in Fig. 3 in main text. The I ds -V gs curves were recorded as made in vacuum (red), after e-annealing in vacuum (orange), after exposed in NH 3 (green) and after e-annealing in NH 3 (blue).

14 References: 1. Li, X., Wang, X. et al. Chemically derived, ultrasmooth graphene nanoribbon semidonductors. Science 319, 1229 (2008). 2. Brintlinger, T. et al. Rapid imaging of nanotubes on insulating substrates. Appl. Phys. Lett. 81, 2454 (2002). 3. Li, X. et al. Highly conducting graphene sheets and Langmuir-Blodgett films. Nature Nanotech. 3, 538 (2008). 4. Solar, J. M. et al. The SIESTA method for ab initio order-n materials simulation. J. Phys.: Cond. Matt. 14, (2002). 5. Kienle, D., Cerda, J. I. and Ghosh, A. W. Extended Huckel theory for band structure, chemistry, and transport. I. Carbon nanotubes. J. Appl. Phys. 100, (2006). 6. Schedin, F. et al. Detection of individual gas molecules adsorbed on graphene. Nature Materials 6, 652 (2007). 7. Zhang, Y., Chen, T., Stormer, H.L. & Kim, P. Experimental Observation of the quantum hall effect and Barry's phase in graphene. Nature 438, 201 (2005). 8. Novoselov, K.S. et al. Two-dimensional gas of massless Dirac fermions in graphene. Nature 438, 197 (2005). 9. Calizo, I. et al. Temperature dependence of the raman spectra of graphene and graphene multilayers. Nano Lett. 7, 2645 (2007). 10. Pop, E. et al. Electrical and thermal transport in metallic single-wall carbon nanotubes on insulating substrates. J. Appl. Phys. 101, (2007). 11. Das, A. et al. Monitoring dopants by Raman scattering in an electrochemically top-gated graphene transistor. Nature Nanotech. 3, 210 (2008). 12. Ni, Z. H. et al. Uniaxial strain on graphene: Raman spectroscopy study and band-gap opening. ACS Nano 2, 2301 (2008).