advances.sciencemag.org/cgi/content/full/2/7/e1600322/dc1 Supplementary Materials for Ultrasensitive molecular sensor using N-doped graphene through enhanced Raman scattering Simin Feng, Maria Cristina dos Santos, Bruno R. Carvalho, Ruitao Lv, Qing Li, Kazunori Fujisawa, Ana Laura Elías, Yu Lei, Nestor Perea-López, Morinobu Endo, Minghu Pan, Marcos A. Pimenta, Mauricio Terrones Published 22 July 2016, Sci. Adv. 2, e1600322 (2016) DOI: 10.1126/sciadv.1600322 This PDF file includes: Supplementary Materials and Methods fig. S1. NG sheets on different substrates and their typical Raman spectra with different synthesis conditions. fig. S2. Raman mapping of PG and NG showing monolayer coverage. fig. S3. XPS spectra (C1s and N1s) of NG synthesized at 850 C. fig. S4. Probing-enhanced Raman scattering effect between NG and PG sheets for different dye molecules. fig. S5. Comparison of Raman spectra with bare SiO2/Si substrates and NG for probing different dye molecules at their resonant condition. fig. S6. Enhancement factors for three different dye molecules between NG and PG. fig. S7. Comparison of GERS and SERS by applying sputtered Au nanoparticles as SERS substrates for comparison. fig. S8. Testing the molecular sensibility of the NG with different laser energies. fig. S9. Photos of RhB in ethanol solution with different concentrations. fig. S10. AFM images of NG samples with different concentrations of RhB. fig. S11. Raman spectrum of 5 10 12 M RhB on NG. fig. S12. Probing GERS effect between NG and PG for additional molecules such as R6G and PPP. table S1. Calculated HOMO-LUMO gap, adsorption data, and resonant Raman laser excitation energy for each molecule.
Supplementary Materials and Methods 1. Optical images of transferred graphene on various substrates and Raman spectra for NG sample with different synthesis conditions. Figure S1A presents as synthesized graphene sample on copper foil. Figure S1 (B and C) present optical images of transferred graphene on SiO2/Si substrate and quartz. As can be seen, the graphene sample on quartz is highly transparent. Figure S1 (D and E) present Raman spectra of the as-grown NG with different synthesis conditions. It is observed that when the synthesis condition changes, the intensity ratio between graphene D-band and G- band considerably changes, indicating N doping level changes. fig. S1. NG sheets on different substrates and their typical Raman spectra with different synthesis conditions. (A) A photograph of the as-grown NG on copper foil. (B) Astransferred NG sheet on a SiO2/Si wafer. The scale unit of the ruler shown in (A) and (B) is in centimeter. (C) As-transferred NG on a quartz slide is highly transparent. The logo of Penn State underneath the NG sheet can be clearly seen. (D) Raman spectra of NG sheets synthesized with constant ammonia reaction time (10 min), but with different reaction
temperature. (E) Raman spectra of NG sheets synthesized with constant ammonia reaction temperature (800 C), but with different reaction time. 2. As synthesized NG and PG sheets monolayer coverage. In order to estimate the uniformity of PG and NG, we performed Raman mapping on PG and NG. The Raman intensity ratio between the graphene 2D-band versus the G-band (I2D/IG) is plotted in fig. S2. From fig. S2A, it can be observed that most of the areas are green-yellow colored, indicating an I2D/IG ratio of 3 or above, and that monolayer graphene (PG) is of high crystalline quality. The blue region shows I2D/IG around 2 and only very few black points gives I2D/IG equal to 1. It can be calculated that more than 94% of the areas have I2D/IG ratio equal to 2 and above, meaning that less than 6% of the surface corresponds to few-layer graphene, which is very similar to NG (fig. S2B). fig. S2. Raman mapping corresponding to the intensity ratio between the graphene 2Dband and the G-band (I2D/IG) for (A) PG and (B) NG. Most of the areas are green-yellow colored, indicating an I2D/IG ratio of 3 or above, and that monolayer graphene is of high crystalline quality. The blue region shows I2D/IG around 2 and only the very few black points gives I2D/IG equal to 1. More than 94% of the area has I2D/IG ratio equal to 2 and above, meaning that less than 6% of the surface consists of few layer graphene.
3. Nitrogen concentration in NG synthesized at 850 o C evaluated by X-ray photoelectron spectroscopy Figure S3 shows C 1s and N 1s region of XPS spectra. As a result of quantization based on XPS, N content was found to be 2.25 at. % for NG sample synthesized at 850 o C. For C 1s region, three peaks were found. The one which has the highest peak intensity and located at 284.6 ev corresponds to graphite-like sp 2 C, whereas the other two (285.8 ev and 288.2 ev) can be attributed to C-N bonding as well as oxygen related functionality. By deconvoluting the peak at N 1s region, we found that most of nitrogen atoms were at substitutional nitrogen (400.6 ev) position rather than pyridinic (398.6 ev) position which consists with the STM measurement. fig. S3. XPS spectra (C1s and N1s) of NG synthesized at 850 o C. The quantification results based on XPS showing that the N content in the sample was around 2.25 at. %. The peak located at 284.6 ev corresponds to graphite-like sp 2 C while the other peaks (285.8 ev and 288.2 ev) in C 1s region can be attributed to C-N bonding as well as oxygen related functionality. The observed two peaks located at 400.6 ev and 398.6 ev in N 1s region, correspond to substitutional and pyridinic nitrogen, respectively.
4. Probing enhanced Raman scattering effect between NG and PG sheets for different dye molecules. Figure S4 presents the enhanced Raman scattering effect between NG and PG sheets for different types of dye molecules with the same concentration (510-5 mol/l). Figure S4A presents the structure of three molecules used for molecular sensing. Figure S4 (B-D) present the Raman spectra when RhB, CRV and MB are used to probe the sensing ability. It can be seen that with PG quenching the fluorescent background, the spectra presents vibrational peaks that correspond to some of the Raman fingerprints of these molecules. Interestingly, when NG sheets were used as substrate, the intensities of all the Raman peaks associated with these molecules are greatly improved and clearly resolved. Furthermore, some small Raman features that cannot be observed with PG as a substrate can now be clearly detected. The intensity of major Raman fingerprints for molecules are on average about 10 times stronger with NG sheets as substrate than with PG sheets. In this way, we can conclude that NG sheets could be considered as an excellent substrate for a unique type of molecular sensing.
fig. S4. Probing-enhanced Raman scattering effect between NG and PG sheets for different dye molecules. (A) Molecular structures of the dye molecules, Rhodamine B (RhB), Crystal Violet (CRV) and Methylene Blue (MB). Color scheme: gray=carbon, red=oxygen, blue=nitrogen, yellow=sulfur, white=hydrogen. The excitation laser lines are 2.41 ev for RhB and CRV, and 1.92 ev for MB. Raman signals of (B) RhB, (C) CRV and (D) MB molecules on PG and NG sheets are shown, respectively. Beyond the typical Raman features (D, G and 2D peaks), additional features correspondent to the Raman signal of the molecules are observed. The inset in (B), (C) and (D) represent the structure of the RhB, CRV, and MB, respectively.
5. Comparison of Raman spectra with bare SiO2/Si substrates and NG for probing different dye molecules at their resonant condition. fig. S5. Comparison of Raman spectra with bare SiO2/Si substrates and NG for probing different dye molecules at their resonant condition. (A, C, and E) shows the Raman
spectra when dye molecules are on top of bare SiO2/Si sheets. (B, D, and F) shows Raman spectra when dye molecules are on top of NG sheets. It is clearly observed that with bare SiO2/Si substrates, Raman spectra presents a huge fluorescent signal and no Raman peak for dye molecules can be observed, while with NG sheets, it quenches the fluorescent signal and Raman signal from dye molecules get enhanced. This demonstrates that the enhancement of the Raman signal is not only the resonance of each dye but from the NG substrate. 6. Comparison of enhancement factor (EF) of NG and PG for different dye molecules. fig. S6. Enhancement factors for three different dye molecules between NG and PG: A) RhB, B) CRV, and C) MB. It is clear that Raman intensities are larger for NG in all cases and range between 2 and 16 depending on the dye molecules and Raman peaks.
7. Comparison of GERS and SERS effects by applying sputtered Au nanoparticles on SiO2 as SERS substrates. fig. S7. Raman spectra comparing MB deposited on NG, PG, and Au nanoparticles. The peak marked as "*" is Si substrate peak. It can be clearly observed that NG performs around 10 times and PG performs around 5 times better than Au nanoparticles. 8. Testing the molecular sensibility of the NG with different laser excitation energies. Figure S8 (A to C) presents the Raman intensity ratio between the highest molecule Raman peaks (1650 cm -1 for RhB, 1625 cm -1 for CRV and 1620 cm -1 for MB) and the graphene G- band. It can be clearly observed that for certain molecules on top of NG sheets, the intensity of Raman fingerprint of those molecules will be most enhanced with certain laser excitation lines, while smaller or no enhancement can be detected with other laser lines.
fig. S8. Testing the molecular sensibility of the NG with different laser excitation energies. It shows the Raman intensity ratio between the strongest Raman peak of (A) RhB 1650 cm -1, (B) CRV 1625 cm -1 and (C) MB 1620 cm -1, and the graphene G-band. It can be clearly observed from the graph that 2.41 ev laser line enhances the Raman signal of RhB and CRV the most, while 1.92 ev laser line enhances the Raman signal of MB the most. 9. Photos of RhB in ethanol solutions with different concentration fig. S9. Photos of RhB in ethanol solution with different concentrations. It should be noted that when the concentration of RhB is below 10-8 mol/l, the solution is almost transparent limiting the detection of the molecules by eyes. 10. AFM images of NG samples with different concentrations of RhB showing clustering effect at high concentration. Our tested dyes are salts, and their interactions are strong, so that when deposited on a surface they will usually diffuse and cluster together. The electronic structure of an aggregate is different from that of a single molecule, as is well documented in the literature. In order to
better understand the clustering effect, we have performed AFM studies of NG samples with different concentrations of RhB. AFM images of NG sample with 1 x 10-5 mol/l RhB is shown in fig. S10A while fig. S10B represents 1 x 10-8 mol/l concentration of RhB on top of NG. It can be seen that even if we have rinsed the sample with ethanol after soaking, there are still many clusters remaining for the high dye concentrations (1 x 10-5 mol/l RhB dyes), while this clustering effect decreases significantly when the concentration drops to 1 x 10-8 mol/l. Therefore, the rinsing does not remove all the clusters that contribute to the diminishing of the Raman signal of the molecules. fig. S10. AFM images of NG samples with different concentrations of RhB. (A) 1 x 10-5 mol/l concentration RhB, and (B) 1 x 10-8 mol/l concentration RhB. It can be observed that even after rinsing the sample with ethanol followed by solution soaking, many clusters remain for the high dye concentrations (e.g. 1 x 10-5 mol/l RhB). Note that the clustering effect decreases significantly when the concentration drops to 1 x 10-8 mol/l. 11. Raman Spectrum of 5x10-12 mol/l RhB on NG. Figure S11 demonstrates the Raman spectrum of 5x10-12 mol/l concentration of RhB on top of NG. In this spectrum, we observed that only one peak of RhB can be clearly observed (marked with a black arrow), and its intensity is very low compared to any graphene Raman peak (1/10 of the intensity when compared to the graphene G-band.), and very close to the noise level. In this context, we assumed this could not be considered as a good spectrum to
detect molecules, so we draw the conclusion that the absolute detection limit of RhB for our NG sample is 5x10-11 mol/l. fig. S11. Raman spectrum of 5 10 12 M RhB on NG sheets. The laser excitation line is 2.41 ev and the integration time is 10 s, where the arrows indicate RhB peak. 12. The HOMO-LUMO gap of molecule with resonant Raman laser excitation energy Table S1 summarizes the value of calculated HOMO-LUMO gap of each molecule, experiment adsorption peak for each molecule and the laser excitation energy when resonant Raman happens with each molecule performed in this work (Fig. 3, fig. S8). It should be noted that the Raman sensing is the strongest when the laser energy is close to the molecular HOMO-LUMO gap. This provides a possible way to determine the HOMO-LUMO gap of a molecule simply by performing resonant Raman measurements using graphene as substrate.
table S1. Calculated HOMO-LUMO gap, adsorption data, and resonant Raman laser excitation energy for each molecule. Dye Molecule HOMO-LUMO gap on PG (calculated) HOMO-LUMO gap on NG (calculated) HOMO-E F gap on NG (calculated) Adsorption peak Resonant Laser excitation energy RhB 2.73 ev 2.65 ev 2.39 ev 2.29 ev 2.41 ev CRV 2.62 ev 2.61 ev 2.41 ev 2.11 ev 2.41 ev MB 2.48 ev 2.38 ev 2.22 ev 1.89 ev 1.92 ev
13. Probing enhanced Raman scattering effect between NG and PG sheets for additional molecules such as R6G and PPP. fig. S12. Raman spectra comparing R6G and PPP molecules on NG and PG using different concentrations (1 10 7 mol/l and 1 10 8 mol/l). We noted that NG performs 6 times better than PG, while for PPP, NG performs 2 times better. The spectra are normalized by graphene 2D-band.