Article pubs.acs.org/JPCC
Two-Dimensional Nitrogen-Enriched Carbon Nanosheets with Surface-Enhanced Raman Scattering Ya-Sen Sun,* Chien-Fu Lin, and Shih-Ting Luo Department of Chemical and Materials Engineering, National Central University, Taoyuan 32001, Taiwan S Supporting Information *
ABSTRACT: We have fabricated two-dimensional nitrogenenriched carbon nanosheets (2D-NECNs) through the pyrolysis of cross-linked poly(4-vinylpyridine) homopolymers as a platform for detecting physically absorbed dye molecules using Raman-scattering spectra. Upon pyrolysis, a polymeric layer consisting of pyridinic rings was converted into a carbonized nanosheet enriched with pyridinic nitrogen (N6), pyrrolic nitrogen (N5), graphitic nitrogen (GN), and nitrogen oxide (NO) groups, the fractions of which were finely controlled through pyrolysis at temperatures selected in the range of 430−550 °C. The effects of temperature on the formation of nitrogen- and carbon-containing species in 2D-NECN were examined by XPS, which showed that N6 and N5 were the dominant species over GN and NO at 430 °C. Increasing the temperature of pyrolysis produced carbonized nanosheets containing more GN and NO generated at the expense of pyridinic groups. Using rhodamine 6G (R6G) and crystal violet (CV) molecules as probes for Raman measurements, we found that the Raman enhancement on 2D-NECN is due to a chemical mechanism (CM) and that the observed enhancement of the Raman intensity of molecules adsorbed on 2D-NECN hence shows a clear dependence on the nitrogen configuration of the four types. Among the nitrogen species, GN dominates the large enhancement. The chemical Raman enhancement is ascribed to the ability of GN to improve the π-conjugated domains and narrow the energy gap in 2D-NECN.
■
INTRODUCTION Raman spectroscopy provides a promising means to detect and identify trace species,1−3 but conventional Raman inelastic scattering spectra exhibit extremely weak signals because of small differential Raman cross sections (dσRaman/dΩ), typically in the range of 10−30−10−25 cm2 sr−1.4,5 For dye molecules, the incident laser light under a resonance condition inevitably produces strong fluorescence (FL) signals. As the FL cross section (dσFL/dΩ ≈ 10−16 cm2 sr−1) is much greater than the corresponding Raman cross section in resonance, extremely weak Raman signals become submerged in the FL background of the spectrum.4,5 This shortcoming increases the difficulty in the analysis of Raman data and hinders the practical use of the technique in the realm of molecular sensing. To address this issue, additional substrates, such as nanostructured metals (copper, silver, and gold nanoparticles)6−9 and semiconductors (silicon and germanium nanostructures),10,11 are desired, because these substrates can magnify the Raman signals immensely, increasing the intensity through an electromagnetic mechanism (EM) and a chemical mechanism (CM).2,3,10 The EM is based on the surface plasmon resonance excited by the incident light amplifying the intensity of the surface-enhanced Raman scattering (SERS) by a factor up to 1010.2,3 The CM is based on a modification of the Raman polarizability tensor upon adsorption of a molecule on the surface of the nanostructure. For comparison, the magnitude of the CM rarely exceeds a factor ∼10.3 Generally, the EM and CM © 2017 American Chemical Society
concurrently enhance Raman signals of organic molecules on nanostructured metals with disparate degrees of influence, whereas only the CM is responsible for the SERS of molecules that undergo physical adsorption or chemical bonding on nanostructured semiconductors through charge transfer (CT).1−3 Recent studies have demonstrated that carbon nanomaterials such as graphene sheets, graphene quantum dots, and sheets of reduced graphene oxide and oxidized graphene can promote efficient charge transfer and also enable a CM-driven SERS effect.1,12−20 The basic idea is that the doping of carbon nanomaterials with heteroatoms (oxygen13,14 or nitrogen17) can cause a modulation of the electronic structures of the nanomaterials to tune the Raman-scattering signal of an adsorbed molecule. Recent efforts have led to the development of heteroatom-doped carbon nanomaterials as efficient substrates for SERS.1,12−20 The improved Raman performance of heteroatom-doped carbon SERS substrates was attributed to the CM through charge transfer between target molecules and the carbon-based substrate. Our previous work demonstrated that hierarchical porous carbon (HPC) nanostructures with bundles of aggregated nanospheres and with nitrogen-rich functional groups fabricated through the pyrolysis of crossReceived: March 27, 2017 Revised: May 9, 2017 Published: June 1, 2017 14795
DOI: 10.1021/acs.jpcc.7b02913 J. Phys. Chem. C 2017, 121, 14795−14802
Article
The Journal of Physical Chemistry C
Figure 1. (a) SEM image (top view), (b) high-resolution TEM image, and (c) magnified HR-TEM image to visualize lattice fringes within a graphitic nanograin for sample 2D-NECN500.
linked polystyrene-block-poly(x-vinylpyridine) (PS-b-PxVP, x = 2 or 4) could serve as substrates with considerable specific surface areas for SERS.21 The abundant nitrogen atoms terminating the surface of HPC nanostructures enables the SERS signals to attain intensities greater than those achievable with bare graphene sheets. The improved Raman performance of the HPC nanostructures was attributed to surface area, graphitic domains, and nitrogen sites. Nevertheless, several important issues remain to be addressed, such as the yields of carbon-containing and nitrogen-containing solids, the nitrogen configuration and content, and the crystallinity of carbonized thin films. Herein, we report a study of two-dimensional nitrogenenriched carbon nanosheets (2D-NECNs) obtained through the pyrolysis of cross-linked poly(4-vinylpyridine) (P4VP) layers on top of a SiOx/Si substrate at a pyrolysis temperature (Tp) selected from within the window of 430−550 °C. The 2DNECN samples had varying nitrogen configurations and contents, both of which depended on Tp. Two dye molecules, R6G and CV, were then deposited on top of the 2D-NECN samples as probes for the measurement of Raman spectra. The 2D-NECNs were used as a systematic basis for an analysis of the impact of the nitrogen configuration and content as functions of Tp on the Raman performance.
copy (HR-TEM; JEM2100, 200 kV). UV−vis absorption spectra of 2D-NECNs supported on quartz were recorded with a UV−vis/near-IR spectrophotometer (JASCO Analytical Instruments, V-670). X-ray microanalysis was performed with an X-ray photoelectron spectroscopy (XPS) system (Thermal VG-Scientific, Sigma Probe, Al Kα radiation) at the Instrument Center of National Central University (NCU). Raman Measurements. R6G molecules (Aldrich) were deposited at various surface coverages onto 2D-NECN-covered SiOx/Si substrates by drop-casting from aqueous solutions of R6G with concentrations ranging from 5 × 10−4 to 10−7 M. After the samples had been allowed to dry naturally, excess free molecules were removed by rinsing with deionized (DI) water, and the substrates were then dried under flowing N2 to avoid the fluorescence signals excited with the green laser light at 532 nm. CV molecules of constant surface coverage, as prepared by drop-casting an aqueous solution (10−5 M) onto SiOx/Si, served as a second probe molecule for Raman measurements. Because of the small fluorescence background,5 the samples with CV molecules were not rinsed with DI water after dropcasting and drying. A Raman microscope (UniRAM system) was used to detect the Raman inelastic scattering signals. A DPSS laser (75 μW before the microscope objective, low-noise, 532 nm) was used to excited the samples with CV and R6G molecules through a 100× (NA = 0.8) objective lens with a laser spot diameter of ∼1 μm. The acquisition period of each Raman spectrum (spectral resolution = 1.2 cm−1) was 10 s with a cooled CCD detection system (DV401-BV, ANDOR) and a grating (1200 grooves per mm). Spectra were averaged from at least three scans. The vibrational band of the silicon wafer at 520 cm−1 was used for wavenumber calibration.
■
EXPERIMENTAL SECTION Preparation of 2D-NECN Materials. Poly(4-vinylpyridine) (Aldrich) with a weight-average molecular weight of 160 000 g/mol was used as received. P4VP powders were dissolved in butanol to prepare solutions (0.3 mass %). The preparation of substrates covered by 2D-NECNs began with the pyrolysis of a cross-linked P4VP layer on top of a silicon substrate with an oxide layer (SiOx/Si, 300 nm) at elevated temperature in a tubular furnace filled with argon gas. A layer of P4VP (thickness of ca. 5.9 nm) was spin-coated from a P4VP solution (0.3 mass %) in butanol onto a SiOx/Si substrate and subsequently exposed to UV irradiation in nitrogen gas (UVIN) for 6 h. To fabricate the 2D-NECN materials, we then pyrolyzed the cross-linked P4VP layers at a target temperature within the range of 430−550 °C for 1 h under an argon atmosphere. The details of the UV treatments and pyrolysis are described elsewhere.22−24 Apparatus and Characterization of 2D-NECN Materials. The 2D-NECN materials were characterized by atomic force microscopy (AFM; Seiko SPA400) in tapping mode, high-resolution field-emission scanning electron microscopy (SEM; JEOL JSM-7600F) using a field-emission source (energy = 10 kV), and high-resolution transmission electron micros-
■
RESULTS AND DISCUSSION Characterization of 2D-NECN Substrates. The preparation of substrates covered with 2D-NECNs began with pyrolysis of a cross-linked P4VP layer on top of a silicon substrate with an oxide layer (300 nm) (SiOx/Si) at elevated temperature in a tubular furnace filled with argon gas. A layer of P4VP (thickness of ca. 5.9 nm) was spin-coated from a P4VP solution (0.3 mass %) in butanol onto the SiOx/Si substrate and subsequently exposed to UVIN for 6 h. The UVIN treatment not only caused inter- and intrachain cross-linking of the P4VP chains through chain scission to form free radicals and recombination of the macroradicals but also strengthened the adhesive force between the polymer layer and the underlying substrate.22−25 Improved interfacial adhesion was essential for the formation of a nitrogen-enriched carbon layer, as pyrolysis was conducted at temperatures above the decomposition 14796
DOI: 10.1021/acs.jpcc.7b02913 J. Phys. Chem. C 2017, 121, 14795−14802
Article
The Journal of Physical Chemistry C
Figure 2. (a) Raman spectra (excited at 532 nm) of 2D-NECN samples supported on a SiOx/Si substrate fabricated from cross-linked P4VP layers subjected to pyrolysis at various temperatures (1 h). The peak marked with an asterisk (*) is from the SiOx/Si substrate; its intensity served as a reference for the normalization of ID and IG. (b) Variation with Tp of normalized values of ID (blue squares), IG (blue triangles), and ID/IG (black circles) for 2D-NECN.
temperature (Td) of the polymer.21,23,24 As a result, UVIN (6 h) significantly improved the solid yields of carbon-containing and nitrogen-containing species. Upon pyrolysis at a constant Tp selected from within the range of 430−550 °C, the layer thickness of the carbon materials, estimated by analysis of the cross-sectional height profiles of the AFM images, was in the range of approximately 0.8−3.8 nm (Figure S1). The layer thickness depended on Tp: As Tp increased within the studied range, the layer thickness decreased. At 550 °C, the cross-linked P4VP layer was significantly degraded, and the chain-scission reaction was dominant, so that the thickness of the sample obtained at that temperature (denoted as 2D-NECN550) was not uniform on top of the SiOx/Si substrate. For brevity, the assynthesized samples are labeled 2D-NECN as a whole or 2DNECNTp if the pyrolysis temperature is specified. Figure 1 shows representative SEM and HR-TEM images of 2DNECN500 for structural characterization. Figure 1a shows that the surface of 2D-NECN500 displayed many small cracks. The presence of these cracks is ascribed to the generation of gaseous products through chain scission and dehydrogenation during pyrolysis: As the gaseous products evaporated, cracks were generated. To understand the graphitic extent, we further selectively analyzed a single graphitic granule (approximate size of a few nanometers). Examination of the HR-TEM image indicates that the graphitic grain crystal had lattice fringes with an average interlattice distance of 0.336 nm (Figure 1c).21 This result indicates that pyrolysis at 500 °C converted the crosslinked P4VP layer into a carbonized layer with polycrystalline graphitic nanograins. The polycrystalline crystallites were embedded within the amorphous carbon matrix. Next, the effects of pyrolysis temperature on the degree of graphitization of the 2D-NECN samples were assessed through the detection of Raman signals (excitation at 532 nm). Figure 2 shows Raman spectra for 2D-NECN samples fabricated at various pyrolysis temperatures (Tp): Two broad distinct bands were observed at 1487 and 1583 cm−1, corresponding to the D and G bands, respectively. The former band is a breathing mode (A1g symmetry) of 6-fold aromatic rings; the latter band is an in-plane bond-stretching vibrational mode of pairs of trigonally bonded C atoms (E2g symmetry26−28). The presence of the two bands has been proposed to be a prominent feature of carbon domains.26−28 Their Raman positions remained independent of Tp, but their relative Raman intensities varied with Tp. On examining the relative intensities of the two bands, one can see a temperature-dependent variation in the degree of
graphitization as well as in the solid yield. In Figure 2, 2DNECN430 exhibits D and G bands with low relative intensities, whereas both 2D-NECN450 and 2D-NECN480 display D and G bands with increased intensities. Further increasing T p produced 2D-NECN samples that exhibited a decreasing trend in the relative intensities of the D and G bands. Two factors determine the relative intensities of the D and G bands: the degree of graphitization and the solid residue. Pyrolysis was demonstrated to cause the conversion of a polymeric substance into a carbon-containing residue after UVIN was imposed to cross-link the polymers.21,23,24,29 The structural characterization with HR-TEM demonstrated that the solid residue contained mixtures of graphitic nanograins and domains of amorphous carbon. We propose that these graphitic nanograins were composed of trigonally bonded carbons with varying degrees of graphitic ordering, whereas the amorphous carbon domains comprised mainly tetrahedrally coordinated sites. Such a carbonaceous solid residue displays an additional prominent feature in the Raman spectrum; that is, in addition to the G and D bands, a small modulated bump from ∼2400 to 3300 cm−1 was observed (Figure S2), unlike in the case of neat graphite, which displays well-defined second-order Raman signals in the same large-wavenumber region.13,14 As the carbonaceous solid contained a mixture of trigonal and tetrahedral sites, increasing numbers of defects would diminish the number of ordered rings, thus resulting in a decreased intensity of the D band (ID). As a result, the intensity ratio of the D band to the G band, ID/ IG, can be taken to indicate the size (Lg) of graphitic nanodomains with ordered rings; that is27,28 ID/IG ∝ Lg 2
(1)
Equation 1 indicates that the square of Lg is proportional to ID/IG. We analyzed the intensity ratio further to understand how Tp influenced the degree of graphitization; Figure 2b shows that the ID/IG ratio increased with Tp. This result indicates that the size of the graphitic nanodomains increased with increasing Tp. Hence, as Tp increased, the degree of graphitization increased. In contrast, the solid residue behaved differently and exhibited a decreasing trend with Tp. The thickness of the 2D-NECNs can serve as an indication of the solid yield. Figure S1 shows that the solid residue decreased with increasing Tp. We recorded XPS survey spectra of the 2D-NECN materials to trace the residual yields of carbon-containing and nitrogencontaining species. Figure S3 shows that two XPS peaks were 14797
DOI: 10.1021/acs.jpcc.7b02913 J. Phys. Chem. C 2017, 121, 14795−14802
Article
The Journal of Physical Chemistry C
Figure 3. (a,b) Nitrogen 1s XPS spectra of (a) 2D-NECN430 and (b) 2D-NECN530. The fitted peaks correspond to N6 (red), N5 (navy), GN (green) and NO (magenta). (c) Relative contents of four types of nitrogen configurations and (d) C/N ratio in 2D-NECN as functions of pyrolysis temperature.
varying contents. Under mild pyrolysis at Tp = 430 °C, P4VP decomposed to produce a mixture of multiple components, of which N6 and N5 were dominant species whereas GN and NO were minor species. Upon pyrolysis at various Tp, whereas both GN and NO gradually increased with increasing Tp, N5 behaved differently. Specifically, N5 decreased with increasing Tp when pyrolysis was carried out at temperatures less than or equal to 500 °C. Nevertheless, N5 decreased instead at Tp = 530 °C. Figure 3d shows Tp-dependent carbon/nitrogen (C/N) atomic ratios in the 2D-NECNs, where it can be seen that the C/N ratio increased with Tp from 12.75 to 18.53. The results indicate that, at Tp = 530 °C, for example, nitrogen species were lost more than carbon species because of poor thermal stability. N5 species were demonstrated to grow simply through the decomposition of pyridinic rings, whereas the formation of GN species involved a more complicated process.30,31 For the latter case, several condensation reactions occurred through structural reorganization of N5 and N6 species and through denitrogenation reactions during pyrolysis.30,31 Pursuing this idea, we propose that the interplay among the reactions (i.e., chain scission, condensation, and denitrogenation) governs the evolution of nitrogen functionalities in the resultant 2DNECN materials. These reactions depend strongly on Tp but to different extents. The increased amount of N5 that grew at the expense of pyridinic rings in 2D-NECN530 is thus ascribed to a dominant chain scission reaction at 530 °C. This interpretation accounts also for the significant losses of carbon and nitrogen species in 2D-NECN550 (Figure S3). In comparison with the evolution of nitrogen functionalities, pyrolysis at Tp values varying in the range of 430−530 °C did not significantly influence the C 1s XPS spectra of the 2D-NECN materials prepared by pyrolysis in an inert environment (Figure S4). Thus, we did not detail and do not discuss the analysis of these data.
present, at approximately 285 and 400 eV. These two signals correspond to carbon 1s (C 1s) and nitrogen 1s (N 1s), respectively.30−32 A peak at 533 eV associated with oxygen 1s (O 1s) resulted mainly from the surface of the SiOx/Si substrate. A decreasing trend with Tp can be clearly observed for the peaks at 285 and 400 eV, indicating that the yields of carbon-containing and nitrogen-containing species decreased with increasing Tp. In particular, the solid yield was significantly degraded for 2D-NECN550, which displayed only a small intensity for the C 1s signal and no intensity for the N 1s signal. As a result, the interplay between the degree of graphitization and the carbonaceous yield accounts for the Tp-dependent variations in the relative intensities of the D and G bands. In addition to the temperature-dependent carbon crystallinity and solid yield, pyrolysis of the cross-linked P4VP layer at various elevated temperatures produced nitrogen configurations of four principal types [pyridinic nitrogen (N6), pyrrolic nitrogen (N5), graphitic nitrogen (GN), and nitrogen oxide (NO], the fractions of which also depended on Tp (Figure 3). Figure 3a shows representative N 1s XPS spectra for 2DNECN430 and 2D-NECN530. To make a quantitative analysis of N 1s XPS peaks in the range of interest, we deconvoluted the N 1s XPS spectra based on the four nitrogen contributions: N6 at 399.0 eV, N5 at 400.3 eV, GN at 401.4 eV, and NO at 403.0 eV.30,31 Note that NO denotes pyridine-N-oxide. 31 A quantitative analysis of the four forms designated in Figure 3a indicates that both N6 (43.8%) and N5 (42.9%) were more prevalent than GN (8.5%) and NO (4.8%) in 2D-NECN430. For comparison, the nitrogen configuration of 2D-NECN530 was dominated by N6 and GN, with relative contents 42.9% and 28.2%, respectively. Figure 3c summarizes the relative integrated intensities of these four spectral components as functions of Tp. Pyrolysis at most accessible Tp produced carbonized P4VP containing N6, N5, GN, and NO species with 14798
DOI: 10.1021/acs.jpcc.7b02913 J. Phys. Chem. C 2017, 121, 14795−14802
Article
The Journal of Physical Chemistry C
Figure 4. (a) Raman spectra (excited at 532 nm) of R6G on 2D-NECN samples fabricated by the pyrolysis of a cross-linked P4VP layer at various Tp on SiOx/Si; the spectra are offset vertically for clarity. The concentration of the R6G solution used for a submonolayer coverage of adsorbed R6G molecules was 10−5 M; rinsing with DI water followed. The position of the second-order silicon Raman line marked with an asterisk served as a reference for normalization of the intensity. (b) Normalized curves of relative Raman intensities of various vibrational modes of R6G on 2D-NECN samples.
Raman Spectra of Adsorbed R6G on 2D-NECN. Figure 4 shows Raman spectra (excited at 532 nm) of R6G dye molecules physically adsorbed at submonolayer coverage on the surface of the 2D-NECN-covered SiOx/Si substrates. The submonolayer coverage was prepared by drop-casting of an aqueous solution (10−5 M) followed by rinsing with DI water (Figure S5). Figure 4a shows that Raman bands at 613, 639, 660, 772, 1127, 1185, 1309, 1361, 1422, 1507, 1537, 1573, and 1648 cm−1 were identified for R6G adsorbed on 2D-NECN samples except for 2D-NECN550.33 The Raman spectra of R6G adsorbed on the 2D-NECN samples were consistent with those reported previously.11,33−36 The vibrational modes of R6G had relative intensities that varied in response to Tp. To observe clearly the variations in the relative intensities of the vibrational modes of R6G on 2D-NECN samples, we quantitatively compared the Raman-scattering profiles further by plotting the relative intensities as a function of vibrational mode after normalization with the band centered at 962.1 cm−1, which is generally regarded as the silicon signal and is unaffected by the vibrational bands of R6G at an identical density of submonolayer (Figure 4b). Estimates of the enhancement factors are shown in Figure S6. For R6G on 2D-NECN430, 2DNECN450, and 2D-NECN480, the Raman bands were weakly present even though the vibrational modes increasingly gained some relative intensity when the pyrolysis temperature for growing 2D-NECNs was gradually increased from 430 to 480 °C. The large-wavenumber bands of R6G were notably superimposed on the D and G bands of carbonized P4VP in the range of 1000−1800 cm−1, so that the intensities of those high-frequency bands appeared greater than for the lowfrequency bands below 1000 cm−1. For R6G on 2D-NECN500 and 2D-NECN530, the bands in the low-wavenumber region gained additional intensity. Because of the interference of the D and G bands of the 2D-NECNs in the Raman intensity, the high-wavenumber bands of R6G revealed a chaotic Raman improvement with Tp. In contrast, only the low-wavenumber bands below 1000 cm−1 showed an increasing trend of Raman enhancement with Tp. In this case, we did not compare the Raman performance of 2D-NECN550 because it showed only a broad fluorescence background without R6G Raman vibrational bands (Figure S7); such a fluorescence background is similar to that of R6G molecules on SiOx/Si.21 This result indicates that, on 2D-NECN550, the Raman signals of R6G would not be enhanced, nor would the fluorescence background be
quenched. Note that a rather large relative increase in the relative intensities of these two lines at 613 and 772 cm−1 could be observed for 2D-NECNS500. This is direct evidence of charge transfer in the enhancement mechanism.34 Considering that, in addition to fluorescence emission, molecular resonance Raman enhancement is involved in the Raman spectrum of R6G with strong absorption at 532 nm, CV with a poor quantum yield for fluorescence emission was thus also utilized as a probe molecule to investigate Raman spectra (excited at 532 nm). Because CV exhibits minimal absorption at 532 nm and a low FL baseline,5 the Raman spectra of CV molecules were collected without rinsing with DI water. Figure 5 shows Raman spectra of CV (10−5 M) on 2D-NECN430 and
Figure 5. Raman spectra (excited at 532 nm) of CV molecules on 2DNECN430 (blue curve), 2D-NECN500 (red curve), and bare SiOx/Si (black curve). The position of the second-order silicon Raman line is marked with an asterisk.
2D-NECN500, in comparison with the Raman spectra of CV on bare SiOx/Si. As can be seen, there are Raman signals for CV molecules on 2D-NECN430 and 2D-NECN500 at 567, 731, (761), 810, 914, 1177, 1297, 1444, 1480, 1535, 1584, and (1620) cm−1. The vibrational bands in parentheses indicate those typical of the totally symmetric (a1) modes.37 Characteristic signals of the nontotally symmetric (e) modes (at 567, 731, 810, 914, 1177, 1297, 1444, 1480, 1535, and 1584 cm−1) are seen to contribute a large fraction of the total enhancement. According to the Herzberg−Teller surface-selection rule,2,37 14799
DOI: 10.1021/acs.jpcc.7b02913 J. Phys. Chem. C 2017, 121, 14795−14802
Article
The Journal of Physical Chemistry C this result suggests that the chemical enhancement from charge transfer plays a key role in the enhancement effect. In a comparison of the Raman spectra of CV on the samples, 2DNECN500 gave greater Raman enhancement than 2D-NECN430, indicating the superior performance of 2D-NECN500. In contrast, under the same excitation conditions, the Raman signals of CV were present with minute intensities on SiOx/Si, so that only the second-order Raman-scattering band of silicon (marked with an asterisk) was clearly observable. In a previous work in which we investigated the Raman signals of adsorbed R6G molecules on hierarchical porous carbon nanostructures fabricated through pyrolysis of crosslinked block copolymer self-assembled nanodomains, we demonstrated that the P2VP or P4VP block plays an important role in CM-driven Raman enhancement.21 Such an enhancement is similar to that of heteroatom-doped (oxygen or nitrogen) graphene-based materials.13,14,17 The improved performance of heteroatom-doped graphene on molecular sensing through Raman spectra might be due to the dopinginduced charge redistribution that further induces a strong surface dipole−dipole interaction with the molecule and breaks the symmetry of the molecule. Such a symmetry-related perturbation leads to an increase of the transition probability, and thus, an increase of the Raman intensity occurs.1,20 This explanation is also responsible for the Raman enhancement observed in the present work. Comparisons of the Raman curves (Figures 4 and 5) of adsorbed R6G and CV molecules with the chemical compositions and graphitization of 2DNECN pyrolyzed at various Tp indicate that the molecular sensing performance might correlate with both the nitrogen configuration and content (Figure 3). Whereas 2D-NECN430 had the lowest extent of graphitization, the lowest content of GN, and the most N5, it exhibited the weakest Raman signals of adsorbed molecules. Note that neither pristine P4VP chains only with N6 nor carbon materials with high contents of N6 fabricated through pyrolysis at low temperature (
