**2. Nitrogen dopants for tuning the electronic and optical properties of graphene**

Graphene is an ideal platform for many optoelectronic devices due to its distinctive combina‐ tion of high electron mobility (μ), optical transparency, and gate/dopant-tunable carrier density [2]. However, to truly harness the potential of this combination and make graphenebased efficient optoelectronic devices a reality, optical and electronic properties of graphene must be tuned via substitutional doping [8]. While doping graphene with boron (B) or nitrogen (N) can tune the Fermi energy (*E*F) and lead to *p*- or *n*-type graphene, it also compromises the inherently high electron mobility in doped graphene, which is useful for electronic applications [11–13]. Furthermore, the recombination rate of photogenerated carriers is also known to decrease with the presence of defects and dopants limiting applications of graphene in optoelectronics [8]. Indeed, the focus of many synthesis efforts in graphene has been to achieve large-area "defect-free" graphene for electronic applications because carrier-defect scattering limits the electron mobility—an important parameter for high-speed electronics [14–17]. Contrary to this conventional wisdom, we recently demonstrated that the defect configuration (in particular, N-dopant configuration in bilayer graphene lattice shown in **Figure 1a**) is more important than the defect concentration for increasing carrier concentration without compro‐ mising photogenerated carrier lifetime [8].

in intriguing observations such as the quantum Hall effect at room temperatures, quantized optical transmittance, nonlocal hot carrier transport, and Klein tunneling [2, 3]. Despite such fundamental breakthroughs, the potential of 2D materials has not yet completely manifested into practical devices due to material limitations [1, 4]. For instance, the lack of a band gap resulted in serious limitations for using graphene in electronics [4]. Defects in material science and engineering are often perceived as performance limiters, but in the case of 2D materials, defect engineering could provide a way to overcome many roadblocks and forge new fron‐ tiers. In this regard, others and we have shown that defects in 2D materials (e.g., dopants, vacancies) can provide an excellent handle to control material properties [5–8]. Specifically, we have shown that defects such as vacancies and N dopants in graphene could be used to control the electron-electron and electron-phonon scattering pathways [8]. These results provided criticalbreakthroughsforimprovingthequantumcapacitanceofgrapheneanddopinggraphene without compromising its intrinsic characteristics [6]. Defects also play a vital role in improv‐ ing the properties of the so-called "beyond graphene" 2D materials. Previously, we used spark plasma sintering (SPS) to introduce charged grain boundaries (GB) in 2D Bi2Te3 for improving its thermoelectric (TE) figure of merit and compatibility factor [5]. Similarly, it has been demonstrated that defect engineering in 2D materials could improve many qualities ranging from electronic levels, conductivity, magnetism, and optics to structural mobility of disloca‐ tionsandcatalyticactivities[9,10].Asdiscussedinthischapter,defectengineeringin2Dmaterials leads to the discovery of potentially exotic properties, which can enable unprecedented technological applications. In particular, we present how dopants and defects in (i) graphene could be used for optical and electrochemical energy storage applications and (ii) 2D Bi2Te3

could be controlled for enhancing its thermoelectric efficiency.

84 Two-dimensional Materials - Synthesis, Characterization and Potential Applications

**graphene**

**2. Nitrogen dopants for tuning the electronic and optical properties of**

Graphene is an ideal platform for many optoelectronic devices due to its distinctive combina‐ tion of high electron mobility (μ), optical transparency, and gate/dopant-tunable carrier density [2]. However, to truly harness the potential of this combination and make graphenebased efficient optoelectronic devices a reality, optical and electronic properties of graphene must be tuned via substitutional doping [8]. While doping graphene with boron (B) or nitrogen (N) can tune the Fermi energy (*E*F) and lead to *p*- or *n*-type graphene, it also compromises the inherently high electron mobility in doped graphene, which is useful for electronic applications [11–13]. Furthermore, the recombination rate of photogenerated carriers is also known to decrease with the presence of defects and dopants limiting applications of graphene in optoelectronics [8]. Indeed, the focus of many synthesis efforts in graphene has been to achieve large-area "defect-free" graphene for electronic applications because carrier-defect scattering limits the electron mobility—an important parameter for high-speed electronics [14–17]. Contrary to this conventional wisdom, we recently demonstrated that the defect configuration (in particular, N-dopant configuration in bilayer graphene lattice shown in **Figure 1a**) is more

**Figure 1.** (a) Possible nitrogen dopant configurations in graphene lattice include pyridinic (green), pyrrolic (purple), and graphitic (red). While graphitic dopants (red) are purely substitutional, non-graphitic dopants (pyridinic and pyr‐ rolic) result in additional defects such as vacancies or pentagons. (b) A schematic representation showing the linear energy dispersion for single-layer graphene and the arrow indicates excitation of electrons from the valence to the con‐ duction band. (c) Under intense photoexcitation, the nonequilibrium carrier distribution (in the E-k space depicted by the energy dispersion shown in (b)) results in an initial rise in the transmitted intensity. The carriers (electron and hole distribution shown in blue and orange, respectively) equilibrate by carrier-carrier scattering on a timescale τ1. Subse‐ quently, the carrier thermalization and decay occur through carrier-phonon scattering on a timescale τ2.

The electronic and optical properties of a single-layer graphene (SLG) can be described in terms of massless Dirac fermions with linear dispersion near the Fermi energy (**Figure 1b**). The semimetallic nature and electronic band structure of SLG allow for the photogeneration of electron-hole pairs at any wavelength in the visible-light spectrum [3]. This property is critical for many wide-bandwidth optoelectronic applications. As shown in **Figure 1b**, incident light excites electrons from the valence band (orange) into the conduction band (purple). Shortly after photoexcitation, incident photon-electron interactions create an out-of-equilibrium electron distribution (purple in **Figure 1c**), which initially relaxes on an ultrafast timescale (τ<sup>1</sup> ~ 100–300 fs) to a hot Fermi-Dirac distribution and subsequently cools via phonon emission or defect scattering (τ2 ~ 1–2 ps) in graphene [8, 18]. In optoelectronic devices, when photoex‐ cited electrons are scattered by phonons or defects, energy transferred to the lattice is dissipated as heat decreasing the net energy transported through charge carriers to drive a circuit. In the current scenario of graphene optoelectronic devices, a critical challenge is to increase the net charge carrier density and quench electron-defect relaxation pathways to extend photogenerated carrier lifetime.

The influence of defects on photogenerated carriers could be accounted by the inclusion of an extra term (A) in the expression for carrier scattering rate τ−1 (N) = A + BN + CN2 , where A represents nonradiative recombination, usually due to defects or traps, B represents radiative recombination, N is the carrier density, and C represents Auger recombination. The photo‐ generated carriers are quickly cooled to ground state through scattering by defects (repre‐ sented by A) in addition to the existing carrier-carrier and carrier-phonon scattering (term B) and Auger recombination (term C). In the context of optoelectronic applications, it is impera‐ tive to identify ideal dopant concentration and configuration in graphene for which A in the carrier scattering rate equation is minimized. As mentioned earlier, heteroatomic doping can tune *E*<sup>F</sup> and alter density of states (DOS) and thereby modify its electronic and optical prop‐ erties. For instance, N-doping in chemical vapor deposition (CVD) graphene resulted in a bandgap due to the suppression of electronic density of states near the Fermi level and a consequent reduction in μ [19, 20]. As mentioned earlier, it is well known that N atoms can be substitutionally doped in the graphene lattice either in the pyridinic, pyrrolic, or graphitic configurations (see **Figure 1a**). Our density functional theory (DFT) calculations previously showed that all the three dopant configurations (viz., pyridinic, pyrrolic, and graphitic) are stable structures with a positive energy (>9.5 eV) released during the formation with graphitic dopants exhibiting the highest stability (**Figure 2**) [8].

**Figure 2.** The lattice structure (top three panels) of graphitic (a), pyridinic (b), and pyrrolic (c) defects in graphene along with their electron density, obtained from our DFT calculations, shown in the bottom panels. The energy re‐ leased on the formation of structure from free atoms for all the configurations was found to be positive (graphitic, 10.22 eV; pyridinic, 9.77 eV; and pyrrolic, 9.55 eV) confirming the stability of N-doped configurations.

#### **2.1. CVD synthesis of N-graphene**

Previously, we employed atmospheric pressure chemical vapor deposition (CVD) method for growing N-graphene [7]. This CVD set up consisted of a Cu foil loaded inside a 1 in. quartz tube at a temperature of 1,000 °C. Methane gas was used as the carbon source for graphene growth, while acetonitrile (AN) and benzylamine (BA) were used as precursors for N dopants in varying concentrations. The reaction was carried out under inert atmosphere by passing a mixture of Ar and H2 through the quartz tube reaction chamber. In particular, 450 sccm of Ar and 50 sccm of H2 were used, and 2 sccm of methane was bubbled through the mixture of BA and AN. The volume percent of BA and AN varied in the ratio of 0:1, 1:1, and 3:1. Accordingly, the obtained samples were labeled S1, S2, and S3, respectively. Interestingly, we found that the N-dopant configuration (viz., graphitic, pyrrolic, and pyridinic) could be controlled using the ratio of BA to AN precursors.

### **2.2. The effects of N dopants in graphene: X-ray and Raman spectroscopy**

The influence of defects on photogenerated carriers could be accounted by the inclusion of an

represents nonradiative recombination, usually due to defects or traps, B represents radiative recombination, N is the carrier density, and C represents Auger recombination. The photo‐ generated carriers are quickly cooled to ground state through scattering by defects (repre‐ sented by A) in addition to the existing carrier-carrier and carrier-phonon scattering (term B) and Auger recombination (term C). In the context of optoelectronic applications, it is impera‐ tive to identify ideal dopant concentration and configuration in graphene for which A in the carrier scattering rate equation is minimized. As mentioned earlier, heteroatomic doping can tune *E*<sup>F</sup> and alter density of states (DOS) and thereby modify its electronic and optical prop‐ erties. For instance, N-doping in chemical vapor deposition (CVD) graphene resulted in a bandgap due to the suppression of electronic density of states near the Fermi level and a consequent reduction in μ [19, 20]. As mentioned earlier, it is well known that N atoms can be substitutionally doped in the graphene lattice either in the pyridinic, pyrrolic, or graphitic configurations (see **Figure 1a**). Our density functional theory (DFT) calculations previously showed that all the three dopant configurations (viz., pyridinic, pyrrolic, and graphitic) are stable structures with a positive energy (>9.5 eV) released during the formation with graphitic

**Figure 2.** The lattice structure (top three panels) of graphitic (a), pyridinic (b), and pyrrolic (c) defects in graphene along with their electron density, obtained from our DFT calculations, shown in the bottom panels. The energy re‐ leased on the formation of structure from free atoms for all the configurations was found to be positive (graphitic,

Previously, we employed atmospheric pressure chemical vapor deposition (CVD) method for growing N-graphene [7]. This CVD set up consisted of a Cu foil loaded inside a 1 in. quartz tube at a temperature of 1,000 °C. Methane gas was used as the carbon source for graphene growth, while acetonitrile (AN) and benzylamine (BA) were used as precursors for N dopants in varying concentrations. The reaction was carried out under inert atmosphere by passing a

10.22 eV; pyridinic, 9.77 eV; and pyrrolic, 9.55 eV) confirming the stability of N-doped configurations.

, where A

extra term (A) in the expression for carrier scattering rate τ−1 (N) = A + BN + CN2

86 Two-dimensional Materials - Synthesis, Characterization and Potential Applications

dopants exhibiting the highest stability (**Figure 2**) [8].

**2.1. CVD synthesis of N-graphene**

We observed a strong correlation between the N-dopant configuration and the accompanying vibrational properties of N-doped CVD graphene: the N atoms bonded in the non-graphitic configurations (pyridinic and pyrrolic, observed using X-ray photoelectron spectroscopy or XPS) resulted in intense Raman disorder bands unlike the N atoms bonded in the graphitic configuration, even though the concentration of N dopants was higher in the latter case [7].

As shown in **Figure 3a**, we identified XPS peaks corresponding to graphitic, pyrrolic, and pyridinic configurations [21]. For pyridinic configuration, the N1s peak positions reported in the literature are usually in the range 398.1–399.3 eV. Similarly, pyrrolic configuration gives rise to peaks in the range 399.8–401.2 eV, while the peak around 400.5 eV (blue colored) is associated with the graphitic configuration. The orange-colored peak at ~401.5 and 406 eV may be attributed to different nitrogenated adsorbents [20, 21]. XPS results confirmed that the atomic percentages of nitrogen in S1, S2, and S3 were 0.2, 2.5, and 3.8 %, respectively [7]. It is important to note that S1 and S3 showed more non-graphitic N dopants com‐ pared to S2, which was purely graphitic doping. In order to further understand the effect of various nitrogen-doping configurations and concentrations on the electronic structure of graphene, we performed Raman spectroscopy of the samples S1, S2, and S3. The Raman spectrum of graphene displays four important bands [22, 23]: (i) the disorder or D band appears ~1350 cm−1 due to the presence of defects such as edges, grain boundaries, or any other type of defects including dopants in the graphene lattice; (ii) in some studies, researchers have also reported the presence of D′-band ~1600–1625 cm−1 in the Raman spectrum of highly disordered graphene [22]; (iii) the graphitic G band ~ 1585 cm−1 arises due to doubly degenerate optical phonon modes at the Brillouin zone center. It is a firstorder Raman scattering process, and (iv) the 2D band ~ 2700 cm−1 is a consequence of secondorder Raman scattering process involving intervalley scattering of in-plane transverse optical (*iTO*) phonons. Unlike the D band, which requires the presence of a defect to conserve the momentum of scattered electron along with an *iTO* phonon, 2D band does not require the presence of any defect due to the involvement of two *iTO* phonons and is always present in Raman spectra of both pristine and doped graphene [22]. The shape and width of the 2D band are sensitive to the number of layers in graphene. For example, 2D band in single-layer graphene (SLG) can be fit into a single Lorentzian peak, while for bilayer graphene, 2D band can be deconvoluted into four sub-peaks [23].

As seen in **Figure 3b**, the Raman spectra of pristine graphene samples did not exhibit strong D band in our studies. While samples S1 and S3, which contain non-graphitic doping config‐ uration of nitrogen, showed strong D bands, the D band in sample S2 (graphitic) is similar to that in pristine sample despite higher dopant concentration (~2.5 %). These results are consistent with our observations in the XPS spectra shown in **Figure 3a**. When nitrogen atoms enter the graphene lattice in non-graphitic configuration, vacancies are needed and result in armchair-type edges. Previous reports showed that armchair edges in graphene allow intervalley scattering of *iTO* phonons in the Brillouin zone unlike zigzag edges [22] and thus increase the intensity of the D band as in samples S1 and S3.

**Figure 3.** (a) XPS spectra for N1s line. The colored peaks represent the deconvolution of N1s peaks. The resolution of the spectrometer was 0.5 eV. The peak at 400.5 eV corresponds to graphitic configuration. Peaks at 398 and 400 eV correspond to non-graphitic. Raman spectra of pristine and N-graphene are shown in (b) and (c). As seen in (b), the D and D′ bands increase in intensity for non-graphitic samples S1 and S3. The deconvolution of 2D band in (c) suggests that graphene samples are bilayer.

From the line-shape analysis of Raman 2D band (**Figure 3c**), we confirmed that our CVD-grown graphene samples are predominantly bilayers. As seen in **Figure 3c**, maximum downshift in 2D band (25 cm−1) was observed for sample S3 with relatively large dopant percentage (~3.5 %). On the other hand, sample S2 (graphitic configuration) showed little downshift in 2D band compared to sample S1 in spite of having higher dopant concentration. 2D band in S1 showed a downshift of ~10–15 cm−1 even in the presence of low dopant concentrations (~0.2 %). These differences in the 2D band shift in the Raman spectra can also be attributed to the nature of the dopant environment. For example, in samples S1 and S3 that are non-graphitic in nature, due to lattice symmetry breaking, electronic structure of graphene is strongly perturbed leading to possible renormalization of electron and phonon energies. Such a renormalization in electron energies results in a concomitant downshift in phonon energies of 2D band [7].

#### **2.3. Nonlinear optical studies of N-graphene**

We further explored the influence of defects on the carrier scattering rate using pump-probe (PP) spectroscopy [8]. The differential transmittance (Δ*T*/*T*) was obtained by taking the ratio of pump-induced change in the probe transmittance (Δ*T*) at a time *t* after the pump excitation to the probe transmittance (*T*) in the absence of a pump (**Figure 4a**). The initial response is an incident pulse-width-limited rise in the transmitted signal immediately after the zero delay (*t* = 0), which eventually decays in an exponential manner. The best fit to the PP data was obtained with a bi-exponentially decaying function, ∆*T*/*T* = *A*1 exp(−*t*/*τ*1) + *A*2 exp(−*t*/*τ*2) with two distinct time scales: a fast component (*τ*1) corresponding to the intraband carrier-carrier scattering and a slower component (*τ*2) corresponding to carrier-phonon scattering (discussed before in **Figure 1b**). Both *τ*1 and *τ*2 values obtained from the best-fitted curves for the samples are tabulated in **Figure 4b**. As shown in **Figure 4b**, the carrier-carrier and carrier-phonon relaxation times do not decrease monotonically with increasing N content, akin to the Raman features described in **Figure 3**. Clearly, the dopants present in the non-graphitic configurations exhibit much faster relaxation times relative to dopants present in S2 with graphitic bonding or pristine graphene. Indeed, carriers in sample S1 decay much faster (within fs), despite lower N content, due to the presence of extended defects that lead to increased contribution from carrier-defect scattering. More importantly, N dopants in graphitic configuration do not affect the carrier recombination times even at >1 % doping, while pyrrolic/pyridinic configurations drastically decrease carrier lifetimes at concentrations as low as ~0.2 %.

**Figure 4.** (a) Time-resolved differential transmission spectra of pristine and N-doped graphene samples S1, S2, and S3, respectively, obtained through pump-probe spectroscopy. The inset shows parabolic energy dispersion for bilayer gra‐ phene, where the arrow indicates excitation of electrons from valence to the conduction band. The solid lines represent bi-exponential fits based on the above equation. (b) Tabulated values of relaxation times τ1 and τ2, corresponding to intraband carrier-carrier scattering and carrier-phonon scattering, respectively, obtained from the fits.
