Intentional doping of graphene with heteroatoms such as nitrogen, boron, or sulfur is a powerful strategy to modify its electronic, chemical, and structural properties. These dopants introduce defects, alter charge distribution, and create active sites, making doped graphene suitable for applications in catalysis, sensing, and electronics. The primary doping methods include substitutional doping, adsorption, and plasma treatments, each offering distinct advantages in terms of dopant concentration, stability, and electronic modulation. Characterization techniques such as X-ray photoelectron spectroscopy (XPS), Raman spectroscopy, and scanning transmission electron microscopy (STEM) are critical for analyzing dopant incorporation and distribution.

Substitutional doping involves replacing carbon atoms in the graphene lattice with heteroatoms. Nitrogen doping is the most studied, where nitrogen atoms can occupy graphitic (substitutional), pyridinic, or pyrrolic sites. Graphitic nitrogen donates electrons to the graphene lattice, increasing n-type conductivity, while pyridinic and pyrrolic nitrogen introduce localized states near the Fermi level, enhancing catalytic activity. Boron doping, in contrast, creates p-type conductivity due to its electron-deficient nature, withdrawing electrons from the graphene network. Sulfur doping is less common but introduces strong spin polarization and charge redistribution due to its larger atomic size and higher electronegativity compared to carbon.

Adsorption-based doping relies on the physical or chemical attachment of dopant atoms or molecules onto the graphene surface without disrupting the lattice. This method is less invasive and allows for reversible doping, but the dopants are often less stable under harsh conditions. Nitrogen-containing precursors like ammonia or amines can adsorb onto graphene defects or edges, while boron-containing molecules such as borane form coordination complexes with carbon atoms. Sulfur doping via adsorption typically uses hydrogen sulfide or thiophene, creating thiol or sulfide linkages. Adsorption doping is advantageous for fine-tuning surface chemistry but lacks the long-term stability of substitutional doping.

Plasma treatments offer a highly controllable and scalable approach to doping graphene. Nitrogen plasma introduces nitrogen radicals that bond to graphene defects or edges, achieving high doping concentrations (up to 10–20 at.% N). Boron and sulfur doping via plasma require careful optimization of precursor gases (e.g., diborane or sulfur hexafluoride) and plasma parameters to avoid excessive damage to the graphene lattice. Plasma treatments can produce both substitutional and adsorbed species, depending on energy and exposure time. The main advantage of plasma doping is its ability to uniformly modify large-area graphene films, but it may introduce additional defects that affect charge carrier mobility.

The electronic properties of doped graphene are highly tunable based on dopant type and concentration. Nitrogen-doped graphene exhibits enhanced n-type behavior, reducing sheet resistance and improving electron transfer kinetics for oxygen reduction reactions in fuel cells. Boron-doped graphene shows p-type conductivity, making it suitable for hole transport layers in electronic devices. Sulfur doping introduces mid-gap states due to its lone pair electrons, which can enhance electrocatalytic activity for hydrogen evolution reactions. The choice of dopant and method depends on the desired electronic structure—whether for metallic, semiconducting, or catalytic applications.

Doped graphene has significant potential in catalysis and sensing. Nitrogen-doped graphene serves as a metal-free catalyst for oxygen reduction, with pyridinic nitrogen sites mimicking platinum-like activity. Boron-doped graphene enhances the sensitivity of gas sensors due to its strong interaction with electron-donating molecules like NO₂. Sulfur-doped graphene shows promise in biosensing due to its affinity for thiol-containing biomolecules. The ability to tailor doping levels allows optimization for specific reactions or detection limits.

Characterization of doped graphene requires multiple techniques to confirm dopant incorporation and distribution. XPS is the most widely used, providing quantitative analysis of dopant species (e.g., distinguishing graphitic from pyridinic nitrogen). Raman spectroscopy detects doping-induced changes in the D, G, and 2D bands—increased D-band intensity indicates defect formation, while G-band shifts reflect charge transfer. STEM with electron energy loss spectroscopy (EELS) offers atomic-resolution mapping of dopant locations, critical for understanding substitutional versus adsorbed configurations. Electrical measurements, such as Hall effect or field-effect transistor (FET) analysis, reveal carrier type and mobility changes due to doping.

The following table summarizes key differences between doping methods:

Method | Dopant Stability | Dopant Concentration | Lattice Disruption | Scalability
----------------- | ---------------- | -------------------- | ------------------ | -----------
Substitutional | High | Moderate (1–10 at.%) | Moderate | Moderate
Adsorption | Low | Low (<5 at.%) | Minimal | High
Plasma Treatment | Moderate | High (5–20 at.%) | High | High

In summary, intentional doping of graphene with nitrogen, boron, or sulfur enables precise control over its electronic and chemical properties. Substitutional doping provides stable and uniform modifications, adsorption allows reversible tuning, and plasma treatments offer high throughput. The choice of method depends on the application requirements, whether for catalysis, sensing, or electronic devices. Advanced characterization techniques are essential to validate doping efficacy and understand structure-property relationships. Future research may focus on optimizing doping processes to minimize defects while maximizing functional performance.