Alkali‑Metal‑Adsorbed Graphene‑Like GaN: Ultra‑Low Work Functions and Tunable Optoelectronic Properties

Background

Traditional bulk GaN, a wide‑bandgap semiconductor, supports operation at extreme voltages, frequencies, and temperatures, and boasts high luminous efficiency, excellent thermal conductivity, chemical resistance, and radiation tolerance. In the realm of two‑dimensional (2D) materials, g‑GaN has recently been synthesized via a migration‑enhanced encapsulated growth method and exhibits a widened bandgap along with enhanced optoelectronic performance compared to its bulk counterpart.

Surface adsorption studies are central to surface physics and nanodevice engineering. Alkali metals readily donate electrons, transforming semiconductors into n‑type materials, lowering their work functions, and modifying optoelectronic properties. Prior work has demonstrated work‑function reduction in graphene and other 2D systems upon alkali‑metal adsorption. However, the full photoelectric response of alkali‑metal‑functionalized g‑GaN remains unexplored.

This work systematically examines the band structure, density of states, work function, and optical response of pristine and alkali‑metal‑adsorbed g‑GaN, providing insights relevant to field‑emission and optoelectronic device fabrication.

Methods

All calculations were performed with the Vienna Ab initio Simulation Package (VASP) using density functional theory. Exchange‑correlation interactions were treated with the generalized gradient approximation (GGA) in the Perdew–Burke–Ernzerhof (PBE) form, known for reliable surface studies. A plane‑wave cutoff of 500 eV, a 20 Å vacuum gap normal to the g‑GaN plane, and a 9 × 9 × 1 Γ‑centered k‑point mesh ensured convergence. Geometry optimizations continued until forces fell below 10⁻⁴ eV/Å and total energy changes were <10⁻⁴ eV.

The adsorption energy (E_ads) was calculated as:

Eads = Eg‑GaN:X − Eg‑GaN − μX

where μ_X is the chemical potential of an isolated alkali atom; a negative E_ads indicates a stable chemisorption.

Charge‑density differences were evaluated via:

Δρ = ρT − ρg − ρx

with ρ_T, ρ_g, and ρ_x representing the total, pristine, and isolated‑atom charge densities, respectively.

Results and discussions

Figure 1 illustrates four potential adsorption sites: T_N (above N), T_Ga (above Ga), T_B (mid‑bond), and T_M (hexagon center). Table 1 shows that all adsorption energies are negative, confirming exothermic chemisorption; the T_M site is energetically favored, so subsequent analyses focus on this configuration.

Model of g‑GaN with four adsorption sites.

Table 2 lists lattice constants, bond lengths, and adsorption heights. Pristine g‑GaN adopts a 3.254 Å lattice, in line with previous reports. Li/Na adsorption slightly contracts the lattice, whereas K/Rb/Cs expand it, reflecting the increasing alkali radius. Correspondingly, N–X and Ga–X bond lengths and adsorption heights rise with heavier alkali atoms.

Band structures: a pristine g‑GaN; b–f Li, Na, K, Rb, Cs adsorbed. The pristine material is a semiconductor (2.1 eV gap). Alkali adsorption raises the Fermi level into the conduction band, imparting metallic character with a residual ~1.92 eV gap.

The density of states (Fig. 3) confirms semiconducting behavior for pristine g‑GaN, with valence‑band maxima dominated by N‑2p and Ga‑4p orbitals. In the adsorbed systems, states near the Fermi level arise mainly from Ga‑4s, N‑2p, and the alkali s orbitals.

Density of states: a pristine, b–f Li, Na, K, Rb, Cs adsorbed.

Charge‑density difference maps (Fig. 4) reveal electron accumulation between the alkali atoms and the three neighboring N atoms, confirming chemisorption and ionic bonding. Bader analysis quantifies charge transfer: Li → 0.88 e, Na → 0.78 e, K → 0.80 e, Rb → 0.79 e, Cs → 0.79 e.

Charge‑density difference for g‑GaN/alkali. Magenta: electron gain; cyan: loss.

The work function drops from 4.21 eV (pristine) to 2.47 eV (Li) down to 0.84 eV (Cs), the lowest among the examined adsorbates. This reduction surpasses that observed in alkali‑metal‑adsorbed GaN nanowires, likely due to the distinct monolayer geometry, and positions Cs‑g‑GaN as an attractive candidate for field‑emission applications.

Work‑function schematic for pristine and alkali‑metal‑adsorbed g‑GaN.

Optical analysis shows that alkali adsorption markedly increases the static dielectric constant (ε₁(0): 1.48 → 2.33–3.81) and shifts key absorption peaks to lower energies, extending the spectrum into the visible range. The absorption edge of pristine g‑GaN starts at 2.77 eV; after adsorption, a new low‑energy peak appears at 1.61 eV. Refractive indices also rise (n₀: 1.22 → 1.53–1.99), and reflectivity peaks shift and diminish, indicating tunable light–matter interaction suitable for optoelectronic devices.

Real (a) and imaginary (b) dielectric functions for pristine and alkali‑adsorbed g‑GaN.

Absorption coefficient (a) and refractive index (b) for pristine and alkali‑adsorbed g‑GaN.

Reflectivity (a), loss‑energy (b), and extinction coefficient (c) for pristine and alkali‑adsorbed g‑GaN.

Conclusions

First‑principles calculations reveal that alkali‑metal adsorption on g‑GaN yields stable, chemisorbed structures with pronounced n‑type doping and dramatic work‑function reduction. Cesium‑functionalized g‑GaN attains a record low work function of 0.84 eV, making it highly suitable for field‑emission devices. Simultaneously, alkali adsorption enhances the static dielectric constant and broadens the absorption spectrum, offering a versatile strategy to engineer g‑GaN’s optoelectronic response for photodetectors, LEDs, and related technologies.

Abbreviations

2D

Two‑dimensional

GGA

Generalized gradient approximation

g‑GaN

Graphene‑like gallium nitride

PBE

Perdew–Burke–Ernzerhof

PDOS

Partial density of states

TDOS

Total density of states