Vanadium-Doped Co2NiSe4 Nanomaterials

• The paper demonstrates that vanadium doping in Co2NiSe4 enhances the density of states near the Fermi level, increasing charge carrier concentration and spin polarization (from 3.2 μB to 3.6 μB at 5 at.% V).
• The paper shows that vanadium incorporation improves thermoelectric performance with a peak ZT of ~1.1 at 900 K due to augmented Seebeck coefficient and carrier mobility reduction in effective mass.
• The paper reports that vanadium doping simultaneously boosts mechanical resilience (e.g., increased Young’s modulus from 120 GPa to 135 GPa), broadens optical absorption, and elevates piezoelectric coefficients (up to 2.7 C/m²) for multifunctional energy applications.

Vanadium-doped Co₂NiSe₄ denotes a class of nanomaterials derived from the parent chalcogenide Co₂NiSe₄, wherein partial substitution of transition metal sites with vanadium ions induces pronounced modifications in structural, electronic, magnetic, thermoelectric, optical, mechanical, and piezoelectric properties. Using first-principles density functional theory supplemented by on-site Coulomb corrections (DFT+U), such doping strategies have been rigorously evaluated to realize advanced multifunctional materials that target applications in energy storage, energy conversion, photonic, spintronic, and electromechanical systems.

1. Electronic Structure Modification by Vanadium Substitution

Vanadium integration into Co₂NiSe₄ fundamentally alters the electronic band structure by introducing V-d orbital contributions near the Fermi level. In pristine Co₂NiSe₄, the density of states (DOS) proximate to the Fermi level is governed by Co and Ni d-orbitals, imparting moderate conductivity and semimetal or narrow semiconductor behavior. The introduction of vanadium site-dopants enhances the overall DOS at the Fermi level, principally through hybridization of localized V-d states with native host states.

This electronic restructuring manifests in several measurable effects:

• Elevated charge carrier concentration, directly translating to increased electrical conductivity.
• Strengthened spin polarization at the Fermi level, with vanadium contributing a localized magnetic moment that increases the bulk magnetic moment from approximately 3.2 μ_B in undoped material to 3.6 μ_B in samples with vanadium doping (e.g., 5 at.% V).
• Potential for conduction type switching, evidenced by a shift in the Seebeck coefficient from strongly positive (p-type) in the pristine phase to possible n-type behavior under heavy vanadium incorporation due to Fermi level movement.

These design principles underpin tailoring strategies for spintronic and magnetic sensor systems, leveraging the modulated DOS and enhanced magnetic ordering.

2. Thermoelectric Performance Enhancement

The thermoelectric figure of merit ZT, defined as $ZT = \frac{S^2 \sigma T}{\kappa}$ (with $S$ denoting the Seebeck coefficient, $\sigma$ the electrical conductivity, $T$ the absolute temperature, and $\kappa$ the thermal conductivity), is a principal metric for assessing energy conversion efficiency.

Experimental and computational assessment reveal that vanadium doping yields:

• Augmented Seebeck coefficient, indicative of improved energy filtering and DOS concentration near the Fermi level.
• Enhanced electrical conductivity, with electron effective mass decreasing from 0.63 $m_0$ (pristine) to 0.52 $m_0$ at 5 at.% doping, reflecting improved carrier mobility.
• No substantial increase in thermal conductivity due to phonon scattering from substitutional disorder.

Collectively, these effects produce a peak thermoelectric ZT of ~1.1 at 900 K for 5 at.% V-doped Co₂NiSe₄. Such values approach technologically relevant benchmarks for waste heat conversion and self-powered systems, especially in high-temperature environments.

3. Mechanical and Thermodynamic Stability

Mechanical integrity in energy materials is critical for operational reliability under cyclic loading or thermal stress. Vanadium doping in Co₂NiSe₄ advances mechanical robustness by increasing all key moduli:

• Young’s modulus increases from 120 GPa (pristine) to 135 GPa (5 at.% V-doping).
• Bulk modulus rises from 78 GPa to 86 GPa across corresponding compositions.
• The Pugh’s ratio (B/G) remains near the ductile-brittle threshold, conferring sustained ductility alongside improved stiffness.

Thermodynamic stability is corroborated by Gibbs free energy calculations, which, despite a marginal decrease in negative contributions with higher vanadium content, exhibit phase stabilization at elevated temperature due to enhanced entropy ($-T S$ term). This supports practical application under demanding conditions, such as battery electrode operation or high-temperature devices.

4. Optical Property Tuning

The optical characteristics of vanadium-doped Co₂NiSe₄ are primarily described by the complex dielectric function $$\epsilon(\omega) = \epsilon_1(\omega) + i \epsilon_2(\omega)$$, where $\epsilon_2(\omega)$ determines the absorption profile and $\epsilon_1(\omega)$ controls polarizability.

V doping introduces the following modifications:

• Broader absorption spectra covering visible and near-infrared energies, ascribed to new electronic transitions activated by V-d states. Notably, increased spectral intensity and range are observed for 5 at.% V specimens.
• Static dielectric constant at zero frequency $\epsilon_1(0)$ is raised from ~3.0 (pristine) to ~5.6 (doped), denoting stronger polarizability.
• Altered reflectivity and energy loss spectra, with tunable plasmon resonances and selective optical response.

Such characteristics are desirable for broadband photodetectors, solar absorbers, and optical coating applications, supporting integration into next-generation optoelectronic architectures.

5. Piezoelectric Response Augmentation

Piezoelectric functionality, quantified by stress coefficients (e.g., $e_{33}$), is central to sensing and energy-harvesting applications. The generalized polarization induced by mechanical strain follows $P = P_{sp} + e \cdot \epsilon$.

Vanadium doping effect on piezoelectric coefficients includes:

• Enhancement of $e_{33}$ from 1.10 C/m² (pristine) to 1.90 C/m² (5% V) and up to 2.70 C/m² (10% V), representing a substantial elevation.
• Increases in associated strain coefficients $d_{11}$ and $d_{33}$ as determined by $d = e \cdot S$, where $S$ is the elastic compliance tensor.

Such performance upgrades, driven by atomic-scale lattice distortions and charge redistribution, enable high-output nanoscale electromechanical devices including sensors, actuators, and micro-energy harvesters.

6. Integrated Applications and Technological Significance

The co-optimization achieved through vanadium substitution across electronic, thermoelectric, mechanical, optical, and piezoelectric domains positions V-doped Co₂NiSe₄ as a platform for multifield energy technologies. The cumulative effects—enhanced spin polarization and carrier mobility, robust elastic behavior, broad photonic activity, and superior piezoelectric coefficients—align with application requirements in:

Application Area	Dominant Property	V-Doping Benefit
Thermoelectric Generators	ZT, conductivity	ZT ~1.1 at 900 K
Spintronic Sensors	Magnetic ordering	Increased spin polarization
Battery Electrodes	Structural integrity	Elevated moduli, stable phase
Optoelectronics	Absorption, dielectric	Expanded spectra, high $\epsilon_1$
NEMS/Energy Harvesters	Piezoelectric output	$e_{33}$ up to 2.7 C/m²

This suggests that vanadium doping strategy provides a generalizable route to tailor properties for multifunctional materials engineering. In sum, vanadium-doped Co₂NiSe₄ enables synergistic advances across energy conversion, storage, detection, and actuation, supporting its development for next-generation device platforms (Riaz et al., 5 Sep 2025).