Few-Layer V2C MXene Research

• Few-layer V2C MXene is a two-dimensional vanadium carbide characterized by mixed surface terminations (-O, -F, -OH) that tune its metallic electronic structure.
• It is synthesized via HF etching of V2AlC followed by intercalation and ultrasonic delamination, yielding transparent, few-atomic-layer flakes with expanded interlayer spacing.
• Its broadband plasmonic response and sub-picosecond relaxation dynamics enable strong, wavelength-dependent saturable absorption for applications in ultrafast fiber lasers and energy storage.

Few-layer $V_2C$ MXene is a two-dimensional vanadium carbide derived experimentally from $V_2AlC$ MAX-phase powder by selective Al removal and subsequent delamination, and represented in first-principles studies as $V_2CT_2$ to denote surface terminations such as $-O$, $-F$, and $-OH$. In the currently documented literature, its defining features are a metallic electronic structure, termination-sensitive interlayer geometry, broadband plasmonic extinction extending into the near-infrared, strong saturable absorption at telecommunication wavelengths, and stacking-dependent ion-intercalation behavior relevant to Na- and Al-based electrochemical storage (Wang et al., 25 Aug 2025, Caffrey, 2018, Nair et al., 3 Mar 2026). A notable boundary of the literature is that direct-synthesis and chemical-vapor-deposition studies of MXenes do not, in the cited record, provide a direct-synthesis or CVD route for $V_2C$ specifically (Wang et al., 2022).

1. Crystallographic identity and surface chemistry

Few-layer $V_2C$ MXene is most commonly discussed as $V_2CT_2$, where $T$ denotes terminating groups created by the chemical etching route. In explicit mixed-termination modeling, the composition $V_2AlC$0 was used with $V_2AlC$1, $V_2AlC$2, and $V_2AlC$3, corresponding to surface coverages of 21%, 59%, and 20%, respectively. There is considerable debate regarding the contribution of these functional groups to the properties of the underlying MXene material, particularly because measured Li or Na capacity is far lower than that predicted by theoretical simulations that generally assume uniformly terminated surfaces; within the reported calculations, weighted averages of uniformly terminated layer properties give excellent approximations to more realistic, randomly terminated structures (Caffrey, 2018).

For uniformly terminated monolayers, a single $V_2AlC$4 layer with $V_2AlC$5 or $V_2AlC$6 termination adopts the hexagonal $V_2AlC$7 symmetry inherited from its parent MAX phase. After full relaxation, monolayer $V_2AlC$8 has an in-plane lattice constant $V_2AlC$9, while $V_2CT_2$0 is effectively the same within $V_2CT_2$1. In fractional coordinates, V occupies $V_2CT_2$2 and $V_2CT_2$3, C occupies $V_2CT_2$4 and $V_2CT_2$5, and the terminal O or F atoms lie at $V_2CT_2$6 with $V_2CT_2$7 for O and $V_2CT_2$8 for F. In few-layer models, the same $V_2CT_2$9-parameter is retained while the out-of-plane repeat distance $-O$0 depends on stacking; typical central-layer C-C interlayer spacings in pristine ZZ-oct $-O$1 converge to $-O$2 (Nair et al., 3 Mar 2026).

Across realistic termination mixtures, few-layer $-O$3 remains metallic. Total and projected density of states show a clear Fermi-level crossing, with states near $-O$4 dominated by V $-O$5 hybridized with C $-O$6, while termination-$-O$7 states lie deeper, below $-O$8 eV. This metallicity is central to both its plasmonic optical response and its use in electrochemical hosts (Caffrey, 2018).

2. Exfoliation route and experimental structural characterization

The reported preparation of few-layer $-O$9 begins from $-F$0 MAX-phase powder with average particle size $-F$1m. HF etching at 49 wt% was carried out at $-F$2, 500-800 rpm, for 48 h to remove Al layers and yield accordion-like multilayer $-F$3. Intercalation then proceeded through successive treatments with 5 wt% TMAOH at room temperature for 12 h and ethanol washes to $-F$4, expanding the interlayer spacing. A LiCl solution flocculation step, performed for $-F$5, replaced $-F$6 with $-F$7 and produced a flocculate that could be stored long-term. Final delamination was achieved by washing with deionized water, re-dispersion in N-methyl-2-pyrrolidone (NMP), 30 min ultrasonication, and 15 min centrifugation at 3000 rpm, yielding stable few-layer $-F$8 dispersions in NMP (Wang et al., 25 Aug 2025).

Transmission electron microscopy shows transparent, sheet-like flakes with lateral sizes ranging from hundreds of nanometers to a few micrometers, indicative of exfoliation into a few atomic layers. Atomic force microscopy was not shown for this system, but on similar MXenes it typically confirms thicknesses of 2-5 nm, corresponding to 3-7 atomic layers. X-ray diffraction shows disappearance of most MAX-phase peaks; only the $-F$9 MXene reflection remains, broadened and shifted from $-OH$0 to $-OH$1, corresponding to an interlayer spacing of $-OH$2 versus $-OH$3 in $-OH$4. Energy-dispersive X-ray spectroscopy gives uniform V, C, F, and O mapping, consistent with successful MXene formation and surface terminations by $-OH$5 and $-OH$6. Raman spectroscopy identifies the in-plane $-OH$7 mode at $-OH$8 and the $-OH$9 mode at $V_2C$0; additional peaks at 411, 526, and $V_2C$1 arise from mixed vibrational modes of $V_2C$2 with $V_2C$3 and $V_2C$4 groups. Abundant $V_2C$5, $V_2C$6, and $V_2C$7 terminations impart hydrophilicity and tune the electronic and plasmonic behavior (Wang et al., 25 Aug 2025).

3. Plasmonic response, metallicity, and ultrafast relaxation

Optically, few-layer $V_2C$8 exhibits broad extinction up to 2000 nm in UV-Vis-NIR spectroscopy, a response described as characteristic of surface plasmon resonance in metallic MXene. Concentration-dependent measurements support broadband plasmonic absorption rather than molecular transitions. Lateral size also modifies the resonance: smaller flakes exhibit broader SPR bands due to increased surface scattering, whereas larger flakes show narrowed plasmon modes, analogous to gold nanoparticle behavior. Although no explicit quality factor was reported, the bandwidth-to-resonance ratio $V_2C$9 at $V_2C$0 nm indicates a low-$V_2C$1, broadband SPR suited to ultrafast applications (Wang et al., 25 Aug 2025).

Pump-probe transient absorption measurements using 325 nm interband excitation and 1300 nm plasmonic excitation show broadband photo-induced absorption below 670 nm, assigned to excited-state absorption, and photobleaching above 670 nm, assigned to ground-state bleaching due to state-filling. In the plasmonic region, time-resolved dynamics at probe wavelength 1100 nm exhibit an ultrafast decay component with $V_2C$2 fs and amplitude $V_2C$3, attributed to hot-electron generation via Landau damping followed by electron-electron scattering. At probe wavelength 500 nm, $V_2C$4 increases to approximately 1 ps and $V_2C$5 decreases to approximately 0.4, indicating dominance of picosecond electron-phonon scattering. Near the SPR, the $V_2C$6 fs hot-electron channel accounts for about 90% of the total relaxation (Wang et al., 25 Aug 2025).

First-principles analysis links these dynamics to the electronic structure of terminated monolayers. Both $V_2C$7 and $V_2C$8 are metallic, with bands crossing $V_2C$9. Parity analysis at the T-point and the joint density of states show a JDOS peak near 4 eV, matching the 325 nm pump and supporting strong interband transitions in the UV, while below 4 eV the JDOS drops, implying that NIR excitation cannot drive direct interband transitions near $V_2CT_2$0. Instead, NIR photons excite collective plasmon modes. Landau damping on a $V_2CT_2$1 fs scale produces nonthermal hot electrons and holes with energies up to the pump photon energy; electron-electron collisions thermalize these carriers within $V_2CT_2$2 fs, and subsequent electron-phonon coupling occurs on a $V_2CT_2$3-5 ps timescale. The reported scaling of low-energy hot-carrier yield as $V_2CT_2$4 provides the stated basis for the giant ultrafast nonlinear response at 1550 nm (Wang et al., 25 Aug 2025).

4. Nonlinear saturable absorption at telecommunication wavelengths

Few-layer $V_2CT_2$5 has been characterized as a saturable absorber by open-aperture Z-scan using 35 fs pulses at 1 kHz, tunable from 800 to 1800 nm, focused through an approximately 1 mm path length of $V_2CT_2$6 dispersion in a quartz cuvette. At all investigated wavelengths, the transmittance displays a symmetric peak at focus, confirming saturable absorption. The fitting procedure uses the standard open-aperture Z-scan formalism with

$V_2CT_2$7

where $V_2CT_2$8 is the on-axis peak intensity and

$V_2CT_2$9

with $T$0 the nonlinear absorption coefficient (Wang et al., 25 Aug 2025).

The extracted $T$1 values are wavelength dependent: $T$2 at 800 nm, $T$3 at 1150 nm, $T$4 at 1300 nm, $T$5 at 1550 nm, and $T$6 at 1800 nm. The strongest saturable absorption therefore occurs at 1550 nm. At that wavelength, $T$7 is reported as roughly twice that of $T$8 at 1200 nm, where the cited value is $T$9 (Wang et al., 25 Aug 2025).

The intensity dependence can be described by the standard two-level saturable-absorber expression

$V_2AlC$00

where $V_2AlC$01 is the non-saturable loss, $V_2AlC$02 is the modulation depth, and $V_2AlC$03 is the saturation intensity. Using the Z-scan $V_2AlC$04 and an assumed $V_2AlC$05, one can estimate $V_2AlC$06, corresponding to a modulation depth $V_2AlC$07-15% and residual loss $V_2AlC$08-90%; the source notes, however, that precise $V_2AlC$09 and $V_2AlC$10 require power-dependent transmission curves (Wang et al., 25 Aug 2025).

5. Integration into erbium-doped fiber lasers

The reported laser implementation uses an all-fiber erbium-doped fiber laser. The pump source is a 976 nm laser diode coupled through a wavelength-division multiplexer, the gain medium is approximately 1.3 m of $V_2AlC$11-doped fiber, and the cavity includes an integrated WDM/pump combiner, a polarization-independent isolator, an output coupler, and polarization controllers. The saturable absorber is a $V_2AlC$12-coated side-polished, or “D-shaped,” fiber segment, which provides evanescent-field interaction without bulk optics. The cavity length is approximately 5 m, giving a fundamental repetition rate of approximately 39.5 MHz (Wang et al., 25 Aug 2025).

Mode locking begins at a pump power of approximately 72 mW and is maintained up to 500 mW, with 12.24 mW average output. Autocorrelation fitted with a $V_2AlC$13 profile gives a deconvolved pulse duration $V_2AlC$14 fs. The optical spectrum has center wavelength $V_2AlC$15 nm and 3 dB bandwidth $V_2AlC$16 nm, with clear Kelly sidebands indicating soliton operation. The measured repetition rate is $V_2AlC$17 MHz, and the RF spectrum shows a signal-to-noise ratio of 92 dB. No significant spectral shift or power degradation is observed over 8 h. From $V_2AlC$18, the pulse energy is approximately $V_2AlC$19 pJ (Wang et al., 25 Aug 2025).

These measurements place few-layer $V_2AlC$20 within the class of plasmonic MXene saturable absorbers operating at the communication band around 1550 nm. In this setting, the material’s low-$V_2AlC$21 broadband SPR and the dominance of the $V_2AlC$22 fs relaxation channel are directly connected to stable femtosecond pulse generation (Wang et al., 25 Aug 2025).

6. Mixed terminations, ion intercalation, and limits of the present synthesis record

For electrochemical modeling, mixed terminations are a primary variable. In the reported Na-intercalation calculations, few-layer $V_2AlC$23 with realistic mixed terminations is metallic under all studied surface chemistries, and the mixed-termination density of states, lattice constants, and work function are well approximated by a weighted average of uniformly terminated systems:

$V_2AlC$24

The sodiation reaction is written as

$V_2AlC$25

For mixed terminations, the computed specific capacities are approximately 17, 48, 86, 155, and 274 mAh g$V_2AlC$26 at $V_2AlC$27, 0.33, 0.56, 1.0, and 2.0, respectively; the corresponding volume changes $V_2AlC$28 are +11%, +13%, +15%, +16%, and +58%, with the double-Na-layer case involving a $V_2AlC$29-axis increase of approximately $V_2AlC$30. The open-circuit voltage is 2.3, 2.5, 2.1, and 1.7 eV at $V_2AlC$31, 0.33, 0.56, and 1.00, respectively. At low Na concentrations, charge transfer is confined to the terminations, with O $V_2AlC$32 F $V_2AlC$33 OH, while V sites are affected only at higher Na concentrations (Caffrey, 2018).

In Al-ion battery modeling, Veluthedath Nair and Caffrey examined four stacking variants—ZZ-oct, ZZ-pris, WS-oct, and WS-pris—and found WS-oct to be the lowest in energy for pristine $V_2AlC$34, with ZZ-oct only $V_2AlC$35 meV/f.u. higher and both prismatic variants approximately 50-66 meV/f.u. higher. For dilute Al intercalation at $V_2AlC$36 Al/f.u., O-terminated $V_2AlC$37 in ZZ-oct shows an interlayer expansion $V_2AlC$38 and average formation energy $V_2AlC$39, whereas ZZ-pris gives $V_2AlC$40 and only slightly negative formation energy. The corresponding Al migration barriers are approximately 1.44 eV for ZZ-oct and 0.50 eV for ZZ-pris, establishing a trade-off between structural stability and ionic mobility. As Al loading increases in ZZ-oct $V_2AlC$41, the open-circuit voltage falls from approximately 1.8 V at $V_2AlC$42 to approximately 0.8 V at $V_2AlC$43, and the maximum specific capacity is approximately 277.6 mAh g$V_2AlC$44. By contrast, $V_2AlC$45 is limited to $V_2AlC$46 Al/f.u., about 0.4 V average voltage, and $V_2AlC$47 mAh g$V_2AlC$48. Bader analysis indicates that each Al atom donates approximately $V_2AlC$49 to the host; in O-terminated $V_2AlC$50 at $V_2AlC$51, about $V_2AlC$52 localize on bonded O sites and about $V_2AlC$53 on the V layers, preserving metallic character (Nair et al., 3 Mar 2026).

A common misconception is that recently reported direct-synthesis or CVD advances for MXenes already include few-layer $V_2AlC$54. The cited record does not support that interpretation. Wang et al., in “Direct synthesis and chemical vapor deposition of 2D carbide and nitride MXenes,” do not report any direct-synthesis or CVD procedures for $V_2AlC$55 MXene; $V_2AlC$56 is mentioned only in passing among the broader family of MXenes, while the reported direct-synthesis and CVD results are limited to Ti- and Zr-based MXenes, specifically $V_2AlC$57, $V_2AlC$58, $V_2AlC$59, and $V_2AlC$60. Consequently, reaction equations, experimental parameters, CVD protocols, layer-thickness data, thermodynamic or kinetic analyses, observed morphologies, and Li-ion performance are not available for $V_2AlC$61 in that publication (Wang et al., 2022).