In recent years, two-dimensional (2D) materials have completely changed how scientists think about light, electronics, and energy. Graphene was the first big star, but it’s far from the only one. A growing family of 2D materials called MXenes has started to attract serious attention, thanks to their unusual combination of metallic conductivity, tunable surfaces, and solution processability.

Among MXenes, niobium carbide (Nb₂C) stands out as a very promising candidate. Its structure, chemistry, and optical behavior make it suitable not just for energy storage and catalysis, but also for photonics and optoelectronics.

The article you shared explores this side of Nb₂C in depth. It shows how few-layer Nb₂C MXene nanosheets can be used in two very different but complementary photonic technologies:

1. Photoelectrochemical (PEC) photodetectors, especially narrow-band UV photodetectors.

2. Ultrafast mode-locked fiber lasers, where Nb₂C acts as a saturable absorber (optical switch) for generating ultrashort laser pulses.

1. A quick primer: MXenes and Nb₂C

2D materials are atomically thin sheets where electrons can move freely in-plane but are tightly confined in the thickness direction. This gives them special optical, electrical, and mechanical properties that differ from the bulk material.

MXenes are a large family of 2D transition-metal carbides and/or nitrides. They’re usually made by selectively etching out a layer (called the “A” layer) from a three-element “MAX phase”, which has the formula:

Mₙ₊₁AXₙ, where

• M = early transition metal (e.g., Ti, Nb, V)

• A = element from group 13 or 14 (e.g., Al, Si)

• X = carbon and/or nitrogen

After etching away the A layer, you get 2D sheets with a general formula:

Mₙ₊₁XₙTₓ,

where Tₓ represents surface terminations such as –F, –OH, –O, etc. These surface groups form during the chemical etching process and strongly influence electronic and optical behavior.

Nb₂C is one such MXene derived from the MAX phase Nb₂AlC by removing the aluminum layer. Previous research has already shown that Nb₂C:

• Can store lithium ions efficiently (battery applications).

• Has interesting photothermal properties (converts light to heat efficiently), useful for cancer therapy.

• Can form composites that work as photocatalysts.

However, its optoelectronic applications—using light and electricity together—are still quite underexplored. This paper specifically asks:

Can few-layer Nb₂C nanosheets work well in photodetectors and ultrafast lasers?

The answer, as the authors show, is yes.

2. Making and characterizing few-layer Nb₂C nanosheets

The authors use a top-down method, starting from bulk Nb₂AlC (the MAX phase) and “peeling it down” to few-layer sheets.

Step 1: Etching to form multilayer Nb₂C MXene

• Nb₂AlC powder is soaked in hydrofluoric acid (HF) for 48 hours at room temperature.

• HF selectively removes the Al layers, leaving behind multilayer Nb₂C MXene.

• After repeated washing and centrifugation, the etching by-products and excess acid are removed.

• At this stage, the material has an accordion-like layered structure, typical of multilayer MXenes.

Step 2: Delamination to few-layer nanosheets

• The multilayer Nb₂C is then treated with tetrapropylammonium hydroxide (TPAOH).

• TPA⁺ ions intercalate between the Nb₂C layers, replacing protons and swelling the structure.

• Upon strong stirring and subsequent washing/centrifugation, the layers separate into few-layer Nb₂C nanosheets (NSs).

Structure and size

From various characterization techniques:

• SEM & TEM show:

  • The initial ceramic Nb₂AlC is dense and layered.

  • After HF treatment: “accordion” style layered Nb₂C.

  • After TPAOH: ultrathin, transparent few-layer nanosheets with typical 2D morphology.

• HRTEM and electron diffraction confirm a crystalline hexagonal structure.

• Size:

  • Lateral sizes from ~30 to 270 nm, with an average around 112 nm (based on TEM).

• AFM thickness:

  • Heights between about 2.4 and 2.9 nm, corresponding to roughly five Nb₂C layers.

Surface chemistry

• EDS and XPS analysis show:

  • Interior elements: Nb and C.

  • Surface terminations: O and F, coming from –OH and –F groups formed during HF etching.

  • The Al signal disappears after etching, confirming successful removal of the Al layer.

Optical absorption

In ethanol, the UV–vis–NIR absorption spectra show:

• Strong absorption in the UV range (200–400 nm).

• Broad absorption extending into the visible and NIR ranges.

• Additional features (enhanced absorption) around ~1500 nm and ~1950 nm.

This combination—strong UV absorption plus broadband NIR response—makes Nb₂C nanosheets promising for:

• UV photodetectors, and

• Ultrafast laser devices in the 1.5–2.0 µm region.

3. Nb₂C-based photoelectrochemical (PEC) UV photodetectors

The first major application explored is photoelectrochemical-type photodetectors (PDs).

Device structure

The photodetector is built in a three-electrode PEC system:

• Working electrode: ITO glass coated with Nb₂C nanosheets (with a small amount of polymer binder).

• Counter electrode: platinum wire.

• Reference electrode: Ag/AgCl.

• Electrolyte: aqueous KOH solution (0.1, 0.5, or 1.0 M).

A light source (xenon lamp) provides:

• Simulated “white” light (a broad band from 350–800 nm).

• Or individual wavelengths using optical filters (e.g., 350, 365, 380, 400, 475 nm, etc.).

Basic electrochemical behavior

Linear sweep voltammetry (LSV) shows no obvious redox peaks between 0 and 0.8 V, meaning:

• Nb₂C is stable in this potential range under these conditions.

• This is important for long-term operation of the detector.

On/off photoresponse and key parameters

Under illumination, the device shows clear on/off switching in current when the light is turned on and off.

The authors study how the performance depends on:

• Light power density (Pλ): how intense the light is.

• KOH concentration: 0.1, 0.5, 1.0 M.

• Bias potential: 0 to 0.8 V vs Ag/AgCl.

They focus on two main parameters:

1. Photocurrent density (P_ph)
   This is essentially the light-induced current per unit area, calculated as the difference between the current in light and in dark.

2. Photoresponsivity (R_ph)
   This is the photocurrent per unit incident optical power:

   R_ph = photocurrent / light power.

It tells you how effectively the device converts light power into electrical signal.

Effect of light intensity and bias

• P_ph increases almost linearly with light intensity: more photons → more excited carriers → higher photocurrent.

• Increasing bias voltage strongly enhances P_ph:

  • At 1.0 M KOH and ~118 mW/cm²:

    • P_ph goes from about 8.3 nA/cm² at 0 V to about 850 nA/cm² at 0.8 V.

    • This is roughly a 100× increase.

• Higher bias helps:

  • Separate photoexcited electrons and holes.

  • Drive electrons towards the electrode and minimize recombination.

Effect of electrolyte concentration

• Higher KOH concentration also increases the photocurrent:

  • Increasing from 0.1 → 0.5 → 1.0 M improves P_ph by factors of about 1.6 and then 4.3.

• Why? Because:

  • More ions improve charge transport in the electrolyte.

  • Better ion environment makes the PEC reactions more efficient.

• Electrochemical impedance spectroscopy (EIS) shows:

  • Interfacial resistance decreases with higher KOH concentration → easier electron transfer.

Overall, both bias voltage and KOH concentration help improve the device performance, but bias has the stronger effect.

Photoresponsivity trends

Interestingly, R_ph behaves differently from P_ph:

• R_ph decreases with increasing light intensity.
  At higher powers, each extra photon contributes proportionally less to the signal, a common behavior in many photodetectors due to saturation-like effects.

• Still, higher KOH concentration and higher bias increase R_ph at a given light intensity.

For example, at low power and 0.8 V in 1.0 M KOH:

• R_ph can reach ≈11.45 μA/W,

• which is about 88× higher than at 0 V under the same conditions.

4. A narrow-band UV photodetector: wavelength selectivity

One of the most interesting findings is that Nb₂C nanosheets behave like a narrow-band UV photodetector.

Wavelength-dependent response

The authors expose the device to different wavelengths (using filters):

• 350, 365, 380, 400, 475 nm, and longer visible wavelengths (520–800 nm).

• Also deep UV at 254 nm from a separate lamp.

They observe:

• Clear on/off photocurrent signals at 350–400 nm.

• Very weak or no response at 475 nm and above.

• No detectable response at 520–800 nm.

• No response at 254 nm, even though Nb₂C itself can absorb there.

Why no response at very short wavelength (254 nm)?

• KOH solution and ITO glass strongly absorb or scatter deep UV light.

• This prevents enough 254 nm light from actually reaching the Nb₂C layer.

• So the lack of signal at 254 nm is mostly due to the environment, not the material itself.

When they plot P_ph and R_ph versus wavelength in the 350–400 nm range:

• Both parameters increase and then decrease, peaking around 365 nm.

• This range (350–400 nm) becomes the effective detection window.

• The Nb₂C device therefore behaves as a narrow-band UV detector, sensitive mainly in this defined region.

Importantly, pure ITO glass shows no photoresponse under the same conditions, confirming that the signal comes from Nb₂C, not from the substrate.

Why narrow-band? A deeper look via DFT

The paper uses density functional theory (DFT) to understand why Nb₂C behaves like this.

Key ideas:

1. Surface terminations matter.
   Nb₂C can have –OH, –F, –O terminations, and these change its optical and electronic properties.

2. The authors calculate:

   • Optical absorption spectra for Nb₂C with different terminations (Nb₂C–OH, Nb₂C–F, Nb₂C–O).

   • Work functions (W_f) for bare and functionalized Nb₂C.

3. The optical calculations show:

   • Nb₂C–OH and Nb₂C–F behave more like the experimental spectrum than Nb₂C–O.

   • This supports the experimental observation that the real samples are mainly terminated with –OH and –F, as also indicated by XPS.

4. Work function argument:

   The work function W_f is the minimum energy needed to pull an electron from the material to vacuum. It’s closely related to how much photon energy is required to liberate or excite electrons efficiently.

   They find:

   • Bare Nb₂C: W_f ≈ 4.59 eV

   • Fully OH-terminated Nb₂C: W_f ≈ 4.36 eV

   • Partially OH-terminated Nb₂C (1 OH per 4 unit cells): W_f ≈ 3.33 eV

   The photon energy of a 365 nm photon is about 3.40 eV, very close to 3.33 eV.

   This suggests:

   • In KOH electrolyte, Nb₂C surfaces get partially functionalized with –OH at a specific coverage.

   • At that coverage, W_f matches the energy of photons around 365 nm.

   • Photons with this energy are particularly effective at exciting electrons across this energy barrier.

   • This makes the detector highly sensitive in the 350–400 nm band, but less so outside it.

So the narrow-band response isn’t an accident. It emerges from the interplay between:

• Surface chemistry (–OH coverage),

• Work function,

• And the energy of incoming photons.

Fast response times

The device also reacts quickly:

• Rise time (t_rise) and recovery time (t_rec) are both < 0.15 s across 350–400 nm.

• This is faster than many other reported PEC photodetectors based on 2D materials and nanostructures.

• Increasing bias can further speed up both rise and decay, as the electric field more efficiently drives carrier separation and recombination.

Stability and FET-type device

• In a long-term cycling test (1000 on/off cycles), the photocurrent barely degrades (only about 0.015% loss per cycle), indicating good stability.

• They also build a field-effect transistor (FET)-type photodetector with Nb₂C deposited on SiO₂/Si:

  • This device shows much larger photocurrents and responsivity, but

  • Much slower response times (several seconds), likely due to environmental oxidation and trapping effects.

Overall, the PEC-type architecture offers a good balance of sensitivity, speed, and stability, with the unique added bonus of narrow-band UV selectivity.

5. Nb₂C as a saturable absorber for ultrafast fiber lasers

The second big part of the paper explores Nb₂C nanosheets as a saturable absorber (SA) in passively mode-locked fiber lasers.

What is a saturable absorber?

A saturable absorber is a material whose absorption decreases at high light intensity:

• At low intensity, it absorbs strongly.

• At high intensity, it becomes more transparent.

This property lets it act like an intensity-dependent switch, allowing high-intensity light pulses to circulate in the laser cavity while suppressing weaker noise. This mechanism enables mode-locking, where many cavity modes synchronize to produce ultrashort pulses.

Because Nb₂C exhibits broadband absorption and metallic-like behavior, it’s a strong candidate for such ultrafast photonics.

Implementing Nb₂C as SA

The authors integrate Nb₂C NSs onto a tapered optical fiber:

• The waist of the tapered fiber is about 12 μm.

• Light traveling in the fiber has an evanescent field extending outside the core.

• Nb₂C deposited on the tapered section interacts with this evanescent field.

• At high intensities, the absorption saturates, enabling mode-locking.

They use this configuration in two fiber laser systems:

1. Erbium-doped fiber laser around 1559 nm (telecom band).

2. Thulium-doped fiber laser around 1882 nm (mid-infrared, ~2 µm).

6. Ultrafast pulses at 1559 nm (Er-doped fiber laser)

In the Er-doped system:

• Total cavity length is designed to give a repetition rate in the MHz range.

• A 980 nm pump powers the Er-doped fiber via a wavelength-division multiplexer.

• Polarization controllers and an isolator help control polarization and direction.

• The Nb₂C-SA is inserted via the tapered fiber section.

• The output is taken from a coupler.

Performance at 1559.98 nm

When mode-locking is reached:

• Central wavelength: ~1559.98 nm.

• 3 dB bandwidth: ~4.6 nm.

• Pulse spacing: ~80.25 ns (seen on oscilloscope).

• Repetition rate: ~12.54 MHz (consistent with cavity length).

• Signal-to-noise ratio (SNR): > 45 dB, indicating very stable pulses.

• Pulse duration: ~603 fs (from autocorrelation, assuming sech² pulse shape).

• Time-bandwidth product (TBP): ~0.341, slightly above the transform limit (0.315), meaning the pulses are slightly chirped.

• Maximum output power at higher pump levels can reach ~9.8 mW before multi-soliton effects appear.

Comparing with other MXene-based SAs (Ti₃C₂, V₂C, etc.), Nb₂C:

• Provides relatively short pulse widths.

• Achieves higher output power at the telecom wavelength.

• Shows good stability over hours of operation.

This demonstrates that Nb₂C MXene is a very competitive material for ultrafast lasers in the 1.55 µm band.

7. Ultrafast and harmonic pulses at 1882 nm (Tm-doped fiber laser)

The authors then push Nb₂C into the mid-infrared (~2 µm) region using a Tm-doped fiber laser.

Basic setup

• Total fiber length ≈ 32 m.

• Gain fiber: ~1.9 m of Tm-doped fiber.

• Pump power:

  • Continuous wave operation starts around 310 mW.

  • Mode-locking occurs when pump reaches ~600 mW.

• Again, Nb₂C on tapered fiber acts as the saturable absorber.

Performance at 1882.13 nm

In the mode-locked regime:

• Central wavelength: ~1882.13 nm.

• 3 dB bandwidth: ~2.16 nm.

• Repetition rate: ~6.28 MHz (matching cavity length).

• Pulse interval: ~158 ns.

• SNR: > 60 dB (even more stable than at 1.55 µm).

• Pulse duration: ~2.27 ps.

• TBP: ~0.442, showing slightly more chirping than the 1559 nm case.

Harmonic mode-locking

By increasing pump power further:

• They achieve harmonic mode-locking, where multiple pulses circulate in the cavity within one round trip.

• The maximum harmonic order reaches 69th, corresponding to a repetition rate of 411 MHz at 1882 nm.

• This is a strong demonstration of the robustness and fast nonlinear response of Nb₂C as a saturable absorber in the mid-IR.

Together, the 1559 nm and 1882 nm results show that few-layer Nb₂C MXene is a broadband, high-performance optical switch suitable for both telecom and mid-IR ultrafast laser systems.

8. Big picture: Why is this important?

This work is significant because it shows that one single 2D material (few-layer Nb₂C) can operate effectively in two very different optoelectronic roles:

1. As the active material in a photoelectrochemical UV photodetector:

   • Narrow-band detection in the 350–400 nm UV range.

   • Tunable performance via bias and electrolyte concentration.

   • Fast response and good stability.

   • Mechanistic understanding via DFT, linking surface termination and work function with spectral selectivity.

2. As a saturable absorber in ultrafast fiber lasers:

   • Femtosecond pulses at 1559 nm.

   • Picosecond pulses and high-order harmonic mode-locking at 1882 nm.

   • High stability and competitive performance compared to other MXenes.

The study highlights several key themes:

• MXenes are not just for batteries and supercapacitors.
  Their metallic nature and tunable surfaces make them powerful tools in photonics and optoelectronics.

• Surface chemistry is crucial.
  The narrow-band UV detection behavior comes directly from how the surface is terminated (e.g., partial –OH coverage) and how that affects the work function.

• Few-layer Nb₂C is highly versatile.
  It can work at high photon energies (UV) and low photon energies (near- and mid-IR), covering a remarkably wide spectral range with different device concepts.

• There is large room for further optimization.
  Future work can tune:

  • Flake size and thickness.

  • Termination chemistry.

  • Device architectures (solid-state PDs, integrated photonic chips, etc.).

  • Laser cavity designs for even shorter pulses or higher repetition rates.

Final takeaway

In simple terms:

The paper shows that few-layer Nb₂C MXene is a powerful and flexible photonic material. It can detect UV light in a very specific band and also act as an optical switch to generate ultrafast laser pulses at telecommunication and mid-infrared wavelengths. This combination of features opens the door to new types of MXene-based photodetectors, lasers, and integrated photonic systems that are compact, tunable, and potentially low-cost to produce.