In this blog post, we’ll walk through a recent and very interesting study about a special 2D material called Nb₂C MXene and how it can be used in two advanced optoelectronic applications:

• Narrow-band UV photodetectors, and

• Femtosecond mode-locked fiber lasers (ultrafast lasers producing extremely short pulses).

The original paper is quite technical and full of experimental details, so the goal here is to explain the main ideas in clear, simple, and logical English, without changing the meaning and without inventing any extra results.

1. Background: What Is Nb₂C MXene and Why Does It Matter?

1.1. 2D materials and MXenes

Since the discovery of graphene in 2004, researchers have become very interested in two-dimensional (2D) materials—materials that are only a few atoms thick. These ultrathin materials often show properties that are very different from their bulk (3D) versions, such as:

• High transparency,

• Very good electrical conductivity,

• Excellent mechanical strength,

• Strong and tunable optical absorption.

Over the years, many types of 2D materials have been developed, including both:

• Elemental 2D materials (like silicene, germanene, phosphorene, etc.), and

• Compound 2D materials (like transition-metal dichalcogenides, layered double hydroxides, graphitic carbon nitride, etc.).

Among these, a particularly important new family is MXenes.

MXenes are 2D transition-metal carbides and/or nitrides. They are usually made from a layered “MAX phase”, where:

• M = early transition metal (like Ti, Nb, V, etc.)

• A = element from group 13 or 14 (like Al, Si, etc.)

• X = carbon and/or nitrogen

The MAX phase has the formula M₂AX, M₃AX₂, etc. When the “A layer” is selectively etched away (chemically removed), what remains is a stack of “M–X” layers, which can be delaminated into 2D sheets called MXenes.

The general formula of a MXene is:

Mn+1XnTx(n=1–4)\text{M}_{n+1}\text{X}_n\text{T}_x \quad (n = 1–4)Mn+1​Xn​Tx​(n=1–4)

where Tₓ represents surface terminations like −F, −OH, −O, −Cl, −Br, etc. These surface groups come from the etching process and strongly affect the material’s properties.

MXenes have already been explored in:

• Energy storage (batteries, supercapacitors)

• Catalysis

• Biomedicine

• Sensors

• Nonlinear optics and photonics

1.2. Nb₂C MXene: a special niobium-based member

Within the MXene family, niobium carbide (Nb₂C) is one of the important members. It is derived from its MAX phase precursor Nb₂AlC.

Historically:

• Around 2013, Nb₂AlC was etched with hydrofluoric acid (HF) to produce Nb₂C MXene.

• Multilayer Nb₂C showed good performance as an electrode in lithium-ion batteries.

• When these multilayer stacks were further delaminated into ultrathin Nb₂C nanosheets (NSs), the electrochemical performance and lithium storage capacity improved significantly.

Later studies showed that Nb₂C MXene also has:

• High lithium storage capacity (strongly affected by surface terminations),

• Very good photothermal conversion efficiency in the NIR (near-infrared) region, useful for tumor therapy,

• Good performance in photocatalytic hydrogen evolution when converted to composites like Nb₂O₅/C/Nb₂C.

So chemically, mechanically, and thermally Nb₂C has been studied quite well.

However, its photoelectronic (optoelectronic) applications—especially photodetectors and ultrafast lasers—are still relatively new and underexplored. That is exactly the gap this paper focuses on.

2. Aim of the Study

The authors set out to:

1. Synthesize few-layer Nb₂C nanosheets (NSs) with controlled thickness and lateral size using a scalable method.

2. Explore their use as:

   • Active material in photoelectrochemical (PEC) photodetectors

   • Saturable absorber (SA) in passively mode-locked fiber lasers in:

     • The telecommunication band (~1.55 μm),

     • The mid-infrared (MIR) region (~1.88 μm).

3. Understand how:

   • Light intensity,

   • Electrolyte concentration, and

   • Applied bias voltage
     influence the photodetector performance.

4. Use density functional theory (DFT) calculations to:

   • Support the interpretation of the narrow-band photodetector behavior,

   • Estimate the work function and discuss the role of surface terminations (OH, F, O).

In simpler terms:

They made few-layer Nb₂C nanosheets and showed that the same material can act as a UV-selective photodetector and as a nonlinear optical switch for generating ultrashort laser pulses at 1.55 and 1.88 μm.

3. How Few-Layer Nb₂C Nanosheets Were Made and Characterized

3.1. Synthesis: from MAX to MXene

The synthesis follows the typical top-down MXene route:

1. Start with Nb₂AlC MAX powder.

2. Etch with HF (40% aqueous HF) at room temperature for 48 hours.

   • This removes the Al layers and produces multilayer Nb₂C MXene with a characteristic accordion-like structure.

3. Wash thoroughly with deionized (DI) water by repeated centrifugation until the pH is about 6.

   • This removes excess HF and byproducts.

4. Delaminate into few-layer nanosheets:

   • The multilayer Nb₂C is dispersed in a 25% tetrapropylammonium hydroxide (TPAOH) solution and stirred vigorously for 72 hours.

   • TPA⁺ ions intercalate between the Nb₂C layers, swelling and weakening their interaction.

   • After washing and centrifugation, the delaminated few-layer Nb₂C nanosheets are obtained as a colloidal suspension.

5. The solvent is then adjusted depending on the application:

   • Water or DMF for some measurements and device fabrication,

   • Ethanol for optical characterization and for preparing the saturable absorber.

This process yields few-layer Nb₂C nanosheets that are stable in solvents like ethanol for several weeks.

3.2. Morphology and structure (SEM, TEM, AFM)

The authors used several standard microscopy methods to check the morphology and thickness:

• SEM (Scanning Electron Microscopy) of HF-etched Nb₂C powder shows an accordion-like, multilayered structure, typical for MXenes after etching.

• TEM (Transmission Electron Microscopy) of the exfoliated material confirms:

  • A clear 2D sheet-like morphology,

  • Transparent, ultrathin flakes (few-layer nature).

• High-resolution TEM (HRTEM) shows:

  • A uniform hexagonal crystal lattice, confirming the structure of Nb₂C.

  • The selected area electron diffraction (SAED) pattern also displays hexagonal symmetry, matching the expected crystal structure.

From TEM images, they measured the lateral size of many nanosheets:

• Size range: ~30–270 nm

• Average lateral size: ~112 nm

To measure thickness, they used AFM (Atomic Force Microscopy):

• Nb₂C flakes are drop-cast or spin-coated on a silicon wafer.

• The measured height of individual sheets is about 2.4–2.9 nm.

• This corresponds to roughly five atomic layers, so the flakes are few-layer, not monolayer.

3.3. Composition and surface chemistry (EDS, XPS)

The composition and surface groups were checked using:

• Energy-dispersive X-ray spectroscopy (EDS):

  • Shows Nb and C from the Nb₂C framework.

  • Detects F and O, which are attributed to surface terminations (−F and −OH/−O) formed during HF treatment and exposure to water/air.

• X-ray photoelectron spectroscopy (XPS):

  • Confirms the disappearance of Al (so the Al layer is successfully removed from Nb₂AlC).

  • Confirms the presence of O and F on the surface, supporting the existence of −OH and −F termination groups.

3.4. Optical absorption (UV–vis–NIR)

The optical absorption of Nb₂C was measured in ethanol:

• Unexfoliated Nb₂C powder shows moderate absorption that changes gradually with wavelength.

• Few-layer Nb₂C nanosheets show:

  • Stronger and more broadband absorption from UV to NIR (200–1400 nm),

  • Particularly strong absorption in the UV region (200–400 nm),

  • Noticeable absorption features near ~1500 nm and ~1950 nm, which become relevant for the laser experiments.

The nanosheets are stable (do not precipitate quickly), as indicated by a clear Tyndall effect in the dispersion.

These optical properties already hint that Nb₂C might be a good candidate for:

• UV photodetectors, and

• NIR / MIR nonlinear optical devices such as saturable absorbers for ultrafast lasers.

4. Nb₂C as a Narrow-Band UV Photodetector

4.1. Device concept: PEC-type photodetector

The authors use photoelectrochemical (PEC) photodetectors, which combine:

• A working electrode coated with Nb₂C nanosheets on ITO glass,

• A counter electrode (Pt wire),

• A reference electrode (Ag/AgCl),

• An electrolyte based on KOH (0.1, 0.5, or 1.0 M),

• A light source (simulated solar light + filters for specific wavelengths),

• And an applied bias potential between 0 and 0.8 V vs Ag/AgCl.

The Nb₂C NSs on ITO act as the light-absorbing active layer. When light hits the electrode:

• Photons excite electrons in Nb₂C,

• The excited carriers participate in electrochemical reactions and/or are driven by the electric field,

• This generates a measurable photocurrent.

The device performance is described through:

• Photocurrent density (Pph),

• Photoresponsivity (Rph),

• Response and recovery times (t₍rise₎, t₍rec₎).

4.2. Role of bias potential and electrolyte concentration

The authors systematically study how the photocurrent changes with:

• Bias voltage (0, 0.3, 0.6, 0.8 V)

• KOH concentration (0.1, 0.5, 1.0 M)

• Light power density (different intensities)

Key observations:

• The Nb₂C-based PD shows a clear on/off photocurrent response when the light is switched on and off.

• Photocurrent increases with light intensity: more photons → more excited electrons → higher current.

• Higher bias voltage → much stronger photocurrent:

  • At 1.0 M KOH and ~118 mW/cm²:

    • Pph increases from about 8.3 nA/cm² at 0 V to about 850 nA/cm² at 0.8 V, nearly 100× increase.

  • The bias creates a stronger electric field, which:

    • Promotes separation of photoexcited charge carriers,

    • Reduces their recombination,

    • Enhances the charge transport.

• Higher KOH concentration → higher photocurrent:

  • Increasing KOH from 0.1 M to 0.5 M and 1.0 M increases Pph by about 1.6× and 4.3×, respectively, at the same bias and light intensity.

  • Electrochemical impedance spectroscopy (EIS) shows that interfacial resistance decreases with higher KOH concentration.

  • Higher ion concentration improves charge transport and accelerates the photoelectrochemical process.

From these trends, the authors conclude:

The photoresponse can be tuned and enhanced by controlling bias voltage and electrolyte concentration.

4.3. Photoresponsivity (Rph)

Photoresponsivity is defined as:

Rph=Ilight−IdarkPλ×sR_{\text{ph}} = \frac{I_{\text{light}} – I_{\text{dark}}}{P_\lambda \times s}Rph​=Pλ​×sIlight​−Idark​​

where:

• IlightI_{\text{light}}Ilight​ and IdarkI_{\text{dark}}Idark​ are the currents under light and dark,

• PλP_\lambdaPλ​ is the light intensity,

• $s$ is the effective device area.

General behavior:

• Rph decreases with increasing light intensity, which is typical for many photodetectors (due to saturation effects).

• Rph increases when:

  • The bias potential is increased, and/or

  • The KOH concentration is increased.

For example (at 26.2 mW/cm² and 0.8 V):

• Rph increases from 2.67 μA/W (0.1 M KOH) to 11.45 μA/W (1.0 M KOH), about 4.3×.

• At 1.0 M KOH and fixed light intensity, increasing bias from 0 to 0.8 V boosts Rph by about 88×.

This confirms that applied bias has a stronger effect on Rph than electrolyte concentration.

4.4. Wavelength-dependent response: narrow-band UV detection

The most interesting part is the wavelength-dependent photoresponse.

The authors use filters to select specific wavelengths from 350 to 800 nm, plus a separate deep-UV source at 254 nm. Under fixed bias (0.6 V) and KOH (1.0 M):

• All measurements are done at a similar intensity level (within practical limitations).

• Clear on/off signals appear only between ~350 and 400 nm.

• At 475 nm, the response is much weaker.

• No detectable photoresponse is observed for:

  • Longer wavelengths from 520 to 800 nm, and

  • The deep-UV wavelength of 254 nm.

From this:

• Both Pph and Rph show a peak around 365 nm, then decrease at shorter and longer wavelengths.

• The Rph values at 350–400 nm are significantly higher than under broad simulated light, indicating selective sensitivity in a narrow UV band.

Why is there no response at 254 nm?

• The KOH solution and ITO glass strongly absorb or scatter deep-UV light,

• So the actual light reaching the Nb₂C layer at 254 nm is strongly reduced.

This combination of:

• High response only in a narrow band (350–400 nm),

• Very low or no response outside that band,

means few-layer Nb₂C works as a narrow-band UV photodetector.

4.5. Understanding the narrow-band behavior: DFT and work function

To explain why Nb₂C responds strongly only over a narrow UV range, the authors use DFT calculations:

• They model few-layer Nb₂C with different surface terminations:

  • −OH, −F, −O, and different coverage ratios.

• They calculate the optical absorption spectra and find:

  • Terminated Nb₂C (especially with −OH and −F) shows trends similar to experiment, even if not identical in magnitude.

• More importantly, they calculate the work function (Wf):

Wf=Evacuum−EFermiW_f = E_{\text{vacuum}} – E_{\text{Fermi}}Wf​=Evacuum​−EFermi​

They find:

• Bare Nb₂C has a Wf ≈ 4.59 eV

• Fully −OH terminated Nb₂C has Wf ≈ 4.36 eV

• Nb₂C with one −OH per four unit cells has Wf ≈ 3.33 eV

Photon energy at 365 nm is about 3.40 eV, which is very close to 3.33 eV.

This suggests:

• The Nb₂C surface in alkaline KOH solution is partially terminated with OH groups, roughly at this coverage.

• The work function (minimum energy to release an electron to the surface) matches the UV photon energy near 365 nm.

• As a result, the material responds most strongly to photons in that energy range, giving a narrow sensitivity window.

In simpler words:

The narrow UV band where the photodetector works best corresponds to the energies that just match the work function of the surface-terminated Nb₂C, so photon energy is used most efficiently there.

4.6. Response speed and stability

Response and recovery times:

• Both t₍rise₎ and t₍rec₎ are < 0.15 s for 350–400 nm under 0.6 V and 1.0 M KOH.

• These times are faster than many other PEC-type photodetectors based on other 2D materials.

Effects:

• KOH concentration: little influence on speed.

• Bias voltage: higher bias makes both t₍rise₎ and t₍rec₎ slightly faster, as electric field helps charge separation and recombination.

Long-term stability:

• The device maintains a stable on/off response over 1000 cycles.

• The decrease in photocurrent per cycle is extremely small, showing good stability in KOH.

They also demonstrate a FET-type Nb₂C photodetector on SiO₂/Si, with much higher Pph and Rph but slower response (several seconds), likely due to surface oxidation and different device geometry.

5. Nb₂C as a Saturable Absorber in Ultrafast Fiber Lasers

Besides being a UV photodetector, Nb₂C also has strong and broadband absorption in the NIR and MIR, especially near ~1.5 μm and ~2 μm. This is ideal for using it as a saturable absorber (SA) in mode-locked fiber lasers.

A saturable absorber is a nonlinear optical element that:

• Absorbs more light at low intensity,

• Becomes more transparent at high intensity.

This intensity-dependent transmission enables the formation of short laser pulses (mode-locking).

5.1. How the SA is made

The Nb₂C NSs are deposited onto a tapered optical fiber:

• A single-mode fiber is tapered down to a waist of about 12 μm over a length of ~5 mm.

• Nb₂C nanosheets are deposited onto this waist region.

• Light propagating through the tapered fiber interacts strongly with the Nb₂C coating.

This structure serves as a fiber-integrated saturable absorber.

5.2. Er-doped fiber laser at ~1.56 μm

In the telecommunication band, they build an Er-doped passively mode-locked fiber laser:

• Pump: 980 nm laser diode.

• Gain fiber: 2.1 m of Er-doped fiber.

• Rest of cavity: ~14.3 m single-mode fiber.

• Total dispersion: anomalous, around −0.373 ps².

• Components: WDM coupler, polarization controller (PC), isolator, output coupler, and the Nb₂C SA.

By tuning the polarization controller and pump power:

• Mode-locked operation is achieved at 70 mW pump power.

• Output pulses:

  • Central wavelength: 1559.98 nm

  • 3 dB bandwidth: 4.6 nm

  • Repetition rate: 12.54 MHz (matches cavity length)

  • Pulse duration: 603 fs (fitted with a sech² profile)

  • Time–bandwidth product: 0.341 (slightly chirped, close to transform-limited)

The signal-to-noise ratio (SNR) of the radio frequency spectrum is >45 dB, confirming stable mode-locking.

The output power can reach 9.8 mW at higher pump power (up to 310 mW). At higher pump powers, multi-soliton and bound states can appear, which is typical for nonlinear fiber laser cavities.

Compared to other MXene-based SAs at 1.55 μm, the Nb₂C SA offers:

• Relatively higher output power,

• Competitive or shorter pulse durations.

This indicates that Nb₂C is an excellent saturable absorber in the telecom band.

5.3. Tm-doped fiber laser at ~1.88 μm (mid-infrared)

The authors also test Nb₂C in a Tm-doped fiber laser in the MIR region (~2 μm):

• Gain fiber: 1.9 m Tm-doped fiber.

• Total cavity length: ~32 m.

• Continuous-wave lasing starts at ~310 mW pump.

• Mode-locked operation appears at 600 mW pump power.

In mode-locked regime:

• Central wavelength: 1882.13 nm

• 3 dB bandwidth: 2.16 nm

• Repetition rate: 6.28 MHz

• Pulse duration: 2.27 ps

• Time–bandwidth product: 0.442 (slightly chirped)

• Average output power: ~12.3 mW

• SNR: >60 dB, indicating very stable operation.

They further increase the pump power and obtain harmonic mode-locking:

• Maximum harmonic order: 69th, corresponding to 411 MHz repetition rate at 1882 nm.

This is a strong demonstration that:

Nb₂C MXene can work as an effective and stable saturable absorber even in the mid-infrared region, supporting both fundamental and high-order harmonic mode-locking.

6. Overall Conclusions and Outlook

Putting everything together, this work shows that few-layer Nb₂C MXene is a powerful and versatile building block for photoelectronic devices:

1. Synthesis & structure

   • Nb₂C MXene is obtained by HF etching of Nb₂AlC followed by TPAOH-assisted delamination.

   • The resulting nanosheets are:

     • Few-layer (~5 layers, ~2.4–2.9 nm thick),

     • Lateral size ~30–270 nm (average ~112 nm),

     • Surface-terminated with −OH and −F groups.

2. Optical and electronic features

   • Strong optical absorption from UV to NIR, with notable peaks near 1500 and 1950 nm.

   • Ultrafast carrier dynamics (from previous work) and metallic character make Nb₂C suitable for high-speed photonics.

3. Narrow-band UV photodetectors

   • In PEC-type photodetectors, Nb₂C shows:

     • Tunable photocurrent and responsivity by adjusting bias potential and KOH concentration.

     • A narrow spectral detection window from 350 to 400 nm, with a peak around 365 nm.

     • Fast response and recovery times (<0.15 s) and good stability over 1000 cycles.

   • DFT calculations reveal that:

     • The work function of partially OH-terminated Nb₂C matches the photon energy around 365 nm.

     • This explains the narrow-band UV sensitivity.

4. Ultrafast mode-locked fiber lasers

   • As a saturable absorber, Nb₂C enables:

     • Sub-picosecond pulses (603 fs) at 1559.98 nm with ~9.8 mW power in an Er-doped fiber laser.

     • Picosecond pulses (2.27 ps) at 1882.13 nm with ~12.3 mW power in a Tm-doped fiber laser.

     • High-order harmonic mode-locking at 1882 nm, up to the 69th harmonic (411 MHz).

   • These performances compare favorably with other MXene-based SAs.

Overall, the study clearly shows that:

• Few-layer Nb₂C MXene can act as both a functional photodetector material and a nonlinear optical switch, using the same basic nanosheet structure.

• It has strong potential for next-generation photoelectronic devices, especially in:

  • Narrow-band UV detection,

  • Ultrafast telecommunications,

  • Mid-infrared laser systems.

As research on MXenes continues, Nb₂C is likely to play an increasingly important role in integrated photonics, optoelectronics, and advanced laser technologies.