Klockmannite CuSe Nanocrystals: Synthesis & Properties

• Klockmannite CuSe nanocrystals are quasi-layered, anisotropic structures exhibiting both semiconducting and metallic behavior across visible and NIR ranges.
• Thiol-free colloidal synthesis using 1-octadecene and oleylamine enables precise control over shape, yielding micron-scale nanosheets and triangular nanoplatelets.
• Their unique optical properties—including hyperbolic dispersion and tunable NIR plasmon resonances—support advanced applications in photonics, sensing, and ultrafast devices.

Klockmannite copper selenide nanocrystals are quasi-layered, anisotropic nanostructures composed of the hexagonal CuSe phase. These nanocrystals (NCs) exhibit a unique combination of semiconducting and metallic behavior across the visible and near-infrared (NIR) spectral ranges. Their intrinsic crystal anisotropy leads to strong optical and photonic effects, including hyperbolic dispersion and NIR-localized surface plasmon resonances (LSPRs). Shape-controlled colloidal synthesis—particularly via thiol-free hot injection—yields two main morphologies: micron-scale nanosheets (NSs) and monocrystalline triangular nanoplatelets (NPLs), which enable sophisticated studies of structure–property relationships, ultrafast carrier dynamics, and advanced optoelectronic phenomena (Parekh et al., 26 Dec 2025).

1. Thiol-Free Colloidal Synthesis and Shape Control

The optimized colloidal synthesis employs a non-coordinating 1-octadecene (ODE) solvent and oleylamine (OLA), which acts simultaneously as a mild reductant and L-type ligand. The protocol omits phosphines and thiols, thereby preventing undesirable Se–S side reactions and eliminating sulphur contamination.

• Precursors and Injection Protocol: The selenium source consists of 4 mmol of Se powder dissolved in an ODE:OLA (3:1) mixture (8–16 mL total volume) at 200 °C under argon. Copper is provided as 0.05–0.20 mmol CuI in 3 mL degassed OLA, rapidly injected at the target synthesis temperature $T_{\text{inj}}$.
• Temperature Dependency:
  • $T_{\text{inj}}$ < 200 °C favors the formation of non-layered, cubic berzelianite Cu$_{2-x}$Se.
  • $200$ °C $\le$ $T_{\text{inj}}$ $\le$ $220$ °C produces pure hexagonal klockmannite CuSe as NSs and NPLs.
  • $T_{\text{inj}}$ > 230 °C leads to mixed-phase products, including Cu$_{1.8}$Se and larger NSs.
• Morphology via Precursor Ratios:
  • High Cu:Se (0.2:4 mmol) yields NSs with lateral dimensions from 0.2–4 μm and thickness of 5–15 nm.
  • Lower ratios (0.1:4 mmol) favor uniform hexagonal NPLs.
  • Very low Cu:Se (0.05:4 mmol) produces monodisperse triangular NPLs (altitude 12–25 nm, thickness ≤5 nm).
• Stoichiometry and Thermodynamics: Synthesis follows the reaction CuI + Se → CuSe + ½ I$_2$. Thermodynamic accounting applies:

$$K = \frac{[\mathrm{CuSe}]}{[\mathrm{Cu}^+][\mathrm{Se}^{2-}]}, \qquad \Delta G^\circ = -RT \ln K, \qquad Q = \frac{[\mathrm{CuSe}]}{[\mathrm{Cu}^+][\mathrm{Se}^{2-}]}, \qquad \Delta G = \Delta G^\circ + RT \ln Q$$

These parameters allow process control and enable systematic optimization of phase purity and morphology (Parekh et al., 26 Dec 2025).

2. Structural and Morphological Characterization

Klockmannite CuSe crystallizes in a hexagonal lattice (space group P6$_3$/mmc), with lattice constants $a=b=3.94$ Å and $c=17.25$ Å. Structural verification employs X-ray diffraction (Cu K$\alpha$, $\lambda=0.154$ nm), which displays characteristic 2$\theta$ peaks:

Peak (2θ, deg)	Miller Index
10.1	(002)
27.8	(102)
31.0	(006)
41.9	(008)
53.1	(0010)

Bragg’s law, $n\lambda=2d\sin\theta$, underpins the indexing.

Transmission electron microscopy (TEM) resolves morphology: NSs exhibit lateral dimensions 0.2–4 μm (thickness 5–15 nm); NPLs show edges of 12–25 nm (thickness ≤5 nm). Selected area electron diffraction (SAED) along [001] confirms single-crystallinity with hexagonal symmetry. High-resolution TEM (HRTEM) on NPLs reveals lattice fringes with $d_{110}$ ≈ 0.20 nm and $d_{020}$ ≈ 0.17 nm (Parekh et al., 26 Dec 2025).

3. Optical and Plasmonic Properties

The optical response is dominated by pronounced anisotropy and strong NIR LSPRs.

• Steady-State Absorption and Band Gaps: Optical band gap $E_g$ for NSs is ≈ 2.2 eV; for NPLs, $E_g$ ≈ 2.5 eV (Tauc plots: $(\alpha h\nu)^2 \propto (h\nu - E_g)$).
• LSPR Behavior:
  • NSs support broad LSPR centered at ≈ 1400 nm.
  • NPLs exhibit narrower LSPR at ≈ 1100 nm.
• Electronic Permittivity and Hyperbolic Dispersion: $ab~initio$ QSGW+RPA calculations yield the complex permittivity tensor $$\varepsilon(\omega)=\varepsilon_1(\omega)+i\varepsilon_2(\omega)$$. Both in-plane ($\varepsilon_\perp$) and out-of-plane ($\varepsilon_\parallel$) components turn negative for $\lambda>900$ nm (metallic behavior). The material manifests a natural hyperbolic domain for $550$ nm < $\lambda$ < $880$ nm, characterized by $$\varepsilon_\perp(\omega)\cdot\varepsilon_\parallel(\omega)<0$$.
• CSDDA++ Simulations: Orientation-averaged LSPR for simulated NPLs (edge 19 nm, thickness 5 nm) appears at ~1045 nm in vacuum, red-shifting to ~1140 nm in hexane ($n=1.45$), matching experimental data. Field mapping identifies dipolar corner hotspots (metallic domain) and a continuous “envelope” mode near the epsilon-near-zero transition (hyperbolic regime).

This pronounced optical anisotropy and the emergence of a hyperbolic spectral window are direct consequences of the quasi-2D crystal symmetry (Parekh et al., 26 Dec 2025).

4. Ultrafast Photophysical Dynamics

Femtosecond pump–probe and transient absorption (TA) studies reveal ultrafast carrier kinetics and coherent phonon generation.

• Experimental Protocol: Pump (460 nm, ~70 fs, NOPA, 1 kHz), probe (white-light continuum, ~100 fs) in cross-polarized configuration. Drop-cast films on quartz ($OD$ 0.15–0.25 at 460 nm).
• TA Signal Decomposition:

$$\Delta A(t) = A_1e^{-t/\tau_1} + A_2e^{-t/\tau_2} + A_3e^{-t/\tau_3} + Be^{-t/\tau_d}\cos(\omega t + \phi)$$

• Hot-hole cooling/trapping: $\tau_1$ ≈ 1.3 ps (NSs), 0.9 ps (NPLs)
• Trap-mediated recombination: $\tau_2$ ≈ 99 ps (NSs), 92 ps (NPLs)
• Long-lived population: $\tau_3$ > 2 ns (both)
• Coherent phonon frequency: $\omega/2\pi$ ≈ 7.6 THz (NSs), 7.8 THz (NPLs); damping $\tau_d$ ≈ 0.5–0.7 ps
  • Phonon Origin and Anisotropy:
• Coherent oscillations arise from displacive excitation of coherent phonons (DECP), confirmed by a slight red-shift relative to ground-state Raman ν = 260 cm⁻¹ (~7.8 THz, Se–Se stretch). Additional low-frequency Raman modes (17, 21, 26, 45 cm⁻¹) signal interlayer shear/breathing in the quasi-2D lattice.
• Anisotropic carrier effective masses ($m^*_{xy} \approx 0.86\,m_e$, $m^*_z \approx 1.05\,m_e$) affect both hyperbolic dispersion and phonon-coupling dynamics (Parekh et al., 26 Dec 2025).

5. Applications and Future Prospects

The unique properties of klockmannite CuSe nanocrystals enable diverse technological platforms:

• Tunable NIR Plasmonics: Applicable to optical communications, chemical and biological sensing, and photothermal therapy via controlled LSPR signatures.
• Natural Hyperbolic Metamaterials: Hyperbolic dispersion permits sub-diffraction imaging, hyperlensing, and the design of planar metamaterials without artificial structuring.
• Ultrafast Photonic Devices: The combination of tunable hot-carrier lifetimes and coherent phonon oscillations suits applications in photodetectors, optical switches, and hot-hole extraction for catalytic and energy conversion processes.
• Flexible Electronics: Large, monocrystalline NSs and NPLs facilitate the construction of conductive films ($\sigma \approx 6.4\times10^2\,\text{S\,cm}^{-1}$ at 293 K), supporting flexible optoelectronic devices (Parekh et al., 26 Dec 2025).

In summary, thiol-free colloidal synthesis produces phase-pure, shape-controlled klockmannite CuSe NCs whose crystal anisotropy manifests in hyperbolic optical response, robust NIR plasmonic activity, and ultrafast carrier/phonon phenomena—enabling next-generation optoelectronic, photonic, and sensing paradigms.