Colloidal semiconductor nanocrystals (NCs) have been recognized as promising solution-processable gain media; however, the lasers with state-of-the-art performance exclusively originate from the cadmium- and lead-based NCs. Herein, we for the first time unravel that high-quality heavy-metal-free ZnSe/ZnS NCs show superior optical gain and lasing performance when the sizes exceed the quantum confinement regime. Corroborated by comprehensive transient spectroscopy, we reveal that the optical gain in large ZnSe/ZnS NCs originates from the novel Coulomb-correlated electron–hole plasma (C-EHP) instead of high-order multi-exciton recombination. Thanks to the formation of a four-level system and the suppression of Auger recombination, the C-EHP renders low gain threshold (9.4 μJ/cm2), high gain coefficient (>6500 cm−1), and long gain lifetime (∼4 ns). Such desirable gain properties compete well with those of classic CdSe NCs and enable the construction of a high-performance laser device. This work represents significant progress toward the development of solution-processable non-heavy-metal nanocrystal lasers.

Colloidal semiconductor nanocrystals (NCs) are recognized as promising candidates for developing next-generation solution-processable laser devices owing to their chemical flexibility, spectral tunability, solution-processability, and exceptional optical properties.1–3 The past decade has witnessed an impressive progress in NC-based optoelectronic devices, including that of electrically driven amplified spontaneous emission (ASE) diodes and optically pumped continuous-wave sub-wavelength lasers.4,5 However, state-of-the-art NC lasers are exclusively constructed from cadmium (Cd) and lead (Pb)-based NCs. Due to the restrictions of heavy metals, these NCs may face challenges to enter the consumer market in the future.6 

To meet the purpose of sustainable development, a number of non-heavy-metal NCs have been explored as the alternative gain media, including AgInS2, Ag2Se, and InP.7–9 Nevertheless, none of them could exhibit gain performance comparable to those of the classic CdSe analogs in terms of gain threshold, gain coefficient, and gain lifetime, due to the tendency of defect formation in ternary NCs and the high-fold degeneracy of optical transition in III–V NCs.6,10 In this regard, the II–VI ZnSe NC can be considered as a promising candidate for heavy-metal-free light-emitters since it possesses a band structure similar to that of CdSe. However, previously fabricated ZnSe NCs typically show poor photoluminescence quantum yield (PLQY) due to the presence of rich defects, such as stacking faults and surface oxidation states, which compromises the optoelectronic aspirations.11–13 

Recent advances in controlling nucleation and shelling processes has made high-quality ZnSe NCs available.14,15 Thanks to the merits endowed by quantum confinement, most of the studies on ZnSe NCs are currently focused on those with dimensions comparable to or much smaller than the exciton in bohr radius.16,17 In such a system, elementary photoexcitation is dominated by the excitons, and the optical gain is attributed to the biexciton and even high-order exciton recombination.18 However, these quantum-confined NCs usually suffer from severe and fast nonradiative Auger recombination, which has been recognized as the main gain dissipating channel in semiconductor NCs.2 Taking CdSe-based NCs as an example, tremendous efforts have been devoted to tackling the Auger loss by engineering complicated nano-heterostructures to achieve improved gain performance, exemplified by the development of giant quasi-type-II CdSe/CdS NCs,19 compositionally graded CdSe/CdZnSe/ZnS NCs,20 and the CdSe core–shell nanoplatelets.21 Nevertheless, such a strategy has not yet been feasible for ZnSe NCs due to the less mature synthetic techniques and limited shelling options.15 As a consequence, it is imperative to find a new scheme for developing high-performance optical gain and lasers from non-heavy-metal NCs.

In this work, we for the first time unravel that heavy-metal-free ZnSe/ZnS NCs without any deliberate heterostructure engineering exhibit superior optical gain performance that is comparable to those of the classic CdSe analogs, including a low gain threshold (9.4 μJ/cm2), high gain coefficient (>6500 cm−1), and long gain lifetime (∼4 ns). Corroborated by the results obtained from systematic wavelength- and power-dependent transient spectroscopy, we find that their desirable gain performance is enabled by a new gain mechanism of Coulomb-correlated electron–hole plasma (C-EHP), rather than high-order multi-exciton recombination. Specifically, the C-EHP forms a four-level gain system that facilitates the build-up of population inversion at low carrier density and nonradiative Auger recombination is significantly suppressed by virtue of the screening effect. Taking advantage of the preferable optical gain, the high-performance vertical cavity surface emitting laser (VCSEL) device featuring a low pump threshold and directional emission output is demonstrated. This work opens a new avenue toward the development of high-performance solution-processable lasers from eco-friendly colloidal NCs.

A series of ZnSe/ZnS NCs with different sizes are synthesized following a recipe slightly modified from the literature (see supplementary material, Note 1 for synthesis details).14,22 The NCs exhibit a high PLQY of 84%–90%, indicating the samples are of high quality. Transmission electron microscope (TEM) images illustrate size ranges from 4 to 16 nm [Fig. 1(a) and Fig. S1]. It is noted that all of the samples possess a similar shell thickness of ∼1 nm controlled by the shelling process. The absorption and emission spectra from the ZnSe/ZnS NC series are shown in Figs. 1(b) and 1(c), respectively. It could be seen that as the NC size increases, the bandgap energy reduces from violet (∼3.1 eV) to a near-bulk range of 2.80 eV. Accordingly, the small and large NCs are termed based on their sizes relative to the exciton in bohr diameter a0 of ∼7.2 nm (Fig. S3).

FIG. 1.

(a) TEM and UV lamp-irradiated fluorescent images of the ZnSe/ZnS NCs. (b) and (c) Absorption and PL spectra of the ZnSe/ZnS NCs as a function of the NC size. (d) Pump intensity-dependent PL spectra from thin film of the ZnSe/ZnS NCs (size: 10 nm) via the stripe pumping configuration. Inset: PL spectra in the linear coordinate. (e) Plot of the FWHM and integrated intensity of the PL peaks as a function of pump fluence. (f) Integrated PL intensity with changing stripe length and constant pump fluence at 34.9 and 167.5 μJ cm−2, respectively.

FIG. 1.

(a) TEM and UV lamp-irradiated fluorescent images of the ZnSe/ZnS NCs. (b) and (c) Absorption and PL spectra of the ZnSe/ZnS NCs as a function of the NC size. (d) Pump intensity-dependent PL spectra from thin film of the ZnSe/ZnS NCs (size: 10 nm) via the stripe pumping configuration. Inset: PL spectra in the linear coordinate. (e) Plot of the FWHM and integrated intensity of the PL peaks as a function of pump fluence. (f) Integrated PL intensity with changing stripe length and constant pump fluence at 34.9 and 167.5 μJ cm−2, respectively.

Close modal

To examine the potential of high-quality ZnSe/ZnS NCs for gain media, ASE characterization is performed through stripe pumping configuration (see supplementary material, Note 1 for synthesis details). Figure 1(d) depicts the pump fluence-dependent PL spectra from large ZnSe/ZnS NCs with diameter of 10 nm, and those for the small analogs are shown in Fig. S5. It is found that a relatively broad spontaneous emission with full-width at half maximum (FWHM) of 12 nm dominates under low pump intensities and a narrow peak with FWHM of 4 nm emerges under high pump fluences, indicating the achievement of ASE [Fig. 1(e)]. A plot of the integrated PL intensity as a function of pump fluence shows a representative threshold-like behavior, further confirming the development of ASE, and the threshold is derived to be ∼10.2 μJ/cm2 [Fig. 1(e)]. Note that the logarithmic coordinate is adopted in Fig. 1(d) to highlight the transition from spontaneous emission to ASE since the linear one [inset in Fig. 1(d)] can only display the ASE and the weak spontaneous emission is covered up. To the best of our knowledge, this threshold is the lowest one for heavy-metal-free NCs, and it is even comparable to those of the classic CdSe NCs with deliberate heterostructure engineering (Fig. S6 and Table S1).2,7–9,13 The model gain coefficient is further derived by variable stripe length (VSL) measurement.23 As shown in Fig. 1(f), a high model gain coefficient of ∼5000 cm−1 is determined at a fluence of 167.5 μJ cm−2. In contrast, the ASE thresholds for small ZnSe/ZnS NCs with strong confinement regime are much higher (∼110 μJ/cm2, Fig. S5), indicating the benefits of large ZnSe/ZnS NCs for light amplification applications.

To understand the seemingly unusual ASE behavior, femtosecond transient absorption (TA) spectroscopy was exploited. Figures 2(a) and 2(b) depict the pseudocolor TA map and the corresponding spectra of the representative ZnSe/ZnS NCs with size of 10 nm at a pump fluence of 6.3 μJ/cm2 (corresponding to a carrier density of n0 ∼ 2.1 × 1018 cm−3, see carrier density calculation in supplementary material, Note 2). The TA spectra are dominated by a broad bleaching band, consisting of heavy hole (HH) and light hole (LH) transitions. With increase in the carrier density, the amplitude of the bleaching band (Δα) increases significantly to overtake the intrinsic absorbance (α0), indicating the achievement of optical gain (Fig. S7). Quantitatively, the optical gain g in NCs is derived from the negative nonlinear absorbance (α = α0 + Δα < 0).24, Figure 2(c) plots the carrier density-dependent nonlinear absorption spectra. It is found that the net gain appears at a low carrier density of ∼3.2 × 1018 cm−3 (corresponding to fluence of 9.4 μJ/cm2), and the gain coefficient at a fluence of 187.2 μJ cm−2 (6.4 × 1019 cm−3) is derived to be ∼5400 cm−1, which are consistent with the ASE results. Remarkably, the gain coefficient could reach up to a high value of >6500 cm−1 with further increase in the carrier density. In contrast, for the quantum-confined ZnSe/ZnS NCs with sizes (4–6 nm) smaller than the bohr radius, the gain occurs at a varied and higher carrier density range of 8.5 × 1018–2.5 × 1019 cm−3, which corresponds to much higher pump fluences of 40–100 μJ/cm2 [Figs. S8(a) and S8(b)]. Interestingly, the occurrence of varied carrier density thresholds in the small ZnSe/ZnS NCs points to a nearly constant ⟨N0⟩ of ∼1.6 (the average electron–hole pairs per NC, ⟨N0⟩), which is consistent with a biexcitonic gain character.22 

FIG. 2.

(a) Pseudocolor TA image of the ZnSe/ZnS NCs with size of 10 nm and n0 ∼ 2.1 × 1018 cm−3. (b) Corresponding spectral distributions at indicated delay times. (c) Nonlinear absorption spectra (α = Δα + α0) at 1 ps for different carrier densities. Inset: Zoomed image of the band edge part, showing a transition from absorption to net gain. (d) Evolution of gain threshold as a function of the NC size and excitation wavelength.

FIG. 2.

(a) Pseudocolor TA image of the ZnSe/ZnS NCs with size of 10 nm and n0 ∼ 2.1 × 1018 cm−3. (b) Corresponding spectral distributions at indicated delay times. (c) Nonlinear absorption spectra (α = Δα + α0) at 1 ps for different carrier densities. Inset: Zoomed image of the band edge part, showing a transition from absorption to net gain. (d) Evolution of gain threshold as a function of the NC size and excitation wavelength.

Close modal
In stark contrast, the large ZnSe/ZnS NCs are found to share a nearly constant carrier density of 2.8 × 1018 cm−3 regardless of the size and excitation wavelength [Fig. 2(d)]. Such a size-independent threshold is similar to the gain characteristics in bulk semiconductors, where the optical gain is derived from an unbound electron–hole plasma (EHP).25–27 However, the persistent excitonic feature above the threshold density [Fig. 2(c)] is inconsistent with the traditional unbound EHP behavior, where the exciton resonance is fully destroyed by the screening of Coulomb interaction.28,29 Similar absorption peaks have also been found in other large NCs and their oscillator strengths monotonously decrease as the size increases (Fig. S10), confirming that they arise from the remaining excitonic absorption. In general, there are two possible origins for the remaining excitonic signature. One is known as the Mahan exciton, resulting from the interaction between electron–hole pairs and electron gas.30 The presence of Mahan exciton would enhance the continuum absorption and blue shift the HH exciton peak, but this is not observed in our experiment. The other is the so-called Coulomb-correlated EHP (C-EHP, also known as the non-degenerate EHP), where the electron–hole pairs in the plasma behave collectively and exhibit long-range correlations.31–33 To verify the C-EHP gain in our case, we tentatively apply the C-EHP model to describe the nonlinear gain spectra g ω:27,
g ω = α ω × f c f v ,
(1)
where α ω denotes the absorption, f c and f v are the Fermi occupation factors for the conduction and valence bands, respectively (see more details in supplementary material, Note 3). As shown in Fig. 3(a), this model reproduces the evolution from net absorption to net gain across the bandgap transitions well, providing strong evidence for the C-EHP gain in large ZnSe/ZnS NCs. Importantly, compared to the unbound EHP with an inherent three-level transition scheme, the C-EHP could create a four-level system and achieve a low-threshold population inversion.31 Specifically, the stimulated emission from the C-EHP is coupled to plasmon resonance, resulting in a plasmon–phonon interactive state [Fig. 3(b)] and relaxing the inversion criterion by32,
E e F n 0 E h F n 0 E g n 0 ω p ,
(2)
where E e , h F n 0 is the quasi-Fermi level of the electron or hole and E g n 0 is the bandgap energy. The plasmon frequency ω p is approximated by ω p = n 0 e 2 ε 0 ε r m r, where ε r and m r are the effective dielectric constant and reduced mass, respectively. Here, the model involving Coulomb interaction is adopted to extract the gain-related parameters, and it gives a theoretical gain threshold of ∼2.5 × 1018 cm−3 [Fig. 3(c), see supplementary material, Note 4 for details].28 This threshold agrees well with the experimental value and is far smaller than that of the unbound plasma (>1019 cm−3), contributing to the low-threshold optical gain from the C-EHP. It is worth noting that different from the bulk semiconductors with quasi-infinite dimension, the sizes of the large NCs are close to the Debye screening length λ S (Fig. S13). This means that the charged carriers in the plasma undergo correlated motions within the restricted range defined by the Debye length, which leads to long-range correlations and, hence, the formation of C-EHP.
FIG. 3.

(a) Theoretical fits to the experimental gain spectra based on the C-EHP gain model. Inset shows the fitted gain spectra by varying the carrier density. (b) Schematic depiction of the gain transition from the exciton to the C-EHP as the NC size increases. (c) Modulations of the Fermi energy, electron–hole pair chemical potential, and transient bandgap energies as a function of carrier density.

FIG. 3.

(a) Theoretical fits to the experimental gain spectra based on the C-EHP gain model. Inset shows the fitted gain spectra by varying the carrier density. (b) Schematic depiction of the gain transition from the exciton to the C-EHP as the NC size increases. (c) Modulations of the Fermi energy, electron–hole pair chemical potential, and transient bandgap energies as a function of carrier density.

Close modal
Importantly, the Auger loss, known as the main gain dissipating channel in semiconductor NCs, is found to be greatly suppressed in large ZnSe/ZnS NCs as reflected by the long gain lifetime of ∼4 ns [Fig. 4(a)]. To quantitatively investigate the Auger recombination, the pump fluence-dependent carrier decay is fitted by the rate equation [Fig. 4(b)].34 It is found that the carrier recombination under high excitation intensities follows a third-order behavior (see more details in supplementary material, Note 5):
d n d t = k 3 n 3 ,
(3)
where k3 is the third-order Auger coefficient and is determined to be a constant of 5 × 10−29 cm6/s at a relatively low density range (<1019 cm−3) [inset in Fig. 4(c)]. Interestingly, with further increase in the carrier density, the Auger coefficient exhibits orders-of-magnitude reduction to 9 × 10−32 cm6/s [Fig. 4(c)], thus confirming the greatly suppressed Auger loss. Such a phenomenon can be understood by analyzing the microscopic dependence of the Auger coefficient on carrier density from a computational perspective. In wide-bandgap ZnSe semiconductors, Auger recombination occurs via a phonon-assisted indirect process that can be evaluated by35,36
k 3 = 2 2 π n 0 3 1234 , v q f 1 f 2 ( 1 f 3 ) 1 f 4 n ν q + 1 2 ± 1 2 M 1234 ; v q 2 δ ϵ 1 + ϵ 2 ϵ 3 ϵ 4 ω ν q ,
(4)
where f i i = 1 , 2 , 3 , 4 are the Fermi occupation factors of the electron/hole states involved in the Auger event [Fig. 4(d)], n ν q are the phonon occupation numbers for the band v at wavevector q, ω v q are the phonon frequencies, and M 1234 ; v q and ϵ i are the matrix elements and eigenvalues of the states, respectively. With the above derivation, it can be seen that the major dependence of the Auger coefficient on carrier density is through the matrix elements M 1234 ; v q (see more details in supplementary material, Note 5).37 The main contribution to M 1234 ; v q comes from screened Coulomb interactions ( W), which reads
W r 1 , r 2 = W r 1 r 2 1 V q W ̃ q e i q r 1 r 2 = 1 V q 1 ε q 4 π e 2 q 2 + κ 2 e i q r 1 r 2 ,
(5)
where V is the cell volume, ε q is the dielectric function in reciprocal space, and κ is the inverse Debye screening length ( 1 / λ S ). As shown in Fig. S13, κ undergoes tremendous enhancement as the carrier density increases, which reduces W and the Auger transition matrix elements. As a consequence, the Auger coefficient decreases abruptly under high carrier densities, contributing to the long-lasting (4 ns) optical gain.
FIG. 4.

(a) Net gain map of the ZnSe/ZnS NCs with size of 10 nm at a carrier density of n0 ∼ 1.1 × 1020 cm−3. (b) Carrier density-dependent TA kinetics at the lowest HH transitions. The curves are fitted by the third-order Auger component. (c) Evolution of the Auger coefficient as a function of carrier density. (d) Schematic depiction of the indirect Auger recombination and carrier screening processes at high carrier density.

FIG. 4.

(a) Net gain map of the ZnSe/ZnS NCs with size of 10 nm at a carrier density of n0 ∼ 1.1 × 1020 cm−3. (b) Carrier density-dependent TA kinetics at the lowest HH transitions. The curves are fitted by the third-order Auger component. (c) Evolution of the Auger coefficient as a function of carrier density. (d) Schematic depiction of the indirect Auger recombination and carrier screening processes at high carrier density.

Close modal

In contrast, as shown in Fig. S14, the Auger rate of the quantum-confined NCs monotonously increases with carrier density, and the gain lifetime is determined to be only 50 ps, which will surely lead to severe Auger loss. These results rationalize the much-improved gain performances from the large ZnSe/ZnS NCs compared to those of the strongly confined analogs.

Taking advantage of the superior gain from ZnSe/ZnS NCs beyond the confinement regime, the VCSEL device is constructed. To do so, large ZnSe/ZnS NCs of 10 nm are spin-coated onto a distributed Bragg reflector (DBR) that is highly reflective around the blue spectral regime [Fig. 5(a)]. Then, the other DBR reflector is brought upside-down to contact well with the NC film following our previously developed method.38 Finally, the device is fixed with glue. Figure 5(b) presents the evolution of PL spectra from the device as a function of pump intensity. It is seen that multiple sharp PL peaks appear under low pump fluence (<8.2 μJ/cm2). These comb-like peaks are not the lasing emission; in fact, they correspond to the modulated spontaneous emission arising from the cavity effect as has been reported for VCSELs.38 With further increase the pump intensity (>13.6 μJ/cm2), the discrete peaks with wavelength close to the stimulated emission spectra grow nonlinearly, while the others saturate in intensity. Simultaneously, a dramatic narrowing of the PL linewidth to ∼0.4 nm is observed, indicating the achievement of lasing action [Fig. S15(a)]. A plot of the PL intensity as a function of the pump fluence shows typical threshold behavior, revealing the lasing threshold to be ∼9.8 μJ/cm2 [Fig. S15(b)]. Moreover, Fig. 5(a) shows a photograph of the output beam from the device. The directional emission beam is clearly observed far (∼50 cm) from the device when the pump fluence is above the threshold. This is the most convincing evidence for the development of lasing action in our device.

FIG. 5.

(a) Schematic configuration and photographs of the VCSEL. The scale bars are 5 cm. (b) Lasing spectra of the VCSEL with large ZnSe/ZnS NCs as the optical gain layer.

FIG. 5.

(a) Schematic configuration and photographs of the VCSEL. The scale bars are 5 cm. (b) Lasing spectra of the VCSEL with large ZnSe/ZnS NCs as the optical gain layer.

Close modal

In conclusion, we demonstrate the superior optical gain from eco-friendly ZnSe/ZnS NCs induced by non-excitonic interaction. Thanks to the formation of a four-level system and the suppression of Auger recombination, the novel C-EHP gain renders low threshold, high coefficient, and long lifetime, which competes well with the properties exhibited by classic CdSe NCs. These results present a new paradigm of eco-friendly NC-based gain media and pave a new direction to develop solution-processable optical amplifiers and lasers.

See the supplementary material for more information on experimental details, TEM images, TA and optical gain spectra of ZnSe/ZnS NCs with different NC size, calculation of carrier density, estimation of chemical potentials, and Debye screening length.

This work is supported by the National Natural Science Foundation of China (Nos. 62274090 and 11904172). The work is supported by “the Fundamental Research Funds for the Central Universities,” No. 30923010101.

The authors have no conflicts to disclose.

Zhigao Huang and Hanchen Shen contributed equally to this work.

Zhigao Huang: Data curation (lead); Formal analysis (lead); Writing – original draft (lead). Hanchen Shen: Resources (lead); Writing – original draft (supporting). Yiming Wu: Validation (supporting). Yuting Wu: Data curation (lead); Visualization (equal); Writing – original draft (equal). Weigao Xu: Software (lead); Supervision (supporting). Xie Zhang: Formal analysis (lead); Methodology (lead); Writing – review & editing (equal). Yue Wang: Funding acquisition (lead); Investigation (lead); Methodology (lead); Supervision (lead); Writing – review & editing (lead).

The data that support the findings of this study are available from the corresponding authors upon reasonable request.

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