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[ChemCommun 2025, 61, 6945.pdf](https://mdr.nims.go.jp/filesets/e0139a0f-fdcd-4832-9c50-a2a4d7a43bbc/download)

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[Hiroyuki Yamada](https://orcid.org/0000-0003-0394-857X), [Tohru Tsuruoka](https://orcid.org/0000-0002-4322-4309), [Tadaaki Nagao](https://orcid.org/0000-0002-6746-2686), [Naoto Shirahata](https://orcid.org/0000-0002-1217-7589)

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[Deep-blue amplified spontaneous emission and lasing in colloidal silicon nanoclusters](https://mdr.nims.go.jp/datasets/af12046c-e108-45a4-bc25-01bc8ae6195e)

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Deep-blue amplified spontaneous emission and lasing in colloidal silicon nanoclustersThis journal is © The Royal Society of Chemistry 2025 Chem. Commun., 2025, 61, 6945–6948 |  6945Cite this: Chem. Commun., 2025,61, 6945Deep-blue amplified spontaneous emission andlasing in colloidal silicon nanoclusters†Hiroyuki Yamada, a Tohru Tsuruoka, *a Tadaaki Nagao ab andNaoto Shirahata *acWe report for the first time the amplified spontaneous emission(ASE) and lasing from blue-emitting silicon nanoclusters. The lasingthreshold is determined as 1.8 mJ per pulse and the emission peaksat 434 nm. The laser-induced emission experiments demonstratedin this study encourage further development of solution-processedfabrication of a colloidal silicon quantum dot laser.Amplified spontaneous emission (ASE) in the blue region of thevisible-spectral wavelength has attracted much interest owingto its potential use for various applications such as full-colorlasers and their spectral superimposition for white lasers.1Colloidal semiconductor quantum dots (CQDs) have beenwidely explored as optical gain media because of their char-acteristic stability against heat and light, along with thesolution-processability of molecular compounds.2 A majorobstacle of the CQDs is nonradiative carrier losses, which arecaused by nonradiative Auger relaxation and recombination atsurface traps or defects.3 The decrease in size of CQDs, whichrequires blue-light emission based on the quantum confine-ment effect, enhances the defect formation because of theincreasing surface–volume ratio, resulting in an increase ofthe nonradiative Auger rate due to increasing Coulomb elec-tron–electron coupling.4 Thus, the presence of the nonradiativerecombination pathways retards the development of lasing atblue wavelengths. There have been several attempts to find agood optical gain medium. For example, nonspherical CQDswith tetrapod- or rod-shape structures have been reported tosuppress Auger recombination rates.5 Organic–inorganic leadperovskite nanoclusters show large absorption coefficients andlow defect densities;6 however their lack of high-thermalstability, ascribed to the organics that are constituents of thehybridized structures, limits their use for photonic applica-tions. Alternatively, fully-inorganic perovskites have beenproposed to overcome the poor stability against heat andlight.7 Here, we report silicon nanoclusters as a new opticalgain medium. Since Kauzlarich’s pioneering work in 1999,8various solution routes have been developed for the synthesis ofblue-emitting silicon nanoclusters.9–11 In accordance with theprotocol we reported,12 a wafer of silicon was ablated in1-decene with a third-harmonic radiation of a Nd:YAG laser(see Fig. 1). As reported, the laser ablation produces siliconnanoclusters through the condensation and rapid quenching ofthe vapor of silicon generated by the ablation. In the absence ofoxygen molecules and moisture, the surface of the nanoclustersremains covered with silicon-centered radicals. The terminala-carbons of 1-decenes surrounding the resultant nanoclustersform covalent bonds between carbon and silicon, yieldingdecane-terminated silicon nanoclusters (see the ESI,† fordetails). Dispersion of the purified product in dichloromethaneappeared transparent and very dilute pale-yellow to the nakedeye. Fig. 1b shows a representative high-angle annular dark-field (HAADF) scanning transmission electron microscopy(STEM) image of the sample. As expected, the nanoclusters,which are contrasted as white dots due to the higher Z-contrastsensitivity of silicon than carbon in the 6-nm-thick amorphouscarbon film covering the copper TEM grid, dispersed withoutaggregation all around. Such a high contrast observed suggeststhat the dots have a crystalline structure rather than anamorphous one.13 The diameters of these dots are close to orless than 1 nm, along with a narrow size-distribution. A typicalbright field image is shown in Fig. 1c. Lattice fringes wereclearly observed as indicated by the open-circles. In addition,the nanocluster surface was not amorphized but faceted, sug-gesting the appearance of an anchor effect of the decane-monolayers, which protects the surface from amorphizationas reported previously for alkyl monolayers.14 Attenuated totalreflection Fourier-transform infrared (ATR-FTIR) spectroscopyindicated the formation of a decane monolayer and partiala Research Center for Materials Nanoarchitectonics (MANA),National Institute for Materials Science (NIMS), 1-1 Namiki, Tsukuba, 305-0044,Japan. E-mail: tsuruoka.tohru@nims.go.jp, shirahata.naoto@nims.go.jpb Graduate School of Science, Hokkaido University, Sapporo 060-0810, Japanc Graduate School of Chemical Sciences and Engineering, Hokkaido University,Sapporo 060-0814, Japan† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d5cc01428jReceived 14th March 2025,Accepted 8th April 2025DOI: 10.1039/d5cc01428jrsc.li/chemcommChemCommCOMMUNICATIONhttps://orcid.org/0000-0003-0394-857Xhttps://orcid.org/0000-0002-4322-4309https://orcid.org/0000-0002-6746-2686https://orcid.org/0000-0002-1217-7589http://crossmark.crossref.org/dialog/?doi=10.1039/d5cc01428j&domain=pdf&date_stamp=2025-04-14https://doi.org/10.1039/d5cc01428jhttps://doi.org/10.1039/d5cc01428jhttps://rsc.li/chemcomm6946 |  Chem. Commun., 2025, 61, 6945–6948 This journal is © The Royal Society of Chemistry 2025oxidation of the nanocluster surface as shown in Fig. 1d (seeFig. S1, ESI†).The optical absorption spectrum shown in Fig. 1e exhibitssimilar characteristics to silicon nanoclusters emitting in theblue spectral region reported elsewhere in the literature.8,11,12Fig. 1f shows PL spectra collected by varying the excitationwavelength from 330 nm to 390 nm in 10 nm increments. Theinset shows digital photographs of the nanoclusters underroom illumination (left) and UV (lem = 365 nm) light irradiation(right), respectively. The blue PL spectrum, peaking at 440 nm,has a long emission tail toward B630 nm. Such a long tailmight not originate from polydispersion of the nanoclustersbut appear due to interfacial-related emission.10 The peak valueof absolute PL quantum yield was 21%, which was as high asthe values of blue-light emissions reported by other papers.15The PL decay dynamics of the nanoclusters was studied at roomtemperature by fluorescence spectroscopy equipped with atime-correlated single photon counting (TCSPC) system. Theeffective values in the PL decay time were 10.5 and 4.2 ns asshown in Fig. 1g and the average decay time was calculated tobe 7.5 ns (see the ESI†).The pioneering work by Pavesi and coworkers reportingASEs in the near-infrared region has raised dreams of realizinga real silicon laser as a monolithic light source compatible withcomplementary metal oxide semiconductor (CMOS) technol-ogy. Initial study focused on the optical cavity consisting of alaser medium of silicon QDs embedded in oxide.15,16 Recently,Veinot and co-workers reported the potential of polymershybridized with red- or yellow-emitting silicon CQDs as a lasermedium.17 They observed the emission of a spectral bandwidthas narrow as ca. 9 nm from the Fabry–Pérot resonator thatincorporates the hybridized active layer sandwiched betweentwo SiO2/Ag mirrors under 351 nm light irradiation using an Arion laser, along with the simultaneous blueish emissionoriginating from the SiO2 protective layer. Observation of theblue emission band inspired us to realize the appearance ofASE in our silicon nanoclusters. The dichloromethane solutionof the nanoclusters was poured into a custom-made quartz-cuvette (see Fig. S2, ESI†). The cuvette had two circular BK-7substrates coated with dielectric multilayers on both sides, andwas placed in front of a cylindrical lens. As guided with a whitearrow in Fig. 2, a laser beam using third harmonic generation(THG, l = 355 nm) was reflected 901 by a mirror, magnified to12 mm in diameter by passing through a beam expander andfocused through the cylindrical lens to produce a beamlinelonger than the cuvette width (B10 mm). The beamline wasirradiated on silicon nanoclusters. In the absence of dielectricmultilayer mirrors, the PL spectrum showed a peak at 440 nm,along with a full-width at a half maximum (fwhm) of ca. 112 nmas indicated by the dotted line in Fig. 3. An optical resonatorwas prepared by sandwiching a quartz cuvette between mirrors.Upon irradiation of the excitation laser, an emission peak at434 nm was obtained. When the excitation density wasincreased, the emission at 434 nm soon became dominantFig. 1 (a) A scheme for the synthesis of decane-terminated silicon nanocrystals. (b) HAADF-STEM image, (c) HR-TEM image, (d) ATR-FT-IR spectrum, (e)UV-vis absorption spectrum, (f) PL spectra at different excitations and (g) PL decay curve of the silicon nanocrystals. The inset of panel (f) shows the digitalphotographs of dichloromethane solution of the silicon nanocrystals under room illumination (left) and 365-nm UV lamp (right). In panel (g), the best fitwas obtained by biexponential decay functions (w2 = 1.12).Fig. 2 Optical pumping apparatus for the characterization of ASE andlaser emission.Communication ChemCommThis journal is © The Royal Society of Chemistry 2025 Chem. Commun., 2025, 61, 6945–6948 |  6947(see Fig. S3, ESI†). Meanwhile, the fwhm of the peak became asnarrow as 3.6 nm (see solid line in Fig. 3). This corresponds to alinewidth 33 times narrower than that observed from thesteady-state PL spectrum. We estimated the average values ofemission intensity with increasing the pumping power, whichresulted in the input–output power curve as shown in the insetof Fig. 3. The laser threshold was observed at a pump powerdensity of approximately 411 mJ cm�2, which is four orders ofmagnitude higher than those of fluorene-based oligomers asgain media for deep blue lasers.18,19Surface trap states (e.g., shallow, midgap and deep energy levels)influence the optical and electric properties in silicon nano-particles.20,21 The existence of oxygen on the silicon surface resultsin oxidation to form an amorphous silica layer. Formation of theinterface between silicon nanoclusters and the surface oxide givesrise to the blue fluorescence originating from the radiative recom-bination between electrons and holes through the silica-relatedemission centers.10,22 According to the analysis of time-resolvedtransient absorption spectral dynamics, the initially photogener-ated carriers are trapped by the silica-related subband energy levelwithin the first B2 ps after excitation, and transit to the localizedstates within B6 ps.21 Further study is needed but the populationinversion and the buildup of optical gain might be generatedbetween the localized state energy level and ground state in asimple three-level system upon the nanoclusters being opticallypumped with a nanosecond laser.In summary, this paper reports the blue-emitting siliconnanoclusters as a new family worthy of inclusion in the libraryof silicon-based laser media. In the past, the lasing in the blue-spectral range has been achieved by amplification of the defect-derived blue-light emission from the protective SiO2 layerconstructed in the optical cavity, but we constructed a multi-energy level system within a nanocluster to facilitate theformation of population inversions under optical pumpingconditions. This finding may pave the way for the developmentof silicon lasers that satisfy the required conditions for theCMOS-compatible monolithic light source.T. T. and N. S. designed the research; H. Y., T. N. and N. S.performed the synthesis and characterization of the siliconnanoclusters; T. T. and N. S. prepared the setup for opticalpumping and ASE evaluation; H.Y., T. T., T. N. and N. S.discussed the results, and H. Y. and N. S. wrote the paper.This work was supported by the WPI program, JSPSKAKENHI (grant no. 24K01462, 24K01278 and 24K21720),Hosokawa powder technology foundation (HPTF24111), andARIM of MEXT (JPMXP1224NM5178). H. Y. thanks theResearch Fellow of JSPS and JSPS KAKENHI Grant-in-Aid(grant no. 23KJ2166).Data availabilityThe data that support the findings of this study are availablefrom the corresponding author upon reasonable request.Conflicts of interestThere are no conflicts to declare.Notes and references1 (a) D. Hahm, V. Pinchetti, C. Livache, N. Ahn, J. Noh, X. Li,J. Du, K. Wu and V. I. Klimov, Nat. Mater., 2025, 24, 48;(b) P. Kambhampati, Nat. Nanotechnol., 2025, 20, 189; (c) X. Lin,Y. Yang, X. Li, Y. Lv, Z. Wang, J. Du, X. Luo, D. Zhou, C. Xiao andK. Wu, Nat. Nanotechnol., 2024, 20, 229; (d) L. N. Quan, B. P. Rand,R. H. Friend, S. G. Mhaisalkar, T. Lee and E. H. Sargent, Chem. Rev.,2019, 119, 7444; (e) H. Dong, C. Zhang, X. Liu, J. Yaoab andY. S. Zhao, Chem. Soc. Rev., 2020, 49, 951.2 (a) N. Ahn, C. Livache, V. Pinchetti and V. I. Klimov, Chem. Rev.,2023, 123, 8251; (b) N. Taghipour, M. Dalmases, G. L. Whitworth,M. Dosil, A. Othonos, S. Christodoulou, S. M. Liga andG. Konstantatos, Adv. 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