# Fileset

[Nanoscale Advances 7 (2025) 4837–4841.pdf](https://mdr.nims.go.jp/filesets/8a7219e1-1c67-4b0f-ab8a-c1fd13500d50/download)

## Creator

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

## Rights



## Other metadata

[Rational impurity doping for enhanced hole mobility in silicon quantum dots for light-emitting diodes](https://mdr.nims.go.jp/datasets/11c3c4ff-7aed-4601-abda-746ec77ec93a)

## Fulltext

NanoscaleAdvancesCOMMUNICATIONOpen Access Article. Published on 08 July 2025. Downloaded on 8/6/2025 12:08:40 AM.  This article is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported Licence.View Article OnlineView Journal  | View IssueRational impurityaResearch Center for Materials NanoarchiMaterials Science (NIMS), 1-1 Namiki, TsukbGraduate School of Chemical Science and ENishi 8, Kita-ku, Sapporo 060-8628, Japan† Electronic supplementary informahttps://doi.org/10.1039/d5na00349kCite this:Nanoscale Adv., 2025, 7, 4837Received 14th April 2025Accepted 26th June 2025DOI: 10.1039/d5na00349krsc.li/nanoscale-advances© 2025 The Author(s). Published bydoping for enhanced holemobility in silicon quantum dots for light-emittingdiodes†Hiroyuki Yamada, *a Tadaaki Nagao a and Naoto Shirahata *abWe report on the impact of impurity doping on the performanceenhancement of silicon quantumdot (SiQD) light-emitting diodes. Theincrease in hole mobility resulting from boron doping increases theexternal quantum efficiency of electroluminescence by a factor of 12and the optical power density by a factor of 2.65.IntroductionThe main features that make the nanocrystal form of silicon (Si)interesting are that Si is naturally abundant and has noknown biological toxicity,1 along with being a typical indirectbandgap semiconductor. In spite of this indirect characteristic,bright uorescence emissions based on quantum connementeffects, which occur when the nanocrystal size is equal to orsmaller than the Bohr radius (∼5 nm), can be observed.2Photoluminescence (PL) spectra are tuneable in the 530–1130nm range by varying the diameter between 1.1 nm and 7.8 nm.3Such spectral tunability has attracted attention for applicationto optically active layers in the current-driven quantum dotlight-emitting diodes (QLEDs). In 2010, Holmes and co-coworkers reported the rst Si-QLED with an electrolumines-cence (EL) spectral peak at l = 868 nm.4 In the following year,several research groups reported red-visible light-emittingdiodes.5,6 The tuning range of EL spectra has also beenexpanded to a broad range between 590 nm and 1000 nm.7,8Moreover, white light emission was also achieved bytaking advantage of the large Stokes shi between the absorp-tion and emission spectra, which is an inherent characteristic ofthe unique energy structure of SiQDs.9 The peak value ofexternal quantum efficiency (EQE) of the red QLEDs based ontectonics (MANA), National Institute foruba 305-0047, Japanngineering, Hokkaido University, Kita 13,tion (ESI) available. See DOI:the Royal Society of ChemistrySiQDs was ∼4% in the early study,6 rising to 6.2% in 2018.10Over the past three years, EQEs have exceeded 10%,11 witha record value of 16.5% achieved in 2025.12 While the EQE hascome within reach of the industrial benchmark of 20%, a newlyemerging concern is the low luminance. Specically, such highEQEs have been observed at a very low applied bias voltage,close to the turn-on voltage. This is a common undesirablecharacteristic of Si-QLEDs: even a slight increase in currentdensity leads to a rapid drop in EQE, unlike QLEDs based onother semiconductors, including CdSe, making it difficult toachieve high luminance while maintaining high efficiency.Watanabe and co-workers noted a lack of balanced energydiagram due to lower carrier mobility for holes compared tothat for electrons in Si-QLEDs, resulting in suppressed lumi-nance with increasing current density.8 To address the chal-lenge mentioned above, many attempts have been made tooptimize the energy band alignment using various materialsincluding molecules for the electron injection/transportationlayer (EIL/ETL) and hole injection/transportation layer (HIL/HTL).6,7,13 Another method is to use short ligands such asallylbenzene or octane as capping molecules on the QDsurface.10,14Heterovalent doping is achieved by intentionally incorpo-rating an impurity ion that has a different valence from the hostcation, providing extra electrons for n-type conductivity or extraholes for p-type conductivity. For Si crystals, boron and phos-phorus are traditional dopants used to provide p- and n-typesemiconductor Si. Recent results showing successful impuritydoping in SiQDs have suggested the potential application of p-type and n-type conducting materials.15 In 2024, we reporteda solution-processing method for constructing a p–n homo-junction architecture and demonstrated the fabrication of a p–nhomojunction photodiode using p- and n-type SiQDs.16 Thisresult inspired us to use the doped QDs as an optically activelayer in QLEDs to enhance carrier mobility. In this study, weattempt to enhance the carrier mobility for holes to achievea balanced energy band diagram.Nanoscale Adv., 2025, 7, 4837–4841 | 4837Nanoscale Advances CommunicationOpen Access Article. Published on 08 July 2025. Downloaded on 8/6/2025 12:08:40 AM.  This article is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported Licence.View Article OnlineResults and discussionIn accordance with our previous protocol,16 boron-doped SiQDscapped with a monolayer of 10-undecanoic acid were prepared,as illustrated in Fig. 1(a) (see the Experimental section, ESI†). Atypical XRD pattern of the hydrogen-capped p-type SiQDs isshown in Fig. 1(b). The diffraction peaks observed at 2q = 28°,47°, 56° were indexed to the (111), (220) and (311) planes ofa diamond cubic Si lattice structure. The mean diametercalculated using theWilliamson–Hall method was 1.9± 0.7 nm.It has been reported that the diameters of SiQDs smaller than 2nm, as estimated from XRD peak broadening, are in goodagreement with those measured by high-resolution trans-mission electron microscopy and small-angle X-ray scattering.3The hydrogen-terminated QD samples were characterized usingvarious analytical techniques. First, there was no visible shi inthe XRD peak positions when compared to the undoped sample(see Fig. S2, ESI†). Second, inductively coupled plasma opticalemission spectrometry (ICP-OES) measurement indicated thatonly 0.21 mol% of boron was present in the sample synthesizedwith a nominal 5 mol% boron content, and 0.23mol%when thenominal concentration was increased to 30 mol% during thesynthesis. Third, the sample synthesized with nominal 5 mol%boric acid was characterized by X-ray photoelectron spectros-copy (XPS). The XPS B1s spectrum of the as-synthesized sampleshows a dominant peak centered at 188 eV (see Fig. S3, ESI†),corresponding to the B–B bond. Aer 5 days of ambient expo-sure, the B0 signal disappeared and was replaced entirely byB2O3 peaks, suggesting that the boron ions detected in the XPSB 1s spectrum resided on the SiQD surface and became oxidizedFig. 1 (a) A schematic representation for the preparation process forhydrogen-capped SiQDs, (c) FTIR spectrum and (d) UV-VIS and PL spec4838 | Nanoscale Adv., 2025, 7, 4837–4841over time in air. Consequently, the fraction of boron substitu-tionally incorporated into the SiQD lattice is assumed to besignicantly low (well below 0.21 mol%), and any slight peakshi arising from minor lattice distortion or trace levels ofsubstitutional boron may be obscured by the intrinsic broad-ness of the XRD peaks.A Fourier transform infrared (FT-IR) spectrum of the p-typeSiQDs terminated with undecanoic acid (UA) monolayers ispresented in Fig. 1(c). The peaks in the range of 2960–2850 cm−1and 1500–1350 cm−1 were attributed to the stretching andbending/scissoring modes of the C–H bond, respectively. Theabsorption in the 1735–1700 cm−1 range was attributed to theC]O stretching mode, indicating the termination of the Sisurface with UA molecules. The broad peak observed at 1100–1000 cm−1 was attributed to the stretching mode of the O–Si–Obond, possibly due to surface oxidation during the thermalhydrosilylation reaction. The small peak observed in the rangeof 2140–2080 cm−1 indicated that Si–Hx bonds remained evenaer the hydrosilylation reaction.8 As shown in Fig. 1(d), the PLspectrum displayed a peak centered at 765 nm with a full widthat half maximum (FWHM) of 165.6 nm. The mean value ofabsolute PLQY was 16.7%. On the other hand, the undopedSiQDs exhibited a PL peak at 755 nm with a FWHM of 143.3 nm(Fig. S4, ESI†) and the mean value of absolute PLQY was 23.6%.Further experiments are needed to clarify the differences inspectral properties. This colloidal ink of p-type SiQDs was usedas the optically active layer for Si-QLEDs.The device architecture and its cross-sectional SEM imageare presented in Fig. 2(a). Formation of the Ag electrode, MoO3HIL and TCTA HTL was performed using a vacuum evaporator,B-doped SiQDs terminated with UA monolayers, (b) XRD pattern oftra of the B-doped SiQDs terminated with UA monolayers.© 2025 The Author(s). Published by the Royal Society of ChemistryFig. 2 (a) Schematic illustration of the Si-QLEDs with an inverted device structure and a cross-sectional SEM image of the device structure witha hybrid organic/inorganic multilayer stack, and (b) flat energy band diagram under unbiased conditions.Communication Nanoscale AdvancesOpen Access Article. Published on 08 July 2025. Downloaded on 8/6/2025 12:08:40 AM.  This article is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported Licence.View Article Onlineas depicted in Fig. S5, ESI.† We found that TCTA forms a at,smooth and amorphous thin lm structure without voids on theluminescent layer at an evaporation rate of 20 Å s−1. Theproposed at energy band diagram under zero applied biasvoltage is shown in Fig. 2(b). The valence bandmaximum (VBM)and conduction band minimum (CBM) of ZnO thin lms wereestimated from UPS and UV-VIS spectra (Fig. S6, ESI†). For theB-doped SiQDs, the VBM energy was 5.3 eV and the Fermi levelposition was close to the VBM, conrming p-type behaviour (seeFig. S7, ESI†). The values of TCTA and MoO3 were taken fromthe literature.17,18 Fig. 2(b) shows the favourable condition foreffective electron injection from the electrode at a low appliedbias voltage owing to the low energy barrier between ITO andZnO. In addition, the deep VBM energy level of ZnO acted asa hole blocking barrier, conning holes within the active layer.Moreover, the high LUMO energy barrier of TCTA layers pre-vented the leakage of electrons to the adjacent anode, leading toenhanced radiative electron–hole recombination in the activelayer for light emission.Fig. 3 exhibits the device characteristics of the Si-QLED withan emission layer consisting of B-doped SiQDs while those ofthe Si-QLED with undoped SiQDs as a control are displayed.Panel (a) shows the current–voltage (I–V) characteristics whilethe optical power density characteristics as a function of voltageare shown in the panel (b). The values of turn-on voltage,dened as a voltage at which the optical power density is greaterthan 0.1 nW cm−2, were 2.62 and 2.55 V for the Si-QLEDs withand without the boron dopant, respectively. The peak opticalpower density of the device with B-doped SiQDs was observed tobe 169 mW cm−2 at 9.1 V. Then, the optical power densitydecreased as the applied voltage exceeded 9.2 V, even thoughthe current density of the device continued to increase. On theother hand, the optical power density of the device with undo-ped SiQDs increased with increasing applied voltage, andalmost saturated at 10 V. The estimated peak optical intensitywas 63.7 mW cm−2. In panel (c), the EQE values of each Si-QLEDare plotted as a function of device current density. For eachdevice, the EQE increased with increasing device currentdensity, reaching peaks of 3.65% at 0.08 mA cm−2 and 0.3% at3.1 mA cm−2, and then began to decrease. This is probably due© 2025 The Author(s). Published by the Royal Society of Chemistryto Auger recombination that dominates the efficiency decreaseat high current densities. We fabricated three Si-QLED devicesof to evaluate the reproducibility of the EQE value. As shown inFig. S8 (ESI),† the peak EQE reached 3.85% (see also Fig. S9,ESI†), while the other two devices exhibited 3.65%, indicatingthe reproducibility of boron doping in enhancing EQE. Panel (d)shows EL spectra of the device with B-doped SiQDs, measured atapplied voltages ranging from 4 V to 9 V. The EL spectral peak at4 V was observed at 765 nm, corresponding to the PL spectralpeak, implying that EL originates from the SiQD layer. The ELspectra shied relatively toward the shorter wavelengths withincreasing applied voltage, and a peak was observed at 733 nmat 9 V. Observation of such a spectral shi is possibly due to thequantum conned Stark effect and/or band-lling effect thatoccur in the high device current density regime as commonlydiscussed in the literature.19 In addition, the absence of para-sitic EL from neighbouring layers even at 9 V indicates that theelectrons injected from the electrode did not radiativelyrecombine in the ETL or HTL, but instead recombined withholes in the SiQDs to produce EL emission. As shown in theinset of panel (d), the luminance of the red EL was clearlyuniform in a 2 × 2 mm region (see Fig. S1, ESI†), and no voidemission was observed with the naked eye. This might be due tothe hydrophilic property of the carboxylic-termination of QDswhich improved the wettability with the underlying ZnO thinlm.In order to assess the electron and hole carrier mobilities ofthe B-doped SiQDs, an electron-only device (EOD) and a hole-only device (HOD) were fabricated in a manner similar to the Si-QLED (see Fig. S10, ESI†). The EOD consisted of ITO/ZnO/QDs/ZnO/Ag while the HOD consisted of ITO/PEDOT:PSS/TFB/QDs/TCTA/MoO3/Ag. The carrier mobilities of both electrons andholes were estimated using the Space-Charge-Limited-Current(SCLC) model.20 The calculation equation is as follows:J ¼ 983r30mV 2d3(1)where J is the device current density, 3r and 30 are the relativeand vacuum dielectric constants, respectively. m represents theeld-independent carrier mobility, and d represents theNanoscale Adv., 2025, 7, 4837–4841 | 4839Fig. 3 Comparison of the device characteristics of B-doped and undoped Si-QLEDs: (a) I–V characteristics, (b) optical power density asa function of voltage, (c) plots of EQE as a function of device current density (the inset shows a zoomed-in view of the EQE dependence atcurrent densities between 0 and 2 mA cm−2) and (d) EL spectra of B-doped Si-QLEDs plotted at different applied voltages. The insets showa photograph of the device operating at 9 V.Nanoscale Advances CommunicationOpen Access Article. Published on 08 July 2025. Downloaded on 8/6/2025 12:08:40 AM.  This article is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported Licence.View Article Onlinethickness of the QD layer. In this study, 3r and 30 are assumed tobe 5.8 and 8.85 × 10−14 F cm−1 based on the literature.20 Thecalculated charge injection ratios are presented in Table 1.The B-doped SiQDs exhibit slightly lower electron mobilitythan undoped SiQDs, which is consistent with the typicalbehaviour observed in B-doped bulk Si. Conversely, the holemobility of the B-doped SiQDs was found to be 3.38 timeshigher than that of the undoped SiQDs. This comparativeanalysis clearly demonstrates a signicant improvementattributable to boron doping in SiQDs. Consequently, thecarrier balance (me/mh) in the B-doped SiQDs was improved bya factor of 6.49 compared to that of the undoped SiQDs.Although boron doping improves the carrier balance by a factorof 6.49, the resulting enhancement in EQE is approximately 12-fold. This discrepancy stems from the difference in currentdensities at which peak EQEs were recorded: 0.08 mA cm−2 forthe B-doped device and 3.1 mA cm−2 for the undoped one. Atsuch a high current density, the undoped device suffers fromTable 1 Electron (me) and hole (mh) mobility results obtained within theSCLC modelSiQD me (cm2 V−1 s−1) mh (cm2 V−1 s−1) me/mhBoron-doped (3.91 � 0.45) × 10−3 (1.74 � 0.29) × 10−5 224.713Undoped (7.50 � 0.10) × 10−3 (5.14 � 0.61) × 10−6 1459.144840 | Nanoscale Adv., 2025, 7, 4837–4841severe carrier imbalance and signicant Auger recombinationlosses, which may result in an EQE that is up to 12 times lowerthan that of the B-doped device. Similarly, the limited increasein maximum optical power density (2.65-fold) in the B-dopeddevice is likely attributable to Auger recombination dominatingat high current densities induced by increased driving voltage.ConclusionsWe introduce a novel method for optimizing carrier balance inthe active layer by incorporating B-doped SiQDs. Compared toSi-QLEDs using undoped SiQDs, those utilizing B-doped SiQDsexhibited signicantly enhanced performance, with an EQEapproximately 12 times higher and an optical power densityabout 2.65 times greater. Analysis of electron-only and hole-onlydevices revealed that boron doping decreases electron mobilityin SiQDs while markedly increasing hole mobility by approxi-mately 3.38-fold. Consequently, boron doping in SiQDs emergesas a promising strategy for adjusting electron and hole mobil-ities, thereby achieving balanced carrier transport within theactive layer.Data availabilityThe data that support the ndings of this study are availablefrom the corresponding author upon reasonable request.© 2025 The Author(s). Published by the Royal Society of ChemistryCommunication Nanoscale AdvancesOpen Access Article. Published on 08 July 2025. Downloaded on 8/6/2025 12:08:40 AM.  This article is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported Licence.View Article OnlineAuthor contributionsH. Y. and N. S. designed the research, H. Y. performed theresearch, T. N. contributed to the analysis of the materials, H.Y., T. N. and N. S. discussed the results, and Y. H. and N. S.wrote the paper.Conflicts of interestThere are no conicts to declare.AcknowledgementsThe authors thank Takeo Ohsawa at NIMS for UPS measure-ments, Yuka Hara at NIMS for SEM observations, TakanobuHiroto at NIMS for powder X-ray measurements and Yu Fujii atNIMS for ICP-OES measurements. This work was supported bythe WPI program, ARIM of MEXT (JPMXP1225NM5200), JSPSKAKENHI grants (24K01462 and 24K21720) and the HosokawaPowder Technology Foundation (HPTF24111). H. Y. thanks JSPSfor the Research Fellowship and KAKENHI Grant-in-Aid (GrantNo. 23KJ2166).Notes and references1 K. Yong, W. Law, R. Hu, L. Liu, M. T. Swihart andP. N. Prasad, Chem. Soc. Rev., 2013, 42, 1236; P. Reiss,M. Carrière, C. Lincheneau, L. Vaure, S. Tamang, B. Bruhn,B. JM Brenny, S. Dekker, I. Doğan, P. Schall andK. Dohnalová, Light Sci. Appl., 2017, 6, e17007;İ. N. G. Özbilgin, T. Yamazaki, J. Watanabe, H. T. Sun,N. Hanagata and N. Shirahata, Langmuir, 2022, 38, 5188;G. Kim, G. Lee, M. Kang, M. Kim, Y. Jin, S. Beck, J. Cheon,J. Sung and J. Joo, Faraday Discuss., 2020, 222, 1236304.2 L. Canham, Faraday Discuss., 2020, 222, 10; U. R. Kortshagen,R. M. Sankaran, R. N. Pereira, S. L. Girshick, J. J. Wu andE. S. Aydil, Chem. Rev., 2016, 116, 11061.3 N. Shirahata, J. Nakamura, J. Inoue, B. Ghosh, K. Nemoto,Y. Nemoto, M. Takeguchi, Y. Masuda, M. Tanaka andG. A. Ozin, Nano Lett., 2020, 20, 1491.4 K. Cheng, R. Anthony, U. R. Kortshagen and R. J. Holmes,Nano Lett., 2010, 10, 1154.5 D. P. Puzzo, E. J. Henderson, M. G. Helander, Z. Wang,G. A. Ozin and Z. Lu, Nano Lett., 2011, 11, 1585; C.-C. Tu,L. Tang, J. Huang, A. Voutsas and L. Y. Lin, Appl. Phys.Lett., 2011, 98, 213102.6 K. Cheng, R. Anthony, U. R. Kortshagen and R. J. Holmes,Nano Lett., 2011, 11, 1952.© 2025 The Author(s). Published by the Royal Society of Chemistry7 F. Maier-Flaig, J. Rinck, M. Stephan, T. Bocksrocker,M. Bruns, C. Kübel, A. K. Powell, G. A. Ozin andU. Lemmer, Nano Lett., 2013, 13, 475; H. Yamada,N. Saitoh, B. Ghosh, Y. Masuda, N. Yoshizawa andN. Shirahata, J. Phys. Chem. C, 2020, 124, 23333.8 J. Watanabe, H. Yamada, H.-T. Sun, T. Moronaga, Y. Ishiiand N. Shirahata, ACS Appl. Nano Mater., 2021, 4, 1165.9 B. Ghosh, Y. Masuda, Y. Wakayama, Y. Imanaka, J. Inoue,K. Hashi, K. Deguchi, H. Yamada, Y. Sakka, S. Ohki,T. Shimizu and N. Shirahata, Adv. Funct. Mater., 2014, 24,7151.10 X. Liu, S. Zhao, W. Gu, Y. Zhang, X. Qiao, Z. Ni, X. Pi andD. Yang, ACS Appl. Mater. Interfaces, 2018, 10, 5959.11 H. Yamada, J. Watanabe, K. Nemoto, H. Sun andN. Shirahata, Nanomater., 2022, 12, 4314; Q. He, K. Wang,D. Li, D. Yang and X. Pi, Adv. Opt. Mater., 2024, 12, 2302422.12 L. Wang, Y. Wada, H. Ueda, T. Hirota, K. Sumida, Y. Oba andK. Saitow, Small Sci., 2025, 2400647.13 L. Yao, T. Yu, L. Ba, H. Meng, X. Fang, Y. Wang, L. Li,X. Rong, S. Wang, X. Wang, G. Ran, X. Pi and G. Qin, J.Mater. Chem. C, 2016, 4, 673–677; K. Saitow, Bull. Chem.Soc. Jpn., 2024, 97, uoad002.14 M. L. Mastronardi, E. J. Henderson, D. P. Puzzo, Y. Chang,Z. Bin Wang, M. G. Helander, J. Jeong, N. P. Kherani, Z. Luand G. A. Ozin, Small, 2012, 8, 3647–3654.15 N. J. Kramer, K. S. Schramke and U. R. Kortshagen, NanoLett., 2015, 15, 5597; T. Velusamy, S. Mitra, M. Macias-Montero, V. Svrcek and D. Mariotti, ACS Appl. Mater.Interfaces, 2015, 7, 28207; Y. Hori, S. Kano, H. Sugimoto,K. Imakita and M. Fujii, Nano Lett., 2016, 16, 2615; Z. Ni,X. Pi, S. Zhou, T. Nozaki, B. Grandidier and D. Yang, Adv.Opt. Mater., 2016, 4, 700; S. Chandra, Y. Masuda,N. Shirahata and F. Winnik, Angew. Chem., Int. Ed., 2017,56, 6157; A. P. Y. Wong, W. Sun, C. Qian, A. A. Jelle, J. Jia,Z. Zheng, Y. Dong and G. A. Ozin, Adv. Sustainable Syst.,2017, 1, 1700118.16 B. Ghosh, H. Yamada, K. Nemoto, W. Jevasuwan, N. Fukata,H. T. Sun and N. Shirahata, Small Sci., 2024, 4, 2400367.17 H. Yamada, J. Watanabe, K. Nemoto, H. Sun andN. Shirahata, Nanomaterials, 2022, 12, 4314.18 H. Peng, W. Wang and S. Chen, IEEE Electron Device Lett.,2015, 36, 369–371.19 D. Wang, J. Chen, J. Zhu, C. Lu and M. Lu, J. NanoparticleRes., 2013, 15, 2063.20 Q. Wan, Z. Li, C. Zhang, W. Zheng, L. Huang, M. Liu,Q. Zhang, B. Wang, W. Liu, L. Kong and L. Li, J. Phys.Chem. C, 2020, 124, 28424.Nanoscale Adv., 2025, 7, 4837–4841 | 4841