10.1117/2.1201606.006525

All-inorganic water-dispersible
silicon quantum dots
Minoru Fujii, Hiroshi Sugimoto, and Shinya Kano
Silicon quantum dots that are co-doped with boron and phosphorus
exhibit bright and stable luminescence in the near-IR wavelength range
and are environmentally friendly.
The rapidly growing research field of biophotonics involves theuse of light-activated processes for several applications, such as
bioimaging, biosensing, medical diagnostics, and light-activatedtherapy. Colloidal silicon (Si) quantum dots (QDs) are a promising candidate to use as probes for these biophotonics purposes
because of their high biocompatibility and biodegradability. In
the conventional synthesis approach, a stable colloidal solution is produced by properly functionalizing the surface of theSi-QDs with organic ligands.1, 2 The luminescence property offunctionalized Si-QDs, however, is often too sensitive to the environment and chemical processes. The high luminescence sensitivity of Si-QDs arises because exciton wavefunctions extend to
the surface molecules. Furthermore, the dispersion of luminescent QDs in aqueous media—which is essential for many biophotonics applications—is very difficult. Although new organic
functionalization processes have been developed to overcome
these problems,3 a more fundamental solution still needs to beexplored so that versatile Si-QDs can be achieved in a variety ofbiophotonics situations.
In our recent work, we have developed a new type of Si-QDs
that do not include organic ligands and that are dispersible inalcohol and water (see Figure 1).4–8 We heavily dope our allinorganic Si-QDs with boron (B) and phosphorus (P) and thus
form a shell that has a high concentration of these elements. Thisshell induces a negative potential on the surface and makes theQDs hydrophilic.9 The shell is also chemically inert—similar toBP10 and related alloy nanocrystals11 —and has a higher resistance to hydrofluoric acid solution than undoped Si-QDs.6 In addition, our co-doped colloidal Si-QDs have several Si–hydrogenand Si–oxygen bonds on their surfaces. By utilizing well-known
reactions, we can therefore functionalize the surfaces with organic molecules.

Figure 1. (a) Photograph of a colloidal solution of boron- andphosphorus-co-doped silicon quantum dots (Si-QDs) in methanol.
(b) High-resolution transmission electron microscope (TEM) image ofa co-doped Si-QD. The lattice fringe corresponds to the f111g plane of
crystalline Si.

Our co-doped Si-QDs exhibit size-controllable photoluminescence over an extremely wide energy range (0.9–1.8eV), as
shown in Figure 2.12 We can control the luminescence energy of
the Si-QDs both above (a typical quantum size effect) and below
the bulk Si band gap (1.12eV at room temperature). We find thatthe luminescence energy of our Si-QDs is always several hundred meV lower than undoped Si-QDs with comparable sizes,
which suggests that the luminescence arises from the donor–acceptor transitions.13 In our work we have also shown that
degradation of the luminescence is almost undetectable when
the Si-QDs are kept in methanol for more than a year. Furthermore, although our co-doped Si-QDs become slowly oxidized inwater, the luminescence is still much more stable than for waterdispersible II-VI quantum dots.14 We believe that the highly stable luminescence characteristics of our Si-QDs originate from theconfinements of photoexcited excitons in the core of the QDs
and that they have little interaction with the surrounding environment. Indeed, we observe very similar luminescence spectrafrom the Si-QDs even when they are in different environments,
including methanol and water,14 hydrofluoric acid solution,6Continued on next page10.1117/2.1201606.006525 Page 2/3

air,15 and human cells. By spin coating the colloidal solution, wecan also produce films with very flat surfaces.16 Such films canbe used as a probe for monitoring the local photonic-mode density of plasmonic substrates.17 To our surprise, we found that theluminescence quantum yield of these completely dried films in
air is almost identical to that of the colloidal solution. As the luminesence of our co-doped Si-QDs is very insensitive to chemical treatments, we can attach different kinds of biosubstances
to the surfaces, without degrading the luminescence properties.
This is a unique feature of our QDs and cannot be achieved withligand-stabilized Si-QDs.
The luminescence insensitivity of our co-doped Si-QDs tothe environment and to chemical processes means that we canuse them to produce a variety of nanocomposites. For example, by simply mixing the solution of co-doped Si-QDs witha solution of positively charged gold (Au) nanoparticles andnanorods, we can form nanocomposites.18, 19 In these nanocomposites the effective excitation cross section18 and the quantum
yield19 of the Si-QDs are enhanced because of the coupling withthe localized surface plasmon resonances of the Au nanoparticles and nanorods. We can also produce nanocomposites via
chemical bonding and by using DNA hybridization. In theseprocesses we can control the distance between the Si-QDs andthe metal nanoparticles (a crucial parameter for maximizing theenhancement of the optical responses) with high accuracy. Furthermore, we can successfully disperse the co-doped Si-QDs inpolymers. Indeed, we observe bright luminescence from cellulose nanofibers in which Si-QDs and Au nanoparticles have been
simultaneously doped.20
The surface of freshly produced co-doped Si-QDs ismainly terminated by hydrogen (H) atoms,21 and the Si–H

Figure 2. Photoluminescence spectra of boron- and phosphorus-codoped Si-QDs that were grown at different temperatures (900–1250◦C)
in methanol (MeOH). The diameter of these Si-QDs ranges from
1 to 9nm.12

Figure 3. (a) High-resolution TEM image of a Si-QD/gold (Au)nanoparticle composite. A Au nanoparticle in the center is surroundedby Si-QDs (marked by yellow circles). (b) Scanning TEM energydispersive x-ray spectroscopy mapping of the hybrid nanoparticle.21bonds are very chemically active. By using the freshly produced H-terminated co-doped Si-QDs, we have explored anew self-limiting solution-synthesis method for Si-QD/metal
nanocomposites.21 In this method, we simply add melt salts toa solution of Si-QDs. In this mixed solution, the co-doped SiQDs act as a reducing agent for the metal salts and also as aprotecting layer to stop metal nanoparticle growth. With thismethod, we have demonstrated the formation of Si-QD/metal(i.e., gold, silver, or platinum) nanocomposites with different
sizes and shapes (see Figure 3).21In summary, we have developed a new approach for the formation of all-inorganic Si-QDs that are co-doped with boron andphosphorus. Our new functional nanomaterials have many superior properties compared with conventional Si-QDs that include organic ligands. Many of the new properties arise fromthe heavily B- and P-doped hydrophilic shell, which protects theSi-QD core from the environment and provides high chemicalstability. The shell also confines excitons to the Si-QD core, andthe luminescence is thus very insensitive to the environment andchemical processes. In addition, we are able to control the luminescence energy of the Si-QDs over a wide wavelength range.
These features, combined with their biocompatibility, make ourco-doped Si-QDs an attractive and environmentally friendly option for a nano-light source in biophotonics and optoelectronics
applications. In the next stages of our research we will study theluminescence property of co-doped Si-QDs and the nanocomposites in cell cultivation media, the cellular uptake processes,and the cytotoxicity. We will also develop the necessary methods to implement the Si-QDs and nanocomposites in biosensingdevices.
Continued on next page10.1117/2.1201606.006525 Page 3/3This work is partly supported by the 2014 Bilateral Joint ResearchProject (between Japan and the Czech Republic) of the Japan Society
for the Promotion of Science (JSPS), the 2015 Japan Science and Technology agency’s Visegrad Group (V4)-Japan Joint Research Project on
advanced materials, and JSPS’s Grants-in-Aid of Scientific Research(KAKENHI) funds (grant 16H03828).
Author Information
Minoru Fujii, Hiroshi Sugimoto, and Shinya Kano
Kobe University
Kobe, Japan
Minoru Fujii is a professor in the Department of Electrical and Electronic Engineering. His research interests include
nanostructured semiconductor and metal materials for photonic
applications.

15. K. Furuta, M. Fujii, H. Sugimoto, and K. Imakita, Energy transfer in silicon
nanocrystal solids made from all-inorganic colloidal silicon nanocrystals, J. Phys. Chem.
Lett. 6, pp. 2761–2766, 2015.
16. M. Sasaki, S. Kano, H. Sugimoto, K. Imakita, and M. Fujii, Surface structure andcurrent transport property of boron and phosphorus co-doped silicon nanocrystals, J. Phys.
Chem. C 120, pp. 195–200, 2016.
17. Y. Wang, H. Sugimoto, S. Inampudi, A. Capretti, M. Fujii, and L. Dal Negro,Broadband enhancement of local density of states using silicon-compatible hyperbolic metamaterials, Appl. Phys. Lett. 106, p. 241105, 2015. doi:10.1063/1.4922874
18. A. Inoue, M. Fujii, H. Sugimoto, and K. Imakita, Surface plasmon-enhanced luminescence of silicon quantum dots in gold nanoparticle composites, J. Phys. Chem. C 119,
pp. 25108–25113, 2015.
19. H. Sugimoto, T. Chen, R. Wang, M. Fujii, B. M. Reinhard, and L. Dal Negro,Plasmon-enhanced emission rate of silicon nanocrystals in gold nanorod composites, ACS
Photon. 2, pp. 1298–1305, 2015.
20. H. Sugimoto, R. Zhang, B. M. Reinhard, M. Fujii, G. Perotto, B. Marelli, F. G.
Omenetto, and L. Dal Negro, Enhanced photoluminescence of Si nanocrystals-doped
cellulose nanofibers by plasmonic light scattering, Appl. Phys. Lett. 107, p. 041111, 2015.
doi:10.1063/1.4927672
21. H. Sugimoto, M. Fujii, and K. Imakita, Silicon nanocrystal-noble metal hybrid
nanoparticles, Nanoscale 8, pp. 10956–10962, 2016.

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c 2016 SPIE