Research

Minoru FUJII （藤井　稔） : Research on Mesoscopic Materials

RESEARCH ACTIVITIES

Colloidal Group IV (Si, Ge) Semiconductor Nanocrystals

		178.　Hiroshi Sugimoto, Minoru Fujii, Kenji Imakita, Shinji Hayashi, and Kensuke Akamatsu, "Codoping n- and p-Type Impurities in Colloidal Silicon Nanocrystals -Controlling Luminescence Energy from below Bulk Band Gap to Visible Range", The Journal of Physical Chemistry C, Vol. 117, No. 22, pp. 11850–11857 (2013).
		We present a novel synthesis of ligand-free colloidal silicon nanocrystals (Si-NCs) that exhibits efficient photoluminescence (PL) in a wide energy range (0.85− 1.8 eV) overcoming the bulk Si band gap limitation (1.12 eV). The key technology to achieve the wide-range controllable PL is the formation of donor and acceptor states in the band gap of Si-NCs by simultaneous doping of n- and p-type impurities. The colloidal Si-NCs are very stable in an ordinary laboratory atmosphere for more than a year. Furthermore, the PL spectra are very stable and are not at all affected even when the colloids are drop-cast on a substrate and dried in air. The engineering of the all- inorganic colloidal Si-NC and its optical data reported here are important steps for Si- based optoelectronic and biological applications.

		175.　Hiroshi Sugimoto, Minoru Fujii, Kenji Imakita, Shinji Hayashi, and Kensuke Akamatsu, "Phosphorus and Boron Co-doped Colloidal Silicon Nanocrystals with Inorganic Atomic Ligands", The Journal of Physical Chemistry C, Vol. 117, No. 13, pp. 6807−6813 (2013).
		The surface structure of P and B codoped colloidal Si- NCs are studied by photoluminescence (PL) in hydrofluoric acid (HF) solution and X-ray photoelectron spectroscopy (XPS). We find that codoped Si-NCs are much more stable in HF solution than undoped, P- doped, and B-doped Si-NCs. The PL study combined with XPS results reveal that a high B concentration layer is formed on the surface of codoped Si-NCs and the layer acts as a kind of inorganic atomic ligands for Si-NCs. The high B concentration layer makes Si-NCs hydrophilic and dispersible in polar liquids. Furthermore, the layer effectively protects Si-NCs from oxidation in solution and in air.

		164.　Hiroshi Sugimoto, Minoru Fujii, Kenji Imakita, Shinji Hayashi,and Kensuke Akamatsu, All-Inorganic Near-Infrared Luminescent Colloidal Silicon Nanocrystals-High Dispersibility in Polar Liquid by Phosphorus and Boron Co-doping The Journal of Physical Chemistry C., Vol.116, No. 33, pp.17969-17974 (2012). (2012).
		We demonstrate the formation of a new type of surfactant-free colloidal silicon nanocrystal (Si-NC). The characteristic structural feature of the Si-NCs is simultaneous doping of phosphorus (P) and boron (B) in and on the surface of Si-NCs. The codoped Si-NCs are stable in methanol for more than a year and exhibit luminescence in the near-infrared range. We perform comprehensive studies on the structure of codoped colloidal Si-NCs and discuss the mechanism of the high solution dispersibility.

		152.　Masatoshi Fukuda, Hiroshi Sugimoto, Minoru Fujii, Kenji Imakita, and Shinji Hayashi, Surfactant-free solution-dispersible Si nanocrystals -surface modification by impurity control　 Optics Letters, Vol. 36, No. 20, pp. 4014-4016 (2011).
		Si nanocrystals (Si-NCs) dispersible in polar liquid without surface functionalization by organic molecules have been realized by simultaneously doping n and p type impurities. We show that the codoped Si-NCs are stable in methanol for more than five months, while intrinsic Si-NCs prepared by the same procedure form large agglom- erates. The different behavior of the intrinsic and codoped Si-NCs in solutions suggests that doped impurities exist on the surface of Si-NCs and the surface potential is large enough to prevent the agglomeration. The colloidal solution of codoped Si-NCs exhibits broad photoluminescence with the maximum in the near infrared range (1:1–1:3 eV).