Synthesis of Assembled Nanocrystalline Si Dots Film by the Langmuir–Blodgett Technique Atsushi TANAKA 1;4;5 , Yoshishige TSUCHIYA 1;4;5 , Koichi USAMI 1 , Shin-ichi SAITO 2;5 , Tadashi ARAI 2;5 , Hiroshi MIZUTA 3;4;5 , and Shunri ODA 1;4;5 1 Quantum Nanoelectronics Research Centre, Tokyo Institute of Technology, 2-12-1 O-okayama, Meguro-ku, Tokyo 152-8552, Japan 2 Central Research Laboratory, Hitachi, Ltd., 1-280 Higashi-Koigakubo, Kokubunji, Tokyo 185-8601, Japan 3 School of Electronics and Computer Science, University of Southampton, Highfield, Southampton SO17 1BJ, U.K. 4 Department of Physical Electronics, Tokyo Institute of Technology, 2-12-1 O-okayama, Meguro-ku, Tokyo 152-8552, Japan 5 SORST, JST (Japan Science and Technology Agency) (Received December 13, 2007; accepted January 27, 2008; published online May 16, 2008) We report on a new bottom-up technique for forming silicon nanostructures based on the assembly of nanocrystalline Si (nc- Si) dots by the Langmuir–Blodgett technique. nc-Si dots with a diameter of 10  1 nm fabricated by a very high frequency (VHF) plasma process are dispersed in solvent and functionalized with an appropriate silane coupling agent. After compression at the surface of a Langmuir trough to form a well-organized two-dimensional array, nc-Si dots are transferred onto Si substrates. We have succeeded in forming a well-assembled nc-Si dot array with an area density of 7:33  10 11 cm 2 . Furthermore, we clarified what happens at the surface of a Langmuir trough based by analyzing surface pressure-area isotherms. [DOI: 10.1143/JJAP.47.3731] KEYWORDS: nanocrystalline silicon, nanoparticle, nanoscale assembly, Langmuir–Blodgett method, –A curve 1. Introduction Over the past few decades, the performance of silicon- based very-large-scale integration (VLSI) circuits has steadily been improved by scaling down device dimensions, and a nearly exponential growth of microelectronics capa- bilities has been achieved. However, maintaining this top- down miniaturization trend is exceedingly difficult due to fundamental physical and technological limitations as well as economical limitations. 1) Recently, bottom-up approaches to form nanometer-scaled silicon structures have attracted us particularly for future applications in nanoelectronics. 2,3) We have succeeded in preparing an assembly of spherical nanocrystalline Si (nc-Si) dots with a diameter of 10 nm and a dispersion of less than 1 nm by the VHF digital plasma process, 4,5) and we have applied the process to a planar cold- electron emitter. 6) In Si nanostructure-based cold electron emitting devices, electron transport along the nc-Si dot chain structures plays an important role in reducing the energy loss of emitted electrons. 7,8) Therefore, a remarkable improve- ment is expected in emission efficiency if an ordered array is introduced to the conducting region of the emitter. In addition, the technologies for formating nc-Si dot arrays provide us with an enormous improvement in flexibility and reproducibility in device design and fabrication, in particular for single electron devices, 9) nano dot memory devices, 10) and even for future quantum information devices. 11) How- ever, in the currently used deposition chamber, individual nc-Si dots are deposited randomly on substrates, and controlling the position of nc-Si dots is a very challenging issue. On the other hand, techniques for fabricating a two- dimensional (2D) ordered array of nanoparticles using the self-assembly of colloidal particles dissolved in a small quantity of liquid have been developed in the field of colloidal particle science. 12) Denkov et al. directly observed the dynamics of 2D array formation of nanoparticles on solid substrate and discussed the mechanism from a view point of lateral capillary force. 13) In our previous report, 14) we tried to assemble our nc-Si dots using the drop-and-evaporation method and succeeded in fabricating a high-density 2D assembly of nc-Si dots, but we also found that the evaporation mechanism is too complicated to control. In this paper, we focus on the Langmuir–Blodgett (LB) technique as a more controllable method which was originally used for assembling organic molecules 15) and recently used as a method by which to assemble gold nanoparticles 16) and silica nanoparticles. 17) We applied the LB technique to form a 2D nc-Si dots array for the first time. In this report, we describe how to prepare a nc-Si dot colloidal solution for use in the LB methods. Results for the nc-Si dot 2D array made by the LB method are shown, and the surface pressure–area (–A) isotherms, which are key features for improving the quality of LB thin films, are discussed. 2. Experimental Methods Nc-Si dots with a diameter of 10  1 nm were deposited on Si substrates in ultra high vacuum (UHV) chamber using VHF plasma decomposition of a pulsed SiH 4 gas supply. 4) Since as-deposited naked dots easily react with pure water and easily go down into water, we tried covering the surface of nc-Si dots with organic molecules to prevent the reaction of the dots with pure water. We chose hexamethyldisilazane (HMDS), a silane coupling agent often used in the silicon process, as a covering molecule which can change a hydrophilic SiO 2 surface into a hydrophobic surface as shown in Fig. 1. We took out the deposited substrate from the chamber and immediately immersed it into HMDS (in about 10 s), We were then able to actually observe bubbles rising; they were caused by the reaction of the surface of HMDS Fig. 1. Schematic image of reaction of HMDS with an nc-Si dot surface. Japanese Journal of Applied Physics Vol. 47, No. 5, 2008, pp. 3731–3734 #2008 The Japan Society of Applied Physics 3731