Over the past decade, one-dimensional inorganic nanostructures have emerged as promising materials for fundamental studies and possible technological applications. [1][2][3][4][5][6][7][8][9][10][11][12][13][14] These structures exhibit physical and chemical properties distinct from their bulk counterparts as a result of radial confinement, while they retain the advantages of wirelike connectivity. In particular, silicon [1,5] and silicide [6] nanowires have received considerable attention owing to their potential ease of integration into conventional silicon-based electronics.
e-FeSi is a narrow-bandgap semiconductor with a cubic structure (space group P2 1 3) that has been classified as a hybridization-bandgap semiconductor or Kondo insulator. [15,16] It has attracted interest for over half a century, mainly because of its unusual magnetic behavior. [17][18][19][20][21] Moreover, a recent study has identified doped iron monosilicides as potential alternatives to (GaMn)As and (GaMn)N in spintronics applications. [22] Attempts to make FeSi thin films [23][24][25] and nanorods [26,27] embedded in silicon substrates have been reported in the past few years. To our knowledge, however, freestanding FeSi nanostructures have never been prepared, nor have magnetic or transport measurements been performed on embedded-rod or thin-film samples.
Here, we report the synthesis of single-crystalline FeSi nanowires and the characterization of their magnetic and electrical properties. The synthesis was performed using a chemical vapor deposition (CVD) method: silicon substrates were cleaned with 1 %-buffered HF and placed in a horizontal-tube furnace, between the center and the downstream end of the alumina tube. Anhydrous FeCl 3 powder (Aldrich, 99.99 %) was placed in an alumina boat upstream from the substrates. An inert atmosphere was maintained with a flow rate of 100 sccm N 2 , and the temperature at the center of the furnace was set to 1100 °C. When the center of the furnace reached the set point, the iron source was hot enough (180-250 °C) to produce vapor-phase FeCl 3 or Fe 2 Cl 6 without thermal decomposition. The precursor vapors were carried by the N 2 flow to the center of the furnace where they reacted with silicon from the substrates. The reaction was held under these conditions for 1-2 h, and then the furnace was allowed to cool to room temperature. When the silicon substrates were taken out for examination, they were covered by a 'fluffy' black powder composed of aggregates of FeSi nanowires.
The morphology of the reaction product was examined using scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Figure 1a shows representative SEM images, illustrating that the reaction produces two kinds of wires: single-stem wires and branched [8,9] wires with triangular 'thorns' on their stems. Both SEM and TEM images reveal that the nanowire diameters range from 5 to about 100 nm and the lengths vary from a few hundred nanometers to tens of micrometers. The stoichiometry of the nanowire samples was assessed by energy-dispersive X-ray (EDX) spectrometry on individual wires using a high-resolution TEM (HRTEM) operating in scanning mode. In all the nanowires analyzed, Fe and Si were present in the correct 1:1 ratio (from 49 to 50.5 at % Si) [28] within instrumental accuracy (see Supporting Information).