Formation of linearly linked Fe clusters on Si(111)-7 × 7-C2H5OH surface
© Ding et al.; licensee Springer. 2014
Received: 30 May 2014
Accepted: 18 July 2014
Published: 3 August 2014
The Fe atoms were deposited on the Si(111)-7 × 7 surface, which has been saturated with the C2H5OH molecules. Then, the Fe clusters were formed on Si(111)-7 × 7-C2H5OH surface and in situ observed by the scanning tunneling microscopy (STM). The STM images showed that with the increase of Fe clusters, the size of clusters was about 5 nm and they self-assembled in straightly linked chain crossing the step to lower or upper terrace. X-ray photoelectron spectroscopy (XPS) was in situ carried out on the surface of Fe/Si(111)-7 × 7-C2H5OH samples before and after the introduction of thin air (4.5 × 10-2 Langmuir) into the STM chamber. The XPS results showed that the Fe clusters are stable in the abovementioned thin air condition at room temperature. Based on the STM and XPS results, the driving force making one-dimensional straightly linked chain structure might be the magnetic force of the Fe clusters. The formation of straightly linked Fe clusters chains suggests the formation of single magnetic domain Fe clusters.
07.79.Cz, 81.15.-z, 75.75.Fk
KeywordsSTM Fe clusters Single magnetic domain
Based on the phenomenological theory of ferromagnetic material, the conception of magnetic domain was first proposed by P. E. Weiss in 1907 , and the structure of magnetic domain based on the interaction of the magneto-static energy was proposed by L. D. Landau and E. M. Lifshitz in 1935 . Recently, it was found that the particles change to single-domain magnetic clusters by decreasing their size [3–5]. Accordingly, the preparation of single magnetic domain clusters is an interesting challenge to magnet materials for high-density magnetic recording medium. So far, the reported critical sizes for single magnetic domains were 85 nm for Ni, 40 nm for Fe3O4, and 16 nm for α-Fe [3–5], and the cluster with a size lower than the critical value displays super paramagnetism, which could not be applied for the magnetic recording medium. To improve the density of magnetic recording without the restriction of super paramagnetism, it is necessary to prepare the single magnetic domain clusters with limited critical size. From this view point, the Fe single magnetic domain clusters have become the research focus, which could be analyzed for the spin in physics, controllable surface reaction in chemistry, for example, FeN and FeO x with the critical size lower than 10 nm.
The Fe clusters were prepared by many techniques, such as chemical precipitation, thermal decomposition, hydrothermal method, sol–gel, and so on [6–9]. The uniformity of cluster size and agglomeration of clusters are difficult to control in these preparation techniques. Therefore, the controlled preparation with uniform size is desired not only for the fundamental studies but also for the application of high-density magnetic recording medium. We intended to prepare the Fe clusters with single magnetic domain by depositing the Fe atoms on Si(111)-7 × 7 surface saturated with ethanol (C2H5OH). A unit cell of Si(111)-7 × 7 surface is composed of triangular-shaped faulted and unfaulted half unit cells. The half unit cell has six Si ad-atoms and three Si-rest atoms. When the clean Si(111)-7 × 7 surface is exposed to C2H5OH, C2H5OH molecules dissociate at the Si ad-atom/Si-rest atom pair sites with almost perfect accuracy, where the Si ad-atom changes to the Si-OC2H5, the Si-rest atom changes to Si-H, and the saturated Si(111)-7 × 7-C2H5OH was formed. The formation of Fe clusters on Si(111)-7 × 7-C2H5OH surface is controlled by the uniformly distributed Si ad-atoms in half unit cells, and we expect the formation of single magnetic domain Fe clusters. In the present work, the Fe atoms were deposited on the surface of Si(111)-7 × 7-C2H5OH at room temperature, then the growth and distribution of Fe clusters were systematically studied.
In our experiments, the Fe clusters were deposited and observed by JSPM-4500S ultra-high vacuum scanning tunneling microscopy (STM) system (JEOL Ltd., Akishima-shi, Japan). The single-crystal n-type Si(111) substrates were firstly ultrasonically pre-cleaned in acetone, ethanol, and deionized water, respectively, and then dried with N2 gas. Finally, the substrates were loaded onto the sample holder and placed into the exchange chamber of STM system. After the base vacuum of exchange chamber was less than 5.0 × 10-4 Pa, the sample holder was transferred into the treatment chamber. After the baking and degas process for 24 h, the sample holder was translated into the main chamber for STM observation, where the vacuum was about 1.0 × 10-8 Pa. Then, the Si(111)-7 × 7-reconstructed surface was obtained according to the standard heating and flashing procedures [10–12]. In order to avoid the chemical reaction of deposited Fe with Si substrate, the substrate surface was passivated by the adsorption of C2H5OH in the main chamber according to the reported procedures . After the preparation of Si(111)-7 × 7-C2H5OH surface, it was translated into the treatment chamber, and the Fe atoms were evaporated by heating a W filament with Fe wire. The different amount of Fe atoms was deposited by controlling the deposition time. After the deposition of Fe atoms, the Fe/Si(111)-7 × 7-C2H5OH sample was translated into the main chamber for STM observation. In order to know the chemical stability of the sample, the sample was exposed to the thin-air condition with 4.5 × 10-2 Langmuir (~10-2 L for O2) in the main chamber by the needle valve. Before and after the exposing, the Fe/Si(111)-7 × 7-C2H5OH sample was translated into the composition test chamber, respectively, where the sample was in situ tested by the GammadataScienta SES-100 X-ray photoelectron spectroscopy (XPS) system (Pleasanton, CA, USA). In our experiments, the XPS spectra were in situ performed with an Alk α line source (hv = 1,486.6 eV) at an incident angle of 45°. Before the measurement, the XPS system was calibrated by the standard Au and Cu samples. In consideration of the signal-to-noise ratio of data, the area of XPS measurement was kept as 100 μm in diameter for all tests. Then, the high-resolution spectra were recorded with 29.35 and 0.125 eV in the pass energy and step, respectively. All spectra were referenced to C 1 s peak of 284.6 eV.
Results and discussion
From the STM and XPS results, one interesting question is the driving force making linked Fe clusters in a straight chain structure. Attractive force forming Fe clusters and the force making straight chain should be different. Based on the theory of total cohesive energy of cluster in the free space, the lowest energy structures for transition metal cluster was not the layer structure, but the polyhedron structure [14, 15]. In fact, the Fe particles prepared at room temperature in the free space have the body-centered cubic structure (<912°C). As the (100) face had the lowest surface energy, the Fe clusters are surround by the (100) face . In this case, the shape of Fe clusters is controlled by the thermodynamic stability of the planes in growth. But if the growth is controlled by the growing rate of crystal planes, the morphology of particles are changed depending on the condition .
In summary, we attained to control the preparation of 5-nm Fe clusters on Si(111)-7 × 7-C2H5OH surface. The Fe cluster is stabilized by the interaction with Si ad-atoms with a dangling bond remained on the Si(111)-7 × 7-C2H5OH surface. The periodical arrangement of Si atoms on Si(111)-7 × 7-reconstructed surface and the periodical surface potential field restrained the growth of Fe clusters with certain periodicity. The XPS results showed that the Fe clusters were stable in the thin-air condition (4.5 × 10-2 Langmuir) at room temperature. When the deposition of Fe atoms was increased, about-5-nm Fe clusters were formed and underwent one-dimensional self-assembly crossing the step onto the upper or lower terrace. The driving force making one-dimensional linked straight chain structure might be the magnetic force of Fe clusters. If so, the Fe cluster takes single magnetic domain with about 5 nm of critical size, and we could expect to lower the single magnetic domain to ca. 5 nm without a change to the super paramagnetic property. Based on our results, the Fe cluster is hopefully to synthesize the strong magnetic FeN x and FeO x particles with 5 nm of critical size in the future. Finally, from the point of applying Fe clusters as the high-density magnetic recording medium, it is interesting to prepare the Fe clusters with a critical size lower than 10 nm. The present work reveals a simple way to realize it as well as the physicochemical mechanism behind it.
scanning tunneling microscopy
X-ray photoelectron spectroscopy
full width at half-maximum
This work was supported by the Nano Project of Saitama Institute of Technology in Japan, the National Natural Science Foundation of China (No. 51102030), Natural Science Foundation of Liaoning Province, China (No. 201202024), and Program for Liaoning Excellent Talents in University (No. LJQ2011043). The first author would like to express his gratitude to the Open Research Center of Saitama Institute of Technology for the financial support during his stay in Japan.
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