CN105702412A - A kind of β-FeSi2 nano-hexahedral particles with light-controlled strong room temperature ferromagnetism and preparation method thereof - Google Patents

Abstract

The invention relates to a beta-FeSi2 nanometre hexahedral particle having strong optical control room-temperature ferromagnetism and a preparation method thereof. A nanometre material has strong room-temperature ferromagnetism (15emu/g); furthermore, the strong room-temperature ferromagnetism can be converted into the paramagnetism in a laser irradiation condition; and thus, adjustment and control of light to the magnetism are realized. The preparation method of the nanometre particle comprises the following steps of: reacting ferrous chloride anhydrous and a silicon wafer by adopting a chemical vapour deposition method at 800 DEG C for 2 h, and naturally cooling. The beta-FeSi2 nanometre hexahedral particle disclosed by the invention has the crystal face-dependent ferromagnetism; the beta-FeSi2 nanometre cubic particle integrates the room-temperature ferromagnetism and the semiconductor properties into a whole; therefore, the beta-FeSi2 nanometre cubic particle is a first-choice material for developing a spin electronic device of a semiconductor; and the beta-FeSi2 nanometre hexahedral particle has the good promotion effect for application and development of spintronics of semiconductors.

Description

A kind of β-FeSi2 nano-hexahedral particles with light-controlled strong room temperature ferromagnetism and preparation method thereof

technical field

The invention relates to a β _- FeSi2 nanometer hexahedral particle with light-controlled strong room-temperature ferromagnetism and a preparation method thereof, belonging to the field of magnetic semiconductors.

Background technique

With the continuous miniaturization of transistors and the miniaturization of electronic components, the size of semiconductor devices is approaching the limit set by quantum coherence effects, which requires the development of new information processing mechanisms. Traditional electronics mainly relies on the charge property of electrons, while spin is another intrinsic property of electrons. The degree of freedom in information processing can be increased through the control of electron spin. Spintronics simultaneously regulates the two properties of charge and spin of electrons for information reading, transmission and processing, which can improve data processing speed, reduce power consumption, increase integration and solve the "volatile" of current information storage. question.

The first condition for realizing semiconductor spin devices is to inject spin-polarized current. Therefore, the preparation of spin-polarized materials is the top priority of spintronics. Due to the different electronic structures of the bulk, surface, and interface of semi-metallic ferromagnets, experiments at the heterojunction interface show that the spin polarizability is much lower than the theoretical value. Ferromagnetic metals have high spin polarizability, but the spins are severely scattered at the interface due to the conductance mismatch with the semiconductor. Magnetic semiconductors are ideal materials for spin-polarized current injection due to their semiconducting conductivity and spin polarization of ferromagnetic materials.

In the study of magnetic semiconductors, the Curie temperature of doped III-V dilute magnetic semiconductors is lower than room temperature, and for wide bandgap magnetic semiconductors, some reports indicate that the Curie temperature can be higher than room temperature, however, different Experimental results vary widely, and there are different opinions on the origin of magnetism. At the same time, due to the low solubility of magnetic impurities, only weaker magnetism can be obtained. In addition, silicon is the current pillar material in the field of semiconductors. It also has low spin-orbit scattering and inverse lattice symmetry, so it has a large spin lifetime and diffusion length, and is expected to develop in spintronics. However, due to the lack of silicon-based magnetic materials, the development of silicon-based spintronics is relatively backward.

Therefore, it is of great significance to search for room-temperature ferromagnetic silicon-based semiconductor materials.

Metal silicides are ubiquitous in today's CMOS microelectronic devices as connections at the silicon-metal interface and are promising materials for building the basis of silicon-based spintronics. Among them, the iron silicide material has high ecological value, the non-toxicity of its constituent elements and the adequacy of reserves on the earth at this stage make it expected to become an important environmentally friendly material. Among iron silicon compounds, the semiconducting phase β _- FeSi2 is particularly attractive due to its wide application in optoelectronic and photovoltaic devices. For semiconductor polyhedral nanoparticles, due to the anisotropy of the crystal, different crystal planes have different surface atomic arrangements and electronic band structures, and the unpaired spin electrons and naturally occurring dangling bond interactions of specific crystal planes can cause magnetic. Therefore, exposing specific crystal faces of nanoparticles is one way to introduce magnetism. In addition, with the continuous improvement of information storage density in recent years, magnetic field-driven writing has not only become more and more difficult, but also has many problems such as slow speed, high noise, and high energy consumption. In most cases, the magnetic properties of ferromagnetic semiconductors are related to carriers. Under light conditions, the change of the carrier concentration on the surface of nanoparticles will regulate the magnetism, which will provide the optical writing of magnetic information. possible. If room-temperature ferromagnetism can be induced through the crystal facet of β _- FeSi2 nanoparticles, not only can a room-temperature magnetic semiconductor be obtained, but also it combines excellent optoelectronic properties and magnetism, which will further expand the use of materials and have great application prospects.

Contents of the invention

Aiming at the shortcomings of existing magnetic semiconductors, the purpose of the present invention is to provide a β _- FeSi2 nano-hexahedral particle and a simple and effective method to prepare a strong room-temperature silicon-based magnetic semiconductor, which is sensitive to light.

The technical scheme of the present invention: a β-FeSi ₂ nanohexahedral particle with light-controlled strong room temperature ferromagnetism, the nanohexahedral particle is a β-FeSi ₂ nanometer particle with two {100} faces and four {011} sides Cubic particles; the nano-hexahedral particles show ferromagnetism up to 15emu/g at room temperature. Under the condition of laser irradiation, the ferromagnetism will become paramagnetic. regulation. The strong room-temperature magnetism of nanoparticles originates from the interaction between the local magnetic moment near the surface iron atomic layer and the surface state itinerant electrons. Its magnetism is shape- and size-dependent, and particles with irregular shapes and smaller than 150nm do not have a magnetic domain structure. .

The preparation method of the β _- FeSi2 nano-hexahedral particles with light-controlled strong room temperature ferromagnetism is characterized in that:

Proceed as follows:

Step 1: Place the cleaned quartz tube with a diameter of 4-10cm horizontally in a tube furnace with two temperature zones with a center distance of 31.5±10cm;

Step 2: Put the quartz boat containing 0.056g of anhydrous ferrous chloride powder with a purity of more than 99.5% and a clean silicon wafer with a resistivity of 25-100Ω·cmP into two temperature zones respectively;

Step 3: The quartz tube is put into the tube furnace as a whole, so that the quartz boat and the silicon wafer correspond to the centers of the upstream and downstream temperature zones;

Step 4: Heat the upstream and downstream temperature zones to 600±20°C and 800±20°C simultaneously within 20 minutes;

Step 5: Under the blowing of argon carrier gas with a flow rate of 100±20 sccm, react for 2 hours;

Step 6: After the reaction is finished, cool down to room temperature naturally, take out the silicon chip, and then generate a film of β _- FeSi2 nanometer hexahedral particles on the surface of the silicon chip.

In particular, the reaction temperature is 800°C and the reaction flow rate is 100 sccm, and a large-density pure β-FeSi ₂ phase is obtained.

The mechanism analysis of the strong light-controlled room temperature ferromagnetism of β-FeSi ₂ nanohexahedral particles is as follows: at the nanoscale, the specific surface area of nanomaterials is greatly increased, the bond state is seriously mismatched, and the degree of disorder increases. Magnetism can be induced by unpaired spin electrons and naturally occurring dangling bond interactions on specific facets of polyhedral nanoparticles. First-principles calculations show that the iron atoms on the {100} and {011} crystal planes of β-FeSi ₂ have large magnetic moments and exhibit ferromagnetic order, while the internal iron atoms have random spins. orientation. The spin density of states in the {100} crystal plane direction shows that there are spin-down partially occupied bands and spin-up empty bands near the Fermi level, the local magnetic moment near the surface iron atomic layer and the surface state itinerant electrons The interaction can induce strong room temperature ferromagnetism. The magnetic properties of ferromagnetic semiconductors are often related to carriers. Under light conditions, the change of carrier concentration on the surface of nanoparticles will play a role in regulating the magnetism.

β-FeSi ₂ nanoparticles have a magnetic domain structure when the size is larger than 150nm and has a hexahedral regular shape. The magnetism of the cubic particles disappears after being oxidized in air for a period of time, indicating that the magnetism originates from the surface of the particles, and also excludes the possibility of magnetic doping and the existence of crystalline iron and iron oxides. In addition, the relevant spectral characterization also shows that the material does not contain impurity phases. In order to prevent oxidation of the surface of the particles, after the reaction for 2 hours, the outlet valve was closed to avoid the pressure drop and air mixing during the cooling process.

Beneficial effects of the present invention: the present invention relates to a β _- FeSi2 nano-hexahedral particle with light-controlled strong room-temperature ferromagnetism and a preparation method thereof. Magnetic measurements show that the nanomaterial has a saturation magnetization as high as 15emu/g. , the material changes from ferromagnetic to paramagnetic. The material combines surface ferromagnetism, semiconductivity and spin polarization properties, and is the best choice for the development of semiconductor spintronics devices, which has a good role in promoting the application and development of semiconductor spintronics. The invention provides a simple, fast and cheap synthesis method, and the magnetism of the synthesized material comes from a specific crystal plane, which provides beneficial enlightenment for other non-magnetic semiconductor materials to introduce magnetism by exposing the specific crystal plane. β-FeSi ₂ is an environmental semiconductor material with excellent performance, and its energy band width of 0.81-0.87eV is suitable for optical fiber communication light source.

Description of drawings

Figure 1(a), (b) and (c) are the SEM images of the hexahedral β-FeSi ₂ nanoparticles at different magnifications, respectively. (d) XRD pattern of the sample.

Figure 2(a) and (b) are the low-magnification transmission electron micrographs and selected area diffraction spots of the as-prepared hexahedral β _- FeSi2 particles along the [0-11] direction, respectively. (c) and (d) are low magnification TEM and selected area diffraction spots along the [0-11] direction.

Figure 3(a) Room temperature MH curves of hexahedral β _- FeSi2 nanoparticles, the square curve is the situation without light, the triangle curve is the situation with light, and the circular curve is the situation after the light is removed. (b) MT curves of β _- FeSi2 nanoparticles in the range of 300K–750K, using mean-field epitaxy to approximate the Curie temperature to be about 800K.

Figure 4(a) and (b) are atomic force and magnetic force micrographs, respectively, of β-FeSi2 particles with a size of about ₂₀₀ nm. (c) and (d) are atomic force and magnetic force micrographs of 150 nm particles, respectively. (e) and (f) are atomic force and magnetic force micrographs of irregularly shaped particles, respectively. (g)(h) and (i)(j) are the atomic force and magnetic force micrographs of untreated and gold-sprayed β _- FeSi2 particles placed in dry air for three weeks, respectively.

Figure 5 XPS spectra of pristine and oxidized samples (a) Fe2p, (b) Si2p, (c) O1s.

Figure 6(a) and (b) are the electronic structures of the flakes with {011} and {100} crystal planes on the upper and lower surfaces of nano-hexahedral particles, respectively. The non-zero Mulliken spin is marked on the iron atom, and the unit is μ _B . Dark and light colored spheres represent iron and silicon atoms, respectively. It can be seen that the iron atoms on the surface of the two crystal planes have ferromagnetic order, and the internal spins have random orientations, indicating that the magnetism originates from a specific crystal plane.

detailed description

A kind of β _- FeSi2 nano-hexahedral particles with light-controlled strong room temperature ferromagnetism and its preparation method, the cleaned quartz tube with a diameter of 6 cm is horizontally placed in a tube furnace with two temperature zones with a distance of 31.5 cm between centers; Clean the P-type silicon wafer (1cm*1cm, resistivity 25-100Ω·cm); put the quartz boat containing 0.056g of anhydrous ferrous chloride powder and the cleaned silicon wafer into a quartz boat with a diameter of 2cm In the tube, the distance is 31.5cm; put the 2cm quartz tube into the quartz tube with a diameter of 6cm in the tube furnace, so that the quartz boat and the silicon wafer correspond to the center of the upstream and downstream temperature zones respectively; exhaust the oxygen in the device; put the upstream and downstream temperature zone Heating to 600°C and 800°C within 20min at the same time; under the blowing of 100sccm carrier gas argon, react for 2h; after the reaction, naturally cool down to room temperature, take out the silicon wafer, and the surface will form β-FeSi ₂ Nano-hexahedral particles film.

The cleaning process of the silicon wafer is as follows: the silicon wafer is ultrasonically cleaned with acetone and ethanol for 10 minutes respectively, and the frequency and power of the ultrasonic waves are set to 40kHz and 150W; the silicon wafer after ultrasonic is rinsed with deionized water; the rinsed silicon wafer is Put it into a mixed solution of hydrochloric acid, hydrogen peroxide and deionized water with a volume ratio of 1:2:5, boil in an oil bath for 3mim; after cooling down to room temperature, rinse the silicon wafer with deionized water; put the rinsed silicon wafer into a mixed solution of ammonia, hydrogen peroxide and deionized water with a volume ratio of 1:2:6, and boiled in an oil bath for 3mim; after cooling down to room temperature, the silicon wafers were rinsed with deionized water; Acid cleaning to remove the oxide layer on the surface; rinse the silicon wafer with deionized water and put it in a drying oven for drying. The silicon wafer used in the present invention is used as a reactant, so that after cleaning, the active sites on the silicon surface can be exposed, and the influence of impurities on the characterization can also be eliminated.

The method of exhausting the oxygen in the device is as follows: Use a mechanical pump plus a molecular pump to pump the air pressure in the quartz tube to 1*10 ^-3 Pa to remove the air; inject 200 sccm of argon to restore normal pressure; open the outlet valve, and then inject argon At the same time, the upstream and downstream temperature zones are heated to 100°C within 10 minutes; at 100°C, argon gas with a flow rate of 200 sccm is continuously introduced for 30 minutes. This reduces the interference of oxygen and avoids the generation of impurity phases.

When the present invention deviates from 800 DEG C, impurity phases such as ε-FeSi will be generated. In order to obtain pure β-FeSi ₂ phases, the method adopted in the present invention is that before the reaction temperature rises to 800 DEG C within 20 min and after the reaction 2 h finishes, all Close the inlet valve to prevent the anhydrous ferrous chloride from reaching the downstream temperature zone to react with the silicon wafer under argon blowing.

Carry out other characterizations to embodiment 1 gained product:

1) X-ray diffraction analysis:

The resulting sample was analyzed by XRD, and the resulting spectrum is shown in Figure 2(d), all diffraction peaks (220), (313), (331), (004), (040) and (442) correspond to orthorhombic For β-FeSi ₂ , there are no peaks of crystalline iron and iron oxide, indicating that the prepared particles do not contain crystalline iron and iron oxide.

2) Scanning electron microscope characterization:

Fig. 1 (a), (b) and (c) are scanning electron micrographs of β-FeSi ₂ nano-hexahedral particles prepared by the present invention, it can be seen that the particles have clear edges and smooth surfaces.

3) Transmission electron microscope characterization:

Fig. 2 is β-FeSi prepared by the present invention ₂ Nano-hexahedral particles transmission electron microscope, (a) and (c) are low magnification images along [0-11] and [100] directions, (b) and (d) The selected area electron diffraction patterns corresponding to (a) and (c), respectively. There is no amorphous diffraction spot in the figure, indicating that the particle is a single crystal. The sharp diffraction spots in picture (b) correspond to (022) and (400) crystal planes, and picture (d) corresponds to (040), (004) and (022) crystal planes. Since the lattice parameters b and c are very close, what we see is basically a square symmetrical diffraction spot. Therefore, among the six faces of the hexahedron, two crystal faces are (100) and (-100), and the remaining four faces are (011), (0-11), (01-1) and (0-1-1 ).

4) Determination of magnetic properties:

The magnetism of the β-FeSi2 _{nanohexahedral} particles prepared in the present invention is measured in a superconducting quantum interference instrument (SQUID), the applied magnetic field is parallel to the silicon substrate, and the final hysteresis loop deducts the diamagnetic signal of the silicon substrate. As shown in Figure 3(a), it can be seen that the magnetization of the sample at room temperature is as high as 15emu/g, and the coercive force is about 123Oe, showing obvious ferromagnetism. When a laser with a power of 100mW and a wavelength of 425nm is introduced into the SQUID through an optical fiber to irradiate the sample, the sample changes from ferromagnetism to paramagnetism, and after the light is removed, the sample returns to ferromagnetism, indicating that the sample has light-regulated ferromagnetism. In order to determine the Curie temperature, due to the limitation of the instrument, the magnetization in the range of 300K-750K was tested. It can be seen that the magnetization is not zero at 750K, and the Curie temperature is about 800K by using mean field approximate epitaxy.

In order to determine the origin of magnetism, magnetic force microscopy experiments were carried out on the samples. Figure 4 (a) and (b) are the atomic force and magnetic force micrographs of β-FeSi2 particles with a size of about ₂₀₀ nm, respectively. These particles have obvious magnetic domains structure. (c) and (d) are atomic force and magnetic force micrographs, respectively, of a particle of about 150 nm, which has no magnetic domain structure. (e) and (f) are atomic force and magnetic force micrographs, respectively, of irregularly shaped particles that also do not show magnetic domain structures. This indicates that the magnetism of β-FeSi ₂ particles is size- and shape-dependent, and the particles smaller than 150nm and with irregular shapes are not ferromagnetic. (g)(h) and (i)(j) are the atomic force and magnetic force micrographs of untreated and gold-sprayed β _- FeSi2 particles placed in dry air for three weeks, respectively. The former has no room temperature ferromagnetism, while the latter retains ferromagnetism. Since the oxidation process is slow and only occurs on the surface, the magnetic properties of β _- FeSi2 particles originate from the surface of the particles. At the same time, the possibility of magnetic doping, crystalline iron and iron oxides is excluded, because if present, the interior of the particle will continue to maintain ferromagnetism.

5) X-ray photoelectron spectroscopy measurement

The XPS test is carried out on the samples prepared by the present invention, one is the initial synthetic sample, and the other is the sample after being oxidized in air for several weeks. As shown in Figure 5, there is no signal of FeOx in the initial sample, and the XPS spectrum of the oxidized sample shows the formation of amorphous iron oxide on the surface. The oxygen in the as-synthesized initial sample came from the oxidation of the silicon wafer, not the iron oxide. It is further illustrated that the initial sample does not contain these impurity phases of iron oxides.

6) First-principle calculations of magnetism introduced by crystal planes

To study the crystal plane dependence of the room temperature ferromagnetism of hexahedral β _- FeSi2 particles, the {011} and {100} crystal planes of the particles were investigated using first-principles, using the generalized gradient approximation, under the plane-wave gauge-conservation pseudopotential Use the CASTEP package. Figure 6 shows the electronic states on the surface of two flakes with {011} and {100} crystal planes. It can be seen that the iron atoms on the surface of the two crystal planes all show ferromagnetic order, while the internal iron atoms on the , the spin arrangement is random, which is consistent with the paramagnetism of the bulk material, indicating that the magnetism comes from the specific crystal plane of the nanoparticle.

Claims (4)

1. a kind of β-FeSi nano-hexahedral particle with light control strong room temperature ferromagnetism, it is characterized in that, described nano-hexahedral particle is β-FeSi with _two {100} faces and four {011} side faces ₂ nanometers Cubic particles; the nano-hexahedral particles show ferromagnetism up to 15emu/g at room temperature. Under the condition of laser irradiation, the ferromagnetism will become paramagnetism. regulation. _2. the β-FeSi nano-hexahedral particles with light-controlled strong room-temperature ferromagnetism according to claim 1, characterized in that, the strong room-temperature magnetism of nanoparticles originates from the local magnetic moment and surface state parade near the surface iron atomic layer The interaction of electrons has a shape and size dependence on its magnetic properties, and particles with irregular shapes and smaller than 150nm do not have a magnetic domain structure. 3. according to claim 1 and ₂ , there is the preparation method of the β-FeSi2nano-hexahedral particle of light-controlled strong room temperature ferromagnetism, it is characterized in that: preparation is carried out as follows: Step 1: Place the cleaned quartz tube with a diameter of 4-10cm horizontally in a tube furnace with two temperature zones with a center distance of 31.5±10cm; Step 2: Put the quartz boat containing 0.056g of anhydrous ferrous chloride powder with a purity of more than 99.5% and a clean silicon wafer with a resistivity of 25-100Ω·cmP into two temperature zones respectively; Step 3: The quartz tube is put into the tube furnace as a whole, so that the quartz boat and the silicon wafer correspond to the centers of the upstream and downstream temperature zones; Step 4: Heat the upstream and downstream temperature zones to 600±20°C and 800±20°C simultaneously within 20 minutes; Step 5: Under the blowing of argon carrier gas with a flow rate of 100±20 sccm, react for 2 hours; Step 6: After the reaction is finished, cool down to room temperature naturally, take out the silicon chip, and then generate a film of β _- FeSi2 nanometer hexahedral particles on the surface of the silicon chip. 4. the preparation method of the β _- FeSi2nano-hexahedral particle with light-controlled strong room temperature ferromagnetism according to claim 3 is characterized in that, 800 ℃ of reaction temperature and reaction flow 100sccm argon gas, obtain the pure β- FeSi ₂ phase.