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Microminiaturization: two-atom magnetic semiconductor



Since the first cell phones, which weighed up to one kilogram and worked on the power for half an hour without recharging, in the technology world there have been many useful improvements, innovative inventions and revolutionary discoveries. Over time, the gadgets that we use almost every day become smaller in size, but more in terms of performance. This process will inevitably move to a standstill, since classic transistors cannot decrease indefinitely, no matter what the designers of new smartphones or tablets require. Therefore, you need to abandon the classics and create something completely new, which was done by scientists from the Stevens Institute of Technology (USA). Today we look at a study in which they describe an atomically thin magnetic semiconductor, capable of not only using an electron charge,but also its spin. What formed the basis of the new semiconductor, how was it created, and how productive is this revolutionary novelty? A report by scientists will tell us about this. Go.

Study basis


Spin is the moment of momentum of elementary particles. Spintronics, in turn, is a branch of quantum electronics that deals with the study of spin current transfer in solid materials. In other words, in contrast to classical electronics, in spintronics, the transfer of information occurs through the spin current.

In the aspect of creating new devices, many scientists are trying to get complete control over spin-polarized charge carriers. One option for achieving this goal is a diluted magnetic semiconductor (DMS or DMS from dilute magnetic semiconductors) A conventional magnetic semiconductor combines the properties of ferromagnets and semiconductors. But a diluted (or semi-magnetic semiconductor) is essentially a non-magnetic semiconductor into which a certain number of paramagnetic atoms are embedded. For example, alloying elements of transition metals, such as iron (Fe) and manganese (Mn), into non-magnetic bulk semiconductors allows one to obtain DMS.

The question is that these DMS, although they show excellent results, but work at very specific temperatures. For example, the Curie point for a diluted magnetic semiconductor (Ga, Mn) As with a doping concentration of 5% with manganese is reached at 110 K, i.e. at -163.15 ° C. In this regard, scientists are trying to get DMS to work at room temperature in order to fully take advantage of their advantages outside the laboratory.

The relatively recent discovery of ferromagnetism in two-dimensional (2D) atomically thin layers, such as CrI 3 (chromium triiodide) and Cr 2 Ge 2 Te 6 (chromium-germanium telluride) has shifted the focus of research from bulk crystals to two-dimensional materials.

Monolayerstransition metal dichalcogenides (DPM) * in the form of atomically thin semiconductors exhibit unique electrical and optical properties that directly depend on the thickness. However, PDM monolayers remain non-magnetic in their own form.
Transition metal dichalcogenides (DPM) * is a thin monolayer semiconductor consisting of a transition metal and chalcogen (oxygen, sulfur, selenium, tellurium, polonium or liverworm). One layer of metal atoms is located between two layers of chalcogen atoms, which can be expressed by the formula MX2 (M - metal and X - chalcogen).
Doping of DPMs, such as vanadium (V), Mn, and Fe, and its transformation into an atomically thin diluted magnetic semiconductor (RMP) would allow us to study magnetic coupling in two-dimensional confined structures. Despite the fact that PDMs are limited by solubility and chemical stability, they can be doped to a certain extent into single-layer RMPs. However, in such experiments, ferromagnetism has not been demonstrated.

Despite this, 2% Mn doping in Mn: MoS 2 monolayers grown on a graphene substrate, and 1% rhenium (Re) doping in the Re: MoS 2 monolayer demonstrated suppression of emission at low temperatures associated with defects. Therefore, there is a chance to fully realize monolayer PDMs.

Earlier studies were conducted in which scientists tried to implement such a difficult task as a diluted magnetic semiconductor based on PDM, but the results of these works were not particularly satisfactory.

In the study we are considering today, we still managed to achieve a positive result in the form of a successful substitutional doping of Fe atoms in the MoS 2 monolayer .

Research results


Doping with iron (Fe) the monolayer of MoS 2 was carried out by growing MoS 2 and Fe 3 O 4 by chemical vapor deposition.

To eliminate the effects of local deformations in the substrate, monolayers of MoS 2 and Fe: MoS 2 were encapsulated in thin-film hBN (hexagonal boron nitride).


Image No. 1

Image 1a shows a SEM (scanning electron microscope) image of Fe: MoS 2 monolayers . We see triangular island-like domains, which is quite typical for similar MoS 2 synthesis methods .

Image 1bshows a schematic representation of the atomic structures of Fe: MoS 2 monolayers (top and side views). Since the substitution of Mo sites for Fe atoms is thermodynamically favorable (i.e., the reaction does not require energy to occur), one Fe dopant atom replaces one Mo atom in the MoS 2 crystal .

Figure 1c shows a PREM (transmission scanning electron microscope) image of a Fe: MoS 2 monolayer. Compared to Mo atoms (Z = 42), Fe (Z = 26) has an atomic number (i.e., the charge number - the number of protons in the atomic nucleus) is 40% less. Since the intensity of scattered electrons depends on the atomic number, it was expected that the Fe atoms would produce a lower relative intensity, which is clearly seen for substituted Fe atoms in the SEM image.

The corresponding PEM scan of the intensity ( 1d ) indicates that the intensity coefficient is 0.38, which is consistent with previous studies.

To accurately confirm the growth of monolayer domains of Fe: MoS 2, the samples were checked using atomic force microscopy (AFM). This technique allowed us to confirm that after wet cleaning and thermal annealing, no Fe3O4 particles remained there after doping on the surface of Fe: MoS 2 .

An optical analysis of Fe: MoS 2 was carried out next , which provided additional evidence that Fe was successfully incorporated into the monolayer lattice. Raman spectroscopy of Fe: MoS 2 demonstrated two typical characteristic vibration modes of MoS 2 monolayers at E 1 2g = 385.4 cm -1 (vibration in the plane of Mo and S atoms) and A 1g = 405.8 cm -1(out-of-plane vibration of S atoms). The introduction of iron causes an extension of the Raman line width from 5.8 ± 0.1 to 7.6 ± 0.1 cm -1 for A 1g and from 4 ± 0.1 to 4.5 ± 0.1 cm -1 for E 1 2g .

Changes in the lattice of the monolayer were additionally studied by comparing the photoluminescence spectra at room temperature of the monolayers MoS 2 and Fe: MoS 2 . The observed strong quenching of photoluminescence is explained by additional nonradiative recombination channels (capture states), which are due to doping, which confirms the successful incorporation of Fe. Evolution of photoluminescence intensity as a function of temperature for Fe: MoS 2 and MoS monolayers2 is shown in 1e and 1f, respectively.


Image No. 2

Image 2a shows low-temperature photoluminescent (PL) emission of Fe: MoS 2 and MoS 2 monolayers in a wider energy range, including the band gap mode. When comparing the PL of the monolayers Fe: MoS 2 and MoS 2 , the emission peak at 2.28 eV becomes obvious.

Graph 2b shows the emission of Fe: MoS 2for three different triangles, showing significant changes in intensity at a peak of 2.28 eV. The reason for these changes may be the difference in the concentration of the dopant (Fe) between these differently oriented triangles.

Further, in order to exclude local vibrational Raman modes associated with Fe as the source of the electron – hole transition * , optical spectra were recorded in the region of the 2.28 eV peak. In this case, the laser wavelength was adjusted from 405 nm ( 2c ) to 532 nm ( 2d ).
The electron-hole transition * is the contact area of ​​two semiconductors with different conductivities - hole (positive) and electron (negative).
A comparison of the results showed that the position of the peak does not change. This confirms that the observed radiation associated with iron is not caused by the Raman vibrational mode, which would shift relative to the laser energy.


Image No. 3

To investigate the origin of the PL peak associated with iron at 2.28 eV, the researchers used DFT (density functional theory) to calculate the electronic structure of Fe: MoS 2 .

An isolated doping Fe impurity was modeled as the replacement of one Mo atom by an Fe atom in a 5 × 5 MoS 2 supercell ( 3a ).

On 3bThe structure of spin-polarized zones for this system is shown, where the area of ​​each blue (or green) circle is proportional to the overlap of the state with spin up (or down) and a sphere of radius 1.3 Å centered in the Fe atom. The graph shows that the presence of Fe introduces states that lie within the untouched band gap of MoS 2 . And the fact that the large blue and green circles inside the forbidden band do not overlap indicates that Fe induces a magnetic moment.

Figure 3c shows a comparison of the spontaneous emission rates of the conduction band with the spin up and the conduction band of pristine MoS 2 . The lowest radiation energy for the initial state of MoS 2is ~ 1.79 eV, which corresponds to a large PL peak at 2a , which arises as a result of relaxation over the entire band gap. The presence of Fe introduces another significant transition with an energy of ~ 2.32 eV, which corresponds to the experimentally observed peak of the PL emission of Fe: MoS 2 at 2.28 eV.

The expected value of the PL peak is much smaller than shown in 3c , since any hole in the valence band remaining from laser excitation relaxes very quickly without radiation to the maximum of the valence band. Therefore, holes will spend very little time in the state of the valence band corresponding to the 2.28 eV transition, which makes this transition much less likely than the transition to the maximum of the valence band.


Image No. 4

At the next stage of the study, the magnetic characteristics of the Fe: MoS 2 monolayers were evaluated .

It is known that optical radiation from transition metal ion complexes usually occurs as a result of charge transfer between ligands and a transition metal. The spin angular momentum of an electron in an ion is highly dependent on polarization due to the rules for selecting the spin of circularly polarized light. Thus, transition metal ions exhibit an unequal amount of light absorption upon excitation with left and right circular polarization.

At the atomic level, light absorption is closely related to magnetically induced Zeeman shifts. Therefore, performing MCD spectroscopy (magnetic circular dichroism) can give an idea of ​​the magnetic properties of the material.

Graphs4a and 4b show the PL spectra of Fe: MoS 2 upon excitation by the opposite circularly polarized light both at 4 K and at room temperature. The radiation associated with Fe shows a strong circular dichroism * (ρ ≈ 40%) both at 4 K and at RT.
Circular dichroism * - the difference between the absorption coefficients of light polarized along the right and left circles.
Considering that the luminescence of transition metals loses its circular dichroism above the Curie temperature, the observation of strong dichroism at 300 K suggests that Fe: MoS 2 remains ferromagnetic at room temperature.

Figure 4c shows Fe-related MCD emission as a function of increasing (blue dots) and decreasing (red dots) magnetic field in the range from −3 T to 3 T at 4 K. The pronounced hysteresis loop clearly identifies the ferromagnetic nature of the PL radiation associated with with iron.

Figure 4d shows that Fe: MoS 2 monolayers exhibit a pronounced M - H hysteresis loop both at 5 K and at room temperature. This confirms that the synthesized monolayers Fe: MoS 2demonstrate ferromagnetism even at 300 K.

In conclusion, scientists carried out magnetometry of Fe: MoS 2 monolayers to estimate the local strength of the ferromagnetic field at room temperature using the ODMR method (optical magnetic resonance detection).


Image No. 5

An exemplary ODMR spectrum for Fe: MoS 2 and MoS 2 is shown in 5a . On 5b shows a histogram of the Zeeman splitting of the energy stored in the 24 and 20 different points monolayer Fe: MoS 2 and MoS undoped 2 respectively. Statistical analysis shows that the average energy splitting on Fe: MoS 2 increased by ~ 11 MHz compared to pure MoS2 . From these data, it was found that the local magnetic field of the sample can reach 0.5 ± 0.1 mT. This indicator is close to that measured in 2D ferromagnets CrI 3 and CrBr 3 at cryogenic temperature.

The fact that Fe: MoS 2 exhibits a large local magnetic field at room temperature is clear evidence that this material has retained its magnetization. Therefore, it can be concluded from the data that Fe: MoS 2 monolayers with embedded Fe atoms act as dilute magnetic semiconductors that exhibit ferromagnetism at room temperature.

For a more detailed acquaintance with the nuances of the study, I recommend that you look into the report of scientistsand additional materials to it.

Epilogue


The development of technologies and related devices is often associated with the comfort of those who use them. Modern devices are becoming smaller, but there is not much compactness. However, no matter how users wish to minimize the dimensions of their favorite gadgets, this process is limited by the dimensions of the interiors of these devices.

The authors of this study note that classical transistors cannot decrease infinitely, which is consistent with both the laws of logic and physics. Nevertheless, if the classics do not work, then you can look in the direction of modernity, which scientists have done. In their work, they described a new type of semiconductor Fe: MoS 2combining the properties of a semiconductor and a ferromagnet. In the process of its creation, iron atoms push molybdenum atoms, so to speak, taking their place. The result of this process is a very thin (only two atoms in thickness) and a flexible material that retains magnetization at room temperature.

As the researchers themselves say, their invention does not obey Moore's well-known law, since it is not related to physical scaling. In their work, they described the ability to use not only the charge of an electron, but also its spin, which expands the possibilities of future technologies.

The foundation of devices of the future, flexible, light and transparent, according to scientists, may be a complete understanding and control of the properties of the materials from which they will be made.

Thank you for your attention, remain curious and have a good working week, guys. :)

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