Magnetic field effects on the crystal structure, morphology, energy gap, and magnetic properties of manganese selenide nanoparticles synthesized by hydrothermal method

In this study, we synthesized manganese selenide under magnetic fields ranging from 0 to 800 gauss and investigated its optical, electrical, and magnetic properties. In the absence of a magnetic field, we observed the formation of MnSe nanorods. As the field strength increased, impurities arose. In the 250 G range, two rock salt structures emerged, altering the morphology from nanorods to cubes. Beyond 250 G, MnSe2 formed, returning to a nanorod morphology. Also, with the increase of the magnetic field, the energy gap of the synthesized compounds increased. To measure the electrical properties of the samples, the synthesized powders were compressed under the same pressure for a certain period of time, and it was observed that the synthesized samples showed insulating behavior in the presence of a magnetic field. For this reason, we performed current–voltage, resistance–temperature, and current–temperature analyses on the synthesized sample, at a constant voltage of 5 eV in the absence of a magnetic field.


Introduction
][3][4][5][6][7][8][9][10][11][12][13][14][15][16] Also, the conductivity behavior of TMC varies from semiconductor to conductor.][19][20][21][22][23] The energy difference between valence and conduction bands, known as the band gap, holds signicant importance in semiconductor physics.For advancing studies in photovoltaic, photocatalytic, and dye-sensitized solar cell technologies, it is essential to create semiconductor materials with small dimensions and reduced bandgap energies.Semiconductors with wider band gaps can efficiently harness UV and solar light through lower-energy absorptions and excitations.Band gap reduction can occur due to various factors, such as temperature, doping, alloying, pressure, lattice volume, and aws.However, when studying materials at the nanoscale, it is crucial to characterize the microstructure, focusing on particle size and microstrain.Lattice strain can signicantly impact and control the optical, electrical, and mechanical properties of nanoparticles, as it plays a fundamental role in determining their behavior. 24][27][28] Researchers have noted its accessibility, affordability, and environmental friendliness, which increase its potential.The compound manganese selenide (MnSe) is a TMC that has captured the interest of many researchers due to its essential magnetic properties.MnSe is a compound that may take on a variety of shapes, such as the wurtzite structure (ZnS), 29 the rock salt structure (NaCl), 30 and the hexagonal structure (NiAs). 31he rock-salt structure is the most stable form of MnSe at standard room temperatures.The Mn and Se atom arrangements in this crystal structure are alternated on a face-centered cubic (fcc) lattice.The direct bandgap of the semiconductor phase of MnSe in the rock-salt crystal structure is about E g = 2 eV. 32nSe may be produced using various techniques, including chemical vapor deposition, solid-state reactions, and procedures that use solutions, such as solvothermal and hydrothermal synthesis. 32,33Due to its many benets over alternative techniquessuch as precise control over particle size and shape, high product purity, process scalability, and minimal environmental impactthe hydrothermal method is an effective strategy for synthesizing MnSe. 34n previous investigations, researchers have explored various methods for synthesizing manganese selenide.For instance, Decker et al. 35 achieved successful synthesis of a-MnSe by subjecting high-purity Mn metal to elemental Se within a quartz tube at a temperature of 900 °C for 20 h, using a catalyst (I 2 ) in

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the process.They also demonstrated the feasibility of obtaining MnSe 2 by employing stoichiometric amounts of Mn and Se at a temperature of 550 °C.In solution-phase synthesis, (NH 4 ) 2 Se or H 2 Se is commonly used as a selenium source, but these procedures oen require advanced equipment, precise reaction conditions, and the use of hazardous and delicate raw materials, which can limit progress in this area.Wu et al., 21 successfully synthesized manganese selenide utilizing Se powder and a hydrazine reducer, heating the mixture to 1800 °C for 12 h to obtain the desired composition.Similarly, Liu et al., 36 achieved successful synthesis using SeO 2 powder and varying concentrations of a hydrazine reducer, with the reaction taking place for 12 h at 1200 °C.
The hydrothermal approach, as documented in previous papers, was applied in this study to create manganese selenide nanorods.Diverse techniques were implemented, encompassing modications to the reducing agent, adjustments in the synthesis temperature, and variations in the synthesis duration, based on available reports.8][39] This research examines how various magnetic elds affect the morphology, energy gap, crystal structure, and magnetic characteristics of MnSe nanoparticles.In order to investigate the impact of magnetic elds on these attributes, MnSe nanoparticles were synthesized under various magnetic eld strengths (ranging from 0 to 800 G).This study provides new insights into the interactions between magnetic elds and the characteristics of MnSe nanoparticles, which could have signicant implications for spintronics and magnetic storage technologies.

Experimental
All chemicals used in this study, including manganese chloride (MnCl 2 $4H 2 O), selenium powder (Se), and sodium borohydride (NaBH 4 ), were acquired from Merck Co (>98%) and utilized without additional purication.To prepare manganese selenide nanorods, MnCl 2 $4H 2 O (3 mmol) was dissolved in 50 ml of deionized water, and this solution was placed under N 2 gas at a pressure of 1 bar for 15 min (Fig. 1a).During the MnCl 2 $4H 2 O test, 6 mmol of the reducing agent NaBH 4 was poured into 5 ml of deionized water and stirred at 500 rpm for 5 min.Aer 5 min, selenium powder was added to the NaBH 4 solution and placed on the stirrer at 600 rpm for 3 min (Fig. 1b).Initially, adding selenium caused the solution to turn black, but aer 3 min, it became colorless.The colorless solution was promptly combined with the manganese chloride solution and subjected to a stirring rate of 700 rpm for 10 min under a N 2 gas atmosphere at a pressure of 1 bar (Fig. 1c).Upon addition of the colorless solution, the overall solution exhibited a creamy appearance.The next phase involved pouring the mixture into the autoclave, which was then heated to 190 °C and le for 12 h (Fig. 1d).The solution was then centrifuged through deionized water and washed with ethanol before being dried in a vacuum oven for 12 h at 70 °C.MnSe nanorods were ultimately produced.
The stages of MnSe synthesis are shown in Fig. 1.These steps were carried out in the presence of an applied magnetic eld of 40, 100, 250, 350 or 800 G.The magnetic eld was meticulously generated using a pair of precisely calibrated magnetic coils.For example, the MnCl 2 $4H 2 O solution was placed under N 2 gas in a magnetic eld of 40 G.The stage combining NaBH 4 with Se was not carried out under a magnetic eld, but then the resulting colorless solution was added to the manganese solution under the presence of a magnetic eld.Finally, aer 10 min, we put the solution in an oven.Syntheses carried out under different magnetic elds revealed changes in the color of the synthesized powders with increasing magnetic eld strength.
According to the syntheses carried out under different magnetic elds, it was observed that the color of the synthesized powders changes with an increase of the magnetic eld.In the synthesis of the original sample, black powder was formed; under a magnetic eld of 40 G the powder was grey; under a 100 G magnetic eld, a combination of cream and brown appeared; under 250 G, the color became creamier; under 350 G, the color of the powder was a light brown; and under a magnetic eld of 800 G, the color of the powder was a burnt brown (Fig. 1e).The dimensions and purity of the resulting materials were assessed utilizing an X-ray diffractometer (XRD) with CuKa radiation (l = 1.54 Å), manufactured by Philips under the XPert MPD model.The morphologies of the materials were investigated using a eld-emission scanning electron microscope (FE-SEM), namely a Hitachi S-4160.Ultraviolet-visible (UV-Vis) absorption behavior was recorded with a Unico 4802 UV-vis spectrophotometer spanning the 200-1000 nm range.Magnetic hysteresis loops of the specimens were measured using a vibrating sample magnetometer (VSM) from MDK-Magnetics.

Results
The X-ray diffraction patterns of the samples synthesized under different magnetic elds are shown in Fig. 2. In the synthesis without the application of a magnetic eld, the peaks are identied as diffraction planes (111), ( 200), ( 220), ( 311), (222), In the sample synthesized under 40 gauss magnetic eld, the mold phase is a-MnSe, but the peaks related to Se and MnSe 2 impurities are increased compared to the synthesis under zero gauss eld.Also, the peaks in this synthesized sample correspond to the original sample and the lattice constant remains unchanged.
Under the magnetic eld of 100 gauss, the mold phase is a-MnSe with a lattice constant of a = 5.90 Å.By increasing the magnetic eld during synthesis, the Mn 3 O(SeO 3 ) 3 impurity can also be seen along with Se and MnSe 2 .In addition, the a-MnSe corresponds to the JCPDS card and number 01-088-2344.
Under the eld of 250 gauss, the mold phase of a-MnSe is with two lattice constants of a = 5.46 Å and a = 5.90 Å, which correspond to JCPDS card numbers 01-088-2344 and 00-011-0683.Also, the impurity peaks in this synthesis are the same as the sample synthesized in the magnetic eld under 100 gauss.
Under the magnetic eld of 350 gauss, with the increase of the magnetic eld, the synthesized compound changed to MnSe 2 , and the mold phase is MnSe 2 , which corresponds to the JCPDS card number 00-020-0722, has a cubic structure with a lattice constant of a = 6.41 Å.Under the 350 G eld, in addition to the impurity peaks that existed in the product under the 100 and 250 G magnetic elds, peaks related to MnSeO 4 also appeared.
Under the magnetic eld of 800 gauss, with the increase of the eld, a Mn 3 O(SeO 3 ) 3 impurity is observed along with MnSeO 4 .Also, it is the MnSe 2 phase, which is compatible with JCPDS card number 01-073-1525, and the lattice constant in this combination is a = 6.41 Å.
Finally, the X-ray diffraction patterns of MnSe samples synthesized under the various magnetic elds show changes in the crystal structure and the impurities that are present.In the absence of a magnetic eld, the sample exhibited a cubic a-MnSe structure, while under increasing magnetic elds, impurity peaks related to Se and MnSe 2 appeared, indicating that the magnetic eld inuenced the formation of impurities in the synthesized samples.The lattice constant of the a-MnSe structure altered as the magnetic eld intensity rose, and nally the synthesized substance shied to the MnSe 2 phase.The appearance of the Mn 3 O(SeO 3 ) 3 impurity under higher magnetic eld strengths, further conrmed the effect of the magnetic eld on the synthesis process.All things considered, the ndings imply that introducing a magnetic eld during synthesis can change the crystal structure and impurity formation in MnSe samples, which may impact their magnetic and electronic characteristics.
The size of the main particles in each sample along with the lattice strain are given in Table 1.
The magnetic eld impacts the crystal lattice during the synthesis process, as the magnetic eld causes a change in the lattice strain.The lattice strain rises as a function of the magnetic eld's intensity since it can result in a more considerable degree of lattice distortion.Furthermore, its interaction with atomic magnetic moments could generate stresses within the crystal lattice.The occurrence of lattice strain might also be affected by the presence of defects or impurities induced by the eld.Additionally, the size reduction of the crystals with an increase in the magnetic eld can be attributed to the eld applied to the particles during the synthesis process, which could impede nucleation and result in smaller crystallite sizes.The magnetic eld might play a crucial role in the alignment and aggregation of particles in the solution, enhancing nucleation efficiency and promoting the growth of smaller particles.Moreover, the magnetic eld may inuence the diffusion and transport of reactants, leading to more uniform nucleation and smaller crystallite sizes.In essence, the magnetic eld can be employed as an adjustable parameter to control the size of the crystals during synthesis.
As shown in Fig. 3, our results clearly demonstrate the signicant impact of the magnetic eld on the nal product's appearance.Crystals with a nanorod shape form when there is no magnetic inuence.However, as the intensity of the magnetic eld increases, it starts affecting crystal growth,  causing the crystals to grow at unexpected orientations.
Consequently, the number of nanorods decreases, and the crystals transform into 'sugar' cubes.
As the magnetic eld intensity continues to rise above 250 G, the cube crystals disappear, and the nanorods reappear, indicating a shi in the direction of crystal growth.The presence of impurities in the morphology inuences the crystal growth process, leading to changes in the crystal shape and size.All these ndings highlight the crucial role the magnetic eld plays in determining the crystal structure and morphology of the synthesized sample.
The results presented in Fig. 4 display variations in the energy gap for the synthesized samples.The energy gap shows an upward trend as the magnetic eld increases.It is wellknown that the application of a magnetic eld impacts electron spin and orbital motion, resulting in modications to the band structure and density of states.In the specic case of MnSe, higher magnetic elds might reduce the overlap between Se (4p) and Mn (3d) orbitals, consequently leading to an increase in the energy gap. 40This effect tends to be more prominent at lower magnetic elds, where the orbital overlap is stronger in the absence of a magnetic eld.
The decrease in the energy gap at high magnetic elds (800 gauss) could be due to the onset of magnetic saturation in the material, which can lead to a reduction in the magnetic moment and a weakening of the effect of the magnetic eld on the electron structure.Additionally, the presence of impurity phases at higher magnetic elds could also contribute to the observed changes in the energy gap.The subsequent steps in our study involved subjecting all synthesized samples to a uniform pressure of 50 kg m −2 for a duration of 24 h.This procedure was crucial for conducting thorough analyses of current-voltage characteristics, temperature-current relationships, and resistance-temperature behaviors.Notably, all samples, with the exception of the one synthesized without applying a magnetic eld, displayed insulating behavior.This observation prompted us to investigate the FE-SEM analysis of all samples.It is worth noting that discontinuities between the crystals of these samples, which show insulating behavior, could be detected.
This nding shows that the insulating behavior observed in the samples can be attributed to the presence of physical discontinuities or fractures in the crystal structure.Such breaks could disrupt the efficient movement of charge carriers, leading to the observed insulating characteristics (Fig. 5).
According to Fig. 6: (a) the exponential increase in current in the low voltage range (0.01-0.1 mA) is likely due to the presence of surface states in the MnSe tablet.These surface states can act as traps for charge carriers, leading to a non-linear response to voltage.As the voltage increases, more charge carriers can overcome these traps and contribute to the current, resulting in an exponential increase in current.The gradual rise in current observed in the upper voltage range (0.1-0.5 mA) is probably attributed to the overall conductivity of the MnSe sample.As the voltage steadily escalates, an increasing number of charge carriers are enabled to partake in the current ow, leading to a linear augmentation in current.
(b) The reduction in resistance as the temperature rises is a widely observed occurrence in most materials and is known as the positive temperature coefficient of resistance (PTCR) phenomenon.The PTCR effect arises due to the thermal activation of charge carriers, leading to an increase in the number of charge carriers available for conduction and, consequently, a decline in resistance. 41owever, within the temperature range 310-320 K, the slope of the resistance-temperature curve differs from that of other temperature ranges.This indicates that factors beyond the thermal activation of charge carriers inuence the resistance within this specic temperature interval.One possible reason for this behavior is that the sample undergoes a structural alteration within this temperature range.Such a transformation might trigger modications in the crystal structure, lattice parameters, or defect concentration, thereby affecting the material's electronic and transport properties.
Another possible explanation is that the sample experiences a phase transition within this temperature range, leading to changes in the material's electronic and transport properties.For instance, this phase transition could bring about alterations in the number or mobility of charge carriers, subsequently impacting the resistance.
(c) The increase in current with temperature can be explained by the increase in thermal energy, which leads to the excitation of more electrons and their promotion to the conduction band.In the range 300-320 K, the increase in current is exponential because the thermal energy is not yet sufficient to overcome the bandgap energy, and only a small number of electrons can be excited to the conduction band.However, as the temperature increases beyond 320 K, the thermal energy becomes sufficient to overcome the band gap energy, and a larger number of electrons can be excited to the conduction band, leading to a linear increase in current.
(d) We observe a clear trend of declining magnetization from the sample synthesized with the lowest magnetic eld of 40 gauss to the one with the highest magnetic eld of 800 gauss.This pattern can be explained by considering the magnetic characteristics of the material and the inuence of external magnetic elds during the synthesis process.
When a magnetic material is exposed to an external magnetic eld, its magnetic domains align with the applied eld, resulting in an increase in magnetization.However, upon removing the external eld, the magnetic domains tend to disperse randomly, potentially causing a partial or complete loss of magnetization.This phenomenon is referred to as magnetic hysteresis.
Regarding the MnSe sample, the original specimen, synthesized without the application of any external magnetic eld, displays the highest magnetization because it was not affected by any external eld that could disturb its magnetic domains.Conversely, the samples synthesized with progressively increasing magnetic elds (ranging from 40 to 800 gauss) encountered progressively stronger external elds during synthesis, which might have disrupted their magnetic domains, leading to lower magnetization values.
It is also worth mentioning that the antiferromagnetic phase observed in all samples indicates that the magnetic moments of Mn and Se ions are oppositely aligned, resulting in a net magnetization of zero.However, an external magnetic eld can induce a nonzero magnetization by breaking the antiferromagnetic alignment, leading to a net magnetic moment in the material.The decreasing trend of magnetization with increasing external eld strength suggests that the external eld was not potent enough to fully align the magnetic moments in the material.

Conclusions
In this research, we investigated the synthesis of manganese selenide nanorods under a range of magnetic elds (0-800 gauss).X-ray diffraction revealed changes in the crystal structure and impurities with increasing eld strength, impacting crystal growth and the energy gap.The magnetization of the samples decreased with stronger magnetic elds, indicating the disruption of magnetic domains.For the synthesized sample without a magnetic eld, the resistance-temperature curve exhibited a positive temperature coefficient of resistance (PTCR) effect.Magnetic elds play a vital role in controlling the crystal structure, impurities, and magnetic properties of MnSe samples.

Fig. 1
Fig. 1 Steps of the synthesis process: (a) adding MnCl 2 $4H 2 O to water under N 2 gas, (b) adding NaBH 4 to deionized water and then adding Se to the solution, (c) the solution containing NaBH 4 and Se is added to the MnCl 2 $4H 2 O solution under N 2 gas, (d) the solution is poured into the autoclave and placed in the oven, (e) color scheme of synthesized solutions under different magnetic fields.

Fig. 2 X
Fig. 2 X-ray diffraction of the synthesized samples in the absence of a magnetic field and magnetic fields of 40, 100, 250, 350 and 800 gauss.

Fig. 3
Fig. 3 FE-SEM analysis in the 500 nm range: (a) sample synthesized without applying magnetic field, (b) sample synthesized under 40 G magnetic field, (c) sample synthesized under 100 G magnetic field, (d) synthesized sample under 250 G magnetic field, (e) the sample synthesized under 350 G magnetic field, and (f) the sample synthesized under the 800 G magnetic field.

Fig. 4
Fig. 4 Energy gap diagrams drawn by the tack plot method.(a) Sample synthesized without applying a magnetic field, (b) sample synthesized under the application of a 40 G magnetic field, (c) sample synthesized under the application of a 100 G magnetic field, (d) sample synthesized under the application of a 250 G magnetic field, (e) sample synthesized under the application of 350 G magnetic field and (f) sample synthesized under the application of an 800 G magnetic field.

Fig. 5
Fig. 5 FE-SEM analysis at 1 mm scale: (a) sample synthesized under 40 G magnetic field, (b) sample synthesized under 100 G magnetic field, (c) sample synthesized under 250 G magnetic field, (d) sample synthesized under 350 G magnetic field, and (e) sample synthesized under 800 G magnetic field.

Table 1
Crystallite size and lattice strain of the synthesised samples in the absence and presence of a magnetic field