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.

Spin Ordering Induced Broadband Photodetection Based on Two-Dimensional Magnetic Semiconductor α-MnSe

Two-dimensional (2D) magnetic semiconductors are considered to have great application prospects in spintronic logic devices, memory devices, and photodetectors, due to their unique structures and outstanding physical properties in 2D confinement. Understanding the influence of magnetism on optical/optoelectronic properties of 2D magnetic semiconductors is a significant issue for constructing multifunctional electronic devices and implementing sophisticated functions. Herein, the influence of spin ordering and magnons on the optical/optoelectronic properties of 2D magnetic semiconductor α-MnSe synthesized by space-confined chemical vapor deposition (CVD) is explored systematically. The spin-ordering-induced magnetic phase transition triggers temperature-dependent photoluminescence spectra to produce a huge transition at Néel temperature (TN  ≈ 160 K). The magnons- and defects-induced emissions are the primary luminescence path below TN and direct internal 4 a T1g →6 A1g transition-induced emissions are the main luminescence path above TN . Additionally, the magnons and defect structures endow 2D α-MnSe with a broadband luminescence from 550 to 880 nm, and an ultraviolet-near-infrared photoresponse from 365 to 808 nm. Moreover, the device also demonstrates improved photodetection performance at 80 K, possibly influenced by spin ordering and trap states associated with defects. These above findings indicate that 2D magnetic semiconductor α-MnSe provides an excellent platform for magneto-optical and magneto-optoelectronic research.

Phosphine-free engineering toward the synthesis of metal telluride nanocrystals: the role of a Te precursor coordinated at room temperature

A colloidal strategy offers opportunities for the rational design and synthesis of metal telluride nanocrystals (NCs) with the desired crystal structure, uniform geometry, and composition. However, it remains a challenge to use the paradigm to construct metal telluride NCs by a phosphine-free synthesis procedure for promising applications such as luminescence, photovoltaics and thermoelectricity. Here, we developed a new strategy for fabricating metal telluride nanocrystals, e.g. CdTe and PbTe NCs, by using a highly reactive phosphine-free Te precursor. The ability to reduce a TeO2 powder with dodecanethiol (DDT) has been achieved in the presence of oleylamine (OLA) to generate a soluble alkylammonium telluride at room temperature. We provide direct experimental evidence that the OLA-Te complexes were formed in an order of second magnitude kinetic process based on an in situ UV-vis absorption test. In the case of the CdTe NC system, the straightforward measurement of luminescence and the fabrication of LED devices are presented that can semiquantitatively assess the quality of preparation and the reactivity of this air-stable precursor. The proposed strategy highlights several unique features of this solution-based green chemistry that can be useful for synthesizing other metal telluride NCs to develop novel functional materials.

Pressure induced photoluminescence modulation in a wide range and synthesis of monodispersed ternary AgCuS nanocrystal based on Ag2S nanocrystals

Binary Ag₂S nanocrystals (NCs) have many potential optical applications because of their low toxicity, narrow direct band gaps and near-infrared photoluminescence (PL) with high emission efficiency. However, due to its small exciton Bohr radius (2.2 nm), the PL spectra of Ag₂S NCs can only be modulated below ∼1200 nm with increasing particle size. Meanwhile, ternary silver copper chalcogenides (AgCuX, X = S, Se) have also attracted increased attention in recent years. Temperature-dependent structural phase transformation leads electrical transport to exhibit fascinating transitions between p and n type conduction, which makes AgCuS and AgCuSe ideal materials for diode or transistor devices. Nevertheless, the traditional method to synthesize these materials is mainly through melting the mixture of Ag, Cu and S/Se powder under extremely high reaction temperatures (973-1373 K) and long reaction time, forming a bulk product. Therefore, the synthesis of high quality monodispersed and size-tunable AgCuS or AgCuSe NCs is still a challenge. To address these issues, in this paper, we report using Ag₂S NCs as a template, a method to synthesize monodispersed and size-tunable β-AgCuS NCs via ion exchange and diffusion processes. Similarly, monodispersed β-AgCuSe NCs were also synthesized by this simple and reproducible strategy. This synthetic method opens new avenues for investigating the size-, morphology- and temperature-dependent phase transitions of these ternary AgCuS and AgCuSe materials. Thus, the corresponding electrical transport between p and n type conduction can be studied in the future. Furthermore, we systematically investigated the pressure-dependent PL properties and band gap modulation of monodispersed Ag₂S NCs using in situ high pressure PL and absorption spectroscopy. We found that the PL peak of 6.0 nm for Ag₂S NCs could be easily adjusted from ∼1200 to 1900 nm with increasing pressure from 0 to 5.1 GPa, which greatly extends the wavelength range of the PL peak beyond that of other approaches.

Solution Synthesis of Nonequilibrium Zincblende MnS Nanowires

Uniform four-coordinate nonequilibrium MnS nanowires mainly in zincblende structure, other than the stable rock-salt phase, are reported for the first time. The MnS nanowires are grown via a solution-solid-solid model from the reaction of a Mn(II) source with dibenzyl disulfide in oleylamine at 180-200 °C catalyzed by Ag₂S nanocrystals in a body-centered cubic (bcc) fast-ionic phase transformed from their low-temperature monoclinic form. Investigations show that most of the zincblende MnS nanowires are grown along the ⟨112⟩ zone axis but a small proportion grow along the ⟨111⟩_ZB/⟨0001⟩_Wur axis with zincblende/defect-section and/or wurtzite/defect-section superlattices connected with the stems along the ⟨112⟩ direction. The nanowires have a tendency to grow straight at relatively low reaction temperature for short reaction times but twist at high temperature for long reaction times. Meanwhile, relatively high temperatures and long times favor the transition of the MnS nanowires in the zincblende phase to the corresponding thermodynamic ones in rock-salt form. Interestingly, even small increases in reaction pressure (1-2 atm) sensitively influence the growth of the MnS nanowires from zincblende to wurtzite form in the present catalytic system although low-pressure changes commonly do not have an obvious effect on condensed matter. In addition, the optical and magnetic properties of the zincblende MnS nanowires were studied, and they are varied largely from the bulk.

Kinetic Growth of Ultralong Metastable Zincblende MnSe Nanowires Catalyzed by a Fast Ionic Conductor via a Solution-Solid-Solid Mechanism

The metastable semiconductor phase allows for the exploration of unusual properties and functionalities of abnormal structures, although it is often difficult to prevent thermodynamic transformations to lower energy structures from higher, unfavored energy states. Here, we show for the first time the preparation of high-quality ultralong metastable zincblende (ZB)-MnSe nanowires with a four-coordinate structure via solution-solid-solid (SSS) growth in a mild solution-phase synthetic environment (120-220 °C) in the presence of a trace amount of Ag(I). The metastable ZB-MnSe nanowires are stabilized kinetically due to the catalysis of early formed body-centered cubic (bcc) fast-ionic (superionic) Ag2Se nanocrystals from the Ag(I) source, and the ZB-MnSe nanowires grow epitaxially along the ⟨110⟩ axis rather than the ⟨111⟩ axis, as commonly observed for typical four-coordinate Grimm-Sommerfeld bonding solids. Our method provides a new route for the growth of metastable nanostructures.

Alternative motif toward high-quality wurtzite MnSe nanorods via subtle sulfur element doping

The manipulated synthesis of high-quality semiconductor nanocrystals (NCs) is of high significance with respect to the exploration of their properties and their corresponding applications. Nevertheless, the preparation of metastable-phase NCs still remains a great challenge due to their high kinetic barriers and harsh synthetic conditions. Herein, we demonstrated the fabrication of high-quality MnSe nanorods with a metastable wurtzite structure via a subtle sulfur-doping strategy. Based on the UV-vis absorption spectra, manganese polysulfide clusters were formed by mixing oleylamine-sulfur and oleylamine-manganese solutions at room temperature. The existence of manganese polysulfide clusters with polymeric sulfur structures makes the system more reactive, inducing fast wurtzite-phase nucleation. This can overcome the natural kinetic barrier of wurtzite MnSe and lead to subsequent growth of targeted NCs. On the other hand, no sulfur doping would produce MnSe NCs in a thermodynamically favorable rock-salt phase. As expected, different doping contents and sulfur sources also resulted in the formation of high-quality wurtzite MnSe nanorods. This success establishes that a facile strategy can be anticipated to synthesize high-quality metal chalcogenide NCs with a metastable phase, especially wurtzite nanorods, for potential applications from spintronics to solar cells.

Physical and biophysical assessment of highly fluorescent, magnetic quantum dots of a wurtzite-phase manganese selenide system

Combining fluorescence and magnetic features in a non-iron based, select type of quantum dots (QDs) can have immense value in cellular imaging, tagging and other nano-bio interface applications, including targeted drug delivery. Herein, we report on the colloidal synthesis and physical and biophysical assessment of wurtzite-type manganese selenide (MnSe) QDs in cell culture media. Aiming to provide a suitable colloidal system of biological relevance, different concentrations of reactants and ligands (e.g., thioglycolic acid, TGA) have been considered. The average size of the QDs is ∼7 nm, which exhibited a quantum yield of ∼75% as compared to rhodamine 6 G dye(®). As revealed from time-resolved photoluminescence (TR-PL) response, the near band edge emission followed a bi-exponential decay feature with characteristic times of ∼0.64 ns and 3.04 ns. At room temperature, the QDs were found to exhibit paramagnetic features with coercivity and remanence impelled by TGA concentrations. With BSA as a dispersing agent, the QDs showed an improved optical stability in Dulbecco's Modified Eagle Media(®) (DMEM) and Minimum Essential Media(®) (MEM), as compared to the Roswell Park Memorial Institute(®) (RPMI-1640) media. Finally, the cell viability of lymphocytes was found to be strongly influenced by the concentration of MnSe QDs, and had a safe limit upto 0.5 μM. With BSA inclusion in cell media, the cellular uptake of MnSe QDs was observed to be more prominent, as revealed from fluorescence imaging. The fabrication of water soluble, nontoxic MnSe QDs would open up an alternative strategy in nanobiotechnology, while preserving their luminescent and magnetic properties intact.

Size-controlled synthesis of bifunctional magnetic and ultraviolet optical rock-salt MnS nanocube superlattices

Wide-band-gap rock-salt (RS) MnS nanocubes were synthesized by the one-pot solvent thermal approach. The edge length of the nanocubes can be easily controlled by prolonging the reaction time (or aging time). We systematically explored the formation of RS-MnS nanocubes and found that the present synthetic method is virtually a combination of oriented aggregation and intraparticle ripening processes. Furthermore, these RS-MnS nanocubes could spontaneously assemble into ordered superlattices via the natural cooling process. The optical and magnetic properties were investigated using measured by UV-vis absorption, photoluminescence spectra, and a magnetometer. The obtained RS-MnS nanocubes exhibit good ultraviolet optical properties depending on the size of the samples. The magnetic measurements suggest that RS-MnS nanocubes consist of an antiferromagnetic core and a ferromagnetic shell below the blocking temperatures. Furthermore, the hysteresis measurements indicate these RS-MnS nanocubes have large coercive fields (e.g., 1265 Oe for 40 nm nanocubes), which is attributed to the size and self-assembly of the samples.