Manganese Chalcogenide Semiconductor Nanocrystals: Synthesis, Properties And Structural Stability Under High Pressure

In recent decades, semiconductor nanocrystals （NCs） have attracted great interestin the areas of both fundamental research and technical application. Manoeuvring theshape and size of the semiconductor NCs has long been considered as a powerfulmethod for tailoring their properties and enhancing their performance in a widevariety of applications, including catalysis, solar cells, light-emitting diodes, Li-ionbatteries and sensors. Besides the shape and size, the crystal structure also caninfluence the chemical and physical properties of NCs. Therefore, simple methods arerequired to be developed in order to easily manipulate the nucleation and growth, andthus to tune the crystal structure of NCs. Moreover, it is well-known that highpressure is an effective approach to change the structure of bulk/nanomaterials.Therefore, high-pressure studies not only enhance the understanding of physicalmechanism but also provide new opportunities to fabricate new crystal structure ofnanomaterials not observed at atmospheric pressure.As an important magnetic semiconductor material, Manganesechalcogenides （MnX, X=S, Se, Te） exhibit a variety of important magneto-opticalproperties that result from their crystal structures. Especially, MnX areantiferromagnetic semiconductors with interesting magnetic ordering, which can betuned with dimensionality, thickness and strain in thin-film superlattices. They cancrystallize into three kinds of structural forms: the rock salt （RS）, the zinc blende （ZB） and the wurtzite （WZ） structures. It is important to investigate and understand thetemperature or pressure induced phase transition processe and mechanism in MnXmaterials. However, the present study for the control of crystal structure of MnX NCsare few, therefore it is needed more experimental and theoretical studies to explore,which makes people have a better understanding for the antiferromagneticsemiconductor NCs. In this paper, we have synthesized MnX NCs by thesolvothermal method. It was found that the size, shape and crystal structure could becontrolled by simply varying the reaction time and temperature. We have alsoinvestigated the influence of size, shape and crystal structure to the optical andmagnetic properties of MnX NCs. Based on synthesis of MnS NCs with differentcrystal structure, we have investigated pressure induced phase transition of MnS NCsby synchrotron radiation X-ray diffraction.We have fabricated ZB-MnS, WZ-MnS, and RS-MnS NCs by one-pot solventthermal approach. It was found that the crystal structure of MnS NCs could becontrolled by simply varying the reaction temperature. Meanwhile, the correspondingmorphologies of MnS NCs also have experienced drastic evolutions, which indicate aclose relationship between the crystal structure and growth behavior ofnanocrystalline. In this process, we systematically explored the formation of bipod（including the ZB-core formation and WZ-arms growth） and the evolution into rodthrough Ostwald ripening processes. This process of structure and shape variationprovides direct evidence for the polymorphism model proposed by Alivisatos and co-workers. Furthermore, we have fabricated RS-MnS nanocubes with tunable sizes bythe one-pot solvothermal method. Their average edge length was found to beincreased from14to40nm with prolonging the reaction time. In our experiments, thestandard deviations （δ） of these NCs sizes were calculated to be79%. In this process, we systematically explored the formation of RS-MnS nanocubes and found that thepresent synthetic method is virtually a combination of oriented aggregation andintraparticle ripening processes. After nature cooling to room temperature, the RS-MnS nanocubes could spontaneously assemble into ordered superlattices. These RS-MnS nanocubes display size-dependent ultraviolet emission （356373nm）. Themagnetic measurements indicate the nanocubes have an antiferromagnetic core/ferromagnetic shell structure and large coercive fields （e.g.,1265Oe for40nmnanocubes）. The assembly of theRS-MnS nanocubes into the superlattices results inthe change of the single domain to the multidomain, leading to the higher coercivity.Using MnCl2and Se as precursors, we have synthesized anisotropic tetrapod andwaterdrop-shaped WZ-MnSe NCs by a one-pot solvothermal method for the first time.For the formation of different anisotropic shaped WZ-MnSe NCs （e.g. tetrapod-,small waterdrop-and large waterdrop-shaped MnSe NCs） by controlling heating rates,oleic acid （OA） and oleylamine （OLA） were employed as the ligand and reactionsolvent. Importantly, this work provides a facile strategy in the synthesis of shape-controlled MnSe NC sthrough simply tuning the heating rates. According to thequantitative analysis of the XRD data and HRTEM image, the tetrapod-shaped MnSeNC in our experiment consists of a ZB core and four arms of WZ structure. So, it issuggested that the growth of tetrapod-shaped MnSe NCs follow the polymorphismmodel proposed by Alivisatos. Furthermore, these WZ-MnSe NCs display blue–violetemission （400405nm）, which may find applications in full-colour display and shortwavelength optoelectronic devices. The magnetic measurements indicate that the Néeltemperature is found to be reduced with decreasing nanocrystal diameter. The size-dependent phase-transition behavior is attributed to the surface effect. Modifying thevolume ratio of the solvents could result in the predominant formation of RS-MnSe NCs. This indicates that RS-MnSe nucleus seeds were formed in the early stage ofnucleation because the relative chemical environment for the configuration of thenuclei was changed. We have also tried to synthesize manganese telluride NCs, andobtained XRD pattern showed that the sample is attributed to MnTe₂NCs.Based on the synthesis of three kinds of crystal structure of high quality MnSNCs, we have explored the stability of MnS NCs under high pressure by using in situsynchrotron radiation X-ray diffraction. We have studied the stability of metastableZB-and WZ-MnS NCs under high pressure and found that ZB-nanoparticles and WZ-nanobipods are stable below their critical pressure,5.3and2.9GPa, respectively.When pressures exceed the critical point, all these metastable MnS NCs directlyconvert to the stable rock salt MnS. By fitting the third-order Birch-Murnaghanequation, we have for the first time obtained the bulk modulus of ZB-and WZ-MnSNCs （49.5and57.2GPa）. We have also studied the RS-MnS NCs under pressure andfound that both RS-MnS nanocubes and nanorods transformed into orthogonal MnPphase at11.6and24.3GPa, respectively. Decompression to ambient pressurethe highpressure phase （orthogonal MnP phase） was retained, which was a new crystalstructure of the MnS nanomaterials firstly prepared by high pressure.