New-phase retention in colloidal core/shell nanocrystals via pressure-modulated phase engineering†

Core/shell nanocrystals (NCs) integrate collaborative functionalization that would trigger advanced properties, such as high energy conversion efficiency, nonblinking emission, and spin–orbit coupling. Such prospects are highly correlated with the crystal structure of individual constituents. However, it is challenging to achieve novel phases in core/shell NCs, generally non-existing in bulk counterparts. Here, we present a fast and clean high-pressure approach to fabricate heterostructured core/shell MnSe/MnS NCs with a new phase that does not occur in their bulk counterparts. We determine the new phase as an orthorhombic MnP structure (B31 phase), with close-packed zigzagged arrangements within unit cells. Encapsulation of the solid MnSe nanorod with an MnS shell allows us to identify two separate phase transitions with recognizable diffraction patterns under high pressure, where the heterointerface effect regulates the wurtzite → rocksalt → B31 phase transitions of the core. First-principles calculations indicate that the B31 phase is thermodynamically stable under high pressure and can survive under ambient conditions owing to the synergistic effect of subtle enthalpy differences and large surface energy in nanomaterials. The ability to retain the new phase may open up the opportunity for future manipulation of electronic and magnetic properties in heterostructured nanostructures.


Synthesis of WZ-type core/shell MnSe/MnS nanorods
Firstly, synthesis of WZ-type MnSe nanorods has been reported in our previous studies. 1 Then, 0.050 g (0.4 mmol) MnCl 2 and 6 mL of oleylamine was loaded to a 50-mL three-necked flask and heated to 300 ℃ under N 2 flow, then, 2 mL unwashed MnSe product was added.
When the solution recovered to 300 ℃, 0.015 g (0.2 mmol) thioacetaminde dissolved in 2 mL oleyamine was added. The reaction was quenched at 5 and 20 mins to get the sample with varied thickness of MnS shell. The resultant WZ-type core/shell MnSe/MnS nanorods were isolated from the growth solution by precipitation with methanol and excess acetone, followed by centrifugation for 10 min at 10000 rpm. Subsequently, the residual samples were redispersed in toluene for characterization.

In situ high-pressure experimental measurements
Electronic Supplementary Material (ESI) for Chemical Science. This journal is © The Royal Society of Chemistry 2021 All in situ high-pressure experiments were implemented using symmetric DAC apparatus furnished with a pair of 400 μm culet diamonds at room temperature. The prepared core/shell MnSe/MnS nanorods were enclosed into a ∼100 μm-diameter hole of the T301 stainless-steel compressible gasket. Silicon oil was utilized as the pressure transmitting medium (PTM) which was purchased from the Dow Corning Corporation (Midland, MI). Pressure determination was achieved by the fluorescence spectrum of the ruby. In situ angle dispersive synchrotron X-ray diffraction (ADXRD) patterns of samples under high pressure were recorded at beamline 15U1, Shanghai Synchrotron Radiation Facility (SSRF), China. The beamline stations at SSRF exploited a monochromatic wavelength of 0.6199 Å. CeO 2 was utilized as the standard sample for the calibration. The pattern of intensity versus diffraction angle 2θ was plotted based on the FIT2D program, which integrated and analyzed the 2D images collected. The in situ high-pressure UV−vis−NIR absorption spectra were measured by a deuterium-halogen light source and recorded with an optical fiber spectrometer (Ocean Optics, QE65000) at room temperature. Magnetic property measurements were conducted on a Quantum Design MPMS superconducting quantum interference device (SQUID) VSM magnetometer. To guarantee the quantity of samples for magnetic performance, highpressure experiments on core/shell MnSe/MnS nanorods were carried out in a Walker-type JLUHC-1000 LVP, subsequently by collecting the quenched products for further magnetic characterization. 2

Theoretical calculations
All the total energy and electronic structure calculations were performed by the Vienna Abinitio Simulation Package (VASP) that uses the projector augmented wave (PAW) formalism of density functional theory (DFT). The Perdew-Burke-Ernzerhof (PBE) generalized gradient approximation for the exchange-correlation function was employed. The energy cutoff for the plane-wave expansion of the wavefunctions was 400 eV for all the calculations. For highly-correlated systems with 3d localized orbitals, the hybrid functional methods are more appropriate than the general DFT. In the present study, HSE method was employed for both geometry optimization and self-consistent calculation. For geometry optimization, the atomic coordinates were relaxed until the Hellmann-Feynman forces were less 0.01 eV/ Å. In order to compare the stability of MnSe under pressure, three structural models of MnSe with phases of WZ, B31 and RS were used. The pressure dependent enthalpies show that the phase WZ is more stable than phase B31 and RS at low pressure (< 1.0 GPa). At high pressure, phase B31 and RS become more stable and the enthalpy difference compared to phase WZ increases with pressure. The enthalpies of phase B31 and RS are very close to each other at all the calculated pressure. However, by taking a closer examination, it can be found the phase B31 is less stable at pressure lower than 30.0 GPa and becomes more stable above 30.0 GPa.       Oe. The small peak around 132 K, suggestive of antiferromagnetic ordering, is resolved in the inset of (c) for the B31-type MnSe/MnS nanorods.

Fig. S8
The phonon spectra of B31-type MnSe. No imaginary phonon frequencies were found in the entire Brillouin zone over the studied pressure range.