Light Emitting Spin Active Electronic States in Ultra-Thin Mn Doped CdSe Layered Nanosheets

Anisotropic Growth of Covalent Inorganic Complexes to Nanoplatelets

Colloidal II-VI semiconductor nanoplatelets (NPLs) provide a new platform in material science due to their unique growth mode and advanced optical properties. However, in contrast to the rapid development of zinc blend structured NPLs, studies on the formation of wurtzite (WZ) NPLs have been limited to the lamellar assembly of specific magic-sized nanoclusters (MSCs). Therefore, the study of new precursors is important for enriching the synthesis strategy, improving the study of two-dimensional (2D) nanocrystal growth mechanisms, and constructing complex nanostructures. Here, we demonstrated that covalent inorganic complexes (CICs), as novel functional intermediates, can be directly used to form NPLs without involving MSCs. Using in situ absorption spectra, we demonstrated that the evolution followed a pseudo-first-order kinetics (kobs = 0.02 min-1 (t1/2 = 34.7 min)). Several types of binary WZ NPLs, including CdSe, CdS, CdTe, and ZnS, have been directly prepared based on this mechanism through the anisotropic growth of CICs. In addition, CICs can also be used to prepare Mn-doped CdSe NPLs. The present study not only affords new precursors for the synthesis of WZ NPLs but also advances our understanding of the synthesis mechanism of nanocrystals.

Size-induced exchange bias in single-phase CoO nanoparticles

The tuning of exchange bias (EB) in nanoparticles has garnered significant attention due to its diverse range of applications. Here, we demonstrate EB in single-phase CoO nanoparticles, where two magnetic phases naturally emerge as the crystallite size decreases from 34.6 ± 0.8 to 10.8 ± 0.9 nm. The Néel temperature (TN) associated with antiferromagnetic ordering decreases monotonically with the reduction in crystallite size, highlighting the significant influence of size effects. The 34.6 nm nanoparticles exhibit magnetization irreversibility between zero-field cooled (ZFC) and field-cooled (FC) states belowTN. With further reduction in size this irreversibility appears well aboveTN, resulting in the absence of true paramagnetic regime which indicates the occurnace of an additional magnetic phase. The frequency-dependent ac-susceptibility in 10.8 nm nanoparticles suggests slow dynamics of disordered surface spins aboveTN, coinciding with the establishment of long-range order in the core. The thermoremanent magnetization (TRM) and iso-thermoremanent magnetization (IRM) curves suggest a core-shell structure: the core is antiferromagnetic, and the shell consists of disordered surface spins causing ferromagnetic interaction. Hence, the EB in these CoO nanoparticles results from the exchange coupling between an antiferromagnetic core and a disordered shell that exhibits unconventional surface spin characteristics.

Enhanced exciton binding energy, Zeeman splitting and spin polarization in hybrid layered nanosheets comprised of (Cd, Mn)Se and nitrogen-doped graphene oxide: implication for semiconductor devices

The exciton properties of (Cd,Mn)Se-NrGO (nitrogen doped reduced graphene oxide) hybrid layered nanosheets have been studied in a magnetic field up to 10 T and compared to those of (Cd,Mn)Se nanosheets. The temperature dependent photoluminescence reveals the hybridization of inter-band exciton and intra-center Mn transition with enhancement of the binding energy of exciton-Mn hybridized state (80 meV with respect to 60 meV in (Cd,Mn)Se nanosheets) and increase of exciton-phonon coupling strength to 90 meV (with respect to 55 meV in (Cd,Mn)Se nanosheets). The circularly polarized magneto-photoluminescence at 2 K provides evidence for magnetic field induced exciton spin polarization and the realization of excitonic giant Zeeman splitting withg_effas high as 165.4 ± 10.3, much larger than in the case of (Cd,Mn)Se nanosheets (63.9 ± 6.6), promising for implementation in spin active semiconductor devices.

Single Mn Atom Doping in Chiral Sensitive Assembled Gold Clusters to Molecular Magnet

Chiral stirred optical and magnetic properties, through the doping of assembled ultrasmall metal clusters (AMCs), are promising discernment to rivet the molecule-like quantum devices. Here, the single manganese (Mn) atom doping and assembly of the gold cluster (Au₈), leading to the chirality driven magnetism, has been achieved through a ligand-mediated growth. The X-ray absorption near edge structure and electron paramagnetic resonance studies corroborate the tetrahedral coordinated local structure of Mn dopant in the Au host. The optical and vibrational circular dichroic analysis affirms the modulation of chirality (negative to positive) in the presence of the Mn. A distinct ferromagnetic hysteresis loop at 300 K shows Mn ridden chiral sensitive ferromagnetism in contrary to the ligand influenced superparamagnetic undoped AMCs. The spin-polarized density functional theory level of calculations reveal the partial overlapping of spin-up and -down density of states in the doped AMCs, attributing to the ferromagnetic nature as like a molecular magnet suitable for the opto-spintronics application.

Intracenter optical transitions in Mn2+ion in CdMnTe crystals: the effect of strong Jahn-Teller coupling in excited state

Intracenter resonance optical transitions between ground and first excited states of a Mn2+ion in bulk CdMnTe crystal are theoretically and numerically studied. An algorithm is proposed to calculate the matrix elements of the optical transitions under the influence of electromagnetic waves of different polarizations from a six-fold degenerate ground state6A1to a twelve-fold degenerate first excited state4T1. The calculations were carried out with the account of a strong Jahn-Teller effect, which is due to the interaction of charge carriers in the excited state of the ion with local tetragonal distortions of the CdMnTe crystal lattice. By comparison with previously obtained experimental data, it is shown that polarization properties of the photoluminescence of manganese ions in such crystals as well as its spectral features could be explained only with the account of strong Jahn-Teller coupling in the excited state. The same calculation technique based on symmetry group analysis combined with numerical methods could be applied to intracenter Mn-transitions in other II-VI materials and it could be extended to other optically addressed atoms with highly degenerate electronic states coupled to vibrational modes.

Energetic hot electrons from exciton-to-hot electron upconversion in Mn-doped semiconductor nanocrystals

Generation of hot electrons and their utilization in photoinduced chemical processes have been the subjects of intense research in recent years mostly exploring hot electrons in plasmonic metal nanostructures created via decay of optically excited plasmon. Here, we present recent progress made in generation and utilization of a different type of hot electrons produced via biphotonic exciton-to-hot electron "upconversion" in Mn-doped semiconductor nanocrystals. Compared to the plasmonic hot electrons, those produced via biphotonic upconversion in Mn-doped semiconductor nanocrystals possess much higher energy, enabling more efficient long-range electron transfer across the high energy barrier. They can even be ejected above the vacuum level creating photoelectrons, which can possibly produce solvated electrons. Despite the biphotonic nature of the upconversion process, hot electrons can be generated with weak cw excitation equivalent to the concentrated solar radiation without requiring intense or high-energy photons. This perspective reviews recent work elucidating the mechanism of generating energetic hot electrons in Mn-doped semiconductor nanocrystals, detection of these hot electrons as photocurrent or photoelectron emission, and their utilization in chemical processes such as photocatalysis. New opportunities that the energetic hot electrons can open by creating solvated electrons, which can be viewed as the longer-lived and mobile version of hot electrons more useful for chemical processes, and the challenges in practical utilization of energetic hot electrons are also discussed.

Slowing down photocarrier relaxation in Dirac semimetal Cd3As2 via Mn doping

In this Letter, we successfully introduce a long-lived non-radiative photocarrier decay component in a Dirac semimetal Cd₃As₂ thin film via Mn doping. The long-lived decay component is found to vary between 200 ps and 2.8 ns with different Mn concentrations and probing wavelengths. Most remarkably, the elongated transients persist over the important mid-infrared wavelengths (observed up to 4 μm). Saturable absorption measurement reveals stronger modulation effects for long-width pulses (∼80  ps) from the Mn-doped samples. Our results provide new insights into the effect of transition-metal doping on the ultrafast optical properties of Dirac semimetal Cd₃As₂ and establish Cd₃As₂ as a highly amendable material for mid-infrared photonic applications.

Carbon impurity-free, novel Mn,N co-doped porous Mo2C nanorods for an efficient and stable hydrogen evolution reaction

Carbon impurity-free, novel Mn,N co-doped porous Mo₂C nanorods reduce the hydrogen adsorption energy, functioning as efficient HER electrocatalysts.