Semiconductor nanostructures and dedicated magneto-optical methodologies

Research

Prof. Lifshitz's group is among the pioneers who initiated the exciting field of nanomaterials for nanotechnology. The group employs a multifaceted approach to studying the low-dimensional semiconductor solids, with a notable contribution to the development of new materials and the in-depth understanding of the fundamental features which govern their size-related physical properties.   

Materials

Our research is based on the investigation of semiconductor nanostructures, which represent a class of luminescent chromophores with quantized electronic states, and tunable intense optical transitions that vary with the material's size, shape, and composition. The efforts are divided into investigating three different material platforms (see Scheme 1). 

The first category of materials we work on belongs to colloidally prepared nanostructures (dots, rods, wires, platelets, and polypods) from the II-VI and IV-VI families and their corresponding core/shell

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hetero-structures. These include (a) colloidal nanostructures of CdSe, CdTe, PbSe, and PbS coated with organic ligands or by an epitaxial layer of another semiconductor, also known as core-shell structures, such as PbSe/PbS, CdSe/CdS, CdTe/CdSe (b) colloidal nanostructures with alloyed composition, such as PbSexS1-x or PbSe/PbSexS1-x core-shell hetero-structures; (c) core and core/shell structures with reduced toxicity, such as SnTe, SnTe/PbSe, InSb, In2S3; (d) transition metal (Mn+2, Ni+2, Cu+1/2) doped core/shell nanostructures (dots, rods, nanoplatelets), with dopants either at the core or at the shell.  

The second type of materials is Van der Waals materials or layered semiconductors, including transition metal dichalcogenides or iodides (SnS2, PbI2, Bi2I3, In2S3) and metal phosphor-tri-chalcogenides (MPX3, M=Mn, Fe, Ni, Mg, Cd) such as MnPS3, FePS3, CoPS3, NiPS3, and ZnPS3, without/with alloying (Mn0,5Fe0,5PS3), mixed oxidation state (Cr0,5Cu0,5PS3) and doping (Mn: ZnPS3), as well as transition metal phosphorous four-chalcogenides, MPX4 (e.g., CrPS4).

We also chemically prepare and study two- and three-dimensional (ABX3, A=Cs+, Ma+; B=Pb, Bi; X=halides) and two-dimensional perovskite materials. All mentioned materials are of enormous interest for implementation in various optoelectronic, spin-electronics, memory, and quantum-computation. 

Properties

The pristine materials, or/and their magnetically doped derivatives, exhibit a unique combination between optical properties and magnetic effects. Our group explores the correlation between intrinsic magnetic fields and the optical properties of nanoparticles.  
A few types of magnetic properties are summarized in Scheme 2: The long-range magnetic order, such as ferromagnetism, anti-ferromagnetism, or unique spin textures (a) are gaining exceptional stabilization by the size confinement, with a special interest in emerging technologies of spin-electronics and quantum computation; The short-range magnetic phenomena, which are intrinsically developed in low dimensional materials, such as Rashba spin-orbit effect (b), nuclear spins (Overhauser effect) (c) and magnetic polarons (d). The intrinsic field leads to a lift of energy or momentum degeneracy at band-edge states, leading to selective spin orientation at the excited state and consequent helicity of a recombination emission. The materials and their properties are investigated by following the behavior of spins as a designation for the electronic properties using the methodologies stated in the following section.

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Experimental Methodologies

Prof. Lifshitz's group developed notable expertise in several distinctive methodologies combining magnetic resonance, cyclotron resonance, magnetic polarization, microwave absorption, and optical spectroscopy, supplying information not revealed by conventional techniques, such as angular momentum of electronic states, g-factors, exchange interaction, Zeeman interaction, diamagnetic shift, crystal field and Rashba effects, all possessing considerable importance in understanding the physical properties of nano-scaled materials. These methodologies include the following: (a) Optically detected magnetic resonance (ODMR); (b) Microwave and thermal modulated photoluminescence; (c) Optically detected cyclotron resonance; (d) Circular polarized photoluminescence in the presence of an external magnetic field; (e) Magneto-optical confocal microscopy, when detecting the photoluminescence of isolated single nanostructure under the influence of an external magnetic field; (f) Atomic force microscopy combined with confocal microscopy for the manipulation and detection of a single nanostructure; (g) Confocal microscopy combined with magnetic resonance measurement at the excited state (viz, optically detected magnetic resonance of an isolated single nanostructure). Some of these techniques are homemade assemblies with a worldwide uniqueness. The experimental set-up or/and representative results are depicted in Figure 1.

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Theoretical and computational studies

The experimental work is corroborated by theoretical modeling, carried out by our group members in a collaboration with expert theoreticians. The electronic band structure calculations of core and core/shell quantum dots and rods are calculated using an effective mass approximation method or DFT means. We included special factors like the interface strain and its relaxation by a soft boundary, dielectric confinement, Coulomb and exchange interactions, and Auger processes (in collaboration with Prof. A. Efros, USA). In another case, electrical conductivity in double quantum dots dependence on inter-dot distance was calculated, showing exceptional phenomena such as induced recoil or negative resistance induced by the application of charge or voltage (in collaboration with Prof. U. Peskin, Technion, Israel). Our most recent study focuses on the influence of spin-orbit coupling and symmetry breaking that formed the Rashba effect, while the last induced electronic band split and the generation of polarized transitions (in collaboration with Prof. Andrew Rappe, Pennsylvania University). Current projects encompass DFT theoretical work on various phenomena in halide perovskite materials (in collaboration with Dr. Liang Tan, LBL, Barkeley and Prof. Leeor Kronik, Weizmann Inst.,). and on the physical phenomena in lamellar MPX3 compounds (in collaboration with and Dr. Magdalena Birowska).

Projects

 (i) Investigating the role of surface-interface centers of the core-shell, doped and alloyed nanocrystals

Colloidal nanocrystals (NCs) are known for their tunable photo-physical properties by size, shape, and composition variation. In this project, we explore the synthesis and characterization of the quantum dots (QDs), seeded nanorods (NRs), nanoplatelets (NPLs), and magnetically doped colloidal nanocrystals of different morphologies, all with core/shell configurations based on different elemental composition with a type-I/ type-II/ quasi-type-II core-shell band alignment. We also explore how the suppression of interband Auger decay, such as biexciton Auger recombination, and multiple excitons in a single particle luminescence, is achieved with the design of hetero-structured core−shell nanocrystals, alloying compositions at the core/shell interface, intentional incorporation of magnetic dopants involving guest-host spin-exchange interaction, thus, consequentially tuning the 

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magneto-optical properties. The confined structures enhance the spin-spin interaction between photo-generated carriers (electron and hole) and spins of the magnetic impurities. The degree of magnetization depends on the quantum confinement, the type of dopant, and its position concerning the host-carrier distribution function.

 (ii) Investigating the Synthesis and properties of branched nanostructures

Branched nanostructures have gained considerable scientific popularity due to their large surface-to-volume ratio, which enforces their applicability in photocatalysis and photovoltaics. This project deals with synthesizing such nanostructures based on metal sulfides, tailoring their shapes (star-like, flower-like, and more as seen in Figure 3) and sizes, understanding the reaction mechanism and the controlling factors that influence and template their morphology. For this purpose, the primary precursors, types of surfactants, precursor-to-surfactant molar ratio, temperature, and the duration of the reaction are varied while examining their effect on the growth and morphology of the nanostructures. Furthermore, intermediate products in their growth reaction are thoroughly examined. Thus, this project aims at gaining fundamental knowledge for designing other branched structures attractive for practical use in catalysis, electrochemistry, and light-harvesting.

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 (iii) Proving anisotropies and Rashba effect in perovskite materials

Metal halide Perovskites (MHP) and hybrid organic-inorganic MHPs have captivated researchers in recent years for their physical properties such as superconductivity, ferroelectricity, piezoelectricity, ferromagnetism, and more. These properties rely on the electronic and spin interactions, which are still poorly understood in these materials. This project includes the investigation of magneto-optical properties utilizing circularly and linearly polarized photoluminescence in the presence of an external magnetic field and optically detected magnetic resonance spectroscopy. Those methodologies should reveal information about selective electronic states with their spin orientation, exposing internal magnetism (e.g., nuclear spin effective fields), g-factor of carriers, and spin-spin interaction. Additionally, the Magneto-optical measurements of MAPbBr3 single crystal show 

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strong evidence of the coupling between a photo-generated carrier and nuclei in the lattice. In this case, the counterbalance between the Rashba, Overhauser, and Zeeman fields is exploited. This new observation further supports the correlation between the formation of the Rashba effect with the breaking of inversion of symmetry. Figure 4 depicts one of our recent observations regarding the Impact of anisotropy in spin-orbit coupling on the magneto-optical properties of bulk lead halide perovskites.

 (vi) Exposing a strong correlation between long-range magnetic order (Ferro- or Anti-ferromagnetic) and optical properties of magnetic van der Waals materials 

Since the discovery of graphene, atomically thin materials such as transition metal dichalcogenides and/or trichalcogenides (MPX3) have sparked much attention due to their honeycomb morphology and variable electronic properties (superconductors, metals, semiconductors). Those based on the first row of transition metals are mainly anti-ferromagnet with three different internal arrangements, Neél, zigzag, and stripe (related to the order of alternating spin 

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directions, shown in Figure 5). Our current research work focuses on the synthesis, exfoliation (mechanical or liquid), and investigation of a vast number of MPX3 compounds and their alloying composition. We mainly explore a correction between magnetism and the electronic/optical properties of the materials. A few intriguing phenomena were uncovered in most recent times, related to magnetic phase transitions which induced unprecedented changes in the electron structure.  The last was pronounced in the appearance of new phenomena in the optical transitions (energy split, polarization, change in lifetime of recombination emission).