Semicondcutor nanostructures and dedicated magneto-optical methodologies

Prof. Lifshitz's group is among the pioneers who initiated the exciting field of nanomaterials for nanotechnology.  The group employs a multifaceted approach for the study of low dimensional semiconductor solids, with a notable contribution in the development of new materials, and moreover, in the depth of understanding of the fundamental features which govern size-related physical properties. In particularly, the group explores the correlation between intrinsic magnetic fields and optical properties of nanoparticles.

The research includes the investigation of semiconductor nano-structures, which represent a class of luminescent chromophores 

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with quantized electronic states, and tunable intense optical transitions that vary with the material’s size, shape and composition. The efforts are divided into the investigation of three different material platforms (see Scheme 1):  (a) Colloidally prepared nanostructures (dots, rods, wires, platelets and polypods) from the II-VI and IV-VI families, and their corresponding core/shell hetero-structures; (b) Van der Waals materials with general chemical formula MPX3, MPX4 and M2X3 (M=transition metals, X=chalcogenides); (c) Two- and three-dimensional perovskite compounds (e.g., ABX3, A=Cs+, Ma+; B=Pb, Bi; X=halides). All mentioned materials are of a large interest for implementation in various opto-electronic, spin-electronics, memory and quantum-computation.

The pristine materials, or/and their magnetically doped derivatives, exhibit a unique combination between optical properties and magnetic effects. A few types of magnetic properties are summarized in Scheme 2:  The long-range magnetic order, such as ferromagnetism, anti-ferromagnetism or special spin textures (a) are gaining special stabilization by the size confinement, with 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

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(d). The mentioned intrinsic field leads to a lift of energy or momentum degeneracy at band-edge states, all leading to selective spin orientation at the excited state and consequent helicity of a recombination emission. The investigations are carried out by following the behavior of spins as a designation for the electronic properties, using complex optical and magneto-optical spectroscopy methodologies, including circularly polarized magneto-photoluminescence, optically detected magnetic resonance (ODMR), thermal and microwave modulated photoluminescence, all when monitoring an ensemble or a single nanostructure at variable temperatures, as well as combining microscopy and spectroscopy e.g., confocal/AFM or confocal/ODMR, some of which are home-made assemblies with a world-wide uniqueness.  The experimental work is corroborated by theoretical modeling, including band structure calculations, spin interactions, Auger processes, in addition to carrier transport via small nanostructure assemblies. The accumulated knowledge is applied to development of prototype nanostructure-based devices, such as solar cells, Q-switch, optical gain, spin-filter and spin FET devices. A few exemplary fundamental projects are discussed below.

Materials

Nanostructures were produced either by colloidal chemical procedures or by vapor transport growth: (a) Colloidal quantum dots, rodes, wires, platelets of CdSe, CdTe, PbSe, PbS coated with organic ligands or by epitaxial layer of another semiconductor, known also 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) special efforts for the preparation of core and core/shell structures with a reduced toxicity, such as SnTe, SnTe/PbSe, InSb, In2S3; (d) Mn+2 doped core/shell nano-structures (dots, rods, nanoplatelets), placing dopants either at the core or at the shell; (e) Layered semiconductors from transition metal dichalcogenides or iodides (SnS2, PbI2, Bi2I3, In2S3) and metal phosphore-tri-chalcogenides (MPX3, M=Mn, Fe, Ni, Mg, Cd); (f) Chemically prepared Perovskites dot, rods and platelets; (g) Spray prepared nanostructures; (h), Metallic/magnetic nanostructures (g-Fe2O3); and their complex structures with CQDs; (i) Ordered or disordered packing of nanostructures, or dispersion with extremely high dilution for the isolation of a single particle.

Methodologies

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 magentic resonance of an isolated single nanostructure). Experimental set-up or/and representative results are depicted in Figure 2.

Theory

The electronic band structure calculations of core and core/shell quantum dot and rods were calculated, using an effective mass approximation method. A distinct treatment beyond state-of-the-art was included, considering strain effects, their relaxation by a soft boundary and graded composition around core/shell interface, finite external barrier and dielectric confinement. Later, Coulomb interactions, exchange interactions, cross section for absorption and complex Auger processes, (in collaboration with Prof. A. Efros, USA), spin alignment and polarization effect were calculated and compared with experimental results. Another study included electrical conductivity in double quantum dots, emphasizing the influence of the inter-dot distance created by the capping ligands and their influence on the connectivity properties, showing exceptional phenomena such as induced recoil or negative resistance induced by application of charge or voltage (in collaboration with Prof. U. Peskin, Technion, Israel). Most recent study, focuses on the influence of spin-obrit coupling and symmetry breaking forming the Rashba effect, which induced electronic band split and the generation of polarized transitions (in collaboration with Prof. Andrew Rappe, Pennsylvania University). The theoretical tools are implemented now a days in various current projects under consideration.