Debangshu Mukherjee

Freestanding van der Waals Heterostructures of Graphene and Transition Metal Dichalcogenides

Vertical stacking of two-dimensional (2D) crystals has recently attracted substantial interest due to unique properties and potential applications they can introduce. However, little is known about their microstructure because fabrication of the 2D heterostructures on a rigid substrate limits ones ability to directly study their atomic and chemical structures using electron microscopy. This study demonstrates a unique approach to create atomically thin freestanding van der Waals heterostructures-WSe2/graphene and MoS2/graphene-as ideal model systems to investigate the nucleation and growth mechanisms in heterostructures. In this study, we use transmission electron microscopy (TEM) imaging and diffraction to show epitaxial growth of the freestanding WSe2/graphene heterostructure, while no epitaxy is maintained in the MoS2/graphene heterostructure. Ultra-high-resolution aberration-corrected scanning transmission electron microscopy (STEM) shows growth of monolayer WSe2 and MoS2 triangles on graphene membranes and reveals their edge morphology and crystallinity. Photoluminescence measurements indicate a significant quenching of the photoluminescence response for the transition metal dichalcogenides on freestanding graphene, compared to those on a rigid substrate, such as sapphire and epitaxial graphene. Using a combination of (S)TEM imaging and electron diffraction analysis, this study also reveals the significant role of defects on the heterostructure growth. The direct growth technique applied here enables us to investigate the heterostructure nucleation and growth mechanisms at the atomic level without sample handling and transfer. Importantly, this approach can be utilized to study a wide spectrum of van der Waals heterostructures.

Polar Oxides without Inversion Symmetry through Vacancy and Chemical Order

One synthetic modality for materials discovery proceeds by forming mixtures of two or more compounds. In transition metal oxides (TMOs), chemical substitution often obeys Vegards principle, and the resulting structure and properties of the derived phase follow from its components. A change in the assembly of the components into a digital nanostructure, however, can stabilize new polymorphs and properties not observed in the constituents. Here we formulate and demonstrate a crystal-chemistry design approach for realizing digital TMOs without inversion symmetry by combining two centrosymmetric compounds, utilizing periodic anion-vacancy order to generate multiple polyhedra that together with cation order produce a polar structure. We next apply this strategy to two brownmillerite-structured TMOs known to display centrosymmetric crystal structures in their bulk, Ca2Fe2O5 and Sr2Fe2O5. We then realize epitaxial (SrFeO2.5)1/(CaFeO2.5)1 thin film superlattices possessing both anion-vacancy order and Sr and Ca chemical order at the subnanometer scale, confirmed through synchrotron-based diffraction and aberration corrected electron microscopy. Through a detailed symmetry analysis and density functional theory calculations, we show that A-site cation ordering lifts inversion symmetry in the superlattice and produces a polar compound. Our results demonstrate how control of anion and cation order at the nanoscale can be utilized to produce acentric structures markedly different than their constituents and open a path toward novel structure-based property design.

26.5 Terahertz electrically triggered RF switch on epitaxial VO2-on-Sapphire (VOS) wafer

An electrically triggered VO2 RF switch with a record switching cut off frequency (FCO) of 26.5THz was demonstrated. The switch exhibits an isolation better than 35dB and a low 0.5dB insertion loss up-to 50GHz. The switch features a highly linear response with 1-dB compression point (PidB) better than 12dBm and output third-order intercept point (OIP3) better than 44dBm. The fast insulator to metal-transition (IMT) of the VO2 enables the switch to have an electrical-turn on delay of less than 25ns.

Aberration Corrected STEM Imaging of Domain Walls in Congruent LiNbO 3

LiNbO3 is a ferroelectric crystal at room temperature belonging to the R3c space group, and has a Curie temperature of 1140C [1]. The spontaneous electrical polarization of LiNbO3 is 71 μC/cm [2] while it’s piezoelectric modulus, the d33 value, is 31.5pm/V [3]. The combination of high Curie temperature, electrical polarization and d33 has led to applications as diverse as pyroelectric sensors, ferroelectric memory, quasi-phase matched second harmonic generators, optical switching etc. A multitude of these applications depend on the precise manipulation and control of domain walls in LiNbO3. The exact structure of the domain wall is however a subject of ongoing controversy on whether it is a pure Ising wall, or has a mixed Ising-Neel-Bloch nature [4]. In this work, for the first time we image domain walls with atomic resolution to measure displacements across them.

Statistical Measurement of Polar Displacements in Complex Oxides

Ferroelectrics are crystals with a spontaneous, switchable electrical polarization [1]. According to the modern theory of ferroelectricity, the polarization is a function of the polar displacement and the Born effective charge [2]. In complex oxides this is defined as second-order Jahn–Teller displacements in proper ferroelectrics, where the primary order parameter is the B site displacement with respect to the oxygen octahedral cage in ABO3 perovskites [3]. In improper ferroelectrics, this is a first order Jahn–Teller distortion, where the rotation of the oxygen octahedral cages results in the absence of inversion symmetry [4]. SubÅngström resolution imaging with aberration correction has allowed the direct measurement of atom positions with scanning transmission electron microscopy [5]. Aberration corrected scanning transmission electron microscopy (STEM) has been used to image the evolution of domain walls in BiFeO3 thin films, and have been used to image the presence of novel phenomena like polarization vortices in oxide superlattices [6, 7].