A domain wall, as a device, can bring about a revolution in developing manipulation of semiconductor heterostructures devices at the atom scale. However, it is a challenge for these new devices to control domain wall motion through insulator-metal transition of correlated-electron materials. To fully understand and harness this motion, it requires visualization of domain wall dynamics in real space. Here, domain wall dynamics in VO2 insulator-metal phase transition was observed directly by in situ TEM at atom scale. Experimental results depict atom scale evolution of domain morphologies and domain wall exact positions in (202) and (040) planes referring to rutile structure at 50°C. In addition, microscopic mechanism of domain wall dynamics and accurate lattice basis vector relationship of two domains were investigated with the assistance of X-ray diffraction, ab initio calculations and image simulations. This work offers a route to atom scale tunable heterostructure device application.
Transition metal dichalcogenides (TMDC's) usually exhibit layered polytypic structures due to the weak interlayer coupling. 2H-NbSe2 is one of the most widely studied in the pristine TMDC family due to its high superconducting transition temperature (Tc = 7.3K) and the occurrence of a charge-density wave (CDW) order below 33 K. The coexistence of CDW with superconductivity poses an intriguing open question about the relationship between Fermi surface nesting and Cooperpairing. Past studies of this issue have mostly been focused on doping 2H-NbSe2 by 3d transition metals without significantly changing its crystal structure. Here we replaced the Se by Te in 2H-NbSe2 in order to design a new 1T polytype layered TMDC NbSeTe, which adopts a trigonal structure with space group P3 ̅ m1. We successfully grew large size and high-quality single crystals of 1T-NbSeTe via the vapor transport method using I2 as the transport agent. Temperature-dependent resistivity and specific heat data revealed a bulk Tc at 1.3 K, which is the first observation of superconductivity in pure 1T-NbSeTe phase. This compound enlarged the family of superconducting TMDC's and provides an opportunity to study the interplay between CDW and superconductivity in the trigonal structure.
High-pressure
solid-state synthesis advances boost discoveries
of new materials and unusual phenomena but endures stringent recipe
conditions, poor yield, and high cost. A methodological approach for
accelerated and precisely high-pressure synthesis is therefore highly
desired. Here, we take the exotic double-perovskite-related nonmagnetic
Li2
B
+4
B′+6O6 as an example to show the pipeline of data-mining,
high-throughput calculations, experimental realization, and chemical
interception of metastable phases. A total of 140 compounds in 7 polymorph
categories were initially screened by the convex hull, which left
∼50% candidates in chemical space on the phase diagram of pressure-dependent
polymorph evolution. Li2TiWO6 and Li2TiTeO6 were singled out for experimental testing according
to the predicted map of crystal structure, function, and synthesis
parameters. Computation on surface energy effect and interfacial chemical
strain suggested that the as-made high-pressure R3-Li2TiTeO6 polymorph cannot be intercepted
below a critical nanoscale but can be stabilized in heterojunction
film on a selected compressive substrate at ambient pressure. The
developed methodology is expected to accelerate the big-data-driven
discovery of generic chemical formula-based new materials beyond perovskites
by high-pressure synthesis and shed light on the large-scale stabilization
of metastable phases under mild conditions.
The
atomic diffusion in transition metal dichalcogenides (TMDs)
van der Waals heterojunctions (HJs) strongly modifies their optoelectronic
properties in the nanoscale. However, probing such localized properties
challenges the spatial resolution and the sensitivity of a variety
of analytic tools. Herein, a multimodal nanoscopy (based on tip enhanced
Raman spectroscopy (TERS) and photoluminescence (TEPL)) combined with
the Kelvin probe force microscopy (KPFM) method was used to probe
such nanoscale localized optoelectronic properties induced by atomic
diffusion. Chemical vapor deposition (CVD)-grown lateral bilayer (2L)
WS2/MoS2 HJs were imaged with a spatial resolution
better than 40 nm via TERS and TEPL mapping by using intrinsic Raman
and photoluminescence (PL) peaks. The contact potential difference
(CPD), capacitance, and PL variation in a nanoscale vicinity of the
HJ interface can be correlated to the local stoichiometry variation
determined by TERS. The diffusion coefficients of W and Mo were obtained
to be ∼0.5 × 10–12 and ∼1 ×
10–12 cm2/s, respectively, by using Fick’s
second law. The obtained results would be useful to further understand
the localized optoelectronic response of the TMDs HJs.
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