Pushing the Band Gap Envelope of Quasi-Type II Heterostructured Nanocrystals to Blue:ZnSe/ZnSe_(1-X)Te_(X)/ZnSe Spherical Quantum Wells

Quasi-type II heterostructured nanocrystals(NCs)have been of particular interest due to their great potential for controlling the interplay of charge carriers.However,the lack of material choices for quasi-type II NCs restricts the accessible emission wavelength from red to near-infrared(NIR),which hinders their use in light-emitting applications that demand a wide range of visible colors.Herein,we demonstrate a new class of quasi-type II nanoemitters formulated in ZnSe/ZnSe_(1-X)Te_(X)/ZnSe seed/spherical quantum well/shell heterostructures(SQWs)whose emission wavelength ranges from blue to orange.In a given geometry,ZnSe_(1-X)Te_(X) emissive layers grown between the ZnSe seed and the shell layer are strained to fit into the surrounding media,and thus,the lattice mismatch between ZnSe_(1-X)Te_(X) and ZnSe is effectively alleviated.In addition,composition of the ZnSe_(1-X)Te_(X) emissive layer and the dimension of the ZnSe shell layer are engineered to tailor the distribution and energy of electron and hole wave functions.Benefitting from the capabilities to tune the charge carriers on demand and to form defectfree heterojunctions,ZnSe/ZnSe_(1-X)Te_(X)/ZnSe/ZnS NCs show near-unity photoluminescence quantum yield(PLQY>90%)in a broad range of emission wavelengths(peak PL from 450nm to 600 nm).Finally,we exemplify dichromatic white NC-based light-emitting diodes(NC-LEDs)employing the mixed layer of blue-and yellow-emitting ZnSe/ZnSe_(1-X)TeX/ZnSe/ZnS SQW NCs.

Quasiparticle interference and impurity resonances on WTe2

Using scanning tunneling microscopy/spectroscopy(STM/STS),we examine quasiparticle scattering and interference properties at the surface of WTe2.WTe2,layered transition metal dichalcogenide,is predicted to be a type-ll Weyl semimetal.The Weyl fermion states in WTe2 emerge as topologically protected touching points of electron and hole pockets,and Fermi arcs connecting them can be visible in the spectral function on the surface.To probe the properties of surface states,we have conducted low-temperature STM/STS(at 2.7 K)on the surfaces of WTe2 single crystals.We visualize the surface states of WTe2 with atomic scale resolution.Clear surface states emerging from the bulk electron pocket have been identified and their connection with the bulk electronic states shows good agreement with calculations.We show the interesting double resonance peaks in the local density of states appearing at localized impurities.The low-energy resonant peak occurs near the Weyl point above the Fermi energy and it may be mixed with the surface state of Weyl points,which makes it difficult to observe the topological nature of the Weyl semimetal WTe2.