Alkalides
with large nonlinear optical (NLO) responses exhibit
broad applications in the electro-optical device field. In the present
work, based on alkali (Li and Na) in conjunction with alkaline-earth
(Ca) atoms doped into facially polarized all-cis 1,2,3,4,5,6-hexafluorocyclohexane
(C6F6H6), we first reported two facially
polarized Janus-type alkalides as an external electric field (EEF)-induced
second order NLO switches M–LCaL–M (M = Li or Na, L
= C6F6H6). The two 4s electrons of
the Ca atom are, respectively, pushed out by the negative fluorocarbon
face of one L and each of them concentrate on one alkali atom and
combine with the s electron of the later to form excess electron pair.
Owing to the two excess electron pairs [highest occupied molecular
orbital (HOMO) and HOMO – 1], the novel alkalides M––LCa2+L–M– is formed.
Interestingly, with continuous increasing of EEF magnitude, the centrosymmetric
M––LCa2+L–M– bearing two excess electron pairs is obviously broken and a long-range
charge transfer is exhibited gradually from one end of the alkali
atom through the middle LCaL to the other end of it. Meanwhile, the influence of EEF brings a large static electronic first hyperpolarizability
from 0 (EEF = 0, off form) to 59 826 (M = Li, EEF = 19 × 10–4 au, on form) or 64 231 au (M = Na, EEF = 12 ×
10–4 au, on form). They also have the largest vibrational
first hyperpolarizabilities (on form). These results show
that alkalides M––LCa2+L–M– have potential application for NLO materials as well
as exhibit advantages such as high sensitivity, being fast, and having
reversible switching.
Being
a metallic transition-metal dichalcogenide, monolayer vanadium
diselenide (VSe2) exhibits many novel properties, such
as charge density waves and magnetism. Its interfaces with other materials
can potentially be used in device applications as well as for manipulating
its intrinsic properties. Here, we present a scanning tunneling microscopy
and synchrotron-based X-ray photoemission spectroscopy study of the
surface charge-transfer doping using efficient electron-withdrawing
and electron-donating materials, that is, molybdenum trioxide (MoO3) and potassium (K), on the molecular beam epitaxy-grown monolayer
VSe2 on highly oriented pyrolytic graphite (HOPG). We demonstrate
that monolayer VSe2 is immune to MoO3- and K-doping
effects. However, at the monolayer edges where the local chemical
reactivity is higher because of Se deficiency, MoO3 is
seen to react with VSe2 to form molybdenum dioxide (MoO2) and vanadium dioxide (VO2). Compared to the obvious
charge-transfer doping effects of MoO3 and K on HOPG, the
electronic structure of monolayer VSe2 is barely perturbed.
This is attributed to the large density of states at the Fermi level
of monolayer VSe2 carrying the metallic character. This
work provides new insights into the chemical and electronic properties
of monolayer VSe2, important for future VSe2-based electronic device design.
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