Single crystals of oxygen-incorporated ZnS (i.e., ZnS
(1–
x
)
O
x
series) are environment-friendly wide-band-gap semiconductors available
for light-emitting devices and solar cell use. The series of materials
has considerable potential for use in visible ultraviolet areas with
flexibility for palette emissions. In this study, we grow oxygen-incorporated
ZnS series crystals by chemical vapor transport method with iodine
(I
2
) as the transport agent. Three different oxygen-incorporated
crystals of undoped ZnS, ZnS
0.94
O
0.06
, and ZnS
0.88
O
0.12
are studied. Through structural studies,
ZnS doped with oxygen crystallizes in the main sphalerite phase and
a little wurtzite structure. The lattice constants of the major cubic
phase are determined to be
a
= 5.43 Å (ZnS),
5.41 Å (ZnS
0.94
O
0.06
), and 5.39 Å
(ZnS
0.88
O
0.12
). Three band-edge excitonic transitions
are simultaneously detected by thermoreflectance measurement for the
ZnS, ZnS
0.94
O
0.06
, and ZnS
0.88
O
0.12
series samples. The energy positions of the band-edge
transitions decrease as the oxygen content increases in the ZnS
(1–
x
)
O
x
series. Defect-state and surface-state emissions, including sulfur
vacancy, oxygen vacancy, zinc interstitial, and so forth, can emit
approximately full-color spectra from the near band edge of the ZnS
(1–
x
)
O
x
series crystals. With adjusting the oxygen content, the ZnS
(1–
x
)
O
x
can be a series of color-palette luminescence matters that applied
for fluorescent display or light-emitting device.
ReS and ReSe have recently been enthusiastically studied owing to the specific in-plane electrical, optical and structural anisotropy caused by their distorted one-layer trigonal (1 T) phase, whereas other traditional transition-metal dichalcogenides (TMDCs, e.g. MoS and WSe) have a hexagonal structure. Because of this special property, more and versatile nano-electronics and nano-optoelectronics devices can be developed. In this work, 2D materials in the series ReS Se (0 ≤ x ≤ 2) have been successfully grown by the method of chemical vapor transport. The direct and indirect resonant emissions of the complete series of layers can be simultaneously detected by polarized micro-photoluminescence (μPL) spectroscopy when the thickness of the ReS Se is greater than ∼70 nm. When it is less than 70 nm, only three direct excitonic emissions-E, E and E-are detected. For the thick (bulk) ReS Se , more stacking of the ReX monolayers even flattens and shifts the valence-band maximum from Γ to the other K- or M-related points, thus leading to the coexistence of direct and indirect resonant light emissions from the c-plane ReX. The transmittance absorption edge of each bulk ReX (a few microns thick) usually has a lower energy than those of the direct E and E excitonic emissions to form indirect absorption. The coexistence of direct and indirect emissions in ReX is a unique characteristic of a 2D layered semiconductor possessing triclinic low symmetry.
Two-dimensional
(2D) semiconductors play a crucial role in high-efficiency
photocatalysts because of their high surface-to-volume ratio. The
surface property is a key part of photocatalysis. In this work, the
enhanced photocatalytic behavior of the layered ReS2 with
optical polarization along the Re4 nano-diamond-chain (DC)
direction (b axis) has been demonstrated. The unpolarized
photoconductivity (PC) response of ReS2 with an applied
bias along the b axis is approximately 1 order higher
than that of the applied bias perpendicular to the b axis. The polarization-dependent PC spectra of E ∥ b also reveal a higher photoresponsivity
with respect to those measured along the E ⊥ b polarization for the layered ReS2. This result
indicates that stronger polarization dipoles as well as a larger amount
of photogenerated carriers and surface states can contribute to the C-plane ReS2 under the illumination of E ∥ b polarized photons. With the
special axial effect, the layered ReS2 2D photocatalyst
shows much faster degradation rates of 5.6 and 12.3 than the other
transition-metal dichalcogenides of TaS2 and MoS2 for the degradation of methylene blue (MNB) solution. For the polarization-dependent
photodegradation test, the degradation rate of illuminated E ∥ b polarized photons is also
approximately 12 times faster than that of the illuminated E ⊥ b polarized light in a 25 μM
MNB solution. The enhanced photocatalytic behavior of ReS2 along the DC also shows a peak photoreponse of ∼25 μV
detected in the polarized photovoltaic spectrum of the 0.5 μM
MNB dye-sensitized solar cell positioned at ∼1.99 eV. The formation
of a nano-DC and a one-layer trigonal crystalline phase is beneficial
for the versatile energy applications of ReS2.
We report on optical
examination of structural properties of cubic-In2O3 (c-In2O3) degenerate-semiconducting-oxide
thin-film nanostructures grown
on Si (100) substrates using vapor transport method driven with vapor–liquid–solid
mechanism. Two different types of dense (A) and loose (B) c-In2O3 thin-film nanorods have been,
respectively, characterized by photoluminescence (PL), time-resolved
photoluminescence (TRPL), thermoreflectance (TR), and Hall and resistivity
measurements. Strong excitonic emissions detected from the PL results
of the A and B samples identify the high-crystalline quality of the
arrow-type c-In2O3 nanorods.
Theoretical modeling fits on the TRPL results of the A and B samples
characterize and identify defect mechanisms of the nanostructured
thin films. Near-band-edge and above-band-edge transitions such as
excitons, direct band gap, and Burstein–Moss shift (above direct
gap) in the degenerate A and B c-In2O3 have also been detected by TR measurement. Field-emission
scanning electron microscopy and field-emission transmission electron
microscopy have been implemented to facilitate the observed optical
measurement results of the c-In2O3. All experimental analyses show that the TR, PL, and TRPL
measurements are powerful techniques for optical characterization
of structure and optoelectronics properties of the c-In2O3 degenerate nanostructured thin film
available for transparent optoelectronics use.
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