5 research outputs found
Preparation of graphene, bismuth chalcogenide and their heterostructures with application in photonics and optoelectronics
Graphene, a novel 2-D
allotrope form of carbon, has triggered intensive research interests in 2-D materials. 2-D materials’ extraordinary properties
promise varies applications, such as electronics, optics and optoelectronics. Particularly,
graphene and bismuth chalcogenides (Bi<sub>2</sub>Se<sub>3</sub>, Bi<sub>2</sub>Te<sub>3</sub> et. al.), which share similar Dirac bandgap
structures and exotic surface states, are outstanding candidates in potential applications of
broadband optoelectronic, plasmonic devices and future on-chip devices. Though researchers have
dedicated their efforts in the 2-D materials, the attention being paid to the graphene and
bismuth chalcogenides based materials remains low, especially in their large production and
optoelectronic device applications. <br>
   This research dissertation starts with the preparation of
high quality graphene, bismuth chalcogenide nanocrystals and their heterostructure. By
taking advantages of the <i>in situ</i> Powder X-ray diffraction technique, better understanding in the
growth mechanism of bismuth chalcogenides nanoplatelets and its graphene heterostructure
has been obtained. Step by step growth mechanism is revealed and discussed. Thus large-scale
prepared graphene and bismuth chalcogenides hybrid material has been integrated into a free
standing thin film, which is further demonstrated as a broadband photodetector. On the
other hand, it is found that the graphene and Bi<sub>2</sub>Te<sub>3</sub> heterostructure films can effectively
enhance plasmon resonance magnitude in its FTIR spectrum by increasing light-matter
interactions. In order to better observe and understand the plasmonic resonance modes on these
materials, the later sections of this dissertation investigated resonance modes on graphene
surfaces from both far-field and near-field. The results show that the light-matter interaction
can be further enhanced by modifying the geometry of the surface and the surface plasmon
can be guided in a controlled manner. It is believed, this dissertation paves way for the
photonic and optoelectronic researches of graphene, bismuth chalcogenides and their heterostructures
Solvothermal Growth of Bismuth Chalcogenide Nanoplatelets by the Oriented Attachment Mechanism: An in Situ PXRD Study
Ultrathin two-dimensional bismuth
chalcogenide materials have received
substantial research attention due to their potential applications
in electronics and optoelectronics. While solvothermal synthesis is
considered to be one of the most promising methods for large-scale
production of such materials, the mechanisms that govern the crystallization
during solvothermal treatment are still poorly understood. In this
work, the solvothermal syntheses of Bi<sub>2</sub>Se<sub><i>x</i></sub>Te<sub>3–<i>x</i></sub> (<i>x</i> = 0–3) hexagonal nanoplatelets were monitored by synchrotron-based
in situ powder X-ray diffraction, which enabled investigation of crystallization
curves, lattice parameters, and crystal size evolution under a variety
of synthesis conditions. On the basis of the crystallization curves
and crystal size evolution, a general 3-step crystallization process
has been deduced: (1) An induction period for the dissolution of the
precursor and nucleation of Bi<sub>2</sub>Se<sub><i>x</i></sub>Te<sub>3–<i>x</i></sub>, followed by (2) rapid
growth of planar crystals through the oriented attachment, and finally
(3) a diffusion-controlled slow growth step consuming the remaining
precursor from the solution. Oriented attachment is very effective
for the growth of binary composites, resulting in a high yield of
large planar crystals; however, it is much less effective for the
growth of ternary composites due to lattice mismatch of the nuclei
formed at the induction period, producing much smaller crystals accompanied
by a limited yield of large planar crystals. Additionally, three intermediate
phases (Bi<sub>2</sub>TeO<sub>5</sub>, Bi<sub>2</sub>SeO<sub>5</sub>, and Na<sub>2</sub>SeO<sub>3</sub>) were observed that played an
important role in controlling the phase separation as well as the
composition of the final ternary compounds
Mechanically-Assisted Electrochemical Production of Graphene Oxide
Graphene
oxide (GO) is promising for a variety of applications
due to its excellent dispersibility and processability. However, current
chemical oxidation routes have several drawbacks, including the use
of explosive oxidizing agents, residual metal ions contaminations,
and the creation of irreparable hole defects on the GO sheet. The
electrochemical exfoliation and oxidation of graphite is a potentially
greener approach without the need for extensive purification steps.
Most reported electrochemical methods employ a single preformed bulk
graphite as electrode, which limits their scalability, reproducibility,
and degree of oxidation. Herein, we reported a novel mechanically
assisted electrochemical method to produce graphene oxide directly
from graphite flakes. The electrochemically derived graphene oxide
(EGO) shows a good degree of oxidation but with less physical defects
than chemically derived graphene oxide (CGO). EGO has good dispersibility
in water and various solvents and, in particular, displays better
long-term stability in ethanol when compared with CGO. Notably, unlike
conventional CGO, EGO can undergo facile thermal conversion at 200
°C in air to conductive thermally processed EGO, which is highly
desirable for heat/chemical-sensitive applications
Two-Dimensional CH<sub>3</sub>NH<sub>3</sub>PbI<sub>3</sub> Perovskite: Synthesis and Optoelectronic Application
Hybrid organic–inorganic perovskite
materials have received
substantial research attention due to their impressively high performance
in photovoltaic devices. As one of the oldest functional materials,
it is intriguing to explore the optoelectronic properties in perovskite
after reducing it into a few atomic layers in which two-dimensional
(2D) confinement may get involved. In this work, we report a combined
solution process and vapor-phase conversion method to synthesize 2D
hybrid organic–inorganic perovskite (<i>i.e.</i>,
CH<sub>3</sub>NH<sub>3</sub>PbI<sub>3</sub>) nanocrystals as thin
as a single unit cell (∼1.3 nm). High-quality 2D perovskite
crystals have triangle and hexagonal shapes, exhibiting tunable photoluminescence
while the thickness or composition is changed. Due to the high quantum
efficiency and excellent photoelectric properties in 2D perovskites,
a high-performance photodetector was demonstrated, in which the current
can be enhanced significantly by shining 405 and 532 nm lasers, showing
photoresponsivities of 22 and 12 AW<sup>–1</sup> with a voltage
bias of 1 V, respectively. The excellent optoelectronic properties
make 2D perovskites building blocks to construct 2D heterostructures
for wider optoelectronic applications
Graphene–Bi<sub>2</sub>Te<sub>3</sub> Heterostructure as Saturable Absorber for Short Pulse Generation
Rapid
progresses have been achieved in the photonic applications
of two-dimensional materials such as graphene, transition metal dichalcogenides,
and topological insulators. The strong light–matter interactions
and large optical nonlinearities in these atomically thin layered
materials make them promising saturable absorbers for pulsed laser
applications. Either Q-switching or mode-locking pulses with particular
output characteristics can be achieved by using different saturable
absorbers. However, it remains still very challenging to produce saturable
absorbers with tunable optical properties, in particular, carrier
dynamics, saturation intensity as well as modulation depth, to suit
for self-starting, high energy or ultrafast pulse laser generation.
Here we report a new type of saturable absorber which is a van der
Waals heterostructure consisting of graphene and Bi<sub>2</sub>Te<sub>3</sub>. The synergetic integration of these two materials by epitaxial
growth affords tunable optical properties, that is, both the photocarrier
dynamics and the nonlinear optical modulation are variable by tuning
the coverage of Bi<sub>2</sub>Te<sub>3</sub> on graphene. We further
fabricated graphene–Bi<sub>2</sub>Te<sub>3</sub> saturable
absorbers and incorporated them into a 1.5 μm fiber laser to
demonstrate both Q-switching and mode-locking pulse generation. This
work provides a new insight for tailoring two-dimensional heterostructures
so as to develop desired photonic applications