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 (Bi₂Se₃, Bi₂Te₃ 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 Bi₂Te₃ 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 Bi₂Se_<i>x</i>Te_3–<i>x</i> (<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 Bi₂Se_<i>x</i>Te_3–<i>x</i>, 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 (Bi₂TeO₅, Bi₂SeO₅, and Na₂SeO₃) 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 CH₃NH₃PbI₃ 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>, CH₃NH₃PbI₃) 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^–1 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–Bi₂Te₃ 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 Bi₂Te₃. 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 Bi₂Te₃ on graphene. We further fabricated graphene–Bi₂Te₃ 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