Systematic studies are performed to explore the electronic, optical, and magnetic features of three different two-dimensional materials, namely, black phosphorus, graphene, and manganese thiophosphite. These three materials have their own advantages and deficiencies. For example, graphene is the most stable one with the highest carrier mobility. However, the application of graphene is limited by the absence of energy band gap leading to a exceptionally low on/off ratio of graphene-based devices and nonappearance of photoluminescence signal. Thus, black phosphorus, which is an emerging two-dimensional material, has attracted attention. Black phosphorus is the most stable allotrope of phosphorus and is predicted to possess high carrier mobility of a few thousands of cm
2V
-1s
-1 at room temperature. More importantly, thickness-dependent direct energy band gaps lead to high on/off ratio (~ 10
5) and tunable optical properties. These characteristics render the black phosphorus an ideal candidate for fundamental studies and engineering applications. Nevertheless, graphene or black phosphorus does not exhibit two-dimensional magnetic properties. Thus, transition metal chalcogenphosphite (TMC), which is a new group of two-dimensional materials, has been explored. TMCs have intrinsic two-dimension magnetic structures. In this thesis, few-layered MnPS
3, which exhibit isotropic Heisenberg type anti-ferromagnetic order, is investigated. The anti-ferromagnetic order and hexagonal lattice structure couples the valley and spin degrees with magnetic orders. This coupling facilitates the use of MnPS
3 as an optimal platform to explore the new degrees of electron freedom. The main results on the three different two-dimensional materials reported in this thesis are summarized as follows.
Charge density wave phase transition is demonstrated on the surface of electrostatically
doped multilayer graphene when the Fermi level approaches the M points in the first
Brillouin zone of graphene band structure. The M points are also known as van Hove
singularities where the densities of states diverge. Ionic liquid gating technique is applied
to tune the Fermi surface to van Hove singularities because of the ultrahigh efficiency of
the technique in inducing charge carriers. The occurrence of charge density wave phase
transition is demonstrated by electrical transport measurements and optical measurements
in electrostatically doped multilayer graphene. A sudden change in the graphene channel
resistance at T
m = 100K and the splitting of Raman G peak (1580 cm
-1) suggests the
charge density wave phase transition. The splitting of the Raman G peak indicates
the lifting of in-plane optical phonon branch degeneracy. The non-degenerate phonon
branches imply a lattice reconstruction of graphene, that is, the charge density wave
phase transition.
A four-step strategy is performed to access the quantum Hall regime in black phosphorus.
The charge carrier scattering mechanism in black phosphorus is first explored, the
localization effects from the charged impurities are found to hinder the high charge carrier
mobility. Then, a new device fabrication process is developed by protecting black phosphorus
flakes with hexagonal boron nitride (BN) under vacuum. A record high mobility of
45000 cm
2V
-1s
-1 is achieved. After that, we successfully control the polarities of black
phosphorus conducting channels by selecting contact metals with matched work function.
Combining the advanced fabrication process with contact metal engineering, high-quality
ambipolar conducting channels are achieved in black phosphorus two-dimensional system.
Finally, ambipolar quantum Hall effect is observed in black phosphorus when the
high-quality ambipolar devices are subjected to a high magnetic field of up to 30 T. Critical
parameters, such as effective mass, Lande g-factor, spin-susceptibility, and quantum
life time, are extracted based on Lifshitz-Kosevich formula. Specifically, an asymmetric
transport behavior is observed between spin-up and spin-down charge carriers in black
phosphorus.
A systematic experimental study on an antiferromagnetic honeycomb lattice of MnPS
3
is performed. The antiferromagnetic honeycomb lattice couples the valley degree of freedom
to a macroscopic antiferromagnetic order. The crystal structure of MnPS
3 is identified
by high-resolution scanning transmission electron microscopy. Layer-dependent angle-resolved polarized Raman fingerprints of the MnPS
3 crystal are obtained. The
Raman peaks at 383 cm
-1 exhibit 100% polarity. Temperature dependences of anisotropic
magnetic susceptibility of MnPS
3 crystal are measured in superconducting quantum
interference device. Magnetic parameters, such as effective magnetic moment, are extracted
from the mean field approximation model. Ambipolar electronic transport channels
in MnPS
3 are realized by liquid gating technique. The conducting channel of MnPS
3
offers a unique platform to explore the spin/valleytronics and magnetic orders in two
dimensions.
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