In the case of single element materials, there are already more than ten allotropes of phosphorus being discovered, and new types of allotropes such as blue phosphorus are constantly being discovered until now. Traditionally, the phosphides involved in III-V compounds, such as indium phosphide (InP), gallium phosphide (GaP) and indium gallium phosphide (InGaP), have been used for high-speed, frequency and power electronic devices. Given the versatile ability of phosphorus, the phosphorus-based materials have potential to be used for a wide range of applications.
However, engineering of phosphorus-based materials such as doping, morphological control and nanostructuring is required to improve the properties resulting in the performance of applications. Additionally, investigating the morphology, composition, and atomic structure of engineered phosphorus-based materials is important to uncover how engineering affects property and performance. In this dissertation, I studied phosphorus-based nanomaterials at the atomic scale with transmission electron microscopy (TEM).
Among the phosphorus-based materials, black phosphorus (BP) and copper phosphide (Cu3P) were mainly investigated. The subtopics of this thesis are 1) angstrom-wide Cu intercalation in BP, 2) investigation of nanobubbles using oxygen absorption capacity of BP and 3) nanostructuring of Cu3P by epitaxial and morphological control. As applications as an anode of Li-ion battery and HER, the Cu3P nanoarchitecture showed superior performance compared to a film architecture.
Introduction
Phosphorus-based nanomaterials
- Two-dimensional black phosphorus
- Transition metal phosphide
Understandably, the electronic properties of phosphorus allotropes are also different including superconductivity in the case of the c-P9-11 structure. Indium phosphide (InP) is one of the most researched phosphides for photonics, high power and high frequency electronics due to its high electron carrier mobility and direct band gap12-14. Compared to other 2D materials such as graphene and semiconducting transition metal dichalcogenides (TMDs), BP has a direct thickness-dependent bandgap and high electron mobility together21-24 (Figure 1.3b and c).
Another fascinating property of BP is the anisotropy that arises from its structure, as shown in Figure 1.3a. Moreover, the anisotropic structure affects all the properties of BP, where the peak electron mobility and electronic band structure are also anisotropy21-22, 29 (Fig. 1.3b and c). For example, lithium and sodium ions were found to be intercalated with respect to the orientation of the BP30 structure (Figure 1.3d).
However, volume expansion that reduces cycle stability inevitably occurs due to the conversion reaction and must be resolved before TMPs can be used for energy storage. Fabrication methods for the nanostructure of transition metal phosphides. a) Fabrication of porous Ni3P nanostructure by using polystyrene beads and (b) Fabrication of Cu3P nanostructure by growing CuCl2 as intermediate material.
Modern transmission electron microscopy (TEM)
- Spherical aberration corrected TEM
- Atom-resolved spectroscopies: Energy dispersive X-ray spectroscopy (EDS) &
- In situ TEM
Anisotropic and angstrom-wide Cu intercalation in black phosphorus
- Background and objective
- Experimental methods
- Atomic structure of angstrom-wide Cu intercalated black phosphorus
- Microstructural effects of Cu intercalation in black phosphorus
- Top-down intercalation pathway of Cu intercalation in black phosphorus: rate-limiting
- Angstrom-wide conductive channels in Cu intercalated black phosphorus
- Summary
A 3D image constructed from atomic resolution HAADF-STEM images of Cu intercalated BP at five different zone axes of the BP. b) Perspective atomic model of Cu intercalated BP. HAADF-STEM image of the plan view showing both the undulating (fully relaxed) and straight (not fully relaxed) morphology of the Cu intercalated structure. Other periodicities of missing Cu atoms. a, b) HAADF-STEM images of Cu-intercalated samples with periodicities where the 11th (a) and 7th (b) copper atoms are missing.
Microstructural effects of Cu intercalation in BP. a) Atomic resolution HAADF-STEM image of strain-induced buckling in a Cu-intercalated structure ([100] zone axis). Pristine atomic structure of BP and Cu intercalated structures densely formed in BP. a) HAADF-STEM image of pristine BP in cross section ([100] zone axis). Four superimposed atomic configurations of Cu intercalated structure in single crystal BP. a) Atomic resolution HAADF-STEM image of two different atomic configurations of Cu-intercalated structure at the [100] zone axis.
Mechanism of Cu intercalation in BP: Top-down intercalation as rate-limiting. a-c) HAADF-STEM images of interrupted Cu-intercalated structures in cross section (a, b) and plan view (c). Angstrom wide conducting channel in Cu intercalated BP. a) Atomic model used to calculate the density of states (DOS) and the electronic band structure. b,c) calculated DOS (b) and electronic band structure (c) with the atomic model.
Gas-filled surface nanobubble in black phosphorus liquid cell
- Background and objective
- Experimental methods
- Highly reproducible assembly method of black phosphorus liquid cell
- Stable gas-filled surface nanobubble under e-beam irradiation
- Dynamics of gas-filled surface nanobubble
- Shrinkage dynamics of gas-filled surface nanobubble
- Pinch-off dynamics of one-dimensional surface nanobubble
- Discussion for driving force of nanobubble dynamics under e-beam irradiation
- Summary
-c) HAADF-STEM images of interrupted Cu intercalation structures in cross-sectional view (a, b) and top view (c). Energy barriers of Cu intercalation in BP. a-c) Calculated energy barriers of Cu intercalation in BP along zigzag (a), armchair (b) and top-down (c) directions. Pinch-off dynamics of one-dimensional surface nanobubble. a-e) Sequential TEM images of one-dimensional surface nanobubble where pinch-off occurs at two points (red and green arrows), and (f) neck diameter as a function of time at the two pinch-off points.
Nanoarchitecture of epitaxially grown copper phosphide (Cu 3 P) nanosheets for energy
Background and objective
The thickness of the Cu3P nanosheet was varied in the range of 30 nm to 400 μm by controlling the chamber pressure during growth. During the heat treatment, red phosphorus vaporized and reacted with the surface of the Cu foil to form Cu3P. The lower DB density of the (001) plane means that this plane has higher surface stability than the (100) plane, indicating that growth perpendicular to (001) is slower than that perpendicular to (100).
EDS, XPS, and HAADF-STEM were performed to study the crystal structure of Cu3P nanosheets in detail. The combined results of EDS, XPS and HAADF-STEM confirm the high crystallinity of the grown Cu3P nanoplates. The thicknesses of both nanoplates are in the same range as measured by SEM.
The two epitaxial relationships explain the two characteristic morphological features of the Cu3P nanosheets nanoarchitecture described in Figure 4.7a. Second, Cu3P (210)/Cu (111) epitaxy results in all the vertices of the truncated hexagonal Cu3P nanosheets having the same direction, as shown in Figure 4.7a. Epitaxial bonding between Cu3P nanosheet and surface of Cu foil. a) SEM image of sparsely grown Cu3P nanosheets on the back of the Cu foil.
Lower right images are digital diffractograms of the TEM images. e) Atomic model corresponding to Figure 4.6c and d. By comparing the performance of the nanoarchitectural form (Cu3P-5) with the film-like morphology of Cu3P (Cu3P-300), the nanoarchitectural effect was clarified. However, the HER efficiency of the nanoarchitecture decreased slightly as the number of cycles increased.
The nanoarchitecture of Cu3P nanosheets changed significantly after 1000 cycles, and the nanoarchitecture was reconstructed to form microparticles (Figure 4.11c). This suggests that the morphology of Cu3P is reconstructed and forms microparticles during HER cycling in H2SO4 solution, regardless of the initial morphology. Our experimental results show that the thickness of Cu3P sheets, whose morphology is attributed to different growth rates of each of the Cu3P facets, can be easily tuned by controlling the chamber pressure.
These two relationships determine, respectively, the growth direction and the peak direction of the Cu3P nanosheets. However, the electrocatalytic efficiency of the nanoarchitecture was higher only in the initial HER cycle, but deteriorated due to morphological reconstruction during cycling.
