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MXene, a recently discovered family of 2D materials, is well known for its application in energy storage areas due to its excellent properties. However, the low capacity of pristine MXene limits its application in energy storage devices such as lithium ion batteries. To solve this problem, we synthesized a new type of N-doped two-dimensional Nb2CTx MXene. The nitrogen content of N-doped MXene was 4.5 at% after thermal reaction with urea. The introduction of nitrogen into MXene nanosheets increases c-lattice parameter of Nb2CTx MXene from 22.32 A to 34.78 A. And the incorporation of nitrogen into MXene nanosheets leads to superior electrochemical performance. For example, the N-doped Nb2CTx shows an increased reversible capacity of 360 mAh /g at 0.2C, which is much higher than that of the un-doped Nb2CTx MXene (190 mAh/g at 0.2C). And the N-doped Nb2CTx MXene also shows excellent cycling stability (288 mAh/g at 0.5C after 1500 cycles). Our results suggest N-doped Nb2CTx MXene is promising for energy storage applications.

Colloidal Quantum dots (QDs) have been considered as attractive candidates for advanced photovoltaics solar cells due to its solution processability and low-cost fabrication procedure as well as the high theoretical efficiency. In this thesis, the potential application of colloidal lead sulfide (PbS) nanoparticles have been systematically studied for application in hot carrier solar cells. The PbS QDs are synthesized with wet-chemistry methods which demonstrate good monodispersity on both sizes and shapes. To be applied as building blocks, a method based on Langmuir-Blodgett (LB) deposition is developed to assemble the colloidal solution-form PbS QDs into uniform closely packed, ordered solid films with controlled surface morphology and thickness. The fabricated LB QD films are also great candidates for the fabrication of energy selective contacts for hot carrier solar cells. Double barrier resonant tunneling diodes fabricated based on the LB QDs monolayer film demonstrated excellent resonant tunneling features both at low temperature and room temperature with narrow full-width half maximum (20 mV) and high quality factor (QF=29000) and hence proved to be employed as promising material and method for energy selective contact fabrication. The carrier cooling dynamics of the PbS QDs solid films are studied with transient and steady-state optical spectroscopy. By using power and temperature dependent continuous wave photoluminescence, the carrier temperature and thermalization parameters of the PbS QDs are estimated. By transient absorption spectroscopy, the hot carrier temperature time evolution, the hot carrier cooling time and relaxation dynamics are estimated. The demonstration of generation of non-equilibrium hot carrier population as well as the slowed hot carrier cooling time on order of 100ps suggests that PbS QDs are attractive candidates for hot carrier absorbers as well.