Solid-state defects have been promising quantum sensors for detecting physical quantities such as magnetic and electric fields, pressure, and temperature. The NV center has been a standout candidate since its first discovery in 1997. Techniques such as Optically Detected Magnetic Resonance (ODMR), Hahn echo measurements, and dynamical decoupling have been developed for sensing purposes. However, due to the presence of spin noise, including spins in the bulk and on the surface, the coherence of the NV center is limited. This limitation affects the sensitivity of the NV center, particularly in magnetic field sensing. After almost 20 years of development, many schemes and techniques have been devised to produce ultra-high-quality NV centers for sensing purposes. In this thesis, we will introduce the basic principles of ODMR sensing with NV centers and their coherence properties. We will also discuss the possible spin noises that cause NV centers to decoherence and introduce some currently existing methods to eliminate spin noise. We then propose our novel method using graphene to eliminate surface free electron spins, showing great compatibility with various quantum sensing applications. Lastly, we will introduce a newly discovered defect, the VB center (VB), which exists in the 2D material h-BN, and discuss its coherence properties.